The Cr2+ and V2+ reduction of µ-carboxylato dicobalt(III) ammine complexes. Part VI. Mechanism of the reduction of µ-p-formlybenzoato- and µ-o-formylbenzoato-complexes

Abstract

The kinetic of the Cr²⁺ and V²⁺ reductios of the µ-p-formylbenzoato- and µ-o-formylbenzoato-di-µ-hydorxobis[triamminecobalt(III)] complexes. µ(pfb) and µ(ofb) respectively, have been studied, I= 1.0M(LiCLO₄), by stopped-flow spectrophotometry. The reactions are first order in oxidant and reductant, and no [H⁺]-dependence is observed, [H⁺]= 0.06–0.60M. Reduction of the first cobalt (III) centre is rate determiniing. At 25 °C with Cr²⁺ as reductant k_cr= 122.5 l mol^–1 s^–1, ΔH^‡= 1.4 ± 0.3 kcal mol^–1, and ΔS^‡=–44.2 ± 0.9 cal K^–1 mol^–1 for the µ(pfb) complex, and k_Cr= 31.6l l mol^–1 s^–1. ΔH^‡=–3.6 ± 0.7 kcal mol^–1, and ΔS^‡=–63.7 ± 0.9 cal K^–1 mol^–1 for the µ(ofb) complex. It is concluded that inner-sphere Cr²⁺ reduction via the remote aldehyde function is occurring in both cases, and that the primary chromium(III) product formed in the initial step aquates rapidly to give [Cr(H₂O)₆]³⁺. Kinetic data (25 °C) with V²⁺ as reductant are k_V= 0.148 l mol^–1 s^–1ΔH^‡= 7.8 ± 1.9 kcal mol^–1, ΔS^‡=–36.4 ± 0.6 cal K^–1 mol^–1 for µ(pfb), and k_V= 0.235 l mol^–1 s^–1, ΔH^‡= 6.1 ± 0.5 kcal mol^–1, and ΔS^‡=–41.0 ± 1.5 cal K^–1 mol^–1 for µ(ofb).