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Reactions of a dinuclear rhodium hydride complex with silanes Rosenberg, Lisa 1994

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REACTIONS OF A DINUCLEAR RHODIUM HYDRIDE COMPLEX WITH SILANES by LISA ROSENBERG B.Sc, Memorial University of Newfoundland, 1986 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Chemistry) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October 1993 © Lisa Rosenberg, 1993 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my wr i t ten permission. (Signature) Department of C H E M I S T R Y The University of British Columbia Vancouver, Canada Date D E C ' Z j l ^ DE-6 (2/88) ii ABSTRACT The reactions of silanes with the complex [(dippe)Rh]2(M--H)2,1, (where dippe = 1,2- bis(diisopropylphosphino)ethane) are presented in this thesis. Addition of a single equivalent of a secondary silane (RR'SiEk), to [(dippe)Rh]2(|i-H)2,1, gives the complexes [(dippe)Rh]2(^i-H)(^-Tj2-H-SiRR'), 2a-c (a, R = R' = Ph; b, R = R' = Me; c, R = Ph, R' = Me). X-ray diffraction studies of 2a-b confirm the presence of a three-centre, two-electron, Rh-H-Si bond. The observed fluxionality of 2a-c in solution is due to exchange of the silicon and rhodium hydrides. Complexes 2a-c reversibly add hydrogen to give the highly fluxional, silyl trihydride species {[(dippe)Rh]2(|i-H)((X-rj2-H-SiRR')},H2, 3a-c, and lose hydrogen in the presence of one equivalent of carbon monoxide to give complexes 4a-c, [(dippe)Rh]2(H-SiRR')(H-CO). A catalytic cycle is proposed for the hydrosilation of ethylene by diphenylsilane to give Ph2SiEt2, which occurs in the presence of 1. The bis(ji-silylene) complexes 6a and 6c, [(dippe)Rh]2(fi-SiRR')2> are prepared by addition of a second equivalent of silane to 2a and 2c, respectively. (When R * R', the complexes exist as cis and trans isomers.) An X-ray diffraction study confirms this structure for 6a and trans-6c. While unreactive toward CO or C2H4, complex 6a does react with an excess of Ph2SiH2, giving an as yet unidentified complex, 7. Based on NMR data for cis- and trans- [(dippe)Rh]2(|i-SiMeTolP)2,6d, a mechanism involving C-H activation of solvent is invoked to explain the equilibrium between the isomers. The unique, butterfly-shaped complex [(dippe)Rh(H)]2(jJ.-Tl2-H-SiMe2)2, **b, *s formed from the addition of a second equivalent of Me2SiH2 to 2b. Thermally unstable, 8b decomposes to either 2b (loss of silane) or [(dippe)Rh]2(H-SiMe2)2, 6b, (loss of hydrogen). *H and 31P{ lH] NMR spectroscopy suggest a fluxional process is occurring for 8b in solution in which both hydrides and phosphines are exchanging between inequivalent sites via a concerted process. Addition of ethylene to 8b causes iii its quantitative conversion to 6b, and 8b undergoes slow exchange of hydrides for deuterides in the presence of deuterium gas. The reaction of 6a with hydrogen to give 2a plus diphenylsilane is proposed as the basis of catalytic exchange of hydrides for deuterides on diphenylsilane in the presence of deuterium gas and catalytic amounts of 1. An intermediate analogous to 8 b is proposed in a catalytic cycle responsible for the dehydrogenative dimerization of diphenylsilane catalyzed by 1. Cis and trans isomers of the bis(n-silylene) complexes [(dippe)Rh]2(|X-SiHR)2,6e-f (e, R = Bun; f, R = TolP) result from the addition of two equivalents of primary silane to 1. Analogues of 2a-c could not be isolated for the primary silanes studied. Complexes 6e-f reversibly add hydrogen to give [(dippe)Rh(H)]2(M.-Ti2-H-SiHR)2,8e-f. Geometric isomers of 8e-f are highly fluxional in solution, undergoing similar exchange processes to 8b. Addition of more than two equivalents of primary silane to 1 gives complexes 9e-f, [(dippe)Rh]2(M--,n2-H- SiHR)2(|i-SiHR). A crystal structure of 9f is the first of a complex containing a metal-metal bond bridged by three silicons. While attempts to oligomerize/Molylsilane using 1 as catalyst gave only dimer plus various silane disproportionation products, n-butylsilane gave coupled products with chains of up to five silicons in the presence of catalytic amounts of 1. The stepwise chain growth in this oligomerization reaction may involve intermediates of the same basic structure as 8, though the involvement of complexes like 9 cannot be ruled out. The results indicate that late transition- metal complexes are potentially most useful for oligomerization of primary alkylsilanes rather than arylsilanes. iv TABLE OF CONTENTS ABSTRACT ii TABLE OF CONTENTS iv LIST OF TABLES x LIST OF FIGURES xi LIST OF ABBREVIATIONS xv ACKNOWLEDGEMENTS xix DEDICATION xx QUOTATION xxi CHAPTER 1 Introduction 1.1 The development of organosilicon chemistry 1 1.2 The organometallic chemistry of silicon 3 1.3 Ligands that bind to transition metals through a metal-silicon bond 5 1.4 General methods for generating transition metal-silicon bonds 8 1.5 Bonding in transition-metal complexes with silicon ligands 11 1.6 Current reasons for study of transition-metal complexes with silicon ligands 13 1.7 The scope of this thesis 16 1.8 References 18 V CHAPTER 2 Reactions of Secondary Silanes with [(dippe)Rh]2(^-H)2: Complexes with a Single Silicon Ligand 2.1 Introduction 23 2.2 Reaction of [(dippe)Rh]2(|i-H)2,1, with one equivalent of R2SiH2 25 2.2.1 Synthesis and properties of dinuclear silyl hydrides 25 2.2.2 Solid-state structure of [(dippe)Rh]2(M-H)(n-Ti2-H-SiR2) (R = Ph, 2a, R = Me,2b) 27 2.2.2.1 Discussion of agostic Si-Hbonds 32 2.2.3 Variable temperature 3lP[ lH} and JH NMR spectroscopy of 2a-c 35 2.2.4 Reaction of the silyl hydride complexes 2a-c with hydrogen 41 2.2.5 Mechanistic considerations in the formation of [(dippe)Rh]2(|i-H)(n-ii2-H-SiPh2),2a 50 2.2.6 Displacement of hydrogen by carbon monoxide in the silyl hydride complexes 2a-c 51 2.3 Catalytic hydrosilation of olefins 52 2.3.1 The hydrosilation of ethylene and 1-butene 52 2.4 Experimental 57 2.4.1 General procedures and reagent syntheses 57 2.4.2 Syntheses of complexes and reactivity studies 59 2.4.3 Catalytic reactions 66 2.4.4 Calculations 69 2.5 References 69 vi CHAPTER 3 Reactions of Secondary Silanes with [(dippe)Rh]2(^-H)2: Complexes with Two Silicon Ligands 3 . 1 Introduction 73 3.2 Reaction of [(dippe)Rh]2(|i-H)2,1, with two equivalents of R2SiH2 74 3 . 2 . 1 Preparation and reactivity of the bis(n-silylene) complex, [(dippe)Rh]20i-SiPh2)2,6a 74 3 .2 .2 The solid-state structure of [(dippe)Rh]2(|J.-SiPh2)2, 6a 78 3 .2 .3 Preparation, solid-state structure and structural isomerism of the bis(|i.-silylene) complex [(dippe)Rh]2(|X-SiMePh)2, 6c 81 3 .2 .4 Preparation of [(dippe)Rh]2(H-SiMeTolP)2,6d, and study of the cis/trans isomerism 86 3 . 2 . 5 Reaction of [(dippe)Rh]2(H-H)2,1. with two equivalents of Me2SiH2 90 3 . 2 . 5 . 1 Formation of a bis(silane) complex, [(dippe)Rh]2»2(Me2SiH2), 8b 90 3 . 2 . 5 . 2 Solid-state structure of [(dippe)Rh(H)]2(^-ri2-H-SiMe2)2, 8b 91 3 . 2 . 5 . 3 Variable temperature NMR spectroscopic studies of [(dippe)Rh(H)]20Ml2-H-SiMe2)2,8b 96 3 . 2 . 5 . 4 Reactivity of [(dippe)Rh(H)]2(^-Tl2-H-SiMe2)2, 8b 101 3 .3 The use of [(dippe)Rh]2(|i-H)2,1, as catalyst precursor for reactions of secondary silanes 103 3 . 3 . 1 Catalytic isotopic exchange between diphenylsilane and deuterium gas 103 3 . 3 . 2 Catalytic dimerization of diphenylsilane 107 3 . 4 Experimental I l l 3 . 4 . 1 General procedures and reagent syntheses I l l 3 .4 .2 Syntheses of complexes and reactivity studies I l l vu 3 .4 .3 Catalytic reactions 119 3 .4 .4 Calculations 123 3 .5 References 123 CHAPTER 4 Reactions of Primary Silanes with [(dippe)Rh]2(H-H)2 4 .1 Introduction 126 4 .2 Stoichiometric reactions of [(dippe)Rh]2(|i-H)2,1, with primary silanes 128 4 . 2 . 1 Preparation of the bis(jx-silylene) complexes [(dippe)Rh]2(|i-SiHR)2, 6e (R = Bun) and 6f (R = TolP) 128 4 . 2 . 1 . 1 Solid-state structure of rra«5-[(dippe)Rh]2(|J.-SiHBun)2,6e 129 4 .2 .1 .2 Room temperature and variable temperature NMR spectra of [(dippe)Rh]2(MiHR)2, 6 e (R = B u " ) md 6 f (R = T o l P ) 131 4 .2 .2 Formation of the bis(silane) complexes [(dippe)Rh(H)]2(|i-r|2-H-SiHR)2, 8e (R = Bun) and 8f (R = TolP) 135 4 . 2 . 2 . 1 Fluxionality and variable temperature NMR spectra of [(dippe)Rh(H)]2(^-Ti2-H-SiHR)2, 8e (R = Bun) and 8f (R = TolP) 136 4 . 2 . 3 Reactions of more than two equivalents of primary silanes with [(dippe)Rh]2(M.-H)2,1 149 4 . 2 . 3 . 1 Solid-state structure of [(dippe)Rh]2(|i-Ti2-H-SiHTolP)2(|i-SiHTolP), 9f 149 4 .2 .3 .2 Room and variable temperature NMR spectra of [(dippe)Rh]2(^-Tl2-H-SiHR)2(^-SiHR), 9e (R = Bun) and 9f (R = TolP) 154 4 . 3 Dehydrogenative silicon-silicon coupling of primary silanes in the presence of catalytic amounts of [(dippe)RhJ2(|i-H)2, 1 163 4.3.1 Mechanistic considerations in the dehydrogenative coupling of primary silanes 164 4.4 Experimental 169 4.4.1 General procedures and reagent syntheses 169 4.4.2 Syntheses of complexes and reactivity studies 169 4.4.3 Catalytic reactions 175 4.4.4 Calculations 176 4.5 References 177 CHAPTER 5 Conclusions and Prospects for Future Studies 5.1 Conclusions 181 5.2 Prospects for future studies 183 5.3 Experimental 186 5.3.1 General procedures and reagent syntheses 186 5.3.2 Reactions of [(dippe)Rh]2(|i-H)2,1, with tertiary silanes 186 5.4 References 187 Appendix A. 1 X-ray Crystallographic Analysis of [(dippe)Rh]2(u-H)(|i-ri2-H-SiPh2), 2a 188 A. 2 X-ray Crystallographic Analysis of [(dippe)Rh]2(|J.-H)(|i-rj2-H-SiMe2), 2b 193 A. 3 X-ray Crystallographic Analysis of [(dippe)RhJ2(|i-SiPh2)2,6a 198 ix A. 4 X-ray Crystallographic Analysis of rra/w-[(dippe)Rh]2(|i-SiMePh)2,6c 203 A.5 X-ray Crystallographic Analysis of [(dippe)Rh(H)]2(^-Ti2-H-SiMe2)2,8b 208 A. 6 X-ray Crystallographic Analysis of rra/i5-[(dippe)Rh]2(|i-SiHBun)2, 6e 214 A.7 X-ray Crystallographic Analysis of [(dippe)Rh]2(^-SiHTolP)(n-ri2-H-SiHTolP)2) 9f 218 X LIST OF TABLES Table 2.1 Selected bond lengths for [(dippe)Rh]20>H)(^-rj2-H-SiPh2), 2a 29 Table 2.2 Selected bond angles for [(dippe)Rh]2(H-H)Qi-ri2-H-SiPh2), 2a 30 Table 2.3 Selected bond lengths for [(dippe)Rh]2([i-H)(|i-Ti2-H-SiMe2), 2b 30 Table 2.4 Selected bond angles for [(dippe)Rh]2(|i-H)(^-r|2-H--SiMe2), 2b 30 Table 3.1 Selected bond lengths for [(dippe)Rh]2(|i-SiPh2)2,6a 78 Table 3.2 Selected bond angles for [(dippe)Rh]2(MiPh2)2,6a 7 8 Table 3.3 Selected bond lengths for rra«5-[(dippe)Rh]2(^i-SiMePh)2,6c 82 Table 3.4 Selected bond angles for fra/w-[(dippe)Rh]2(|J.-SiMePh)2, 6c 83 Table 3.5 Selected bond lengths for [(dippe)Rh(H)]2(n-Tl2-H-SiMe2)2, 8b 93 Table 3.6 Selected bond angles for [(dippe)Rh(H)]2(^-r|2-H-SiMe2)2, 8b 93 Table 4.1 Selected bond lengths for rrans-[(dippe)Rh]2(H-SiHBun)2,6e 130 Table 4.2 Selected bond angles for rran5-[(dippe)Rh]2(|i-SiHBun)2,6e 131 Table 4.3 Selected bond lengths for [(dippe)Rh]2(^-Tl2-H-SiHTolP)2(H-SiHTolP), 9f. 150 Table 4.4 Selected bond angles for [(dippe)Rh]2(^-Tl2-H-SiHTolP)2(^-SiHTolP), 9f. 150 xi LIST OF FIGURES Figure 1.1 Some of the principal ligands that bind to metal centres through a silicon atom 5 Figure 1.2 Examples showing the reactivity of [(dippe)Rh]2(M.-H)2,1, with a variety of small organic molecules 17 Figure 2.1 300 MHz *H NMR spectrum of [(dippe)Rh]2*(Ph2SiH2), 2a, in CGD6 26 Figure 2.2 Possible structure in solution of complexes 2a-c 27 Figure 2.3 Molecular structure of [(dippe)Rh]2(H-H)(|i-T|2-H-SiPh2), 2a 28 Figure 2.4 Molecular structure of [(dippe)Rh]2(H-H)(|n-Ti2-H-SiMe2), 2b 29 Figure 2.5 Variable temperature 121.4 MHz 31P{ JH} NMR spectra of [(dippe)Rh]2(H-H)(n-Ti2-H-SiPh2), 2a, in C7D8. The "*" marks an impurity, complex 1 36 Figure 2.6 Variable temperature 300 MHz *H NMR spectra (hydride region) of [(dippe)Rh]2(^-H)(n-ri2-H-SiPh2), 2a, in C7D8 38 Figure 2.7 121.4 MHz 31p{lH} NMR spectrum of [(dippe)Rh]2(|i-H)(u-ri2-H-SiMePh), 2c, in C7D8 at -84°C 39 Figure 2.8 Variable temperature 300 MHz *H NMR spectra (hydride region) of {[(dippe)Rh]2ai-H)ai-rt2-H-SiPh2)} «H2,3a, in C7D8 43 Figure 2.9 Variable temperature 121.4 MHz 31P{ JH} NMR spectra of {[(dippe)Rh]2(|!-H)(n-T|2-H-SiPh2)}«H2,3a, in C7D8 45 Figure 2.10 Variable temperature 121.4 MHz 31P{ *H} NMR spectra of {[(dippe)Rh]20i-H)(|i-Ti2-H-SiMePh)}«H2,3c, in C7D8. The "*" marks signals due to an impurity, the hydrogen adduct of 1 46 xn Figure 2.11 Variable temperature 121.4 MHz 31P{ 1H} NMR spectra of {[(dippe)Rh]2(H-H)(u-Ti2-H-SiMe2)}*H2,3b, in C7D8. The "*" marks an impurity, [(dippe)Rh(H)]2(|x-T|2-H-SiMe2)2,8b (see Chapter 3) 47 Figure 2.12 A possible catalytic cycle for the hydrosilation of ethylene based on the dinuclear active catalyst, [(dippe)Rh]2(|i-H)(|j.-ri2-H-SiPh2), 2a. (The chelating phosphine ligands have been omitted to simplify the diagram.) 55 Figure 3.1 121.4 MHz 31P{ !H} NMR spectrum of 6a in C6D6, an AA'A"A,"XX' pattern 75 Figure 3.2 121.4 MHz 3*P{ JH} spectrum of compound 7, resulting from the addition of excess Ph2SiH2 to 6a 76 Figure 3.3 Some possible structures for compound 7, the product from the reaction of 6a with excess diphenylsilane 77 Figure 3.4 Two views of the molecular structure of [(dippe)Rh]2(jx-SiPh2)2. 6a 79 Figure 3.5 Molecular structure of rra/w-[(dippe)Rh]2(u.-SiMePh)2, 6c 83 Figure 3.6 121.4 MHz 31P{ lH} spectrum of a mixture of the cis and trans isomers of [(dippe)Rh]2(H-SiMePh)2,6c, in C6D6 84 Figure 3.7 Possible structures of bis(|i.-silylene) complexes leading to an AA'A"AmXX'pattern in the 31P{1H} NMR spectrum 85 Figure 3.8 300 MHz *H NMR spectrum of [(dippe)Rh]2»2(Me2SiH2), 8b, in C7D8 91 Figure 3.9 Molecular structure of [(dippe)Rh(H)]2(H-T|2-H-SiMe2)2,8b 92 Figure 3.10 Side view, looking along the Rh-Rh axis, of the proposed intermediate XIII, demonstrating steric interactions of the substituents on silicon. (Phosphine ligands and agostic hydrides are omitted for clarity.) 96 Figure 3.11 Variable temperature 121.4 MHz 31P{ !H} NMR spectra of [(dippe)Rh(H)]2(H-'n2-H-SiMe2)2, 8b, in C7D8 97 xni Figure 3.12 Variable temperature 300MHz *H NMR spectra (hydride region) for [(dippe)Rh(H)]2(|i-Ti2-H-SiMe2)2,8b, in C7D8. The "*" marks an impurity, {[(dippe)Rh]2(^-H)(u-Ti2-H-SiMe2)} *H2,3b 99 Figure 4.1 Molecular structure of rra/w-[(dippe)Rh]2(M--SiHBun)2, 6e 130 Figure 4.2 Variable temperature 121.4 MHz 31P{ !H} NMR spectra of [(dippe)Rh]2(n-SiHBun)2, 6e, in C7D8 133 Figure 4.3 Rotation of the /j-tolyl group around the Si-Cipso bond in [(dippe)Rh]20i-SiHTolP)2,6f. (P = PPri2) 134 Figure 4.4 Variable temperature 121.4 MHz 31P{ 1U} NMR spectra of [(dippe)Rh(H)]2(n-T|2-H-SiHBun)2,8e, in C7D8 137 Figure 4.5 Hydride region of the variable temperature 300 Mz *H NMR spectra of [(dippe)Rh(H)]2(|i-Tl2-H-SiHBun)2,8e, in C7D8 139 Figure 4.6 Side views of the bis(silane) complexes 8e-f (e, R = Bun, f, R = TolP), showing the possible geometric isomerism. (The agostic hydrides and phosphine ligands are omitted for clarity.) 140 Figure 4.7 Silyl hydride region of the 300 MHz *H NMR spectrum of 8e at -25°C. 29Si satellites for the signal due to the cis isomer are indicated by the "*" 141 Figure 4.8 Twisting mechanism which would exchange the two Rh-H sites and the two phosphine sites in compound 8. (Substituents on silicon are omitted for clarity.) 142 Figure 4.9 a) Side views of 8e-f demonstrating the "flapping" mechanism which would exchange the syn and anti substituents on silicon. (The phosphines are omitted for clarity.) b) A front view of the "flapping" mechanism, showing how the two types of Rh-H and the two types of phosphine in 8 are exchanged by this mechanism. (Substituents on phosphorus and silicon are omitted for clarity.) 143 Figure 4.10 Variable, high temperature 300 MHz *H NMR spectra of 8e in C7D8, showing the silyl hydride region 145 XIV Figure 4.11 Variable temperature 121.4 MHz 31P{JH} NMR spectra of [(dippe)Rh(H)]2(H-Tl2-H-SiHTolP)2, 8f, in C7D8 148 Figure 4.12 Molecular structure of [(dippe)Rh]2(n-Tl2-H-SiHTolP)2(|i-SiHTolP), 9f. Isopropyl groups on phosphorus and p-tolyl groups on silicon are represented by single carbons 151 Figure 4.13 Room temperature 300 MHz *H NMR spectrum of [(dippe)Rh]2(^-Tl2-H-SiHR)2(^-SiHR), 9e, in C6D6 155 Figure 4.14 Variable temperature 121.4 MHz 31P{1H} NMR spectra of [(dippe)Rh]2(^i-ri2-H-SiHBun)2(|i-SiHBun), 9e, in C7D8 157 Figure 4.15 Simulated 121.4 MHz 3lp{lH} NMR spectrum of [(dippe)Rh]2(^-Tl2-H-SiHBun)2(|i-SiHBun), 9e, at -24°C. (See Figure 4.14 for the measured spectrum.) 158 Figure 4.16 Variable temperature 121.4 MHz 31P{ ̂ H} NMR spectra of [(dippe)Rh]2(^-Tl2-H-SiHTolP)2(MiHTolP), 9f, in C7D8 160 Fi gu r e 4.17 Mechanism for the dehydrogenative coupling of silanes catalyzed by late transition-metal complexes, based on oxidative addition and reductive elimination steps 165 Figure 4.18 Mechanism for the dehydrogenative coupling of primary silanes catalyzed by [(dippe)Rh]2(|i-H)2,1, based on oxidative addition and reductive elimination steps at the dinuclear centre 167 XV LIST OF ABBREVIATIONS The following abbreviations, most of which are commonly found in the literature, are used in this thesis. A Anal. atm avg br brd br d mult brmult br s brt Bun BuO1 <C 13C{*H} Calcd cm-1 Cp Cp* Cy d dmult d sept angstrom (10"10 m) analysis atmosphere average broad broad doublet broad doublet of multiplets broad multiplet broad singlet broad triplet n-butyl group, -CH2CH2CH2CH3 tertiary butoxy group, -OC(CH3)3 degrees Celsius observe carbon while decoupling proton calculated wave number cyclopentadienyl group, -C5H5 pentamethylcyclopentadienyl group, -05(013)5 cyclohexyl group, -C^Hn doublet doublet of multiplets doublet of septets dd deg(or°) A AG* AH0 dippe dippp Do dppm EI equiv GC GC-MS 1H{31P} 2H HETCOR HMPA Hm Ho Hp hi-* Hz IR "JA-B K kcal doublet of doublets degrees heat free energy of activation enthalpy of reaction 1,2 - bis(diisopropylphosphino)ethane l,3-bis(diisopropylphosphino)propane donor ligand bis(diphenylphosphino)methane electron ionization equivalent(s) gas chromatography gas chromatography - mass spectrometry observe proton while decoupling phosphorus deuterium heteronuclear chemical shift correlation hexamethylphosphoramide /we/a-hydrogen ortho-hydrogen para-hydrogen per hour Hertz, seconds"1 infrared n-bond scalar coupling constant between A and B nuclei Kelvin kilocalories equilibrium constant M M+ mass spec Me mg MHz min"1 mL mm mmol MO mol ms mult NMR OEt OMe OPri ORTEP OTf 31p{lH} pent Ph pmdeta ppm P r i Pt central metal atom (or "molar", when referring to concentration) parent ion mass spectrometry methyl group, -CH3 milligram(s) megaHertz per minute millilitre millimetre millimole(s) molecular orbital mole millisecond(s) multiplet nuclear magnetic resonance ethoxy group, -OCH2CH3 methoxy group, -OCH3 isopropoxy group, -OCH(CH3)2 Oakridge Thermal Ellipsoid Plotting Program triflate group, -OSO2CF3 observe phosphorus while decoupling proton pentet phenyl group, -C6H5 pentamethyldiethylenetriamine parts per million isopropyl group, -CH(CH3)2 pentet of triplets xviii q R RI RT s sept 29SipH} T t Ti THF TMS TolP tt VT wi/2 X xs quartet alkyl, aryl or alkoxy group relative intensity room temperature singlet (or "strong", for infrared data) septet observe silicon while decoupling proton temperature triplet spin-lattice relaxation time tetrahydrofuran tetramethylsilane p-tolyl group, -C6H4CH3 triplet of triplets variable temperature linewidth at half-height anionic, unidentate donor ligand excess inch(es) xix ACKNOWLEDGEMENTS I would like to thank my supervisor, Mike Fryzuk, for his advice, encouragement and patience throughout this project. I extend thanks also to Steve Rettig, who solved all of the crystal structures in this thesis. For their help during the writing of this thesis, my thanks go to Cindy Longley, Charles Stone, and Guy Clentsmith. I have greatly appreciated the help of the technical staff in the Chemistry Department of UBC; I also wish to thank members of the Fryzuk research group, past and present, for their assistance. Thanks especially to Dave McConville, Deryn Fogg, and Murugesapillai Mylvaganam for many helpful discussions, and warmest thanks to Greg Spohr for his love and support. I am grateful for financial support from the Natural Sciences and Engineering Research Council of Canada and the Department of Chemistry at the University of British Columbia. XX For Rose XXI "... it really is simple- seeing clearly a good picture is simply seeing- seeing clearly a struggling picture is a struggle." Barbara Spohr, photographer. Chapter 1 1 CHAPTER 1 Introduction 1.1 The development of organosilicon chemistry Historically the development of the organic chemistry of silicon has lagged behind that of carbon. This has been mainly due to the assumption that silicon should have similar reactivity to carbon, given their proximity in the periodic table. The two elements actually have very different patterns of reactivity, which are probably best explained by the differences in their size and electronegativity. With a covalent radius half as large again as that of carbon, silicon tends to form less stable rc-bonds and has lower barriers to rotation around its bonds to other elements. With its much lower electronegativity (carbon is 2.5, whereas for silicon it is 1.8, which is closer to that of the first row transition metals) silicon tends to have more polar bonds to carbon, nitrogen and oxygen.1 Both carbon and silicon are found in the tetrahedral tetravalent form in their organic derivatives, but multiple bonds to silicon are rare. On the other hand, silicon is known to form derivatives such as compound I with coordination numbers greater than four,2 while for carbon such "hypervalent" species are rarely isolated and are generally unstable. Me While the lack of multiple bonding in silicon compounds limits the traditional organic synthetic methods available, pathways such as nucleophilic substitution reactions, which are based on the formation of hypervalent species, become more useful in organosilicon chemistry. References pi 8 Chapter 1 2 Among the barriers to the development of organic silicon chemistry was the scarcity of useful starting materials. The element is found naturally in the form of silica and silicates, and the only silicon-element bond stronger than the Si-0 bond found in these materials is the Si-F bond. However, by the mid-1800s methods had been found to produce SiCU and Si(OEt)4, and subsequent attempts to prepare compounds with Si-C bonds generally involved these starting compounds, with limited success.1 Intensive investigations of the organic chemistry of silicon began around the turn of the century, but in 1937 one of the key investigators of the time, F. S. Kipping, was forced to conclude that the chemistry of silicon was very limited relative to that of carbon and that major advances in this field were unlikely.3 The production of organosilicon compounds really began to accelerate around 1940-1945 with the development of the Rochow, or "direct synthesis" process.4 This process involves the reaction of an alkyl or aryl halide in the gas phase with elemental silicon at high temperatures in the presence of a metal catalyst, usually copper. The composition of the product mixture depends upon the reactants and the reaction conditions used (the quality of the elemental silicon, type of catalyst, temperature, etc.) and can be shifted to give specific major products by the variation of these factors.5 r.- 250-550 °C _ _ . v nRX + Si — *~ RnSiX4.n [ M ] Thus in the past 50 years, with the availability of more and more starting materials containing Si-C bonds, organosilicon chemistry has developed enormously, giving birth to many new industrial applications such as the production of silicone rubbers, oils and resins. A portion of a typical silicone structure is shown below.6 References pi8 Chapter 1 3 I I —Si-0-Si-O-Si-O-Si—0- - I / I • -Si—0-Si—O-Si-0-Si—0 - The preparation of silicones is based on the reactions of siloxanes (-SiOSi-). Also an important field in organosilicon chemistry is the study of compounds where silicon is bound to such heteroatoms as nitrogen, phosphorus and sulfur.7,8 Further discussion of these areas is outside the scope of this introduction though, since the work described in this thesis is mainly concerned with Si-H, Si-C and Si-Si bonds. Though the differences in the behaviour of silicon and carbon are now more clearly recognized, an ongoing challenge is the preparation of silicon analogues of unsaturated carbon compounds. Recent developments include the preparation of stable multiply-bonded silicon molecules, including disilenes (R2Si=SiR2)9,10 and silenes (R2Si=CR'2).11,12 These molecules are characterized by very bulky substituents, which protect the double bond from addition reactions. Also the first structurally characterized example of a tricoordinate silicon cation has recently been reported.13 Much more susceptible to nucleophilic attack than the analogous carbocations (R3C+), this Et3Si+ ion is stabilized by a large, non-coordinating borate anion, B(C6F5)4-. 1.2 The organometallic chemistry of silicon To a large extent the field of organometallic chemistry has grown out of the discovery that transition metals are catalysts for many important processes in organic chemistry, such as hydrogenation of unsaturated bonds, hydroformylation, hydrocyanation, and polymerization.14 References pi 8 Chapter J In an attempt to understand the mechanisms involved in the catalysis and to improve selectivity and control in catalytic reactions, the interactions of a myriad of small organic molecules with a plethora of transition-metal complexes have been and continue to be probed. What was originally initiated by an interest in optimizing useful organic processes has become an exercise in understanding fundamental concepts in the bonding of transition metals to carbon-containing ligands. Initial interest in the reactions of transition metals with silanes came with the discovery that metals could catalyze the hydrosilation of unsaturated organic molecules. R3SiH + X C = C / Cata'ySt » R3Si - C - C - H [1-2] The hydrosilation of olefins catalyzed by metals was first discovered in 1947,15 and has become an important method for the formation of Si-C bonds. Hydrosilation reactions have been extended to alkynes, conjugated dienes, carbonyl compounds and even C-N multiple bonds.16 In the case of carbonyl compounds, the hydrosilated product contains a Si-O bond that can easily be hydrolyzed, providing an alternative method for the reduction of carbonyl functionalities. / * " catalyst • H20 ri-31 R3SiH + 0=Q — R3Si - O - C - H »- R3S1OH + R"R'HCOH R1 I R' Some common hydrosilation catalysts are chloroplatinic acid (H2PtCl6), [Pt(C2H4)Cl2]2, Rh(PPh3)3Cl, and Co2(CO)8-17 Countless other compounds have been found to catalyze hydrosilation, including some early-metal and lanthanide complexes, though none have the activity of the late-metal catalysts mentioned above. The hydrosilation process, which will be discussed further in Chapter 2, requires both activation of the Si-H bond by the metal centre and coordination of the unsaturated species. References pi 8 Chapter 1 The discovery of hydrosilation was eventually followed in 1956 by the preparation of the first characterized complex containing a metal-silicon bond (vide infra)}* This was followed by a brief lull in organometallic silicon chemistry, which picked up again in 1965 with the preparation of some cobalt-silicon complexes and the study of oxidative addition of Si-H bonds to metal centres.19"21 These studies led to the proposal of a general mechanism for the hydrosilation reaction, and from that point, the organometallic chemistry of silicon ligands has progressed to become a highly diverse field of synthetic and mechanistic chemistry. 1.3 Ligands that bind to transition metals through a metal-silicon bond Some of the principal types of ligands that bind to metal centres through a silicon atom are described here and shown in Figure 1.1. While this list is not exhaustive, it includes the most commonly found silicon-containing ligands and those that are the most popular targets synthetically. & D o Si I / \ LnM-SiR3 LnM MLn Ln M~SiR 2 silyl u-silylene donor-, or base-, stabilized silylene LnM—SiR2 LnM—SiR2 \ / \ / CH2 SiR2 ri2-silene n2-disilene Figure 1.1 Some of the principal ligands that bind to metal centres through a silicon atom. The most common type of transition metal-silicon bond is the metal-silyl bond, where the silyl ligand -SiR3 is directly analogous to an alkyl group. The silyl ligand can be considered formally as a two-electron donor with a charge of-1 (R3Si_), or as a neutral radical (R3Si»)- The References pl8 Chapter 1 first characterized example of a silyl ligand bound to a transition-metal centre is the iron complex II shown below.18 Many more have been prepared since then with a wide variety of substituents on silicon ranging from alkyl and aryl groups to hydrides, alkoxides, halides and other silyl groups. Generally, transition-metal silyl complexes have tended to contain carbon monoxide, phosphines or cyclopentadienyl groups as ancillary ligands,14 and the majority of silyl ligands used have been tertiary, with no silicon hydrides. This subject has been reviewed extensively.17-22-30 Na4 + Me3SiCI NaCl THF [1-4] A second type of silicon-containing ligand is the silylene fragment, :SiR2- Silylenes are high energy species which have not been isolated but are frequently inferred on the basis of the products isolated from their reactions. They can be trapped using various reagents such as Et3SiH; they can also be trapped, or stabilized, by transition-metal complexes. The silylene ligand is directly analogous to the carbene or alkylidene ligand which can be considered formally as a four-electron donor with a -2 charge, as a neutral diradical (triplet state) or as a neutral species with a lone pair of electrons (singlet state). Complexes with terminal silylene ligands are extremely rare, and those that have been prepared are generally stabilized by donor ligands on silicon, as in compound III.31 References pi8 Chapter 1 7 OC CO Do M >' O C - F e ^ S L . ^ C R o III R = OBu' Do = HMPA Dinuclear complexes with bridging silylene ligands are far more common; reactions intended to produce monomeric transition-metal silylene complexes tend to give the more stable dimers instead, as shown in Equation 1-5.32 lowT Na2Cr(CO)5 + Me2SiCI2 T H F - - 2NaCI OC CO THF O C — C r — S i - ocfco \ '•»## Me Me T > -40°C -THF -CO Me2 Si 1/2 (OC)4Cr \ r ( C O ) 4 Si Me2 [1-5] Two other types of complex involving a transition metal-silicon bond include silicon that is involved in multiple bonding, either to carbon or to a second silicon. One type is the T]2-silene (or silaethene) complex where a Si=C double bond is bound to the metal centre in a side-on fashion, and an example is complex IV.33 The other is an T|2-disilene (or disilaethene) complex where a Si=Si double bond is bound to the metal centre, again in a side-on fashion. An example References pl8 Chapter 1 8 of an Ti2-disilene complex is complex V.34 These ligands are analogous to side-bound olefins, such as r|2-C2H4. Ph2 j A / CH2 ^ P ^ 2 n 2  Ph2 IV V R = Me, Ph R = Me, Pr' 1.4 General methods for generating transition metal-silicon bonds There are two general routes to the formation of transition metal-silicon bonds. The first is a metathesis reaction, exemplified by the reactions shown in Equations 1-4 and 1-5. Shown is the metathesis of an anionic metal complex and a silyl halide (or halide equivalent). This reaction is driven by the elimination of a salt, which in these cases is sodium chloride. The second, and by far the most common, general method for making metal-silicon bonds is by the addition of a Si-H bond to a metal centre. There are many variations on this method, and often the addition is accompanied by the elimination of a small molecule from the metal centre, such as H2, CH4 or HC1. An example is shown below.35 (Ph3P)3CoH3 + HSiR3 • - (Ph3P)3CoH2(SiR3) + H2 [1-6] Preparing metal-silicon bonds by the metathesis method is somewhat limited by the availability of appropriate anionic reagents. For instance this method has been most useful in the addition of silyl ligands to carbonyl complexes, as these are the most common type of transition- metal anions. However a range of metathesis reactions using transition-metal anions has been demonstrated, as shown in Equations 1-7 through 1-9.3638 References pi8 Chapter 1 9 < ^ > - I K Film .Nb - - *• PMe3 PMe3 THF < ^ > - Nb PMe3 / PMe3 PMe3 < ^ > - MeaSiCI | .Nb THF P M C / ^ S i M e 3 PMe3 PMe3 [1-7] ., „,,, BuLi(pmdeta) . Me3SiOTf . r , on Cp*lrH4 Z L ^ [Cp lrH3]Li(pmdeta) : • Cp lrH3SiMe3 [1-8] (pmdeta = pentamethyldiethylenetriamine) Li[Cp(NO)(PPh3)Re] + Me2SiHCI - • Cp(NO)(PPh3)ReSiMe2H [1-9] The metathesis can also involve the addition of an anionic silyl reagent to a transition-metal halide, Equation l-lO,39 though these main-group silyl reagents can be difficult to prepare.29 Cp2MCI2 + LiSiPh3 -LiCI M = Zr, Hf Cp2M(SiPh3)CI [l-lO] Metathesis has also been used to prepare monomeric silylene complexes, using dianionic metal complexes and dihalosilanes in the presence of bases. This was the method used to prepare the iron silylene complex, III, shown in Section 1.3.31 The addition of Si-H bonds to transition-metal centres is a general process occurring for both early and late transition-metal complexes. For late transition metals the silyl complexes References pl8 Chapter! 10 generated are the result of oxidative addition to low-valent species that possess a vacant coordination site. In Equation 1-11 the addition is straightforward, with a change from Ru(II) to Ru(IV) and the formation of a Ru-Si and a Ru-H bond.40 Cp Ru(PPr'3)CI + RSiH3 * - Cp Ru(PPr'3)(SiH2R)(H)CI „ n ] R = Ph, Tolp In some cases the addition is accompanied by dissociation of an ancillary ligand to provide the necessary vacant coordination site, as shown in Equation 1-12.41 - PPh3 + R3SiH (Ph3P)3lr(CO)H - j — » - (Ph3P)2lr(CO)H ^ z £ : (Ph3P)2lr(CO)H2(SiR3) [1-12] + PPh3 . R3siH As the cobalt complex in Equation 1-6 demonstrates, the oxidative addition of the Si-H bond at a metal centre often induces the simultaneous reductive elimination of a small molecule such as hydrogen, giving an overall ligand exchange reaction. For the more electropositive, early transition metals, oxidative addition reactions are less viable routes to transition-metal silyls because the metals are generally already in high oxidation states. Metathesis reactions have been more useful for preparing silyl complexes of these metals. However Si-H cleavage with formation of M-Si and M-H bonds does occur for the early transition metals, as shown in Equation 1-13.42 In this case it is presumed that there is initial reductive elimination of hydrogen, followed by oxidative addition of the Si-H bond. A,-H2 Cp2MH3 + PhMe2SiH • Cp2M(SiMe2Ph)H2 n_i3] M = Nb, Ta On the other hand, kinetic studies of the reactions of hafnocene hydride and silyl complexes with phenylsilane indicate that in this case the cleavage of the Si-H bond of the silane proceeds via a- References pi 8 Chapter 1 11 bond metathesis. This concerted process requires an intermediate, four-centre transition state (see Equation 1-14), and there is no change in the coordination number or the formal oxidation state of the hafnium.43 .CI / PhSiH3 CpCp'Hf i * . H SiPhH; CpCpXIHf^' Vl V ^ C p C P * H < C I [ M 4 ] SiPhH2 Some routes to metal silicon bonds other than the two described above have been reported but few of these have general applicability.29,30 Of interest is the fact that oxidative additions of silyl halides (Si-X) to transition metals are extremely rare, unlike the alkyl halides for which oxidative addition provides a facile route to the formation of metal-carbon bonds.29 1.5 Bonding in transition-metal complexes with silicon ligands As is the case for all second-row (and heavier) elements in the p-block, a recurring question arises as to the extent of participation of the d-orbitals in the bonding of silicon, both in silicon-silicon bonds and in silicon-heteroatom bonds. Certainly hybridization at silicon involving d-orbitals, as was originally suggested by Pauling,44 seems a straightforward way to explain some structural features of silicon compounds. In particular, silicon has a much greater tendency than carbon to form stable "hypervalent" compounds with more than four bonds, which could be explained by the involvement of dsp3 or d2sp3 hybrid orbitals. In the case of the silicon-transition metal bond, participation of the d-orbitals on silicon as 7t-acceptors has frequently been invoked as a means of explaining unusually short metal-silicon distances in many complexes containing silyl ligands. This effect is prominent in electron-rich, late transition-metal complexes. Silyl complexes of d° metals should not exhibit this d^-d^ bonding, as the d-orbitals on the metal are unoccupied. M-Si bond lengths for many early transition-metal References pi 8 Chapter 1 12 silyls are close to those predicted from covalent radii, indicating that there is indeed no backbonding occurring. This observation has been cited as support for the idea of d-orbital involvement in 7t-bonding in the late-metal complexes, where the distances are shorter than predicted from covalent radii.29 % - back-donation —*- -*—o - donation However, numerous theoretical studies carried out on silicon compounds have suggested that participation of the d-orbitals in the bonding of silicon is minimal. Ab initio calculations can accurately predict most structural characteristics of organosilicon compounds without including the d-orbitals in the basis sets used.45 These calculations suggest that for hypervalent silicon compounds a more realistic picture of the bonding is based on a model where three-centre, four- electron, or other multi-centre bonding is in effect.46,47 R three-centre, four electron bonding Other theoretical studies suggest that while it is rc-backbonding that gives the unexpectedly short M-Si bonds for late transition-metal complexes, the rc-acceptor orbital on the silyl ligands in the complexes is actually a o* orbital, not a d-orbital as was originally proposed.29 References pi 8 Chapter 1 13 7i - back-donation-*- - * - G - donation 1.6 Current reasons for study of transition-metal complexes with silicon ligands Contemporary organometallic chemistry of silicon is fueled by several different areas of interest, all of which relate to attaining a better understanding of the nature of the transition metal-silicon interaction. For instance, great attention continues to be paid to the synthesis of silicon analogues of some established carbon ligands, including the preparation of terminal silylenes free of stabilizing donor atoms, and the study of coordinated silenes and disilenes. The reactivity of these molecules should shed light on their possible involvement as intermediates in various catalytic processes. Recently an interest has developed in silicon polymers, or polysilanes. These are long chains of organic silicon moieties linked by Si-Si bonds. The polysilanes have applications in the preparation of silicon carbide fibres, which are precursors for ceramics. Unusual photochemical properties and electronic properties arising from extensive electron derealization through the a-bonded backbones of polysilanes make them useful in the area of microelectronics, as photoresists, and also as photoinitiators for radical reactions. Other potentially commercial uses for polysilanes include their use as photoconductors in photographic processes and the use of their nonlinear optical properties in optical technologies.48,49 While in carbon chemistry addition reactions of unsaturated molecules (whether nucleophilic, electrophilic or radical addition) provide a wide range of methods for the References pl8 Chapter 1 14 production of C-C bonds, the instability of the unsaturated silicon analogues rules out their usefulness as precursors in the formation of Si-Si, or Si-C bonds. Instead, much of organosilicon chemistry relies on substitution reactions or on the reduction chemistry of silyl halides. Hence the most common way to prepare oligomers or higher polymers containing silicon chains is using the Wurtz coupling method,50 whereby silyl halides react with electropositive metals such as sodium or lithium to give polysilanes and alkali salts. R1 , „ Na,>100°C / I \ ^ '-1"15-' nR1R2SiCI2 — — » - 4 - S i - ) - + 2n NaCI c  hydrocarbon \ i /n solvent 1 2 This method has been very successful for the production of many polysilanes, including higher polymers with molecular weights up to several million,49,51 and most studies on the possible applications described above have been for silicon polymers prepared by Wurtz coupling techniques. However, contending with large amounts of molten alkali metals and contaminated salt waste are among the reasons for finding other synthetic routes to these polymers. Another reason is the difficulty in synthesizing polymers with useful functional groups in the harsh, reducing conditions of the Wurtz coupling reactions. In recent years research into the use of transition-metal catalysts for the dehydrogenative coupling of silanes has received renewed attention. This methodology involves the activation of Si-H bonds in silanes by a metal centre, followed by the formation of Si-Si bonds and concomitant elimination of H2. Essentially, this is a condensation reaction. R1 . , • I [1-16] catalyst / I  N nR1R2SiH? • H-fSi-)-H + (n-1)H2 R2 The catalysis of dehydrogenative silicon-silicon coupling by a transition-metal complex was first noted in 1970 with the report of disilanes (RMe2Si-SiMe2H, where R = Me or H) being References pi 8 Chapter 1 15 converted to a mixture of oligomers with chains up to six silicons long in the presence of (Et3P)2PtCl2-52 In 1973 Wilkinson's catalyst was found to give low conversions of various secondary silanes to mixtures of di- and trisilanes.53 In fact, a number of platinum-group metals will catalyze the formation of oligomers (principally dimers and trimers) by dehydrogenative coupling.54"57 A problem with the use of these late transition-metal catalysts for Si-Si coupling (apart from the short chain lengths achieved) is that they also catalyze the redistribution of Si-C bonds in silanes,58 giving rise to unwanted disproportionation products.53,54,57 Somewhat more promising results have been obtained through the use of early transition- metal complexes, in particular with titanocene and zirconocene derivatives. In 1986 the oligomerization of primary silanes using Cp2TiMe2 was reported, the products having an average silicon chain length of ten.59 The activity of a range of related metallocene complexes of both titanium and zirconium has been evaluated.60,61 Most of the complexes surveyed have at least some activity, though the size of oligomers achieved varies considerably depending on the functionality of the silanes (primary, secondary, tertiary), what type of cyclopentadienyl ligand is present (Cp, Cp*, etc.) and what other ligands are attached to the metallocenes (alkyl, silyl, hydrido). The most active catalyst is CpCp*Zr[Si(SiMe3)3]Me which with phenylsilane, under certain conditions, will give linear poly(phenylsilylene) samples with average molecular weights of about 5300, corresponding to roughly 44 silylene units.61 The mechanisms by which these dehydrogenative coupling reactions occur have been a subject of some debate. Initially, for both early and late transition-metal catalysts, mechanisms were proposed that involved the intermediacy of transition-metal silylene derivatives.53,55 However, further research which included careful stoichiometric reactivity studies of the interactions of silanes with transition-metal centres, has shown that mechanisms involving terminal silylenes are unlikely. The currently accepted mechanism for Si-Si coupling catalyzed by zirconocene and titanocene catalysts suggests there is stepwise growth of the silicon chains via four-centre transition states such as the one shown for hafnium in Equation 1-14. The References pl8 Chapter! 