Functionalization in Flexible Porous Solids: Effects on the Pore Opening and the Host−Guest Interactions

The synthesis on the gram scale and characterization of a series of flexible functionalized iron terephthalate MIL-53(Fe) type solids are reported. Chemical groups of various polarities, hydrophilicities, and acidities (−Cl, −Br, −CF₃, −CH₃, −NH₂, −OH, −CO₂H) were introduced through the aromatic linker, to systematically modify the pore surface. X-ray powder diffraction (XRPD), molecular simulations, thermogravimetric analyses, and in situ IR and ⁵⁷Fe Mössbauer spectrometries indicate some similarities with the pristine MIL-53(Fe) solid, with the adoption of the narrow pore form for all solids in both the hydrated and dry forms. Combined XRPD and computational structure determinations allow concluding that the geometry of the pore opening is predominantly correlated with the intraframework interactions rather than the steric hindrance of the substituent. Only (MIL-53(Fe)-(CF₃)₂) exhibits a nitrogen accessible porosity (S_BET ≈ 100 m² g⁻¹). The adsorption of some liquids leads to pore openings showing some very specific behaviors depending on the guest−MIL-53(Fe) framework interactions, which can be related to the energy difference between the narrow and large pore forms evaluated by molecular simulation.