One-electron oxidation of an oxoiron(IV) complex to form an [O═FeV═NR]+ center.

Oxoiron(V) species are postulated to be involved in the mechanisms of the arene cis-dihydroxylating Rieske dioxygenases and of bioinspired nonheme iron catalysts for alkane hydroxylation, olefin cis-dihydroxylation, and water oxidation. In an effort to obtain a synthetic oxoiron(V) complex, we report herein the one-electron oxidation of the S = 1 complex [Fe(IV)(O)(TMC)(NCCH(3))](2+) (1, where TMC is tetramethylcyclam) by treatment with tert -butyl hydroperoxide and strong base in acetonitrile to generate a metastable complex 2 at -44 °C, which has been characterized by UV-visible, resonance Raman, Mössbauer, and EPR methods. The defining spectroscopic characteristic of 2 is the unusual x/y anisotropy observed for the (57)Fe and (17)O A tensors associated with the high-valent Fe═O unit and for the (14)N A tensor of a ligand derived from acetonitrile. As shown by detailed density functional theory (DFT) calculations, the unusual x/y anisotropy observed can only arise from an iron center with substantially different spin populations in the d(xz) and d(yz) orbitals, which cannot correspond to an Fe(IV)═O unit but is fully consistent with an Fe(V) center, like that found for [Fe(V)(O)(TAML)](-) (where TAML is tetraamido macrocyclic ligand), the only well-characterized oxoiron(V) complex reported. Mass spectral analysis shows that the generation of 2 entails the addition of an oxygen atom to 1 and the loss of one positive charge. Taken together, the spectroscopic data and DFT calculations support the formulation of 2 as an iron(V) complex having axial oxo and acetylimido ligands, namely [Fe(V)(O)(TMC)(NC(O)CH(3))](+).

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, electron paramagnetic resonance, and density functional theory studies of synthetic S = 1/2 Fe(III)-O-Fe(IV)═O complexes. Superexchange-mediated spin transition at the Fe(IV)═O site.

Previously we have characterized two high-valent complexes [LFe(IV)(µ-O)(2)Fe(III)L], 1, and [LFe(IV)(O)(µ-O)(OH) Fe(IV)L], 4. Addition of hydroxide or fluoride to 1 produces two new complexes, 1-OH and 1-F. Electron paramagnetic resonance (EPR) and Mössbauer studies show that both complexes have an S = 1/2 ground state which results from antiferromagnetic coupling of the spins of a high-spin (S(a) = 5/2) Fe(III) and a high-spin (S(b) = 2) Fe(IV) site. 1-OH can also be obtained by a 1-electron reduction of 4, which has been shown to have an Fe(IV)═O site. Radiolytic reduction of 4 at 77 K yields a Mössbauer spectrum identical to that observed for 1-OH, showing that the latter contains an Fe(IV)═O. Interestingly, the Fe(IV)═O moiety has S(b) = 1 in 4 and S(b) = 2 in 1-OH and 1-F. From the temperature dependence of the S = 1/2 signal we have determined the exchange coupling constant J (ℋ = JS(a)·S(b) convention) to be 90 ± 20 cm(-1) for both 1-OH and 1-F. Broken-symmetry density functional theory (DFT) calculations yield J = 135 cm(-1) for 1-OH and J = 104 cm(-1) for 1-F, in good agreement with the experiments. DFT analysis shows that the S(b) = 1 → S(b) = 2 transition of the Fe(IV)═O site upon reduction of the Fe(IV)-OH site to high-spin Fe(III) is driven primarily by the strong antiferromagnetic exchange in the (S(a) = 5/2, S(b) = 2) couple.

Million-fold activation of the [Fe(2)(micro-O)(2)] diamond core for C-H bond cleavage.

In biological systems, the cleavage of strong C-H bonds is often carried out by iron centres-such as that of methane monooxygenase in methane hydroxylation-through dioxygen activation mechanisms. High valent species with [Fe(2)(micro-O)(2)] diamond cores are thought to act as the oxidizing moieties, but the synthesis of complexes that cleave strong C-H bonds efficiently has remained a challenge. We report here the conversion of a synthetic complex with a valence-delocalized [Fe(3.5)(micro-O)(2)Fe(3.5)](3+) diamond core (1) into a complex with a valence-localized [HO-Fe(III)-O-Fe(IV)=O](2+) open core (4), which cleaves C-H bonds over a million-fold faster. This activity enhancement results from three factors: the formation of a terminal oxoiron(iv) moiety, the conversion of the low-spin (S = 1) Fe(IV)=O centre to a high-spin (S = 2) centre, and the concentration of the oxidizing capability to the active terminal oxoiron(iv) moiety. This suggests that similar isomerization strategies might be used by nonhaem diiron enzymes.

