Metal Ion Extraction With New Di-ionizable Ligands

This study seeks to evaluate several series of novel di-ionizable multidentate ligands for the separation of metal ions by solvent extraction. The ligands studied included calix[4]arenes, calix[4]arene-crown ethers, and acyclic polyether ligands, each containing either carboxylic acid functionalities or “acidity-tunable” N-(X)sulfonyl carboxamide moieties in which the X-group was varied. Both single species and competitive extractions of metal cations were performed using solutions of the ligands in chloroform.
The study of novel di-ionizable calix[4]arene ligands sought to determine the effect of several structural variations on the ligands’ efficiency to extract lead(II), mercury(II), alkali metal cations, and alkaline earth metal cations from an aqueous phase into chloroform. These structural variations included ligand conformation, upper and lower rim functionalization, addition of a crown-ether ring to the lower rim, and identity of the ionizable groups. The ionizable groups were located either on the upper or lower rim of the calix[4]arene structure. The pH dependence of the ligand extraction efficiency was also determined in order to compare ligand acidities. The results of the study indicate that all of these structural variations have an effect on the efficiency and selectivity with which the ligands extract metal ions.
Novel di-ionizable acyclic polyether ligands were utilized in solvent extraction studies of divalent metal cations. Three series of ligands containing various ionizable groups were used, in which systematic structural variations were made to determine the effect of the end-group identity and placement on ligand performance. The pH dependence of the ligand extraction behavior was investigated to elucidate information about ligand acidity. It was determined from the results of these studies that the these ligands do have a propensity to complex Pb2+ and Hg2+ in single ion extractions, and exhibit selectivity for Ba2+ in competitive extractions of alkaline earth metal cations. The identity and attachment site of the end group, as well as the identity of the ionizable groups, did have an effect on extraction efficiency, selectivity, and acidity of the ligands studied.