Cation-cation interactions: crystal structures of neptunyl(V) selenate hydrates, (NpO2)2(SeO4)(H2O)n (n=1, 2, and 4)

Green crystals of (NpO(2))(2)(SeO(4))(H(2)O)(4), (NpO(2))(2)(SeO(4))(H(2)O)(2), and (NpO(2))(2)(SeO(4))(H(2)O) have been prepared by hydrothermal methods. The structures of these compounds have been characterized by single-crystal X-ray diffraction. (NpO(2))(2)(SeO(4))(H(2)O)(4), isostructural with (NpO(2))(2)(SO(4))(H(2)O)(4), is constructed from layers comprised of corner-sharing neptunyl(V) pentagonal bipyramids and selenate tetrahedra that are further linked by hydrogen bonding with water molecules. Each NpO(2)(+) cation binds to four other NpO(2)(+) units through cation-cation interactions (CCIs) to form a distorted "cationic square net" decorated by SeO(4)(2-) tetrahedra above and below the layer. Each selenate anion is bound to two neptunyl(V) cations through monodentate linkages. (NpO(2))(2)(SeO(4))(H(2)O)(2) is isostructural with the corresponding sulfate analogue as well. It consists of puckered layers of neptunyl(V) pentagonal bipyramids that are further connected by selenate tetrahedra to form a three-dimensional framework. The CCI pattern in the neptunyl layers of dihydrate is very similar to that of tetrahydrate; however, each SeO(4)(2-) tetrahedron is bound to four NpO(2)(+) cations in a mondentate manner. (NpO(2))(2)(SeO(4))(H(2)O) crystallizes in the monoclinic space group P2(1)/c, which differs from the (NpO(2))(2)(SO(4))(H(2)O) orthorhombic structure due to the slightly different connectivities between NpO(2)(+) cations and anionic ligands. The structure of (NpO(2))(2)(SeO(4))(H(2)O) adopts a three-dimensional network of distort neptunyl(V) pentagonal bipyramids decorated by selenate tetrahedra. Each NpO(2)(+) cation connects to four other NpO(2)(+) units through CCIs and also shares an equatorial coordinating oxygen atom with one of the other units in addition to the CC bond to form a dimer. Each SeO(4)(2-) tetrahedron is bound to five NpO(2)(+) cations in a monodentate manner. Magnetic measurements obtained from the powdered tetrahydrate are consistent with a ferromagnetic ordering of the neptunyl(V) spins at 8(1) K, with an average low temperature saturation moment of 1.98(8) μ(B) per Np. Well above the ordering temperature, the susceptibility follows Curie-Weiss behavior, with an average effective moment of 3.4(2) μ(B) per Np and a Weiss constant of 14(4) K. Correlations between lattice dimensionality and magnetic behavior are discussed.     

An isomorphous series of cubic, copper-based triazolyl isophthalate MOFs: linker substitution and adsorption properties

An isomorphous series of 10 microporous copper-based metal-organic frameworks (MOFs) with the general formulas (∞)(3)[{Cu(3)(μ(3)-OH)(X)}(4){Cu(2)(H(2)O)(2)}(3)(H-R-trz-ia)(12)] (R = H, CH(3), Ph; X(2-) = SO(4)(2-), SeO(4)(2-), 2 NO(3)(2-) (1-8)) and (∞)(3)[{Cu(3)(μ(3)-OH)(X)}(8){Cu(2)(H(2)O)(2)}(6)(H-3py-trz-ia)(24)Cu(6)]X(3) (R = 3py; X(2-) = SO(4)(2-), SeO(4)(2-) (9, 10)) is presented together with the closely related compounds (∞)(3)[Cu(6)(μ(4)-O)(μ(3)-OH)(2)(H-Metrz-ia)(4)][Cu(H(2)O)(6)](NO(3))(2)·10H(2)O (11) and (∞)(3)[Cu(2)(H-3py-trz-ia)(2)(H(2)O)(3)] (12(Cu)), which are obtained under similar reaction conditions. The porosity of the series of cubic MOFs with twf-d topology reaches up to 66%. While the diameters of the spherical pores remain unaffected, adsorption measurements show that the pore volume can be fine-tuned by the substituents of the triazolyl isophthalate ligand and choice of the respective copper salt, that is, copper sulfate, selenate, or nitrate.     

