Axial Oxo Ligand Research Articles

Extremely low barrier activation of methane on spin-polarized ferryl ion [FeO]2+ at the four-membered ring of zeolite

The Panov's α-O center in ZSM-5 zeolite revealing extraordinary activity in the oxidation of methane to methanol has been a subject of intense experimental and theoretical studies. Consensus on the structure and the origin of α-O superactivity seems to be achieved only in that α-O represents the ferryl group [FeO]2+ (bound by four lattice oxo sites) in cation-exchange position at some zeolite ring containing two framework Al atoms. Among rings with various sizes only three ones (four-, five-, and six-membered) can provide the ferryl ion with the necessary four number of oxo ligands (in the “plane” of ring) at reasonable Fe-O distance of about 2.0 Å. The six-membered ring is considered as the most probable one due to the 29Si MAS NMR data on Al pairs distribution on zeolite rings. However, the six-membered model in ZSM-5 has not been checked on behalf of the possibility of axial Fe-O coordination along Fe and terminal oxygen axis. By means of plane-wave DFT modeling, the present work shows that this coordination in fact takes place for six-membered ring unlike the four- and five-membered ones in which the Fe cation appears well above the plane of ring making the axial coupling impossible. Therefore, by known strong negative effect of axial oxo ligand on the ferryl species oxidizing activity, the six-membered model has to be discarded. Instead, a model of [FeO]2+ at four- or five-membered ring is to be put forward. In this work the four-membered model is shown to form the oxyl ground state Fe(III)–O• with the radical-like β-spin polarized oxygen. Calculated barrier of hydrogen abstraction from methane on this center is as low as 1.7 kcal/mol. The obtained prediction opens a way for other reactions to be probed on this center, particularly, the condensation of methane and oxidative dehydrogenation of hydrocarbons at low temperatures.

Creation of [OUF]+ using gas-phase reactions of [UO2(C6F5)

While the strong axial UO bonds confer high stability and inertness to UO22+, it has been shown that the axial oxo ligands can be eliminated or replaced in the gas-phase using collision-induced dissociation (CID) reactions. In this study, CID of the pentafluorobenzoate precursor ion [UO2(O2C–C6F5)]+ was used to produce the organo-uranyl ion [UO2(C6F5)]+ by decarboxylation. Subsequent CID of [UO2(C6F5)]+ created [UO2(F)]+ by fluoride transfer and elimination of C6F4, rather than UO2+ by elimination of pentafluorophenyl radical (as has been observed for similar species). Moreover, upon reaction of [UO2(C6F5)]+ with H2O, apparent substitution of OH for F to create [UO3HC6F4]+ is favored over hydrolysis to produce [UO2(OH)]+ and (neutral) C6F5H. Subsequent CID of [UO3HC6F4]+ generates [UO2(F)]+ and [OUF]+. When [OUF]+ is isolated to react with H2O and O2 in the ion trap, the principal product ions observed are [UO2(F)]+ and [UO2(OH)]+. Experiments conducted with labeled reagent suggest that reaction with H218O leads to exchange of the oxo ligand and incorporation of the 18O label into [OUF]+ while reaction with O2 likely creates [UO2(F)]+. Formation of [UO2(OH)]+ from [OUF]+ is the result of a cascade of reactions, with initial formation of [UO2(F)]+ by reaction with O2, followed by hydrolysis to create the hydroxide species.

Destruction and reconstruction of UO22+ using gas-phase reactions.

