. n as o. 1 a8“... . i. .151. .. this“ a! \lllmlllllllllllli This is to certify that the dissertation entitled Excited State Chemistry and Mechanistic Studies of Quadruply Bonded Bimetallic Systems presented by Tsui-Ling Carolyn Hsu has been accepted towards fulfillment of the requirements for PhD degree in Chemis try Major professor Se t. 22 1995 Date p ’ MSU is an Affirmative Action/Equal Opportunity Institution 0-12771 LIBRARY l Michigan Statel University | PLACE ll RETURN BOX to move this checkout from your ncord. TO AVOID FINES return on or bdoro onto duo. DATE DUE DATE DUE DATE DUE MSU loAnNflrmotlvoActlorVEouol Opportunitylnotituion mm: EXCITED STATE CHEMISTRY AND MECHANISTIC STUDIES OF QUADRUPLY BONDED BIMETALLIC SYSTEMS By Tsui-Ling Carolyn Hsu A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1995 ABSTRACT EXCITED STATE CHEMISTRY AND MECHANISTIC STUDIES OF QUADRUPLY BONDED BIMETALLIC SYSTEMS By Tsui-Ling Carolyn Hsu Multielectron reactions are fundamental to promoting energy conversion transformations such as the oxidation of water, and the reduction of oxygen and nitrogen. This research effort focuses on using the electronic excited state of specially designed transition metal complexes to harness the energy of a photon to drive oxidation-reduction reactions useful for energy storage. Can excited states directly participate in multielectron reactions initiated by visible light? We address this issue by using quadruply bonded metal-metal compounds (MA-M) as photoreagents. These systems possess low energy excited states localized at a coordinately unsaturated, electron rich core, which is essential for small molecule activation. The lowest energy 1(52—)SS*) transition of MAM complexes, corresponding to a metal-to-metal charge transfer (MMCT) transition, produces a two-electron mixed-valence excited state (i.e. Mn—MII —-) MI—Mm), which is predisposed for multielectron photochemistry. Two-electron reductions of substrate may be promoted at the MI site whereas oxidation may occur at the MIII site. The quadruply bonded metal-metal systems chosen to study are Moz[02P(OC6H5)2]4 (D4h), M2Cl4(dppm)2 (D211) and M2C14(PR3)4 (Du), where M2: M02, W2; dppm = bis(diphenylphosphino)methane, PR3 = PMe3, PMezPh, PBu3. Moz[02P(OC6H5)2]4 photoreduces 1,2-dichlorocarbons and produces a nixed reveal are re monci subsr edge-s anCh photot~ fincfir wflh di Vvfilll consist excited Vll'llle 4 lepres. Photo: System Such & and W mixed-valence M02[02P(OC6H5)2]4C1. Analysis of the organic photoproducts reveals that dichlorocarbons such as 1,2-dichloroalkanes and 1,2-dichloroalkenes are reduced via an initial chloride atom abstraction to yield olefins and monohalogenated alkenes, respectively, depending on the nature of the substrates. Excitation of M2Cl4(dppm)2 in the presence of PhSSPh affords an edge-sharing bioctahedral M2(III,III) addition product, M2Cl5(dppm)2(SPh). The wavelength dependence of quantum yield studies for the M2Cl4(dppm)2 photochemistry and the presence of a long-lived transient suggest that the reactivity is derived from metal-localized excited states. W2Cl4(PR3)4 photoreacts with dihalocarbons upon near-ultraviolet excitation and affords a mixed-valence W2(II,III) photoproduct, W2C15(PR3)3. The photoreaction of the Du complexes is consistent with LMCT excited state parentage as opposed to the metal-localized excited states of D2,l counterparts. The critical mixed-valence Ml—MIII excited state may be stabilized by virtue of the asymmetry of the bimetallic core. Heterobimetallic Moi-W systems represent a straightforward means to further study the. multielectron photochemistry. Prior to undertaking photochemical experiments of these systems, their photophysics have been explored. The excited state properties such as energy and lifetime of Moi-W species fall in the range of their Mo—4—Mo and W—‘LW analogues. Dedicated to my dearest parents, Tsang-Sui and Hui-Yuan whose endless love and support were essential to this accomplishment. iv ACKNOWLEDGMENTS First and foremost, I would like to thank my present group members, especially the metal-metal quadruple bond subgroup, Ann Macintosh, Sara Helvoigt, Dan Engebretson for their numerous helpful discussions and long-term friendship. I am greatly indebted to my former group collaborators 1-] y. Chang, Colleen Partigianoni, Claudia Turro for generously sharing their experience with me during my early years. My other fifteen labmates present and past, too many to be mentioned here, have furnished this research lab with a tremendous family atmosphere, which warmed my heart and got me going through the difficult times over the past several years. It is always a great joy working in Nocera's group. I wish to express my sincere thanks to all who help me learn the instruments for taking physical measurements, in particular, Dr. Rui Huang, Dr. Zhi-Heng Huang, Professor Doug Gage, and Ms. Bev Chamberlin in Mass Spectrometry Facility for introducing me to the Mass spec field; Dr. Yvonne Gindt, Dr. Kurt Wamcke and people in Professor Gerald Babcock's group for training me on EPR spectroscopy; Jim Roberts and Dr. Neils van Dantzig, in my group, for luminescence lifetime measurements, and Dr. Long Le for his expertise in polishing my NMR skills. In the absence of their assistance, the work presented here would be impossible. The chemistry department at Michigan State has provided me with a very pleasant working environment and I am most grateful to the machine shop, the glass shop, and electronic shop for their. friendly technical help. My deepest appreciation goes to my special friends, Stacey Bernstein, Ying Liang, Zoe V PflJBI Wu ht with t chenu KWOD‘; Isish b any mVflu. once, enthus VVHhOI unders nextst {Of Che Pikramenou, I. P. Kirby, Wen Lian Lee, Kuolih Tsai, Hui-Lein Tsai and Chun-Guey Wu here for giving me strength and encouragement during my graduate years. Other contributions to this work come from many stimulating discussions with the chemistry professors, including Dr. Kim Dunbar on metal-metal bond chemistry, Dr. Peter Wagner on organic photochemistry, Dr. Ned Jackson, Dr. Chi Kwong Chang and Dr. George Leroi for their thoughtful supports in many ways. I wish to show my gratitude to them. The most memorable person in my graduate life who can not be forgotten is my academic father, Professor Daniel Nocera. Dan's scientific insight and invaluable guidance have lead me to see the big picture of science. More than once, I would like to express my profound appreciations for his patience, enthusiasm and concerns by taking his time to train me as a good scientist. Without his intense criticisms, I would not be able to accomplish this work. His understanding and inspiration have always ensured me to move steadily on to the next step of my academic career. With all my heart, I gratefully acknowledge him for cherishing my scientific growth! vi LIST LIST CRAP CHAP TABLE OF CONTENTS Page LIST OF TABLES ............................................................ xi LIST OF FIGURES ........................................................... xii CHAPTER I INTRODUCTION 1. Photosynthesis ....................................... 3 II. Small Molecule Activation by Multielectron Transfer .............................................. 5 III. Photochemical Conversion ........................... 6 . IV. Metal-Metal Binuclear Complexes ................... 12 V. Quadruply Bonded Metal-Metal Systems ............ 17 VI. Thesis Outline ........................................ 23 CHAPTER II EXPERIMENTAL I. General Procedures ................................... 26 11. Synthesis ............................................. 27 A. Ten-akis(diphenylphosphate) Dimolybdenum Complexes ........................................ 27 B. Homonuclear Dimolybdenum and Ditungsten Complexes ........................................ 23 C Heteronuclear Molybdenum-Tungsten Complexes ........................................ 29 III. Reactions ............................................. 34 A. Photochemistry of Moz[02P(OC6H5)2]4 with Dihalocarbons .................................... 34 B. Photochemistry of MozCl4(dppm)2 with PhSSPh. 34 vii CHAPTER III CHAPTER IV C. Thermal Reactions of M02Cl4(dppm)2 with PhSSPh ........................................... D. Photochemistry of W2C14(PR3)4 with CH2C12. .. E. Thermal Oxidations of W 2Cl4(PR3)4 with PhIC12 Page 35 35 36 F. Thermal Oxidations of W2Cl4(PR3)4 with NOBF4 36 IV. Instrumentation and Methods ........................ A. Absorption Specu'oscopy ......................... B. Photolysis. ....................................... C. Mass Spectrometry ............................... D. Nuclear Magnetic Spectroscopy (NMR) .......... E. Electron Paramagnetic Spectroscopy (EPR) ...... F. Steady-State Luminescence Spectroscopy ........ G. Time-Resolved Laser Spectrosc0py .............. H. Crystallography ................................... PHOTOREDUCI'ION OF 1,2-DICHLOROALKANES AND 1,2-DICHLOROALKENES BY TETRAKIS (DIPHENYLPHOSPHATO)-DIMOLYBDENUM(II,II). I. Background .......................................... II. Results and Discussion ............................... A. Photochemistry ................................... B. Quantum Yields .................................. C Mechansirn ....................................... PHOTOREDUCI'ION OF DIARYL DISULFIDES BY QUADRUPLY BONDED DIMOLYBDENUM ANDDITUNGSTEN COMPLEXES I. Background .......................................... II. Results and Discussion ............................... viii 37 37 37 38 40 4o 41 41 42 45 so so 61 64 69 72 CHAPTER V. CHAPTER IV A. Photochemistry ................................... B. Thermal Chemistry ............................... C. Quantum Yields .................................. D. Transient Absorption Studies ..................... E. Proposed Mechanism. ............................ 111. Conclusion ........................................... CHARGE TRANSFER PHOTOCHEMISTRY OF QUADRUPLY BONDED DITUNGSTEN HALOPHOSPHINE COMPLEXES I. Background .......................................... II. Results ............................................... A. Photochemistry of W2Cl4(PR3)4 ................. B. Comparison of Thermal Oxidation Chemistry. . .. C. Organic Photoproduct Analysis .................. D. Photophysics and Transient Absorption Studies. . 111. Discussion ........................................... IV. Conclusion ........................................... FINAL REMARKS: SPECTROSCOPIC STUDIES OF QUADRUPLY BONDED HETEROBIMETALLIC MOLYBDENUM-TUNGSTEN COMPLEXES ........... I. Background ............................. t ............. II. Results and Discussion ............................... A NMR Studies of MoWCl4(PR3)4 and MoWCl4(dppm)2 ................................. B. Electronic Absorption Specstroscopy ............ C Luminescence Spectroscopic Studies ............ 111. Conclusion ........................................... ix Page 72 89 91 95 104 105 107 110 110 119 124 124 130 135 137 138 141 141 153 153 162 Page LIST OF REFERENCES ...................................................... 165 3.1 4.1 4.2 4.3 4.4 4.5 5.1 6.1 3.1 3.2 4.1 4.2 4.3 4.4 4.5 5.1 6.1 6.2 LIST OF TABLES Formal Reduction Potentials of the M02(II,III)/(II,II) and Moz(I[I,III)/(II,III) Couples of Dimolybdenum Tetrakis Sulfate, Phosphate, and Diphenylphosphate Complexes. ........................... Quantum Yield Data for the Photoreaction between M02[02P(OC6H5)2]4 and 1,2-Dichlorocarbon in Various Nonaqueous Solvents. ....................................................... Crystallographic Data for MozCl4(dppm)2(lt-Cl)(u-SPh)02C7H3. Positional Parameters and Equivalent Isotropic Displacement Parameters (A2) for M02Cl4(dppm)2(|.t-Cl)(u-SPh)-2C7H3 .......... Selected Bond Lengths (A) and The Standard Deviations for for M02Cl4(dppm)2(u-Cl)(u-SPh)'2C7H3. .................................. Selected Bond Angles (deg) and The Standard Deviations for MozCl4(dppm)2(p.-Cl)(u-SPh)-2C7H3. ................................... Wavelength Dependence of Quantum Yields for Photoreaction of M02Cl4(dppm)2 and W2Cl4(dppm)2 with PhSSPh. ................... Photophysical properties of W2Cl4(PR3)4 complexes in benzene and dichloromethane at room temperature. ............................. 319 NMR Chemical Shift and coupling constants (J) of M2Cl4(PR3)4 (M2 = M02, MOW, WZ) ....................................................... Luminesence Lifetimes and Emissive Quantum Yields of 55* Excited State of M2Cl4(PR3)4 (M2 = M02 MoW, W7) Complexes. .. xi Page 60 62 76 77 81 82 92 126 146 161 1.7 3.1. 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 3.1. LIST OF FIGURES Solar spectral distribution outside the Earth's atmosphere (air mass 0) and at normal incidence to the Earth's surface (air mass 1). Shaded area is where is used for photosynthesis. (ref. 26) ............. Schematic representation of photoinduced electron flow in Photosystems I and II. .......................................................................... Molecular chemical device based on photoinduced electron transfer. Pel represents a photosensitizer whereas Rel represents an electron transfer relay. - - ....... Photosplitting of water on (a) composite catalyst (b) catalyst with sacrificial donor (c) catalyst with sacrificial acceptor. (ref. 46) ...... Schematic illustration of an alcohol-splitting cycle for conversion of isopropanol to acetone and hydrogen catalyzed by polyoxometalates. ....... . ........ ....... Photooxidation mechanisms for the reaction of an organic substrate (RX) with the photogenerated (a) "dissociative diradical" of d7 and (b) "associative diradical" of d8 binuclear complexes. Localized two-electron mixed-valence excited state of binuclear metal complexes is produced upon metal-to-metal charge transfer (MMCT). ................................................................................................ MO diagram of quadruply bonded MAM binuclear metal complexes. ............................................................................................. Energy level diagram for 8/5* manifold of MAM complexes in valence bond description of the electronic states formed by the dxy orbitals with the corresponding MO formalism. ............................... Energy level diagram summarizing the mechanism for the photochemistry of the quadruply bonded M02[HP04]44- in acidic solution. .................................................................................................. Page 11 14 16 19 21 3.5 3.6 3.7. 3.8 3.9 3.2. 3.3 3.4. 3.5. 3.6 3.7. 3.8 3.9 3.10 4.1 Electronic absorption spectra of (a) Moz[OzP(OC6H5)2]4 (_—) in CH2C12 solution and (b) Moz(HPO4)4" (- - —) in 2 M H3PO4 at 25 Spectral changes resulting from the photolysis (I. > 495 nm) of CszClz solution of M02[02P(OC6H5)2]4. ........................................ Valence bond description of the electronic states formed by the dxy orbitals with the corresponding MO formalism (left). With the removal of one electron, a transition of doublet states is observed (right). ..................................................................................................... Mass spectra of the organic products resulting from the photoreaction of trans-dichlorocyclohexane and Moz[02P(OC6H5)2]4 with cyclohexene obtained as the photoproduct. [M]+ denotes parent ion peak mass fragment. ....... Mass spectra of the organic products resulting from the photoreaction of o—dichlorobenzene and Moz[OzP(OC6H5)2]4 with (a) chlorobenzene obtained as the primary product and (b) tri- chlorobiphenyl obtained as a secondary product in ~3% yield. Mass spectra of the organic products resulting from the photoreaction of cis-dichloroethylene and Moz[02P(OC6H5)2]4 with chloroethylene obtained as the photoproduct. The star mark * indicates the presence of 02 as a background from the air. ............ Computer generated space-filling models of (a) M02[02P(OC6H5)214'2THF and (D) M02[02P(OC6I'I5)2]4. .............. Energy level diagram summarizing the mechanism for the photochemical reduction of saturated 1,2-dichlorocarbons by M02102P(OC6Hs)2]4. ................................................. Energy level diagram summarizing the mechanism for the photochemical reductionof unsaturated 1,2-dichlorocarbons by M02102P(OC6H5)2]4. ............................................................................ Electronic absorption spectral changes during the photolysis (Itexc > 435 nm) of M02C14(dppm)2 with a twenty-fold excess PhSSPh in deoxygenated dichloromethane at 16°C. Spectra were recorded at O to 240 mins in 30 min intervals. ....................................................... Page 49 51 53 55 56 57 63 65 67 4.4 4.5 EN] .. .2 . A 4.6 4.7 8 4 4.9 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 (a) Electronic absorption spectrum of MozCl4(dppm)2(rr-Cl)(tt—8Ph) in CHzClz solution at room temperature. (b) FAB-MS spectrum of MozCl4(dppm)2(tt-Cl)(tl-SPh). ............................................................. ORTEP view of molecular structure of photoproduct M02C14(dppm)2(u-Cl)(u-SPh)-2C7H8. Thermal ellipsoids are drawn at50% probability. ............................................................................... Electronic absorption spectral changes during the photolysis (Inc > 365 nm) of toluene solution of MozCl4(dppm)2 with excess PhSSPh at 8 °C; specu'a were recorded at 0, 36, 70, 105, 140, 185, 235 and 300 mins. The irradiation was interrupted when the 600- nm band had attained a maximum absorption value. ....................... FAB-MS spectrum of the product isolated from the photolyzed (lac > 365 nm) toluene solutions of M02C14(dppm)2 showing the presence of M02Cl4(dppm)2(SPh)2. The asterisk is MOZCl4(dppm)2(SPh). .......................................................................... (a) Electronic absorption spectrum of MozCl4(dppm)2(tt-Cl)(u-SEt) in CH2C12 solution at room temperature. (b) FAB-MS spectrum of the product isolated from the photolyzed (lac > 235 nm) CH2C12 solutions of M02Cl4(dppm)2 with 40-fold excess of EtSSEt, consistent with M02Cl4(dppm)2(u-Cl)(u-8Et). .................................. Electronic absorption spectrum of the isolated brown product from reactions of MozCl4(dppm)2 with (a) excess PhSSPh refluxing in toluene solutions for 12 hours (b) excess EtSSEt refluxing in toluene solutions for 36 hours. ........................................................... Electronic absorption spectra of M02Cl4(dppm)2 (.) and W2Cl4(dppm)2 (— — -) complexes in toluene. .................................... Time evolution of the disappearance of the picosecond transient absorption of MozCl4(dmpm)2 in dichloromethane. The spectra were obtained 2, 20 and 50 ps after a 600 nm, 3 ps excitation pulse. The inset shows a plot of the ln(AOD) for the transient absorption at 630 nm vs time. (ref. 164) ................................................................ xiv Page 74 75 85 86 87 9O 93 4.1 4.1 5.1 5.3 5.4 5.5 4.10 4.11 5.1 5.2 5.3 5.4 5.5 Transient difference spectra (0) of a deoxygenated (a) dichloromethane solution of MozCl4(dmpm)2 recorded 1 us after 355 nm, 10 ns excitation, and electronic absorption spectrum of M02C16(dmpm)2 (—) in dichloromethane, (b) benzene solution of W2Cl4(dppm)2 collected 100 ns after 532 nm, 10 ns excitation, and electronic absorption spectrum of W2C15(dppm)2 (—) in dichloromethane. (ref. 164) .................................................................. Energy level diagram summarizing the formation of the edge- sharing bioctahedral distortion of the 1(1!:8'“) (or l(61l:"‘)) excited state of the M2Cl4(PP)2 complexes to stabilize the charge transfer mixed-valence state. ............................................................................. Electronic absorption spectral changes during the photolysis (Am: > 375 nm) of W2Cl4(PMe3)4 in deoxygenated dichloromethane at 15.0 °C at ~5 h intervals. Panel (a) shows the photolysis reaction proceeding to a maximum absorption in the near-IR region. Panel (b) shows the spectral changes occurring with continued photolysis of the solution. The absorbance scale of the near- infrared relative to the visible spectral region in both panels is expanded by a factor of 6. ................................................................... FAB-MS spectrum of the final products resulting from the photoreaction of W2Cl4(PBu3)4 with dichloromethane. ................. 31P NMR spectrum of the final products resulting from the photoreaction of W2Cl4(PBu3)4 with dichloromethane-dz. ............ Electronic absorption spectral changes during the photolysis (hem > 375 nm) of W2Cl4(PMe2Ph)4 in deoxygenated dichloroethane at 15.0 °C at ~6 h intervals. Panel (a) shows the photolysis reaction proceeding to a maximum absorption in the near-IR region. Panel (b) shows the spectral changes occurring with continued photolysis of the solution. ................................................................... The X-band EPR spectrum of W2Cl4(PMe3)4 in frozen CH2C12/2- MeTHF solutions (T = -170 °C) exhibiting the maximum near- infrared absorption in panel (a) of Figure 5.1. .................................. XV Page 100 103 113 114 115 117 5.6 5.8 5.9 5.10 5.11 5.15 h 5.6 5.7 5.8 5.9 5.10 5.11 5.12 Electronic absorption spectra of the product resulting from the thermal oxidation of W2Cl4(PMe3)4 by one equivalent of (a) PhIC12 in dichloromethane (—) (b) NOBF4 in CHzClleeOH solutions (- The X-band EPR spectrum of the product resulting from the thermal oxidation of W2Cl4(PMe3)4 by one equivalent of (a) PhIClz in frozen CH2C12/2-MeTHF solutions (b) NOBF4 in frozen CHzClleeOH solutions. Both spectra are obtained at T = —l70 The X-band EPR spectrum of the product resulting from the thermal oxidation of W2Cl4(PMe2Ph)4 by one equivalent of (a) PhIC12 in frozen CH2C12/2-MeTHF solutions (b) NOBF4 in frozen CH2C12/MeOH solutions. Both spectra are obtained at T = —170 The X-band EPR spectrum of frozen CH2C12/2-MeTHF solutions (T = —l70 °C) of (a) the photoproduct exhibiting the maximum near-infrared absorption, and of (b) the product resulting from the thermal oxidation of W2Cl4(PBu3)4 by one equivalent of PhIClz. Gas chromatogram (inset) of (a) the volatile organic products resulting from the photolysis reaction of W2Cl4(PMe3)4 (1.5 mM) in CHzClz. (b) the coupling product resulting from dirneration of chloromethyl radicals at higher concentrations. The retention times during which the CH2C12 solvent peak elutes from the GC column is shown on the side. ............................................................................ Transient difference spectra of W2Cl4(PMe3)4 in deoxygenated dichloromethane (O) and benzene (0) recorded immediately after a 650 nm, 8 ns laser excitation pulse. .................................................... Transient difference spectra of W2Cl4(PBu3)4 in deoxygenated (a) dichloromethane and (b) benzene recorded 50 ns after a 355 nm, 10 ns laser excitation pulse. The bottom trace in panels (a) and (b) is the difference spectrum and the top trace is the transient’s absorption spectrum generated by the addition of the ground state absorption spectrum of W2Cl4(PBu3)4 to the difference spectrum. xvi Page 120 121 122 123 125 128 129 5.1 5.1 6.1 6.3 6.4 6.5 6.6 6.7 6.8 6.9 5.13 5.14 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 Qualitative correlation MO diagram of D211 and D2d geometries based on the theoretical calculation of the model compound M02Cl4(PH3)4. (ref. 163) ...................................................................... Electronic absorption spectra of (a) MoZCl4(PMe3)4 (——) and WZCI4(PMC3)4 (- - -) in benzene. ...................................................... Schematic potential energy curves of the mixed-valence states of Mom—wan) vs. Mo(III)-W(I) in heterobirnetallic systems. ........... Proposed scheme for the formation of the Mo(l)—W(III) excited state. Upon 58* excitation, the charge transfer mixed-valence state can be stabilized without utilizing the edge-sharing bioctahedral distorted intermediate. .......................................................................... 31P{1H} NMR spectrum of MoWCl4(PMePh2)4 at 202.4 MHz in benzene-d6 at room temperature. The asterisk indicates the presence of its M02 congener with approximately 6.8%. ............... ”Pl 1H} NMR spectrum of MoWCl4(PMe2Ph)4 at 202.4 MHz in benzene-d6 at room temperature. The asterisk indicates the presence of its M02 congener with approximately 1.2%. ............... 3114111} NMR spectrum of MoWCl4(PMe3)4 at 202.4 MHz in benzene-d6 at room temperature. The asterisk indicates the presence of its M02 congener with approximately 4.4%. ............... 3114111} NMR spectrum of MoWCl4(dppm)2 at 202.4 MHz in CD2C12 at —40 °C. The asterisk indicates the presence of its M02 congener with approximately 8%. ............................................ 1H NMR spectrum of MoWCl4(PMePh2)4 at 300 MHz in benzene- d6 at room temperature. ........................................................................ 1H NMR spectrum of MoWCl4(PMe2Ph)4 at 300 MHz in benzene- d6 at room temperature. ........................................................................ 1H NMR spectrum of MoWC14(PMe3)4 at 300 MHz in benzene—d6 at room temperature. ............................................................................. xvii Page 133 134 139 140 142 143 144 145 148 149 150 6.11 6.11 6.12 6.13 6.14. 6.15 6.16 6.17 6.10 6.11 6.12 6.13 6.14. 6.15 6.16 6.17 1H NMR spectrum of MoWCl4(PMe3)4 at 500 MHz in benzene-d6 at room temperature. ............................................................................. 1H NMR spectrum of MoWCl4(dppm)2 obtained at 300 MHz in @202 at -40 °C. ................................................................................. Electronic absorption spectra of (a) MoWCl4(PMe3)4 (-- -) (b) MoWCl4(PMe2Ph)4 ( —) (c) MoWCl4(PMePh2)4 (----) in benzene solution at room temperature. .............................................................. Electronic absorption spectra of (a) M02Cl4(PMe2Ph)4 (-- -) (b) MoWCl4(PMe2Ph)4 (—)(c) W2Cl4(PMe2Ph)4 (....) in benzene solution at room temperature. .............................................................. Absorption and emission spectra of 52 H 58* transitions of MoWCl4(PMePh7)4 in benzene solution at room temperature. ..... Excitation, absorption (- - -) and emission spectra of 52 H 88* transitions of MoWCl4(PMe2Ph)4 in benzene solution at room temperature. ........................................................................................... Excitation, absorption (- - -) and emission Spectra of 82 H 55* transitions of MoWCl4(PMe3)4 in benzene solution at room temperature. ........................................................................................... Emission Spectra of 55"'—)82 transitions of MoWCl4(PMePh2)4 in solid state (a) at room temperature and (b) at 77 K. ......................... xviii Page 151 152 154 155 156 157 158 160 Imific ChCrg PTOdL “3&H;' CHAPTER I INTRODUCTION Seeking and developing alternate sources of energy is a necessary endeavor as global needs continue to deplete fossil fuels. The energy resource for nature is photosynthesis. Solar energy provides the driving force in photosynthesis for the continuous cycling of atmospheric C02 and 02 through the biosphere to the high energy fuels, sugar and water, respectively. About 50% of the incident radiation on the earth is used photosynthetically (350 to 700 nm of the entire solar spectral distribution, which ranges from 350 to 2500 um; see Figure 1.1). Each year at least 3-6 x 1017 kcal of free energy from sunlight is captured by photosynthetic organisms and used for biosynthesis.l This is more than ten times the amount of energy derived from the annual global usage of fossil fuels. In addition to its abundance, solar energy is also a pollution-free source of energy. A challenging goal confronting chemists is to develop an artificial photosynthetic system for the conversion of light energy into chemical energy, thereby providing an alternate process for energy storage and production.2 Accordingly, mimicking the highly efficient initial reactions of natural photosynthesis is a central goal of photochemical research.3‘6 UV MSIBLE|<—— INFRARED 4 1" 0 /AIR MASS ZERO (Outside Atmospherel _L a) 1 AIR MASS ONE .3 N I .0 oo Solar Spectral lrradiance / Wm-an-t O is l 1111 1 1'1 1 r 400 800 1200 1600 2000 2400 711nm Figure 1.1 Solar spectral distribution outside the Earth's atomsphere (air mass 0) and at normal incidence to the Earth's surface (air mass 1). Shaded area is where is used for photosynthesis. (ref. 26) Wile PlTU into “her Phot. 04m: Cl1101 1101' 1 . Q n. r'df I. Photosynthesis The development of efficient, artificial systems for the conversion and storage of solar energy requires an understanding of the mechanism of photosynthesis. Natural photosynthesis occurs not only in green plants but in lower microorganisms such as algae, cyanobacteria, and green sulfur bacteria. Plants and bacterial photosynthesis are fundamentally similar processes and differ by the hydrogen donors that they employ. The overall reaction of photosynthesis can be written in a general form: 21420 + co2 2; > (01120) + H20 +20 (1.1) where Chl = chlorophyll; H2D = H20, H28, or lactate; D = oxygen, sulfur, or pyruvate for green plants, green sulfur bacteria and bacteria, respectively.7 The reaction that has been most intensively investigated in green plants involves the absorption and transduction of light to chemical energy as follows, hv Chl 2H20 + A > 2H2A +02 (1.2) where A is the electron acceptor. This process is performed with two photosystems. Each of them contains a photoreaction center working in series, namely P700 in Photosystem I and P680 in Photosystem 11 (Figure 1.2). The chlorophyll molecule in each of the photochemical reaction centers captures the photon energy with 100% quantum efficiency to yield a high energy electronic excited state. This initial absorption event initiates electron/hole charge separation via a series of electron transfer Steps within the reaction center. In \1 ‘ d 28 H mEosmmeoFE 5 >6: 5:88 eoozefieoga mo confinemeqoc cumEonum «A «Sufi _ 52:335. .. Eofi>mo~ono A v ? > > ?N W J N J .5 + No 5.950 m. o 1 £2 oazm To: 9 a 2. acmmn— <1; :9 Pb CC nh P11 P6 cor ore fou yiel ther NA] CO- 11. ‘ PTOCt SCpaI addlt: found N03~ t5.110.. (flhlc 1110113- 15311; 1’- 11117: [‘9 5 Photosystem I, electrons are passed from the excited state of the P700 reaction center through a series of carriers to ferredoxin (FRS), which in turn reduces nicotinamide adenine dinucleotide phosphate (NADP+) to NADPH. In Photosystem II, excited P680 transfers an electron to a series of phe0phytin/plastoquinone pigments (PQ) to create a charge-separated P680+IPQ' pair. The P680"’ acquires'an electron from a tetranuclear Mn cluster containing protein complex to return to its ground state. By turning this cycle over in successive one-electron steps, the tetranuclear Mn cluster is oxidized by four electrons whereupon it catalyzes the oxidation of two water molecules to yield four electrons, four protons and a molecule of oxygen.8 The overall process therefore uses light energy to energetically run electrons uphill from H20 to NADP“; the biological electron acceptor, to yield NADPH, which in turn reduces C02 to give energy-rich carbohydrates. 11. Small Molecule Activation by Multielectron Transfer Photosynthesis provides us with the two fundamental bioenergetic processes in nature: conversion of light into chemical energy by charge separation and multielectron transformations to activate small molecules. In addition to Photosystem 11, many examples of multielectron redox reactions are found in enzymatic systems.9‘“ These include 8032‘ -+ HS" (sulfite reductase),12 NO3' —) NOf (nitrate reductase),13 NOf —> NH3 (nitrite reductase),“ 02 -9 H20 (cytochrome c oxidase),15 02 —) H20 (blue copper oxidases),16 N2 —) NH3 (nitrogenase),17'18 and 2H+ -) H2 (hydrogenase).19 In each case, the enzyme is thought to have one or more metals at the active site to promote the multielectron transformation. The mechanisms have been addressed by in vitro and in functional model studies for redox processes.”21 Bioinorganic and bioorganic C111 bit 616 p0 mu cor actl 11th 111. excl: cxcit SL113 Flora ‘ ’ ‘h l JCS: . 6 chemistry has sought to emulate the overall multielectron transformations of 22‘2“ such as the direct four-electron biology with a number of metal complexes, electrocatalytic reduction of 02 to H20 by ruthenated cyanophenyl cobalt porphyrins.25 However, these studies have yet to successfully drive the multielectron transformations with light, a key step in designing energy conversion schemes. Accordingly, the development of new small molecule activation reactions by light-sensitive coordination compounds are important to investigate, especially those involving multielectron transfer reactivity. III. Photochemical Conversion The addition of a photon to a molecule in its ground state raises it to its excited state where the incident energy is stored. As a result, the electronically excited molecule is a stronger oxidant and a stronger reductant than its ground state parent. For this simple reason, a photon can be harvested by a molecule to effect the reactions that are kinetically or thermodynamically inaccessible from the ground state. Nevertheless, in order to mimic the high efficiency of sunlight conversion of the photosynthesis system, certain criteria of the excited state molecule are required.26 The excited state energy must be effectively translated into a thermodynamically unfavorable chemical reaction at high photochemical quantum yield. Thus energy degrading processes such as intramolecular nonradiative decay and energy-wasting back electron-transfer reactions must be In order to fulfill these criteria, many schemes rely on organizing the photosensitiZer, donors, and acceptors within the organized assemblies provided 27,28 29,30 31,32 3,5,33,34 by polymer-films, membranes, vesicles, molecular systems. These artificial photosynthetic systems have the advantage that specific so 01 [Ill 510 CDC 0113 few long ph0t mm with - Water 7 pr0perties of the photoinduced electron transfer reaction can be tuned by synthetically designing sensitizers, charge-relays, donors, bridges and acceptors spatially juxtaposed within the assembly.”36 Figure 1.3 shows the general strategy of the photochemical molecular device.37 Light may be absorbed by a diverse array of photoactive redox metal systems ranging from metalloproteins to mononuclear metal complexes such as the ruthenium polypyridine complex?”0 Parallel to the function of chlorophyll in the reaction center of the green leaf, an electron is transferred from the photosensitizer one at a time to an electron storage center, where electrons are passed to reduce substrate. A frequently encountered problem in designing an artificial photosynthetic system is the fast charge recombination of the energy-releasing process in the charge separation state has to be prevented before the chemistry can be achieved. A highly efficient forward reaction when combined with a slow back electron transfer, results in a long-lived, charge separation state, which determines the overall efficiency of the photochemical process. The time scales of the forward and back reactions can effectively be managed in the heterogeneous environment!”44 Ti02 semiconductor particles with two metal/metal-oxide sites on a surface have been explored as promising water-splitting photocatalysts.“5‘47 Figure 1.4a shows how water is split on the surface of TiOz. Upon excitation across the band gap of the photocatalyst, electron-hole pairs are generated; negatively charged electrons get injected into the conduction band (CB) and positively charged holes remain in the valence band (VB). Electrons trapped in the metal site 1 (e.g. Pt cathOde) reduce water to H2, and holes trapped in the metal site 2 (e.g. Rqu anode) oxidize water to 02. The system is driven by a sacrificial electron donor such as MeOH, which is oxidized to C02 (Figure 1.4b). In contrast, if the metal modified -c< $22 8.3:“: 5:83 EN 35352 EM 3233 5555.885 a 2:8an Em commas: .8583 8035923 co 3me 83% 30:55 33882 n fl 2:»:— E coz . .. 5 -o_~.>>_ E .Im anti . 4.1.2,. I ”zeroes: :0 L23. +1 0 fermion: :o -932: is chi sm pic [1111 12 is the same, evolving little since the initial discovery of excited state redox chemistry over thirty years ago. Namely, the photoactive metal complexes undergo single electron transfer to or from their excited states. By itself, the single electron transfer step is limited inasmuch as most important reactions including small molecule activation processes involve two or more electrons. Thus, as is prevalent in the systems discussed heretofore, the single electron transfer step must be coupled to achieve the multielectron reactivity. We wondered if an excited state could be designed to undergo a direct multilelectron reaction. The realization of this goal is important because it fundamentally represents a new reaction of excited states and therefore opens new avenues in the design of energy storage schemes. Because we wished to observe a multielectron reaction, we thought it would be logical to consider binuclear redox systems where the redox activity of the two metal subunits might be exploited for a rich redox chemistry/.6345 IV. Metal-Metal Binuclear Complexes The photochemistry of binuclear complexes was popularized with the d7— (17 and d9—d9 metal-metal compounds.“‘7° The lowest energy transitions in these complexes are metal-localized ab —9 do“ and drt“ —> do and their irradiation usually results in the cleavage of the metal-metal bond (Figure 1.6a). From the 1(02 —> 00*) singlet excited state, metal-metal bond breaking correlates to M(CO)5+ and M(CO)5' disproportionation products. Alternatively, if the singlet State nonradiatively decays to the lower lying 3(OO'*) triplet state, dissociation produces M(CO)5 radicals. More generally, homolysis of the metal-metal bond to generate reactive radical species is the general rule for binuclear metal-metal single bonded complexes. This process is also observed for ds—d5 (Cp2M2(CO)6, [th sysl com (Figl 8161‘ 13 M: Mo, W), d9—d9 (C02(CO)6(PR3)2), d5—d7 (CpM(CO)2M’(CO)5, M = Mo, w; M’ = Mn, Re), d7—d9 (CpFe(CO)2M’(CO)3(PR3)3, M = Mn, Re), and d5—d9 (CpM(CO)3Co(CO)4, M = M0, W) complexes.“ The disadvantage of these metal- metal single bonded systems is that the energy of the photon is diverted to metal- metal bond breaking. This results in the loss of polynuclearity in the excited state. Hence, the uncoupled, selective multielectron activation of substrates by these complexes is difficult to control. Conversely, the structural integrity of metal-metal core can be preserved in the excited states of d8-~d3,7"7‘ own-d10 bimetallic.75 or d1°---d3 heterobimetallic systems." Excitation of the lowest energy do" -)po transition of these complexes yields an associative diradical species with a long-lived excited state (Figure l.6b)."2 The excited state chemistry of this class of compounds is exemplified by the dgmd8 binuclear compound Pt2(P205H2)44', which consists of two square-planar tetracoordinated Pt(II) metal units held together in a face-to- face orientation by the four bridging PZOSHZZ‘ bidentate ligands.- The d} and p2 orbitals of the platinum centers overlap with each other to give do/do’ and polpo'“ bonding/antibonding orbitals. Since do and do’ are filled, the ground state is expected to be nonbonding, although spectroscopically it has been shown there is a weak interaction between the two metal units. Upon the lowest energy do" —> po excitation, an electron is promoted from a localized antibonding orbital on the exterior of the M2 unit (do* orbital) to a localized bonding orbital in the interior of the dinuclear cage (p0 orbital). This electron promotion results in the formation of a net metal-metal bonding interaction in the excited state. Chemically, the excitation creates a hole on each of the coordinatively unsaturated metal centers thereby generating a very reactive associative diradical ( [dB—(181* = [oM—M-]* ). The 3A2“ excited state of l4 Figure 1.6 Photooxidation mechanisms for the reaction of an organic substrate (RX) with the photogenerated (a) "dissociative diradical" of d7 and (b) "associative diradical" of d8 dinuclear complexes. Ill 3 efl su dcl pht pht mu. r11 sing the at 11.: 1116 2 Can t. 15 Pt2(P205H2)44‘, which exhibits a long lifetime, 9 yrs, and high quantum yield ((116 = 0.5), enables Pt2(P205H2)44‘ to undergo excited state birnolecular reactions." The triplet diradical activates organic substrate via single-electron or atom transfer to effect the overall two-electron photocatalytic reduction of the organic substrates such as the conversion of isopropyl alcohol to acetone71 and the dehydrogenation of selected hydrocarbons to olefins-”'78 The above systems illustrate a general characteristic of classic photosystems: triplet excited states correlate to triplet-spin type primary photoproducts by means of electron transfer or atom abstraction.79 Alternatively, multielectron processes such as oxidative-addition, reductive-elimination or atom- transfer, correlate to singlet products. Thus, spin conservation arguments suggest that successful multielectron photoreagents will acquire their reactivity from the singlet electronic excited states. The above analogy between one-electron chemistry and biradical excited states made us speculate as to whether multielectron transfer might be emphasized for excited states in which the two metal-localized electrons were not triplet coupled on individual metal atoms (as in the d8---d8 complexes) but rather singlet coupled on a single center of the binuclear core (i.e. :M‘—M*). The preparation of this type of excited state, called a zwitterionic excited state, seems to suggest that the metal orbitals should be weakly coupled. In this case, the multielectron. zwitterionic excited state may be prepared by exciting a metal-to-metal charge transfer (MMCT) (Figure 1.7). Here electrons originally localized on the individual metal centers of a bimetallic core in the ground state, are paired upon the absorption of a photon to produce an excited state. The two-electron reduction of the substrate is anticipated to occur at the :M' Site, and the two-electron oxidation occurs at the M+ site. In this way, the zwitterionic excited state, possessing two-electron mixed-valence character, can realize direct multielectron reactivity from a discrete excited state. 16 +M—M:-] (= Mn+1 Mn“) hv MMCT M—M (= M"—Mn ) Figure 1.7 Localized two-electron mixed-valence excited state of binuclear metal complexes is produced upon metal-to-metal charge transfer (MMCT). get 05: 20c Th: pht‘ 11118 [Co p13: feat M) + 17 Can stable zwitterionic excited states be elaborated so that their chemistry may be explored? A cursory glance at the problem suggests that the metal centers should be widely separated to achieve zwitterionic character upon MMCT excitation. Precedent for this approach comes from Taube and coworkers’ benchmark studies of mixed-valency in which electron localization between two metal centers may be observed80 with the judicious choice of a bridging ligand (e.g. [(NH3)5Mm-L-M"(NH3)5]5+; M = Ru, Os; L = bipyrazine),81 However, in general these complexes will not be practical excited state reagents because the oscillator strength for the MMCT transitions is too small and the metals are coordinatively saturated. Also the back electron transfer often occurs rapidly.82 The oscillator strength can be increased with cyanide-bridged complexes, [(NC)5Mn(u-NC)Com(CN)5]6‘ (M: Ru, Os, Fe). But the resulting redox isomeric photoproduct [(NC)5Mm(u-NC)Con(CN)5]6' generated by IT excitation is unstable and often leads to the subsequent dissociation to [Mm(CN)6]3‘ and [Con(CN)5,]3'.83 In perhaps one of the great ironies of inorganic chemistry, one place to find two electrons in weakly coupled metal orbitals is in complexes that feature the shortest distances between metals, quadruple bonded metal-metal (M4 M) complexes. V. Quadruply Bonded Metal-Metal Systems Quadruply bonded metal-metal (MA-M) complexes84 offer an unique opportunity to explore excited state oxidation-reduction chemistry. These systems possess several other attractive features as polynuclear multielectron . photoreagents. First, the presence of an open coordinate site in the axial position of the metal-metal bond provides a site for initial substrate activation. Second, the metal-metal multiple bond is an electron rich core that can provide many electrons 18 in an oxidation-reduction transformation. Third, the lowest energy transitions are metal localized and lie in the visible spectral region. Fourth, the strong metal-metal bond makes photodissociative pathways less likely. Both theoretical calculations and experimental spectroscopic studies show that the bond strength of the metal- metal quadruple bond is in the range of 127-190 kcal/mol. Even under intense UV . radiation, the metal-metal bond is usually retained.”86 Finally, and perhaps most important. with regard to the theme of this thesis, the lowestenergy transitions produce a long-lived zwitterionic excited state of : "—M* character. This excited state is of singlet character and unlike most singlet excited states, which typically exist for a few nanoseconds or even shorter,”88 this excited state can be quite long-lived to permit its birnolecular reaction. A general molecular orbital diagram for M-‘l-M complexes is shown in Figure 1.8. Quadruply bonded MAM dimers are composed of two ML4 fragments with four electrons delocalized in four degenerate d orbitals from each fragment to give a 021:482 ground state electronic configuration. The dzz orbitals overlap with each other to form a metal-metal a bond, while dn and dyz orbitals give two metal-metal 1t bonds. The dxy orbitals on each metal are parallel to each other and consequently only weakly interact to give a 8 bond. For two (14 metals, as is the case with MoII and W", the 5 orbital is the highest occupied molecular orbital (HOMO), while the 8* orbital is the lowest unoccupied molecular orbital (LUMO). Therefore the lowest energy, electronically allowed, transitions involve 5 —> 8*, it -) 8*, 8 —-) 1t* promotions, and ligand-to-metal charge transfer (LMCT) or metal- to-ligand (MLCT) transitions lie to higher energy. Because the metal localized transitions are in the visible region, the photochemistry of these complexes may be studied by utilizing visible light. While the molecular orbital model conceptually describes the qualitative bonding diagram for M-‘i-M complexes, a valence bond model gives us a better 19 ' \ I \ I \ I, ‘\ I \ I * \ \ I I “ \ ’ ’l \‘ \ \ \ S‘ “‘ \ dzz \\ ‘ \ \ \ \‘ ‘ s I I \ \ I ' \ \ I ' \ \ ' I \ \ I ' \ ‘ I ' \ X ' ~ It ' \eu ' \ I \ I \ I \ I \ I I I Figure 1.8 MO diagram of quadruply bonded M-‘-M binuclear metal complexes. 