Skye Fortier

Synthesis of a Nitrido-Substituted Analogue of the Uranyl Ion, [N═U═O]+

Addition of 0.5 equiv of NaN(3) to U[NR(2)](3) (R = SiMe(3)) affords the metallacycle [Na(DME)(2)(TMEDA)][(NR(2))(2)U(mu-N)(CH(2)SiMe(2)NR)U(NR(2))(2)] (1) in 69% yield. Complex 1 is readily oxidized by 0.5 equiv of I(2), generating the mixed valent U(IV/V) nitrido complex (NR(2))(2)U(mu-N)(CH(2)SiMe(2)NR)U(NR(2))(2) (2). Alternatively, oxidation of 1 with 1 equiv of Me(3)NO affords the oxo-nitrido complex [Na(DME)(2)][(NR(2))(2)(O)U(mu-N)(CH(2)SiMe(2)NR)U(NR(2))(2)] (3) in good yield. The solid-state molecular structures of 1-3 have been determined by X-ray crystallography. A notable feature of these complexes are the inequivalent U=N-U bonding interactions. Moreover, 3 contains a trans oxo-nitrido [O=U=N](+) moiety with metrical parameters approximating those of the uranyl cation, UO(2)(2+). The magnetic properties of 1-3 were investigated by SQUID magnetometry.

Synthesis, Molecular and Electronic Structure of U-V(O) N(SiMe3)(2) (3)

Addition of 1 equiv of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) to U(NR(2))(3) in hexanes affords U(O)(NR(2))(3) (2), which can be isolated in 73% yield. Complex 2 is a rare example of a terminal U(V) oxo complex. In contrast, addition of 1 equiv of Me(3)NO to U(NR(2))(3) (R = SiMe(3)) in pentane generates the U(IV) bridging oxo [(NR(2))(3)U](2)(μ-O) (3) in moderate yields. Also formed in this reaction, in low yield, is the U(IV) iodide complex U(I)(NR(2))(3) (4). The iodide ligand in 4 likely originates from residual NaI, present in the U(NR(2))(3) starting material. Complex 4 can be generated rationally by addition of 0.5 equiv of I(2) to a hexane solution of U(NR(2))(3), where it can be isolated in moderate yield as a tan crystalline solid. The solid-state molecular structures and magnetic susceptibilities of 2, 3, and 4 have been measured. In addition, the electronic structures of 2 and 3 have been investigated by density functional theory (DFT) methods.

Probing the reactivity and electronic structure of a uranium(V) terminal oxo complex

Treatment of the U(III)-ylide adduct U(CH(2)PPh(3))(NR(2))(3) (1, R = SiMe(3)) with TEMPO generates the U(V) oxo metallacycle [Ph(3)PCH(3)][U(O)(CH(2)SiMe(2)NSiMe(3))(NR(2))(2)] (2) via O-atom transfer, in good yield. Oxidation of 2 with 0.85 equiv of AgOTf affords the neutral U(VI) species U(O)(CH(2)SiMe(2)NSiMe(3))(NR(2))(2) (3). The electronic structures of 2 and 3 are investigated by DFT analysis. Additionally, the nucleophilicity of the oxo ligands in 2 and 3 toward Me(3)SiI is explored.

Synthesis of a Phosphorano-Stabilized U(IV)-Carbene via One-Electron Oxidation of a U(III)-Ylide Adduct

Addition of the Wittig reagent Ph(3)P═CH(2) to the U(III) tris(amide) U(NR(2))(3) (R = SiMe(3)) generates a mixture of products from which the U(IV) complex U═CHPPh(3)(NR(2))(3) (2) can be obtained. Complex 2 features a short U═C bond and represents a rare example of a uranium carbene. In solution, 2 exists in equilibrium with the U(IV) metallacycle U(CH(2)SiMe(2)NR)(NR(2))(2) and free Ph(3)P═CH(2). Measurement of this equilibrium as a function of temperature provides ΔH(rxn) = 11 kcal/mol and ΔS(rxn) = 31 eu. Additionally, the electronic structure of the U═C bond was investigated using DFT analysis.

Homoleptic uranium(IV) alkyl complexes: synthesis and characterization.

