Oxidation of Methanesulfinate by Hexachloroiridate(IV) and the Standard Electrode Potential of the Aqueous Methanesulfonyl Radical.

Abstract

The aqueous reaction of [IrCl6]2- with CH3SO2- is biphasic and yields a 1:1 mixture of [IrCl6]3- and [IrCl5(H2O)]2- and CH3SO2Cl in the initial rapid phase. The next slow phase corresponds to the hydrolysis of CH3SO2Cl to yield CH3SO3- and Cl-. The initial phase shows kinetic inhibition by [IrCl6]3- that can be minimized by the addition of the radical scavenger propiolic acid. A detailed analysis of the kinetics indicates a mechanism with reversible outer-sphere electron transfer from CH3SO2- to [IrCl6]2- as the first step, followed by the irreversible inner-sphere oxidation of CH3SO2• by [IrCl6]2- to yield [IrCl5(H2O)]2- and CH3SO2Cl. Analysis of the inhibition by [IrCl6]3- and the kinetic effects of propiolic acid enable the determination of the equilibrium constant for the first electron-transfer step. This equilibrium constant then yields E° (CH3SO2•/CH3SO2-) = 1.01 V vs NHE at 25 °C. This is the first report of a standard potential for an alkanesulfonyl radical.