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

Abstract

The reaction of (C5Me5)2U(CH3)2 with 2 equiv of N[triple bond]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 (phi=53.7-89.4 degrees) 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 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 [-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 (pi*) charge-transfer and interconfiguration (5f2-->5f16d1) transitions, respectively. Variable-temperature magnetic susceptibility data for these complexes indicate a temperature-independent paramagnetism (TIP) below approximately 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.

Comparison of the Reactivity of 2‐Li‐C6H4CH2NMe2 with MCl4 (M=Th, U): Isolation of a Thorium Aryl Complex or a Uranium Benzyne Complex

“…This behavior is fully consistent with the proposed 4 + oxidationstate assignment. [16] The NIR spectrum of 2 is also consistent with the presence of the U IV ion. [15a, 17] Interestingly, complex 2 exhibits considerable thermal sensitivity, which partially explains the low yield isolated.…”

Comparative Study of f-Element Electronic Structure across a Series of Multimetallic Actinide and Lanthanoid-Actinide Complexes Possessing Redox-Active Bridging Ligands

“…29,30 The appearance of two, well-resolved tpy-based reduction waves in 5a separated by 210 mV indicates step-wise reduction of these groups and substantial electronic interaction between them in this complex. 39 The uranium analogue 5b also exhibits significant separation of the tpy-group reduction waves; however, the presence of a U IV/III reduction sandwiched between the tpy-reduction processes precludes direct comparison of the extent of electronic interaction between the tpy moieties in 5a versus 5b.…”

“…Data for the uranium analogue 5b is also given in the Figure 5, and is similar to 5a with the addition of two new redox processes at -0.38 and -2.42 V. These waves were assigned to U V/IV and U IV/III redox couples, consistent with benchmark data reported for uranium(IV) bis(ketimide) complexes. 29,30 Square-wave voltammetry affords clear resolution of the three reduction waves observed for 5b between -2.00 and -2.75 V and assignment of the wave at -2.42 to the U IV/III couple can be made definitively by noting the separation (∆E 1/2 = 2.04 V)…”

“…This observation is consistent with our studies of monometallic (C 5 Me 5 ) 2 An IV (An = Th and U) ketimide complexes in which the participation of the metal 5f-orbitals in metal-ketimide bonding is clearly manifest in both spectroscopic and electrochemical properties and is greater for uranium than thorium. 7,8,[29][30][31] The role of the bridging actinide moiety in enhancing communication in these heterotrimetallic systems is acutely illustrated by comparison to the behavior of the structurally analogous {(C 5 Me 5 ) 2 Yb} 2 (µ-1-Me-3,5-tpy 2 -C 6 H 2 ) (15) system in which the (C 5 Me 5 ) 2 An(ketimide) 2 bridge is replaced with a simple phenyl bridge. This latter system, even with occupied π* orbitals on the tpy moieties, shows no measurable splitting of the lanthanide-based reduction waves.…”

“…Complexes 4a-b and 5a-b were prepared by insertion of benzonitrile and 4'-cyano-2,2':6',2''-terpyridine, respectively, into the actinide-C benzyl bonds of (C 5 Me 5 ) 2 An(CH 2 C 6 H 5 ) 2 (An = Th, U). 6,25 We have observed that 4a-b and 5a-b and related systems show distinct characteristics of metal-ketimide ligand covalency in bonding as judged experimentally through ground-and excited-state electronic absorption spectroscopy, 8,[29][30][31][32] electrochemistry, 30 and magnetic susceptibility studies, 30 complimented by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. 7,30,31 The terpyridyl groups in complexes 5a-b provide a platform for the introduction of additional metal ions.…”