Synthesis and metal ion complexation behavior of polycarboxylate 18-crown-6 ethers derived from tartaric acid

The metal ion complexation behavior of four 18-crown-6 ethers derived from (+)- and meso-tartaric acid is examined. Preparations of a meso-crown ether diacid and of a crown ether hexacid from three units of (+)-tartaric acid are described. Acidity constants and stability constants for complexation of metal cations in aqueous solution were determined by potentiometric titration. The complexes are substantially stabilized by favourable electrostatic interactions and are of similar stability to complexes of cryptands and EDTA. The complexation behavior of the series can be rationalized in terms of electrostatic interactions, direct coordination of the cations by at least one carboxylate from the crown ether periphery, and rigidification of the ligands as the anionic charge increases. Distributions of charge influence the relative stability of isomeric complexes. Highly charged polycarboxylate crown ethers are effective, but relatively unselective, cation complexing agents for a range of cations. The complexes are stable to pH 3 and the ligands can be used as simultaneous pH and metal ion buffers.