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New Insight into the Formation Mechanism of Imidazolium-based Ionic Liquids from N-alkyl Imidazoles and Halogenated Hydrocarbons: A Polar Microenvironment Induced and Auto-promoted Process Xueli Mu, Nan Jiang, Chengbu Liu, and Dongju Zhang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b11610 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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The Journal of Physical Chemistry

New Insight into the Formation Mechanism of Imidazolium-based Ionic Liquids from N-alkyl Imidazoles and Halogenated Hydrocarbons: A Polar Microenvironment Induced and Auto-promoted Process

Xueli Mu,† Nan Jiang,‡ Chengbu Liu,*,† and Dongju Zhang *,†

†

Institute of Theoretical Chemistry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China ‡

Dongguan Institute of Advanced Technology, Dongguan 523808, China

Corresponding Authors: [email protected] [email protected]

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ABSTRACT To illustrate the formation mechanism of imidazolium-based ionic liquids (ILs) from N-alkyl imidazoles and halogenated hydrocarbons, density functional theory calculations have been carried out on a representative system, the reaction of N-methyl imidazole with chloroethane to form 1-ethyl-3-methyl imidazolium chloride ([Emim]Cl) IL. The reaction is shown to proceed via a SN2 transition state with a free energy barrier of 34.4 kcal/mol in gas phase and 27.6 kcal/mol in toluene solvent. The reaction can be remarkably promoted by the presence of ionic products and water molecules. The calculated barriers in toluene are 22.0, 21.7, 19.9 kcal/mol with presences of 1-3 ionic pairs of [Emim]Cl, and 23.5, 21.3, and 19.4 kcal/mol with presences of 1-3 water molecules, respectively. These ionic pairs and water molecules do not participate directly in the reaction, but provide a polar environment which favors to stabilize the transition state with large charge separation. Hence we propose that the synthesis of imidazolium-based ILs from N-alkyl imidazoles and halogenated hydrocarbons is an auto-promoted process and a polar microenvironment induced reaction, and the existence of molecular water (a highly polar solvent) in the reaction may be mainly responsible for the initiation of reaction.

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INTRODUCTION

Ionic liquids (ILs), typically organic-based molten salts,1 have attracted tremendous enthusiasm due to their numerous unique physicochemical properties, such as low melting point, high electric conductivity, fine solubility, good thermal stability, negligible vapor pressure and etc.1-6 In the past three decades, ILs have been explored in almost all areas of chemistry and chemical engineering as solvents,5 catalysts,6 electrolytes,7 and new materials.8-9 Among all ILs, those based on imidazolium salts represent one of the most important sub-classes of ILs. As the most extensively investigated ILs, they are used widely in various chemical process, including organic synthesis,10 liquid-liquid extraction,11 and electrochemical studies.12 Researchers have paid more and more attention to their macroscopic properties,13-14 microscopic structures,15-17 catalytic performances,18-20 capabilities of tuning reactivity,21 as well as synthesis strategies.22-26 The most utilized route to prepare imidazolium-based ILs is carried through two successive steps: i) Menshutkin reaction between N-alkyl imidazoles and halogenated hydrocarbons (Scheme 1) to yield first-generation halide ILs,3,12 and ii) the metathesis of halide with a required anion to obtain second-generation halogen-free and stable ILs.3 To understand the particular Menshutkin reaction of preparing imidazolium-based ILs at the atomic level, several published works have computed the relevant energetics and molecular mechanism.12,27-29 However, in most of the previous studies, the reaction was usually mimicked using the most simple supermolecular approach with the presence of only two reactant molecules. Moreover, it was still unrevealed about how the reactivity changed during the reaction. With the simplistic model, the calculated energy barriers of

