Keggin-type Polyoxotungstates Research Articles

Polyoxometalate-Ionic Liquids for Mitigating the Effects of Iron Gall Ink Corrosion on Cellulosic Supports.

Iron gall ink (IGI), renowned for its indelibility, was the most important writing ink in the Western world from the 15th to the late 19th century. However, it is now known that IGIs induce acid-catalyzed hydrolysis and iron-catalyzed oxidation of the cellulose in historical paper documents. These mechanisms of deterioration cause significant damage to the writing support materials, including color alteration and burn-through appearance, and in the worst scenarios, physical disintegration of the supports. Minimally invasive, long-term effective conservation treatments that tackle the underlying mechanisms of IGI degradation and their corrosion effects are yet to be developed. This study introduces the deployment of hydrophobic and anticorrosive polyoxometalate-ionic liquids (POM-ILs) as colorless coatings to counteract IGI-corrosion of cellulosic supports. Model IGI-containing papers (mockups) were prepared, coated with POM-ILs, and artificially aged to assess the compatibility of POM-ILs with IGI-containing documents. Comprehensive monitoring using colorimetric and scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM/EDS) analyses showed minimal interference with the aesthetic properties and morphology of the IGI mockups. In addition, polyoxometalates (POMs) with vacant metal atom sites in the cluster shell can be used to coordinate free transition metal ions. The ability of a monolacunary Keggin-type polyoxotungstate to coordinate free Fe(II) from IGI solution was demonstrated using UV-vis analysis. This led to the formation of a dimeric species, [(SiW11O39Fe)2O]K12·28H2O, which was characterized by single-crystal X-ray diffraction. Altogether, this study points to POM-ILs as promising protective coatings for effectively preserving historical IGI-written heritage.

Selective Hydrogenation of Azobenzene to Hydrazobenzene via Proton-Coupled Electron Transfer from a Polyoxotungstate Cluster.

In this report, we describe proton-coupled electron transfer (PCET) reactivity at the surface of the Keggin-type polyoxotungstate cluster [nBu4N]3[PWVI12O40] (PW12) in acetonitrile. Bond dissociation free energies (BDFEs) of the O-H groups generated upon reduction of PW12 in the presence of acid are determined through the construction of a potential-pKa diagram. The surface O-H bonds are found to be weak (BDFE(O-H)avg < 48 kcal mol-1), comparable to the BDFE of H2. This is consistent with the observed formation of H2 upon addition of a suitably strong organic acid, H2NPh2+ (pKaMeCN = 5.98), to the reduced form of the cluster. The one-electron reduced form of PW12 is isolated and used in conjunction with acid to realize the stoichiometric semihydrogenation of azobenzene via PCET from the surface of the reduced cluster.

Effective Combination of the Metal Centers in MOF-Based Materials toward Sustainable Oxidation Catalysts.

A successful encapsulation of Keggin-type polyoxomolybdate (H3[PMo12O40], PMo12) into metal-organic framework (MOF) materials with an identical framework but distinct metal centers (ZIF-8 with Zn2+ and ZIF-67 with Co2+) was accomplished by a straightforward room-temperature procedure. The presence of Zn2+ in the composite material PMo12@ZIF-8 instead of Co2+ in PMo12@ZIF-67 caused a remarkable increase in the catalytic activity that achieved a total oxidative desulfurization of a multicomponent model diesel under moderate and friendly conditions (oxidant: H2O2 and solvent: ionic liquid, IL). Interestingly, the parent ZIF-8-based composite with the Keggin-type polyoxotungstate (H3[PW12O40], PW12), PW12@ZIF-8, did not show the relevant catalytic activity. The ZIF-type supports present an appropriate framework to accommodate active polyoxometalates (POMs) into their cavities without leaching, but the nature of the metallic center from the POM and the metal present in the ZIF framework were vital for the catalytic performance of the composite materials.