Fe(3+) coordinated to amino-functionalized MCM-41: an adsorbent for the toxic oxyanions with high capacity, resistibility to inhibiting anions, and reusability after a simple treatment

Fe(3+) coordinated to amino ligands fixed on MCM-41 mesoporous silica works as a strong adsorbent for toxic oxyanions. The maximum adsorption amounts were 1.56, 0.99, 0.81, and 1.29 mmol g(-1) for arsenate, chromate, selenate, and molybdate, respectively. When the initial concentrations of oxyanions were less than 1 mmol l(-1), they were removed completely by adsorption and the distribution coefficients K(d) were found to be more than 200000. Inhibition of oxyanion adsorption by abundant competing anions found in nature, NO(3)(-), SO(4)(-), PO(4)(3)(-) and Cl(-), was evaluated at adsorption saturation. Among these anions, the adsorption of the oxyanions was inhibited most in the presence of PO(4)(3)(-) , with which the selectivities for the target oxyanions were still more than 80%. The other coexisting anions, NO(3)(-), SO(4)(-), PO(4)(3)(-) and Cl(-), had little influence on adsorption of the oxyanions except in the case of selenate removal from sulfate solution. The high ability and selectivity to the target oxyanions are attributed to specific interactions between Fe(3+) and the oxyanions. The acid treatment and re-coordination of Fe(3+) lead to a reactivation of the used adsorbent, in which 87-90% of the original adsorption capacity was obtained and the oxyanion/Fe stoichiometries were not changed.     

Oxyanion selectivity in sulfate and molybdate transport proteins: an ab initio/CDM study

A striking feature of sulfate (SO(4)(2-)) and molybdate (MoO(4)(2-)) transport proteins, such as SBP and ModA, which specifically bind SO(4)(2-) and MoO(4)(2-), respectively, is their ability to discriminate very similar anions with the same net charge, geometry, and hydrogen-bonding properties. Here, we determine to what extent (1) oxyanion-solvent interactions, (2) oxyanion-amino acid interactions, and (3) the anion-binding pocket sizes of the cognate protein contribute to the anion selectivity process in SO(4)(2-) and MoO(4)(2-) transport proteins by computing the free energies for replacing SO(4)(2-) with MoO(4)(2)(-)/WO(4)(2-) in model SO(4)(2-)-binding sites of varying degrees of solvent exposure using a combined quantum mechanical/continuum dielectric approach. The calculations reveal that MoO(4)(2-) transport proteins, such as ModA, specifically bind MoO(4)(2-)/WO(4)(2-) but not SO(4)(2-), mainly because the desolvation penalty of MoO(4)(2-)/WO(4)(2-) is significantly less than that of SO(4)(2-) and, to a lesser extent, because the large and rigid cavity in these proteins attenuates ligand interactions with SO(4)(2-), as compared to MoO(4)(2-). On the other hand, SO(4)(2-) transport proteins prefer SO(4)(2-) to MoO(4)(2-)/WO(4)(2-) because the small anion-binding pocket characteristic of these proteins inhibits binding of the larger MoO(4)(2-) and WO(4)(2-) anions. The calculations also help to explain the absence of positively charged Lys/Arg side chains in the anion-binding sites of SBP and ModA. During evolution, these transport proteins may have excluded cationic ligands from their binding sites because, on one hand, Lys/Arg do not contribute to the selectivity of the binding pocket and, on the other, they substantially stabilize the complex between the oxyanion and protein ligands, which in turn would prohibit the rapid release of the bound oxyanion at a certain stage during the transport process.     

Sulfates of the refractory metals: crystal structure and thermal behavior of Nb2O2(SO4)3, MoO2(SO4), WO(SO4)2, and two modifications of Re2O5(SO4)2

