Redox transformations and cation-cation interactions of neptunium in organic solutions

Neptunium is a minor actinide with diverse redox chemistry and high tendency to form unique non-covalent interactions. As a result, the fate of neptunium during nuclear fuel reprocessing remains underexplored. In this work we aim to understand neptunium chemistry in organic solutions with tetradentate N,O-donor ligand that is pre-organized to stabilize 5f-elements in contact with concentrated nitric acid solutions. Crystal structures of neptunium(V) and (VI) complexes with ligand revealed the formation of three types of species: [LNpVIO2NO3][NpVIO2(NO3)3], [LNpVIO2NO3]2[NpVIO2(NO3)4] ion pairs, and [LNpVO2NO3]2⋅NpVIO2(NO3)2. The latter one is a mixed-valence NpV-NpVI complex with ligand-coordinated neptunium(V), which stabilizes poorly extractable neptunium(V) in organic solution. Neptunium speciation in organic solutions after extraction from neptunium(V) and (VI) aqueous solutions was investigated using a combination of XAS and 1H-NMR spectroscopies, and computational studies. The results demonstrate that neptunium(V) is extracted as a complex with ligand and then oxidized to form [LNpVIO2NO3]NO3. Extraction of neptunium(VI) leads to the formation of ion pairs. The formation of mixed-valence NpV-NpVI complexes was not observed in solvent extraction systems, but proved to be stable in acetonitrile solution. This study provides valuable insights into the chemical processes behind the extraction of neptunium(V) and (VI) under conditions relevant to spent nuclear fuel reprocessing.