Theoretical Study for Pyridinium-Based Ionic Liquid 1

ARTICLE pubs.acs.org/JPCA

Theoretical Study for Pyridinium-Based Ionic Liquid 1-Ethylpyridinium Trifluoroacetate: Synthesis Mechanism, Electronic Structure, and Catalytic Reactivity Xueying Zhu, Peng Cui, Dongju Zhang,* and Chengbu Liu* Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan, 250100, People's Republic of China ABSTRACT: By performing density functional theory calculations, we have studied the synthesis mechanism, electronic structure, and catalytic reactivity of a pyridinium-based ionic liquid, 1-ethylpyridinium trifluoroacetate ([epy]+ [CF3COO]). It is found that the synthesis of the pyridinium salt follows a SN2 mechanism. The electronic structural analyses show that multiple H bonds are generally involved in the pyridinium-based ionic liquid, which may play a decisive role for stabilizing the ionic liquid. The cationanion interaction mainly involves electron transfer between the lone pair of the oxygen atom in the anion and the antibonding orbital of the C*H bond (C* denotes the carbon atom at the ortho-position of nitrogen atom in the cation). This present work has also given clearly the catalytic mechanism of [epy]+[CF3COO] toward to the DielsAlder (D-A) reaction of acrylonitrile with 2-methyl-1, 3-butadiene. Both the cation and anion are shown to play important roles in promoting the D-A reaction. The cation [epy]+, as a Lewis acid, associates the CtN group by CtN 3 3 3 H H bond to increase the polarity of the CdC double bond in acrylonitrile, while the anion CF3COO links with the methyl group in 2-methyl-1,3-butadiene by CH 3 3 3 O H bond, which weakens the electron-donating capability of methyl and thereby lowers the energy barrier of the D-A reaction. The present results are expected to provide valuable information for the design and application of pyridinium-based ionic liquids.

1. INTRODUCTION Room temperature ionic liquids (RTILs), have received increasing attention in both academia and industry because of their attractive physicochemical properties,1,2 such as low melting point, powerful dissolving capacity, vanishingly small vapor pressure, and high thermal stability. Furthermore, these features can easily be tailored by varying cation, anion, or alkyl substituent on the cation for a given purpose. Therefore, RTILs have been regarded as “designer solvents”.3 In recent years, they have been used as alternative solvents in many fields, including organic catalysis,4,5 electrochemistry,6,7 material engineering,8 and biochemistry.911 Pyridinium-based salts are a kind of widely used RTILs in experiment. The majority of published works have focused on their syntheses,1214 macroscopic properties,1517 and applications.1822 For example, Prausnitz et al.23,24 investigated the polarities and KamletTaft parameters of pyridinium-based ionic liquids (PBILs), and Khupse et al,25,26 researched solutesolvent interactions in the binary mixtures of PBIL with some molecular solvents including water, methanol, and dichloromethane. In the area of organic synthesis, PBILs are used not only as catalysts2730 but also as reactants to prepare disubstituted and trisubstituted fluorous pyridines.31 Compared with extensive experimental research on PBILs, theoretical studies are relatively laggard and knowledge at the molecular level is still fragmentary. To design more effective PBILs, predict their properties and even r 2011 American Chemical Society

choose a more suitable one for a specific application, theoretical study at the molecular level seems to be extraordinarily necessary. In this work, by performing density functional theory (DFT) calculations, we carried out theoretical studies on a PBIL, 1-ethylpyridinium trifluoroacetate ([epy]+[CF3COO]), to gain insight into the general synthesis mechanism, electronic structure, and catalytic reactivity of pyridinium salts. The reason we chose [epy]+[CF3COO] is that this ionic liquid was experimentally found to be the most effective catalyst on the Diels Alder (D-A) reaction between acrylonitrile and 2-methyl-1, 3-butadiene, which is studied in the present work as a representative of D-A reactions.

