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)](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.