An Fe IV O complex of a tetradentate tripodal nonheme ligand

Abstract

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.