Crystal structure and replacement reaction of coordinated water molecules of the heteropoly compounds of sandwich-type tungstoarsenates

Heteroelements in polyoxometalates: a study on the influence of different group 15 elements on polyoxometalate formation

In the field of polyoxometalate (POM) chemistry, different heteroelements are integrated into the cage-like structures, to obtain different structural types of so-called heteropolyanions (HPAs). While it is generally accepted, that some elements favor certain types of structure, a systematic study is still missing. In this article, we present a systematic investigation of the influence of the group 15 elements nitrogen, phosphorous, arsenic, and antimony on the formation of different POM structure types. Our study is comprised of DFT calculations and corresponding experimental structural analysis. In this context, the DFT study establishes the thermodynamics of formation of different coordination geometries with various heteroelements on two POM structure types, the Keggin and the Anderson-Evans structures. Our POM synthesis experiments were performed at two different pH values (1 and 5) and resulted in a variety of heteropolytungstates, which were identified and characterized by elemental analysis as well as single crystal X-ray diffraction and vibrational spectroscopy. With these methods, we were able to establish a clear trend, showing that heavier elements lead to formation of different structure types than lighter elements. These results signify a large step towards a better understanding of POM formation specifically with respect to the choice of heteroelement.

Investigating catalytic oxidative desulfurization of model fuel using hollow PW12/TiO2@MgCO3 and performance optimization via box-behnken design

A facile and eco-friendly synthesis of PW12/TiO2@MgCO3 hollow tubes (PW12·∼· H3[PW12O40] = polyoxometalate) using a soluble and reusable MgCO3·3H2O micro-rods template was reported for the first time. The resultant hollow tubes were characterized by Fourier transform infrared spectroscopy (FT-IR), UV-visible spectroscopy, powder X-ray diffraction (PXRD), energy-dispersive X-ray spectroscopy (EDX), and scanning electron microscopy (SEM), which indicated that the [PW12O40]3- structure remained intact within the hollow tubes. Furthermore, the specific surface area (88.982 m2/g) and average pore size (2.6 nm) of the PW12/TiO2@MgCO3 hollow tubes were calculated using the Brunauer-Emmett-Teller (BET) analysis. This study explored the catalytic performance of PW12/TiO2@MgCO3 hollow tubes using a three-level Box-Behnken design (BBD), through which optimization curves were designed. The desulfurization of model fuel using hollow tubes was optimally performed when the catalyst dose, time, temperature, and oxidant/sulfur (O/S) were 20-80 gm, 80-120 min, 25-80 °C and 3-8 molar ratio, respectively. These results were further processed, and the experiments were replicated twenty-nine times using a model based on two quadratic polynomials to create a response surface methodology (RSM). This permits a mathematical correlation linking the desulfurization and experimental parameters. The optimal performance of reaction mixture was evaluated to be 80 mg for catalyst concentration, 25 °C of temperature, reaction time of 100 min, and 5.5 for oxidant/sulfur molar ratio from 20 mL of octane simulation oil containing 350 ppm dibenzothiophene (DBT). The predicted desulfurization rate of the model fuel under these optimal conditions was 95.3%. The correspondence between the experimental results and predicted values was verified based on regression analysis, with an R2 value greater than 0.99. These hollow tubes could be used for their desulfurization properties ten times a row without significantly reducing catalytic activity.

Electrochemical, Electrocatalytic, and Magnetic Properties of Vanadium-Containing Polyoxometalates

Mono-substituted vanadium-containing Dawson-type polyoxometalates having the general formula α1-[VIVW17X2O62]8− and α2-[VIVW17X2O62]8−, with X = As or P, were synthesised and subject to a comprehensive electrochemical study comprising the pH dependency. These POMs exhibit an electrocatalytic behaviour towards the oxidation of thiols (namely cysteine), rendering them interesting species for mimicking oxidative stress situations, at physiological pH values. The efficiency of the electro-oxidation was assessed with thiols of different nature, and the substrate that responded best was used to compare the electrocatalytic capabilities of the POM series. The magnetic behaviour of these POMs was also evaluated and compared to their analogues, α1- and α2-[VVW17X2O62]7− (X = As or P), at low temperatures and showed, as expected, a paramagnetic behaviour of VIV based compounds.

