ABSTRACT
The magnetic exchange coupling between two diuranium(V) ions exhibiting the 5f1-5f1 configuration in diimide-bridged complexes [Cp3UV]2(µ-L) (L = stilbene-, naphthalene-diimide) has been investigated theoretically using relativistic ZORA/DFT calculations. Using two different hybrid PBE0 and B3LYP functionals, combined with the broken symmetry (BS) approach, we found that the BS states of both naphthalene and stilbene complexes have lower energy than the corresponding high-spin (HS) triplet ones. The B3LYP/BS estimated exchange coupling J constants (- 16.1 vs. - 9.0 cm-1 respectively for the naphthalene and stilbene complexes) corroborate well with those obtained previously for other pentavalent diuranium(V) diimide-bridged systems. The computed J value is found to be sensitive to π-network linking the two magnetic U(V) centers. The natural spin density distributions and molecular orbital analyses explain well the antiferromagnetic character of these compounds and clarify the crucial role of the π aromatic spacer in promoting spin polarization and delocalization favoring the magnetic coupling. Furthermore, the effective involvement of the 6d/5f metal orbitals in metal-ligand bonding plays an important role for the magnetic communication between the two active U(V) 5f electrons. Graphical abstract.
ABSTRACT
The uranyl(V) complexes [UO2(dbm)2K(18C6)]2 (dbm = dibenzoylmethanate) and [UO2(L)]3(L = 2-(4-tolyl)-1,3-bis(quinolyl)malondiiminate), exhibiting diamond-shaped U2O2 and triangular-shaped U3O3 cores respectively with 5f1-5f1 and 5f1-5f1-5f1 configurations, have been investigated using relativistic density functional theory (DFT). The bond order and QTAIM analyses reveal that the covalent contribution to the bonding within the oxo cores is slightly more important for U3O3 than for U2O2, in line with the shorter U-O distances existing in the trinuclear complex in comparison to those in the binuclear complex. Using the broken symmetry (BS) approach combined with the B3LYP functional for the calculation of the magnetic exchange coupling constants (J) between the magnetic centers, the antiferromagnetic (AF) character of these complexes was confirmed, the estimated J values being respectively equal to -24.1 and -7.2 cm-1 for the dioxo and trioxo species. It was found that the magnetic exchange is more sensitive to small variations of the core geometry of the dioxo species in comparison to the trioxo species. Although the robust AF exchange coupling within the UxOx cores is generally maintained when small variations of the UOU angle are applied, a weak ferromagnetic character appears in the dioxo species when this angle is higher than 114°, its value for the actual structure being equal to 105.9°. The electronic factors driving the magnetic coupling are discussed.
