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.