English

• 1. INTRODUCTION

  The increase of carbon dioxide (CO₂) has caused great damage to the environment^[1]. With the continuous rise of earth temperature, people have paid extensive attention to the reduction of CO₂ emission^[2] and utilization of CO₂ as a carbon source^[3]. So, various chemicals such as methanol^[4], urea, salicylic acid, formic acid, oxidant^[5], and organic carbonates^{[6, 7]} have been produced in the past decades. Among these reactions, the coupling reaction of CO₂ and epoxides leading to cyclic carbonate is one of the most promising ways for effective utilization of CO₂^[8-11].

  In recent years, ionic liquids (ILs)^[12-14] have been extensively investigated to catalyze the conversion of CO₂ to cyclic carbonate. However, the requirement for high CO₂ pressure and reaction temperature hinders the large scale application of this reaction. The efficient single component ILs for the coupling reaction of CO₂ and epoxides is still an important research topic^{[15, 16]} along with the low cost, easy preparation and other advantages. In general, the involvement of active hydrogen atom can improve the catalytic activity of ILs effectively by forming hydrogen bonds between ILs and epoxides. Protic ILs, that involved active hydrogen atom, have shown unique catalytic activity in organic reaction, and have been used to catalyze the conversion of CO₂ to cyclic carbonate^[17]. In 2013, Aoyagi et al.^[18] found that hydriodate of secondary and primary amines could effectively catalyze the reaction of CO₂ and epoxides under mild conditions such as ordinary pressure and ambient temperature, to obtain cyclic carbonates to high yields. Later, Xiao et al.^[19] prepared a series of protic imidazolium ILs as catalyst for the cycloaddition reaction of CO₂ and PO with satisfied product yield.

  Ionic liquids have been widely used in functional materials, reaction media, reaction catalysts and other fields, and the researches of crystal structure are crucial to its application^[20]. Therefore, the research on the structural characteristics of ionic liquids has gotten wide attention. However, the melting point of ionic liquids is generally low, and most ionic liquids are liquid at room temperature. It is not easy to study the structure, especially the crystal structure of ionic liquids. So, compared with the data of synthesized ionic liquids, the single crystal data of ionic liquid are very few. Gordon et al.^[21] and Roche et al.^[22] have reported the crystal structures of [C₁₂-mim][PF₆], [C₁₄-mim][PF₆] and [C₁₆-mim][PF₆] successively. Abdallah et al.^[23] determined the neat phase behavior of 28 phosphonium salts (nPmA), each with three equivalent long n-alkyl chains containing m carbon atoms and one shorter chain containing n carbon atoms or a benzyl group. In 2004, Hardacre et al.^[24] prepared the crystals of [C₁₈-mim]Cl·H₂O and [C₁₄-dmim]Cl·H₂O, and found that these long chain alkyl imidazole chloride salts are stable in the form of monohydrate chloride salt at room temperature because of the strong hydrogen bond between chloride ion and water molecule. In 2016, Wang et al.^[25] determined the crystal structure of 1,3-bis(carboxymethyl)imidazolium bis(trifluoromethylsulfonyl)imide (BCITFSI). Relatively speaking, the crystal structures of proton ionic liquids, especially pyrazole ionic liquids, are rarely reported.

  Recently, we have synthesized some pyrazolium ILs and protic pyrazolium ILs and investigated their catalytic activities for the cycloaddition of PO and CO₂^[26-29]. In this work, two protic pyrazolium ILs are synthesized and characterized by mass spectra (MS), ¹H NMR, ¹³C NMR, and single-crystal X-ray diffraction. Then, their catalytic activities for the cycloaddition of PO and CO₂ are investigated.

  2. EXPERIMENTAL

  2.1 Instruments and reagents

  Mass spectra (MS) were determined with an Agilent 1100LC-MS mass spectrometer. 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. Intensity data were collected on a Bruker Apex Ⅱ CCD detector. All chemicals and solvents are of commercial reagent grade and used without further purification.

  2.2 Synthesis of protic pyrazolium ILs

  Hydrobromic acid (24 mmol) was added dropwise to 3,5-dimethylpyrazole (20 mmol) or 1,3,5-trimethylpyrazole (20 mmol), and the mixtures were stirred for 15 h at 40 ℃ to ensure that all of the base were reacted. Then the water was removed by cyclohexane under heating condition. Finally, the residue was washed repeatedly with ethyl acetate in reflux condition and dried in vacuum to obtain protic ionic liquid, protic 3,5-dimethylpyrazolium bromide (HDMPzBr) or protic 1,3,5-trimethyl-pyrazolium bromide (HTMPzBr). While the single crystal specimens of HDMPzBr and HTMPzBr were acquired via slow evaporation of their methanol solutions at room temperature.