2. COMPUTATIONAL DETAILS Throughout this work, we used the popular B3LYP functional, which has been confirmed to give results for both geometries and energies as good as the more expensive methods,32 for instance, MP methods33,34 and coupled-cluster methods.35,36 The standard 6-31G(d,p) basis set3739 was used in the calculations, because it is a moderate basis set and generally gives a good compromise between accuracy and computational cost. No symmetry Received: February 7, 2011 Revised: June 14, 2011 Published: June 14, 2011 8255

dx.doi.org/10.1021/jp201246j | J. Phys. Chem. A 2011, 115, 8255–8263

The Journal of Physical Chemistry A

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Scheme 1. Sketch Map Indicating the Synthesis of Pyridinium-Based Halide Ionic Liquid

Figure 1. Optimal structures of the reactant-like precursor (R), transition state (TS1 and TS2), and product (P) along the two pathways. The distances are in angstroms.

constraint was imposed on any initial structures. Frequency calculations at the same level of theory were carried out to verify all stationary points as minima (zero imaginary frequencies) or first-order saddle points (one imaginary frequency) and to provide the zero-point vibrational energy (ZPE) corrections. With the aim of confirming that the transition states actually connect the two corresponding minima, intrinsic reaction coordinates (IRC) calculations were performed to trace the reaction pathways. The electronic properties for the relevant stationary points were discussed based on the natural bond orbital (NBO) analysis40,41 and the frontier molecular orbital theory.42,43 All calculations were carried out using Gaussian 03 program package.44 2.1. Synthesis Mechanism. As shown in Scheme 1, PBILs are generally prepared via a two-step process: the first step involves a Menshutkin reaction4549 of pyridine with alkyl halide to produce N-alkylpyridinium halide, and the second step is a simple substitution of halide anion by a functional anion. Clearly, the Menshutkin reaction is a crucial step for the synthesis of PBILs. However, as far as we know, the detail of the mechanism has not been elucidated yet at the molecular level. In a recent work,46 we studied the formation mechanism of imidazolium-based halide salts via the Menshutkin reaction between N-alkylimidazole and alkyl halide. It was found that the reaction proceeds via a SN2 mechanism, which is energetically much more favorable than the five-membered transition state mechanism, proposed in the early literature. Does the SN2 mechanism also apply to the

Figure 2. Calculated potential energy profiles for the synthesis of [epy]+[Cl]. The profile linked by a dash line is for the first pathway and that linked by a solid line is for the second pathway. The relative energies are in kcal/mol.

Menshutkin reaction for preparing pyridinium salts? To answer this question, herein, as an example, we studied the Menshutkin reaction between pyridine and chloroethane. The optimized structures involved in the reaction are shown in Figure 1, and the calculated potential energy surface profiles along the reaction coordinate are given in Figure 2. We denote the reactant-like precursor as R and the product as P in Figure 1. In these two structures, H-bonds are formed: the C12H 3 3 3 N H-bond with a distance of 2.42 Å in R, and the furcated H-bond with a distance of 2.07 Å for the C2H 3 3 3 Cl branch and a distance of 2.41 Å for the C12H 3 3 3 Cl branch in P. The overall reaction is found to be endothermic by 11.06 kcal/mol. From the present calculations, we located two transition states (TS1 and TS2) connecting R and P. In TS1, the nitrogen atom in pyridine attacks the positively charged C12 atom in R, leading to forming the C12N1 bond (2.37 Å) and breaking the C12Cl bond (from 1.83 to 2.71 Å). This fact can be confirmed by the transition vector corresponding to the imaginary frequency (427i cm1) and the bond orders of N1C12 and C12C1 bonds (0.154 and 0.127, respectively). The two CH 3 3 3 Cl H-bond distances are calculated to be 2.34 and 2.29 Å. TS1 is found to be less stable than R in energy by 53.95 kcal/mol. Alternatively, by placing the N1 atom at the back of the Cl atom relative to the C12 atom, we located the other transition state (TS2), which presents a typical SN2-type structure. From the geometrical parameters of TS2, it is clear that the C12Cl bond is breaking and the N1C12 bond is forming, as confirmed by the calculated relative bond orders (0.19 for the C12Cl 8256

dx.doi.org/10.1021/jp201246j |J. Phys. Chem. A 2011, 115, 8255–8263


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Figure 3. The stable geometries for [epy]+ and ion pair [epy]+[CF3COO] (ag). Bond lengths are in angstroms. The values in parentheses are the relative energies (in kcal/mol).

