Unique properties and reactivity of high-valent manganese-Oxo versus manganese-hydroxo in the salen platform

Article abstract of DOI:10.1021/ic100673b

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-Bu₂C₆H ₂OH (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 secondorder rate kinetics. Mn^Iv(salen)(OH) reacts with 4-CH₃CO-2,6-tert-Bu ₂C₆H₂OH (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 ₂C₆H₂OH (BDEOH=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)(0) 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.