5151 u...» \ . .20 and more quantitative understanding of the states and their energies, especially for those associated with the 5 orbitals. Because the overlap of the dxy orbitals is weak, the 8 bond of MAM complexes is similar to the bonds formed from the two-electron, two-center weakly coupled orbitals of organic diradicals, such as twisted ethylene, and the stretched 0' bond of a hydrogen molecule.”‘92 The large energies associated with pairing electrons in weakly coupled d,‘y orbitals give rise to four states: two low-energy diradical states and two high-energy zwitterionic states as shown Figure 1.9. The two diradical states come from an electron in each singly occupied orbital of the metal site with spins opposed to give the singlet 11A18(in D4,, symmetry) 1(52) ground state, and with spins parallel to yield the triplet 3A2u 3(85*) excited state. The upper zwitterionic excited states arise from the symmetric 21A]g and antisymmetric 1A2n linear combination of d,‘y orbitals to form the corresponding l(8"'8"') and 1(88*) states with two electrons paired in either orbital of one metal site. As shown by Hay’s general valence bond calculation of Re2Clgz‘,93 the energetic disposition of the diradical excited states is far below the ionic states, whereas the energy gaps between 11A13—3A2u diradical states and 1A2u—21A1g ionic states are small and equal. Electronically, the singlet zwitterionic states correlate to a MIMIII charge-separated center whereas the diradical states correlate to a symmetrical MUMII configuration. Thus excitation of transitions associated with the 8 manifold (eq. 1.3) amounts to MMCT in which an excited state of considerable :M‘—M+ zwitterionic character is achieved. M=M A» ':M=M* d4—d4 d5—d3 (1‘3) 21 25" 21A..(‘A) 155 . (g m (g 20 _ I (Nocera) I 'r i 1A... (*8...) g g: (Gray) 8 :5 o J: 5 >. i 3. o) . *2 1 :37 ”J i 5.5 i > i 5— .c. 3A 33 3 * -;-— at 2) as 3 (Gray, Cotton) 0_ _'_L 11A,g (1A,) ‘52 (Cotton) Figure 1.9 Energy level diagram for 8/8* manifold of M-i-M complexes in valence bond description of the electronic states formed by the dxy orbitals with the corresponding MO formalism 22 The existence of zwitterionic character for MAM complexes has been spectroscopically verified by recent two-photon spectroscopic experiments carried out in our laboratory.94 By identifying the 1(5*8*) excited State, a 4,890 -1 cm AE(E 1(8"‘8"')-E 1(8*8*) = 4,890 cm‘!). This gap is small, which should be the energy gap has been determined for the two zwitterionic excited states (i.e., case for a zwitterionic state. Moreover, as mentioned above, the diradical energy gap, 3A2u—1Alg, should be equivalent, to a first approximation, to that for the zwitterionic states, 21Alg—11A18. This is observed experimentally. Cotton and coworkers have extrapolated their experimental measurements of the 3A2u—1A1 8 gap for twisted MA—M complexes to a torsional angle of x = 0° and obtained an energy gap splitting of 4840 cm‘l,” in excellent agreement with the 21A“;— llAlg gap measured by two-photon spectroscopy. The spectroscopic results unequivocally establish that transitions associated with the 8-orbital manifold are MMCT in character, and yield an electron pair, singlet coupled, (:M-—M+ = d5—d3) within the binuclear core. More quantitatively, the spectroscopically ascertained values of AW and K (determined from the difference between the 1(82 -> 88*) and 1(82 —) 5*8*) energy gaps) in conjunction with the effective dxy(A)—dxy(B) overlap (as evaluated from the oscillator strength of the 8 —->8* transition“) reveals that the 52 ground state possesses 34% ionic character, which increases significantly to 68% in the excited state. Nevertheless, the presence of a long-lived, ionic, excited state in MiM complexes does not in itself ensure multielectron photochemistry. The 155* (‘I’_) and 15*8“ (‘I’+) zwitterionic states are described by the following linear combinations, ‘13 = :M"—M+ i Mt—Mr (1.4) [d5—d3] i [d3—ds] d1: P0 111C 5111 23 Although these states are ionic, they are nonpolar because :M —M+ and M"— M:' contribute equally to the linear combination as long as a center of inversion is maintained within the molecule and its environment. However, intermolecular or intramolecular perturbations that remove the center of inversion will lead to dissimilar contributions of the :M'—M+/M*—M:" states in eq. (1.4), thereby polarizing the system. VI. Thesis Outline The zwitterionic nature of MAM excited states presents these complexes as logical candidates for multielectron photochemistry. Unlike the singlet states of most transition metal complexes, the 1(55*) excited state of M—‘LM dimers are )97 in some cases to permit bimolecular sufficiently long-lived (1: ~ 100 ns reactivity. The longevity of the MMCT state most probably arises from the large 1(85"')——3(88"') energy gap which inhibits intersystem crossing. Chapter III outlines mechanistic studies of the photoreactions of a dimolybdenum tetraphosphate, Moz[02P(OC6H5)2]4, in the presence of dihalocarbons. In these complexes, the zwitterionic excited state can not be trapped by intramolecular ligand rearrangement because the diary] phosphate ligands are too bulky and in their bidentate coordination geometry, structurally inflexible. Mechanistic studies reveal that the reaction proceeds by two sequential one-electron steps at the axial coordination site of the bimetallic core to yield. the overall two-electron transformation. This chemistry has been generalized by reacting the MiM complex with unsaturated dihalocarbons as well as saturated halocarbons as substrates. In Chapters IV and V complexes are chosen with ligands that can stabilize the mixed-valence MI—MIII intermediate coordination by rearranging to edge in or pr ca 01; 51. dift mix the will M 0 81:11: low 24 bridging positions. The terminal halide monodentate ligands of M2Cl4(PR3)4 (with D211 geometry) and M2C14(PP)2 (with D21l geometry) complexes (where M = M0 or W) give the structural flexibility to form edge-bridging intermediates. We proposed that discrete multielectron transformations might be achieved in these cases. Chapter IV details the photochemistry of the M2Cl4(PP)2 complexes with diaryl/dialkyl disulfides and Chapter V presents the photochemistry of the M2C14(PR3)4 complexes with dihalocarbons. For M2Cl4(PP)2 compounds, the photochemistry proceeds with visible excitation of MMCT states that are immediately adjacent in. energy to the 88* excited state. In contrast, these transitions are obscured in the M2C14(PR3)4 (M = Mo, W) complexes by LMCT transitions. Consequently, the photochemistry for these two family of compounds differs. Whereas the approach taken in Chapters IV and V is to trap the critical mixed-valence MI—MIII intermediate by an internal ligand arrangement to break the symmetry of the bimetallic core, Chapter VI attempts to trap the intermediate with the asymmetry induced by a heterobimetallic core. Our hypothesis is that Mo-fi-W birnetallics may exhibit better energy-Storage behavior in the excited state because the mixed-valence character of a MoI—Wm excited state will be lower than the MoIII —-WI counterpart. Chapter VI will compare the photophysical properties of Moi-W species with their Moi—Mo and W-‘l-W analogs, and examine the influence of the asymmetric core on the photoreactivity of this class of photoreagents. In summary, we have systematically explored the multielectron photochemistry of a unique class of photoreagents, provided by a systematic approach to the realization of the first excited state multielectron pathways in chemistry. The nature of the substrates and the M-‘i-M electronically excited 25 complexes, and the affect of environment in contributing to the novel photochemical observations will be rationally developed in this dissertation. CHAPTER II EXPERIMENTAL I. General Procedures All manipulations were performed under the argon atmosphere of a Vacuum Atmospheres drybox or the manifold of a Schlenk line. For the most air- sensitive compounds, syntheses were performed on the manifold of a high vacuum line (10’6 to 10’7 torr). Syntheses of quadruply bonded metal-metal complexes were accomplished with standard Schlenk techniques. K4M02Clg,98 (N H4)5M02C19,99 M02(02CCH3)4,‘°° and iodobenzene dichloride (PhIC12)1°‘ were prepared by published procedures. Tungsten(IV) tetrachloride (WCl4) was also prepared according to a literature method102 but was further purified by sublimating the product from tungsten oxide impurities. Molybdenum(V) pentachloride (MozClm), tungsten(VI) hexachloride (W C16), diphenylphosphate, bidentate phosphines, chlorotrimethylsilane (Me3SiCl), dialkyl disulfides and NOBF4 were purchased from Aldrich Chemicals and used without further purification as were trialkylphosphines, which were purchased from Strem Chemicals. Diphenyl disulfide was recrystallized and dried under vacuum. Solvents used for synthesis were dried by general procedures.103 Toluene was freshly distilled from sodium/potassium benzophenone ketyl, and dichloromethane was distilled from P205, then stored over 4-A molecular sieves in a l-liter flask equipped with a Kontes high-vacuum valve. The methanol used for 26 .27 the syntheses of dimolybdenum tetrakis(diphenylphosphates) was refluxed over NaOMe for no less than 6 h and freshly distilled prior to use; the amount of Na added to MeOH was 20% more than the amount required to react with the water contained in a freshly Opened bottle. All other chemicals were reagent grade and used as received unless otherwise noted. All solvents used for spectroscopic and photochemical experiments were purchased from Burdick and Jackson Laboratories (Spectroscopic grade). Purified solvents were stored in glass containers that consisted of a l-liter flask equipped with a Kontes high-vacuum valve. 1,2-Dichloroethane, 1,2- dichloroethylene, and 1,2-dichlorocyclohexane were degassed by seven freeze- pump-thaw cycles and stored over activated (heated to 250 °C under 10"5 torr dynamic vacuum for 12 h) Linde 4 A molecular sieves contained in a storage flask; ortho-dichlorobenzene was refluxed over P205 under a nitrogen atmosphere. Tetrahydrofuran, toluene and benzene were subjected to seven freeze-pump-thaw cycles and subsequently distilled into flasks containing sodium-potassium alloy with a small amount of benzophenone; the purple ketyl form of benzophenone formed over a two-day period. 11. Synthesis A. Tetrakis(diphenylphosphate) Dimolybdenum Complexes The Mo2(II,II) tetrakis(diphenylphosphate) was prepared by a simple ligand substitution reaction. The anion (C6H50)2P02‘ was generated by stirring (C6H50)2P(O)OH (3.25 g, 13 mmol) with NaOMe (0.702 g, 13 mmol) in 60 mL of MeOH. After 10 min, (NH4)5M02C19-H20 (0.5 g, 0.8 mmol) was added to the above solution, which was then heated to reflux for 3 h. The pink precipitate, 28 formed during refluxing, was collected by suction filtration and washed with three 15 mL portions of MeOH. Aliquots for all washings were delivered by cannula. The solid was dried in vacuo (yield: 0.67 g, 70%). The M02":II tetrakis(diphenylphosphate) complex is extremely air-sensitive, decomposing within seconds upon exposure to air. The compound, though not soluble in most organic solvents, exhibits fairly good solubilities in halocarbons and THF. UV-vis, A...” (e) in cnzch: 515 nm (156 M-lcm-l) and 404 nm (38 M-lcrn-l). B. Homonuclear Dimolybdenum and Ditungsten Complexes. Bole Syntheses Of MOZCI4(PR3)4 (PR3 = PMC3, PMezPh, PMCPI'I;)104 Complexes of M02C14(PR3)4 were prepared typically by the following procedure. The K4M02Clg or (NH4)5Mo2C19 (0.5g, 0.8 mmol) salt was dissolved in dry methanol (25 mL), followed by adding four equivalents of trialkylphosphines. The reaction solution was refluxed for 4 h to form a green precipitate, which was then filtered off, washed with water and methanol, and dried in vacuo. The blue solid, M02C14(PR3)4, can be purified by filtering a CH2C12 solution of M02C14(PR3)4 through a Florisil column under nitrogen; the solvent was removed under reduced pressure. Compounds were identified by UV-vis, 1H and 31P NMR. B.2 Syntheses of M02C14(dppm)21°5 MozCl4(dppm)2 was prepared by the same method described for the M02Cl4(PR3)4 complexes but the bidentate phosphine ligand, dppm, was used in place of PR3. UV-vis, it,” (e) in CHZClZ: 634 run (2490 M-lcm-l), 462 nm (900 hi-lcm-l) and 325 nm (5600 M“ em-l). 29 B.3. Syntheses of wzchwng, (PR3 = PMe3, PMezPh, PMeth, PBu3)1°6 Sodium amalgam (0.41%, 90 g, 16.04 mmol of Na) was freshly prepared in a three-neck round bottom flask under argon. To this liquid, 135 mL of dry THF was introduced into the flask, which was cooled to —78 °C with a dry-ice/acetone, and tungsten(IV) tetrachloride (2.6 g, 8.0 mmol ) was added followed by the addition of two equivalents of phosphine, PR3. The reaction solution was stirred at -30 °C (chloroform/ liq. N2 bath) for half an hour, then slowly warmed to 0 °C with an ice bath and, finally warmed to room temperature over an hour upon the removal of the ice bath. The deep brown-green solution was filtered through Celite and the solvent was removed from the filtrate in vacuo. The residue was extracted with hexane (or CH2C12 when PR3 = PMePh2 owing to the poor sloubility of W2Cl4(PMePh2)4 ). The extract was filtered and concentrated to 3 mL. The dark green W2Cl4(PR3)4 complex was precipitated out by adding MeOH to the concentrated filtrate. (Note: different W2Cl4(PR3)4 complexes have different solubilities in THF and CHzClz. The longer the alkyl chain, the better the solubility in these solvents.) Compounds were identified by UV-vis, 1H and 319 NMR. C. Heteronuclear Molybdenum-Tungsten Complexes c.1. Synthesis of Mom‘-PhPMePh)(PMePh2)3‘°7 Molybdenum(V) pentachloride (0.65 g, 2.38 mmol ) was dissolved in a 135 mL of dry THF solution containing methyl(diphenyl)phosphine (2.4 g, 11.9 mol) at 0 °C under argon atmosphere. (Note: Argon is necessary in this case since nitrogen is found to act as a ligand and coordinates to the metal site.) Excess Grignard magnesium (1.26 g of Mg tumings) was slowly added to the ice-cold 30 orange THF solution over one-hour with vigorous stirring. The reaction solution was kept stirring at 0 °C for an additional hour and warmed up to room temperature over half an hour. The reaction solution changed from rusty orange to olive green after ca. 30 min and then turned orange. This orange solution was filtered through Celite by cannula techniques. The filtrate was concentrated to 30 mL under vacuum but not too concentrated to induce the formation of oligomers, which is indicated by the solution turning black-green. An orange powder was precipitated by the dropwise addition of MeOH and filtered, followed by three MeOH washings to remove excess phosphine from the solid. The product was recrystallized from benzene/MeOH in order to remove oligomer impurities. Mo(n6-PhPMePh)(PMePh2)3 is extremely air-sensitive especially when the mother liquid is around. It is better to dry the product by evaporating MeOH under an argon stream ovemight before drying under high vacuum (yield: 0.84 g, 40%). 1H NMR(C6D6) 8 7.55 (m, 6 H, o-PhP—Mo), 7.3-7.2(m, 9 H, m,p—PhP—Mo), 4.42 (m. 2 H, o-né-Ph), 4.16 (m, 1 H, pn6-Ph). 3.72 (m, 2H, m-né-Ph). 3.60 (m, 2H, m-né-Ph). 1.77 (br, 9H, MezP-Mo), 1.20 (d, 6H, 21,“: 4 Hz, MezP-né-Ph). 31? NMR(C6D6): 8 15.8 (s, 3P, P—Mo), -46.7 (s, 1 P.116-Ph-P). V C.2. Synthesis of Mo(11‘-PhPMe2)(PMe2ph) 3108 The experimental set-up is similar to Mo(n6-PhPMePh)(PMePh2)3 except that the reaction requires heat. Molybdenum pentachloride (0.50 g, 1.83 mmol) was dissolved in 45 mL of dry THF containing dimethyl(phenyl)phosphine (1.3 g, 9.4 mol) at room temperature under an argon atmosphere. (Caution: water must be rigorously excluded from the reaction set-up because free dimethyl(phenyl)— phosphine reacts with water vigorously and will ignite in the presence of flammable material). Excess Grignard magnesium (2 g of Mg tumings) was slowly 31 added to the THF solution with vigorous stirring. The reaction solution was then heated to 70 °C for 2h. The solution was filtered through Celite and concentrated. Precipitation was induced by the dropwise addition of MeOH. The resulting filtrate was kept at -20 °C overnight to induce more precipitate. The bright-orange solid, which formed on the sides of the reaction flask, was washed with MeOH. Mo(n6-PhPMe2)(PMe2Ph)3 is even more air-sensitive than the its PMePh2 analog, both in solid state and in solution. lH NMR(C6D6) 5 7.5 (m, 6 H, o-PhP-Mo), 7.3-7.2 (m, 9 H, m,p-PhP-Mo), 4.0 (m, 2 H, o-nG-Ph), 3.4 (m, l H, p- 115-Pb), 3.2 (m, 2H, m-né-Ph), 1.5 (br, 1.8H, Mer—Mo), 1.0 (d. 6H, 21,9: 4 Hz, MezP-nG-Ph). 31? NMR(C6D6): 5 15.8 (s, 3P, P—Mo), -46.7 (s, 1 P, nG-Ph—P). (2.3. Synthesis of WC14(PPh3)2‘°9 Tungsten(VI) hexachloride (5.6 g, 14.1 mmol) and dry granulated mossy zinc (7.5g) were placed in round bottom flask. The mossy Zn amalgam was prepared prior to its use by the dropwise addition of 2 mL of 12 M HCl (aq) containing HgO (0.2 g, Matheson, Coleman and Bell) to mossy Zn in 80 mL H20 solution. The amalgamated Zn was filtered, washed with H20 and acetone and dried in the oven. Dichloromethane (90 mL) was added to flask containing the mossy zinc and tungsten complex under argon via a side-arm. The solution was mixed well by shaking the sealed flask for 1 min and the pressure was released through the side-arm stopcock. Triphenylphosphine (11.25g, 42.9 mmol) was slowly (~3 min) added to the saturated solution of WC16. The flask was shaken and the build-up in pressure was relieved periodically until the tungsten hexacholride had totally reacted. The resulting yellowish-orange solid was filtered and the co-deposited zinc was removed by picking out the pieces under an argon flow. The solid was washed with CH2C12 and MeOH to remove the 32 tungsten oxide impurities. The product was dried in vacuo (yield: 5.4g, 45%). UV- vis (crrzcrz): 420 nm. IR(Nujol mull, Csl) v(M-Cl): 317 em-1 (strong) and 325 cm-1 (shoulder). 111 NMR(CDC13) 8 11.2 (d, 2 H, o-PhP), 8.2 (d, 2 H, m-PhP), 7.88 (t. 1 H, p-PhP). C.4. Synthesis of MoWCl4(PMePh2)4“° In “a glove bag inside the dry box, a 40 mL benzene solution of Mom 6- PhPMePh)(PMePh2)3 (0.9 g, 1.0 mmol) in a dropping funnel was added over 30 min to a 100 mL suspension of benzene solution containing yellow WC14(PPh3)2 (2.25 g, 2.7 mmol). This part of the reaction is very moisture-sensitive and all the glassware must be flamed dried under 10‘3 torr vacuum; otherwise the product yield will drop to near zero and impurities, such as WOC12(PMePh2)3 , will be produced in significant yields. The entire reaction set-up was removed from the dry box and subsequent manipulations were carried out with Schlenk techniques. After an additional half hour of stirring, the resulting blue-green solution was filtered through Celite by the cannula technique and the solvent was removed from the filtrate. The residue was redissolved in toluene and filtered to remove excess WCl4(PPh3)2. The toluene filtrate was then concentrated to 5 mL and MeOH was added to induce precipitation. The blue-green precipitate was filtered, washed with MeOH three times, and dried in vacuo (Yield: 0.6 g, 49%). UV-vis, 3...,“ (c) in cgrlg: 650 run (2609 M-lcm-l), 460 nm (394 M-lcm-l) and 320 11m (7133 M-lcm-l). 1H NMR(C6D6): 8 8.0—7.5 (m, 16 H, ortho-PhP-M, M= Mo, W), 7.0-6.8 (m, 24 H, m,p-PhP-M, M = M0, W), 2.1 (t, virtual coupling, 6 H, MeP-W, J = 4 Hz), 1.9 (t, virtual coupling, 6 H, MeP-Mo, J = 3 Hz). 31P NMR(C6D(,): 5 —12.05 (t, 2 x 0.86 P, P-Mo-W (1 = 0), 3r”, = 23.5 Hz), 4205 (dt. 2 x 0.14 P, P- 33 Mo-183w, 219,, : 47 Hz, 31”, : 23.5 Hz), 23.03 (t, 2 x 0.86 P, P—W (I : 0), 31m, : 23.5 Hz), 23.03 (dt, 2 x 0.14 P, P-Mo—133w, 11p, : 266 Hz, 31”, : 23.5 Hz). C.5. Synthesis of MoWC14(PMe2Ph)4“°'111 Mo(n6-PhPMe2)(PMe2Ph)3 (60 mg, 0.093 mmol) was added dropwise into a benzene solution of WCl4(PPh3)2 (80 mg, 0.094 mmol) and dark green MoWCl4(PMe2Ph)4 was produced (yield: 48 mg, 53%). The solid was isolated as described in Section 3.4. UV-Vis .(CgHg): 630 nm. 1H NMR(cgog): 6 7.7, 7.23 (dm. 8 H, ortho—PhP-M, M = M0, W), 7.0, 6.9 (d,m, 12 H, m,p—PhP—M, M = M0, W), 1.9, 1.8 (d,t, virtual coupling, 12 H, MeP—Mo, J : 3.6 Hz). 31? NMR(C6D6): 6 -19.62 (t. 2P, P—Mo, 31m, : 24.0 Hz), 17.84 (t, 2 x 0.86 P, P—W (1 : 0). 31w : 24.0 Hz). 17.84 (dt, 2 x0.14 P, P—183w, 11,, = 280 Hz, 31”, = 24.0 Hz). C.6. Synthesis of MoWCl4(PMe3)4”° Trimethylphosphine (40 mg, 0.53 mmol) was added to a benzene solution of MoWCl4(PMePh2)4 (130 mg, 0.106 mmol). The reaction solution was sealed under vacuum and heated to 60 °C for 3 h thereby ensuring complete ligand exchange. The ligand PMeth, excess PMe3, and the solvent were removed under vacuum. The resulting blue residue was washed with MeOH to yield the dark blue-green product, MoWCl4(PMe3)4 (yield: 20 mg, 26%). UV-Vis (CgHg): 630 nm. lH NMR(C6D6): 6 1.5 (m, 36 H, MeP—M, M: Mo. W). 31? NMR(C6D6): 6 -27.4 (t, 2P, P-Mo, 3Jpp = 24.4 Hz), 10.9 (t, 2 x 0.86 P, P—W (I: 0), 3Jpp = 24.4 Hz), 10.9 (dt, 2 x 0.14 P, P—mW, 11p, : 269 Hz, 31”, : 24.4 Hz). C.7. Synthesis of MoWC14(dppm)2112 _34 MoWCl4(PMePh2)4 (70 mg, 0.057 mmol) and bis(diphenylphosphino)- methane (90 mg, 0.234 mmol) were suspended in dry l-propanol (20 mL). The blue-green suspension solution was refluxed overnight to form a green precipitate, which was collected by filtration, washed with hexanes, and dried in vacuo (Yield: 44 mg, 63%). UV-Vis (THF): 675, 317 nm. 1H NMR(CD2C12, -40 °C): 8 7.5—7.0 (m. 40 H, PhP—M, M = Mo, W), 4.7 (m, 4 H, CHzP-M, M = Mo, W). 31? NMR(CD2C12, —40 °C) exhibits a AA’BB’X pattern: 6 —2.5 (m, 2P, P—Mo), 35.4 (m, 2 P, P-W). III. Reactions A. Photochemistry of Moz[OzP(OC6H5)2]4 with Dihalocarbons Dichlorocarbon solutions containing 2-4 mgS of M02[02P(0C6H5)2]4 were irradiated at h>495 nm and the photolysis was monitored by UV-vis spectra. Photolysis was terminated when the near-IR absorption reached a maximum, indicating the formation of the mixed-valence M02“:III complex. UV-vis, km, in CH2C12: 1492, 542 and 420 nm. B. Photochemistry of MozCl4(dppm)2 with PhSSPh M02C14(dppm)2 (0.05 g, 0.045 nrrnol) was dissolved in CHZCIZ containing a twenty-fold molar excess of diphenyl disulfide and irradiated at I. > 435 nm (16 i 0.5 °C). The solution was concentrated to 5 mL and the photoproduct was precipitated by adding hexane to the reaction solution. The compound was further purified by chromatography on a Florisil packed column with CH2C12/CH3CN as an eluent. FAB/MS displayed a mass ion cluster with a peak at 1246 amu, corresponding to the molecular weight of M02C15(dppm)2(SPh). 