Audria Stubna

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.

Axial Ligand Effects on the Geometric and Electronic Structures of Nonheme Oxoiron(IV) Complexes

A series of complexes [Fe(IV)(O)(TMC)(X)](+) (where X = OH(-), CF3CO2(-), N3(-), NCS(-), NCO(-), and CN(-)) were obtained by treatment of the well-characterized nonheme oxoiron(IV) complex [Fe(IV)(O)(TMC)(NCMe)](2+) (TMC = tetramethylcyclam) with the appropriate NR4X salts. Because of the topology of the TMC macrocycle, the [Fe(IV)(O)(TMC)(X)](+) series represents an extensive collection of S = 1 oxoiron(IV) complexes that only differ with respect to the ligand trans to the oxo unit. Electronic absorption, Fe K-edge X-ray absorption, resonance Raman, and Mossbauer data collected for these complexes conclusively demonstrate that the characteristic spectroscopic features of the S = 1 Fe(IV)=O unit, namely, (i) the near-IR absorption properties, (ii) X-ray absorption pre-edge intensities, and (iii) quadrupole splitting parameters, are strongly dependent on the identity of the trans ligand. However, on the basis of extended X-ray absorption fine structure data, most [Fe(IV)(O)(TMC)(X)](+) species have Fe=O bond lengths similar to that of [Fe(IV)(O)(TMC)(NCMe)](2+) (1.66 +/- 0.02 A). The mechanisms by which the trans ligands perturb the Fe(IV)=O unit were probed using density functional theory (DFT) computations, yielding geometric and electronic structures in good agreement with our experimental data. These calculations revealed that the trans ligands modulate the energies of the Fe=O sigma- and pi-antibonding molecular orbitals, causing the observed spectroscopic changes. Time-dependent DFT methods were used to aid in the assignment of the intense near-UV absorption bands found for the oxoiron(IV) complexes with trans N3(-), NCS(-), and NCO(-) ligands as X(-)-to-Fe(IV)=O charge-transfer transitions, thereby rationalizing the resonance enhancement of the nu(Fe=O) mode upon excitation of these chromophores.

A diiron(IV) complex that cleaves strong C-H and O-H bonds.

The controlled cleavage of strong C-H bonds like those of methane poses a significant challenge for chemists. In nature methane is oxidized to methanol by soluble methane monooxygenase via a diiron(IV) intermediate called Q. To model the chemistry of MMO-Q, an oxo-bridged diiron(IV) complex has been generated by electrochemical oxidation and characterized by several spectroscopic methods. This novel species has an Fe(IV/III) redox potential of +1.50 V vs. ferrocene (>2 V vs. NHE), the highest value thus far determined electrochemically for an iron complex. This species is quite an effective oxidant. It can attack C-H bonds as strong as 100 kcal mol(-1) and reacts with cyclohexane a hundred- to a thousand-fold faster than mononuclear Fe(IV)=O complexes of closely related ligands. Strikingly, this species can also cleave the strong O-H bonds of methanol and tert-butanol instead of their weaker C-H bonds, representing the first example of O-H bond activation for iron complexes.

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.

Mössbauer Evidence for an Exchange-Coupled {[Fe4S4]1+ Nip1+} A-Cluster in Isolated α Subunits of Acetyl-Coenzyme A Synthase/Carbon Monoxide Dehydrogenase

The active site A-cluster in the alpha subunit of the title enzyme consists of an Fe4S4 cluster coordinated to a [Nip Nid] subcomponent. The cluster must be activated for catalysis using low-potential reductants such as Ti(III) citrate. Relative to the inactive {[Fe4S4]2+ Nip2+ Nid2+} state, the activated state appears to be 2-electrons more reduced, but the location of these electrons within the A-cluster is uncertain, with {[Fe4S4]2+ Nip0 Nid2+} and {[Fe4S4]1+ Nip1+ Nid2+} configurations proposed. Recombinant apo-alpha subunits oligomerize after activation with NiCl2. The dimer fraction, upon reduction with excess Ti(III)citrate, exhibited Mössbauer spectra consisting of two quadrupole doublets representing 51% and 21% of the Fe, with parameters indicating [Fe4S4]1+ states. Spectra recorded in strong magnetic fields were typical of diamagnetic systems, indicating an exchange-coupled S = 0 {[Fe4S4]1+ Nip1+} state. Additional treatment with CO altered the doublet Mössbauer parameters, suggesting an interaction with CO, but maintaining the cluster in the {[Fe4S4]1+ Nip1+} state. Reduction with substoichiometric equivalents of Ti(III) citrate afforded an EPR signal typical of Ni1+ ions, with g parallel = 2.10 and g perpendicular = 2.02. Addition of more Ti caused the signal intensity to decline, suggesting that it arises from the semireduced {[Fe4S4]2+ Nip1+} state.

Mössbauer and EPR Study of Recombinant Acetyl-CoA Synthase from Moorella thermoacetica†

Mössbauer and EPR spectroscopies were used to study the electronic structure of the A-cluster from recombinant acetyl-CoA synthase (the alpha subunit of the alpha2beta2 acetyl-CoA synthase/CO dehydrogenase). Once activated with Ni, these subunits have properties mimicking those associated with the alpha2beta2 tetramer, including structural heterogeneities. The Fe4S4 portion of the A-cluster in oxidized, methylated, and acetylated states was in the 2+ core oxidation state. Upon reduction with dithionite or Ti3+ citrate, samples of Ni-activated alpha developed the ability to accept a methyl group. Corresponding Mössbauer spectra exhibited two populations of A-clusters; roughly, 70% contained [Fe4S4]1+ cubanes, while approximately 30% contained [Fe4S4]2+ cubanes, suggesting an extremely low [Fe4S4](1+/2+) reduction potential for the 30% portion (perhaps <-800 mV vs NHE). The same population ratio was observed when Ni-free unactivated alpha was used. The 70% fraction exhibited paramagnetic hyperfine structure in the absence of an applied magnetic field, excluding the possibility that it represents an [Fe4S4]1+ cluster coupled to a (proximal) Ni(p)1+. EPR spectra of dithionite-reduced, Ni-activated alpha exhibited features at g = 5.8 and g(ave) approximately 1.93, consistent with a physical mixture of {S = 3/2; S = 1/2} spin-states for A-clusters containing [Fe4S4]1+ clusters. Incubation of Ni-activated alpha with dithionite and CO converted 25% of alpha subunits into the S = 1/2 A(red)-CO state. Previous correlation of this state to functional A-clusters suggests that only the 30% fraction not reduced by dithionite or Ti3+ citrate represents functional A-clusters. Comparison of spin states in oxidized and methylated states suggests that two electrons are required for reductive activation, starting from the oxidized state containing Ni(p)2+. Refitting published activity-vs-potential data supports an n = 2 reductive activation. Enzyme starting in the methylated state exhibited catalytic activity in the absence of an external reductant, suggesting that the two electrons used in reductive activation are retained by the enzyme after each catalytic cycle and that the enzyme does not have to pass through the A(red)-CO state during catalysis. Taken together, our results suggest that a Ni(p)0 state may form upon reductive activation and reform after each catalytic cycle.