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.

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.

Facile synthesis of Au-nanoparticle/polyoxometalate/graphene tricomponent nanohybrids: an enzyme-free electrochemical biosensor for hydrogen peroxide.

A green, facile, one-pot synthesis of well-defined Au NPs@POM-GNSs tricomponent nanohybrids is reported (POM stands for polyoxometalate and GNSs for graphene nanosheets). The synthesis is convenient, rapid and environmentally friendly. The POMs serve as both reducing, encapsulating molecules, and bridging molecules; this avoids the introduction of other organic toxic molecules. Characterization using transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy analysis is performed, and the structure of the prepared nanohybrids of Au NPs@POM-GNSs is verified. Most importantly, the amperometric measurements show the Au NPs@POM-GNSs nanohybrids have high catalytic activity with good sensitivity, good long-term stability, wide linear range, low detection limit, and fast response towards H(2)O(2) detection for application as an enzyme-free biosensor. Transformation of the POMs during H(2)O(2) detection does not affect the catalytic activities of the nanohybrids. Thus, the synergistic effect of Au NPs and GNSs in the nanohybrids leads to the enhanced catalytic property.

Polyoxometalate-based metal organic frameworks (POMOFs): structural trends, energetics, and high electrocatalytic efficiency for hydrogen evolution reaction.

The grafting of the triangular 1,3,5-benzene tricarboxylate linkers (denoted trim) on tetrahedral ε-Keggin polyoxometalates (POMs) capped by Zn(II) ions, formed in situ under hydrothermal conditions, has generated three novel POM-based metal organic frameworks (POMOFs). (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)][C(6)H(3)(COO)(3)](4/3)·6H(2)O (ε(trim)(4/3)) is a 3D open-framework built of molecular Keggin units connected by trim linkers, with channels occupied by tetrabutylammonium (TBA) counterions. ε(trim)(4/3) is a novel (3,4)-connected net, named ofp for open-framework polyoxometalate, and computer simulations have been used to evaluate its relative stability in comparison with ctn- and bor-like polymorphs, showing the stability of this novel phase directly related to its greatest density. A computational study was also undertaken with the aim of locating TBA molecules, the positions of which could not be deduced from single crystal X-ray diffraction, and further rationalizes their structure directing role. In (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)][C(6)H(3)(COO)(3)] (ε(2)(trim)(2)), the building unit is not the molecular Keggin but a dimerized form of this POM. Their connection via trim linkers generates a 3D framework with channels filled by TBA cations. In (TBA)(3)[PMo(V)(8)Mo(VI)(4)O(37)(OH)(3)Zn(4)][C(6)H(3)(COO)(3)]·8H(2)O ([ε(trim)](∞)), zigzag chains are connected via the organic linkers, forming 2D grids. Modified electrodes were fabricated by direct adsorption of the POMOFs on glassy carbon or entrapment in carbon paste (CPE). A remarkable electrocatalytic hydrogen evolution reaction (HER) was detected with a yield greater than 95%, and a turnover number as high as 1.2 × 10(5) was obtained after 5 h. The reported POMOF-based electrodes are more active than platinum, with a roughly 260 mV anodic shift. Finally, the electrocatalytic activities of ε(trim)(4/3)/CPE electrodes in various XCl (X = Li, Na, K, Cs) media have been studied. This allowed us to detect a cation effect and propose an electrocatalytic mechanistic pathway for the HER.

Assembly of a magnetic polyoxometalate on SWNTs.

Recently, the organisation of magnetic molecules on carbon nanotubes has raised much interest due to their possible interesting contribution to molecular spintronics. In this paper, we describe the assembly on SWNTs of a magnetic polyoxometalate encompassing a single cobalt ion (CoPOM) and its isostructural diamagnetic zinc analogue (ZnPOM). The simple magnetic behaviour of CoPOM and the availability of its diamagnetic counterpart render these POM@NTs systems interesting model compounds for the study of molecular electronics devices based on carbon nanotubes and magnetic molecules. The success and rate of the grafting have been investigated by electron microscopy, electron energy loss spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, Raman scattering and magnetisation measurements. These characterisations altogether demonstrate the preservation of the structural and magnetic properties of the molecules upon functionalisation and the existence of an electronic communication between the molecules and the nanotubes.

