Kinetics and mechanism of the oxidation of sulfite by trans-[Ru(tmc)O₂]²⁺ (tmc = 1,4,8,11-tetramethyl-1,4,8,11- tetraazacyclotetradecane)

Abstract

The kinetics of the oxidation of SO₃^2- by trans-[Ru(tmc)O₂]²⁺ (tmc = 1,4,8,11-tetramethyl-1,4,8,11- tetraazacyclotetradecane) has been studied in aqueous solution at 25.0 °C. The reaction has the following stoichiometry: trans-[Ru^VI(tmc)O₂]²⁺ SO₃^2- + H₂O → trans-[Ru^IV(tmc)O(OH₂ )]²⁺ + SO₄^2-. Sulfite and sulfur-containing products were analysed by ion chromatography. No S₂O₆^2- could be detected. The rate law is -d[Ru^VI]/dt = k/{1 + ([H⁺]/K)}[Ru^VI][S^IV] with k = (7.0 ± 1.4) × 10⁴ dm³ mol^-1 s^-1 and K = (3.4 ± 1.0) × 10^-7 mol dm^-3 at I = 1.0 mol dm^-3. The value of k is more than two orders of magnitude greater than predicted by Marcus theory, and an inner-sphere mechanism involving the intermediate [O=Ru^VI=O–SO₃] is proposed. This may decompose by one- or two-electron pathways.

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