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

The formation of dimer [(μ-Cl)Rh-( κ 3 ( P , Si , Si )PhP( o -C 6 H 4 CH 2 Si i Pr 2 )( o -C 6 H 4 CH 2 Si i Pr n Pr))] 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 -C 6 H 4 CH 2 SiH i Pr 2 ) 2 , L1. The major product of the reaction was monomeric 14-electron Rh(III) complex [ClRh( κ 3 ( P , Si , Si )PhP( o -C 6 H 4 CH 2 Si i Pr 2 ) 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( κ 3 ( P , Si , Si )PhP( o -C 6 H 4 CH 2 SiMe 2 ) 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{ κ 3 ( P , Si , C )PPh( o -C 6 H 4 CH 2 Si i PrO Si i Pr 2 CH- o -C 6 H 4 )}] 2 , Rh-4, is generated from the reaction of Rh-1 with adventitious water as a result of intramolecular C-H activation.