The sulfates Nb(2)O(2)(SO(4))(3), MoO(2)(SO(4)), WO(SO(4))(2,) and two modifications of Re(2)O(5)(SO(4))(2) have been synthesized by the solvothermal reaction of NbCl(5), WOCl(4), Re(2)O(7)(H(2)O)(2), and MoO(3) with sulfuric acid/SO(3) mixtures at temperatures between 200 and 300 °C. Besides the X-ray crystal structure determination of all compounds, the thermal behavior was investigated using thermogravimetric studies. WO(SO(4))(2) (monoclinic, P2(1)/n, a = 7.453(1) ?, b = 11.8232(8) ?, c = 7.881(1) ?, β = 107.92(2)°, V = 660.7(1) ?(3), Z = 4) and both modifications of Re(2)O(5)(SO(4))(2) (I: orthorhombic, Pba2, a = 9.649(1) ?, b = 8.4260(8) ?, c = 5.9075(7) ?, V = 480.27(9) ?(3), Z = 2; II: orthorhombic, Pbcm, a = 7.1544(3) ?, b = 7.1619(3) ?, c = 16.8551(7) ?, V = 863.64(6) ?(3), Z = 4) are the first structurally characterized examples of tungsten and rhenium oxide sulfates. Their crystal structure contains layers of sulfate connected [W═O] moieties or [Re(2)O(5)] units, respectively. The cohesion between layers is realized through weak M-O contacts (343-380 pm). Nb(2)O(2)(SO(4))(3) (orthorhombic, Pna2(1), a = 9.9589(7) ?, b = 11.7983(7) ?, c = 8.6065(5) ?, V = 1011.3(1) ?(3), Z = 4) represents a new sulfate-richer niobium oxide sulfate. The crystal structure contains a three-dimensional network of sulfate connected [Nb═O] moieties. In MoO(2)(SO(4)) (monoclinic, I2/a, a = 8.5922(6) ?, b = 12.2951(6) ?, c = 25.671(2) ?, β = 94.567(9)°, V = 2703.4(3) ?(3), Z = 24) [MoO(2)] units are connected through sulfate ions to a three-dimensional network, which is pervaded by channels along [100] accommodating the terminal oxide ligands. In all compounds except WO(SO(4))(2), the metal ions are octahedrally coordinated by monodentate sulfate ions and oxide ligands forming short M═O bonds. In WO(SO(4))(2), the oxide ligand and two monodentate and two bidentate sulfate ions build a pentagonal bipyramid around W. The thermal stability of the sulfates decreases in the order Nb > Mo > W > Re; the residues formed during the decomposition are the corresponding oxides.     

Pt- (and Co-) Promoted Molybdena-Alumina Catalysts: Analysis of the Impregnation Steps

The equilibrium parameters for the adsorption of Mo(VI) on gamma-Al(2)O(3) and of Co(II) and Pt(IV) on MoO(3)/gamma-Al(2)O(3) were determined. The adsorption isotherms were performed from aqueous solutions of the corresponding precursors on two different alumina supports. According to the classification given by Giles, L-type-shaped, subgroup 2, adsorption curves were found for the system Mo on gamma-Al(2)O(3), L-type, subgroup 1, for the Pt on MoO(3)/gamma-Al(2)O(3), and S-type for Co on the MoO(3)/gamma-Al(2)O(3) system. Numerical calculations were carried out for all the isotherms to find the equilibrium parameters. These constants are being used to model the development of Pt, Co, and Mo profiles on MoO(3)/gamma-Al(2)O(3) or gamma-Al(2)O(3) extrudates, respectively, which belong to the new generation of noble-metal-MoO(3)/gamma-Al(2)O(3)-supported catalysts to be used in oil-refining processes. Copyright 2001 Academic Press.     

Thorium(IV)-selenate clusters containing an octanuclear Th(IV) hydroxide/oxide core

Four Th(IV) hydroxide/oxide clusters have been synthesized from aqueous solution. The structures of [Th(8)(μ(3)-O)(4)(μ(2)-OH)(8)(H(2)O)(15)(SeO(4))(8)·7.5H(2)O] (1), [Th(8)(μ(3)-O)(4)(μ(2)-OH)(8)(H(2)O)(17)(SeO(4))(8)·nH(2)O] (2), [Th(9)(μ(3)-O)(4)(μ(2)-OH)(8)(H(2)O)(21)(SeO(4))(10)] (3), and Th(9)(μ(3)-O)(4)(μ(2)-OH)(8)(H(2)O)(21)(SeO(4))(10)·nH(2)O (4) were determined using single crystal X-ray diffraction. Each structure consists of an octanuclear core, [Th(8)O(4)(OH)(8)](16+), that is built from eight Th(IV) atoms (four Th in a plane and two up and two down) linked by four "inner" μ(3)-O and eight "outer" μ(2)-OH groups. Compounds 3 and 4 additionally contain mononuclear [Th(H(2)O)(5)(SeO(4))(4)](4-) units that link the octamers into an extended structure. The octanuclear units are invariably complexed by two selenate anions that sit in two cavities formed by four planar Th(IV) and four extra-planar Th(IV) atoms, thus making [Th(8)O(4)(OH)(8)(SeO(4))(2)](12+) a common building block in 1-4. However, changes in hydration as well selenate coordination give rise to structural differences that are observed in the extended structures of 1-4. The compounds were also characterized by Raman spectroscopy. Density functional theory calculations were performed to predict the geometries, vibrational frequencies, and relative energies of different structures. Details of the calculated structures are in good agreement with experimental results, and the calculated frequencies were used to assign the experimental Raman spectra. On the basis of an analysis of the DFT results, the compound Th(8)O(8)(OH)(4)(SeO(4))(6) was predicted to be a strong gas phase acid but is reduced to a weak acid in aqueous solution. Of the species studied computationally, the dication Th(8)O(6)(OH)(6)(SeO(6))(6)(2+) is predicted to be the most stable in aqueous solution at 298 K followed by the monocation Th(8)O(7)(OH)(5)(SeO(6))(6)(+).     

