Are Ionic Liquids Chemically Stable? - Chemical Reviews (ACS

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Are Ionic Liquids Chemically Stable? Binshen Wang,†,‡ Li Qin,†,‡ Tiancheng Mu,*,§ Zhimin Xue,∥ and Guohua Gao*,† †

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, China § Department of Chemistry, Renmin University of China, 59 Zhongguancun Street, Beijing 100872, China ∥ Beijing Key Laboratory of Lignocellulosic Chemistry, College of Materials Science and Technology, Beijing Forestry University, 35 Tsinghua East Road, Beijing 100083, China ABSTRACT: Ionic liquids have attracted a great deal of interest in recent years, illustrated by their applications in a variety of areas involved with chemistry, physics, biology, and engineering. Usually, the stabilities of ionic liquids are highlighted as one of their outstanding advantages. However, are ionic liquids really stable in all cases? This review covers the chemical stabilities of ionic liquids. It focuses on the reactivity of the most popular imidazolium ionic liquids at structural positions, including C2 position, N1 and N3 positions, and C4 and C5 positions, and decomposition on the imidazolium ring. Additionally, we discuss decomposition of quaternary ammonium and phosphonium ionic liquids and hydrolysis and nucleophilic reactions of anions of ionic liquids. The review aims to arouse caution on potential decomposition of ionic liquids and provides a guide for better utilization of ionic liquids.

CONTENTS 1. Introduction 2. Reactions of Imidazolium Ionic Liquids 2.1. Reactions at C2 Position 2.1.1. Reactions with Organic Compounds 2.1.2. Reactions with Biomass 2.1.3. Reactions with CO2, CS2, OCS, and Chalcogenides 2.1.4. Reactions with Oxidants 2.2. Cleavage of C−N Bond at N1 and N3 Positions 2.3. Reactions at C4 and C5 Positions 2.4. Ring-Opening Reactions on the Imidazolium Ring 3. Decomposition of Quaternary Ammonium and Phosphonium Ionic Liquids 3.1. Decomposition of Quaternary Ammonium Ionic Liquids 3.2. Decomposition of Quaternary Phosphonium Ionic Liquids 4. Hydrolysis and Nucleophilic Reactions of Anions of Ionic Liquids 5. Conclusions and Outlook Author Information Corresponding Authors ORCID Author Contributions Notes Biographies Acknowledgments Abbreviations References

© XXXX American Chemical Society

1. INTRODUCTION Ionic liquids are a subset of molten salts with melting points at or below 100 °C. In 1914, Walden1 prepared the first ionic liquid, ethylammonium nitrate (EAN), by neutralizing ethylamine with concentrated HNO3. EAN exhibits similar physical properties to water as it is clear, colorless, odorless and has reasonably high viscosity. Most importantly, the conductivity of EAN was consistent with a composition of purely anions and cations.1−3 In 1951, Hurley and Wier4 reported the second generation of ionic liquids by mixing alkylpyridinium chlorides with AlCl3. Unfortunately, most of the second-generation ionic liquids are not stable in the presence of moisture, and their acidity/basicity is not easy to regulate.5 In 1992, Wilkes and Zaworotko6 prepared moisture- and air-stable ionic liquids based on imidazolium and tetrafluoroborate. The distinct advantages of ionic liquids enabled them to receive extensive attention from both academia and industry. Ionic liquids, due to their low volatility, are considered as promising alternative solvents to replace traditional volatile organic compounds (VOCs), spurred by the green chemistry movement.7−9 Indeed, most research efforts have been devoted to applying ionic liquids as green solvents7,10−12 and electrolytes13,14 at the initial stages. However, ionic liquid chemistry has progressed by leaps and bounds in the past dozen years. Nowadays, ionic liquids have become a hot research topic among multidisciplinary areas including chemistry, physics, biology, and engineering. Various exciting new applications of ionic liquids have been developed continuously, for example,

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Special Issue: Ionic Liquids Received: August 30, 2016

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DOI: 10.1021/acs.chemrev.6b00594 Chem. Rev. XXXX, XXX, XXX−XXX

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Scheme 1. Reactions of Imidazolium Ionic Liquids at Different Positions

metallic catalysis, with ionic liquids as solvents, first aroused attention. In 2000, Xiao and co-workers39 first reported firm evidence that imidazolium ionic liquids can react with a transition-metal catalyst to give metal−carbene complexes by deprotonation of C2−H in imidazolium in the Heck reaction. Soon after, a number of studies also showed the formation of metal−carbene complexes by reactions of metal complexes and ionic liquids.40−43 Dupont and Spencer44 highlighted the noninnocent nature of imidazolium ionic liquids in 2004, which mainly focused on the formation of carbenes and carbene complexes in the reactions where ionic liquids were used as solvents. In 2005, Scammells et al.45 addressed the neglected issues of ionic liquids: purity, stability, biodegradability, and toxicity. In 2007, Scott and co-workers46 summarized reactivity of ionic liquids based on the types of reactions. Chu and coworkers47 reviewed the noninnocent nature of ionic liquids in 2009, which emphasized the possibility of ionic liquids to influence the mechanisms of organic reactions and hence affect yields and selectivities. This review attempts to comprehensively summarize the chemical stability of ionic liquids, covering the recent literature up to early 2016. It focuses on the reactivity of the most popular imidazolium ionic liquids at different positions of the imidazolium ring, including C2 position (section 2.1), N1 and N3 positions (section 2.2), and C4 and C5 positions (section 2.3). It also includes the decomposition of quaternary ammonium and phosphonium ionic liquids (section 3) and hydrolysis and nucleophilic reactions of anions of ionic liquids (section 4). The decomposition mechanisms based on structure positions are also discussed. The main aim of this review is to advise a more cautious approach when applying ionic liquids as solvents, electrolytes, catalysts, etc., and to make better use of ionic liquids.

gas adsorbents,15,16 lubricants,17 catalysts,18−20 extractants,21 ionic liquid crystals,22,23 and explosives and propellant fuels.24 Ionic liquids have brought about a revolution in the scientific community, deriving from their unique properties such as extremely low vapor pressure, large liquid range, tunable polarity, nonflammability, wide solubility, and high stability. Especially, the nature of high stability has been highlighted by a growing number of publications concerning ionic liquids.25−29 Definitively, the stability of ionic liquids plays a crucial role in their applications. Ionic liquids were considered as nonparticipative or inert solvents in the last century. However, are ionic liquids really stable? This is an issue that should be noticed by every researcher who uses ionic liquids in a variety of fields, as any decomposition of ionic liquids may lead to negative impacts on his/her studies. Before now, the thermal stability of ionic liquids had already attracted considerable investigation. In 2000, Ngo et al.30 published the first systematic investigation on the thermal stability of ionic liquids, in which the thermal stability was characterized by onset decomposition temperature (Tonset). However, Fox et al.31 in 2003, Kosmulski et al.32 in 2004, and Scott and co-workers33 in 2004 proposed the concept of long-term thermal stability and pointed out that the thermal stability of ionic liquids was severely overrated by the Tonset derived from a fast scan in thermogravimetric analysis (TGA). Also, many works on the thermal stability of ionic liquids including the characterization methods, decomposition temperature, kinetics, and mechanism have been carried out.34,35 We also provided a comprehensive study on the thermal stability of various ionic liquids.36 Recently, a distinguished review on thermal stability of ionic liquids has been published by Stevens and co-workers.37 Additionally, the electrochemical stability of ionic liquids has also raised many concerns and was well reviewed in 2014.38 In terms of the chemical stability of ionic liquids, the formation of N-heterocylic carbene complexes in organoB


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2. REACTIONS OF IMIDAZOLIUM IONIC LIQUIDS Imidazolium ring is a planar, rigid, π-conjugated and fivemembered heterocyclic structure. The unique structure gives imidazolium salts attractive physicochemical properties and potential applications. Imidazolium salts are the most popular investigated ionic liquids and have been applied in many fields, such as solar cell electrolytes, carbon material precursors, and alkaline anion-exchange membranes. Generally, imidazolium ionic liquids are considered to be inert. However, this is not always the case. This section covers reactions of imidazolium ionic liquids at C2 position (section 2.1), N1 and N3 positions (section 2.2), C4 and C5 positions (section 2.3), and decomposition on the imidazolium ring (section 2.4) (see Scheme 1).

