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

Abstract

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)(OUVIOUIV)(THF)(L)]2+ has high reaction free energy (ΔrG), and its replacement with electron-donating iodine promotes the reaction thermodynamics to obtain uranyl iodide [(I)(OUVIOUIV)(I)(L)]2+ (UVI-UIV). 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 UVI-AnIV. As a consequence, the reaction free energy is appreciably lowered, suggesting the thermodynamic feasibility for the experimental synthesis of these bimetallic complexes. Among all UVI-AnIV, the electron-spin density and high-lying occupied orbitals of UVI-PaIV show a large extent of electron transfer from electron-rich Pa(IV) to electron-deficient U(VI), leading to a more stable UV-PaV oxidation state. Additionally, the shortest bond distance and the comparatively negative Eint of the Pa-Oendo bond suggest more positive and negative charges (Q) of Pa and endo-oxo atoms, respectively. As a result of the enhanced Pa-Oendo bond and strong CCI in UVI-PaIV 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.