The Manganese-Containing Polyoxometalate, [WZnMnII2(ZnW9O34)2]12-, as a Remarkably Effective Catalyst for Hydrogen Peroxide Mediated Oxidations

Abstract

The disubstituted manganese polyoxometalate, [WZ1Mn2(ZnW9034)2]'~-, 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 H 2 0 2 ) 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 tumovers 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 [WZnMn112(ZnW9034)2]'2-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 [ W Z I I M~~~~( Z~W~O~~)~]~~-anion with other mono-, tri-, or tetrasubstituted manganesepolyoxometalates 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 tumovers. IR spectra demonstrated that the original catalyst reacted with hydrogen peroxide to form a peroxo intermediate with a typical absorbance at 8 18 cm-'. After completion of the reaction, the original IR spectrum was measured again. ESR and atomic aborption spectroscopy also revealed that the [WZnMnn~(ZnW9034)z1'*-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.

Remarkable enhancement of catalytic activity of a 2 : 1 complex between a non-planar Mo(v)–porphyrin and a ruthenium-substituted Keggin-type heteropolyoxometalate in catalytic oxidation of benzyl alcohols

“…However, the catalytic reactivity of such a metalloporphyrin-POM assembly has yet to be examined. 10 We report herein formation of a 2:1 complex between a nonplanar Mo(V)-porphyrin, ( …”

“…Polyoxometalates and heteropolyoxometalates (POMs) are widely known as functional materials in catalytic oxidation 25 reactions, [1][2][3][4][5][6][7][8][9][10][11][12][13][14] metallorganic-POM hybrid catalysts. 9 In particular, the facile incorporation of transition-metals into lacunary-type POMs has attracted significant interest for preparation of oxidation catalysts with high activity.…”

Microwave-Assisted Oxidation of Alcohols Using Aqueous Hydrogen Peroxide

“…4, 13 We have shown that in the microwave-assisted reactions there is no need for the preparation of such ammonium salts, and they could be successfully substituted with commercially available TBAHS. However, the test of other common PTC catalysts showed that, like under conventional conditions, the oxidation reactions occur most efficiently in the presence of hydrogen sulfate ions probably because of their sufficient acidity.…”

Simple Heteropoly Acids as Water-Tolerant Catalysts in the Oxidation of Alcohols with 34% Hydrogen Peroxide, A Mechanistic Approach

“…[8][9][10][11][12][13][14][15][16] However, much of these methods involve the use of expensive reagents, harsh reaction conditions, and leading to the generation of a large amount of toxic waste. 17,18 Because of these facts, there is still a need to develop new environmentally benign routes that meet industrial demands for this transformation.…”