Hétéropolyanions de type Dawson. 2. Synthèses de polyoxotungstoarsénates lacunaires dérivant de l'octadécatungstodiarsénate. Étude structurale par RMN du tungstène-183 des octadéca(molybdotungstovanado)diarsénates apparentés

Abstract

The di-, tri-, and hexavacant lacunary tungstoarsenates α-[HAs2W16O59]11−, α-[As2W15O56]12−, and α-[H2As2W12O48]12− were prepared by controlled alkaline degradation of the Dawson-type anion α-[As2W18O62]6−. The relationships between all these anions and the two α-[As2W17O61]10− isomers were established. The mixed saturated derivatives α-[As2VxW18−xO62](6+x)− (x = 1,2,3) and α-[As2MoxW18−xO62]6− (x = 1, 3) were obtained by refilling the lacunas with vanadium and molybdenum respectively. The new mixed anion α2-[As2Mo2W15O61]10− was used concurrently with α-[HAs2WI6O59]11− to obtain α-[As2Mo2W16O62]6−; in both cases minor parts of α-[As2MoxW18−xO62]6− (x = 1 and 3) were obtained simultaneously. All species have been characterized by polarography and by IR spectrometry. Proposed structures, based on the similarity of the redox properties and syntheses processes to those of the corresponding Dawson-type lacunary and mixed tungstophosphates, were given for all these species. The structures of the "saturated" tungstovanado- and molybdotungstoarsenates were confirmed by 183W solution NMR. In addition the 183W solution NMR spectra of all these latter species are discussed in relation to those of the analogous phosphoric derivatives. Finally, the main differences with the corresponding tungstophosphoric anions lie in (i) the existence of [HAs2W16O59]11−, whose protonation satisfies Lipscomb's rule, (ii) the lack of a stable oligomer like [P8W48O184]40−, and (iii) the lower stability of the octadeca(molybdotungsto)diarsenates. Key words: heteropolyanions, tungstoarsenate, molybdotungstoarsenate, tungstovanadoarsenate, l83W NMR.