Quantum Chemistry Study of Uranium(VI), Neptunium(V), and Plutonium(IV,VI) Complexes with Preorganized Tetradentate Phenanthrolineamide Ligands

The preorganized tetradentate 2,9-diamido-1,10-phenanthroline ligand with hardsoft donors combined in the same molecule has been found to possess high selectivity toward actinides in an acidic aqueous solution. In this work, density functional theory (DFT) coupled with the quasi-relativistic small-core pseudopotential method was used to investigate the structures, bonding nature, and thermodynamic behavior of uranium(VI), neptunium(V), and plutonium(IV,VI) with phenanthrolineamides. Theoretical optimization shows that Et-Tol-DAPhen and Et-Et-DAPhen ligands are both coordinated with actinides in a tetradentate chelating mode through two N donors of the phenanthroline moiety and two O donors of the amide moieties. It is found that [AnO(2)L(NO3)](n+) (An = U-VI, Np-V, Pu-VI; n = 0, 1) and PuL(NO3)(4) are the main 1:1 complexes. With respect to 1:2 complexes, the reaction [Pu(H2O)(9)](4+) (aq) + 2L(org) + 2NO(3)(-) (aq) -> [PuL2(NO3)(2)](2+) (org) + 9H(2)O(aq) might be another probable extraction mechanism for Pu-IV. From the viewpoint of energy, the phenanthrolineamides extract actinides in the order of Pu-IV > U-VI > Pu-VI > Np-V, which agrees well with the experimental results. Additionally, all of the thermodynamic reactions are more energetically favorable for the Et-Tol-DAPhen ligand than the Et-Et-DAPhen ligand, indicating that substitution of one ethyl group with one tolyl group can enhance the complexation abilities toward actinide cations (anomalous aryl strengthening).