SINGLE-SITE MOLYBDENUM(IV) MEDIATED BOND CLEAVAGE REACTIONS AND LIGAND PARAMETERIZATION USING A CR(VI) NITRIDO PLATFORM By Stephen Andrew DiFranco A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of Chemistry-Doctor of Philosophy 2013 ABSTRACT SINGLE-SITE MOLYBDENUM(IV) MEDIATED BOND CLEAVAGE REACTIONS AND LIGAND PARAMETERIZATION USING A CR(VI) NITRIDO PLATFORM By Stephen Andrew DiFranco The following pages comprise various studies in the area of organometallic chemistry of the transition metals and may be separated into two main projects; exploration of novel molybdenum(IV) bispyrrolide complexes with implications toward the mechanism(s) of catalytic dinitrogen fixation, and electronic and steric parameterization of ligands using a chromium(VI) nitride complexes. − Chapter 1 discusses a novel 2e reduction the N-N bond in disubstuted hydrazines that proceeds via a Mo-hydrazido(1-) moetiy to produce a Mo(VI) nitride and a free secondary amine from N-N bond cleavage. Kinetic studies on this and an analogous Zr(IV) system are presented. The related N-O bond cleavage of o-benzyl hydrazine is also reported. Such single-site bond cleavage reactions may be operative in the enzymatic reduction of dinitrogen, and my find use in future catalytic systems toward the same end. Chapter 2 focuses on the reactivity of the same molybdenum(IV) bispyrrolide with alcohols, giving either mono or multiple substitutions resulting in Mo(IV) alkoxides, or radical C-O bond cleavage commensurate with Mo(VI) bisoxo and C-C bond formation. Additional reactivity and structural studies of molybdenum(IV) bispyrrolides are presented in Chapter 3. Chapter 4 presents the synthesis of 28 new chromium nitride complexes of the form i NCr(NPr 2)2X where X is a halide or anionic ligand bound through O, N or S. A method for using Spin Saturation Magnetization Transfer to measure the kinetics of diisopropylamide rotation is discussed. These barriers of rotation are indicative of the donor ability of the X ligand, and have been reported as the Ligand Donor Parameter (LDP) of each compound. Structural analysis is included of each compound, as it steric analysis using two different systems. Additionally, correlations to Hammett parameters, pKa‘s, 13 C NMR data, electronic spectra, and i the angular overlap model are presented. Chapter 5 explores similar NCr(NPr 2)2X compounds where X is cyclopentadienyl and related compounds. Further substitution induces a hapticity i shift in the bound Cp. Chapter 6 presents a new set of compounds based on the NCr(NPr 2)2X platform where X is an organometallic ligand bound through carbon. Synthesis, structures, LDP‘s and steric analysis are presented. Chapter 7 discusses synthesis of cationic Cr(VI) nitridos i − of the form [NCr(NPr 2)2L][A] where L is a neutral 2e donor and A is a weakly coordinating anion. LDP‘s and a correction for anion effects are presented. Temperature dependent equilibrium in cation formation is reported. Structural and steric analysis of many of the compounds are included. Chapter 8 focuses on the synthesis and structure of a 3-ferrocenyl substituted pyrrolide ligand. Structural, steric and LDP analysis is discussed. In total a series of 58 ligands bound through O, N, P, S and C and halides have been parameterized via the LDP methodology. This list spans both neutral and anionic ligands. Crystal structures and steric analysis for many of these are included as to comprise a comprehensive map of ligation toward transition metals. ACKNOWLEDGEMENTS While the following gives an account of the scientific progress made during my tenure at Michigan State University I would find it incomplete without dedicating at least a few pages to those that helped make it all possible. I would like to start by thanking my family. Their love and encouragement throughout the years has allowed me to go places and do things I never thought I could. I just hope that I can pay dividends on their investment in my future. Next, I would like to thank Dr. Brenna Flannery for her love and support, and being my best friend and sounding board throughout this process. I would like to thank my advisor, Aaron Odom for giving me a great opportunity to join his lab and explore my passion for chemistry. The lessons learned in his lab are invaluable to my past and future successes. His patience and guidance will not soon be forgotten. Likewise, I would like to thank those who served on my committee, Prof. Jim McCusker, Prof. Babak Borhan, Prof. Mitch Smith, and Prof. Viktor Poltavets, for their continuing guidance, support and providing me perspective throughout my graduate career. I am indebted to the Odom lab members who have come before me; Bala Kapil, Doug, Supriyo, Eyal, and Sam. You have taught me to think and act like a scientist. A large thank you to Amila and Nick for being such wonderful colleagues. I will miss your companionship and the long hours together in the lab. I also wish to thank Medha and other students from around the department, especially the McCusker and Poltavets lab members. iv I would like to acknowledge the role of the support staff in the Department of Chemistry for their technical assistance, especially that of Dr. Dan Holmes and Kermit Johnson who were vital to the success of the LDP project. Lastly, I would like to thank Dr. Richard Staples for the many hours spent together in the XRD lab. The technical support you have provided me is secondary only to mentorship you offer. In the words of our craft—without you I would have crashed from solution long ago. –Steve. v TABLE OF CONTENTS LIST OF TABLES………………………………………………………….………………..….xi LIST OF FIGURES…………………………………………………………...…………..……xiii LIST OF SCHEMES…………………………………………………………………..….......xxii LIST OF ABBREVIATIONS………………………………………………….………...…....xxiii Chapter 1: Single-Site N-N Bond Cleavage by Mo(IV) to Produce a Terminal Nitrido………....1 ABSTRACT…………………………………………………………………………………...…..1 1.1 Introduction………………………………………………………………………………........2 1.2 Synthesis and Characterization of Compounds…………...………………………………......4 1.3 Mechanistic Investigations…………………………………………………………..............15 1.4 Conclusions………………………………………………………………………...........…...23 1.5 Experimental……………………………………………………………………………....…25 mes Synthesis of H2dpma ……………………………………………………………..….25 Mo(NMe2)2(dpma mes ) (1)……………………………………………………...…...…..26 t Mo(NMe2)2(CNBu )(dpma Mo(N)(NMe2)(dpma Mo(N)(NMe2)(dpma Zr(NMe2)2(dpma mes t ) (1•CNBu )……………………………………….…....27 mes ) (anti-2)…………………………………………………..…..28 mes ) (syn-2)…………………………………………………….....28 mes ) (3)………………………………………………………….….....29 2 mes Zr(η -NHNMe2)2(dpma ) (4)………………………………………….……….….…30 APPENDIX A……………………………………………………...…………………………….31 REFERENCES…………………………………………………………………………………..73 Chapter 2:Molybdenum (IV) Promoted Radical Bond Cleavage of Alcohols…….……..……..78 ABSTRACT………………………………………………………………………………….…..78 2.1 Introduction………………………………………………………………………..……..….79 mes 2.2 Reactivity of Mo(dpma )(NMe2)2 (1) with benzyl alcohol, trityl alcohol, and pinicol………………………………………………………………………….………...81 mes 2.3 Structural Comparisons and Synthesis of Mo(dpma )(NTol)2 (3)………………...........83 2.4 Radical Reactivity in Related Complexes …………………………………………..……...89 mes 2.5 Reactivity of Mo(dpma )(NMe2)2 (1) with Phenols ……………………………..….….93 2.6 Mechanism and Structure of OMo(dpma mes )(OBn) (5)…………………………………….94 mes 2.7 Reactivity of Mo(dpma )(NMe2)2 (1) with 1-Adamantanol ……………………………101 2.8 Conclusion…………………………………………………...………………….…….……104 2.9 Experimental……………………………………………………………………….………105 mes Synthesis of O2Mo(dpma ) (2)……………………………………………….….….105 mes Synthesis of Mo(NTol)2(dpma ) (3)………………………………………….….….106 vi Synthesis of Mo(OPh)4(NMe2) (4)………………………………………………....…107 mes Synthesis of Mo(dpma )(NMe2)(OAd) (6)…………………………………….…..107 REFERENCES………………………………………………………………………………..108 mes Chapter 3: Additional Reactivity of Mo(dpma )(NMe2)2…………………………..……...110 ABSTRACT……………………………………………………………………………….…...110 3.1 Introduction……………………………………………………………………………..….110 mes 3.2 Oxidation of Mo(dpma )(NMe2)2 (1) in presence of Carbon-Halogen and Silicon-Halogen Bonds …………………………………………………………….......…111 mes 3.3 Synthesis and Structure of Mo(dpma )(NBn)(H2NBn)(Cl) (3)………………..……....113 mes 3.4 Insertions into the Metal-Amide bond of Mo(dpma )(NMe2)2 (1)…………………....116 3.5 Conclusion………………………………………………………………………………....120 REFERENCES...…………………………………………………………………………….…121 Chapter 4: Evaluation of Donor and Steric Properties of Anionic Ligands on High Valent Transition Metals ……………………………………….…………….……….....123 ABSTRACT…………………………………………………………………………………...123 4.1 Introduction…………………………………………………………………………….…..124 4.2 The System Used for Ligand Parameterization ……………………………………...…....127 i 4.3 Preparation of NCr(X)(NPr 2)2 Complexes and Characterization ………………........….133 i 4.4 Synthesis of NCr(NPr 2)3 (1) by Nitrogen Atom Transfer ………………………....…….133 4.5 Syntheses Using Protonolysis with Lutidinium Halides……………………………….….134 4.6 Syntheses Using Direct Protonolysis with HX…………………………………………..…134 4.7 Syntheses Using Exchange with Thallium Salts……………………………………….…..135 4.8 Syntheses Using Exchange with Lithium Salts……………………………………….……137 4.9 Syntheses Using Exchanges with Sodium Salts………………………………………..…..137 4.10 Ligand Exchange with Zinc Transmetallation………………………………………....…138 4.11 Tin(IV)-Catalyzed Decomposition of a Cationic BF4 Salt ………………..……..……...138 4.12 Single-Crystal X-ray Diffraction Studies ……………………………………………..….139 4.13 Measurement of Amido Rotational Barriers and the Ligand Donor Parameters (LDP) ………………………………………….……….….....145 4.14 Steric Properties of the Ligands……………………………………………………..……153 4.15 Applications and Comparisons with These New Electronic Parameters …………....….157 4.16 Comparison of LDPs with pKa Values of the HX Compounds…………………………..158 4.17 Phenoxides: An LDP Comparison with Hammett Parameters…………………………...160 13 4.18 Evaluation of C NMR Chemical Shifts in Tungsten Metallacycles using LDP……....162 4.19 Comparison Between the AOM of Cr(III) Complexes and the SST Determined Donor Values …………………………………………….………..…..164 4.20 Comparison Values from Electronic Spectra of Cp*2TiX Complexes…………….……167 4.21 Concluding Remarks ……………………………………………………………….….…171 APPENDIX B…………………………………………………………………………….….…173 REFERENCES………………………………………………………………………….……...230 vii Chapter 5: Synthesis, Structure, and Hapticity Changes in Chromium (VI) Nitrido Complexes Bearing Cp and Related Ligands…………………………………………….………………...238 ABSTRACT…………………………………………………………………………………...238 5.1 Introduction……………………………………………………………………………..….238 i 1 5.2 Synthesis and Structure of NCr(NPr 2)2(η -Cp)……………………………………….….240 i i 5.3 Synthesis of [NCr(NPr 2)2DBN][I] (2), NCr(NPr 2)2(Ind) (4), and i NCr(NPr 2)2(Flu) (5) and Synthesis and Structure of Thallium(I)indenide .…………….242 i i 5.4 Structures of NCr(NPr 2)2(Ind) (4) and NCr(NPr 2)2(Flu) (5) and Steric Evaluation ……………………………………………………………………..……246 5.5 LDP determination of compounds 3-5……………………………………………….....…248 i 3 5.6 Synthesis, Structure and LDP determination of NCr(NPr 2)(O2CPh)(η -Cp) (6)……..…251 i 3 5.7 FT-IR Studies of NCr(NPri2)2(O2CPh) and NCr(NPr 2)(O2CPh)(η -Cp) (6)…………..254 5.8 Hapticity Shift as a function of π-loading in 6…………………………………………..…255 5.9 Conclusion …………………………………………………………………………...…….259 5.10 Experimental……………………………………………………………………….….….260 Synthesis of thallium(I) indenylide…………………………………...………..……...260 i Synthesis of [NCr(NPr 2)2(DBN)][I] (2)………………………………….…………..260 i Synthesis of NCr(NPr 2)2(Cp) (3)………………………………………….………….261 i Synthesis of NCr(NPr 2)2(indenyl) (4)………………………………………………..262 i Synthesis of NCr(NPr 2)2(fluorenyl) (5)………………………………………..…..…262 i Synthesis of NCr(NPr 2)(O2CPh)(Cp) (6)……………………………………….…….263 General Procedure for FT-IR Denticity Determination………………………………..……....264 REFERENCES………………………………………………………………………………...265 Chapter 6: Synthesis, Structure, and LDP Determination of Organometallic Ligands on Chromium(VI) Nitrido Complexes………………………………………………………..…..268 ABSTRACT………………………………………………………………………………..….268 6.1 Introduction ………………………………………………………………………….……268 i 6.2 Synthesis of NCr(NPr 2)2(X) X = Alkyl Complexes ………………………………….….271 i 6.3 Synthesis of NCr(NPr 2)2(X) X = Alkynyl Complexes…………………………........…..274 i 6.4 Synthesis and Structure of [NCr(NPr 2)2(C2Ph)]2 (9)…………………….………...……275 i 6.5 LDP Determination of NCr(NPr 2)2(X) Organometallic Complexes ………...…….……278 i 6.6 Structure and Steric Analysis of NCr(NPr 2)2(X) Complexes……………………….……283 6.7 Conclusion…………………………………………………………………………….…...289 6.8 Experimental………………………………………………………………………….……290 i Synthesis of NCr(NPr 2)2(CH2SiMe3) (4)………………………………………….…290 i Synthesis of NCr(NPr 2)2(CH2C(Me)2Ph) (5)…………………………….…….…….291 i Synthesis of NCr(NPr 2)2 (CH2CMe3) (6)……………………………………….…...292 i i Synthesis of NCr(NPr 2)2(CCSiPr 3) (7)………………………………………….…..292 viii i t Synthesis of NCr(NPr 2)2(CC Bu) (8)……………………………………………..…..293 i Synthesis of [NCr(NPr 2)2(C2Ph)]2 (9)………………………………………….……294 i Synthesis of LiCCSi( Pr)3……………………………………………………………..294 t Synthesis of LiCC Bu……………………………………………………………….…294 REFERENCES…………………………………………………………………………..…….296 Chapter 7: Synthesis, Structure, and Ligand Evaluation of Cationic Chromium(VI) Nitrido Complexes…………………………………………………………………………...…299 ABSTRACT………………………………………………………………………………...…..399 7.1 Introduction ……………………………………………………………………………..…300 7.2 Synthesis of Chromium Starting Materials …………………………………………….…304 7.3 Synthesis of Cr(VI) Cations …………………………………………….……………...….304 7.4 Synthesis of Zwitterionic Cr …………………………………………………………….....315 7.5 LDP determination of Cr(VI) Cations ………………………………………...……..……316 7.6 Counter Ion Effect and Correction …………………………………………………......…319 7.7 In situ generation, temperature dependent equilibrium, and solvent effect on LDP ………………………………………………………………….….324 7.8 Structure of Cr(VI) Cations …………………………………………………………….....329 7.9 Steric Analysis of Cr(VI) Cations ………………………………………………….……..336 7.10 Series of Nitrogen Bound Ligands…………………………………………………….…340 7.11 Conclusion……………………………………………………………………….…….…345 7.12 Experimental ………………………………………………………………………..……346 Procedure for Equilibrium Studies………………………………………………….…346 i [NCr(NPr 2)2(DMAP)][I] (4-I)………………………………………………..……….347 i Synthesis of [NCr(NPr 2)2(DBN)][I] (2-I)………………………………………….…347 i General Synthesis of [NCr(NPr 2)2(DBN)][A] A=BF4, PF6, SbF6 (2-BF4, 2-PF6, 2-SbF6)……………………………………………………………..….348 i [NCr(NPr 2)2(DBN)][BF4] (2-BF4)………………………………………………..….349 i [NCr(NPr 2)2(DBN)][PF6] (2-PF6)……………………………………………..……..349 i [NCr(NPr 2)2(DBN)][SbF6] (2-SbF6)…………………………………………..……..350 i Synthesis of [NCr(NPr 2)2(DBN)][BPh4] (2-BPh4)………………………….……….350 i Synthesis of [NCr(NPr 2)2 (DBN)][OTf] (2-OTf)……………………………….……351 i F4 F4 Synthesis of [NCr(NPr 2)22(DBN)][BAr ] (2-BAr )………………………………351 i Synthesis of [NCr(NPr 2)2 (Quin)][PF6] (3)…………………………………….…….352 i Synthesis of [NCr(NPr 2)2 (DMAP)][BF4] (4-BF4)…………………………….…….353 i Synthesis of [NCr(NPr 2)2 (NHC)][PF6] (5)…………………………………….……354 i Synthesis of [NCr(NPr 2)2 (HMPA)][PF6] (6)…………………………………….….355 i Synthesis of [NCr(NPr 2)2(P(Me)2Ph)][PF6] (7)………………………………….….355 i Synthesis of [NCr(NPr 2)2(THF)][PF6] (8)……………………………………….…..356 ix i t Synthesis of [NCr(NPr 2)2( BuNC)][I] (9)……………………………………….……357 i Synthesis of [NCr(NPr 2)2(PzB(Et3)) (10)……………………………………………357 REFERENCES ……………………………………………………………………………...…359 Chapter 8: Synthesis and Evaluation of 3-Ferrocenylpyrrole as a Ligand…………...….….363 ABSTRACT………………………………………………………………………………...….363 8.1 Introduction……………………………………………………………………….….…….363 8.2 Electron Deficient Pyrrolides in Catalysis………………………………………..………..367 8.3 Redox Active Ligands Bearing Cationic Charge…………………………………....…….374 8.4 3-Ferrocenylpyrrole……………………………………………………………………..…375 8.5 Synthesis of 3-Ferrocenylpyrrole………………………………………………………..…379 i Fc 8.6 Synthesis of NCr(NPr 2)2(Pyr ) …………………………………………………….…...383 i Fc 8.7 Structure and Steric Evaluation of NCr(NPr 2)2(Pyr )……………………...……….….384 i Fc 8.8 LDP Determination of NCr(NPr 2)2(Pyr )………………………………………….……388 i Fc 8.9 Attempted Chemical Oxidation of NCr(NPr 2)2(Pyr )……………………………….….390 8.10 Conclusion …………………………………………………………………………….…390 8.11 Experimental…………………………………………………………………….…….….391 Synthesis of 3-ferrocenyl-N-BOC-pyrrole……………………………………….……391 Deprotection of 3-ferrocenyl-N-BOC-pyrrole……………………………….…….…..392 i Fc Synthesis of NCr(NPr 2)2(Py )………………………………………………………392 REFERENCES ………………………………………………………………………..………394 x LIST OF TABLES Table 1.1: Rates and yields of the reactions of various substrates with 1 to form 2…………………………………………………………………….14 Table A.1: Rate dependence on the concentration of 1………………………………………..40 Table A.2: Rate constants at different temperatures for the reaction of 3 with 2 equivalents of H2NNMe2 ……………………………………………………...….61 Table 2.1: Table of substrates and organic byproducts of the molybdenum mediated C-O bond cleavage……………………………………………………….82 Table 2.2: Comparison of dpmames N-Methyl orientation toward the metal-ligand 1 multiple bond with the geminal H NMR coupling of the methylenes. …………...88 ‡ Table 4.1: Values for LDP (kcal/mol) and ∆S rot (cal/mol•K) for 1-31 and 33………….….150 Table B.1: Thermodynamic parameters determined for compounds 1-31…………………....200 Table B.2: R-factors of the fits of the literature data with LDP using different ‡ ΔS terms………………………………………………………………….…..….201 Table 5.1: Chromium-carbon bond distances and average angle around C1 in compounds 3-5. Steric parameters of %Vbur and Gm(L) of the X ligand, both as percentages……………………………………………………..247 Table 5.2: Experimentally determined LDP values of 3-5…………………………………...250 Table 6.1: Ligand Donation Parameter, Cr1-C1 bond distance, and the steric parameters %Vbur and Gm(L) of compounds 3-9………………………………..282 Table 7.1: Corrected and uncorrected Ligand Donor Parameters of 3-10………………..…..318 Table 7.2: Evaluation of Ti(IV) Hydroamination Catalysts and their corresponding ligands evaluated by the LDP method………………………………………….…321 i Table 7.3: Ligand Donor Parameters for [NCr(NPr 2)2DBN][A] (2-A) complexes with anion corrective factors and calculated anion radii……………………….…323 i Table 7.4: Temperature based equilibrium of NCr(NPr 2)2I (1) and i [NCr(NPr 2)2DMAP][I] (4-I) in various solvents…………………………..……327 xi i Table 7.5: LDP‘s of [NCr(NPr 2)2DMAP][I] (4-I) in each solvent along with their dielectric constants…………………………………………………………..328 i Table 7.6: The steric parameters %Vbur and Gm(L) of [NCr(NPr 2)2L][A] complexes, both given as a percent……………………………………………………………337 Table 7.7: Cr1-N1 bond lengths and LDP‗s of all structurally characterized i i NCr(NPr 2)2(X) and [NCr(NPr 2)2L][A] complexes with ligands bound through nitrogen. …………………………………………………………….......341 Table 8.1: LDP and steric parameters of pyrrolyl compounds……………………………….387 xii LIST OF FIGURES mes Figure 1.1: Synthesis and structure of Mo(NMe2)2(dpma ) (1). Hydrogens and a toluene solvent molecule found in the lattice are omitted from the structure. For interpretation of the references to color in this and all other figures, the reader is referred to the electronic version of this dissertation….……………………………………………………………...…..…...5 Figure 1.2: Fit of the Me3SiPh methyl group chemical shifts to Equation 1. The values for the fit 6 parameters are C = (2.31 ± 0.61)×10 , ∆G = 3278 ± 202 cal/mol, and δ‘ = 18.3 ± 2.0 Hz. ……………………………………………………………..….6 Figure 1.3: Synthesis 1•CNBut ,of the tert-butylisonitrile adduct of 1………………..…….......8 Figure 1.4: ORTEP diagram at the 50% probability level for the structures of mes mes anti-Mo(N)(NMe2)(dpma ) (anti-2, top) and syn-Mo(N)(NMe2)(dpma ) (syn-2, bottom) as determined by single crystal X-ray diffraction. H-atoms omitted. …………………………………………………………….……12 Figure 1.5: Synthetic route to the hydrazido(1–) zirconium complex 4 and ORTEP diagram at the 2 mes 50% probability level for the structure of Zr(η -NHNMe2)2(dpma ) (4) as determined by single crystal X-ray diffraction. H-atoms (pink spheres) are omitted except on the hydrazido(1–) nitrogens N4 and N6…………….……16 Figure 1.6: Reductive addition via nucleophilic attack of an amine on a terminal nitride…......23 Figure A.1: A representative second order fit of 1 with disubstituted hydrazine ………..…...34 Figure A.2: A representative pseudo-1st order fit of 1 with disubstituted hydrazine……..…..35 Figure A.3: Initial rate data for the conversion of 1 to 2-anti as a function of metal concentration……………………………………………………………………...37 Figure A.4: Fits of rate at 10% of conversion of 1 to 2-anti along with tabulated data……….38 Figure A.5: Initial rate data of conversion of 1 to 2-anti as a function of the concentration of 1…………………………………………………………………41 Figure A.6: Thermal conversion of 2-syn into 2-anti……………………………………..……43 Figure A.7: UV-Vis trace of 2-anti formation using H2NNMe2 and 1 (60 s intervals) showing a clear isobestic point, thus no long lived intermediates………………..44 xiii Figure A.8: UV-Vis trace of 2-anti formation using N-Amino piperdine showing the production of other metal containing species, likely the results of product inhibition………………………………………………………………………..….46 Figure A.9: Experimental and calculated X-ray powder pattern for 1……………………....…..48 Figure A.10: Eyring plot of ln(k/T) vs 1/T with a linear fit giving activation parameters…...….50 Figure A.11: α,β-proton migration as the rate determining step. Mes groups removed for clarity…………………………………………………………...….....52 Figure A.12: Alternate mechanism of bond cleavage going through a hydrazido(2−) pathway……………………………………………………….....…55 Figure A.13: Reaction of 3 with 2 equivalents of dimethylhydrazine giving 4……………......58 Figure A.14: A representative 2 nd order fir of reaction of 3 with disubstituted hydrazine……59 1 Figure A.15: Stacked H NMR plots of a reaction of 3 with disubstituted hydrazine over 20 h with traces taken every 20 min (top), and the first 160 min of the same run with traces taken every 2 min (bottom)……………………….....60 Figure A.16: Eyring plot of 3 with H2NNMe2……………………………………………...…...62 Figure A.17: Overlaid crystal structure renderings of Mo(dpma Zr(dpma mes mes )(NMe2)2 (1) in purple )(NMe2)2 (3) in blue…………………………………………..……..63 1 Figure A.18: H NMR of 2-syn with the assigned proton resonanceses on the inset structure ……………………………………………………………......…..65 1 Figure A.19: A region of the homo decoupled spectra H NMR of 2-syn showing the changing multiplicity of each peak when saturating another. ………………………...….…66 1 Figure A.20: A region of the homo decoupled H NMR spectra of 2-syn showing the changing multiplicity of each peak when saturating another…………………………...…....67 mes Figure 2.1: N-N and N-O bond cleavages mediated by Mo(dpma )(NMe2)2 (1) and the major byproduct of each reaction……………………………………………………..…...79 Figure 2.2: Organic byproducts identified in the reaction of 1 with O-benzyl hydroxylamine……………………………………………………..……81 xiv 1 mes Figure 2.3: Methylene H NMR resonances of the bound dpma ligand where the N-methyl is anti or syn to the nitride………………………………………………84 Figure 2.4: Synthesis of Mo(NTol)2(dpma mes ) (3)…………………………………………...…85 mes Figure 2.5: Crystal structure rendering of the Mo(VI) bis-imido Mo(NTol)2(dpma )…….....86 Figure 2.6: The radical forming step in the reaction of 1 with benzylic alcohols (top) along with t the expulsion of a butyl radical initiated by a Ti(III) radical source (below)…..…..90 Figure 2.7: Radical attack on a terminal oxo to form a M-O-R linkage…………………...….....92 Figure 2.8: Crystal structure rendering of Mo(OPh)4(HNMe2)2 (4)…………………………….93 Figure 2.9: Possible products from alternate ligand substitution pathways……………......…….96 Figure 2.10: Concerted C-O bond cleavage of benzyl alcohols………………………….…..….98 Figure 2.11: Crystal structure rendering of the Mo(V) intermediate mes OMo(dpma )(OBn) (5)……………………………………………………...…100 Figure 2.12: Stoichiometric reaction of 1 with 1-adamantanol yielding mes Mo(dpma )(NMe2)(OAd) (6)……………………………………….......……..102 Figure 2.13: Crystal structure rendering of the Mo(IV) complex mes Mo(dpma )(NMe2)(OAd) (6) Thermal ellipsoids at 50% probability with H atoms removed for clarity.………………………...……...…..103 Figure 3.1: Crystal structure rendering of the oxidized product mes Mo(dpma )(NMe2)2Cl (2)…………………………………………..……...….112 mes Figure 3.2: Crystal structure rendering of Mo(dpma )(NBn)(H2NBn)(Cl) (3) showing a doubly and datively bonded benzylamine ligand……………...……...114 mes Figure 3.3: A space filling model of Mo(dpma )(NBn)(H2NBn)(Cl) (3) viewed down the Mo1-N5 bond axis showing the sterically shielded benzylammine ligand (highlighted in red)…………………………………....…..116 Figure 3.4: The related complex Mo(dpma)(NMe2)2(NHMe2) showing [1,2]-insertion of an alkyne into a Mo-NMe2 bond……………………………………………......……117 xv mes Figure 3.5: Crystal structure rendering of Mo(dpma )(S2CNMe2)(NMe2) (4) Atom positions at 50% probability. H atoms and two toluene molecules of crystallization removed for clarity..………..........................................................118 Figure 4.1: DFT B3LYP/aug-cc-pVQZ computational results for NCr(NH2)3. On the bottom are two views of the computed ground state, with the right view looking down one of the three equivalent amido-chromium vectors. In the middle is a plot of the hybridization parameter (λ) vs the calculated enthalpic energy of the complex with the ground state set to 0 kcal/mol. On the top are two views of the transition state structure found for rotation of one amido ligand, with the right view looking down the rotated amido-chromium vector. …………………………………………………………………………..…..130 i Figure 4.2: Spacefilling views of NCr(NPr 2)2(carbazolyl) (26)…………………………..…..141 i Figure 4.3: Plot of average Cr–NPr 2 distance (Å) vs the donor ability of X (LDP in kcal/mol)…...……………………………………………………….….....144 i Figure 4.4: The Ligand Donor Parameters (kcal/mol) for various X in NCr(NPr 2)2X with the associated errors For interpretation of labels deemed illegible the reader is refered to Figure 3 of DiFranco, S. A.; Maciulis, N. A.; Staples, R. J.; Batrice, R. J.; Odom, A. L. Inorganic Chemistry 2012, 51, 1187–200.…………………….148 Figure 4.5: Space filling model of relatively small chloro 3 (top) and large indolyl 25 (bottom). The inscribed orange sphere shows the 3.5 Å radius limit………..…...152 Figure 4.6: The %Vbur for the ligands used in this study. Values are for the percentage volume occupied by the ligand in a sphere of radius 3.5 Å from the chromium center………………………………………………….154 Figure 4.7: The Solid Angle Model from the Solid G program for 25 with indolyl (green), diisopropylamido (yellow and blue), and nitrido (red)……………………...…….155 Figure 4.8: The percentage of the chromium coordination sphere shielded, GM(L), from the Solid G program for the ligands used in this study……………….....….156 Figure 4.9: Plot of pKa in water versus LDP. The inset is an expansion of the region containing the phenoxides………………………………………………….…......159 Figure 4.10: Plot of LDP vs Hammett parameters for the aryloxide complexes…………...…161 xvi Figure 4.11: Plot of the 13 C NMR chemical shift for the carbon directly bonded to tungsten in W(=C8H12=C8H12=NAr)(NAr)X2 versus the donor ability of X found in this work with a linear fit…………………………………………...163 Figure 4.12: Plot of eπ + eζ for chromium(III) complexes from experimentally determined AOM values versus the LDP for X (top). Plot of eζ (blue) and eπ (red) parameters versus the LDP for X (bottom)…………………………………….165 –1 Figure 4.13: Plot of ∆Exz in wavenumbers (cm )…………………….………………….….169 Figure B.1: %Vbur calculated for each ligand in 1-31, and 33 in a 3.5 Å sphere………….....203 Figure B.2: %Vbur calculated for select ligands as a function of coordination sphere size…………………………………………………………………..…..205 Figure B.3: Space-filling models of 8 and 21 with a coordination sphere (yellow) with a radius of 3.5 Å……………………………………………………………...…..207 Figure B.4: Percentage of the sphere shielded by a given ligand as calculated by Solid G……………………………………………………………………...…...210 i Figure B.5: Percentage of free space not shielded by any ligand of NCr(NPr 2)2(X) …….…212 Figure B.6: 2D NOESY of 2 at –60 °C ………………………………………….……...……214 1 Figure B.7: H Homodecoupling Experiment of 2 at 25 ˚C……………………………….…216 Figure B.8: Kinetic Scheme for Two-Site Exchange………………………………………....224 i Figure B.9: 1H NMR Spectrum of NCr(NPr 2)2I (2) at 25 ˚C……………………………….225 Figure B.10: ln(1/T1) (blue) and ln (1/apparent T1) (red) of 2 at different temperatures.…....227 Figure 5.1: A Ti(IV) bis-Cp complex and an isolobal chromium nitride…………………..…239 i 1 Figure 5.2: Synthesis of NCr(NPr 2)2(η -Cp)……………………………………………......240 i Figure 5.3: A crystal structure rendering of NCr(NPr 2)2(η1-Cp) 3 Thermal ellipsoids at 50% probability and H atoms removed except for calculated H on C1.…………………………………………………………………………...…242 xvii Figure 5.4: Crystallograpically determined interatomic Cr-C distances (blue) and C-C bond distances (red) in compounds 3 and 6, with free C5H6 for comparison…………………………………………………………………….254 Figure 5.5: Synthesis of [NCr(NPri2)2DBN][I] (2)……………………………………...…...243 Figure 5.6: Synthesis of 4 using thallium(I) indenide……………………………………..….244 i Figure 5.7: Overlaid structures of NCr(NPr 2)2(X) where X = Cp (3) (red), Indenyl (4) (blue), and fluorenyl (5) (green…………………………………...….245 i 3 Figure 5.8: Synthesis of compound NCr(NPr 2)(O2CPh)(η -Cp) 6…………………………251 i 3 Figure 5.9: A crystal structure rendering of NCr(NPr 2)(O2CPh)(η -Cp) (6)…………….…253 i 3 Figure 5.10: A space filling representation of NCr(NPr 2)(O2CPh)(η -Cp) (6)……………..258 Figure 6.1: Hyperconjugation as seen in β-substituted carbocations……………………..…..269 Figure 6.2: The ylide like resonance forms possible in β-silicon substituted metal alkyls……………………………………………………...…..269 i Figure 6.3: Synthesis of [Cr(μ-N)(NPr 2)2]2, the Cr(V)-Cr(V) dimer by reduction with sodium amalgam…………………………………………………………………..272 i Figure 6.4: Synthesis of NCr(NPr 2)2(CH2SiMe2(Ph)) (3) and i NCr(NPr 2)2(CH2CMe2(Ph)) (5)……………………………………………...…271 i i Figure 6.5: Synthesis of NCr(NPr 2)2(CH2CMe3) (6) and NCr(NPr 2)2(CH2SiMe3) (4)…...274 i t i i Figure 6.6: Synthesis of NCr(NPr 2)2(CC Bu) (8) and NCr(NPr 2)2(CCSi(Pr 3)) (7)………..275 i Figure 6.7: A crystal structure rendering of [NCr(NPr 2)2(C2Ph)]2 (9) showing a dimeric structure symmetric about the midpoint of the bridging allenyl moiety…………276 Figure 6.8: Ligand Donation Parameters of all C-bound X-type ligands evaluated with associated experimental errors. Lower barriers of amide rotation (stronger donors) displayed on the left…………………………………………………………...….281 xviii i Figure 6.9: Crystal structure overlay of NCr(NPr 2)2(CH2SiMe3) (4) (green) and i NCr(NPr 2)2(CH2CMe3) (6) (purple) showing a close structural similarity between the complexes…………………………………………………………..284 i Figure 6.10: Crystal structure overlay of NCr(NPr 2)2(CH2SiMe2(Ph)) (3) i (blue) and NCr(NPr 2)2(CH2CMe2(Ph)) (5) (red)……………………………...285 Figure 6.11: %Vbur Calculated for each ligand in 3-8 in a 3.5 Å sphere…………………..…287 Figure 6.12: Percentage of the sphere shielded by the C bound ligand in 3-8 as calculated by Solid G………………………………………………………...…288 i Figure 7.1: Synthesis of [NCr(NPr 2)2DBN][I] 2-I…………………………………………..305 i Figure 7.2: Synthesis of [NCr(NPr 2)2DMAP][BF4] 4 using silver tetrafluoroborate……….306 Figure 7.3: Synthesis of compounds 3 and 6-8 using AgPF6 to abstract a halide from 1….…308 Figure 7.4: In situ generation of a persistent carbene and its reaction with 1 to make i [NCr(NPr 2)2NHC][PF6] 5…………………………………………………......310 Figure 7.5: Counter ion substitution of 2-I using sodium salts…………………………...…..311 Figure 7.6: Synthesis of 2-OTf by anion exchange with an ammonium salt…………………312 Figure 7.7: Synthesis of 2- BAr F4 F4 by anion exchange with an Ag(BAr )…………..………313 Figure 7.8: Displacement of a weaker donating ligand (LW) by a stronger neutral donor (LS)………………………………………………………………..314 i t Figure 7.9: In situ synthesis of [NCr(NPr 2)2( BuNC)][I] (9) by ligand substitution……...…314 i Figure 7.10: Synthesis of NCr(NPr 2)2(PzB(Et3)) (10) with resonance structures………...…316 i Figure 7.11: LDP‗s of [NCr(NPr 2)2DBN][A] (2-A) complexes versus the approximated anion size…………………………………………………………………...…...322 i Figure 7.12: Reaction of [NCr(NPr 2)2DMAP][BF4] (4-BF4) with silver triflate……….…..325 i i Figure 7.13: NCr(NPr 2)2I (1) and [NCr(NPr 2)2DMAP][I] (4-I) in a temperature dependent equilibrium………………………………………………………………………326 xix i Figure 7.14: Crystal structure rendering of [NCr(NPr 2)2DBN][I] (2-I) with thermal probabilities at 50% and H atoms omitted for clarity. Bond lengths within the DBN ligand are shown in the inset in Å (blue)………………………………....330 Figure 7.15: Resonance structures of 2-I, 4-BF4, and 6. Weakly coordinating anion omitted for clarity……………………………………………………...…331 i Figure 7.16: Crystal structure renderings for the cations of [NCr(NPr 2)2DMAP][BF4] (4-BF4) i (left) and [NCr(NPr 2)2quin][PF6] (3) (right)…………………………………...332 i Figure 7.17: Crystal structure renderings of the cation of [NCr(NPr 2)2(HMPA)][PF6] 6 i (left) and [NCr(NPr 2)2(THF)][PF6] 8 (right). Atoms positions at 50% probability and H atoms removed for clarity…………………………………...334 i Figure 7.18: Crystal structure rendering of the cation of [NCr(NPr 2)2NHC][PF6] (5). Atomic positions at 50% probability and H atoms removed for clarity……..….335 i Figure 7.19: Space filling models of [NCr(NPr 2)2HMPA][PF6] (6) (left) and i [NCr(NPr 2)2quin][PF6] (3) (right) with a 3.5 Å radius sphere approximating the first coordination sphere of the Cr(VI) center…………..…..339 Figure 7.20: LDP values versus the crystallographically determined bond lengths from chromium to the ligated nitrogen atom…………………………….………..….344 Figure 8.1: The Chauvin and Bergman mechanisms for transformations catalyzed by d0 transition metals, both containing a [2+2]-cycloaddition step to a metal ligand multiple bond………………………………………………………………...…..365 Figure 8.2: The hydroamination of 1-phenylpropyne with aniline under pseudo-first order conditions with vaious catalysts…………………………………………………368 Figure 8.3: Limiting resonance structures of an η1 bound pyrrolide to a metal capable of accepting pπ-dπ donation ranging from ligand to metal charge transfer (top) to a fully aromatic pyrrolyl (bottom)……………………………….…….....371 Figure 8.4: Synthesis of 2-aryl pyrroles, condensation into the bidentate ligand H2dpmX, and transamination onto titanium(IV)………………………………………...…..372 Figure 8.5: Representative synthesis of transition metal complexes bearing ligands that have cationic charge………………………………………………………….375 Figure 8.6: The non-classical reductive elimination of two alkynylferrocene ligands induced by chemical oxidation……………………………………………………377 xx Figure 8.7: Resonance contributors in HPyr Fc upon 1e− oxidation……………………….…378 Figure 8.8: Failed cross coupling strategies in the production of HPyrFc……………….…..380 Figure 8.9: A new synthesis for 3-Ferrocenyl-N-BOC-pyrrole using Pd catalyzed cross-coupling and subsequent deprotection……………………………………..382 i Fc Figure 8.10: Synthesis of NCr(NPr 2)2(Pyr ) using an in situ generated thallium(I) transmetalating reagent…………………………………………………………384 i Fc Figure 8.11: Crystal structure rendering of NCr(NPr 2)2(Pyr ) with thermal ellipsoids at 50%. Probability. H atoms removed for clairity……………………………...386 i i Fc Figure 8.12: Overlaying crystal structures of NCr(NPr 2)2(Pyr) (blue), NCr(NPr 2)2(Pyr ) i C6F5 i C6H3(CF3)2 (orange), NCr(NPr 2)2(Pyr ) (green), and NCr(NPr 2)2(Pyr ) (purple) showing similarities in the orientation of the pyrrolyl based ligand……………………………………………………...…... 388 xxi LIST OF SCHEMES Scheme 1.1: Two catalytic cycles often discussed for N–N bond cleavage. The exact steps forelectron transfer are left ambiguous…………………………..………….3 Scheme 1.2: Synthesis of anti-2 and syn-2 by N–N or N–O bond cleavage with isolated yields for the complexes. See Table 1.2 for hydrazines and yields of anti-2. Yields of by-products in the O-benzylhydroxylamine reaction are by GC-FID………………………………………………………….10 Scheme 1.3: Proposed mechanism for the reaction of 1 with H2NNMe2. Mesityl groups on the dpma mes ligand were omitted for clarity……………………….…21 30 Scheme 1.4: Poulos-Kraut mechanism for heterolytic O–O cleavage and ferryl generation in peroxidase……………………………………………………….…25 i Scheme 4.1: Synthesis of NCr(N Pr2)2(F)(33)………………………………………………..139 xxii LIST OF ABBREVIATIONS %Vbur Percent Buried Volume anth 2-anthrycenyl AOM Angular Overlap Model BOC tert-butyloxycarbonyl t 3,3-dimethylbut-1-yn-1-ide CC Bu i CCSi(Pr 3) (triisopropylsilyl)ethyn-1-ide COD 1,5-cyclooctadiene Cp cyclopentadienyl F BAr 4 tetrakis(3,5-bis(trifluoromethyl)phenyl)borate d day DBN 1,5-diazabicyclo[4.3.0]non-5-ene dip 2,6-diisopropylphenyl DMAP 4-dimethylaminopyridine DME dimethoxyethane DMSO dimethyl sulfoxide Fc ferrocenyl GC Gas Chromatograph Flu fluorenyl Gm(L) Solid angle h hour H2dpma N-((1H-pyrrol-2-yl)methyl)-N-methyl-1-(1H-pyrrol-2yl)methanamine H2dpmames 1-(5-mesityl-1H-pyrrol-2-yl)-N-((5-mesityl-1H-pyrrol-2yl)methyl)-N-methylmethanamine HMPA hexamethylphosphoramide xxiii ind indenyl LDP Ligand Donor Parameter NHC 1,3-dimethylimidazol-3-ium-2-ide min minute mes mesityl NMR Nuclear Magnetic Resonance i NPr 2 diisopropylamide Ad 1-adamantyl Bn benzyl t OBu F6 1,1,1,3,3,3-hexafluoro-2-methylpropan-2-olate OPht 1,3-dioxoisoindolin-2-olate OTf trifluoromethanesulfonate Pyr pyrrolyl C6F5 3-(perfluorophenyl)pyrrol-1-ide C6H3(CF3)2 3-(3,5-bis(trifluoromethyl)phenyl)pyrrol-1-ide Fc 3-ferrocenyl pyrrolyl Pyr Pyr Pyr PzB(Et)3 2-(triethylboranyl)--pyrazolyl quin quinuclidine SST or SSMT Spin Saturation Magnetization Transfer t BuNC 2-isocyano-2-methylpropane THF tetrahydrofuran tol tolyl xxiv Chapter 1: Single-Site N–N Bond Cleavage by Mo(IV) to Produce a Terminal Nitrido ABSTRACT mes Mo(NMe2)4 and the tridentate, dipyrrolyl ligand H2dpma coordinate Mo(NMe2)2(dpma mes ) (1), which exhibits spin-crossover behaviour in solution. The complex is a ground state singlet with a barrier of 1150 cm in d8-toluene. The were found to form 5- complex reacts with –1 for production of the triplet 1,1-disubstituted benzylhydroxylamine to produce nitrido MoN(NMe2)(dpma hydrazines or O- mes ). The mechanism of the 1,1- dimethylhydrazine reaction with 1 was examined along with the mechanism of substitution of NMe2 with H2NNMe2 in a diamagnetic zirconium analogue. The proposed mechanism involves production of a hydrazido(1–) intermediate, Mo(NMe2)(NHNMe2)(dpma mes ), which undergoes an α,β-proton shift and N–N bond cleavage with metal oxidation to form the nitrido. This conversion from hydrazido(1–) to nitrido is less discussed in the dinitrogen activation literature but is somewhat analogous to the proposed mechanism for O–O bond cleavage in some peroxidases. 1 1. 1 Introduction The reduction of dinitrogen is arguably one of the most important reactions ever 1 discovered and is the starting point for the production of the vast majority of nitrogencontaining compounds with applications from fertilizers to pharmaceuticals. In accordance with the importance of the reaction, numerous studies have been carried out on the biological systems 2 3 responsible (nitrogenases), the industrial process for production of ammonia (Haber-Bosch), and other systems capable of nitrogen reduction. 1 The naturally occurring systems can, but do not always, contain molybdenum in the active site of the cofactor but do include an iron-sulfur cluster. 4 The mechanism of the N–N cleavage has been divided into two general forms depending largely on the sites of protonation, which have been dubbed the distal and alternating mechanisms. In Scheme 1.1 are some of the steps commonly attributed to these cycles. 5 Frequently in molybdenum- and tungsten-based systems, the cleavage of the nitrogen6 nitrogen single bond required in these reactions is proposed to occur through a hydrazido(2–) intermediate that becomes protonated to an ammonium imido (hydrazidium) complex (Distal Cycle of Scheme 1.1). If the metal center has the two electrons required for N–N bond cleavage, this can occur through simple N–N bond scission in Mo(IV) and W(IV) hydrazidium intermediates. Conversely, some molecular iron-based systems for nitrogen reduction often have been suggested to proceed through diazene intermediates (Alternating Cycle of Scheme 1.1). 2 7 Scheme 1.1: Two catalytic cycles often discussed for N–N bond cleavage. The exact steps for electron transfer are left ambiguous. In this report, we demonstrate facile single-site cleavage of an N–N bond through 3 production of a Mo(IV) hydrazido(1–). The data suggest that molybdenum systems can proceed from hydrazido(1–) to nitrido without the intermediacy of a hydrazido(2–) by way of α,β-proton 8 migration. Consequently, pathways that include complexed diazene, hydrazido(1–), and nitrido without the intermediacy of hydrazido(2–) appear to be viable for Mo-based N–N bond cleavage. 1.2 Synthesis and Characterization of Compounds The ancillary ligand chosen was a sterically more substantial version of the pyrrole-based N,N-di(pyrroyl-α-methyl)-N-methylamine, dpma, which we have employed in several catalytic 9 and stiochiometric studies previously. For this chemistry, mesityl groups were installed 10 into mes the remaining α-positions of the pyrroles. The synthesis of the new ligand, H2dpma , is shown in Figure 1.1 along with the synthesis and structure of the molybdenum dimethylamido 11 derivative prepared by transamination on Mo(NMe2)4. The structure of 5-coordinate 1 is very nearly halfway between trigonal bipyramidal (tbp) and square pyramidal (sp). The largest angle subtended at molybdenum is 164.9(1)° (α) for N3Mo1-N5 and the second largest is 129.1(1)° (β) for N1-Mo1-N2. The value for η from η = (α – β)/60 = 0.60, where a value of 1 is for tbp and a value of 0 is for sp. 4 12 mes Figure 1.1: Synthesis and structure of Mo(NMe2)2(dpma ) (1). Hydrogens and a toluene solvent molecule found in the lattice are omitted from the structure. For interpretation of the references to color in this and all other figures, the reader is referred to the electronic version of this dissertation. Due to weaker donation of the pyrroles to the Mo center relative to the dimethylamidos, 13 metal–N(pyrrole) distances are usually significantly longer than metal–NMe2 distances, and that is the case here. The average Mo–N(pyrrole) distance in the structure was determined to be 2.091(2) Å. The average Mo–NMe2 distance was 1.917(2) Å. The much weaker donor nitrogen of the dpma mes had an Mo(1)-N(3) distance of 2.401(2) Å. 5 Magnetic susceptibility measurements in solution (Evan‘s method at 29.7 °C in d814 toluene) 2 provided a μeff = 1.18 μB, well below the spin-only moment for high-spin d of 2.87 μB and well below the value for the high spin tert-butylisonitrile adduct of the same compound t 1•CNBu (vide infra). SQUID magnetometry on 1 in the solid state suggests the compound is a ground-state singlet. Even at room temperature, the compound exhibited no detectable paramagnetism as a solid. This suggests that the compound has a thermally accessible triplet state in solution but not in the solid state, likely because an isomerization is required to access the higher spin state. Figure 1.2: Fit of the Me3SiPh methyl group chemical shifts to Equation 1. The values for the fit 6 parameters are C = (2.31 ± 0.61)×10 , ∆G = 3278 ± 202 cal/mol, and δ‘ = 18.3 ± 2.0 Hz. 6 Examination of the magnetism in solution was done over the accessible temperature range (~230-350 K), bound on the lower end by the solubility of the compound and on the higher end by its stability in solution. The data were fit using the expression of Gütlich and coworkers (Eqn. 1). 15 1 In our case, none of the resonances in the H NMR could be followed over the entire temperature range due to broadening. Consequently, we used an internal standard of PhSiMe3 in the solution with a capillary containing d8-toluene and reference PhSiMe3 to follow the contact shift due to the paramagnetic species. ( ) (1) The value δ is the difference in ppm between the chemical shift of the methyl groups in the PhSiMe3 reference (inside the capillary) and PhSiMe3 in solution with 1. In Eqn 1, C is a constant, T is the temperature in Kelvin, and R is the Gas Constant. In this case ∆GSC, the free energy associated with the spin crossover may contain components associated with the unidentified isomerization. Fitting the data to this equation gives the plot shown in Figure 1.2. From the fit, ∆GSC = –1 3.3 kcal/mol (1150 cm ). This is similar to values reported by Rothwell and coworkers for a large series of W(IV) complexes that exhibited spin crossover in solution. 16 For these cyclometallated 2,6-diphenylphenol compounds, W(OC6H3Ph-C6H4-)2L2, the energy difference –1 varied from 358-1205 cm , where L was a host of different pyridine derivatives. Similarly, Schrock and co-workers have reported singlet-triplet spin crossover for the Mo(IV) species 17 [(Me3SiNCH2CH2)3N]MoNMe2. The ∆GSC value reported for this related system was ~1800 7 cm –1 based on reported enthalpy and entropy values. Interestingly, there are now three different magnetic behaviours for reported Mo(IV) bis(dimethylamido) complexes bearing derivatives of the dpma ligand. In our paper using the less substituted pyrrolyl-based ligand, 18 6-coordinate Mo(NMe2)2(HNMe2)(dpma) was reported as a paramagnetic complex with a magnetic moment close to the spin-only value for two unpaired electrons. For this study, we confirmed that measurement on a freshly prepared sample; the value for this Mo(IV) 6-coordinate compound was found to be 2.48-2.40 μB from 210-300 K. Ar Schrock and coworkers recently reported a related 5-coordinate compound Mo(NMe2)2(tpa ), Ar where tpa = tris[2-(3,5-trifluoromethylphenyl)pyrrolylmethyl)amine. 19 In this compound, which is structurally similar to 1, two pyrrolyl substituents are bound to the metal center and the third is a ―dangling‖ NH-pyrrole group. Interestingly, this compound is reported to have a mixture of broad and sharp lines in the NMR that were ―slightly paramagnetically shifted‖. Consequently, this complex, contrary to 1, seems to have a preponderance of the singlet complex in solution. In this report, 1 is a spin-crossover compound in solution and apparently diamagnetic in the solid state. t Figure 1.3: Synthesis 1•CNBu ,of the tert-butylisonitrile adduct of 1. 8 To examine the relationship between the electronic structure of 1 and its coordination number further, we t prepared mes Mo(NMe2)2(CNBu )(dpma (Equation 1.3) the tert-butylisonitrile adduct t ) (1•CNBu ). This adduct is quite similar structurally to 1 with the t CNBu ligand trans to one of the dimethylamido ligands and with the donor amine of the mes dpma trans to the other NMe2 group. The complex is high spin with μeff = 2.47 μB like the previously reported 6-coordinate Mo(NHMe2)(NMe2)2(dpma). The isonitrile adduct of 1 was also structurally characterized (See the Appendix A for details). 9 Scheme 1.2: Synthesis of anti-2 and syn-2 by N–N or N–O bond cleavage with isolated yields for the complexes. See Table 1.2 for hydrazines and yields of anti-2. Yields of by-products in the O-benzylhydroxylamine reaction are by GC-FID. In an attempt to prepare the Mo(IV) terminal hydrazido(2–) complex, dimethylhydrazine was added to 1. A diamagnetic product was obtained, the nitrido complex antiMo(N)(NMe2)(dpma methyl of the dpma mes mes ) (anti-2) shown in Scheme 1.2. The product has the nitrido nitrogen and ligand on opposite sides of the plane defined by the Mo–N1(pyrrolyl)– 1 N2(pyrrolyl) atoms. The yield of anti-2 in this reaction by H NMR was 75%, and the isolated yield was 74% (Table 1.1). 10 The structure of anti-2 is shown in Figure 1.4 (top). The structure of the Mo(VI) nitrido is best approximated as square pyramidal (η = 0.02). The expectation in proceeding from the formally Mo(IV) complex 1 to the Mo(VI) complex 2 is that the bond distances should shorten. However, the Mo1-N(pyrrolyl) average distance in 2 is 2.127(3) Å, slightly longer than the 2.091(2) Å found in 1. This lengthening of the pyrrolyl distance in 2 vs 1 may be due to the rigidity of the dpma mes ligand in this square planar derivative and the widening of the N(pyrrolyl)–Mo–N(pyrrolyl) angle from 129.06(7)° in 1 to 145.0(1)° in anti-2. The Mo1-N3(donor amine) distance in higher valent 2 was found to be 2.274(3) Å, whereas in 1 it was a much longer 2.401(2) Å. The Mo1-NMe2 distance in 2 is 1.906(3) Å, which is not statistically different from the 1.917(2) Å average distance in 1. The Mo–N(nitrido) distance in 2 was found to be 1.647(3) Å. 11 Figure 1.4: ORTEP diagram at the 50% probability level for the structures of antiMo(N)(NMe2)(dpma mes ) (anti-2, top) and syn-Mo(N)(NMe2)(dpma mes ) (syn-2, bottom) as determined by single crystal X-ray diffraction. H-atoms omitted. Other 1,1-disubstituted hydrazines react with 1 to give the same product (Scheme 1.2 and Table 1.1). Consequently, only one dimethylamido in 1 is involved in the production of the nitrido. 12 Addition of O-benzylhydroxylamine (Scheme 1.2) led to formation of the syn-isomer of 2, where the amine donor methyl of the dpma mes and nitrido nitrogen are on the same side of the Mo–N1(pyrrolyl)–N2(pyrrolyl) plane. The isomer syn-2 was isolated in 31% yield. Also found in the reaction mixture were benzyl alcohol (51% yield), benzaldehyde (11% yield), and 1,2diphenylethane (6% yield), where the yields are relative to internal standard (dodecane) from GC-FID. The syn-isomer also was structurally characterized (Figure 1.4 bottom). The structure of syn-2 is, like the structure of 1, in between sp and tbp with η = 0.61. The Mo–N(pyrrolyl) and Mo–NMe2 distances in syn-2 are the same within error as in 1. The Mo1–N3(donor) and Mo1– N5(nitrido) distances are the same as in anti-2. 13 Substrate (H2NX) %Yield of a 2 1 ( H NMR) Byproduct %Yield HX (GC-FID) b kobs –1 –1 –3 c (M s •10 ) 75 – 873 ± 5 18 31 684 ± 2 81 – 146.8 ± 0.1 H2NN(Ph)Me 82 91 135.8 ± 0.2 H2NNPh2 76 98 21.1 ± 0.1 H2NOBn 31 51 H2NNMe2 b e d – a b The product is 2-anti except for the reaction with H2NOBn which gave 2-syn. By-product c yield was not determined. Errors are from the fits and then propagated through the equations. d e Reaction was too fast to measure using the methods employed here. Yield given is for benzyl alcohol, but 3 by-products were identified. Table 1.1: Rates and yields of the reactions of various substrates with 1 to form 2. A computational study was carried out on the two isomers using Density Functional Theory with the LANL2DZ basis set as implemented in Gaussian09. 20 The difference in energy (∆H) between the syn and anti derivatives was found to be extremely small with syn-2 being more stable than anti by 2 kcal/mol using B3LYP as the functional. Using B3PW91 as the functional a similar value of 2 kcal/mol was found with syn more stable than anti. 14 Experimentally, heating syn-2 in toluene at 100 °C for 24 h led to some conversion to the anti-2 isomer (see Appendix A for more details); however, the conversion did not continue to completion and some decomposition also occurred. Only about 9% anti was produced during the 1 heating of the syn isomer. Alternatively, heating anti-2 did not result in detectable ( H NMR) amounts of syn-2; only decomposition was observed. It seems that the energies of the isomers are very comparable but kinetic barriers hamper the equilibrium. The isomer, syn or anti, produced in the reaction of 1 and hydrazine or hydroxylamine is determined kinetically. Since the hydrazido(1–) derivatives are unstable intermediates in the case of Mo(IV), in order to examine their structure, we prepared the Zr(IV) analogues where no bond cleavage can occur. The zirconium bis(dimethylamido) complex Zr(NMe2)2(dpma mes by addition of H2dpma mes ) (3) is cleanly produced to Zr(NMe2)4; 3 was also structurally characterized (see Appendix A). The addition of one equivalent of H2NNMe2 to 3 provides mixtures of the bis(hydrazido(1– )) complex Zr(NHNMe2)2(dpma mes ) (4) and a trace of a compound not fully characterized that 1 has a H NMR spectrum as expected for the mono(hydrazido(1–)) complex. It appears that the second addition of hydrazine may have a similar rate constant to the first. The complex 4 was prepared cleanly by addition of 2 equivalents of the hydrazine (Figure 1.5). 1.3 Mechanistic Investigations The reaction between molybdenum-containing 1 and hydrazines was not amenable to st typical pseudo-1 order conditions for the examination of the reaction kinetics. Using either the metal complex or the hydrazine in large excess led to very low yields of the nitrido product, and we were unable to isolate and characterize the products under these conditions. However, the nitrido product does not react with excess hydrazine on the timescale of the hydrazine reactions 15 with 1. Figure 1.5: Synthetic route to the hydrazido(1–) zirconium complex 4 and ORTEP diagram at 2 the 50% probability level for the structure of Zr(η -NHNMe2)2(dpma mes ) (4) as determined by single crystal X-ray diffraction. H-atoms (pink spheres) are omitted except on the hydrazido(1–) nitrogens N4 and N6. 16 Excess hydrazine or 1 in the N–N cleavage reactions leads to unidentified by-product formation; however, we were able to vary the hydrazine concentrations and examine initial rates st for the loss of 1. These experiments suggest a 1 order dependence on hydrazine concentration. st Similar initial rate experiments changing metal concentration suggest a 1 order dependence on the concentration of 1. Kinetics using 1:1 hydrazine to 1 provided clean second order behaviour. Considering the st st 1 order dependence of the reaction on hydrazine, 1 order dependence on 1, and 2 nd order dependence overall, the rate law is assigned as –d[1]/dt = kobs[1][dimethylhydrazine]. The reaction to form 2 was carried out with a variety of different substrates (Table 1.1). With all the hydrazine derivatives investigated, anti-2 was the product. There was a dramatic affect of the substituents on the rate of nitrido formation; however, the cause of that dependence seems complex and is likely due to a mixture of factors including sterics constraints of the incoming reactant. Reactions with all of the hydrazines were followed by UV-Vis absorption spectroscopy and fit 2 nd order kinetics. Using the G3 method 21 implemented in Gaussian09, we calculated the Bond Dissociation Enthalpies (BDEs) associated with some of the substrates in Table 1.1. The BDE of Me2NNH2 for the N–N bond was calculated as 60.2 kcal/mol, whereas the experimental BDE for this compound is 59.0 ± 2. 