Showing papers on "Polyoxometalate published in 1996"

A Novel Optical Complex between an Organic Substrate and a Polyoxometalate. Crystal and Molecular Structure of α-H4SiW12O40·4HMPA·2H2O (HMPA = Hexamethylphosphoramide)

Abstract: A solvated complex of α-H4SiW12O40·4HMPA·2H2O composed the heteropolytungstate α-H4SiW12O40 and the organic substrate hexamethylphosphoramide (HMPA) has been synthesised, purified, and characterized. The electronic spectra (λ = 220−500 nm) as well as the 1H NMR spectra for the title compound dissolved in CD3CN establish that this complex dissociates into free SiW12O404- and HMPA moieties in solution unless the organic substrate HMPA is present in very high concentrations. The solid reflectance electronic spectra and IR spectra indicate that there is interaction between the α-H4SiW12O40 and the organic substrate. The complex has no photosensitivity under irradiation of sunlight, but under the near-UV light result in a charge transfer by oxidation of the HMPA and the reduction of the polyoxometalate. Light yellow polyhedrons of the title compound crystallize from the aqueous solvent of acetonitrile and aqueous solution as the formula of α-H4SiW12O40·4HMPA·2H2O in the monoclinic, space group P21. The unit ce...

Layered Double Hydroxides Intercalated by Polyoxometalate Anions with Keggin (α-H2W12O406-), Dawson (α-P2W18O626-), and Finke (Co4(H2O)2(PW9O34)210-) Structures

Abstract: The pillaring of Mg3Al layered double hydroxides (LDHs) by the title polyoxometalates (POMs) was accomplished by ion exchange reaction of the LDH−hydroxide and −adipate precursors with the POM anion at ambient or refluxing temperatures The structural, thermal and textural properties of the LDH−POM intercalates were elucidated based on XRD, FTIR, TEM, EDS, and N2 adsorption−desorption studies A gallery height of ∼10 A was observed for the LDH intercalated by the symmetrical Keggin POM, whereas two different gallery heights were found for the cylindrical Dawson (145 and 128 A) and Finke (133 and 126 A) anions, depending on the preparation temperature The differences in POM orientations were rationalized in terms of different electrostatic and hydrogen-bonding interactions between the POM pillars and the LDH layers Upon thermal treatment at ≥100 °C, the intercalated Dawson and Finke POM ions exhibited only one gallery orientation, regardless of synthesis conditions The crystalline microporous struct

Polyoxometalate Derivatives with Multiple Organic Groups. 2. Synthesis and Structures of Tris(organotin) Α, Β-Keggin Tungstosilicates

Abstract: Eight tris(organotin)-substituted Keggin tungstosilicate heteropolyanions have been synthesized and characterized by elemental analysis, infrared and Mossbauer spectroscopy, multinuclear NMR, and X-ray crystallography. The new anions contain alpha- or beta-SiW(9)O(34)(10)(-) moieties and are of two structural types, [(RSn)(3)(SiW(9)O(37))](7)(-) (R, isomer: Ph, alpha-, 1; n-Bu, alpha-, 2; Ph, beta-, 3; n-Bu, beta-, 4) and [(RSnOH)(3)(SiW(9)O(34))(2)](14)(-) (Ph, alpha-, 5; n-Bu, alpha-, 6; Ph, beta-, 7; n-Bu, beta-, 8). Crystals of Cs(4)H(3)[(PhSn)(3)(SiW(9)O(37))].8H(2)O (anion 3) are monoclinic, space group C2/c, with lattice constants a = 48.91(2) A, b = 12.111(3) A, c = 20.334(9) A, beta = 102.30 degrees, and Z = 8. The anion has nominal C(3)(v)() symmetry and has a structure with three corner-shared WO(6) octahedra of the beta-Keggin anion replaced by three PhSnO(5) groups. Crystals of Cs(9)H(5)[(BuSnOH)(3)(SiW(9)O(34))(2)].36H(2)O (anion 6) are tetragonal, space group Pf2(1)m, with lattice constants a = b = 29.005(4) A, c = 13.412(4) A, and Z = 4. The anion has the anticipated D(3)(h)() symmetry and contains three BuSnOH groups sandwiched between A,alpha-SiW(9)O(34)(10)(-) anions.