Influence of the heteroatom size on the redox potentials of selected polyoxoanions.

The apparent formal potentials for the one-electron redox process of most Keggin-type heteropolytungstates, XW(12)O(40)(q-), have long been shown to linearly depend on their overall negative charges, in the absence of proton interference in the process. However, for a given overall negative charge, these formal potentials are also shown here to depend on the specific central heteroatom X. In the present work, cyclic voltammetry was used to study a large variety of Keggin-type anions, under conditions where their comparisons are straightforward. In short, apparent potential values get more negative (the clusters are more difficult to reduce) for smaller central heteroatoms within a given family of Keggin-type heteropolyanions carrying the same overall negative charge. Density functional theory calculations were performed on the same family of Keggin compounds and satisfactorily reproduce these trends. They show that internal XO(4) units affect differently the tungstate oxide cage. The electrostatic potential created by each internal anionic unit in a fragment-like approach (XO(4)(q-)@W(12)O(36)) was analyzed, and it is observed that X atoms of the same group show slight differences. Within each group of the periodic table, X atoms with lower atomic numbers are also smaller in size. The net effect of such a tendency is to produce a more negative potential in the surroundings and thus a smaller capacity to accept electrons. The case of [BW(12)O(40)](5-) illustrates well this conclusion, with the smallest heteroatom of the Keggin series with group III central elements and a very negative reduction potential with respect to the other elements of the same group. Particularly in this case, the electronic structure of the Keggin anion shows the effects of the small size of boron: the highest occupied molecular orbitals of [BW(12)O(40)](5-) appear to be approximately 0.35 eV higher than those in the other clusters of the same charge, explaining that the BO(4) unit is more unstable than AlO(4) or GaO(4) despite carrying the same formal charge.

Polyoxopalladates encapsulating yttrium and lanthanide ions, [X(III)Pd(II)12(AsPh)8O32]5- (X=Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu).

A series of novel yttrium- and lanthanide-containing heteropolyoxopalladates have been prepared and isolated as hydrated sodium salts, Na(5)[X(III)Pd(II)(12)(AsPh)(8)O(32)]y H(2)O (X=Y (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7), Dy (8), Ho (9), Er (10), Tm (11), Yb (12), Lu (13); y=15-27). The polyanions [X(III)Pd(II)(12)(AsPh)(8)O(32)](5-) consist of a cuboid framework of twelve Pd(II) ions with eight phenylarsonate heterogroups located at the vertices and a central guest ion X. The compounds 1-13 have been prepared in a simple one-pot self-assembly reaction of Pd(CH(3)COO)(2), phenylarsonic acid and the respective salt of the element X in 0.5 M aqueous sodium acetate solution (pH 6.9), and characterized in the solid state by single-crystal X-ray diffraction, elemental and thermogravimetric (TGA) analyses, and IR spectroscopy. It was demonstrated that small, medium, and also large lanthanide ions can be incorporated in the center of the novel heteropolypalladate [X(III)Pd(II)(12)(AsPh)(8)O(32)](5-). The Ln-O bond lengths follow the expected trend decreasing from left to right in the lanthanide series. This indicates that the {Pd(II)(12)O(32)} shell can adjust to the coordination requirements of the encapsulated guest cation. Compounds 3 and 5 were selected for electrochemical studies. Their cyclic voltammetry in a lithium acetate buffer at pH 5.9 showed a Pd(0) deposition process on the glassy carbon electrode surface. Coulometry indicated that all Pd(II) centers were reduced to Pd(0). The film was stable and could be taken out of the deposition medium and characterized in pure pH 5.9 buffer. Magnetic susceptibility and EPR measurements were carried out on 5 and 6. The former was confirmed to be diamagnetic and the latter strongly paramagnetic with a S=7/2 ground state. DFT calculations for some of the polyoxometalates have been also performed.

Molecular interaction between europium decatungstate and histone H1 and its application as a novel biological labeling agent.

