Applications of vanadium-51 and phosphorus-31 nuclear magnetic resonance spectroscopy to the study of iso- and hetero-polyvanadates

TLDR: In this article, a number of iso-and hetero-polyvanadate anions are reported, including vanadium-51 n.p.m. and tetrahedral vanadium.

Abstract: Vanadium-51 n.m.r. spectra of a number of iso- and hetero-polyvanadate anions are reported. Spectra of solutions containing [V10O28]6–(pH 6.5–3.5) are consistent with the known solid-state structure of this anion. The [VO2]+ cation in aqueous solution almost certainly has an octahedral cis-dioxo-structure as its n.m.r. linewidth is comparable with those for the cis compounds [VO2(04C2)2]3– and [VO2(edta)]3–(edta = ethylenediaminetetra-acetate), which are broader than those for pseudo-tetrahedral [VCl2O2]– and [VF2O2]–. Chemical shifts of pseudo-octahedral vanadium atoms in [VxW6–xO19]n–. [PVxW12–xO40]n–. [PMo12–xVxO40]n–, and [VVxW12–xO40]n– anions range from 506 to 545 p.p.m. (VCl3O reference) and linewidthsfrom 60 to 200 Hz (2.6–9 p.p.m.). Multiple lines in the specta of the 1 : 12 (Keggin) heteropolyanions confirm the existence of geometrical isomers distinguished by the relative positions of two or more vanadium atoms in the polyanion structure. Phosphorus-31 n.m.r. spectra confirm that most if not all of the possible isomers of this type areformed. The chemical shifts of 31P in [PMo12040]3– and [PW12040]3– are ca. 11 and 21 p.p.m. upfield from the unprotonated [PO4]3– ion (–6 with respect to 85% H,PO,). Introduction of V atoms in the heteropoly structures causes a progressive decrease in the chemical shift, enabling mixtures of such Keggin anions to be analyzed by n.m.r. A parallel variation in 51V chemical shift is found for [VO4]3–(536) and the tetrahedral vanadium in [VV3W9040]6–(553) and [VV2W10O40]5–(556 p.p.m.).