Synthesis, structure, electrochemistry and magnetism of cobalt-, nickel- and zinc-containing [M4(OH)3(H2O)2(α-SiW10O36.5)2]13- (M = Co2+, Ni2+, and Zn2+).

Interaction of the trilacunary 9-tungstosilicate [A-α-SiW9O34]10- with cobalt(ii), nickel(ii) and zinc(ii) ions in pH 9 aqueous medium at room temperature led to the formation of the respective M4-containing heteropolytungstates [M4(OH)3(H2O)2(α-SiW10O36.5)2]13- (M = Co2+ (1), Ni2+ (2), and Zn2+ (3)). Polyanions 1-3 were characterized in the solid state by single-crystal XRD, FT-IR spectroscopy, and thermogravimetric and elemental analyses. Electrochemical studies showed that the Co2+ ions in 1 can be oxidized to Co3+ and the CVs of the WVI centers of the polyanions feature well-defined and chemically reversible reduction waves. Magnetic measurements on 1 and 2 showed paramagnetism with complex ferromagnetic and antiferromagnetic interactions. A model was presented for extracting the exchange constants for the magnetic exchange interaction.

15-Copper(ii)-containing 36-tungsto-4-silicates(iv) [Cu15O2(OH)10X(A-α-SiW9O34)4]25- (X = Cl, Br): synthesis, structure, magnetic properties, and electrocatalytic CO2 reduction.

The 15-copper(ii)-containing 36-tungsto-4-silicates [Cu15O2(OH)10X(A-α-SiW9O34)4]25- (X = Cl, 1; Br, 2) have been prepared in 70% yield by reaction of the trilacunary 9-tungstosilicate precursor [A-α-SiW9O34]10- with Cu2+ ions in aqueous pH 8 medium. Both polyanions 1 and 2 were isolated as hydrated mixed potassium/sodium salts and characterized in the solid state by FT-IR, TGA, single-crystal XRD, and elemental analysis. DC magnetic susceptibility measurements from 1.8-300 K established the ground state to be paramagnetic with a magnetic moment corresponding to 15 uncoupled Cu2+ (S = 1/2) ions. EPR measurements and simulations were consistent with this analysis. Electrochemical studies were performed for polyanions 1 and 2 dissolved in solution to elucidate the electroactivity of both copper and tungstate sites. Using 2 as a representative example, the electrocatalytic activity towards CO2 reduction upon deposition on a glassy carbon electrode surface, while retaining selectivity relative to hydrogen evolution, was demonstrated.

Simple and efficient polyoxomolybdate-mediated synthesis of novel graphene and metal nanohybrids for versatile applications.

The application of nanohybrids based on polyoxomolybdates, reduced graphene oxide (rGO) and/or metal nanoparticles (NPs) high-performance electrode materials in electrocatalysis and energy storage devices is promising but still limited due to the complexity and the cost of the synthesis. Here we introduce a simple polyoxomolybdate, [MoV4O8(OH)2(H2O)2(C4O4)2]2- (MoS), as reducing and stabilizing agent for the facile and one-pot syntheses of large quantity of highly stable MoS/rGO and MoS/Au NPs nanohybrids in aqueous solution without any catalyst or toxic co-solvent. They were characterized by various physical techniques and electrochemistry which confirm strong interaction between MoS and rGO sheets. We also used DFT calculations to investigate the affinity between MoS or its neutral form with graphene. The adsorption energy for the most stable configuration is -1.97 eV, indicating a strong adsorption process of MoS, which can also be confirmed by the distance (3.04 Å) and the charge transfer (0.86 e) between MoS and graphene. These observations are also consistent with the electrochemical results which underscore the excellent redox properties and high stability of MoS/rGO. Importantly, the MoS/rGO nanohybrids are excellent noble metal-free electrocatalysts for hydrogen peroxide reduction with high sensitivity, large detection range and low detection limit. Finally, the preliminary tests reveal that the electrode materials based on MoS/rGO and a low-cost carbon cloth (CC) composite MoS/rGO/CC may have a potential for an application in energy storage as performant and flexible supercapacitor, showing specific capacitance as high as 870 F g-1 at 10 mV s-1 and excellent stability after 5000 cycles.

