Equilibria between and Isomers of Keggin Hetero
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- 作者：Ira A. Weinstock ; Jennifer J. Cowan ; Elena M. G. Barbuzzi ; Huadong Zeng ; and Craig L. Hill
- 刊名：Journal of the American Chemical Society
- 出版年：1999
- 出版时间：May 19, 1999
- 年：1999
- 卷：121
- 期：19
- 页码：4608 - 4617
- 全文大小：181K
- 年卷期：v.121,no.19(May 19, 1999)
- ISSN：1520-5126
文摘
Conventional wisdom maintains that 
isomers of fully oxidized Keggin heteropolytungstates,[Xn+WVI12O40](8-n)- (X = main-group or transition-metal cation), are unstable with respect to 
structuressuch that isomeric rearrangements all occur in the direction 
. Contrary to this view, equilibria between and forms of the Keggin anion [AlIIIW12O40]5- (- and -1) have now been observed. Moreover, a trendin kinetic and thermodynamic stabilities of isomers in the order X = Al(III) > Si(IV) > P(V) has beenestablished, and the difference in energy between and isomers (- and -1) has been quantified for thefirst time. Mild acid condensation of WO42-, followed by addition of Al(III), gave [Al(AlOH2)W11O39]6-(2)-three -isomer derivatives, 1 (Cs symmetry), 2 (C1), and 3 (Cs), with the derivative (Cs) a minorproduct-in nearly quantitative yield by 27Al NMR spectroscopy. Acidification of the reaction mixture to pH0 and refluxing cleanly converted 2 to H5[AlIIIW12O40] (1)-mostly -1 (yellow, C3v), with -1 (white, Td) aminor product. Samples of each isomer were isolated by fractional crystallization and characterized by 27Aland 183W NMR, IR, and UV-vis spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction. TheAl-O bond length in the Td AlO4 group at the center of -1 (hydrated potassium salt of -1; final R1 =3.42%) establishes a trend in X-O bond lengths in the [Xn+O4](8-n)- groups of -Keggin anions of 1.74(1),1.64(2), and 1.53(1) Å, respectively, for X = Al(III), Si(IV), and P(V). Equilibria between isomers of 1 wereobserved by heating separate 0.1 M aqueous solutions of either pure or anions under identical conditions.The progress of the reaction was measured, and the relative concentrations of the and isomers present atequilibrium were determined by 27Al NMR spectroscopy. First-order rate constants for approach to equilibriumof - and -1 at 473 K were k1() = 7.68 × 10-7 s-1 and k-1() = 6.97 × 10-6 s-1. The equilibrium ratioof -1 to -1 (k1/k-1) was Keq(473 K, 0.1 M 1) = 0.11 ± 0.01. From 
G = -RT ln Keq, -1 is more stable than-1 by 2.1 ± 0.5 kcal mol-1. Controlled hydrolysis of -1 gave the monolacunary derivative -Na9[AlW11O39](-3; Cs); hydrolysis of -1 gave 2-3 (C1) as the major product. Thermal equilibration of the lacunary Kegginheteropolytungstates could also be achieved: Independently heated solutions of either -3 or 2-3 (0.13 M ofeither isomer in D2O at 333 K; natural pH values of ca. 7) both gave solutions containing -3 (60%) and asingle -3 isomer of Cs symmetry (40%). Using Keq = 1.5, the two isomers differ in energy by 0.3 kcal mol-1.
isomers of fully oxidized Keggin heteropolytungstates,[Xn+WVI12O40](8-n)- (X = main-group or transition-metal cation), are unstable with respect to structuressuch that isomeric rearrangements all occur in the direction . Contrary to this view, equilibria between and forms of the Keggin anion [AlIIIW12O40]5- (- and -1) have now been observed. Moreover, a trendin kinetic and thermodynamic stabilities of isomers in the order X = Al(III) > Si(IV) > P(V) has beenestablished, and the difference in energy between and isomers (- and -1) has been quantified for thefirst time. Mild acid condensation of WO42-, followed by addition of Al(III), gave [Al(AlOH2)W11O39]6-(2)-three -isomer derivatives, 1 (Cs symmetry), 2 (C1), and 3 (Cs), with the derivative (Cs) a minorproduct-in nearly quantitative yield by 27Al NMR spectroscopy. Acidification of the reaction mixture to pH0 and refluxing cleanly converted 2 to H5[AlIIIW12O40] (1)-mostly -1 (yellow, C3v), with -1 (white, Td) aminor product. Samples of each isomer were isolated by fractional crystallization and characterized by 27Aland 183W NMR, IR, and UV-vis spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction. TheAl-O bond length in the Td AlO4 group at the center of -1 (hydrated potassium salt of -1; final R1 =3.42%) establishes a trend in X-O bond lengths in the [Xn+O4](8-n)- groups of -Keggin anions of 1.74(1),1.64(2), and 1.53(1) Å, respectively, for X = Al(III), Si(IV), and P(V). Equilibria between isomers of 1 wereobserved by heating separate 0.1 M aqueous solutions of either pure or anions under identical conditions.The progress of the reaction was measured, and the relative concentrations of the and isomers present atequilibrium were determined by 27Al NMR spectroscopy. First-order rate constants for approach to equilibriumof - and -1 at 473 K were k1() = 7.68 × 10-7 s-1 and k-1() = 6.97 × 10-6 s-1. The equilibrium ratioof -1 to -1 (k1/k-1) was Keq(473 K, 0.1 M 1) = 0.11 ± 0.01. From G = -RT ln Keq, -1 is more stable than-1 by 2.1 ± 0.5 kcal mol-1. Controlled hydrolysis of -1 gave the monolacunary derivative -Na9[AlW11O39](-3; Cs); hydrolysis of -1 gave 2-3 (C1) as the major product. Thermal equilibration of the lacunary Kegginheteropolytungstates could also be achieved: Independently heated solutions of either -3 or 2-3 (0.13 M ofeither isomer in D2O at 333 K; natural pH values of ca. 7) both gave solutions containing -3 (60%) and asingle -3 isomer of Cs symmetry (40%). Using Keq = 1.5, the two isomers differ in energy by 0.3 kcal mol-1.