Urothermal synthesis of metal-organic frameworks

While ionothermal synthesis using deep eutectic solvents based on the combination of choline chloride and urea derivatives has been widely explored for metal-organic framework (MOF) synthesis, the alternative approach consisting in using urea derivatives on their own as solvents, albeit promising, remains comparatively underemployed. This highlight article aims to review the field of urothermal synthesis, covering the state of the art of this approach and its potential development. The use of e-urea (2-imidazolidinone, ethyleneurea), the most extensively employed species in this context, is detailed, showing its ability to play diverse roles in MOF construction. Beyond its role as solvent and soft regulator of solution acidity, it can be present in the pore or as a ligand, most commonly in a bridging mode with divalent metal cations via coordination of the carbonyl group assisted by hydrogen bonding of the NH moieties, or yield ethylenediamine as a decomposition product incorporated in the MOF. Furthermore, urothermal synthesis has demonstrated potential for the preparation of chiral architectures and their enantio-enrichment. Alternatives to e-urea in pure form or as a hemihydrate are also presented. The combination of e-urea with other organic solvents or the use of co-ligands have been shown to modulate its tendency to act as a bridging ligand, while fully N-alkylated urea derivatives represent appealing solvents. They have low melting point or can even be liquid at room temperature, making them media of choice, prone to ligation to the metal center in a terminal fashion given the absence of hydrogen bonding donor, favoring removal towards activation. The structures of the materials reported under urothermal conditions are described as well as their properties and applications. The use of urea derivatives as solvents for the preparation of metal-organic frameworks is reviewed.