Polyoxometalates

An efficient and robust heterogeneous electrocatalyst, FSWZ-8 ((Fe-(salen)(OH) + H 4 [SiW 12 O 40 ]·HCl)@ZIF-8) for oxygen evolution reaction (OER) at the neutral pH, was developed by coencapsulation of Fe-salen (i.e., Fe(salen)Cl) and SiW 12 (i.e., H 4 [SiW 12 O 40 ]) inside the cavity of zeolitic imidazolate framework-8 (ZIF-8) material by an in situ synthesis. Here ZIF-8 functions as a host, Fe-salen as the active catalyst, and SiW 12 helps in the charge transport by lowering the overall electrical resistance of the resulting composite system. High turnover frequency (∼5 s −1) and high Faradaic efficiency (∼94%) make the concerned composite an efficient catalyst toward water oxidation. This is the first report of one of the simplest known metal complexes, Fe-salen, to perfrom electrocatalytic OER as a heterogeneous catalyst in the neutral pH. This work also highlights the benefits of coencapsulation of the Keggin polyoxometalate (POM) along with the active catalyst Fe-salen species. Encapsulation of SiW 12 results in (i) faster formation of FSWZ-8 composite, (ii) higher loading of Fe-salen, and, most importantly, (iii) lowering of required overpotential for electrochemical OER by more than 150 mV. The Keggin POMs, located as discrete molecular oxides inside the cavity of ZIF-8 as well as on the surface of ZIF-8, facilitate electrical charge conduction in the ZIF-8 matrix and lower the overall charge-transfer resistance.

Two new polyoxometalate (POM)-based hybrid monomers (Bu 4 N) 5 (H)[P 2 V 3 W 15 O 59 {(OCH 2) 3 CNHCO(CH 3)C=CH 2 }] (2) and (S(CH 3) 2 C 6 H 4 OCOC(CH 3) = CH 2) 6 [PV2Mo 10 O 40 ] (5) were developed by grafting polymerizable organic units covalent-ly or electrostatically onto Wells–Dawson and Keggin-type clusters and were characterized by analytical and spectro-scopic techniques including ESI-MS and/or single-crystal X-ray diffraction analyses. Radical initiated polymerization of 2 and 5 with organic monomers (methacryloyloxy)phenyldi-methylsulfonium triflate (MAPDST) and/or methylmethacry-late (MMA) yielded a new series of POM/polymer hybrids that were characterized by 1 H, 31 P NMR and IR spectroscopic techniques, gel-permeation chromatography as well as thermal analyses. Preliminary tests were conducted on these POM/polymer hybrids to evaluate their properties as photo-resists using electron beam (E-beam)/extreme ultraviolet (EUV) lithographic techniques. It was observed that the POM/polymer hybrid of 2 with MAPDST exhibited improved sensitivity under EUV lithographic conditions in comparison to the MAPDST homopolymer resist possibly due to the efficient photon harvesting by the POM clusters from the EUV source.

Ternary phthalocyanine–metal–polyoxometalate (Pc–M–POM)
complexes were synthesized and characterized. Group (IV) Hf or
Zr ions reside outside the plane of the macrocycle and are
coordinated to both the phthalocyanine and the lacunary
polyoxometalate. The metal ions mediate the electronic communication between the Pc and the POM.

Herein, we want to report a simple and convenient way for protection-deprotection of alcohols in the presence of ammonium decatungstocerate(IV) {(NH4)8[CeW10O36]·20H2O} as catalyst under ambient temperature in CH3CN. Using 0.002 mmol of the catalyst, various alcohols and phenols were transformed easily to the corresponding TMS ethers in excellent yields. In the second part, various TMS ethers were successfully converted to the parent hydroxyl compounds in the presence of the ammonium decatungstocerate(IV) catalyst. Copyright © 2010 John Wiley & Sons, Ltd.

Stable anchoring of polyoxometalate H3PMo12O40 onto in-situ Reduced Graphene Oxide leads to a hybrid electrode with combined capacitive (GO) and faradaic (H3PMo12O40) charge storage mechanisms featuring increased capacitance, energy density and extended cyclability.

