Showing papers on "Polyoxometalate published in 2010"

Hybrid Organic−Inorganic Polyoxometalate Compounds: From Structural Diversity to Applications

Abstract: Polyoxometalates (POMs) are discrete anionic metaloxygen clusters which can be regarded as soluble oxide fragments. They exhibit a great diversity of sizes, nuclearities, and shapes. They are built from the connection of {MOx} polyhedra, M being a d-block element in high oxidation state, usually VIV,V, MoVI, or WVI.1 While these species have been known for almost two centuries, they still attract much interest partly based on their large domains of applications. They play a great role in various areas ranging from catalysis,2 medicine,3 electrochemistry,4 photochromism,5 to magnetism.6 This palette of applications is intrinsically due to the combination of their added value properties (redox properties, large sizes, high negative charges, nucleophilicity...). Parallel to this domain, the organic-inorganic hybrids area has followed a similar expansion during the last 10 years. The concept of organic-inorganic hybrid materials * To whom correspondence should be addressed. E-mail: dolbecq@ chimie.uvsq.fr. Chem. Rev. 2010, 110, 6009–6048 6009

Cubic Polyoxometalate−Organic Molecular Cage

Abstract: Tris(hydroxymethyl)aminomethane (Tris) was successfully grafted onto the surface of a Ni6-substituted polyoxotungstate formed in situ to further generate a three-connected polyoxometalate building block. The cooperative assembly of Tris functionalized three-connected building blocks and rigid 1,3,5-benzenetricarboxylate gave rise to an unprecedented cubic polyoxometalate−organic molecular cage with high thermal and hydrothermal stability.

Graphene-Based Nanoporous Materials Assembled by Mediation of Polyoxometalate Nanoparticles

Abstract: A kind ofgraphene-based nanoporous material is prepared through assembling graphene sheets mediated through polyoxometalate nanoparticles. Owing to the strong interaction between graphene and polyoxometalate, 2D graphene sheets with honeycomb-latticed carbon atoms could assemble into a porous structure, in which 3D polyoxometalate nanoparticles serve as the crosslinkers. Nitrogen and hydrogen sorption analysis reveal that the as-prepared graphene-based hybrid material possesses a specific surface area of 680 m 2 g -l and a hydrogen uptake volume of 0.8-1.3 wt%. Infrared spectrometry is used to probe the electron density changes of polyoxometalate particle in the redox-cycle and to verify the interaction between graphene and polyoxometalate. The as-prepared graphene-based materials are further characterized by Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy.

Activation of molecular oxygen, polyoxometalates, and liquid-phase catalytic oxidation.

Abstract: In this Forum Article, we discuss the use of dioxygen (O2) in oxidations catalyzed by polyoxometalates. One- and two-electron-transfer oxidation of organic substrates is catalyzed by H5PV2Mo10O40 and often occurs via an outer-sphere mechanism. The reduced polyoxometalate is reoxidized in a separate step by O2 with the formation of water. H5PV2Mo10O40 also catalyzes electron transfer−oxygen transfer reactions. Here, in contrast to the paradigm that high-valent oxo species are often stronger oxygen-transfer species than lower-valent species, the opposite occurs. Thus, oxygen transfer from the catalyst is preceded by electron transfer from the organic substrate. The monooxygenase-type reduction of O2 with polyoxometalates is also discussed based on the formation of a stable iron(III) hydroperoxide compound that may have implications for the oxidation of other lower-valent polyoxometalates such as vanadium(IV)- and ruthenium(II)-substituted polyoxometalates. Finally, the formation of hybrid compounds through ...

Terbium polyoxometalate organic complexes: correlation of structure with luminescence properties.

Abstract: Light up the POMs: A luminescent lanthanoid complex with polyoxometalate (POM) and organic ligands has been structurally characterized (see picture). Comparison of this octanuclear TbIII complex of 2-picolinate and tungstoarsenate ligands with a dinuclear relative reveals the role of the organic ligands as chromophores, identifies the luminescent Tb centers, and determines the relationship between POM coordination mode and luminescence quenching.

