Enhancing Lithium Binding Affinity, Exchange Rate, and Hard–Soft Selectivity of Catenane Ligands by Post-Interlocking Modification

Abstract

A Li+-selective catenane host featuring a small, tetrahedral binding cavity lined with pyridine N-oxide motifs was synthesized through post-interlocking modification of a readily accessible bipyridine-derived catenane. The incorporation of N-oxide groups enhances binding strength and hard–soft selectivity by optimizing the hard–soft acid–base interactions with the hard Li+ ion, while the mechanically constrained catenane results in partially unsaturated coordination sites for Na+ and K+, further favoring Li+ binding. Exchange kinetics of Li+ binding are also accelerated with more rapid association/dissociation dynamics. The strong and dynamic Li+ binding enables the catenane to selectively extract the alkali metal from a solid mixture of alkali metal chlorides. Overall, the design underscores the potential of mechanically interlocked molecules with tailored cavities for selective alkali metal ion separation.