Spectroscopic and DFT study on the complexation of Np(v) with amidoxime-derived ligands in aqueous solutions: speciation and structural optimization

Abstract

The pentavalent neptunyl ion (Np(V)) is the most stable form of neptunium in aqueous solutions. Studying its complexation is essential for developing ligands for Np separation and environmental management. This work investigates Np(V) complexation with three aromatic substituted amidoxime ligands: Benzamidoxime (BzAO), 2-Pyridylamidoxime (Py-2-AO), and 3-Pyridylamidoxime (Py-3-AO). Speciation and stability constants were determined by spectrophotometric titrations, while coordination structures were optimized using DFT calculations. The coordination strength follows the order: Py-2-AO > Py-3-AO > BzAO. Py-2-AO exhibits the strongest coordination due to a synergistic chelation from its pyridine and amidoxime N atoms in the equatorial plane. DFT results confirm that Py-2-AO forms the most stable complex via a five-membered chelate ring, displacing two equatorial water molecules. In contrast, BzAO and Py-3-AO primarily bind in a bidentate fashion through the amidoxime O and N atoms, wherein each ligand also displaces two water molecules. These findings offer fundamental insights for designing effective Np(V) ligands for separation and remediation.