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.