Intermediates in the oxygenation of a nonheme diiron(II) complex, including the first evidence for a bound superoxo species

AUTOR(ES)

Shan, Xiaopeng

FONTE

National Academy of Sciences

RESUMO

The reaction of [Fe2(μ-OH)2(6-Me3-TPA)2]2+ (1) [6-Me3-TPA, Tris(6-methyl-2-pyridylmethyl)amine] with O2 in CH2Cl2 at –80°C gives rise to two new intermediates, 2 and 3, before the formation of previously characterized [Fe2(O)(O2)(6-Me3-TPA)2]2+ (4) that allow the oxygenation reaction to be monitored one electron-transfer step at a time. Raman evidence assigns 2 and 3 as a diiron–superoxo species and a diiron–peroxo species, respectively. Intermediate 2 exhibits its ν(O–O) at 1,310 cm–1 with a –71-cm–1 18O isotope shift. A doublet peak pattern for the 16O18O isotopomer of 2 in mixed-isotope Raman experiments strongly suggests that the superoxide ligand of 2 is bound end-on. This first example of a nonheme iron–superoxo intermediate exhibits the highest frequency ν(O–O) yet observed for a biomimetic metal–dioxygen adduct. The bound superoxide of 2, unlike the bound peroxide of 4, is readily reduced by 2,4-di-tert-butylphenol via a proton-coupled electron-transfer mechanism, emphasizing that metal–superoxo species may serve as oxidants in oxygen activation mechanisms of metalloenzymes. The discovery of intermediates 2 and 3 allows us to dissect the initial steps of dioxygen binding at a diiron center leading to its activation for substrate oxidation.

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