Synergetic Catalysis of Copper and Iron in Oxidation of Reduced Keggin Heteropolytungstates by Dioxygen

Polyoxometalates (POMs) and in particularly Keggin heteropolytungstates are much studied and commercially important catalysts for dioxygen-based oxidation processes. The rate-limiting step in many POM-catalyzed O2-based oxidations is reoxidation of the reduced POM by O2. We report here that this reoxidation process, as represented by the one-electron-reduced Keggin complexes POMred (α-PW12O40 4– and α-SiVW11O40 6–) reacting with O2, is efficiently catalyzed by a combination of copper and iron complexes. The reaction kinetics and mechanism have been comprehensively studied in sulfate and phosphate buffer at pH 1.8. The catalytic pathway includes a reversible reaction between Cu­(II) and Fe­(II), followed by a fast oxidation of POMred by Fe­(III) and Cu­(I) by O2 to regenerate Fe­(II) and Cu­(II). The proposed reaction mechanism quantitatively describes the experimental kinetic curves over a wide range of experimental conditions. Since the oxidized forms, α-PW12O40 3– and α-SiVW11O40 5–, are far better oxidants of organic substrates than the previously studied POMs, α-SiW12O40 4– and α-AlW12O40 5–, this synergistic Fe/Cu cocatalysis of reduced-POM reoxidation could well facilitate significant new O2/air-based processes.