High Oxygen Reduction Reaction Performances of Cathode Materials Combining Polyoxometalates, Coordination Complexes, and Carboneous Supports.

A series of carbonaceous-supported precious-metal-free polyoxometalate (POM)-based composites which can be easily synthesized on a large scale was shown to act as efficient cathode materials for the oxygen reduction reaction (ORR) in neutral or basic media via a four-electron mechanism with high durability. Moreover, exploiting the versatility of the considered system, its activity was optimized by the judicious choice of the 3d metals incorporated in the {(PW9)2M7} (M = Co, Ni) POM core, the POM counterions and the support (thermalized triazine-based frameworks (TTFs), fluorine-doped TTF (TTF-F), reduced graphene oxide, or carbon Vulcan XC-72. In particular, for {(PW9)2Ni7}/{Cu(ethylenediamine)2}/TTF-F, the overpotential required to drive the ORR compared well with those of Pt/C. This outstanding ORR electrocatalytic activity is linked with two synergistic effects due to the binary combination of the Cu and Ni centers and the strong interaction between the POM molecules and the porous and highly conducting TTF-F framework. To our knowledge, {(PW9)2Ni7}/{Cu(ethylenediamine)2}/TTF-F represents the first example of POM-based noble-metal-free ORR electrocatalyst possessing both comparable ORR electrocatalytic activity and much higher stability than that of Pt/C in neutral medium.

Synthesis, Structure, and Antibacterial Activity of a Thallium(III)-Containing Polyoxometalate, [Tl2{B-ß-SiW8O30(OH)}2]12.

We have synthesized and structurally characterized the first discrete thallium-containing polyoxometalate, [Tl2{B-ß-SiW8O30(OH)}2]12- (1). Polyanion 1 was characterized in the solid-state and shown to be solution-stable by 203/205Tl NMR, electrospray ionization mass spectrometry, and electrochemical studies. The antibacterial activity of 1 was also investigated.

Mixed-Valent Mn16-Containing Heteropolyanions: Tuning of Oxidation State and Associated Physicochemical Properties.

The two 16-manganese-containing, Keggin-based 36-tungsto-4-silicates [Mn(III)10Mn(II)6O6(OH)6(PO4)4(A-α-SiW9O34)4](28-) (1) and [Mn(III)4Mn(II)12(OH)12(PO4)4(A-α-SiW9O34)4](28-) (2) have been prepared by reaction of the trilacunary Keggin precursor [A-α-SiW9O34](10-) with either Mn(OOCCH3)3·2H2O (for 1) or MnCl2·4H2O (for 2), in aqueous phosphate solution at pH 9. Polyanions 1 and 2 comprise mixed-valent, cationic {Mn(III)10Mn(II)6O6(OH)6}(24+) and {Mn(III)4Mn(II)12(OH)12}(24+) cores, respectively, encapsulated by four phosphate groups and four {SiW9} units in a tetrahedral fashion. Both polyanions were structurally and compositionally characterized by single-crystal XRD, IR, thermogravimetric analysis, and X-ray absorption spectroscopy. Furthermore, studies were performed probing the magnetic, electrochemical, oxidation catalytic, and Li-ion battery performance of 1 and 2.

Platinum-Containing Polyoxometalates: syn- and anti-[Pt(II)2(α-PW11O39)2](10-) and Formation of the Metal-Metal-Bonded di-Pt(III) Derivatives.

