Sandwich polyoxometalates

FIGURE 16.1 Structure of the [WZn3(ZnW9034)2] sandwich polyoxometalate anion. 

FIGURE 16.J Conversion and selectivity after 4 h versus QCl/W ratio in the self-assembled sandwich polyoxometalate (0.0005 mol% 0.01 mol% W)-catalyzed epoxidation of geraniol with 1.06 equiv 50% H2O2 at 60 °C in toluene. Reprinted with permission from [10]. Copyright 2004 American Chemical Society. 

FIGURE 16.4 Effect of 1,2- and/or 1,3-allylic strain on the diastereoselectivity observed with sandwich polyoxometalate (POM)-catalyzed epoxidations of chiral allylic alcohols. 

FIGURE 16.7 Sandwich polyoxometalate (POM) catalyst recycling through nanofiltration based on a ceramic alumina membrane. 

Neumann R, Khenkin AM (1996) Anewdinuclearrhodium(III) sandwich polyoxometalate, [(WZnRH j)(ZnW,03,)J . Synthesis, characterization and catalytic activity. J Mol Catal A Chem 114 169-180... 

The formation of sandwich-type POMs from solutions of metal cations and triva-cant heteropolytungstate species may alternatively be viewed as means to sequester cations from solution [116]. The question then arises if reversible dissociation of the metal cations from the multidentate ligands is possible as well. The sequestration and subsequent release of metal cations under appropriate conditions is of considerable interest in waste decontamination technology as well as in medicine [124]. Following these ideas, the demetallation process of ten multi-iron Wells-Dawson polyoxometalates is studied by cyclic... 

A polyoxometalate is also at the heart of an enantioselective epoxidation of allylic alcohols using a C-2 symmetric chiral hydroperoxide 39 derived from l,l,4,4-tetraphenyl-2,3-0-isopropylidene-D-threitol (TADDOL). Thus, in the presence of the oxovanadium(IV) sandwich-type POM [ZnW(V0)2(ZnW9034)2]12- and stoichiometric amounts of hydroperoxide 39, the dienol 40 is converted to the (2R) epoxide 41 in 89% yield and 83% ee. The proposed catalytic cycle invokes a vanadium(V) template derived from the POM, substrate, and hydroperoxide, a hypothesis supported by the lack of enantioselectivity with unfunctionalized alkenes. The catalytic turnover is remarkably high at about 40,000 TON <03OL725>. 

The structure of [(Y0H2)3(C03)(A-a-PW9034)2]" [25]. (Reprinted with permission from T.M. Anderson, W.A. Neiwert, and C.L. HiU, Yttrinm polyoxometalates. Synthesis and characterization of a carbonate-encapsulated sandwich-type complex, Inorganic Chemistry, 42, no. 26, 8600-8602 (Figure 1), 2003. 2003 American Chemical Society.)... 

Fan, L., Xu, L., Gao, G et al. (2006) A novel polyoxometalate chain constructed from sandwich lanthanide-containing polyanion ICe(PWn039)2] and sodium ion linker. Inorganic Chemistry Communications, 9, 1308-1311. 

Chen, W, Li, Y, Wang, Y. et al. (2007) An inorganic aggregate based on a sandwich-type polyoxometalate with lanthanide and potassium cations from ID chiral ladder-like chains to a 3D open framework. European Journal of Inorganic Chemistry, 2216-2220. 

M. D. Ritorto, T. M. Anderson, W. A. Neiwert, C. L. Hill, Decomposition of A-type sandwiches. Synthesis and characterization of new polyoxometalates incorporating multiple d-electron-centered units, Inorg. Chem. 43 (2004) 44. 

W. Adam, P. L. Alsters, R. Neumann, C. R. Saha-Moller, D. Seebach, A. K. Beck, R. Zhang, Chiral hydroperoxides as oxygen source in the catalytic stereoselective epoxidation of allylic alcohols by sandwich-type polyoxometalates Control of enantioselectivity through a metal-coordinated template, /. Org. Chem. 68 (2003) 8222. 

P. T. Witte, S. Roy Chowdhury, J. E. ten Elshof, D. Sloboda-Rozner, R. Neumann, P. L. Alsters, Highly efficient recycling of a sandwich" type polyoxometalate oxidation catalyst using solvent resistant nanofiltration, Chem. Commun. (2005) 1206. 

Neumann and co-workers have used sandwich-type polyoxometalates as catalysts for such reactions.305 The sandwich consists of transition metal ions between two Keggin anions, as in Ki2WZnMn(II)2(ZnW9034)2. In some cases, quaternary ammonium salts were used as the cations to take the catalyst into organic solvents. Sulfides were oxidized to sulfoxides with 30% aqueous hydrogen peroxide in 85-90% yields, with some sulfone also being formed. The system was more than 99% selective in the conversion of cyclooctene to its epoxide. The system also shows good selectivity between double bonds (6.57), probably the result of the bulky anion and the increased electron density in alkyl-substituted double bonds. 

Some of these surfactant-encapsulated polyoxometalate complexes exhibited liquid-crystaUine behavior. Paul and coworkers obtained room temperature LCs by complexation of the sandwiched europium-containing polyoxometalate [Eu(SiW9Mo2039)2] with a series of cationic surfactants (Zhang et al., 2005). For the quaternary ammonium cations with one long chain a hexagonal (columnar ) phase and possibly a cubic phase were observed, whereas for the quaternary ammonium cation with two long chains... 

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