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

[Bu₄N]₂[Pt₁₂(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 k₁ [S] + k₂, indicating that there are two catalytic cycles. In one of them, the formation of a complex between S and [Pt₁₂(CO)₂₄]^2- in the rate determining step (rate constant k₁) 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 [Pt₁₂(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⁺, MV²⁺ and [Fe(CN)₆]^3- the contribution of the second cycle is about 99%, 55%, 77% and 97%, respectively. Both k₁ and k₂ 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 [Pt₁₂(CO)₂₄]^2- to [Pt₉(CO)₁₈]^2-.