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

摘要

[Bu4N](2)[Pt-12(CO)(24)] (1) catalyses the selective reduction of electron acceptors (S), methylene blue (MB+), safranine O (Saf(+)), methyl viologen (MV2+) 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 k2) 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+, Sar, MV2+ 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-). @ 2006 Elsevier B.V. All rights reserved.