Homoleptic Uranium(IV) Alkyl Complexes: Synthesis and Characterization

Progress in Nonaqueous Molecular Uranium Chemistry: Where to Next?

There is long-standing interest in nonaqueous uranium chemistry because of fundamental questions about uranium's variable chemical bonding and the similarities of this pseudo-Group 6 element to its congener d-block elements molybdenum and tungsten. To provide historical context, with reference to a conference presentation slide presented around 1988 that advanced a defining collection of top targets, and the challenge, for synthetic actinide chemistry to realize in isolable complexes under normal experimental conditions, this Viewpoint surveys progress against those targets, including (i) CO and related π-acid ligand complexes, (ii) alkylidenes, carbynes, and carbidos, (iii) imidos and terminal nitrides, (iv) homoleptic polyalkyls, -alkoxides, and -aryloxides, (v) uranium-uranium bonds, and (vi) examples of topics that can be regarded as branching out in parallel from the leading targets. Having summarized advances from the past four decades, opportunities to build on that progress, and hence possible future directions for the field, are highlighted. The wealth and diversity of uranium chemistry that is described emphasizes the importance of ligand-metal complementarity in developing exciting new chemistry that builds our knowledge and understanding of elements in a relativistic regime.

An Allyl Uranium(IV) Sandwich Complex: Are ϕ Bonding Interactions Possible?

A method to explore head‐to‐head ϕ back‐bonding from uranium f‐orbitals into allyl π* orbitals has been pursued. Anionic allyl groups were coordinated to uranium with tethered anilide ligands, then the products were investigated by using NMR spectroscopy, single‐crystal XRD, and theoretical methods. The (allyl)silylanilide ligand, N‐((dimethyl)prop‐2‐enylsilyl)‐2,6‐diisopropylaniline (LH), was used as either the fully protonated, singly deprotonated, or doubly deprotonated form, thereby highlighting the stability and versatility of the silylanilide motif. A free, neutral allyl group was observed in UI₂(L1)₂ (1), which was synthesized by using the mono‐deprotonated ligand [K][N‐((dimethyl)prop‐2‐enyl)silyl)‐2,6‐diisopropylanilide] (L1). The desired homoleptic sandwich complex U[L2]₂ (2) was prepared from all three ligand precursors, but the most consistent results came from using the dipotassium salt of the doubly deprotonated ligand [K]₂[N‐((dimethyl)propenidesilyl)‐2,6‐diisopropylanilide] (L2). This allyl‐based sandwich complex was studied by using theoretical techniques with supporting experimental spectroscopy to investigate the potential for phi (ϕ) back‐bonding. The bonding between U^IV and the allyl fragments is best described as ligand‐to‐metal electron donation from a two carbon fragment‐localized electron density into empty f‐orbitals.

Synthesis of a heterobimetallic actinide nitride and an analysis of its bonding

Reaction of [K(DME)][Th{N(R)(SiMe₂ CH₂)}₂(NR₂)] (R = SiMe₃) with 1 equiv. of [U(NR₂)₃(NH₂)] (1) in THF, in the presence of 18-crown-6, results in formation of a bridged uranium-thorium nitride complex, [K(18-crown-6)(THF)₂][(NR₂)₃U^IV(μ-N)Th^IV(NR₂)₃] (2), which can be isolated in 48% yield after work-up. Complex 2 is the first isolable molecular mixed-actinide nitride complex. Also formed in the reaction is the methylene-bridged mixed-actinide nitride, [K(18-crown-6)][K(18-crown-6)(Et₂O)₂][(NR₂)₂U(μ-N)(μ-κ²-C,N-CH₂SiMe₂NR)Th(NR₂)₂]₂ (3), which can be isolated in 34% yield after work-up. Complex 3 is likely generated by deprotonation of a methyl group in 2 by [NR₂]^-, yielding the new μ-CH₂ moiety and HNR₂. Reaction of 2 with 0.5 equiv. of I₂ results in formation of a U^V/Th^IV bridged nitride, [(NR₂)₃U^V(μ-N)Th^IV(NR₂)₃] (4), which can be isolated in 42% yield after work-up. The electronic structure of 4 was analyzed with EPR spectroscopy, SQUID magnetometry, and NIR-visible spectroscopy. This analysis demonstrated that the energies of 5f orbitals of 4 are largely determined by the strong ligand field exerted by the nitride ligand.

