Iron(IV) or iron(V)? Heterolytic or free radical? Oxidation pathways of a TAML activator in acetonitrile at −40 °C

Abstract

The oxidation of TAML complex (1) by various oxidants is explored in MeCN with 0.2% water at −40 °C, where the iron(V)oxo complex is stable enough for reliable spectral identification. The iron(V)oxo state is achieved using NaClO even faster than in the case of previously explored m-chloroperoxybenzoic acid. In contrast, H2O2 and organic peroxides (benzoyl peroxide, tert-butylperoxide, and tert-butylhydroperoxide) all convert 1 into the corresponding diiron(IV)-μ-oxo dimer (2) under the same conditions. The latter does not form when (NH4)2[Ce(NO3)6] is employed and the FeIV product obtained does not seem to contain an oxo moiety. In contrast to all other oxidants, the conversion of 1 by tBuOOH into 2 is characterized by non-conventional kinetics, and therefore this reaction was explored in some detail. The evidence is presented that this light-, O2-, TEMPO-, and base-dependent reaction is a free radical process under the conditions used.