Polyoxometalate salt

Neumann and Fish have studied the novel polyoxometalate salt 25, which features 12 fluorous ammonium cations [12]. This material was insoluble in EtOAc (and toluene) at room temperature, but dissolved at 80 °C to give an effective catalyst system for the oxidation of alkenes and alcohols by 30% aqueous H2O2. CooHng precipitated the catalyst, which was reused. Additional examples of thermomorphic fluorous catalysts have been briefly described in meeting abstracts [60,61] and will Hkely soon appear in the peer-reviewed literature. 

Many polyoxometalate salts have been shown to be biologically active. Snch activity inclndes a highly selective inhibition of enzyme Ihnction (phosphatases, dehydrogenases, isomerases), especially with the smaller vanadate and molybdate oligomers (V4, M04, M05). In some cases, the selectivity seems to depend npon shape. 

Solid-state reference electrodes for potentiometric sensors are currently under research. The main problem to be faced in developing this type of electrode lies in connecting the ionic conducting (usually aqueous) solution with an electronic conductor. Since the reference electrode has to maintain a defined potential, the electrochemical reaction with components of the electrolyte has to be avoided. Oxides, mixed oxides, and polyoxometalate salts of transition elements can be proposed for preparing solid-state reference electrodes. Tested compounds include tungsten and molybdenum oxides (Guth et al., 2009). 

Lyons, J., Volpe, A., EUis, P, and Karmakar, S. Conversion of alkanes to unsaturated carboxylic acids over heteroploy acids supported on polyoxometallate salts. U.S. patent 5,990,348, November 23,1999. 

It is shown that both Sb(III) and Bi(III) can speed reduction of 12-molybdophosphate (12-MPC) to the corresponding heteropoly blue (12-MPB) by ascorbic acid (AA). It is found that mixed polyoxometalates can be formed in solution which reduce considerably more rapidly than 12-MPC. Complete formation of mixed POM is observed only if significant excess of Me(III) ions is used in the reaction. POM responsible for blue color was synthesized by selective extraction. Chemical analysis of tetrabutyl-ammonium salt is in accordance with formula of (TBAl PMeflllfMo O j (Me = Sb, Bi). IR spectmm of mixed POM is identical to 12-MPC. 

A convenient application uses an inorganic salt (e.g., a polyoxometallate)" or a semiconductor oxide as the sensitizer. Such materials are often chemically more stable with respect to organic molecules, and again can be conveniently used for the generation of a radical from unconventional precursors. 

SCHEME 85. Epoxidation of alkenes and alkenols with a perfluorinated quaternary ammonium salt of a polyoxometalate catalyst... 

The present chapter reviews the structural chemistry (Section 2) and properties and applications (Section 3) of polyoxometalates that incorporate one or more rare-earth elements. In most cases these are discrete anionic entities within the crystal and in solution, but there are also extended lattices in which POM groups are linked by rare-earth cations. Solids which can best be described as mixed oxides, or which appear to be salts of common polyoxometalate architectures such as the... 

The first systematic studies of polyoxometalate—rare-earth complexes were published in 1971 (Peacock and Weakley, 1971a, 1971b). Two classes of polytungstate complexes were described and these can now be seen to have common structural features. The complexes of Y, La, Ce(III and IV), Pr, Nd, Sm, Ho, Er, and Yb in which R W = 1 10 were readily formed by reaction of WO - with the appropriate rare-earth salt, but are stable only within the pH range 5.5-8.5. More recent 183W-NMR studies (Inoue et al., 2003) indicate that in solution the anions containing the heaviest R (Tm, Yb and Lu) are partially decomposed. 

Recently, Hill and coworkers [69] have developed a homogeneous Co catalyst that self-assembles in water from inorganic salts (Co, W, and P). It employs a polytungstate polyoxometalate (POM) ligand, which is oxidatively resistant, thus enhancing... 

Hobson, S.T., Braue, E.H., Lehnert, E.K. (2002a). Active topical skin protectants using combinations of reactive nanoparticles and polyoxometalates or metal salts. U.S. Patent No. 6,410,603, US Patent and Trademark Office, Washington, DC. 

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