Formation, isomerization, and derivatization of Keggin tungstoaluminates

Trends in the stability of alpha- and beta -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 AI(III) and W(VI) (2:11 molar ratio) in water gives a pH 7 solution containing beta (1) and beta (2) isomers of [Al(AlOH2)W11O39](6-) (beta (1)- and beta (2)-1). Partial equilibration of this kinetic product Mixture by gentle heating (2 h at 100 degreesC) or, alternatively, co-condensation of Al(III) and W(VI) for 2.5 h at 100 degreesC both give mixtures of beta (2)-, beta (3)-, and alpha -1. Full equilibration, by prolonged heating (25 days at 100 degreesC). gives an isomerically pure solution of alpha -1. thus demonstrating that isomerization occurs in the direction beta (1) --> beta (2) --> beta (3) --> alpha. Furthermore, kinetically controlled conversions of 1 to H-5[AlW12O40] (2)-achieved by heating pH 0-0.2 solutions of 1 for 5 days at 100 degreesC-occur with retention of isomeric integrity, such that alpha -1 is converted to alpha -2 (92%: 8% beta), while mixtures of beta (2)- and beta (3)-1 are converted to beta -2 (87%: 13% alpha), These data. when combined with previously reported observations (equilibria between alpha- and beta -2, kinetically controlled hydrolyses of alpha -2 to alpha-[AlW11O39](9-) (alpha -3) and of beta -2 to beta (2)-3, and equilibria between beta (3)- and alpha -3), provide a comprehensive Picture regarding the roles of kinetic and thermodynamic control. Finally. a general method for preparation of the isomerically pure derivatives alpha -K9-n[AlMn+W11O39] (4), Mn+ = Al(III). [(VO)-O-IV](2+), [(VO)-O-V](3+). Mn(II). Mn(III). Mn(IV), Cot(II), and Co(III), is provided. The presence of Mn(IV) is confirmed by cyclic voltammetry. pK(a) values of the aquo ligands oil 4 are determined by pH titration, and the isomeric structure of these derivatives is established by Al-27, V-51. and W-183 NMR and IR spectroscopies and X-ray crystallography.