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

摘要

The reaction of (C5Me5)_2U(CH3)_2 with 2 equiv of N ident to C-Ar_F gives the fiuorinated uranium(IV) bis(ketimide) complexes (C5Me5)_2U[-N=C(CH3)(Ar_F)]_2 [where Ar_F=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 ~(19)F NMR,cyclic voltammetry,UV-visible-near-lR 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 fiuorinated aryl (Ar_F) group out of the plane defined by the N=C(CMe)(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 {=53.7-89.4°) between the planes formed by ketimide atoms N=C(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°.Complex 4 with a single o-fluorine substituent has intermediate values of phi=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 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 fiuorinated 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 ~50 K that results from admixing of low-lying crystal-field excited states derived from the symmetry-split ~3H_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.