Novel, recyclable supramolecular metal complexes for the synthesis of cyclic carbonates from epoxides and CO2 under solvent-free conditions

Journal of CO2 Utilization 17 (2017) 243–255

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Novel, recyclable supramolecular metal complexes for the synthesis of cyclic carbonates from epoxides and CO2 under solvent-free conditions Jing Penga,b , Hai-Jian Yanga,* , Yongchao Genga , Zidong Weib,* , Lihua Wanga , Cun-Yue Guoc,* a Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, Key Laboratory of Analytical Chemistry of the State Ethnic Affairs Commission, College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan, 430074, PR China b The State Key Laboratory of Power Transmission Equipment & System Security and New Technology, College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, PR China c School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing, 100049, PR China

A R T I C L E I N F O

Article history: Received 13 February 2016 Accepted 27 October 2016 Available online xxx Keywords: Chemical fixation of carbon dioxide Epoxide Cyclic carbonate Supramolecular metal complexes Solvent-free conditions

A B S T R A C T

A series of novel Zn, Cu, Fe supramolecular complexes were designed and synthesized. The structures of these compounds have been confirmed by IR, NMR, EA and X-ray crystallography. The complexes in combination with tetrabutylammonium bromide were studied as a binary catalyst system for CO2fixation in the context of organic carbonate formation under solventless condition. The effects of various co-catalysts, co-catalyst concentration, reaction temperature, CO2 pressure, reaction time, various substituents on ligand and centre metal of complexes have been investigated systematically. The binary catalyst system also showed broad substrate scope of epoxide. The catalysts can be easily recovered and reused without significant loss of activity and selectivity. Comparison of different two-component dinuclear metal complexes catalysts reported by other scientists showed that the dinuclear metal complexes reported herein were competitive with most catalysts currently available. On the basis of the experimental results, the mechanism for the reaction was proposed. © 2016 Published by Elsevier Ltd.

1. Introduction Nowadays, carbon dioxide (CO2) has become the major manmade greenhouse gas, which could lead to disaster, such as desertification of land, Arctic ice disappearing, sea level rising, and so on [1]. On the other hand, its nontoxic, nonflammable, inexpensive, and large atmospheric abundance makes it a viable alternative to other depleting substances [2]. From an environmental chemical standpoint, CO2 is an invaluable chemical resource because of its recovery property as a by-product of many industrial processes and its capability to replace toxic chemicals in various synthetic processes [3]. As a consequence, the chemical fixation of CO2 is attracting interest to reduce the concentration of CO2 in the atmosphere recently [4]. An important way of CO2 fixation is its reaction with epoxides which can be controlled to produce either cyclic carbonates [5] or

* Corresponding authors. E-mail addresses: [email protected], [email protected] (H.-J. Yang), [email protected] (Z. Wei), [email protected] (C.-Y. Guo). http://dx.doi.org/10.1016/j.jcou.2016.10.013 2212-9820/© 2016 Published by Elsevier Ltd.

polycarbonates [6]. Polycarbonates have gained considerable momentum as replacements for petrochemically derived polymers. Cyclic carbonates have numerous applications such as electrolytes for lithium ion batteries [7], polar aprotic solvents [8] and chemical intermediates for drug synthesis [9]. The cycloaddition between epoxides and CO2 is generally carried out by various homogeneous and heterogeneous catalysts, including metalloporphyrins complexes [10], metal complexes [11], metal oxides [12], molecular sieves [13], metal-organic frameworks [14], Lewis acids or bases [15], ion-exchange resins [16], nanoparticles [17], ionic liquid [18], functionalized polymers [19], biopolymer-supported catalys [20] and so on. Among these catalysts, metal complexes have been of significant interest due to their easy-to-synthetize and excellent stability against moisture and air [21]. Because of the potential advantages than mononuclear complexes as catalysts, binuclear or multinuclear complexes have attracted considerable attention among these metal complexes [22]. Supramolecular catalysts offer chemists precise spatial control over chemical transformations [23]. The allosteric supramolecular catalyst with active sites could be opened and closed by effecting

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exceptionally active catalyst system for the synthesis of cyclic carbonates from terminal epoxides under mild solvent-free conditions (Scheme 1). Furthermore, the insoluble nature of the complexes in the reactants and products made separation of catalysts much easier, which also leads to 6 times catalyst recyclability without any significant loss of reactivity. Additionally, a proposed mechanism was given. 2. Experimental section 2.1. Chemicals and analytical methods

Scheme 1. Cycloaddition of PO and CO2 to give PC and novel binuclear supramolecular Zn, Cu, Fe complexes 4a–4e and dimer zinc catalyst 4f used in this study.

chemistry at one or more distal sites within the structure. This, in turn, could modulate the catalytic activity of the complex [24]. Although many catalyst systems had been designed, synthesized and used for the coupling reaction of epoxides and CO2, there are few literature reports about the application of supramolecular catalyst reported in this area to the best of our knowledge [25]. In this work, three hexadentate ligands containing two sets of ONO donor atoms have been designed and synthesized. These ligands were reacted with metal salts to prepare six new dinuclear metal supramolecular complexes in moderate yields. The catalytic performance of the supramolecular catalysts has been systematically investigated for the coupling reaction of epoxides and CO2, the results indicated that the combination of metal complex with two bimetallic cores and tetrabutylammonium bromide formed an

