Unraveling the dual nature of covalent and electrostatic binding in americyl capture by a phenanthroline-based covalent organic framework (DAPhen)

Abstract

The separation of minor actinides, especially americium, from lanthanides in spent nuclear fuel remains a critical challenge in nuclear waste management, primarily due to their nearly identical chemical behavior in the trivalent state. To address this, we target the linear dioxo configuration of pentavalent americium (AmO2+), which offers distinct steric and electronic features compared to spherical trivalent lanthanides. This work investigates Am(V) adsorption using a phenanthroline-based covalent organic framework (DAPhen-COF), where the pre-organized N,O-donor environment from the phenanthroline-amidine motif is designed for strong actinide coordination. Multiscale computations show that DAPhen-COF forms a highly stable complex with AmO2+. Density of states analysis reveals strong orbital hybridization between Am-5f and ligand N/O-2p states, underscoring substantial covalent interaction. Topological analysis of electron density confirms the existence of bonds with pronounced covalent character within highly polarized coordination environments, while electrostatic potential analysis verifies complementary electrostatic contributions. Energy decomposition analysis further quantifies the binding as a cooperative interplay between orbital and electrostatic forces. Quantitative adsorption energy calculations further corroborate these findings, revealing that DAPhen-COF exhibits a strong affinity for Am(V) (−151.9 kcal/mol) and clear selectivity over Eu(III), with the most stable configuration arising from cooperative actinide-actinide interactions within the confined COF interlayer space. This study not only sheds light on the unique coordination chemistry of pentavalent americium but also provides a robust theoretical foundation for designing ligand architectures capable of distinguishing actinides from lanthanides based on oxidation-state-specific motifs.