Synthesis of tris-(azacrown) ethers for carboxylic acid recognition

The synergistic enhancement of metal ion extraction by azacrown ethers in the presence of carboxylic acids has been attributed to a ligand assembly effect in which these two ligands form a complex, facilitated by proton transfer, prior to complexation of the metal ion. In order to investigate the first steps in this multi-component complexation procedure, six tris-(azacrown) ethers were synthesised in high yields and their ability to complex mono- and tri-carboxylic acids was investigated by 1H NMR in methanol-d4. All six compounds bound to benzoic acid with 1:3 host-guest stoichiometry and four of them bound tricarboxylic acids with 1:1 host-guest stoichiometry, providing good support for the proposed first step in the ligand assembly effect.