Ligand Design for Small Cations: The Li⁺/14-Crown-4 System

This review describes progress in the authors' research toward understanding factors related to the design of ligands for the solvent extraction of lithium. Recent work has demonstrated that 14-crown-4 ethers can be effective extractants for the separation of lithium from mixtures of alkali metal salts. It has also been shown that substituents on the macrocyclic ring of 14-crown-4 ethers can have a large effect on both extraction efficiency (i.e. strength) and selectivity (i.e. discrimination or recognition). The calculated strain induced in the ligand by the conformational reorganization of the ligand upon cation complexation correctly predicts the ordering of lithium extraction efficiencies in a series of substituted 14-crown-4 ethers. More detailed extraction experiments have revealed the component equilibrium processes underlying the overall extraction. A profound dependence of extraction efficiency on the nature of the diluent has been noted and related largely to the solvation of the coextracted anion. A thermochemical scheme has been proposed to rationalize the effect of ligand conformational strain on the net extraction process within the context of other terms sensitive to ligand structure and solvation.