Investigação vibracional dos complexos catiônicos de Al(III) em solução etérea e suas misturas binárias de solventes

Abstract

Aluminum chloride (AlCl₃) has several applications in industry and secondarily used in rechargeable Mg battery electrolytes because it plays an important role for formation of species responsible for the charge transport. Nevertheless, Al(III) speciation in non-aqueous media is still a big challenge from a vibrational spectroscopic point of view. In this sense, the current work studied the speciation of Al(III) complexes in solutions containing AlCl₃/ether and AlCl₃/acetonitrile (ACN) and its binary mixtures by using Raman and infrared spectroscopies, supplemented by AIMD simulations (ab initio molecular dynamics). The findings showed that low dielectric constant ethereal solvents, such as tetrahydrofuran (THF) e tetraglyme (G4), preferentially stabilize AlCl3 and [AlCl₂]⁺, respectively, but the larger dielectric constant of ACN leads to the formation of higher cationic complexes, assigned to [AlCl(ACN)₅]²⁺ and [Al(ACN)₆]³⁺ species. Addition of G4 caused an increase of intensity of the 419 e 407 cm⁻¹ bands, reported for the first time for ACN-based systems and related to the [AlCl₂]⁺ cation, and, at the same time, the vanish of the bands associated to higher solvates. The changes observed for ν₂ (C≡N stretching) and ν₄ (C–C stretching) modes evidenced gradual substitution of ACN for G4 molecules in the inner solvation sphere of Al(III). The results demonstrate that the monovalent cation may be formed in MACC-kind electrolytes (Magnesium Aluminum Chloride Complex), if the MgCl₂:AlCl₃ ratio is not correctly tunned in ethereal medium, thus confirming literature data on its corrosive nature at the Mg anode. The investigation helps understand fundamental aspects on coordination chemistry of Al(III) and next-generation electrolytic solutions.