Showing papers on "Polyoxometalate published in 1999"
TL;DR: In polyoxometalate chemistry, a large variety of compounds, clusters and solid-state structures can be formed by linking together metal-oxygen building blocks as discussed by the authors, resulting for instance in the high-yield synthesis of inorganic superfullerenes and giant ring-shaped, electron-rich, mixed-valence polyoxomolybdates with nanosized cavities.
295 citations
TL;DR: In this article, the authors discuss protonation, oligomerization, and condensation reactions of vanadate(V), molybdate(VI), and tungstate(VI).
Abstract: Publisher Summary This chapter discusses protonation, oligomerization, and condensation reactions of vanadate(V), molybdate(VI), and tungstate(VI). The elements vanadium, molybdenum, and tungsten in their high oxidation states form the stable oxyanions [VO4]3-, [MoO4]2-, and [WO4]2-. In aqueous solution, these ions are easily protonated and show a strong tendency to form polyoxoanions by oxygen bridging, and the release of water molecules. Depending on pH and concentration, a great variety of ions can form in each case, giving rise to complex systems of simultaneous equilibria, which have proved difficult to characterize with certainty. The molybdate(VI) system, in particular, needs further clarification regarding the existence of some polyions. More kinetic and thermodynamic data would also help to improve the understanding of these systems and lead to a general inclusive explanation of the mechanism of polyoxoanion formation. In this respect, new information about some structural preferences of the different metals in mixed polyoxoanions is a useful addition to known facts regarding polyoxometalate structures.
210 citations
TL;DR: The first polyoxometalates incorporating uranyl hetero groups have been synthesized by reaction of Na9(A-PW9O34) with UO2(NO3)2 as discussed by the authors.
Abstract: The first examples of polyoxometalates incorporating uranyl hetero groups have been synthesized by reaction of Na9(A-PW9O34) with UO2(NO3)2. Sodium, ammonium, and potassium salts of (M2(UO2)2- (PW9O34)2) 12- (M ) Na, NH4, K) were characterized by P and W NMR spectroscopy and single-crystal structural analysis. The sodium salt (1) contains an anion of Ci symmetry in which two PW9O34 groups sandwich two UO2 2+ and two Na + cations. The uranium atoms have pentagonal-bipyramidal coordination, achieved by three equatorial bonds to one PW9O34 and two bonds to the other. The sodium cations have approximate tetrahedral coordination geometry. Single-line P and five-line W NMR spectra confirm that the anion structure is maintained in solutions containing excess Na + . The ammonium and potassium salts contain anions in which the UO2 2+ cations (pentagonal bipyramids) are unsymmetrically sandwiched between the PW9O34 groups, as are two closely associated cations, one internal and one external. In solution both, salts give two-line P and nine-line W NMR spectra that are consistent with the solid-state structures. Solution equilibria are rapidly established between the two structure types and are dependent upon the concentrations and identities of the cations. Based on integrated P NMR spectra, the equilibrium constants for (Na2(UO2)2(PW9O34)2) 12- + 2M + T (M2(UO2)2(PW9O34)2) 12- + 2Na + are 128 ( 12 (M ) K + ) and 1.8 ( 0.8 (M ) NH4 + ). The requirement of the second cation in order to satisfy the mass-action expression for Keq, coupled with a W NMR spectrum that implies Cs symmetry, demonstrates that the external ion-paired cation undergoes rapid exchange between several surface sites of the heteropolyanion. Solutions of 1 are stable at pH 7 in the presence of excess sodium cations, and the anion is transferable into toluene by phase-transfer techniques. Addition of calcium ions to solutions of 1 generates a new one-line P NMR spectrum of a Ca derivative which is assumed to be isostructural with 1.
132 citations
TL;DR: In this article, the synthesis and characterization of a tetrameric complex from Keggin ions has been described, which is stable under physiological conditions of the stomach (pH 1-2) and is one of the least toxic and yet most potent antiviral agents of the more than 300 polyoxometalates investigated biologically and pharmacologically.
Abstract: Giant polyoxometalates with catalytic, magnetic, and antiviral properties, which are in part attributable to their structures, are currently of great interest. Herein is described the synthesis and characterization of 1, a structurally novel tetrameric complex from Keggin ions (see picture). This complex is stable under the physiological conditions of the stomach (pH 1-2), which is interesting since related keggin anions are among the least toxic and yet most potent antiviral agents of the more than 300 polyoxometalates investigated biologically and pharmacologically.