Polyoxometalates (POMs) show promising biological activities, but the mechanism of potential therapeutic effects remains to be elucidated at a molecular level. As a step toward the elucidation of the mechanistic pathways governing the bioactivity of POMs, the interaction between Eu-containing decatungstate [EuW10O36]9- (EuW10) and histone H1 has been studied. Fluorescence/luminescence analysis showed the existence of a strong interaction between EuW10 and histone H1. This interaction has key effects both on the luminescence of EuW10 and on the structure of histone H1. A gradual and intense enhancement of EuW10 luminescence was observed upon addition of increasing concentrations of histone H1. Circular dichroism investigations indicated that the binding of EuW10 significantly alters the secondary structure of histone H1. The present work is meaningful in finding novel labeling agents for fluorescence/luminescence or solid-state bioimaging.

Cobalt, manganese, nickel, and vanadium derivatives of the cyclic 48-tungsto-8-phosphate [H(7)P(8)W(48)O(184)](33-).

The cobalt(II) containing tungstophosphate [Co(4)(H(2)O)(16)P(8)W(48)O(184)](32-) (1) has been synthesized by addition of Co(2+) ions to an aqueous solution of [H(7)P(8)W(48)O(184)](33-) (P(8)W(48)) and characterized by single-crystal XRD, IR, and UV-vis spectroscopy, elemental analysis, electrochemistry, and magnetochemistry. The novel polyanion 1 is a derivative of the superlacunary P(8)W(48) with four cobalt(II) ions coordinated to the rim of the central cavity and two additional cobalt(II) ions linked on the outside bridging neighboring polyanions. Using similar synthetic procedures, but adding a few drops of H(2)O(2), we isolated the manganese(II) derivative [Mn(4)(H(2)O)(16)(P(8)W(48)O(184))(WO(2)(H(2)O)(2))(2)](28-) (2) and its nickel(II) analogue [Ni(4)(H(2)O)(16)(P(8)W(48)O(184))(WO(2)(H(2)O)(2))(2)](28-) (3). Both polyanions have picked up two equivalents of tungsten resulting in the unprecedented {P(8)W(50)} host framework. We also made the vanadium(V) derivative [(VO(2))(4)(P(8)W(48)O(184))](36-) (4), with four tetrahedral vanadate groups grafted to the P(8)W(48) host. The voltammetric patterns associated with the W-centers in polyanions 1, 2, and 4 display enough distinct features allowing for a qualitative classification according to relative basicity of the reduced polyanions: 2 > P(8)W(48) > 1 > 4. The electrochemistry of 1 offers a new example for detection of the Co(2+) centers in a multicobalt containing polyanion. During a study of the Mn(2+) centers of 2 at pH 5, a film deposition is observed. The vanadium(V) centers of 4 are well-behaved in a pH 0.33 medium. Temperature and magnetic field dependence of the magnetic moment of 1-3 were performed on a SQUID magnetometer over the temperature range 1.8-250 K and field range 0-7 T. The results are consistent with the model of noninteracting 3d metal ions. Variable temperature (4-295 K) and variable frequency (34-413 GHz) EPR measurements support the magnetic susceptibility results. The zero-field splitting D and g values obtained for 1-3 are in agreement with those reported for high-spin Co(2+), Mn(2+), and Ni(2+) ions in axially distorted octahedral environments.

A multitechnique study of europium decatungstate and human serum albumin molecular interaction.

Polyoxometalates (POMs) show promising biological activities, but the mechanism of these potential therapeutic effects remains to be elucidated at a molecular level. As a step in this direction, the interaction between the Eu-containing decatungstate [EuW(10)O(36)](9-) and human serum albumin (HSA) has been studied by several techniques. Fluorescence/luminescence analysis showed the existence of a strong interaction between the POM and HSA. This interaction has key effects both on luminescence of the POM and on the behaviours of HSA. An enhancement of the POM luminescence is observed upon interaction. The presence of increasing concentrations of the POM results in the progressive quenching of the fluorescence of the single tryptophan of HSA. Circular dichroism led to the conclusion that the binding of the POM did not alter the secondary structure of HSA. Isothermal titration calorimetry revealed an enthalpy-driven binding reaction between HSA and the POM, resulting in the formation of a 1:1 complex. The present work is meaningful in finding novel solid state bio-image or fluorescence/luminescence labelling agents.

Nitrite sensor based on multilayer film of Dawson-type tungstophosphate alpha-K7)[H4PW18O62]x18H2O immobilized on glassy carbon.