bond and 0.38 for the N1C12 bond), changes of bond lengths (from 3.51 to 1.90 Å for the N1C12 bond and from 1.84 to 2.53 Å for the C12Cl bond), and the transition vector corresponding to the imaginary frequency (415i cm1). The Walden inversion of the two hydrogen atoms on C12 atom is observed by determining the relevant dihedral angles. For example, the DH14C12C13H15 angle changes from 179° in isolated chloroethane to 162° in TS2. As is shown by the solid line profile in Figure 2, the barrier to get across TS2 is 34.41 kcal/mol, which is lower by 19.54 kcal/mol than that to get across TS1, indicating that the SN2-type pathway is energetically more favorable. This result is in accord with easy syntheses of N-alkylpyridinium halide salts.49 Clearly, the formation of N-alkylpyridinium halide salt also follows a SN2 mechanism, i.e., N1 atom in pyridine, as a nucleophilic center, attacks the positively charged C12 atom at the back of a Cl atom in R to produce N-entylpyridinium cation, which induces the heterolytic cleavage of the C12Cl bond to obtain an isolated Cl anion. 2.2. Electronic Structure of the Ion Pair. To better understand the properties of PBILs, it is essential to know the microstructures of the ion pair at atomic and molecular levels. Herein, we present our calculated results for the chosen ion pair [epy]+[CF3COO]. The optimized geometry of the isolated cation [epy]+ with main bond parameters is depicted in Figure 3. The cation has Cs symmetry and the pyridinium ring is almost planar, where all lengths of CC and CN bonds are between the corresponding

single bonds and double bonds (cf. 1.53 Å for a CC single bond, 1.33 Å for a CdC double bond, 1.47 Å for a CN single bond, and 1.27 Å for a CdN double bond) and their bond orders are in the range of 1.051.17. To obtain stable structure of ion pair [epy]+[CF3COO], the anion CF3COO was initially put at several possible sites around the cation [epy]+. Optimized geometries for seven stable structures (ag) are shown in Figure 3, with the relative energies and detailed geometrical parameters. The common geometrical features in the seven structures of the ion pair [epy]+[CF3COO] are that multiple H-bonds are formed between the anion and cation, and the anion prefers to near the C2H7 bond. Note that C2 is identified with C6 due to the Cs systemmetry of the cation [epy]+, so our discussion focuses on the C2H7 moiety in the pyridinium ring. For example, in the most stable structure a, two H-bonds are formed between the anion and cation: the C2H7 3 3 3 O H-bond with a distance of 1.78 Å and the C12H13 3 3 3 O H-bond with a length of 1.99 Å, respectively. It is also found that the lengths of C2H7 and C12H13 bonds are 1.11 and 1.10 Å, which are larger by 0.03 and 0.01 Å than those in the isolated cation, as confirmed by the substantial red shifts of the CH stretch vibrational frequencies. The calculated IR spectra of the cation [epy]+ and the most stable structure of ion pair [epy]+[CF3COO] (structure a in Figure 3) are summarized in Table 1, where the C2H7 and C12H13 normal modes occur at ∼3250 and 3103 cm1 in the isolated cation. However, they shift to 2802 and 2984 cm1 in the ion pair, respectively. The C2H7 3 3 3 O H-bond is also found in structures b, c, and d, which are slightly 8257