Scheme 3. Reaction of [Bmim][Cl] with Benzaldehyde in the Baylis−Hillman Reaction

Scheme 4. Reactions of [Emim][X] with Aldehydes in the Horner−Wadsworth−Emmons Reaction

2.1. Reactions at C2 Position

The C2 position of imidazolium ionic liquids is one of the active sites, mainly due to the weak acidity of the C2 proton. Various compounds could react with imidazolium salts at C2 position, especially in the presence of bases. This section covers reactions at the C2 position of imidazolium salts with organic compounds (section 2.1.1), with biomass (section 2.1.2), with inorganic compounds (CO2, CS2, OCS, and chalcogenides; section 2.1.3), and with oxidants (section 2.1.4). The formation of N-heterocyclic carbenes and corresponding metal−Nheterocyclic complexes has already been well reviewed44,48−51 and will not be included in this review. 2.1.1. Reactions with Organic Compounds. 1,3Disubstituted imidazolium ionic liquids have been extensively employed as solvents in organic reactions. Although they are commonly considered as inert, many studies have reported that they can participate in reactions with organic compounds at the C2 position, especially in the presence of bases (section 2.1.1.1) and transition-metal complexes (section 2.1.1.2). 2.1.1.1. Reactions with Organic Compounds in the Presence of Bases. In the presence of bases, imidazolium salts are deprotonated at the C2 position to generate reactive nucleophiles. The generated nucleophiles can further react with organic electrophiles, such as aldehydes, alkyl halides, ethyl formate, acetic anhydride and isothiocyanates, to form C2adduct imidazolium compounds (Scheme 2).

Subsequently, Chu and co-workers53 also found that [Bmim][PF6] reacted with aldehyde under basic conditions in the Baylis−Hillman reaction. To overcome the defect, they used [Bdmim][PF6] as an alternative solvent; they found that [Bdmim][PF6] was inert and the Baylis−Hillman reaction could proceed smoothly with higher yield (Scheme 5). Scheme 5. Reaction of [Bmim][PF6] with Aldehyde in the Baylis−Hillman Reaction

Scheme 2. Reactions of Imidazolium Salts at the C2 Position with Organic Electrophiles in the Presence of Bases

Additionally, Welton and co-workers54 found that the reaction of [Emim][OAc] with paraformaldehyde gave 1ethyl-2-(hydroxymethyl)-3-methylimidazolium acetate at 80 °C for 24 h (Scheme 6). The alkalinity of acetate promoted the reaction going smoothly. Ricciardi and Joullié55 observed that [Mmim][I] was deprotonated to generate a reactive nucleophile in the presence of K2CO3. The nucleophile further reacted with acetic anhydride or ethyl formate, affording 2-formyl-1,3-dimethyli-

While studying the Baylis−Hillman reaction catalyzed by organic bases such as 1,4-diazabicyclooctane (DABCO) and 3hydroxyquinuclidine in an imidazolium-based ionic liquid ([Bmim][Cl]), Aggarwal and co-workers52 found that the conversion of benzaldehyde was much higher than the yield of product (Scheme 3). Under mild organic basic conditions, imidazolium salts were deprotonated to form reactive carbenes, which reacted with aldehyde, leading to low yields. Similar results were also observed in the Horner−Wadsworth− Emmons reaction when [Emim][BF4] and [Emim][PF6] were employed as solvents (Scheme 4).52

Scheme 6. Reaction of [Emim][OAc] with Paraformaldehyde

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midazolium iodide (78% yield) or 2-acetyl-1,3-dimethylimidazolium iodide (68% yield), respectively (Scheme 7).

Scheme 10. Alkylation Reactions of Imidazolium Ionic Liquids with Alkyl Halides

Scheme 7. 2-Acylations of [Mmim][I] with Acetic Anhydride or Ethyl Formate

During studies of the synthesis of tetrahydro-β-carbolinequinoxalinones, Chu and co-workers59 investigated the nucleophilic aromatic substitution reactions catalyzed by dimethylaminopyridine (DMAP) in various ionic liquids. Those reactions in C2-substituted imidazolium ionic liquids, [Bdmim][NTf2] and [Bdmim][PFBuSO3], afforded the desired products in yields of 74% and 93%. In contrast, the reaction gave a lower yield (approximately 20%) in [Bmim][PF6]. This was ascribed to a side reaction between [Bmim][PF6] and 1-fluoro-2nitrobenzene in the presence of DMAP (Scheme 11).

Cheng and co-workers56 reported that imidazolium bromides reacted with phenyl isothiocyanate in the presence of NaH to afford 2-phenylthiocarbamoylimidazolium zwitterion as the sole product in yields of 73−89% (Scheme 8). Therein, NaH was used for deprotonation of imidazolium to form corresponding carbenes, which underwent further reaction with phenyl isothiocyanate to give the final products.

Scheme 11. Reaction of [Bmim][PF6] with 1-Fluoro-2nitrobenzene in the Synthesis of Tetrahydro-βcarbolinequinoxalinones

Scheme 8. Reactions of Imidazolium Bromides with Phenyl Isothiocyanate

In the presence of strong bases, imidazolium salts also undergo reactions with alkyl halides. Handy and Okello57 found that alkylation of 3-butyl-5-(hydroxymethyl)-1-methylimidazolium iodide with MeI in the presence of NaH gave 3-butyl-2ethyl-5-(methoxymethyl)-1-methylimidazolium iodide. The formation of 3-butyl-2-ethyl-5-(methoxymethyl)-1-methylimidazolium iodide may be via the intermediate of 3-butyl-5(methoxymethyl)-1,2-dimethylimidazolium iodide which underwent further methylation by MeI. These observations indicate that a methyl group in the C2 position can also be subjected to deprotonation under mild conditions (Scheme 9).

