An Investigation of the Symmetric and Asymmetric Cleavage Products in the System Aluminum Trihalide/1‐Butylimidazole

Abstract

Mixtures of AlX₃ (X=Cl, Br) with 1-butylimidazole (BuIm) in various ratios were investigated. The mixtures were characterized by multinuclear (¹ H, ²⁷ Al, ¹³ C, and ¹⁵ N) NMR, IR, and Raman spectroscopy and in part by single-crystal X-ray diffraction. Depending on the molar fraction x(AlBr₃ ) of the AlBr₃ -based mixtures, the cationic aluminum complexes [Al(BuIm)₆ ]³⁺ and [AlBr₂ (BuIm)₄ ]⁺ , the neutral adduct [AlBr₃ (BuIm)], as well as the anions Br^- , [AlBr₄ ]^- , and [Al₂ Br₇ ]^- could be identified as the products of the symmetric and asymmetric cleavage of dimeric Al₂ Br₆ . Furthermore, there are hints at the formation of [AlBr₂ (BuIm)₂ ]⁺ or related cations. Comparison of the AlBr₃ /BuIm system with AlCl₃ -based mixtures revealed the influence of the halide: In contrast to AlBr₃ , the trication [Al(BuIm)₆ ]³⁺ could not be detected as main product in a 1:6 mixture of AlCl₃ and BuIm. Additionally, [AlCl₃ (BuIm)] crystallizes from a mixture with x(AlCl₃ )=0.60 at room temperature, whereas the corresponding AlBr₃ -based mixture remains liquid even at +6 °C. Three AlBr₃ -based mixtures are liquid at room temperature, whereas all other mixtures are solids with melting points between 46 and 184 °C. The three liquid mixtures exhibit medium to high viscosities (117 to >1440 mPa s), low conductivities (0.03-0.20 mS cm^-1 ), but high densities (1.80-2.21 g cm^-3 ). Aluminum could be successfully deposited from one of the neat Lewis acidic mixtures of the AlBr₃ -based system.