The detailed information of the materials used in present work is listed in Table 1. The epoxides were distilled from CaH2 before use. NMR spectra was recorded on a Bruker Al–400 MHz instrument using trimethylsilane (TMS) as an internal standard. IR spectra was recorded on a Perkin-Elmer 2000 FT-IR spectrometer. Elemental analysis was conducted on a PE 2400 series II CHNSO elemental analyser. Melting point was obtained from X-4-type digital micromelting point apparatus. X-ray diffraction studies were performed on a Bruker-APEX diffractometer equipped with a CCD area detector, MoKa-radiation (l= 0.71073 Å), and a graphite monochromator. All spectra were recorded at room temperature. All the known compounds were identified by comparison of their physical and spectral data with those in previous reports. 2.2. Synthesis of ligands and complexes The synthesis procedure of ligands and complexes was shown in Scheme 2. for comparision, a one-active zinc centre conplex 4f and its coresponding ligand 3f were also designed and synthesized as shown in Scheme 2.

Table 1 Provenance and mass fraction purity of the materials. Materials

Mass fraction purity

CAS

Provenance

2-Amino-1-hydroxybenzene 2-Amino-4-t-butylphenol 2-Amino-4-nitrophenol Propylene oxide 1,2-Epoxyethylbenzene Epichlorohydrin Carbon dioxide Nitrogen Salicylaldehyde 1,2-Epoxyhexane Glycidyl isopropyl ether Isobutylene oxide Calcium hydride Chloroform-d Tetra-n-butylammonium briomide Tetra-n-butylammonium iodide Tetra-n-butylammonium chloride Potassium iodide Dimethyl sulfoxide-d6 Pyridine-d5 Paraformaldehyde Pyridine Ethyl ether Methanol Acetone Tetrahydrofuran Ethanol Petroleumether Ethyl acetate Dichloromethane

0.98 0.98 0.98 0.99 0.99 0.99 0.9999 0.9999 0.99 0.99 0.99 0.97 0.97 99.8atom%D 0.99 0.99 0.99 0.985 0.9999 99.5atom%D 0.94 A. R. grade A. R. grade A. R. grade A. R. grade A. R. grade A. R. grade A. R. grade A. R. grade A. R. grade

95-55-6 1199-46-8 99-57-0 16088-62-3 96-09-3 106-89-8 124-38-9 7727-37-9 90-02-8 1436-34-6 4016-14-2 558-30-5 7789-78-8 865-49-6 1643-19-2 311-28-4 1112-67-0 7681-11-0 2206-27-1 7291-22-7 30525-89-4 110-86-1 60-29-7 67-56-1 67-64-1 109-99-0 64-17-5 64742-49-0 141-78-6 75-09-2

Shanghai Darui Finechemical Co., Ltd Shanghai Darui Finechemical Co., Ltd Shanghai Darui Finechemical Co., Ltd Shanghai Darui Finechemical Co., Ltd Shanghai Darui Finechemical Co., Ltd Shanghai Darui Finechemical Co., Ltd Sichuan Tianyi Science & Technology Co., Ltd Sichuan Tianyi Science & Technology Co., Ltd J&K Chemica Co J&K Chemica Co J&K Chemica Co J&K Chemica Co J&K Chemica Co J&K Chemica Co Tianjin Bodi Chemical Engineering Co., Ltd Tianjin Bodi Chemical Engineering Co., Ltd Tianjin Bodi Chemical Engineering Co., Ltd Tianjin Standard Science & Technology Co., Ltd Tianjin Standard Science & Technology Co., Ltd Deuterium Laboratory Peking University Dabei Sinopharm Chemical Reagent Co., Ltd Sinopharm Chemical Reagent Co., Ltd Sinopharm Chemical Reagent Co., Ltd Sinopharm Chemical Reagent Co., Ltd Sinopharm Chemical Reagent Co., Ltd Sinopharm Chemical Reagent Co., Ltd Sinopharm Chemical Reagent Co., Ltd Sinopharm Chemical Reagent Co., Ltd Sinopharm Chemical Reagent Co., Ltd Sinopharm Chemical Reagent Co., Ltd Sinopharm Chemical Reagent Co., Ltd

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Scheme 2. Synthesis of ligands 3a-3f and complexes 4a–4f.

