Optimized Synthesis of Tetrafluoroterephthalic Acid: A Versatile Linking Ligand for the Construction of New Coordination Polymers and Metal-Organic Frameworks

Pure 2,3,5,6-tetrafluoroterephthalic acid (H2tfBDC) is obtained in high yields (95%) by reacting 1,2,4,5-tetrafluorobenzene with a surplus (>2 equiv) of n-butyllithium in tetrahydrofuran (THF) and subsequent carbonation with CO2 without any extensive purification procedure. A single crystal X-ray structure analysis of H2tfBDC (1) confirms former data obtained for a deuterated sample (P1̅, Z = 1). Recrystallization from water/acetone leads to single crystals of H2tfBDC·2H2O (2, P21/c, Z = 2), where an extensive hydrogen bonding network is found. By reacting H2tfBDC with an aqueous ammonia solution, single crystals of (NH4)2tfBDC (3, C2/m, Z = 2) are obtained. 3 is thermally stable up to 250 °C and shows an enhanced solubility in water compared to H2tfBDC. Monosubstituted 2,3,5,6-tetrafluorobenzoic acid (H2tfBC, 4) is obtained by reacting 1,2,4,5-tetrafluorobenzene with stoichiometric amounts (1 equiv) of n-butyllithium in THF. Its crystal structure (Fdd2, Z = 16) shows dimeric units as characteristic structural feature.