Synthesis and Characterization of Bulky Cationic Arylalkylaluminum Compounds

The cationic m-terphenyl-substituted organoaluminum compounds [Dipp*AlEt][CH₆B₁₁Cl₆] (3; Dipp* = 2,6-(2,6-i-Pr₂C₆H₃)₂C₆H₃−), [DcpAlEt][CH₆B₁₁Cl₆] (4; Dcp = 2,6-(2,6-Cl₂C₆H₃)₂C₆H₃−), [Dipp*AlEt][CH₆B₁₁I₆] (5), and [DcpAlEt][CH₆B₁₁I₆] (6) have been obtained by ethide elimination through trityl or preferably silylium salts from the precursors Dipp*AlEt₂ (1) and DcpAlEt₂ (2). The crystal structures of compounds 3−5 reveal the presence of cation−anion adducts involving two halides of the carborane anions, and the NMR spectroscopic data and solubility properties indicate such an interaction for 6. The interactions of the hexaiodocarborane anion with the [Dipp*AlEt]⁺ cation are stronger than those of the hexachlorocarborane anion. Interestingly, cation···anion contacts are preferred to intramolecular Al···Cl interactions in the Dcp species 4. Compound 4 forms the bis-amine adduct [DcpAlEt(NH₂t-Bu)₂][CH₆B₁₁I₆] (8) upon exposure to tBuNH₂, and compounds 3 and 4 slowly catalyze the alkylation of benzene with 1-hexene. The compounds have been characterized by ¹H, ¹³C, and ¹¹B NMR spectroscopy and by X-ray crystallography in the case of 1−5, 7, and 8.