16 mechanism is less well understood for the platinum-group catalysts that dehydrogenatively oligomerize silanes, but is thought to proceed via a series of oxidative addition and reductive elimination steps, including the reductive elimination of a Si-Si bond from the metal centre. Most of the steps in this mechanism have some precedent in stoichiometric chemistry.62 This will be discussed further in Chapters 3 and 4. 1.7 The scope of this thesis. This thesis describes the reactions of silanes with the electron-rich, coordinatively unsaturated dinuclear rhodium hydride, [(dippe)Rh]2(|i-H)2, 1 ( where dippe = 1,2- bis(diisopropylphosphino)ethane). H  A Complex 1, a dimer with 28 valence electrons, is an extremely reactive species, with a rich and varied chemistry, some of which is illustrated in Figure 1.2.63"74 In addition to oxidative addition reactions, it undergoes insertion of unsaturated species such as olefins,63,67,69 dienes,64,67,68 nitriles72 and imines66,70 into the bridging Rh-H bonds. Generally, dinuclear products are isolated from reactions of 1 with small organic and organometallic molecules. Also, [(dippe)Rh]2(fi-H)2, 1, is a catalyst for the hydrogenation of olefins.69 The original impetus for the study of this dimer was to discover fundamental reactions occurring at the dinuclear centre that are distinct from the traditional types of processes which occur at a single metal centre.75 Given the reactivity of the dimer towards both olefins and hydrogen it seemed natural to extend References pi 8 Chapter 1 17 reactivity studies to silanes and to determine the capability of 1 as a hydrosilation catalyst. In fact, as the work presented in this thesis will show, there is good reason to suspect the involvement of the dinuclear centre, with the two metals behaving cooperatively, in various reactions of silanes that are catalyzed by 1. " H ^ ^ R K f p R h ^ r Rh^ ^ R h H H R H Rh Rh + Rh Rh ^ C ^ C - H Figure 1.2 Examples showing the reactivity of [(dippe)Rh]2(|i-H)2,1, with a variety of small organic molecules. Chapter 2 describes the complexes formed by the addition of one equivalent of various secondary silanes, R2SiH2, to [(dippe)Rh]2(|i-H)2, 1. The fluxionality and reactivity of these References p!8 Chapter 1 18 complexes are discussed, and a description of their possible involvement in the hydrosilation of olefins is presented. Chapter 3 includes descriptions of complexes formed from the addition of two equivalents of secondary silane to 1, and of their reactivity. The stoichiometric chemistry observed is used to discuss qualitatively the mechanisms of two reactions of secondary silanes that are catalyzed by the addition of 1: exchange of Si-H bonds with D2 and dimerization of diphenylsilane. Extension of the above chemistry to primary silanes, RSiH3, is presented in Chapter 4. The differences in reactivity arising from the decreased steric requirements of primary silanes are highlighted. Some of the complexes prepared are fluxional in solution. Some proposed mechanisms of fluxionality, based on variable temperature NMR studies, are presented. Oligomerization reactions of /z-butylsilane that are catalyzed by 1 are discussed in light of postulated mechanisms for this dehydrogenative coupling process. Conclusions about the work described in this thesis, including a discussion of the apparent dependence of the reactions of silanes with 1 on steric factors is given in Chapter 5. With these conclusions in mind, possibilities for future studies involving silanes and dinuclear rhodium hydride dimers are presented. 1.8 References (1) Corey, J. Y. In The Chemistry of Organic Silicon Compounds; S. Patai and Z. Rappoport, Ed.; John Wiley and Sons Ltd: New York, 1989; pp 1-56. (2) Breliere, C; Carre, F.; Corriu, R. J. P.; Poirier, M.; Royo, G. Organometallics 1986,5, 388. (3) Kipping, F. S. Proc. R. Soc. London, Ser. A 1937,159, 139. References pi 8 Chapter 1 19 (4) Rochow, E. G. / . Am. Chem. Soc. 1945,67,963. (5) Haiduc, I.; Zuckerman, J. J. Basic Organometallic Chemistry; Walter de Gruyter and Co: Berlin, 1985. (6) Elschenbroich, G; Salzer, A. Organometallics: A Concise Introduction, 2nd Ed.; VCH Publishers Inc.: New York, 1992. (7) Borisov, S. N.; Voronkov, M. G.; Lukevits, E. Y. Organosilicon Heteropolymers and Heterocompounds; Plenum Press: New York, 1970. (8) The Chemistry of Organic Silicon Compounds; Patai, S.; Rappoport, Z., Ed.; John Wiley and Sons Ltd.: New York, 1989. (9) West, R.; Fink, M. J.; Michl, J. Science 1981,214,1343. (10. (11 (12. (13 (14. (15 (16 (17 (18 (19 (20 (21 West, R. Angew. Chem., Int. Ed. Engl. 1987,26, 1201. Brook, A. G.; Nyburg, S. C ; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury, R. K. M. R.; Poon, Y. C; Chang, Y. M.; Wong-Ng, W. / . Am. Chem. Soc. 1982,104,5667. Brook, A. G. / . Organomet. Chem. 1986,300,21. Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science 1993,260,1917. Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. 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Reviews on Silicon, Germanium, Tin and Lead Compounds 1981,5,67. Colomer, E.; Corriu, R. J. P. Top. Curr. Chem. 1981,96,79. Aylett, B. J. Adv. Inorg. Chem. Radiochem. 1982,25,1. Tilley, T. D. In The Chemistry of Organic Silicon Compounds; S. Patai and Z. Rappoport, Ed.; John Wiley and Sons Ltd.: New York, 1989; pp 1415-1477. Tilley, T. D. In The Silicon-Heteroatom Bond; S. Patai and Z. Rappoport, Ed.; John Wiley and Sons Ltd: 1991; pp 309-364. Zybill, C.; Miiller, G. Angew. Chem., Int. Ed. Engl. 1987,26, 669. Zybill, C. Top. Curr. Chem. 1992,160, 1. Campion, B. K.; Heyn, R. H.; Tilley, T. D. J. Am. Chem. Soc. 1988,110, 7558. Pham, E. K.; West, R. / . Am. Chem. Soc. 1989, 111, 7667. Archer, N. J.; Haszeldine, R. N.; Parish, R. V. / . Chem. Soc, Chem. Commun. 1971, 524. Green, M. L. H.; O'Hare, D.; Watkin, J. G. / . Chem. Soc, Chem. Commun. 1989, 698. Gilbert, T. M.; Hollander, F. J.; Bergman, R. G. / . Am. Chem. Soc. 1985,107, 3508. Crocco, G. L.; Young, C. S.; Lee, K. E.; Gladysz, J. A. Organometallics 1988, 7,2158. Kingston, B. M.; Lappert, M. F. / . Chem. Soc, Dalton Trans. 1972, 69. Campion, B. K.; Heyn, R. H.; Tilley, T. D. / . Chem. Soc, Chem. Commun. 1988, 278. Harrod, J. F.; Gilson, D. F. R.; Charles, R. Can. J. Chem. 1969,47, 2205. References pl8 Chapter 1 21 Curtis, M. D.; Bell, L. G.; Butler, W. M. Organometallics 1985,4,701. Woo, H. G.; Walzer, J. F.; Tilley, T. D. / . Am. Chem. Soc. 1992,114,7047. Pauling, L. The Nature of the Chemical Bond, 3rd Ed.; Cornell University Press: Ithaca, NY, 1960. Ponec, R. In Carbon-Functional Organosilicon Compounds; V. Chvalovsky and J. M. Bellama, Ed.; Plenum Press: New York, 1984; pp 233-298. Dewar, M. J. S.; Healy, E. Organometallics 1982,1,1705. Kutzelnigg, W. Angew. Chem. Int. Ed. Engl. 1984,23,272. West, R. / . Organomet. Chem. 1986,300, 327. West, R. In The Chemistry of Organic Silicon Compounds; S. Patai and Z. Rappoport, Ed.; John Wiley and Sons Ltd: New York, 1989; pp 1207-1240. Morrison, R. T.; Boyd, R. N. Organic Chemistry; Allyn and Bacon, Inc.: Boston, 1959. West, R. In Comprehensive Organometallic Chemistry; E. Abel, Ed.; Pergamon: Oxford, England, 1982; Vol. 2; pp 365-397. Yamamoto, K.; Okinoshima, H.; Kumada, M. / . Organomet. Chem. 1970,23, C7. Ojima, I.; Inaba, S.; Kogure, T. / . Organomet. Chem. 1973,55, C7. Lappert, M. F.; Maskell, R. K. / . Organomet. Chem. 1984,264, 111. Corey, J. Y.; Chang, L. S.; Corey, E. R. Organometallics 1987,6,1595. Chang, L. S.; Corey, J. Y. Organometallics 1989,8,1885. Brown-Wensley, K. A. Organometallics 1987,6,1590. Curtis, M. D.; Epstein, P. S. Adv. Organomet. Chem. 1981,19, 213. Aitken, C. T.; Harrod, J. F.; Samuel, E. / . Am. Chem. Soc. 1986,108,4059. Harrod, J. F.; Mu, Y.; Samuel, E. Polyhedron 1991,10, 1239. References pi 8 Chapter 1 Tilley, T. D. Ace. Chem. Res. 1993,26,22. Tilley, T. D. Comments Inorg. Chem. 1990,10, 37. Fryzuk, M. D.; Jones, T.; Einstein, F. W. B. Organometallics 1984,3, 185. Fryzuk, M. D.; Jones, T.; Einstein, F. W. B. / . Chem. Soc, Chem. Commun. 1984, 1556. Fryzuk, M. D.; Jang, M.; Jones, T.; Einstein, F. W. B. Can. J. Chem. 1986,64,174. Fryzuk, M. D.; Piers, W. E. Organometallics 1988, 7, 2062. Fryzuk, M. D.; Piers, W. E. Polyhedron 1988, 7, 1001. Fryzuk, M. D.; Piers, W. E.; Rettig, S. J.; Einstein, F. W. B.; Jones, T.; Albright, T. A. / . Am. Chem. Soc. 1989, 111, 5709. Fryzuk, M. D.; Piers, W. E.; Einstein, F. W. B.; Jones, T. Can. J. Chem. 1989, 67, 883. Fryzuk, M. D.; Piers, W. E. Organometallics 1990, 9, 986. Fryzuk, M. D.; Rosenberg, L.; Rettig, S. J. Organometallics 1991,10,2537. Fryzuk, M. D.; Piers, W. E.; Rettig, S. J. Can. J. Chem. 1992, 70, 2381. Fryzuk, M. D.; McConville, D. H.; Rettig, S. J. Organometallics 1993,12,2152. Fryzuk, M. D.; McConville, D. H.; Rettig, S. J. / . Organomet. Chem. 1993,445, 245. Piers, W. E. Ph.D. Thesis, University of British Columbia, 1988. References pi8 Chapter 2 CHAPTER 2 Reactions of Secondary Silanes with [(dippe)Rh]2(ji-H)2: Complexes with a Single Silicon Ligand 2.1 Introduction Most studies of the reactivity of silanes toward rhodium complexes have arisen from investigation of their activity as hydrosilation catalysts. In particular, Wilkinson's catalyst ((PPh3)3RhCl) has received much attention as a catalyst for the hydrosilation of various unsaturated organic molecules,1 so it is not too surprising that the vast majority of known rhodium silyl complexes are derivatives of Wilkinson's catalyst and its analogues. (PR'3)3RhCI + HSiR3 - — i * ~ R3SiRhH(CI)(PR,3)2 [2-1] Silyl complexes of the formula shown in Equation 2-1 are known for a wide variety of substituents on silicon and for many different phosphine ligands.2"6 Another series of rhodium silyl complexes are derivatives of rhodium analogues of Vaska's complex ((PPh3)2lrCl(CO)).2 (PR'3)2RhCI(CO) + HSiR3 »~ R3SiRhH(CI)(CO)(PR'3)2 [2-2] As in the examples shown above, most rhodium silyl complexes are formed from the oxidative addition of a Si-H bond (usually from a tertiary silane) to a Rh(I) centre, thus the complexes generally contain rhodium in the +3 oxidation state. Exceptions include a few examples of Rh(I) silyl complexes,7,8 including the one shown in Equation 2-3.8 (PMe3)4Rh(CH3) + HSiPh3 " ^ »• (PMe3)3RhSiPh3 [2-3] - PMe3 References p69 Chapter 2 Also, reaction of silanes with some "half-sandwich", rhodium cyclopentadienyl complexes have produced silyl complexes (shown below) with rhodium in the rare +5 oxidation state.9,10 7 Rh Et3Si' fl H'SiEtg Et3Si fl H SiEt3 There are not many examples in the literature of dinuclear rhodium silyl complexes. In fact, addition of silanes to dinuclear rhodium complexes such as Rh2(PF3)s and [(CgHi2)RhCl]2 generates mononuclear products.11,12 Studies of the dinuclear rhodium complex Rh2H2(CO)2(dppm)2, however, have yielded dinuclear silyl complexes, including the one shown below.13 (dppm = bis(diphenylphosphino)methane.) Et Et H -4 k H oc\ P\ /co In this chapter the reactions of a single equivalent of a variety of secondary silanes with the dinuclear rhodium hydride complex [(dippe)Rh]2(|i-H)2,1, are presented, and the characterization and reactivity of the ensuing complexes are discussed. Complexes that include a single silyl ligand bridging two rhodium centres are discussed in this chapter, whereas those derivatives incorporating two silicon-containing ligands are presented in the next chapter, Chapter 3. The use of 1 as a catalyst precursor for the hydrosilation of olefins by diphenylsilane is also described. A possible hydrosilation catalytic cycle and likely mechanisms are proposed. References p69 Chapter 2 2.2 Reaction of [(dippe)Rh]2(|i-H)2, 1, with one equivalent of R2SiH2 2.2.1 Synthesis and properties of dinuclear silyl hydrides The dinuclear rhodium hydride complex [(dippe)Rh]2(|i-H)2,1, reacts rapidly with one equivalent of diphenylsilane to afford the dinuclear product of empirical formula [(dippe)Rh]2*Ph2SiH2, 2a, in good yield, with concurrent evolution of one equivalent of hydrogen.14 This reaction may be extended to dimethylsilane and methylphenylsilane, generating the analogous complexes 2b-c. - H 2 [(dippe)Rh]2(^i-H)2 + RR*SiH2 • • [(dippe)Rh]2»RR'SiH2 [2-4] 1  r, ™ r.u 2 a " C a: R = R = Ph b: R = R = Me c: R - Me, R" = Ph Isolated yields of these complexes vary from 60 to 85% depending on the scale used, due to the high solubility of these complexes in organic solvents. The complexes are air and moisture sensitive both in solution and in the solid state; they are thermally stable in solution as judged by the fact that heating to 80°C in ds-toluene in a sealed NMR tube for several days caused no decomposition, as determined by 31P{1H} NMR spectroscopy. References p69 Chapter 2 26 j J . u JL I 111 i i M 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 M 1 1 1 M 1 1 1 1 1 1 1 1 1 1 1 1 M I i i i i i i i i M [ M i 8 6 A 2 0 - 2 - 4 - 6 PPM Figure 2.1 300 MHz *H NMR spectrum of [(dippe)Rh]2»(Ph2SiH2), 2a, in C6D6. The room temperature lH and 31P{!H} NMR spectra of complexes 2a-c suggest highly symmetric structures in solution, where all four phosphorus centres are chemically and magnetically equivalent, as are both rhodium centres. A single rhodium hydride resonance, a sharp multiplet with a relative intensity of two, is observed in each *H NMR spectrum (see Figure 2.1). Symmetric doublet or doublet-of-multiplet patterns are observed in the 31P{1H} NMR spectra.15 These results are consistent with the structure shown in Figure 2.2, with two bridging hydrides and one bridging silylene ligand. However the broadness of the doublets observed in the 31P{1H} NMR spectra of 2a and 2c suggest that these compounds are fluxional; variable References p69 Chapter 2 27 temperature 31P{1H} and *H NMR spectroscopic studies have confirmed that some fluxional process is occurring in which the two hydrides exchange between inequivalent sites. Indeed some fluxional process must also be occurring to exchange the methyl and phenyl substituents on silicon for 2c, to give an average structure in solution with four equivalent phosphines. This is discussed in more detail in Section 2.2.3. R R' V _ p r i 2 / \ ^k a:R=R'=ph / r"".. . R £ ^ p u- l r t ' \ b: R = R = Me X - ^ p ^ ^ H I } - H n ^ ^ p ^ * c:R = Me,R = Ph Pr'2 \ ' H ^ P * Figure 2.2 Possible structure in solution of complexes 2a-c. 2.2.2 Solid-state structure of [(dippe)Rh]2(^-H)(^-ri2-H-SiR2) (R = Ph, 2a, R = Me, 2b) The solid-state structure of the dinuclear complex [(dippe)Rh]2(M--H)(p.-rj2-H-SiPh2), 2a, was evident from a single-crystal X-ray diffraction analysis; the analogous structure of [(dippe)Rh]2(M-H)(|i-T|2-H-SiMe2), 2b was also determined by this method. Both structures are different from the solution structures proposed in the previous section. ORTEP diagrams of the dinuclear complexes are shown in Figures 2.3 and 2.4, and selected bond distances and bond angles for the two structures are shown in Tables 2.1 - 2.4. In these structures the two hydride ligands were both located and refined. References p69 Chapter 2 28 Figure 2.3 Molecular structure of [(dippe)Rh]2(^-H)(^-T|2-H-SiPh2), 2a. References p69 Chapter 2 29 Figure 2.4 Molecular structure of [(dippe)Rh]2(|i-H)(|i-T|2-H-SiMe2), 2b. Table 2.1 Selected bond lengths for [(dippe)Rh]2(^i-H)({i-r|2-H-SiPh2), 2a. Bond RM-Rh2 R h l - P l R M - P 2 Rh l -S i R h l - H l Length (A) 2.937(1) 2.213(2) 2.295(2) 2.298(2) 1.71(6) Bond Rh2-P3 Rh2-P4 Rh2-Si Rh2-Hl Rh2-H2 Length (A) 2.221(2) 2.233(2) 2.487(2) 1.90(6) 1.61(6) Bond Si - C29 Si-C35 Si-H2 Length (A) 1.907(5) 1.902(5) 1.66(6) References p69 Chapter 2 Table 2.2 Selected bond angles for [(dippe)Rh]2(^-H)(^-ri2-H-SiPh2), 2a. Bonds P l - R h l - P 2 P 3 - R h 2 - P 4 R h l - S i - R h 2 Angle (deg) 87.02(6) 86.60(6) 75.60(5) Bonds R h l - H l - R h 2 R h 2 - H 2 - S i C 2 9 - S i - C 3 5 Angle (deg) 109(3) 99(3) 104.0(2) Table 2.3 Selected bond lengths for [(dippe)Rh]2([i-H)(ii-ri2-H-SiMe2), 2b. Bond R h l - R h 2 R h l - P l R h l - P 2 R h l - S i R h l - H l Length (A) 2.8835(6) 2.2738(9) 2.2154(9) 2.308(1) 1.65(3) Bond R h 2 - P 3 Rh2-P4 Rh2-S i R h 2 - H l Rh2-H2 Length (A) 2.246(1) 2.2165(9) 2.463(1) 1.91(3) 1.51(4) Bond S i -C13 S i -C14 S i - H 2 Length (A) 1.899(4) 1.899(4) 1.73(4) Table 2.4 Selected bond angles for [(dippe)Rh]2(^-H)(^i-r|2-H-SiMe2), 2b. Bonds P 1 - R M - P 2 P 3 - R h 2 - P 4 R h l - S i - R h 2 Angle (deg) 87.45(3) 86.71(3) 74.29(3) Bonds R h l - H l - R h 2 R h 2 - H 2 - S i C 1 3 - S i - C 1 4 Angle (deg) 108(2) 99(2) 99.8(2) The most interesting feature of these two structures is that while one hydride ligand bridges the two Rh centres, the other is found bridging a Rh-Si bond. The average lengthening of the bridged Rh-Si bond relative to the unbridged bond in the two molecules is 0.17 A, which is References p69 Chapter 2 31 consistent with the presence of the bridging hydride. The Si-H distances are only roughly 0.2 A longer than a typical Si-H bond length in free silane, 1.48 ± 0.02 A,16 and the Rh-H distances of 1.51(4) and 1.61(6) A are within the normal range for terminal or bridging hydrides on rhodium. As discussed below, three-centre, two-electron bonds such as these are no longer considered rare for Si-H interactions with transition metals. They are commonly viewed as coordinated Si-H a- bonds - essentially arrested oxidative additions. They can be referred to as "agostic" Si-H bonds, a term originally coined to describe the coordination of a C-H o-bond from an already-coordinated ligand to the metal centre.17 The strong trans influence of the silyl ligand (SiR3~) manifests itself in the two structures as a lengthening of the P-Rh distances for those phosphines trans to the silicon, relative to the other P-Rh distances in the structures.18 The most noticeable effects of the trans influence are in the structure of 2a, where P2-RI11 is -0.08 A longer than Pj-Rhj and P3~Rh2. In the structure of 2b Pi-Rhi is 0.06 A longer than P2-RI11 and P4-RI12. It could be argued that these effects are simply steric in origin, but in that case one would expect the trend to be reversed. For example, in 2a one would expect Pi and P3 to be pushed away from the phenyl groups on silicon, resulting in longer Pi-Rhi and P3-RI12 bond lengths. Instead these are the shortest P-Rh bond lengths. If the Si-H G-interaction with rhodium is viewed as a single ligand, the geometry at each of the rhodium centres in complexes 2a-b is roughly square planar, though the two planes are twisted relative to each other. The dihedral angles between the coordination planes defined by Pi, Rhi, P2 and by P3, Rh2, P4 in the two structures are 116.6° and 133.8° for 2a and 2b, respectively. This twisting of the chelate rings relative to each other is probably due mainly to the size of the bridging rj2-H-SiR2 ligand. The Rh-Rh separations observed in these structures are similar to a range of distances seen for the complexes [(P2)Rh]2(ji-H)(p,-X) (where P2 = dippe or (P(OMe)3)2; X = r]2- CR=CHR). For example, in the vinyl hydride complex [(dippe)Rh]2(fi-H)(|i-Ti2-CH=CH2), 5, the Rh-Rh distance is 2.8655(5) A19 while for [(P(OMe)3)2Rh]2(^-Ti2-C(TolP)=CH(TolP))(^-H) the separation is 2.936(1) A.20 References p69 Chapter 2 2.2.2.1 Discussion of agostic Si-H bonds The most extensively studied complexes containing three-centre, two-electron M-H-Si bonds are those of the formula Cp(CO)(L)Mn(r|2-H-SiR3). The preparation and characterization of this series have been reviewed, along with most other examples of this type of bonding.16 Some complexes of this formula have been the focus of theoretical studies. The three-centre, two- electron bond is generally viewed theoretically in the same way as coordinated H-H and C-H bonds: a-donation occurs from the filled a-molecular orbital (MO) of the Si-H bond to an empty orbital at the metal centre and "back-bonding", 7t-donation, occurs from a filled d-orbital on the metal to the a* (antibonding) MO of the Si-H bond.16,21,22 This has been demonstrated by extended Hiickel calculations using Cp(CO)2Mn(r|2-H-SiH3) as a model.23 -*— a - donation n - donation -*- Another study combines the results of Fenske-Hall MO calculations and valence photoelectron spectroscopy on the Cp(CO)(L)Mn(rj2-H-SiR3) series, where L, the silane, and the type of Cp have all been varied. These studies show that the extent of Si-H interaction with the metal centre is affected more by substitutions at silicon than at manganese.24 While not enough coordinated Si-H bonds have been discovered to determine whether a continuum exists between the extreme cases of the dissociated Si-H bond and the oxidatively added Si-H bond, this certainly seems to be the case for coordinated C-H.21 It is generally assumed that the same situation exists for the Si-H ligand, and that these coordinated a-bonds represent the transition state for oxidative addition of the Si-H bond. Interestingly, one theoretical study seems to suggest that for some LnMHSiR3 complexes, when the Si-H bond is completely <w- References p69 Chapter 2 33 broken there is no change in the formal oxidation state of the metal, and instead the bonding involves formal H" and SiR3+ ligands. The authors attribute this non-oxidative addition to intermixing of the four relevant MOs, due to an absence of symmetry in these complexes.23 As well as the extensive family of manganese complexes with agostic Si-H bonds, a series of (T|6-arene)(CO)2Cr(T|2-H-SiHPh2) complexes has been reported,25,26 along with a number of dinuclear compounds containing coordinated Si-H bonds, some of which are shown below.27"31 In these structures the features of the three-centre, two-electron bond are analogous to those in mononuclear complexes, wherein the second metal centre can simply be considered as a third R- group on silicon. H Ph H Ph y y Si"—"" ̂  Si"""—^ Cp/i i i .^Tf \ j j . . . . , . n iCp C p m ^ ^ j j / >.y.....uttCp C p ^ \ / ^ * C p C P \ \ / ^ * C p H li^Si A Ph H VI27  V I I 2 7 Me, R 2 S i I2-H H -.Si H PR'; / W PR'3 . I X X I .Pt - P f " Cp Ru , , H»». . . . ^ R u — V ^ S i W-CeHii | \ / H  Ms„ H -Si R = Et" e2 VIII28 IX31 R2 It is of interest to estimate the extent of oxidative addition which has taken place in complexes with agostic Si-H bonds. While spectroscopic studies can give some information on this topic,16 X-ray crystallography gives the most reliable indication. In particular the M-Si distance is sensitive to the electronic distribution among MOs involving the three centres. The References p69 Chapter 2 34 longer the M-Si distance is, relative to "normal" M-Si single bonds, the less oxidatively added is the Si-H bond, and conversely, the closer this distance is to "normal" the more oxidatively added is the Si-H bond. For the series of manganese complexes described above, the Fe-Si distance in Cp(CO)2FeSiFPh2 was chosen as the standard M-Si distance for comparison,32 based on the similarity of the two metal fragments, and allowing for the increased bonding radius of the metal.16 For dinuclear complexes where the agostic Si-H ligand is in a bridging position the conventional two-centre, two-electron bond between Si and the second metal centre provides an "internal standard". The difference in M-Si bond lengths within these complexes directly reflects the different types of bonding involved. For example, in the titanium complex VI shown above, the conventional, two-centre Ti-Si bond is 2.61(2) A long and the Ti-Si distance for the three-centre bond is 2.78(2) A, a lengthening of 6.5%. The largest bond lengthening observed for any of the mononuclear manganese complexes is 4%. Thus, in the dinuclear titanium complex, addition of the silane to the metal is arrested at a much earlier stage than in any of the manganese complexes. For the second titanium complex shown, VII, the average bond lengthening is actually 10%; the coordinated silane in this complex is the closest to being dissociated that is known. Comparison of these bond lengths in the structure of [(dippe)Rh]2(|i-H)Qi-r|2-H-SiMe2), 2b, (a lengthening of 6.7%) with the corresponding lengths observed for other dinuclear complexes with agostic Si-H bridges suggests that the oxidative addition of the Si-H bond to rhodium in 2b has been arrested at an intermediate stage. For 2a the difference between bridged and unbridged Rh-Si bond lengths within the molecule is 0.19 A (or a lengthening of 8.2%), which is larger than for 2b. Thus for 2a the oxidative addition of the Si-H bond appears to have been arrested at a relatively early stage. It should be noted that the Si-H bond distances in agostic Si-H ligands also tend to reflect the extent of oxidative addition of the Si-H bond; in most cases where there is a decrease in the M- Si distance throughout a series of similar complexes, there is a corresponding increase in the Si-H distance. However the Si-H bond lengths are generally not known as accurately as the M-Si bond lengths, and therefore are not so sensitive as indicators of electronic structure. References p69 Chapter 2 35 2.2.3 Variable temperature 3 1P{1H} and *H NMR spectroscopy of 2a-c The variable temperature 3 1 P{ 1 H} and lH (hydride region) NMR spectra of [(dippe)Rh]2(|J.-H)(|j.-Ti2-H-SiPh2), 2a, are shown in Figures 2.5 and 2.6. While at room temperature the 31P{ lH} NMR spectrum of 2a is a slightly broad doublet, at lower temperatures the spectrum becomes more complex, and at -96°C the spectrum shows a set of signals attributable to the presence of four inequivalent phosphines, which is consistent with the solid state structure of 2a. The presence of a single hydride resonance for both the Si-H and the Rh-H-Rh moieties in the room temperature *H NMR spectrum suggests that the fluxionality in 2a involves exchange of the bridging hydride Hb and the silicon "hydride" Ha. A possible mechanism for this exchange (see Scheme 2.1) involves the complete oxidative addition of Si-Ha to give a bridging silylene fragment and a terminal hydride which subsequently swings into a bridging position, yielding a symmetric intermediate with the structure X. The reverse of these steps, with Hb swinging out to the terminal position instead of Ha, would exchange the two hydrides.33 A complex with similar geometry to the proposed intermediate X, [(P(OPri)3)2Rh]2((i-H)2([i-CO), has been observed spectroscopically during addition of one equivalent of carbon monoxide to an analogous dinuclear hydride incorporating monodentate phosphite ligands, [(P(OPr')3)2Rh](|0.-H)2.34 The changes in geometry at the rhodium centres required to accommodate this hydride exchange process are the most likely cause for the complexity of these temperature dependent 31P{ 1H] spectra. References p69 Chapter 2 * J \. 20°C 45°C 76°C 55°C 86°C JJ^J uut -66°C i ) i ] i i i i i m i | i i i i | i i i i | i i i i | i i i i | i i i i | i n i | 110 100 90 80 PPM 70 | U l l ] l l l l | l l l l | i l l l | l l l l | l l l l | l l l l | l l l l | l l i l | 110 100 90 80 PPM 70 Figure 2.5 Variable temperature 121.4 MHz 31P{1H} NMR spectra of [(dippe)Rh]2(^-H)(|i- ri2-H-SiPh2), 2a, in C7D8. The "*" marks an impurity, complex 1. References p69 Chapter 2 Ph Ph V Ha^rSi Pr'o A p * \>i K P r i Ph Ph u \ / \ ,Rh Rh Pr'2 P. 2P^V X / \ o p- Pr'2 5 * Ph Ph .Si Ph Ph u Hb-rSi Pr». •P. V-̂ Rh "Rh"*' \ Prj, Hs Hi A Ph Ph Prj2 P- P r 2 p ^ y \ / \ ^ O Hs P* Prj2 Rh / \.s PH, X Scheme 2.1 Rh.  P Pr'c The variable temperature *H NMR spectra shown in Figure 2.6 are also consistent with the hydride fluxionality described: the spectrum at the low temperature limit shows two distinct hydride signals at -5.28 ppm and -6.26 ppm. A rough JH-P value of 40 Hz observed for the lower field signal (-5.28 ppm) is larger than the usual cis H-P coupling but smaller than most trans coupling constants measured in similar compounds. Because of the broadening due to coupling to phosphorus and rhodium no 29Si satellites are seen for either of these hydride signals. It seems reasonable to conclude that the three-centre, two-electron Rh-H-Si bond observed in the solid state References p69 Chapter 2 38 structure of 2a does exist in solution but is labile with respect to complete oxidative addition of the Si-H bond to the Rh centre, rendering the silyl hydride complex fluxional. A AG$(193K) of 8.6±0.2kcal/mole for this fluxional process was calculated for [(dippe)Rh]2(n-H)(^-T|2-H- SiPh2), 2a, using the coalescence of the hydride signals in the 1H NMR. 20°C -66°C •71 °C -55°C -76°C 91 °C -61 °C -81 °C mnTmirTninirnnrpnirrmrmT inirrjiTnTrriqTrnninrrnTmnjTrrr i [ 1111111 n p-rrrrrrriTn r -< -S -fc -7 PPM -4 -B -* - 7 PPM - 4 - 6 HI Figure 2.6 Variable temperature 300 MHz *H NMR spectra (hydride region) of [(dippe)Rh]20i-H)(^-T|2-H-SiPh2), 2a, in C7D8. y I7TIJ1111J PPM -• For the methylphenylsilane analogue 2c, [(dippe)Rh]2(M--H)(p.-T|2-H-SiMePh), the activation barrier for exchange of the hydrides must be lower, since a low temperature limit for the hydride signals was not achieved even at -84°C. As shown in Figure 2.7, at that temperature the References p69 Chapter 2 39 31P{1H}NMR spectrum of 2c shows three signals: a broad doublet at 97.8 ppm, a doublet at 92.8 ppm and a broad singlet at 87.2 ppm, in a 2:1:1 ratio. i i i i M J I i 1 i i i I | M 11 j I I I I I I I 1 I I I I I ) I I I I i j I 1 I i I i ] M I 115 110 105 100 95 90 85 80 75 PPM 70 Figure 2.7 121.4 MHz 3lp{lH} NMR spectrum of [(dippe)Rh]2(^-H)(^-ri2-H-SiMePh), 2c, in C7D8 at -84°C. A similar spectrum (three doublets of integral ratio 2:1:1) is observed for 2a at -55°C, with almost identical chemical shift separations between the peaks. The *H and 31P{1H) NMR spectra of [(dippe)Rh]2(H-H)(|i-T|2-H-SiMe2), 2b, show even less change at low temperature, than those for the methylphenylsilyl analogue. For 2b there is only a slight broadening of the doublet observed in the 31P{ !H} NMR spectrum and of the hydride signal at -6.3 ppm in the *H NMR spectrum at -85°C. Given that all three complexes 2a-c show resonances in their room temperature 31P{1H}NMR spectra with practically identical chemical shifts, and given the similar shifts observed in the low temperature 31P{ *H} NMR spectra of 2a and 2c, it is likely that the low temperature limit for the 31P{ 1H} spectra of all three complexes would show peaks with similar shifts. Thus the differences in coalescence temperatures for 2a-c result mainly from differences in the activation barrier to hydride exchange. There is a steady reduction of this activation barrier to References p69 Chapter 2 exchange in complexes 2a-c which corresponds to the stepwise replacement of bulky phenyl groups with methyl groups; AG^a > AG^c > AG^b f° r the process. The exchange mechanism in effect for these molecules in solution does not explain why the room temperature 3 1P{ !H} NMR spectrum for [(dippe)Rh]2(H-H)(|i-ri2-H-SiMePh), 2c, indicates the presence of four equivalent phosphines despite the unsymmetrically-substituted silicon in the average structure. This result requires the substituents on silicon to be exchanging very rapidly. One explanation for this is that while the complex is in the ground state the coordinated Si-H bond is rapidly dissociating from and re-associating with the metal centre, allowing rotation around the remaining Rh-Si bond and inversion at the Si centre. Another explanation for the exchange of substituents on silicon in 2c is that the coordinated Si-H bond is oxidatively adding to the Rh2 core, generating a Rh-Si double bond and a terminal hydride. Rotation around the Rh-Si double bond, followed by reductive elimination of the Si-H completes the exchange. (Rh-Si double bonds are discussed further in Section 3.2.4, in Chapter 3.) Both of these possible mechanisms are shown in Scheme 2.2. If this process is occuring for 2c, it is probably occurring for 2a-b as well. References p69 Chapter 2 41 Me Ph Rh Rri \ Me Ph V H~-Si PH2 ^ r/ \ ^p' -Rh Rh \ Ph Me V H*-S i PH2 Rh .Rh \ PrU PH, V - ^ P r i Prv MePh P. C 5S >*P\. ^ •Rh :Rh ^ ^ Pr«a P PH5 Scheme 2.2 2.2.4 Reaction of the silyl hydride complexes 2a-c with hydrogen When solutions of 2a-c are placed under an atmosphere of hydrogen the solution colour changes somewhat from a dark red to a lighter red. Changes in the JH and 31P{ *H} NMR spectra indicate that a hydrogen adduct has formed (3a-c). For example, in the room temperature *H NMR spectrum of 3a the hydride resonance has shifted, changing from a sharp septet of relative Referencesp69 Chapter 2 intensity two (for 2a) to a broad singlet at -10 ppm surrounded by less intense, broad singlets at -6.3 ppm and -13.4 ppm, with a total relative intensity of four. This and similar features in the *H NMR spectra of 3b-c show the presence of four hydrides per dinuclear unit, indicating that a single equivalent of dihydrogen has been added. The same spectra are obtained whether the samples have been sealed under one atmosphere or four atmospheres of hydrogen. Removal of the hydrogen atmosphere from the "silyl trihydrides" 3a-c regenerates the corresponding silyl hydrides 2a-c. + H 2 [(dippeJRhk^-HX^-Tf-H-SiRR')  g » {[(dippe)Rh]2(ji-H)(^-ri2-H-SiRR,)}»H2 [2-5] 2a-c -H2 a: R = R' = Ph b: R = R1 = Me c: R = Me, R' = Ph 3a-c The reaction shown in Equation 2-5 is directly analogous to the reaction observed upon addition of hydrogen to the starting dimer, [(dippe)Rh]2(|i-H)2,1. The hydrogen adduct of 1 is a fluxional tetrahydride species, and variable temperature *H and 31P{1H} NMR spectroscopic studies led to the proposed mechanism of hydride exchange shown in Scheme 2.3.35 H2 Re p A / \ Rh R9 -: R = Prj. D2 I D2 ^•Rh^ffeRh-'' Hb" ^ P Ht R, Hh. A X <s R2 ..->Rh 3K R2 pt/ \ / \"R H, R2 & Scheme 2.3 References p69 Chapter 2 43 24<C 25°C 55°C l^JKs^h -44°C -85°C | l l l l l l l l l | l l l l | l l l l | l l l l | l l l l | l l l l [ l l l l | l l l l | l l l l | | l l l l | ) l l l | l l l l | l l l l | I i l l | l l l l | I H I | l l l l | l l l l | l l l l - 6 - B - 1 0 - 1 2 P P M - 1 4 - 6 - B - 1 0 - 12 PPM-14 Figure 2.8 Variable temperature 300 MHz !H NMR spectra (hydride region) of {[(dippe)Rh]2(u-H)(u-Ti2-H-SiPh2)}»H2, 3a, in C7D8. References p69 Chapter 2 The hydrogen adducts 3a-c, like the silyl hydrides 2a-c and the tetrahydride species (1»H2) described above, are highly fluxional in solution, as demonstrated by variable temperature NMR studies. Temperature dependent NMR spectra for 3a are shown in Figures 2.8 (*H NMR spectra) and 2.9 (31P{ lH} NMR spectra).The mechanism of this fluxionality is difficult to deduce from the highly complex hydride signals in the *H NMR spectra observed at low temperature. Spin-lattice relaxation times (Ti's) have been measured for the hydride signals observed for 3a at -85°C, where the signals are best separated, and all fall within a range of 300 - 400 ms. These values are typical for classical metal hydrides,36,37 and therefore no ri2-H2 ligands are suspected. In the room temperature 31P{ *H} NMR spectrum of 3a the slightly broad doublet seen at room temperature for 2a has changed to an unsymmetric, broad peak shifted from 93.4 ppm to 96.4 ppm. The 31P{1H} NMR spectra at low temperatures indicate a lack of symmetry in the adducts but are too complex to provide much information about the structures in solution. In fact the low temperature 31P{ lH} NMR spectra of both 3a and 3c seem to suggest that more than one species is present in solution; perhaps one major and one minor isomer is forming. (The temperature dependent 31P{1H) NMR spectra for 3c are shown in Figure 2.10.) Apart from the complexity of the hydride region of the low temperature spectra, the variable temperature *H NMR spectra of 3a show no evidence for the presence of more than one isomer in solution. Low temperature *H NMR spectra for 3c, however, do show the signal due to the ortho protons splitting into two signals of the integral ratio 2.5:1. While the hydride region of the variable temperature *H NMR spectra of [(dippe)Rh]2(|i-H)(|i-Ti2-H-SiMe2), 3b, is directly analogous to that seen for 3a and 3c, the variable temperature 31P{ *H} spectra for 3b (shown in Figure 2.11) are considerably simpler and are consistent with the presence of a single compound in solution. References p69 Chapter 2 45 -5°C I w -25°C | i i i i | i i i i i 1 1 I l I 1 1 i i I i i i I I 1 1 I i | 110 105 100 95 90 85 PPMBO I -45°C Ui -65°C JW -84°C I i i I I I i i I | i 1 1 1 1 1 1 1 I | I I i I I 1 1 i 1 105 100 95 90 85 PPM Figure 2.9 Variable temperature 121.4 MHz 31P{!H} NMR spectra of {[(dippe)Rh]2(H-H)( î- Ti2-H-SiPh2)}«H2, 3a, in C7D8. References p69 Chapter 2 46 _J«i r^J __r ^j^ -70°C I I j I 1 1 1 I 11 I 1 1 I I 1 1 M 1 I I j i I I I I M l 110 105 100 85 SO 85 PPM I j I I I I I I I 1 1 I I I I I 111 ! I I I 1 1 I I I I 110 105 100 85 80 85 P PH Figure 2.10 Variable temperature 121.4 MHz 31p{lH} NMR spectra of {[(dippe)Rh]2( î- H)(^-Ti2-H-SiMePh)}»H2,3c, in C7Dg. The "*" marks signals due to an impurity, the hydrogen adduct of 1. References p69 Chapter 2 47 M M ' J "° -41 °C 55<C -10°C J \ -24°C A/1 -86°C | i i i i | i i i i l i i i i i ) i i ] | i i i i | i i n ] i i i i | ) i i i | i i ) i i n i ) l | ! i i i j i i i i | i ] i i ] i i i i j i i i i | i i i J | i i i i j i i i i | i n i | i i i i 110 100 90 BO 70 PPM 110 100 SO 80 70 PPM Figure 2.11 Variable temperature 121.4 MHz 3 IP{ 1H} NMR spectra of {[(dippe)Rh]2(^i- H)((i-ri2-H-SiMe2)}*H2,3b, in C7D8. The "*" marks an impurity, [(dippe)Rh(H)]2(^-Tl2-H-SiMe2)2,8b (see Chapter 3). References p69 Chapter 2 48 Possible solution structures and fluxionality of these silyl trihydride species, based mainly on analogy to the tetrahydride complex described above (1»H2) are shown in Schemes 2.4 and 2.5. The structures in these schemes have less symmetry when the silicon is unsymmetrically substituted, as for methylphenylsilane. The 31P{ 1H} NMR spectra for 3c are consistent with this loss of symmetry, as they show more peaks than those for 3a-b. R FT V Prj: .P« PH2 Pr'2 P«»... P * ^ PH. HbR Hb bi»«'Ha R h - f ' ^ - R h : I R>H b^ Pr'. •»w Ht 1 R R' 'P PH2 SiHRR' '"'••• Rh-, ,%'~'b'''.Rh'" i t « Pr". Ht 3 a: 3 b: 3 c: R« R. R = = R' - Ph • ff. • Me s M e . R - Ph A Pr'2 P •P Pr1. H, JSi-H \ .;>Hb^v HRR'Si t n Rh PH. Pr1? ...Rh £/ V \s \ / H ^ . Pr1? ...Rh ^ R h Pr1? a, v ^ Scheme 2.4 References p69 Chapter 2 RR' V: PrV*:Rvh. .->Rh Prj2 RR' \ PrU A? Prj2 3 w ^ . • / \ f ^ P ^ 3a: R = R" = Ph 3b: R = R' = Me 3c: R = Me,R' = Ph P r ^ R h ..vRh Pr1! " " H ^ \ H, P ^ .•* D ^ H, R R' PH2 \! P O / \ < ^ P PiV*Rh . . R h p r L Scheme 2.5 Interestingly enough, even at low temperature there are no signals in the *H NMR spectra of 3a-c in the region where signals due to hydrides bound to silicon are normally observed. This seems to suggest that the hydride exchange mechanisms which are the expected source of fluxionality in the hydrogen adducts do not include the species with a terminal -SiR2H group shown in Scheme 2.4. While this might indicate that dissociation of the agostic Si-H bond from the metal does not occur during the exchange process, it could also mean that the low temperature limit for the exchange process as monitored by NMR was not achieved. There is no evidence that the addition of excess hydrogen to the silyl hydrides 2a-c causes dissociation of the silane and formation of the tetrahydride species 1»H2- No free silane is detected in any of the XH NMR spectra of 3a-c; apparently the addition of silane to 1 is not a reversible reaction. When degassed solutions of 2a-c are placed under an atmosphere of deuterium for 24 hours, analogues can be isolated that show 90% exchange of the hydrides for deuterides, as determined by *H NMR spectroscopy. Undoubtedly a deuterated version of the fluxional References p69 Chapter 2 hydrogen adduct described above forms initially, and in the presence of excess D2 intermolecular exchange is occurring to give the completely deuterated complex. Removal of the deuterium gas generates the complexes d2-2a-c. 2.2.5 Mechanistic considerations in the formation of [(dippe)Rh]2(y.-H)(|i-Ti2-H- SiPh2) , 2a The reaction of [(dippe)Rh]2(|!