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

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 [V_xW_6–xO₁₉]^n–. [PV_xW_12–xO₄₀]^n–. [PMo_12–xV_xO₄₀]^n–, and [VV_xW_12–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.).