Cryptands plateau 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). 

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). 

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