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

Abstract

A porous coordination polymer was prepared using pyrazine-2,3-dicarboxylate (pzdc) and a dipyridyl ligand that contains −CH2CH2CH2– 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, P21/c, a = 13.300(3), b = 13.112(7), c = 10.808(5) Å, β = 101.28(4)°) composition is Cu2(pzdc)2(bpp))·4H2O, 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 Cu2(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 CO2 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 Cu2(pzdc)2(bpy) (bpy: 4,4′-bipyridine). The phenomenon is amplified at 298 K and remains at pressures up to 50 atm of CO2. 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 Cu2(pzdc)2(dpyg) (dpyg: 1,2-di(4-pyridyl)-glycol), Cu2(pzdc)2(bpp) appears to have a stronger interaction with CO2 as evidenced by the isosteric heats of adsorption profiles and probably due to the flexibility introduced by the bpp ligand.