Iron(II) Bis-CNN Pincer Complex-Catalyzed Cyclic Carbonate

Research Article pubs.acs.org/journal/ascecg

Iron(II) Bis-CNN Pincer Complex-Catalyzed Cyclic Carbonate Synthesis at Room Temperature Fei Chen, Ning Liu,* and Bin Dai* School of Chemistry and Chemical Engineering, The Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, Shihezi University, North Fourth Road, Shihezi, Xinjiang 832003, China S Supporting Information *

ABSTRACT: A series of unsymmetrical pyridine-bridged bispincer-type iron(II) complexes were synthesized, and the complexes were used as catalysts for the cycloaddition of CO2 and epoxides. At ambient temperature, the combined use of iron complexes and tetrabutyl ammonium bromide (TBAB) resulted in an efficient catalytic system for the synthesis of cyclic carbonates under low CO2 pressure (0.5 MPa) and solvent-free conditions. More importantly, in the absence of TBAB, propylene oxide was also easily converted to the target products when the temperature was increased to 100 °C, and a turnover frequency (TOF) value of 7900 h−1 was achieved at 120 °C. An elevated reaction temperature (80 °C) was required to achieve high conversion of challenging internal epoxides and oxetanes to their respective carbonates with good yields. Moreover, the iron catalyst can be easily recycled six times via simple filtration without any significant loss of activity. KEYWORDS: Carbon dioxide, Cyclic carbonate, Epoxide, Pincer-type ligand, Iron

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INTRODUCTION Carbon dioxide (CO2) is an abundant, inexpensive, nontoxic, and renewable C1 building block for constructing organic molecules.1−7 The synthesis of cyclic carbonates is one of the most promising reaction routes for CO2 utilization, because of its 100% atom economy.8−15 Such carbonates have been widely used as raw materials for the preparation of polycarbonates,16−18 in synthetic organic chemistry19−21 and the synthesis of biologically active molecules,22−24 and as electrolytes for lithium-ion secondary batteries.25,26 Given the importance of cyclic carbonates, many organocatalytic systems for this transformation have been developed, including imidazolium salts,27−33 ammonium salts,34−37 phosphonium salts,38−42 organic amines,43−45 phenols,46−49 and binary systems.50−53 Despite the growing number of organocatalysts, metal-based catalysts provide advantages: high activity, low catalyst loading, and milder reaction conditions, because of the presence of a Lewis acid metal center that promotes opening of the epoxide ring. However, most metalbased catalytic systems focus on the Al,54−56 Mg,57,58 Co,59−61 Cr,62,63 Zn,64−67 bimetallic complexes,68,69 and metal-organic frameworks (MOFs).70−72 Iron is one of the most ideal metals as a catalyst for CO2 fixation, because it is abundant and cost-effective.73 However, one-compound iron systems are inactive and rely on the cooperation of other active metals.74 The limitations of iron catalysts have been minimized through the pioneering work of Kleij,75−77 Williams,78 and Rieger.79 There is now considerable © 2017 American Chemical Society

interest in iron complexes for the cycloaddition of CO2 and epoxides.80−87 However, in most cases, organic solvents and/or harsh reaction conditions are often needed. In addition, the poor activities of substrates and challenging internal epoxides and oxetane limit their broad use. In our previous work, we synthesized unsymmetrical pyridine-bridged pincer-type imidazolium salts, but found that these organocatalysts had poor catalytic activity under mild reaction conditions (atmospheric pressure and room temperature).88 To overcome these challenges, we designed and synthesized iron(II) bis-CNN pincer complexes, which are highly active catalysts for the cycloaddition of CO2 and epoxides. They are particularly useful for the conversion of sterically hindered internal epoxides and oxetanes under solvent-free and mild reaction conditions.