Fensterbank and co-workers60,61 found that imidazolium reacted with borane−tetrahydrofuran (THF) complex in the presence of KO tBu or sodium bis(trimethylsilyl)amide (NaHMDS) to give carbene−borane complexes (Scheme 12). Scheme 12. Reactions of Imidazolium with Borane−THF Complex

Scheme 9. Alkylation Reactions of Imidazolium Iodides with Methyl Iodide

During the synthesis of carbon ylide metal complexes, Fürstner et al.62 found that 1,3-dimethyl-2-methyleneimidazoline was generated by deprotonation of 1,2,3-trimethylimidazolium iodide in the presence of potassium hydride (Scheme 13a). Lu and co-workers63 reported treatment of 1,3disubstituted-2-methylimidazolium iodides with KH also afforded N,N′-disubstituted 2-methyleneimidazolines, which provided a procedure for the preparation of N-heterocyclic olefin−CO2 adduct (Scheme 13b). These results indicate that C2-substituted imidazolium salts are also not completely stable under strong base conditions. 2.1.1.2. Reactions with Organic Compounds in the Presence of Transition-Metal Catalysts. Imidazolium ionic

The same group58 also found that reactions of imidazolium with other alkyl halides in the presence of NaH afforded the corresponding 2-alkylimidazolium ionic liquids effectively (Scheme 10). The length of the alkyl halide, such as ethyl, butyl, hexyl, heptyl, or decyl, seemed not to have a significant effect on the reactions, as all alkyl halides gave similar yields. On the other hand, alkyl chlorides gave higher yields than bromo and iodo analogues. This procedure provided a new method to synthesize C2-substituted imidazolium ionic liquids. D


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Scheme 13. Deprotonation of 1,3-Disubstituted 2Methylimidazolium Iodides in the Preparation of Gold Complexes or N-Heterocyclic Olefin−CO2 Adducts

Scheme 15. Intramolecular Coupling Reactions of AlkenylSubstituted Azolium Ionic Liquids

liquids have been widely applied as solvents for organometallic catalysis,64 and a few reports have found that imidazolium reacted with substrates in the presence of organometallic catalysts. Cavell and co-workers65,66 found that imidazolium salts underwent coupling reactions with various 1-alkenes in the presence of zerovalent Ni catalysts to give 2-alkylimidazolium salts in yields of 35−100% under mild conditions. Other azolium salts, such as thiazolium, benzothiazolium, oxazolium, benzoxazolium, and 1,3-dimethyltriazolium, also gave 2substituted products in high yields (Scheme 14). Therein, the

Scheme 16. Proposed Reaction Mechanism for Imidazolium and Alkene in the Presence of Transition-Metal Complex

Scheme 14. Coupling Reactions of Azolium Ionic Liquids and Alkenes

conventional organic solvents, presenting a major challenge for utilization of this biomass. In 2002, Rogers and co-workers68 first reported that ionic liquids could be used as nonderivatizing solvents for cellulose. After that, many studies on the application of ionic liquids in biomass chemistry were reported.69−72 Although the contribution started a novel development in cellulose research, several studies demonstrated that imidazolium ionic liquids reacted with cellulose and its derivatives. Rosenau and co-workers 73 studied the reactions of imidazolium ionic liquids with cellulose and its derivatives. They mixed 13C-labeled α- and β-glucopyranoside with [Bmim][OAc] at room temperature for 7 days and found that the reaction of [Bmim][OAc] with D-glucose took place through electrophilic attack from C2 of the imidazolium to the anomeric carbon of glucopyranoside to afford corresponding imidazolium adducts bearing C2 hydroxyalkyl substituents (Scheme 17). Similar results were obtained after 2 h in the

triazolium formed doubly substituted products. Although the procedure provides a catalytic route for the preparation of 2alkylazolium ionic liquids, these results have also direct ramifications for the utilization of azolium salts as solvents in organometallic catalysis. Later, the same group67 investigated the intramolecular coupling reactions of N-but-3-enyl-substituted imidazolium or thiazolium salts catalyzed by zerovalent Ni and Pd complexes to give fused-ring imidazolium or thiazolium salts (Scheme 15). This procedure may have potential synthetic value in the preparation of chiral ionic liquids. The authors also proposed a mechanism for the imidazolium/alkene coupling reaction (Scheme 16). Oxidative addition of imidazolium to low-valent metal complex generates a carbene−Ni−H species. Alkene insertion into the M−H bond gives an alkyl metal intermediate, followed by reductive elimination to afford 2-alkylimidazolium and re-form M0Ln. 2.1.2. Reactions with Biomass. As the most abundant renewable feedstock, cellulose is insoluble in water and most

Scheme 17. Reactions of [Bmim][OAc] with D-Glucose at Room Temperature

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of the Emim+COO− zwitterion, the [Emim]+ cation, and the [H(OAc)2]− anion (Scheme 20). Besnard et al.78 also found

presence of catalytic amounts of bases such as triethylamine (TEA), imidazole, and 1-methylimidazole. Reactions between cellulose and [Bmim][OAc] or [MNapmim][OAc] were also detected by 13C NMR and fluorescence spectroscopy (Scheme 18).73 In the exploration of new cellulose solvents and chemical

Scheme 20. Reaction of [Emim][OAc] with CO2

Scheme 18. Reactions of Imidazolium-based Acetate Ionic Liquids with Cellulose formation of Bmim+COO− by the reaction of CO2 and [Bmim][OAc] at ambient temperature and pressure, which was confirmed by NMR (1H, 13C, 15N) spectroscopy, Raman and infrared spectroscopy, and density functional theory (DFT) calculations. While studying the staged precipitation of cellulose from [Bmim][OAc] by compressed CO2, we79 also detected formation of Bmim+COO− by NMR spectroscopy. Imidazolium ionic liquids with neutral anions (e.g., [Cl]−, [NTf2]−) also undergo reactions with CO2 in the presence of inorganic and organic bases. Tommasi and Sorrentino80 reported direct carboxylation of imidazolium chlorides with CO2 (under 5 MPa) in the presence of Na2CO3, which afforded the corresponding 1,3-dialkylimidazolium-2-carboxylates in good yields and selectivity at 80 °C (Scheme 21). A similar

modification agents, Liebert and co-workers74,75 investigated solubility of cellooligomers in various ionic liquids, such as [Bmim][Cl], [Emim][Cl], and [Emim][OAc]. They analyzed the mixture of cellooligomers and [Emim][OAc] by 13C NMR spectroscopy and found that C1 signals of the reducing end of cellooligomers were not observed. In contrast, these signals were still detected in the spectrum when the oligomers were dissolved in other ionic liquids. The results illustrated that [Emim][OAc] could react with the reducing end of cellooligomers. The high reactivity of [Emim][OAc] was attributed to abstraction of the C2 proton of imidazolium by alkaline acetate, which led to formation of the active carbene. Following the above important discoveries, Welton and coworkers54 investigated the reaction mixtures between carbohydrates and imidazolium carboxylate ionic liquids in detail by NMR, HPLC, and LC/MS techniques. They found that [Emim][OAc] and carbohydrates underwent reaction at the C2 position of imidazolium to form different carbon chain (C6, C4, C3, C2, C1) imidazolium adducts (Scheme 19). The reactions of cellulose with imidazolium ionic liquids prevent the recycling of ionic liquids and may reduce the quality and quantity of the cellulose.

Scheme 21. Reactions of Imidazolium Chloride Ionic Liquids with CO2 in the Presence of Na2CO3

result was also observed by Louie and co-workers.81 They investigated the carboxylation of imidazolium salts with CO2 in the presence of KHMDS and found that a series of imidazolium-2-carboxylates with different N-substituents could be generated cleanly at atmospheric pressure and room temperature. While studying CO2 capture by imidazolium ionic liquids and organic superbases, Li and co-workers82 found that imidazolium ionic liquids reacted with CO2 to form imidazolium-2carboxylates in the presence of an equimolar superbase. Their results show that the ionic liquid−superbase can rapidly and reversibly capture equal equivalents of CO2 and provide an alternative to volatile amines and alkanols (Scheme 22).

