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

George A. Olah, Omar Farooq, S. Morteza F. Farnia, Mark R. Bruce, Francoise L. Clouet, Peter R. Morton, G. K. Surya Prakash, Raymond C. Stevens, Robert Bau, Koop Lammertsma, Sefik Suzer, Lester Andrews

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28 Citations (Scopus)

Abstract

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-xClx 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.

Original languageEnglish
Pages (from-to)3231-3238
Number of pages8
JournalJournal of the American Chemical Society
Volume110
Issue number10
DOIs
Publication statusPublished - May 1988
Externally publishedYes

ASJC Scopus subject areas

  • Catalysis
  • General Chemistry
  • Biochemistry
  • Colloid and Surface Chemistry

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