[H4AsW18O62](7-), A novel Dawson heteropolyanion and two of its sandwich-type derivatives [Zn-4(H2O)(2)(H4AsW15O56)(2)](18-), [Cu-4(H2O)(2)(H4AsW15O56)(2)](18-): cyclic voltammetry and electrocatalytic properties towards nitrite and nitrate

The cyclic voltammetry behaviours of the new Dau son heteropolyanion (HPA) octadecatungstomonoarsenate [H(4)ASW(18)O(62)] were studied as a function o f pH. The electrochemistry of the fully symmetrical octadecatungstodiarsena te [As2W18O62](6-) in a pH 0.3 medium is constituted mainly by four reducti on steps, the first two featuring reversible one-electron processes. The co mparison of the cyclic voltammetry behaviours of two complexes reveals, int erestingly, that the merging of the first two waves of the monoarsenate occ urs in a less acidic pH medium than for the diarsenate. The voltammograms o f the two corresponding trivacant species. [H(4)AsW(15)square O-3(56)](12-) and [AS(2)W(15)square O-3(56)](12-), in which the symbol square represents the vacancy. were also studied and compared. The comparison was extended t o the Zn- and Cu-sandwich-type derivatives, xith particular emphasis on the compounds of the monoarsenate series. [Zn4H2O)(2)(H4AsW15O56)(2)](18-) and [CU4(H2O)(2)(H4AsW15O56)(2)](18-) All these species prove efficient in the electrocatalytic reduction of nitrite. but only copper-substituted derivat ives are active for the electrocatalytic reduction of nitrate. To our knowl edge, this observation constitutes the first example of electrocatalysis of nitrate reduction by a sandwich complex of the polyoxometalate family. (C) 2001 Elsevier Science B.V. All rights reserved.