Synthesis and electronic structure analysis of the actinide allenylidenes, [{(NR2)3}An(CCCPh2)]- (An = U, Th; R = SiMe3)

The reaction of [AnCl(NR₂)₃] (An = U, Th, R = SiMe₃) with in situ generated lithium-3,3-diphenylcyclopropene results in the formation of [{(NR₂)₃}An(CH[double bond, length as m-dash]C[double bond, length as m-dash]CPh₂)] (An = U, 1; Th, 2) in good yields after work-up. Deprotonation of 1 or 2 with LDA/2.2.2-cryptand results in formation of the anionic allenylidenes, [Li(2.2.2-cryptand)][{(NR₂)₃}An(CCCPh₂)] (An = U, 3; Th, 4). The calculated ¹³C NMR chemical shifts of the C_α, C_β, and C_γ nuclei in 2 and 4 nicely reproduce the experimentally assigned order, and exhibit a characteristic spin-orbit induced downfield shift at C_α due to involvement of the 5f orbitals in the Th-C bonds. Additionally, the bonding analyses for 3 and 4 show a delocalized multi-center character of the ligand π orbitals involving the actinide. While a single-triple-single-bond resonance structure (e.g., An-C[triple bond, length as m-dash]C-CPh₂) predominates, the An[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]CPh₂ resonance form contributes, as well, more so for 3 than for 4.

Actinide arene-metalates: ion pairing effects on the electronic structure of unsupported uranium-arenide sandwich complexes

Addition of [UI₂(THF)₃(μ-OMe)]₂·THF (2·THF) to THF solutions containing 6 equiv. of K[C₁₄H₁₀] generates the heteroleptic dimeric complexes [K(18-crown-6)(THF)₂]₂[U(η⁶-C₁₄H₁₀)(η⁴-C₁₄H₁₀)(μ-OMe)]₂·4THF (118C6·4THF) and {[K(THF)₃][U(η⁶-C₁₄H₁₀)(η⁴-C₁₄H₁₀)(μ-OMe)]}₂ (1THF) upon crystallization of the products in THF in the presence or absence of 18-crown-6, respectively. Both 118C6·4THF and 1THF are thermally stable in the solid-state at room temperature; however, after crystallization, they become insoluble in THF or DME solutions and instead gradually decompose upon standing. X-ray diffraction analysis reveals 118C6·4THF and 1THF to be structurally similar, possessing uranium centres sandwiched between bent anthracenide ligands of mixed tetrahapto and hexahapto ligation modes. Yet, the two complexes are distinguished by the close contact potassium-arenide ion pairing that is seen in 1THF but absent in 118C6·4THF, which is observed to have a significant effect on the electronic characteristics of the two complexes. Structural analysis, SQUID magnetometry data, XANES spectral characterization, and computational analyses are generally consistent with U(iv) formal assignments for the metal centres in both 118C6·4THF and 1THF, though noticeable differences are detected between the two species. For instance, the effective magnetic moment of 1THF (3.74 μ_B) is significantly lower than that of 118C6·4THF (4.40 μ_B) at 300 K. Furthermore, the XANES data shows the U L_III-edge absorption energy for 1THF to be 0.9 eV higher than that of 118C6·4THF, suggestive of more oxidized metal centres in the former. Of note, CASSCF calculations on the model complex {[U(η⁶-C₁₄H₁₀)(η⁴-C₁₄H₁₀)(μ-OMe)]₂}²⁻ (1*) shows highly polarized uranium–arenide interactions defined by π-type bonds where the metal contributions are primarily comprised by the 6d-orbitals (7.3 ± 0.6%) with minor participation from the 5f-orbitals (1.5 ± 0.5%). These unique complexes provide new insights into actinide–arenide bonding interactions and show the sensitivity of the electronic structures of the uranium atoms to coordination sphere effects.