Influence of the cation size on the framework structures and space group centricities in AMo2O5(SeO3)2 (A = Sr, Pb, and Ba)

Two new quaternary mixed-metal selenites, SrMo(2)O(5)(SeO(3))(2) and PbMo(2)O(5)(SeO(3))(2), have been synthesized as crystals and pure polycrystalline phases by standard solid-state reactions using SrMoO(4), PbO, MoO(3), and SeO(2) as reagents. The crystal structures of the reported materials have been determined by single-crystal X-ray diffraction. SrMo(2)O(5)(SeO(3))(2) and PbMo(2)O(5)(SeO(3))(2) are isostructural and crystallized in the triclinic centrosymmetric space group P1? (No. 2). The reported materials exhibit chain structures consisting of MoO(6) octahedra and asymmetric SeO(3) polyhedra. Complete characterizations including IR spectroscopy and thermal analyses for the compounds are also presented, as are dipole moment calculations. In addition, the powder second-harmonic-generating (SHG) properties of noncentrosymmetric polar BaMo(2)O(5)(SeO(3))(2) have been measured using 1064 nm radiation. Through powder SHG measurement, we are able to determine that BaMo(2)O(5)(SeO(3))(2) has a SHG efficiency of approximately 80 times that of α-SiO(2). Additional SHG measurements reveal that the material is phase-matchable (type 1). A detailed cation size effect on the symmetry and framework structure is discussed.     

Competitive Adsorption of Ferricyanide and Ferrocyanide on gamma-Al(2)O(3) Surface

In the past 3 decades, research has proven the significance of competitive adsorption in the equilibrium of pollutants between solid and liquid phases. However, studies on the competitive adsorption of complex ions are very limited in spite of its important role in transporting pollutants in the natural environment. The objective of this study is to derive the thermodynamic parameters of the competitive adsorption between ferricyanide and ferrocyanide from the modified Langmuir isotherm and the triple-layer model (TLM) to determine the location of adsorption. The effects of pH, temperature, and ion concentration on competitive adsorption onto gamma-Al(2)O(3) were investigated. The results demonstrate that ferrocyanide is more competitive than ferricyanide. By comparing the derived K(app) with K(int), we inferred that the adsorption of ferricyanide and ferrocyanide onto gamma-Al(2)O(3) was achieved through outer-sphere complexation. The negative DeltaH degrees indicated that the adsorption was exothermic. The positive entropy (Delta S degrees ) was caused by the replacement and release of a greater number of smaller surface ions by adsorbed ferricyanide and ferrocyanide ions of larger size. Copyright 2000 Academic Press.     

Structural,vibrational and thermal properties of a new inorganic compound: Tl0.92(NH4)0.08(SO4)0.65(SeO4)0.35Te(OH)6

Research on Chemical Intermediates - A new compound thallium ammonium sulfate selenate tellurate: Tl0.92(NH4)0.08(SO4)0.65(SeO4)0.35Te(OH)6 (TlNSSeTe) has been synthesized by slow evaporation at...     

Syntheses, crystal structures and SHG properties of a series of polar alkali-metal molybdenum(VI) selenites based on strandberg-type [Mo5O15(SeO3)2]4- polyanion

