Following the heme paradigm, it is often proposed that dioxygen activation by nonheme monoiron enzymes involves an iron(IV)=oxo intermediate that is responsible for the substrate oxidation step. Such a transient species has now been obtained from a synthetic complex with a nonheme macrocyclic ligand and characterized spectroscopically. Its high-resolution crystal structure reveals an iron-oxygen bond length of 1.646(3) angstroms, demonstrating that a terminal iron(IV)=oxo unit can exist in a nonporphyrin ligand environment and lending credence to proposed mechanisms of nonheme iron catalysis.

Rates of exchange: Evidence that nonheme oxoiron(IV) complexes exchange their oxygen atoms with H218O was obtained for the first time, by monitoring electrospray ionization mass spectral changes of the oxoiron(IV) species. The oxygen‐atom exchange depended markedly on the amounts of H218O present and reaction temperatures but not on the presence of trans axial ligand (see scheme).

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