Catalytic activation of dioxygen by oximatocobalt(II) and oximatoiron(II) complexes for catecholase-mimetic oxidations of o-substituted phenols

Bis(dimethylglyoximato)cobalt(II) and -iron(II) complexes, referred to as cobaloxime(II) and ferroxime(II), respectively, containing rigid equatorial macrocycles stabilized by hydrogen bonding, are functional catecholase and phenoxazinone synthase models, but show no catechol dioxygenase type activity. We have studied the oxidation of 3,5-di-tert-butylcatechol (DBCatH(2)) and 2-aminophenol (AP) as model substrates with the objective of elucidating the mechanisms of these reactions, using a combination of techniques including identification of free-radical intermediates by ESR spectroscopy and UV-vis spectrophotometry of semiquinone anion radical adducts of cobaloxime and ferroxime species. Detailed kinetic studies of the catecholase-mimetic oxidations reveal a general mechanistic pattern involving reversible formation of ternary catalyst-substrate-dioxygen complexes, which are key intermediates capable of H-atom abstraction from the substrates. The resulting semiquinone anion-radical intermediate and its adducts with the catalyst complexes have been detected by ESR spectroscopy. They contain a unidentate catecholato ligand as shown by the adduct of cobaloxime(H), which has been isolated and characterized by X-ray diffraction. Our work has led to the conclusion that the lack of catechol dioxygenase activity in the case of ferroxime(II) is due to the rigid equatorial macrocycle, which prevents bidentate catechol coordination. To further test this hypothesis, we have synthesized and studied analogous iron(II) complexes with flexible quadridentate and quinquedentate dioximato Schiff-base ligands. In line with expectations, these new complexes exhibit both catecholase and catechol dioxygenase activity. (C) 2003 Elsevier B.V. All rights reserved.