Uranium complexes play an increasingly important role in the fields of power resource, environment and medical science. As the most stable and the most prevalent formation of uranium, hexavalent uranyl species (UO₂²⁺) are widely present in the natural water system and the nuclear fuel cycle. Since 2005, the isoelectronic analogue of the uranyl, U(NR)₂²⁺, (R=alkyl and aryl) has been a burgeoning area of research. Many bis-imido uranium complexes have been synthesized and investigated for their structural, reactivity and spectroscopic properties. It is found that the bis-imido uranium(VI) complex is capable of undergoing imido exchange reaction with oxo group, but the contrary reaction can not occur. Recently, a flexible polypyrrolic macrocycle (H₄L) has been widely used to complexate hexavalent UO²⁺. An interesting Pacman-like complex, [(THF)(U^VIO₂)(H₂L)], was obtained, where the uranyl ion is accommodated by one N₄-donor compartment and the other compartment remains vacant. In the equatorial plane of linear uranyl ion, one THF solvent serves as the fifth coordination. Notably, two hydrogen bonds are formed between the endo-oxo of uranyl and remaining hydrogen atoms of two pyrrolides in the second compartment of macrocycle. In this work, a series of uranium complexes of the polypyrrolic macrocycle with the mixed oxo and imido groups, [(THF)(OUE)(A₂L)] (E=NH, NMe and NPh; A=H and Li; labeled as UE-A) were designed, on the basis of their dioxo analogues UO-A (E=O) where the UO-H was experimentally synthesized and characterized. Their structures, Infrared (IR) vibrational spectra and oxo-imido exchange reaction were examined by the scalar relativistic density functional theory (DFT). The U=O_endo bond lengths of UE-A were optimized to be within 1.84～1.89 Å, longer than those of known uranyl complexes which possess regular pentagonal dipyramidal structure. This is related to the interaction between the A and endo-oxo atoms. The calculated U=N distances range from 1.87 to 1.90 Å, which are affected by various R substituent (direct effect) and different A atom that is bonding to endo-oxo atom (indirect effect). All the calculated U=O/U=N distances fall well within the range of experimental values. The partial triple character is unraveled for U=O_exo and U=N bonds, but a modest one between single and double is assigned to U=O_endo of UE-Li. Frequency calculations find the close bonding properties for O=U=NH and O=U=O, both of which show symmetrical and asymmetrical stretching vibrational bands between 700 and 900 cm^-1. The introduction of steric substituents of Me and Ph leads to two greatly separate peaks of U=O and U=N-C. The strong coupling of Me and Ph with U=N bonding makes the U=N-C vibrations present in the high-frequency region from 1166 to 1266 cm^-1, which are comparable to experimental values of 1170～1270 cm^-1 for U=N-R (R=^tBu and Ph). The variation of the A atom from H to Li significantly redshifts the U=O and U=N-C stretches. When carefully selecting the A atom and the R substituent, the Pacman-like complex UO-A would be easier to undergo the oxo exchange with the imido group, compared with regular pentagonal dipyrimid complex. This study is expected to provide theoretical support for experimental study of mixed oxo-imido uranium complex.

Key words: mixed oxo-imido uranium complexes, IR vibrational spectra, electronic structure, exchange reaction, relativistic DFT