Spectroscopic Properties of a Biologically Relevant [Fe2 (μ-O)2 ] Diamond Core Motif with a Short Iron-Iron Distance

Diiron cofactors in enzymes perform diverse challenging transformations. The structures of high valent intermediates (Q in methane monooxygenase and X in ribonucleotide reductase) are debated since Fe-Fe distances of 2.5-3.4 Å were attributed to "open" or "closed" cores with bridging or terminal oxido groups. We report the crystallographic and spectroscopic characterization of a Fe^III ₂ (μ-O)₂ complex (2) with tetrahedral (4C) centres and short Fe-Fe distance (2.52 Å), persisting in organic solutions. 2 shows a large Fe K-pre-edge intensity, which is caused by the pronounced asymmetry at the T_D Fe^III centres due to the short Fe-μ-O bonds. A ≈2.5 Å Fe-Fe distance is unlikely for six-coordinate sites in Q or X, but for a Fe₂ (μ-O)₂ core containing four-coordinate (or by possible extension five-coordinate) iron centres there may be enough flexibility to accommodate a particularly short Fe-Fe separation with intense pre-edge transition. This finding may broaden the scope of models considered for the structure of high-valent diiron intermediates formed upon O₂ activation in biology.

[2Fe-2S] Cluster Supported by Redox-Active o-Phenylenediamide Ligands and Its Application toward Dinitrogen Reduction

As prevalent cofactors in living organisms, iron-sulfur clusters participate in not only the electron-transfer processes but also the biosynthesis of other cofactors. Many synthetic iron-sulfur clusters have been used in model studies, aiming to mimic their biological functions and to gain mechanistic insight into the related biological systems. The smallest [2Fe-2S] clusters are typically used for one-electron processes because of their limited capacity. Our group is interested in functionalizing small iron-sulfur clusters with redox-active ligands to enhance their electron storage capacity, because such functionalized clusters can potentially mediate multielectron chemical transformations. Herein we report the synthesis, structural characterization, and catalytic activity of a diferric [2Fe-2S] cluster functionalized with two o-phenylenediamide ligands. The electrochemical and chemical reductions of such a cluster revealed rich redox chemistry. The functionalized diferric cluster can store up to four electrons reversibly, where the first two reduction events are ligand-based and the remainder metal-based. The diferric [2Fe-2S] cluster displays catalytic activity toward silylation of dinitrogen, affording up to 88 equiv of the amine product per iron center.

Localized Electronic Structure of Nitrogenase FeMoco Revealed by Selenium K-Edge High Resolution X-ray Absorption Spectroscopy

The size and complexity of Mo-dependent nitrogenase, a multicomponent enzyme capable of reducing dinitrogen to ammonia, have made a detailed understanding of the FeMo cofactor (FeMoco) active site electronic structure an ongoing challenge. Selective substitution of sulfur by selenium in FeMoco affords a unique probe wherein local Fe-Se interactions can be directly interrogated via high-energy resolution fluorescence detected X-ray absorption spectroscopic (HERFD XAS) and extended X-ray absorption fine structure (EXAFS) studies. These studies reveal a significant asymmetry in the electronic distribution of the FeMoco, suggesting a more localized electronic structure picture than is typically assumed for iron-sulfur clusters. Supported by experimental small molecule model data in combination with time dependent density functional theory (TDDFT) calculations, the HERFD XAS data is consistent with an assignment of Fe2/Fe6 as an antiferromagnetically coupled diferric pair. HERFD XAS and EXAFS have also been applied to Se-substituted CO-inhibited MoFe protein, demonstrating the ability of these methods to reveal electronic and structural changes that occur upon substrate binding. These results emphasize the utility of Se HERFD XAS and EXAFS for selectively probing the local electronic and geometric structure of FeMoco.

Hard versus soft: zero-field dinuclear Dy(iii) oxygen bridged SMM and theoretical predictions of the sulfur and selenium analogues

Two dinuclear lanthanide complexes (Gd and Dy) were prepared and characterized by X-ray, magnetic and computational methods. The Dy analogue shows SMM behavior with an energy barrier of 98.4 K in the absence of an applied dc field. Theoretical calculations were performed on model complexes which support the hypothesis that the energy barrier will increase if the soft-donor atoms S and Se are used in lieu of an O donor.

Strong Ferromagnetic Exchange Coupling Mediated by a Bridging Tetrazine Radical in a Dinuclear Nickel Complex

The radical bridged compound [(Ni(TPMA))₂-μ-bmtz^•-](BF₄)₃·3CH₃CN (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA = tris(2-pyridylmethyl)amine) exhibits strong ferromagnetic exchange between the S = 1 Ni^II centers and the bridging S = 1/2 bmtz radical with J = 96 ± 5 cm^-1 (-2J_Ni-radS_NiS_rad). DFT calculations support the existence of strong ferromagnetic exchange.

