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Protic pyrazolium ionic liquids: An efficient catalyst for conversion of CO in the absence of metal and solvent 2

Tengfei Wang, Danning Zheng, Jingshun Zhang, Baowan Fan, Yuan Ma, Tiegang Ren, Li Wang, and Jinglai Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04051 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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ACS Sustainable Chemistry & Engineering

Protic pyrazolium ionic liquids: An efficient catalyst for conversion of CO2 in the absence of metal and solvent Tengfei Wanga,b, Danning Zhengb, Jingshun Zhanga,b, Baowan Fana,b, Yuan Maa,b, Tiegang Ren*a,b, Li Wang*b, Jinglai Zhang*b a

Engineering Laboratory for Flame Retardant and Functional Materials of Henan Province,

b

College of Chemistry and Chemical Engineering, Henan University, Jinming Avenue,

Longting District, Kaifeng, Henan 475004, PR China

ABSTRACT

A series of novel protic pyrazolium ionic liquids are firstly synthesized and utilized as catalysts for cycloaddition of carbon dioxide and epoxides to form cyclic carbonates under metal- and solvent-free conditions. The new developed protic pyrazolium ionic liquids present excellent catalytic activity towards the fixation of carbon dioxide. More importantly, they would be prepared by a facile two-step reaction from cheap raw starting materials with a yield more than 90%. The influence of catalyst dosage, reaction temperature, carbon dioxide pressure, and reaction time

*

Corresponding author E-mail: [email protected]

*

Corresponding author E-mail: [email protected]

*

Corresponding author E-mail: [email protected] 1

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on the synthesis of cyclic carbonates is investigated to identify the optimal reaction conditions. Under the optimum condition, the catalyst suitability is studied. Additionally, the possible reaction mechanism is investigated by the Double-IL model to elucidate the synergistic effects of electrostatic and weak interaction in catalytic process.

KEYWORDS: protic pyrazolium ionic liquids, carbon dioxide, epoxide, cyclic carbonate, mechanism

INTRODUCTION The exhausting fossil fuels and rapidly deteriorating environment are two vital issues confronted by the whole world. Development of clean and renewable sources has become the focuses of both academic and industrial communities.1 Carbon dioxide (CO2) is an ideal carbon source since it is inexpensive, nontoxic, and abundant.2 Moreover, reduction of CO2 emission would be beneficial to ameliorating the greenhouse effect. As a result, numerous methods have been developed to utilize CO2 to produce various chemicals such as methanol,3 oxidant,4 and organic carbonates.5,6 The coupling reaction of CO2 with epoxides leading to cyclic carbonate is one of the most efficient routes because of the 100% atom utilization and negligible by-product. In addition, the cyclic carbonate would be applied as electrolytes, polar aprotic solvents, and intermediates.7,8 However, the cycloaddition of CO2 would not 2


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be performed in a benign condition since CO2 is in the highest oxidation state. Involvement of catalyst is a popular method to refine the reaction condition and improve the product yield. Although lots of catalysts have been developed in past two decades such as, transition metal complexes,9,10 schiff bases,11 ion-exchange resins,12 and functional polymers,13,14 they confront some common problems including harsh reaction conditions, requirement of co-catalyst and organic solvent, and sensitive to moisture.15,16 Ionic liquids (ILs)17,18 are extensively investigated to catalyze the coupling reaction of CO2 and epoxides in recent years since additional organic solvent is not required. Moreover, ILs would also be taken as the efficient catalysts in the reaction of alkoxycarbonylation19 and alcoholysis.20 However, the co-catalyst, normally metal, is still necessary in many cases.21 The high-efficiency single-component IL without solvent and co-catalyst is still not enough to meet the requirement. As early as 1982, imidazolium ILs have been synthesized.22 However, they have not been applied to catalyze the reaction of CO2 and propylene oxide (PO) until 2001.23 Later, hydroxyl-functionalized imidazolium ILs have been developed by Zhang,24 which present better catalytic activity than room temperature imidazolium ILs. The theoretical study has testified that the improved catalytic activity of hydroxyl-functionalized imidazolium ILs is related with the high active hydrogen atom included in hydroxyl group.25 Involvement of active hydrogen atom is one of the most

effective

methods

to

further

improve

3


the

catalytic

efficiency.


