Spectroscopic investigation of concentrated solutions of gallium(III) chloride in mesitylene and benzene

Abstract

The metal–arene interaction in concentrated binary mixtures of GaCl3 and mesitylene or benzene has been investigated using multinuclear (1H, 13C, 71Ga) NMR, Raman and UV/VIS spectroscopy as well as liquid X-ray scattering. The effects of hydrolysis and radiolysis of such mixtures have also been studied using UV/VIS, EPR and IR spectroscopy. Gallium(III) chloride has been found to be monomeric in mesitylene and to form η6 complexes with this arene. A structural determination of this complex in solution yielded a Ga–C distance of 2.20 A and a GaCl3 moiety with C3v symmetry and Ga–Cl distances of 2.10 A. In benzene, GaCl3 exists as a mixture of the Cl2Ga(µ-Cl)2GaCl2 dimer and one or several other species, containing terminal GaCl3 groups of C3v symmetry, e.g. the dimer isomer Cl2Ga(µ-Cl)GaCl3 and monomeric GaCl3. The complex with benzene is weaker than with mesitylene and the co-ordination mode cannot be unambiguously elucidated. Hydrolysis of the GaCl3–C6H6 system produced a darkly coloured precipitate showed by IR spectroscopy to be a mixture of poly(p-phenylene) and other polymeric species, probably meta-substituted, chlorinated and hydroxylated polyphenylenes. On the basis of 71Ga NMR results, it is suggested that the formation of this polymer proceeds via protonated Wheland intermediates, rather than direct oxidation of benzene by GaIII. Radiolysis of the mixtures of GaCl3 with mesitylene and benzene yielded radicals, according to EPR spectroscopy. In benzene solution the formation of radicals is accompanied by reduction of GaIII to GaI. No formation of solid polymers could be detected.