93 citations
TL;DR: In this article, the authors describe the strong and spontaneous adsorption of different polyoxometalates on a carbon filled polycarbonate electrode and on glassy carbon, and the modification of the surface occurs by dipping the electrode in a polyoxometricalate solution at open circuit.
82 citations
TL;DR: The polyoxometalate [H2PW9Ni4O34(OH)3(H2O)6]2- (1) has been synthesized and characterized by IR spectroscopy, polarography, elemental analysis, and magnetic measurements as mentioned in this paper.
Abstract: The novel polyoxometalate [H2PW9Ni4O34(OH)3(H2O)6]2- (1) has been synthesized and characterized by IR spectroscopy, polarography, elemental analysis, and magnetic measurements. An X-ray single-crystal analysis was carried out on Cs2[H2PW9Ni4O34(OH)3(H2O)6]·5H2O, which crystallizes in the orthorhombic system, space group Pbcn, with a = 24.3131(3) A, b = 21.5400(1) A, c = 19.8936(3) A, V = 10418.4(2) A3, Z = 8, and ρcalcd = 3.767 g/cm3. Structural analysis based on 14 734 independent reflections leads to a solution, R = 8.04% (I > 2σ(I)). Anion 1 consists of a Ni4O4 cubane fragment which is inserted into a trivacant lacunary B-α-[PW9O34]9- Keggin unit. Magnetic measurements show that the Ni−Ni exchange interactions are ferromagnetic.
64 citations
TL;DR: A new metathetical route to diazoalkane-polyoxometalate complexes is described which allows the introduction of such ligands into previously inaccessible environments.
Abstract: A new metathetical route to diazoalkane complexes is described which allows the introduction of such ligands into previously inaccessible environments. The method, which involves the exchange of oxo and [N2 CR2 ] ligands, is illustrated by the preparation of the first diazoalkane-polyoxometalate complex 1.
61 citations
TL;DR: The impregnation of H5PV2Mo10O40 polyoxometalate onto MCM•41 and amino modified MC•41 materials provided mesoporous active catalysts with large surface areas for aerobic hydrocarbon oxidation using isobutyraldehyde as a reducing agent as mentioned in this paper.
Abstract: The impregnation of H5PV2Mo10O40 polyoxometalate onto MCM‐41 and amino‐modified MCM‐41 materials provided mesoporous active catalysts with large surface areas for aerobic hydrocarbon oxidation using isobutyraldehyde as a reducing agent. The results of the oxidation of alkenes and alkanes gave product selectivities similar to those observed in the corresponding homogeneous reaction although catalytic activity was somewhat reduced. Under appropriate experimental conditions there was no leaching and the solid catalyst could be recovered and reused without loss in activity.
53 citations
TL;DR: In this paper, the authors showed that three hydroxylated donor molecules derived from tetrathiafulvalene (TTF) or ethylenedithiotetrathyfulvalenes (EDT-TTF), whose crystal structures have been solved by X-ray diffraction, formed complex hydrogen bond networks in the solid state where the oxygen atoms of both the hyroxyl groups and the [Mo6O19]2]-anions act as hydrogen bond acceptors.
Abstract: Electrocrystallization of three hydroxylated donor molecules derived from tetrathiafulvalene (TTF) or ethylenedithiotetrathiafulvalene (EDT-TTF), i.e. Me3TTF-CH2OH, EDT-TTF(CH2OH)2 and TTF(CH2OH)4, in the presence of [n-Bu4N+]2[Mo6O19]2– afforded 2∶1 cation radical salts, [donor+˙]2[Mo6O19]2–, whose crystal structures have been solved by X-ray diffraction. In the three different salts complex hydrogen bond networks develop in the solid state where the oxygen atoms of both the hydroxyl groups and the [Mo6O19]2– anions act as hydrogen bond acceptors. The observed hydrogen bonding directed toward one surface, bridging oxygen atom of [Mo6O19]2– is rationalized by an analysis of ab initio calculations of the distribution of electrostatic potentials.
43 citations
TL;DR: In this article, the epoxidation of alkenes with hydrogen peroxide or molecular oxygen, in the presence of aldehyde catalyzed by heteropolyoxometalate (HPOM) salts and layered double hydroxides (LDHs) containing HPOM anions as pillars, has been investigated.
