Showing papers on "Polyoxometalate published in 2007"

Noncovalently Connected Frameworks with Nanoscale Channels Assembled from a Tethered Polyoxometalate–Pyrene Hybrid

Abstract: Polyoxometalate (POM) clusters are of interest since their assembly can bridge multiple length scales from the assembly of sub-nanoscale to protein-sized molecules and even colloidal aggregates of clusters many hundreds of nanometers in size. Therefore the ability to bridge such length scales, coupled with their attractive electronic and molecular properties that give rise to a variety of applications in diverse fields, such as catalysis, medicine, or materials science offers interesting and exciting perspectives for the design of new materials, especially framework materials. Perhaps one of the most interesting aspects of POM chemistry lies with the fact that the clusters can be viewed as transferable building blocks. As such, the controlled assembly of polyoxometalate-based building blocks defines a crucial challenge to engineer the POM building blocks so they can assemble into novel architectures with functionality. An important extension to this buildingblock concept is realized by the use of POMs to form organic– inorganic hybrid compounds which comprise covalently connected cluster and organo fragments, thereby allowing the intermit combination of the properties of the metal–oxo and organic building blocks; this has been used to prepare polymers, dendrimers, and macroporous materials. It is apparent now that organic components can dramatically influence the microstructures of inorganic oxides, thus providing a way for the design of novel materials, and this has been shown in the natural world. In this respect, we are interested in the combination of highly conjugated organic molecules with POMs since this class of materials has hardly been explored, and should allow the assembly of interesting systems. Such organic–inorganic hybrid materials will not only combine the advantages of organic molecules, such as structural fine tuning, but also the close interaction and synergistic effects of organic group and inorganic cluster. However, the major limitation is that the covalent functionalization of POMs in general is not straightforward, depending critically upon the building blocks chosen. One route to achieve this goal is to use a flexible synthetic strategy, in which the organic linker between the POMs and the organic site is designed to be bifunctional. Therefore we utilized Tris (tris(hydroxymethyl)aminomethane, (HOCH2)3NH2), in combination with a manganese-Anderson-type (Mn-Anderson) POM and this was inspired by the work of Hasenknopf, Gouzerh et al. , who first demonstrated the utility of this approach to construct potential hybrid POM–organic units. Herein, we demonstrate the tethering of highly conjugated and planar pyrene units, derivatized with Tris yields [(HOCH2)3CNH-CH2-C16H9] (1) and that the reaction of 1 with [N(C4H9)4]4[a-Mo8O26] and Mn(OAc)3 in CH3CN yields a novel building block that comprises a Mn-Anderson cluster tethered to two pyrene units through the Tris linker to give [Mn-Anderson(Tris–pyrene)2] 3 , that is, [MnMo6O18{(OCH2)3CNH-CH2-C16H9}2] 3 (2a ; Scheme 1; compound 2 is 2a+ counterions). By tethering the highly delocalized aromatic pyrene moiety covalently to the Mn-Anderson-type cluster through the Tris connector, we have been able to modify the physical properties of the Mn-Anderson cluster dramatically, and this is shown in the UV/Vis absorption spectrum (Figure 1) and also in the fluorescence emission spectrum (see the Supporting Information). In comparison to the UV/Vis spectrum of the MnAnderson cluster itself, which only shows absorption at 217 nm and a shoulder peak at 245 nm, the absorption spectrum of 2 is dominated by pyrene vibronic progression. The steady-state fluorescence emission spectrum of the compound 2 shows one sharp peak at 380 nm with another very broad peak (see the Supporting Information) to lower energy at 413 nm (by ca. 2000 cm ). This peak is assigned to emission from the lowest excited single state of the pyrene chromophore and it shows that 2 is highly fluorescent compared to the underivatized Mn-Anderson cluster. In addition, we show that the assembly in the presence of tetrabutylammonium (TBA) cations facilitates the construction of an unprecedented framework material [N(C4H9)4]3[MnMo6O18{(OCH2)3CNH-CH2-C16H9}2]·2DMF·3H2O (2) with nanoscale channels that can reversibly bind aromatic guest molecules. This situation is remarkable since only very weak supramolecular interactions between the anionic building blocks, 2a (this building block itself is built from a combination of coordinative and covalent interactions), and TBA, such as van der Waals and C H···O=Mo hydrogen bonds are responsible for the structural integrity of the overall framework which is stable up to 240 8C. The framework material 2 was isolated in 19% yield as very long needle crystals after diethyl ether diffusion into a DMF solution of the compound for one week. The crystals were characterized [*] Dr. Y.-F. Song, Dr. D.-L. Long, Prof. L. Cronin WestCHEM Department of Chemistry The University of Glasgow Glasgow, G12 8QQ (UK) Fax: (+44)141-330-4888 E-mail: l.cronin@chem.gla.ac.uk

Chiral polyoxometalate-induced enantiomerically 3D architectures: a new route for synthesis of high-dimensional chiral compounds.

