Reaction of O2 with a high-spin mononuclear iron(II) complex supported by a five-azole donor set yields the corresponding mononuclear non-heme iron(III)-superoxo species, which was characterized by UV/Vis spectroscopy and resonance Raman spectroscopy. (1)H NMR analysis reveals diamagnetic nature of the superoxo complex arising from antiferromagnetic coupling between the spins on the low-spin iron(III) and superoxide. This superoxo species reacts with H-atom donating reagents to give a low-spin iron(III)-hydroperoxo species showing characteristic UV/Vis, resonance Raman, and EPR spectra.

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