  HDMPzBr: white solid (yield: 94.2%), ¹H NMR (400 MHz, DMSO-d₆) δ (ppm) 6.35 (s, 1H), 2.33 (s, 6H). ¹³C NMR (101 MHz, DMSO-d₆) δ (ppm) 145.26, 106.83, 11.19. MS (ESI): m/z 97.11 [M-Br]⁺.

  HTMPzBr: white solid (yield: 96.4%) ¹H NMR (400 MHz, DMSO-d₆) δ (ppm): 6.27 (s, 1H), 3.82 (s, 3H), 2.31 (s, 3H), 2.27 (s, 3H). ¹³C NMR (101 MHz, D₂O) δ (ppm): 144.36, 137.55, 106.94, 35.53, 12.33, 10.95. MS (ESI): m/z 111.09 [M-Br]⁺.

  2.3 Cycloaddition reaction of CO₂ with epoxides catalyzed by HTMPzBr and HDMPzBr

  An epoxide/catalyst mixture was introduced in the high pressure cell (100 mL) at ambient temperature, then CO₂ (2.0 MPa) was added to the cell and heated up to the desired temperature. The reaction was stirred at 110~130 ℃ for 4 h. After this reaction, the reactor was cooled to room temperature and the pressure was released. And the product PC was obtained as a colorless liquid by distillation under a vacuum.

  2.4 Single-crystal X-ray crystallography

  Single-crystal X-ray diffraction measurements of compounds HTMPzBr and HDMPzBr were carried out on a Bruker Apex Ⅱ CCD X-ray single-crystal diffractometer. The reflection data were collected at 300 K in an ω-2θ scan mode with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) as the excitation source. The reflections of single crystal HTMPzBr were measured in a range of 3.528º~25.074º and 2.896º~25.093º for single crystal HDMPzBr; and 786 and 1375 independent reflections were measured for HTMPzBr and HDMPzBr, respectively. SADABS multi-scan empirical absorption corrections were adopted for data processing. The crystal structure was solved by direct methods and refined based on full-matrix least-squares on F². All non-hydrogen atoms were refined anisotropically using the SHELXTL program^[30]. The hydrogen atoms of organic ligands were generated geometrically for HTMPzBr and HDMPzBr. The final least-squares cycle of refinement for HTMPzBr gave R = 0.0196 and wR = 0.0497; and that for HDMPzBr gave R = 0.0374 and wR = 0.1015. The crystallographic data and structural refinements for HTMPzBr and HDMPzBr are listed in Table 1.

  Table 1

  

  Table 1.  Crystallographic Data and HTMPzBr and HDMPzBr

  DownLoad: CSV

  Item	HTMPzBr	HDMPzBr
  CCDC deposit no.	1550043	1551485
  Empirical formula	C6H11BrN2	C5H9BrN2
  Formula weight	191.08	177.05
  Temperature/K	300	300
  Radiation	MoKα (λ = 0.71073 Å)	MoKα (λ = 0.71073 Å)
  Crystal system	Orthorhombic	monoclinic
  Space group	Cmca	C2/c
  a/Å	6.7985(6)	6.994(6)
  b/Å	16.2100(16)	16.555(15)
  c/Å	14.8447(13)	13.394(10)
  α (º)	90	90
  β (º)	90	94.71(3)
  γ (º)	90	90
  Z	8	8
  Volume/Å3	1635.9(3)	1545(2)
  Density (calculated)	1.552 g/cm3	1.522 g/cm3
  F(000)	768	704
  Crystal size/mm3	0.21 × 0.15 × 0.12	0.12 × 0.15 × 0.19
  Range for data collection (º)	3.528~25.074	2.896~25.093
  Index ranges	–8≤h≤7, –19≤k≤19, –17≤l≤17	–8≤h≤8, –19≤k≤19, –15≤l≤15
  Reflections collected	7991	20070
  Independent reflections	786 (Rint = 0.0210)	1375 (Rint = 0.0536)
  Data/restraints/parameters	786/0/58	1375/0/69
  Goodness-of-fit on F2	1.085	1.041
  Final R indexes (I > 2σ(I))	R = 0.0196, wR = 0.0497	R = 0.0374, wR = 0.1015
  Final R indexes (all data)	R = 0.0225, wR = 0.0512	R = 0.0506, wR = 0.1103
  Largest diff. peak/hole (e·Å–3)	0.24, –0.38	0.556, –0.446