Abstract: The epoxidation of alkenes with hydrogen peroxide or molecular oxygen, in the presence of aldehyde catalyzed by heteropolyoxometalate (HPOM) salts and layered double hydroxides (LDHs) containing HPOM anions as pillars, has been investigated. In the case of oxidation with hydrogen peroxide, a lacunary Keggin-ion, SiW 11 O 39 8− , was more active than SiW 12 O 40 4− . For this reaction system using hydrogen peroxide, intercalated SiW 11 O 39 8− showed higher stability in structure and activity than free K 8 SiW 11 O 39 . For the oxidation using molecular oxygen and aldehyde, SiW 12 O 40 4− showed higher activity than SiW 11 O 39 8− . It was confirmed that SiW 12 O 40 4− was intercalated between the brucite-type layers of the LDHs without change in structure. Ni-containing LDHs without any interstitial polyoxometalate showed high activity for the oxidation of cyclohexene with the combination of O 2 and aldehyde.
39 citations
TL;DR: In this article, a chain-like polymer, [Mo 36 O 108 (NO) 4 (MoO) 2 La 2 (H 2 O) 28 ] n ·56 n H 2 O, was proposed, in which the units are linked to each other by two parallel La atoms.
TL;DR: Combination of anionic [V10O28]6- and cationic [AlO4Al12(OH)24(H2O)12]7+ polyoxometalate clusters in an aqueous solution resulted in immediate hydrogel precipitation formation as discussed by the authors.
Abstract: Combination of anionic [V10O28]6- and cationic [AlO4Al12(OH)24(H2O)12]7+ polyoxometalate clusters in an aqueous solution resulted in immediate hydrogel precipitation formation. This gel has the clu...
TL;DR: In this article, a ring-shaped polyoxometalate and ligand-protected metal chalcogenide clusters provide versatile models for the study of molecular growth processes, especially at the interface to solid-state chemistry.
Abstract: Novel nanosized ring-shaped polyoxometalate and ligand-protected metal chalcogenide clusters provide versatile models for the study of molecular growth processes, especially at the interface to solid-state chemistry. Besides their unusual electronic properties, their use as synthons or precursors for solid-state structures with tailored properties, such as mesoporosity, raises special interest for materials science (polyoxometalate case).
TL;DR: In this paper, the recent achievements of the group in the synthesis of molecular and polymeric materials combining conductivity and magnetism are presented, and the results can be grouped in three main areas: (1) molecular magnetic conductors based upon TTF-type donors and discrete metal complexes and clusters (metal halides, metal cyanides), metal oxalates, polyoxometalate clusters, and (2) conducting molecular ferromagnets based upon extended magnetic layers incorporating donors in between the inorganic layers.
TL;DR: The mixed-valence compound, [C10H10N]4PMo12O40, has been synthesized from N-methylisoquinoline iodide and molybdophosphoric acid, and characterized and recrystallized from DMF solution by diffusing acetone vapor into the solution as mentioned in this paper.
Abstract: The mixed-valence compound, [C10H10N]4PMo12O40, has been synthesized from N-methylisoquinoline iodide and molybdophosphoric acid, and characterized and recrystallized from DMF solution by diffusing acetone vapor into the solution The anion has the Keggin structure Esr data indicate that the single electron in the title compound is delocalized at room temperature, but localized on one molybdenum atom at low temperature Xps data indicate that the binding energy of the elements in the anion decreases after salt formation
TL;DR: In this paper, Zirconium-supported Me(VI) oxide (Me = Cr, Mo, W) was prepared by the equilibrium adsorption method at pH values 2 and 8 by using as a support either hydrous [ZrO2(383)] or thermally pretreated ZirConium oxide [ZRO2T] and the prepared solids were characterized by X-ray photoelectron spectroscopy and analytical methods.
Abstract: Zirconium-supported Me(VI) oxide (Me = Cr, Mo, W) was prepared by the equilibrium adsorption method at pH values 2 and 8 by using as a support either hydrous [ZrO2(383)] or thermally pretreated zirconium oxide [ZrO2(T)]. The prepared solids were characterized by X-ray photoelectron spectroscopy, surface area determinations, electron scanning microscopy and analytical methods. Samples of definite grain size ranges of ZrO2(383) were also compared. The results show that the W-containing samples based on ZrO2(383) differed from Cr- and Mo-samples in the uptake dependence on pH. Despite its higher specific surface area, ZrO2(383) had a smaller W uptake than ZrO2(T). This behavior depends on the polyoxometalate anion size and the microporous structure of hydrous zirconium oxide. The depth of penetration of the Cr(VI), Mo(VI), and W(VI) anionic species in the grains of the ZrO2(383) support was estimated by quantitative XPS measurements and by electron microscopy determinations. The nuclearity of the adsorbed sp...
TL;DR: In this article, the coordination geometry around Cu(II) in the tetrabutylammonium (TBA) salts, with X=P or B, is square pyramidal, with copper bound to the five oxygen donor atoms of the polyoxometalate, whereas for the corresponding potassium salt, a tetragonally elongated pseudo-octahedral geometry was found.
