Thermally Induced Changes in a Porous Coordination Polymer {Cu2(pyrazine-2,3-dicarboxylate)2(4,4′-bipyridine)} Studied via in Situ X-ray Diffraction and 13C Cross-Polarization Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy

A nonreversible thermally induced pore contraction has been observed in the coordination polymer {Cu-2(pzdc)(2)(bpy)} (pzdc = 2,3-pyrazinedicarboxylate; bpy = 4,4'-bipyridine), known as CPL-2. In an effort to understand this phenomenon, we have employed coupled differential scanning calorimetry and X-ray diffraction (DSC-XRD) and in situ C-13 cross-polarization magic angle spinning nuclear magnetic resonance (CP MAS NMR) measurements. DSC data showed no energetic transitions that would suggest a structural rearrangement of the crystalline sorbent, while small changes in the corresponding XRD patterns were probably related to the structural breathing mode upon the removal of water molecules. On the other hand, in situ high-temperature C-13 CP MAS NMR evidenced displacement of pzdc carbons upon an increase in temperature. Moreover, bpy pillar ligand resonance signals were only observed for samples pretreated at 423 K, both ex situ and in situ, probably indicating nonreversible local-range changes in the 1D channels of CPL-2. This temperature is well below the decomposition point of the framework, evidencing how sensible the material is to thermal preactivation. Careful analysis of the implications of employing thermal treatments for coordination polymers is crucial to elucidate the potential of these materials for adsorption and catalysis applications.