An Fe^IV=O complex of a tetradentate tripodal nonheme ligand

The reaction of [Fe^II(tris(2-pyridylmethyl)amine, TPA)(NCCH₃)₂]²⁺ with 1 equiv. peracetic acid in CH₃CN at -40°C results in the nearly quantitative formation of a pale green intermediate with γmax at 724 nm (ε ≈ 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)(ClO₄)]⁺ ion. The Mössbauer spectra recorded in zero field reveal a doublet with ΔE_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 [Fe₂(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.