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Cr(CO)6 System, While for the Mo(CO)6 and W(CO)E

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Cr(CO)6 System, While for the Mo(CO)6 and W(CO)E The photochemistry of M(CO)6 and (rj6-pyridine)Cr(CO )3 (M = Cr, Mo, or W) and related systems. This thesis is presented to Dublin City University for the degree, Doctor of Philosophy, by Ciara Breheny BSc Supervisor Dr Conor Long School of Chemical Sciences, Dublin City University 1996 Declaration This thesis has not been submitted as an exercise for a degree at this or at any other university. Except as otherwise stated, this work has been carried out by the author alone. Signed Ciara Breheny. Dedication This thesis is dedicated to my family, Mum, Dad, Conor, and Saibh HI Acknowledgements I would like to say a most sincere thank you to all the following people, Dr Conor Long, for his constant support, help, and advice over the past few years All members past and present of the CLRG, namely Irene, Mick, Maureen, Celia, Mary, Siobhan, and Deirdre Everyone in AG07 who made the past few years enjoyable and unforgettable All members of the chemistry department, especially the technicians, who were always at hand to help when a problem arose (as it invariably did) A word of thanks to my friends outside DCU without whom the past few years would not have been the same, namely, Mane, Ger, Siobhan, Orla, Greg, Ciaran, Shivaun, Ciara, Teresa, Monica, Susan, Bronagh, Anna, Dawn, and Fiona Also a special thanks to Paul for his support over the past year Finally, to my family for their never-ending patience with the seemingly endless student life I have undertaken Without their love and support these past few years would have been a lot more difficult A special thanks to anyone else I have forgotten IV Abstract The reaction of M(CO)s(solvent) with CO has been investigated in a range of alkane solvents, where M = Cr, Mo, or W The kinetic and activation parameters have been determined for Reaction 1 M(CO)5(S) + CO - > M(CO)6 Reaction 1 For chromium hexacarbonyl the AH* is constant (22 ± 2 kJ mol'1), the rate of Reaction 1 increases with the lengthening as the alkane chain, this is reflected in the AS* term, which becomes less negative as the alkane chain length mcreases For the larger metals the variation in the kinetic and activation parameters is less significant The kinetic and activation parameters indicate that solvent displacement by CO involves an interchange mechanism for the Cr(CO)6 system, while for the Mo(CO)6 and W(CO)e systems the mechanism is more associative in character The photochemistry of (rj6-2,6-X2C5H3N)Cr(CO)3 was investigated both in low-temperature matrices (X = H, (CH3)3Si) and in room temperature solution ( X = H, CH3, or (CH3)3Si) Room temperature photolysis (k ^ c > 410 nm) of (t]6-pyndine)Cr(CO)3 m CO saturated methanol yielded ('n1-pyndme)Cr(CO)5 which subsequently formed Cr(CO)6 in a secondary photochemical process The efficiency of this reaction is reduced in cyclohexane, or when X = CH3 rather than X = H Photolysis in low temperature matrices resulted in an i f to r\l haptotropic rearrangement of pyridine (Acxc - 460 nm, X = H) Visible irradiation in a CO-doped methane matrix produced (t|1-pyndine)Cr(CO )5 while m N2 matrix/ac-(Vi>yndine)(N2)2Cr(CO)3 is formed Irradiation with A*« = v 308 nm produced both the nng-slip product and also the CO-loss product (rj6-pyndine)Cr(CO ) 2 Time resolved infrared spectroscopy in cyclohexane revealed only the CO-loss product = 308 nm, X = H) The apparent difference in room- temperature and low-temperature photochemistry is explamed by the rapid regeneration of the parent species from the r| ^intermediate This explanation is supported by the laser flash photolysis experiments = 355 nm) in CO-saturated cyclohexane (S), where the recovery of the (r|6-pyndine)Cr(CO ) 3 absorption follows a biphasic time profile, whereby the faster process was assigned to the r|l to rf rearrangement and the slower to the reaction of (r|6-pyndine)Cr(CO) 2(S) with CO When the matrix photochemistry was investigated in a CO-doped matrix, where X = (CH3)3Si, no pentacarbonyl or hexacarbonyl species were obtained Both the nng-slip and the CO-loss product were formed following irradiation with = 308 nm, and both were sensitive to photoreversal with white light, which may explain the lack of photoproducts following long wavelength photolysis Crystals of (ti6-2,6-X2C5H3N)Cr(CO)3 (X - H, CH3, or (CH3)3Si) were characterised by X-Ray diffraction When X = H or CH3 an eclipsed conformation is adopted, where the carbonyl ligands eclipse the carbon atoms When X = (CH3)3 Si the more common conformation where the carbonyl groups are in a staggered conformation and eclipse the ring bond centres is adopted In all the compounds the chromium atom is located directly below the arene ring, which is essentially planar VI TABLE