Aluminum Dichloride and Dibromide. Preparation, Spectroscopic (Including Matrix Isolation) Study, Reactions, and Role (Together with Alkyl(Aryl)Aluminum Monohalides) in the Preparation of Organoaluminum Compounds

Anhydrous aluminum trichloride or bromide when heated in a 2:1 molar ratio with aluminum powder as a suspension in dry n-heptane or methylcyclohexane was found to be partially reduced to aluminum dichloride or dibromide. Ultrasound treatment (sonication) significantly promotes the reaction. Aluminum dichloride in higher purity was obtained by the reaction of gaseous aluminum trichloride with aluminum metal in a high-vacuum reactor, allowing subsequent investigation by IR spectroscopy. An aluminum sub-halide of the form Al₂(i-Bu)_4-xCl_x was also prepared through the reaction of tetraisobutyldialane and HCl at low temperature. Both materials were investigated by IR spectroscopy and compared to AlCl₂ prepared and isolated through the codeposition of aluminum atoms and molecular chlorine in a solid argon matrix. The matrix study characterized AlCl₂ together with AlCl and AlCl₃, which were also formed in the system. The paramagnetic aluminum dihalides, i.e. AlCl₂ and AlBr₂, are associated in the condensed state (except under matrix isolation conditions where they are monomeric). An ESR study of the pyridinium complex of AlCl₂ was carried out and showed its paramagnetic nature. In the present study, for simplicity, the reactions of aluminum dihalides are considered as those of the dimers but could involve higher associated oligomers. MNDO calculations on the heats of formation of several possible isomeric structures of Al₂Cl₄ indicate the preference for both halogen bridging and significant Al—Al bonding in the dimer. Reaction of AlCl₃ + Al with ethylene, the Hall and Nash reaction, was reinvestigated by ¹³C and ²⁷Al NMR spectroscopy. The reaction was found to give, besides ethylaluminum sesquichloride, 1,2-and 1,1-bis(dichloroaluminio)ethanes. Cyclohexene in a similar reaction gives, although less readily, 1,2-bis(dichloroaluminio)cyclohexane. The reactions are indicative of addition of (AlCl₂)₂ to the olefins. Alkyl-and arylaluminum monohalides are intermediately formed in the reaction of alkyl halides or halobenzenes with active aluminum powder. These divalent aluminum halides are also considered to be dimeric in nature and immediately react with excess of the alkyl (aryl) halides to form the corresponding sesquihalides. In contrast, aluminum dihalides formed in the aluminum trihalide—aluminum metal systems react with alkyl or aryl halides to give alkyl(aryl)aluminum dihalides. Sonication was found to significantly promote these reactions.