Systematic Assessment of the Structural Impact of Linker Fluorination in CeIV-Based UiO-66

Abstract

Incorporating fluorine atoms into the organic linker is a widely adopted strategy to tune the properties of Metal–Organic Frameworks (MOFs). However, identifying a trend in how fluorine atoms modulate the structure of porous frameworks remains an unaddressed topic. We report a systematic investigation of how stepwise fluorination of 1,4-benzenedicarboxylic acid linkers (Fx-H₂BDC) influences the structural features of the Ce^IV-based UiO-66 architecture. The complete series of fluorinated Fx_UiO-66(Ce) materials (where x = 0, 1, 2, 3, 4) was synthesized using acetic acid as a crystallization modulator in a mixed DMF:H₂O solvent and fully characterized. ¹H and ¹⁹F liquid-state NMR spectroscopy of digested samples, supported by thermogravimetric analysis, confirmed that all fluorinated derivatives are defect-free, whereas the nonfluorinated analogue contains a small amount of missing-linker defects. N₂ adsorption measurements revealed a decreasing trend in BET surface areas with increasing fluorination. 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 Ce^IV–O_carboxylate bonds, while the aromatic rings are progressively twisted out of the plane of the carboxylates and a water molecule is incorporated into the coordination sphere of Ce^IV.