2.3. Synthesis of compound 2 (5,50 -methylene-bis-salicylaldehyde) [26] To a solution of 23 mL (0.218 mol) of salicylaldehyde and 2.40 g (0.08 mol) of paraformaldehyde in 17 mL of glacial acetic acid, a mixture of 0.2 mL of concentrated sulfuric acid and 0.8 mL of glacial acetic acid was added slowly with magnetic stirring in a nitrogen atmosphere at 90  C. After being stirred for 22 h at 90  C, the reaction mixture was then poured into 500 mL of ice-water and allowed to stand overnight. The deposited solid was filtered and washed twice with 100 mL of diethyl ether. The solvent was then removed under vacuum. Recrystallization from 150 mL of acetone and dichloromethane (cooling overnight) furnished 9.30 g of pure 5,50 -methylene-bis-salicylaldehyde (45.4% yield), m.p. 141–142  C. 1 H NMR (400 MHz, DMSO) d 10.51 (d, J = 48.1 Hz, 2H), 10.22 (s, 2H), 7.67 (s, 2H), 7.44 (d, J = 28.1 Hz, 1H), 7.38 (d, J = 8.2 Hz, 1H), 6.94 (d, J = 8.4 Hz, 2H), 3.85 (s, 2H). Anal. Calcd. for C15H12O4: C, 70.31; H, 4.69%. Found: C, 70.34; H, 4.64%. 2.4. General procedure for the preparation of the hexadentate ligands 3a-3c [27] A mixture of one equivalent of 5,50 -methylenebis(salicylaldehyde) and 2.5 equivalents of the corresponding -aminophenol was dissolved in ethanol. The solution was refluxed for 7 h under N2 protection, then the mixture was filtered and the solvent was removed under vacuum. The residue was washed with diethyl ether (30 mL) to give compounds 3a-3c as red solids, which were used for the subsequent reaction without further purification. Ligand 3a was prepared from 2.00 g (7.8 mmol) of 5,50 methylenebis(salicylaldehyde) and 2.13 g (19.5 mmol) of 2-amino-1-hydroxybenzene. A red solid was obtained in a yield of 86% (2.98 g). M.p. 228–230  C. IR (KBr) y: 3405, 3048, 2896, 2557, 1631 (C¼N), 1590, 1526, 1492, 1458, 1369, 1304, 1273, 1244, 1138, 823, 797, 746 cm1. 1H NMR (400 MHz, DMSO-d6) d 13.60 (s, 2H), 9.71 (s, 2H), 8.93 (s, 2H), 7.47 (s, 2H), 7.35 (d, J = 7.6 Hz, 2H), 7.27 (d, J = 8.6 Hz, 2H), 7.12 (t, J = 7.9 Hz, 2H), 6.99–6.82 (m, 6H), 3.89 (d, J = 10.8 Hz, 2H). 13C NMR (101 MHz, DMSO) d 161.04, 159.46, 152.48, 142.10, 135.43, 133.19, 131.27, 129.85, 124.69, 123.65, 120.12, 118.39,

117.38, 45.98. Anal. Calcd. for C27H22N2O4: C, 73.95; H, 5.05; N, 6.38%. Found: C, 73.96; H, 5.08; N, 6.33%. Ligand 3b was prepared from 2.00 g (7.8 mmol) of 5,50 methylenebis(salicylaldehyde) and 2.58 g (19.5 mmol) of 2-amino-4-t-butylphenol. A red solid was obtained in a yield of 84% (3.63 g). M.p. 248–250  C. IR (KBr) y: 3448, 2959, 2373, 1627 (C¼N), 1516, 1494, 1363, 1278, 1243, 1148, 817 cm1. 1H NMR (400 MHz, DMSO-d6) d 13.71 (s, 2H), 9.48 (s, 2H), 8.96 (s, 2H), 7.50 (s, 2H), 7.36–7.23 (m, 4H), 7.13 (m, 2H), 6.88 (m, 4H), 3.90 (s, 2H), 1.28 (s, 18H). 13C NMR (101 MHz, DMSO) d 160.21, 158.37, 148.69, 144.38, 140.95, 135.20, 132.86, 130.39, 125.11, 121.38, 120.04, 116.72, 117.16, 45.65, 41.48, 32.56. Anal. Calcd. for C35H38N2O4: C, 76.33; H, 6.95; N, 5.08%. Found: C, 76.37; H, 6.95; N, 5.07%. Ligand 3c was prepared from 2.00 g (7.8 mmol) of 5,50 methylenebis(salicylaldehyde) and 3.01 g (19.5 mmol) of 2-amino-4-nitrophenol. A red solid was obtained in a yield of 93% (3.83 g). M.p. 267–269  C. IR (KBr) y: 3442, 2976, 1628 (C¼N), 1589, 1519, 1492, 1340, 1293, 1133, 892, 831 cm1. 1H NMR (400 MHz, DMSO-d6) d 13.02 (s, 2H), 11.35 (s, 2H), 9.06 (d, J = 5.8 Hz, 2H), 8.26 (d, J = 2.7 Hz, 2H), 8.07 (m, 2H), 7.58–7.26 (m, 4H), 7.12 (d, J = 9.0 Hz, 2H), 6.94 (t, J = 8.9 Hz, 2H), 3.90 (d, J = 14.9 Hz, 2H). 13C NMR (101 MHz, DMSO) d 192.18, 164.54, 158.14, 140.68, 138.00, 134.48, 132.04, 129.15, 122.45, 119.63, 117.88, 117.28, 116.70, 40.53. Anal. Calcd. for C27H20N4O8: C, 61.36; H, 3.79; N, 10.61%. Found: C, 61.37; H, 3.82; N, 10.62%. 2.5. Synthesis of ligand 3f A mixture of 2.00 g (16.4 mmol) of salicylaldehyde and 2.15 g (19.7 mmol) of 2-amino-1-hydroxybenzene was dissolved in ethanol. The solution was refluxed for 7 h, then the mixture was filtered and the solvent was removed under vacuum. A red solid was obtained in a yield of 74% (2.60 g). M.p. 191–193  C. IR (KBr) y: 3445, 3049, 2700, 1632 (C¼N), 1531, 1463, 1308, 1276, 1225, 1140, 743 cm1. 1H NMR (400 MHz, DMSO-d6) d 13.81 (s, 1H), 9.76 (s, 1H), 8.97 (s, 1H), 7.61 (m, 1H), 7.43–7.33 (m, 2H), 7.18–7.09 (m, 1H), 7.01– 6.84 (m, 4H). 13C NMR (101 MHz, DMSO) d 40.52, 117.26, 119.17, 120.01, 120.18, 128.54, 128.58, 132.79, 133.27, 135.52, 151.72, 161.43,

