Water-induced phased 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 on a crystal-to-crystal transformation found for a CuII-based SCF. Thus, 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 loss of water. 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 provokes 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 (2D for 1 and 0D for 2). In fact, the wavy nature of the planes in 1 becomes sharper in 2, producing an oscillation of the framework: i.e., open (1) and close (2) accordion. The crystal-to-crystal transformation is reversible (1↔2) and hysteresis has been observed associated to it. Quantum-mechanical calculations based on the density functional theory (DFT) show that the 1↔2 structural rearrangement involves a high amount of energy, meaning that the role of the coordinated molecule of water exceeds the mere formation of hydrogen bonds.