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Isolable crystals of the cesium salt of an anionic Jahn-Teller-active Cu complex, initially formed by reaction in solution, are spontaneously consumed in a solvent-mediated crystal-to-crystal transformation that produces a final product with a four-coordinate Cu center. Depending on the size of the crystals of the intermediate and the evaporation rate of the solvent, the transformation from intermediate to final product occurs in a two-week time frame. The crystalline Jahn-Teller Cu intermediate presents a noteworthy difference in stability compared to its non-Jahn-Teller Ni-centered isomorph. It is proposed that a Jahn-Teller intermediate may precede the formation of other four-coordinate Cu complexes., This work was supported by the Spanish Ministerio de Ciencia e Innovación (Grant PGC2018–093451-B-I00), the European Union Regional Development Fund, (FEDER), and the Diputación General de Aragón, Project M4, E11_20R. The authors acknowledge the use of the Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza.

The combination of strong coordination bonds and hydrogen bonding interactions were used to generate a series of supramolecular coordination materials (SCMs), which was achieved by reacting a bis-pyridyl amide ligand, namely N-(4-pyridyl)nicotinamide (4PNA) with copper(II), zinc(II), and cadmium(II) benzoates. The SCMs were structurally characterized using X-ray diffraction and the key intermolecular interactions were identified via Hirshfeld surface analysis. The role of solvent molecules on the supramolecular architecture was analyzed by synthesizing the SCMs in different solvents/solvent mixtures. A solvent-mediated solid-state structural transformation was observed in copper(II) SCMs and we were able to isolate the intermediate form of the crystal-to-crystal transformation process. The luminescence experiments revealed that complexation enhanced the fluorescence properties of 4PNA in the zinc(II) and cadmium(II) SCMs, but a reverse phenomenon was observed in the copper(II) SCMs. This work demonstrated the tuning of supramolecular assembly in coordination compounds as a function of solvents for generating SCMs with diverse properties.

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. Ministerio de Economía y Competitividad(MAT2013-42092-R), Gobierno Vasco (Basque University System Research Group, IT-630-13) and UPV/EHU (UFI 11/15) and Ministerio de Ciencia e Innovación (BES-2011-045781).

The chain polymer [{Cd(μ-X)2py2}1∞] (X = Cl, Br, py = pyridine) undergoes a fully reversible phase transition between a monoclinic low-temperature and an orthorhombic high-temperature phase. The transformation can be directly monitored in single crystals and can be confirmed for the bulk by powder diffraction. The transition temperature can be adjusted by tuning the composition of the mixed-halide phase: Transition temperatures between 175 K up to the decomposition of the material at ca. 350 K are accessible. Elemental analysis, ion chromatography and site occupancy refinements from single-crystal X-ray diffraction agree with respect to the stoichiometric composition of the samples.