Solvent influence on imidazolium based ionic liquid contact pairs
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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