Formation Mechanisms and Defect Engineering of Imine-Based Porous Organic Cages

Syntheses of porous organic cages (POCs) represent an important synthetic puzzle in dynamic covalent chemistry-based self-sorting. Improved understanding of the formation mechanisms of POCs can lead to control and rational design of cages with desired functionality. Herein, we explore the formation mechanisms of imine-based POCs using time-resolved electrospray mass spectrometry and electronic structure calculations at the density functional theory and correlated molecular orbital theory levels. We found that the synthesis of the [4 + 6] cycloimine cage CC3-R and the [2 + 3] cycloimine cage CC-pentane both proceed through similar intermediates via a series of consecutive reactions. The proposed reaction mechanisms are supported by electronic structure calculations. On the basis of our observations from both experiments and calculations, we propose a comprehensive method for designing and predicting new POC species. In addition, the observation of stable incomplete cages during CC3-R synthesis inspired us to design intentionally defective cages. These missing-linker-type molecular defects were installed into CC3-R via nonsolvent induced crystallization. The defective CC3-R materials were found to have enhanced CO2 interaction and improved CO2 uptake capacity due to the additional functional groups present within the CC3 crystals.