Four new quaternary molybdenum selenites, namely, HRb(3)(Mo(5)O(15))(SeO(3))(2)(H(2)O)(2)1, α-Rb(4)Mo(5)O(15)(SeO(3))(2)(H(2)O)(2)2, β-Rb(4)Mo(5)O(15)(SeO(3))(2)(H(2)O)(2)3 and K(4)Mo(5)O(15)(SeO(3))(2)(H(2)O)(2)4 were synthesized by hydrothermal reactions. All of the four compounds feature a zero-dimensional (0D) [(Mo(5)O(15))(SeO(3))(2)](4-) anionic unit composed of a five-member MoO(6) octahedral ring capped by two SeO(3)(2-) trigonal pyramids, with the Rb(+)/K(+) or/and H(+) cations and water molecules acting as spacers and keeping charge balance. Although these compounds exhibit similar chemical formula, their structures are slightly different. HRb(3)(Mo(5)O(15))(SeO(3))(2)(H(2)O)(2)1 crystallizes in a polar space group (Pca2(1)). α-Rb(4)Mo(5)O(15)(SeO(3))(2)(H(2)O)(2)2 crystallizes in a centrosymmetric (CS) space group (P2(1)/n) whereas β-Rb(4)Mo(5)O(15)(SeO(3))(2)(H(2)O)(2)3 and K(4)Mo(5)O(15)(SeO(3))(2)(H(2)O)(2)4 are isomorphous, crystallize in a chiral space group (C2). The chiral structures of 3 and 4 contain two similar polyanions of [Mo(5)O(15)(SeO(3))(2)](4-) with opposite handedness. Second-harmonic-generation (SHG) measurements indicate that 1, 3 and 4 are all SHG-active. Compound 1 displays a weak SHG response of about 20% of that of KDP (KH(2)PO(4)) and is phase-matchable whereas the SHG responses of 3 and 4 are very weak (less than 5% of that of KDP). Thermal analyses and optical property measurements have also been performed.     

Influence of lattice interactions on the Jahn-Teller distortion of the [Cu(H2O)6]2+ ion: dependence of the crystal structure of K2[Cu(H2O)6](SO4)(2x)(SeO4)(2-2x) upon the sulfate/selenate ratio

The temperature dependence of the structure of the mixed-anion Tutton salt K2[Cu(H2O)6](SO4)(2x)(SeO4)(2-2x) has been determined for crystals with 0, 17, 25, 68, 78, and 100% sulfate over the temperature range of 85-320 K. In every case, the [Cu(H2O)6]2+ ion adopts a tetragonally elongated coordination geometry with an orthorhombic distortion. However, for the compounds with 0, 17, and 25% sulfate, the long and intermediate bonds occur on a different pair of water molecules from those with 68, 78, and 100% sulfate. A thermal equilibrium between the two forms is observed for each crystal, with this developing more readily as the proportions of the two counterions become more similar. Attempts to prepare a crystal with approximately equal amounts of sulfate and selenate were unsuccessful. The temperature dependence of the bond lengths has been analyzed using a model in which the Jahn-Teller potential surface of the [Cu(H2O)6]2+ ion is perturbed by a lattice-strain interaction. The magnitude and sign of the orthorhombic component of this strain interaction depends on the proportion of sulfate to selenate. Significant deviations from Boltzmann statistics are observed for those crystals exhibiting a large temperature dependence of the average bond lengths, and this may be explained by cooperative interactions between neighboring complexes.     

Synthesis, characterization, and structure-property relationships in two new polar oxides: Zn2(MoO4)(SeO3) and Zn2(MoO4)(TeO3)

Two new noncentrosymmetric (NCS) polar oxide materials, Zn(2)(MoO(4))(AO(3)) (A = Se(4+) or Te(4+)), have been synthesized by hydrothermal and solid-state techniques. Their crystal structures have been determined, and characterization of their functional properties (second-harmonic generation, piezoelectricity, and polarization) has been performed. The isostructural materials exhibit a three-dimensional network consisting of ZnO(4), ZnO(6), MoO(4), and AO(3) polyhedra that share edges and corners. Powder second-harmonic generation (SHG) measurements using 1064 nm radiation indicate the materials exhibit moderate SHG efficiencies of 100 × and 80 × α-SiO(2) for Zn(2)(MoO(4))(SeO(3)) and Zn(2)(MoO(4))(TeO(3)), respectively. Particle size vs SHG efficiency measurements indicate the materials are type 1 non-phase-matchable. Converse piezoelectric measurements resulted in d(33) values of ～14 and ～30 pm/V for Zn(2)(MoO(4))(SeO(3)) and Zn(2)(MoO(4))(TeO(3)), respectively, whereas pyroelectric measurements revealed coefficients of -0.31 and -0.64 μC/m(2) K at 55 °C for Zn(2)(MoO(4))(SeO(3)) and Zn(2)(MoO(4))(TeO(3)), respectively. Frequency-dependent polarization measurements confirmed that all of the materials are nonferroelectric; that is, the macroscopic polarization is not reversible, or "switchable". Infrared, UV-vis, thermogravimetric, and differential thermal analysis measurements were also performed. First-principles density functional theory (DFT) electronic structure calculations were also done. Crystal data: Zn(2)(MoO(4))(SeO(3)), monoclinic, space group P2(1) (No. 4), a = 5.1809(4) ?, b = 8.3238(7) ?, c = 7.1541(6) ?, β = 99.413(1)°, V = 305.2(1) ?(3), Z = 2; Zn(2)(MoO(4))(TeO(3)), monoclinic, space group P2(1) (No. 4), a = 5.178(4) ?, b = 8.409(6) ?, c = 7.241(5) ?, β = 99.351(8)°, V = 311.1(4) ?(3), Z = 2.     

