Oxidation of thiourea by peroxomonosulfate ion catalyzed by a ruthenium(III) complex: kinetic and mechanistic studies

The complex [RuIII(edta)(H2O)]− (edta4− = ethylenediaminetetraacetate) catalyzes the oxidation of thiourea (TU) by peroxomonosulfate ion (HSO5−). The kinetics of the catalytic oxidation process was studied by using stopped-flow and rapid-scan spectrophotometry as a function of [RuIII(edta)(H2O)−], [HSO5−] and [TU] at a constant pH of 6.2 (phosphate buffer) and temperature of 25 °C. Spectral and kinetic data are suggestive of a catalytic pathway involving rapid formation of a [RuIII(edta)(TU)]− intermediate complex by reaction of [RuIII(edta)(H2O)]− with TU, followed by the oxidation of the coordinated TU in which HSO5− reacts directly with the S atom of TU coordinated to the RuIII(edta) complex. Analysis of the reaction mixture at the end of the reaction revealed the formation of formamidine disulfide (TU2) as a major product; however, thiourea dioxide (TUO2) and sulfate were also observed if the reaction mixture was kept for longer time periods. A detailed mechanism in agreement with the spectral and kinetic data is presented.