Supercapacitors operating in physiological electrolytes are of great relevance for both their environmentally
friendly aspect as well as the possibility to be employed for powering implantable microelectronic
devices using directly biological fluids as electrolytes. Polyoxometalate (POMs) have been
proven to be useful for supercapacitors in acidic media. However, in neutral pH, POMs are usually not
stable. One relevant alternative is to stabilize POMs by pairing them with organic moieties to form hybrids.
In this study, we combined K6P2Mo18O62$12H2O (P2Mo18) with Ru(bpy)3Cl2.6H2O (Ru(bpy)). The
synthesis was carried out with and without the mild reducing agent KI. The hybrids were characterized
by CHN analysis, TEM, FT-IR, XRD, TGA and cyclic voltammetry. CHN elemental analysis revealed that one
mole [P2Mo18O62]6 is paired with 3 mol [Ru(bpy)3]2þ to form [Ru(bpy)3]3PMo18O62$nH2O. With KI
present, [P2Mo18O62]6 is linked to 3.33 mol to yield [Ru(bpy)3]3.33PMo18O62$mH2O. Excess of Ru(bpy) in
[Ru(bpy)3]3.33PMo18O62$mH2O was further confirmed by TEM, FT-IR, XRD, TGA and cyclic voltammetry.
In turn, hybrid composition is found to strongly influence the supercapacitor behavior. The hybrid rich in
Ru(bpy) is found to perform better for supercapacitors in physiological electrolytes. 125 F g1 and
68 F g1 are the capacitance values obtained with [Ru(bpy)3]3.33PMo18O62$mH2O and [Ru(bpy)3]3PMo18O62$
nH2O, respectively. In terms of specific energy densities, 3.5 Wh kg1 and 2 Wh kg1 were
obtained for both hybrid simultaneously. The difference in supercapacitor performance between both
hybrids is also noticed in impedance spectroscopy which showed that [Ru(bpy)3]3.33PMo18O62$mH2O has
lower electron transfer resistance if compared to [Ru(bpy)3]3PMo18O62$nH2O. Finally, if compared of
parent K6P2Mo18O62$12H2O, the stability of both hybrids is found to be highly improved

Composites of Keggin heteropolymetalates have been prepared and characterized by SEM, XPS, 31P-magic angle spinning/nuclear magnetic resonance and differential scanning calorimetry. The phosphotungstic acid and phosphomolybdic acid form nanocomposites where the oxocluster structure is preserved and is highly dispersed over the material, probably inserted between the polymeric chains. Phosphovanadotungstic clusters suffer partial decomposition during composite formation, resulting in a biphasic microcomposite. The decomposition product, phosphotungstic acid presents surface segregation as shown by comparison of XPS and nuclear magnetic resonance results. Copyright © 2002 John Wiley & Sons, Ltd.

A new [P 2 V 3 W 15 O 62 ] 9− cluster based covalent polyoxometalate-organic hybrid (P1) containing a naphthol-amide-Tris derivative has been synthesized and characterized using standard analytical and spectroscopic techniques including single crystal X-ray diffraction analyses. P1 exhibits interesting self-assembly behaviour leading to the generation of ~200 nm spherical aggregates in solutions as revealed by DLS and TEM analyses. Standard in vitro experiments revealed that P1 is capable of exhibiting moderate antioxidant properties under standard experimental conditions.

The Dawson tungstovanadate [P2W15V3O62]9− can be grafted to secondary diolamides. The electron-withdrawing character of the polyanion increases the acidity of the amide proton, leading to an organo-polyoxometalate, which can be used as a Brønsted organocatalyst. High-field NMR and DFT modeling indicate that the amide proton stays on the nitrogen and that the exalted acidity derives from the interaction between the organic and inorganic parts of the organo-polyoxometalate. The amide-inserted vanadotungstates thus form a new family of (hybrid) heteropolyacids, offering new perspectives for the application of POM-based catalysis in organic synthesis.

Hybrid electroactive materials based on Polyoxometalate – adsorbed Activated Carbons have been developed for the first time as electrodes for Supercapacitors. By combining the double layer capacitance of a high–surface area material with the reversible redox activity of a polyoxometalate (POM), enhanced electrochemical performance is obtained.

In this paper, phosphomolybdic acid H3PMo12O40 (PMo12) was anchored to an activated carbon providing 160 Fg- 1 and 183 Fg- 1 for the positive and negative hybrid electrodes, respectively (at 2 Ag- 1), substantially larger than the 136 Fg- 1 of the unmodified activated carbon. This translates to a cell capacitance for the AC–POM supercapacitor is 45 Fg- 1 versus 35 Fg- 1 for the AC supercapacitor, i.e. a 29% increase after POM integration into the AC matrix.

Organic-inorganic hybrid materials are an upcoming class of materials that attract increasing research interest. In the present manuscript, we report on the combination of a particularly versatile organic matrix – the conductive polymer polyaniline (PAni) – with a special type of polyoxometalates (POMs), namely a lacunary Keggin anion, [AsW9O33]9–. Although the high application potential and the manifold structural motifs of POMs provide an almost infinite number of options for composite materials, up to now, only few POM-polymer combinations have been explored in more detail. Thus, we have investigated the interaction of PAni with the unique reactivity of [AsW9O33]9– that combines building-block facilities with the presence of a lone pair on the heteroatom. The resulting composite material was characterized with a wide spectrum of analytical techniques and the effect on the reaction conditions on the nanoscale structuring was investigated. Furthermore, the newly synthesized POM-PAni composites display a stable temperature-dependent resistivity that renders them promising for further applications.