While the strong axial U[double bond, length as m-dash]O bonds confer high stability and inertness to UO22+, it has been shown that the axial oxo ligands can be eliminated or replaced in the gas-phase using collision-induced dissociation (CID) reactions. We report here tandem mass spectrometry experiments initiated with a gas-phase complex that includes UO22+ coordinated by a 2,6-difluorobenzoate ligand. After decarboxylation to form a difluorophenide coordinated uranyl ion, [UO2(C6F2H3)]+, CID causes elimination of CO, and then CO and C2H2 in sequential dissociation steps, to leave a reactive uranium fluoride ion, [UF2(C2H)]+. Reaction of [UF2(C2H)]+ with CH3OH creates [UF2(OCH3)]+, [UF(OCH3)2]+ and [UF(OCH3)2(CH3OH)]+. Cleavage of C-O bonds within these species results in the elimination of methyl cation (CH3+). Subsequent CID steps convert [UF(OCH3)2]+ to [UO2(F)]+ and similarly, [U(OCH3)3]+ to [UO2(OCH3)]+. Our experiments show removal of both uranyl oxo ligands in "top-down" CID reactions and replacement in "bottom-up" ion-molecule and dissociation steps.

Exploring the Oxidation States of Neptunium with Schiff Base Coordination Complexes.

A pair of neptunium Schiff base coordination complexes, NpVIO2L(MeOH) and NpIVL2 {H2L = N,N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine}, have been synthesized and analyzed by several characterization methods including single-crystal X-ray diffraction, electronic absorption, 1H NMR, cyclic voltammetry, and theoretical interpretation. Structural analysis reveals that NpVIO2L(MeOH) and NpIVL2 are isomorphous with the previously reported UVIO2L(MeOH) and MIVL2 (M = Pu, Ce, U, Th) complexes, respectively, allowing for a direct comparison across the series. The reduction of NpVIO2L(MeOH) in situ or direct synthesis from a (NpVO2)+ source shows evidence of a pentavalent neptunyl (NpVO2L)xn- species as determined by UV/vis/NIR and 1H NMR spectroscopy. The synthesis of (NpVO2L)xn- directly from a (NpVO2)+ starting material gives a similar spectrum. Theoretical analysis offers insight into the electronic structure for a better understanding of the bonding patterns and relative stability of the different oxidation states. Computational results show that the Np-L covalent interactions in NpIVL2 are similar to those in the NpVIO2L(MeOH) complex, indicating that neither the presence of the axial oxo ligands nor the oxidation state significantly modify the nature of the Np-L bonds.

Features of Vibrational and Electronic Structures of Decavanadate Revealed by Resonance Raman Spectroscopy and Density Functional Theory.

Polyoxometalates are known to be inhibitors of a diverse collection of enzymes, although the specific interactions that lead to this bioactivity are still unclear. Spectroscopic characterization may be an invaluable if indirect tool for remedying this problem, yet this requires clear, cogent assignment of polyoxometalate spectra before the complicating effect of their binding to large biomolecules can be considered. We report the use of FT-IR and resonance Raman spectroscopies alongside density functional theory to describe the vibrational and electronic structures of decavanadate, [V10O28]6-. Our computational model, which reproduced the majority of vibrational features to within 10 cm-1, was used to identify an axial oxo ligand as the most likely position of the acidic proton in the related cluster [HV10O28]5-. As resonance Raman spectroscopy can directly interrogate chromophores embedded in complex systems, this approach may be of general use in answering structural questions about polyoxometalate-enzyme systems.

Crystal Growth of Cu6(Ge,Si)6O18·6H2O and Assignment of UV-VIS Spectra in Comparison to Dehydrated Dioptase and Selected Cu(II) Oxo-Compounds Including Cuprates

Low-dimensional quantum spin systems with the Cu2+ central ion are still in the focus of experimental and theoretical research. Here is reported on growth of mm-sized single-crystals of the low-dimensional S = 1/2 spin compound Cu6(Ge,Si)6O18·6H2O by a diffusion technique in aqueous solution. A route to form Si-rich crystals down to possible dioptase, the pure silicate, is discussed. Motivated by previously reported incorrect assignments of UV-VIS spectra, the assignment of dd excitations from such spectra of the hexahydrate and the fully dehydrated compound is proposed in comparison to dioptase and selected Cu(II) oxo-compounds using bond strength considerations. Non-doped cuprates as layer compounds show higher excitation energies than the title compound. However, when the antiferromagnetic interaction energy as Jz·ln(2) is taken into account for cuprates, a single linear relationship between the Dqe excitation energy and equatorial Cu(II)-O bond strength is confirmed for all compounds. A linear representation is also confirmed between 2A1g energies and a function of axial and equatorial Cu-O bond distances if auxiliary axial bonds are used for four-coordinated compounds. The quotient Dt/Ds of experimental orbital energies deviating from the general trend to smaller values indicates the existence of H2O respectively Cl− axial ligands in comparison to oxo-ligands, whereas larger Dt/Dqe values indicate missing axial bonds. The quotient of the excitation energy 2A1g by 2·2Eg-2B2g allows checking for correctness of the assignment and to distinguish between axial oxo-ligands and others like H2O or Cl−.

