Oxidation of Ru(III)-Bound Thiocyanate with Peroxomonosulfate: Kinetic and Mechanistic Studies

The reaction of [Ru-III(edta)(SCN)](2-) (edta(4-) = ethylenediaminetetraacetate; SCN- = thiocyanate ion) with the peroxomonosulfate ion (HSO5-) has been studied by using stopped-flow and rapid scan spectrophotometry as a function of [Ru-III(edta)], [HSO5-], and temperature (15-30oC) at constant pH 6.2 (phosphate buffer). Spectral analyses and kinetic data are suggestive of a pathway in which HSO5- effects the oxidation of the coordinated SCN- by its direct attack at the S-atom (of SCN-) coordinated to the Ru-III(edta). The high negative value of entropy of activation (S = -90 +/- 6 J mol(-1) deg(-1)) is consistent with the values reported for the oxygen atom transfer process involving heterolytic cleavage of the O-O bond in HSO5-. Formation of SO42-, SO32-, and OCN- was identified as oxidation products in ESI-MS experiments. A detailed mechanism in agreement with the spectral and kinetic data is presented.