TL;DR: A review of the literature relating to the bonding in polyoxometalate ions is presented in this article, where the author's opinions are presented and expanded to give a bond model based on the following commonly used concepts and principles: Lewis's octet rule, extended to the decet and dodecet rule for MV and MVI.
Abstract: The literature relating to the bonding in polyoxometalate ions is reviewed. The author's opinions are presented and expanded to give a bond model for polyoxometalate ions of the early transition elements (groups V and VI) composed of MO6 octahedra (MOk polyhedra with k>4). This bond model concerns in particular the bond valence (bond lengths) and charge distribution in the polyoxometalate ions and the factors modifying them. It is based on the following commonly used concepts and principles as applied to polyoxometalate ions:
Lewis's octet rule, extended to the decet and dodecet rule for MV and MVI;
pπ-dπ M=O double bond and the coordinate bond (dative bond);
the resonance concept;
the resonance bond number (or the bond valence concept and the valence sum rule);
polydentate ligands and the chelate effect;
the larger space requirements of unshared electron pairs (cf. the VSEPR model);
the model of multicenter pπ-dπ multiple bonds for certain μ-oxo bridges between metal atoms;
Bronsted's acid/base concept and acid/base equilibria;
Pauling's rules for the acid/base strength of (monomeric) oxoacids/oxoanions;
the law of mass action (Le Chatelier's principle), and others.
Journal Article•
TL;DR: The chemistry of heteropolyanions of tungsten and molyb-denum continues to attract much attention, particularly with respect to potential catalytic activity, but also with regards to biological and medical applications.
Abstract: C). This may be attributed mainly to the different positions of the two protons in the most stable (gauche)conformer.IntroductionThe chemistry of heteropolyanions of tungsten and molyb-denum continues to attract much attention, particularly withrespect to potential catalytic activity, but also with respect tobiological and medical applications.
TL;DR: In this paper, the title compound (C 16 H 36 N) 4 [(p-NC-C 6 H 4 As) 4 Mo 12 O 46 ] or (C 7 H 4 N)] was synthesized by the reaction of p-cyanophenylarsonic acid and sodium molybdate at low pH.
Abstract: Crystals of the title compound, (C 16 H 36 N) 4 [(p-NC-C 6 H 4 As) 4 Mo 12 O 46 ] or (C 16 H 36 N) 4 [{As(C 7 H 4 N)} 4 Mo 12 O 46 ], were synthesized by the reaction of p-cyanophenylarsonic acid and sodium molybdate at low pH. With nitrile groups pointed in tetrahedral directions, these new organically modified polyoxometalate clusters are potential building blocks in coordination networks.
TL;DR: The polyoxometalate Na6H4 [Bi2Ni2W20O70(H2O)6] was obtained by substitution of Ni2+ in preformed aqueous [Bi 2W22O74(OH)2] with two β-B-BiW9O33 units sharing vertices of octahedra with two WO6 and two NiO3(H 2O)3 bridging groups as mentioned in this paper.
Abstract: The polyoxometalate Na6H4[Bi2Ni2W20O70(H2O)6].36H2O prepared by substitution of Ni2+ in preformed aqueous [Bi2W22O74(OH)2] contains two β-B-BiW9O33 units sharing vertices of octahedra with two WO6 and two NiO3(H2O)3 bridging groups. The anion was previously prepared from aqueous Bi3+, Ni2+, and WO4 as the Na10 salt [Rodewald & Jeannin (1998). C. R. Acad. Sci. Ser. IIc, 1, 175–181].
01 Jan 1999
TL;DR: A new delignification technology based on the use of polyoxometalates (POMs) is under development at the USDA Forest Service, Forest Products Labotatory (FPL), in Madison, Wisconsin this article.
