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