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

A three-dimensional (3D) robust porous coordination polymer, {[Cu( azpy )( glut )](H 2 O) 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 Cu 2 (CO 2 ) 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 )](H 2 O) 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 )](H 2 O) 2 } n upon exposure to water vapor ( 1a ), suggesting complete reversibility upon dehydration and rehydration. Dehydrated compound 1a shows remarkable CO 2 selectivity over N 2, a typical type I profile with MeOH and EtOH, and gated adsorption behavior with H 2 O. 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.