Mechanistic studies on the intra-molecular electron transfer in the adduct species of some oxo-centred trinuclear iron(III)/chromium(III) cations and l-ascorbic acid in aqueous acetate buffer

The kinetics of the intra-molecular electron transfer of the adduct of l-ascorbic acid and a series of mixed metal Cr2Fe oxo-centred trinulear complexes, of the general formula [Cr2Fe(μ3-O)(μ2-RCO2)6(H2O)3]+ (where R = H, CH3, CH3CH2 and (CH3)2CH) were investigated spectrophotometrically, in aqueous carboxylate buffer. The reactions were studied over the ranges 2.34 ⩽ pH ⩽ 4.45, 25.0 ⩽ θ ⩽ 35.0 °C at an ionic strength of 0.50 mol dm−3 (NaClO4). The reaction involves an equilibrium step between both triaqua and diaquahydroxo species of the complex cations and l-ascorbic acid, resulting in the formation of an adduct. These equilibrium steps are followed by a slower one-electron oxidation step, corresponding to the reduction of the iron(III) centre(s) which is proceeded by rapid decomposition of the oxo-centred trinuclear core to yield aqueous iron(II), and dinuclear chromium(III). The dinuclear chromium(III) further decomposes by aquation to give an aqueous solution of the metal ions and free ligands.

Graphical abstractThe kinetics of the intra-molecular electron transfer of an adduct of l-ascorbic acid and a series of mixed metal Cr2Fe oxo-centred trinulear complex cations, of the general formula [Cr2Fe(μ3-O)(μ2-RCO2)6(H2O)3]+ (where R = H, CH3, CH3CH2 and (CH3)2CH) in aqueous acetate buffer was studied spectrophotometrically, over the ranges 2.34 ⩽ pH ⩽ 4.86, 25.0 ⩽ θ ⩽ 35.0 °C, at an ionic strength of 0.50 mol dm−3 (NaClO4).Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The mechanisms of reduction of trimeric mixed metal complexes by ascorbate ion are reported. ► The adduct formation has been found to be the prior requirement for electron transfer. ► Selective electron transfer occurs between metal centre and the reductant. ► Both the kinetics and electrochemical studies indicate electron transfer in the adduct.