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the Menshutkin reactions are unreasonably high (generally > 30 kcal/mol),27 which is not compatible with the experimental observation that imidazolium-based ILs are usually prepared under mild reaction conditions.31-32 It is known that the synthesis of ILs is generally carried out in conventional organic solvents, i.e. the initial reaction occurs in a molecular solvent. However, as the reaction proceeds, uncharged reactants are continuously converted to the ionic

products, leading to an ever-changing

microenvironment around the active sites of the reaction. Molecular interactions in the newly formed ionic product, including strong ion-ion Coulombic interactions, and weak H-bonding interactions, are expected to have a significant effect on the reactivity. In this work, density functional theory (DFT) calculations are performed to reexamine the molecular mechanism of synthesizing imidazolium-based ILs from N-alkyl imidazoles and halogenated hydrocarbons by considering the effect of different microenvironments near the active site on the reactivity. As one of the most simple examples, the reaction of synthesizing 1-ethyl-3-methyl imidazolium chloride ([Emim]Cl) from N-methyl imidazole (A) and chloroethane (B), as depicted in Scheme 1, is chosen as the subject of study in this work. This reaction has been experimentally studied by Pant et al.,32 and was smoothly carried out in toluene at 110 °C and atmospheric pressure. Our calculations for this system are performed by considering several different situations at the active site: i) without presence of any ionic product in toluene to mimic the reaction at the initial stage, ii) with presences of 1-3 ionic pairs in toluene to understand the reactivity change as increasing concentration of the ionic product, iii) with presences of 1-3 water molecules in toluene to examine the effect of polar microenvironment on the

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reaction, and iv) with presence of 3 ionic pairs in 1,2-dichloroethane to discribe the reactivity towarding the final stage where the concentration of the ionic product around the active site is large enough to provide a very different solvent surrounding in comparison with that of the inital stage in toluene solvent around the active site. Here, 1,2-dichloroethane was used to model the solvent effects of ILs, because it has similar dielectic properties (ε = 10.125) with ILs.29 Our main goal is to learn more about the reactivity change as the reaction progresses in the important Menshutkin reaction. As far as we know, such a topic has not been addressed in the previous investigations. This work presents the first example that adresses such a topic. Based on the results of DFT calculations at molecular level, we show different reactivities of the Menshutkin reaction at different reaction satges. We expect that theoretical results would provide a guidance for the preparation of imidazolium-based ILs from N-alkyl imidazoles and halogenated hydrocarbons.

Scheme 1. Schemtic diagram for the synthesis of 1-ethyl-3-methyl imidazolium chloride ([Emim]Cl) from N-methyl imidazole (A) and chloroethane (B).

COMPUTATIONAL DETAILS

All calculations are carried out using the Gaussian 03 program33 in the framework of DFT,34-37 which is known to be an efficient and inexpensive alternative to the 5



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time-consuming ab initio based methods.38 The hybrid B3LYP functional34,37,38 as the most commonly employed DFT model, and the medium size 6-31+G(d,p) basis set are chosen for calculations. Such a functional/basis set combination represents a medium level methodology, and has been proven to sucessfully provide a qualitative understanding of the structures and properties of IL systems.40 Geometry optimizations for all structures involved in each case described below are carried out in both the gas phase without any symmetry constraint and in different solvents (toluene and 1,2-dichloroethane) using the polarizable continuum model (PCM) with UAKS radii.41-42 The vibrational frequencies are calculated at the same level of theory to verify all stational points as local minima (zero imaginary frequencies) or first-order saddle points (one imiganaty frequency) and to provide the Gibbs free energies at 298 K and 1 atm. The reaction pathways have been traced by performing intrinsic reaction coordinate (IRC)43 calculations to confirm that the optimized transition states connect the forward and reverse minima. The claculated energy barriers are reported in both the relative electronic energies (∆E#, including zero-point-energy corrections) and relative Gibbs free energies (∆G#), however, only the latters are used to discuss the reactivity.

RESULTS AND DISCUSSION Genernal energetic profile without presence of any explicit ionic product. To mimic the initial stage of the Menshutkin reaction, we first calculated the potential energy surface (PES) profile of the reaction without presence of any explicit ionic pair. Both situations in gas phase and in toluene solvent have been considered in the present work.

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The calculated results are schematically shown in Figure 1, where R0 denotes the binding complex between N-methyl imidazole (A) and chloroethane (B), TS0 is the transition state, and P0 represents the binding complex between the cation and anion of [Emim]Cl. In these symbols, the subscript “0” means the situation without presence of ionic product [Emim]Cl. Similarly, for the following systems with presence of 1-3 ionic pairs of [Emim]Cl or 1-3 water molecules, the corresponding structures are marked with subscripts 1-3 or 1w-3w.

