How Does the Fluorination of the Linker Affect the Structural Chemistry of Trimesate-Based Metal-Organic Frameworks (MOFs)?

FMOFs, i.e. metal-organic frameworks with linkers with fluoro substituents, were supposed to show enhanced thermal and chemical stability as well as high gas affinity and hydrophobicity. However, at least for aromatic carboxylate ligands it was shown in a subsequent work that fluoro substituents weaken the C(phenyl)-COO^– bond and thus facilitate decarboxylation leading to a decreased chemical and thermal stability. Nonetheless, it was concluded that linker fluorination leads to a rich structural chemistry, as the torsion angle between the phenyl ring and the carboxylate group is significantly increased in these compounds. Here, we present the very first examples of four MOFs all based on Sr²⁺ cations and trimesate-based linkers with three different degrees of fluorination as well as the known non-fluorinated counterpart: Sr(HL)(H₂O)] · n H₂O 1 : L = mF-BTC^3–, n = 0.5, P1 , Z = 2; 2 : L = dF-BTC^3–, n = 0.5, C2/c, Z = 8; 3 : L = pF-BTC^3–, n = 1.5, C2/c, Z = 8; 4 : L = BTC^3–, n = 0.5, P1 , Z = 2; BTC^3– ≡ 1,3,5-benzenetricarboxylate (trimesate); mF-BTC^3– ≡ monofluorinated trimesate, dF-BTC^3– ≡ difluorinated trimesate, pF-BTC^3– ≡ per-(tri-)fluorinated trimesate]. Whereas 1 and known 4 are found to crystallize in isotypic structures and 2 in a very similar structural arrangement all CN(Sr²⁺) = 9], 3 with the highest degree of fluorination exhibits a completely different crystal structure CN(Sr²⁺) = 8], which is already obvious from the different composition. It is shown that the torsion angles between the phenyl ring and the carboxylate groups play an important structure-directing role. DSC/TGA investigations confirm that with increasing fluorination the thermal stability is decreased. However, the release temperature of water, i.e. the affinity to water, increases with the number of fluoro substituents.