Uranium(iv) alkyl cations: synthesis, structures, comparison with thorium(iv) analogues, and the influence of arene-coordination on thermal stability and ethylene polymerization activity

Abstract

Reaction of [(XA₂)U(CH₂SiMe₃)₂] (1; XA₂ = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) with 1 equivalent of [Ph₃C][B(C₆F₅)₄] in arene solvents afforded the arene-coordinated uranium alkyl cations, [(XA₂)U(CH₂SiMe₃)(η ⁿ -arene)][B(C₆F₅)₄] {arene = benzene (2), toluene (3), bromobenzene (4) and fluorobenzene (5)}. Compounds 2, 3, and 5 were crystallographically characterized, and in all cases the arene is π-coordinated. Solution NMR studies of 2-5 suggest that the binding preferences of the [(XA₂)U(CH₂SiMe₃)]⁺ cation follow the order: toluene ≈ benzene > bromobenzene > fluorobenzene. Compounds 2-4 generated in C₆H₅R (R = H, Me or Br, respectively) showed no polymerization activity under 1 atm of ethylene. By contrast, 5 and 5-Th (the thorium analogue of 5) in fluorobenzene at 20 and 70 °C achieved ethylene polymerization activities between 16 800 and 139 200 g mol^-1 h^-1 atm^-1, highlighting the extent to which common arene solvents such as toluene can suppress ethylene polymerization activity in sterically open f-element complexes. However, activation of [(XA₂)An(CH₂SiMe₃)₂] {M = U (1) or Th (1-Th)} with [Ph₃C][B(C₆F₅)₄] in n-alkane solvents did not afford an active polymerization catalyst due to catalyst decomposition, illustrating the critical role of PhX (X = H, Me, Br or F) coordination for alkyl cation stabilization. Gas phase DFT calculations, including fragment interaction calculations with energy decomposition and ETS-NOCV analysis, were carried out on the cationic portion of 2'-Th, 2', 3' and 5' (analogues of 2-Th, 2, 3 and 5 with hydrogen atoms in place of ligand backbone methyl and tert-butyl groups), providing insight into the nature of actinide-arene bonding, which decreases in strength in the order 2'-Th > 2' ≈ 3' > 5'.

Fig. 1. Crystallographically-characterized early transition metal and f-element alkyl cations which exist as arene-solvent-separated ion pairs (R = Me or Br and R′ = H, or R = R′ = Me; Ar = C₆H₃ⁱPr₂-2,6 or C₆H₂ⁱPr₃-2,4,6).

Scheme 1. Synthesis of monoalkyl uranium(iv) cations 2 and 3 (Ar = 2,6-diisopropylphenyl).

Fig. 2. X-ray crystal structure of [(XA₂)U(CH₂SiMe₃)(η⁶-C₆H₆)][B(C₆F₅)₄]·2 benzene (2·2 benzene), with thermal ellipsoids at 50% probability. Hydrogen atoms, the borate anion, and two non-coordinated benzene solvent molecules are omitted for clarity. Selected bond distances (Å) and angles (°): U(1)–O(1) 2.440(2), U(1)–N(1) 2.236(2), U(1)–N(2) 2.223(2), U(1)–C(48) 2.365(3), U(1)–C(52) 3.249(3), U(1)–C(53) 3.187(3), U(1)–C(54) 3.108(3), U(1)–C(55) 3.097(3), U(1)–C(56) 3.159(3), U(1)–C(57) 3.243(3), U(1)–C_arene ave. 3.17, U(1)–Centroid 2.86, U(1)–C(48)–Si(1) 133.8(2), O(1)–U(1)–C(48) 87.22(9), N(1)–U(1)–N(2) 124.29(8).

Fig. 3. Literature examples of crystallographically characterized uranium alkyl cations.

Fig. 4. X-ray crystal structure of [(XA₂)U(CH₂SiMe₃)(η³-C₆H₅Me)][B(C₆F₅)₄]·toluene (3·toluene), with thermal ellipsoids at 50% probability. Hydrogen atoms, the borate anion and a non-coordinated toluene solvent molecule are omitted for clarity. Selected bond distances (Å) and angles (°): U(1)–O(1) 2.417(9), U(1)–N(1) 2.22(1), U(1)–N(2) 2.21(1), U(1)–C(48) 2.36(2), U(1)–C(52) 3.05(2), U(1)–C(53) 3.13(2), U(1)–C(54) 3.47(2), U(1)–C(55) 3.78(2), U(1)–C(56) 3.70(2), U(1)–C(57) 3.36(2), U(1)–C_arene ave. 3.42, U(1)–Centroid 3.14, C(55)–C(58) 1.46(3), U(1)–C(48)–Si(1) 136.8(7), O(1)–U(1)–C(48) 88.8(4), N(1)–U(1)–N(2) 128.0(4).