Figure 1. Calculated relative energy profiles (electronic energy ∆E#, and Gibbs free energy ∆G#) with schematic geometries (distances are in angstroms) for the synthesis of [Emim]Cl from N-methyl imidazole (A) and chloroethane (B) with the optimized geometries of intermediate (R0), transition state (TS0) and product (P0) in vacuum (a) and in toluene (b).

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As indicated by calculated geometrical parameters (Figure 1a), the weak H-bonding interaction between two substrates (A and B) leads to the formation of an initial bimolecular complex R0 with a H-bonding distance of 2.445 Å. Along the reaction coordinate, R0 evolves into ionic product P0 via transition state TS0, which features a SN2-type transition state. In TS0, N1 atom in A acting as a nucleophile attacks C2 atom in B, inducing the formation of N1-C2 bond (1.957 Å) and the breaking of C2-Cl (2.533 Å) bond in a simultaneous process. Expected as a SN2-type transition state, TS0 presents a quasilinear structure with the Cl-C2-N1 angle being 160.3°. This quasilinear characteristic of TS0 is in agreement with the computational results in the Menshutkin reactions between 2-amino-1-methylbenzylimidazole and iodomethane reported by Melo et al.44 and with those between N-methylimidazole and benzyl halides.29 However, it is quite different from the computational results found in similar systems with bent transition state structures calculated by Li et al.27 In the ionic product complex [Emim]Cl, there exists H-bonding interactions between the cation and anion besides the strong ion-ion interaction, as indicated in Figure 1a by the bifurcate H-bonding of Cl- with the hydrogen atom at C2 position in 1.997 Å and the methylene hydrogen atom in 2.639 Å. The reaction is calculated to be endothermic by 4.7 kcal/mol with a Gibbs free energy barrier of 34.4 kcal/mol. This barrier seems to be somewhat high for a thermal reaction carried out at 110 °C.32 Furthermore, we estimate the solvent effect of toluene on the reactivity using the PCM with UAKS radii. It is found that the structures of the binding complex and transition state are very similar to those in the gas phase with only small differences in bond

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distances. Upon inclusion of the solvent effect, the reaction is calculated to be exothermic by 1.9 kcal/mol, and the free energy barrier is reduced to 27.6 kcal/mol (Figure 1b). This is attributed to the fact that the transition state and product are more polar than the reactant complex, as indicated by calculated dipole moments, 15.5 Debye for TS0, 14.8 Debye for P0, and 3.8 Debye for R0. It is well known that the inclusion of solvent usually decreases the barrier for SN2 reactions as the transition state is more polar than the separated reactants.45-46 The calculated energy profile in toluene can be used to describe the initial stage of the reaction. However, once the uncharged reactants are converted to ionic products, the microenvironment around the active sites of the reaction is changed/reorganized in the course of the reaction. The newly formed ionic products may have a significant effect on the reactivity. In order to understand the reactivity change with increasing concentration of the ionic product, we mimic the reaction in toluene by including one to three ionic pairs of the products in the following section. Reactions in toluene with presence of 1-3 ionic pairs of [Emim]Cl. The initial supermolecular structures with presence of ionic pair(s) are generally obtained based on a combination of chemical and electrostatic senses. For example, in the situation of one ionic pair, the cation docks with the Cl atom in B, the chloride anion approaches the acidic C2 proton in A, and the electrostatic interaction occurs between the cation and anion. Although this procedure does not guarantee finding the global energetic minimum in mechanistic pathways, it provides a reasonable approach to approximately describe the reaction under normal circumstance. The calculations for the system with presence of 9



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explicit ionic pairs have considered the solvent effect of toluene and all structures involved have been fully optimized using the continuum model. Thus the calculated barriers of the reaction with presence of 1-3 ionic pairs are comparable with those of the corresponding reaction without presence of any ionic pair.

Figure 2. Calculated relative energy profiles (electronic energy ∆E#, and Gibbs free energy ∆G#) with schematic geometries (distances are in angstroms) for the formation of [Emim]Cl from N-methyl imidazole (A) and chloroethane (B) in toluene with presence of one ionic pair of [Emim]Cl.