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Table 1. Calculated Vibrational Frequencies (cm1) for the Isolated Cation [epy]+ and the Most Stable Ion Pair [epy]+[CF3COO] cation

a c

ion paira

cation

ion paira

Table 2. The NBO Charges in the Most Stable Structure of Ion Pair [epy]+[CF3COO] atoms

a

N1

0.293

1128

C2 C3

0.088 0.252

1681

1161

C4

0.165

3066

1179

C5

0.264

40 41

3104c 3140

1181 1182

C6

0.054

H7

0.293

113

42

3152

1194

H8

0.276

560

154

43

3164

1217

H9

0.263

9

659

159

44

3225

1230

10

696

252

45

3236

1262

C2H7 C3H8

0.401 0.016

11

771

257

46

3239

1294

C4H9

0.098

12

790

275

47

3246b

1382

13 14

808 877

319 404

48 49

3249b

1417 1427

15

972

414

50

1442

16

977

425

51

1454

17

1001

432

52

1499

18

1045

490

53

1511

19

1047

510

54

1527

20

1079

566

55

1530

21 22

1096 1109

587 660

56 57

1562 1629

23

1172

707

58

1688

24

1186

719

59

1729

25

1198

769

60

2802b

26

1240

790

61

2984c

27

1281

793

62

3050

28

1373

834

63

3117

29 30

1387 1397

849 899

64 65

3128 3149

31

1438

972

66

3211

32

1500

989

67

3230

33

1503

1027

68

3234

34

1515

1039

69

3246

35

1526

1078

1

52

13

36

1544

1098

2

135

19

37

1632

3

226

35

38

4

317

71

39

5 6

395 404

90 106

7

489

8

Structure a in Figure 3. b The C2H7 bond stretching vibration. The C12H13 bond stretching vibration.

less stable than structure a by 1.29, 1.79, and 4.57 kcal/mol, respectively. These three structures involve furcated H-bonds: the C2H7 3 3 3 O15 (2.09 Å), C2H7 3 3 3 O16 (2.36 Å), and C12 H13 3 3 3 O (2.12 Å) H-bonds for structure b, the C14H 3 3 3 O (2.34 Å), C2H7 3 3 3 O (1.74 Å), and C3H8 3 3 3 O (2.28 Å) H-bonds for structure c, and C2H7 3 3 3 O (1.72 Å) and C12H13 3 3 3 O (2.19 Å) H-bonds for structure d. In these three structures, the C2H7 3 3 3 O H-bond is always stronger than the other H-bonds, as shown by the shorter H-bond distances. Structures e, f, and g are three relatively less stable geometries, where no C2H7 3 3 3 O H-bond is formed between the anion and cation. Their relative energies are considerably higher by 9.79, 16.20, and 16.99 kcal/mol compared with structure a, respectively. So we believe that the C2 H7 3 3 3 O H-bond plays a crucial role in stabilizing ion pair

[epy]+[CF3COO] and even the ionic liquid system. In addition, in structures f and g, not only the CH 3 3 3 O H-bond but also the CH 3 3 3 F H-bond is formed. However, the CH 3 3 3 F H-bond (2.39 Å in f and 2.32 Å in g) is much longer than the CH 3 3 3 O H-bonds (1.69 Å in f and 1.70 Å in g), suggesting the former plays a less important role in stabilizing the ion pair. The interaction energy between the cation and anion is an important index for evaluating the stability of an ion pair. The calculated interaction energies for the structures ag are 100.13, 98.84, 98.34, 95.56, 90.34, 83.93, and 83.14 kcal/mol, respectively. Obviously, the stability decreases from a, via bf to g. These great energy values can provide reasonable explanations for low melting point and thermal stability of PBILs. NBO analyses for ion pair [epy]+[CF3COO] were carried out to know the charge distribution and the intrinsic property of the interactions between the anion and cation. From NBO atomic charges shown in Table 2, most of the negative charge is located on heavy atoms except C2 and C6 atoms in the pyridinium ring, and the positive charge is focused on the peripheral hydrogen atoms. For the total CH units in the ring, the charge on C2H7 unit (0.401 e) is much more than those on C3H8 unit (0.16 e) and C4H9 unit (0.098 e). Obviously, the positive charge of the cation concentrates on the C2H7 group, implying nucleophilic reagent should easily attack this C2H7 bond in nucleophilic substitution reactions. From the electronic structural analyses above, the anion preferentially approaches the positively charged C2H7 group, indicating that the electrostatic interaction between the anion and cation is dominative for the formation of ion pair. Previous studies5055 on imidazoliumbased ionic liquids indicate that the C*H bond in the cation (C* denotes the carbon atom linking two nitrogen atoms) plays an important role in the cationanion interaction and the stability of the ILs. It was also reported that the greater the charge on the CH unit in the imidazolium ring, the stronger the acidity of the corresponding hydrogen atom. These findings are expected to also apply to PBILs. Thus H7 atom at the orthoposition of nitrogen atom in the pyridinium ring should have the largest acidities because of the most charge on the C2H7 unit. Determining the relative acidity of a hydrogen atom in the pyridinium ring of the cation [epy]+ is significant for estimating the solubility of a potential solute in PBILs and the active site of pyridinium ring in some chemical reactions when PBILs act as 8258



