Keggin structures

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... 

Fig. 8. Polyhedral representation of the Keggin structure (central tetrahedral atom not shown). The lines seen in the center are the edges of the V30i3 subunit at the back (cf. also Fig. 21). Black dots indicate where one of the two five-coordinate capping units are attached in [Vi5042]9 .

The protons in HvW,vO,vl i i are more accessible to water than in the dodecamer with Keggin structure [(H2)Wi204o]6, and a rapid exchange can be expected (145). Parenthesis are used in the Keggin formula to indicate the internal protons where necessary. The acceptance of a proton by paratungstate B, leads to the formation of three different species (141), its protonated form and the monoprotonated a- and /3-Keggin ions... 

Fig. 21. Polyhedral representation of the a- and /3-Keggin structures of [(H2)Wi2 O40]6, also known as metatungstate and tungstate X respectively. The numbers show where the W30i3 unit (60° rotation involved) is attached to build the structures.

Recently, Corma et al. have patented a process of oxidizing cycloalkane with molecular oxygen to produce cycloalkanol and/or cycloalkanone in the presence of hydrotalcite-intercalated heteropoly anion [Co MnCo (H20)039] (M = W or Mo), which comprised one cobalt as a central atom and another as a substitute of a W=0 fragment in the Keggin structure [98]. At 130 °C and 0.5 MPa, 64 and 24% selectivity to cyclohexanone and cyclohexanol, respectively, was achieved at cyclohexane conversion about 5%. This catalytic system could be of practical importance provided a true heterogeneous nature of catalysis and good catalyst recyclability had been proved. Unfortunately, this information was lacking in [98]. 

Four-coordinate Primary Heteroatoms 3813,LI The Keggin structure and derivatives... 

Several deficit or lacunary derivatives of the Keggin structure exist as individual species or as fragments of other heteropolyanion structures. Removal of one M06 octahedron (stoichiometric loss of MO"+) from a- or /3-Keggin anions leads to isomers of [XM11O39]"-. Removal of three adjacent octahedra leads to A- and B-type XM9034 anions. The structures of these are shown in Figure 11. 

A modification of the Keggin structure is seen in the heteropolyvanadates, [XV14042]9 (X = P, As). In these anions two extra V03+ groups are attached to opposite sides of die Keggin core forming a cluster of reduced charge (see Figure 13). 68... 

Several other heteropolytungstates are known or presumed to have the Keggin structure (Table 6). Two 12-tungstoborates have been known for over a century, but these prove not to be simple a or (S isomers. The anion a -[BWi2O40]5 " seems to be an undoubted Keggin species based on isomorphism, vibrational spectrum and its behavior upon reduction. Tlie second isomer ( h ), crystallizing as the acid in hexagonal crystals, clearly is not a /3 isomer for its... 

The 12-tungstates of Se1, Telv, Sbm and Bira are unlikely to have an undistorted Keggin structure in view of the unshared pair of electrons on the heteroatoms. No detailed structural studies have been made. There are however several characterized heteropolytungstates of Asm and Sbm in which the lone pair is stereochemically active. These are summarized in Table 8. Most of these contain B-ar-XW9033 units (see Figure 11), linked by extra W06 or WOs polyhedra. See Figure 14 for an example. 

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