Pairing of a Keggin or Lindqvist polyoxometalate (POM) anion with an appropriate tetraalkylphosphonium cation is shown to yield the first members of a new family of ionic liquids (ILs). Detailed characterization of one of them, an ambient-temperature “liquid POM” comprising the Lindqvist salt of the trihexyl(tetradecyl) phosphonium cation, by voltammetry, viscometry, conductimetry, and thermal analysis indicates that it exhibits conductivity and viscosity comparable to those of the one previously described inorganic−organic POM-IL hybrid but with substantially improved thermal stability.

Tetraalkylphosphonium Polyoxometalate Ionic Liquids: Novel, Organic−Inorganic Hybrid Materials†

Spherical polyoxometalates (POMs) such as M6O192- and SiM12O404- (with M = Mo or W) and planar arene donors (anthracenes and pyrenes) can be cocrystallized (despite their structural incompatibility) by attaching a cationic “anchor” onto the arene which then clings to the POM anion by Coulombic forces. As a result, novel charge-transfer (CT) salts are prepared from arene donors and Lindqvist-type [M6O19]2- and Keggin-type [SiM12O40]4- acceptors with overall 2:1 and 4:1 stoichiometry, respectively. The CT character of the dark-colored (yellow to red) crystalline materials is confirmed by the linear Mulliken correlation between the CT transition energies and the reduction potentials of the POM acceptors, as well as by the transient (diffuse reflectance) absorption spectra (upon picosecond laser excitation) of anthracene or pyrene cation radicals (in monomeric and π-dimeric forms). X-ray crystallographic studies reveal a unique “dimeric” arrangement of the cofacially oriented arene couples which show contact ...

/pdf/novel-charge-transfer-materials-via-cocrystallization-of-52jt2xgu3i.pdf

Novel Charge-Transfer Materials via Cocrystallization of Planar Aromatic Donors and Spherical Polyoxometalate Acceptors

Transition metal (TM) cations and clusters have been widely used to functionalize polyoxometalates (POMs) of tungsten, molybdenum and vanadium due to their acidic nature, which made them compatible with addendum TM cations. Together with the good understanding of their solution behavior, a number of TM-based POMs have been achieved, which exhibited potential applications in many fields. Comparably, fewer polyoxoniobates (PONs) were obtained due to the poor understanding of their solution behavior, and they could only exist in strong alkaline aqueous solutions, where most of the TM cations precipitate because of the formation of hydroxide or oxide. Recently, TM cations were introduced into PONs by protection of chelate organic ligands or via selection of alkaline-soluble TM cations. Further, lacunary POMs, composed of early TM cations, were used as polydentate ligands to isolate PONs, and they make the PON clusters stably exist in acidic solutions. The acidic nature of tungstoniobate (W–Nb) clusters allowed acid-soluble TM cations to directly functionalize the Nb–W clusters. This highlight focuses on discussing the roles of TM centers in constructing PON-based materials, including TM-cluster-containing PONs, TM–organic group-decorating PONs and niobate clusters isolated by lacunary POMs. This systematic compilation of TM-PON studies will reveal a promising direction for further functionalization of PONs, and it will disclose a richer and more diverse set of structures in PON chemistry.

Recent progress in polyoxoniobates decorated and stabilized via transition metal cations or clusters

The deoxygenation of the mixed-metal polyoxoanion [(C4H9)4N]3NbW5O19 with benzoyl chloride in dichloromethane forms quantitatively the condensed polyoxanion [(C4H9)4N]4(NbW5O18)2O, in which two polyoxoanion fragments are linked together by a Nb−O−Nb oxo bridge. The product is characterized by a strong IR band at 692 cm-1 assigned to a Nb−O−Nb stretch and a broad single 93Nb NMR resonance at 975 ppm. Partial hydrolysis of [(C4H9)4N]4(NbW5O18)2O to NbW5O19O3- in wet acetonitrile was observed by IR and 17O NMR spectroscopy. The reaction of [(C4H9)4N]4(NbW5O18)2O with a variety of alcohols and phenol forms alkoxide-derivatized polyoxoanions [(C4H9)4N]2Nb(OR)W5O18 (R = methyl, ethyl, isopropyl, cholesteryl, phenyl). The similarity of the IR spectra of these deriviatives suggests that functionalization occurs at the terminal NbO oxygen. A crystallographic study of [(C4H9)4N]4(NbW5O18)2O revealed a crystallographically imposed linear Nb−O−Nb oxo bridge (Nb−Obridge = 1.887(3) A) and a structure in which the termi...

Deoxygenation of Polynuclear Metal−Oxo Anions: Synthesis, Structure, and Reactivity of the Condensed Polyoxoanion [(C4H9)4N]4(NbW5O18)2O

Two isostructural polyoxotungstates, K6Na4H8[Eu6(H2O)38(P2W15Nb3O62)4]·45H2O (1) and K6Na4H8[Ce6(H2O)38(P2W15Nb3O62)4]·56H2O (2), have been synthesized by reactionof saturated Nb/W mixed-addendum polyoxometalate [P2W15Nb3O62]9– and lanthanide ions in acidic solution. They are the first examples of lanthanide derivatives based on saturated Dawson-type Nb/W mixed-addendum polyoxometalates. All the compounds were fully characterized by single-crystal X-ray diffraction, IR spectroscopy, thermogravimetric analysis, elemental analysis, and electrochemistry. XRD analysis reveals that 1 and 2 display one-dimensional chains constructed from sandwich-type fragment {Ln3(P2W15Nb3O62)2} linked by alternating Ln–O–W bridging bonds. In 1 and 2, all the lanthanide ions selectively bind to Ot(Nb) of {P2W15Nb3O62} fragments (Ot: terminal oxygen atoms), indicating the high nucleophilicity of Ot(Nb). The photoluminescence behavior of 1 in the solid state was investigated at room temperature; it exhibits the characteristic transition of Eu under the excitation wavelength (394 nm). Additionally, the cyclic voltammogram of 2 indicates good electrocatalytic activity towards the reduction of nitrite.

Mongi Dabbabi

Papers

Propriétés électrochimiques de polyanions mixtes de composition NbnW6−nO19(2+n)−(n=0, 1, 2, 3 et 4)