Polyoxometalate Anions Research Articles

Two hybrid polyoxometalate-pillared metal–organic frameworks

Two polyoxometalate-pillared 3D compounds, {Cu(5)(2-ptz)(6)(H(2)O)(4)(SiW(12)O(40))}·4H(2)O 1 and {Cu(9)(2-ptz)(12)(H(2)O)(6)(PMo(12)O(40))(2)}·H(2)O 2 (2-ptz = 5-(2-pyridyl)tetrazole) have been constructed based on different polyoxometalate anions, and copper-organic coordination polymer sheets by a hydrothermal method. Magnetic investigations reveal that both 1 and 2 exhibit antiferromagnetic coupling between the Cu(II) ions. Structural studies show the compound 1 exhibits a typical pcu-type net with the Schälfli symbol of {4(12)·6(3)}, and that compound 2 is a (3,4,6)-connected framework with novel {4(4)·6(10)·10}{6(3)}(2){6(5)·8} topology which has not been reported to date.

Novel intercluster compound between a heptakis{triphenylphosphinegold(i)}dioxonium cation and an α-Keggin polyoxometalate anion

A novel intercluster compound, [{{Au(PPh(3))}(4)(μ(4)-O)}{{Au(PPh(3))}(3)(μ(3)-O)}][α-PW(12)O(40)]·EtOH (1) constructed between a heptakis{triphenylphosphinegold(i)}dioxonium cation and an α-Keggin polyoxometalate (POM) is synthesized and unequivocally characterized by elemental analysis, TG/DTA, FTIR, X-ray crystallography, solid-state CPMAS (31)P NMR and solution ((1)H, (31)P{(1)H}) NMR. The heptagold(i) cluster was formed during the course of carboxylate elimination of a monomeric phosphinegold(i) carboxylate precursor, i.e., [Au((RS)-pyrrld)(PPh(3))] ((RS)-Hpyrrld = (RS)-2-pyrrolidone-5-carboxylic acid), in the presence of the sodium salt of an α-Keggin POM, Na(3)[α-PW(12)O(40)]·9H(2)O. Compound 1 was formed by ionic interaction between the heptagold(i) cluster cation and the α-Keggin POM anion. The heptagold(i) cluster unit was formed by four inter-cationic aurophilic interactions between the tetragold(i) cluster unit and trigold(i) cluster unit. The tetragold(i) cluster unit and trigold(i) cluster unit contained μ(4)-O and μ(3)-O atoms, respectively.

Cobalt porphyrin–tungsten polyoxometalate anion as non-noble metal cathode catalyst in a fuel cell

A mixture of a cationic cobalt 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin (CoPP) and an anionic metatungstate (POM) supported on graphite carbon was developed as a high performance novel cathode catalyst using a rotating ring-disk electrode for the oxygen reduction reaction (ORR) with high current density and low hydrogen peroxide production in a fuel cell. The CoPP-POM/carbon catalysts were characterized by XRD, X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy and BET surface area measurements. The 823 K-treated CoPP-POM/carbon catalyst exhibited the highest activity for the ORR at 0.85 V (RHE) at −0.005 mA cm−2 and a current density of 1.55 mA cm−2 at 0.8 V (RHE). The CoPP-POM/carbon exhibited an exchange current density of 2.28 × 10−9 mA cm−2 and a H2O2 percentage of less than 5%. The metatungstate anion was coordinated with the cobalt porphyrin compound and adsorbed on the surface of the Vulcan carbon. The active species of the catalyst for the ORR were combined with the Co ions in the cobalt porphyrin compound and the metatungstate anion on the graphite.