22 The N–N and N–O BDEs for N-aminopyrrole and H2NOMe (as a model for H2NOCH2Ph) were calculated as 34.9 and 54.4 kcal/mol, respectively. As a result, it appears that the rate of bond cleavage is not correlated with the N–N or N–O BDE. The only species observed by UV-Vis absorption spectroscopy for all of the hydrazine 17 substrates, except N-aminopiperidine, over the course of the reactions are the starting material 1 and the product 2. These reactions show a clean isosbestic point (see Appendix A). However, the reaction with N-aminopiperidine is complicated by reactions of the piperidine by-product with starting material, which is likely the cause of the low yields for this particular substrate. All other by-products do not react on the timescales of nitrido formation with either the starting material or product. No product inhibition was found for the hydrazine reactions except for addition of piperidine to reactions of N-aminopiperidine with 1. Other by-products were tested with up to 10 equivalents of the corresponding amine and gave the same rate constant for disappearance of 1 and provided clean formation of 2. The reaction with O-benzylhydroxylamine liberates benzyl alcohol as the by-product. The nitrido product 2 does not react with benzyl alcohol. The starting material 1 does react rapidly with benzyl alcohol using a radical pathway. 22 We examined the temperature behaviour of the rate of 1,1-dimethylhydrazine reactions ‡ with 1. An Eyring plot of ln(kobs/T) vs 1/T was linear and provided ∆H = +7 kcal/mol and ∆S ‡ = –35 cal/mol•K. These parameters are consistent with a very modest enthalpic barrier and a very ordered activated complex. The parameters are similar to many known activation parameters for ligand additions to metal complexes. 23 The data above did not conclusively identify the rate-determining step in the reaction. In order to further investigate the NMe2 for NHNMe2 exchange as a possible rate-determining step, we used the zirconium complex 3 and its reaction with H2NNMe2 as a model. The reaction 1 between 3 and two equivalents of dimethylhydrazine was followed by H NMR and showed 18 second-order kinetics like its molybdenum analogue. Examination of the second order rate ‡ constant versus temperature for the zirconium reaction gave activation parameters, ∆H = +6.4 ‡ kcal/mol and ∆S = –45 cal/mol•K, similar to the molybdenum system. We propose that the rate-determining step in the hydrazine reaction with 1 is the dimethylamido substitution step. In zirconium-containing 3, the second replacement of NMe2 has a similar rate as the first replacement with dimethylhydrazine (vide supra). A second NMe2 replacement is not observed in the reaction with the molybdenum(IV) analogue. Since the N–N bond cleavage in the reaction of 1 with dimethylhydrazine would then be faster than the substitution of dimethylamido, reaction of the first equivalent of dimethylhydrazine with 1 gives the mono(dimethylamido) complex 2. The nitrido 2 is then inert to NMe2 replacement by dimethylhydrazine. In other words, the unimolecular N–N bond cleavage occurs much faster than the bimolecular reaction of hydrazine and the unobserved hydrazido(1–) intermediate Mo(NHNMe2)(NMe2)(dpma mes ). In light of the data above, we propose the N–N cleavage mechanism illustrated in Scheme 1.3. (For discussions of alternative mechanisms see Appendix A.) One of the dimethylamido ligands in 1 is protolytically replaced with a hydrazido(1–) ligand. We speculate that the unobserved hydrazine adduct A adopts a geometry reminiscent of previously reported 18 Mo(NMe2)2(NHMe2)(dpma) where the donor nitrogen of NHMe2 is trans to the donor nitrogen of the dpma ancillary. It appears that it is this bimolecular coordination of the hydrazine (or hydroxylamine) derivative to the metal that is rate determining. The alternative rate determining steps are proton migration (conversion from A to B) or the coordination and proton migration occurring in a 19 concerted fashion. We assign the RDS as the coordination based on the similarity of the activation parameters to other associative substitutions. 20 24 Scheme 1.3:. Proposed mechanism for the reaction of 1 with H2NNMe2. Mesityl groups on the mes dpma ligand were omitted for clarity. 21 In examining various donor ligands with 1, flat and cylindrically symmetric donors t (CNBu , pyridine, DMAP, and 2-picoline) react extremely quickly with reactions being done faster than samples can be taken. Larger donors such as the hydrazone formed from benzaldehyde and 1,1-dimethylhydrazine, Me2NN=C(H)Ph, reacted very slowly over the course of days as judged by disappearance of the UV-Vis bands in 1. Again, steric constraints of the incoming donor ligand are one factor in the rate of reaction in the system. After protolytic cleavage of a dimethylamido and dimethylamine loss, formation of the hydrazido(1–) ligand follows. The experiments with the zirconium hydrazido model suggest that 2 the hydrazido(1–) is η in this intermediate. In the next step, the β-nitrogen of the hydrazido acts as a proton acceptor during the α,βproton shift. The N–N bond cleavage could occur concomitant with proton migration (Path A) or through an intermediate ammonium hydrazido(2–), sometimes called a hydrazidium (Path B). (See Appendix A for additional discussion.) The reaction mechanism proposed here is an oxidative elimination from a metalappended nitrogen atom, where the metal is oxidized by elimination of substituents to form a i metal ligand multiple bond. In a process that might involve the microscopic reverse of the hydrazine cleavage reaction described, the nucleophilic addition of an amine to a nitrido with concomitant metal reduction has been reported by Meyer and coworkers. example is shown, where tpm = tris(1-pyrazolyl)methane. 26 25 In Figure 1.6 one Several related reductive additions, where the metal center is reduced by addition of substituents to metal ligand multiple bonds have 27 been reported. 22 Figure 1.6: Reductive addition via nucleophilic attack of an amine on a terminal nitrido. Whether the nitrido ends up syn or anti with respect to the methyl on the dpma mes donor nitrogen may be determined by which dimethylamido is kinetically preferred for protolytic replacement in intermediate A of Scheme 1.3. Considering the similarity between the two dimethylamidos, we postulate that this is determined by steric interactions between the complex and hydrazine/hydroxylamine substrate leading to the difference in products observed when using hydrazines and O-benzylhydroxylamine. Alternatively, the sites of initial coordination for the larger H2NNR2 compounds could be as shown in A, while the smaller H2NOBn may bind in t a site similar to the isonitrile in 1•CNBu , trans to a dimethylamido (Figure 1.3). 1.4 Conclusions We have described a new 5-coordinate Mo(IV) complex with spin crossover behaviour in –1 solution. The free energy barrier for the spin state change was measured as 1150 cm . The singlet is the ground state in solution and was the only species observed in the solid, suggesting that molecular dynamics unavailable in the lattice are required for the spin equilibrium. The Mo(IV) compound reacts in a unimolecular fashion with hydrazines and O-alkyl hydroxylamines to give the Mo(VI) nitrido complex, NMo(NMe2)(dpma mes ) (2). Depending on the nature of the nitrogen atom donor molecule (hydrazines or O-benzylhydroxylamine), two different isomers of 2 were isolated, one isomer where the methyl group of the dpma 23 mes ligand is syn to the nitrido and one where the methyl is anti. We propose a mechanism where the hydrazido(1–) complex undergoes an α,β-proton shift either concerted with N–N bond cleavage or in a stepwise fashion with an unstable hydrazidium, ammonium hydrazido(2–), intermediate. While the N–N cleavage mechanism proposed in Scheme 1.3 does not fall into either of the commonly discussed pathways in Scheme 1.1, there is precedent for this type of mechanism out of the peroxidase literature for O–O bond cleavage. 28 It is proposed that peroxidase uses an α,β-proton shift in a heme iron peroxide to generate a histidine-stabilized hydrogen isoperoxide complex. Cleavage of the O–O bond, which computationally occurs simultaneously with proton migration (cf. Path A in Scheme 1.3), liberates water and generates the ferryl iron(IV) with a porphyrin radical cation (Compound I). 29 We propose a very close nitrogen analogue of the peroxidase mechanism is the low energy pathway that leads to facile N–N bond cleavage in our system and is an important possible pathway for N2 activation in general. 24 Scheme 1.4: Poulos-Kraut mechanism 30 for heterolytic O–O cleavage and ferryl generation in peroxidase. 1.5 Experimental General experimental details and a more thorough discussion on how the kinetic data were collected and the results can be found in Appendix A. H2dpma mes In a 125 mL Erlenmeyer flask methylamine hydrochloride (0.911 g, 13.5 mmol, 1 equiv.) was dissolved in formaldehyde solution (37% v/v) (2.19 g, 27.0 mmol, 2 equiv.) and 25 EtOH (20 mL). The solution was transferred to a 100 mL Schlenk tube, sealed, and stirred for 10 min in a 55 C oil bath. 2-Mesitylpyrrole (5.00 g, 27.0 mmol, 2 equiv.) in EtOH (30 mL) was added to the Schlenk tube, and the headspace was evacuated. The solution continued to stir at 55 C for 7 h, during which a white precipitate formed. The Schlenk tube was cooled to room temperature, and the precipitate was collected on a glass frit and washed with EtOH (3 × 20 mL). The solids were basified with aq. NaOH (1 M, 150 mL) and extracted with CH2Cl2 (3 × 100 mL). The organic layers were combined and dried under reduced pressure, yielding H2dpma mes 1 as a white powder (3.97 g, 9.32 mmol, 69%). H NMR (500 MHz, CDCl3): 8.01 (s br, 2H, N-H), 6.89 (s, 4H, aromatic C-H), 6.07-6.05 (m, 2H, pyrrole C-H), 5.92-5.90 (m, 2H, pyrrole C-H), 3.53 (s, 4H, CH2), 2.28 (s, 6H Ar-p-CH3), 2.18 (s, 3H, NCH3), 2.11 (s, 12H, Ar-o-CH3). 13 1 C{ H} NMR (125 MHz, CDCl3): 138.3, 137.4, 130.9, 129.1, 128.0, 127.9, 108.0, 107.7, 53.5, 41.9, 21.0, 20.6. Anal. Calcd. for C29H35N3: C, 81.84; H, 8.29; N, 9.87. Found: C, 81.53; H, 8.32; N, 9.67. Mp: 82-84 C. Mo(NMe2)2(dpma mes ) (1) In a glove box under an N2 atmosphere, a 100 mL Schlenk tube was loaded with a stir bar and a solution of Mo(NMe2)4 11 (1.00 g, 36.7 mmol, 1 equiv.) in toluene/hexane (1:4, 5:20 mL). To the Schlenk tube, H2dpma mes (1.56 g, 3.67 mmol, 1 equiv.) in toluene (8 mL) was added. The headspace was evacuated, and the vessel was sealed with a Teflon stopcock. The tube was removed from the box and was placed into a 55 C oil bath for 10 h, while stirring vigorously. After this time, the vessel was allowed to cool to room temperature and was taken back inside the dry box. The volatiles were removed in vacuo, and the solids were washed with hexane (10 mL). The solids were dissolved in a minimal amount of toluene and 26 held at −35 C yielding 1 as bright green crystals (1.78 g, 2.94 mmol, 80%). Magnetic susceptibility (Evan‘s method, 29.7 C): μeff = 1.178 μB. TOF-MS ES+ calcd (found): 608.68 –1 –1 (609.2). UV-Vis [toluene, 25 C] λmax in nm (ε in cm M ): 643.9 (498.7), 786.9 (187.4). M.p.: 138-144 C (d). The molecule contains a disordered toluene in the lattice as crystallized. Attempts to obtain elemental analysis were not satisfactory unless toluene was included with occupancy of 0.3. Anal. Calcd for C34H45MoN5•0.3C7H8: C, 66.36; H, 7.52; N, 11.02. Found: C, 66.60; H, 7.26; N, 11.39. t Mo(NMe2)2(CNBu )(dpma mes t ) (1•CNBu ) Under an N2 atmosphere, a scintillation vial was loaded with a stir bar, 1 (0.1 g, 0.165 mmol, 1 equiv.) and toluene (5 mL). To this, tertbutylisonitrile (0.1 g, 0.165 mmol, 1 equiv.) in toluene (1 mL) was added. The solution was stirred and rapidly turned dark red. After 1 h, the volatiles were removed in vacuo. The residue was extracted with toluene (2 mL). The solution was filtered through Celite and layered with an t equal volume of pentane. Crystallization at −35 C gave dark red crystals of 1•CNBu in 40% yield (0.045 g, 0.066 mmol). Magnetic susceptibility (Evan‘s method, 28.2 C): μeff = 2.469 μB. The molecule contains a toluene in the lattice as crystallized. Attempts to obtain elemental analysis were not satisfactory unless toluene was included with full occupancy. Anal. Calcd for C38H54MoN6•C7H8: C, 69.03; H, 7.98; N, 10.73. Found: C, 68.89; H, 8.17; N, 10.61. Mp: 128–1 –1 130 C (dec). UV-Vis [toluene, 25 C] λmax in nm (ε in cm M ): 372.2 (6588), 490 (3042). Crystals for X-ray diffraction grown from toluene gave poor structural results, and the crystals were regrown from Et2O. 27 Mo(N)(NMe2)(dpma mes ) (anti-2) Under an N2 atmosphere, a scintillation vial was loaded with a stir bar, 1 (0.650 g, 1.07 mmol, 1 equiv.) and toluene (8 mL). To the stirring solution of 1, a solution of N,N-dimethylhydrazine (0.64 g, 1.07 mmol, 1 equiv) in toluene (1 mL) was added. Upon addition, the solution turned brown and an orange precipitate formed. After 1 h, the volatiles were removed in vacuo. The residue was stirred in pentane (2 mL) for 5 min, and the suspension was filtered on a glass frit. The solids were collected and dried in vacuo yielding the title compound as an orange powder (0.459 g, 0.795 mmol, 75%). Diffraction quality crystals 1 were grown from a concentrated toluene solution layered in pentane held at −35 C. H NMR (500 MHz, C7D8): 6.70 (s, 2H, aromatic C-H), 6.58 (s, 2H, aromatic C-H), 6.37 (d, JHH = 3.0 Hz, 2H, pyrrole C-H), 6.17 (dd, JHH = 0.5 Hz, JHH = 3.0 Hz, 2H, pyrrole C-H), 4.53 (d, JHH = 13.0 Hz, 2H, CH2), 3.52 (s, 3H, NCH3), 3.34 (d, JHH = 12.5 Hz, 2H, CH2), 2.43 (s, 6H, Ar-pCH3), 2.18 (s, 3H N(CH3)2), 2.12 (s, 3H N(CH3)2), 2.05 (s, 6H, Ar-o-CH3), 2.04 (s, 6H, Ar-oCH3). 13 1 C{ H} NMR (125 MHz, C6D6): 140.8, 140.2, 139.2, 138.4, 136.8, 136.6, 111.2, 106.8, 63.4, 46.5, 43.6, 21.2, 21.0, 20.9. Anal. Calcd for C31H39MoN5: C, 64.46; H, 6.81; N, 12.12. Found: C 64.35; H, 6.72; N, 12.08. Mp: 264-270 C (dec). Mo(N)(NMe2)(dpma mes ) (syn-2) Under an N2 atmosphere, a scintillation vial was loaded with a stir bar, 1 (0.123 g, 0.202 mmol, 1 equiv.), and toluene (8 mL). To the stirring solution of 1, a solution of O-(benzyl)hydroxylamine (0.750 mL, 0.269 M, 0.202 mmol, 1 equiv.) in toluene (1 mL) was added. Upon addition, the solution turned brown. After 2 h, the volatiles were removed in vacuo. The residue was taken up in Et2O (5 mL) and filtered through Celite. The solution was concentrated in vacuo to 2 mL and held at −35 C, which crystalized 2-syn as orange blocks 28 1 (0.0356 g, 0.062 mmol, 30.7%). H NMR (500 MHz, C6D8): 6.79 (s, 2H, aromatic C-H), 6.63 (s, 2H, aromatic C-H), 6.32 (d, JHH = 3.0 Hz, 2H, pyrrole C-H), 6.26 (d, JHH = 3.0 Hz, 2H, pyrrole C-H), 3.66 (dd, JHH = 0.5 Hz, JHH = 14.0 Hz, 2H, CH2), 3.50 (dd, JHH = 0.5 Hz, JHH = 14.0 Hz, 2H, CH2), 3.20 (d, JHH = 1.0 Hz, 3H, N(CH3)2), 2.74 (s, 3H, NCH3), 2.60 (s, 6H, Ar-pCH3), 2.50 (d, JHH = 1.0 Hz, 3H N(CH3)2), 2.11 (s, 12H, Ar-o-CH3). 13 1 C{ H} NMR (125 MHz, CDCl3): 139.72, 139.62, 137.23, 136.86, 136.54, 134.77, 128.42, 127.45, 112.15, 106.10, 63.88, 59.31, 53.13, 47.56, 21.35, 20.98, 20.84. Mp: 117-123 C (dec). By-products of this reaction include benzyl alcohol (51.2%), benzaldehyde (10.7%) and 1,2-diphenylethane (5.9%) as determined by GC-MS/GC-FID; the yields are relative to dodecane internal standard for calibrated samples of those compounds. Zr(NMe2)2(dpma mes ) (3) Under an N2 atmosphere, a 100 mL Schlenk tube was loaded with a stir bar, Zr(NMe2)4 (0.851 g, 3.18 mmol, 1 equiv.), toluene (6 mL), and Et2O (1 mL). To the pressure tube was added H2dpma mes (1.35 g, 3.18 mmol, 1 equiv.) in toluene (1 mL). The headspace was evacuated, and the tube was sealed with a Teflon stopcock and removed from the dry box. The solution was stirred in a 70 °C oil bath for 48 h. The tube was taken back into the dry box, and the solution was filtered through Celite. The volatiles were removed in vacuo 1 yielding 3 as a yellow powder (1.75 g, 2.89 mmol, 91% yield). H NMR (500 MHz, C6D6): 6.82-6.81 (m, 2H, aromatic C-H), 6.74-6.73 (m, 2H, aromatic C-H), 6.30 (dd, JHH = 2.75 Hz, JHH = 0.5 Hz, 2H, pyrrole C-H), 6.16 (d, JHH = 3.0 Hz, 2H, pyrrole C-H), 4.05 (d, JHH = 13.5 Hz, 2H, CH2), 3.40 (d, JHH = 13.5 Hz, 2H, CH2) 2.59 (s, 6H, N(CH3)2), 2.28 (s, 6H, N(CH3)2), 29 2.26 (s, 3H, NCH3), 2.12 (s, 6H, Ar-o-CH3), 2.10 (s, 6H, Ar-o-CH3), 2.02 (s, 6H, Ar-p-CH3). 13 1 C{ H} NMR (125 MHz, CDCl3): 139.07, 138.61, 138.26, 136.03, 135.94, 135.92, 128.02, 127.47, 127.37, 109.25, 104.71, 59.39, 43.25, 40.63, 39.16, 21.17, 21.01, 20.75. Mp: 262-270 °C. 2 Zr(η -NHNMe2)2(dpma mes ) (4) Under an N2 atmosphere, a scintillation vial was loaded with 3 (0.100 g, 0.166 mmol, 1 equiv.), a stir bar, and a mixture of toluene and Et2O (1:1 v:v, 8 mL). The solution was rapidly stirred and a 0.712 M toluene solution of 1,1-dimethylhydrazine was added dropwise (466 μL, 0.332 mmol, 2 equiv.). DME (1 mL) was added. The solution stirred for 16 h, and the volatiles were removed in vacuo. 4 was obtained in 86% yield as an off-white powder (0.090 g, 1.42 mmol, 86% yield). Diffraction quality crystals of 4 were obtained from a 1 −35 °C concentrated toluene solution layered with Et2O. H NMR (500 MHz, C7D8): 6.83 (br s, 2H, aromatic C-H), 6.74 (br s, 2H, aromatic C-H), 6.25 (d, JHH = 3.0 Hz, 2H, pyrrole C-H), 6.14 (dd, JHH = 1.0 Hz, JHH = 3.0 Hz, 2H, pyrrole C-H), 4.30 (s, 1H, NH), 4.17 (d, JHH = 13.5 Hz, 2H, CH2), 4.10 (s, 1H, NH), 3.68 (d, JHH = 13.5 Hz, 2H, CH2), 2.48 (s, 3H, NCH3), 2.30 (s, 6H N(CH3)2), 2.22 (s, 6H, N(CH3)2), 2.12 (s, 6H, Ar-o-CH3), 1.91 (s, 3H, Ar-o-CH3), 1.57 (s, 6H, Ar-p-CH3). 13 1 C{ H} NMR (125 MHz, C6D6): 140.12, 139.04, 138.91, 138.89, 136.41, 135.63, 128.54, 111.29, 108.21, 104.08, 62.57, 53.49, 50.91, 42.62, 22.44, 21.49, 21.07. Mp: 216-218 °C (dec). 30 APPENDIX A 31 APPENDIX A A.1: General Considerations for the Experiments Reactions and manipulations of air sensitive materials were carried out in an MBraun glovebox under a nitrogen atmosphere and/or using standard Schlenk techniques. Ethereal solvents, pentane, hexane, and toluene were purchased from Aldrich Chemical Co. and purified through alumina columns to remove water after sparging with dinitrogen to remove oxygen. Other compounds, such as formalin and methylamine hydrochloride, were purchased from commercial sources and used as received. All NMR solvents were purchased from Cambridge Isotopes Laboratories, Inc. Deuterated toluene and benzene were distilled from sodium benzophenone ketyl. Deuterated chloroform was distilled from CaH2 under dry a dinitrogen atmosphere. The NMR solvents were stored in the glovebox in glass containers with a stopcock. Spectra were taken on Varian instruments located in the Max T. Rogers Instrumentation Facility at Michigan State University. 32 A.2: Determination of Second-Order Rate Constants for Table 1.1 The reactions of 1 with various hydrazines were followed over ~3 half-lives. All kinetic data were obtained on an Evolution 600 model Thermo UV-Vis spectrometer. Temperature control was possible through a Peltier sample holder which maintained ± 0.2 °C throughout the run. Hydrazine was injected through a septum directly into an airtight UV-Vis absorption cell. Reactions were performed in toluene (3.5 mL) with constant stirring throughout the run. Single wavelength absorbance measurements were taken at 651 nm. This point was chosen due to the large difference in the absorbance of 1 (starting material) and 2-anti (product) at this wavelength (ε651 = 621.46 for 1 vs ε651 = 24.23 for 2). Absorbance data were fit to the Second-order Equation below using OriginPro 7.5 software, where Abs = absorbance, y∞ and y0 are floating 1 parameters corresponding to the ending and starting absorptions respectively. (The expression is only applicable when rate = k[A]2 or when 1 rate = k[ A][B] for [A] = [B].) A minimum of 3 runs was used to obtain the rate constants in Table 1.1 of the manuscript, and the errors are at the 99% confidence level. ( ( ) [ ] ) Shown below is one of the fits to absorption data for one of the kinetics runs where the hydrazine derivative was 1-aminopyrrole as a representative example. The black line is for the data, and the red line is the fit to the 2 nd order equation above. 33 Figure A.1: A representative second order fit of 1 with disubstituted hydrazine st It is also useful to consider how well the 1 order expression fits the same curve. The first 1 order equation used is shown below where the parameters have the same meaning as above. The nd fit (red line in the plot) is much worse in comparison with the fit to the 2 ( ) 34 ( ) order expression. st Figure A.2: A representative pseudo-1 order fit of 1 with disubstituted hydrazine 35 A.3: Initial Rates Experiments on 1,1-Dimethylhydrazine Concentration To determine the order in hydrazine, considering pseudo-first order conditions were not possible with the system, we used initial rates. All kinetic data were obtained on an Evolution 600 model Thermo UV-Vis spectrometer. Temperature control was possible through a Peltier sample holder which maintained ± 0.2 °C throughout the run. 1,1-Dimethylhydrazine was injected through a septum directly into an airtight UV-Vis absorption cell. Reactions were performed in toluene (3.5 mL) with constant stirring throughout the run. Single wavelength absorbance measurements were taken at 651 nm. This point was chosen due to the large difference in the absorbance of 1 (starting material) and 2-anti (product) at this wavelength (ε651 = 621.46 vs ε651 = 24.23 respectively). The data were truncated around 10% conversion of 1 as judged by the decrease from the absorbance at t = 0. The initial rate is determined from the slope 1 of the linear fit to the line. The initial absorbances, intercept of the lines, were allowed to fit and did vary slightly from run to run for this extremely reactive and air-sensitive compound even though reactions were run 36 from stock solutions. The [1] vs time plot is shown below with the linear fits. Figure A.3: Initial rate data for the conversion of 1 to 2-anti as a function of metal concentration. 37 The tabulated hydrazine concentrations and rates are shown in the figure and table below. Figure A.4: Fits of rate at 10% of conversion of 1 to 2-anti along with tabulated data. 38 Since the reaction is cleanest when the [1] to [hydrazine] are 1:1 and the reaction produces unidentified by-products when the [hydrazine] goes either higher or lower than [1], we chose to use Run D, which has these two concentrations closest, as the most accurate run for the analysis below. For any two reactions in the set, the following equation should apply where rateD is the rate of reaction D determined from the slopes above, power of a, and [1]a D is the concentration of 1 in Run D to the [hydrazine]b is the concentration of the hydrazine in Run D to the power of D b. a b rateD kobs [1] D [hydrazine] D = rateX kobs [1]a [hydrazine]b X X If we assume that kobs [1]a ~ kobs [1]a , those terms can be eliminated. Rearrangement and D X using ln to isolate the order gives the equation below where the order in hydrazine, b, is related to the rates and concentrations of any two runs. b= æ rateD ö ln ç è rate ÷ ø X æ [hydrazine]D ö ln ç è [hydrazine] X ÷ ø To find the orders in hydrazine relative to Run D (where the reagents are ~1:1) we applied the above expression. The run with the lowest order was Run B displaying an apparent order of 0.45; 2 this run also had the worst R factor for the fit. The highest order obtained was 1.35 for Run E. 39 The average order for all of the runs was b = 0.90, and the reaction was assigned as first order in hydrazine. A.4: Initial Rates Experiments on 1 Concentration As a final check on the rate law, we did briefly examine the dependence of the reaction on the concentration of 1. The conditions and procedure were very similar to the procedure above for the initial rates experiments on hydrazine concentration. The exception being that raising the metal concentration much over that used in the hydrazine initial rates experiments leads to solutions that are too concentrated for accurate absorption measurement. Consequently, the concentrations of the hydrazine and 1 were lowered somewhat relative to those above. The raw data are tabulated below with the initial concentration of 1 and the rate of the disappearance of the 1. The plots and linear fits are given. [1] (M) [hydrazine] (M) Rate (10–7) J 0.00122 0.000607 –3.44 K 0.00122 0.000866 –4.13 L 0.00142 0.0011 –6.52 M 0.00132 0.0011 –4.87 Table A.1:Rate dependence on the concentration of 1. 40 Figure A.5: Initial rate data of conversion of 1 to 2-anti as a function of the concentration of 1. 41 As before, the analysis was done using Run D, where the concentrations of 1 and hydrazine are equal, as the reference Run. We then assumed that the order in hydrazine, exponential b, was unity as found in the experiments described previous. This gives the expression below for the order in 1, a. a b rateD kobs [1] D [hydrazine] D = rate X kobs [1]a [hydrazine]bX X æ rateD ö ln ç ÷ è rate X ø a= æ [1] D ö ln ç ÷ è [1] X ø Again, the metal complex is extremely reactive and there is some variation in the data. However, for the four Runs the orders in [1], a in the expression above, were found to be 0.77, 1.11, 0.97, and 1.48 for Runs J–M respectively. This gives an average order of 1.08. Based on this data, the reaction being 2 nd order overall, and 1 st assigned as 1 order in metal. 42 st order in hydrazine, the reaction was A.5: Thermal Conversion of syn-2 into anti-2 Figure A.6: Thermal conversion of 2-syn into 2-anti. 43 A.6: UV-Vis trace of anti-2 formation using H2NNMe2 and 1 (60 s intervals) Figure A.7: UV-Vis trace of 2-anti formation using H2NNMe2 and 1 (60 s intervals) showing a clear isobestic point, thus no long lived intermediates. 44 The trace shows an isobestic point, as do all of the substrates with 1 with the exception of Naminopiperidine. It was shown independently that the piperidine by-product reacts rapidly with 1 on the time scale of the experiment, which leads to its 45 anomalous behavior. Figure A.8: UV-Vis trace of 2-anti formation using N-Amino piperdine showing the production of other metal containing species, likely the results of product inhabition. 46 A.7: Powder diffraction on samples of 1 Presumably due to the high reactivity of 1 and it‘s tendency to retain toluene in the lattice, we had difficulty getting an adequate elemental analysis on the paramagnetic compound. One method used to examine the purity of new samples of the compound was to calculate the powder structure of the complex from the single crystal X-ray diffraction experiment and to examine the new sample by power diffraction. One 47 such comparison is shown below. Figure A.9: Experimental and calculated X-ray powder pattern for 1. 48 A.8: Eyring Plot for H2NNMe2 reaction with 1 The reaction of 1,1-dimethylhydrazine with 1 was carried out over as large a temperature range accessible. The Eyring plot of ln(k/T) vs 1/T is shown below with the linear fit. 49 Figure A.10: Eyring plot of ln(k/T) vs 1/T with a linear fit giving activation parameters. 50  k  H  k  S ln     ln  B   R T   h  RT ‡ ‡ (1) H ‡  m  R (2)   k  S ‡  R b  ln  B   h   (3) Equation 1 above is the Eyring Equation itself, with R = gas constant, h = Planck‘s constant, ‡ kB = Boltzmann constant, T = Temperature in Kelvin, and k = observed rate constant. The ∆H ‡ and ∆S are found from the slope (m) and intercept (b) of the plot using Equations 2 and 3. Using R = 1.986 cal/K•mol, kB = 1.381 ´ –23 10 J/K, and h = 6.626 ´ –24 10 as below. DH ‡ = -(–3556.3 ´ 1.986) DH ‡ = +7.1 kcal/mol é æ 1.381´ 10-23 ö ù DS = 1.986 ê6.08 - ln ç -34 ú è 6.626 ´ 10 ÷ ú øû ê ë DS ‡ = 1.986 é6.08 - 23.76 ù ë û ‡ DS ‡ = -35.1 cal/mol•K 51 J•s, the equations set up A.9: Additional Mechanistic Discussions As is usually the case, there are some other mechanisms for the N–N and N–O bond cleavage reaction that are difficult to rule out entirely. Here are a few additional arguments for the mechanistic proposal and some of the other possible mechanisms considered. Coordination/Proton Transfer as Rate Determining Step Figure A.11: α,β-proton migration as the rate determining step. Mes groups removed for clarity. From the activation parameters, the transition state for the rate-determining step is extremely ordered. The very ordered step proposed for the rate-determining step is the addition of a ligand 52 (NH2X = hydrazine or hydroxylamine) to 5-coordinate 1 in an associative process. This could occur either in conjunction with proton transfer to the dimethylamido or stepwise. The activation parameters for ligand additions to metals in an associative process can be of ‡ similar magnitude to the parameters observed in our reaction. In our system ∆H = +7 kcal/mol ‡ and ∆S = –35 cal/mol•K. For example, the second-order, associative exchanges of TaCl5•L with ‡ additional L (L = SMe2, SeMe2, TeMe2) have activation parameters of ∆H = 5.5 to 5.7 ± 0.3 ‡ kcal/mol and ∆S = –25.8 to –22.7 ± 1.2 cal/mol•K. Also for reference, the associative exchange ‡ ‡ of SMe2 to trans-Pd(SMe2)2Cl2 has activation parameters of ∆H = +9.2 kcal/mol and ∆S = – 18 cal/mol•K where the complex goes from 4-coordinate to 5-coordinate. Small positive enthalpies and large negative entropies of activation seem to be common for ligand association reactions in such cases. Considering the addition of H2NNMe2 to the zirconium analogue of 1, also has similar activation parameters and no possibility of doing the other steps (namely the N–N bond cleavage), we assigned the coordination of hydrazine as the rate-determining step. Whether the coordination occurs with concomitant proton transfer is unknown. Attempts to produce the very dry D2NN(Me)Ph necessary for kinetic isotope studies did not lead to a pure isotopomer. Kinetics using about 50% labeled 1-methyl-1-phenylhydrazine did not show a kinetic isotope effect outside the error bars. However, we are uncertain as to the definitiveness of the results considering H/D-exchange in the various intermediates with the hydrazine and by-product amine may mask the KIE. 53 Hydrazido(2–) Formation with HNMe2 Back Reaction Another alternative pathway to 2 would involve formation of a hydrazido(2–) intermediate with loss of both dimethylamido ligands from the starting material 1. We know that NMe2 is present in the final product 2 regardless of the nature of X, but this may occur through loss of NMe2 followed by regaining the NMe2 by protonation of X in a fast step. The slow step might then be oxidative addition of N–X to the metal center through an transition state like the one below. Hydrazido(2–) complexes like the one shown in the transition state above have been proposed and even observed in the presence of Lewis acids for Zr and Hf complexes where X = NPh2. However, the reaction requires the replacement of X protolytically with HNMe2. We have examined reactions of 2 with a variety of potential proton donors. The position occupied by NMe2 in 2 is flanked by two mesityl groups that seem to very competently shield the nitrogen from protonation. For example, the dimethylamido group in 2 does not react with piperidine, Nmethylaniline, diphenylamine, or benzylalcohol under any conditions we have found. In other words, the product nitrido is quite resistant towards all of the by-products of the N–N and N–O bond cleavage reactions not only on the order of the reaction but also on the order of days! 54 Figure A.12: Alternate mechanism of bond cleavage going through a hydrazido(2−) pathway 55 Consequently, it seems unlikely that X in the same position as NMe2 in the nitrido would be readily replaced by HNMe2 in a reaction fast enough that NMo(dpma mes )X is never observed, which makes this mechanism seem unlikely. This replacement would have to occur very rapidly in the reaction regardless of X substituent as well, e.g., HNMe2 would have to rapidly protonate the alkoxide intermediate to release HOBn. Nature of the N–N bond cleavage Because the N–N bond cleavage occurs after rate-determining hydrazine addition information as to its molecularity is more difficult to gather. However, we proffer a unimolecular bond cleavage mechanism. If the α,β-proton migration is not unimolecular and is aided by either another metal center or by another molecule of hydrazine, that bimolecular reaction must be much faster than bimolecular reaction of the Mo(NMe2)(NHNMe2)(dpma replace NMe2 considering no NMo(NHNMe2)(dpma mes ) intermediate with hydrazine to mes ) is observed. It seemed more reasonable to assert that a proton migration, either with concomitant N–N bond cleavage or without, is unimolecular and fast. In addition, the N–N cleavage mechanism proposed is the microscopic reverse of known reactions in the literature and has analogies to known O–O bond cleavage mechanisms as discussed in the paper. 56 A.10: Kinetics on the reaction of Zr(NMe2)2(dpma mes ) (3) with H2NNMe2 Kinetic data for the reaction of 3 with hydrazine was obtained on a Varian 600 MHz NMR instrument. A stock solution of N,N-dimethylhydrazine (2 equiv.) in C6D6 was injected through a septum into an 5 mm NMR tube containing 3 (1 equiv.) in C6D6 (1.2 mL). The reaction was 1 monitored by the H NMR integration of the methylene resonances of 3 and 4. There was an observed intermediate with steady-state behavior with NMR resonances suggestive of Zr(NMe2)(NHNMe2)(dpma mes ). nd Data was fit using the OriginPro 7.5 software package to a 2 -order equation, where Yt is the methylene integral value for 3, Y∞ is the integral value at ―infinite‖ time, and Y0 is the initial integral value. The factor ∆0 is the difference in concentration between dimethylhydrazine and 3 at the start of the reaction. The initial dimethylhydrazine concentration, [hydrazine]0, and zirconium concentration, [3]0, were 0.0633 M and 0.0317 M respectively, giving ∆0 = 0.0317. { ( [ [ [ ] [ ] ] ) } ] The proposed reaction pathway involves reaction of 3 with hydrazine in a stepwise fashion. An intermediate assigned as mono(dimethylamido) (I) is observable at low concentrations (~steady state) throughout the reaction. 57 Figure A.13: Reaction of 3 with 2 equivalents of dimethylhydrazine giving 4 If a second order rate law is applied to the formation of 4 and the steady-state approximation is used to eliminate the concentration of the intermediate I, it is simply found that the rate law should be d[4]/dt = kobs[3][hydrazine] consistent with experiment. 58 nd One of the 2 -order fits is shown below as an example. Figure A.14: A representative 2 nd order fir of reaction of 3 with disubstituted hydrazine 1 Below is a stacked H NMR of the methylene region in the 25 °C kinetics run. The run lasted 1 around 20 h with traces every 20 min. The second stacked H NMR is the same run showing the first 160 min of the reaction 59 with traces every 2 min. Figure A.15: Stacked 1H NMR plots of a reaction of 3 with disubstituted hydrazine over 20 h with traces taken every 20 min (top), and the first 160 min of the same run with traces taken every 2 min (bottom). 60 The reaction rate was examined versus temperature as well. The results are tabulated below along with a ln(k/T) vs 1/T (Eyring) plot. Error in -2 2 Temp(°C) kobs (×10 ) kobs (×10 ) 26.54 1.736 0.017 40.71 2.191 0.061 54.41 3.449 0.037 67.68 7.434 0.137 Table A.2: Rate constants at different temperatures for the reaction of 3 with 2 equivalents of H2NNMe2 ‡ ‡ The calculated activation parameters from the Eyring plot are ∆H = +6 kcal/mol and ∆S = – 45 cal/mol•K. These are similar to the activation parameters for the molybdenum analog of ∆H ‡ = +7 kcal/mol and ∆S = –35 cal/mol•K. 61 ‡ Figure A.16: Eyring plot of 3 with H2NNMe2 The reactions with the zirconium analog are somewhat slower and there are some differences in the structures for the two compounds, most of which are discussed in the text. An overlay of the two structures is 62 provided below. Figure A.17: Overlaid crystal structure renderings of Mo(dpma Zr(dpma mes )(NMe2)2 (3) in blue 63 mes )(NMe2)2 (1) in purple 1 A.11: H NMR Assignment of (2-syn) via HOMO Decoupling 1 Compound (2-syn) shows interesting couplings in its room temperature H NMR spectrum. 1 To investigate these small couplings a series of homo decoupled H NMR spectra were taken. In each spectrum a single resonance was saturated resulting in a change in multiplicity of any other resonance coupled to it. This allowed the assignment of two distinguishable dimethylamido proton environments (3.20 and 2.50 ppm) with a 1 Hz coupling. These methyl protons are likely too far from the N-methyl protons of the dpma mes ligand (5.45 Å syn and 5.55 Å anti in the solid state) to be coupling to those, and no 2D NOESY cross-peak was identified. Furthermore, the methylene resonances from the backbone in the dpma mes ligand displayed a small coupling (JHH = 0.5 Hz) in addition to the geminal coupling caused by hindered rotation upon chelation (JHH = 14.0 Hz). Saturation of each of the β-pyrrolyl resonances showed the origin of this coupling and allowed for the assignment pyrrolyl resonances. 64 1 Figure A.18: H NMR of 2-syn with the assigned proton resonanceses on the inset structure 65 1 Figure A.19: A region of the homo decoupled spectra H NMR of 2-syn showing the changing multiplicity of each peak when saturating another. 66 1 Figure A.20: A region of the homo decoupled H NMR spectra of 2-syn showing the changing multiplicity of each peak when saturating another 67 REFERENCES 73 REFERENCES (1) For some reviews and selected seminal papers see (a) M. D. Fryzuk, Acc. Chem. Res. 2009, 42, 127. (b) P. J. Chirik, Organometallics 2010, 29, 1500. (c) S. Gambarotta, J. Scott, Angew. Chem. Int Ed 2004, 43, 5298. (d) N. Hazari, Chem. Soc. Rev. 2010, 39, 4044. (e) Z. Jonas, P. Sobota, Coord. Chem. Rev. 2005, 249, 2144. (f) K. H. Horn, N. Bores, N. Lehnert, K. Mersmann, C. Nather, G. Peters, F. Tuczek, Inorg. Chem. 2005, 44, 3016. (g) A. Dreher, K. 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Chem., 1978, 56, 2844-2852. (f) M. Kato, S. Aibara, Y. Morita, H. Nakatani, K. Hiromi, J. Biochem. 1984, 95, 861. (g) L. Gebicka, J. Pawlak, J. Molecular Cat. B:Enzymatic, 1997, 2, 185. (30) (a) T. L. Poulos, J. Kraut J. Biol. Chem. 1980, 255, 8199. (b) E. Derat, S. Shaik, J. Phys. Chem. B 2006, 110, 10526. 77 Chapter 2: Molybdenum (IV) Promoted Radical Bond Cleavage of Alcohols ABSTRACT An intriguing product, 1,2 diphenylethane was recovered from the reaction of mes Mo(dpma )(NMe2)2 (1) and O-benzyl hydroxylamine. This was found to be a result of the by- product benzyl alcohol reacting with 1. Subsequently, 1 was treated with 2 equiv. of benzyl or trityl alcohol giving organic products resulting from substitution chemistry followed by βhydride elimination, and the C-C bond forming coupling of radicals. The Mo(VI) bis-oxo mes complex O2Mo(dpma ) (2) is formed as a result of C-O bond cleavage. For comparison the bis-imido analogue Mo(NTol)2(dpma mes ) (3) was independently synthesized via salt metathesis 1 after instillation of the imidos, and was structurally characterized. H NMR analysis of other Mo(VI) complexes bearing the dpma NMo(dpma mes ligand, anti-NMo(dpma mes )(NMe2) and syn- mes )(NMe2) along with 1 and 3 show a link between the germinal coupling on the methylene of the ligand and the orientation of the N-methyl group relative to the metal-ligand multiple bond. The reaction intermediate OMo(dpma mes )(OBn) (5) was isolated from the treatment of 1 with benzyl alcohol and was structurally characterized. This asserts that the C-O bond braking is the result of two successive 1e − steps. Reaction of 1 with one or more equivalents of 1-adamantanol result in the mono-substituted paramagnetic complex mes Mo(dpma )(NMe2)(OAd) (6), which has been determined to be competent for the N-N bond cleaving chemistry like that of 1. 78 2.1 Introduction In probing the reactivity of pyrrolide-supported molybdenum(IV) complexes, which were recently shown to mediate the scission of an N-N single bond, we postulated that mes Mo(dpma )(NMe2)2 (1) could also induce other bond cleavages. Expanding the substrate scope to O-substituted hydroxylamines yielded the products expected from N-O bond cleavage, though in drastically reduced yield (Figure 2.1). Figure 2.1: N-N and N-O bond cleavages mediated by Mo(dpma byproduct of each reaction. 79 mes )(NMe2)2 (1) and the major Despite the lower yield, the cleavage of hydroxylamines was much faster than the analogous cleavage of hydrazines. The heterolytic breaking of N-N and N-O bonds in these − 2 substrates are 2e processes, where the necessary electrons are supplied from the d metal center and donated into a ζ* orbital based on the hydrazido(1-) or N bound hydroxylamido(1-) moiety. This pattern of reactivity is in part rationalized by the thermodynamic stability imparted by the 0 creation of a metal-ligand multiple bond supporting the now Mo(VI), d metal center. Upon further investigation of the cleavage of O-benzyl hydroxylamine, two additional byproducts were identified, the results of product inhibition (Figure 2.2). Benzaldehyde was identified in the reaction mixture by GC-MS. The most likely origin of this is the ligand substitution of 1 with the benzyl alcohol produced by N-O cleavage, and subsequent β-hydride elimination. Analogous imine products (which would result from β-hydride elimination of an amide ligand) are not detected in the case of N-N bond scission of disubstituted hydrazines. In all the hydrazines substrates tried, only the piperdine formed by cleavage of N-amino-piperdine was competent to substitute the dimethylamide ligands of 1. The presence of 1,2-diphenylethane was also detected, the result of a C-C bond forming side reaction. This indicated that Mo(dpma mes )2(NMe2)2 (1) was viable for C-O bond cleavage ( 1 Figure 2.2). H NMR analysis showed a diamagnetic metal containing product, determined to be mes the bis-oxo complex O2Mo(dpma ) (2). This result was striking, as the formation of Mo(VI) − was the result of a series of two 1e processes. This shows that a homolytic pathway is also - available in the Mo(IV) system, still resulting in net 2e oxidation at the metal center. 80 Figure 2.2: Organic byproducts identified in the reaction of 1 with O-benzyl hydroxylamine. Calibrated GC-FID yields are given. Toluene presumed to be created during the reaction was undetected above the background, as it was also the solvent for the reaction. 2.2 Reactivity of Mo(dpma mes )(NMe2)2 (1) benzyl alcohol, trityl alcohol and pinicol To further investigate this new C-C coupling reaction, Mo(dpma mes )2(NMe2)2 (1) was treated with 2 equiv. of benzyl alcohol in toluene, similar conditions used in that of hydroxylamine cleavage. The reaction rapidly turned from dark green to orange-brown, characteristic of the of the other Mo(IV) to Mo(VI) transitions seen with 1. Both benzaldehyde from the β-hydride elimination pathway as well as 1,2 diphenylethane were produced (Table 81 2.1). Additionally the reaction of 1 with 2 equiv. of benzyl alcohol was conducted as an NMR tube reaction in C6D6, confirming the production of toluene. a Substrate Yield of mes O2Mo(dpma ) b (2) Byproduct(s) Yield c 31.6% 24% Not observed above background 53.9% 22% 41.6% 10% _ _ a Use of 2 equiv. of substrate for benzyl and trityl, and 1 equiv. of pinicol 1 b Mo containing product yield determined by H NMR c Organic product yield determined by GC-FID relative to cymene or dodecane as an internal standard Table 2.1: Table of substrates and organic byproducts of the molybdenum mediated C-O bond cleavage. 82 In an attempt to probe the substrate scope of the above reaction a variety of alcohols were screened. Simple aliphatic alcohols such as 4-methylpentan-1-ol gave complex mixtures of 1 organic products, however low yields of 2 were identified by H NMR. Diols, such as pinicol reacted slowly also giving low yields of the metal containing product 2. Using 2 equiv. of triphenylmethanol proved to be most fruitful, yielding triphenylmethane and Gomberg‘s dimer, as quantified by GC-FID. The increased yield when using the radical stabilizing substrate triphenylmethanol, as well as the detection of a dimer of the trityl radical implicates a radical mechanism may be operative in the homolytic cleavage of C-O bonds. 2.3 Structural Comparisons and Synthesis of Mo(dpma mes )(NTol)2 (3) Unfortunately, the Mo(IV) hydride product formed from the β-hydride elimination of a benzyloxy ligand was not recovered, nor were the metal-containing decomposition products of this pathway. The only diamagnetic molybdenum-containing product from the reaction of 1 with benzyl or trityl alcohol recovered showed only resonances of bound mes dpma 1 in H NMR spectroscopy. Upon tridentate coordination, the four magnetically equivalent methylene protons of mes H2dpma NMo(dpma split to form a characteristic pair of doublets. In the related case of anti- mes )(NMe2) where N-methyl on the ligand‘s backbone is on the opposite side of the plane defined by the pyrrolides as the metal-ligand multiple bond, this presents itself as a pair of doublets with a large coupling constant in toluene-d8 of JHH = 12.5 Hz, and a separation of mes 591.3 Hz (1.18 ppm) between the two sets. Conversely, the isomer syn-NMo(dpma 83 )(NMe2) with the N-methyl on the same side as the nitride, shows a doublet of doublets, with couplings of JHH = 14.0 Hz and 82.0 Hz (0.16 ppm) (Figure 2.3). 1 Figure 2.3: Methylene H NMR resonances of the bound dpma mes ligand where the N- methyl is anti or syn to the nitride. To discern if these vastly different geminal couplings correspond to the N-methyl orientation in bound dpma mes , the bis-imido analogue of 2 was independently prepared. Synthesis of bis-imido complexes of Mo(VI) have been well established, mainly because they are a route to the Mo(VI) alkylidene-imido complexes active for olefin metathesis (i.e. Schrock‘s catalyst). Reaction of (NH4)2Mo2O7 with the desired aniline and SiMe3Cl in the presence of DME and triethylamine affords complexes of the type Mo(NAr)2Cl2(DME) where Ar = an aryl 1 group. For our purposes the p-tolyl imido was chosen for spectroscopic reasons, as well as to minimize steric congestion close to the metal center. Halide for pyrrolide substitution was 84 achieved in high yield by treatment of Mo(NTol)2Cl2(DME) with 1 equiv. of H2dpma mes in toluene, in the presence of a slight excess of triethylamine (Figure 2.4). Figure 2.4: Synthesis of Mo(NTol)2(dpma The single crystals of Mo(NTol)2(dpma mes mes ) (3). n ) (3) grown from a concentrated heptane solution held at −35 °C show 3 as nearly square pyramidal (η = 0.12), with one imido taking the apical position (Figure 2.5). Mo1-N4 and Mo1-N5 bond distances are 1.750(2) Å and 1.760(2) Å 85 respectively, and are similar to other 5 coordinate imidos of this type. Notably, the Mo1-N3 bond distance (2.281(2) Å) is nominally longer than the corresponding Mo1-N3 bond in antiNMo(dpma mes )(NMe2) (2.275(3) Å), nicely illustrating the larger trans-influence of an imido over an amido ligand. 2 mes Figure 2.5: Crystal structure rendering of the Mo(VI) bis-imido Mo(NTol)2(dpma (3). Thermal ellipsoids at 50% probability with H atoms removed for clarity. 86 ) 1 mes H NMR analysis of 3 shows a set of methylene doublets for bound dpma , of with geminal couplings of JHH = 12.5 Hz and a shift between the sets of doublets of 500.0 Hz (1.0 1 ppm). This establishes a causal link between the solid state structure and the H resonance multiplicity in solution for 5 coordinate Mo centers of dpma of O2Mo(dpma mes . Unfortunately, suitable crystals mes ) (2) have not been grown for structural analysis, but the above allows for the formulation of a diamagnetic Mo(VI) complex bearing only dpma to an oxo bond similar to that of syn-NMo(dpma mes 87 mes where the N-methyl is syn )(NMe2). (Table 2.2) antiCompound NMo(dpma mes syn-NMo(dpma mes )(NMe2) Mo(dpma mes )(NTol)2 (3) O2Mo(dpma mes ) (2) )(NMe2) Structure Geminal 12.5 14.0 12.5 14.0 591.3 (1.18) 82.0(0.16) 500.0(1.00) 100.5(0.20) JHH Coupling (Hz) δ difference between doublets Hz (ppm) Table 2.2: Comparison of dpma mes 1 N-Methyl orientation toward the metal-ligand multiple bond with the geminal H NMR coupling of the methylenes. 88 2.4 Radical Reactivity in Related Complexes Though rare in the literature, cleavage of C-O bonds of a molybdenum bound ligands 3 have been reported for alcohols. One such transformation reported by Cummins et. al. involves t the reaction of Ti(NRAr)3 [R = C(CD3)2(CH3), Ar = C6H3Me2 with NMo(O Bu)3 which expels a tert-butyl radical to produce a Mo-oxo species. The reaction proceeds through initial formation 1 of a μ-Nitrido bridging a Mo(V) and Ti(IV) center as detected by H NMR and EPR. From here t hemolytic cleavage from the butoxide ligand forms a stabilized 3° radical resulting in a 1e − reduction to Mo(VI). Rapid arrangement reinstates a terminal nitride and the formation of a μoxo. This reaction however, differs from the observed C-O cleavage in 1 in that the starting 0 2 − Mo(VI) nitride is d whereas 1 is d , thus the 1e ultimately comes from an external open shell source not the Mo center itself (Figure 2.6). 89 Figure 2.6: The radical forming step in the reaction of 1 with benzylic alcohols (top) along with t the expulsion of a butyl radical initiated by a Ti(III) radical source (below). 90 Although the radical source is titanium(III), the microscopic reverse is precedented in the 4 case of vanadium oxo in its highest oxidation state. Also reported by Cummins and coworkers attack of a Ti radical on a metal-ligand multiple bond results in one electron reduction of the vanadium center and the formation of a bridging μ-oxo. Similar to the Mo(VI) nitride system the initial step involves the bridging of the two metal centers by the multiply bounded atom, what 5 may be described as incomplete O atom transfer. However, this system does not cleave off an organic radical. In context of the reactivity seen in 1 the resulting structure may be thought of a ‗titanoxide‘ ligand as noted by the authors. Thus, such a transformation satisfies the concept of microscopic reversibility (Figure 2.7). 91 Figure 2.7: Radical attack on a terminal oxo to form a M-O-R linkage. 92 2.5 Reactivity of Mo(dpma mes )(NMe2)2 (1)with Phenols In further studies aromatic alcohols were screened as substrates with 1. Upon treatment of 1 with 2 equiv. of phenol in toluene, a rapid reaction occurred, turning the reaction mixture dark 1 mes brown. Upon H NMR analysis only free H2dpma was identified, with no metal containing, diamagnetic products detected. Crystallization of the reaction mixture from a concentrated toluene solution held at −35 °C gave single crystals suitable for diffraction. Figure 2.8: Crystal structure rendering of Mo(OPh)4(HNMe2)2 (4). Thermal ellipsoids at 50% probability with carbon bound H atoms removed for clarity. 93 These crystals were determined to be the Mo(IV) complex Mo(OPh)4(HNMe2)2 (4) (Figure 2.8). This structure is nearly identical to that of Mo(OTol)4(HNMe2)2 and related complexes bearing para-substituted phenoxides previously reported by Green and coworkers. 6 Here, the reaction of Mo(NMe2)4 with 5 equiv. of cyclopentyldiene gave the compound t Mo(C5H5)(NMe2)3 in good yield. Subsequent treatment with 4 equiv. of p-methyl or p- butyl phenol was sufficient to replace all 4 ligands via protonolysis giving a paramagnetic 6 coordinate complex in much the same way as 4. This finding confirmed that treatment of 1 with 2 or more equivalents of phenol decompose the Mo(IV) complex, an additional pathway dependent on the alcohol chosen. To date phenol is the only substrate known to be able to remove the tridentate ligand from mes Mo(dpma )(NMe2)2 (1) via protonlysis. 2.6 Mechanism and Structure of OMo(dpma mes )(OBn) (5) Although the increase in reactivity upon stabilizing the proposed radical formed in the transition state was telling, mechanistic questions still remained. Namely, was replacement of both dimethylamide ligands necessary before bond cleavage could occur? One possible consequence of a slow ligand replacement (i.e. stepwise) and a rapid C-O bond breaking step may be the production of ethylbenzene concurrent with the formation of a mixed oxomethylimido product (Figure 2.9). In every reaction conducted, neither ethylbenzene nor a Mo(VI) oxo/imido product was seen. This may however simply be the stability of the 94 dimethylamido ligand toward forming a methyl radical, as this decomposition pathway was not seen in samples of 1. 95 Figure 2.9: Possible products from alternate ligand substitution pathways. 96 Additionally, if di-substitution of dimethylamide ligands for alkoxides precedes bond cleavage, Is the cleavage step a concerted scission of two bonds or stepwise (Figure 2.10)? In other words, does Mo(IV) mediate the expulsion of radical fragments from the bound ligands or directly couple C-C bonds whilst breaking C-O bonds in alkoxides? In attempts to purify and structurally characterize the bis-oxo 2, a reaction mixture from treatment of 1 with 2 equiv. of benzyl alcohol was dried under vacuum and extracted with Et2O. These extracts were filtered and held at −35 °C, forming a small amount of an orange precipitate after several days. This precipitate was collected and re-dissolved in minimal Et2O, which yielded crystals after several days at −35 °C. 97 Figure 2.10: Concerted C-O bond cleavage of benzyl alcohols. 