INVESTIGATIONS OF HI GH VALENT METAL REAC TIVITY USING THE LIG AND DONOR PARAMETER By Brennan Shay Billow A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of Chemistry Docto r of Philosophy 2018 ABSTRACT INVESTIGATIONS OF HI GH VALENT METAL REAC TIVITY USING THE LIG AND DONOR PARAMETER By Brennan Shay Billow Understanding the chemistry of high valent metals stands to open the door to a new realm of chemical transformations. Un fortunately, a relative lack of research into chemical processes using high valent metals has led to a dearth of inf ormation in comparison to processes involving low valent metals. Recently, our group has set about developing new tools for the development of high valent metal - based catalysts. Specifically, development of the ligand donor parameter (LDP) stands to uncove r a wealth of information involving high valent metal - ligand interactions. In the following chapters the application of LDP in chromium - , ti tanium - , and uranium - based systems will be discussed. These studies have led to better perception of metal - ligand bo nds, improved understanding of titanium - based catalysis, and discovery of exciting new uranium complexes. iii To My Family and Friends iv ACKNO WLEDGMENTS I hope that all who have assisted me in my pursuit of academic achievement already know they have my unending gratitude. Still, I would be remiss if I did not address those who have been most influential to my growth as a scientist and as a per son. I owe t he most to my parents and family , without their support and encouragement I could not have done this. Despite their less - than - hieve more. I was also able to find a home away from home in the chemistry department, complete with an make up what I consider to be my immediate academic family. The discussions , advice, and encouragement they provided is what kept chemistry exciting. Of course, there were others in the group as well. Without the other past and current group members, the extended academic family, none of the progress I have made would have been p ossible . Especially, Dr. Ross Bemowski and Dr. Amrendra Singh, both of whom helped jump start both my graduate studies, as well as, my 4 th floor aviation experiments . The list of those I consider to be extended academic family goes on . I owe t hanks to my committee, Professor Mitch Smith, Professor Tom Hamann, and Professor Ben Levine for their insight and criticism. I wish there were more opportunities to meet with everyone, our few meetings were both productive and enjoyable. Furthermore, t he (formerly ) open door policy in the 4 th and 5 th floor of the chemistry building led to a truly collaborative and friendly environment. I that atmosphere . v I was also fortunate enough to s pend time in the lab of Dr. James (Jim) Boncella. Everyone at LANL welcomed me as both a co - worker and friend, so much so that I decided to go back. To Dr. Richard Staples and Dr. Dan Holmes I am also especially grateful. The hundreds of hours I spent in t he X - ray lab and NMR facilities might have been thousands if not for you r expertise and guidance. Besides that , your counsel and willingness to listen won t be forgotten. Further, the support staff in the chemistry building are far too often under apprecia ted. With out the folks like Glenn, Scott, and Bob, our lab would have ceased to function a long time ago. I also owe thanks to Oscar, while not in the Odom group, I consider you as much of a family member as anyone. A ll of my future employers have you to t hank for my (at times distracting) skiing addiction. But , as other group members can confirm, it helped keep me sane during even the most stressful times. Of course, I also need to thank those who most influenced my interest in chemistry in the first place . Ms. Win czewski, Dr. Dan Adsmond, and Dr. Colleen Partigianoni, without the passion each of you displayed for chemistry, my interests in the field might have never been realized. vi TABLE OF CONTENTS LIST OF TABLES ................................ ................................ ................................ ....................... viii LIST OF FIGURES ................................ ................................ ................................ ........................ x LIST OF SCHEMES ................................ ................................ ................................ .................... xxi KEY T O SYMBOLS AND ABBREVIATIONS ................................ ................................ ....... xxii Introduction ................................ ................................ ................................ ............... 1 1.1 Background and Motivation ................................ ................................ ............................. 1 1.2 ................................ ............................. 1 1.3 Advancements in Modeling Ancillary Ligands ................................ ............................... 3 1.4 Co nsiderations for High Valent Metals ................................ ................................ ............ 6 1.5 Titanium Catalysis ................................ ................................ ................................ ............ 7 1.6 Ligand Donor Parameter ................................ ................................ ................................ .. 9 1.7 Quantitatively Modeling Reactions ................................ ................................ ................ 14 1.8 Conclusions ................................ ................................ ................................ .................... 14 REFERENCES ................................ ................................ ................................ .......................... 16 Investigations of Chromium(VI) Nitrido Cyclopentadienyl Bonding System ....... 19 2.1 Introduction ................................ ................................ ................................ .................... 19 2.2 Synthesis and Characterization ................................ ................................ ...................... 20 2.3 Bonding Analysis ................................ ................................ ................................ ........... 24 2.4 Conclusions ................................ ................................ ................................ .................... 28 2.5 Experimental ................................ ................................ ................................ .................. 28 REFERENCES ................................ ................................ ................................ .......................... 44 Analysis of Phosphines as Ligand on High Valent Transition Metal s ................... 47 3.1 Introduction ................................ ................................ ................................ .................... 47 3.2 Synthesis ................................ ................................ ................................ ......................... 47 3.3 Initial LD P Analysis ................................ ................................ ................................ ....... 48 3.4 Anion and Solvent Dependence ................................ ................................ ..................... 50 3.5 Entropy Analysis ................................ ................................ ................................ ............ 54 3.6 Phosphine Analysis ................................ ................................ ................................ ........ 58 3.7 Ph osphine Bonding Analysis ................................ ................................ ......................... 61 3.8 Conclusions ................................ ................................ ................................ .................... 65 3.9 Experimental ................................ ................................ ................................ .................. 65 REFERENCES ................................ ................................ ................................ ........................ 145 Developing a Model for the Optimization and Development of Titanium Catalyze d Hydroamination ................................ ................................ ................................ .......................... 148 4.1 Introduction ................................ ................................ ................................ .................. 148 4.2 Ligand Analysis Considerations ................................ ................................ ................... 149 4.3 Synthesis and Characterization of the Chromium Complexes ................................ ..... 152 vii 4.4 Modeling the Hydroamination Kinetics ................................ ................................ ....... 154 4.5 Te sting Ligand Variety ................................ ................................ ................................ . 161 4.6 Investigating Anomalies ................................ ................................ ............................... 165 4.7 Other Applications of the Model ................................ ................................ .................. 168 4.8 Conclusions ................................ ................................ ................................ .................. 173 4.9 Experime ntal ................................ ................................ ................................ ................ 175 REFERENCES ................................ ................................ ................................ ........................ 238 Preliminary Investigations of Uranium - Ligand Interactions ................................ 242 5.1 Introduction ................................ ................................ ................................ .................. 242 5.2 Reacti on Desi gn ................................ ................................ ................................ ........... 244 5.3 Catalyst Design and Synthesis ................................ ................................ ..................... 246 5.4 Catalysis ................................ ................................ ................................ ....................... 252 5.5 Future Wor k ................................ ................................ ................................ ................. 254 5.6 Conclusions ................................ ................................ ................................ .................. 257 5.7 Experimental ................................ ................................ ................................ ................ 258 REFERENCES ................................ ................................ ................................ ........................ 277 Synthesis and Characterization of Uranium - Terphenyl Complexes ..................... 280 6.1 Investigating Imido Synthesis ................................ ................................ ...................... 280 6.2 Steric Bulk as a Method to Slow Ligand Redistribution ................................ .............. 281 6.3 Comparisons Between Amide and Imides of Bulky Ligands ................................ ...... 283 6.4 Bis(Amide) Species as a Way to Access Low Valent Uranium ................................ .. 290 6.5 Generation of a Neutral Uranium(II) ................................ ................................ ........... 294 6.6 Oxidation of the Uranium(II) ................................ ................................ ....................... 297 6.7 Spectroscopic Analysis ................................ ................................ ................................ 299 6.8 Conclusions ................................ ................................ ................................ .................. 301 6.9 Experime ntal ................................ ................................ ................................ ................ 301 REFERENCES ................................ ................................ ................................ ........................ 351 Reduction of Dinitrogen to Hydrazine ................................ ................................ .. 354 7.1 Introduction ................................ ................................ ................................ .................. 354 7.2 Ammonia as an Energy Storage Solution ................................ ................................ .... 356 7.3 Alternative Methods for Nitrogen Fixation ................................ ................................ .. 357 7.4 ................................ ................................ ................. 359 7.5 Monitoring Hydrazine Formation ................................ ................................ ................ 360 7.6 Moving to More Complex Systems ................................ ................................ ............. 362 7.7 Catalyst Synthesis ................................ ................................ ................................ ........ 363 7.8 Looking Forward ................................ ................................ ................................ .......... 364 7.9 Conc lusions ................................ ................................ ................................ .................. 365 7.10 Experimental ................................ ................................ ................................ ................ 365 REFERENCES ................................ ................................ ................................ ........................ 370 viii LIST OF TABLES Table 3 - 1. Initial LDP measurements for PR 3 ligands. All compounds used SbF 6 as the counter ion. All values are based on at least one good LDP measurement performed, but results here were not necessarily performed in triplica te due to decomposition of the complexes. As such, the values are reported only to the tenths. ................................ ................................ .......................... 49 Table 3 - 2. LDP measurements of [ NCr(N i Pr 2 ) 2 (PPhMe 2 )] + X - . ................................ .................... 52 Table 3 - 3. Summary of Eyring analyses of the various phosphine complexes. The entropy analysis error w as determined from a linear lea st squares fit of the experimentally data as reported by Lente. 17 a Values in CD 3 CN use SbF 6 as an anion. These H values include the, admittedly crude, assumption of a constant entropy of - 9 e.u. b These values were run at 3 constant temperatures, as such the precision is high and the error for the measurement is low, this does not necessarily mean the H values are more accurate than the other values here, especially in light of the entropy assumption. c Values in CDCl 3 use BAr F 24 as the anion. H values are from the Eyring analysis. ................................ ................................ ................................ .............. 56 Table 3 - 4. Parameters used to model the trialkyl phosphines against H . H values taken from Table 3 - 3. ................................ ................................ ................................ ................................ ...... 59 Table 3 - ) for [NCr(N i Pr 2 ) 2 PE 3 ]SbF 6 salts in CD 3 CN . a value in CD 3 CN was measured via in situ generated species stabilized with excess phosphine. b was taken in multiple trials, taking a single measurement on three different samples due to compound instability. As a result, three different temperatures were calibrated, one for each sep arate run. The reported G is approximate, as it is an average from 3 (close) temperatures. ................................ ................................ ................................ ..................... 75 Table 4 - 1. Summary of the titanium catalysts use for hydroamination, the chromium complexes used to parameterize the ligands, and the rates of catalysis. ................................ ....................... 156 Table 4 - 2. Summary of the validation set of bis - aryloxide ligands. ................................ ........... 162 Table 4 - 3. Table showing the fitting parameters and rate constant for catalyst 3. ..................... 165 Table 4 - 4. Table summarizing the rate constant and fitting parameters for catalyst 6 . .............. 167 Table 4 - 5. Spin saturation transfer data. a Determined fro m the rate constant for isopropyl group exchange using the Eyring Equation with the assumption that the transmission coefficient is unity. b = 1 1 . c Pyrr = pyrrolide, Ind = indolide, tol = p - tolyl. d Determined by l ine shape analysis. e The 6 - Br - SNap ligand was only used as a surrogate for 2 - napthylthiolate (SNap) to obtain the %V bur value. Consequently, its LDP was not employed in this study; however, we include the LDP for completeness. ................................ ...................... 175 ix Table 6 - 1. Selected bond lengths and angles. a The centroid refers to the Cp* centroid - U - Arene centroid angle in 2 - 6 , and the Arene centroid - U - Arene centro id angle in 7 . .............................. 329 Table 6 - 2. Selected bond lengths and angles. a The white rows are the values from the diethyl ether crystallized structure, the grey row s are from the hexane crystallized structure. .............. 330 x LIST OF FIGURES Figure 1 - 1. Representation of Tolman Cone Angle 1 ................................ ................................ ...... 3 Figure 1 - 2. Comparison of different NHC ligand profiles and resulting cone angle. .................... 4 Figure 1 - 3. Top: Graphical representation of %V bur . Black shaded area represents buried volu me. Bottom: Steric map output from SambVca 2.0 program. 9 The x and y axis are distance from center in Å. The ma p cuts off at 3.5 Å due to our defined radius. The colored contour represents the distance in angstroms in the z direction (perpendicular to the plane of the page). ................... 5 Figure 1 - 4. Hydroamination catalytic cycle. 19 ................................ ................................ ................ 8 Figure 1 - 5. Representative chromium structure for LDP deter mination. 24 ................................ .... 9 Figure 1 - 6. LDP values of previously reported ligands. 24 ................................ ............................ 13 Figure 2 - 1. Representation of the three typical binding modes of cyclopentadienyl. .................. 19 Figure 2 - 2. Crystals structures of 1 , 3 , 4 , and 5 . Ellipsoids displayed at the 50% probability level. All hydrogens, solvents in the lattice, and counterion from 5 , removed for clarity. .................... 22 Figure 2 - 3. Mayer bond order analysis of 1 - 5 . ................................ ................................ ............. 25 Figure 2 - 4. Cr - C bond lengths and averages for compound s 1 - 5 . ................................ ................ 25 Figure 2 - 5. Ma 15, 20 - 21 .................. 26 Figure 2 - 6. M - C bond lengths a nd averages for the ................... 27 Figure 2 - 7. NCr(N i Pr 2 ) 2 (Cp) ( 1 ) 1 H NMR ................................ ................................ .................... 33 Figure 2 - 8. NCr(N i Pr 2 ) 2 (Cp) ( 1 ) 13 C NMR ................................ ................................ ................... 34 Figure 2 - 9. NCr(N i Pr 2 ) 2 (Ind) ( 2 ) 1 H NMR ................................ ................................ ................... 35 Figure 2 - 10. NCr(N i Pr 2 ) 2 (Ind) ( 2 ) 13 C NMR ................................ ................................ ................ 36 Figure 2 - 11. NCr(N i Pr 2 )(O 2 CPh)(Cp) ( 3 ) 1 H NMR (13 °C) ................................ ......................... 37 Figure 2 - 12. NCr(N i Pr 2 )(O 2 CPh)(Cp) ( 3 ) 13 C NMR (13 °C) ................................ ....................... 38 Figure 2 - 13. NCr(N i Pr 2 )(Cp)Cl ( 4 ) 1 H NMR ................................ ................................ ................ 39 Figure 2 - 14. NCr(N i Pr 2 )(Cp)Cl ( 4 ) 13 C NMR ................................ ................................ .............. 40 xi Figure 2 - 15. [NCr(N i Pr 2 ) (Cp)(NCMe)][SbF 6 ] ( 5 ) 1 H NMR (In Situ Reaction) ........................... 41 Figure 2 - 16. [NCr(N i Pr 2 )(Cp)(NCMe)][SbF 6 ] ( 5 ) 13 C NMR (In Situ Reaction) .......................... 42 Fig ure 2 - 17. [NCr(N i Pr 2 )(Cp)(NCMe)][SbF 6 ] ( 5 ) 19 F NMR (In Situ Reaction ) .......................... 43 Figure 3 - 1. Various anions explored in this study. ................................ ................................ ....... 50 Fig ure 3 - 2. 14 N NMR analysis of [NCr(N i Pr 2 ) 2 (PMePh 2 )] + X - in CDCl 3 where X = SbF 6 (right) and PF 6 (left). The peak at 309 ppm (labelled with a *) represents dissolved N 2 , which was referenced as an internal standard. ................................ ................................ ................................ 51 Figure 3 - 3. Plot of H vs model predicted H using H values determine from CD 3 CN with an assumption of constant S . ................................ ................................ ................................ .......... 59 Figure 3 - 4. Lewis structures depicting the typical resonance forms of a low valent metal - ........ 61 Figure 3 - 5. Resonance forms discovered in NRT analysis of [NCr(NH 2 ) 2 (P(OMe) 3 ] + . .............. 63 Figure 3 - 6. NRT analysis of [NCr(NH 2 ) 2 (PPhMe 2 )] + ................................ ................................ ... 64 Figure 3 - 7. Eyring Plot for NCr(N i Pr 2 ) 2 I in CD 3 CN. The value obtained was - 1 e.u. (± 0.5). ..... 76 Figure 3 - 8. Eyring Plot analysis of NCr(N i Pr 2 ) 2 I in CDCl 3 . The value obtained for S was - 0.6(0.3). ................................ ................................ ................................ ................................ ......... 77 Figure 3 - 9. Eyring Plot analysis of NCr(N i Pr 2 ) 2 OPh in CDCl 3 . The value obtained for S was - 3.1(0.5). ................................ ................................ ................................ ................................ ......... 78 Fi gure 3 - 10. Eyring Plot analysis of NCr(N i Pr 2 ) 3 in CDCl 3 . The value obtained for S was - 5.7(0.7) ................................ ................................ ................................ ................................ .......... 79 Figure 3 - 11. Eyring Plot analysis of NCr(N i Pr 2 ) 2 Pyrr 3C6H3(CF3)2 in CDCl 3 . The value obtained for S was - 3.7(0.4) ................................ ................................ ................................ ........................... 80 Figure 3 - 12. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (PMe 3 )]SbF 6 ( 3a ) in CD 3 CN .......................... 81 Figure 3 - 13. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (PMe 3 )]SbF 6 ( 3a ) in CD 3 CN ......................... 82 Figure 3 - 14. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (PMe 3 )]SbF 6 ( 3a ) in CD 3 CN ......................... 83 Figure 3 - 1 5. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (PMe 3 )]SbF 6 ( 3a ) in CD3CN ........................ 84 Figure 3 - 16. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (P n Bu 3 )]SbF 6 ( 3b ) in CDCl 3 ........................... 85 Figure 3 - 17. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (P n Bu 3 )]SbF 6 ( 3b ) in CDCl 3 .......................... 86 Figure 3 - 18. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (P n Bu 3 )]SbF 6 ( 3b ) in CDCl 3 .......................... 87 xii Figure 3 - 19. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (P n Bu 3 )]SbF 6 ( 3b ) in CDCl 3 .......................... 88 Figure 3 - 20. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (P i Bu 3 )]SbF 6 ( 3c ) in CDCl 3 ............................ 89 Figure 3 - 21. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (P i Bu 3 )]SbF 6 ( 3c ) in CDCl 3 ........................... 90 Figure 3 - 22. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (P i Bu 3 )]SbF 6 ( 3c ) in CDCl 3 ........................... 91 Figure 3 - 23. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (P i Bu 3 )]SbF 6 ( 3c ) in CDCl 3 ........................... 92 Figure 3 - 24. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (P i Pr 3 )]SbF 6 ( 3d ) in CDCl 3 ............................. 93 Figure 3 - 25. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (P i Pr 3 )]SbF 6 ( 3d ) in CDCl 3 ............................ 94 Figure 3 - 26. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (P i Pr 3 )]SbF 6 ( 3d ) in CDCl 3 ............................ 95 Figure 3 - 27. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (P i Pr 3 )]SbF 6 ( 3d ) in CDCl 3 ............................ 96 Figure 3 - 28. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (PCy 3 )]SbF 6 ( 3e ) in CDCl 3 ............................. 97 Figure 3 - 29. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (PCy 3 )]SbF 6 ( 3e ) in CDCl 3 ............................ 98 Figure 3 - 30. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (PCy 3 )]SbF 6 ( 3e ) in CDCl 3 ............................ 99 Figure 3 - 31. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (PCy 3 )]SbF 6 ( 3e ) in CDCl 3 .......................... 100 Figure 3 - 32. 14 N NMR Spectrum for [NCr(N i Pr 2 ) 2 (PCy 3 )]SbF 6 ( 3e ) in CDCl 3 ......................... 101 Figure 3 - 33. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhMe 2 )]SbF 6 ( 3e ) in CDCl 3 ...................... 102 Figure 3 - 34. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhMe 2 )]SbF 6 ( 3e ) in CDCl 3 ..................... 103 Figure 3 - 35. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhM e 2 )]SbF 6 ( 3e ) in CDCl 3 ..................... 104 Figure 3 - 36. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhMe 2 )]SbF 6 ( 3e ) in CDCl 3 ..................... 105 Figure 3 - 37. 1 H NMR Spect rum for [NCr(N i Pr 2 ) 2 (PPh 2 Me)]SbF 6 ( 3g ) in CDCl 3 ...................... 106 Figure 3 - 38. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 Me)]SbF 6 ( 3g ) in CDCl 3 ..................... 107 Figure 3 - 39. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 Me)]SbF 6 ( 3g ) in CDCl 3 ..................... 108 Figure 3 - 40. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 Me)]SbF 6 ( 3g ) in CDCl 3 ..................... 109 Figure 3 - 41. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhEt 2 )]SbF 6 ( 3h ) in CDCl 3 ....................... 110 Figure 3 - 42. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhEt 2 )]SbF 6 ( 3h ) in CDCl 3 ...................... 111 Figure 3 - 43. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhEt 2 )]SbF 6 ( 3h ) in CDCl 3 ...................... 112 xiii Figure 3 - 44. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhEt 2 )]SbF 6 ( 3h ) in CDCl 3 ....................... 113 Figure 3 - 45. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 Et)]SbF 6 ( 3i ) in CDCl 3 ........................ 114 Figure 3 - 46. 13 C NMR Spectrum for [NCr( N i Pr 2 ) 2 (PPh 2 Et)]SbF 6 ( 3i ) in CDCl 3 ....................... 115 Figure 3 - 47. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 Et)]SbF 6 ( 3i ) in CDCl 3 ........................ 116 Figure 3 - 48. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 Et)]SbF 6 ( 3i ) in CDCl 3 ........................ 117 Figure 3 - 49. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 n Bu)]PF 6 ( 3j ) in CDCl 3 ........................ 118 Figure 3 - 50. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 n Bu)]PF 6 ( 3j ) in CDCl 3 ....................... 119 Figure 3 - 51. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 n Bu)]PF 6 ( 3j ) in CDCl 3 ....................... 120 Figure 3 - 52. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 n Bu)]PF 6 ( 3j ) in CDCl 3 ....................... 121 Figure 3 - 53. 14 N NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 n Bu)]PF 6 ( 3j ) in CDCl 3 ...................... 122 Figure 3 - 54. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 Cy)]SbF 6 ( 3k ) in CDCl 3 ...................... 123 Figure 3 - 55. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 C y)]SbF 6 ( 3k ) in CDCl 3 ..................... 124 Figure 3 - 56. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPh 2 Cy)]SbF 6 ( 3k ) in CDCl 3 ..................... 125 Figure 3 - 57. 19 F NMR Spect rum for [NCr(N i Pr 2 ) 2 (PPh 2 Cy)]SbF 6 ( 3k ) in CDCl 3 ..................... 126 Figure 3 - 58. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhCy 2 )]SbF 6 ( 3l ) in CDCl 3 ....................... 127 F igure 3 - 59. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhCy 2 )]SbF 6 ( 3l ) in CDCl 3 ...................... 1 28 Figure 3 - 60. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhCy 2 )]SbF 6 ( 3l ) in CDCl 3 ....................... 129 Figure 3 - 61. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhCy 2 )]SbF 6 ( 3l ) in CDCl 3 ....................... 130 Figure 3 - 62. 14 N NMR Spectrum for [NCr(N i Pr 2 ) 2 (PPhCy 2 )]SbF 6 ( 3l ) in CDCl 3 ...................... 131 Figure 3 - 63. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (P(OEt) 3 )]SbF 6 ( 3m ) in CDCl 3 ..................... 132 Figure 3 - 64. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (P(OEt) 3 )]SbF 6 ( 3m ) in CDCl 3 .................... 133 Figure 3 - 65. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (P(OEt) 3 )]SbF 6 ( 3m ) in CDCl 3 .................... 134 Figure 3 - 66. 19 F NMR Spectrum for [N Cr(N i Pr 2 ) 2 (P(OEt) 3 )]SbF 6 ( 3m ) in CDCl 3 .................... 135 Figure 3 - 67. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (P(O i Pr) 3 )]SbF 6 ( 3n ) in CDCl 3 ..................... 136 Figure 3 - 68. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (P(O i Pr) 3 )]SbF 6 ( 3n ) in CDCl 3 .................... 137 xiv Figure 3 - 69. 31 P NMR Spectrum for [NCr(N i Pr 2 ) 2 (P(O i Pr) 3 )]SbF 6 ( 3n ) in CDCl 3 .................... 138 Figure 3 - 70. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (P(O i Pr) 3 )]SbF 6 ( 3n ) in CDCl 3 .................... 139 Figure 3 - 71. 14 N NMR Spectrum for [NCr(N i Pr 2 ) 2 (P(O i Pr) 3 )]SbF 6 ( 3n ) in CDCl 3 .................... 140 Figure 3 - 72. 1 H NMR Spectrum for [NCr(N i Pr 2 ) 2 (NCCH 3 )]SbF 6 ( 2 ) in CDCl 3 ........................ 141 Figure 3 - 73. 13 C NMR Spectrum for [NCr(N i Pr 2 ) 2 (NCCH 3 )]SbF 6 ( 2 ) in CDCl 3 ....................... 142 Figure 3 - 74. 19 F NMR Spectrum for [NCr(N i Pr 2 ) 2 (NCCH 3 )]SbF 6 ( 2 ) in CDCl 3 ........................ 143 Figure 3 - 75. 14 N NMR Spectrum for [NCr(N i Pr 2 ) 2 (NCCH 3 )]SbF 6 ( 2 ) in CDCl 3 ....................... 144 Figure 4 - 1. Plot displaying the calculate vs. experimental rate constant. The y - axis was calculated by using the experimental LDP and %V bur values in the mode l d escribed above. The error bars are displayed at the 95% confidence level. ................................ ................................ 160 Figure 4 - 2.Plot displaying the calculate vs. exp erimental rate constant for the full set of ligands tested. The y - a xis was calculated by using the experimental LDP and %Vbur values in the model described above. The error bars are displayed at the 95% confidence level. The grey square are the aryloxide p oints and were not included in the regression. ................................ .................... 163 Figure 4 - 3. Expanded model displaying the poor fit for catalyst 3 . ................................ ........... 166 Figure 4 - 4. Representations of the proposed possible catalyst structures th e hydroamination reaction using Ti(NMe 2 ) 4 as a catalyst. ................................ ................................ ....................... 169 Figure 4 - 5. Series of ligands with the computationally modelled LDP fit against the experimental LDP. The oran ge point is the predic ted imido value fit to the best fit line of the model. .......... 170 Figure 4 - 6. Fit of the computed and experimental LDP values including a steric term to account for the truncated chromium molecule in the calculations. The orange point is the imido theoretical value fit to the best fit line. ................................ ................................ ....................... 172 Figure 4 - 7. Plot of the predicted imido value to our model. The orange diamond is - Ti(NPh), the g reen diamond is - Ti(NHPh) 2 , and the purple diamond is - Ti(NMe 2 ) 2 . ................................ ..... 173 Figure 4 - 8. DOSY spectrum of Ti(dithioBINAP)(NMe 2 ) 2 (6) at 25 °C. ................................ .... 177 Figure 4 - 9. Molecular weight calibration of Ti(dithioBINAP)(NMe 2 ) 2 (6) at 25 °C. ................ 178 Figure 4 - 10. Molecular weight calibration of Ti(dithioBINAP)(NMe 2 ) 2 (6) at 50 °C. .............. 180 Figure 4 - 11. Molecular weight calibration of Ti(dithioBINAP)(NMe 2 ) 2 (6) with addition of aniline (4 equiv.) at 50 °C. ................................ ................................ ................................ .......... 181 Figure 4 - 12. Represe ntative Plots for Kinetics Plot of [1 - phenylpropyne] vs time with Ti(NMe 2 ) 2 ( bis - phenoxide 2tBu - 4Me ) ( 5 ) ................................ ................................ ......................... 194 xv Figure 4 - 13. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me ) in CDCl 3 . ................................ ........... 195 Figure 4 - 14. 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me ) in CDCl 3 . ................................ .......... 196 Figure 4 - 15. 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me ) in CDCl 3 . ................................ .......... 197 Figure 4 - 16. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me5F ) in in CDCl 3 . ................................ .... 198 Figure 4 - 17. 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me5F ) in in CDCl 3 . ................................ ... 199 Figure 4 - 18. 19 F NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me5F ) in in CDCl 3 . ................................ ... 200 Figure 4 - 19. 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me5F ) in in CDCl 3 . ................................ ... 201 Figure 4 - 20. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Me ) in CDCl 3 . ................................ .......... 202 Figure 4 - 21. 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Me ) in CDCl 3 . ................................ .......... 203 Figure 4 - 22. 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Me ) in CDCl 3 . ................................ ......... 204 Figure 4 - 23. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr 2 - [C6H3(CF3)2] ) in CDCl 3 . ............................ 205 Figure 4 - 24. 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr 2 - [C6H3(CF3)2] ) in CDCl 3 . ........................... 206 Figure 4 - 25. 19 F NMR S pectrum of NCr(N i Pr 2 ) 2 (Pyrr 2 - [C6H3(CF3)2] ) in CDCl 3 ............................. 207 Figure 4 - 26. 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr 2 - [C6H3(CF3)2] ) in CDCl 3 . ........................... 208 Figure 4 - 27. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Ph ) in CDCl 3 . ................................ ............ 209 Figure 4 - 28. 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Ph ) in CDCl 3 . ................................ .......... 210 Figure 4 - 29. 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Ph ) in CDCl 3 . ................................ .......... 211 Figure 4 - 30. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Tol ) in CDCl 3 . ................................ ........... 212 Figure 4 - 31. 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Tol ) in CDCl 3 . ................................ .......... 213 Figure 4 - 32. 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Tol ) in CDCl 3 . ................................ ......... 214 Figure 4 - 33. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2,4 - diMe ) in CDCl 3 . ................................ .... 215 Figure 4 - 34. 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2,4 - diMe ) in CDCl 3 . ................................ ... 216 Figure 4 - 35. 14 N NMR Spect rum of NCr(N i Pr 2 ) 2 (OPh 2,4 - diMe ) in CDCl 3 . ................................ ... 217 Figure 4 - 36. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - Me ) in CDCl 3 . ................................ . 218 Figure 4 - 37. 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - Me ) in CDCl 3 . ................................ 219 xvi Figure 4 - 38. 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - Me ) in CDCl 3 . ................................ 220 Figure 4 - 39. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2,4,5 - triMe ) in CDCl 3 . ................................ .. 221 Figure 4 - 40. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2,4,5 - tr iMe ) in CDCl 3 at 29 °C. .................. 222 Figure 4 - 41. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2,4,5 - triMe ) in CDCl 3 at 29 °C. .................. 223 Figure 4 - 42. 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2,4,5 - triMe ) in CDCl 3 . ................................ . 224 Figure 4 - 43. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4,5 - diMe ) in CDCl 3 . ............................. 225 Figure 4 - 44. 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4,5 - diMe ) in CDCl 3 . ............................ 226 Figure 4 - 45. 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4,5 - diMe ) in CDCl 3 . ............................ 227 Figure 4 - 46. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - Me - 4 - OMe ) in CDCl 3 . ................................ 228 Figure 4 - 47. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - Me - 4 - OMe ) in CDCl 3 at 26 °C. ................ 229 Figure 4 - 48. 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - Me - 4 - OMe ) in CDCl 3 . ............................... 230 Figure 4 - 49. 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - Me - 4 - OMe ) in CDCl 3 . .............................. 231 Figure 4 - 50. 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - OMe ) in CDCl 3 . ............................... 232 Figure 4 - 51. 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - OMe ) in CDCl 3 . .............................. 233 Figure 4 - 52. 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - OMe ) in CDCl 3 . .............................. 234 Figure 4 - 53. 1 H NMR Spec trum of NCr(N i Pr 2 ) 2 (SNap) in CDCl 3 . ................................ ............ 235 Figure 4 - 54. 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (SNap) in CDCl 3 . ................................ ........... 236 Figure 4 - 55. 14 N NMR Spec trum of NCr(N i Pr 2 ) 2 (SNap) in CDCl 3 . ................................ ........... 237 Figure 5 - 1. Plot of the LDP versus the E 1/2 values of a U(V)/U(VI) redox couple for a series of ligands. ................................ ................................ ................................ ................................ ........ 244 Figure 5 - 2. Comparison of the olefinic region of the 1 H NMR spectrum before (top) and after (bottom) catalysis showing complete cyclization. ................................ ................................ ...... 249 Figure 5 - 3. Structures of top : Cp*UI 2 (PyPyr Me3 )(thf) ( 3a ), Cp*UI 2 (PyPyr Me2 )(thf) ( 5 ), b ottom : Cp*UCl 2 (PyPyr Me2Tol )(thf) ( 4a ), and Cp*UI 2 (PyPyr Me2Tol )(thf) ( 4b ), Hydrogens and co - crystallized solvents removed for clarity. ................................ ................................ ................... 251 Figure 5 - 4. Comparison of the solid - 2 (N - C) - CH 2 SiMe 2 NTMS)U(Cp*)(PyPyr Me3 ) ( 6 2 (N - C) - CH 2 SiMe 2 NTMS)U(Cp*)( PyPyr Me2 ) ( 7 ) (right). ................................ ................................ ....... 254 xvii Figure 5 - 5. Crystals s truc tures of U(PyPyr Me3 ) 4 and U(PyPyr Me2Cl ) 4 byproducts. The structure of U(PyPyr Me3 ) 4 crystallizes with four - fold symmetry, the grown structure is shown. Hydrogens and co - crystallized solvents removed for purity. ................................ ................................ ............... 255 Figure 5 - 6. Crystal structure of UI 2 (PyPyr tBu2 ) 2 ( 6 ). Hydrogens and co - crystallized diethyl ether molecules removed for clarity. ................................ ................................ ................................ ... 256 Figure 5 - 7. Crude 1 H NMR of [N(TMS) Cy] 2 U(PyPyr Me2 ) 2 , 1 . ................................ .................. 264 Figure 5 - 8. Crude 1 H NMR of [N(TMS)Cy]U(PyPyr Me2 ) 3 , 2 . ................................ .................... 265 Figure 5 - 9. Crude 1 H NMR of U(PyPyr Me2 ) 4 . Ther e are still signals present for 2 , but the majority of the N(TMS)Cy amides have been displaced. ................................ ................................ ......... 266 Figure 5 - 10. 1 H NMR of Cp*UCl 2 (PyPyr Me3 )(thf) ( 3a ). ................................ ............................ 267 Figure 5 - 11. Best 1 H NMR of Cp*UI 2 (PyPyr Me3 )(thf), ( 3b ). The spectrum contains impurities from unknown species that are not removed by recrystallization. ................................ .............. 268 Figure 5 - 12. 1 H NMR of Cp*UCl 2 (PyPyr Me2Tol )(thf), ( 4a ). ................................ ........................ 269 Figure 5 - 13. 1 H NMR of Cp*UI 2 (PyPyr Me2Tol )(thf) ( 4b ). ................................ ........................... 270 Figure 5 - 14. 1 H NMR o f Cp*UI 2 (PyPyr Me2 )(thf) ( 5 ). ................................ ................................ . 27 1 Figure 5 - 15. 1 2 (N - C) - CH 2 SiMe 2 NTMS)U(Cp*)(PyPyr Me3 ) ( 6 ). ........................... 272 Figure 5 - 16. Crude 1 H NMR 2 (N - C) - CH 2 SiMe 2 NTMS)U(Cp*)(PyPyr Me2 ) ( 7 ). ................ 273 Figure 5 - 17. Arrayed Spectra from the catalysis of 6 (10 mol%, 65 °C, C 6 D 6 ) with DPAP. Each spectrum was taken in 10 - minute intervals. The rea ction appears to be complete after ~150 minutes (spectrum 15). ................................ ................................ ................................ ................ 274 Figure 5 - 18. Arrayed Spectra from the catalysis of 6 (5 mol%, 65 °C, C 6 D 6 ) with DPAP. Each spectrum was taken in 10 - minute int ervals. The reaction appears to be complete after ~330 minutes (spectrum 33). ................................ ................................ ................................ ................ 275 Figure 5 - 19. Arrayed Spectra from the catalysis of 7 (5 mol%, 65 °C, C 6 D 6 ) with DPAP. Each spectrum was taken i n 10 - minute intervals. The reaction appears to be complete after ~190 minutes (spectrum 19). ................................ ................................ ................................ ................ 276 Figure 6 - 1. Single crystal structure of (Cp*)UI(NAr iPr6 ) ( 2 ). Solvent molecules and hydrogens re moved for clarity. ................................ ................................ ................................ ..................... 281 Figure 6 - 2. Single crystal structure of (Cp*)UI(NHAr iPr6 ) ( 3 ). Hydrogens (except NH) and solvent removed for clarity. ................................ ................................ ................................ ........ 284 Figure 6 - 3. Single crystal structure of (Cp*)UI(NHAr Me6 ) ( 5 ). Hydrogens (except NH) removed for clarity. ................................ ................................ ................................ ................................ .... 285 xviii Figure 6 - 4. Single crystal structure of (Cp*)UI(NAr Me6 )(thf) 2 , 4 . Hyd rogens (except NH) removed for clarity. ................................ ................................ ................................ ..................... 287 Figure 6 - 5. Single crystal structure of (Cp*)U(NHAr Me6 ) 2 , 6 . Hydrogens (except NH) removed for clarity. ................................ ................................ ................................ ................................ .... 289 Figure 6 - 6. Top : Single crystal structure of 7 Bottom : Synthesis of UI(NHAr Me6 ) 2 7 . ................................ ................................ ....................... 290 Figure 6 - 7. Top : Crystal structure of UI(NHAr iPr6 ) 2 8 . Hydrogens (excepts N - molecule removed for clarity. Bottom : Synthesis of (I)U(NHAr iPr6 ) 2 ( 8 ). ................................ .. 292 Figure 6 - 8. Co ntrasting solid state structures of 8 when crys tallized form diethyl ether (green) and n - hexane (light blue). ................................ ................................ ................................ ........... 293 Figure 6 - 9. Crystal structure of U(NHAr iPr6 ) 2 ( 9 ). Hydrogens, except N - molecule removed for clarity. The stru cture is grown to show the full molecule, but 9 crystallizes as half of the molecule with a 2 - fold rotational axis. ................................ ................................ .. 295 Figure 6 - 10. Top : Crystal structure of [U(NHAr iPr6 ) 2 ][BAr F 24 ] ( 10 ) Hydrog ens, except N - anion, disorder, and solvent molecule removed for clarity. Bottom : Space filling structures of 8 (left) and 10 (right). ................................ ................................ ................................ ..................... 297 Figure 6 - 11. EPR spectra of (a) (I)U(NHAr iPr6 ) 2 ( 8 ) and (b) [U(NHAr iPr6 ) 2 ][BAr F 24 ] ( 10 ). Measurement parameters for both spectra were: microwave frequency, 9.40 GHz, microwave power, 0.79 mW; field modulation amplitude, 1 mT; and sample temperature, 6 K. ................ 298 Figure 6 - 12. Temperature dependence of the magnetic susceptibility for UI(NHAr iPr6 ) 2 ( 8 ) (black squares), U(NHAr iPr6 ) 2 ( 9 ) (red circles), and [U(NHAr iPr6 ) 2 ][BAr F 24 ] ( 10 ) (blue diamonds) collected at 5000 Oe. ................................ ................................ ................................ ................... 301 Figure 6 - 13. 1 H NMR spectrum of (Cp*)UI(NAr iPr6 ) in C 6 D 6 ................................ ................... 309 Figure 6 - 14. 1 H NMR spectrum of (Cp*)UI(NHAr iPr6 ) in C 6 D 6 ................................ ................. 310 Figure 6 - 15. 1 H NMR spectrum of (Cp*)UI(NAr Me6 ) in C 6 D 6 ................................ ................... 311 Figure 6 - 16. Crude 1 H NMR spectrum of (Cp*)UI(NHAr Me6 ) in C 6 D 6 ................................ ..... 312 Figure 6 - 17. Crude 1 H NMR of (Cp*)U(NHAr Me6 ) 2 in C 6 D 6 . Inset shows the broad peaks attributed to fluxionality. ................................ ................................ ................................ ............ 313 Figure 6 - 18. 1 H NMR spectrum of UI(NHAr Me6 ) 2 in C 6 D 6 ................................ ........................ 314 Figure 6 - 19. 1 H NMR Spectrum of UI(NHAr iPr6 ) 2 ( 8 ) at 30 °C in toluene - d 8 .......................... 315 Figure 6 - 20. 1 H NMR Spectrum of UI(NHAr iPr6 ) 2 ( 8 ) at ambient temperature in toluene - d 8 .... 316 Figure 6 - 21. 1 H NMR Spectrum of U(NHAr iPr6 ) 2 ( 9 ) at 25 °C in toluene - d 8 ............................. 317 xix Figure 6 - 22 . 1 H NMR Spectrum of [U(NHAr iPr6 ) 2 ] + [BAr F 24 ] ( 10 ) at 25 °C in THF - d 8 ........... 318 Figure 6 - 23. UV - vis/NIR spectrum of 2 in 1 mm cuvette ................................ .......................... 319 Figure 6 - 24. UV - vis/NIR spectrum of 3 in 1 mm cuvette ................................ .......................... 320 Figure 6 - 25. UV - vis/NIR spectrum of 6 in 1 mm cuvette. ................................ ......................... 320 Figure 6 - 2 6. UV - vis/NIR spectrum of 8 at low concentration (~0.5 mM). ................................ 321 Figure 6 - 27. NIR spectrum of 8 at high concentration (~10 mM). ................................ ............. 321 Figure 6 - 28. UV - vis/NIR spectrum of 9 at low concentration (~0.5 mM). ................................ 322 Figure 6 - 29. UV - vis/NIR spectrum of 10 at low concentration (~0.5 mM). .............................. 322 Figure 6 - 30. EPR spectra of (I)U(NHAr iPr6 ) 2 ( 8 ), U(NHAr iPr6 ) 2 ( 9 ), and U(NHAr iPr6 ) 2 + ( 10 ). Measurement parameters for both spectra were: microwave frequency = 9.40 GHz, microwave power = 0.79 mW, field modulation amplitude = 1 mT, and s ample temperature = 6 K. .......... 324 Figure 6 - 31. Parallel and perpendicular EPR spectra for 8 . Instrument conditions for these scans were: (a) perpendicular mode: microwave frequency = 9.643 GHz, and (b) parallel mode: microwave frequency = 9.441 GHz. Conditions common to the two spectra are: microwa ve power = 1.0 mW, field modulation amplitude = 1.6 mT, field modulation frequency = 10 kHz, and sample temperature = 5 K. ................................ ................................ ................................ ... 325 Fi gure 6 - 32. Temperature dependence of magnetic susceptibility (µ eff ) for 8 collected at 5000 Oe. ................................ ................................ ................................ ................................ ..................... 327 Figure 6 - 33. Temperature depen dence of magnetic suscepti bility (µ eff ) for 9 collected at 5000 Oe. ................................ ................................ ................................ ................................ ..................... 327 Figure 6 - 34. Temperature dependence of magnetic susceptibility (µ eff ) for 10 collected at 5000 Oe. ................................ ................................ ................................ ................................ ............... 328 Figure 6 - 35. Temperature dependence of solution state magnetic susceptibil ity ( µ eff ) for 9 . .... 328 Figure 6 - 36. Full structure of ( 2 ) including solvent ................................ ................................ .... 331 Figure 6 - 37. Full structure of ( 3 ) including solvent and molecular disorder .............................. 333 Figure 6 - 38. Full structure of (4) ................................ ................................ ................................ 335 Figure 6 - 39. Full structure of ( 5 ) ................................ ................................ ................................ 337 Figure 6 - 40. Full structure of ( 6 ) ................................ ................................ ................................ 339 Figure 6 - 41. Full str ucture of ( 7 ) ................................ ................................ ................................ 341 xx Figure 6 - 42. Full structure of ( 8 ) solvent and molecular disorder ................................ .............. 343 Figure 6 - 43. Full structure of ( 8 ) includi ng solvent and molecular disorder .............................. 345 Figure 6 - 44. Full structure of ( 9 ) including solvent. ................................ ................................ ... 347 Figure 6 - 45. Full structure of ( 10 ) including anion, solvent, and molecular disorder ................ 349 Figure 7 - 1. Concentration of CO 2 in the atmosphere measured from 1958 to 2018 at the Manua Loa Observatory. Figure taken from reference 3. ................................ ................................ ....... 354 Figure 7 - 2. Heterometallic cluster reported by Shilov to be the active species in his dinitrogen reduction system. 36 ................................ ................................ ................................ ...................... 358 Figu re 7 - 3. Molybdenum cluster isolated from attempts at catalyst synthesis. Hydrogens are removed from the tetrabutyl ammonium for clarity. ................................ ................................ .. 364 Figure 7 - 4. GC/MS trace of an aliquot of the reactio n solution from a reaction that produced hydrazine according to the indica tor solution. 48 The indicated peak at 13.424 represents the peak for stilbene. ................................ ................................ ................................ ................................ . 368 Figure 7 - 5 GC/MS trace of an equal a liquot fro m the same solution shown in Figure 7 - 4 spiked with 0.25 g hydrazine (added as hydrate). 48 The indicated peak at 13.424 represents the peak for stilbene. ................................ ................................ ................................ ................................ ....... 369 xxi LIST OF SCHEMES Scheme 1 - 1. General synthesis of chromium starting materials for LDP analysis. a Synthesized using literature procedure. 26 ................................ ................................ ................................ .......... 12 Scheme 2 - 1. Synthesis of NCr(N i Pr 2 ) 2 (Cp), 1 , a nd NCr(N i Pr 2 )(Cp)(O 2 CPh), 3, from NCr(N i Pr 2 )I. ................................ ................................ ................................ ................................ .. 21 Scheme 2 - 2. Synthesis of NCr(N i Pr 2 )(Cp)(Cl), 4 . ................................ ................................ ........ 23 Scheme 2 - 3. In situ synthesis of NCr(NPr i 2 )Cp(NCMe), 5 . ................................ ........................ 24 Scheme 3 - 1. General procedure for the synthesis of PR 3 complexes from NCr(N i Pr 2 ) 2 I. X = SbF 6 , PF 6 , BAr F 24 , BAr F 20 , BPh 4 , Al(O t Bu F 9 ) 4 ................................ ................................ ....................... 48 Scheme 4 - 1. General procedures f or synthesis of the chromium complexes. ............................ 154 Scheme 4 - 2. Reaction scheme for the hydroamination kinetics experiments. ........................... 155 Scheme 4 - 3. Comparison of the molecular weight of the monomer and dimerized catalyst. .... 179 Scheme 5 - 1. Example of an intramolecular hy droamination. ................................ ..................... 246 Scheme 5 - 2. Proposed synthesis of the uranium precatalysts. ................................ .................... 247 Scheme 5 - 3. Attempted synthesis of U(PyPyr) 2 Cl 2 from UCl 4 . ................................ ................. 248 Scheme 5 - 4. Synthesis of mixed amide PyPyr compounds. ................................ ....................... 248 Scheme 5 - 5. Synthesis of Cp*UX 2 (PyPyr)(thf) from UX 4 star ting materials. ........................... 252 Scheme 5 - 2 (N - C) - CH 2 SiMe 2 NTMS)U(Cp*)(PyPyr Me3 ) ( 6 ) from UCl 4 . ........ 253 Scheme 6 - 1. Reaction of (Cp*)U(I) 2 (thf) 3 with DiPPN 3 . ................................ ........................... 280 Scheme 6 - 2. Top. Discovery of (Cp*)UI(NAr i Pr ) ( 2 ) from (Cp*)U(I) 2 (thf) 3 and N 3 Ar iPr6 disproportionation. Middle. Rational synthesis of 2 . Bottom. Synthesis of (Cp*)UI(NHAr i Pr6 ) ( 3 ) from (Cp*)U(I) 2 (thf) 3 and NaNAr iPr6 ................................ ................................ .......................... 282 Scheme 6 - 3. Synthesis of (Cp*)UI(NAr Me6 ) ( 4 ) and (Cp*)UI(NHAr Me6 ) ( 5 ). ........................... 286 Scheme 6 - 4. Synthesis of (Cp*)U(NHAr Me6 ) 2 ( 6 ). ................................ ................................ ..... 288 Scheme 6 - 5. Synthesis of U(NHAr iPr6 ) 2 ( 9 ) from 8 . ................................ ................................ ... 294 Scheme 6 - 6. Synthesis of [U(NHAr iPr6 ) 2 ][BAr F 24 ] ( 10 ) from 9. ................................ ................. 297 xxii KEY TO SYMBOLS AND ABBREVIATIONS LDP Ligand Donor Parameter IR Infrared Radiation Tolman Cone Angle Å Angstrom Tolman Electronic Parameter %V bur Percent Buried Volume NHC N - Heterocyclic Carbene HVM High Valent Metal Cp Cyclopentadiene NBO Natural Bond Orbital NMR Nuclear Magnetic Resonance THF Tetr ahydrofuran HPCC High Power Computing Cluster MBO Mayer Bond Order G09 Gaussian 09 DOSY Diffusion Ordered Spectroscopy ROESY Rotating - frame Overh auser Spectrosco py e.u. Entropy unit d Corrected phosphine electronic parameter H Enthalpy of Activation S Entropy of Activation xxiii G Free Energy of Activation DFT Density Functional Theory NRT Natural Resonance Theory AN Acceptor Number Std. Dev. Standard Deviation Pyrr. Pyrrole Et Ethyl Ph Phenyl Nap Napthyl h hour(s) H 2 dpm 5,5 - dimethyldipyrrolylmethane dim Diindolylmethane SST Spin Saturation Transfer DOE Department of Energy SCGSR Scie nce Graduate Student Research LANL Los Alamos National Laboratory PyPyr 2 - - pyridylpyrrol e TMS trimethylsilyl RT Room Temperature Cp* Pentamethylcyclopentadiene UV Ultraviolet Radiation Vis Visible Radiation DPAP 2,2 - diphenyl - 1 - amino - 4 - pentene xxiv DiPP Diisopropylphenyl Ar iPr6 2,6 - bis(2,4,6 - triisopropylphenyl) - phenyl Ar Me6 2,6 - bis(2,4,6 - trimethylphenyl) - phenyl SQUID Superconducting Quantum Interference Device Ppm Parts Per Million PDMAB p - dimethylaminobenzaldehyde PC L - - dipalmitoylphosphatidylcholine Equiv. Equivalents Cy Cyclohexyl GC Gas Ch romatography GC/MS Gas Chromatography/Mass Spectrometry 1 Introduction 1.1 Background and Motivation Transition metal catalyzed reactions find use in all scales of modern chemistry. Whether it is an academic ian using a metathesis reaction for the final step in a natural product synthesis, or multibillion - dollar polymerizations, the catalysts employed need to work well . One of the most common endeavors to improve those catalysts is to modify the ancillary ligand s ( a ligand that does not directly participate i n bond making and bond breaking in the catalytic cycle). Ligand manipulations allow a researcher to tune subtle electronic and steric factors that can control everything from catalytic rates and product selectivity , to substrate scope and catalyst stabilit y. For those who have been in the field for many years, and have years of knowledge and experience, making productive changes to a ligand is often based on intuition. Unfortunately, in many cases, that wealth of knowledge from experience is not always avai lable. Young graduate students charged with designing new catalysts, new industry researchers trying to make processes more energy efficient and cheaper, or new catalyst design where little literature background is available, are all examples of this knowl edge gap . That considered, it i s a wonder there are not more tools available for researchers to use in catalyst development. 1.2 Cone Angle and Electronic Parameter When Chadwick Tolman published his methods of quantitively characterizing various pho sphine ligands, it was a huge advance in catalyst development. 1 Parameterizing phosphine ligands based on quantitative values for size and donor ability made it possible to develop educated ligand design ideas with minimal prior knowledge. based on Ni(CO) 3 L complexes , where L represents various phosphine - type ligands. 2 Using IR spectroscopy , Tolman measured the A 1 stretching frequencies of the CO ligands to gain insight into how much 2 electron density the Ni is gaining from the L donation. When a ligand, L , donates mo re electron density to the Ni, the Ni can donate density orbital of the CO ligands. This back donation causes a measurable change in the IR stretch of the CO ligands. So, as a PR 3 donates decreases. The change in this frequency , when referenced to (CO) 3 NiP( t Bu) 3 i . is a simple and effective method for comparing the sterics of various phosphines. 1,3 Tolman made models of each phosphine and arranged the mo dels in a steric conformation that was as small as possible. The cone was then generated by making a cylindrical cone with its apex 2.28 Å from the P atom and the sides of the cone just touching the edges of the outermost atoms of the substituents. The mea A diagram is shown in Figure 1 - 1 below . Folding the ligand into the smallest possible confirmation allowed the comparisons between phosphines to be systematic rather than random due to orientation of the model. The 2.28 Å distance effectively adjusted the measurements to a P - Ni bond length. Both assumptions are quite crude, but considering the simplicity of the measurement, the Tolman cone angle has proven to be urning qualitative descriptions of ligand size into a quantitative measure . 3 Figure 1 - 1 . Representation of Tolman Cone Angle 1 1.3 Advancements in Modeling Ancillary Ligands system. The most useful advances have been modelling sterics based on experimental structural data, rather than models, and measuring sterics of ligands with less regular shapes. 4 - 8 One method that has proven to be useful to our research is percent buried volume (%V bur ) . 4,9 T his program allows a user to define a radius from the metal center ( 3.5 Å is the program default ) and project a sphere around the metal center at that radius . The program calculates the percentage of the sphere occupied by a given ligand. In thi s way, percent buried volume differs from the Tolman cone angle in that only atoms within the sphere contrib ute sterically, rather than a steric projection of the entire ligand. igands that are less symmetric than phosphines. Take, for example, N - heterocyclic carbene (NHC) ligands. Thi s class of ligands is becoming more prevalent . F reported by Hoveyda uses an NHC ligand. 10 These ligands are far from cylindrical. If we were to try to define a cone angle for NHC ligands, which prof ile should be used? Figure 1 - 2 shows how choosing a different profile drastically changes the output. 4 Figure 1 - 2 . Comparison of different NHC ligand profiles and resulting co ne angle. Because of the enormous difference in profiles, it is likely that neither extreme is accurate by s system to evaluate asymmetric phosphines, which is - group, cannot be applied to NHC ligands. A program like %V bur is much more useful in situations like this. Figure 1 - 3 shows how the buried volume program differs. The ligand in the figure is a 2 - aryl substituted pyrrole, b ecause %V bur simply measures sterics of the liga nd close to the metal, the shape of the ligand is irrelevant. As mentioned, the program also uses structural data such as a crystal structure or the oretically optimized structures to calculate the steric parameter. The recent update to the program has also made it possible to get a multidimensional steric map to see where the most sterically crowded areas of the ligand are (Figure 1 - 3, bottom). 9 5 Figure 1 - 3 . Top: Graphical representation of %V bur . Black shaded area represents buried volume. Bottom: Steric map output from SambV ca 2 .0 program. 9 The x and y axis are distance from center in Å. The map cuts off at 3.5 Å due to our defined radius. The colored conto ur represents the distance in angstroms in the z direction (perpendicular to the plane of the page). Like the steric parameter, th with the measurement ( the starting material Ni(CO) 4 is very toxic), and its application to modelin g reactions. 11 - 13 Despite the innovations, i f a chemist wants to develop a reaction based on a metal in a high oxidation state, Ti(IV) for example, phosphines are rarely going to be the ligand of choice. Other met potentia support a wider variety of ligands. 12,14 In Lev 6 wide variety of L - type lig ands. In either case though, the numbers are still not necessarily applicab le to metals in higher oxidation states. This is primarily due to the differences in bonding between a ligand and a metal in a high or low oxidation state. More specifically, the hi gh electronegativity of HVMs, and the availability of acceptor orbitals, me ans bonding between ligands and HVMs is very dependent on ligand to metal - interactions. In contrast, - interactions in low valent metal systems are usually metal to ligand based. Because of this, the preferred ligand choice in each system is quite differ ent. Low valent metals often bond more strongly to good - accepting ligands while high valent metals typically prefer - donating ligands. 1.4 Considerations for High Valent Metals The difference in metal - ligand interaction leads to a vastly different set of a ncillary ligands in HVM - based catalysts. Ancillary ligands vary from alkoxides (e.g. Kulinkovich cyclopropanation hydroamination catalysts), to cyclopentadienyl (Cp) rings and amides (e.g. ) . 15 - 19 Th e wide variety of ligands employed in HVM catalysis poses the most significant hurdle in comparing one ligand set to another. Traditionally, the only way to compare ligands against each other was to use metrics such as p K a , or Hammett parameters. 20 - 21 Such metrics deployed in discussions about interactions of the ligand with a proton do not accurately represent the interaction of the same ligand with a HVM. Much like the differences between low and high valent metals, interactions between a ligand and proton cannot - e ffects of the ligand. 7 1.5 Titanium Catalysis The lack of tools to develop high valent metal catalyzed reactions is unfortunate. There are many advantages to using high valent metals in catalysis. One such advantage is the abun dance of early transition metals. 19 While metals such as Pd , Pt, Ru, and Ir catalyze a number of incredibly useful reactions, the cost associated with using these elements can be a deterrent. That alo ne, though, does not warrant investigation of alternatives. The real driving force for using early transition metal s is the difference in reactivity they display. Our group has developed a number of interesting transformations based on a series of titaniu m catalysts, but this dissertation will focus on only hydroamination. 19, 22 A general mechanism for hydroamination is shown in Figure 1 - 4. This mechanism is adapted from a report by Bergman where many intermediates in the reaction were isolable through stoichiometric reactions. 23 8 Figure 1 - 4 . Hydroamination c atalytic cycle. 19 The mechanism for hydroamination that Bergman elucidated (shown above), proposed a rate - - elimination of amine re sults in regeneration of a metal imide. 23 This step is dependent on proton transfer from one coordinated amide to another. Making the m etal center more Lewis acidic may increase th is rate by increasing the acidity of the protons on the dative amine, thereby increasing the rate of protonation. The first step to make a catalyst more Lewis acidic, is to adjust the ancillary ligands. Here aga in, though, what changes can be made to make the metal center more Lewis acidic? 9 1.6 Ligand Donor Parameter In 2011 the Odom group answered this question with a report measuring various ligand donation abilities for HVM systems. 24 The system that was developed employed a Cr(VI) metal center as the reporter. Because the output is derived from a d 0 metal, the results are better suited for application in HVM catalysis. The ligand donation parameter, LDP, is a measurement system that could be regarded as the high valent analogue to t he Tolman electronic parameter. In place of the traditional carbonyl A 1 stretching frequency as a reporter, the LDP system uses a Cr - N bond rotation to measure donation from the ligand under investigation. A mod el complex is show in Figure 1 - 5 below. Figure 1 - 5 . Representative chromium structure for LDP determination. 24 The LDP system has been described in det ail in the original publication, but we will briefly discuss it here. 24 The LDP system functions through a competition for electron density. The highly electronegative - acceptor orbitals. Depending on the X ligand under study, availability of the acceptor orbitals is variable. When X is a poor donor, the acceptor orbitals are more vacant, allowing the N i Pr 2 groups to donate the ir nitrogen lo ne pair from the nitrogen to the chromium. This donation creates d ouble bond character in the Cr - N bond, hindering rotation about it . Thus, when X is a poor donor the barrier to rotation about the Cr - N bond is high and, if X is a strong donor, the Cr - N bon d has mostly single bond in character, and the barrier to rotation is low. 10 The barrier to rotation is measured using a simple series of NMR experiments to determine a rate of rotation about the Cr - N bond. 24 - 25 It is important to note that this is only possible because the hinder ed rotation of the Cr - N bond creates unique chemical shifts for the syn - and anti - isopropyl groups , and the rate of rotation about the Cr - N bonds l ie within the window of the NMR time scale. Using the Eyring equation, we can convert the rate of rotation i nto a free energy of activation, and, using an assumption about the entropy, we can convert the free energy value to a temperature independent enthalpy of activation, H . In short, an X ligan d that donates substantial electron density to a metal center will result in a low H and an X ligand that is a poor electron density donor will have a high H , or LDP. It is worth noting that in this system, the competition for electron density is based on both and effects , so the LDP is a sum of all electron density donation. It is worth stating that we measure and report LDP values to two decimal places, for example the LDP of pyrrole is 13.64 kcal/mol. This is not meant to imply that we can accura tely measure a t o the hundredth of a kcal. Rather, this is indicative of the precision with which we can perform these measurements. Different researchers on different instruments can reproduce LDP values with standard deviatio ns of only calories. Thus , the difference between ligands is considered quite accurate. Much like the Tolman system, the Odom group aimed to provide a complete characterization of the ligands studied. This means providing a steric analysis of the ligands a s well. Fortunately, almo st all of the chromium complexes synthesized are crystalline, so detailed steric analysis was also performed. 24 11 In general, the synthesis of the chromium complexes used in the LDP analysis is quite straightfo rward. Scheme 1 - 1 highlig hts the most common synthetic strategies to access molecules to study. 12 Scheme 1 - 1 . General synthesis of chromium starting materials for LDP analysis . a Synthesized using literature procedure. 26 13 O ver the years since the original LDP publication, a vast array of ligands has been investigated using the LDP method. The values that were determined at the time of the original publication are displayed in Figure 1 - 6. Figure 1 - 6 . LDP values of previously reported ligands. 24 As can be seen in Figure 1 - 6, the halides increase in donor ability as they decrease in size, with fluoride being the strongest donor. This ef fect correlates dire ctly with the orbital overlap from NBO calculations. 27 Additionally, it was shown that the series of para - substituted phenols fit well against their respective Hammett para meters. 24 In short, the values derived from the chromium system were benchmarked against a variety of other metrics including orbital overlap, angular 8 9 10 11 12 13 14 15 16 NMe2 OAd NPhMe NiPr2 OBn Carbazole OPhOMe OPhtBu OPh OPhSMe OPhF OPhCl OPhCF3 OSiPh3 OPhtBu F Indole OtBuF6 NO3 OC6F5 Pyrrole SPh PyrC6H5 PyrAr(CF3)2 CN CO2Ph NCO NCS Cl Br OTf I LDP (kcal/mol) 14 overlap model, Hammett parameters, 13 C NMR data, spectrochemical data, and p K a s . 24, 27 All of ability to a high valent transition metal, but they under sell the utility of the LDP system. 1.7 Quantita tively Modeling Reactions The goal for the ligand donor parameterization is to analyze reactions such as HVM catalyzed transformations, and use the knowledge gained to improve the reactions. As Tolman highlighte d, it is simple to model a property of a comp ound, Z (IR stretching frequencies, reaction ratees, etc.) as a function of sterics or a function of electronics. It is also quite easy to measure the same property as a function of both parameters though. Equat ion 1 below highlights the simplest case wher e a property, Z, shows dependence on both sterics and electronics. (1) In this equation, a , b, and c are all fitting parameters that scale with the relative magnitude of the effects of the electronics and sterics. For example, if a is a large value rel ative to b , it implies that the electronics have a large effect on property Z. Furthermore, the sign of the fitting parameters gives information abo ut how each parameter affects Z. If, for instance, Z represents a reaction rate constant and b is a negative number, it implies that as the sterics increase, the rate of reaction is slowed. 1.8 Conclusions Reaction models of this type can be usefu l at any point in the phases of catalyst development. Models like Equation 1 can provide a wealth of information about a reaction, as they detail what property of the ligand should be given highest priority, and how it should be adjusted. This allows a res earcher to screen a few reactions, analyze and model the results, and then make educated decisions about what changes to make and new catalysts to design. There is no need for prior 15 knowledge, only the need to make educated interpretations of data, elimina ting dependence on years of experience to successfully develop catalysts. The models, though, are not just useful to init ial stages of development. As will be discussed in detail in the coming chapters, these equations can also be deployed in more complex situations and used to discover much more information about a reaction. 16 REFERENCES 17 REFERENCES 1. To lman, C. A., Chem . Rev . 1977, 77 (3), 313 - 348. 2. Tolman, C. A., J . Am . Chem . Soc . 1970, 92 (10), 2953 - 2956. 3. Tolman, C. A., J . Am . Chem . Soc . 1970, 92 (10), 2956 - 2965. 4. Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo , L., Eur. J. Inorg. Chem. 2009, 2009 (13), 1759 - 1766. 5. Taverner, B. C., J . Comput . Chem . 1996, 17 (14), 1612 - 1623. 6. Guzei, I. A.; Wendt, M., Dalton Trans . 2006, (33), 3991 - 399 9. 7. White, D. P.; Anthony, J. C.; Oyefeso, A. O., J. Org. Chem. 1999, 64 (21), 7707 - 7716. 8. Harper, K. C.; Bess, E. N.; Sigman, M. S., Nat . Chem . 2012, 4 (5), 366 - 3 74. 9. Falivene, L.; Credendino, R.; Poater, A.; Petta, A.; Serra, L.; Oliva, R.; Scarano, V.; Cavallo, L., Organometallics 2016, 35 (13), 2286 - 2293. 10. Kingsbury, J. S.; Harrity, J. P. A.; Bonitatebus, P. J.; Hoveyda, A. H., J . Am . Chem . Soc . 1999, 121 (4), 791 - 799. 11. Liu, H. Y.; Eriks, K.; Prock, A.; Giering, W. P., Organometallics 1990, 9 (6), 1758 - 1766. 12. Gusev, D. G., Organometallics 2009, 28 (3), 763 - 770. 13. Nelson, D. J.; Nolan, S. P., Chem. Soc. Rev. 2013, 42 (16), 6723 - 67 53. 14. Lever, A. B. P., Ino rg Chem 1990, 29 (6), 1271 - 1285. 15. Kulinkovich, O. G.; Sviridov, S. V.; Vasilevski, D. A., Synthesis - Stuttgart 1991, 1991 (3), 234 - 234. 16. Kulinkovich, O. G., Chem . Rev . 2003, 103 (7), 2597 - 2 632. 17. Finn, M. G.; Sharpless, K. B., J . Am . Chem . Soc . 1991, 113 (1), 113 - 126. 18. Klosin, J.; Fontaine, P. P.; Figueroa, R., Acc. Chem. Res. 2015, 48 (7), 2004 - 20 16. 19. Odom, A. L.; McDaniel, T. J., Acc. Chem. Res. 2015, 48 (11), 2822 - 28 33. 20. Hammett, L. P., J . Am . Chem . Soc . 1937, 59 (1), 96 - 103. 21. Hansch, C.; Leo, A.; Taft, R. W., Chem . Rev . 1991, 91 (2), 165 - 195. 18 22. Odom, A. L., Dalton Trans . 2005, (2), 225 - 2 33. 23. Walsh, P. J.; Baranger, A. M.; Bergman, R . G., J . Am . Chem . Soc . 1992, 114 (5), 1708 - 17 19. 24. DiFranco, S. A.; Maciulis, N. A.; Staples, R. J.; Batrice, R. J.; Odom, A. L., Inorg . Chem . 2012, 51 (2), 1187 - 1 200. 25. Jarek, R. L.; Flesher, R. J.; Shin, S. K., J. Chem. Educ. 1997, 74 (8), 978 - 982. 26. Chiu, H. T.; Chen, Y. P.; Chuang, S. H.; Jen, J. S.; Lee, G. H.; Peng, S. M., Chem. Commun. 1996, (2), 139 - 140. 27. Bemowski, R. D.; Singh, A. K.; Bajorek, B. J.; DePorre, Y.; Odom, A. L., Dalton Trans . 2014, 43 (32), 12299 - 12 305. 19 Investigations o f Chromium (VI) Nitrido Cyclopentadienyl Bonding System 2.1 Introduction One aspect of organometallic chemistry w h ere the utility of the LDP framework can be exploited is in analysis of the cyclopentadienyl ligand. Cyclopentadienyl, or Cp, has been a staple lig and in organometallic and inorganic chemistry for many years and arguably started the field of organometallic chemistry altogether . 1 - 5 The Cp ligand can bind to a metal center in a variety of ways, including 5 3 1 binding modes as shown i n Figure 2 - 1 . In many cases i t i s obvious what confirmation the Cp ring takes , but, sometimes , Cp binding is rather ambiguous. Using a few compounds synthesized previously in the g roup , in addition to new synthe ses, we set out to shed some light on binding modes of the cyclopentadienyl ring. 6 Figure 2 - 1 . Representation of the three typical binding modes of cyclopentadienyl. The compoun ds under investigation are a rare class of complexes bearing a nitride and a Cp ligand. It is surprising how few NMCp complexes were known at the time of this study. In fact, prior to this work, only four complexes bea ring both a nitride and Cp ligand had been structurally 20 characterized. 7 - 10 This might seem like a specific molecule moiety to compar e, but when we considered the prevalence of group (IV) metallocene complexes in polymerizations, it is surprising that 2 framework. 11 - 14 2.2 Synthesis and Characterization Previously , a few compounds using the NCr(N i Pr 2 ) 2 fragment and Cp - type ligands were made. 6 Namely the NCr( N i Pr 2 ) 2 Cp ( 1 ) a nd NCr( N i Pr 2 ) 2 Indenyl ( 2 ) molecules. Compound 1 was produced through reaction of NCr( N i Pr 2 ) 2 I with a solution of NaCp in THF (scheme 2 - 2). Compound 2 was produced similarly using metathesis of Li - Indenyl. Interestingl y , the preferred binding mode for the Cp ring in 1 and 2 is quite clearly 1 . In this mode , the C p ring formally donates two electrons, acting only as a - donor . It was unclear whether the Cp adopted the confirmation due to electronic or steric effects. We were surprised to see, though, that in the 1 H NMR, the Cp ring in 1 app ears as a sharp singlet, even a Fe(CO) 2 5 - Cp)( 1 - Cp) system reported by Wilkinson. 15 Compound 2 also shows interesting charac t eristics in the 1 H NMR. The resonance for protons located on the 1 and 3 position of the ring are equivalent due to rapid exchange. We were unable to reach slow exchange on our instrumentation, but the coalescence point of the signal occurs at - 50 °C. To t est whether we could induce a haptotropic shift of the Cp ring in 1 , we began attempting to change the ligand framework around the chromium center. Reaction of 1 with benzoic acid results in protonation of an amide and formation of NCr(N i Pr 2 )(Cp)(O 2 CPh), 3 (Scheme 2 - 1). 21 Scheme 2 - 1 . Synthesis of NCr(N i Pr 2 ) 2 (Cp), 1 , and NCr(N i Pr 2 )(Cp)(O 2 CPh), 3, f ro m NCr(N i Pr 2 )I . With substitution of a diisopropylamido ligand by the benzoate, t is reduced. 16 As a result , 3 conformation. Importantly, the reduced electron density is not the only factor changing. The steric bulk is significantly reduced in the substitution process as well. Regardless of the cause, there is a signi ficant difference in the Cp ring binding mode between 1 and 3 , which is obvious even with visual comparison of the solid - state structures, see Figu re 2 - 2 below. 22 Figure 2 - 2 . Crystals structures of 1 , 3 , 4 , and 5 . Ellipsoids displayed at the 50% probability level. All hydrogens, solvents in the lattice , and counterion from 5, removed for clarity. Further reduction of both electron density and steric hinderance can be achieved by protonation of 1 with etherea l hydrochloric acid yielding a substitution of a diisopropylamido with a chloride to make NCr(N i Pr 2 )(Cp)(Cl) ( 4 ) ( Scheme 2 - 2) . Since chromium is even more electron deficient , and less sterically hindered, with a chloride than the benzoate , w e postulated th e Cp ring would slip closer to chromium. 16 In the solid state structures, the Cp rings of 3 and 4 a re essentially equal in orientation with respect to the Cr metal center. 23 Scheme 2 - 2 . Synthesis of NCr(N i Pr 2 )(Cp)(Cl), 4 . We began to wonder if we could force the Cp to bind even tighter to the Cr. We thought it 5 confirmation since the resulting structures are isoele ctronic with classical group(IV) metallocene - type molecules, which are ubiq uitous in polymerization catalysis. 11 - 14 Treatment of the 4 with silver hexafluoroantimonate, or a similar noncoordinating anion, in acet onitrile yields the NCr(N i Pr 2 )Cp(NCMe ) ( 5 ), S cheme 2 - 3 . Compound 5 was only p roduced in situ due to its low thermal stability. We tried a variety of other methods to generate 5 including other noncoordinating anions, super acids (such as HSbF 6 ), and other metathesis reactions. In most cases 5 was generated (indicated by NMR spectro scopy), but not in isolable quantities. Despite the instability and small reaction scale we were still able to produce X - ray quality single crystals as shown in Figure 2 - 2. We we re somewhat surprised to find that complex 5 coordinated an equivalent of acetonitrile. 5 - interaction does not provide the electron density necessary to stabilize a three coordinate Cr(VI) cation. Still, d ue to the poorer donation ability of the coordinated a cetonitrile , relative to Cl, the Cp ring is once again brought slightly closer to chromium to stabilize the loss of electron density . 17 24 Scheme 2 - 3 . In situ s ynthesis of NCr(N i Pr 2 )Cp(NCMe) , 5 . Thus far, the discussion about the coordination mode has been qualitative based on solid state structure inspection. We wondered if the visual analysis could be a result of packing forces in the solid state. As such, we turned to theory to calculate b ond o rders of the Cr - C bonds to compare 1 - 5 . 2.3 Bonding Analysis For complexes 1 and 2 , where both diisopropylamido ligands are present to compete for electron density and space , Mayer bond order shows a strong bond (>0.7) to only one carbon , while the other four remain essentially nonbonding 1 - Cp binding . 18 - 19 When electron density is removed from the metal center, as shown in 3 and 4 , three carbons are involved in bonding and the remaining two have only rel atively weak interaction with chromium. This appears t 3 - interaction. E ven when we have removed as much electron density from chromium as possible and made as much space around the metal center as we could, in compound 5 , the Cp ring barely slid cl 5 conf i rmation. Consequently, cation ic compound 5 also has the highest average bond order for all five carbons at 0.32 but remains in what appears to be an 3 confirmation. The Mayer bond order analysis is shown in Figure 2 - 3. 25 Figure 2 - 3 . Mayer bond order analysis of 1 - 5 . Figure 2 - 4 . Cr - C bond lengths and averages for compounds 1 - 5 . Cp is an example of a ligand that can adapt and stabilize changin g electronic conditions on a metal . It was quite ap parent that as the L ewis acidity and available space, of the Cr center was 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1 2 3 4 5 Bond Order Mayer Bond Order C1 C2 C3 C4 C5 Average BO 1.5 2 2.5 3 3.5 4 1 2 3 4 5 Bond Length (Å) Cr - C Bond Lengths C1 C2 C3 C4 C5 Average 26 increased , the donation of the Cp ring compensated accordingly. It is interesting that in the reaction to form 5 , the complex coor dinated an acetonitrile rather than allowing the Cp ring to form a full 5 - interaction. Maybe the Cp ring in 5 is in fact as close to a 5 interaction as the molecule can achieve. To try and get a better understanding, we decided to benchmark our series o f coordination modes of 1 - 5 Cp. Figure 2 - 5 . Mayer bond order analysis of the 15 , 20 - 21 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 FpCp ( FpCp ( WCp2(CO)2 ( WCp2(CO)2 (eta3) Bond Order Mayer Bond Order Analysis of Literature Compounds C1 C2 C3 C4 C5 Average BO 27 Figure 2 - 6 . M - W e selected two complexes from the literature that represent what could be considered as standards for the 5 3 1 coordination modes. The Fe complex Fe( 5 - Cp)( 1 - Cp)(CO) 2 , [Fe], has ideal examples of the 1 and 5 coordination modes. 15, 20 The tungsten complex W( 5 - Cp)( 3 - Cp)(CO) 2 , [W], o n the other hand, has ideal examples of both the 5 and 3 coordination modes. 21 A MBO analysis of those model complexes is shown in Figure 2 - 5. It is immediately obvious when comparing 1 and 2 to [Fe] that, in the chromium complexes bearing both diisopropylamide ligands, the Cp and indenyl rings are in a 1 confirmation. Both the Mayer bond order and crystallographic bond lengths agree well on that point. The remaining complexes 3 , 4 and 5 , however, are clearly 1 5 conformations of the [Fe] complex. Comparing to the [W] complex , the Cp rings in 3 and 4 appear quite close to an 3 confirmation, which agrees well with the crystallographic bond lengths. Complex 5 on the other hand does not 1.5 2 2.5 3 3.5 4 FpCp ( FpCp ( WCp2(CO)2 ( WCp2(CO)2 ( Bond Length (Å) M - C Bond Lengths C1 C2 C3 C4 C5 Average 28 make the distinction so clear. The distribution of bond lengths, especially, looks more like th e 5 coordinated Cp ring in the [W] molecule, while the Mayer bond order calculations more closely resemble the 3 mode. When compared to the [Fe] 5 ring, it seems unreasonable to classify it as 5 ring. The Cp ring in complex 5 is likely best de sc 3 interaction. 2.4 Conclusions As with most concepts in chemistry, these various coordination modes are simply not be considered as the only allo we d modes, and, in fact, should be considered a continuum as highlighted with complex 5 here, which seems intermediate to 5 and 3 . Regardless, this study highlights the flexibility of the Cp ligand to stabilize a variety of states at a metal center. It c an compensate for both steric and electronic perturbations at the metal center, which is a likely reason for its deployment in so many organometallic systems. 3 - 5, 11 - 13 2.5 Experimental Experimental taken from recent p ublication: This can be located at: Organometallics , 2015 , 34 (18), 4567 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, and toluene were purchased from Aldrich Chemical Co. and purified by passing through alumina columns to remove water after sparging with dinitrogen to remove oxygen. Silver hexfluoroantimonate was purchased from Aldrich Chemical Co. and used as re ceived. Tert - butanol was purchased f rom Jade Chemical Co. and dried over 3 Å molecular sieves to remove water after being sparged with dry nitrogen to remove oxygen. Trimethylsilyl iodide was purchased from 29 Oakwood Chemical and distilled under dry nitrogen . FpCp was prepared using the litera ture procedure. 9 All NMR solvents were purchased from Cambridge Isotopes Laboratories, Inc. Deuterated chloroform and acetonitrile were dried over 3 Å sieves and freeze pump thaw degassed. The NMR solvents were stored in the glove box in glass containers w ith 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 - field - gr adient (PFG) switchable broadband pr obe and operating at 499.955 MHz (1H) and 125.77 ( 13 C). 