Intrinsic properties and reactivities of mononuclear nonheme iron-oxygen complexes bearing the tetramethylcyclam ligand

Iron-oxygen species, such as iron(IV)-oxo, iron(III)-superoxo, iron(III)-peroxo, and iron(III)-hydroperoxo complexes, are key intermediates often detected in the catalytic cycles of dioxygen activation by heme and nonheme iron enzymes. Our understanding of the chemistry of these key intermediates has improved greatly by studies of the structural and spectroscopic properties and reactivities of their synthetic analogues. One class of biomimetic coordination complexes that has proven to be particularly versatile in studying dioxygen activation by metal complexes is comprised of Fe ^IVO and Fe ^IIIO ₂(H) complexes of the macrocyclic tetramethylcyclam ligand (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). Several recent advances have been made in the synthesis and isolation of new iron-oxygen complexes of this ligand, their structural and spectroscopic characterization, and elucidation of their reactivities in various oxidation reactions. In this review, we summarize the chemistry of the first structurally characterized mononuclear nonheme iron(IV)-oxo complex, in which the Fe ^IVO group was stabilized by the TMC ligand. Complexes with different axial ligands, [Fe ^IV(O)(TMC)(X)] ⁿ⁺, and complexes of other cyclam ligands are discussed as well. Very recently, significant progress has also been reported in the area of other iron-oxygen intermediates, such as iron(III)-superoxo, iron(III)-peroxo, and iron(III)-hydroperoxo complexes bearing the TMC ligand. The present results demonstrate how synthetic and mechanistic developments in biomimetic research can advance our understanding of dioxygen activation occurring in mononuclear nonheme iron enzymes.