Review of the Extraction Behavior of Metal Cations with Calixarene Derivatives

Introduction Calixarenes, classified as metacyclophanes and prepared by the condensation reaction of p-alkylphenol with formaldehyde, are also macrocyclic oligomers. They consist of units of phenols and crosslinked methylenes, the cavity size being different by unit number (n) as shown in Fig.1. After a single step large scale synthesis was established in 1981 by Gutsche et al. [1], calixarenes attracted much attention by many chemists. In order to improve the functionality of the extractants, chemical modifications are necessary. There are two features to modify calixarenes as follows [2]; 1. They possess phenolic hydroxyl groups which permit easy introduction of functional groups. 2. Aromatic substitution reactions are facilitated by dealkylation at the p-position. Consequently, both chemical modifications at the lower (OH side) and upper (alkyl branch side) rims of calixarene are possible. The calixarene framework as a “platform” provides the space to accept metal ions. The remarkable features of discrimination for appropriately modified calixarenes are listed as follows; 3. They possess different sized cavities consisting of the units of phenol and methylene. 4. They provide two frames of different rim size at their upper and lower sides. 5. They can be fixed at different conformations by introducing functional groups of suitable length. 6. They show symmetry. 7. They provide aggregation of multifunctionality. 8. They may provide allosteric effects. R