Uranium(IV) and Thorium(IV) Coordination Polymers Based on Tritopic Carboxylic Acids

Nine new coordination polymers based on U(IV) and Th(IV) were synthesized solvothermally utilizing four different trianionic carboxylates (H₃BHTC = biphenyl-3,4',5-tricarboxylic acid, H₃NTB = 4,4',4″-nitrilotribenzoic acid, H₃BTB = 4,4',4″-benzene-1,3,5-triyl-tris(benzoic acid), H₃BTE = 4,4',4″-(1,3,5-benzenetriyltri-2,1-ethynediyl)trisbenzoic acid). The influence of the ligand architecture, the pH, the stoichiometry, the nature of the metal, and the concentration on the structure and dimensionality of the final actinide assembly is discussed. The H₃BHTC ligand allowed the synthesis of a cationic three-dimensional (3D) framework [U(BHTC)(DMF)₃]I (1), which is the first example of a cationic U(IV) polymer. The H₃NTB ligand yielded the 3D neutral polymer [U₃(NTB)₄] (2) or the two-dimensional (2D) cationic polymer [U(NTB)(NMP)₃]I (3), depending on the solvent. When conditions leading to (2) were used with a Th(IV) precursor, the 2D neutral polymer [Th(NTB)(DMF)₃Cl] (4) was obtained. The ligand H₃BTB allowed the synthesis of two 3D cationic networks [U(BTB)(DMF)₂]I (5) and [U(BTB)(DMF)₃]I (7) or the neutral 3D analogue [U₃(BTB)₄] (6), depending on the precursor's oxidation state and the acidity of the reaction mixture. The ligand H₃BTE allowed the synthesis of the anionic 3D [(CH₃)₂NH₂][U₂(BTE)₃] (8) framework featuring large accessible pores, and under the same conditions, an isostructural Th(IV) was also obtained [(CH₃)₂NH₂][Th₂(BTE)₃] (8-Th). All isolated coordination polymers were characterized by single-crystal X-ray diffraction (SCXRD). The Langmuir surface areas of the U(IV) polymers (2), (7), and (8) increased from 140 to 310 m²/g owing to the increasing size of the linker, with polymer (8) showing a value that is comparable to the highest surface area reported to date. The effect of the postsynthetic solvent substitution was also studied, revealing a crystal-to-crystal transformation of the cationic framework (7) to the neutral framework [U(BTB)(THF)I] (7c).