Three-dimensional robust porous coordination polymer with schiff base site on the pore wall: synthesis, single-crystal-to-single-crystal reversibility, and selective CO₂ adsorption

Abstract

A three-dimensional (3D) robust porous coordination polymer, {[Cu(azpy)(glut)](H₂O)₂}_n (1) (azpy = N,N′-bis-pyridin-4-ylmethylene-hydrazine, glut = glutarate), has been synthesized and structurally characterized. Single crystal X-ray diffraction analysis reveals that each of paddle-wheel Cu₂(CO₂)₄ units is connected with glutarates in the crystallographic bc plane to form a two-dimensional (2D) sheet which is pillared by an azpy linker to afford a 3D porous framework. Controlled heating of the as-synthesized crystal 1 at ∼150 °C under reduced pressure causes a color change of {[Cu(azpy)(glut)]}_n (1a) from deep green to light green. The structure determination of the dehydrated compound shows the same framework structure as that of {[Cu(azpy)(glut)](H₂O)₂}_n (1) with the only difference of the nonexistence of lattice water molecules resulting in a large void in the framework. The dehydrated light green crystal 1a regenerates the virgin as-synthesized crystal 1 with the formula of {[Cu(azpy)(glut)](H₂O)₂}_n upon exposure to water vapor (1a), suggesting complete reversibility upon dehydration and rehydration. Dehydrated compound 1a shows remarkable CO₂ selectivity over N₂, a typical type I profile with MeOH and EtOH, and gated adsorption behavior with H₂O. The selectivity in gas uptake and interesting vapor adsorption profiles was correlated with the polarity of the pore surface in 1a to the corresponding adsorbates molecules.