Equilibria between α and β isomers of Keggin heteropolytungstates

Conventional wisdom maintains that beta isomers of fully oxidized Keggin heteropolytungstates, [(Xn+W12O40)-O-VI]((8-n))(-) (X main-group or transition-metal cation), are unstable with respect to alpha structures such that isomeric rearrangements all occur in the direction beta --> alpha. Contrary to this view, equilibria between alpha and beta forms of the Keggin anion [(AlW12O40)-W-III](5-) (alpha- and beta-1) have now been observed. Moreover, a trend in kinetic and thermodynamic stabilities of beta isomers in the order X = AI(III) > Si(IV) > P(V) has been established, and the difference in energy between alpha and beta isomers (alpha and beta-1) has been quantified for the first rime. Mild acid condensation of WO42-, followed by addition of Al(III), gave [Al(AlOH2)W11O39](6-) (2)-three beta-isomer derivatives, beta(1) (C-s symmetry), beta(2) (C-1), and beta(3) (C-s), With the a derivative (C-s) a minor product-in nearly quantitative yield by Al-27 NMR spectroscopy. Acidification of the reaction mixture to pH 0 and refluxing cleanly converted 2 to H-5[(AlW12O40)-W-III] (1)-mostly beta-1 (yellow, C-3v), With alpha-1 (white, T-d) a minor product. Samples of each isomer were isolated by fractional crystallization and characterized by Al-27 and W-183 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 alpha-1 (hydrated potassium salt of alpha-1; final R-1 = 3.42%) establishes a trend in X-O bond lengths in the [Xn+O4]((8-n))- groups of alpha-Keggin anions of 1.74(1), 1.64(2), and 1.53(1) Angstrom, 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 alpha or beta anions under identical conditions. The progress of the reaction was measured, and the relative concentrations of the alpha and beta isomers present at equilibrium were determined by Al-27 NMR spectroscopy, First-order rate constants for approach to equilibrium of alpha- and beta-1 at 473 K were k(1)(alpha-beta) = 7.68 x 10(-7) s(-1) and k(-1)(beta-->alpha) 6.97 x 10(-6) s-(1). The equilibrium ratio of alpha- to beta-1 (k(1)/k(-1)) was K-eq(473 K, 0.1 M1) = 0.11 +/- 0.01 From Delta G = -RT In K-eq, alpha-1 is more stable than beta-1 by 2.1 +/- 0.5 kcal mol(-1). Controlled hydrolysis of alpha-1 gave the monolacunary derivative alpha-Na-9[AlW11O39] (alpha-3; C-s); hydrolysis of beta-1 gave beta(2)-3 (C-1) as the major product. Thermal equilibration of the lacunary Keggin heteropolytungstates could also be achieved: Independently heated solutions of either alpha-3 or beta(2)-3 (0.13 M of either isomer in D2O at 333 K; natural pH values of ca. 7) both gave solutions containing alpha-3 (60%) and a single beta-3 isomer of C-s symmetry (40%). Using K-eq = 1.5, the two isomers differ in energy by 0.3 kcal mol(-1).