Carboxyl-functionalized and bifunctionalized imidazolium ILs are developed in succession.26,27 Besides variation of functional groups, protic ILs are an alternative choice to afford active hydrogen atom. In 2014, Xiao et al.28 developed a series of protic imidazolium ILs and applied them to catalyze the coupling reaction of CO2 and PO with satisfied product yield. In general, involvement of the active hydrogen atom is a feasible method to refine the catalytic activity of ILs with the same cation. Alternatively, the catalytic activity of imidazolium ILs is normally better than quaternary ammonium ILs,29 quaternary phosphonium ILs,21 and others,30 which would be attributed to the existence of five-member ring in imidazolium ILs. As the molecular isomer, pyrazolium ILs also have the similar five-member ring. However, less attention has been focused on the latter. As reported, pyrazolium ILs have been applied as surfactant,31 electrolyte,32 and catalyst for esterification reaction.33 However, they have never been employed to catalyze the fixation of CO2. Recently, we firstly synthesized some pyrazolium ILs and testified that they present excellent catalytic activity for the fixation of CO2.34 According to the aforementioned analysis, it is easy to refer that the catalytic activity of protic pyrazolium ILs should be higher than that of pyrazolium ILs. In this work, six protic pyrazolium ILs are firstly synthesized. Then, their catalytic activity for the cycloaddition of PO and CO2 is investigated without any co-catalyst and solvent. The influence of alkyl chain length in cation and different anions on the catalytic performance is explored. Moreover, the optimal reaction condition including catalyst amount, reaction temperature, CO2

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pressure, reaction time, and generality of catalyst are also considered. Finally, the reaction mechanism is investigated by density functional theory (DFT) method with Double-IL model to uncover the difference of catalytic activity from atomic level. The ring-opening of PO is synergetic effects of electrophilic activation from cation and nucelophilic activation from anion.35,36 Our ultimate goal is not limited to explore an efficient single-component IL for the chemical fixation of CO2 with metal- and solvent-free30,34 but it could be facilely synthesized from cheap raw materials with a high product yield.

EXPERIMENT SECTION

Materials and Instruments. The pyrazole was purchased from Shanghai Macklin Biochemical Co. Ltd.. Oxirane, epichlorohydrin, and styrene oxide were purchased from Aladdin Industrial Corporation. Propylene oxide and other organic compounds were produced by Sinopharm Chemical Reagent Co. Ltd. and all of reagents were used without further purification. The CO2 (99.9%) was obtained from Kaifeng Xinri Gas Co. Ltd.. The 1H NMR (400 MHz) and 13C NMR (100 MHz) were run on a Bruker AVANCE III HD spectrometer with TMS as the internal standard. High resolution mass spectra (HRMS) were measured in Agilent 1290 Infinity LC with 6224 TOF MSD. The thermal decomposition temperature was analyzed with a thermal gravimetric analyzer (Mettler Toledo TGA/SDTA851e). GC analyses were performed on Agilent GC–7890 Busing a flame ionization detector.

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Preparation of Protic Pyrazolium ILs. Six protic pyrazolium ILs (Figure 1) were synthesized by only two steps including alkylation reaction and protonation reaction. For example, 1-ethylpyrazole was prepared by the following method37 (See Scheme 1):

Pyrazole (3.40 g, 0.05 mol) and powdered KOH (2.80 g, 0.05 mol) were added in 10 mL of DMSO. Then, the mixtures were stirred at 80℃ for 30 minutes. After that, bromoethane (5.45 g, 0.05 mol) was added in three equal portions every 30 minutes. Stirring was continued for 24 h at 80℃, then, solution was cooled down to room temperature. The mixtures were filtered and then chloroform (20 mL) was added in the filtrate. Next, the solution was washed with water (5×20 mL), dried over calcium chloride and evaporated in vacuo to obtain colorless liquid 1-ethylpyrazole. 1-propylpyrazole, 1-butylpyrazole, and 1-amylpyrazole were synthesized by the similar procedure to that of 1-ethylpyrazole.

The obtained alkylpyrazole can be directly used for the next reaction without further treatment, then protic pyrazolium ILs were synthesized according to the literature.38 Hydrobromic acid (3.102 g, 18 mmol) was added dropwise to 1-ethylpyrazole (1.44 g, 15 mmol), and the mixtures were stirred for 15 h at 40℃ to ensure that all of the substrate were reacted. Then water was removed by cyclohexane under the heating condition. Finally, the residue was washed repeatedly with ethyl acetate in reflux condition and dried in vacuum to obtain 1-ethylpyrazolium bromide (HEPzBr). 6


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Then, 1-propylpyrazolium bromide (HPPzBr), 1-butylpyrazolium bromide (HBPzBr), and 1-amylpyrazolium bromide (HAPzBr) were synthesized by the similar procedure to that of HEPzBr.