35 Suitable single crystals of M02C15(dppm)2(SPh)-2CyHg for X-ray structural determination were obtained by layering a toluene solution of the photoproduct with cyclohexane in a Schlenk tube. The same photoproduct was obtained for an excitation wavelength range of 305-435 nm either in CH2C12 or toluene; only the yield was perturbed by the nature of the solvent (33% for A>435 nmin CH2C12 and 52% for A>335 nm in toluene). FAB/MS of M02C15(dppm)2(SPh): 1246 ([M]+); 1211 ([M-Cl]*) and 1137 ([M-SC6H5]+) amu. UV-vis for M02C15(dppm)2(SPh) in CH2C12: 527 (752), 457 (1120), and 407 nm (898 M’lcm‘ 1). C. Thermal Reactions of M02Cl4(dppm)2 with PhSSPh113 MozCl4(dppm)2 (0.06 g, 0.054 mmol) and PhSSPh (0.06 g, 0.28 mmol) were mixed in toluene and rigorously shielded from light. No reaction was observed at room temperature. Under reflux conditions for 12 h, an unidentified orange-brown solid precipitated out of the solution and the product was isolated by washing with hexane to remove excess disulfide. Parent ion clusters at 1263 and 1279 amu in the FAB/MS are consistent with a trinuclear cluster product. UV-vis (CH2C12): 460 (m) and 375 nm (sh). D. PhOtOChemIStI’y Of W2CI4(PR3)4 With CH2C12 Dichloromethane solutions containing 2-4 mgs of W2Cl4(PR3)4 were irradiated at wavelength A>375 nm and the photolysis was monitored by UV-vis spectra. Photolysis was terrr'rinated when the near-IR absorption reached a maximum, indicating the formation of the mixed-valence WZIIJII complex (UV- vis(CH2C12): ragga,” : 1336.419. 383 and 315 nm for PR3 : PMe3; nabs”, = 36 1360, 427, and 344 nm for PR3 = PMezPh; and ham“, = 1410, 400, and 334 nm for PR3 = PBu3). The photolyzed solution (1 mL) was transferred to an EPR tube, followed by adding 2-methyl tetrahydrofuran (2—MeTHF) to form a glass at -—170 °C and was characterized by EPR (gu = 1.950, 1.950, and 1.941 and g j = 1.855, 1.847, and 1.856 for PR3 = PMe3, PMezPh, PBu3, respectively). Continued irradiation of the remaining solution led to the disappearance of the near-infrared absorption with the concomitant appearance of an absorption at ~480 nm. E. Thermal oxidations of W2C14(PR3)4 with PhICl2 Thermal oxidatiOns of W2Cl4(PR3)4 were accomplished with PhIC12 or NOBF4. For the former, 0.01 mmol of PhIC12 was added dropwise to an equivalent amount of W2Cl4(PR3)4.in 5 m1. of CH2C12. The color of the solution changed from green to red-brown instantly (UV-vis(CH2C12): labs,“ = 1340, 469, 428, 376, and 315 nm for PR3 = PMe3; labs’m = 1366, and 421 nm for PR3 = PMth; and MSW = 1415, 507, 439, 403 and 331 nm for PR3 = PBu3. EPR (CH2C12/2- MeTHF, —170 °C): gll = 1.952, 1.952 and 1.940 and g j = 1.857, 1.854 and 1.852 for PR3 = PMe3, PMezPh, PBu3, respectively). F. Thermal oxidations of W2C14(PR3)4 with NOBF4 The oxidation of W2Cl4(PMe3)4 (4 mg) in CH2C12 was also carried out by dropwise addition of NOBF4 in MeOH. The solution promptly turned from green to yellow-orange, and NO was removed by several freeze-pump-thaw cycles. The compound is extremely air-sensitive, but microcrystalline solid can be obtained by slowly removing solvent (U V-vis(CH2C12): Imam = 1468, 474, and 375; 3.3mm : 1505. 497, and 398 nm; and nabs,“ = 1514, 483. and 388 nm for PR3 : PMe3, Si 1. 37 PMth, PBu3, respectively. EPR (CH2C12/2-MeTHF, -170 °C): g" = 1.980, 1.985 and 1.980 and g j = 1.840, 1.836 and 1.834 for PR3 = PMe3, PMezPh, PBu3, respectively). When solvent is completely removed, the compound is susceptible to rapid oxidation, and blue tungsten oxo compounds are obtained. IV. Instrumentation and Methods A. Absorption Spectroscopy Absorption spectra were recorded on Cary 17D or Cary 2300 spectrometers. Extinction coefficients were determined by using high-vacuum cells consisting of a 1-cm quartz cuvette and a 10-mL side arm. These two chambers were separated with two Kontes high-vacuum quick-release teflon stopcocks. For measurements of molar absorptivity coefficients, weighed samples were placed in the cuvette and isolated by the Kontes valve. The appropriate high purity solvent was transferred to the 10-mL side arm by bulb-to-bulb vacuum distillation or by syringe from a calibrated volumetric vessel, and three subsequent freeze-pump-thaw cycles were performed before mixing with the sample. Extinction coefficients were calculated from Beer-Lambert plots composed of at least seven points. B. Photolysisl ‘4 Sample irradiations for photochemical experiments were executed by using a Hanovia 1000-W Hg/Xe high-pressure lamp. The beam was collimated and passed through a 10-cm circulating water filter. Photolysis experiments were performed in two-arm evacuable cells muipped with Kontes quick-release teflon valves. Solutions were prepared by bulb-to—bulb distillation of solvent on a high- 38 vacuum manifold. Sample temperatures were thermostatted at constant temperature (15.021: 0.5 °C) in all photoreactions. Excitation wavelengths in the ultraviolet and visible spectral regions were selected by using colored glass high- energy cutoff filters from Schott. For quantum yield measurements, the excitation wavelength was isolated by using an interference filter purchased from Oriel Corporation with a half-width of less than 10 nm at the given mercury line. Absorption spectra for photolysis and quantum yield experiments were recorded on a Cary 2300 or Cary 17D spectrometer. Quantum yields for dimolybdenum diphenylphosphate photochemistry were determined on 7 M solutions of the dichlorocarbon in benzene by monitoring the disappearance of M02[02P(OC6H5)2]4 at 515 nm and appearance of Mo2[02P(OCgH5)2]4+ at 1494 nm (e = 362 M‘1 cm'l). The absolute quantum yield for photoreduction of 1,2- dichloroethane was measured by using a ferrioxalate actinometenm’ the quantum yields for reaction of all other halocarbon solutions were measured relative to 1,2- dichloroethane. Quantum yields for the photochemistry of M2Cl4(dppm)2 with disulfides were determined on CH2C12 solutions containing 1.4x 10’3 M M2Cl4(dppm)2 and a twenty-fold excess of diphenyl disulfide. The quantum yield, determined by monitoring the disappearance of the 82-)188* transition of M2Cl4(dppm)2, was standardized by using a ferrioxalate actinometer. C. Mass Spectrometry C.1 GCIMS Volatile organic photoproducts were identified by GC/MS on a JEOL JMS-AXSOSH double focusing MS coupled to a Hewlett-Packard 5890J GC. Separations were achieved on a Poraplot U fused silica capillary column from Chromp' organic photoch temperat produc t! ranged f 40 to 13 headspac O photore GCfl'RI. 39 Chrompack (25 m length, 0.32 mm id, IQ-pm film thickness). For gas phase organic photoproducts obtained from dimolybdenum diphenylphosphate photochemistry, the m/z scans ranged from 15 to 400 and the GC column temperature was programmed from 30 to 200 °C at 10 °C/min. For gas phase products isolated from ditungsten LMCT photoreacted solutions, the scans ranged from 0 to 200 m/z and the GC column temperature was programmed from 40 to 180 °C at 10 °C/min. The volatile sample was introduced from a 2 mL headspace by splitless injection. Organic liquid products isolated from dimolybdenum diphenylphosphate photoreacted solutions by vacuum distillation were analyzed on a HP 5890 GC/TRIO-l MS instrument up to masses of one thousand. The column is an SE- 54 support-coated open tubular capillary column from Alltech with 30 m length (0.32 mm id, 0.25-rrm film thickness). The mass speCtrometer ion source was maintained at 200 °C , and the GC column temperature was programmed from 35 to 200 °C at 5 °C/min. Organic liquid photoproducts separated from ditungsten LMCT photoreacted solutions by vacuum distillation were analyzed on a DB— SMS J&W fused silica capillary column from Alltech (30 m length, 0.32 mm id, 0.25-rrm film thickness). The GC column temperature was programmed at 40 °C for 2 min, and then it was ramped from 40 to 200 °C at 10 °Can. The mass spectrometer ion source was maintained at 280 °C and the scans ranged from 20 to 400 m/z. For both instruments, the helium carrier gas was set at a flow rate of 1 mL/min. 1 11L of liquid sample was employed by splitless injection. C.2. FAB/MS Fast atom bombardment mass spectrometric analyses (FAB/MS) were performed on a JEOL HX-llO double focuSing mass spectrometer housed in Nationa Facility. instrumc CJES Finnega mobile vaporiza Nitrogen D. Nuc sPietrom CXtCrnall. dcuterate “rider hlgl E. Elec EPI "Sing a X amplitud:J 110m (1er l freqUenC‘ 40 National Institutes of Health/Michigan State University Mass Spectrometry Facility. Samples were dissolved in 2-(octyloxy)nitrobenzene matrices and the instrument was operated in the positive ion detection mode. C.3 ESIMS Electrospray mass spectrometric (ES/MS) analyses were obtained with a Finnegan mat (San Jose, CA) quadrupole mass spectrometer using a CH3CN mobile phase. A CH3CN solution of the sample was infused directly into the vaporization nozzle of the electrospray ion source at a flow rate of 3 [IL min'l. Nitrogen was used as the nebulizing gas at a pressure of 35 PSI. D. Nuclear Magnetic Spectroscopy (NMR) 1H and 31P{1H} NMR spectra were obtained on a Varian VXR-300 spectrometer, with the latter experiment recorded at 121.4 MHz and referenced externally to 85% H3PO4. Deuterated methylene chloride (Aldrich, 99.6+%) and deuterated chloroform (99.8%, Cambridge Isotope Laboratories) were dried under high vacuum conditions as described in Section A of Chapter II. E. Electron Paramagnetic Spectroscopy (EPR) EPR spectra were recorded at -170 °C on a Varian E-4 spectrometer by using a X-band TE 102 cavity with 100 kHz field modulation, a 1.0 G modulation amplitude, and a microwave power of 19.8 mW. The g values were determined from direct measurement of the magnetic field strength and the microwave frequency, which was measured with a Hewlett-Packard 5245 counter/5255 3- 12 GHz converter. F. Ste: E Michigan 01 the di the ditur samples standard quantum Where x the avera Corrected the soluh' G- Tim Lit i“Winner were mac‘l LAXER (lF Michigan and 650 Quantap; Quam‘a‘R; 41 F. Steady-State Luminescence Spectroscopy Emission spectra were recorded on a spectrometer constructed at Michigan State University.116 The R1104 Hamamatsu PMT was used in the case of the dimolybdenum complexes and the R316 Hamamatsu PMT in the case of the ditungsten complexes. Absolute emission quantum yields of optically dilute samples (A<0.2) were measured using M02C14(PMe3)4 as a quantum yield standard (96m = 0.013 at kg“: 585 nm in 2-methylpentane at 300 K).117 The quantum yield was calculated from the following equation:118 _ x Aru'r) X T1:12 x Dx “’*"”' 14.0.)1 111.21 191 ‘2'” where x and r designate the unknown and standard solutions, respectively, n is the average refractive index of the solution, D is the integrated area under the corrected emission spectrum, and A0.) is the absorbance per unit length (cm) of the solution at the exciting wavelength 71.. G. Time-Resolved Laser Spectroscopy Lifetimes were measured on a time-resolved picosecond lifetime instrument.119 Picosecond and nanosecond transient absorption measurements were made with the pulse-probe technique with instrumentation120 housed in the LASER (Laser Applications in Science and Engineering Research) Laboratory at Michigan State University. Transient absorption spectra were collected at 355 and 650 nm. The latter excitation wavelength was achieved by pumping a Quanta-Ray PDL-2 dye laser (DCM, Exciton) with the second harmonic of a Quanta-Ray DCR-2A Nd:YAG laser. The transient spectra were collected using a SPEX If elsewher the Nd:) sample c circumve lnstrume intensificl Princetor allowed range apfi H. Cry: A- Data x- M02131.1( 1 Structure ( Michigan I 11611011116: The inten Scans at ; 42 SPEX 1680A monochromator and Hamamatsu R928 photomultiplier as described elsewhere.120 Excitation at 355 nm was provided by third harmonic generation of the Nd:YAG laser. UV excitation led to high background fluorescence of the sample cells, thus precluding the use of the photomultiplier for detection. To circumvent these difficulties, the monochromator and PMT were replaced by an Instruments SA HR-320 spectrograph and Princeton Instruments IRY-1024 intensified photodiode array detector. The intensifier was driven by an EG&G Princeton Applied Research model 1302 fast pulser for 20 ns. This combination allowed the transient absorption measurements to be acquired over a 150 nm range approximately 50 us after laser excitation. H. Crystallography A. Data collection of M02Cl4(dppm)2(tt-Cl)(tt-SPh) X-ray quality crystals were obtained by layering a toluene solution of M02Cl4(dppm)2(tr-Cl)(tr-SPh) with cyclohexane in a Schlenk tube. Crystal structure determinations were performed in the X-ray Crystal Structure Facility at Michigan State University. Preliminary examination and data collection were performed with Mo Kor radiation (A = 0.71073 A) on a Nicolet P3P diffractometer. The intensity data were collected at 202(3)K by using Wyckoff omega(a)) scans at a rate of 4°lmin (in (1)) to a maximum 20 of 40°. The ratio of peak counting time to background counting time was 1:1. The diameter of the incident beam collimator was 1.5 mm and the crystal to detector distance was 19 cm. The crystal parameters and details of intensity collection and refinement are listed in Table 4.1, and complete tables of positional parameters, bond distances, bond angles, anisotropic thermal parameters, and structure factors are given in Table 4.2 t0 Tat senor puma dnmn onent inane sever whh absel 13111'): IL I 1101 5 on t Tang P012 C0€l Inte are 43 to Table 4.4 discussed in Chapter IV. All calculations were performed on a VAX station 3100 computer by using SDPNAX.121 A deep maroon rod-shaped crystal of bis(diphenylphosphinomethane) pentacholro(tt-phenylsulfido) dimolybdenum(III,III) having approximate dimensions of 0.20 x 0.20 x 0.45 mm, was mounted on a glass fiber in a random orientation using Exxon “Paratone-N" oil. Cell constants and an orientation matrix for data collection were obtained from least—squares refinement, by using the setting angles of 25 reflections in the range 15°< 20 <20°. The monoclinic cell parameters and calculated volume are: a = 15.270(14), b = 17.926(15), c = 23.88101) A; B : 9233(5); v = 6531.5(6)A3. For 2 : 4 and F.W. = 1431.40 the calculated density is 1.46 g/cm3. As a check on crystal quality, omega scans of several intense reflections were measured; the width at half-height was 0.26° with a take-off angle of 60°, indicating good crystal quality. From the systematic absences of h01 (l = 2n) and 0k0 (k = 2n), and from subsequent least-squares refinement, the space group was determined to be P21/C (No. 14). B. Data Reduction A total of 6670 reflections were collected, of which 6092 were unique and not systematically absent (RMERGEO): 0.007). A linear decay correction, based on three representative reflections, was applied; the correction factors on F ranged from 0.997 to 1.000 with an average value of 0.998. Lorentz and polarization corrections were applied to the data. The linear absorption coefficient is 7.5 cm’1 for Mo Kor radiation. No absorption correction was made. Intensities of equivalent reflections were averaged. The agreement factors for the averaging of the 294 observed and accepted reflections was 4.3% based on intensity and 3.0% based on F0.122 44 C. Structure Solution and Refinement of M02C14(dppm)2(p- Cl)(tt-SPh) The structure was solved by using the direct methods program SHELXS86.123 A total of 22 atoms were located from an E-map; the remaining atoms were located in succeeding difference Fourier syntheses. Hydrogen atoms were refined isotropically and included in the refinement but restrained to ride on the atom to which they are bonded. The structure was refined by full-matrix least- squares where the function minimized was 2‘.w(lFol-|Fcl)2 and the weight w is defined as 4F02/o(F02)2. Scattering factors were taken from Cromer and Waber.m Anomalous dispersion effects were included in F5125 the values for Af’ and Af” were those of Cromer.‘2‘5 Only the 3160 reflections having intensities greater than 2.0 times their standard deviation were used in ‘ the refinements. The final cycle of refinement, which included 389 variable parameters, converged (largest parameter shift was 0.03 times is esd) with R = 0.082 and RW = 0.071. The standard deviation of an observation of unit weight was 1.79. The highest peak in the final difference Fourier had a height of 0.90 e/A3 with an estimated error based on oFm of 0.19; the minimum negative peak had a height of .094 e/A3 with an estimated error based on oF of 0.19. Plots of 2w(lFol-|Fcl)2 vs. lFol, reflection order in data collection, sine/7t, and various classes of indices showed the 16 “worst” reflections with IF°|< 335 nm) of phosphoric acid solutions of M02(HPO4)44' leads to the production of H2 and the tripled bond dimer, M02(HPO4)42‘. Further studies shows the one-electron oxidized complex, M02(HPO4)43’, as the primary photoproduct, which undergoes a secondary photolysis to give the two-electron fully oxidized complex M02(HP04)42’. hva hv‘ Mo HPO ‘- Mo HPO 3- Mo HPO ' 3.3 21 414 ask“? 2( 474 $3 2( 414 ( ) a 400 > 1. > 254 nm "2 H2 "2 H2 Careful examination of the electronic absorption and action spectra of the M02(HPO4)4“' series of complexes reveals that the photochemistry is derived from the 7: —9 7t* metal-localized transition. Photoreduction from a high energy 1(“111'“) excited state conforms well with M02(SO4)44' 311d Mo2(aq)x4+ 611' am Mt rec prt by. by b0| C0 occ lyil ina pro the phc que PFC; Pro 47 photochemistry described by eqs. (3.1) and (3.2), which also is derived when the energy of irradiation is coincident with the 1(1|:7t"') excited state of M02(SO4)44' and M02(aq)x4"' complexes. Figure 3.1 summarizes the proposed mechanism for the photoprocess of M0203 complexes in acidic aqueous solution. Excitation produces the powerfully reducing l(7t71:*) excited state of Mozog, which is capable of directly reducing protons to H atoms. Hydrogen production comes from either annihilation of two hydrogen atoms, or sequential oxidation of the M0203 starting reactant by a hydrogen atom to yield H' followed by protonation. A general to emerge from these photochemical studies is that quadruply bonded M0203 systems do not exhibit appreciable lifetimes in acidic solution. Consequently, photoactivation of substrates by M-4-M complexes typically occurs from metal-localized states that are high in energy and not from the low- lying 5* metal localized excited statesm'129 One reason for 1(88*) photo- inactivity is the propensity of this excited state to be efficiently quenched by protons in acidic aqueous solutions.131 This suggested to us that the chemistry of the 1(88*) excited state could be exploited by undertaking studies of M02”:11 phosphates in aprotic environments, where the energy-wasting, proton- quenching reactions of the 1(55*) excited state would be circumvented thus preserving it for reaction with substrate. In this manner, substrate activation processes of M—4-M species might be achieved with visible light. On this premise, Dr. I-Jy Chang in our research group synthesized and structurally characterized the dimolybdenum diphenylphosphate, Moz[02P(OC6H5)2]4. As shown in Figure 3.2, comparison of the absorption spectra of M02[02P(OC6H5)2]4 in CH2C12 and aqueous solutions of M02(HPO4)44’ reveals that substitution of HPO42' by diphenyl phosphate does little to perturb the electronic structure of the quadruple bonded metal-metal r - - 1.2600119). J 1111 1 +I 48 doze—8 220m 5 reason—ESE e623 beaten—c 65 Lo EEEBQEEQ 65 com Emmcmnoofi 65 mafia—«ESE :8..me ~65— »wcecm fin «Sufi $93 1>1 .. 0.0 Se :31 c r a: r1? 1 o.N m2 111 Se 35 .I . 52.8585. 1. .1 NI. 1 3. 42.061122 .1 lvavOn—IVNOE + .I A v v N 1 Asp—Dav OF(ocgri5)2]4+’*) = 2.24 eV),132 engenders the photochemical activation of dichloroethane upon low-energy excitation of the quadruple bond complex. We set out to generalize the chemistry of this system with dihalocarbons, and to provide mechanistic insight into the photoredox process. II. Results and Discussion A. Photochemistry Whereas 1.2-dichloroethane solutions of M02[02P(OC6H5)2]4 are indefinitely stable in the absence of light, we have observed prompt reaction of dihalocarbon solutions of Mo2[OzP(OC6H5)2]4 upon visible irradiation with wavelengths of light energetically coincident with the 58* transition. The two- electron conversion of 1,2-dichloroethane to ethylene occurs by the following reaction,‘33 2M02[02P(OC6H5)2]4 + CICI'I2CH2CI L 2M02[02P(OC6H5)2]4CI + CH2CH2 (3.4) 51 The Moz[02P(OC6H5)2]4 dimer also reacts with other 1,2-dihalocarbons including 1,2-dichlorocyclohexane (DCC), cis-1,2-dichloroethylene (DCEE), and o—dichlorobenzene (DCB). Figure 3.3 shows an example of the spectral change of the M02[02P(OC6H5)2]4 photochemistry in DCEE. Irradiation at I.) 495 nm causes the disappearance of the 1(55*) absorption band at 515 nm and an increase of absorption in the near-infrared spectral region. Isosbestic points are observed in the visible region during the initial stages of photolysis. The appearance of a vibrationally structured absorption in the near-IR region (71..., = 1494 nm, 8 = 362 M'lcm'l) is consistent with the production of a mixed-valence Mozu'm complex, M02[02P(OC6H5)2]4CI, which has been identified by FAB/MS.133 The absorption bands of the Mozn'III photoproduct at 600 nm and 1494 nm are comparable in energy and intensity to the corresponding 2(n: -) 8) and 2(6 -+ 5*) transitions of Mo2[ozl>(ocgns)2]481=4,123-13‘ which has been structurally and spectroscopically characterizedm A red shift in the 2(8 -) 8*) absorption band with respect to the BF; salt is attributed to the presence of chloride ion in photolyzed solutions.135 The 308 cm‘1 progression on the the near-IR 2(5 —) 8*) transition of the M02":111 complex conforms well with the symmetric metal-metal stretching vibration. As with dimolybdenum phosphate and sulfate,”‘5 the large red-shift of the 2(5 -) 8*) transition is not due to a lengthening of the Mo-—Mo bond length, which increases by only 0.05 A upon one-electron oxidation of the metal-metal core (2.141 A and 2.191 A for quadruple bond and mixed-valence complexes, respectively).137 Rather, the shift is explained by the absence of two electron spin pairing terms for a (3271:45l ground state configuration shown in Figure 3.4.96 This spectral shift is also observed in the mixed-valence dimolybdenum sulfate system. 52 Absorbance l l l l 1300 1400 1500 1600 711nm Absorbance 400 500 600 700 800 711nm Figure 3.3 Spectral changes resulting from the photolysis (7» > 495 nm) of CZHZCIZ solution of M02[02P(O6CH5)2]4. 53 .0:th 823.20 mm 823 85:8 .8 action: a .8583 use we 3.682 65 5;? .905 Emszoz— 02 wficeommoboo 05 5:: £338 on 65 .3 3an mafia Baotou—o 65 .8 53:86.6 econ oo:o_m> in 2:3...