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Scheme 2. Possible Pathways for the DielsAlder Reaction of 2-Methyl-1,3-butadiene with Acrylonitrile

Figure 4. The frontier molecular orbitals for the most stable structure a. Isosurface calculated at 0.02.

solutions or/and catalysts. We calculated the acid equilibrium constants (pKa) of the hydrogen atoms on the pyridinium ring. According to the previous literature,5658 pKa can be calculated by eq 1 pKa ¼

ΔGaq RT ln 10

ð1Þ

where ΔGaq can be obtained according eq 2 ΔGaq ¼

∑GaqðproductsÞ ∑Gaq ðreactantsÞ

ð2Þ

Here, all the Gibbs free energies (Gaq) for products and reactants are calculated using the polarized continuum model (PCM). The estimated pKa values of H7, H8, and H9 are 49.86, 56.59, and 56.39, respectively, indicating the acidity order is H7 > H8 > H9, which is in good agreement with the results from NBO analysis. To better understand the intrinsic property of the cation anion interactions, we further calculated the second-order perturbation stabilization energy E(2) between the donor NBO and the acceptor NBO, using the second-order perturbation theory.5961 A larger E(2) value indicates a stronger orbital interaction. The calculated results of structure a show that, the largest E(2) value is 22.97 kcal/mol for the interaction between the antibonding orbital of C2H7 bond and the lone pair * interacorbital of O15 (LPO15). Thus the LPO15σO2H7 tion determines the stability of the ion pair, which accords with the stronger C2H7 3 3 3 O15 H-bond discussed above. Such a LPO15σ*O2H7 interaction results in the electron transfer from the proton acceptor (LPO15) to the proton donor * ) and thereby stabilizes ion pair [epy]+[CF3COO]. (σO2H7 The second largest E(2) value is 10.37 kcal/mol for the LPO16σ*O12H13 interaction, which associates with the formation of the C12H13 3 3 3 O16 H-bond. The cationanion interaction in ion pair [epy]+[CF3COO] can be depicted visually by the contour plots of the frontier orbitals (see Figure 4). The HOMO of structure a has σ-type * interaction, which could symmetry and exhibits LPO15σO2H7 be crucial to stabilize ion pair [epy]+[CF3COO]. For HOMO-2 and HOMO-3, two σ-type interactions are related with LPO15σ*C2H7 and LPO16σ*C2H13 interactions due to the formation of C2H7 3 3 3 O15 and C12H13 3 3 3 O16 H-bonds. 2.3. The Catalytic Reactivity of [epy]+[CF3COO] toward a Typical D-A Reaction. D-A reactions are known as one of the most powerful methods for synthesizing six-membered cyclic compounds.62 Many catalysts have been explored to enhance reaction rate, regioselectivity, and stereoselectivity, and most of them are Lewis acids.6365 Recently, ILs have received more and more attention in the chemical field and have been endorsed as useful solvents and catalysts for many reactions.6668 Malhotra et al.30 reported that [epy]+[CF3COO] ionic liquid is an

