Ru(EDTA) mediated partial reduction of O₂ by H₂S

Abstract

An effective procedure for selective reduction of O2 to H2O2 exploring the use of hydrogen sulfide, an obnoxious industrial pollutant as reductant is reported herein. The reduction of [Ru(III)(EDTA)pz](-) (EDTA(4-) = ethylenediaminetetraacetate; pz = pyrazine) by hydrogen sulfide resulting in the formation of a red [Ru(II)(EDTA)pz](2-) complex (λmax = 462 nm) has been studied spectrophotometrically and kinetically using both rapid scan and stopped-flow techniques. The time course of the reaction was followed as a function of [HS(-)]i, pH (5.5-8.5), and temperature. Alkali metal ions were found to have a positive influence (K(+) > Na(+) > Li(+)) on the reaction rate. Kinetic data and activation parameters are interpreted in terms of a mechanism (admittedly speculative) involving outer-sphere electron transfer between the reaction partners. Reaction of the red [Ru(II)(EDTA)pz](2-) complex with molecular oxygen regenerates the [Ru(III)(EDTA)pz](-) species in the reacting system along with the formation of H2O2, a partially reduced product of dioxygen (O2) reduction. A detailed reaction mechanism in agreement with the spectral and kinetic data is presented.

Electron Transfer Reactions of RuIII(edta) Containing the N-Heterocyclic Ligand Pyrazine: Kinetic and Mechanistic Studies

“…[28,29] The use of hydrogen sulfide, an obnoxious industrial pollutant as an effective electron transfer agent in the reduction of [Ru III (edta)(pz)]has been reported quite recently. [30] Detailed kinetic and mechanistic studies of the reduction of [Ru III (edta)(pz)]to [Ru II (edta) (pz)] 2with hydrogen sulfide was performed strictly under argon atmosphere.…”

“…It had been reported that the rate of reduction increased linearly with increasing concentration of HSat a constant pH (8.5). [30] The observed decrease in the rate of the reduction of [Ru III (edta)(pz)]at lower pH could be explained in terms of the lower reducing ability of hydrogen sulfide (which is the major reacting species at pH < 7.0) than its conjugate base HS -(pK 1 of H 2 S is 7.02). [31] Considering that HSis the predominant reacting species at pH 8.5, the overall experimental observations [30] can be accounted for in terms of the following reaction scheme (Scheme 6).…”

“…[Ru III (edta)(pz)] -+ HS -[Ru II (edta)(pz)] 2-+ HS (13) The rate determining step (13) proposed in the above mechanism involves an one-electron transfer from HSto the Ru(III) complex in an outer-sphere manner to yield the Ru(II) complex and the HS Reprinted with permission from ref. [30] Copyright the Royal Society of Chemistry. in the reaction system was reportedly noticed at the end of the reaction.…”

“…in the reaction system was reportedly noticed at the end of the reaction. [30] However, formation of colloidal sulfur may also take place via dimerization of the HS • radical to yield H 2 S 2 (Eq. 15), followed by the decomposition of H 2 S 2 to produce H 2 S and S° (Eq.…”

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

“…Moreover, the Ru III (edta) complex affords a range of stable and accessible oxidation states. Our recent research interests lie in studying the catalytic properties of Ru(edta)-type complexes toward oxidation of sulfur-containing molecules of biological importance [5][6][7][8]. Recently, we have shown that the Ru III (edta) complex can efficiently catalyze the oxidation of the thiocyanate ion (SCN − ) using H 2 O 2 as a precursor oxidant [9].…”