-D)2, d2-l, with one equivalent of diphenylsilane yielded a mixture of do-, di- and d2-2a, as determined by *H and 2H NMR. This result is consistent with a mechanism in which initially a Si-H bond from diphenylsilane oxidatively adds to the RI12H2 centre (or RI12D2), generating an intermediate silyl trihydride species. If this species is the same as the hydrogen adduct 3a formed upon addition of H2 to 2a, it is highly fluxional with all three hydrides and the silyl hydride exchanging. The exchange of the hydrides must be occurring more quickly than the elimination of dihydrogen to form the silyl hydride, resulting in scrambling of the deuterium label in the reaction using d2-l (see Schemes 2.4 and 2.5 and Equation 2-6). [(dippe)Rh]2(n-H)2 + 1Ph2SiH2 1 s> X 2 ; r >7 PiS P r ^ R h J \  Pri H - H 2 [(dippe)Rh]2(^-H)(^-if-H-SiPh2) 2a [2-6] References p69 Chapter 2 51 2.2.6 Displacement of hydrogen by carbon monoxide in the silyl hydride complexes 2a-c Dinuclear rhodium complexes with bridging carbonyl and silylene units (4a-c) are generated by the addition of approximately one equivalent of CO to the silyl hydrides 2a-c. Complex 4a has been isolated in pure form, and complexes 4b-c have been observed in solution, though they were not isolated pure in the solid state. Addition of an excess of CO to the silyl hydride complexes gives a mixture of products other than 4a-c which are difficult to separate from each other and which have not been identified. The approximately 1:1 reactions of the silyl hydrides with CO are of interest though, because they demonstrate the ease with which hydrogen is removed from the Rh2Si core. The complete oxidative addition of the second Si-H bond of diphenylsilane to Rh is facile, as is the subsequent elimination of H2. [(dippe)Rh]2(^-H)(|i-r|2-H-SiR2) + 1-2CO ""* » FT R 2a ' C  PH A  p i ,--£/ \ _ J X [2-7] a:R = R' = Ph2 P r i \ X p r l b:FUR = Me2 • '2 \ r P f ' 2 c: R = Me, R' = Ph O 4a-c The 31P{1H} NMR spectra of complexes 4a-c show simple doublets of multiplets, indicating that in solution all four phosphines are chemically equivalent. A structure which is consistent with the NMR data contains tetrahedral rhodium. For the unsymmetrically substituted fi-methylphenylsilylene complex, 4c, the simple 31P{1H} NMR spectrum must be due to fluxionality of the complexes in solution: a twisting of the phosphine chelate rings would exchange the phosphine in each ring which is proximal to the methyl group on silicon for the one which is proximal to the phenyl group on silicon, and vice versa. Referencesp69 Chapter 2 2.3 Catalytic hydrosilation of olefins The complex [(dippe)Rh]2(n-H)2,1, has been shown to be an efficient catalyst precursor for the hydrogenation of olefins at ambient temperature and pressure. For example, its use as a catalyst precursor for the hydrogenation of 1-hexene (with a substrate/catalyst ratio of approximately 1700) gives turnover numbers in the range of 850 - 950 hr1.35 Given the reactivity of this complex towards both hydrogen and olefins, one of the initial goals of this project was to confirm that 1 is also a catalyst precursor for the hydrosilation of olefins. Many Rh(I) complexes are catalysts for hydrosilation reactions; examples include RhCl(PPh3)3, RhCl(CO)(PPh3)2, HRh(CO)(PPh3)3, [Rh(CO)2Cl]2 and [Rh(C2H4)2Cl]2.1 Then- activities are high, though they are consistently lower than that of chloroplatinic acid (H2PtCl6#6H20), which is the most commonly used catalyst for the hydrosilation of a wide range of olefinic substrates.1 For Rh(I) catalysts containing tertiary phosphine ligands it has been found that addition of oxidants to the catalysts will accelerate the hydrosilation. This is thought to be due to the oxidation of excess phosphines, generating active species such as Rh(PPh3)yX (y < 2).38 Complexes like 1 which contain chelating bis-phosphine ligands and are coordinatively unsaturated already fit the formula proposed for the active species in these catalytic cycles, thus no oxidants need be added for control of the concentration of phosphine in the catalytic cycle. 2.3.1 The hydrosilation of ethylene and 1-butene The complex [(dippe)Rh]2(H-H)2,1, was found to be a catalyst precursor for the hydrosilation of both ethylene and 1-butene by diphenylsilane. The product of hydrosilation of ethylene is the double-addition product diethyldiphenylsilane, Et2SiPh2. None of the single addition product, ethyldiphenylsilane, was observed in reaction mixtures, even before completion (ie. disappearance of diphenylsilane), so it must proceed rapidly through the second hydrosilation References p69 Chapter 2 reaction. When this catalytic reaction was monitored by *H NMR, a turnover number of 960 h r 1 was calculated. The product of hydrosilation of 1-butene is simply /i-butyldiphenylsilane, BunSiHPh2, the expected anti-Markovnikov addition product.39 Stoichiometric reactivity studies relevant to the hydrosilation of ethylene catalyzed by the presence of small amounts of 1 were carried out to determine if the active species in the catalytic cycle could be identified. The reactions studied are summarized in Equations 2-5 through 2-8. When ethylene is added to [(dippe)Rh]2(|i-H)2,1, a vinyl hydride complex, [(dippe)Rh]2(n-H)(|i- 712-CH=CH2), 5, is formed.4 H [(dippe)Rh]2(ji-H)2 + xs C2H4 1 - H 2 Prj2 JZ-H Prj2 \ / \ / ^ Rh Rh [2-8] Pr': Pr'; When ethylene is added to [(dippe)Rh]2(H-H)(|i-ri2-H-SiPh2), 2a, the same vinyl hydride complex is isolated, along with diethyldiphenylsilane. However when diphenylsilane is added to the vinyl hydride complex, 5, the silyl hydride complex 2 a is regenerated. No diethyldiphenylsilane is observed in the *H NMR spectrum of this reaction, shown in Equation 2- 6; it is assumed that addition of the silane to the vinyl hydride causes elimination of ethylene. [(dippe)Rh]2(n-H)(ii-rf-H-SiPh2) + xs C2H4 2a [2-9] [(dippe)Rh]2(n-H)(^-riz-CH=CH2) + Ph2SiEt2 References p69 Chapter 2 [(dippe)Rh]2(^-H)(^-Ti2-CH=CH2) + iPh2SiH2 — — [2-10] [(dippe)Rh]2(n-H)([i-Ti -H-SiPh2) 2a The bis(|j.-silylene) complex [(dippe)Rh]2(|J.-SiPh2)2,6a, is certainly present in any mixture of 1 and excess silane (see Chapter 3 for more on this complex). No reaction is observed between 6a and ethylene, which rules out the possible participation of the bis(ji-silylene) complex in the hydrosilation cycle. Pr1 Pr'5 Ph2 p " Si p„ \ / \ / Rh Rh D / \ / \ P. Si P' + xs C2H4 •*- No Reaction [2-11] r1, Ph2 6a Pr1, The above reactions suggest that the hydrosilation of ethylene catalyzed by 1 could involve 2a as the active catalyst. A possible catalytic cycle for the hydrosilation of ethylene based on this dinuclear active species is shown in Figure 2.12. If, in the presence of excess silane, the catalyst is converted to 6a, this would "shut down" the hydrosilation. However, with every oxidative addition of Ph2SiH2 to either 1 or 2a, hydrogen is evolved. Reactions described in Chapter 3 verify that in the presence of hydrogen, the bis(fi-silylene) complex, 6a, is converted back to the silyl hydride complex, 2a. Thus, in the catalytic reaction mixture where hydrogen is constantly being evolved, there should always be local concentrations of hydrogen that are enough to regenerate 2a and continue the catalytic cycle. References p69 Chapter 2 55 Rh- -Rh ph2siEt2r> ^ p h 2 s r CH5 ,CH2 Ph2SiH2 y y H / \ Rh Rh + Ph2SiH2 V Ph2 H—,Si Rh Rh 2a CH; Ph 2 s j^C H= / Rh- N/ •Rh C,H 2 n 4 Ph2 H---Si Rh Rh VI insertion of olefin into Rh-H bond, addition of agostic Si-H to Rh centre Ph2 A Rh Rh H CH2 CH3 reductive elimination of SJ-CH2CH3 Figure 2.12 A possible catalytic cycle for the hydrosilation of ethylene based on the dinuclear active catalyst, [(dippe)Rh]2(n-H)(|i-r|2-H-SiPh2), 2a. (The chelating phosphine ligands have been omitted to simplify the diagram.) References p69 Chapter 2 The rhodium complex that would form after reductive elimination of the second Si-C bond is an unbridged Rh(0) dimer. This species has never been isolated or detected spectroscopically in reactions of 1, but has been proposed as an intermediate in some reactions of the dimer.40,41 The steps in this catalytic cycle parallel those in the cycle originally proposed for hydrosilation catalyzed by mononuclear platinum-group metal complexes in that the olefin inserts into a metal-hydride bond and the silicon-carbon bond is formed by reductive elimination of adjacent silyl and alkyl groups 4 2 R3 R3 Si Si [2-12] U H „ I Ln M — • Ln M—CH2CH3 ^ LnM + R3SiCH2CH3 Recent evidence, however, suggests that for some catalysts the silicon-carbon bond-forming reaction is an insertion of the olefin into the metal-silicon bond, which is then followed by reductive elimination of 6-silyl-alkyl and hydride ligands.43 R3 Si H [2-13] Ln M — +- Ln M-CH2CH2SiR3 »- LnM + R3SiCH2CH3 In the system studied here, there is no evidence supporting one mechanism over the other, except that no vinylsilanes were conclusively identified in the product mixture. Vinylsilanes might be expected as side products if the olefin inserts into the M-Si bond during the catalytic cycle, so their absence supports the catalytic cycle shown in Figure 2.12. Of course, the fragmentation of the dinuclear silyl hydride complex into very reactive mononuclear species that carry out the catalysis before recombining to give the observed dinuclear complexes is also a mechanistic possibility that cannot be ruled out. To discover if the silyl hydride complex 2a undergoes fragmentation/recombination reactions in solution a mixture of 2a and [(dippp)Rh]2(|i-H)(ji-r|2-H- SiPh2), 2g, in toluene was stirred and heated to 100°C. No signals due to the crossover product [(dippe)Rh](|i-H)(|!-r|2-H-SiPh2)[Rh(dippp)] were observed in the 31P{1H} NMR spectrum of References p69 Chapter 2 the residues of this reaction. Either no fragmentation occurred, or fragmentation occurred only for one of the two complexes; as such this result is inconclusive. Given the observed stoichiometric chemistry of 2a, though, a catalytic cycle for hydrosilation based on the dinuclear complex seems reasonable. 2.4 Experimental 2.4.1 General procedures and reagent syntheses All manipulations were performed under prepurified nitrogen in a Vacuum Atmospheres HE-553-2 workstation equipped with an MO-40-2H purification system or using Schlenk-type glassware. The term "reactor bomb" refers to a cylindrical, thick-walled Pyrex vessel equipped with a 5 mm (or 10 mm, for larger bombs) Kontes teflon needle valve and a ground glass joint for attachment to a vacuum line. NMR tubes used for preparing sealed samples are 8" or 9" Wilmad 507 PP NMR tubes with a female bl4 joint attached by glassblowing. Toluene and hexanes were pre-dried over CaH2 then deoxygenated by distillation from molten sodium and sodium-benzophenone ketyl, respectively, under argon. Pentane and diethyl ether were dried and deoxygenated by distillation from sodium-benzophenone ketyl under argon. Di-H-butyl ether was dried by refluxing over CaH2- Deuterated benzene (C&e, 99.6 atom % D) and deuterated toluene (C7D8,99.6 atom % D) were purchased from MSD Isotopes and dried over 4A molecular sieves. The dried, deuterated solvents were then degassed using three "freeze-pump- thaw" cycles and were vacuum transferred before use. Carbon monoxide, ethylene and 1-butene were purchased from Matheson Gas Products. Deuterium gas was purchased from Matheson and passed through a glass coil immersed in liquid nitrogen to remove any traces of water and oxygen. Hydrogen gas was purified by being passed through a column packed with activated 4A molecular sieves and MnO. References p69 Chapter 2 lH NMR spectra were recorded on Varian XL-300, Bruker WP-200, Bruker WH-400, or Broker AMX-500 spectrometers. With d6-benzene as solvent the spectra were referenced to C6D5H at 7.15 ppm and with ds-toluene as solvent the spectra were referenced to the CD2H residual proton at 2.09 ppm. 31P{1H} NMR spectra were recorded at 121.4 MHz on the Varian XL-300 or at 202.3 MHz on the Bruker AMX-500 and were referenced to external P(OMe)3 at +141.0 ppm relative to 85% H3PO4. !H{31P} NMR spectra were recorded on the Bruker AMX- 500. 2H{ lH] NMR spectra were run in C6H6 at 46.0 MHz on the Varian XL-300 and were referenced to residual solvent deuterons at 7.15 ppm. ^ S i ^ H } and 29Si NMR were run at 59.6 MHz on the Varian XL-300 and were referenced to external TMS at 0.0 ppm. Elemental analyses were carried out by Mr. P. Borda of this department. Gas chromatography/mass spectrometry was carried out by Ms. L. Madilao of this department. Literature methods were used to prepare [(dippe)Rh]2(M--H)2, l,19*44 [(dippe)Rh]2(|i-D)2, d2-l,19 and [(dippp)Rh]2(^-H)2.44 Ph2SiH2 and MePhSiH2 were purchased from the Aldrich Chemical Company, dried by refluxing over calcium hydride overnight and distilled. Me2SiH2 was prepared by reduction of Me2SiCl2, purchased from Aldrich, using LiAlH4, in the modified procedure described below.45 Me2SiH2. The solvent, Bun20 (350 mL) was distilled under nitrogen into a 500 mL, three-neck round-bottom flask containing LiAlH* (4.1 g, 31.8 mmol) and a stir bar. The flask was attached via a dry ice/acetone condenser to a T-joint. Attached to the other side of the T-joint was a 250 mL reactor bomb which had already been evacuated, sealed and cooled in liquid nitrogen. The entire apparatus was connected to a vacuum line through the T-joint. The round-bottom flask was cooled to -78°C and, under a strong flow of nitrogen, Me2SiCl2 (6.86 g, 53.2 mmol) was added by syringe. The flask was then frozen in liquid nitrogen, and the head space was evacuated. The entire apparatus was then placed under static vacuum, the reactor bomb was opened to the T-joint, and the reaction mixture was allowed to warm to room temperature slowly. As warming References p69 Chapter 2 59 commenced, the system was evacuated several times to remove nitrogen formerly dissolved in the solvent. By the time the mixture reached -10°C a white precipitate had formed. The flask was heated in a warm water bath for 15 minutes to ensure complete reduction of the dichlorosilane, then the reactor bomb was sealed to trap the product silane. The dimethylsilane was purified by vacuum transfer away from the reactor bomb, which had been cooled to approximately -15°C. A sample was prepared for NMR analysis by condensing a small amount of the product into a sealable NMR tube containing dg-benzene; the product was pure, by *H NMR. Yield: 70% (2.24 g) colourless gas/liquid. *H NMR (C6D6, ppm) Si-// 3.93 (septet, 2H, 3jH_H = 4.0 Hz, lJSi-H = 190 Hz (from 29Si satellites)); S\(CHj)2 -0.03 (t, 6H, 2JSi_H = 120 Hz (from 29si satellites)). to vacuum line 2.4.2 Syntheses of complexes and reactivity studies [(dippe)Rh]2(|i-H)(|i-Ti2-H-SiPh2), 2a. To a stirred, dark green solution of [(dippe)Rh]2(|J.-H)2,1, (150 mg, 0.205 mmol) in toluene (3 mL) was added dropwise a solution of diphenylsilane (38 mg, 0.206 mmol) in toluene (2 mL) to give a dark red solution. The toluene was removed under vacuum and the residue was dissolved in hexanes. After filtration of the solution through a celite pad, red crystals were obtained from a minimum volume of hexanes (or pentane) by cooling to -40°C. Yield: 77% (145 mg). *H NMR (C6D6, ppm) Hortho 8.06 (dd, References p69 Chapter 2 4H, 3JHm-Ho = 7.9 Hz, 4JHp-Ho = 1.4 Hz); Hm e t a 7.27 (mult, 4H); Hpara 7.14 (tt, 2H, 3JHpHm = 6.6 Hz); C//(CH3)2 1.96 (mult, 8H, 3JH-H = 6.8 Hz (from *H{31P} NMR)); PC//2C//2P 1-25 (d, 8H, 2 J P _ H = 13.1 Hz); CH(C//3)2 1-05 (dd, 24H, 3 J P H = 15.1 Hz, 3JHH = 7.1 Hz); CH(C//3)2 0.95 (dd, 24H, 3JP_H = 12.2 Hz, 3 J H H = 6.9 Hz); Rh-H -6.17 (sept, 2H, JRh_H = 14 Hz = Jp_H). 3 1P{1H} NMR (C6D6 , ppm) 94.0 (br d, ^Rh-p = 155 Hz). 29Si NMR (C7D8, ppm) 137-141 (br mult, at room temperature), 129-143 (br mult, at -89°C). Anal. Calcd for C40H76P4Rh2Si: C, 52.52; H, 8.37. Found: C, 52.32; H, 8.45. [(dippe)Rh]2( | i -H)( | i -Ti2-H-SiMe2), 2b. One equivalent of dimethylsilane (0.139 mmol, 103 mmHg in a 25.2 mL constant volume bulb) was vacuum-transferred to a dark green solution of [(dippe)Rh]2(p.-H)2,1, (102 mg, 0.139 mmol) in frozen toluene (8 mL) at -196°C. The solution was allowed to warm to room temperature by which time the solution had changed to a deep red colour. The toluene was removed under vacuum and the residue was dissolved in hexanes. After filtration of the solution through a celite pad, red crystals were obtained from a minimum volume of hexanes (or pentane) by cooling to -40°C. Yield: 85% (93 mg). *H NMR (C6D6, ppm) C//(CH3)2 2.05 (overlapping d sept, 8H); PC//2C//2P 1-33 (d, 8H, 2Jp_H = 12.9 Hz); CH(C//3)2 1.23 (dd, 24H, 3 J P . H = 15.0 Hz, 3JH-H = 7.2 Hz); Si(C//3)2 1.12 (s, 6H); CH(C//3)2 1.03 (dd, 24H, 3 J P . H = 12.3 Hz, 3JH-H = 6.9 Hz); Rh-H -6.25 (mult, 2H). Note: For 2b in d8-toluene the Si(C//3)2 resonance is seen at 1.02 ppm, a solvent-related, upfield shift of 0.10 ppm. 31P{1H} NMR (C6D6, ppm) 94.2 (d mult, jRh_p = 159 Hz). 29Si NMR (C7D8, ppm) 163 (mult). Anal. Calcd for C3()H72P4Rh2Si: C, 45.57; H, 9.18. Found: C, 45.70; H, 9.23. [(dippe)Rh]2(|J.-H)(|i-Ti2-H-SiMePh), 2c. This complex was prepared by the same method as for 2a (210 mg, 0.287 mmol 1; 35 mg, 0.286 mmol MePhSiH2). Dark, reddish- brown crystals were obtained in 62% yield (151 mg). *H NMR (C6D6, ppm) Hortho 8.00 (d, 2H, 3JHm-Ho = 6.6 Hz); Hm e t a 7.29 (t, 2H, 3J a v g Hp-Hm, Ho-Hm = 7.3 Hz); Hpara 7.15 (mult, 1H); C//(CH3)2 1.97 (mult, 8H); PC//2C//2P, SiC//3 1.28 - 1.36 (overlapping d and s, 11H); References p69 Chapter 2 61 CH(C//3)2 1-20 (dd, 12H, 3JP_H = 14.8 Hz, 3JH-H = 7.0 Hz); CH(C//3)2 1.12 - 0.94 (mult, 36H); Rh-// -6.00 (pt, 2H, JP_H = 19.5 Hz, JRh-H = 15.1 Hz). 31P{*H} NMR (QD6, ppm) 94.3 (d mult, ^Rh-p = 160 Hz). Anal. Calcd for C35H74P4Rh2Si: C, 49.30; H, 8.75. Found: C, 49.62; H, 8.86. [(dippp)Rh]2(H-H)(p.-ri2-H-SiPh2), 2g. This complex was prepared by the same method as for 2a (96 mg, 0.13 mmol [(dippp)Rh]2(*i-H)2; 22 mg, 0.12 mmol Ph2SiH2). Dark, reddish-brown crystals were obtained in 64% yield (72 mg). *H NMR (C7D8, ppm) Hortho 8.12 (d, 4H, 3JH m-Ho = 7.8 Hz); H m e t a 7.23 (t, 4H); H p a r a 7.15 (t, 2H, 3JHm-Hp = 6.3 Hz); PCH2Ci/2CH2P, C//(CH3)2 1.90 - 1.60 (overlapping mult, 12H); PC//2CH2C//2P 1.18 (br mult, 8H); CH(C//3)2 1-09 (dd, 24H, 3jH_P = 14.4 Hz, 3JH_H = 7.2 Hz); CH(C//3)2 0.99 (dd, 24H, 3JH-P = 11.8 HZ, 3JH-H = 6.4 Hz); Rh-// -8.55 (pt, 2H, JP_H = 20.9 Hz, JRh_H = 14.6 Hz). 31P{1H} NMR (C7Dg, ppm) 37.2 (d, ^Rh-p = 155 Hz). Anal. Calcd for C42H80P4Rh2Si: C, 53.50; H, 8.55. Found: C, 53.16; H, 8.71. Note: Heating complexes 2a-c to reflux in ds-toluene causes exchange of the hydrides for deuterides, as determined by the disappearance of the hydride signals at around -6 ppm in the *H NMR spectra. This solvent-complex isotopic exchange has been observed previously for imido- hydride complexes formed from the addition of nitriles to 1, 4 6 and also for complex 1 itself.19 The exchange is thought to occur by the oxidative addition of a C-D bond from toluene to one rhodium centre in the dinuclear complexes (alkyl and aryl C-H bonds in toluene appear to be equally activated), followed by rapid exchange of the hydrides and deuteride, and finally elimination of the toluene molecule. Reaction of [(dippe)Rh]2(n-H)(^-T|2-H-SiPh2), 2a, with H 2 . [(dippe)Rh]2(H- H)(n-r|2-H-SiPh2), 2a, (25 mg, 0.0027 mmol) was dissolved in ds-toluene in a sealable NMR tube. The tube was attached to a vacuum line by a needle valve adapter and slightly less than one atmosphere of hydrogen was introduced. The tube was tapped for five to ten minutes to encourage References p69 Chapter 2 diffusion of the gas into the dark red solution, which lightened in colour slightly. The tube was sealed and NMR spectra (*H and 31P{ lH}, variable temperature) of the sample were run. (In another experiment the sample was sealed under slightly less than four atmospheres of H2(g) by introducing the gas and sealing the tube while it was cooled to -196°C, then allowing the tube to warm to room temperature. The results were similar for both experiments.) *H NMR (C7D8, ppm) H0rtho 8.03 (dd, 4H, ^Hm-Ho = 7.8 Hz, 4JHp-Ho = 1-3 Hz); Hm e t a 7.22 (t, 4H); Hpara 7.12 (tt, 2H, 3JHm-HP = 7.5 Hz); C//(CH3)2 1.78 (br s, 8H); PO/ 2 C/ / 2 P, CH(C//3)2 1-62 - 0.62 (overlapping mult, 56H); Rh-H, -6.28 (br s, wi/2 = 190 Hz), -9.95 (br s, wi/2 = 120 Hz), -13.40 (br s, wi/2 = 190 Hz); total relative intensity of Rh-H = 4H. 31P{1H} NMR (C7D8, ppm) 100.5 - 88.5 (overlapping br mult). Variable temperature NMR data is presented in Section 2.2.4. Reaction of [(dippe)Rh]2(|i-H)(|i-Ti2-H-SiMe2), 2b, with H2. This reaction was carried out in the same manner as the preceding reaction. *H NMR (C7D8, ppm) C//(CH3)2 1.95 (mult); PC//2C//2P, S1CH3, CH(C//3)2 1.64 - 0.78 (overlapping br mult, 62H); Rh-H, -7.72 (br s, wi/2 = 150 Hz); -10.04 (br s, wi/2 = 96 Hz); -13.73 (br s, wi/2 = 150 Hz); total relative intensity of Rh-H = 4H. 31p{lH} NMR (C7Dg, ppm) 97.2 (br d, ijRh-p = 146 Hz). Variable temperature 31P{1H} NMR data is presented in Section 2.2.4. Reaction of [(dippe)Rh]2(|i-H)(n-Ti2-H-SiMePh), 2c, with H2. This reaction was carried out in the same manner as the preceding reaction. *H NMR (C7D8, ppm) Hortho 7.92 (br s, 2H, wi/2 = 21 Hz); Hm e t a 7.15 (t, 2H, 3 j a v g = 7.5 Hz); Hp a r a 7.03 (mult, 1H); C//(CH3)2, PCH2CH2R, SiC//3, CH(C//3)2 2.74 - 0.06 (overlapping mult, 67H); Rh-H, -7.21 (br s, wi/2 = 90 Hz); -10.16 (br s, wi/2 = 180 Hz); -13.60 (br s, w i ^ = 90 Hz); total relative intensity of Rh-H = 4H. 3 1 P{ 1 H} NMR (C 7D 8 , ppm) 104.8 - 86.7 (overlapping br mult). Variable temperature 31P{1H} NMR data is presented in Section 2.2.4. References p69 Chapter 2 63 [(dippe)Rh]2(|i-D)(n-r|2-D-SiPh2), d2-2a. A solution of [(dippe)Rh]2(|i-SiPh2)2, 6a, (75 mg, 0.068 mmol) (See Chapter 3) in 10 mL toluene in a thick-walled reactor bomb was degassed by two freeze-pump-thaw cycles, then cooled to -196°C. Deuterium gas was introduced to one atmosphere pressure and the reactor bomb was sealed. The mixture was allowed to warm to room temperature, giving a D2 pressure of four atmospheres. The solution was stirred at room temperature for 12-16 hours, during which time the bright orange changed to a deep red colour. An identical workup procedure to that described for 2a was followed, giving red crystals in 64% yield (40 mg). In the *H NMR spectrum of the product the signal due the bridging hydrides was absent. 2 H NMR (C6H6, ppm) -5.92 (br s, wi/2 = 14 Hz). Reaction of [(dippe)Rh]2(p.-D)2 with Ph2SiH2 . An identical procedure to that described for the preparation of [(dippe)Rh]2(^.-H)((i-Ti2-H-SiPh2), 2a, was followed using di-\ (61 mg, 0.083 mmol). *H and 2H NMR spectroscopy suggested a product mixture of do-, dj- and d2- 2a, by comparison with spectra of authentic d^"2a and d2-2a complexes. Reaction of [ (d ippe)Rh] 2 ( | i -H)(^ i -Ti 2 -H-SiPh 2 ) , 2a, with D 2 . Enough [(dippe)Rh]2(n-H)(|i-r|2-H-SiPh2), 2a, for two NMR samples (50 mg, 0.055 mmol) was dissolved in 10 mL toluene in a thick-walled reactor bomb. The solution was degassed by three freeze-pump-thaw cycles then placed under an atmosphere of deuterium gas. The dark red solution lightened slightly in colour. The mixture was stirred for 24 hours then the excess deuterium and the solvent were removed in vacuo and the residues were dissolved in hexanes. After filtration of the solution through a celite pad, red crystals were obtained from a minimum volume of hexanes by cooling to -40°C. Comparison of the *H and 2H NMR spectra with those of authentic do- and d2- samples indicated almost complete exchange of the hydrides for deuterides in this compound (<10% hydride left according to lH NMR). References p69 Chapter 2 Reaction of [(dippe)Rh]2(^-H)( | i -r t2-H-SiMe2) , 2b, with D2 . This reaction was carried out in the same manner as the preceding reaction, with similar results. 2H NMR (C6H6, ppm) -5.89 (br s, wi/2 = 14 Hz). Reaction of [(dippe)Rh]2(n-H)(n.-r|2-H-SiMePh), 2c, with D2 . This reaction was carried out in the same manner as the preceding reaction, with similar results. 2H NMR (C6H6, ppm) -5.90 (br s, wi/2 = 14 Hz). [(dippe)Rh]2(n-SiPh2)(^-CO), 4a. To a red solution of [(dippe)Rh]2(^-H)(^i-T|2- H-SiPh2), 2a, (85 mg, 0.092 mmol) in toluene (3 mL) in a thick-walled reactor bomb was added approximately two equivalents of carbon monoxide (0.184 mmol, 136 mmHg in a 25.2 mL bulb) by vacuum transfer. After the solution warmed to room temperature the toluene was removed under vacuum. The rust-brown residues were reprecipitated from a minimum volume of hexanes at -40°C; a brown powder was obtained. Yield: 70% (61 mg). *H NMR (C6D6, ppm) Honho 8.29 (d, 4H, 3jHm_H0 = 7.5 Hz); Hm e t a 7.30 (t, 4H); Hp a r a 7.15 (mult, 2H); C//(CH3)2 2.11 (br s, 8H); ?CH2CH2P 1.35 (d, 8H, 2 j P _ H = 13.2 Hz); CH(C//3)2 1.07 (dd, 24H, 3jp_H = 14.7 Hz, 3 J H - H = 6.9 Hz); CH(C#3)2 0.90 (dd, 24H, 3 j p _ H = H.5 Hz, 3 j H . H = 7.1 Hz). 31P{1H} NMR (C6D6, ppm) 79.8 (d mult, lJR h-p = 161 Hz). IR (KBr pellet) v C o = 1703.4 cm"1 (s). Anal. Calcd for C4lH740iP4Rh2Si: C, 52.34; H, 7.93. Found: C, 52.21; H, 8.07. [(dippe)Rh]2(jj.-SiMe2)(M.-CO), 4b. This complex was prepared using the same method as for 4a and has been identified in solution using NMR spectroscopy, but could not be isolated in a pure form. *H NMR (C6D6, ppm) C//(CH3)2 2.14 (mult, 8H); PC//2C//2P 1.40 (d, 8H, 2 j P _ H = 13.2 Hz); Si(C//3)2 1-28 (s, 6H); CH(C//3)2 1.15 (dd, 24H, 3 j P _ H = 15.1 Hz, 3 J H - H = 7.0 Hz); CH(C//3)2 1.04 (dd, 24H, 3 j P _ H = 11.8 Hz, 3 j H . H = 7.1 Hz). 31P{!H} NMR (C6D6, ppm) 82.9 (br d, ^Rh-P = 172 Hz). [(dippe)Rh]2(|j.-SiMePh)(|i-CO), 4c. This complex was prepared using the same method as for 4a and has been identified in solution using NMR spectroscopy, but could not be References p69 Chapter 2 isolated in a pure form. JH NMR (C6D6, ppm) Hortho 7.94 (d, 2H, 3JHm-Ho = 6.0 Hz); Hmeta 7.33 (t, 2H); Hp a r a 7.15 (t, 1H, 3JHm-Hp = 7.5 Hz); C//(CH3)2 2.23 (mult, 8H); PC//2C//2P 1.70 (s, 4H); PCH2CH2P, S1CH3, CH(C//3)2 1.58 - 1.17 (overlapping mult, 19H); CH(C//3)2 1.03 (mult, 36H). 31P{lH} NMR (C6D6, ppm) 81.6 (br d mult, ^Rh-p = 172 Hz). Reaction of [(dippe)Rhh(^-H)(|i-r|2-H-SiPh2) with C2H4. A dark red solution of [(dippe)Rh]2(H-H)(|i-r)2-H-SiPh2), 2a, (25 mg, 0.027 mmol) in ds-toluene (0.6 mL) was placed in a sealable NMR tube. The tube was attached to a vacuum line by a needle valve adapter and slightly less than one atmosphere of ethylene was introduced. The tube was cooled to -196°C, then sealed. After one week the solution colour had changed to a lighter orange, and *H and 3 1P{!H} NMR spectra showed the presence of [(dippe)Rh]2(^-H)0i-rt2-CH=CH2), 5,1 9 dissolved ethylene (XH NMR (ppm, C7D8) 5.4 (s)), and the presence of some compound containing the SiPh2 fragment, as determined by the presence of signals in the aromatic region of the *H NMR spectrum. The sealed NMR tube was cut open and the solvent and excess ethylene were removed from the sample in vacuo. The residues were dissolved in hexanes and passed down a small column of alumina to remove the vinyl hydride complex, 5. A *H NMR spectrum of the eluted compound showed it to be diethyldiphenylsilane, by comparison to an authentic sample prepared by the addition of excess EtMgBr to dichlorodiphenylsilane. *H NMR (06^6. ppm) Hortho 7.62 - 7.53 (mult, 4H); H m e t a , Hpara 7.38 - 7.16 (mult, 6H); CH2CH3 1.18 - 0.88 (mult, 10H). 13C{1H} NMR (CDCI3, ppm relative to CDCI3 at 77.0) Cipso 136.26; Cmeta 134.97; Cortho, Cpara 129.10, 127.80; Cmethyl 7.47; Cmethylene 3.96. A HETCOR (13C, *H) experiment carried out on the Bruker AMX-500 machine shows the alkyl resonances in the *H NMR spectrum to be: CH2CK3 1-004 (q, 4H, 3 J H - H = 8 Hz); CH2C//3 0.982 (t, 6H). Mass spec (EI): 240 (M+), 211, 183, 159, 131, 105,79. Reaction of [ (d ippe)Rh]2(n- r i 2 -CH = CH2) , 5, with one equivalent of Ph 2SiH 2 . To a solution of [(dippe)Rh]2(^-H)(M.-ri2-CH=CH2), 5, (24 mg, 0.032 mmol) in References p69 Chapter 2 toluene (5 mL) was added a solution of one equivalent of diphenylsilane (6 mg, 0.032 mmol) in toluene (3 mL). The originally pale orange solution immediately darkened to a deep red-orange colour. The solvent was removed in vacuo and the residues were dissolved in de-benzene for analysis by NMR spectroscopy. The only product observed in the 31P{ ̂ K} NMR spectrum was [(dippe)Rh]2(fi-H)(|j.-ri2-H-SiPh2), 2a. In the *H NMR spectrum the expected resonances due to 2a are observed, along with small multiplets at 3.43 ppm and 2.21 ppm, due to an unidentified species. No signals due to Ph2SiEt2 were observed. Reaction of [(dippe)Rh]2(H-Ti2-CH = CH2), 5, with two equivalents of Ph2SiH2. To a solution of [(dippe)Rh]2(^-H)(n-r|2-CH=CH2), 5, (16 mg, 0.021 mmol) in toluene (5 mL) was added a solution of one equivalent of diphenylsilane (8 mg, 0.021 mmol) in toluene (3 mL). The originally pale orange solution immediately darkened to a deep red-orange colour, then with 10 seconds of stirring lightened somewhat. The final solution colour was darker than the original orange. The solvent was removed in vacuo and the residues were dissolved in de- benzene for analysis by NMR spectroscopy. The only products observed in the 31P{ lH} NMR spectrum were [(dippe)Rh]2(}i-H)(|j.-r)2-H-SiPh2), 2a, and [(dippe)Rh]2(H-SiPh2)2, 6a (see Chapter 3), in a 3:1 ratio. In the *H NMR spectrum the expected resonances due to 2a and 6a were observed, along with two small multiplets at 3.43 ppm and 2.21 ppm and another multiplet at 5.06 ppm, due to unidentified species. No signals due to Ph2SiEt2 were observed. 2.4.3 Catalytic reactions Typical conditions for the hydrosilation of ethylene by diphenylsilane using [(dippe)Rh]2(|i--H)2, 1, as catalyst. 253 mg (1.38 mmol) of diphenylsilane were dissolved in 2.5 mL of toluene and placed in a 50 mL reactor bomb with a stir bar. The reactor bomb was attached to the vacuum line and degassed by evacuation of the head space, then placed under one atmosphere of ethylene. The catalyst (1, 0.014 mmol, 0.25 mL of a 0.55 M solution in toluene) References p69 Chapter 2 67 was added to the substrate solution by syringe under a strong flow of ethylene. The reactor bomb was left open to the manifold full of ethylene, which was kept at one atmosphere by periodic addition of more gas from the cylinder. The mixture was stirred for 24 hours, at which time it was taken into the glovebox and the catalyst was removed on a Florisil column. The product mixture was analyzed by both GC-MS and *H NMR spectroscopy, which showed that all of the diphenylsilane had been consumed. The major product was diethyldiphenylsilane (estimated at > 90%). The GC trace shows the presence of very small amounts of several other species, and the *H NMR spectrum shows three very small AB quartet resonances from 6.5 to 5.7 ppm. These may be due to species containing vinyl groups on silicon, though no conclusive "matches" were made with the GC-MS data. Mass spectrometric data for diethyldiphenylsilane are listed above in the description of the reaction of ethylene with 2a, as are the *H and 13C{ 1H} NMR spectroscopic data. Progress of the reaction monitored by *H NMR. Diphenylsilane (126 mg, 0.700 mmol) was dissolved in 3.0 mL of d6-benzene and placed in a 250 mL reactor bomb with a stir bar. The reactor bomb was attached to the vacuum line and degassed by evacuation of the head space, then placed under one atmosphere of ethylene. This substrate solution was stirred for 15 minutes before the addition of the catalyst, to ensure saturation of the solution with ethylene. The catalyst (1, 0.014 mmol, 10 mg in 1.0 mL of d6-benzene) was added by syringe to the substrate solution under a strong flow of ethylene. The reactor bomb was left open to the manifold full of ethylene, which was kept at one atmosphere by periodic addition of more gas from the cylinder. Samples of the mixture were withdrawn periodically by syringe, under a strong flow of ethylene. *H NMR spectra were run within 10 minutes of removal of the samples from the mixture, and in some cases 31P{ !H} NMR spectra were run afterwards. The first sample was withdrawn from the catalytic mixture three minutes after the catalyst was added to the substrate. Its *H NMR spectrum showed that 100% conversion of Ph2SiH2 to Ph2SiEt2 had already occurred. Three more samples removed from the reaction mixture over the References p69 Chapter 2 next two hours showed the same ratio of product to starting material. The assumption that the hydrosilation reaction finished in three minutes leads to a turnover number of 16 mol»min_1 per mole of catalyst (or 16 min-1, which is 960 hr1) . The 31P{ lH) NMR spectra of the samples all showed [(dippe)Rh]2(^-H)(|i-r|2-H-SiPh2), 2a, to be the major complex in solution. The hydrosilation of 1-butene by diphenylsilane using [(dippe)Rh]2(n-H)2, 1, as catalyst. In a glovebox, diphenylsilane (248 mg, 1.35 mmol) was dissolved in 1.5 mL of d6-benzene and placed in a reactor bomb with a stir bar. The catalyst [(dippe)Rh]2(M.-H)2,1, (2 mg, 0.003 mmol) was added to the diphenylsilane solution. The dark green solid immediately changed to orange and began to dissolve, and bubbles were evolved. The bomb was attached to a vacuum line and the head space was evacuated before the bomb and the vacuum manifold were filled with one atmosphere of 1-butene. The mixture was allowed to stir for 24 hours, then the head space was evacuated and a *H NMR spectrum was run. The initial spectrum showed the reaction had not gone to completion. To remove the volatile starting material, the NMR sample was placed under vacuum while being cooled in an ice bath. *H NMR spectroscopy showed the only silicon-containing product to be n-butyldiphenylsilane and a very small amount of triphenylsilane (a product of disproportionation of diphenylsilane, which probably formed before the 1-butene was added to the reaction mixture). For H-butyldiphenylsilane: *H NMR (C6D6, ppm) Hortho 7.64 (mult, 4H); Hmeta, Hpara 7.24 (mult, 6H); Si-/ / 5.19 (t, 1H, 3 j H _ H = 3.2 Hz, ^s i -H = 192 Hz (measured from 2 9Si satellites)); SiCH2C//2CH2CH3 1.54 (mult, 2H); SiCH2CH2C//2CH3 1.40 (mult, 2H); SiC//2CH2CH2CH3 1.17 (mult, 2H); SiCH2CH2CH2C//3 0.88 (t, 3H, 3 j H _ H = 7.3 Hz). Mass spec (EI): 240 (M+), 183, 162, 134, 105, 84. References p69 Chapter 2 2.4.4 Calculations Calculation of Ti values. The calculation of Ti values at various temperatures was carried out on the Varian XL-300 NMR machine using the inversion - recovery method.47 Calculation of AG* for the hydride exchange in 2a. AG* was calculated using the value for the rate constant48 kc (where kc = TiAvcKl)1^) in the Eyring equation AG* = -RTcln[(kch)/(kBTc)]; where R = the gas constant, Tc = temperature of coalescence, A\)c = peak separation at the low T limit, h = Planck's constant and ke = Boltzmann constant. For the hydride resonances in the variable temperature lH NMR spectra of 2a, Tc = 193K (-80°C) and A\)c = 309 Hz. The coalescence temperature was estimated visually from the spectra and has an error of approximately ±5K. 2.5 References (1) Ojima, I. In The Chemistry of Organic Silicon Compounds; S. Patai and Z. Rappoport, Ed.; John Wiley and Sons Ltd: New York, 1989; pp 1479-1526. (2) de Charenteney, F.; Osborn, J. A.; Wilkinson, G. J. Chem. Soc. A 1968, 787. (3) Haszeldine, R. N.; Parish, R. V.; Parry, D. J. / . Chem. Soc. A 1969, 683. (4) Glockling, F.; Hill, G. C. / . Chem. Soc. A 1971, 2137. (5) Ojima, I.; Nihonyanagi, M. / . Chem. Soc, Chem. Comm. 1972, 938. References p69 Chapter 2 (6) Haszeldine, R. N.; Parish, R. V.; Taylor, R. J. / . Chem. Soc, Dalton Trans. 1974, 2311. (7) Joslin, F. L.; Stobart, S. R. / . Chem. Soc, Chem. Commun. 1989, 504. (8) Thorn, D. L.; Harlow, R. L. Inorg. Chem. 1990,29, 2017. (9) Fernandez, M.; Bailey, P. M.; Bentz, P. O.; Ricci, J. S.; Koetzle, T. F.; Maitlis, P. M. / . Am. Chem. Soc 1984,106, 5458. (10) Duckett, S. B.; Perutz, R. N. / . Chem. Soc, Chem. Commun. 1991, 28. (11) Bennett, M. A.; Patmore, D. J. Inorg. Chem. 1971,10, 2387. (12) Aylett, B. J. / . Organomet. Chem. 1980, 9, 327. (13) Wang, W.; Eisenberg, R. / . Am. Chem. Soc 1990,112, 1833. (14) Here, and elsewhere in this thesis, "hydrogen" refers to molecular hydrogen, H^. Similarly, "deuterium" refers to D2. (15) The doublet signals arise because of coupling to 103Rh, which has a spin of 1/2. (16) Schubert, U. Adv. Organomet. Chem. 1990,30, 151. (17) Brookhart, M.; Green, M. L. H. / . Organomet. Chem. 1983,250, 395. (18) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, California, 1987.pp. 242-243 (19) Fryzuk, M. D.; Jones, T.; Einstein, F. W. B. Organometallics 1984,3, 185. (20) Burch, R. R.; Shusterman, A. J.; Muetterties, E. L.; Teller, R. G.; Williams, J. M. / . Am. Chem. Soc. 1983,105, 3546. (21) Crabtree, R. H.; Hamilton, D. G. Adv. Organomet. Chem. 1988,28, 299. (22) Jessop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992,121, 155. (23) Rabaa, H.; Saillard, J.; Schubert, U. / . Organomet. Chem. 1987,330, 397. References p69 Chapter 2 71 Lichtenberger, D. L.; Rai-Chaudhuri, A. Inorg. Chem. 1990,29, 975. Matarasso-Tchiroukhine, E.; Jaouen, G. Can. J. Chem. 1988,66, 2157. Matarasso-Tchiroukhine, E. / . Chem. Soc, Chem. Commun. 1990, 681. Aitken, C. T.; Harrod, J. F.; Samuel, E. / . Am. Chem. Soc. 1986,108, 4059. Auburn, M.; Ciriano, M.; Howard, J. A. K.; Murray, M.; Pugh, N. J.; Spencer, J. L.; Stone, F. G. A.; Woodward, P. / . Chem. Soc, Dalton Trans. 1980, 659. Bennett, M. J.; Simpson, K. A. / . Am. Chem. Soc. 1971, 93, 7156. Carreno, R.; Riera, V.; Ruiz, M. A.; Jeannin, Y.; Philoche-Levisalles, M. J. Chem. Soc, Chem. Commun. 1990, 15. Suzuki, H.; Takao, T.; Tanaka, M.; Moro-oka, Y. / . Chem. Soc, Chem. Commun. 1992, 476. Schubert, U.; Kraft, G.; Walther, E. Z. Anorg. Allg. Chem. 1984,519, 96. Fryzuk, M. D.; Rosenberg, L.; Rettig, S. J. Organometallics 1991,10, 2537. Burch, R. R.; Muetterties, E. L.; Schultz, A. J.; Gebert, E. G.; Williams, J. M. / . Am. Chem. Soc 1981,103, 5517. Fryzuk, M. D.; Piers, W. E.; Einstein, F. W. B.; Jones, T. Can. J. Chem. 1989, 67, 883. Crabtree, R. H.; Lavin, M.; Bonneviot, L. / . Am. Chem. Soc. 1986,108, 4032. Bautista, M. T.; Earl, K. A.; Maltby, P. A.; Morris, R. H.; Schweitzer, C. T.; Sella, A. / . Am. Chem. Soc 1988,110, 7031. Dickers, H. M.; Haszeldine, R. N.; Malkin, L. S.; Mather, A. P.; Parish, R. V. / . Chem. Soc, Dalton Trans. 1980, 308. Elschenbroich, G; Salzer, A. Organometallics: A Concise Introduction, 2nd Ed.; VCH Publishers Inc.: New York, 1992 Fryzuk, M. D.; Piers, W. E. Polyhedron 1988, 7, 1001. References p69 Chapter 2 72 (41) Fryzuk, M. D.; Piers, W. E.; Rettig, S. J.; Einstein, F. W. B.; Jones, T.; Albright, T. A. / . Am. Chem. Soc. 1989, 111, 5709. (42) Chalk, A. J.; Harrod, J. F. / . Am. Chem. Soc. 1965, 87, 16. (43) Tilley, T. D. In The Chemistry of Organic Silicon Compounds; S. Patai and Z. Rappoport, Ed.; John Wiley and Sons Ltd.: New York, 1989; pp 1415-1477. (44) Fryzuk, M. D.; McConville, D. H.; Rettig, S. J. / . Organomet. Chem. 1993,445, 245. (45) Doyle, M. P.; DeBruyn, D. J.; Donnelly, S. J.; Kooistra, D. A.; Odubela, A. A.; West, C. T.; Zonnebelt, S. M. / . Org. Chem. 1974,39, 2740. (46) Piers, W. P. Ph.D. Thesis, University of British Columbia, 1988, p. 140. (47) Sanders, J. K. M.; Hunter, B. K. Modern NMR Spectroscopy, 2nd Ed.; Oxford University Press: Oxford, 1993 (48) Thomas, W. A. Annu. Rev. NMR Spectrosc. 1968,1, 43. References p69 Chapter 3 CHAPTER 3 Reactions of Secondary Silanes with [(dippe)Rh]2([i-H)2: Complexes with Two Silicon Ligands 3.1 Introduction In this chapter the reactions of two equivalents of a variety of secondary silanes with the dinuclear rhodium hydride complex [(dippe)Rh]2(|i-H)2,1, are presented, and the characterization and reactivity of the ensuing complexes are discussed. The use of 1 as a catalyst precursor for isotopic exchange between diphenylsilane and deuterium gas and for the dehydrogenative coupling of diphenylsilane is also described. Hypotheses about the possible active species in these catalytic cycles and likely mechanisms are proposed. There are many examples in the literature of dinuclear transition-metal complexes containing two silicon ligands, which are generally found in bridging modes. Three such structures have already been presented in Chapter 2 (Section 2.2.2.1), all of which contain bridging silyl ligands bound to one of the metal centres through a three-centre, two-electron M-H- Si bond.1"3 However the most common structure for dinuclear metal complexes containing two silicon ligands is a dimeric structure, [LnMSiR2j2, where two identical metal fragments are bridged by two silylene ligands, :SiR.2.4'5 Often these structures arise from the addition of silanes to mononuclear precursors. The complexes have planar M2Si2 cores and are typically characterized by long Si-Si distances, short M-M distances and acute M-Si-M angles. References on pl23 Chapter 3 3.2 Reaction of [(dippe)Rh]2(|i-H)2, 1, with two equivalents of R2SiH2 3.2.1 Preparation and reactivity of the bisQi-silylene) complex, [(dippe)Rh]2((i- SiPh2h, 6a Addition of one equivalent of diphenylsilane to [(dippe)Rh]2(M.-H)(|i-T|2-H-SiPh2), 2a, (or addition of two equivalents of the silane to 1) causes the formation of the bis(|i.-silylene) complex [(dippe)Rh]2((i-SiPh2)2» 6a, with simultaneous loss of two equivalents of hydrogen. [(dippe)Rh]2(|i-H)(^-Ti2-H-SiPh2) + 1 Ph2SiH2 ~^- 2 a  Ph Ph \ / " [3-1] Prj2 .Si PH 2 ^ N P ^ \ / " * * " P / V Prj2 Si Pr!2 Ph *Ph 6a There are some difficulties in preparing this compound in a pure state; these are described in Section 3.4.2. The complex is practically insoluble in hexanes and halogenated solvents, but, when pure, is soluble in aromatic solvents. Compound 6a is air and moisture sensitive in solution, though in the solid state it is air stable for short periods of time (~hours). References on p!23 Chapter 3 75 1 I I I ] ! M I j ! ; i I j J I I ! j I I 1 I ] ! I I ! J ! I I J ] 1 ! 1 I I M M J I I I I j I I ! I J I I 1 I j J 1 I I | I I I I 84 62 60 78 76 74 PPM Figure 3.1 121.4 MHz 3lp{lH} NMR spectrum of 6a in C6D6, an AA'A"A"'XX' pattern. The 31P{!H} NMR spectrum of 6a is shown in Figure 3.1. This centre-symmetric doublet of multiplets is typical of an AA'A"A'"XX' system, and is similar to many others observed for the dinuclear rhodium complexes studied, including that of [(dippe)Rh]2(H-H)2,1. Thus all four phosphines are in chemically equivalent but magnetically inequivalent environments in the molecule, as are both rhodiums. Probably in solution the rhodiums have a square planar arrangement of ligands, with the substituents on the tetrahedral silicons projecting into and out of the plane. The *H NMR spectrum of 6a is consistent with this structure. The bis(n-silylene) complex 6a does not react with CO or C2H4, but does react with hydrogen, as will be described in Section 3.3.1. Complex 6a reacts only very slowly with one equivalent of diphenylsilane. However in the presence of excess diphenylsilane (4-5 equivalents) an orange solution of 6a in benzene reacts to give a yellow solution. The 31P{ *H} NMR spectrum References on pJ23 Chapter 3 76 of this solution, shown in Figure 3.2, is complex but may be attributed to a single complex in solution, compound 7. I [ l 1 I 1 I 1 1 I l I I I I I I I I I 1 I M I 1 I I 1 I I I I I I I I I I I I I )5 100 95 90 65 60 75 70 PPM 6! ^ Figure 3.2 121.4 MHz 31P{ !H} spectrum of compound 7, resulting from the addition of excess Ph2SiH2 to 6a. The JH NMR spectrum of the reaction mixture indicates the presence of both rhodium hydrides and terminal silicon hydrides in 7, and also the presence of unreacted diphenylsilane and tnphenylsilane. The expected by-product of the disproportionation reaction that must produce the tnphenylsilane, phenylsilane, is not observed. These spectra have not been conclusively assigned to a specific structure; several possibilities have been considered and are shown in Figure 3.3. Complex 7 has not yet been isolated in a pure state. References on pi 23 Chapter 3 PhH PhH Ph H \f !