Scheme 19. Reaction of [Emim][OAc] with D-(+)-Glucose

Scheme 22. Reaction of [Bmim][NTf2] with CO2 in the Presence of Superbase

2.1.3. Reactions with CO2, CS2, OCS, and Chalcogenides. Under basic conditions, as summarized above, imidazolium salts can react with organic electrophiles. Analogously, the deprotonated imidazolium salts could also react with inorganic electrophiles, such as CO2, CS2, and OCS (section 2.1.3.1) and chalcogenides (S, Se) (section 2.1.3.2). 2.1.3.1. Reactions with CO2, CS2, and OCS. Imidazolium ionic liquids with basic carboxylate-derived anions have been widely applied for chemisorption of CO2. In order to clarify the mechanism, Rogers and co-workers76,77 investigated the reactions between 1,3-dialkylimidazolium acetate and CO2. Upon bubbling CO2 at atmospheric pressure and room temperature for 24 h, [Emim][OAc] was converted to 1ethyl-3-methylimidazolium-2-carboxylate (Emim+COO−). The structure of imidazolium-2-carboxylate was confirmed by singlecrystal X-ray diffraction analysis; the asymmetric unit consisted

Deprotonation of imidazolium salts can be also achieved by electroreduction. Devillers and co-workers83 reported electrosynthesis of imidazolium-2-carboxylates by electroreduction of the corresponding imidazolium ionic liquids under CO2 atmosphere (Scheme 23). The method provides a simple, versatile, and environmentally benign way to synthesize pure imidazolium-2-carboxylates in comparable yields with the common dimethyl carbonate or basic deprotonation routes. In their studies of electrochemical reduction of CO2 in a diluted F


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OCS was used as starting material to react with imidazolium chlorides and imidazolium tetrafluoroborates in the presence of KHMDS, imidazolium-2-thiocarboxylates were generated smoothly at room temperature (Scheme 27).96 Strong bases

Scheme 23. Electrochemical Reduction of Imidazolium Salts with CO2

Scheme 27. Reactions of Imidazolium Salts with OCS solution of [Emim][BF4], Asadi et al.84,85 also found the production of Emim+COO− zwitterion. These studies also demonstrate that cautious use of imidazolium ionic liquids as electrolyte in reactions involving CO2, which has been extensively reported,86,87 is necessary. In order to make use of these imidazolium-2-carboxylates, Rogers and co-workers88−90 investigated the reactions of imidazolium-2-carboxylates with protic acids, such as HPF6, H2SO4, HCl, picric acid, and H2CO3, to afford corresponding ionic liquids in high yields with release of CO2 (Scheme 24).

play an important role in these reactions by deprotonation of imidazolium ionic liquids at the C2 position to form free carbenes, which further reacted with CS2 and OCS. 2.1.3.2. Reactions with Chalcogenides. Reactions of imidazolium salts at the C2 position with chalcogenides (S, Se) to generate imidazole-2-chalcogenones have been conducted in basic environments or under electrochemical and ultrasound irradiation-assisted conditions. Rogers and co-workers97 reported spontaneous reactions between imidazolium acetate ionic liquids and elemental chalcogens. Upon stirring of [Emim][OAc] and S8 at 25 °C for 24 h, 1-ethyl-3-methylimidazole-2-thione was afforded in around 50% yield. Similarly, reaction of [Emim][OAc] and Se also gave 1-ethyl-3-methylimidazole-2-selone in 90% yield at 75 °C (Scheme 28). In contrast, when other imidazolium ionic

Scheme 24. Reactions of Imidazolium-2-carboxylates with Protic Acids

Moreover, similar reactions of imidazolium-2-carboxylates with azoles91,92 and ammonium perchlorate93 have also been used for preparing imidazolium ionic liquids via green and efficient synthetic routes. As isoelectronic molecules of CO2, both CS2 and OCS demonstrated comparable ability to react with imidazolium salts at the C2 position. Cabaço et al.94 reported that CS2 and OCS reacted spontaneously with basic [Bmim][OAc] to afford 1butyl-3-methylimidazolium-2-dithiocarboxylate and 1-butyl-3methylimidazolium-2-thiocarboxylate (Scheme 25). Ionic liquids with other carboxylate anions such as lactate, malonate and hexanoate also gave similar results.

Scheme 28. Reactions of Imidazolium Ionic Liquids with Chalcogenides

liquids containing various anions (e.g., [Cl]−, [HSO4]−, [SCN]−, [CH3SO4]−, [CH3C6H4SO3]−, [CF3SO3]−, and [CF3COO]−) were used, no reaction with sulfur was observed. These results implied that imidazolium acetate ionic liquids could produce the carbenes in situ, which reacted with sulfur to afford the thiones. In the presence of strong bases, imidazolium salts with neutral anions (e.g., [Cl]−, [BF4]−, and [NTf2]−) also undergo reactions with chalcogenides. Tian et al.98 investigated the reactions of imidazolium salts with Se in water in the presence of Na2CO3 under refluxing. As shown in Scheme 29, chloride, bromide, iodide, and tetrafluoroborate ionic liquids were easily converted to the corresponding imidazole-2-selenones. Steric

Scheme 25. Reactions of Imidazolium Salts with CS2 /OCS

Although imidazolium salts with neutral anions (e.g., [OTf]−, [BF4]−, [NTf2]−) cannot react with CS2 and OCS spontaneously,94 they can react with CS2 and OCS under strong base conditions. Delaude et al.95 investigated the reactions of imidazolium chlorides and CS2 in the presence of NaH. Various imidazolium-2-dithiocarboxylates were produced in yields of 55−89% at room temperature (Scheme 26). When

Scheme 29. Reactions of Imidazolium Salts with Se in the Presence of Na2CO3

Scheme 26. Reactions of Imidazolium Chlorides with CS2

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When studying the generation of superoxide ion (O2• −) by electrochemical reduction of oxygen in ionic liquids, AlNashef and co-workers105,106 observed that reactions of imidazolium ionic liquids with O2• − gave the corresponding 2-imidazolones in excellent yields at room temperature and atmospheric pressure (Scheme 33).

hindrance of substituents on N3 position did not influence the reactions significantly. Wasserscheid and co-workers99 also found reactions of imidazolium halide ionic liquids with sulfur in the presence of NaOMe to form 1-alkyl-3-methylimidazole2-thiones, which had been used as starting materials for synthesis of cyclic thiouronium ionic liquids. During the synthesis of organochalcogen ligands, Jin and coworkers100,101 investigated the reactions of methylene- and ethylene-bridged imidazolium dibromides with chalcogenides in the presence of K2CO3 and found the corresponding bidentate organochalcogen compounds were obtained in yields of 40−84% (Scheme 30). Streubel and co-workers102 also reported similar reactions between various imidazolium halides and S8 in the presence of NaH or KOtBu at ambient temperature.

