Unique Properties and Reactivity of High-Valent Manganese−Oxo versus Manganese−Hydroxo in the Salen Platform

Abstract

To gain an understanding of oxidation reactions by Mn(III)(salen), a reaction of Mn(III)(salen) with m-chloroperoxybenzoic acid in the absence of a substrate is investigated. UV-vis, perpendicular- and parallel-mode electron paramagnetic resonance, and X-ray absorption spectroscopy show that the resulting solution contains Mn(IV)(salen)(O) as a major product and Mn(IV)(salen)(OH) as a minor product. Mn(IV)(salen)(O) readily reacts with 4-H-2,6-tert-Bu(2)C(6)H(2)OH (homolytic bond dissociation energy of an OH bond, BDE(OH) = 82.8 kcal mol(-1)), 4-CH(3)CO-2,6-tert-Bu(2)C(6)H(2)OH (BDE(OH) = 83.1 kcal mol(-1)), and 4-NC-2,6-tert-Bu(2)C(6)H(2)OH (BDE(OH) = 84.2 kcal mol(-1)) at 203 K, following second-order rate kinetics. Mn(IV)(salen)(OH) reacts with 4-CH(3)CO-2,6-tert-Bu(2)C(6)H(2)OH (BDE(OH) = 83.1 kcal mol(-1)) much more slowly under identical conditions than Mn(IV)(salen)(O) and does not react with 4-NC-2,6-tert-Bu(2)C(6)H(2)OH (BDE(OH) = 84.2 kcal mol(-1)), suggesting that the thermodynamic hydrogen-atom-abstracting ability of Mn(IV)(salen)(OH) is about 83 kcal mol(-1). The rate constant for reactions of Mn(IV)(salen)(OH) with phenols is not dependent on the concentration of phenols, suggesting that Mn(IV)(salen)(OH) might bind phenols prior to the rate-limiting oxidation reactions. Quantum chemical calculations are carried out for Mn(IV)(salen)(O) and Mn(IV)(salen)(OH), both of which well reproduce the extended X-ray absorption fine structures as well as the electronic configurations. It is also indicated that protonation of Mn(IV)(salen)(OH) induces a drastic electronic structural change from manganese(IV) phenolate to a manganese(III) phenoxyl radical, which is also consistent with the experimental observation.