title: The ball-shaped heteropolytungstates [[Sn(CH3)2(H2O)]24[Sn(CH3)2]12(A-XW9O34)12]36- (X=P, As): stability, redox and electrocatalytic properties in aqueous media.
abstract: The electrochemical behavior of the ball-shaped heteropolytungstates [[Sn(CH(3))(2)(H(2)O)](24)[Sn(CH(3))(2)](12)(A-XW(9)O(34))(12)](36-) (X=P, 1; As, 2) was examined in aqueous electrolytes by redissolution of their respective mixed cesium-sodium salts Cs(14)Na(22)[[Sn(CH(3))(2)(H(2)O)](24)[Sn(CH(3))(2)](12) (A-PW(9)O(34))(12)]149 H(2)O (Cs(14)-1) and Cs(14)Na(22)[[Sn(CH(3))(2)(H(2)O)](24)[Sn(CH(3))(2)](12)(A-AsW(9)O(34))(12)]149 H(2)O (Cs(14)-2). In the studied media, Cs(14)-2 is readily soluble in contrast to the significantly less soluble Cs(14)-1. The solubility of Cs(14)-1 is increased by the presence of Li(+) ions in solution. Gel filtration studies with 1 and 2 rule out a decay of the dodecameric spherical assemblies to Keggin-based monomers on the timescale of the experiment. By UV/Vis spectroscopy and cyclic voltammetry, 2 was found to be significantly less stable than 1 and both polyanions also show rather different decomposition pathways. Polyanion 1 collapses first into Keggin-type monomers which might contain the trilacunary [A-alpha-PW(9)O(34)](9-). The final monomeric species obtained from 1 appears to be very similar to [PW(11)O(39)](7-), which is the final transformation product of [A-alpha-PW(9)O(34)](9-) in the same media. In contrast, 2 does not seem to follow an analogous transformation pathway as that of the trilacunary [A-alpha-AsW(9)O(34)](9-). Importantly, stabilization of 1 is observed in chloride media. The fairly long-term stability of 1 in 1 M LiCl, pH 3, has allowed for its electrochemical study to be carried out. The solid-state cyclic voltammogram of 1 entrapped in a carbon paste electrode shows the same characteristics as 1 dissolved in chloride solutions, thus supporting the conclusion that the polyanion is stable in these environments. Controlled potential coulometry on 1 indicates that the number of electrons consumed in the first wave is larger than twenty. To our knowledge, 1 constitutes the first example of a molecule that can take up such a large number of electrons resulting in a chemically reversible W-wave. These properties show promise for future fundamental and applied studies. Polyanion 1 is also efficient in the electrocatalytic reduction of NO(x), including nitrate. Finally, a remarkable interaction was found between 1 and NO, a highly promising feature for biomimetic applications.
authors: Laboratoire de Chimie Physique, UMR 8000, CNRS, Equipe d'Electrochimie et Photoélectrochimie, Université Paris-Sud, Bâtiment 420, 91405 Orsay Cedex, France.
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