The photoluminescence study of an organic–inorganic hybrid Keggin-type polyoxotungstate: A better excitation efficiency between polyoxometalate and thulium

An organic–inorganic hybrid thulium-based polyoxometalate was synthesized and further studied using several physicochemical methods and photoluminescence. • an organic–inorganic hybrid Keggin-type polyoxotungstate was synthesized. • its photoluminescence is studied in detailed. • energy transfer efficiency form POM fragment to Tm 3+ ion can be found. An organic–inorganic hybrid lanthanide-based POM (Ln-POM) [N(CH 3 ) 4 ] 3 K 2 [Tm(C 7 H 5 O 3 )(H 2 O) 2 (α-PW 11 O 39 )]·10H 2 O ( 1 ) was synthesized and further characterized by single crystal X-ray diffraction analysis and other analyses. The physicochemical analyses indicate that POM fragments, organic ligand and Tm 3+ ion exist in compound 1 . The photoluminescence studies show that the photoexcitation O → M ligand to metal charge transfer (LMCT) of POM fragments can motivate the emissions of Tm 3+ ions within 1 . A much better excitation efficiency between POM fragment and Tm 3+ ion can be found in this organic–inorganic hybrid Ln-POM, and the unusual phenomenon can be explained that the emission of POM fragment can match well with the emission of Tm 3+ ion, yet the emission of organics is not consistent with emission of Tm 3+ ion in 1 .

Hybrid Antimicrobial Films Containing a Polyoxometalate-Ionic Liquid.

The increasing resistance of pathogenic microorganisms against common treatments requires innovative concepts to prevent infection and avoid long-term microbe viability on commonly used surfaces. Here, we report the preparation of a hybrid antimicrobial material based on the combination of microbiocidal polyoxometalate-ionic liquids (POM-ILs) and a biocompatible polymeric support, which enables the development of surface coatings that prevent microbial adhesion. The composite material is based on an antibacterial and antifungal room-temperature POM-IL composed of guanidinium cations (N,N,N′,N′-tetramethyl-N″, N″-dioctylguanidinum) combined with lacunary Keggin-type polyoxotungstate anions, [α-SiW11O39]8–. Integration of the antimicrobial POM-IL into the biocompatible, flexible, and stable polymer poly(methyl methacrylate) (PMMA) results in processable films, which are suitable as surface coatings or packaging materials to limit the proliferation and spread of pathogenic microorganisms (e.g., on public transport and hospital surfaces, or in ready-to-eat-food packaging).

Basicity of isostructural porous ionic crystals composed of Nb/Ta-substituted Keggin-type polyoxotungstates

Three isostructural porous ionic crystals (PICs) based on Keggin-type POMs with different compositions but equal negative charge ([BW12O40]5−, [SiW11NbO40]5−, and [SiW11TaO40]5− ) are synthesized.

Keggin型ポリオキソタングステートの合成経路に関する研究

Synthetic pathways of Keggin-type polyoxotungstates (POTs), which are expected to catalyze multi-electron transfer reactions, were investigated by first-principles calculations. The reaction pathways for the formation of new Keggin-type POTs from existing ones via ion exchange were explored by the Nudged Elastic Band (NEB) method, and the effects of the kind of heteroatom and the relative permittivity of the solvent were discussed.

Merging the chemistry of metal–organic and polyoxometalate clusters to form enhanced photocatalytic materials

The synergistic combination of a zirconium metal–organic cluster and a Keggin type polyoxotungstate led to a chemically stable and photoactive porous material.

Synthesis, characterization and photoluminescence properties of an organic–inorganic hybrid monolacunary Keggin-type polyoxotungstate

An organic–inorganic hybrid lanthanide-based polyoxometalate (Ln-POM) [N(CH3)4]3K2[Tb(C7H5O3)(H2O)2(α-PW11O39)]·10H2O (1) was synthesized and further characterized by single crystal X-ray diffraction analysis, elemental analyses, Inductive Coupled Plasma Emission Spectrometer (ICP), powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy, solid-state UV/vis spectrum and thermogravimetric analyses (TGA). Single-crystal X-ray diffraction analyses indicated that both organic ligand and [α-PW11O39]7− group directly bind Tb3+ center for the formation of polyanion in 1. The photoluminescence property of 1 was investigated at room temperature, and the results showed that compound 1 exhibits lower energy transfer efficiency from POM fragment and organic ligand to Tb3+ ion. Furthermore, time-resolved emission spectroscopy indicates that the photoexcitation O → M ligand to metal charge transfer (LMCT) of POM fragments and organic ligand can sensitize the Tb3+ ions through intramolecular energy transitions in 1.

Hofmeister effect in the Keggin-type polyoxotungstate series

The chaotropic character of Keggin-type polyoxotungstate anions was evaluated with respect to their ability to bind to γ-cyclodextrin (γ-CD) by varying the global charge density of the nanometer-sized polyanion.

