Electron and oxygen transfer in polyoxometalate, H5PV2Mo10O40, catalyzed oxidation of aromatic and alkyl aromatic compounds: Evidence for aerobic Mars-van Krevelen-type reactions in the liquid homogeneous phase

The mechanism of aerobic oxidation of aromatic and alkyl aromatic compounds using anthracene and xanthene, respectively, as a model compound was inves tigated using a phosphovanadomolybdate polyoxometalate, H5PV2Mo10O40, as ca talyst under mild, liquid-phase conditions. The polyoxometalate is a solubl e analogue of insoluble mixed-metal oxides often used for high-temperature gas-phase heterogeneous oxidation which proceed by a Mars-van Krevelen mech anism. 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 H5PV2Mo10O40 to the hy drocarbons was studied using various techniques to show that commonly obser ved liquid-phase oxidation mechanisms, autoxidation, and oxidative nucleoph ilic substitution were not occurring in this case. Techniques used included (a) use of O-18-labeled molecular oxygen, polyoxometalate, and water; (b) carrying out reactions under anaerobic conditions; (c) performing the react ion with an alternative nucleophile (acetate) or under anhydrous conditions ; and (d) determination of the reaction stoichiometry. All of the experimen ts pointed against autoxidation and oxidative nucleophilic substitution and toward a Mars-van Krevelen mechanism. Second, the mode of activation of th e hydrocarbon was determined to be by electron transfer, as opposed to hydr ogen atom transfer from the hydrocarbon to the polyoxometalate. Kinetic stu dies showed that an outer-sphere electron transfer was probable with format ion of a donor-acceptor complex. Further studies enabled the isolation and observation of intermediates by ESR and NMR spectroscopy. For anthracene, t he immediate result of electron transfer, that is formation of an anthracen e radical cation and reduced polyoxometalate, was observed by ESR spectrosc opy. The ESR spectrum, together with kinetics experiments, including kineti c isotope experiments and IH 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 no t be observed. Instead, the initial radical cation undergoes fast additiona l proton and electron transfer (or hydrogen atom transfer) to yield a stabl e benzylic cation observable by IH NMR. Again, kinetics experiments support the notion of an oxygen-transfer rate-determining step between the xanthen yl cation and the polyoxometalate, with formation of xanthen-9-one as the o nly product. Schemes summarizing the proposed reaction mechanisms are prese nted.