Does Fluorine Make a Difference? How Fluoro Groups Influence the (Structural) Properties of MOFs

Abstract

Herein, the influence of fluorine substituents on the structural and physical properties of MOFs with aromatic carboxylate ligands is investigated. To achieve this, the focus is limited to a few widely used ligand classes, e. g. terephthalates or trimesates. It is shown that the torsion angle between the phenyl ring and the carboxylate groups plays a decisive role. This can also be transferred to more complex ligands. These considerations clearly show why there are isostructural variants of some MOFs with perfluorinated ligands known (e. g. MIL-53), while for others no successful synthesis has been reported up to now (e. g. HKUST-1). In order to investigate the influence of the fluorination of the linker on the properties of the corresponding MOFs, isostructural systems were analysed more specifically. It was found that the thermal and chemical stability decreases with fluorination, while the absorption of CO₂ in particular is enhanced. With regard to the specific surface area (S_BET), the results are contradictory.