Relativistic DFT Probe for Reaction Energies and Electronic/Bonding Properties of Polypyrrolic Hetero-Bimetallic Actinide Complexes: Effects of Uranyl endo-Oxo Functionalization

A series of hetero-bimetallic actinide complexes of the Schiff-base polypyrrolic macrocycle (L), featuring cation-cation interactions (CCIs), were systematically investigated using relativistic density functional theory (DFT). The tetrahydrofuran (THF) solvated complex [(THF)((OUOUIV)-O-VI)(THF)(L)](2+) has high reaction free energy (Delta(r)G), and its replacement with electron-donating iodine promotes the reaction thermodynamics to obtain uranyl iodide [(I)((OUOUIV)-O-VI)(I)(L)](2+) (U-VI-U-IV). Retaining this coordination geometry, calculations have been extended to other An(IV) (An = Th, Pa, Np, Pu), i.e., for the substitution of U(IV) to obtain U-VI-An(IV). As a consequence, the reaction free energy is appreciably lowered, suggesting the thermodynamic feasibility for the experimental synthesis of these bimetallic complexes. Among all U-VI-An(IV), the electron-spin density and high-lying occupied orbitals of U-VI-Pa-IV show a large extent of electron transfer from electron-rich Pa(IV) to electron-deficient U(VI), leading to a more stable U-V-Pa-V oxidation state. Additionally, the shortest bond distance and the comparatively negative E-int of the Pa-O-endo bond suggest more positive and negative charges (Q) of Pa and endo-oxo atoms, respectively. As a result of the enhanced Pa-O-endo bond and strong CCI in U-VI-Pa-IV along with the corresponding lowest reaction free energy among all of the optimized complexes, uranyl species is a better candidate for the experimental synthesis in the ultimate context of environmental remediation.

Automated summary

A series of hetero-bimetallic actinide complexes with cation-cation interactions were systematically investigated using relativistic density functional theory. The study showed that replacing certain components in the complexes can significantly lower the reaction free energy, indicating the thermodynamic feasibility of experimental synthesis of these bimetallic complexes.