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A Systematic Theoretical Study on the Acidities for Cations of Ionic Liquids in Dimethyl Sulfoxide Zhen Wang, Yongjun Zheng, Yong Zheng, Pengju Ji, and Xiao-Song Xue J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02265 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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The Journal of Physical Chemistry

A Systematic Theoretical Study on the Acidities for Cations of Ionic Liquids in Dimethyl Sulfoxide Zhen Wang†,§, Yongjun Zheng†, Yong Zheng†, Xiao-Song Xue*,†,§, Pengju Ji*,‡ †

School of Chemical and Environmental Engineering, Anyang Institute of Technology,

Anyang 455000, China ‡

Center of Basic Molecular Science, Department of Chemistry, Tsinghua University, Beijing

100084, China §

State Key Laboratory on Elemento-organic Chemistry, College of Chemistry, Nankai

University, Tianjin 300071, China

ABSTRACT The acidities of 40 commonly seen cations of ionic liquids (CILs) in DMSO were investigated by a well-established theoretical method SMD/M06-2X/6-311++G (2df,2p)//B3LYP/6-31+G(d). The calculated pKas agree excellently with the available experimental data and range from 20.0 to 45.8 with an acidity order of 1,3-dialkylimidazolium > amidinium > pyridinium > tetraalkylphosphonium > morpholinium > pyrrolidinium ≈ piperidinium > guanadinim cations. The established acidity scale in this work provides a useful tool, as verified by the acidity comparisons, to assess the stability of ILs under various extent basic conditions, and also reveals the relative basicity of several classical N-heterocyclic carbenes and olefins as well as ylides.

INTRODUCTION Being composed of entirely ions, ionic liquids (ILs) exhibit a number of remarkable properties, such as negligible vapor pressure, high thermal stability and relatively wide electronic windows, etc., which are distinctive from these of conventional molecular solvents.1 Largely due to these favorable features, ILs are labelled as green solvent and have been widely employed in the fields of gas capture, biomass

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transformations, synthesis and catalysis, pharmaceutical industry, battery materials, energy storage and conversion.2-10 As well known, cations and anions, the elementary components of ILs, are crucial for dictating properties and applications of ILs, in fact, a rational development of chemistry in ILs relies heavily on our understanding of how the cations and anions of ILs interact with each other and with substrates. In this respect, the knowledge on the acidities of cations and basicities of anions are very helpful to evaluate a number of fundamental issues for ILs, such as viscosity, stability, degree of proton transfer, etc. 11 -14 Furthermore, accurate acidity and basicity data for cations and anions, especially these systematically measured in an absolute sense or calculated with a high level of accuracy, are highly valuable and may potentially serve as a tool to analyze and understand these interactions between various entities, such as hydrogen bonding and ion-association, ionicity of ILs, etc.15-22 in a quantitative way. Figure 1 shows several commonly seen cations of ILs (CILs), as can be seen from the structures of these CILs, due to the inductive effect of positively charged heteroatoms, hydrogens on the α-carbon (carbon acid) may exhibit a certain extent of acidity, depending on the structure of these CILs. Therefore, the ILs composed with these cations may be potentially deprotonated under various degree of basic conditions, leading to the degradation of ILs. Thus, the acidities for these CILs are crucial to evaluate the stability of ILs under certain basic conditions. R2

imidazolium

N

R16

piperidinium

P

R10

R7

N R1

R8

R9 N R11 R12

pyridinium

phosphonium

ammonium

R17

N

R20

N

O R15

R6 R5

N R2

R1 N

R18

morpholinium

N R19 amidinium

N

N

N R13

pyrrolidinium

R21

N N N R22

N

guanadinium

R14

cyclic guanadinium

Figure 1 Commonly used cations of ILs.