/ \r Pri2 / S i — S \ PH2 Prj2 / S l \ *| Pr'2 PT^ \ Rh<Tp-^ ^ R ^ R h - ^ Ph Ph Ph PhH PhH Ph H >v \F r/ . V Pr i /> PH2 H 7 S i - S | -H P r i 2 J I 2 H / S i \ \ . ^ H N s i u Pl"2 p . H—Si" H bi H  p r ,2 | - H Ph' 'ph P h H,Ph P \ f UH Si—H * PrL H—Si pr j2 2  Ph2 Figure 3.3 Some possible structures for compound 7, the product from the reaction of 6a with excess diphenylsilane. Evidently the formation of complex 7 has something to do with the addition of three or more equivalents of Ph2SiH2 to the dinuclear rhodium core, as signals due to 7 are observed in the NMR spectra of other reaction mixtures as well. For example, it is a minor product in the reaction of [(dippe)Rh]2(|i-H)2,1, with an excess of Ph2SiH2 under an atmosphere of D2(g). Complex 7 has also appeared as a product in reactions relevant to catalytic isotopic exchange between Ph2SiH2 and D2 and the catalytic dimerization of Ph2SiH2. These reactions are described in Section 3.4.2. References on pl23 Chapter 3 3.2.2 The solid-state structure of [(dippe)Rh]2(n-SiPh2)2, 6a A single-crystal X-ray analysis of 6a confirmed the dimeric structure of this molecule with bridging silylene groups. Two views of the molecular structure of 6a are shown in Figure 3.4 and some bond distances and angles are shown in Tables 3.1 and 3.2, respectively. Table 3.1 Selected bond lengths for [(dippe)Rh]2(|i-SiPh2)2,6a. Bond R h l - R h l * R h l - P l Rhl - P2 Length (A) 2.921(2) 2.295(2) 2.288(2) Bond R h l - S i l R h l - S i 2 Length (A) 2.357(2) 2.340(2) Bond S i l - C 1 5 Si2-C21 Length (A) 1.911(5) 1.898(6) * Refers to symmetry operation: 1-x, y, 1/2-2. Table 3.2 Selected bond angles for [(dippe)Rh]2(p.-SiPh2)2, 6a. Bonds P l - R h l - P 2 S i l - R h l - S i 2 R h l - S i l - R h l * R h l - S i 2 - R h l * Angle (deg) 86.34(6) 103.09(5) 76.57(7) 77.25(8) Bonds R h l - S i l - C 1 5 R h l - S i l - C 1 5 * C 1 5 - S H - C 1 5 * Angle (deg) 121.7(2) 115.5(2) 105.0(4) * Refers to symmetry operation: 1-2C, y., l/2-£. References on pi 23 Chapter 3 79 C 2 3 C 1 4 C18 C17 C24 Figure 3.4 Two views of the molecular structure of [(dippe)Rh]2(M--SiPh2)2,6a. References on pl23 Chapter 3 Each rhodium centre in the dimer has a distorted square planar geometry (if one ignores the Rh-Rh bond); when viewed down the Rh-Rh axis the two square planes are skewed relative to each other, with an angle between the two mean square planes of 46.95°. There is a C2 axis running through both Si atoms. The observed diamagnetism of 6a requires a formal single bond between the two rhodium atoms, and this is borne out by the Rh-Rh separation of 2.921(2) A and the acute silylene bridge angles of 77.25(8)° and 76.57(7)°. The Si-Si distance across the dinuclear rhodium centre is approximately 3.71 A,6 which precludes any interaction between the two silicons. In fact this complex fits perfectly the general characteristics of dimeric M2Si2 complexes as described in Section 3.1. A recent study compared theoretical evaluations of the platinum complex [(PEt3)2Pt(SiPhCl)]2 and the dimeric manganese complex, [(CO)4Mn(SiPh2)J2, using the atom superposition and electron delocalization molecular orbital (ASED-MO) theory.7 CL Ph P t \ fh ? / \ (Et3P)2Pt ! R(PEta)2 (OC)4Mr/ -Mn(CO)4 Nr \ / Si SI A A Ph CI Ph Ph The platinum complex is one of a series of Pt2Si2 complexes of the formula [(Et3P)2Pt]2(SiPhX)(SiPhY), where X=Y=H, X=Y=C1, or X=H and Y=C1. The crystal structures of these complexes show short cross-ring Si-Si distances within the known range of Si- Si bonds,8 long M-M distances (e.g. 3.973 A for the complex where X=Y=C1) and acute Si-M-Si angles.9 The short Si-Si distances could represent nascent bond formation between the silicons, and suggest that these complexes are perhaps better viewed as dinuclear platinum species bridged by 7c-bound, T|2-disilene species. The MO studies were carried out by interacting the appropriate disilene fragments ([R2Sih) with M-M bonded transition-metal dimers ([Pt(PH3)2]2 and References on pi 23 Chapter 3 81 [Mn(CO)4]2- Essentially this study confirms that for the manganese complex, where each Mn in the Mn2 fragment is d7, the Mn-Mn bond order is one and there is little interaction between the two silicons. For the platinum complex, where each Pt in the Pt2 fragment is d10, the Pt-Pt bond order is zero and there is substantial interaction between the two silicons. Based on these studies the authors generalize about other M2Si2 rings, based on the number of d-electrons on each M of the M2 fragments. For two d9 centres such as would be found in the [(dippe)Rh]2 fragment a structure with a single M-M bond is predicted, with strong Si-M interactions and large Si-Si separations. This is consistent with the dimensions of the Rh2Si2 ring observed in the solid-state structure of [(dippe)Rh]2(|i-SiPh2)2, 6a. 3.2.3 Preparation, solid-state structure and structural isomerism of the bis((i- silylene) complex [(dippe)Rh]2(|i-SiMePh)2, 6c The methylphenylsilylene analogue of 6a, [(dippe)Rh]2(|i-SiMePh)2, 6c, has also been prepared and was isolated as an orange powder. This complex is less soluble than 6a, making purification and spectroscopic characterization quite difficult. However, enough dissolves in deuterated benzene or toluene to give analyzable *H and 31P{1H} NMR spectra. The 31P{ *H} spectrum shows that a mixture of two products is formed in this reaction; assuming a structure in solution that is similar to the solid-state structure of the diphenylsilane analogue, the two products are believed to be geometric isomers arising from the possibility of the methyl groups on the two silylene units being either cis or trans to each other across the Rh2Si2 plane. (See Equation 3-2.) References on pi 23 Chapter 3 82 [(dippe)Rh]2(|i-H)(^-Ti -H-SiMePh) + 1 MePhSiH2 -2H2 A: P?2 ' '» . . 2c Ph Me Pr1- Rh Rh • . \ / '"" . Pr1, Si Pr1; iV Ph Me c is-6 c P P Me Ph V Prj2 / S i Rh Rh \ / Pr1, Si Ph Me trans-6c Pr*2 PrU [3-2] It is not obvious from *H and 31P{1H} NMR spectra of mixtures of cis- and trans-6c which isomer is which, and the two could not be separated by crystallization. In fact, study of this isomerism and determination of the initial ratio of isomers formed has been hampered by the very low solubility of these compounds, even in halogenated solvents. However, slow evaporation of solvent from a very dilute solution of 6c in hexanes did yield crystals of the complex suitable for an X-ray diffraction study. The structure obtained was of the trans isomer. (See Figure 3.5 for an ORTEP diagram of trans-6c and Tables 3.3 and 3.4 for some relevant bond distances and bond angles, respectively.) Table 3.3 Selected bond lengths for rra/w-[(dippe)Rh]2(|i-SiMePh)2, Bond Length (A) R h l - R h l * 2.8925(9) R h l - P l 2.273(1) R h l - P 2 2.2760(8) Bond Length (A) R h l - S i l 2.345(1) R h l - S i l * 2.3531(8) Bond S i l - C 1 5 S i l - C 1 6 6c Length (A) 1.908(2) 1.902(2) * Refers to symmetry operation: 1-x, 1-y., 1-z. References on pl23 Chapter 3 83 Table 3.4 Selected bond angles for rran.y-[(dippe)Rh]2(M--SiMePh)2,6c. Bonds Angle (deg) P l - R h l - P 2 86.83(3) S i l - R h l - S i l * 104.00(3) R h l - S i l - R h l * 76.00(3) R h l - S i l - C 1 5 126.37(8) * Refers to symmetry operation: 1-x, 1-y, 1 Bonds Angle (deg) R h l - S i l - C 1 6 112.81(8) R h l * - S i l - C 1 5 127.01(8) R h l * - S i l - C 1 6 111.02(8) -z- Figure 3.5 Molecular structure of fra/2s-[(dippe)Rh]2(u-SiMePh)2,6c. The dimensions of the molecular structure of trans-6c are analogous to those observed for [(dippe)Rh]2(^-SiPh2)2, 6a, and are as expected for a dimeric M2Si2 complex. The single crystal used for the diffraction experiment was dissolved in ds-toluene and its *H NMR spectrum was References on pi 23 Chapter 3 84 obtained, allowing assignment of the peaks due to the trans isomer in spectra of mixtures of the two isomers. The other peaks were assigned to the cis isomer. The 31P{1H} NMR spectra for the cis and trans isomers of 6c are centre-symmetric doublets of multiplets, typical of AA'A"A'"XX' spin systems. | J l II111111 j l J1111111111111111j11111111111111j111111111j111111111 j 11111111111111 j ! 11111 j BE 65 84 83 82 Bl 80 79 78 PPM Figure 3.6 121.4 MHz 31P{1H} spectrum of a mixture of the cis and trans isomers of [(dippe)Rh]2((i-SiMePh)2, 6c, in C6D6. For complexes where the two R groups on silicon are the same an AA'A"A'"XX' pattern could arise from either a structure where the four phosphorus atoms are coplanar with the Rh2Si2 plane (structure XI in Figure 3.7) or a structure where the four phosphorus atoms occupy a plane which is perpendicular to the Rh2Si2 plane (structure XII in Figure 3.7). References on pi 23 Chapter 3 85 R R \ / \ / Rh -Rh y v V P^2 fV Pr*2 R R X I P[P] Pr1. P/A R R \ ^ .Si ">., PH2 ^Rh- / \ R R XII Rh PH2 ....!»«« P • P Pr1, P[P] Figure 3.7 Possible structures of bis(|i-silylene) complexes leading to an AA'A"A'"XX' pattern in the 31P{ ^H} NMR spectrum. When the two R groups on silicon are inequivalent only a structure like XI would still render all four phosphorus atoms chemically equivalent. Examination of the molecular structures of both [(dippe)Rh]2(H-SiPh2)2> 6a, and [(dippe)Rh]2(|i-SiMePh)2, 6c, show that in the solid state neither meets the structural requirements for an AA'A"A'"XX' system; both have a distorted square planar geometry at the Rh centres (if one ignores the Rh-Rh bond) and the two phosphine chelate rings are twisted relative to each other by roughly 45°. For a complex with unsymmetrically substituted silicons a structure such as this would generate more complex AA'BB'XX' patterns for both cis and trans isomers in the 31P{1H} NMR spectra. A fluxional process which would render the phosphines in these structures equivalent in solution is the twisting of the chelate rings in and out of the plane containing the Rh2Si2 core of each molecule, as depicted in Scheme 3.1. While for the bis(|i-diphenylsilylene) complex 6a an average structure resembling either XI or XII would generate the observed spectrum, for the unsymmetrically References on pi23 Chapter 3 substituted silylene the average structure in solution must be like structure XI, with all four phosphorus coplanar with the core of the molecule. It is assumed that 6a also has this planar, average structure in solution. P[P] - ^ - Scheme 3.1 3.2.4 Preparation of [(dippe)Rh]2(|i-SiMeTolP)2, 6d, and study of the cisltrans isomerism In an effort to produce a more soluble cisltrans isomer pair of a bis(|0.-silylene) complex, reactions with methyl-p-tolylsilane were carried out. The complex [(dippe)Rh]2(|J.-H)(p.-r|2-H- SiMeTolP), 2d, shows similar details in its *H and ^Pf^H} NMR spectra to those seen for [(dippe)Rh]2((i-H)(^.-,ri2-H-SiMePh), 2c, and reacts with a second equivalent of methyl-/?- tolylsilane to produce a mixture of cis- and trans-6d. These isomers are initially formed in a trans/cis ratio of roughly 2:1. The trans isomer (designated trans based on comparison with NMR spectra for trans-6c) is far less soluble than the cis isomer, and has been isolated in a pure form by fractional crystallization. This isomer is stable in solution at room temperature but when heated to 70 - 130°C begins to convert to the cis isomer. At these high temperatures an equilibrium between the isomers is established, with 64% trans to 36% cis being the highest conversion observed. References on pi23 Chapter 3 It seems probable that the mechanism of conversion from trans-6d to cis-6d must involve the change of one silylene group from a bridging mode to terminal mode of bonding; a terminal silylene would have a rhodium-silicon double bond. Rotation around the Rh=Si double bond and a return to the bridging mode would interchange the methyl and /?-tolyl groups on the silicon, as shown in Scheme 3.2. Me TolP \r ToJP Me Pr1, Pw Prj '"/i V J  U \r ^ ^ ^Rh :Rh P Si 4\ TolP Me trans-6d Pr'. Pr'2 Si Pr'< y v TolP Me c/s-6d PH2 V / < ^5t TolP Me 1/ Si •Rh" P Pri2 V TolP Me Scheme 3.2 Pr'2 o PH2 Terminally-bound silylene ligands are elusive species that have been proposed as intermediates in dehydrogenative silicon-silicon coupling reactions catalyzed by late transition-metal complexes.10' 12  As described in Chapter 1, only a few examples have been isolated, which are generally stabilized by the presence of donor ligands.13 Terminal silylenes have never been observed spectroscopically in coupling reaction mixtures.14'15 If the proposed mechanism for cis/trans isomerism could be proven kinetically, indicating the formation of a terminally-bound silylene ligand as an intermediate, this might shed some light on the mechanisms involved in reactions of References on pl23 Chapter 3 silanes catalyzed by [(dippe)Rh]2(|i-H)2,1. However, attempts to study the kinetics of the approach to equilibrium from pure trans-6d using NMR were unsuccessful. While the runs carried out at higher temperatures generally seemed to achieve equilibrium faster than those runs carried out at lower temperature, the reaction curves and final equilibrium values varied wildly from run to run and showed no steady progression for an increase in temperature, indicating that the straightforward formation of a terminal silylene is not occurring. The possibility that the dinuclear units are dissociating to mononuclear units and then recombining to give a mixture of isomers is unlikely, given that a Rh-Rh bond is present. A more appealing proposal to explain the interconversion of the isomers of the bis(|i- silylene) complex 6d involves participation of the solvent. As described in Section 2.4.2, the silyl hydride complexes 2a-c undergo isotopic exchange of hydrides for deuterides when they are heated to reflux in deuterated toluene. The exchange with the solvent is thought to occur by oxidative addition of a C-D bond from toluene and subsequent exchange of hydrides and deuterides around the dinuclear centre. It is entirely likely that at the temperatures where the trans Zeis equilibrium is rapidly established (90-130°C, 4-8 hours) the solvent may be involved as a source of hydride ligands, facilitating the conversion. As shown in Scheme 3.3, a hydride at rhodium could be involved in the reductive elimination of Si-H from one rhodium centre, leaving a terminal R2SiH group bound to the second rhodium centre. Free rotation around the Rh-Si bond and inversion at the Si centre, followed by an oxidative addition of the Si-H bond to the initial rhodium centre would exchange the alkyl and aryl substituents on silicon. References on pi 23 Chapter 3 89 Me TOP \r Prj2 Si TolP Me frans-6d RrT PH2 iP* O* fP PH2 Me ToP PHo _ Pr>2 P 2 Si \ / \ Rh ^ R h Prj2 -/ "V ToP Me / ««P Me ToP \r Prj2 Si ^ < _ "Rh :Rh ^P ' PH2 PH. V yv Me TolP " " / / p / PH2 X c/s-6d PH2 • R Me TpP  P r i V/ p>1 Me TolP pH? PH2 \ t  7 p -n VA/J Rh Rh PH2 / ToP Scheme 3.3 This proposal obviates the original suggestion of a high energy, terminal silylene species as an intermediate, and instead invokes the activation of aromatic C-H bonds, a better known process,16 as its central premise. The fact that a mixture of the two isomers which has been heated shows a References on pi 23 Chapter 3 twofold excess of the trans isomer is probably due to the trans isomer being a slighdy lower energy structure than the cis isomer. 3.2.5 Reaction of [(dippe)Rh]2(|i-H)2, 1, with two equivalents of Me2SiH2 3.2.5.1 Formation of a bis(silane) complex, [(dippe)Rh]2»2(Me2SiH2), 8b Addition of two equivalents of dimethylsilane to [(dippe)Rh]2(|i-H)2,1, (or addition of one equivalent of dimethylsilane to [(dippe)Rh]2(|X-H)(fi-T|2-H-SiMe2), 2b) in hexanes gives rise to a pale, golden-yellow solution from which yellow crystals can be obtained of the formula [(dippe)Rh]2'2(Me2SiH2), 8b. -H 2 [(dippe)Rh]2(^-H)2 + 2Me2SiH2 - [(dippe)Rh]2»2(Me2SiH2) [3-3] 1 8b The solution *H NMR spectrum of 8b is simple, the most notable feature being a hydride signal observed at -11.7 ppm (See Figure 3.8). This hydride resonance is a second order multiplet of relative intensity four (based on the ligand protons' integration, assuming a dinuclear complex); there are no other signals attributable to hydrides in either the Rh-H region or the Si-H region of the spectrum. The signal at -11.7 ppm is shifted upfield from the rhodium hydride signal for the silyl hydride complex, [(dippe)Rh]2(|i-H)(|i-r|2-H-SiMe2), 2b. Normally terminal hydrides on rhodium resonate at higher field than do those which bridge two rhodiums, suggesting that the hydrides in this complex might be terminally bound. The 31P{1H} NMR spectrum at room temperature is simply a broad doublet at 77 ppm, indicating the equivalence of all four phosphines, and suggesting that the molecule is fluxional in solution. References on pi 23 Chapter 3 n / l _ ] ) I I I I I | I I 1 I j n i I | I I 1 I I i n I | I I I 1 1 1 I 1 1 ] I I I I I 1 1 i ) 1 1 I I 1 1 u I I | I I I 1 I I I I I 1 1 1 1 1 ] i 2 0 - 2 - 4 -6 -8 -10 PPM -12 Figure 3.8 300 MHz !H NMR spectrum of [(dippe)Rh]2»2(Me2SiH2), 8b, in C7D8. 3.2.5.2 Solid-state structure of [(dippe)Rh(H)]2(n-rt2-H-SiMe2)2, 8b An X-ray crystallographic study was carried out on a single crystal of [(dippe)Rh]2*2(Me2SiH2), 8b. In Figure 3.9 an ORTEP diagram shows the molecular structure of 8b, and pertinent bond lengths and bond angles are shown in Tables 3.5 and 3.6. The hydrides attached to rhodium and silicon were located and refined isotropically. References on pl23 Chapter 3 92 C21 C25 Figure 3.9 Molecular structure of [(dippe)Rh(H)]2(|i-r|2-H-SiMe2)2, 8b. References on pi23 Chapter 3 Table 3.5 Selected bond lengths for [(dippe)Rh(H)]2(|i-'n2-H-SiMe2)2,8b- Bond R h l - R h l - R h l - R h l - R h l - R h l - R h l - -Rh2 -PI -P2 -Sil -Si2 -HI -H3 Length (A) 2.8575(5) 2.391(1) 2.269(1) 2.526(1) 2.337(1) 1.69(4) 1.52(5) Bond R h 2 - P 3 R h 2 - P 4 Rh2 - Sil Rh2-Si2 Rh2-H2 R h 2 - H 4 S i l - C 2 9 Length (A) 2.279(1) 2.364(1) 2.324(1) 2.474(1) 1.70(4) 1.52(4) 1.910(5) Bond S i l - C 3 0 S i l - H I Si2-C31 Si2-C32 S i2 -H2 Length (A) 1.889(5) 1.67(5) 1.896(5) 1.892(5) 1.72(4) Table 3.6 Selected bond angles for [(dippe)Rh(H)]2(li-ri2-H-SiMe2)2, 8b. Bonds P l - R h l - P 2 P 3 - R h 2 - P 4 R h l - S i l - R h 2 R h l - S i 2 - R h 2 S i l - R h l - S i 2 S i l - R h 2 - S i 2 Angle (deg) 86.72(4) 86.88(4) 72.06(3) 72.82(3) 80.17(4) 81.52(4) Bonds R h 2 - R h l - P 2 P l - R h l - S i 2 H l - R h l - H 3 R h l - R h 2 - P 3 P 4 - R h 2 - S i l H 2 - R h 2 - H 4 Angle (deg) 156.18(3) 162.05(4) 170(2) 157.52(3) 162.41(4) 177(2) Complex 8b contains two unsymmetric, bridging silyl groups, each being bound to one rhodium centre through a covalent Si-Rh bond and to the other rhodium centre through a Rh-H-Si three-centre, two-electron bond. Each rhodium atom is also bound to one terminal hydride. The geometry around each Rh centre is roughly octahedral. The core of the structure consists of two edge-sharing planes, each of which contains the two rhodium atoms, a silicon atom and the "agostic" hydride associated with that silicon. The two planes are almost orthogonal to each other, References on pi 23 Chapter 3 the dihedral angle between them being 74.5°. This core shape is referred to as a "butterfly" structure. Me2 74.5° Me2 H M P . " P — / K H Me2 Pr'2 H — S i u • \ / \ " PA •P -Rh rRh R Phosphine ligands omitted for clarity Me2 * r r 2 8b The observed diamagnetism of [(dippe)Rh(H)]2(|x-T|2-H-SiMe2)2» 8b, requires the presence of a single Rh-Rh bond, which is borne out by the Rh-Rh separation of 2.8575(5) A. While the Si-Si separation in 8b of 3.135 A17 is sufficiently long to preclude any bonding interaction (the normal range for a Si-Si single bond is 2.33-2.70 A8), this distance is much shorter than those normally observed for dimeric complexes containing bridging silylene ligands (3.852-4.225 A9). Obviously the shorter distance is largely due to steric considerations in the butterfly-shaped structure but it does give rise to the possibility of elimination of a silicon-silicon bonded entity from the dinuclear centre. This is discussed further in Section 3.3.2. The features relating to the two "agostic" Si-H bonds in the structure of 8b are very similar to those observed for the silyl hydride complexes 2a-b (Chapter 2). The Rh-Si distances within the three-centre, two-electron interactions are 0.20 A and 0.13 A longer than the corresponding unbridged Rh-Si bonds. By comparison to other dinuclear complexes with agostic Si-H bonds, these bond "lengthenings" represent oxidative additions of the Si-H bond to rhodium which have been arrested at an early and an intermediate stage, respectively. Structurally, 8b is unique, though two similar compounds (VII and IX, shown below) have been reported and were described in Chapter 2. References on pi 23 Chapter 3 H Ph R S i -—H H—;S i H ^ - l - ^ • . s . , A V R = Et, Ph H -y\ Ph H H Si R2 VII IX The titanium complex VII2 shows a similar arrangement to 8b of two agostic Si-H bonds around the dinuclear centre with the corresponding variations in M-Si bond lengths, but contains no terminal hydride ligands. The ruthenium complex IX3 also contains two agostic Si-H interactions, but in this structure both involve the same metal centre instead of being symmetrically disposed across the metal dimer. Like 8b, this complex also contains two other hydride ligands, though only one is terminal and the other is bridging the two ruthenium centres. The biggest difference between the structure of 8b and the two structures shown above is that both structures VII and IX have planar M2Si2 cores, which is in keeping with all other structures involving an M2Si2 tetranuclear centre,5'7 whereas 8b has the tetrahedral, butterfly-shaped core. Formation of the bis(n-silylene) complexes 6a,c-d probably proceeds through an initial bis(silane) adduct XIII, which is analogous to 8b. Elimination of two equivalents of H2 from an intermediate such as XIII would generate a bis(|i-silylene) complex. H  Prj2 R-ff-Ph I P R' XIII Rh R - Pr' r  ~. ™ « - l -7™I r v R - Me, R' - Ph Pr'2 l S \ s . ^ 2 \ > R-Me.R.ToP H S i — H \ / References on pl23 Chapter 3 The butterfly-shaped core of these proposed complexes would be sterically hindered relative to 8b; these higher-energy species have not been isolated for the diaryl and arylalkyl silanes. H "H Figure 3.10 Side view, looking along the Rh-Rh axis, of the proposed intermediate XIII, demonstrating steric interactions of the substituents on silicon. (Phosphine ligands and agostic hydrides are omitted for clarity.) 3.2.5.3 Variable temperature NMR spectroscopic studies of [(dippe)Rh(H)]2(|i- r | 2 -H-SiMe 2 )2 , 8b The complex [(dippe)Rh(H)]2(|i.-r|2-H-SiMe2)2, 8b , is fluxional with an average structure in solution that is symmetric; all four phosphines are chemically equivalent, as are the four hydrides. It is possible to "freeze" this fluxionality, as observed in the solution NMR spectra of 8b, by cooling the sample. Shown in Figure 3.11 are the variable temperature 31P{1H} NMR spectra for 8b in ds-toluene. The simple doublet observed at room temperature attests to the equivalence of the four phosphines in the average structure. At approximately -29°C a coalescence point has been reached and at -54°C two signals of equal intensity have been resolved. Thus a process which exchanges one set of two phosphines for another, inequivalent, set of two phosphines has been "frozen" at this stage. A value for AG*(244K) of 10.2 ± 0.2 kcal/mole was calculated for this process based on the coalescence of the signals observed at -54°C. Further cooling of 8b to -94°C shows the emergence of four signals in the 31P{1H} NMR spectrum, all broad doublets. However, the changes occurring in these spectra between -54°C and -94°C do not P References on pl23 Chapter 3 97 look like a normal decoalescence; rather, it looks as though some new species, with four inequivalent phosphines, is forming as the temperature of the sample is lowered. It may be that an equilibrium exists in solution between a symmetric species with two sets of two equivalent phosphines and some less symmetric species, where all four phosphines are inequivalent 20°C 14°C -44°C 24°C I -54°C •34°C -94°C [lllJ|imjllll|llll|llll|llll|mi|llll|llll|llll|IHI|IHI Jlllllllll[l[ll|lllllllll|llll|lll)|llll|llll|llll|llll|ll)l |IIII|IIII|IIII|IIII|IIII|IIIIIIIII|IIII|IIII|IIIIIIIII|IIII 110 100 90 80 70 60 PPM 110 100 SO 80 70 60 PPM 110 100 B0 80 70 60 PPM Figure 3.11 Variable temperature 121.4 MHz 31P{!H} NMR spectra of [(dippe)Rh(H)]2(^i- Tl2-H-SiMe2)2, 8b, in C7D8. To a certain extent, it is possible to correlate the changes in the variable temperature 31P{ *H} NMR spectra of 8b with fluxional processes occurring for a solution structure that is References on pi 23 Chapter 3 98 analogous to the solid-state structure of the complex shown in Figure 3.9. The solid-state structure of [(dippe)Rh(H)]2(M--Tl2-H-SiMe2)2» 8b, shows an arrangement of phosphines where the two chelate rings are perpendicular to each other. One phosphine from each bis(phosphine) ligand is trans to the Rh-Rh bond (P2 and P3 in the diagram below). Both of the two remaining phosphines are trans to an unbridged Rh-Si bond, one (Pi) in the xy plane and level with the P2~Rh-Rh-P3 axis, the other (P4) in the xz plane and below the P2-Rh-Rh-P3 axis. Me2 H 1 >S i H„ \ ^ ^ P 2 — : R h -Rh P3 ^-—* 8b Pi and P4 are chemically equivalent, as are P2 and P3. As mentioned in Chapter 2, silyl ligands have a strong trans influence. For the solid-state structure of 8b, this influence is manifested in the lengthening of the Rh-P bond lengths by 0.1 A for the phosphines bound trans to the silyl ligands relative to the Rh-P bonds for the phosphines trans to the Rh-Rh bond. Empirical studies indicate that ligands which have a strong trans influence can also affect the 31p{lH} NMR spectra of complexes containing them.18 In particular the signals for phosphorus nuclei trans to silyl ligands tend to be shifted to higher field, and their jRh-p values tend to be significantly reduced. The 31P{ *H} NMR spectrum for 8b at -54°C is consistent with the solid-state structure: the signal at 84 ppm (^Rh-p =150 Hz) is due to the phosphines trans to the Rh-Rh bond, and the signal at 72 ppm (!JRh-p = 97 Hz) is due to the phosphines trans to the Rh-Si bonds. It should be noted that the above structure also gives rise to two different types of hydride ligand; two are involved in the agostic Si-H bonds to rhodium (Hi and H2), the others are terminal rhodium hydrides (H3 and H4). The variable temperature *H NMR spectra (Figure 3.12) do not show conclusively that the agostic Si-H interactions observed in the solid-state structure of 8b exist in the solution structure; it is possible that in solution the agostic Si-H bonds have completely References on pi23 Chapter 3 oxidatively added to give four terminal hydride ligands while the geometry of the complex remains the same. This situation would still give rise to two types of hydride ligand. |iiii|iiiijiiii|iMijiiii|iiiiimi|iiii|iMi|mi| iiiii|i)ii|iiii|iiii|nii|iiii|iiii|iiiiiiiii|iii) |iiii|)iiiiiiii|iiii|iiii|[iii|iiii|iiiiimi|iiii| -9 -10 -11 -12 -13 PPM-14 -9 -10 -11 -12 -13 PPM -B -10 -11 -12 -13 PPH-14 Figure 3.12 Variable temperature 300MHz *H NMR spectra (hydride region) for [(dippe)Rh(H)]2(^-Tt2-H-SiMe2)2,8b, in C7D8. The "*" marks an impurity, {[(dippe)Rh]2(H-H)(n-rj2-H-SiMe2)}«H2,3b. The hydride region of the variable temperature !H NMR spectra of [(dippe)Rh(H)]2(p.-ri2- H-SiMe2)2. 8b, is shown in Figure 3.12. The multiplet seen at -11.7 ppm in the room temperature spectrum splits into two broad singlets as the sample is cooled to -64°C, with coalescence of the signals occurring at -39°C. This indicates that the two types of hydrides in the complex are being exchanged between inequivalent sites; the process responsible for this exchange has AG*(234 K) = 10.2 ± 0.2 kcal/mol, as calculated from the observed coalescence. The hydride signals observed in the spectrum at -64°C are consistent with the structure shown in the diagram of 8b above, where there are two types of hydride, H1/H2 and H3/H4. If the agostic Si-H interactions do persist in solution, the signal at -10.6 ppm is probably due to the agostic hydrides, References on pi 23 Chapter 3 100 and the signal at -12.7 ppm is due to the terminal hydrides. Similar to what was seen in the 31P{1H} NMR spectra, when the sample is cooled to lower temperatures new signals emerge which appear to be due to four inequivalent hydrides in a less symmetric structure. Again, the changes in these spectra at low temperature do not look like typical decoalescences, but more like the formation of a new species in solution. The structure of this unsymmetric compound which forms at low temperature in a solution sample of 8b is not obvious. Perhaps the symmetric structure shown above is in equilibrium with a conformational isomer where the phosphine ligands, and hence the hydrides, are less symmetrically disposed. The equilibrium would lie toward the higher symmetry isomer at higher temperatures and shift toward the lower symmetry isomer at low temperature. Me2 < \ / \ •Po W l Rh—P3, H3 ST"2 P4-7 H Me2 •& H . P P2' i 4 h < — ^ H5 Me2 P1,2.3,4 = PPr2 Me; S i — - H 4 P.—Rh ^Rh- y —P2 Me2 H2 Scheme 3.4 A possible mechanism for the fluxionality in 8b which could explain the exchange of both phosphines and hydrides between two inequivalent sites in the symmetric solution structure proposed above involves a concerted twisting of the axes at each rhodium centre, without References on pi 23 Chapter 3 101 movement of the bridging silicon ligands. Essentially, Scheme 3.4 shows that the axis containing two hydride ligands trans across a rhodium centre rotates by 90°, thus moving the terminal hydrides into agostic positions and vice verse. There is a simultaneous twisting of the phosphine chelate rings by 90° which exchanges the phosphorus atoms between the two different sites. 3.2.5.4 Reactivity of [(dippe)Rh(H)]2(^-Tl2-H-SiMe2)2» 8b The complex [(dippe)Rh(H)]2(^i-T|2-H-SiMe2)2J 8b, tends to decompose, particularly in solution at room temperature or above, to give both [(dippe)Rh]2(M--SiMe2)2. 6b, and [(dippe)Rh]2(H-H)(^-Ti2-H-SiMe2),2b. ? A [(dippe)Rh(H)]2(n-Ti -H-SiMe2)2 •- [(dippe)Rh]2(fi-SiMe2)2 + 2H2 + [3.4] 8b 6b [(dippe)Rh]2(^-H)(n-ri2-H-SiMe2) + Me2SiH2 2b These correspond to the loss of hydrogen and the loss of silane, respectively, from 8b. When sealed NMR samples of 8b are heated, the 31P{ *H} NMR spectra show signals appearing for both 6b and the hydrogen adduct of 2b, {[(dippe)Rh]2(|i-H)(|j.-Ti2-H-SiMe2)}*H2,3b. A controlled experiment was carried out in which an NMR sample was heated at 45°C while open to a nitrogen manifold and bubbler to allow any volatile products to escape the NMR tube. 31P{1H} NMR spectra, run periodically, showed the slow formation of [(dippe)Rh]2(jx-H)(fj.-T|2-H-SiMe2), 2b. The positive pressure of nitrogen appears to discourage formation of the bis(fi-silylene) complex, 6b, and it may be that a slight vacuum, as is present in sealed NMR samples, is required to initiate the loss of hydrogen from 8b. References on pi 23 Chapter 3 102 Addition of ethylene to a solution of [(dippe)Rh(H)]2(|J.-Ti2-H-SiMe2)2. 8b, causes the quantitative formation of [(dippe)Rh]2(|i-SiMe2)2,6b. [(dippe)Rh(H)]2(^-rt -H-SiMe2)2 *- [(dippe)Rh]2(n-SiMe2)2 + 2C2H6 [3.5] 8b 6b (or + C2H4 + 2H2) It is not clear whether or not the ethylene in this reaction is being hydrogenated by 8b, or if the reversible coordination of ethylene simply prompts the elimination of hydrogen from the bis(silane) complex. A similar reaction was carried out using styrene (Ph(H)C=CH2); the hydrogenated olefin, ethylbenzene, should be easier to detect in the reaction mixture than ethane. However, this reaction did not proceed at room temperature, and heating the mixture caused decomposition of 8b to 2b and 6b. No ethylbenzene was detected. Placing a solution of 8b under an atmosphere of hydrogen leaves the molecule unchanged, as shown by NMR spectroscopy on a sample sealed under a hydrogen atmosphere. The only change observed in the spectra is a slight broadening of the hydride resonance in the *H NMR. This may be due to some exchange of the hydrides for free hydrogen, presumably occurring by a dissociative mechanism. There is some evidence for this exchange: an NMR sample of 8b sealed under deuterium gas was monitored by *H NMR spectroscopy. There was a slow decrease in intensity of the hydride resonance relative to the ligand resonances in the spectra, and after two weeks approximately 50% of the hydrides had been exchanged for deuterium. References on pi 23 Chapter 3 103 3.3 The use of [(dippe)Rh]2([i-H)2, 1, as catalyst precursor for reactions of secondary silanes 3.3.1 Catalytic isotopic exchange between diphenylsilane and deuterium gas The complex [(dippe)Rh]2(|i-H)2,1, is an effective catalyst for the isotopic exchange of hydrides for deuterides on diphenylsilane, with deuterium gas as a source of deuterides. D2,1 atm Ph2SiH2 »- Ph2SiD2 [3-6] 1, R r The reaction has been carried out under a variety of conditions; the cleanest reaction (95% exchange, and no side products detected) involved a dilute solution of diphenylsilane in hexane and a 100:1 substrate to catalyst ratio. Higher substrate concentrations and the use of aromatic solvents appear to encourage a side reaction, the disproportionation of diphenylsilane to triphenylsilane and phenylsilane, though monitoring the reaction in d6-benzene by *H NMR spectroscopy indicates that the exchange reaction occurs more quickly than the disproportionation reaction. (The disproportionation of organosilicon compounds in the presence of transition metals, which evidently requires activation of both Si-H and Si-C bonds, is quite common.11 However, the mechanism for this redistribution of Si-C bonds catalyzed by late transition-metal complexes is not well understood.) 1, RT 2Ph2SiH2 - Ph3SiH + PhSiH3 [3-7] Very high concentrations of diphenylsilane, or the addition of the catalyst to the diphenylsilane prior to the introduction of deuterium, tends to give small amounts of 1,1',2,2'- tetraphenyldisilane, the product of dehydrogenative dimerization of diphenylsilane. 1, RT 2Ph2SiH2 Ph2HSi-SiHPh2 [3-8] - H 2 References on pi 23 Chapter 3 104 Though the isotopic exchange and dimerization reactions might be competitive reactions, because the dimerization requires elimination of hydrogen (or deuterium, as the case may be) it is supressed in the presence of excess hydrogen or deuterium. Therefore care was taken to saturate the substrate solutions with deuterium gas before the addition of the catalyst, in order to minimize formation of the disilane. This dimerization reaction will be discussed further in the following section. Minor products which have been observed in some of the catalytic isotopic exchange reactions are probably siloxanes which can form during workup of the product mixture. Some transition-metal complexes are known to catalyze the reactions of silanes with trace amounts of water or oxygen.19,20 For example, diphenylsilane reacts with water in the presence of catalytic amounts of BzCr(CO)2(n2-H-SiHPh2) to produce a variety of siloxanes and silanediols.21 Decomposition has been minimized by carrying out the reaction workup under nitrogen in a glovebox. Isotopic exchange between methylphenylsilane and deuterium gas was also attempted using 1 as catalyst. However, disproportionation products are formed in the reaction mixture, as determined by *H NMR spectroscopy. Interestingly, when this reaction was carried out in d6- benzene and monitored by *H NMR by the periodic withdrawal of samples, it was observed that virtually all of the methylphenylsilane had been deuterated within the first ten minutes after addition of the catalyst to the deuterium-saturated substrate solution. While the Si-Me region of the spectrum indicated that some disproportionation had occurred, essentially all of the Si-H resonance for methylphenylsilane at 4.5 ppm had disappeared due to exchange with deuterium, and no other Si-H signals were observed. These results suggest that the isotopic exchange on methylphenylsilane preceded its disproportionation reaction. Thus for mixtures of methylphenylsilane and deuterium gas, hydride-deuteride exchange is fast relative to the disproportionation reaction, whereas for diphenylsilane these reactions appear to be proceeding at competitive rates. This isotopic exchange reaction was not attempted for dimethylsilane or methyl- p-tolylsilane. References on pi 23 Chapter 3 105 There are only three other examples in the literature of transition-metal complexes homogeneously catalyzing the exchange of hydrides for deuterides on silanes, with deuterium gas as the source of deuterides.22"24 Previous to these examples was a report of the use of nickel surfaces to heterogeneously catalyze this reaction.25 Two of the reported homogeneous catalysts are phosphine complexes of Group 8 metals. The first is a cobalt complex, CoH(N2)(PPh3)3.22 This complex easily loses the N2 ligand, generating the 16-electron complex, CoH(PPh)3. As shown in Scheme 3.5 this coordinatively unsaturated intermediate reacts rapidly with D2 to give the d2-trihydride complex. CoH(N2)(PPh3)3 -N2 SiR3D CoH(PPh3)3 D2 CoH(D)(SiR3)(PPh3)3 | R = F, Me, Et,"oET| CoHD2(PPh3)3 SiR3H HD CoD(PPh3)3 Scheme 3.5 Subsequent loss of HD from this molecule generates CoD(PPh)3, which then reacts with silane (only tertiary silanes were used in these experiments). The elimination of SiR3D from the resulting complex regenerates the unsaturated CoH(PPh)3 complex and returns the cycle to its starting point. Elimination of the silane is thought to be the rate-determining step in the cycle. The second example of homogeneously catalyzed isotopic exchange between silanes and deuterium gas involves the use of IrCl(CO)(PPh3)2, IrH3(CO)(PPh3)2, or IrH[Si(OEt)3](CO)(PPh3)2.23 The key to the catalytic cycle for each of these catalyst precursors is thought to be the addition of silane (again only a tertiary silane has been studied, HSi(OEt)3) to an intermediate of formula References on pi 23 Chapter 3 106 IrD(CO)(PPh3)2. Based on the reactivity described above, one would expect that any metal hydride system that can reversibly add both H2 and silane should act as a catalyst for the isotopic exchange between silanes and deuterium gas. However, the isotopic exchange on diphenylsilane and methylphenylsilane that is catalyzed by [(dippe)Rh]2(|i-H)2,1, is the first reported example of such an exchange occurring for secondary silanes, R.2SiH2. The reaction shown in Equation 3-9 forms the basis for the catalytic cycle responsible for the observed isotopic exchange of hydrides for deuterides on diphenylsilane in the presence of deuterium gas. Solutions of the bis(n-silylene) complex [(dippe)Rh]2(H-SiPh2)2, 6a, in hexanes or toluene left stirring under an atmosphere of hydrogen react to give principally [(dippe)Rh]2(|i- H)((i-r|2-H-SiPh2), 2a, plus diphenylsilane, as determined by 2H and ^ P ^ H } NMR spectroscopy. No [(dippe)Rh]2(|i-H)2,1, is observed in these reactions; this is in keeping with the observation (Chapter 2) that addition of hydrogen to [(dippe)Rh]2((i-H)(n-r)2-H-SiPh2), 2a, does not cause the elimination of diphenylsilane. The addition of one equivalent of diphenylsilane to [(dippe)Rh]2(H-H)2,1, is an irreversible reaction. [(dippe)Rh]2Oi-SiPh2)2 1 ^ » [(dippe)Rh]2(^-H)(^-Ti2-H-SiPh2)2 + Ph2SiH2 [3-9] 6 a RT ' 2 a (When this reaction is carried out in a sealed NMR tube the principal product is the hydrogen adduct of 2a, 3a.) Although 1 in the presence of excess diphenylsilane gives a mixture of 2a, 6a and 7, under an atmosphere of hydrogen or deuterium any 6a formed is eventually converted back to 2a (or d2-2a), releasing Ph2SiH2 (or Ph2SiD2). Thus, a probable catalytic cycle for isotopic exchange between deuterium and diphenylsilane catalyzed by 1, based on the stoichiometric chemistry described, is shown in Scheme 3.6. Presumably the same cycle is in effect for the isotopic exchange on methylphenylsilane as well. References on pl23 Chapter 3 107 H / \ Rh* Rh* \ / H •Ph2SiH2 H2 Ph2 ( D ) H ^ . S i Rh* Rh* v (D) 2a Ph2SiH2 ( Rh* = (dippe)Rh I 2H2 (2[H2 + HD + D^) .2 Ph2SiD2 2D2 Scheme 3.6 Ph2 / \ Rh* Rh* v Ph2 6a 3.3.2 Catalytic dimerization of diphenylsilane Dehydrogenative dimerization of diphenylsilane is observed in the presence of catalytic amounts of 1. (See Equation 3-8.) This behaviour is not too surprising, given the activity of other late transition-metal catalysts towards this reaction.19,26,27 The mechanism of the dehydrogenative silicon-silicon coupling as catalyzed by the late transition metals has not been conclusively established, with mechanisms having been proposed based either on the insertion of silylene fragments into M-Si bonds or on the reductive elimination of Si-Si bonds from bis(silyl) complexes. A mechanism involving a metal hydride catalyst and relying solely on oxidative addition and reductive elimination steps has become popular recently (see Scheme 3.7);n most of the steps in this cycle have been observed in stoichiometric reactions of various mononuclear, late transition-metal systems.28 References on pi 23 Chapter 3 108 FfeHSiSiHRz H >SiHR2 L n M—SiHR 2 R2SiH2 LpM-H L n M-S iHR 2 Scheme 3.7 FfeSiH2 H L n M — S i H R 2 Hz It is possible that for the dimerization of diphenylsilane catalyzed by [(dippe)Rh]2(H-H)2, 1, the active catalyst is dinuclear rather than mononuclear. Certainly in the stoichiometric chemistry of this system described so far there is evidence of oxidative addition of silane to a metal hydride complex (Section 2.2.1), oxidative addition of silane to a metal silyl complex (Section 3.2.1) and elimination of hydrogen from hydride and silyl complexes (Sections 2.2.1, 2.2.4, 3.2.1 and 3.2.5.4). All of these reactions occur at dinuclear centres and generate dinuclear products. The only step in the above catalytic cycle that has not been directly observed to occur at the dinuclear rhodium centre is the reductive elimination of a Si-Si bond. However, there is evidence that the reverse reaction occurs, that is, oxidative addition of a Si-Si bond to [(dippe)Rh]2(|i-H)2,1. When a mixture of l,r,2,2'-tetraphenyldisilane and 1 in toluene is heated to 70°C overnight the major product observed by 31P{!H} NMR is [(dippe)Rh]2(fi-H)(|i-Ti2-H- SiPh2), 2a, the formation of which requires the cleavage of the Si-Si bond of the disilane. A, 70°C [ (d ippe)Rh] 2 (u -H) 2 + P h 2 S i H - H S i P h 2 toluene [3-10] [(dippe)Rh]2(|i-H)(n-ri2-H-SiPh2)2 + 1/2Ph2SiH-HSiPh2 2a The principal silicon-containing compound in this reaction mixture is unreacted disilane. While in the absence of any kinetic data for this system it is impossible to know conclusively whether the References on pl23 Chapter 3 109 active species in this dimerization reaction is mononuclear or dinuclear, the stoichiometric chemistry of the system does lend some weight to the possibility of its being dinuclear. As described in Section 3.2.2, crystallographic studies of some dinuclear Pt complexes containing bridging silylene units show very short Si-Si distances across the dinuclear Pt centres suggestive of nascent Si-Si bond formation. However, the bis(n-silylene) derivative, [(dippe)Rh]2(|J.