Catalysis of hydrosilylation by well-defined rhodium siloxide complexes immobilized on silica

Abstract

Rhodium surface siloxide complexes were prepared directly by condensation of the molecular precursors ([{Rh( μ-OSiMe 3)(cod)} 2], [{Rh( μ-OSiMe 3)(tfb)} 2], [{Rh( μ-OSiMe 3)(nbd)} 2]) with silanol groups on silica surface (Aerosil 200 and SBA-15) and their structures were characterized by 13C and 29Si CP/MAS NMR spectroscopy. Such single-site complexes were tested for their activity in hydrosilylation of carbon–carbon double bonds with triethoxysilane, heptamethyltrisiloxane and poly(hydro,methyl)(dimethyl)siloxane. The best catalyst appeared to be cyclooctadiene ligand-containing rhodium siloxide complex immobilized on Aerosil which was recycled as many as 20 times without loss of activity and selectivity in hydrosilylation of vinylheptamethyltrisiloxane with heptamethyltrisiloxane. On the ground of CP/MAS NMR measurements it was established that the mechanism of hydrosilylation catalyzed by silica-supported rhodium siloxide complexes is different from that for the complexes in the homogeneous system.