— Afireua L 54 With respect to the organic photoproducts, the DCC and DCE photochemistry is analogous inasmuch as GC/MS shows the exclusive production of the unsaturated olefin (Figure 3.5: cyclohexene, [M]+ = 82 m/z, ([M] — CH3)+ : 67 m/z, ([M] —c2H.,)+ = 54 m/z, ([M] - c3H3)+ : 41 m/z and ([M] - C3H5)+ = 39 m/z).138 However, when the substrate is an unsaturated dichlorocarbon the organic photoproduct is not the olefinic hydrocarbon. Consider the photochemistry of DCB. Elimination of chlorine from DCB would result in benzyne, which is efficiently trapped by furan to yield or-naphthol. Yet when Mo2[02P(OC6H5)2]4 is irradiated in the presence of DCB and furan, a- naphthol is not detected; GC/MS reveals chlorobenzene as the principal photoproduct (Figure 3.6a). In addition to chlorobenzene, a high boiling fraction is also isolated from the photoreaction mixture; its GC/MS (Figure 3.6b) shows it to be primarily tri-chlorobiphenyl. The photoproduction of mono-chlorinated product is consistent with chlorine abstraction by the photoexcited quadruply bonded species, but subsequent elimination of chlorine from the incipient chlorobenzyl radical does not occur. The appearance of polychlorinated biphenyls is consistent with initial chlorine atom abstraction owing to the known reaction of the chlorobenzyl radical with DCB,139 gel 1 :1 _’ awe! * H' (3.5) CI CI 0| g + H' ——> @E (3.6) ' H Cl Cl Cl @- + . —» e—o Cl LoI 0,.IS: .55 45895 :88 x3e com Evan 88:3 LS: 3388.23 05 3 .6858 868666 5:: 43:68:65: e5 652268626665 .6 8382205 2: 80¢ wen—:8: 3262a cameo 65 Me 8.8% 822 Wm edema NE. om cm on om om o.V _ _ _ . — P _ -— _ _ _ — _—:_ _nq ___._._q___ :4 .. cm H LmImOH 1 who. on to: M. LmImO=r mg. m. roe w mm em .. m... +2 armors; .8 A 100.. me .mzotz_ 100, ‘ A‘ld 0 5 fi ‘ b79335 0>_um_®I q 100- nw 5 55:02: ®>.:m<®t n01 Phi (a; (b. 56 (a) W” 112 100- ,; [NI—CI]+ 8 (D ‘ 77 g 1 09504 + 1% q[M—Cl —CZH2] a; i 51 m t ‘LILL'"ll1‘l“!IrI'l'l'l'l‘j 4O 60 80 100 120 140 m/z (b) [MH - CI — HCI]+ 186 100- [MH]+ ‘ 257 E / (I) 8 + E: 50- [MH - 3Cl] a) ‘ 152 .g 221/222 ‘3 a: 1 m '1 140 180 220 260 m/z Figure 3.6 Mass spectra of the organic products resulting from the photoreaction of o-dichlorobenzene and M02[02P(OC6H5)2]4 with (a) chlorobenzene obtained as the primary product and (b) tri-chlorobiphenyl obtained as a secondary product in ~3% yield. 57 Eu 05 Eat 33.303 a 8 NO .8 8:085 05 3:36.: ... “.38 .83 2:. 33238:; 05 a 8598 8258820 53 inmeuozmoaaz 2a oaoasoeozeuac do 55888.5 2: 80¢ was—=8. 32605 $530 05 mo «58% $22 sum 2:3,.— us our cor om om ow om _..._cLL_.r_L.L..._I ._ _.... =1. _____7_fii_d__ .— tom 8 w. R 18 m... +__o-_2_ m -8 m U m... tom ,M The p1 with t chloro summz 3103K saturat monoh 01' species COmpl‘ the trip rations the r “02K reduc: Mozngh much . ~1.0\ 58 The photoreactivity Moz[02P(OC6H5)2]4 with DCEE is similar to that of DCB with the monohalohgenated olefin observed as the photoproduct (Figure 3.7: chloroethylene, [M]+ = 62 m/z; ([M] - Cl)+ = 27 m/z). The general result to emerge from our product distribution studies is summarized by eqs. 3.8 and 3.9. Initial chlorine abstraction by photoexcited M02[02P(OC6H5)2]4 leads to the olefinic hydrocarbon when the substrate is a saturated dichlorocarbon whereas unsaturated dihalocarbons react to produce monohalogenated olefin. Cl H hv (7L > 495 nm) : >____: 495 nm) H—— : ; >——W—' : (3.9) CI M°2[02P(0P W214 CI H Cl In both cases, the inorganic photoproduct is the mixed-valence Mozn'III species; the photoreaction terminates at the one-electron oxidized M02IIJII metal complex rather than fully oxidized Mozm'III species. The difficulty in obtaining the triply bonded dirnolybdenyum sulfate and diphenylphosphate systems can be rationalized on the basis of the oxidation-reduction chemistry. Table 3.1 collects the reduction potentials for M02( S O4)44‘, M02( HPO4)44‘, and M02[02P(OC(,I-I5)2]4.”9'136 As compared with previously reported oxidation- reduction properties of multiply bonded “M020 3” complexes, the M02n,m/M02II,II reduction potentials of the sulfate and diphenylphosphate are much the same, and significantly positive to that for the phosphate compound by ~l.0 V shift, in spite of the structural and electronic similarities among HPO42’, 59 so}- and 02P(OR)2' ligands. The HPO42' is distinguished from 02P(OR)2" and 8042‘ by its chemical properties. An equilibrium with protons of the phosphate ligand can be established in aqueous solution,129 whereas the SO42“ and 02P(OR)2" ligands are not affected by proton association. Consequently, the inability to generate the triply bonded sulfate complex is not due to the intrinsic instability of this species, but rather because the reduction potential for the generation of the triply bonded complex lies positive of the potential for water oxidation. Similarly, attempts to prepare the triply bonded dimolybdenum diphenylphosphate are also frustrated by the very positive M02111.III/M02n,m couple. Although oxidation of the quadruple bond complex by N OBF4 proceeds smoothly to the M02[02P( OC6H 5)2]4+, subsequent oxidation to M02[02P(OC6H5)2]42" can not be achieved by NOBF4 (the NOVNO couple is estimated to be ~O.85 to 1.0 V vs. SCE in nonaqueous solution‘”), not even by stronger one-electron oxidants, such as tris(4-bromophenyl)aminium hexachloroantimonate (E1 ,2 = 1.04 V vs. SCE) or dichloroiodobenzene (estimated 0.9 to 1.2 V vs. SCE). This may be a result from kinetic barriers for generating the dication that cannot be overcome by carrying out the reaction at elevated temperatures owing to decomposition of the mixed-valence dimer upon heating. Alternatively, electrochemical methods may offer the triply bonded dication species. Bulk electrolysis of solution by holding the potential of platinum electrodes at potentials as high as 1.7 V vs. SCE led to the disappearance of the near-infrared absorption of the mixed-valence dimer and the appearance of weak absorptions in the visible.“ Although the species in solution failed to crystallize, the spectral changes accompanying bulk electrolysis are consistent with the formation of the triply bonded dimer, and we suspect that the electrochemical preparation of this triply bonded species is plausible. Tabh Mozl i Phosp’ a refer: 60 Table 3.1 Formal Reduction Potentials of the M02(II,III)/(II,II) and M02(III,III)/(II,III) Couples of Dimolybdenum Tetrakis Sulfate, Phosphate, and Diphenylphosphate Complexes Em/V vs SCE Em/V vs SCE C°mplex M02(II,III)/(II,II) M02(III,III)/(II,III) . Sulfate +0.25a «.120d Phosphate -0.67 b - -0.24 b Diphenylphosphate +0.06 C +1 .00 c areference 136. b reference 129. c ref 135 d Estimated see c for reference B. Q! organi selects consis dihalo dihalol less th barnet lll'lSZiiU Moz[( quantt ability transiw Ifansfe metal CC>0rdi likely Clequ the CI} FiEUrc MC’JC IHSPCC insular 61 B. Quantum Yields Table 3.2 summarizes the organic reactants, the corresponding principle organic phOtoproducts and the quantum yields of the photoreaction for the selected substrates with Moz[02P(OC6H5)2]4. The quantum yield studies are consistent with the nature of the organic photoproducts. For saturated dihalocarbons, the quantum yield falls in the range of 0.04. For unsaturated dihalocarbons, the quantum yields drops off dramatically to 10'2 to 10“ times less than that of saturated dihalocarbons. This may be due to the greater kinetic barrier for breaking the more thermodynamically stable C—Cl bond of the unsaturated halocarbon. The quantum yields for photoreduction of dihalocarbons by M02[02P(OC6H5)2]4 is also solvent dependent. As shown in Table 3.2, the quantum yields for photoreaction decrease dramatically with the increasing ability of solvent to ligate the metal core. The reduction of alkyl halides by transition metal donors can occur by outer-sphere”0 or inner-sphere electron “NM“ with the latter being especially important for transition transfer pathways, metal reductants featuring open coordination sites. In view of the vacant axial coordination sites of the metal-metal core, an inner-sphere reaction pathway is likely to play a significant role‘ in the photoreduction of alkyl halides by the electronically excited Mozn'II diphenylphosphate complex. In the case of THF, the crystal structure of M02[02P(OC6H5)2]4° 2THF142 supports this contention. Figure 3.8a shows the space filling computer generated model of M02[02P(0C6H5)2]4-2THF, situated just off-axis of its metal-metal bond.131 Inspection of the model reveals that the ligating solvent molecules completely insulate the metal-metal core from substrate. Alternatively, with the solvent 'Tat hio‘ SOI\ aQUm 'wnhc Cone .62 Table 3.2 Quantum Yield Data for the Photoreaction between Moz[02P(OC6H5)2]4 and 1,2-Dichlorocarbon in Various Nonaqueous Solvents. 1,2-Dichlorocarbon Solvent ¢p a 1 ,2-dichloroethane (3qu 0.029 1 ,2-dichloroethane THF 0.014 1,2-dichloroethane CH3CN 0.0012 1 ,2-dichlorocyclohexane C6116 0.040 1,2-dichloroethylene Cst 5 .5 x 10‘6 o-dichlorobenzene C6146 1.4 x 10'4 aQuantum yield for the photoreaction (ken = 546 nm) of M02[02P(OC6H5)2]4 with dichlorocarbon as determined by using a ferrioxalate actinometer. Concentration of chlorocarbon is 7 M. 63 98 mpg. .sEnmeUOvaOwo—z A8 AuAnEeUOENOHNoE As Mo 3808 mfizmboam cBEocow 523800 a n 9.sz ICIDO‘ shou‘ l103[| phou: THF. ' anacl shout (L h min} Phou exch Chk) Prod 310s eChm Phou Speci 64 removed, the metal-metal core is clearly visible to axially approaching substrates shown in Figure 3.8b. Consequently, the photoreduction of DCE by M02[02P(OC6H5)2]4 is severely impeded. Therefore, the quantum yield for the photoreduction of DCE by M02[02P(OC6H5)2]4 is significantly diminished in THF. These data suggest that the photoreaction is confined predominantly to attack by substrate at the axial coordination site of the metal-metal core as shown in eq. (3.10), C. Mechanism The mechanism for the reduction of saturated dihalocarbons is consistent with the reaction pathways shown in Figure 3.9. In contrast to the M02(HPO4)44' photochemical scheme of Figure 3.1, the visible photoreaction from the 55* excited state of Moz[02P(OC6H5)2]4 is observed in nonaqueous solutions. Chlorine abstraction by the electronically excited M02[02P(OC6H5)2]4 complex II.III produces ~‘CHCHC1~ and the mixed-valence M02 complex, Moz[02P(OC6H5)2]4Cl. Subsequent reaction of the radical with another 11,11 reactant, which is in excess, directly yields the observed equivalent of the M02 photoproducts. The photogenerated chloroalkane radical is a very reactive species. It can either react within or external to the solvent cage of the primary «ESSS IN \ IQ.© 65 $560563: 3 8358266-? Baas... Co :28:on 32826223 05 Ba Ema—«:88 05 wcmutafiea 883% _26— 3.25 an 953..— _o_._e_2__e_2 m _o___o_2__o_2 + szouszo __o_>___o_2 _.o_2__o_2 NIouszo + _o___os___.os__o _o___o_2__o_2 + 5:96; £95 -o.o _0 693 2.620% mmmo Emeom g8 =~o_ .8005“ :3” 0.53..— ..z... .3 550.110.)» (SOHO? .1 I. a 2.0.1.0 1 + Ase .5 _o___o_2__o_2 + .2. 2.0:“st _o__.o_2_.o_2 loo _0 _0 £935. 0 __0_>___0_2 :_o_2___e_2_0. émuo 2.0 145058.83 £0 3.5 K _o .0 _O CE BS It 0.0 ON. 0.? 06 Ae/Afileua 68 On the other hand, if the carbanion is the important intermediate, then it is conceivable that the production of the radical is circumvented and the carbanion is produced directly. It should be noted that the photoreduction of dichlorocarbons by M02[02P(OC6H5)2]4 can be correlated with the general reaction chemistry observed in the electroreduction of 1,2-dihaloalkanes and 1,2- dihaloalkenes.” For instance, the early studies of von Stackelburg and Stracke established that the electroreduction of 1,2-dihalides afforded olefinic hydrocarbons.”5 Reductions of 1,2-dihalobenzenes may take place via a carbanion, which is then trapped by a proton to produce the monosubstituted halo-benzenes.““S Also, the two-electron electrochemical reduction of cis-2,3- dichloroacrylonitrile to 2-chloroacyrlonitrile is believed to occur via the carbanion.“7 Recently, aromatic and vinylic dibromides have been shown to react with low valent metallo-porphyrins by X” atom departure to produce a carbanion.140° This E2 mechanism presumably is preferred owing to the presence of the 1r* orbital. This observation implies that the first halogen is eliminated as a cation and the carbanion is produced directly. However, in this case, the direct production of the carbanion may be precluded by the high potential for production of Mozm'm, which is necessary for direct X+ transfer. Alternatively, a carbanion may be produced by electron transfer of the chloroalkene radical ~.C=CC1~ with the Mozu'n starting reagent. In either case, the mono- halogenated product is obtained by protonation of the carbanion. While the above studies paint a general picture as to the mechanism of dihalocarbon reductions by M02[02P(OC6H5)2]4, a more precise mechanism for the photoreduction of aromatic and vinylic dihalides should be garnered when quantum yields are measured (if the reaction of the primary photoproduct is rate determining) and HID isotope studies are performed for photoreactions carried out in the presence of proton donors and radical scavengers. ‘5 ‘1 "5.7"" .4 LB photc the p suno bidet photc from react path I Coon Mo Mo:I 0nd; redu; of th; Obtai CHAPTERIV PHOTOREDUCTION OF DIARYL DISULF IDES BY QUADRUPLY BONDED DIMOLYBDENUM AND DITUNGSTEN COMPLEXES I. Background The M208 studies of Chapter III indicate that two-electron photoprocesses to be exceptional reaction pathways and the precise nature of the photooxidation product depends intimately on the coordination environment surrounding the bimetallic core.“8 When the metal core is strapped by four bidentate ligands (Du), d3—d4 mixed-valence species are the common photoproduct. Although the photoactivation range of M-4-M dimers has moved from UV to visible in the M02 diphenylphosphate system, the excited—state reaction chemistry is still limited primarily to sequential single electron transfer pathway. The structural inflexibility of the four bidentate ligands renders the coordination sphere incapable of accommodating the increased charge of a Mozm'III core and hence the reaction terminates at the one-electron oxidized Mozn'III metal complex. As we have discussed in Chapter III, a two-electron oxidized photoproduct may be accessed if the bimetallic core is made more reducing. Substitution of 02P(OR)2‘ by HPO42‘ increases the reduction potential of the dimolybdenum core by 1.0 V and, accordingly, a Mozm'III tetraphosphate is obtained on the photochemical reduction of protons to hydrogen.129 However, 69 d‘——< core i Inden hahdt bndg plane oxkhi additi reduc photo and ( subsu chlon thetei Here, fhcflit meta] ESBC SpeCle 7O d4—d4—> d3—d3 photoconversions of this type are unusual when the bimetallic core is spanned by four bidentate ligands. Two-electron chemistry is typically observed when two of the four bidentate ligands of a MAM core are replaced by monodentate ligands, usually halides. Quadruply bonded binuclear systems M2X4(PP)2 (M2 = M02, W2; PP = bridging phosphine; X = halide)84 feature sterically uncongested equatorial planes and the terminal halides can easily move to edge-bridging positions upon oxidation of the metal core to form edge-sharing bioctahedral (ESBO) species. In addition, the bidentate or chelating function of PP ligands in these complexes reduces the possibility of circumventing photoredox chemistry by photosubstitution of the phosphine ligands. It is well established that M2X4(P P)2 and (PP = bidentate phosphine) complexes thermally reduce a variety of substrates (YZ), such as diselenides,“3”9'15° halogens,151 and hydrogen chloridem'”3 These two-electron thermal reactions involve the rearrangement of the terminal halide ligands to yield M2X4YZ(P P)2 ESBO products,l52154°155 l_—_| I | P P P II3 CI Cl YZ .. s .. our.» ...Y>M'!.'.scu (M (M M-M w 0' ‘2 ‘0: (4'1) Cl I CI I ‘ °' I P P P P I_._J l____l Here, the ability of the ligand coordination sphere to stabilize an ESBO structure facilitates the reaction by enforcing an octahedral geometry about the individual metal centers of the oxidized bimetallic core. For the same reason, our group has previously shown that formation of ESBO species is also prominent in the two-electron photochemistry of these species.156 Early work in our group established the two-electron photochemical oxid; the e W 2 C anal) The t direc edge of h phou phott the f corn; smdi tth the 5 biocr term path‘ rt‘rpre II. 1 CH2: 71 oxidative-addition of CH3] to W2Cl4(dppm)2.‘57 This reaction is also attributed to the excitation of the metal-localized 5n* and 1:5* transitions. The photoproduct, W2Cl4(dppm)2(CH3)(I), was characterized by chemical and spectroscopic analyses, and l3C-NMR shows the methyl group to be in the terminal position. The cis-addition to one end of the tungsten bimetallic core is consistent with the direct addition of the substrate to the open coordination site of a photogenerated edge-sharing bioctahedral intermediate. The observation of simple photoaddition of Mel to a terminal coordination site of the ditungsten core suggests a photoreaction pathway in which substrate is activated at the :M' site of the photoactivated :M"—M+ core. This reaction is important as it represents one of the first examples of a discrete multielectron photoreaction of a transition metal complex. We wished to generalize this type of photoreactivity with further studies of M2X4(dppm)2 (M = Mo or W, dppm = diphenyl(phosphino)mthane) in the presence of other substrates such as RSSR (R = Et, Ph). Terminal coordination of the methyl group is consistent with addition of the substrate to an open coordination site of a photogenerated edge—sharing bioctahedral intermediate. The observation of simple photoaddition of Mel to a terminal coordination site of the ditungsten core suggests a photoreaction pathway in which substrate is activated at a distinct tungsten atom, and represents one of the first examples of a discrete multielectron photoreaction. II. Results and Discussion A. Photochemistry Figure 4.1 shows the spectral changes associated with the irradiation of CH2C12 solutions (7t>435 nm) of M02Cl4(dppm)2 and PhSSPh. While isosbestic 72 points are maintained during the course of the photoreaction in the low energy absorption range for the disappearance of the 1(52-955'") absorption band, isosbestic behavior in the ultraviolet spectral region is not observed. The photoproduct is isolated with a yield of 33%. The absorption spectrum of M02C15(dppm)2(SPh) compares well to that of the final photoreaction (Amx = 527 and 457 nm) as shown in Figure 4.2. When the solvent is toluene, irradiation of M02Cl4(dppm)2 with excess PhSSPh from low (k>435 nm) to high energy (70335 nm) under the same conditions produces the same final product in 52% isolated yields. The identification of the M02C15(dppm)2(SPh) ESBO, with SPh' in the bridging position of the Mozm'III core, as the final photoproduct is confirmed by X-ray crystallography. Specifically, the toluene co-solavte was obtained, M02C15(dppm)2(SPh)-C7H3. The molecular structure is similar to that previously reported by Cotton and coworkers,113 and differs only in the solvent packing within the lattice. The structure of M02C15(dppm)2(SPh) 0 C7H3 is shown in Figure 4.3. Deep maroon rod-shaped crystals of the bis(diphenylphosphinomethane)- pentachlorom-phenylsulfido) dimolybdenum(III,III) of the oxidized complex are monoclinic with a P21/C space group. Tables 4.1 and 4.2 gives a summary of crystal data and the selected positional parameters, respectively; selected bond angles and bond distances are listed in Tables 4.3 and 4.4. The Mo—Mo bond distance of 2.787(3) A for the oxidized dimer is 0.649 A greater than its quadruple bond parent complex, M02Cl4(dppm)2 (2.138 A). Because of the 73 £323.: 58 cm 5 8:: ova 8 o a 3282 Be? £8on .002 3 2852:2256 335350“. E .Emmnm 3088 28-553. a 5:5 #:2333082 .8 AE: mmv A 33.6 mam—Sosa 05 meta. 89:50 358% 5:98.? £5585 g .9 95»...— 2:2 com cos . com com cow eoueqlosqv PHCU r ZIMC rlwew >zmt®tt ®>2m-®m 74 Figure 4.2 (a) Electronic absorption spectrum of M02C14(dppm)2(tt-Cl)(tt-SPh) in CHZCIZ solution at room temperature. (b) FAB-MS spectrum of M02C14(dppm)2(u-Cl)(tt-SPh). 12 e (a) £3 8 '2 N ,0. ED 4 O '- I l J I 400 500 600 700 800 lt/nm (b) 2: '7) g; [Mt-(mp- .s m 12112 .2 . ‘5 I as l c: I 12412 1100 '1150 1200 1300 1350 m/z 75 Figure 4.3 ORTEP view of molecular structure of phot0product M02CL,(dppm)2(tt-Cl)(tt-SPh)02C7H3. Thermal ellipsoids are drawn . at 50% probability. Tab 76 Table 4.1. Crystallographic Data for M02C14(dppm)2(u-Cl)(tt-SPh)-2C7H3 chem formula fw space group a b N <~< U Q 7» (Mo Ka) Pcalc u R(Fo) Rw(Fo) C56H49C15M02P4S: 2C7Hg 1431.40 P21/c (No. 14) 1527004) A 1792605) A 23.88101) A 90. 92.33(5)° 90. 6531.5(6) A3 4 202(3) K 0.71073 A 1.46 g cm"3 7.5 cm‘1 0.082al 0.071b “R: -):IIF l—IFCII/ZIFOI. bR..,=[Zw(l1=,I-|I= |)2/2w|1=o I21‘”:w =1/(oleol). .77 Table 4.2 Positional Parameters and Equivalent Isotropic Displacement Parameters (A2) for M02Cl4(dppm)2(tt-Cl)(tt-SPh)-2C7H3 atom x y z Beq, A2 MO(1) 0.2552(1) 0.4268(1) 0. 14521 (8) 1.67(4) MO(2) 0.3166(1) 0.3275(1) 0.22759(8) 1.75(4) C1(1) 0.2899(4) 0.4639(3) 0.0499(2) 2.5(1) C1(2) 0.4289(4) 0.2293(3) 0.2371 (2) 2.9(1) C1(3) 0.1425(4) 0.5196(3) 0.1317(2) 2.9(2) C1(4) 0.2936(4) 0.2991 (3) 0.3239(2) 2.4(1) C1(5) 0.3665(3) 0.3360(3) 0.1343(2) 2.1(1) 3(1) 0.2166(4) 0.4282(3) 0.2417(2) 2.2(1) P(1) 0.3636(4) 0.5299(4) 0.1784(3) 2. 1 (1) P(2) 0.4383(4) 0.4187(4) 0.2640(2) _ 2. 1 (1) P(3) 0.1440(4) 0.3302(4) 0.1036(2) 2.2(1) P(4) 0.1994(4) 0.2255(3) 0.1991(2) 1.9(1) C(l) 0.403(1) 0.516(1) 0.2511(8) 1.9(5)* C(2) 0.105(1) 0.269(1) 0.1602(8) 2.2(5)* C(11) 0.465(1) 0.548(1) 0.1416(7) 1.0(4)* C(12) 0.509(1) 0.611(1) 0.1529(9) 2.7(5)* C(13) 0.588(1) 0.627(1) 0.1236(8) 3.0(6)* C(14) 0.617(1) 0.579(1) 0.0850(9) 3.3(6)* C(15) 0.567(1) 0.514(1) 0.0722(9) 3.0(5)* C(16) 0.497(1) 0.499(1) 0.1028(8) 2.5(5)* C(21) 0.318(1) 0.621(1) 0.1804(9) 3.0(6)* 78 Table 4.2 Positional Parameters and Equivalent Isotropic Displacement Parameters (A2) for M02C14(dppm)2(tt-Cl)(u-SPh)-2C7Hg (cont'd) atom x y z Beg, A2 C(22) 0.296(2) 0.656(2) 0.129(1) 7.2(9)* C(23) 0.260(2) 0.727(2) 0.123(1) 7.5(9)* C(24) 0.246(2) 0.764(2) 0.171(1) 7.3(9)* C(25) 0.251(2) 0.731(2) 0.222(1) 10(1)* C(26) 0.296(2) 0.659(2) 0.226(1) 6.0(7)* C(31) 0.549(1) 0.417(1) 0.2392(8) 2.3(5)* C(32) 0.576(1) 0.369(1) 0.1979(9) 3.5(6)* C(33) 0.662(1) 0.371(1) 0.1790(9) 3.7(6)* C(34) 0.721(2) 0.420(2) 0.204(1) 5.2(7)* C(35) 0.695(2) 0.466(1) 0.2456(9) 4.2(6)* C(36) 0.61 1 (1) 0.467(1) 0.2634(9) 3.2(6)* C(41) 0.459(1) 0.413(1) 0.3401 (9) 2.8(5)* C(42) 0.405(2) 0.456(1) 0.3752(9) 3.9(6)* C(43) 0.423(2) 0.450(1) 0.435(1) 5.0(7)* C(44) 0.487(2) 0.400(1) 0.454(1) 4.7(7)* C(45) 0.531(2) 0.353(1) 0.419(1) 4.6(7)* C(46) 0.513(1) 0.359(1) 0.3609(9) 4.0(6)* C(51) 0.040(1) 0.368(1) 0.0728(8) 2.1(5)* C(52) -0.026(1) 0.385(1) 0.107(1) 3.6(6)* C(53) -0. 