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

Abstract

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.