Polyoxometalate Derivatives with Multiple Organic Groups. 3. Synthesis and Structure of Bis(phenyltin) Bis(decatungstosilicate), [(PhSnOH2)2(γ-SiW10O36)2]10-

Abstract: A new phenyltin tungstosilicate derivative, [(PhSnOH2)2(γ-SiW10O36)2]10- (1), has been prepared by reaction of phenyltin trichloride with K8[γ-SiW10O36]·xH2O. The new heteropolyanion was characterized by elemental analysis, infrared spectroscopy, multinuclear NMR, and X-ray crystallography. The crystals of Cs9H[(PhSnOH2)2(γ-SiW10O36)2]·16H2O (Cs salt of 1) are triclinic, space group P1, with lattice constants a = 12.401(3) A, b = 13.832(3) A, c = 16.313(3) A, α = 96.17(2)°, β = 109.73(2)°, γ = 97.13(2)°, V = 2579.9(10) A, and Z = 1. Anion 1 has a structure of virtual C2h symmetry with two phenyltin groups sandwiched between two γ-SiW10 groups. Such a structure is different from all previously reported polytungstates derived from [γ-SiW10O36]8- lacunary anions.

A new dinuclear rhodium(III) ‘sandwich’ polyoxometalate, [(WZnRHIII2)(ZnW9O34)2]10−. Synthesis, characterization and catalytic activity

Abstract: The rhodium containing polyoxometalate, [(WZnRhIII2)(ZnW9O34)2]10− was prepared by reaction of [(WZn3)(ZnW9O34)2]12− with RhCl3. The X-ray diffraction showed this compound to be isostructural to other compounds of this series. This was confirmed by the IR spectra. The elementary analysis and cyclic voltammetry measurements indicated that the polyoxometalate was of high purity. Epoxidation of alkenes with 30% aqueous H2O2 in 1,2-dichloroethane/water biphasic systems using [(WZnRhIII2)(ZnW9O34)2]10− as catalyst showed high overall activity as compared to other noble metal substituted polyoxometalates of this series. The reactions were highly selective to epoxidation of cyclohexene and similar to previously reported [(WZnMnII2)(ZnW9O34)2]12−, except that hydrogen peroxide yields were measurably higher in the rhodium case. The [(WZnRhIII2)(ZnW9O34)2]10− catalyst appears to be stable under turnovers conditions. Using IR spectroscopy as a probe to compare active polyoxometalates (Rh and Mn) versus an inactive one (Zn), showed that in the former case peroxo-tungstate intermediates could be identified whereas for the inactive compound no such intermediate was observed. A possible mechanism taking into account reactivity, steric constraints and IR spectra assuming structural integrity under catalytic turnover conditions was put forth.

Oxidative dehydrogenation of isobutyric acid: Characterization and modeling of vanadium containing polyoxometalate catalysts

Abstract: The catalyzed oxidative dehydrogenation of isobutyric acid by H4[PMo11VO40] and VOH[PMo12O40] was studied at 593 K. Thermal evolution of H4[PMo11VO40] at 593 K leads to a complex mixture of different species with or without vanadium inside the Keggin structure. After reaction, the one and only heteropolyanion is partly reduced [PMo12O40]3− and vanadium is present as vanadyl cations. Vanadyl cations are bound to the oxygen atoms of the heteropolyanion at 593 K and enhance the selectivity to methacrylic acid with respect to acetone. On the other hand, copper cations improve the reoxidation of the catalyst and increase the IBA conversion. Synergism between copper and vanadyl cations was observed.

Preparation and photocatalytic studies on a novel Ti-substituted polyoxometalate

Abstract: The crystal structure of a Ti-substituted Dawson-type polyoxotungstate K 19 H 9 [P 4 W 32 Ti 6 O 132 ] has been determined in the presence of coordinated oxalate anions. The K + salt is water soluble and the [ n -Bu 4 N] + salt is soluble in acetonitrile and acetone. The anion exists as a dimer of two [P 2 W 16 Ti 2 O 62 ] 10− units connected via two external TiO 6 octahedra. The photocatalytic activity of the [ n -Bu 4 N] + salt toward the oxidation of cyclohexanol, 1-hexanol, and 2-hexanol gave pseudo first-order rate constants of 0.03 h −1 , 0.01 h −1 , and 0.01 h −1 , respectively. The relative photocatalytic activities for the oxidation of cyclohexanol by ( n -Bu 4 N) 9 H 19 [P 4 W 32 Ti 6 O 132 ] and various polyoxotungstates of similar structure are compared.

