posted on 2023-12-04, 19:00authored byJonas Gosch, Virginia Guiotto, Felix Steinke, Erik Svensson Grape, Cesare Atzori, Kalle Mertin, Tobias Otto, Niklas Ruser, Christoph Meier, Diletta Morelli Venturi, A. Ken Inge, Kirill A. Lomachenko, Valentina Crocellà, Norbert Stock
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 (SDB2–) and 4,4′-(hexafluoroisopropylidene)bis(benzoate)
(hfipbb2–). 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[CeIV(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.