Abstract: Two new enantiomerically chiral 3D architectures, l-(Zn(H2O)3)3[MnMo9O32]·4H2O and d-(Zn(H2O)3)3[MnMo9O32]·4H2O, have been produced from a new way. The compounds were composed of the discrete chiral [MnMo9O32]6- polyoxoanions linked by zinc(II) atoms to a 3D framework with a 2-fold interpenetrating net, which is the first examples of POM-based chiral 3D frameworks consisting of only inorganic components.

Polyoxometalates with internal cavities: redox activity, basicity, and cation encapsulation in [Xn+P5W30O110](15-n)- Preyssler complexes, with X = Na+, Ca2+, Y3+, La3+, Ce3+, and Th4+.

Abstract: The Preyssler anion, of general formula [Xn+P5W30O110](15-n)-, is the smallest polyoxometalate (POM) with an internal cavity allowing cation exchange with the solution. The Preyssler anion features a rich chemistry evidenced by its ability to accept electrons at low potentials, to selectively capture various metal cations, and to undergo acid−base reactions. A deep understanding of these topics is herein provided by means of DFT calculations on the title series of compounds. We evaluate the energetics of the release/encapsulation process for several Xn+ cations and identify the effect of the encapsulated ion on the properties of the Preyssler anion. We revisit the relationship between the internal cation charge and the electrochemical behavior of the POM. A linear dependence between the first one-electron reduction energies and the encapsulated Xn+ charge is found, with a slope of 48 mV per unit charge. The protonation also shifts the reduction potential to more positive values, but the effect is much lar...

Tetraalkylphosphonium Polyoxometalate Ionic Liquids: Novel, Organic−Inorganic Hybrid Materials†

Abstract: Pairing of a Keggin or Lindqvist polyoxometalate (POM) anion with an appropriate tetraalkylphosphonium cation is shown to yield the first members of a new family of ionic liquids (ILs). Detailed characterization of one of them, an ambient-temperature “liquid POM” comprising the Lindqvist salt of the trihexyl(tetradecyl) phosphonium cation, by voltammetry, viscometry, conductimetry, and thermal analysis indicates that it exhibits conductivity and viscosity comparable to those of the one previously described inorganic−organic POM-IL hybrid but with substantially improved thermal stability.

Characterization and Electrochemical Properties of Molecular Icosanuclear and Bidimensional Hexanuclear Cu(II) Azido Polyoxometalates

Abstract: Two new Cu(II) azido polyoxometalates compounds have been synthesized, and their structures were determined by X-ray crystallography. The compound Na14[SiW9O34Cu3(N3)2(OH)(H2O)]2·24H2O (1) is built from two [SiW9O34Cu3(μ1,1,3-N3)2(μ-OH)(H2O)]7- subunits where the copper centers, connected by two azido ligands and one hydroxo group, form a nearly equilateral triangle. The two subunits are related by an inversion center and connected via the two μ1,1,3-N3 ligands in an end-to-end fashion, affording a hexanuclear Cu(II) cluster. Linkage of these fragments via CuOW bonds leads to a bidimensional arrangement of the polyoxometalate units. The complex LiK14Na9[P8W48O184Cu20(N3)6(OH)18]·60H2O (2) consists of two {Cu5(OH)4}6+ and two {Cu5(OH)2(μ1,1,3,3-N3)}7+ subunits connected via four μ-OH and four μ1,1-N3 additional ligands, the 20 copper centers being encapsulated in the [P8W48O184]40- crown polyoxotungstate ligand. 1 represents the first multidimensional compound based on azido polyoxometalate (POM) units, an...

