Iron(V)-oxo species have been proposed as key reactive intermediates in the catalysis of oxygen-activating enzymes and synthetic catalysts. Here, we report the synthesis of [Fe(TAML)(O)]- in nearly quantitative yield, where TAML is a macrocyclic tetraamide ligand. Mass spectrometry, Mössbauer, electron paramagnetic resonance, and x-ray absorption spectroscopies, as well as reactivity studies and density functional theory calculations show that this long-lived (hours at -60 degrees C) intermediate is a spin S = 1/2 iron(V)-oxo complex. Iron-TAML systems have proven to be efficient catalysts in the decomposition of numerous pollutants by hydrogen peroxide, and the species we characterized is a likely reactive intermediate in these reactions.

Mössbauer spectra of [LFe(II)X](0) (L = beta-diketiminate; X = Cl(-), CH(3)(-), NHTol(-), NHtBu(-)), 1.X, were recorded between 4.2 and 200 K in applied magnetic fields up to 8.0 T. A spin Hamiltonian analysis of these data revealed a spin S = 2 system with uniaxial magnetization properties, arising from a quasi-degenerate M(S) = +/-2 doublet that is separated from the next magnetic sublevels by very large zero-field splittings (3/D/ > 150 cm(-1)). The ground levels give rise to positive magnetic hyperfine fields of unprecedented magnitudes, B(int) = +82, +78, +72, and +62 T for 1.CH(3), 1.NHTol, 1.NHtBu, and 1.Cl, respectively. Parallel-mode EPR measurements at X-band gave effective g values that are considerably larger than the spin-only value 8, namely g(eff) = 10.9 (1.Cl) and 11.4 (1.CH(3)), suggesting the presence of unquenched orbital angular momenta. A qualitative crystal field analysis of g(eff) shows that these momenta originate from spin-orbit coupling between energetically closely spaced yz and z(2) 3d-orbital states at iron and that the spin of the M(S) = +/-2 doublet is quantized along x, where x is along the Fe-X vector and z is normal to the molecular plane. A quantitative analysis of g(eff) provides the magnitude of the crystal field splitting of the lowest two orbitals, /epsilon(yz) - epsilon(2)(z)/ = 452 (1.Cl) and 135 cm(-1) (1.CH(3)). A determination of the sign of the crystal field splitting was attempted by analyzing the electric field gradient (EFG) at the (57)Fe nuclei, taking into account explicitly the influence of spin-orbit coupling on the valence term and ligand contributions. This analysis, however, led to ambiguous results for the sign of epsilon(yz) - epsilon(2)(z). The ambiguity was resolved by analyzing the splitting Delta of the M(S) = +/-2 doublet; Delta = 0.3 cm(-1) for 1.Cl and Delta = 0.03 cm(-)(1) for 1.CH(3). This approach showed that z(2) is the ground state in both complexes and that epsilon(yz) - epsilon(2)(z) approximately 3500 cm(-1) for 1.Cl and 6000 cm(-1) for 1.CH(3). The crystal field states and energies were compared with the results obtained from time-dependent density functional theory (TD-DFT). The isomer shifts and electric field gradients in 1.X exhibit a remarkably strong dependence on ligand X. The ligand contributions to the EFG, denoted W, were expressed by assigning ligand-specific parameters: W(X) to ligands X and W(N) to the diketiminate nitrogens. The additivity and transferability hypotheses underlying this model were confirmed by DFT calculations. The analysis of the EFG data for 1.X yields the ordering W(N(diketiminate)) < W(Cl) < W(N'HR), W(CH(3)) and indicates that the diketiminate nitrogens perturb the iron wave function to a considerably lesser extent than the monodentate nitrogen donors do. Finally, our study of these synthetic model complexes suggests an explanation for the unusual values for the electric hyperfine parameters of the iron sites in the Fe-Mo cofactor of nitrogenase in the M(N) state.

Figure 1. Mössbauer spectra of a sample at 4.2 K containing 250 mm Z, prepared by the rapid freeze-quench technique (see Supporting Information). The solid lines (red for Z, representing ca. 50 % of total Fe) are simulations based on Equation (2). The contribution from [Fe(H 2 O) 6 ] 3+ is shown in blue. We found the following parameters for Z: D = 9.7(7) cm À1 , A x /g n b n = A y / g n b n = À20.3(3) T, DE Q = À0.33(3) mm s À1 , d = 0.38(2) mm s À1. For the 8.0-T spectrum, the theoretical curves for Z and [Fe(H 2 O) 6 ] 3+ were added (black).

