An Experimental and Theoretical Study of the Aluminium Species Present in Mixtures of AlCl3 with the Ionic Liquids [BMP]Tf2N and [EMIm]Tf2N

It is known that nano- or microcrystalline aluminium may be electrodeposited from mixtures of AlCl3 and the ionic liquids 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([BMP]Tf2N) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMIm]Tf2N), and that two phases form with higher formal concentrations of AlCl3 (at 1.6 mol L-1 (x(A1) = 0.33) and 2.5 mol L-1 (x(A1) = 0.39), respectively). This account analyzes the hitherto unknown molecular nature of these mixtures by a detailed experimental (multinuclear NMR and Raman spectroscopies) and theoretical study (BP86/TZVP DFT calculations, including COSMO solvation energies). The addition of AlCl3 to the two liquids first leads to complexation with [Tf2N](-) and then disproportionation of the initial [AlClx(Tf2N)(y)](-) complexes give Al(Tf2N)(3) and [AlCl4](-). At high concentrations of AlCl3, the lower phase consists almost completely of Al(Tf2N)(3), whereas in the upper phase [AlCl4](-) is the dominant species. Electrodeposition of aluminium in the upper phase occurs from mixed AlClx- (Tf2N)(y) species, most likely from [AlCl2(Tf2N)(2)](-) formed in small concentrations at the phase boundary between the [AlCl4](-) and the Al(Tf2N)(3) layers. All the findings are supported by DFT calculations as well as an Xray crystal structure determination of Al(Tf2N)(3). The latter was separated from the mixture by sublimation on a preparative scale. It was independently prepared from AlEt3 and HNTf2 and fully characterized. Moreover, the ionic liquids [BMP]AlCl4 (m.p. 74 degrees C) and [EMIm]AlCl4 (m.p. -7 degrees C), which mainly form the upper layer in the biphasic regime, were independently prepared and also fully characterized.