Equilibria between {alpha} and {beta} isomers of Keggin heteropolytungstates
- Emory Univ., Atlanta, GA (United States). Dept. of Chemistry
Efforts to rationalize the structure and dynamics of Keggin anions, [X{sup n+}W{sup VI}{sub 12}O{sub 40}]{sup (8{minus}n){minus}} (X = main-group or transition-metal cation), have occupied chemists for over a century. Nonetheless, a lack of agreement between theory and experiment regarding relative energies of the {alpha} and {beta} isomers of these cluster anions has remained unresolved. Conventional wisdom maintains the {beta} isomers of fully oxidized Keggin heteropolytungstates, [X{sup n+}W{sup VI}{sub 12}O{sub 40}]{sup (8{minus}n){minus}} (X = main-group or transition-metal cation), are unstable with respect to {alpha} structures such that isomeric rearrangements all occur in the direction {beta} {r_arrow} {alpha}. Contrary to this view, equilibria between {alpha} and {beta} forms of the Keggin anion [Al{sup III}W{sub 12}O{sub 40}]{sup 5{minus}} ({alpha}- and {beta}-1) have now been observed. Moreover, a trend in kinetic and thermodynamic stabilities of {beta} isomers in the order X = Al(III) > Si(IV) > P(V) has been established, and the difference in energy between {alpha} and {beta} isomers ({alpha} and {beta}-1) has been quantified for the first time. Mild acid condensation of WO{sub 4}{sup 2{minus}}, followed by addition of Al(III), gave [Al(AlOH{sub 2})W{sub 11}O{sub 39}]{sup 6{minus}} (2) -- three {beta}-isomer derivatives, {beta}{sub 1} (C{sub s} symmetry), {beta}{sub 2} (C{sub 1}), and {beta}{sub 3} (C{sub s}), with the {alpha} derivative (C{sub s}) a minor product -- in nearly quantitative yield by {sup 27}Al NMR spectroscopy. Acidification of the reaction mixture to pH 0 and refluxing cleanly converted 2 to H{sub 5}[Al{sup III}W{sub 12}O{sub 40}] (1) -- mostly {beta}-1 (yellow, C{sub 3v}), with {alpha}-1 (white, T{sub d}) a minor product. Samples of each isomer were isolated by fractional crystallization and characterized by {sup 27}Al and {sup 183}W NMR, IR, and UV-vis spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction. The Al-O bond length in the T{sub d} AlO{sub 4} group at the center of {alpha}-1 (hydrated potassium salt of {alpha}-1; final R{sub 1} = 3.42%) establishes a trend in X-O bond lengths in the [X{sup n+}O{sub 4}]{sup (8{minus}n){minus}} groups of {alpha}-Keggin anions of 1.74(1), 1.64(2), and 1.53(1) {angstrom}, respectively, for X = Al(III), Si(IV), and P(V). Equilibria between isomers of 1 were observed by heating separate 0.1 M aqueous solutions of either pure {alpha} or {beta} anions under identical conditions. The progress of the reaction was measured, and the relative concentrations of the {alpha} and {beta} isomers present at equilibrium were determined by {sup 27}Al NMR spectroscopy. First-order rate constants for approach to equilibrium of {alpha}- and {beta}-1 at 473 K were k{sub 1({alpha}{r_arrow}{beta})} = 7.68 {times} 10{sup {minus}7} s{sup {minus}1} and k{sub {minus}1({beta}{r_arrow}{alpha})} = 6.97 {times} 10{sup {minus}6} s{sup {minus}1}. The equilibrium ratio of {beta}-1 to {alpha}-1 (k{sub 1}/k{sub {minus}1)} was K{sub eq(473 K, 0.1 M 1)} = 0.11 {+-} 0.01. From {Delta}G = {minus}RT ln K{sub eq}, {alpha}-1 is more stable than {beta}-1 by 2.1 {+-} 0.5 kcal/mol. Controlled hydrolysis of {alpha}-1 gave the monolacunary derivative {alpha}-Na{sub 9}[AlW{sub 11}O{sub 39}] ({alpha}-3; C{sub s}); hydrolysis of {beta}-1 gave {beta}{sub 2}-3 (C{sub 1}) as the major product. Thermal equilibration of the lacunary Keggin heteropolytungstates could also be achieved: Independently heated solutions of either {alpha}-3 or {beta}{sub 2}-3 (0.13 M of either isomer in D{sub 2}O at 333 K; natural pH values of ca. 7) both gave solutions containing {alpha}-3 (60%) and a single {beta}-3 isomer of C{sub s} symmetry (40%). Using K{sub eq} = 1.5, the two isomers differ in energy by 0.3 kcal/mol.
- Sponsoring Organization:
- USDOE, Washington, DC (United States); National Science Foundation, Washington, DC (United States)
- DOE Contract Number:
- FC36-95GO10090
- OSTI ID:
- 682057
- Journal Information:
- Journal of the American Chemical Society, Vol. 121, Issue 19; Other Information: PBD: 19 May 1999
- Country of Publication:
- United States
- Language:
- English
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