A Temporarily Pore-Openable Porous Coordination Polymer for Guest Adsorption/Desorption

Flexible porous coordination polymers (PCPs)/metal-organic frameworks are unique materials that have potential applications as components of highly efficient separation, sensor, and actuator systems. In general, the structures of flexible PCPs drastically change upon guest loading. In this investigation, we uncovered the rare one-dimensional PCP [Cu₂(bza)₄(2-apyr)] (1; bza = benzoate and 2-apyr = 2-aminopyrimidine), which exhibits a unique type of flexibility involving temporary pore opening. Single-crystal X-ray diffraction analysis revealed that desolvated 1 and ethyl acetate (AcOEt)-loaded (1·AcOEt) and CO₂-loaded (1·CO₂) 1 have isolated pores. In the case of 1, the pore structure prevents guest penetration. In addition, the isolated pore structures of 1·AcOEt and 1·CO₂ block guest release. However, 1 participates in reversible adsorption/desorption of AcOEt and CO₂ because pore opening occurs temporarily. The CO₂ adsorption/desorption isotherms of 1 are type I and dissimilar to those observed in traditional flexible PCPs with adsorption/desorption hysteresis. The lesser conventional flexibility displayed by 1 could offer new insight into the design of flexible PCPs.