Azacalix[6]arene hexamethyl ether: Synthesis, structure, and selective uptake of carbon dioxide in the solid state

To investigate dynamic solid-state complexation hitherto unexplored in nitrogen-bridged calixarene analogues, azacalix[6]arene hexamethyl ether has been prepared in three steps by applying a 5+1 fragment-coupling approach by using a Buchwald-Hartwig aryl amination reaction with the aid of our previously devised temporal N-silylation protocol. X-ray crystallographic analysis and NMR spectroscopic measurements have revealed that the azacalix[6]arene is well endowed with hydrogen-bonding ability, by which both the molecular and crystal structures are controlled. The azacalix[6]arene is conformationally flexible in solution on the NMR time scale, whereas it adopts a definite 1,2,3-alternate conformation with S-2 symmetry in the solid state as a result of intramolecular bifurcated hydrogen-bonding interactions. In the crystal, molecules of the azacalix[6]arene are mutually interacted by intermolecular hydrogen bonds to establish one-dimensional hexane-filled nanochannel crystal architecture. Although the single crystal was broken after desolvation, the resultant polycrystalline powder material was capable of selectively adsorbing CO2 among the four main gaseous components of the atmosphere. In contrast, carbocyclic p-tert-butylcalix[6]arene hexamethyl ether, the crystal structure of which was also elucidated for the first time in the present study, gave rise to almost no uptake of CO2. Additional solid-gas adsorption experiments for another three gases, such as N-2, O-2, and Ar, suggested that quadrupole/induced-dipole interactions and/or hydrogen-bonding interactions played an important role in permitting the observed selective uptake of CO2 by this new azacalix[6]arene in the solid state.