Cryptands and Cryptate Complexes

This remarkable group of compounds was introduced by Lehn and his co-workers in Strasbourg . They are macrobi-, macrotri-, or macropolycyclic compounds which are similar to the in-out amines illustrated above, but they have crown-like bridges throughout. Because of their ability not only to complex cations but to encapsulate 

For the purposes of nomenclature, the simple cryptands are assumed to be macro-bicyclic and nitrogen is assumed to be the bridgehead atom. The different cryptands are designated by assigning numbers according to the number of heteroatoms in each ethylenoxy chain. The three cryptands shown below are designated [ 1.1.1 ]-cryptand (9), [2.2.1 ]-cryptand (10) and [3.2.2]-cryptand (11), respectively. 

As with the crowns, the situation becomes more complicated when there are other heteroatoms or substituents in one of the cryptand bridges. The symbol B is used to designate a benzo- or catechol unit in the bridge and subscripts are used to designate heteroatoms when non-oxygen heteroatoms are present. Examples of this are shown in structures 12 and 13 above. 

An even more complicated nomenclature problem arises with the closely related all-oxygen cryptands. These compounds do not utilize nitrogen as the three-chain junction. Most examples of this class of compounds have utilized pentaerythritol or glycerol as the junction. This naturally imparts a somewhat lower flexibility to the molecule than would be present in the nitrogen-containing cases. Structures of two such molecules are illustrated below. 

The disadvantages of these names are manifest, but one shudders to think about naming them systematically. It seems likely that for the foreseeable future, nomenclature of crowns, cryptands and related substances will bear a semblance to heiro-glyphics. 

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