Elsevier

Journal of Molecular Liquids

Volume 342, 15 November 2021, 117553
Journal of Molecular Liquids

Thiazolium-based ionic liquids: Synthesis, characterization and physicochemical properties

https://doi.org/10.1016/j.molliq.2021.117553Get rights and content

Highlights

  • Six new thiazolium-based ILs were prepared and characterized.

  • Thiazolium ILs have low melting points and high thermal stability.

  • Thiazolium ILs exhibit moderate density, refractive index and surface tension.

  • The temperature-dependent viscosity and conductivity of thiazolium ILs obey fractional Walden’s rule.

Abstract

Thiazolium salts are structurally analogous to the most common imidazolium-based ionic liquids (ILs), and are supposed to be a new class of ILs. However, there are up to now only a few reports and absence of systematic study on thiazolium ILs. In this work, several thiazolium-based ILs were prepared, characterized and comparatively studied. Most of the thiazolium ILs are liquid at room temperature, with melting points below 43.0 °C. Good thermal stability was observed for thiazolium ILs, resulting in thermal decomposition temperatures up to 264 ∼ 358 °C. In comparison with the conventional imidazolium ILs, thiazolium ILs exhibit moderate density, refractive index and surface tension, which all generally decrease with temperature. According to the temperature-dependent viscosity and conductivity, thiazolium ILs follow fractional Walden’s rule, indicating strong ion-ion coupling. Besides, thiazolium ILs display relatively narrow electrochemical window (3.1 ∼ 4.0 V).

Introduction

Ionic liquids (ILs), a special category of organic salts with melting points below 100 °C, combine two apparently contradictory features, i.e. “ionic” and “liquid”. In order to lower melting points, the ions of ILs are often incorporated with high charge delocalization to weaken the interionic Coulomb force, and low symmetry to increase lattice mismatch [1]. Compared to conventional molecular organic solvents, ILs have many attractive properties notably non-volatility and non-flammability, which avoid their release to the atmosphere and the risk of exposure [2], [3]. As a result, ILs have emerged as an environmentally-favorable platform for various applications, such as solvents, electrolytes, functional materials and many more [4], [5]. The ever-increasing applications also benefited from ILs’ structural diversity and versatile designability, based on the nearly limitless combinations of ions and functional groups [6].

The number of theoretically possible ILs was estimated to be as high as 1018, and only several thousands have been synthesized and studied [7]. However, most of the available ILs are highly viscous, and have to be used in form of solutions in organic solvents, instead of neat ILs with non-volatile and non-flammable characteristics [8]. Generally, the viscosity of a liquid increases significantly with molecular weight [9], Therefore the ions of low-viscous ILs should not only bear the charge delocalization and unsymmetrical structures, but also be as small in size as possible [10]. Such type of ions are actually very few in number [11]. Typically, the unsymmetrical aromatic imidazolium cation-based ILs are low-melting and low-viscous, which are the most popular and widely studied class of the vast family of ILs [12].

Thiazolium heterocycle is structurally similar to imidazolium, with a sulfur atom replacing one of the nitrogens in imidazolium. It is thus expected that thiazoliums are appropriate in both structure and size to construct low-melting and low-viscous ILs. Nevertheless, the reports on thiazolium-based ILs are fairly rare [13]. There are generally two main approaches to prepare thiazolium salts, namely Hantzsch reaction and quaternization. Hantzsch reaction, discovered in 1889, is a classical route to form heterocycles, including thiazoles (by cyclization of thioamides with α-halocarbonyl compounds) [14], [15], [16]. Another more commonly used route to thiazolium salts is quaternization of thiazoles with alkylation agents, such as halohydrocarbons, dialkyl sulfates, alkyl triflates, alkyl p-toluenesulfonates and so on [17], [18]. The nucleophilic attack ability of thiazoles towards alkylation agents, i.e. the primary determinant of reaction rate, is evidently weaker than pyridine and only comparable with quinolone [19]. This might be related to the enhanced aromaticity caused by the inclusion of S atom in the five-membered heterocycle, leading to low nucleophilicity [20]. As a result, pKa values of protic thiazolium salts (ca. 17 ∼ 19) are significantly lower than their imidazolium analogues (ca. 21 ∼ 24) [21]. Moreover, the thiazolium sulfur atom may cause unique interactions, such as sigma (σ)-hole type interaction with bromide ion, affecting the properties in both the solid and liquid state [17].

As a subclass of ILs, thiazolium-based ILs were also routinely used as solvents in the fields of extraction [22], gas separation [18], lubrication [23] and so on. Besides, Yuan et al [24], [25] reported a thiazolium-type polymeric binder by polymerization of 3,4-dimethyl-5-vinylthiazolium bis(trifluoromethyl)sulfonylimide. The poly(ILs) was directly used as a binder in Li-ion battery, resulting in specific capacity and cycling stability outperforming the traditional PVDF binder (ca. 140 vs. 120 mAh/g at 1C). However, for a long time thiazolium salts were mostly applied as organocatalysts in various reactions for carbon–carbon and carbon-heteroatom bond formation, such as benzoin condensation reaction [26], Stetter reaction [27] and so on, because of their tendency to form N-heterocyclic carbenes [28], [29], [30].

Although thiazolium cations are promising to be an important subclass of ILs, in consideration of their quite analogous structures to imidazolium-based ILs, there are only a handful of relevant reports and absence of systematic study on thiazolium ILs. In this study, a series of thiazolium-based ILs were synthesized and characterized, and the physicochemical properties were measured and systematically analyzed, in comparison with imidazolium ILs.

Section snippets

Synthesis and structure characterization

Thiazole (99%) and 4-methylthiazole (99%) were purchased from Sigma-Aldrich. Alkyl iodides (99%) were obtained from J&K Scientific Ltd and distilled before use. Lithium bis(trifluoromethane)sulfonamide (LiNTf2, 99%) was supplied by Aladdin Reagent Company. Lithium difluoro(oxalato)borate (LiDFOB, 98%) was offered by Chemlin Chemical Industry Co., Ltd.

Generally, ILs used in this study were prepared via two steps, as shown in Fig. 1. Firstly, the cation precursors (iodides) were obtained by the

Thermal properties

The DSC curves of ILs are shown in Fig. 3, and the corresponding glass transition temperature (Tg), crystallization temperature (Tc), melting point (Tm), polymorphic transition temperature (Tp) and associated enthalpy change (ΔH) are summarized in Table 1. Accordingly, four types of phase transition behavior were observed during a cooling and heating cycle. Firstly, [C2-Thz]NTf2 started to crystallize at −29.0 °C on cooling and melt at 30.8 °C on heating. As the crystallization was a very fast

Conclusions

In conclusion, a series of thiazolium-based ILs were prepared, characterized and comparatively studied with the analogous imidazolium-based ILs. The structure and composition of the prepared ILs were confirmed by 1H NMR and ESI-MS. Thermal characterization results revealed that these thiazolium ILs have low melting points ranging from −4.5 to 43.0 °C and high thermal decomposition temperatures ranging from 264 to 358 °C. Compared with the analogous imidazolium ILs, thiazolium ILs show moderate

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.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21905069), the Marine Biomaterials Research Joint Lab of ZIAT and Dangan Town, the Zhuhai Science and Technology Department Project (ZH22036207200025PWC, ZH22017003200028PWC, ZH22017001200078PWC), the Graphene Manufacture Innovation Center of Economic, Trade and Information Commission of Shenzhen (201901161514), and the Natural Science Basic Research Plan in Shaanxi Province (2019JM248).

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