104(1) 0.416(1) 0.0813(9) 3.5(6)* C(54) -0.1 12(2) 0.426(1) 0.025(1) 4.9(7)* 79 Table 4.2 Positional Parameters and Equivalent Isotropic Displacement Parameters (A2) for M02C14(dppm)2(tt-Cl)(tt-SPh)-2C7H3 (cont'd) atom x y z Beg, A2 C(55) -0.046(2) 0.405(2) 0.007(1) 7.0(9)* C(56) 0.035(2) 0.378(1) 0.016(1) 4.9(7)* C(61) 0.182(1) 0.266(1) 0.0482(8) 2.3(5)* C(62) 0.251(1) 0.280(1) 0.0185(8) 2.3(5)* C(63) 0.271(2) 0.231(1) -0.0248(9) 4.1(6)* C(64) 0.219(1) 0.171(1) -0.0378(9) 3.7(6)* C(65) 0.142(2) 0.156(2) -0.008(1) 6.2(8)* C(66) 0.120(1) 0.206(1) 0.0344(9) 3.3(6)* C(71) 0.145(1) 0.178(1) 0.2573(8) 2.4(5)* C(72) 0.057(1) 0.183(1) 0.2658(8) 2.2(5)* C(73) 0.023(1) 0.143(1) 0.3105(8) 3.3(6)* C(74) 0.080(1) 0.105(1) 0.3477(9) 3.2(6)* C(75) 0.167(1) 0.101(1) 0.3394(9) 3.2(6)* C(76) 0.206(1) 0.138(1) 0.2934(7) 1.4(4)* C(81) 0.229(1) 0.147(1) 0.1557(8) 1.7(5)* C(82) 0.309(1) 0.139(1) 0.1339(8) 2.3(5)* C(83) 0.331(2) 0.076(1) 0.1015(9) 3.8(6)* C(84) 0.266(2) 0.024(1) 0.088(1) 4.5(7)* ‘ C(85) 0.187(2) 0.029(1) 0.1120(9) 4.2(6)* C(86) 0.166(1) 0.091(1) 0.1441(9) 3.1(6)* C(91) 0.106(1) 0.414(1) 0.2633(9) 2.9(5)* 80 Table 4.2 Positional Parameters and Equivalent Isotropic Displacement Parameters (A2) for M02C14(dppm)2(tt-Cl)(tt-SPh)-2C7H3 (cont'd) atom x y z Beq, A2 C(92) 0.045(1) 0.464(1) 0.2444(9) 3.1(6)* C(93) 0.041(2) 0.461(1) 0.2642(9) 4.3(6)* C(94) 0.063(2) ' 0.403(1) 0.2988(9) 4.1(6)* C(95) 0.001(2) 0.353(1) 0.319(1) 4.6(7)* C(96) 0.084(2) 0.356(1) 0.2983(9) 4.4(6)* C(101) 0.404(2) 0.777(1) 0.075(1) 4.1(6)* C(102) 0.487(2) 0.754(1) 0.0559(9) 3.7(6)* C(103) 0.490(2) 0.693(1) 0.0184(9) 3.9(6)* C(104) 0.419(2) 0.663(2) 0.002(1) 5.4(7)* C(105) 0.340(2) 0.689(2) 0.015(1) 5.5(7)* C(106) 0.332(2) 0.747(2) 0.052(1) 6.4(8)* C(107) 0.403(2) 0.840(2) 0.116(1) 8.1(9)* C(lll) 0.115(2) 0.634(2) 0.461(1) 10(1)* C(112) 0.111(2) 0.689(2) 0.421(1) 7.1(8)* C(113) 0.118(2) 0.672(2) 0.365(1) 10(1)* C(114) 0.130(3) 0.600(2) 0.348(2) 14(2)* C(l 15) 0.130(3) 0.548(3) 0.386(2) 15(2)* C(116) 0.115(3) 0.559(2) 0.444(2) 15(2)* C(117) 0.107(2) 0.655(2) 0.522(1) 12(1)* Starred atoms were refined isotropically. Anisotropically refined atoms are given in the form of the isotropic equivalent thermal parameter defined as: (4/3) [32B11 + b2B22 + c2B33 + ab(cos y)Blz + ac(cos B)B13 + bc(cos a)B23] 81 Table 4.3 Selected Bond Lengths (A) and The Standard Deviations for for M02C14(dppm)2(tt-Cl)(u-SPh)-2C7l-I3 Bond Lengths _ Bond Lengths Mo(1)—Mo(2) 2.787(3) S(1)—C(91) 180(2) Mo(1)——C1(1) 2.450(6) P(1)—C(1) 1.83(2) Mo(1)—-Cl(3) 2.407(6) P(1)—C(1 1) ‘ 184(2) Mo(1)—-Cl(5) 2.376(6) P(1)——C(21) 1.79(2) Mo(1)—S(l) 2.401(6) P(2)—C(1) 185(2) Mo(1)—P(l) 2.584(6) P(2)—C(31) 182(2) Mo(l)—P(3) 2.595(6) P(2)—C(4l) 183(2) Mo(2)—C1(2) 2.462(6) P(3)——C(2) 186(2) Mo(2)—Cl(4) 2.395(6) P(3)-—C(51) 185(2) Mo(2)—Cl(5) 2.389(5) P(3)—C(61) 186(2) Mo(2)—S(1) 2.397(6) P(4)-C(2) 186(2) Mo(2)—P(2) 2.599(6) P(4)—C(71) 185(2) Mo(2)—P(4) 2.628(6) P(4)—C(81) 181(2) 82 Table 4.4 Selected Bond Angles (deg) and The Standard Deviations for M02C14(dppm)2(u-Cl)(u-SPh)-2C7Hs Bond Angles Bond Angles Mo(2)—Mo(1)—Cl(1) 138.6(2) Mo(l)—Mo(2)—Cl(2) 137.3(2) Mo(2)-—Mo(1)——C1(3) 139.0(2) Mo(l)—-Mo(2)—Cl(4) 139.0(2) Mo(2)—Mo(1)—Cl(5) 54.4(1) Mo(1)——Mo(2)—Cl(5) 54.0(1) Mo(2)—Mo(1)—S(l) 54.4(2) Mo(1)-Mo(2)—S(l) 54.6(1) Mo(2)—Mo(1)-—P(1) 92.7(2) Mo(1)—Mo(2)—P(2) 92.9(2) Mo(2)—Mo(l)—P(3) 92.3(2) Mo(1)—Mo(2)—P(4) 93.2(2) Cl(l)—Mo(1)—-Cl(3) 82.4(2) Cl(2)—Mo(2)—Cl(4) 83.6(2) C1(1)—Mo(1)——Cl(5) 84.4(2) Cl(2)—Mo(2)——Cl(5) 83.3(2) C1(1)—Mo(1)—S(1) 163.6(2) Cl(2)—Mo(2)——S(1) 166.2(2) Cl(1)—Mo(1)—P(1) 86.0(2) , Cl(2)—Mo(2)—P(2) 86.1(2) Cl(1)—Mo(1)—P(3) 89.3(2) C1(2)—Mo(2)—P(4) 89.5(2) Cl(3)—Mo(1)—Cl(5) 166.0(2) Cl(4)—-Mo(2)—Cl(5) 166.9(2) Cl(3)—Mo(1)—S(l) 85.4(2) Cl(4)—Mo(2)—S(1) 84.6(2) C1(3)—Mo(1)—P(1) 89.6(2) Cl(4)—Mo(2)—P(2) 86.7(2) C1(3)—-Mo(1)—P(3) 87.5(2) Cl(4)—Mo(2)—P(4) 88.6(2) Cl(5)—Mo(1)—S(1) 108.5(2) C1(5)—Mo(2)—S(1) 108.2(2) Cl(5)—Mo(l)—P(1) 94.1(2) Cl(5)—Mo(2)—P(2) 91 .2(2) Cl(5)—-Mo(1)—P(3) 87.7(2) Cl(5)—Mo(2)—P(4) 92.5(2) S(1)—Mo(1)—P(1) 83.0(2) S(l)—Mo(2)——P(2) 86.0(2) S(1)—Mo(1)—P(3) 101.1(2) S(1)——Mo(2)—P(4) 97.4(2) P(l)—Mo(1)—P(3) 174.8(2) P(2)—Mo(2)—P(4) 173.9(2) 83 Table 4.4 Selected Bond Angles (deg) and The Standard Deviations for M02C14(dppm)2(u-Cl)(tt-SPh)-2C7Hg (cont'd) Bond Angles . Bond Angles Mo(1)—Cl(5)—Mo(2) 71.6(2) Mo(l)—P(3)-C(61) 117.6(7) Mo(l)—S(1)—Mo(2) 71 .0(2) ‘ Mo(2)—P(2)—C(l) 109.8(7) Mo(1)—S(1 )_C(91) 122.9(7) Mo(2)—P(2)—C(31) 122.9(7) Mo(2)—S(1)-—C(91) 122.8(8) Mo(2)—P(2)—C(41) 112.7(8) Mo(1)—P(1)—C(1) 112.1(7) Mo(2)—P(4)—C(2) 110.2(7) Mo(1)—P(l)—C(ll) 121.2(6) Mo(2)—P(4)—C(71) 116.5(7) Mo(1)—P(1)—C(21) 114.7(7) Mo(2)—P(4)—C(81) 120.3(7) Mo(1)—P(3)—C(2) 109.9(9) P(1)——C(1)—P(2) 112(1) Mo(l)—P(3)—C(51) 116.5(7) P(3)—C(2)-—P(4) 110(1) .84 longer metal-metal bond distance in the triply bonded species, there is less steric strain associated with the bridging bidentate phosphine ligands. The Mo2P2 segments of the five-membered M02(P2C) rings are planar and orthogonal to each other (dihedral angle of 93° average), compared to 96° and 100° of the parent complex. In the equatorial plane, terminal chlorine atoms trans to the bridging sulfur atom have longer Mo—Clt bond lengths «Mo—CL)“: 2.456 A ) than those cis to the bridging sulfur atom (ave. 2.401 A), while the bridging chlorine atoms have even shorter Mo—Clb, bond distances (d(Mo—Clb,)m = 2.383 A). This Mo—Cl bond length difference within the binuclear core can be attributed to the trans effect of SR’ group, which is a stronger 7t donor as compared to Cl'. Support for this intrepretation comes from the crystal structure of M02C16(dppm)2, which shows similar Mo—Cl bond distances for the bridging and terminal chlorines. There are no significant differences between the Mo—P bond distances in the crystal structures of Mo2C15(dppm)2(SPh) and Mo2C16(dppm)2. Whereas MozCl4(dppm)2 converts to Mo2C15(dppm)2(SPh) in CHzClz, evidence for reactive intermediates is provided when the photolysis is halted prior to its completion in toluene. In contrast to the isoenergetic disappearance of the 1(52--) 58*) absorption band in Figure 4.1, toluene solutions display a 600-nm absorption in the low energy spectral region (Figure 4.4). The compound responsible for this absorption has not been successfully crystallized, even at low temperature. However, FAB/MS and ESIMS analysis of solids isolated during intermediate stages of the photolysis showed the presence of Mo2C14(dppm)2(SPh)2 as the major product, with M02C15(dppm)2(SPh) present as well (Figure 4.5). Partially photolyzed solutions containing M02C14(dppm)2(SPh)2, left standing in the dark, converted completely to M02Cl5(dppm)2(SPh) as monitored by UV-vis spectroscopy and FAB/MS. We believe that Mo2Cl4(dppm)2(SPh)2 may be present in both the CHzClz and 85 .029, 8:988 8288:. a 8538 was 9:3 8:58 05 5:3 8&2..an as? 8:06.05 2E. .mEE com 98 mmm .9: 63 .2: .2. dm 6 8 8282 203 880% "Do w “a Ema—E $858 53, AEQQEeGNoE me 8:28 0:032 .8 A8: mom A 236 cabaoga on. warn—o mow—8:0 158% 8:953“ £2585 9.9 2:»...— EC \ 5A com CON 000 com DOV _ _ _ _ eoueqlosqv 86 .Emmvxeaaaxoaoz 8 0.883 ofi.£:mmvnAEamcv:U~oE me 8888 05 wfiaofi AEaneVe—UNOE we £5328 0532 AE: mom A 986 moi—Sosa 05 Soc coda—0mm 8:88 05 .«o 8.58% m2-m260 nm, and two additional bands at 510 nm and 420 nm appear (Figure 4.6a). The photoproduct is confirmed to be M02C15(dppm)2(SEt) by FAB/MS; spectra display a parent ion mass cluster at 1200 amu. (Figure 4.6b). During the early course of the reaction, an isosbestic point is. observed at 595 nm, but it is not maintained with continued photolysis, therefore suggesting that the mechanism proceeds by more than one step. Toluene solutions produce the same result at a solvent cutoff wavelength A>285 nm with the exception that small amounts of a brown precipitate form. (b) Relative Intensity 88 Figure 4.6 (a)Electrcr1ic absorption spectrum of M02C14(dppm)2(u-Cl)(u-SEt) in CHzClz solution at room temperature. (b) FAB-MS spectrum of the product isolated from the photolyzed (Aw: > 235 nm) CHzClz solutions of MozCh(dppm)2 with 40-fold excess of EtSSEt, consistent with M02C14(dppm)2(tt-Cl)(u-SEt). d) 0 c: (U E O to .D 495 nm) reacted toluene solutions of W2Cl4(dppm)2 and PhSSPh are identical to samples of the ESBO, W2Cl4(dppm)2(SPh)2, which have been previously synthesized and characterized.113 Although the same product is obtained in both cases, the thermal reaction occurs over days as opposed to only hours for the photolysis. 90 Absorbance fl l J l l l . l 300 400 . 500 600 700 800 Nnm Figure 4.7 Electronic absorption spectrum of the isolated brown product from reactions of M02C14(dppm)2 with (a) excess PhSSPh refluxing in toluene solutions for 12 hours (b) excess EtSSEt refluxing in toluene solutions for 36 hours. q 91 C. Quantum Yields Table 4.5 summarizes the wavelength dependence of the photoreaction quantum yield, 1%. The action spectrum of M2Cl4(dppm)2/PhSSPh photochemistry parallels its absorption spectrum, shown in Figure 4.8 and is consistent with reactivity originating from the metal complex. The onset of MozCl4(dppm)2 photoreactivity occurs at A = ~436 nm (op = 0.01), and dip increases monotonically between 320-380 nm, asymptotically approaching a limiting quantum yield of 0.27 at l<313 nm. The band centered at 330 nm appears to be responsible for the photoreactions. The action spectrum tracks the rising absorption edge of transitions immediately to higher energy of the 1(52—155“) transition (hm, = 635 nm). This coincidence between the wavelengths of absorptions immediately to higher energy of 1(5 2—9 58*) and the action spectrum of Mo2Cl4(dppm)2 is preserved in W2Cl4(dppm)2 photochemistry. The ~50 nm red-shift of the W2Cl4(dppm)2 action spectrum as compared to its M02C14(dppm)2 homolog is consistent with a respective 50 nm red-shift of the absorption spectrum of the ditungsten complex (see Figure 4.8). When the substrate is diethyl disulfide, the quantum yields decreases dramatically to 0.0014 and 0.061 for the corresponding wavelength 365 and 313 nm. The result can be rationalized by the increased bond strength of AlkS—SAlk (Alk = alkyl group) substrates as compared to PhS—SPh (bond dissociation energies are ~55 and 74 kcal, respectively).159 The increased activation barrier to sulfur-sulfur bond claevage in AlkS—SAlk substrates is sufficient to significantly perturb the efficiency of the photoreaction. The wavelength dependence of M2Cl4(dppm)2/PhSSPh photochemistry excludes the possibility of a reaction derived from direct homolysis of the disulfide bond to produce RS- radicals. Organic disulfides add to acetylenes to 92 Table 4.5 Wavelength Dependence of Quantum Yields for Photoreaction of M02C14(dppm)2 and W2C14(dppm)2 with PhSSPh. ten/um ¢pIM02C14(dPPm)2]° <1, [W2C14(dppm)zl" 546 <10-5 0.025 436 0.010 0.048 405 0.11 0.075 365 0.23 0.15 313 0.27 0.39 a'r = 16 °C in CH2C12; b T = 0 °C in toluene. 93 .0529 E mono—9:8 Allv £838.59? 28 Ale «As—334.082 no 58% 8:98am 038805 3. 953% E:\& ocm can com com oov com 94 afford 1.2—diarylmercaptoethenes in high yields via a free radical pathway, resulting from the photochemically induced cleavage of the disulfide bond, but the reaction occurs at higher wavelengths than reported here.“50 Benzene solutions of PhSSPh in either the presence of methylacetylene or l-heptyne are irradiated with wavelengths coincident with M2Cl4(dppm)2 photochemistry. No reaction is observed for A > 380 nm and only trace amount of reaction is observed at lower excitation wavelengths. With decreasing wavelength, the 1:1 adduct is observed with the highest yields occurring for irradiation wavelengths coincident with the absorption maximum of PhSSPh at A = 240 nm, a result that is consistent with Heiba and Dessau’s original observations of free radical addition of PhSSPh to these alkynes.161 The parallels between the quantum yields listed in Table 4.5 and the absorption profiles of the quadruply bonded metal-metal complexes, and their disparity with the action spectrum of RSSR photochemistry, establish that M2Cl4(dppm)2/RSSR photochemistry is derived from the quadruple bond complex. A comparison of the action spectra of dimolybdenum and ditungsten complexes provides additional insight into the excited state responsible for the observed photochemistry. Substitution of Mo by W in quadruply bonded metal- metal halophosphine complexes is known to lead to a blue shift in ligand-to-metal transitions owing to the greater difficulty associated with reducing W113“ Conversely, metal-localized transitions exhibit a red shift owing to decreased two-electron contributions to the overall transition energy of the tungsten complex.163 Figure 4.8 shows the absorption spectra of M02C14(dppm)2 and W2Cl4(dppm)2. As expected on the basis of these arguments, the metal-localized 1(82—) 58*) transition shifts from 635 nm for MozCl4(dppm)2 to 710 nm for W2Cl4(dppm)2. More pertinent to the photochemistry described here, a red shift is 95 clearly observed in the excitation window of the photochemistry. That this Mo -) W red shift is accomapnied by a similar red shift in the action spectra implies that the parentage of M2Cl4(dppm)2 (D3,) photochemistry is from a metal-localized excited state. D. Transient Absorption Studies Photochemical studies were complemented by the investigations of the transient absorption spectroscopy of M2X4(PP)2 complexes upon visible and near-ultraviolet excitation.164 In the case of M = Mo, a complete wavelength study could not be achieved owing to stimulated emission of dppm (Am, = 460 nm, 1 ~ 1 ns)‘°5 when samples were excited with high energy light (ken = 355 nm). Specifically, simulated emission in the 450-520 nm spectral range, although weak, was problematic because the AODs for the transient absorption bands of M02C14(dppm)2 in this range were small. The stimulated emission was eliminated upon replacement of the dppm by dmpm (bis(dimethylphosphino)methane) thereby allowing us to obtain sufficiently high signal-to-noise to detect absorptions with AODs<0.05. Because MozCl4(dmpm)2 is structurally and electronically analogous to M02C14(dppm)2, we believe the transient spectroscopy of the dmpm system to be representative of M02C14(dppm)2. Evidence for the electronic similarity of the two complexes comes from the absorption profile of M02C14(dmpm)2 and MozCl4(dppm)2, which are nearly identical but blue-shifted by ~40 nm for the former complex.166 Moreover, M02C14(dmpm)2 photochemistry is parallel to that of M02C14(dppm)2 with the production of M02C15(dmpm)2(SPh) as the photoproduct. For the case of W2Cl4(dppm)2, these complicating problems do not arise. A red-shift in the electronic absorption spectrum of the complexes transitions with regard to the 96 intraligand transitions of the dppm permitted us to obtain transient spectra at high and low energy excitation without interference from stimulated emission from the sample. Figure 4.9 shows transient absorption profiles for M02C14(dmpm)2 following a 3 ps excitation pulse at 600 nm. A prominent feature at 460 nm decays monoexponentially to ground state with a lifetime of 40(8) ps. Concurrent with this absorption decay is the recovery of the 1(56*) bleach at 630 nm on the same time scale thereby establishing the assignment of the transient absorption in Figure 4.9 to the 1(5151') excited state. The excitation of the 1(1551') state of W2Cl4(dppm)2 also affords a short-lived intermediate (< 1 ns), which is generally true of all M2X4(PP)2 complexes that we have surveyed to date. The short lifetimes of the 1(88*) transient accounts for the absence of M2C14(dppm)2/RSSR photochemistry upon 1(5 2—9 66*) excitation. A long-lived intermediate is observed, however, when the absorption profile immediately to higher energy of the 1(5 2—> 85*) transition is excited. High energy excitation of MozCl4(dmpm)2 in dichloromethane at he“ = 355 nm displays a transient absorption profile at Am, = 520 nm with a lifetime of 5 us)“ which is spectrally similar to the absorption spectrum of the edge-sharing bioctahedron, M02C16(dmpm)2, as shown in Figure 4.10a1°7'1°° A similar result is obtained for W2Cl4(dppm)2 in benzene at room temperature. Figure 4.10b shows that the transient absorption, which decays even more lewly (t = 46 us) than that observed for its molybdenum congener, is also similar to w2C16(dPPm)2,150'167 97 Figure 4.9 Time evolution. of the disappearance of the picosecond transient absorption of M02C14(dmpm)2 in dichloromethane. The spectra were obtained 2, 20 and 50 ps after a 600 nm, 3 ps excitation pulse. The inset shows a plot of the ln(AOD) for the transient absorption at 630 nm vs time. (ref. 164) .98 A34 8 0.04 git-8 4.2 1 l 0 10 20 30 Time / ps 0.02 20 ps CD) 0.00 50 ”3 <1 0.02 -— 50 p3 20 ps I t l l l 400 500 600 700 800 A/nm Figure 4.9 99 Figure 4.10 Transient difference spectra (0) of a deoxygenated (a) dichloromethane solution of M02C14(dmpm)2 recorded 1 us after 355 nm, 10 ns excitation, and electronic absorption spectrum of M02C16(dmpm)2 (—) in dichloromethane, (b) benzene solution of W2Cl4(dppm)2 collected 100 ns after 532 nm, 10 ns excitation, and electronic absorption spectrum of W2C16(dppm)2 (—) in dichloromethane. (ref. 164) ' A
U' (D O 3' m 3 O (D (9 c? (b) O O > a. <-—— 00 8 3' Q) 3 C (D 000 l l l I l l 400 500 600 700 A/nm Figure 4.10 101 The correlation between the spectra of the long-lived non-luminescent transients and the edge-sharing bioctahedra suggests that the transient may be derived from a chemically distorted ESBO-intermediate shown in eq. (4.2), T T P T Cl Cl 1’ r hv , 'h——'_—_' . —» CW ;‘°'*~ ' (4.2) I Cl l c" Cl Cl l. p 5: Although the electron counts of the chemically distorted ESBO transient (le'm = d5—d3) and M2X6(PP)2 complexes (MzmiIII = d3—d3) are different, the spectra of the two species may be similar for several reasons. The energy ordering of the molecular orbitals of an ESBO are 0' << 1: < 5*~5 < n* << 0* with a very small 5*l5 splitting.“9' 168' 1‘9 Consequently, transitions such as 1: —> 5*, 1t —) 5 and 5* -> it“ in d3—d3 are predicted to be energetically similar to 5 -) 11:" and 5* -) n" in a chemically distorted d5—d3 ESBO. Even greater similarities between the spectra of the two intermediates will result if the two additional electrons of the chemically distorted intermediate occupied the in-plane norbital of the ESBO as a lone pair. In this case, the electron count of a chemically distorted ESBO would be d3—d3 (iti,,.,,.m,¢)2 and metal-based transitions within the metal-based d-orbitals would be similar to the native d3—d3 ESBO. The formation of such an intermediate is consistent with the electronic structure of quadruply bonded metal-metal complexes. Metal-localized excited states associated with promotion of electrons to and from the 5 and 5* orbitals exhibit charge transfer character (i.e. Mzn'n" Ele'm).9"9° Intramolecular chemical distortion to an ESBO can stabilize this charge transfer within the 102 bimetallic core by providing cooperative stabilization with an octahedral geometry about a partially oxidized metal center and diminished donation of electron density from the halides about the partially reduced metal center. Whereas the activation barrier associated with the intramolecular ligand rearrangement may prevent access to an ESBO-type intermediate from the 1(55"') excited state, population of the higher energy metal-localized transitions can lead to diminished metal-metal 1t bonding relative to that in the ground state molecule (Figure 4.11). This feature is expected to enhance the formation of a bioctahedral intermediate because interactions of the metal dyz (or dxz) orbitals with those of ligands in the equatorial plane of an edge-sharing bioctahedron occur at the expense of M—M 1t interactions. Notwithstanding, the generation of long-lived intermediates upon excitation of the absorption profile to higher energy of the 1(52--) 55*) transition is consistent with our observations of M2Cl4(dppm)2 photochemistry over this same excitation range. The photochemistry of M2Cl4(dppm)2 is analogous in many ways to the thermal oxidative-addition chemistry of mononuclear d8 square planar metal complexes. In the d8 archetype, Vaska’s complex, trans-IrCl(CO)(PR3)2, a reduced, coordinatively unsaturated ML4 center adds substrate to yield an octahedral, two-electron oxidized metal center.17° The same is true of M2Cl4(dppm)2 inasmuch as the ML4 fragments composing the quadruply bonded metal-metal core are oxidized to yield a two-electron oxidized ESBO. The analogy is even more striking when a chemically-distorted ESBO is considered. A ML4 fragment provides the reducing equivalent to and accepts two terminal ligands from its ML4 neighbor thereby creating a single active site susceptible to substrate addition. More practically, the presence of a chemically distorted intermediate is necessary for M2Cl4(dppm)2 photochemistry. As transient spectroscopy shows, the 1(55") excited state is too short for its reaction with 103 \ 'U—g. Figure 4.11 Energy level diagram summarizing the formation of the edge-sharing bioctahedral distortion of the l(1t5"‘) (or l(51t"')) excited state of the M2C14(PP)2 complexes to stabilize the charge transfer mixed-valence state. 104 substrate. However a chemically distorted intermediate provides a means of trapping the charge separated character of the metal-localized excited states of quadruply bonded metal—metal complexes at sufficiently long lifetime to permit their reaction with substrate. E. Proposed Mechanism Although the characteristic 2(5 -) 5*) absorption of the mixed-valence Man species in the near-infrared.region is not observed during the course of the photoreaction of MozCl4(dppm)2 with PhSSPh, this does not necessarily exclude the possibility of a one-electron reaction sequence. It has been shown that the photoreaction of M02Cl4(dppm)2 in the presence of PhSSPh and TolSSTol (Tol = CH3C 6H 5) yields a cross-over product, M02Cl4(dppm)2(STol)(SPh), in addition to the formation of M02Cl4(dppm)2(SPh)2 and M02Cl4(dppm)2(STol)2,‘3' The presence of the mixed crossover product M02C14(dppm)2(SPh)(STol) appears to support a radical pathway as follows, M2"'"C|4(dppm)2 A» M2"".'C|4(dppm)2‘ (43) M2"'"Cl4(dppm)2* + RSSR —> M2"""Cl4(dppm)2(SR)+-SR - (4.4) '3” M2"""'C|4(dppm)a(SR)a+M2"'"Q|t(dppm)a (45) 2M2"""CI.(dppm)2(sn) M2""'"Cl5(dppm)2(SR) + M2"-"Cl3(dpprn)2(SR) (4.6) The primary step involves one-electron oxidation of electronically excited Mzn'nCl4(dppm)2 by the substrate RSSR to yield the mixed valence Mzn'mCl4(dppm)2(SR) intermediate. Disproportionation upon Cl atom abstraction yields Mzu'mC15(dppm)2(SR) and Mzu'nCl3(dppm)2. Alternatively, 1,05 disproportionation by ~SR atom transfer generates Mzm'mCl4(dppm)2(SR)2 and Mzn'nCl4(dppm)2. The formation of the M02C15(dppm)2(SR) is consistent with what we have previously observed for the formation of W2C15(dppm)2Y and W2Cl4(dppm)2Y2 photoproducts upon the photolysis of the parent quadruple bond complex in the presence of substrate Y—Y (e.g. alkyl iodides).