Abstract: A new delignification technology based on the use of polyoxometalates (POMs) as delignification agents is under development at the USDA Forest Service, Forest Products Labotatory (FPL), in Madison, Wisconsin. These reagents are chlorine free and can be used under conditions wherein they oxidize lignin and chromophores in wood pulp fibers while leaving the cellulose undamaged. Their promise is enhanced by the fact that they can be re-activated with oxygen under conditions that result in oxidation of the organic byproducts of the delignification process. Thus, they can be continuously recycled in a closed system that promises to provide the basis for a new class of closed mill technologies in which the consumable oxidant is oxygen, and the primary byproducts are carbon dioxide and water. The first POM systems investigated included α-(PV2Mo10O40] 5– α-[SiVW11O40] 5– . The former is effective in both delignification and wet oxidation, but it is not stable at pH levels greater than 4 and can result in unacceptable hydrolytic degradation of cellulose. The latter, α-[SiVW11O40] , is more effective at delignification and is useable at pH levels up to 6 or 7, but it is more difficult to re-oxidize. A new group of POMs are now under development; they combine the most attractive features of each of the members of the first group. In addition, they are more easily synthesized and they have an inherent self buffering capacity. Three members of this new group of POMs are under investigation and each has been found to be effective in both delignification and the regeneration/wet-oxidation. The new generation of POMs includes α-[SiV2W10O40] 6– and α-[A1VW11O40] 6–, as well as a mixed tungstatemolybdate system. The presentation will provide an overview of the development of these new delignification systems and their potential as the basis for new pulping and bleaching technologies. The POMs have now also been applied to linerboard pulps at kappa levels of 65. They reduced the kappa to below 10, while maintaining the viscosity above 20 mPa•s. Therefore, in addition to providing an alternative closed-mill technology for traditional delignification stages in bleaching, POM-based processes can also be considered as a basis for accepting the pulp in the bleach plant at higher kappa levels. This possibility allows for reduction of the load on the digesters and recovery boiler, thereby increasing the capacity of kraft mills at a significantly lower level of investment.
TL;DR: Combination of anionic [V10O28]6- and cationic [AlO4Al12(OH)24(H2O)12]7+ polyoxometalate clusters in an aqueous solution resulted in immediate hydrogel precipitation formation as discussed by the authors.
Abstract: Combination of anionic [V10O28]6- and cationic [AlO4Al12(OH)24(H2O)12]7+ polyoxometalate clusters in an aqueous solution resulted in immediate hydrogel precipitation formation. This gel has the clu...
Patent•
11 Mar 1999
TL;DR: In this article, a method of delignifying lignocellulosic fibers is disclosed, which comprises the steps of combining a polyoxometalate complex with aluminum heteroatom.
Abstract: not available for EP1017899Abstract of corresponding document: WO9911857A method of delignifying lignocellulosic fibers is disclosed. In one embodiment, the method comprises the steps of combining a polyoxometalate complex with aluminum heteroatom of the formula [Al1VmMonWoNbpTaq(TM)rOs] where 1 is 1-6, m is 0-18, n is 0-40, o is 0-40, p is 0-10, q is 0-10, r is 0-9, and TM is a d-electron-containing transition metal ion, where l+m+n+o+p+q >/= 4, and s is sufficiently large that x > 0, with a lignocellulosic pulp, wherein the pH of the combination is between 6 and 11 and the consistency of the combination is 1-20 %; and heating the combination in a temperature-controlled and pressure-controlled vessel under conditions of temperature and time wherein the polyoxometalate is reduced and delignification occurs.
TL;DR: In this paper, a ring-shaped polyoxometalate and ligand-protected metal chalcogenide clusters provide versatile models for the study of molecular growth processes, especially at the interface to solid-state chemistry.
Abstract: Novel nanosized ring-shaped polyoxometalate and ligand-protected metal chalcogenide clusters provide versatile models for the study of molecular growth processes, especially at the interface to solid-state chemistry. Besides their unusual electronic properties, their use as synthons or precursors for solid-state structures with tailored properties, such as mesoporosity, raises special interest for materials science (polyoxometalate case).
TL;DR: The polyoxometalate [H2PW9Ni4O34(OH)3(H2O)6]2- (1) has been synthesized and characterized by IR spectroscopy, polarography, elemental analysis, and magnetic measurements as mentioned in this paper.
Abstract: The novel polyoxometalate [H2PW9Ni4O34(OH)3(H2O)6]2- (1) has been synthesized and characterized by IR spectroscopy, polarography, elemental analysis, and magnetic measurements. An X-ray single-crystal analysis was carried out on Cs2[H2PW9Ni4O34(OH)3(H2O)6]·5H2O, which crystallizes in the orthorhombic system, space group Pbcn, with a = 24.3131(3) A, b = 21.5400(1) A, c = 19.8936(3) A, V = 10418.4(2) A3, Z = 8, and ρcalcd = 3.767 g/cm3. Structural analysis based on 14 734 independent reflections leads to a solution, R = 8.04% (I > 2σ(I)). Anion 1 consists of a Ni4O4 cubane fragment which is inserted into a trivacant lacunary B-α-[PW9O34]9- Keggin unit. Magnetic measurements show that the Ni−Ni exchange interactions are ferromagnetic.