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191.97. Anal. Calcd. for C13H11NO2: C, 73.24; H, 5.16; N, 6.57%. Found: C, 73.27; H, 5.13; N, 6.55%. 2.6. General procedure for the preparation of the complexes 4a-4c [28] To a solution of one equivalent of the ligand 3 in 50 mL of methanol, 4.0 equivalents of the zinc acetate dehydrate (Zn (CH3COO)22H2O) were added in batches. The solution was

refluxed for 2 h, then the mixture was filtered, the residue was washed with dichloromethane, acetone, methyl alcohol, tetrahydrofuran and water in sequence to give solid complexes 4. Complex 4a was prepared from 2.00 g (5.2 mmol) of ligand 3a and 4.57 g (20.8 mmol) of zinc acetate dehydrate. A yellow solid was obtained in a yield of 89% (2.30 g). M.p. >320  C. IR (KBr) y: 3394, 1616 (C¼N), 1589, 1541, 1476, 1384, 1291, 1165, 1048, 747 cm1. 1H NMR (400 MHz, Pyridine-d5) d 10.52 (s, 1H), 10.22

Fig. 1. Molecular structures for complexes 4a, 4d and 4f.

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(s, 2H), 9.18–9.11 (m, 1H), 9.00–8.77 (m, 5H), 8.69 (s, 4H), 6.44 (s, 4H), 1.49 (s, 1H). 13C NMR (101 MHz, Pyr) d 172.47, 158.74, 150.88, 137.04, 136.40, 135.94, 130.36, 128.28, 124.23, 121.92, 121.15, 115.94, 114.26, 41.71. Anal. Calcd. for C27H18N2O4Zn2: C, 57.37; H, 3.21; N, 4.96%. Found: C, 57.35; H, 3.24; N, 4.93%. Complex 4b was prepared from 2.30 g (5.2 mmol) of ligand 3b and 4.57 g (20.8 mmol) of zinc acetate dehydrate. A yellow solid was obtained in a yield of 86% (2.54 g). M.p. >320  C. IR (KBr) y: 2961, 1616 (C¼N), 1535, 1486, 1380, 1276, 1158, 1048, 828, 511 cm1. 1 H NMR (400 MHz, Pyridine-d5) d 10.71 (d, J = 14.4 Hz, 2H), 9.39– 9.31 (m, 2H), 9.06 (d, J = 2.3 Hz, 6H), 9.02–8.77 (m, 4H), 5.45–5.35 (m, 2H), 2.89–2.74 (m, 18H). 13C NMR (101 MHz, Pyr) d 172.32, 158.21, 151.41, 150.87, 135.67, 133.72, 127.55, 125.27, 122.01, 119.21, 118.68, 118.34, 112.40, 41.86, 35.56, 32.90. Anal. Calcd. for C35H34N2O4Zn22H2O: C, 58.92; H, 4.80; N, 3.93%. Found: C, 58.19; H, 4.83; N, 3.90%. Complex 4c was prepared from 2.75 g (5.2 mmol) of ligand 3c and 4.57 g (20.8 mmol) of zinc acetate dehydrate. A yellow solid was obtained in a yield of 92% (3.13 g). M.p. >320  C. IR (KBr) y: 2930, 1616 (C¼N), 1543, 1481, 1387, 1292, 1155, 1089, 834, 645 cm1. 1H NMR (400 MHz, Pyridine-d5) d 10.56 (s, 2H), 9.85 (d, J = 9.0 Hz, 2H), 8.91 (s, 2H), 8.68 (d, J = 12.2 Hz, 8H), 5.51 (s, 2H). 13 C NMR (101 MHz, Pyr) d 173.36, 161.94, 151.14, 150.59, 137.30, 135.93, 128.66, 125.41, 124.78, 124.24, 121.29, 119.85, 112.88, 41.59. Anal. Calcd. for C27H16N4O8Zn2: C, 49.49; H, 2.46; N, 8.55%. Found: C, 49.47; H, 2.49; N, 8.56%. 2.7. General procedure for the preparation of the complexes 4d and 4e To a solution of one equivalent of the ligand 3a in 50 mL of methanol, 4.0 equivalents of the corresponding metal salt were added. The solution was refluxed for 2 h, then the mixture was filtered, the residue was washed with dichloromethane, acetone, methyl alcohol, tetrahydrofuran and water in sequence to give corresponding complexes. Complex 4d was prepared from 2.00 g (5.2 mmol) of ligand 3a and 4.15 g (20.8 mmol) of copper(II) acetate monohydrate (Cu (CH3COO)2H2O). A dark green solid was obtained in a yield of 89%

