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

Ruthenium complexes, mer-[RuX3(MeCN)(3)] and cisltrans-[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 CO2. 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 Me3SiH. The key steps are: Transfer of the Me3Si moiety to a coordinated halide ligand, resulting in an LnRuH-(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)(2)O as a minor byproduct which arises from the condensation of two initially formed Me2SiOH molecules.