98 X-ray diffraction revealed the Mo(V) complex to be OMo(dpma mes )(OBn) (5) (Figure 2.11). The structure of 5 has many interesting features. Firstly, the replacement of dimethylamide by an oxo places molybdenum in the 5+ oxidation state. This signifies that stepwise 1e − chemistry is occurring. The Mo1-O1 bond length of 1.677(3) Å is typical for a Mo(V) oxo. MoNpyrrolide bond lengths are 2.044(3) Å and 2.051(3) Å, slightly shorter than those in the Mo(IV) complex 1 (2.099(2) Å and 2.082(2) Å) and those in the Mo(VI) complex synNMo(dpma mes )(NMe2) at 2.109(2) Å and 2.111(2) Å. Likewise a contraction of bond lengths is seen in the Mo1-N3 distance of 2 (2.248(3) Å), statistically shorter than that bond distance in syn-NMo(dpma mes )(NMe2) (2.294(2) Å), and well shorter than the 2.401(2) Å Mo1-N3 bond distance in 1. This trend in bond length is most likely attributed not only to the change in oxidation state of Mo center, but to the donor properties and steric effects of the other ligands accompanying the dpma mes . The Mo1-O2 distance of 1.892(2)Å in 5 is similar to the mes molybdenum-oxygen bond in Mo(dpma mes dpma )(NMe2)(OAd) (6) (vida infra) at 1.858(2)Å. The framework in 5 is distorted in much the same way as it is in syn- NMo(dpma mes )(NMe2), with a O1-Mo1-N3-C33 torsional angle of −34.3(2)° as compared to −35.6(2)° for N4-Mo1-N3-C33 in the nitride. 99 Figure 2.11: Crystal structure rendering of the Mo(V) intermediate OMo(dpma mes )(OBn) (5). Thermal ellipsoids at 50% probability with carbon H atoms removed for clarity. Interestingly, the N-methyl in the ligand backbone of 5 is syn to the oxo bond. This implies mes that the stereochemistry seen in O2Mo(dpma ) (2) is set before the installation of the second metal-ligand multiple bond, barring rapid and complete isomerization. Additionally, the 5coordinate structure of 5 is closer to a square pyramidal geometry (with a continuous structure 100 parameter of η = 0.62) reminiscent of the structure of syn-NMo(dpma mes )(NMe2). (η = 0.61), likely a consequence of the strong trans-influence common to metal-ligand multiple bonds. 7,8 Lastly, the isolation of the intermediate 5 shows that the formation of the bis-oxo mes O2Mo(dpma ) (2) is a result of stepwise C-O bond breaking via radical formation. The isolation of the mono oxo shows that C-O cleavage is not dependent upon C-C coupling. Therefore the transformation is not a concerted mechanism that directly couples the two organic fragments cleaved off of the alkoxides, as is the case in the McMurry reaction. 2.7 Reactivity of Mo(dpma 9 mes )(NMe2)2 (1) with 1-Adamantanol With substantial evidence of a radical mechanism in hand, we turned our attention to another substrate, 1-adamantanol. Since moving from benzyl to trityl alcohol (thus increasing the stability of the formed radical) a modest increase in reactivity was seen, an opposite effect was expected in the case of 1 with 1-adamantanol. This is due to cage structure of the adamantly 2 fragment prohibiting a flat sp center, thus resisting radical formation on the quaternary carbon. 101 Figure 2.12: Stoichiometric reaction of 1 with 1-adamantanol yielding mes Mo(dpma )(NMe2)(OAd) (6). Treatment of 1 with 2 equiv. of 1-adamantanol in toluene affords the bright green Mo(IV) compound Mo(dpma mes )(NMe2)(OAd) (6). Holding a concentrated toluene solution layered in an equal volume of pentane at −35 °C gives diffraction quality crystals in good yield. Structural analysis shows a distorted 5 coordinate Mo(IV) center between square pyramidal and trigonal bipyramidial (η = 0.53), similar to the structure of 1 (Figure 2.13). 102 Figure 2.13: Crystal structure rendering of the Mo(IV) complex Mo(dpma mes )(NMe2)(OAd) (6). Thermal ellipsoids at 50% probability with H atoms removed for clarity. Reaction of 1 with a stoichiometric amount of HOAd gave 6 in 68% after crystallization (Figure 2.12). To date, substitution of both dimethylamides in 1 have been unsuccessful, with no reaction beyond mono-substitution seen even at elevated temperatures and with excess equivalents of 1-adamantanol. This ligand exchange whilst leaving the dpma 103 mes framework mes intact shows yet a third substrate dependent pathway in the reactivity of Mo(dpma )(NMe2)2 (1) with alcohols. Due to the similarities of Mo(dpma mes mes )(NMe2)(OAd) (5) with Mo(dpma )(NMe2)2 (1), the magnetic properties of 5 were investigated in solution via Evan‘s method NMR. 10, 11 The paramagnetic nature of complex 5 varied only modestly with temperature, unlike that of 1, with the effective magnetic moment varying from 1.07 μB to 1.21 μB over a range of 231.2298.8 K. This does not exclude 5 from similar spin state transitions as 1, but may suggest a higher energy barrier between S = 0 and S = 1, thus a higher crossover temperature. Despite this difference, the treatment of 5 with di-substituted hydrazines gave a 1 to 1 mixture of the syn and anti nitrido product NMo(dpma mes )(NMe2) under similar reaction conditions. This argues against the spin-crossover nature of 1 being absolutely necessary for the N-N bond cleavage reaction. The treatment of Mo(dpma mes )(NMe2)(OAd) (5) with hydroxylamines have yet to be determined and the loss of anti selectivity with hydrazines are still being investigated. 2.8 Conclusion The Mo(IV) complex Mo(dpma mes )(NMe2)2 (1) has been shown to cleave the C-O mes bonds in benzylic alcohols, concurrent with the formation of O2Mo(dpma ) (2). The use of triphenylmethanol as a substrate leads to cleaner reactions and higher conversion. Similar treatment of 1 with 1-adamantanol shows only the replacement of an amide ligand, and treatment with phenol results in decomposition of the metal complex, representing 3 pathways based on the alcohol used. These results are reflective of the stabilities of the radical (or that would be 104 formed) formed after C-O bond cleavage, thus evidence for their inclusion in the reaction mechanism. Isolation of the intermediate OMo(dpma mes )(OBn) (5) shows the reaction to proceed via a stepwise mechanism, where carbon-based radical fragments are expelled and later terminate by coupling to each other or abstracting H from other organics in solution. Combined with the preceding study, we now have evidence for the Mo(IV) center in 1 to cleave N-N, N-O, and C-O bonds in both a homolytic and heterolytic fashion. 2.9 Experimental Synthesis of O2Mo(dpma mes ) (2) Under an inert atmosphere a scintillation vial was loaded with 1 (80 mg, 0.132 mmol, 1 equiv.), a stir bar and toluene (10 mL). The solution was cooled for 10 min in a liquid nitrogen cooled cold well. The solution was moved to a stir plate and a toluene solution of benzyl alcohol (586 μL, 0.45 M, 0.263 mmol, 2 equiv.) was added dropwise over 5 min. The solution stirred for a further 6 h. The volatiles were removed under vacuum, and the residue was extracted with toluene (2 x 3 mL). The extracts were filtered through Celite. The solution was concentrated to 2 mL and placed in a –35 °C freezer. After 3 d the mother liquor was removed from the orange precipitate and dried under reduced pressure yielding 5. This was taken up with minimal amounts of Et2O and filtered through Celite. Holding a concentrated Et2O solution at –35 °C gave crystals of OMo(dpma mes )(OBn) (5) in low yield. These crystals mes were filtered from solution, and the solution concentrated again. Treating Mo(dpma mes with 2 equiv. of the following alcohols gave O2Mo(dpma 1 )(NMe2)2 ) (2) in the following yields as judged by H NMR: benzyl Alcohol 24%, triphenylmethanol 22% pinicol 10%. Yields of the 105 byproduct of C-C bond formation were determined by GC-FID calibrated with dodecane as an 1 internal standard as given in Table 2.1. H NMR (500 MHz, toluene-d8, 25 °C): 6.75 (s, 2H, ArH), 6.72 (s, 2H, Ar-H), 6.41 (d JHH = 3.0 Hz, 2H, Pyr-H), 6.313 (d JHH = 3.0 Hz, 2H, Pyr-H), 3.76 (d, JHH = 14 Hz, 2H, CH2), 3.56 (d, JHH = 13.5 Hz, 2H, CH2), 2.72 (s, 3H, N-CH3), 2.26 (s, 6H, Ar-CH3), 2.20 (s, 6H, Ar-CH3), 2.12 (s, 6H, Ar-CH3). Synthesis of Mo(NTol)2(dpma mes ) (3) Under an inert atmosphere a scintillation vial was loaded with Mo(Ntol)2(DME)Cl2 (0.126 mg, 0.27 mmol, 1 equiv.), a stir bar and toluene (5 mL). In a separate vial H2dpma mes (115 mg, 0.270 mmol, 1 equiv.) in toluene (5 mL) was added along with triethylamine (60 mg, 0.593 mmol, 2.2 equiv.). This solution was added to the stirring molybdenum solution. The solution stirred for 4 d forming dark solids. The volatiles were removed under vacuum. The residue was extracted with pentane until the pentane was clear. The extracts were filtered through Celite. The solution was dried under vacuum and dissolved with minimal amounts of n-heptane and held at –35 °C yielding crystals of 3 (79 mg, 0.108 mmol, 1 40% yield). H NMR (500 MHz, toluene-d8, 25 °C): 6.68 (d JHH = 8.5 Hz, 2H, NC6H4-CH3), 6.50 (dd JHH = 1 Hz JHH = 8.5 Hz, 2H, NC6H4-CH3), 6.14 (q JHH = 8.5 Hz JHH = 17.5 Hz, 4H, NC6H4-CH3) 6.08 (dd JHH = 1 Hz JHH = 3.0 Hz, 2H, Pyr-H), 6.04 (s, 2H, Ar-H), 5.84 (s, 2H, Ar-H), 5.82 (d JHH = 3.0 Hz, 2H, Pyr-H), 4.53 (d, JHH = 13.5 Hz, 2H, CH2), 3.53 (d, JHH = 12.5 Hz, 2H, CH2), 2.36 (s, 3H, N-CH3), 2.14 (s, 6H, Ar-CH3) 2.13 (s, 3H, NC6H4-CH3), 2.06 (s, 3H, NC6H4-CH3), 2.04 (s, 6H, Ar-CH3), 1.85 (s, 6H, Ar-CH3). 106 Synthesis of Mo(OPh)4(NMe2) (4) Under an N2 atmosphere, a scintillation vial was loaded with a stir bar, 1 (0.045 g, 0.074 mmol, 1 equiv.) and toluene (5 mL) and was cooled in a liquid nitrogen cooled cold well (10 min). The vial was removed to a stir plate and the solution stirred rapidly. To this vial phenol (0.014 g, 0.148 mmol, 2 equiv.) was added as a toluene solution (5 mL). Solution stirred 16 h turning brown. The volatiles were removed under vacuum, and the residue was extracted with pentane and filtered through Celite. Holding this concentrated pentane solution at −35 C yielded brown crystals of 4 (as the only tractable product) suitable for diffraction in less than 10% yield. Synthesis of Mo(dpma mes )(NMe2)(OAd) (6) Under an N2 atmosphere, a scintillation vial was loaded with a stir bar, 1 (0.100 g, 0.165 mmol, 1 equiv.) and toluene (5 mL) and was cooled in a liquid nitrogen cooled cold well (10 min). To this 1-adamantanol (0.025 g, 0.165 mmol, 1 equiv.) in toluene (8 mL) was added and the reaction was allowed to come to room temperature while stirring. After 16 h the solution was dried in vacuo yielding a light green powder. Crystalization at from a concentrated toluene solution layered in pentane held at −35 C gave green, hexagonal crystals suitable for diffraction in 68% yield (0.080 g, 0.112 mmol). Magnetic susceptibility (Evan‘s method, toluene-d8 −42C-+26 C) μeff = 1.07-1.21 μB. UV-Vis (toluene): λmax=636.0 ε=191.1, λmax=765.7 ε=163.8. Mp:145-150 C (d). 107 REFERENCES 108 REFERENCES (1) Fox, H. H.; Yap, K. B.; Robbins, J.; Cai, S.; Schrock, R. R. Inorganic Chemistry 1992, 31, 2287–2289. (2) Gibson, V. C.; Redshaw, C.; Clegg, W.; Elsegood, M. R. J.; Siemeling, U.; Türk, T. Polyhedron 2004, 23, 189–194. (3) Peters, J. C.; Johnson, A. R.; Odom, A. L.; Wanandi, P. W.; Davis, W. M.; Cummins, C. C. Journal of the American Chemical Society 1996, 118, 10175–10188. (4) Wanandi, P. W.; Davis, W. M.; Cummins, C. C.; Russell, M. A.; Wilcox, D. E. Journal of the American Chemical Society 1995, 117, 2110–2111. (5) Woo, L. K. Chemical Reviews 1993, 93, 1125–1136. (6) Yan, X.; Chernega, A. N.; Metzler, N.; Green, M. L. H. Journal of the Chemical Society, Dalton Transactions 1997, 2091–2100. (7) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; John Wiley & Sons: New York, 1988. (8) Griffith, W. P. Coordination Chemistry Reviews, 1972, 8, 369–396. (9) McMurry, J. E. Chemical Reviews 1989, 89, 1513–1524. (10) Evans, D. F. Journal of the Chemical Society 1959, 2003. (11) Grant, D. H. Journal of Chemical Education 1995, 72, 39. 109 Chapter 3: Additional Reactivity of Mo(dpma mes )(NMe2)2 ABSTRACT The Mo(IV) complex Mo(dpma mes )(NMe2)2 (1) which mediates the scission of N-N, N- O and C-O bonds decomposes in the presence of ethereal and halogenated solvents. Isolation and structural characterization of the reaction of 1 with CH2Cl2 shows the six-coordinate product mes Mo(dpma )(NMe2)2Cl (2), likely through a radical process. A high yielding synthesis of 2 has yet to be achieved. Reaction of 1 with 2 equiv. of benzylamine in the presence of CH2Cl2 gives mes the complex Mo(dpma )(NBn)(NH2Bn)Cl (3). X-Ray diffraction shows that 3 contains a Mo- N double bond and Mo-N dative bond originating from the same amine, a unique structural feature in transition metal chemistry. Complex 1 also exhibits [1,2]-insertion chemistry into one Mo-NMe2 upon treatment with excess CS2, giving the structurally characterized thiocarbamate product, Mo(dpma mes )(S2CNMe2)(NMe2) (4). 3.1 Introduction Exploring the chemistry of Mo(dpma mes )(NMe2)2 (1) has demonstrated the bond − cleaving potential of Mo(IV) complexes. As seen in di-substituted hydrazines, a 2e pathway results in N-N bond cleavage as well as direct oxidation from Mo(IV) to Mo(VI). Conversely, − complex 1 promotes the C-O cleavage of alcohols via a 1e radical mechanism. Shown to be 2 discrete elementary steps. This oxidation also takes Mo(IV) to Mo(VI) while passing through a 110 Mo(V) intermediate. With both pathways accessible, the coordinative unsaturated complex 1 was screened with a variety of substrates, in hopes that new patterns of reactivity would emerge. 3.2 Oxidation of Mo(dpma mes )(NMe2)2 (1) in presence of Carbon-Halogen and Silicon- Halogen Bonds The above factors lead to the extreme sensitivity of 1 towards atmospheric oxygen and water. Additionally, Mo(dpma mes )(NMe2)2 (1) was observed to decompose rapidly in ethereal (Et2O, THF, DME) as well halogenated (CHCl3, CH2Cl2, chlorobenzene). Likewise, attempts at substitution of the dimethylamides of 1 with halides by treatment with MeI or SiMe3Cl were unsuccessful, giving dark solutions predominately of paramagnetic complexes, and insoluble metal containing products. Though the decomposition product(s) of 1 in ethers remain elusive, single crystals of an oxidized product was grown from the treatment of 1 with CH2Cl2. Upon mixing, a cold toluene solution of 1 rapidly turns dark brown upon treatment with 10 equiv. of CH2Cl2. After 1h the volatiles were removed under dynamic vacuum. The residue was extracted with Et2O and filtered through a frit with Celite as a filtering agent. The solution was then concentrated and held at −35 mes Mo(dpma °C. After several days microcrystals were harvested from solution giving )(NMe2)2Cl (2) as small plates in low yield. 111 Figure 3.1: Crystal structure rendering of the oxidized product Mo(dpma mes )(NMe2)2Cl (2). Atom positions at 50% probability and H atoms removed for clarity. mes Mo(dpma )(NMe2)2Cl (2) exhibits several interesting structural features. The distorted octahedron has bond angles of N1-Mo1-N1# 150.2(4)°, N4-Mo1-N3179.8(6)°, and N5-Mo1-Cl1 175.7(6)°. This is very similar to the 6-coordinate Mo(IV) compound t mes Mo(NMe2)2(CNBu )(dpma t t ) (1•CNBu ). Also as seen in (1•CNBu ) the two dimethylamidos of 2orientate themselves such that the plane defined by each C-N-C unit is orthogonal to each other. This orientation imparts C1v symmetry to the molecule such that the lone pair of each NMe2 unit donates into different d orbitals. The Mo-pyrrolide bond distance of 2.109(7)Å is t statistically the same as both the Mo(IV) 5-coordinate 1 and the 6-coordinate 1•CNBu (however 112 these two bond distances are outside of esd‘s). Mo(dpma mes )(NMe2)2Cl (2) has two significantly different NMe2bond lengths with a short 1.813(16)Å Mo-N4 contact trans to N3 and a longer 1.985(12)Å contact trans to Cl1, with a Mo-Cl1 bond of 2.407(4)Å. To further our understanding of M(V) bis-pyrrolides attempts at a higher yielding synthesis of Mo(dpma mes )(NMe2)2X compounds were made. Treatment of 1 with 1 or more equiv. of tert-butylchloride did give 2 after crystallization, but in equally poor yields. Other halogenating oxidants such as 1,2-dibromoethane and I2failed to give any tractable products. 3.3 Synthesis and Structure of Mo(dpma mes )(NBn)(H2NBn)(Cl) (3) The presently known reactivity of Mo(dpma mes − )(NMe2)2 (1) is dominated by 1e and − 2e bond cleavage reactions. Since 1 reacts with various substrates to afford N-N, N-O, and C-O bond scission, attempts to synthesize Mo(IV) amides were made to see if C-N bond breaking would follow. Treatment of 1 with 1 or 2 equiv. of benzylamine or N-methyl benzylamine at − room temperature in toluene did not induce radical or 2e bond cleavage. Products from these reactions are paramagnetic in nature and have UV-Vis spectra consistent with Mo(IV) bis-amide of the form Mo(dpma mes )(NR2)2 where R = H, Me, or Bn. Serendipitously a reaction of 1 with 2 equiv. of benzylamine in toluene was contaminated with trace amounts of CH2Cl2. Crystallization of the mixture from a cold toluene solution held at −35 °C gave dark purple mes crystals of Mo(dpma )(NBn)(H2NBn)(Cl) (3). This compound is a structural curiosity in that 113 it prefers one doubly bounded and one datively bonded benylimide/benzylammine ligand in the ground state as opposed to two singlelly bound amide. Figure 3.2: Crystal structure rendering of Mo(dpma mes )(NBn)(H2NBn)(Cl) (3) showing a doubly and datively bonded benzylamine ligand. Thermal ellipsoids at 50% probability and nonN bound H atoms and a toluene solvent of crystallization removed for clarity. The 6-coordinate Mo(V) complex 3 shows a octahedral coordination similar to that of 2 with bond angles defined by N1-Mo1-N2, N5-Mo1-N3, and N4-Mo1-Cl1 at 154.6(1)°, 165.4(1)°, and 114 169.2(1)° respectively. Mo-Npyrrolide distances of 2.114(2)Å and 2.137(2)Å are also typical of the other 6-coordinate Mo(V) (dpma mes ) compounds known. The Mo1-N4-C41 unit is nearly linear at 172.9(2)° with an Mo1-N4 bond distance of 1.730(2)Å. These parameters are consistent with the bis-arylimido Mo(NTol)2(dpma mes ), where the two analogous angles observed are 153.3(2)° and 177.9(2)°. Likewise the Mo-Nimido distance of 3 is closer to the more linear of the mes two arylimdos of Mo(NTol)2(dpma ) at 1.761(2)Å and 1.750(2)Å. Even though complex 3 is a lower formal oxidation state and has a ligand Mo(NTol)2(dpma trans to the imido unlike the mes ), the imido length in 3 is markedly shorter outside of esd‘s. This may be due to the increased availability of the lone pair in an alkylimido nitrogen over an arylamido nitrogen. The observed Mo1-Cl1 bond of 2.525(1)Å is over 0.1Å longer than that observed in 2 suggesting an increased competitive interaction for d-orbitals. The Mo1-N5 linkage at 2.214(2)Å is similar to the Mo1-N3 bond (2.221(2)Å), thus was assigned it to be a dative bond. Crystal quality of 3 allowed for the refinement of the hydrogen atoms H5A and H4B. The mechanism by which 3 forms has yet to be determined. Benzylimde for bisdimethylamide substitution before oxidation by CH2Cl2 is likely. The structure of 3 differs from 2 in that the N-methyl of the dpma mes backbone is anti to the chloride ligand. If 3 was the substitution product of 2 an isomerization process would be necessary to give the observed compound. Likewise, the preference for a double and dative bond in place of two single bonds to benzylamine has not been explored. Intramolecular proton transfer from N5 to N4 likely has a 115 high kinetic barrier, as it would require rotation of the Mo1-N4 bond. A space filling model of 3 illustrates the highly constrained pocket formed by the flanking mesityl groups. Figure 3.3 A space filling model of Mo(dpma mes )(NBn)(H2NBn)(Cl) (3) viewed down the Mo1-N5 bond axis showing the sterically shielded benzylammine ligand (highlighted in red). 3.4 Insertions into the Metal-Amide bond of Mo(dpma mes )(NMe2)2 (1) A 2002 paper by Odom and coworkers reported the insertion of an electron rich alkyne 1 into a Mo-NMe2 bond (Figure 3.4). This departs from the reactivity of the Bradley compound 2 Mo(NMe2)4 which oligermerizes 3-hexyne at room temperature. At room temperature only a 116 single insertion is observed with Mo(dpma)(NMe2)2(NHMe2) in the presence of excess alkyne. Catalytic schemes using excess alkyne and a secondary amine were unsuccessful for the hydroammination reaction, and elevated temperatures led to decomposition. It was our hope that the increased steric protection afforded by the substituted dpma framework in 1 would allow for alkyne insertion while preventing decomposition. When 1 is treated with stoichiometric amounts of 3-hexyne an orange paramagnetic product is obtained. However, to date suitable crystals and identification of this material have been unobtainable. Figure 3.4: The related complex Mo(dpma)(NMe2)2(NHMe2) showing [1,2]-insertion of an alkyne into a Mo-NMe2 bond. mes The complex Mo(dpma )(NMe2)2 (1) does show [1,2] insertion reactivity similar to other molybdenum amides. The homoleptic Mo(NMe2)4 undergoes quantitative conversion to 2 Mo(S2CNMe2)4 when refluxed in benzene with CS2. Similarly Green et. al. reported multiple 3 CS2 insertions into Mo(η -C5H5)(NMe2)3 concurrent with a Cp hapticity shift to form − 5 Mo(η -C5H5)(S2CNMe2)3. IR analysis of this complex suggests an 18e metal center ligated by 117 3 1 monodentate and 2 bidentate S2CNMe2 moieties. When 1 is treated with a 10 equiv. excess of mes CS2 in cold toluene the paramagnetic complex Mo(dpma )(S2CNMe2)(NMe2) (4) immediately forms. Crystallization from a concentrated toluene solution held at −35 °C gave single crystals suitable for diffraction. Figure 3.5: Crystal structure rendering of Mo(dpma mes )(S2CNMe2)(NMe2) (4). Atom positions at 50% probability. H atoms and two toluene molecules of crystallization removed for clarity. 118 The 6-coordinate Mo thiocarbamate complex is best described as a distorted octahedron with the largest bond angles N1-Mo1-N2, S1-Mo1-N3, and S2-Mo1-N4 close to 180°, at 154.68(5)°, 160.62(4)°, and 162.28(5)° respectively. With a distance of 2.4434(4)Å the Mo-S1 mes bond trans to the donor ammine of dpma is shorter than the Mo-S2 bond (2.5796(4)Å) trans to the dimethyamide due to the greater trans influence of the anion. These lengths are consistent 2 with a delocalized thiocarbimate anion bound  to molybdenum. Mo-pyrrolide bond distances of 2.123(1)Å and 2.134(1)Å are slightly longer than the 2.082(2) Å and 2.099(2)Å found in 1. Likewise the Mo-NMe2 bond distance in 4 of 1.936(1)Å is lengthened from the average bond mes distance of 1.918(2)Å seen in Mo(dpma )(NMe2)2 (1). The like 2 the Mo-N distances in 4 are consistent with the corresponding bond lengths in the 6-coordinate Mo(IV) complex t mes Mo(NMe2)2(CNBu )(dpma t ) (1•CNBu ). Steric analysis of complexes 1 and 4 were calculated from the crystal structures using 4 Solid G program (For a detailed description please see Appendix B). In this method the metal center is treated in some respects as a point light source. Each ligand blocks conical access to this center from incoming substrates. When a sphere of arbitrary radius is circumscribed about the complex each ligand projects a ‗shadow‘ on its surface. The area of this ‗shadow‘ is calculated and given as a percentage of the spheres total area. When calculated this way the higher coordinate mes Mo(dpma Mo(dpma mes )(S2CNMe2)(NMe2) (4) is sterically more shielded than )(NMe2)2 (1) at 97.5% and 92.1% respectively. When subdivided the NMe2 ligand of 4 (20.3%) is smaller than the S2CNMe2 unit (26.9%) by ~6.6%. Although it is unknown if the 119 reaction of 1 with excess CS2 gives the mono insertion product 4 as its only product, insertion into both Mo-amide bonds seems sterically unfavored, as substrate access is greatly diminished upon mono substitution. Moreover, to a first approximation the bis-insertion product mes Mo(dpma )(S2CNMe2)2 would have a Solid G measurement at or over 100% assuming an mes isosteric dpma ligand. When the area projected by each ligand is summed a value over 100% is representative of areas of the metal center shielded by more than ligand and unfavorable intramolecular close contacts between ligands. Considering 4 is coordinatively unsaturated unlike the other literature examples of CS2 insertion into Mo-N bonds, the mono insertion observed is likely to be kinetically controlled. 3.5 Conclusions In addition to the Mo(IV) to Mo(VI) transformations accessible through N-N, N-O, and C-O bond scission, Mo(dpma mes )(NMe2)2 (1) forms stable Mo(V) complexes through radical C- Cl bond cleavage. Further investigations of Mo(V)(dpma mes ) are merited as the framework displays rich structural chemistry. Like other molybdenum dimethylamido compounds mes Mo(dpma 2 )(NMe2)2 (1) undergoes insertion of CS2 into a Mo-N bond to produce a  thiocarbamate ligand. 120 REFERENCES 121 REFERENCES (1) Katayev, E.; Li, Y.; Odom, A. L. Chemical Communications 2002, 2, 838–839. (2) Bradley, D. C.; Chisholm, M. H. Journal of the Chemical Society A: Inorganic, Physical, Theoretical 1971, 2741. (3) Yan, X.; Chernega, A. N.; Metzler, N.; Green, M. L. H. Journal of the Chemical Society, Dalton Transactions 1997, 2091–2100. (4) Guzei, I. A.; Wendt, M. Dalton transactions 2006, 3991–9. 122 Chapter 4: Evaluation of Donor and Steric Properties of Anionic Ligands on High Valent Transition Metals ABSTRACT Synthetic protocols and characterization data for a variety of chromium(VI) nitrido i i compounds of the general formula NCr(NPr 2)2X are reported, where X = NPr 2 (1), I (2), Cl (3), t Br (4), OTf (5), 1-adamantoxide (6), OSiPh3 (7), O2CPh (8), OBu F6 (9), OPh (10), O-pt (OMe)C6H4 (11), O-p-(SMe)C6H4 (12), O-p-(Bu )C6H4 (13), O-p-(F)C6H4 (14), O-p-(Cl)C6H4 (15), O-p-(CF3)C6H4 (16), OC6F5 (17), (O)-N-oxy-phthalimide (18), SPh (19), OCH2Ph (20), NO3 (21), pyrrolyl (22), 3-C6F5-pyrrolyl (23), 3-[3,5-(CF3)2C6H3]pyrrolyl (24), indolyl (25), carbazolyl (26), N(Me)Ph (27), (N)-NCO (28), (N)-NCS (29), CN (30), NMe2 (31), F (33). Several different techniques were employed in the syntheses, including nitrogen-atom transfer i for the formation of 1. A cationic chromium complex [NCr(NPr 2)2(DMAP)]BF4 (32) was used as an intermediate for the production of 33, which was produced by tin-catalyzed degredation of the salt. Using spin saturation transfer or line shape analysis, the free energy barriers for diisopropylamido rotation were studied. It is proposed that the estimated enthalpic barriers, Ligand Donor Parameters (LDPs), for amido rotation can be used to parameterize the donor abilities of this diverse set of anionic ligands toward transition metal centers in low d-electron counts. The new LDPs do not correlate well to the pKa value of X. Conversely, the LDP values of phenoxide ligands do correlate with Hammett parameters for the para-substituents. Literature 123 data for 13 C NMR chemical shifts for a tungsten-based system with various X ligands plotted versus LDP provided a linear fit. In addition, the angular overlap model derived e + e values for chromium(III) ammine complexes correlate with LDP values. Also discussed is the correlation with XTiCp*2 spectroscopic data. X-ray diffraction has been used used to characterize 31 of the compounds. From the X-ray diffraction data, steric parameters for the ligands using the Percent Buried Volume and Solid Angle techniques were found. 4.1 Introduction One of the most important methods for controlling reaction pathways in many transition metal catalyzed systems is through the steric and electronic adjustment of ancilliary ligands. Choosing from the extensive gallery of possible ligand choices is often done by (1) analogy with similar reactions already in the literature (2) picking readily available ancillaries in the investigator’s laboratory or (3) making an educated guess based on experience in the field. Once some of the desired reactivity is found, reactions are optimized through similar procedures involving available ligand sets and trying to encourage hypothesized processes with slow reaction rates. Finally, a reaction may be deemed interesting enough to warrant full mechanistic investigations through experimental and computational techniques that can often lead to improved catalyst designs. In this process of taking new reactions from conception to fruition, the donor properties and steric profiles of the ancillary ligands, along with reaction conditions, provide the major tools for optimization. Simple metrics for donor and steric properties have proven to be powerful tools for catalyst optimization, especially in late transition metal chemistry. Perhaps one of the most familiar citations in chemistry is by Chadwick Tolman published in Chemical Reviews in 124 2 1977 on steric and electronic effects in phosphine chemistry. Tolman’s cone angle gave a reasonable one-parameter metric for sterics. The energy of the totally symmetric carbonyl vibration in Ni(CO)3(PR3) complexes gave a useful single-parameter metric for donor properties. Using CO stretching frequencies to parameterize later transition metal ligand effects predates this Tolman review, however. For example, Wilkinson and co-workers in 1959 reported the IR stretching frequencies of transition metal carbonyls bearing amine/phosphine donors and + – reported ―the resulting negative charge on the metal atom R3N –M may…be dissipated by 3 increasing the bond orders in the M–C–O system.‖ They reported a steady rise in carbonyl stretching frequencies on replacement of phenyl groups in (Ph3P)3Mo(CO)3 with chlorine until 4 reaching (Cl3P)3Mo(CO)3. Parameterization methods have been extensively used in a large variety of low-valent and 5 late transition metal catalyses. Similar quantitative measures are a mainstay of physical organic chemistry; for example, the reactivity of compounds with pendant aryl groups is often predicted 6 or explained by parameters developed by Hammett, Taft, and others. Quantitative structureactivity relationships (QSAR) have developed into a powerful tool for other areas as well, e.g., pharmaceutical design. 7 In contrast, methods for determining donor properties of ligands on metal complexes in higher formal oxidation states are less well known. This is despite the fact that the donor properties of common ligands on earlier, higher-valent metals are likely to be quite different 125 from later, lower valent metals in many cases due to differences in the number and type of empty acceptor orbitals. If the ligands in question are members of a closely related series, e.g., para-substituted phenoxides, one can try to draw analogies to pKa or Hammett parameters; however, the investigator is left wondering if these are good measures for the transition metal system in question. This problem is only exacerbated if the ligands are more dissimilar, such as comparing phenoxide to iodide to indolyl. Different donor atoms, e.g., oxygen versus nitrogen, or even different hybridization of the same donor atom may affect radial extensions for the orbitals even if the frontier orbitals are of similar shape, which could lead to quite different bonding properties due to the changes in overlap integrals and energies. While QSAR has been done on early to middle transition metal complexes, these are often studies of specific systems with limited applicability to high valent metals in general. For example, extensive QSAR has also been done in recent computationally driven studies like the one published by Jensen and coworkers on Grubbs’s catalyst. parameterizations have been applied to metallocene 9 8 Steric and electronic and nonmetallocene 10 polymerization catalysts using a variety of techniques varying from simply categorizing ligand types to numerical quantization of ligand properties. In addition, electronic influence of substituents on ansa-metallacene complexes has been examined in great detail. For our investigations in titanium catalysis, 12 11 we sought a method for the comparison of a large variety of monodentate ligands on early metals to aid in ligand design and for understanding spectroscopic and reactivity trends within various transition metal systems. 126 0 In this study, we discuss the use of a large selection of monodentate ligands on the d i metal complex, NCr(NPr 2)2X, where X is an adjustable monodentate ligand. As will be shown, the synthetic versatility of this framework allows synthesis of a series of compounds for evaluation. In this manuscript, we limit the discussion to monoanionic X; 13 however, these range from common ancillaries used in organometallic chemistry like amido and alkoxide, to classical Werner-type ligands such as halides, cyanide, and thiocyanate. The system’s design lends itself to a one parameter quantification of donor properties similar to the Ni(CO)3L system commonly used for late transition metal ligands. Steric metrics for the ligands are provided, and possible steric interference is discussed. 4.2 The System Used for Ligand Parameterization The method chosen here for the experimental parameterization of ligand donor properties on high valent metal centers involves the use of chromium(VI) nitrido complexes with i diisopropylamido ancillaries. All of the compounds in this study are of the type NCr(NPr 2)2X, where X is a monoanionic ligand. These complexes are readily prepared, as will be shown in the next Section. In addition, the amido ligands display variable rotation rates dependent upon the donor properties of X. The rotation of the diisopropylamido ligand in these systems has a rate 1 that is readily measured by H NMR spectroscopy. In this study, spin saturation transfer was the standard method for ligand rotation rate determination; however, line-shape analysis can also be used and was used for some compounds (vide infra). Since the compounds are pseudo-tetrahedral, the system is not orthoaxial, and the - and -orbitals mix during the bonding interactions with the ligands. This is exemplified in the 127 Angular Overlap Model (AOM) Parameters for a rigorously tetrahedral compound by the energy of the t2* orbital being parameterized as et2* = 4/3 e + 8/9 e where the ligand’s - and donor parameters both contribute to the energy of the triply degenerate orbital. Lowering the symmetry, as is done here, will lead to further mixing and a single parameter for ligand donor properties is the result. The highest symmetry available in the nitrido compounds here would be C 3v with a formula of NCrX3, where the chromium-nitrido bond is along the z-axis. In C3v, the dxy/dx2–y2orbitals comprise an e-set, with the px/py-orbitals having the same symmetry. These e-sets act as both - and -acceptor orbitals for the basal amido ligands. In addition to the -accepting e-set near the xy-plane, there is an e-set comprised of the dxz/dyz-orbitals that are involved in strong interactions with the nitrido. 14 In other words, rotation of the amido ligands 90° from where they -donate into acceptor orbitals near the xy-plane to where they could donate into the dxz/dyz orbitals along the nitrido vector causes them to compete with the very strongly donating nitrido. As a result, there is an i electronic barrier to rotation around the Cr–NPr 2 bond determined by the energy difference between the geometries where the amido CrNR2 plane is parallel with the Cr–N(nitrido) vector and where it is perpendicular. Increasing the donor abilities of the ligands in the basal set reduces this difference somewhat, decreasing the barrier for amido rotation through ground-state destabilization. 128 i In short, the stronger a donor X is in a compound like NCr(NPr 2)2X, the smaller the barrier to amido rotation is expected to be. Because the - and -systems are strongly mixed, the - and -donor properties of X both contribute to the size of the diisopropylamido rotational barrier. We propose that this isomerization barrier can be used as a measure of the donor ability of X in high valent transition metal systems. The above arguments can be illustrated using Density Functional Theory on the model system NCr(NH2)3 using B3LYP as the functional. Our initial exploration with this molecule used 6-31G**, but the calculations were extended to the much larger aug-cc-pVQZ basis set, which provided similar results. Optimization provided a ground state structure where all of the amido ligands are planar and the Cr–NH2 planes are parallel to the nitrido vector as expected (Figure 4.1, bottom). 129 Figure 4.1. DFT B3LYP/aug-cc-pVQZ computational results for NCr(NH2)3. On the bottom are two views of the computed ground state, with the right view looking down one of the three equivalent amido-chromium vectors. In the middle is a plot of the hybridization parameter () vs the calculated enthalpic energy of the complex with the ground state set to 0 kcal/mol. On the top are two views of the transition state structure found for rotation of one amido ligand, with the right view looking down the rotated amido-chromium vector. 130 If NCr(NH2)3 is reoptimized while restricting the dihedral angle in one of the amido ligands to induce rotation, the rotating nitrogen pyramidalizes as the lone pair on the amido approaches the nitrido -orbitals. 15 In other words, the nitrido prohibits significant -donation from the rotating amido when it would donate into the same orbital. The energy of the complex  2 increases with increasing amido nitrogen hybridization parameter ( in sp ) from  = 2, i.e. sp , in the ground state to around  = 2.8 (Figure 4.1, plot) at the transition state for the rotation. 16 ‡ From the calculations an enthalpic barrier, H , of 5.7 kcal/mol was found using the aug-ccpVQZ basis set. Meanwhile, the Cr–N(nitrido) bond distance seems virtually unaffected by the rotation, varying by less than 0.01 Å over the entire course of the rotation. The transition state for amido rotation, which had a single negative vibration, was found at 61° in the N(nitrido)-Cr–N(amido)–H dihedral. Figure 4.1 has images of the ground state and transition state structures. Also in Figure 4.1 is a plot of  versus enthalpy. The hybridization parameter increases fairly smoothly up to the transition state, consistent with competition with the nitrido -donation being the cause of amido pyramidalization. The amido distance does seem to change slightly with rotation due to bond order effects with the chromium, but the relationship is complicated by electronic adjustments made by the other amido ligands. The average Cr–NH2 distance in the ground state from the calculations is 1.83 Å. In the transition state, the pyramidalized (rotating) amido distance increases to 1.90 Å, but the amidos not undergoing rotation shorten their distance to chromium to 1.81 Å. As a result, the average Cr–NH2 distance in the transition state is 1.84 Å, essentially identical to the ground state. In other words, rotating one ligand causes ripples of change through the other ligands. It is 131 these indirect changes in the amido ligands due to compensating effects around the metal that we are measuring in this study. In the actual diisopropylamido complexes used in the experimental studies, this degree of pyramidalization may not be possible for steric reasons. However, the hybridization of the amido nitrogen in the model is illustrative of the type of electronic changes expected on rotation. Donation into the same orbitals as the strongly donating nitrido is energetically unfavorable, and, in the model at least, this competition for the metal’s acceptor orbitals manifests as amido rehybridization. i In this study, the NCr(NPr 2)2 fragment is held constant, and the barrier to rotation of the diisopropylamido ligands in this constant fragment are what is being measured. The X substituents affect the amido barrier to rotation only indirectly, and the only changes from one i complex to another are the electronic and steric components of X in NCr(NPr 2)2X. This system has several advantages for this type of study. First, the compounds prepared thus far have good to excellent thermal stability. Second, the complexes are diamagnetic, allowing easy use of NMR for evaluation. Third, the Cr(VI) nitrido compounds tend to be pseudotetrahedral; we have not observed dimers with bridging X ligands in this system, for example. Fourth, ligands tend to be monodentate on the metal allowing a more uniform comparison between various ligand sets. Even ligands such as carboxylate, with a strong tendency to have higher hapticity in most complexes, only show what appear to be weak i secondary interactions with the metal if any (vide infra). Fifth, the NCr(NPr 2)2X complexes are readily prepared from inexpensive reagents with an extraordinary variety of X as will be described next. 132 i 4.3 Preparation of NCr(X)(NPr 2)2 Complexes and Characterization Here, we begin by discussing a new synthetic protocol based on nitrogen-atom transfer i for the formation of NCr(NPr 2)3 (1). All other complexes are prepared by modification of 1. i The synthetic protocols for the production of the other NCr(X)(NPr 2)2 complexes will be divided into 7 categories: protonolysis with lutidinium halides, protonolysis with HX, exchange using thallium salts, exchanges between lithium salts and the chromium phenoxide, metathesis using sodium salts, ligand exchange with lithium to zinc transmetallation, and tin(IV)-catalyzed decomposition of a cationic BF4 salt. i 4.4 Synthesis of NCr(NPr 2)3 (1) by Nitrogen Atom Transfer i The starting material for all of this chemistry is the Bradley complex, Cr(NPr 2)3, i prepared on large scales from CrCl3 and LiNPr 2 in ethereal solvent. 17 The 3-coordinate compound is soluble in hydrocarbons and crystallizes as large black plates. In the previously i i reported synthesis of nitrido NCr(NPr 2)3 (1), black solutions of Cr(NPr 2)3 reacted with NO gas i to form orange ONCr(NPr 2)3, which was deoxygentated with vanadium(III) to form the terminal nitrido. 18 i For this work, a somewhat more straightforward synthesis was used where Cr(NPr 2)3 t i was treated with NCr(OBu )3 to give NCr(NPr 2)3 (1) through a nitrogen-atom transfer (Eqn 1). t Yellow NCr(OBu )3 is available from chromyl chloride in a one-pot procedure published by 133 Chiu and coworkers. 19 t 20 The nitrido was readily separated from the oily Cr(OBu )3 byproduct by washing with acetonitrile. 4.5 Syntheses Using Protonolysis with Lutidinium Halides i Dark beet-red 1 was converted to orange NCr(I)(NPr 2)2 (2) with 2,6-lutidinium iodide using the published procedure. 21 Using procedures similar to the iodide synthesis, the chloride (3) and bromide (4) complexes were prepared. The syntheses involved the addition of anhydrous 2,6-lutidenium halide to 1 in chloroform at 60 °C (Eqn 2). 4.6 Syntheses Using Direct Protonolysis with HX For this study, a total of 15 complexes were prepared using the direct addition of HX, where X is the new desired ancillary ligand (Eqn 3). For the synthesis of most alcohols, silanols, carboxylates, and thiolates, direct protonolysis on 1 turned out to be the most convenient and highest yielding methodology. The reactions were carried out using toluene as the solvent for all of these cases, with the exception of triflate where DME/pentane was employed. The other conditions required for the syntheses varied widely depending on the substrate HX. Some reactions, such as with triflic acid, worked best when started at near frozen temperatures with short stirring times at room temperature. Other substrates were heating for 134 several days to get good conversion, e.g., the reaction with 1-adamantanol (HOAd) required heating at 90 °C for 3 days. Two of the compounds in Eqn 3, 9 and 10, have been previously reported. 22 4.7. Syntheses Using Exchange with Thallium Salts Thallium salts were advantageous in several cases for the production of new i NCr(NPr 2)2X complexes. The reactions with iodide (2), led to rapid precipitation of TlI, which is readily removed by filtration. The reactions were generally clean, and the use of thallium avoids unwanted reduction processes found using some other reagents (vide infra). Some obvious disadvantages for thallium are the toxicity of the metal and lack of stability with some X substituents. Thallium was employed in the preparation of five of the complexes (Eqn 4). 135 Protonolysis of 1 with HOBn proved to be slow and not very clean. Using TlOBn, i NCr(NPr 2)2(OBn) (19) was prepared in good yield from iodide 2. Hexanes or toluene were used for the majority of these transmetallation reactions. i Likewise, commercially available TlNO3 gave NCr(NPr 2)2(NO3) (20); in this case, THF was advantageous due to the low solubility of the thallium salt in most other solvents. Thallium was especially useful for pyrrolyl and pyrrolyl derivatives. The thallium salts were readily available by simple reaction of TlOEt with the NH-pyrrole. The thallium pyrrolyls seemed at best sparingly soluble in any solvent with which they didn’t react, as evinced by being 1 H NMR silent as saturated solutions in several solvents, but the compounds reacted readily and cleanly with the iodide 2. The chromium complexes of pyrrolyl (22) and two different 3-arylpyrroles (23 and 24, Eqn 4) were prepared using this procedure. 136 4.8 Syntheses Using Exchange with Lithium Salts When using lithium reagents, reduction of iodide 2 to the known -nitrido chromium(V) 20 dimer i [NCr(NPr 2)2]2 was evident. Transmetallation using the phenoxide 10 was often more successful with these reagents, and this method was used to prepare the indolyl (25), carbazolyl (26), and N-methylanilide (27) complexes (Eqn 5) from their respective lithium salts. A similar i i method was used in the conversion of NCr(OPh)2(NPr 2) to NCr(CH2SiMe3)2(NPr 2) in work from the Cummins laboratory. 21 4.9 Syntheses Using Exchanges with Sodium Salts i The three complexes NCr(NPr 2)2(X), where X = NCO (28), NCS (29), and CN (30), i were prepared (Eqn 6) from the commercially available NaX salts and NCr(NPr 2)2(I) (2). The main difficulty with the reactions was the low solubility of these reagents in organic solvents. Acetonitrile was used as the solvent and reactions required relatively long reaction times and/or mild heating. In the case of NaCN, one equivalent of 15-crown-5 was advantageous. 137 4.10 Ligand Exchange with Zinc Transmetallation For probing steric effects in the barriers to rotation (vide infra), it was desirable to i synthesize the dimethylamido complex NCr(NPr 2)2(NMe2) (31) for comparison with i NCr(NPr 2)3 (1). The most fruitful route (Eqn 7) we discovered to 31 involves treating ZnCl2 with LiNMe2 in DME/THF solvent. Presumably, a Zn(NMe2)2 solvate or perhaps an amidocontaining zincate complex is prepared. This mixture does transmetallation with iodide 2 more cleanly than with the lithium salt alone. Attempts to prepare 31 directly from 2 with LiNMe2 resulted largely in reduction. 4.11 Tin(IV)-Catalyzed Decomposition of a Cationic BF4 Salt Many different methods were tried in attempts to prepare the fluoro complex. Success was finally found when we generated a cationic complex by treatment of the iodo 2 with AgBF4 i in the presence of DMAP to form [NCr(NPr 2)2(DMAP)]BF4 (32). 12 Thermal decomposition of i this complex does form small amounts of fluoride NCr(NPr 2)2(F) (33) but also gives a large amount of unidentified side-products. In one attempt to form the fluoride by transmetallation, we n reacted 32 with FSnBu 3, which gives fluoro 33 relatively cleanly. Subsequently, we found that n FSnBu 3 could be used catalytically. It appears that Sn(IV) complexes can catalyze the decomposition of 32, as can some other mild Lewis acids. 138 23 The expected byproduct, DMAP•BF3, was easily detected in the 19 F NMR spectrum of a reaction to form 33 carried out 24 in an NMR tube. i Scheme 4.1. Synthesis of NCr(NPr 2)2(F) (33). 4.12 Single-Crystal X-ray Diffraction Studies i All of the compounds of the formula NCr(NPr 2)2(X), where X is an anionic substituent, have been structurally characterized. 25 As might be expected, the Cr–N distances, both nitrido and amido, are not exceedingly sensitive to changes in X outside the error limits of the X-ray diffraction experiment. All of the compounds exhibit diisopropylamido ligands with the Cr–NC2 amido planes parallel to the Cr–N(nitrido) vector, as expected from the electronic structure (vide supra). Discussion of the X-ray structure of each compound would be gratuitous; however, a few of the more salient features will be addressed in this section. The structural characterization on so large a number of derivatives was carried out predominately to facilitate the steric analyses discussed below, to determine if there were any 139 secondary interactions (e.g., bidentate ligands), and to ascertain if any of the structural parameters might reliably correlate with the electronic and steric features. The donor abilities of the X ligands did not have a large impact on the chromium-nitrido distance. Indeed, the nitrido distance is very similar for all the complexes measured thus far. For example, the nitrido distance in the poorly donating triflate (5), strongly donating and relatively small benzyloxy (19), and the large and strongly donating diisopropylamido (1) were found to be 1.543(3), 1.543(2), and 1.544(3) Å, respectively. The full range of Cr–N(nitrido) values is 1.524(3) in nitrate 21 to 1.553(4) Å in O-p-(CF3)C6H4 16. However, there is no obvious correlation between this distance—or, for that matter, any other metric parameters investigated— and any of the steric or electronic parameters derived. The two chromium-diisopropylamido bond distances were generally the same within error. The only exception in this list of compounds was for carbazolyl 26, which had Cr– N(diisopropylamido) distances of 1.796(2) and 1.833(2) Å. The carbazolyl plane is tilted from the Cr–N(nitrido) vector (Figure 4.2) and, according to the space filling models, there is steric clash between the aromatic ring of the carbazolyl anti to the nitrido and a diisopropylamido i ligand. The longer Cr–NPr 2 distance is associated with the amido closer to the tilted carbazolyl i on the side anti to the nitrido (the left NPr 2 group in the top of Figure 4.2). However, the i average Cr–NPr 2 distance in 26 is similar to the other compounds, and it appears that the steric influence of the carbazolyl is mostly to differentiate the two diisopropylamidos in the solid state. (The two diisopropylamido groups are equivalent in solution.) i The NCr(NPr 2)2 molecular fragment showed some variability in its metric parameters. For example, the amido-chromium-amido angle varied from 116.1(5)° (X = N(Me)Ph 27) to 140 124.9(1)° (X = CN 30), and the average Cr–N(amido) distances varied from 1.805(3) Å (X = i OTf 5) to 1.842(2) Å (X = NPr 2 1). While attempted correlations with these metric parameters and the donor parameters are suggestive, plots of N(amido)-Cr–N(amido) angles and Cr– N(amido) distances with either the steric parameters or donor properties gleaned from the NMR data showed no strong correlations. i Figure 4.2. Spacefilling views of NCr(NPr 2)2(carbazolyl) (26). The top view is looking down the Cr–N(carbazolyl) bond showing the tilting of the heterocyclic framework. The bottom view is anti to the nitrido and shows the tilted carbazolyl ring’s close contacts with one of the isopropyl groups. 141 Steric factors seem evident in the tris(diisopropylamido) complex 1 according to other data (vide infra), but it is difficult to discern this from the X-ray diffraction studies alone. The average Cr–N(amido) distance in the published structure for 1 is 1.842(3) Å. This distance in 1 is somewhat larger than many of the derivatives prepared. For example, the average Cr– N(diisopropylamido) distances for a few derivatives are: Cl 3 1.813(2), OBn 20 1.823(1), OAd 6 1.822(7), N(Me)Ph 27 1.830(2), and OTf 5 1.805(3) Å. However, the average diisopropylamido distance in 1 is very much in line with the sterically less encumbered NMe2 31 with average Cr– N distances of 1.842(4) Å; incidentally, 31 was one of the compounds examined that displayed full molecule disorder in the X-ray diffraction experiments. The disorder was fully modelled. A plot (Figure 4.3) of the average diisopropylamido distance versus the Ligand Donor Parameter (LDP) described below shows no clear correlation. This may be due to the errors in the structural parameters considering the less donating ligands (towards the right in the plot) do, generally speaking, seem to have shorter Cr–N(amido) distances and the more donating ligands seem to have generally longer distances. However, the scatter in the data is far too large to make anything resembling an accurate correlation. In fact, the shortest Cr–N(amido) averages are found for two aryloxide derivatives with moderate LDP values (vide infra) for this series. 26 Two compounds, benzoate 8 and nitrate 21, possibly show weak secondary interactions between Cr and the X ligand. For nitrate containing 21, the Cr–O distance is 1.973(3) Å, and there is a possible weak interaction with a second oxygen of the nitrate nearly trans to the nitrido, which is quite long at over 2.7 Å. For benzoate 8, which has a similar structure near the metal center, the short Cr–O distance is 1.924(1) Å with a possible interaction with the second carboxylate oxygen that is around 3.0 Å away. The contributions from these secondary 142 interactions of X to the measured donor abilities are unlikely to be large at those distances but are not known. 143 i Figure 4.3. Plot of average Cr–NPr 2 distance (Å) vs the donor ability of X (LDP in kcal/mol). For interpretation of data labels the reader is refered to Figure 3 of DiFranco, S. A.; Maciulis, N. A.; Staples, R. J.; Batrice, R. J.; Odom, A. L. Inorganic Chemistry 2012, 51, 1187–200. 