Coordination polymer of uranyl nitrate with 4,4,10,10-Tetramethyl-1,3,7,9-Tetraazaspiro[5.5]undecane-2,8-dione (Spirocarbone, Sk): Synthesis and study of molecular and crystal structures

The coordination polymer {[UO2(NO3)2(C11H20N4O2)] · 2H2O}n (I) was obtained and examined by X-ray diffraction. The crystals are monoclinic, space group C2/c; a = 23.1386(13), b = 5.41575(15), c = 19.7769(11) A, β = 125.285(8)°, V = 2023.01(17) A3, ρcalcd = 2.20 g/cm3, Z = 4. The U atom occupies a special position in the center of inversion. Its coordination polyhedron is a distorted hexagonal bipyramid with axial oxo ligands. In the equatorial plane, the U(1) atom is coordinated by four O atoms of two bidentate nitrate anions and two O atoms of two carbonyl groups of organic spirocarbone (Sk) molecules, which are related by the symmetry operation (0.5 − x, 0.5 − y, −z). In the crystal, polymer chains are parallel to the direction (101). Water molecules are hydrogen-bonded to the N(1) atom of ligand Sk; in addition, they are linked together by the intermolecular hydrogen bonds O(6)-H(6d)…O(6)i(i1/2 − x, −1/2 + y, 1/2 − z); H…O 2.11 A angle O-H…O 160°) and to the nitrate anions by the hydrogen bonds O(6)-H(6e)…O(3)i; H…O 2.29 A; the angle O-H…O 149°). In the crystal, hydrogen-bonded water molecules form chains along the axis y that are perpendicular to the coordination polymers. To verify the purity of complex I, the Rietveld refinement of its X-ray powder diffraction pattern was performed. At room temperature, the unit cell parameters are a = 23.2965(6), b = 5.51225(15), c = 19.8588(6) A, β = 125.6063(17)°, V = 2073.40(10) A3.

Structural Studies of the Molybdenum Center of Mitochondrial Amidoxime Reducing Component (mARC) by Pulsed EPR Spectroscopy and 17O-Labeling

Mitochondrial amidoxime reducing components (mARC-1 and mARC-2) represent a novel group of Mo-containing enzymes in eukaryotes. These proteins form the catalytic part of a three-component enzyme complex known to be responsible for the reductive activation of several N-hydroxylated prodrugs. No X-ray crystal structures are available for these enzymes as yet. A previous biochemical investigation [Wahl, B., et al. (2010) J. Biol. Chem., 285, 37847-37859 ] has revealed that two of the Mo coordination positions are occupied by sulfur atoms from a pyranopterindithiolate (molybdopterin, MPT) cofactor. In this work, we have used continuous wave and pulsed electron paramagnetic resonance (EPR) spectroscopy and density functional theoretical (DFT) calculations to determine the nature of remaining ligands in the Mo(V) state of the active site of mARC-2. Experiments with samples in D(2)O have identified the exchangeable equatorial ligand as a hydroxyl group. Experiments on samples in H(2)(17)O-enriched buffer have shown the presence of a slowly exchangeable axial oxo ligand. Comparison of the experimental (1)H and (17)O hyperfine interactions with those calculated using DFT has shown that the remaining nonexchangeable equatorial ligand is, most likely, protein-derived and that the possibility of an equatorial oxo ligand can be excluded.

