Characterization of the binding of six actinyls AnO22+/+ (An = U/Np/Pu) with three expanded porphyrins by density functional theory

Abstract

We reported a density functional theory study on the complexation of six hydrated actinyl cations, AnO₂(H₂O)₅^2+/+ (aq) (An = U/Np/Pu), with three expanded porphyrins, amethyrin (H₄L1), oxasapphyrin (H₂L2), and grandephyrin (H₃L3). The geometries have been fully optimized and analyzed, and the electronic structures, the binding free energies, and the NMR properties were calculated. Natural population analysis and Quantum Theory of Atoms in Molecules (QTAIM) topology analysis techniques were applied to understand the interaction modes between two entities of each complex. The calculations show that for the same ligand, PuO₂²⁺ and NpO₂²⁺ display stronger binding affinity than UO₂²⁺, UO₂⁺, NpO₂⁺, and PuO₂⁺, and among the three ligands tested, L2²⁻ fits better with the actinyl cations than L3³⁻ and H₂L1²⁻. The redox process was observed in the complexation of PuO₂²⁺ and NpO₂²⁺ with specific ligands, which agrees with the experimental results. In the characterization of the nature of the coordination bonding interactions, QTAIM gives a consistent description with the natural population analysis method and shows that the interaction between An and the electron donor atoms in the first coordination shell has a strong ionic feature, while the interaction between An and O^yl atoms of the actinyls in the complexes remains covalent. This work complements the earlier experimental studies by providing a molecular level of understanding on the interaction between actinyls and expanded porphyrins, and is expected to contribute to the communities of the chemistry of actinides and expanded porphyrins.