144 4.13 Measurement of Amido Rotational Barriers and the Ligand Donor Parameters (LDP) i Most of the NCr(NPr 2)2(X) complexes employed in this study exhibit two distinct methyne peaks for the diisopropylamido ligands at room temperature assigned as being syn and anti to the nitrido substituent. The methyne that is anti is assigned as being deshielded relative to the syn methyne resonance on the basis of 2D NMR experiments on iodo 2 (see the Supporting Information). A few of the compounds where X is a strong donor ligand, e.g., dimethylamido and 1-adamantoxide, have the methynes at or near the coalescence point at room temperature. One of the reasons this system was chosen was that the rate of diisopropylamido rotation was 1 easily measured by H NMR spectroscopy. Once the rate constants were known, the Eyring ‡ equation was used to determine the free energy barriers to rotation, G rot, relative to X. The rate constant for the exchange of the two methynes in the isopropyls, in the majority 1 of cases, was measured using Spin Saturation Transfer (SST) in the H NMR. 27 The ideal temperature for the SST experiment was found to be between –56 °C and +27 °C, depending on the rate of rotation for the particular complex being studied. Detailed descriptions of how the SST experiments and error analyses were done can be found in the Supporting Information. In addition, there is a detailed discussion of how the T1 values were found and the types of T1 values to use in Appendix B. i For one complex, NCr(NPr 2)2(NMe2) (31), due to instrument limitations, we were unable to reach the slow exchange temperature. Line Shape Analysis (LSA) was used to determine the barrier for the isopropyl exchange rather than SST. 145 The X = 1-adamantoxide 6 complex was studied using both LSA and SST for ‡ comparison. Both gave the same value to the nearest tenth of a kcal/mol at G rot = 12.8 kcal/mol. It was expected that the entropy associated with the diisopropylamido rotation would be more or less constant over the series. To investigate this assertion, Eyring plots were done on several of the compounds. The plots were done over as large a temperature range allowable by the kinetics of rotation and our instrumentation. In addition, we explored compounds that varied ‡ in G rot and sterics as much as possible. Consequently, the Eyring plots using SST were i determined for iodo 2, benzyloxy 20, NPr 2 1, and O-p-SMe-C6H4 12. For these four ‡ compounds, S rot was found to be –9, –6, –5, and –3 cal/mol•K, respectively. Consequently, the entropy values appear to be small and negative. The entropy values were found from variable temperature (VT) experiments over temperature ranges of 47, 36, 26, and 44 K, respectively. Additional information on the entropy measurements is found in Appendix B. ‡ It is most desirable to place the rotational barriers in terms of H rot, to remove some of the temperature dependence associated with the measurements. Each compound had to be measured at a temperature best suited for its particular rotation kinetics in order to measure the rate constant as accurately as possible. Consequently, the SST and LSA data were collected at different temperatures for each complex. Here, we are always measuring the kinetics for the rotation of a diisopropylamido ligand i ‡ in a NCr(NPr 2)2(X) complex. We assume that the S rot values for the compounds will all be 146 similar. Even in cases where the X ligand is quite large and steric effects are likely, i.e., for X = i ‡ NPr 2, we have not observed large deviations in S rot values. 28 The most reliable measurement of entropy, based on where the slow and fast exchange limits occur relative to our available instrumentation, appears to be the value for iodo 2, which was done over a 47 K interval. As a result, –9 cal/mol•K was used as the entropy barrier for most of the compounds. 29 The only compound calculated differently is X = NMe2 31, where the activation barriers were found using a different technique (LSA). The experimental barriers for ‡ ‡ 31 were determined to be S rot = –4 cal/mol•K and H rot = 9.3 kcal/mol. 30 Under the assumption that entropy differences are minimal, a set of values approximating ‡ H rot is obtained. Considering the values are an approximation based on the assumption of ‡ S rot = –9 cal/mol•K and their uses for the purposes of this study are more dependent on their relative rather than absolute magnitudes, we call each value a Ligand Donor Parameter (LDP). The LDPs are collected in Figure 4.4 with horizontal error bars allowing quick distinguishing of ligands that are different outside error limits. The current best numerical parameters are collected in Table 4.1. 147 i Figure 4.4. The Ligand Donor Parameters (kcal/mol) for various X in NCr(NPr 2)2X with the associated errors. 148 The values in Figure 4.4 and Table 4.1 constitute our current best measurements on this particular series for the enthalpies of amido rotation at this time. There are a number of interesting series that one can look at qualitatively. Quantitative comparisons will be discussed in sections 4.15-4.20 after discussion of steric influences on rotational barriers. 149 X= LDP 29 a ‡ b NMe2 (31) OAd (6) N(Me)Ph (27) i NPr 2 (1) OBn (20) e S rot b –4 ± 1 9.34 ± 0.32 10.83 ± 0.24 10.86 ± 0.23 d c 11.12 ± 0.23 –5 ± 2 11.15 ± 0.23 –6 ± 5c Carbazolyl (26) 12.04 ± 0.25 O-p-(OMe)C6H4 (11) 12.14 ± 0.24 t O-p-(Bu )C6H4 (13) 12.18 ± 0.25 OPh (10) 12.38 ± 0.25 c O-p-(SMe)C6H4 (12) 12.51 ± 0.26 –3 ± 4 12.64 ± 0.23 O-p-(F)C H (14) 6 4 O-p-(Cl)C6H4 (15) 12.81 ± 0.23 O-p-(CF3)C6H4 (16) 13.00 ± 0.28 OSiPh3 (7) OPht (18) F (33) Indolyl (25) t OBu F6 (9) 13.35 ± 0.23 13.39 ± 0.27 13.40 ± 0.25 13.89 ± 0.26 NO3 (21) Pyr (22) SPh (19) OC6F5 (17) C6F5 Pyr Pyr (23) C6H3(CF3)2 CN (30) O2CPh (8) NCO (28) NCS (29) Cl (3) Br (4) OTf (5) I (2) (24) 13.28 ± 0.27 14.15 ± 0.29 14.16 ± 0.28 14.22 ± 0.27 14.32 ± 0.28 14.33 ± 0.28 14.36 ± 0.28 14.40 ± 0.27 14.45 ± 0.28 14.51 ± 0.29 14.86 ± 0.30 15.05 ± 0.29 15.45 ± 0.30 15.75 ± 0.29 15.80 ± 0.30 –9 ± 5 c a Average value from at least 3 measurements. Entropy values assumed to be –9 cal/mol•K b c except for 31 where the variable temperature LSA value was used. Value from VT LSA. Value d e from VT SST. Steric effects are quite likely contributing to this LDP. Experimental entropy values in cal/mol•K. Errors are from the fits to the Eyring plots. ‡ Table 4.1. Values for LDP (kcal/mol) and S rot (cal/mol•K) for 1-31 and 33. 150 The halides are in the expected order with iodide being the least donating and fluoride the most. Looking at some monoanionic nitrogen-based heterocycles, it was found that pyrrolyl was a far poorer donor than indolyl, which was a poorer donor than carbazolyl. This is consistent with the expected availability of the nitrogen lone pair for donation in these particular heterocycles. The pyrrolyl ring’s aromaticity is dependent upon use of the nitrogen lone pair to reach the 6 -electrons required by the Hückel rule for aromaticity. As a consequence, the aromatic stabilization energy of pyrrole directly competes with -donation, which leads to pyrrolyl being a poor -donor. 31 For indolyl and even more so for carbazolyl, the aromaticity of the 5-membered heterocycles must compete with the 6-membered carbocycle(s) in resonance form contributions to the aromaticity. 32 As a result, the nitrogens in indolyl and carbazolyl seem to donate more strongly to the metal center than pyrrolyl due to the greater availability of their nitrogen-based lone pairs. 151 Figure 4.5. Space filling model of relatively small chloro 3 (top) and large indolyl 25 (bottom). The inscribed orange sphere shows the 3.5 Å radius limit. The strongest donors explored thus far are dialkylamido and alkoxides, which were thought previously to be strong - and -donors. The weakest donors in the series are those with poor overlap due, in all likelihood, to poor size matches between orbitals such as in iodo and thiophenolate, cf. phenolate, or where the X ligand has a competing -system that limits donation such as NCS, benzoate, and pyrrolyl. 152 4.14 Steric Properties of the Ligands Coming up with a single parameter for the steric properties of a diverse set of ligands is an inherently inaccurate exercise. One is using a single parameter to describe a 3-dimensional object, which unless that object is a perfect sphere is an incomplete description. However, the 1 Tolman cone angle is quite successful and gives researchers a parameter for initial optimization of reactions. More recently, Percent Buried Volume (%Vbur) calculations have proven useful in determining steric parameters for ancillary ligand sets. 33 Encouragingly, the %Vbur of phosphine ligands agrees nicely with Tolman’s cone angles for many standard phosphines. Furthermore, %Vbur are easily calculated from crystallographic data using Cavallo and coworkers web-based utility, SambVca, and have been used extensively for ligand types like Nheterocyclic carbenes. 34 In order to determine the %Vbur, the ligands are placed in a sphere so that the ligand is the Cr–X bonding distance away from the center. The sphere size is an adjustable parameter meant to approximate the size where ligands affect the primary coordination sphere of the metal. In most instances, a sphere radius of 3.5 Å is used, 35 which is the default in Cavallo’s program as well as the distance used by Nolan and coworkers. In Figure 4.5, there are two examples showing space filling models; the orange sphere is 3.5 Å and shows how much of each ligand is included in the calculation. For a discussion and a plot of sphere radius’s effect on %Vbur for a selection of ligands, see the Supporting Information. The results of the ligand parameterization using this method are shown in Figure 4.6. 153 Figure 4.6. The %Vbur for the ligands used in this study. Values are for the percentage volume occupied by the ligand in a sphere of radius 3.5 Å from the chromium center. 154 In addition to %Vbur, we also investigated another steric parameterization, the Solid Angle Steric Parameter using the Solid G program. 36 The computational technique works directly from the X-ray diffraction data. The central metal is viewed in some respects as a point source of light, and the ligands block conal access to a sphere around the molecule. An example of the Solid Angle Model is shown in Figure 4.7 for indolyl 25, where the molecule is in a similar orientation as in the bottom of Figure 4.5. The Solid Angle Steric Parameters for the series of X ligands are shown in Figure 4.8. The x-axis values are in percentage of the sphere occupied by the X ligand. 34 Figure 4.7. The Solid Angle Model from the Solid G program for 25 with indolyl (green), diisopropylamido (yellow and blue), and nitrido (red). 155 Figure 4.8. The percentage of the chromium coordination sphere shielded, GM(L), from the Solid G program for the ligands used in this study. 156 The orderings for many of the ligands change somewhat using the different methods. For example, the Solid G method gives a halide ordering of F > Br > I > Cl due to a mixture of bond distance and radii effects. The halide steric ordering from %Vbur is I > Br > Cl > F and seems to be most greatly affected by atomic radius. 37 i That the complex with X = NPr 2 1 has steric influences on its rotational barrier seems i likely. One would expect NPr 2 to be a similar or better donor than NMe2, and yet it has a higher LDP by almost 2 kcal/mol. It appears that sterics are raising the barrier for rotation in this i system, and X = NPr 2 is the largest ligand investigated by far using both steric metrics. The only compound where steric effects seem certain to be playing a role in the i measurement of LDP is NPr 2 1. All other compounds are assumed to have LDPs predominately associated with the electronic barrier to amido rotation as there is currently no compelling evidence to suggest otherwise. Some of the most likely ligands to have unresolved steric effects t are OSiPh3, OBu F6, OAd, N(Me)Ph, indolyl, and carbazolyl, but all of these have steric metrics i below NPr 2. If other ligands do have steric effects on the amido rotational barrier, their observed LDPs are likely upper limits, and they are electronically more donating than observed. 4.15 Applications and Comparisons with These New Electronic Parameters We conclude the Results and Discussion Section by comparing the LDP data quantitatively with different systems from the literature. In these types of applications, the LDP values in Figure 4.4 and Table 4.1 are used much as CO stretching frequencies may be used in 157 alternative late transition metal applications, as arbitrary numbers (in the case of LDP inversely) proportional to the donor ability of the ligand of interest. 4.16 Comparison of LDPs with pKa Values of the HX Compounds First, we investigated if the LDPs found in this study correlated to the pKa of the substituents used. As shown in Figure 4.9, there is no strong relationship. This is to be expected considering the numerous size and orbital make-up differences between the proton and the transition metal system under study. At best, pKa can give one a sense of the -donor ability of the X ligand when attached to a proton. Perhaps this is the reason for the linear correlation for pKa and LDP that seems to exist in the plot between the heavier halides I, Br, and Cl where -effects to the metal are likely minimal. However, pKa would be expected to give no information on -donor ability, which is perhaps why fluoride is not on the same 158 line with its heavier congeners. Figure 4.9. Plot of pKa in water versus LDP. The inset is an expansion of the region containing the phenoxides. 159 Other closely related series, such as the phenoxides (inset in Figure 4.9) may show some trend with pKa. However, relating ligand acidity to ligand donor ability is likely to be of dubious quality, especially if strong -effects are present. 4.17 Phenoxides: An LDP Comparison with Hammett Parameters Hammett parameters are a reliable and well-worn method for examining electronic 5 effects in a variety of systems with, in large part, para-substitution on an arene. In order to determine if there was a correlation between LDP and Hammett p, we generated the set of para-substituted phenoxides 10-16. The full range of LDP differences in the series studied from para-OMe to para-CF3 is 12.14 to 13.00 kcal/mol with errors around 0.25 kcal/mol. While values at the extremes of this subset are different outside the errors, many of the values within the series are not distinguishable with these error limits. However, plotting this series of LDP values versus the Hammett Parameters (p) for the substituents shows a good linear correlation considering the error bars on the LDP (Figure 4.10). 160 Figure 4.10. Plot of LDP vs Hammett parameters for the aryloxide complexes. 161 4.18 Evaluation of 13 C NMR Chemical Shifts in Tungsten Metallacycles using LDP In some cases, spectroscopic data known to change with donor properties of ligands may be correlated with LDP. In 2008, our research group published a study on the reactivity and properties of an unusual class of metallacycles with tungsten-carbon double bond character. 38 Included in this study were NMR spectroscopic data for the series and reactivity in carbonyl olefination reactions for the chloride complex with various additives. The addition of 2 equivalents of cyclooctyne to W(NAr)2Cl2(DME) results in the formation of W(=C8H12=C8H12=NAr)(NAr)Cl2 (Figure 4.11). Substitution of the chlorides for other X ligands leads to structural changes in the complexes and changes in the 13 C NMR chemical shift of the carbon bonded to tungsten. These changes may be viewed as being the result of differing contributions between the alkylidene-imine (left) and alkyl-amido (right) resonance forms. In the paper, we simply stated that the values for the chemical shifts changed as 37 ―might be expected‖ with OEt higher than O-p-(OMe)C6H4 higher than OC6F5. Whereas, for chloride and triflate, also included as X ligands in the paper, it was more difficult to discern their donor properties versus these alkoxides. Using the LDP and plotting versus the 13 C chemical shifts, one sees a good correlation between the ligand donor ability as measured in this work across the available ligand sets in the 39 tungsten system and the NMR data reported (Figure 4.11). The chemical shifts of the - carbons in the metallacycles do indeed correlate with the donor abilities of the X ligands across the entire range of compounds produced in the tungsten study. The linear relationship between LDP and the 13 2 C NMR chemical shift is exceptionally good (R = 0.996). 162 Figure 4.11. Plot of the 13 C NMR chemical shift for the carbon directly bonded to tungsten in W(=C8H12=C8H12=NAr)(NAr)X2 versus the donor ability of X found in this work with a linear fit. 37-8 163 4.19 Comparison Between the AOM of Cr(III) Complexes and the SST Determined Donor Values In this study, we were able to include several classic Werner-type ligands and determine their donor properties in this Cr(VI) system. In these traditional coordination compounds, the donor properties are usually determined using visible absorption data in conjunction with Ligand Field Theory. The values can also be parameterized using the Angular Overlap Model as - and -donor energies, e and e respectively, 164 40 of individual X. Figure 4.12. Plot of e + e for chromium(III) complexes from experimentally determined AOM values versus the LDP for X (top). Plot of e (blue) and e (red) parameters versus the LDP for X (bottom). 165 Figure 4.12 cont: Plot of e + e for chromium(III) complexes from experimentally determined AOM values versus the LDP for X (top). Plot of e (blue) and e (red) parameters versus the LDP for X (bottom). In Figure 4.12 is a plot of e + e for chromium(III) complexes from the experimentally determined AOM values 41 versus the donor ability of X found in this study, which shows a good 166 linear correlation between the two parameterization systems. Also in Figure 4.12 is a plot of the individual e + e parameters versus LDP (bottom). The correlation with either the e or e parameter alone is not nearly as good as their sum. 4.20 Comparison Values from Electronic Spectra of Cp*2TiX Complexes In 1996, Lukens, Smith, and Andersen 42 reported a ―-donor spectrochemical series for X‖ in Cp*2TiX titanium(III) compounds with a large number of X ligands. The study employed 1 EPR and absorption spectroscopy to elucidate the electronic structure of d titanium complexes. Of specific interest in the context of this paper, Andersen and coworkers report the singly occupied a1 to b2 energy gap, which ―depends directly upon the -donor ability of X‖. Mach and coworkers have since extended the system to include additional alkoxide ligands. A plot of the energy gap between a1 (approximately nonbonding) 44 43 and the b2 - antibonding orbital (Exz) in Cp*2TiX Andersen complexes versus LDP for all X in common between the two studies is shown in Figure 4.13 (blue and red circles). In the case of X = OMe, the value for Exz was correlated with the LDP value for OBn in the plot (blue line). The obvious outlier is X = N(Me)Ph (red circle), which is well away from what seems to be a linear correlation between the Cp*2TiX spectroscopic data and LDP. Andersen and coworkers centered much of their discussion on the differences between X = N(Me)Ph and the other compounds, and this is quite obvious in Figure 4.13 as well. Also plotted in Figure 4.13 are 167 Mach’s data (green squares) on Cp*2TiX, where we used our X = OAd data for their X = OBu example. 168 t –1 Figure 4.13. Plot of Exz in wavenumbers (cm ) [Andersen data 42 Mach’s data 41 (red and blue circles), (green squares)] versus LDP (kcal/mol) for X. For the data represented by circles, methylcyclohexane was the solvent. The data represented by green squares were taken in either hexane (‡) or toluene (†). 169 45 There are several indications that the X = N(Me)Ph in Cp*2TiX has little or no -effects to the nitrogen; although there are indications of agostic effects to the methyl. 41 In the structure from X-ray diffraction, the Cp*(centroid)–Ti–N–Me average dihedral in the X = N(Me)Ph complex is 86.9°. In other words, the large N(Me)Ph ligand rests in the plane bisecting the Cp *Ti-Cp* unit, and the nitrogen lone pair is orthogonal to the empty orbital of appropriate symmetry to act as an acceptor. Consequently, the experimental Ti–N bond distance is quite long at 2.054(2) Å. This is similar to Ti–N(pyrrolyl) distances, 30 usually a much weaker donor than N(Me)Ph (vide supra). This distance is also much closer to the Ti–N single bond distance of 2.07 Å than the Ti=N distance of 1.77 Å using Pyykkö’s radii. where there is a strong dative -bond, are typically ~1.90 Å. 46 In contrast, Ti–NMe2 distances, 46 It can be concluded that the lack of correlation for X = N(Me)Ph is due to a deficiency of -bonding in the Cp*2TiX system due to steric effects that do not allow the amido to reach the electronically preferred geometry, a fact readily seen in both the X-ray diffraction study and in correlations with LDP. If one examines the X = N(H)Me complex of Cp*2TiX, the amide is rotated much closer to where maximal overlap with the -acceptor orbital (b2) would be possible. The Cp*(centroid)–Ti–N–Me dihedral for this compound is 13.2°. However, the Ti–N bond, 170 1.955(5) Å, is slightly longer than the average Ti–NMe2 bond in the CSD database. lengthened bond may be due to steric clash with the Cp* ligands. 47 This 48 Overall, the LDP correlate fairly well with the Cp*2TiX spectroscopic data in cases where steric effects are not apparent, i.e., all X ligands in common between the two studies except for where the X ligand is an amido derivative. 4.21 Concluding Remarks Changing metals, changing formal oxidation state, and other ligands on the metal can greatly alter donor properties of ligands. These types of single parameter studies should not replace full mechanistic and computational studies for systems; instead, this is a quick technique that will hopefully be useful in the discussion of properties and mechanisms for metal complexes at low d-electron counts. If a series of ligands for a particular system correlate well to LDP and one does not, it might indicate steric influences, hapticity differences between the chromium system here and the system under study, or other effects are important for that particular compound (for an example see X = N(Me)Ph in section 4.20 above). If the system under study does not correlate at all with LDP, there is any number of possible explanations ranging from differences in ligand donor properties, differences in metal acceptor properties, steric interactions, or simply a lack of correlation of the property being measured with ligand donor ability. In the last segment, we attempted to correlate these new LDP values with numbers from the literature. In the cases discussed above, the values that do correlate, i.e., those other than pKa, did so linearly. It should be kept in mind, however, that there is no reason to assume that all 171 correlations between LDP parameters and data determined using numerous techniques on various systems will always be linear. In addition, more involved methods than the single parameter %Vbur and Solid Angle methods may be required for accurate comparisons of sterics in some systems as well. The absolute values above constitute our current best evaluations of these ligands. Improved techniques and instrumentation for the determination of ligand donor abilities in this system may lead to improved values in the future. There are obvious ligand types that would be useful to include in a series of this type that have not yet been prepared for parameterization of their donor properties. We are continuing to expand the series presented here. Current plans include the characterization of cationic chromium(VI) nitrido systems with neutral X ligands and a selection of organometallic ligands, which are being prepared for evaluation. Parameterizations of this type have seen some historical success in explaining reaction mechanisms and trends in reactivity. For example, Basolo, Pearson, Burdett, and many others have used the Angular Overlap Model extensively in this regard especially for later transition metal systems. 49 It is hoped that this method of ligand parameterization will be useful in catalysis studies ongoing in our group and others on high valent metal complexes. Appendix B contains experimental details and characterization for the production of all the compounds used in the study and data for the X-ray diffraction experiments. Plot of radius versus %Vbur for several ligands. Details on the spin saturation transfer experiments. Details on propagation of error to find LDP values. 2D NMR on 2 and assignments. Details on %Vbur and Solid G calcs. 172 APPENDIX B 173 APPENDIX B B.1 Experimental Procedures for the Preparation of Compounds General Considerations: All reactions and manipulations were carried out in an MBraun glovebox under a nitrogen atmosphere and/or using standard Schlenk techniques. Ethereal solvents, pentane, hexanes, toluene, and benzene were purchased from Aldrich Chemical Co. and purified through alumina columns to remove water after sparging with dinitrogen to remove oxygen. HCl in diethyl ether was purchased from Aldrich Chemical Co. and used as received. The literature preparations were used for Iodo 2, 20 t 21 OBu F6 9, 21 and OPh 10. The reagent 15- crown-5 was dried by making a toluene solution and refluxing with a Dean-Stark trap overnight. Lutidinium iodide was prepared using the literature procedure. 3-C6F5 Hpyr and C6H3-3,5-(CF3)2 Hpyr , where prepared 20 The 3-substituted pyrroles, as (Pyrrolyl)thallium(I) was prepared similar to the literature procedure previously 52 reported. 5051 using 1.1 equivalents of freshly filtered TlOEt in ether, which was added to cold pyrrole in ether. The product precipitates as a colorless solid with low solubility in common organic solvents. All NMR solvents were purchased from Cambridge Isotopes Laboratories, Inc. Deuterated toluene and benzene were distilled from sodium benzophenone ketyl. Deuterated chloroform was distilled from calcium hydride under dry a dinitrogen atmosphere. The NMR solvents were stored in the glovebox in glass containers with a stopcock. Spectra were taken on Varian instruments located in the Max T. Rogers Instrumentation Facility at Michigan State University. These include a UNITYplus 500 spectrometer equipped with a 5 mm Pulsed-Field1 Gradient (PFG) switchable broadband probe and operating at 499.955 MHz ( H) and 125.77 174 13 1 1 ( C), and a UNITYplus 300 spectrometer operating at 299.976 MHz( H). H NMR chemical shifts are reported relative to residual CHCl3 in CDCl3 as 7.24 ppm. are reported relative to 13 CDCl3 as 77.0 ppm. 13 C NMR chemical shifts 19 F NMR chemical shifts are relative to external, neat FC6H5 as –113.15 ppm. The resonances for the quaternary carbons for CN, NCS, and NCO in 13 C NMR spectroscopy have very long relaxation times, requiring the delay time to be set to at least 15 s for the acquisition. Computational Methods for the NCr(NH2)3 Model System: DFT calculations were done using the Gaussian03 software package. 53 The initial optimizations were done using the B3LYP functional and the 6-31G** basis set on a Macintosh computer. The amido rotation was examined by restricting an H–N–Cr–N(nitrido) dihedral angle in 10° increments from 0° (in the the ground state) to 90°, which is past the transition state for rotation. The structures were then reoptimized with B3LYP 54 and the aug-cc-pVQZ basis set 55 using the computational facilities at the High Performance Computing Center (HPCC) at MSU. The highest energy structure investigated through incrementing the dihedral had an angle of 60°; this structure was used as the approximate transition state in a Quadratic Synchronous Transit (QST) optimization to find the transition state. The transition state had a dihedral of 61° and one negative vibrational frequency associated with the amido rotation. Synthesis of (benzyloxide)thallium(I): 56 In the dry box, a scintillation vial was loaded with benzyl alcohol (0.150 g, 1.39 mmol, 1 equiv), pentane (3 mL), and a stirbar. The solution was cooled to near frozen in a liquid nitrogen cooled cold well. The vial was moved to a stir plate, and freshly filtered TlOEt (0.349 g, 1.40 mmol, 1.01 equiv) in pentane (3 mL) was added 175 dropwise. The reaction was allowed to come to room temperature with stirring. After 2 h, the volatiles were removed in vacuo yielding TlOBn as a white powder (0.415 g, 1.33 mmol, 96%). 1 H NMR (500 MHz, C6D6, 25 °C): 4.92 (s, 2H, PhCH2O), 7.06-7.09 (m, 1H, p-Ar-H), 7.19-7.21 (m, 2H, m-Ar-H), 7.23-7.25 (m, 2H, o-Ar-H). 13 1 C{ H} NMR (125 MHz, C6D6, 25 °C): 66.58, 127.07, 127.81, 128.86, 146.53. M.p. 74-76 °C (Lit. 57 74-78 °C). Synthesis of 2,6-lutidenium chloride: In an oven dried 125 mL Erlenmeyer flask was loaded dry 2,6-lutidine (1.00 g, 9.33 mmol, 1 equiv.) and dry pentane (50 mL). The solution was cooled in an ice water bath. A stir bar was added to the flask, and HCl in Et2O (10.26 mL, 1.0 M, 1.1 equiv.) was added dropwise with stirring. The bath was removed, and the mixture was allowed to come to room temperature. After 1 h the solids were collected on a glass frit and washed with pentane (3 × 10 mL). The solids were dried in vacuo yielding lutidenium chloride as an off white powder (1.27 g, 8.87 mmol, 95% yield). M.p. 237-240 °C. Synthesis of 2,6-lutidenium bromide: In a fumehood, an oven dried 250 mL Schlenk flask with 2 side arms and one ground-glass 24/40 joint was charged with NaBr (30 g, 4.5 equiv). An inlet N2 line (on a mercury bubbler) was connected to one of the arms. To the other side arm, Tygon tubing was attached. The other end of the Tygon tubing was connected to a glass tube. The ground glass joint was sealed with a rubber septum. The glass tube was inserted through a rubber stopper, which was placed in another 250 mL Schlenk flask. This second flask was charged with 2,6-lutidine (7.00 g, 65.3 mmol, 1 equiv.) in freshly distilled THF (60 mL) and a stir bar. The side arm of the flask containing the lutidine solution was connected via Tygon tubing to a gas trap containing a saturated NaOH aqueous solution. While vigorously stirring the lutidine solution, concentrated H2SO4 (5 mL, 93.8 mmol, 1.44 equiv.) was slowly added via 176 syringe onto the solid NaBr. After about 0.5 mL was added, the glass rod was dipped below the surface of the lutidine in THF solution. The remaining H2SO4 was added over a period of 10 min. After addition, the flowing N2 was turned off, and the reaction stirred for a further 30 min during which the THF solution turned cloudy white. The suspension was then filtered on a frit and washed with pentane (3 × 20 mL). The solids were collected and dried in vacuo yielding 2,6lutidenium bromide as a white powder (1.28 g, 6.79 mmol, 10% yield). If desired, unreacted 2,6lutidine can be recovered from the filtrate via distillation. Exposure of the salt to air should be minimized to avoid possible hydrates. The reagent was stored in the dry box. Mp: 206-210 °C. 2,6-Lutidinium bromide can also be prepared from trimethylsilylbromide using a procedure analogous to that used for production of 2,6-lutidinium iodide. 20 i Synthesis of NCr(NPr 2)3 (1): Under an inert N2 atmosphere, a 250 mL Erlenmeyer flask i was loaded with Cr(NPr 2)3 (1.15 g, 3.26 mmol, 1 equiv.) and pentane (~25 mL). In a separate t flask, a pentane solution (50 mL) of freshly sublimed NCr(OBu )3 (0.931 g, 3.26 mmol, 1 equiv.) t was prepared. The yellow solution of NCr(OBu )3 was added slowly in portions over ~10 min to i the rapidly stirring Cr(NPr 2)3 solution. The solution rapidly turned beet red, and stirring was continued for 1.5 h after addition was complete. The volatiles were removed in vacuo, and acetonitrile (100 mL) was added. After stirring for 5 min, the mixture was filtered through a fritted glass funnel, and the solids were washed with acetonitrile (2 × 10 mL). The solids were transferred to a vial and dried in vacuo yielding the title compound as dark red microcrystals (1.06 g, 2.90 mmol, 89% yield). If necessary, 1 can be further purified by recrystallization from 1 concentrated pentane solution at −35 °C. H NMR (500 MHz, CDCl3, −30 °C): 4.33 (br sept, 177 3H, CH(CH3)2), 3.42 (br sept, 3H, CH(CH3)2), 1.43 (br d, 14H, CH(CH3)2), 1.05 (br d, 14H, CH(CH3)2). Melting point and room temperature NMR spectroscopy were in agreement with 17a literature values. i Synthesis of NCr(NPr 2)2(Cl) (3): Under an inert atmosphere a pressure tube was loaded with 1 (0.400 g, 1.09 mmol, 1 equiv.), 2,6-lutidinium chloride (0.392 g, 2.73 mmol, 2.5 equiv.), and a stirbar. CHCl3 (~35 mL) was added. The tube was sealed and removed from the drybox. The tube was set in a 60 °C oil bath, and the reaction was stirred for 12 h. The tube was taken back into the drybox, and the volatiles were removed in vacuo. The residue was extracted with pentane and filtered. The solvent was removed yielding 3 as an orange powder (0.270 g, 0.895 mmol, 82% yield). Diffraction quality crystals were obtained from a concentrated pentane 1 solution at −35 °C. H NMR (500 MHz, CDCl3, 25 °C): 5.24 (sept, JHH = 6.61, 2H, CH(CH3)2), 3.82 (sept, JHH = 6.21, 2H, CH(CH3)2), 1.91 (d, JHH = 6.42, 6H, CH(CH3)2), 1.49 (d, JHH = 6.28, 6H, CH(CH3)2), 1.25 (d, JHH = 6.33, 6H, CH(CH3)2), 1.13 (d, JHH = 6.57, 6H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, 0 °C): 59.20, 57.10, 30.36, 29.92, 21.45, 19.90. Mp: 157-158 °C. i Synthesis of NCr(NPr 2)2(Br) (4): Under an inert atmosphere, a pressure tube was loaded with 1 (0.120 g, 0.328 mmol, 1 equiv.), 2,6-lutidinium bromide (0.092 g, 0.49 mmol, 1.5 equiv.), and a stirbar. CHCl3 (~25 mL) was added. The tube was sealed and removed from the drybox. The tube was set in a 60 °C oil bath and stirred for 12 h. The tube was moved back into the drybox, and the volatiles were removed in vacuo. The residue was extracted with pentane and 178 filtered through Celite. The solvent was removed in vacuo yielding 4 as an orange powder (0.060 g, 0.17 mmol, 52% yield). Diffraction quality crystals were obtained from a concentrated 1 pentane solution at −35 °C. H NMR (500 MHz, CDCl3, 25 °C): 5.28 (sept, JHH = 6.51, 2H, CH(CH3)2), 3.81 (sept, JHH = 6.31, 2H, CH(CH3)2), 1.89 (d, JHH = 6.31, 6H, CH(CH3)2), 1.50 (d, JHH = 6.26, 6H, CH(CH3)2), 1.28 (d, JHH = 6.40, 6H, CH(CH3)2), 1.14 (d, JHH = 6.64, 6H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, −5 °C): 59.34, 57.40, 30.20, 29.54, 21.29, 19.78. Mp: 160-164 °C. i Synthesis of NCr(NPr 2)2(OTf) (5): Under an inert atmosphere, a scintillation vial was loaded with 1 (0.30 g, 0.82 mmol, 1 equiv.), pentane (10 mL), and a stir bar. The solution was cooled to near frozen in a liquid nitrogen cooled cold well. To the rapidly stirring solution, 1.55 M triflic acid in a DME (533 μL, 0.826 mmol, 1.01 equiv.) was added dropwise. The reaction was allowed to come to room temperature and stirred for 4 h. The volatiles were removed in vacuo, and the residue was taken up in a minimal amount of pentane (2 × 10 mL) and filtered through Celite. The filtrate was concentrated in vacuo. Cooling the pentane solution to −35 °C 1 yielded 5 as red-orange crystals (0.238 g, 0.573 mmol, 70% yield). H NMR (300 MHz, CDCl3, 25 °C): 5.31 (sept, JHH = 6.66, 2H, CH(CH3)2), 3.94 (sept, JHH = 6.35, 2H, CH(CH3)2), 2.02 (d, JHH = 6.35, 6H, CH(CH3)2), 1.48 (d, JHH = 6.35, 6H, CH(CH3)2), 1.31 (d, JHH = 6.35, 6H, CH(CH3)2), 1.16 (d, JHH = 6.35, 6H, CH(CH3)2). Mp: 198 °C (sub). 179 19 F NMR (564 MHz, CDCl3, 25 °C): −76.65. i Synthesis of NCr(NPr 2)2(OAd) (6): Under an inert atmosphere, a pressure tube was loaded with 1-adamantanol (0.042 g, 0.27 mmol, 1 equiv.), toluene (10 mL), and a stirbar. To this solution, 1 (0.10 g, 0.27 mmol, 1 equiv.) in toluene (8 mL) was added. The pressure tube was sealed and placed in a 90 °C oil bath. The reaction stirred at this temperature for 3 d. The tube was taken back into the glove box, and the volatiles were removed in vacuo. The residue was extracted with pentane and filtered through Celite. The solution was concentrated to ~5 mL and placed in a −35 °C freezer yielding red-orange crystals of 6 (0.080 g, 0.191 mmol, 70% 1 yield). H NMR (500 MHz, CDCl3, −40 °C): 4.75 (br sept, 2H, CH(CH3)2), 3.54 (br sept, 2H, CH(CH3)2), 2.08 (app s, 3H, Ad CH), 1.71 (d, JHH = 2.5 Hz, 6H, Ad CH2), 1.63 (d, JHH = 5.5 Hz, 6H, CH(CH3)2), 1.54 (app s, 6H, Ad CH2), 1.40 (d, JHH = 5.5 Hz, 6H, CH(CH3)2), 1.071.02 (m, 12H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, −35 °C): 74.0, 57.4, 53.4, 47.0, 36.2, 31.0, 30.0, 29.3, 21.0, 19.5. Mp: 120-125 °C. i Synthesis of NCr(NPr 2)2(OSiPh3) (7): Under an inert atmosphere, a scintillation vial was loaded with 1 (0.70 g, 0.19 mmol, 1 equiv.), toluene (~5 mL), and a stirbar. A solution of HOSiPh3 (0.053 g, 0.19 mmol, 1 equiv.) in toluene (5 mL) was added slowly. As the reaction stirred it gradually turned from the beet color of the starting material to orange. After 16 h, the solution was dried in vacuo. The residue was extracted with pentane and filtered through Celite. The filtrate was concentrated (~5 mL) in vacuo. The pentane solution yielded orange crystals of 1 7 (0.075 g, 0.14 mmol, 72% yield) at −35 °C. H NMR (500 MHz, CDCl3, 0 °C): 7.64 (d, JHH = 6.34, 6H Ar-C-H), 7.49-7.26 (m, 9H Ar-C-H), 5.01 (sept, JHH = 6.61, 2H, CH(CH3)2), 3.63 180 (sept, JHH = 6.20, 2H, CH(CH3)2), 1.60 (d, JHH = 6.39, 6H, CH(CH3)2), 1.39 (d, JHH = 6.29, 6H, CH(CH3)2), 1.09 (d, JHH = 6.53, 6H, CH(CH3)2), 1.00 (d, JHH = 6.29, 6H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, 25 °C): 138.7, 135.3, 128.9, 127.2, 58.4, 55.4, 30.2, 29.5, 21.4, 20.6. Mp: 115-120 °C. i Synthesis of NCr(NPr 2)2(O2CPh) (8): Under an inert atmosphere a scintillation vial was loaded with 1 (0.150 g, 0.409 mmol, 1 equiv.), a stir bar, and toluene (8 mL), and placed in a liquid nitrogen cooled cold well until nearly frozen. Benzoic acid (0.050 g, 0.41 mmol, 1 equiv.) in toluene (1 mL) was added. The reaction was allowed to warm to room temperature and was stirred for 6 h. Over that time the solution changed from the beet color of the starting material to dark orange. The volatiles were removed in vacuo, and the residue was extracted with pentane (2 × 5 mL) and filtered through Celite. Concentrated solutions cooled to −35 °C yielded 8 as red1 orange crystals (0.140 g, 0.360 mmol, 88% yield). H NMR (500 MHz, CDCl3, 0 °C): 8.02-8.00 (m, 2H Ar-o-C-H), 7.45-7.41 (m, 1H Ar-p-C-H), 7.37-7.34 (m, 2H Ar-m-C-H), 5.60 (sept, JHH = 6.29, 2H, CH(CH3)2), 3.86 (sept, JHH = 6.47, 2H, CH(CH3)2), 1.94 (d, JHH = 6.31, 6H, CH(CH3)2), 1.53 (d, JHH = 6.37, 6H, CH(CH3)2), 1.18 (d, JHH = 6.40, 6H, CH(CH3)2), 1.13 (d, JHH = 6.46, 6H, CH(CH3)2). 13 C NMR (125 MHz, CDCl3, 0 °C): 171.5, 133.5, 131.4, 129.9, 127.9, 58.2, 57.0, 30.7, 30.1, 22.2, 21.7. Mp: 121 °C (dec). i Synthesis of NCr(NPr 2)2(O-p-(OMe)C6H4) (11): Under an inert atmosphere, a scintillation vial was loaded with 1 (0.15 g, 0.41 mmol, 1 equiv.), toluene (5 mL), and a stirbar. The solution was frozen in a liquid nitrogen cooled cold well, then removed to a stir plate. To the 181 thawing solution of 1 was added a solution of HO-p-(OMe)C6H4 (0.051 g, 0.41 mmol, 1 equiv.) in toluene (5 mL) over 5 min. The reaction was stirred and was allowed come to room temperature. After 1.5 h, the orange solution was dried in vacuo. The residue was extracted with pentane and filtered through Celite. The filtrate was concentrated (~5 mL) in vacuo. Cooling concentrated pentane solutions to −35 °C yielded dark orange crystals of 11 (0.139 g, 0.356 1 mmol, 87% yield). H NMR (500 MHz, CDCl3, −38 °C): 6.89 (d, JHH = 8.50, 2H, Ar-m-C-H), 6.71 (d, JHH = 9.00, 2H, Ar-o-C-H), 4.99 (sept, JHH = 6.50, 2H, CH(CH3)2), 3.72 (s, 3H, Ar-pOCH3), 3.71 (sept, JHH = 6.50, 2H, CH(CH3)2), 1.82 (d, JHH = 6.00, 6H, CH(CH3)2), 1.43 (d, JHH = 6.00, 6H, CH(CH3)2), 1.15-1.13 (m, 12H, CH(CH3)2). 13 C NMR (125 MHz, CDCl3, −35 °C): 161.3, 152.3, 117.6, 113.5, 58.0, 55.5, 54.9, 30.3, 29.9, 21.3, 21.0. Mp: 102-104 °C. i Synthesis of NCr(NPr 2)2(O-p-(SMe)C6H4) (12): Under an inert atmosphere, a scintillation vial was loaded with 1 (0.15 g, 0.41 mmol, 1 equiv.), toluene (5 mL), and a stirbar. The solution was frozen in a liquid nitrogen cooled cold well, then removed to a stir plate. To the thawing solution was added HO-p-(SMe)C6H4 (0.057 g, 0.41 mmol, 1 equiv.) in toluene (5 mL) over 5 minutes. The reaction was stirred and allowed to come to room temperature. After 1.5 h the orange solution was dried in vacuo. The residue was extracted with pentane and filtered through Celite. The filtrate was concentrated (~5 mL) in vacuo. Cooling concentrated pentane 1 solutions to −35 °C yielded dark orange crystals of 12 (0.153 g, 0.376 mmol, 92% yield). H NMR (500 MHz, CDCl3, −10 °C): 7.14 (d, JHH = 8.64, 2H, Ar-m-C-H), 6.88 (d, JHH = 8.64, 2H, Ar-o-C-H), 5.02 (sept, JHH = 6.21, 2H, CH(CH3)2), 3.73 (sept, JHH = 6.09, 2H, CH(CH3)2), 182 2.40 (s, 3H SCH3), 1.83 (d, JHH = 6.08, 6H, CH(CH3)2), 1.44 (d, JHH = 6.18, 6H, CH(CH3)2), 1.14 (d, JHH = 6.00, 12H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, −10 °C): 165.7, 130.2, 125.7, 118.2, 58.2, 55.3, 30.3, 30.0, 23.3, 21.3, 21.0, 18.6. Mp: 112-115 °C. i t Synthesis of NCr(NPr 2)2(O-p-(Bu )C6H4) (13): Under an inert atmosphere, a scintillation vial was loaded with 1 (0.15 g, 0.41 mmol, 1 equiv.), toluene (5 mL), and a stirbar. The solution was frozen in a liquid nitrogen cooled cold well, then removed to a stir plate. To the t thawing solution was added HO-p-( Bu)C6H4 (0.061 g, 0.41 mmol, 1 equiv.) in toluene (5 mL) over 5 min. The reaction was stirred and allowed to come to room temperature. After 1.5 h, the orange solution was dried in vacuo. The residue was extracted with pentane and filtered through Celite. The filtrate was concentrated (~5 mL) in vacuo. Cooling concentrated pentane solutions 1 to −35 °C yielded dark orange crystals of 13 (0.16 g, 0.385 mmol, 94% yield). H NMR (500 MHz, CDCl3, −25 °C): 7.16 (d, JHH = 8.62, 2H, Ar-m-C-H), 6.88 (d, JHH = 8.62, 2H, Ar-o-CH), 5.00 (sept, JHH = 6.35, 2H, CH(CH3)2), 3.71 (sept, JHH = 6.23, 2H, CH(CH3)2), 1.82 (d, JHH = 6.18, 6H, CH(CH3)2), 1.44 (d, JHH = 6.35, 6H, CH(CH3)2), 1.24 (s, 9H C(CH3)3), 1.13 (d, JHH = 6.35, 12H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, −35 °C): 164.4, 141.3, 125.5, 116.5, 58.1, 58.0, 55.0, 33.9, 31.5, 30.3, 30.0, 21.2, 20.9. Mp: 188-190 °C. i Synthesis of NCr(NPr 2)2(O-p-(F)C6H4) (14): Under an inert atmosphere, a scintillation vial was loaded with 1 (0.15 g, 0.41 mmol, 1 equiv.), toluene (5 mL), and a stirbar. The solution was frozen in a liquid nitrogen cooled cold well, then removed to a stir plate. To the thawing solution was added HO-p-(F)C6H4 (0.046 g, 0.41 mmol, 1 equiv.) in toluene (5 mL) over 5 min. 183 The reaction was stirred and was allowed to come to room temperature. After 1.5 h, the orange solution was dried in vacuo. The residue was extracted with pentane and filtered through Celite. The filtrate was concentrated (~5 mL) in vacuo. Cooling concentrated pentane solutions to 1 −35 °C yielded dark orange crystals of 14 (0.136 g, 0.360 mmol, 88% yield). H NMR (500 MHz, CDCl3, −30 °C): 7.38 (d, JHH = 8.63, 2H, Ar-m-C-H), 6.95 (d, JHH = 8.63, 2H, Ar-o-CH), 5.06 (sept, JHH = 6.48, 2H, CH(CH3)2), 3.75 (sept, JHH = 6.42, 2H, CH(CH3)2), 1.84 (d, JHH = 6.35, 6H, CH(CH3)2), 1.45 (d, JHH = 6.43, 6H, CH(CH3)2), 1.16-1.11 (m, 12H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, 0 °C): 163.2, 157.3, 117.9 (d, JCF = 7.8), 114.9 (d, JCF = 22.4), 58.3, 55.3, 30.3, 30.0, 21.3, 21.1. 19 F NMR (564 MHz, CDCl3, 25 °C): –126.8. Mp: 81 °C (dec). i Synthesis of NCr(NPr 2)2(O-p-(Cl)C6H4) (15): Under an inert atmosphere, a scintillation vial was loaded with 1 (0.15 g, 0.41 mmol, 1 equiv.), toluene (5 mL), and a stirbar. The solution was frozen in a liquid nitrogen cooled cold well, then removed to a stir plate. To the thawing solution was added HO-p-(Cl)C6H4 (0.053 g, 0.41 mmol, 1 equiv.) in toluene (5 mL) over 5 min. The reaction was stirred and was allowed to come to room temperature. After 1.5 h, the orange solution was dried in vacuo. The residue was extracted with pentane and filtered through Celite. The filtrate was concentrated (~5 mL) in vacuo. Cooling concentrated pentane solutions to 1 −35 °C yielded dark orange crystals of 15 (0.137 g, 0.348 mmol, 85% yield). H NMR (500 MHz, CDCl3, −40 °C): 7.06 (d, JHH = 8.99, 2H, Ar-m-C-H), 6.84 (d, JHH = 8.76, 2H, Ar-o-CH), 5.01 (sept, JHH = 6.26, 2H, CH(CH3)2), 3.73 (sept, JHH = 6.26, 2H, CH(CH3)2), 1.82 (d, 184 JHH = 6.29, 6H, CH(CH3)2), 1.42 (d, JHH = 6.04, 6H, CH(CH3)2), 1.12 (d, JHH = 6.57, 6H, CH(CH3)2), 0.996 (d, JHH = 6.18, 6H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, −20 °C): 165.3, 128.5, 123.1, 118.7, 58.3, 55.4, 44.8, 30.3, 30.1, 23.2, 21.3, 21.0. Mp: 124-125 ºC. i Synthesis of NCr(NPr 2)2(O-p-(CF3)C6H4) (16): Under an inert atmosphere, a scintillation vial was loaded with 1 (0.15 g, 0.41 mmol, 1 equiv.), toluene (5 mL), and a stirbar. The solution was frozen in a liquid nitrogen cooled cold well, then removed to a stir plate. To the thawing solution was added HO-p-(CF3)C6H4 (0.066 g, 0.41 mmol, 1 equiv.) in toluene (5 mL) over 5 min. The reaction was stirred and was allowed to come to room temperature. After 1.5 h, the orange solution was dried in vacuo. The residue was extracted with pentane and filtered through Celite. The filtrate was concentrated (~5 mL) in vacuo. Cooling concentrated pentane 1 solutions to −35 °C yielded dark orange crystals of 16 (0.143 g, 0.336 mmol, 82% yield). H NMR (500 MHz, CDCl3, 3 °C): 7.37 (d, JHH = 8.58, 2H, Ar-m-C-H), 6.95 (d, JHH = 8.58, 2H, Ar-o-C-H), 5.07 (sept, JHH = 6.29, 2H, CH(CH3)2), 3.76 (sept, JHH = 6.22, 2H, CH(CH3)2), 1.85 (d, JHH = 6.04, 6H, CH(CH3)2), 1.46 (d, JHH = 6.03, 6H, CH(CH3)2), 1.15 (d, JHH = 3.72, 6H, CH(CH3)2), 1.02 (d JHH = 6.34, 6H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, 3 °C): 169.1, 126.3 (quar, JCF = 3.79), 117.7, 117.6, 58.5, 55.7, 30.4, 30.2, 23.3, 21.3, 21.1. 19 F NMR (564 MHz, CDCl3, 25 °C): −64.24. Mp: 131-132 ºC. Anal. Calcd: C, 53.38; H, 7.56; N, 9.82. Found: C, 53.40; H, 7.77; N, 9.80. 185 i Synthesis of NCr(NPr 2)2(OC6F5) (17): Under an inert atmosphere, a scintillation vial was loaded with 1 (0.15 g, 0.41 mmol, 1 equiv.), toluene (5 mL), and a stirbar. The solution was frozen in a liquid nitrogen cooled cold well, then removed to a stir plate. To the thawing solution was added HOC6F5 (0.075 g, 0.41 mmol, 1 equiv.) in toluene (5 mL) over 5 minutes. The reaction was stirred and was allowed to come to room temperature. After 1.5 h, the orange solution was dried in vacuo. The residue was extracted with pentane and filtered through Celite. The filtrate was concentrated (~5 mL) in vacuo. Cooling concentrated pentane solutions to 1 −35 °C yielded dark orange crystals of 17 (0.16 g, 0.385 mmol, 76% yield). H NMR (500 MHz, CDCl3, −20 °C): 5.12 (sept, JHH = 6.50, 2H, CH(CH3)2), 3.81 (sept, JHH = 6.50, 2H, CH(CH3)2), 1.87 (d, JHH = 6.50, 6H, CH(CH3)2), 1.41 (d, JHH = 6.00, 6H, CH(CH3)2), 1.26 (d, JHH = 6.50, 6H, CH(CH3)2), 1.16 (d, JHH = 6.00, 6H, CH(CH3)2). CDCl3, -20 ºC): 143-137, 58.7, 56.4, 30.4, 29.8, 21.6, 20.6. 19 13 1 C{ H} NMR (125 MHz, F NMR (564 MHz, CDCl3, 25 °C): −161.70 (dd, JFF = 37.22 Hz, 13.54 Hz, 2F), −167.25 to −167.46 (m, 2F), −173.58 (tt, JFF = 44.56 Hz, 13.54 Hz, 1F). Mp: 129-132 ºC. i Synthesis of NCr(NPr 2)2(OPth) (18): Under an N2 atmosphere a scintillation vial was loaded with N-(hydroxy)phthalimide (HOPth, 0.081 g, 0.494 mmol, 1 equiv), CHCl3 (5 mL), and a stir bar. To this slurry was added 1 (0.181 g, 0.494 mmol, 1 equiv) in CHCl3 (5 mL). The solution turned orange and stirred for 16 h at room temperature. The volatiles were removed in vacuo, and the residue was extracted with toluene (3 × 5 mL). This solution was filtered through 186 Celite, and the filtrate was concentrated to 8 mL. Diffraction quality crystals of 18 were grown 1 from toluene solution at −35 °C (0.110 g, 0.257 mmol, 52%). H NMR (500 MHz, CDCl3, −2 °C): 7.68-7.67 (m, 2H, Phth), 7.59-7.57 (m, 2H, Phth), 5.13 (sept, 2H, CH(CH3)2, J HH = 6.5 Hz), 3.82 (sept, 2H, CH(CH3)2, J HH = 6.5 Hz), 1.93 (d, 6H, CH(CH3)2, J HH = 6.5 Hz), 1.411.38 (m, 12H, CH(CH3)2), 1.16 (d, 6H, CH(CH3)2, JHH = 6.5 Hz). 13 1 C{ H} NMR (125 MHz, CDCl3, −2 °C): 20.38, 21.66, 56.10, 58.52, 122.32, 129.72, 133.30, 163.75. M.p. 179 °C (dec). i Synthesis of NCr(NPr 2)2(SPh) (19): Under an inert atmosphere, a pressure tube was loaded with 1 (0.10 g, 0.27 mmol, 1 equiv.), toluene (5 mL), and a stirbar. To the stirring solution of 1 was added thiophenol (0.030 g, 0.27 mmol, 1 equiv.) in toluene (5 mL). The pressure tube was sealed, placed in a 65 °C oil bath, and stirred for 20 h. The reaction was taken back under an inert atmosphere, and the volatiles were removed in vacuo. The residue was extracted with pentane and filtered through Celite. The solution was concentrated to ~5 mL and placed in a −35 °C freezer yielding red-purple crystals of 19 (0.077 g, 0.21 mmol, 75% yield). 1 H NMR (500 MHz, CDCl3, −4 °C): 7.62-7-60 (d, 2H, Ar-o-C-H), 7.12-7.09 (t, 2H, Ar-m-C-H), 6.99-6.96 (t, 1H Ar-p-C-H), 5.23-5.18 (sept, 2H, CH(CH3)2), 3.72-3.67 (sept, 2H, CH(CH3)2), 1.75-1.731 (d, 6H, CH(CH3)2), 1.49-1.47 (d, 6H, CH(CH3)2), 1.13-1.11 (d, 12H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, −4 °C): 141.95, 132.58, 127.87, 123.94, 59.00, 55.95, 30.34, 29.91, 21.96, 20.38. Mp: 118-120 ºC. i Synthesis of NCr(NPr 2)2(OBn) (20): Under an inert atmosphere, a scintillation vial was loaded with 2 (0.075 g, 0.19 mmol, 1 equiv.) and hexane (10 mL). This was cooled to near 187 frozen in a liquid nitrogen cooled cold well. In a separate vial, TlOBn (0.065 g, 0.21 mmol, 1.1 equiv.) was slurried in THF (2 mL), and a stir bar was added. The solution of 2 was then added dropwise over 5 min to the rapidly stirring slurry. The reaction was allowed to come to room temperature and stir for 16 h, during which yellow TlI precipitated. The volatiles were removed in vacuo. The residue was extracted with pentane and filtered through Celite. The filtrate was concentrated in vacuo. Cooling a concentrated pentane solution to −35 °C yielded 20 as orange 1 crystals (0.641 g, 0.172 mmol, 90% yield). H NMR (500 MHz, CDCl3, −45 °C): 7.55 (d, 2H, Ar-o-CH, JHH = 7.0), 7.30 (app t, 2H, Ar-m-CH, JHH = 7.5), 7.20 (t, 1H, Ar-p-CH, JHH = 7.5), 5.47 (s, 2H, CH2), 4.75 (sept, 2H, CH(CH3)2, JHH = 6.0), 3.59 (sept, 2H, CH(CH3)2, JHH = 6.0), 1.61 (d, 6H, CH(CH3)2, JHH = 6.0), 1.35 (d, 6H, CH(CH3)2, JHH = 6.0), 1.04 (br s, 12H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, –40 ºC): 143.9, 127.8, 127.3, 126.6, 80.3, 57.5, 54.1, 30.1, 29.4, 21.3, 20.7. Mp: 139-140 ºC. Anal. Calcd: C, 61.09; H, 9.46; N, 11.24. Found: C, 60.87; H, 9.16; N, 11.22. i Synthesis of NCr(NPr 2)2(NO3) (21): Under an inert atmosphere, a scintillation vial was loaded with TlNO3 (0.203 g, 0.763 mmol, 3 equiv.), a stir bar, and THF (8 mL). To the slurry of TlNO3 was added 2 (0.10 g, 0.25 mmol, 1 equiv.) in THF (5 mL). The reaction was stirred for 16 h at room temperature, after which time the volatiles were removed in vacuo. The residue was extracted with pentane (3 × 10 mL) and filtered through Celite. Removal of volatiles in vacuo yielded the title compound as a red-orange powder (0.063 g, 0.19 mmol, 75% yield). Diffraction 1 quality crystals were obtained from a pentane at −35 °C. H NMR (600 MHz, CDCl3, 25 °C): 188 5.54 (sept, JHH = 5.35, 2H, CH(CH3)2), 3.92 (sept, JHH = 6.41, 2H, CH(CH3)2), 1.95 (d, JHH = 6.21, 6H, CH(CH3)2), 1.55 (d, JHH = 6.23, 6H, CH(CH3)2), 1.22 (d, JHH = 6.35, 6H, CH(CH3)2), 1.13 (d, JHH = 6.32, 6H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, 25 °C): 59.80, 58.12, 31.06, 30.00, 22.41, 22.03. Mp: 77 ºC (dec). i Synthesis of NCr(NPr 2)2(pyrrolyl) (22): Under an inert atmosphere, a scintillation vial was loaded with 2 (0.100 g, 0.254 mmol 1 equiv.), a stir bar, and toluene (8 mL). A slurry of freshly made thallium pyrrole (0.695 g, 0.257 mmol, 1.01 equiv.) in Et2O (5 mL) was added to the stirring solution. The reaction was allowed to stir for 20 h at room temperature, during which yellow TlI precipitated. The precipitate was removed by filatration, and the volatiles were removed in vacuo. The residue was extracted with pentane (3 × 10 mL) and concentrated in vacuo to ~5 mL. Cooling the concentrated solution to −35 °C yielded red-orange crystals of 22 1 (0.068 g, 0.20 mmol, 80% yield). H NMR (500 MHz, CDCl3, −10 °C): 6.94-6.81 (m, 2H, pyrC-H), 6.26-6.17 (m, 2H, pyr-C-H), 5.10 (sept, JHH = 6.09, 2H, CH(CH3)2), 3.77 (sept, JHH = 5.52, 2H, CH(CH3)2), 1.83 (d, JHH = 4.64, 6H, CH(CH3)2), 1.55 (d, JHH = 5.03, 6H, CH(CH3)2), 1.16 (d, JHH = 5.16, 6H, CH(CH3)2), 1.05 (d, JHH = 4.30, 6H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, −10 °C): 129.2, 107.3, 58.0, 56.0, 30.3, 30.1, 22.0, 21.3. Mp: 125-6 ºC. i Synthesis of NCr(NPr 2)2(Pyr C6F5 ) (23): Under an inert atmosphere a scintillation vial was loaded with 2 (0.100 g, 0. 254 mmol 1 equiv.), a stir bar, and toluene (8 mL). A slurry of freshly made Tl(Pyr C6F5 ) (0.112 g, 0.257 mmol, 1.01 equiv.) in Et2O (5 mL) was added to the 189 stirring solution. The reaction was allowed to stir for 20 h at room temperature, during which yellow TlI precipitated. The precipitate was removed by filatration, and the volatiles were removed in vacuo. The residue was extracted with pentane (3 × 10 mL) and concentrated in vacuo to ~5 mL. Cooling the concentrated solution to −35 °C yielded red-orange crystals of 23 1 (0.080 g, 0.16 mmol, 63% yield). H NMR (500 MHz, CDCl3, 0 °C): 7.35-7.32 (m, 2H, pyr-CH), 6.91-6.88 (m, 2H, pyr-C-H), 5.15 (sept, JHH = 6.28, 2H, CH(CH3)2), 3.82 (sept, JHH = 6.37, 2H, CH(CH3)2), 1.87 (d, JHH = 6.07, 6H, CH(CH3)2), 1.57 (d, JHH = 6.07, 6H, CH(CH3)2), 1.18 (d, JHH = 6.29, 6H, CH(CH3)2), 1.11 (d, JHH = 6.33, 6H, CH(CH3)2). 13 C{H} NMR (125 MHz, CDCl3, 25 °C): 143.76 (dm, JCF = 255.75 Hz), 137.90 (dm, JCF = 248.5 Hz), 137.33 (dm, JCF = 243.13 Hz), 130.98, 130.63, 112.47, 108.97, 58.46, 56.59, 30.37, 30.24, 22.10, 21.29. 19 F NMR (564 MHz, CDCl3, 25 °C): −142.47 to −142.57 (m, 2F), −163.35 (t, JFF = 42.86 Hz, 1F), −164.77 to −164.95 (m, 2F). Mp: 169-171 °C. i Synthesis of NCr(NPr 2)2(Pyr C6H3(CF3)2 ) (24): Under an inert atmosphere, a scintillation vial was loaded with 2 (0.100 g, 0. 