The disubstituted manganese polyoxometalate, [WZnMn2(ZnW9O34)2]12-, has been used as a catalyst for the epoxidation of alkenes and the oxidation of secondary alcohols to ketones in biphasic (water—organic) reaction media using hydrogen peroxide (30% aqueous H2O2) as the oxygen donor. At subambient temperatures, epoxidations are highly selective with little dismutation of hydrogen peroxide by homolysis or formation of side products by allylic oxidation. Thus, cyclohexene was oxidized to cyclohexene oxide at 2 °C with 99% selectivity. At low catalyst loadings and depending on the substrate and reaction temperature, hundreds to thousands of catalytic turnovers were obtained with only a 2-fold excess of hydrogen peroxide over substrate. Kinetic measurements on a model epoxidation of cyclooctene showed that the reaction is first order in cyclooctene, hydrogen peroxide, and [WZnMnII2(ZnW9O34)2]12- catalyst. Magnetic susceptibility measurements along with ESR and atomic absorption spectroscopy reveal that the manganese atom is in a terminal position coordinated by five bridging oxygen atoms and one labile aquo ligand which is disassociated upon transfer of the polyoxometalate anion into an organic phase. Comparison of the [WZnMnII2(ZnW9O34)2]12- anion with other mono-, tri-, or tetrasubstituted manganese polyoxometalates or catalysts having no manganese atoms showed that the catalytic activity of the former was unique. Kinetic evidence exhibited the absence of an induction period for catalyst activation or catalyst deactivation over a period of 12500 turnovers. IR spectra demonstrated that the original catalyst reacted with hydrogen peroxide to form a peroxo intermediate with a typical absorbance at 818 cm-1. After completion of the reaction, the original IR spectrum was measured again. ESR and atomic aborption spectroscopy also revealed that the [WZnMnII2(ZnW9O34)2]12- anion is solvolytically stable to aqueous hydrogen peroxide. Cyclic voltammetry, IR and UV-vis, and a comparative reaction with iodosobenzene as the oxygen donor seem to exclude a high valent manganese—oxo compound as the reactive intermediate. Rather, high reactivity is probably due to a tungsten—peroxo intermediate somehow uniquely activated by an adjacent manganese atom.
ASJC Scopus subject areas
- Colloid and Surface Chemistry