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.

The reaction of [Fe ^II (tris(2-pyridylmethyl)amine, TPA)(NCCH ₃ ) ₂ ] ²⁺ with 1 equiv. peracetic acid in CH ₃ CN at −40°C results in the nearly quantitative formation of a pale green intermediate with λ _max at 724 nm (ɛ ≈ 300 M ⁻¹ ⋅cm ⁻¹ ) formulated as [Fe ^IV (O)(TPA)] ²⁺ by a combination of spectroscopic techniques. Its electrospray mass spectrum shows a prominent feature at m / z 461, corresponding to the [Fe ^IV (O)(TPA)(ClO ₄ )] ⁺ ion. The Mössbauer spectra recorded in zero field reveal a doublet with Δ E _Q = 0.92(2) mm/s and δ = 0.01(2) mm/s; analysis of spectra obtained in strong magnetic fields yields parameters characteristic of S = 1 Fe ^IV O complexes. The presence of an Fe ^IV O unit is also indicated in its Fe K-edge x-ray absorption spectrum by an intense 1-s → 3-d transition and the requirement for an O/N scatterer at 1.67 Å to fit the extended x-ray absorption fine structure region. The [Fe ^IV (O)(TPA)] ²⁺ intermediate is stable at −40°C for several days but decays quantitatively on warming to [Fe ₂ (μ - O)(μ - OAc)(TPA) ₂ ] ³⁺ . Addition of thioanisole or cyclooctene at −40°C results in the formation of thioanisole oxide (100% yield) or cyclooctene oxide (30% yield), respectively; thus [Fe ^IV (O)(TPA)] ²⁺ is an effective oxygen-atom transfer agent. It is proposed that the Fe ^IV O species derives from O—O bond heterolysis of an unobserved Fe ^II (TPA)-acyl peroxide complex. The characterization of [Fe ^IV (O)(TPA)] ²⁺ as having a reactive terminal Fe ^IV O unit in a nonheme ligand environment lends credence to the proposed participation of analogous species in the oxygen activation mechanisms of many mononuclear nonheme iron enzymes.

Thiolate-ligated oxoiron(IV) centers are postulated to be the key oxidants in the catalytic cycles of oxygen-activating cytochrome P450 and related enzymes. Despite considerable synthetic efforts, chemists have not succeeded in preparing an appropriate model complex. Here we report the synthesis and spectroscopic characterization of [Fe ^IV (O)(TMCS)] ⁺ where TMCS is a pentadentate ligand that provides a square pyramidal N ₄ (SR) _apical , where SR is thiolate, ligand environment about the iron center, which is similar to that of cytochrome P450. The rigidity of the ligand framework stabilizes the thiolate in an oxidizing environment. Reactivity studies suggest that thiolate coordination favors hydrogen-atom abstraction chemistry over oxygen-atom transfer pathways in the presence of reducing substrates.