The structures of protic pyrazolium ILs were determined by NMR and HRMS, and the data was provided as follows (See Figure S1): HEPzBr: Light yellow liquid (yield: 97.06%), 1H NMR (400 MHz, DMSO-d6): δ (ppm) 8.18 (d, J=2.4 Hz, -CH-, 1H), 7.97 (d, J=2.4 Hz, -CH-, 1H), 6.52 (t, J=2.4 Hz, -CH-, 1H), 4.33 (q, J=7.3 Hz, -CH2-, 2H), 1.42 (t, J=7.3 Hz, -CH3, 3H).

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C NMR

(100 MHz, D2O): δ (ppm) 134.82, 134.15, 107.72, 46.99, 14.06. MS (ESI): [C5H9N2]+ m/z 97.51. HPPzBr: White solid (yield: 90.57%), 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.73 (d, -CH-, 1H), 7.46 (d, -CH-, 1H), 6.24 (t, J=2.1 Hz, -CH-, 1H), 4.06 (m, -CH2, 2H), 1.76 (m, J=7.2 Hz, -CH2, 2H), 0.80 (t, J=7.4 Hz, -CH3, 3H). 13C NMR (100 MHz, D2O): δ (ppm) 135.51, 134.58, 107.82, 53.31, 22.47, 9.94. HR-MS (QTOF) calcd. for [C6H11N2]+ m/z 111.0917, found 111.0919. HBPzBr: Light yellow solid (yield: 93.31%), 1H NMR (400 MHz, D2O) δ (ppm) 7.87 (d, J=2.3 Hz, -CH-, 1H), 7.63 (d, J=2.0 Hz, -CH-, 1H), 6.33 (t, J=2.2 Hz, -CH-, 1H), 4.16 (t, J=7.0 Hz, -CH2-, 2H), 1.75 (m, -CH2-, 2H), 1.21 (m, J=7.4 Hz, -CH2-, 2H), 0.87 (t, J=7.4 Hz, -CH3, 3H).

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C NMR (100 MHz, D2O): δ (ppm) 135.23,

134.58, 107.66, 51.52, 30.72, 18.77, 12.56. HR-MS (QTOF) calcd. for [C7H13N2]+ 7



m/z 125.1073, found 125.1091. HAPzBr: White solid (yield: 91.13%), 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.83 (d, J=2.2 Hz, -CH-, 1H), 7.57 (d, J=2.0 Hz, -CH-, 1H), 6.29 (t, J=2.2 Hz, -CH-, 1H), 4.12 (t, J=7.0 Hz, -CH2-, 2H), 1.75 (m, J=7.2 Hz, -CH2-, 2H), 1.26 (m, -CH2-, 2H), 1.15 (m, -CH2-, 2H), 0.83 (t, J=7.2 Hz, -CH3, 3H). 13C NMR (100 MHz, D2O): δ (ppm) 135.29, 134.66, 107.73, 51.84, 28.43, 27.56, 21.32, 13.10. MS (ESI): [C8H15N2]+ m/z 139.17. Determining the Acidic Strength of Protic Pyrazolium ILs. The acidity of protic pyrazolium ILs was determined by the Hammett acidity function (H0) with employment of 4-nitroaniline as the basic indicator.28 Actually, the protonation extent of uncharged 4-nitrilaniline (named I) in water is evaluated by the measurable ratio [I]/[IH+]. The maximal absorbance of the unprotonated 4-nitrilaniline was observed at 380 nm in water (See Figure 2) and it was decreased in carrying degrees when protic pyrazolium ILs was added into the mixture. Taken the total unprotonated indicator as the initial reference (ILs were not added to the solution), the [I]/[IH+] ratio was determined from the measured absorbance after treating by protic pyrazolium ILs, then the Hammett function (H0) of protic pyrazolium ILs was calculated by the equation: H0 = pK(I)aq + log ([I]/[IH+]). Coupling of PO and CO2 to Form Propylene Carbonate (PC). The coupling reaction was carried out in a 100 mL stainless steel autoclave equipped with a

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magnetic stirrer. For each typical reaction process (Scheme 2): ionic liquid (0.85 mmol) and propylene oxide (6.0 mL, 85 mmol) were charged into the reactor vessel without any co-solvent and co-catalyst. The reactor vessel was placed under a constant pressure of carbon dioxide and the reaction was carried out in accordance with the designed temperature and time. After the reaction, the reactor was cooled to ambient temperature, and the remaining CO2 was slowly released. Finally, the product PC was obtained as a colourless liquid by distillation under vacuum.

