Plateau selectivity

An extensive survey has been carried out by McKervey and coworkers [7], who prepared the carbo-alkoxymethyl ethers of p-tert-h x y calix[4]arene, p-/< r/-butyl calix[6]arene, p-tert-bu y calix[8]arene, ca-lix[4]arene, calix[6Jarene, and calix[8]arene, and measured their abilities to extract cations from the aqueous phase into the nonaqueous phase. They concluded the following general aspects for the phase-transfer experiments (1) the calix[4]arene compounds show the greatest selectivity for Na (2) phase-transfer of Li is inefficient with all of the compounds (3) the calix[6]arene compounds show less affinity for Na than for K, with plateau selectivity for Rb" and Cs (4) the calix[8]ar-ene compounds are the least efficient of the cyclic oligomers, showing low levels of transport and low discrimination for all five cations (5) the calix[6]arene... 

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. 

Fig. 9. Selectivity behaviour of a typical rigid ligand (e. g. 30 peak selectivity) (left), and of a typical flexible ligand (e.g. 33 plateau selectivity) (right)...

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). 

Number of donor atoms. In general, supramolecular interactions are additive, hence we would expect the larger crown ethers to bind more strongly to metal cations as long as all of the donor atoms can fit around the metal. This contributes to the plateau selectivity seen for most cations on the right-hand side of Figure 3.20. 

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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