Investigation of the electronic ground states for a reduced pyridine(diimine) uranium series: evidence for a ligand tetraanion stabilized by a uranium dimer.

The electronic structures of a series of highly reduced uranium complexes bearing the redox-active pyridine(diimine) ligand, (Mes)PDI(Me) ((Mes)PDI(Me) = 2,6-(2,4,6-Me3-C6H2-N═CMe)2C5H3N) have been investigated. The complexes, ((Mes)PDI(Me))UI3(THF) (1), ((Mes)PDI(Me))UI2(THF)2 (2), [((Mes)PDI(Me))UI]2 (3), and [((Mes)PDI(Me))U(THF)]2 (4), were examined using electronic and X-ray absorption spectroscopies, magnetometry, and computational analyses. Taken together, these studies suggest that all members of the series contain uranium(IV) centers with 5f (2) configurations and reduced ligand frameworks, specifically [(Mes)PDI(Me)](•/-), [(Mes)PDI(Me)](2-), [(Mes)PDI(Me)](3-) and [(Mes)PDI(Me)](4-), respectively. In the cases of 2, 3, and 4 no unpaired spin density was found on the ligands, indicating a singlet diradical ligand in monomeric 2 and ligand electron spin-pairing through dimerization in 3 and 4. Interaction energies, representing enthalpies of dimerization, of -116.0 and -144.4 kcal mol(-1) were calculated using DFT for the monomers of 3 and 4, respectively, showing there is a large stabilization gained by dimerization through uranium-arene bonds. Highlighted in these studies is compound 4, bearing a previously unobserved pyridine(diimine) tetraanion, that was uniquely stabilized by backbonding between uranium cations and the η(5)-pyridyl ring.

Harnessing redox activity for the formation of uranium tris(imido) compounds.

Classically, late transition-metal organometallic compounds promote multielectron processes solely through the change in oxidation state of the metal centre. In contrast, uranium typically undergoes single-electron chemistry. However, using redox-active ligands can engage multielectron reactivity at this metal in analogy to transition metals. Here we show that a redox-flexible pyridine(diimine) ligand can stabilize a series of highly reduced uranium coordination complexes by storing one, two or three electrons in the ligand. These species reduce organoazides easily to form uranium-nitrogen multiple bonds with the release of dinitrogen. The extent of ligand reduction dictates the formation of uranium mono-, bis- and tris(imido) products. Spectroscopic and structural characterization of these compounds supports the idea that electrons are stored in the ligand framework and used in subsequent reactivity. Computational analyses of the uranium imido products probed their molecular and electronic structures, which facilitated a comparison between the bonding in the tris(imido) structure and its tris(oxo) analogue.

Variation of electronic transitions and reduction potentials of cerium(IV) complexes.

The trivalent compound K[Ce[N(SiHMe2)2]4] was synthesized and oxidized, providing a convenient route to the reported cerium(IV) compound Ce[N(SiHMe2)2]4. Protonolysis reactions of Ce[N(SiHMe2)2]4 with tert-butanol, substituted benzyl alcohols, and 2,6-diphenylphenol yielded the neutral tetravalent compounds Ce(O(t)Bu)4(py)2, Ce2(OCH2C6R5)8(thf)2 (R = Me, F), and Ce(Odpp)4 (dpp = 2,6-(C6H5)2-C6H3). Spectroscopic and electrochemical characterization of the monometallic cerium(IV) silylamide, alkoxide, and aryloxide compounds revealed variable ligand-to-metal charge transfer transitions and metal-based reduction potentials. Computational bonding analyses were performed to complement the physical characterization of the complexes.

Synthesis and analysis of a family of cerium(IV) halide and pseudohalide compounds.

The first complete series of isostructural cerium(IV) halide complexes in a conserved ligand framework was isolated by halogen-exchange reactions of CeF[N(SiMe3)2]3 with Me3SiX (X = Cl(-), Br(-), I(-)). The use of Me3SiX reagents represents a useful method for obtaining cerium(IV) complexes. Spectroscopic, electrochemical, and computational analyses were used to describe the effects of halide coordination on the cerium(IV) metal center. Cerium(IV) complexes of the pseudohalide ligands: N3(-) and NCS(-) were also synthesized and evaluated in comparison to the halide congeners. The results showed that the complexes exhibited reduction potentials and electronic absorption energies that varied with the identity of the halide or pseudohalide ligand.