Computational Details. The geometric parameters of all stationary points were optimized by the Becke's three parameter exact exchange-functional combined with Perdew and Wang (B3PW91)39,40 method with the 6-31G(d,p) basis set.41 Starting from the transition state, the minimum-energy path (MEP) was constructed by intrinsic reaction coordinate (IRC) method42 to confirm that two desired minima were connected with the transition state. On the basis of the optimized structures, the energy profiles were further refined at the M06/6-311+G(2d,2p) level43 associated with the polarized continuum model (PCM) in ethyl ether (Et2O) solvent.44,45 Abovementioned electronic calculations were performed by the Gaussian 09 program.46

RESULTS AND DISCUSSION Synthesis and Thermostability of Protic Pyrazolium ILs. The synthetic route of protic pyrazolium ILs is shown in Scheme 1. The target ILs are synthesized by only

9



two steps from inexpensive raw materials, including alkylation reaction from pyrazole with halohydrocarbon and protonation reaction from alkylpyrazole and hydrobromic acid at 40℃. The yield of protic pyrazolium ILs is higher than 90%. And best of all, the purification process is very simple. The water in ILs was removed by cyclohexane and ILs was washed with ethyl acetate. The target product was characterized by NMR and MS (ESI) indicating that the desired structure is correct. In general, the new synthesized protic pyrazolium ILs present some excellent advantages, including simple synthetic process, cheap raw materials, easy purification, relative low reaction temperature, and high product yield. If they would present excellent catalytic activity for the coupling reaction of CO2 and PO, it is possible to apply them in a larger scale. In order to investigate the thermal stability of protic pyrazolium ionic liquids, the thermogravimetic analysis (TGA)was performed (See Figure S2).

Effect of Catalysts. In the presence of a series of protic pyrazolium ILs, the coupling reaction of PO and CO2 to form PC was carried out without additional co-catalyst and organic solvent. The corresponding results are summarized in Table 1. For the same counterpart anion, the effect of alkyl chain length in the cation on the catalytic activity is obvious (entries 1, 2, 3, and 4). The highest PC yield approaches 84.71% with > 99% selectivity in the presence of HEPzBr under 130℃ and 2.0 MPa CO2 pressure during 4.0 h. The product yield is decreased from 84.71% (entry 1) to 60.30% (entry 4) when the alkyl substitution in the cation varies from ethyl (HEPzBr) to amyl (HAPzBr). It is attributed to the larger steric bulk of latter, which is 10


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detrimental for the electrophilic activation of PO that is completed by cation. Taken the HEPzBr as an example, other two protic pyrazolium ILs, HEPzCl (entry 5) and HEPzI (entry 6) are synthesized to further explore the influence of anion. The catalytic activity decreases in the order of HEPzI (entry 6) > HEPzBr (entry 1) > HEPzCl (entry 5), which is totally consistent with the nucleophilic ability of halogen anions. Although the nucleophilic attack of Cl- anion is weaker than that of Br- anion, the product yield catalyzed by HEPzCl (entry 5) is higher than that catalyzed by HAPzBr (entry 4) indicating that the elongation of alkyl chain length plays the more important role in affecting the catalytic activity.

Effect of Acidic Strength. Besides the structure of cation, the acidity is another factor to affect the catalytic activity. The determined acidity increases in the order of HAPzBr < HBPzBr < HPPzBr < HEPzBr (See Table 2), which is consistent with the sequence of their catalytic activity. Therefore, the enhanced acidity is beneficial for the improvement of catalytic activity, which is aroused by two factors. One side, the stronger acidity is favorable for the electrophilic attack to promote the ring-opening of PO. On the other side, the stronger acidity is helpful to strengthen the hydrogen bond formed between H atom in cation and O atom of PO, which is also a vital item to activate the PO.47 The similar conclusion is also suitable for the protic imidazolium ILs.28

Effect of Reaction Parameters. According to the results in Table 1, HEPzBr with the highest catalytic activity is employed as an example to explore the optimal 11



reaction conditions including catalyst amount, reaction temperature, CO2 pressure, and reaction time. First, the effect of catalyst amount on PC product yield is carried out and the corresponding results are shown in Figure 3. There is a sharply improvement for the PC product yield from 42.6% to 85.9% when the catalyst loading is improved from 0.33 mol% to 1.0 mol%. After that the yield of PC is almost kept till the catalyst amount reaches 2.0 mol%.

