Humidity-Responsive ON/OFF Switching of Gas Inclusion by Using Cooperative Opening/Closing of Heterogeneous Crystalline Cavities in a Peptide NiII Macrocycle.

Humidity-responsive ON/OFF switching of CO2 inclusion, associated with cooperative structural changes of heterogeneous cavities in crystalline peptide NiII macrocycles, 1⋅NO3 , is demonstrated. The structural transformation between an "open form" and a "closed form" was caused by reversible water uptake into one of the cavities. The adsorption behavior of CO2 was studied with and without water vapor, combined with gas-composition analysis by using gas chromatography. The studies revealed that the open form of 1⋅NO3 adsorbs CO2 gas up to 34 g mol-1 , whereas the closed form adsorbs only a negligible amount of CO2 gas. Also, structural studies (X-ray and IR spectroscopies) and investigation of water-content dependence of CO2 adsorption revealed that the switching of gas inclusion occurred abruptly over a narrow range of water-vapor pressure and was caused by the cooperative opening of vacant space to give the open form due to the structural transformation by water uptake.

Selective CO2 gas adsorption in the narrow crystalline cavities of flexible peptide metallo-macrocycles.

Crystalline peptide Ni(ii)-macrocycles (BF4(-) salt) exhibited moderate CO2 gas adsorption (ca. 6-7 CO2 molecules per macrocycle) into very narrow cavities (narrowest part <2 Å), accompanied by the expansion of the cavities. The BF4(-) salt demonstrated selective uptake of CO2 gas in preference to CH4 and N2 gases.