Exchangeable oxygens in the vicinity of the molybdenum center of the high-pH form of sulfite oxidase and sulfite dehydrogenase.

The electron spin echo envelope modulation (ESEEM) investigation of the high-pH (hpH) form of sulfite oxidase (SO) and sulfite dehydrogenase (SDH) prepared in buffer enriched with H(2)(17)O reveals the presence of three types of exchangeable oxygen atoms at the molybdenum center. Two of these oxygen atoms belong to the equatorial OH ligand and the axial oxo ligand, and are characterized by (17)O hyperfine interaction (hfi) constants of about 37 MHz and 6 MHz, respectively. The third oxygen has an isotropic hfi constant of 3-4 MHz and likely belongs to a hydroxyl moiety hydrogen-bonded to the equatorial OH ligand. This exchangeable oxygen atom is not observed in the ESEEM spectra of the Y236F mutant of SDH, where the active site tyrosine has been replaced by phenylalanine.

Structural studies of the molybdenum center of the pathogenic R160Q mutant of human sulfite oxidase by pulsed EPR spectroscopy and 17O and 33S labeling.

Electron paramagnetic resonance (EPR) investigation of the Mo(V) center of the pathogenic R160Q mutant of human sulfite oxidase (hSO) confirms the presence of three distinct species whose relative abundances depend upon pH. Species 1 is exclusively present at pH < or = 6, and remains in significant amounts even at pH 8. Variable-frequency electron spin echo envelope modulation (ESEEM) studies of this species prepared with (33)S-labeled sulfite clearly show the presence of coordinated sulfate, as has previously been found for the "blocked" form of Arabidopsis thaliana at low pH (Astashkin, A. V.; Johnson-Winters, K.; Klein, E. L.; Byrne, R. S.; Hille, R.; Raitsimring, A. M.; Enemark, J. H. J. Am. Chem. Soc. 2007, 129, 14800). The ESEEM spectra of Species 1 prepared in (17)O-enriched water show both strongly and weakly magnetically coupled (17)O atoms that can be assigned to an equatorial sulfate ligand and the axial oxo ligand, respectively. The nuclear quadrupole interaction (nqi) of the axial oxo ligand is substantially stronger than those found for other oxo-Mo(V) centers studied previously. Additionally, pulsed electron-nuclear double resonance (ENDOR) measurements reveal a nearby weakly coupled exchangeable proton. The structure for Species 1 proposed from the pulsed EPR results using isotopic labeling is a six-coordinate Mo(V) center with an equatorial sulfate ligand that is hydrogen bonded to an exchangeable proton. Six-coordination is supported by the (17)O nqi parameters for the axial oxo group of the model compound, (dttd)Mo(17)O((17)Otms), where H2dttd = 2,3:8,9-dibenzo-1,4,7,10-tetrathiadecane; tms = trimethylsilyl. Reduction of R160Q to Mo(V) with Ti(III) gives primarily Species 2, another low pH form, whereas reduction with sulfite at higher pH values gives a mixture of Species 1 and 2, as well as the "primary" high pH form of wild-type SO. The occurrence of significant amounts of the "sulfate-blocked" form of R160Q (Species 1) at physiological pH suggests that this species may be a contributing factor to the lethality of this mutation.

Syntheses and structures of 2-acetylpyridine-(2-amino-benzoylhydrazone) and its dioxouranium(VI) complex

We describe the synthesis and characterization of 2-acetylpyridine-(2-aminobenzoylhydrazone) (HL1) and its first dioxouranium complex. The single crystal X-ray structural analysis of the hydrazone, HL1, reveals an almost planar structure with intermolecular hydrogen bonds. The complex [UO2(L1)(dmf)2](NO3) is formed by refluxing a suspension of HL1 with uranyl nitrate in methanol in the presence of a small quantity of triethylamine. Recrystallisation from dmf results in single crystals suitable for X-ray crystallography. The single crystal X-ray study of the complex shows a distorted pentagonal-bipyramidal coordination geometry around the uranium(VI) center, determined by two axial oxo ligands, two nitrogen and one oxygen atoms from a chelating monodeprotonated hydrazone and the oxygen donor atoms of two dimethylformamide molecules.

