Remarkable CO-catalyst effect of gold nanoclusters on olefin oxidation catalyzed by a manganese-porphyrin complex

The effect of dodecanethiolate-protected metallic nanoclusters of gold (Au:SC12, 1), silver (Ag:SC12), palladium (Pd:SC12), and platinum (Pt:SC12) on the catalytic activity of Mn(TPP)Cl (TPP = tetraphenylporphinato) was investigated in styrene oxidation with iodosylbenzene. Among the four metal clusters, only Au:SC12 led to appreciable acceleration of the catalytic reaction. The major role of the Au cluster was to regenerate the active catalytic path involving Mn(III) and Mn(V) from the deactivated Mn(IV) species. The binary 1/Mn(TPP)Cl catalyst system showed an absorption spectrum characteristic of Mn(III)-porphyrin after reaction, whereas a catalytically ineffective Mn(IV) species was observed as the sole porphyrin species in the absence of the Au cluster or in the presence of Pd, Ag, and Pt clusters. Accordingly, the slow oxidation reaction with Mn(TPP)Cl was accelerated by the addition of Au:SC12, and complete conversion of Mn(IV) into Mn(III) was observed in the absorption spectrum. 1H NMR inspection of the reaction of Au:SC12 and iodosylbenzene revealed that the surface dodecyl groups were partially oxidized into dodecanal and eliminated from the cluster surface, thereby producing unprotected gold sites on the surface. A reactivation mechanism involving the reaction of the Mn-porphyrin and the oxidant activated on the gold surface is proposed.