Reactions of a dinuclear rhodium hydride complex with silanes

• Main Author: Rosenberg, Lisa
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/6946

The reactions of silanes with the complex [(dippe)Rh]₂(μ--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'SiH₂), to [(dippe)Rh]₂(μ-H)₂,1, gives the complexes [(dippe)Rh]₂(μ-H)(μ-n²-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(μ-H)((μ-n²-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(μ-SiRR')(μ-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(μ-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(|μ-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(μ-n²H-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(μ-SiMe₂)₂, 6b, (loss of hydrogen). ¹H and ³¹P{¹H} 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 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(μ-silylene) complexes [(dippe)Rh]₂(μ-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(μ-n²-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(H)]₂(μ-n²-H-SiHR)₂(μ-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.