254 mmol 1 equiv.), a stir bar, and toluene (8 mL). A slurry of freshly made Tl(Pyr C6H3(CF3)2 ) (0.124 g, 0.257 mmol, 1.01 equiv.) in Et2O (5 mL) was added. The reaction was allowed to stir for 20 h at room temperature, during which yellow TlI precipitated. The precipitate was removed by filatration, and the volatiles were removed in vacuo. The residue was extracted with pentane (3 × 10 mL) and concentrated in vacuo to ~5 mL. Cooling the concentrated solution to −35 °C yielded red-orange crystals of 24 (0.098 g, 0.18 1 mmol, 71% yield). H NMR (500 MHz, CDCl3, 0 °C): 7.85 (s, 2H, Ar-o-CH), 7.49 (s, 1H, Ar-p- 190 CH), 7.29-7.28 (m, 1H, pyr-CH), 6.88-6.87 (m, 1H, pyr-CH), 6.52-6.52 (m, 1H, pyr-CH), 5.15 (sept, 2H, CH(CH3)2, JHH = 6.0), 3.81 (sept, 2H, CH(CH3)2, JHH =6.0), 1.86 (d, 6H, CH(CH3)2, JHH = 6.0), 1.57 (d, 6H, CH(CH3)2, JHH = 6.0), 1.20 (d, 6H, CH(CH3)2, JHH = 6.0), 1.11 (d, 6H, CH(CH3)2, JHH = 6.0). 13 C{H} NMR (125 MHz, CDCl3, 25 °C): 138.84, 131.27 (q, JCF = 32.5 Hz), 130.63, 128.45, 124.26, 123.72 (q, JCF = 270.9 Hz), 121.84, 117.30 (s, br) 106.02, 58.28, 56.43, 30.37, 30.22, 21.99, 21.35. 19 F NMR (564 MHz, CDCl3, 25 °C): −62.85 (s). Mp: 116-122 °C. i Synthesis of NCr(NPr 2)2(indolyl) (25): Under an inert atmosphere, a scintillation vial was loaded with 10 (0.100 g, 0.278 mmol, 1 equiv.), a stir bar, and hexanes (8 mL). This was cooled to near frozen in a liquid cooled nitrogen cold well. Freshly prepared lithium indolide (0.034 g, 0.28 mmol, 1.00 equiv.), in toluene (5 mL) was added dropwise over 5 min to the thawing solution of 10. The reaction was allowed to stir for 20 h while warming to room temperature. The volatiles were removed in vacuo. The residue was extracted with pentane (3 × 10 mL) and filtered through Celite. The filtrate was concentrated to ~5 mL and cooled to −35 °C, which provided crystals of 25. The crystals were redissolved in cold pentane and filtered to remove remaining lithium salts. Recrystalization at –35 °C from pentane yielded pure 25 as 1 purple crystals (0.045 g, 0.12 mmol, 42% yield). H NMR (500 MHz, CDCl3, -21 ºC) 8.06 (d, JHH = 8.5, 1H, H-7 ind), 7.55 (d, JHH = 8, 1H, H-4 ind), 7.37 (d, JHH = 3, 1H, H-2 ind), 7.15 (t, JHH = 6.8, 1H, H-5 ind), 7.03 (t, JHH = 6.8, 1H, H-6 ind), 6.54 (d, JHH = 3, 1H, H-3 ind), 5.18 (sept, JHH = 6.50, 2H, CH(CH3)2), 3.74 (sept, JHH = 6.50, 2H, CH(CH3)2), 1.73 (d, JHH = 6.50, 191 6H, CH(CH3)2), 1.59 (d, JHH = 6.50, 6H, CH(CH3)2), 1.19 (d, JHH = 6.50, 6H, CH(CH3)2), 0.97 (d, JHH = 6.50, 6H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, –21 ºC): 144.5, 133.4, 128.8, 120.6, 119.1, 118.7, 115.9, 102.3, 58.1, 55.8, 30.6. Mp: 194-196 °C. i Synthesis of NCr(NPr 2)2(carbazolyl) (26): Under an inert atmosphere a pressure tube was loaded with 10 (0.150 g, 0.417 mmol 1 equiv.), a stir bar, and hexanes (8 mL). Freshly prepared lithium carbazolide (0.072 g, 0.42 mmol, 1.00 equiv.) in toluene (5 mL) was added to the solution of 10. The vessel was sealed, removed from the box, and stirred in a 45 °C oil bath for 16 h. The pressure tube was taken back into the dry box, and the volatiles were removed under reduced pressure. The residue was extracted with pentane, and filtered through Celite. The filtrate was concentrated to ~5 mL and cooled to −35 °C, which provided crystals of 26. The crystals were redissolved in cold pentane and filtered to remove remaining lithium salts. 1 Recrystalization at –35 °C from pentane yielded pure 26 (0.063 g, 0.146 mmol, 35% yield). H NMR (500 MHz, CDCl3, −20 °C): 8.02 (d, JHH = 7.58, 2H), 7.97 (d, JHH = 8.33, 2H), 7.40 (d, JHH = 7.71, 2H), 7.17 (d, JHH = 7.28, 2H), 5.30 (sept, JHH = 6.18, 2H, CH(CH3)2), 3.79 (sept, JHH = 6.21, 2H, CH(CH3)2), 1.69 (d, JHH = 6.07, 6H, CH(CH3)2), 1.62 (d, JHH = 6.21, 6H, CH(CH3)2), 1.21 (d, JHH = 6.08, 6H, CH(CH3)2), 0.96 (d, JHH = 6.29, 6H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, −20 °C): 149.28, 125.02, 124.78, 119.15, 118.76, 114.96, 58.09, 55.81, 31.00, 29.47, 22.69, 22.26. Mp: 158-160 ºC. i Synthesis of NCr(NPr 2)2[N(Ph)Me] (27): Under an inert atmosphere, a scintillation vial was loaded with 10 (0.150 g, 0.417 mmol 1 equiv.), a stir bar, and hexanes (8 mL). This was 192 cooled to near frozen in a liquid nitrogen cooled cold well. Freshly prepared lithium N-methyl anilide (0.047 g, 0.42 mmol, 1.00 equiv.) in toluene (5 mL) was added dropwise over 5 min. The reaction was allowed to stir for 20 h with warming to room temperature. The volatiles were removed in vacuo. The residue was extracted with pentane and filtered through Celite. The filtrate was concentrated to ~5 mL and cooled to −35 °C, which provided crystals of 27. The crystals were recrystallized from cold pentane to obtain dark purple crystals of pure 27 (0.068 g, 1 0.184 mmol, 44% yield). H NMR (500 MHz, CDCl3, –10 ˚C): 7.45-7.33 (m, 2H, Ar-C-H), 7.32-7.24 (m, 3H, Ar-C-H), 5.10 (sept, JHH = 6.09, 2H, CH(CH3)2), 3.77 (sept, JHH = 5.52, 2H, CH(CH3)2, 1.83 (d, JHH = 4.64, 6H, CH(CH3)2), 1.55 (d, JHH = 5.03, 6H, CH(CH3)2), 1.16 (d, JHH = 5.16, 6H, CH(CH3)2), 1.05 (d, JHH = 4.30, 6H, CH(CH3)2). 13 C NMR (125 MHz, CDCl3, −55 °C): 158.56, 127.95, 118.76, 115.46, 57.49, 53.21, 41.69, 29.99, 29.32, 22.02, 21.25. Mp: 194-6 °C. i Synthesis of NCr(NPr 2)2(NCO) (28): Under an inert atmosphere a pressure tube was loaded with sodium cyanate (0.083 g, 1.271 mmol, 5 equiv), 1,4-dioxane (8 mL), and a stir bar. To the stirring cyanate solution was added 2 (0.100 g, 0.254 mmol, 1 equiv), in acetonitrile (~8 mL). The pressure tube was sealed and placed in a 45 °C oil bath and stirred for 20 h. The tube was returned to the dry box, and the volatiles were removed in vacuo. The residue was extracted with pentane and filtered through Celite. The solution was concentrated to ~5 mL and placed in a 1 −35 °C freezer, which yielded light orange needles of 28 (0.047 g, 0.153 mmol, 60% yield). H NMR (500 MHz, CDCl3, 25 °C): 5.04 (sept, JHH = 6.34, 2H, CH(CH3)2), 3.81 (sept, JHH = 6.28, 2H, CH(CH3)2), 1.90 (d, JHH = 6.36, 6H, CH(CH3)2), 1.47 (d JHH = 6.29, 6H, 193 CH(CH3)2), 1.23 (d, JHH = 6.34, 6H, CH(CH3)2), 1.10 (d, JHH = 6.50, 6H, CH(CH3)2). 13 C NMR (125 MHz, CDCl3 i Synthesis of NCr(NPr 2)2(NCS) (29): Under an inert atmosphere, a scintillation vial was loaded with 2 (0.100 g, 0.254 mmol, 1 equiv.), toluene (5 mL), and a stirbar. To this solution, sodium thiocyanate (0.062 g, 0.763 mmol, 3 equiv.) in acetonitrile (10 mL) was added. The reaction stirred at room temperature for 3 d. The volatiles were removed in vacuo. The residue was extracted with pentane, and filtered through Celite. Cooling concentrated pentane solutions of the crude product to −35 °C yielded yellow-orange needles of 29 (0.043 g, 0.132 mmol, 52% 1 yield). H NMR (500 MHz, CDCl3, −13 °C): 5.10 (sept, JHH = 6.31, 2H, CH(CH3)2), 3.86 (sept, JHH = 6.40, 2H, CH(CH3)2), 1.92 (d, JHH = 6.23, 6H, CH(CH3)2), 1.45 (d, JHH = 6.23, 6H, CH(CH3)2), 1.28 (d, JHH = 6.23, 6H, CH(CH3)2), 1.11 (d, JHH = 6.29, 6H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, −13 °C): 189.8, 59.3, 57.9, 30.7, 30.4, 22.0, 21.6. Mp: 138- 142 °C. i Synthesis of NCr(NPr 2)2(CN) (30): Under an inert atmosphere, a scintillation vial was loaded with sodium cyanide (10.6 mg, 0.216 mmol, 1 equiv.) in acetonitrile (~10 mL), freshly dried 15-crown-5 (47.6 mg, 0.216 mmol, 1 equiv.), and a stir bar. After stirring for 5 min, 2 (0.085 g, 0.216 mmol, 1 equiv.) in acetonitrile (5 mL) was added. The reaction stirred at room temperature for 6 h. The volatiles were removed in vacuo, and the residue was extracted with pentane and filtered through Celite. Cooling concentrated pentane solutions of the crude product 1 to −35 °C yielded orange crystals of 30 (28.7 mg, 0.093 mmol, 43% yield). H NMR (500 MHz, 194 CDCl3, −6 °C): 5.13 (sept, JHH = 6.29, 2H, CH(CH3)2), 3.88 (sept, JHH = 6.04, 2H, CH(CH3)2), 1.89 (d, JHH = 5.63, 6H, CH(CH3)2), 1.54 (d, JHH = 5.63, 6H, CH(CH3)2), 1.36 (d, JHH = 5.84, 6H, CH(CH3)2), 1.13 (d, JHH = 5.84, 6H, CH(CH3)2). 13 C NMR (125 MHz, CDCl3, −6 °C): 143.9, 58.5, 57.9, 57.7, 31.1, 31.0, 30.7, 30.5, 23.1, 22.6, 22.4, 21.9. IR: C–N stretch appears at –1 2172 cm . M.p.: 180 °C (dec). i Synthesis of NCr(NPr 2)2(NMe2) (31): Under an inert atmosphere, a vial was loaded with ZnCl2 (0.293 g, 2.15 mmol, 4.23 equiv), a stirbar, and THF (15 mL). This was cooled in a liquid cooled nitrogen cold well for 10 min. The vial was moved to a stir plate, and a chilled solution of LiNMe2 (0.220 g, 4.31 mmol, 8.47 equiv) in THF (4 mL) and DME (4 mL) was added dropwise. The reaction stirred for 1 h and was allowed to come to room temperature, during which the mixture turned cloudy white. To this suspension was added a solution of 2 (0.200 g, 0.509 mmol, 1 equiv) in THF (2 mL) dropwise. The reaction stirred at room temperature for 4 h and turned bright red. The volatiles were removed in vacuo. The residue was extracted with pentane and filtered through Celite. The pentane was removed in vacuo. The complex was recrystallized from a minimum of acetonitrile (~4 mL) and red crystals of 31 were isolated (0.110 g, 0.354 mmol, 1 70%). H NMR (500 MHz, CDCl3, 25 °C): 3.89 (sept, JHH = 6.43, 4H, CH(CH3)2), 3.55 (s, 3H, N(CH3)2), 1.31 (d, JHH = 6.27, 12H, CH(CH3)2), 1.23 (d, JHH = 6.34, 12H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, 25 °C): 54.5, 53.7, 26.4, 25.1. Mp: 52-57 °C. i Synthesis of NCr(NPr 2)2(F) (33): Under an N2 atmosphere a scintillation vial was loaded with 2 (0.177 g, 0.450 mmol, 1 equiv), 4-dimethylaminopyridine (0.055 g, 0.450 mmol, 1 195 equiv), and a stir bar. To this vial, CHCl3 (8 mL) was added, and the solution was stirred for 10 min. A solution of AgBF4 (0.096 g, 0.495 mmol, 1.1 equiv) in acetonitrile (4 mL) was added over 5 min. The reaction stirred at room temperature for 1 h. The brown suspension was filtered through a glass frit with Celite as a filtering agent. The filtrate was dried in vacuo and washed with pentane (2 mL). The residue was extracted with CHCl3 (2 × 5 mL). These extracts were i filtered through Celite and dried under vacuum yielding [NCr(NPr 2)2(DMAP)]BF4 (32) (0.124 1 g, 0.261 mmol, 58%). This was used without further purification in the synthesis of 33. H NMR (500 MHz, CDCl3, 0 °C): 8.21 (d, 2H, Ar-H, JHH = 7.0 Hz), 6.68 (d, 2H, Ar-H, JHH = 6.5 Hz), 5.50 (sept, 2H, CH(CH3)2, JHH = 6.0 Hz), 3.93 (sept, 2H, CH(CH3)2, JHH = 6.0 Hz), 3.12 (s, 6H, N(CH3)2) 1.86 (d, 6H, CH(CH3)2, JHH = 6.5 Hz), 1.55 (d, 6H, CH(CH3)2, JHH = 6.5 Hz), 1.23 (d, 6H, CH(CH3)2, JHH = 6.5 Hz), 1.15 (d, 6H, CH(CH3)2, JHH = 6.5 Hz). 19 F NMR (564 MHz, CDCl3, 25 °C): –151.9 ppm. Under an N2 atmosphere, a scintillation vial was loaded with n FSnBu 3 (3.38 mg, 0.011 mmol, 10 mol%), THF (1 mL), and a stir bar. A solution of i [NCr(NPr 2)2(DMAP)]BF4 (0.052 g, 0.109 mmol, 1 equiv.) from the previous step in THF (8 mL) was added. The reaction stirred for 4 h at room temperature. The volatiles were removed in vacuo, and the residue was extracted with pentane (2 × 5 mL) and filtered through Celite. The pentane solution was concentrated to ~5 mL under vacuum, and held at −35° C yielding 33 as 1 red-orange crystals (0.015 g, 0.054 mmol, 49%). H NMR (500 MHz, CDCl3, 25 °C): 5.08 (sept, 2H, CH(CH3)2, JHH = 6.5 Hz), 3.81 (sept, 2H, CH(CH3)2, JHH = 6.0 Hz), 1.94 (d, 6H, 196 CH(CH3)2, JHH = 6.5 Hz), 1.44 (d, 6H, CH(CH3)2, JHH = 6.5 Hz), 1.23 (d, 6H, CH(CH3)2, JHH = 6.0 Hz), 1.12 (d, 6H, CH(CH3)2, JHH = 6.5 Hz). 21.21, 21.45, 30.15, 30.22, 56.63, 58.65. 19 13 1 C{ H} NMR (125 MHz, CDCl3, 25 °C): F NMR (564 MHz, CDCl3, 25 °C): −145.24. M.p. 100-102 °C. 197 ‡ B.2 The Effect of ∆S Choice on LDP Values For this project we sought to put the LDP values in the form of enthalpies. In order to accomplish this, it was necessary to measure the entropy for some of the compounds using variable temperature SST in conjunction with the Eyring equation. The entropy is the intercept in such plots, and the extrapolation must often be done over a large temperature range making the i uncertainty in the entropy large. Experimental entropy values were obtained for X = NPr 2 1 (in two solvents), OBn 20, O-p-(SMe)C6H4 12, and I 2. Most of the values are fairly small and negative, varying from –1 to –16. ‡ The entropy for CN 30 was obtained as the large and negative value of ∆S = –16 ± 10 cal/mol•K, but the fit had a large error. The entropy for OAd 6 was also measured experimentally, but the temperature range accessible was very small at 10 K. The value was –10 cal/mol•K, but a number determined from such a small temperature range should be considered dubious. Even the error is misleadingly ‡ small because it is a linear fit to only 3 points. LSA was also done on 6, which gave a ∆G = 12.8 ± 1.3 kcal/mol and was very comparable to the SST value of 12.90 kcal/mol. The entropy from the LSA was –1 ±4 cal/mol•K i The most accurate experimental determination (outside of the sterically very large NPr 2 1 system) of the entropy was for I 2, which was done over the largest temperature range of 47 K. ‡ The value found was ∆S = –9 ± 5 cal/mol•K, where the error was simply taken from the fit. We chose to use the Iodo value of –9 cal/mol•K as the standard entropy because it was near the 198 middle of the experimentally determined values, had a relatively small error for the fit, was a relatively small system unlikely to have steric effects, and the value was determined over the largest temperature range. 199 ‡ ‡ ‡ b X= ∆G from SST Ave. Temp. (K) ∆H (∆S = –9) NMe2 (31) OAd (6) -- -- 9.34 ± 0.32 10.83 ± 0.24 12.90±0.96 12.82±0.86 N(Me)Ph (27) i NPr 2 (1) OBn (20) Carbazolyl (26) O-p-(OMe)C6H4 (11) 13.30±0.78 13.07±0.89 14.23±0.79 14.07±0.85 t a 224.1 237.4 233.9 231.4 248.0 235.7 10.86 ± 0.23 d 11.12 ± 0.23 11.15 ± 0.23 12.04 ± 0.25 12.14 ± 0.24 ‡ c ∆S rot (cal/mol•K) a –4 ± 1 –10 ± 1 (–1 ± a 4) 12.18 ± 0.25 O-p-(Bu )C6H4 (13) OPh (10) O-p-(SMe)C6H4 (12) 14.24±0.88 14.43±0.63 14.69±0.68 247.1 254.3 262.7 O-p-(F)C6H4 (14) 14.41±0.84 232.1 12.64 ± 0.23 O-p-(Cl)C6H4 (15) 14.59±0.83 233.9 12.81 ± 0.23 O-p-(CF3)C6H4 (16) 15.43±0.72 275.8 13.00 ± 0.28 OSiPh3 (7) OPht (18) F (33) Indolyl (25) t OBu F6 (9) 15.78±0.47 15.76±0.56 15.73±0.75 15.82±0.68 274.6 274.3 254.3 274.4 256.9 13.28 ± 0.27 NO3 (21) Pyr (22) SPh (19) OC6F5 (17) 16.82±0.76 16.44±0.63 16.56±0.56 14.15 ± 0.29 16.76±0.55 276.3 274.7 270.2 286.8 Pyr 16.57±0.66 274.8 d –5 ± 2 (–2 ± 3) –6 ± 5 14.33 ± 0.28 C6F5 (23) C6H3(CF3)2 Pyr a 16.19±0.51 12.38 ± 0.25 12.51 ± 0.26 –3 ± 4 13.35 ± 0.23 13.39 ± 0.27 13.40 ± 0.25 13.89 ± 0.26 14.16 ± 0.28 14.22 ± 0.27 14.32 ± 0.28 14.36 ± 0.28 272.6 CN (30) 265.2 14.40 ± 0.27 –16 ± 10 14.45 ± 0.28 279.8 O2CPh (8) 16.99±0.78 NCO (28) 16.53±0.55 286.7 14.51 ± 0.29 NCS (29) 17.79±0.31 292.3 14.86 ± 0.30 Cl (3) 17.56±0.43 300.8 15.05 ± 0.29 Br (4) 18.36±0.49 285.8 15.45 ± 0.30 OTf (5) 18.15±0.31 301.5 15.75 ± 0.29 I (2) 18.62±0.36 301.1 15.80 ± 0.30 –9 ± 5 Table B.1: Thermodynamic parameters determined for compounds 1-31 (24) 16.56±0.51 16.81±0.60 b Values from LSA. Enthalpy of activation for the amido rotation at that entropy value in c d kcal/mol. Values in CDCl3. Value in d8-toluene. 200 ‡ Above are the values from the SST fits for ∆G and the average temperatures at which the experiments were done. All the free energies are from at least three measurements. More details for how the SST experiments were carried out and how the error analysis was done are found later in the SI. ‡ ‡ Naturally, the ∆H values shift in magnitude with the ∆S value chosen. However, the correlations with other systems, the most important aspect, do not change significantly with ‡ reasonable changes in ∆S value. The fits for the correlations in this paper are shown in the table ‡ below. Some of the R-factors for the linear fits are marginally better with ∆S = –9 cal/mol•K, ‡ the value in the paper, and some are better with ∆S = –3. The differences in the fits are quite small, however. Correlation AOM (eσ+eπ) Andersen data Mach data Hammett σp W metallacycle 13 C NMR ‡ ‡ ‡ ‡ R-factor (∆H with ∆S = –9) R-factor (∆H with ∆S = –3) 0.991 0.988 0.987 0.989 0.995 0.994 0.993 0.969 0.996 0.998 ǂ Table B.2: R-factors of the fits of the literature data with LDP using different ∆S terms. 201 B.3 Steric Evaluation of Monodentate Ligands In seeking to quantify the steric profile of each member in such a diverse set of ligands two independent methods were employed. Each method treated steric bulk differently, but gave a single parameter output as a percent. Although quantifying a 3-dimensional object in one parameter is often an incomplete description, some useful information can be extracted, especially in the case of ligands of largely different shape, where a qualitative comparison might not be intuitive. %Vbur Volume Calculation The system developed by Nolan and coworkers was chosen. 58 This methodology evaluates each ligand in isolation from the other ligands in the coordination sphere. Since the atomic positions of the ligand are taken directly from the crystal structure, ligand−ligand interactions in our system are in some ways accounted for, in that distortions in the ligand of interest as a consequence of steric crowding from the others will be reflected in the X-ray data. Likewise, metal−donor atom bond lengthening due to steric congestion close to a metal center will be accounted for, as this bond length is a major component in the calculation. Although the system nicely describes a single ligand in its coordination environment, the incorporation of other ligands by using multiple %Vbur calculations is less instructive. In the instance of the tris(diisopropylamido) 1, each of the three amides are calculated to occupy 29.1% of a 3.5 Å sphere. When added to the nitride volume of 13.76% calculated by equation 1 (vida infra), the total suggests that the ligand set occupies over 100% of the coordination sphere. 202 Figure B.1. %Vbur calculated for each ligand in 1-31, and 33 in a 3.5 Å sphere. 203 It would be of use to identify any system specific trends in our data if parameters from our series are to be applied to other systems. One such variable may be the difference in the size of the first coordination sphere of chromium(VI) upon moving to another formal oxidation state or metal. To investigate this we looked at %Vbur as a function of the defined coordination sphere size for a selection of ligands. 204 Figure B.2. %Vbur calculated for select ligands as a function of coordination sphere size. 205 Above is a plot of %Vbur versus the radius used in the SambVca program for 16 of the ligands used in this study. The plot in the article has the radius set to 3.5 Å, the default setting in the web application. All of the ligands evaluated certainly show significant changes in %Vbur with radius. In our system, many of the orderings do not change with distance but some do, e.g., OTf and NMe2. This suggests that judicious choice of the defined coordination sphere size may be necessary if one is to extract meaningful parameters on a system by system basis. One trend evident in the %Vbur is that this system assigns a smaller steric parameter than one might expect to ligands where the bulk is significantly far from the coordinated metal. Surprisingly, this regime determined OSiPh3 to be the same size or a slightly smaller ligand than NMe2 at 22.2% and 22.4%, respectively. The effect of coordination sphere size and the distance from the steric bulk to the coordinated metal are nicely illustrated in compounds 8 and 21, determined to have the same %Vbur of 19.7%. 206 Figure B.3. Space-filling models of 8 and 21 with a coordination sphere (yellow) with a radius of 3.5 Å Although NO3 and O2CPh ostensibly are different sizes, in terms of the occupation of a 3.5 Å coordination sphere they look identical to the metal center. %Vbur Volume Calculation Procedure General procedure: To assess the ligand sterics, the %Vbur was calculated using the SambVca web-based application. 59 Prior to upload all atoms not in the ligand of interest were removed from the .cif file. The default coordination sphere of 3.5 Å was used as well as the default mesh size of 0.050 Å. Hydrogen atoms were included in each volume calculation. For each compound the crystallographically determined Cr–X bond distance was input to define the distance of the coordinating atom to the center of the sphere. The volume for each input atom was defined by the Bondi radius scaled by a factor of 1.17 Å. 207 Amides and Heterocycles: For compounds 1, 21-26, and 30 the coordinating atom was input to define the distance to the center of the sphere as above. To define the Cr–X bond axis the atoms α to the X atom were used. Phenoxides, Alkyoxides, and Triflate: For compounds 5-20 the crystallographic positions were imported into a Gaussview3. The ligand was rotated along the Cr–X bond axis 180° from the observed N≡Cr-X-Y torsion angle (where X is the coordinating atom and Y is any atom α to it). A new set of atom coordinates was generated from this, and the new position of Y, Y′ was manually input into the original list of positions. This new list was opened in Mercury and saved as an .xyz file, allowing it to be further altered so that the new atom Y′ position was defined as ‗X‘, an atom with zero volume (i.e. a point in space equidistant from the coordinating atom as the α atom, but rotated 180°). In addition, all other atoms not included in the ligand of interest were removed from the file. This was then uploaded to the SambVca application and the positions of Y and Y′ were used to define the Cr–X bond axis. By defining Y′ as an ‗X‘ atom no additional volume was introduced into the ligand. Halides and Pseudohalides: For the %Vbur of compounds 2-4, 27-29 and 33 SambVca was not used since the application did not define all the atom types required, or the evaluation of single atom ligands was problematic. For these calculations a formula for the calculation of the volume of two intersecting spheres (Equation 1) was used Vol   (R  (r  d 2 ))(d 2  2dr  3r 3  2dR  6rR  3R 3 ) 12d Equation 1 where R is the radius of the coordination sphere (3.5 Å), r is the Bondi radius scaled by 1.17 of the  coordinating atom, and d is the crystallographically determined Cr–X bond distance, giving the volume occupied in the intersection of two spheres. This volume was then divided by the 208 volume of a complete sphere with radius 3.5 Å to give the %Vbur. For pseudohalides, only the volume of coordinating atom was taken into account, as the rest of the ligand falls outside of the 3.5 Å sphere of enclosure. Calculation on the chloride 3 using this method agreed with the SambVca application, which defines the Cl radius. Steric Evaluation using Solid G In addition to the %Vbur calculations, where each ligand is treated individually, another independent steric parameter was used. The Solid-G program was selected as it evaluates the sterics of the entire ligand set using solid angles. 60 For each compound, the crystallographically determined atom positions were imported as an .xyz file. For compounds 6, 9, 13, 16, and 31 with disordered structures, the conformations that had the highest refined occupancies were selected for evaluation. Table 4 of the output file gave G(L), the percentage of the sphere shielded by each ligand, in addition to the solid angle Omega(L) in Steradians, and the equivalent cone angle. 209 Figure B.4. Percentage of the sphere shielded by a given ligand as calculated by Solid G 210 Additionally, the G (complex) gave the amount of the sphere shielded as if all the ligands were treated as one. The difference between G (complex) and the sum of each of the individual ligand G(L)‘s, SUM (G(L)), is due to areas of overlap of multiple ligands. The G (complex) was subtracted from 100 to give the available free space around the metal center as a percentage. 211 i Figure B.5. Percentage of free space not shielded any ligand of NCr(NPr )2(X) 212 Concluding Remarks As stated before, the property of sterics is complex and often under-described in quantitative terms using a single parameter. Using both Solid G and %Vbur with such a vast array of ligands highlights some of the inherent shortcomings in describing a 3-dimensional object in one number. That being said, both systems seem to have a place in steric discussions. It is suspected that, while %Vbur may accurately predict the steric environment close to a metal thus being applicable for intramolecular processes, the Solid G parameter may be more useful in describing intermolecular reactions such as substrate binding. 213 1 B.4 H NMR assignment of 2 i Spectral Assignment of NCr(X)(NPr 2)2 Complexes with Iodo 2 as an Example Figure B.6: 2D NOESY of 2 at –60 °C 214 i Assignment of the chemical shifts for HA and HB of NCr(NPr 2)2I (2) are based upon the 2D NOESY spectrum above. In the spectrum, positive resonances are in red, while negative are in blue. Thus, for this small molecule, red cross-peaks represent exchange and blue cross-peaks represent NOE‘s. The blue arrows on the structure show observed NOE interactions between the methyne and methyl protons. The resonance at 3.834 ppm is assigned to HA since it shows NOE interactions with all four methyl resonances. HB is assigned to 5.346 ppm and has two NOE interations. The above spectrum is of 2 at –60 °C, where no chemical exchange (due to amido rotation) between the methyne peaks occurs. There is free rotation about the N—C bond bearing HA allowing interaction with all methyl groups, both syn and anti, with the syn methyne. HB cannot rotate as freely around the N–C bonds to larger steric constraints anti to the nitrido due to the N–Cr–amido angles. In other words, rotating the anti iso-propyl group to where the methyl is under the metal is sterically more difficult, which makes the conformation with hydrogen of the iso-propyl group adjacent to the metal strongly preferred. As a result, the methynes HB do not come within the NOE limit of ~5 Å for the methyl groups syn to the nitrido. Furthermore, it is plausible that since HB spends more time closer to the metal center, it experiences a greater downfield shift due to the deshielding zone near the metal center. 215 1 Figure B.7: H Homodecoupling Experiment of 2 at 25 ˚C 216 The above spectra indicate the coupling of the methyne proton with the methyl protons on the isopropyl groups. Both the homodecoupling experiment and the 2D NOESY experiment are in agreement in assignment of the methyne and methyl protons. The methyne peak at 5.316 ppm is deshielded due to being closer to the chromium center, and the two more shielded doublet peaks, at 1.148 ppm and 1.335 ppm, are anti to the nitrido, where they spend more time pointed away from the metal center. The iso-propyl group syn to the nitrido is freely rotating and has a methyne peak at 3.783 ppm. The methyls of the iso-propyl group syn to the nitrido spend more time pointed toward the metal center and are therefore deshielded relative to the other doublets at 1.519 ppm and 1.852 ppm. In short, the less shielded (higher ppm) methyne septet and more shielded (lower ppm) methyl doublets are assigned to the diisopropyl groups anti to the nitrido. The more shielded methyne septet and less shielded methyl doublets are assigned to the diisopropyl groups syn to the nitrido. 217 B.5 Spin Saturation Magnetization Transfer Experiment General Considerations for the SST The rate constant for the exchange of the two methynes in the isopropyls, in the majority 1 of cases, was measured using Spin Saturation Transfer (SST) in the H NMR. 61 The ideal temperature for the SST experiment was found to be between –56 °C and +27 °C depending on the rate of rotation for the particular complex being studied. A methanol standard was used to measure the temperature of the probe before and after each experiment. The T1 values for the compounds were determined using the inversion recovery method. The concentration of the analyte was 0.02-0.03 M in CDCl3 for all of the compounds, and solutions were degassed. Sample Preparation Since T1‘s are concentration dependent, it is important to have a consistent concentration for all experiments. In all the cases, the goal was to maintain a concentration between 0.0222 M and 0.0333 M. The solution was dispensed into an oven dried JY tube and sealed. The sample was then degassed using the freeze-pump-thaw method. General Considerations for SSMT Experiment For temperatures colder and warmer than room temperature, the sample was allowed 15 min to equilibrate to the new temperature before the experiment began. After every spin saturation magnetization experiment, a methanol standard was placed into the NMR and left to equilibrate at the set temperature for 15 minutes before the temperature was measured. The number reported for the barrier of rotation is the average of at least 3 runs attempted at the same temperature (within 1-2 °C). 218 The Experiment 1 First, a standard H NMR spectrum was obtained followed by determination of the 90° pulse width of the exchanging peaks (the methyne peaks in this case). The sample was scanned with the new pulse width with 64 transients, shimmed, and referenced. Then the T1 of the septets was determined using the inversion recovery method. The T1 value is used to set the saturation delay and for calculating the rate of exchange. Now the spin saturation magnetization experiment 1 can be started. A quality non-spinning H spectrum was acquired with 64 transients using the newly acquired 90° pulse width. The spectrum was expanded upon the region of the two methynes, and two experiments were performed. One experiment was carried out in which one methyne was saturated, and, in the second experiment, an offsite point was saturated as a control that is equidistant from exchanging resonance in order to compensate for decoupler sidebands. The peak to be saturated was set to the decoupler frequency. It was made sure that the offsite point chosen did not lie on a peak of exchange. Saturation power was set to -4, the saturation delay was set to 5 × T1 value and the experiment was acquired with 64 transients. Integration of the peak with saturation and without saturation was performed and used to determine the rate of exchange between the methynes using the equation below. kobs  1  M0   1  T1  M  where MOA is integration before spin saturation magnetization transfer and MA is after integration after exchange. Then the T1 of the exchanging peak was determined using the 219 inversion recovery method. By knowing the temperature of the experiment the barrier of rotation of the compound can be determined using the Eyring equation shown below.  k BT   G‡ / RT kobs    e  h  Where kb is the Boltzmann constant, T is temperature (K), h is Plank‘s constant, κ is the transmission coefficient (assumed to be 1), kobs is the rate of exchange determined using -3 -1 -1 equation above, and R is the gas constant (1.987 × 10 kcal·mol ·K ). Control Experiments To test the consistency of the values under different conditions, several control experiments were carried out involving solvent, NMR field, and the effect of paramagnetic impurities. The effect of solvent on the barrier was examined. The free energy barrier for amido ‡ i rotation in NCr(NPr 2)3 (1) was measured in both CDCl3 and d8-toluene; the values for ∆G rot determined in these two solvents were quite close at 12.6 and 12.5 kcal/mol. In addition, the i t value for NCr(NPr 2)2(OBu F6) (9) in CDCl3 was found to be 16.1 kcal/mol, which was ‡ 62 consistent with the literature value in C6D6 of ∆G rot = 16 kcal/mol. The consistency of the SST values at different fields was also explored using i NCr(NPr 2)2(I) (2) as the analyte. The standard experiments for the compounds in this paper ‡ were performed on a 500 MHz instrument. For 2 at that field, the measured barrier was ∆G rot = 18.2 kcal/mol. The same compound at 300 MHz had a measured barrier of 18.3 kcal/mol. 220 To test the effect of possible paramagnetic impurities, a common paramagnetic relaxation agent, Cr(acac)3, was added in high concentration. In CDCl3, the SST experiment was carried ‡ i out on NCr(NPr 2)2(I) (2) in the presence of 9 equivalents of Cr(acac)3, which gave ∆G rot = 18.4 versus 18.2 kcal/mol in the absence of the paramagnetic compound. Consequently, small amounts of paramagnetic impurites possibly present are unlikely to have a significant effect on these SST measurements. 221 B.6 Propagation of Error in the Spin Saturation Transfer Experiments The equation for the relating the experimental observables to the rate constant, k, is Eqn 1 below. In Eqn 1, M0A is the integral before irradiation, which was set to 100, and MA is the observed integral after irradiation. The T1 were found the inversion recovery method, and the error in T1 is that calculated by the Varian software VNMR 61c or VNMR J22d, both software packages gave very similar results. The error in the integrals were set to 0.1 with the integration of the peak before irradiation set to 100. The error in temperature, εT, was ±1 °C. propagation of error in this system, error in k (εk), is found using Eqn 2. 1 æ M0A ö ç M - 1÷ T1 è A ø k=   k  k  1 / T1  T  1 (1)   M0A / M A     M /M   A    0A 2 2 (2) Where, ε1/T1 and εM0A/MA are found using Eqns 3 and 4, respectively. 1 / T 1 1  T1   T1  T  1 222     2 (3) 63 The  M   M   0.1    0 A    A   1106   M   M  M   A  0A   A  2 M 0A /MA 2 2 (4) The free energy of the amido rotation was found using the Eyring equation in the form shown in Eqn 5. The error in ∆G was calculated as shown in Eqn 6. k  G  RT ln  B   RT ln T  RT ln k  h  (5) 2  G   k     T R ln  B    ( R T ln T ) 2  ( R T ln k ) 2    h         T ln T  T    T   T ln T   T  2  T ln T       T ln k  k    T   k ln k   T  2  T ln k 223 2 2 (6) T1s, Apparent T1s, and NOE Effects of Exchanging Resonances In spin saturation magnetization experiments involving exchange between two sites, the rate is determined by the ratio of the fractional decrease in integration of one site in the presence of saturation of the other relative to its T1. In a system experiencing two site exchange, the relaxation of the spins for peak A is dependent on the rate of decay from the excited state A* to the ground state A; the same is true for peak B. When T1A = T1B, the rate of relaxation is the same. In this case, the T1 of the peak experiencing exchange is measured before saturation. 64,65,66 But if T1A and T1B differ more than ~30% or if more than two peaks are experiencing exchange, the T1 of the peak experiencing exchange in the presence of saturation is measured, which is known as the apparent T1. 67,68 In essence, the apparent T1app is the average T1 for the two exchanging sites. Figure B.8: Kinetic Scheme for Two-Site Exchange In our chromium nitrido system, both T1 and apparent T1 of the less shielded septet were measured, e.g., the septet at 5.32 ppm in iodo 2 (see below). 224 1 i Figure B.9: H NMR Spectrum of NCr(NPr 2)2I (2) at 25 ˚C 225 This peak was chosen because it is easily saturated in a selective fashion without effecting the peak due to the exchanging site. Data for both the T1 and apparent T1 of the peak at 5.316 ppm are shown below for 2. The relationship between T1 and apparent T1 (T1app) is M zA   T1app  A T1 M z 0 (1) where Mz(∞) is the intensity (or integrated area) of the resonance upon saturation (applied for, at least, 5 × T1) of the exchanging site and Mz(0) is the intensity of the resonance with no saturation. 226 Figure B.10: ln(1/T1) (blue) and ln (1/apparent T1) (red) of 2 at different temperatures 227 At 285 K and below the molecule does not experience exchange of the isopropyl groups as seen by the lack of change in chemical shifts and linewidths of the exchanging sites. At this extreme, Eqn 1 above simplifies so that T1app = T1. This was observed in our experiments (see plot above) as a region to the right in the plot (large 1/T) where the red and blue points converge at the same line. In this region, the Kinetic Scheme is dominated by the vertical equilibria because there is no chemical exchange. Above 335 K, exchange becomes fast on the NMR timescale (k >> δa – δb) and individual peaks are not observable. In the temperature regime where the exchange can be measured, T1 increases with temperature. The opposite is observed for the apparent T1, as would be expected from Eqn 1. In this region, the Kinetic Scheme is dominated by chemical exchange, the horizontal equilibria. Between 285 and 335 K, the T1app ≠ T1, and T1app becomes linked with the exchange process. In other words, the entire Kinetic Scheme becomes relevant, and the horizontal and vertical equilibria all have similar rates. Use of apparent T1 in cases of this type, where T1A and T1B have very similar values that are also similar to the exchange rate, leads to a situation where it can be difficult to extricate the exchange rate from the T1 value. They become linked. The relationship between T1 and temperature should be linear when 1/Temp versus ln(1/T1) is plotted (see the graph above) provided that the apparent T1 is not contaminated by exchange processes. 19 As shown, this is the case for T1 (blue points) but it is not the case for 228 T1app (red points). Presumably, the deviation from linearity in the plot above for T1app is due to it becoming linked with the chemical exchange (amido rotation) process. Another possible complication to getting very accurate exchange values is that NOE effects in the saturated site and the exchange site can interfere with the measurement of the chemical exchange kinetics. We examined our system using 2D NOESY spectroscopy, and NOE effects between the chemically exchanging sites measured in the SST were negligible. It is possible to separate the NOE and exchange phenomena using this technique, which fortunately was unnecessary for our system. 229 REFERENCES 230 REFERENCES (1) Tolman, C. A. Chem. 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(12) Some cationic complexes using this framework with neutral ligands in place of X have already been prepared and many others are being developed. It is hoped that this series will allow more overlap with the Tolman ligand maps, which involve mostly neutral ligands. One such complex, 32, is used as an intermediate to the fluoride reported here. Unpublished results DiFranco, S.; Maciulis, N.; Odom, A. L. (13) The orbital labels are an approximation. In actuality, because the dxy/dx2-y2 and dxz/dyz esets have the same symmetry, they will mix as well. (14) In the calculations, two minima can sometimes be found in the model: one where the lone pair is syn to the nitrido and one where it is anti. The anti isomer is typically the global minimum, which is what is used here. (15) The hybridization at nitrogen was calculated by taking the calculated angles around the nitrogen and dividing by 3 to get an average hybridization for all the bonds. From the average angle, Coulson’s equation for the Directionality Theorem, cos wij = - 1 ( l i l j )1/ 2 , can be used to get the hybridization parameter. In this case where we are simply looking at the average angle, effectively average orbital hybridization, 1 the equation can be simplified to l = for looking at equivalent hybrids. (a) cos w McWeeny, R. Coulson’s Valence, 1979, Oxford University Press: Oxford. (b) Weinhold, F.; Landis, C. R. Valency and Bonding, 2005, Cambridge University Press: Cambridge. (16) (a) Bradley, D. C.; Hursthouse, M. B.; Newing, C. W.; Welch, A. J. Chem. Commun. 1971, 411. (b) Bradley, D. C.; Copperthwaite, R. G. Inorg. Synth. 1978, 18, 112. (c) Bradley, D. C.; Newing, C. W. Chem. Commun. 1970, 219. (17) (a) Odom, A. L.; Cummins, C. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 1995, 117, 6613. For related nitrosyl deoxygenation studies see (b) Veige, A. S.; Slaughter, L. M.; Lobkovsky, E. B.; Matsunaga, N.; Decker, S. A.; Cundari, T. R.; Wolcanski, P. T. Inorg. Chem. 2003, 42, 6204. (18) Chiu, H.-T.; Chen, Y.-P.; Chuang, S.-H.; Jen, J.-S.; Lee, G.-H.; Peng, S.-M. Chem. Commun. 1996, 139. t (19) The byproduct Cr(OBu )3 is believe to be a soluble dimer. Bradley, D. C.; Mehrotra, R.; Rothwell, I.; Singh, A., Alkoxo and Aryloxo Derivatives of Metals, Academic Press: San Diego, 2001. (20) Odom, A. L.; Cummins, C. C. Organometallics 1996, 15, 898. (21) Odom, A. L.; Cummins, C. C. Polyhedron 1998, 17, 675. 232 (22) For a similar decomposition of a DMAP containing BF4 salt see Huynh, K.; Rivard, E.; Lough, A. J.; Manners, I. Chem. Eur. J. 2007, 13, 3431. In addition, the tetraalkyltin n SnBu 3(allyl), which we had on hand, was an active catalyst. It is likely that Sn(IV) is acting as a mild Lewis acid in the reaction, and other acids may catalyzed the decomposition of 32. (23) The chemical shift in the 19 F NMR of the byproduct in the reaction to produce 33 was identical to the product of the reaction between 1 equivalent each of BF3•OEt2 and DMAP in the same solvent. (24) In this work, 31 of the compounds have been structurally characterized; this number i i includes the previously reported structures for NCr(NPr 2)3 (1) and NCr(NPr 2)2(I) (2). The only compound not structurally characterized is compound 32, which was only used as an intermediate in production of the fluoride 33. (25) For related studies on aryloxides where no correlation was found between M–O–Ar angle and M–O bond distance see (a) Steffey, B. D; Fanwick, P. E., Rothwell, I. P. Polyhedron 1990, 9, 963-968. (b) Howard, W. A.; Trnka, T. M.; Parkin, G. Inorg. Chem. 1995, 34, 5900. (c) Wolczanski, P. T. Polyhedron 1995, 14, 3335. For systems where alkoxide bond angle does seem to correlate with bond distance see (d) Tomaszewski, R.; Arif, A. M.; Ernst, R. D. J. Chem. Soc., Dalton Trans. 1999, 1883-90. (e) Huffman, J. C.; Moloy, K. G.; Marsella, J. A.; Caulton, K. G. J. Am. Chem. Soc. 1980, 102, 3009. (26) (a) Jarek, R. L.; Flesher, R. J.; Shin, S. K. J. Chem. Ed. 1997, 74, 978. (b) Perrin C. L.; Thoburn, J. D.; Kresge, A. J. J. Am. Chem. Soc. 1992, 114, 8800. (c) Dahlquist, F. W.; Longmuir, K. J.; Du Vernet, R. B. J. Mag. Resonance 1975, 17, 406. i (27) We measured the variable temperature SST data for X = Pr in two different solvents. In ‡ toluene, the activation parameters for rotation were found to be H = 12.4 kcal/mol and ‡ S = –2 cal/mol•K over a temperature range of 30 °C. In CDCl3, the usual solvent employed for the studies in this paper, the activation parameters for rotation were found to ‡ ‡ be H = 11.6 kcal/mol and S = –5 cal/mol•K over a temperature range of 26 °C. The ‡ value of H = 11.12 ± 0.23 kcal/mol in Table 4.1 is from multiple measurements, consistent with the majority of data in that table. (28) The amido entropic barriers found here are quite similar to values in the literature for organic amides of various types. For example, the amide rotational barrier in nicotinamide was measured using several different techniques and values from –3 to –10 cal/mol•K were obtained. Olsen, R. A.; Liu, L.; Ghaderi, N.; Johns, A.; Hatcher, M. E.; Mueller, L. J. J. Am. Chem. Soc. 2003, 125, 10125. For some other examples see Drakenberg, T.; Dahlqvist, K.-I.; Forsén, S. J. Phys. Chem. 1972, 76, 2178. (29) Because the value for the NMe2 derivative was found using a different technique from all the other values, we view its absolute value relative to all the other LDP numbers with 233 some suspicion. However, we have little doubt in its placement as the most donating ligand investigated in this study. (30) This can be seen in the metal-nitrogen bond distances of pyrrole versus dialkylamidos. See Harris, S. A.; Ciszewski, J. T.; Odom, A. L. Inorg. Chem. 2001, 40, 1987 and references therein for examples. (31) For a mixed experimental theoretical treatment of aromaticity in indoles and pyrroles see (a) Hou, D.-R.; Wang, M.-S.; Chung, M.-W.; Hsieh, Y.-D.; Tsai, H.-H. G. J. Org. Chem. 2007, 72, 9231. For a review on aromaticity of heterocycles see (b) Balaban, A. T.; Oniciu, D. C.; Katritzky, A. R. Chem. Rev. 2004, 104, 2777. (32) For a review concerning the application of %Vbur see Clavier, H.; Nolan, S. P. Chem. Commun. 2010, 46, 841. (33) (a) Hillier, A. C.; Sommer, W. J.; Yong, B. S.; Petersen, J. L.; Cavallo, L.; Nolan, S. P. Organometallics 2003, 22, 4322. (b) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 1759. (c) http://www.molnac.unisa.it/OMtools/sambvca.php (34) It is certainly possible that no single parameter steric scheme will accurately encompass the influences of a diverse set of ligands like the one found here even if the methodologies are applicable to a relatively narrow group like phosphines and N-heterocyclic carbenes. Included in the Supporting Information is a plot of sphere radius versus %Vbur for some ligands. As might be expected, the ligands show different maxima and some of the lines do cross, suggesting the ideal radius is likely system and application dependent. The values here are intended to give a rough measure of sterics for the various ligands. (35) Guzei, I. A.; Wendt, M. Dalton Trans. 2006, 3991. (36) The online program for %Vbur, SambVca, does not include all of the halides, and some of these were calculated using an overlapping spheres methods at the appropriate distances. See Supporting Information for details. (37) (a) Lokare, K. S.; Odom, A. L. Inorg. Chem. 2008, 47, 11191. See also (b) Lokare, K. S.; Ciszewski, J. T.; Odom. A. L. Organometallics 2004, 23, 5386. (38) The value used in the plot for the mixed X ligand set (Cl)(OTf) is the average of the two SST values for the two different ligands. The SST value for OBn was used as the parameter for the ethoxide ligand. (39) A note of caution for the use of AOM values, the e values are often found using the assumption that e for NH3 is equal to zero due to over-parameterization within the AOM model. The validity of this assumption has been called into question for some coordination complexes, most notably Pt(II) where the e value for NMe3 was determined to be well above zero. It is not believed that NH3 actually does have -effects; instead, this is 234 attributable to the e parameter not being truly zero within the model. For further discussion see reference 39 and (a) Chang, T.-H.; Zink, J. I. J. Am. Chem. Soc. 1987, 109, 692. (b) Bridgeman, A. J.; Gerlock, M. Prog. Inorg. Chem. 1997, 45, 179. (c) Schaffer, C. E.; Anthon, C.; Bendix, J. Coord. Chem. Rev. 2009, 253, 575. (40) AOM values were taken from Hoggard, P. E. Structure and Bonding 2004, 106, 37, which were calculated from splittings in spin-allowed transitions of the corresponding Cr(III) ammine complexes. (41) Lukens, W. W., Jr.; Smith, M. R., III; Andersen, R. A. J. Am. Chem. Soc. 1996, 118, 1719. (42) (a) Gyepes, R.; Varga, V.; Horacek, M.; Kubista, J.; Pinkas, J.; Mach, K. Organometallics 2010, 29, 3780. (b) Varga, V.; Cisarova, I.; Gyepes, R.; Horacek, M.; Kubista, J.; Mach, K. Organometallics 2009, 28, 1748. (43) That the quantity measured is exclusively determined by the -donor ability of X is of course an approximation. The a1 is weakly -bonding but largely nonbonding. The b2 orbital is orthogonal and its energy is largely determined by the -bonding of X. For a discussion of the bonding see Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1729. (44) The data for X = N(Me)Ph was not used in determining the linear fit to the Andersen data (blue line) shown in Figure 4.10. The exact cause for the two different slopes of the Andersen and Mach data fits is unknown; however, there are several small differences in the data sets like different solvents, slightly different instrumentation, and possible differences in concentration. In addition, the Andersen data are relative to Cp*2TiH as Exz = 0, and we did not adjust the baseline in the Mach data similarly. If the data from the two groups are taken all together and linearly fit, the line obtained is y = 20761 – 1205x 2 2 with a much worse R of 0.93 versus the R = ~0.99 obtained fitting the data from the two groups independently. This fit for Mach’s data is for only four points from the four new alkoxides in common between our study and Mach’s reports. In addition, one of the four points was done with a substitution in the LDP value, tert-butoxide for 1-adamantoxide. (45) Calculated using Year 2009 single-, double- and triple-bond covalent radii from a private communication. (a) http://www.chem.helsinki.fi/~pyykko/ (b) Pyykkö, P.; Atsumi, M. Chem. Eur. J. 2009, 15, 12770. (46) The average Ti–NMe2 bond in CSD version 5.32 Nov 2010 is 1.897 Å. It should be noted, however, that the majority of those complexes are Ti(IV), and the distance in titanium(III) Me(H)NTiCp*2 may be somewhat longer just due to the difference in oxidation state. There are a few Ti–NH2 cyclopentadienyl complexes in the CSD database, which should allow the Ti–N bond to shorten if electronically favorable. These do tend to have slightly shorter titanium-amido bond distances. For example, amidobis[1,3bis(trimethylsilyl)cyclopentadienyl]titanium(III) has a Ti–NH2 distance of 1.933(3) Å. (a) Sofield, C. D.; Walter, M. D.; Andersen, R. A. Acta Cryst. C 2004, 60, M465. The 235 compound H2NTiCp*2 has a Ti–N distance of 1.944(2) Å. (b) Brady, E.; Telford, J. R.; Mitchell, G.; Lukens, W. Acta Cryst C 1995, 51, 558. Neither of these NH2 complexes has a statistically significant difference in the Ti–N distance from the value in Me(H)NTiCp*2, however. (47) The methyl group is rotated in the Me(H)N–TiCp*2 complex so that the amido methyl is directed at one of the Cp* ligands. The Solid-G program calculates the G(gamma) value to be 3.03 in this compound, which is a measure of how much of the sphere is shielded by more than one ligand. For the complex Me(Ph)N–TiCp*2, were the amido methyl and phenyl groups rest between the Cp*s, the G(gamma) value is 2.19, suggesting there is less ligand overlap. In other words, the small amido is nesting with the methyls of the Cp* groups in order to reach this electronically preferred but sterically encumbered position. For the structure of Me(Ph)NTiCp*2 see: Feldman, J.; Calabrese, J. C. Chem. Commun. 1991, 1042. nd (48) For examples see (a) Basolo, F.; Pearson, R. G. 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E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J. V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, and J.A. Pople, Gaussian, Inc., Wallingford CT, 2004. (52) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785. 236 (53) For the lighter elements, this basis set is found in the G03 software. For chromium, see the Basis Set Exchange website https://bse.pnl.gov/bse/portal. For the reference on the chromium basis see Balabanov, N. B.; Peterson, K. A. J. Chem. Phys 2005, 123, 064107. (54) Burke, P. J.; Matthews, R. W.; Gillies, D. G. Dalton Trans 1980, 1439. (55) Sidgwick, N. V.; Sutton, L. E. J. Chem. Soc. 1930, 1461 (56) Clavier, H.; Nolan, S. P. Chem. Commun 2010, 46, 841-861. (57) https://www.molnac.unisa.it/OMtools/sambvca.php (58) Guzei, I. A.; Wendt, M. Dalton Trans 2006, 3991–3999. (59) (a) Jarek, R. L.; Flesher, R. J.; Shin, S. K. J. Chem. Ed. 1997, 74, 978. (b) Perrin C. L.; Thoburn, J. D.; Kresge, A. J. J. Am. Chem. Soc. 1992, 114, 8800. (c) Dahlquist, F. W.; Longmuir, K. J.; Du Vernet, R. B. J. Mag. Resonance 1975, 17, 406. (60) Odom, A. L.; Cummins, C. C. Polyhedron 1998, 17, 675. (61) Van Geet, A. L. Analytical Chem 1968, 40, 2227. For a more recent study see Findeisen, M.; Brand, T.; Berger, S. Mag. Reson. Chem. 2007, 45, 175. (62) Forsen, S., Hoffman, R. A. J. Chem. Phys. 1963, 39, 2892-2901. (63) Jarek, R. L., Flesher, R. J., Shin, S. K. J. Chem. Educ. 1997, 74, 978-982. (64) Dahlquist F. W., Longmuir, K. J., Du Vernet, R. B. J. Magn. Reson. 1975, 17, 406-410. (65) Perrin, C. L., Johnston, E. R. J. Magn. Reson. 1979, 33, 619-626. (66) Mann, B. E. J. Magn. Reson. 1977, 25, 91-94. (67) Klochkov, V. V.; Shaikhutdinov, R. A.; Khairutdinov, Klimovitskii, E. N.; Findelsen, M.; Berger, S. Appl. Mag. Reson. 2003, 24, 97. 237 Chapter 5: Synthesis, Structure, and Hapticity Changes in Chromium (VI) Nitrido Complexes Bearing Cp and Related Ligands. ABSTRACT Synthesis and characterization of new chromium(VI) nitride compounds of the form i NCr(NPr 2)2X, where X = Cp (3), indenyl (4), and fluorenyl (5) are reported. Crystal structures 1 of complexes 1-3 show an η -interaction to X in the solid state. Evaluation of the donor properties of X is parameterized by measurement of the free energy of rotation about the Cramide bond, and is reported as the LDP. Treatment of 3 and 5 with benzoic acid gives i i NCr(NPr 2)(O2CPh)(Cp) (6) and NCr(NPr 2)(O2CPh)(Fluorenyl) (7) respectively. A crystal 1 3 structure of 6 shows a Cp hapticity change from η to η upon replacement of an amide ligand 1 from 3. IR carbonyl stretches of the benzoate ligand of 6 show it to be  . Also reported is the i synthesis and structure of a cationic chromium(VI) compound [NCr(NPr 2)2DBN]I (2) used in the synthesis of 4 and 5, as well as thallium indenidide. LDP‘s of complexes 2, 4 and 5 are also reported. Steric analysis of all 2-6 are reported as a %Vbur as well using the method of solid G. 5.1 Introduction Perhaps the quintessential organometallic complex, cyclopentyldienyl appended transition 1 metals have been known since the 1950‘s. Interest in metallocene complexes, however has not waned, due in part to their high catalytic activity in a variety applications. Common in once such 238 2 application, the polymerization of olefins, researchers have successfully employed Cp and derivative ligands to support early transition metal based Ziegler-Natta systems. Moreover, tuning of the highly amendable Cp framework has allowed for control over reaction parameters, which give rise to specific tacticity, polymers. 3,4 poly despersity, 5 and the formation of co-block 6 7 Recently, our work with chromium(VI) nitrido complexes led us to draw a parallel to the titanium and zirconium(IV) catalysts frequently used in olefin polymerizations (Figure 5.1). Figure 5.1: A Ti(IV) bis-Cp complex and an isolobal chromium nitride. 5 Both nitride and the η -cyclopentadieneyl anion require three orbitals to accommodate 2ζ and 4π electrons. In the case of Cr(VI) the more electronegative nitride may support a more electron 0 deficient, formally d metal center. Furthermore, the sterically smaller nitride (vide infra) may provide better metal access to incoming substrates. Lastly, substituting nitride in place of one Cp ligand in a metallocene might have interesting structural consequences due in part to the much stronger trans influence of a metal-ligand multiple bond. form NCr(Cp)(X)2 warranted additional study. 