THE MANGANESE-CONTAINING POLYOXOMETALATE, [WZNMN2II(ZNW9O34)(2)](12-), AS A REMARKABLY EFFECTIVE CATALYST FOR HYDROGEN-PEROXIDE MEDIATED OXIDATIONS

The disubstituted manganese polyoxometalate, [WZnMn2(ZnW9O34)(2)](12-) , has been used as a catalyst for the epoxidation of alkenes and the o xidation of secondary alcohols to ketones in biphasic (water-organic) reaction media using hydrogen peroxide (30% aqueous H2O2) as the oxyge n donor. At subambient temperatures, epoxidations are highly selective with little dismutation of hydrogen peroxide by homolysis or formatio n of side products by allylic oxidation. Thus, cyclohexene was oxidize d to cyclohexene oxide at 2 degrees C with 99% selectivity. At low cat alyst 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 measurem ents on a model epoxidation of cyclooctene showed that the reaction is first order in cyclooctene, hydrogen peroxide, and [WZnMn2II(ZnW9O34) (2)](12-) catalyst. Magnetic susceptibility measurements along with ES R 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 pol yoxometalate anion into an organic phase. Comparison of the [WZnMn2II( ZnW9O34)(2)](12-) anion with other mono-, tri-, or tetrasubstituted ma nganese polyoxometalates or catalysts having no manganese atoms showed that the catalytic activity of the former was unique. Kinetic evidenc e 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 peroxid e to form a peroxo intermediate with a typical absorbance at 818 cm(-1 ). After completion of the reaction, the original IR spectrum was meas ured again. ESR and atomic aborption spectroscopy also revealed that t he [WZnMn2II(ZnW9O34)(2)](12-) anion is solvolytically stable to aqueo us hydrogen peroxide. Cyclic voltammetry, IR and W-vis, and a comparat ive reaction with iodosobenzene as the oxygen donor seem to exclude a high valent manganese-oxo compound as the reactive intermediate. Rathe r, high reactivity is probably due to a tungsten-peroxo intermediate s omehow uniquely activated by an adjacent manganese atom.