UoC-3: A Metal–Organic Framework with an Anionic Framework Based on Uranyl UO<sub>2</sub><sup>2+</sup> Nodes and Partly Fluorinated Benzene-1,3,5-Tribenzoate Linkers
Ronja Christoffels, Carina Breitenbach, Jean Patrick Weber, Lisa Körtgen, Christian Tobeck, Michael Wilhelm, Sanjay Mathur, Jörg‐Martin Neudörfl, Majied Sadegh Zadeh Farid, Melisa Maslo, Erik Strub, Uwe Ruschewitz
Abstract
The reaction of UO2(NO3)2·6H2O with partly fluorinated H3-3F-BTB (BTB3–: benzene-1,3,5-tribenzoate) in N,N-dimethylformamide (DMF) leads to the crystallization of the metal–organic framework (MOF) [(CH3)2NH2][UO2(3F-BTB)]·xDMF, named UoC-3 (UoC: University of Cologne). X-ray single-crystal structure analysis (Pnna, Z = 4) reveals that an anionic framework is formed, in which UO22+ nodes are connected by 3F-BTB3– ligands. Because of the fluorination of the inner ring of the linker, its three benzoate groups are tilted to an “out of plane” arrangement, which leads to the formation of a three-dimensional structure with large pores. This is in contrast to a known uranyl coordination polymer with the unfluorinated BTB3– linker, where an almost coplanar arrangement of the linker leads to graphene-like layers. The high porosity of UoC-3 was confirmed by N2 gas sorption measurements, resulting in SBET = 4844 m2/g. The charge compensating [(CH3)2NH2]+ cation is formed by hydrolysis of DMF. Direct addition of [(CH3)2NH2]Cl to the reaction carried out in ethanol/H2O (v:v, 5:1) leads to the same MOF but with lower crystallinity. When using solvents, which hydrolyze to larger cations (e.g., N,N-diethylformamide (DEF): [(C2H5)2NH2]+ and N,N-di(n-butyl)formamide (DBF): [(C4H9)2NH2]+), again the formation of UoC-3 was found, as confirmed by X-ray single-crystal analysis and X-ray powder diffraction. Thus, no templating effect was achieved with these cations. The exchange of the organic cations by K+ turned out to be successful, as revealed by XPS analysis. UoC-3 was also successfully tested to remove approximately 96% radioactive 137Cs+ from aqueous solutions (93% after one regeneration cycle) while retaining its crystal structure.