Fluoroarene Separations in Metal–Organic Frameworks with Two Proximal Mg2+ Coordination Sites

Fluoroarenes are widely used in medicinal, agricultural, and materials chemistry, and yet their production remains a critical challenge in organic synthesis. Indeed, the nearly identical physical properties of these vital building blocks hinders their purification by traditional methods, such as flash chromatography or distillation. As a result, the Balz–Schiemann reaction is currently employed to prepare fluoro­arenes instead of more atom-economical C–H fluorination reactions, which produce inseparable mixtures of regioisomers. Herein, we propose an alternative solution to this problem: the purification of mixtures of fluoro­arenes using metal–organic frameworks (MOFs). Specifically, we demonstrate that controlling the interaction of fluoro­arenes with adjacent coordinatively unsaturated Mg2+ centers within a MOF enables the separation of fluoro­arene mixtures with unparalleled selectivities. Liquid-phase multicomponent equilibrium adsorption data and breakthrough measurements coupled with van der Waals-corrected density functional theory calculations reveal that the materials Mg2(dobdc) (dobdc4– = 2,5-dioxidobenzene-1,4-dicarboxylate) and Mg2(m-dobdc) (m-dobdc4– = 2,4-dioxidobenzene-1,5-dicarboxylate) are capable of separating the difluoro­benzene isomers from one another. Additionally, these frameworks facilitate the separations of fluoro­anisoles, fluoro­toluenes, and fluoro­chlorobenzenes. In addition to enabling currently unfeasible separations for the production of fluoro­arenes, our results suggest that carefully controlling the interaction of isomers with not one but two strong binding sites within a MOF provides a general strategy for achieving challenging liquid-phase separations.