Influence of a Bridging Group and the Substitution Effect of Bis(1,2,4-triazine) N‑Donor Extractants on Their Interactions with a Np<sup>V</sup> Cation

The present theoretical study provides a realistic evaluation of the equilibrium structure, reaction modes, and bonding characteristics of a variety of neptunyl complexes formed with bis­(triazinyl) N-donor extractants, which differ in their bridging groups such as pyridine, bipyridines, and orthophenanthroline, corresponding to the ligands (L) of tridentate bis­(triazinyl)­pyridines and tetradentate bis­(triazinyl)­bipyridines and bis­(triazinyl)-1,10-phenanthrolines (BTPhens), respectively. Our calculations show that coordination of [NpO<sub>2</sub>]<sup>+</sup> to tetradentate ligands is more favorable than that to tridentate ones no matter in a gas, aqueous, or organic phase. The presence of nitrate ions can enhance the coordination ability of neptunyl and stabilize the neutral NpO<sub>2</sub>L­(NO<sub>3</sub>) complexes in thermodynamics. Our studies indicate that the complexation reaction mode [NpO<sub>2</sub>(H<sub>2</sub>O)<sub><i>n</i></sub>]<sup>+</sup> + L + NO<sub>3</sub><sup>–</sup> → NpO<sub>2</sub>L­(NO<sub>3</sub>) + <i>n</i>H<sub>2</sub>O is the most probable at the interface between water and the organic phase. The contribution of an orthophenanthroline bridging group is relatively more pronounced compared to its pyridine counterpart in ligand-exchange reaction. Complexation reactions of hydrated neptunyl with C2-BTPhen and BTPhen assisted by a nitrate ion are favorable thermodynamically, resulting from the least deformation of the ligand and strong complexation stability. The quantum theory of atoms-in-molecules and charge decomposition analysis suggest that electron delocalization and charge transfer are the main reasons responsible for stabilization of the tetradentate complexes and reveal a strong ionic feature of the Np–ligand bonds. Inspection of the frontier molecular orbitals reveals a distinct 5f orbital (Np) interaction with ligand atoms, implying the extent of f-based covalency. Our study may facilitate the rational design of ligands toward the improvement of their binding ability with Np<sup>V</sup> and more efficient separation of Np in spent nuclear fuels.