Scheme 33. Oxidation of Imidazolium-based Ionic Liquids with Superoxide Ion

Scheme 30. Reactions of Methylene- and Ethylene-Bridged Methylimidazolium with Chalcogens

Recently a new class of hypergolic ionic liquids that work by reaction with oxidants has been developed.24,107 These hypergolic ionic liquids commonly contain fuel-rich cations and/or anions. The most popularly used azolium ionic liquids are listed in Scheme 34. Upon contact with an oxidant such as fuming nitric acid, red-fuming nitric acid, N2O4, or H2O2, these energetic ionic liquids can exhibit fast spontaneous combustion. The active hypergolicity of ionic liquids grants them great potential to be used as green hypergolic fuels in the field of space propulsion.107−115

Inesi and co-workers103 developed an efficient route combining electrochemical and ultrasound conditions for preparation of imidazole-2-thiones from imidazolium ionic liquids and S8. Electrochemical reductions of imidazolium ionic liquids, which were used as solvents, supporting electrolytes, and reagents, gave the corresponding N-heterocyclic carbenes, which subsequently reacted with S8 under ultrasound irradiation to afford target products in high yields (Scheme 31).

2.2. Cleavage of C−N Bond at N1 and N3 Positions

Cleavage of C−N bond at N1 and N3 positions of dialkylimidazolium salts leads to the removal of N-substituents along with the generation of neutral imidazole compounds. This kind of decomposition has been observed under specific physical and chemical conditions due to the electrophilicity of imidazolium. While investigating a procedure applying thiophenol in [Bmim][BF4] under both microwave and conventional heating, Glenn and Jones116 found the generation of methyl phenyl sulfide and n-butyl phenyl sulfide. Subsequently, they investigated the reactions between [Bmim][BF4] and other nucleophiles. When sodium thiophenolate in neat [Bmim][BF4] was heated to 225 °C for 10 min via microwave, methyl phenyl sulfide and butyl phenyl sulfide were formed in yields of 39% and 7%, respectively, which showed the reactions occurred via SN2 nucleophilic attack of sodium thiophenolate on the alkyl substituents of imidazolium at high temperature (Scheme 35). Other nucleophiles, such as sodium phenoxide, aniline, thiophenol, and sodium benzoate, also reacted with imidazolium to afford similar alkylation products in combined yields of 2−40%. The product distribution showed that methyl group on imidazolium underwent nucleophilic attack more facilely than butyl group. The authors also investigated the relative reactivity of the imidazolium versus conventional electrophiles and found that the reaction of imidazolium as an electrophile is competitive with that of a typical alkyl halide. These results indicate that the stabilities of imidazolium ionic liquids are decreased significantly in the presence of nucleophiles. Therefore, it is better to be cautious when nucleophilic substitution or addition reactions are conducted in ionic liquids. In their study of protection/deprotection of imidazole, Malachowski and co-workers117 found that 3-benzyl-1-(2cyanoethyl)-4-phenylimidazolium bromide reacted with NaOH in methanol to form 1-benzyl-5-phenylimidazole in 78% yield at room temperature (Scheme 36). Katritzky et al.118

Scheme 31. Reactions of Imidazolium Ionic Liquids and Sulfur under Electrochemical and Ultrasound Conditions

2.1.4. Reactions with Oxidants. Imidazolium ionic liquids have been reported to undergo oxidation reactions at the C2 position with oxygen. Farmer and Welton104 employed imidazolium ionic liquids as alternative solvents for the selective oxidation of alcohols to aldehydes and ketones, catalyzed by Pr4NRuO4 (TPAP) in combination with CuCl and 2aminopyridine as cocatalysts. They found that [Bmim][NTf2] was oxidized to 1-butyl-3-methylimidazolidone and 1-butyl-3methylimidazolidine-2,4,5-trione (Scheme 32). In contrast, C2substituted [Bdmim][NTf2] was not oxidized under identical reaction conditions. Scheme 32. Oxidation of [Bmim][NTf2] with Oxygen

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Scheme 34. Most Popularly Used Azolium Ionic Liquids with Fuel-Rich Cations and/or Anions

position occurred primarily at the N1-benzylic position, followed by the N3-methyl position (Scheme 39). They further

Scheme 35. Reaction of [Bmim][BF4] with Sodium Thiophenolate

Scheme 39. Decomposition of Imidazolium Salts in the Presence of KOH

Scheme 36. Decomposition of 3-Benzyl-1-(2-cyanoethyl)-4phenylimidazolium Bromide in the Presence of NaOH

evaluated the effect of N1 and N3 substituents on the stability of imidazolium in various concentrations of base and found that the larger substituents improved imidazolium stability (N3butyl > N3-ethyl > N3-methyl). C4 and C5 substitution also played a crucial role in the stability of the imidazolium salts, and methyl groups gave a slight improvement in stability compared to phenyl groups (Scheme 40). Although most of the imidazolium salts they investigated cannot be regarded as conventional ionic liquids, all the conventional imidazolium ionic liquids should be less stable in basic environments. It is advised to be cautious when imidazolium ionic liquids are used in basic environment. Furthermore, acid may also lead to the cleavage of C−N bond of imidazolium at N1 and N3 positions. Ohta and coworkers121 found that decomposition of 1-benzyl-3-{[2(trimethylsilyl)ethoxy]methyl}imidazolium bromides in 10% aqueous hydrogen chloride solution gave the corresponding 1benzylimidazoles in yields of 84−86% at 80 °C (Scheme 41).

also reported the decomposition of 1-methyl-4-[2-(3-methylimidazolium-1-yl)ethyl]pyridinium diiodide to 1-methylimidazole and 1-methyl-4-vinylpyridinium iodide in the presence of K2CO3 at 20 °C (Scheme 37). Scheme 37. Decomposition of 1-Methyl-4-[2-(3methylimidazolium-1-yl)ethyl]pyridinium Diiodide in the Presence of K2CO3

During studies of an anion-exchange membrane based on imidazoliums, Zhang and co-workers119 investigated alkaline stability of imidazolium and found that a C2-substitutent on imidazolium greatly improved its stability. However, when 1benzyl-3-butyl-2-methylimidazolium bromide was added in an aqueous solution of NaOH (1.0 M) at 60 °C for 48 h under N2, 1-benzyl-2-methylimidazole was generated by dealkylation at the N3 position of imidazolium (Scheme 38).

2.3. Reactions at C4 and C5 Positions

Laali and Gettwert122 studied electrophilic nitration of aromatics in ionic liquids. When NO2BF4 was used as nitrating agent and [Emim][BF4] or [Emim][PF6] as solvent, the C4 and C5 positions of imidazolium ring were nitrified at room temperature (Scheme 42). The increasing viscosity of nitrated imidazolium salts limited their utilization in the nitration reactions. Moreover, oxidations at C4 and C5 positions of the imidazolium ring also occurred in the presence of O2 (Scheme 32).104 The acidities of C4 proton and C5 proton in imidazolium ionic liquids are weaker than that of C2 proton; however, reactions could still occur at C4 and C5 positions. Nyulászi and co-workers123 investigated the reaction between [Emim][OAc] and CO2 under relatively harsh conditions (10 MPa, 125 °C). They found that not only the known 2-carboxylate isomer but also the 4- and 5- carboxylate isomers were obtained in yields of 18%, 8%, and 7%, respectively, after 1.5 h (Scheme 43). This finding was in accordance with the observation that reaction at

Scheme 38. Decomposition of 1-Benzyl-3-butyl-2methylimidazolium Bromide in Aqueous 1.0 M NaOH

