σ-Silane Platinum(II) Complexes as Intermediates in C-Si Bond-Coupling Processes

Platinum complexes [Pt(NHC ')(NHC)][BArF] (in which NHC ' denotes a cyclometalated N-heterocyclic carbene ligand, NHC) react with primary silanes RSiH3 to afford the cyclometalated platinum(II) silyl complexes [Pt(NHC-SiHR ')(NHC)][BArF] through a process that involves the formation of C-Si and Pt-Si bonds with concomitant extrusion of H-2. Low-temperature NMR studies indicate that the process proceeds through initial formation of the sigma-SiH complexes [Pt(NHC ')(NHC)(HSiH2R)][BArF], which are stable at temperatures below -10 degrees C. At higher temperatures, activation of one Si-H bond followed by a C-Si coupling reaction generates an agostic SiH platinum hydride derivative [Pt(H)(NHC '-SiH2R)(NHC)][BArF], which undergoes a second Si-H bond activation to afford the final products. Computational modeling of the reaction mechanism indicates that the stereochemistry of the silyl/hydride ligands after the first Si-H bond cleavage dictates the nature of the products, favoring the formation of a C-Si bond over a C-H bond, in contrast to previous results obtained for tertiary silanes. Furthermore, the process involves a trans-to-cis isomerization of the NHC ligand before the second Si-H bond cleavage.