Organometallic Uranium(V)−Imido Halide Complexes: From Synthesis to Electronic Structure and Bonding

Reaction of (C5Me5)2U(N-2,4,6-tBu3-C6H2) or (C5Me5)2U(N-2,6-iPr2-C6H3)(THF) with 5 equiv of CuXn (n = 1, X = Cl, Br, I; n = 2, X = F) affords the corresponding uranium(V)−imido halide complexes, (C5Me5)2U(N-Ar)(X) (where Ar = 2,4,6-tBu3-C6H2 and X = F (3), Cl (4), Br (5), I (6); Ar = 2,6-iPr2-C6H3 and X = F (7), Cl (8), Br (9), I (10)), in good isolated yields of 75–89%. These compounds have been characterized by a combination of single-crystal X-ray diffraction, 1H NMR spectroscopy, elemental analysis, mass spectrometry, cyclic voltammetry, UV–visible−NIR absorption spectroscopy, and variable-temperature magnetic susceptibility. The uranium LIII-edge X-ray absorption spectrum of (C5Me5)2U(N-2,4,6-tBu3-C6H2)(Cl) (4) was analyzed to obtain structural information, and the UNimido (1.97(1) Å), U−Cl (2.60(2) Å), and U−C5Me5 (2.84(1) Å) distances were consistent with those observed for compounds 3, 5, 6, 8–10, which were all characterized by single-crystal X-ray diffraction studies. All (C5Me5)2U(N-Ar)(X) complexes exhibit UV/UIV and UVI/UV redox couples by voltammetry, with the potential separation between these metal-based couples remaining essentially constant at ∼1.50 V. The electronic spectra are comprised of π→π* and π→nb5f transitions involving electrons in the metal−imido bond, and metal-centered f−f bands illustrative of spin–orbit and crystal-field influences on the 5f1 valence electron configuration. Two distinct sets of bands are attributed to transitions derived from this 5f1 configuration, and the intensities in these bands increase dramatically over those found in spectra of classical 5f1 actinide coordination complexes. Temperature-dependent magnetic susceptibilities are reported for all complexes with μeff values ranging from 2.22 to 2.53 μB. The onset of quenching of orbital angular momentum by ligand fields is observed to occur at ∼40 K in all cases. Density functional theory results for the model complexes (C5Me5)2U(N-C6H5)(F) (11) and (C5Me5)2U(N-C6H5)(I) (12) show good agreement with experimental structural and electrochemical data and provide a basis for assignment of spectroscopic bands. The bonding analysis describes multiple bonding between the uranium metal center and imido nitrogen which is comprised of one σ and two π interactions with variable participation of 5f and 6d orbitals from the uranium center.