Synthesis, bonding, and reactivity of a cerium(IV) fluoride complex.

Oxidation of Ce[N(SiMe3)2]3 in the presence of PF6(-) or BF4(-) afforded isolation of CeF[N(SiMe3)2]3. Structural and electrochemical characterization shows that this compound is in its tetravalent oxidation state and contains a terminal fluoride ligand. Spectroscopy and density functional theory have been used to characterize the Ce-F bond as ionic, which is reinforced by an initial reactivity study that demonstrates the nucleophilicity of the fluoride ligand.

Tetrakis(bis(trimethylsilyl)amido)uranium(IV): synthesis and reactivity.

The synthesis of the sterically saturated uranium(IV) complex U[N(SiMe3)2]4 (1) is demonstrated from the one-electron oxidation of U[N(SiMe3)2]3 with a variety of oxidants in THF. A high yielding synthesis of 1 directly from UI3(THF)4 is provided.

Single crystal to single crystal transformation and hydrogen-atom transfer upon oxidation of a cerium coordination compound.

Trivalent and tetravalent cerium compounds of the octamethyltetraazaannulene (H2omtaa) ligand have been synthesized. Electrochemical analysis shows a strong thermodynamic preference for the formal cerium(IV) oxidation state. Oxidation of the cerium(III) congener Ce(Homtaa)(omtaa) occurs by hydrogen-atom transfer that includes a single crystal to single crystal transformation upon exposure to an ambient atmosphere.

A comparison of the effects of symmetry and magnetoanisotropy on paramagnetic relaxation in related dysprosium single ion magnets.

Dysprosium complexes of the tmtaa(2-) ligand were synthesized and characterized by X-band EPR and magnetism studies. Both complexes demonstrate magnetoanisotropy and slow paramagnetic relaxation. Comparison of these compounds with the seminal phthalocyanine complex [Dy(Pc)(2)](-) shows the azaannulide complexes are more susceptible to relaxation through non-thermal pathways.

Uranium pyrrolylamine complexes featuring a trigonal binding pocket and interligand noncovalent interactions.

The syntheses of tri- and tetravalent uranium complexes of the Ar(F)(3)TPA(3-) ligand [Ar(F) = 3,5-bis(trifluoromethyl)phenyl; TPA = tris(pyrrolyl-α-methylamine)] are described. Interligand noncovalent interactions between arene groups within the complexes are detected both in the solid state and in solution.

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.

A general and modular synthesis of monoimidouranium(IV) dihalides.

The conproportionation reaction between the dimeric diimidouranium(V) species [U(N(t)Bu)(2)(I)((t)Bu(2)bpy)](2) ((t)Bu(2)bpy = 4,4'-di-tert-butyl-2,2'-bipyridyl) and UI(3)(THF)(4) in the presence of additional (t)Bu(2)bpy yields U(N(t)Bu)(I)(2)((t)Bu(2)bpy)(THF)(2) (2), an unprecedented example of a monoimidouranium(IV) dihalide complex. The general synthesis of this family of uranium(IV) derivatives can be achieved more readily by adding 2 equiv of MN(H)R (M = Li, K; R = (t)Bu, 2,6-(i)PrC(6)H(3), 2-(t)BuC(6)H(4)) to UX(4) in the presence of coordinating Lewis bases to give complexes with the general formula U(NR)(X)(2)(L)(n) (X = Cl, I; L = (t)Bu(2)bpy, n = 1; L = THF, n = 2). The complexes were characterized by (1)H NMR spectroscopy and single-crystal X-ray diffraction analysis of compounds 2 and {U[N(2,6-(i)PrC(6)H(3))](Cl)(2)(THF)(2)}(2) (4). (The X-ray structures of 5 and 6 are reported in the Supporting Information.)