1 H NMR chemical shifts are reported relative to residual CHCl 3 in CDCl 3 as 7.26 ppm. 13 C NMR chemical shifts are reported relative to 13 C in CDCl 3 as 77.0 ppm. Sing le crystal X - ray diffraction data wa s collected in the Center for Crystallographic Research at MSU. Synthesis of NCr(N i Pr 2 ) 2 (Cp) ( 1 ) : Under an inert atmosphere, a scintillation vial was loaded with NCr(N i Pr 2 ) 2 (I) (0.500 g, 01.271 mmol, 1 equiv.), 5 mL THF, and a stirbar. To this a 3.5 M THF solution of sodium cyclopentadienide (1.089 mL, 3.813 mmol, 3 equiv.) was added, and the solution was rapidly stirred for 20 h. The volatiles were removed in vacuo, and the residue was extracted with pentane (3 × 25 mL) and filtered through Celite. The vol atiles were removed in vacuo yielding 1 as a brown powder. Diffraction quality crystals were obtained from a concentrated pentane solution of NCr(N i Pr 2 ) 2 (Cp) held at 30 °C (0.280 g, 1.051 mmol, 82% yield). 1 H NMR (CDCl 3 , 60 °C, 500 MHz): 6.16 (s, 5 H, Cp), 4.88 (sept, J HH = 6.4, 2H, C H (CH 3 ) 2 ), 3.59 (sept, J HH = 6.3, 2H, C H (CH 3 ) 2 ), 1.72 (d, J HH = 6.3, 6 H, CH(C H 3 ) 2 ), 1.42 (d, J HH = 6.4, 6H, CH(C H 3 ) 2 ), 1.04 (d, J HH = 6.2, 6H, CH(C H 3 ) 2 ). 13 C NMR (CDCl 3 , 25 °C, 125 MHz): 115. 0, 58.3, 55.2, 30.4, 30.3, 23.3, 18.0. Anal. Calcd. for C 17 H 33 CrN 3 : C, 61.60; H, 10.03; N, 12.68. Found: C, 61.59; H, 9.97; N, 12.65. Mp: 90 92 °C (sub). 30 Synthesis of NCr(N i Pr 2 ) 2 (Ind) ( 2 ) : To a partially frozen solution of NCr(N i Pr 2 ) 2 (I) (100 mg, 0.254 mmo l, 1 equiv.) in ether (3 mL), a suspension of lithiated indene (34.2 mg, 0.280 mmol, 1.1 equiv.) in ether (2 mL) was added This dark mixture was allowed to warm to room temperature and st irred for 18 h. The volatiles were then removed in vacuo, and the re sidue extracted with pentane , filtered through Celite, and evaporated to a dark orange/brown solid. The solids were dissolved in a minimal amount of pentane and chilled to 30 °C overnight providing 2 as dark orange crystals (26 mg, 27% yield). 1 H NMR (CDC l 3 , 25 °C, 500 MHz): 7.59 (dd, J HH = 5.6, 3.2, 2 H, Ar), 7.02 7.07 (m, 3 H, Ar and ß - Ind), 5.65 (br, 2 H, Ar), 4.82 (sept, J HH = 6.3, 2H, C H (CH 3 ) 2 ), 3.48 (sept, J HH = 6.3, 2 H, C H (CH 3 ) 2 ), 1.46 (d, 12 H, CH(C H 3 ) 2 ), 1.04 (d, J HH = 6.4, 6 H, CH(C H 3 ) 2 ), 0.90 ( d, J HH = 6.2, 6 H, CH(C H 3 ) 2 ). 13 C NMR (CDCl 3 , 25 °C, 125 MHz): (6 of the signals for the indenyl ligand are not observed due to broadening from fluxionality on the 13 C NMR time scale) 136.7, 123.1, 122.4, 57.2, 54.8, 30.7, 29.3, 23.4, 19.9 14.2. Anal. Calc d. for C 21 H 35 CrN 3 : C, 66.11; H, 9.25; N, 11.01. Found: C, 65.68; H, 9.60; N, 10.95. Mp: 131 - 133 °C (dec.) Synthesis of NCr(N i Pr 2 )(O 2 CPh)(Cp) ( 3 ) : Under an inert atmosphere a sci ntillation vial was loaded with 1 (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, 1 equiv.) in toluene (6 mL) was added dropwise over 5 min. The solution turned dark red and was allowed to stir at room temperature for 2 h. The volatiles were removed in vacuo, and the residue was dissolved in 2 mL of toluene. The solution was filtered over Celite , l 3 (0 . 117 g, 0.333 mmol, 62%). 1 H NMR (500 MHz, CDCl 3 , 13 °C): 7.95 (dd, J HH = 8.25 Hz, J HH = 1.5 Hz, 2 H, Ph), 7.41 (tt, J HH = 7.0 Hz, J HH = 2.5 Hz, 1 H, Ph), 7.34 (t, J HH = 7. 5 Hz, 2 H, Ph), 31 6.14 (s, 5 H, C 5 H 5 ), 5.56 (sept, J HH =6.0 Hz, 1 H, NCH(CH 3 ) 2 ), 4.31 (sept, J HH = 6.0 Hz, 1 H, NCH(CH 3 ) 2 ), 2.11 (d, J HH = 6.0 Hz, 3 H, NCH(CH 3 ) 2 ), 1.75 (d, J HH = 6.0 Hz, 3 H, NCH(CH 3 ) 2 ), 1.29 (d, J HH = 6.0 Hz, 3 H, NCH(CH 3 ) 2 ), 1.11 (d, J HH = 6.0 Hz, 3 H, NCH(CH 3 ) 2 ). 13 C NMR (125 MHz, CDCl 3 , 13 °C): 170.74, 135.26, 130. 77, 129.66, 127.83, 108.22, 73.71, 63.71, 31.06, 29.83, 20.64, 20.15. FT - IR (KBr): 1639.2 cm 1 ( s CO 2 ), 1415.5 cm 1 ( a CO 2 ). Satisfactory elemental analysis was not obtained after several attempts. Synthesis of NCr(N i Pr 2 )(Cp)Cl ( 4 ) : Under an inert atmosphere , a Schlenk flask was loaded with 1 (50 mg, 0.151 mmol, 1 equiv.) and ether (5 mL). To the solution of 1 , 2.0 M HCl (0.226 mL, 0.453 mmol, 3 equiv.) in ether was added ra pidly. The dark mixture turns reddish, and some precipitate forms during addition. This mixture was stirred at room temperature for 30 min. The volatiles were then removed in va cuo, and the residue was washed with pentane (2 × 5 mL). The solid was then ext racted with ether, filtered through Celite, and concentrated to ~2 mL. This dark solution was cooled to 30 °C overnight providing 4 as dark red crystals (24 mg, 59%). 1 H NMR (C DCl 3 , 25 °C, 500 MHz): 6.09 (s, 5 H, Cp), 5.23 (sept, J HH = 6.5, 1 H, C H (CH 3 ) 2 ) , 4.36 (sept, J HH = 6.3, 1 H, C H (CH 3 ) 2 ), 2.17 (d, J HH = 6.3, 3 H, CH(C H 3 ) 2 ), 1.80 (d, J HH = 6.4, 3 H, CH(C H 3 ) 2 ), 1.26 (d, J HH = 6.5, 3 H, CH(C H 3 ) 2 ), 1.20 (d, J HH = 6.5, 6 H, CH( C H 3 ) 2 ). 13 C NMR (CDCl 3 , 25 °C, 125 MHz): 108.9, 74.7, 64.4, 30.8, 29.3, 20.2, 17.8. Anal. Calcd. for C 11 H 19 ClCrN 2 : C, 49.53; H, 7.18; N, 10.50. Found: C, 49.50; H, 7.56; N, 10.45. Mp: 121 - 123 °C (dec.) Synthesis of [NCr(N i Pr 2 )(Cp)(NCMe)][SbF 6 ] ( 5 ) : Under a n inert atmosphere, a scintillation vial was loaded with 4 (25 mg, 0.124 mmol, 1 equiv.) and CD 3 CN (1 mL). To this, a solution of AgSbF 6 (85 mg, 0.248 mmol, 2 equiv.) in CD 3 CN (1 mL) was added. The reaction was allowed to proceed and was monitored by 1 H NM R. After 4 d it was observed that all starting material peaks had disappeared. The volatiles were removed in vacuo, and the residue was washed with ether (5 32 mL). The solids were extracted with chloroform (1 mL), filtered through Celite, layered with ether, a nd chilled to 30 °C for recrystallization. Despite being of quality for single crystal diffraction, the crystals obtained by this technique were unstable, and the bulk material was consistently impure. As a result, only the in situ 1 H NMR and single cry st al X - ray diffraction were successful for the characterization of this compound. 1 H NMR (CD 3 CN, 25 °C, 500 MHz): 6.24 (s, 5 H, Cp), 5.33 (sept, J HH = 6.3, 1 H, C H (CH 3 ) 2 ), 4.67 (sept, J HH = 6.2, 1 H, C H (CH 3 ) 2 ), 2.15 (d, J HH = 6.2, 3 H, CH(C H 3 ) 2 ), 1.78 (d, J H H = 6.2, 3 H, CH(C H 3 ) 2 ), 1.28 (d, J HH = 6.8, 3 H, CH(C H 3 ) 2 ), 1.22 (d, J HH = 6.4, 3 H, CH(C H 3 ) 2 ). 13 C NMR (CD 3 CN, 25 °C, 125 MHz): 129.19, 110.30, 78.55, 67.60, 49.45, 31.81, 30.41, 20.99, 19.06. 19 F NMR (CD 3 CN, 25 °C, 470 MHz): 113.65 to 134.23 (m). Ge neral Procedure for FT - IR Carboxylate 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 carbo ny l stretches were identified by comparison to its isotopologue, 13 C labeled at the carbonyl carbon. Difference s = 1415.5 cm 1 a = 1594.8 cm 1 ) stretc he s in a sample of sodium benzoate in KBr, which 2 CPh) of 179.3 cm 1 . Computational Details : All calculations were done at the High - Performance Computing Center (HPCC) at Michigan State University. The optimization of structures was done using G09 with DFT and the B3PW91 functional. Due to the size of the structures, only double zeta basis sets were used in most cases with 6 - 31G** used for all th e chromium and iron complexes. In the case of the molybdenum and tungsten, the SDD basis was u sed. The Mayer Bond Order calculations were done on a departmental cluster using BORDER. In the case of the chromium complexes, the basis set dependence of the Ma yer Bond Orders was examined. For example, for compound 1 the 33 calculation was carried out with 3 - 21G, 6 - 31G, 6 - 31G**, and SDD. Bond orders using these different basis sets were generally comparable and either 6 - 31G** or SDD were employed for all the comple xes. For example, the Cr N(nitrido) bond orders in 1 with the different basis sets were 2.71, 2.72, 2.68, and 2.89, respectively. The highest bond order between Cr - C(Cp) in 1 was 0.60, 0.71, 0.73, 0.71, respectively. NMR Spectra Figure 2 - 7 . NCr(N i Pr 2 ) 2 (Cp) ( 1 ) 1 H NMR 34 Figure 2 - 8 . NCr(N i Pr 2 ) 2 (Cp) ( 1 ) 13 C NMR 35 Figure 2 - 9 . NCr(N i Pr 2 ) 2 (Ind ) ( 2 ) 1 H NMR 36 Figure 2 - 10 . NCr(N i Pr 2 ) 2 (Ind ) ( 2 ) 13 C NMR 37 Figure 2 - 11 . NCr(N i Pr 2 )(O 2 CPh)(Cp) ( 3 ) 1 H NMR (13 °C) 38 Figure 2 - 12 . NCr(N i Pr 2 )(O 2 CPh)(Cp) ( 3 ) 13 C NMR (13 °C) 39 Figure 2 - 13 . NCr(N i Pr 2 )(Cp)Cl ( 4 ) 1 H NMR 40 Figure 2 - 14 . NCr(N i Pr 2 )(Cp)Cl ( 4 ) 13 C NMR 41 Figure 2 - 15 . [ NCr(N i Pr 2 )(Cp)(NCMe)][SbF 6 ] ( 5 ) 1 H NMR (In Situ Reaction) 42 Figure 2 - 16 . [ NCr(N i Pr 2 )(Cp)(NCMe)][SbF 6 ] ( 5 ) 13 C NMR (In Situ Reaction) 43 Figure 2 - 17 . [ NC r(N i Pr 2 )(Cp)(NCMe)][SbF 6 ] ( 5 ) 19 F NMR (In Situ Reaction ) 44 REFERENCES 45 REFERENCES 1. Kealy, T. J.; Pauson, P. L., Nature 1951, 168 (4285), 1039 - 1040. 2. Wilkinson, G.; Rosenblum, M.; Whiting, M. C.; Woodward, R. B., J. Am. Chem. Soc. 1952, 74 (8), 2125 - 2126. 3. Field, L. D.; Lindall, C. M.; Masters, A. F.; Clentsmith, G. K. B., Coord. Chem. Rev. 2011, 255 (15 - 16), 1733 - 1790. 4. Cotton, F. A., J. Organomet. Chem. 2001, 637 - 639 (0), 18 - 26. 5. MacDonald, M. R.; Fies er, M. E.; Bates, J. E.; Ziller, J. W.; Furche, F.; Evans, W. J., J. Am. Chem. Soc. 2013, 135 (36), 13310 - 1331 3. 6. DiFranco, S. A. Single - Site Molybdenum(Iv) Mediated Bond Cleavage Reactions and Li gand Parameterizaton Using a Cr(Vi) Nitrido Platform. Mich igan State University, 2013. 7. Shin, J. H.; Bridgewater, B. M.; Churchill, D. G.; Baik, M. - H.; Friesner, R. A.; Parkin, G., J. Am. Chem. Soc. 2001, 123 (41), 10111 - 10112. 8. Koch, J. L.; Shapley, P . A., Organometallics 1997, 16 (19), 4071 - 4076. 9. Johnson , C. E.; Kysor, E. A.; Findlater, M.; Jasinski, J. P.; Metell, A. S.; Queen, J. W.; Abernethy, C. D., Datlton Trans. 2010, 39 (14), 3482 - 348 8. 10. Miyazaki, T.; Tanaka, H.; Tanabe, Y.; Yuki, M.; Nak ajima, K.; Yoshizawa, K.; Nishibayashi, Y., Angew. Chem. I nt. Ed. 2014, 53 (43), 11488 - 114 92. 11. Bochmann, M., J. Chem. Soc. Dalton. Trans. 1996, (3), 255 - 270. 12. Alt, H. G.; Koppl, A., Chem. Rev. 2000, 100 (4), 1205 - 1221. 13. Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F., Chem. Rev. 2000, 100 (4), 1253 - 1 346. 14. Hoffmann, R., Angew. Chem. Int. Ed. 1982, 21 (10), 711 - 724. 15. Piper, T. S.; Wilkinson, G., J. Inorg. Nucl. Chem. 1956, 3 (2), 104 - 124. 16. DiFranco, S. A.; Maciu lis, N. A.; Staples, R. J.; Batrice, R. J.; Odom, A. L., Inorg. Chem. 2012, 51 (2), 1187 - 1 200. 17. Aldrich, K. E.; Billow, B. S.; Holmes, D.; Bemowski, R. D.; Odom, A. L., Organometallics 2017, 36 (7), 1227 - 1237. 18. Mayer, I., J. Comput. Chem. 2007, 28 (1 ), 204 - 2 21. 46 19. Mayer, I., Border 1.0 , 2005. 20. Billow, B. S.; Bemowski, R. D.; DiF ranco, S. A.; Staples, R. J.; Odom, A. L., Organometallics 2015, 34 (18), 4567 - 4573. 21. Huttner, G.; Brintzinger, H. H.; Bell, L. G.; Friedrich, P.; Bejenke, V.; Neugebaue r, D., J. Organomet. Chem. 1978, 145 (3), 329 - 333. 47 Analysis of Phosphines as Li gand on High Valent Transition Metals 3.1 Introduction Phosphines are one of the most frequently employed ligands in catalysis. As we discussed in chapter 1, much effort has gone into the development and understanding of phosphine - metal interactions. 1 - 4 The sheer number of different phosphine ligands makes thes e research efforts necessary, as the choice of phosphine can dramatically affect the outcome of a catalytic reaction. Take for example Grubbs first generation ca talyst. 5 The choice of phosphine between two seemingly similar ligands, PPh 3 and PCy 3, is the difference betw een an inactive species and a commercially available olefin metathesis catalyst. The choice of phosphine become s even more complex when choosing between the extremely wide variety of ligands designed for cross coupling reactions. 6 - 7 A highlight of th e research discussed in chapter 2 was the realization of Cr(VI) cations. Former group members had some results synthesizing cationic complexes, but very little had been accomplished with them . 8 In an effort to learn more a bout the donor ability of neutral donor ligands, such as phosphines, to HVMs we began investigating them using the LDP method. Since phosphines are one of the most prevalent ligands in catalysis , and our system is esse ntially the high valent analogue to th e Tolman electronic parameter , we set out to help improve the understanding of phosphine - metal bonding. 3.2 Synthesis This project was undertaken in close collaboration with Kelly Aldrich . Much of the work performed in th is chapter was a shared effort, but th e ion pairing analysis and computational 48 explicitly give credit to Kelly, but all of the contents of this chapter were a col laborative effort to some degree. Natu rally, analysis of L - type ligands (neutral, two - electron donor ligands) in the LDP system requires formation of a cationic chromium complex . It is preferable to use a noncoordinating ion for charge balance to avoid com peting exchange reactions with the L - t ype ligands. There are a number of noncoordinating ions to choose from and synthesis of cationic chromium complexes works reasonably well with most of them. One notable exception is triflate. The triflate anion is actu ally quite coordinating in this system and can easily outcompete L - type ligands. The synthesis of the chromium phosphine complexes is quite simple. Addition of AgX or TlX, where X = SbF 6 , PF 6 , BAr F 24, BAr F 20 , BPh 4 , Al(O t Bu F 9 ) 4 , to an acetonitrile solution of NCr(N i Pr 2 ) 2 I results in immediate precipitation of either AgI o r TlI, generating [NCr(N i Pr 2 ) 2 (MeCN)] + X - . Addition of a slight excess of PR 3 to the resulting solution generates the desired [NCr(N i Pr 2 ) 2 PR 3 ] + X - complex. The reaction s equence is shown in S cheme 2 - 1 . Scheme 3 - 1 . General procedure for the s ynthesis of PR 3 complexes from NCr(N i Pr 2 ) 2 I. X = SbF 6 , PF 6 , BAr F 24 , BAr F 20 , BPh 4 , Al(O t Bu F 9 ) 4 3.3 Initial LDP Analysis We realized that changing the LDP framework f ro m a neutral species to an ionic one was likely to cause some complications, but we began by analyzing the complexes using our standard method. 49 On synthesizing the first few complexes, we r ealized immediately there were some surprises in the LDP assessments of the phosphines. The first issue we encountered was the high barriers to rotation of the Cr - N bond. Because the phosphine complexes have extremely high barriers compared to the previous ly studied ligands, values >17 kcal/mol, the measurements required e levated temperatures. This was an immediate problem as triplicate LDP results usually require ~1.5 to 2 h at temperature , and several of the compounds were not stable on that timescale. Fo r all of the measurements in T able 3 - 1 below, the complexes had at l east one good LDP measurement performed, but triplicate results could no t be easily obtained. Still though, the small series of phosphine complexes we analyzed did not follow the expected trend s . Phosphine Group LDP ( k cal/mol) PMe 3 17.2 PPhMe 2 17.0 PMe Ph 2 16.2 PCy 3 17.2 PBu 3 17.3 Table 3 - 1 . Initial LDP measurements for PR 3 ligands. All compounds used SbF 6 as the counter ion. All values are based on at least one good LDP measurement performed , but results here were not necessarily performed in triplicate due to decomposition of the complexes . As such, the values are reported only to the tenths. It is import ant to realize there are a number of things that could potentially affect the numbers, but the values for the cursory analysis highlighted some surprising results. The LDP of the PR 3 alues. 9 The trialkyl phosphines are reported to be the best electron density donors, and the more aryl subst ituents that are added, the worse donors they become. We see the opposite trend in our data. Before d rawing conclusions, we decided to make sure our measurements were actually an accurate measure of the PR 3 donor ability. 50 3.4 Anion and Solvent Dependence The f irst thing we investigated was the ionic species dependence on the counterion. To test this, we synth esized a series of [NCr(N i Pr 2 ) 2 (PPhMe 2 )] + X - complexes with a series of counterions, X = SbF 6 , PF 6 , BAr F 24, BAr F 20 , BPh 4 , Al(O t Bu F 9 ) 4 . The anions are displayed below in Figure 3 - 1 . Figure 3 - 1 . Various anions explored in this study. Even before performing an LDP analysis on the series of [Cr]PR 3 + X - salts , we realized the anions were not equal . Surprisingly, the various ions did not give equivalent NMR spectra. Specifically, in the 14 N NMR spectrum, the signal for the nitride was missing in the c omplexes synthesized using the SbF 6 anion . W hen we synthesized the PF 6 version of the same phosphine complex es , the signal for the nitride was apparent ( Figure 3 - 2 ). We hypothesized that the anions were in such close proximity to the chromium fragment in solution, that the quadripolar Sb nucleus was broadening the nitride resonance into the baseline . In other words, we thought the SbF 6 anion 51 was ion paired with the Cr cation. This ion pai ring, if it was the cause, should be solvent dependent. 10 Figure 3 - 2 . 14 N NMR analysis of [NCr(N i Pr 2 ) 2 (PMePh 2 )] + X - in CDCl 3 where X = SbF 6 (right) and PF 6 (left). The peak at 309 ppm (labelled with a *) represents dissolved N 2 , which was referenced as an internal standard. To test this theory, w e ran LDP analyses of the anion series in two different solvents, CDCl 3 which is ou r standard solvent for the measurement, and CD 3 CN which is a more polar solvent. The two solvents tested were chose n to highlight any differences in ion pairing between the cation and anion. The results are shown in T able 3 - 2. 52 The results of the series were interesting. The LDP values with different anions in CDCl 3 were all quite different. This led us to believe that ion pairing in solution does indeed affect the Cr - N bond rotation, and it does so in a non - systematic way. For example, the SbF 6 anion in CDCl 3 has an LDP nearly 0.5 kcal/mol greater than that of the BAr 4 anions. This lends some support to what we saw in the 14 N NMR spectrum. If the SbF 6 anion is close enough to the cation to hi nder bond rotation, it could be close enough to broaden the nitride resonance in the 14 N NMR spectrum. Surprising was the contrasting results between the two solvents. The same measurements in CD 3 CN led to much more con sistent results. We postulated that t his was due to the increased solvation of the ions by the more polar solvent. If the ions were separated in solution by the acetonitrile molecule, the anion w ould not be in a proximity where the Cr - N bond rotation is hindered. Table 3 - 2 . LDP measurements of [NCr(N i Pr 2 ) 2 (PPhMe 2 )] + X - . We wondered how much the charge distribution in the noncoordinating anion affected the ion pairing . The BAr 4 anions have the nega tive charge highly delocalized, but there are known anions with even more charge distribution. 11 - 12 At the time of the study, some of the weakest coordinating anion were carborane s and perfluoroaluminates . 13 The extremely charge delocalized carborane anion , though, was explosive. Fortunately , the Al( O t Bu F 9 ) 4 anion has nearly the same charge distribution and, as such, is considered equally noncoordinating thanks to the 36 F atoms around Anion LDP (kcal/mol) in CDCl 3 LDP (kcal/mol) in CD 3 CN SbF 6 16.99 16.53 PF 6 16.96 1 6.53 BAr F 24 16.60 16.58 BAr F 20 16.64 16.62 BPh 4 16.57 16.47 Al( O t Bu F 9 ) 4 16.66 16.62 53 the periphery. These a nions are easily prepared and are extremely stable. 12 Moreover, they provide interesting contr ast to the BAr 4 anions since they are based on an alkyl fluorocarbon pe riphery rather than an aryl one. Despite all of this, the aluminate anion hindered Cr - N bond rotation slightly more than the BAr 4 anions. Presumably, the flexible alkyl substituents ca n interact with the i Pr groups of the amides easier than the rigid aryl substituents. We attempted synthesis of the hydrocarbon analogue of the aluminate, Al(O t Bu) 4 , to test that theory, but we could only produce the lithium and potassium salts, which were unsuccessful in the synthesis of [NCr(N i Pr 2 ) 2 (P R 3 )] + (Al(O t Bu ) 4 ¯ . The results of testing the series of anions suggested that , regardless of how delocalized the charge was in the anion, LDP alone was probably not going to be an adequate indication of ion interaction. In light of this, Kelly tested the ion paring dependenc e on solvent and anion identity using a series of detailed analyses including diffusion ordered spectroscopy (DOSY) NMR, LDP, and DFT. 14 - 15 . The D OSY study showed equal diffusion coefficients of the cation and anion in solution. This suggests the ion s are diffusing together in solution, or in other words, they are tightly ion paired. DFT analysis even suggested ions lik e SbF 6 paired to the chromium complex in a specific way, directly above the nitride, confirming our suspicions fr o m the 14 N NMR. When the ions are in this position, the electronegative fluorides can hydrogen bond to the protons on the i Pr groups of the amides. This means the SbF 6 - anio n might be able to hinder bond rotation both sterically and through noncovalent bonding interactio ns. In CD 3 CN, the DOSY experiment showed different diffusion rates for each cation and anion. As we suspected, the higher polarity of acetonitrile meant the i ons could be completely solvated, separating the cation from the anion. This is consistent with th e lower LDP values in the more polar solvent. 54 Kelly also studied the ion pairing using ROESY NMR. 14 The ROESY experiment shows through space correlations in the 1 H NMR. The larger BAr F 24 ion was ion paired in CDCl 3 , but the pairing was non - specific, meaning the aromatic 1 H signal from the anion correlated weakly with all of the proton signals in the chromium complex. T he lower effect of the LDP value in CDCl 3 for the BAr 4 io ns may be due to the ion spending some time by the amides, some by the phosphine ligand, and some solvated . In CD 3 CN, though, there were no correlation signals. While this is a negative result, i t is what we would expect if the acetonitrile completely solv ates the ions. nclusive. The easie st modification we could make to the LDP method was switching to CD 3 CN as a solvent. This allows us our choice of anions, and for synthetic ease, we chose the SbF 6 anion. 3.5 Entropy Analysis Because LDP analysis of the cationic complexes required using a different solvent, we realized that one of the assumptions we had made in all LDP measurements in the past may not translate to the PR 3 complex es. In the previous LDP studies, the assumption was that the entropy of activation, S , was equal to - 9 e.u. This number was established for the original LDP publication by investigating a number of chromium complexes with Eyring plot analysis in CDCl 3 . A ll of the values determined were small, negative values. The number that was establish ed over the largest temperature range was that using the NCr(N i Pr 2 ) 2 I complex . 16 Since this value had the most reliable data backing the value, the S for the iodide complex has been used for all ligands. Throughout the studies we have done since the original publication, we have not had cause to question that value. Even in situations where it would have become obvious that the assumed value was inc orrect, specifically doing LDP measurements at different temperatures for the sam e ligand, the assumption seemed to hold. 55 We first became suspicious of the entropy assumption when we investigated some of the phosphine complexes at vari ed temperatures. The LDP values determined in CD 3 CN at varying temperatures were not self - consistent. Considering we changed our analysis from chloroform to acetonitrile and the complexes from neutral to ionic , it seemed prudent to reinvestigate our original assumption. We pe rformed numerous analys e s of the PR 3 complexes and the results are outlined in T able 3 - 3. 56 Table 3 - 3 . Summary of Eyring analyses of the various phosphine complexes. The entropy analysis error was determined from a linear least squares fit of the experimentall y d ata as reported by Lente. 17 a Values in CD 3 CN use SbF 6 as an anion. These H values include the, admittedly crude, assumption of a constant entropy of - 9 e.u. b These values were run at 3 constant temperatures, as such the precision is high an d the error for the measurement is low, this does not necessarily mean the H values are more accurate than the other values here, especially in light of the entropy assumption. c Values in CDCl 3 use BAr F 24 as the anion. H values are from the Eyring analysis. 57 We decided it would also be sensible to reevaluate the anionic ligands to confirm the assumption of a constant S value , especially since our techniques for the LDP measurement ha d improved since the o riginal publica tion. 18 For this analysis we determined S for the NCr(N i Pr 2 ) 2 I, NCr(N i Pr 2 ) 2 OPh, NCr(N i Pr 2 ) 2 Pyrr 3 - C6H3(CF3)2 and NCr(N i Pr 2 ) 3 which represent a wide range of dono r ability and buried volumes. The results for those four experiments are shown at the bottom of T able 3 - 3. The results line up well with those previously reported. Th is r esult w as unsurprising given the successful deplo yment of the LDP system in modelling . We averaged the data from the four points to come to a S value of - 3.5 e.u. Given the circumstances, the value of the entropy is inconsequential as long as we can assume the value is constant for all of the ligands m easured. The accuracy of the H value is not the critical factor in the LDP assessment of li gands. As mentioned in chapter 1, the important aspect of the LDP measurement is precision and the relative difference from one ligand to another. Since the value seems reasonably constant for the anionic ligands , our assumption of a constant S does not seem unreasonable. It is important to note too , the entropy values for the PR 3 complexes in chloroform match quite well to the anionic ligand values . A gain , this supports our assumption about constant S values in CDCl 3 . Unfortunately, the Cr - N bond rota tion is hindered in chloroform by intermolecular interactions from ion pairing , precluding the use of CDCl 3 fo r the phosphine LDP measurements. The entropy values in acetonitrile , though, seem to vary widely. We wondered if we could shed some light on the cause of the fluctuating entropy. To test whether entropy was dependent on solvent, we analyzed the S of NCr(N i Pr 2 ) 2 I in CD 3 CN. The value, de termined over a temperature range of 41 K , matched almost exactly the value we determined from CDCl 3 at - 1 .3 (0.5) e.u. Since the value of a neutral complex in CD 3 CN matches 58 the values of both ionic and neutral sp ecies in CDCl 3 , we suspect the substantial changes in S comes from varying solvation spheres surrounding the cationic chromium molecule. 3.6 Phosphine Analysis Unfortunately, the investigation into the entropy of our LDP system only highlighted the cause of the mismatching PR 3 values at different te mperatures, it did not solve the problem. As a result, direct, quantitative comparison of neutral donors is still not achievable using the LDP method . We were still able to make qualitative comparisons between the phosphines, though, but we will discuss th e donor ability in terms of H to emphasize these values do not fall on the previously discussed LDP scale. We could not analyze the entropy value of the alkyl phosphines due to extremely limited temperature windows where the complexes wer e stable. Since the complexes were similar a nd the LDP measurements were performed at similar temperatures , we thought the entropy differ ences between them might be negligible. We compared the set of trialkyl phosphine complexes to see if an analysis was possible. We fit the H data from CD 3 CN to t he Tolman type equation below. 1 (1) In E d is the adapted Tolman electronic parameter by Prock and Giering. 19 The values scale like - effects and are meant to be a true - donor value. 3 We decided this would be t he simplest model for the trialkylphosphines since there should be minimal influence - effects. Modelling the data in T able 3 - 4 using E quation 1 with a least squares fit gives E quation 2 below. (2) 59 Ligand H (CD 3 CN ) H (CDCl 3 ) TCA (°) d PMe 3 16.64 18.71 118 8.55 P n Bu 3 16.77 18.91 136 5. 25 P i Bu 3 17.13 17.76 143 5.70 P i Pr 3 17.17 19.47 160 3.45 PCy 3 17.2 7 19.46 170 1.40 Table 3 - 4 . Parameters used to model the trialkyl phosphines against H . H values taken from T able 3 - 3 . One test of the fit can be seen in Figure 3 - 3 . The plot is the model predicted H plotted against the experimentally measured H values. Plots of this type give an indication of how well the data is modelled based on the regression. A good linear fit means there is good correlation between the H d values. Fortunately, this model gives quite a good fit for the alkyl phosphines. The best fit line for the plot shows that there is a n almost exact 1:1 ratio from the model predicted and actual H values with a decent fit. Figure 3 - 3 . Plot of H vs model predicted H using H values determine from CD 3 CN with an assumption of constant S . y = 1x - 1E - 11 R² = 0.9503 16.6 16.7 16.8 16.9 17 17.1 17.2 17.3 17.4 16.6 16.7 16.8 16.9 17 17.1 17.2 17.3 17.4 Model Predicted vs. Model Predicted 60 While this is not quantitative, the model can tell us a bit about the H measurement of the PR 3 complexes . In the model there is a dependence on both sterics and electronics. Equation 2 highlights some phenomena that agree, logically, with what we expect to see. First, the a term is 0.135. The magnitude is somewhat meaningle ss, but the sign is important. The positive correlation between H d d ), the H also reflects a better donor ability (smaller H ). Conversely b term indicate s that steric hin derance increases the barrier to Cr - N bond rotation, shown in the positive correlation between H . When we extended the model in E quation 2 to the whole series of PR 3 ligands, there was no correlation. This is unsurprising since we specifically chos d the trialkylphosphines and the other PR 3 ligands have groups that are likely heavily influenced by , as well as different values of S . We were surprised to see that the series of the alkyl/aryl phosphin es s till followed the trend that adding aryl substituents increased donor ability (lower H ) regardless of the conditions ( T able 3 - 3). Even more shocking, though, was the trend of the P(OR) 3 ligands. Much like the alkyl vs. aryl phosphines, the phosphites are expected to be far worse - donors due to the electron withdrawing substituents. Every number we have measured with these ligands suggests they are, in fact, better donors to Cr(VI). It s eems unlikely that the difference in H between the values for the p hosphites and phosphines are within the error of even these crude measurements. For example, in CDCl 3 where only steric interactions of the anion affect the measurement , the P(OEt 3 ) 3 comp lex has a H of 16.99 kcal/mol compared to a H of 18.71 kcal/mol f or PMe 3 . 61 We can say with some degree of certainty that the sterics of the PR 3 ligands are not the cause of this difference in donation . While it is PMe 3 is larger than that of P(OEt 3 ) 3 , 118° and 109°, respectively, t he P(O i Pr) 3 than PMe 3 . Since we could only get qualitative data from experiment, we sought an explanation through theory. 3.7 Phosphine Bonding Analysis To gain better understanding of the difference in H , K elly analyzed the phosphine complexes using natural bond orbital theory (NBO) with DFT optimized structures. The findings were quite surprising. Decades of information about phosphines involved in late metal catalysts have led to a wealth of information ab out phosphine bonding to metal centers. 20 - 22 The resonance forms shown in Figure 3 - 4 highlight the accepted bonding modes of a phosphine and a late transition metal. Figure 3 - 4 . Lewis structures depicting the typical resonance forms of a low valent metal - phosphine interaction. The 62 As described in the introduction, Cr(VI) i s far from a low valent metal. Still, the interactions between chromium and an alkyl phosphine ligand are not substantially different than those expect ed for a metal like Ni or Pd. The resonance forms that contributed to the Cr - P bond were simple Lewis pai r resonance forms. The contributions to the resonance form, cal culated by NRT, (the resonance forms are defined in the figures) . T he may account for the high H we see in the alkyl phosphines, as well as their instability at elevated temperature. When the same analysis was performed on the phosphite ligand, there were additional resonance forms found to contribute. Figure 3 - 5 below details the findings, but in short, the phosphite ligand was found to interact through hyperconjugative resonance forms that re semble the phosphite have an available lone pair, they can donate into a P - O * orbital , resonance form . Form is unsurprising a s these resonance forms ar e well established as a resonance phenomenon within the phosphi t e ligand itself and have little dependence on M . 23 - 24 What was shocking was the phosphite was also found to have a resonance form where a lone pair fr om o xygen donates into a Cr - P * orbital, resulting in what could be considered a net reduction of Cr. We postulate the contribution s from , resulting in an overall higher bond order with chrom ium. 63 Figure 3 - 5 . Resonance forms discovered in NRT analysis of [NCr(N H 2 ) 2 (P(OMe) 3 ] + . Still, though, this resonance form is very unexpected. Why would the resonance form account for 1 4 % of the bondi ng interaction between Cr and P? The answer could lie in the electronegativity of the formally d 0 Cr(VI) atom. Because of the high formal charge, the Cr could potentially compete for electron density with the O atoms. It is diff icult to assign an electrone gativity to the chromium atom in our system, especially since the coordination environment should vary the electronegativity, but the Sanderson electronegativity of Cr(VI) is 3.37 , between Cl (3.48) and Br (3.21) . 25 To get a better measure on the Lewis acidity of the chromium system, we turned to a method developed by Gutmann. 26 - 27 The acceptor number (AN) system is based on the 31 P NMR shift of a coordinated OPEt 3 ligand . A spectrum is acquired on a sample of free phosphine oxide in dichloroethane, th en remeasured with the test species present. The shift of the free phosphine oxide versus the coordinated one is then compared to a scale of previousl y measured standards. The scale 64 is based on a range of 0 - 100 where 0 is hexane and 100 is SbCl 5 . Our chrom ium system has an of 100, equal to that of SbCl 5 . For comparison, other common Lewis acids like B(C 6 F 5 ) 3 and BBr 3 have ANs of 78 and 90.3, respectivel y. 28 As we suspected, the high oxidation state an d poor donor ability of the phosphine ligands means that the chromium atom in our system is extremely Lewi s acidic. This comes as no surprise since d 0 metals, TiCl 4 for example, have been used as Lewis acids in organic synthesis. 29 These findings lend some support to reson ance form as a reasonable resonance form in the chromium system. Figure 3 - 6 . NRT analysis of [NCr(NH 2 ) 2 (PPhMe 2 )] + Further NRT analysis of the arylphosphines and tri s amidoph osphines yielded related results to the phosphite. In the analysis o f PPhMe 2 as a ligand, NRT highlighted the same resonance form, where the phenyl group delocalized a positive charge in the ring ( Figure 3 - 6 ). This resonance form could explain why we see when more phenyl groups are added to the phosphin e. 65 3.8 Conclusions Regrettably, we were unable to perform a full, quantitative bonding analysis of the phosphine ligands. The hurdle that the inconsistent S value in acetonitrile and the in consistent interactions of the anion in chloroform imposed , provided no workaround. Despite this, our qualitative investigations led to some exciting results. The estimated values we observed from the H led us to investigate the Cr(VI) - P bonding computat ionally. In doing so we discovered contrasting bonding pictures between high and low valent metals. The resonance form provides an explanation for the surprisingly strong electron donation from aryl phosphines and phosphites. O ur phosphine study was qui te different than the M - PE 3 phosphine studies that Tolman and others have studied. 1 A s such , the observed diffe rences in M - P bonding are not shocking. W e propose that many systems that could broadly be classified as mid - valent, may incorporate both sorts of metal - phosphine bonding pictures to varying degrees. As such, we hope these bonding descriptors find use in a wide variety of phosphine selection considerations. 3.9 Experimental This experimental was taken from our recent publications. These can be located at: Organometallics , 2017 , 36 (7), 1227 - 1237 And Polyhedron , 2018 , accepted All syntheses were carried out unde r an N 2 atmosphere, using standard Schlenk techniques or in an MBraun glovebox. All reagents were stored in a glovebox after purification. Diethyl ether, acetonitrile, and dichloromethane were purified by passing them over a neutral alumina column under N 2 and stored over 3 Å molecular sieves. Chloroform was distilled from P 2 O 5 under N 2 and 66 stored over molecular sieves. Deuterated chloroform from Cambridge Isotope Laboratories was distilled from P 2 O 5 under N 2 and stored over m olecular sieves. Deuterated ace tonitrile from Cambridge Isotope Laboratories was distilled under N 2 from calcium hydride and stored over 3 Å molecular sieves. The complex NCr(N i Pr 2 ) 2 I was prepared according to the literature procedure. 18 Trimethyl - , dimethylphenyl - , and diphenylmethylphosphine were purchased from Aldrich Chemical Co. and used as received. Triethylphosphite and triisopropylphosphite were purchased from Aldrich Chemical Co. and distilled from Na 2 SO 4 under reduced pressure. Triisobutylphosphine, diphenylcyclohexylphosphine, and phenyldicyclohexylphosphine were purchased from Strem Chemical Co. and used as received. Triisopropylphosphine purchased from Stre m Chemical Co. was distilled from a 10 wt% solution in hexanes and stored over 3 Å molecular sieves. Tri( n - butyl)phosphine purchased from Strem Chemical Co. was distilled under purified N 2 and stored over 3 Å molecular sieves. Triphenyl - , phenyldiethyl - , d iphenylethyl - , and tricyclohexylphosphine were purchased from Alfa Aesar and used as received. Silver hexafluoroantimonate and thallium hexafluorophosphate were purchased from Sigma - Aldrich Chemical Co. and used as received. Thallium(I) BAr F 24 , where BAr F 24 = B [ 3,5 - (CF 3 ) 2 C 6 H 3 ] 4 , was prepared us ing the literature procedure. 30 The KBAr F 20 was supplied as a gift from Boulder Chemical Co. and was used as received. AgAl(O t BuF 9 ) 4 was prepared following the literature procedure. 12 Diphenyl( n - butyl)phosphine was synthesized by adding 1.7 M n BuLi solution (1 equiv) to PPh 2 Cl (1 equiv) in cold ( 78 °C) diethyl ether. 31 In the literature preps for these phosphines, they were purified by distillation. However, the syntheses were carried out on much smaller scales than was conducted in the literature. Thus as an alternative method of purification, the diphenyl( n - 67 butyl)phosphine, were run over a short plug of alumina for pur ification, which provided colorless oils pure by multi - nuclear NMR spectroscopy. Adequate CHN was not obtained on the c omplexes under study despite many attempts. The cationic Cr(VI) complexes have been characterized by NMR ( 1 H, 13 C, 31 P, 14 N, and 19 F), X - ray diffraction, and melting point. All experiments carried out with the chromium complexes in this study were conducte d with X - ray quality single crystals in an attempt to ensure purity. Synthetic Details The preparation of the phosphines with various cat ions followed essentially the same procedure as that below. The exception is the synthesis of BAr F 24 salts, which were synthesized using Tl in DCM using the same general procedure as that below General Procedure for the Synthesis of [ NCr(N i Pr 2 ) 2 PE 3 ] + SbF 6 : A 20 mL scintillation vial was charged with 1 equiv of NCr(N i Pr 2 ) 2 I, 18 acet onitrile (3 mL), and a Teflon - coated stir bar. This mixture was stirred at room temperature giving a dark red - orange solution. Separately, a solution of AgSbF 6 (1 equiv) was prepared in acetonitrile (1 - 2 mL). The AgSbF 6 solution was then added dropwise to the stirred solution o f NCr(N i Pr 2 ) 2 I . Upon addition, copious amounts of off - white precipitate formed, and the solution became dark brown. The resultant mixture was stirred for 20 min after complete addition of the Ag solution. The mixture was then filtered over Celite to remove the precipitate. The dark brown solution was once again stirred at room temperature and a solution of PR 3 (1 - 2 equiv) in acetonitrile (1 - 2 mL) was added. (1 equiv of the phosphine was used if it was a solid or high - boiling liquid pho sphine that is difficu lt to remove by recrystallization. 2 equiv of phosphine were used if PR 3 is a low - boiling liquid easily removed in vacuo .) Upon addition of PR 3 , the solution quickly became yellow - orange. The reaction solution was stirred for 1 h at r oom 68 temperature. The volatiles were then removed in vacuo to give a dark residue. This residue was rinsed with small aliquots of cold Et 2 O (3 x 1 mL) to remove any unreacte d material . The residue was once more dried in vacuo. The residue was dissolved in a min imal amount of CH 2 Cl 2 or CHCl 3 and layered with Et 2 O or pentane. The layered solution was then stored overnight at 35 °C to yield yellow - orange X - ray quality crystals. Synthesis of [NCr(N i Pr 2 ) 2 P Me 3 ] + SbF 6 : Following the general procedure, the reactio n wa s carried out with NCr(N i Pr 2 ) 2 I ( 89 mg, 0.2 26 mmol), AgSbF 6 ( 78 mg, 0.2 26 mmol), and P Me 3 ( 35 mg, 0. 46 mmol). Yield ( 68.2 mg, 52.4 %). Note: Synthesis for the other anions of this complex were achieved using the same generic procedure. M.p.: 111 - 113 °C. 1 H NMR (500 MHz, CDCl 3 ) 5.29 (sept, 2H), 4.08 3.93 (sept, 2H), 1.86 (t, J = 4.6 Hz, 6H), 1.69 (d, J = 10.9 Hz, 9H), 1.61 (d, J = 6.3 Hz, 6H), 1.39 (d, J = 6.3 Hz, 6H), 1.27 (d, J = 9.0 Hz, 6H). 13 C NMR (126 MHz, CD 3 6 (s), 30.44 (s), 23 .20 (d), 16.79 (s), 16.14 (s), 15.89 (s). 19 F (470 MHz, CDCl 3 - 123.1 (d). 31 P NMR (202 MHz, CDCl 3 Synthesis of [ NCr(N i Pr 2 ) 2 P n Bu 3 ] + SbF 6 : Following the general procedure, the reaction was carried out with NCr(N i Pr 2 ) 2 I (1 00 mg, 0.254 mmol), AgSbF 6 (87 mg, 0.254 mmol), and P n Bu 3 (51.4 mg, 0.51 mmol). Yield (83.1 mg, 46.4%). M.p.: 50 °C (dec.). 1 H NMR (500 MHz, CDCl 3 ): 7.70 (s, 1H), 7.52 (s, 1H), 7.47 (ddd, J = 19.8, 9.9, 5.1 Hz, 1H), 4.95 (sept, J = 12.8, 6.4 Hz, 1H), 3.88 (sept, J = 12.5, 6.3 Hz, 1H), 1.87 (d, J = 10.3 Hz, 1H), 1.65 (d, J = 6.3 Hz, 1H), 1.56 (d, J = 6.3 Hz, 1H), 1.15 (d, J = 6.4 Hz, 1H), 1.10 (d, J = 6.4 Hz, 1H). 13 C NMR (126 MHz, CDCl 3 ): 59.85 (d), 58.92, 32.48, 30.30, 25.79 (d), 24.92 (d), 24.56 (d), 23.87, 23.79, 13.85. 31 P NMR (202 MHz, CDCl 3 ): 30.0. 19 F NMR (470 MHz, CDCl 3 ): 106.55 to 137.63 (m). Synthesis of [ NCr(N i Pr 2 ) 2 P i Bu 3 ] + SbF 6 : Following the general procedure, the reaction was carried out with NCr(N i Pr 2 ) 2 I (50 mg, 0.127 mmol), AgSbF 6 (44 mg, 0.127 mmol), and P i Bu 3 (36 69 mg, 0.254 mmol). Yield (58.6 mg, 64.7%). M.p.: 150 °C (dec.). 1 H NMR (500 MHz, CDCl 3 ): 5.36 (sept, J = 12.4, 6.2 Hz, 2H), 4.00 (sept, J = 12.4, 6.2 Hz, 2H), 2 .16 (dt, J = 18.8, 6.3 Hz, 3H), 1.97 (dd, J = 8.5, 6.3 Hz, 6H), 1.84 (d, J = 6.3 Hz, 6H), 1. 