How Does the Fluorination of the Linker Affect the Stability of Trimesate-Based Coordination Polymers and...

The syntheses and crystal structures of the monopotassium salts of difluorinated and tri/perfluorinated trimesic acid (1,3,5-benzenetricarboxylic acid ≡ H₃BTC) are presented, namely, K(H₂ dF-BTC) ( Fdd2, Z = 16) and K(H₂ pF-BTC) ( Cc, Z = 4). For the first time, together with already known K(H₂ mF-BTC), all fluorination degrees of trimesic acid are accessible and can be used for a systematic study of the influence of fluorination on the stability of the resulting coordination polymers and metal-organic frameworks (MOFs). The monopotassium salts show a decreasing (chemical) stability in water upon heating, as well as a decreasing thermal stability, as evidenced by differential scanning calorimetry/thermogravimetric analysis (DSC/TGA). A similar decreasing thermal stability is found for two series of isostructural coordination polymers (UHM-33 topology: _∞²[Cu₂(L)₂(DMA)₂]·2DMA with L^2- = H mF-BTC^2- and H dF-BTC^2-) and MOFs (_∞³[Ba(L)(H₂O)₂]·¹/₂H₂O with L^2- = HBTC^2-, H mF-BTC^2- and H dF-BTC^2-). Remarkably, while the decomposition temperatures decrease with increasing fluorination of the linker, the releasing temperatures for embedded solvent molecules (DMA and H₂O, respectively) increase. To identify possible candidates for the synthesis of isostructural coordination polymers and MOFs with BTC^3- ligands with different degrees of fluorination, a database-adapted approach was developed, which utilizes the increased torsion angle between the carboxylate groups and the phenyl rings in these materials as a structure-determining parameter.