Homoleptic Uranium(IV) Alkyl Complexes: Synthesis and Characterization

Carbon-carbon reductive elimination from homoleptic uranium(IV) alkyls induced by redox-active ligands

The synthesis, characterization, and reactivity of the homoleptic uranium(IV) alkyls U(CH(2)C(6)H(5))(4) (1-Ph), U(CH(2)-p-CH(3)C(6)H(4))(4) (1-p-Me), and U(CH(2)-m-(CH(3))(2)C(6)H(3))(4) (1-m-Me(2)) are reported. The addition of 4 equiv of K(CH(2)Ar) (Ar = Ph, p-CH(3)C(6)H(4), m-(CH(3))(2)C(6)H(3)) to UCl(4) at -108 °C produces 1-Ph in good yields and 1-p-Me and 1-m-Me(2) in moderate yields. Further characterization of 1-Ph by X-ray crystallography confirmed η(4)-coordination of each benzyl ligand to the uranium center. Magnetic studies produced an effective magnetic moment of 2.60 μ(B) at 23 °C, which is consistent with a tetravalent uranium 5f(2) electronic configuration. Addition of 1 equiv of the redox-active α-diimine (Mes)DAB(Me) ((Mes)DAB(Me) = [ArN═C(Me)C(Me)═NAr]; Ar = 2,4,6-trimethylphenyl (Mes)) to 1-Ph results in reductive elimination of 1 equiv of bibenzyl (PhCH(2)CH(2)Ph), affording ((Mes)DAB(Me))U(CH(2)C(6)H(5))(2) (2-Ph). Treating an equimolar mixture of 1-Ph and 1-Ph-d(28) with (Mes)DAB(Me) forms the products from monomolecular reductive elimination, 2-Ph, 2-Ph-d(14), bibenzyl, and bibenzyl-d(14). This is confirmed by (1)H NMR spectroscopy and GC/MS analysis of both organometallic and organic products. Addition of 1 equiv of 1,2-bis(dimethylphosphino)ethane (dmpe) to 1-Ph results in formation of the previously synthesized (dmpe)U(CH(2)C(6)H(5))(4) (3-Ph), indicating the redox-innocent chelating phosphine stabilizes the uranium center in 3-Ph and prevents reductive elimination of bibenzyl. Full characterization for 3-Ph, including X-ray crystallography, is reported.

Probing the 5f orbital contribution to the bonding in a U(V) ketimide complex

Reaction of UCl(4) with 5 equiv of Li(N═C(t)BuPh) generates the homoleptic U(IV) ketimide complex [Li(THF)(2)][U(N═C(t)BuPh)(5)] (1) in 71% yield. Similarly, reaction of UCl(4) with 5 equiv of Li(N═C(t)Bu(2)) affords [Li(THF)][U(N═C(t)Bu(2))(5)] (2) in 67% yield. Oxidation of 2 with 0.5 equiv of I(2) results in the formation of the neutral U(V) complex U(N═C(t)Bu(2))(5) (3). In contrast, oxidation of 1 with 0.5 equiv of I(2), followed by addition of 1 equiv of Li(N═C(t)BuPh), generates the octahedral U(V) ketimide complex [Li][U(N═C(t)BuPh)(6)] (4) in 68% yield. Complex 4 can be further oxidized to the U(VI) ketimide complex U(N═C(t)BuPh)(6) (5). Complexes 1-5 were characterized by X-ray crystallography, while SQUID magnetometry, EPR spectroscopy, and UV-vis-NIR spectroscopy measurements were also preformed on complex 4. Using this data, the crystal field splitting parameters of the f orbitals were determined, allowing us to estimate the amount of f orbital participation in the bonding of 4.

