UoC-3: A Metal–Organic Framework with an Anionic Framework Based on Uranyl UO₂²⁺ Nodes and Partly Fluorinated Benzene-1,3,5-Tribenzoate Linkers

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.