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

In aqueous acidic media containing an excess of Hbipy⁺–bipy buffer in the pH 3.5–4.5 range, the complex ion [(bipy)₂Mn^III(-O)₂Mn^IV(bipy)₂]³⁺(1) coexists in rapid equilibrium with its diaqua derivative [Mn^III,IV₂ (-O)₂(bipy)₃(H₂O)₂]³⁺(1a) (bipy = 2,2-bipyridine). An excess of N₂H₅⁺ quantitatively reduces the mixture to Mn^II, itself being oxidised to N₂. The first order rate constant, k_o decreases with increasing C_bipy (C_bipy = [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 k_o 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.