Subsequently, the influence of reaction temperature on the yield of PC is determined (See Figure 4). When the reaction is below 140℃, the PC product yield is enhanced step by step with the increase of reaction temperature. Further increasing the temperature, there is a slight decrease for the PC yield, which is possible due to the generation of by-production at the higher temperature. According to the previous research results, the possible by-products are acetone, propylene glycol, and polycarbonate, which is attributed to the possible side reactions including PO isomerization, hydrolysis of PO, or PC polymerization.48,49 In general, the PC yield is positive to the reaction temperature. Although the 140℃ is the optimal temperature, it is difficult to be reached. When the reaction temperature increases from 120℃ to 130℃, the enhancement of PC yield is 20.26%. When the reaction temperature is further increased by 10℃ (from 130℃ to 140℃), the increase extent of PC yield is only 9.81% that is smaller than the former. To get a good compromise between PC yield and cost, it is better to perform reaction under 130℃. Next, the influence of initial CO2 pressure on coupling of CO2 with PO is 12


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investigated (See Figure 5). The PC yield rises at the beginning to approach the highest peak under 2.0 MPa and then reduces. The more CO2 would be introduced into the reaction system with the improvement of CO2 pressure leading to the more product yield. However, too much CO2 would retard the interaction between PO and the catalyst, thus resulting in a low yield. The concentration of CO2 would be increased with the enhancement of CO2 pressure resulting in the improvement of product yield. However, the extended pressure of CO2 would also reduce the concentration of PO, which is unfavorable for the reaction. Therefore, a suitable CO2 pressure rather than the very high CO2 pressure is necessary to enhance the PC product yield.50 The 2.0 MPa is an equilibrium point to have the highest product yield.

Finally, the influence of reaction time is explored (See Figure 6). During the first 4.0 hours, the PC yield increases rapidly up to 94.52%. Then, there is only a mild enhancement when another hour is prolonged. The 4.0 h is appropriate for the synthesis of PC in the presence of HEPzBr.

In general, the optimal reaction condition is the catalyst dosage of 1.0 mol%, reaction temperature of 140℃, CO2 pressure of 2.0 MPa and reaction time of 4.0 h. There is a common feature for aforementioned reaction parameters, i.e., the PC selectivity kept over 99%, which is independent with the variation of reaction parameters.

Recyclability of the Catalyst. The reusability is also an important criterion to

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judge the performance of catalyst, especially for the homogeneous catalyst. Initially, the coupling reaction of CO2 and PO is performed under the reaction conditions (130℃, 1.0 mol% catalyst, 2.0 MPa CO2, 4.0 h) catalyzed by HEPzBr. After the reaction, HEPzBr is separated from the reaction mixture by distillation to be utilized next run. And the residue is washed by ethyl acetate. The results are shown in Figure 7. The catalyst could be utilized for five times with only slight loss of PC yield (1.0~3.0%), which suggested the recyclability of HEPzBr is good enough for the cycloaddition reaction of CO2 with PO. Coupling CO2 and Other Epoxides. Under the optimum reaction conditions, the suitability of HEPzBr for other epoxides is studied. The corresponding results are summarized in Table 3. The selectivity of cyclic carbonate is over 99% for all investigated substrates. The product yields of 2a and 2b are comparable and much higher than other items, which is attributed to the smaller steric bulk of 2a and 2b. It is also testified that the nucleophilic attack is responsible for promoting the ring-opening of PO. Reaction Mechanism. To uncover the different catalytic activity of various protic pyrazolium ILs with different alkyl chain lengths from the microscopic viewpoint, the detailed reaction mechanisms catalyzed by HEPzBr, HPPzBr, HBPzBr, and HAPzBr are investigated by density functional theory (DFT) method. According to our experience,47 Double-IL model is more reliable to determine the relative catalytic activity than Single-IL model. Therefore, only the former is employed to investigate the reaction mechanism and to elucidate the reason resulted in the 14