Palladium(II)-Containing Tungstoarsenate(V), [PdII4(As2W15O56)2]16-, and Its Catalytic Properties

We have synthesized and structurally characterized the 4-palladium(II)-containing 30-tungsto-4-arsenate(V), [Pd₄(As₂W₁₅O₅₆)₂]^16- (1), which represents the first palladium(II)-containing tungstoarsenate(V). The title polyanion 1 was prepared by a simple one-pot procedure in aqueous medium and characterized by single-crystal X-ray diffraction (XRD), thermogravimetric analysis (TGA), cyclic voltammetry, elemental analysis, and ¹⁸³W nuclear magnetic resonance (NMR), infrared (IR), and ultraviolet-visible (UV-vis) spectroscopies. Polyanion 1 consists of four Pd²⁺ ions that are coordinated in a square-planar geometry to two trilacunary [As₂W₁₅O₅₆]^12- Wells-Dawson fragments resulting in a sandwich-type assembly. Catalytic studies on 1 revealed that it is an efficient catalyst precursor for the Suzuki-Miyaura cross-coupling reactions of various aryl halides in aqueous and nonaqueous media.

Synthesis, Crystal Structure, Electrochemistry and Electro-Catalytic Properties of the Manganese-Containing Polyoxotungstate, [(Mn(H2O)3)2(H2W12O42)]6−

We present the synthesis and structural characterization of the manganese-containing polyoxotungstate, [(Mn(H2O)3)2(H2W12O42)]6− (1), obtained by reaction of MnCl2 with six equivalents of Na2WO4 in the presence of Zn(CH3COO)2 in acetate medium (pH 4.7). This has been assessed by various techniques (FTIR, TGA, UV-Visible, XPS, elemental analysis, single crystal X-ray and electrochemistry). Single-crystal X-ray analyses showed that, in the solid state, 1 forms a 2-D network in which [H2W12O42]10− fragments are linked in pairs via Mn2+ ions, leading to linear chains of the form [(Mn(H2O)3)2(H2W12O42)]n6n−. The connection between chains occurs also via Mn2+ ions which bind [H2W12O42]10− fragments belonging to two adjacent chains, forming an infinite 2-D network. A complete electrochemical study was done in aqueous solution where 1 is stable in the pH range 1 to 6. This complex undergoes multiple electron-transfer processes that lead to the electro-generation of manganese high oxidation state species that catalyse water electro-oxidation. 1 is also effective in the electro-catalytic reduction of nitrite and dioxygen.

A novel polyoxometalate-based metal–organic nanotube framework templated by twin-Dawson clusters: synthesis, structure and bifunctional electrocatalytic properties

A novel polyoxometalate-based metal–organic nanotube framework templated by twin-Dawson clusters was synthesized, which exhibits high-performance bifunctional electrocatalytic activities toward reduction of bromate and oxidation of ascorbic acid.

Synthesis and characterization of self-assembled inorganic-organic hybrid arsenotungstates

A series of 12 inorganic-organic hybrid 4f-substituted arsenotungstates decorated with 3d-based complexes along with acetate bridging, viz. {[Cu₂(H₂O)(1,10-phen)₂(μ-CH₃COO)₂]₄[Ln₂(H₂O)₂(μ-CH₃COO)₂(α-AsW₁₁O₃₉)₂]}^2- (Ln^III = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu), have been synthesized. These compounds were isolated using a one-pot synthetic strategy with copper, lanthanoid, trilacunary arsenotungstate and 1,10-phenanthroline as precursors. They were characterized using various analytical techniques, such as single-crystal X-ray diffraction (SC-XRD), powder XRD (PXRD), Fourier transform IR (FT-IR) spectroscopy, UV-Vis spectroscopy and thermogravimetric analysis (TGA). In all these compounds, a dinuclear Ln₂(ac)₂ moiety is sandwiched between two monolacunary [AsW₁₁O₃₉]^7- polyoxometalates (POMs), where the Ln₂(ac)₂ moiety is comprised of two acetate ligands in a μ-κO,O':κO fashion. Moreover, the anion is decorated by a copper-phenanthroline-acetate complex through a terminal O atom of the polyanion.

