Systematic Studies of Early Actinide Complexes: Uranium(IV) Fluoroketimides

The reaction of (C5Me5)2U(CH3)2 with 2 equiv of N⋮C−ArF gives the fluorinated uranium(IV) bis(ketimide) complexes
(C5Me5)2U[−NC(CH3)(ArF)]2 [where ArF = 2-F-C6H4 (4), 3-F-C6H4 (5), 4-F-C6H4 (6), 2,6-F2-C6H3 (7), 3,5-F2-C6H3
(8), 2,4,6-F3-C6H2 (9), 3,4,5-F3-C6H2 (10), and C6F5 (11)]. These have been characterized by single-crystal X-ray
diffraction, 1H and 19F NMR, cyclic voltammetry, UV−visible−near-IR absorption spectroscopy, and variable-temperature
magnetic susceptibility. Density functional theory (DFT) results are reported for complexes 6 and 11 for comparison
with experimental data. The most significant structural perturbation imparted by the F substitution in these complexes
is a rotation of the fluorinated aryl (ArF) group out of the plane defined by the NC(CMe)(Cipso) fragment in complexes
7, 9, and 11 when the ArF group possesses two o-fluorine atoms. Excellent agreement is obtained between the
DFT-calculated and experimental crystal structures for 11, which displays the distortion, as well as for 6, which
does not. In 7, 9, and 11, the out-of-plane rotation results in large angles (φ = 53.7−89.4°) between the planes
formed by ketimide atoms NC(CMe)(Cipso) and the ketimide aryl groups. Complexes 6 and 10 do not contain
o-fluorine atoms and display interplanar angles in the range of φ = 7−26.8°. Complex 4 with a single o-fluorine
substituent has intermediate values of φ = 20.4 and 49.5°. The distortions in 7, 9, and 11 result from an unfavorable
steric interaction between one of the two o-fluorine atoms and the methyl group [−NC(CH3)] on the ketimide
ligand. All complexes exhibit UV/UIV and UIV/UIII redox couples, although the distortion in 7, 9, and 11 appears to
be a factor in rendering the UIV/UIII couple irreversible. The potential separation between these couples remains
constant at 2.15 ± 0.03 V. The electronic spectra are dominated by unusually intense f−f transitions in the near-IR
that retain nearly identical band energies but vary in intensity as a function of the fluorinated ketimide ligand, and
visible and near-UV bands assigned to metal (5f)-to-ligand (π*) charge-transfer and interconfiguration (5f2 → 5f16d1) transitions, respectively. Variable-temperature magnetic susceptibility data for these complexes indicate a
temperature-independent paramagnetism (TIP) below ∼50 K that results from admixing of low-lying crystal-field
excited states derived from the symmetry-split 3H4 5f2 manifold into the ground state. The magnitude of the TIP is
smaller for the complexes possessing two o-fluorine atoms (7, 9, and 11), indicating that the energy separation
between ground and TIP-admixed excited states is larger as a consequence of the greater basicity of these ligands.