Dawson structures

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). 

DAF-1, butene diffusion in, 42 36 Dangling bonds, 34 138 Data analysis, in extended X-ray absorption fine structure studies, 35 31-33 Dawson structure... 

Several lacunary derivatives of the Dawson structure can be synthesized by controlled alkalinization of aqueous solutions, as shown in Scheme 4. 

Wells-Dawson structure enclosing eight potassium cations (gray). 

In our attempts to design functional clusters we have focused on substitution of the heteroanions within the Wells-Dawson structure to create nonconventional Dawson clusters incorporating two pyramidal anions. Our design rationale was based on the idea that such clusters may exhibit unprecedented properties arising from the intramolecular electronic interactions between the encapsulated anions (in this case we aimed to engineer between S S atoms of two encapsulated sulfite ions), thus providing a novel route to manipulate the physical properties of... 

The Keggin and Dawson structures incorporate internal XO4 tetrahedra that are attached to the oxometalate shell through each oxygen atom (/X4 type), but structures are also known in which a tetrahedral coordination... 

A more extensive set of actinide complexes is formed with tungstates of the Keggin and Dawson structure, An[XWi 1039)2 and AnpC2Wi706i]2 (X = P, Si, B, As An = Th, U, Np, jj ggg ligands form very stable complexes of tetravalent lanthanides and actinides. 

Figure 12 Trivacant a-Keggin (A- and B-type) and Wells-Dawson structures. 

A closely related series of sandwich complexes (also shown in Table 15) is formed with the anions derived from the Wells-Dawson structure, [X2Wi5056]12 (X = P, As), since these possess a lacunary surface identical to that of B-[PW9034]9-. Since the precursor lacunary anions are easily isolated, the synthesis of the sandwich complexes is straightforward. Although several Baker Figgis-type isomeric possibilities exist for anions of this composition,233 until recently all confirmed structures have been found to be identical. The exception is [(P2W15056)2FeIII2Na2(H20)2]16-, which has a different connectivity of the lacunary components.235... 

Figure 1 Representative POM structures in polyhedral notation. Top left the common Keggin structure top right the hexametalate structure bottom left the Wells-Dawson structure bottom right the Pope-Jeannin-Preyssler structure. In polyhedral notation, the center of each M06 unit is a metal nucleus and each...

---