Cryptands peak selectivity

In general, the cryptands (213) show a stronger correlation between thermodynamic stability and match of the metal ion for the cavity. Thermodynamic data for complexation of the alkali metal ions with a number of cryptands is summarized in Table 6.2. The data for the smaller (less flexible) cryptands 2.1.1, 2.2.1, and 2.2.2 illustrate well the occurrence of peak selectivity. 

In contrast to the peak selectivity just discussed, there is evidence that the larger, more flexible, ligands tend to exhibit plateau selectivity - a reflection that a number of the larger metal ions are accommodated by the cryptand without major variation in binding energy. 

A more precise cavity control is found with the three-dimensional cryptands and Figure 26 shows the variation of Ks with alkali and alkaline earth metal cations for a range of cryptands.29 Two types of selectivity are displayed. Peak selectivity occurs when there is an excellent correlation between cavity and cation radius (Table 8) and so a sharp discrimination can occur (Figure 26). Plateau selectivity occurs when there is little discrimination between cations of higher radius but discrimination against cations of small radius which are not well accommodated (Figure 26). 

In contrast to the crown ethers, the three-dimensional cryptands display peak selectivity. The macrobicyclic cavities are more rigid and restricted so that they are unable to constrict sufficiently... 

Cryptands display enhanced size selectivity with respect to alkali metal ions. Figure 17 refers to 2.2.2-crypt, which shows peak selectivity for K+, in a 95/5 MeOH/water mixture (circles in Fig. 17) [38]. This effect is much more pronounced than that exerted by 18-crown-6 in pure MeOH (triangles in Fig. 17), a behaviour which may reflect the higher degree of steric restraint granted by a cage compared to a ring. 

Macrocycles of the rigid type (e.g., small cryptands and other preorganized macrocycles) discriminate between cations that are either smaller or larger than the one that exactly fits into the cavity (peak selectivity). Macrocycles of the flexible type (e.g., larger polyether crowns and cryptands) discriminate principally among smaller cations (plateau selectivity) (Vogtle and Weber, 1989). 

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. 

Details about the chemical nature and the structure of the ligands involved can be found in Section 5. The data in the tables of Section 2.3. reflect the main issue of our discussion, such as selectivity, which sometimes — as e.g. for valinomycin or for certain specially designed cryptands — shows peak values for Li+, Na+ or K+, orders of magnitude above the other members of the alkali series. At the same time they demonstrate that the rank order of selectivity within this series might vary considerably. 

Peak thermodynamic selectivity can occur for a given metal as the cavity size progressively increases across the series of cryptands of type 14. Thus, the 1 1 stability constants (log values) for the interaction of K+ with 2.1.1, 2.2.1, 2.2.2, and 3.2.3 are <2.0, 4.0, 5.4, and 2.2 respectively, reflecting that the match of the cavity size and the number of donor sites in 2.2.2 provide optimal complementarity for this cation. 

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