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 Å and a GaCl₃ moiety with C_3v symmetry and Ga–Cl distances of 2.10 Å. 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 C_3v 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 Ga^III. 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 Ga^III to Ga^I. No formation of solid polymers could be detected.