A reinvestigation of isomerism in the Dawson structure: syntheses and 183W NMR structural characterization of three new polyoxotungstates [X2W18O62]6− (X=PV, AsV)
1993; Elsevier BV; Volume: 212; Issue: 1-2 Linguagem: Inglês
10.1016/s0020-1693(00)92306-5
ISSN1873-3255
AutoresRoland Contant, René Thouvenot,
Tópico(s)Metal-Organic Frameworks: Synthesis and Applications
ResumoThree 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(WW) greater than 21 Hz) and β-XW9 (Δδ about 80 ppm for X=P and 55 ppm for X=As; 2J(WW) less than 21 Hz).
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