57 (d, J = 6.2 Hz, 6H), 1.37 (d, J = 6.2 Hz, 6H), 1.32 (d, J = 6.2 Hz, 6H), 1.10 (d, J = 6.6 Hz, 18H). 13 C NMR (126 MHz, CDCl 3 ): 59.50 (d), 59.09, 32.56 (d), 29.40, 25.02, 24.87, 23.94 (d), 23.74, 19.46. 31 P NMR (202 MHz, CDCl 3 ): 32.8. Synthesis of [ NCr(N i Pr 2 ) 2 P i Pr 3 ] + SbF 6 : Following the general procedure, the reaction was carried out with NCr(N i Pr 2 ) 2 I (52 mg, 0.132 mmol), AgSbF 6 (45 mg, 0.132 mmol), and P i Pr 3 (33.5 mg, 0.210 mmol). Yield (33.5 mg, 40%). M.p.: 155 °C (dec.). 1 H NMR (500 MHz, CDCl 3 ): 5.41 ( sept, J = 12.5, 6.2 Hz, 2H), 4.02 (sept, J = 12.6, 6.3 Hz, 2H), 2.46 - 2.34 (m, 3H), 1.84 (d, J = 6.4 Hz, 6H), 1.58 (d, J = 6.3 H z, 6H), 1.43 (d, J = 7.2 Hz, 9H), 1.40 (d, J = 6.9 Hz, 15H), 1.34 (d, J = 6.2 Hz, 6H). 13 C NMR (126 MHz, CDCl 3 ): 59.71, 59.24, 32 .74, 29.58, 25.12 (d), 24.14, 23.94, 19.65. 31 P NMR (202 MHz, CDCl 3 19 F NMR (470 MHz, CDCl 3 ): 105.15 to 139.23 (m, J = 1809.1, 1654.8, 1457.8, 1258.2 Hz). Synthesis of [ NCr(N i Pr 2 ) 2 PCy 3 ] + SbF 6 ) : Following the general procedure, the reaction was carried out with NCr(N i Pr 2 ) 2 I (55 mg, 0.140 mmol), AgSbF 6 (48 mg, 0.140 mmol) and PCy 3 (40.5 mg, 0.140 mmol). Yield (66 mg, 60%). M.p.: 117 °C (dec.). 1 H NMR (500 MHz, CDCl 3 ): 5.35 (sept, J = 12.4, 6.2 Hz, 2H), 4.02 (sept, J = 12.6, 6.3 Hz, 2H), 2.06 (dt, J = 21.0, 10.5 Hz, 6H), 1.99 - 1.86 (m, 16H), 1.83 (d, J = 6.3 Hz, 8 H), 1.78 (s, 4H), 1. 58 (d, J = 6.3 Hz, 10H), 1.56 - 1.41 (m, 6H), 1.39 (d, J = 6.2 Hz, 8H), 1.36 (d, J = 6.2 Hz, 8H), 1.35 - 1.11 (m, 16H). 13 C NMR (126 MHz, CDCl 3 ): 59.84, 59.22, 34.84 (d), 32.70, 29.75, 29.65, 27.60 (d), 25.84, 24.30, 23.75. 31 P NMR (202 MHz , CDCl 3 ): 56.9. 19 F NMR (470 MHz, CDCl 3 ): - 123.19 (d, J = 3323.8 Hz). Synthesis of [NCr(N i Pr 2 ) 2 PPhMe 2 ] + SbF 6 : Following the general procedure, the reaction was carried out with NCr(N i Pr 2 ) 2 I (50 mg, 0.127 mmol), AgSbF 6 (44 mg, 0.127 mmol), and PPhMe 2 70 (35 m g, 0.253 mmol). Yield (56.4 mg, 68.4%). Note: Synthesis for the other anions of this complex were achieved using the same generic procedure. M.p.: 152 - 154°C (dec.). 1 H NMR (500 MHz, CDCl 3 J = 8.7, 3.1 Hz, 2H), 7.53 (s, 3H), 5.33 (sept, J = 23.2 Hz, 2H), 3.91 (dt, J = 12.0, 5.9 Hz, 2H), 1.99 (d, J = 10.3 Hz, 6H), 1.57 (dd, J = 11.1, 6.2 Hz, 12H), 1.31 1.17 (m, 13H). 13 C NMR (126 MHz, CDCl 3 ), 59.81 (d), 58.73 (s), 32.05 (s), 29.68 (s), 23.40 (s), 22.63 (s), 14.56 (s), 14.32 (s). 19 F NMR (470 MHz, CDCl 3 - 122.13 (d, J = 5077.9 Hz). 31 P NMR (202 MHz, CDCl 3 14 N NMR (36 MHz CDCl 3 448.8 (s). Synthesis of [ NCr(N i Pr 2 ) 2 PPh 2 M e ] + SbF 6 : Following the genera l procedure, the reaction was carried out with NCr(N i Pr 2 ) 2 I (50 mg, 0.127 mmol), AgSbF 6 (43 mg, 0.127 mmol), and PPh 2 Me (50 mg, 0.250 mmol). Yield (50.1 mg, 54.9%). M.p.: 138 °C. 1 H NMR (500 MHz, CDCl 3 ): 7.94 - 7.27 (m, 10H), 5.40 (sept, J = 11.6, 5.7 Hz, 2H), 3.89 (sept, J = 12.0, 5.9 Hz, 2H), 2.34 (d, J = 9.2 Hz, 3H), 1.89 (s, 1H), 1.55 (dd, J = 15.3, 6.2 Hz, 12H), 1.40 (s, 3H), 1.27 (d, J = 6.0 Hz, 7H), 1.07 (d, J = 6.0 Hz, 6H). 13 C NMR (126 MHz, CDCl 3 ): 132.42 (d), 132.09, 131.79 (d), 130.70, 129.92 (d), 129.21, 128.31, 127.93, 59.97 (d), 58.89, 31.99, 29.11, 23.05, 22.25, 12.67 (d). 31 P NMR (202 MHz, CDCl 3 ): 20.7. 19 F NMR (470 MHz, CDCl 3 ): 107.41 to 139.91 (m). The X - ray diffraction study was carrie d out on the BAr F 24 sa lt, which gave single crystals and was made analogously to the SbF 6 salt. Synthesis of [ NCr(N i Pr 2 ) 2 PPhEt 2 ] + SbF 6 : Following the general procedure, the reaction was carried out with NCr(N i Pr 2 ) 2 I (50 mg, 0.127 mmol), AgSbF 6 (43 mg, 0.127 mmol), and PPhEt 2 (34 mg, 0.246 mmol). Yield (28.1 mg 33.1%). M.p.: 115 °C (dec.). 1 H NMR (500 MHz, CDCl 3 ): 7.64 - 7.50 (m, 5H), 5.21 (sept, J = 12.5, 6.3 Hz, 2H), 3.90 (sept, J = 12.5, 6.3 Hz, 2H), 2.48 - 2.33 (m, J = 15.0, 10.7, 7.5 Hz, 2H), 2.33 - 2.21 (m, 2H), 1.65 (d, J = 6. 3 Hz, 6H), 1.57 (d, J = 6.3 Hz, 71 6H), 1.31 - 1.18 (m, 14H), 1.14 (d, J = 6.2 Hz, 6H). 13 C NMR (126 MHz, CDCl 3 ): 132.09, 130.87 (d), 130.25 (d), 59.65, 58.51, 31.88, 23.53, 22.34, 17.60 (d), 7.16. 31 P NMR (202 MHz, CDCl 3 ): 34.9. 19 F NMR (470 MHz, CDCl 3 ): 123. 30 (d, J = 5888.3 Hz). Synthesis of [ NCr(N i Pr 2 ) 2 PPh 2 Et ] + SbF 6 : Following the general procedure, the reaction was carried out with NCr(N i Pr 2 ) 2 I (100 mg, 0.254 mmol), AgSbF 6 (87 mg, 0.254 mmol), a nd PPh 2 Et (63 mg, 0.298 mmol). Yield (94 mg, 51.6%). M.p.: 15 0 °C (dec). 1 H NMR (500 MHz, CDCl 3 ): 7.60 (dt, J = 11.6, 6.1 Hz, 6H), 7.45 - 7.28 (m, 4H), 5.20 (sept, J = 6.3 Hz, 2H), 3.93 (sept, J = 6.4 Hz, 2H), 2.62 (p, J = 7.3 Hz, 2H), 1.70 (d, J = 6.2 Hz, 6 H), 1.56 (d, J = 6.2 Hz, 6H), 1.24 (d, J = 6.3 Hz, 6H), 1.11 (dt, J = 18.0, 7.4 Hz, 3H), 1.01 (d, J = 6.3 Hz, 6H). 13 C NMR (126 MHz, CDCl 3 ): 132.98 (d), 132.85 (d), 130.20 (d), 125.36 (d), 60.06, 59.07, 32.31, 29.49, 23.39, 22.60 (d), 22.34, 7.98 (d). 31 P NMR (202 MHz, CDCl 3 ): 35.3. 19 F NMR (470 MHz, CDCl 3 ): 122.12 (d, J = 5841.6 Hz). Synthesis of [ NCr(N i Pr 2 ) 2 PPh 2 n Bu ] + PF 6 : A 20 mL scintil lation vial was charged with NCr(N i Pr 2 ) 2 I (100 mg, 0.254 mmol), CH 2 Cl 2 (5 mL), PPh 2 n Bu (61 mg, 0. 252 mmol), and a Teflon - coated stir bar. This solution was stirred at room tempera ture to give a dark red - orange solution. Separately, a suspension of TlPF 6 wa s prepared in 2 mL of CH 2 Cl 2 . The TlPF 6 suspension was then added dropwise to the stirred solution of NCr(N i Pr 2 ) 2 I and PPh 2 n Bu . A yellow precipitate began to form on addition. Aft er addition, the solution was stirred 3 h at room temperature. Then, the reaction mixture was filtered through Celite to remove the precipitate, and the bright orange filtrate was collected. The volatiles were removed from the filtrate in vacuo, leaving a dark residue. The residue was washed with cold Et 2 O (3 x 1 mL), and the s olution was again dried in vacuo. The residue was dissolved in a minimal amount of CH 2 Cl 2 and layered with pentane. The layered solution was stored at 35 °C overnight to get X - ray qu ality orange crystals. Yield (104 mg, 63%). 72 M.p.: 74 - 77 °C (dec). 1 H NMR (500 MHz, CDCl 3 ): 7.78 - 7.47 (m, 10H), 5.22 (sept, J = 6.3 Hz, 2H), 3.93 (sept, J = 6.4 Hz, 2H), 2.53 (q, J = 7.8 Hz, 2H), 1.69 (d, J = 6.3 Hz, 6H), 1.56 (d, J = 6.2 Hz, 6H), 1.38 (dq, J = 23.9, 8.0, 7.6 Hz, 4H), 1.25 (d, J = 6.3 Hz, 6H), 0.99 (d, J = 6.3 H z, 6H), 0.84 (t, J = 7.1 Hz, 3H). 13 C NMR (126 MHz, CDCl 3 ): 132.89 (d), 132.80 (d), 130.23 (d), 125.81 (d), 60.11, 59.04, 32.29, 29.44, 29.05 (d), 25.67 (d), 23.99 (d), 23.33, 22.32, 13.60. 31 P NMR (202 MHz, CDCl 3 ): 32.7. 19 F NMR (470 MHz, CDCl 3 ): 73.41 ( d, J = 712.4 Hz). Synthesis of [ NCr(N i Pr 2 ) 2 PPh 2 Cy ] + SbF 6 : Following the general procedure, the reaction was carried out with NCr(N i Pr 2 ) 2 I (75 mg, 0.191 mmol), AgSbF 6 (68 mg, 0.195 mmo l), and PPh 2 Cy (63 mg, 0.230 mmol). Yield (62.5 mg, 63.8%). M.p.: 168 °C (dec). 1 H NMR (500 MHz, CDCl 3 ): 7.63 (m, J = 15.0, 7.8 Hz, 10H), 4.94 (sept, J = 6.2 Hz, 2H), 3.92 (sept, J = 6.3 Hz, 2H), 2.50 2.33 (m, 1H), 2.31 2.14 (m, 2H), 1.93 1.77 (m, 2H), 1.7 2 (d, J = 6.3 Hz, 6H), 1.57 (d, J = 6.2 Hz , 6H), 1.45 1.33 (m, 6H), 1.20 (d, J = 6.3 Hz, 6H), 1.02 (d, J = 6.2 Hz, 6H), 0.93 0.72 (m, 2H). 13 C NMR (126 MHz, CDCl 3 ): 133.77 (d, J = 10.2 Hz), 132.89, 129.85 (d, J = 10.2 Hz), 123.03 (d, J = 42.8 Hz), 59.92 (d , J = 1.8 Hz), 58.90, 36.94 (d, J = 22.4 H z), 32.14 (d, J = 1.8 Hz), 29.05, 26.63 (d, J = 12.5 Hz), 25.46, 23.34, 22.39. 31 P NMR (202 MHz, CDCl 3 ): 45.2. 19 F NMR (470 MHz, CDCl 3 ): 106.76 to 136.05 (m). The X - ray diffraction study was done with the BPh 4 s alt, which gave single crystals and was ma de analogously to the SbF 6 salt. Synthesis of [ NCr(N i Pr 2 ) 2 PPhCy 2 ] + SbF 6 : Following the general procedure, the reaction was carried out with NCr(N i Pr 2 ) 2 I (52 mg, 0.132 mmol), AgSbF 6 (45 mg, 0.132 mmol), and PPhCy 2 (48.3 mg, 0.176 mmol). Yield (58.2 mg, 56.1%). M.p.: 115 °C (dec). 1 H NMR (500 MHz, CDCl 3 ): 7.79 - 7.37 (m, 5H), 5.38 (sept, J = 12.2, 6.1 Hz, 2H), 3.99 (sept, J = 12.2, 6.0 Hz, 2H), 2.34 (d, J = 8.7 Hz, 2H), 1.91 (d, J = 21.6 Hz, 8H), 1.77 (d, J = 6.2 Hz, 8H), 1.60 (d, J = 6.2 Hz, 6H), 1.44 - 1.05 (m, 24H). 13 C NMR (126 MHz, CDCl 3 ): 132.89 (d), 132.49, 130.01 (d), 122.36 (d), 59.96, 59.02, 73 34.31 (d), 32.28, 29.37, 28.32, 28.04, 27.17 - 26.56 (m), 25.52, 23.50, 23.19. 31 P NMR (202 MHz, CDCl 3 ): 49.8. 19 F NMR (470 MHz, CDCl 3 ): 108.70 to 142.59 (m). Synthesis of [ NCr(N i Pr 2 ) 2 P(OEt) 3 ] + SbF 6 : Following the general procedure, the reaction was carried out with NCr(N i Pr 2 ) 2 I (100 mg, 0.254 mmol), AgSbF 6 (87 mg, 0.254 mmol), and P(OEt) 3 (43 mg, 0.26 mmol). Yield (48.2 mg , 28.3%). M.p.: 149 - 150 °C. 1 H NMR (500 MHz, CDCl 3 ): 5.36 (sept, J = 12.6, 6.3 Hz, 2H), 4.24 (p, J = 7.1 Hz, 6H), 4.03 (sept, J = 12.5, 6.1 Hz, 2H), 1.87 (d, J = 6.3 Hz, 6H), 1.60 (d, J = 6.3 Hz, 6H), 1.45 - 1.35 (m, 18H), 1.28 (d, J = 6.3 Hz, 9H) . 13 C NMR ( 126 MHz, CDCl 3 ): 65.06 (d), 60.32 (d), 59.19, 32.14, 30.46, 23.17, 22.81, 16.29 (d). 31 P NMR (202 MHz, CDCl 3 ): 122.6. 19 F NMR (470 MHz, CDCl 3 ): 122.86 (d, J = 5081.0 Hz). Synthesis of [ NCr(N i Pr 2 ) 2 P(O i Pr) 3 ] + SbF 6 : Following the general procedure, th e rea ction was carried out with NCr(N i Pr 2 ) 2 I (50 mg, 0.127 mmol), AgSbF 6 (43 mg, 0.127 mmol), and P(O i Pr) 3 (38 mg, 0.182 mmol). Yield (44.3 mg, 51%). M.p.: 138 - 140 °C. 1 H NMR (500 MHz, CDCl 3 ): 5.35 (dt, J = 12.6, 6.3 Hz, 2H), 4.89 4.70 (m, 3H), 4.05 (dt, J = 12 .5, 6.2 Hz, 2H), 1.92 (d, J = 6.3 Hz, 6H), 1.56 (d, J = 6.3 Hz, 6H), 1.41 (d, J = 6.2 Hz, 18H), 1.38 (d, J = 6.3 Hz, 6H), 1.30 (d, J = 6.3 Hz, 6H). 13 C NMR (126 MHz, CDCl 3 ): 75.15 (d), 59.98 (d), 59.26, 32.48, 29.97, 24.11 (d), 23.00 (d). 31 P NMR (202 MHz, CDCl 3 ): 119.1. Synthesis of [ NCr(N i Pr 2 ) 2 NCCH 3 ] + SbF 6 : A 20 mL scintillation vial was charged with NCr(N i Pr 2 ) 2 I (50 mg, 0.127 mmol), a Teflon - coated stir bar, CH 2 Cl 2 (4 m L), and acetonitrile (60 dark red - orange solution. Separately, AgSbF 6 (43 mg, 0.125 mmol) was suspended in CH 2 Cl 2 (2 mL). The AgSbF 6 suspension was then added dropwise to the chromium solution, re sulting in rapid formation of an off - white precipitate. Upon complete addition, the solution was stirred 3 h at room temperature. Then, the reaction mixture was filtered over Celite to remove the precipitate, and the red filtrate was collected. The 74 volatil es were removed from the filtrate in vacuo, leaving a dark brown residue. The residue was washed with cold Et 2 O (3 x 1 mL), and once again, the volatiles were removed in vacuo. The residue was dissolved in a minimal amount of CH 2 Cl 2 and layered with Et 2 O. This solution was stored overnight at 35 °C to give X - ray quality red - orange crystals (30.9 mg, 43.5%). M.p.: 126 - 129 °C (dec.). 1 H NMR (500 MHz, CDCl 3 ): 5.57 (sept, J = 7.5, 6.7 Hz, 2H), 4.07 (sept, J = 6.2 Hz, 2H), 2.58 (s, 3H), 2.02 (d, J = 6.2 Hz, 6H) , 1.52 (d, J = 6.2 Hz, 6H), 1.39 (d, J = 6.2 Hz, 6H), 1.20 (d, J = 6.3 Hz, 6H). 13 C NMR (126 MHz, CDCl 3 ): 60.34 (d), 59.87, 31.15, 30.91, 22.40, 22.25. 19 F NMR (470 MHz, CDCl 3 ): 123.79 (d, J = 6244.7 Hz). Instrumentation and Facilities All NMR spectra, in cluding LDP and routine characterization data, were recorded utilizing the Max T. Rogers NMR Facility at Michigan State University. These include a UNITYplus 500 spectrometer equipped with a 5 mm switchable broadband probe operating at 36.12 MHz ( 14 N); a V arian Inova 500 spectrometer equipped with a 5 mm Pulse Field Gradient (PFG) switchable broadband probe operating at 499.84 MHz ( 1 H) and 470.28 MHz ( 19 F); a Varian Inova 600 spectrometer equipped with a 5 mm PFG switchable broadband probe operat ing at 599. 89 MHz ( 1 H) and 564.30 MHz ( 19 F); and an Agilent DDR2 500 MHz NMR spectrometer equipped with a 5 mm PFG OneProbe operating at 499.84 MHz ( 1 H) , 125.73 MHz ( 13 C), and 202.35 ( 31 P) . 1 H NMR chemical shifts are reported relative to residual CHCl 3 in CDCl 3 as 7. 26 ppm. 13 C NMR chemical shifts are reported relative to natural abundance 13 CDCl 3 in d - chloroform as 77.0 ppm. Single crystal X - ray diffraction data was collected in the Center for Crystallographic Research at MSU. 75 Phosphine Rate C onstant (s 1 ) (kcal/mol) H (kcal/mol) Std. Dev. T emperature (°C) PMe 3 0.58 19.59 16.64 0.03 54.31 P n Bu 3 0.36 19.69 16.77 0.005 51.13 P i Pr 3 0.46 20.14 17.13 0.02 62.9 0 P i Bu 3 0.52 20.19 17.17 0.02 60.72 PCy 3 0.43 20.32 17.29 0.03 62.96 PPhMe 2 0.57 19.46 16.53 0.01 52.04 PPh 2 Me 0.33 18.96 16.16 0.01 34.71 PPhEt 2 0.45 19.58 16.65 0.02 51.46 PPh 2 Et 1.16 19.09 16.15 0.02 53.62 PPh 2 n Bu 0.4 19.16 16.31 0.008 43.71 PPh 2 Cy 1.01 19.41 16.43 0.02 57.36 PPhCy 2 0.54 19.26 16.37 0.02 48.25 P(OEt) 3 0.48 18.5 15.73 0.0 2 34.88 P(O i Pr) 3 1.15 18.81 15.91 0.04 48.82 Table 3 - 5 . Experimental H ) for [NCr(N i Pr 2 ) 2 PE 3 ]SbF 6 salts in CD 3 CN . a H value in CD 3 CN was measured via in situ generated species stabilized with excess phosphine. b Measured H was taken in multiple trials, taking a single measurement on three differen t samples due to compound instability. As a result, three different temperatures were c alibrated, one for each separate run. The reported G is approximate, as it is an average from 3 (close) temperatures. 76 Figure 3 - 7 . Eyring Plot for NCr(N i Pr 2 ) 2 I in CD 3 CN . The value obtained was - 1 e.u. (± 0.5). y = - 9171.9x + 23.099 R² = 0.9988 -10.5 -9.5 -8.5 -7.5 -6.5 -5.5 -4.5 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034 0.00345 0.0035 0.00355 0.0036 ln(k/T) 1/T(K) NCr(N i Pr 2 ) 2 I in CD 3 CN 77 Figure 3 - 8 . Eyring Plot analysis of NCr(N i Pr 2 ) 2 I in CDCl 3 . The value obtained for S was - 0.6(0.3). y = - 9084.7x + 23.484 R² = 0.9993 -9 -8.5 -8 -7.5 -7 -6.5 -6 -5.5 -5 -4.5 -4 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034 0.00345 0.0035 0.00355 ln(k/T) 1/T NCr(N i Pr 2 ) 2 I in CDCl 3 78 Figure 3 - 9 . Eyring Plot analysis of NCr(N i Pr 2 ) 2 OPh in CDCl 3 . The value obtained for S was - 3.1(0.5). y = - 6793.7x + 22.183 R² = 0.9981 -7.5 -7 -6.5 -6 -5.5 -5 -4.5 -4 -3.5 -3 0.0037 0.0038 0.0039 0.004 0.0041 0.0042 0.0043 ln(k/T) 1/T NCr(N i Pr 2 ) 2 OPh in CDCl 3 79 Figure 3 - 10 . Eyring Plot analysis of NCr(N i Pr 2 ) 3 in CDCl 3 . T he value obtained for S was - 5.7(0.7) y = - 5778.2x + 20.905 R² = 0.9966 -6 -5.5 -5 -4.5 -4 -3.5 -3 -2.5 0.0041 0.0042 0.0043 0.0044 0.0045 0.0046 0.0047 ln(k/T) 1/T NCr(N i Pr 2 ) 3 in CDCl 3 80 Figure 3 - 11 . Eyring Plot analysis of NCr(N i Pr 2 ) 2 Pyrr 3C6H3(CF3)2 in CDCl 3 . The value obtained for S was - 3.7(0.4) y = - 7800.8x + 21.888 R² = 0.9989 -7.5 -7 -6.5 -6 -5.5 -5 -4.5 -4 -3.5 -3 0.00325 0.0033 0.00335 0.0034 0.00345 0.0035 0.00355 0.0036 0.00365 0.0037 0.00375 ln(k/T) 1/T NCr(N i Pr 2 ) 2 Pyrr 3C6H3(CF3)2 in CDCl 3 81 NMR Spectr a F igure 3 - 12 . 1 H NMR Spectr um for [NCr(N i Pr 2 ) 2 ( P Me 3 )]SbF 6 ( 3 a ) in CD 3 CN 82 Figure 3 - 13 . 1 3 C NMR Spectr um for [NCr(N i Pr 2 ) 2 ( P Me 3 )]Sb F 6 ( 3 a ) in CD 3 CN 83 Figure 3 - 14 . 31 P NMR Spectr um for [NCr(N i Pr 2 ) 2 ( P Me 3 )]SbF 6 ( 3 a ) in CD 3 CN 84 Figure 3 - 15 . 19 F NMR Spectr um for [NCr(N i Pr 2 ) 2 ( P Me 3 )]SbF 6 ( 3 a ) in CD3CN 85 Figure 3 - 16 . 1 H NMR Spectr um for [NCr(N i Pr 2 ) 2 (P n Bu 3 )]SbF 6 ( 3b ) in CDCl 3 86 Figure 3 - 17 . 1 3 C NMR Spectr um for [NCr(N i Pr 2 ) 2 (P n Bu 3 )]SbF 6 ( 3b ) in CDCl 3 87 Figure 3 - 18 . 31 P NMR Spectr um for [NCr(N i Pr 2 ) 2 (P n Bu 3 )]SbF 6 ( 3b ) in CDCl 3 88 Figure 3 - 19 . 1 9 F NMR Spectr um for [NCr(N i Pr 2 ) 2 (P n Bu 3 )]SbF 6 ( 3b ) in CDCl 3 89 Figure 3 - 20 . 1 H NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P i Bu 3 ) ] SbF 6 ( 3c ) in CDCl 3 90 Figure 3 - 21 . 1 3 C NMR Sp ectr um for [ NCr(N i Pr 2 ) 2 (P i Bu 3 ) ] SbF 6 ( 3c ) in CDCl 3 91 Figure 3 - 22 . 31 P NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P i Bu 3 ) ] SbF 6 ( 3c ) in CDCl 3 92 Figure 3 - 23 . 1 9 F NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P i Bu 3 ) ] SbF 6 ( 3c ) in CDCl 3 93 Figure 3 - 24 . 1 H NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P i Pr 3 ) ] SbF 6 ( 3d ) in CDCl 3 94 Figure 3 - 25 . 1 3 C NMR Spectr um for [ NC r(N i Pr 2 ) 2 (P i Pr 3 ) ] SbF 6 ( 3d ) in CDCl 3 95 Figure 3 - 26 . 31 P NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P i Pr 3 ) ] SbF 6 ( 3d ) in CDCl 3 96 Figure 3 - 27 . 1 9 F NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P i Pr 3 ) ] SbF 6 ( 3d ) in CDCl 3 97 Figure 3 - 28 . 1 H NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Cy 3 ) ] SbF 6 ( 3e ) in CDCl 3 98 Figure 3 - 29 . 1 3 C NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Cy 3 ) ] SbF 6 ( 3e ) in CDCl 3 99 Figure 3 - 30 . 31 P NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Cy 3 ) ] SbF 6 ( 3e ) in CDCl 3 100 Figure 3 - 31 . 1 9 F NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Cy 3 ) ] Sb F 6 ( 3e ) in CDCl 3 101 Figure 3 - 32 . 1 4 N NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Cy 3 ) ] SbF 6 ( 3e ) in CDCl 3 102 Figure 3 - 33 . 1 H NMR Spectr um for [ NCr (N i Pr 2 ) 2 (P PhMe 2 ) ] SbF 6 ( 3 e ) in CDCl 3 103 Figure 3 - 34 . 1 3 C NMR Spectr um for [ NCr (N i Pr 2 ) 2 (P PhMe 2 ) ] SbF 6 ( 3 e ) in CDCl 3 104 Figure 3 - 35 . 31 P NMR Spectr um for [ NCr (N i Pr 2 ) 2 (P PhMe 2 ) ] SbF 6 ( 3 e ) in CDCl 3 105 Figure 3 - 36 . 19 F NMR Spectr um for [ NCr (N i Pr 2 ) 2 (P PhMe 2 ) ] SbF 6 ( 3 e ) in C DCl 3 106 Figure 3 - 37 . 1 H NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 Me ) ] SbF 6 ( 3g ) in CDCl 3 107 Figure 3 - 38 . 1 3 C NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 Me ) ] SbF 6 ( 3g ) in CD Cl 3 108 Figure 3 - 3 9 . 31 P NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 Me ) ] SbF 6 ( 3g ) in CDCl 3 109 Figure 3 - 40 . 1 9 F NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 Me ) ] SbF 6 ( 3g ) in CDC l 3 110 Figure 3 - 41 . 1 H NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P PhEt 2 ) ] SbF 6 ( 3h ) in CDCl 3 111 Figure 3 - 42 . 1 3 C NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P PhEt 2 ) ] SbF 6 ( 3h ) in CDC l 3 112 Figure 3 - 43 . 31 P NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P PhEt 2 ) ] SbF 6 ( 3h ) in CDCl 3 113 Figure 3 - 44 . 1 9 F NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P PhEt 2 ) ] SbF 6 ( 3h ) in CDCl 3 114 Figure 3 - 45 . 1 H NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 Et ) ] SbF 6 ( 3i ) in CDCl 3 115 Figure 3 - 46 . 1 3 C NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 Et ) ] SbF 6 ( 3i ) in CDCl 3 116 Figure 3 - 47 . 31 P NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 Et ) ] SbF 6 ( 3i ) in CDCl 3 117 Figure 3 - 48 . 1 9 F NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 Et ) ] SbF 6 ( 3i ) in CDCl 3 118 Figure 3 - 49 . 1 H NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 n Bu) ] P F 6 ( 3j ) in CDCl 3 119 Figure 3 - 50 . 1 3 C NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 n Bu) ] P F 6 ( 3j ) in CDCl 3 120 Figure 3 - 51 . 31 P NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 n Bu) ] P F 6 ( 3j ) in CDCl 3 121 Figure 3 - 52 . 1 9 F NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 n Bu) ] P F 6 ( 3j ) in CDCl 3 122 Figure 3 - 53 . 1 4 N NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 n Bu) ]P F 6 ( 3j ) in CDCl 3 123 Figure 3 - 54 . 1 H NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 Cy ) ] SbF 6 ( 3k ) in CDCl 3 124 Figure 3 - 55 . 1 3 C NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 Cy ) ] SbF 6 ( 3k ) in CDCl 3 125 Figure 3 - 56 . 31 P NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 Cy ) ] SbF 6 ( 3k ) in CDCl 3 126 Figure 3 - 57 . 1 9 F NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P Ph 2 Cy ) ] SbF 6 ( 3k ) in CDCl 3 127 Figure 3 - 58 . 1 H NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P PhCy 2 ) ] SbF 6 ( 3l ) in CDCl 3 128 Figure 3 - 59 . 1 3 C NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P PhCy 2 ) ] SbF 6 ( 3l ) in CDCl 3 129 Figure 3 - 60 . 31 P NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P PhCy 2 ) ] SbF 6 ( 3l ) in CDCl 3 130 Figure 3 - 61 . 1 9 F NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P PhCy 2 ) ] SbF 6 ( 3l ) in CDCl 3 131 Figure 3 - 62 . 1 4 N NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P PhCy 2 ) ] SbF 6 ( 3l ) in CDCl 3 132 Figure 3 - 63 . 1 H NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P (OEt) 3 ) ] SbF 6 ( 3m ) in CDCl 3 133 Figure 3 - 64 . 1 3 C NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P (OEt) 3 ) ] SbF 6 ( 3m ) in CDCl 3 134 Figure 3 - 65 . 3 1 P NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P (OEt) 3 ) ] SbF 6 ( 3m ) in CDCl 3 135 Figure 3 - 66 . 1 9 F NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P (OEt) 3 ) ] SbF 6 ( 3m ) in CDCl 3 136 Figure 3 - 67 . 1 H NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P (O i Pr) 3 ) ] SbF 6 ( 3n ) in CDCl 3 137 Figure 3 - 68 . 1 3 C NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P (O i Pr) 3 ) ] SbF 6 ( 3n ) in C DCl 3 138 Figure 3 - 69 . 31 P NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P (O i Pr) 3 ) ] SbF 6 ( 3n ) in CDCl 3 139 Figure 3 - 70 . 1 9 F NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P (O i Pr) 3 ) ] SbF 6 ( 3n ) in CDCl 3 140 Figure 3 - 71 . 1 4 N NMR Spectr um for [ NCr(N i Pr 2 ) 2 (P (O i Pr) 3 ) ] SbF 6 ( 3n ) in CDCl 3 141 Figure 3 - 72 . 1 H NMR Spectr um for [ NCr(N i Pr 2 ) 2 ( NCCH 3 ) ] SbF 6 ( 2 ) in CDCl 3 142 Figure 3 - 73 . 1 3 C NMR Spectr um for [ NCr(N i Pr 2 ) 2 ( NCCH 3 ) ] SbF 6 ( 2 ) in CDCl 3 143 Figure 3 - 74 . 1 9 F NMR Spectr um for [ NCr(N i Pr 2 ) 2 ( NCCH 3 ) ] SbF 6 ( 2 ) in CD Cl 3 144 Figure 3 - 75 . 1 4 N NMR Spectr um for [ NCr(N i Pr 2 ) 2 ( NCCH 3 ) ] SbF 6 ( 2 ) in CDCl 3 145 REFERENC E S 146 REF ERENCES 1. 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Kermagoret, A.; Braunstein, P., Dalton Trans. 2008, (6), 822 - 831. 148 Developing a Model for the Optimization and Development of Titanium Catalyzed Hydroami nation 4.1 Introduction We have discussed in the previous chapters how LDP can be used to not only parameterize ligands, but also interrogate ligand - metal bonding interactions. We wanted to prove, though, that LDP could also be used to design and optimize hig h valent metal catalysts. Being the first endeavor in modeling catalysis with the LDP system, w e decided it would be wise to start with a catalyst that we knew behaves well and has some simple ancillary ligand variability. With the combined knowledge from our group, the Bergman group , and other s, we decided t itanium - catalyzed hydroamination made a good platform from which to start . 1 - 6 As mentioned in chapter 1, more electronically deficient ligands put on the Ti ce nter should yield faster catalysis due to an inc reased acidity of the coordinated amide . 7 - 9 Surprisingly, i n a study published in 2006, it was observed that adding electron withdrawing substituents to the 2 - positio n of pyrrole slowed the rate of hy droamination . 9 In a 2011 study , Swartz again added similar electronic withdrawing arenes to the ligand, but this time to the 3 - position of the pyrroles. 7 With the withdrawing groups in the 3 - position, the rate of catalysis was increased . The se observations were attributed to the steric profile of the ligands. When the p yrrole is substituted in the 2 - pos ition, the large profile of the electron deficient arenes slows the rate of catalysis. The 3 - posistion on the other hand, meant the large substituents were pointing away from the metal center and did not increase the steri c profile of the ligand. The series of studies reported by Swartz highlights the difficulty in designing new and improved catalysts. There is a delicate balance between adding electron withdrawing groups and keeping the steric profile of the ligand small e nough. Simply 149 guessing at how to change the ligand structure can be productive, but it is often a long, intensive process. We hoped we could model the titanium hydroamination catalysis in a way that allowed quantitative understanding of the steric and elec tronic effects of the ligands. 4.2 Ligand Analysis Considerations The first hurdle in modeling the catalysis was the ligand choice. Since Dr. Doug Swartz, a former Odom group member, had results with both monodentate and bidentate pyrrole li gands, and there se emed to be an easily observable rate difference between pyrrole substitutions, we chose pyrrole - based ligands. But we were hesitant to use monodentate ligands. It has been established that titanium systems undergo ligand redistribution r eactions and, from experience in our group, monodentate ligands make this process more facile. 10 - 13 Using the ( X 2 ) Ti ( NMe 2 ) 2 ligands as a platform, we could easily manipulate the catalyst by exchanging the bidentate X 2 ancillary ligan d. 8 - 9, 14 The pyrrole platform is an excellent system to manipulate the sterics and electronics of the catalysts. Substitution of the 2 - , 3 - , and 4 - positions of the pyrrole with both steric ally and electronically v aried groups provides a wide variety of potential catalytic activities. We hoped these types of substitutions would allow enough ligand variation to produce a sound model for the reaction without the need to include any monodentate ligands. Importantly , th e analogous monodentate pyrrole fragments are also isolable and easy to measure with the LDP system. 15 Due to the nature of the LDP system, we are only able to parameterize monodentate ligands. We postulated this problem could be solved by simply measuring one half of the ligand fragment (ignoring the linker) to establish values for steric and electronic parameters. 150 This brings us to the other critical factor of the ligand a nalysis, sterics. The system Tolman developed for sterics was incredibly simple. 16 - 17 By forming a cone around the ligand, he simplified the complicated steric system of a phosphine ligand into a single valu e . This single value has found much use in the literature and is still commonly used today. Unfortunately, though, the simplicity of is a limitation on its implementation to other systems. The three - fold symmetry of simple PR 3 ligands means that a cone a round the ligands can be a reasonable approximation of the steric profile. Moving to ligands with less regular profiles means that th e conical approximation becomes a less accurate description. Moreover, as substituents are added to the ligands which make them more unsymmetrical , the approximation fails even faster. As such , we needed to find a way to quantitatively compare the steric profiles of the ligands we planned to study. In the original LDP publication, two steric descriptor s were used to compare t he ligands reported. 15 The first program treats the metal center as a point light source. 18 - 19 The program then uses the ligands as solid objects which, when shined with light from the metal center, create shadows. The output for the steric parameter is the percentage of a sphere around the metal this, later named, solid angle program is that the ligands essentially become t wo dimensional. This has two primary issues. The first, s ubstituents such as a phenyl group are treated as a function of the orientation of the group. For e xample, if a phenyl group is in an orientation where the vector connecting its centroid and the meta l center is perpendicular to the aromatic plane, the phenyl group appears large . If the same vector lies in the plane of the phenyl ring, then the group app ears small. The second problem is, like the orientation of the ligand, the distance to the metal has a dramatic effect on the measured steric parameter. Small ligands close to the metal are measured as being large while large radii atoms far from the metal center are measured as being quite small. In some cases this can be a true measure of sterics, 151 for example O t Bu is larger than S t Bu, but the program seems to treat this scenarios inconsistently This leads to strange results such as the ordering of the ha lides of F > Br > I > Cl on Cr(VI). 15 A better option for comparing the s teric profile of ligands is the percent buried volume program, %V bur , developed by Cavallo. 20 - 2 1 In this program t he first coordination sphere around the metal is set to an arbitrary value, generally a radius of 3.5 Å. The p rogram then creates a shell around the metal center at this radius. The volume of th e shell occupied by the ligand under investigation is the %V bu r . The calculation of the %V bur of a ligand is based on a crystal structure, and as such, the values are much less approximate than, for example, which is based on molecular models. It is important to note, though, that since hydrogen positions are calculated in the crystal structures, they are often remove d in the %V bur calculations , but for our system, they were included . The advantages of the b uried volume program were discussed in more detail in chapter 1, but, in brief, it allows a single parameter that accounts for size of ligand, irregularity in shape, and proximity of the ligand to the metal. For each ligand studied , the chromium complex wa s used to determine the steric parameters. We decided this was the most reasonable estimation of the ligands sterics. This is in part because we wanted to ignore contributions of the linker in the bidentate l igands to test metallocycle ring size effects, w hich will be discussed later . But, more importantly, determining the steric parameters from Ti would have meant accepting some inherent, known inaccuracy. Reactive species are, by definition, unstable. So, th e structure of the catalyst employed in the rate limiting step is rarely an isolable species and it is likely to be different from the precatalyst structure . By fully parameterizing the ligands from our Cr system, we have a systematic platform to compare a ll aspects of various ligands against one anothe r. In the titanium systems, some catalysts have ligands that vary in hapticity depending on the substitution. For example, the crystal 152 structure of (NMe 2 ) 2 1 through only th e nitrogen and the other 5 through the aromatic system; other dipyrrole ligands systems have both sides bound 1 through the pyrrole. 14 Since we predicted the ligand was fluctional, it i s possible that modeling the sterics from titanium would be inaccurate. Besides that, some of the precatalysts have varying coordination number due to dative coordination of HNMe 2 ligands. Mea suring the %V bur from the titanium complexes would have meant we were comparing both changes in ligand and changes in the coordination environment of the metal center. If, on the other hand, we measure the st erics from the chromium crystal structures, the coordination environment is much more constant. Additionally, b ecause of the sterics at the metal, 1 configuration. We think this is a more accurate representation for the active species of the Ti complexes , but that will be discussed in detail later. One might be concerned that the steric bulk of the diisopropyl groups would affect the orientation of the ligand on Cr. While this is a possibility, in some sense it relates ou r system back to the parameter . Tolman configured the phosphines into their smallest possible config uration before measuring . 16 This achieves a more constant orientation, rather than leaving the R groups arra nged at random, providing a more systematic comparison between ligands. 4.3 Synthesis and Characterization of the Chromium Complexes This project was undertaken as close collaboration with Tanner McDaniel. The ligands presented in this study that were not com mercially available were sy nthesized by him. 22 - 23 Tanner also synthesized the titanium catalysts and did kinetic analysis o f each catalyst. My part in this investigation was the synthesis of the chromium complexes, evaluation of the LDP parameters, and modeling the experimental data. 153 The synthesis and characterization of the chromium species in this study were quite straightf orward. Most procedures are either analogous to those from the original LDP publication or subtle derivations thereof. 15, 24 The gen eral procedures for the synthesis of the chromium complexes is shown in Scheme 4 - 1. For the electron deficient pyrrole ligands, the pyrroles were reacted with TlOEt in ether to produce the Tl - Pyrrole. The Tl - p yrrole can then be added to a solution of NCr(N i Pr 2 ) 2 I in ether, precipitating TlI, and generating the desired chromium pyrrole species. For the methyl - substituted pyrroles and indoles it was necessary to use transmetalation via ZnX 2 and NCr(N i Pr 2 ) 2 I or LiX and NCr(N i Pr 2 ) 2 OPh. Synthesis of the aryloxid e chromium complexes was achieved using an acid base reaction of the phenol and NCr(N i Pr 2 ) 3 . 24 All of the complexes except NCr(N i Pr 2 ) 2 ( SNap ) were crystalline and produced X - ray quality single crystals. Crystallogra phic analysi s of all compounds provided the structures used in the percent buried volume analysis. The settings for the buried volume program, SambVca 2, were left to the default settings. 25 The sph ere was given a radius of 3.5 Å, hydrogens were not included in the calculation, and the %V bur calculation used the Bondi radii of the atoms scaled by 1.17 with a 0.10 mesh size. The ligand donor parameters were established using our standard m ethod. 15, 26 The bidentate aryloxide ligands used for the catalysis featured a very large t Bu group in the 2 - position. The monodentate version of these ligands were bound to chromium easily enough, but the LDP valu es were not in line with what we expected. The values determined were >13 kcal/mol. When compared to unsubstituted phenol, 11.98 kcal/mol, these values did not seem logical. We synthesized the 2 - Me substitut ed versions of each phenol as electronic surrogat es and determined the LDP values to be lower than phenol. This is expected as adding alkyl or alkoxy groups to the 154 15, 27 We s uspect that, like the discussion of bul ky anions and phosphine ligands in chapter 3, the steric bulk of the 2 - t Bu phenols sterically crowds the i Pr groups of the amides and hinders rotation, artificially inflating the LDP value. 28 As such, we decided to use the LDP valu es of the methyl - substituted aryloxides for our modeling, but the %V bur was calculated using the t Bu substituted ligand. Scheme 4 - 1 . General procedures for synthesis of the chromium complexes. 4.4 Modeling the Hydroamination Kinetics With the methods to quantitatively compare sterics and electronics of our chosen ligands we set out to begin modeling kinetics. As mentioned above, we aimed to s tudy intermolecular hydroamination. Th e catalys e s w ere run with a 10 - fold excess of aniline to produce pseudo first order kinetics. The overall reaction is shown in Scheme 4 - 2 below. By monitoring consumption of 1 - phenylpropyne, we were able to model reaction progress while ignoring things lik e regioisomer mixtures. 155 Scheme 4 - 2 . Reaction scheme for the hydroamination kinetics experiments. We started the process with a training set of ligands for the catalysis. The training set allows for deve lopment of a model based o n a series of ligands , which can then be confirmed for accuracy later using another set of different ligands . The advantage of the training set is that we are able to confirm the accuracy of the model r ather than influencing the r egression by including all data points . For the training set we began with pyrrole - and indole - based bidentate ligands, including the dpm ligand. The initial complexes are shown in Table 4 - 1 below. 156 Ti Catalyst Cr Complex for L DP (kcal/mol) %V bur from Cr k obs x 10 4 (s 1 ) Ti(NMe 2 ) 2 (dpm) (1a) 13.64 20.4 4.16 Ti(NMe 2 ) 2 (dim 3Me ) (2a) 12.49 22.6 0.6 62 Ti(NMe 2 ) 2 (dpm 2Me ) (1b) 13.46 23.7 1.35 Ti(NMe 2 ) 2 (dpm 2phenyl ) (1c) 14.03 27.1 0.522 Ti(NMe 2 ) 2 (dpm 2tolyl ) (1d) 13.91 26.7 0.552 Table 4 - 1 . Summary of the titanium catalysts use for hydroamination, the chromium complexes used to parameterize the ligands, an d the rates of catalysis. 157 Table 4 - Ti Catalyst Cr Complex for LDP (kcal/mol) %V bur from Cr k obs x 10 4 (s 1 ) Ti(NMe 2 ) 2 (dpm 2 - [C6H3(CF3)2] ) (1e) 14.32 27.9 0.581 Ti(NMe 2 ) 2 (dim 3Me5F ) (2b) 12.66 22.6 1.08 It is worth restating that both the sterics and electronics were measured using the monodentate ligand ignoring the linker. As such the bidentate ligands that w ere synthesized using different carbonyl groups ( via condensation with acetone or benzaldehyde) are all treated identically. 23 The complexes were modeled using the simplest equation that a ccounts for both sterics and electronics. (1) In Equation 1 above , k is the pseudo first order rate constant of the catalysis, a and b are fitting constants that scale the contributions of LDP and %V bur , respectively, and c is simply an intercept in the linear fit. Two different forms of the data can be modeled, and both will be discussed. The first way is to use natural variables, or the actual measured %V bur and LDP. These natural variables are the eas iest way to generate a model that can predict ideal sterics and electronics of future 158 catalysts. The other method is to use scaled variables. While t he modeling is complicate d by scaling the variables , the resulting equation is more informative. The scalin g was done using Equation 2 below, where x i = scaled variable, u i = natural variable, u i 0 = midp oint of the range of the natural variables, and u i = the difference between the midpoint and the high value (half the full range). (2) The equations for the calculation of u i 0 and u i are shown below. (3) (4) Using either the scaled or natural variables, a least squares fit to the data was done by solving Equation 5 . In E quation 5 , y = single column matrix of the rate constants, b = single column matrix of the coefficients X = the m odel matrix which consists of the scaled or unscaled variables, X t = transform of the model matrix. The equation below provides the least square values without being prone to local minima like iterative methods can be and required nothing more than an Exce l spreadsheet to calculate th e set of coefficients. (5) 159 The equation with the natural variables in the matrices is shown in Equation 6 . In order to get the c - coefficient in the fit, a row of ones was added after the LDP and %V bur data. (6) Solution of the Equation 6 gives: a = 1.75, b = - 0.635, and c = 6.88. The scaled variable coefficients were found , using the same method , to be: a s = 1.61, b s = - 2.25, and c s = 1.34. While the natural variables give some insight into how each variable affects the model, they are mostly meaningless due to the relati ve magnitude of the electronic and steric measurements. Scaling the variables affords more information from a s , b s , and c s . Both equations are displayed below where Equation 7 is made from the natural variables and Equation 8 is made using the scaled varia bles. (7) (8) By scaling the changes made to electronics and sterics to an equivalent range, we can see that the steric term, b s is larger in magnitude than the ele ctronic term, a s . In other words, the sterics play a slightly more significant role in the rate determining step of the hydroamination. Moreover, b s is negative, implying that as the steric term increases (ligands become larger) the rate of catalysis is sl owed. The ele ctronic term on the other hand is positive. This means that as we increase the 160 LDP of the ancillary ligand (the ligand is less electron donating) the rate of catalysis increases. In short, small electron deficient ligands yield faster catalysi s. A graph of the model is shown Figure 4 - 1 below. The graph shows the model predicted rate constant vs. the experimental rate constant. A good linear fit implies good correlation between our combination of sterics and electronics and the observed rate. Figure 4 - 1 . Plot displaying the calculate vs. experimental rate constant. The y - axis was calculated by using the experimental LDP and %V bur values in the model described above. The error bars are displayed at the 95 % confidence level. It is incredibly easy to use this model qualitatively, as we already mentioned, the model tells us that smaller electron deficient ligands are better. We can use this model more quantitively , though. We can take any ligand from t he seri es of ligands we have examined and use the LDP and %V bur to calculate a theoretical rate constant. For example, we know the LDP and %V bur of 2,4 - diMe - pyrrole is 12.81 kcal/mol and 22.8%, respectively. Inputting those values into Equation 7 0.0E+00 5.0E-05 1.0E-04 1.5E-04 2.0E-04 2.5E-04 3.0E-04 3.5E-04 4.0E-04 4.5E-04 0.0E+00 5.0E-05 1.0E-04 1.5E-04 2.0E-04 2.5E-04 3.0E-04 3.5E-04 4.0E-04 4.5E-04 Calculated k obs Mean k obs Calculated vs. Experimental Rate Constants 161 gives a p redicte d rate of 1.094*10 - 5 s - 1 . In other words, without doing any chemistry, we know that the 2,4 - diMe - pyrrole version of dpm s hould be slower than the unsubstituted dpm. With the training set developed, we began testing other varieties of ligand sets, sp ecifica lly we moved to aryloxide based bidentate ligands. We believed that the switch to a different ligand motif was a good test of whether our model would be expandable to other ligand types. Since, i f the model was only functional in the range of pyrrol e deriv atives, it would not be especially useful for catalyst design. 4.5 Testing Ligand Variety We set about making a variety of aryloxide derivatives. Unfortunately, the aryloxide - based titanium catalysts are unstable and quite dependent on having steric bul k. As s uch we were limited in the ligand design. The three catalysts shown in the Table below were synthesized , and the rate determined for the hydroamination reaction . This set of ligands could be considered a validation set for the model. 162 Ti Catalyst C r Complex for LDP (kcal/mol) %V bur from Cr k obs x 10 4 (s 1 ) Ti (NMe 2 ) 2 ( bis - phenoxide 2 - tBu - 4 - Me ) (5) 11.98 21.6 0.432 Ti (NMe 2 ) 2 ( biphenol 2 - tBu - 4,5 - diMe ) (4a) 11.87 21.5 0.244 Ti(NMe 2 ) 2 (biphenol 2 - tBu - 4 - OMe ) (4b) 11.82 21.5 0 .0546 Table 4 - 2 . Summary of the validation set of bis - aryloxide ligands. The aryloxide catalysts tested three primary aspects of the model . First, the switch to an oxygen - based ligand using th e model derived from pyrrole derivatives. This tests whether there is a dependence on what type of donor is used for the ligand. Second, the ligands form a variety of metall a cycle ring sizes. This tests whether metallacycle ring size is an important factor in the catalysis. Third, the aryloxide ligands are all locked into a single conf o rmation, i.e. no haptotropic shifts are possible. This tests our theory about the pyrrole ring in (NMe 2 ) 2 Ti(dpm) undergoing a haptotropic shift during the reaction. 