Equilibria between α and β isomers of Keggin heteropolytungstates

Conventional wisdom maintains that β isomers of fully oxidized Keggin heteropolytungstates, [X(n+)W(VI)₁₂O₄₀/(t)]((8-n)-) (X = main-group or transition-metal cation), are unstable with respect to α structures such that isomeric rearrangements all occur in the direction β → α. Contrary to this view, equilibria between α and β forms of the Keggin anion [Al(III)W₁₂O₄₀]^5- (α- and β-1) have now been observed. Moreover, a trend in kinetic and thermodynamic stabilities of β isomers in the order X = Al(III) > Si(IV) > P(V) has been established, and the difference, in energy between α and β isomers (α- and β-1) has been quantified for the first time. Mild acid condensation of WO₄^2-, followed by addition of Al(III), gave [Al(AlOH₂)W₁₁O₃₉]^6- (2)-three β-isomer derivatives, β₁ (C(s) symmetry); β₂ (C₁), and β₃ (C(s)), with the α derivative (C(s)) a minor product-in nearly quantitative yield by ²⁷Al NMR spectroscopy. Acidification of the reaction mixture to pH 0 and refluxing cleanly converted 2 to H₅[Al(III)W₁₂O₄₀] (1)-mostly β-1 (yellow, C(3v)), with α-1 (white, T(d)) a minor product. Samples of each isomer were isolated by fractional crystallization and characterized by ²⁷Al and ¹⁸³W NMR, IR, and UV-vis spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction. The Al-O bond length in the T(d) AlO₄ group at the center of α-1 (hydrated potassium salt of α-1; final R₁ = 3.42%) establishes a trend in X-O bond lengths in the [X(n+)O₄]((8-n)-) groups of α-Keggin anions of 1.74(1), 1.64(2), and 1.53(1) Å, respectively, for X = Al(III), Si(IV), and P(V). Equilibria between isomers of 1 were observed by heating separate 0.1 M aqueous solutions of either pure α or β anions under identical conditions. The progress of the reaction was measured, and the relative concentrations of the α and β isomers present at equilibrium were determined by ²⁷Al NMR spectroscopy. First-order rate constants for approach to equilibrium of α- and β-1 at 473 K were k(1(α-β)) = 7.68 x 10^-7 s^-1 and k(-1(β-α)) = 6.97 x 10^-6 s^-1. The equilibrium ratio of β-1 to α-1 (k₁/k_-1) was K(eq(473 K. 0.1 M 1)) = 0.11 ± 0.01. From ΔG = -RT in K(eq), α-1 is more stable than β-1 by 2.1 ± 0.5 kcal mol^-1. Controlled hydrolysis of α-1 gave the monolacunary derivative α-Na₉[AlW₁₁O₃₉] (α-3; C(s)); hydrolysis of β-1 gave β₂-3 (C₁) as the major product. Thermal equilibration of the lacunary Keggin heteropolytungstates could also be achieved: Independently heated solutions of either α-3 or β₂-3 (0.13 M of either isomer in D₂O at 333 K; natural pH values of ca. 7) both gave solutions containing α-3 (60%) and a single β-3 isomer of C(s) symmetry (40%). Using K(eq) = 1.5, the two isomers differ in energy by 0.3 kcal mol^{- 1}.