Stoichiometric Formation of an Oxoiron(IV) Complex by a Soluble Methane Monooxygenase Type Activation of O₂ at an Iron(II)-Cyclam Center

Abstract

In soluble methane monooxygenase enzymes (<i>s</i>MMO), dioxygen (O₂) is activated at a diiron(II) center to form an oxodiiron(IV) intermediate <b>Q</b> that performs the challenging oxidation of methane to methanol. An analogous mechanism of O₂ activation at mono- or dinuclear iron centers is rare in the synthetic chemistry. Herein, we report a mononuclear non-heme iron(II)-cyclam complex, <b>1</b>-<i>trans</i>, that activates O₂ to form the corresponding iron(IV)-oxo complex, <b>2</b>-<i>trans</i>, via a mechanism reminiscent of the O₂ activation process in <i>s</i>MMO. The conversion of <b>1</b>-<i>trans</i> to <b>2</b>-<i>trans</i> proceeds via the intermediate formation of an iron(III)-superoxide species <b>3</b>, which could be trapped and spectroscopically characterized at -50 °C. Surprisingly, <b>3</b> is a stronger oxygen atom transfer (OAT) agent than <b>2</b>-<i>trans</i>; <b>3</b> performs OAT to <b>1</b>-<i>trans</i> or PPh₃ to yield <b>2</b>-<i>trans</i> quantitatively. Furthermore, <b>2</b>-<i>trans</i> oxidizes the aromatic C-H bonds of 2,6-di-<i>tert</i>-butylphenol, which, together with the strong OAT ability of <b>3</b>, represents new domains of oxoiron(IV) and superoxoiron(III) reactivities.