Formation of Ni(ii)-phenoxyl radical complexes by O2: a mechanistic insight into the reaction of Ni(ii)-phenol complexes with O2

Abstract

A reaction of Ni(ClO₄)₂·6H₂O with a tripodal ligand having two di(tert-butyl)phenol moieties, H₂tbuL, and 1 equivalent of triethylamine in CH₂Cl₂/CH₃OH (1 : 1, v/v) under N₂ gave a Ni^II-(phenol)(phenolate) complex, [Ni(HtbuL)(CH₃OH)₂]ClO₄. The formation of the Ni^II-phenoxyl radical complex by O₂ was observed in the reaction of this complex in the solid state. On the other hand, the Ni^II-phenoxyl radical complex [Ni(Me₂NL)(CH₃OH)₂]ClO₄ was obtained by the reaction of H₂Me₂NL having a p-(dimethylamino)phenol moiety with Ni(ClO₄)₂·6H₂O in a similar procedure under O₂, through the oxidation of the Ni^II-(phenol)(phenolate) complex. However, a direct redox reaction of the Ni^II ion could not be detected in the phenoxyl radical formation. The results of the reaction kinetics, XAS and X-ray structure analyses suggested that the O₂ oxidation from the Ni^II-(phenol)(phenolate) complex to the Ni^II-phenoxyl radical complex occurs via the proton transfer–electron transfer (PT–ET) type mechanism of the phenol moiety weakly coordinated to the nickel ion.