Formation, isomerization, and derivatization of Keggin tungstoaluminates

Trends in the stability of α and β-Keggin heteropolytungstates of the second-row main-group heteroatoms Al(III), Si(IV), and P(V) are elaborated by data that establish the roles of kinetic and thermodynamic control in the formation and isomerization of Keggin tungstoaluminates. Slow, room- temperature co-condensation of Al(III) and W(VI) (2:11 molar ratio) in water gives a pH 7 solution containing β₁ and β₂ isomers of [Al(AlOH₂)W₁₁O₃₉]^6- (β₁- and β₂-1). Partial equilibration of this kinetic product mixture by gentle heating (2 h at 100 °C) or, alternatively, co-condensation of Al(III) and W(VI) for 2.5 h at 100 °C both give mixtures of β₂-, and β₃-, and α-1. Full equilibration, by prolonged heating (25 days at 100 °C), gives an isomerically pure solution of α-1, thus demonstrating that isomerization occurs in the direction β₁ → β₂ → β₃ → α. Furthermore, kinetically controlled conversions of 1 to H₅[AlW₁₂O₄₀] (2)-achieved by heating pH 0-0.2 solutions of 1 for 5 days at 100 °C-occur with retention of isomeric integrity, such that α-1 is converted to α-2 (92%; 8% β), while mixtures of β₂- and β₃-1 are converted to β-2 (87%; 13% α). These data, when combined with previously reported observations (equilibria between αand β-2, kinetically controlled hydrolyses of α-2 to α-[AlW₁₁O₃₉]^9- (α-3) and of β-2 to β₂-3, and equilibria between β₃- and α-3), provide a comprehensive picture regarding the roles of kinetic and thermodynamic control. Finally, a general method for preparation of the isomerically pure derivatives α-K₉-_n [AlMⁿ⁺W₁₁O₃₉] (4), Mⁿ⁺ = Al(III), [V^IVO]²⁺, [V^VO]³⁺, Mn(II), Mn(III), Mn(IV), Co(II), and Co(III), is provided. The presence of Mn(IV) is confirmed by cyclic voltammetry, pK_a values of the aquo ligands on 4 are determined by pH titration, and the isomeric structure of these derivatives is established by ²⁷Al, ⁵¹V, and ¹⁸³W NMR and IR spectroscopies and X-ray crystallography.