Superoxide Oxidation by a Thiolate-Ligated Iron Complex and Anion Inhibition
Maksym A. Dedushko, Jessica H. Pikul, Julie A. Kovacs
Abstract
Superoxide (O2•–) is a toxic radical, generated via the adventitious reduction of dioxygen (O2), which has been implicated in a number of human disease states. Nonheme iron enzymes, superoxide reductase (SOR) and superoxide dismutase (SOD), detoxify O2•– via reduction to afford H2O2 and disproportionation to afford O2 and H2O2, respectively. The former contains a thiolate in the coordination sphere, which has been proposed to prevent O2•– oxidation to O2. The work described herein shows that, in contrast to this, oxidized thiolate-ligated [FeIII(SMe2N4(tren)(THF)]2+ (1ox-THF) is capable of oxidizing O2•– to O2. Coordinating anions, Cl– and OAc–, are shown to inhibit dioxygen evolution, implicating an inner-sphere mechanism. Previously we showed that the reduced thiolate-ligated [FeII(SMe2N4(tren))]+ (1) is capable of reducing O2•– via a proton-dependent inner-sphere mechanism involving a transient Fe(III)-OOH intermediate. A transient ferric-superoxo intermediate, [FeIII(SMe2N4(tren))(O2)]+ (3), is detected by electronic absorption spectroscopy at −130 °C in the reaction between 1ox-THF and KO2 and shown to evolve O2 upon slight warming to −115 °C. The DFT calculated O–O (1.306 Å) and Fe–O (1.943 Å) bond lengths of 3 are typical of ferric-superoxo complexes, and the time-dependent DFT calculated electronic absorption spectrum of 3 reproduces the experimental spectrum. The electronic structure of 3 is shown to consist of two antiferromagnetically coupled (Jcalc = −180 cm–1) unpaired electrons, one in a superoxo π*(O–O) orbital and the other in an antibonding π*(Fe(dyz)–S(py)) orbital.