163 Figure 4 - 2 . Plot displaying the calculate vs. experimental rate constant for the full set of liga nds tested . The y - axis was calculated by using the experimental LDP and %Vbur values in the model described above. The error bars are displayed at the 95% confidence level. The grey square s are the aryloxide points and were not included in the regression. As shown in Figure 4 - 2, the rate constant of the three aryloxide catalysts (grey points) is small due to both the ster ic bulk required to prevent side reactions and the strong donor ability of the oxygen - based ligands compared to the pyrroles. Nevertheles s, they seem to fit the model very well a nd, in fact, we were able to draw some conclusions as a result. With regards to the first point mentioned previously , we have show n in Figure 4 - 2 that all three aryloxide ligands tested fit the training set model quite well. This supports that the reactivity of the (NMe 2 ) 2 Ti(X 2 ) type catalysts for hydroamination are sensitive to th e Lewis acidity of the metal center and the sterics of the X 2 regard less of what the X 2 is. There seems to be no dependence on the identity of the donor atom. Next, we wondered if changing the metall a cycle ring size would have an effect that was unaccounte d for in the model. In other words, we wondered if measuring the ste rics fr o m the Cr complex was a poor representation of the steric profile of the ligands in 0 0.00005 0.0001 0.00015 0.0002 0.00025 0.0003 0.00035 0.0004 0.00045 0 0.00005 0.0001 0.00015 0.0002 0.00025 0.0003 0.00035 0.0004 0.00045 Calculated k obs Mean k obs Calculated vs. Experimental Rate Constants 164 varying ring sizes. In the full set of ligands tested, we interrogated 6 - , 7 - , and 8 - membered rin gs and , regardless of the ring size, the ligands fit the model. The final of the three points that the aryloxide derivatives address is the hapticity change. As we have seen (NMe 2 ) 2 5 configuration, bound through the aromatic ring. 14 I f the ring stayed in that conf o rmation , the %V bur would be very different f ro 1 conf o rmation and the LDP would likely be very differe nt as well. The aryloxides, and the indoles as well, specifically address this point since the only available coordination mode of the aryloxide ligands is 1 . Since all of our catalysts fit one model , it strongly suggests that the complexes which feature 5 ring in the crystal structure must undergo a haptotropic shift during the catalysis to 1 . If they did not, the LDP system would not accurately represent the electronic and steric paramete rs and the catalysts would not fit to the model. 165 4.6 Investigati ng Anomalies Ti Catalyst Cr Complex for LDP (kcal/mol) %V bur from Cr k obs x 10 4 (s 1 ) Ti(NMe 2 ) 2 (dpm 3 - [C6H3(CF3)2] ) (3) 14.0 6 20.3 7.32 Table 4 - 3 . Table showing the fitting parameters and rate constant for catalyst 3. One of the catalysts tested previously - aryl substituted dpm ligand. 7 By adding electron withdrawing groups to the backside of the ligand, the ligand becomes more electron deficient to much faster turnover for hydroamination. 7 very well. In fact, even using 95% confidenc e level error bars, the cata lyst was well off the line (orange point in Figure 4 - 3). 166 Figure 4 - 3 . Expanded model displaying the poor fit for catalyst 3 . When we saw how poorly 3 fit, we wondered if the model was better described by a c urved fit rather than a linear one. No matter how we tried to fit the data though, the correlation was always poor. We decided to investigate in more detail, what was happening with catalyst 3 . After running a kinetics trial , we checked the solution by GC/ MS. The solution had many assorted products. At most , we expect signals from the product isomers, free ligand, and possibly starting materials. But what we found did not match the mass of any of those. The product peak (MW = 209 ) was the majority of the mi xture, but there were additional , much heavier peaks (MW = 325). The heavy peaks match the mass of the product of coupling aniline with two equivalents of 1 - phenyl propyne. While the byproduct could never be isolated, and 1 H NMR was u nhelpful due to the ma ssive excess of aniline, this product lines up well with our modeling problem. Since our rates are measured by alkyne consumption, i f the catalyst is incorporating extra equivalents of alkyne into the hydroamination product, we would expect to see an artif icially faster rate due to alkyne being our 0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 0.0009 Calculated k obs Mean k obs Calculated v. Experimental Rate Constants 167 handle for measuring the kinetics. What the model has accomplished is alerting us to a change in the chemistry happening in the reaction. Often, small amounts of by - products in reactio ns might go overlooked, but by using the model, we knew something was wrong , investigated the reaction, and discovered some potentially exciting chemistry. From just this short series of tests we were able to learn an immense amount of information about an already well - studied catalytic system. We wanted to go further, though, and use the model to design a catalyst that would display a faster turnover for hydroamination. We also wanted to stick to a relatively simple ligand framework, even the 3 - substituted pyrrole in complex 3 was several steps to make and can be a chore to isolate. 7, 23 That said , we consulted our table of LDP values and scanned the less electron - donating ligands. One type of ligand that stood out immediately was thiols. The congeners of the aryl o xide - based ligands are poorer donors due to orbital overlap. 29 They also have the advantage of having a longer bond length due to the larger ionic radius, which makes the effective steric profile significantly smaller . Ti Catalyst Cr Complex for LDP (kcal/mol ) %V bur from Cr k obs x 10 4 (s 1 ) Ti(NMe 2 ) 2 (dithioBINAP) (6) 13.99 22.3 a 0.0794 Table 4 - 4 . Table summarizi ng the rate constant and fitting parameters for catalyst 6 . Catalyst 6 was synthesized and tested for hydroamination. Surprisingly , th is catalyst also did not fit the model, in fact, it was far slower than pre dicted . The predicted rate of hydroamination wa s 3.48*10 - 4 and the experimental rate was far slower at 0.0 794*10 - 4 s - 1 . 168 When we investigated the catalyst more , we discovered the problem. By routine spectroscopy, the catalyst appeared clean, but, when Dr. McDaniel analyzed the solution behavior using d iffusion ordered spectroscopy (DOSY) NMR, we discovered the catalyst is a dimer in solution. Crystallization of the complex confirmed this finding. Even heating the complex with an excess of aniline in solutio n did not make a difference, the compound still behaved as a dimer. The long S - Ti bonds and available lone pair of the sulfur atom mean bridging two metal centers is very favorable, so using thiols that are not extremely sterically protected is unlikely to be successful. This example highlights anothe r benefit of using a model such as the one we developed. Had LDP and the model not existed, we might have assumed that all thiols were poor catalysts for the react ion due to a strong donor ability . We would have never had cause to investigate the molecular structure of the catalyst to learn why it was such a poor catalyst. Instead, we know that with proper ligand development of, say, an unsymmetrical ligand bearing one thiol and one other donor, the catalyst may potentially be improved. We tried several oth er ligand ideas as well. Monodentate ligands, halogenated pyrroles, and indoles, and smaller aryloxides all failed for one reason or another. As the ligands become more electron deficient and smaller, stability of the precatalyst suffers as well, making is olation of a new, improved catalyst quite challenging. 4.7 Other Applications of the Model Since designing a new catalyst that was an improvement over (NMe 2 ) 2 Ti(dpm) w as unsuccessful, we wondered if we might be able to use to the model for another purpose. It is known that Ti(NMe 2 ) 4 is a precat a lyst for hydroamination. 30 What is unknown, is the identity of the active species. When Ti (NMe 2 ) 4 is added to a solution of alkyne and amine, what reactions happen to 169 form an active c atalyst ? Are all the dimethylamines replaced by anilides? Is imido formation the first step? What ancillary ligands are on the metal center? To answer this questio n, we needed to compile a set of possibilities. Our three most likely conjectures at the stru cture of the catalyst in the slow step were: - Ti(NMe 2 ) 2 , - Ti(NPh), or - Ti(NHPh) 2 (Figure 4 - 4) . Both LDP and %V bur had previously been investigated for NMe 2 and NHP h using the LDP system, so those were easy to compare. By inputting the values into our model , we had predicted rates for each of the two possibilities. The imido was a little bit more difficult. Figure 4 - 4 . Representations of the proposed possible catalyst structur es the hydroamination reaction using Ti(NMe 2 ) 4 as a catalyst. We had previously synthesized the [NCr(N i Pr 2 ) 2 (NPh)] - K + complex. Unfortunately, the donor ability of the imido ligand made rotation so rapid, the rate of rotation could not be monitored using so lution state NMR. We turned to theory to get a prediction of the LDP value. For the calculations , all chromium molecu les were truncated to NCr(NH 2 ) 2 X for ease of optimization. The optimizations of structures were done using DFT and were performed using the M06L functional with the TZVP basis set. Optimizations of transition states were done using the Berny algorithm moni toring vibrational frequencies to confirm minima. There was a single negative vibration in the transition state corresponding to Cr N rotat ion. The ligand donation parameter calculations were attempted by optimizing the ground state geometry and the transi tion state in which one amido ligand is rotated ~90°, which caused amido pyramidalization. The electronic energy difference between the gro und state and transition state 170 was calculated and plotted against the experimental LDP values. We selected a variety of ligands including halogens, heterocycles, and phenoxide to include differences in size, a wide variety of electronics, and structures th at are simple enough to optimize by computation. These points were used to generate a line relating the computational barrier and the experimental values. The computational barrier for the imido ligand was then fit to this line, and the LDP for the ligand approximated. The fits for the lines generated were compared across multiple levels of theory, revealing that the com putational values generated using the B3PW91/cc - pvtz and M06L/TZVP were the best fits. See Figure 4 - 5 for the B3PW91/cc - pvtz plot (the imid o is the orange point). Figure 4 - 5 . Series of ligands with the comp utationally modelled LDP fit against the experimental LDP. The orange point is the predicted imido value fit to the best fit line of the mo del. The fits generated just comparing the calculated barrier to experimental LDP are passable , but the correlation o f the model could be improved . If theory accurately calculated the LDP , the slope of the line would be exactly one , and the intercept would be zero. So, as an additional measure, fits were generated using the NCr(NH 2 ) 2 X model that included the %V bur of the ligand to account y = 1.0748x - 4.2265 R² = 0.8392 3 5 7 9 11 13 15 6 8 10 12 14 16 Modeled LDP Experimental LDP Modeled LDP v. Experimental LDP 171 for the switch to a much smaller ancillary ligand. We have established that in some cases, large X ligands can affect th e pure electronic measure of the LDP system. As such, since we truncated the Cr molecule for the calculations, we thought including a steric term might help our calculated approximations better match the experimental values. By fitting the computed barrier with a linear regression using the LDP and %V bur , a fit was generated that more accurately related the comput ationally generated energy barriers to the experimentally derived LDP values. The fit including sterics using M06L/TZVP is displayed below in Figu re 4 - 6 . Compared to the fit using the computed barrier and the experimental LDPs, the values from this fit cor relate more accurately; the slope of the line is nearly one, and the intercept nearly zero. Again, the orange point in the figure represents the imido fit to the model. The values for both methods were similar, 8.1 kcal/mol for the firs t fit and 7.9 kcal/ mol for the model including sterics, and the value that included sterics was used due to the higher quality of the fit. 172 Figure 4 - 6 . Fit of the computed and experimental LDP values including a steric term t o account for the truncated chromium molecule in the calculations. The orange point is the imido theoretical value fit to the best fit line. When we compared the model calculated rate constant values for - Ti(NMe 2 ) 2 , - Ti(NPh), or - Ti(NHPh) 2 as catalysts, th e results were surprising. Neither - Ti(NMe 2 ) 2 n or - Ti(NHPh) 2 were even close to matching the value we determined by kinetics. Both ligands are muc h stronger donors than all the ligands we used in the training set and validation set , so the predicted rates were slow . The imido, on the other hand, is also a strong donor, but the steric profile of this one dianionic ligand is small when compared to two monoanionic ligands , meaning that the predicted rate is actually quite fast. Figure 4 - 7 below is a graphical representation of the results. y = 0.989x + 0.1236 R² = 0.9919 6 8 10 12 14 16 18 6 7 8 9 10 11 12 13 14 15 16 Modeled LDP Experimental LDP Modeled LDP v. Experimental LDP 173 Figure 4 - 7 . Plot of the predicted imido value to our model . The orange diamond is - Ti(NPh), the green diamond is - Ti(NHPh) 2 , and the purple diamond is - Ti(NMe 2 ) 2 . Clearly the imido is the best prediction for the active species in the Ti(NMe 2 ) 4 - catalyzed hydroamination. While we are aware this does not definitivel y prove the mechanism by which the catalysis happens , this is compelling evidence in support of the active species that we w ould not have without the model. 4.8 Conclusions It is unfortunate we were not able to use the full potential of the model to design an improved hydroamination catalyst immediately. Instead, we have learned that pyrrole is a special ligand. It can stabilize a Lewis acidic metal center through hapticity c hanges to provide a stable, easy to handle precatalyst that converts to a highly active catalyst. In hindsight, we were never likely to improve much on dpm as a ligand. The ease of synthesis and low cost associated with the ligand make it the ideal partner to a cheap, earth abundant catalyst. Even if a faster catalyst is found, matching the rate of catalysis and the simplicity of (NMe 2 ) 2 Ti(dpm) will be no small feat. y = 0.9877x + 2E - 06 R² = 0.9877 0 0.00001 0.00002 0.00003 0.00004 0.00005 0.00006 0.00007 0.00008 0.00009 0.000035 0.00004 0.000045 0.00005 0.000055 0.00006 0.000065 0.00007 Model Predicted Rate Constant Experimental Rate Constant Predicted v. Experimental Rate - Ti(NPh) - Ti(NPh) 2 - Ti(N Me 2 ) 2 174 Even still, the LDP driven model of titanium - catalyzed hydroamination proved to be an incr edibly useful tool. The results of our study provided a means to screen potential catal ysts while saving the time and energy associated with synthesizing, testing, and modifying a series of molecules. It uncovered a simple method of spotting side reactions that could have easily gone unnoticed. We were able to glean mechanistic information a bout the haptotropic shift using pyrrolyl - based catalysts. We were even able to shed light on a reaction that was published more than 15 years ago and identify the activ e species. This study illustrates how LDP can provide an immense amount of information about high valent catalyzed reaction. And as demonstrated, whether the catalyst is brand new or has been used for 15 years, models of this kind can be informative. Hopef ully, other high valent metal chemists can employ our methods and use the knowledge gai ned to transform the way we approach catalyst optimizations. 175 4.9 Experimental This experimental was taken from our recent publication. This be accessed at: Nat. Chem. , 201 7 , 9 (9), 837 Ligand Donation Parameter Considerations X Ligand c Experimental Rate (s 1 ) (kcal/mol) LDP b (kcal/mol) Standard Deviation in LDP (kcal/mol) Temp (°C) Pyrr 0.89 16.11 13.64 0.01 1.7 Pyrr 3 - [C6H3(CF3)2] 0.42 16.54 14.06 0.04 1.9 Ind 3Me 2.93 14.87 12.49 0.002 - 8.4 Ind 3Me5F 2.09 15.05 12.66 0.01 - 8.4 Pyrr Me 1.47 15.95 13.46 0 .01 3.5 Pyrr 2 - [C6H3(CF3)2] 1.63 16.98 14.32 0.0003 21.8 Pyrr Ph 0.67 16.54 14.03 0.003 6.2 Pyrr tol 0.55 16.38 13.91 0.01 1.9 OPh 2,4 - diMe 1.05 14.17 11.98 0.01 - 29.0 OPh 2 - tBu - 4 - Me 0.86 15.52 13.14 0.01 - 8.3 OPh 2,4,5 - triMe 2.35 14.11 11.87 0.003 - 23.3 OPh 2 - tBu - 4,5 - diMe 1.89 13.89 11.70 0.004 - 28.9 OPh 2 - Me - 4 - OMe 3.07 15.55 13.06 0.003 3.7 OPh 2 - tBu - 4OMe 1.95 14.05 11.82 0.01 - 26 SNap 1.87 16.60 13.99 .002 17.0 6 - Br - SNap e 1.56 16.77 14.15 0.007 18.0 Table 4 - 5 . Spin saturation transfer data. a Determined from the rate constant for isopropyl group exchange using the Eyring Equation with the assumption that the t ransmission coefficient is unity. b = 1 1 . c Pyrr = pyrrolide, Ind = ind olide, tol = p - tolyl. d Determined by line shape analysis. e The 6 - Br - SNap ligand was only used as a surrogate for 2 - napthylthiolate (SNap) to obtain the %V bu r value. Consequently, its LDP was not employed in this study; however, we include the LDP for com pleteness. General LDP Procedure The rate constant for the exchange of the two methyne hydrogens of the isopropyl groups was measured using 1 H NMR spin satur ation transfer (SST). The temperature chosen for each experiment was based on that required to rea ch the slow exchange limit of the complex under 176 inversion recovery m ethod. Samples were made between 0.02 0.03 M in CDCl 3 for this 1 1 for NCr(N i Pr 2 ) 2 I and assumed to be the same for the other compounds. 15 General Percent Buried Volume Considerations In calculating percent buried volume, the ligand structure was taken from the chromium complex crysta l structure. The NCr(N i Pr 2 ) 2 fragment was deleted, leaving just the X ligand for analysis. This fragment, in conjunction with the bond length from the chromium crystal structure (Cr - X bond length) were used in the Samb V ca 2.0 program to calculate the %V bur . 25 This method was used for several reasons. First, the chromium molecules are easily crystallizable and provided an experimental basis for the measurement (as opposed to modeling compu tationally). Second, the chr omium molecule is already the model for the electronic term, it was logical to use the same molecule to determine the steric factor. Finally, using just the ligand fragment eliminates as much bias for bonding angles, twisting, a nd torsions from sterics and crystal packing as possible. In other words, it is a better measure of the ligands sterics towards a more general set of high valent metals. The sphere radius was left at the default 3.5 Å. Other radii were tested, but the defa ult gave the best correlatio n. Mesh spacing was left at the default value of 0.10, the atomic radii were used as the default Bondi radii scale by 1.17, and for all ligands the H atoms were included in the calculations. DOSY Analysis The DOSY NMR experimen ts were recorded with a Vari an Inova 600 spectrometer equipped with a 5 mm PFG switchable broadband probe operating at 599.89 MHz ( 1 H). The Varian 177 Dbppste_cc (DOSY bipolar pulse pair simulated spin echo convection corrected) pulse sequence was utilized for all experiments. Following literature methods , 31 - 33 the molecular weight of Ti(dithioBINAP)(NMe 2 ) 2 ( 6 ) was analyzed using DOSY techniques. The internal molecular weight standards chosen for this experiment include d ferrocene (FeCp 2 ), tetrakis(trimethylsilyl)silane (Si(TMS) 4 ), and toluene. The experiment that was performed at room temperature was carried out in a threaded J. Young tube that was sealed with a Teflon stopper. The experiments that were performed at 50 °C were carried out u tilizing a capillary tube (2 mm) to reduce and convection errors in the experiments and improve accuracy. An example of the DOSY spectrum obtained by this method is shown below in Figure 4 - 8 . Figure 4 - 8 . DOSY spectrum of Ti(dithioBINAP)(NMe 2 ) 2 (6) at 25 °C. Si (TMS) 4 Toluene FeCp 2 178 Figure 4 - 9 . Molecular weight calibration of Ti(dithioBINAP)(NMe 2 ) 2 (6) at 25 °C. The log of diffusion coefficient vs log molecul ar weight plots for the internal standards FeCp 2 , toluene, and Si(TMS) 4 (show as the blue diamonds) and Ti(dithioBINAP)(NMe 2 ) 2 (shown as the orange square) in toluene - d 8 . The calibrated molecular weight of Ti(dithioBINAP)(NMe 2 ) 2 a t room temperature is 639. 06 ± 51.52 g/mol. The expected molecular weights for the monomer (shown as the gre y triangle) is 452.46 g/mol and the dimer (shown as the yellow circle) is 904.91 g/mol. Unfortunately, the results of this experiment were inconclusive as to which species i s predominant in solution. However, in the solid state, Ti(dithioBINAP)(NMe 2 ) 2 was found to be dimeric. Based on these observations, Ti(dithioBINAP)(NMe 2 ) 2 could possibly be in equilibrium between the monomeric and dimeric complexes, as shown in the scheme below: y = - 0.457x + 2.3 R² = 0.9766 0.9 1 1.1 1.2 1.3 1.4 1.5 1.85 2.05 2.25 2.45 2.65 2.85 3.05 Log (Diffusion Coefficient) Log (Molecular Weight) Ti(dithioBINAP)(NMe 2 ) 2 Internal Standards Predicted Monomer Dimer 179 Scheme 4 - 3 . Comparison of the molecular weight of the monomer and dimerized catalyst. Heppert and co - workers have shown an analogou s titanium binaphtholate complex t o be dimeric at low temperatures, but upon heating undergoes rapid conversion to the monomeric complex. 34 Intrigued by these resu lts an elevated temperature DOSY e xperiment was conducted. 180 Figure 4 - 10 . Molecular weight calibration of Ti(dithioBINAP)(NMe 2 ) 2 (6) at 50 °C. The log of diffusion coefficient vs log molecular weight plots for the internal standards FeC p 2 , toluene, and Si(TMS) 4 (show as the blue diamonds) and Ti(dithioBINAP)(NMe 2 ) 2 (shown as the orange square) in toluene - d 8 . The predicted molecular weight of Ti(dithioBINAP)(NMe 2 ) 2 at 50 °C is 450.82 ± 22.76 g/mo l. The expected molecular weights for the m onomer (shown as the gre y triangle) is 452.46 g/mol and the dimer (shown as the yellow circle) is 904.91 g/mol. The results of this experiment suggest that, when Ti(dithioBINAP)(NMe 2 ) 2 is heated to 50 °C, the pred ominant species in solution is monomeric. I n order to get a better understanding of what was occurring during the kinetic experiments another DOSY experiment was conducted to better mimic the kinetic conditions. y = - 0.4112x + 2.4635 R² = 0.9917 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.9 2.1 2.3 2.5 2.7 2.9 Log(Diffusion coefficient) Log(Molecular Weight) Ti(dithioBINAP)(NMe 2 ) 2 Internal Standards Predicted Monomer Dimer 181 Due to error caused by thermal convection, this experiment could not be measured at t he same temperature as the kinetic conditions. Instead, the DOSY experiment was conducted at 50 °C, like the previous experiment. Also, in the kinetic experiments 10 equivalents of aniline is used, however, using this much aniline, the 1 H NMR signals for t he titanium complex and the standards are too obscured to measure an accurate diffusion coefficient. Lessening the amount of excess aniline (to 4 equivalents), the 1 H NMR signals are not obscured while still provi ding an environment similar to the kinetic conditions. The data collected from this DOSY experiment is shown in Figure 4 - 11 . Figure 4 - 11 . Molecular weight calibration of Ti(dithioBINAP)(NMe 2 ) 2 (6) with addition of aniline (4 equiv.) at 50 °C. The lo g of diffusion coefficient vs log molecular weight plots for the internal standards FeCp 2 , n - hexane, and Si(TMS) 4 (show as the blue diamonds) and Ti(dithioBINAP)(NMe 2 ) 2 (shown as the orange square) in toluene - d 8 . The predicted molecular weight of Ti(dithio BINAP)(NMe 2 ) 2 at 50 y = - 0.387x + 2.4072 R² = 0.9999 1.2 1.25 1.3 1.35 1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.8 2 2.2 2.4 2.6 2.8 3 3.2 Log (Diffusion Coefficient) Log (Molecular Weight) Ti(dithioBINAP)(NMe 2 ) 2 Internal Standards Predicted Monomer Dimer 182 °C is 992.41 ± 48.35 g/mol. The expected molecular weight of the monomer Ti(dithioBINAP)(NHPh) 2 is 548.54 g/mol (shown as the gre y triangle). The expected molecular weight of the dimer [Ti( dithioBINAP)(=NPh)] 2 is 910.83 g/mol (shown as the yellow circle). The results of this experiment suggest that even at an elevated temperature (50 °C) in the presence of excess aniline the titanium species in solution is dimeric. If the titanium species is dimeric under the kinetic conditions then that species could be experiencing increased sterics around the metal center, as well as different electronic effects. These added effects would drastically change the catalysis and are not accounted for in our mo del, which is likely why it does not correlate. While some dimerization occurs with the titanium imide active species with all the catalysts, the dimerization of the thiolate species in the presence of a large excess of amine is far more profound, which le ads to the large inhibition. Synthetic Procedu res General Considerations All reactions and manipulations were carried out in an MBraun glovebox under a nitrogen atmosphere and/or using standard Schlenk techniques. Diethyl ether, pentane, acetonitrile, te trahydrofuran, benzene, n - hexane, and toluene w ere purchased from Aldrich Chemical Company. Diethyl ether, pentane, acetonitrile, and toluene were purified by passing through alumina columns to remove water after being sparged with dry nitrogen to remove o xygen. Tetrahydrofuran, benzene, and hexane wer e sparged with dinitrogen to remove oxygen and distilled from sodium and benzophenone. NCr(N i Pr 2 ) 3 , NCr(N i Pr 2 ) 2 (OPh), NCr(N i Pr 2 ) 2 I, NCr(N i Pr 2 ) 2 (Pyrr), and NCr(N i Pr 2 ) 2 ( Pyrr 3 - [C6H3(CF3)2] ) were prepared using th e previously reported procedures. 15, 29 Procedure s for generation of thallium - pyrrole salts were modified from the 183 literature. 15 2,4 - dimethyl phenol, 2 - tert - butyl - 4 - - methylenebis(6 - tert - butyl - 4 - methylphenol, 2 - methy l - 4 - methoxyphenol and pyrrole - 2 - carboxyaldeh yde were purchased from Aldrich Chemical Company and used as received. 3 - methylindole and 2,4,5 - trimethylphenol were purchased from Alfa Aesar and used as received. 3,3' - di - tert - butyl - 5,5' - dimethoxy - [1,1' - bipheny l] - 2,2' - diol was purchased from Strem Chemic als Inc. and used as received. 3 - tert - butyl - 4 - hydroxyanisole was purchased from TCI America and was used as received. 2 - phenylpyrrole, 2 - tolylpyrrole, 2 - (3,5 - trifluoromethylphenyl)pyrrole, H 2 dpm 2 - [C6H3(CF3)2] , Ti (NMe 2 ) 2 (dpm 2 - [C6H3(CF3)2] ), H 2 dpm 3 - [C6H3(CF3 )2] , Ti(NMe 2 ) 2 (dpm 3 - [C6H3(CF3)2] ), H 2 dpm, Ti(NMe 2 ) 2 (dpm), 2 - methylpyrrole, H 2 dpm 2Me , 3 - methyl - 5 - fluoroindole and di(3 - methylindol - 2 - yl)phenylmethane, 2 - tert - butyl - 4,5 - - binaphthalene - - dith iol, 3,3' - di - tert - butyl - 5,5' - dimethoxy - [1,1' - biphenyl] - 2,2' - diol and 6 - bromo - 2 - napthalenthiol were prepared following their literature procedures.( 2, 7, 9, 35 - 43 ) Generation of lithium salts was performed by slow a ddition of 2.5 M n - butyl lithium in hexanes to a nearly frozen solution of the ligand in ether. The lithium salts were then isolated as solids and used without further purification. To remove all water and oxygen, all ligands were dissolved in benzene, spa rged with nitrogen, and refluxed in a Dean - Stark trap overnight. The benzene was then removed in vacuo, and the solids were taken into the nitrogen glov e box. Ti(NMe 2 ) 4 was purchased from Gelest and used as received. In many cases, due to the sensitivity o f the reported complexes to air and moisture, elemental analysis could not be accurately performed. In these cases, bulk purity of the compound was dete rmined by 1 H NMR. CDCl 3 , C 6 D 6 , and toluene - d 8 were purchased from Cambridge Isotopes Laboratories, Inc. Toluene - d 8 and C 6 D 6 were sparged with dry dinitrogen and dried over 3 Å molecular sieves. CDCl 3 was sparged with dinitrogen and distilled from P 2 O 5 prior to use. All NMR solvents were stored under an inert atmosphere. Spectra were taken on Varian instrume nts located in the Max T. Rogers 184 Instrumentation Facility at Michigan State University. These in clude an Agilent DDR2 500 spectrometer equipped with a 5 mm pulsed - field - gradient (PFG) OneProbe and operating at 499.955 MHz ( 1 H) and 125.77 MHz ( 13 C), a Varia n Inova 600 spectrometer equipped with a 5 mm PFG switchable broadband probe operating at 599.89 MHz ( 1 H) and 564.30 MHz ( 19 F), a UNITY plus 500 spectrometer equipped with a 5 mm Pulsed - Field - Gradient (PFG) switchable broadband probe and operating at 499.9 55 MHz ( 1 H) and 125.77 ( 13 C), as well as a Varian Unity Plus 500 spectrometer with a low gamma b roadband probe operating at 36 MHz ( 14 N). NMR chemical shifts are reported in ppm and referenced to the solvent peaks for 1 H NMR (CDCl 3 6 D 6 7.16 ppm; toluene - d 8 13 C NMR (CDCl 3 6 D 6 128.06 ppm; toluene - d 8 14 N NMR chemical shifts are reported in ppm and referenced to the dinit rogen gas dissolved in solvents (CDCl 3 ppm), which in turn has been externally referenced against neat CH 3 NO 2 as 381.6 ppm ; this procedure places NH 3 as 0 ppm. Single crystal X - ray diffraction data was collected in the Center for Crystallographic Research at MSU. Synthesis of NCr(N i Pr 2 ) 2 (Ind 3Me ): To a solution of NCr(N i Pr 2 ) 2 OPh (75 mg, 0.209 mmol) in toluene, freshly prepared Li - Ind 3Me (86 mg, 0.626 mmol) was added slowly. The reaction was heated to 40 °C for 18 h whereupon color changes to a dark purple. The volatiles were removed in vacuo and the residue extracted with pentane. The extracts were then filtered through Celite and concentrated in vacuo . Recrystallization was achieved by cooling the concentrated solution at 30 °C overnight. (25 mg, 0 .063 mmol, 30.2%). 1 H NMR (500 MHz, CDCl 3 J = 8.2 Hz, 1H, Ind - H), 7.48 7.42 (m, 1H, Ind - H ), 7.18 (d, J = 0.9 Hz, 1H, Ind - H), 7.15 (ddd, J = 8.2, 7.0, 1.3 Hz, 1H, Ind - H), 7.06 (ddd, J = 7.8, 7.1, 1.0 Hz, 1H, Ind - H), 5.18 (br. sept., 2H, CH( CH 3 ) 2 ), 3.74 (br. sept., 2H, CH(CH 3 ) 2 ), 2.33 (d, J = 1.0 Hz, 3H, Ind - CH 3 ), 1.74 (d, J = 1.74 Hz, 6H, 185 CH(CH 3 ) 2 ), 1.62 (d, J = 1.68 Hz, 6H, CH(CH 3 ) 2 ), 1.21 (d, J = 6.9 Hz, 6H, CH(CH 3 ) 2 ), 1.00 (s, 6H, CH(CH 3 ) 2 ). 13 C NMR (125 MHz, CDCl 3 29.17, 120.78, 118.76, 116.84, 115.77, 111.61, 57.83, 55.59, 30.60, 30.09, 21.85, 9.96. 14 N NMR (36 MHz, C DCl 3 ): 998.7, 390.5, 208.6. Elemental Analysis: Calcd. C, 63.61; H, 9.15; N, 14.13. Found: C, 63.26; H, 9.44; N, 13.97 M.p.: 200 - 201 °C. Synthesis of NCr(N i Pr 2 ) 2 (Ind 3Me5F ): To a solution of NCr(N i Pr 2 ) 2 OPh (75 mg, 0.191 mmol) in toluene, freshly prepared Li - Ind 3Me5F (89 mg, 0.572 mmol) was added slowly. The reaction was heated to 40 °C for 18 h whereupon color changed to a dark purplish color. The volat iles were removed in vacuo and the residue extracted with pentane. The extracts were then filter ed through Celite and concentrated in vacuo . Recrystallization was achieved by cooling the concentrated solution at 30 °C overnight. (51.9 mg, 0.125 mmol, 65.7 %). 1 H NMR (500 MHz, CDCl 3 7.94 (dd, J = 8.9, 4.7 Hz, 1H, Ind - H), 7.20 (s, 1H, Ind - H), 7.0 7 (dd, J = 9.7, 2.6 Hz, 1H, Ind - H), 6.97 (d, J = 7.7 Hz, 1H, Ind - H), 6.87 (td, J = 9.2, 2.6 Hz, 1H, Ind - H), 5.17 (br. sept., 2H, CH(CH 3 ) 2 ), 3.76 (br. sept., 2H, CH(CH 3 ) 2 ), 1.69 (d, J = 59.8 Hz, 12H, CH(CH 3 ) 2 ), 1.22 (s, 6H, CH(CH 3 ) 2 ), 1.01 (s, 6H, CH(CH 3 ) 2 ) . 13 C NMR (125 MHz, CDCl 3 133.45, 129.70, 129.62, 116.78, 116.70, 112.19, 112.15, 109.06, 108.85, 102.18, 101.99, 58.40, 56.14, 31 .04, 30.48, 22.24, 21.99, 10.29. 19 F NMR (470 MHz, CDCl 3 126.65 (td) 14 N NMR (36 MHz, CDC l 3 ): 998.1, 392.6, 204.2 Note: Despite multiple attempts, adequate elemental analysis could not be obtained. M.p.: 206 - 207 °C. Synthesis of NCr(N i Pr 2 ) 2 (Pyrr Me ): A suspension of ZnCl 2 (52.0 mg, 0.381 mmol) in THF was chilled to near freezing temperatures in the cold well. To this cold, stirring solution was added freshly prepared Na - Pyrr Me (79 mg, 0. 763 mmol) as a chilled solution in THF. This was left to react f or 2 h. After the 2 h, the solution was chilled again, and a chilled solution of NCr(N i Pr 2 ) 2 I 186 (75 mg, 0.191 mmol) in THF was added slowly. This reaction was left to stir for 4 h, yielding a dark reddish orange solution. The volatiles were removed in vacuo and the residue extrac ted with pentane. The extracts were then filtered through Celite and concentrated in vacuo . Recrystallization was achieved by cooling the concentrated solution at 30 °C overnight. (39 mg, 0.113 mmol, 59.0%). 1 H NMR (500 MHz, CDCl 3 ): - H), 6.12 (t, J = 2.6 Hz, 1H, Pyrr - H), 5.91 (s, 1H, Pyrr - H), 5.16 (br. sept., 2H, CH(CH 3 ) 2 ), 3.77 (br. sept., 2H, CH(CH 3 ) 2 ), 2.51 (s, 3H, Pyrr - CH 3 ), 1.80 (d, J = 5.5 Hz, 6H, CH(CH 3 ) 2 ), 1.60 (d, J = 5.5 Hz, 6H, CH(CH 3 ) 2 ), 1.20 (d, J = 5 .6 Hz, 6H, CH(CH 3 ) 2 ), 1.06 (d, J = 5.6 Hz, 6H, CH(CH 3 ) 2 ). 13 C NMR (125 MHz, CDCl 3 14 N NMR (36 MHz, CDCl 3 Elemental Analysis: Calcd. C, 58.93; H, 9.89; N, 16.17. Found: C, 58.93; H, 10.08; N, 15.96. M.p.: 151 - 152 °C. Synthesis of NCr(N i Pr 2 ) 2 (Pyrr 2 - [C6H3(CF3)2] ): To a solution of NCr(N i Pr 2 ) 2 I (132 mg, 0.336 mmol) in ether, a suspension of freshly prepared Tl - Pyrr 2 - [C6H3(CF3)2] (170 mg, 0.352 mmol ) in ether was adde d slowly. Immediately a yellow precipitate formed. The reaction was allowed to stir for 2 h whereupon it was filtered through Celite, dried in vacuo , and the reddish orange solids extracted with pentane. Recrystallization was achieved by cooling a concentr ated solution of the compound in HMDSO at 30 °C overnight. (80 mg, 0.147 mmol, 44%). 1 H NMR (500 MHz, CDCl 3 8.04 (s, 2H, Ar - H), 7.64 (s, 1H, Ar - H), 7.01 (s, 1H, Pyrr - H), 6.37 (s, 2H, Pyrr - H), 5.17 (m, J = 12.7, 2H, CH(CH 3 ) 2 ), 3.70 (m, J = 12.5, 2H, CH(CH 3 ) 2 ), 1.53 (d, J = 6.3 Hz, 6H, CH(CH 3 ) 2 ), 1.48 (d, J = 6.2 Hz, 6H, CH(CH 3 ) 2 ), 1.15 (d, J = 6.4 Hz, 6H, CH(CH 3 ) 2 ), 1.06 (d, J = 6.3 Hz, 6H, CH(CH 3 ) 2 ). 13 C NMR (125 MHz, CDCl 3 128.20, 119.11, 119.08, 119.0, 119.3, 110.89, 110.22, 59.04, 56.67, 30.26, 22.39, 21.96. 19 F NMR 187 62.86. 14 N NMR (36 MHz, CDCl 3 ): 1006.5, 404.4, 208.9. Elemental Analysis: Calcd. C, 52.94; H, 6.29; N, 10.29. Found: C, 52.73; H, 6.43; N, 10.15 . M.p.: 40 - 42 °C. Synthesis of NCr(N i Pr 2 ) 2 (Pyrr Ph ): T o a solution of NCr(N i Pr 2 ) 2 I (75 mg, 0.191 mmol) in ether, a suspension of freshly prepared Tl - Pyrr Ph (46.3 mg, 0.133 mmol) in ether was added slowly. Immediately a yellow precipitate formed. Th e reactio n was allowed to stir for 2 h before being filtered through Celite. The filtrate was dried in vacuo , and the reddish orange solids extracted with pentane. The extracts were then concentrated in vacuo . Recrystallization was achieved by cooling the concentra ted solution at 30 °C overnight. (52 mg, 0.127 mmol, 67%). 1 H NMR (500 MHz, CDCl 3 - H), 7.30 (m, 2H, Ph - H), 7.19 (t, J = 9.2, 4.3 Hz, 1H, Ph - H), 6.94 (t, J = 2.4, 1.4 Hz, 1H, Pyrr - H), 6.33 (t, 1H, Pyrr - H), 6.27 (m, J = 3.1, 1. 4 Hz, 1H, Pyrr - H), 5.13 (sept, 2H, CH(CH 3 ) 2 ), 3.70 (sept, 2H, CH(CH 3 ) 2 ), 1.54 (d, J = 25.2, 6.3 Hz, 6H, CH(CH 3 ) 2 ), 1.50 (d, J = 25.2, 6.4 Hz, 6H, CH(CH 3 ) 2 ), 1.14 (d, J = 6.4 Hz, 6H, CH(CH 3 ) 2 ), 1.11 (d, J = 6.4 Hz, 6H, CH(CH 3 ) 2 ). 13 C NMR (CDCl 3 , 25 ºC, 125 125.62, 109.70, 108.25, 58.49, 56.13, 29.97, 29.83, 22.6, 21.57. 14 N NMR (36 MHz, CDCl 3 1007.8, 402.0, 213.8. Elemental Analysis: Calcd. C, 64.84; H, 8.88; N, 13.71. Found: C, 64.89; H, 8.81; N, 13.74 . M.p.: 133 - 135 °C. Synthesis of NCr(N i Pr 2 ) 2 (Pyrr Tol ): To a solution of NCr(N i Pr 2 ) 2 I (75 mg, 0.191 mmol) in ether, a suspension of freshly prepared Pyrr Tol (72.2 mg, 0.200 mmol) in ether was added slowly. Immediately a yellow precipitate is formed. The re action was allowed to stir for 2 h before being filtered through Celite. The filtrate was dried in vacuo, and the dark orange solids extracted with pentane. The extracts were then concentrated in vacuo. Recrystallization was achieved by cooling the concent rated solution at 30 °C overnight. (70 mg, 0.166 mmol, 87%). 1 H NMR (500 MHz, 188 CDCl 3 J = 7.6 Hz, 2H, Tol - H), 7.12 (d, J = 7.6 Hz, 2H, Tol - H), 6.95 (s, 1H, Pyrr - H), 6.33 (s, 1H, Pyrr - H), 6.25 (s, 1H, Pyrr - H), 5.17 (m, J = 12.5, 6.2 Hz, 2H, CH (CH 3 ) 2 ), 3.73 (m, J = 12.4, 6.1 Hz, 2H, CH(CH 3 ) 2 ), 2.35 (s, 3H, Tol - CH 3 ), 1.58 (d, J = 6.2 Hz, 6H, CH(CH 3 ) 2 ), 1.53 (d, J = 6.2 Hz, 6H, CH(CH 3 ) 2 ), 1.16 (d, J = 6.3 Hz, 6H, CH(CH 3 ) 2 ), 1.10 (d, J = 6.2 Hz, 6H, CH(CH 3 ) 2 ). 13 C NMR (CDCl 3 1 43.61, 135.47, 135.04, 129.18, 127.93, 126.23, 109.36, 107.75, 58.38, 56.08, 29.97, 29.86, 22.04, 21.57, 21.14. 14 N NMR (36 MHz, CDCl 3 Elemental Analysis: Calcd. C, 65.37; H, 9.06; N, 13.26 Found: C, 65.33; H, 9.11; N, 13.21. M. p.: 143 - 144 °C. Synthesis of NCr(N i Pr 2 ) 2 (OPh 2,4 - diMe ): To a nearly frozen, stirring solution of NCr(N i Pr 2 ) 3 (75 mg, 0.205 mmol) in ether, an ethereal solution of the HOPh 2,4 - diMe (25 mg, 0.205 mmol) was added dropwise. The solution began to change to an or ange color rapidly. After 2 h of stirring, the volatiles were removed in vacuo , and the residue extracted with pentane. The extracts were filtered over Celite and concentrated fo r recrystallization at 30 °C overnight. (32.5 mg, 0.084 mmol, 41%). 1 H NMR (5 00 MHz, CDCl 3 J = 7.9 Hz, 1H, Ar - H), 6.85 (s, 1H, Ar - H), 6.83 (s, 1H, Ar - H), 5.04 (br. sept., 2H, CH(CH 3 ) 2 ), 3.75 (br. sept., 2H, CH(CH 3 ) 2 ), 2.22 (s, 3H, Ar - CH 3 ), 2.14 (s, 3H, Ar - CH 3 ), 1.84 (br. s, 6H, CH(CH 3 ) 2 ), 1.47 (br. s, 6H, CH(CH 3 ) 2 ), 1.25 (br. d, J = 61.3 Hz, 12 H CH(CH 3 ) 2 ). 13 C NMR (125 MHz, CDCl 3 125.33, 117.62, 58.64, 55.49, 30.71, 21.77, 20.92, 17.20. 14 N NMR (36 MHz, CDCl 3 379.3. Elemental Analysis: Calcd. C, 61.99; H, 9.62; N, 1 0.84. Found: C, 61.70; H, 9.31; N, 10.79 M.p.: 116 - 117 °C. Synthesis of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - Me ): To a nearly frozen, stirring solution of NCr(N i Pr 2 ) 3 (75 mg, 0.205 mmol) in ether, an ethereal solution of the HOPh 2 - tBu - 4 - Me (33.6 mg, 0.205 mmol) was adde d dropwise. After 16 h of stirring, the volatiles were removed in vacuo . The orange, extremely 189 solu ble residue was then extracted with pentane. The extracts were filtered over Celite and concentrated for recrystallization at 30 °C overnight. (53 mg, 0.123 mmol, 60%). 1 H NMR (500 MHz, CDCl 3 J = 8.1 Hz, 1H, Ar - H), 6.98 (s, 1H, Ar - H), 6.90 (d, J = 8.0 Hz, 1H, Ar - H), 5.05 (s, 2H, CH(CH 3 ) 2 ), 3.77 (s, 2H, CH(CH 3 ) 2 ), 2.27 (s, 3H, Ar - CH 3 ), 1.86 (s, 6H, CH(CH 3 ) 2 ), 1.54 (s, 6H, CH(CH 3 ) 2 ), 1.37 (s, 9H, t Bu), 1.18 (s, 12H, CH(CH 3 ) 2 ). 13 C NMR (125 MHz, CDCl 3 ): 4, 119.96, 58.81, 55.76, 35.12, 30.92, 30.44, 30.17, 22.82, 22.25, 21.28. 14 N NMR (36 MHz, CDCl 3 Due to the high affinity for solv ents, adequate elemental analysis could not be obtained. M.p.: 188 - 191 °C. Synthesis of NCr(N i Pr 2 ) 2 (OPh 2,4,5 - triMe ): To a nearly frozen, stirring solution of NCr(N i Pr 2 ) 3 (150 mg, 0.409 mmol) in ether, an ethereal solution of the HOPh 2,4,5 - triMe (55.7 mg, 0 .409 mmol) was added dropwise. The solution began to change to an orange color rapidly. After 1.5 h of stirring, the volatiles were removed in vacuo , and the residue extracted with pentane. The extracts were then filtered over Celite and concentrated for r ecrystallization at 30 °C overnight. (129 mg, 0.321 mmol, 79%). 1 H NMR (500 MHz, CDCl 3 (s, 1H, Ar - H), 6.81 (s, 1H, Ar - H), 5.05 (br. sept., 2H, CH(CH 3 ) 2 ), 3.78 (br. sept., 2H, CH(CH 3 ) 2 ), 2.17 (s, 3H, Ar - CH 3 ), 2.15 (s, 3H, Ar - CH 3 ), 2.13 (s, 3H, A r - CH 3 ), 1.86 (br. s, 6H, CH(CH 3 ) 2 ), 1.31 (br. s, 6H, CH(CH 3 ) 2 ), 1.21 (apt. t, 12H, CH(CH 3 ) 2 ). Note: Due to rapid exchange in the system, the integral values on the room temperature spectrum are inaccurate. As such, an integrated low temperature spectrum is included in the spectra below. 13 C NMR (125 MHz, CDCl 3 6, 119.10, 58.41, 55.21, 30.61, 21.42, 19.61, 18.87, 16.42. 14 N NMR (36 MHz, CDCl 3 375.8. Elemental Analysis: Calcd. C, 62.81; H, 9.79; N, 10.46. Found: C, 62.67; H, 9.90; N, 10.52. M.p.: 127 - 128 °C . 190 Synthesis of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4,5 - diMe ): To a nearly frozen, stirring solution of NCr(N i Pr 2 ) 3 (100 mg, 0.273 mmol) in THF, a THF solution of the HOPh 2 - tBu - 4,5 - diMe (48.6 mg, 0.273 mmol) was added dropwise. The solution was loaded into a pressure tube and sealed. The reaction was the heated on an aluminum heating block at 50 °C for 3 h, whereupon the color changed from beet to orange. After the 3 h, the volatiles wer e dried in vacuo , and the dark residue was extracted with pentane. The extracts were th en filtered over Celite and concentrated for recrystallization at 30 °C overnight. (67 mg, 0.151 mmol, 55%). 1 H NMR (500 MHz, CDCl 3 Ar - H), 6.93 (s, 1 H, Ar - H), 5.05 (s, 2H, CH(CH 3 ) 2 ), 3.79 (s, 2H, CH(CH 3 ) 2 ), 2.18 (app. d, 6H, Ar - CH 3 ), 1. 89 (d, J = 4.2 Hz, 6H, CH(CH 3 ) 2 ), 1.55 (d, J = 4.3 Hz, 6H, CH(CH 3 ) 2 ), 1.37 (s, 9H, t Bu), 1.19 (s, 12H, CH(CH 3 ) 2 ). 13 C NMR (125 MHz, CDCl 3 26.77, 126.59, 121.54, 58.52, 55.49, 34.52, 30.69, 30.13, 30.20, 22.58, 21.99, 19.30, 1 9.15. 14 N NMR (36 MHz, CDCl 3 Elemental Analysis: Calcd. C, 64.98; H, 10.22; N, 9.47. Found: C, 64.92; H, 10.33; N, 9.63. M.p.: 107 - 109 °C. Synthesis of NCr(N i Pr 2 ) 2 (OPh 2 - Me - 4 - OMe ): To a nearly frozen, stirring solution of NCr(N i Pr 2 ) 3 (1 00 mg, 0.273 mmol) in ether, an ethereal solution of the HOPh 2 - Me - 4 - OMe (37.7 mg, 0.273 mmol) was added dropwise. After stirring for 1.5 h, the color had changed from beet to orange. The volatiles were dried in vacuo , and the dark residue was extracted wit h pentane. The extracts were filtered over Celite and concentrated for recrystallization at 30 °C overnight. (44 mg, 0.109 mmol, 40%). 1 H NMR (500 MHz, CDCl 3 J = 8.6 Hz, 1H, Ar - H), 6.61 (m, 1H, Ar - H), 5.03 (br. sept., 2H, CH(CH 3 ) 2 ), 3.76 (br. sept., 2H, CH(CH 3 ) 2 ), 3.73 (s, 3H, O - CH 3 ), 2.17 (s, 3H, Ar - CH 3 ), 1.8 5 (br. s, 6H, CH(CH 3 ) 2 ), 1.45 (br. s, 6H, CH(CH 3 ) 2 ), 1.23 (br. s, 12H, CH(CH 3 ) 2 ). Note: Due to rapid exchange in the system, the room temperature integral values in the spectrum are inaccurate. As such, an integrated low temperature spectrum is included in the spectra below. 13 C 191 NMR (125 MHz, CDCl 3 2, 111.55, 58.37, 55.89, 30.18, 21.53, 17.31. 14 N NMR (36 MHz, CDCl 3 Elemental Analysis: Calcd. C, 59.53; H, 9.24; N, 10.41. Found: C, 59.73; H, 9.7 8; N, 10.24. M.p.: 158 - 160 °C. Synthesis of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4OMe ): To a nearly fro zen, stirring solution of NCr(N i Pr 2 ) 3 (100 mg, 0.273 mmol) in ether, an ethereal solution of the HOPh 2 - tBu - 4OMe (49.2 mg, 0.273 mmol) was added dropwise. The solution wa s allowed to stir for 18 h with little noticeable color change. After that time, the vo latiles were dried in vacuo , and the dark residue was extracted with pentane. The solution was filtered over Celite and once again dried. The extremely soluble residue was dissolved in a minimal amount of HMDSO for recrystallization at 30 °C overnight. (6 1 mg, 0.137 mmol, 50%). 