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(mu-tBu2BAP)2(2), and CoIII(tBu2AP)3(3), wheretBu2APH = 2-amino-4,6-di-tert-butylphenol,tBu2AP = 2-amino-4,6-di-tert-butylphenolate, and mu-tBu2BAP = bridging 2-amido-4,6-di-tert-butylphenolate. Stoichiometric reactivity studiesof these well-defined complexes demonstrate the catalyticcompetency of both cobalt(II) and cobalt(III) complexes in theaerobic oxidative cyclization oftBu2APH withtert-butylisonitrile.Reactions with O2reveal the aerobic oxidation of the cobalt(II)complex1to generate the cobalt(III) species2and3.UV-visible time-course studies and electron paramagnetic resonancespectroscopy indicate that this oxidation proceeds through a ligand-based radical intermediate. These studies represent thefirstexample 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