excellent solvent and catalyst for the D-A reaction between acrylonitrile and 2-methyl-1,3-butadiene. They found that the yield is as high as 99% and the stereoselectivity is also great (the rate of para to meta product is 75:25). To obtain an insight into the catalytic mechanism of PBILs toward D-A reactions, herein, as an example, we examined the D-A reaction of acrylonitrile (denoted R1) and 2-methyl-1,3-butadiene (denoted R2) without and with the presence of [epy]+[CF3COO] ionic liquid. 2.3.1. The D-A Reaction without the Catalyst. As shown in Scheme 2, for the D-A reaction between R1 and R2, there are four possible pathways, which lead to trans-para, trans-meta, cispara, and cis-meta products, respectively. The optimized structures for the intermediates (IMTPIMCM), transition states (TSTPTSCM), and products (PTPPCM) along the four reaction pathways are shown in Figure 5, where the subscripts TP, TM, CP, and CM represent the trans-para, trans-meta, cis-para, cis-meta pathways, respectively. The activation energies (ΔE, the relative energies of the transition states with respect to the reactants) and the total reaction heats (ΔH, the relative energy of products with respect to the reactants) are summarized in Table 3. IRC calculations show that IMTPIMCM are converted into PTPPCM via TSTPTSCM (in Figure 5) with the energy barriers of 19.67, 20.53, 20.50, and 21.19 kcal/mol (see Table 3) along trans-para, trans-meta, cis-para, and cis-meta pathways, respectively. Exothermic heats of the reaction are calculated to be in range of 35 to 37 kcal/mol. It is clear that the trans-para pathway is the energetically most favorable, which can be explained as follows. On the one hand, the electron-withdrawing CtN group in R1 leads to the polarization of C1dC2 double bond and makes C1 and C2 atoms carry partial positive and negative charges, respectively. On the other hand, the electrondonor methyl group on C6 atom in R2 makes C5 atom carry more negative charge, and thereby the positively charged C1 atom easily bonds with the negatively charged C5 atom to form para product. Additionally, considering the steric effect between the CtN group of R1 and the methyl of R2, the trans-para pathway is more favorable than the cis-para pathway. Due to the calculated largest difference in energy barriers being less than 2 kcal/mol, the four nearly parallel pathways may be competitive in a state of thermal equilibrium at room temperature. From 8259



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Figure 5. Optimal structures for the intermediates, transition states, and products involved in the DielsAlder reaction of 2-methyl-1,3-butadiene with acrylonitrile in the absence of [epy]+[CF3COO] salt. The bond distances are in angstroms.

Table 3. The Energy Barriers (ΔE) and the Reaction Heats (ΔH) for the D-A Reaction along the Four Different Pathways in the Absence and Presence of the Catalyst (in kcal/mol) ΔE

ΔH

uncatalyzed

19.67

36.57

catalyzed

18.23

35.10

uncatalyzed

20.53

36.42

catalyzed

18.63

35.94

cis-para

uncatalyzed

20.50

36.55

cis-meta

catalyzed uncatalyzed

17.74 21.19

30.30 35.94

catalyzed

18.78

32.43

reaction pathway trans-para trans-meta

geometrical parameters of transition states shown in Figure 5, it is clear that the D-A reaction is a concerted but asynchronous elementary reaction. For example, compared to free reactants, the distances between C1 and C5 and C2 and C8 atoms in TSTP are shortened to 2.050 and 2.578 Å for the formation of C1C5 and C2C8 bonds between R1 and R2, and the lengths of C1C2, C5C6, C6C7, and C7C8 bonds vary from 1.338, 1.341, 1.478, and 1.337 to 1.401, 1.398, 1.416, and 1.371 Å, respectively. The asynchronicity is confirmed by the length differences (0.528, 0.472, 0.461, and 0.369 Å) between two new forming bonds in transition states along trans-para, trans-meta, cis-para, and cis-meta pathways, respectively. 2.3.2. The D-A Reaction Catalyzed by [epy]+[CF3COO] Ionic Liquid. We further studied the D-A reaction between R1 and R2 catalyzed by ion pair [epy]+[CF3COO]. The optimized structures for intermediates (IMTP0 IMCM0 ), transition states

