Polyoxometalates (POMs) are a unique class of molecular metal-oxygen clusters. They consist of a polyhedral cage structure or framework bearing a negative charge and centrally located heteroatom(s) surrounded by the cage framework. The negative charge is balanced by cations that are external to the cage. Generally, suitable heteroatoms include Group IIIa-VIa elements such as phosphorus, antimony, silicon, selenium and boron. The framework of polyoxometalates is usually comprised of edge- or corner-shared MO6 octahedra, where M represents a transition metal (addenda). Due to appropriate cation radius and good π-electron acceptor properties, the addenda metal is substantially limited to a few metals including Group Vb or VIb transition metals in their highest oxidation state (e.g. V5+, Nb5+, Ta5+, W6+, Mo6+).
A major subclass of polyoxometalates is constituted by Keggin type POMs. These polyoxoanions generally consists of 12 framework metals and 40 oxygen atoms symmetrically arranged around a central atom X and thus can be represented by the formula XnM12O40(8−n)−. If the central atom X is a heteroatom with a lone pair of electrons (e.g. AsIII, SbIII), the formation of such closed Keggin units is not allowed. In fact, most of these POMs consist of dimeric adducts of incomplete (lacunary) Keggin fragments joined together by extra framework or heteroatoms.
For example, Krebs et al. (Chem. Eur. J. 1997, 3, 1232; Inorg. Chem. 1999, 38, 2688) describe the dimeric structural type [(WO2)4(OH)2(β-XW9O33)2]12− (X═SbIII, BIII). Moreover, the authors were also able to substitute the two external tungsten atoms by first-row transition metals resulting in transition metal substituted polyoxometalates (TMSPs) represented by the formula [(WO2)2M2(H2O)6(β-XW9O33)2](14−2n)− (X═SbIII, Mn+=Mn2+, Fe3+, Co2+, Ni2+; X═BiIII, Mn+═Fe3+, Co2+, Ni2+, Cu2+, Zn2+).
Moreover, Kortz et al. report on tetrasubstituted dimeric polyoxotungstates which consist of two [β-XW9O33]n− (n═9, X═AsIII, SbIII; n═8, X═SeIV, TeIV) moieties linked by four Fe3+ ions having terminal H2O ligands (Inorg. Chem. 2002, 41, 783). These authors were also able to substitute the iron centers in this structure by a large number of other 1st, 2nd and 3rd row transition metals (e.g. Mn2+, Co2+, Ni2+, Cd2+, Hg2+).
Due to their size, shape, charge density and redox-active nature, POMs and in particular TMSPs have attracted continuously growing attention in the area of oxidation catalysis.
Several oxidation reactions of organic substrates using polyoxometalates are known. For instance, Neumann et al. describe the oxidation of alkenes and cycloalkanes using a ruthenium-substituted sandwich type polyoxometalate and hydrogen peroxide or molecular oxygen as an oxygen donor (Angew. Chem. Int. Ed. Engl. 1995, 34, 1587; Inorg. Chem. 1995, 34, 5753; and J. Am. Chem. Soc. 1998, 120, 11969). In addition, they report on the epoxidation of chiral allylic alcohols (J. Org. Chem. 2003, 68, 1721-1728). Cavani et al. disclose the oxidation of isobutane to methacrylic acid (Topics in Catalysis 2003, 23, 141-152) and Kamat et al. describe the epoxidation of various olefins using hydrogen peroxide and a silicotungstate compound (Science 2003, 300, 964-966). Further, WO 03/028881 discloses a process for the selective oxidative dehydrogenation of alkanes to produce olefins using certain polyoxometalate catalysts.
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However, with respect to the catalytic performance of polyoxometalates in oxidation reactions and in particular in the selective oxidation of alkanes no satisfactory results have been reported up to now.
Therefore, it is the object of the present invention to provide polyoxometalates showing an improved catalytic performance in oxidation reactions of organic substrates and in particular attaining high conversions when used for the selective oxidation of alkanes.