Aluminium Speciation in 1-Butyl-1-Methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide/AlCl3 Mixtures

Electrodeposition of aluminium is possible from solutions of AlCl3 dissolved in the 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (C(4)mpyrNTf(2)) ionic liquid. However, electrodeposition is dependant oil the AlCl3 concentration as it only occurs at concentrations >1.6 mol L-1. At these relatively high AlCl3 concentrations the C(4)mpyrNTf(2)/AlCl3 mixtures exhibit biphasic behaviour. Notably, at 1.6 mol L-1 AlCl3, aluminium can only be electrodeposited from the upper phase. Conversely, we found that at 3.3 mol L-1 aluminium electrodeposition can only occur from the lower phase. The complex chemistry of the C(4)mpyrNTf(2)/AlCl3 System is described and implications of aluminium speciation in several C(4)mpyrNTf(2)/AlCl3 mixtures, as deduced from Raman and Al-27 NMR spectroscopic data, are discussed. The Al-27 NMR spectra of the C(4)mpyrNTf(2)/AlCl3 mixtures revealed the presence of both tetrahedrally and octahedrally coordinated aluminium species. Raman spectroscopy revealed that the level of uncoordinated NTf2- anions decreased with increasing AlCl3 concentration. Quantum chemical calculations using density functional and ab initio theory were employed to identify plausible aluminium-containing species and to calculate their vibrational frequencies, which in turn assisted the assignment of the observed Raman bands. The data indicate that the electroactive species involved are likely to be either [AlCl3(NTf2)](-) or [AlCl2(NTf2)(2)](-).