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

Superoxide (O 2 •- ) is a toxic radical, generated via the adventitious reduction of dioxygen (O 2 ), which has been implicated in a number of human disease states. Nonheme iron enzymes, superoxide reductase (SOR) and superoxide dismutase (SOD), detoxify O 2 •- via reduction to afford H 2 O 2 and disproportionation to afford O 2 and H 2 O 2 , respectively. The former contains a thiolate in the coordination sphere, which has been proposed to prevent O 2 •- oxidation to O 2 . The work described herein shows that, in contrast to this, oxidized thiolate-ligated [Fe III (S Me2 N 4 (tren)(THF)] 2+ ( 1 ox -THF ) is capable of oxidizing O 2 •- to O 2 . 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 II (S Me2 N 4 (tren))] + ( 1 ) is capable of reducing O 2 •- via a proton-dependent inner-sphere mechanism involving a transient Fe(III)-OOH intermediate. A transient ferric-superoxo intermediate, [Fe III (S Me2 N 4 (tren))(O 2 )] + ( 3 ), is detected by electronic absorption spectroscopy at -130 °C in the reaction between 1 ox -THF and KO 2 and shown to evolve O 2 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 ( J calc = -180 cm -1 ) unpaired electrons, one in a superoxo π*(O-O) orbital and the other in an antibonding π*(Fe(d yz )-S(p y )) orbital.