Recently, Coates and co-workers120 systematically studied the influence of imidazolium substituents on alkaline stability. They found that 1-benzyl-2-(2,6-dimethylphenyl)-3-methyl-4,5diphenylimidazolium iodide suffered from decomposition by SN2 attack of CD3OH at N-substituents. Dimethyl ether, benzyl methyl ether, and imidazoles were observed by 1H NMR spectra after 3 months in KOH/CD3OH (1 M) solution at 80 °C. The distribution of products demonstrated that decomI


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Scheme 40. Decomposition of Imidazolium Salts in the Presence of KOH: Effect of N1 and N3 Substituents

2.4. Ring-Opening Reactions on the Imidazolium Ring

Scheme 41. Decomposition of 1-Benzyl-3-{[2(trimethylsilyl)ethoxy]methyl}imidazolium Bromide in the Presence of HCl

Reactions on the imidazolium ring, which are also regarded as ring-opening decomposition, are mainly triggered by nucleophilic attack of strong bases (e.g., [OH]− and [OMe]−) at the C2 position of the imidazolium ring. Nyulászi and coworkers124 investigated the hydrolysis of imidazol-2-ylidenes, which were generated by the abstraction of C2 proton of imidazolium by strong bases, to form the ring-opening products (Scheme 44). These results imply that imidazolium-based ionic liquids may undergo ring-opening reactions in a basic aqueous solution.

Scheme 42. Reactions of Imidazolium Ionic Liquids with NO2BF4

Scheme 44. Ring-Opening Reactions of Imidazolium Salts in Basic Aqueous Solution

Scheme 43. Reaction of [Emim][OAc] with CO2

Subsequently, in studies of alkaline stability of imidazoliumbased anion-exchange membranes, Yan and co-workers125,126 observed that imidazolium-based ionic liquids underwent ringopening reactions in KOH solution at 80 °C. [OH]− attacked the imidazolium at the C2 position to form a neutral alcohol intermediate. Then the C2−N bond in the alcohol intermediate broke, giving a more energetically favorable ring-opened carbonyl compound (Scheme 45). They found that the alkaline stabilities of C2-substituted imidazolium salts were higher than those of C2-unsubstituted imidazolium salts. The stabilities of the imidazolium salts were in the order [Edmim][Br] > [EiPrmim][Br] > [EPhmim][Br] > [Emim][Br]. The same group127 further investigated the effect of substitution positions and C2-substituents on the alkaline stability of imidazolium, and they found that imidazolium was more stable with strong electron-donating groups at the N1/N3 positions than at the C2 position. Zhang and co-workers119 also observed that

the C2 position with CO2 could take place at room temperature, whereas reactions at C4 and C5 positions could occur at higher temperature only. Similarly, Tommasi and Sorrentino80 reported carboxylation of imidazolium chlorides with CO2 (5 MPa) in the presence of Na2CO3 in anhydrous N,N-dimethylformamide (DMF). They found formation of 4and 5-carboxylate imidazolium products in moderate yields at 135 °C after 8 h. J


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began to decompose upon heating to 30 °C. The reactions produced corresponding alkenes and trimethylamine below 95 °C, which were consistent with Hofmann elimination as the sole decomposition pathway (Scheme 47).

Scheme 45. Ring-Opening Reactions of Imidazolium Salts in the Presence of KOH

Scheme 47. Decomposition of Quaternary Ammonium Salts with β-Hydrogen through Hofmann Elimination

imidazolium ionic liquids underwent a ring-opening reaction in NaOH/H2O/MeOH (1 M) solution. Polymerized ionic liquids, which were used as anionexchange membranes, have also been found to decompose on the imidazolium ring under relative vigorous conditions, for example, at high temperature (80 °C) in dry environment (10% relative humidity) or at high alkaline concentrations ([KOH] > 1 M).128

In the absence of β-hydrogen, SN2 attack and ylide routes were primary pathways for decomposition of quaternary ammonium salts.138 When benzylalkylammonium hydroxide aqueous solution was heated at 60 °C, the major identifiable products were alkylamines and benzyl alcohol.139 These products could be generated via nucleophilic attack of [OH]− at the benzylic carbon (Scheme 48 a, SN2 attack) and by

3. DECOMPOSITION OF QUATERNARY AMMONIUM AND PHOSPHONIUM IONIC LIQUIDS

Scheme 48. Decomposition of Quaternary Ammonium Salts in the Absence of β-Hydrogen through (a) SN2 Attack and (b) Ylide Route

3.1. Decomposition of Quaternary Ammonium Ionic Liquids

Quaternary ammonium ionic liquids are applied in extensive fields, such as catalysts,129 solvents,130 electrolytes,131 and gas absorption.132 The low cathodic potential of quaternary ammonium provides a wide electrochemical window, which may allow these salts to be used as supporting electrolytes to improve the safety of high-energy density devices such as Li batteries and electrochemical capacitors.133 Recently, quaternary ammonium salts have been used to prepare anionexchange membranes for alkaline fuel cells.134 Hence, much attention has been paid to alkaline stability of quaternary ammonium salts. It is found that quaternary ammonium salts undergo different decomposition mechanisms depending on their structures (Scheme 46).

reaction between water and nitrogen ylide, which was generated through abstraction of α-hydrogen at benzylic carbon (Scheme 48 b, ylide route). In addition, it should be noted that quaternary ammonium salts with β-hydrogen at sterically hindered carbons might undergo the above three decomposition mechanisms simultaneously in the presence of [OH]−.

Scheme 46. Decomposition of Quaternary Ammonium and Phosphonium Salts

3.2. Decomposition of Quaternary Phosphonium Ionic Liquids

Quaternary phosphonium ionic liquids have superior properties compared to nitrogenium ionic liquids in many applications, such as extractants, solvents, and electrolytes.140 Quaternary phosphoniums are not completely inert (Scheme 46), though they exhibit higher chemical stability compared with imidazolium ionic liquids in some cases. Chu and co-workers141 found that H/D exchange reaction occurred at the α-CH2 proton under basic conditions, which demonstrated protons on the phosphoniums were indeed accessible. They further investigated the chemical reactivity of quaternary phosphonium in the presence of nucleophiles. They found that reactions of trihexyl(tetradecyl)phosphonium chloride and substituted sodium benzoates afforded aryl ketones via microwave heating (30 W) at 180 °C (Scheme 49). The yields of aryl hexyl ketones (3.8−7.6%) were higher than those of aryl tetradecyl ketones (2.1−3.5%), which might be attributed to the higher molarity of hexyl groups in the starting phosphonium salt. During the preparation of Grignard reagents in ionic liquids, Walsby and co-workers142 found that phosphonium with decanoate as anion showed superior performance as a solvent

Quaternary ammonium salts with β-hydrogen were not stable and Hofmann elimination was expected to mainly contribute to decomposition through the attack of [OH]− on β-hydrogen, leading to the formation of alkenes and tertiary amines.135,136 Boncella and co-workers137 found that ethyltrimethylammonium deuteroxide, n-propyltrimethylammonium deuteroxide, and isobutyltrimethylammonium deuteroxide K


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Scheme 49. Reactions of Trihexyl(tetradecyl)phosphonium Chloride with Substituted Sodium Benzoates

for Grignard reagents, which may result from coordination between the magnesium of the Grignard reagent and the oxygen of the carboxylate anion. However, when the phosphonium salts with other anions, such as chloride, dicyanamide, and trifluoromethanesulfonimide, were applied as solvents, decomposition occurred and gave a variety of products, for example, tetradecyl(dihexyl)phosphine and hexene (Scheme 50). Formation of these products was derived

