A Raman Study on the Speciation of Different Metal Ions in an AlCl3-Based Ionic Liquid

The speciation of Cr, Zn and Sn in AlCl3/1-ethyl-3-methylimidazolium chloride containing CrCl2, ZnCl2 and SnCl2, respectively, has been studied by cyclic voltammetry (CV), Raman spectroscopy and density functional theory (DFT) calculations. Addition of the respective metal salt causes the current waves in the CV to decrease, indicating a reaction of the metal salts with Al2Cl7 (-). Compared to the neat electrolyte, the Raman peaks of Al2Cl7 (-) decrease while the AlCl4 (-) peak increases in intensity, broadens and shifts towards lower wavenumbers. Calculated wavenumbers of metal complexes [Me(AlCl4)(3)](-) reflect these observations. DFT calculations of the Gibbs free energies of formation, solvation and reaction support the formation of the proposed complexes. The central ions are coordinated by three bidentate AlCl4 (-) ligands that are arranged planar-trigonally. Due to the occupied Sn-5s orbital, repulsive forces cause a trigonal-pyramidal geometry in case of the Sn complex. Based on the similarities in the experimental observations and the orbital configuration of Zn2+ compared to Cr2+, the spontaneous formation of the species [Cr(AlCl4)(3)](-) can be assumed.

Automated summary

The speciation of Cr, Zn, and Sn in AlCl3/1-ethyl-3-methylimidazolium chloride solution has been investigated using CV, Raman spectroscopy, and DFT calculations. The addition of metal salts leads to a decrease in current waves in CV, indicating reaction with Al2Cl7(-). Raman spectroscopy shows a decrease in Al2Cl7(-) peaks and an increase in AlCl4(-) peaks, suggesting the formation of metal complexes [Me(AlCl4)(3)](-). DFT calculations support the proposed complexes by calculating Gibbs free energies.