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

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).