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Title: Formation, isomerization, and derivatization of keggin tungstoaluminates.

Author: Cowan, Jennifer J.; Bailey, Alan J.; Heintz, Robert A.; Do, Bao T.; Hardcastle, Kenneth I.; Hill, Craig L.; Weinstock, Ira A.;

Date: 2001

Source: Inorganic chemistry. Vol. 40, no. 26 (2001). Pages 6666-6675.

Publication Series: Miscellaneous Publication

Description: 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-K9-n[AlMn+W11O39] (4), Mn+ = 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, pKa 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.

Keywords: keggin tungstoaluminates, formation, isomerization, derivation.

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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.

 


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