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(2.60 g). M.p. >320  C. IR (KBr) y: 3059, 1613 (C¼N), 1530, 1475, 1374, 1296, 1155, 830, 743, 627 cm1. However, a resolvable NMR spectrum could not be measured owing to paramagnetism [29]. Anal. Calcd. for C27H18O4N2Cu2: C, 57.75; H, 3.23; N, 4.99%. Found: C, 57.72; H, 3.24; N, 4.95%. Complex 4e was prepared from 2.00 g (5.2 mmol) of ligand 3a and 5.62 g (20.8 mmol) of iron(III) chloride hexahydrate (FeCl36H2O). A black solid was obtained in a yield of 19% (0.61 g). M.p. >320  C. IR (KBr) y: 2366, 1611 (C¼N), 1535, 1472, 1373, 1289, 1134, 839, 748, 612 cm1. However, a resolvable NMR spectrum could not be measured owing to paramagnetism [29]. Anal. Calcd. for C27H18N2O4Cl2Fe2: C, 52.56; H, 2.94; N, 4.54%. Found: C, 52.55; H, 2.93; N, 4.56%. 2.8. Synthesis of complex 4f To a 50 mL methanol solution of 1.11 g (5.2 mmol) ligand 4f, 2.81 g (10.4 mmol) of the zinc acetate dehydrate were added in batches. The solution was refluxed for 2 h, then the mixture was filtered, the residue was washed with dichloromethane, acetone, methyl alcohol, tetrahydrofuran and water in sequence to give a yellow solid with 91% yield (0.65 g). M.p. >320  C. IR (KBr) y: 3445, 1613 (C¼N), 1544, 1472, 1444, 1391, 1295, 1154, 1125, 751 cm1. 1H NMR (400 MHz, Pyridine-d5) d 10.55 (s, 1H), 9.15 (m, 1H), 9.01–8.78 (m, 5H), 8.19 (m, 2H). 13C NMR (101 MHz, Pyr) d 173.889, 166.11, 158.60, 151.68, 137.06, 136.57, 134.64, 130.51, 125.28, 124.79, 122.61, 115.96, 114.25. Anal. Calcd. for C13H9NO2Zn: C, 56.44; H, 3.28; N, 5.06%. Found: C, 56.44; H, 3.27; N, 5.07%. 2.9. Cycloaddtion procedure A typical procedure for the coupling reaction of CO2 and epoxide was as following: A stainless steel autoclave (250 mL) was linked to CO2 cylinders. A prescribed amount of epoxide was added with a hypodermic syringe. The catalysts were successively charged into the reactor without using any additional solvent. The reactor vessel was sealed and immersed into an oil bath at the desired temperature under stirring. Then, the CO2 was pressurized

Table 2 Crystallographic data for compounds 4a, 4d and 4f. Identification code

4a

4d

4f

Empirical formula Formula weight (g mol1) Crystal size (mm3) Crystal system Space group

C52H45N7O5Zn2 978.69 0.13 x 0.12  0.10 Monoclinic P21/c

C47H38Cu2N6O4 877.91 0.10  0.05  0.03 Monoclinic P21/c

C46H38N6O4Zn2 869.56 0.13  0.12  0.10 Triclinic P1

Unit cell dimensions a (Å) b (Å) c(Å) að Þ bð Þ g ð Þ

24.117(2) 10.5010(11) 19.2996(19) 90 90 110.243(2) 90

24.298(8) 10.347(3) 19.197(6) 90 90 108.575(6) 90

9.4564(13) 10.0300(14) 11.8005(16) 112.481(2) 112.481(2) 101.031(2) 94.373(2)

4585.8(8) 4 1.418 1.103

4575(3) 4 1.275 0.977

1001.2(2) 1 1.442 1.251

36849 9943(0.0550) 583 0.0495 0.1437 1024

8055 8055(0.0000) 532 0.0637 0.1529 0.889

7972 4206(0.0166) 262 0.0346 0.1319 1.128

Volume (Å3) Z DCalc(g/cm3)

s

ðmm  1Þ

Collected refl. Independent refl. (Rint) Parameters R1[/> 2s (/)] wR2 (all data) GOF

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into the reactor to the given pressure and the reaction started. After the given time, the reaction was stopped and cooled to ambient temperature, the result mixture was transferred to a 50 mL round bottom flask. The unreacted propylene oxide was removed in vacuo. The product was obtained as a pale yellow liquid. All the cyclic carbonates were identified by GC/MS (HP6890/5973) and 400 MHz NMR. 3. Results and discussion 3.1. X-ray crystallographic studies It was worth noting that all of these complexes were insoluble in common organic solvents, such as water, alcohol, dimethyl formamide, dimethylsulfoxide, and so on. On the basis of literatures [30], this kind of insoluble property may be attributed to the formation of supramolecular layered structure, which could be destroyed by pyridine. That is to say, pyridine molecule could insert into the supramolecular structure to break the spatial network structure because of its stronger nucleophilicity to the metal centre. The solubility of complexes in pyridine confirmed the deduction. The compounds 4a and 4d could be crystallized by slow evaporation of concentrate pyridine, and their structures were determined by single crystal X-ray diffraction analyses (Fig. 1). The most relevant crystallographic data are summarized in Table 2, and the selected bond distances and angles are summarized in Table 3. Both 4a and 4d systems crystallized in the monoclinic space group P21/c and only small differences in their molecular structures were observed in Table 3, which indicated that the two complexes were in the same coordination structure. Viewing the crystal structures of 4a and 4d, each of the two metal atoms (zinc, copper) has a coordination number of five, resulting from the bonding to one nitrogen and two oxygen atoms from the ligand,

