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A Tetratopic Phosphonic Acid for the Synthesis of Permanently Porous MOFs: Reactor Size-Dependent Product Formation and Crystal Structure Elucidation via Three-Dimensional Electron Diffraction

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journal contribution
posted on 01.09.2020 by Stephan Wöhlbrandt, Christoph Meier, Helge Reinsch, Erik Svensson Grape, A. Ken Inge, Norbert Stock
Following the strategy of installing porosity in coordination polymers predefined by linker geometry, we employed the new tetratopic linker molecule 1,1,2,2-tetrakis­[4-phosphonophenyl]­ethylene (H8TPPE) for the synthesis of new porous metal phosphonates. A high-throughput study was carried out using Ni2+ and Co2+ as metal ions, and a very strong influence of the reactor size on the product formation is observed while maintaining the same reaction parameters. Using small autoclaves (V = 250 μL), single crystals of isostructural mononuclear complexes of the composition [Ni­(H3DPBP)2(H2O)4] (1) and [Co­(H3DPBP)2(H2O)4] (2) are formed. They contain the linker molecule H4DPBP (4,4′-diphosphonobenzophenone), which is formed in situ by oxidation of H8TPPE. Using autoclaves with a volume of V = 2 mL, two new 3D metal–organic frameworks (MOFs) of composition [Ni2(H4TPPE)­(H2O)6]·4H2O (CAU-46) and [Co2(H4TPPE)­(H2O)4]·3H2O (CAU-47) were isolated in bulk quantities, and their crystal structures were determined from three-dimensional electron diffraction (3D ED) and powder X-ray diffraction data. Using even larger autoclaves (V = 30 mL), another 3D MOF of the composition [Co2(H4TPPE)]·6H2O (Co-CAU-48) was obtained, and a structure model was established via 3D ED measurements. Remarkably, the isostructural compound [Ni2(H4TPPE)]·9H2O (Ni-CAU-48) is only obtained indirectly, i.e., via thermal activation of CAU-46. As the chosen linker geometry leads to the formation of MOFs, topological analyses were carried out, highlighting the different connectivities observed in the three frameworks. Porosity of the compounds was proven via water sorption experiments, resulting in uptakes of 126 mg/g (CAU-46), 105 mg/g (CAU-47), 210 mg/g (Ni-CAU-48), and 109 mg/g (Co-CAU-48).