A nitrite sensor based on immobilized Dawson-type tungstophosphate alpha-K(7)[H(4)PW(18)O(62)].18H(2)O (PW(18)) in multilayers of charged polyelectrolyte poly(allylamine hydrochloride) (PAH) on a glassy carbon electrode is described. A nitrite sensor manufactured with 10 layers has a sensitivity of approximately 4 nA/microM nitrite, fast response time (<6s), low detection limit ( approximately 0.1 microM), high selectivity towards endogenous interferences such as nitrate and molecular oxygen, a linear range from 0.1 microM to at least 20mM nitrite and was stable for at least 2 months. In addition, such nitrite sensors can operate in a pH range from 1 to 9, and the sensitivity can be increased by increasing the number of layers at the expense of increasing the response time.

Capture of the [Mo3S4]4+ cluster within a {Mo18} macrocycle yielding a supramolecular assembly stabilized by a dynamic H-bond network.

The use of the [Mo(3)S(4)(Hnta)(3)](2-) complex (nta(3-) = nitrilotriacetate) as structuring agent toward the self-condensation process of the [Mo(2)O(2)S(2)(OH(2))(6)](2+) cation leads to the largest oxothiomolybdenum ring. In the solid state, X-ray diffraction analysis reveals the presence of the targeted molecular compound (noted 1a), which consists of the {Mo(18)O(18)S(18)(OH)(18)} host templated by the [Mo(3)S(4)(Hnta)(3)](2-) guest. Nevertheless, the structure shows an additional molecular moiety corresponding to a dinuclear unit {Mo(2)O(2)S(2)} coordinated to two nta(3-) ligands, mutually arranged in a cis fashion (1b). In the solid state, both entities interact through two short hydrogen bonds to give a striking supramolecular adduct, noted {1a-1b}. Synthetic procedures to prepare the individual species as pure compounds were reported. 1a was obtained as a pure mixed Cs(+)/NMe(4)(+) salt while the dinuclear unit [Mo(2)O(2)S(2)(Hnta)(2)](2-) was obtained as mixed K(+)/Na(+) crystals. X-ray diffraction study of the latter reveals a trans isomer (noted 1b'), characterized by the specific coordination of both nta(3-) ligands. All the compounds were characterized in solution (D(2)O or DMSO) by multiexperiment (1)H NMR (1D, COSY, NOESY, and DOSY). The overall results were consistent with the retention of the adduct {1a-1b} which exhibits a supramolecular reactivity. The dinuclear individual species in solution gave rise to cis-trans equilibrium, while in the presence of the oxothiomolybdenum ring 1a, the dinuclear unit is maintained as a frozen cis complex. DOSY NMR provides a definitive argument for the integrity of the supramolecular assembly. In addition, preliminary electrochemical study of 1a is also reported.

Zeolitic polyoxometalate-based metal-organic frameworks (Z-POMOFs): computational evaluation of hypothetical polymorphs and the successful targeted synthesis of the redox-active Z-POMOF1.

The targeted design and simulation of a new family of zeolitic metal-organic frameworks (MOFs) based on benzenedicarboxylate (BDC) as the ligand and epsilon-type Keggin polyoxometalates (POMs) as building units, named here Z-POMOFs, have been performed. A key feature is the use of the analogy between the connectivity of silicon in dense minerals and zeolites with that of the epsilon-type Keggin POMs capped with Zn(II) ions. Handling the epsilon-Keggin as a building block, a selection of 21 zeotype structures, together with a series of dense minerals were constructed and their relative stabilities computed. Among these Z-POMOFs, the cristobalite-like structure was predicted to be the most stable structure. This prediction has been experimentally validated by the targeted synthesis of the first experimental Z-POMOF structure, which was strikingly found to possess the cristobalite topology, with three interpenetrated networks. Crystals of [NBu(4)](3)[PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)(BDC)(2)].2H(2)O (Z-POMOF1) have been isolated under hydrothermal conditions from the reduction of ammonium heptamolybdate in the presence of phosphorous acid and Zn(II) ions. Tetrabutylammonium cations play the role of counterions and space-filling agents in this tridimensional interpenetrated framework. Moreover, the electrochemistry of the epsilon-Keggin POM is maintained and can be exploited in the insoluble Z-POMOF1 framework, as demonstrated by the electrocatalytic reduction of bromate.