The addition of 4.5 equiv of LiCH(2)SiMe(3) to [Li(THF)](2)[U(O(t)Bu)(6)], in the presence of LiCl, results in the formation of the homoleptic uranium(IV) alkyl complex [Li(14)(O(t)Bu)(12)Cl][U(CH(2)SiMe(3))(5)] (1) in low yield. Complex 1 has been characterized by X-ray crystallography. As a solid, 1 is thermally stable for several days at room temperature. However, 1 rapidly decomposes in C(6)D(6), as indicated by (1)H and (7)Li{(1)H} NMR spectroscopy, owing to the lability of the [Li(14)(O(t)Bu)(12)Cl](+) cation. To avoid the formation of the [Li(14)(O(t)Bu)(12)Cl](+) counterion, alkylation of UCl(4) was investigated. Treatment of UCl(4) with 5 equiv of LiCH(2)SiMe(3) or LiCH(2)(t)Bu at -25 degrees C in THF/Et(2)O affords [Li(DME)(3)][U(CH(2)SiMe(3))(5)] (2) and [Li(THF)(4)][U(CH(2)(t)Bu)(5)] (3), respectively, in good yields. Similarly, treatment of UCl(4) with 6 equiv of MeLi or KCH(2)C(6)H(5) generates the U(IV) hexa(alkyl) complexes [Li(TMEDA)](2)[UMe(6)] (4) and {[K(THF)](3)[K(THF)(2)][U(CH(2)C(6)H(5))(6)](2)}(x) (5) in 38% and 70% yields, respectively. The structures of 3-5 have been confirmed by X-ray crystallography. Complexes 2, 3, and 5 are thermally stable solids which can be stored at room temperature for several days, whereas 4 decomposes upon warming above -25 degrees C. The electronic and magnetic properties of 2, 3, and 5 were also investigated by NIR spectroscopy and SQUID magnetometry.

High-Valent Uranium Alkyls: Evidence for the Formation of UVI(CH2SiMe3)6

Oxidation of [Li(DME)(3)][U(CH(2)SiMe(3))(5)] with 0.5 equiv of I(2), followed by immediate addition of LiCH(2)SiMe(3), affords the high-valent homoleptic U(V) alkyl complex [Li(THF)(4)][U(CH(2)SiMe(3))(6)] (1) in 82% yield. In the solid-state, 1 adopts an octahedral geometry as shown by X-ray crystallographic analysis. Addition of 2 equiv of tert-butanol to [Li(DME)(3)][U(CH(2)SiMe(3))(5)] generates the heteroleptic U(IV) complex [Li(DME)(3)][U(O(t)Bu)(2)(CH(2)SiMe(3))(3)] (2) in high yield. Treatment of 2 with AgOTf fails to produce a U(V) derivative, but instead affords the U(IV) complex (Me(3)SiCH(2))Ag(μ-CH(2)SiMe(3))U(CH(2)SiMe(3))(O(t)Bu)(2)(DME) (3) in 64% yield. Complex 3 has been characterized by X-ray crystallography and is marked by a uranium-silver bond. In contrast, oxidation of 2 can be achieved via reaction with 0.5 equiv of Me(3)NO, producing the heteroleptic U(V) complex [Li(DME)(3)][U(O(t)Bu)(2)(CH(2)SiMe(3))(4)] (4) in moderate yield. We have also attempted the one-electron oxidation of complex 1. Thus, oxidation of 1 with U(O(t)Bu)(6) results in formation of a rare U(VI) alkyl complex, U(CH(2)SiMe(3))(6) (6), which is only stable below -25 °C. Additionally, the electronic properties of 1-4 have been assessed by SQUID magnetometry, while a DFT analysis of complexes 1 and 6 is also provided.

Quantifying the σ and π interactions between U(V) f orbitals and halide, alkyl, alkoxide, amide and ketimide ligands

f Orbital bonding in actinide and lanthanide complexes is critical to their behavior in a variety of areas from separations to magnetic properties. Octahedral f(1) hexahalide complexes have been extensively used to study f orbital bonding due to their simple electronic structure and extensive spectroscopic characterization. The recent expansion of this family to include alkyl, alkoxide, amide, and ketimide ligands presents the opportunity to extend this study to a wider variety of ligands. To better understand f orbital bonding in these complexes, the existing molecular orbital (MO) model was refined to include the effect of covalency on spin orbit coupling in addition to its effect on orbital angular momentum (orbital reduction). The new MO model as well as the existing MO model and the crystal field (CF) model were applied to the octahedral f(1) complexes to determine the covalency and strengths of the σ and π bonds formed by the f orbitals. When covalency is significant, MO models more precisely determined the strengths of the bonds derived from the f orbitals; however, when covalency was small, the CF model was better than either MO model. The covalency determined using the new MO model is in better agreement with both experiment and theory than that predicted by the existing MO model. The results emphasize the role played by the orbital energy in determining the strength and covalency of bonds formed by the f orbitals.

Synthesis and Spectroscopic and Computational Characterization of the Chalcogenido-Substituted Analogues of the Uranyl Ion, [OUE]2+ (E = S, Se)

Addition of E (E = 0.125S8, Se) to [Cp*2Co][U(O)(NR2)3] (R = SiMe3) in THF results in the isolation of the chalcogen-substituted uranyl analogues [Cp*2Co][U(O)(E)(NR2)3] [E = S (1), Se (2)] in good yields. Similarly, addition of 1 equiv of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) to [Cp*2Co][U(O)(NR2)3] affords the uranyl complex [Cp*2Co][UO2(NR2)3] (3). All of the complexes were fully characterized, including analysis by X-ray crystallography. They were also analyzed by density functional theory calculations to probe the changes in the U-E bond as group 16 is descended.

