Mixed Formate-Dicarboxylate Coordination Polymers with Tetravalent Uranium: Occurrence of Tetranuclear {U4O4} and Hexanuclear {U6O4(OH)4} Motifs

Two new three-dimensional coordination networks with tetravalent uranium have been solvothermally synthesized by associating UCl4 and a combination of two carboxylate-based ligands in N,N-dimethylformamide (DMF). The mixture of terephthalate (bdc) and formate (form) resulted in the formation MOF-type structure (Hdma)­[U4O2­(bdc)3(form)7] based on the tetranuclear uranium-centered motif {U4O4}. This inorganic building block is relatively rare in the actinides chemistry and generates here a negatively charged framework trapping dimethylammonium cations resulting from the in situ decomposition of DMF. The association of formate (form) and fumarate (fum) led to the isolation of the hexameric building block {U6O4(OH)4} in a three-dimensional framework (U6O4(OH)4­(fum)5(form)2­(H2O)2­·3DMF) closely related to the topology observed with single ditopic ligand in the UiO-66 series. The porous solid contained two types of cages with estimated free pore diameters of 6.3 × 6.7 Å and 4.7 Å.