Kinetic investigations of the mechanism of dihydrogen driven catalytic reduction of methylene blue, safranine O, methyl viologen and ferricyanide using platinum carbonyl cluster anions (Chini-clusters) as catalyst

Abstract

[Bu 4N] 2[Pt 12(CO) 24] ( 1) catalyses the selective reduction of electron acceptors ( S), methylene blue (MB +), safranine O (Saf +), methyl viologen (MV 2+) and ferricyanide by dihydrogen. Macroscopic rate investigations for the cationic substrates in DMF, and for ferricyanide in DMSO have been carried out. In all cases, k obs is given by k 1 [ S] + k 2, indicating that there are two catalytic cycles. In one of them, the formation of a complex between S and [Pt 12(CO) 24] 2− in the rate determining step (rate constant k 1) is followed by electron transfer and/or other fast steps. In the other catalytic cycle, the rate determining step (rate constant k 2) involves formation of the solvated cluster anion [Pt 12(CO) 24] 2−. The solvated cluster then undergoes fast reduction by dihydrogen and other reactions. The relative contributions of these two cycles depend on the substrate, and for MB +, Saf +, MV 2+ and [Fe(CN) 6] 3− the contribution of the second cycle is about 99%, 55%, 77% and 97%, respectively. Both k 1 and k 2 of ferricyanide are about three orders of magnitude smaller than those of the cationic electron acceptors. The rates of reduction of MB + and Saf + have also been studied in the presence of added water. Rates increase as the presence of water provides an additional pathway for the reduction of [Pt 12(CO) 24] 2− to [Pt 9(CO) 18] 2−.