On the other hand, these CILs showed in Figure 1 also have practical applications. For examples, these CILs are widely used as the key components of alkaline anion exchange membranes (AEMs) in fuel cells,23 furthermore, they are the precursor

2


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acids of N-heterocyclic carbenes (NHCs) and olefins (NHOs) or ylides, which have been extensively employed as organocatalysts or as ligands in organometallic catalysis.24-26 Literature survey shows that there is only a little work have been dedicated to the CILs.27-28 Previous acidity studies for CILs were mainly concentrated on imidazolium salts in water or DMSO. For examples, Alder et al. measured the pKa of 1,3-di-isopropyl-4,5-dimethylimidazol-2-ylidene by NMR method.29 Streitwieser et al. reported the acidity of 1,3-di-tert-butylimidazol-2-ylidene in THF.30 Amyes and Richard measured the pKa values of some imidazolium cations by a kinetic deuterium exchange method in aqueous solution,31 later O’Donoghue et al. determined the acidity for a large series of NHCs’ conjugate acids in water, using the same kinetic method. 32-33 Yates and coworkers calculated the basicity of several nucleophilic carbenes through theoretical computations.34 Cheng et al. measured the acidities of a series of 1,3-dialkylimidazolium by overlapping indicator method in DMSO.35 Fu and co-workers predicted the pKa values of structurally unrelated ylide precursors in DMSO by theoretical protocol.36 Harper et al. systemically determined the pKa values of more than 25 imidazolium salts in DMSO. 37 More recently, Bryantsev and coworkers investigated the acidities of an array of organic cations, which could be used as the anion-exchange groups in polymer membrane, in water and DMSO by theoretical methods.38 These pioneer work have provided us a general knowledge of how acidic some CILs would be in molecular solvents, however, the substrates involved were mostly limited to the precursors of imidazolium salts, with little work focused on the acidities of other kinds of CILs. The reason for this is that the most of CILs are rather weak carbon acids (C-H acids), and the experimental determination of their pKas in molecular solvents is impossible or very difficult due to the levelling effects. However, a systematic acidity scale for these CILs is highly valuable to access the stability of ILs as well as catalytic ability of organocatalysts. However, there have been many work focused on estimating the acidities of small organic acids and bases by theoretical method in the past few years.39-60 Therefore, alternatively, we employ a 3



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sophisticated theoretical calculation, which was calibrated by the experimental results, to establish a comprehensive acidity scale for a variety of CILs, including phosphonium, ammonium, imidazolium, pyridinium, pyrrolidinium, piperidinium, morpholinium, guanadinium and amidinium cations in DMSO. The calculated acidities for different class of CILs were compared with each other and also with the precursor acidities of several weak organic anions and some superbases. The purpose of this work is to present the relative acidity scale of CILs in one work, which enables a direct comparison between these CILs, and moreover, the acidity scale of CILs may facilitate a rational design of ILs for task-specific purpose or selection of catalyst with appropriate acidity.

RESULTS AND DISCUSSION Cheng and co-workers calculated the acidities of chiral phosphoric acids (O-H acids) in DMSO by SMD/M06-2X/6-311++G(2df,2p)//B3LYP/6-31+G(d) method,61 which is based on the solvation model density (SMD) method62 developed by Cramer, Truhlar and coworkers, with a precision of ca. 0.5 pK units. Later the same method was successfully extended to the acidities of N-H acids in DMSO.63 We found that the same method is applicable to the acidities of the charged species, i.e., CILs in DMSO as well. In specific, the reliability of Cheng and co-workers’ method for predicting pKas of CILs in DMSO was tested by a series of imidazolium cations (1-8 in Figure 1), of which experiment data are available.35 As shown in Table 1, the pKas calculated by the SMD/M06-2X/6-311++G(2df,2p)//B3LYP/6-31+G(d) method agrees well with the experimental data, with a mean unsigned error (MUE) of 0.4 pK units.

By

comparison,

the

calculated

SMD/M06-2X/6-311++G(2df,2p)//B3LYP-D3/6-31+G(d)

pKas

using and

SMD/M06-2X/6-311++G(2df,2p) methods exhibit similar MUEs, suggesting that geometry optimizations with dispersion contribution have a negligible influence on the accuracy of pKa predictions for these compounds in DMSO. Additionally, the calculated pKas show an MUE of 0.5 pK units when one DMSO molecule is explicitly considered, indicating the specific solvent-solute interactions only have a minor effect

4


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on

calculation

accuracy.