1 H NMR (500 MHz, CDCl 3 J = 8.7 Hz, 1H, Ar - H), 6.79 (d, J = 3.1 Hz, 1H, Ar - H), 6.64 (dd, J = 8.7, 3.1 Hz, 1H, Ar - H), 5.03 (m, 2H, CH(CH 3 ) 2 ), 3.77 (m, 2H, CH(CH 3 ) 2 ), 3.75 (s, 3H, O - CH 3 ), 1.87 (d, J = 5.4 Hz, 6H, CH(CH 3 ) 2 ), 1.53 (d, J = 5.5 Hz, 6H , CH(CH 3 ) 2 ), 1.37 (s, 9H t Bu), 1.18 (app. t, 12H, CH(CH 3 ) 2 ). 13 C NMR (125 MHz, CDCl 3 159.60, 152.49, 137.10, 119.48, 112.54, 110.34, 58.38, 55.62, 55.26, 34.92, 30.51, 29.99, 29.54, 22.39, 21.82. 14 N NMR (36 MHz, CDCl 3 = 100 4.4, 379.2. Elemental Analysis: Calcd. C, 61.99; H, 9.73; N, 9.43. Found: C, 61.66; H, 9.82; N, 9.53. M.p.: 97 - 100 °C. Synthesis of NCr(N i Pr 2 ) 2 (SNap): To a nearly frozen, stirring solution of NCr(N i Pr 2 ) 3 (100 mg, 0.273 mmol) in THF, a THF solution of the HSNap (45.9 mg, 0.286 mmol) was added dropwise. The solution was allowed to stir for 18 h at 55 °C upon which time the color changed to a reddish - purple color. After that time, the volatiles were dried in vacuo , and the dark residue was extracted with pentane. The solution was filtered over Celite and the filtrate concentrated. The pentane solution was then left in the freezer for recrystallization at 30 °C overnight. (63.9 mg, 0.150 mmol, 55%). 1 H NMR (50 0 MHz, CDCl 3 - H), 7.76 (dd, J = 8.6, 1.8 Hz, 1H, Ar - 192 H), 7.72 (d, J = 8.1 Hz, 1H, Ar - H), 7.63 (dd, J = 24.8, 8.3 Hz, 2H, Ar - H), 7.41 7.29 (m, 2H, Ar - H), 5.28 (m, 2H, CH(CH 3 ) 2 ), 3.74 (m, 2H, CH(CH 3 ) 2 ), 1.81 (d, J = 6.2 Hz, 6H, CH(CH 3 ) 2 ), 1.54 (d, J = 6.2 Hz, 6H, CH(CH 3 ) 2 ), 1.16 (dd, J = 6.1, 2.7 Hz, 12H, CH(CH 3 ) 2 ). 13 C NMR (125 MHz, CDCl 3 140.42, 134.08, 131.91, 131.25, 130.50, 127.77, 127.20, 126.99, 126.05, 124.77, 59.39, 56.39, 30.69, 30.32, 22.79, 20.79. 14 N NMR (36 MHz, CDCl 3 ): 406.3. Elemental Analysis: Calcd. C, 62.09; H, 8.29; N, 9.87. Found: C, 61.96; H, 8.15; N, 9.76. M.p.: 126 - 127 °C. Synthesis of NCr(N i Pr 2 ) 2 (6Br - SNap) :To a nearly frozen, stirring solution of NCr(N i Pr 2 ) 3 (125 mg, 0.340 mmol) in THF, a THF solution of the 6B r - HSNap (87 mg, 0.360 mmol) was added dropwise. The solution was allowed to stir for 18 h at 55 °C upon which time the color changed to a reddish - purple color. After that time, the volatiles were dried i n vacuo , and the dark residue was extracted with ethe r. The solution was filtered over Celite and the filtrate concentrated. The ether solution was then left in the freezer for recrystallization at 30 °C overnight. The LDP value, which was measured but no t used in this particular study, is 14.15 kcal/mol. ( 120 mg, 0.264 mmol, 78%). 1 H NMR (500 MHz, CDCl 3 - H), 7.88 (s, 1H, Ar - H), 7.80 - 7.78 (m, 1H, Ar - H), 7.53 - 7.51 (m, 2H, Ar - H), 7.46 7.44 (m, 1H, Ar - H), 5.34 - 5.26 (sept, J = 6.2 Hz, 2H, CH(CH 3 ) 2 ), 3.79 - 3.71 (sept, J = 6.3 Hz, 2H, CH(CH 3 ) 2 ), 1.81 - 1.80 (d, J = 6.3 Hz, 6H, CH(CH 3 ) 2 ), 1.55 - 1.54 (d, J = 6.3 Hz, 6H, CH(CH 3 ) 2 ), 1.18 - 1.15 (dd, J = 6.5, 4.5 Hz, 12H, CH(CH 3 ) 2 ). 13 C NMR (125 MHz, CDCl 3 141.06, 132.58, 132.11, 131.89, 130.09, 129.51, 129.11, 128.29, 125.92, 118.14, 59.21, 56.20, 30.43, 30.06, 21.99, 20.52. 14 N NMR (36 MHz, CDCl 3 M.p.: 149 - 151 °C. General Procedure for Kinetics All manipulations were done in an inert atmosphere drybox. A 2 mL volumetric flask was loaded the catalyst (10 mol%, 0.1 mmol) and ferr ocene (0.0560 g, 0.3 mmol) as an internal 193 standard. Next, 0.75 mL of toluene - d 8 was added to the volumetric flask and the solution was mixed by swirling the flask until all solids were dissolved. Once al l solids were dissolved, aniline (911 µL, 10 mmol) an d 1 - phenylpropyne (125 µL, 1.0 mmol) were added respectively to the volumetric flask. Lastly, the solution was diluted to 2 mL with toluene - d 8 . The solution was mixed via pipette (i.e. the solution was d rawn up into the pipette and dispensed back into the volumetric flask) five times to ensure the solution was well - mixed. An ample amount of solution (~0.75 mL) was loaded into a threaded J. Young tube that was sealed with a Teflon stopper. The tube was rem oved from the dry box and was heated at 75 °C in the NMR spectrometer ( Varian Inova 600 spectrometer). The relative 1 - phenylpropyne versus ferrocene concentration was monitored as a function of time. The fits are to the exponential decay of the starting ma terial using the scientific graphing program Origin. The exact expression used to fit the data is shown below: ( 44 ) Where Y = [1 - phenylpropyne] at time t (Y t ), infinity (Y ), or at the start of the reaction (Y 0 ). The variables Y , Y 0 , k obs , were optimized in the fits. Each kinetic experiment was completed in triplicate. 194 Figure 4 - 12 . Representative Plots for Kinetics Plot of [1 - phenylpropyne ] vs time with Ti(NMe 2 ) 2 ( bis - phenoxide 2tBu - 4Me ) ( 5 ) 195 Figure 4 - 13 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me ) in CDCl 3 . 196 Figure 4 - 1 4 . 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me ) in CDCl 3 . 197 Figure 4 - 15 . 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me ) in CDCl 3 . 198 Figure 4 - 16 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me5F ) in in CDCl 3 . 199 Figure 4 - 17 . 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me5F ) in in CDCl 3 . 200 Figure 4 - 18 . 19 F NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me5F ) in in CDCl 3 . 201 Figure 4 - 19 . 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (Ind 3Me5F ) in in CDCl 3 . 202 Figure 4 - 20 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Me ) in CDCl 3 . 203 Figure 4 - 21 . 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Me ) in CDCl 3 . 204 Figure 4 - 22 . 14 N NMR Spect rum of NCr(N i Pr 2 ) 2 (Pyrr Me ) in CDCl 3 . 205 Figure 4 - 23 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr 2 - [C6H3(CF3)2] ) in CDCl 3 . 206 Figure 4 - 24 . 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr 2 - [C6H3(CF3)2] ) in CDCl 3 . 207 Figure 4 - 25 . 19 F NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr 2 - [C6H3(CF3)2] ) in CDCl 3 208 Figure 4 - 26 . 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 ( Pyrr 2 - [C6H3(CF3)2] ) in CDCl 3 . 209 Figure 4 - 27 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Ph ) in CDCl 3 . 210 Figure 4 - 28 . 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Ph ) in CDCl 3 . 211 Figure 4 - 29 . 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Ph ) in CDCl 3 . 212 Figure 4 - 30 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Tol ) in CDCl 3 . 213 Figure 4 - 31 . 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Tol ) in CDCl 3 . 214 Figure 4 - 32 . 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (Pyrr Tol ) in CDCl 3 . 215 Figure 4 - 33 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2,4 - diMe ) in CDCl 3 . 216 Figure 4 - 34 . 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2,4 - diMe ) in CDCl 3 . 217 Figure 4 - 35 . 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2,4 - diMe ) in CDCl 3 . 218 Figure 4 - 36 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - Me ) in CDCl 3 . 219 Figure 4 - 37 . 13 C NM R Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - Me ) in CDCl 3 . 220 Figure 4 - 38 . 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - Me ) in CDCl 3 . 221 Figure 4 - 39 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2,4,5 - triMe ) in CDCl 3 . 222 Figure 4 - 40 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2,4,5 - triMe ) in CDCl 3 at 29 °C . 223 Figure 4 - 41 . 1 H NMR Spectrum of N Cr(N i Pr 2 ) 2 (OPh 2,4,5 - triMe ) in CDCl 3 at 29 °C . 224 Figure 4 - 42 . 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2,4,5 - triMe ) in CDCl 3 . 225 Figure 4 - 43 . 1 H NMR Spectrum of NC r(N i Pr 2 ) 2 (OPh 2 - tBu - 4,5 - diMe ) in CDCl 3 . 226 Figure 4 - 44 . 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4,5 - diMe ) in CDCl 3 . 227 Figure 4 - 45 . 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4,5 - diMe ) in CDCl 3 . 228 Figure 4 - 46 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - Me - 4 - OMe ) in CDCl 3 . 229 Figure 4 - 47 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OP h 2 - Me - 4 - OMe ) in CDCl 3 at 26 °C . 230 Figure 4 - 48 . 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - Me - 4 - OMe ) in CDCl 3 . 231 Figure 4 - 49 . 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - Me - 4 - OMe ) in CDCl 3 . 232 Figure 4 - 50 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - OMe ) in CDCl 3 . 233 Figure 4 - 51 . 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - OMe ) in CDCl 3 . 234 Figure 4 - 52 . 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (OPh 2 - tBu - 4 - OMe ) in CDCl 3 . 235 Figure 4 - 53 . 1 H NMR Spectrum of NCr(N i Pr 2 ) 2 (SNap) in CDCl 3 . 236 Figure 4 - 54 . 13 C NMR Spectrum of NCr(N i Pr 2 ) 2 (SNap) in CDCl 3 . 237 Figure 4 - 55 . 14 N NMR Spectrum of NCr(N i Pr 2 ) 2 (SNap) in CDCl 3 . 238 REFERENCES 239 REFERENCES 1. 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A major challenge in cleaning or recycling the waste pro du ced by nuclear reactors is the difficulty in separating the components of the mixture. Because there is often a complex mixture of lanthanides, actinides, and many other decay products, isolation of radioactive components can be extremely costly. 1 However, if the bonding prefere nces of the waste components, such as uranium, were better understood, a better procedure might be developed to separate and recycle the waste mixtures. As such, bonding interactions between f - block elements and ligands have become a popular research subje ct in recent decades. In contrast to transition metal systems, where bonding involving primarily d - and s - orbitals is understood . B onding in the actinides involves varying degrees of f - and d - orbital participation depending on orbital extension and energy in the complexes making the bonding trends quite c omplex . Orbital participation is further complicated when considering the varying degree of covalency and ionicity in actinide bonding as well. In 2017, a collaboration with Dr. James Boncella at Los Alamo s National Lab was started through a DOE SCGSR fellowship. This collaboration was based on a proposal to treat a series of uranium catalysts as we treated the titanium hydroamination catalysts in chapter 4. Using the LDP met hod with systematic variation of ligand sets we aim ed to elucidate some of the intricacies involved in actinide - light element bonding . 243 - acceptor orbitals, it was proposed that our system of ligand parameterization, based on Cr(VI), might be applicable. The electronic values are based on a metal in which bonding is heavily covalent, so the numbers could show correlation in uranium systems where covalency between uranium and its ligands is important. Interestingly, we have show n in correlation between LDP and uranium is possible. The plot in Figure 5 - 1 shows correlation between our LDP values and the predicted reduction potentials of OU(NMe 2 ) 3 X. 2 For the OMe value, we used the LDP from OEt as an electronic surrogate. The number for CN is the only point that seems to be a poor fit. It may be that since the X ligand in t he Schelter system is trans to a - - effects would not be present. The correlation for all of the other X ligands suggests the LDP may indeed be an adequate measure of small atom donation to a uranium center. 244 Figure 5 - 1 . Plot of the LDP versus the E 1/2 value s of a U(V)/U(VI) redox couple for a series of ligands. By developing a model for uranium catalysis similar to the reported titanium model, we proposed that evaluation of actinide reactivity as a function of ligand properties and covalency could be possibl e. 3 5.2 Reaction Desig n The decision to use catalysis to study uranium as oppose d to other types of studies was an obvious one. Moni toring catalysis has many advantages over other studies. Primarily, small electronic effects at a metal center, as a result of ligand electronic changes, are hard to measure. Theory can give some insight, and there are advanced spectroscopies that would al low comparisons between ligand sets as well, but these studies are generally limited to simple systems due to complexities in the electronic structure of the actinides. 4 - 10 Rather than try to replace these types of studies, our aim was to bridge these studies to real - world reactivity using uranium - based catalysis. NMe 2 OMe F SPh Cl Br I CN y = 11.404x + 20.203 R² = 0.9192 8 9 10 11 12 13 14 15 16 17 -1 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 LDP (kcal/mol) Calculated E 1/2 U V/VI (eV) LDP vs Calculated Reduciton Potential 245 When we study a reaction using a metal complex as a catalyst, we see the direct effects of ancillary ligand changes in the reaction. And, because the c omp lex is used for the same reaction over and over, the effects of those changes are amplified several times. For example, if we were to consider a ligand exchange reaction instead of a catalyzed transformation, the reaction happens once at each metal cent er. To study that reaction, we only get to observe one transformation at each metal - containing molecule. This can be more difficult to detect, but, more importantly, slight changes between reactions can go unnoticed. If we compare that to a reaction where a u ranium complex catalyzes a reaction at 1 mol%, now we can observe each molecule doing one hundred reactions. The result is that any differences between various uranium catalysts are amplified by one hundred times, making those same slight differences mo re obvious. This makes studying small amounts of material, such as radioactive actinides, easier to do. This approach could allow observation of the effects that minor electronic changes from the ancillary ligands have on the overall reactivity of the cata lys t, providing a new mechanism of study for the tendencies of the actinide series. Since using LDP allows quantitative measure of the changes in both steric and electronic perturbations, studying the resulting effects on reactivity should be simplified. T his could allow development of ideal complexing ligands for such processes as nuclear power waste remediation or medical isotope chelation. Typically, new reactions are more exciting and, therefore, more desirable. For this study though, the reaction is so mew hat unimportant. It was most critical for us to know that the reaction can work with minimal complications. Therefore, we chose to study intramolecular hydroamination (Scheme 5 - 1) because it is well established . Hydroamination of alkynes is a relatively si mple reaction, but using the intramolecular hydroamination avoids side reactions like 246 alkyne oligomerizations. 11 Another ad vantage of hydroamination for this study is the existing literature precedence. Scheme 5 - 1 . Example of an intramolecular hydroamination. Several groups have reported hydroam ination using uranium catalysts , unfortunately none of the reported complexes would work well with the LDP system. 11 - 15 The large ionic radius and lability of uranium typically requires the ancillary ligands to be quite bulky. As we have discussed in the previous chapters, that limits our options fo r LDP evaluation of the ligands. In order to keep our study quantitative, we decided to design new uranium catalysts that allowed interrogation through the LDP system. 5.3 Ca talyst Design and Synthesis Our first design was a new series of uranium catalysts bas ed on the 6 , 5 ' - pyridylpyrrole, or PyPyr, ligands. The pyrrole fragment allows for easy substitution o n various positions of the ring. 16 In this way, we were able to easily manipulate both the size of the ligand, and, more importantly, the electronic properties of the ligand. Additionally, f rom our experience with titanium, PyPyr ligands are typically easy to bind to the meta l center through acid - base reactions with M(NR 2 ) 4 starting materials , which are known starting materials for U as well. 17 - 20 Lik e the linker of the ligand for our titanium catalysts, we hoped that we would be able to ignore any contributions from the pyridine, considering it as a constant. 3 If true, we could quantitatively measure changes to the catalyst by measuring the LDP differences of the pyrrol e rings. Since our focus is the electronic bonding interactions, we also sought to keep sterics as constant as possible. To achieve this, we 247 use d 2,4 - dimethyl substituted pyrrole rings. This leaves the 3 - position open for substitution of electronically dif ferent groups but maintains an almost constant steric profile at the metal center. When this project began , we envisioned complexes analogous to our previously reported titanium catalysts , U(PyPyr) 2 (NR 2 ) 2 type compounds (Scheme 5 - 2) . 21 Since the known U(N(SiMe 3 ) 2 ) 4 is reported ly very inert , and the smaller amide based starting materials are not readily available, we targeted other synthetic routes . 18, 22 We decided, instead of using acid - base type reaction s , it might be easier to u tiliz e transmetalation s using sodium or potassium salts of the ligands with the halide based U starting materials . 23 - 24 Scheme 5 - 2 . Proposed synthesis of the uranium precatalysts. The sodium salt of PyPyr Me 2 , the PyPyr derivative bearing a 2,4 - d imethyl pyrrole , can be easily gen erated by deprotonation of the ligand with NaCH 2 TMS or N aN(TMS) 2 . Addition of two equivalents of NaPyPyr Me 2 to UCl 4 , or UI 4 , was unsuccessful. D ue to the lability of uranium complexes , the metal tends to redistribute ligands quite easily. These rearrangeme nt reactions cause difficulties in synthesizing pure mol ecules. For example, when making UCl 2 (PyPyr Me 2 ) 2 from UCl 4 and two equivalents of NaPyPyr Me 2 , the major products of the reaction are U(PyPyr Me 2 ) 4 and UCl 4 regardless of addition rate, reaction time, e tc . (Scheme 5 - 3). 248 Scheme 5 - 3 . Attempted synthesis of U(PyPyr) 2 Cl 2 from UCl 4 . Fortunately, Dr. Aaron Tondreau had recently prepared a series of uranium amides that were capa ble of producing U(NR 2 ) 2 Cl 2 molecules . 20 In addition to the U(NR 2 ) 2 Cl 2 molecule, the U(NR 2 ) 3 Cl and U(NR 2 ) 4 molecules were also stable. To us, this suggested the N(TMS)Cy amides might be ideal as co - li gand in our PyPyr based catalysts. If the N(TMS)Cy amides produced stability with chlorides, it would be likely to work with our PyPyr ligands too. Scheme 5 - 4 . Synthesis of mixed amide PyPyr compounds. Stirring two equivalents of HPyPyr Me2 and U [ N(TMS)Cy ] 4 in THF overnight at 40 C changed the reaction color fr om light tan to dark red (Scheme 5 - 4) . Crude NMR of the mixture indicat ed a relatively clean reaction, though , no X - ray quality crystals could be isolated from the reaction and paramagnetic shifts of in the NMR precluded adequate assignment . We decided to test the crude product, presumably U(PyPyr Me 2 ) 2 (N(TMS)Cy) 2 ( 1 ) for cata lytic activity towards intramolecular hydroamination using 2,2 - diphenyl - 1 - amino - 4 - pentene (DPAP) . At 60 C, the c atalyst cyclizes >95% of the DPAP in less than 18 hours by 1 H NMR (Figure 5 - 2) . The cyclized product was 249 isolated by passing the react ion mixture through alumina with hexane , resulting in a colorless oil after removal of the volatiles , confirming conversion to the heterocycle. Figure 5 - 2 . Comparison of the olefinic region of the 1 H NMR spectrum before (top) and after (bottom) catalysis showing complete cyclization. As a control, we added three equivalents of HPyPyr Me2 to U(N(TMS)Cy) 4 following the same procedure as that for 1 (Scheme 5 - 4). When the hydroamination reaction is perfo rmed with in situ generated U(PyPyr Me 2 ) 3 [ N(TMS)Cy ] ( 2 ) the reaction does not proceed . Due to the nature of the I nitial Spectrum NaCH 2 TMS h 18 h 250 reaction we are performing, these results sugg est that we are, indeed, generating 1 and 2 in the synthesis . The difference in two or three equiva lents of PyPyr Me 2 is significant . If we assume the reaction follows the Bergman hydroamination mechanism (which Eisen suggests their U species do , but there exists some debate) the reaction needs two proteolytically cleavable sites for imido formation. 11, 13 - 15, 25 I n this case the N(TMS)Cy ligands are the only sites basic enough to be deprotonated by the primary amine , so the reaction cannot occur with U(PyPyr Me 2 ) 3 [ N(TMS)Cy ] . The difficulty in isolating pure cataly st, however, makes a kinetic study of the reaction dubious, especially when considering that hydroaminati on with U [ N(TMS)Cy ] 4 also result ed in the cyclization of DPAP, albeit at a much slower rate and with formation of byproducts by 1 H NMR. 251 Figure 5 - 3 . Structures of t op : Cp*U I 2 (PyPyr Me3 ) (thf) ( 3a ) , Cp*U I 2 (PyPyr Me2 ) (thf) ( 5 ), b ottom : Cp*U Cl 2 (PyPyr Me2Tol ) (thf) ( 4a ), and Cp*U I 2 (PyPyr Me2Tol ) (thf) ( 4 b ), Hydrogens and co - crystallized solvents removed for clarity . By replacing one of the ancillary PyPyr ligands with a pentamethylcyclopentadiene, or Cp*, we postu lated that we would increase both the stability and crystallinity of the catalyst. Beginning with U Cl 4 , addition of NaCp*, or KCp*, in a THF solution presu mably generates Cp*U Cl 3 . The generated Cp*U Cl 3 is not isolable and decomposes upon attempts at isolat ion, but subsequent addition of NaPyPyr (Scheme 5 - 5) to the solution produces a reddish orange color , which, after workup, can be crystallized to yield Cp* U Cl 2 (PyPyr) (thf ) (PyPyr Me3 3a , PyPyr Me2Tol 4a ). Likewise, the same reaction procedure starting from UI 4 produces Cp*UI 2 (PyPyr)(thf), (PyPyr Me3 3 b , PyPyr Me2Tol 4 b , PyPyr Me2 5 ), also shown in Scheme 5 - 5. Surprisingly, despite lower yield relative 252 to the chlo ride compounds, the Cp*U I 2 (PyPyr) molecules crystallized easier with the methyl substi tuted PyPyr ligands, the structures are shown in Figure 5 - 3 . Scheme 5 - 5 . Synthesis of C p*U X 2 (PyPyr) (thf) from UX 4 starting materials. We then attempted substitution of the halides to create active sites for the catalysis. Attempts with small amides, bulky amides, alkyl groups, and alkoxy groups were all met with failure . Attempted formation of an imido group was equally unsuccessful due to ligand redistr ibution reactions. These redistribution reactions led to the exciting chemistry that will be discussed in chapter 6, but never to a competent catalyst. 5.4 Catalysis We finally found success using Na N(TMS) 2 and NaCH 2 TMS . Sequential addition of NaCp*, NaPyPyr Me 3 , Na N(TMS) 2 , and NaCH 2 TMS to a solution of UCl 4 in THF results in a cyclometalated, 2 (N - C) - N(TMS)(CH 2 SiMe 2 ) ligand (Scheme 5 - 6) . The resulting 2 (N - C) - CH 2 SiMe 2 NTMS ) U ( Cp * )( PyPyr Me3 ) ( 6 ) was more crystalline than the bis - PyPyr based catalysts , and X - ray quality crystals were isolated. 253 We were unsure what to expect for the competency of these catalysts towards hydroamination. Surprisingly, with 10 mol% catalyst loading at 65 °C the catalysis was rather rapid, with the reaction reaching completion in under 3 h. Using 6 at 5% catalyst loading and a reaction temperature of 65 C , the reactio n reached completion (by 1 H NMR) in under 6 hours. Analysis of the 1 H NMR spectrum indicates that, after heating, the catalyst structure seems to remain constant (i. e. there appear to be no peaks for disproportionation products). Scheme 5 - 6 2 (N - C) - CH 2 SiMe 2 NTMS ) U ( Cp * )( PyPyr Me3 ) ( 6 ) from UCl 4 . Having established a baseline with a catalyst that seemed stable with regard to ligand redistribution, we sought to produ ce a derivative using a different PyPyr ligand. Using the same 2 (N - C) - CH 2 SiMe 2 NTMS ) U ( Cp* ) (PyPyr Me2 ) ( 7 ) can be produced. Interestingl y, with just a change in the 3 - position of the pyrrole from Me to H, the catalyst has a different co ordination geometry due to incorporation of an equivalent of THF. The crystal structures of 6 and 7 are compared in Figure 5 - 4. 254 Figure 5 - 4 . Comparison of the solid - 2 (N - C) - CH 2 SiMe 2 NTMS ) U ( Cp * )( PyPyr Me3 ) ( 6 ) (left) and 2 (N - C) - CH 2 SiMe 2 NTMS ) U ( Cp* ) ( PyPyr Me2 ) ( 7 ) (right). Rather than the pseudo - square pyramidal geometry observed in 6 , the structure in 7 has a pseudo - octahedral ligand arrangement. The coordination of THF is not surprising, b ut the position relative to the other ligands is. If we define a z - axis along the U - C p* bond vector, rather than filling in the open axial coordination site of the square pyramid trans to the Cp* ligand (which is the location of the THF in 3 - 5 and the open site in 6 ), the THF molecule is bound in the equatorial plane. This places the N3 am ide trans to the Cp* ligand, effectively putting the two strongest donors trans to one another. 26 This phenomenon is not necessarily uncommon in actinide chemistry, but might be a significant difference between 6 and 7 . 27 - 31 When the hydroamination of DPAP was r un with 7 , using 5 mol% loading at 65 °C , the catalysis was complete in just over 3 hours. It is worth restating, t he only difference between the ancillary ligands is a Me group in the 3 - position of the pyrrole ring , yet the catalysis is nearly twice as fast . Importantly, though, the two PyPyr ligands remain es sentially isosteric. If we assume the TH F in 7 dissociates during the catalysis, t his is the same trend seen with the titanium catalysis. Minor changes in the electronic structure of the ligand play large roles in reactivity differences. While drawing conc lusions based on two qualitative points would be careless , the results of the experiment were exciting . We also attempted to produce the analogous catalysts of 6 and 7 using halides and aryl groups in the 3 - position of the pyrrole ring, but isolation of th ose catalysts was unsuccessful. 5.5 Future Work Regrettably, synthesis of the catalysts is not very reproducible. The yields of the reaction vary and, after moving back to MSU, we were unable to isolate the species from UI 4 starting materials. One issue affec ting the isolation of the catalysts from UI 4 starting materials is ligand redistribution 255 reactions. Reactions where the catalysts were made using the one pot procedure always show production of redistribution reaction products, commonly U(PyPyr) 4 and Cp*U( NTMS 2 2 (N - C) - CH 2 SiMe 2 NTMS ). Even stepwise synthesis reactions to produce the isolable intermediate Cp*U(PyPyr) I 2 results in formation of a significant quantity of the U(PyPyr) 4 compounds shown in Figure 5 - 5, which may explain the poor yield of 3b . Perhaps sim ply acquiring some UCl 4 is the solution to our issues, but currently that is not a material we have access to. Figure 5 - 5 . Crystals structures of U(PyPyr Me3 ) 4 and U(PyPyr Me 2Cl ) 4 byproducts. The structure of U(PyPyr Me3 ) 4 crystallizes with four - fold symmetry, the grown structure is shown. Hydrogens and co - crystallized solvents removed for purity. 256 Figure 5 - 6 . Crystal structure of UI 2 (PyPyr tBu2 ) 2 ( 6 ). Hydrogens and co - crystallized diethyl ether molecules removed for clarity. We wondered if adding more steric bulk to the PyPyr in the 2 - position might slow some of the redistribution of ligands. To achieve this, we synthesized the 2,4 - di t Bu - PyPyr (PyPyr tBu2 ) ligand. 16 The pyrrole was then deprotonated and added with NaCp* to UI 4 (thf) 2 following the analogous procedure to synthesize 3 - 5 . Still though, significant quantities of unwanted byproducts were produced, and the primary isolated species was UI 2 (PyPyr tBu2 ) 2 , 8 (Figure 5 - 4). We have not thoroughly explored or characterized 8 , but P yPyr tBu2 could potentially work as an ancillary lig and in the (PyPyr) 2 U(NR 2 ) 2 - type catalysts due to the increased steric bulk. In trying to sort out the synthesis of adequate catalysts, we produced a number of interesting U - PyPyr molecules. An in - depth an alysis of some of the molecules discovered may prov e useful in moving forward. Specifically, compounds 3 - 5 are quite interesting. The 1 H NMR spectra of 3a / 4a and 3b / 4b show a significant dependence on halide bound to uranium. It could be that making a seri es of halides, including Br, with various PyPyr lig ands might give another indication on how the electronic structure of the uranium atom is affected as a function of LDP. Since the 257 NMR data are quite clean, NMR may be an easy way to investigate the changi ng structure similar to the correlations that have been shown with NMR shift and ligand donor abilities previously. 26, 32 By comparing both the series of PyPyr ligands against one another for each type of dihalide ( i.e. comparing compounds 3a and 4a ) and comparing the halides with each PyPyr ligand (i.e. comparing compounds 3a and 3b ), we could interrogate the U atom as a function of the LDP of each the PyPyr ligands and changing the halide. While this does not use catalysis, it might provide more insight into the effects of the PyPyr substituents on the electronic structure of uranium. Additionally, when more catalysts can be prepared, in addition to determin ing typical kinetic data, such as rate dependence on substrate and catalyst, a dependence on THF should als o be 2 (N - C) - CH 2 SiMe 2 NTMS ) UCp*( PyPyr Me2 ) ( 7 ) is significant, or if it is simply displaced during the catalysis. 5.6 Conclusions The synthesis of complexes 6 and 7 , and t heir reactivity towards cyclization of the hydroamination substrate, was a minor success. Unfortunately, time and equipment constraints at Los Alamos, a nd starting material limitations at MSU meant those were the only active molecules isolated. Looking for ward, more derivatives of each catalyst are necessary to confirm reactivity trends. Moreover, the catalyst behavior needs to be thoroughly examined duri ng the reaction. The ligand redistribution reactions that we have observed, and the lability of uranium necessitate detailed analysis to be sure the hydroamination reaction is proceeding normally and catalyst decomposition is not an issue. In the two catal ytic runs we performed in this study, no hints of catalyst side reactions were observed, but more eviden ce is necessary to prove the differences in reaction rate are a function of ligand substitutions and not adventitious reactions. 258 5.7 Experimental All reacti ons and manipulations were carried out in an MBraun glovebox under a n inert atmosphere and/or using stan dard Schle nk techniques. Diethyl ether , pentane, tetrahydrofuran, and hexane were purc hased from Aldrich Chemical Company. Diethyl ether and pentane wer e purified by passing through alumina columns to remove water after being sparged with dry nitrogen to r emove oxygen. Tetrahydrofuran and hexane were sparged with dinitrogen to remove oxygen and distilled from sodium and benzophenone. C 6 D 6 , THF - d 8 , and t ol uene - d 8 were purchased from Cambridge Isotopes Laboratories, Inc or Aldrich Chemical Company . Toluene - d 8 and THF - d 8 were sparged with dry dinitrogen and dried over 4 Å molecular sieves . Before use, each solvent was passed through a plug of activated alumina to filter the solvent and to ensure dryness . C 6 D 6 was sparged with dry dinitrogen and distilled from C aH 2 before use. All NMR solvents were stored under an inert atmosphere away from light . Depleted uranium turnings were purchased from Ma nufacturing Sciences Corporation. Synthesis of UCl 4 , UI 4 (dioxane) 2 , PyPyr ligands, U(N(TMS)Cy) 4 , NaCH 2 TMS, and 2,2 - diphe nyl - 1 - amino - 4 - pentene were prepared according to literature procedures. 16 , 20, 23 - 24, 33 - 34 The PyPyr and Cp* ligands were deprotonated in hexane with stoichiometric NaCH 2 TMS of NaN(TMS) 2 over 16 h. The reaction ge nerates an off - white precipitate which can be collected by filtration. The precipitat e was washed with several aliquots of hexane and used as is. Elemental Analysis was performed by Atlantic Microlab in Norcross, GA using a He filled glovebag to handle the compounds. Synthesis Caution! Depleted uranium (primary isotope 238 U) is a weak - emitter (4.197 MeV) with a half - l ife of 4.47 x 10 9 years; manipulations and reactions should be carried out in monitored fume 259 hoods or in an inert atmosphere drybox in a radi - - counting equipment Oxide - Free Uranium Meta l Turnings: Cautionary Note: Uranium is pyrophoric when finely divided; caution is recommended in the washing process to avoid exposure to air . This is a modification of the literature procedure. 23 Depleted uranium turnings were receive d in mineral oil from Manufacturing Sciences Corporation . The t urnings (~10 g) wer e carefully transferred to a 500 mL side arm flask which was fitted with a hose flowing dry dinitrogen or dry argon gas. The turnings were washed with hexanes (3 x 150 mL), a cetone (3 x 150 mL), then water (3 x 150 mL). The flask was then filled with enoug h water to fully cover the turnings (~100 mL). Concentrated nitric acid was then added by pipette while gently swirling the flask until removal of the black oxide layer was a chieved and the turnings became metallic in color. The amount of concentrated acid necessary can vary significantly depending on the quality of turnings, but typically ~20 mL is sufficient. Cleaning of the oxide layer can be accompanied by warming of the s olution as well as NO 2 gas generation, observable as a brown gas. Once the turning s appear shiny and metallic, the acid solution is carefully decanted. The turnings are then washed again with water (3 x 150 mL) and acetone (3 x 150 mL). After the final was h, the turnings were dried under reduced pressure and transferred to the drybox. I n - situ preparation of U(N(TMS)Cy) 2 (PyPyr Me2 ) 2 ( 1 ): In a 20 mL scintillation vial, U(N(TMS)Cy) 4 ( 200 mg, 0. 218 mmol ) was loaded with a stir bar and THF (5 mL). The solution w as stirred with a magnetic stir plate. To the stirring solution was added HPyPyr Me2 ( 75 mg, 0.435 mmol ). The vial was heated to 40 °C. The vial was then capped and allowed to stir for 18 h, during which, the solution changed color from tan to dark red. 1 w as never isolat ed as a pure product, but the crude NMR is displayed in Figure 5 - 7 , below. 260 In - situ preparation of U(PyPyr Me2 ) 3 (N(TMS)Cy) ( 2 ): In a 20 mL scintillation vial, U(N(TMS)Cy) 4 ( 200 mg, 0.218 mmol ) was loaded with a stir bar and THF (5 mL). The sol ution was stirr ed with a magnetic stir plate. To the stirring solution was added HPyPyr Me2 ( 112 mg, 0.653 mmol ). The vial was heated to 40 °C. The vial was then capped and allowed to stir for 18 h, during which, the solution changed color from tan to dark red. 2 was neve r isolated as a pure product, but the crude NMR is displayed in Figure 5 - 8 , below. Synthesis of Cp*UCl 2 PyPyr Me3 (thf) ( 3 a ): In a 20 mL scintillation vial, UCl 4 (200 mg, 0.527 mmol) was loaded with a stir bar and THF (5 mL). The solution was s tirred with a magnetic stir plate. To the stirring solution was added NaCp*( 83 mg, 0. 527 mmol). The solution rapidly darkened. Next, NaPyPyr Me3 ( 109 mg, 0. 527 mmol ) was adde d as a solution in THF (1 mL). The solution was left to stir for 2 h whereupon the color had turned to a dark brown/yellow. The solution was then dried of the volatiles. The solids were extracted with diethyl ether (~10 mL) and filtered using Celite as a f ilter ing agent. The filtrate was concentrated to ~1 mL and put in the freezer at - 3 0 °C overnight for recrystallization. 1 H NMR ( 4 00 MHz , C 6 D 6 49.28 ( s, 2H, thf) , 48.32 ( s, 2H, thf) , 24.26 (s, 4H thf), 9.37 (s, 15H, Cp*), 1.46 (s, 3H, Me), 1.02 (s, 3 H, Me), 1.03 (s, 1H, Ar), 5.29 (s, 1H, Ar), 14.15 (s, 1H, Ar), 25.89 (s, 3H, Me), 67.31 (s, 1H, Ar). Synthesis of Cp*UI 2 PyPyr Me3 (thf) ( 3 b ): In a 20 mL scintillation vial, U I 4 (dioxane) 2 (2 50 mg, 0. 271 mmol) was loaded with a stir bar and THF (5 mL). The solution was stirred over a magnetic stir plate. To the stirring solution was added NaCp* (43 mg, 0.271 mmol). The solution rapidly darkene d. Next, NaPyPyr Me3 (56 mg, 0.271 mmol ) was added as a solution in THF (1 mL). The solution was left to stir for 2 h whereupon the color had turned to a dark brown/yellow. The solution was then dried of the volatiles. The solids were extracted with diethyl ether (~10 mL) and filtered using Celite as a filter ing agent. The filtrate was concentrated to ~1 mL and put in the 261 freezer at - 30 °C overnight for recrystallization. Yield: 19 mg (8%). 1 H NMR ( 6 00 MHz , toluene - d 8 71.76 ( s, 2H, thf) , 68.72 ( s, 2H, thf) , 35.38 (s, 4H thf), 15.04 (s, 15H, Cp*), 0.42 (s, 3H, Me), 1.29 (s, 3H, Me), 3.72 (s, 1H, Ar), 7.95(s, 1H, Ar), 17.78 (s, 1H, Ar), 41.07 (s, 3H, Me), 88.90 (s, 1H, Ar). Synthesis of Cp*UCl 2 PyPyr Me2Tol (thf) ( 4a ): In a 20 mL scintillation vial, U Cl 4 (200 mg, 0.527 mmol) was loaded with a stir bar and THF (5 mL). The solution was stirred over a magnetic stir plate. To the stirring solution was added NaCp*( 83 mg, 0. 527 mmol). The solution rapidly darkened. Next, NaPyPyr Me 2Tol (150 mg, 0.527 mmol) wa s added as a solution in THF (1 mL). The solution was left to s tir for 2 h whereupon the color had turned to a dark brown/yellow. The solution was then dried of the volatiles. The solids were extracted with diethyl ether (~10 mL) and filtered using Celite as a filter agent. The filtrate was concentrated to ~1 mL and p ut in the freezer at - 30 °C overnight for recrystallization. 1 H NMR ( 4 00 MHz , C 6 D 6 52.76 ( s, 2H, thf) , 51.81 ( s, 2H, thf) , 26.14 (s, 4H thf), 9.84 (s, 15H, Cp*), 6.38 (s, 2H, Tol), 5.97 (s, 2H, Tol), 1.61 (s, 3H, Me), 0.00 (s, 3H, Me), 1.17 (s, 1H, Ar), 5.95 (s, 1H, Ar), 14.08 (s, 1H, Ar), 28.81(s, 3H, Me), 68.57 (s, 1H, Ar) . Synthesis of Cp*UI 2 PyPyr Me2Tol (thf) ( 4b ): In a 20 mL scintillation vial, U I 4 (dioxane) 2 (2 50 mg, 0. 271 mmol) was loaded with a stir bar and THF (5 mL). The solution was stirred over a magnetic stir plate. To the stirring solution was added NaCp* (43 mg, 0 .271mmol). The solution rapidly darkened. Next , NaPyPyr Me 2Tol ( 77 mg, 0.271 mmol ) was added as a solution in THF (1 mL). The solution was left to stir for 2 h whereupon the color had turned to a dark brown/yellow. The solution was then dried of the volatil es. The solids were extracted with diethyl eth er (~10 mL) and filtered using Celite as a filter ing agent. The filtrate was concentrated to ~1 mL and put in the freezer at - 30 °C overnight for recrystallization producing small quantities of X - ray quality 262 cr ystals . 1 H NMR ( 4 00 MHz , C 6 D 6 75.40 ( s, 2H, thf) , 72.49 ( s, 2H, thf) , 37.29 (s, 4H thf), 15.88 (s, 15H, Cp*), 5.25 (d, J = 8.4 Hz, 2H, Tol), 3.50(d, J = 8.2 Hz, 2H, Tol), 0.90 (s, 3H, Me), 0.55 (s, 3H, Me), 4.05 (s, 1H, Ar), 8.81 (d, J = 9.9 Hz, 1H, Ar), 17.94 (s, 1H, Ar), 44.22 (s, 3H, Me), 90.74 (d, J = 6.8, 1H, Ar). Synthesis of Cp*UI 2 PyPyr Me2 (thf) ( 5 ): In a 20 mL scintillation vial, U I 4 (dioxane) 2 (2 50 mg, 0. 271 mmol) was loaded with a stir bar and THF (5 mL). The solution was stirred over a magnetic stir plate. To the s tirrin g solution was added NaCp* (43 mg, 0.271 mmol). The solution rapidly darkened. Next, NaPyPyr Me 2 ( 40 mg, 0. 271 mmol ) was added as a solution in THF (1 mL). The solution was left to stir for 2 h whereupon the color had turned to a dark brown/yellow. Th e solu tion was then dried of the volatiles. The solids were extracted with diethyl ether (~10 mL) and filtered using Celite as a filter agent. The filtrate was concentrated to ~1 mL and put in the freezer at - 30 ° C overnight for recrystallization producing small quantities of X - ray quality crystals . 1 H NMR ( 4 00 MHz , C 6 D 6 74.81 ( s, 2H, thf) , 71.79 ( s, 2H, thf) , 36.88 (s, 4H thf), 15.57 (s, 15H, Cp*), 0.98 (s, 1H, Ar), 0.63 (s, 3H, Me), 4.49 (s, 1H, Ar), 9.24 (d, J = 9.7 Hz, 1H, Ar), 18.17 (s, 1H, Ar), 41.09 (s, 3H, Me), 90.42 (d, J = 7.3 Hz, 1H, Ar). Prelimina ry synthesis 2 (N - C) - CH 2 SiMe 2 NTMS) U ( Cp* )( PyPy r Me3 ) ( 6 ): In a 20 mL scintillation vial, UCl 4 (200 mg, 0.527 mmol) was loaded with a stir bar and THF (5 mL). The solution was stirred with a magnetic stir plate. To the stirring solution was added NaCp*(83 mg , 0.527 mmol). The solution rapidly darkened. Next, NaPyPyr Me3 (109 mg, 0.527) was added as a solution in THF (1 mL). Following that, NaHMDS (96 mg, 0.527 mmol) was added as a solid. Finally, NaCH 2 TMS (58 mg, 0.526 mmol) was added as a solution in THF (1 m L). The solution was left to stir for 18 h whe reupon the color had turned from green to dark brown/red. The solution was then dried of the volatiles in vacuo . The solids were extracted with n - hexane (~10 mL) and 263 filtered using Celite as a filter ing agent. The filtrate was concentrated to ~1 mL and put in the freezer at - 30 °C overnight for recrystallization. Crystals grown were of X - ray quality and contained no solvent, however, extra unassignable peaks in the 1 H NMR spectrum suggest in solution 5 m ay coord inate a solvent molecule. Yield 188 mg (50%) 1 H NMR ( 4 00 MHz , C 6 D 6 = 32.71 ( s, 9H) , 21.68 ( s, 3H) , 21.46 (s, 3H), 10.29 (s, 15H, Cp*), 7.43 (s, 1H), 3.04 (s, 1H), 2.87 (s, 1H), 2.12 (s, 1H), 0.13 (s, 1H), 5.79 (br s, 3H), 9.03 (br s, 3H), 1 0.58 (br s, 3H), 33.89 (br), 54.71 (br). Preliminary synthesis 2 (N - C) - CH 2 SiMe 2 NTMS) U ( Cp* )( PyPyr Me2 ) (thf) ( 7 ): In a 20 mL scintillation vial, UCl 4 (200 mg, 0.527 mmol) was loaded with a stir bar and THF (5 mL). The solution was stirred with a magnetic stir plate. To the stirring solution was added NaCp*(83 mg, 0.527 mmol). Th e solution rapidly darkened. Next, NaPyPyr Me2 (102 mg, 0.527) was added as a solution in THF (1 mL). Following that, NaHMDS (96 mg, 0.527 mmol) was added as a solid. Finally, NaCH 2 TMS (58 mg, 0.526 mmol) was added as a solution in THF (1 mL). The solution was left to stir for 18 h whereupon the color had turned from green to dark brown/red. The solution was then dried of the volatiles in vacuo . The solids were extracted with n - hexane (~10 mL) and filtered using Celite as a filter ing agent. The filtrate was concentrated to ~1 mL and put in the freezer at - 30 °C overnight for recrys tallization. Yield 102 mg (25%) 1 H NMR ( 4 00 MHz , C 6 D 6 ): 33.41 ( s, 9H) , 21.76 ( s, 3H) , 21.63 (s, 3H), 10.84 (br s, 15H, Cp*), 7.43 (s, 1H), 2.12 (s, 1H), 2.05 (s, 1H), 1.79 (s, 1H), 1.69 (s, 1H), 0.12 (s, 1H), 0.02 (s, 1H), 5.94 (br, 3H), 10.32 (br, 3H), 10.58 (br s, 3H), 33.25 (br), 54.47 (br). 264 Figure 5 - 7 . Crude 1 H NMR of [N(TMS)Cy] 2 U(PyPyr Me2 ) 2 , 1 . 265 Figure 5 - 8 . Crude 1 H NMR of [N(TMS)Cy] U(PyPyr Me2 ) 3 , 2 . 266 Figure 5 - 9 . Crude 1 H NMR of U(PyPyr Me2 ) 4 . There are still signals present for 2 , but the majority of the N(TMS)Cy amides have been displaced. 267 Figure 5 - 10 . 1 H NMR of Cp*UCl 2 (PyPyr Me3 )(thf) ( 3a ). 268 Figure 5 - 11 . Best 1 H NMR of Cp*U I 2 (PyPyr Me3 )(thf), ( 3 b ) . The spectrum contains impurities from unknown species that are not removed by recrystallization. 269 Figure 5 - 12 . 1 H NMR of Cp*UCl 2 (PyPyr Me 2Tol )(thf), ( 4 a ). 270 Figure 5 - 13 . 1 H NMR of Cp*U I 2 (PyPyr Me 2Tol )(thf) ( 4b ). Toluene 271 Figure 5 - 14 . 1 H NMR of Cp*UI 2 (PyPyr Me2 )(thf) ( 5 ). 272 Figure 5 - 15 . 1 2 (N - C) - CH 2 SiMe 2 NTMS ) U ( Cp* )( PyPyr Me3 ) ( 6 ). 273 Figure 5 - 16 . Crude 1 H NMR of 2 (N - C) - CH 2 SiMe 2 NTMS ) U ( Cp* )( PyPyr Me 2 ) ( 7 ). 274 Figure 5 - 17 . Arrayed Spectra from the catalysis of 6 ( 10 mol%, 65 °C, C 6 D 6 ) with DPAP. Each spectrum was taken in 10 - minute intervals. The reaction appears to be complete after ~ 150 minutes (spectrum 15 ). 275 Figure 5 - 18 . Arrayed Spectra from the catalysis of 6 (5 mol%, 65 °C, C 6 D 6 ) with DPAP. Each spectrum was taken in 10 - minute interval s. The reaction appears to be complete after ~ 330 minutes (s pectrum 33 ). 