Ionothermal synthesis of calcium-based metal-organic frameworks in a deep eutectic solvent

The unprecedented use of a deep eutectic solvent (DES) for the ionothermal synthesis of Ca(ii) based metal-organic frameworks (MOFs) has been explored. The 1 : 2 choline chloride : e-urea DES (e-urea = 2-imidazolidinone, ethylene urea) has been successfully employed for the preparation of Ca-MOFs with a series of dicarboxylic acid ligands. These materials have been structurally characterized by single-crystal and powder X-ray diffraction techniques as well as by thermo-gravimetric analysis and infrared, UV-visible and emission spectroscopy techniques in the crystalline state. The MOFs obtained have been found to be three-dimensional with the presence of channels occupied by coordinated e-urea solvent molecules. While the latter solvent acts as a bridging ligand in the case of terephthalate and isophthalate based MOFs, it behaves as a terminal ligand in the MOFs isolated with 2,5-dihydroxyterephthalate (dobdcH(2)(2-)). With this ligand, two phases differing in the coordination mode of the dobdcH(2)(2-) anion have been obtained. Interestingly, for one of them formulated as [Ca(dobdcH(2))(e-urea)(2)], the crystals rapidly lost their transparency and luster upon exposure to air. This phenomenon could be rationalized by X-ray diffraction to result from the uptake of water molecules from the ambient atmosphere leading to the replacement of one e-urea molecule by H2O in the Ca(ii) cation coordination sphere. These results demonstrate that the 1 : 2 choline chloride : e-urea DES can be considered as an effective solvent for the synthesis of water-sensitive Ca-MOFs.

Automated summary

The study investigates the use of a deep eutectic solvent for the ionothermal synthesis of Ca(ii) based metal-organic frameworks, successfully synthesizing a series of Ca-MOFs with different dicarboxylic acid ligands. The structural characterization and solvent coordination analysis of these materials were carried out, showing potential for the deep eutectic solvent to be an effective solvent for the synthesis of water-sensitive Ca-MOFs.