New Multidentate Ligand Carriers for Macrocycle-Facilitated Metal Ion Transport Across Liquid Membranes

Abstract

To probe the influence of structural variation within macrocyclic multidentate compounds upon their ability to function as carriers for macrocycle-facilitated, metal ion transport across liquid membranes, a variety of new crown and lariat ether componds have been evaluated in a bulk liquid membrane system. For crown ethers substituted with one or more linear alkyl groups, the effect of varying the number and attachment sites for the lipophilic groups and replacement of one or more ring oxygen atoms with sulfur or nitrogen atoms has been investigated. For lariat ethers with dibenzo-16-crown-5 rings, the influence of amide-and thioamide-containing side arms has been assessed.In the search for improved metal ion separation schemes, considerable attention has been focused upon the use of liquid membranes (7,2). In a liquid membrane system, a liquid or quasi-liquid phase separates two other liquid phases in which the membrane is immiscible. In the most common arrangement, a hydrophobic liquid phase, such as chloroform or toluene, separates two aqueous phases. If chemical species have some solubility in the membrane, they may pass from one aqueous phase through the membrane into the second aqueous phase by simple diffusion. More frequently, a carrier molecule which resides in the membrane provides carrierfacilitated transport of metal ions across the membrane. Compared with simple diffusion, the carrier-facilitated transport is usually more efficient and selective.Due to their ability to form complexes selectively with metal cations and thereby solubilize such cations in liquid media of low dielectric constant, macrocyclic multidentate ligands have been widely investigated as carriers in metal ion transport processes for the past two decades (3-9). Such macrocycles include crown ethers, lariat ethers (crown ethers with a side arm which contains potential binding sites) (10), calixarenes, and some cryptands. Studies of structural variations within a given 0097-6156/%/0642-O155$15.00/0