17O ESEEM Evidence for Exchange of the Axial Oxo Ligand in the Molybdenum Center of the High pH Form of Sulfite Oxidase

A 17O ESEEM investigation of the high pH form of chicken sulfite oxidase using hyperfine sublevel correlation (HYSCORE) spectroscopy at 29.25 GHz has revealed a new type of exchangeable 17O ligand that is characterized by a significantly smaller hyperfine interaction ( approximately 5 MHz) than that previously detected by CW EPR. This new type of exchangeable oxygen ligand is assigned to the axial oxo group of the Mo(V) center.

A versatile entry into aqueous niobium chemistry: isolation and structure of the intermediate Nb 4(μ 2-O) 2(μ 2-OC 2H 5) 4Cl 4(OC 2H 5) 4(HOC 2H 5) 4

The reduction of ethanolic solutions of niobium pentachloride with zinc, followed by treatment with aqueous acids serves as a versatile entry into the aqueous solution chemistry of niobium. From the zinc-reduced solution, the major intermediate, Nb 4(μ 2-O) 2(μ 2-OC 2H 5) 4Cl 4(OC 2H 5) 4(HOC 2H 5) 4, was isolated and the crystal structure determined by X-ray crystallography. The complex crystallizes in the orthorhombic space group Pccn, with Z=4, a=21.0105(9), b=11.0387(5), c=19.1389(8), V=4438.9(3) Å 3, M r=1090.19, R 1=0.0327 and wR 2=0.0876. The structure revealed a centrosymmetric tetrameric Nb(IV) complex, consisting of a pair of edge-sharing bi-octahedral Nb 2(μ 2-OC 2H 5) 4Cl 2(OC 2H 5) 2(HOC 2H 5) 2 units that are joined by two axial oxo ligands. The Nb–Nb distance of 2.7458(3) Å is consistent with a single metal–metal bond.

A versatile entry into aqueous niobium chemistry: isolation and structure of the intermediate Nb4($mu;2-O)2($mu;2-OC2H5)4Cl4(OC2H5)4(HOC2H5)4*1

The reduction of ethanolic solutions of niobium pentachloride with zinc, followed by treatment with aqueous acids serves as a versatile entry into the aqueous solution chemistry of niobium. The major intermediate Nb 4 (μ 2 -O) 2 (μ 2 -OC 2 H 5 ) 4 Cl 4 (OC 2 H 5 ) 4 (HOC 2 H 5 ) 4 was isolated and the crystal structure reported. The reduction of ethanolic solutions of niobium pentachloride with zinc, followed by treatment with aqueous acids serves as a versatile entry into the aqueous solution chemistry of niobium. From the zinc-reduced solution, the major intermediate, Nb 4 (μ 2 -O) 2 (μ 2 -OC 2 H 5 ) 4 Cl 4 (OC 2 H 5 ) 4 (HOC 2 H 5 ) 4 , was isolated and the crystal structure determined by X-ray crystallography. The complex crystallizes in the orthorhombic space group Pccn , with Z =4, a =21.0105(9), b =11.0387(5), c =19.1389(8), V =4438.9(3) Å 3 , M r =1090.19, R 1 =0.0327 and wR 2 =0.0876. The structure revealed a centrosymmetric tetrameric Nb(IV) complex, consisting of a pair of edge-sharing bi-octahedral Nb 2 (μ 2 -OC 2 H 5 ) 4 Cl 2 (OC 2 H 5 ) 2 (HOC 2 H 5 ) 2 units that are joined by two axial oxo ligands. The Nb–Nb distance of 2.7458(3) Å is consistent with a single metal–metal bond.

