Conventional wisdom maintains that β isomers of fully oxidized Keggin heteropolytungstates, [X n +WVI 12O40](8- n )- (X = main-group or transition-metal cation), are unstable with respect to α structures such that isomeric rearrangements all occur in the direction β → α. Contrary to this view, equilibria between α and β forms of the Keggin anion [AlIIIW12O40]5- (α- and β-1) have now been observed. Moreover, a trend in kinetic and thermodynamic stabilities of β isomers in the order X = Al(III) > Si(IV) > P(V) has been established, and the difference in energy between α and β isomers (α- and β-1) has been quantified for the first time. Mild acid condensation of WO4 2-, followed by addition of Al(III), gave [Al(AlOH2)W11O39]6- (2)three β-isomer derivatives, β1 (C s symmetry), β2 (C 1), and β3 (C s ), with the α derivative (C s ) a minor productin nearly quantitative yield by 27Al NMR spectroscopy. Acidification of the reaction mixture to pH 0 and refluxing cleanly converted 2 to H5[AlIIIW12O40] (1)mostly β-1 (yellow, C 3 v ), with α-1 (white, T d ) a minor product. Samples of each isomer were isolated by fractional crystallization and characterized by 27Al and 183W NMR, IR, and UV−vis spectroscopy, cyclic voltammetry, and single-crystal X-ray diffraction. The Al−O bond length in the T d AlO4 group at the center of α-1 (hydrated potassium salt of α-1; final R 1 = 3.42%) establishes a trend in X−O bond lengths in the [X n +O4](8- n )- groups of α-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 were observed by heating separate 0.1 M aqueous solutions of either pure α or β anions under identical conditions. The progress of the reaction was measured, and the relative concentrations of the α and β isomers present at equilibrium were determined by 27Al NMR spectroscopy. First-order rate constants for approach to equilibrium of α- and β-1 at 473 K were k 1( α → β ) = 7.68 × 10-7 s-1 and k - 1( β → α ) = 6.97 × 10-6 s-1. The equilibrium ratio of β-1 to α-1 (k 1/k - 1) was K eq(473   K,  0.1   M   1 ) = 0.11 ± 0.01. From ΔG = −RT ln K eq, α-1 is more stable than β-1 by 2.1 ± 0.5 kcal mol-1. Controlled hydrolysis of α-1 gave the monolacunary derivative α-Na9[AlW11O39] (α-3; C s ); hydrolysis of β-1 gave β2-3 (C 1) as the major product. Thermal equilibration of the lacunary Keggin heteropolytungstates could also be achieved:  Independently heated solutions of either α-3 or β2-3 (0.13 M of either isomer in D2O at 333 K; natural pH values of ca. 7) both gave solutions containing α-3 (60%) and a single β-3 isomer of C s symmetry (40%). Using K eq = 1.5, the two isomers differ in energy by 0.3 kcal mol-1.

Although many enzymes can readily and selectively use oxygen in water-the most familiar and attractive of all oxidants and solvents, respectively-the design of synthetic catalysts for selective water-based oxidation processes utilizing molecular oxygen remains a daunting task. Particularly problematic is the fact that oxidation of substrates by O2 involves radical chemistry, which is intrinsically non-selective and difficult to control. In addition, metallo-organic catalysts are inherently susceptible to degradation by oxygen-based radicals, while their transition-metal-ion active sites often react with water to give insoluble, and thus inactive, oxides or hydroxides. Furthermore, pH control is often required to avoid acid or base degradation of organic substrates or products. Unlike metallo-organic catalysts, polyoxometalate anions are oxidatively stable and are reversible oxidants for use with O2 (refs 8,9,10). Here we show how thermodynamically controlled self-assembly of an equilibrated ensemble of polyoxometalates, with the heteropolytungstate anion [AIVVW11O40]6- as its main component, imparts both stability in water and internal pH-management. Designed to operate at near-neutral pH, this system facilitates a two-step O2-based process for the selective delignification of wood (lignocellulose) fibres. By directly monitoring the central Al atom, we show that equilibration reactions typical of polyoxometalate anions keep the pH of the system near 7 during both process steps.

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