Using Flexible and Rigid Organic Ligands to Tune Topology Structures Based on Keggin Polyoxometalates

Abstract: Three inorganic−organic hybrids based on polyoxometalate (POM), [Cu6(bbtz)6(HPM12O40)]·2H2O (M = Mo, 1; W, 2) and [Cu6(trz)2(bbtz)2(SiW12O40)] (3) (trz = 1-H-1,2,4-triazole, bbtz = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene), were synthesized under hydrothermal conditions, and structurally characterized. Through the use of the flexible ligand bbtz, isostructural compounds 1 and 2 with interpenetrating structures were obtained. In compound 1, ladder-like chains exist, in which the PMo12 anions act as “middle rails”. These chains are linked by wave-like [Cu(bbtz)]nn+ lines to construct a three-dimensional (3D) framework. Two such frameworks penetrate each other to construct a 2-fold interpenetrating structure. By introducing the rigid ligand trz, compound 3 with an un-interpenetrating structure is obtained. In compound 3, two-dimensional (2D) (63)2 metal organic framework (MOF) layers exist, which are linked by Keggin anions to construct a 3D (44·62)(63)2 framework. The differences between these compounds sho...

Self-Assembly of a Nanosized, Saddle-Shaped, Solution-Stable Polyoxometalate Anion Built from Pentagonal Building Blocks: [H34W119Se8Fe2O420]54−

Abstract: Precise control of the “one-pot” reaction conditions is employed for the assembly of a nanosized 2.8 nm cluster, [H34W119Se8Fe2O420]54− (1a), which is templated using eight selenite units and two iron(III) centers. Not only is this cluster characterized structurally using X-ray crystallography, it is the largest polyoxotungstate to be observed intact in solution using ESI-MS, with a molecular mass of ca. 30 000 Da.

Synthesis and photocatalytic properties of mixed polyoxometalate-porphyrin copolymers obtained from Anderson-type polyoxomolybdates.

Abstract: Hybrid polyoxometalate-porphyrin copolymeric films are obtained by the electro-oxidation of zinc octaethylporphyrin (ZnOEP) and zinc 5,15-dipyridinium octaethylporphyrin (5,15-ZnOEP(py)22+) in the presence of the polyoxometalate [MnMo6O18{(OCH2)3CNHCO(4-C5H4N)}2]3− (Py-POM-Py). These films allow the photocatalytic reduction of AgI2SO4 under visible irradiation in air in the presence of propan-2-ol at the 2D interface between water and the copolymeric films. The formation of metallic Ag0 nanowires and triangular nanosheets is observed.

Electron Transfer−Oxygen Transfer Oxygenation of Sulfides Catalyzed by the H5PV2Mo10O40 Polyoxometalate

Abstract: The oxygenation of sulfides to the corresponding sulfoxides catalyzed by H5PV2Mo10O40 and other acidic vanadomolybdates has been shown to proceed by a low-temperature electron transfer−oxygen transfer (ET−OT) mechanism. First, a sulfide reacts with H5PV2Mo10O40 to yield a cation radical−reduced polyoxometalate ion pair, R2+•,H5PVIVVVMo10O40, that was identified by UV−vis spectroscopy (absorptions at 650 and 887 nm for PhSMe+• and H5PVIVVVMo10O40) and EPR spectroscopy (quintet at g = 2.0079, A = 1.34 G for the thianthrene cation radical and the typical eight-line spectrum for VIV). Next, a precipitate is formed that shows by IR the incipient formation of the sulfoxide and by EPR a VO2+ moiety supported on the polyoxometalate. Dissolution of this precipitate releases the sulfoxide product. ET−OT oxidation of diethylsulfide yielded crystals containing [V(O)(OSEt2)x(solv)5−x]2+ cations and polyoxometalate anions. Under aerobic conditions, catalytic cycles can be realized with formation of mostly sulfoxide (90...

