Kinetic investigations on the redox reactions of platinum carbonyl clusters with dihydrogen and acid

Abstract

Redox reactions of the Chini clusters, [Pt₃(CO)₆]_n^2- (n = 3, 4, 5), with dihydrogen and acid have been kinetically studied. The reduction of the tetramer to the trimer by dihydrogen is found to be second order with respect to the cluster concentration. Within the pressure range of 1-7 bar, the rate is independent of hydrogen pressure. The acid-induced oxidations of the trimer to the tetramer in DMF and the tetramer to the pentamer in both DMF and acetonitrile proceed through fast protonation equilibria followed by rate-determining steps. The consecutive oxidation of the trimer → tetramer → pentamer can be accurately simulated by assuming a three rate constant kinetic model. While the oxidation of the trimer involves more than one protonation equilibria between the free cluster and the protonated species, for the tetramer there is one equilibrium with a 1:1 cluster to proton molar ratio. Cyclic voltammetric studies on the trimer in DMF and the tetramer in acetonitrile show three and two anodic responses in the potential ranges -0.42 to -0.08 and 0.4 to 0.75 V, respectively. The kinetic findings and literature-reported spectroscopic and other observations are consistent with a mechanism that involves the formation of weak aggregates of the clusters and disproportionation of the aggregates.