Therefore,

in

the

present

work,

the

SMD/M06-2X/6-311++G(2df,2p)//B3LYP/6-31+G(d) method were selected to investigate the acidity of CILs for the economic considerations. It worth noting that experimental evidence shows that the anions of ILs have no or negligible influence on the acidities of the cations in DMSO, 35,64 we therefore did not consider the counter-anion effects on the acidities of these CILs in the present calculation model. Table 1. The predicted pKas with different method and the available experimental data in DMSO. Acids

pKa(calcd.)a

pKa(calcd.)b

pKa(calcd.)c

pKa(calcd.)d

pKa (exptl.)e

1

21.5

21.2

21.5

21.9

22.2

2

21.6

22.0

21.7

22.9

22.1

3

21.9

22.4

21.9

24.0

22.0

4

20.1

20.6

20.0

20.5

20.5

5

20.0

20.1

20.0

20.1

19.7

6

21.2

20.7

21.4

21.6

21.6

7

23.9

24.0

23.9

23.5

23.3

23.2

23.1

23.2

23.5

23.4

0.4

0.5

0.4

0.5

8 MUE a

f

Calculated

by

SMD/M06-2X/6-311++G(2df,2p)//B3LYP/6-31+G(d).

SMD/M06-2X/6-311++G(2df,2p). d

c

Calculated

by

b

Calculated

by

SMD/M06-2X/6-311++G(2df,2p)//B3LYP-D3/6-31+G(d).

Calculated by SMD/M06-2X/6-311++G(2df,2p)//B3LYP/6-31+G(d) with the consideration of an explicit DMSO

molecule. eExperimental data from ref. 35. fMUE is the mean unsigned error.

Having

verified

the

reliability

of

the

SMD/M06-2X/6-311++G(2df,2p)//B3LYP/6-31+G(d) method for predicting pKas of CILs in DMSO, we next applied it to the acidities of other families of CILs (9-40, Figures 2-4). As shown in Figures 2-4, the calculated pKa scale for various families of CILs spans a wide range of about 26 pK units, i.e., from 20.0 (5) to 45.8 (38), and the acidities change in an order of 1,3-dialkylimidazolium > amidinium > pyridinium > tetra-alkyl-phosphonium > morpholinium > pyrrolidinium ≈ piperidinium > guanadinium cations, suggesting an opposite order for the basicities of their corresponding conjugated bases. Figure 2 shows imidazolium cations (1-10) have pKas in a range between 20 to 30 in DMSO, depending on their structure. The 5



Page 6 of 17

substitution of methyl group at 1-N position in 1 with other alkyl group, such as ethyl (2), n-butyl (3) and benzyl (6) shows a little influence on the pKa values (within 0.5 pK units), while the substitution with an aromatic ring, for example, phenyl group (5) leads to a 1.5 pK units decrease. This should be mainly due to the resonance stabilization effect of the π-system in 5. Similarly, the acidity of 4 is 1.4 pK units stronger than 1 can be understood in terms of the greater electronegativity of oxygen atom in the side chain. On the other hand, the substitution of methyl group at C4 and C5 positions (8) causes 1.7 pK units decrease in acidity, which can be rationalized as a result of the electron-donating property of methyl group in solution. H

H N

nBu

N Et

N

N

H

H N

N

N

N

N Ph

N

O

N

H

H

H

H

N

tBu

Ph

N

N

tBu

N

N

1

2

3

4

5

6

7

8

21.5

21.6

21.9

20.1

20.0

21.2

23.9

23.2

H

H N

N

N

N

N

N H

N H

N

H

N H

H

9

10

11

12

13

14

15

23.9

30.3

29.6

28.9

30.5

31.8

31.4

Figure 2 The structure and pKa values of imidazolium and pyridinium cations in DMSO. The most acidic hydrogen for the individual compound is highlighted in red color

Replacement of C2-H of 1 with alkyl groups leads to a pronounced increase in basicity of their corresponding conjugate bases (9, 10), for examples, a replacement of C2-H with methyl (9) or i-propyl group (10) brings about a basicity enhancement of 2.4 and ~9 pK units, respectively. It is worth noting that the deprotonation of 1,3-dialkylimidazolium and 1,2,3-trialkylimidazolium cations yields NHCs and NHOs, respectively, here our predicted pKa data for these cations enable a straightforward comparison of basicity between these two kinds of well-known organic catalysts. The N-alkyl substituted pyridines (pyridinium cations, 11-15), with a pKa value between 29.6 to 31.8, is ca. 8 pK units higher than imidazolium cation 1. In addition, the replacement of C4-H of 11 and 12 with a methyl group (14-15) causes more than 2 pK units increase of the basicity of pyridinium ylides. As shown in Figure 3, due to the lack of resonance stabilization of carbanions by the aromatic system, the acidity of quaternary ammoniums (16-27) are much weaker 6


