Trevor W. Hayton

Synthesis of Imido Analogs of the Uranyl Ion

Here we describe the synthesis of two imido analogs of the uranyl ion, UO2+2, in which the oxygens are replaced by divalent alkyl or aryl nitrogen groups: U(NtBu)2I2(THF)2 (1) and U(NPh)2I2(THF)3 (2) (where tBu is tert-butyl and THF is tetrahydrofuran). Both compounds have been fully characterized by standard analytical techniques, including x-ray crystallography, and the chemical bonding between the metal center and the nitrogen ligands was quantified by using hybrid density functional theory calculations. As expected for a uranyl analog, these complexes exhibit linear N-U-N linkages and very short U-N bonds. In addition, the theoretical calculations show strong involvement of the 5f and 6d electrons in the U-N bonding.

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.

Synthesis, Characterization, and Reactivity of a Uranyl β-Diketiminate Complex

Addition of 1 equiv of Li(Ar2nacnac) (Ar2nacnac = (2,6-(i)Pr2C6H3)NC(Me)CHC(Me)N(2,6-(i)Pr2C6H3)) to an Et2O suspension of UO2Cl2(THF)3 generates the uranyl dimer [UO2(Ar2nacnac)Cl]2 (1) in good yield. A second species can be isolated in low yield from the reaction mixtures of 1, namely [Li(OEt2)2][UO2(Ar2nacnac)Cl2] (2). The structures of both 1 and 2 have been confirmed by X-ray crystallography. Complex 1 reacts with Ph3PO to generate UO2(Ar2nacnac)Cl(Ph3PO) (3). In addition, 1 reacts with AgOTf and either 1 equiv of DPPMO2 or 2 equiv of Ph2MePO to provide [UO2(Ar2nacnac)(DPPMO2)][OTf] (4) and [UO2(Ar2nacnac)(Ph2MePO)2][OTf] (5), respectively. Both 4 and 5 have been fully characterized, including analysis by X-ray crystallography and cyclic voltammetry. Reduction of 4 with Cp2Co provides UO2(Ar2nacnac)(CH{Ph2PO}2) (6), a uranyl(VI) complex that is generated by the formal loss of H* from the DPPMO2 ligand. Labeling studies have been performed in an attempt to elucidate the mechanism of hydrogen loss. In contrast, reduction of 5 with Cp2Co provides UO2(Ar2nacnac)(Ph2MePO)2 (7), a rare example of a uranyl(V) complex. As expected, the solid-state molecular structure of 7 reveals slightly longer U-O(oxo) bond lengths relative to 5. Furthermore, complex 7 can be converted back into 5 by oxidation with AgOTf in toluene.

Tetrahalide complexes of the [U(NR)2]2+ ion: synthesis, theory, and chlorine K-edge X-ray absorption spectroscopy.

Synthetic routes to salts containing uranium bis-imido tetrahalide anions [U(NR)(2)X(4)](2-) (X = Cl(-), Br(-)) and non-coordinating NEt(4)(+) and PPh(4)(+) countercations are reported. In general, these compounds can be prepared from U(NR)(2)I(2)(THF)(x) (x = 2 and R = (t)Bu, Ph; x = 3 and R = Me) upon addition of excess halide. In addition to providing stable coordination complexes with Cl(-), the [U(NMe)(2)](2+) cation also reacts with Br(-) to form stable [NEt(4)](2)[U(NMe)(2)Br(4)] complexes. These materials were used as a platform to compare electronic structure and bonding in [U(NR)(2)](2+) with [UO(2)](2+). Specifically, Cl K-edge X-ray absorption spectroscopy (XAS) and both ground-state and time-dependent hybrid density functional theory (DFT and TDDFT) were used to probe U-Cl bonding interactions in [PPh(4)](2)[U(N(t)Bu)(2)Cl(4)] and [PPh(4)](2)[UO(2)Cl(4)]. The DFT and XAS results show the total amount of Cl 3p character mixed with the U 5f orbitals was roughly 7-10% per U-Cl bond for both compounds, which shows that moving from oxo to imido has little effect on orbital mixing between the U 5f and equatorial Cl 3p orbitals. The results are presented in the context of recent Cl K-edge XAS and DFT studies on other hexavalent uranium chloride systems with fewer oxo or imido ligands.

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.