Synthesis and characterization of an octanuclear nickel(II) polyoxometalate cluster

A high-nuclearity nickel-substituted polyoxotungstate, Na6K5.5H4.5 {[Ni8(OH)6(H2O)2](CO3)3(SiW9O34)2}·22H2O (1), has been obtained by the reaction of Na10[α-SiW9O34]·18H2O, Ni(NO3)2·6H2O and K2CO3 in aqueous solution and characterized by IR spectroscopy, single crystal X-ray diffraction, thermal gravimetric analysis and elemental analysis. Carbonate is not only used to adjust the pH of the solution, but also is a structure-stabilizing agent. The polyoxoanion {[Ni8(OH)6(H2O)2](CO3)3(SiW9O34)2}16- (1a) can be regarded as two {[Ni4(OH)3(H2O)](SiW9O34)} subunits connected by three carbonate groups, which represents the first polyanion containing an octanuclear nickel cluster based on trivacant Keggin-type polyoxotungstate {XW9} (X = heteroatom). The subunit {[Ni4(OH)3(H2O)](SiW9O34)} can be viewed as a tetranuclear nickel cluster linked to a trivacant Keggin-type polyoxotungstate {SiW9O34}. Furthermore, magnetic measurements show that 1 exhibits antiferromagnetic interactions.

Synthesis, catalytic activity and the structural transformation of dimeric mono-Fe (Ⅲ)-substituted Keggin-type polyoxotungstates in the oxidation of cyclohexanol with H2O2

Three dimeric mono-Fe(III)-substituted polyoxotungstates [C16H33(CH3)3N]10[(PW11FeO39)2O]·3.5H2O (1), [(C2H5)4N]10[(PW11FeO39)2O]·3H2O (2) and [(CH3)4N]10[(PW11FeO39)2O]·4H2O (3) were synthesized and characterized by elemental analysis, TGA-DSC, FT-IR, UV–vis and SEM. The catalytic activities of them were detected in the oxidation of cyclohexanol with H2O2 as oxidant. 1 with the longest carbon chain of organic countercations showed the best catalytic activity and exhibited a reaction-controlled phase transfer behavior. Many kinds of alcohols can be also efficiently oxidized into corresponding aldehydes or ketones based on 1. The fresh catalyst, the recovered one and the catalyst 1 treated with H2O2 were characterized by IR, UV and MALDI-TOF mass spectroscopy. The results manifested that the dimeric 1 decomposed into monomeric [C16H33(CH3)3N]4PW11Fe(H2O)O39·H2O (4) after the reaction. The temperature and H2O2 had an influence on the dimer-monomer transformation process. With the addition of H2O2, the solubility of 1 in the reaction solution was enhanced and 1 gradually decomposed into 4 with the temperature increasing, until decomposed completely at high temperature (70, 80 and 90 °C), eg., the dimer-monomer transformation completed at the first 3 h of the reaction at 70 °C. The mechanism of 1-catalyzed oxidation of cyclohexanol system with H2O2 was mainly a radical-chain mechanism. It was found that a faster reaction rate was observed during the transformation process based on 1 compared to 4 at the same amount of Fe in the catalyst. This suggested that there should exist the equilibrium of [(PW11FeO39)2O]10−, [PW11Fe(OH)O39]5- and [PW11Fe(H2O)O39]4- anions during the transformation process based on the radical decomposition of H2O2, which all played a catalytic role in the oxidation of cyclohexanol. But after the transformation, the [(PW11FeO39)2O]10- anion entirely decomposed into [PW11Fe(H2O)O39]4- anion until the reaction ended.

Keggin-type polyoxotungstates as mushroom tyrosinase inhibitors - A speciation study