239 8, 9 Thusly, we felt complexes of the i 1 5.2 Synthesis and Structure of NCr(NPr 2)2(η -Cp) 3 Access into high valent chromium nitrides of the form NCr(X)3 was achieved through a facile i 10 nitrogen atom transfer reaction onto Cr(NPr 2)3. Subsequent treatment with Lutidenium Iodide i gave NCr(NPr 2)2(I) (1) as reported by Cummins and coworkers. i 11 Substitution of the halide 1 ligand using 3 equivalents of NaCp gave NCr(NPr 2)2(η -Cp) cleanly in good yield (Figure 5.2). i 1 Figure 5.2: Synthesis of NCr(NPr 2)2(η -Cp). 1 H NMR analysis of 3 shows a sharp singlet corresponding to the aromatic protons of Cp, which does not resolve even at −30 °C. Single crystal Xray diffraction of 3 is consistent with an 1 η -Cp, with double bond character localized between C2-C3 and C4-C5 (Figure 5.3). This C2C5 moiety is very similar in bond lengths to that of free C5H6, suggesting that the Cp ring of 3 is not largely aromatic (Figure 5.4). 12 C1 adopts a typical tetrahedral arrangement with a bond length to chromium of 2.115(2) Å, considerably shorter than the other Cr-C interatomic distances (Figure 5.4). This bond distance is also significantly shorter than those in all other 240 1 crystallographically characterized examples of η -Cp complexes of chromium (2.195(2) Å 2.262(5) Å). 14 13 and 1 5 This is likely due to the higher oxidation state of 3 in comparison to the η , η bis-Cp complexes of Cr(II). 1 Furthermore planarity of the 5 member ring is maintained in the η coordination mode as judged by the angle between the planes defined by C1 C2 C5 and C2 C3 C4 C5, intersecting at just 3.5° (often reported in indenide literature as the hinge angle). Coordination in a η 1 15 − fashion maintains a 16e , assuming strong π-donation from the 0 diisopropylamide ligand to a d Cr center. This situation puts additional interaction with the Cp ligand in direct competition with strong amide donors as well as the strongly π-donating nitrido. 241 i 1 Figure 5.3: A crystal structure rendering of NCr(NPr 2)2(η -Cp) 3. Thermal ellipsoids at 50% probability and H atoms removed except for calculated H on C1. i i i 5.3 Synthesis of [NCr(NPr 2)2DBN][I] (2), NCr(NPr 2)2(Ind) (4), and NCr(NPr 2)2(Flu) (5) and Synthesis and Structure of Thallium(I)indenide We were similarly interested in the electronic and structural features of indenyl and fluorenyl complexes of Cr(VI) nidrides. Synthesis of these complexes through similar methods as 3 proved to be low yielding. Production of 4 and 5 was achieved by using the cationic Cr(VI) complex i [NCr(NPr 2)2DBN][I] (2). 242 Compound 2 is generated in high yield by treatment of 1 with one equivalent of 1,5Diazabicyclo[4.3.0]non-5-ene (DBN), which readily displaces an iodine ligand into the outer coordination sphere (Figure 5.5). Diffraction quality crystals of 2 were grown from a cold THF solution. Analysis shows a Cr-N4 bond length of 1.9728(18) Å. This contact is surprisingly short for a formally dative bond when compared to the average Cr-N amide bond length in 2 (1.8156 Å) and the Cr-N pyrrolide i bond distance in NCr(NPr 2)2(Pyr) (1.946(2) Å) where little π-bonding to empty d orbitals is observed. i Figure 5.5: Synthesis of [NCr(NPr 2)2DBN][I] (2). 13 C NMR analysis shows a downfield shift of the imine carbon resonance in 2 when compared to free DBN (169.30 ppm and 160.16 ppm respectively). This deshielding effect suggests that a resonance form that delocalizes cationic charge from the Cr(VI) on the carbon is a significant - contributor to the structure of 2. Consequences of the I anion on the properties of 2 are unknown but are currently being investigated. 243 i Synthesis of NCr(NPr 2)2(ind) (4) (ind = indenide) was realized by a salt metathesis reaction of 2 with a slight excess of Tl indenide in THF (Figure 5.6). Figure 5.6: Synthesis of 4 using thallium(I) indenide. Thallium indenide was first generated by reaction of thallium(I) ethoxide with freshly distilled indene in pentane. The yellow-orange powder was unstable at room temperature for extended periods and readily precipitated Tl metal. Interestingly, the bulk solid was observed to be thermochromic, shifting from yellow to orange in the temperature range −35 °C – 25 °C. This property has also been briefly noted by Janiak for the related TlCp. 16 In attempt to determine the structural nature of this effect, crystals of Tlind were grown from a concentrated THF solution held at −35 °C as yellow needles. Single crystal diffraction carried out at 173 K revealed an asymmetric unit containing 2 atoms of thallium, 2 disordered indenides, 1 and one unbound THF as a solvent of crystallization. Indenides appear to be bound η via the 5 member ring to one Tl (with an average bond distance of 2.99 Å), with the six member ring 244 datively bound to the other Tl. This differs significantly from the known crystal structures of TlCp 17 18 5 and TlCp* , both η polymeric half sandwich complexes with average Tl-Cp bond distances of 2.73 Å and 2.96 Å respectively. Crystals of Tlind also exhibit a close inter atomic distance between Tl centers of 3.6207 (17) Å, shorter than the sum of the Bondi radii for Tl (3.92 19 Å). i Figure 5.7. Overlaid structures of NCr(NPr 2)2(X) where X = Cp (3) (red), Indenyl (4) (blue), and fluorenyl (5) (green). Unfortunately, variable temperature data collection was unsuccessful, bound by the lower limit of our instrumentation, and by poor diffraction intensity at higher temperatures. The asymmetric unit however remained unchanged when the crystal was cycled from 298 K back to 173 K, 245 which may rule out the exclusion of the solvent of crystallization as giving rise to the thermochromicity. i Similarly NCr(NPr 2)2(flu) (5) (flu = fluorenyl) was synthesized in moderate yield from the reaction of 2 with one equiv. of LiFlu generated in situ from stoichiometric amounts of fluorene n and BuLi in THF. i i 5.4 Structures of NCr(NPr 2)2(Ind) (4) and NCr(NPr 2)2(Flu) (5) and Steric Evaluation Compounds 4 and 5 show structural similarities to 3 in the solid state, with the bound 5 1 member ring adopting an η configuration in each (Figure 5.7). Alternating bond lengths within the 5 member rings of 4 and 5 are similar to those of Cp suggesting the localization of multiple 1 3 bonding character typical of the η binding mode. Likewise the sp hybridization around C1 5 2 suggests this binding mode as all the carbons in an aromatic η Cp must be sp . Summarized in Table 5.1, Cr-C bond lengths to the bound carbon is the same within ESDs for each, and thus not a reliable indicator of the electronic differences within the series. 246 Compound Cr-C1 Avg. Bond angle distance around C1 (Å) i NCr(NPr 2)2(Cp) (3) i NCr(NPr 2)2(Ind) (4) i NCr(NPr 2)2(Flu) (5) i NCr(NPr 2)(O2CPh)(Cp) (6) a %Vbur Gm(L) 2.115(2) 109.3° 24.88 20.87 2.108(4) 109.3° 25.19 22.31 2.102(7) 109.4° 26.73 23.48 2.228(3) 119.73° 36.08 31.83 a Only H1-C1-C2, H1-C1-C5, and C2-C1-C5 angles included. Table 5.1: Chromium-carbon bond distances and average angle around C1 in compounds 3-5. Steric parameters of %Vbur and Gm(L) of the X ligand, both as percentages. Granted the binding of each ligand may be similar, the overall steric environment in 3-5 varies. To quantify this property, two individual methods were used in describing the added bulk away from the metal center. In addressing sterics, the Tolman cone angle 20 traditionally used to evaluate ligand size has expounded upon in recent years with the need to describe unsymmetrical and unusually shaped ligands. One such system developed by Nolan and coworkers places the ligand of interest the correct bond distance (in this case crystallographically determined) from the center point of a 21 sphere. This sphere represents the first coordination sphere of a given metal, usually with a radius of 3.5 Å. Atomic radii of each atom within this 3.5 Å sphere are then used to calculate the 247 total volume of space occupied by the ligand. This is then compared to the total volume of the sphere and given as a single parameter, the percentage buried volume (%Vbur). Additionally, the sterics in compounds 3-5 were evaluated using the system of Soild Angles. 22 In this treatment the crystollographically determined positions of each compound are inscribed in a sphere of arbitrary radius. The metal center is treated analogous to a point source of light. Each ligand blocks a given amount of this ‗light‘, casting a ‗shadow‘ that is projected on the inside of the sphere. This area is compared to the total area of the sphere and is also given as a single parameter as a percentage, Gm(L). Results of these analyses summarized in Table 5.1 illustrate the differences within the ligand series as well as between the two systems of steric evaluation. Expectedly both systems track with increasing ring sizes. However, %Vbur only increases moderately, as not all of the the extended ring system of 4 and 5 are within the defined 3.5 Å coordination sphere. Likewise, the small increase seen in the Solid G analysis suggests that the subtle differences in sterics, and thus substrate access to a metal center are non-trivial, since large changes in reactivity are seen among catalysts bearing a series of Cp and derivative ligands. 5.5 LDP determination of compounds 3-5 1 With compounds 3-5 in hand we sought to evaluate the electronic differences between η -Cp, indenyl, and fluorenyl. Although they are sterically different (vida supra), the similar bond distances and exclusively ζ-bonded coordination mode allows for a good comparison of their relative donor abilities to be made. 248 2 2 In C3v compounds such as those above the similar symmetry of the dxy and dx -y with the px and py orbitals form an pair of e-sets, assuming a z-axis along the Cr-nitrido bond vector. These orbitals act to accept ζ and π electron density from the three ligands in the basal plane of i 2 2 NCr(NPr 2)2(X). Since the strongly donating nitride also π-donates into the dxy /dx -y e-set, a situation arises whereas a 90° rotation of a basal ligand places π electrons in direct competition to donate into the same set of orbitals. This is manifested in a kinetically unfavorable barrier to i bond rotation along the Cr-NPr 2 bond. To date all solid (ground) state geometries of i NCr(NPr 2)2X compounds have diisoproplylamido ligands aligned such that the plane defined by Cmethine-N-Cmethine is nearly parallel to the Cr-nitrido bond (the defined z-axis). The observed i barrier of rotation along either Cr-NPr 2 therefore approximates the energy difference between i the ground state geometry and one in which a 90° rotation of the NPr 2 ligand causes the unfavorable competition with the multiply bonded nitrido. It has further been established that greater donor ability of the X ligand in the basal set slightly attenuates this energy difference via ground state destabilization. To quantify the effect of the X ligand on the system the barrier to Cr-amide rotation was observed in solution. The 1D 23, 24 (SSMT) 1 H NMR technique of Spin Satuaration Magnetic Transfer was employed to gauge the kinetics of the interchange between the magnetically ǂ i inequivalent methine resonances of the NPr 2 ligand. Assuming an entropy of the ∆S = −9 i cal/mol K for each compound (experimentally derived for other NCr(NPr 2)2X compounds) the 249 ǂ enthalpy of the transition state of amide rotation, ∆H was used as a measure of the donor ability of X, defined as the Ligand Donor Parameter (LDP). Due to orbital mixing, ζ and π contributions from the X ligand are also mixed (as in the case in most real world catalytic systems). This methodology has been shown to give a single, meaningful parameter of electronic donation in a highly diverse series of anionic monodentate ligands. Although the above system does not give a definitive amount, as absolute ligand donor ability could/should vary from system to system, the relative ordering in such series is chemically relevant. Below are the experimentally derived LDPs of 3-5 (Table 5.2). Compound LDP (kcal/mol) i (4) NCr(NPr 2)2(ind) (3) NCr(NPr 2)2(Cp) (5) NCr(NPr 2)2(flu) i i 13.76±0.270 13.91±0.277 14.16±0.301 Table 5.2. Experimentally determined LDP values of 3-5. 1 The following series of η -bound five member rings of Cyclopentadienyl, indenyl, and fluorenyl has closely spaced LDPs, giving the ordering of donor ability as Ind>Cp>Flu. However, these values are indistinguishable within experimental errors. This however does not impugn the usefulness of this type of donor evaluation, as small changes in the LDP in the test complexes can translate into significant differences in reactivity. LDP values on the order of 13.8 1 to 14.2 kcal/mol places the donor ability of these ligands similar to pyrrolyl (also bound η ) 250 which has little π-donor ability from the bound N. In comparison, the indenlyl ligand is very t similar to OBu F6 (LDP = 13.89±0.26), typically regarded as an electron poor alkoxide. Slight variation in the series may be attributed to the available resonance structures in 5 member and higher ring systems. Furthermore the increased steric interaction of the larger rings may increase the barrier of rotation making the LDP parameter a lower bound to the donor ability. i 3 5.6 Synthesis, Structure and LDP determination of NCr(NPr 2)(O2CPh)(η -Cp) 6 We were further interested in the substitution reactivity of compounds 3-5. Treatment of 3 with excess lutidenium iodide and other halogenating agents did not yield mono or bis-halide products, even at elevated temperatures. Substitution of a single diisopropylamide ligand is possible by treatment of 3 with 1 equiv. of benzoic acid (Figure 5.8) yielding clean i 3 NCr(NPr 2)(O2CPh)(η -Cp) after crystallization from toluene layered with pentane. Attempts at multiple substitutions with additional benzoic acid led to no reaction. i 3 Figure 5.8: Synthesis of compound NCr(NPr 2)(O2CPh)(η -Cp) 6. Upon substitution a hapticity change in the Cp ligand was observed in the X-ray structure of 6 which crystalized as enantiomers in the space group P21/c (Figure 5.9). Carbon atoms C1, C2, 251 and C3 adopt a geometry reminiscent of a π-allyl system, with C4 and C5 acting as an olefinic unit. This configuration was first described by Huttner and coworkers in the complex 25 W(CO)2(Cp)2. Bond distances within the 5 member ring as well as interatomic distances to the bound metal are given in Figure 5.4, showing much closer contacts from the Cr to the carbons of the allylic unit. Planes defined by C1, C2, C3 and C2, C3, C4, C5 intersect at a 2.4° angle (as compared to 19.6° the corresponding atoms in W(CO)2(Cp)2) showing little deformation of the ring from planarity. In light of this planarity despite the distribution in chromium-carbon distances, the Cp 3 of 6 might be best described as in a ‗slipped‘ η bonding mode. In drawing the analogy to a bonding mode more often encountered with indenyl ligands, a commonly used metric can be used to describe the amount of ‗slippage‘ versus an idealized pentohapto mode. Defined by the equation below, where M-C(X) is metal-carbon distance. 15 The Cp in 6 has a of ∆M-C = 0.215 5 Å, whereas typical η complexes have a ∆M-C under 0.1 Å. ( ( ) ( ) 252 ) ( ( ) ( ) ) i 3 Figure 5.9. A crystal structure rendering of NCr(NPr 2)(O2CPh)(η -Cp) (6). Thermal ellipsoids at 50% probability. Hydrogen atoms omitted ford for clarity. Moreover similarities to can be drawn to W(CO)2(Cp)2 as the authors note the 6π aromatic − 5 system of an η -Cp is broken to maintain an 18e complex rather than dissociation of a CO − ligand. While formally a 16e complex, compound 6 shows a preference not to adopt an η 5 − configuration realizing a full 18e . This is not so surprising when viewed in the context of other i nitride complexes of Cr(VI) such as the previously reported NCr(NPr 2)2(O2CPh), where the 1 − benzoate ligand remains  , maintaining a 16e electronic structure. 253 Figure 5.4. Crystallograpically determined interatomic Cr-C distances (blue) and C-C bond distances (red) in compounds 3 and 6, with free C5H6 for comparison. 1 Furthermore, the benzoate ligand in complex 6 also remains  in the solid state with a Cr-O1 i distance of 1.928(2) Å and a Cr-O2 distance of 3.169(2) Å (compared to NCr(NPr 2)2(O2CPh), with Cr-O1 and Cr-O2 distances of 1.924(1) Å and 2.9664(15) Å respectively). i i 3 5.7: FT-IR Studies of NCr(NPr 2)2(O2CPh) and NCr(NPr 2)(O2CPh)(η -Cp) (6) As seen in compounds of this type ligand bond distances are not always reliable to judge the nature of interactions to the metal. As such we investigated the coordination of the benzoate ligand in 6 by FT-IR spectroscopy. It has been noted by Deacon and Phillips that the stretches of 254 a metal bound carboxlate can be used to judge its denticity. 26 In this system a ∆ value is − − determined by the difference of the symmetric, νs(CO2 ) and asymmetric stretch, νa(CO2 ) according the the formula below. [ ( ) ( )] Comparison of this value to the determined ∆ value of the corresponding sodium or potassium salt of the ligand are made. Monodentate carboxalate ligands exhibit a ∆ greater than their ionic (sodium) salts. 27 Likewise, bidentate ligands are marked by their significantly lower ∆ values. i i 3 To identify these stretches isotopolagues of NCr(NPr 2)2(O2CPh) and NCr(NPr 2)(O2CPh)(η Cp) (6) were synthesized, with a 13 C label at the carbonyl carbon on the benzoate ligand. Values -1 -1 of ∆ in both cases were less than the measured ∆NaO2CPh = 179.3 cm (at 295.1 cm and 223.7 cm -1 1 respectively) assigning both as  . This is in agreement with the single crystal data, however both are solid state measurements. Attempts at solution IR measurements of the same type were unsuccessful due to solubility issues with the above complexes and the reference sodium benzoate. 5.8 Hapticity Shift as a function of π-loading in 6 Overall the Cp hapticity shift is consistent with the determined LDP values for the diisopropylamide and benzoate ligands, along with the penchant for Cr(VI) nitrides to adopt a − 16e configuration. With LDP values of 11.12±0.23 and 14.45±0.28 kcal/mol respectively, much 1 less electron density is donated to the metal in the case of a  benzoate. Although the additive effects of LDP are still under investigation, to a first approximation dividing the 2 values 255 suggests that the amide is about 1.3 times a better donor than benzoate. Thus replacement of i NPr 2 with O2CPh would result in a more electron deficient metal center. Since the complex also contains a ligand capable of a hapticity shift (which upon doing so increases its formal 3 contribution in electron counting) the η -Cp may be compensating for the loss of electron density. Related complexes bearing multiple cyclopentyldienyl ligands of the form M(Cp)3X where M = Ti, Zr, or Hf and X = H, CH3, OH, or NH2, have been investigated by Bursten and 28 coworkers. The DFT calculations have determined that the resulting hapticity of the Cp is dependent on the ligand X, with lower hapticities favored as the π-donor ability of X increases. Such competition for orbital occupation can be described as π-loading, with filled-filled orbital interactions destabilizing coordinately saturated complexes. 3 The Mo(IV) complex Mo(η -Cp)(NMe2)3 described by M. L. H. Green and coworkers displays a similar hapticity shift in the Cp ring upon ligand substitution. 1 29 Upon replacement of one dimethylamide ligand by a much less donating alkoxide ligand the Cp moiety adopts an η 5 − coordination. Assuming each amide formally donates 3e , whereas alkoxides only contribute − − 5 1e , an 18e complex is maintained after the transformation to Mo(η -Cp)(NMe2)2(OR) where i t R = Pr or Bu . Such an effect may be thought of as ‗π-unloading‘, with a hapticity shift compensating for loss of electron density. 256 3 Moreover, the complex Mo(η -Cp)(NMe2)3 undergoes [1,2]-insertion of CS2 giving the 5 1 2 complex Mo(η -Cp)(S2CNMe2)3, with a  and two  thiocarbamate ligands. Resulting in an − 2 18e complex (assuming 1ζ and 2π electrons from each  -thiocarbamate ligand), the compound − avoids a 20e configuration by not binding the last S moiety. This is very similar to the case of 6, where increased hapticity (thus a greater electronic donation) of Cp out competes a ligand 1 2 capable of  or  coordination. Additionally, replacement of the bulky amide in complex 6 for the less sterically demanding benzoate relaxes congestion around the metal and may allow for the larger sterics of a higher hapticity Cp. As before both the methods of Solid G and %Vbur (31.83 and 36.08% respectively) 3 1 3 suggests that the η -Cp in 6 is much bigger than the η -Cp of 3. A %VBur of 36.08% places η i Cp as the largest ligand thus far encountered in our investigations of NCr(NPr 2)2(X) complexes, i with the next largest being N Pr2 (29.1%). This is due not only to Cp‘s size but its proximity to the metal in 6, as the large but distal sterics of OSiPh3 have %Vbur of 22.2% (Figure 5.10). Comparing the Gm(L) of 3 and 6 gives an increase of 10.96. To put this number into perspective a bound Cl ion has a Gm(L) of 15.14. 257 i 3 Figure 5.10: A space filling representation of NCr(NPr 2)(O2CPh)(η -Cp) (6). The inscribed st orange sphere with radius 3.5 Å approximating the 1 coordination sphere. i Since complex 6 also shows magnetically inequivalent resonances for the NPr 2 methines, SSMT was conducted giving a LDP of 16.53±0.30 kcal/mol. This comparatively large barrier to amide rotation is a function both of the new Cp coordination mode but also the loss one of the i i electron donating NPr 2 ligands. However, when compared to the LDP of NCr(NPr 2)2(O2CPh) i (14.45±0.28 kcal/mol), only one ligand in the basal set has changed (i.e. replacement of NPr 2 by 3 η -Cp). Attempts at the synthesis of the indenyl analogue of 6 yielded only intractable products. i Limited success in the synthesis of NCr(NPr 2)(O2CPh)(flu) (7) through the same methods as 6 was low yielding. Attempts at isolation and purification via crystallization have yet to be 258 successful; however, the crude product was suitable to be evaluated by SSMT. The observed i LDP of 14.68±0.30 kcal/mol is significantly greater than that of NCr(NPr 2)2(flu), it is drastically lower than that of compound 6. Currently the relationship between contributions from multiple ligands as well as the alteration of the model system are not well understood. It is our hope that these effects can be derived from comparisons of compounds bearing two or more ligands in common from within in our series. 5.9 Conclusion i A series of complexes of the form NCr(NPr 2)2(X) whereas X = Cp (3), indenyl (4), and 1 fluorenyl (5) were synthesized and structurally characterized, each having an η interaction to the metal. In doing so a new reagent thallium indenylide was synthesised and structurally i characterized. Additionally, the cationic Cr(VI) complex [NCr(NPr 2)2DBN][I] (2) proved to be a useful synthon, as well as an interesting compound which will be fully reported on elsewhere. Sterics in 3-5 were evaluated by the method of Solid G and %VBur with both systems showing the expected trend of increasing size CpCp>Flu although these are equivalent with error. Substitution of a i diisopropylamide ligand on 3 affords the compound NCr(NPr 2)(O2CPh)( Cp) (6), which shows 3 a slipped η -Cp in the solid state. This structural change can explained in terms of π-loading on 1 the chromium center. FT-IR analysis reveals a  benzoate ligand in 6 and the related compound i NCr(NPr 2)2(O2CPh), both in agreement with their solid state X-ray structures. Additional 259 investigations are underway to understand the individual ligand contributions to the overall measured LDPs. 5.10 Experimental Synthesis of thallium(I) indenylide Under an inert atmosphere a scintillation vial was loaded with freshly distilled indene (1.48 g, 12.71 mmol, 1 equiv.), 8 mL pentane and a stir bar. The solution was cooled 10 min in a liquid nitrogen cooled cold well. To this vial thallium(I) ethoxide (3.17 g, 12.71 mmol, 1 equiv.), was added as a pentane solution (1 mL) and the solution turned yellow. The reaction was stirred 2 h at which time the volatiles were removed under vacuum. The solid was titrated twice with Et2O, and left under dynamic vacuum for several hours yielding Thallium Indenylide as an orange-yellow powder (3.28 g, 10.27 mmol, 81% yield). Diffraction quality crystals were grown from a concentrated THF solution of 1 held at −35 °C. Mp: 114 °C (dec.) i Synthesis of [NCr(NPr 2)2(DBN)][I] (2) Under an N2 atmosphere a 125 mL Erlenmeyer flask i was loaded with NCr(NPr 2)2(I) (2.05 g, 5.20 mmol, 1 equiv.) a stirbar and 50 mL Et2O. To this an Et2O (10 mL) solution of DBN (0.646 g, 5.20 mmol, 1 equiv.) was added drop wise to the flask over 5 min. The solution stirred for 1.5 h during which time the product precipitated. The suspension was filtered on a glass frit and the washed with pentane (2 × 10 mL). The solids were collected and dried under reduced pressure yielding 2 as an orange powder (2.195 g, 4.24 mmol, 82% yield). Diffraction quality crystals were grown from a concentrated THF solution of 2 held 1 at −35 °C. H NMR (500 MHz, CDCl3, 25 °C): 5.32 (sept. JHH = 6 Hz, 2H, (CH3)2CH), 3.87 (sept. JHH = 6.5 Hz, 2H, (CH3)2CH), 3.75 (t JHH = 7.5 Hz, 2H, DBN), 3.63 (t JHH = 5 Hz, 2H, 260 DBN), 3.48 (t JHH = 6 Hz, 2H, DBN), 3.10 (t JHH = 8 Hz, 2H, DBN), 2.11-2.04 (multi., 4H, DBN), 1.80 (d JHH = 6 Hz, 6H, (CH3)2CH), 1.52 (d JHH = 6.5 Hz, 6H, (CH3)2CH), 1.30 (d JHH 13 1 = 6.5 Hz, 6H, (CH3)2CH), 1.19 (d JHH = 6.5 Hz, 6H, (CH3)2CH). C{ H} NMR (125 MHz, CDCl3, 25 °C): 169.30, 59.44, 57.41, 55.15, 47.56, 43.68, 36.26, 30.89, 30.97, 22.79, 22.29, 20.50, 18.36. Anal. Calcd. for C19H40CrIN5: C, 44.10; H, 7.79; N, 13.53. Found: C, 44.02; H, 7.63; N, 13.47. Mp: 168-170 °C. i Synthesis of NCr(NPr 2)2(Cp) (3) Under an inert atmosphere a scintillation vial was loaded i with NCr(NPr 2)2(I) (1) (0.257 g, 0.654 mmol, 1 equiv.), 8 mL THF, and a stirbar. The solution was cooled to near frozen in a liquid nitrogen cooled cold well. To this a 2.0 M THF solution of sodium cyclopentadienide (0.98 mL, 1.96 mmol, 3 equiv.) was added, and the solution was rapidly stirred for 20 h. The volitiles were removed in vacuo, and the residue was extracted with pentane (2 × 10 mL) and filtered through Celite. The volitiles were removed under vacuum yielding 3 as a brown powder. Diffraction quality crystals were obtained from a concentrated 1 pentane solution of 3 held at −35 °C (0.152 g, 0.459 mmol, 70% yield). H NMR (500 MHz, CDCl3, −30 °C): 6.14 (s, 5H, C5H5), 4.89-4.85 (sept, 2H, CH(CH3)2), 3.59-3.55 (sept, 2H, CH(CH3)2), 1.72-1.71 (d, 6H, CH(CH3)2), 1.43-1.41 (d, 6H, CH(CH3)2), 1.05-1.01 (m, 12H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, −30 °C): 114.73, 57.98, 54.87, 30.25, 30.17, 23.10, 17.49. Anal. Calcd. for C17H33CrN3: C, 61.60; H, 10.03; N, 12.68. Found: C, 61.59; H, 9.97; N, 12.65. Mp: 90-92 C (sub). 261 i Synthesis of NCr(NPr 2)2(indenyl) (4) Under an inert atmosphere a scintillation vial was loaded with 2 (0.122 g, 0.237 mmol, 1 equiv.), 4 mL THF, and a stirbar. To this a THF solution (5 mL) of freshly prepared thallium(I) indenylide (0.076 g, 0.237 mmol, 1 equiv.) was added. The reaction stirred rapidly for 1 h. The volatiles were removed under reduced pressure. The residue was extracted with pentane until the extracts were clear. The extracts were filtered on a fritted funnel with Celite as a filtering agent. The pentane solution was concentrated under reduced 1 pressure to 5 mL and held at −35 °C yielding crystals of 4 (0.049 g, 0.128 mmol, 54%). H NMR (500 MHz, CDCl3, 25 °C): 7.53-7.50 (multi, 2H, C9H9), 6.98-6.95 (multi, 3H, C9H9), 5.58 (s br. 2H C9H9), 4.75 (sept JHH = 6.4 Hz, 2H, CH(CH3)2), 3.41 (sept JHH = 6.1 Hz, 2H, CH(CH3)2), 1.39 (dd JHH = 6.1 Hz JHH = 13.9 Hz, 12H, CH(CH3)2), 0.97 (d JHH = 6.3 Hz, 6H, CH(CH3)2), 0.83 (d JHH = 6.1 Hz, 6H, CH(CH3)2). Mp: 138-140 C. i Synthesis of NCr(NPr 2)2(fluorenyl) (5) Under an inert atmosphere a scintillation vial was loaded with a fluorene (0.048 g, 0.287 mmol, 1 equiv.), a stir bar, and THF (8 mL). The vial was cooled in liquid nitrogen cooled cold well for 10 min. The vial was moved to a stir plate. To the n rapidly stirring solution an BuLi (0.179 mL, 0.287 mmol, 1 equiv.) was added drop wise over 5 min as a 1.6 M solution in hexane. The reaction was allowed to come to room temperature and stirred for 30 min, turning bright yellow. To the vial a solution of 2 (0.149 g, 0.287 mmol, 1 equiv.) in THF (5mL) was added. The reaction stirred for an additional 2 h. The volitiles were removed under vacuum. The residue was extracted with pentane until the extracts were clear. The extracts were filtered on a fritted funnel, with Celite as a filtering agent. The pentane 262 solution was concentrated under reduced pressure to 5 mL and held at −35 °C yielding crystals of 1 5 (0.039 g, 0.092 mmol, 32%). H NMR (500 MHz, CDCl3, 25 °C): 7.79 (d, JHH = 8.9 Hz, 2H, C13H9), 7.74 (d, JHH = 8.9 Hz, 2H, C13H9), 7.25 (t, JHH = 7 Hz, 2H, C13H9), 7.15 (t, JHH = 7.4 Hz, 2H, C13H9), 4.67 (s br., 1H, C13H9), 4.55 (sept JHH = 6.4 Hz, 2H, CH(CH3)2), 3.43 (sept JHH = 6.2 Hz, 2H, CH(CH3)2), 1.44 (d JHH = 6.3 Hz, 6H, CH(CH3)2), 1.32 (d JHH = 6.3 Hz, 6H, CH(CH3)2), 0.98 (d JHH = 6.3 Hz, 6H, CH(CH3)2), 0.76\ (d JHH = 6.3 Hz, 6H, CH(CH3)2). i Synthesis of NCr(NPr 2)(O2CPh)(Cp) (6) Under an inert atmosphere a scintillation vial was loaded with 3 (0.178 g, 0.537 mmol, 1 equiv.), a stir bar, and toluene (4 mL). The vial was moved to a liquid nitrogen cooled cold well for 10 min. The reaction was stirred vigorously and benzoic acid (0.066 mg, 0.537 mmol, 1equiv.) in toluene (6 mL) was added drop wise over 5 min. The solution turned dark red and was allowed to stir at room temperature for 2 h. The volatiles were removed under reduced pressure and the residue was dissolved in 2 mL of toluene. The solution was filtered and layered with an equal volume of pentane and held at −35 °C 1 yielding crystals of 6 (0117 g, 0.333 mmol, 62%). H NMR (500 MHz, CDCl3, 13 °C): 7.95 (dd JHH = 8.25 Hz, JHH = 1.5 Hz , 2H, Ar-H), 7.41 (tt JHH = 7.0 Hz, JHH = 2.5 Hz, 1H, Ar-H), 7.34 (t JHH = 7.5 Hz, 2H, Ar-H), 6.14 (s, 5H, C5H5), 5.56 (sept. JHH =6.0 Hz ,1H, NCH(CH3)2), 4.31 (sept. JHH = 6.0 Hz ,1H, NCH(CH3)2), 2.11 (d JHH = 6.0 Hz ,3H, NCH(CH3)2), 1.75 (d JHH = 6.0 Hz ,3H, NCH(CH3)2), 1.29 (d JHH = 6.0 Hz ,3H, NCH(CH3)2), 1.11 (d JHH = 6.0 Hz ,3H, 263 NCH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, 13 °C): 170.74, 135.26, 130.77, 129.66, -1 127.83, 108.22, 73.71, 63.71, 31.06, 29.83, 20.64, 20.15. FT-IR (KBr): 1639.2 cm , -1 1415.5 cm . General Procedure for FT-IR Denticity Determination All FT-IR analysis was done on a Mattson Galaxy Series FTIR 3000 spectrometer. Samples were prepared by pressing ~10 mg of each compound into anhydrous KBr. The symmetric and asymmetric carbonyl stretches were identified by comparison to its isotopologue, 13 C labeled at the carbonyl carbon. Difference between the stretches in the sample were compared to the difference in the symmetric (νs = -1 -1 1415.5 cm ) and asymmetric (νa = 1594.8 cm ) stretches in a sample of sodium benzoate in -1 KBr, which had a value for ∆NaO2CPh of 179.3 cm . 264 REFERENCES 265 REFERENCES (1) Kealty, T. J.; Pauson, P. L. Nature 1951, 168, 1039–1040. (2) McKnight, A. L.; Waymouth, R. M. Chemical Reviews 1998, 98, 2587–2598. (3) Miller, S. A.; Bercaw, J. E. Organometallics 2004, 23, 1777–1789. (4) Brintzinger, H. H.; Fischer, D.; Mülhaupt, R.; Rieger, B.; Waymouth, R. M. Angewandte Chemie International Edition in English 1995, 34, 1143–1170. 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Coordination Chemistry Reviews 2002, 233-234, 157–176. (16) Janiak, C. Journal fur praktische Chemie Chemiker-Zeitung 1998, 340, 181–183. (17) Frasson, E.; Menegus, F.; Panattoni, C. Nature 1963, 199, 1087. 266 (18) Kraus, H. J.; Otto, H.; Werner, H. Journal of Organometallic Chemistry 1986, 315, C57– C60. (19) Bondi, A. Journal of Physical Chemistry 1965, 68, 441–451. (20) Tolman, C. A. Chemistry Reviews 1976, 77, 313–348. (21) Clavier, H.; Nolan, S. P. Chemical Communications 2010, 46, 841–61. (22) Guzei, I. A.; Wendt, M. Dalton Transactions 2006, 3991–9. (23) Jarek, R. L.; Flesher, R. J.; Shin, S. K. Journal of Chemical Education 1997, 74, 978–982. (24) Perrin, C. L.; Thoburn, J. D.; Kresge, A. J. Journal of the American Chemical Society 1992, 114, 8800–8807. (25) Huttner, G.; Brintzinger, H. H.; Bell, L. G. Journal of Organometallic Chemistry 1978, 145, 329–333. (26) Deacon, G. B.; Phillips, R, J. Coordination Chemistry Reviews 1980, 33, 227–250. (27) Nakamoto, K. IR and Raman spectra of Inorganic and Coordination Compounds 4ed; 1986. (28) Palmer, E. J.; Bursten, B. E. Polyhedron 2006, 25, 575–584. (29) Yan, X.; Chernega, A. N.; Metzler, N.; Green, M. L. H. Journal of the Chemical Society, Dalton Transactions 1997, 2091–2100. 267 Chapter 6: Synthesis, Structure, and LDP Determination of Organometallic Ligands on Chromium(VI) Nitrido Complexes ABSTRACT In expanding the series ligand parameterization using a Cr(VI) nitride platform i complexes of the form NCr(NPr 2)2(X) i where X = CH2SiMe3 (4), CH2CMe2(Ph) (5), t CH2CMe3 (6), CCSi(Pr 3) (7), and CC Bu (8) were synthesized. Using the technique of Spin Saturation Magnetization Transfer the kinetics of diisopropylamide rotation were determined and used to parameterize the donor ability of the organometallic ligand X. This ligand Donor i Parameter (LDP) is given for complexes 4-8 and the known NCr(NPr 2)2(CH2SiMe2(Ph)) (3). Crystal structures of compounds 3-8 were used to quantify the sterics of each X ligand using the method of Percent Buried Volume and Solid Angle techniques. The complex i [NCr(NPr 2)2(C2Ph)]2 (9) was also synthesized and structurally characterized showing a cumulated 4 carbon chain between two metal centers. 6.1 Introduction In an effort to expand the scope of the preceding studies into the electronic and steric profiles of commonly used ligands, our attention turned toward X-type ligands bound through a metal carbon bond. Metal alkyls and aryls show many interesting features specific to ligand of the type. Our interest in this study had two main goals. Firstly, to find the effect of the hybridization of the bound carbon on the ligand‘s donor properties. Secondly, to determine if substituting a non-carbon atom β to the bound carbon had a measurable effect on its donor ability 268 toward a metal center. Well studied in the realm of organic chemistry, placement of a Si atom adjacent to a carbocation stabilizes the positive charge through hyperconjugation (Figure 6.1). 1 Figure 6.1: Hyperconjugation as seen in β-substituted carbocations. Such stabilization may also be gained by orbital overlap of the empty p orbital of the carbocation with the larger filled σ orbital of a Si-C bond. Dubbed the β atom effect, the effect of a silicon atom in this position on a transition metal bound ligand is less well known despite been employed in many common organometallic ligands. Although Si is less electronegative 2 than C any inductive effect is thought to be overshadowed by a larger resonance effect, where the Si can go hypervalent, competing for the electron density donating into the transition metal. It follows that this may weaken the metal carbon linkage to some degree (Figure 6.1). Figure 6.2: The ylide like resonance forms possible in β-silicon substituted metal alkyls. Additionally, metal-carbon linkages are common in the intermediates and transition states of many metal-catalyzed organic transformation. Perhaps most notably M-C bonds are sequentially made and broken in group 10 cross-coupling chemistry, ubiquitous in modern 269 organic synthesis. 3 In rationally designing more efficient catalyst systems, knowing the nature of M-C bonded intermediates (i.e. measuring relative bond strengths) may be useful in preventing yield attenuating side reactions and catalyst decomposition pathways. The possible utility of measuring a standardized parameter such as the Ligand Donor Parameter (LDP) for these metal-carbon bonds is twofold. Although there are very important differences between the late metals typically employed in cross-coupling and the early metals most relevant to or previous studies, we felt a 0 8 series of LDP‘s measured on a d system would be applicable to d metal alkyl complexes. While the presence of metal based electrons occupying d orbitals may impede the usefulness of LDP with π –donor ligands such as amides and alkoxides, carbon bound organometallic ligands are ζ only. As measured, LDP mixes ζ and π electron donation from the ligand into one parameter. To a first approximation the LDP‘s of complexes bearing ligands with no π contribution will likely be invariant in correlations to known systems with d-orbital occupancy. Perhaps more ambitiously, there has been a push in recent years toward developing early transition metal analogues to platinum group metals that can catalyze the cross-coupling of organic substrates. One such proof of concept system by Heyduk and coworkers uses zirconium bearing redox active ligands as a stand in for palladium. Here redox chemistry on the formally d 4 0 5 metal supports the oxidative addition and reductive elimination steps typically seen by the Pd(0)/Pd(II) redox couple in more orthodox systems. While more conventional early metal 6 systems are known to react via ζ bond metathesis (yielding products as if oxidative addition and 270 reductive elimination were happening concurrently), this is a concerted reordering of bonds, thus unlikely to be useful in directed coupling of two different substrates. To this end, a series of commonly employed organometallic ligands were affixed a i Cr(VI) nitride complex of the form NCr(NPr 2)2(X) so that their electronic and steric profiles could be quantified. i 6.2 Synthesis of NCr(NPr 2)2(X) X = Alkyl Complexes i Complexes of the type NCr(NPr 2)2(X) where X is the monodentante, monoanionic 7 ligand of interest are accessible through a variety of established methods. In general complexes of this type bearing organometallic (i.e. carbon bound) X-type ligands are less stable than those bearing amide, halides, and alkoxydes. Specifically, two main decomposition products have been i seen in the synthesis of virtually all of the NCr(NPr 2)2(X) compounds reported; from trace amounts to the major product of reaction. Firstly, the highly electron deficient Cr(VI) center in such complexes is prone to reduction by reagents appropriate in other systems for simple substitution chemistry. First 8 − described by Odom and coworkes, 1e reduction to Cr(V) results in the bridging of nitridos i between molecules, giving [Cr(µ-N)(NPr 2)2]2. This product was first seen as a decomposition product of thermally unstable Cr(VI) alkyls, and then intentionally synthesized with Na amalgam (Figure 6.3). 271 i Figure 6.3: Synthesis of [Cr(µ-N)(NPr 2)2]2, the Cr(V)-Cr(V) dimer by reduction with sodium amalgam. The reducing power of many lithium and sodium alkyl reagents was found to be sufficient to reduce typical Cr(VI) nidrido starting materials appropriate for similar metathesis reactions with lithiumi alkyoxide or amide reagents. This issue was sidestepped by disfavoring electron transfer by using less polar solvents, lowering temperatures, and using more electron rich Cr(VI) alkyoxides as synthons in place of Cr(VI) halides. Thus judicious choice of both the reagent used to install the X ligand as well as the Cr(VI) nitride source is necessary. Secondly, the imine product N-(propan-2-ylidene)propan-2-amine was recovered in many failed syntheses of Cr(VI) alkyls. We surmise that this is likely the result of the abstraction of the i methine proton on one NPr 2 ligand by the incoming carbanion, as the corresponding product of Li- or Na-alkyl protonalysis was also observed. Conversely, this product may be formed after the desired substitution has occurred, thus an endemic limitation of our system with specific X ligands. 272 In circumventing these synthetic pitfalls we found that the preparation of organometallic Cr(VI) nitridos are highly sensitive to reaction conditions, such as solvent, temperature, time, and the transmetallation reagents chosen. One of the earliest Cr(VI) alkyls known was prepared by the Cummins group using 9 CH2SiMe2(Ph), commonly known as the silyneophyl ligand (Figure 6.4). starting point, the neophyl ligand, 11 8,10 Using this as a CH2CMe2(Ph) ligand was installed (Figure 6.4). i Figure 6.4: Synthesis of NCr(NPr 2)2(CH2SiMe2(Ph)) (3) and i NCr(NPr 2)2(CH2CMe2(Ph)) (5). i The synthesis of NCr(NPr 2)2(CH2CMe3) (6) and its silicon substituted analogue i NCr(NPr 2)2(CH2SiMe3) (4) were accessible through the above procedure, however yield and i purity was low, as NCr(NPr 2)2(I) (1) is reduced to the Cr(V) dimer under the attempted conditions. Instead for 4 and 6 the use of the more electronically rich alkoxides i 10 NCr(NPr 2)2(OPh) (2) i and NCr(NPr 2)2(OAd) where Ad = 1-adamantanyl 273 were a more advantageous starting materials with the reducing neopentyl 12 and trimethylsilymethyl lithium (Figure 6.5). i i Figure 6.5: Synthesis of NCr(NPr 2)2(CH2CMe3) (6) and NCr(NPr 2)2(CH2SiMe3) (4). i 6.3 Synthesis of NCr(NPr 2)2(X) X = Alkynyl Complexes In expanding our series, alkynyl substituents were installed onto Cr(VI) to judge the effect of hybridization of the bound C atom upon overall ligand donor ability. The Cr-alkynyl i t i i complexes NCr(NPr 2)2(CC Bu) (8) and NCr(NPr 2)2(CCSi(Pr 3)) (9) were synthesized in good yield from treatment of 2 the corresponding lithium alkynyls (Figure 6.6). 274 i t i i Figure 6.6: Synthesis of NCr(NPr 2)2(CC Bu) (8) and NCr(NPr 2)2(CCSi(Pr 3)) (7). i 6.4 Synthesis and Structure of [NCr(NPr 2)2(C2Ph)]2 (9) Interestingly, a similar reaction to those above with an in situ generated Li-alkynyl of i phenyl acetylene gave the dimer [Cr(µ-N)(NPr 2)2]2, as well as various other intractable metal containing species. An alternative synthetic strategy using a presumed in situ generated zincate also failed give the desired product, but gave an intriguing product found to be i [NCr(NPr 2)2(C2Ph)]2 (9) (Figure 6.7). 275 i Figure 6.7: A crystal structure rendering of [NCr(NPr 2)2(C2Ph)]2 (9) showing a dimeric structure symmetric about the midpoint of the bridging allenyl moiety. Atom positions shown at 50% probability with H atoms removed for clarity. As seen in other syntheses using Zn(II) or Mg(II) as transmettalating reagents for Cr(VI), i both do not give appreciable amounts of the desired NCr(NPr 2)2(X) product when a halide source is present (i.e. Grignard reagents of the form MgRX where X = Cl or Br will selectively i i transmetallate a halide ligand to NCr(NPr 2)2(I) (1) or NCr(NPr 2)2(OPh) (2) in most cases). This has also been observed with Zn reagents generated in situ from ZnCl2. In fact, treatment of i 1 with 1 equiv. of ZnCl2 gives NCr(NPr 2)2(Cl) in nearly quantitative yield. 276 To promote transfer of the desired R group, ZnCl2 was treated with 2.02 equiv. of (phenylethynyl)lithium in cold THF. Transmetalation onto Zn(II) ostensibly creates the dialkynyl zincate, which upon reaction with 1 produces an unusual Cr(VI)-Cr(VI) dimer bridged through a cumulated ligand ligand. A concentrated Et2O solution held at –35 ºC yielded small orange crystals of 9 in the space group P1−, with half of the symmetric dimer in the asymmetric unit. Each Cr1-C2 bond 3 length is 2.017(5) Å, statically shorter than the average corresponding Cr-C(sp ) bonds of 3-6 at 2.047 Å and longer than the Cr-C(sp) bonds of 7-8 at 1.986 Å (vida infra). The bridging ligand consists of four nearly linear C atoms, with a torsional angle of C2-C1-C1‘-C2‘ of 180°. Bond angles in the linkages of C2-C1-C1‘ and C2‘-C1‘-C1 are 174.75° and symmetric about the molecules‘ inversion center, midway between C1 and C1‘. The C-C bonds are best described as double bonds, with a length of 1.324(4) Å between C2 and C1 and 1.270(6) Å between C1 and C1‘. These bonds are characteristic of an allene like structure closely matching the corresponding lengths seen in Ph(H)C=C=C=C(H)Ph of 1.343(6) Å and 1.322(6) Å for terminal C=C bonds and 1.256(6)Å for the internal C=C linkage. 13 2 This alternation of longer Csp -Csp bonds with the 14 shorter interior Csp-Csp bond has been noted in the case of butatrienes. Although the reaction by which 9 forms is currently not fully understood at present, the C4 diacetylene moiety has been known to undergo redox chemistry when bound to transition metals. 15 277 i 6.5 LDP Determination of NCr(NPr 2)2(X) Organometallic Complexes i With a broad ranging series of NCr(NPr 2)2(X) compounds in hand, the kinetics of diisopropylamide rotation for the complexes bearing various organometallic ligands were 7 evaluated via the method of Spin Saturation Magnetization Transfer as outlined in Chapter 4. ǂ Assuming a ∆S of −9 cal/mol K the Ligand Donation Parameters (LDPs) derived for 3-8 are shown in Table 6.1. With an LDP of 13.30±0.26 kcal/mol compound 7 is the most donating (i.e. has the lowest barrier to amide rotation) of all the organometallic ligands surveyed. This value places i the alkynyl CCSi(Pr 3) as donating as the siloxide ligand OSiPh3 (13.28±0.27 kcal/mol). Although the coordinated oxygen of OSiPh3 has a lone pair of appropriate symmetry to donate into the empty d orbitals of Cr(VI) the Lewis acidic Si atom is thought to compete for this density, lowering the amount of π-donation (thus LDP) relative to other alkoxides. i 16 t It is not surprising that 7 and the alkynyl NCr(NPr 2)2(CC Bu) (8) (LDP = 13.73±0.14 kcal/mol) were relatively better donors than most organometallic ligands in the study, despite 3 being bound via an sp carbon versus an less electronegative sp carbon. Scholarship into the nature of metal-acetylide bonding has shown that the filled d orbital-π* interactions which play an important part in the bonding of the isoelectronic cyanide ligand are not predominate with acetylides. 17 Studies using a variety of techniques have suggested acetylide ligands act as weak π-donors toward metal centers with d orbital vacancies. 278 18,19 Homoleptic Cr(III), Fe(II), and Co(III) complexes bearing alkynyl ligands have been prepared by Long and coworkers which show three strong charge transfer bands, assigned as one LMCT and two MLCT, suggesting 20 alkynyls ability to also act as a π-donor. In the case of Cr(III) the authors propose an estimate -1 of the ligand field splitting parameter of ∆o = 20,200 cm , putting CCSiMe3 after methyl (∆o = -1 21 20,800 cm ) -1 22 but before chloride (∆o = 18,700 cm ) in the spectrochemical series. This -1 23 value differs significantly from that of cyanide (∆o = 26,600 cm ) also suggesting a π- donating as opposed to a π-accepting role. Computational work by Floriani and coworkers on dinuclear transition metal species bridged by an C 2- unit also suggests that this behavior is more typical for early first row transition metals than for late, as the empty d orbitals are of a more appropriate energy to interact with the filled π-orbitals of the ligand. 24 As it relates to complex 7 and 8, the predominate metal-ligand π interaction (if any) is of the ligand acting as a π-donor as there is formally no metal based electron density to back donate. Graphically represented in Figure 6.8, the majority of 3-8 exhibited donor abilities closely spaced and within experimental error of each other. Of note however, the silyl analogues 3 and 4 were nominally stronger donors than their carbon congeners 5 and 6. This might be a manifestation of the electronegativity differences between a carbon ligated methylene and a silyl ligated methylene playing more of a role in the LDP than hypervalent or hyperconjugated resonance structures. Likewise, the expected trend in hybridization (i.e. more electronegative C atoms would be weaker donors) is not seen within the series beyond error. In fact the presumed most electronegative Csp bound ligands (7 and 8) are at the stronger donating end of the series. This 279 may be due to π-effects as previously discussed, suggesting this added effect may trump the effect of the change in electronegativity of C with hybridization. 280 a Proposed but not fully characterized. B See Chapter 5. C See Chapter 4. Figure 6.8: Ligand Donation Parameters of all C-bound X-type ligands evaluated with associated experimental errors. Lower barriers of amide rotation (stronger donors) displayed on the left. 281 LDP (kcal/mol) Compound Cr1-C1 a %Vbur Gm(L) 1.997(7) 17.31 18.15 2.046(2) 24.35 22.67 1.979(8) 17.43 18.57 13.78±0.27 2.061(3) 24.12 21.87 13.79±0.28 2.041(5) 23.94 22.35 13.96±0.26 2.040(3) 24.62 24.75 - 2.017(5) - - Bond Length (Å) i i NCr(NPr 2)2(CCSi(Pr 3)) (7) i NCr(NPr 2)2(CH2SiMe3) (4) i t NCr(NPr 2)2(CC Bu) (8) i NCr(NPr 2)2(CH2CMe3) (6) i NCr(NPr 2)2(CH2SiMe2(Ph)) (3) i NCr(NPr 2)2(CH2CMe2(Ph)) (5) i [NCr(NPr 2)2(C2Ph)]2 (9) a 13.30±0.26 b 13.71±0.27 13.73±0.14 b b LDP calculated from the average of three SSMT runs. LDP calculated from the average of six SSMT runs. Table 6.1: Ligand Donation Parameter, Cr1-C1 bond distance, and the steric parameters %Vbur and Gm(L) of compounds 3-9. 282 i 6.6 Structure and Steric Analysis of NCr(NPr 2)2(X) Complexes i Like most of the previously synthesized NCr(NPr 2)2(X) complexes those bearing organometallic ligands crystalize well from cold concentrated pentane solutions, sans for the highly lipophilic neopentyl compound (6). Along with the previously reported 3, single crystal X-ray diffraction of 4-8 was collected to provide structural as well as quantitative steric assessment of each X ligand. As is the case with the other X-type ligands previously evaluated, no correlation was seen between Cr1-C donar atom bond length and LDP throughout the series, reiterating that caution is necessary in comparing the relative donor properties of ligands solely by crystallographic parameters. Structural comparison of 4 and 6 suggest that that Si for C substitution at the β has little i effect upon the solid state structure. The average Cr-N Pr2 bond distance in 4 of 1.815(3) Å is indistinguishable within ESD to that of 6 at 1.822(3) Å. Cr-C bond distances of 2.046(2) Å in 4 and 2.061(3) Å in 6 are statistically different, and do follow the initial expectation that stronger donors will have shorter Cr1-C bonds. The trimethylsilymethyl compound 4 has a N1-Cr1-C1Si1 dihedral angle of 1.64(2)°, slightly smaller than the corresponding dihedral angle of the neopentyl 6 at −12.02(3)°. This structural similarity, especially of distal steric bulk in the tert-Si of C suggests that the barriers to rotation reported do not contain significantly different steric components when comparing 4 and 6. 283 i Figure 6.9: Crystal structure overlay of NCr(NPr 2)2(CH2SiMe3) (4) (green) and i NCr(NPr 2)2(CH2CMe3) (6) (purple) showing a close structural similarity between the complexes. The structures of the silyneophyl 3 and neophyl 5 complexes exhibit much less agreement upon the phenyl for methyl substitution on C2/Si1. Complexes 3 and 5 have nearly identical Cr1-C1 bond lengths but differing N1-Cr1-C1-C2/Si1tortional angles of 6.1(3)° and −26.22(3)° respectively. This change in orientation may be a manifestation of the unsymmetrical bulk held closer to Cr1 in 5 than in 3 due to the relatively shorter C1-C2 bond distance in 5 (1.535(5) Å) versus the C1-Si1 bond distance in 3 (1.832(4) Å). 284 i Figure 6.10: Crystal structure overlay of NCr(NPr 2)2(CH2SiMe2(Ph)) (3) (blue) and i NCr(NPr 2)2(CH2CMe2(Ph)) (5) (red). To address the overall steric profile of 3-8 quantitatively, two separate methods were employed using the atomic positions gained from single crystal x-ray diffraction. For an in depth discussion of the method of solid angles using the Solid G 26 occupied volume, %Vbur 25 program, and a calculation of please refer to Appendix B. Tabulated data are listed in Table 6.1. When addressing the amount of space occupied by each alkyl or alkynyl ligand via the method of %Vbur, a sphere of 3.5 Å is circumscribed around the metal center approximating the 285 first coordination sphere of the Cr(VI) center. The volume of this sphere occupied by the constituents of a given ligand are then compared to the total volume and given as a percentage. t This regime sharply discriminates between ligands like the CC Bu in 8 (17.43%) and the t neopentyl of 6 (24.12%). Although they both contain a tert-butyl moiety, and CC Bu even contains one more C atom it occupies a significantly lower percentage. This is due to the linear t orientation of the CC Bu placing the bulk of the sterics outside the sphere of enclosure, in spite of the shorter Cr-C1 bond distance in the alkynyl relative to the alkyl. Illustrated in Figure 6.11, this rift is apparent for the other alkynyls and alkyls in the series. When compared to other ligands treated in the same manner (Chapter 4 and Appendix B), the common alkyl ligands tested t rank among some of the largest in terms of %Vbur falling between OBu F6 (23.6%) and NMe(Ph) (25.9%). Conversely, the alkynyls tested, sit at the low end of all the ligands tested, about the size of chloride (16.8%). When approached from a solid angle standpoint, the description of sterics is geared toward substrate access to the metal center. By treating the Cr(VI) center as a point source of light each bound ligand casts a ‗shadow‘ on an arbitrarily sized sphere inscribing the complex. The area of this shadow is calculated and also given as a percentage of the total area of the sphere. The steric parameter calculated in this way by the Solid G program for 3-8 are shown in Figure 6.12. While the alkynyl substituents are still smaller, the effect of moving steric bulk distal to the metal is attenuated. In the context of other ligands examined organometallic ligands i span the range of G(L), from CCSi(Pr 3) (18.15%) comparable to CN (17.08%) to neophyl t (24.75%) like the large OBu F6 (24.71%). 286 Figure 6.11: %Vbur Calculated for each ligand in 3-8 in a 3.5 Å sphere. 287 Figure 6.12: Percentage of the sphere shielded by the C bound ligand in 3-8 as calculated by Solid G. 288 6.7 Conclusion i The above NCr(NPr 2)2(X) complexes bearing Cr-C linkages round out a fairly comprehensive series of common monoanionic, mondentate ligands evaluated in both electronics and sterics. Though our system mixes both σ and π effects into a single parameter the subset of alkyl complexes are σ only, thus their LDP may find broader usage across transition metals of various d-counts. In realizing the series, new synthetic methodologies for problematic substitution reactions have been developed. LDP determinations on complexes 3-8 give the t following order of increasing donor ability CH2CMe2(Ph)Csp >Csp. As seen in NCr(NPr 2)2(X) where X = NPr 2, a steric limit exists where a one to one comparison between ligands breaks down when their bulk becomes 289 appreciably large. Since the LDP parameter is empirically derived from the kinetics of amide rotation any hindrance to this molecular motion will also be included. The above studies make i the assumption that the barrier to diisopropylamide bond rotation is a function of the Cr-NPr 2 bond order and that any steric hindrance (thus increase to that barrier) is negligible or consistent i for all the compounds. Though crystals of NCr(NPr 2)2(anth) suitable for diffraction have not yet been obtained, it is reasonable to assume a large steric bulk by either measure, Solid G or %Vbur. However, even if the sterically large 2-anthrycenyl hindered rotation this would serve to increase the LDP, artificially making the ligand look like a worse donor. This is the opposite of what is observed. Like was suggested for alkynyl ligands donation of ligand based π-electrons may account for this aberration warranting further study. 6.8 Experimental i Synthesis of NCr(NPr 2)2(CH2SiMe3) (4) In a 20 mL scintillation vial equipped with a stir bar i was loaded with NCr(NPr 2)2(OPh) (2) (0.050 g, 0.140 mmol, 1.0 equiv.) and 8 mL of pentane. This solution was placed into a liquid nitrogen cooled cold well to cool for 5 min. To this cold, stirring solution was added 0.139 mL of 1.0 M LiCH2SiMe3 dropwise. The solution was allowed to warm up to room temperature and stir for 1 h. The solution turned from orange-red to a yellow-brown color. The pentane solution was cooled and then filtered through Celite to remove LiOPh. The solution was concentrated in vacuo and placed in a freezer yielding yellow-orange 1 crystals (0.036 g, 0.10 mmol, 73%). H NMR (500 MHz, CDCl3, 1.67 ºC): 4.93 (sept, JHH = 6.0 Hz, 2H, CH(CH3)2), 3.52 (sept, JHH = 6.0 Hz, 2H, CH(CH3)2), 1.60 (d, JHH = 6.5 Hz, 6H, 290 CH(CH3)2), 1.41 (d, JHH = 6.0 Hz, 6H, CH(CH3)2), 1.14 (d, JHH = 6.5 Hz, 6H, CH(CH3)2), 1.12 (d, JHH = 6.5 Hz, 6H, CH(CH3)2), 0.056 (s, 9H, Si(CH3)3), -0.033 (s, 2H, CH2Si(CH3)3). 13 1 C{ H}NMR (125 MHz, CDCl3, 10 ºC): 56.1, 53.5, 30.8, 29.4, 29.0, 22.4, 20.1, 1.6. 29 Si NMR (119.16 MHz, CDCl3, 25 ºC): 2.078 (s, CH2Si(CH3)3). Mp: 90-92 ºC. i Synthesis of NCr(NPr 2)2(CH2C(Me)2Ph) (5) A 20 mL scintillation vial equipped with a stir bar was loaded with 1 (0.020 g, 0.10 mmol, 1 equiv.) and 2 mL of diethylether. This solution was placed into a liquid nitrogen cooled cold well to cool for 5 min. To this cold stirring solution was added a cold solution of Mg(CH2C(Me)2Ph)2 (0.011 g, 0.04 mmol, 0.7 equiv.) in 2 mL of diethylether dropwise. This was allowed to warm to room temperature and stir for 1.5 h. A white solid precipitated, and the solution turned yellow-brown. The solvent was removed in vacuo and the brown solid dissolved in hexane. The solution was cooled to –35 ºC and filtered through Celite to remove MgI2. The solution was concentrated in vacuo and placed in a freezer yielding yellow-orange crystals (0.019 g, 0.048 mmol, 95% yield). 1 H NMR (500 MHz, CDCl3, 25 ºC): 7.33 (d, JHH = 7.5 Hz, 2H, ortho), 7.13 (app t, JHH = 8.0 Hz, 2H, meta), 6.98 (app t, JHH = 7.0 Hz, 1H, para), 4.74 (sept, JHH = 6.5, 2H, CH(CH3)2), 3.40 (sept, JHH = 6.5 Hz, 2H, CH(CH3)2), 1.49 (s, 6H, C(CH3)2Ph), 1.47 (s, 2H, CH2C(CH3)2Ph), 1.45 (d, JHH = 6.0 Hz, 6H, CH(CH3)2), 1.32 (d, JHH = 6.0 Hz, 6H, CH(CH3)2), 1.05 (d, JHH = 6.0 Hz, 6H, CH(CH3)2), 1.01 (d, JHH = 6.0 Hz, 6H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, 25 ºC): 154.7, 127.7, 125.5, 124.5, 64.7, 55.8, 53.3, 39.6, 32.0, 31.0, 28.9, 22.7, 19.7. Mp: 71-73 ºC. 291 i Synthesis of NCr(NPr 2)2(CH2CMe3) (6) A 20 mL scintillation vial equipped with a stir bar i was loaded with NCr(NPr 2)2(OAd) (0.027g, 0.065 mmol, 1.0 equiv.) and 4 mL of pentane. This was placed into a liquid nitrogen cold well to cool for five min. To this cold stirring solution was added LiCH2CMe3 (0.005 g, 0.065 mmol, 1.0 equiv.) in 3 mL of cold ether dropwise. Over a period of 1 h a white solid precipitated. The solution was pumped dry, and the product was dissolved in pentane. The pentane solution was cooled to –35 ºC and filtered through Celite to remove LiOAd. The solution was concentrated in vacuo and stored in the freezer yielding 1 crystals of 6. H NMR (500 MHz, CDCl3, 2 ºC): 4.85 (sept, JHH = 6.5 Hz, 2H, CH(CH3)2), 3.49 (sept, JHH = 6.5 Hz, 2H, CH(CH3)2), 1.58 (d, JHH = 6.0 Hz, 6H, CH(CH3)2), 1.41 (d, JHH = 6.0 Hz, 6H, CH(CH3)2), 1.26 (s, 2H, CH2C(CH3)3), 1.14 (d, JHH = 6.5 Hz, 6H, CH(CH3)2), 1.11 (d, JHH = 6.0 Hz, 6H, CH(CH3)2), 1.09 (s, 9H, CH2C(CH3)3). i i Synthesis of NCr(NPr 2)2(CCSiPr 3) (7) A 20 mL scintillation vial equipped with a stir bar was loaded with 2 (0.050 g, 0.140 mmol, 1.0 equiv.) and 5 mL of hexane. This was placed in a liquid nitrogen cooled cold well to cool for 5 min. To this cold stirring solution was added a cold i solution of LiCCSiPr 3 (0.026 g, 0.140 mmol, 1.0 equiv.) in 5 mL of hexane. The reaction was allowed to warm up to room temperature and stir for 1.5 h. The solution was cooled to –35 ºC and filtered through Celite to remove LiOPh. This solution was concentrated in vacuo and then 1 placed in the freezer yielding orange crystals (0.052, 0.115 mmol, 83% yield). H NMR (500 MHz, CDCl3, –18 ºC): 5.05 (sept, JHH = 6.0 Hz, 2H, CH(CH3)2), 3.72 (sept, JHH = 6.5 Hz, 2H, 292 CH(CH3)2), 1.80 (d, JHH = 6.0 Hz, 6H, CH(CH3)2), 1.48 (d, JHH = 6.5 Hz, 6H, CH(CH3)2), 1.25 (d, JHH = 6.5 Hz, 6H, CH(CH3)2), 1.10 (d, JHH = 6.5 Hz, 6H, CH(CH3)2), 1.00 (br s, 21H, Si(CH(CH3)2)3. 13 1 C{ H} NMR (125 MHz, CDCl3, –20 ºC): 150.4, 119.0, 57.5, 55.8, 30.6, 30.4, 21.9, 21.4, 18.8, 11.4. 29 i Si NMR (119.16 MHz, CDCl3, 25 ºC): –6.108 (s, CCSi Pr3). Mp: 109- 110 ºC. i t Synthesis of NCr(NPr 2)2(CC Bu) (8) In a 20 mL scintillation vial equipped with a stir bar was loaded 2 (0.037 g, 0.102 mmol, 1.0 equiv.) and 7 mL of hexanes. This was placed in a liquid nitrogen filled cold well to cool for five min. To this cold stirring solution was added a cold t solution of LiCC Bu (0.009 g, 0.1 mmol, 1.0 equiv.) in 2 mL of ether. The reaction was allowed to warm to room temperature and stir for 1 h. Over this period, the solution turned from dark-red to light orange-red. Then the solvent was removed in vacuo. The product was dissolved in pentane, chilled, and filtered through Celite to remove LiOPh. The cold solution was concentrated and placed in a freezer at –35 ºC yielding bright red-orange crystals (0.035 g, 0.101 1 mmol, 98% yield). H NMR (500 MHz, CDCl3, 1 ºC): 5.07 (sept, JHH = 6.5 Hz, 2H, CH(CH3)2), 3.69 (sept, JHH = 6.0 Hz, 2H, CH(CH3)2), 1.80 (d, JHH = 6.5 Hz, 6H, CH(CH3)2), 1.46 (d, JHH = 6.5 Hz, 6H, CH(CH3)2), 1.25 (d, JHH = 6.5 Hz, 6H, CH(CH3)2), 1.17 (s, 9H, C(CH3)3), 1.10 (d, JHH = 6.0 Hz, 6H, CH(CH3)2). 13 1 C{ H} NMR (125 MHz, CDCl3, 25 ºC): 127.0, 114.7, 57.4, 55.6, 31.7, 30.6, 30.3, 28.9, 21.7, 21.1. Mp: 110-112 ºC. 293 i Synthesis of [NCr(NPr 2)2(C2Ph)]2 (9) Under an N2 atmosphere a 25 mL Erlynmeyer flask was loaded with a stir bar (phenylethynyl)lithium (0.073 g, 0.679 mmol, 2 equiv.) and THF (8 mL). The flask was cooled in a liquid nitrogen cooled cold well for 10 min. The flask was removed and stirred on a plate vigorously. To this ZnCl2 (0.046 g, 0.340 mmol, 1 equiv.) was added as a solid. The solution came to room temperature and stirred for 1h. To the stirring mixture was added 1 (0.134 g, 0.340 mmol, 1 equiv.) in THF (5 mL). The reaction stirred for 12 h. The volatiles we removed under reduced pressure. The residue was extracted with pentane until the extracts were clear. The combined extracts were filtered through a fritted funnel using Celite as a filtering agent. The filtrate was dried under reduced pressure and dissolved in minimal amounts of Et2O (~5 mL) held at −35 °C yielding small orange crystals of 9 in low yield. i Synthesis of LiCCSi( Pr)3 In a 20 mL scintillation vial equipped with a stir bar was loaded tri(isopropyl)silylacetylene (0.20 g, 1.1 mmol, 1 equiv.) and 8 mL of hexane. The solution was placed in a liquid nitrogen cooled cold well. To this cold stirring solution was added 1.6 M nbutyl lithium in hexanes (0.625 mL, 1.10 mmol, 1.0 equiv.) by syringe. The solution was allowed to warm to room temperature and stir for 1 h. The solvent was removed in vacuo leaving a sticky oil. This was used directly in the next reaction. (0.181 g, 0.9 mmol, 87.4% yield). This is a slight modification of the literature procedure. 27 t Synthesis of LiCC Bu In a 20 mL scintillation vial equipped with a stir bar was loaded 3,3dimethyl-1-butyne (0.100 g, 1.21 mmol, 1 equiv.) and 5 mL of hexane. This solution was cooled for 5 min in a liquid nitrogen cooled cold well. To this cold stirring solution was added 1.6 M nbutyl lithium in hexanes (0.760 mL, 1.21 mmol, 1.0 equiv.) and was allowed to warm to room 294 temperature and stir for 1 h. Over this period the solution turned cloudy and a white precipitate formed. The solvent was removed in vacuo leaving a white solid. (0.101 g, 1.1 mmol, 96% yield). The melting point matched the reported literature value. 295 28 REFERENCES 296 REFERENCES (1) Wierschke, S. G.; Chandrasekhar, J.; Jorgensen, W. L. 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ABSTRACT Synthesis and characterization of cationic Cr(VI) nitride complexes of the form i t [NCr(NPr 2)2(L)][A] where L = THF, DMAP, HMPA, qunicludine, BuNC, DBN, P(Me)2Ph, F NHC and A = I, OTf, BF4, PF6, SbF6, BPh4, and BAr 4 are reported. New synthetic i methodologies were developed to access [NCr(NPr 2)2(L)][A] compounds including i displacement of iodine from NCr(NPr 2)2I (1), halide abstraction using silver salts of weakly coordinating anions, substitution of L, and substitution of A. The neutral complex i NCr(NPr 2)2(PzB(Et3)) (10) bearing a mono pyrazolyl borate is also reported. The kinetics of 1 diisopropylamide rotation was measured via Spin Saturation Magnatization Transfer in the H NMR for each complex to assess the donor abilities of L. Collected into a single parameter, the Ligand Donor Parameter (LDP) of each are reported. In order to correct these LDP‘s for i differing anions between each complex a series of [NCr(NPr 2)2(DBN)][A] compounds were F made where A = I, OTf, BF4, PF6, SbF6, BPh4, and BAr 4, and a linear correction factor applied t giving the donor series PzB(Et3)>THF>NHC>DMAP> BuNC>DBN>HMPA>P(Me)2Ph>quin. i When NCr(NPr 2)2I (1) was treated with DMAP iodine was displaced into the outer sphere i producing [NCr(NPr 2)2(DMAP)][I] (4-I) in a temperature dependent equilibrium, favoring i cation formation at low temperature. The LDP was determined for [NCr(NPr 2)2(DMAP)][I] (4299 I) in CDCl3, CD3CN, and THF-d8 with LDP increasing with increasing dielectric constant of the solvent. The compounds i [NCr(NPr 2)2(DBN)][I] i [NCr(NPr 2)2(DMAP)][BF4] i (2-I), i [NCr(NPr 2)2(quin)][PF6] i (4-BF4), [NCr(NPr 2)2(NHC)][PF6] (3), (5), i [NCr(NPr 2)2(HMPA)][PF6] (6), and [NCr(NPr 2)2(THF)][PF6] (8) are structurally characterized via single crystal x-ray diffraction and give evidence for cationic charge delocalization onto the ligands. The steric profiles of 2-I, 3-6 and 8 were calculated through the methods of %Vbur and Solid G. Lastly, Cr-Nligand bond distances were discussed as a function of LDP for the X- and Ltype ligands bound through nitrogen. 7.1 Introduction In the development of transition metal chemistry formal oxidation state assignments have provided a solid foundation in describing metal-ligand bonding interactions. However, with increasing ligand complexity and ligands with multiple coordination modes, a need arose to classify covalent interactions toward metal ions based upon the origin of the electrons involved. Introduced by M. L. H. Green, the Covalent Bond Classification Method (CBC method) provided such a construct, dividing metal-ligand interactions into permutations of 3 categories, 1 X-type, L-type, and Z-type. − The L-type interaction is a dative interaction where the 2e forming the bond are provided from the ligand. Often these ligands are neutral molecules with the electrons originating from occupied orbitals such as a non-bonding lone pair (PR3 ,NH3, CO) or σ a interaction to 300 double bond (C2H4, side on N2, aromatic systems). X-type interactions are those involving − − singly occupied orbitals (radicals) where 1e is provided from the metal and 1e from the ligand (Cl, H, alkyls). Lastly, Z-type interactions are also defined as dative interactions with both electrons originating from the metal donating into an empty ligand based orbital of a Lewis acid (BF3, AlR3). Much work has, and will continue to go into expanding these qualitative concepts in a quantitative fashion. Systematic evaluations of ligand properties in the literature however, have been predominated of L-type ligands. Perhaps the most notable example of ligand parameterization followed an observation by Wilkinson and coworkers that successive substitutions in the R groups of the phosphine in (R3P)3Mo(CO)3 perturbs the bond orders in the Mo-CO moiety, which can be quantified by an increase in the IR stretching frequency of the carbonyl. 2 In his seminal work Chadwick Tolman profiled both the steric and electronic properties of commonly used phosphines. This exhaustive study was predicated upon the previously mentioned observation that perturbations in the symmetric IR stretch of a carbonyl in Ni(CO)3L varied regularly with the nature of L. This construct allowed for the unprecedented ordering of L-type ligands used in catalytic systems based on their donor strength toward late transition metals in a quantitative way. Tolman‘s work also addressed the issue of ligand sterics within the aforementioned series. To quantitate the size of the various L groups the concept of ligand cone angle was developed. In this formalism a cone whose vertex lies at the metal center is 301 circumscribed around the ligand. The cone angle is thus described as the angle of the smallest cone which fully encapsulates the ligand. While incredibly useful for its intended purpose, Tolman‘s assessment focused exclusively on L-type ligands evaluated on low valent Pt group metals, largely due to their ease of synthesis and the rapid data collection afforded by IR. This methodology limited the platform for electronic parameterization to L-type ligands, and moreover only phosphines in the initial studies. Additionally, the concept of cone angle was also designed around phosphine ligands. As more elaborate and highly asymmetrical ligands have been developed, the metric of cone angle has struggled to adequately describe their ‗size‘ in one parameter. More recently there has been electronic evaluations of N-heterocyclic carbene ligands (NHC‘s) by the groups of Crabtree 3,4 5 and Nolan using a (NHC)Ir(CO)2Cl system. Although the Ir(I) system shows good linear correlation to Tolman‘s original work this system still relies on the installation of CO ligands as a reporting unit. Furthermore, to correlate to ―Tolman‘s electronic parameter‖ (TEP) based off the A1stretching mode of CO, the average of the 2 CO stretching frequencies of (NHC)Ir(CO)2Cl are used, as the synthetic route to complexes of the type do not produce C2 symmetric systems. DFT investigations by Gusev comparing both NiCO3L and various Ir(I) systems suggest that the average stretching of (L)Ir(CO)2X complexes does not linearly correlate to the average CO bond distance, thus may not be an appropriate measure of metal to ligand back-bonding and the overall electronic ‗richness‘ of the metal. 6 Furthermore, although metrics such as TEP may be meaningful within a series, direct comparisons to other series (i.e. phosphines to NR3 ligands or NHC‘s) may breakdown. 302 Similar ligand parameterizations have been conducted by experimental methods beyond IR stretching frequencies, including electrochemical 7,8 and electronic spectrum analysis. Notably, a 1996 report by Anderson et. al. evaluated a series of π-donor, monodentate X-type 9 1 ligands using the Cp*2TiX platform. This departure toward early d metals employed EPR and absorption spectroscopy, circumventing the need to install a ‗reporting group‘. Powerful in their own right, when examining these disparate methods in the context of each other, inter system comparisons become tenuous at best and indiscernible at worst. Often an insufficient amount of ligands overlap between series. Furthermore, the utility of computational methods of parameterization are dependent on the extent they describe real-world phenomena. Empirical data is essential to their development and is not always obtainable for the ligand in question. Recently the steric and electronic profiles of a wide set of monodentate X-type ligands were experimentally evaluated using a Cr(VI) nitride system. 10 Compounds of the form i NCr(NPr 2)2X were synthesized in good yields, and the rotation kinetics of the diisopropylamide group were used to probe the ligand donor ability (LDP) of the X ligand via the method of Spin 1 Saturation Magnetization Transfer (SST) in the H NMR. 11-12 i Analogous Cr(VI) cations have been synthesized of the form [NCr(NPr 2)2L][A] where L − is a 2e donor and A is an anion. Herein we report the LDP and steric profiles of several L-type ligands featuring O, P, N and C donor atoms using the method of SST as before. This provides a quantitative series that spans both X- and L-type ligands reported as a single parameter (LDP), 303 facilitating direct comparison along the continuum. Furthermore, a general procedure for future ligand parameterization studies has been delineated using facile data collection via readily 1 available H NMR spectroscopy. The chromium nitride platform employed proves to be highly amenable to a wide variety of reagents and synthetic strategies. Beyond the scope of this work we hope the model will facilitate ligand parameterization for other researchers who are curious about their own specialized monodentate ligand, be it X- or L-type. 7.2 Synthesis of Chromium Starting Materials − i Access into complexes of the form [NCr(NPr 2)2L][A] where L is a 2e donor and A is a weakly-coordinating anion can be achieved through various synthetic routes. Using the method i of Bradley and coworkers, Cr(NPr 2)3 can be obtained in high yield using anhydrous CrCl3 and i 13 LiNPr 2. t Chiu and coworkers published a one-pot synthesis of NCr(O Bu)3 starting from chromyl chloride. 14 i t A stoichiometric mixture of Cr(NPr 2)3 and NCr(O Bu)3 results in complete i N-atom transfer affording NCr(NPr 2)3 in high yield. i 15 i Subsequent treatment of NCr(NPr 2)3 by 2,6-lutidenium iodide in chloroform gives NCr(NPr 2)2I (1) as reported by Cummins. 16 7.3 Synthesis of Cr(VI) Cations i Synthesis of [NCr(NPr 2)2L][A] was accomplished through a variety of techniques and will be addressed individually. In general Cr(VI) cations seem more sensitive to temperature, solvent, and reagents than their Cr(VI) nitride counterparts. However, in situ synthesis from 304 known precursors proved to be adequate for the SST experiment, allowing electronic evaluation of in-isolatable species. Synthesis by displacement of an X-type ligand i While typical syntheses of NCr(NPr 2)2X complexes were performed in non-polar (toluene) or weakly polar (Et2O) solvents, preliminary experiments using largely insoluble sodium or silver salts required the use of THF and DME. Analysis of these reaction mixtures showed a trace amount of a then unidentified chromium bisdiisopropylamide compound. - Subsequent investigations revealed that strongly coordinating solvents could act as 2e donors i and displace the iodide of NCr(NPr 2)2I (1) into the outer sphere. When a stoichiometric amount i of DBN was added to NCr(NPr 2)2I (1) in Et2O an orange precipitate rapidly formed, found to i be [NCr(NPr 2)2DBN][I] 2-I when isolated in 82% yield (Figure 7.1). i Figure 7.1: Synthesis of [NCr(NPr 2)2DBN][I] 2-I. 305 Curiously, the same reaction ran in THF gave a mixture of 1 and 2-I when evaporated to dryness, and using pentane as a solvent failed to give more than a trace amount of the product. This suggests that L-type for X-type ligand substitution in these systems is reversible as well as solvent dependent. Synthesis by halide abstraction While the synthesis of 2-I was straightforward other complexes of the form i [NCr(NPr 2)2L][A] with other L-type ligands were not satisfactorily generated by halide displacement. It is likely that some of the L ligands chosen for study are either not competent to initially displace the iodide of 1, or that the product generated is unstable toward the back reaction. As such replacement of an outer sphere iodide with a counter ion traditionally viewed 17 as ‗non-coordinating‘ was sought to alleviate this restriction. Treatment of 1 with 1 equiv. of 4-Dimethylaminopyridine (DMAP) in chloroform followed by the addition of 1.1 equiv. of i AgBF4 in acetonitrile produced [NCr(NPr 2)2DMAP][ BF4] 4 in 58% yield after crystallization from a concentrated toluene solution held at −35 °C (Figure 7.2). i Figure 7.2: Synthesis of [NCr(NPr 2)2DMAP][ BF4] 4 using silver tetrafluoroborate. 306 Attempted syntheses with other L-type ligands using AgBF4 and similar conditions resulted largely in decomposition into insoluble products. Synthesis where L = quinuclidine (quin) was successful by switching counter ions, using silver hexafluorophosphate for halide abstraction (Figure 7.3). 307 Figure 7.3: Synthesis of compounds 3 and 6-8 using AgPF6 to abstract a halide from 1. i Compounds of the form [NCr(NPr 2)2L][A] where L = hexamethylphosphoramide (HMPA) 6 and THF 8 we also accessible using AgPF6 (Figure 7.3). While reaction of 1 with 308 P(Me)2Ph followed by treatment with AgPF6 did give the desired product, treatment with 1 i equiv. of AgBPh4 resulted in a much cleaner reaction yielding [NCr(NPr 2)2P(Me)2Ph][BPh4] (7). Greater yields of 6 and 7 were obtained by using an excess of the desired ligand, as the Ag(I) ion scavenged some of the L reactant in solution. Unlike the quinuclidine in 3 or the DMAP in 4 residual HMPA or P(Me)2Ph was readily removed by evaporation or lyophilzation i with the reaction solvent. Synthesis of [NCr(NPr 2)2THF][PF6] 8 was also attempted using 10 equiv. of THF and with THF as a solvent. Although product was formed, compound 8 is able to polymerize any excess THF giving poor yields and complicating purification. For completeness of our series a representative N-heterocyclic carbine ligand was sought. 1,3-dimethylimidizoium iodide was obtained by treatment of N-methylimidazole with MeI. 18 Generation of persistent (Arduengo) carbenes are typically obtained from such precursors t 19,20,21 by in situ deprotonation using a strong base such as KO Bu. Treatment of N,N- t dimethylimidizoium iodide with KO Bu followed successively by the addition of 1 and silver i t salts failed to give the desired product, yielding NCr(NPr 2)2O Bu as the only tractable product. Instead, deprotonation of 1,3-dimethylimidizoium iodide by 1 equiv. benzyl potassium gave only toluene as a byproduct. After 2 h the carbene in toluene solution was filtered through celite in a fritted funnel to remove KI and any unreacted benzyl potassium. Addition of the filtrate to 1 i followed by the addition of 1.1 equiv. AgPF6 gave [NCr(NPr 2)2NHC][PF6] 5 in 40% isolated yield (Figure 7.4). 309 Figure 7.4: In situ generation of a persistent carbene and its reaction with 1 to make i [NCr(NPr 2)2NHC][PF6] 5 . Synthesis by substitution of A Although many L-type ligands form stable complexes with the PF6 anion suitable for i evaluation, the target [NCr(NPr 2)2DMAP][PF6] could not be obtained by halide abstraction or by ligand exchange. Additionally, the effect of the counter ion upon LDP was unknown. Thus a 310 i series of [NCr(NPr 2)2L][A] complexes was synthesized where L is held constant and A is a variety of weakly-coordinating anions. For this study the L chosen was DBN, as it readily i produces 2-I where A is an outer sphere iodide. The [NCr(NPr 2)2DBN] cation also forms stable complexes with BF4, PF6, SbF6, and BPh4 by treating 2-I with 1 equiv. of the corresponding sodium salt (Figure 7.5). Figure 7.5: Counter ion substitution of 2-I using sodium salts i The compound [NCr(NPr 2)2DBN][OTf] (2-OTf) was synthesized by treatment of 2-I with excess NBu4OTf in toluene. This represents a unique example of Cr(VI) cation, as the DBN ligand is not displaced by triflate in solution to produce the previously described i NCr(NPr 2)2OTf (Figure 7.6). 311 Figure 7.6: Synthesis of 2-OTf by anion exchange with an ammonium salt. Lastly, treatment of 2-I with F 22 trifluoromethyl)phenyl]borate (Ag(BAr 4)) i F one equiv. of silver tetrakis[(3,5- in toluene afforded the highly lipophilic F [NCr(NPr 2)2DBN][BAr 4] 2-BAr 4 (Figure 7.7). 312 F F Figure 7.7: Synthesis of 2- BAr 4 by anion exchange with an Ag(BAr 4. Synthesis by substitution of L In the generation of the series of Cr(VI) cation with various weakly-coordinating counterions it was noticed that some ligand-counterion combinations are inaccessible by the i treatment of NCr(NPr 2)2I (1) with the appropriate silver slat in the presence of the L-type ligand. Attempts at synthesis of these molecules by directly substituting a coordinated L with another (stronger) L-type ligand gave only trace amounts of the desired product, and quickly 313 decomposed upon isolation. This suggests that these combinations are kinetically accessible but are thermodynamically unstable complexes (Figure 7.8). Figure 7.8: Displacement of a weaker donating ligand (LW) by a stronger neutral donor (LS). One such exchange reaction however, yielded the desired product in situ. Treatment of t 2-I in with an excess of tert-butlyisonitrile ( BuNC) in chloroform afforded a mixture of i i t [NCr(NPr 2)2DBN][I] 2-I and a complex presumed to be [NCr(NPr 2)2( BuNC)][I] (9) as 1 determined by H NMR. i t Figure 7.9: In situ synthesis of [NCr(NPr 2)2( BuNC)][I] (9) by ligand substitution. 314 Further analysis via FT-IR on this this mixture showed the CN stretch of both unbound -1 -1 t (2140.6 cm ) and coordinated (2254.3 cm ) BuNC. Isoelectronic to the CO ligand, a reduction of the CN stretching frequency is typically observed upon isonitrile coordination to transition metals due to filled d-orbital donation into ligand π*. In 9 however the increase observed is likely due to the removal of electron density from a ligand based σ* orbital to form the Cr-C bond, along with the (formal) lack of d-electrons which typically back-donate to populate the ligand π* orbital. 23 Thus far attempts to isolate 9 have been unsuccessful, leading to insoluble i decomposition products and residual NCr(NPr 2)2I (1). When isolated coordinated nitrile and isonitrile complexes like 9 may be invaluable to the further development of this model, providing an independent metric of metal center electronics via their IR stretching frequencies. 7.4 Synthesis of Zwitterionic Cr Due to their widespread use in transition metal chemistry, a model for the trispyrazolylborate (Tp) ligand was sought. A monodentate pyrazolylborate was chosen for consistency‘s sake for 24 triethylpyrazolylborate evaluation on the chromium nitride i platform. was added to 2-I in THF to afford NCr(NPr 2)2(PzB(Et3)) (10). 315 Sodium i Figure 7.10: Synthesis of NCr(NPr 2)2(PzB(Et3)) (10) with resonance structures. While Tp and similar pyrazolyl borate ligands are formally regarded as a (1−) anion 25 Zwitterionic resonance structures of 10 exist, placing a cationic charge on chromium with a dative bond to a trivalent pyrazolyl nitrogen. 7.5 LDP determination of Cr(VI) Cations i With a fairly comprehensive series of [NCr(NPr 2)2L][A] complexes where L is bound through an N, O, C or P atom, the kinetics of diisopropylamide inchange were used quantify the electronic ligand donation to the metal center in 2-10 via the method of Spin Saturation Magnetization Transfer (SST) 11,12,26 1 in the H NMR as outlined in Chapter 4. Assuming an 316 ǂ i 9 entropy for the rotation of ∆S of −9 cal/mol K as determined for NCr(NPr 2)2X complexes the ǂ i Cr-(NPr 2) bond enthalpy denoted ∆H was calculated for 2-10, and presented in Table 7.1 as the Ligand Donation Parameter (LDP). Unlike the previously reported X-type ligands which were measured at room temperature down to −48 °C, SST measurements on the series of neutral donors generally required temperatures above room temperature (up to +47 °C) to obtain satisfactory results. This large temperature range of −48 °C to +47 °C was predicated on the molecular dynamics of the i NCr(NPr 2)2 moiety with each specific ligand. As donor ability of X or L decreases bond order i between Cr and NPr 2 increases slowing rotation and the chemical exchange of the methine protons. Since accurate measurement of the kinetics of site exchange depends largely on the 1 difference in the H NMR integration between the resonances of the two sites, a balance is needed. If the site exchange is too fast both sites are saturated or can give a signal obscured by the signal to noise of the baseline. If exchange is too slow the error associated with the integration is as large as the measurement itself. A judicious choice of temperature for accurate results can be estimated by a comparison to the other ligands already in the series. Thus the constraint of temperature let us determine a priori that the series of L-type ligands were far worse donors than the anions previously tested. Unfortunately, this also sets a technical limit on the L-type ligands we are able to evaluate. To obtain adequate site exchange for the poorest donating of all the ligands thus far i studied, the sample of [NCr(NPr 2)2quin][PF6] (3) required heating to near the boiling point of 317 the NMR solvent (CDCl3). This precludes SST measurement on any ligand more poorly donating than quinuclidine unless a higher boiling solvent (and a solvent correction) is used. LDP (kcal/mol) i t [NCr(NPr 2)2 BuNC][I] (9) i [NCr(NPr 2)2HMPA][PF6] (6) 16.08±0.30 15.85±0.30 i [NCr(NPr 2)2DMAP][BF4] (4) 15.70±0.30 15.37±0.30 i [NCr(NPr 2)2NHC][PF6] (5) 15.61±0.30 15.37±0.30 i [NCr(NPr 2)2THF][PF6] (8) 16.87±0.30 16.55±0.32 i [NCr(NPr 2)2P(Me)2Ph][BPh4] (7) 16.95±0.32 i [NCr(NPr 2)2quin][PF6] (3) a - 15.28±0.30 i NCr(NPr 2)2(PzB(Et3)) (10) Corrected LDP (kcal/mol) 14.35±0.30 Compound 18.67±0.32 16.88±0.32 17.33±0.32 19.01±0.32 a See Table 7.2 for a list of corrective factors added to each LDP based upon anion Table 7.1: Corrected and uncorrected Ligand Donor Parameters of 3-10. Suitably, the series of L-type ligands had much higher barriers to diisopropylamide rotation than the majority of X-type ligands previously tested. In fact most were worse donors i than the poorest anions in the NCr(NPr 2)2X system where X = iodide (15.80±0.30 kcal/mol), triflate (15.65±0.29 kcal/mol) and bromide(15.45±0.30 kcal/mol). 318 i Interestingly, NCr(NPr 2)2(PzB(Et3)) (10) with an LDP of 14.35±0.30 kcal/mol, acted characteristically of a poorly donating anion, much like OC6F5 (14.32±0.28 kcal/mol) or the electron deficient pyrrole Pyr C6H3(CF3)2 (14.36. ±0.28 kcal/mol). As such, the resonance form which places cationic charge β to the coordinated atom (i.e. an electron deficiency on at the 2position nitrogen of the pyrazole ring) may be the dominate resonance form leading to poor electronic donation from a anionic ligand. 7.6 Counter Ion Effect and Correction The series of LDP presented in Table 7.1 however was compiled from SST measurements on complexes with varying anions as well as ligands. Though they are all poorer donors than the anions tested, the ordering within the series appears problematic. Suspiciously compounds 4 and 5 both have LDP‘s of 15.37±0.30 kcal/mol. This seems counterintuitive that an NHC would be as strongly donating as a heterocyclic amine in DMAP. Thus a correction was sought to normalize these numbers, and allow for direct comparison in spite of the inability to synthesize some ligand-anion pairs. To this end a series of complexes all bearing the DBN ligand was made using a variety of weakly coordinating anions. Presented in Table 7.3 the LDP‘s of each complex (2-A) was F measured as above. Upon inspection the experimental LDP of (2-BAr 4) is more than 1 kcal/mol greater than (2-I) at 16.44±0.30 and 15.42±0.30 kcal/mol respectively. This energy difference is approximately the difference in the LDP‘s of OC6F5 and OSiPh3 or F and OPh anionic ligands. Although a small numerical difference, an energy of this magnitude in LDP can translate to largely different chemical properties and patterns of reactivity when the ligand is 319 used in catalytic processes. For example studies by Swartz et. al. showed that the Ti(IV) catalyzed hydroamination of 1-phenylpropyne with aniline is over 3.5 times faster when using a 27, 28 3-substitued bispyrrolide ligand (Table 7.2). As previously reported this specific 3- subsitituted pyrrolide has an LDP just 0.2 kcal/mol greater than unsubstituted pyrrolyl (although they are ‗indistinguishable‘ within experimental error). 320 Hydroamination Catalyst Comparable Ligand LDP (Kcal/mol) 866±94 NMe2 9.34±0.32 1976±130 Pyr 14.16±0.28 Kobs(× 10 -7 -1 s ) C6H3(CF3)2 Pyr 6963±582 14.36±0.28 Table 7.2: Evaluation of Ti(IV) Hydroamination Catalysts and their corresponding ligands evaluated by the LDP method. Within the LDP‘s of 2-A complexes a general trend immerges as LDP increases with larger (less-coordinating) cations, implying closer bound cation-anion pairs reduce the barrier to amide rotation in complexes of the same L. In other words, as charge separation increases Crdiisopropylamide bond order increases, as cationic charge builds at the metal center. In exploring this phenomenon the average radius of each anion was tabulated from a listing of all the structures deposited in the Cambridge Structure Database 29 containing an uncoordinated anion of interest, as at present only 2-I has been suitable to structurally characterize. Bond length and angle data was used in conjunction with the atomic radii reported by Bondi 321 30 to calculate the radius of the smallest sphere possible which completely encloses the anion. This approximation is tabulated in Table 7.2 and graphically presented below. 17.0 16.8 16.6 PF6 LDP (kcal/mol) 16.4 SbF6 16.2 BArF4 16.0 15.8 BPh4 15.6 OTf 15.4 BF4 15.2 I 15.0 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 Calculated Radius of A (Å) i Figure 7.11: LDP‘s of [NCr(NPr 2)2DBN][A] (2-A) complexes versus the approximated anion size. Figure 7.11 shows a general approach toward an asymptote in the LDP of i [NCr(NPr 2)2DBN][A] (2-A) complexes. Thus to quantify the anion perturbation upon LDP a i F F linear correction was applied using [NCr(NPr 2)2DBN][BAr 4] (2-BAr 4) as an endpoint, assuming total dissociation between the anion and the chromium nitride cation. By subtracting i F the LDP‘s of each [NCr(NPr 2)2DBN][A] complex the from that of 2-BAr 4 an corrective factor was calculated for each anion as given in Table 7.3. 322 LDP (kcal/mol) i [NCr(NPr 2)2DBN][I] (2-I) i [NCr(NPr 2)2DBN][BF4] (2-BF4) i [NCr(NPr 2)2DBN][OTf] (2-OTf) i [NCr(NPr 2)2DBN][SbF6] (2-SbF6) i [NCr(NPr 2)2DBN][BPh4] (2-BPh4) i [NCr(NPr 2)2DBN][PF6] (2-PF6) i F F [NCr(NPr 2)2DBN][BAr 4] (2-BAr 4) Corrective Factor (kcal/mol) Calculated Anion Radius (Å) 15.42±0.30 Compound 1.025 2.32 15.73±0.30 0.718 3.08 15.83±0.30 0.609 3.60 15.87±0.30 0.577 3.57 16.06±0.30 0.387 6.73 16.11±0.31 0.334 3.29 16.44±0.30 - 7.93 i Table 7.3: Ligand Donor Parameters for [NCr(NPr 2)2DBN][A] (2-A) complexes with anion corrective factors and calculated anion radii. These factors are then added to the experimentally determined LDP of the various L-type ligands based upon the anion employed. In this way we can project what the LDP of each i F [NCr(NPr 2)2L][A] complex would be if anion A were the BAr 4 anion. This works well to a first approximation, as the corrected LDP places DMAP as a worse donor than the NHC. Barring the strongly donating NHC and THF this correction places all the L-type ligands worse than the tested X-type ligands. Furthermore, this system places the strongest donor as NHC (5) and the poorest as a tertiary amine (3). 323 7.7 In situ generation, temperature dependent equilibrium, and solvent effect on LDP. Although the counter anion correction is appropriate for comparison and implementation of our system, information about the system can be gleaned from the synergistic effect of ligand and anion. With an (uncorrected) LDP of 15.42±0.30 kcal/mol it is self-consistent that DBN can displace iodine (15.80±0.30 kcal/mol) to form thermodynamically stable 2-I. i In a set of related studies NCr(NPr 2)2I (1) was treated with excess DMAP in an NMR i tube in CDCl3. Although the corrected LDP for DMAP based on [NCr(NPr 2)2DMAP][BF4] (4BF4) is 16.08±0.30 kcal/mol, the DMAP ligand is competent to displace the iodine into the outer i i sphere creating a mixture of [NCr(NPr 2)2DMAP][I] (4-I) and NCr(NPr 2)2I + DMAP based on i temperature (vida infra). Separately a sample of [NCr(NPr 2)2DMAP][BF4] (4-BF4) was treated with 1 equiv. of AgOTf in toluene. This gave a complex mixture which contained unreacted i i i [NCr(NPr 2)2DMAP][BF4] (4-BF4), [NCr(NPr 2)2DMAP][OTf] (4-OTf), and NCr(NPr 2)2OTf in a roughly 1:2:6 ratio, along trace amounts of NCr(DMAP)4OTf and other unidentified side products. Together these experiments show that in our system DMAP with an uncorrected LDP of 15.37±0.30 kcal/mol can displace an iodide ligand (15.80±0.30 kcal/mol) but resists displacement by triflate anions (15.75±0.29 kcal/mol) forming an equilibrium mixture between i i [NCr(NPr 2)2DMAP][OTf] (4-OTf) and NCr(NPr 2)2OTf (Figure 7.12). Although triflate and iodide are indistinguishable within error, the pattern of reactivity with DMAP may show a manifestation of the 0.05 kcal/mol discrepancy in their LDP parameters. 324 i Figure 7.12: Reaction of [NCr(NPr 2)2DMAP][BF4] (4-BF4) with silver triflate i i In situ generation of [NCr(NPr 2)2DMAP][I] (4-I) by treating NCr(NPr 2)2I (1) with 1 equiv. DMAP in CDCl3 was monitored via i i 1 H NMR. This gave a mixture of [NCr(NPr 2)2DMAP][I] and NCr(NPr 2)2I (1) in a ratio based upon the solution‘s temperature (Figure 7.12). 325 i i Figure 7.13: NCr(NPr 2)2I (1) and [NCr(NPr 2)2DMAP][I] (4-I) in a temperature dependent equilibrium. Data was collected from −41 °C to +27 °C in CDCl3 and −30 °C to +20 °C in actetonitrile-d3. In both instances this equilibrium mixture showed no memory, as the temperature was cycled back between temperatures giving the same results (Table 7.4). In both solvents the equilibrium shifts favoring the cation at low temperature. This behavior may be linked to the solvents dielectric constant as the Keq(CD3CN)>Keq(CDCl3) at any given temperature. Furthermore, as temperature increases the dielectric constant of both solvents decrease, with acetonitrile going from 39.7-39.1 on the range 5-60 °C and chloroform going from 6.5-3.7 on the range of −61.5-100 °C. 31,32 In testing this hypothesis temperature dependent i equilibrium data was sought in THF-d8. Although this caused [NCr(NPr 2)2DMAP][I] (4-I) 1 formation, H NMR spectroscopy at 25 °C showed no 1 present. Inspection of the NMR tube showed 4-I precipitating from solution, precluding the accurate measurement of an equilibrium constant. Other solvents resulted in either decomposition of 1 (DMSO) or the inability to support Cr(VI) cation formation (toluene). 326 Acetonitrile-d3 Temperature (°C) Chloroform-d1 Keq(CD3CN) Temperature (°C) Keq(CDCl3) −30.4 0.243 −41.1 0.236 −14.5 0.173 −30.7 0.134 −3.7 0.109 −4.2 0.068 7.8 0.015 10.8 0.029 20.3 0.005 14.9 0.004 - - 27.1 0.001 i Table 7.4: Temperature based equilibrium of NCr(NPr 2)2I (1) and i [NCr(NPr 2)2DMAP][I] (4-I) in various solvents. From a synthetic standpoint the balance of solubility versus solvent polarity seems general to the other Cr(VI) cations with L-type ligands. Synthesis (and purification) of i [NCr(NPr 2)2DBN][I] (2-I) by precipitation from a mixture of 1 with DBN in Et2O proceeded smoothly, although incompletely. While changing the solvent to THF produced more 2-I the increased solubility of 2-I in THF prohibited precipitation making isolation from 1 difficult. Conversely, in pentane complex 1 was very soluble whereas 2-I is completely insoluble but would not form in such low polarity conditions. Although solubility issues precluded the determination of an equilibrium constant of the i in situ generated [NCr(NPr 2)2DMAP][I] (4-I) in THF-d8, the LDP was measured giving a 327 barrier to rotation of 15.77±0.30 kcal/mol. When compared to the LDP of i [NCr(NPr 2)2DMAP][I] (4-I) in acetonitrile-d3 of 15.98±0.28 kcal/mol the DMAP ligand appears to be a stronger donor in THF. Attempts to obtain an LDP for (4-I) in CDCl3, unfortunately were unsuccessful as a suitable temperature is not available where both enough of the compound is generated in the equilibrium and the kinetics of diisopropylamide rotation are fast enough to measure. However, if we use the corrected LDP for DMAP (ostensibly what we i F would observe for the compound [NCr(NPr 2)2DMAP][BAr 4]) and subtract the corrective i factor for the iodide anion a reasonable estimate for [NCr(NPr 2)2DMAP][I] (4-I) in CDCl3 can be made at 15.07 kcal/mol. These LDP figures track well with the dielectric constants of the three solvents (Table 7.5). Solvent LDP (kcal/mol) Dielectric Constant 15.98±0.28 37.5 Tetrahydrofuran 15.77±0.30 7.58 Chloroform 15.07 a Acetonitrile b 4.81 a Dielectric constants are given at 25 °C. b Calculated from anion corrections. i Table 7.5: LDP‘s of [NCr(NPr 2)2DMAP][I] (4-I) in each solvent along with their dielectic constants. Such a large solvent dependence of LDP is unique to the cationic complexes with neutral donors, as little to no solvent dependence was observed when evaluating anionic ligands. A 328 i t previous SST measurement of NCr(NPr 2)2OBu F6 by the Cummins group placed the free ǂ 16 energy of diisopropylamide rotation at ∆G = 16 kcal/mol at room temperature in C6D6. ǂ Assuming the same entropy term of ∆S = −9 cal/mol K this translates to an LDP of 13.54 kcal/mol versus an LDP of 13.89±0.26 kcal/mol when evaluated in CDCl3. This is likely within error but still follows the observed trend as benzene‘s dielectric constant is 2.27 at 25 °C. Further work in this area is needed to fully understand and correct for the solvents effect upon LDP. 7.8 Structure of Cr(VI) Cations In addressing the steric profiles of the L-type ligands single crystal X-ray structures of 2I, 3-6 and 8 were obtained via the slow evaporation of concentrated solutions of each at −35 °C. Generally the chromium nitride cationic complexes do not crystalize as well as the neutral i NCr(NPr 2)2X complexes previously investigated. Chromium-Nnitrido bond distances for the Cr(VI) cations range from 1.533(2) Å to 1.546(2) Å and show no significant difference from the i i Cr-Nnitrido NCr(NPr 2)2X complexes. Similarly, the average Cr-NPr 2 bond distance was 1.813(3) Å in the Cr(VI) cations, close to the average bond distance observed in the neutral complexes. As before no correlation between the Cr-Nnitrdo bond distance or the average Cri NPr 2 bond distance and the LDP value was observed. Several structural features of these complexes are worth noting and will be discussed individually below. 329 i Figure 7.14: Crystal structure rendering of [NCr(NPr 2)2DBN][I] (2-I) with thermal probabilities at 50% and H atoms omitted for clarity. Bond lengths within the DBN ligand are shown in the inset in Å (blue). i The complex [NCr(NPr 2)2DBN][I] (2-I) represents the only Cr(VI) cation of this type to be isolated and structurally characterized with an iodine counter ion. The Cr-N3 bond distance of 1.973(2) Å is markedly short for a formally dative bond, approaching that of the analogous i C6H3(CF3)2 Cr-N bond distance of 1.966(4) Å observed in NCr(NPr 2)22Pyr . Comparison to the 33 structurally characterized 9-hydroxymethyl-1,5-Diazabicyclo[4.3.0]non-5-ene show that the N4-C41 bond in 2-I is lengthened while the N5-C41 bond has shortened from the corresponding bond lengths (1.279 Å and 1.343 Å respectively). Likewise the analogous N4-C41 bond length in DBN∙HCl (protonated at the 1 position of DBN i.e. N4) is 1.377 Å, with the N5-C41 bond shortened to 1.269 Å. These lengths suggest an intermediary between the two structures and provide evidence of a resonance structure in 2-I that places cationic charge on N5 (Figure 7.15). 330 Additionally the 13 C NMR of 2-I shows a downfield shift of the imine carbon resonance when compared to free DBN (169.30 ppm and 160.16 ppm respectively) suggesting a deshielding of this position. Further study of the 13 C NMR of the DBN ligand may be instructive among the entire series of 2-A complexes as it may correlate to the observed LDP‘s. Figure 7.15: Resonance structures of 2-I, 4-BF4, and 6. Weakly coordinating anion omitted for clarity. 331 i Figure 7.16: Crystal structure renderings for the cations of [NCr(NPr 2)2DMAP][BF4] (4-BF4) i (left) and [NCr(NPr 2)2quin][PF6] (3) (right). Atom positions at 50% probability. H atoms and a solvent of crystallization (toluene) removed in the case of 4-BF4. 2 Although both are bound to chromium through a sp nitrogen, compound 4-BF4 has a significantly longer Cr-N4 bond length to that of 2-I at 2.001(2) Å. Some bond localization within the 6-membered ring of DMAP is observed with slightly shorter C41-C42 (1.359(4)Å) and C45-C44 (1.354(4)Å) bonds compared to 1.381(3)Å and 1.375(4)Å in free DMAP. 34 Additionally, the C41-N4 (1.359(3)Å) and C45-N4 (1.357(3)Å) distances are lengthened from those of unbound DMAP at 1.337(3)Å and 1.335(4)Å). Bond contraction is also observed in the C43-N5 bond length of 4-BF4 at 1.342(3)Å versus the 1.367(3)Å for the corresponding bond in DMAP. These bond lengths are similar to those in 4-Dimethylaminopyridine hydrochloride 332 dehydrate, with localized bonding with the ring and a contracted C-NMe2 bond length corresponding to C43-N5 of 1.340(3) Å. 35 Compound 4-BF4 maintains co-planarity of the 6 membered ring and the NMe2 group with a dihedral angle defined by C42-C43-N5-C51 of 2.46°. 1 H NMR of 4-BF4 shows a single resonance for the N-methyl groups of DMAP at 0 °C (the lowest temperature obtained), as does the related 4-I down to −40 °C suggesting a low barrier to rotation about the Cr1-N4 bond. Like 2-I it is likely that the cationic charge of 4-BF4 is delocalized onto the ligand (Figure 7.15) affording stability to the molecule. i Unlike 2-I and 4-BF4, [NCr(NPr 2)2quin][PF6] (3) exhibits a crystallographic mirror plane along the N1-Cr1-N3 plane. The Cr-N3 bond distance in 3 of 2.068(4) Å is statistically shorter than the other chromium sp 2 nitrogen bonds evaluated. Though electronegativity 3 increases with increasing s-character, the less electronegative sp nitrogen of 3 is the weakest donating of all N bound ligands tested. The hybridization of nitrogen does not appear to be a predominate factor in the LDP‘s or bond lengths to each ligand. Moreover, LDP and bond lengths within 2-I, 3, and 4-BF4 do not correlate to pKa values for the conjugate acids of DBN, quinicludine, and DMAP. As was noted in Chapter 4, metal-ligand bond lengths are not i i predictive of LDP‘s in the NCr(NPr 2)2X and [NCr(NPr 2)2L][A] systems, reiterating the need for multifaceted approaches toward ligand evaluation . 333 i Figure 7.17: Crystal structure renderings of the cation of [NCr(NPr 2)2(HMPA)][PF6] 6 (left) i and [NCr(NPr 2)2(THF)][PF6] 8 (right). Atom positions at 50% probability and H atoms removed for clarity. The oxygen bound ligands in i [NCr(NPr 2)2(HMPA)][PF6] 6 and i [NCr(NPr 2)2(THF)][PF6] 8 contain especially short Cr1-O1 bond lengths bond at 1.888(4) Å and 1.957(4) Å respectively. These bonds represent the closest Cr-O contacts crystallographically known for formally dative interactions. As shown in Figure 7.15 a likely resonance structure in 6 delocalizes cationic charge onto phosphorus. This manifests as a weakening of the P-O bond which averaged 1.517(4) Å in the crystal structure of 6 as compared to 1.478(2)Å in unbound HMPA. 36 Further evidence for charge delocalization comes from the 334 31 P NMR resonance of bound HMPA in 6 at 37.86 ppm, shifted downfield of free HMPA 29.30 ppm when added into the same sample. Unlike 6 complex 8 has no stable resonance forms that can delocalize the cationic charge residing on the metal center. However, complex 8 is competent to polymerized THF, likely by ring opening an oxonium species. 37,38 1 Inspection of the H NMR resonances of the coordinated THF in 8 shows a downfield shift of 0.426 ppm for the protons attached to C1/C4 and 0.303 ppm for the protons on C2/C3 when compared to unligated THF. As before this effect is consistent with oxonium formation via the transfer of positive charge from the chromium onto the ligand. 39 i Figure 7.18: Crystal structure rendering of the cation of [NCr(NPr 2)2NHC][PF6] (5). Atomic positions at 50% probability and H atoms removed for clarity. i [NCr(NPr 2)2NHC][PF6] (5) is of particular interest as it is the first structurally characterized chromium N-heterocyclic carbene known in the Cr(VI) oxidation state. 335 Crystalizing in the space group Pnma, the compound has a mirror plane bisecting the molecule down the plane of the NHC and adopts a distorted tetrahedral geometry about the chromium center. The plane defined by the 5-membered NHC ring is parallel to the Cr-Nnitrido axis, with a dihedral angle defined by N1-Cr1-C1-N3 of 0.00°.Complex 5 has a Cr-C1 bond length of 2.081(4) Å, 0.111 Å longer than predicted by Pyykkö‘s table of covalent radi for a chromium carbon single bond. 40 This length is markedly shorter than the Cr(0) NHC‘s reported (which average 2.143 Å), and most Cr(II) and Cr(III), averaging 2.126 Å and 2.158 Å respectively. A 2011 paper by Wang and coworkers reports the Cr(V) nitride NCrPh2(IMP)2, where IMP = 1,341 diisopropyl-4,5-dimethylimidazole-2-yl-idene. This complex has an average Cr-CIMP bond 1 length of 2.111(2) Å, statistically longer than that in 5. The H NMR of complex 5 has equivalent resonances for the methyl groups on N3 and N4 at room temperature indicating a low barrier to rotation about the Cr-C1 bond at this temperature. 7.9 Steric Analysis of Cr(VI) Cations i In quantitatively addressing the overall steric profile of the [NCr(NPr 2)2L][A] complexes structurally characterized two separate methods were employed using the atomic positions gained from the single crystal x-ray diffraction of 2-I, 3-6 and 8. For an in depth discussion of the method of solid angles using the Solid G 43 occupied volume, %Vbur 42 program, and a calculation of please refer to Appendix B. Tabulated data for each complex are listed in Table 7.6. 336 Compound i [NCr(NPr 2)2THF][PF6] (8) i [NCr(NPr 2)2NHC][PF6] (5) i [NCr(NPr 2)2DMAP][BF4] (4-BF4) i [NCr(NPr 2)2DBN][I] (2-I) i [NCr(NPr 2)2HMPA][PF6] (6) i [NCr(NPr 2)2quin][PF6] (3) %Vbur Gm(L) 20.4 18.4 25.0 23.4 20.8 19.1 25.3 22.8 22.8 24.3 26.8 22.6 i Table 7.6: The steric parameters %Vbur and Gm(L) of [NCr(NPr 2)2L][A] complexes, both given as a percent. When addressing the amount of space occupied by each L-type ligand via the method of %Vbur, a sphere of 3.5 Å is circumscribed around the metal center approximating the first coordination sphere of the Cr(VI) center. The volume of this sphere occupied by the constituents of a given ligand are then compared to the total volume and given as a percentage. For this treatment the anion of each complex was ignored, as none showed any close contacts to the Cr(VI) cation. The calculated %Vbur of the L-type ligands span a relatively narrow range when viewed in context of the anionic ligands already reported. On the small end THF at 20.4% is identical to that calculated for pyrrolide. While most of the L-type ligands presented here are largely planer, 337 the ridged structure of quinicudine places it as the second largest ligand evaluated among anions i and neutral donors, with a %Vbur of 26.8%. Only NPr 2 at 29.1% was larger. Although 1,3 dimethyl imidazolide was chosen as it is the smallest 1,3 disubstituted NHC available, the 25.0% buried volume suggests some steric interaction may hinder the diisopropylamide rotation in 5 This may also be the case with DBN (25.3%) as noted before with the anionic carbazolyl ligand (25.0%). In fact among the two series, the L-type ligands presented here represent 4 of the 10 largest ligands evaluated by %Vbur. When approached from a solid angle standpoint, the description of sterics is focused more on access to the metal center by an incoming substrate. By treating the Cr(VI) center as a point source of light each bound ligand casts a ‗shadow‘ on an arbitrarily sized sphere inscribing the complex. The area of this ‗shadow‘ is calculated and also given as a percentage of the total area of the sphere. The steric parameter Gm(L) calculated in this way by the Solid G program for 2-I, 3-6 and 8 are shown in Table 7.6. Unlike the series of %Vbur the Gm(L) parameters span a large range, with THF (18.4%) th similar in size to that of Br (18.6%) to HMPA (24.3%) the 4 largest ligand evaluated similar to that of 1-adamantoxy (24.3%). Additionally, the ordering of ligand size in Gm(L) within the series of HMPA>NHC>DBN>quin>DMAP>THF differs from that calculated by %Vbur, quin>DBN>NHC>HMPA>DMAP>THF. This reordering is likely attributed to the different way in which each system treats steric bulk distal to the metal center. For example, while HMPA contains more non-hydrogen atoms than quinuclidine, the majority of the bulk in the NMe2 338 groups is held over 3.5 Å away from the metal center, thus is not included in the in %Vbur calculation. Quinuclidine however is bound through a tertiary nitrogen, placing much of the bulk with the sphere of enclosure leading to a larger %Vbur (Figure 7.19). Conversely, when calculated in by the Solid G the branched HMPA ligand shields more of the metal center than the compact quinuclidine ligand. i Figure 7.19: Space filling models of [NCr(NPr 2)2HMPA][PF6] (6) (left) and i [NCr(NPr 2)2quin][PF6] (3) (right) with a 3.5 Å radius sphere approximating the first coordination sphere of the Cr(VI) center. 