DAWSON TYPE HETEROPOLYANIONS .2. SYNTHESIS OF LACUNARY POLYOXOTUNGSTOARSENATES FROM OCTADECATUNGSTODIARSENATE - NMR OCTADECA (MOLYBDOTUNGSTOVANADO) DIARSENATES

The di-, tri-, and hexavacant lacunary tungstoarsenates alpha-[HAs2W16O59]11-, alpha-[As2W15O56]12-, and alpha-[H2As2W12O48]12- were prepared by controlled alkaline degradation of the Dawson-type anion alpha-[As2W18O62]6-. The relationships between all these anions and the two alpha-[As2W17O61]10- isomers were established. The mixed saturated derivatives alpha-[As2VxW18-xO62](6+x)-(x = 1,2,3) and alpha-[As2MoxW18-xO62]6- (x = 1, 3) were obtained by refilling the lacunas with vanadium and molybdenum respectively. The new mixed anion alpha-2-[As2Mo2W15O61]10- was used concurrently with alpha-[HAs2W16O59]11- to obtain alpha-[As2Mo2W16O62]6-; in both cases minor parts of alpha-[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 W-183 solution NMR. In addition the W-183 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.