Figure 2 shows the calculated results for the reaction with presence of one ionic pair of [Emim]Cl at the active site. It is found that the intrinsic nature of the SN2 reaction keeps unchanged, i.e. the introduced ionic pair does not directly participate in the reaction. However, its presence changes the microenvironment of the active site. Throughout the reaction process, both the cation and anion show substantial H-bonding interactions with the reactants. Indicated by the optimized geometrical parameters, it is obvious that the 10


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H-bonding interactions in the transition state (TS1) are stronger than those in the initial binding complex (R1) due to its larger polarity. By H-bonding interactions and Coulombic interactions of the ionic pair with substrates, the barrier of the reaction is decreased to 22.0 kcal/mol from 27.6 kcal/mol. Clearly, the effect of the ionic pair on the reactivity is signiﬁcant because of the ionic character of the SN2-type transition state. As a result, TS1 with a larger charge separation is much more stabilized by the polar solvent provided by the ionic pair than the reactant complex R1.

Figure 3. Calculated relative energy profiles (electronic energy ∆E#, and Gibbs free energy ∆G#) with schematic geometries (distances are in angstroms) for the formation of [Emim]Cl from N-methyl imidazole (A) and chloroethane (B) in toluene with presence of two ionic pairs of [Emim]Cl.

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Figure 4. Calculated relative energy profiles (electronic energy ∆E#, and Gibbs free energy ∆G#) with schematic geometries (distances are in angstroms) for the formation of [Emim]Cl from N-methyl imidazole (A) and chloroethane (B) in toluene with presence of three ionic pairs of [Emim]Cl.

Similarly, we introduce the second and third ionic pairs into the active site of the system to study the reactivity change with increase of the ionic product. The calculated PES profiles with schematic geometries are given in Figures 3 and 4. The interactions between the components differ in these two relative larger systems. Electrostatic interactions between cation and anion, π-π stacking interactions between imidazolium rings of the cations, and particularly, the H-bonding interactions among these components, make the systems a complicated interaction network. These systems are expected to be more realistic and might describe the observed reaction in a more precise way than the

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conditions without presence of any ionic pairs. Figure 5 shows the evolution of the free energy barrier as increasing the ionic pair number. Without presence of the ionic pair, the free energy barrier is 27.6 kcal/mol. When the first ionic pair is introduced into the active site, in contrast, the free energy barrier of this reaction is reduced to 22.0 kcal/mol, decreasing by 5.6 kcal/mol. This fact indicates that the effect of the first ionic pair on the reactivity is significant, which might be attributed to the substantially different microenvironment change of two systems at the active site. The ionic pair provides a favorable polar environment which importantly stabilizes the transition state having significant charge separation. The introductions of the second and third ionic pairs further reduce the barrier to 21.7 and 19.9 kcal/mol. However, the barrier downtrend becomes slower due to the relatively small microenvironment change of the active site resulted from the introduction of the second and third ionic pairs.

Figure 5. Change of the free energy barrier with the increasing number (n) of ionic pairs.

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From these results, it is very clear that the initial reaction is expected to be a little difficult due to lack of a polar environment. However, the reaction would be remarkably promoted by the newly formed ionic product, which provides a favorable polar microenvironment for the following reaction. In this sense, we refer the reaction as an auto-promoted process. Reaction in toluene with presence of 1-3 water molecules. In view of the substantial effect of the polar environment on the reactivity, we conjecture that the initiation of reaction may be related to the existence of trace amounts of water, a highly polar solvent with a dielectric constant of 78, because the reaction was carried out experimentally in an open system which might bring in trace water. To confirm this conjection, calculations have been performed on the systems where 1-3 water molecules are contained in active site. The calculated PES profiles are shown in Figure 6, and the evolution of the barrier with the number of water molecules are shown in Figure 7. Similar to the stiuations with presence of the ionic pair(s), the water molecules at the active site assemble to the substrates through intermolecular H-bonding interactions. The calculated barriers are 23.5, 21.3, and 19.4 kcal/mol for the one-, two-, and three-water-containing systems, respectively. Figure 7 shows the largerst barrier change when one water molecule is introduced into the active site, 4.1 kcal/mol lower than the correspoding one without presence of any water molecule in toluene. On the other hand, the barriers of the systems with presence of water molecules are comparable with the corresonding those with presence of ionic pairs, 23.5 vs. 22.0, 21.3 vs. 21.7, and 19.4 vs. 19.9 kcal/mol. Water molecules as a polar solvent can also promote the reaction. In this sense, the reaction 14


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under consideration can also be referred as a polar microenvironment induced reaction.

