Is the formation of N-heterocyclic carbenes (NHCs) a feasible mechanism for the distillation of imidazolium ionic liquids?

Abstract

We describe the synthesis of two tetrachloroindate ionic liquids used as probes to study the involvement of NHCs (N-heterocyclic carbenes) in the distillation of imidazolium derivatives. Atmospheric-pressure chemical ionization mass spectrometry (APCI-MS), electrospray ionization mass spectrometry (ESI-MS), atmospheric-pressure thermal desorption ion mass spectrometry (APTDI-MS) and laser-induced acoustic desorption (LIAD) were used to depict the possibility of the involvement of NHCs during the distillation process. Each type of imidazolium derivative showed a particular mechanism of distillation, pointing firmly to the dependence of both the cation and the anion natures to distil as ion pairs or NHCs. Ionic liquid 1-n-butyl-3-methylimidazolium tetrachloroindate (1a) exhibited a preference to distil as ion pairs, whereas 3,3'-(ethane-1,2-diyl)bis(1-methyimidazolium)bis-tetrachloroindate (1b) may react with the Lewis acid anion, affording a bidentate NHC complex to distil. Thermodynamics, quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses of the ionic liquid 1a were also conducted and helped understand the preference for ion pairs instead of NHCs. The performed theoretical calculations did not forwent the possibility of NHC formation; however, they clearly indicated the high stability of the anions (Lewis acids in nature) and also indicated that the possible reaction between NHC and the anion is not favoured. The calculated thermodynamic values were in accordance with the features observed by MS and indicated ion pairs as the feasible species for the distillation of imidazolium-based ionic liquids.

Charge‐Tagged N‐Heterocyclic Carbenes (NHCs): Revealing the Hidden Side of NHC‐Catalysed Reactions through Electrospray Ionization Mass Spectrometry

“…Accordingly, Diniz et al applied the tagging strategy to study the carbene role in the distillation process, demonstrating that both the cation and anion nature can affect the process. [53] In particular, anions from Lewis acids ([InCl 4 ] À ), that commonly prevent the acid-base reactions responsible for the NHC formation in solution, favoured the distillation of ionic liquids as ion pairs, by preserving the cation-anion interaction. As far as the influence of the cation is concerned, the same authors suggested that a dicationic moiety could react with the [InCl 4 ] À anion, allowing the ionic liquid to distillate as a bidentate NHC complex.…”

“…As far as the influence of the cation is concerned, the same authors suggested that a dicationic moiety could react with the [InCl 4 ] À anion, allowing the ionic liquid to distillate as a bidentate NHC complex. [53] Turning to the negative ion mode, Lalli et al succeeded for the first time in 2012 in the isolation of N-heterocyclic carbenes bearing negative charge tags. [54] In this case, deprotonation of imidazolium parent cation does not occur through the intervention of the basic counter anion, as in the positive ion mode, but by adding an excess of a strong base (KOtBu).…”

Rapid carbene formation increases ion diffusivity in an imidazolium acetate ionic liquid confined between polar glass plates

“…This reaction is a typical deprotonation of the C2-H group of the [EMIM] + cation by a basic counter-ion leading to the spontaneous formation of NHCs, as observed previously. [28][29][30][31][32] Therefore, the products of dissociation of [EMIM][OAc] IL on the glass surfaces are N-heterocyclic carbene (NHC) and acetic acid.…”

“…The possibility of deprotonation of the C2-H group of the [EMIM] + cation by means of a basic counter-ion is known to spontaneously lead to the formation of N-heterocyclic carbene (NHC). [28][29][30][31][32] It has been suggested that the distillation process of [EMIM][OAc] is driven by the formation of NHCs. 28 Both theoretical 32 and spectroscopic 29,30 studies of carbene formation in the gas phase have been presented.…”

“…[28][29][30][31][32] It has been suggested that the distillation process of [EMIM][OAc] is driven by the formation of NHCs. 28 Both theoretical 32 and spectroscopic 29,30 studies of carbene formation in the gas phase have been presented. However, the presence of carbene in a liquid has been observed only indirectly, via ''trapping'' reactions.…”