A molecular entity comprising a cyclic or polycyclic assembly of binding sites that contains three or more binding sites held together by covalent bonds, and which defines a molecular cavity in such a way as to bind (and thus "hide" in the cavity) another molecular entity, the guest (a cation, an anion or a neutral species), more strongly than do the separate parts of the assembly (at the same total concentration of binding sites). The adduct thus formed is called a "cryptate". The term is usually restricted to bicyclic or oligocyclic molecular entities.
Example
Corresponding monocyclic ligand assemblies (crowns) are sometimes included in this group, if they can be considered to define a cavity in which a guest can hide. The terms "podand" and "spherand" are used for certain specific ligand assemblies. Coplanar cyclic polydentate ligands, such as porphyrins, are not normally regarded as cryptands. DIETRICH, LEHN and SAUVAGE (1969). See also host. For a contribution to the evolving terminology, see also WEBER and VÖGTLE (1980).
