Oxidation of iodide with a mononuclear manganese(IV) complex ion: Mechanistic investigation of autocatalytic behaviour

Abstract

One example of rare water soluble Mn(IV) species is the chelate complex formed between Mn(IV) and biguanide, the tris(biguanide)manganese(IV) complex ion, [Mn(LH2)3]4+ (1), LH2 = biguanide (C2N5H7). When this Mn(IV) complex is reacted with iodide ion in mildly acidic solution, reaction profiles of an autocatalytic nature are observed. These profiles show an initial slow reduction followed by a faster and nearly linear progress, and then become curved again near the end of the reaction. The observed kinetic behaviour is explained by a mechanism involving the reduction of Mn(IV) to Mn(II) via Mn(III) and a comproportionation between Mn(IV) and Mn(II) to regenerate Mn(III). The Mn(III) species is believed to be a steady state intermediate for the most part of the reaction. This sequence of reaction explains the observed profiles pretty well and the second order rate constants for successive electron transfers were estimated using KINSIM simulations. The effect of variation of pH and added biguanide on the reaction rates are small and accounted for logically, assuming the involvement of a ligand dissociation equilibrium for the Mn(III) complex.