The mechanism of aerobic oxidation of aromatic and alkyl aromatic compounds using anthracene and xanthene, respectively, as a model compound was investigated using a phosphovanadomolybdate polyoxometalate, H(5)PV(2)Mo(10)O(40), as catalyst under mild, liquid-phase conditions. The polyoxometalate is a soluble analogue of insoluble mixed-metal oxides often used for high-temperature gas-phase heterogeneous oxidation which proceed by a Mars-van Krevelen mechanism. The general purpose of the present investigation was to prove that a Mars-van Krevelen mechanism is possible also in liquid-phase, homogeneous oxidation reactions. First, the oxygen transfer from H(5)PV(2)Mo(10)O(40) to the hydrocarbons was studied using various techniques to show that commonly observed liquid-phase oxidation mechanisms, autoxidation, and oxidative nucleophilic substitution were not occurring in this case. Techniques used included (a) use of (18)O-labeled molecular oxygen, polyoxometalate, and water; (b) carrying out reactions under anaerobic conditions; (c) performing the reaction with an alternative nucleophile (acetate) or under anhydrous conditions; and (d) determination of the reaction stoichiometry. All of the experiments pointed against autoxidation and oxidative nucleophilic substitution and toward a Mars-van Krevelen mechanism. Second, the mode of activation of the hydrocarbon was determined to be by electron transfer, as opposed to hydrogen atom transfer from the hydrocarbon to the polyoxometalate. Kinetic studies showed that an outer-sphere electron transfer was probable with formation of a donor-acceptor complex. Further studies enabled the isolation and observation of intermediates by ESR and NMR spectroscopy. For anthracene, the immediate result of electron transfer, that is formation of an anthracene radical cation and reduced polyoxometalate, was observed by ESR spectroscopy. The ESR spectrum, together with kinetics experiments, including kinetic isotope experiments and (1)H NMR, support a Mars-van Krevelen mechanism in which the rate-determining step is the oxygen-transfer reaction between the polyoxometalate and the intermediate radical cation. Anthraquinone is the only observable reaction product. For xanthene, the radical cation could not be observed. Instead, the initial radical cation undergoes fast additional proton and electron transfer (or hydrogen atom transfer) to yield a stable benzylic cation observable by (1)H NMR. Again, kinetics experiments support the notion of an oxygen-transfer rate-determining step between the xanthenyl cation and the polyoxometalate, with formation of xanthen-9-one as the only product. Schemes summarizing the proposed reaction mechanisms are presented.

The polyoxometalates substituted with noble metals, Pd(II), Pt(II) and Ru(IH), KnftWZnPdSf^OhKZnW^^}' 38H20, Ki2{[WZnPtn2(H20)2](ZnW9034)2}-36H20, and Nan{[WZnRunl2(0H)(H20)](ZnW9034)2}-42H20, were prepared by exchange of labile zinc atoms with noble metal atoms from the isostructural starting material, Nai2-{[WZn3(H20)2](ZnW9034)2]},46H20. The X-ray crystal structure of the ruthenium compound shows a structure compatible with a sandwich-type structure type with a WRuZnRu (Ru and W, Zn at opposing sides) ring between two B-XW9O34 units. Magnetic susceptibility studies as a function of temperature provide convincing evidence of two ruthenium (III) centers with no magnetic interaction between them. The EPR spectrum is supportive of this formulation showing an anisotropic spectrum of a ruthenium (III) atom (5 = '/2) in an octahedral field. The IR and UV-vis spectra of the ruthenium compound as well as of the diamagnetic palladium and platinum compounds are consistent with an isostructural series of compounds. The water soluble polyoxometalates may be extracted into an organic phase e.g. 1,2-dichloroethane by the addition of methyltricaprylammonium chloride to form their quaternary ammonium salts. The catalytic activity of these compounds was tested for the oxidation of alkenes and alkanes using aqueous 30% hydrogen peroxide and 70% fert-butyl hydroperoxide as oxidants. The alkene oxidation proceeded in high reactivity and moderate selectivity to the epoxide product using 30% H202. Kinetic profiles as well as UV-vis and IR spectra before, during and after the reaction indicate that the catalysts are stable throughout the reaction. Formation of epoxides rather than ketonization in the reaction of terminal alkenes as well as low reactivity with iodosobenzene indicates that the reaction is tungsten centered and not noble metal centered. Oxidation of alkenes with ferf-butyl hydroperoxide gave mostly allylic oxidation and/ or addition of tert-butyl alcohol to the double bond. Oxidation of cyclic alkanes such as cyclohexane and adamantane was successful with rerf-butyl hydroperoxide with catalytic activity 10 times higher than previously found for transition metal substituted Keggin compounds. Ratios of hydroxylation of adamantane at tertiary vs secondary positions indicates different active species in the palladium-, platinum-, and ruthenium substitutedpolyoxometalates.

The history of aerobic catalytic oxidation mediated by a subclass of polyoxometalates, the phosphovanadomolybdates of the Keggin structure, [PV(x)Mo(12-x)O40](3+x)-, is described. In the earlier research it was shown that phosphovanadomolybdates catalyze oxydehydrogenation reactions through an electron-transfer oxidation of a substrate by the polyoxometalate that is then reoxidized by oxygen. These aerobic oxidations are selective and synthetically useful in various transformations, notably diene aromatization, phenol dimerization and alcohol oxidation. Oxygen transfer from the polyoxometalate to arenes and alkylarenes was also discussed as a homogeneous analog of a Mars-van Krevelen oxidation. "Second generation" catalysts include binary complexes of the polyoxometalate and a organometallic compound useful, for example, for methane oxidation and nanoparticles stabilized by polyoxometalates effective for aerobic alkene epoxidation.