Cryptates: macropolycyclic inclusion complexes
Macropolycyclic ligands containing appropriate binding sites and cavities of suitable size and shape, may be designed so as to display molecular recognition in the formation of selective inclusion complexes, cryptates, with metal cations, but also with anions and molecules. Macrobicyclic ligands B form highly stable and selective alkali and alkaline-earth cryptates, displaying a pronounced crypt4te effect on both stability and selectivity with respect to macromonocyclic ligands. Toxic heavy metal cations and transition netal cations may be selectively complexed by replacing oxygen by nitrogen binding sites. Alkali cryptates may also be considered as a class of very large counterions which enable the isolation of unusual species like alkali metal anions or cluster anions. Cylindrical macrotricyclic ligands C contain two metal cation sites and form both mononuclear and binuclear metal cation cryptates. Their structure, the relative position of the cations and the complexation properties may be monitored via ligand design. Spheroidal macrotricyclic ligands D effect tetrahedral recognition of small molecules or ions; they may be considered as topologically. optimal receptors for the ammoniuni ion, the water molecule, the halide ions, with which they form cryptates where the substrate is held in the intramolecular cavity by a tetrahedral array of hydrogen bonds. Prospects for the design of other macropolycyclic receptors, of polynuclear complexes and of "cascade" complexes are analyzed.