Surfactant-Encapsulated Polyoxometalates as Immobilized Supramolecular Catalysts for Highly Efficient and Selective Oxidation Reactions

Abstract: A type of interesting immobilized supramolecular catalysts based on surfactant-encapsulated polyoxometalates has been developed for oxidation reactions. Through a sol-gel process with tetraethyl orthosilicate, hydroxyl-terminated surfactant-encapsulated polyoxometalate complexes have been covalently and uniformly bound to a silica matrix with unchanged complex structure. The formed hybrid catalysts possess a defined hydrophobic nano-environment surrounding the inorganic clusters, which is conducive to compatibility between the polyoxometalate catalytic centres and organic substrates. The supramolecular synergy between substrate adsorption, reaction, and product desorption during the oxidation process has been found to have an obvious influence on the reaction kinetics, with the activity of the catalyst being greatly improved. The supramolecular catalysts performed effectively in the selective oxidation of several different kinds of organic compounds, such as alkenes, alcohols, and sulfides, and the main products were the corresponding epoxides, ketones, sulfoxides, and sulfones. More significantly, the catalyst could be easily recovered by simple filtration, and the catalytic activity was well retained for at least five cycles. Finally, the present strategy has proved to be a general route for the fabrication of supramolecular hybrid catalysts containing common polyoxometalates suitable for various purposes.

Reaction controlled assemblies of polyoxotungstates (-molybdates) and coordination polymers.

Abstract: To investigate the influence of reaction conditions on the compound structures, five polyoxometalate (POM)-supported inorganic−organic hybrid compounds, [CuI3(L1)4][PW12O40] (1), [CuI3(L1)4][PMo12O40] (2), [CuI3(L1)4][PW12O40] (3), [CuI3(L1)3][PW12O40] (4), and [CuI3(L2)3][PMo12O40] (5) [L1 = 1,4-bis(pyrazol-1-ylmethyl)benzene, L2 = 4,4′-bis(pyrazol-1-ylmethyl)biphenyl], were hydrothermally synthesized by tuning the reactant species and molar ratio and reaction temperature and were characterized by elemental analysis, IR spectroscopy, PXRD, XPS, electrochemistry, SPS, and X-ray crystallography. Compounds 1 and 2 were synthesized in the isostructural state at 150 °C, in which L1 ligands link CuI ions, generating a cationic 2D 63 (hcb) skeleton {[Cu3(L1)4]3+}n-like sheet that further connects with POM anions, forming a neutral 2D (3,4)-connected network with a Schlafli symbol of (53)2(54;82). In contrast, compound 3 was synthesized at a relatively lower reaction temperature (130 °C) than that for the synthe...

Structural and catalytic performance of a polyoxometalate-based metal-organic framework having a lanthanide nanocage as a secondary building block.

Abstract: A polyoxometalate-based lanthanide−organic framework was achieved using the {[Ho4(dpdo)8(H2O)16BW12O40] (H2O)2}7+ nanocage as a secondary building block for the heterogeneous catalysis of phosphodiester cleavage in an aqueous solution.

Green Oxidation Reactions by Polyoxometalate-Based Catalysts: From Molecular to Solid Catalysts

Abstract: This article summarizes our recent developments for H2O2- and O2-based green oxidation reactions by polyoxometalates (POMs) and related compounds. POM-based structurally controlled molecular catalysts exhibit high catalytic performance for epoxidation and sulfoxidation with H2O2. These molecular catalysts can be immobilized on the organic–inorganic hybrid support with covalently anchoring N-octyldihydroimidazolium cation fragments via the anion exchange. In addition, we have developed supported metal hydroxide catalysts such as Ru(OH)x/Al2O3 and Cu(OH)x/TiO2 on the basis of the information of the catalytically active sites with POMs. By using these supported metal hydroxide catalysts, novel aerobic oxidative synthesis of nitriles and amides, and efficient aerobic oxidative homocoupling of alkynes can be realized.

