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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
journal contribution
posted on 2014-08-04, 00:00 authored by Xia Yang, Yanni Liang, Songdong Ding, Shoujian Li, Zhifang Chai, Dongqi WangThe
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.
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nH 2Ocharge transfertetradentate ligandsligand atomsNpV CationThebinding abilitytridentate onesNpO 2L complexesequilibrium structuretetradentate complexescalculations showComplexation reactionscomplexation stabilitynitrate ionsreaction modesBTPhenSubstitution Effectcharge decomposition analysiscomplexation reaction modenitrate ion5 fneptunyl complexespyridine counterpartorthophenanthrolinequantum theoryelectron delocalizationphaseBridging Groupcoordination ability
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