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-Bu₂C₆H₂OH (homolytic bond dissociation energy of an OH bond, BDE_OH = 82.8 kcal mol⁻¹), 4-CH₃CO-2,6-tert-Bu₂C₆H₂OH (BDE_OH = 83.1 kcal mol⁻¹), and 4-NC-2,6-tert-Bu₂C₆H₂OH (BDE_OH = 84.2 kcal mol⁻¹) at 203 K, following second-order rate kinetics. Mn^IV(salen)(OH) reacts with 4-CH₃CO-2,6-tert-Bu₂C₆H₂OH (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-Bu₂C₆H₂OH (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.

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