Aerobic Catechol Oxidation Catalyzed by a Bis(μ-oxo)dimanganese(III,III) Complex via a Manganese(II)−Semiquinonate Complex

A 3,5-di-tert-butyl-1,2-semiquinonato (DTBSQ) adduct of Mn(II) was prepared by a reaction between MnII(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 [MnII(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
[MnIII2(μ-oxo)2(TPA)2]2+ (2). The reaction of 2 with 3,5-di-tert-butylcatechol (DTBCH2) quantitatively afforded two
equivalents of 1 under anaerobic conditions. The highly efficient catalytic oxidation of DTBCH2 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.