Catalytic consequences of composition in polyoxometalate clusters with keggin structure

Abstract: Reliable correlations among structure, composition, and function in heterogeneous catalysis require well-defined atomic connectivity within active structures and the assessment of the specific elementary steps and reaction intermediates responsible for the relevant catalytic function. The non-uniform nature of typical active structures creates significant challenges because probes of structure and function average such heterogeneity in complex ways. Polyoxometalate (POM) clusters with stable Keggin structures and well-defined atomic connectivity provide the compositional diversity required for a rigorous assessment of the consequences of composition on catalytic reactivity. We describe herein the effects of the central atom X (P, Si, Al, and Co) in Keggin-type POM clusters (H8 nX W12O40; H8 nXW) on acid strength based on calculated deprotonation enthalpies, which reflect intrinsic acid strength, and reactivity, based on a rigorous analysis of elementary rate constants, using 2-butanol dehydration as a probe reaction. Previous studies have not reported intrinsic acid properties for these materials and treated reactivity merely in terms of measured rates without the mechanistic interpretations required for meaningful composition–function relations. The ubiquitous aggregation and incomplete and environment-dependent accessibility of POM clusters was minimized by dispersing them onto SiO2 supports. The number of accessible protons, required for rigorous measurements of turnover rates, was determined by titration with pyridine during catalysis. We conclude that C O bond breaking in chemisorbed butanol monomers is the kinetically relevant step, while butanol dimers that form by solvation of adsorbed butanol with another butanol molecule are unreactive spectators. Measured turnover rates depend on the rate constant for C O cleavage and on the equilibrium constant for dimer formation; their values were obtained from the measured effects of 2-butanol pressure on dehydration rates. Both constants increased with increasing valence of the central atom, as the deprotonation enthalpy—a measure of the relative stability of the conjugate base—decreased. Supported POM clusters catalyze 2-butanol dehydration at low temperatures (333–373 K) without detectable deactivation or structural changes. Reaction rates decreased sharply with increasing 2-butanol pressure (Figure 1) on all POM

Assembly of Multitrack Cu−N Coordination Polymeric Chain-Modified Polyoxometalates Influenced by Polyoxoanion Cluster and Ligand

Abstract: By introducing different long rigid ligands into polyoxometalate (POM) systems, four novel inorganic–organic hybrids containing multitrack Cu−N coordination polymeric chain-modified POMs, {[Cu(4,4′-bipy)]3[HGeMo12O40]}·0.5H2O (1), [Cu(4,4′-bipy)]{[Cu(4,4′-bipy)]2[W6O19]}·4H2O (2) (bipy = bipyridine), [Cu(bpe)]{[Cu(bpe)]2[GeMo12O40(VO)2]} (3) (bpe = bis(4-pyridyl)ethylene), and [Cu2(phnz)3]2[SiW12O40] (4) (phnz = phenazine), have been hydrothermally synthesized and characterized by routine physical methods. The crystal structures of 1–4 have been established by single crystal X-ray diffraction. Compound 1 exhibits a tritrack Cu−N chain-modified Keggin POM 1D structure. Compound 2 represents the first example of a 2D framework constructed from a Lindquist polyoxoanion modified by double bitrack Cu−N chains via its terminal oxygen and bridge oxygen atoms. The rail-like 1D structure of compound 3 is formed from Keggin clusters modified by double monotrack Cu−N chains. Compound 4 shows a pseudo-1D structure of...

Breaking Symmetry: Spontaneous Resolution of a Polyoxometalate

Abstract: A chiral polyoxometalate [Hf(PW11O39)2]10- (1) has been prepared and structurally characterized. It crystallizes in the chiral space group P2(1)2(1)2, as a conglomerate of two enantiomerically pure crystals in the absence of any chiral source. The absolute configuration of 1 was determined from the Flack parameter by X-ray crystallography. The structure of 1 comprises two lacunary [PW11O39]7- units, each functioning as a tetra-dentate ligand sandwiching an 8-coordinate HfIV centre in a distorted square antiprismatic geometry. Optically active crystals of both enantiomers were spectroscopically distinguishable by means of solid state circular dichroism spectroscopy. This hafnium-substituted polyoxometalate (POM), 1, shows that spontaneous chiral resolution, a rare phenomenon, can be operable in POM systems.

Oxidation on zerovalent iron promoted by polyoxometalate as an electron shuttle.

