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

A series of flexible bidentate pyridyl ligands (L1–10), 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.