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difference of various protic pyrazolium ILs. The coupling reaction of CO2 with PO catalyzed by ILs includes three steps: ring-opening of PO, CO2 insertion, and ring-closure to form PC along with catalyst regeneration (See Scheme 3). Only ring-opening of PO is considered since it is the rate-determining step. It is well known that the ring-opening of PO is completed by both electrophilic attack from cation and nucleophilic attack from anion.51 The more active the hydrogen atom in cation is, the stronger the electrophilic attack is. Consequently, the activation of PO is easier since they have the same anion indicating the same nucleophilic attack. The protic hydrogen atom, H1 atom (See Figure 1), is more active than other hydrogen atoms in cation. Therefore, it is employed to activate the O atom of PO. Correspondingly, the C atom of PO is activated by Br- anion. The other protic pyrazolium IL is taken as a bridge to connect cation and anion by hydrogen bonds, which is also beneficial to stabilize the whole system. The corresponding schematic structures for transition states are plotted in Figure 8. The confirmed transition states are similar with that catalyzed by 1,5-dimethylpyrazolium bromide (HMM5PzBr), which has been testified to be the most active form in our previous work. Although electrophilic attack would be stronger if two cations activate PO at the same time, the steric hindrance is still enhanced. Moreover, no one is taken to stabilize the anion, which is also not favorable to lower the barrier height. Therefore, it is not considered in this work. Following the same model of TS1, the reaction mechanism catalyzed by HPPzBr (TS2), HBPzBr (TS3), and HAPzBr (TS4) is determined, respectively. The corresponding potential surfaces are also plotted in Figure 8. The barrier heights decrease in the order of route 4 (HAPzBr, 9.57 kcal/mol) > route 3 (HBPzBr, 8.87 kcal/mol) > route 2 (HPPzBr, 7.91 kcal/mol) > route 1 (HEPzBr, 4.93 kcal/mol), 15



which agrees well with the sequence of experimental product yields. The interaction between HEPzBr and PO is further confirmed by the NMR measure. The 1H NMR spectra of HEPzBr with and without PO are shown in Figure 9. The chemical shift for the H attached to the C atom on the pyrazole ring is almost the same with a slight change. However, the signal at 13.18 ppm corresponding to protic H on N2 atom of pyrazole ring is greatly increased to 5.07 ppm after that HEPzBr is mixed with PO suggesting the enhancement of interaction energy.52 It is mainly attributed to the formation of hydrogen bond between protic H on N2 atom and O atom of PO, which is beneficial for the ring-opening of PO.

AIM and NBO Analysis. Not only electrophilic attack but also nucleophilic attack is the synergetic result of both electrostatic and non-covalent interaction. To deeply understand the role of non-covalent interaction played in transition states, they are analyzed by AIM (See Table 4 and NCI (See Figure 10) methods. The N1-H1…O and C1…Br1 are two most important interactions related with electrophilic and nucleophilic attack, respectively, in the catalytic central region. The C1…Br1 interaction is non-covalent interaction since its ∇2ρ value is positive. Moreover, it belongs to the classical halogen bond since both ρ (0.02135-0.02501) and ∇2ρ (0.05643-0.06316) values are in a reasonable range. In contrast, the N1-H1…O interactions are covalent interaction since they have the negative ∇2ρ values. The C1…Br1 interaction of TS1 is smaller than that of TS2. However, the N1-H1…O interaction in TS1 is stronger than that in TS2 indicating the stronger electrophilicity. Moreover, the electrostatic interactions in TS1 are stronger than that in TS2 (See 16


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Figure S3) resulting in the highest catalytic activity for HEPzBr. While HPPzBr (TS2) and HBPzBr (TS3) have the similar interactions for electrostatic interaction, C1…Br1 interaction, and N1-H1…O interaction indicating the similar catalytic activity, which is consistent with the experimental results. The product yields of PC catalyzed by HPPzBr and HBPzBr are 80.26% and 78.96%, respectively. The difference between TS2 and TS3 is too small to differentiate them. It is attributed to two factors: one is that there were some errors for both theoretical and experimental, which could not be eliminated totally; the other is that the ∇2ρ value could not be directly compared with the experimental product yield. In nutshell, the catalytic activity of HPPzBr (TS2) and HBPzBr (TS3) is very close. Neither electrostatic interaction nor non-covalent interaction for TS4 (HAPzBr) is larger than that for TS3 (HBPzBr), which is consistent with its highest barrier height (9.57 kcal/mol) and lowest product yield (60.30%).

