Allyldiamidinium and Diamidinium Salts: Are Dicationic Ionic Liquids in Fact Superionic?

This work reports on novel acid-base conjugate pairs of monocationic allyldiamidinium and dicationic diamidinium salts, some of which are ionic liquids (ILs) at ambient temperatures. A series of allyldiamidinium salts of the general formula [C3H(NRMe)4]X (R = Me, Et, Pr, allyl, CH2CH2OMe; X = Cl, bistriflamide, dicyanamide) were prepared from C3Cl4 or C3Cl5H and the appropriate secondary amine RNMeH. Alkylated ethylenediamines similarly give bicyclic allyldiamidinium salts, whereas longer diamines H2N(CH2)nNH2 (n = 3, 4, 5) were isolated as their conjugate acids, the diamidinium dicationic salts [C3H2(HN(CH2)nNH)2]X2. The salts were characterized by NMR, ES-MS, DSC, TGA, and miscibility or solubility studies. Additionally, the ILs were characterized by their viscosity. The conductivities of the diamidinium ILs were also measured, and this allowed an investigation of their Walden parameters. In contrast to expectations, since ion pairing and clustering would be expected to be significant, this showed them to be “superionic”. Previous reports of Walden plots of dicationic ILs were found to be erroneous and a reanalysis of the literature data found that all reported dicationic, and even tetracationic, ILs can be classified as superionic. The salts [C3H(NMe2)4]Cl, [C3H(EtN(CH2)2NEt)2]OTf and [C3H2(HN(CH2)nNH)2]Cl2 (n = 3, 4, 5) were also characterized by single-crystal X-ray diffraction.