Structural and Bonding Properties of Uranium Complexes With Phenanthroline Ligands

In the field of spent nuclear fuel reprocessing, the recovery and utilization of uranium is of significant importance. Current research focuses primarily on the development of highly efficient actinide extractants. Actinide coordination computational chemistry provides a novel investigative space for this development work. In ligand design, phenanthroline-based tetradentate ligands have become a research hotspot due to their combined soft-hard donor properties. Within this context, this study employs density functional theory(DFT) and a series of chemical bond analysis methods to conduct a detailed investigation into the ground-state structures and bonding characteristics of uranyl-phenanthroline-type complexes. The research first examines the coordination behavior of uranyl with a phenanthroline amide ligand(DAP) and two novel phenanthroline-based organophosphorus ligands(PIP and BPP). Nitrogen and oxygen atoms from these ligands participate in coordination, forming four-coordinate complexes with the uranyl ion in a tetradentate inserted mode. Among them, the phosphine oxide ligand, by introducing the P=O oxygen donor group, exhibits enhanced binding affinity for uranyl compared to the phenanthroline amide ligand (DAP). Bond critical point and bond order analyses indicate that the covalent character is stronger for U-O_L(ligand oxygen) bonds than for U-N bonds. The U-O_L bond length follows the order: BPP＜PIP＜DAP, with the shortest bond and highest bond order observed in the BPP complex, underscoring the crucial role of U-O_L covalency in its stability. The infrared spectral predictions show that the BPP complex exhibits the most pronounced red-shift in the antisymmetric stretching frequency(ν₃) of the uranyl U-O_yl bonds, confirming its strongest electron-donating capability. The results of energy decomposition analysis(EDA) reveal that the interaction between uranyl and the phenanthroline ligand is primarily contributed by electrostatic interactions, with orbital interactions also playing a significant role in stabilizing the complex. Extended transition state-natural orbitals for chemical valence(ETS-NOCV) analysis reveals that the σ-donation from ligand N/O 2p orbitals to uranium 6d and 5f orbitals constitutes the primary orbital interaction. Charge analysis(Mülliken) shows that replacing the amide C=O with a P=O group increases the charge density on the donor oxygen atom. Furthermore, orbital energy analysis indicates that ligand coordination raises the energy of uranium’s d_δ orbital, with the most pronounced effect in the BPP complex. In conclusion, this work highlights the potential of soft-hard donor ligands incorporating phosphorus, particularly those with P=O groups, for uranium separation. The increased basicity and charge density on the oxygen donor atom, achieved by introducing phosphorus-containing substituents, can enhance the selectivity of phenanthroline-derived ligands for uranyl ions. These findings provide valuable theoretical guidance for developing novel soft-hard donor extractants with broad practical application prospects.