and two nitrogen atoms from the solvent pyridine molecules. As observed, the oxygen atoms are in axial position; the nitrogen atom forms a coordination bond with the zinc/copper atom and it is found in equatorial position, as well as the two nitrogen atoms from the solvent pyridine molecules. It is interesting to be found that most of the molecules of 4a and 4d do not possess ideal or approximate mirror or C2 symmetry, as it might have been expected from the spectroscopic data. Apparently, the flexibility present in the  CH2  moiety of the systems allows the complexes to adopt different orientations when relating the two diorganometal moieties [28,31]. In consequence, the angles between the planes of the two salicylidene rings attached to the bridging methylene group have value of 113.8 (Table 3). To understand the supramolecular structure, an analogue single zinc centre complex 4f was synthesized (Scheme 2) and its structure was also determined by X-ray diffraction analysis and shown in Fig. 1. Unlike 4a and 4d, the molecule of 4f is located at a crystallographic mirror plane (Table 3). Complex 4f formed a dimeric structure, in which one of the oxygen atoms of each ligand unit forms a bridge between two zinc centres. From the molecular structure in Fig. 1 and crystallographic details in Table 2, pyridine was found as axial ligand bound to the zinc metal, which clearly demonstrates that axial ligand pyridine is located in relatively more accessible voids inside the crystal and hence no significant steric repulsion is present [32]. Thus, the possible supramolecular structures of 4a–4e could be deducted as shown in Scheme 3: the supramolecular layered structure was formed by the weak interaction between metal atom and oxygen atom (the red dotted line in Scheme 3). In Fig. 1, the pyridine molecules were found as axial ligand bound to each zinc centre of 4f, which clearly demonstrated that the addition of pyridine allowed the isolation of the supramolecular complex of which the pyridine ligands can be irreversibly decomplexed by addition of strong N-donor ligand.

Table 3 Selected bond distances (Å) and angles ( ) for compounds 4a, 4d and 4f. 4a Bond distances(Å) Zn1-N1/Zn2-N2 Zn1-O1/Zn2-O4 Zn1-O2/Zn2-O3 N1-C7/N2-C21 Bond angles( ) O1-Zn1-N1/O4-Zn2-N2 O1-Zn1-O2/O3-Zn2-O4 O2-Zn1-N1/O3-Zn2-N2 C12-C14-C15

2.072(3)/2.090(3) 2.054(3)/2.012(2) 1.989(3)/1.979(2) 1.295(4)/1.290(4)

1.956(4)/1.953(4) 1.925(3)/1.950(4) 1.901(3)/1.907(3) 1.275(6)/1.289(6)

80.04(11)/80.62(10) 161.22(12)/155.47(12) 89.23(11)/88.55(10) 113.8(3)

Bond angles( ) O1-Cu1-N1/O4-Cu2-N2 O1-Cu1-O2/O3-Cu2-O4 O2-Cu1-N1/O3-Cu2-N2 C12-C14-C15

83.98(15)/83.86(17) 161.22(12)/155.47(12) 93.10(15)/93.11(16) 113.8(4)

4fa Bonddistances(Å) Zn1  N1a=Zn1aN1

2.1116(18)

Zn1  O2a=Zn1aO2

1.9798(19)

Bondangles( ) O2aZn1  N1a=O2  Zn1aN1

a

2.061(2)

Zn1  O1a=Zn1aO1 N1  C7=N1aC25

4d Bond distances(Å) Cu1-N1/Cu2-N2 Cu1-O1/Cu2-O4 Cu1-O2/Cu2-O3 N1-C7/N2-C21

1.292(3)

91.85(8)

O2aZn1  O1a=O2  Zn1aO1

1163.31(9)

O1aZn1  N1a=O1  Zn1aN1

78.50(7)

Zn1  O1Zna=Zn1aO1aZn1

n1102.27(8)

O2aZn1  O1=O2Zn1aO1a

100.11(8)

O1aZn1  O1=O1  Zn1aO1a

77.73(8)

Only one value is described due to the mirror symmetry of the molecule.

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249

Scheme 3. Schematic view of the higher-order supramolecular structures 4a–4e, and the disaggregation of the supramolecular after adding proper amount of pyridine.

3.2. Catalytic performances of 4a with various additives

3.3. The effect of the cocatalyst amount

The coupling of CO2 and epoxides catalyzed by complexes 4a was investigated systematically under different reaction conditions. As expected, the addition of moderate amounts of cocatalyst could greatly improve the productivity of propylene carbonate (PC) and TOF value (entries of 1–7 in Table 4), especially for the quaternary ammonium salts. The results from the entries 7–9 in Table 4 also indicated that the quaternary ammonium salts with Br were better than those with Cl and I for improving the yield and TOF value. It is seemed that Br has a comprehensive strong leaving and nucleophilic abilities, which are favorable for propylene oxide (PO) activation [33].

The effect of the cocatalyst tetrabutylammonium bromide (TBAB) amount on the catalytic performance in the cycloaddition of CO2 to PO has been summarized in Fig. 2. The increase in TBAB loading led to the growth of the reaction rate at first, however, a large excess of TBAB led to a slight decrease in catalyst activity. Therefore, in the absence of TBAB cocatalyst, a molar ratio of TBAB/ 4a = 1 was found to be optimal.

Table 4 Synthesis of PC from PO and CO2 catalyzed by 4a and various cocatalysts.a

Entry

Catalyst

Cocatalyst

Yield (%)b

TOF (h1)c

1 2 3 4 5 6 7 8 9

4a – 4a – 4a – 4a 4a 4a

– DMAPd DMAP KIe KI TBABf TBAB TBACg TBAIh

1.2 2.2 29.4 9.2 70.3 25.4 95.4 61.3 85.1

2 4 59 18 141 51 191 123 170

a Catalyst (0.214 mmol), cocatalyst (0.214 mmol), PO (15 mL, 0.214 mol), CO2 (5 MPa), 5 h, 130  C, the selectivity to products are all > 99%. b Isolated yields. c Moles of PC produced per mole of catalyst per hour. d DMAP = 4-dimethylaminopyridine. e KI = potassium iodide. f TBAB = tetrabutylammonium bromide. g TBAC = tetrabutylammonium chloride. h TBAI = tetrabutylammonium iodide.

