Equilibrating metal-oxide cluster ensembles for oxidation reactions using oxygen in water

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^1-4 remains a daunting task^5,6. 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 degradation⁵ by oxygen-based radicals, while their transition-metalion active sites often react with water to give insoluble, and thus inactive, oxides or hydroxides⁷. 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^8,9 for use with O₂ (refs 8-10). Here we show how thermodynamically controlled self-assembly of an equilibrated ensemble of polyoxometalates, with the heteropolytungstate anion^11,12 [AIV^VW₁₁O₄₀]^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 A1 atom, we show that equilibration reactions typical of polyoxometalate anions^13,14 keep the pH of the system near 7 during both process steps.