Antibacterial Activity of Polyoxometalates Against Moraxella catarrhalis

The antibacterial activity of 29 different polyoxometalates (POMs) against Moraxella catarrhalis was investigated by determination of the minimum inhibitory concentration (MIC). The Preyssler type polyoxotungstate (POT) [NaP₅W₃₀O₁₁₀]¹⁴⁻ demonstrates the highest activity against M. catarrhalis (MIC = 1 μg/ml) among all tested POMs. Moreover, we show that the Dawson type based anions, [P₂W₁₈O₆₂]⁶⁻, [(P₂O₇)Mo₁₈O₅₄]⁴⁻, [As₂Mo₁₈O₆₂]⁶⁻, [H₃P₂W₁₅V₃O₆₂]⁶⁻, and [AsW₁₈O₆₀]⁷⁻ are selective on M. catarrhalis (MIC range of 2-8 μg/ml). Among the six tested Keggin type based POTs ([PW₁₂O₄₀]³⁻, [H₂PCoW₁₁O₄₀]⁵⁻, [H₂CoTiW₁₁O₄₀]⁶⁻, [SiW₁₀O₃₆]⁸⁻, [SbW₉O₃₃]⁹⁻, [AsW₉O₃₃]⁹⁻), only the mono-substituted [H₂CoTiW₁₁O₄₀]⁶⁻ showed MIC value comparable to those of the Dawson type group. Polyoxovanadates (POVs) and Anderson type POMs were inactive against M. catarrhalis within the tested concentration range (1-256 μg/ml). Four Dawson type POMs [P₂W₁₈O₆₂]⁶⁻, [(P₂O₇)Mo₁₈O₅₄]⁴⁻, [As₂Mo₁₈O₆₂]⁶⁻, [H₃P₂W₁₅V₃O₆₂]⁶⁻ and the Preyssler POT [NaP₅W₃₀O₁₁₀]¹⁴⁻ showed promising antibacterial activity against M. catarrhalis (MICs < 8 μg/ml) and were therefore tested against three additional bacteria, namely S. aureus, E. faecalis, and E. coli. The most potent antibacterial agent was [NaP₅W₃₀O₁₁₀]¹⁴⁻, exhibiting the lowest MIC values of 16 μg/ml against S. aureus and 8 μg/ml against E. faecalis. The three most active compounds ([NaP₅W₃₀O₁₁₀]¹⁴⁻, [P₂W₁₈O₆₂]⁶⁻, and [H₃P₂W₁₅V₃O₆₂]⁶⁻) show bacteriostatic effects in killing kinetics study against M. catarrhalis. We demonstrate, that POM activity is mainly depending on composition, shape, and size, but in the case of medium-size POTs (charge is more than −12 and number of addenda atoms is not being higher than 22) its activity correlates with the total net charge.

Synthesis, Structure, and Magnetic Electrochemical Properties of a Family of Tungstoarsenates Containing Just CoII Centers or Both CoII and FeIII Centers

The three polyoxotungstates [(NaOH₂)₂Co^II₂(As₂W₁₅O₅₆)₂]^18- (1), [(NaOH₂)(Co^IIOH₂)Co^II₂(As₂W₁₅O₅₆)₂]^17- (2), and [(Co^IIOH₂)₂Co^II₂(As₂W₁₅O₅₆)₂]^16- (3) have been prepared in aqueous solution upon mixing cobalt(II) salts with the ligand [As₂W₁₅O₅₆]^12-. The reaction of 1 or 2 with the Fe³⁺ ion leads invariably to the same species [(Fe^IIIOH₂)(Co^IIOH₂)Co^II₂(As₂W₁₅O₅₆)₂]^15- (4) possessing three cobalt atoms and a single iron atom. However, if the Fe-containing homologue of compound 1, that is, the polyoxotungstate [(NaOH₂)₂Fe^III₂(As₂W₁₅O₅₆)₂]^16- (5), is employed instead to react with the Co²⁺ ion, the species [(Co^IIOH₂)₂Fe^III₂(As₂W₁₅O₅₆)₂]^14- (6) is obtained, having two cobalt atoms and two iron atoms. The compounds 1, 2, 3, 4, and 6 are described for the first time and have been characterized by several physicochemical methods such as FTIR, UV-visible, ATG, and elemental analysis. Structural analysis by single-crystal X-ray diffraction has been carried out with compounds 2 (monoclinic space group P2₁/c, a = 17.0622(5) Å, b = 15.0828(4) Å, c = 32.0872(8) Å, β = 91.170(1)°, and Z = 2) and 3 (triclinic space group P1̅, a = 13.6137(7) Å, b = 13.8836(8) Å, c = 22.9276(6) Å, α = 89.906(3)°, β = 78.356(2)°, γ = 61.451(2)°, and Z = 1). Electrochemical studies undertaken with all the above-mentioned compounds and some of their homologues shed light on the influence of the chemical composition on their electrocatalytic properties toward substrates such as the nitrite ion and dioxygen. Magnetic measurements evidence anisotropic ferromagnetic interactions between Co²⁺ ions and antiferromagnetic interactions between Fe³⁺ ions. The nature and the strength of the Co²⁺-Fe³⁺ interactions depend on the relative orientations of their 3d orbitals. The effective magnetic moment of the Co²⁺ ions varies with the temperature and with the distortion of the octahedral sites in which they are located.