Fe III -O 2 •− intermediates are well known in heme enzymes, but none have been characterized in the nonheme mononuclear Fe II enzyme family. Many steps in the O 2 activation and reaction cycle of Fe II -containing homoprotocatechuate 2,3-dioxygenase are made detectable by using the alternative substrate 4-nitrocatechol (4NC) and mutation of the active site His200 to Asn (H200N). Here, the first intermediate (Int-1) observed after adding O 2 to the H200N-4NC complex is trapped and characterized using EPR and Möss-bauer (MB) spectroscopies. Int-1 is a high-spin ( (1)(2)(3)(4)(5)(6)(7)(8). Internal electron transfer to form an Fe III -superoxo species converts the kinetically inert triplet ground state of O 2 to a doublet that can participate in the many types of chemistry characteristic of this mechanistically diverse group of enzymes. The same strategy is usually employed by heme-containing oxygenases and oxidases, leading in some cases to comparatively stable Fe III -superoxo intermediates that have been structurally and spectroscopically characterized (9-12). Instability of the putative superoxo intermediate in all mononuclear nonheme iron-containing enzymes has prevented similar characterization, although a superoxide level species has been reported for the dinuclear iron site of myo-inositol oxygenase (13).In recent studies of the nonheme Fe II -containing homoprotocatechuate 2,3-dioxygenase (2,3-HPCD), we have shown that three intermediates of the catalytic cycle can be trapped in one crystal for structural analysis (14). One of these intermediates has been proposed to be an Fe II -superoxo species based on the long Fe-O bond distances and an unexpected lack of planarity of the aromatic ring of the alternative substrate 4-nitrocatechol (4NC), which chelates the iron in ligand sites adjacent to that of the O 2 . In accord with the mechanism postulated for this enzyme class as illustrated in Scheme 1 (1, 8, 15-21), we have proposed that net electron transfer from 4NC through the Fe II to O 2 forms adjacent substrate and oxygen radicals (Scheme 1B). Recombination of the radicals would begin the ring cleavage and oxygen insertion reactions of this enzyme that eventually yield a muconic semialdehyde adduct as the product. A localized radical on the 4NC semiquinone at the incipient position of oxygen attack would account for the lack of ring planarity. Although this is the only structurally characterized nonheme Fe-superoxo species, the iron oxidation state differs from all of the other postulated Fe-superoxo intermediates.The mechanism that emerges from the structural and kinetic studies does not require a change in metal oxidation state to form a reactive intermediate (22). However, our studies of 2,3-HPCD in which Fe II is replaced with Mn II suggest that transient formaScheme 1. Proposed mechanism for extradiol dioxygenases. In the case of 2,3-HPCD, R is −CH 2 COO − and B is His200. When R is −NO 2 and His200 is changed to Asn, the reaction stalls before reaching intermediate C. Peroxide is slowly released a...

Keywordsbioinorganic chemistry iron-oxo; nonheme iron complexes; high-valent compounds; enzyme models High-valent oxoferryl intermediates have been proposed as the active oxidants in the catalytic cycles of a wide range of mononuclear non-heme oxygen activating enzymes.[1] These high-valent species have now been spectroscopically characterized for four enzymes and were found in all instances to contain high-spin (S = 2) iron(IV) centers.[2] Contemporaneously, the first examples of the existing family of synthetic nonheme oxoiron(IV) complexes were characterized, [3][4][5] which are exclusively octahedral and in all but one case exhibit the S = 1, rather than S = 2, spin-state. Given that DFT suggests higher reactivity for an S = 2 oxoiron(IV) unit, [6,7] it is perhaps not surprising that there is a scarcity of such complexes. Indeed, the only example to date is [Fe IV (O)(H 2 O) 5 ] 2+ (1),

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.

High-spin oxoiron(IV) species are often implicated in the mechanisms of nonheme iron oxygenases, their C-H bond cleaving properties being attributed to the quintet spin state. However, the few available synthetic S = 2 Fe(IV)═O complexes supported by polydentate ligands do not cleave strong C-H bonds. Herein we report the characterization of a highly reactive S = 2 complex, [Fe(IV)(O)(TQA)(NCMe)](2+) (2) (TQA = tris(2-quinolylmethyl)amine), which oxidizes both C-H and C═C bonds at -40 °C. The oxidation of cyclohexane by 2 occurs at a rate comparable to that of the oxidation of taurine by the TauD-J enzyme intermediate after adjustment for the different temperatures of measurement. Moreover, compared with other S = 2 complexes characterized to date, the spectroscopic properties of 2 most closely resemble those of TauD-J. Together these features make 2 the best electronic and functional model for TauD-J to date.

Recently, we reported the characterization of the S = (1)/ 2 complex [Fe (V)(O)B*] (-), where B* belongs to a family of tetraamido macrocyclic ligands (TAMLs) whose iron complexes activate peroxides for environmentally useful applications. The corresponding one-electron reduced species, [Fe (IV)(O)B*] (2-) ( 2), has now been prepared in >95% yield in aqueous solution at pH > 12 by oxidation of [Fe (III)(H 2O)B*] (-) ( 1), with tert-butyl hydroperoxide. At room temperature, the monomeric species 2 is in a reversible, pH-dependent equilibrium with dimeric species [B*Fe (IV)-O-Fe (IV)B*] (2-) ( 3), with a p K a near 10. In zero field, the Mössbauer spectrum of 2 exhibits a quadrupole doublet with Delta E Q = 3.95(3) mm/s and delta = -0.19(2) mm/s, parameters consistent with a S = 1 Fe (IV) state. Studies in applied magnetic fields yielded the zero-field splitting parameter D = 24(3) cm (-1) together with the magnetic hyperfine tensor A/ g nbeta n = (-27, -27, +2) T. Fe K-edge EXAFS analysis of 2 shows a scatterer at 1.69 (2) A, a distance consistent with a Fe (IV)O bond. DFT calculations for [Fe (IV)(O)B*] (2-) reproduce the experimental data quite well. Further significant improvement was achieved by introducing hydrogen bonding of the axial oxygen with two solvent-water molecules. It is shown, using DFT, that the (57)Fe hyperfine parameters of complex 2 give evidence for strong electron donation from B* to iron.