Figure 6. Calculated relative energy profiles (electronic energy ∆E#, and Gibbs free energy ∆G#) with schematic geometries (distances are in angstroms) for the investigated systems in toluene with presence of one (a), two (b) and three (c) water molecules. 15



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Figure 7. Change of the free energy barrier with increasing number (n) of water molecules at the active site.

Reaction in 1,2-dichloroethane with presence of three ionic pairs. Our attention now focuses on the final stage of the reaction, where the ionic products distribute around the active site, providing a very different solvent surrounding in comparison with that of the initial stage with only toluene solvent. At this stage, the concentration of ionic products around the active site is so high that the reaction can be approximately regarded as occurring in an ionic liquid environment. Therefore, for the system with presence of three ionic pairs, we re-performed calculations using 1,2-dichloroethane as the implicit solvent, which has similar dielectic properties with ILs and is generally used to model the solvent effect of ILs.29 The calculated free energy barrier is found to be 22.4 kcal/mol, which is 2.5 kcal/mol larger than that (19.9 kcal/mol) for the same model system but using toluene as the implicit solvent. We conjecture that the larger free energy barrier in

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1,2-dichloroethane may be due to the stronger interactions of the explicite ionic pairs with 1,2- dichloreetnane than with toluene, which weaken the interactions between the ionic pairs and the active site. This conjecture is supported by the calculated larger interaction energies of the ionic pair with 1,2-dichlorpethane (6.33 kcal/mol) than with toluene (3.41 kcal/mol). These results imply that at the final stage, the reaction might be suppressed by superabundant ionic products. Therefore, an appropriate concentration of the ionic product around the reactive site is very crucial: neither a low nor a high one is favorable for the Menshutkin reaction. Based on the discussion above, we can propose a suitable strategy for synthesizing imidazolium-based ILs from N-alkyl imidazoles and halogenated hydrocarbons at mild reaction conditions, such as at atmospheric pressure and near room temperature conditions. On one hand, adding trace amounts of water to the system induces the initial reaction. And on the other hand, extracting the superabundant ionic products accumulated during the reaction using a suitable solvent so as to keep the ionic concentration around the active site at an appropriate level, which would promote the reaction proceeds smoothly. Our proposal might be informative for the efficient synthesis of imidazolium-based ILs and even other ILs which are synthesized through Menshutkin reactions.

CONCLUSIONS

In summary, we have presented a systematic theoretical study of the synthesis mechanism of imidazolium-based ILs from N-alkyl imidazoles and halogenated hydrocarbons. Based on the theoretical results, we refer the reaction as a polar microenvironment induced and 17



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auto-promoted process. By introducing the explicit ionic pair(s) to the active site, the free energy barrier of the reaction is reduced to 19.9 kcal/mol from 27.6 kcal/mol. Such a moderate barrier is remarkably much lower than those (generally > 30 kcal/mol) in literature, and is well compatible with the experimental observation that the reaction was carried out under mild conditions. In addition, we present a theoretical proposal of efficiently synthesizing ILs through the Menshutkin reaction: inducing the initiation by adding trace amounts of water to the system, and extracting the superabundant ionic products accumulated during the reaction using a suitable solvent so as to keep the ionic concentration around the active site at an appropriate level. We hope that the present results would deepen our understanding for the Menshutkin reaction between N-alkyl imidazoles and halogenated hydrocarbons.

■ ASSOCIATED CONTENT Supporting Information The Cartesian coordinates of all stationary points located in this work and complete citation for Gaussian 03 (Frisch et al., 2004). This material is available free of charge on the ACS Publication website at DOI:

.

Corresponding Authors *

C. B.Liu. E-mail: [email protected].

*

D. J. Zhang. E-mail: [email protected].

Notes The authors declare no competing financial interest.

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ACKNOWLEDGEMENTS

This work was jointly supported by National Basic Research Program of China (973 Program, 2013CB934301) and National Natural Science Foundation of China (Nos. 21433006 and 21273131).

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