The first examples of dimeric, di-Pt(II) -containing heteropolytungstates are reported. The two isomeric di-platinum(II)-containing 22-tungsto-2-phosphates [anti-Pt(II)2(α-PW11O39)2](10-) (1 a) and [syn-Pt(II)2(α-PW11O39)2](10-) (2 a) were synthesized in aqueous pH 3.5 medium using one-pot procedures. Polyanions 1 a and 2 a contain a core comprising two face-on PtO4 units, with a Pt⋅⋅⋅Pt distance of 2.9-3 Å. Both polyanions were investigated by single-crystal XRD, IR, TGA, UV/Vis, (31) P NMR, ESI-MS, CID-MS/MS, electrochemistry, and DFT. On the basis of DFT and electrochemistry, we demonstrated that the {Pt2(II)} moiety in 1 a and 2 a can undergo fully reversible two-electron oxidation to {Pt2(III)}, accompanied by formation of a single Pt-Pt bond. Hence we have discovered the novel subclass of Pt(III)-containing heteropolytungstates.

Ti2-Containing 18-Tungsto-2-Arsenate(III) Monolacunary Host and the Incorporation of a Phenylantimony(III) Guest.

The novel Ti2-containing, sandwich-type 18-tungsto-2-arsenate(III) [(Ti(IV)O)2(α-As(III)W9O33)2](14-) (1) was successfully synthesized by the reaction of [TiO](2+) species with [α-As(III)W9O33](9-). The monolacunary polyanion 1 is solution-stable, and a further reaction with 1 equiv of phenylantimony(III) dichloride resulted in [C6H5Sb(III)(Ti(IV)O)2(α-As(III)W9O33)2](12-) (2). Both polyanions 1 and 2 were structurally characterized in the solid state and solution. Electrochemical studies were also performed on both polyanions.

Sequential Synthesis of 3 d-3 d, 3 d-4 d, and 3 d-5 d Hybrid Polyoxometalates and Application to the Electrocatalytic Oxygen Reduction Reactions.

The Co7 (AlePy)2 polyoxometalate, which encloses a {(PW9 )2 Co(II) 7 } core covalently bound to two free aminopyridine groups through bisphosphonate ligands (AlePy), has been isolated. It can be used as a precursor, allowing the synthesis of heterometallic hybrid compounds, as illustrated by the characterization of cobalt/zinc (Co7 (AlePyZn)2 ), cobalt/palladium (Co7 (AlePyPd)2 ), and cobalt/platinum (Co7 (AlePyPt)2 ) species. A composite based on the water-insoluble precious metal-free Co7 (AlePyZn)2 compound and the low-cost carbon material Vulcan XC-72 has been selected as a cathode material (Co7 Zn/C) for oxygen reduction reaction studies. The electrocatalytic performances of the Co7 Zn/C hybrids were assessed at neutral and basic pH, showing that Co7 Zn/C exhibits high selectivity for the four-electron reduction of O2 . Moreover, its durability is superior to that of a commercial Pt/C catalyst with 20 % loading. Also, comparative studies performed in the presence of methanol have indicated that Co7 Zn/C has a much better tolerance to the crossover effect than Pt/C. Altogether, these results indicate for the first time that, even in neutral media, polyoxometalate/carbon composites can represent low-cost oxygen reduction catalysts that can function stably, for a long time, and with high performance.

Synthesis and characterization of multinuclear manganese-containing tungstosilicates.

The five manganese-containing, Keggin-based tungstosilicates [Mn(II)3(OH)3(H2O)3(A-α-SiW9O34)](7-) (1), [Mn(III)3(OH)3(H2O)3(A-α-SiW9O34)](4-) (2), [Mn(III)3(OH)3(H2O)3(A-ß-SiW9O34)](4-) (3), [Mn(III)3Mn(IV)O3(CH3COO)3(A-α-SiW9O34)](6-) (4), and [Mn(III)3Mn(IV)O3(CH3COO)3(A-ß-SiW9O34)](6-) (5) were synthesized in aqueous medium by interaction of [A-α-SiW9O34](10-) or [A-ß-SiW9O34H](9-) with either MnCl2 (1) or [Mn(III)8Mn(IV)4O12(CH3COO)16(H2O)4] (2-5) under carefully adjusted reaction conditions. The obtained salts of these polyanions were analyzed in the solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric analysis. The salts of polyanions 1, 2, and 4 were further characterized in the solid state by magnetic studies, as well as in solution by electrochemistry.