Use of Chatt metal-arene protocols with uranium leads to the synthesis of the first well-characterized, unsupported actinide–arenide sandwich complexes. The electronic structures of the actinide centres show a key sensitivity to ion pairing effects.

Homoleptic Perchlorophenyl "Ate" Complexes of Thorium(IV) and Uranium(IV)

The reaction of AnCl₄(DME)_n (An = Th, n = 2; U, n = 0) with 5 equiv of LiC₆Cl₅ in Et₂O resulted in the formation of homoleptic actinide-aryl "ate" complexes [Li(DME)₂(Et₂O)]₂[Li(DME)₂][Th(C₆Cl₅)₅]₃ ([Li][1]) and [Li(Et₂O)₄][U(C₆Cl₅)₅] ([Li][2]). Similarly, the reaction of AnCl₄(DME)_n (An = Th, n = 2; U, n = 0) with 3 equiv of LiC₆Cl₅ in Et₂O resulted in the formation of heteroleptic actinide-aryl "ate" complexes [Li(DME)₂(Et₂O)][Li(Et₂O)₂][ThCl₃(C₆Cl₅)₃] ([Li][3]) and [Li(Et₂O)₃][UCl₂(C₆Cl₅)₃] ([Li][4]). Density functional calculations show that the An-C_ipso σ-bonds are considerably more covalent for the uranium complexes vs the thorium analogues, in line with past results. Additionally, good agreement between experiment and calculations is obtained for the ¹³C_ipso NMR chemical shifts in [Li][1] and [Li][3]. The calculations demonstrate a deshielding by ca. 29 ppm from spin-orbit coupling effects originating at Th, which is a direct consequence of 5f orbital participation in the Th-C bonds.

Structure and magnetism of a tetrahedral uranium(iii) β-diketiminate complex

We describe the functionalisation of the previously reported uranium(iii) β-diketiminate complex (BDI)UI₂(THF)₂ (1) with one and two equivalents of a sterically demanding 2,6-diisopropylphenolate ligand (ODipp) leading to the formation of two heteroleptic complexes: [(BDI)UI(ODipp)]₂ (2) and (BDI)U(ODipp)₂ (3). The latter is a rare example of a tetrahedral uranium(iii) complex, and it shows single-molecule magnet behaviour.

The Exceptional Diversity of Homoleptic Uranium-Methyl Complexes

Homoleptic σ-bonded uranium-alkyl complexes have been a synthetic target since the Manhattan Project. The current study describes the synthesis and characterization of several unprecedented uranium-methyl complexes. Amongst these complexes, the first example of a homoleptic uranium-alkyl dimer, [Li(THF)₄ ]₂ [U₂ (CH₃ )₁₀ ], as well as a seven-coordinate uranium-methyl monomer, {Li(OEt₂ )Li(OEt₂ )₂ UMe₇ Li}_n were both crystallographically identified. The diversity of complexes reported herein provides critical insight into the structural diversity, electronic structure and bonding in uranium-alkyl chemistry.

