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Issue 38, 2018
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Is the formation of N-heterocyclic carbenes (NHCs) a feasible mechanism for the distillation of imidazolium ionic liquids?

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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.

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

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Publication details

The article was received on 07 Jun 2018, accepted on 04 Sep 2018 and first published on 04 Sep 2018


Article type: Paper
DOI: 10.1039/C8CP03609H
Citation: Phys. Chem. Chem. Phys., 2018,20, 24716-24725
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    Is the formation of N-heterocyclic carbenes (NHCs) a feasible mechanism for the distillation of imidazolium ionic liquids?

    J. R. Diniz, T. B. de Lima, R. Galaverna, A. L. de Oliveira, D. A. C. Ferreira, F. C. Gozzo, M. N. Eberlin, J. Dupont and B. A. D. Neto, Phys. Chem. Chem. Phys., 2018, 20, 24716
    DOI: 10.1039/C8CP03609H

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