Synthesis and Crystal Structure of Two Fe(III) Pyrazole-3,5-Dicarboxylates

A systematic screening of synthesis conditions using the V-shaped linker pyrazole-3,5-dicarboxylic acid (H₃PzDC, C₅H₄N₂O₄) and various iron salts by high-throughput (HT) methods led to the discovery of the new porous metal–organic framework [Fe₃(μ₃-O)(SO₄)(CH₃CO₂)(HPzDC)₂(H₂O)₃]·10.5H₂O, denoted CAU-56as, and the coordination polymer [Fe(HPzDC)(OH)(H₂O)], denoted Fe-CP. The crystal structures were determined by 3D electron diffraction (3DED) and single-crystal X-ray diffraction (SCXRD), respectively, and bulk phase purity was confirmed by refinements against powder X-ray diffraction data. CAU-56as is the first porous Fe(III)-MOF with pyrazole-3,5-dicarboxylate ions and contains the well-known trinuclear inorganic building unit (IBU) in addition to two coligands (SO₄^2– and CH₃CO₂^– ions). Bridging of the trinuclear units through the pyrazole-3,5-dicarboxylate ions leads to the formation of layers which are interconnected through hydrogen bonds to a 3D framework with a modulated 3D pore system. Stirring of CAU-56as in water leads to ligand exchange of acetate ions by one water and one hydroxide molecule each and the composition [Fe₃(μ₃-O)(SO₄)(OH)(HPzDC)₂(H₂O)₄]·8H₂O (CAU-56w). A combination of characterization techniques was used to confirm the composition of the title compounds. Microporosity of CAU-56as and CAU-56w was confirmed by N₂ and H₂O adsorption measurements at 77 and 298 K, respectively. Specific surface areas of a_s,BET = 727 and 699 m²/g and water uptake of 264 and 300 mg/g (at p/p₀ = 0.85) for CAU-56as and CAU-56w are found, respectively. Thermal stabilities of up to 300 °C in air and chemical stability in water at pH = 2 to 12 were also observed.

A permanently porous Fe(III)-MOF was synthesized using the V-shaped linker pyrazole-3,5-dicarboxylic acid. The structure was determined by 3D electron diffraction and reveals the presence of trinuclear [Fe₃(μ₃-O)] clusters, whose connectivity is reduced from six to four by coordination with sulfate and acetate coligands. The coordinated acetate ions can be replaced by hydroxide ions and water molecules.