A quantitative study of the aluminium trichloride–acetonitrile system using X-ray crystallography, electrical conductivity, aluminium-27 and chlorine-35 nuclear magnetic resonance and Raman spectroscopy. The characterization of the pentakis(acetonitrile)chloroaluminium(III) ion in the solid state and in solution

Abstract

The compounds MCl₃(M = Al, Ga, or In) yield electrically conducting solutions in acetonitrile. Boron trichloride gives non-conducting solutions and, contrary to previous work, this is interpreted as due to the presence of a molecular solute BCl₃·MeCN. The electrical-conductivity data for AlCl₃ in acetonitrile are discussed in detail and it is shown that from the results obtained it is not possible to differentiate between 1 : 1 and 1 : 2 electrolyte behaviour. Quantitative Raman and ²⁷Al n.m.r. spectra demonstrate that ca. 70% of the aluminium in solutions of AlCl₃ in acetonitrile is present in the form of [AlCl₄]^–. An X-ray single-crystal study of the solid adduct AlCl₃·2MeCN crystallizing from such a solution shows that this adduct is correctly formulated as the auto-complex [AlCl(NCMe)₅]²⁺2[AlCl₄]^–·MeCN. Further ²⁷Al n.m.r. studies on solutions of Al[ClO₄]₃ in acetonitrile and of the solute AlCl[ClO₄]₂, in conjunction with the work on AlCl₃ demonstrate that [AlCl(NCMe)₅]²⁺ is the major cationic constituent of aluminium trichloride solutions in acetonitrile. The electrical-conductivity, Raman, and n.m.r. data on these solutions are all satisfactorily interpreted by the principal ionization scheme [AlCl(NCMe)₅]²⁺2[AlCl₄]^– which is the formulation found for the crystal. The ionization of AlCl₃, but the non-ionization of BCl₃, in solution in acetonitrile is attributed principally to the ability of aluminium to adopt a co-ordination number of greater than four in ions such as [AlCl(NCMe)₅]²⁺.