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

Authors:
Publisher website
Google Scholar
Abstract
To gain an understanding of oxidation reactions by MnIII(salen), a reaction of MnIII(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 MnIV(salen)(O) as a major product and MnIV(salen)(OH) as a minor product. MnIV(salen)(O) readily reacts with 4-H-2,6-tert-Bu2C6H2OH (homolytic bond dissociation energy of an OH bond, BDEOH = 82.8 kcal mol−1), 4-CH3CO-2,6-tert-Bu2C6H2OH (BDEOH = 83.1 kcal mol−1), and 4-NC-2,6-tert-Bu2C6H2OH (BDEOH = 84.2 kcal mol−1) at 203 K, following second-order rate kinetics. MnIV(salen)(OH) reacts with 4-CH3CO-2,6-tert-Bu2C6H2OH (BDEOH = 83.1 kcal mol−1) much more slowly under identical conditions than MnIV(salen)(O) and does not react with 4-NC-2,6-tert-Bu2C6H2OH (BDEOH = 84.2 kcal mol−1), suggesting that the thermodynamic hydrogen-atom-abstracting ability of MnIV(salen)(OH) is about 83 kcal mol−1. The rate constant for reactions of MnIV(salen)(OH) with phenols is not dependent on the concentration of phenols, suggesting that MnIV(salen)(OH) might bind phenols prior to the rate-limiting oxidation reactions. Quantum chemical calculations are carried out for MnIV(salen)(O) and MnIV(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 MnIV(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.

This publication has 92 references indexed in Scilit: