Electron-Transfer-Enhanced Cation-Cation Interactions in Homo- and Heterobimetallic Actinide Complexes: A...

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)(OAn^VIO) 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 U^III to the electron-deficient An^VI of the actinyl moiety, leading to a more stable An^V-U^IV 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., O_exo-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 O_exo-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 O_exo-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)(OU^VIO)(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.