CONCLUSION

A series of protic pyrazolium ILs are firstly synthesized and employed as catalysts for the cycloaddition reaction of CO2 with PO to produce PC without any solvent and co-catalyst. The synthetic yield of protic pyrazolium ILs is over 90% with simple purification process. HEPzBr presents the best catalytic activity with the product yield of 84.71% under the identical reaction conditions. Moreover, the generality of HEPzBr is explored. Finally, the reaction mechanism of reaction catalyzed by HEPzBr, HPPzBr, HBPzBr, and HAPzBr is elucidates by the Double-IL model. The 17



calculated sequence of barrier heights is totally consistent with the experimental result. On the basis of AIM and NCI analysis, the ring-opening of PO is the synergetic result of electrostatic interaction, C1…Br1 interaction, and N1-H1…O interaction.

ASSOCIATED CONTENT

Supporting Information Detailed 1H NMR and 13C NMRspectrum, TGA curves and Optimized geometries for the intermediates and transition states. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT

We thank the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center) for providing computational resources and softwares. This work was supported by the National Natural Science Foundation of China (21376063, 21476061, 21503069, 21676071), Key Scientific Research Project Plan of Colleges and Universities in Henan Province (18A150024).

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Figure 1. Several protic pyrazolium ionic liquids

29



1.0

indicator HEPzBr HPPzBr HBPzBr HAPzBr

0.8

0.6

Indicator

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 50

0.4

0.2

0.0

300

350

400

450

500

Wavelength (nm)

Figure 2. UV-vis absorption spectra of 4-nitroaniline in water

30


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100 90

Yield (%) and Selctivity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 70 60 50

Yield (%) Selctivity (%)

40 0.0

0.5

1.0

1.5

2.0

Weight (mol%)

Reaction condition: PO (100 mmol), pressure (2.0 MPa), time (4.0 h), temperature (130℃)

Figure 3. Effect of HEPzBr dosage on PC yield.

31



100 90


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 50

80 70 60 50 40 30


20 10

100

110

120

130

140

150

o

Temperature ( C)

Reaction condition: PO (100 mmol), HEPzBr (1.0 mol%), pressure (2.0 MPa), time (4.0 h).

Figure 4. Effect of temperature on yield of PC.

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100


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90

80

70

60

50

Yield (%) Selctivity (%) 1.0

1.5

2.0

2.5

3.0

Pressure (MPa)

Reaction condition: PO (100 mmol), HEPzBr (1.0 mol%), temperature (130℃), time (4.0 h).

Figure 5. Effect of pressure on yield of PC.

33



100 90


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 70 60 50 40


30 20

1

2

3

4

5

Time (h)

Reaction condition: PO (100 mmol), HEPzBr (1.0 mol%), temperature (130℃), pressure (2.0 MPa).

Figure 6. Effect of time on yield of PC.

34


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100

Yield

80

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40 20 0 1

2

3

4

5

Reuse Figure 7. The recycling test of HEPzBr: PO 100 mmol, HEPzBr 1.0 mol%, 130℃, CO2 pressure 2.0 MPa, time 4.0 h.

35



N E 15

2.96 2.12 0

PO+Cat+CO2

N11.09 O H1 H

C1 2.89 H 2.72 2.70 Br1 Br H 3.10 2.68 3.02 H H 1.98 H2 N2N route 1 1.10 O N1 N H1 H C1 2.81 2.70 H 2.81 2.49 Br12.85 Br H 1.91 3.14 H H2 N2 N

route 1(HEPzBr) route2(HPPzBr) route 3(HBPzBr) route 4(HAPzBr) 4.47

Relative Energy (kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

-1.07

route 3

-5.10 -5.79

-24.23

-5.91

-24.51

-6.00

Page 36 of 50

N11.10 O H1 C1 2.82 H H H 2.70 Br1 2.47 2.78 Br 2.85 H 1.90 3.15 H H2 N2 N N

route 2 1.10 O N1 N H1 C1 2.81 2.69 H H 2.83 Br12.87 2.49 Br H 1.90 3.14 H H N2 N route 4

-25.15 PC+Cat

-26.91 -30.00 CO2 insertion

ring-opening

ring-closure

Figure 8. Potential surfaces and schematic structures of transition states for the ring-opening step along routes 1-4 calculated at the M06/6-311+G(2d,2p) (PCM)//B3PW91/6-31G(d,p) level. 36


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Figure 9. The 1H NMR spectrum of IL HEPzBr with and without PO.