Facets of Induced Internal Redox Involving WS42 - and WSe42 - : Synthesis and X-Ray Crystal Structures of syn-W2S2(μ-S)2(S2PPh2)2 and WO(Se2)(S2CNEt2)2

Reaction of (NEt4)2[WS4] and [S2PPh2]2 in acetonitrile results in the formation of diamagnetic, orange syn-W2S2(µ-S) 2(S2PPh2)2 by an induced internal redox process. Crystals of the toluene solvate W2S2(µ-S) 2(S2PPh2)2 . C7H8 are triclinic and belong to space group P1– with a 13.845(5), b 16.536(7), c 8.387(2) Å, α 99.75(3), β 95.51(3), γ 83.08(3)˚, and Z 2. Refinement of 5445 data converged at R 0.038 and Rw 0.038. The complex possesses a syn-[W V 2S2(µ-S)2]2+ core with a W–W bond of 2.8161(5) Å and two terminal W=S bonds of 2.097(3) and 2.096(3) Å. The W-(µ-S) bond distances are in the range 2.310(2)–2.315(2) Å. Each of the five-coordinate, square-pyramidal tungsten atoms is further coordinated by a bidentate dithiophosphinate ligand with W–S bond distances ranging from 2.464(2) to 2.481(2) Å. Extended reaction of (NH4)2[WSe4] and [S2CNEt2]2 in acetonitrile produces green WS(Se2)(S2CNEt2)2 and red WO(Se2)(S2CNEt2)2 in 40% and 15% yields, respectively. Crystals of WO(Se2)(S2CNEt2)2 are monoclinic and belong to space group P21/n with a 9.609(6), b 15.053(7), c 13.120(9) Å, β 100.08(4)˚, and Z 4. Refinement of 2473 data converged at R 0.039 and Rw 0.039. The seven-coordinate, pentagonal bipyramidal complex features C1 symmetry, an axial oxo ligand (W=O 1.705(6) Å), an equatorial, side-on bonded diselenido ligand (W–Se(1) 2.509(1), W–Se(2) 2.547(1) Å, Se(1)–Se(2) 2.306(2) Å) and two bidentate dithiocarbamate ligands.

N,N'-Ethylenebis(3-tert-butyl-5-methylsalicylideneaminato)]oxovanadium(IV)

[VO(C26H34N2O2)] was synthesized in order to study its reactivity in the oxygen-transfer catalysis of alkyl­hydroperoxides to a wide array of substituted olefins. The geometry about the v center is nearly square pyramidal with an axial oxo ligand and a four-coordinate substituted Schiff base ligand. The axial V–O bond distance of 1.592 (3) Å is indicative of double-bond character.

Molecular stereochemistry of μ-oxo-bis[5,10,15,20-tetra(4-chlorophenyl)-porphinatoiron(III)]—A model compound of cytochrome P450

A model compound of cytochrome P450, μ-oxo-bis[5,10,15,20-tetra (4-chlorophenyhl) porphinatoiron (III)] [(PCTPPFe) 2O], has been synthesized; its crystal and molecular structures have been determined by the single-crystal diffraction technique. The crystal belongs to the monoclinic system, space group C 2/c, with a = 23.202(6), b = 23.007(9), c = 20.006(9) A ̊ , β = 125.30(3)°, V = 8715.7 A ̊ 3, Z = 4, D c = 1.241 d/cm 3, μ = 6.255 cm −1, F(000) = 3312 . The structure was solved by direct methods and Fourier syntheses and refined by the block diagonal and full matrix least-squares procedures to a final R of 0.082 for the 1698 independent reflections. The FeN p averaged bond distance of 2.08(2) Å is at the long end of the range of high-spin ferric porphyrin, the FeO bond length of 1.743(3) Å is unusually small, and the FeOFe angle of 180° shows a exact linear oxo bridge. The ferric ion is 0.529 Å away from the porphyrin and mean plane toward the axial oxo ligand. A noncoplanar conjugation effect has been found. Comparison of some structure parameters among (TP-PFe) 2O, (TPP(F 5)Fe) 2O, and (PCTPPFe) 2O has been made, and the electronic effect influencing the catalytic activity is discussed.