Preparation of organocobalt(iii) complexes via O2 activation

Abstract

The coupling of selective C–H activation with O₂ activation is an important goal for organic synthesis. New experimental and computational results, along with the results from experimental work accumulated over many decades, now unequivocally link O₂ activation with C–H activation by the classic Co(salen) complexes. A common holistic mechanistic framework can rationalise the formation of ostensibly diverse peroxo, superoxo, organo and alkoxide complexes of Co^III(salen). DFT calculations show that cobalt(III)superoxo, dicobalt(III)peroxo and cobalt(III)hydroperoxo complexes are all viable intermediates as participants in hydrogen atom transfer reactions, whereas a Co(IV)oxo intermediate is unlikely. The reaction conditions will determine the pathway followed and all pathways are initiated through the initial formation of a superoxo complex, Co^III(salen)(O₂˙)(MeOH) (EPR: g = 2.025, A = 19 G). Organo and alkoxide ligands are derived from solvent media and the trends in reactivity reveal that combination of the pK_a and BDE of the C–H of the respective solvent substrates are important. These data explain why landmark, structurally characterized, μ₂–η¹,η²-peroxide and η¹-superoxide Co(salen)–O₂ adducts were predominantly isolated from solvents with high C–H pK_a values (DMSO, DMF, DMA).