Sandwich-type polyoxometalate

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.)... 

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. 

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. 

Beyond the two mechanistic themes presented above, that is, autooxidation and redox-type reactions involving electron transfer, a ruthenium-substituted sandwich -type polyoxometalate was shown to be a catalyst for oxidation by a dioxygenase -type mechanism as outlined in Scheme 9.13 [125]. 

Figure 5.1 General structure of the Keggin-type (a) and sandwich-type (b) transition metal substituted polyoxometalates (TMSPs) used in catalysis W (green) O (red) heteroatom X (yellow) transition metal M (pink). See insert for color representation of the figure.)...

Neumann, R. and Khenkin, A. (1995). Noble Metal (Ru, Pd, Pt ) Substituted Sandwich Type Polyoxometalates Preparation, Characterization, and Catalytic Activity in Oxidations of Alkanes and AUcenes by Peroxides, Inorg. Chem., 34, pp. 5753—5760. 

Adam, W., Alsters, R, Neumann, R., et al. (2003). A Highly Chemoselective, Diastereose-lective, and Regioselective Epoxidation of Chiral AllyUc Alcohols with Hydrogen Peroxide, Catalyzed by Sandwich-Type Polyoxometalates Enhancement of Reactivity and Control of Selectivity by the Hydroxy Group through Metal-Alcoholate Bonding, J. Org. Chem., 68, pp. 1721-1728. 

Enantioselective oxidation catalysis to yield chiral products from prochiral substrates using polyoxometalate catalysts has not been observed until recently. However, in a combined effort of several research groups it has been shown that the racemic vanadium-substituted sandwich type polyoxometalate, [(V 0)2ZnW(ZnW9034)2] , is an extremely effective catalyst (up to 40 000 turnovers) at near ambient temperatures, for the enantioselective epoxidation of aUyHc alcohols to the 2R,3R-ejx)xyalcohol with the sterically crowded chiral hydroperoxide, TADOOH, as oxygen donor [31], Scheme 8.6. 

Hydrogen peroxide is often used as a green oxidant in aqueous alcohol oxidation reactions. A good catalyst are water-soluble polyoxometalates [160]. Eor example, sandwich -type polyoxometalate Naj2[WZn3(H20)2][ZnW9034]2] was shown to... 

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. 

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