Influence of a Bridging Group and the Substitution Effect of Bis(1,2,4-triazine) N‑Donor Extractants on Their Interactions with a NpV 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 [NpO2]+ 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 NpO2L­(NO3) complexes in thermodynamics. Our studies indicate that the complexation reaction mode [NpO2(H2O)n]+ + L + NO3 → NpO2L­(NO3) + nH2O 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 NpV and more efficient separation of Np in spent nuclear fuels.