Michael R. Bukowski

An FeIV=O complex of a tetradentate tripodal nonheme ligand.

The reaction of [FeII(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 λmax at 724 nm (ɛ ≈ 300 M−1⋅cm−1) formulated as [FeIV(O)(TPA)]2+ by a combination of spectroscopic techniques. Its electrospray mass spectrum shows a prominent feature at m/z 461, corresponding to the [FeIV(O)(TPA)(ClO4)]+ ion. The Mössbauer spectra recorded in zero field reveal a doublet with ΔEQ = 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 FeIVO complexes. The presence of an FeIVO 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 [FeIV(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 [FeIV(O)(TPA)]2+ is an effective oxygen-atom transfer agent. It is proposed that the FeIVO species derives from O—O bond heterolysis of an unobserved FeII(TPA)-acyl peroxide complex. The characterization of [FeIV(O)(TPA)]2+ as having a reactive terminal FeIVO 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.

A thiolate-ligated nonheme oxoiron(IV) complex relevant to cytochrome P450.

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 [FeIV(O)(TMCS)]+ where TMCS is a pentadentate ligand that provides a square pyramidal N4(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.

AN FEIVO COMPLEX COMPLEX OF A TETRADENTATE TRIPODAL NONHEME LIGAND

The reaction of [FeII(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 λmax at 724 nm (ɛ ≈ 300 M−1⋅cm−1) formulated as [FeIV(O)(TPA)]2+ by a combination of spectroscopic techniques. Its electrospray mass spectrum shows a prominent feature at m/z 461, corresponding to the [FeIV(O)(TPA)(ClO4)]+ ion. The Mössbauer spectra recorded in zero field reveal a doublet with ΔEQ = 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 FeIVO complexes. The presence of an FeIVO 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 [FeIV(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 [FeIV(O)(TPA)]2+ is an effective oxygen-atom transfer agent. It is proposed that the FeIVO species derives from O—O bond heterolysis of an unobserved FeII(TPA)-acyl peroxide complex. The characterization of [FeIV(O)(TPA)]2+ as having a reactive terminal FeIVO 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.

Sulfur versus Iron Oxidation in An Iron-Thiolate Model Complex

In the absence of base, the reaction of [Fe(II)(TMCS)]PF6 (1, TMCS = 1-(2-mercaptoethyl)-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane) with peracid in methanol at -20 °C did not yield the oxoiron(IV) complex (2, [Fe(IV)(O)(TMCS)]PF6), as previously observed in the presence of strong base (KO(t)Bu). Instead, the addition of 1 equiv of peracid resulted in 50% consumption of 1. The addition of a second equivalent of peracid resulted in the complete consumption of 1 and the formation of a new species 3, as monitored by UV-vis, ESI-MS, and Mössbauer spectroscopies. ESI-MS showed 3 to be formulated as [Fe(II)(TMCS) + 2O](+), while EXAFS analysis suggested that 3 was an O-bound iron(II)-sulfinate complex (Fe-O = 1.95 Å, Fe-S = 3.26 Å). The addition of a third equivalent of peracid resulted in the formation of yet another compound, 4, which showed electronic absorption properties typical of an oxoiron(IV) species. Mössbauer spectroscopy confirmed 4 to be a novel iron(IV) compound, different from 2, and EXAFS (Fe═O = 1.64 Å) and resonance Raman (ν(Fe═O) = 831 cm(-1)) showed that indeed an oxoiron(IV) unit had been generated in 4. Furthermore, both infrared and Raman spectroscopy gave indications that 4 contains a metal-bound sulfinate moiety (ν(s)(SO2) ≈ 1000 cm (-1), ν(as)(SO2) ≈ 1150 cm (-1)). Investigations into the reactivity of 1 and 2 toward H(+) and oxygen atom transfer reagents have led to a mechanism for sulfur oxidation in which 2 could form even in the absence of base but is rapidly protonated to yield an oxoiron(IV) species with an uncoordinated thiol moiety that acts as both oxidant and substrate in the conversion of 2 to 3.