Lani A. Seaman

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.

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.

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

Why do U react like that? Reaction of 2-Li-C6H4CH2NMe2 with [MCl4(DME)n] (M=Th, n=2; M=U, n=0) results in the formation of a thorium aryl complex, [Th(2-C6H4CH2NMe2)4] or a uranium benzyne complex, [Li][U(2,3-C6H3CH2NMe2)(2-C6H4CH2NMe2)3]. A DFT analysis suggests that the formation of a benzyne complex with U but not with Th is a kinetic and not thermodynamic effect.

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.

Synthesis, structure and bonding of hexaphenyl thorium(IV): observation of a non-octahedral structure.

We report herein the synthesis of the first structurally characterized homoleptic actinide aryl complexes, [Li(DME)3]2[Th(C6H5)6] (1) and [Li(THF)(12-crown-4)]2[Th(C6H5)6] (2), which feature an anion possessing a regular octahedral (1) or a severely distorted octahedral (2) geometry. The solid-state structure of 2 suggests the presence of pseudo-agostic ortho C-H···Th interactions, which arise from σ(C-H) → Th(5f) donation. The non-octahedral structure is also favoured in solution at low temperatures.