An amphiphilic catalyst based on sandwich-type polyoxometalate for deep desulfurization of fuels in ionic liquid

A cobalt-based polyoxometalate, (STA)₁₀[Co₄(H₂O)₂(PW₉O₃₄)₂] (SEP) (STA = stearyltrimethylammonium), was prepared by ionic exchange of K₁₀[Co₄(H₂O)₂(PW₉O₃₄)₂] and STA·Cl.

pH-controlled and sulfite anion-directed assembly of a family of cerium(III)-containing polyoxotungstates clusters

A versatile one-pot strategy was employed to synthesize five cerium(III)-containing polyoxotungstate nanoclusters through pH-controlled and sulfite anion-directed assembly: [C2H8N]3Na7[Ce2(H2O)6W22O72(OH)4]·20H2O (1) at pH 5.0; [C2H8N]8Na16[Ce4(H2O)12W44O144(OH)12]·23H2O (2) at pH 4.5; [C2H8N]2Na4Ce2[Ce2(H2O)10W28O92(OH)2]·27H2O (3) at pH 2.8-3.3; [C2H8N]2Na7[{α-SW7O28}{Ce2(H2O)6}(W3O6){α-SW9O32}{α-SW9O31(OH)}]·18H2O (4) at pH 2.5; [C2H8N]2Na18[Ce2(H2O)9W36O110(OH)12]2·30H2O (5) at pH 1.5. These compounds were characterized by single-crystal X-ray structure analysis, IR spectroscopy, thermogravimetric (TG) analysis, X-ray photoelectron spectroscopy (XPS), and electrospray ionization mass spectrometry (ESI-MS). Moreover, their electrochemical properties were investigated. Single-crystal X-ray structure analysis revealed that 1 and 2 were di- and tetra-cerium(III)-bridged polyoxotungstates, constructed from two different types of lacunary {W11} units. 3 composed of the well-known cerium(III)-stabilized {W28} unit and organic amine-sodium-cerium cations, was isolated in the pH range 2.8-3.3. In this reaction system, the SO3(2-) anion acted as a heteroanion template at a lower pH 2.5. 4 was isolated by the combination of cerium(III) centers and SO3(2-) heteroanion template, which is the first lanthanide-containing polyoxotungstates with sulfur heteroatoms, and the 4f metal cerium(III) centers in 4 both have eight-coordinated modes and the SO3(2-) heteroanion templates display μ7 and μ9 coordination modes. At a much lower pH 1.5, the polyanion of 5 was obtained, two triangular-shaped {W36} subunits were bridged by the cerium(III) ions, resulting in the largest lanthanide-containing iso-polyoxotungstates known to date.

Multinuclear cobalt(II)-containing heteropolytungstates: structure, magnetism, and electrochemistry

Interaction of the trilacunary Keggin polyanions [A-α-XW9O34](10-) (X = Si(IV), Ge(IV)) with Co(II) and phosphate ions in aqueous, basic media and under mild heating leads to the formation of the tetrameric, Co16-containing heteropolytungstates [{Co4(OH)3PO4}4(A-α-XW9O34)4](32-) (X = Si(IV), Ge(IV)). Both polyanions were characterized in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric and elemental analyses. Furthermore, the electrochemical and magnetic properties of these isostructural polyanions were investigated.

A new 3D framework based on reduced Wells-Dawson arsenotungstates as eight-connected linkages

A compound representing the first example of high dimensional and high connected hybrid based on reduced Wells-Dawson arsenotungstates has been synthesized, and its electrocatalytic and photocatalytic properties have been investigated.