Superoxide Oxidation by a Thiolate-Ligated Iron Complex and Anion Inhibition.

Superoxide (O) is a toxic radical, generated via the adventitious reduction of dioxygen (O), which has been implicated in a number of human disease states. Nonheme iron enzymes, superoxide reductase (SOR) and superoxide dismutase (SOD), detoxify O via reduction to afford HO and disproportionation to afford O and HO, respectively. The former contains a thiolate in the coordination sphere, which has been proposed to prevent O oxidation to O. The work described herein shows that, in contrast to this, oxidized thiolate-ligated [Fe(SN(tren)(THF)] () is capable of oxidizing O to O. Coordinating anions, Cl and OAc, are shown to inhibit dioxygen evolution, implicating an inner-sphere mechanism. Previously we showed that the reduced thiolate-ligated [Fe(SN(tren))] () is capable of reducing O via a proton-dependent inner-sphere mechanism involving a transient Fe(III)-OOH intermediate. A transient ferric-superoxo intermediate, [Fe(SN(tren))(O)] (), is detected by electronic absorption spectroscopy at -130 °C in the reaction between and KO and shown to evolve O upon slight warming to -115 °C. The DFT calculated O-O (1.306 Å) and Fe-O (1.943 Å) bond lengths of are typical of ferric-superoxo complexes, and the time-dependent DFT calculated electronic absorption spectrum of reproduces the experimental spectrum. The electronic structure of is shown to consist of two antiferromagnetically coupled ( = -180 cm) unpaired electrons, one in a superoxo π*(O-O) orbital and the other in an antibonding π*(Fe(d)-S(p)) orbital.