Crown ethers as solvent extraction reagents: where do we stand?

It is perhaps prophetic that among the first applications of crown ethers was their use as solvent extraction reagents. In 1962 the compound 2,3,11,12-dibenzo- 1,4,7,10,13,16-hexaoxacycloocta-2, 11-diene, now mercifully called dibenzo-18-crown-6, was identified and was found to form alkali-metal-salt adducts that were soluble in organic solvents. These serendipitously discovered extraction properties of crown ethers were later used to compare the complex-formation properties of metal ions with crown ethers, and it was observed that salts of organophilic anions were more readily transferred to aprotic solvents. Picrate salts were extensively used in this role. However, those seeking analytical or process applications of the unusual complexing abilities of macrocycles needed the aqueous anion to be one of the more common mineral acid anions such as chloride, nitrate, or sulfate. This problem was circumvented in the late 1970's when it was discovered that crown ethers mixed with an organophilic cation exchanger such as alkyl phosphoric, alkyl-aryl sulfonic, or carboxylic acid, would produce synergistic extraction systems having excellent and selective affinities for a large number of metal ions. In 1981, macrocyclic compounds with attached carboxylic groups were synthesized and found to be excellent extractants. In addition to liquid-liquid extraction, much work has been done using macrocycles as separation agents in membranes and resin beads. Fundamental research is now beginning to provide the stoichiometries and structures of some of the organic-phase complexes produced and an understanding of the reactions in these extraction systems. At the same time, some potential uses of these systems are beginning to be identified