Oxidation of alcohols with hydrogen peroxide in water catalyzed by recyclable keggin-type tungstoborate catalyst

Abstract: Different kinds of alcohols were efficiently transformed into corresponding ketones catalyzed by a Lacunary Keggin polyoxometalate of K 8 [BW 11 O 39 H]·13H 2 O in water with hydrogen peroxide. The system not only catalyzes the oxidation of alcohols but also avoids the use of organic solvent. The reactions were carried out in an aqueous/oil system. The catalyst was reused for the next cycle from the organic product by separation of the aqueous phase. High yields of ketones were obtained in oxidation of secondary alcohols. Benzyl alcohol was converted to benzaldehyde in high conversion and selectivity.

Influence of Transition Metal Coordination Nature on the Assembly of Multinuclear Subunits in Polyoxometalates-Based Compounds

Abstract: Four inorganic−organic hybrids based on Keggin-type polyoxometalate (POM), [Ag3(bmte)2(PMo12O40)] (1), [CuI3(bmte)3(PMo12O40)] (2), [CuII(bmte)3(HPMo12O40)] (3), and [Ni(bmte)3(HPMo12O40)] (4) (bmte = 1,2-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)ethane), have been synthesized under hydrothermal conditions by tuning the coordination nature of transition metal (TM) ions. The transformation of coordination modes of TM ions has a crucial influence on the multinuclear structures of this series. In compound 1, binuclear subunits [Ag(1)2(bmte)2]2+ occur, which is connected by PMo12O403− (PMo12) anions to construct a one-dimensional (1D) chain. Furthermore, these chains are linked by Ag(2) ions acting as fuses to construct a two-dimensional (2D) gridlike sheet. Compound 2 shows trinuclear clusters, [CuI3(bmte)3]3+, which are also connected by PMo12 anions to construct a 1D chain. These two neighboring chains arrange like a “zipper”. Compounds 3 and 4 are isostructural. In compound 3, the metal−organic subunit is...

A Noble‐Metalate Bowl: The Polyoxo‐6‐vanado(V)‐7‐palladate(II) [Pd7V6O24(OH)2]6−

Abstract: The new polyoxometalate (IV) crystallizes in the monoclinic space group P21/n with Z = 4 (single crystal XRD).

Polyoxometalate-based N-heterocyclic carbene (NHC) complexes for palladium-mediated C-C coupling and chloroaryl dehalogenation catalysis.

