Theoretical study of structural, spectroscopic and reaction properties of trans - Bis (imido) uranium(VI) complexes

To advance the understanding of the chemical behavior of actinides, a series of trans-bis(imido) uranium(VI) complexes, U(NR)₂(THF) ₂(cis-I₂) (2R; R = H, Me, ^tBu, Cy, and Ph), U(NR)₂(THF)₃(trans-I₂) (3R; R = H, Me, ^tBu, Cy, and Ph) and U(N^tBu)₂(THF) ₃(cis-I₂) (3^tBu′), were investigated using relativistic density functional theory. The axial U =N bonds in these complexes have partial triple bonding character. The calculated bond lengths, bond orders, and stretching vibrational frequencies reveal that the U =N bonds of the bis-imido complexes can be tuned by the variation of their axial substituents. This has been evidenced by the analysis of electronic structures. 2H, for instance, was calculated to show iodine-based high-lying occupied orbitals and U(f)-type low-lying unoccupied orbitals. Its U =N bonding orbitals, formed by U(f) and N(p), occur in a region of the relatively low energy. Upon varying the axial substituent from H to ^tBu and Ph, the U =N bonding orbitals of 2^tBu and 2Ph are greatly destabilized. We further compared the U =E (E = N and O) bonds of 2H with 3H and their uranyl analogues, to address effects of the equatorial tetrahydrofuran (THF) ligand and the E group. It is found that the U =N bonds are slightly weaker than the U =O bonds of their uranyl analogues. This is in line with the finding that cis-UNR ₂ isomers, although energetically unfavorable, are more accessible than cis-UO₂ would be. It is also evident that 2H and 3H display lower U =(NH) stretching vibrations at 740 cm^-1 than the U =O at 820 cm^-1 of uranyl complexes. With the inclusion of both solvation and spin-orbit coupling, the free energies of the formation reactions of the bis-imido uranium complexes were calculated. The formation of the experimentally synthesized 3Me, 3Ph, and 2^tBu are found to be thermodynamically favorable. Finally, the absorption bands previously obtained from experimental studies were well reproduced by time-dependent density functional theory calculations.