Sandwich-type POM

The ruthenium-substituted sandwich-type POM WZnRu2(OH)(H20)(A W9034)2]11 (X- Zn2+ or Co2+, Figure 13.1) catalyzed the selective hydroxylation of adamantane with Oz as an oxidant[58,59] ... 

The metal-catalyzed asymmetric epoxidation of allylic alcohols with various enan-tiomerically pure hydroperoxides has been studied by several groups. This approach has been employed in the Ti- and V-mediated epoxidation of this class of substrates, in the presence of different achiral additives with modest enantioselectivities (ee ee < 46% ), which turned satisfactory (ee 72%) in the presence of the TADDOL-derived hydroperoxide TADOOH 73 . This oxidant has been recently employed in the oxovanadium sandwich-type POM [ZnW(V0)2(ZnW9034)2] catalyzed epoxidation of various allylic alcohols with very high catalytic efficiency (42000 turnovers) and enantiomeric ratios up to 95 5 98. 

Electron Transfer Behaviors of Multi-iron Sandwich-type POMs. .. 649... 

Transition metal-substituted POMs (TM-SPs) can be considered as the largest subclass of polyanions [6]. Within the class of TMSPs, the sandwich-type compounds represent the largest family. In short, sandwich-type POMs are formed by the fusion of two trivacant a-XW9 034" ... 

In parallel with the explosion of sandwich-type POMs synthesis and characterization, their electrochemistry is studied, both on... 

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... 

The oxidized forms of these two sandwich-type POMs are stable from pH 3 to at least pH 7. Their characterization by cyclic voltammetry revealed the stepwise reduction of the Gu " " centers within the POMs, before Gu" deposition on the glassy carbon electrode surface [115]. Phenomena are described mainly for the P-derivative and are the same for the As-analog. 

Reaction of the sandwich-type POM [(Fc(0H2)2)j(A-a-PW9034)2 9 with a colloidal suspension of silica/alumina nanopartides ((Si/A102)Cl) resulted in the production of a novel supported POM catalyst [146-148]. In this case, about 58 POM molecules per cationic silica/alumina nanoparticle were electrostatically stabilized on the surface. The aerobic oxidation of 2-chloroethyl ethyl sulfide (mustard simulant) to the corresponding harmless sulfoxide proceeded efficiently in the presence of the heterogeneous catalyst and the catalytic activity of the heterogeneous catalyst was much higher than that of the parent POM. In addition, this catalytic activity was much enhanced when binary cupric triflate and nitrate [Cu(OTf)2/Cu(N03)2 = 1.5] were also present [148],... 

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>. 

---