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.

Synthesis, magnetism, and electrochemistry of the Ni14- and Ni5-containing heteropolytungstates [Ni14(OH)6(H2O)10(HPO4)4(P2W15O56)4]34- and [Ni5(OH)4(H2O)4(ß-GeW9O34)(ß-GeW8O30(OH))]13-.

The two Ni(2+)-containing heteropolytungstates [Ni14(OH)6(H2O)10(HPO4)4(P2W15O56)4](34-) (Ni14) and [Ni5(OH)4(H2O)4(ß-GeW9O34)(ß-GeW8O30(OH))](13-) (Ni5) have been successfully synthesized in aqueous, basic media under conventional reaction conditions, and they were characterized by single-crystal X-ray diffraction, IR spectroscopy, thermogravimetric and elemental analyses, electrochemistry, and magnetic studies. The cyclic voltammetry (CV) patterns of Ni14 and Ni5 showed chemically reversible multielectronic waves for slow scan time scales. For Ni14, an important acidity inversion effect between its reduced forms was observed. Magnetic studies revealed dominant ferromagnetic interactions among the nickel(II) ions in both polyanions.

Properties of a tunable multinuclear nickel polyoxotungstate platform.

A series of hybrid nickel bisphosphonate (BP) polyoxometalates (POMs) has been isolated. The complexes NaK-Ni7-Ale2 (Ale = [H2O3PC(C3H6NH2)(OH)PO3H2]) and NaNH4-Ni7-Ale2 are both made of two {PW9O34} fragments enclosing a heptanuclear Ni(II) core connected to two alendronate ligands. By pre-functionalising the alendronate moiety through the amino group, the naphthalene (napht) derivative Ni7-(AleNapht)2 and the compounds Ni7-(AlePy2 Ni)2 (py = pyridyl) and Ni7-(AleAc2Ni)2 (Ac = acyl) have been obtained. Compared with the NaK-Ni7-Ale2 species, these last two complexes contain two additional Ni(II) centres connected through two bis(2-pyridylmethyl)amine or two iminodiacetate groups, respectively. This results show that it is possible to functionalise the heptanuclear POM in a controlled manner. Quantification of the magnetic interactions in NaK-Ni7-Ale2 revealed that in the {Ni7} core, ferromagnetic interactions are predominant, with a S = 5 ground state. Magnetisation versus dc field sweeps on a single crystal of NaK-Ni7-Ale2 exhibited hysteresis at low temperature. (1)H and (31)P NMR studies in aqueous solution performed on NaK-Ni7-Ale2 and Ni7-(AleNapht)2 evidenced that the PW9/Ni7/bisphosphonate assembly is stable in solution. This was completed by (31)P magic angle spinning (MAS) investigations and confirmed by (1)H DOSY experiments. The electrochemistry of these compounds proceeds through two well-defined four-electron chemically reversible waves in a medium at pH 6. NaK-Ni7-Ale2 proved to be efficient for the electrocatalytic reduction of nitrate, nitrite and nitrous oxide. Remarkably, its electrocatalytic efficiency for nitrate reduction is approximately three times higher than those previously reported for POMs in a medium at pH >4 under the same potential. The catalytic properties of two representatives of the hybrid family were also examined. It is shown that these nickel bisphosphonate polyoxotungstates are pre-catalysts for the oxidation of alcohols into ketones or carboxylic acids, depending on the classes of alcohols considered, the stoichiometric oxidant used being H2O2. Noticeably, it has been found that an analogous cobalt bisphosphonate polyoxotungstate complex does not present any related activity, highlighting the crucial role of the 3d cations on the catalytic process.

Polyoxopalladates encapsulating 8-coordinated metal ions, [MO8Pd(II)12L8]n- (M = Sc3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Lu3+; L = PhAsO3(2-), PhPO3(2-), SeO3(2-)).

