Ester, ketone, and amide functions have been introduced on the ‘narrow rim’ of calix[n]arenes (Fig. 11.4) in order to mimic natural substrates (Chang

11.4 Examples of functionalized calix[n]arenes at the narrow rim.

and Cho, 1986, 1987, Arnaud-Neu et al., 1989). The trends observed while investigating the complexing properties of these ligands can be summed up as follows, although the experimental conditions often differ:

• The blocked cone conformation and the four ‘hard’ donor functions (bearing oxygen atoms) pointing in the same direction confer the sub­stituted calix[4]arenes interesting complexation and extraction proper­ties toward sodium cation, in the following order: esters < ketones < amides (Arduini et al., 1988, Schwing et al., 1989, Arnaud-Neu et al., 1991, 1995, Gradny et al., 1996). The stability constants of the 1 : 1 (metal : ligand) complexes measured in solution are greater than those observed with crown ethers, almost challenging those of cryptands, thus enabling the use of these substituted calix[4]arenes as active agents for chemical sensors (Forster et al., 1991, Brunink et al., 1991, Ganjali et al., 2006).

• The ester derivatives of calix[6]arene extract potassium better than sodium and show a selectivity plateau for voluminous alkali cations. The amide derivatives of calix[6]arene extract the alkaline-earth elements better than the alkali elements, with a preference for calcium and strontium.

• Calix[8]arene derivatives are the least efficient extractants of the series.