Heteropoly-13-palladates(II) [Pd(II)(13)(As(V)Ph)(8)O(32)](6-) and [Pd(II)(13)Se(IV)(8)O(32)](6-).

Two discrete anionic palladium(II)-oxo clusters have been prepared: [Pd(13)(As(V)Ph)(8)O(32)](6-) (1) and [Pd(13)Se(IV)(8)O(32)](6-) (2) were synthesized in one-pot self-assembly reactions of Pd(OAc)(2) with PhAsO(3)H(2) and SeO(2) and characterized by single-crystal X-ray analysis, IR, thermogravimetric analysis, elemental analysis, magnetic and electron paramagnetic resonance measurements, and electrochemistry.

Organo-ruthenium supported heteropolytungstates: synthesis, structure, electrochemistry, and oxidation catalysis.

The reaction of [Ru(arene)Cl(2)](2) (arene = benzene, p-cymene) with [X(2)W(22)O(74)(OH)(2)](12-) (X = Sb(III), Bi(III)) in buffer medium resulted in four organo-ruthenium supported heteropolytungstates, [Sb(2)W(20)O(70)(RuC(6)H(6))(2)](10-) (1), [Bi(2)W(20)O(70)(RuC(6)H(6))(2)](10-) (2), [Sb(2)W(20)O(70)(RuC(10)H(14))(2)](10-) (3), and [Bi(2)W(20)O(70)(RuC(10)H(14))(2)](10-) (4), which have been characterized in solution by multinuclear ((183)W, (13)C, (1)H) NMR, UV-vis spectroscopy, electrochemistry, and in the solid state by single-crystal X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis. Polyanions 1, 2, and 4 crystallize in the triclinic system, space group P1 with the following unit cell parameters: K(5)Na(5)[Sb(2)W(20)O(70)(RuC(6)H(6))(2)] x 22 H(2)O (KNa-1), a = 12.1625(2) A, b = 13.1677(2) A, c = 16.0141(3) A, alpha = 78.9201(7) degrees, beta = 74.4442(8) degrees, gamma = 78.9019(8) degrees, and Z = 1; Cs(2)Na(8)[Bi(2)W(20)O(70)(RuC(6)H(6))(2)] x 30 H(2)O (CsNa-2), a = 11.6353(7) A, b = 13.3638(7) A, c = 16.7067(8) A, alpha = 79.568(2) degrees, beta = 71.103(2) degrees, gamma = 80.331(2) degrees, and Z = 1; Na(10)[Bi(2)W(20)O(70)(RuC(10)H(14))(2)].35H(2)O (Na-4), a = 15.7376(12) A, b = 15.9806(13) A, c = 24.2909(19) A, alpha = 92.109(4) degrees, beta = 101.354(4) degrees, gamma = 97.365(3) degrees, and Z = 2. Polyanions 1-4 consist of two (L)Ru(2+) (L = benzene or p-cymene) units linked to a [X(2)W(20)O(70)](14-) (X = Sb(III), Bi(III)) fragment via Ru-O(W) bonds resulting in an assembly with idealized C(2h) symmetry. Polyanions 1-4 are stable in solution as indicated by the expected (183)W, (13)C, and (1)H NMR spectra. The electrochemistry of 1-4 is described by considering the reduction and the oxidation processes. The nature of the arene in Ru(arene) has practically no influence on the formal potentials of the W-centers, which are more sensitive to the Sb or Bi hetero atoms. The results suggest that the respective Sb- and Bi derivatives have very different pK(a) values, with the reduced form of 1 being the most basic, thus permitting the observation of two well-developed voltammetric waves at pH 6. In contrast, the identity of the arene influences the oxidation processes, thus permitting to distinguish them. A strong electrocatalytic water oxidation peak is observed that is more positive than the one corresponding to the Ru(arene) oxidation process. Also a stepwise oxidation of the Ru(benzene) group could be observed at pH 3. The catalytic efficiency, on the other hand, of 1-4 toward the oxidation of n-hexadecane and p-xylene illustrated the effect of ruthenium substitution on the polyanion catalytic performance.