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(~1015-1020 times) than these for the precursors of NHCs and NHOs (Figure 2, 1-10) in DMSO. The acidity for tetramethylammonium (16) was calculated as 41.0, while 17 and 18 with increased length of alkyl chains are ca. 4 pK units weaker than that of 16. Pyrrolidiniums (19-21) and piperidiniums (22-24) have a similar pKa value of around 42, and are about 20 pK units higher than these of 1,3-dialkylimidazolium cations. Changing the nearby alkyl substituent of 1-N-methyl pyrrolidinium and piperidinium has practically no effect on the acidities of these CILs (20-21 and 23-24). The presence of a strong electronegative O atom in morpholiniums (25-27) causes an acidity increasing about 3 pK units, compared with pyrrolidinium and piperidinium cations, however, different from pyrrolidinium and piperidinium cations, it is the methylene at C3 position shows the strongest acidity (25-27), as a balanced result of inductive effects of O atom and charged nitrogen atom. H

H N

H

N

N

N

N H

H

N

nBu

16

17

18

19

20

21

41.0

45.2

44.3

42.6

42.1

42.5

O H

N

O

O H

N

H

N

H

H

N

H

P

N

H

N

22

23

24

42.1

42.5

42.9

H

H P

H

nBu

H

P

P

nBu

25

26

27

28

29

30

31

39.7

39.3

39.6

32.8

33.5

33.8

35.9

Figure 3 The structures and acidities of ammonium and phosphonium cations in DMSO

Figure 3 also shows that the acidities of tetraalkylphosphonium cations (28-31) are ca. 10 pK units stronger than these of tetramethylammoniums (16-18) in DMSO. This could be explained by considering the relative larger size of phosphonium and thus easier to delocalize charge, which leads to a stronger stabilization of the incipient carbanion. The acidity of tetramethylphosphonium cation (28) is ca. 10 pK units higher than the classical ylide triphenylphosphonium cations (e.g., Ph3P+CH3 has a pKa of 22.3 in DMSO65). In addition, the pKa values increase appreciably as the methyl group in 28 is replaced by bulkier substituents. For example, the pKa value of tetrabutylphosphonium cations (31) is ca. 3 pK units higher than 28.

7



Interestingly,

1,8-diazabicyclo[5.4.0]undec-7-ene

Page 8 of 17

(DBU)

derived

cations

(amidinium, 32-34) are more acidic than most of CILs involved in this work. The hydrogen on the C10 position of DBU ring exhibits the strongest acidity, this indicates an excellent stabilization of negative charge by amidinium system (as also shown in Figure 4). Replacing the methyl to a longer alkyl group results a notable acidity decrease (33-34). N

N

N N

H

H

H

32

33

23.0

23.8

N N nBu

N

N

N

N

H

24.1

45.2 H

H

N N

N

N

H

45.6

H

H

H

N

N N

36

35

34

H

N

N N

N N

N

N

nBu

H 37

38

39

40

44.7

45.8

45.1

45.4

Figure 4 The acidities of amidiniums and guanadinium cations in DMSO

The guanadinium cations (35-40) show an acidity around 45 in DMSO, which are among the weakest acids in these commonly used CILs. The acidities of acyclic guanadinium [tetramethylguanidine (TMG) derived cations, 35-37] and cyclic guanadinium [1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2-a]pyrimidine (MTBD) derived cations, 38-40] show no marked difference. However, it worth noting that our calculation indicates the hydrogen in N-methyl of TMG derived cations (35-37) exhibits the strongest acidity, while the most acidic hydrogen of MTBD derived cations is on C-4 or C-5 position (38-40). N

N

N H

H N

H N

H N H

H

N

41

42

43

44

45

46

6.4

3.4

4.0

8.4

9.0

11.1

H

N

H

H

H

H

N

N

N O

N

N

H

H N

H

47

48

49

10.9

9.2

13.9

50 13.2

Figure 5 The structures and acidities of conjugate acids of amines and superbases27 in DMSO

