Kinetics of oxidation of hydrazine by a dioxo-bridged dimanganese(III,IV) complex ion

Abstract

In aqueous acidic media containing an excess of Hbipy+–bipy buffer in the pH 3.5–4.5 range, the complex ion [(bipy)2MnIII(μ-O)2MnIV(bipy)2]3+(1) coexists in rapid equilibrium with its diaqua derivative [MnIII,IV2 (μ-O)2(bipy)3(H2O)2]3+(1a) (bipy = 2,2′-bipyridine). An excess of N2H5+ quantitatively reduces the mixture to MnII, itself being oxidised to N2. The first order rate constant, ko decreases with increasing Cbipy (Cbipy = [Hbipy+] + [bipy]) but increases with increasing [N2H5+] and [H+]. The observed kinetic dependence can be explained in terms of a reaction between (1a) and N2H5+. Replacement of solvent H2O with D2O decreases ko substantially and the effect suggests simultaneous transfer of an electron and a proton in the rate-determining step. The relevance of this observation to the delayed oxidation of H2O in the hydrazine-treated photosystem II is discussed.