A B S T R A C T Using Density Functional Theory (DFT) calculations, we studied the electrochemistry of polyoxome-talates (POMs), specifically the redox properties of Mn in tri-Mn-substituted W-based Keggin ions. For direct comparison with recent cyclic voltammetry results [J. Friedl et al. Electrochim. Acta, 141 (2014) 357], we estimated the reversible half-wave potentials of proton-and cation-coupled electron transfer for Mn(IV/III) and Mn(III/II), respectively. The calculated reversible potentials agree well with experiment, reproducing the trend with pH for Mn(IV/III). For adequate DFT energies, it is crucial to apply a reliable description of the electrolyte environment of the POM, accounting also for their rather high charges, up to À7 e. To this end, we included the Li + counterions, required for charge neutralization, directly in the quantum chemical models which were embedded in a polarizable continuum. We explored various arrangements of the Li + ions around the POMs and their effect on both structural parameters and electrochemical properties of the POMs. Hybrid functionals (TPSSh, B3LYP, PBE0) overestimate the experimental reduction potentials: the larger the exact-exchange contribution, the larger the resulting reduction potential. The best agreement with experiment is achieved with the PBE approach, likely due to fortuitous error cancellation. The results of the present work indicate that a more sophisticated (atomistic) representation of the electrolyte environment will be beneficial for predicting redox potentials in better agreement with experiment.

Small-angle X-ray scattering (SAXS) studies of aqueous [Ta6O19]8−
compared to prior studies of aqueous [Nb6O19]8− reveals key
differences in behaviour, which is likely at the root of the difficultly
in developing polyoxotantalate chemistry. Specifically, where
contact ion-pairing dominates between [Nb6O19]8− and its
counterions, solvent-separated ion-pairing between [Ta6O19]8−
and its counterions has been unveiled in the current study.

While sophisticated computational methods can predict 31P NMR spectra of phosphorus atoms encapsulated within Keggin-derived heteropoly tungstate and molybdate cluster anions, calculated and experimental chemical shift values typically deviate considerably from one another. Motivated by the observation that experimentally determined 31P chemical shift values within a series of water-soluble plenary and metal-cation substituted lacunary Keggin anions, [PMnW11O39](7–n)– (Mn = Ag+, Zn2+, Nb5+, W6+) and [(PW11O39)2Mn](14–n)– (Mn = Y3+, Zr4+), varied as a linear function of the oxidation states, n, of the complexed Mn cations, a linear correlation was sought between observed chemical shift values and the net Coulombic forces experienced by the encapsulated phosphorus atoms. The Coulombic model based on Shannon radii, published electronegativity values, and bond angles from X-ray crystallographic data remarkably accounted for the relative 31P chemical shift values of phosphorus atoms in over 50 metal-oxide cluster anions, including large structures comprised of up to four Keggin-derived fragments with an overall R2 value of 0.974. With the model being applied here to three cluster anions whose 31P chemical shift values are reported here for the first time, predicted and experimental values differed by only ±0.4 ppm.

A new series of class II polyoxometalate–organic hybrids based on Mn-Anderson type polyoxometalate cluster and aromatic organic moieties has been synthesized and characterized through various analytical and spectroscopic techniques including single crystal X-ray analyses in some cases. The genotoxic effects of these covalent hybrids were evaluated by studying their effects on Allium cepa cells which revealed their low toxic nature as compared to the parent polyoxometalate cluster.

Transition metal heteropolyanions have been used to catalyze a variety of organic oxidations but have not previously been used for O2 generation, despite sharing some structural similarities with dioxoruthenium water-oxidation catalysts. In this study, we report that the di-Ru-substituted polyoxometalate (POM) [Ru2Zn2(H2O)2(ZnW9O34)2]14- can be used to catalyze the electrochemical generation of O2. By comparing the behavior of this compound to that observed using a mono-Ru-substituted POM catalyst, we show that adjacent Ru sites are necessary to observe O2 generation. These observations suggest a reaction pathway involving two Ru-bound oxygen species combining to form O2 and are consistent with the accepted mechanism of electrochemical oxygen evolution. Finally, analysis of the observed electrode kinetics yields a Tafel slope of roughly 120 mV, which is similar to values reported previously for perovskite anodes.