Abstract: Most studies on zerovalent iron (ZVI) were mainly focused on the reductive transformation of halo- or nitro-compounds. Oxidation reactions occurring on ZVI have been recently recognized. In this study, we demonstrate that the oxidation pathways on ZVI can be accelerated by the presence of polyoxometalate (POM: nanosized metal−oxygen cluster anion) serving as an electron shuttle. The ions, SiW12O404- and PW12O403-, can mediate the electron transfer from the Fe0 surface to O2 while enhancing the production of H2O2, which subsequently initiates the OH radical-mediated oxidation through a Fenton-type reaction. The oxidation reaction was completely quenched by adding methanol as an OH radical-scavenger. On the other hand, PMo12O403- completely inhibited the oxidative degradation by irreversibly scavenging an electron and holding it. We systematically investigated the effects of iron loading, the concentration of POM, and pH on the oxidative degradation kinetics of 4-chlorophenol in the POM-mediated ZVI system...

From polyoxometalate building blocks to polymers and materials: the silver connection

Abstract: Molecular growth processes utilizing polyoxometalate-based building blocks with silver connecting units have been used to produce four new materials: 1, [Ag3(bhepH)8(W115Na05O40P)2]·8H2O (bhep = N,N′-bis(2-hydroxyethyl)piperazine); 2, [Ag4(DMSO)8(Mo8O26)]n; 3, {Ag3[MnMo6O18{(OCH2)3CNH2}2(DMSO)5]·3(DMSO)}n; 4, {Ag3[MnMo6O18{(OCH2)3CNH2}2(DMSO)6(CH3CN)2]·DMSO}n The compounds were characterised using single crystal X-ray crystallography, elemental analysis, IR, TGA, DSC, and compounds 2–4 were imaged on silicon substrates using scanning electron microscopy Compound 1 represents a 0D dimer connected by silver ions but was also found to form channels facilitated by the hydrogen bonding between the protonated [bhepH]+ ligand and the cluster, and complex 2 forms 2D layered networks, whereas both compounds 3 and 4 form 1D networks; these networks are all connected by Ag(I) ions Thermal studies show that the stabilities of compounds 2–4 are affected by the linking Ag(I) ions and the DMSO ligands and EM studies on compounds 3 and 4 showed the formation of fibres on silicon substrates

MoV-MoVI mixed valence polyoxometalates for facile synthesis of stabilized metal nanoparticles : Electrocatalytic oxidation of alcohols

Abstract: MoV−MoVI mixed valence polyoxometalates (POMs), H7[β-P(MoV)4(MoVI)8O40] (1), (NH4)10[(MoV)4(MoVI)2O14(O3PCH2PO3)2(H2OPCH2PO3)2]·15H2O (2), and [e-P(MoV)8(MoVI)4O36(OH)4{La(H2O)2.5Cl1.25}4]·27H2O (3), respectively, were selected because they possess built-in reducing abilities. They were used for the synthesis of Pt0 and Pd0 nanoparticles (NPs) in water at room temperature without UV irradiation. Small size NPs (1.7−4 nm) were obtained, their stabilization following the order 1 > 2 ≫ 3. They were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electrochemistry. Detection of molybdenum in XPS and electrochemistry supports the hypothesis that each polyoxometalate serves both as a reductant and as a protective layer, in agreement with the propensity, particularly of Mo-based POMs, to self-assemble on metal and other solid surfaces. The Pt0 and Pd0 NPs were tested for the electrocatalytic oxidation of methanol and ethanol, respectively, and proved efficient ...

Polyoxometalate Clusters, Nanostructures and Materials: From Self-Assembly to Designer Materials and Devices

Abstract: Polyoxometalates represent a diverse range of molecular clusters with an almost unmatched range of physical properties and the ability to form structures that can bridge several length scales. The new building block principles that have been discovered are beginning to allow the design of complex clusters with desired properties and structures and several structural types and novel physical properties are examined. In this critical review the synthetic and design approaches to the many polyoxometalate cluster types are presented encompassing all the sub-types of polyoxometalates including, isopolyoxometalates, heteropolyoxometalates, and reduced molybdenum blue systems. As well as the fundamental structure and bonding aspects, the final section is devoted to discussing these clusters in the context of contemporary and emerging interdisciplinary interests from areas as diverse as anti-viral agents, biological ion transport models, and materials science.