Abstract: Molecular hybrids are of particular interest in catalysis, due to the interplay of joint organic–inorganic domains with very diverse functional environments. The hybrid strategy is expected to allow for fine-tuning of the stereo-electronic properties of the catalyst through a tailored choice of the interacting organic–inorganic moieties. Such a hybrid upgrade of the catalytic system can be readily accessed, through the covalent functionalization of molecular polyoxometalates (POMs). Indeed, covalent POM hybrids are characterized by discrete, nanosized, multi-metal oxides as polyanionic scaffolds, which allow for anchoring on-surface organic pendants, including chiral residues. The hybrid structures display a remarkable stability, and resist harsh catalytic conditions, enabling a multi-turnover performance and sequential recycling in ionic-liquid media exposed to MW irradiation. Interestingly, the hybrid modification of the POM framework has been successfully used to expand the coordination potential by introduction of tailored organic domains. In this respect, polydentate salen-type or thiol/phosphineterminated ligands have been immobilized on the POM surface to bind Mn, Pd or Rh ions, or to implement the stabilization of Pd colloids. We present herein the synthesis, characterization, and catalytic applications of new POM-appended N-heterocyclic carbene (NHC) palladium complexes and their remarkable performance in catalyzing C C crosscoupling and aromatic dehalogenation reactions. To this end, imidazolium moieties have been successfully grafted on the defect site of the divacant Keggin polyanion [gSiW10O36] 8 . These organic domains are useful precursors of N-heterocyclic (NHC) carbenes, which are known to provide strong M NHC bonds, thus imparting high thermal robustness and stability to organometallic intermediates in multi-turnover palladium catalysis. In addition to the specifically designed Pd binding site, such POM-based hybrids display an extended polyanionic surface. This setup is expected to contribute sterically and by virtue of electrostatic repulsions to prevent agglomeration of incipient Pd, which generally deteriorates the turnover efficiency of the system. The molecular nature of such composite structures is a further point of interest, offering a single-site model for extended materials. Specifically, the molecularly defined structure of the hybrid allows for selective tailoring of the active site as well as for detecting subtle changes by solution characterization techniques, including heteronuclear NMR spectroscopy and ESI-MS. The synthetic route to Keggin-type hybrids involves the reaction of organosilane reagents with the nucleophilic oxygen atoms that border a defect site on the POM surface. Grafting strategies have been optimized in acetonitrile in which the presence of excess nBu4NBr promotes the solubilization of the POM by counterion metathesis (Scheme 1). Under these conditions, decoration of the lacunary POM is known to yield hybrids with two surface-anchored organosilyl (RSi-) groups, each one linked to two oxygen atoms of two edge-shared WO6 octahedra. [4,10] A first attempt focused on a divergent approach, including the covalent attachment of the triethoxysilyl-functionalized 4,5-dihydro-imidazolium bromide (1) to the divacant decatungstosilicate. The hybrid POM (2) was isolated and displayed spectroscopic data (H, C, Si and W NMR, FT-IR) that are in agreement with the expected bis-functionalized structure. However, palladation of 2 met with little success (Scheme 1). Neither direct metalation using Pd ACHTUNGTRENNUNG(OAc)2 nor transmetalation using Ag2O resulted in the [a] Dr. S. Berardi, Dr. M. Carraro, Dr. A. Sartorel, Prof. G. Scorrano, Dr. M. Bonchio ITM-CNR and Department of Chemical Sciences University of Padova, via Marzolo 1, 35131 Padova (Italy) Fax: (+39)0498275239 E-mail : marcella.bonchio@unipd.it [b] Dr. M. Iglesias, Prof. M. Albrecht Department of Chemistry, University of Fribourg Current address: School of Chemistry and Chemical Biology University College Dublin, Belfield, Dublin 4 (Ireland) Fax: (+353)17162501 E-mail : martin.albrecht@ucd.ie Supporting information for this article is available

Nanocasting of ordered mesoporous Co3O4-based polyoxometalate composite frameworks

Abstract: We report the synthesis of highly ordered mesoporous frameworks consisting of nanocrystalline Co3O4 and Keggin-type tungstophosphoric acid (HPW12O40, HPW) compounds using the hard-templating method. The resulting materials feature a Co3O4/HPW solid solution structure with different HPW loadings, i.e., 6, 11, 15, and 36 wt %. Characterization by small-angle X-ray scattering (SAXS), high-resolution transmission electron microscopy (TEM), and N2 physisorption measurements reveal that all mesoporous frameworks possess a three-dimensional cubic symmetry with large internal Brumauer−Emmett−Teller (BET) surface area (87−141 m2g−1) and narrow sized pores (ca. 4 nm). The Keggin structure of the incorporated PW12O403− clusters within the composite frameworks was confirmed with X-ray diffused scattering and atomic pair distribution function (PDF) analysis, X-ray photoelectron spectroscopy (XPS), infrared (IR), and diffuse-reflectance UV/vis spectroscopy. Catalytic studies have indicated that these Co3O4/HPW composit...

Nanoparticles of Pd on Hybrid Polyoxometalate−Ionic Liquid Material: Synthesis, Characterization, and Catalytic Activity for Heck Reaction

Abstract: By partial exchange of surface protons of a polyoxometalate (POM; H5PO40V2Mo10) by an ionic liquid (IL; butylmethylimidazolium, bmim+), a protonic IL-POM ([bmim]4HPO40V2Mo10) was obtained. Further exchange of this material with Pd2+ yields a final solid that HRTEM and EXAFS-XANES techniques show to be formed by very regular nanoparticles of PdO with ∼2 nm diameter, which are supported on 1 μm particles of IL-POM hybrid material. The PdO nanoparticles can be easily reduced to Pd, and the resultant material is an active and selective catalyst for the Heck reaction. The catalyst can be recycled, though metal leaching occurs in polar solvents. This can be an interesting material for performing Pd-catalyzed reactions in nonpolar solvents or for working in gas-phase reactions.

