Oxidation of Reduced Keggin Heteropolytungstates by Dioxygen in Water Catalyzed by Cu(II)

The reaction of reduced polyoxometalates (POMs) with dioxygen is centrally important in POM catalysis. We report that this process, as represented by the one-electron-reduced Keggin complexes POMred (alpha-AlW12O406-, and alpha-PW12O404-), is efficiently catalyzed by copper complexes. The Cu-catalyzed pathway is dominant in the presence of as low as similar to 0.1 mu M of Cu, a copper concentration that is typically lower than the copper ion contamination in aqueous solutions. The reaction kinetics and mechanism have been comprehensively studied in sodium sulfate buffer at pH 2.0. The catalytic pathway includes a reversible reduction of Cu(II) by POMred, followed by a fast reoxidation of Cu(I) by O-2 to regenerate Cu(II). The rate constants of the first catalytic steps were determined by three approaches and found to be (1.8 +/- 0.3) X 10(5) and 57 +/- 15 M-1 s(-1) for SiW12O405- and PW12O404-, respectively. These reactions are thermodynamically more favorable and therefore proceed significantly more quickly than those for the direct outer-sphere electron transfer to O-2. The proposed reaction mechanism quantitatively describes the experimental kinetic curves over a wide range of experimental conditions.