276 Figure 5 - 19 . Arrayed Spectra from the catalysis of 7 (5 mol%, 65 °C, C 6 D 6 ) with DPAP. Each spectrum w as taken in 10 - minute intervals. The reaction appears to be complete after ~190 minutes (spectrum 19). 277 REFERENCES 278 REFERENCES 1. Economic Assessment of Used Nuclear Fuel Management in the United States. 2006 . 2. Lewis, A. J.; Carroll, P. J.; Schelter, E. J., J. Am. Chem. Soc. 2013, 135 (35), 13 185 - 13192. 3. Billow, B. S.; McDaniel, T. J.; Odom, A. L., Nat. Chem. 2017, 9 , 837 - 842 . 4. Pepper, M.; Bursten, B. E., Chem. Rev. 1991, 91 (5), 719 - 741. 5. Allen, P. G.; Shuh, D. K.; Bucher, J. J.; Edelstein, N. M.; Reich, T.; Denecke, M. A.; Nitsche, H., Inorg. Chem. 1996, 35 (3), 784 - 787. 6. Denecke, M. A., Coord. Chem. Rev. 2006, 250 (7), 73 0 - 754. 7. Graves, C. R.; Yang, P.; Kozimor, S. A.; Vaughn, A. E.; Clark, D. L.; Conradson, S. D.; Schelter, E. J.; Scott, B. L.; Thompson, J. D.; Hay, P. J.; Morris, D. E.; Kiplinger, J. L., J. Am. Chem. Soc. 2008, 130 (15), 5272 - 5285. 8. 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Clegg, W.; Conway, B.; Kennedy, A. R.; Klett, J.; Mulvey, R. E.; Russo, L., Eur. J. of Inorg. Chem. 2011, 2011 (5), 721 - 726. 34. Mo tolko, K. S. A.; Emslie, D. J. H.; Jenkins, H. A., Organometallics 2017, 36 (8), 1601 - 1608. 280 Synthesis and Characterization of Uranium - Terphenyl Complexes 6.1 Investigating Imido Synthesis As was mentioned briefly in chapter 5, we were attempting experiment s aimed at isolating uranium(IV) monoimido compounds . More specifically, w e were attempting diisopropylphenyl (DiPP) imido synthesis using azide and KC 8 , analogous to th e reports recently published by the Bart group. 1 - 2 Starting from a THF solution of ( Cp* ) U (I) 2 (thf) 3 , where Cp* = pentamethylcyclopentadiene, N 3 DiPP was added (Scheme 6 - 1) . Scheme 6 - 1 . Reaction of ( Cp* ) U ( I ) 2 (thf) 3 with DiPPN 3 . In contrast to reports from the Bart group, where U ( I ) 3 (thf) 4 and azide can be added together without reaction , when azide is added to (Cp*) U ( I ) 2 (thf) 3 , immediate, vigorous bubbling occurs. We expected formation of a U(V) imido compound since U(III ) was reacting with just N 3 DiPP . Surprisin gly, crystals isolated from the reaction mixture were identified by X - ray di ffraction as (Cp*) U ( I ) 3 (thf) 2 ( 1 ). Presumably, the strongly electron donating Cp* makes the U(III) metal center reducing enough to react w ith the azide without the addition of KC 8 . The compounds should have reacted to form a U(V) imido, but perhaps the in stability of the product caused disproportionat ion into a mixture of compounds. Unfortunately, no other products were isolable. 281 This react ion made us wonder if it would be possible to stabilize the U(V) imido ([ Int ] in Scheme 6 - 1) using sterically crowded ligands. We proposed that if the aryl azide were much larger, the intermediate, [ Int ], of the reaction shown in Scheme 6 - 1 might be isolab le. Toward this end, we began investigations using the extremely bulky terphenyl ligands. 3 There are surprisingly few reported compounds of uranium with terphenyl substituents. 4 - 6 However, as will be discussed in detail below, these terphenyl ligands proved to be ideal ligands for uranium, and provided access to some exciting low valent uranium species. 6.2 Steric Bulk as a Method to Slow Ligand Redistr ibution Figure 6 - 1 . Single crystal structure of (Cp*) UI ( NAr iPr6 ) ( 2 ) . Solvent molecules and hydrogens removed for clarity. Following the same procedure in Scheme 6 - 1, when 2,6 - bis(2,4,6 - triisopropylphenyl) - phenyl azide (N 3 Ar i Pr6 ) was added to a THF solution of (Cp*) U ( I ) 2 (thf) 3 (Scheme 6 - 2, top), the reaction mixture immediately bubbles and changes color from turquoise to a deep red. Upon crystallization of the product, deep red colored blocks were obtained. Single crystal X - ray diffraction analysis of these crystals identified the product as (Cp*) UI ( NAr i Pr6 ) ( 2 ), Figure 6 - 1. 282 Scheme 6 - 2 . Top. Discovery of (Cp*) UI ( NAr i Pr ) ( 2 ) f rom (Cp*)U(I) 2 (thf) 3 and N 3 Ar iPr6 disproportionation. Middle. Rational synthesis of 2 . Bottom. Synthesis of (Cp*) UI ( N H Ar i Pr6 ) ( 3 ) from (Cp*)U(I) 2 (thf) 3 and NaNAr iPr6 Unfortunately, the bulky azide was only partially successful at stopping the disproportion ation. Electron transfer reaction s must have still occurred since the oxidation state of the uranium center in 2 is U(IV). But, in contrast to the reactions with N 3 DiPP from Scheme 6 - 1, the ligand stoichiometry was retained. Rational synthesis of 2 was ach ieved by adding an equivalent of KC 8 to (Cp*) U ( I ) 2 (thf) 3 before addition of the N 3 Ar i Pr6 . Using this procedure, the U(IV) imido complex, 2 , could be isolated in reasonable yields (Scheme 6 - 2, middle) . 283 In the crystal structure, 2 has a pronounced 6 - interac tion between the ortho - arene substituent of the Ar i Pr6 and uranium. This interaction gives the molecule a geometry similar to classic (Cp*) 2 U ( X ) 2 compounds. 7 Because of t he 6 - arene interaction , the U - N - C bond angle of the imido is uncharacteristically bent at 145 .0(3) ° . 1 - 2, 8 - 9 The N - U bond length of 1.9 77(4) Å, however, is consistent with the assignment of N1 as an imido. UV - vis/NIR spectroscopy is also consistent with assignment of 2 as a U(IV) imido. It has long been established th at uranium species can be stabilized through interaction with arenes. 10 - 19 Given the definite a rene interaction in compound 2 , we postulated the steric bulk and arene capping ability of the Ar iPr6 ligand may lead to stabilization of some exciting ura nium species. Specifically, w e wondered if the se bulky nitrogen - based ligands presented here would al low access to low oxidation state uranium species. 6.3 Comparisons Between Amide and Imides of Bulky Ligands In order to compare 2 to an analogous uranium Ar i Pr6 amide as well as producing uranium compounds in lower oxidation states , the synthesis of (Cp*) UI ( NHAr i Pr6 ) ( 3 ) was pursued. Simple precipitation of NaI by reacting (Cp*) U ( I ) 2 (thf) 3 and NaNHAr i Pr6 in THF results in a color change from turquoise to black , producing 3 ( Scheme 6 - 2 , bottom). In the crystal structure of 3 (Figure 6 - 2) , the U - N - C bond is substantially more bent at 132.9 (2) °, and the U - N bond is significantly longer, 2.2 77(3) Å compared to 1.9 77(4) Å in the imido. Compound 3 displays a similar 6 - interaction between the ortho - arene substituent of the Ar i Pr6 and the uranium atom. Aside from the bond length differences, the crystal structures of the two molecules are quite similar, and unit cell dimensions indicate the two molecules are isostructu ral despite the difference in formal oxidation state. Interestingly, the 6 - arene in 3 has a dista nce from uranium to the centroid of the arene of 2. 482(2) Å, compared to the U - arene interaction in 2 which is substantially longer, at 2. 594(1) Å. 284 While ther e are a number of possible explanations for this phenomenon, to us, this suggests a stronger back bonding interaction from U(III) to the arene. 15, 19 Figure 6 - 2 . S ingle crystal structure of (Cp*)UI(NHAr iPr6 ) ( 3 ). Hydrogens (except NH) and solvent removed for clarity. Initial reactivity studies of both 2 and 3 show that the Ar i Pr6 substituent confers a r emarkable degree of kinetic stability to these molecules , which could be ideal to access low valent uranium . Attempts at reduction of 2 using KC 8 and decamethylcobaltacene did result in color change of the solution s , but no clean products were isolable ; 1 H NMR indicated messy reactions, and the only isolable crystals o f X - ray quality were the starting material, 2 . Likewise, e ven simple deprotonations of the 3 were unsuccessful with a number of bases, presumably due to the extraordinary steric bulk of the A r i Pr6 substituent. These results were promising insomuch that the terphenyl group may stabilize what would normally be very reactive uranium species, but, ironically, those same groups were also hindering our ability to make the reactive species in the fir st place. 285 Figure 6 - 3 . Single crystal structure of (Cp*)UI(NHAr Me6 ) ( 5 ) . Hydrogens (except NH) removed for clarity. We postulated that perhaps reducing the steric bulk of the ligand somewhat but maintaining an 6 - arene interaction might allow for a balance between ligand redistribution reactivity, stability of the molecules, and subsequent reactivity. By exchanging the Ar i Pr6 group for a smaller terphenyl group, NAr Me6 , we reduced steric bulk on the ligand, but retained 2,6 - arene substituents available for 6 - arene intera ctions with the metal center. Syntheses similar to those for 2 and 3 produced the analogous (Cp*)UI(NAr Me6 ) ( 4 ) and (Cp*)UI(NHAr Me6 ) ( 5 ) Ar Me6 compounds (Scheme 6 - 3) . 286 Scheme 6 - 3 . Synthesis of (Cp*)UI(NAr Me6 ) ( 4 ) and (Cp*)UI(NHAr Me6 ) ( 5 ) . As expected, 5 , shown in Figure 6 - 3, has the same basic geometry as 2 and 3 with a U - N - C bond angle of 132.4 (3) ° and a U - N bond length of 2.3 15(4) Å. The centroid of the arene in 5 is 2.50 1(3 ) Å from the uranium center, almost identical to 3 . This suggests similar uranium - arene interaction. In contrast, compound 4 , shown in Figure 6 - 4, has no 6 - arene interaction. Instead, two THF molecules are bound to the uranium and the U - N - C angle is 16 9 . 3(2) °, an angle much more typical of uranium imidos. Still, the U - N bond length of 4 is typical of similar to 2 at 2.006(3) Å . In an effort to encourage an 6 - arene interaction and geometry analogous to 2 , attempts were made to synthesize the adduct - free a nalogue of 4 by performing the reaction in solvents other than THF . When the reaction was performed in diethyl ether, 4 was still the primary product. Seemingly, the THF molecules bound to the starting (Cp*) U ( I ) 2 (thf) 3 are never displaced. 287 Figure 6 - 4 . Single crystal structure of (Cp*)UI(NAr Me6 )(thf) 2 , 4 . Hydrogens (except NH) removed for clarity. In an effort to test the reactivity of the m - terphenyl amides, we attempted deprotonation of 5 usin g (trimethy lsilyl)methyl sodium. Instead of the desired U(III) imido, we isolated (Cp*) U(NHAr Mes2 ) 2 ( 6 ). 6 can be generated through a more rational route by adding two equivalents of NaNHAr Mes2 to a stirring solution of (Cp*) U ( I ) 2 (thf) 3 in THF (Scheme 6 - 4) , but the r eaction always contains impurities of free H 2 NAr Me6 , among other things, which are not removed by recrystallization . 288 Scheme 6 - 4 . Synthesis of (Cp*)U(NHAr Me6 ) 2 ( 6 ). Interesti ngly, in the crystal structure of 6 (Figure 6 - 5) , the two amide ligands are inequivalent. The U - N1 - C bend is similar to the other U(III) amides, 3 and 5 , at 132.1 (2) o . Compound 6 also has an 6 - mesityl group bound to the uran ium center making the coordinat ion sphere of 6 similar to the above compounds 2 , 3 , and 5 . Here the centroid of the 6 - mesityl is again at a distance of 2. 497(3) Å, identical to the 6 - arene - U distances in 3 and 5 . This observation is surprising considering the large difference between an ancillary iodide in 3 and 5 and the ancillary m - terphenyl amide in 6 . The N2 amide of 6 , however, is surprisingly linear , likely as a result of the steric hindrance around the ura nium center with a U - N2 - C angle of 162.4 (2) . Despite this linear bond angl e, the bond length of 2.3 36(3) Å suggests this nitrogen is still a monoanionic amide ligand. Additionally, UV - vis/NIR spectroscopy of a crude sample of 6 is consistent with the assig nment of the metal center as a U(III), supporting the identity of both N1 and N2 as mono - anionic amide ligands. 20 Still, since analytically pure 6 could not be generated, these as sertions should not be considered definite. 289 Figure 6 - 5 . Single crystal structure of (Cp*) U(NHAr Me6 ) 2 , 6 . Hydrogens (except NH) removed for clarity. Nevertheless, discovery of 6 was very unexpected. We kne w the ionic radius of uranium is large and can accommodate very bulky ligands; still we did not expect that two terphenyl groups could fit. This realization led to an idea, if we could fit two terphenyl groups around uranium, we might also be able to encou rage a bis( 6 - arene) - U complex with two amides. This way, we would produce a coordinatively saturated (formally 8 coordinate) uranium species with only two anionic ligands. 290 6.4 Bis(Amide) Species as a Way to Access Low Valent Uranium Figure 6 - 6 . Top : Single crystal structure of 7 . Hydrogens (except NH removed for clarity. Bottom : Synthesis of UI(NHAr Me6 ) 2 7 . A ddition of two equivalents of NaNHAr Mes2 to UI 3 (thf) 4 in THF give s U I (NHAr Mes2 ) 2 ( 7 ), in decent yield (Figure 6 - 6) . The single crystal X - ray diffraction structure of 7 displays the postulated bis( 6 - arene) coordination. Unlike the analogous ( Cp* ) 2 complexes, the 6 - arene substituents adopt a more classical sandwich stru cture, i.e. , the centroid - U - centroid angle is 174.5 (1) °. As a result of the orientation of the ligands, 7 is almost C 2 symmetric through the U - I bond vector. In the solid state the two amides are slightly inequivalent. The bond length of N1 - U1 is 2.3 16(6) Å compared to the N2 - U2 length of 2.3 49(6) Å. The U - N - C bond angles, though, are identical at 291 136.5 (5) °. The largest disparity between the two amides is in the uranium - arene distances. Despite the slightly longer N - U bond, the 6 - mesityl group from N2 is ~ 0.08 Å closer to uranium than the arene from N1, 2. 754 (3) Å compared to 2.8 25(3) Å. The 1 H NMR spectrum of 7 displays very few well - resolved signals. Many of the signals observed are very broad, likely due to fluctuation in the U - arene bonds. This could be consistent with the inequivalent bond lengths to the arenes of N1 and N2 in the solid state. By performing a reaction analogous to that shown in Figure 6 - 6 with the bulkier Ar iPr6 amide, we were able to produce the analogous bis - (Ar iPr6 - arene) complex. R eacti on of UI 3 (thf) 4 with two equivalents of NaNHAr i Pr 6 in THF overnight, produces UI(NHAr i Pr6 ) 2 ( 8 ) (Figure 6 - 7, Bottom ) . I n the crystal structure of 8 , the 6 - arene substituents are farther from the metal than in 7 at 2.788(1) Å and 2.897(1) Å for the ar enes coordinated to N1 and N2, respectively. This is likely a result of the increased steric bulk of the isopropyl groups on the arene substituents in 8 relative to 7 . Additionally, the ligands are arranged in a fashion typical of U ( X ) 2 (Cp*) 2 compounds, wi th the centroids ma king an angle of 158. 78(2) ° with respect to uranium . T his is likely a result of the closely packed arrangement of isopropyl groups surrounding the iodide ligand. The N - U bond distances are similar to 7 with both N1 - U and N2 - U at 2.354(2) Å and 2.365(2) Å. The U - N - C bond angles are slightly more obtuse than 7 at 138. 7(2) ° and 139.5 (2) ° for N1 and N2, respectively, which is expected given the greater arene - U distances. In the solid state, 8 appears C 2 symmetric with the rotational axis alon g the U - I bond vect or ( Figure 6 - 7 ). The two amide nitrogens and the iodide constitute an equatorial plane with the sum of angles totaling 359.88(5)°. Additionally, one triisopropylphenyl group from each amide substituent is 6 - arene capped in an axial posi tion. The U - N - C bond angles and U - N bond lengths 292 are all consistent with monoanionic amide ligands. Furthermore, the quality of the data obtained in the crystal structure allowed location and refinement of the N - H hydrogen pos itions. Figure 6 - 7 . Top : Crystal structure of U I (NHAr iPr6 ) 2 8 . Hydrogens ( excepts N - ) and solvent molecule removed for clarity. Bottom : Synthesis of (I)U(NHAr iPr6 ) 2 ( 8 ). Surprisingly, t he structure of 8 varies somewhat depending on the solvent of crystallization. As mentioned above, w hen crystallized from diethyl ether, 8 gives a solid - state structure with a molecule of ether in the asymmetric unit, and the distance from the metal center to arene cen troids are significantly different, at 2.788(1) Å and 2.897(1) Å , similar to what was s een in 7 . However, when the crystals are grown from concentrated n - hexane and a molecule of hexane appear s in the 293 asymmetric unit, the cell changes, and the U - centroid d istances are similar at 2.790(1) Å and 2.776(1) Å. The U - C bonds to the arene are also different by a significant amount from an average of 3.120(2) Å and 3.217(2) Å to 3.122(4) Å and 3.111(4) Å. This phenomenon is consistent with weak U - arene interactions , as small packing forces with the different solvent molecules are apparently enough to disrupt the structure slightly, see Figure 6 - 8. Figure 6 - 8 . Contrasting solid state structures of 8 when crystallized form diethyl ether (green) and n - hexane (light blue). The weak interaction is confirmed in the solution state. Ambient temperature 1 H NMR only shows signals for the solvent that crystallizes in the lattice, a singlet at 12.97 ppm, a broad singlet at 9.1 pp m, and a very broad signal from ~6 to 10 ppm. The signals are even less resolved than in the NMR of 7 . However, on cooling the solution to 30 °C, the expected number of signals for the C 2 symmetric molecule become distinguishable between 87 ppm and 82 p pm. As we 294 suspected, t his behavior is consistent with fluxionality of the arene substit uents resulting from a weak uranium - arene interaction. Compound 8 was also investigated with absorption spectroscopy in the visible and near - infrared. A large absorptio n in the visible region along with broad but distinct f - f transitions in the near - IR ar e consistent with this assignment of 8 as a U(III) species ( vide infra ) . 20 6.5 Generation of a Neutral Uranium(II) Scheme 6 - 5 . Synthesis of U(NHAr iPr6 ) 2 ( 9 ) from 8 . Common techniques for abstracti on of iodide from 8 , such as precipitation by silver or sodium salts, were fruitless. However, reduction of 8 with excess KC 8 in THF ( Scheme 6 - 5 ) results in pr ecipitation of KI and graphite with generation of green U(NHAr iPr6 ) 2 ( 9 ). This is in contrast to the reactions performed with (Cp*) UI ( NAr i Pr6 ) ( 2 ) , where no tractable products could be obtained. Compound 9 was the first example of a neutral uranium organometallic species in the +2 - oxidation state. 21 - 24 Compoun d 9 was also the third U(II) moiety. This is particularly significant due to reported ly differing valence electron configurations in the original two studies . In the tris(cyclopentadienyl) systems reported by the Evans group, the geom etry of the complex en forces a 5 f 3 d 1 electron configuration. 24 In the tris(aryloxide) - arene system reported by Meyer, a 5 f 4 6 d 0 electronic c onfiguration was deter mined. 19 Compound 9 is essentially set up to be a tie breaker to confirm the more commo n valence electron configuration of U(II). 295 Figure 6 - 9 . Crystal structure of U(NHAr iPr6 ) 2 ( 9 ). Hydrogens, except N - clarity. The structure is grown to show the full molec ule , but 9 crystallizes as half of the molecule with a 2 - fold rotational axis. Fortunately, the structural data of 9 was also high enough quality to locate and refine the N - H hydrogen. This combined with, the U - N bond of 2.330(2) Å and U - N - C bond ang le of 130.2(2)° are consistent with the assignment of N1 as a mono anionic amide ligand. The geometry of 8 ( Figure 6 - 7 ) and 9 ( Figure 6 - 9 ) vary significantly in the solid - state structures. The U(II) compound 9 crystallizes as a C 2 symmetric molecule with only half of the molecule occupying the asymmetric unit. Upon removal of iodide and formal reduction of the complex, the U - centroid bond distances shortened significantly, from 2.843(1) Å (avg.) in 8 to 2.405(1) Å in 9 . The contrac tion is consistent with i ncreased backbonding interactions between a formally reduced metal center and the arene. An alternative explanation involves increased ionic interaction between a reduced arene and the metal center; however, the data presented here are more consistent with a U(II) species interacting with a neutral arene ( vide infra ). Indeed, analysis of the C - C bond lengths in the coordinated arene of 9 shows an avg. C - C bond length of 1.415(4) Å. This is essentially unchanged from both 8 (avg. C - C 1.402(5) Å) and from fre e H 2 NAr i Pr6 . 25 296 Also noteworthy in the com parison of the solid - stat e structures of 8 and 9 are the differences in arene - U - arene angles. Going from 8 to 9 , the centroid - U - centroid angle decreases from 158.785(2)° to 134.240(9)°, respectively. Additionally, the N - U - N angle decreases from 149.92(7)° to 99.22(11)°. The 1 H NMR of 9 displays sharp signals at room temperature, confirming stronger interaction in the U - arene bonds relative to 8 . Assignment of the NMR spectrum of 9 was not possible due to the paramagnetic shifts, but the number of peaks is c onsistent with a C 2 symme tric molecule with static U arene bonds on the NMR timescale and diastereotopic methyl groups in the iso - propyl groups, consistent with the solid - state structure. Absorption spectroscopy in the visible to near - IR on 9 in diethyl et her is mostly featureless except for strong absorption at 400 nm and a very broad absorption at ~600 nm. The spectrum of 9 is similar to that observed by the Meyer group in their report of a uranium(II) species. 19 Compared to previous reports of U(II) species, 9 is more robust. 1 9, 24 The room temperature synthesis and characterization of 9 display reasonable thermal stability, for instance, the visible to near - IR absorption spectrum was taken by serial dilution over ~5 h at room temperature with no signs of decomposition. Exten ded storage of 9 in the glovebox freezer ( - 35 °C) is also possible, provided the sample is pure. 297 6.6 Oxidation of the Uranium(II) Scheme 6 - 6 . Synthesis of [U(NHAr iPr6 ) 2 ][BAr F 24 ] ( 1 0 ) from 9 . Reaction of 9 in ether with [FeCp 2 ][BAr F 24 ] (Scheme 6 - 6) results in a rapid color change from green to brown. After removal of volatiles and a hexane wash, [U(NHAr iPr6 ) 2 ][BAr F 24 ] ( 1 0 ) can be isolated. Recrystallization from concentra ted ether results in diffraction quality crystals of 1 0 . Figure 6 - 10 . Top : Crystal structure of [U(NHAr iPr6 ) 2 ][BAr F 24 ] ( 10 ) Hydrogens, except N - solvent molecule removed for clarity. Bottom : Space filling structures of 8 (left) an d 10 (right). 298 Structurally ( Figure 6 - 10 Top ), 1 0 is between 8 and 9 . The distance between uranium and the 6 - arene centroid s are 2.570(3) and 2.583(3) Å. The centroid - U - centroid and N - U - N angles also falls between those of 8 and 9 at 145.8(1)° and 111.2(2)°, respectively. The uranium arene distances in 1 0 confirm that bot h the large radius of the iodide and the bulk of the Ar i Pr6 groups play a significant role in the large change in uranium - arene distance s between compounds 8 to 1 0 ( Figure 6 - 10 Bottom ). The 1 H NMR spectrum of 1 0 , contrary to 8 , shows the expected resonanc es at room temperature. It is worth mentioning, however, that the solubility of 1 0 required the use of THF - d 8 , and, over the course of extended experiments, the compound reacts with this solvent. Figure 6 - 11 . EPR spectra of (a) ( I ) U(NHAr iPr6 ) 2 ( 8 ) and (b) [U(NHAr iPr6 ) 2 ][BAr F 24 ] ( 1 0 ). Measurement parameters for both spectra were: microwave frequency, 9.40 GHz, microwave power, 0.79 mW; field modulation amplitude, 1 mT; and sampl e temperature, 6 K. Dissolvin g 1 0 in THF at room temperature results in an extremely viscous liquid, this is not altogether unexpected. Surprisingly though, dissolving 1 0 in toluene results in a color change from 299 brown to bright red. The cause of the color change is unclear; however, attempts at crystallization of the products did yield colorless crystals containing the BAr F 24 anion with an unresolvable cation, as well as crystals of free H 2 NAr iPr6 . 6.7 Spectroscopic Analysis The X - band EPR spectrum of 8 collected at 6 K (Figure 6 - 11a) sh ows well - resolved peaks at g = 5.17 and g = 4.56. These were the only peaks resolved over a field range that extended from 50 - 850 mT (g = 13 to g = 0.8). While broad features at higher fields w ere observed in our spectra (Figure 6 - 11a), they could not be d istinguished from typical baseline distortions that remain after subtracting background contributions. No EPR response was detected for 9 in either perpendicular or parallel detection modes at 6 K. Absence of a signal here supports our assignment of 9 as a U(II) species with neutral arene substituents . The EPR spectrum of 10 collected at 6 K is provided in Figure 6 - 11 and shows a broad peak at g = 4.3 with a shoulder at g = 3.6. A resonance wit h a narrower line shape was also resolved at g = 2.003. The res onance at g = 2.003 is unusual because its narrow line shape makes it unlikely that it arises from the U(III) paramagnetic center. This signal was observed in three separate preparations of 10 that were carried out in two different solvents. It is possible that oxidation of 9 resulted in a species best described as a U(III) center, but with a small contribution of U(II) and ligand radical, or a small contribution from solvent radical. Solution state magnetic susceptibility studies were carried out on 9 usi ng the Evans method. 30 The resulting temperature dependent paramagnetism was determined in 10 K intervals from 299 - 219 K in toluene - d 8 with hexamethyldisiloxane a s a reference. At room temperature the effective 300 magnetic momen t value of 9 is 0.78 cm 3 Kmol - 1 ( µ eff = 2.50), which decreases slightly to 0.72 cm 3 Kmol 1 ( µ eff = 2.40) at 219 K. Solid - state magnetic properties of 8 - 10 were also probed by SQUID magnetometry ( Figure 6 - 5). 21, 23 F ocusing on the divalent species, 9 , at 300 K M T = 0.67 cm 3 K mol 1 ( µ eff = 2.32). Upon decreasing the temperat ure, the M T value decreases monotonically until ~100 K, where the decrease becomes more dramatic; at 2 K, M T = 0 cm 3 K mol 1 ( µ eff = 0). The temperature dependent profile of 9 tracks lower in the solid state than in solution. Differences observed between solution and solid - state behavior are common given the very different environments. For the one other U(II) species where magnetic studies were done in solution and the solid state, the sol id state susceptibilities also tracked lower than the solution val ues. 21 The solid - state behavior is similar to what was observed for previously published U(II) complexes, although the downturns for the other U(II) complexes are observed at lower temperatures (~15 - 20 K) compared to 9 . 21, 23 Similar to the other two complexes reported previously, the magnetic susceptib ility approaches 0 cm 3 K mol 1 ( µ eff = 0) at low temperature, suggestive of an integer spin system where spin - orbit coupling leads to a ground state singlet. This agrees with the absence of an EPR signal in 9 . While not diagnostic , the temperature profile for 9 is qualitatively similar to Me f 4 6 d 0 electronic configuration. 21 301 Figure 6 - 12 . Temperature dependence of the magnetic susceptibility for UI(NHAr iPr6 ) 2 ( 8 ) (black squares), U(NHAr iPr6 ) 2 ( 9 ) (red circles), and [U(NHAr iPr6 ) 2 ][BAr F 24 ] ( 1 0 ) (blue diamonds) collected at 5000 Oe. 6.8 Conclusions In summary, the terphenyl groups proved to be ideal ligand partners for low valent uranium . T he Ar iPr6 ligands allowed isolation of a neutral U(II) complex , U(NHAr iPr6 ) 2 ( 9 ), where the large arenes act as 6 - donors towards the metal center . Analysis of 9 by visible - NIR absorption spectroscopy, SQUID magnetometry, and EPR spectroscopy are consistent with a 5 f 4 6 d 0 electron configura tion. EPR spectroscopy and magnetometry studies clearly display integer spin p roperties expected from a U(II) center. This suggests that the three - fold symmetrical tris(cyclopentadienyl) coordination environments employed roup may impart the unexpected 5 f 3 6 d 1 electronic configuration in U(cyclopentadienyl) 3 . 22 - 24 Additionally, we synthesized a series of U(IV), U(III), and U(II) arene complexes. The uranium(II) complex has metrical parameters consistent with much stronger U - arene interactions than the U(III) complexes, which, l ikewise, are stronger than the U(IV) arene interactions. This is presumably due to stronger metal - arene backbonding in the lower oxidation state complexes. Hopefully, the complexes presented here will pave the way to new and exciting low valent uranium rea ctivity. 6.9 Exp erimental 302 All reactions and manipulations were carried out in an MBraun glovebox under a nitrogen atmosphere and/or using standard Schle nk techniques. Diethyl ether , pentane, tetrahydrofuran, and hexane were purc hased from Aldrich Chemical Comp any. Diethyl ether and pentane were purified by passing through alumina columns to remove water after being sparged with dry nitrogen to remove oxygen. Tetrahydrofuran and hexane were sparged with dinitrogen to remove oxygen and distilled from sodium and b enzophenone. C 6 D 6 , THF - d 8 , and t oluene - d 8 were purchased from Cambridge Isotopes Laboratories, Inc or Aldrich Chemical Company . Toluene - d 8 and THF - d 8 were sparged with dry dinitrogen and dried over 4 Å molecular sieves . Before use, each solvent was passed through a plug of activated alumina to filter the solvent and to ensure dryness . C 6 D 6 was sparged with dry dinitrogen and distilled from CaH 2 before use. All NMR solvents were stored under an inert atmosphere away from light . Depleted uranium turnings were purchased from Manufacturing Sciences Corporation. Synthesis of UI 3 (THF) 4 , KC 8 , IAr iPr6 , N 3 Ar iPr6 , H 2 Ar iPr6 , and [FeCp 2 ] + BAr F 24 was done according to the literature procedures. 3, 31 - 33 H 2 Ar iPr6 was deprotonated in hexane with stoichiometric NaCH 2 TMS over 16 h. The reaction generates an off - white precipitate which can be collected by filtration. The precipitate was washed with several aliquots of hexane and used as is. Elemental Analysis was performed by Atlantic Microlab in Norcross, GA using a He filled glovebag to handle the compounds. Synthesis of Compounds Caution! Depleted uranium (primary isotope 238 U) is a w - emitter (4.197 MeV) with a half - l ife of 4.47 x 10 9 years; manipulations and reactions should be carried out in monitored fume - - counting equipment 303 Oxide - Free Uranium Metal Turnings : Cautionary Note: Uranium is pyrophoric when finely divided; caution is recommended in the washing process to avoid exposure to air . This is a modification of the literature procedure. 31 Depleted uranium turnings were received in mineral oil from Manufacturing Sciences Corporation. The turnings (~10 g) were carefully transferred to a 500 mL side armed flask which was fitted with a hose flowing dry dinitrogen or dry argon gas. The turnings were washed with hexanes (3 x 150 mL), acetone (3 x 150 mL) , then water (3 x 150 mL). The flask was then filled with enough water to fully cover the turnings (~100 mL). Concentrated nitric acid was then added by pipette while gently swirling the flask until removal of the black oxide layer was achieved and the tur nings were becoming metallic in color. The amount of conc entrated acid necessary can vary significantly depending on the quality of turnings, but typically ~20 mL is sufficient. Cleaning of the oxide layer can be accompanied by warming of the solution as w ell as NO 2 gas generation. Once the turnings appear shiny and metallic, the acid solution is carefully decanted. The turnings are then washed again with water (3 x 150 mL) and acetone (3 x 150 mL). After the final wash, the turnings were dried under reduce d pressure and transferred to the drybox. Synthesis of (C p*) UI ( NAr iPr6 ) ( 2 ) : A 20 mL scintillation vial was loaded with (Cp*) U ( I ) 2 (thf) 3 (300 mg, 0.356mmol), 4 mL THF, and a magnetic stir bar. The solution was stirred vigorously. To the stirring solution w as added KC 8 (48 mg, 0.356 mmol) as a solid in small port ions. The solution immediately darkened in color. Next, N 3 Ar iPr6 (187 mg, 0.356 mmol) was added as a solid in small portions, causing immediate, vigorous bubbling, and turning the solution a dark red color. The reaction was loosely capped and left to stir for approximately 30 minutes before being dried of the volatiles under reduced pressure. The remaining residue was extracted with approximately 5 mL n - hexane. The extracts were filtered through Celit e and the filtrate concentrated to a volume of 304 approximat ely 1 mL. The deep red solution was chilled in the freezer at - 30 ° C overnight yielding small dark red X - ray quality crystals ( 270 mg, 76 %). Additional crops of crystals can be obtained by concentrat ing the mother liquor further and chilling. Elemental Ana lysis Calculated C 46 H 64 IN 2 U (0.5 C 6 H 14 from X - ray) : C 55.47(56.59), H 6.48(6.98), N 1.41(1.35); Found C 56.22, H 7.06, N 0.94. 1 H NMR ( 4 00 MHz , 25 °C , C 6 D 6 17.04 ( br s) , 16.11 (br s), 14.38 (br s), 12.62 (br s), 10.10 (s), 7.92 (s), 4.30 (br), 3.24 (br), 3.00 (s), 2.84 (s), 1.26 (s), 0.46 (s), 1. 21 (s), 3.22 (s), 3.79 (s), 7.14 (s), 8.57 (br), 15.47 (br), 20.48 (br s), 34.14 (br s), 51.20 (br s), 59.44 (br). Synthesis of (Cp*) UI ( NHAr iPr6 ) ( 3 ) : A 20 mL scintillation vial was loaded with (Cp*) U ( I ) 2 (thf) 3 (300 mg, 0.356mmol), 3 mL THF, and a magnetic stir bar. The solution was stirred vigorously. To the stirring solution was added NaNHAr iPr6 (18 5 mg, 0.356 mmol) in THF dropwise. The solution slowly darkened in color to black. The solution was left to stir for approximately 6 h before being dr ied of volatiles under reduced pressure. The remaining dark residue was extracted with approximately 4 mL n - hexane. The extracts were filtered through Celite and concentrated to a volume of approximately 1 mL. The concentrated solution was put in the freezer and chilled at - 30 ° C overnight, yielding dark black X - ray quality crystals (171mg, 54%) Elemental Anal y sis Calculated C 46 H 65 INU: C 55.42, H 6.57, N 1.40; Found C 54.24, H 6.57, N 1.20. 1 H NMR ( 4 00 MHz , 25 °C, C 6 D 6 62.13 ( br) , 51.90 (br d), 29.09 (br s), 27.20 (s), 17.35 (br s), 14.30 (br s), 12.78 (br), 9.88 (br), 8.36 (br), 6.93 (s), 2.26 (br), 2.87 (br), 4.76 (br), 7.99 (br), 21.84 (br), 37.51 (br), 74.50 (br), 100.56 (br). Synthesis of (Cp*) UI ( NAr Me 6 ) (thf) 2 ( 4 ) : A 20 mL scintillation vial was loaded with (Cp*) U ( I ) 2 (thf) 3 (300 mg, 0.356mmol), 4 mL THF, and a magnetic stir bar. The solution was stirred vigorously. To the stirring solution was added KC 8 (48 mg, 0.356 mmol) as a solid in small 305 portions. The solution immediately darkened in color. Next, N 3 Ar Me6 (127 mg, 0.356 mmol) was added as a solid in small portions, causing immediate, vigorous bubbling, and turning the solution a dark red color. The reaction was loosely capped and left to stir for app roximately 30 minutes before being dried of the volatiles under reduced p ressure. The remaining residue was extracted with a 1:1 mixture of n - hexane: diethyl ether. The extracts were filtered through Celite and the volatiles were removed once again. The re sidue was dissolved in a minimal amount of diethyl ether, approximately 1 mL, and chilled in the freezer at - 30 ° C overnight for recrystallization yielding deep red X - ray quality crystals (144 mg, 41.7%). Elemental Analysis Calculated C 42 H 56 IN O 2 U : C 51.91, H 5.81, N 1.41; Found C 49.98, H 5.77, N 1.41. 1 H NMR ( 4 00 MHz , 25 °C, C 6 D 6 48.03 (br), 24.86 (br), 13.40 (s), 12. 83 (s), 5.83 (s), 2.17 ( m ), 2.91 (s), 3.60 (s), 7.44 ( br). Synthesis of (Cp*) U ( I )( NHAr Me6 ) ( 5 ) : A 20 mL scintillation vial was loade d with (Cp*) U ( I ) 2 (thf) 3 (300 mg, 0.356mmol), 3 mL THF, and a magnetic stir bar. The solution was stirred vigorously. To the stirring solution was added a solution of NaNHAr Me6 (185 mg, 0.356 mmol) in THF dropwise. The solution slowly darkened in color to a black solution. The so lution was left to stir for approximately 6 h before being dried of volatiles under reduced pressure. The remaining dark residue was extracted with approximately 4 mL of 1:1 n - hexane: diethyl ether. The extracts were filtered through Celite and again, the volatiles were removed. The residue was dissolved in a minimal amount of diethyl ether, approximately 1 mL, and chilled in the freezer at - 30 ° C overnight for recrystallization yielding dark black X - ray quality crystals (160 mg, 54.3 %). Analytically pure s ample could not be obtained. Crude 1 H NMR (400 MHz, 25 °C, C 6 D 6 15.35 (br s), 11.10 (br), 10.86 (br i ), 8.48 (br s), 5.74 (s), 2.36 (s), 2.14 (br), 0.25 ( s ), 3.05 (s), 5.48 ( m ), 10.16 ( br d ), 27.68 ( br s), 38.55 ( br s ), 56.85 (br s ), 58.71 (br s). 306 Synthesis of (Cp*) U(NHAr Me6 ) 2 ( 6 ): A 20 mL scintillation vi al was loaded with (Cp*) U ( I ) 2 (thf) 3 (300 mg, 0.356 mmol) , 3 mL THF, and a magnetic stir bar. The solution was stirred with a stir plate. To the stirring solution was added NaNHAr Me 6 (251 mg, 0.712 mmol) in THF (~2 mL) dropwise. The solution was allowed to stir at room temperature for approximately 18 h before being dried in vacuo. The remaining dark residue was extracted with n - hexane. The extracts were filtered through Celite, and t he volatiles removed in vacuo. The remaining solids were dissolved in a min imal amount of hexane for recrystallization. Synthesis of (Cp*) U(NHAr Me6 ) 2 without impurity was never achieved. Crude yield of (Cp*) U(NHAr Me6 ) 2 was 92 mg (27.4%). Synthesis of U I (NH Ar Me6 ) 2 ( 7 ): In the drybox, UI 3 (THF) 4 (250 mg, 0.276 mmol ) was weighed into a 20 mL glass scintillation vial. The vial was charged with ~5 mL diethyl ether and a magnetic stir bar. The vial was then placed in a liquid nitrogen cooled cold well until frozen. Once frozen, the vial was removed from the cold well and sus pended above a magnetic stir plate. When the solution had thawed enough to stir, a solution of NaNHAr Me6 (194 mg, 0.551 mmol ) in diethyl ether (~2 mL) was added dropwise. The solution was left to warm to room temperature and stirred for 16 h, whereupon the solution cha nged color from dark blue to dark purple. The volatiles were then removed under reduced pressure. The residue was then extracted with several aliquots of n - hexane until colorless. The extracts were then filtered using Celite as a filtering age nt. The filtr ate was then concentrated to ~1 - 2 mL under reduced pressure and chilled to 35 °C overnight to produce dark crystals of 7 ( 184 mg, 6 5.2 %). X - ray quality crystals were grown from a concentrated solution of 7 in a 35 °C freezer overnight. Eleme ntal Analysis Calculated C 48 H 52 I N 2 U : C 56.42, H 5.13, N 2.74; Found C 56.40, H 5.24, N 2.73. 1 H NMR ( 600 MHz, 25 °C, C 6 D 6 27.76 ( br) , 307 20.09 (br), 13.33 (br s), 8.87 (s), 6.88 (s), 5.92 (br), 3.28 (s), 3.26 (s), 2.21 (s), 2.13 (s), 1.77 (v br), 14.89 (br), 19.29 (br), 28.31 (br), 43.66 (br). Synthesis of UI(NHAr iPr6 ) 2 ( 8 ): In the drybox, UI 3 (THF) 4 (250 mg, 0.276 mmo l) was weighed into a 20 mL glass scintillation vial. The vial was charged with ~5 mL diethyl ether and a magnetic stir bar. The vial was then placed in a liquid nitrogen cooled cold well until frozen. Once frozen, the vial was removed from the cold well a nd sus pended above a magnetic stir plate. When the solution had thawed enough to stir, a solution of NaNHAr iPr6 (287 mg, 0.551 mmol) in diethyl ether (~2 mL) was added dropwise. The solution was left to warm to room temperature and stirred for 16 h whereup on the solution changed from dark blue to dark purple. The volatiles were then removed under reduced pressure. The residue was then extracted with several aliquots of n - hexane until colorless. The extracts were then filtered using Celite as a filtering age nt. Th e filtrate was then concentrated to ~1 - 2 mL under reduced pressure and chilled to 35 °C overnight to produce dark crystals of 1 0 (262 mg, 66.5%) . Note: Crystals of 8 contain 1 molecule of solvent depending on which solvent it is crystallized from, h exane or ether. X - ray quality crystals were grown from a concentrated solution of 1 0 in a 35 °C freezer overnight. Elemental Analysis Calculated C 72 H 100 IN 2 U: C 63.66, H 7.42, N 2.06; Found C 63.08, H 7.75, N 1.90. 1 H NMR ( 600 MHz, 30 °C , t oluene - d 8 = 87.0 2 ( br) , 56.22 (br), 38.01 (br s), 35.92 (s), 28.04 (s), 24.73 (s), 24.06 (br), 20.85 (br), 16.88(s), 15.56 (s), 10.65 (s), 9.42 (s), 6.87 (s), 6.51 (s), 1.18 (s), 1.58 (s), 3.24 (br), 5.30 (br), 11.76 (s), 22.44 (s), 31.96 (br), 43.51 (br), 5 0.74 ( br), 82.02 (br). Synthesis of U(NHAr iPr6 ) 2 ( 9 ): In the drybox, crystals of 8 (210 mg, 0.147 mmol) were weighed into a 20 mL glass scintillation vial. The vial was charged with ~5 mL THF and a magnetic stir bar. The vial was then placed in a liquid n itrogen cooled cold well until the solution froze. Once frozen, the vial was removed from the cold well and suspended above a magnetic stir plate. When 308 the s olution had thawed enough to stir, a suspension of KC 8 (40 mg, 0.296 mmol) in THF (1 mL) was added. The solution turned color from deep purple to dark green rapidly. The solution was stirred for 1 h at room temperature. The volatiles were then removed unde r reduced pressure, and the remaining residue dissolved in ~5 mL diethyl ether. The ether solution was stirred for ~5 min before being filtered using Celite as a filtering agent. The remaining residue was washed with an additional aliquot of diethyl ether (~3 mL) and the extracts filtered. The combined filtrate was then dried of the volatiles. The remai ning dark green residue was dissolved in n - pentane and filtered using Celite as a filtering agent once more. Crude 9 can be isolated and used for further rea ctivity by removal of the pentane (145 mg, 80.3%). Otherwise, X - ray quality single crystals can be produced by chilling a concentrated pentane solution of 9 in a - 35 °C freezer overnight. The yield of recrystallization is only slightly worse over two crops (130mg, 80%). Elemental Analysis Calculated C 72 H 100 N 2 U: C 70.21, H 8.18, N 2.27; Found C 68.66, H 8.38, N 2.13. 1 H NMR (600 MHz, 25 °C , t oluene - d 8 = 12.38 (s), 9.55 (s), 8.85 (d, J = 7.4 Hz), 7.37 (s), 6.82 (s), 6.54 (s), 5.44 (s), 4.41 (s), 3.59 3. 5 0 (m), 3.48 (br), 3.34 (t, J = 7.4 Hz), 2.74 (s), 1.82 (d, J = 10.2 Hz), 1.75 (d, J = 6.9 Hz), 1.62 (s), 1.26 (dd, J = 19.3, 7.0 Hz), 1.18 (d, J = 7.0 Hz), 0.75 ( s ), 0.52 (d, J = 118.4 Hz), - 0.38 (d, J = 42.2 Hz), - 1.55 (d, J = 131.5 Hz), - 6.52 (s), - 14.97 (s), - 25.82 (s). Synthesis of ( 10 ): In the drybox, 9 (70 mg, 0.057 mmol) wa s weighed in to a 20 mL glass scintillation vial. The vial was charged with diethyl ether (~3 mL) and a magnetic stir bar. The solution was then stirred on a magnetic stir plate. To the stirring solution, [FeCp 2 ] + BAr F 24 (60 mg, 0.057 mmol) in diethyl ethe r (1 mL) was added dropwise. The green solution rapidly turned a reddish - brown color. The solution was left to stir for 30 min. The volatiles we re then removed in vacuo . The residue was then washed with hexane (3 mL) , and the residue dried again in vacuo . Diethyl ether (~3 mL) was then added to the vial. The solution was then 309 filtered using Celite as a filter ing agent . The filtrate was concentrate d to ~1 mL and chilled in a 35 °C freezer overnight. X - ray quality crystals were observed over three crops of c rystals totaling 94 mg (79%). Elemental Analysis Calculated C 104 H 112 BF 24 N 2 U : C 59.63, H 5.39, N 1.34; Found C 53.82, H 4.85, N 1.33. 