Cluster anion substitution tunes flexibility and porosity of MIL-53(Sc)-fum conformational isomers isolated by solvent-induced MOF-to-MOF self-assembly

Metal–organic frameworks (MOFs) of the type MIL-53(M)-fum, where M³⁺ cations are connected by fumarate (fum) linkers in the canonical MIL-53 topology, have been investigated as possible adsorbents due to their structural rigidity, high porosity, and biologically endogenous organic linker. Examples reported to date are limited to those comprised of p-block metal ions (e.g., Al³⁺, Ga³⁺, and In³⁺); d-block metal ions (e.g., Fe³⁺ and Sc³⁺) tend to form MIL-88A(M) materials with the fumarate linker. Herein, we describe the solvent-induced self-assembly of MIL-53(Sc)-fum, by immersing MIL-88A(Sc) in water or aqueous solutions of N,N-dimethylformamide (DMF). Single-crystal X-ray diffraction confirms the solvent-selective formation of two different conformational isomers of MIL-53(Sc)-fum in this unusual MOF-to-MOF synthesis, and both are found to exhibit structural breathing, in contrast to the predominantly rigid p-block metal homologues. Furthermore, we show that the extent of flexibility can be tuned by postsynthetic cluster anion substitution. Refluxing MIL-53(Sc)-fum samples in methanol results in 40–50% exchange of μ₂–OH units for μ₂-OCH₃ groups, where the steric bulk holds the MOFs in a slightly more open pore configuration, enabling significant N₂ adsorption at low pressures in contrast to the fully closed, unfunctionalized precursors. This study provides significant insight into MOF-to-MOF self-assembly, conformational isomerism in MOFs, and the effect of both metal ion and cluster anion substitution on structural flexibility, enabling careful tuning of highly porous MOFs from simple components.