Peroxomolybdate(VI)-citrate and -malate complex interconversions by pH-dependence. Synthetic, structural and spectroscopic studies

The reaction of potassium molybdate(VI) with biologically relevant ligands, citric and malic acids, in the presence of H2O2 was investigated for the effect of pH variations on the product pattern. That with citric acid led to the formation of the monomeric complex K4[MoO(O2)2(cit)].4H2O (1) in the pH range 7-9, and dimer K5[MoO(O2)(2-)(Hcit)H(Hcit)(O2)2OMo].6H2O (2) (H4cit = citric acid) at pH 3-6 through carboxylate-carboxylic acid hydrogen bonding. The relation with the previously identified K4[MoO3(cit)].2H2O (4) and K4[Mo2O5(Hcit)2].4H2O (5) were shown. These and other intermediates were shown to react in the pH range 3-6 to give a more stable species 2; the reaction sequence was demonstrated either by the protonation from 1 or the deprotonation of [MoO(O2)2(H2cit)](2-) (8). Evidence that 2 exists as a dimer in solution is presented. The reaction with (S)-malic acid afforded Delta-K(2n)[MoO(O2)2((S)-Hmal)]n.nH2O (3) (H3mal = malic acid) that was oxidized further to oxalato molybdate (11) by H2O2. The three complexes 1-3 were characterized by elemental analysis, UV, IR and NMR spectroscopies, in addition to the X-ray structural studies that show citrate and malate being coordinated as bidentate ligands via alpha-alkoxyl and alpha-carboxylate groups. The formation of these complexes is dictated by pH and their thermal stabilities varied with the coordinated hydroxycarboxylate ligands.     

Influence of the crystal field stabilization energy of metal(II) ions on the structural distortion of matrix-isolated SO4(2-) guest ions in selenate matrices

Infrared spectra of metal(II) selenate hydrates (MeSeO4.nH2O and Na2Me(SeO4)2.2H2O; n=6, 5, 4, 1; Me=Mg, Mn, Co, Ni, Cu, Zn, Cd) containing matrix-isolated SO42- guest ions are reported and discussed with respect to the S-O stretching modes 3 and 1. An adequate measure for the SO42- guest ion distortion is the site group splitting deltanuas (deltanuab and deltanuac in the case of a doublet and a triplet for 3, respectively; a, being the highest wavenumbered component of nu3) and deltanumax (the difference between the highest and the lowest wave numbered S-O stretching modes). It has been shown that the SO42- guest ion distortion depends on both the number of the sulfate oxygen atoms involved in coordinative bonds with the metal(II) ions and the electronic configuration of the metal(II) ions, i.e. their crystal field stabilization energy (CFSE) additionally to the site symmetry and the local potential at the lattice site of the host lattice. The SO42- guest ions matrix-isolated in MeSeO4.H2O (Me=Mn, Co, Zn) and in Na2Me(SeO4)2.2H2O (Me=Mn, Cu, Cd) exhibit three bands corresponding to the nu3 modes as deduced from the site group analysis and deltanuab approximately equal to deltanubc. When SO42- guest ions are incorporated in the triclinic Na2Me(SeO4)2.2H2O host lattices (Me=Co, Ni, Zn) the nu3 stretching region resembles a higher local symmetry of the SO42- guest ions (an approximate (A1 + E) splitting) than the crystallographic one (i.e. deltanuab>deltanubc instead of deltanuab approximately equal to deltanubc) and, hence, the ratio deltanuab/deltanubc has to be taken into account (the higher value of the ratio deltanuab/deltanubc, the weaker is the distortion of the SO42- guest ions). The SO42- guest ions incorporated in MeSeO4.nH2O (n=6, 5, 4) exhibit a higher local symmetry of the guest ions than that deduced from the site group analysis (D2d for the SO42- guest ions in MeSeO4.5H2O, MeSeO4.4H2O and in the monoclinic MeSeO4.6H2O host lattices and close to Td in the tetragonal MeSeO4.6H2O host lattices). The analysis of the infrared spectra of selenate host lattices containing SO42- guest ions reveals that the guest ions are stronger distorted when the adjacent metal(II) ions have CFSE not equal to 0. These ions are more resistant to angular deformations of the MeO6-octahedra (i.e. changes in the O-Me-O bond angles), thus facilitating the SO42- guest ion distortion as compared to those having CFSE=0 which allow stronger angular deformations of the respective metal octahedra. Infrared spectra of kieserite-type compounds MeSeO4.H2O (Me=Mn, Co, Zn) containing matrix-isolated SO42- guest ions and Me'2+ guest ions different from those of the host ions (i.e. Me'SO4.H2O in MeSeO4.H2O) are also presented and discussed (double matrix-spectroscopy).     

