Mixed-addendum Polyoxometalate Research Articles

Confined PMo6W6 for energy storage

Mixed-addenda polyoxometalate (e.g., PMoxW12-x) by “orbital engineering” allows the functionalization of the single-addenda cluster surface, which offers new electronic properties. However, the well-known agglomeration phenomenon greatly limits the full understanding of its unique redox properties. It makes sense to fully stimulate the intrinsic multi-electron activity of mixed-addenda polyoxometalate by confining engineering to apply in energy technology. With the verification of potential candidate PMo6W6 possessing remarkable stability with fully exposed activity sites in a confined state by theoretical analysis, we achieve the precise confinement of the single PMo6W6 molecule in porous carbon (PC) with a matched pore aperture (PMo6W6@PC). As a result, PMo6W6@PC-based supercapacitor shows high energy densities of 0.308 mWh cm−2 at power densities of 43.2 mW cm−2, outperforming most polyoxometalate-based supercapacitors. Moreover, the device exhibits a capacity retention of over 80.4 % at 8 mA cm−2 after 8000 cycles. This improved electrochemical redox activity may be ascribed to the strong orbital electronic coupling between W and Mo atoms of PMo6W6 by confinement engineering. This work proves that the confined PMo6W6 can maximize the advantages of PMo12O40 and PW12O40, which provides a theoretical basis for other mixed-addenda polyoxometalate species.

Silver-modified polyniobotungstate for the visible light-induced simultaneous cleavage of C–C and C–N bonds

Silver-modified polyniobotungstate based on Nb/W mixed-addendum polyoxometalate with formula Ag₉[P₂W₁₅Nb₃O₆₂]·21H₂O (<strong>Ag-Nb/W</strong>) was synthesized and then characterized by various analytical and spectral techniques. <strong>Ag-Nb/W</strong> was proven to be an efficient photocatalyst for the oxidative ring opening of 2-phenylimidazo[1,2-<i>a</i>]pyridine via the simultaneous cleavage of C–C and C–N bonds. Under visible light (430–440 nm) and with oxygen as an oxidant at room temperature, <strong>Ag-Nb/W</strong> can catalyze the rapid transformation of various 2-phenylimidazo[1,2-<i>a</i>]pyridine derivatives to produce the corresponding oxidative ring-opening product N-(pyridin-2-yl) amides in good isolated yields ranging from 65% to 78%. As a heterogeneous photocatalyst, <strong>Ag-Nb/W</strong> showed excellent sustainability and recyclability in the recycling experiments. Infrared (IR) spectroscopy and X-ray diffraction (XRD) analysis indicated that <strong>Ag-Nb/W</strong> could retain its integrity after catalysis. A possible mechanism involving the singlet oxygen for the catalytic reaction was proposed.

A Polyniobotungstate-Based Hybrid for Visible-Light-Induced Phosphorylation of N-Aryl-Tetrahydroisoquinoline.

A new organic-inorganic hybrid based on a Nb/W mixed-addendum polyoxometalate with the formula H14[(Co(H2O)3)2(C10H8N2)4(P4W30Nb6O123)]·4(C10H8N2)·8H2O (Co-POM) has been synthesized by the solvothermal method and characterized by single-crystal X-ray diffraction (XRD), powder X-ray diffraction (PXRD), elemental analysis, FTIR spectroscopy, UV-vis absorption spectrum, and thermogravimetric analysis (TGA). Importantly, visible-light-absorption peaks around 525 nm for Co-POM indicated that this material should have potential in visible-light-induced organic reactions. Herein, we disclosed visible-light-promoted phosphorylation of N-aryl-tetrahydroisoquinoline using Co-POM as an efficient heterogeneous photocatalyst. In this procedure, diverse phosphorus reagents are compatible at room temperature and in an O2 atmosphere, giving the corresponding products in good to excellent yields (up to 97%). Simultaneously, this heterogeneous photocatalyst can be recycled up to ten times with a negligible decrease in yield, showing outstanding sustainability and recyclability.

A Giant Mo/Ta/W Ternary Mixed-Addenda Polyoxometalate with Efficient Photocatalytic Activity for Primary Amine Coupling.

