Unexpected alkyl isomerization at the silicon ligand of an unsaturated Rh complex: combined experiment and theory

Abstract

The formation of dimer [(μ-Cl)Rh-(κ³(P,Si,Si)PhP(o-C₆H₄CH₂SiⁱPr₂)(o-C₆H₄CH₂SiⁱPrⁿPr))]₂ (Rh-3) with an n-propyl group on one of the silicon atoms as a minor product was affected by the reaction of [RhCl(COD)]₂ with proligand PhP(o-C₆H₄CH₂SiHⁱPr₂)₂, L1. The major product of the reaction was monomeric 14-electron Rh(III) complex [ClRh(κ³(P,Si,Si)PhP(o-C₆H₄CH₂SiⁱPr₂)₂)] (Rh-1). Computations revealed that the monomer–dimer equilibrium is shifted toward the monomer with four isopropyl substituents on the two Si atoms of the ligand as in Rh-1; conversely, the dimer is favored with only one n-propyl as in Rh-3, and with less bulky alkyl substituents such as in [ClRh(κ³(P,Si,Si)PhP(o-C₆H₄CH₂SiMe₂)₂]₂ (Rh-2). Computations on the mechanism of formation of Rh-3 directly from [RhCl(COD)]₂ are in agreement with the experimental findings and it is found to be less energetic than if stemming from Rh-1. Additionally, a Si–O–Si complex, [μ-Cl-Rh{κ³(P,Si,C)PPh(o-C₆H₄CH₂SiⁱPrO SiⁱPr₂CH-o-C₆H₄)}]₂, Rh-4, is generated from the reaction of Rh-1 with adventitious water as a result of intramolecular C–H activation.