RESUMEN
The synthesis, X-ray crystal structures, cyclic voltammetry, and IR and (31)P NMR characterizations of five new Wells-Dawson-type polyoxometalates are reported, the formulas of which being [(Cu(H(2)O)(4))(2.5)][HP(2)W(18)O(62)].11.33H(2)O (4), [(Cu(H(2)O)(4))(2.4)][H(1.2)P(2)W(12)Mo(6)O(62)].11.4H(2)O (5), [(Cu(H(2)O)(4))(2.3)][H(1.4)P(2)W(15)Mo(3)O(62)].10.1H(2)O (6), [Co(H(2)O)(4))(2.3)][H(1.4)P(2)W(14)Mo(4)O(62)].18.8H(2)O (7), and [Ni(H(2)O)(4))(1.85)][H(2.3)P(2)W(15)Mo(3)O(62)].20.6H(2)O (8). These compounds are obtained as crystalline materials from the corresponding acids by substitution of a transition metal in aqueous solution. Compounds 4, 5, and 6 are isostructural; their crystal structure consists of 2D polymeric hexagonal layers where each Wells-Dawson anion is connected to six neighbors by [CuO(6)] octahedra. On the other hand, compounds 7 and 8, which are also isostructural, present a 3D structure where the Mo-substituted [P(2)W(14)Mo(4)O(62)] units are connected by [MO(6)] octahedra (M = Co(II), Ni(II)). Different types of substitution modes of Mo ions are observed within polyanions. Cyclic voltammetry shows that Cu(II) derivatives have similar behavior but different from that of the starting compounds. No electrochemical activity has been detected for the Ni-containing compound 8.
RESUMEN
Electrochemical behaviors of selected Dawson-type polytungstates including 2-K10[P2W15Mo2O61box] where the symbol [box] designates a vacant site, alpha2-K7[Fe(OH2)P2W15Mo2O61], alpha2-K8[Cu(OH2)P2W15Mo2O61], alpha1- and alpha2-K8[Cu(OH2)P2W17O61], alpha2-K8[Cu(OH2)P2W13Mo4O61], and alpha2-K8[Cu(OH2)P2W12Mo5O61] were investigated by cyclic voltammetry (CV) coupled with the electrochemical quartz microbalance (EQCM), and the results were completed by atomic force microscopy (AFM) observations of the electrodeposited films. The electrocatalytic abilities of these polyoxometalates (POMs) in the reduction of dioxygen, hydrogen peroxide, and NOx were also assessed by CV and EQCM. It turns out that the remarkable electrocatalysis obtained at the reduction potential of Mo centers within alpha2-K8[Cu(OH2)P2W15Mo2O61], but in a domain where Cu2+ is not deposited, benefits from the assistance of the copper center because such catalysis could not be observed in the absence of Cu2+. EQCM confirms that no copper deposition occurs under the experimental conditions used. Analogous behaviors are encountered in the electrocatalytic reduction of nitrite where assistance by the presence of the Cu2+ center induced the observation of catalysis at the potential location of Mo centers. Finally, the reduction of nitrate is triggered by electrodeposited copper but was remarkably favored by the presence of molybdenum atoms within these polyoxometalates (POMs). All of the results converge to indicate a cooperative effect between the Mo and Cu centers within these POMs. The various results suggest that copper deposition from these POMs should give morphologically different surfaces. AFM studies confirm this expectation, and the observed morphologies and sizes of particles were rationalized by taking into account the role of the POM skeleton and its atomic composition in the electrodeposition process.