Sites of protonation and copper(II)-complexation in protic ionic liquids comprised of N-hexylethylenediaminium cation

Protonation sites of a chelating mono-protic ionic liquid (PIL) comprising N-hexylethylenediaminium (= HHexen+) depending on the counter anion were studied by 15N and 13C NMR spectroscopy and DFT calculation. In the case of the bis(trifluoromethanesulfonyl) amide (= TFSA−) PIL, the protonation occurred almost at the secondary amine, whereas the protonation occurred preferentially at the primary amine in the trifluoroacetate (= TFA−) PIL. The formation of a chelating copper(II) complex with the cationic unit of the PIL despite the electrostatic repulsion was revealed using the paramagnetic broadenings of the 13C NMR spectra of the PILs.