Electron-Transfer-Enhanced Cation-Cation Interactions in Homo- and Heterobimetallic Actinide Complexes: A Relativistic Density Functional Theory Study.

To provide deep insight into cation-cation interactions (CCIs) involving hexavalent actinyl species that are major components in spent nuclear fuel and pose important implications for the effective removal of radiotoxic pollutants in the environment, a series of homo- and heterobimetallic actinide complexes supported by cyclopentadienyl (Cp) and polypyrrolic macrocycle (H4L) ligands were systematically investigated using relativistic density functional theory. The metal sort in both parts of (THF)(H2L)(OAnVIO) and (An')IIICp3 from U to Np to Pu, as well as the substituent bonding to Cp from electron-donating Me to H to electron-withdrawing Cl, SiH3, and SiMe3, was changed. Over 0.70 electrons are unraveled to transfer from the electron-rich UIII to the electron-deficient AnVI of the actinyl moiety, leading to a more stable AnV-UIV isomer; in contrast, uranylneptunium and uranylplutonium complexes behave as electron-resonance structures between VI-III and V-IV. These were further corroborated by geometrical and electronic structures. The energies of CCIs (i.e., Oexo-An' bonds) were calculated to be -19.6 to -41.2 kcal/mol, affording those of OUO-Np (-23.9 kcal/mol) and OUO-Pu (-19.6 kcal/mol) with less electron transfer (ET) right at the low limit. Topological analyses of the electron density at the Oexo-An' bond critical points demonstrate that the CCIs are ET or dative bonds in nature. A positive correlation has been built between the CCIs' strength and corresponding ET amount. It is concluded that the CCIs of Oexo-An' are driven by the electrostatic attraction between the actinyl oxo atom (negative) and the actinide ion (positive) and enhanced by their ET. Finally, experimental syntheses of (THF)(H2L)(OUVIO)(An')IIICp3 (An' = U and Np) were well reproduced by thermodynamic calculations that yielded negative free energies in a tetrahydrofuran solution but a positive one for their uranylplutonium analogue, which was synthetically inaccessible. So, our thermodynamics would provide implications for the synthetic possibility of other theoretically designed bimetallic actinide complexes.