  3. RESULTS AND DISCUSSION

  3.1 Synthesis and characterization of protic pyrazolium ILs

  Two protic pyrazolium ILs are synthesized from alkylpyrazole and hydrobromic acid at 40 ℃. The yield of two protic pyrazolium ILs is higher than 90%. The reaction takes place under mild conditions, and the purification process is very simple. In general, the new synthesized reaction possesses some obvious advantages including simple synthetic process, low cost raw materials, easy purification, relative low reaction temperature, and high product yield. Two protic pyrazolium ILs are characterized by ¹H NMR, ¹³C NMR, and MS (ESI), indicating that the desired structure is correct. Particularly, the crystal structure data of two ionic liquids were obtained. It is clear that the proton had transferred from the acid to the base completely.

  3.2 Crystal structure analysis

  The crystallographic data, selected bond lengths, and bond angles for as-synthesized compounds HTMPzBr and HDMPzBr are listed in Tables 2 and 3, and structure perspective of HTMPzBr and HDMPzBr is in Figs. 1 and 3; cell stacking diagram is in Figs. 2 and 4, respectively. In compound HTMPzBr, the N–C distances (1.337(3) Å for N(1)–C(1) and 1.339(3) Å for N(2)–C(3)) lie between those of typical double C=N (1.287 Å) and single C–N (1.471 Å) bonds, suggesting that intraannular π conjugated systems occur; Particularly, all of the C and N atoms in HTMPzBr molecule are almost in the same plane (Fig. 1), and a similar situation exists in the HDMPzBr molecule. For HTMPzBr, there are intramolecular hydrogen bonds between Br^– and N(2)–H (Table 4, H–Br 2.36 Å, N(2)–H⋅⋅⋅Br(1) 172.0°). Unlike HTMPzBr, in HDMPzBr two N in cation are connected to H atoms, and a dimmer is thus formed through the hydrogen bond of two Br^– and H in N of different cations (Table 4), as shown in Fig. 3. Besides, there are no hydrogen bonds between the adjacent molecules of HTMPzBr and the adjacent dimmer of HDMPzBr. There are two different ways of π-π stacking in HTMPzBr and HDMPzBr, and the distance between the pyrazole ring planes of two adjacent molecules is 3.4579(3) Å in HTMPzBr, but in HDMPzBr, the data are 3.6299(3) and 3.5264(3) Å, respectively. As a result, strong π-π interactions take compounds HTMPzBr and HDMPzBr to extend into a one-dimensional columnar structure along the a axis, as shown in Figs. 2 and 4.

  Table 2

  

  Table 2.  Selected Bond Lengths (Å) and Band Angles (°) for HTMPzBr

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.
  N(1)–N(2)	1.350(3)		N(2)–C(3)	1.339(3)
  N(1)–C(1)	1.337(3)		N(1)–C(6)	1.462(3)
  C(3)–C(5)	1.490(4)		C(3)–C(2)	1.380(4)
  C(1)–C(4)	1.480(4)		C(1)–C(2)	1.387(3)
  Angle	(°)		Angle	(°)
  C(1)–C(2)–C(3)	107.4(2)		N(2)–C(3)–C(5)	122.1(2)
  N(2)–C(3)–C(2)	107.0(2)		C(2)–C(3)–C(5)	130.9(2)
  C(1)–N(1)–N(2)	108.9(2)		N(1)–C(1)–C(2)	107.2(2)
  N(2)–N(1)–C(6)	121.0(2)		N(1)–C(1)–C(4)	122.0(2)
  C(3)–N(2)–N(1)	109.4(2)		C(2)–C(1)–C(4)	130.7(2)
  C(3)–N(1)–C(6)	130.1(2)

  Table 3

  

  Table 3.  Selected Bond Lengths (Å) and Band Angles (°) for HDMPzBr

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.
  N(2)–C(4)	1.329(5)		C(2)–C(3)	1.365(6)
  C(1)–C(2)	1.497(7)		N(1)–N(2)	1.345(5)
  C(3)–C(4)	1.373(6)		C(4)–C(5)	1.492(7)
  N(1)–C(2)	1.347(5)
  Angle	(°)		Angle	(°)
  N(2)–C(4)–C(3)	107.5(4)		N(2)–C(4)–C(5)	120.4(4)
  C(2)–C(3)–C(4)	108.1(4)		C(3)–C(4)–C(5)	132.0(4)
  N(1)–C(2)–C(1)	121.2(4)		C(3)–C(2)–C(1)	132.4(4)
  C(4)–N(2)–N(1)	108.6(3)		N(1)–C(2)–C(3)	106.4(4)

