Speciation of aluminium in mixtures of the ionic liquids [C3mpip][NTf2] and [C4mpyr][NTf2] with AlCl3: an electrochemical and NMR spectroscopy study.

This paper reports on the electrodeposition of aluminium on several substrates from the air- and water-stable ionic liquids 1-propyl-1-methylpiperidinium bis(trifluoromethylsulfonyl)amide ([C(3)mpip][NTf(2)]) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([C(4)mpyr][NTf(2)]), which contain anhydrous AlCl(3). At an AlCl(3) concentration of 0.75 molal, no evidence for aluminium electrodeposition was observed in either system at room temperature. However, aluminium electrodeposition becomes feasible upon heating the samples to 80 degrees C. Aluminium electrodeposition from bis(trifluoromethylsulfonyl)amide-based ionic liquids that contain AlCl(3) has previously been shown to be very dependent upon the AlCl(3) concentration and has not been demonstrated at AlCl(3) concentrations below 1.13 molal. The dissolution of AlCl(3) in [C(3)mpip][NTf(2)] and [C(4)mpyr][NTf(2)] was studied by variable-temperature (27)Al NMR spectroscopy to gain insights on the electroactive species responsible for aluminium electrodeposition. A similar change in the aluminium speciation with temperature was observed in both ionic liquids, thereby indicating that the chemistry was similar in both. The electrodeposition of aluminium was shown to coincide with the formation of an asymmetric four-coordinate aluminium-containing species with an (27)Al chemical shift of delta=94 and 92 ppm in the [C(3)mpip][NTf(2)]-AlCl(3) and [C(4)mpyr][NTf(2)]-AlCl(3) systems, respectively. It was concluded that the aluminium-containing species that give rise to these resonances corresponds to the electroactive species and was assigned to [AlCl(3)(NTf(2))](-).