Layer-by-layer assembled MoO2–graphene thin film as a high-capacity and binder-free anode for lithium-ion batteries

Thin films of MoO(2) nanoparticles and graphene sheets are created by layer-by-layer (LBL) assembly as binder-free anodes for lithium-ion batteries. Both anionic polyoxometalate clusters and graphene oxide nanosheets with oxygen functional groups on both basal planes and edges are assembled into LBL films with the aid of a cationic polyelectrolyte. After a subsequent thermal treatment in an Ar-H(2) atmosphere, hybrid MoO(2)-graphene films with three-dimensionally interconnected nanopores are formed, which comprise ultrafine MoO(2) nanoparticles homogeneously embedded in the porous network of graphene nanosheets. When used as an anode for lithium-ion batteries, the MoO(2)-graphene thin-film electrode shows superior electrochemical performance with high specific capacity and excellent cyclability. A high specific capacity of 675.9 mA h g(-1) after 100 discharge-charge cycles is achieved, indicating a promising anode candidate for lithium-storage applications.

Intriguing Role of a Quaternary Ammonium Cation in the Dissociation Chemistry of Keggin Polyoxometalate Anions

The gas-phase fragmentations of a series of Keggin polyoxometalate anions with molecular formula of TBA(n)[XM(12)O(40)] (X = P, Si; M = Mo, W) were studied by electrospray ionization tandem mass spectrometry. The bare polyoxoanions [XM(12)O(40)](n-) as well as the non-covalent complexes {TBA[XM(12)O(40)]}((n-1)-) and {TBA(m)[XM(12)O(40)](2)}(3-) displayed characteristic dissociation pathways. Fragmentation of [XM(12)O(40)](n-) led to pairs of complementary product anions whose total stoichiometry and charge matched those of the precursor anion, consistent with the previous study by Ma et al. The nature of the non-covalent interaction between [XM(12)O(40)](n-) and TBA(+) was addressed in detail via the example of {TBA[XM(12)O(40)]}((n-1)-). The non-covalent interaction [1] primarily dominated by the Coulombic attraction of the opposite charges completely changed the dissociation chemistry of [XM(12)O(40)](n-). The non-covalent complexes {TBA[XM(12)O(40)]}((n-1)-) and {TBA(m)[XM(12)O(40)](2)}(3-), formed by the charge reduction during the electrospray process, underwent distinct dissociation routes: {TBA[XM(12)O(40)]}((n-1)-) fragmented to give rise to its product ion {(C(4)H(9))[XM(12)O(40)]}((n-1)-) by cleaving the N-C covalent bond inside the TBA(+) cation whereas {TBA(m)[XM(12)O(40)](2)}(3-) dissociated into a pair of product ions, {TBA(i)[XM(12)O(40)]}(2-) and {TBA(m-i)[XM(12)O(40)]}(-), by breaking the non-covalent bond between [XM(12)O(40)](n-) and TBA(+). In addition, energy-variable CID was used to map the relative stabilities of the ion clusters in the gas phase, which was in excellent agreement with the relative orders of thermal stability in the condensed phase.

From 1D to 3D Single-Crystal-to-Single-Crystal Structural Transformations Based on Linear Polyanion [Mn4(H2O)18WZnMn2(H2O)2(ZnW9O34)2]4–

A 1D anionic polyoxometalate, [Mn(4)(H(2)O)(18)WZnMn(2)(H(2)O)(2)(ZnW(9)O(34))(2)](4-), undergoes 1D to 3D single-crystal-to-single-crystal structural transformations that are induced by transition-metal cations (Co(2+) and Cu(2+)) and solvent molecules. These solid materials present interesting catalytic activity for the oxidative aromatization of Hantzsch 1,4-dihydropyridines that is dependent on the inserted heterogeneous metal cations.

ChemInform Abstract: Counterion Interaction and Association in Metal‐Oxide Cluster Macroanionic Solutions and the Consequent Self‐Assembly

AbstractReview: 80 refs.

Polyoxometalate and copolymer-functionalized ionic liquid catalyst for esterification

A new room-temperature ionic liquid (RTIL) consisting of a polyoxometalate (POM) anion and tri-block copolymer (P123)-functionalized imidazolium cation was synthesized and utilized as a halogen-free catalyst for esterification. The catalytic system was a homogeneous solution at the beginning of the reaction, but an emulsion formed during the course of the reaction, and a progressive phase separation of the catalyst occurred at 0 °C over the course of 3 h. Dynamic light scattering (DLS), transmission electron microscopy (TEM), and Fourier transform/infrared spectroscopy (FT/IR) have been used to characterize the properties of the IL during the reaction. The new IL catalyst was found to be highly efficient in the esterification of various alcohols and can be recycled at least seven times.

