Polyoxometalate complexes

Figure 2.9 Schematic structure of two dif- (d) cube. The dpp parameter defines the aver-ferent polyoxometalate complexes with inter- age distance between the two oxygen-based esting coordination symmetry (a) LnW10 and square planes. The djn is the average 0-0 (b) [LnPd"12(AsvPh)8032]5-, and their coor- distance within the oxygen-based square dination polyhedra (c) square antiprism and planes.

Goals and five limitations in conjunction with the development of selective catalytic homogeneous oxidation systems are evaluated. Systems are presented that address several of the problems or goals. One involves oxidation of alkenes by hypochlorite catalyzed by oxidatively resistant d-electron-transition-metal-substituted (TMSP) complexes. A second involves oxidation of alkenes by H2O2 catalyzed by specific TMSP complexes, and a third addresses functionalization of redox active polyoxometalate complexes with organic groups. 

Hydrogen peroxide is an inexpensive oxidant, but it requires a catalyst to effect oxidation of an alcohol to the ketone. Removal of the catalyst then becomes an issue. Ronny Neumann of the Weizmann Institute of Science reports (J. Am. Chem. Soc. 2004,126, 884) the development of a hybrid organic-tungsten polyoxometalate complex that is not soluble in organic solvents, but that nonetheless catalyzes the hydrogen peroxide oxidation of alcohols to ketones. The solid catalyst is removed by filtration after the completion of the reaction. The catalyst retained its activity after five recyles. 

Neumann, Ronny, Polyoxometallate Complexes in Organic Oxidation Chemistry 47 317... 

In acetonitrile solution the luminescence of [Eu(H20)4(o i-P2Wi706i)]7 increases with the addition of 1,10-phenanthroline or 2,2-bipyridine and reaches a limiting value with a 2 1 ratio of the bidentate ligand to polyoxometalate complex, consistent with the replacement of the four aqua ligands of the Eu cation (Boglio et al., 2006). 

Interest in polyoxometalate complexes of the rare earths has been driven to a large extent by their photophysical and photochemical properties. Table 1 lists several reviews. In general, photoexcitation into LMCT (O -> W, O -> Mo) bands results in intramolecular energy transfer to the rare earth with subsequent emission and luminescence. 

Table 22 Recent examples of Weakley-type polyoxometalate complexes, [(Ln,An)L2] . ...

R. Neumann, Polyoxometalate complexes in organic oxidation chemistry. Prog. Inorg. Chem. 47 (1998) 317. 

J. M. Clemente-Juan and E. Coronado Magnetic Clusters from Polyoxometalate Complexes Coord. Chem. Rev. 193-195 361 (1999). 

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