Scheme 52. Hydrolysis and Nucleophilic Reactions of Anions of Ionic Liquids

Scheme 50. Reaction of Trihexyl(tetradecyl)phosphonium Chloride with PhMgBr Scheme 53. Hydrolysis of [Bmim][PF6], [Bmim][BF4], and [Bmim][SbF6] from reduction of the phosphonium. Furthermore, triphenylethylphosphonium bromide can be deprotonated by phenylmagnesium bromide solutions in a mixture of phosphonium decanoate and hexane to form nonstabilized phosphorenes, known as Wittig reagents (Scheme 51). The resulting ylide was synthetically useful and could be employed in Wittig reactions. Scheme 51. Reaction of Phosphonium Salt with PhMgBr To Generate Wittig Reagent hydrofluoric acid in highly concentrated nitric acid. Extreme caution is needed when [PF6]−-based ionic liquids are used to replace conventional organic solvents in extraction processes, as [Bmim][PF6] becomes monophasic rather quickly when it comes in contact with high concentrations of nitric acid. The same group157 also found hydrolysis of [PF6]− in the synthetic procedures of [Bmim][PF6] by metathesis of [Bmim][Cl] with aqueous HPF6. The afforded [Bmim][F]·H2O was separated from the mixture and identified by single-crystal X-ray diffraction. While studying enzymatic esterfication with [Bmim][PF6] as solvent, Gubicza et al.158 also found hydrolysis of [PF6]− by the side product of water. Subsequently, Fernandes and co-workers159 studied the hydrolysis of [PF6]−- and [BF4]−-based imidazolium ionic liquids under different pH and/or temperature conditions. They found that [PF6]− would undergo hydrolysis at high temperature (70 and 100 °C) or in acidic conditions (pH = 3), while hydrolysis of [BF4]− happened even at room temperature. The extent of hydrolysis of both anions increased with the size of the alkyl side-chain substituent of imidazolium, because of a decrease in cation−anion interaction that further facilitated interaction between the anions and water. While assessing the toxicity of imidazolium ionic liquids using the alga Selenastrum capricornutum, Yun and co-workers160 studied the hydrolysis of [Bmim][PF6], [Bmim][BF4], and [Bmim][SbF6]. They found that hydrolysis of [SbF6]− was easier than that of [BF4]− and [PF6]− under identical conditions. Aqueous extraction has been widely used to recycle ionic liquids from reaction mixtures;

4. HYDROLYSIS AND NUCLEOPHILIC REACTIONS OF ANIONS OF IONIC LIQUIDS The anions of ionic liquids are either organic or inorganic moieties and can regulate the water miscibility of ionic liquids. Anions also play an important role in CO2 absorption, which makes ionic liquids promising candidates as energy-saving absorbents in CO2 capture.143−148 We and other groups have reported that anions of ionic liquids were able to activate nucleophiles serving as hydrogen-bond acceptors in catalyzing organic reactions.149−155 However, the chemical stability of several popular anions in ionic liquids has been highlighted, including hexafluorophosphate ([PF6]−), tetrafluoroborate ([BF4]−), hexafluoroantimonate ([SbF6]−), trifluoromethanesulfonate ([NTf2]−), and acetate ([OAc]−). This section covers the hydrolysis of fluorine-based anions, which is easily neglected, and nucleophilic reactions of anions of ionic liquids (Scheme 52). The hydrolysis of fluorine-based anions of ionic liquids has received much attention recently (Scheme 53). Rogers and coworkers156 studied the extraction of Na+, Cs+, and Sr2+ from aqueous solutions by a series of ionic liquids and found that [PF6]−-based ionic liquids underwent hydrolysis to give hydrophilic phosphate ([PO4]3−) and an abundance of L


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however, these studies illustrate that specific caution should be taken to avoid potential hydrolysis when handling aqueous solutions of [SbF6]−, [BF4]−, and [PF6]− ionic liquids.161 Other anions, such as [NTf2]− and [OAc]−, would undergo reactions with different substrates. While ionic liquids were used as solvents in the synthesis of SF5 aromatics from 4(pentafluorosulfanyl)phenyldiazonium tetrafluoroborate (pSF5-C6H4NBF4), Laali and co-workers162 found that [NTf2]− reacted with p-SF5-C6H4NBF4 at 50 °C for 17 h to give p-SF5C6H4OS(O)(CF3)NTf (27%) and p-SF5-C6H4NTf2 (4%). The yields improved with increasing temperature or prolonged reaction time. This finding was ascribed to solvolytic dediazoniation of p-SF5-C6H4NBF4 by nucleophilic attack of [NTf2]− (Scheme 54).

Scheme 56. Reaction of Acetate Ionic Liquids with CS2 and OCS

chemical stabilities of ionic liquids are neglected in most cases. This review is committed to provide an overview on chemical stabilities of ionic liquids. Imidazolium ionic liquids have plenty of reactive sites, owing to their structural characteristics. Reactions can occur at different positions of imidazolium, including C2 position, N1 and N3 positions, C4 and C5 positions, and on the ring. Without any additive, imidazolium is electrophilic and can react with nucleophilic reagents, such as aniline, sodium thiophenolate, sodium phenoxide, thiophenol, and sodium benzoate. However, under moderate basic conditions, imidazolium can undergo deprotonation at the weakly acidic C2 position, and the generated nucleophilic N-heterocyclic carbenes can react with aldehydes, alkyl halides, CO2, CS2, and chalcogenides. Imidazolium ionic liquids also react with biomass to generate C2 hydroxyalkyl adducts through nucleophilic addition from C2 of the imidazolium to anomeric carbons of biomass. Under strong basic conditions, imidazolium ionic liquids suffer from ring-opening decomposition. In the presence of low-value transition-metal catalysts, imidazolium ionic liquids may undergo coupling reactions with alkenes. Although C2substituted imidazolium salts are more stable toward Brönsted bases than C2-unsubstituted ones, C2-substituted imidazolium salts remain unstable to nucleophilic attack. This, coupled with their considerably higher melting points, leads to narrow liquid ranges that usually are not convenient for utilization. Decomposition of quaternary ammonium and phosphonium ionic liquids affords corresponding neutral products via SN2 attack, ylide routes, and/or Hofmann elimination in principle. Therein, quaternary ammonium compounds with β-hydrogen are most unstable because Hofmann elimination takes place more easily through the attack of [OH]− on β-hydrogen. Additionally, hydrolysis of fluorine-based anions in ionic liquids is prominent, and nucleophilic substitutions of bis(trifluoromethylsulfonyl)imide or acetate with electrophilic substrates also can easily occur. To be sure, the overactive reactivity limits the application of ionic liquids. When ionic liquids are applied as solvents, gas adsorbents, and electrolytes, the reactions of ionic liquids and substrates should be taken with extra caution, especially under basic conditions or in the presence of low-value transition-metal catalysts. The ionic liquid-derived byproducts from those reactions change the chemical and physical properties of ionic liquids, such as viscosity, conductivity, and dissolving and adsorption capacities, and they prevent the recycling of ionic liquids. Furthermore, when used as solvents for biomass, ionic liquids are demonstrated to be not completely inert solvents. The reactions of ionic liquids with cellulose not only affect rheological properties of ionic liquids but also degrade the cellulose fibers and reduce the quality and quantity of the cellulose product. When ionic liquids, such as [BF4]−-, [SbF6]−-, and [PF6]−-based ionic liquids, are applied in organic/aqueous extraction, one should be especially cautious about hydrolysis of anions. The hydrolysis of fluorine-based anions gives an abundance of toxic acid (HF), which results in environmental pollution and changes the acidity of solvent and the catalytic ability. This review provides special advice for

