Aerobic Oxidation Reactivity of Well-Defined Cobalt(II) and Cobalt(III) Aminophenol Complexes

This paper describes the synthesis and reactivity studies of three cobalt complexes bearing aminophenol-derived ligands without nitrogen substitution: CoII(tBu2APH)2(tBu2AP)2 (1), CoIII2(tBu2APH)2(tBu2AP)2(μ-tBu2BAP)2 (2), and CoIII(tBu2AP)3 (3), where tBu2APH = 2-amino-4,6-di-tert-butylphenol, tBu2AP = 2-amino-4,6-di-tert-butylphenolate, and μ-tBu2BAP = bridging 2-amido-4,6-di-tert-butylphenolate. Stoichiometric reactivity studies of these well-defined complexes demonstrate the catalytic competency of both cobalt(II) and cobalt(III) complexes in the aerobic oxidative cyclization of tBu2APH with tert-butylisonitrile. Reactions with O2 reveal the aerobic oxidation of the cobalt(II) complex 1 to generate the cobalt(III) species 2 and 3. UV-visible time-course studies and electron paramagnetic resonance spectroscopy indicate that this oxidation proceeds through a ligand-based radical intermediate. These studies represent the first example of well-defined cobalt aminophenol complexes that participate in catalytic aerobic oxidation reactions and highlight a key role for a ligand radical in the oxidation sequence.