Heteropoly blue-intercalated layered double hydroxides for cationic dye removal from aqueous media

Heteropoly blue-intercalated layered double hydroxide (HB-LDH) was obtained by aqueous ion exchange of a Zn–Al LDH precursor in nitrate form with the reduced polyoxometalate anions [PW 10Mo 2O 40] 5−. The physicochemical properties of the product were characterized by the methods of powder X-ray diffraction, infrared spectroscopy, and cyclic voltammetry. The HB-LDH has been used for the removal of cationic dye methylene blue (MB) from aqueous solutions via adsorption. The intercalation of large cluster anion [PW 10Mo 2O 40] 5− into LDH could induce the adsorption to cationic dye of MB, obviously. The HB-LDH shows much higher cationic dye adsorption capacity than pure LDH and the maximum adsorption capacity Q max of MB onto ZnAl–PW 10Mo 2 is 30.87 mg/g.

ChemInform Abstract: Nanoscale Polyoxometalate‐based Inorganic/Organic Hybrids

AbstractReview: 107 refs.

ChemInform Abstract: Ionic Liquid‐Enhanced Photooxidation of Water Using the Polyoxometalate Anion [P2W18O62]6‐ as the Sensitizer.

AbstractThe photochemistry of K6[P2W18O62] is studied in the presence of water and the aprotic ionic liquid (bmim)[BF4] and the protic ionic liquid diethanolamine hydrogen sulfate.

Electron transfer behavior at polyoxometalate-adsorbed alkanethiol self-assembled monolayers

The interaction between polyoxometalate (POM) anions, SiMo 12O 40 4−, and a self-assembled monolayer (SAM) of dodecanethiol (DT) on Au surfaces was investigated using electrochemical methods, X-ray photoelectron spectroscopy, and scanning probe microscopy. The SiMo 12O 40 4− ions adsorb on the SAM of DT on Au to form a composite organic–inorganic hybrid layer. The adsorbed SiMo 12O 40 4− ion on the SAM layer shows its characteristic redox waves with an electron transfer rate slower than that on a bare Au electrode. The electron transfer behavior at DT−SAM could be regulated by the adsorption of SiMo 12O 40 4− depending on the charge of the investigated electroactive species: a significant increase toward a positively charged Ru(NH 3) 6 3+ ion, a moderate increase toward a neutral 1,1′-ferrocenedimethanol molecule and a slight decrease toward a negatively charged Fe(CN) 6 3− ion. The effect of the chain length of alkanethiols on the adsorption of SiMo 12O 40 4− ion was also investigated: as the chain length decreases, the amount of the adsorbed POM increases and the electron transfer rate through the composite layers increases. The nature of SiMo 12O 40 4− ions adsorbed on the SAMs of alkanethiols on Au is discussed in detail.

Nanoscale polyoxometalate‐based inorganic/organic hybrids

The latest advances in the area of polyoxometalate (POM)-based inorganic/organic hybrid materials prepared by self-assembly, covalent modification, and supramolecular interactions are presented. This Review is composed of five sections and documents the effect of organic cations on the formation of novel POMs, surfactant encapsulated POM-based hybrids, polymeric POM/organic hybrid materials, POMs-containing ionic crystals, and covalently functionalized POMs. In addition to their role in the charge-balancing, of anionic POMs, the crucial role of organic cations in the formation and functionalization of POM-based hybrid materials is discussed.