Synthesis, characterization, and multielectron reduction chemistry of uranium supported by redox-active α-diimine ligands

Uranium compounds supported by redox-active α-diimine ligands, which have methyl groups on the ligand backbone and bulky mesityl substituents on the nitrogen atoms {(Mes)DAB(Me) = [ArN═C(Me)C(Me)═NAr], where Ar = 2,4,6-trimethylphenyl (Mes)}, are reported. The addition of 2 equiv of (Mes)DAB(Me), 3 equiv of KC(8), and 1 equiv of UI(3)(THF)(4) produced the bis(ligand) species ((Mes)DAB(Me))(2)U(THF) (1). The metallocene derivative, Cp(2)U((Mes)DAB(Me)) (2), was generated by the addition of an equimolar ratio of (Mes)DAB(Me) and KC(8) to Cp(3)U. The bond lengths in the molecular structure of both species confirm that the α-diimine ligands have been doubly reduced to form ene-diamide ligands. Characterization by electronic absorption spectroscopy shows weak, sharp transitions in the near-IR region of the spectrum and, in combination with the crystallographic data, is consistent with the formulation that tetravalent uranium ions are present and supported by ene-diamide ligands. This interpretation was verified by U L(III)-edge X-ray absorption near-edge structure (XANES) spectroscopy and by variable-temperature magnetic measurements. The magnetic data are consistent with singlet ground states at low temperature and variable-temperature dependencies that would be expected for uranium(IV) species. However, both complexes exhibit low magnetic moments at room temperature, with values of 1.91 and 1.79 μ(B) for 1 and 2, respectively. Iodomethane was used to test the reactivity of 1 and 2 for multielectron transfer. While 2 showed no reactivity with CH(3)I, the addition of 2 equiv of iodomethane to 1 resulted in the formation of a uranium(IV) monoiodide species, ((Mes)DAB(Me))((Mes)DAB(Me2))UI {3; (Mes)DAB(Me2) = [ArN═C(Me)C(Me(2))NAr]}, which was characterized by single-crystal X-ray diffraction and U M(4)- and M(5)-edge XANES. Confirmation of the structure was also attained by deuterium labeling studies, which showed that a methyl group was added to the ene-diamide ligand carbon backbone.

Actinide metals with multiple bonds to carbon: synthesis, characterization, and reactivity of U(IV) and Th(IV) bis(iminophosphorano)methandiide pincer carbene complexes

Treatment of ThCl(4)(DME)(2) or UCl(4) with 1 equiv of dilithiumbis(iminophosphorano) methandiide, [Li(2)C(Ph(2)P═NSiMe(3))(2)] (1), afforded the chloro actinide carbene complexes [Cl(2)M(C(Ph(2)P═NSiMe(3))(2))] (2 (M = Th) and 3 (M = U)) in situ. Stable PCP metal-carbene complexes [Cp(2)Th(C(Ph(2)P═NSiMe(3))(2))] (4), [Cp(2)U(C(Ph(2)P═NSiMe(3))(2))] (5), [TpTh(C(Ph(2)P═NSiMe(3))(2))Cl] (6), and [TpU(C(Ph(2)P═NSiMe(3))(2))Cl] (7) were generated from 2 or 3 by further reaction with 2 equiv of thallium(I) cyclopentadienide (CpTl) in THF to yield 4 or 5 or with 1 equiv of potassium hydrotris(pyrazol-1-yl) borate (TpK) also in THF to give 6 or 7, respectively. The derivative complexes were isolated, and their crystal structures were determined by X-ray diffraction. All of these U (or Th)-carbene complexes (4-7) possess a very short M (Th or U)═carbene bond with evidence for multiple bond character. Gaussian 03 DFT calculations indicate that the M═C double bond is constructed by interaction of the 5f and 6d orbitals of the actinide metal with carbene 2p orbitals of both π and σ character. Complex 3 reacted with acetonitrile or benzonitrile to cyclo-add C≡N to the U═carbon double bond, thereby forming a new C-C bond in a new chelated quadridentate ligand in the bridged dimetallic complexes (9 and 10). A single carbon-U bond is retained. The newly coordinated uranium complex dimerizes with one equivalent of unconverted 3 using two chlorides and the newly formed imine derived from the nitrile as three connecting bridges. In addition, a new crystal structure of [CpUCl(3)(THF)(2)] (8) was determined by X-ray diffraction.