3.4. The effect of reaction temperature Fig. 3 showed the effect of reaction temperature on 4a/TBABcatalyzed synthesis of PC at 5 MPa, 5 h and a 4a/TBAB molar ratio of 1. With the increase of reaction temperature from 70  C to 130  C, the PC yield increased significantly. As a result, the TOF value also increased rapidly. It is also interesting to note that no by-product polyether was detected even at very low temperature for this

Fig. 2. Activity of 4a/TBAB as a function of TBAB concentration in the coupling reaction of CO2 and PO.

250

J. Peng et al. / Journal of CO2 Utilization 17 (2017) 243–255

Fig. 3. The effect of reaction temperature on PC yield and TOF value.

catalytic reaction, which indicated that the catalyst 4a had a good selectivity for the production of PC [34]. 3.5. The effect of reaction pressure The influence of CO2 pressure on the cycloaddition was also examined, the results were presented in Fig. 4. As in the case of the epoxide/CO2 coupling chemistry [35], the reaction yield and TOF value increased as a function of increasing pressure up to a certain point and then slowly dropped off. It could be speculated that at high CO2 pressures, the amount of available TBAB for catalysis is reduced due to its reaction with CO2 to form a zwitterionic complex that is not active as a cocatalyst [36]. 3.6. The effect of reaction time The dependence of the PC yield on reaction time at 130  C and 5 MPa was shown in Fig. 5. The yield of PC increased smoothly with the reaction time, but the TOF value decreased with reaction time. It illustrated that the reaction rate was faster in the initial stage and remained almost invariant after 5 h, which indicated that nearly all the PO could be converted within 5 h.

Fig. 5. The effect of reaction time on PC yield and TOF value.

3.7. Activities of various catalysts for the cycloadditon reaction of CO2 and PO To evaluate the synthetic catalysts described here, the activities of various catalysts 4a-4f for the cycloaddition of CO2 to PO were examined under the same reaction conditions (130  C, 5 MPa, 3 h). Among 4a-4c (Table 5, entries 1–3), the zinc complex 4c with nitro group exhibited high catalytic activity (92.8% PC yield), while the activity of the zinc complex 4b with tertiary butyl group was much lower. It is suggested that the stronger electron-withdrawing NO2 group could induce a more electropositive metal centre suitable for coordination to PO [37]. It also can be observed that all the three metal (Zn, Cu, Fe) complexes showed very low catalytic activities without cocatalyst, even prolonged the reaction time (Table 5, entries 4–6 and 9). Comparing with the zinc complex 4a, the iron complex 4e exhibited higher catalytic activity (Table 5, entry 8), while copper complex 4d was slightly less active (Table 5, entry 7). The results suggested that the activity of the catalyst largely varied with the change of metal centre in the substituent, which results in change in the bond lengths, bond angles and the stereochemical arrangements [38]. In addition, the higher activity of iron complex

Table 5 Synthesis of PC from PO and CO2 catalyzed by various catalysts.a

Fig. 4. The effect of CO2 pressure on PC yield and TOF value.

Entry

Catalyst

Cocatalyst

t

Yield (%)b

TOF (h1)c

1 2 3 4 5 6 7 8 9 10

4a 4b 4c 4a 4d 4e 4d 4e 4f 4f

TBAB TBAB TBAB – – – TBAB TBAB – TBAB

3 3 3 20 20 20 3 3 20 3

85.1 83.8 92.8 21.4 3.3 24.7 48.6 96.8 1.8 60.2

284 279 309 43 6 49 162 323 1 201

a Catalyst (0.214 mmol), TBAB (0.214 mmol), PO (15 mL, 0.214 mol), CO2 (5 MPa), 3 h, 130  C, the selectivity to products are all >99%. b Isolated yields. c Moles of propylene carbonate produced per mole of catalyst per hour.

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251

Table 6 Cycloaddition between CO2 and various epoxides catalyzed by 4c in the presence of TBAB.a Yield (%)b

TOF (h1)c

1

86.2

172

2

60.8

122

3

53.7

107

4

48.4

97

5

21.5

43

6

14.1

28

Entry

Epoxide

Product

Fig. 6. Catalyst recycling for cycloaddition reaction.

4e may be attributed that the axial chlorides of 4e are be benefit to the rate-determined step in the coupling reaction, which could activate and ring-open epoxides [39], but 4e was not be used in follow-up experiments because of its low yield of synthesis (19%). Thus, as a typical sample, the supramolecular dinuclear zinc catalyst 4a had been used for the recycled runs. Comparing with the mononuclear complex 4d, the dinuclear zinc supramolecular complex 4a was more active (Table 5, entries 1, 10). Besides, the heterogeneous dinuclear complex 4a could be reused, which has significant application value and development potentials in industry.

a Catalyst 4c (0.214 mmol), TBAB (0.214 mmol), epoxide (0.214 mol), CO2 (5 MPa), 3 h, 130  C, the selectivity to products are all >99%. b Isolated yields. c Moles of propylene carbonate produced per mole of catalyst per hour.

times without dramatic activity loss, while the selectivity of the product remained same (>99%). The results also indicated the high activity and stability of the developed catalysts. More interesting, the PC yield even slightly increased during the catalytic recycling of 4a/TBAB, which probably due to the binding of tertiary amines of TBAB to the zinc compounds during the reaction [40].