-SiPh2)2,6a, with a Si-Si separation on the order of 3.7 A,6 is not analogous and is not thought to be the active species in the formation of disilane in this catalytic cycle. Speculation as to the nature of the active species in the catalytic dimerization of diphenylsilane by 1 leads back to the reaction shown in Equation 3-10. Shown in Scheme 3.8 is a possible structure for the initial adduct of the disilane with [(dippe)Rh]2(|i-H)2,1, which is directly analogous to the structure of [(dippe)Rh(H)]2(|i-ri2-H-SiMe2)2, 8b. As mentioned in Section 3.2.5.2, this compound (XIII) has never been isolated and has not been detected spectroscopically. Complex XIII could react further in several different ways: a) the reverse reaction of its formation could occur, regenerating disilane and 1, b) one equivalent of silane could be eliminated, generating diphenylsilane and 2a and c) hydrogen could be eliminated, generating 6a. In this reaction mixture there is probably an equilibrium established between the starting materials and XIII. Elimination of diphenylsilane from XIII is facile, especially at higher temperatures, but in the presence of equilibrium amounts of 1, diphenylsilane reacts instantly and irreversibly to give 2a plus hydrogen. This reaction consumes 1, shifting the initial equilibrium to the left so that more of 1 and the disilane are formed from XIII. Thus for each molecule of XIII that loses one equivalent of diphenylsilane to form 2a, another molecule of XIII loses one equivalent of the disilane to form 1. It is the fact that the formation of 2a from 1 and diphenylsilane is irreversible that is responsible for the final product mixture of 2a and l,r,2,2'-tetraphenyldisilane. Hydrogen is also being eliminated from XIII to give 6a, but in the presence of hydrogen (generated by the reaction of 1 with diphenylsilane) this reaction is easily reversed, again especially at high temperatures. References on pi 23 Chapter 3 110 Pr'2 Pr j 2 H p * Rh Rh Pr*2 PH2 1 + [Ph2SiH]2 - [Ph2SiH]2 H- Ph2 Pr*2 V"~^S ,v •P =Rh -Rh- H P h > BJ Prv, "2 XIII — R h R. + Ph2SiH2 -H5 Ph2SiH2 Ph2 n i H ^ S i PH2 <y v \ J V ^ DrL DrL 2 a •H ; + Ph2SiH2 + H, Ph2 .Si Pr'; > < P ^ R h < — > h ^ P V ^ N P ^ \ / V * * p / V Pr'2 Si Prl, Ph2 6a Scheme 3.8 This is a very complicated explanation for the results of the reaction shown in Equation 3-10; the most important point is that the observed results require the formation of an intermediate XIII and also require that its formation be reversible, as shown in Equation 3-11. [(dippe)Rh(H)]20i-ri2-H-SiPh2)2- X I I I -*- [(dippe)Rh]2(n-H)2 + Ph2HSi-SiHPh2 1 Thus the complex XIII is a plausible intermediate in the catalytic dehydrogenative coupling of diphenylsilane to give l,r,2,2'-tetraphenyldisilane. References on pi 23 Chapter 3 111 3.4 Experimental 3.4.1 General procedures and reagent syntheses The general procedures and equipment used are outlined in Chapter 2. The silane MeTolPSiH2 was prepared by reduction of MeTolPSiCl2, purchased from Petrarch Systems, using UAIH4.29 3.4.2 Syntheses of complexes and reactivity studies [(dippe)Rh]2((i-H)(|i-ri2-H-SiMeTolP), 2d. This complex was prepared by the same method as for 2b (described in Section 2.4.2) (113 mg, 0.154 mmol 1; 21 mg, 0.154 mmol methyl-p-tolylsilane). Reddish-brown crystals were obtained in 76% yield (102 mg). *H NMR (C6D6, ppm) Hortho 7.93 (d, 2H, 3JHm-Ho = 7.3 Hz); Hm e t a 7.13 (d 2H); SiCeH5CH3 2.17 (s, 3H); C//(CH3)2 1.98 (mult, 8H); PC//2C//2P, SiC//3 1.26 - 1.36 (overlapping d and s, 11H); CH(C//3)2 1.21 (dd, 12H, 3 j P _ H = 14.5 Hz, 3jH-H = 7.0 Hz); CH(C//3)2 1.12 - 0.94 (mult, 36H); Rh-H -6.00 (pt, 2H, JP_H = 20.6 Hz, ^Rh-H = 15.7 Hz). Note: For 2d in dg-toluene the singlet resonance due to SiCH^ shifts to 1.24 ppm and the PC//2C//2P resonance is seen as a doublet at 1.33 ppm (2JP_H = 12.6 Hz). 31p{lH} NMR (C6D6, ppm) 94.3 (d mult, JRh_p = 159 Hz). 29si NMR (C7D8, ppm) 157 - 166 (br mult). Anal. Calcd for C36H76P4Rh2Si: C, 49.88; H, 8.84. Found: C, 50.20; H, 8.86. [(dippe)Rh]2(M--SiPh2)2, 6a. This compound can be prepared by two methods: 1) To a red solution of [(dippe)Rh]2(|i-H)(n-r|2-H-SiPh2), 2a, (275 mg, 0.300 mmol) in hexanes (5 mL) was added dropwise a solution of diphenylsilane (55 mg, 0.300 mmol) in hexanes (3 mL). The supernatant was decanted from the bright yellow-orange precipitate which formed and the precipitate was washed with cold hexanes to remove any unreacted silyl hydride complex. The yellow solid was then stirred in a large volume of toluene (25 to 30 mL) for several days until the most of the powder had dissolved to give a bright orange solution. After filtration of the References on pi 23 Chapter 3 112 solution through a celite pad, orange crystals were obtained from a minimum volume of toluene by cooling to -40°C. Yield: 58% (191 mg). 2) An identical reaction to that described above was carried out in toluene instead of hexanes. The dark red solution of 2a changed to a bright orange colour upon addition of the diphenylsilane. After filtration of the solution through a celite pad, orange crystals were obtained from a minimum volume of toluene by cooling to -40°C. While the second method described is faster and more straightforward, it tends to give lower yields and second crops can be contaminated with any excess 2a left in solution. The identity of the yellow powder formed from the first method has not yet been established. 1H NMR (C6D6, ppm) Ho r t ho 8.31 (dd, 4H, 3jHm-Ho = 7.2 Hz, 4jHp-Ho = 1.3 Hz); Hm e t a , Hpara 7.38 - 7.20 (overlapping mults, 6H); C//(CH3)2 1.96 (mult, 8H); PC//2C//2P 1.30 (d, 8H, 2Jp_H = 15.2 Hz), CH(C//3)2, 0.97 - 0.74 (mult, 48H). SlP^H} NMR (QD6, ppm) 79.3 (d mult, jRh-P = 134 Hz). Anal. Calcd for C52H84P4Rh2Si2: C, 57.03; H, 7.73. Found: C, 56.79; H, 7.72. Reaction of [(dippe)Rh]2(|i-SiPh2)2> *>a, with excess Ph2SiH2. A solution of [(dippe)Rh]2((i-SiPh2)2,6a, (10 mg, 0.0091 mmol) in dg-benzene was added to diphenylsilane (7-mg, 0.037 mmol). Over one day the solution colour changed from bright orange to yellow. 31P{ !H} NMR spectroscopy showed the presence of a single product, complex 7. Signals due to (presumably) 7 as well as Ph2SiH2 and Ph3SiH were observed in the *H NMR spectrum, along with an unassigned signal at 5.93 ppm. For complex 7: *H NMR (C6D6, ppm) (The complex was never isolated free of silicon- containing by-products, so some of the peaks in the aromatic region were buried under peaks due to the other products, and relative integrals for the aromatic protons were unobtainable. Relative integrals and peak assignments are tentative.) Si-// 8.56 (br s, 2H, wi/2 = 30 Hz); Hortho 8.27 (d, References on p!23 Chapter 3 113 3JHm-Ho = 6.9 Hz); Hortho 8.04 (dd, 3JHm-Ho = 7.6 Hz, 3JHp-Ho = 1.6 Hz); H meta 7.44 (mult); C//(CH3)2 2.54 (mult, 4H); C//(CH3)2 2.11 (mult, 4H); CH(C//3)2, 1.65 (dd, 12H, 3JP_H = 13.5 Hz, 3 j H _ H = 7.2 Hz); PC//2C//2P, CH(Ci/3)2 1.52 - 0.49 (overlapping mult, 32H); CH(C//3)2 , 0.25 (dd, 12H, 3 j P _ H = 15.6 Hz, 3 j H _ H = 7.2 Hz); Rh-// -4.46 (br s, 2H, wi/2 = 55 Hz); Rh-H -6.79 (br s, 1H, wi/2 = 60 Hz); Rh-H -13.05 (br s, 1H, wi/2 = 25 Hz); Rh-H -13.48 (br s, 1H, wi/2 = 25 Hz). 31p{lH} NMR (C6D6, ppm) 98.1 (d mult, IP); 85.1 (d mult, IP); 82.9 (d mult, IP); 72.4 (mult, IP). Reaction of [(dippe)Rh]2(|i-SiPh2)2, 6a, with H 2 . [(dippe)Rh]2(MiPh2)2, 6 a ' (25 mg, 0.023 mmol) was dissolved in ds-toluene (0.6 mL) in a sealable NMR tube. The solution was degassed by one freeze-pump-thaw cycle. One atmosphere of hydrogen gas was introduced and the sample was sealed. The sample was then frozen in liquid nitrogen while being transported to the NMR spectrometer, then thawed (the orange solution did not change in colour) and placed in the spectrometer. The initial *H NMR spectrum showed signals due to the starting bis(n-silylene), 6a, [[(dippe)Rh]2(|J.-H)(|i-ri2-H-SiPh2)],H2, 3a, Ph2SiH2 and an unknown species (Hortho at 8.15 ppm). The initial 31P{ lH.} NMR spectrum showed signals due to 6a and 3a and also small peaks at 90 ppm (br d) and at 78.8 (d). The spectra were run again after the sample had sat at room temperature for two days. The 31P{ ^H) NMR spectrum showed the presence of only two products, 3a and the unknown complex, 7. The *H NMR spectrum showed also the presence of both Ph2SiH2 and Ph3SiH, along with a signal (overlapping with that due to dissolved hydrogen) that might be due to [PhSiH2]2- Reaction of [(dippe)Rh]2(|i-SiPh2)2, 6a, with D2. This reaction was described in the experimental section of Chapter 2 as a means of preparing d2-[(dippe)Rh]2(|i-H)(p.-rj2-H- SiPh2), 2a. The silicon-containing by-product is Ph2SiD2, though longer reaction times produce a considerable amount of Ph3SiD (and presumably PhSiD3, which is lost in workup). References on pi 23 Chapter 3 114 [(dippe)Rh]2(|i-SiMePh)2, 6c. This complex can be prepared by the same methods as for 6b (96 mg, 0.113 mmol 2b; 14 mg, 0.113 mmol MePhSiH2; Yield: 65% (72 mg)). Unfortunately even when the reaction is carried out in toluene the product tends to precipitate quickly, giving a bright orange powder which has low solubility in most common solvents. However, slow evaporation of a very dilute toluene solution did eventually yield crystals suitable for an X-ray crystallographic study. This complex has two geometric isomers, cis and trans, which could not be completely separated by crystallization, despite the fact that the trans isomer is less soluble than the cis isomer. trans-6c: *H NMR (C6D6, ppm) Honho 7.71 (dd, 4H, 3JHm-Ho = 6.6 Hz, 4JHp-Ho = 1.5 Hz); Hmeta 7.17 (mult, 4H); Hpara 7.08 (mult, 2H); C#(CH3)2 2.38 (mult, 8H); SiC//3 1.71 (s, 6H); PC/ / 2 C// 2 P 1.64 - 1.47 (mult, 4H); CH(C//3)2 1.19 (dd, 12H, 3 j P _ H = 17.1 Hz, 3 j H _ H = 7.2 Hz); PCH2CH2P, CH(C//3)2 1.14 - 0.92 (mult, 28H); CH(C//3)2 0.53 (dd, 12H, 3JP_H = 15.6 Hz, 3 j H _ H = 7.3 Hz). 31p{lH} NMR (Q5D6, ppm) 82.8 (d mult, lJRh_P = 162 Hz). cis-6c: 1H NMR (C6D6, ppm) Ho r t h o 8.03 (dd, 4H, 3JHm-Ho = 7.8 Hz, 4JHp-Ho = 1.2 Hz); Hmeta 7.33 (t, 4H, Javg = 7.2 Hz); Hpara 7.19 (mult, 2H); C#(CH3)2 2.27 (br s, 4H); C//(CH3)2 2.06 (br s, 4H); S1CH3, PC//2C//2P 1.43 - 1.30 (overlapping s and d, 14H); CH(C//3)2 1.17 - 0.91 (mult, 48H); 31p{lH} NMR (C6D6, ppm) 80.9 (d mult, lJRh_P = 163 Hz). Anal. Calcd for C42HsoP4Rh2Si2 • 0.3(C7H8): C, 52.85; H, 8.29. Found: C, 52.84; H, 8.30. [ ( d i p p e ) R h ] 2 ( H - S i M e T o l P ) 2 , 6d. To a stirred, dark green solution of [(dippe)Rh]2((i-H)2,1, (107 mg, 0.146 mmol) in hexanes (5 mL) was added dropwise a solution of methyl-p-tolylsilane (40 mg, 0.292 mmol) in hexanes (10 mL) to give a bright orange solution. After filtration of the solution through a celite pad, orange crystals were obtained from a minimum volume of hexanes by cooling to -40°C. Yield: 71% (104 mg). While both cis and trans isomers are formed from this reaction, the less soluble trans isomer can be isolated by fractional crystallization. References on pi 23 Chapter 3 115 trans-6d: *H NMR (Q>D6, ppm) Hortho 7.64 (d, 4H, 3JHo-Hm = 7.5 Hz); Hm e t a 7.03 (d, 4H), C//(CH3)2 2.50 - 2.29 (overlapping mult, 8H); SiC6H5C#3 2.13 (s, 6H); SiC//3 1.73 (br s, 6H); PCH2CH2P 1.63 - 1.45 (mult, 4H); CH(C//3)2 1.22 (dd, 12H, 3JH-P = 18.3 Hz, 3JH-H = 7.5 Hz); PC//2C//2P, CH(C//3)2 1.13 - 0.94 (mult, 28H); CH(C//3)2 0.57 (dd, 12H, 3JH-P = 15.3 Hz, 3 j H _ H = 7.2 Hz). 31P{1H} NMR (C6D6, ppm) 82.7 (d mult, JP_Rh = 161 Hz). cis-6d: 1H NMR (C7D8, ppm) Hortho 7.86 (d, 4H, 3JHo-Hm = 7.8 Hz); Hm e t a 7.11 (d, 4H); C//(CH3)2 2.37 - 1.92 (overlapping mult, 8H); SiC6H5C//3 2.19 (s, 6H); PC//2C//2P, S1CH3 1.38 - 1.27 (overlapping s and d, 14H); CH(C//3)2 1.15 - 0.85 (mult, ASH). 31P{1H} NMR (C6D6, ppm) 80.8 (d mult, Jp_Rh = 163 Hz). Anal. Calcd for C44H84P4Rh2Si2: C, 52.90; H, 8.47. Found: C, 52.66; H, 8.40. Attempts to monitor the kinetics of the conversion of trans-6d to cis-6d by *H NMR. A solution of rra/u-[(dippe)Rh]2Qi-SiMeTolP)2,6d, (23 mg, 0.023 mmol) in ds-toluene (0.6 mL) was placed in an NMR tube with a teflon screw cap. The tube was closed and heated to the appropriate temperature (70,90,110, or 130°C) in an oil bath. The tube was removed from the oil bath once every 30 minutes, and was placed in an ice bath to quench the equilibration reaction. A lH NMR spectrum of the sample was run before returning the sample to the oil bath. Appearance of the cis isomer in solution was monitored by integration of the Hortho signals at 7.64 ppm (trans) and 7.86 ppm (cis). The samples were heated over 8-10 hour periods. [(dippe)Rh(H)]2(|i-r|2-H-SiMe2)2, 8b. Dimethylsilane (0.840 mmol, 2.1 equiv, 271 mmHg in a 57.5 mL constant volume bulb) was vacuum-transferred to a dark green solution of [(dippe)Rh]2(H-H)2,1, (293 mg, 0.400 mmol) in frozen hexanes (5 mL) at -196°C. The solution was allowed to warm to slightly below room temperature (ice water bath) by which time the solution had changed to a pale, golden-yellow colour. Yellow crystals were obtained from a minimum volume of hexanes by cooling to -40°C. Yield: 62% (211 mg). The product is obtained in purest form if a minimum of hexanes is used from the start of the reaction and if the solution is References on pi 23 Chapter 3 116 kept cool throughout. If solvent has to be removed from the product solution to induce crystallization there is normally some decomposition of the product to 2b and [(dippe)Rh]2(p.- SiMe2)2> 6b; ie. loss of one equivalent of silane or one equivalent of hydrogen. This same decomposition occurs slowly if solutions of 8b are allowed to stand at room temperature for periods of time longer than 5-10 minutes. Complex 8b can also be prepared by the addition by vacuum transfer of one equivalent of dimethylsilane to a solution of 2b in hexanes, with similar yields. *H NMR (C7Dg, ppm) C//(CH3)2 1.91 (mult, 8H, 3 j H _ H = 7.0 Hz); PC//2C//2P 1-25 (d, 8H, 2 j P _ H = 10.4 Hz); CH(C//3)2 1.16 - 1.02 (two dd); Si(C//3)2 0.88 (s, 12H); Rh-H -11.59 (mult (second order pattern), 4H). Note: In d6-benzene the Si(C//3)2 resonance is seen as two singlets at 0.94 and 0.98 ppm; it is not known why the two inequivalent signals arise. 31P{1H} NMR ( C 6 D 6 , ppm) 77.3 (br d, J R h _p = 118 Hz). A n a l . Calcd for C32H80P4Rh2Si2: C, 45.17; H, 9.48. Found: C, 45.30; H, 9.60. The rma l decomposi t ion of [ ( d i p p e ) R h ( H ) ] 2 ( ^ - r i 2 - H - S i M e 2 ) 2 » 8b. Approximately 25 mg (0.029 mmol) of [(dippe)Rh(H)]2(|i-ri2-H-SiMe2)2, 8b, was dissolved in dg-toluene in an NMR tube equipped with a teflon screw cap/gas inlet adaptor. An initial 31P{ !H} NMR spectrum was run to establish the purity of the sample, then the sample was attached to a nitrogen manifold. The sample was heated in an oil bath at 45°C while open to the nitrogen manifold and bubbler, to allow any volatile products to escape the NMR tube. The progress of the reaction was checked periodically by 31P{ lH} NMR spectroscopy. After four hours a substantial amount of [(dippe)Rh]2(|i-H)(|i-T|2-H-SiMe2), 2b, had formed, with 23% conversion of 8b to 2b. After 24 hours all of 8b had disappeared, giving 2b as the principal product, along with some decomposition products due to traces of air and water in the system. Reaction of [(dippe)Rh(H)]2(H-ri2-H-SiMe2)2> 8b, with H2. Approximately 25 mg (0.029 mmol) of [(dippe)Rh(H)]2(|i-T|2-H-SiMe2)2, 8b, was dissolved in ds-toluene in a sealable NMR tube. The tube was attached to a vacuum line by a needle valve adapter and slightly less than one atmosphere of hydrogen was introduced. No colour change was observed. The *H References on pi 23 Chapter 3 117 and 31P{ !H} NMR spectra of the sealed sample revealed that no reaction with the hydrogen had occurred. In the *H NMR spectrum the hydride resonance at -11.59 ppm had lost resolution, which might indicate that hydride exchange with hydrogen in solution was occurring. Reaction of [(dippe)Rh(H)]2(n-T|2-H-SiMe2)2, 8b, with D2. This reaction was carried out in the same manner as the preceeding reaction, except that the sealable NMR tube containing solid 8b was cooled in an ice bath before the addition of the ds-toluene and the solution was kept cool while the deuterium gas was added and the tube was sealed. The initial 31P{ *£!} and *H NMR spectra showed no reaction with the deuterium. The sample was monitored by NMR spectroscopy over a two week period, after which time approximately 50% of the hydride in the sample had been exchanged for deuteride, as determined from integration of the JH NMR spectrum. [(dippe)Rh]2(|i-SiMe2)2» 6b. This complex is formed by dissociation of two equivalents of H2 from 3b if it is left in solution at room temperature or is heated. However it is difficult to get complete conversion to 6b by this method; the product is often mixed with leftover 8b and with 2b formed by a concurrent dissociation of dimethylsilane. A reaction which gives pure 6b as product is that of 8b with an excess of ethylene. Solid [(dippe)Rh(H)]2(|i-T|2-H- SiMe2)2. 8b, (58 mg, 0.068 mmol) was weighed into a thick-walled reactor bomb. The bomb was cooled to 0°C in an ice bath and, under a strong flow of nitrogen, toluene (5 mL) was added, giving a pale, golden yellow solution. The head space above the solution was evacuated and one atmosphere of ethylene was introduced. The solution was allowed to stir at 0°C for 15 minutes, during which time the solution colour changed to orange-red. Orange-red crystals were obtained from a minimum volume of toluene by cooling to -40°C. Yield: 68% (39 mg). lH NMR (C6D6. ppm) C//CH3)2 2.18 (mult, 8H); PC//2CH2P 1.36 (d, 8H, 2JP_H = 15.3 Hz); Si(C//3)2 1.14 (s, 12H) overlapping with CH(C//3)2 1.18-1.02 (overlapping mult, 48H). Note: For 6 b in dg-toluene the Si(C//3)2 resonance is seen at 1.04 ppm, a solvent-related, upfield shift of References on pl23 Chapter 3 118 0.10 ppm. ^ P ^ H } NMR (C6D6, ppm) 83.3 (d mult, ^Rh-P = 167 Hz); Anal. Calcd for C32H80P4Rh2Si2: C, 45.17; H, 9.48. Found: C, 45.30; H, 9.60. Reaction of [(dippe)Rh]2(jx-SiMe2)2> 6b, with H2. Approximately 25 mg -(0.030 mmol) of [(dippe)Rh]2(|i-SiMe2)2.6b, was dissolved in ds-toluene in a sealable NMR tube. The tube was attached to a vacuum line by a needle valve adapter and slightly less than one atmosphere of hydrogen was introduced. The tube was tapped for five to ten minutes to encourage diffusion of the gas into the orange-red solution, which changed to a pale yellow colour. The tube was sealed and NMR spectra (}H and 31P{ lH}) of the sample were run. The product of the reaction was [(dippe)Rh(H)]2(|i-'n2-H-SiMe2)2, 8b, as determined by comparison with NMR spectra of an authentic sample. Reaction of [(dippe)Rh]2(H-H)2, 1, with Ph2SiH-HSiPh2. To a dark green solution of [(dippe)Rh]2(M.-H)2,1, (37 mg, 0.051 mmol) in toluene (5 mL) in a small reactor bomb was added l,r,2,2'-tetraphenyldisilane (19 mg, 0.051 mmol) in toluene (3 mL). Initially there was no colour change. The mixture was heated overnight at 70°C, during which time the solution colour changed to red. The toluene was removed under vacuum and d6-benzene was added to the residues to make up an NMR sample. The 31P{ *H} NMR spectrum showed the principal product to be [(dippe)Rh]2(M.-H)(|i-T|2-H-SiPh2), 2a (76%). Signals due to small amounts of [(dippe)Rh]2(H-SiPh2)2> 6a, (13%) and the unknown complex, 7, (11%) were also observed. The lH NMR spectrum showed the principal silicon-containing product to be 1,1',2,2'- tetraphenyldisilane. Reaction of [ (dippe)Rh] 2 (^-H) 2 , 1, with P h 2 S i H - H S i P h 2 under an atmosphere of H2. A mixture of [(dippe)Rh]2(|i-H)2,1, (33 mg, 0.045 mmol) and Ph2SiH- HSiPh2 (17 mg, 0.045 mmol) were dissolved in ds-toluene (0.6 mL) in a sealable NMR tube. The dark green liquid was frozen in liquid nitrogen, and just under one atmosphere of hydrogen was added to the tube, which was then sealed. After the solution thawed (changing to a greenish- brown colour), *H and 31P{1H} NMR spectra of the sample were run. The major species in References on pi 23 Chapter 3 119 solution was the hydrogen adduct of 1, a tetrahydride complex. A small amount of 3b, the hydrogen adduct of [(dippe)Rh]2(|J.-H)(n-'n2-H-SiPh2) was also detected. The sample was then heated to 60°C in an oil bath for three days, during which time the solution colour changed to red. NMR spectrocopy indicated the presence of 3b, complex 7 and Ph3SiH in solution. Reaction of [(dippe)Rh]2(n-H)(n-ri2-H-SiPh2)2, 2a, with Ph2SiH-HSiPh2 . To a red solution of [(dippe)Rh]2(^-H)(^i-T|2-H-SiPh2), 2a, (30 mg, 0.033 mmol) in d6-benzene (1 mL) was added an excess of Ph2SiH-HSiPh.2 (25 mg, 0.068 mmol). No initial colour change was observed and *H and 31P{ lH} NMR spectroscopy indicatedthat no reaction had occured. The mixture was placed in a small reactor bomb with a stir bar and was stirred for one week at room temperature, during which time the solution colour changed to a light orange. lH and 31P{ ̂ H} NMR spectra of the solution showed the major products to be complex 7 and Ph3SiH. Small amounts of [(dippe)Rh]2(|i-H)(|i-r|2-H-SiPh2), 2a, and [(dippe)Rh]2(|i-SiPh2)2, 6a, were also observed by 31P{ XH} NMR. Note: Complex 6a, [(dippe)Rh]2(H-SiPh2)2, does not react with Ph2SiH-HSiPh2. 3.4.3 Catalytic reactions Isotopic exchange between Ph2SiH2 and D2. A typical reaction procedure is as follows: A solution of diphenylsilane (165 mg, 0.895 mmol) in spectroscopic grade hexane (6 mL) was placed in a small reactor bomb equipped with a stirbar. The solution was degassed by evacuation of the head space and one atmosphere of deuterium gas was introduced. The substrate solution was stirred under deuterium for 30 minutes to ensure its saturation with the gas. The catalyst (1, 7 mg, 0.0138 mmol, in 4 mL hexane) was then added to the substrate solution by syringe under a strong flow of deuterium. After 18-24 hours the mixture was taken into a glovebox, where the catalyst was removed on a Florisil column. The product mixture was analyzed by *H NMR spectroscopy (and by GC-MS), and showed up to 95% exchange of References on pi23 Chapter 3 120 hydrides for deuterides on diphenylsilane, as determined by integration of the *H NMR spectrum. For these conditions, no disproportionation of diphenylsilane to phenyl- and triphenylsilane was observed. The disproportionation was observed to occur with higher concentrations of substrate and also when aromatic solvents were used. When the reaction was carried out using neat diphenylsilane (or very high concentrations of the substrate), some of the dimerization product, l,l',2,2'-tetraphenyldisilane, was observed, as was the case when the catalyst was added to the diphenylsilane before the introduction of deuterium. Progress of the reaction monitored by lH NMR. In the glovebox, a solution of diphenylsilane (126 mg, 0.68 mmol) in dg-benzene (3 mL) was placed in a 250 mL thick-walled reactor bomb, with a small stir bar. The bomb was taken out of the glovebox and attached to a vacuum line. The head-space gas was removed and the solution was placed under an atmosphere of deuterium gas. The bomb was left open to the manifold full of deuterium gas for 1/2 hour, the solution stirring, to ensure D2-saturation of the substrate solution. The catalyst solution was also prepared in the glovebox: [(dippe)Rh]2(M--H)2,1, (10 mg, 0.014 mmol) in d6-benzene (1 mL). (A 50:1 substrate to catalyst ratio.) The dark green catalyst solution was added to the substrate by syringe under a slight positive pressure of deuterium gas. The mixture immediately changed from clear to orange, then within 30 seconds lightened to a pale yellow. The bomb was left open to the manifold full of deuterium gas throughout the experiment. Samples were withdrawn periodically by syringe and analyzed by *H NMR spectroscopy. The resulting spectra indicated that disproportionation of Ph2SiH2 to Ph3SiH and PhSiH3 is occurring as well as isotopic exchange of H for D on Ph2SiH2, though the isotopic exchange is occurring more quickly. The results are summarized in the table below: References on pl23 Chapter 3 121 time elapsed since addition of catalyst to substrate (min) 5 40 94 130 171 % Ph2SiH2 (do-, in solution 80 64 54 48 45 d2-) % of Ph2SiH2 which isd2- 18 97 99 99 100 % of Ph2SiH2 which has disproportionated to give Ph3SiH and PhSiH3 20 36 46 52 55 Isotopic exchange between MePhSiH2 and D2: progress of the reaction monitored by 1H NMR. This reaction was carried out in exactly the same manner as described above for diphenylsilane, using methylphenylsilane (83 mg, 0.68 mmol), and samples were withdrawn periodically by syringe for analysis by *H NMR spectroscopy. The first sample, withdrawn 11 minutes after the catalyst was added to the substrate solution, showed that the major species in solution was methylphenylsilane (76%), 99% of which was d2-MePhSiH2. The small Si-H signal at 4.5 ppm due to non-deuterated MePhSiH2 was the only signal observed in the Si-H region of the spectrum. Disproportionation of 24% of the d2-MePhSiH2 was evident from signals due to di-MePh2SiH. No signals due to the gas, MeSiH3 (the other disproportionation product) were observed. Subsequent NMR samples removed from the catalytic mixture showed a steady increase in the amount of di-MePh2SiH. Four hours after the addition of the catalyst to the substrate, 60% of the d2-MePhSiH2 had disproportionated. Dimerization of Ph2SiH2. A typical reaction procedure is as follows: A solution of diphenylsilane (254 mg, 1.38 mmol) in toluene (1.25 mL) was placed in a 50 mL round-bottom flask equipped with a stirbar and condenser. The catalyst (1, 0.25 mL of a 0.055 M solution in toluene, 0.014 mmol) was added to the substrate solution by syringe under a strong flow of nitrogen. The solution was degassed by evacuation of the head space and then heated to 60 °C References on pi 23 Chapter 3 122 under nitrogen, open to a bubbler. After 18-24 hours the mixture was taken into a glovebox, where the catalyst was removed on a Florisil column. The product mixture was analyzed by lH NMR spectroscopy and by GC-MS. *H NMR spectra showed the disilane, [Ph2SiH]2 to be the principal product, with yields as high as 78%. Also seen in the spectra were small signals due to unreacted Ph2SiH2 and to Ph3SiH. A small singlet at 4.85 ppm was due to an unidentified product. The GC-MS analysis confirmed the presence of [Ph2SiH]2, Ph2SiH2, Ph3SiH and PhSiH3 in these mixtures, but no other peak was observed which might be due to the unidentified product observed in the *H NMR spectra. Parent ions observed for these products in the mass spectra are shown in the table below. Products M+(m/e) PhSiH3 107 Ph2SiH2 184 [PhSiH2]2 366 Ph3SiH 260 No PhSiH3 was detected in the *H NMR spectra because toluene was removed from the reaction mixture under vacuum, so the volatile PhSiH3 was removed at the same time. The disilane [Ph2SiH]2 could be separated from these reaction mixtures by recrystallization from hexanes. The dimerization reaction does proceed at room temperature, but with lower yields of disilane. Progress of reaction monitored by 1H NMR. To monitor the Ph2SiH2 dimerization reaction by *H NMR the same procedure was used as for monitoring the preceding isotopic exchange reactions by *H NMR. The only difference in the procedure was that the mixture was kept under an atmosphere of nitrogen instead of an atmosphere of deuterium. Samples were withdrawn from the reaction mixture periodically, the first being taken five minutes after addition of the catalyst to the substrate, the final being taken three hours later. The final JH NMR spectrum showed an 19% yield of [Pl^SiHfc. References on pi 23 Chapter 3 123 3.4.4 Calculations Calculation of AG* for the phosphine and hydride exchange in 8b. AG* was calculated using the value for the rate constant30 kc (where kc = izAx>c/(2)lty in the Eyring equation AG* = -RTcln[(kch)/(kBTc)]; where R = the gas constant, Tc = temperature of coalescence, Ave = Pea^ separation at the low T limit, h = Planck's constant and kfi = Boltzmann constant. For the phosphorus resonances in the variable temperature 31P{!H} NMR spectra of 8b, Tc = 244K (-29°C) and Avc = 1590 Hz. For the hydride resonances in the variable temperature !H NMR spectra of 8b, Tc = 234K (-39°C) and ADC = 705 Hz. The coalescence temperatures were estimated visually from the spectra and have an error of approximately ± 5K. 3.5 References (1) Auburn, M.; Ciriano, M.; Howard, J. A. K.; Murray, M.; Pugh, N. J.; Spencer, J. L.; Stone, F. G. A.; Woodward, P. / . Chem. Soc, Dalton Trans. 1980, 659. (2) Aitken, C. T.; Harrod, J. F.; Samuel, E. / . Am. Chem. Soc. 1986,108, 4059. (3) Suzuki, H.; Takao, T.; Tanaka, M.; Moro-oka, Y. / . Chem. Soc, Chem. Commun. 1992, 476. (4) Hencken, G.; Weiss, E. Chem. Ber. 1973,106, 1747. (5) Nicholson, B. K.; Mackay, K. M.; Gerlach, R. F. Reviews on Silicon, Germanium, Tin and Lead Compounds 1981,5, 67. (6) The coordinates for the molecular structure of 6a were entered into the Chem 3D Plus program through a Molecule Editor using the Cache system by Techtronix, and the Si-Si distance was calculated using the Molecule Editor. References on pi23 Chapter 3 124 (7) Anderson, A. B.; Shiller, P.; Zarate, E. A.; Tessier-Youngs, C. A.; Youngs, W. J. Organometallics 1989,8, 2320. (8) Pham, E. K.; West, R. Organometallics 1990,9, 1517. (9) Zarate, E. A.; Tessier-Youngs, C. A.; Youngs, W. J. / . Am. Chem. Soc. 1988,110, 4068. (10) Corey, J. Y. In Advances in Silicon Chemistry; G. L. Larson, Ed.; JAI Press Inc.: Greenwich, Connecticut, 1991; Vol. 1; pp 327-387. (11) Curtis, M. D.; Epstein, P. S. Adv. Organomet. Chem. 1981,19, 213. (12) Lee, K. E.; Arif, A. M.; Gladysz, J. A. Chem. Ber. 1991,124, 309. (13) Zybill, C. Top. Curr. Chem. 1992,160, 1. (14) In the study of homogeneous catalytic systems, the inability to spectroscopically observe suspected intermediates in the reaction mixture does not preclude the activity of that species in the catalytic cycle! For example, exhaustive kinetic studies on the use of Wilkinson's catalyst in the hydrogenation of olefins showed that the catalysis was in fact being carried out by a series of complexes that, although related through equilibria to detected or isolated complexes, were not direcdy observed. (See the following reference.) (15) Halpern, J. Inorg. Chim. Acta. 1981,50, 11. (16) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987 (17) The coordinates for the molecular structure of 8b were entered into the Chem 3D Plus program through a Molecule Editor using the Cache system by Techtronix, and the Si-Si distance was calculated using the Molecule Editor. (18) Meek, D. W.; Mazanec, T. J. Ace. Chem. Res. 1981,14, 266. (19) Lappert, M. F.; Maskell, R. K. / . Organomet. Chem. 1984,264, 111. (20) Chang, L. S.; Corey, J. Y. Organometallics 1989,8, 1885. <21) Matarasso-Tchiroukhine, E. J. Chem. Soc, Chem. Commun. 1990, 681. References on pi 23 Chapter 3 125 (22) Archer, N. J.; Haszeldine, R. R ; Parish, R. V. J. Chem. Soc, Dalton Trans. 1979, 2509. (23) Blackburn, S. R ; Haszeldine, R. R ; Parish, R. V.; Setchfield, J. H. J. Organomet. Chem. 1980,192, 329. (24) Paonessa, R. S.; Prignano, A. L.; Trogler, W. C. Organometallics 1985,4, 647. (25) Sommer, L. H.; Lyons, J. E.; Fujimoto, H.; Michael, K. W. / . Am. Chem. Soc. 1967, 89, 5483. (26) Brown-Wensley, K. A. Organometallics 1987,6, 1590. (27) Ojima, I.; Inaba, S.; Kogure, T. J. Organomet. Chem. 1973,55, C7. (28) Tilley, T. D. Comments Inorg. Chem. 1990,10, 37. (29) Benkeser, R. A.; Landesman, H.; Foster, D. J. J. Am. Chem. Soc. 1952, 74, 648. (30) Thomas, W. A. Annu. Rev. NMR Spectrosc. 1968,1, 43. References on pi 23 Chapter 4 126 CHAPTER 4 Reactions of Primary Silanes with [(dippe)Rh]2(|i-H)2 4.1 Introduction The vast majority of silanes used in reactions with transition metals have been tertiary (R.3SiH) or secondary (R.2SiH2) silanes; there have been few reported reactions of primary silanes (RSiH3) with metal complexes.1"7 The recent surge of interest in transition-metal silicon chemistry has been largely due to a desire to find catalysts for the preparation of Si-Si bonded polymers. In the case of early transition-metal catalysts (e.g. zirconocene and titanocene), investigators have shown that the reactivity and product chain length increased for different silanes in the order 3° < 2° < l0.8 For this reason, attention has turned to the interaction of primary silanes with these metals, and derivatives have now been isolated which contain SiH2R ligands, including the two examples shown below.9,10 The titanocene complex was isolated from a catalytic reaction mixture producing poly(phenylsilylene). H Ph sir-H CI / W / CP2TL JiCp2 CpCp*Zr' H—Si 4\ Ph H SiPhH. The use of late transition-metal catalysts for Si-Si bond-forming reactions has principally been limited to reactions with secondary silanes.11"13 Dimers and trimers are the largest oligomers formed in these reactions. Tertiary silanes tend to be very unreactive towards dimerization, though traces of hexaethyldisilane were observed in reactions of a range of hydrosilation catalysts with triethylsilane.12 Also the dehydrogenative coupling of PhMe2SiH to References pi 77 Chapter 4 127 (PhMe2Si)2 catalyzed by platinum complexes has been reported.14 These oligomers are frequently accompanied by products of disproportionation reactions which are also catalyzed by late transition metals, particularly with arylsilanes. As seen in Chapter 3, the rhodium hydride system studied in this thesis behaves quite typically in this respect, with triphenylsilane being a ubiquitous side product in the preparation of l,l',2,2'-tetraphenyldisilane using [(dippe)Rh]2(|i-H)2,1, as a catalyst The above results demonstrate that for both early and late transition-metal catalysts in dehydrogenative silicon-silicon coupling, sterics play an important role, yet not many examples exist of late transition-metal catalysts being used to oligomerize primary silanes. A study using Wilkinson's catalyst (RhCl(PPh3)3) did examine the behaviour of phenylsilane towards coupling, but found it behaved much as the secondary silanes did, giving dimers and trimers. Also observed were some products arising from coupling between phenylsilane and the disproportionation product, diphenylsilane.11 Recently, though, there has been more interest in the reactions of late transition-metal complexes with primary silanes. Shown below are some complexes which are formed from the reaction of primary silanes with late transition-metal centres. Both of the platinum compounds were formed from mononuclear precursors, and both are catalyst precursors for at least slow dimerization of phenylsilane.15,16 The rhodium complex arises from the addition of primary silane to a dinuclear hydride precursor. This complex is the subject of the only previous, in-depth study of the interactions of primary silanes with rhodium complexes. It is not, however, a catalyst precursor for dehydrogenative silicon coupling17 u  Du Et Et \ „ . .. * S i - ___ „ „ -si Si E,3\ s-K /PE'3 ^ A H \ £L °°\ ZS-\ /co Df^ i ^o* J rn....Dt____ •_. M Vu...t»r—v DU ZDk Et3P Si X p E t 3 Me2 Phi \ Me s A Ph H Ph References pi 77 Chapter 4 128 This chapter describes the reactivity of the primary silanes p-tolylsilane (TolPSiH3) and n-butylsilane (BunSiH3) with [(dippe)Rh]2(|i-H)2, l . 1 8 Some of the chemistry described parallels that observed for the secondary silanes studied and some of the chemistry is unique to these primary silane substrates. The use of 1 as a catalyst for the dehydrogenative coupling reactions of these silanes is also described. Possible mechanisms for these catalytic cycles are discussed based on the stoichiometric chemistry observed and on product analysis of catalytic runs. 4.2 Stoichiometric reactions of [(dippe)Rh]2(|i-H)2,1, with primary silanes 4.2.1 Preparation of the bis(ji-silylene) complexes [(dippe)Rh]2(|i-SiHR)2,6e (R = Bun) and 6f (R = TolP) Addition of two equivalents of primary silane to [(dippe)Rh]2(ji-H)2,1, initially causes a colour change from dark green to yellow. Over 10-15 minutes the solution colour gradually darkens to orange, and if the mixture is left at room temperature for several hours or more it eventually changes to red (for n-butylsilane) or red-orange (p-tolylsilane). Red-orange crystals of the bisQi-silylene) complexes [(dippe)Rh]2(fi-SiHR)2 (R = Bun, 6e; R = TolP, 6f) are isolated from these solutions. -3H2 [(dippe)Rh]2(n-H)2 + 2RSiH3 »- [(dippe)Rh]2(n.-SiHR)2 [4-1] 1 6e-f Silyl hydride complexes of the formula [(dippe)Rh]2(M--H)(^i-ri2-H-SiHR), where R = Bun or TolP, could not be isolated. Addition of a single equivalent of primary silane to a dark green toluene solution of [(dippe)Rh]2(|i-H)2,1, generates a brownish-green solution. *H and 31P{1H} NMR spectra indicate that the major species in this reaction mixture are the starting hydride dimer and a mixture of isomers of the bis(p--silylene) complex [(dippe)Rh]2(M--SiHR)2, 6. References pi 77 Chapter 4 129 The same results were achieved even when the reaction was carried out at -70°C and the mixture was allowed to warm to room temperature slowly. Undoubtedly the silyl hydride complexes [(dippe)Rh]2(|i-H)(|i-ri2-H-SiHR), 2e-f, or their hydrogen adducts, are forming upon addition of silane to the hydride dimer, but evidently they are extremely reactive species and react more quickly with free silane than does the hydride dimer, 1. This contrasts with the behaviour of the analogous complexes formed with secondary silanes; the silyl hydride complexes 2a-d are quantitatively produced from the addition of a single equivalent of silane to 1. [(dippe)Rh]2(n-H)2 + 1RSiH3 X " [(dippe)Rh]2(>i-H)(^-ri2-H-SiHR) 1 [4-2] I »- 1/2[(dippe)Rh]2(|i-H)2 + 1/2 [(dippe)Rh]2Oi-SiHR)2 6e-f e: R = Bun f: R = Tolp The bis(|i-silylene) complexes 6e-f are air and moisture sensitive, even in the solid state. While the analogous complexes prepared with secondary silanes are somewhat air stable in the solid state, crystals of complexes 6e-f become discoloured in air in less than a minute, much like the silyl hydrides 2a-d. This is probably due to the dinuclear cores of 6e-f being less sterically congested than those of 6c-d, and thus more susceptible to attack by oxygen. Complexes 6e-f are also more soluble in aliphatic and aromatic solvents than are their secondary silane analogues. 4.2.1.1 Solid-state structure of fra/w-[(dippe)Rh]2(^L-SiHBun)2,6e An X-ray crystallographic analysis was carried out on a crystal of [(dippe)Rh]2(|i-SiHBun)2, 6e, which turned out to be the trans geometric isomer. An ORTEP References pi 77 Chapter 4 130 diagram of the molecular structure of trans-6e is shown in Figure 4.1, and some relevant bond lengths and bond angles are shown in Tables 4.1 and 4.2 respectively. Table 4.1 Selected bond lengths for fraftS-[(dippe)Rh]2([i-SiHBun)2,6e. Bond R h l - R h l * R h l - P l R h l - P 2 Length (A) 2.8850(7) 2.266(1) 2.272(1) Bond R h l - S i l R h l - S i l * S i l - H l Length (A) 2.334(1) 2.335(1) 1.45(3) Bond S i l - C 1 5 C15-C16 Length (A) 1.905(5) 1.519(6) * Refer to symmetry operation: l-£, 1-y., l-£. Figure 4.1 Molecular structure of rran.y-[(dippe)Rh]2(|i-SiHBun)2, <>e. References pi 77 Chapter 4 131 Table 4.2 Selected bond angles for rrfl«y-[(dippe)Rh]2(|l-SiHBun)2,6e. Bonds P 1 - R M - P 2 . S i l - R h l - S i l * Rhl - S i l - R h l * Angle (deg) 87.64(4) 103.66(4) 76.34(4) Bonds R h l - S i l - C 1 5 R h l * - S i l - C 1 5 Angle (deg) 117.2(1) 116.9(1) * Refer to symmetry operation: 1-x, l-y_, 1-z. The dimensions of this bisQi-silylene) structure are analogous to those for [(dippe)Rh]2(n-SiPh2)2, 6a, and rrans-[(dippe)Rh]2(|i-SiMePh)2, 6c, shown in Chapter 3. The Rh-Rh separation of 2.8850(7) A in the planar core of trans-6e is just slightly shorter than that for trans-6c, which is consistent with the replacement of an R group on silicon with the much smaller H. All other bond distances are as expected, with the acute Rh-Si-Rh angles indicating the presence of a Rh-Rh bond. 4.2.1.2 Room temperature and variable temperature NMR spectra of [(dippe)Rh]2(|i- SiHR)2,6e (R = Bun) and 6f (R = ToIP) The room temperature 31P{ 1H) NMR spectra of complexes 6e-f show signals attributable to the presence of two isomers in solution. The signals have been assigned to cis and trans geometric isomers of 6e and 6f, based on analogy with the methylphenylsilane and methyl- p-tolylsilane complexes 6c and 6d, discussed in Chapter 3. If the assignments are correct the ratio of trans to cis isomers is roughly 9:1 for both 6e and 6f. Heating these samples in d$- toluene at 80°C for a week showed no change in this ratio. As for complexes 6c-d, the centre-symmetric doublets of multiplets observed in the 31P{1H) NMR spectra of 6e-f correspond to AA'A"A"'XX' spin systems. In Chapter 3 it was pointed out that this type of pattern must arise from fluxionality in the molecules, with twisting of the chelating phosphine rings to give an average structure for each isomer where the References pi 77 Chapter 4 132 phosphines are contained in the same plane as the Rh2Si2 core. For all of the bis(ji-silylene) complexes studied, this fluxionality is occurring so rapidly that the AA'A"A'"XX' patterns in the 31P{1H} NMR spectra persist even when the samples are cooled to -85°C. However, as shown in Figure 4.2, the room temperature 31P{1H} NMR spectrum for [(dippe)Rh]2(|J.-SiHBun)2, 6e, shows broad, overlapping signals at room temperature which can be resolved at lower temperatures, indicating some other fluxional process is occurring as well, possibly having to do with movement of the butyl chain on each silicon. At -35 °C two sharp AA'A"A'"XX' signals are seen in an 85:15 ratio at 89.3 and 86.7 ppm, respectively. At temperatures lower than -35°C the signals broaden slightly, but they are still well separated at -83°C. The room temperature 31P{ *H} NMR spectrum of 6f shows the two signals attributable to trans and cis isomers as sharp doublets of multiplets at 91.0 and 88.2 ppm, respectively. At lower temperatures these signals do broaden as they do for 6e; in fact the resonance for the cis isomer broadens to such an extent that it disappears into the base line at -57°C. References pi 77 Chapter 4 133 trans X-. 20°C jiu CIS -36°C JJ\ 7°C JL •76°C JL -5°C -83°C i J i I I J J i I 1 i i J I i i i l I i l i l I ] I l J l I l l l J ] 1 J J J J l i 1 I I 95 90 95 BO PPM 75 100 95 90 85 80 Figure 4.2 Variable temperature 121.4 MHz 31P{ *H} NMR spectra of [(dippe)Rh]2(M.