Solvent influence on imidazolium based ionic liquid contact pairs

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Highlights

  • The association of imidazolium cations with the imidazolate anion has been studied in different solvents.

  • The local solvent structure around the ion pair plays an important role on ion association.

  • NMR and molecular dynamic simulations corroborate that the ion pairs are more stable in acetone than chloroform.

Abstract

The interaction and solvent influence on two different imidazolium based ion pairs have been investigated by molecular dynamics simulations and Nuclear Magnetic Resonance. The cations 1,2,3,4,5-pentamethyl imidazolium and 1,3,4,5-tetramethyl imidazolium were considered with the imidazolate anion to evaluate the influence of the acidic hydrogen at the position 2 of the imidazolium ring on the ion pair formation in different solvents. The selected solvents are chloroform, dichloromethane, acetone, dimethylsulfoxide, and water, covering a broad range of polarity and permittivity. The binding free energy of the ion pair was computed by umbrella sampling. We observed that, with the increase of the dielectric constant, the ion pairs become more transient being separated in water. The free energies of binding corroborate ion pair stabilization by the hydrogen bond at carbon 2 of the imidazolium cations. In dichloromethane, we obtained weaker bound ion pairs than in acetone due to intercalation of dichloromethane into the ion pair. Thus, the ion pair stability is not only a consequence of the solvent's dielectric constant, but also due to local structural details.

Introduction

Bulk ionic liquids (ILs) have received a lot of attention in the past few decades due to their large versatility as chemical green solvents [[1], [2], [3], [4]]. In addition to the advantageous chemical properties typical for ILs (low volatility, fairly high conductivity, high thermal stability), imidazolium based ILs also offer the possibility of tailoring polar and non-polar domain formation by adequate alkyl substitutions permitting to adjust their properties as desired [[5], [6], [7]].

The analysis of the interactions and physical properties of ILs mixtures with other solvents became a focus of recent researches. Several studies in the literature report the geometrical structures, interactions and influences of solvent impurities, mainly water or chloroform, on ILs [[8], [9], [10], [11], [12]]. Ionic liquid ion pairs (ILIPs) offer a broad range of applications, such as ion pair chromatography, biphasic reaction catalysis, ion batteries and energy generation, among others [[13], [14], [15], [16]]. Also, the phenomenon of ion pair formation is of fundamental importance in many chemical fields, as its occurrence may influence reaction rates and intermediate stabilities.

The contact ion pair can favour chemical reactions, as previously reported by Dupont and co-workers [[17], [18], [19]] describing unusual H/D exchange reactions in methyl substitutions at the imidazolium ring in less polar solvents, in which the contact ion pairs are maintained. On the other hand, only small degrees of deuteration have been detected for the same ion pair in more polar solvents as a consequence of ion pair separation. Zanatta et al. reported that an ILIP may act as a neutral base when the contact ion pair is maintained [8]. Moreover, Swatloski et al. presented that the dissolution of cellulose depends on the presence of charged and neutral aggregates in solution [20].

As the physical and chemical properties of ILs can change drastically depending on the environment in which they are diluted, it becomes of interest to understand the formation and behaviour of contact ion pairs in different solvents. Several works in the literature can be found on ion pair formation and cation-anion interaction, mainly for halide anions (Cl, Br, F) or BF4, in one or two solvents, often chloroform and water [21]. In this work, five different solvents have been chosen spanning a wide range of polarity: chloroform (εr: 5), dichloromethane (εr: 8.9), acetone (εr: 17.9), DMSO (εr: 47.2) and water (εr: 80). Moreover, motivated by earlier studies from our group [8,17], we selected ion pairs containing small imidazolium cations combined with the organic aromatic imidazolate anion. We focus on the structure of the ILIP using molecular dynamics computer simulations (MD) combined with NMR data from the Nuclear Overhauser Effect (NOE) in the different solvents. The stability of the ion pair is revealed by the free energy for the ion pair separation by MD.

Section snippets

Materials and methods

To evaluate the solvent influence, both experimentally and theoretically, two ILIPs, 1,3,4,5-tetramethylimidazolium·imidazolate (TetMI·Im) and 1,2,3,4,5-pentamethylimidazolium·imidazolate (PMI·Im), were synthetised and studied by MD and NMR. The TetMI·Im and PMI·Im structures are represented in Fig. 1, with the corresponding atom numbers used throughout the article.

Molecular dynamic simulations

The formation of stable ion pairs between cations and anions is governed by electrostatic attraction between the charge species. Thus, introducing more dielectric surroundings to the ion pairs is expected to weaken the interaction between the charged species turning the ion pairs more transient [6,34]. As a measure for the life time of the ion pairs, we monitored the minimum distance between any atom of the cations and the anions illustrated in the Supplementary information (Fig. SI 19). We

Conclusions

The ion pair formation of the ILs 1,2,3,4,5-pentamethyl imidazolium (PMI·Im) and 1,3,4,5-tetramethyl imidazolium (TetMI·Im) imidazolates has been investigated in the solvents chloroform, dichloromethane, acetone, DMSO, and water by computational and experimental procedures. Structural aspects and ion interactions were evaluated by RDFs, SDFs, and the association free energy. We found ion pairs that become more transient when the polarity of the solvent is increased. An exception is represented

CRediT authorship contribution statement

Chiara Valsecchi: Methodology, Formal analysis, Data curation, Writing - original draft, Writing - review & editing, Visualization. Marcileia Zanatta: Investigation, Formal analysis, Data curation, Writing - original draft. Jessé Neumann: Visualization, Investigation. Graciane Marin: Data curation, Investigation. Jairton Dupont: Supervision, Validation. Francisco P. dos Santos: Supervision, Investigation. Hubert K. Stassen: Conceptualization, Validation, Supervision, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors acknowledge financial support from the Brazilian agencies CNPq (169462/2017-0), CAPES (financial code 0001), and FAPERGS (8887.195052/2018-00).

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