Wells—Dawson structures

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... 

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...

Four of the possible six possible skeletal isomers having the Wells-Dawson structure, as exemplified by [EgWigOgz] (E = P or As), have been structurally identified. The so-called Preyssler anion was structurally identified as [ Na(H2 0) P5W3oOiio] , itbeing shown that the Na(H2 0)+ group was tightly bound in the center of the cluster. However, under forcing conditions, the aqua-sodium cation could be replaced by Ca +, trivalent lanthanide cations and some tetravalent actinide cations, all of which have similar ionic radii... 

The presence of the ammonium ion after the exchange and the stability of the Wells-Dawson structure were followed through infrared spectroscopy. The analysis was performed with FTIR Bruker IFS 66 equipment under ambient conditions. The solid samples were diluted with KBr and pressed into thin wafers. 

The purity of the Wells-Dawson structure was followed through nuclear magnetic resonance of phosphorous. Liquid NMR spectra were performed with a Bruker AM 500 spectrograph both in DjO and deutered methanol CDjOD under ambient conditions. The equipment operates at a frequency of 202.458 MHz with 11.3 ts pulses. The analysis involved 8-800 pulse responses with a resolution of 0.25 Hz per point. 

The influence of the degree of hydration on the amount of accessible active acid sites of complex Wells-Dawson structures... 

The amount of intermediate isopropoxy species toward propylene is directly influenced by the hydration water since fully hydrated Wells-Dawson acid produces twice the amount of propylene (52 ftmol/g) than the anhydrous acid (25 p.mol/g). Moreover, a continuous drop of the amount of propylene is observed with the decrease in the degree of hydration of the Wells-Dawson structure. 

Keggin structure) (Wells-Dawson structure) (mixture of isomers)... 

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