Atoms-in-molecules analysis of extended hypervalent five-center, six-electron (5c-6e) C(2)Z(2)O interactions at the 1,8,9-positions of anthraquinone and 9-methoxyanthracene systems

To clarify the nature of five-center, six-electron (5c-6e) C(2)Z(2)O interactions, atoms-in-molecules (AIM) analysis has been applied to an anthraquinone, 1,8-(MeZ)(2)ATQ (1 (Z=Se), 2 (Z=S), and 3 (Z=O)), and a 9-methoxyanthracene system, 9-MeO-1,8-(MeZ)(2)ATC (4 (Z=Se), 5 (Z=S), and 6 (Z=O)), as well as 1-(MeZ)ATQ (7 (Z=Se), 8 (Z=S), and 9 (Z=O)) and 9-MeO-1-(MeZ)ATC (10 (Z=Se), 11 (Z=S), and 12 (Z=O)). The total electronic energy density (H(b)(r(c))) at the bond critical points (BCPs), an appropriate index for weak interactions, has been examined for 5c-6e C(2)Z(2)O and 3c-4e CZO interactions of the n(p)(O)sigma*(Z--C) type in 1-12. Some hydrogen-bonded adducts were also re-examined for convenience of comparison. The total electronic energy densities varied in the following order: OO (3: H(b)(r(c))=0.0028 au)=OO (6: 0.0028 au)>OO (9: 0.0025 au)> or =NNHF (0.0024 au)> or =OO (12: 0.0023 au)>H(2)OHOH (0.0015 au)>SO (8: 0.0013 au)=SO (2: 0.0013 au)> or =SO (11: 0.0012 au)=SO (5: 0.0012 au)>HFHF (0.0008 au)=SeO (10: 0.0008 au)=SeO (4: 0.0008 au)> or =SeO (1: 0.0007 au)> or =SeO (7: 0.0006 au)>HCNHF (-0.0013 au). H(b)(r(c)) values for SO were predicted to be smaller than the hydrogen bond of H(2)OHOH and H(b)(r(c)) values for SeO are very close to or slightly smaller than that for HFHF in both the ATQ and 9-MeOATC systems. In the case of Z=Se and S, H(b)(r(c)) values for 5c-6e C(2)Z(2)O interactions are essentially equal to those for 3c-4e CZO if Z is the same. The results demonstrate that two n(p)(O)sigma*(Z--C) 3c-4e interactions effectively connect through the central n(p)(O) orbital to form the extended hypervalent 5c-6e system of the sigma*(C--Z)n(p)(O)sigma*(Z--C) type for Z=Se and S in both systems. Natural bond orbital (NBO) analysis revealed that n(s)(O) also contributes to some extent. The electron charge densities at the BCPs, NBO analysis, and the total energies calculated for 1-12, together with the structural changes in the PhSe derivatives, support the above discussion.     

Adsorption mechanisms of selenium oxyanions at the aluminum oxide/water interface

Sorption processes at the mineral/water interface typically control the mobility and bioaccessibility of many inorganic contaminants such as oxyanions. Selenium is an important micronutrient for human and animal health, but at elevated concentrations selenium toxicity is a concern. The objective of this study was to determine the bonding mechanisms of selenate (SeO4(2-) and selenite (SeO3(2-) on hydrous aluminum oxide (HAO) over a wide range of reaction pH using extended X-ray absorption fine structure (EXAFS) spectroscopy. Additionally, selenate adsorption on corundum (alpha-Al2O3) was studied to determine if adsorption mechanisms change as the aluminum oxide surface structure changes. The overall findings were that selenite forms a mixture of outer-sphere and inner-sphere bidentate-binuclear (corner-sharing) surface complexes on HAO, selenate forms primarily outer-sphere surface complexes on HAO, and on corundum selenate forms outer-sphere surface complexes at pH 3.5 but inner-sphere monodentate surface complexes at pH 4.5 and above. It is possible that the lack of inner-sphere complex formation at pH 3.5 is caused by changes in the corundum surface at low pH or secondary precipitate formation. The results are consistent with a structure-based reactivity for metal oxides, wherein hydrous metal oxides form outer-sphere complexes with sulfate and selenate, but inner-sphere monodentate surface complexes are formed between sulfate and selenate and alpha-Me2O3.     

