Azacalix[7]arene Heptamethyl Ether: Preparation, Nanochannel Crystal Structure, and Selective Adsorption of Carbon Dioxide

Abstract

To investigate the solid-state complexation of nitrogen-bridged calixarene analogues, azacalix[7]arene heptamethyl ether 1 has been prepared by applying a "5 + 2"-fragment coupling approach using Buchwald-Hartwig aryl amination reaction aided by our previously devised temporal N-silylation protocol. X-ray crystallographic analysis revealed that azacalix[7]arene 1 adopted a highly distorted 1,2-alternate conformation in the solid state as a result of intramolecular NH/O hydrogen bonding interactions and steric repulsion between the methoxy groups. In the crystal, molecules of 1 are mutually interacted by intermolecular NH/O and CH/pi interactions to establish one-dimensional (1D) hexane-filled nanochannel crystal architecture. Similarly to our recently reported azacalix[6]arene 2, the desolvated crystalline powder material of 1 was capable of selectively and rapidly adsorbing CO2 among the four main components of the atmosphere. The adsorption capacity of 1 for CO2 nearly doubled as compared to that of 2 because of the formation of the 1D nanochannel with almost twice the volume of the latter.