Roland Contant

A reinvestigation of isomerism in the Dawson structure : syntheses and 183W NMR structural characterization of three new polyoxotungstates [X2W18O62]6- (X=PV, AsV)

Abstract Three new Dawson-type heteropolyoxotungstates β- and γ-[AS2W18O62]6− and γ-[P2W18O62]6− have been prepared and identified. A comparative study of these three compounds and of the four other already known [X2W18O62]6− anions (α and β for X=Pv, and α and γ* for X=Asv) has been performed by spectroscopic (IR, Raman, UV-Vis, 183W and, eventually, 31P NMR), polarographic and kinetic measurements, and some erroneous literature assignments have been rectified. The stabilities of the different isomers are discussed in relation with their conditions of formation: the isomerization proceeds along the path γ → β → α; the relatively high stability of the γ* anion is also discussed on the basis of the geometrical features of the α- and β-XW9 half-anion. A rational nomenclature is proposed for the staggered belt junction with respect to the eclipsed one by adding a star, as in γ*-[As2W18O62]6−. 183W solution NMR easily distinguishes α-XW9 (Δδ about 40 ppm for X=P and 20 ppm for X=As; 2J(Wue5f8W) greater than 21 Hz) and β-XW9 (Δδ about 80 ppm for X=P and 55 ppm for X=As; 2J(Wue5f8W) less than 21 Hz).

Efficient conversion of NO into N2O by selected electroreduced heteropolyanions

It is shown that a series of one- and two-electron-reduced heteropolyanions of the Keggin- and Dawson-types convert quantitatively NO into N2O in acidic aqueous media. The corresponding oxidized forms of the heteropolyanions are fully regenerated. Unsubstituted and lacunary heteropolyanions, as well as the substituted heteropolyanions of these two groups, prove active in this context. Transformaion quantitative de NO en N2O par des he′te′ropolyanions e′lectrore′duits. De nombreux he′te′ropolyanions ayant la structure de Keggin ou celle de Dawson pre′sentent un comportement catalytique vis a vis de la re′duction de NO, au potentiel de leur premiere vague de re′duction. Les compose′s sature′s ou lacunaires, et les substitue′s appartenant a ces deux groupes de structures, se re′velent efficaces pour cette re′duction. Sur quelques exemples dhe′te′ropolyanions, dont le premier couple re′dox est soit monoe′lectronique, soit bie′lectronique, il a e′te′ montre′ que la transformation de NO en N2O est quantitative.

Solid-state31P magic-angle spinning NMR relaxation study of silica-supported polyoxoanions of the Keggin and Dawson structures

Abstract Solid-state31P magic-angle spinning NMR investigations performed on silica-supported phosphopolyoxoanions [PMo11Vo40]4−, [PW12O40]3− and [P2W18O62]6− reveal a similar trend as already observed with [PMo12O40]3− — the31P spin-lattice relaxation rate increases with decreasing concentration of the polyanions at the silica surface. However the relaxation time T1 decreases with increasing the age of the samples, especially for the tungstic compounds. More generally T1 depends on the history of the sample after having been supported on silica; a vacuum treatment increases T1 whereas exposure to water vapour has an opposite effect, showing the influence of water molecules assisting the relaxation. In addition the chemical shift anisotropy for the less symmetrical anions ([PMo11Vo40]4− and [P2W18O62]6−) is partially averaged after water exposure, which could be explained by a “pseudoliquid phase”, already invoked for the interpretation of the “spillover” effect at the silica surface.