Synthesis and physical properties of tris(dialkylamino)cyclopropenium bistriflamide ionic liquids

The synthesis and properties of 23 tris(dialkylamino)cyclopropenium (TDAC) cations with the bistriflamide anion, NTf2−, are described. D3h- and C3h-symmetric cations ([C3(NR2)3]NTf2 (R = Me, Et, Pr, Bu, Pent, Hex, Dec) and [C3(NRMe)3]NTf2 (R = Et, Bu, St), respectively) were synthesised by reaction of C3Cl5H with the corresponding amine. Reaction of alkoxydiaminocyclopropenium salts ([C3(NMe2)2(OMe)]+ and [C3(NEt2)2(OMe)]+) with amines led to two series of C2v-symmetric salts ([C3(NMe2)2(NR2)]NTf2 (R = Et, Pr, Bu, Hex) and [C3(NEt2)2(NR2)]NTf2 (R = Me, Bu, Hex), respectively) and two series of Cs-symmetric salts ([C3(NMe2)2(NRMe)]NTf2 (R = Et, Pr, Bu, Hex) and [C3(NEt2)2(NRMe)]NTf2 (R = Bu, Hex), respectively). In addition to characterisation by NMR, mass spectrometry and microanalysis, the salts were characterised by DSC, TGA, density, viscosity, conductivity and miscibility/solubility studies. Along with molecular weight, symmetry plays a significant role in determining melting points and viscosity, whereas density was found to depend only on molecular weight. Methyl groups were found to significantly decrease thermal stability, while increasing the size of the other alkyl groups was found to increase stability; this increase in stability is contrary to observations with other classes of ionic liquids and indicates an associative decomposition mechanism. Walden plots indicated that these are “good ionic liquids” but that significant ion-pairing occurs when at least two alkyl chains of size C6 or larger are present. Diffusion coefficients of [C3(NBu2)3]NTf2 revealed a relatively small loss of conductivity due to ion correlations. The chemical stability of [C3(NEt2)3]NTf2 to various reagents (acid, base, redox) was investigated at 25 °C and 60 °C. Cyclic voltammetry indicated a relatively small electrochemical window of 3.6 V (due to a relatively low oxidation potential of 1.2 V). The X-ray structures of [C3(NMe2)3]NTf2 and [C3(NPr2)3]NTf2 are reported.