Triggered Exchange of Anionic for Neutral Guests inside a Cationic Coordination Cage

Molecular encapsulation processes
under the control of an external
trigger play a major role in biological signal transduction processes
and enzyme catalysis. Here, we present an artificial mimic of a controllable
host system that forms via self-assembly from a simple bis-monodentate
ligand and Pd­(II) cations. The resulting interpenetrated double cage
features three consecutive pockets which initially contain one tetrafluoroborate
anion, each. Activation of this host system with two halide anions
triggers a conformational change that renders the central pocket susceptible
to the uptake of small neutral guest molecules. Thereby, the pentacationic
cage expels the central anion and replaces it with a neutral molecule
to give a hexacationic species. The cage structures prior and after
the halide triggered binding of benzene were examined by X-ray crystallography,
ESI MS, and NMR techniques. The kinetics and thermodynamics of the
encapsulation of benzene, cyclohexane, and norbornadiene are compared.