Ionic Liquid-Enhanced Photooxidation of Water Using the Polyoxometalate Anion [P2W18O62]6–as the Sensitizer

Simple polyoxometalate anions are known to be photoreduced in molecular solvents in the presence of 2-propanol or benzyl alcohol. The use of ionic liquids (ILs) as the solvent is now reported to also allow the photooxidation of water to be achieved. In particular, the photochemistry of the classic Dawson polyoxometalate salt K(6)[P(2)W(18)O(62)] has been studied in detail when water is present in the aprotic IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF(4)]) and the protic IL, diethanolamine hydrogen sulfate (DEAS). In these and other ILs, irradiation with white light (wavelength 275-750 nm) or UV light (wavelength 275-320 nm) leads to overall reduction of the [P(2)W(18)O(62)](6-) anion to [P(2)W(18)O(62)](7-) and concomitant oxidation of water to dioxygen and protons. The modified structure of bulk water present in ILs appears to facilitate its oxidation. Analogous results were obtained in aqueous solutions containing the protic IL as an electrolyte. The photoproducts (reduced polyoxometalate anion, dioxygen, and protons) were identified by, respectively, voltammetry, a Clark electrode, and monitoring of pH. The formal reversible potentials E(0)(F) for [P(2)W(18)O(62)](6-/7-/8-/9-/10-) couples are much more positive than in molecular solvents. The [P(2)W(18)O(62)](8-) and more reduced anions, if formed as intermediates, would efficiently reduce photoproducts H(+) or dioxygen to produce [P(2)W(18)O(62)](7-), rather than reform to [P(2)W(18)O(62)](6-). Thus, under photoirradiation conditions [P(2)W(18)O(62)](7-) acts as a kinetic sink so that in principle indirect splitting of water to produce dioxygen and dihydrogen can be achieved. The equivalent form of photooxidation does not occur in liquid water or in molecular solvents such as MeCN and MeCN/CH(2)Cl(2) containing added water, but does occur for solid K(6)[P(2)W(18)O(62)] in contact with water vapor.

Surface immobilisation of transition metal substituted Krebs type polyoxometalates, [X 2W 20M 2O 70(H 2O) 6] n− (X = Bi or Sb, M = Co 2+ or Cu 2+), by the layer by layer technique

A series of transition metal (i.e. Cu 2+ and Co 2+) substituted Krebs type polyoxometalates (POMs), of the general formula [X 2W 20M 2O 70(H 2O) 6] n− , X = Sb or Bi, M = Co(II) or Cu(II), have been successfully immobilised onto carbon electrode surfaces through the employment of the layer-by-layer (LBL) technique. This involved the construction of alternating anionic POM, [X 2W 20M 2O 70(H 2O) 6] n− , layers and the cationic metallodendrimer, Ru(II)-metallodendrimer as the cationic layers, in addition to a [poly(diallyldimethylammonium chloride)] PDDA base layer. Stable multielectron redox couples associated with the W–O framework, for the Krebs type POMs, and the Ru(III/II) for the metallodendrimer, were clearly observed upon layer construction and redox switching within the pH domain of 2–6.5. The constructed multilayer assemblies exhibited pH dependent redox activity and thin layer behaviour up to 100 mV s −1. The porosity and permeability of the individual multilayer assemblies towards an anionic probe were determined by AC impedance and cyclic voltammetry. The surface morphology of each multilayer was also determined by Atomic Force Microscopy (AFM).

A 12-molybdophosphoric acid supramolecular compound: Synthesis, crystal structure and photochromic properties

A new 12-molybdophosphoric acid supramolecular compound based on Keggin polyoxometalate building blocks, (C 6H 6NO 2)·(C 5H 6N) 2·[PMo 12O 40]·(C 6H 5NO 2)·2H 2O ( 1), has been conventionally synthesized and structurally characterized by elemental analyses, the powder X-ray diffraction (XRD), TG, IR, and single crystal X-ray diffraction, respectively. Compound 1 is composed of polyoxometalate anions [PMo 12O 40] 3−, pyridinium cations, 4-carboxylpyridinium cation and lattice water. A three-dimensional network is formed by N–H⋯O, C–H⋯O and O–H⋯O hydrogen-bond interactions. In addition, the supramolecular compound shows photochromism.