OF CONTENTS (a) Title Page 1 (b) Declaration 11 (c) Dedication 111 (d) Acknowledgements iv (e) Abstract v (f) Table o f Contents vu INTRODUCTION: PAGE 1 (i) A brief history of organometalhc chemistry 2 (u) Definition of an organometalhc compound 4 (m) Organometalhc bonds encountered in this study 5 (iv) Excited states encountered in this study 10 (v) Uses o f organometalhc compounds 11 (vi) The nature o f the interaction between solvents and metal centres 13 (vu) Techniques employed for studying organometalhc transient species 14 References 23 V ll CHAPTER 1: PAGE 25 I Literature survey 26 111 Early low temperature studies o f the photochemistry ofM(CO)6 26 112 Room temperature studies o f the photochemistry ofM(CO)e 27 II 3 An in depth study of the photochemistry in the matrix 29 1 1 4 Solution phase photochemistry m weakly coordinating solvents 3 5 115 The photochemistry of M(CO) ó m liquefied noble gases 36 1 1 6 Investigation o f the dynamics o f solvation employing picosecond and femtosecond flash photolysis 37 1 1 7 The photochemistry ofM(CO)t in the gas phase 41 1 1 8 Time resolved photoacoustic calorimetry experiments 45 1 1 9 Mechanistic information obtained from solution phase experiments 47 1110 Photochemistry o f M(CO) 6 m the presence o f bidentate ligands 49 1 2 Results and discussion 52 121 Possible mechanisms for solvent dissociation 52 12 2 Spectroscopic information 54 12 3 Measurement o f rate constants 57 12 4 Activation parameters 59 12 6 Discussion 62 127 Conclusions 65 References 67 V1U CHAPTER 2: PAGE 72 2 1 Literature survey 73 2 1 1 Photochemistry of (7f-arene)M(C0) 3 complexes 73 2 1 2 Thermal chemistry of (rf-arene)M(C0)3 complexes 75 2 1 3 Haptotropic rearrangements 78 2 1 4 Molecular orbitals for rf pyridine vs r\l pyridine 81 2 1 5 Pyridine as a r f ligand 83 2 1 6 Pyridine as an rf-ligcmd 84 2 2 Results and discussion 87 2 21 Spectroscopic information 88 2 2 2 Steady state photolysis of (rf-pyridine) Cr(CO) 3 92 2 2 3 Matrix Isolation Experiments 101 2 2 4 Time Resolved Infrared Spectroscopy Experiments 110 2 2 5 Ultraviolet/visible Flash Photolysis Experiments 112 2 2 6 Discussion 126 2 2 7 Conclusions 135 References 136 CHAPTER 3: PAGE 141 3 1 Literature survey 142 311 X-ray single crystal diffraction 142 IX 3 1 2 Crystal structures obtained of related compounds 143 3 2 Results and Discussion 146 3 2 1 Data collection for (rf-pyridine)Cr(C 0)3 146 3 2 2 Molecular structure of (rf-pyridine)Cr(CO)3 147 3 2 3 Data collection fo r (rf- 2y 6-dimethylpyridine)Cr(C0) 3 149 3 2 4 Molecular structure of (rf- 2,6-dimethylpyridine)Cr(CO )3 150 3 2 5 Data collection for (rf- 2,6-bis(trimethylsilyl)pyridine)Cr(C0) 3 153 3 2 6 Molecular structure o f (rf- 2, 6-bis(trimethylsilyl)pyndine)Cr(CO ) 3 154 3 2 7 Discussion 156 3 2 8 Conclusions 159 References 160 CHAPTER 4: PAGE 162 4 Experimental 163 4 1 Materials 163 4 2 Equipment 163 4 3 Synthesis of (rf-2,6-bis(trimethylsilyl)pyndine)Cr(CO)3 164 4 4 Synthesis of (if-pyndine)Cr(CO) 3 165 4 5 Synthesis of (rf-2 ,6-dimethylpyndme)Cr(CO) 3 165 4 6 Attempted synthesis o f molybendum and tungsten analogues 166 4 7 Laser flash photolysis 167 4 7 1 Sample preparation for laser flash photolysis with UV/vis detection 167 x 1 4 7 2 Laser flash photolysis with UV/vis detector apparatus 169 4 7 3 Laser flash photolysis with TRIR detection apparatus 170 4 8 Matrix isolation instrumentation 171 4 9 Steady state photolysis with NMR monitoring 171 4 10 Determination of extinction coefficients 111 4 11 Determination o f the concentration o f CO in an alkane solvent 173 412 Determination o f the activation parameters 173 References 175 Appendix 1: PAGE 176 A Extinction coefficients of M(CO )6 177 B Arrhenius and Eyring data for the desolvation of the M(CO) s(S) species 182 Appendix 2: PAGE 198 A (i) Extinction coefficients for the (rf-arene)Cr(CO) 3 compounds 199 (11) Spectroscopic spectra for the (rf~arene)Cr (CO) 3 compounds 205 B Supphmentary spectra from the matrix isolation experiments 212 Appendix 3: PAGE 216 Crystal data , intensity measurements , and refinements for the (rf-arene)Cr(CO) 3 compounds 217 Structural parameters for (rf- 2,6-dimethylpyridine)Cr(CO) 3 219 XI Observed and calculated structure factors for ( r f -2,6-dimethylpyndine)Cr(C0) 3 225 Structural parameters for ( r f -2,6-(TMS)pyridine)Cr(C0) 3 236 Observed and calculated structure factors for ( r f - 2, 6-(TMS)pyrtdme)Cr(C0) 3 242 Structural parameters for (rf-pyridine)Cr(CO )3 257 Observed and calculated structure factors for ( 7]6-pyridine)Cr(CO )3 262 XII INTRODUCTION 1 This introduction attempts to provide an overview of the relevant chemistry and techniques which are explored m this thesis It is not an extensive review of the relevant literature, this is provided at the beginning of the each chapter (i) A brief history of organometallic chemistry The history
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