Controlled Interactions between Anhydrous Keggin-Type Heteropolyacids and Silica Support: Preparation and Characterization of Well-Defined Silica-Supported Polyoxometalate Species

Abstract: Anhydrous Keggin-type phosphorus heteropolyacids were deposited on partially dehydroxylated silica by using the surface organometallic chemistry (SOMC) strategy. The resulting solids were characterized by a combination of physicochemical methods including IR, Raman, 1D and 2D 1H, and 31P MAS NMR, electron microscopy experiments and density functional theory (DFT) calculations. It is shown that the main surface species is [≡Si(OH...H+)]2[H+]1[PM12O403−] where the polyoxometalate is linked to the support by proton interaction with two silanols. Two other minor species (10% each) are formed by coordination of the polyoxometalate to the surface via the interaction between all three protons with three silanol groups or via three covalent bonds formed by dehydroxylation of the above species. Comparison of the reactivity of these solids and of compounds prepared by a classical way shows that the samples prepared by the SOMC approach contain ca. 7 times more acid sites.

Modular Inorganic Polyoxometalate Frameworks Showing Emergent Properties: Redox Alloys

Abstract: Alloy, alloy: A pure cobalt-based Keggin network linked by W-O-Co interactions is “alloyed” with a pure manganese-based Keggin network, linked by W-O-Mn interactions. These isostructural alloys are solid solutions of the Co and Mn nets. The alloys are prepared by the stoichiometric mixing of the components of each discrete network, leading to a series of single-crystalline mixed transition-metal frameworks with novel redox properties.

Thermotropic Liquid Crystals of a Non-Mesogenic Group Bearing Surfactant-Encapsulated Polyoxometalate Complexes

Abstract: A tri(ethylene oxide)octadecyldimethylammonium p-toluenesulfonate (C18NEO3·Ts) amphiphile was employed to encapsulate Keggin-type polyoxometalates by ion metathesis reactions with phosphotungstic acid, silicotungstic acid, pentapotassium dodecatungstoborate(III), and phosphomolybdic acid, giving the surfactant-encapsulated polyoxometalates (SEPs) in SEP-P, SEP-Si, SEP-B, and SEP-PMo, respectively. Meanwhile, a C18NEO3·PF6 amphiphile was prepared by substituting the p-toluenesulfonate of C18NEO3·Ts with hexafluorophosphate. The chemical composition of all SEPs and amphiphiles was characterized through 1H NMR, infrared spectroscopy, mass spectroscopy, elemental analysis, and thermogravimetric analysis. The thermal properties of the SEPs and the amphiphiles were investigated by differential scanning calorimetry, polarized optical microscopy, and variable-temperature X-ray diffraction. The types of anions have a remarkable influence on the thermal properties of the prepared compounds or complexes. C18NEO3·Ts ...

Aerobic oxidation of primary aliphatic alcohols to aldehydes catalyzed by a palladium(II) polyoxometalate catalyst

Abstract: A hexadecyltrimethylammonium salt of a "sandwich" type polyoxometalate has been used as a ligand to attach a palladium(II) center. This Pd-POM compound was an active catalyst for the fast aerobic oxidation of alcohols. The unique property of this catalyst is its significant preference for the oxidation of primary versus secondary aliphatic alcohols. Since no kinetic isotope effect was observed for the dehydrogenation step, this may be the result of the intrinsically higher probability for oxidation of primary alcohols attenuated by steric factors as borne out by the higher reactivity of 1-octanol versus 2-ethyl-1-hexanol. The reaction is highly selective to aldehyde with little formation of carboxylic acid; autooxidation is inhibited. No base is required to activate the alcohol. The fast reactions appear to be related to the electron-acceptor nature of the polyoxometalate ligand that may also facilitate alcohol dehydrogenation in the absence of base.