New supporting ligands in actinide chemistry: tetramethyltetraazaannulene complexes with thorium and uranium

We report the synthesis, characterization, and preliminary reactivity of new heteroleptic thorium and uranium complexes supported by the macrocyclic TMTAA ligand (TMTAA = Tetramethyl-tetra-aza-annulene). The dihalide complexes Th(TMTAA)Cl₂(THF)₂ (1), [UCl₂(TMTAA)]₂ (2) and U(TMTAA)I₂ (3) are further functionalized to the Cp* derivatives ThCp*(TMTAA)Cl (4), UCp*(TMTAA)Cl (5) and UCp*(TMTAA)I (6) (Cp* = pentamethylcyclopentadienide). Compounds 4-6 are also obtained through a one-pot reaction from standard thorium(iv) and uranium(iv) starting materials, Li2TMTAA and KCp*. Complexes 1-6 function as valuable starting materials for salt metathesis chemistry. Treatment of precursors 4 or 5 with trimethylsilylmethyllithium (LiCH₂TMS) results in the new actinide TMTAA alkyl complexes ThCp*(TMTAA)(CH₂TMS) (7) and UCp*(TMTAA)(CH₂TMTS) (8), respectively. The TMTAA-derived alkyl complexes (7 and 8) show unexpected stability and are stable for several weeks at room temperature in solution and in the solid-state. Additionally, double substitution of the halide ligands in 1-3 shows a strong dependence on the nucleophile used. While weaker nucleophiles, such as amides, and more sterically demanding nucleophiles, such as Cp (Cp = cyclopenadienide), favour the formation of bis-TMTAA "sandwich" complexes [An(TMTAA)₂] (An = Th (9) and An = U (10)), the use of oxygen-functionalized ligands like the ODipp anion (Dipp = diisopropylphenyl) results in the formation of the doubly substituted species Th(ODipp)₂TMTAA (11) and U(ODipp)₂TMTAA (12). We also describe the divergent reactivity of the TMTAA ligand towards uranium(iii). Unlike the syntheses of actinide(iv) TMTAA complexes, the synthesis of a uranium(iii) TMTAA was not successful and only uranium(iv) species could be obtained.

Uranium(iii) and thorium(iv) alkyl complexes as potential starting materials

The synthesis and characterisation of a rare U(iii) alkyl complex, U[η⁴-Me₂NC(H)C₆H₅]₃, using the dimethylbenzylamine (DMBA) ligand has been accomplished. While attempting to prepare the U(iv) compound, reduction to the U(iii) complex occurred. In the analogous Th(iv) system, C-H bond activation of a methyl group of one dimethylamine was observed yielding Th[η⁴-Me₂NC(H)C₆H₅]₂[η⁵-(CH₂)MeNC(H)C₆H₅] with a dianionic DMBA ligand. The utility of these complexes as starting materials has been analyzed using a bulky dithiocarboxylate ligand to yield tetravalent actinide species.

A Homoleptic Uranium(III) Tris(aryl) Complex

The reaction of 3 equiv of Li-C₆H₃-2,6-(C₆H₄-4-^tBu)₂ (Terph-Li) with UI₃(1,4-dioxane)_1.5 led to the formation of the homoleptic uranium(III) tris(aryl) complex (Terph)₃U (1). The U-C bonds are reactive: treatment with excess ⁱPrN═C═NⁱPr yielded the double-insertion product [TerphC(NⁱPr)₂]₂U(Terph) (2). Complexes 1 and 2 were characterized by X-ray crystallography, which showed that the U-C bond length in 2 (2.624(4) Å) is ∼0.1 Å longer than the average U-C bond length in 1 (2.522(2) Å). Thermal decomposition of 1 yielded Terph-H as the only identifiable product; the process is unimolecular with activation parameters ΔH^⧧ = 21.5 ± 0.3 kcal/mol and ΔS^⧧ = -7.5 ± 0.8 cal·mol^-1 K^-1, consistent with intramolecular proton abstraction. The protonolysis chemistry of 1 was also explored, which led to the uranium(IV) alkoxide complex U(OCPh₃)₄(DME) (3·DME).

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.

