Proton-Ionizable Crown Ethers. A Short Review

Abstract

Abstract : Since Pederson reported the synthesis and cation-complexing characteristics of the crown ethers in 1967, there has been increasing interest in these compounds as complexing agents for various metal and organic cations. Many different modifications of the crowns have been made to enhance their cation-complexing properties. Some of these modifications involve the use of alkyl substituents, aromatic subcyclic units, nitrogen and/or sulfur atoms substituted for oxygen in the macroring and other changes which provide crowns with unique complexing properties. Macrocyclic crown ether ligands containing proton-ionizable groups are of interest because they alleviate the need for a counter anion in metal cation transport through a liquid membrane or in solvent extraction experiments. Often the cation-crown complex stability is increased when the crown is ionized. Transport of cations is, in many cases, also tied to the pH of the aqueous solutions in the liquid membrane system so that transport of cations can, in effect, be turned on and off by adjusting the pH. Proton- ionizable crown ligands have also been used spectrophotometrically to determine parts per million quantities of alkaline earth metal cations in solution. Having macrocycles which show selectivity in complexation and transport of cations is also an objective in the syntheses of these types of crown ligands. This short review summarizes the syntheses of proton-ionizable crown ether ligands (OICE) to the end of 1988, gives a listing of these compounds and briefly reports on some of their uses.