A total of 16 discrete polyoxopalladates(II) [MO(8)Pd(II)(12)L(8)](n-), with a metal ion M encapsulated in a cuboid-shaped {Pd(12)O(8)L(8)} cage, have been synthesized: the phenylarsonate-capped series (1) L = PhAsO(3)(2-), M = Sc(3+) (ScPhAs), Mn(2+) (MnPhAs), Fe(3+) (FePhAs), Co(2+) (CoPhAs), Ni(2+) (NiPhAs), Cu(2+) (CuPhAs), Zn(2+) (ZnPhAs); the phenylphosphonate-capped series: (2) L = PhPO(3)(2-), M = Cu(2+) (CuPhP), Zn(2+) (ZnPhP); and the selenite-capped series (3) L = SeO(3)(2-), M = Mn(2+) (MnSe), Fe(3+) (FeSe), Co(2+) (CoSe), Ni(2+) (NiSe), Cu(2+), (CuSe), Zn(2+) (ZnSe), Lu(3+) (LuSe)). The polyanions were prepared in one-pot reactions in aqueous solution of [Pd(3)(CH(3)COO)(6)] with an appropriate salt of the metal ion M, as well as PhAsO(3)H(2), PhPO(3)H(2), and SeO(2), respectively, and then isolated as hydrated sodium salts Na(n)[MO(8)Pd(II)(12)L(8)]·yH(2)O (y = 10-37). The compounds were characterized in the solid state by IR spectroscopy, single-crystal XRD, elemental and thermogravimetric analyses. The solution stability of the diamagnetic polyanions ScPhAs, ZnPhAs, ZnPhP, ZnSe, and LuSe was confirmed by multinuclear ((77)Se, (31)P, (13)C, and (1)H) NMR spectroscopy. The polyoxopalladates ScPhAs, MnPhAs, CoPhAs, and CuPhAs were investigated by electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS). Electrochemical studies on the manganese- and iron-containing derivatives demonstrated that the redox properties of the Mn(2+), Fe(3+), and Pd(2+) centers in the polyanions are strikingly influenced by the nature of the capping group. These results have subsequently been verified by density functional theory (DFT) calculations. Interestingly, electron paramagnetic resonance (EPR) measurements suggest that the coordination geometry around Mn(2+) is dynamically distorted on the EPR time scale (∼10(-11) s), whereas it appears as a static ensemble with cubic symmetry on the X-ray diffraction (XRD) time-scale (10(-15) s). The octacoordinated Cu(2+) cuboid is similarly distorted, in good agreement with DFT calculations. Interestingly, g(∥) is smaller than g(⊥), which is quite unusual, needing further theoretical development.

Photo- and electrochromic properties of covalently connected symmetrical and unsymmetrical spiropyran-polyoxometalate dyads.

Polyoxometalates covalently linked to one or two spiropyran entities have been isolated. These organic-inorganic hybrids exhibit multi-electrochromic and photochromic properties.

The selenite-capped polyoxo-4-aurate(III), [Au(III)4O4(Se(IV)O3)4]4-.

The discrete heteropolyaurate [Au(III)(4)O(4)(Se(IV)O(3))(4)](4-) (Au(4)Se(4)) represents only the second member of this class ever reported, and was synthesized via one-pot room temperature condensation in aqueous medium. was structurally characterized in the solid state and in solution by single-crystal XRD, TGA, FT-IR, (77)Se NMR, mass spectrometry and electrochemistry.

ε-Keggin-based coordination networks: Synthesis, structure and application toward green synthesis of polyoxometalate@graphene hybrids.

