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 Fe^IV 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.

Graphical abstract

Keywords:

• Iron complex
• High-valent
• TAML
• tert-Butylhydroperoxide
• Kinetics
• Mechanism

Acknowledgments

Operating support is acknowledged from the Heinz Endowments (T.J.C.), and the Environmental Protection Agency (grant RD 83 to T.J.C.) and the Steinbrenner Institute for a Steinbrenner Institute Graduate Fellowship. NMR instrumentation at CMU was partially supported by NSF (CHE-0130903 and CHE-1039870). The authors thank Dr Karla Arias-Salazar and Dr Soumen Kundu for guidance in the development of this project.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Environmental Protection Agency [grant number RD 83] and the Steinbrenner Institute for a Steinbrenner Institute Graduate Fellowship. NMR instrumentation at CMU was partially supported by NSF [grant number CHE-0130903], [grant number CHE-1039870].