Dual Cation/Anion Binding in Crown Ether-Based Coordination Cages

A series of flexible bidentate pyridyl ligands (L^1-10), comprising crown ether-based backbones with ring sizes between 15 and 30 atoms, was synthesized. Depending on their size, geometry, and flexibility, palladium-mediated self-assembly yielded mononuclear PdL₂ chelate complexes or lantern-shaped Pd₂L₄ coordination cages, able to bind guests in their macrocyclic and/or self-assembled cavities. Cage self-assembly, as well as the interdependent binding of cationic and anionic guests, was monitored by NMR and mass spectrometry, and five cages were characterized by X-ray crystallography. In the case of cage Pd₂L⁹₄, featuring a dibenzo-24-crown-8 backbone, pseudorotaxanation with cationic guest dibenzylammonium (DBA) was found to control both cage formation and anionic guest binding. Guest displacement and cage-to-cage transfer experiments revealed an allosteric enhancement of the cage's affinity toward guest naphthalene-1,5-disulfonate through DBA pseudorotaxanation, supported by theoretical calculations. Integrating crown ether motifs into the walls of transition metal-based coordination cages opens potential for the development of stimuli-responsive catalysts, selective multitopic receptors, and ion-conducting materials.