Mushroom tyrosinase abPPO4 is a commercially relevant polyphenol oxidase and has been being targeted for numerous inhibition studies including polyoxometalates (POMs). In the present work, its diphenolase activity was inhibited at pH 6.8 by a series of structurally related polyoxotungstates (POTs) of the α-Keggin archetype, exhibiting the general formula [Xn+W12O40](8−n)− in order to elucidate charge-dependent activity correlations. Kinetic data were obtained from the dopachrome assay and 183W NMR was applied to obtain crucial insights into the actual Keggin POT speciation in solution, facilitating a straightforward assignment of inhibition effects to the identified POT species. While [PW12O40]3− was completely hydrolyzed to its moderately active lacunary form Hx[PW11O39](7−x)− (Ki = 25.6 mM), [SiW12O40]4− showed the most pronounced inhibition effects with a Ki of 4.7 mM despite of partial hydrolysis to its ineffective lacunary form Hx[SiW11O39](8−x)−. More negative Keggin cluster charges of 5− and 6− generally resulted in preclusion of inhibitory efficacy as well as hydrolysis, but with the Ni-substituted cluster [PW11O39{Ni(H2O)}]5− enzymatic inhibition was clearly restored (Ki = 9.7 mM). The inhibitory capacity of the structurally intact Keggin POTs was found to be inversely correlated to their net charge. The here applied speciation strategy is of utmost importance for any biological POM application to identify the actually active POM species.

Mo(VI) and W(VI) complexes as heterogeneous catalysts for degradation of azo dyes

Oxomolybdenum(VI) and polyoxotungstate(VI) complexes, [C2H10N2][(MoO2)4O3(Hcit)2].6H2O (1) and Al2W24O85C96H64Cl1.33Cu5 (2) (H4cit = citric acid) were synthesized and structurally characterized by single crystal X-ray diffraction analysis. Compound 1 is a tetranuclear oxomolybdenum complex and 2 is a Keggin-type polyoxotungstate hybrid containing phen ligand (phen = 1,10-phenanthroline). The obtained materials were further characterized by powder X-ray diffraction (PXRD), inductive coupled plasma optical emission spectroscopy (ICP-OES) and Fourier transformation infrared (FT-IR). The prepared complexes exhibit good activity for the catalytic wet air oxidation (CWAO) and catalytic wet oxidation (CWO) of Bismarck Brown (BB) and Direct Blue 71 (DB) as typical examples of di- and three-azo dye pollutants in water. The degradation products were analyzed by ion chromatography (IC) and the catalysts could be reused without observable loss of their activity.

Keggin type butterfly core phosphate-bridged poly-nuclear copper (II) complex photo/bi-functional electro-catalytic material

A new polyoxometalate (POM)-based inorganic-organic hybrid, [Cu4(HPO4)(PO4)(H2O)2(PBI)4][PW12O40]·H2O-(1), constructed from the Keggin-type polyoxotungstate and chelating ligand 2‑(pyridine‑2‑yl)‑1H‑benzo[d]imidazole (PBI) ligand linked by phosphate-bridged poly-nuclear copper (II) clusters has been obtained. The structure was characterized by X-ray single-crystal diffraction, IR spectroscopy, powder X-ray diffraction, thermogravimetric analysis, and elemental analysis. Compound 1 comprises of a rarely explored uncommon cationic moiety [Cu4(HPO4)(PO4)(H2O)2(PBI)4]3+ and the Keggin-type {PW12O40}3− anion, leading to three-dimensional (3-D) supra-molecular framework with compact interstices to access the guest molecules. Compound 1 displays efficient photo-catalytic performance for Methylene Blue (MB) degradation under UV light and dual function electro-catalytic properties.

Novel tetra-propyl/butylammonium encapsulated Keggin-type polyoxotungstates: synthesis, structural characterization, and catalytic capability in oxidative cleavage of unsaturated fatty acids.

Four tungsten oxide (TO)-based catalysts, (1) TO (without any organic moiety), (2) TO-TMA, (3) TO-TPA, and (4) TO-TBA (TMA: tetramethylammonium, TPA: tetrapropylammonium, and TBA: tetrabutylammonium cations) were straightforwardly prepared via a mild hydrothermal approach, based on oxidative dissolution of tungsten bare powder, followed by a facile encapsulation of the tetraalkylammonium cations (NR4+). The significant impact of the NR4+ cations on crystallization of the amorphous and metastable peroxotungstic species, formed during the synthesis, has been discussed through different characterization techniques including XRD, FTIR, TGA and DTG, TEM, EDS, N2 adsorption/desorption isotherms, zeta potential measurement, and elemental analysis. TO-TPA and TO-TBA were composed of two novel hybrid inorganic-organic polyoxotungstates, [(CH3CH2CH2)4N]2.75[H5.25W12O40]·7.42H2O and [(CH3CH2CH2CH2)4N]3.31[H4.69W12O40]·1.08H2O, respectively. The capability of the synthesized catalysts in oxidation of unsaturated fatty acids (UFAs) has been examined in the reaction of oleic acid, the most abundant UFA, with H2O2. This is the first report on employing heterogeneous Keggin clusters of tungsten oxide in the oxidative cleavage of UFAs. The catalysts exhibited excellent efficiencies, particularly TO-TPA and TO-TBA, owing to the strong and organo-protected acidity of their Keggin structures, which resulted in full conversion of oleic acid and superior yields of the desired products. Despite their pseudo-homogeneous behavior in the reaction, the NR4+-encapsulated catalysts demonstrated efficient recovery and steady reusability without any significant decay in their activities up to at least 4 cycles.

