An FeIV=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 degrees C results in the nearly quantitative formation of a pale green intermediate with lambda(max) at 724 nm ( epsilon approximately 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 mz 461, corresponding to the [Fe(IV)(O)(TPA)(ClO(4))](+) ion. The Mössbauer spectra recorded in zero field reveal a doublet with DeltaE(Q) = 0.92(2) mms and delta = 0.01(2) mms; 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 ON scatterer at 1.67 A to fit the extended x-ray absorption fine structure region. The [Fe(IV)(O)(TPA)](2+) intermediate is stable at -40 degrees C for several days but decays quantitatively on warming to [Fe(2)(mu-O)(mu-OAc)(TPA)(2)](3+). Addition of thioanisole or cyclooctene at -40 degrees 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 OO 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.

Fig 1.

(A) Conversion of 2 mM 1 to 2 in CH₃CN at −40°C by addition of 1 equiv. CH₃CO₃H (32 wt %), as monitored by UV-visible spectroscopy. (B) Subsequent conversion of 2 to 3 at 10°C. For reasons yet undetermined, the addition of 5 μl of H₂O to the 3-ml solution increased the stability of 2 by a factor of 3 without affecting λ_max and ɛ_max; thus, all subsequent samples were prepared in this manner.

Fig 2.

Electrospray mass spectrum of a solution of 1a in CH₃CN maintained at −40°C 3 min after addition of CH₃CO₃H.

Fig 3.

The 4.2-K Mössbauer spectra were recorded in zero field. (A) Spectrum of 2 from the reaction of 1b with CH₃CO₃H. The solid line is a spectral simulation using the ΔE_Q and δ values of Table 1. Eighty percent of the Fe in the sample belongs to 2, and the remainder is a diiron(III) species with properties identical to that observed in B. The spectrum in B was observed after 2 was allowed to decay. This species, 3, is diamagnetic according to a spectrum (not shown) recorded in an applied field of 7.0 T; ΔE_Q = 1.48(5) mm/s, δ = 0.45(2) mm/s, η = 0.5 (approximately equivalent sites). (C) Spectrum observed after addition of thioanisole to 2. The major doublet (75%) is a low-spin iron(II) (S = 0) species with ΔE_Q = 0.35 mm/s and δ = 0.44 mm/s. The remainder of the absorption is mainly a diiron(III) species, most likely the same species present in the sample before addition of thioanisole.

Fig 4.

Mössbauer spectra of 2 (same sample as that used for Fig. 3A) recorded in magnetic fields applied parallel to the observed γ-rays at temperatures and fields indicated. Solid lines are spectral simulations based on Eq. 1 using the parameters listed in Table 1.

Fig 5.

X-ray absorption near-edge features (Fe K-edge, fluorescence excitation) of 2 (solid line), [Fe^IV(O)(TMC)(CH₃CN)](OTf)₂ (dotted line), and [Fe^IV₂(μ-O)₂(BPMCN)₂](OTf)₄ (dashed line).

Fig 6.

Fourier transform of the Fe K-edge EXAFS data [k³χ(k)] and Fourier-filtered EXAFS spectrum [k³χ′(k), Inset] of 6 mM 2 in frozen CH₃CN solution at T = 13(1) K, obtained by fluorescence detection and prepared from a 2:1 mixture of [Fe(TPA)(OTf)₂] and [⁵⁷Fe(TPA)(OTf)₂]. This same sample was used for the Mössbauer analysis shown in Figs. 3A and 4. Fourier-transform range k = 2–15 Å⁻¹; back-transformation window indicated by dashed vertical lines; experimental data (dotted line) and best fit (solid line). Fitting: one O/N at 1.67 Å (Δσ², 0.0004 Ų), four N/O at 1.99 Å (0.0016), one N/O at 2.20 Å (0.003), six C at 2.89 Å (0.0028).

Scheme 1.