Selective coordination behaviors of Uranium(VI) with novel asymmetrical tetra-alkylcarbamides

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In this work, the solvent extraction and complexation behaviors of U(VI) by two novel asymmetrical tetra-alkylcarbamides, N,N’-butyl-N,N’-hexylurea (L1: SBHU) and N,N’-butyl-N,N’-pentylurea (L2: SBPU) in HNO3 media were systematically investigated. Both two ligands extract U(VI) more effectively than other metals such as Pu(IV), Np(IV), and Th(IV) in a wide range of acidity. Under 5 mol/L HNO3, DU reached 5.25, while DPu = 1.08, DNp = 0.11, DTh = 0.13.. Additionally, no third-phase was present by the extraction of highly uranium concentration solution with SBHU even under 7 mol/L HNO3 after four cycles demonstrating good acid resistance and resistance to the formation of the third phase. Slope analyses and FT-IR indicate that the U(VI) complexes are a form of UO2(NO3)2·2L for the two ligands during the extraction. Additionally, based on the DFT calculations, the NBO analysis, and topological analysis were analyzed in detail. The results demonstrate that the interactions between ligands and U(VI) are shown mainly as ionic characteristics, and the ligand-to-metal charge transfer was also observed. This study enriches the coordination chemistry of actinides: Th(IV), U(VI), Np(IV), Am(III), and Pu(IV) by tetra-alkylcarbamides, which may shed light on the design and synthesis of novel U(VI) selective ligands.