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.