Spherands alkali metal complexes

The types of alkali and alkaline earth metal complexes subjected to molecular mechanics modeling fall into four categories crown ethers1255-2 ], cryptands1262 263 , spherands[264 2651, and other biologically important ligands such as ionophores and cyclic antibiotics1266-2691. 

These macrocycles show the highest stability constants known for certain alkali metal ions, and remarkable selectivity. For example, when the ionic diameter (1) for the guest metal cations becomes too large for the preformed cavity no complexation occurs. The three spherands (84)-(86) bind only Na" and Li no binding to could be detected (81). This behaviour is in contrast to that of crowns and cryptands in which size-related peak selectivity is found. 

The first studies mainly concerned the complexation of cations which is based on the relatively strong ion-dipole interactions. Lehn and Cram introduced their cryptands [3] and spherands [4] which show substantially higher affinities for alkali and alkaline-earth metal cations compared to simple crown ethers. With these new macrocyclic host molecules they exemplified the importance of preorganization in the host and complementarity between host and guest [5]. 

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