Mechanistic insights into the co-catalyst effect of Au clusters in Mn-porphyrin-catalyzed olefin oxidation

Abstract

Styrene oxidation with iodosylbenzene catalyzed by a Mn-porphyrin/Au-cluster binary system was investigated from several aspects to disclose the mechanism of unique and prominent positive effects of Au clusters. In the catalytic system utilizing a fixed amount of Mn(TPP)Cl (TPP = tetraphenylporphinato) and a varying amount of dodecanethiolate-capped Au cluster (Au:SC12, 1), the reaction rate was increased sigmoidally with the increase of the charged amount of 1. From these profiles together with the results of absorption spectral studies, a possible mechanism involving two cluster molecules is proposed. Comparative studies utilizing Mn(TPP)Cl and Mn(TMP)Cl (TMP = tetramesitylporphinato) showed that the latter is superior in terms of the robustness towards oxidative degradation and catalytic activity. Furthermore, at a certain condition, a characteristic induction period was observed only for the reaction with conformationally rigid and sterically hindered Mn(TMP)Cl. Analogous Au clusters protected by non-thiolate ligands such as triphenylphosphine and poly(N-vinyl-2-pyrrolidone) also promoted the catalytic oxidation by Mn(TMP)Cl. Based on these observations, we discuss the critical paths and factors that govern the catalytic activity of the present binary catalyst system.