Highly selective and efficient extraction of two Pb2+ ions with a p-tert-butylcalix[6]arene hexacarboxylic acid ligand: an allosteric effect in extraction

Solvent extraction behavior of p-tert-butylcalix[6]arene hexacarboxylic acid towards Pb2+ and other divalent transition metal ions has been studied to investigate the selectivity, extraction mechanism, stoichiometry of complexation and composition of the complex in the organic phase. Extractability and selectivity of the cyclic ligand were analyzed in comparison with the acyclic analog. The macrocyclic effect was genuinely operative and the size-match effect was the governing factor for cation selectivity. Accordingly, the cyclic ligand selectively extracted the Pb2+ ion in a pH range fairly more acidic than the acyclic ligand. The cations were extracted by ion exchange mechanism, and the electroneutral complexes were resulted by the exchange of two protons for a divalent cation. A molecule of p-tert-butyl calix[6]arene hexacarboxylic acid ligand extracted two Pb2+ ions. One Pb2+ was extracted due to the size-fit effect inside the hydrophilic cavity of calix[6]arene composed by phenoxy and carbonyl oxygen atoms. The macrocyclic ring inversion was obstructed due to guest encapsulation, and the encapsulated Pb2+ ion acted as a template for aggregation of the carboxyl groups. This binding event caused a positive allosteric effect and led to the extraction of the second Pb2+ ion at the aggregated functional group site.