Discovery and In Situ Crystallization Studies of Cerium-Based Metal-Organic Frameworks with V-Shaped Linker Molecules

We report the discovery and characterization of two porous Ce(III)-based metal-organic frameworks (MOFs) with the V-shaped linker molecules 4,4'-sulfonyldibenzoate (SDB^2-) and 4,4'-(hexafluoroisopropylidene)bis(benzoate) (hfipbb^2-). The compounds of framework composition [Ce₂(H₂O)(SDB)₃] (1) and [Ce₂(hfipbb)₃] (2) were obtained by using a synthetic approach in acetonitrile that we recently established. Structure determination of 1 was accomplished from 3D electron diffraction (3D ED) data, while 2 could be refined against powder X-ray diffraction (PXRD) data using the crystal structure of an isostructural La-MOF as the starting model. Their framework structures consist of chain-like inorganic building units (IBUs) or hybrid-BUs that are interconnected by the V-shaped linker molecules to form framework structures with channel-type pores. The composition of both compounds was confirmed by PXRD, elemental analysis, as well as NMR and IR spectroscopy. Interestingly, despite the use of (NH₄)₂[Ce^IV(NO₃)₆] in the synthesis, cerium ions in both MOFs occur exclusively in the + III oxidation state as determined by X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopy (XPS). Thermal analyses reveal remarkably high thermal stabilities of ≥400 °C for the MOFs. Initial N₂ sorption measurements revealed the peculiar sorption behavior of 2 which prompted a deeper investigation by Ar and CO₂ sorption experiments. The combination with nonlocal density functional theory (NL-DFT) calculations adds to the understanding of the nature of the different pore diameters in 2. An extensive quasi-simultaneous in situ XANES/XRD investigation was carried out to unveil the formation of Ce-MOFs during the solvothermal syntheses in acetonitrile. The crystallization of the two Ce(III)-MOFs presented herein as well as two previously reported Ce(IV)-MOFs, all obtained by a similar synthetic approach, were studied. While the XRD patterns show time-dependent MOF crystallization, the XANES data reveal the presence of Ce(III) intermediates and their subsequent conversion to the MOFs. The addition of acetic acid in combination with the V-shaped linker molecule was identified as the crucial factor for the formation of the crystalline Ce(III/IV)-MOFs.