An Inorganic Aggregate Based on a Sandwich‐Type Polyoxometalate with Lanthanide and Potassium Cations: From 1D Chiral Ladder‐Like Chains to a 3D Open Framework

Abstract: The reaction of sandwich-type polyanion [Mn4Si2W18O68(H2O)2]12–, ceric sulfate, and potassium citrate in an acidic aqueous solution led to the isolation of a new compound K4Na2[{Ce(H2O)7}2Mn4Si2W18O68(H2O)2]·21.5H2O (1), which crystallizes in the space group P212121 with a = 12.040(2) A, b = 26.094(5) A, c = 30.043(6) A, V = 9439(3) A3, Z = 4, and flack parameter x = 0.63(1). X-ray diffraction analysis reveals that 1 consists of novel chiral ladder-like chains based on sandwich-type polyanions and lanthanide cations, which are further connected into a three-dimensional open framework by potassium cations.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Tetrairon and hexairon hydroxo/acetato clusters stabilized by multiple polyoxometalate scaffolds. Structures, magnetic properties, and chemistry of a dimer and a trimer.

Abstract: Investigation of the catalytically relevant γ-diiron(III) Keggin complexes in aqueous acetate buffer leads to a dimeric C2v-symmetric polyanion, [{Fe(OH)(OAc)}4(γ-SiW10O36)2]12- (3) and a trimeric ...

Novel copper-complex-substituted tungstogermanates.

Abstract: Two copper-complex-substituted tungstogermanates [Cu5(2,2‘-bpy)6(H2O)][GeW8O31]·9H2O, 1, and {[Cu5(2,2‘-bpy)5(H2O)][GeW9O34]}2·7H2O, 2 (2,2‘-bpy = 2,2‘-bipyridyl), have been hydrothermally synthesized and structurally characterized. X-ray structural analyses show that 1 is a rare tetralacunary Keggin anion of [B-β-GeW8O31]10- substituted by copper complexes, while dimer 2 consists of two trilacunary [B-α-GeW9O34]10- Keggin units bridged by copper complexes. Magnetic measurement for 2 shows the presence of antiferromagnetic interactions within the dinuclear Cu2+ cations.

An Unprecedented Fivefold Interpenetrating Network Based on Polyoxometalate Building Blocks

Abstract: The first three-connected, 5-fold interpenetrating network based on polyoxometalate building blocks and metal−organic frameworks that represents the highest degree of interpenetration presently known for 3D polyoxometalate systems is reported.

Investigation of the photocatalytic activity of TiO2–polyoxometalate systems for the oxidation of methanol

Abstract: We have developed a TiO2–POM co-catalyst system for use in a hybrid photo-electrochemical cell to oxidize methanol. We employed a simple -Keggin-type polyoxometalate (PW12O40 3− ) that is known to adsorb to the surface of positively charged TiO2 particles at low pH. The use of a co-catalyst dramatically improves the separation of photogenerated electron–hole pairs in suspended TiO2 nanoparticles, leading to a 50-fold increase in the observed photocurrent compared to the use of TiO2 alone. The photocurrent densities we observed exceeded those recently reported by Kamat and co-workers [K. Drew, G. Girishkumar, K. Vinodgopal, P.V. Kamat, J. Phys. Chem. B 109 (2005) 11851]. Due to the improved charge separation offered by the co-catalyst system, electron transfer from the reduced polyoxometalate to the anode now appears to be the kinetic limitation. In addition, we find that the polyoxometalate itself can oxidize methanol efficiently in the absence of TiO2. However, photogenerated holes in the POM are unable to oxidize either formic acid or acetic acid, in contrast to the behavior of TiO2. The dependence of the photocurrent on [PW12O40 3− ] was also investigated. We find that the optimum [PW12O40 3− ] is approximately 2 mM.

A DFT study on the effect of metal, anion charge, heteroatom and structure upon the relative basicities of polyoxoanions

Abstract: In many reactions catalyzed by polyoxoanions, the first step involves attaching a proton at the anion so that the activated species can react with the substrate. The present study focuses on this first step and makes a detailed analysis of the factors that affect the basicity of the distinct oxygen sites in a polyoxometalate. Several prototypical polyoxoanions were studied in solution using the conductor-like screening model (COSMO) to model the solution. The nucleophilicity of a given site depends mainly on the coordination type, the metal to which the oxygen is linked, the charge of the anion and the bonding angle in the bridging oxygen sites. The coordinated metal and the coordination type are the most important. We discuss in detail the effect of the charge and of the charge density of the anion on the basicity of the polyoxoanion.