  Figure 1

  

  Figure 1.  Molecular structure of HTMPzBr

  DownLoad: Full-Size Img PowerPoint

  Figure 2

  

  Figure 2.  Stacking diagram of HTMPzBr

  DownLoad: Full-Size Img PowerPoint

  Figure 3

  

  Figure 3.  Molecular structure of HDMPzBr

  DownLoad: Full-Size Img PowerPoint

  Figure 4

  

  Figure 4.  Stacking diagram of HDMPzBr

  DownLoad: Full-Size Img PowerPoint

  Table 4

  

  Table 4.  Hydrogen Bonds for HTMPzBr and HDMPzBr

  DownLoad: CSV

  	D–H⋅⋅⋅A	d(D–H)/Å	d(H⋅⋅⋅A)/Å	d(D⋅⋅⋅A)/Å	D–H⋅⋅⋅A/°
  HTMPzBr	N(2)–H(2)⋅⋅⋅ Br(1)ⅰ	0.86	2.36	3.214(2)	172.0
  HDMPzBr	N(1)–H(1)⋅⋅⋅Br(1)	0.86	2.32	3.170(4)	172.4
  	N(2)– H(2)⋅⋅⋅ Br(1)	0.86	2.37	3.206(4)	165.6
  ⅰ1/2–x, 1/2–y, –z

  3.3 Catalytic properties for CO₂ cycloaddition

  Preliminary experimental results show that HTMPzBr and HDMPzBr can catalyze the cycloaddition reaction of CO₂ effectively and show strong catalytic activity above 110 ℃ (Table 5). Relatively, the catalytic activity of HDMPzBr is better than that of HTMPzBr. In general, when the reaction condition is the catalyst dosage of 1.0 mol%, reaction temperature of 130 ℃, CO₂ pressure of 2.0 MPa and reaction time of 4.0 h, the PC yields are 83.1% and 90.4% for HTMPzBr and HDMPzBr, respectively. The experimental results showed that the differences in the structures of the two ionic liquids have important influence on their catalytic activity, which is aroused by two factors. On one side, HDMPzBr with H atom attached to the N(1) atom is twice as likely to form hydrogen bond with epoxy compound as that with -CH₃ attached to N(1) atom in HTMPzBr; On the other side, compared with one hydrogen bond between Br^– and NH in the HDMPzBr molecule, the formation of two hydrogen bonds between Br^– and NH in HDMPzBr molecule can stabilize Br^- preferably, which is more conducive to the nucleophilic attack of Br^– on epoxides. Therefore, the difference in the structures of the two ionic liquids leads to a great difference in their catalytic activity.

  Table 5

  

  Table 5.  Catalytic Properties of HTMPzBr and HDMPzBr for CO₂ Cycloaddition^a

  DownLoad: CSV

  T (℃)	HTMPzBr	HDMPzBr
  110	41.0%	49.7% (1.5 MPa, 2.0 h)
  120	73.1%	82.1% (1.5 MPa, 2.0 h)
  130a	83.1%	90.4%
  a Reaction condition : CO2 pressure of 2.0 MPa, reaction time of 4.0 h, and catalyst dosage of 1.0 mol%.

  4. CONCLUSION

  Two protic pyrazolium ILs are synthesized and their crystal structures are measured. These ILs can catalyze the cycloaddition reaction of CO₂ with PO to produce PC without any solvent and co-catalyst and show strong catalytic activity when the reaction temperature is over 110 ℃.

  
  
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• 

  Figure 1  Molecular structure of HTMPzBr

  下载: 全尺寸图片 幻灯片
  

  Figure 2  Stacking diagram of HTMPzBr

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Molecular structure of HDMPzBr