3.8. Catalyst recovery and reuse 3.9. Substrate scope A series of repeated reactions was also carried out to investigate the constancy of the catalyst activity. After the completion of the reaction, the catalyst was filtered, washed with ethanol, dried under vacuum, and reused for the next time. The results in Fig. 6 showed that the catalyst 4a could be reusable for at least up to 6

The scope of the substrates was further explored as shown in Table 6. Several terminal epoxides were chosen to be tested under the optimized reaction conditions. The catalyst (4c) was found to be applicable to a variety of terminal epoxides, yielding the

Table 7 Comparison of dcoifferent two-component dinuclear metal complexe catalysts for cycloaddition of CO2 to PO. Comparison of different two-component dinuclear metal complexe catalysts for cycloaddition of CO2 to PO.Entry

Cocatalyst Solvent Cat:Cocat:PO

P T t Yield (MPa) ( C) (h) (%)

Reference

1

TBABa

–

1:1:1000

5

130

5

95.4

This work

2

TBAB

–

1:1:1000

5

130

3

92.8

This work

3

TBAIb

–

0.1:5:1000

1

85

2

86

[22c]

Catalyst

252

J. Peng et al. / Journal of CO2 Utilization 17 (2017) 243–255

Table 7 (Continued) Comparison of different two-component dinuclear metal complexe catalysts for cycloaddition of CO2 to PO.Entry

Catalyst

Cocatalyst Solvent Cat:Cocat:PO

P T t Yield (MPa) ( C) (h) (%)

Reference

4

TBAB

–

1:1:1000

5

130

3

48.6

This work

5

TBAB

–

10:50:1000

0.4

35

24

90

[25d]

6

TBAB

–

10:50:1000

0.4

60

10

95

[25d]

7

TBAB

–

1:1:1000

5

130

5

96.8

This work

8

TBAB

–

0.25:2.5:1000 2

120

1

95.2

[11b]

9

TBAB

MEKc

5:50:1000

0.2

25

18

65

[11e]

10

TBAI

MEK

5:50:1000

0.2

25

18

74

[11e]

11

PPNCld

–

5:10:1000

0.1

25

48

91

[6f]

a b c d

TBAB = tetrabutylammonium bromide. TBAI = tetrabutylammonium iodide. MEK = methyl ethyl ketone. PPNCl = bis(triphenylphosphine)iminium chloride.

corresponding cyclic carbonates with more than 99% selectivity. Among the mono-substituted terminal epoxides, it was found that with the growth of the substituent chain, the activity sequence of epoxides was propylene oxide > epichlorohydrin > 1,2-epoxyhexane. On the other hand, glycidyl phenyl ether and styrene oxide were reasonable substrates to give cyclic carbonates in good yield. However, in the case of isobutylene oxide and cyclohexene oxide, the reactions were carried out with relative low yield, which was

probably due to the low reactivity of electron-rich substrates and bigger steric hindrance [41]. 3.10. Comparison of different two-component dinuclear metal complexes catalysts Table 7 summarized the catalytic performances of recent reported two-component dinuclear metal (Zn, Cu, Fe) complexes catalysts on cycloaddition reaction of CO2 to PO. In general, since

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activity, and all the complexes maintained superior selectivity for PC (>99%). The catalyst was reused several times without significant loss of activity and selectivity. This heterogeneous, environmental friendly and active catalyst system was suitable for the production of cyclic carbonates from epoxides. This type of supramolecular metal complexes could find promising utilization in area of CO2 fixation. Acknowledgement We are grateful to National Natural Science Foundation of China (No. 51073175) and Natural Science Foundation of Hubei province, P. R China (No. 2016CKB704). Authors also want to appreciate the valuable help from Prof. Buxing Han and the financial support of Beijing National Laboratory for Molecular Sciences (BNLMS). References

Scheme 4. Proposed mechanism for the cycloaddition reaction of CO2 with PO.

different reaction conditions such as cocatalysts, catalysts loading, CO2 pressure, reaction temperature and time were used, it is hard to critically evaluate and compare the efficacy of these dinuclear metal complexes. Nevertheless, the dinuclear metal complexes catalysts reported upon herein were competitive with most catalysts currently available. Notably, there is few literature reports about the application of stable, reusable Cu and Fe complexe catalysts for CO2 chemical fixation. 3.11. Proposed mechanism Considering the mechanism of coupling reaction of PO and CO2 reported before [42], a general mechanism was proposed in Scheme 4. Firstly, PO was activated by the coordination to the two Lewis acidic metal centres, and then the PO mediator was attacked by nucleophilic reagent Br of TBAB to generate the requisite metal heteroatom alkoxide intermediate, which then was inserted by CO2 to produce PC [43]. This mechanism indicated that the presence of a leaving group, like halide, was important for the coupling reaction and the catalytic activity of the halide was determined by a balance between its nucleophilicity and leaving-group ability: a good nucleophile would favour the initial opening of the epoxide ring, but it would also be a worse leaving group in the last step of the reaction, i.e. in the back-biting reaction leading to the formation of cyclic carbonate [44], which was consistent with the results in Table 5. 4. Conclusions In summary, a series of easily accessible and novel supramolecular dinuclear metal (zinc, copper, iron) complexes were synthesized and had been demonstrated to be highly active and versatile catalysts for coupling reaction of CO2 and epoxides without any organic solvents. Among these catalysts, the zinc complex with electron-withdrawing group NO2 showed highest

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