Deep eutectic solvents for the preparation and post-synthetic modification of metal- and covalent organic frameworks

Abstract

Deep eutectic solvents (DESs) represent an emerging class of solvents featuring some characteristics of their ionic liquid cousins – low vapor pressure, relatively wide liquid range, non-flammability and the ability to dissolve polar species – along with unique specificities, such as their limited toxicity and an improved biocompatibility. Among the numerous applications these solvents may find, their use as media for the ionothermal preparation and post-synthetic modification of metal- and covalent organic frameworks (MOFs and COFs) has been recently explored. This contribution aims at providing an overview of the state of the art in this area focusing on the synthetic aspects as well as on the applications of the materials and composites reported. In particular, the different roles DESs can play in the MOF/COF formation are highlighted. Indeed, while DESs act as solvents, they may also behave as structure-directing agents. Thus, one of their components or decomposition products formed under the ionothermal conditions may be present in the architectures, either as a ligand coordinated to the metal centers or in the pores of the MOFs, yielding unknown crystalline systems. Removal of the DES components from the MOF has been shown as an efficient strategy for the preparation of novel porous architectures. Furthermore, DESs can be employed for the conversion of one MOF into another. DESs show clear advantages as media for COF synthesis, such as the possibility to run reactions at lower temperature, in open-air vessels, with shortened reaction time or to elaborate COFs otherwise difficult to access and with a large variety of linkages. Beyond their solvent role, DESs can also be employed for post-synthetic modification, affording composites or hybrids benefiting from the properties of these media combined with the ones of the porous materials. Applications in catalysis for a diversity of reactions, in gas sorption or for the sensing/extraction of various analytes have been demonstrated.