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

The oxidation of TAML complex [Fe-III{C6H4-1,2-(NCOCMe2NCO)(2)CMe2}OH2](-) (1) by various oxidants is explored in MeCN with 0.2% water at -40 degrees 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)-mu-oxo dimer (2) under the same conditions. The latter does not form when (NH4)(2)[Ce(NO3)(6)] is employed and the Fe-IV product obtained does not seem to contain an oxo moiety. In contrast to all other oxidants, the conversion of 1 by (BuOOH)-Bu-t into 2 is characterized by non-conventional kinetics, and therefore this reaction was explored in some detail. The evidence is presented that this light-, O-2-, TEMPO-, and base-dependent reaction is a free radical process under the conditions used.