Electron transfer through a carbon–carbon triple bond: kinetics and mechanisms of the reductions of µ(carboxyacetylenecarboxylato-OO′)-di-µ-hydroxo-bis[triamminecobalt(III)] and related complexes by chromium(II) and vanadium(II)

Abstract

Four polynuclear cobalt(III)ammine complexes with bridging carboxylic acids containing a carbon–carbon triple-bond have been prepared and characterized. The kinetics of the reduction of the µ-(carboxyacetylemecarboxylato-OO′)-di-µ-hydroxo-bis[triamminecobalt(III)] complex, (I), by Cr^II has been studied in perchloric acid media. Reduction of the first Co^III is rate determining and dependent on [H⁺]. It is shown that both the protonated and unprotonated form of complex (I) are reduced by an inner-sphere mechanism with attack of the first Cr²⁺ at the uncomplexed remote carbonyl oxygen and transfer of an electron through the ligand. The detection of a binuclear chromium(III) complex incorporating the organic ligand as a product of the reaction is strong evidence that the second Co^III is reduced via an inner-sphere mechanism too, most probably by attack of a second Cr²⁺ at the carbonyl oxygen adjacent to co-ordinated Co^III. The assignments are supported by reactivity patterns observed for the reductions of similar complexes: di-µ-hydroxo-µ-(methoxycarbonylacetylenecarboxylato-OO′)-bis-[triamminecobalt(III)], (II), reacts with Cr²⁺ by an inner-sphere mechanism with remote attack, but the rate of reduction is independent of [H⁺]; V²⁺ reacts via an outer-sphere mechanism. The reductions of complexes µ-(acetylenedicarboxylato-O¹O¹′ : O²O²′)-bis{di-µ-hydroxo-bis[triamminecobalt(III)]}, (III), and di-µ-hydroxo-µ-(propiolato-OO′)-bis[triamminecobalt(III)], (IV), by Cr²⁺ and V²⁺ occur by outer-sphere mechanisms, respectively, because a suitable ‘lead-in’ functional group for the reductants is not available. No evidence for direct interactions of the reductants with the CC triple-bonds has been found.