37


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Figure 10. NCI plots for HEPzBr, HPPzBr, HBPzBr, and HAPzBr. The corresponding 3D plots are displayed right with blue regions representing strong 39



electrostatic interactions and green regions representing more dispersive attractive interactions.

40


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Scheme 1. Synthesis of protic pyrazolium ionic liquids

41



Scheme 2. Synthesis of PC in the presence of protic pyrazolium ionic liquids

42


Page 42 of 50

Page 43 of 50

O O

O N

N

n

n

NH X

-

-

NH X

X

O

O

O O

N H N

X

H H

H

N N

X -

-

O C

N H

H n

N

O

X

O

H

n

n=0, 1, 2, 3 X=Cl-, Br-, I-

X

n

n

H H

H

-

N H N

n

O

n

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H

N N

X -

Scheme 3. The proposed reaction mechanism

43


N N


Page 44 of 50

Table 1. The effect protic pyrazolium ionic liquids on the synthesis of PCa.

a

Entry

Catalyst

Yieldb(%)

TOF (h-1)

1

HEPzBr

84.71

21.05

2

HPPzBr

80.26

20.07

3

HBPzBr

78.96

19.74

4

HAPzBr

60.30

15.08

5

HEPzCl

74.90

18.73

6

HEPzI

90.21

22.55

Reactionconditions: propylene oxide 100mmol, catalystHEPzBr 1.0 mol%, CO2 pressure 2.0 MPa,

reaction time 4.0 h and temperature 130 ℃. b

Isolated yield and the selectivity>99%.

44


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Table 2. Calculation and comparison of Hammett functions of protic pyrazolium ionic liquids (5.8×10-5mol/L) in the watera.

a

ILs

Amax

[I](%)

[IH+](%)

H0

Blank

0.901

100

0

-

HEPzBr

0.780

86.6

13.4

1.80

HPPzBr

0.782

86.8

13.2

1.81

HBPzBr

0.800

88.8

11.2

1.89

HAPzBr

0.844

93.6

6.4

2.15

H0=pK(I)+log([I]/[IH+]); indicator: 4-nitroaniline, pKA=0.99.

45



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Table 3. Coupling of CO2 and various epoxides with HEPzBra.

Entry

Epoxide

Cyclic carbonate

Selectivity (%)

Yieldb (%)

>99

93.45

>99

94.57

>99

80.38

>99

68.10

>99

88.03

1 1a

2a

2 1b 2b

3 1c

2c

4 1d

2d

5 1e 2e

46


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6

>99

74.47

1f 2f a

Reaction conditions: expoxides 100 mmol, catalyst HEPzBr 1.0 mol%, CO2 pressure

2.0 MPa, reaction time 4.0 h , temperature 140 ℃. b

Isolated yield.

Table 4. Selected topological parameters in the most stable transition state calculated at the B3PW91/6-31G(d,p) level of theory.

Transition state

X-Y…Z

Sign(λ2)ρ

ρ

∇2ρ

N1-H1…O

-0.24159

0.24159

-0.99621

C1…Br1

-0.02135

0.02135

0.05643

N1-H1…O

-0.23905

0.23905

-0.95606

C1…Br1

-0.02489

0.02489

0.06271

N1-H1…O

-0.23908

0.23908

-0.95611

C1…Br1

-0.02501

0.02501

0.06298

N1-H1…O

-0.23637

0.23637

-0.93265

TS1 (HEPzBr)

TS2 (HPPzBr)

TS3 (HBPzBr)

TS4 (HAPzBr)

47



C1…Br1

-0.02495

0.02495

48


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0.06316

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Table of Contents (TOC) Graphic

Synopsis: The cycloaddtion reaction of CO2 and PO catalyzed

by

without solvent and co-catalyst.

49


protic

pyrazolium

ILs


222x157mm (96 x 96 DPI)


Page 50 of 50

Protic Pyrazolium Ionic Liquids: An Efficient ... - ACS Publications

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