The reactivity and properties of polyoxometalates (POMs) vary remarkably as a function of the kind of addenda atoms, so the design and synthesis of new mixed-addenda POMs is a promising approach for the further development of the POM-related areas. In the present work, the first Mo/Ta/W ternary mixed-addenda POM (NH4)41H7[K3(H2O)3(P2W15Ta3O62)6(Mo2O4CH3CO2)3(MoO3)2]·85H2O (1), which is composed of 6 {P2W15Ta3O62} linked by 3 {MoV2O4(OOCCH3)+} and 2 {MoVIO3} via 18 novel Mo-O-Ta bridges has been synthesized. The precursor {P2W15Ta3}, which has lower redox potential, is crucial for the formation of 1. The red-brown solid sample of 1 shows strong absorption in the visible region. The visible-light responsive charge transfer from benzylamine to 1 was observed experimentally. 1 was proved to be an efficient photocatalyst under simulated sunlight (AM 1.5G) radiation for the oxidative coupling of primary amines to imines using atmospheric O2.

An unprecedented 3D architecture based on a Nb/W mixed-addendum polyoxometalate and lanthanide–organic complex: synthesis, crystal structure and properties

For the first time, a 3D framework based on lanthanide-containing Nb/W mixed-addendum polyoxometalate has been obtained by utilizing the steam-assisted conversion method.

A novel Ta/W mixed-addendum polyoxometalate with photocatalytic properties.

An unprecedented polytantalotungstate (POTT), Cs12.5K4.5H[Ta12Si4W37O158]·25H2O (1), based on the {SiW9Ta3O40}7- cluster was hydrothermally synthesized. A photocatalytic study revealed that 1 exhibits significant photocatalytic water splitting activity.

Self-assembly and photocatalytic properties of Ta/W mixed-addendum polyoxometalate and transition-metal cations

Two nanoscale Ta/W mixed-addendum polyoxometalates, Cs5K2[(Si2W18Ta6O78)Cr(H2O)4]·8H2O and Cs3K4H2[(Si2W18Ta6O78)FeCl2(H2O)2]·15H2O involving [SiTa3W9O40]7− fundamental building units, have been synthesized, both of which show photocatalytic hydrogen evolution activity.

Self-assembly and photocatalytic hydrogen evolution of a niobium-containing polyoxometalate

A tetra-Keggin polyoxometalate (POM), Cs19K2[Nb4O6(SiW9Nb3O40)4]Cl·27H2O (1) with a photocatalytic property has been synthesized by utilizing the self-assembly of W/Nb mixed-addendum POM [SiW9(NbO2)3O37]7− under hydrothermal conditions. Its structure was determined by single-crystal X-ray diffraction, and further characterized by IR spectroscopy, thermogravimetric analysis, and cyclic voltammetry. Moreover, the photocatalytic activity of 1 for H2 evolution from water was observed under UV irradiation, suggesting that 1 might be a promising photocatalytic catalyst.

Mixed addenda polyoxometalate “solutions” for stationary energy storage

A series of redox flow batteries utilizing mixed addenda (vanadium and tungsten), phosphorus-based polyoxometalates (A-α-PV3W9O40(6-), B-α-PV3W9O40(6-), and P2V3W15O62(9-)) were prepared and tested. Cyclic voltammetry and bulk electrolysis experiments on the Keggin compounds (A-α-PV3W9O40(6-) and B-α-PV3W9O40(6-)) established that the vanadium centers of these compounds could be used as the positive electrode (PV(IV)3W(VI)9O40(9-)/PV(V)3W(VI)9O40(6-)), and the tungsten centers could be used as the negative electrode (PV(IV)3W(VI)9O40(9-)/PV(IV)3W(V)3W(VI)6O40(12-)) since these electrochemical processes are separated by about 1 V. The results showed that A-α-PV3W9O40(6-) (where A indicates adjacent, corner-sharing vanadium atoms) had coulombic efficiencies (charge in divided by charge out) above 80%, while the coulombic efficiency of B-α-PV3W9O40(6-) (where B indicates adjacent edge-sharing vanadium atoms) fluctuated between 50% and 70% during cycling. The electrochemical yield, a measurement of the actual charge or discharge observed in comparison with the theoretical charge, was between 40% and 50% for A-α-PV3W9O40(6-), and (31)P NMR showed small amounts of PV2W10O40(5-) and PVW11O40(4-) formed with cycling. The electrochemical yield for B-α-PV3W9O40(6-) decreased from 90% to around 60% due to precipitation of the compound on the electrode, but there were no decomposition products detected in the solution by (31)P NMR, and infrared data on the electrode suggested that the cluster remained intact. Testing of P2V3W15O62(9-) (Wells-Dawson structure) suggested higher charge density clusters were not as suitable as the Keggin structures for a redox flow battery due to the poor stability and inaccessibility of the highly reduced materials.

