Selective carbonyl insertion and ethene hydroformylation on a [Ru6C(CO)16Me]––SiO2 catalyst

Abstract

Stoichiometric acetaldehyde formation by insertion of CO into the methyl ligand and catalytic ethene hydroformylation on the cluster, [Ru₆C(CO)₁₆Me]^– supported on silica at 373–473 K have been investigated to understand the effects of the catalysis on the metal cluster framework and also to develop new catalytic systems on a molecular scale. Two elementary steps for stoichiometric acetaldehyde formation, (i) from methyl to acetyl and (ii) from acetyl to acetaldehyde, were observed by Fourier-transform IR spectroscopy. The rate of (i) in CO + H₂ was faster than that in CO, suggesting a hydride-promoted mechanism for carbonyl insertion (acetyl formation). The hydride promotion and hydrogen pressure dependence suggested dissociative adsorption of H₂ so as to bridge a Ru-Ru and the incorporation of the multi-Ru sites in the acetaldehyde formation mechanism. The reductive elimination of hydride and methyl ligands upon methane formation was much slower than the reductive elimination of H and MeO for acetaldehyde formation as well as the insertion of CO (methyl migration) for acetyl formation. In terms of this specific feature the catalytic hydroformylation of ethene was found to proceed on the catalyst with nearly 100% selectivity at 398 K in the case of highly dehydrated SiO₂(823 K). The retention of the cluster framework under the reaction conditions was confirmed by extended X-ray adsorption fine structure curve-fitting analysis. On the contrary, [Ru₆C(CO)₁₆Me]^– in a homogeneous system did not catalyse this reaction and conventional impregnation Ru–SiO₂ catalysts showed only 0–0.09% selectivities. A reaction mechanism is presented.