Four coordination networks based on the {ε-PMo(V)(8)Mo(VI)(4)O(40)(OH)(4)Zn(4)} Keggin unit (εZn) have been synthesized under hydrothermal conditions. (TBA)(3){PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)}[C(6)H(4)(COO)(2)](2) (ε(isop)(2)) is a 2D material with monomeric εZn units connected via 1,3 benzenedicarboxylate (isop) linkers and tetrabutylammonium (TBA) counter-cations lying between the planes. In (TPA)(3){PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)}[C(6)H(3)(COO)(3)] (TPA[ε(trim)](∞)), 1D inorganic chains formed by the connection of εZn POMs, via Zn-O bonds, are linked via 1,3,5 benzenetricarboxylate (trim) ligands into a 2D compound with tetrapropylammonium (TPA) cations as counter-cations. (TBA){PMo(V)(8)Mo(VI)(4)O(40)Zn(4)}(C(7)H(4)N(2))(2)(C(7)H(5)N(2))(2)·12H(2)O (ε(bim)(4)) is a molecular material with monomeric εZn POMs bound to terminal benzimidazole (bim) ligands. Finally, (TBA)(C(10)H(10)N(4))(2)(HPO(3)){PMo(V)(8)Mo(VI)(4)O(40)Zn(4)}(2)(C(10)H(9)N(4))(3)(C(10)H(8)N(4)) (ε(2)(pazo)(4)) is a 1D compound with dimeric (εZn)(2) POMs connected by HPO(3)(2-) ions and terminal para-azobipyridine (pazo) ligands. In this compound an unusual bond cleavage of the central N[double bond, length as m-dash]N bond of the pazo ligand is observed. We report also a green chemistry-type one-step synthesis method carried out in water at room temperature using ε(2)(pazo)(4) and ε(isop)(2) as reducing agent of graphite oxide (GO) to obtain graphene (G). The POM@G hybrids were characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, powder X-ray diffraction, energy dispersive X-ray analysis, infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and cyclic voltammetry.

Functionalized polyoxometalates with covalently linked bisphosphonate, N-donor or carboxylate ligands: from electrocatalytic to optical properties.

The structures of two families of hybrid organic-inorganic polyoxometalates (POMs) functionalized by covalently grafted carboxylate or bisphosphonate ligands are overviewed. The first family concerns the so-called POMOF materials, built from the connection of mixed-valent ε-Keggin type polyoxomolybdates via N-donor or carboxylate organic molecules coordinated to transition metal ions (Zn(II), Co(II), Ni(II)) grafted at the surface of the POMs. The simulation of the hypothetical zeolitic-like POMOF structures is presented and compared to the experimental ones. The second family gathers the various molecular Mo(V), Mo(VI) and W(VI) POMs incorporating directly into their architecture bisphosphonate (BP) ligands. The potentiality of this family of hybrid POMs comes from the organic group grafted on the carbon atom which bears the two phosphonate groups. For both families, besides the structural description, synthetic trends and an overview of their properties are presented. Namely, the electrocatalytic properties (production of H(2) and reduction of BrO(3)(-)) of the ε-Keggin type polyoxomolybdates are described. The optical and biological activities of the POM-BP compounds as well as their ability to form nanosystems are also reported.

{W48} ring opening: Fe16-containing, Ln4-stabilized 49-tungsto-8-phosphateopen wheel [Fe16O2(OH)23(H2O)9(P8W49O189)Ln4(H2O)20]11-.

Wheely: For the first time and very unexpectedly, a rupture of the very stable {P(8)W(48)} wheel was observed in aqueous solution at pH 4 and 80 °C in the presence of Fe(III), Eu(III)/Gd(III), and H(2)O(2). This inorganic ring opening is unprecedented in polyoxometalate chemistry.

3 d metal ions in highly unusual eight-coordination: the phosphate-capped dodecapalladate(II) nanocube.

Trapped in a noble cube: A novel family of noble metalates has been discovered in which a 3d metal ion M (M = Mn(II), Fe(III), Co(II), Cu(II), Zn(II)) is encapsulated by a 12 palladium-oxo cage {Pd(12)O(32)}, which is capped by eight phosphate groups. Such discrete nanocubes were further investigated by EPR spectroscopy, electrochemistry, and in homogeneous hydrogenation catalysis.