8


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Thermodynamically, a CIL should be stable in the presence of basic anions or bases whose conjugated acids have an acidity at least 3 pK units lower than that for the CIL in solution, otherwise the CIL would be partially or completely deprotonated, resulting an undesirable degradation of CIL. Now with the availability of calculated acidity scale for CILs at hands, it is possible to evaluate the stability of ILs under certain basic conditions by simply comparing the acidities of CILs with the available acidity data in DMSO.27 For example, amines and superbases are frequently employed in organic synthesis, whether a specific ILs would survive or not in the presence of base can be easily told from the acidity gap between the CIL and conjugated acids of the base (Figure 5, 41-50). As can be seen from Figure 5, the acidity of the conjugated acids of amines and superbases is much stronger than these for the CILs in DMSO. In general, through a simple acidity comparison, the ILs with all the CILs involved in this work will be stable when co-exist with these superbases and amines (41-50) in solution, even under a high concentration of the bases. Anions can be potentially employed as nucleophiles, catalysts in organic transformations in ionic media or as counter-anions of ILs, however, they can be sufficiently basic and may destroy the ionic media used, the knowledge of basicities of these anions therefore are crucial for the selection and design of ILs. For example, the task-specific ILs with strong basic hydroxide as counter-anion (i.e., [CILs][OH]) have been used as strong basic ILs which show extraordinary catalytic performance in synthetic processes.66-68 In addition, as the key components of fuel-cells, AEMs which contain the cationic moieties similar to CILs, they are usually performed under a high concentration of OH−.23 A comparison between the acidities of the CILs involved in current work and that of H2O in DMSO (water has a pKa = 31.4 in DMSO27) reveals that imidazolium, amidinium and pyridinium cations will be deprotonated in the presence of hydroxide in DMSO, while phosphonium, morpholinium, pyrrolidinium, piperidinium and guanadinium are sufficiently suitable as the cations or cationic moiety for the task-specific ILs and AEMs.

9



CONCLUSIONS The acidities of 40 typical CILs in DMSO were investigated by a theoretical method, i.e., SMD/M062-2x/6-311++G(2df,2p)//B3LYP/6-31+G(d). The predicted pKa’s agree well with the available experiment values with a deviation within ca. 0.5 pK units. The pKa values of these CILs ranges from 20.0 to 45.8, and the acidity of these CILs follows an order of 1,3-dialkylimidazolium > amidinium > pyridinium > tetraalkylphosphonium > morpholinium > pyrrolidinium ≈ piperidinium > guanadinium cations in DMSO. These acidity data are highly useful and may potentially serve as a practical tool for the rational design of ILs and selection of appropriate bases for reactions in ILs.

COMPUTATIONAL METHODS The protocol used in predicting the acidities of O-H and N-H acids61,63 was applied herein for predicting pKa values of CILs (C-H acids) in DMSO. In brief, as shown in Scheme 1, the heterolytic dissociation energy of an acid HA (here refers to the CILs in this work) in DMSO, ∆G*solv(HA), can be derived from the thermodynamic cycle (Scheme 1, eqn. 1). Then, pKa value of HA can be obtained through the relationship shown in eqn. 2. The asterisks (*) in eqn. 1 and eqn. 2 indicate the standard state of 1 mol/L in any phase.

Scheme 1. The principle of pKa calculation via a direct method

The structures of all species were carried out with the Gaussian 09 suite of programs.69 Geometry optimizations were conducted at the B3LYP/6-31+G(d) level. The nature of the stationary points was confirmed by frequency calculations at the same level of theory. The solution phase electronic energy calculations were performed at the SMD/M06-2X/6-311++G(2df,2p) level with the B3LYP/6-31+G(d) structures.

10


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ASSOCIATED CONTENT Supporting Information Cartesian coordinates of optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected], [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is dedicated to Prof. Jin-Pei Cheng on the occasion of his 70th Birthday. We are grateful for the financial grants from National Natural Science Foundation of China (Nos. 21702005, 21672124, 21406002). We also would like to sincerely thank the reviewer's suggestive comments on the manuscript.

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