Equilibria between alpha and beta isomers of Keggin heteropolytungstates

Conventional wisdom maintains that beta isomers of fully oxidized Keggin he teropolytungstates, [(Xn+W12O40)-O-VI]((8-n))(-) (X main-group or transitio n-metal cation), are unstable with respect to alpha structures such that is omeric rearrangements all occur in the direction beta --> alpha. Contrary t o 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 ord er 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 quantifie d 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 deriva tive (C-s) a minor product-in nearly quantitative yield by Al-27 NMR spectr oscopy. 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), Wit h alpha-1 (white, T-d) a minor product. Samples of each isomer were isolate d by fractional crystallization and characterized by Al-27 and W-183 NMR, I R, and UV-vis spectroscopy, cyclic voltammetry, and single-crystal X-ray di ffraction. The Al-O bond length in the T-d AlO4 group at the center of alph a-1 (hydrated potassium salt of alpha-1; final R-1 = 3.42%) establishes a t rend in X-O bond lengths in the [Xn+O4]((8-n))- groups of alpha-Keggin anio ns 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 heatin g separate 0.1 M aqueous solutions of either pure alpha or beta anions unde r identical conditions. The progress of the reaction was measured, and the relative concentrations of the alpha and beta isomers present at equilibriu m 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 eq uilibrium 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 mo nolacunary derivative alpha-Na-9[AlW11O39] (alpha-3; C-s); hydrolysis of be ta-1 gave beta(2)-3 (C-1) as the major product. Thermal equilibration of th e lacunary Keggin heteropolytungstates could also be achieved: Independentl y 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).