- SiHBun)2,6e, in C7D8. The room temperature *H NMR spectra of these complexes are consistent with the bis(|i- silylene) structure, with resonances due to the terminal silicon hydrides occurring between 8.5 and 7.5 ppm. For both 6e and 6f the cis Si-H resonances are at higher field (by roughly 0.5 ppm) than the trans Si-H resonances. These *H NMR spectra are slightly complex because there is a mixture of approximately 85-90% trans isomer and 10-15% cis isomer in solution. It is not References pi 77 Chapter 4 134 possible to pick out all peaks due to the cis isomers because of overlap with the signals due to the trans isomers. For 6e the room temperature spectrum is broad and poorly resolved. This may be due to some fluxionality arising from the motion of the n-butyl chain. Cooling the sample does not substantially improve the resolution. For 6f, the aromatic region in the JH NMR spectrum is of interest. The signals for the ortho and meta protons on the p-tolyl groups and for the Si-H's are found in this region. All of the complexes containing phenyl or p-to\y\ rings that have been described so far show a single signal in their *H NMR spectra for the ortho protons and one for the meta protons, because the aryl groups are spinning rapidly around the Si-Cipso bond, creating average environments for these protons. (See Figure 4.3.) CH3 Ho Rh -Rh N/ V \ ^ A HToP Figure 4.3 Rotation of the/?-tolyl group around the Si-Cipso bond in [(dippe)Rh]2(}i- SiHTolPh, 6f. (P = PPri2) However, at lower temperatures this spinning appears to be restricted in 6f, generating four separate environments for the protons on the aromatic ring. This is manifested in the *H NMR spectrum by the splitting of the ortho and meta proton signals for the trans isomer from two broad doublets (each with a relative intensity of two) into four separate signals (each with a relative intensity of one) when the sample is cooled to -60°C. The corresponding signals due to the cis isomer also broaden as 6f is cooled, but the splitting is not obvious, as the signals are mostly lost in the base line of the spectrum at -60°C. Thep-tolyl methyl signal remains a singlet. References pl77 Chapter 4 135 While rotation of the ^-tolyl group around the silicon-carbon bond should have a very low energy barrier in an unconstricted environment, it is probably a higher energy process in this molecule because of the steric influences of the ligand isopropyl groups on the dinuclear core. It is unclear why the Si-Cipso rotation is restricted in 6f but not in 6a, 6c or 6d. 4.2.2 Formation of the bis(silane) complexes [(dippe)Rh(H)]2(^--n2-H-SiHR)2,8e (R = Bun) and8f(R= TolP) As was mentioned in Section 4.2.1, when two equivalents of a primary silane are added to [(dippe)Rh]2(|i-H)2, 1, the solutions initially turn yellow, then gradually darken to red (6e) or red-orange (6f). The initial, yellow product is thought to be a bis(silane) adduct, directly analogous to the bis(dimethylsilane) complex 8b which was previously isolated (Chapter 3). This adduct would form via addition of a second equivalent of silane to the highly reactive silyl hydride complex [(dippe)Rh]2(|H-H)(ji-r|2-H-SiHR). Loss of two equivalents of hydrogen from the reactive intermediates 8e-f would generate the observed bis(ji-silylene) complexes 6e and 6f, respectively. [(dippe)Rh]2(n-H)2 + 2RSiH3 ~"2 » [(dippe)Rh(H)]2(|i-ri2-H-SiHR)2 1 8e-f -2H 2 e: R = Bu n f: R = Tolp [4-3] [(dippe)Rh]2(n-SiHR)2 6e-f The bis(silane) complexes 8e-f can be regenerated by placing solutions of 6e-f under an atmosphere of hydrogen. The resulting solutions are yellow, and their *H NMR spectra show resonances in the rhodium hydride region. If the excess hydrogen is removed the solutions References pi 77 Chapter 4 136 gradually return to their original colours, and after several hours the major species in solution are again the bis((i-silylene) complexes 6e-f. + 2H2 [(dippe)Rh]2(u,-SiHR)2 m *• [(dippe)Rh]2(H)2(^-ri2-H--SiHR)2 [4-4] 6e-f "2H2 8e-f e: R = Bun f: R = Tolp 4.2.2.1 Fluxionality and variable temperature NMR spectra of [(dippe)Rh(H)]2(^i-Tl2-H- SiHR)2,8e (R = Bun) and 8f (R = TolP) The room temperature 31P(1H} spectrum of [(dippe)Rh(H)]2(M--Tl2-H-SiHBun)2, 8e, shows a single resonance, a doublet at 84 ppm, whose broadness suggests that the molecule is fluxional in solution on the NMR time scale. The broad singlet seen in the high field, rhodium hydride region of the room temperature *H NMR spectrum of 8e also supports its possible fluxionality. Variable, high temperature 31P{1H} NMR spectra confirm this fluxionality, with a sharp doublet observed at 60°C illustrating the high temperature limit for the process. Low temperature NMR studies indicate complex behaviour of this compound in solution. These *H and 31P{1H} spectra are complicated and somewhat difficult to interpret; in the following discussion an attempt is made to present the most likely mechanisms for exchange and isomerism processes which are occurring. References pi 77 Chapter 4 137 | i i i i | i i i i j i i i i | i i i i | ) iu|) i i i | i i i i | i i i i | i i i i i i i i i | i i i i | i i i i | i in | i i i i | i i i i imi |U)i | i i i i | i i i i i i i i ) | i i i i i i i i i | i i i i j i i i i | i i i !j i i i i | i i i i i i i i i | i i ! ij i i i] | 100 90 BO 70 PPBO 100 80 BO 70 PPBO 100 90 BO 70 PPBO Figure 4.4 Variable temperature 121.4 MHz 31P{ lH} NMR spectra of [(dippe)Rh(H)]2( î-T|2- H-SiHBun)2,8e, in C7D8. Figure 4.4 shows the variable temperature 31P{ *H} spectra for 8e. The spectra show that there are actually two species in solution, and that they are both fluxional, though the coalescence point for each of the two species occurs at a different temperature. At -15°C the broad doublet observed at room temperature has changed to two signals: a broad signal in the baseline stretching from 65 to 100 ppm and a slightly broad doublet still at 84 ppm but reduced to roughly 1/3 of the original peak height observed at room temperature. Thus the signals due to the major isomer in solution are approaching a coalescence point. This point occurs somewhere between -15°C and -25°C, where two new peaks have begun to appear on either side of the now References pi 77 Chapter 4 138 broader doublet at 84 ppm. The integral ratios of the peaks observed at -25°C suggest that the two isomers observed exist in a ratio of slightly more than 3:1. The peaks which coalesce at roughly -20°C reach their low temperature limiting spectrum at -65°C, where two sharp doublets at 93.6 and 75.5 ppm are observed, with ^Rh-p = 150 and 100 Hz, respectively. This pattern is similar to the one observed in the 31P{1H} NMR spectrum of [(dippe)Rh(H)]2(|i-Ti2-H-SiMe2)2» 8b, at -54°C (See Chapter 3). It corresponds to there being two different types of phosphines in the complex: the signal at lower field represents the two phosphines which are trans to the Rh- Rh bond and the signal at higher field represents the two phosphines which are trans to Rh-Si bonds. A value for AG* of 10.5 ± 0.2 kcal/mol for the process exchanging the two different types of phosphine ligands in this major isomer was calculated based on a coalescence temperature of -20 ± 5°C. R H V Pr'2 H — S i H — P fih-° ^Rh P. Prt I \V.X^i \ \ ' PH2 Be: R = Bun 8f: R = Tolp The 31P{1H} signal due to the minor isomer of 8e reaches its coalescence point at approximately -55°C. As the low temperature limit is not reached by -95°C for these signals, and the resolution of the spectra becomes poor from -85°C down, no activation parameters were calculated for the exchange process occurring in this isomer. References pi 77 Chapter 4 139 V 20^ /I A -35°C VjHlWIW* mmJ ******* *mm -45<C -75*0 -55<C -85°C as trans CIS -25°C - 6 5 ^ -A—AT [l)ll|l)ll|l)]J|Hlllll[l|IJ|][llJl|i)ll|l)ll|l)ll|llll| |J|]]|!ll]jlJlt|lll]jJlllj|]lljllll|lJlJ]lllljllllJUJ!] |1]!I|1IIIJ!!IIJIII!]]I!I|1!!I|I]II]!II!|]IIIJIIII|I!II|I - 9 - 1 0 - 1 1 - 1 2 - 1 3 PP*W - 9 - 1 0 - 1 1 - 1 2 - 1 3 PPMJ - 9 - 1 0 - 1 1 - 1 2 - 1 3 PPM» Figure 4.5 Hydride region of the variable temperature 300 Mz ]H NMR spectra of [(dippe)Rh(H)]2(|i-ri2-H-SiHBun)2,8e, in C7D8. Figure 4.5 shows the rhodium hydride region from the variable temperature 1H NMR spectra of 8e. Coincidentally, these patterns look remarkably similar to those seen in the low temperature 31P{1H} NMR spectra. A broad singlet in the room temperature spectrum at -11.85 ppm collapses as the temperature is lowered to give signals due to the major and minor isomers. The signals due to the major isomer coalesce at roughly -10°C and are most clearly resolved as two doublets due to two different types of hydride at -55°C. As for References pi 77 Chapter 4 140 [(dippe)Rh(H)]2(|i-Tl2-H-SiMe2)2, 8b, it is not certain whether the agostic Si-H interactions in 8e persist in solution or if the Si-H bonds have been oxidatively added. Both possibilities would, however, give rise to two different types of hydride ligands which are trans to each other at each rhodium centre. A value for AG* (263K) of 11.4 ± 0.2 kcal/mol was calculated for the process exchanging these two hydrides in the major isomer of 8e. Signals due to the hydrides of the minor isomer of 8e, which coalesce at about -45°C, do not reach their low temperature limit in the temperature range studied. No AG* was calculated for exchange of hydrides in this isomer. As shown in Figure 4.6, two structural isomers are possible if we assume that complexes 8e-f have the same structure as the dimethylsilane analogue 8a. Hand R- syn H •H R I H Si K Si H H' H H' H trans cis Monti H I I syn NN .'/ I "central allyl Figure 4.6 Side views of the bis(silane) complexes 8e-f (e, R = Bun, f, R = TolP), showing the possible geometric isomerism. (The agostic hydrides and phosphine ligands are omitted for clarity.) By analogy to allyl ligands, the substituents on silicon can be labelled syn or anti, based on the view along the Rh-Rh axis. The substituents which point to the centre, or inside, of the v-shaped core can be labelled anti; they are anti to the terminal hydrides that are trans to the bridging silyl References pi 77 Chapter 4 141 groups. The substituents which point away from the core of the molecule can be labelled syn, as they are syn to the terminal hydrides. Therefore the cis isomer has two possible structures, where like substituents on the silicons are either both syn or both anti. The trans isomer has one R group and one hydride in an anti site and one of each in a syn site. The sterically most favoured structure, where the R groups on silicon are furthest away from each other, would probably be the cis isomer in which the Si-H's are anti and the R groups are syn. The trans isomer should follow, with the sterically most disfavoured structure being the cis isomer where the R groups are anti and the Si-H's are syn. free H2 trans — 1 — i — 1 — i — i — 1 — i — 1 — i — 1 — I — i — i — i — i — I — i — 1 — 1 — r ~ 6 . 0 5 . 5 5 . 0 4 . 5 PPM Figure 4.7 Silyl hydride region of the 300 MHz *H NMR spectrum of 8e at -25°C. 29Si satellites for the signal due to the cis isomer are indicated by the "*". At room temperature the silyl hydride region of the *H NMR spectrum shows a broad singlet for the major isomer at 5.4 ppm and two broad singlets for the minor isomer at 5.1 and 4.3 ppm. These signals do not vary in their ratio as the sample is cooled; the only change is that the peaks sharpen to a minimum linewidth at -25°C (see Figure 4.7), then lose resolution gradually as the sample is further cooled. The major isomer must be a cis isomer (two Si-H's in References pi 77 Chapter 4 142 equivalent sites), probably with and Si-H's. The minor isomer is the trans isomer, with one syn Si-H (4.3 ppm) and one anti Si-H (5.1 ppm). The phosphine and hydride ligands in both isomers of 8e could be exchanged by a mechanism involving simultaneous twisting of the two rhodium coordination spheres, as shown in Figure 4.8, and as described for [(dippe)Rh(H)]2(^-Tl2-H--SiMe2)2,8b, in Chapter 3. H 1 — > S i , <I/M •P9— :Rh Y .P, Rh ^Rh P P1,2,3,4 = PPfj I~1 n i i - r Pl»... h -pRh Figure 4.8 Twisting mechanism which would exchange the two Rh-H sites and the two phosphine sites in compound 8. (Substituents on silicon are omitted for clarity.) At each rhodium centre, the H-Rh-H axis rotates through 90°, with simultaneous breaking of one agostic Si-H bond and the formation of another. This process exchanges the terminal hydrides on rhodium and the agostic silicon hydrides. There is a concurrent twisting of the phosphine chelate rings through 90°, which exchanges the two types of phosphorus on each ligand. As depicted in Figure 4.9b, another mechanism that would exchange the phosphine and hydride sites in both isomers of 8e is a "flapping" process, whereby the "V"-shaped core of the References pi 77 Chapter 4 143 butterfly structure inverts, leaving the phosphines and hydrides stationary while the silicons and their substituents move. This mechanism would also exchange the syn and anti sites in the complexes, as shown in the side views of the isomers in Figure 4.9a. a) Hanti R I I R—Si S i — H H H II b) -<i?Si\73  p •P3—-Rh - R h — P 2 . H, S S i < H ' , \ _ > RanO' I H—Si H H II R I S i — H ) H H—Si syn I Ranti H I) ^ S i — R | 1 H H R—Si syn j Hanti ,* I) ^ S i — R | 1 H H 2 u Pp -P4——Rh - ^ R h — P ( Figure 4.9 a) Side views of 8e-f demonstrating the "flapping" mechanism which would exchange the syn and anti substituents on silicon. (The phosphines are omitted for clarity.) b) A front view of the "flapping" mechanism, showing how the two types of Rh-H and the two types of phosphine in 8 are exchanged by this mechanism. (Substituents on phosphorus and silicon are omitted for clarity.) Whether it is twisting or flapping that is actually causing exchange in [(dippe)Rh(H)]2(^- T]2-H-SiHBun)2, 8e, can be determined based on the behaviour of the Si-H resonances observed in the variable temperature *H NMR spectra. If both cis and trans isomers in solution are "flapping", then Si-H's in syn and anti sites would be exchanging and should give a single, References pi 77 Chapter 4 144 average signal at the high temperature limit for each isomer. However, as described earlier, at room temperature this region of the *H NMR spectrum shows a broad singlet at 5.4 ppm due to the cis isomer and two broad singlets at 5.1 and 4.3 ppm due to the trans isomer. That these signals do not change at low temperature and do not vary in their ratio even when the sample is cooled to -95°C suggests that it is the twisting mechanism that best explains the data, exchanging hydrides and phosphines without exchanging the uncoordinated Si-H's. Figure 4.10 shows the silyl hydride region of the variable, high temperature *H NMR spectra for 8e. The signals for Si-H(trans) do broaden as the sample temperature is raised, which could be considered to be a coalescence caused by exchange of syn and anti sites in the trans isomer. However both the Si-H(cw) signal and the sharp singlet at 4.5 ppm (due to free H2 dissolved in solution) also broaden when the sample is heated. The broad peak seen at roughly 4.5 ppm in the spectrum generated at 61°C is not a coalescence of Si-H(trans) signals but is due to free H2 in solution which is exchanging with both the cis and trans isomers of 8e. References pi 77 Chapter 4 145 CIS 24<C free H2 trans k trans ] 1 I ! I I 1 I 1 i 1 I I I I j M I I j 6.0 5.5 5.0 4.5 PPM4.0 I I I I 1 M i 1 1 I 1 I I I I I I I I I 6.0 5.5 5.0 4.5 PPM4.0 Figure 4.10 Variable, high temperature 300 MHz *H NMR spectra of 8e in C7D8, showing the silyl hydride region. Generally when two different species in solution are in equilibrium the equilibrium constant, Keq, is temperature dependent. Thus NMR spectra from a range of sample temperatures should show varying ratios of the two species, and integration should allow calculation of Keq for each temperature. Signals for the two isomers of 8e appear to remain in References pi 77 Chapter 4 146 the same ratio from +60°C to -95 °C, which suggests either that the two species are not in equilibrium or that Keq for the isomers has a very small temperature dependence. One experiment performed suggests that cis- and transSe should be in equilibrium. A solution containing [(dippe)Rh]2(H-SiHBun)2,6e, was placed under an atmosphere of D2. The originally red solution immediately turned yellow, indicating the formation of d4-8e, [(dippe)Rh(D)]2((i-T|2-D-SiHBun)2. The solvent was removed under vacuum and d6-benzene was added to the residues to make an NMR sample. Interestingly, the yellow colour of the product darkened only slightly throughout this process: the relative slowness of this conversion of 8e to the red [(dippe)Rh]2(M.-SiHBun)2, 6e, is probably caused by a deuterium isotope effect on the rate of elimination of D2 from d4-8e. The important result, though, was that the *H NMR spectrum of this sample showed signals for both silyl hydrides and rhodium hydrides on 8e. If there was no exchange between terminal hydrides bound to silicon and the rhodium hydrides (generated by addition of H2 or D2 to 6e) then we should see only Si-H signals in this spectrum, and no rhodium hydride signals. Evidently these sites are exchanging, though the exchange must be occurring relatively slowly, as the variable temperature *H NMR spectra give no evidence for it. The most likely mechanism for the exchange involves dissociation of the agostic Si-H's to give terminal SiRH2 groups on rhodium (see Scheme 4.1). (This is perhaps evidence that the three-centre, two-electron Si-H interaction does exist in solution for 8e.) Rotation around the Rh-Si bond and re-association of a different Si-H bond would exchange the terminal and "bridging" Si-H's. Note that this mechanism also changes the relative position of the R- group, interchanging the cis and trans isomers. Thus there should be an equilibrium between the isomers. The temperature invariance of the equilibrium constant, Keq, as observed in the variable temperature NMR studies of 8e, may indicate that AH0 = 0 for the conversion of one isomer to the other. References pi 77 Chapter 4 147 Bun Ht V Hb—Si H Rh*-^ Rh* H | H Bun Bu? Ht Rh*-Q Rh* Bun Rh* = (dippe)Rh Hb Bun M Ht—Si H Rh*-G Rh* %<A Bun H Scheme 4.1 The variable temperature iH and 31P{1H} NMR spectra of [(dippe)Rh(H)]2(|J.-T|2-H- SiHTolP)2, 8f, undergo the same sort of transformations as do those for 8e, but the spectra of 8f are much more complex. For example, from examination of the variable temperature 31P{1H} NMR spectra of 8f (see Figure 4.11), it is obvious that again there are two isomers in solution, in roughly the same ratio as they were in 8e, presumably with the cis being the major isomer again. Both isomers are fluxional: the peaks due to the cis isomer coalesce at approximately -40°C and are most clearly resolved at -65°C. The peaks due to the trans isomer coalesce between -50 and -65°C, and their low temperature limit is not reached by -95°C. However, in this case the complexity of the spectra would suggest a lower symmetry in these molecules. There are more signals, and their splitting patterns indicate that 31p.3lp coupling is occurring between the now inequivalent phosphines. These spectra are too complicated to assign; it is assumed that the decreased symmetry in 8f arises from steric constraints imposed by the /7-tolyl groups interacting with the isopropyl substituents on the dippe ligands. The *H NMR spectra (not shown here) are correspondingly more complex in the hydride region, though Si-H signals similar to those for 8e are seen, and the changes occurring with changes in temperature correspond to the transformations observed in the variable temperature *H NMR spectra of 8e. It is assumed that similar exchange processes are at work in 8f. References pi 77 Chapter 4 148 Hi -JU -94°C i i i i | i i i i l i i i i | i i i i | i i i i | i i i i | i i i i | i i i i | i i i i | in i | i i i i ] i i i i i i i i i j i i i i | i i i i | i i in i i i i | i i i i in i i | i i i i 110 100 90 60 70 PPM 110 100 90 60 70 PI PM Figure 4.11 Variable temperature 121.4 MHz 31P{ *H} NMR spectra of [(dippe)Rh(H)]2(u.- T|2-H-SiHTolP)2,8f, in C7D8. References pi 77 Chapter 4 149 4.2.3 Reactions of more than two equivalents of primary silanes with [(dippe)Rh]2(|i-H)2,1 When four or five equivalents of a primary silane are added to [(dippe)Rh]2(|i-H)2,1, a new silicon-containing species is formed. These species, 9e-f (where e = Bun and f = TolP), show complex *H and 31P{ lH} NMR spectra, both at room temperature and at low temperature. Elemental analyses for the two compounds suggest the presence of three silicons per dinuclear rhodium centre for both complexes, and the resonances observed in the rhodium hydride region of the *H NMR spectra and the patterns which emerge at low temperature in the 31P{ lH} NMR spectra of the two compounds suggest that their structures are similar. -3H2 [(dippe)Rh]2(^-H)2 + 4-5 equiv RSiH3 * • [(dippe)Rh]2»(RSiH)3(H)2 [4-5] 1  e: R-Bu" • • • » f: R = Tolp 4.2.3.1 Solid-state structure of [(dippe)Rh]2(^-Tj2-H-SiHTolP)2(n-SiHTolP), 9f The complexes 9e-f are unusual in this system of compounds in that they can be isolated as crystalline solids which are air-stable; the crystals show only slight discoloration after several days in air. These complexes are very soluble in aromatic and aliphatic solvents. In particular, 9e is so soluble that its isolation requires the cooling of extremely concentrated pentane solutions to -40°C. The pale yellow crystals of 9e tend to be soft and waxy, and they easily desolvate, becoming opaque. Crystals of the p-tolyl analogue, 9f, are light orange needles which are more easily isolated. An X-ray crystallographic study was carried out on a single crystal of 9f to determine its structure; the results of this study are presented below. Figure 4.12 shows an ORTEP diagram of the solid-state structure of [(dippe)Rh]2(^-Tl2-H-SiHTolP)2(^-SiHTolP), 91 References pi 77 Chapter 4 150 Tables 4.3 and 4.4 show some of the relevant bond lengths and bond angles. The silicon and rhodium hydrides were located in the Fourier difference map but were not refined. Table 4.3 Selected bond lengths for [(dippe)Rh]2(|i-ri2-H-SiHTolP)2(MiHTolP),9f. Bond Length (A) R h l - R h 2 2.8499(9) R h l - P l 2.409(2) R h l - P 2 2.309(2) R h l - S i l 2.336(2) Rhl - Si2 2.349(2) R h l - S i 3 2.444(2) Bond Length (A) Rh2-P3 2.379(2) Rh2-P4 2.299(2) Rh2-S i l 2.356(2) Rh2 - Si2 2.477(2) Rh2 - Si3 2.350(2) R h l - H 4 1.49 Bond Length (A) Rh2-H2 1.47 S i l - H l 1.41 S i2 -H2 1.47 S i2 -H3 1.31 S i3 -H4 1.43 S i3 -H5 1.39 Table 4.4 Selected bond angles for [(dippe)Rh]2(^-T|2-H-SiHTolP)2(^-SiHTolP), 9f. Bonds P 1 - R M - P 2 P 3 - R h 2 - P 4 P I - R h l - S i l P 3 - R h 2 - S i l R h l - S i l - R h 2 Angle (deg) 84.84(7) 86.10(7) 173.13(7) 175.38(7) 74.79(6) Bonds R h l - S i 2 - R h 2 R h l - S i 3 - R h 2 C 2 9 - S i l - H l C 3 6 - S i 2 - H 3 C 4 3 - S i 3 - H 5 Angle (deg) 72.34(5) 72.92(6) 97.9 100.6 72.3 References pi 77 4 C16 ure 4.12 Molecular structure of [(dippe)Rh]2(|i-Tl2-H-SiHTolP)2(u-SiHTolP), 9f. Isopropyl groups on phosphorus andp-tolyl groups on silicon are represented by single carbons. References pi 77 Chapter 4 152 In the solid state this molecule consists of the dinuclear [P2Rh]2 unit with two rhodiums bridged by three silicon-containing ligands. One of these (Sii in Figure 4.12) is a bridging silylene unit. The other two ligands are silyl groups (Si2 and Si3> which are bound to one rhodium through a direct Si-Rh bond and to the other rhodium through participation of the Si-H bond in a three-centre, two-electron interaction with the metal centre. Examination of Rh-Si distances for each of the agostic silyl ligands in 9f and comparison with other dinuclear complexes incorporating agostic silyl ligands suggests that oxidative addition of the Si-H bonds is arrested at an intermediate stage for Si2 and at a later stage for Si3. However, it is possible that the steric crowding at the core of 9f as well as electronic factors could play a large role in bond length variations. The substituents on the three bridging silicons encircle the dinuclear rhodium core with alternating silicon hydride and /Molyl groups. The two chelating phosphine rings are almost coplanar, there being a dihedral angle of 170.35° between the planes containing Pi, P2, Rhi and P3, P4, RI12. The rings are tilted away from the bridging silylene ligand such that Pi and P3 are trans to Sii. The trans influence of Sii is manifested in the Pi-Rhi and P3-Rh2 bond lengths being an average of 0.09 A longer than the P2-Rhi and P4-RI12 bond lengths, respectively. It is increasingly obvious that within this system of dinuclear rhodium silyl complexes the agostic Si-H interaction is not an exceptional phenomenon, but a standard mode of bridging ligation for silyl ligands in the solid state and probably in solution as well. These three-centre, two-electron interactions allow the Rh centres to increase their coordination spheres (become more coordinatively saturated) without increasing their formal oxidation state. For T|2-H2 complexes it has been noted that very specific electronic conditions at the metal centre appear to be required for the stabilization of the r|2-H2 form versus the dihydride formulation.19 If the metal fragment is too basic, or electron-rich, rc-donation from the metal into the c* antibonding orbitals on H2 will reduce the H-H bond order such that oxidative addition occurs to give two M-H bonds. However, if this back donation is absent due to electron deficiency of the metal fragment, the coordination of the the H-H a-bond is destabilized and dissociation of the References pi 77 Chapter 4 153 hydrogen molecule occurrs. The scarcity of confirmed examples of rj2-H-SiR3 ligands seems to indicate that special electronic environments are also essential for their stabilization. Evidently the dimeric [(dippe)Rh]2 fragment provides that environment, perhaps due to the basicity of the chelating phosphines with their isopropyl substituents. It is interesting to note that most of the T|2-H-SiR3 complexes (in the system studied here, one "R" is (dippe)Rh) that have been characterized contain strongly donating ligands such as trialkylphosphine ligands or coordinated arenes which are often alkyl-substituted.20,21 While there are some structures of dinuclear complexes reported in the literature which have two bridging silyl groups having agostic Si-H interactions (see Chapter 3), 9f represents the first structurally characterized example of a dinuclear transition-metal centre having two bridging, agostic silyl ligands and a third, bridging silylene ligand as well. The ruthenium complex with three bridging dimethylsilylene ligands shown in Equation 4-6 was characterized by infrared spectroscopy and mass spectrometry, by analogy with dimethylgermylene analogues of ruthenium and iron.22 It is the only other example of a metal-metal bond bridged by three silicon-containing ligands. Me3 Me2 Me2 (OC)4Ru Ru(CO)4 + xs Me2SiH-SiHMe2 • (OC)3Ru'^- ^Ru(CO)3 I \ / [4-6] Sl X S i J  Me2 Two crystallographically characterized complexes contain dinuclear platinum centres with bridging ligands containing a total of four silicons bound to the two metal centres; these both contain bridging disilane ligands.16,23 ^R. ^Pv Si—Si .P^ K-7] PtH2+ H3SiSiH3 »- P C ^2 Cy = cyclohexyl Cy2 \ H2 / ' - Pt S Cy2 V H2 H2 - \ Cy2 /S Pt / -Si H2 V Cy2 References pi 77 Chapter 4 154 H Ar A r ^ s | _ S I ^ H Me2 /\\ \ Me2 [(dmpe)Pt(SiH2Ar)2]2(|i-dmpe) A' 6°°C,. y — p ^ P \ ^ \ , J i ; ^ P v ^ ~ ^ S . t4"8] Ar = phenyl, p-tolyl M e 2 A r > \ Me2 Ar 4.2.3.2 Room and variable temperature NMR spectra of [(dippe)Rh]2(|i-r|2-H-SiHR)2([i- SiHR), 9e (R = Bun) and 9f (R = TolP) NMR spectroscopic studies of 9e-f have been of limited use in discerning the solution structures of these complexes. While for 9e the spectra are generally consistent with a structure analogous to the solid-state structure of 9f, the spectra for 9f are somewhat more complex and difficult to assign. It is likely that 9f does have a structure in solution which is similar to its solid-state structure, and that the complexities in its NMR spectra arise from a loss of symmetry in the complex due to the steric interactions of the p-tolyl groups on silicon with the isopropyl groups on the phosphine ligands. The *H NMR spectrum of 9e in solution is temperature invariant down to -85°C and, as mentioned above, the structural information it provides about the complex is consistent with a structure analogous to the solid-state structure of 9f shown in Figure 4.12. Figure 4.13 shows the room temperature *H NMR spectrum of 9e. References pi 77 Chapter 4 155 n!jlJ]JJ]llJ]JllJJ]J]!jl)liJJIiJJ]ll!JllliJJIlllllllJlJll|llll]l - 7 .8 -8.0 - 8 . 2 -8 .4 - 8 . 8 -FW* i i11111111;1111111)111111]ij))n1111111 i l l I) i J ) j) i J i M i) 11)) 111 i i i) | ; i i i 11 i | i i i n j 11 n 11 111! I I I I j 1 I I I I ) I I I j 1 1 I I I PPM Figure 4.13 Room temperature 300 MHz ! H NMR spectrum of [(dippe)Rh]2(|i.-T|2-H- SiHR)2((i-SiHR), 9e, in (^De- Five separate resonances due to hydrides in the molecule are observed, each of relative intensity one. Three of these signals occur between 6.8 and 4.5 ppm, where Si-H resonances normally occur. The other two signals appear in the high field region normally associated with Rh-H resonances. This is consistent with a structure containing three SiHBu" fragments and two hydrides which are bound to rhodium. The shifts of the silyl hydride resonances (6.78, 6.52 and 4.59 ppm) suggest that there are two Si-H's in similar chemical environments and one in a different chemical environment. *H NMR homonuclear decoupling experiments have shown that none of the five hydrides are coupled to each other. The rhodium hydride resonances are multiplets which show second order patterns. A !H{31P} NMR spectrum of 9e shows a simple References p] 77 Chapter 4 156 doublet for each rhodium hydride signal, indicating that each hydride is coupled to just one ^ 3Rh. (In the same ^ { ^ P } NMR spectrum the Si-H resonances, seen as broad singlets in the 31P-coupled spectrum, have slightly smaller wi/2 values but no coupling is resolved.) A computer simulation of the rhodium hydride signals observed in the lH NMR confirms that each hydride is coupled to a single rhodium and also to two different phosphorus nuclei. The 2 J H - P values range from 11 to 13 Hz, which are classic values for cis coupling.24 While it is assumed that 9f has the same structure in solution as 9e (i.e. the same as the solid-state structure of 9f) the room temperature spectrum of 9f is more complex, particularly in the aromatic and Si-H regions. The changes in this spectrum as the temperature is lowered, however, are somewhat informative. A single p-tolyl methyl peak seen in the room temperature spectrum splits into three separate signals at low temperature, indicating the inequivalent environments for the three /Kolyl groups. The ortho and meta proton signals split accordingly in the low temperature spectrum. One of the three /7-tolyl groups experiences restricted rotation around its Si-Cipso bond, as one of the three sets of aromatic proton signals further splits from two into four separate signals when the sample is cooled. The Si-H signals become correspondingly complex with the decrease in temperature, but the rhodium hydride signal of relative intensity two (a single multiplet of the same second order pattern as is seen for the hydrides in 9e) does not change as the temperature is lowered, apart from broadening slightly. The two rhodium hydrides must be exchanging rapidly, giving an average signal. References pi 77 Chapter 4 157 —AV/V 20°C f\Z l l JUb _JUI jl Jl_r= JlA -24°C JV1 ] l ! l J | l l J ] j l l l J | i l l i | l ! U | J J I ! j l H J | l ! H | SO BO 70 60 PPM 63°C 83°C _AJL_ l ) ) ) l l l l i l ] ] l l l l l l l ) l [ l l l l l l l l l l ) n l l ) i ) l ) 90 80 70 60 PPM Figure 4.14 Variable temperature 121.4 MHz 31p{lH} NMR spectra of [(dippe)Rh]2(u.-T|2-H- SiHBun)2(u-SiHBun), 9e, in C?D8. The 31P{1H} NMR spectrum of 9e at room temperature shows peaks due to four inequivalent phosphines that are somewhat broad and unresolved. Figure 4.14 shows the variable, low temperature 31P{ JH} NMR spectra of 9e. The spectrum generated at -24°C shows clearly the four signals. Those between 60 and 65 ppm have *JRh-p values of 105 Hz and are due to the two phosphines trans to the bridging silylene ligand. Those at lower field with ^Rh-p References pi 77 Chapter 4 158 values of about 160 Hz correspond to the phosphines which are cis to the bridging silylene ligand, and at this temperature the coupling between these cis phosphines is clearly resolved (3Jp_P = 53 Hz). A computer simulation of the 31P{ *H} NMR spectrum of 9e at -24°C confirms these asignments and is shown in Figure 4.15. J V w L 90.0 85.0 80.0 75.0 70.0 65.0 60.0 Figure 4.15 Simulated 121.4 MHz 3lp{ lH} NMR spectrum of [(dippe)Rh]2(n-T|2-H- SiHBun)2(|i-SiHBun), 9e, at -24°C. (See Figure 4.14 for the measured spectrum.) As the sample is further cooled to a minimum of -83°C more splitting of the four peaks is observed. This splitting may be due to an equilibrium which is established at low temperature between two different conformers in solution. No corresponding splitting is observed in the variable temperature !H NMR spectra, so either the changes in conformation do not significantly alter the proton environments in 9e, or the shift differences for the *H NMR signals due to the two conformers are too small for separate peaks to be resolved at low temperature. References pi 77 Chapter 4 159 While the broadness of the peaks in the room temperature 31P{ lH} NMR spectrum of 9e might suggest that the four magnetically inequivalent phosphorus nuclei are exchanging, in fact the peaks are simply broad and do not show a chemical shift dependence on temperature. At higher temperatures, 31P{1H} NMR spectra (not shown here) show the four signals getting slightly broader, then sharpening as the sample is heated to 70°C. Evidently the broadening of the 31P{ *H} NMR signals is due to some other process (perhaps having to do with movement of the n-butyl groups on silicon) than exchange of the phosphine ligands. The variable, low temperature 31P{1H} spectra of [(dippe)Rh]2(|i-Ti2-H-SiHTolP)2(M.- SiHTolP), 9f, are shown in Figure 4.16. The room temperature spectrum shows two very broad peaks superimposed on each other, centred at 70 ppm. Each peak represents two phosphines which are exchanging with each other. Lowering the temperature to -30 to -45°C causes these two signals to separate into four distinct signals in a pattern almost identical to that of the spectrum of 9e at -24°C, indicating that the two molecules have fairly similar structures in solution, with four inequivalent phosphines. As the temperature of 9f is lowered further to -88°C, though, the peaks behave differently from those due to 9e. The two low field signals initially show splitting which indicates that those two phosphines are coupled to each other; these are the phosphines which are cis to the bridging silylene ligand. However, as the sample temperature is lowered the upfield signal of this pair and the upfield signal of the other pair of signals begin to broaden and become slightly skewed. As in the spectra of 9e or 8e (Section 4.2.2.1), it looks as though signals due to a new species are growing in, but in this case the downfield signals of each pair remain perfectly sharp, with no loss of coupling. It is almost as though only one chelating phosphine ligand is undergoing conformational exchange while the other remains fixed, though it is difficult to imagine how any conformational change on one side of the molecule could leave the other side unaffected. The behaviour of this complex in solution is somewhat of a mystery. References pi 77 Chapter 4  1 6 0 |!ll!jl!llj]l!!llll]j|]!!Jl!IIJllll]IJlljllil|llll]llll|llli jllll|llll|IIII|llll|llHJIIIl|Illl|l)lljiHI|llll|l)ll|llll 100 90 80 70 60 50 PPM 100 80 80 70 60 50 PPM Figure 4.16 Variable temperature 121.4 MHz 31p{ lH} NMR spectra of [(dippe)Rh]2(^i-Tl2-H- SiHTolP)2(^-SiHTolP), 9f, in C7D8. It is possible that the exchange of phosphorus nuclei indicated by the variable temperature 31P{ !H} NMR spectra of 9f are caused simply by the fluxionality of the rhodium hydrides in this complex. If 9f does have the same structure in solution as it does in the solid state, then there are two types of phosphine in the complex: one type (P2, P4) is cis to the References pi 77 Chapter 4 161 bridging silylene ligand (Sii), the other type (Pi, P3) is trans to the silylene ligand. Within each category the only difference between the two phosphorus nuclei is their spatial relationship with the substituents on the two bridging silyl ligands. view down view down this axis this axis substituents on phosphorus and silicon omitted for clarity This is difficult to visualize, given the two-fold symmetry of the dinuclear P4RI12 unit juxtaposed against the three-fold symmetry of the three bridging silicon ligands. Shown below are side views of the complex, looking along the two Prrany-Rh-Sisilylene axes. These views demonstrate that each of Pi and P3 are cis to an agostic Si-H bond, but the arrangement of the Si-R and uncoordinated Si-H groups on the agostically-bound silicon is different for these two views. H- TOP H 1  H H . ' - Sl3~ i - < ^ S i 2 "~T0|P w phosphine ligands omitted for clarity H ToP I H H , ! ToP—Si2-H2 l4»-s,- — H W If the hydrides (presumably agostic in solution, or, if terminal, cis to a particular bridging silicon) shift from being associated with one silyl ligand to the other, this would exchange Pi and P3 between these two inequivalent magnetic environments, as shown in Scheme 4.2. Thus Pi and P3 are exchanged by this simple hydride shift, as are P2 and P4. The only problem with this proposed mechanism of fluxionality for 9f in solution is the lack of supporting evidence in the !H NMR spectra. References pi 77 Chapter 4 162 TolP H H TolP H — S i 3 ^ 4 ^ S i 2 — - T o P T o P — S i 2 - , H 2 j ^ S i 3 — H substituents on phosphorus omitted for clarity TolP H H TolP H —Si 3 ^2H4£S i 2 —-ToP T O P — S i 2 — f c t ' z j S i a — H Scheme 4.2 Hydride exchange around the entire core may be responsible for the different types of structure observed by 3lp{lH} NMR spectroscopy for complexes 9e-f in solution at low temperature, though the fact that the rhodium hydride signals in the *H NMR spectra do not change for either of 9e-f at low temperature is not consistent with this exchange. Another possibility is simply that, at low temperature in solution, the interaction of the isopropyl groups on the chelating phosphines with the substituents on silicon in these complexes causes them to "lock" into two subtly different conformations where the phosphines are twisted slightly and uniquely relative to the ligands bridging the two rhodiums. References pi 77 Chapter 4 163 4.3 Dehydrogenative silicon-silicon coupling of primary silanes in the presence of catalytic amounts of [(dippe)Rh]2(|i-H)2,1 The activity of primary silanes toward Si-Si coupling using [(dippe)Rh]2(|J.-H)2, 1, as catalyst was investigated using p-tolylsilane and n-butylsilane as substrates. The two silanes behaved very differently under the catalytic conditions, and the results of these studies suggest that more attention should be paid to the use of late transition-metal catalysts for the coupling reactions of primary alkylsilanes as opposed to the arylsilanes normally used. When a catalytic amount of [(dippe)Rh]2(^-H)2,1, is added to a solution of p-tolylsilane in toluene, the dark green colour of the catalyst precursor immediately changes to a golden colour, and bubbles are evolved (H2). The formation of bubbles slows after several hours. Analysis of the reaction mixture by GC-MS shows very similar results to those observed for the catalytic reaction of phenylsilane with Wilkinson's catalyst; that is, very low conversion of the monomer to the disilane and various products from simultaneous disproportionation reactions. RSiH3 *- [RSiH2]2 + R2SiH2 + R3S1H + R2SiH-SiH2R [4_Q] R = Tolp Obviously SiH4 must be forming as well, as a disproportionation product, but is evolved along with the hydrogen formed during the coupling reactions. The reaction of rz-butylsilane with catalytic amounts of 1 does not, under mild conditions, give any disproportionation products. Instead, dehydrogenative coupling proceeds cleanly, giving much higher conversions of monomer (63%), and giving polysilane products with chains up to five silicons long. References pi 77 Chapter 4 164 RSiH, - [RSiH2]2 + [RSiH2]2RSiH + [4.101 hexane L 18.9% 29.4% R = Bun [RSiH2]2[RSiH]2 + [RSiH2]2[RSiH]3 12.9% 1.8% Though the yield of the higher chains is low, these are the longest chains obtained from coupling of a monosilane using a platinum-group catalyst.25,26 4.3.1 Mechanistic considerations in the denydrogenative coupling of primary silanes The mechanism for the dehydrogenative coupling of silanes catalyzed by late transition- metal complexes is not known, but is presumed to proceed via oxidative addition and reductive elimination steps rather than by the G-bond metathesis processes that have been shown to be in effect in the coupling mechanism for early transition metals.27 Figure 4.17 shows a possible catalytic cycle involving late transition-metal hydride complexes.28 Obviously the metal hydride species must be coordinatively unsaturated for the cycle to proceed. All of the steps in this cycle have at one time or another been observed to occur stoichiometrically, though not all have been observed for one metal system.29 Given that at some point in the catalytic cycle both of the silanes which are coupling to each other must be bound to the metal centre it is easy to see that the steric requirements of the silane will have a large effect on the progress of the reaction. This explains why secondary silanes usually give only dimers in these reactions, as the formation of longer chains demands the coordination of both a secondary silane and a tertiary disilane to the metal centre. Similarly, it seems more likely that a primary silane would undergo coupling reactions with its dimers and trimers to give longer chains; this only requires simultaneous activation of a primary silane and a secondary polysilane. Thus formation of longer polysilane References pi 77 Chapter 4 165 chains should be possible using late transition-metal catalysts, as long as the monomer chosen is not inclined to participate in the competing disproportionation reactions, as are many arylsilanes. R2HSiSiHR2_ LnM-H ^-R2SiH2 H H | ,.SiHR2 | %H LnM'—SiHR2 LnM—SiHRg R2SiH2^ LnM-SiHR2 H2 [For primary silanes R2 = HRJ Figure 4.17 Mechanism for the dehydrogenative coupling of silanes catalyzed by late transition-metal complexes, based on oxidative addition and reductive elimination steps. It is interesting to speculate on the possible involvement of a dinuclear rhodium species in the catalytic silicon-silicon coupling cycle for the system studied here. The stoichiometric chemistry of silanes with [(dippe)Rh]2(|i-H)2,1, contains examples of almost all the steps in the catalytic cycle shown in Figure 4.17, yet they occur at the dinuclear centre and generate dinuclear products in all cases: oxidative addition of a silane to a metal hydride, hydrogen elimination, and addition of a silane to a metal silyl complex. The last reaction occurs twice for primary silanes, as the complexes 9 almost certainly form from the addition of silane to a bis(|i- silylene) complex, 6, or a bis(silane) complex, 8. The only step which has not been clearly observed to occur stoichiometrically in this system (apart from the addition of a single equivalent of primary silane to 1) is the reductive elimination of a Si-Si bond, though the reverse reaction has been observed in the addition of l,l',2,2'-tetraphenyldisilane to 1, where cleavage of the Si- Si bond gives [(dippe)Rh]2()i-H)(fi-Ti2-H-SiPh2), 2a. Certainly there has been no direct evidence of any mononuclear species forming in any of the reactions of 1 with silanes. This does References pl77 Chapter 4 166 not preclude the possible fragmentation of dinuclear compounds into highly active mononuclear species which recombine after participation in the catalytic cycle, but the fact that dinuclear complexes are capable of almost all the steps required for a catalytic cycle does support the idea of a dinuclear catalyst. The low yields of the longer chains and the fact that odd-numbered chainlengths occur suggest that the chain growth is stepwise. For chains longer than two silicons, this requires addition of both a monomer and an oligomer Si-H bond to the active catalyst. This could be facilitated by having two Rh centres in the catalyst at which the additions could occur. In Chapter 3 the possibility that a dinuclear bis(silane) complex could be involved in the dimerization of diphenylsilane was discussed. XIV 9 It is possible that a similar intermediate could be involved in the further coupling of a primary silane, for example where one bridging group is an T| ^disilane as in compound XIV above. Elimination of a Si-Si bond from this intermediate would generate the trimer plus 1, which would react rapidly with more silane. Figure 4.18 shows a possible mechanism for the dehydrogenative coupling of a primary silane catalyzed by 1. References pi 77 Chapter 4 167 R'RSiH-SiH2R RSiH3 (R'-H) 1 2H2 R'RSiH2 ( R" = H, RSiH2, etc.) I H 9 Rh* = (dippe)Rh Figure 4.18 Mechanism for the dehydrogenative coupling of primary silanes catalyzed by [(dippe)Rh]2(p.