Dioxygen adducts of cobalt(II) complexes of 6,6′-bis(p-substituted benzoylamino)-2,2′-bipyridines and their catalytic activities in oxygenation of 2,6-di-tert-butylphenol

Abstract

The oxygen affinities and properties of oxygen adducts of complexes [CoL][H₂L = 6,6′-bis(benzoyl-amino)-2,2′-bipyridine and its 4-methyl-, -tert-butyl or -chloro-benzoylamino analogues] has been studied by cyclic voltammetry and IR, electronic and ESR spectroscopy. The complexes exhibited oxygen-binding ability in the presence of an appropriate axial base ligand (B), and the oxygen adducts were of the end-on bonded Co^III(O₂^–) type [CoL(B)(O₂)]. More complete electron transfer from the metal centre to the oxygen was indicated compared to that with [Co(salen)][H₂salen =N,N′-ethylenebis(sali-cylideneimine)]. Electron-donating substituents gave higher concentrations of the oxygen adducts. The superior catalytic activity of these complexes, especially the Me- and Bu^t-substituted derivatives, in the oxygenation of 2,6-di-tert-butylphenol to the corresponding quinone is interpreted in terms of the reactivity of the co-ordinated oxygen and the oxygen affinity of the complexes.