Cryptate spherical

Cryptands, 42 122-124, 46 175 nomenclature, 27 2-3 topological requirements, 27 3-4 Cryptate, see also Macrobicyclic cryptate 12.2.2], 27 7-10 applications of, 27 19-22 cylindrical dinuclear, 27 18-19 kinetics of formation in water, 27 14, 15 nomenclature, 27 2-3 spherical, 27 18 stability constants, 27 16, 17 Crystal faces, effect, ionic crystals, in water, 39 416... 

Cryptands 7-9 thus function as receptors for spherical cations. Their special com-plexation properties result from their macropolycyclic nature and define a cryptate effect characterized by high stability and selectivity, slow exchange rates, and efficient shielding of the bound ion from the environment [2.17,2.27]. 

Spherical recognition of halide ions is displayed by protonated macropolycyclic polyamines. Thus, macrobicyclic diamines yield katapinates [3.9]. Anion cryptates are formed by the protonated macrobicyclic 16-6H+ [2.52] and macrotricyclic 21-4H+ [2.97] polyamines, with preferential binding of F and Cl- respectively in an octahedral and in a tetrahedral array of hydrogen bonds. 

H+ binds Cl- very strongly and very selectively compared with Br- and other types of anions, giving the [Cl- c (21-4H+)] cryptate 26. Quaternary ammonium derivatives of oxygen free macrotricycles of type 21 also bind spherical [3.10a] and other [3.10b] anions. 

Linear recognition is displayed by the hexaprotonated form of the ellipsoidal cryptand bis-tren 33, which binds various monoatomic and polyatomic anions and extends the recognition of anionic substrates beyond the spherical halides [3.11, 3.12]. The crystal structures of four such anion cryptates [3.11b] provide a unique series of anion coordination patterns (Fig. 4). The strong and selective binding of the linear, triatomic anion N3" results from its size, shape and site complementarity to the receptor 33-6H+. In the [N3 pyramidal arrays of +N-H "N- hydrogen bonds, each of which binds one of the two terminal nitrogens of N3-. 

The non-complementarity between the ellipsoidal 33-6H+ and the spherical halides results in much weaker binding and appreciable distortions of the ligand, as seen in the crystal structures of the cryptates 35 where the bound ion is F , Cl-, or Br-. In these complexes, F- is bound by a tetrahedral array of hydrogen bonds whereas Cl- and Br- display octahedral coordination (Fig. 4). Thus, 33-6H+ is a molecular receptor for the recognition of linear triatomic species of a size compatible with the size of the molecular cavity [3.11]. 

Macrotricyclic cryptates may have either spherical or cylindrical topology (90). The cylindrical ligands, such as 5, are formed by linking together two macrocycles and define three cavities, two lateral and one central cavity inside the macrotricycle (18,19,91, 92). The macrotricyclic ligands of spherical topology are Particularly well suited for com-plexation of alkali cations as they define a spherical cavity (93, 94). Ligand 10, for example, contains a spherical cavity (diameter 3.4 A)... 

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