Systematic studies of early actinide complexes: Uranium(IV) fluoroketimides

The reaction of (C5Me5)(2)U(CH3)(2) with 2 equiv of NC-Ar-F gives the fluorinated uranium(IV) bis(ketimide) complexes (C5Me5)(2)U[-NC(CH3)(Ar-F)](2) [where Ar-F = 2-F-C6H4 (4), 3-F-C6H4 (5), 4-F-C6H4 (6), 2,6-F-2-C6H3 (7), 3,5-F-2-C6H3 (8), 2,4,6-F-3-C6H2 (9), 3,4,5-F-3-C6H2 (10), and C6F5 (11)]. These have been characterized by single-crystal X-ray diffraction, H-1 and F-19 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 (Ar-F) group out of the plane defined by the NC(C-Me)(C-ipso) fragment in complexes 7, 9, and 11 when the Ar-F 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 (phi = 53.7-89.4 degrees) between the planes formed by ketimide atoms NC(C-Me)(C-ipso) and the ketimide aryl groups. Complexes 6 and 10 do not contain o-fluorine atoms and display interplanar angles in the range of phi = 7-26.8 degrees. Complex 4 with a single o-fluorine substituent has intermediate values of phi = 20.4 and 49.5 degrees. 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 [-NC(CH3)] on the ketimide ligand. All complexes exhibit U-V/U-IV and U-IV/U-III redox couples, although the distortion in 7, 9, and 11 appears to be a factor in rendering the U-IV/U-III 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 (pi*) charge-transfer and interconfiguration (5f(2) -> 5f(1)6d(1)) transitions, respectively. Variable-temperature magnetic susceptibility data for these complexes indicate a temperature-independent paramagnetism (TIP) below similar to 50 K that results from admixing of low-lying crystal-field excited states derived from the symmetry-split H-3(4) 5f(2) 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.