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Stacking diagram of HDMPzBr

  下载: 全尺寸图片 幻灯片

  Table 1.  Crystallographic Data and HTMPzBr and HDMPzBr

  Item	HTMPzBr	HDMPzBr
  CCDC deposit no.	1550043	1551485
  Empirical formula	C6H11BrN2	C5H9BrN2
  Formula weight	191.08	177.05
  Temperature/K	300	300
  Radiation	MoKα (λ = 0.71073 Å)	MoKα (λ = 0.71073 Å)
  Crystal system	Orthorhombic	monoclinic
  Space group	Cmca	C2/c
  a/Å	6.7985(6)	6.994(6)
  b/Å	16.2100(16)	16.555(15)
  c/Å	14.8447(13)	13.394(10)
  α (º)	90	90
  β (º)	90	94.71(3)
  γ (º)	90	90
  Z	8	8
  Volume/Å3	1635.9(3)	1545(2)
  Density (calculated)	1.552 g/cm3	1.522 g/cm3
  F(000)	768	704
  Crystal size/mm3	0.21 × 0.15 × 0.12	0.12 × 0.15 × 0.19
  Range for data collection (º)	3.528~25.074	2.896~25.093
  Index ranges	–8≤h≤7, –19≤k≤19, –17≤l≤17	–8≤h≤8, –19≤k≤19, –15≤l≤15
  Reflections collected	7991	20070
  Independent reflections	786 (Rint = 0.0210)	1375 (Rint = 0.0536)
  Data/restraints/parameters	786/0/58	1375/0/69
  Goodness-of-fit on F2	1.085	1.041
  Final R indexes (I > 2σ(I))	R = 0.0196, wR = 0.0497	R = 0.0374, wR = 0.1015
  Final R indexes (all data)	R = 0.0225, wR = 0.0512	R = 0.0506, wR = 0.1103
  Largest diff. peak/hole (e·Å–3)	0.24, –0.38	0.556, –0.446

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  Table 2.  Selected Bond Lengths (Å) and Band Angles (°) for HTMPzBr

  Bond	Dist.		Bond	Dist.
  N(1)–N(2)	1.350(3)		N(2)–C(3)	1.339(3)
  N(1)–C(1)	1.337(3)		N(1)–C(6)	1.462(3)
  C(3)–C(5)	1.490(4)		C(3)–C(2)	1.380(4)
  C(1)–C(4)	1.480(4)		C(1)–C(2)	1.387(3)
  Angle	(°)		Angle	(°)
  C(1)–C(2)–C(3)	107.4(2)		N(2)–C(3)–C(5)	122.1(2)
  N(2)–C(3)–C(2)	107.0(2)		C(2)–C(3)–C(5)	130.9(2)
  C(1)–N(1)–N(2)	108.9(2)		N(1)–C(1)–C(2)	107.2(2)
  N(2)–N(1)–C(6)	121.0(2)		N(1)–C(1)–C(4)	122.0(2)
  C(3)–N(2)–N(1)	109.4(2)		C(2)–C(1)–C(4)	130.7(2)
  C(3)–N(1)–C(6)	130.1(2)

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  Table 3.  Selected Bond Lengths (Å) and Band Angles (°) for HDMPzBr

  Bond	Dist.		Bond	Dist.
  N(2)–C(4)	1.329(5)		C(2)–C(3)	1.365(6)
  C(1)–C(2)	1.497(7)		N(1)–N(2)	1.345(5)
  C(3)–C(4)	1.373(6)		C(4)–C(5)	1.492(7)
  N(1)–C(2)	1.347(5)
  Angle	(°)		Angle	(°)
  N(2)–C(4)–C(3)	107.5(4)		N(2)–C(4)–C(5)	120.4(4)
  C(2)–C(3)–C(4)	108.1(4)		C(3)–C(4)–C(5)	132.0(4)
  N(1)–C(2)–C(1)	121.2(4)		C(3)–C(2)–C(1)	132.4(4)
  C(4)–N(2)–N(1)	108.6(3)		N(1)–C(2)–C(3)	106.4(4)

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  Table 4.  Hydrogen Bonds for HTMPzBr and HDMPzBr

  	D–H⋅⋅⋅A	d(D–H)/Å	d(H⋅⋅⋅A)/Å	d(D⋅⋅⋅A)/Å	D–H⋅⋅⋅A/°
  HTMPzBr	N(2)–H(2)⋅⋅⋅ Br(1)ⅰ	0.86	2.36	3.214(2)	172.0
  HDMPzBr	N(1)–H(1)⋅⋅⋅Br(1)	0.86	2.32	3.170(4)	172.4
  	N(2)– H(2)⋅⋅⋅ Br(1)	0.86	2.37	3.206(4)	165.6
  ⅰ1/2–x, 1/2–y, –z

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  Table 5.  Catalytic Properties of HTMPzBr and HDMPzBr for CO₂ Cycloaddition^a

  T (℃)	HTMPzBr	HDMPzBr
  110	41.0%	49.7% (1.5 MPa, 2.0 h)
  120	73.1%	82.1% (1.5 MPa, 2.0 h)
  130a	83.1%	90.4%
  a Reaction condition : CO2 pressure of 2.0 MPa, reaction time of 4.0 h, and catalyst dosage of 1.0 mol%.

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