Orthometallation of N-substituents at the NHC ligand of [Rh(Cl)(COD)(NHC)] complexes: its role in the catalytic hydrosilylation of ketones

The rhodium(I)-NHC (NHC = N-heterocyclic carbene) complex [Rh(Cl)(COD)(2-methoxyphenyl-NHC-(CH2)(3)Si(OiPr)(3))] (2a) catalyzes the solvent-free homogeneous hydrosilylation of acetophenone with HSiMe(OSiMe3)(2). Kinetic studies show that 2a behaves differently to the related homogeneous catalysts [Rh(Cl)(COD)(R-NHC-(CH2)(3)Si(OiPr)(3))] (R = 2,6-diisopropylphenyl (2b); R = 2-methoxyethyl (2c)). This behavior could be attributable to the participation of different catalytic active species. Indeed, H-1 NMR studies of the reaction of 2a with HSiMe(OSiMe3)(2) evidenced the formation of a new hydrido-bridged binuclear complex, namely {[Rh(SiMe(OSiMe3)(2))(kappa-C,C'-R-NHC-(CH2)(3)Si(OiPr)(3))](2)(mu-H)(2)} (R = 2-methoxyphenyl, 3), featuring orthometallated NHC and terminal silyl ligands, which has been proposed as the resting species in the hydrosilylation of acetophenone with HSiMe(OSiMe3)(2) catalyzed by 2a. Moreover, the heterogeneous catalyst 2a-MCM-41 evidenced a behavior similar to the homogeneous catalyst 2a in the solvent-free hydrosilylation of acetophenone with HSiMe(OSiMe3)(2)