Spontaneous and Selective CO2 Sorption under Ambient Conditions in Seemingly Nonporous Molecular Crystal of Azacalix[5]arene Pentamethyl Ether

Described are the syntheses, crystal structures, and solid−gas adsorption behaviors of azacalix[4]arene tetramethyl ether and azacalix[5]arene pentamethyl ether. While the former compound exhibited no adsorption of four main atmospheric components, the latter selectively and rapidly adsorbed CO2 at ambient temperature and pressure. X-ray crystallographic and potential energy distribution analysis revealed that azacalix[5]arene created an energetically favorable space for CO2 in its seemingly nonporous crystal, leading to the observed selective CO2 uptake under ambient conditions.