Chirality technology in metal separation: Stereochemical design of Ag+ ion-specific ionophores for practical membrane separation

New approaches to the development of specific ionophores for use in membran e separation are reviewed: design of ligand geometry and optimization of mo lecular chirality. A new series of Ag+ ion-specific ionophores was successf ully prepared so that three pyridine donors cooperatively bind a guest Agion in a tridentate fashion. The tridentate ligand geometry offered a perfe ct Ag+ ion selectivity in binding, extraction, and transport processes. Fur thermore, the stereocontrolled substitution of two chiral centers close to the pyridine binding sites enhanced Ag+ ion binding ability of the podand-t ype ionophores. Because the present type of Ag+ ion-specific podands mediat ed membrane transport of an Ag+ ion with practical selectivity and efficien cy, such a stereochemical approach can be viewed as a new means to create m etal ion-specific ionophores of the podand type.