Scheme 54. Reactions of [Bmim][NTf2] with 4(Pentafluorosulfanyl)benzenediazonium Tetrafluoroborate

Acetate-based ionic liquids are widely applied in biomass chemistry54,70−75,163,164 and catalysis151,153−155 In a study of alkoxycarbonylation of cellulose with dimethyl carbonate using [Emim][OAc] or [P8881][OAc] as solvents, King and coworkers165 noticed that acetate could react with dimethyl carbonate to form methyl acetate (Scheme 55). This is Scheme 55. Reactions of Acetate Ionic Liquids with Dimethyl Carbonate

obviously problematic for the sustainability of the process. To avoid this decomposition and fully recover the ionic liquids, a workup of the reaction under mild conditions was advised. Cabaço and co-workers94,166 investigated the reactions of acetate-based ionic liquids with CS2 and found formation of [CH3COS]−. This species was formed via exchange between an oxygen atom of the acetate anion and a sulfur atom of CS2, leading also to formation of the OCS molecule. Furthermore, the afforded OCS would sequentially react with acetate to give [CH3COS]− and release CO2 (Scheme 56). These results show that acetate-based ionic liquids are difficult to recycle in the reaction with CS2 as reagent and solvent.

5. CONCLUSIONS AND OUTLOOK Nowadays, ionic liquids are being applied in various fields irreplaceably. There is no doubt that ionic liquid chemistry has made great contributions to social progress. Nevertheless, the M


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using ionic liquids under different conditions. It is hoped that this review will play an imperative and stimulating role in understanding chemical stabilities of ionic liquids, thus encouraging the design and selection of appropriate ionic liquids to meet the application requirements.

interests cover green chemistry, ionic liquids, biomass treatment, and so on. Professor Guohua Gao received his Ph.D. from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, under the supervision of Professor Yuanqi Yin in 1993. From 1993 to 2003, he worked in the Research Institute of Beijing Yanshan Petrochemical Corporation (SINOPEC), National University of Singapore, Hokkaido University, and University of Ottawa. He is currently a full professor in the Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University. He also serves as a member of the Ionic Liquids Professional Committee of the Chemical Industry and Engineering Society of China. His research concerns imidazolium chemistry, especially catalysis by imidazolium ionic liquids and recognition by imidazolium-based receptors.

AUTHOR INFORMATION Corresponding Authors

*(T.M.) Telephone +86-010-62514925; e-mail [email protected]. cn. *(G.G.) Telephone +86-021-62233323; e-mail ghgao@chem. ecnu.edu.cn. ORCID

Tiancheng Mu: 0000-0001-8931-6113 Zhimin Xue: 0000-0001-6554-8788 Guohua Gao: 0000-0003-4016-394X

ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grants 21573072, 21473252, and 21273078) and Shanghai Leading Academic Discipline Project (Grant B409) for financial support.

Author Contributions ‡

B.W. and L.Q. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ABBREVIATIONS [Bdmim][NTf2

Biographies

1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [Bdmim][PF6 1-butyl-2,3-dimethylimidazolium hexafluorophosphate [Bdmim][PFBuSO3] 1-butyl-2,3-dimethylimidazolium perfluorobutylsulfonate [Bmim][Cl] 1-butyl-3-methylimidazolium chloride [Bmim][F]·H2O 1-butyl-3-methylimidazolium fluoride monohydrate [Bmim][OAc] 1-butyl-3-methylimidazolium acetate [Bmim][PF6] 1-butyl-3-methylimidazolium hexafluorophosphate [Bmim][TFA] 1-butyl-3-methylimidazolium trifluoroacetate [BmPyrro][OAc] 1-butyl-3-methylpyrrolidinium acetate DABCO 1,4-diazabicyclooctane DMAP dimethylaminopyridine EAN ethylammonium nitrate [Edmim][Br] 1-ethyl-2,3-dimethylimidazolium bromide [Emim][BF4] 1-ethyl-3-methylimidazolium tetrafluoroborate [Emim][Br] 1-ethyl-3-methylimidazolium bromide [Emim][Cl] 1-ethyl-3-methylimidazolium chloride [Emim][OAc] 1-ethyl-3-methylimidazolium acetate [Emim][PF6] 1-ethyl-3-methylimidazolium hexafluorophosphate [EPhmim][Br] 1-ethyl-2-phenyl-3-methylimidazolium bromide [EiPrmim][Br] 1-ethyl-2-isopropyl-3-methylimidazolium bromide KHMDS potassium bis(trimethylsilyl)amide [Mmim][I] 1,3-dimethylimidazolium iodide [MNapmim][OAc] 1-(2-naphthylmethyl)-3-methylimidazolium acetate [P8881][OAc] trioctylmethylphosphonium acetate TEA triethylamine

Binshen Wang was born in Sichuan, China, in 1989. He earned a bachelor’s degree in bioengineering at Chongqing University in 2011 and completed his Ph.D. in chemistry under the supervision of Professor Guohua Gao in 2016 at East China Normal University. In 2014−2015, he joined the group of Professor Markus Antonietti at Max Planck Institute of Colloids and Interfaces as a visiting student. Since 2016, he has worked in the group of Professor Qinghua Zhang as a postdoctoral fellow at the Institute of Chemical Materials, China Academy of Engineering Physics. His research interests focus on ionic liquids chemistry and energetic materials. Li Qin was born in Sichuan, China, in 1992. She earned a bachelor’s degree in chemistry at Sichuan Normal University in 2014. Since 2014, she has worked under the supervision of Professor Guohua Gao as a Ph.D student at Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University. Her research interests focus on ionic liquids chemistry and poly(ionic liquid) materials. Dr. Tiancheng Mu received his Ph.D. in physical chemistry from the Institute of Chemistry, Chinese Academy of Sciences under the supervision of Academician Buxing Han in 2004. He worked in the Department of Industrial Chemistry, Oldenburg University, as a postdoctoral fellow from 2005 to 2007. He is currently an associate professor in the Department of Chemistry, Renmin University of China. His research interests involve ionic liquids, chemical thermodynamics, and green chemistry. He has authored over 100 peer-reviewed scientific publications. Now he also serves as an associate editor for RSC Advances, a member of the Ionic Liquids Professional Committee of the Chemical Industry and Engineering Society of China, and a member of the Green Chemistry Professional Committee of the Chinese Chemical Society. Dr. Zhimin Xue received her Ph.D. from Renmin University of China in 2014 under the co-guidance of Academician Buxing Han, Professor Jianling Zhang, and Dr. Tiancheng Mu. She is currently an assistant professor in the College of Materials Science and Technology, Beijing Forestry University. She has published more than 30 scientific papers in journals including Angewandte Chemie, International Edition in English, Green Chemistry, Chemical Communications, Catalysis Science & Technology, and ACS Sustainable Chemistry & Engineering. Her research N


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