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

The reaction of [Fe-2(mu-OH)(2)(6-Me-3-TPA)(2)](2+) (1) [6-Me-3-TPA, Tris(6-methyl-2-pyridylmethyl)amine] with O-2 in CH2Cl2 at -80 degrees C gives rise to two new intermediates, 2 and 3, before the formation of previously characterized [Fe-2(O)(O-2)(6-Me-3-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 nu(O-O) at 1,310 cm(-1) with a -71-cm(-1) O-18 isotope shift. A doublet peak pattern for the (OO)-O-16-O-18 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 nu(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.