Kinetic Studies of Electron-Transfer Reactions Induced by the Reductive Quenching of Tris(2,2′-bipyrazine)ruthenium(II)

Abstract

Abstract The oxidation reaction of the oxalate ion (C2O42−) by the peroxodisulfate ion (S2O82−) is greatly accelerated by irradiation with visible light in an aqueous solution containing tris(2,2′-bipyrazine)ruthenium(II) ([Ru(bpz)3]2+). The mechanism constitutes a chain reaction initiated by a reductive quenching of photoexcited [Ru(bpz)3]2+ with C2O42−. The rate of reaction increases with an increase of the [Ru(bpz)3]2+ concentration or incident light intensity. On the other hand, the reaction rate hardly depends not only on the concentrations of C2O42− and S2O82−, but also on the ionic strength and temperature. The [Ru(bpz)3]2+ acts as a photo-catalyst, although the absorption spectra of the reaction solution are slightly changed during the photoreaction. An analysis of the absorption spectra by a graphic method indicates that another absorbing species than [Ru(bpz)3]2+ is formed in the reaction solution. Such a complex has no photocatalytic effect for the redox reaction between S2O82− and C2O42−. The concentration of the produced complex is always coincident with ca. 0.7% of that of the S2O82− reacted. The reaction mechanism and rate law are clarified to account for the results obtained.