Constructing nanosized polyanions with diverse structures by the self-assembly of W/Nb mixed-addendum polyoxometalate and lanthanide ion

The reaction activity of W/Nb mixed-addendum POMs towards lanthanide has been demonstrated by experimental and computational methods. Utilizing the nucleophilicity of Ot(Nb) (Ot: terminal oxygen), six Nb–O–Eu bridge-based POMs with different structures have been isolated by the reactions of {GeW9Nb3O40} and lanthanide ions Eu3+ under different conditions, including a dimmer Cs3K4[(Ge2W18Nb6O78)Eu(H2O)4]·23H2O (1), three tetramers Cs8K9[(Ge4W36Nb12O156)Eu(H2O)3]·25H2O (2), Cs12K2[Cs(GeW9Nb3O40)4(SO4)Eu5(H2O)36]·61H2O (3), Cs12.5K1.5[Cs2(GeW9Nb3O40)4Eu4(H2O)22]·28H2O (4), a one-dimensional compound Cs11.5[(GeW9Nb3O40)4Eu5.5(H2O)26]·24H2O (5), and a two-dimensional compound CsK2.25[GeW9Nb3O40Eu1.25(H2O)12]·8H2O (6). In the polyanions of 1–6 (1a–6a), all the EuIII atoms are selectively bonded to Ot(Nb) of {GeW9Nb3O40} fragments, indicating the high nucleophilicity of Ot(Nb). The Density Functional Theory (DFT) calculation on anion [GeW9Nb3O40]7− further approves this. Comparing with the (Nb)O–Eu bonds, the formation of Nb–O–Nb bridges needs stronger acidity. Thus, weaker acidity (pH>1.5) is favorable to the former. The Cs+ cation plays an important templating role in the formation and stability of polyanions 1a–5a. The photoluminescence behaviors of 1 and 2 have been investigated and compound 1 exhibits selective luminescence response to malonic acid.

Luminescent multilayer film based on mixed-addenda polyoxometalates and polyethyleneimine by layer-by-layer assembly

A luminescent multilayer film has been fabricated from mixed-addenda polyoxometalate anion [Eu(SiW 4Mo 7O 39) 2] 13− (EuSiWMo) and polyethyleneimine (PEI) through electrostatic layer-by-layer self-assembly (LBL). The fabrication processes of the film have been monitored by UV-Vis absorption spectra. The results indicated that the composite film was formed by the alternate adsorption of EuSiWMo and PEI, and the layer growth was linear and highly reproducible from layer-to-layer. The elemental composition of the film was obtained from X-ray photoelectron spectrum (XPS). The data of XPS confirmed the presence of the expected elements. The film was smooth and homogeneous as indicated by atomic force microscopy images (AFM). The close-packed nanoclusters of the conglomeration of EuSiWV anions, with a mean grain size of ca. 60 nm, are distributed on the surface uniformly. The mean interface roughness estimated by AFM is about 3.8 nm in an area of 2.0×2.0 μm 2. The film exhibited characteristic emission bands of rare-earth ions, consisting with those of the solid sample. In addition, the electrochemical behavior of the film was investigated by cyclic voltammetry, indicating that the electrochemical property of EuSiWMo was fully maintained in the LBL film.