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: Co II ( t Bu2 APH) 2 ( t Bu2 AP) 2 ( 1 ), Co III 2 ( t Bu2 APH) 2 ( t Bu2 AP) 2 (μ- t Bu2 BAP) 2 ( 2 ), and Co III ( t Bu2 AP) 3 ( 3 ), where t Bu2 APH = 2-amino-4,6-di- tert -butylphenol, t Bu2 AP = 2-amino-4,6-di- tert -butylphenolate, and μ- t Bu2 BAP = 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 t Bu2 APH with tert -butylisonitrile. Reactions with O 2 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.