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

Reaction of (C5Me5)2Th(CH3)2 with 2 equiv of N⋮C−ArF gives the corresponding fluorinated
thorium(IV) bis(ketimide) complexes (C5Me5)2Th[−NC(CH3)(ArF)]2 (where ArF = 3-F-C6H4 (4), 4-F-C6H4
(5), 2-F-C6H4 (6), 3,5-F2-C6H3 (7), 3,4,5-F3-C6H2 (8), 2,6-F2-C6H3 (9), 2,4,6-F3-C6H2 (10), and C6F5 (11)).
The complexes have been characterized by a combination of single-crystal X-ray diffraction, cyclic
voltammetry and NMR, and UV−visible absorption and low-temperature luminescence spectroscopies.
Density functional theory (DFT) and time-dependent DFT (TD-DFT) results are reported for complexes 5,
11, and (C5Me5)2Th[−NC(Ph)2]2 (1) for comparison with experimental data and to guide in the interpretation
of the spectroscopic results. The most significant structural perturbation imparted by the fluorine substitution
in these complexes is a rotation of the fluorophenyl group (ArF) out of the plane defined by the NC(CMe)(Cipso) fragment in complexes 9−11 when the ArF group possesses two ortho fluorine atoms. Excellent
agreement is obtained between the optimized ground state DFT calculated structures and crystal structures
for 11, which displays the distortion, as well as 5, which does not. In complexes 9−11, the out-of-plane
rotation results in large interplanar angles (φ) between the planes formed by ketimide atoms NC(CMe)(Cipso) and the ketimide aryl groups in the range φ = 49.1−88.8 °, while in complexes 5, 7, and 8, φ =
5.7−34.9 °. The large distortions in 9−11 are a consequence of an unfavorable steric interaction between
one of the two ortho fluorine atoms and the methyl group [−NC(CH3)] on the ketimide ligand. Excellent
agreement is also observed between the experimental electronic spectroscopic data and the TD-DFT
predictions that the two lowest lying singlet states are principally of nonbonding nitrogen p orbital to
antibonding CN π* orbital (pN→π*CN or nπ*) character, giving rise to moderately intense transitions in
the mid-visible spectral region that are separated in energy by less than 0.1 eV. Low-temperature (77 K)
luminescence from both singlet and triplet excited states are also observed for these complexes. Emission
lifetime data at 77 K for the triplet states are in the range 50−400 μs. These emission spectral data also
exhibit vibronic structure indicative of a small Franck−Condon distortion in the ketimide M−NC(R1)(R2)
linkage. Consistent with this vibronic structure, resonance enhanced Raman vibrational scattering is also
observed for (C5Me5)2Th[−NC(Ph)(CH2Ph)]2 (2) when exciting into the visible excited states. These
systems represent rare examples of Th(IV) complexes that engender luminescence and resonance Raman
spectral signatures.