基于Keggin 型多酸和金属有机配合物的超分子化合物

A supramolecular compound, [Cu2(HL)7(H2O)2(SiW12O40)3]·(H2L)0.5·20.5H2O, based on Keggin-type polyoxometalate anions and metal-organic cations and non-linear 1,3,4-oxadiazole derivatives was obtained via hydrothermal reactions. We structurally characterized the compound using infrared spectroscopy, thermogravimetric analysis and single-crystal X-ray diffraction. The electrochemical properties of a bulk modified carbon-paste electrode based on the supramolecular compound were studied in 1 mol/L H2SO4 aqueous solution. The results indicate that the compound is a monoclinic system with a C2/c space group. The basic building blocks of the compound are Keggin-type SiW12O404-anions; metal-organic units, [Cu2(HL)7(H2O)2]11+; a discrete protonated ligand, 2-(3-pyridyl)-5-(4-pyridyl)-1,3,4-oxadiazole; and water molecules. The supramolecular compound has a high level of electrocatalytic activity.

Reactivity of one-, two-, three- and four-electron reduced forms of α-[P2W18O62]6− generated by controlled potential electrolysis in water

One, two, three and four electron reduced forms of α-[P2W18O62]6− in aqueous acidic electrolyte media have been selectively generated by bulk electrolysis from a solution that has an initial pH of 3.6. The reactivities of the reduced polyoxometalate anions and identities of products formed in the presence and absence of dioxygen have been assessed via oxygen and hydrogen Clark-type electrodes, a pH electrode and rotating disk electrode voltammetry. [P2W18O62]7− is stable to protons but is slowly oxidized by dioxygen (timescale: hours to days) back to [P2W18O62]6−. In contrast, [P2W18O62]8− reacts more rapidly with O2 and slowly with H+, whereas generation of the [P2W18O62]9− and [P2W18O62]10− anion is accompanied by a large increase in pH and rapid reaction with O2 or, in its absence, with H+. Consequently, it is concluded that photocatalytic reactions based upon [P2W18O62]6− chemistry are only likely to be of significance if [P2W18O62]9− or more highly reduced species are generated and form part of the catalytic cycle.

Synthesis and characterization of hybrid materials based on 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid and Dawson-type tungstophosphate K 7[H 4PW 18O 62]·18H 2O and K 6[P 2W 18O 62]·13H 2O

In this study, we synthesized hybrid materials using well-Dawson polyoxometalates (POMs), K 7[H 4PW 18O 62]·18H 2O or K 6[P 2W 18O 62]·13H 2O and a room temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF 4]). K, W, P and CHN elemental analysis showed that one mole of [H 4PW 18O 62] 7− reacts with 6 moles of BMIM + and one mole of [P 2W 18O 62] 6− reacts with 4 moles of BMIM + to form, respectively, K[BMIM] 6H 4PW 18O 62 and K 2[BMIM] 4P 2W 18O 62. X-ray diffraction illustrated amorphous structure of the hybrid materials. FT-IR spectra showed the presence of both 1-butyl-3-methylimidazolium cation and the Dawson anion. TG analysis displayed a relative thermal stability of the hybrid materials compared to the parents Dawson POMs. Cyclic voltammetry showed that the reduction peak potentials of the Dawson anion in the hybrid materials shift towards negative values and the shift is more pronounced for K[BMIM] 6H 4PW 18O 62 compared to K 2[BMIM] 4P 2W 18O 62. This was attributed to a decrease in the acidity of the Dawson POM anion in the hybrid material.

Counterion Interaction and Association in Metal‐Oxide Cluster Macroanionic Solutions and the Consequent Self‐Assembly

AbstractThe interaction between large metal‐oxide polyanions and their counterions is unique. Owing to their size disparity, there is a moderate ion‐pairing effect and loose distribution of counterions around macroions, which leads to the unique solution behavior and the self‐assembly of the macroions in polar solvents, and the counterion exchange capability around macroions. Furthermore, the macroion–counterion interaction also affects the catalytic behavior of the polyoxometalate (POM) clusters. Replacement of functionalized cations helps to modify the POM anions through static charge attraction. At the same time, the strong POM–counterion interaction can also lead to counterion‐dependent synthesis. Recent developments on theoretical simulations help to understand this interaction at the molecular scale. This review summarizes the chronological progress of the exploration of macroion–counterion interaction (both theoretically and experimentally) and its impact on related research fields.