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 (SO42– 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 as,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.