Molecular recognition by azacalix[3]arenes

The azacalix[3]arene macrocycles 1 are ideal hosts for the binding of metal ions and alkylammonium ions due to the presence of heteroatoms in the macrocycle ring and the ease of modifying their structure through the R, R', and R substituents. Synthetic routes to the azacalix[3]arene macrocycles will be discussed from the perspective of their ability to generate the azacalix[3]arenes 1 free from the corresponding azacalix[4]arenes 2. Azacalix[3]arene macrocycle 1a exhibits the ability to bind trivalent metal ions and form complexes where the macrocycle is either a neutral (H3L) or a trianionic (L3-) donor. Crystal structures of well-defined yttrium(III) [3: Y(H3L)Cl3] and lanthanum(III) [4: La(L)] complexes are reported. Molecular recognition studies have been performed on the azacalix[3]arenes 1. In contrast with the O-unsubstituted macrocycles (1, R= H) which exhibit no detectable binding of alkali or ammonium ions, the O-methylated macrocycle 1j is observed to extract alkali metal and alkylammonium picrates.