Scheme 2. In situ generation of C₆D₅Br-coordinated cation 4-d₅ (Ar = 2,6-diisopropylphenyl); although bromobenzene is depicted as π-coordinated, κ¹-coordination via bromine cannot be ruled out.

Fig. 5. ²H NMR spectra of (a) 2-d₆ in C₆H₅Br containing 5 equiv. of C₆D₆, (b) 3-d₈ in C₆H₅Br containing 5 equiv. of toluene-d₈, and (c) 4-d₅ in neat C₆D₅Br.

Scheme 3. Synthesis of fluorobenzene-coordinated uranium alkyl cation 5 (Ar = 2,6-diisopropylphenyl), which in solution, on the NMR timescale, undergoes rapid migration of the alkyl group from one side of the plane of the ligand backbone to the other.

Fig. 6. X-ray crystal structure of [(XA₂)U(CH₂SiMe₃)(η³-C₆H₅F)][B(C₆F₅)₄]·fluorobenzene (5·fluorobenzene), with thermal ellipsoids at 50% probability. Hydrogen atoms, the borate anion, and non-coordinated fluorobenzene lattice solvent are omitted for clarity. Selected bond distances (Å) and angles (°): U(1)–O(1) 2.431(3), U(1)–N(1) 2.215(3), U(1)–N(2) 2.217(3), U(1)–C(48) 2.351(4), U(1)–C(52) 3.126(5), U(1)–C(53) 3.296(5), U(1)–C(54) 3.527(5), U(1)–C(55) 3.594(5), U(1)–C(56) 3.434(5), U(1)–C(57) 3.215(6), U(1)–F(1) 4.528(4), U(1)–C_arene ave. 3.37, U(1)–Centroid 3.08, C(55)–F(1) 1.362(7), U(1)–C(48)–Si(1) 134.9(2), O(1)–U(1)–C(48) 89.4(1), N(1)–U(1)–N(2) 127.6(1).

Fig. 7. Fluoroarene complexes of electrophilic transition metals. (a) [(Cp*)₂Ti(κ¹-FC₆H₅)][BAr′₄] (Ar′ = Ph or C₆F₅), (b) [(Cp*)₂Sc(κ¹-FC₆H₅)₂][BPh₄], and (c) [(nacnac)TiNAr(κ¹-FC₆H₅)][B(C₆F₅)₄] (nacnac = {CH(C(^tBu)NAr)₂}⁻; Ar = 2,6-diisopropylphenyl).

Fig. 8. Deformation density contributions Δρ₁, Δρ₂, Δρ₃, Δρ₄, and Δρ₅ (each Δρ_n figure is the sum of the α and β contributions) to bonding between the (XA_2′)U(CH₂SiMe₃)⁺ and arene fragments in [(XA_2′)U(CH₂SiMe₃)(benzene)]⁺ (2′), [(XA_2′)U(CH₂SiMe₃)(toluene)]⁺ (3′), and [(XA_2′)U(CH₂SiMe₃)(fluorobenzene)]⁺ (5′). Increased (green) and decreased (yellow) electron density is presented relative to the fragments, and isosurfaces are set to 0.0003 (Δρ₁ and Δρ₂), 0.00005 (Δρ₃), and 0.001 (Δρ₄ and Δρ₅). Dashed boxes at the top of the figure show the main α-spin fragment orbital contributors for Δρ₁, Δρ₂, and Δρ₃ in 2′ (filled orbitals are shaded dark blue and red, whereas virtual orbitals are shaded in pale blue and orange; isosurfaces are set to 0.03). The character (% 5f, % 6d etc.) of uranium atomic orbitals contributing to (XA_2′)U(CH₂SiMe₃)⁺ fragment orbitals in 2′ is provided (only values ≥ 3% are included) – these values are normalized to the total of all uranium contributions (those contributing 1% or more to the total for the fragment orbital).