Structure of a Porous Cu₂(pzdc)₂(bpp) (pzdc: Pyrazine-2,3-dicarboxylate, bpp: 1,3-Bis(4-pyridyl)propane) Coordination Polymer and Flexibility upon Concomitant Hysteretic CO₂ Adsorption

A porous coordination polymer was prepared using pyrazine-2,3-dicarboxylate (pzdc) and a dipyridyl ligand that contains −CH 2 CH 2 CH 2 – as a spacer, namely, 1,3-bis(4-pyridyl)propane (bpp). The material was thoroughly tested for determination of textural and adsorption properties in an attempt to elucidate structural flexibility in the absence of a gate opening pressure phenomenon. The periodic framework (monoclinic, P 2 1 / c, a = 13.300(3), b = 13.112(7), c = 10.808(5) Å, β = 101.28(4)°) composition is Cu 2 (pzdc) 2 (bpp))·4H 2 O, with an overall structural arrangement similar to those present in other two-dimensional copper + pzdc based coordination polymers, but showcasing a heavily distorted, parallelogram-shaped gallery along c . The structure appears stable up to 510 K based on thermogravimetric analysis and in situ high temperature X-ray diffraction data; complete elimination of water takes place at 373 K. Upon activation, the material effective surface area and pore volume are much smaller than those of other Cu 2 (pzdc) 2 structures, probably because of the constricted void space. Still, a corrected Horvath–Kawazoe pore size distribution analysis method points to a 4.2 Å average pore size. Uptake of CO 2 at 194.5 K revealed a hysteretic adsorption–desorption phenomenon, probably due to a concomitant pore width expansion process that is analogous to the one reported for Cu 2 (pzdc) 2 (bpy) (bpy: 4,4′-bipyridine). The phenomenon is amplified at 298 K and remains at pressures up to 50 atm of CO 2 . Furthermore, it appears that the dynamics of structural changes are slower compared to those of the adsorption process; equilibrium seems to take place at an equilibration time interval of at least 120 s. In contrast with isostructural Cu 2 (pzdc) 2 (dpyg) (dpyg: 1,2-di(4-pyridyl)-glycol), Cu 2 (pzdc) 2 (bpp) appears to have a stronger interaction with CO 2 as evidenced by the isosteric heats of adsorption profiles and probably due to the flexibility introduced by the bpp ligand.