Aluminum-based heteropolytungstates of the Keggin structure: Synthesis, isomerism, and application

A new method of preparation of the aluminum-centered tungstate of the Keggin structure, [AlW₁₂O₄₀]5−, was accomplished. Condensation of Al(III) and tungstate in boiling pH 7.7 solution leads to the formation of a mixture of α, β₂, and β₃-[Al(AlOH₂)W₁₁O₃₉] 6−. Further condensation of this isomeric mixture leads to the formation of α- and β-[AlW₁₂O₄₀]5− , in an overall crude yield of 95% for the two steps. Equilibrium between α- and β-[AlW₁₂O₄₀]5− was observed by 27Al NMR spectra of 0.1 M aqueous solutions of either pure α-H₅[AlW₁₂O₄₀] or β-H₅[AlW₁₂O₄₀] heated at 473K. From K_eq and ΔG = −RT ln K_eq, α-[AlW₁₂O₄₀]5− is more stable than β-[AlW₁₂O₄₀] 5− by 2.1 ± 0.5 kcal/mol at the given conditions. The monolacunary derivative of [AlW₁₂O₄₀]5− , [AlW₁₁O₃₉]9−, was prepared by selective base hydrolysis of either the α- or β-isomer of [AlW ₁₂O₄₀]5−. Equilibrium between α-[AlW ₁₁O₃₉]9− and β₃-[AlW ₁₁O₃₉]9− was observed; K_eq (also determined from 27Al NMR spectroscopy experiments) gives a difference in energy between these two isomers of 0.3 kcal/mol (333 K, 0.13 M). Isomeric equilibration studies of [Al(AlOH₂)W₁₁O ₃₉]6− showed that the α-isomer is sufficiently more stable than any of the β-isomers that solutions of mixtures of [Al(AlOH ₂)W₁₁O₃₉]6− isomers may be converted quantitatively to the α-isomer. The α-isomer of [AlW ₁₁O₃₉]9− may be prepared selectively under certain conditions, from either α- or β-[AlW₁₂O ₄₀]5−, as the potassium salt, α-K₉[AlW ₁₁O₃₉]. While itself possessing marginal solubility, slurries of α-K₉[AlW₁₁O₃₉] react with aqueous metal cations to form more soluble transition-metal substituted POMs (TMSPs). These reactions result in the formation of anions of the formula [AlM n+W₁₁O₃₉](9−n)−, where Mn+ = Al(III), Co(II), Co(III), Mn(II), Mn(III), Mn(IV), [VIVO]2+, and [VVO]3+ . These anions were characterized by 27Al, 183W, and 51V NMR (as appropriate), infrared spectroscopy, UV-Visible spectroscopy, cyclic voltammetry, pH titration (to determine the pK_a of water ligand on the transition metal, as appropriate), and elemental analysis. α-[AlVW₁₁O₄₀]6− was used as an oxidant for a phenolic lignin model compound, 1-(3,5-dimethoxy-4-hydroxyphenyl)-2-(phenoxy)propane-1,3-diol.