A-Keggin anions

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.)...

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

The two known isomers of the (a)-Keggin anion with Csv (P) and C2v (y) symmetry (Figure 5a) also have accessible lacunary derivatives, but the only one of relevance to present discussion is the divacant y-[SiWio036] (Figure 5b). 

Two metastable aqueous polyvanadates have been identified. Acidification of neutral vanadate solutions yields a solution with the expected V and O NMR spectra of the decavanadate anion and with additional resonances (two V, 1 12 three O, 1 1 1) that decay with a half-life of 80 min at 25 °C. The spectra are consistent with either a protonated a-Keggin anion or its e-isomer, [II12V13O40]3, 8 9 Reaction of V02+ with 50% H202 and subsequent treatment with tetramethyl-ammonium ion leads to black (NMe -jI I6[V, 5O4J in 36% yield. The anion has a trans bicapped Keggin structure analogous to that of [PV 4042]9 (see below) and has the expected four-line 51V NMR spectrum (1 2 8 4). At pH 3.5, solutions of the anion are stable for a week at room temperature, and exhibit an initial reduction potential of 0.23 V vs. Ag/AgCl. Acidification leads irreversibly to VO2+.X0... 

Another possibility exists for incorporating sulfur in a Keggin anion. When the size of the [M202S2] " thio-fragment (M = Mo, W) and that of the cavity of the lacunary oxo-precursor are not compatible, the direct stereospecific addition is no longer possible. In that case the acido-basic reaction gives sandwich-type compounds. This is typically the case of the reaction between the trivacant... 

Fig. 12.1 The a-Keggin anions, shown in coordination-polyhedron notation, are 1.12 nm in diameter, and possess tetrahedral (J ) symmetry. In each polyanion, 12 W addendum atoms are at the center of WOe polyhedra, which each possess Qv symmetry and have O atoms at their vertices. At the center of this cluster (shaded) is a tetrahedral metalate oxoanion, X"+04...

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