(TSTP0 TSCM0 ), and products (PTP0 PCM0 ) are shown in Figure 6, and the calculated relative energies are summed in Table 3. The calculated results show that ion pair [epy]+[CF3COO] reduces the energy barrier and enhances the stereoselectivity but almost does not change the potential energy profile of the D-A reaction. Along the four pathways (trans-para, trans-meta, cis-para, and cis-meta), we found that the D-A reaction catalyzed by ion pair [epy]+[CF3COO] is also concerted but asynchronous. The calculated energy barriers (shown in Table 3) from TSTP0 TSCM0 to the corresponding intermediates IMTP0 IMCM0 are 18.23, 17.51, 17.74, and 18.78 kcal/mol, which are lower by 1.44, 1.90, 2.76, and 2.41 kcal/mol than the uncatalyzed pathways, respectively. Clearly, ion pair [epy]+[CF3COO] can promote the D-A reaction through reducing the energy barrier. From the calculated barriers, it is found that the cis-para pathway is the energetically most favorable among these four pathways, and the para pathway is more favorable than the meta pathway, which accords with more para product (the rate of “para” to “meta” is 75:25) in the experiment.30 To understand how ion pair [epy]+[CF3COO] controls the D-A reaction, we carried out structural analyses of TSTP0 TSCM0 (Figure 6). It is noted that multiple H-bonds, particularly the C*H 3 3 3 N H-bond, where the H atom is the most acidic hydrogen atom in the pyridinium ring, always exist in each transition state. The formation of the C*H 3 3 3 N H-bond can effectively polarize the C1C2 bond and thus make the reaction proceed more easily. The distances of C*H 3 3 3 N H-bonds are 2.646, 2.532, 2.337, and 2.348 Å in TSTP0 , TSTM0 , TSTP0 , and TSCM0 , respectively. Evidently, the strongest C*H 3 3 3 N H-bond lies in the most stable structure TSCP0 , which is in 8260



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Figure 6. Optimal structures for the intermediates, transition states, and products involved in the DielsAlder reaction of 2-methyl-1,3-butadiene with acrylonitrile in the presence of [epy]+[CF3COO] salt. The bond distances are in angstroms.

Table 4. The Partial NBO Charges for Free Reactants and the Intermediates (IMTP0 IMCM0 ) of the D-A Reaction with [epy]+[CF3COO] Salt C1

C2

C3

N4

C5

C8

reactants

0.344

0.350

0.255

0.299

0.423

0.414

IMTP0

0.337

0.367

0.308

0.397

0.455

0.420

IMTM0

0.306

0.379

0.317

0.383

0.437

0.419

IMCP0 IMCM0

0.303 0.304

0.398 0.391

0.315 0.319

0.399 0.401

0.443 0.434

0.445 0.401

agreement with the calculated lowest barrier along the cis-para pathway. Thus we conjecture that this H-bond may be important for stabilizing the transition states. Additionally, only in TSCP0 , the hydrogen atom of methyl group in R2 can interact with the anion CF3COO] via the CH 3 3 3 O H-bond, which will weaken even eliminate the electron-donating capability of methyl to C5dC6 double bond, making the C1 atom easily bond with the C5 atom to obtain the para product. From the above discussion, we have seen that both the cation and anion of the ion pair [epy]+[CF3COO] play important roles in the catalyzed D-A reaction. To examine the change of atomic charges and bonds in intermediates, NBO analyses of reactants R1 and R2 and intermediates (IMTP0 IMCM0 ) involved in the D-A reaction have been carried out. As shown in Table 4, with the presence of ion pair [epy]+[CF3COO], the formation of C*H 3 3 3 N H-bond makes negative charges on N atom increase from 0.299 in R1 to 0.397, 0.399, 0.383, and 0.401 e in IMTP0 IMCM0 along transpara, trans-meta, cis-para, and cis-meta pathways, respectively.