‘57'm The W2Cl4(dppm)2 complex photoreacts with substrate to produce a mixed-valence primary photoproduct that disproportionates by either chlorine or Y atom transfer. A similar disproportionation mechanism involving halogen atom transfer between mixed-valence intermediates for the photochemical reaction of diplatinum pyrophosphite with aryl halides has been proposed.”2 For the case here, only M02C15(dppm)2(SR) is observed in the disproportionation due to the limited stability of M02Cl4(dppm)2(SR)2, which further converts to M02C15(dppm)2(SR). Nevertheless, the possibility for direct addition to the bimetallic core in a two elecuon step. While the crossover experiments with TolS—STol and PhS— SPh appear to exclude this possibility, this experiment should be carried out again under more careful conditions. The crossover photoproducts were isolated only by FAB/MS. Moreover, TolS—STol and PhS—SPh will form STol—SPh standing in room light. Until these experiments are carefully performed, the precise mechanism remains undefined. 111. Conclusion From the observations above, we conclude that the photochemical pathways differ from the thermal reaction routes. M02C15(dppm)2(SR) compounds can be prepared cleanly by photolysis of M02Cl4(dppm)2 in the presence of RSSR. The photoreaction to produce these compounds in high yields 106 is completely general. We believe that the previous preparation113 of M02C15(dppm)2(SPh) was actually obtained photochemically and not thermally since no product formation is observed when the reaction is carried out in complete absence of light, even when solutions are heated to the boiling point of toluene. The production of Mozm'mC 15( d p p m ) 2 (SR) and Mozm’mCh(dppm)2(SR)2 may occur by the free radical chemistry of eqs. 4.3-4.6. Alternatively these two products could form from the direct addition of persulfide to the bimetallic core. Notwithstanding these intimate mechanistic details, the wavelength dependence of quantum yields coupled with transient studies clearly indicates that the photochemistry is derived from metal-localized MMCT excited state of the bimetallic complex. If _ _ 7'" CHAPTER V CHARGE TRANSFER PHOTOCHEMISTRY OF QUADRUPLY BONDED DITUNGSTEN HALOPHOSPHINE COMPLEXES I. Background A comaprison of the photochemistry of the M0203 and M2X4(PP)2 compounds shows that the ligands about the bimetallic core play an important role in determining the oxidation-reduction photochemistry of M—4-M complexes. The photochemistry of these classes of M-4-M complexes is the scheme below, M —4—M (D4,, ) Complexes TL/TP TL. ,L “Tit-E; ’ —>hv ATE """ ’3"in -¥—R —» Wig —y + Fi- L RY L L L L L’L/ L L1” - d M J—M (D 2h) Complexes r-—-I P w r—-I T T T T T T I pl II II III I P: hv on... not. I RY on... or. v Cl If}? —’ Cl PVC. T ’ Cl Cr R P P P P P |___rl _ _ L—J 107 108 The bimetallic core is oxidized by either One or two electrons, depending on the coordination environment of the MiM center. A Mzm'm core necessarily demands that the coordination number of the bimetallic core increase. This can not be accommodated by the ‘lantern’ geometry of . the M0203 compounds and the M—4-M photoproduct is oxidzed by a single electron. However, when two of the bidentate ligands are replaced by terminal ligands of the D21I complexes, complementary redox reactivity of both metal centers is achieved. As described in Chapter IV, two-electron photoreduction of substrate (RY) is accomplished by the D2,, class of MAM complexes, M2X4(P P)2, to yield a d3—d3 edge-sharing bioctahedron species (ESBO).”‘ Here, the photochemistry is facilitated by the ability of the ligand coordination sphere to stabilize the d3—d3 core by providing an octahedral geometry about each of the oxidized metal centers. Can the multielectron photochemistry of MJ-M compounds be generalized toclasses of compounds other than the D2,I systems? We decided to investigate the M2Cl4(PR3)4 complexes, which are structurally congruent to M2X4(P P)2, r'__l P P Cl P ll Igull‘cl ”|,.P||lp' 'T'eT « a CI Cl P, P, To: p D2h 02d As with their D21, M2X4(P P)2 homologs, the M2X4(PR3)4 complexes may also undergo two-electron redox chemistry. These complexes are capable of sustaining bioctahedral geometries in either edge-sharing (ESBO) or face-sharing (FSBO) conformations,173 109 T 1 III In L~QI L5,,” :3”: LKM,‘ .Latl. L" \L/M MmeL WL l‘ L L \L L L ESBO FSBO These structural types are prevalent in the thermal oxidative-addition chemistry of M2X4(PR3)4 species. For instance, oxidiation of M02X4(PMe3)4 by I2 (X = 0,173” or PhIC12(X= Cl)‘37’ results in formation of the F830 complex, M02X7(PMe3)2" with the loss of two phosphines. In the case of labile phosphines such as PEt3, P(n-Pr)3, M02C14(PR3)4 thermal reacts with C0,, and to yield M02C193‘.”‘ Reaction of dimolybdenum complexes to ESBOs by simple oxidative-addition is more unusual. Attempts to generate these species by oxidation of M02Cl4(PR3)4 by NOPF6‘75 or electrolysis often leads to decomposition of the oxidized product.176 In contrast, ditungsten ESBO complexes do exist, but they are obtained from reduction of WCl4 with one equivalent of sodium amalgam.177 Attempts to convert WCl3(PR3)3 to either the known W2C16(PR3)4 ESBO complex or the known W2Cl6(PR3)3 FSBO complex have also failed.173C The parallels in the reaction chemistry of M2X4(PR3)4 (D24) and M2X4(PP)2 (Dzh) complexes to produce bioctahedral, two-electron products might suggest that these complexes will exhibit similar photochemistry. However such a correlation is based on ground state reactivity alone andit does not consider the nature of the excited state. Although the D2d and D23, MAM halophosphine complexes possess 1(55"') as their lowest energy excited state, the excited states to higher energy are of different parentages for the two classes of complexes. The absorption spectra for M2X4(PR3)4 complexes are dominated by ligand-to-metal charge transfer (LMCT) transitions in the near-ultraviolet spectral region162 110 whereas M2X4(P P)2 complexes display metal localized, near-ultraviolet absorptions.163 Accordingly replacement of these metal-localized states by the LMCT states of the D24 complexes may be manifested by significantly divergent photochemistry. Accordingly, studies were undertaken to compare the photochemistry of MAM Dzd complexes to their D21. counterparts. Specifically, we have investigated the visible and near-ultraviolet photochemistry of W2Cl4(PR3)4 (PR3 = PMe3, PMezPh, PBu3) in CH2C12 and benzene. The photochemistry of MozCl4(PR3)4 (PR3 = PEt3, PEtth, PBu3) complexes has previously been investigated,85 but the excitation wavelengths were in the hard ultraviolet (lac = 254 nm). Under these high energy conditions, photodegradation of the complex was observed to yield M02C193‘. Rupture of the metal-metal bond is not found in the case of MozCl4(PR3)4 in halocarbon solvents, either, despite the existence of the stable MoCl4.,,(PR3)2+x (x = 2, 3, 4) monomers.178 In our studies, by moving to lower energy excitation wavelengths, W2Cl4(PR3)4 smoothly converts to photo- oxidized bimetallic products. The observed photochemistry is correlated with photophysic studies of time-resolved absorption and emission spectroscopies. These results are compared to those obtained from our photochemical studies of the W2Cl4(P P)2 homologs described in Chapter IV. 11. Results A. Photochemistry of W2Cl4(PR3)4 Dichlorocarbon solutions of W2Cl4(PR3)4 are indefinitely stable in the absence of light. However, irradiation in the near ultraviolet (300 < 7cm. < 400 nm) engenders prompt reaction at room temperature. The photochemistry is similar for the three phosphine complexes, and it is exemplified by the absorption spectral 111 changes for W2Cl4(PMe3)4, shown in Figure 5.1. The disappearance of the 1(5 2—> 55*) band at 655 nm is accompanied by the appearance of an intense near infrared absorption at 1340 nm and weaker absorption bands at 419 and 383 nm. Isosbestic points are maintained at 584 and 313 nm during the early stages of the photolysis. The near-infrared absorption is a signature of the 2(5-9 5*) transition for a szIJII mixed-valence compound, as described in Chapter 111. With continued photolysis, the near-infrared and visible absorptions completely disappear with the concomitant growth of an absorption at 483 nm. The isolated product exhibits a parent ion peak of 808 amu in the FAB/MS and 31P NMR spectra of the final photoproduct show two singlets with a 2:1 intensity, consistent with the production of the previously characterized W2C16(PMe3)3 face-sharing bioctahedron.m'”9 Photochemistry of W2Cl4(PBu3)4 shows the same results. The final product is also characterized by FAB/MS and 31? NMR spectra shown in Figures 5.2 and 5.3. When the dihalocarbon is dichloroethane, similar W2C14(PMe2Ph)4 photochemistry is observed with the photoreduction of dichloroethane signified by the disappearance of the 1(5 2—> 55*) transition at 667 nm and the concomitant growth of the mixed-valence Wzn'm absorption at 1360, 427 and 343 nm as shown in Figure 5.4. As inferred from electronic absorption spectra, solutions displaying the near-infrared absorption band are paramagnetic. The X-band EPR spectrum of a frozen CH2C12/2sMeTHF solution (T = -170 °C) collected at an intermediate stage of the photolysis (Figure 5.5) displays an axial doublet. gn = 1.950 and g .L = 1.855. Similar signals have been observed from many mixed-valence MAM species such as Moziozmocéugzif, Mo2(HPo4)43- and M02(SO4)43‘ and they indicate a species in which the unpaired electron is coupled between two equivalent tungsten nuclei.180 The disappearance of the near-infrared band is accompanied by a similar disappearance of the EPR signal. .112 Figure 5.1 Electronic absorption spectral changes during the photolysis (hem > 375 nm) of W2C14(PMe3)4 in deoxygenated dichloromethane at 15.0 °C at ~5 h intervals. Panel (a) shows the photolysis reaction proceeding to a maximum absorption in the near-IR region. Panel (b) shows the spectral changes occurring with continued photolysis of the solution. The absorbance scale of the near-infrared relative to the visible spectral region in both panels is expanded by a factor of 6. 113 com F oov F com P 3 seem...— Ec\& 0009 com com com 90¢ com _ d _ E aoueqlosqv aoueqrosqv 114 nfimw nmNF N\E mam? m? P Ls; .oceeoeeozoe .23 £5343? we 58388.3 05 :25 mat—32 Seneca Rec 05 we 8.58% m2-m 375 nm) of W2C14(PMe2Ph)4 in deoxygenated dichloroethane at 15.0 °C at ~6 h intervals. Panel (a) shows the photolysis reaction proceeding to a maximum absorption in the near-IR region. Panel (b) shows the spectral changes occurring with continued photolysis of the solution. 117 1m 9:53 5:2 cow com com 00¢ 00 Door Dov? oom— coop d aoueqrosqv 118 l l l l 3000 3200 3400 3600 3800 4000 H/Gauss Figure 5.5 The X-band EPR spectrum of W2Cl4(PMe3)4 in frozen CHZCIZIZ-MeTHF solutions (T = —170 °C) exhibiting the maximum near-infrared absorption in panel (a) of Figure 5.1. 1.19 B. Comparison to Thermal Oxidation Chemistry The spectroscopic results suggesting the presence of a Wzn’m species during the initial stages of photolysis are substantiated by the thermal oxidation chemistry of W2Cl4(PR3)4 complexes. Dichloromethane solutions of W2Cl4(PMe3)4 are efficiently oxidized by one equivalent of PhIClz or NOBF4. The absorption spectrum and EPR spectrum of the product obtained by PhIC12 oxidation (labsmax = 1340, 428, 376, and 315 nm; g” = 1.952 and g i = 1.857) were identical to the spectra obtained by W2Cl4(PMe3)4 photolysis shown in Figures 5.6a and 5.7a. Recently, Cotton and coworkers have characterized the reaction chemistry of W2Cl4(PMe3)4 with PhIC12 by isolating and structurally characterizing the mixed-valence complex, 1,3,6-W2Cl5(PMe3)3.181 Here, oxidation of the W?” core is accompanied by chloride substitution of phosphine to maintain the overall neutral charge of the complex. An axial doublet EPR signal and near-infrared absorption profile are also observed for the NOBF4 oxidation product of W2Cl4(PMe3)4. However, the spectra of this oxidation reaction, shown in Figures 5.6b and 5.7b, are not the same (21mm = 1468, 474, and 375; 8n = 1.980 and g i = 1.840) as those obtained from photolysis and Pthlz thermal oxidation. EPR spectra of one-electron oxidized mixed-valence WZIIJII species for other phosphines, PMezPh and PBu3, are also shown for comparison in Figures 5.8 and 5.9. As Shrock has shown,106 NO+ oxidation of W2Cl4(PR3)4 affords the unsubstituted one-electron oxidized W2Cl4(PR3)4” complex in the absence of anionic, coordinating ligands. 120 Absorbance u; " E 0.0.0.0.... .u. o-o‘oo'.. L l l l l J r 1 l t l l r r r l a r l l l 400 600 800 1000 1200 9» /nm Figure 5.6 Electronic absorption spectra of the product resulting from the thermal oxidation of W2Cl4(PMe3)4 by one equivalent of l l l l l 1400 1600 (a) PhIC12 in dichloromethane (——) (b) NOBF4 in CH2C12 IMeOH solutions(— - - -). 121 g” = 1.980 (b) I g_L = 1.840 l l l L 3000 3200 3400 3600 3800 4000 H/Gauss Figure 5.7 The X-band EPR spectrum of the product resulting from the thermal oxidation of W2Cl4(PMe3)4 by one equivalent of (a) PhIC12 in frozen CH2C12/2-MeTHF solutions (b) NOB F4 in frozen CH2C12/MeOH solutions. Both spectra are obtained at T = —170 °C. 122 l l I l 3000 3200 3400 3600 3800 4000 H/Gauss Figure 5.8 The X-band EPR spectrum of the product resulting from the thermal oxidation of W2Cl4(PMe2Ph)4 by one equivalent of (a) PhIC12 in frozen CHzCl2/2-MeTHF solutions (b) N OBF4 in frozen CHzClleeOH solutions. Both spectra are obtained at T = -170 °C. 123 l I l l 3000 3200 3400 3600 3800 4000 H/Gauss Figure 5.9 The X-band EPR spectrum of frozen CH2C12/2-MeTHF solutions (T = -l70 °C) of (a) the photoproduct exhibiting the maximum near-infrared absorption, and of (b) the product resulting from the thermal oxidation of W2Cl4(PBu3)4 by one equivalent of PhIClz. 124 C. Organic Photoproduct Analysis Organic photoproducts of the photoreaction were analyzed by gas chromatography/ mass spectrometry (GC / MS). The inset of Figure 5.10a shows a typical GC of the gas collected above photolyzed solutions. Besides a solvent peak (not shown), a single product is observed to elute from the GC column whose mass spectrum is identical to chloromethane (CH3Cl: [M]+ = 50 m/z, [M— Cl] * = 15 mlz).182 An additional product is observed for sufficiently concentrated solutions of the W2Cl4(PR3)4 photoreactant (> 8 mM). Photolyzed solutions exhibit a peak in the gas chromatogram whose intensity increases as the concenu'ation of the photoreactant is increased; GC/MS (Figure 5.10b) shows the exclusive production of dichloroethane (C1CH2CH2C1: [M]+ = 98 m/z, [M - HCl]+ = 62 m/z, [M - HCl— Cl]+ = 27 mlz). No photochemical reaction was observed upon irradiation of W2Cl4(PR3)4 complexes in neat CH3CN or benzene. D. Photophysics and Transient Absorption Studies The W2Cl4(PR3)4 complexes all exhibit a red luminescence, which decays monoexponentially, upon excitation of the 1(5 2-9 55*) transition. Table 5.1 lists the luminescence maxima and the lifetime decay constants in benzene and 01202.1” The excited state lifetimes are typical of 1(523") excited states, as is the slight shift of the emission band to lower energies and increased bandwidth with increasing solvent polarity. Solvent dependent photophysical properties of the 184 1(55"') excited state have been observed previously and they are consistent with solvent stabilizing the zwitterionic character of excited states associated with the population and depopulation of the 5/5“ levels.94 The correlation of the shorter lifetimes with decreasing emission energy in Table 5.1 follows expected 125 50 M+. i ‘l 5. 6. 2 mm 100 ~ (3) ( T TIC 80 r WWW M” g: ml: 50 U) 8, so . T .E . . 9 52 200 300T 460 560 sob 700 E 40 9”" 6‘6 5 20 ; . l j 1.4. i 1 - J ‘11 [1'1 lrlll l t {1%1 i 1111 .T lull If! . l I- l' 0 20 40 60 80 100 m/z 62 RT min 1 2 100 “ 9136—" (b) ,, 80 LL "”298 ’5 . m/262 C . Q) E 60 . 41 0 1 1 40 .5 27 O scan g . 7% 40 * a 98 (We) “5 : 20 j 41 l . ll 15' 111‘ L tl I” ll 1' r. 1" I. v 20 40 60 80 100 120 m/z Figure 5.10 Gas chromatogram (inset) of (a) the volatile organic products resulting from the photolysis reaction of W2C14(PMe3)4 (1.5 mM) in CHzClz. (b) the coupling product resulting from dimeration of chloromethyl radicals at higher concentrations. The retention times during which the CH2C12 solvent peak elutes from the GC column is shown on the side. 126 Table 5.1 PhotOphysical properties of W2Cl4(PR3)4 complexes in benzene and dichloromethane at room temperature. 1D DCDZCD 1D CH2C12 W2C14(PR3)4 1mm I run a ‘t/ns lemma, / nm a rim PMe3 7 17 (850) 50 719 (961) 41 PMezPh 728 (676) 43 731 (749) 43 PBu3 723 (784) 54 723 (859) 44 a The values in parantheses indicate the emission band’s full-width at half- maximum in cm“. 127 energy gap considerations,185 which predict an increase in the nonradiative decay rate constant with decreasing energy gap. The transient absorption spectra of W2Cl4(PR3)4 complexes have been measured in dihalocarbon (e.g. CHZClz) and hydrocarbon (e.g. benzene) solvents for excitation of the 1(5 2-9 55*) transition and excitation of states lying in the near-ultraviolet spectral region. Figure 5.11 shows the transient absorption profiles for W2Cl4(PMe3)4 following a 8 ns excitation pulse at 650 nm. The same transient absorption profile is obtained for either solvent. A prominent feature at 350 nm decays monoexponentially to ground state with a lifetime similar to that measured for the luminescence decay of the 1(55") excited state (see Table 5.1). Concurrent with this absorption decay is the recovery of the 1(5 2—9 55*) bleach at 700 nm over the same time scale thereby establishing the assignment of the transient absorption in Figure 5.11 to the 1(55"') excited state. Solvent dependent behavior of W2Cl4(PR3)4 transient spectra is observed for solutions excited at higher energy wavelength 355 nm. When the solvent is CH2C12, W2Cl4(PR3)4 exhibits a transient absorption spectrum after the 155* excited state has decayed to ground state. As discussed below, the ‘transient’ appears to result from laser photolysis, which occurs readily for W2Cl4(PMe3)4, thereby complicating the collection of transient absorption data. Because laser photolysis of the sample was diminished for W2Cl4(PBu3)4, we were able to signal average transient data reliably at 355 nm excitation of this complex. As shown in Figure 5.12, a transient absorption maximizing at 370 nm and a transient bleaching at A > 500 nm are observed 50 ns after the excitation pulse. The transient spectrum at the longer wavelengths is consistent with the bleach of the l(52—>1t5"') (A = 540 nm) absorption of the ground state molecule. Addition of the W2Cl4(PBu3)4 ground state absorption spectrum to the difference spectrum 128 0.12 0.1 - o 0.08 — o 00o 0.06 — , AOD 0.04 - 0.02 — 00 099° 0 - on _002 l I l l l 300 350 400 450 500 550 600 k/nm Figure 5.11 Transient difference spectra of W2Cl4(PMe3)4 in deoxygenated dichloromethane (o) and benzene (0) recorded immediately after a 650 nm, 8 ns laser excitation pulse. 129 (106 (104 (102 AC”) <3 -4102 -{L04 (102 ACE) c: -{102 300 400 500 600 Mnm Figure 5.12 Transient difference spectra of W2Cl4(PBu3)4 in deoxygenated (a) dichloromethane and (b) benzene recorded 50 us after a 355 nm, 10 ns laser excitation pulse. The bottom trace in panels (a) and (b) is the difference spectrum and the top trace is the transient’s absorption spectrum generated by the addition of the ground state absorption spectrum of W2Cl4(PBu3)4 to the difference spectrum. 130 shows the transient to possess two absorption bands between 370 and 420 nm followed by a monotonic decrease to an absorbance = O at 7t > 500 nm. An overlay of the absorption spectrum of photolyzed solutions of W2Cl4(PBu3)4 in CH2C12 (same spectrum as in Figure 5.1) reveals that the transient absorption is in fact the primary photoproduct. Conversely, benzene solutions of W2Cl4(PBu3)4 excited at 355 nm exhibit no transient absorption at times longer than the 155* excited state lifetime. Figure 5.12b shows only a slight bleach at 1. > 500 nm, and addition of the W2Cl4(PBu3)4 ground state absorption profile to the transient difference spectrum yields a baseline trace. 111. Discussion Photolysis of W2Cl4(PMe3)4 in CH2C12 gives a one-electron oxidized Wzn'm core. and subsequent reaction yields a fully oxidized wzmtIII product. Comparison of the absorption and EPR spectra of photolyzed solutions to those obtained from independently oxidized solutions of W2Cl4(PMe3)4 reveal the primary photoproduct to be W2C15(PMe3)3, which reacts over time to produce the face-sharing bioctahedron, W2C16(PMe3)3. These results, in conjunction with analysis of the organic products, give rise to the following photoreaction scheme, Cl PR3 (IDI (IDI \PR3 ,CI ’PFla ’Cl (WT (W. hvéAH>g|75 rim)» 1‘”.-.ng _* WZCIB(PR3)3 R3P lot I 2x R3P ICI I Cl PR3 oCHZCI Cl P83 '1" .CH2CI CHac CICHZCHZ or 131 The chlorohydrocarbon photoproducts are consistent with initial chlorine atom abstraction by the photoexcited quadruple bond species to produce the chloromethyl radical. At low concentrations of the primary photoproduct, hydrogen abstraction from solvent appears to be sufficiently efficient to compete with dirnerization and chloromethane is the only product observed by GC/MS. As the concentration of the primary photoproduct is increased (with increasing concentration of the MAM photoreagent), the chloromethyl radicals couple to produce dichloroethane. The above reaction scheme is consistent with LMCT photochemistry for the W2C14( PR3)4 complexes. The strong near ultraviolet absorption of W2Cl4(PMe3)4 (1m, = 293 nm) has been previously assigned to the e(o(MP) -> 5*) LMCT transition based on the absorption band’s strong .L z-polarization, a vibronic progression in a1v(MP), and the relative insensitivity of the transition with respect to halide.162 Excitation into this LMCT manifold will generate a highly reducing bimetallic Wzl'n core, which is capable of reducing the C—X bond in chlorohydrocarbon solvents thereby leading to the photoinitiated radical chemistry of the above scheme. The LMCT excited state appears to be short— lived, and hence it is able to react only with substrates in high concentration, as is the case here where the substrate is solvent. Consequently, the observed ‘transient’ spectrum in Figure 5.12 is not that of the native LMCT excited state but rather correlates directly with the primary product of the photolysis reaction. In the absence of the C—X bond, reduction of substrate is circumvented and the excited state converts smoothly and rapidly back to ground state. Hence absorptions attributable to a photo-oxidized species are not observed in the laser flash photolysis spectrum of W2Cl4(PBu3)4 in benzene, and correspondingly no photoreaction is observed. Moreover, the 1(55*) excited state does not appear to be reducing enough to activate the C—X or C—H bonds of solvent. Therefore 132 excitation of the 1(55"') excited state W2Cl4(PR3)4 in CH2C12 or benzene yields the same transient species, which is unreactive and decays to ground state at the 1(55*) luminescence decay rate. The transient spectroscopy and LMCT photochemistry of W2Cl4(PR3)4 is in direct contrast to that observed for W2Cl4( P P);, which derives its photoreactivity from MMCT excited states. These differences can be explained by the different electronic structures of the two classes of compounds. Figure 5.13 shows the qualitative correlation MO diagram163 of D2,, and Du geometries based on the theoretical calculation of the model compound M02C14(PI-I3)4.”