Electrochemical and Computational Study of Ion Association in the Electroreduction of PW12O403–

Insights into ion pairing effects on the redox properties of the Keggin-type polyoxotungstate PW12O403– are gained by combining electrochemical experiments and density functional theory (DFT) calculations. Such effects have been reported to affect the performance of these species as molecular electrocatalysts. Experimental square wave voltammetry (SWV) of the two-electron reduction of PW12O403– in acetonitrile evidences that the reduced forms PW12O404– and PW12O405– can be significantly stabilized by ion association. The strength and stoichiometry of the corresponding aggregates are estimated as a function of the nature of the cation (lithium, sodium, and tetramethylammonium) and the oxidation state of the polyoxometalate. The results obtained in combination with DFT enable us to examine the roles of the cation solvation and the charge number and distribution of the polyanions.

Design of Metal-to-Metal Charge-Transfer Chromophores for Visible-Light Activation of Oxygen-Evolving Mn Oxide Catalysts in a Polymer Film

To construct photoresponsive unidirectional charge transfer units for the activation of oxygen-evolving manganese oxide (MnOx) catalyst, metal-oxide nanoclusters consisting of cerium (CeIII) or cobalt (CoII) ions and Keggin-type polyoxotungstate (PW12O403–) were synthesized in a polymer matrix as visible-light-absorbing chromophores. The utilization of the polymer matrix enabled the molecularly dispersed PW12O403– states and was advantageous to achieve product-separable energy conversion systems. The reaction of PW12O403– with Ce(NO3)3 or CoCl2 in the polymer matrix generated the new broad absorption tails extending from the UV to visible region, assignable to metal-to-metal charge transfer (MMCT) transitions of oxo-bridged binuclear WVI–O–CeIII and WVI–O–CoII units. Cyclic voltammetry analysis of the oxo-bridged binuclear units in the polymer membrane revealed that the CoIII/CoII couple had a 300 mV more positive redox potential than that of CeIV/CeIII and was capable of extracting electrons from MnOx ca...

Zn-containing double complex salts formed by Keggin type polyoxotungstates: Synthesis and crystal structure

Novel double complex salts, [Zn(DMF)6]2[SiW12O40] · 2H2O (I) and [Zn(H2O)2(DMF)4][Zn(DMF)6]2[PW12O40]2 · 6DMF (II) (DMF = N,N-dimethylformamide), were prepared by the reaction of Zn2+ and heteropoly acids Hx[EW12O40] · xH2O (E = P, X = 3, E = Si, X = 4) in DMF. Compounds I and II were studied by X-ray diffraction (СIF files CCDC nos. 1497570 (I) and 1497571 (II)) and IR spectroscopy.

Polyoxometalate silver salts loaded with silver nanoparticles as new photocatalysts under visible light

A series of polyoxometalate silver salts (Ag n POM) with low solubility were prepared by mechanically grinding the dry solid mixture of Keggin-type polyoxotungstates with different central heteroatoms and the silver nitrate. The as-prepared Ag n POM solids were further UV-irradiated, resulting in the generation of metallic silver nanoparticles (NPs) on the surface of the Ag n POM salts. Thus, a series of composite Ag n POM materials loaded with different amount of Ag NPs (Ag0 x /AgI n POM) were obtained. The UV-Vis diffuse reflectance spectra show that the intensities of absorbing bands of the composite materials in visible area have been enhanced due to the plasmon resonance of Ag NPs. Such composite materials exhibit enhanced visible light-driven photocatalytic performances for dye degradation and the reduction of Cr(VI). Furthermore, the loading amount of Ag NPs on the photocatalysts can be tuned by changing the negative charge of POM anions. As a result, the relevant photocatalytic activity of such composite materials can be also adjusted.