Formation, isomerization, and derivatization of keggin tungstoaluminates.

Abstract

Trends in the stability of ¥á and ©¬-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 ©¬₁ and ©¬₂ isomers of [Al(AlOH₂)W₁₁O₃₉]^6-(©¬_1- and ©¬₂=1). Partial equilibration of this kinetic product mixture by gentle heating (2 h at 100¡ÆC) or, alternatively, co-condensation of Al(III) and W(VI) for 2.5 h at 100¡ÆC both give mixtures of ©¬_2-, ©¬_3-, and ¥á-1. Full equilibration, by prolonged heating (25 days at 100¡ÆC), gives an isomerically pure solution of ¥á-1, thus demonstrating that isomerization occurs in the direction ©¬₁ -> ©¬₂ -> ©¬₃ -> ¥á. 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¡ÆC--occur with retention of isomeric integrity, such that ¥á-1 is converted to ¥á-2 (92%; 8% ©¬), while mixtures of ©¬_2- and ©¬_3-1 are converted to ©¬-2 (87%; 13% ¥á). These data, when combined with previously reported observations (equilibria between ¥á and ©¬-2, kinetically controlled hydrolyses of ¥á-2 to ¥á[AlW₁₁O₃₉]^9- (¥á-3) and of ©¬-2 to ©¬₂-3, and equilibria between ©¬_3- and ¥á-3), provide a comprehensive picture regarding the roles of kinetic and thermodynamic control. Finally, a general method for preparation of the isomerically pure derivatives ¥á-K_9-n[AlMⁿ⁺W₁₁O₃₉] (4), Mⁿ⁺ = Al(III), [V^IVO]²⁺, [V^VO]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 pectroscopies and X-ray crystallography.

Titles contained within Formation, isomerization, and derivatization of keggin tungstoaluminates.

• Introduction
• Impacts of forest and land management on biodiversity and carbon

Citation

Cowan, Jennifer J.; Bailey, Alan J.; Heintz, Robert A.; Do, Bao T.; Hardcastle, Kenneth I.; Hill, Craig L.; Weinstock, Ira A. 2001. Formation, isomerization, and derivatization of keggin tungstoaluminates. Inorganic chemistry. Vol. 40, no. 26 (2001). Pages 6666-6675.