°'m The relative energetics of the metal-metal based orbitals are displayed as a function of a C4 rotation of one MClsz unit about the metal-metal bonded axis where a P—M-M-P torsion angle of 0° defines the D2,I structure. At 90° rotation, the DM conformation is obtained. The cylindrically symmetric M-M 0' bond is not affected by internal rotation about the M—M axis and hence the energy of these orbitals remain relatively constant in the correlation diagram. But it components are affected considerably. In the DE limiting structure, the n orbitals are degenerate by symmetry considerations. However these orbitals lose their degeneracy upon rotation with a maximum splitting obtained for the D2,, structure. The spiltting is due to the difference in it bonding ability of the halide and phosphine ligands. The 5/5" energy gap is also affected by rotation. As a trans-MX2P2 unit rotates away from either of the eclipsed conformations, the dxy orbital overlap decreases until it reaches zero at the staggered conformations (45° rotation angle). However, when the MClsz units are eclipsed, the dxy overlap is maximized and to a first approximation the 5 orbitals are energetically similar in the D2.l and D2,! structures. Representative ground state absorption spectra of Mo and W homologs for the D2d system, shown in Figure 5.14, conform to the predictions of the 133 $2 gob Ania—EDS: 25388 338 05 .«o cots—=28 39:28.: 05 :o 3me 852:8» 8Q 98 an we Efiwfiu OE coca—oboe o>m§=azo nfim 9...»:— nNO NO .50 a r... t t. s s s 0—2 x. x_n__ x_a... x_x_ x. a a x a x a a a 134 65589 E A- - .3 squeegee? can TV Amozaveuaoz 3 mo 8.8% 5:982. 38:85 Sm 9...»...— l|"' 8:2 com com 00? com _ I"" aoueqrosqv 135 correlation diagram. The spectroscopic assignment of the D2,, complexes has been intensively studied for the series of M2X4(PMe3)4 (M = M0, W; X = Cl, Br, 0.16“” The first absorption band in the 550 - 650 nm region and the second small band in the 400 - 500 nm region have been assigned to be the 5 -—) 5*and it -> 5* transitions, respectively, and the strong absorption at around 300 nm has been correlated with a 0(MP) —-> 5* transition. A comparsion of the absorption spectra for the M2C14(PR3)4 in Figure 5.2 and M2Cl4(P P)2 (M = Mo, W) in Figure 4.9 reveals that both the D2d and D2,, complexes display a red-shift in the 1(5 2—> 55*) and 1052—) Ito") transitions. The red shift upon substitution of w for Mo is an indication of metal-localized transitions as discussed in Chapter IV. However the near-ultraviolet profiles exhibit disparate behavior. For M2CI4(PR3)4 complexes, a distinct blue shift specific to LMCT transitions is observed in the near-ultraviolet spectral region with replacement of a metal-localized Mo by W.162 This blue-shift is due to the destabilization of the metal-metal levels relative to the ligand levels. This is in contrast to the red-shift in the spectra of M2Cl4(P P); in the same region resulting from metal localized absorptions. Accordingly replacement of these metal-localized states of the D2,, complexes by the LMCT states of the Du complexes is manifested in the significantly divergent photochemistry between these two families of M-4-M compounds. IV. Conclusion The formation of a mixed valence photoproduct, W2C15(PR3)3 and the organic photoproduct analysis reveal that W2Cl4(PR3)4 photochemistry with dihalocarbons occurs via single electron pathways originating from the excitation of' a LMCT absorption manifold. Because near-ultraviolet excitation of M2Cl4(P P)2 produces a long-lived ESBO transient derived from a discrete excited 136 state of the M—4-M complex, the photochemistry is not confined to solvent and the MiM (D2h) halophosphine systems exhibit multielectron activation of a wide variety of substrates. However, this is not the case for MiM (132d) complexes. The metal localized excited states that lead to discrete multielectron photoreactivity of W2Cl4(P P); are obscured by the LMCT transitions in the absorption spectra of W2Cl4(PR3)4 complexes. The consequences of this state inversion in the D2,, complexes is that excitation in the near-ultraviolet produces a short-lived, highly reduced metal core that is only able to react with solvent via a free radical pathway. Hence, substrate activation occurs in one electron steps that are difficult to control with M2X4(PR3)4 as the photoreagent. Accordingly, these studies suggest that the multielectron activation of substrates by MiM halophosphine excited states is best accomplished with M2Cl4(P P)2 binuclear complexes or with M2X4L4 complexes in which the L-> M charge transfer states are removed to high energies. CHAPTERVI SPECTROSCOPIC STUDIES OF QUADRUPLY BONDED HETEROBIMETALLIC MOLYBDENUM-TUNGSTEN COMPLEXES I. Background The zwitterionic character of MAM complexes can be accessed chemically only upon low symmetry distortion within the molecule or its environment. As described in Chapters IV and V, this low symmetry distortion is intramolecular in nature. In the zwitterionic excited state of the M-iM complexes, one center of the binuclear core is oxidized thereby driving distortion to a coordinatively unsaturated ESBO intermediate as described by eq. (4.2). Such a ligand rearrangement provides cooperative stabilization of a charge-separated core by achieving an octahedral geometry about the oxidized MIn center and by diminishing the donation of electron density from the halides about the reduced MI center. While this intramolecular ligand distortion traps the zwitterionic character of MAM excited states, it also stabilizes the excited state thereby lowering the energy and hence reactivity of the photogenerated intermediate. This may explain why our studies of M—‘r—M complexes to date show us that these complexers can photoactivate only weak substrate bonds such as C—I of alkyl iodides or S—S of persulfides. 137 138 The heterobimetallic cores of MoWC14(PR3)4 and MoWC14(PP)2 complexes present a alternative and straightforward means to trap the charge transfer excited character of M-4-M metal-localized excited states without relying on an energy dissipating intramolecular rearrangement of the ligating coordination sphere. Because tungsten is easier to oxidize than molybdenum, the energy wells of MoIWIII and MomWI are asymmetric, and therefore the energy of MoIWIII charge separated state should be stabilized with respect to MomWI; this is schematically represented in Figure 6.1. With the low symmetry distortion intrinsic to the nature of the heterobimetallic core, the zwitterionic state may be stabilized without the need for a ligand distortion. As diagrammed on the state diagram of Figure 6.2, the Moi-W bimetallic compounds may exhibit better energy-storage behavior in the excited state and therefore may provide us with a more activated photogenerated intermediate than can be obtained from the homobinuclear M02, and W2 analogs. Thus we plan to undertake the systematic exploration of the two-electron photooxidation chemistry of MOS-W heterobimetallics and compare the results of these photosensitizers to the phototransformations of the analogous Moi-Mo and W-4—W complexes. The nature of the addition product is especially intriguing because the formation of a Mo‘WIII edge-sharing intermediate should be reflected in a product displaying substrate addition at the Mo center. Thus a simple photoproduct characterization will in itself be enlightening. However before a systematic photochemical study can be meaningfully undertaken, the excited state dynamics of Moi-W heterobimetallics need to be defined. This is the focus of the work in this Chpater. The spectroscopic properties of heteronuclear quadruply bonded MonWII complexes in DM and D21, symmetries are presented and compared with the homonuclear Mozn'II and W?” congeners. 139 Potential Energy Mo(lll)W(l) Mo(l)W(lll) Nuclear Coordinates Figure 6.1 Schematic potential energy curves of the mixed-valence states of Mo(I)-W(III) vs. Mo(III)-W(I) in heterobimetallic systems. Figure 6.2 Proposed scheme for the formation of the Mo(I)-W(III) excited state. Upon 55* excitation, the charge transfer mixed-valence state can be stabilized without utilizing the edge-sharing bioctahedral distorted intermediate. 141 II. Results and Discussion A. NMR Studies of MoWCl4(PR3)4 and MoWCl4(dppm)2 The 31P NMR spectra of shows characteristic signatures of MoHWn cores, and therefore 31? NMR is an important tool in the chemistry of these species. Figures 6.3, 6.4, and 6.5 show the 31? NMR spectra of MoWCl4(PR3)4 (132d) complexes (PR3 = PMeth, PMezPh, PMe3) in C6D6 at room temperature. Two intense triplets are observed for molybdenum and tungsten isotopes with I: 0 due to equivalent 31P nuclei coupling across the quadruple bond (Jpp ~ 24 Hz). The downfield resonance is assigned to the tungsten site based on 183W—31P coupling, which gives rise to a doublet of triplets with coupling lJpw = 280 Hz for the 31P nuclei on W and a doublet of triplets (only the outer satellite peaks are observed) with coupling 21(‘83W-31P) ~ 47 Hz for those on Mo. The results clearly show that the phosphines on the tungsten atoms are less shielded with respect to those on molybdenum atoms in the heteronuclear species. Due to the sample solubility, 31P NMR spectrum of MoWCl4(dppm)2 (D2h) complex was taken in CDzClz but recorded at -40 °C to prevent the oxidation of the sample in the halogenated solvent}.12 As was the case for the D2d complexes, two resonancesassociated with the Mo and W centers are observed in the 31P NMR spectrum of MoWCl4(dppm)2 (Figure 6.6). However, the spectrum is more complicated than its D2,, counterpart, exhibiting an AA’BB’X spin pattern. These 31P NMR spectra of the heterobimetallics are clrearly distinguished from the singlet observed in the spectra of the M02 and W2 congeners. A comparison of the chemical shifts and coupling constants (J) for these heterobimetallics vs. the homobimetallics is listed in Table 6.1. These results indicate that the electron density on the molybdenum is higher than that on the 142 some 3395x053 ~23 cosowcou «32 a: no 8585 05 8:365 xmtamm 05. 832382 :88 a 6.8353 5 ~22 vdom 3 ease—025.5322 .8 8.58% £22 5. 5: n6 azure Ema OT. ml 0 m 0“ ma om mm .om _ p—r—_-—_—P——_Ppb———- 143 com.— bofiemxoama .23 3.59.8 No: a. me 3:80.... 05 3.865 vita"... 05. 2222—52 Soc. 3 6-2.05.3 5 ~52 imam .a .Emaozmv._o>>oz .«o 88.8% 522 E. E: 9.9 95w:— Eau mfil ofil ml 0 m o« ma om ——h——_—-—h_——P—___—_————_TPbb—-_-—__pr—p—r_F-— - P. 144 omega 32.8883“ 53 8888 Nas. 8 Mo 8888 o... 8.868 328.8 2? 8.58882 82: 8 6.8.8889 8 N22 VNON .m Anozquoaos. 8 8.88% M22 E. 7... m6 982..— Eaa OMI mm! oml mat. Cal m... o m 0a ma __pP———_————_-—— —-————-pr————p—p—-p—_—_————P——-—————-pl—lt 145 P Ema .L_ , 58w a_e.a8mxo8% 5.3 8.888 62 a: mo 8888 on. 8.868 x828 2:. .u. 91.. 8.8 e. E: 88 a ..e....3..u3o2 co ease... ”:22 E. r... 3. one»... o m CH ma om mm om p—b—p-—-—r—L———— 146 Q: ._ 2 .o: moucohomoc «:32 2332: 2.. .23 .5328 2a 3% 82F. .0. 912528 550 s 853% x £55530: 3 5.58%. 3028 £090.. 2028 6.2.8.55 d: 5 mm .5328 9:325 2: can Ema 5 mm :5. E5 .3585 2:5 .vOmnz $3 9 08:05.2 at? 332.25. 3 I 3.2 I 8w 3.2 .. 3% we 25% N: wm 3 02 3. 0% NR 2: S: v.8- 2:- 8:: Ma 3 .02 3. 1% _R a: 3 92- 2- .582; MN _m § 2‘. 08 Sm 9mm 8 _.~_- 3 2:02; E? 383? 323... ABA—Vb .3? 38...? 9.7% >53? 8.24.5 32:me IIII. AMEEUNE cNZI 930—2 082 035 .322 .32 M «Sc vAmmn—vvfivfiz mo $3 35338 $5950 cam eaaEm 32825 ~=22 m: —.e 035% .147 tungsten. In the case of Du heterobimetallic series, the chemical shift difference between the phosphorus ligands on the two metals increases slightly with an increase of the phosphine basicity, suggesting that the electron-donating abilities support an effective charge separation within the heterobimetallic core. The difference in electron density on the two metal atoms of the heterobimetallic core also results in chemical shift differences in 1H NMR spectra of MoWCl4(PR3)4. Figures 6.7, 6.8, and 6.9 for PR3 = PMeth, PMeQPh, PMe3, respectively, reveal two separate methyl resonances associated with two types of protons on the molybdenum site vs the tungsten site. The downfield resonances are attributed to the methyl groups of the phosphines bound to the tungsten and the upfield resonances are the resonances for the methyl groups of the phosphines bound to molybdenum. This trend is consistent with that observed for the 31F chemical shifts of these complexes. The different substituent groups on phosphine ligands exhibit some interesting features. A broad unresolved singlet was obtained for the methyl groups on MoWCl4(PMePh2)4 and a triplet pattern for those on MoWCl4(PMe2Ph)4 and MoWCl4(PMe3)4. While the methyl resonance on MoWCl4(PMe3)4 appears as a semi-quartet at 121.4MHz (recorded at Varian VXR—300), two resolved triplets can be observed when the magnetic field is increased to 202.4 MHz (recorded at Varian VXR-SOO); this result is shown in Figure 6.10. In contrast, the resonances of the 1H NMR spectrum for MoWCl4(dppm)2 complex appear to be complex multiplets (Figure 6.11). It should be noted that the protons on the bridging methylene position are affected by the diamagnetic anisotropy of the quadruple bond and the 1H NMR chemical differences for a series of MoWCl4(LL)2 have been used to estimate the value for diamagnetic anisotropy.l ”"89 148 éaflomaa :58 S 6.88:3 5 ~52 com a .Aunmozn—Euaoz «o 89:89. «22 I. he 2:»:— 149 2.52382 :52 “a 6-83:3 5 ~52 8m 3 .EmfloZBAUBoE «c 62.58% «22 I. a.» «.5»:— Eaa m m v m m h m P p n P — — - — — - — p p p p — n — p - — — F . — — F p p n — — — — — p — — — q p 28 mm: \ \. Eaa m.m «K m.\. wk. NR ———_———_p——b—»——-——————_————————-———-————_-——_———F. 150 223383 :88 3 6-33:3 5 ~32 8m 8 Amos—$40302 .«o 8.53% $22 I. a.» 2:3,..— Eaa mvé mm.“ mm; EEELEEEEEFE. 92825.8 Soc. 8 8.8352 5 N32 com 3 AJEAEUBoE .3 8.58% 522 3. :3. Earn Ema « m m w m m m __—____ — I— — p— p— _- — — I— - _— h b— b p— _— — u— - — 151 Ema o«v.« mmvé 03V.“ 4 W 152 .u. 9.. a .650 a £2 8m 3 853% .Aaaedacz .3 5.58% «22 m. :6 25w...— e% as m& m.m\ . —————~_——-———~———————————————-————E. __- 153 B. Electronic Absorption Spectroscopy The visible absorption spectra of MoWCl4(PR3)4 (PR3 = PMeth, PMth, PMe3) complexes are shown in Figure 6.12. The assignment of their electronic transitions is consistent with the metal-localized transitions of their M02 and W2 homologs. The 1(52 —-) 55‘) transition for MoWCl4(PR3)4 lies between the energies of the M02 and W2 congeners, but it is closer to the W2 values (see Figure 6.13). The energies of 1(52 —> 55") transitions of the Mowch,(i>1?.3)4 complexes blue-shift with increasing basicity of the phosphine ligands, PMeth < PMezPh < PMe3 (Figure 6.12).190 Such a shift is expected and it can be attributed to an increase in the electron density at the metals and in the Mo—W bond, in agreement with the NMR results above. C. Luminescence Spectroscopic Studies Solids and benzene solutions of MoWCl4(PR3)4 complexes exhibit intense red luminescence both at room and low temperatures upon excitation of the 1(52—> 85*) transition. The emission spectra for this series of compounds in benzene at room temperature are shown in Figures 6.14, 6.15 and 6.16. In all cases, electronic absorption and emission spectra are essentially mirror images with a small Stoke shift. This implies that the emission arises from an excited state complex geometrically similar to the ground state structure. Figures 6.14, 6.15 and 6.16 also display the excitation profiles of MoWCl4(PR3)4 obtained by monitoring the 1(55*-) 52) emission. The transitions are energetically coincident with absorptions in their ground state spectra, indicating that MoWCl4(PR3)4 complexes fluoresce directly from the lowest-lying singlet 155* excited state in which D2d symmetry is preserved; chemically distorted excited states do not 154 .2383an 808 a 5:38 83:3 s A. V efimozevgoaoz 6v AI v «2.325403% é T tnvéozevsoaoz 3 .8 «saw eoeeoee 28:85 «3 2:3..— FEE“ com 005 com com oov com ................ _ _ --~ _ ---- _ eoueqiosqv 155 .2883an :88 3 8:28 83:3 a Ase ..Aésozesuaa Q T. v ..Emeozefiaoz as T ....V sense—25582 3 mo seam 8:989. geesooa Se sour.— EC)“ com com com oov com IIIII I Futon-000000000 .\ I I I not... hool- I I — I o t It... ... r \ o a S do '0 s 0 Mi s o . s . o s v . ~ . ~ . s o .. . . . u a; ‘x .o n. V . a\\ . ..\I q . .. ... . s .. m . ,. O. u . B u . U ... 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This has been observed previously for the homobimetallic species and the observation is consistent with the charge seprarted character of a zwitterionic excited states. The emission profile also exhibits a pronounced temperature dependence as illustarted in Figure 6.17 for solid MoWCl4(PMePh2)4 over a 77 - 300 K temperature range. The emission bandwidth decreases with increasing temperature; surprisingly in view of the emission spectra of the homobimetallics vibrational fine structure is not resolved even at low temperature. Lifetimes of MoWCl4(PR3)4 complexes in benzene and in the solid state were measured by time-correlated photon counting spectroscopy. The emission signals show a biexponential decay. The shorter lifetime component arises from the 188* excited state while the origin of the other component remains unknown. Table 6.2 lists the luminescence maxima and the lifetimes of MoWCl4(PR3)4 complexes, along with a comparison to the emission data of their homonuclear counterparts. MoWCl4(PR3)4 complexes exhibit typical 155* excited state lifetimes with values that are closer to the W2 congeners. An exceptionally long lifetime is observed for MoWCl4(PMe3)4, but this observation is consistent with the lifetime for D2d phosphine compunds. It has been postulated that the lifetime is lengthened by the reduced steric of the smaller phosphine PMe3 ligand,191 or owing to the absence of additional nonradiative decay pathways” arising from the existence of long-lived transient species.192 Absolute emission quantum yields for M02C14(PMePh2)4, MoWCl4(PMePh2)4, and W2Cl4(PMePh2)4 complexes are 0.011, 0.036 and 0.044, respectively; this trend is consistent with an increasing emission lifetime along this series. 160 lem l J L l I - 600 700 800 900 1000 k/nm Figure 6.17 Emission spectra of 58*—>82 transitions of MoWCl4(PMePh2)4 in solid state (a) at room temperature and (b) at 77 K. 1.61 Table 6.2 Luminesence Lifetimes and Emissive Quantum Yields of 65* Excited State of M2Cl4(PR3)4 (M2 = Mo2 MoW, W2) Complexes. Tem (item)a M2C14(PR3)4 Mo2 ' MoW w2 PMePh2 11.4 ns (0.011) 54 ns (0.036)a 65 ns (0.044) PMth 16.4 ns (0,036)a 38 ns 43 ns PMe3 135 ns (0.259)b 59 ns 50 ns 3 Emission quantum yield was mearured in dilute benzene solution of the M2Cl4(PR3)4 (absorbance = 0.2) as determined by using a 2-methylpentane solution of M02Cl4(PMe3)4 at Aemm = 585 nm at 300K. bref.117 162 The D2,, MoWCl4(dppm)2 complexes exhibits no appreciable emission in benzene solution at room temperature, The same is true for the M02 and W2 homologs. This is in contrast to the luminescence previously observed for D2 staggered complexes of M02Cl4(dppe)2 (dppe = bis(diphenylphosphino)ethane) and M02Cl4(dmpe)2 (dmpe = bis(dimethylphosphino)ethane) (torsion angle 1 = 30° and 40°, respectively).114 The trend of decreasing luminescence along the series of eclipsed D2d > D2 > D2,, may be explained by the lifting of the an and “y: orbital degenracy with rotation. The increase in the 1t/1t:* splitting with increasing torsion angle results in the decreased energy gaps between 1: and 5 states. Thus the presence of additional nonradiative decay pathways may arise from the proximity of the 1(55*) excited state to 1: based states of the metal-metal bond. 111. Conclusion The NMR and electronic absorption studies reveal the presence of a ground state of (5')Mo—W(5+) with a partial dipole on each metal. Luminescence spectroscopic studies show that the geometry of the excited state is preserved. Based on this, we believe that the Mo—‘t—W bimetallic compounds should have structurally undistorted MOI-WIII excited states that can be accessed from the 1(55"‘) excited state. This is in contrast to the homobimetallics which need to eneter a charge-separated excited state from higher energy excited state (see Figure 4.11). In these cases charge transfer within the bimetallic core does not appear to be solely sufficient to promote rearrangement because the structurally distorted transient intermediate is not observed to form upon 58* excitation, but rather is observed when energy metal localized transitions (e.g., 5 —> 113*, It —) 8*) immediately to higher energy of 5 —9 5* are excited. These states not only possess charge transfer character, but their population leads to diminished metal-metal 1t 163 bonding relative to that in the ground state molecule. This feature is needed to enhance formation of a bioctahedral intermediate because interactions of the metal dyz (or dxz) orbitals with those of ligands in the equatorial plane of an edge- shan‘ng bioctahedron occur at the expense of M—M 1t interactions. Because a intramolecularly distorted intermediate is not needed to stabilize charge separation within the bimetallic core of homobimetallics, the excited states of these complexes may be more potent photoreductants of substrates from the 1(58*). If photochemistry is to occur from higher energy excited states, then we expect the photoreactivity to parallel that observed in Chapters IV and V. We come to this conclusion on the basis of the photophysical and spectroscopic properties of Moi-W species, which are intermediate to those for the Moi-M o and WA-W analogues. Consequently, the D2d systems would be expected to exhibit LMCT photoreactivity whereas the D21. MoWCl4(dppm)2 complexes would be expected to exhibit photoreactivity from metal-localized excited states. For this reason, the latter complexes are the more promising multielectron photoreagents. From a practical standpoint, it has been previously shown that MoWCl4(dppm)2 reacts halocarbons thermally. Thus the photochemistry of MoWCl4(dppm)2 with YZ is best investigated at low temperatures to prevent the thermal reaction or decomposition of the photoproducts. Finally it should be noted that asymmetry about the bimetallic core may also be induced by the ligand coordination sphere. Two ground state mixed- 193 and M02(CHZSiMe3)2[(CH;3)2SiMe2](PMe3)3.194 The former contains a Mo()MoIV valence compounds are currently known, M02(OPri)4(dmpe)2 bimetallic core and the latter contains a MoIMoIII core formed from a dative component of quadruple bond, MOIE Mom. It might be interesting to 164 investigate the photophysical and photochemical properties of these compounds in a comparative study to the Mo—W systems. S99.“ 10. 11. LIST OF REFERENCES Parmon, V. N.; Zamareav, K. I. In Photocatalysis: Fundamentals and Applications; Serpone, N.; Pelizzetti, E. Eds.; Wiley-Interscience: New York, 1989, pp. 565. (a) Photochemical Energy Conversion; Norris, J. 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