Spheroidal cryptand

The spherand prepared by Cram and coworkers was designed to have a relatively small molecular cavity and appeared to prefer complexation with Li and Na over larger cations like K", Rb, etc. Tlie spheroidal cryptand prepared by Lehn ° involved strategy employed previously but the spherand 24 was prepared by quite a different approach. 

Thus, it is the polyazamacrocyclic ligands developed by Lehn which have proved to be the most successful and versatile cryptands, rather than those carbon-bridgehead cryptands sought initially by Stoddart (25-27) which define a spheroidal cavity (e.g., 6). On the other hand, the bridged macrocyclic polyethers developed by Parsons (28), such as 7, show high complexing ability with alkali metal cations (28-30)... 

Stability and selectivity. The pH-metric titration method is usually used to determine the stability constants of cryptates (47). For alkali and alkaline earth metal cations, high stability constants are generally observed. As with the monocyclic polyethers, the most stable complex results when the ionic radius of the metal cation best matches the radius of the cavity formed by the cryptand on complexation. Because the cryptand host cavity is three-dimensional and spheroidal in shape, it is well adapted for a ball-like guest metal cation. Hence they have more pronounced recognition receptor... 

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