Thermal multi-electron transfer catalysis by polyoxometalates. Application to the practical problem of sustained, selective oxidation of hydrogen sulfide to sulfur

Abstract: Novel polyoxometalate-based catalytic systems for the effective and selective aerobic oxidation of H2S to S(0) are reported. The base stable catalysts TMA9[Nb3P2W15O62], 1, Li33[H7P8W48O184], 2, K10[Zn4P2W18O68], 3, and K4[Nb2W4O19] produce 17, 135, 5.7 and 8.8 turnovers of S8, respectively (23°C; 1.0 atm O2; 4 h; sealed vessel). The stability of these catalysts during sulfide oxidation are evaluated. The redox potentials of these polyoxometalates and the primary fragmentation products pertinent to sulfide oxidation are given. Early time kinetics of the reduction of 1 by sulfide proceeds according to the rate law + d[Nb3P2W15O10−62]/dt = k[1][HS−], with a complex pH dependence. The buffering agent is shown to be non-innocent and at parity of reaction conditions, the reduction of K14[NaP5W30O110], 4, by sulfide in media buffered with PO3−4, CO2−3, B4O2−10, and OH− (a blank) results in 40, 9.0, 49, and 10 turnovers of S8, respectively.

Oxidative delignification of wood or wood pulp by transition metal-substituted polyoxometalates

Abstract: A method for delignifying wood pulp and fiber is disclosed. The method comprises the steps of obtaining a wood pulp and exposing the wood pulp to a polyoxometalate of the formula Vl Mom Wn Nbo Tap (TM)q Xr Os !x- where l is 0-18, m is 0-40, n is 0-40, o is 0-10, p is 0-10, q is 0-9, r is 0-6, TM is a d-electron-containing transition metal ion, and X is a heteroatom, which is a p or d block element, where l+m+n+o+p≧4, l+m+q>0 and s is sufficiently large that x>0. The exposure is under conditions wherein the polyoxometalate is reduced. In a preferable form of the invention, the method additionally comprises the step of reoxidizing the polyoxometalate.

Catalytic oxidation of cyclohexene with molecular oxygen by polyoxometalate-intercalated hydrotalcites

Abstract: Cyclohexene was oxidized with molecular oxygen over transition-metal-substituted polyoxometalate-intercalated hydrotalcites to produce 2-cyclohexene-1-one and 2-cyclohexene-1-ol with high selectivity under mild reaction conditions

Spectroscopic Studies of Polyoxometalates and their Complexes with Lanthanide(III) Ions in Solution

Abstract: The results of investigations related to physico-chemical properties, synthesis and structure of polyoxometalates and their lanthanide(III) complexes are briefly reviewed. Useful techniques for the verifrcation of polyoxometalate compositions and the determination of components are compared. Complexation of metal ions, especially of lanthanide(III) ions, with polyoxometalates using absorption (in the UV-visible and the near IR region) and luminescence spectroscopy methods are presented. Absorption hypersensitive transitions of certain lanthanide(III) ions and laser-induced europium(III) ion luminescence spectroscopy are shown to be useful in complexation investigations. The significance of some polyoxometalate complexes as potential magnetic resonance imaging agents and in biological studies as antiviral agents are discussed. PACS numbers: 42.62.Hk, 78.40.—q, 78.40.Ha, 78.55.-m

Silicone-Polyoxometalate (SiPOM) Hybrid Compounds

Abstract: Oligomeric and polymeric SiPOM materials have been prepared by reacting trichloro endblocked PDMS [degree of polymerization, ∼ 14 (I) and 60 (II)] with α-K 8 SiW 11 O 39 . NMR and IR spectroscopy show that the POM units are bonded to the siloxane via Si-O-W bonds. The reaction of the short PDMS segment (I) with the POM produces powdery materials, (isolated as tetrabutylammonium salts) while the longer PDMS (II) forms elastomeric materials. For the latter the POM anions act as reinforcing fillers. Other important properties of the hybrid materials are their UV absorbency, their electrochromicity, their electrical properties (volume resistivity, ∼10 11 ohm-cm; low loss tangent, memory capability) and their film formation.