Theoretical Study Of Structures And Reaction Properties Of Binuclear Actinyl Complexes Supported By Schiff-base Polypyrrolic Macrocycles

Actinide elements play a centering role in the production of nuclear power,the manufacturing of nuclear weapon and the medical science.However,the generated nuclear waste has brought about serious environmental concerns,so its long-term safe disposal becomes an important subject in the fundamental science.In the processing of nuclear fuel and spent nuclear fuel,the U?VI?uranyl ion [UO2]2+ is the most prevalent species due to its extraordinarily chemically robust axial U=O bonds.Its U?V?analogue readily disproportionates into U?IV?uranium species and U?VI?[UO2]2+ in an aqueous environment.In contrast,the An?V?transuranium actinyl ions [Np O2]+ and [Pu O2]+ are stable and relatively mobile in the environment.In addition,the common Pu?VI?actinyl complexes are highly soluble and mobile in water.The safe immobilization of radionuclides and elimination of environmental contamination of nuclear waste requires an in-depth understand of the structures,reaction behavior and redox property of uranium and transuranium complexes.To avoid experimental difficulties of handling actinides such as high radiotoxicity and materials' scarcity,all-electron relativistic density functional theory?DFT?has been employed to examine the interaction between Schiff-base polypyrrolic macrocycles and high-valent actinyl ions,and to calculate geometrical/electronic structures of formed stable complexes,bonding,Infrared vibrational spectra,redox properties and thermodynamic reactions.Mono-and bis-actinyl complexes supported by an anthracenyl-linked Schiff-base polypyrrolic macrocycle were systematically investigatged,aiming to explore the effects of the fifth equatorial donor site?pyridine solvent molecule?of the actinyl ions on their structures and relevant properties.Comparison was further made with pyridine-free analogues.The results show that it is the key to explicitly include the fifth-coordination pyridine,allowing theoretical structures in good agreement with experimental determinations of X-ray single crystal diffraction.The calculated reduction potentials?E0?,while incorporating both solvation and spin-orbit coupling effects,show the order of Np > Pu > U.Unlike other mononuclear penta-and hexavalent U,Np and Pu complexes,the theoretically predicted mononuclear Pu?V?complex of the anthracenyl-linked polypyrrolic possesses a peculiarly twisted structure.Calculated symmetric/asymmetric An=O stretching vibrational frequencies display a decreasing trend along U,Np and Pu,and the reduction from hexa-to pentavalent complexes cause pronounced redshift of An=O vibrational frequencies.The binuclear hexavalent uranium complex has low-lying U?5f?-character virtual orbitals,where f???and f???orbitals occur in low-energy region and ?-type ones are residing further high;the ?*?U=O?and ??U=O?orbitals are significantly split over 7 e V.The experimental observation that gives the bis-uranyl complex as the major product has been corroborated by our thermodynamic calculations,and moreover a possible reaction process has been proposed.Due to the large space of the aforementioned anthracenyl-linked polypyrrolic macrocycle,formed binuclear complexes present a paralleled arrangement of two linear actinyl units.In contrast,the utilization of a small-space phenyl-linked polypyrrolic macrocycle allows to form cation-cation interaction?CCI?between actinyls in the complex,and eventually stabilizes the molecular system.On the basis of the mononuclear uranyl?VI?complex of the phenyl-linked polypyrrolic macrocycle,we firstly studied uranyl endo-oxo-UIV complexes with variations of surrounding donors;and the UVI-UIV-type iodides are revealed to be the most thermodynamically stable.Then,we changed the low-valent actinide center in the UVI-UIV-type iodide from Th to Pu;the UVI-PaIV₂I complex was calculated to show a large electron transfer and lead to a V-V complex in reality,while other complexes retain VI-IV actinide oxidation states.Secondly,chemical modification of exo-oxo with Al and alkali metal ions form a series of stable CCI complexes;these complexes were unraveled to display higher reduction potentials than the mononuclear uranyl?VI?complex without CCI modification,and consequently showed the raised reducing ability of uranium center?VI?V?.In brief,theoretically calculated structures,reaction energies and redox properties of uranium and transuranium complexes provide a theoretical guide for the synthesis of low-valence actinyl Schiff-base polypyrrolic complexes.Comparison with available experimental results,on the one hand,presents the test of theoretical methods and then improve them in return;on the other hand,calculations based on accurate density functional theory also pose new clues for the functionalization of thermodynamically stable and kinetically inert the axial oxygen of hexavalent uranyl group.