Phosphine Oxide Group Enhances the In-Plane σ-Bonding Covalency in Actinyl Phenanthroline-Derivative Complexes

The separation of nuclear waste is one of the urgent problems to be solved in the nuclear industry; therefore, designing a new and promising extractant to deal with nuclear waste is a desirable but difficult task. Here, using quantum-mechanical calculations at the scalar-relativistic and spin-orbit coupling levels, we assessed the complexation of a series of actinyl moieties by phenanthroline-derived organophosphorus ligands. Specifically, we considered the [UO₂(L)]²⁺ and [AnO₂(BPP)]²⁺, An = U, Np, Pu, and L = DAP, BPP, and PIP species. First, we find that ligand BPP binds more strongly to the uranyl dication than DAP or PIP, and similarly, BPP binds more strongly to uranyl and neptunyl than plutonyl, as a result of the significant ionic bonding and covalent bonding in the equatorial plane. The results of this work generate a fundamental and qualitative understanding of bonding properties in actinyl complexes, suggest phosphine oxide-derived phenanthroline ligands as a potential extractant for uranyl, and provide some knowledge when designing efficient ligands for the extraction behaviors toward actinide elements.