-H)2,1, based on oxidative addition and reductive elimination steps at the dinuclear centre. Though [(dippe)Rh]2(fX-T|2-H-SiHR)2(^-SiHR), 9, is shown as being outside the catalytic cycle in Figure 4.18 this compound cannot be ruled out as a possible intermediate in the catalytic cycle. The silicons are in close proximity in this molecule, and elimination of disilane from this crowded complex could possibly drive the catalytic cycle. References pi 77 Chapter 4 168 In support of the notion of dinuclear centres being involved in catalytic coupling of silanes it should be noted that many reactions of silanes with mononuclear metal fragments give dinuclear metal complexes as products.9,16,23,30"33 Of note are the dinuclear platinum complexes described in Chapter 3 in which the bridging silylene groups are close enough to each other across the four-membered metallocycle that nascent Si-Si bond formation has been invoked.33 Also of interest is the dinuclear platinum complex shown in Equations 4-8 and 4-11.16 H Ph Ph<£j  S j -^H 4PhSiH3 SiPhH2 t4-1 1 ! Me9 / u \ Me2 - [Ph^Si]2 * Me2 | ^SiPhH2 ^-P...?..p{ V H V . . . . P - V y—P-^Pt—SiPhH2 Me2 Ph* S{ M e 2 « '"* Me 2 H % p h -2PhSiH3 Stoichiometric Si-Si bond formation is only seen when this dinuclear "basket" compound is formed, or upon production of the corresponding dinuclear bis(jx-silylene) complexes. The cycle shown in Equation 4.11 could be a catalytic cycle, as both of the complexes shown do act as catalyst precursors for the slow dimerization of phenylsilane. It is important to re-emphasize that while some stoichiometric chemistry may implicate dinuclear complexes as the active species in the dehydrogenative Si-Si coupling reactions observed for this dinuclear rhodium system, these complexes may actually be side products in the catalytic cycle which must fragment in order to participate in the cycle. In the absence of kinetic data to support the activity of the dinuclear species towards the dehydrogenative coupling it is impossible to prove the involvement of dinuclear complexes in the catalytic cycle. References pi 77 Chapter 4 169 4.4 Experimental 4.4.1 General procedures and reagent syntheses The general laboratory procedures and purification of standard solvents and reagents were described in Section 2.4.1 (Chapter 2). The primary silanes n-butylsilane34 and p-tolylsilane35 were prepared by the IJAIH4 reduction of the corresponding trichloroalkylsilanes. n-Butyltrichlorosilane and /7-tolyltrichlorosilane were purchased from Petrarch Systems and used without further purification. Hexane used in the catalytic runs was spectroscopic grade, dried over sodium benzophenone ketal and distilled under nitrogen. 4.4.2 Syntheses of complexes and reactivity studies [(dippe)Rh]2(|i-SiHBun)2, 6e. To a stirred, dark green solution of [(dippe)Rh]2(ji-H)2, 1, (94 mg, 0.128 mmol) in hexanes (4 mL) was added dropwise a solution of w-butylsilane (23 mg, 0.257 mmol) in hexanes (4 mL) to give a brownish-red solution, which lightened in colour over 10 seconds, giving a golden-yellow solution. The volume of the solution was reduced by one third. Slow evaporation of the solvent at room temperature overnight caused the solution to become red. Red crystals were obtained by cooling to -40°C. Yield: 67% (78 mg). NMR characterization showed these crystals to be a mixture of trans (85%) and cis (15%) isomers. The two isomers were inseparable by crystallization techniques, and heating or cooling solutions containing this mixture did not change the ratio of trans to cis. *H NMR (C7D8, ppm) Si-// {trans) 8.07 (br s, wi/2 = 38 Hz); Si-/ / (cis) 7.58 (br s); total Si-/ / = 2H; C//(CH3)2, PC//2C//2P, CH(C//3)2, CH2CH2CH2CH3 3.0 - 0.0 (overlapping br s and mult, 64H); distinguishable in this region was one set of signals due to CH(C//3)2: 1.22 (dd, 3Jp_H = 13.1 Hz, 3 j H . H = 7.1 Hz). 31p{lH} NMR (C7D8, ppm, T= -35.5°C) trans 89.3 (d mult, ljRh_P = References pi 77 Chapter 4 170 160 Hz); cis 86.7 (d mult, ^Rh-p = 158 Hz). Anal. Calcd for C36H84P4Rh2Si2: C, 47.89; H, 9.38. Found: C, 48.23; H, 9.49. [(dippe)Rh]2(|i-SiHTolP)2, 6f. To a stirred, dark green solution of [(dippe)Rh]2(M.-H)2, 1, (131 mg, 0.179 mmol) in toluene (6 mL) was added dropwise a solution of p-tolylsilane (42 mg, 0.340 mmol, 1.9 equiv) in toluene (6 mL) to give a brownish-red solution, which lightened in colour over 10 seconds, giving a light orange solution. The volume of the solution was reduced by one third, during which time the solution colour darkened to a medium orange. Red-orange powder was obtained by cooling to -40°C. Yield: 65% (108 mg). Use of two, or slightly more than two, equivalents of the silane in this reaction tended to give small amounts of an impurity, [(dippe)Rh]2(|i-r|2-H-SiHTolP)2(|i-SiHTolP), 9f. NMR characterization showed 6f to be a mixture of trans (90%) and cis (10%) isomers. The two isomers were inseparable by crystallization techniques, and heating or cooling solutions containing this mixture did not change the ratio of trans to cis. trans-6f: *H NMR (QD6, ppm) Si-// 8.47 (mult, 2H); Hortho 7.60 (br d, 4H); Hmeta 6.99 (d, 4H, 3 j H _ H = 7.8 Hz); C//(CH3)2 2.80 (sept, 4H, 3 j H _ H = 6.9 Hz); SiC6H5C//3 2.07 (s, 6H); C//(CH3)2 1.95 (mult, 4H); CH(C//3)2 1.45 (dd, 12H, 3JH-p = 13.6 Hz, 3jH_H = 6.8 Hz); PC//2C//2P 1.14 (mult, 8H); CH(C//3)2 1.02 (dd, 12H, 3 j H _ p = 9.2 Hz, 3JH-H = 6.9 Hz); CH(C//3)2 0.86 (dd, 12H, 3jH_P = 13.8 Hz, 3 j H . H = 6.1 Hz); CH(C//3)2, 0.60 (dd, 12H, 3jH-p = 14.8 Hz, 3 j H . H = 6.9 Hz). 31p{lH} NMR (C6D6, ppm) 91.0 (d mult, IjRh-P = 159 Hz). cis-6f: *H NMR (C7D8, ppm) This compound was never isolated separately from the trans isomer, and because it is present as approximately 10% of the cisltrans mixture, many of its signals in the *H NMR are buried beneath signals due to the trans isomer. The following were the only assignments made: Hortho 7.99 (d, 4H, 3JHo-Hm = 7.5 Hz); Si-// 7.83 (br s, 2H). 3lp{lH} NMR (C7D8, ppm) 88.2 (d mult, Up.Rh = 153 Hz). Anal. Calcd for C42H80P4Rh2Si2: C, 51.95; H, 8.30. Found: C, 51.61; H, 8.23. References pi 77 Chapter 4 111 Reaction of [(dippe)Rh]2(|i.-H)2,1, with one equivalent /i-butylsilane. To a solution of [(dippe)Rh]2(|i-H)2,1, (42 mg, 0.057 mmol) in toluene (3 mL) was added BunSiH3 (7 mg of a 70% solution in toluene, 0.056 mmol) in toluene (2 mL), dropwise, with stirring. The solution changed from a dark green to an orange-brown colour. The toluene was removed under vacuum and the residues were dissolved in d6-benzene to make an NMR sample. The principal products, as determined by 31P{1H} NMR spectroscopy, were [(dippe)RhJ2(|i-SiHBun)2, 6e, and [(dippe)Rh]2((i-H)2,1. Repeating the reaction at low temperature (-70°C) and warming the mixture to room temperature slowly did not change the results. Reaction of [(dippe)Rhh(|i-H)2,1, with one equivalent p-tolylsilane. This reaction was carried out in the same manner as for the preceeding reaction (76 mg, 0.10 mmol [(dippe)Rh]2(M--H)2,1; 13 mg, 0.10 mmol TolPSiH3), with similar results. Repeating the reaction at low temperature (-70°C) did not change the results. [(dippe)Rh(H)]20i-ri2-H-SiHBun)2, 8e. [(dippe)Rh]2(n-SiHBun)2, 6 e, (22 mg, 0.024 mmol) was dissolved in ds-toluene in a sealable NMR tube. The tube was attached to a vacuum line by a needle valve adapter and slightly less than one atmosphere of hydrogen was introduced, after the solution was degassed by one freeze-pump-thaw cycle. The tube was tapped for five to ten minutes to encourage diffusion of the gas into the red solution, which instantly lightened in colour to pale yellow. The tube was sealed and NMR spectra (JH and 31P{1H}, variable temperature) of the sample were run. The *H NMR spectrum indicated the presence of two isomers in solution in approximately a 3:1 ratio. Because of fluxional processes which appear to be occurring, some of the rhodium hydride signal is not apparent in the room temperature spectrum. Also, no separate alkyl resonances for the minor (trans) isomer were observed; these resonances overlap with those for the major (cis) isomer. The *H NMR signals reported here are for the cw-isomer, unless otherwise indicated. *H NMR (C7D8, ppm) Si-// 5.42 (br s, wi/2 = 27 Hz); Si-// 5.03,4.29 (br s, trans isomer) (total Si-//= 2H); C//(CH3)2 1.96 (d sept, 8H, 3 J H _H = 6.8 Hz); SiCH2C//2CH2CH3 1.83 (mult, 4H); SiCH2CH2C//2CH3 1.58 References pi 77 Chapter 4 111 (mult, 4H); PC//2C//2P1.28 (d, 8H, 2jP_H = 10.7 Hz); CH(C//3)2 1.20 (dd, 24H, 2 j p _ H = 14.7 Hz); CH(C//3)2 1.05 (dd, 24H, 2JP_H =11.1 Hz); (these latter two signals cover those for n-butyl resonances totalling 10H); Rh-//, -11.85 (br s, 4H, wi/2 = 90 Hz). 31P{1H} NMR (C7D8, ppm) 84.1 (br d, ^Rh-P =117 Hz). Variable temperature NMR data is presented in Section 4.2.2.1. [(dippe)Rh(H)]2(|i-T|2-H-SiHTolP)2,8f. This complex was prepared in situ in the same manner as described above for 8e. The solution colour of the product is a slightly darker yellow colour than for 8e. The product is approximately 75% cis and 25% trans isomer, as determined by *H NMR spectroscopy. Signals due to both isomers in the *H NMR spectrum are reported together. Due to the fluxionality of the isomers, accurate integrals of the Si-H signals could not be measured. *H NMR (C7D8, ppm) Hortho 8.00 (d, 3 JH-H = 7.8 Hz, 4H); Si-H 8.33 - 7.62 (br s buried under Hortho signal); Hmeta 7.08 (d, 4H); Si-H 5.36 (br s, wi/2 = 27 Hz); H2(free), Si-H 4.51 (overlapping s and br s); S i C ^ C / ^ 2.15 (s, 6H); C//(CH3)2 2.39 - 2.10 (br mult, 4H); C//(CH3)2, PC//2C//2P, CH(C//3)2 1.95 - 0.34 (overlapping br s and mult, 60H); Rh-//, -5.12 (br s, wl/2 = 190 Hz); Rh-//, -11.23 (br s, wl/2 = 45 Hz); Rh-//, -12.45 (br s, wl/2 = 190 Hz); total Rh-// = 4H. 31P{1H} NMR (C7D8, ppm) Major isomer (cis, by analogy with 8e): 102.5 (br s, wi/2 = 450 Hz, 2P); 81.0 (br s, wi/2 = 450 Hz). Minor isomer (trans, by analogy with 8e): 85.3 (asymmetric d mult). Variable temperature 31P{1H} NMR data is presented in Section 4.2.2.1. Reaction of [(dippe)Rh]2(|i-SiHBun)2, 6e, with deuterium. A solution of [(dippe)Rh]2(|i-SiHBun)2, 6e, (35 mg, 0.039 mmol) in toluene (6 mL) was placed in a small reactor bomb and degassed by one freeze-pump-thaw cycle. Deuterium gas was introduced to one atmosphere. Over 10-15 seconds the solution colour changed from red-orange to a pale yellow. The solution was stirred under deuterium for five minutes, then the deuterium and toluene were removed under vacuum. The yellow residues were dissolved in d6-benzene to make an NMR sample. Addition of the solvent caused the colour of the residues to darken References pi 77 Chapter 4 173 slightly; 31P{1H} NMR spectroscopy confirmed the presence of a small amount of 6e, though the bis(silane) complex 8e was the major product in solution. *H NMR spectroscopy showed the the presence of small amounts of both silicon and rhodium hydrides in the sample, indicating the scrambling of the rhodium deuterides and the silicon hydrides from the original sample of 6e. [(dippe)Rh]2(^-Tt2-H-SiHBun)2(^-SiHBun), 9e. To a solution of [(dippe)Rh]2(^i-H)2, 1, (137 mg, 0.187 mmol) in pentane (5 mL) was added five equivalents of /i-butylsilane (83 mg, 0.935 mmol) in pentane (4 mL). The colour of the solution immediately changed from dark green through brown to an intense yellow. Yellow powder was obtained from a minimum volume of pentane by cooling to -40°C. Yield: 62% (115 mg). *H NMR (C6D6, ppm, 500 MHz) Rh-H-Si//Bun 6.78 (br s, IH, wi/2 = 47 Hz, ^ S i - H = 173 Hz*); Rh-H-Si//Bun 6.52 (br s, IH, wi/2 = 38 Hz, 1J29Si-H = 173 Hz*); ^-Si//Bun 4.59 (br s, IH, wi/2 = 35 Hz, !J29Si-H = 165 Hz*); SiCH2C//2CH2CH3 2.58 - 2.37 (overlapping mult, 6H); SiCH2CH2C//2CH3, CHiCHih 2.25 - 1.84 (overlapping mult, 14H); SiC//2CH2CH2C//3, PC//2C//2P, CH(C//3)2 1.80 - 0.99 (overlapping mult, 71H); Rh-// -8.03 (mult (second order), IH, !JRh-H = 24 Hz*); Rh-// -8.48 (mult (second order), IH, ^Rh-H = 22 Hz*). Parameters used in the simulation of the hydride region of 9e:36 Ha -8.04 (linewidth 5 Hz), Hb -8.48 (linewidth 4 Hz); jRhX-Ha = 24 Hz, JpA-Ha = 11 Hz, JpA-RhX = 100 Hz, JpM-Ha = 13 Hz, JpM-RhX = 160 Hz, JRhY-Hb = 22 Hz, Jpfi-Hb = 13 Hz, JpB-RhY = 100 Hz, JpN-Hb = 12 Hz, JpN-RhY = 160 Hz. P M ^ R h / ^ S i ^ . .^N PA^- X V / j Y ^ P B S i - H b 31P{1H} NMR (C6D6. ppm, 121.4 MHz, room temperature) 87.4 (br d mult, IP, URJI-P = 160 Hz); 82.7 (br d mult, IP, l j R h _ P = 155 Hz); 62.8 (br d, IP, ^Rh-p = 102 Hz); 60.6 (br d, IP, JjRh-P = 104 Hz). Variable temperature 31P{1H} NMR spectra are presented in Section References pi 77 Chapter 4 174 4.2.3.2. Parameters used for simulation of the 31P{ !H} NMR spectrum observed for 9e in C7D8 at -24°C: PA 63.21 (linewidth 7 Hz), PB 60.76 (linewidth 5 Hz), P M 88.56 (linewidth 10 Hz), P N 83.13 (linewidth 10 Hz); JXA = 106 Hz, JYB = 105 Hz, J A B = 10 Hz, JXM = 163 Hz, J Y N = 161 Hz, JMN = 53 Hz. 29si NMR (C^D6, ppm) 125.5 (br t, 2Si, Un-Si avg = 180 Hz); 90.2 (br mult, ISi). Anal. Calcd for C40H96P4Rh2Si3: C, 48.47; H, 9.76. Found: C, 48.77; H, 9.89. * Measured from the !H{31P} NMR spectrum recorded on a 500 MHz spectrometer. [(dippe)Rh]2(^-ri2-H-SiHToIP)2(^-SiHTolP), 9f. To a solution of [(dippe)Rh]2(P--H)2, 1, (86 mg, 0.117 mmol) in hexanes (4 mL) was added four equivalents of p-tolylsilane (57 mg, 0.466 mmol) in pentane (3 mL). The colour of the solution immediately changed from dark green through brown to a light golden orange. Light orange needles were obtained from a minimum volume of hexanes by cooling to -40°C. Yield: 84% (108 mg). 1H NMR (C7D8, ppm) Hortho 8.15 (br d, 6H); Hmeta 7.07 (d, 6H, 3JHo-Hm = 7.5 Hz); Si-H 7.58-6.41 (br s, 3H); SiC6H5C//3 2.18 (s, 9H); C//(CH3)2, PCH2CH2P, CH(C//3)2 2.07 - 0.61 (overlapping mult, 64H) [distinguishable in this region are: C//(CH3)2 1.80 (mult); CH(C//3)2 1.38 (dd, 3JP_H = 14.7 Hz, 3 j H _ H = 6.9 Hz); CH(C//3)2 0.76 (dd, 3JP_H =11.8 Hz, 3JH-H = 6.7 Hz)]; Rh-H -6.89 (mult (second order), 2H, ^H-Rh = 25 Hz*). *H NMR, (C7D8, ppm, T = -45°C) Hortho 8.74 (br d, IH); Si-H, Hortho 8.56 - 8.26 (overlapping br mult and br d, 4H, 3JHo-Hm = 6.3 Hz); Hortho 8.12 (br d, 2H, 3JHo-Hm = 6.6 Hz); Hortho 7.95 (br d, IH, 3JHo-Hm = 6.9 Hz); Hmeta 7.45 - 6.90 (overlapping br d, 6H); Si-H 5.74 (br s, IH, wi / 2 = 33 Hz); SiC6H5C//3 2.26 (s, 3H); SiC6H5C//3 2.18 (s, 3H); SiC6H5C//3 2.215 (s, 3H); C//(CH3)2, FCH2CH2P, CH(C//3)2 1-98 - 0.43 (overlapping mult, 64H); Rh-H -6.91 (br mult, 2H). 31P{*H} NMR (C7D8, ppm) 90 - 55 (br, 2P); 70.6 (br s, 2P, wi/2 = 760 Hz). 31p{lH} NMR (C7D8, ppm, T = -30°C) 85.0 (dd, IP, 2JRh-P = 158 Hz, 3JP_p = 24 Hz); 77.6 (dd, IP, !jRh-p = 1170 Hz); 64.8 (d, IP, ^Rh-P = 121 Hz); 55.4 (d, IP, ^Rh-p = 121 Hz). Other variable temperature 31P{1H} NMR spectra are References pi 77 Chapter 4 175 presented in Section 4.2.3.2. 29Si NMR (C6D6, ppm) 142 - 100 (br). Anal. Calcd for C49HQ()P4Rh2Si3: C, 53.84; H, 8.30. Found: C, 53.89; H, 8.24. * Measured from the 1H{31P} NMR spectrum recorded on a 500 MHz spectrometer. 4.4.3 Catalytic reactions Catalytic coupling of/7-tolylsiIane. A typical procedure for the coupling reaction is as follows: In a 75 mL reactor bomb was placed /j-tolylsilane (168 mg, 1.38 mmol) in 2.50 mL toluene, along with a stir bar. Addition by syringe of a dark green solution of [(dippe)Rh]2(|J.-H)2,1, (0.25 mL of a 0.055 M solution in toluene, 0.014 mmol) to the substrate solution gave an immediate colour change to golden, accompanied by the evolution of bubbles (H2). The mixture was allowed to stir for approximately two days open to the nitrogen manifold, which was vented to a nujol bubbler to allow H2 produced in the reaction to escape. Two days after the catalyst was added to the substrate solution the bomb was taken into a glovebox, where the mixture was passed down a Florisil column to remove the catalyst. The eluted reaction products were analyzed by GC - MS. The consumption of monomer was low, 36% based on products observed in the GC trace. The major product was 1,2-di- p-tolyldisilane; other products included di-/?-tolylsilane, tri-p-tolylsilane and 1,1,2-tri- p-tolyldisilane. Parent ions observed for these products in the mass spectra are shown in the table below. Products M+(m/e) TolPSiH3 122 (TolP)2SiH2 212 [TolPSiH2]2 242 (TolP)3SiH 302 TolPSiH2- (TolPhSiH 332 Evidently SiH* is forming during the disproportionation reactions but is carried away by the hydrogen evolved during the reaction. References pi 77 Chapter 4 176 Catalytic coupling of /t-butylsilane. A typical procedure for the coupling reaction is as follows: In a 75 mL reactor bomb was placed [(dippe)Rh]2(|i-H)2,1, (14 mg, 0.019 mmol) in 2.0 mL hexane, along with a stir bar. Addition of BunSiH3 (165 mg, 1.87 mmol) in 2.0 mL hexane to the dark green catalyst solution gave an immediate colour change to yellow, accompanied by the evolution of bubbles (H2). The head space in the bomb was evacuated several times, causing an increase in the bubbling which continued (with successive head space evacuation) for several hours. The mixture was allowed to stir overnight open to the nitrogen manifold, which was vented to a nujol bubbler to allow H2 produced in the reaction to escape. 24 hours after the substrate was added to the catalyst the bomb was taken into a glovebox, where the mixture was passed down a Florisil column to remove the catalyst. The eluted reaction products were analyzed by GC - MS. The consumption of monomer was 63%, and the products were obtained in the following quantities: 18.9% disilane, 29.4% trisilane, 12.9% tetrasilane (two diastereomers in approximately a 5:1 ratio) and 1.8% pentasilane (two peaks in a 1:1 ratio, probably both linear products). The mass spectral data was in agreement with that reported from an earlier study of the polymerization of n-butylsilane.37 The GC response factors for these polysilanes have not been measured, so the relative integrals from the GC trace, and therefore the yields, are uncorrected. 4.4.4 Calculations. Calculation of AG* for the phosphine and hydride exchange in 8e. AG* was calculated using the value for the rate constant38 kc (where kc = JtA'Oc/(2)1/2) in the Eyring equation AGt = -RTcln[(kch)/(kBTc)]; References pi 77 Chapter 4 111 where R = the gas constant, Tc = temperature of coalescence, AVQ = peak separation at the low T limit, h = Planck's constant and kg = Boltzmann constant For the phosphorus resonances due to the major isomer in the variable temperature 31p{ lH} NMR spectra of 8e, Tc = 253 K (-20°C) and Avc = 2.00 x 103 Hz. The coalescence temperature was estimated visually from the spectra and has an error of approximately ± 5 K. For the hydride resonances due to the major isomer in the variable temperature *H NMR spectra of 8e, Tc = 263 K (-10°C) and Avc = 797 Hz. The coalescence temperature was estimated visually from the spectra and has an error of approximately ± 5 K. 4.5 References (1) Young, J. F. Adv. Inorg. Chem. Radiochem. 1968,11, 92. (2) Ang, H. G.; Lau, P. T. Organomet. Chem. Rev., Sect. A 1972, 8, 235. (3) Cundy, C. S.; Kingston, B. M.; Lappert, M. F. Adv. Organomet. Chem. 1973,11,253. (4) Hofler, F. Top. Curr. Chem. 1974,50, 129. (5) Bonny, A. Coord. Chem. Rev. 1978,25,229. (6) Aylett, B. J. / . Organomet. Chem. 1980,9, 327. (7) Aylett, B. J. Adv. Inorg. Chem. Radiochem. 1982,25,1. (8) Corey, J. Y. In Advances in Silicon Chemistry; G. L. Larson, Ed.; JAI Press Inc.: Greenwich, Connecticut, 1991; Vol. 1; pp 327-387. (9) Aitken, C. T.; Harrod, J. F.; Samuel, E. / . Am. Chem. Soc. 1986,108,4059. References pi 77 Chapter 4 (10) Woo, H. G.; Heyn, R. H.; Tilley, T. D. J. Am. Chem. Soc. 1992,114, 5698. (11) Ojima, I.; Inaba, S.; Kogure, T. / . Organomet. Chem. 1973,55, C7. (12) Brown-Wensley, K. A. Organometallics 1987,6, 1590. (13) Corey, J. Y.; Chang, L. S.; Corey, E. R. Organometallics 1987,6,1595. (14) Tanaka, M.; Kobayashi, T.; Hayashi, T.; Sakakura, T. Appl. Organomet. Chem. 1988,2, 91. (15) Zarate, E. A.; Tessier-Youngs, C A.; Youngs, W. J. / . Am. Chem. Soc. 1988,110,4068. (16) Heyn, R. H.; Tilley, T. D. / . Am. Chem. Soc. 1992,114, 1917. (17) Wang, W.; Eisenberg, R. / . Am. Chem. Soc. 1990,112,1833. (18) Preliminary studies of the reactions of phenylsilane with 1 yielded highly insoluble complexes, so further reactivity studies with this silane were not pursued. (19) Jessop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992,121,155. (20) Schubert, U. Adv. Organomet. Chem. 1990,30,151. (21) Suzuki, H.; Takao, T.; Tanaka, M.; Moro-oka, Y. / . Chem. Soc, Chem. Commun. 1992, 476. (22) Brookes, A.; Knox, S. A. R.; Stone, F. G. A. / . Chem. Soc. (A) 1971, 3469. (23) Michalczyck, M. J.; Recatto, C. A.; Calabrese, J. C ; Fink, M. J. / . Am. Chem. Soc. 1992, 114, 7955. (24) Piers, W. E. Ph.D. Thesis, University of British Columbia, 1988. References pi 77 Chapter 4 (25) There is a report of the use of chloroplatinic acid as a catalyst for dehydrogenative coupling of hexylsilane to give polysilanes up to five silicons long, but the reaction conditions include oxygen. Given the tendency of late metals to catalyze the reaction of polysilanes with oxygen, it seems as though the products of this reaction must be polysiloxanes rather than polysilanes. Further verification is required. (See the following reference.) Onopchenko, A.; Sabourin, E. T. / . Org. Chem. 1987,52,4118. Tilley, T. D. Ace Chem. Res. 1993,26, 22. Curtis, M. D.; Epstein, P. S. Adv. Organomet. Chem. 1981,19,213. Tilley, T. D. Comments I norg. Chem. 1990,10, 37. Auburn, M.; Ciriano, M.; Howard, J. A. K.; Murray, M.; Pugh, N. J.; Spencer, J. L.; Stone, F. G. A.; Woodward, P. / . Chem. Soc., Dalton Trans. 1980, 659. Bennett, M. J.; Simpson, K. A. / . Am. Chem. Soc. 1971,93, 7156. Crozat, M. M.; Watkins, S. F. / . Chem. Soc, Dalton Trans. 1972,2512. Zarate, E. A.; Tessier-Youngs, C. A.; Youngs, W. J. / . Chem. Soc, Chem. Commun. 1989,577. Doyle, M. P.; DeBruyn, D. J.; Donnelly, S. J.; Kooistra, D. A.; Odubela, A. A.; West, C. T.; Zonnebelt, S. M. / . Org. Chem. 1974,39,2740. Benkeser, R. A.; Landesman, H.; Foster, D. J. / . Am. Chem. Soc. 1952, 74, 648. The simulated spectrum of 9e was calculated using the NMR" program (version 1.0). Campbell, W. H.; Hilty, T. K.; Yurga, L. Organometallics 1989,8, 2615. References pi 77 Chapter 4 180 (38) Thomas, W. A. Annu. Rev. NMR Spectrosc. 1968,1,43. References pi 77 Chapter 5 181 CHAPTER 5 Conclusions and Prospects for Future Studies 5.1 Conclusions In this thesis the characterization of a series of new, dinuclear rhodium silyl and silylene complexes is described. Some of these complexes are implicated as possible active catalysts for the hydrosilation, dimerization and isotope exchange reactions of silanes. This system of dinuclear complexes is distinctive in the variety of reactions that it undergoes, though some of the observed reactions occur for other late transition-metal complexes as well. The solid-state structures of the complexes [(dippe)Rh]2(p.-H)(|i-ri2-H-SiR2), 2a-b, are unique among late transition-metal silyl complexes, and the crystal structures of [(dippe)Rh(H)]2(M'-'n2-H-SiMe2)2, 8b, and [(dippe)Rh]2(^i-T|2-H-SiHTolP)2(MiHTolP), 9f, are simply unique. One feature that dominates the chemistry described in this thesis is the impact of the steric requirements of the silanes on their observed reactivity with [(dippe)Rh]2(|i-H)2,1, and on the resulting behaviour of the complexes isolated. Simply within the range of secondary silanes studied this impact is apparent. In Chapter 2, the variable temperature NMR studies of the silyl hydride complexes [(dippe)Rh]2((i-H)(p.-T|2-H-SiRR'), 2a-c, indicate that the exchange process rendering these molecules fluxional is slowed by the presence of bulkier groups on silicon. In fact, addition of the bulky di-r-butylsilane to 1 gave no reaction at all, even when the reaction mixture was heated.1 It is obvious, from reactions described in Chapter 3, that steric factors play a role in the type of complex isolated from the addition of two equivalents of silane to 1. For dimethylsilane the butterfly-shaped complex [(dippe)Rh(H)]2(|i-T|2-H-SiMe2)2» 8b, is isolated. Although the addition of two equivalents of the bulkier secondary silanes almost certainly proceeds through an intermediate such as 8, these intermediates are not stable and lose two molecules of References pi87 Chapters 182 hydrogen to give the bis(|i-silylene) complexes 6. The reactions of 1 with primary silanes, described in Chapter 4, serve to further illustrate the importance of steric requirements within this system. Addition of a single equivalent of a primary silane to 1 apparently generates a silyl hydride complex which is so reactive that it reacts more quickly with any remaining silane in solution than does 1. And, for «-butylsilane and /Molylsilane, the bis(silane) complexes [(dippe)Rh(H)]2(M--rl2-H-SiHR)2, 8e-f, can be generated in situ and characterized spectroscopically, though these complexes are less stable towards the loss of molecular hydrogen than the dimethylsilane analogue, 8b. Novel complexes containing three silicon ligands bridging a metal-metal bond are isolated from the reaction of 1 with more than two equivalents of primary silane: [(dippe)Rh]2(fJ.-Ti2-H-SiHR)2(^-SiHR), 9. These complexes are far too sterically hindered for analogues with two substituents on silicon to be prepared. Attempts to add the Si-H bonds in tertiary silanes to complex 1 were unsuccessful. For example, triethylsilane gave no reaction at all. Preliminary studies have shown that a mixture of either triphenylsilane or diphenylmethylsilane and 1, when heated, does give a colour change from dark green to red. 31P{ !H) NMR spectroscopy shows the principal product in either case to be the silyl hydride complex [(dippe)Rh]2(|i-H)(ja.-T|2-H-SiRR'), 2. For triphenylsilane [(dippe)Rh]2((i-H)(p:-Ti2-H-SiPh2), 2a, was the product, and for diphenylmethylsilane, [(dippe)Rh]2(fi-H)((i-Ti2-H-SiMePh), 2c, was the product. Ph2RSiH „ [(dippe)Rh]20i-H)2 ^ [(dippe)Rh]2(^-H)(^n2-H-SiPhR) [5-1] 1 2 2a: R = Ph 2c: FUMe The products of these reactions are due to Si-C activation rather than Si-H activation, though perhaps some association of the Si-H bond with the metal centre is required before the disproportionation reaction can occur. The only other tertiary silane that was added to complex 1 References pi87 Chapter 5 183 was l,r,2,2'-tetraphenyldisilane and, as described in Chapter 3, it reacts to give the Si-Si bond cleavage product, 2a, rather than an Si-H addition product. The steric effects described above are due mainly to the interaction of the substituents on silicon with the isopropyl groups on the chelating phosphine ligands, which surround the dinuclear rhodium core. As described in Chapter 4, silicon-silicon coupling reactions catalyzed by 1 are affected by this steric congestion at the dinuclear core as well: the apparent stepwise growth of the silicon oligomers requires coordination of progressively longer and longer silicon chains to the rhodium centres. The advantage of having two rhodium centres to which the monomer and oligomers can bind is offset by the presence of the "isopropyl cloud" surrounding the binuclear core, when chains longer than four silicons are formed. So while the results obtained in Si-Si coupling reactions of a primary alkylsilane catalyzed by 1 are encouraging (five-membered chains are the largest silicon oligomers obtained from a monomer using a late transition-metal catalyst), and provide some insight into possible mechanisms for the dehydrogenative coupling, clearly the system studied has limitations. 5.2 Prospects for future studies Given the importance of steric considerations in the system studied in this project, future studies of the reactions of silanes with this type of dinuclear rhodium hydride might involve a variation in the chelating phosphine ligands used. For instance, complexes could be prepared with the l,2-bis(dimethylphosphino)ethane (dmpe) ligand, replacing isopropyl groups on phosphorus with methyl groups. Use of this less obtrusive ligand might, for instance, allow isolation of analogues of 9 for secondary silanes, and could increase silicon chain lengths obtained from dehydrogenative coupling of primary alkylsilanes. Previous studies compared the reactivity of rhodium hydride dimers containing phosphine ligands of varying chelate ring size toward small organic molecules.2 Complexes with phosphines containing two carbons in the "backbone" were References pi87 Chapters 184 found to be more reactive than those with three or four carbons. The smaller bite angle of the five- membered chelate ring (relative to the six- or seven-membered rings) pulls the isopropyl substituents on phosphorus back and away from the rhodium hydride core of the dimer, allowing small organic molecules easier access to this reactive site. For the larger chelate rings, the substituents on phosphorus are pushed forward around the reactive core of the molecule. Thus one would expect that using a ligand with a one carbon backbone instead of the two carbon backbone in the dippe ligand might increase the reactivity of the hydride dimer even more. However, attempts to prepare analogous complexes to 1 using ligands containing a single CH2 unit in the chelate ring on rhodium have instead resulted in polyhydride complexes where the bis- phosphine ligand is dinucleating instead of chelating.3 Prj2 R PH2 Rh Rh ^y \ / \y H 1 H. H" Rh: .H. •H' :Rh: .H •H R - Pr, Cyclohexyl Future research involving complex 1 without any variation in the phosphine ligand should probably focus on the reactions of other primary silanes with 1. For instance, the oligomerization reactions of smaller alkylsilanes such as MeSiH3 or EtSiH3 might give longer silicon chains. Also, complexes 9e-f have not yet been thoroughly studied and may present interesting modes of reactivity. It would be of interest to evaluate their activity as catalyst precursors for the coupling reactions. There has been some interest in controlling the stereochemistry of polysilanes as a means of gaining further control over their physical properties.4 Given the steric influence of the chelating phosphine ligand in [(dippe)Rh]2((^-H)2,1, on its reactions with silanes, it is possible that replacing dippe with a chiral phosphine ligand might influence the stereochemistry of oligomers References pi 87 Chapter 5 185 produced by silicon coupling catalyzed by 1. The chiral ligand, A, shown below is almost analogous to dippe, except that the PPr^ groups are replaced by dimethyl-substituted phospholane rings.5 On a dinuclear rhodium hydride complex like 1, the ligand A could influence the stereochemical outcome of Si-Si coupling reactions at the dinuclear centre. The phospholane rings on A would also have the effect of pulling the substituents on phosphorus away from the RI12H2 core, and thus potentially increasing the reactivity of the complex toward bulky silanes. A dippe The homogeneous catalysis by [(dippe)Rh]2(|i-H)2,1, of two other reactions of silanes could be explored. These are the silane alcoholysis and silane ammonolysis reactions, dehydrogenative condensation reactions, which are principally of interest for the formation of Si-O and Si-N bonds in organic synthesis.6 catalyst R3S1H + R'OH »- R3SiOR' [5-2] -H2 catalyst R3SiH + R"R'NH — R3SiNR'R" [5-3] -H2 Complex 1 has been shown to react with Bronsted acids such as amines and alcohols to give complexes of the formula [(dippe)Rh]2(|X-H)(|J.-X), where X = NR2 or OR.7,8 It appears that oxidative addition of the N-H and O-H bonds to rhodium is the initial step in these reactions, which parallels the observed chemistry of silanes with the hydride complexes. The reactions of silanes with [(dippe)Rh]2(|i-H)2,1, described in this thesis illustrate the variety of silicon chemistry available using a dinuclear, late transition-metal complex. This References pi 87 Chapter 5 186 research should provide a basis for more research exploring the fundamental interactions of silanes with transition-metal centres. 5.3 Experimental 5.3.1 General procedures and reagent syntheses The general laboratory procedures and purification of standard solvents and reagents were described in Section 2.4.1 (Chapter 2). Bu^Sit^ was prepared by the LiAltL* reduction of BulClSiH2,9 which was purchased from Petrarch Systems and used without further purification. Et3SiH, Ph3SiH and Ph2MeSiH were purchased from Aldrich, dried over calcium hydride and distilled. 5.3.2 Reactions of [(dippe)Rh]2(n-H)2, 1, with tertiary silanes Reaction of [(dippe)Rh]2(|i-H)2, 1, with Ph3SiH. To a stirred, dark green solution of [(dippe)Rh]2(|i-H)2,1, (25 mg, 0.034 mmol) in toluene (3 mL) was added dropwise a solution of triphenylsilane (18 mg, 0.068 mmol) in toluene (2 mL). Initially no colour change was observed. The mixture was placed in a reactor bomb and heated to 50°C overnight, during which time the solution colour changed from dark green to red. The solvent was then removed under vacuum and the residues used for an NMR sample. The 31P{ 1H} NMR spectrum of these residues showed the product to be [(dippe)Rh]2(|i-H)(|i-r|2-H-SiPh2), 2a. The !H NMR spectrum showed signals due to 2a and unreacted Ph3SiH. No other peaks due to silicon- containing products were discernible among these signals, though presumably Ph4Si is the other product of the disproportionation of Ph3SiH. References pi87 Chapters 187 Reaction of [(dippe)Rh]2(fi-H)2, 1, with Ph2MeSiH. This reaction was carried out in the same manner as the preceding reaction (25 mg, 0.034 mmol [(dippe)Rh]2(|i-H2), 1; 13 mg, 0.068 mmol Ph2MeSiH). The 31P{ 1H) NMR spectrum of the red reaction residues showed the product to be [(dippe)Rh]2(|i-H)(p.-ri2-H-SiMePh), 2c. The ! H NMR spectrum showed signals due to 2c and unreacted Ph2MeSiH. No other peaks due to silicon-containing products were discernible among these signals, though presumably Ph3MeSi is the other product of the disproportionation of Ph2MeSiH. 5.4 References (1) See Section 5.3 for experimental details of this and other reactions described for the first time in this chapter. (2) Piers, W. E. Ph.D. Thesis, University of British Columbia, 1988. (3) Fryzuk, M. D.; McConville, D. H.; Rettig, S. J. / . Organomet. Chem. 1993,445, 245. (4) Banovetz, J. P.; Stein, K. M.; Waymouth, R. M. Organometallics 1991,10, 3430. (5) Burk, M. J.; Feaster, J. E.; Harlow, R. L. Organometallics 1990,9, 2653. (6) Corey, J. Y. In Advances in Silicon Chemistry; G. L. Larson, Ed.; JAI Press Inc.: Greenwich, Connecticut, 1991; Vol. 1; pp 327-387. (7) Fryzuk, M. D.; Jang, M.; Jones, T.; Einstein, F. W. B. Can. J. Chem. 1986, 64, 174. (8) Fryzuk, M. D.; Piers, W. E., unpublished results. (9) Benkeser, R. A.; Landesman, H.; Foster, D. J. / . Am. Chem. Soc. 1952, 74, 648. References pi87 Appendix Appendix A.1 X-ray Crystallographic Analysis of [(dippe)Rh]2(u.-H)(u.-ri2-H-SiPIi2), 2a. Z22. CIS C19 EXPERIMENTAL DETAILS A. Crystal Data Empirical Formula Formula Weight Crystal Color, Habit Crystal Dimensions (mm) Crystal System No. Reflections Used for Unit Cell Determination (26 range) Omega Scan Peak Width at Half-height Lattice Parameters: C40H76P4Rh2Si 914.83 red-orange, irregular 0.200 X 0.350 X 0.500 monoclinic 25 ( 31.2 - 44.7°) 0.36 a - 19.338 (8)A b - 11.223 (4)A c - 22.934 (9)A 3 - 112.39 (3)° V - 4602 (6)A3 Appendix Space Group P 2 1 / n (#14) Z v a l u e 4 D , 1 . 3 2 0 g/cm c a l c Tnnn 1920 000 "(MoKa) 8 . 9 3 cm" 1 B. intensity Measurements Diffractometer Rigaku AFC6S Radiation MoKa (X « 0.71069 A) Temperature 21°C Take-off Angle 6.0e Detector Aperture 6.0 mm horizontal 6.0 mm vertical Crystal to Detector Distance 28.5 cm Scan Type co-28 Scan Rate 32.0°/min (in omega) (8 rescans) Scan Width (1.31 + 0.35 tan6)° 29 55 0° max :>:5*u No. of Reflections Measured Total: 11434 Unique: 11112 (» i n t - Corrections Lorentz-polarization Absorption (trans, factors: 0.91 C. Structure Solution and Refinement Structure Solution Patterson Method Hydrogen Atom Treatment Refined or included in calculated positions (dC-H " °-98A> Refinement Full-matrix least-squar Function Minimized I w (|Fo| - |Fc|) Least-squares Weights 4Fo2/a2(Fo2) p-factor 0.03 Appendix Anomalous Dispersion All non-hydrogen atoms No. Observations (X>3.00?(X)) 5752 No. Variables 432 Reflection/Parameter Ratio 13.31 Residuals: R; Rw 0.037; 0.043 Goodness of Fit Indicator 1.46 Max Shift/Error in Final Cycle 0.03 Kaximum Peak in Final Diff. Map 0.78 e~/A^ Minimum Peak in Final Diff. Map -0.52 e~/A Table . Final atomic coordinates (fractional) and B(eq). atom x y z B(eq) R h ( l ) Rh(2) P ( l ) P (2 ) P (3 ) P ( 4 ) Si C ( l ) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C ( l l ) C(12) C(13) C(14) 0 . 4 9 3 6 7 ( 2 ) 0 . 5 4 3 6 1 ( 2 ) 0 . 4 3 4 9 0 ( 8 ) 0 . 4 6 9 5 5 ( 8 ) 0 . 5 4 9 4 1 ( 8 ) 0 . 6 3 5 0 5 ( 8 ) 0 . 4 9 3 6 3 ( 7 ) 0 . 3 9 9 7 ( 3 ) 0 . 4 1 0 9 ( 3 ) 0 . 6 0 6 6 ( 4 ) 0 . 6 6 6 6 ( 4 ) 0 . 3 4 3 1 ( 3 ) 0 . 4 8 4 2 ( 4 ) 0 . 4 2 3 5 ( 3 ) 0 . 5 3 7 0 ( 4 ) 0 . 5 6 8 6 ( 4 ) 0 . 4 5 9 2 ( 4 ) 0 . 7 2 4 6 ( 3 ) 0 . 6 0 9 6 ( 4 ) 0 . 2 9 1 6 ( 3 ) 0 . 3 0 3 6 ( 4 ) 0 . 1 3 7 8 2 ( 4 ) 0 . 2 2 7 3 0 ( 4 ) 0 . 0 0 5 7 ( 1 ) 0 . 2 7 4 5 ( 1 ) 0 . 2 7 5 5 ( 2 ) 0 . 3 5 9 9 ( 1 ) 0 . 0 2 9 1 ( 1 ) 0 . 2 0 5 7 ( 5 ) 0 . 0 7 2 3 ( 5 ) 0 . 4 1 0 9 ( 6 ) 0 . 4 2 2 2 ( 6 ) - 0 . 0 5 1 0 ( 5 ) - 0 . 1 3 1 3 ( 6 ) 0 . 4 1 4 6 ( 5 ) 0 . 3 3 0 7 ( 8 ) 0 . 1 6 7 1 ( 7 ) 0 . 3 2 1 ( 1 ) 0 . 3 0 8 4 ( 6 ) 0 . 4 9 1 4 ( 6 ) 0 . 0 5 1 2 ( 6 ) - 0 . 1 3 9 1 ( 6 ) 0 . 1 9 6 5 3 ( 2 ) 0 . 3 2 6 4 3 ( 2 ) 0 . 1 2 2 5 9 ( 6 ) 0 . 1 1 6 7 5 ( 6 ) 0 . 4 2 2 1 3 ( 6 ) 0 . 3 4 2 0 2 ( 6 ) 0 . 2 8 1 4 3 ( 6 ) 0 . 0 4 4 4 ( 2 ) 0 . 0 4 3 3 ( 2 ) 0 . 4 5 0 0 ( 3 ) 0 . 4 2 2 9 ( 3 ) 0 . 1 1 7 7 ( 2 ) 0 . 1 1 4 3 ( 3 ) 0 . 1 2 4 6 ( 3 ) 0 . 0 8 2 6 ( 4 ) 0 . 4 8 8 2 ( 3 ) 0 . 4 2 6 3 ( 3 ) 0 . 3 3 8 8 ( 3 ) 0 . 2 8 9 6 ( 3 ) 0 . 1 1 6 4 ( 3 ) 0 . 0 6 5 1 ( 3 ) 3 . 0 1 ( 1 ) 3 . 1 1 ( 1 ) 3 . 7 9 ( 5 ) 3 . 8 8 ( 5 ) 4 . 8 4 ( 6 ) 3 . 9 4 ( 5 ) 3 . 2 9 ( 5 ) 4 . 8 ( 2 ) 4 . 6 ( 2 ) 6 . 5 ( 3 ) 5 . 9 ( 3 ) 4 . 9 ( 2 ) 5 . 8 ( 3 ) 5 . 3 ( 3 ) 8 . 3 ( 4 ) 7 . 4 ( 4 ) 8 . 5 ( 4 ) 5 . 6 ( 3 ) 6 . 3 ( 3 ) 6 . 1 ( 3 ) 7 . 6 ( 3 ) Appendix atom C(15) C(16) C(17) C(18) C(19) C(20) C(21) C(22) C(23) C(24) C(25) C(26) C(27) C(28) C(29) COO) C(31) C(32) C(33) C(34) C(35) C(36) C(37) C(38) C(39) C(40) H ( l ) H(2) x 0 . 5 6 2 1 ( 5 ) 0 . 4 8 8 9 ( 5 ) 0 . 3 6 8 9 ( 4 ) 0 . 3 8 5 0 ( 4 ) 0 . 5 8 7 8 ( 5 ) 0 . 5 7 9 6 ( 5 ) 0 . 5 8 7 5 ( 6 ) 0 . 6 6 6 3 ( 4 ) 0 . 4 1 6 5 ( 5 ) 0 . 4 1 0 6 ( 5 ) 0 . 7 1 4 8 ( 4 ) 0 . 7 5 4 8 ( 4 ) 0 . 5 4 4 1 ( 6 ) 0 . 6 7 6 3 ( 6 ) 0 . 4 0 8 9 ( 3 ) 0 . 3 4 4 3 ( 3 ) 0 . 2 7 9 8 ( 3 ) 0 . 2 7 9 0 ( 4 ) 0 . 3 4 1 0 ( 5 ) 0 . 4 0 6 5 ( 3 ) 0 . 5 7 6 6 ( 3 ) 0 . 6 4 1 0 ( 3 ) 0 . 7 0 5 3 ( 3 ) 0 . 7 0 5 8 ( 4 ) 0 . 6 4 4 3 ( 4 ) 0 . 5 7 8 8 ( 3 ) 0 . 5 4 2 ( 4 ) 0 . 4 7 9 ( 3 ) y - 0 . 1 0 2 1 ( 8 ) - 0 . 2 2 3 1 ( 6 ) 0 . 3 9 2 2 ( 7 ) 0 . 4 8 6 2 ( 6 ) 0 . 2 2 9 ( 1 ) 0 . 4 3 7 ( 1 ) 0 . 2 0 7 ( 1 ) 0 . 1 2 8 7 ( 8 ) 0 . 4 0 7 ( 1 ) 0 . 2 1 1 ( 1 ) 0 . 2 7 7 5 ( 8 ) 0 . 2 0 3 3 ( 7 ) 0 . 5 5 3 6 ( 7 ) 0 . 5 7 5 5 ( 7 ) - 0 . 0 5 5 0 ( 5 ) 0 . 0 0 2 8 ( 5 ) - 0 . 0 5 5 5 ( 7 ) - 0 . 1 7 5 7 ( 8 ) - 0 . 2 3 7 9 ( 6 ) - 0 . 1 7 8 3 ( 5 ) - 0 . 0 6 3 1 ( 5 ) - 0 . 0 6 9 0 ( 5 ) - 0 . 1 2 5 5 ( 6 ) - 0 . 1 7 8 6 ( 7 ) - 0 . 1 7 7 4 ( 7 ) - 0 . 1 2 1 4 ( 6 ) 0 . 2 5 2 ( 6 ) 0 . 1 3 6 ( 5 ) z 0 . 1 1 7 6 ( 3 ) 0 . 1 6 4 2 ( 3 ) 0 . 1 5 7 1 ( 4 ) 0 . 0 6 3 6 ( 3 ) 0 . 0 7 8 6 ( 5 ) 0 . 1 1 2 0 ( 5 ) 0 . 5 5 0 6 ( 4 ) 0 . 4 9 5 0 ( 3 ) 0 . 3 7 3 7 ( 5 ) 0 . 4 2 2 8 ( 5 ) 0 . 2 7 2 0 ( 4 ) 0 . 3 8 0 2 ( 4 ) 0 . 2 9 6 2 ( 4 ) 0 . 2 9 8 1 ( 5 ) 0 . 2 8 3 4 ( 2 ) 0 . 2 7 8 3 ( 3 ) 0 . 2 7 4 6 ( 3 ) 0 . 2 7 5 5 ( 4 ) 0 . 2 8 0 1 ( 4 ) 0 . 2 8 4 4 ( 3 ) 0 . 3 3 3 8 ( 2 ) 0 . 3 2 1 1 ( 2 ) 0 . 3 6 1 2 ( 3 ) 0 . 4 1 4 9 ( 3 ) 0 . 4 2 8 7 ( 3 ) 0 . 3 6 8 3 ( 3 ) 0 . 2 4 4 ( 3 ) 0 . 3 2 6 ( 3 ) B(eq) 8 . 7 ( 4 ) 7 . 7 ( 4 ) 7 . 5 ( 4 ) 7 . 8 ( 4 ) 1 1 . 2 ( 6 ) 1 1 . 7 ( 6 ) 1 4 . 4 ( 7 ) 9 . 1 ( 4 ) 1 2 . 6 ( 7 ) 1 6 . 