Rubidium chalcogenido diferrates(III) containing dimers [Fe2 Q 6]6− of edge-sharing tetrahedra (Q=O, S, Se)

The two isotypic rubidium chalcogenido diferrates Rb12[Fe2 Q 6](Q 2)3 (Q=S/Se), which both form needles with green-metallic lustre, were synthesized from Rb2S, elemental iron, rubidium and sulfur (Q=S) or from the pure elements (Q=Se) at maximum temperatures of 500–800°C. Their triclinic crystal structures were determined by means of X-ray single crystal data (space group P1̅, a=863.960(10)/903.2(3), b=942.790(10)/982.1(3), c=1182.70(2)/1227.4(4) pm, α=77.4740(10)/77.262(6), β=71.5250(10)/71.462(6), γ=63.7560(10)/63.462(5)°, Z=1, R1=0.0308/0.0658 for Q=S/Se). The structures contain isolated dinuclear anions [FeIII 2 Q 6]6− composed of two edge-sharing [FeQ 4] tetrahedra (dFe −Q =223.4–232.3/236.2–244.8 pm), which are also found in the two polymorphs of the pure alkali diferrates Rb6[Fe2 Q 6]. The diferrate ions are arranged in layers running in the a/b plane around z=0. Inbetween (around z ≈ 1 2 $z \approx {1 \over 2}$ ), two crystallographically different disulfide/diselenide ions Q 2 2 − $Q_2^{2 - }$ (dQ −Q =211.1–213.4/237.9–241.1 pm), which are arranged in slightly puckered 36 nets, are intercalated. The intra-anionic distances and angles, the Rb coordination numbers and the molar volumes of these two ‘double-salts’ are in accordance with their corresponding reference compounds, Rb6[Fe2 Q 6] and Rb2 Q 2. In addition, the two polymorphs of Rb6[Fe2Se6], which are both isotypic with the sulfido analogous (Cs6[Ga2Se6]-type, monoclinic, space group P21 /c, a=827.84(5), b=1329.51(7), c=1074.10(6) pm, β=127.130(5)°, R1=0.0443 and Ba6[Al2Sb6]-type, orthorhombic, space group Cmce, a=1963.70(3), b=718.98(3), c=1348.40(7) pm, R1=0.0264) were prepared and characterized to complete the series of alkali diferrates(III) with oxido, sulfido and selenido ligands. The electronic band structures of the three Rb salts Rb6[Fe2 Q 6], which have been calculated within the GGA+U approach applying an AFM spin ordering in the dimers and appropriate Hubbard parameters, allow a comparison of the chemical bonding characteristics (e.g. covalency) and the magnetic properties (magnetic moments) within the series of chalcogenido ligands. An analysis of the spin densities enables a comparative consideration of the mechanisms crucial for the magnetic ordering in chalcogenido ferrates. Ultimately, the electronic structure of the new compound Rb12[Fe2S6](S2)3 nicely compares with those of the S2-free reference compound Rb6[Fe2S6].

Quasi-three-coordinate iron and cobalt terphenoxide complexes {Ar(iPr8)OM(μ-O)}2 (Ar(iPr8) = C6H-2,6-(C6H2-2,4,6-(i)Pr3)2-3,5-(i)Pr2; M = Fe or Co) with M(III)2(μ-O)2 core structures and the peroxide dimer of 2-oxepinoxy relevant to benzene oxidation

The bis(μ-oxo) dimeric complexes {Ar(iPr8)OM(μ-O)}2 (Ar(iPr8) = C6H-2,6-(C6H2-2,4,6-(i)Pr3)2-3,5-(i)Pr2; M = Fe (1), Co (2)) were prepared by oxidation of the M(I) half-sandwich complexes {Ar(iPr8)M(η(6)-arene)} (arene = benzene or toluene). Iron species 1 was prepared by reacting {Ar(iPr8)Fe(η(6)-benzene)} with N2O or O2, and cobalt species 2 was prepared by reacting {Ar(iPr8)Co(η(6)-toluene)} with O2. Both 1 and 2 were characterized by X-ray crystallography, UV-vis spectroscopy, magnetic measurements, and, in the case of 1, Mössbauer spectroscopy. The solid-state structures of both compounds reveal unique M2(μ-O)2 (M = Fe (1), Co(2)) cores with formally three-coordinate metal ions. The Fe···Fe separation in 1 bears a resemblance to that in the Fe2(μ-O)2 diamond core proposed for the methane monooxygenase intermediate Q. The structural differences between 1 and 2 are reflected in rather differing magnetic behavior. Compound 2 is thermally unstable, and its decomposition at room temperature resulted in the oxidation of the Ar(iPr8) ligand via oxygen insertion and addition to the central aryl ring of the terphenyl ligand to produce the 5,5'-peroxy-bis[4,6-(i)Pr2-3,7-bis(2,4,6-(i)Pr3-phenyl)oxepin-2(5H)-one] (3). The structure of the oxidized terphenyl species is closely related to that of a key intermediate proposed for the oxidation of benzene.

Oxidized and reduced [2Fe-2S] clusters from an iron(I) synthon

Synthetic [2Fe-2S] clusters are often used to elucidate ligand effects on the reduction potentials and spectroscopy of natural electron-transfer sites, which can have anionic Cys ligands or neutral His ligands. Current synthetic routes to [2Fe-2S] clusters are limited in their feasibility with a range of supporting ligands. Here, we report a new synthetic route to synthetic [2Fe-2S] clusters, through oxidation of an iron(I) source with elemental sulfur. This method yields a neutral diketiminate-supported [2Fe-2S] cluster in the diiron(III)-oxidized form. The oxidized [2Fe-2S] cluster can be reduced to a mixed valent iron(II)-iron(III) compound. Both the diferric and reduced mixed valent clusters are characterized using X-ray crystallography, Mössbauer spectroscopy, EPR spectroscopy and cyclic voltammetry. The reduced compound is particularly interesting because its X-ray crystal structure shows a difference in Fe-S bond lengths to one of the iron atoms, consistent with valence localization. The valence localization is also evident from Mössbauer spectroscopy.

Modulating the magnetic behavior of Fe(ii)–MOF-74 by the high electron affinity of the guest molecule

The O₂ adsorption on Fe–MOF-74, which has high electron affinity, can induce a largely enhanced FM coupling between the intrachain Fe centers in Fe–MOF-74 through the superexchange interaction compared to Bare Fe–MOF-74, and it also leads to a different intrachain magnetic behavior in comparison with the olefin adsorbed Fe–MOF-74.