Systematic assessment of the structural impact of linker fluorination in CeIV-based UiO-66

Incorporating fluorine atoms in the organic linker is a widely adopted strategy to tune the gas adsorption properties of Metal-Organic Frameworks (MOFs). However, identifying a consistent trend in how fluorine atoms modulate the structure and host-guest interactions within porous frameworks remains an open challenge. Here, we report a systematic investigation of how stepwise fluorination of 1,4-benzenedicarboxylic acid linkers (Fx-H2BDC) influences the structural features of the CeIV-based UiO-66 architecture. The complete series of fluorinated Fx_UiO-66(Ce) materials (where x = 0; 1; 2; 3; 4) was synthesised using acetic acid as a crystallisation modulator in a mixed DMF:H₂O solvent system and fully characterised. 1H and 19F liquid-state NMR spectroscopy of digested samples, supported by thermogravimetric analysis, confirmed that all fluorinated derivatives are defect-free, whereas the non-fluorinated analogue contains a small amount of missing-linker defects. N2 adsorption measurements revealed a decreasing trend in BET surface areas with increasing fluorination, likely due to steric effects and reduced pore accessibility. Rietveld refinements of high-resolution synchrotron powder X-ray diffraction data indicate a gradual expansion of the unit cell as the degree of fluorination increased, along with the elongation of the CeIV–Ocarboxylate bonds, while the aromatic rings are progressively twisted out of the plane of the carboxylates. Notably, the presence of a water molecule coordinated to CeIV was observed in all fluorinated frameworks, suggesting enhanced Lewis acidity of CeIV sites, induced by the electron-withdrawing nature of the fluorinated linkers.