Vanadium polyoxometalate

A more recent approach to the epoxidation of allylic alcohols makes use of a vanadium polyoxometallate, together with a sterically demanding chiral tartrate (or TADDOL) -derived hydroperoxide, to give a highly chemoselective, regiose-lective and enantioselective outcome (Figure 11.2). ... 

An isopolyoxovanadate catalyst, [C8Hi7N(CH3)3]3H3Vio028, showed high catalytic activity in the oxidation of dibenzothiophene to its corresponding sulfone using molecular oxygen as oxidant under mild reaction conditions. A radical chain mechanism has been ruled out. Lewis bases suppress the vanadium-polyoxometalate catalysed dioxygenase reaction and catalysed the oxidation reaction of 3,5-di-t-butylcatechol selectively. The rate of autoxidation is increased by the polyoxometalate catalyst. ... 

Figure I. Oxygen uptake by supported clusters and vanadium oxide. Samples were pre-reduced and re-oxidized at the temperatures indicated on the abscissa. Silica-supported polyoxometalates PVl ( ), PV3 (A), PVI4 ( ). Bulk V2O5 (+, after [10])...

Marchal-Roch, C. Bayer, R. Moisan, J.F. Teze, A. Herve, G. Oxidative dehydrogenation of isobu-tyric acid Characterization and modeling of vanadium containing polyoxometalate catalysts. Top. Catal. 1996, 5,407-419. 

Other classes of catalysts investigated include supported Keggin-type P/Mo/V/O polyoxometalates [14-16], and titania-supported vanadium oxide or V/P/(Mo)/0... 

Huang, W., L. Todaro, L.C. Francesconi, and T. Polenova. 2003. 51V Magic angle spinning NMR spectroscopy of six-coordinate Lindqvist oxoanions A sensitive probe for the electronic environment in vanadium-containing polyoxometalates. Counterions dictate the 51V fine structure constants in polyoxometalate solids. J. Am. Chem. Soc. 125 5928-5938. 

Polyoxometalates are important catalysts but they are also finding application in optical, electrical, and magnetic devices. Mixed-metal polyoxometalates with vanadium(V) in the polyoxoanion core confers enhanced properties to such structures, principally in their ability to form essentially infinite networks that can be utilized as coatings or as other thin film materials. Additionally, these materials have tunable electromagnetic and photochromic properties. In combination with organic polymers, so-called hybrid polymers, special electrochemical properties are conferred, making possible such electrical storage devices such as capacitors and batteries that utilize the redox properties of the polyoxometalate [7],... 

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>. 

Like many other polyoxometalates (POMs), vanadium-containing POM systems can also be potent anti-viral drugs. A vanadium-substituted polyoxotungstate has anti-viral in vitro activity against viruses causing influenza, parainfluenza. Dengue fever, HIV-1 and A possible mechanism of action is the (inhibitory) interference with... 

E. Cadot, C. Marchal, M. Fournier, A. Teze G. Herve, Role of Vanadium in Oxidation Catalysis by Heteropoly anions, in Polyoxometalates M. T. Pope A. Muller, Ed., Kluwer Academic Publisher 1994, S. 315-326. 

Another catalyst type is the heterogeneous catalyst, which remains as a solid and promotes chemistry at the surface. To function well, they require high surface areas per unit mass. Metal oxides and hydroxides are common examples. A vanadium(V) oxide is employed in the formation of ammonia from nitrogen and hydrogen under elevated temperature and pressure, for example. Polyoxometallate metal clusters, which are oxo-ligand coordination complexes employing dominantly 02 and HO as ligands, have some catalytic roles. 

Polyoxometalates and their derivatives can be amalgamated with a variety of ligands. An impressive array of vanadium-oxide-phosphate based materials with new electronic and structural properties has been prepared by combining the tetrahedral PO4 ligand with oxovanadate moieties. Some of these materials have open-framework containing very large cavities and channels similar to those observed in conventional aluminosilicate-based zeolites . 

Hendrickson, and G. Christou, Tetranuclear and pentanuclear vanadium(lV/V) carboxylate complexes. [V408(N03)(02Cr)4] and [V509X(02Cr)4] (X=cL, Br") salts, Inorg. Chem., 35 6450 (1996) andb) E. Coronado and C. J. Gomez-Garcia, Polyoxometalate-based molecular materials, Chem. Rev., 98 273 (1998). 

Many highly oxidizing polyoxometalates, particularly those containing vanadium(V), have long been known to directly oxidize a range of organic substrates, R, (equation 8) and that reduced forms could be reoxidized by a variety of oxidants, OX, and under some conditions, by dioxygen (OX = O2), equation 9. The net reaction, equation 10, (the sum of equations 8 and 9) is... 

CLH wishes to thank the National Science Foundation (grant CHE-9022317), the Army Research Office (DAAL03-87-K-0131), and Interox Corporation for support of the different research programs addressed in this article. We thank Mark Weeks in our group for the synthesis of several vanadium containing polyoxometalates. R. Carlisle Chambers in our group initiated the work on the kinetics of the Venturello-Ishii chemistry. 

The vanadium-substituted polyoxometalates are applied to chemical pulps as stoichiometric oxidants, much as elemental chlorine is currently used. The reduced polyoxometalate bleaching solutions are regenerated in a second step by treatment with chlorine-free terminal oxidants, preferably air or dioxygen. By exclusion of dioxygen during the bleaching step, exposure of the pulp to non-selective species such as hydroxyl radicals is avoided entirely. 

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