Redox equilibration observed for the reduction of a ruthenium(III) complex by ascorbate under low driving force conditions

Abstract

A detailed kinetic study of the reduction of trans-[RuCl2(dipicOEt)2]–, where dipic OEt = dipicolinate monoethyl ester anion, by L-ascorbic acid that leads to the formation of the corresponding RuIIcomplex, was carried out spectrophotometrically using the stopped-flow technique. The reaction was studied as a function of [AscH2]Tand pH. The observed kinetic traces could only be fitted by a three-exponential function, characteristic of three parallel reaction paths. The complex was isolated as {[Na(H2O)2][trans-RuCl2(dipicOEt)2]}n, of which a single-crystal X-ray diffraction structure was determined. Detailed spectroscopic studies on the complex in aqueous solution showed that, under the selected experimental conditions, only a single complex species is present in solution. The observed complication is suggested to arise from the low driving force of the reaction during which the reoxidation of RuIIby the semi-oxidized-ascorbic acid, that is, ascorbyl radical Asc·–, accounts for the apparent three-exponential behavior of the reaction.The results are in excellent agreement with those of a recent study on the same reaction forcis-dichloridobispicolinato-ruthenate(III)