Direct spectroscopic observation of large quenching of first-order orbital angular momentum with bending in monomeric, two-coordinate Fe(II) primary amido complexes and the profound magnetic effects of the absence of Jahn- and Renner-Teller distortions in rigorously linear coordination.

The monomeric iron(II) amido derivatives Fe{N(H)Ar*}(2) (1), Ar* = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Pr(i)(3))(2), and Fe{N(H)Ar(#)}(2) (2), Ar(#) = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Me(3))(2), were synthesized and studied in order to determine the effects of geometric changes on their unusual magnetic properties. The compounds, which are the first stable homoleptic primary amides of iron(II), were obtained by the transamination of Fe{N(SiMe(3))(2)}(2), with HN(SiMe(3))(2) elimination, by the primary amines H(2)NAr* or H(2)NAr(#). X-ray crystallography showed that they have either strictly linear (1) or bent (2, N-Fe-N = 140.9(2) degrees ) iron coordination. Variable temperature magnetization and applied magnetic field Mossbauer spectroscopy studies revealed a very large dependence of the magnetic properties on the metal coordination geometry. At ambient temperature, the linear 1 displayed an effective magnetic moment in the range 7.0-7.50 mu(B), consistent with essentially free ion magnetism. There is a very high internal orbital field component, H(L) approximately 170 T which is only exceeded by a H(L) approximately 203 T of Fe{C(SiMe(3))(3)}(2). In contrast, the strongly bent 2 displayed a significantly lower mu(eff) value in the range 5.25-5.80 mu(B) at ambient temperature and a much lower orbital field H(L) value of 116 T. The data for the two amido complexes demonstrate a very large quenching of the orbital magnetic moment upon bending the linear geometry. In addition, a strong correlation of H(L) with overall formal symmetry is confirmed. ESR spectroscopy supports the existence of large orbital magnetic moments in 1 and 2, and DFT calculations provide good agreement with the physical data.

A synthetic precedent for the [FeIV2(mu-O)2] diamond core proposed for methane monooxygenase intermediate Q.

Intermediate Q, the methane-oxidizing species of soluble methane monooxygenase, is proposed to have an [Fe(IV)(2)(mu-O)(2)] diamond core. In an effort to obtain a synthetic precedent for such a core, bulk electrolysis at 900 mV (versus Fc(+/0)) has been performed in MeCN at -40 degrees C on a valence-delocalized [Fe(III)Fe(IV)(mu-O)(2)(L(b))(2)](3+) complex (1b) (E(1/2) = 760 mV versus Fc(+/0)). Oxidation of 1b results in the near-quantitative formation of a deep red complex, designated 2b, that exhibits a visible spectrum with lambda(max) at 485 nm (9,800 M(-1).cm(-1)) and 875 nm (2,200 M(-1).cm(-1)). The 4.2 K Mössbauer spectrum of 2b exhibits a quadrupole doublet with delta = -0.04(1) mm.s(-1) and DeltaE(Q) = 2.09(2) mm.s(-1), parameters typical of an iron(IV) center. The Mössbauer patterns observed in strong applied fields show that 2b is an antiferromagnetically coupled diiron(IV) center. Resonance Raman studies reveal the diagnostic vibration mode of the [Fe(2)(mu-O)(2)] core at 674 cm(-1), downshifting 30 cm(-1) upon (18)O labeling. Extended x-ray absorption fine structure (EXAFS) analysis shows two O/N scatterers at 1.78 A and an Fe scatterer at 2.73 A. Based on the accumulated spectroscopic evidence, 2b thus can be formulated as [Fe(IV)(2)(mu-O)(2)(L(b))(2)](4+), the first synthetic complex with an [Fe(IV)(2)(mu-O)(2)] core. A comparison of 2b and its mononuclear analog [Fe(IV)(O)(L(b))(NCMe)](2+) (4b) reveals that 4b is 100-fold more reactive than 2b in oxidizing weak C H bonds. This surprising observation may shed further light on how intermediate Q carries out the hydroxylation of methane.