New Routes to Synthetically Useful, Sterically Encumbered Arylaluminum Halides and Hydride Halides

Abstract

The synthesis and structural characterization of the compounds Mes*AlCl₂(THF) (1), Mes*AlCl₂ (2), Mes*Al(H)Cl(THF) (3a), Mes*Al(H)Cl (4a), and (Mes*AlH₂)₂ (5) (Mes* = 2,4,6-t-Bu₃C₆H₂^-) are described as well as those for two compounds 3b and 4b that are analogs of 3a and 4a but have H:Cl ratios that are less than 1:1. All compounds were characterized by ¹H, ¹³C NMR, and IR spectroscopy, and 1, 2, 3a, and 4b were additionally characterized by X-ray crystallography. Compound 1 is best synthesized by the reaction of [(THF)₂LiH₃AlMes*]₂ (6) with 6 equiv of Me₃SiCl. A more conventional route involving the addition of (THF)₂LiMes* to 2 equiv of AlCl₃ in toluene usually affords a mixture of 1 and AlCl₃·THF. Recrystallization of 1 from n-hexane results in a species that has less than 1 equiv of THF per Mes*AlCl₂. The THF free complex 2 may be obtained in quantitative yield by heating 1 for 20 min at 90 °C under reduced pressure. Compound 3a may be obtained by treating a 1:1 mixture of Mes*Li(THF)₂ and LiAlH₄ with 2 equiv of Me₃SiCl or by the addition of slightly less than 4 equiv of Me₃SiCl to 6. The THF can be removed from 3a by sublimation to give 4a. The related compounds 3b and 4b, which display an aluminum-bound H:Cl ratio that is deficient in H, can be obtained by reactions with slightly more than 2 equiv of Me₃SiCl. Crystal data at 130 K with Cu Kα (λ = 1.541 78 Å) radiation: 1, C₂₂H₃₇AlCl₂O, a = 11.889(3) Å, b = 9.992(3) Å , c = 19.704(5) Å, orthorhombic, space group Pca2₁, Z = 4, R = 0.068 for 1556 (I > 2σ(I)) data; 2, C₁₈H₂₉AlCl₂, a = 12.147(5) Å, b = 18.042(6) Å, c = 17.771(7) Å, β = 95.77(3)°, monoclinic, space group P2₁/n, Z = 8, R = 0.032 for 4610 (I > 2σ(I)) data; 3a, C₂₂H₃₈AlClO, a = 16.887(7) Å, b = 16.333(6) Å, c = 8.739(3) Å, β = 101.41(3)°, monoclinic, space group P2₁/c, Z = 4, R = 0.073 for 2752 (I > 2σ(I)) data; 4b, C₁₈H_29.64AlCl_1.36, a = 12.077(3) Å, b = 17.920(3) Å, c = 17.634(5) Å ; β = 95.21(2) Å, monoclinic, space group P2₁/n, Z = 8, R = 0.070 for 4261 (I > 2σ(I)) data.