Fine-Tuning Apertures of Metal–Organic Cages: Encapsulation of Carbon Dioxide in Solution and Solid State

Metal–organic cages are potential
artificial models for mimicking biological functions due to their
capability of selective encapsulation for certain guest molecules.
In this work, we designed and synthesized a series of rhombic dodecahedral
Ni-imidazolate cages (Ni14L24) with precisely
controlled aperture for CO2 encapsulation. The aperture
of the cages can be tuned by the strategies of ligand decoration and
metal-ion hybridization. Similar to the breathing function of alveoli,
CO2 passes through the dynamic aperture into the cages
under a pressure of 2.0–3.0 bar in methanol solution, and slowly
move out of the cages when the pressure goes down. In the solid state,
CO2 is encapsulated and prisoned in the cages under a high
pressure of 15.0–30.0 bar or supercritical conditions. By replacing
the square-coordinated Ni2+ with Cu2+, the resulting
Ni–Cu heteronuclear cage lost the capability of physically
encapsulating CO2 even though the aperture’s size
is only slightly changed.