NMR and DFT studies of the aggregation behavior of dicationic dialkyl DABCO bistriflimide salts in solution

In this study, various derivatives of 1,4-dialkyl-diazabicyclo[2.2.2]octane-1,4-diium bis(trifluoromethanesulfonyl)imide salts, [Cn.DABCO.Cn][Tf2N]2, with different alkyl groups, were synthesized and characterized by their melting points and NMR spectra. Moreover, various solutions of [C4.DABCO.C4][Tf2N]2 and [C8.DABCO.C8][Tf2N]2 salts in deuterated methanol and acetone in different concentrations were prepared. The effect of solvent polarity, concentration and charge density of the cation on the aggregation behavior of these salts were investigated using ESI-mass spectrometry, 1HNMR, DOSY NMR experiments and DFT calculations. The obtained results showed that in methanol, a protic and very polar solvent, the chemical shift of DABCO protons and the diffusion coefficient of the cation are almost independent of the concentration, except for relatively high concentrations. Thus, in this solvent and in the low concentration regime, aggregation is not favored and in high concentration small aggregates can be formed. In contrast, when dissolved in acetone, a less polar solvent, aggregates formation is favored. As a result, chemical shift of DABCO protons and diffusion coefficient of the cation in acetone are more strongly dependent on the concentration.