Relativistic DFT investigation for reaction energies and electronic/bonding properties of Schiff-base polypyrrolic uranyl(V) complexes: effects of group 14-functionalized uranyl exo-oxo group

Context

The safe immobilization of radionuclides and the removal of nuclear waste contamination from environment require a thorough understanding of the structures, reaction behaviour and bonding properties of uranium complexes. The cation-cation interaction (CCI), which is also known as the direct actinyl-actinyl bonding interaction, is common only for An(V). A series of binuclear uranyl compounds of Schiff-base polypyrrolic macrocycle (H₄L), [{(Me₃R)OU^VO}₂(L)] (R = C (1), Si (2), Ge (3), Sn (4) and Pb (5)), featuring CCIs, were systematically investigated by relativistic density functional theory (DFT). Three electronic states of singlet (f^αβ), symmetry-broken (f^αf^β), and triplet (f^αf^α) were calculated, which are labeled as s, s′ and t, respectively. Calculations show that the latter two electronic states are energy-degenerate, and much lower in energy than the singlet state. Along compounds 1 t to 5 t, R − O_exo bonds gradually decrease in strength, while U − O_exo bond gradually increases. The quantum theory of atoms in molecule (QTAIM) analyses show that the R − O_exo bond is a covalent one for 1 t, and it turns a covalent/ionic mixed bond in 2 t and 3 t, and is attributed to a dative bond for 4 t and 5 t. From 1 t to 4 t, the HOMO and H-1 orbitals, as well as the π(R − O_exo) and π(U − O_exo) orbitals ascend to the higher energy level. In addition, the shortest bond distance, the maximum vibration wavenumber and the most negative interaction energy E_int of R − O_exo bond result in the strongest CCI in 1 t among 1 t − 5 t, along with the corresponding lowest reaction free energy. Our calculations reveal that the CCIs are instrumental in enhancing the stability of 1 t − 5 t.

Methods

Structural optimizations of all compounds were performed in the gas phase using the Priroda code. A generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhoff (PBE) functional was used. All-electron correlation-consistent double-ς polarized quality basis sets were used in all calculations. Single point calculations have been performed by using the ADF 2012 code on the basis of optimized geometries from Priroda code. The scalar relativistic zero-order regular approximation (ZORA) method and Slater-type triple-zeta polarization (TZP) basis sets were employed. Solvation effects were considered with the Conductor-Like Screening Model (COSMO) and spin–orbit coupling (SOC) effects were explicitly included in the calculations. Single-point calculations were carried out using the Gaussian09 program. Stuttgart relativistic large-core effective core potentials (RLC-ECPs) and corresponding basis sets were applied for U, def2SVP for Sn and Pb, and 6-31G* for other atoms. Then, the quantum theory of atoms in molecules (QTAIM) data were computed with the Multiwfn 3.3.3 package.