A Rare Uranyl(VI)–Alkyl Ate Complex [Li(DME)1.5]2[UO2(CH2SiMe3)4] and Its Comparison with a Homoleptic Uranium(VI)–Hexaalkyl

Beginning with the Manhattan Project, uranium–alkyl complexes have been a coveted target in the actinide chemistry. The initial interest in these complexes was due to their anticipated volatility, which would permit their use in gas-phase isotope separation. In the 70 years since the Manhattan Project, the organometallic chemistry of uranium has flourished, however, this chemistry has been dominated by the 3 + and 4 + oxidation states. In contrast, the organometallic chemistry of the 5 + and 6 + states has lagged behind considerably. This is especially true for the uranyl ion, the most studied fragment in uranium chemistry. Despite the fact that the preparation of organometallic uranyl complexes has been attempted for over 150 years, the first uranyl–hydrocarbyl complex was only reported in 2002. It is now clear that many initial synthetic attempts resulted only in uranyl reduction, and not in alkylation. For example, Ephritikhine and co-workers demonstrated that reaction of one equivalent of Li(CH2SiMe3) with [UO2I2(THF)3] or [UO2(OTf)2] in pyridine resulted in the formation of a uranyl(V) complex, while Seyam observed formation of UO2 and biphenyl upon reaction of UO2Cl2 with two equivalents of phenyllithium. Only recently have chemists developed strategies for inhibiting the reduction of uranyl. For example, Sarsfield et al. used a chelating bis(iminophosphorano) ligand to stabilize the U C bond in [(BIPMH)UO2Cl(THF)] (A, BIPMH = HC(PPh2NSiMe3)2; Scheme 1). [9, 13] This strategy was later employed in the synthesis of the first uranyl–methanediide complex, [UO2(SCS)(py)2] (B, SCS 2 = [C(Ph2PS)2] 2 ). Also notable is the synthesis of the first cyclopentadienyl complex of uranyl, [NEt4]2[(C5Me5)UO2(CN)3] (C), [15] formed by oxygen atom transfer to a U–cyclopentadienyl precursor. Finally, our research group demonstrated that uranyl carbon s bonds could be stabilized by steric saturation of the uranium coordination sphere (so-called “ate” complex formation), as in the case of the bis(imidazolyl) complex, [Li(MeIm)][(UO2(Ar2nacnac)(C4H5N2)2] (D), [16] which exhibits unusual thermal stability for a U–hydrocarbyl complex. The strategy of “ate” complex formation has also allowed us to stabilize several homoleptic uranium–alkyl complexes in the 4 + , 5 + , and 6 + oxidation states, as well as several homoleptic thorium–alkyl complexes. 17,18] Their synthesis and characterization, in conjunction with quantum-chemical calculations, has enabled the study of the contribution of the 6d and 5f valence orbitals in actinide carbon bonds. These results inspired us to explore the synthesis of a uranyl–alkyl “ate” complex to gain further insight into the U C bonding interactions. Herein, we describe the synthesis of the rare uranyl(VI)–alkyl complex [Li(DME)1.5]2[UO2(CH2SiMe3)4] (1) and an analysis of its electronic structure by relativistic Scheme 1. Schematic structure of the recently synthesized uranyl complexes with direct U C s bonds. MeIm= 1-methylimidazole, py = pyridine.

Comparison of the redox chemistry of primary and secondary amides of U(IV): isolation of a U(VI) bis(imido) complex or a homoleptic U(VI) amido complex.

Reaction of UCl4 with 6 equiv of LiNH(t)Bu generates the U(IV) homoleptic amide complex [Li(THF)2Cl]2[Li]2[U(NH(t)Bu)6] (1 · THF) in 57% yield. In the solid-state, 1 · THF exists as a one-dimensional coordination polymer consisting of alternating [Li]2[U(NH(t)Bu)6] and [Li(THF)2Cl]2 building blocks. Recrystallization of 1 · THF from DME/hexanes affords the monomeric DME derivative, [Li(DME)2ClLi]2[U(NH(t)Bu)6] (1 · DME), which was also characterized by X-ray crystallography. The oxidation of 1 · THF with 1 equiv of AgOTF generates the U(VI) bis(imido) complex [Li(THF)]2[U(N(t)Bu)2(NH(t)Bu)4] (2) in low yield. In contrast, oxidation of 1 · THF with 1 equiv of I2, in the presence of excess tert-butylamine, cleanly affords the U(VI) bis(imido) U(N(t)Bu)2(NH(t)Bu)2(NH2(t)Bu)2 (3) in 78% yield. We have also explored the reactivity of UCl4 with the lithium salt of a secondary amide. Thus, reaction of 6 equiv of (LiNC5H10) (HNC5H10 = piperidine) with UCl4 in DME produces the U(IV) amide, [Li(DME)][U(NC5H10)5] (4). Oxidation of this material with 0.5 equiv of I2, followed by addition of Li(NC5H10), produces [Li(DME)3][U(NC5H10)6] (5) in moderate yield. Oxidation of 5 with 0.5 equiv of I2 generates U(NC5H10)6 (6) in good yield. The structures of 4-6 were elucidated by X-ray crystallographic analysis, while the magnetic properties of 4 and 5 were investigated by SQUID magnetometry. Additionally, the solution phase redox properties of 5 were examined by cyclic voltammetry.