Analogue reaction systems of selenate reductase

Analogue reaction systems of selenate reductase, which reduces substrate in the overall enzymatic reaction SeO4(2-) + 2H+ + 2e- --> SeO3(2-) + H2O, have been developed using bis(dithiolene) complexes of Mo(IV) and W(IV). On the basis of the results of EXAFS analysis of the oxidized and reduced enzyme, the minimal reaction Mo(IV)OH + SeO4(2-) --> Mo(VI)O(OH) + SeO3(2-) is probable. The square pyramidal complexes [M(OMe)(S2C2Me2)2](1-) (M = Mo, W) were prepared as structural analogues of the reduced enzyme site. The systems, [ML(S2C2Me2)2](1-)/SeO4(2-) (L = OMe, OPh, SC6H2-2,4,6-Pr(i)3) in acetonitrile, cleanly reduce selenate to selenite in second-order reactions whose negative entropies of activation implicate associative transition states. Rate constants at 298 K are in the 10(-2)-10(-4) M(-1) s(-1) range with DeltaS++ = -12 to -34 eu. When rate constants are compared with previous data for the reduction of (CH2)4SO, Ph3AsO, and nitrate by oxygen atom transfer, reactivity trends dependent on the metal, axial ligand L, and substrate are identified. As in all other cases of substrate reduction by oxo transfer, the kinetic metal effect k(2)W > k(2)Mo holds. A proposal from primary sequence alignments suggesting that a conserved Asp residue is a likely ligand in the type II enzymes in the DMSO reductase family has been pursued by synthesis of the [Mo(IV)(O2CR)(S2C2Me2)2](1-) (R = Ph, Bu(t)) complexes. The species display symmetrical eta2-carboxylate binding and distorted trigonal prismatic stereochemistry. They serve as possible structural analogues of the reduced sites of nitrate, selenate, and perchlorate reductases under the proposed aspartate coordination. Carboxylate binding has been crystallographically demonstrated for one nitrate reductase, but not for the other two enzymes.     

Explorations of new quaternary phases in the Ln(III)-V(V)(d0)-Se(IV)-O (Ln = Nd, Eu, Gd, Tb) systems

Systematic explorations of new phases in the Ln(III)-V(V)-Se(IV)-O systems by hydrothermal syntheses led to four new quaternary compounds, namely, Nd(2)(V(V)(2)O(4))(SeO(3))(4)·H(2)O (1), Ln(V(V)O(2))(SeO(3))(2) (Ln = Eu 2, Gd 3, Tb 4). The structure of Nd(2)(V(V)(2)O(4))(SeO(3))(4)·H(2)O features a 3D framework composed of the 2D layers of [N d(SeO(3))](+) bridged by the infinite [VO(2)(SeO(3))](-) chains with the lattice water molecules located at the 6-membered ring tunnels formed. The structure of Ln(V(V)O(2))(SeO(3))(2) (Ln = Eu, Gd, Tb) also features a 3D framework composed of 2D layers of [Ln(SeO(3))](+) bridged by the infinite [(VO(2))(SeO(3))](-) double chains. The 1D vanadium oxide selenite chain of 1 differs significantly from those in compounds 2-4 in terms of the coordination modes of the selenite groups and the connectivities between neighbouring VO(6) octahedra. Luminescent and magnetic properties of these compounds were also measured.     

N-heterocycle chelated oxomolybdenum(VI and V) complexes with bidentate citrate

A 1,10-phenanthroline (phen) chelated molybdenum(VI) citrate, [(MoO2)2O(H2cit)(phen)(H2O)2] x H2O (1) (H4cit = citric acid), is isolated from the reaction of citric acid, ammonium molybdate and phen in acidic media (pH 0.5-1.0). A citrato oxomolybdenum(V) complex, [(MoO)2O(H2cit)2(bpy)2] x 4H2O (2), is synthesized by the reduction of citrato molybdate with hydrazine hydrochloride in the presence of 2,2'-bipyridine (bpy), and a monomeric molybdenum(VI) citrate [MoO2(H2cit)(bpy)] x H2O (6) is also isolated and characterized structurally. The citrate ligand in the three neutral compounds uses the alpha-alkoxy and alpha-carboxy groups to chelate as a bidentate leaving the two beta-carboxylic acid groups free, that is different from the tridentate chelated mode in the citrato molybdate(VI and V) complexes. 1 and in solution show obvious dissociation based on 13C NMR studies.