Uranium(iv) terminal hydrosulfido and sulfido complexes: insights into the nature of the uranium-sulfur bond

Herein, we report the synthesis and characterization of a series of terminal uranium(iv) hydrosulfido and sulfido complexes, supported by the hexadentate, tacn-based ligand framework (^Ad,MeArO)₃tacn^3- (= trianion of 1,4,7-tris(3-(1-adamantyl)-5-methyl-2-hydroxybenzyl)-1,4,7-triazacyclononane). The hydrosulfido complex [((^Ad,MeArO)₃tacn)U-SH] (2) is obtained from the reaction of H₂S with the uranium(iii) starting material [((^Ad,MeArO)₃tacn)U] (1) in THF. Subsequent deprotonation with potassium bis(trimethylsilyl)amide yields the mononuclear uranium(iv) sulfido species in good yields. With the aid of dibenzo-18-crown-6 and 2.2.2-cryptand, it was possible to isolate a terminal sulfido species, capped by the potassium counter ion, and a "free" terminal sulfido species with a well separated cation/anion pair. Spectroscopic and computational analyses provided insights into the nature of the uranium-sulfur bond in these complexes.

A Mononuclear Uranium(IV) Single-Molecule Magnet with an Azobenzene Radical Ligand

A tetravalent uranium compound with a radical azobenzene ligand, namely, [{(SiMe2 NPh)3 -tacn}U(IV) (η(2) -N2 Ph2 (.) )] (2), was obtained by one-electron reduction of azobenzene by the trivalent uranium compound [U(III) {(SiMe2 NPh)3 -tacn}] (1). Compound 2 was characterized by single-crystal X-ray diffraction and (1) H NMR, IR, and UV/Vis/NIR spectroscopy. The magnetic properties of 2 and precursor 1 were studied by static magnetization and ac susceptibility measurements, which for the former revealed single-molecule magnet behaviour for the first time in a mononuclear U(IV) compound, whereas trivalent uranium compound 1 does not exhibit slow relaxation of the magnetization at low temperatures. A first approximation to the magnetic behaviour of these compounds was attempted by combining an effective electrostatic model with a phenomenological approach using the full single-ion Hamiltonian.

The Renaissance of Non-Aqueous Uranium Chemistry

Prior to the year 2000, non-aqueous uranium chemistry mainly involved metallocene and classical alkyl, amide, or alkoxide compounds as well as established carbene, imido, and oxo derivatives. Since then, there has been a resurgence of the area, and dramatic developments of supporting ligands and multiply bonded ligand types, small-molecule activation, and magnetism have been reported. This Review 1) introduces the reader to some of the specialist theories of the area, 2) covers all-important starting materials, 3) surveys contemporary ligand classes installed at uranium, including alkyl, aryl, arene, carbene, amide, imide, nitride, alkoxide, aryloxide, and oxo compounds, 4) describes advances in the area of single-molecule magnetism, and 5) summarizes the coordination and activation of small molecules, including carbon monoxide, carbon dioxide, nitric oxide, dinitrogen, white phosphorus, and alkanes.

Achievements in uranium alkyl chemistry: celebrating sixty years of synthetic pursuits

This Perspective highlights the synthesis, characterization, and reactivity of significant organouranium complexes with σ-bonded alkyl ligands.

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 complete family of terminal uranium chalcogenides, [U(E)(N{SiMe3}2)3]- (E = O, S, Se, Te)

Addition of 1 equiv of E (E = 0.125 S(8), Se, Te) to U(H(2)C═PPh(3))(NR(2))(3) (R = SiMe(3)) (1) in Et(2)O results in generation of the terminal chalcogenide complexes, [Ph(3)PCH(3)][U(E)(NR(2))(3)] (E = S, 2; Se, 3; Te, 4; R = SiMe(3)), in modest yield. Complexes 2-4 represent extremely rare examples of terminal uranium monochalcogenides. Synthesis of the oxo analogue, [Cp*(2)Co][U(O)(NR(2))(3)] (5), was achieved by reduction of [U(O)(NR(2))(3)] with Cp*(2)Co. All complexes were fully characterized, including analysis by X-ray crystallography. In the solid state, complexes 2-5 feature short U-E bond lengths, suggestive of actinide-ligand multiple bonding.