Aerobic catechol oxidation catalyzed by a bis(μ-oxo)dimanganese(III,III) complex via a manganese(II)-semiquinonate complex

A 3,5-di-terf-butyl-1,2-semiquinonato (DTBSQ) adduct of Mn(II) was prepared by a reaction between Mn^II(TPA)Cl₂ (TPA = tris(pyridin-2-ylmethyl)amine) and DTBSQ anion and was isolated as a tetraphenylborate salt. The X-ray crystal structure revealed that the complex is formulated as a manganese(II)-semiquinonate complex [Mn^II(TPA) (DTBSQ)]⁺ (1). The electronic spectra in solution also indicated the semiquinonate coordination to Mn. The exposure of 1 in acetonitrile to dioxygen afforded 3,5-di-tert-butyl-1,2-benzoquione and a bis(μ-oxo)dimanganese(III, III) complex [Mn^III₂(μ-oxo)₂(TPA) ₂]²⁺ (2). The reaction of 2 with 3,5-di-terf-butylcatechol (DTBCH₂) quantitatively afforded two equivalents of 1 under anaerobic conditions. The highly efficient catalytic oxidation of DTBCH ₂ with dioxygen was achieved by combining the above two reactions, that is, by constructing a catalytic cycle involving both manganese complexes 1 and 2. It was revealed that dioxygen is reduced to water but not to hydrogen peroxide in the catalytic cycle.