1 H NMR (600 MHz, 25 °C , THF - d 8 = 37.22 (br s), 26.72 (br s), 18.42 (s), 14.90 (s), 13.68 (s), 11.52 (br), 10.82 (s), 8. 49 (s), 8.28 (br s), 7.57 (s), 7.40 (s), 5.97 (br), 2.63 (br), 1.30 (br), 1.11 (br), 0.89 (br), 0.48 (br), 6.69 (br), 6.94 (br), 7. 66 (br), 13.25 (s), 16.16 (br), 17.11 (br s), 21.57 (s), 26.89 (br s), 33.91 (br s). 1 H NMR Spectra Figure 6 - 13 . 1 H NMR spectrum of (Cp*)UI(NAr iPr6 ) in C 6 D 6 310 Figure 6 - 14 . 1 H NMR spectrum of (Cp*)UI(NH Ar iPr6 ) in C 6 D 6 311 Figure 6 - 15 . 1 H NMR spectrum of (C p*)UI(NAr Me6 ) in C 6 D 6 312 Figure 6 - 16 . Crude 1 H NMR spectrum of (Cp*)UI(NHAr Me6 ) in C 6 D 6 313 Figure 6 - 17 . Crude 1 H NMR of (Cp*)U(NHAr Me6 ) 2 in C 6 D 6 . Inset shows the broad peaks attributed to fluxionality. 314 Figure 6 - 18 . 1 H NMR spectrum of UI(NHAr Me6 ) 2 in C 6 D 6 315 Figure 6 - 19 . 1 H NMR Spectrum of U I (NHAr iPr6 ) 2 ( 8 ) at 30 °C in toluene - d 8 316 Figure 6 - 20 . 1 H NMR Spectrum of U I (NHAr iPr6 ) 2 ( 8 ) at ambient temperature in toluene - d 8 Hexane 317 Figure 6 - 21 . 1 H NMR Spectrum of U( NHAr iPr6 ) 2 ( 9 ) at 25 °C in toluene - d 8 318 Figure 6 - 22 . 1 H NMR Spectrum of [U(NHAr iPr6 ) 2 ] + [ BAr F 24 ] ( 10 ) at 25 °C in THF - d 8 Diethyl ether 319 Vis - near IR Spectra Preparation of samples was performed in the glovebox using dry, degassed diethyl ether. Spectra are baseline corrected for a blank sample of diethyl ether in the quartz cuvette used for data collection. Data were collected at ambie nt temperature. All molar absorptivity values should only be considered approximate as the values are based off of a single concentration rather than averaged over a series of concentrations. Figure 6 - 23 . U V - vis/NIR spectrum of 2 in 1 mm cuvette 0 200 400 600 800 1000 1200 400 600 800 1000 1200 1400 1600 1800 Molar Absorptivity (M - 1 cm - 1 ) Wavelength (nm) (Cp*)UI(NAr iPr6 ) 50 mM UV - vis/NIR 320 Figure 6 - 24 . UV - vis/NIR spectrum of 3 in 1 mm cuvette Figure 6 - 25 . UV - vis/NIR spectrum of 6 in 1 mm cuvette. 0 200 400 600 800 1000 1200 400 600 800 1000 1200 1400 1600 1800 Molar Absorptivity (M - 1 cm - 1 ) Wavelength (nm) (Cp*)UI(NHAr iPr6 ) 50 mM UV - vis/NIR 0 200 400 600 800 1000 1200 400 600 800 1000 1200 1400 1600 1800 Molar Absorptivity ( M - 1 cm - 1 ) Wavelength (nm) (Cp*)U(NHAr Me6 ) 2 50 mM UV - vis/NIR 321 Figur e 6 - 26 . UV - vis/NIR spectrum of 8 at low concentration (~0.5 mM). Figure 6 - 27 . NIR spectrum of 8 at high concentration (~10 mM). 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 400 600 800 1000 1200 1400 1600 Molar Absorptivity (M - 1cm - 1) Wavelength (nm) UI(NHAr iPr6 ) 2 0.5mM UV - vis/NIR 0 50 100 150 200 250 800 900 1000 1100 1200 1300 1400 1500 1600 Molar Absorptivity (M - 1cm - 1) Wavelength (nm) UI(NHAr iPr6 ) 2 10 mM NIR 322 Figure 6 - 28 . UV - vis/NIR spectrum of 9 at low concentration (~0.5 mM). Figure 6 - 29 . UV - vis/NIR spectrum of 10 at low concentration (~0.5 mM). 0 500 1000 1500 2000 2500 3000 3500 4000 4500 425 625 825 1025 1225 1425 1625 Molar Absorptivity (M - 1cm - 1) Wavelength (nm) U(NHAr iPr6 ) 2 (0.5mM) UV - vis/NIR 0 500 1000 1500 2000 2500 3000 3500 4000 400 600 800 1000 1200 1400 1600 Molar Absorptivity(M - 1cm - 1) Wavelength (nm) [U(NHAr iPr6 ) 2 ] + [BAr F 24 ] 0.5mM UV - vis/NIR 323 EPR Spectroscopy Samples were prepared for EPR spectroscopy in the drybox using either dry degassed diethyl ether or toluene. The samples were loaded as a solution into a quartz EPR tube and sealed using a rubber septum and PTFE tape. The samples were then rapidly taken from the drybox and subm erged into a Dewar flask containing liquid nitrogen. The X - band EPR spectra were collected at 6 K. For all scans, the data were corrected for background contributions by subtracting EPR spectra obtained for a blank quartz tube containing only solv ent. For some scans, the baselines were leveled by subtracting a first or second - degree polynomial after the background correction was done. The data for 9 collected using a SHQ - E probe are shown below plotted on the same scale as 8 and 10 shown in the man uscript. The data for 9 were collected under the same conditions: microwave frequency, 9.40 GHz; microwave power, 0.8 mW; field modulation amplitude, 1.0 mT; and sample temperature, 6K. We collected these data on the second day that we ran in July. The dat a were co llected on a Bruker E - 680X EPR spectrometer equipped with an Oxford ESR - 900 liquid helium cryostat. 324 Figure 6 - 30 . EPR spectra of ( I ) U(NHAr iPr6 ) 2 ( 8 ), U(NHAr iPr6 ) 2 ( 9 ), and U(NHAr iPr6 ) 2 + ( 10 ). Measurement pa rameters for both spectra were: microwave frequency = 9.40 GHz, microwave power = 0.79 mW, field modulation amplitude = 1 mT, and sample temperature = 6 K. The data collected with the DM - 4116 parallel mode cavity are shown below for both para llel and perpe ndicular modes. For the perpendicular mode spectrum, the background contribution from the cryostat was subtracted and the baseline was then corrected by subtracting a second - degree polynomial. For parallel mode, no background correction was n eeded, but a 3 rd degree polynomial was subtracted from the baseline to flatten it. 325 Figure 6 - 31 . Parallel and perpendicular EPR spectra for 8 . Instrument conditions for these scans were: (a) perpendicular mode: microwave frequency = 9.643 GHz, and (b) parallel mode: microwave frequency = 9.441 GHz. Conditions common to the two spectra are: microwave power = 1.0 mW, field modulation amplitude = 1.6 mT, field modulation frequency = 10 kHz, and s ample temperature = 5 K. Magne tism Details SQUID magnetization data were recorded using a Quantum Design SQUID magnetometer at 5 kOe. Data were recorded at 1 K intervals from 2 to 20 K, 5 K intervals from 20 to 100 K, and 10 K intervals from 100 to 300 K. Each measurement was checked b y following the same temperature program in reverse. Several batches of the samples were measured, and the most self - consistent data was chosen for this publication. Data were corrected for magnetization of the sample holder b y subtracting the susceptibili ty of an empty container and for diamagnetic contributions of the 34 Sample s for SQUID magnetometry were all prepared using the following method. Using a vacuum sealer, vacuum - sealable polyethylene bags (bags with opposing diagonal lines) were 326 sealed to form narrow bags approximately 1 cm in width an d 8 cm in length. In an inert atmosphere glovebox, the uranium samples were loaded into the bottom of the narrow bags using a straw to avoid contamination on the sides of the bag. Once loaded, the bags were vacuum sealed at the end of the bag, leaving a su bstantial amount of bag betwee n where the vacuum is pulled and the sample. With the bag sealed under vacuum, the sample chamber was reduced in size by sealing the bag as close to the sample as possible while avoiding sealing any sample in the seam. The exc ess bag was then cut off and t he size of the bag was reduced to less than 1 cm 2 (by mass the bags weighed ~57 mg). The bag was then folded into a small ball, loaded into a straw, and transferred to the SQUID magnetometer. Solution state magnetic susceptibility studies were carried out on 9 using the Evans method. The resulting temperature dependent paramagnetism was determined in 10 K intervals from 299 - 219 K. The sample of 2 was 20 mM in toluene - d 8 with a sealed capillary containing 20 mM hexamet hyldisiloxane as a reference. At room t emperature the effective magnetic moment value of 2 is 0.78 cm 3 Kmol - 1 ( µ eff = 2.50), which decreases slightly to 0.72 cm 3 Kmol 1 ( µ eff = 2.40) at 219 K. Data were corrected for diamagnetic contributions of the sample u 34 No approximation was included for the temperature dependence of toluene - d 8 dens ity. 327 Figure 6 - 32 . Temperature dependence of magnetic susceptibility (µ eff ) for 8 collected at 5000 Oe. Figure 6 - 33 . Temperature dependence of magnetic susceptibility (µ eff ) for 9 collected at 5000 Oe. 328 Figure 6 - 34 . Temperature dependence of magnetic susceptibility (µ eff ) for 10 collected at 5000 Oe. Figure 6 - 35 . Temperature dependence of so lution state magnetic susceptibility ( µ eff ) for 9 . 329 X - ray Crystallographic Details Complex (Cp*) U I( NAr iP r6 ) 2 (Cp*) UI ( NHAr iP r6 ) 3 (Cp*) UI ( NAr Me6 ) (t hf) 2 4 (Cp*) UI ( NHAr M e6 ) 5 (Cp*) U(NHAr Me 6 ) 2 6 U I (NHAr Me6 ) 2 7 U - N 1.977(4) 2.277(3) 2.006(3) 2.315(4) 2.3 5 6(3) , 2.336(3) 2.349(6), 2.316(6) U - N - C 145.0(3) 132.9(2) 163.9(2) 132.4(3) 162.4(2), 132.1(2) 136.5(5), 136.5(5) N - U - N -- -- -- -- 110.8(1) 113.2(2) U - Ar c ent 2.594(1) 2.482(2) -- 2.501(3) 2.497(3) 2.754(3), 2.843(3) a Cent - U - Cent 136.4(3) 137.2(08) -- 136.9(1) 125.9(08) 174.5(1) Cp* cent - U 2.462(2) 2.469(2) 2.500(4) 2.466(3) 2.496(2) -- Table 6 - 1 . Selected bond lengths and angles. a The centroid refers to the Cp* centroid - U - Arene centroid angle in 2 - 6 , and the Arene centroid - U - Arene centroid angle in 7 . 330 Complex U I (NHAr iPr6 ) 2 8 a U(NHAr iPr6 ) 2 9 U(NHAr iPr6 ) 2 BAr F 10 U - N 2.390(3), 2.372(3) 2.330(2) 2.283(6), 2.282(6) 2.355(2), 2.366(2) U - N - C 137.5(2), 135.6(2) 1 30.26(2) 133.5(5), 131.6(5) 139.43(2), 138.53(2) N - U - N 149.07(1) 99.24(1) 111.2(2) 149.92(7) U - Ar c ent 2.777(1), 2.790(1) 2.405(1) 2.573(3), 2.583(3) 2.8968(9), 2.7878(9) Ar cent - U - Ar cent 158.28(4) 134.23(5) 145.8(1) 158.78(3) U - C arene 3. 147(3), 3.208(3), 3.143(3), 3.080(3), 3.043(3), 3.109(3), 3.147(3), 3.178(3), 3.135(3), 3.069(3), 3.020(3), 3.114(3) 2.735(3), 2.730(3), 2.869(3), 2.725(3), 2.770(3), 2.903(3) 2.945(7), 3.008(7), 2.953(7), 2.902(7), 2.842(7), 2.908(7), 3.061(9), 2.993(8), 2.940(7), 2.828(6), 2.869(7), 2.947(8) 3.182(2), 3.177(2), 3.091(2), 3.039(2), 3.058(2), 3.175(2), 3.314(2), 3.310(2), 3.191(2), 3.106(2), 3.116(2), 3.264(2) Avg. C Ar - C Ar bond 1.402 1.415333333 1.403 1.403 Table 6 - 2 . Selected bond lengths and angles. a The white rows are the values from the diethyl ether crystallized structure, the grey rows are from the hexane crystallized structure. 331 Single crystal diffraction data were collected Bruker APEX - II CCD di ffractometers using either or . Single crystals were mounted on glass fibe r loops using either N - paratone oil or Krytox grease. Data collection was done at either 100 K or 173 K under a liquid nitrogen cold stream. Using Olex2, the structures were solved with the ShelXT sol ution program using intrinsic phasing and refined with t he XL refinement package using least squares minimization. 35 - 37 Crystal data and structure refinement for (Cp*) U I (NAr iPr6 ) ( 2 ) Figure 6 - 36 . Full structure of ( 2 ) including solvent Identification code (Cp*) UINTriPP 332 Empirical formula C 49 H 71 INU Formula weight 1038.99 Temperature/K 100.0 Crystal system monoclinic Space group P2 1 /c a/Å 17.797(11) b/Å 17.303(11) c/Å 16.558(10) 90 103.395(7) 90 Volume/Å 3 4960(5) Z 4 calc g/cm 3 1.391 - 1 3.924 F(000) 2068.0 Crystal size/mm 3 0.27 × 0.25 × 0.06 Radiation /° 5.058 to 54.206 Index ranges - - - Reflections collected 54710 Independent reflections 10926 [R int = 0.0564, R sigma = 0.0438] Data/restraints/parameters 10926/2/479 Goodness - of - fit on F 2 1.029 Final R R 1 = 0.0326, wR 2 = 0.0779 Final R indexes [all data] R 1 = 0.0523, wR 2 = 0.0869 333 Largest diff. peak/hole / e Å - 3 1.46/ - 0.97 Crystal data and structure refinement for (Cp*) U I (NHAr iPr6 ) ( 3 ) Figure 6 - 37 . Full structure of ( 3 ) including solvent and molecular disorder Identification code CpstarUINHTriPP Empirical formula C 52 H 79 INU Formula weight 1083.09 Temperature/K 103.0 Crystal system monoclinic Space group P2 1 /c a/Å 18.0138(13) 334 b/Å 17.3681(13) c/Å 16.4608(12) 90 103.183(2) 90 Volume/Å 3 5014.3(6) Z 4 calc g/cm 3 1.435 - 1 3.885 F(000) 2172.0 Crystal size/mm 3 0.42 × 0.42 × 0.11 Radiation 5.204 to 55.752 Index ranges - - - Reflections collected 76155 Independent reflections 11949 [R int = 0.0465, R sigma = 0.0294] Data/restraints/parameters 11949/4/544 Goodness - of - fit on F 2 1.024 R 1 = 0.0250, w R 2 = 0.0528 Final R indexes [all data] R 1 = 0.0395, wR 2 = 0.0579 Largest diff. peak/hole / e Å - 3 1.16/ - 0.66 Crystal data and structure refinement for ( Cp *) UI(NAr Me6 ) (thf) 2 ( 4 ) 335 Figure 6 - 38 . Full st ructure of (4) Identification code Mes2ArNUCpstarIthf2 Empirical formula C 42 H 53 INO 2 U Formula weight 968.83 Temperature/K 296.15 Crystal system monoclinic Space group P2 1 /c a/Å 10.6120(18) b/Å 12.157(2) c/Å 29.815(5) 90 95.774(2) 336 90 Volume/Å 3 3826.9(11) Z 4 calc g/cm 3 1.6814 - 1 5.085 F(000) 1850.6 Crystal size/mm 3 0.19 × 0.17 × 0.07 Radiation 4.96 to 55.66 Index ranges - - - Reflections collected 44555 Independent reflections 9053 [R int = 0.0432, R sigma = 0.0331] Data/restraints/parameters 9053/0/434 Goodness - of - fit on F 2 1.063 R 1 = 0.0258, wR 2 = 0.0568 Final R indexes [all data] R 1 = 0.0319, wR 2 = 0.0592 Largest diff. peak/hole / e Å - 3 1.01/ - 1.25 Crystal data and structure refinement for ( Cp*) U I (NHAr Me 6 ) ( 5 ) 337 Figure 6 - 39 . Full structure of ( 5 ) Identification code Mes2ArNHUICpstar Empirical formula C 34 H 41 INU Formula weight 828.61 Temperature/K 100.0 Crystal system monoclinic Space group P2 1 /c a/Å 21.313(2) b/Å 8.7053(10) c/Å 17.0261(19) 90 101.810(2) 90 Volume/Å 3 3092.0(6) 338 Z 4 calc g/cm 3 1.780 - 1 6.270 F(000) 1588.0 Crystal size/mm 3 0.34 × 0.33 × 0.02 Radiation 5.28 to 57.452 Index ranges - - - Reflections collected 22223 Independent reflections 7990 [R int = 0.0412, R sigma = 0.0498] Data/restraints/parameters 7990/0/348 Goodness - of - fit on F 2 1.030 R 1 = 0.0347, wR 2 = 0.0706 Final R indexes [all data] R 1 = 0.0517, wR 2 = 0.0780 Largest diff. peak/hole / e Å - 3 2.84/ - 1.79 339 Crystal data and structure refinement for ( Cp*) U(NHAr Me 6 ) 2 ( 6 ) Figure 6 - 40 . Full structur e of ( 6 ) Identification code CpstarUNArMes2_2 Empirical formula C 58 H 67 N 2 U Formula weight 1030.22 Temperature/K 100.01 Crystal system monoclinic Space group P2 1 /c a/Å 11.2677(12) b/Å 27.403(3) c/Å 15.9408(17) 90 340 110.049(2) 90 Volume/Å 3 4623.7(9) Z 4 calc g/cm 3 1.4798 - 1 3.552 F(000) 2044.1 Crystal size/mm 3 0.17 × 0.06 × 0.05 Radiation 4.86 to 55.98 Index ranges - - - Reflections collected 65480 Independent reflections 11109 [R int = 0.0593, R sigma = 0.0388] Data/restraints/parameters 11109/0/566 Goodness - of - fit on F 2 1.056 R 1 = 0.0330, w R 2 = 0.0693 Final R indexes [all data] R 1 = 0.0478, wR 2 = 0.0749 Largest diff. peak/hole / e Å - 3 2.00/ - 0.99 Crystal data and structure refinement for U I (NHAr Me 6 ) 2 ( 7 ) 341 Figure 6 - 41 . Full structure of ( 7 ) Identification code Mes2ArNH2UI Empirical formula C 48 H 52 IN 2 U Formula weight 1021.89 Temperature/K 104.34 Crystal system monoclinic Space group P2 1 /c a/Å 19.2948(18) b/Å 13.1778(12) c/Å 17.2207(16) 90 104.958(2) 90 Volume/Å 3 4230.2(7) Z 4 342 calc g/cm 3 1.6044 - 1 4.602 F(000) 1954.5 Crystal size/mm 3 0.19 × 0.19 × 0.15 Radiation 5.8 to 52.72 Index ranges 0 - - Reflections collected 9882 Independent reflections 8465 [R int = 0.0000, R sigma = 0.0532] Data/restraints/parameters 8465/0/480 Goodness - of - fit on F 2 1.054 R 1 = 0.0503, wR 2 = 0. 1049 Final R indexes [all data] R 1 = 0.0727, wR 2 = 0.1145 Largest diff. peak/hole / e Å - 3 2.55/ - 1.61 343 Crystal data and structure refinement for U I (NHAr iPr6 ) 2 ( 8 ) crystallized from ether Figure 6 - 42 . Full structure of ( 8 ) solvent and molecular disorder Identification code TriPPNH2UI Empirical formula C 74 H 105 IN 2 O 0.5 U Formula weight 1395.52 Temperature/K 100(2) Crystal system monoclinic Space group P2 1 /n a/Å 15.2986(7) b/Å 23.85 67(11) c/Å 18.6523(9) 90 96.0070(10) 90 Volume/Å 3 6770.2(5) Z 4 calc g/cm 3 1.369 344 - 1 2.895 F(000) 2848.0 Crystal size/mm 3 0.18 × 0.15 × 0.15 Radiation 4.71 2 to 56.564 Index ranges Reflections collected 82390 Independent reflections 16801 [R int = 0.0388, R sigma = 0.0323] Data/restraints/parameters 16801/48/803 Goodness - of - fit on F 2 1.014 Final R indexe R 1 = 0.0268, wR 2 = 0.0552 Final R indexes [all data] R 1 = 0.0401, wR 2 = 0.0594 Largest diff. peak/hole / e Å - 3 0.70/ 0.54 345 Crystal data and structure refinement for UI(NHAr iPr6 ) 2 ( 8 ) with n - hexane co - crystallized . Figure 6 - 43 . Full structure of ( 8 ) including solvent and molecular disorder Identification code TriPPNH2UI_hexane Empirical formula C 78 H 114 IN 2 U Formula weight 1444.64 Temperature/K 296.15 Crystal system monocl inic Space group P2 1 /n a/Å 13.3008(15) b/Å 15.0013(17) c/Å 36.190(4) 90 94.3230(10) 90 Volume/Å 3 7200.4(14) Z 4 346 calc g/cm 3 1.333 - 1 2.724 F(000) 2964.0 Crystal size/mm 3 0.22 × 0.22 × 0.07 Radiation 4.74 to 54.314 Index ranges 46 Reflections collected 80270 Independent reflections 15912 [R int = 0.0470, R sigma = 0.0376] Data/restraints/parameters 15912/21/743 Goodness - of - fit on F 2 1.037 R 1 = 0.0345, wR 2 = 0.0770 Final R indexes [all data] R 1 = 0.0464, wR 2 = 0.0812 Largest diff. peak/hole / e Å - 3 1.70/ 0.85 Crystal data and structure refinement for U(NHAr iPr6 ) 2 ( 9 ). 347 Figure 6 - 44 . Full stru cture of ( 9 ) including solvent. Identification code UNHTriPP2 Empirical formula C 76 H 106 N 2 OU Formula weight 1301.65 Temperature/K 100(2) Crystal system monoclinic Space group C2/c a/Å 18.0536(13) b/Å 16.8949(12) c/Å 22.7572(16) 90 106.9720(10) 90 Volume/Å 3 6638.9(8) Z 4 calc g/cm 3 1.302 - 1 2.489 F(000) 2704.0 348 Crystal size/mm 3 0.26 × 0.22 × 0.12 Radiation 4.718 to 55.67 Index ranges Reflections collected 39160 Independent reflections 7874 [R int = 0.0396, R sigma = 0.0311] Data/restraints/parame ters 7874/66/390 Goodness - of - fit on F 2 1.072 R 1 = 0.0275, w R 2 = 0.0690 Final R indexes [all data] R 1 = 0.0318, wR 2 = 0.0711 Largest diff. peak/hole / e Å - 3 1.06/ 0.66 349 Crystal data and structure refinement for [U(NHAr iPr6 ) 2 ] + [ BAr F 24 ] ( 1 0 ) . Figure 6 - 45 . F ull structure of ( 10 ) including anion, solvent, and molecular disorder Identification code UNHTriPP2BArF Empirical formula C 105 H 114.5 BF 24 N 2 O 0.25 U Formula weight 2113.32 Temperature/K 172.98 Crystal system monoclinic Space group P2 1 /c a/ Å 14.7775(2) b/Å 16.4035(2) c/Å 41.8881(6) 90 90.8930(10) 90 Volume/Å 3 10152.6(2) 350 Z 4 calc g/cm 3 1.383 - 1 5.277 F(000) 4294.0 Crystal size/mm 3 0.258 × 0.106 × 0.062 Radiation e for d ata collection/° 4.22 to 144.264 Index ranges 51 R eflections collected 78623 Independent reflections 19461 [R int = 0.1307, R sigma = 0.0952] Data/restraints/parameters 19461/934/1530 Goodness - of - fi t on F 2 1.014 R 1 = 0.0572, wR 2 = 0.1242 Final R indexes [all data] R 1 = 0.0954, wR 2 = 0.1415 Largest diff. peak/hole / e Å - 3 1.40/ 1.23 351 REFERENCES 352 REFERENCES 1. Anderson, N. H.; Xie, J.; Ray, D.; Zeller, M.; Gagliardi, L.; Bart, S. C., Nat. Chem. 2017, 9 , 850. 2. Anderson, N. H.; Odoh, S. O.; Yao, Y.; Williams, U. J.; Schaefer, B. 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Crystallogr. 2009, 42 (2), 339 - 341. 354 Reduction of Dinitrogen to Hydrazine 7.1 Introduction In recent years the temperature of the planet has begun rising steadily. 1 Both land and ocean temperatures have seen record high temperatures, and, in fact, all five h ottest years on record (since 1880) have occurred sinc e 2010. 1 High ca rbon dioxide (CO 2 ) concentrations in the atmosphere are likely a contributing factor. CO 2 is a very efficient greenhouse gas due to its high transparency to solar radiation and its high absorptivity of infrared radiation emitted from the earth. 2 Over the l ast 420,000 years, CO 2 concentrations in the atmosphere have been in equilibrium between 200 - 300 ppm. 3 Recently, this equilibri um has been shattered as concentrations of CO 2 have climbed over 400 ppm ( Figure 7 - 1). 4 Figure 7 - 1 . Concentration of CO 2 in the atmosphere measured from 1958 to 2018 at the M anua Loa Observatory. Figure taken from reference 3. 355 These recent increases in concentration are almost certainly due to human influence. One indication of human influ ence is that the increase in concentration of CO 2 is accompanied by a reduction in the concentration of 13 C in the atmosphere, likely from burning 13 C deficient fossil fuels. 5 - 8 The trend in using fossil fuels as an energy source has shown little indication of slowing. 9 Fossil fuels are predicted to accou the onl y fossil fuel source with a predicted plateau in growth rate is coal, while natural gas is predicted to grow even faster than renewable energy sources. A major hurdle in implementation o f renewable energy sources is the often - intermittent energy supply. Fo r example, solar power may be a worthwhile investment in southern states with consistent exposure to sun, but in the northern states short and cloudy days during winter make solar energy unreliable. 10 We need to find a method to make renewable energy sources, like solar, viable in all areas of the wo rld. In order to solve this problem, we need to find a method of energ y storage that has a similar to fossil fuels. Better energy storage solutions would allow full utilization of things like solar energy when they are available. For example, we could use solar energy to charge a battery during the day, then use the battery to power lights at night. One potential source we could take advantage of is hydrogen gas (H 2 ). H 2 is a clean energy source since the only combustion byproduct is water (H 2 O). The issue when considering H 2 as a fuel source is its energy density (per unit v olume). Even using the best - case scenario storage tanks for fuel cell - based vehicles, the maximum energy density of H 2 is 5.3 MJ/L. 11 Whe n compared to something like gasoline, which has an energy density of 34.4 MJ/L, H 2 is hardly viable. 11 356 7.2 Ammonia as an Energy Storage S olution One possible solution to the energy density of H 2 is ammonia (NH 3 ). Ammonia can be considered a carrier of H 2 but has a higher energy density. In fact, NH 3 has a higher energy d ensity than even natural gas, at 13.6 MJ/L for ammonia compared to 10. 4 MJ/L in natural gas. 11 Ammonia is already produced, stored, and t ransported on a massive scale. Holding tanks on the 50,000 - ton scale, dedicated pipelines, and easy conversion of natural gas infrastructure are all a testament to the ease with which we could convert to ammonia based fuels. 12 Even aspects such as safety assess ments of NH 3 as a fuel source have been conducted, concluding equal or lower risk when compared to current fossil fuels. 13 - 14 Part of what makes this infrastructure possible is the simple liquification of NH 3 . At just under 10 atm of pressure NH 3 liquifies, producing high energy density compared to H 2 . This liquified ammonia could then be easily transported and used ei ther directly as a fuel source or split into H 2 and N 2 so that the H 2 can be used as fuel. 15 - 16 One might have cause to wonder, if NH 3 is such a perfect solution, why is it not already in use? To answer that questi on, we must look to the production methods currently employed for ammonia synthesis: the Haber - Bosch process. 17 - 18 Currently the Haber - Bosch process is run on a ~175 million metric ton sca le annually to meet demand s of compounds like ammonium salts, nitrates, and ureas. 19 But, due to the high activation energy of the N - N triple bond in dinitrogen, this process is extremely energy intensive. At current production levels, the Haber - Bosch process produces 1 - 2% of the en ses. 20 Its worth mentioning, though, that the Haber - production, so the energy demand is justified. 21 Still, the demand for nitrogen fixation is 357 extraordinary and will only con tinue to grow. 22 Even if NH 3 demand for fertilizers is unlikely to stop growing. 20 - 21 7.3 Alter native Methods for Nitrogen Fixation In recent years there has been a surge in efforts to fix dinitrogen using a variety of catalysts. 19, 23 - 29 In particular, the data presented by Shilov is very intriguing. Shilov number of research articles spanning several decades highlighting their work on fixatio n of dinitrogen. Their report in Nature in 1971 highlighted a few notable experiments. 30 demonstrated N 2 could be reduced to hydrazine or a mmonia by a mixture of V(II) in H 2 O or Cr(II) in MeOH/H 2 O. Both of these experiments produce hydrazine in subst oichiometric amounts. In the same report though, they produce hydrazine using Ti(III) as a reductant. In this system, when there is Ti(III), MgCl 2 , and KOH at elevated pressure and temperature, very small quantities of hydrazine are produced. When Mo is ad ded, in the form of MoOCl 3 or MoO 4 2 - , the reaction produces noticeably more hydrazine, and, in fact, is catalytic in Mo obtaining ~87 turnovers. 30 In the following years, the Shilov system developed and the reaction changed in almost every aspect. They changed Ti(I II) to sodium - mercury amalgam or an electrode, 31 - 32 added L - - dipalmitoylphosphatidylcholine (PC) as a surfactant, 33 added phosphine to the system, 32 reduced the extremity of the conditions to ambient temperature and pressure, and, in the process, increased the yield of hydrazine produced per Mo to ~120 turnovers. 29, 34 - 35 Probably the most notable advancement in the Shilov system was the discovery of an active heterometallic Mo 8 Mg 2 cluster. 36 - 37 The cluster, shown in Figure 7 - 2, is a dianionic cluster consisting of bridging oxygen - based ligands ranging from datively c oordinated methanol to oxo ligands, and has an outer sphere magnesium ion. 358 Figure 7 - 2 . Heterometallic cluster reported by Shilov to be the active species in his dinitr ogen reduction system. 36 Using the Mo 8 Mg 2 cluster above, Shilov was able to increase the production of hydrazine up to 1600 turnovers per Mo at 1 atm N 2 and ambient temperature. 35 The number of turnovers could b e increased up to 10 4 turnovers if the system was pressurized with 70 atm N 2 . 29 While the Shilov system does not reliably produce NH 3 , initial cleavage of the N - N triple bond is, perhaps, the most difficult step, e specially since hydrazine cleavage has been reported. 38 - 40 Part of the energy i ntensity of the Haber - Bosch process is that it requires a stream of H 2 gas. Since the Shilov system only requires N 2 and aqueous media, the proton source is presumably H 2 O. 29, 34 Unfortunately, implementation of the Shilo v system has not been achieved to date. One f the cluster synthesis, the procedure reads as follows: - VI) complex a sampl e was used containing 30% of Mo(VI). The sample was left standing in the presence of air for a long time and presumably Mo(VI) - A. E. Shilov 1989 359 The above procedure is the method to produce the starting Mo compound for synthesis of the cluster. There is no analysis of the resulting compound, only an estimated composition of Mo(V) vs. Mo(VI weeks, or longer. There are no details about the humidity (MoCl 5 reacts vigorously with H 2 O from the starting material is. In brief, there is no detail at all. The lack of detail continues through the rest of the experimental in 30 - 33, 36 - 37 The lack of experimental rigor is exacerbated by the sensitivity of the cluster synthesis. As evidenced by recent reports, an extremely wide variety of products can be obtained through similar reactions. 41 - 45 7.4 problems to be solved, we set out to tr y and replicate the results Shilov had reported. We avoided, at first, using the com plete system including the phosphatidylcholine and amalgam to try and simplify and more systematically characterize the system. Nat ur e , Hil l and Richards published a report using 15 N 2 to Mo and V as catalysts for N 2 reduction. 46 Confirming the result s reported by Shilov, the system produced 15 N containing hydrazine. 30 Like the original repor t by Shilov, they found the reaction was substoichiometric, but went on to conclude that modification of the Shilov system could yield useful systems for dinitrogen activat ion. 30, 46 Because the system was less complex than the later Shilov systems, and because there were additional reports of its success, we set out to study the TiCl 3 reductant system. 360 Due to the high pressure of N 2 required for the reactions, Shilov emp loyed a specialized reactor. 33, 47 The dia grams of the reactor the y employed were difficult to interpret. Given that their N 2 pressures exceeded 100 atm, we used a Parr pressure reactor. The reactor was fit with a Teflon - coated mechanical stirrer, a Teflon liner, thermocouple, two pressure sensors (one electronic and one standard gauge), a burst disc set to 2000 psi, and a custom fit heating mantle. We hoped using Teflon liners wherever possible would avoid the corrosive solution leaching any metal from the reactor walls. For the thermocouple, whic h could not be coated wi th Teflon due to thermal conductivity concerns, we used a narrow glass sleeve filled with MeOH. This way, the thermocouple maintained thermal contact with the solution but was isolated from the reaction mixture. The system was set u p so that reactions coul d be transported to and from a glove bag while remaining sealed under a nitrogen atmosphere. Over a series of attempts to produce hydrazine using the general method reported by Shilov, we did eventually manage to find a system that produced some quantities of N 2 H 4 . Our reaction was done using 12% TiCl 3 in HCl with a solution of 15 mL H 2 O and 100 mL MeOH, KOH, MgCl 2 · 6H 2 O, MoCl 5 (see experimental for details). Over a number of reactions, heating the Parr bomb containing those reagents to 85 °C with >800 psi N 2 produced small, but measurable quantities of N 2 H 4 . We were unable to detect any amount of NH 3 in any of the reactions. Th is reaction, when performed with the specific sequence outlined in the experimental, was reproducible (in th e sense that , qualitatively, detectable hydrazine was produced each time) by our group, as well as Dr. Dan Little and Dillon Edwards following the s ame procedure in the Hamann lab. 7.5 Monitoring Hydrazine Formation We attempted quantification of the hydrazin e using the p - dimethylaminobenzaldehyde (PDMAB) indicator solution, but the solution was unreliable, especially with the complex, colored 361 reaction mixture. 48 From our expe rience, the i ndicator can reliably be used to produce qualitative color changes which can indicate the presence of N 2 H 4 , but quantitation was not possible. As a substitute, we explored other aldehydes that might allow isolation of the hydrazone product. We attempted hydrazine to hydrazone conversion using several different benzaldehydes, including phthaldialdehyde (o - benzene - 1,2 - dicarboxaldehyde). Unfortunately, most reactions with the aldehydes we tested did not seem to lead to quantitative conversion to the hydrazon e, or, as was the case with phthaldialdehyde, the reaction was rather slow. To our surprise, when a small aliquot of the reaction mixture from the Parr reactor is added to a solution containing benzaldehyde, GC/MS analysis of the resulting mix ture indicate s the presence of stilbene. When reaction mixtures from the reactor that do not show production of N 2 H 4 by the PDMAB indicator are added to the benzaldehyde solution, no stilbene is observed. We thought this might be a possible method of quant ification, so we took two samples from the reactor and added each to separate vial s containing a solution of benzaldehyde. The first was analyzed by GC/MS as is, the second was spiked with a known quantity of hydrazine hydrate. From the relative difference in peak area s in the GC traces, we estimated the amount of hydrazine present in the first vial. than the PDMAB indicator solution, but given the complexity of the transfor mations, it is unlikely to be a perfect quantitative measure. Attempts at elucidating the mechanism of conversion of the aldehyde to stilbene were fruitless as r eaction of hydrazine hydrate and benzaldehyde in the presence of anything other th an the comple te reaction mixture , failed to produce stilbene . Still, our estimated production of hydrazine was usually substoichiometric relative to Mo. Only in one experiment were we able to produce N 2 H 4 in excess of the amount of Mo added (0.09 mmol N 2 H 4 relative to 0.08 mmol Mo added). Given that we cannot assign error bars to the measurement, 362 we hesitate to call this a turnover of Mo. Regardless, the system using Mo and Ti, as Shilov reported, does reduce N 2 to N 2 H 4 . 7.6 Moving to More Complex Systems In l ight of the s uccess with the Ti system, we sought to move to the more complex Shilov systems that were more productive. 32, 49 Unfortunately, as m entioned above, this meant the reaction mixture had to be even more c omplicated. Specifically, in addition to the amounts of Mo, Mg, KOH, MeOH:H 2 O ratio, pressure, stir rate, and temperature, we now also had to worry about amalgam concentration, phosphine concentration, phospholipid choice (since changing the structure of t he phospholipid can completely deactivate catalysis), and catalyst preparation. 49 Unsurprisingly, despite a wide variety of con ditions, the more complex reaction mixture was completely unsuccessful in every attempt. e ach. Take for example the phospholipid. The proposal Shilov put forwa rd for the action of PC in the reaction mixture was as a surfactant to keep the amalgam surface area high. 34, 49 They report that when PC is added to the solutions containing amalgam, they get finely divided beads of amalgam that are stable enough to be measured for size distribution. Yet, somehow, the in creased activity on addition of phosphine ligand to the solution is dependent on the presence of the phospholipid, and the addition of phosphatidylcholine suppresse s production of NH 3 , making N 2 H 4 the sole product. 49 The reported procedure for the catalyst, too, is disturbing (this procedur e was published before the isolation of the Mo 8 Mg 2 cluster discussed above but is equally un helpful). In the report they suggest that the catalyst solution was vastly improved by subsequent acidification by HCl and basification by NaOCH 3 . In the process, m olybdenum is reported to precipitate, but the remaining solution displayed more catalytic ac tivity. 49 Much like the other 363 exper imental reports, concentrations of the acid and base are not given, pH of the solution at an y point is not given, reaction times are not given, and Mo concentration after the precipitation is unknown. In an effort with the Hamann group, we also tried to re place chemical reductants with electrodes. Here too, all efforts at dinitrogen reduction wer e unsuccessful despite a variety of electrodes, potentials, and catalyst preparations. 7.7 Catalyst Synthesis In our opinion, the most room for error in the systems whe re we saw hydrazine production is in the catalyst solution preparation. In our procedure, we make a methanolic solution of MgCl 2 and add it to MoCl 5 . There is a rapid series of color changes and evolution of HCl gas. This was our best guess at mimicking th e Shilov procedure, and it seemed to work to some degree, but given how rapid the reaction i s, it almost certainly leads to a variety of products. We cannot even say what the approximate concentrations of catalyst in our experiments are because we have no idea what the active species is and how much is present. Much like Shilov, we sought to pre - form a catalyst to use in these reactions. 36 In an ideal situation, we could isolate a catalyst, then add it directly to the reductant solution, and start the reaction. We also hoped we could more completely characterize the co mplex and develop a reproducible method for its production. As mentioned earlier, though, we are not the first to attempt synthesis of Mo - Mg clusters in methanolic media. The Bazhenova and Kuznetsov group have published a series of reports outlining just h ow many products are possible. 41 - 45, 50 We made several attempts at isolation of complexes. We used anhydrous MeOH and Mg Cl 2 , more rigorous air - free techniques, and controlled reaction times. In most cases, the resulting 364 mixtures were unstable species that w ere intractable. One minor success was had in the isolation of crystals of the complex in Figure 7 - 3. While the complex bears resemblance to those in the recent literature, it is not identical. Figure 7 - 3 . Molybdenum cluster isolated from attempts at catalyst synthesis. Hydrogens are removed from the tetrabutyl ammonium for clarity. anion pair. We hoped we could make more stable complexes using discrete cation units such as NBu 4 + . Besides the incorporation of the ammonium, the compl ex is far from that reported by Shilov. Among other things, it is not heterometallic, it did not achieve complete hydrolysis, and the charge is not correct. Not surprisingly, the cluster in Figure 7 - 3 is not competent for the catalysis. 7.8 Looking Forward It clusters should be the primary goal in this research. There are simply too many other variables at work in the complete reaction mixture to analyze each part systematical ly. That said, cluster 365 synthesis should be approached with caution. Most importantly, I think, the pH of the solution should be controlled carefully. The chemistry of these cluster formations is likely to be extremely sensitive to pH, solution conductivity , and concentration. In hindsight, I suspect this was my biggest shortcoming in attempts at cluster synt hesis. Additionally, if possible, a different Mo source may afford more control over the reaction. MoCl 5 is extremely reactive with methanol and water. Because of the immediate release of HCl on contact with the solvents, control over the reaction propert ies mentioned above will be difficult. We attempted some synthesis from other Mo starting materials, such as amides and oxides, but only briefly and wit h little success. 7.9 Conclusions The Shilov group published a number of reports spanning several decades. 29 We have managed to lend only some to the simplest research presented in those reports. Unfortunately, a lack of experimental detail a nd an overwhelming complexity preclude d reproduction of the interesting results. Still, the challenge of nitrogen fixation is yet to be solved, as such, every effort to solve this problem is welcome and necessary. 7.10 Experimental Gene ral Considerations TiCl 3 solution, MgCl 2 hydrate, KOH pellets, benzaldehyde, and hydrazine hydrate were purchased form Sigma Aldrich as used as received. MoCl 5 was purchased from Strem and used as received. Methanol was distilled from Mg before use. All pr oductive reactions follow ed the general procedure below. 366 All required chemical, glassware, and reactor vessel were loaded into a glovebag. The glovebag was purged several times with dry N 2 gas before being sealed. Once sealed, 12% TiCl 3 in HCl (9.6 mL, ~7. 5 mmol) was loaded into the Teflon lined reactor. In a vial, KOH (0.702 g, 12.5 mmol) was dissolved in 15.5 mL H 2 O. The contents of the vial were then added to the reactor. In a separate vial, MgCl 2 6H 2 O (0.771 g, 3.8 mmol) was dissolve d in MeOH (15 mL). T his solution was added to another vial containing MoCl 5 (20.5 mg, 0.08 mmol). The solution rapidly changed color and white fumes were emitted from the vial. This solution was capped and swirled by hand gently before being added to the r eactor. A narrow tub e filled with clean MeOH was also loaded into the reactor. The reactor cap was the put on, being careful to keep the thermocouple in the MeOH filled tube away from the corrosive reaction mixture. The reactor was sealed and transported t o the holder. A line from a N 2 tank was fixed while flowing <14 psi N 2 gas so the line was purged of any air or moisture. The vessel was filled to 50 psi of N 2 gas and the headspace was carefully purged using the pressure relief valve. Caution was taken to avoid the pressure falling below 50 psi (this was again to ensure a clean atmosphere of N 2 gas in the reactor. Once purged, the vessel was sealed, and the thermocouple, stirrer, and pressure sensor were fitted. The heating mantle was put in place and the stirrer was switched on. The vessel was then slowly pressurized with N 2 and heated. For reactions that were heated, the timing was started once they reached temperature. All reactions were allowed to cool once the reaction time was complete by removing the heating mantle. Cau tion is emphasized in opening the reactor due to the corrosivity of the mixture. Qualitative analysis is performed by adding a small drop to the hydrazine indicator solution. 48 Mass Spec Analysis of the Reaction Solution 367 Two 500 L aliquots of reaction solution was loaded into two separate GC vials. To each vial was added a THF solution containing excess benzaldehyde. The first vial was analyzed by GC/MS as is while the second vial was spiked with 4 L of 50 ppm hydrazine hydrate s olution before analysis. In the GC trace the peak for stilbene was integrated in each spectrum. The concentration of hydrazine in the unspiked vial was solved using the difference in peak area between the first and second vial as the known amount of hydra z ine added. The number of mols in the GC vial sample was then scaled to the volume of the reactor solution (125 mL) to solve for the total hydrazine production. Cluster Synthesis In a glovebag under nitrogen, MgSO 4 (13.22 mg, 0.183 mmol) was dissolved in M eOH (~5 mL) . The solution was added to a vial containing MoCl 5 (50 mg, 0.183 mmol) . Immediately a green color was formed as well as smoke, presumably HCl gas. The solution was stirred for ~10 minutes. The solution was then neutralized (determined by sampl i ng on a pH test strip) with a 1M methanolic solution of NaOH whereupon the color changed from green to orange/red. After neutralization NBu 4 Cl (203.4 mg, 0.732 mmol) . The solution was concentrated, h exanes added, and the white precipitate filtered. Ether w as added to the filtrate causing a n orange precipitate to crash out, the solution was centrifuged and the solvent decanted. The solid was washed with 50:50 methanol: ether , and centrifuged and decanted again. The remaining solid was dissolved in minimal a m ounts of MeOH and put in a - 20 °C f reezer for recrystallization . Orange X - ray quality crystals were produced after 2 days (30 mg, 10%). The X - ray analysis was the only characterization technique performed as the synthesis was not reproducible. 368 Representat i ve GC/MS t races for the successful N 2 reduction reactions. Figure 7 - 4 . GC/MS trace of an aliquot of the reaction solution from a reaction that produced hydrazine according to the indicator solution. 48 T he indicated peak at 13.424 represents the peak for stilbene . Integral = 267351776 369 Figure 7 - 5 GC/MS trace of an equal aliquot from the same solution shown in Figure 7 - 4 spik ed with 0.25 g hydrazine (added as hydrate). 48 The indicated peak at 13 .424 represents the peak for stilbene. Integral = 318242896 370 REFERENCES 371 REFERENCES 1. State of the Climate: Global Climate Report for August 2018. National Ocean and Atmospheric Administration Nat ional Centers for Environmental Information: 2018 . 2. Harries, J. E.; Brindley, H. E.; Sagoo, P. J.; Bantges, R. J., Nature 2001, 410 , 355 - 357 . 3. Petit, J. R.; Jouzel, J.; Raynaud, D.; Barkov, N. 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