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-, 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 α- and 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 [AlM n +W11O39] (4), M n + = Al(III), [VIVO]2+, [VVO]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.