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

Abstract

The rhodium(I)-NHC (NHC = N-heterocyclic carbene) complex [Rh(Cl)(COD)(2-methoxyphenyl-NHC-(CH₂)₃Si(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)₃Si(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, ¹H 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)(κ-C,C′-R-NHC-(CH₂)₃Si(OiPr)₃)]₂(μ-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.