These increased negative charges are favorable to the electron transfer from C1 to C2 and hence enhance the polarity of C1dC2 bond. The charge on C1 atom decreases from 0.344 in R1 to 0.337, 0.306, 0.303, and 0.304 e in IMTP0 , IMTM0 , IMCP0 , and IMCM0 , and that on C2 increases from 0.350 in R1 to 0.367, 0.379, 0.398, and 0.391 e in these four intermediates, respectively. The large polarity of C1dC2 bond makes C1 and C2 atoms of R1 easily bond with C5 and C8 atoms of R2, and hence the reaction easily happens. Clearly, [epy]+[CF3COO] acts as a Lewis acid to catalyze the D-A reaction. In addition, a comparison of charge differences between C1 and C2 atoms for the four intermediates shows that the polarity of C1dC2 double bond in IMCP0 is the largest, which is consistent with the shortest distance of the C*H 3 3 3 N H-bond and the lowest barrier along the cispara pathway. Thus we believe that the strength of C*H 3 3 3 N H-bond plays an important role in the catalyzed D-A reaction. The catalytic role of ion pair [epy]+[CF3COO] can also be understood by performing the frontier molecular orbital (FMO) analysis. According to the FMO theory, a smaller energy difference (LUMOHOMO gap) between the highest occupied molecular orbital (HOMO) of the electron donor and the lowest unoccupied molecular orbital (LUMO) of the electron acceptor would determine a larger reactivity between two molecules. As shown in Figure 7, in the case without the presence of the ion pair, the HOMOLUMO gaps of the four intermediates IMTP, IMTM, IMCP, and IMCM are 5.237, 4.758, 4.819, and 5.298 eV. In contrast, with the presence of the ion pair, the corresponding gap values of IMTP0 , IMTM0 , IMCP0 , and IMCM0 are reduced to 3.128, 3.222, 3.210, and 3.238 eV, respectively. This fact clearly indicates that the catalyzed reaction can take place more easily than the uncatalyzed reaction. 8261



Figure 7. Energies of the HOMOs and LUMOs for intermediates in the absence (IMTPIMCM) and presence (IMTP0 IMCM0 ) of ion pair [epy]+[CF3COO]. The relative energies are in electronvolts.

3. CONCLUSIONS In this work, the synthesis mechanism, electronic structures, and catalytic reactivity of the pyridinium-based ionic liquid 1-ethyl-pyridinium trifluoroacetate ([epy]+[CF3COO]) have been investigated at the B3LYP/6-31G(d,p) level of theory. The calculated results show the synthesis of [epy]+[CF3COO] proceeds via a SN2 mechanism. In ion pair [epy]+[CF3COO] , ̅ there exist multiple H-bonds, where the C*H 3 3 3 O H-bond (C* denotes the carbon atom at the ortho-position of nitrogen atom in the [epy]+ cation) plays a decisive role in stabilizing the ion pair and even the ionic liquid system. It is also shown that the most acidic hydrogen atom in the pyridinium ring is decisive for the catalytic reactivity of pyridinium-based ionic liquids. Our calculations have also shown clearly the mechanism details of [epy]+[CF3COO] as a catalyst on the D-A reaction. The notable catalytic activity may originate from the cooperative actions of the cation and anion. The cation [epy]+, as a Lewis acid, associates with the CtN group by CtN 3 3 3 H H-bond to increase the polarity of CdC double bond in acrylonitrile, while the anion CF3COO interacts with the methyl group in 2-methyl1,3-butadiene by CH 3 3 3 O H-bond, which weakens the electrondonating capability of methyl and leads to a lower energy barrier of the D-A reaction. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT We thank the grant from the National Natural Science Foundation of China (Nos. 20873076, 20873075), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 200804220009), and the Natural Science Foundation of Shandong Province (No. Z2008B02). ’ REFERENCES (1) Welton, T. Chem. Rev. 1999, 99, 2071–2083. (2) Brennecke, J. F.; Maginn, E. J. AIChE J. 2001, 47, 2384–2389. (3) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 1391–1398.

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Theoretical Study for Pyridinium-Based Ionic Liquid 1

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