0 ( 8 ) 8 . 8 ( 4 ) 7 . 5 ( 4 ) 9 . 6 ( 5 ) 1 1 . 6 ( 6 ) 3 . 7 ( 2 ) 4 . 8 ( 3 ) 6 . 6 ( 3 ) 7 . 5 ( 4 ) 7 . 2 ( 4 ) 5 . 3 ( 3 ) 3 . 8 ( 2 ) 4 . 1 ( 2 ) 5 . 7 ( 3 ) 6 . 7 ( 3 ) 6 . 9 ( 3 ) 5 . 7 ( 3 ) 1 0 ( 2 ) 8 ( 2 ) Appendix Table . Selected bond lengths (A) with estimated standard deviations in parentheses. atom P(2) P(2) P(3) P(3) P(3) P(4) P(4) P(4) Si Si Si C(l) C(3) atom C(7) C(B) C(3) C(9) C(10) C(4) C(ll) C(12) C(29) C(35) H(2) C(2) C(4) distance 1.850(6) 1.868(7) 1.844(7) 1.864(7) 1.855(7) 1.855(6) 1.854(6) 1.849(7) 1.907(5) 1.902(5) 1.66(6) 1.515(8) 1.516(9) atom Kh(l) Rh(l) Rh(l) Kh(l) Rh(l) Rh(2) Rh(2) Rh(2) Rh(2) Rh(2) P(l) P(l) Pd) P{2) atom Rh(2) P(l) P(2) Si H(l) P(3) P(4) Si H(l) H(2) C(2) C(5) C(6) C(l) distance 2.937(1) 2.213(2) 2.295(2) 2.298(2) 1.71(6) 2.221(2) 2.233(2) 2.487(2) 1.90(6) 1.61(6) 1.854(5) 1.848(6) 1.856(6) 1.862(5) Table . Selected bond angles (deg) with estimated standard deviations in parentheses. atom P(l) Pd) P(l) P(2) P(2) Si P(3) P(3) P(3) P(4) P(4) H(l) atom Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Rh(2) Rh(2) Rh(2) Rh(2) Rh(2) Rh(2) atom P(2) Si H(l) Si H(l) H(l) P(4) H(l) H(2) H(l) H(2) H(l) angle 87.02(6) 97.33(6) 170(2) 164.73(5) 84(2) 92(2) 86.60(6) 157(2) 85(2) 77(2) 172(2) 111(3) atom Rh(2) Rh(2) Rh(2) C(3) C(3) C(9) Rh(2) Rh(2) Rh(2) C(4) C(4) C(ll) atom P(3) P(3) P(3) P(3) P(3) P(3) P(4) P(4) P(4) P(4) P(4) P(4) atom C(3) C(9) C(10) C(9) C(10) C(10) C(4) C(ll) C(12) C(ll) C(12) C(12) angle 110.0(2 119.7(3 115.1(2 104.6(3 101.9(4 103.7(4 110.5(2 118.7(2 115.4(2 101.6(3 104.7(3 104.1(3 Appendix 193 atom Rh(l) Rh(l) Rh(l) C(2) C(2) C(5) Rh(l) Rh(l) Rh(l) C(l) C(l) C(7) atom P(l) P(l) P(l) P(l) P(l) P(l) P(2) P{2) P(2) P(2) P(2) P(2) atom C(2) C(5) C(6) C(5) C(6) C(6) C(l) C(7) C(B) C(7) C(8) C(8) angle 110.2(2) 118.6(2) 120.0(2) 101.7(2) 100.3(3) 103.2(3) 106.9(2) 116.7(2) 126.7(3) 103.1(3) 98.7(3) 101.1(4) atom Rh(l) Rh(l) Rh(l) C(29) C(29) C(35) P(2) P(D P(3) P(4) Rh(l) RH(2) atom Si Si Si Si Si Si C(l) C(2) C(3) C(4) H(l) H(2) atom C(29) C(35) H(2) C(35) H(2) H(2) C(2) C(l) C(4) C(3) Rh(2) Si angle 124.1(2) 123.5(2) 101(2) 104.0(2) 90(2) 108(2) 111.4(4) 111.5(4) 112.2(4) 111.5(4) 109(3) 99(3) A.2 X-ray Crystallographic Analysis of [(dippe)Rh]2(|i-H)(^-r)2-H-SiMe2), 2b. EXPERIMENTAL DETAILS A. Crystal Data Empirical Formula c30H72P4Rh2Si Formula Weight 790.69 Crystal Color, Habit orange, prism Appendix 194 Crystal Dimensions (mm) Crystal System No. Reflections Used for Unit Cell Determination (29 range) Omega Scan Peak Width at Half-height Lattice Parameters: 0.080 X 0.150 X 0.450 monoclinic 25 ( 25.2 - 33.1°) 0.36 a - 13.386 (3)A b - 17.067 (4)A c - 17.862 (4)A P - 102.69 (2)° V - 3981 (1)A3 Space Group Z value Dcalc F000 ''(MoKa) B. DiffTactometer Radiation Temperature Take-off Angle Detector Aperture Crystal to Detector Scan Type Scan Rate Scan Width 2 9 _ Intensity Distance P21/n (#14) 4 1.319 g/cm3 1664 10.21 cm"1 Measurements Rigaku AFC6S MoKo (X - 0.71069 A) 21CC 6.0° 6.0 mm horizontal 6.0 mm vertical 285 mm <«>-26 16.0°/min (in omega) (8 rescans) (0.89 + 0.35 tan9)° 60.1° max No. of Reflections Measured Total: 12412 Unique: 11943 ( R i n t ,036) Appendix Corrections Lorentz-po lar izat ion Absorption ( t rans , f a c t o r s : 0.90 - 1.00) Decay ( -2.70% decline) C. Structure Solution and Refinement Structure Solution Refinement Function Minimized Least-squares Weights p-factor Anomalous Dispersion No. Observations (I>3 . 00<r( I)) No. Variables Reflection/Parameter Ratio Residuals: R; R Goodness of Fit Indicator Max Shift/Error in Final Cycle Maximum Peak in Final Diff. Map Minimum Peak in Final Diff. Map Patterson Method Full-matrix least-squares I w (|Fo| - |Fc|)2 4Fo 2 / a 2 (Fo 2 ) 0.01 All non-hydrogen atoms 6473 342 18.93 0.029; 0.027 1.35 0.19 0.37 e~/kl -0.33 e"/A3 Table atom Rh(l) Rh(2) P(l) P(2) P(3) P(4) Si(l) C(l) C(2) C(3) . Final atomic X 0.42457(2) 0.60307(2) 0.29058(6) 0.34204(6) 0.54615(7) 0.75903(6) 0.58926(6) 0.2123(2) 0.2103(2) 0.6567(3) coordinates y 0.15166(1) 0.21278(1) 0.10156(5) 0.11725(5) 0.32026(5) 0.24973(5) 0.14550(5) 0.0805(2) 0.0440(2) 0.3814(2) (fractional) and z 0.22821(1) 0.18262(1) 0.14046(5) 0.31788(5) 0.11382(5) 0.17440(4) 0.30189(5) 0.2721(2) 0.1935(2) 0.1041(2) Beq <A2>* Beg 2.349(9) 2.408(9) 2.79(3) 3.02(3) 3.35(3) 2.70(3) 2.73(3) 3.8(2) 3.8(1) 4.0(2) Appendix 196 atom C(4) C(5) C(6) C(7) C(8) C(9) C(10) C ( l l ) C(12) C(13) C(14) C(15) C(16) C(17) C(18) C(19) C(20) C(21) C(22) C(23) C(24) C(25) C(26) C(27) C(28) C{29) C{30) H ( l ) H(2) X 0 . 7 5 1 5 ( 3 ) 0 . 3 2 8 4 ( 3 ) 0 . 1 9 1 9 ( 3 ) 0 . 3 9 0 9 ( 3 ) 0 . 3 1 1 5 ( 3 ) 0 . 4 7 1 6 ( 3 ) 0 . 4 6 7 0 ( 3 ) 0 . 8 5 0 6 ( 2 ) 0 . 8 3 5 5 ( 3 ) 0 . 6 5 9 8 ( 3 ) 0 . 6 4 2 9 ( 3 ) 0 . 3 7 2 3 ( 3 ) 0 . 4 0 6 4 ( 3 ) 0 . 1 1 2 9 ( 3 ) 0 . 1 3 8 5 ( 3 ) 0 . 3 2 9 5 ( 4 ) 0 . 4 0 5 9 ( 3 ) 0 . 4 0 6 4 ( 3 ) 0 . 2 6 5 5 ( 4 ) 0 . 5 2 9 1 ( 4 ) 0 . 4 3 7 4 ( 4 ) 0 . 5 1 6 8 ( 4 ) 0 . 3 6 0 2 ( 3 ) 0 . 8 0 6 5 ( 3 ) 0 . 9 5 7 4 ( 3 ) 0 . 8 7 7 6 ( 3 ) 0 . 7 7 1 3 ( 3 ) 0 . 4 6 5 ( 2 ) 0 . 6 6 2 ( 3 ) y 0 . 3 3 2 2 ( 2 ) 0 . 0 2 6 9 ( 2 ) 0 . 1 5 8 2 ( 2 ) 0 . 0 3 8 5 ( 2 ) 0 . 1 9 5 6 ( 2 ) 0 . 3 0 8 3 ( 2 ) 0 . 3 8 7 6 ( 2 ) 0 . 2 8 4 8 ( 2 ) 0 . 1 7 5 0 ( 2 ) 0 . 2 0 5 9 ( 2 ) 0 . 0 4 4 7 ( 2 ) 0 . 0 6 5 1 ( 3 ) - 0 . 0 3 0 4 ( 2 ) 0 . 1 1 0 8 ( 3 ) 0 . 2 1 6 6 ( 2 ) 0 . 0 2 2 4 ( 3 ) - 0 . 0 3 7 2 ( 2 ) 0 . 2 1 9 9 ( 3 ) 0 . 2 6 7 1 ( 3 ) 0 . 2 5 6 5 ( 3 ) 0 . 3 8 5 2 ( 3 ) 0 . 4 0 3 5 ( 3 ) 0 . 3 5 3 7 ( 3 ) 0 . 3 5 4 2 ( 2 ) 0 . 3 0 4 5 ( 3 ) 0 . 1 1 1 7 ( 3 ) 0 . 1 3 7 4 ( 3 ) 0 . 1 7 8 ( 2 ) 0 . 1 5 0 ( 2 ) z 0 . 1 0 6 0 ( 2 ) 0 . 0 7 6 7 ( 2 ) 0 . 0 7 0 8 ( 2 ) 0 . 3 8 6 2 ( 2 ) 0 . 3 8 1 8 ( 2 ) 0 . 0 1 3 3 ( 2 ) 0 . 1 5 7 0 ( 3 ) 0 . 2 6 1 3 ( 2 ) 0 . 1 3 6 2 ( 2 ) 0 . 3 8 6 3 ( 2 ) 0 . 3 3 2 8 ( 2 ) 0 . 0 1 3 0 ( 2 ) 0 . 1 2 1 6 ( 2 ) 0 . 0 1 4 3 ( 2 ) 0 . 1 1 4 2 ( 2 ) 0 . 4 4 9 9 ( 2 ) 0 . 3 4 6 4 ( 2 ) 0 . 4 4 0 9 ( 2 ) 0 . 3 3 5 2 ( 3 ) - 0 . 0 3 1 2 ( 2 ) - 0 . 0 2 9 2 ( 3 ) 0 . 2 4 0 6 ( 3 ) 0 . 1 5 1 0 ( 3 ) 0 . 2 9 7 1 ( 2 ) 0 . 2 4 9 7 ( 2 ) 0 . 1 9 5 6 ( 3 ) 0 . 0 6 4 0 ( 3 ) 0 . 1 5 1 ( 2 ) 0 . 2 3 3 ( 2 ) B eq 3 . 6 ( 1 ) 3 . 6 ( 1 ) 4 . 0 ( 2 ) 3 . 9 ( 2 ) 4 . 3 ( 2 ) 5 . 1 ( 2 ) 5 . 3 ( 2 ) 3 . 5 ( 1 ) 4 . 1 ( 2 ) 4 . 4 ( 2 ) 4 . 5 ( 2 ) 5 . 5 ( 2 ) 5 . 1 ( 2 ) 6 . 1 ( 2 ) 5 . 5 ( 2 ) 6 . 2 ( 2 ) 4 . 7 ( 2 ) 6 . 0 ( 2 ) 6 . 6 ( 2 ) 7 . 0 ( 2 ) 8 . 4 ( 3 ) 7 . 4 ( 3 ) 7 . 1 ( 3 ) 4 . 9 ( 2 ) 5 . 4 ( 2 ) 6 . 3 ( 2 ) 6 . 2 ( 2 ) 5 . 3 ( 8 ) 7 ( 1 ) *B - ( 8 / 3 ) n 2 r r u i j a i * a . * ( a i - a . ) Appendix Table . Selected bond lengths (A) and angles (deg) with estimated standard deviations in parentheses. Bh ( i : Rh( l ] R h ( l ] R h ( i ; Rh( l ] Rh(2! Rh(2: Rh(2) Rh(2) Rh(2) Rh(2) P ( l ) P ( l ) P ( l ) P ( 2 ) P ( 2 ) S i ( l R h d ! R h ( i ; R h d R h d : R h d ; P ( 3 ) P ( 3 ) P (3 ) P ( 3 ) Rh(2) P ( l ] P ( 2 ! S i d ) H(l i P(3 P(4 R h d R h d R h d Rh( l R h d Rh( l Rh( l R h d Rh( l R h d Rh(2 Rh(2 Rh(2 Rh(2 Rh(2 Rh(2 R h d Rh(2 Rh(2 2.6835(6) 2 . 2 7 3 8 ( 9 ) 2 . 2 1 5 4 ( 9 ) 2 . 3 0 8 ( 1 ) 1 . 6 5 ( 3 ) 2 . 2 4 6 ( 1 ) 2 . 2 1 6 5 ( 9 ) P d ) P ( 2 ) S i ( l ) H ( l ) P ( 2 ) S i ( l ) H ( l ) S i d ) H d ) H ( l ) P ( 3 ) P ( 4 ) S i ( l ) H ( l ) H(2) P ( 4 ) S i d ) H ( l ) H(2) 1 2 1 . 2 3 ( 3 ) 1 5 1 . 1 4 ( 3 ) 5 5 . 3 0 ( 3 ) 3 9 ( 1 ) 8 7 . 4 5 ( 3 ) 1 5 2 . 3 6 ( 3 ) 8 3 ( 1 ) 9 8 . 1 2 ( 4 ) 1 7 0 ( 1 ) 9 2 ( 1 ) 1 0 4 . 3 2 ( 3 ) 1 6 6 . 5 7 ( 2 ) 5 0 . 4 1 ( 2 ) 3 3 ( 1 ) 8 6 ( 1 ) 8 6 . 7 1 ( 3 ) 1 4 0 . 6 5 ( 3 ) 8 4 ( 1 ) 1 6 8 ( 1 ) Rh(2) S i ( l ) Rh(2) H d ) Rh(2) H(2) S i d ) C(13) S i d ) C(14) S i d ) H(2) 2 . 4 6 3 ( 1 ) 1 . 9 1 ( 3 ) 1 . 5 1 ( 4 ) 1 . 8 9 9 ( 4 ) 1 . 8 9 9 ( 4 ) 1 . 7 3 ( 4 ) P ( 4 ) P ( 4 ) P ( 4 ) S i d ) S i d ) H ( l ) R h d ) R h d ) R h ( l ) R h ( l ) Rh(2) Rh(2) Rh(2) C(13) C(13) C(14) R h d ) Rh(2) Rh(2) Rh(2) Rh(2) Rh(2) Rh(2) Rh(2) S i ( l ) S i ( l ) S i d ) S i d ) S i d ) S i ( l ) S i d ) S i d ) S i d ) S i d ) H ( l ) H(2) S i ( l ) H ( l ) H(2) H ( l ) H(2) H(2) Rh(2) C(13) C(14) H(2) C(13) C(14) H(2) C(14) H(2) H(2) Rh(2) S i d ) 1 1 6 . 1 6 ( 3 1 5 9 ( 1 ) 8 2 ( 1 ) 8 2 ( 1 ) 4 4 ( 1 ) 1 0 7 ( 2 ) 7 4 . 2 9 ( 3 1 3 2 . 4 ( 1 ) 1 1 7 . 4 ( 1 ) 1 0 2 ( 1 ) 1 0 8 . 4 ( 1 ) 1 2 5 . 3 ( 1 ) 3 7 ( 1 ) 9 9 . 8 ( 2 ) 1 0 6 ( 1 ) 9 1 ( 1 ) 1 0 8 ( 2 ) 9 9 ( 2 ) Appendix 198 A.3 X-ray Crystallographic Analysis of [(dippe)Rh]2(|i-SiPh2)2,6a. C18 C17 CI7 EXPERIMENTAL DETAILS A. Crystal Data Empirical Formula Formula Weight Crystal Color, Habit Crystal Dimensions (mm) Crystal System No. Keflections Used for Unit Cell Determination (29 range) Omega Scan Peak Width at Half-height Lattice Parameters: C52H84P4Rh2Si2 1095.11 brown, prism 0 . 0 5 0 X 0 . 1 5 0 X 0 .300 monoclinic 20 ( 10.9 - 24.6°) 0.37 a « b - c - e - 12.933 (6)A 22.538 (6)A 18.875 (8)A 99.99 (4)" 5418 (4)A3 Space Group C2/c (#15) Appendix 199 Z va lue D c a l c Fooo ' '(MoKo) B. DiffTactometer Radiation Temperature Take-off Angle Detector Aperture Crystal to Detector Distance Scan Type Scan Rate Scan Width 26 max No. of Reflections Measured Corrections 1.342 g/cnT 2296 7.91 cm"1 Intensity Measurements Rigaku AFC6S MoKet (X - 0.71069 A) 21°C 6.0" 6.0 mm horizontal 6.0 mm vertical 285 mm w-26 16.0°/min (in omega) (8 rescans) (1.21 + 0.35 tan6)° 55.0° Total: 6675 Unique: 6392 (Rint - .048) Lorentz-polarization Absorption (trans, factors: 0.63 - 1.00) Decay ( -6.40% decline) Structure Solution and Refinement Structure Solution Refinement Function Minimized Least-squares Weights p-factor Anomalous Dispersion No. Observations (I>3.00ff(I)) No. Variables Reflection/Parameter Ratio Patterson Method Full-matrix least-squares I w (|Fo| - |Fc| ) 2 4Fo2/ff2(Fo2) 0.00 All non-hydrogen atoms 2756 272 10.13 Appendix 200 Residuals: R; Rw 0.036; 0.028 Goodness of Fit Indicator 1.45 Max Shift/Error in Final Cycle 0.16 Maximum Peak in Final Diff. Map 0.39 e~/A^ Minimum Peak in Final Diff. Map -0.41 e~/k Table . Final atomic coordinates (fractional) and B (A )* atom Rh(l) P(D P(2) Si(l) Si(2) C(l) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(ll) C(12) C(13) C(14) C(15) C(16) C(17) C(18) C(19) X 0.39013(3) 0.2539(1) 0.2764(1) 1/2 1/2 0.1363(4) 0.1613(4) 0.2101(4) 0.2570(4) 0.3206(5) 0.2141(7) 0.2377(4) 0.0919(5) 0.1735(6) 0.2568(5) 0.2325(6) 0.3912(5) 0.1494(6) 0.191(1) 0.5450(4) 0.5085(4) 0.5437(6) 0.6152(6) 0.6516(5) y 0.26609(2) 0.19959(7) 0.32713(7) 0.3482(1) 0.1850(1) 0.2354(3) 0.2818(3) 0.1782(2) 0.1265(3) 0.3548(3) 0.3934(4) 0.2256(3) 0.1626(3) 0.0801(3) 0.1389(3) 0.3736(3) 0.3098(3) 0.3753(3) 0.4435(4) 0.3998(2) 0.4572(3) 0.4909(3) 0.4680(3) 0.4112(3) z 0.25910(2) 0.23737(8) 0.30471(9) 1/4 1/4 0.2591(3) 0.3185(4) 0.1421(3) 0.2863(3) 0.3972(3) 0.2557(4) 0.0916(3) 0.1195(3) 0.2599(4) 0.3656(4) 0.4380(4) 0.4419(3) 0.1839(4) 0.2810(6) 0.3297(3) 0.3372(3) 0.3981(4) 0.4528(4) 0.4472(4) Beq 2.53(1) 3.34(7) 3.86(8) 2.9(1) 3.0(1) 5.5(3) 6.1(4) 4.0(3) 4.8(3) 5.0(3) 8.8(5) 5.0(3) 6.5(4) 8.2(5) 7.1(4) 7.5(4) 6.7(4) 8.2(5) 17.0(9) 3.1(3) 4.3(3) 5.8(4) 6.1(4) 5.6(4) Appendix atom C(20) C(21) C(22) C(23) C(24) C{25) C(26) 0.6165(5) 0.4749(4) 0.4461(5) 0.4285(7) 0.4373(7) 0.4627(6) 0.4807(5) 0.3777(2) 0.1359(3) 0.0763(3) 0.0461(3) 0.0750(4) 0.1340(4) 0.1632(3) 0.3870(3) 0.1671(3) 0.1612(4) 0.0958(5) 0.0347(5) 0.0364(4) 0.1022(4) Beq 4.3(3) 3.7(3) 5.9(4) 7.9(5) 8.2(6) 7.0(4) 5.2(3) *Beg - (8/3)n2rruija.*aj*(a..aj) Table Bond lengths (A) with estimated standard deviatio atom atom Rh(l) Rh(l)* Rh(l) P(l) Rh(l) P(2) Rh(l) Si(l) Rh(l) Si(2) P(D C(l) P(l) P(l) P(2) P(2) P(2) C(3) C(4) C(2) C(5) C(6) Si(l) C(15) Si(2) C(21) C(l) C(2) C(3) C(7) C(3) C(8) C(4) C(9) distance 2.921(2) 2.295(2) 2.288(2) 2.357(2) 2.340(2) 1.831(6) 1.852(6) 1.886(6) 1.860(6) 1.848(6) 1.864(7) 1.911(5) 1.898(6) 1.525(8) 1.515(7) 1.556(7) 1.523(8) atom C(4) C(5) C(5) C(6) C(6) atom C(10) C(ll) C(12) C(13) C(14) C(15) C(16) C(15) C(20) C(16) C(17) C(17) C(18) C(18) C(19) C(19) C(20) C(21) C(22) C(21) C(26) C(22) C(23) C(23) C(24) C(24) C(25) C(25) C(26) distance 1.523(8) 1.542(8) 1.519(8) 1.52(1) 1.28(1) 1.392(7) 1.388(7) 1.387(6) 1.363(9) 1.375(8) 1.374(8) 1.394(7) 1.383(8) 1.39(1) 1.35(1) 1.37(1) 1.388(8) •Here and elsewhere, refers to symmetry operation: 1-x, y, 1/2-z. Table . Bond angles (deg) with estimated standard deviations. atom Rh(l)* Rh(l)* Rh(1) * Rh(l)* P d ) P(l) Pd) P(2) P(2) Sid) Rh(l) Rh(l) Rh(l) Cd) Cd) C(3) Rh(l) Rh(l) Rhd) C(2) C(2) C(5) Rh(l) Rhd) Rhd) C(15) Rhd) Rhd) Rh(l) C(21) atom Rh(l Rh(l) Rhd Rhd Rhd Rh(l] Rhd Rhd Rh(l Rh(l P d ) P d ) P d ) P d ) Pd) P d ) P(2) P(2) P(2) P(2) P(2) P(2) Si(l) Sid) Sid) Sid) Si(2) Si(2) Si(2) Si(2) atom Pd) P(2) Sid) Si(2) P(2) Si(l) Sid) Sid) Si(2) Si(2) Cd) C(3) C(4) C(3) C(4) C(4) C(2) C(5) C(6) C(5) C(6) C(6) Rh(l)* C(15) C(15)* C(15)* Rh(l)* C(21) C(2D* C(2D* angle 135.34(4) 138.26(5) 51.71(4) 51.37(4) 86.34(6) 162.41(5) 86.21(6) 89.45(6) 159.30(5) 103.09(5) 108.3(2) 116.0(2) 122.2(2) 101.9(3) 102.9(3) 103.0(3) 107.7(2) 116.4(2) 123.1(3) 100.2(3) 102.8(4) 103.5(3) 76.57(7) 121.7(2) 115.5(2) 105.0(4) 77.25(8) 119.8(2) 114.4(2) 108.7(4) atom Pd) P(2) Pd) P(l) C(7) Pd) Pd) C(9) P(2) P(2) C(ll) P(2) P(2) C(13) Si(l) Sid) C(16) C(15) C(16) C(17) C(18) C(15) Si(2) Sid) C(22) C(21) C(22) C(23) C(24) C(21) atom C(l) C(2) C(3) C(3) C(3) C(4) C(4) C(4) C(5) C(5) C(5) C(6) C(6) C(6) C(15) C(15) C(15) C(16) C(17) C(18) C(19) C(20) C(21) C(21) C(21) C(22) C(23) C(24) C(25) C(26) atom C(2) Cd) C(7) C(8) C(8) C(9) CdO) C(10) C(ll) C(12) C(12) C(13) C(14) C(14) C(16) C(20) C(20) C(17) C(18) C(19) C(20) C(19) C(22) C(26) C(26) C(23) C(24) C(25) C(26) C(25) angle 112.8(4) 110.3(4) 111.2(4) 116.4(4) 108.3(5) 119.1(5) 108.5(5) 109.0(5) 115.5(5) 111.0(4) 109.3(6) 110.6(5) 129.1(8) 116.0(8) 125.7(4) 117.8(4) 116.4(5) 121.7(6) 120.2(6) 119.4(6) 120.4(6) 121.9(6) 129.2(5) 116.3(5) 114.4(6) 122.8(7) 119.8(8) 120.3(8) 119.0(8) 123.7(6) Appendix 203 A.4 X-ray Crystallographic Analysis of tomH(dippe)Rh]2(|i-SiMePh)2,6c. C I * EXPERIMENTAL DETAILS A. Crystal Data Empirical Formula Formula Weight Crystal Color, Habit Crystal Dimensions (mm) Crystal System No. Reflections Used for Unit Cell Determination (29 range) Omega Scan Peak Width at Half-height Lattice Parameters: Space Group Z value C42H80P4Rh2Si2 970.97 yellow-orange, prism 0.100 X 0.150 X 0.400 triclinic 25 ( 26.5 29.4°) 0.36 a - 11.307 (3)A b - 12.036 (3)A c - 10.798 (2)A o - 104.26 (2)° 6 - 117.38 (2)° y - 67.77 (2)° V - 1203.2 (5)A3 Pi (#2) 1 Appendix D c a l c Fooo ^ ( M O K B ) B. Intensity Me DiffTactometer Radiation Temperature T*Ve-off Angle Detector Aperture Crystal to Detector Distance Scan Type Scan Rate Scan Width 26 max No. of Reflections Measured Corrections C. Structure Solution Structure Solution Refinement Function Minimized Least-squares Weights p-factor Anomalous Dispersion No. Observations (I>3.00a(I)) No. Variables Reflection/Parameter Ratio 1.340 g/cm3 510 8.81 cm isurements Rigaku AFC6S MoKa (X - 0.71069 A) 21°C 6.0" 6.0 mm horizontal 6.0 mm vertical 285 mm u-26 16.0e/min (in omega) (8 rescans) (1.00 + 0.35 tan6)° 65.0° Total: 9114 Unique: 8726 (Rint - .028) Lorentz-polarization Absorption (trans, factors: 0.90 - 1.00) Secondary Extinction (coefficient: 0.92(2) E-06) and Refinement Patterson Method Full-matrix least-squares I w (|Fo| - |re|)2 4Fo2/o2(Fo2) 0.00 All non-hydrogen atoms 6177 227 27.21 Appendix 205 Residuals: R; Rw 0.030; 0.029 Goodness of Fit Indicator 1.68 Max Shift/Error in Final Cycle 0.08 Maximum Peak in Final Diff. Map 0.51 e~/A Minamum Peak in Final Diff. Map -0.35 e~/A Table . Final atomic coordinates (fractional) and B (A )* atom R h ( l ) P ( l ) P ( 2 ) S i ( l ) C ( l ) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C ( l l ) C(12) C(13) C(14) C(15) C(16) C(17) C(18) C(19) X 0 . 3 7 7 3 6 ( 2 ) 0 . 1 8 1 9 3 ( 6 ) 0 . 2 9 0 6 7 ( 6 ) 0 . 5 2 3 8 4 ( 6 ) 0 . 0 6 1 2 ( 3 ) 0 . 1 3 9 3 ( 3 ) 0 . 0 6 9 6 ( 3 ) 0 . 1 9 5 8 ( 3 ) 0 . 4 0 3 3 ( 3 ) 0 . 2 2 0 7 ( 3 ) - 0 . 0 8 9 1 ( 3 ) 0 . 1 0 1 3 ( 4 ) 0 . 0 6 3 4 ( 4 ) 0 . 2 6 8 5 ( 4 ) 0 . 5 1 4 3 ( 3 ) 0 . 3 2 6 9 ( 3 ) 0 . 1 1 9 9 ( 3 ) 0 . 1 5 7 0 ( 4 ) 0 . 6 1 6 0 ( 3 ) 0 . 4 3 8 9 ( 2 ) 0 . 3 6 1 4 ( 3 ) 0 . 2 9 9 0 ( 4 ) 0 . 3 1 3 7 ( 4 ) y 0 . 5 9 9 9 0 3 ( 1 5 ) 0 . 7 1 0 3 6 ( 6 ) 0 . 7 1 6 0 6 ( 5 ) 0 . 4 6 1 4 9 ( 5 ) 0 . 8 0 6 9 ( 2 ) 0 . 8 4 2 7 ( 2 ) 0 . 6 2 7 1 ( 3 ) 0 . 8 1 7 4 ( 2 ) 0 . 7 9 7 0 ( 2 ) 0 . 6 4 9 0 ( 2 ) 0 . 6 9 0 2 ( 4 ) 0 . 5 0 2 7 ( 4 ) 0 . 8 8 6 6 ( 3 ) 0 . 9 0 4 6 ( 3 ) 0 . 8 1 0 6 ( 3 ) 0 . 9 1 9 9 ( 3 ) 0 . 5 8 4 8 ( 3 ) 0 . 7 2 8 6 ( 3 ) 0 . 4 9 9 2 ( 2 ) 0 . 3 4 9 3 ( 2 ) 0 . 2 9 9 6 ( 2 ) 0 . 2 1 5 2 ( 3 ) 0 . 1 7 7 5 ( 3 ) z 0 . 4 9 7 8 1 ( 2 ) 0 . 3 3 2 0 8 ( 6 ) 0 . 6 5 6 5 4 ( 6 ) 0 . 6 6 7 4 5 ( 6 ) 0 . 4 1 5 7 ( 3 ) 0 . 5 6 7 3 ( 3 ) 0 . 1 8 3 9 ( 3 ) 0 . 2 4 5 4 ( 3 ) 0 . 8 0 8 4 ( 3 ) 0 . 7 3 6 4 ( 3 ) 0 . 1 2 7 4 ( 4 ) 0 . 2 2 3 1 ( 3 ) 0 . 1 3 1 5 ( 4 ) 0 . 3 5 5 5 ( 4 ) 0 . 7 7 6 2 ( 3 ) 0 . 8 6 5 5 ( 3 ) 0 . 6 2 2 5 ( 3 ) 0 . 8 3 6 8 ( 3 ) 0 . 6 6 3 6 ( 2 ) 0 . 6 6 0 6 ( 2 ) 0 . 5 2 9 9 ( 3 ) 0 . 5 1 5 5 ( 4 ) 0 . 6 3 4 1 ( 4 ) B eq 2 . 2 4 1 ( 7 ) 3 . 2 5 ( 3 ) 2 . 8 6 ( 2 ) 2 . 4 9 ( 2 ) 4 . 2 ( 1 ) 4 . 1 ( 1 ) 4 . 8 ( 1 ) 4 . 6 ( 1 ) 3 . 8 ( 1 ) 3 . 8 ( 1 ) 8 . 7 ( 2 ) 6 . 2 ( 2 ) 8 . 2 ( 2 ) 6 . 7 ( 2 ) 5 . 3 ( 2 ) 5 . 6 ( 2 ) 5 . 3 ( 2 ) 5 . 6 ( 2 ) 3 . 4 ( 1 ) 3 . 1 ( 1 ) 4 . 3 ( 1 ) 5 . 5 ( 2 ) 6 . 0 ( 2 ) Appendix 206 atom C(20) C(21) 0.3877(4) 0.4494(3) 0.2243(3) 0.3089(2) *B e g - (8/3)n,irruijai*a.*(ai'a.) 0.7634(4) 0.7771(3) B eq 5.6(2) 4.3(1) Table Bond lengths (A) with estimated standard deviations. atom Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) P(D P(D P(l) P(2) P(2) P(2) Sid) Sid) Cd) atom Rhd)* P d) P(2) Sid) Sid)* Cd) C(3) C(4) C(2) C(5) C(6) C(15) C(16) C(2) distance 2.8925(9) 2.273(1) 2.2760(8) 2.345(1) 2.3531(8) 1.856(3) 1.866(3) 1.856(3) 1.848(3) 1.868(3) 1.865(3) 1.908(2) 1.902(2) 1.502(4) atom C(3) C(3) C(4) C(4) C(5) C(5) C(6) C(6) C(16) C(16) C(17) C(18) C(19) C(20) atom C(7) C(8) C(9) CdO) Cdl) C(12) C(13) C(14) C(17) C(21) C(18) C(19) C(20) C(21) distance 1.537(4) 1.522(5) 1.522(4) 1.526(5) 1.519(4) 1.537(4) 1.522(4) 1.514(4) 1.400(4) 1.396(3) 1.386(4) 1.362(5) 1.365(5) 1.383(4) * Symmetry operation: 1-x, l-y.» 1~1- Appendix 207 Table . Bond angles (") with e atom Rh(1) * Rh(1) * Rh(l)* Rhd)* P d ) P(l) Pd) P(2) P(2) Sid) Rhd) Rh(l) Rh(l) Cd) C(l) C(3) Rhd) Rhd) Rhd) C(2) C(2) C(5) Rh(l) Rh(l) Rhd) atom Rhd Rhd Rhd Rhd Rhd Rhd Rhd Rhd Rhd Rh(l Pd) Pd) Pd) Pd) Pd) Pd) P(2) P(2) P(2) P(2) P(2) P(2) Sid Sid Sid atom P d ) P(2) Sid) Sid)* P(2) Sid) Sid)* Sid) Si(l)* Sid)* C(l) C(3) C(4) C(3) C(4) C(4) C(2) C(5) C(6) C(5) C(6) C(6) Rh(1) * C(15) Cd6) angle 136.27(2) 136.89(2) 52.12(2) 51.88(2) 86.83(3) 160.92(3) 86.93(3) 87.07(3) 161.52(2) 104.00(3) 107.93(8) 117.3(1) 119.8(1) 101.9(1) 103.9(1) 103.8(1) 107.85(8) 117.98(9) 119.07(9) 101.7(1) 103.9(1) 104.1(1) 76.00(3) 126.37(8) 112.81(8) Btimated standard deviations. atom Rh(1) * Rh(l)* C(15) P d ) P(2) Pd) Pd) C(7) P(l) Pd) C(9) P(2) P(2) Cdl) P(2) P(2) C(13) Si(l) Sid) C(17) C(16) C(17) C(18) C(19) C(16) atom Sid) Sid) Sid) Cd) C(2) C{3) C(3) C(3) C(4) C(4) C(4) C(5) C(5) C(5) C(6) C(6) C(6) C(16) C(16) C(16) C(17) C(18) C(19) C(20) C(21) atom C(15) C(16) C(16) C(2) C(l) C(7) C(8) C(8) C(9) C(10) C(10) C(ll) C(12) C(12) C(13) C(14) C(14) C(17) C(21) C(21) C(18) C(19) C(20) C(21) C(20) angle 127.01(8) 111.02(8) 102.4(1) 111.8(2) 111.9(2) 116.6(2) 110.1(2) 109.3(3) 118.2(3) 109.9(2) 110.1(3) 110.7(2) 117.2(2) 109.1(2) 109.9(2) 118.6(2) 110.3(2) 118.7(2) 125.3(2) 115.9(2) 122.5(3) 119.2(3) 119.9(3) 120.6(3) 121.8(3) Appendix A.5 X-ray Crystallographic Analysis of [(dippe)Rh(H)]2(u-r|2-H-SiMe2)2,8b. C23 C23 C26 C2S C21 EXPERIMENTAL DETAILS A. Crystal Data Empirical Formula Formula Height Crystal Color, Habit Crystal Dimensions (mm) Crystal System No. Reflections Used for Unit Cell Determination (26 range) Omega Scan Peak Width at Half-height Lattice Parameters: Space Group Z value c32H80P4Rh2Si2 850.86 yellow, prism 0.200 X 0.250 X 0.450 orthorhombic 25 ( 35.2 - 40.6e) 0.35 a - b - c - 16.869 19.741 12.862 (2)A (3)A (2)A V - 4283 (2)A3 P212121 (#19) Appendix 209 Dcalc Fooo ''(MoKa) B. Intensity Me DiffTactometer Radiation Temperature Take-off Angle Detector Aperture Crystal to Detector Distance Scan Type Scan Rate Scan Width 26 max No. of Reflections Measured Corrections C. Structure Solution Structure Solution Refinement Function Minimized Least-squares Weights p-factor Anomalous Dispersion No. Observations (I>3.00a(D) No. Variables Reflection/Parameter Ratio 1.319 g/cm3 1800 9.80 cm"1 surements Rigaku AFC6S MoKa (X - 0.71069 A) 21°C 6.0° 6.0 mm horizontal 6.0 mm vertical 265 mm w-26 32.0°/min (in omega) (8 rescans) (1.31 + 0.35 tanG)0 60.0° Total: 6883 Lorentz-polariration Absorption (trans, factors: 0.93 - 1.00) Secondary Extinction (coefficient: 0.43900E-07) and Refinement Patterson Method Full-matrix least-squares r w (|Fo| - |Fc|)2 4Fo2/e2(Fo2) 0.02 All non-hydrogen atoms 4936 388 12.72 Appendix 210 Residuals: R; R Goodness of Fit Indicator Wax Shift/Error in Final Cycle Maximum Peak in Final Diff. Kap Minimum Peak in Pinal Diff. Map 0.025; 0.026 1.18 0.04 0.28 t~/k\ -0.34 e~/A3 Table Final atomic coordinates (fractional) and B (A2)* atom B_ R h ( l ) Rh(2) P ( l ) P ( 2 ) P ( 3 ) P ( 4 ) S i ( l ) S i ( 2 ) C ( l ) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C{11) C(12) C(13) C(14) C(15) C(16) 0 . 5 4 3 7 1 ( 2 ) 0 . 4 5 1 9 7 ( 2 ) 0 . 6 8 2 0 4 ( 7 ) 0 . 5 6 8 5 7 ( 6 ) 0 . 3 7 7 4 6 ( 6 ) 0 . 4 7 8 5 4 ( 6 ) 0 . 4 6 7 3 6 ( 7 ) 0 . 4 1 9 5 9 ( 7 ) 0 . 7 1 8 2 ( 3 ) 0 . 6 7 6 8 ( 3 ) 0 . 3 6 3 5 ( 3 ) 0 . 4 3 6 1 ( 3 ) 0 . 7 5 1 5 ( 3 ) 0 . 7 1 4 5 ( 4 ) 0 . 5 3 0 3 ( 3 ) 0 . 5 4 2 8 ( 3 ) 0 . 2 7 2 2 ( 3 ) 0 . 4 1 5 2 ( 3 ) 0 . 4 3 4 8 ( 3 ) 0 . 5 8 1 1 ( 3 ) 0 . 7 4 0 0 ( 4 ) 0 . 8 3 9 7 ( 3 ) 0 . 7 1 0 1 ( 4 ) 0 . 7 7 8 3 ( 7 ) J 0 . 1 3 8 7 6 ( 1 ) 0 . 1 2 9 7 3 ( 1 ) 0 . 1 1 4 1 7 ( 7 ) 0 . 1 6 5 7 0 ( 6 ) 0 . 0 7 9 9 0 ( 6 ) 0 . 2 1 6 6 3 ( 5 ) 0 . 0 3 9 2 9 ( 5 ) 0 . 1 8 9 2 4 ( 6 ) 0 . 1 1 1 1 ( 3 ) 0 . 1 6 1 7 ( 3 ) 0 . 1 4 2 5 ( 2 ) 0 . 1 8 7 1 ( 2 ) 0 . 1 7 4 2 ( 4 ) 0 . 0 2 9 5 ( 4 ) 0 . 1 0 9 5 ( 2 ) 0 . 2 5 1 3 ( 2 ) 0 . 0 5 6 0 ( 3 ) 0 . 0 0 4 0 ( 2 ) 0 . 3 0 2 7 ( 2 ) 0 . 2 3 7 5 ( 3 ) 0 . 2 4 5 2 ( 4 ) 0 . 1 5 6 6 ( 5 ) 0 . 0 2 7 4 ( 4 ) - 0 . 0 1 3 9 ( 5 ) 0 . 3 5 9 3 1 ( 2 ) 0 . 1 7 3 0 5 ( 2 ) 0 . 3 4 4 5 7 ( 8 ) 0 . 5 2 7 6 8 ( 8 ) 0 . 0 4 6 5 7 ( 8 ) 0 . 0 5 1 4 7 ( 8 ) 0 . 2 8 6 9 3 ( 8 ) 0 . 3 3 6 9 2 ( 9 ) 0 . 4 8 0 6 ( 4 ) 0 . 5 5 1 4 ( 3 ) - 0 . 0 5 8 6 ( 3 ) - 0 . 0 7 4 3 ( 3 ) 0 . 2 8 0 0 ( 4 ) 0 . 2 9 6 8 ( 5 ) 0 . 6 3 3 4 ( 3 ) 0 . 5 7 9 2 ( 3 ) 0 . 0 7 0 0 ( 4 ) - 0 . 0 2 3 5 ( 3 ) 0 . 0 6 5 1 ( 4 ) 0 . 0 1 2 3 ( 4 ) 0 . 3 1 9 5 ( 5 ) 0 . 2 8 3 5 ( 6 ) 0 . 1 7 7 4 ( 5 ) 0 . 3 4 2 8 ( 7 ) eg 2 . 2 5 ( 1 ) 2 . 1 0 ( 1 ) 3 . 7 0 ( 5 ) 2 . 9 1 ( 4 ) 2 . 7 6 ( 4 ) 2 . 8 8 ( 4 ) 2 . 7 3 ( 4 ) 2 . 8 1 ( 5 ) 5 . 0 ( 3 ) 4 . 3 ( 2 ) 3 . 6 ( 2 ) 3 . 5 ( 2 ) 5 . 7 ( 3 ) 7 . 5 ( 4 ) 3 . 9 ( 2 ) 4 . 2 ( 2 ) 4 . 5 ( 2 ) 3 . 6 ( 2 ) 4 . 4 ( 2 ) 4 . 6 ( 2 ) 7 . 1 ( 4 ) 1 0 . 8 ( 5 ) 7 . 2 ( 4 ) 6 . 6 ( 6 ) occ. 0.68(2) Appendix atom C(16A) C(17) C(18) C(19) C(20) C{21) C(22) C(23) C(24) C(25) C(26) C(27) C(28) C(29) COO) C(31) C(32) X 0 . 7 0 4 ( 1 ) 0 . 4 4 1 1 ( 3 ) 0 . 5 5 5 5 ( 4 ) 0 . 5 6 7 0 ( 4 ) 0 . 5 7 6 1 ( 4 ) 0 . 2 6 3 9 ( 4 ) 0 . 2 2 7 1 ( 3 ) 0 . 3 6 3 5 ( 4 ) 0 . 5 0 0 3 ( 3 ) 0 . 4 5 9 1 ( 4 ) 0 . 3 4 5 0 ( 4 ) 0 . 6 2 2 0 ( 3 ) 0 . 6 2 6 3 ( 3 ) 0 . 3 8 5 2 ( 3 ) 0 . 5 1 2 4 ( 3 ) 0 . 3 2 7 8 ( 3 ) 0 . 4 1 0 5 ( 4 ) y - 0 . 0 2 2 ( 1 ) 0 . 1 1 4 6 ( 3 ) 0 . 0 3 6 3 ( 3 ) 0 . 2 6 3 9 ( 3 ) 0 . 3 0 7 3 ( 3 ) - 0 . 0 1 0 9 ( 4 ) 0 . 1 1 0 7 ( 4 ) - 0 . 0 2 1 2 ( 3 ) 0 . 0 1 1 9 ( 3 ) 0 . 3 5 4 7 ( 3 ) 0 . 2 9 8 7 ( 3 ) 0 . 2 7 5 9 ( 3 ) 0 . 1 7 4 7 ( 3 ) 0 . 0 0 0 3 ( 3 ) - 0 . 0 4 1 8 ( 2 ) 0 . 1 6 8 6 ( 3 ) 0 . 2 8 4 7 ( 2 ) z 0 . 3 4 8 ( 2 ) 0 . 6 4 2 7 ( 4 ) 0 . 6 1 9 6 ( 4 ) 0 . 6 9 3 5 ( 4 ) 0 . 5 0 9 8 ( 4 ) 0 . 1 2 5 7 ( 5 ) 0 . 1 2 8 5 ( 5 ) - 0 . 1 1 3 3 ( 4 ) - 0 . 0 6 0 5 ( 4 ) - 0 . 0 1 8 3 ( 5 ) 0 . 0 7 0 4 ( 5 ) 0 . 0 9 8 9 ( 5 ) - 0 . 0 1 6 8 ( 5 ) 0 . 3 6 9 8 ( 4 ) 0 . 2 3 6 2 ( 4 ) 0 . 4 1 5 8 ( 4 ) 0 . 3 4 0 7 ( 4 ) B « g 6 ( 1 ) 5 . 2 ( 3 ) 5 . 4 ( 3 ) 6 . 0 ( 3 ) 5 . 8 ( 3 ) 7 . 3 ( 4 ) 6 . 8 ( 3 ) 5 . 3 ( 3 ) 5 . 2 ( 3 ) 6 . 6 ( 3 ) 6 . 3 ( 3 ) 6 . 4 ( 3 ) 6 . 5 ( 3 ) 4 . 4 ( 2 ) 4 . 3 ( 2 ) 5 . 2 ( 3 ) 4 . 8 ( 2 ) kBeq " ( 8 / 3 ) n 2 J r U i j a i * a j * < » i , » j ) Hydrogen atom coordinates ( f r a c t i o n a l ) and B, ISO atom x y z B, i s o H(l) 0 .528(3) 0 .056(2) 0 .386(3) 5(1) H{2) 0 .371(2) 0 .169(2) 0 .224(3) 2.6(B) H(3) 0 .563(3) 0 .209(2) 0 .317(4) 6(1) H(4) 0 .527(3) 0 .096(2) 0 .132(3) 5(1) Appendix 212 Table . Selected bond distances (A). atom Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Rh(2) Rh(2) Rh(2) atom Rh(2) Pd) P(2) Si(l) Si(2) H(l) H(3) P(3) P(4) Si(l) distance 2.8575(5) 2.391(1) 2.269(1) 2.526(1) 2.337(1) 1.69(4) 1.52(5) 2.279(1) 2.364(1) 2.324(1) atom Rh(2) Rh(2) Rh(2) Si(l) Si(l) Si(l) Si(2) Si(2) Si(2) atom Si(2) H(2) H(4) C(29) C(30) H(l) C(31) C(32) H(2) distance 2.474(1) 1.70(4) 1.52(4) 1.910(5) 1.889(5) 1.67(5) 1.896(5) 1.892(5) 1.72(4) Table . Selected bond angles (deg). atom Rh<2) Rh(2) Rh(2) Rh(2) Rh(2) Rh(2) P(D Pd) Pd) Pd) Pd) P(2) P(2) P(2) P(2) Sid) Si(l) atom Rh(l Rh(l Rh(l Rh(l Rh(l Rh(l Rh(l Rh(l Rh(l Rh(l Rh(l Rh(l Rh(l Rh(l Rh(l Rh(l Rh(l] atom > Pd) 1 P(2) Si(l) > Si(2) > H(D H(3) P(2) Sid) Si(2) H(l) H(3) Sid) Si(2) H(l) H(3) Si(2) HID angle 116.71(3) 156.18(3) 50.69(3) 55.81(3) 92(2) 83(2) 86.72(4) 108.10(4) 162.05(4) 89(2) 87(2) 128.92(4) 100.56(4) 93(1) 95(2) 80.17(4) 41(2) atom Rh(l) P(3) P(3) P(3) P(3) P(3) P(4) P(4) P(4) P(4) Sid) Sid) Sid) Sid) Sid) H(2) Rh(l) atom Rh(2 Rh(2 Rh(2 Rh(2 Rh(2 Rh(2 Rh(2 Rh(2 Rh(2 Rh(2 Rh(2 Rh(2 Rh(2 Rh(2] Rh(2 Rh(2 Si(l] atom ) H(4) I P(4) Si(l) Si(2) H(2) > H(4) Sid) Sid) H(2) H(4) Si(2) H(2) H(4) H(2) H(4) H(4) Rh(2) angle 82(2) 86.88(4 100.37( 133.73( 92(1) 91(2) 162.41( 105.12( 94(1) 86(2) 81.52(4 101(1) 78(2) 44(1) 133(2) 177(2) 72.06(3 Appendix 213 atom atom atom angle atom atom atom angle S i d ) S i d ) S i d ) H ( l ) R h d ) R h ( l ) R h ( l ) R h ( l ) R h ( l ) R h ( l ) R h d ) R h ( l ) R h d ) R h d ) R h d ) R h d ) R h ( l ) R h ( l ) R h d ) R h d ) R h d ) R h d ) R h d ) R h d ) S i d ) S i d ) S i d ) S i d ) S i d ) S i d ) H(3) H ( l ) H(3) H(3) P ( 3 ) P ( 4 ) S i d ) S i d ) H(2) R h d ) C(31) C{32) B ( 2 ) C(31) C(32) 1 3 3 ( 2 ) 1 0 7 ( 2 ) 7 6 ( 2 ) 1 7 0 ( 2 ) 1 5 7 . 5 2 ( 3 ) 1 1 3 . 9 9 ( 3 ) 5 7 . 2 5 ( 3 ) 5 1 . 3 7 ( 3 ) 9 5 ( 1 ) 7 2 . 8 2 ( 3 ) 1 2 5 . 1 ( 2 ) 1 1 9 . 6 ( 2 ) 1 1 6 ( 1 ) 1 2 2 . 3 ( 2 ) 1 2 0 . 6 ( 2 ) R h ( l ) R h ( l ) R h ( l ) Rh(2) R h d ) R h d ) C(29) C(29) C(30) R h d ) C(31) C(31) C(32) R h ( l ) Rh(2) S i d ) s i d ) S i d ) S i d ) S i d ) S i d ) S i d ) S i d ) S i d ) S i d ) S i ( 2 ) S i ( 2 ) S i d ) H ( l ) H(2) C(29) C(30) H ( l ) C(29) C(30) H ( l ) C(30) H ( l ) H ( l ) H(2) C(32) H(2) H(2) S i d ) S i ( 2 ) 1 1 B . 5 ( 2 1 2 5 . 5 ( 2 4 2 ( 2 ) 1 2 5 . 5 ( 2 1 1 8 . 6 ( 2 1 1 4 ( 2 ) 9 8 . 2 ( 2 ) 9 5 ( 2 ) 1 0 1 ( 2 ) 4 3 ( 1 ) 9 7 . 7 ( 3 ) 9 1 ( 1 ) 1 0 2 ( 1 ) 9 7 ( 2 ) 9 3 ( 2 ) Appendix 214 A.6 X-ray Crystallographic Analysis of *ra/w-[(dippe)Rh]2(|i-SiHBun)2,6e. Cl C2 C12 C14 EXPERIMENTAL DETAILS A. Crystal Data Empirical Formula Formula Weight Crystal Color, Habit Crystal Dimensions (mm) Crystal System No. Reflections Used for Unit Cell Determination (26 range) Omega Scan Peak Width at Half-height Lattice Parameters: Space Group Z value Dcalc C36H84P4Rh2Si2 902.94 red, prism 0.060 X 0.250 X 0.300 monoclinic 25 ( 23.2 - 28 .0° ) 0.41 a - b - c • e - 10.869 (1)A 13.105 (2)A 16.676 (2)A 104.06 (l)e V - 2308.3 (6)A3 P2 2 1. !1/n 299 (#14) g/cm3 Appendix 215 ' 0 0 0 "(MoKcc) DiffTactometer Radiation Temperature Take-off Angle Detector Aperture 956 -1 9.13 cm B. Intensity Measurements Rigaku AFC6S MoKa (X - 0.71069 A) 21°C 6.0° 6.0 mm horizontal 6.0 mm vertical Crystal to Detector Distance Scan Type Scan Rate Scan Width 26 max No. of Reflections Measured Corrections C. Structure Solution Structure Solution Refinement Function Minimized Least-squares Weights p-factor Anomalous Dispersion No. Observations (I>3.00<r( I)) No. Variables Reflection/Parameter Ratio 285 mm u>-26 16.0°/min (in omega) (8 rescans) (1.26 + 0.35 tan6)° 60.0° Total: 7348 Unique: 7017 (Rint - .045) Lorentz-polarization Absorption (trans, factors: 0.89 - 1.00) Secondary Extinction (coefficient: 0.25252E-06) and Refinement Patterson Method Full-matrix least-squares I w (|Fo| - |Fc|)2 4Fo2/c2(Fo2) 0.00 All non-hydrogen atoms 3184 204 15.61 Residuals: R; R, w 0.035; 0.030 Appendix 216 Goodness of Fit Indicator 1.59 Max Shift/Error in Final Cycle 0.005 Maximum Peak in Final Diff. Hap 0.44 e~/A Minimum Peak in Final Diff. Map -0.49 e~/A Table . Final atomic coordinates (fractional) and B._ (A )* eg atom x y z B R h ( l ) P ( l ) P (2 ) S i ( l ) C ( l ) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C ( l l ) C(12) C(13) C(14) C(15) C(16) C(17) C(18) 0 . 5 0 5 8 3 ( 3 ) 0 . 3 8 3 3 ( 1 ) 0 . 6 3 9 3 ( 1 ) 0 . 5 8 6 8 ( 1 ) 0 . 4 4 1 5 ( 5 ) 0 . 5 8 2 1 ( 5 ) 0 . 2 1 5 3 ( 4 ) 0 . 3 6 9 2 ( 4 ) 0 . 6 6 5 9 ( 4 ) 0 . 8 0 4 5 ( 4 ) 0 . 2 0 1 9 ( 5 ) 0 . 1 2 2 7 ( 5 ) 0 . 4 9 9 0 ( 5 ) 0 . 2 8 8 7 ( 6 ) 0 . 5 4 0 3 ( 5 ) 0 . 7 6 3 5 ( 5 ) 0 . 8 0 4 9 ( 5 ) 0 . 8 8 9 6 ( 5 ) 0 . 5 1 1 7 ( 5 ) 0 . 3 6 8 6 ( 5 ) 0 . 3 1 1 8 ( 6 ) 0 . 1 7 0 3 ( 6 ) 0 . 4 4 0 7 3 ( 2 ) 0 . 4 2 2 0 3 ( 9 ) 0 . 3 2 3 4 ( 1 ) 0 . 3 9 9 4 ( 1 ) 0 . 3 0 7 2 ( 4 ) 0 . 2 9 3 7 ( 4 ) 0 . 3 8 3 5 ( 4 ) 0 . 5 1 9 8 ( 3 ) 0 . 1 9 3 8 ( 3 ) 0 . 3 6 6 1 ( 4 ) 0 . 3 1 0 2 ( 5 ) 0 . 4 7 1 9 ( 5 ) 0 . 5 5 5 0 ( 4 ) 0 . 4 6 8 9 ( 4 ) 0 . 1 3 8 6 ( 4 ) 0 . 1 2 6 9 ( 4 ) 0 . 4 7 0 9 ( 4 ) 0 . 3 6 6 9 ( 5 ) 0 . 2 8 7 0 ( 3 ) 0 . 2 7 7 2 ( 3 ) 0 . 1 8 2 6 ( 4 ) 0 . 1 7 6 8 ( 5 ) 0 . 4 2 8 1 3 ( 2 ) 0 . 2 9 8 1 4 ( 6 ) 0 . 3 9 5 3 6 ( 7 ) 0 . 5 6 7 0 7 ( 7 ) 0 . 2 5 2 7 ( 3 ) 0 . 2 8 3 3 ( 3 ) 0 . 2 8 5 4 ( 3 ) 0 . 2 1 5 4 ( 3 ) 0 . 4 4 1 9 ( 3 ) 0 . 3 9 9 7 ( 3 ) 0 . 3 5 2 3 ( 4 ) 0 . 2 8 2 4 ( 3 ) 0 . 2 0 8 8 ( 3 ) 0 . 1 2 9 0 ( 3 ) 0 . 4 2 9 5 ( 3 ) 0 . 4 1 3 1 ( 3 ) 0 . 3 6 0 2 ( 4 ) 0 . 4 8 6 7 ( 4 ) 0 . 6 0 9 7 ( 3 ) 0 . 5 8 2 1 ( 3 ) 0 . 6 1 1 2 ( 4 ) 0 . 5 7 7 4 ( 4 ) 3 . 0 0 ( 1 ) 3 . 8 1 ( 5 ) 4 . 0 0 ( 5 ) 3 . 6 7 ( 5 ) 5 . 6 ( 2 ) 5 . 6 ( 2 ) 5 . 1 ( 2 ) 4 . 7 ( 2 ) 4 . 8 ( 2 ) 5 . 6 ( 3 ) 8 . 0 ( 3 ) 7 . 2 ( 3 ) 6 . 3 ( 3 ) 7 . 6 ( 3 ) 6 . 3 ( 3 ) 7 . 4 ( 3 ) 8 . 5 ( 4 ) 8 . 9 ( 4 ) 4 . 6 ( 2 ) 5 . 0 ( 2 ) 7 . 0 ( 3 ) 7 . 8 ( 3 ) * Beq " (8/3>n2rruijai*aj*<«i*«j> Appendix Hydrogen atom coordinates (fractional) and B. (A ) atom x y z B i g Q H(l) 0.716(3) 0.370(3) 0.603(2) 4.1(6) Table . Bond lengths (A) with estimated standard deviations. atom Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Pd) P(l) P(l) P(2) P(2) P(2) Si(l) Si(l) * Here 1-x, atom Rh(l)* P(l) P(2) Sid) Si(l)* C(l) C(3) C(4) C(2) C(5) C(6) C(15) H(l) distance 2.8850(7) 2.266(1) 2.272(1) 2.334(1) 2.335(1) 1.864(5) 1.860(5) 1.862(4) 1.862(5) 1.860(5) 1.868(5) 1.905(5) 1.45(3) and elsewhere, refer 1 ~1> 1 _ i ' atom C(l) C(3) C(3) C(4) C(4) C(5) C(5) C(6) C(6) C(15) C(16) C(17) to symmetry ope atom C(2) C(7) C(8) C(9) C(10) C(ll) C(12) C(13) C(14) C(16) C(17) C(18) ration: distance 1.501(6) 1.506(7) 1.529(7) 1.516(6) 1.547(6) 1.516(6) 1.541(6) 1.524(7) 1.520(7) 1.519(6) 1.516(6) 1.509(7) Table . Bond angles atom atom atom Rh(l)* Rh(l) P(l) Rh(l)* Rh(l) P(2) Rh(l)* Rh(l) Si(l) Rh(l)* Rh(l) Si(l)* P(l) Rh(l) P(2) P(l) Rh(l) Sid) (deg) with estimated angle atom 135.94(3) C(5) 136.41(3) Rhd) 51.85(3) Rh(l) 51.81(3) Rh(l)* 87.64(4) P(l) 156.56(5) P(2) standard deviations. atom atom angle P(2) C(6) 101.9(2) Sid) Rh(l)* 76.34(4) Si(l) C(15) 117.2(1) Si(l) C(15) 116.9(1) C(l) C(2) 112.1(3) C(2) Cd) 112.4(3) Appendix 218 atom Si(l)* Si(l) Si(D* Si(D* C(l) C(3) C(4) C(3) C(4) C(4) C(2) C(5) C(6) C(5) C(6) angle 88.34(4) 88.42(4) 157.30(5) 103.66(4) 107.3(1) 118.2(1) 123.9(2) 99.0(2) 103.3(2) 101.6(2) 107.8(2) 124.2(2) 117.0(2) 102.0(2) 100.7(2) atom P(l) P(l) C(7) P(l) P(l) C(9) P(2) P(2) C(ll) P(2) P(2) C(13) Si(l) C(15) C(16) atom C(3) C(3) C(3) C(4) C(4) C(4) C(5) C(5) C(5) C(6) C(6) C(6) C(15) C(16) C(17) atom C(7) C(8) C(8) C(9) C(10) C(10) C(ll) C(12) C(12) C(13) C(14) C(14) C(16) C(17) C(18) angle 111.0(3) 114.8(3) 109.0(5) 110.6(3) 115.8(3) 109.9(4) 109.6(3) 116.0(3) 110.7(4) 110.9(4) 113.4(4) 109.9(5) 117.0(3) 116.1(4) 112.4(5) R h d ) S i ( l ) H ( l ) 1 2 5 ( 1 ) R h ( l ) s i ( l ) H ( l ) 1 2 8 ( 1 ) C d 5 ) S i ( l ) H ( l ) 9 6 ( 1 ) •ray Crystallographic Analysis of [(dippe)Rh]2(^SiHToIP)(^Ti2.H-SiHTolP)2, 9f. atom P ( l ) P ( 2 ) P ( 2 ) S i ( l ) R h ( l ) R h ( l ) R h ( l ) C ( l ) C ( l ) C ( 3 ) R h ( l ) R h ( l ) R h ( l ) C ( 2 ) C ( 2 ) atom R h ( l ) R h ( l ) R h ( l ) R h ( l ) P ( l ) P ( l ) P ( l ) P ( l ) P ( l ) P ( l ) P ( 2 ) P ( 2 ) P ( 2 ) P ( 2 ) P ( 2 ) EXPERIMENTAL DETAILS A. Crystal Data Empirical Formula Formula Weight Crystal Color, Habit Crystal Dimensions Crystal System Lattice Type No. of Reflections Used for Unit Cell Determination (26 range) Omega Scan Peak Width at Half-height Lattice Parameters C49H9oP4Rh2Si3 1093.21 orange, prism 0.10 X 0.18 X 0.35 Monodinic P 25 ( 24.0-36.1° ) 0.30° a = 25.839(4) A b = 13.084(3) A c = 17.133(3) A 3= 107.10(1)° Space Group Z value DcJc Fooo ^ ( M O K Q ) V = 5535(1) A3 P2!/a(#14) 4 1.312 g/cm3 2304 67.56 cm - 1 B. Intensity Measurements Diffractometer Radiation Take-off Angle Detector Aperture Crystal to Detector Distance Temperature Scan Type Rigaku AFC6S CUKQ (A = 1.5417S.4) graphite monochromated 6.0° 6.0 mm horizontal 6.0 mm vertical 285 mm 21.0°C u-26 Appendix 220 Scan Rate Scan Width 2&mar No. of Reflections Measured Corrections C. Structure Solution Refinement Function Minimized Least Squares Weights p-factor Anomalous Dispersion No. Observations (l>3.00o-(I)) No. Variables Reflection/Parameter Ratio Residuals: R: Rw Goodness of Fit Indicator Max Shift/Error in Final Cycle Maximum peak in Final Diff. Map Minimum peak in Final Diff. Map le.C/min (in omega) (8 rescans) (0.90 + 0.20 tan 0)° 120.2° Total: 8873 Unique: 8657 (R™, = 0.037) Lorentz-polarization Absorption (trans, factors: 0.81 - 1.00) Decay (-32.5% decline) Secondary Extinction (coefficient: 9.4(8) x 10"8) Solution and Refinement Direct Methods Full-matrix least-squares Eu»(|Fo| - |Fc|)2 i i£sL- 0.00 All non-hydrogen atoms 5997 524 11.44 0.047 ; 0.060 2.13 0.02 1.11 e~/A3 -0.83 e- /X3 Table II. Atomic coordinates and Be , atom Rh(l) Ri(2) P(l) P(2) P(3) P(4) Si(l) Si(2) Si(3) C(l) C(2) C(3) C(4) G(5) C(6) C(7) C(8) C(9) C(10) C(ll) C(12) C(13) C(14) C(15) C(16) C(17) C(18) C(19) C(20) X 0.19907(2) 0.17244(2) 0.28240(8) 0.15925(8) 0.22886(8) 0.10523(8) 0.11339(7) 0.19254(8) 0.21970(8) 0.2617(4) 0.2124(4) 0.3314(4) 0.3245(4) 0.1094(4) 0.1302(5) 0.3046(6) 0.3812(4) 0.3419(10) 0.3602(5) 0.1316(6) 0.0872(5) 0.0725(5) 0.1383(6) 0.1825(3) 0.1336(3) 0.2649(3) 0.2803(3) 0.0678(3) 0.0479(3) y 0.11436(4) 0.31972(4) 0.0388(2) -0.0329(1) 0.4648(1) 0.4141(2) 0.1789(2) 0.2484(2) 0.2054(1) -0.0828(7) -0.1286(7) -0.0042(8) 0.0894(9) -0.1002(7) -0.0289(7) -0.067(1) -0.0609(10) 0.039(2) 0.1772(9) -0.1211(8) -0.2001(8) 0.0094(8) -0.117(1) 0.5705(6) 0.5362(6) 0.5238(6) 0.4774(6) 0.3718(7) 0.4617(6) z 0.21085(3) 0.23905(3) 0.2031(1) 0.1461(1) 0.2839(1) 0.2678(1) 0.2031(1) 0.1172(1) 0.3410(1) 0.1479(7) 0.1619(6) 0.3008(7) 0.1390(8) 0.1852(7) 0.0311(6) 0.3530(8) 0.2939(9) 0.087(1) 0.1784(7) 0.2755(8) 0.1405(10) 0.0052(7) -0.0173(7) 0.2879(6) 0.3142(5) 0.2166(5) 0.3863(5) 0.3403(5) 0.1810(5) B«, 3.41(1) 3.41(1) 5.04(5) 4.54(5) 4.39(4) 4.47(5) 3.62(4) 3.63(4) 3.76(4) 7.7(3) 6.9(3) 7.5(3) 8.8(4) 7.1(3) 7.6(3) 11.8(5) 10.5(4) 24(1) 8.8(4) 10.0(4) 11.7(5) 10.5(4) n.2(4) 5.7(2) 5.7(2) 5.7(2) 5.2(2) 5.4(2) 5.6(2) Appendix 222 atom C(21) C(22) C(23) C(24) C(25) C(26) C(27) C(28) C(29) C(30) C(31) C(32) C(33) C(34) C(35) C(36) C(37) C(38) C(39) C(40) C(41) C(42) C(43) C(44) C(45) C(46) C(47) C(48) C(49) X 0.2294(4) 0.3147(4) 0.2552(4) 0.3112(4) 0.1077(4) 0.0264(4) 0.0676(3) 0.0069(3) 0.0677(3) 0.0844(3) 0.0488(3) -0.0055(3) -0.0230(3) 0.0127(3) -0.0444(3) 0.1464(3) 0.1638(3) 0.1300(4) 0.0778(4) 0.0610(3) 0.0930(3) 0.0407(4) 0.2942(3) 0.3133(3) 0.3688(4) 0.4057(4) 0.3869(3) 0.3322(3) 0.4654(4) y 0.5410(7) 0.4610(7) 0.4537(7) 0.5785(7) 0.3420(8) 0.4484(8) 0.5164(7) 0.3794(7) 0.1257(5) 0.0939(7) 0.0531(7) 0.0441(6) 0.0727(7) 0.1135(6) -0.0002(7) 0.2499(5) 0.2119(6) 0.2085(6) 0.2431(6) 0.2841(6) 0.2868(6) 0.2369(7) 0.2199(5) 0.2103(6) 0.2199(7) 0.2387(7) 0.2509(6) 0.2416(6) 0.2451(9) z 0.1293(6) 0.2157(6) 0.4549(5) 0.4029(6) 0.4233(6) 0.3533(7) 0.1155(6) 0.1426(6) 0.2619(4) 0.3428(5) 0.3814(5) 0.3404(5) 0.2602(5) 0.2212(5) 0.3830(5) 0.0079(4) -0.0548(5) -0.1354(5) -0.1566(5) -0.0941(5) -0.0148(5) -0.2420(5) 0.3997(4) 0.4838(5) 0.5252(5) 0.4827(6) 0.4002(5) 0.3588(4) 0.5292(8) B„ 7.4(3) 6.5(3) 6.5(2) 7.2(3) 7.3(3) 8.5(3) 6.2(2) 6.8(3) 3.9(2) 5.2(2) 5.5(2) 5.0(2) 5.2(2) 4.8(2) 6.1(2) 4.2(2) 4.9(2) 5.6(2) 5.3(2) 5.4(2) 4.8(2) 7.1(3) 3.8(2) 5.2(2) 6.4(2) 6.1(2) 5.2(2) 4.6(2) 9.8(4) Blq = -n (lu(aa-)2 + U„(bb'f + U33(bb-f + 2U17aa-bb' cos7 + 2r 1 3aa- c f- cos 3 + 2U23bfcc- coso) Appendix Table V. Hydrogen atom coordinates and B,,0 atom H(l) H(2) H(3) H(4) H(5) X 0.0750 0.1621 0.2378 0.2071 0.2242 y 0.1820 0.3231 0.2814 0.1085 0.2020 z 0.1255 0.1499 0.1064 0.3005 0.4239 B,. 5.9 5.9 5.9 5.9 5.9 Table 3 III. Bond Lengths^) atom Rh(l) Rh(l) Rh(l) Rh(2) Rh(2) Rh(2) P(l) P(2) P(2) P(3) P(4) P(4) Si(2) C(l) C(3) C(4) C(5) C(6) C(17) C(18) atom Rh(2) P(2) Si(2) P(3) Si(l) Si(3) C(3) C(2) C(6) C(17) C(16) C(20) C(36) C(2) C(8) C(10) C(12) C(14) C(21) C(23) distance 2.8499(9) 2.309(2) 2.349(2) 2.379(2) 2.356(2) 2.350(2) 1.865(10) 1.819(9) 1.891(10) 1.850(9) 1.837(8) 1.870(8) 1.902(7) 1.49(1) 1.52(1) 1.50(1) 1.54(1) 1.47(1) 1.52(1) 1.53(1) atom Rh(l) Rh(l) Rh(l) Rh(2) Rh(2) P(l) P(l) P(2) P(3) P(3) P(4) Si(l) Si(3) C(3) C(4) C(5) C(6) C(15) C(17) C(18) atom P(l) Si(l) Si(3) P(4) Si(2) C(l) C(4) C(5) C(15) C(18) C(19) C(29) C(43) C(7) C(9) C(ll) CH3) C(16) C(22; C(24) distance 2.409(2) 2.336(2) 2.444(2) 2.299(2) 2.477(2) 1.848(9) 1.88(1) 1.842(9) 1.844(8) 1.870(8) 1.868(8) 1.895(7) 1.902(7) 1.52(2) 1.29(2) 1.51(2) 1.51(1) 1.53(1) 1.53(1) 1.53(1) atom atom distance atom atom distance C(19) C(20) C(29) C(30) C(32) C(33) C(36) C(38) C(39) C(43) C(44) C(46) C(47) Rh(2) Si(2) Si(3) C(25) C(27) C(30) C(31) C(33) C(34) C(41) C(39) C(42) C(44) C(45) C(47) C(48) H(2) H(2) H(4) 1.54(1) 1.54(1) 1.389(10) 1.388(10) 1.37(1) 1.394(10) 1.403(10) 1.37(1) 1.50(1) 1.384(10) 1.41(1) 1.36(1) 1.386(10) 1.47 1.47 1.43 C(19) C(20) C(29) C(31) C(32) C(36) C(37) C(39) C(40) C(43) C(45) C(46) Rh(l) Si(l) Si(2) Si(3) C(26) C(28) C(34) C(32) C(35) C(37) C(38) C(40) C(41) C(48) C(46) C(49) H(4) H(l) H(3) H(5) 1.53(1) 1.52(1) 1.397(9) 1.38(1) 1.52(1) 1.375(9) 1.40(1) 1.38(1) 1.37(1) 1.393(9) 1.38(1) 1.52(1) 1.49 1.41 1.31 1.39 Table IV. Bond Angles(°) atom Rh(2) Rh(2) Rh(2) P(l) P(l) P(2) Si(l) Si(2) Rh(l) Rh(l) P(3) atom Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Rh(2) Rh(2) Rh(2) atom P(l) Si(l) Si(3) Si(l) Si(3) Si(2) Si(2) Si(3) P(4) Si(2) P(4) angle 131.36(6) 52.92(5) 52.02(5) 173.13(7) 106.96(7) 111.24(7) 79.33(7) 102.22(7) 141.06(6) 51.75(5) 86.10(7) atom Rh(2) Rh(2) P(l) P(l) P(2) P(2) Sid) Rh(l) Rhd) Rh(l) P(3) atom Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Rh(l) Rh(2) Rh(2) Rh(2) Rh(2) atom P(2) Si(2) P(2) Si(2) Si(l) Si(3) Si(3) P(3) Sid) Si(3) Sid) angle 140.59(6) 55.91(5) 84.84(7) 98.89(7) 89.61(7) 142.37(7) 79.90(7) 130.65(5) 52.29(5) 55.05(5) 175.38(7) Appendix 225 atom P(3) P(4) P(4) Si(l) Rh(l) Rh(l) C(l) Rh(l) Rh(l) C(2) Rh(2) Rh(2) C(15) Rh(2) Rb(2) C(16) Rh(l) Rh(2) Rh(l) Rh(l) Rh(2) P(2) P(l) P(l) C(9) P(2) P(2) C(13) P(4) P(3) P(3) atom Rh(2) Rh(2) Rh(2) Rh(2) P(l) P(l) P(l) P(2) P(2) P(2) P(3) P(3) P(3) P(4) P(4) P(4) Si(l) Si(l) Si(2) Si(3) Si(3) C(2) C(3) C(4) C(4) C(5) C(6) C(6) C(16) C(17) C(18) atom Si(2) Si(l) Si(3) Si(3) C(l) C(4) C(4) C(2) C(6) C(6) C(15) C(18) C(18) C(16) C(20) C(20) Rh(2) C(29) C(36) Rh(2) C(43) C(l) C(8) C(9) C(10) C(12) C(13) C(14) C(15) C(22) C(23) angle 108.21(7) 90.24(7) 116.27(7) 81.46(7) 104.6(3) 124.6(4) 97.7(5) 107.0(3) 117.5(3) 103.0(5) 105.5(3) 124.9(3) 100.9(4) 109.2(3) 118.6(3) 100.1(4) 74.79(6) 127.2(2) 125.4(2) 72.92(6) 123.6(2) 110.0(7) 116.4(9) 127(1) 113(1) 115.0(8) 111.7(8) 112.8(9) 112.8(5) 111.1(6) 110.9(5) atom P(3) P(4) Si(l) Si(2) Rh(l) C(l) C(3) Rh(l) C(2) C(5) Rh(2) C(15) C(17) Rh(2) C(16) C(19) Rh(l) Rh(l) Rh(2) Rh(l) P(D P(l) C(7) P(l) P(2) C(l l) P(2) P(3) P(3) C(21) P(3) atom Rh(2) Rh(2) Rh(2) Rh(2) P(l) P(l) P(l) P(2) P(2) P(2) P(3) P(3) P(3) P(4) P(4) P(4) Si(l) Si(2) Si(2) Si(3) C(l) C(3) C(3) C(4) C(5) C(5) C(6) C(l 5) C(17) C(17) C(18) atom Si(3) Si(2) Si(2) Si(8) C(3) C(3) C(4) C(5) C(5) C(6) C(17) C(17) C(18) C(19) C(19) C(20) C(29) Rh(2) C(36) C(43) C(2) C(7) C(&) C(10) C(l l) C(12) C(14) C(16) C(21) C(22) C(24) angle 97.63(7) 137.89(7) 76.40(6) 101.19(7) 117.2(3) 102.6(5) 105.9(6) 119.0(3) 101.2(5) 106.6(5) 120.3(3) 99.2(4) 101.4(4) 123.7(3) 100.9(4) 100.6(4) 124.4(2) 72.34(5) 126.9(2) 116.5(2) 113.4(7) 112.5(8) 109.0(10) 113.1(8) 111.4(8) 109.3(10) 119.2(8) 113.1(6) 114.0(6) 109.4(8) 116.0(6) Appendix 226 atom C(23) P(4) P(4) C(27) Sid) C(29) C(31) C(33) C(29) Si(2) C(36) C(38) C(40) C(36) Si(3) C(43) C(45) C(47) C(43) Rh(2) P(2) Si(2) Rh(l) P(4) Si(2) Rh(l) C(29) Rh(l) atom C(18) C(19) C(20) C(20) C(29) C(30) C(32) C(32) C(34) C(36) C(37) C(39) C(39) C(41) C(43) C(44) C(46) C(46) C(48) Rh(l) Rh(l) Rh(l) Rh(2) Rh(2) Rh(2) Si(l) SKI) Si(2) atom C(24) C(26) C(27) C(28) C(34) C(31) C(33) C(35) C(33) C(41) C(38) C(40) C(42) C(40) C(48) C(45) C(47) C(49) C(47) H(4) H(4) H(4) H(2) H(2) H(2) H(l) H(l) H(3) angle 110.4(7) 115.1(7) 112.4(6) 110.7(8) 118.6(5) 122.4(7) 118.5(7) 121.1(7) 121.6(7) 123.9(5) 121.9(7) 115.8(7) 121.6(8) 121.7(7) 120.5(5) 120.9(8) 118.4(8) 122.4(10) 121.5(7) 80.8 111.1 134.6 80.3 106.8 32.4 117.2 97.9 117.1 atom P(4) C(25) P(4) Si(l) C(30) C(30) C(31) C(32) Si(2) C(37) C(37) C(38) C(39) Si(3) C(44) C(44) C(45) C(46) P(l) Si(l) Si(3) P(3) Si(l) Si(3) Rh(2) Rh(l) Rh(2) atom C(19) C(19) C(20) C(29) C(29) C(31) C(32) C(33) C(36) C(36) C(38) C(39) C(40) C(43) C(43) C(45) C(46) C(47) atom C(25) C(26) C(28) C(30) C(34) C(32) C(35) C(34) C(37) C(41) C(39) C(42) C(41) C(44) C(48) C(46) C(49) C(48) angle 110.5(5) 109.8(8) 113.4(6) 125.6(5) 115.8(6) 120.5(7) 120.4(7) 121.1(7) 120.7(6) 115.4(7) 122.1(7) 122.7(8) 123.0(8) 122.5(6) 117.0(7) 120.8(8) 119.2(10) 121.4(8) Rh(l) Rh(l) Rh(l) Rb(2) Rh(2) Rh(2) Si(l) Si(2) Si(2) H(4) H(4) H(4) H(2) H(2) H(2) H(l) H(2) H(2) 100.4 85.4 32.6 102.6 81.1 133.3 116.6 100.1 32.6 Appendix 227 atom Rh(2) C(36) Rh(l) Rh(2) C(43) H(4) atom Si(2) Si(2) Si(3) Si(3) Si(3) Si(3) atom H(3) H(3) H(4) H(4) H(4) H(5) angle 114.3 100.6 34.1 102.0 113.2 113.8 atom C(36) H(2) Rh(l) Rh(2) C(43) P(l) atom Si(2) Si(2) Si(3) Si(3) Si(3) C(l) atom H(2) H(3) H(5) H(5) H(5) H(6) angle 96.8 115.8 147.8 130.4 72.3 108.5

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