Experimental and theoretical investigations on the catalytic hydrosilylation of carbon dioxide with ruthenium nitrile complexes

Article abstract of DOI:10.1002/chem.200600396

Ruthenium complexes, mer-[RuX₃(MeCN)₃] and cis/trans-[RuX₂-(MeCN)₄] with X = Br, Cl, were investigated as precatalysts in homogeneously catalyzed hydrosilylation of CO₂, The oxidation state of ruthenium and nature of the halide in the precatalysts were found to influence the catalytic activity in the conversion of Me₂PhSiH to the formoxysilane Me₂PhSiOCHO, with Ru ^III having chloride ligands being most active. Monitoring the reactions by in-situ IR spectroscopy in MeCN as the solvent indicates an interaction of the precatalyst with the silane prior to activation of CO ₂. In the absence of CO₂, hydrosilylation of the MeCN solvent occurs. Catalytic activity in CO₂ hydrosilylation is enhanced by Me₂PhSiCl, generated during reduction of Ru^III in mer-[RuX₃(MeCN)₃] to Ru^II or, when added as promoter to Ru^II precatalysts. The reaction mechanism for the catalytic cycle has been calculated by DFT methods for the reaction of Me ₃SiH. The key steps are: Transfer of the Me₃Si moiety to a coordinated halide ligand, resulting in an L_nRuH-(XSiMe₃) intermediate → CO₂ coordination → Me₃Si transfer to CO₂ → reductive elimination of formoxysilane product. This reaction sequence is more favorable energetically for chloride complexes than for the analogous bromide complexes, which accounts for their differences in catalytic activity. Calculations also explain the rate increase observed experimentally in the presence of Me₂PhSiCl. A parallel reaction pathway leads to (Me₃Si)₂O as a minor byproduct which arises from the condensation of two initially formed Me₃SiOH molecules.