339 7.10 Series of Nitrogen bound ligands With the addition of 2-I, 3 and 4-BF4 fourteen complexes of the form i i NCr(NPr 2)2(X) or [NCr(NPr 2)2L][A] where X or L are bound by a nitrogen atom have been synthesized, structurally characterized, and evaluated by the LDP methodology. This subset spans the range of observed LDP‘s, with NMe2 being the strongest donor evaluated and quinuclidine the weakest, at 9.34±0.32 kcal/mol and 19.01±0.32 kcal/mol respectively. Furthermore, the series of N-bound ligands also covers a wide range of steric profiles. When i evaluated by %Vbur the largest ligand evaluated was NPr 2 (29.1%) ranging to the second i smallest ligand NCO at 13.4%. When evaluated by the Solid G system NPr 2 remains the largest ligand at 26.4%, with NCO (18.0%) also remaining the smallest N-bound ligand (although now the fourth smallest in the entire series). The crystallographically determined Cr-Nligand bond lengths are equally as varied, with a difference of 0.225 Å between the longest and shortest contact observed. (Table 7.7). Of the observed bond lengths the longest belongs to i i [NCr(NPr 2)2quin][PF6] (the worst donor), and the shortest to NCr(NPr 2)3 or within error i NCr(NPr 2)2NMe2 (the best donor). 340 Compound LDP (Corrected) (kcal/mol) Cr-N Bond distance (Å) i NCr(NPr 2)3 11.12±0.23 1.842(3) i NCr(NPr 2)2NMe2 9.34±0.32 1.845(6) i NCr(NPr 2)2N(Me)Ph 10.86±0.23 1.889(2) i NCr(NPr 2)2indolyl 13.40±0.24 1.933(2) i Fc a NCr(NPr 2)2Pyr 13.45±0.27 1.938(3) i NCr(NPr 2)2Pyr 14.16±0.28 1.946(2) i NCr(NPr 2)2NCS 14.86±0.30 1.946(2) i NCr(NPr 2)2carbazolyl 12.04±0.25 1.947(2) i C6F5 NCr(NPr 2)2Pyr 14.33±0.28 1.951(1) i C6H3(CF3)2 NCr(NPr 2)2Pyr 14.36±0.28 1.966(4) i [NCr(NPr 2)2DBN][I] (2-I) 15.42±0.30 (16.44±0.30) i NCr(NPr 2)2NCO 14.51±0.29 i [NCr(NPr 2)2DMAP][BF4] 15.37±0.30 (4-BF4) (16.08±0.30) i [NCr(NPr 2)2quin][PF6] (3) 18.67±0.32 (19.01±0.32) 1.973(2) 1.980(3) 2.001(2) 2.067(4) a See Chapter 8 i Table 7.7: Cr1-N1 bond lengths and LDP‘s of all structurally characterized NCr(NPr 2)2(X) and i [NCr(NPr 2)2L][A] complexes with ligands bound through nitrogen. 341 Traditional thinking has argued that metal-ligand bond lengths should correlate to bond order. The series above provides a unique opportunity to investigate that argument, as bond lengths and the empirically derived electronic nature of the Cr-Nligand bonds are known i quantitatively. Although our LDP parameter is a measurement of the Cr-NPr 2 bond order and not the Cr-Nligand bond order, a one-to-one correlation exists. Since both bond orders are of noninteger value the numerical value is less important than the ordering since no zero-point exists (i.e. we would have to designate one Cr-Nligand bond length as having a bond order of exactly 1). Below the series of N-bound ligands is presented graphically (Figure 7.20). The dataset shows a loose linear correlation between LDP and bond length running i through the best and worse donors of the series. Although the NCr(NPr 2)2 fragment is largely isosteric among the compounds the steric influence of the ligand be the source of deviation from the trend line. The LDP as defined is dependent on the rotation of very bulky diisopropylamide ligands. When the X or L in question is sufficiently large steric influence may cause an increase i i in the observed NPr 2 barrier of rotation in addition to the barrier imposed by the Cr-NPr 2 bond ǂ order. This may or may not be included in the entropy term (∆S ) which has not been independently determined for all compounds. i Curiously, the data points furthest off the line are that of carbazolyl and NPr 2, the largest by far and third largest of all ligands evaluated by the %Vbur system. When these two points are 2 i excluded from the graph the R value increases to 0.95. Though NPr 2 is also the largest by Solid 342 G carbazolyl is nominally bigger than NMe2. This may indicate that in fact %Vbur is a better measure of intermolecular processes (i.e. bond rotation) than Solid G, as argued in Appendix B. i When comparing the solid state Cr-Nligand bond lengths of NMe2 and NPr 2 they are indistinguishable within esd. Their LDP‘s however are ~1.78 kcal/mol different. From a purely i electronic standpoint one might expect NPr 2 to be more electron rich by the increased inductive 0 effect of an extend carbon change, and thus a better donor than NMe2 toward a d metal center. i This follows as the pKa of NPr 2 (11.05) is greater than that of NMe2 (10.64). 44 However, the opposite ordering in LDP‘s is observed, as the measurement is taken in solution with molecular dynamics occurring. It is very likely that the large difference in LDP (stemming from the larger i barrier to rotation) is steric in origin and that in the absence of this effect the LDP of NPr 2 would be slightly lower or equal to that of NMe2. The same argument may be true of the other members of the series to a lesser extent. Where this threshold of sterics exists merits further study, but may be a system specific effect. 343 Figure 7.20: LDP values versus the crystallographically determined bond lengths from chromium to the ligated nitrogen atom. 344 7.11 Conclusion In expanding the methodology for electronically parameterizing ligand donor abilities of neutral donors several cationic Cr(VI) complexes bearing L-type ligands were synthesized. New i synthetic methods were developed including displacement of iodine from NCr(NPr 2)2I (1), halide abstraction using silver salts of weakly coordinating anions, substitution of L, and i substitution of A to form complexes of the type [NCr(NPr 2)2(L)][A] where L = THF, DMAP, t HMPA, qunicludine, BuNC, DBN, P(Me)2Ph, NHC and A is an dissociated anion. Also, the i complex NCr(NPr 2)2(PzB(Et3)) (10) bearing a mono pyrazolyl borate was synthesized. The kinetics of diisopropylamide rotation was measured via Spin Saturation Magnatization Transfer 1 in the H NMR for each complex. In correcting these LDP‘s for anion effects a series of i [NCr(NPr 2)2(DBN)][A] compounds were made where A = I, OTf, BF4, PF6, SbF6, BPh4, and F BAr 4, and a linear correction factor applied t giving PzB(Et3)>THF>NHC>DMAP> BuNC>DBN>HMPA>P(Me)2Ph>quin. the When donor series i NCr(NPr 2)2I was treated with DMAP iodine was displaced into the outer sphere producing i [NCr(NPr 2)2(DMAP)][I] in a temperature dependent equilibrium, favoring cation formation at i low temperature. The LDP was determined for [NCr(NPr 2)2(DMAP)][I] in CDCl3, CD3CN, and THF-d8 with LDP increasing with increasing dielectric constant of the solvent. Compounds 2-I, 3-6 and 8 were structurally characterized via single crystal x-ray diffraction, showing short Cr-O bonds and giving evidence for cationic charge delocalization onto the ligands. The steric profiles of 2-I, 3-6 and 8 were calculated through the methods of %Vbur and Solid G. Lastly, Cr-Nligand 345 bond distances were discussed as a function of LDP for the X- and L-type ligands bound through nitrogen. 7.12 Experimental Procedure for Equilibrium Studies Under an N2 atmosphere a JY tube was loaded with 1 (26.3 mg, 0.067 mmol, 1 equiv.), 4-Dimethylaminopyridine (8.17 mg, 0.067 mol, 1equiv.) and a deuterated solvent (0.75 mL). The tube was sealed with a teflon stopcock and shaken to ensure mixing. The tube was removed from the drybox and was degased using the freeze-pump-thaw 1 method. H NMR measurements were taken on a Varian Inova 500 MHz spectrometer. At each data point the temperature was allowed to stabilize for 10 min. After collection the temperature of the spectrometer was taken with a methanol or ethylene glycol standard allowed to equilibrate for 10 min. 45 After initial collection the high and low temperature data points were taken again to ensure that a fully reversible equilibrium was occurring. Concentrations of 1 and 4-I were measured by the relative proportions of the integrals of their methine resonances normalized to the initial concentration. This was fit to the following equation to find the Keq at each temperature. [ ) ( ( [ ) ][ ( 346 )] [ ] ] i 1 [NCr(NPr 2)2(DMAP)][I] (4-I) H NMR (500 MHz, CDCl3, −25 °C): 8.49 (d JHH = 6.8 Hz, 2H, NC5H4N(CH3)2), 6.68 (d JHH = 6.6 Hz, 2H, NC5H4N(CH3)2), 5.81 (sept JHH = 5.9 Hz, 2H, N(CH(CH3)2)2), 3.90 (sept JHH = 6.1 Hz, 2H, N(CH(CH3)2)2), 3.11 (s, 6H, NC5H4N(CH3)2), 1.83 (d JHH = 6.5 Hz, 6H, N(CH(CH3)2)2), 1.52 (d JHH = 6 Hz, 6H, N(CH(CH3)2)2), 1.22 (d JHH = 6 Hz, 6H, N(CH(CH3)2)2), 1.15 (d JHH = 6 Hz, 6H, N(CH(CH3)2)2). 1 H NMR (500 MHz, CD3CN, −30 °C): 8.26 (d JHH = 6.3 Hz, 2H, NC5H4N(CH3)2), 6.84 (d JHH = 6.1 Hz, 2H, NC5H4N(CH3)2), 5.61 (sept JHH = 6.2 Hz, 2H, N(CH(CH3)2)2), 4.19 (sept JHH = 5.9 Hz, 2H, N(CH(CH3)2)2), 3.24 (s, 6H, NC5H4N(CH3)2), 2.01 (d JHH = 5.6 Hz, 6H, N(CH(CH3)2)2), 1.69 (d JHH = 6.1 Hz, 6H, N(CH(CH3)2)2), 1.36 (d JHH = 6.1 Hz, 6H, N(CH(CH3)2)2), 1.26 (d JHH = 6.1 Hz, 6H, N(CH(CH3)2)2). 1 H NMR (500 MHz, THF, 25 °C): 8.10 (d JHH = 3 Hz, 2H, NC5H4N(CH3)2), 6.50 (d JHH = 3.5 Hz, 2H, NC5H4N(CH3)2), 5.37 (sept JHH = 5.5 Hz, 2H, N(CH(CH3)2)2), 3.86 (sept JHH = 5 Hz, 2H, N(CH(CH3)2)2), 2.96 (s, 6H, NC5H4N(CH3)2), 1.82 (d JHH = 5 Hz, 6H, N(CH(CH3)2)2), 1.54 (d JHH = 5 Hz, 6H, N(CH(CH3)2)2), 1.33 (d JHH = 5 Hz, 6H, N(CH(CH3)2)2), 1.18 (d JHH = 5.5 Hz, 6H, N(CH(CH3)2)2). i Synthesis of [NCr(NPr 2)2(DBN)][I] (2-I) Under an N2 atmosphere a 125 mL Erlenmeyer flask was loaded with 1 (2.05 g, 5.20 mmol, 1 equiv.) a stirbar and 50 mL Et2O. To this an Et2O (10 347 mL) solution of DBN (0.646 g, 5.20 mmol, 1 equiv.) was added drop wise to the flask over 5 min. The solution stirred for 1.5 h during which time the product precipitated. The suspension was filtered on a glass frit and the washed with pentane (2 × 10 mL). The solids were collected and dried under reduced pressure yielding 2-I as an orange powder (2.195 g, 4.24 mmol, 82% yield). Diffraction quality crystals were grown from a concentrated THF solution of 2-I held at 1 −35 °C. H NMR (500 MHz, CDCl3, 25 °C): 5.32 (sept. JHH = 6 Hz, 2H, (CH3)2CH), 3.87 (sept. JHH = 6.5 Hz, 2H, (CH3)2CH), 3.75 (t JHH = 7.5 Hz, 2H, DBN), 3.63 (t JHH = 5 Hz, 2H, DBN), 3.48 (t JHH = 6 Hz, 2H, DBN), 3.10 (t JHH = 8 Hz, 2H, DBN), 2.11-2.04 (multi., 4H, DBN), 1.80 (d JHH = 6 Hz, 6H, (CH3)2CH), 1.52 (d JHH = 6.5 Hz, 6H, (CH3)2CH), 1.30 (d JHH = 6.5 Hz, 6H, (CH3)2CH), 1.19 (d JHH = 6.5 Hz, 6H, (CH3)2CH). 13 1 C{ H} NMR (125 MHz, CDCl3, 25 °C): 169.30, 59.44, 57.41, 55.15, 47.56, 43.68, 36.26, 30.89, 30.97, 22.79, 22.29, 20.50, 18.36. Anal. Calcd. for C19H40CrIN5: C, 44.10; H, 7.79; N, 13.53. Found: C, 44.02; H, 7.63; N, 13.47. Mp: 168-170 °C.Anal. Calcd. for C19H40CrIN5: C, 44.10; H, 7.79; N, 13.53. Found: C, 44.02; H, 7.63; N, 13.47. Mp: 168-170 °C i General Synthesis of [NCr(NPr 2)2(DBN)][A] A=BF4, PF6, SbF6 (2-BF4, 2-PF6, 2-SbF6) Under an N2 atmosphere a scintillation vial was loaded with 2-I (1 equiv.), a stirbar and toluene (5 mL). To this solution the desired sodium salt was added drop wise as a toluene solution (5 mL) over 5 min. The reaction was stirred vigorously for 1.5 h at which time the reaction mixture 348 was filtered through Celite. The volatiles were removed under vacuum yielding 2 as an orange powder. i [NCr(NPr 2)2(DBN)][BF4] (2-BF4) 1 H NMR (300 MHz, CDCl3, 25 °C): 5.20 (sept. JHH = 6.3 Hz, 2H, (CH3)2CH), 3.87 (sept. JHH = 6.3 Hz, 2H, (CH3)2CH), 3.68 (t JHH = 7.3 Hz, 2H, DBN), 3.52-3.46 (multi, 2H, DBN), 3.39 (t JHH = 6.9 Hz, 2H, DBN), 3.08 (t JHH = 7.9 Hz, 2H, DBN), 1.89-1.78 (multi., 4H, DBN), 1.80 (d JHH = 6.3 Hz, 6H, (CH3)2CH), 1.52 (d JHH = 6.3 Hz, 6H, (CH3)2CH), 1.27 (d JHH = 6.3 Hz, 6H, (CH3)2CH), 1.18 (d JHH = 6.3 Hz, 6H, (CH3)2CH). 19 F NMR (282 MHz, CDCl3, 25 °C):−153.50. i [NCr(NPr 2)2(DBN)][PF6] (2-PF6) 1 H NMR (500 MHz, CDCl3, 30 °C): 5.33 (sept. JHH = 6.5 Hz, 2H, (CH3)2CH), 3.88 (sept. JHH = 6 Hz, 2H, (CH3)2CH), 3.74 (t JHH = 7.5 Hz, 2H, DBN), 3.64 (t JHH = 5.5 Hz, 2H, DBN), 3.50-3.43 (multi, 4H, DBN), 3.11 (t JHH = 8 Hz, 2H, DBN), 2.10-2.02 (multi., 2H, DBN), 1.81 (d JHH = 6.5 Hz, 6H, (CH3)2CH), 1.53 (d JHH = 6 Hz, 6H, (CH3)2CH), 1.31 (d JHH = 6 Hz, 6H, (CH3)2CH), 1.20 (d JHH = 6.5 Hz, 6H, (CH3)2CH). °C):−70.87, -73.39. i [NCr(NPr 2)2(DBN)][SbF6] (2-SbF6) 349 19 F NMR (282 MHz, CDCl3, 25 1 H NMR (500 MHz, CDCl3, 25 °C): 5.19 (sept. JHH = 6.5 Hz, 2H, (CH3)2CH), 3.87 (sept. JHH = 6.5 Hz, 2H, (CH3)2CH), 3.75-366 (multi, 4H, DBN), 3.49-3.43 (multi, 2H, DBN), 3.09 (t JHH = 8 Hz, 2H, DBN), 2.02-1.95 (multi., 2H, DBN), 1.80 (d JHH = 6 Hz, 6H, (CH3)2CH), 1.53 (d JHH = 6.5 Hz, 6H, (CH3)2CH), 1.26 (d JHH = 6.5 Hz, 6H, (CH3)2CH), 1.19 (d JHH = 6 Hz, 6H, (CH3)2CH). 19 F NMR (282 MHz, CDCl3, 25 °C):−72.19, −74.68. i Synthesis of [NCr(NPr 2)2(DBN)][BPh4] (2-BPh4) Under an N2 atmosphere a scintillation vial was loaded with 2-I (0.044 g, 0.085 mmol, 1 equiv.), a stirbar and toluene (5 mL). To this solution sodium tetraphenylborate (0.029 g, 0.02 mmol, 1 equiv.) in toluene (5 mL) was added. The reaction was stirred for 1.5 h. The volatiles were removed under vacuum. The residue was extracted with diethyl ether (5 mL) and filtered through a frit of Celite. The remaining residue was extracted with THF (8 mL) and filtered through Celite. The organic extracts were combined causing precipitate to form. The solids were collected on a frit and dried under vacuum giving 21 BPh4 as an orange powder. (0.038 g, 0.053 mmol, 62% yield). H NMR (500 MHz, CDCl3, 25 °C): 7.39 (s br., 8H, B(C6H5)4), 7.01 (t JHH = 7 Hz, 8H B(C6H5)4), 6.87 (t JHH = 6.5 Hz, 8H B(C6H5)4), 4.92 (sept. JHH = 6.5 Hz, 2H, (CH3)2CH), 3.84 (sept. JHH = 6 Hz, 2H, (CH3)2CH), 3.14 (t JHH = 7.5 Hz, 2H, DBN), 2.97-2.92 (multi, 4H, DBN), 2.58 (t JHH = 5.5 Hz, 2H, DBN), 1.89-1.81 (multi, 2H, DBN), 1.78 (d JHH = 6.5 Hz, 6H, (CH3)2CH), 1.52 (d JHH = 6.5 Hz, 6H, (CH3)2CH), 1.39-1.33 (multi, 2H, DBN), 1.18 (dd JHH = 3 JHH = 3 Hz, 12H, (CH3)2CH). 350 13 1 C{ H} NMR (125 MHz, CDCl3, 25 °C):169.08, 136.24, 125.49, 121.65, 59.36, 57.35, 54.68, 46.75, 42.76, 36.24, 30.84, 30.08, 22.28, 22.17, 19.74, 18.06. i Synthesis of [NCr(NPr 2)2(DBN)][OTf] (2-OTf) Under an N2 atmosphere a scintillation vial was loaded with 2-I (0.050 g, 0.097 mmol, 1 equiv.), a stirbar and toluene (8 mL). The solution was rapidly stirred and tetrabutlyammonium triflate (0.113 g, 0.290 mmol, 3 equiv.) was added as a solid. The vial was stirred 3 h at which time the volatiles were removed under vacuum. The residue was washed with diethyl ether (2 × 3 mL) to remove unreacted starting materials. The residue was extracted with THF (2 × 5 mL) and filtered through Celite. The volatiles were removed under vacuum yielding 2-OTf as an orange powder (0.XXX g, 0.XXX mmol, XX% 1 yield). H NMR (500 MHz, CDCl3, 25 °C): 5.30 (sept. JHH = 6.5 Hz, 2H, (CH3)2CH), 3.90 (sept. JHH = 6.5 Hz, 2H, (CH3)2CH), 3.78 (t JHH = 7.5 Hz, 2H, DBN), 3.63-3.59 (multi, 2H, DBN), 3.49 (t JHH = 5.5 Hz, 2H, DBN), 3.37-3.33 (multi, 2H, DBN), 3.13 (t JHH = 8 Hz, 2H, DBN), 2.03-1.97 (multi., 2H, DBN), 1.84 (d JHH = 6.5 Hz, 6H, (CH3)2CH), 1.56 (d JHH = 6.5 Hz, 6H, (CH3)2CH), 1.32 (d JHH = 6.5 Hz, 6H, (CH3)2CH), 1.22 (d JHH = 6.5 Hz, 6H, (CH3)2CH). 19 F NMR (282 MHz, CDCl3, 25 °C):−78.32. i F F Synthesis of [NCr(NPr 2)2(DBN)][BAr 4] (2-BAr 4) Under an N2 atmosphere a scintillation vial was loaded with 2-I (0.038 g, 0.074 mmol, 1 equiv.), a stirbar, and toluene (5 mL). To the rapidly stirring solution silver tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (0.072 g, 0.074 mmol, 1 equiv.) was added in THF (5 mL). The reaction stirred for 2 h, and the volatiles were removed under vacuum. The residue was extracted with toluene until the toluene was clear. The 351 F extracts were filtered through Celite and then dried under vacuum yielding (2-BAr 4) as an 1 orange powder. (0.XXX g, 0.XXX mmol, XX% yield). H NMR (500 MHz, CDCl3, 25 °C): 7.67 (s br., 8H, B(C6H3(CF3)2)4), 7.52 (s, 4H, B(C6H3(CF3)2)4), 4.98 (sept. JHH = 6.5 Hz, 2H, (CH3)2CH), 3.84 (sept. JHH = 6 Hz, 2H, (CH3)2CH), 3.56 (t JHH = 7.5 Hz, 2H, DBN), 3.50 (t JHH = 7.5 Hz, 2H, DBN), 3.29-321 (multi, 4H, DBN), 3.14-3.04 (multi, 4H, DBN), 1.77 (d JHH = 6 Hz, 6H, (CH3)2CH), 1.53 (d JHH = 6 Hz, 6H, (CH3)2CH), 1.15 (dd JHH = 6.3 Hz JHH = 5.1 Hz, 12H, (CH3)2CH). i Synthesis of [NCr(NPr 2)2(Quin)][PF6] (3) Under an N2 atmosphere a scintillation vial was loaded with 1 (0.108 mg, 0.275 mmol, 1 equiv.), a stirbar, and chloroform (8 mL). To this quinuclidine (0.031 g, 0.275 mmol, 1 equiv.) was added as a solid. The solution stirred for 5 min at which time silver hexafluorophosphate (0.069 g, 0.275 mmol, 1 equiv.) in acetonitrile (4 mL) was added drop wise over 10 min. The reaction stirred for 2 h forming a tan precipitate. The reaction was filtered, and filtrate was collected. The volitiles were removed under vacuum. The residue were washed with pentane (3 mL) and then extracted with toluene until the extracts were clear. The extracts were filtered through Celite and concentrated under vacuum to half their volume. The concentrated toluene solution was held at −35 °C yielding crystals of 3 (0.039 g, 1 0.074 mmol, 27% yield). H NMR (500 MHz, CDCl3, 25 °C): 5.49 (sept JHH = 6.3 Hz, 2H, N(CH(CH3)2)2), 3.98 (sept JHH = 6.3 Hz, 2H, N(CH(CH3)2)2), 3.32 (t, JHH = 8.1 Hz, 6H, N(CH2CH2)3CH), 1.94 (d JHH = 6.6 Hz, 6H, N(CH(CH3)2)2), 1.88-1.78 (multi, 6H, 352 N(CH2CH2)3CH), 1.48 (d JHH = 6.6 Hz, 6H, N(CH(CH3)2)2), 1.37 (d JHH = 6.6 Hz, 6H, N(CH(CH3)2)2), 1.27 (s, 1H, N(CH2CH2)3CH), 1.23 (d JHH = 6.3 Hz, 6H, N(CH(CH3)2)2). 19 F NMR (282 MHz, CDCl3, 25 °C): –72.08, –72.61. Mp: 218-221°C i Synthesis of [NCr(NPr 2)2(DMAP)][BF4] (4-BF4) Under an N2 atmosphere a scintillation vial was loaded with 1 (0.177 g, 0.450 mmol, 1 equiv), 4-dimethylaminopyridine (0.055 g, 0.450 mmol, 1 equiv.), and a stir bar. To this vial, CHCl3 (8 mL) was added, and the solution was stirred for 10 min. A solution of AgBF4 (0.096 g, 0.495 mmol, 1.1 equiv.) in acetonitrile (4 mL) was added over 5 min. The reaction stirred at room temperature for 1 h. The brown suspension was filtered through a glass frit with Celite as a filtering agent. The filtrate was dried in vacuo and washed with pentane (2 mL). The residue was extracted with CHCl3 (2 × 5 mL). These extracts were filtered through Celite and dried under vacuum yielding 4 (0.124 g, 0.261 mmol, 58% yield). Diffraction quality crystals of 4 were obtained by holding a concentrated toluene 1 solution of 4 at −35 °C. H NMR (500 MHz, CDCl3, 0 °C): 8.21 (d, 2H, Ar-H, JHH = 7.0 Hz), 6.68 (d, 2H, Ar-H, JHH = 6.5 Hz), 5.50 (sept, 2H, CH(CH3)2, JHH = 6.0 Hz), 3.93 (sept, 2H, CH(CH3)2, JHH = 6.0 Hz), 3.12 (s, 6H, N(CH3)2) 1.86 (d, 6H, CH(CH3)2, JHH = 6.5 Hz), 1.55 (d, 6H, CH(CH3)2, JHH = 6.5 Hz), 1.23 (d, 6H, CH(CH3)2, JHH = 6.5 Hz), 1.15 (d, 6H, CH(CH3)2, JHH = 6.5 Hz). 19 F NMR (282 MHz, CDCl3, 25 °C): –151.9 ppm. 353 i Synthesis of [NCr(NPr 2)2(NHC)][PF6] (5) Under an N2 atmosphere a scintillation vial was loaded with 1,3 dimethylimidazolium iodide (0.070 g, 0.312 mmol, 1.0 equiv.), a stir bar, and toluene (2 mL). In a separate vial benzyl potassium (0.041 g, 0.312 mmol, 1 equiv.) was slurried in toluene (8 mL). The suspended benzyl potassium was added to the imidazolium salt dropwise over 5 min. The combined mixture stirred vigorously for 2 h, turning from redish to clear with an off white precipitate. The solution was filtered through Celite to remove KI and any unreacted starting materials. The filtrate was then added to a new vial containing 1 (0.123, 0.312 mmol, 1 equiv.) and a stir bar. The solution stirred for 10 min and lightened slightly. To this solution, AgPF6 (0.087 g, 0.343 mmol, 1.1 equiv.) in acetonitrile (4 mL) was added over 5 min. The reaction stirred for 2 h, forming a tan precipitate. The volatiles were removed in vacuo and the residue was extracted with Et2O (3 × 5 mL). These extracts were filtered through Celite and the volatiles were removed under vacuum. Crystals of 5 were grown from the concentrated 1 dichloromethane of 5 held at −35° C (0.063 g, 0.125 mmol, 40%). H NMR (500 MHz, CDCl3, 25 °C): 7.39 (s, 1H, NHC-H), 7.29 (s, 1H, NHC-H), 5.46 (sept JHH = 6.5 Hz, 2H, N(CH(CH3)2)2), 4.07 (s, 6H, NHC-(CH3)2), 3.98 (sept JHH = 6.5 Hz, 2H, N(CH(CH3)2)2), 1.74 (d JHH = 6 Hz, 6H, N(CH(CH3)2)2), 1.71 (d JHH = 6 Hz, 6H, N(CH(CH3)2)2), 1.31 (d JHH = 6.5 Hz, 6H, N(CH(CH3)2)2), 1.26 (d JHH = 6.5 Hz, 6H, N(CH(CH3)2)2). 13 CDCl3, 25 °C): 260.6, 244.0, 124.5, 59.6, 57.9, 39.4, 31.2, 30.5, 23.0. CDCl3, 25 °C):−71.80, −74.32. 31 1 C{ H} NMR (125 MHz, 19 F NMR (282 MHz, P (202 MHz, CDCl3, 25 °C):−143.78 (sept JPF = 711.04 Hz, PF6). Mp: 218-221 °C (dec.). 354 i Synthesis of [NCr(NPr 2)2(HMPA)][PF6] (6) Under an N2 atmosphere a scintillation vial was loaded with 1 (0.118 g, 0.300 mmol, 1 equiv.), a stirbar, and chloroform (5 mL). To the solution hexamethylphosphoramide (0.054 g, 0.300 mmol, 1 equiv.) was added. After stirring 5 min a solution of silver hexafluorophosphate (0.083 g, 0.330 mmol, 1.1 equiv.), and hexamethylphosphoramide (0.108 g, 0.600 mmol, 2 equiv.) in acetonitrile (5 mL) was added drop wise over 10 min. The reaction was stirred for 2 h. The volatiles were removed under vacuum. The residue was extracted with diethyl ether until the extracts were clear. The extracts were filtered through Celite, and concentrated under vacuum to 5 mL. The concentrated solution 1 was held at −35° C yielding orange needles of 6 (0.056 mg, 0.095 mmol, 31.6%). H NMR (500 MHz, CDCl3, 25 °C): 5.24 (sept JHH = 6 Hz, 2H, N(CH(CH3)2)2), 3.93 (sept JHH = 6 Hz, 2H, N(CH(CH3)2)2), 2.68 (d JHH = 10 Hz, 18H, OP(NCH3)2)3), 1.94 (d JHH = 6 Hz, 6H, N(CH(CH3)2)2), 1.43 (d JHH = 6.5 Hz, 6H, N(CH(CH3)2)2), 1.25 (d JHH = 6.5 Hz, 6H, N(CH(CH3)2)2), 1.20 (d JHH = 6 Hz, 6H, N(CH(CH3)2)2). °C):−71.48, −74.00. 19 F NMR (282 MHz, CDCl3, 25 31 P (121 MHz, CDCl3, 25 °C):37.86 (s, OP(NCH3)2)3), -143.11 (sept JPF = 675.3, PF6). i Synthesis of [NCr(NPr 2)2(P(Me)2Ph)][PF6] (7) Under an N2 atmosphere a scintillation vial was loaded with 1 (0.090 g, 0.229 mmol, 1 equiv.), a stir bar, and acetonitrile (5 mL). To this dimethyl(phenyl)phosphine (0.035 g, 0.252 mmol, 1.1 equiv.) in acetonitrile (1 mL) was added, and the solution stirred for 5 min. A solution of silver tetraphenylborate (0.098 g, 0.229 mmol, 1 equiv.) in acetonitrile (3 mL) was added drop wise over 5 min. The reaction stirred for 2 h. The 355 volatiles were removed under vacuum. The residue was extracted with THF until the extracts were clear. The extracts were filtered through Celite and concentrated to ~3 mL. The solution was then dropped into hexane (10 mL) slowly forming a precipitate. The precipitate was 1 collected on a frit, and dried under vacuum yielding 7 (0.XXX g, 0.XXX mmol, XX% yield). H NMR (300 MHz, CDCl3, 25 °C): 7.51 (s br., 8H, B(C6H5)4), 7.43 (t br. JHH = 9 Hz, 2H, P(CH3)2C6H5), 7.35 (s br., 2H, P(CH3)2C6H5), 7.32 (s br., 1H, P(CH3)2C6H5), 7.05 (s br., 8H, B(C6H5)4), 6.89 (s br., 4H, B(C6H5)4), 4.81 (sept JHH = 6.6 Hz, 2H, N(CH(CH3)2)2), 3.80 (sept JHH = 6.3 Hz, 2H, N(CH(CH3)2)2), 3.51-3.44 (multi, 6H P(CH3)2Ph), 1.14 (d JHH = 6 Hz, 6H, N(CH(CH3)2)2), 1.04 (d JHH = 6 Hz, 6H, N(CH(CH3)2)2), 0.86 (d JHH = 7.2 Hz, 6H, N(CH(CH3)2)2), 0.76 (d JHH = 6.3 Hz, 6H, N(CH(CH3)2)2). 11 B (96 MHz, CDCl3, 25 °C):−6.62. i Synthesis of [NCr(NPr 2)2(THF)][PF6] (8) Under an Under an N2 atmosphere a scintillation vial was loaded with 1 (0.196 g, 0.498 mmol, 1 equiv.), a stirbar, and chloroform (5 mL). To this tetrahydrofuran (0.036 g, 0.498 mmol, 1 equiv.) was added, and the reaction was placed in a liquid N2 cooled cold well for 10 min. A separate vial was loaded with AgPF6 (0.127 g, 0.503 mmol, 1.01 equiv.), THF (3 drops) and acetonitrile (5 mL). The acetonitrile solution was added dropwise to the stirring chloroform solution over 5 min. After stirring an additional 15 min the volitiles were removed under vacuum. The residue was extracted with chloroform (~5 ml) and filtered through Celite on a fritted funnel. The solution was dried under vacuum and the residue extracted with Et2O until the extracts were clear. The combine extracts were filtered through 356 Celite on a fritted funnel and dried under reduced pressure giving 8 as an orange powder. A saturated pentane solution (5mL) of 8 with cholorform (3 drops) was held at −35 °C giving 1 orange needles suitable for diffraction. H NMR (300 MHz, CDCl3, −40 °C): 5.57 (s br., 2H, N(CH(CH3)2)2), 4.15 (s, 4H, THF) 4.03 (s br., 2H, N(CH(CH3)2)2), 2.14 (s, 4H, THF), 1.98 (d JHH = 5.5 Hz, 6H, N(CH(CH3)2)2), 1.47 (d JHH = 5.5 Hz, 6H, N(CH(CH3)2)2), 1.35 (d JHH = 5 Hz, 6H, N(CH(CH3)2)2), 1.18 (d JHH = 5.5 Hz, 6H, N(CH(CH3)2)2). i t Synthesis of [NCr(NPr 2)2( BuNC)][I] (9) Under an N2 atmosphere a scintillation vial was loaded with 2-I (0.110 g, 0.213 mmol, 1 equiv.), a stirbar, and THF (8 mL). To this tert-butyl isonitrile (0.019 g, 0.234 mmol, 1.1 equiv.) in THF (1 mL) was added. The reaction stirred vigorously for 1.5 h. The volatiles were removed under vacuum. The residue was titrated with 1 pentane (2 × 5 mL) yielding an orange powder (0.XXX g, 0.XXX mmol, XX% yield). H NMR (500 MHz, CDCl3, 25 °C): 5.31 (sept JHH = 6.4 Hz, 2H, N(CH(CH3)2)2), 3.86 (sept JHH = 6.4 Hz, 2H, N(CH(CH3)2)2), 1.80 (d JHH = 6.4 Hz, 6H, N(CH(CH3)2)2), 1.52 (d JHH = 6.4 Hz, 6H, N(CH(CH3)2)2), 1.29 (d JHH = 6.4 Hz, 6H, N(CH(CH3)2)2), 1.19 (d JHH = 6.4 Hz, 6H, N(CH(CH3)2)2). FT-IR (N≡C): 2254.3 cm -1 i Synthesis of [NCr(NPr 2)2(PzB(Et3)) (10) Under an N2 atmosphere a scintillation vial was loaded with 2-I (0.194 g, 0.375 mmol, 1 equiv.), a stir bar, amd THF (8 mL). To this Sodium triethylpyrazolylborate (0.071 g, 0.375 mmol, 1 equiv.) in THF (3 mL) was added drop wise over 5 min. The reaction was stirred for 12 h. The volatiles were removed under vacuum. The residue 357 was extracted with pentane until the extracts were clear. These extracts were filtered through Celite and the volitiles removed under vacuum. The product was dissolved in benzene (2 mL) and frozen solid. The frozen solution was lyophilized under dynamic vacuum yielding 10 and a 1 red orange powder (0.XXX g, 0.XXX mmol, XX% yield). H NMR (500 MHz, CDCl3, 25 °C): 7.67 (d JHH = 1.7 Hz, 2H, N2C3H3), 6.18 (t JHH = 1.7 Hz, 1H, N2C3H3) 5.59 (sept JHH = 6.5 Hz, 2H, N(CH(CH3)2)2), 3.76 (sept JHH = 6.35 Hz, 2H, N(CH(CH3)2)2), 1.79 (d JHH = 6 Hz, 6H, N(CH(CH3)2)2), 1.54 (d JHH = 5.5 Hz, 6H, N(CH(CH3)2)2), 1.15 (d JHH = 6.5 Hz, 6H, N(CH(CH3)2)2), 0.95 (d JHH = 6.5 Hz, 6H, N(CH(CH3)2)2), 0.63 (s br., 9H, B(CH2CH3)3), 0.29-0.15 (multi, 6H, B(CH2CH3)3). 358 REFERENCES 359 REFERENCES (1) Green, M. L. H. Journal of Organometallic Chemistry 1995, 500, 127–148. (2) Abel, E. W.; Bennett, M. A.; Wilkinson, G. Journal of the Chemical Society (Resumed) 1959, 2323. (3) Chianese, A. R.; Li, X.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics 2003, 22, 1663–1667. (4) Perrin, L.; Clot, E.; Eisenstein, O.; Loch, J.; Crabtree, R. H. Inorganic Chemistry 2001, 40, 5806–5811. (5) Kelly III, R. 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Journal of the American Chemical Society 1957, 256, 1955–1958. (45) Van Geet, A. L. Analytical Chemistry 1968, 40, 2227–2229. 362 Chapter 8: Synthesis and Evaluation of 3-Ferrocenylpyrrole as a Ligand ABSTRACT A new synthesis for 3-ferrocenyl pyrrole is reported using a palladium catalyzed cross coupling method. Metallation onto thallium(I) using TlOEt and subsequent reaction with i i Fc NCr(NPr 2)2(I) affords NCr(NPr 2)2(Pyr ) in 70% yield. X-ray diffraction studies on i Fc 1 NCr(NPr 2)2(Pyr ) reveal an η -bound pyrrolide in the solid state. Steric analysis was conducted using the %Vbur and Solid G steric parameters giving 20.5% and 19.95%, respectively. The Ligand Donation Parameter (LDP) was determined via the method of SSMT and determined to be 13.45±0.27 kcal/mol. Attempts at chemical oxidation of the ferrocenyl Fc moiety failed with various silver salts, returning only the protonated HPyr . 8.1 Introduction Undoubtedly, chemical mechanism plays a central role in optimizing catalyzed transformations. In the case of transition metals, the nature of the metal employed and the supporting ligand ultimately determine the observed reactivity. Lumped under the ubiquitous and broad terms of Sterics and Electronics, these characteristics can be manipulated to great effect by judicious choice of supporting ligands. It is through rational design of these two ligand properties that allows for faster reaction times, milder conditions, lower catalyst loadings, and larger substrate scopes in many transformations. Specifically, we are interested in optimizing early transition metal mediated hydroamination; the formal addition of an N-H moiety across an unsaturated bond and related 363 0 reactions. It has been determined that one viable pathway for d Ti(IV) to catalyze such reactions proceeds through the [2 + 2] cycloaddition of an alkyne to an in situ generated Ti-imido bond 1,2 (Figure 8.1). Dubbed the Bergman mechanism, this pathway shares several similarities to the 0 3 related reaction of olefin metathesis, catalyzed by a d molybdenum or tungsten center. Here the reactivity of a metal ligand multiple bond is also exploited to form a 4 membered metallacyclic intermediate. These [2 + 2]-cycloadditions, orbitally disallowed in traditional organic chemistry are accessible by use of the metal‘s d manifold. 364 4 Figure 8.1: The Chauvin and Bergman mechanisms for transformations catalyzed by d transition metals, both containing a [2+2]-cycloaddition step to a metal ligand multiple bond. 365 0 Figure 8.1(cont’d): The Chauvin and Bergman mechanisms for transformations 0 catalyzed by d transition metals, both containing a [2+2]-cycloaddition step to a metal ligand multiple bond. Studies done by Schrock and coworkers have shown that increasing the Lewis acidity of the metal center (i.e. a more electron poor Mo(VI) or W(VI) center) greatly increases the rate by 5 which olefin metathesis occurs. In some cases this accelerated rate allows for the transformation of substrates inert to previous catalysts. Due to the similarities between the Chauvin mechanism of olefin metathesis and the Bergman mechanism of hydroamination, an analogous effect was envisioned in the latter case. 366 8.2 Electron Deficient Pyrrolides in Catalysis Studies by the Odom lab demonstrate that pyrrolide supported Ti(IV) catalysts are 6 competent to add primary amines across a wide range of alkynes. Kinetic data suggests that the simple dipyrrolylmethane appended Ti(dpm)2(NMe2)2 7 (where dpm is 5,5- dimethyldipyrrolylmethane ) shows higher rates of catalysis than that of sterically similar Cp and indolyl-based systems (Figure 8.2). 8 367 [ Catalyst ] Temperature (°C) Kobs(× 10 -7 -1 s ) 75 866±94 75 1976±130 75 20±16 Figure 8.2: The hydroamination of 1-phenylpropyne with aniline under pseudo-first order conditions with vaious catalysts. 368 Catalyst Temperature (°C) Kobs(× 10 -7 -1 s ) 75 403±80 75 6963±582 Figure 8.2(cont’d): The hydroamination of 1-phenylpropyne with aniline under pseudo-first order conditions with vaious catalysts. 369 Catalyst Temperature (°C) Kobs(× 10 -7 -1 s ) 75 780±30 75 6225±614 Figure 8.2(cont’d): The hydroamination of 1-phenylpropyne with aniline under pseudo-first order conditions with vaious catalysts. Moreover, homoleptic Ti(IV) bearing dimethylamide ligands are sluggish to catalyze the hydroamination of 1-phenylpropyne with aniline under similar conditions. This is arguably a manifestation of the same effect as seen in electron poor olefin metathesis catalysts, and 370 confirmed by comparison of the LDP of dimethylamide and pyrrolide (9.34±0.32 and 9 14.16±0.28 kcal/mol respectively). One rationalization for the greatly reduced donor ability of pyrrolyl versus other amides is that lone pair normally localized on the N in traditional amides is delocalized over a 5 atom ring, and contributes to the aromaticity of pyrrole (Figure 8.3). This creates a situation where the π-donation from the N into empty metal d orbitals would dearomatize the 5 membered pyrrolide ligand at an energy cost approaching the aromatic stabilization energy of pyrrole (c.a. 24 kcal/mol). Additionally, significant anionic charge is localized on the α and β-positions of the pyrrolide ion. This is manifest in the organic heterocycle chemistry of pyrrole, as the α position is the preferred nucleophilic site. 10 1 Figure 8.3: Limiting resonance structures of an η bound pyrrolide to a metal capable of accepting pπ-dπ donation ranging from ligand to metal charge transfer (top) to a fully aromatic pyrrolyl (bottom). 371 Further exploiting this effect for greater catalyst performance, a systematic study was conducted using 2-substituted pyrroles. Synthesis of such pyrroles was facilitated by a modified Negishi coupling, a method first reported by Sadaghi and coworkers. 11 Installation of electronically withdrawing aryl moieties, as well as isosteric (yet largely electronically neutral) substituents was achieved in moderate to high yields (Figure 8.4). X Figure 8.4: Synthesis of 2-aryl pyrroles, condensation into the bidentate ligand H2dpm , and transamination onto titanium(IV) 372 These 2-substituted pyrroles were easily fabricated into ligands with the dpm framework via an acid-catalyzed condensation reaction with acetone (Figure 8.4). Kinetic profiles of a test hydroamination reaction under pseudo-first order conditions revealed that the increased steric inhibition caused by these appended dpm ligands offsets any increase in reactivity that incorporation of electron-deficient groups has. 12 With this balance of sterics and electronics in mind, attention was focused on synthesis of 3-substituted, electron poor-pyrroles, which proved to be much less accessible than their 2substituted isomers. Although similar kinetic studies on the hydroamination reaction do show an increase in the rate constant, the difficulty in synthesis and purification of 3-substituted dpm ligands precludes their practical adoption. 13 Moreover, the observed rate increase was moderate at best, as was the variability between electron-deficient pyrroles bearing different 3-aryl rings. As reflected in their LDP values, C6F5 perfluoronated Pyr and the bistrifluoromethyl substituted pyrrolyl, Pyr C6H3(CF3)2 (14.33±0.28 and 14.36±0.28 kcal/mol respectively) are indistinguishable within error. Similarly, the complexes Ti(NMe2)2(dpm C6H3(CF3)2 C6H2F3 ) and Ti(NMe2)2(dpm ) exhibit equivalent rate constants with the 99% confidence limit for the hydroamination reaction tested. In light of the above study, 12 we surmised that the optimal place for the elaboration of pyrrolyl moieties in the dpm framework is at the 3-position of pyrrole due to steric congestion around the substrate binding pocket. Additionally, while the reaction rate did increase roughly three-fold, the effect of inductively removing electronic density from bound pyrrolides upon reaction rates fell short of our expectations. In turn, focus shifted to methods of creating 373 electronically poor pyrroles that would communicate this deficiency to the metal center via resonance. 8.3 Redox Active Ligands Bearing Cationic Charge One such class of ligands that may fit the criteria desired, redox active ligands, has recently come into favor in a variety of applications. While supporting unprecedented redox chemistry in a wide variety of main group 14-15 16 and early 17 to late transition metals , our interest in these ligands is somewhat different. Redox active ligands, which can electronically communicate cationic charge to low valent metal centers, can increase their Lewis acidity dramatically. Sadow and coworkers have cited a related effect in the intramolecular hydroaminiation of alkenes, in which cationic charge is localized on the d zwitterionic complex [{PhB(C5H4)(Ox Me2 18-19 )2}Zr(NMe2)2]. 374 0 metal of the Figure 8.5: Representative synthesis of transition metal complexes bearing ligands that have cationic charge. The Sanford lab has been investigating one such framework for Pt and Pd catalyzed H/D exchange in arenes and oxidation chemistry. 20 Computational studies have implicated a protonated (thus cationic) ligand as the active catalyst for the C-H activation in the conversion of methane to methanol. 21 Here dicationic, bidentate bipyridyl ligands support the highly electrophilic group 10 metal centers necessary for high reactivity without the need for an acidic medium (Figure 8.5). 8.4 3-Ferrocenylpyrrole Due to the large number of pyrrolide supported titanium complexes active for hydroamination, as well as the synthetic versatility of substituted pyrroles into bi- and tridentate 375 ligands, we thought the incorporation of redox active units onto known architectures would be fruitful. The redox couple of ferrocene/ferrocenium was chosen due to its presumed stability in typical reaction conditions. Moreover in a related 1996 communication by Wakatsuki et. al., a Ti(IV) complex bearing a ferrocenyl moiety in conjugation with the metal center shows unique reactivity upon oxidation. 22 Here titanocene derivatives were appended with ferrocenyl units placed in conjugation with the metal via alkynyl units. Upon treatment with 2 equiv. of the 1 − e oxidant AgPF6 the Ti-alkynyl yielded TiCp2F2 as well as a coupled alkynylferrocene compound (Figure 8.6). 376 Figure 8.6: The non-classical reductive elimination of two alkynylferrocene ligands induced by chemical oxidation. It was our hope that Ti(IV) supported by a bi- or tridentate ligand framework such as dpm would be stable toward elimination, yet still delocalize the cationic charge of a ferrocenium moiety in the 3-position, similar to the ferrocenyldiketonates of Brown and coworkers. 377 23 Figure 8.7: Resonance contributors in HPyr 3-Ferrocenylpyrrole has already been reported The oxidation potential of HPyr Fc 24 Fc − upon 1e oxidation. and is used in polymer applications. 25 is decreases with respect to unsubstituted ferrocene, possibly due to the delocalization of charge through the π-system. The site of initial oxidation may be from ferrocene (Path A) or pyrrole (Path B). Though HPyr Fc is much easier to oxidize than free pyrrole, it has yet to be determined. Electron delocalization results in a planar structure described as ‗fulvalene‘-like by the authors. Furthermore, the effect of N-metallation on the resonance structures of HPyr Fc is unknown, but may lead to pyrrolide displaying characteristics indicative of an L type ligand as opposed to an anionic X type ligand. 378 8.5 Synthesis of 3-Ferrocenylpyrrole Synthesis of 3-ferrocenylpyrrole was first attempted via the method outlined by Rose and coworkers (Figure 8.5). 24 Although this method did produce the desired product, it was very low yielding and required an excess of toxic KCN, chromatography using a benzene eluent, and reduction using pyrophoric DiBAl-H; thus an improved method was sought out. At the outset, cross-coupling strategies were very attractive due their direct nature. Initial attempts sought to couple mono-borylated ferrocene with a brominated, protected pyrrole generated by reaction with NBS. This compound proved hard to separate from βbrominated, and multiply brominated N-TIPS-pyrrole, typical cross-coupling conditions involving ferrocene. 25 and was highly unstable towards the 26-27 Mono-borylated ferrocene 28 can be converted to bromo-ferrocene by treatment with N-bromosuccinimide, circumventing the difficulty in accessing monohalogenated ferrocene through more traditional routes. However, the coupling between the brominated ferrocene and the previously reported 3-BPin-N-Boc-pyrrole gave only trace amounts of the desired product (Figure 8.8). 379 Fc Figure 8.8: Failed cross coupling strategies in the production of HPyr . A succinct albeit moderate yielding synthesis for 3-ferrocenylpyrrole was finally achieved through the Pd catalyzed cross coupling with of 3-BPin-N-Boc-pyrrole and iodoferrocene (Figure 8.9). High yielding synthesis of iodoferrocene was accomplished using the method of Kubiak, through treatment of a small excess of ferrocene with tert-butyllithium in cold THF, followed by the slow addition of I2 in toluene. 29 The basic biphasic system employed was sufficient to partially remove the BOC protecting group from 3-BPin-N-Boc-pyrrole as well as 3-ferrocenyl-N-BOC-pyrrole during the reaction. An excess of 3-BPin-N-Boc-pyrrole (from 1.5-2 equiv.) was necessary for good yields, although unreacted 3-BPin-N-Boc-pyrrole as well as 3-BPin-N-H-pyrrole were seen in GC-FID traces taken after 12 h. Thus far extended reaction times have not increased conversion. Likewise, the premature deprotection of 3-BPin-N-Bocpyrrole before coupling is exacerbated by elevated temperatures, lowering yields. 3-ferrocenyl380 N-BOC-pyrrole was purified by column chromatography on silica with 9:1 hexane:ethyl acetate as an eluent. Deprotection was achieved by treatment of 3-ferrocenyl-N-BOC-pyrrole with 3 equiv. of sodium methoxide in 1:1 (v/v) THF:methanol at room temperature for 45 min giving 3ferrocenyl-pyrrole in near quantitative yield. 381 Figure 8.9: A new synthesis for 3-Ferrocenyl-N-BOC-pyrrole using Pd catalyzed cross-coupling and subsequent deprotection. 382 i Fc 8.6 Synthesis of NCr(NPr 2)2(Pyr ) i X Analogous to the synthesis of other pyrrolide compounds of the form NCr(NPr 2)2(Pyr ), the thallium(I) salt of the pyrrolide was first generated by treating freshly purified HPyr Fc with 1.05 equiv. thallium(I)ethoxide in cold diethylether. After 3 h the volatiles were removed under reduced pressure, and the residue was titrated with pentane (3 × 1 mL). A stoichiometric amount i i Fc of NCr(NPr 2)2(I) was added in THF, affording NCr(NPr 2)2(Pyr ) as a red-orange powder in 70% yield (Figure 8.10). Further purification via crystallization from pentane held at −35 °C gave crystals suitable for diffraction. 383 i Fc Figure 8.10: Synthesis of NCr(NPr 2)2(Pyr ) using an in situ generated thallium(I) transmetalating reagent. i Fc 8.7 Structure and Steric Evaluation of NCr(NPr 2)2(Pyr ) 1 Single crystal X-ray diffraction shows an η -bound pyrrolide, akin to other known i NCr(NPr 2)2(X) complexes, where X = pyrrolyl and 3-substituted pyrrolyl (Figure 8.11). The determined Cr-Pyr Fc bond distance of 1.938(3) Å is just within error of the Cr-Pyr bond distance C6F5 (1.946(2) Å) in the unsubstituted pyrrolide, but significantly different than that of the Cr-Pyr C6H3(CF3)2 and Cr-Pyr bond lengths (1.951(1) Å and 1.966(4) Å respectively). Although Cr-X bond lengths in the previously reported ligand series are not indicative of the corresponding LDP 384 values, as they are complicated by other factors, this ordering makes sense for the subseries of structurally similar pyrrolides (Figure 8.12). Namely, the most donating pyrrolyl, Pyr Fc (vida supra) has a bond length statistically shorter than the next two most donating pyrrolides, Pyr and C6F5 Pyr , whose Cr-N bond lengths are the same within ESD, and whose LDP values overlap accounting for experimental error. Of the pyrrolyls examined, the poorest donating, CrC6H3(CF3)2 Pyr , has both a statically longer bond length and larger LDP value than that of i Fc NCr(NPr 2)2(Pyr ). This trend also follows that ferrocenyl substituents are generally regarded as electron donating, whereas trifluoromethyl-substituted aryls tend to be withdrawing in organic systems. Curiously, the Cr-Pyr Fc bond distance is identical to the Cr-indolyl distance (1.933(2) i Å) of NCr(NPr 2)2(indolyl), as well as overlapping LDP values. 385 i Fc Figure 8.11: Crystal structure rendering of NCr(NPr 2)2(Pyr ) with thermal ellipsoids at 50% probability. H atoms removed for clarity. Features within the Pyr Fc unit are largely unremarkable for substituted ferrocene, with Fe1-Cpcentroid distances of 1.645 Å and a Cpcentroid-Fe1-Cpcentroid angle of 177.15°. The dihedral angle defined by C1-C2-C5-C6 at −19.06° is relatively shallow as compared to other known 3-ferrocenyl pyrroles, suggesting a larger amount of communication between the π i Fc systems on the adjoining 5 membered rings in NCr(NPr 2)2(Pyr ). Likewise, the C2-C5 linkage is 1.466(6) Å, slightly shorter than 386 the Cp-CH2 bond distance in FeCp(C5H4(CH2C(CH2OH)2CH3)) at 1.501(1) Å, implying some amount of double bond character. Compound i LDP (kcal/mol) %V Bur Gm(L) NCr(NPr 2)2(Pyr ) Fc 13.45±0.27 20.5 19.95 NCr(NPr 2)2(Pyr) i 14.17±0.27 20.4 19.47 14.08±0.28 20.4 19.44 14.25±0.28 20.3 19.30 i C6F5 NCr(NPr 2)2(Pyr i NCr(NPr 2)2(Pyr ) C6H3(CF3)2 ) Table 8.1: LDP and steric parameters of pyrrolyl compounds. i Fc The sterics of NCr(NPr 2)2(Pyr ) were also evaluated by the method of Solid G 31 %Vbur 30 and and are summarized in Table 8.1. At the defined radius of 3.5 Å, approximating the i Fc first coordination sphere, the %Vbur parameter of NCr(NPr 2)2(Pyr ) is largely unaffected regardless of the 3-substitution as the entirety of the Fc unit is outside the area of enclosure, such is the case for all the 3-substituted pyrrolides examined. Slight differences may reflect the closer Cr1-pyrrolyl bond distances, lending to more of the ligand being enclosed in the first coordination sphere. This is a very promising feature in terms of implementation of the PyrFc ligand into real catalyst systems, as the sterics are largely unaffected regardless of the 3substituant used to tune the electronics. Comparisons, among the Gm(L) parameter reflect only a 387 modest shielding of the metal center upon substituting bound pyrrolyl, likely due to the Fc moiety overlapping area already shielded by a diisopropylamide ligand. Again, this bodes well for applications where barriers to substrate binding upon increased sterics may become an issue. i i Fc Figure 8.12: Overlaying crystal structures of NCr(NPr 2)2(Pyr) (blue), NCr(NPr 2)2(Pyr ) i (orange), NCr(NPr 2)2(Pyr C6F5 i C6H3(CF3)2 ) (green), and NCr(NPr 2)2(Pyr ) (purple) showing similarities in the orientation of the pyrrolyl based ligand. i Fc 8.8 LDP Determination of NCr(NPr 2)2(Pyr ) 0 In addressing the effect of redox changes at the iron site upon the d Cr center, a baseline determination of the donor ability of the ferrocenyl(II)pyrrolide ligand was needed. Through the method of Spin Saturation Magnetization Transfer as discussed earlier, the LDP was found to be 388 13.45±0.27 kcal/mol. This places the overall donor ability above that of unsubstituted pyrrolide (14.17±0.27 kcal/mol) and close to that of indolyl (13.76±0.27 kcal/mol). This decrease in the barrier of amide rotation clearly shows that the electron donating effect of ferrocenyl (with a Hammett parameter of ζp = −0.15) in the 3-position of a bound pyrrolyl inductively perturbs the electronics of the metal center. Assuming the Hammett parameters are an accurate assessment of the Fc unit‘s effect on the pyrrolyl ring, it is not necessary that these correlate with the observed LDPs. LDPs are a reflection of the overall electronics at the metal, and are likely complicated by structural features, such as ligand conformation, other molecular motions, and orbital overlap. Additionally, the observed LDP is a function of all the ligands in the basal set. Granted the two diisopropylamide groups are consistent among the test compounds, the contribution of subtle i changes in Cr-NPr 2 bonding induced by the variable X ligand cannot be experimentally separated. Although absolute correlation with Hammett parameters and other linear free-energy relationships developed for organic systems fail to predict ligand donation, they are qualitatively useful in making predictions in closely related subsets of the ligands explored thus far. Making this assumption, it follows that upon oxidation the ferrocenium substituent would likely be inductively withdrawing from the pyrrolyl ring as judged by the change in sign of its Hammett parameter (ζp = 0.29) as compared to that of ferrocene. Our real hope was that any resonance effects in combination with inductive effects would lead to a very electron deficient system, putting cationic charge on the metal center via delocalization from iron(III) through the pyrrolyl linkage. 389 i Fc 8.9 Attempted Chemical Oxidation of NCr(NPr 2)2(Pyr ) i Fc The chemical oxidation of NCr(NPr 2)2(Pyr ) was attempted using silver(I) salts of AgPF6, AgSbF6, and AgB(Ph)4 in various solvents. Unfortunately, no oxidized products were identified. While a rapid color change from orange to deep purple did occur, all chromiumcontaining products were either insoluble or paramagnetic, suggesting Cr(V). Interestingly, significant amounts of the N-H pyrrole, HPyr Fc were recovered along with N-(propan-2- ylidene)propan-2-amine, both likely β-hydride elimination products from a diisopropylamide i Fc ligand of NCr(NPr 2)2(Pyr ). While discouraging, conditions may be found which yield the desired product through milder oxidants or electrochemical means. 8.10 Conclusions A new cross-coupling synthesis of 3-ferrocenyl pyrrole is reported, one which affords higher yield (62%) and utilizes milder reagents. Transmetallation from thallium(I) gives the test i Fc complex NCr(NPr 2)2(Pyr ), which was structurally characterized. Steric analysis was conducted using the %Vbur and Solid G steric parameters giving 20.5% and 19.95%, respectively. The Ligand Donation Parameter (LDP) was determined via the method of SSMT and determined to be 13.45±0.27 kcal/mol, placing Pyr Fc as slightly more donating than pyrrole. Thus far attempts at chemical oxidation of the ferrocenyl moiety induce decomposition, yielding Fc only protonated Pyr . Multidentate ligands containing 3-ferrocenyl pyrrole may ultimately prove stable in Ti complexes, competent to catalyze the hydroamination reaction. 390 8.11 Experimental Synthesis of 3-ferrocenyl-N-BOC-pyrrole Under an inert atmosphere of N2 a 1L Schlenk tube was loaded with 3-BPin-N-BOC-pyrrole (4.32 g, 14.74 mmol, 1.9 equiv.), K3PO4 (4.94 g, 23.27 mmol, 3 equiv.), toluene (10 mL) and a stirbar. In a separate 50 mL Erylenmeyer flask a 20 mL of a toluene solution of Pd(OAc)2 (87.0 mg, 0.388 mmol, 0.05 equiv.) and [1,1'-biphenyl]-2yldicyclohexylphosphine (272 mg, 0.775 mmol, 0.1 equiv.) was mixed. After 5 min the toluene solution was added to the Schlenk tube, and iodoferrocene (2.42 g, 7.76 mmol, 1 equiv.) was added. The tube was sealed with a Teflon stopcock and removed from the drybox. The stopcock was replaced by a septum under flowing N2. Distilled water (30 mL) was degassed through 3 freeze-pump-thaw cycles and cannulated into the Schlenk tube. The septum was replaced by the Teflon stopcock, and the headspace was evacuated. The tube was placed in a 60 °C oil bath for 12 h with very vigorous stirring. The tube was removed from the oil bath and allowed to cool to room temperature, at which time the solution was filtered through a fritted funnel full of silica. The filtrate was rotovapped to near dryness. The residue was extracted with Et2O (3 × 75 mL) and brine, and dried over Na2SO4. The solution was concentrated and loaded onto a silica column using 9:1 (v/v) hexane:ethyl acetate as an eluent. The product containing fractions were rotovapped to dryness, yielding 3-ferrocenyl-N-BOC-pyrrole as an orange powder (1.68 g, 4.78 1 mmol, 61.7%). H NMR (500 MHz, CDCl3, 25 °C): 7.17 (s br.,1H, Pyr-H), 6.31 (s br., 1H, PyrH), 6.18 (s br., 1H, Pyr-H), 4.44 (s br., 2H, Cp‘-H), 4.20 (s br., 2H, Cp‘-H), 4.04 (s, 5H, C5H5), 1.59 (s, 9H, O-C(CH3)3). MP 86-70 °C. 391 Deprotection of 3-ferrocenyl-N-BOC-pyrrole A 100 mL round bottom flask was loaded with 3-ferrocenyl-N-BOC-pyrrole (1.68 g, 4.78 mmol, 1 equiv.), a stirbar, methanol (20 mL) and THF (20 mL). To the stirring round bottom flask, sodium methoxide (0.775 g, 14.35 mmol, 3 equiv.) was added. The reaction stirred for 45 min. The volatiles were removed under reduced pressure, Fc and the residue was titrated with pentane (3 × 5 mL). 3-ferrocenyl-N-H-pyrrole (HPyr ) was recovered in near quantitative yield. Spectroscopy and melting point of the 3-ferrocenyl-N-Hpyrrole was identical to those reported in literature. i Fc Synthesis of NCr(NPr 2)2(Py ) Under an inert N2 atmosphere a scintillation vial was loaded with HPyr Fc (0.090 g, 358 mmol, 1 equiv.), Et2O (10 mL), and a stirbar. The vial was cooled in a liquid N2 cooled cold well for 10 min. The vial was removed from the cold well, and thallium(I)ethanolate (0.094 g, 0.376 mmol, 1.05 equiv.) was added dropwise. The mixture was rapidly stirred for 3 h. The volatiles were removed under vacuum, and the residue was titrated i with pentane (3 × 1 mL). The residue was dissolved in THF (5 mL), and NCr(NPr 2)2(I) (0.141 g, 0.358 mmol, 1 equiv.) was in an additional 5 mL of THF. The reaction stirred for 20 h at room temperature, with a yellow precipitate of thallium(I)iodide forming. The reaction mixture was evaporated to dryness, and extracted with Et2O until the ether ran clear. The extracts were filtered through a fritted funnel with Celite as a filtering agent. The volitiles were removed under i Fc vacuum affording NCr(NPr 2)2(Pyr ) as a red-orange powder (0.130 g, 0.251 mmol, 70%). Further purification via crystallization from a concentrated pentane solution held at −35 °C gave 1 crystals suitable for diffraction. H NMR (500 MHz, CDCl3, 4 °C): 6.93 (t JHH = 1.5 Hz, 1H, 392 Pyr-H), 6.82 (t JHH = 2.0 Hz, 1H, Pyr-H), 6.25-6.24 (m, 1H, Pyr-H), 5.09 (sept JHH = 6.0 Hz, 2H, NCH(CH3)2), 4.40 (t JHH = 1.7 Hz, 2H, Cp‘-H), 4.01 (t JHH = 1.7 Hz, 2H, Cp‘-H), 3.96 (s, 5H, C5H5), 3.77 (sept JHH = 6.0 Hz, 2H, NCH(CH3)2), 1.84 (d JHH = 6.1 Hz, 6H, NCH(CH3)2), 1.56 (d JHH = 6.35 Hz, 6H, NCH(CH3)2), 1.17 (d JHH = 6.35 Hz, 6H, NCH(CH3)2), 1.08 (d JHH = 6.6 Hz, 6H, NCH(CH3)2). 13 1 C { H} NMR (125 MHz, CDCl3, 4 °C) 129.86, 125.41, 120.41, 106.80, 83.14, 69.25, 67.03, 65.69, 57.99, 55.91, 30.26, 30.15, 21.88, 21.44. 393 REFERENCES 394 REFERENCES (1) Bisamides, Z.; Walsh, P. 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