A MIXED LEWIS ACID BRONSTED ACID AMBIENT-TEMPERATURE IONIC LIQUID - AN ELECTROCHEMICAL AND NMR-STUDY OF DIMETHYLANILINE

A mixed Lewis-Bronsted acid ambient temperature chloroaluminate molten salt, in which both the Lewis and Bronsted acidity may be varied, has been investigated making use of a probe solute molecule. The interaction of the proton and molten salt solvent with N,N-dimethylaniline has been investigated making use of electrochemistry and NMR spectroscopy. The ambient temperature chloroaluminate molten salt, composed of 1-ethyl-3-methylimidazolium chloride, ImCl, and aluminium chloride, is the underlying Lewis acid substrate, to which a proton source, ImHCl2, is added. DMA is oxidized in both the basic and acidic melts in the absence of proton and forms an AlCl3 adduct in an acidic (excess AlCl3) melt; the potential for oxidation in the acidic melt is considerably more positive than that in the basic melt. Upon addition of proton, the DMA oxidation wave disappears. In a basic melt (excess ImCl), an amperometric titration indicates that DMA reacts quantitatively with protons, added as ImHCl2, to form DMAH+, which is not oxidized in the melt window. C-13 and H-1 NMR spectroscopy show changes in chemical shifts consistent with these observations. DMAH+ shows a reduction wave at potentials more negative than that shown by ''free'' proton reduction in the basic melt; reverse pulse voltammetry from the limiting plateau for this reduction wave shows a H-2 oxidation wave and reappearance of the DMA oxidation wave. These results indicate that the process corresponds to the reduction of the proton from the DMAH+ to yield H-2 and DMA at the electrode surface. In an acidic melt, DMA forms an AlCl3 adduct; on addition of proton, a DMAH+ species, which cannot be oxidized in the melt window, is formed. However, the reaction of the DMA-AlCl3 adduct in the acidic melt with proton appears to be less quantitative than the reaction of proton with DMA in the basic melt. Again, the C-13 and H-1 NMR support these observations.