A-Keggin

The Dawson structure, C, can be visualized as being formed by removing the three basal octahedra from each of two a-Keggin ions which are then fused together. By omitting the four octahedra at the front, the oxygens associated with the 2 heteroatoms can be seen more clearly (D). 

Several studies have addressed the cubic NLO properties of salts of heteropolymolybdate/ tungstate cluster anions.360,557-561 The tetraanion in a-H4SiW12O40,4HMPA,2H2O (HMPA= hexamethylphosphoramide) has a Keggin structure which has idealized Td point symmetry... 

The preference of the vanadium atoms to be mutual adjacent is confirmed by a range of a-Keggin ions with vanadium occupying the center (172). A metatungstate Keggin ion, a-[H2WnVO40]7, with one... 

In 2002, Ichihara and coworkers reported on the utilization of a Keggin-type phos-phomolybdate (NH4)3PMoi2O40 on apatite as catalyst for the solvent-free epoxidation of olefins (see Section III.A.3.c) and the oxidation of sulfldes and sulfoxides in the presence of urea hydrogen peroxide (Scheme 104) . Chemoselectivities of the oxidation of the sulfides were good [product ratios (sulflde/sulfoxide) 84/16 up to 91/9] depending on the substrate, temperature and reaction time. 

The possibilities of POMs compositions and structures are virtually enormous, and the number of these molecules actually prepared and characterized continues to grow unabated. However, examination of their short electrochemical studies, which are becoming a usual part of their characterization, shows that a limited selection of representative groups of POMs is sufficient for a description of the main electrochemical behaviors of this class of chemicals. Therefore, a-Keggin- and Dawson-type heteropolyanions of phosphotungstate, silicotungstate, phosphomolybdate, and silicomolybdate mixed addenda heteropolyanions, and transition-metal substituted heteropolyanions including sandwich-type derivatives were selected for the present article. 

Figure 3.30. Heteropoly molybdate catalysts (a) Keggin anion (PMoi204o) (the central PO4 tetrahedron is shaded and the M0O6 octahedra are unshaded) and (b) selectivity (S) against conversion (C) data for acrolein to acrylic acid over KjcH3 PMoi2O40 (with x = 1-3) at " 350 °C show high selectivities. (After Black et al 1987, Gai et al 1983.)...

Figure 44 Examples of heteropolyanions (A) Keggin-type polyanion, for example 0 -PW,2O403-, (B) Dawson-type polyanion, for example P2Wn80626 (C) disubstituted polyanion, for example >-SiW,oFe204o,0. Taken from Ref. [216]. Reproduced by permission of The Royal Society of Chemistry.

The Keggin-structure anions q -[XMoi204o]" are frequently investigated because of their potential and real catalytic applications. A number of capped Keggin structures have been identified in which the a-Keggin core... 

Mo-heteropolyanions with a Keggin structure are readily reduced to give heteropoly blue in solution. A trend exists toward anions with X of higher charge (small value of n) being more easily reduced, while the reduction potential decreases in the order V > Mo > W. In the case of XMnWn04oH2", the Mn + in the anion is oxidized by O2. 

RE-tungstate clusters. [B-a-X(III)W9033] is derived from an a-Keggin anion [XWi204o]" by removal of three edge-sharing WOe octahedra. 

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