Water-induced phase transformation of a CuII coordination framework with pyridine-2,5-dicarboxylate and di-2-pyridyl ketone: synchrotron radiation analysis

Abstract

Phase transformations in solid coordination frameworks (SCFs) are of interest for several applications, and this work reports a crystal-to-crystal transformation found for hydrogen-bonded Cu^II-based solid coordination frameworks (SCFs). Thus, the combination of PDC and (py)₂C(OH)₂ ligands, where PDC is pyridine-2,5-dicarboxylate and (py)₂C(OH)₂ is the derivative gem-diol of di-2-pyridyl ketone ((py)₂CO), produces [Cu(PDC)((py)₂C(OH)₂)(H₂O)] (1). Compound 1 transforms into [Cu(PDC)((py)₂C(OH)₂)] (2) by thermally-induced dehydration. Characterization of both compounds has been carried out by means of IR spectroscopy, single crystal and powdered sample X-ray diffraction (XRD) through conventional and synchrotron radiation, thermogravimetry (TG), X-ray thermodiffractometry (TDX), and scanning electron microscopy (SEM). Since the molecules of water in 1 are coordinated to the metal ions, their removal induces local distortions on the coordination sphere (square pyramidal for 1 and square planar for 2), which extend through the whole framework, affecting the hydrogen bond system and the packing. In fact, the wavy nature of the planes in 1 becomes sharper in 2, producing an oscillation of the framework, i.e., from open (1) to close (2) waves. The crystal-to-crystal transformation is reversible (1 ↔ 2) and hysteresis has been observed associated with it. Quantum-mechanical calculations based on density functional theory (DFT) show that the 1 ↔ 2 structural rearrangement involves a high amount of energy, which means that the role of the coordinated molecule of water exceeds the mere formation of hydrogen bonds.