Abstract
Aluminium speciation: Aluminium speciation in NTf~2~ ionic liquids has a strong influence on its electrodeposition from the liquid mixture. This work probed the nature of these species and proposes that the electroactive species involved are either [AlCl~3~(NTf~2~)]^−^ or [AlCl~2~(NTf~2~)~2~]^−^ (e.g., see figure).magnified image
Electrodeposition of aluminium is possible from solutions of AlCl~3~ dissolved in the 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (C~4~mpyrNTf~2~) ionic liquid. However, electrodeposition is dependant on the AlCl~3~ concentration as it only occurs at concentrations >1.6 mol L^−1^. At these relatively high AlCl~3~ concentrations the C~4~mpyrNTf~2~/AlCl~3~ mixtures exhibit biphasic behaviour. Notably, at 1.6 mol L^−1^ AlCl~3~, 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~/AlCl~3~ system is described and implications of aluminium speciation in several C~4~mpyrNTf~2~/AlCl~3~ mixtures, as deduced from Raman and ^27^Al NMR spectroscopic data, are discussed. The ^27^Al NMR spectra of the C~4~mpyrNTf~2~/AlCl~3~ mixtures revealed the presence of both tetrahedrally and octahedrally coordinated aluminium species. Raman spectroscopy revealed that the level of uncoordinated NTf~2~^−^ anions decreased with increasing AlCl~3~ 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 [AlCl~3~(NTf~2~)]^−^ or [AlCl~2~(NTf~2~)~2~]^−^.