Anchoring rhodium(I) on thiourea-functionalized silica xerogels and silsesquioxanes .2. Matrix effects on the selectivity in the hydroformylation of styrene

Three thiourea-functionalized siloxane materials, 5SiO(2) . SiO3/2(CH2)(3)NHC(S)NHPh (XGphtn), SiO3/2(CH2)(3)NHC(S)NHPH (XGphtu*) and p-{SiO3/2(CH2)(3)NHC(S)NH}(2)C6H4 (XGphenditu*) were prepared. They are able to anchor Rh(I) species giving supported complexes that are very active recoverable catalysts for the hydroformylation of styrene. Some of these materials show regioselectivity variation when used in consecutive catalytic runs. The recovered catalysts have been investigated by XPS and EDX and the change in regioselectivity has been ascribed to matrix effects. In fact, the surface rhodium leaching apparently forces the catalytic process to move in the inside of the materials causing the substrate to experience the inner matrix environment. Furthermore, the non-siloxanized thioureas PhNHC(S)NHPh (Phtu) and p-{PrNHC(S)NH}(2)C6H4 (Phenditu), which give discrete molecular Rh(I) complexes, were studied as models for the surface binding functions. The structure of [Rh(cod)Cl(Phtu)] (cod=1,5-cyclooctadiene) has been determined by X-ray diffraction methods. (C) 1997 Published by Elsevier Science S.A.