An approach to the O2 activating mononuclear non-heme Fe enzymes: structural characterization of Fe(II)–acetato complex and formation of alkylperoxoiron(III) species with the highly hindered hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate

Structural characterization of an Fe(II)–acetato complex and attempts to synthesize mononuclear Fe(III) dioxygen complexes bearing the highly sterically demanding TptBu,iPr (=hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate) ligand have been investigated. X-ray crystallography reveals that the acetato complex consists of the distorted square pyramidal Fe(II) center as found for the previously reported O2-reactive TpiPr2 derivative. In contrast to the less hindered TpiPr2, (=hydrotris(3,5-diisopropyl-1-pyrazolyl)borate) complexes, oxidative addition of O2 to the coordinatively unsaturated Fe(II) centers of the acetato and a hydroxo complexes with TptBu,iPr has never been observed in any conditions. Reaction of the ferrous hydroxo complex with ROOH (R=H, alkyl) results in the formation of the thermally unstable intermediates. Especially, the Fe(III)-alkylperoxo complex is characterized by UV–Vis, ESR and resonance Raman spectroscopy. The extremely bulky TptBu,iPr ligand hinders the approach of the exogenous O2 molecule to the Fe(II) centers but stabilizes the unstable Fe(III)alkylperoxo intermediate enough to be detected.