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EXPERIMENTAL SECTION

General Procedure for the Synthesis of Iron Catalysts. The pyridine-bridged pincer-type imidazolium salts were initially prepared according to the literature.88 The imidazolium salts (0.6 mmol), and FeX2 (X = Cl, Br, and I, 0.3 mmol) were successively put into a 25 mL Schlenk tube and dissolved in 2 mL of dimethyl sulfoxide (DMSO) under a nitrogen atmosphere. The mixtures were subsequently heated to 110 °C for 24 h. The reaction mixture was added to brine (15 mL) and extracted three times with dichloromethane (3 × 15 mL). The Received: June 18, 2017 Revised: September 3, 2017 Published: September 7, 2017 9065

DOI: 10.1021/acssuschemeng.7b01990 ACS Sustainable Chem. Eng. 2017, 5, 9065−9075

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ACS Sustainable Chemistry & Engineering

8.08 (s, 1H), 7.75 (t, J = 8.0 Hz, 1H), 7.68 (d, J = 8.4 Hz, 1H), 7.63 (d, J = 8.0 Hz, 1H), 7.54 (t, J = 8.6 Hz, 1H), 7.45−7.38 (m, 2H), 3.28− 3.04 (m, 2H), 0.82−0.53 (m, 2H), 0.30 (t, J = 7.2 Hz, 3H). 13C NMR (100 MHz, DMSO-d6): δ 208.3, 154.1, 151.5, 142.5, 142.0, 138.3, 136.0, 131.3, 130.3, 126.2, 125.1, 124.6 (d, J = 31.2 Hz), 122.0, 112.0, 111.6, 111.2, 108.7, 106.6, 47.9, 22.5, 10.4. Elemental analysis: calcd for C44H38FeN10Cl2: C 63.40, H 4.59, N, 16.80; found C 63.34, H 4.51, N 16.72. HRMS (MALDI) calcd for C44H38FeN10Cl2 [M−Cl]+ 797.2315, found 797.2306. Iron Complex 1e. Iron complex 1b (510.5 mg, 0.502 mmol) and KPF6 (140.8 mg, 0.76 mmol) were successively placed into a 100 mL flask and dissolved in 20 mL of H2O. The mixtures were subsequently stirred at room temperature for 48 h. The raw products were filtered from the reaction solution. The products were isolated by flash chromatography (DCM/MeOH = 15:1, Rf = 0.56). Compound 1e was isolated as a brown, air-stable, and moisture-stable powder (115 mg, 35%). 1H NMR (400 MHz, DMSO-d6): δ 8.84 (d, J = 8.0 Hz, 1H), 8.77−8.67 (m, 2H), 8.65 (dd, J = 8.8, 4.8 Hz, 2H), 8.06 (d, J = 0.8 Hz, 1H), 7.81−7.74 (m, 1H), 7.70 (d, J = 8.0 Hz, 1H), 7.63−7.53 (m, 2H), 7.49−7.39 (m, 2H), 3.25−3.08 (m, 2H), 0.86−0.52 (m, 2H), 0.33 (t, J = 7.3 Hz, 3H). 13C NMR (100 MHz, DMSO-d6): δ 208.3, 154.2, 151.6, 142.2, 142.1, 138.3, 136.0, 131.3, 130.3, 126.2, 124.8, 124.6, 124.5, 122.0, 111.9 111.5, 111.1, 108.3, 106.3, 47.93, 22.5, 10.4. 19 F NMR (376 MHz, DMSO-d6): δ −69.18, −71.03. Elemental analysis: calcd for C44H38FeN10P2F12: C 50.21, H 3.64, N 13.31; found C 50.26, H 3.59, N 13.37. HRMS (MALDI) calcd for C44H38FeN10P2F12 [M−PF6]+ 907.2268, found 907.2259. Iron Complex 1f. 1-(6-(1H-Indazol-1-yl)pyridin-2-yl)-5,6-dimethyl3-propyl-1H-benzo[d]imidazol-3-ium iodide (305.3 mg, 0.598 mmol), and FeI2 (93.7 mg, 0.301 mmol) were successively placed into a 25 mL Schlenk tube and dissolved in 2 mL of DMSO under a nitrogen atmosphere. The mixtures were subsequently heated to 110 °C for 24 h. The solvent was concentrated under vacuum and the products were isolated by flash chromatography (DCM/MeOH = 5:1, Rf = 0.33). Compound 1f was isolated as a brown, air-stable, and moisture-stable powder (104 mg, 31%). 1H NMR (400 MHz, DMSO-d6): δ 8.93−8.84 (m, 1H), 8.76−8.63 (m, 3H), 8.47 (s, 1H), 8.05 (d, J = 0.5 Hz, 1H), 7.81−7.72 (m, 1H), 7.69 (d, J = 4.0 Hz, 1H), 7.43−7.41 (m, 2H), 3.20−3.00 (m, 2H), 2.47 (s, 3H), 2.29 (s, 3H), 0.84−0.54 (m, 2H), 0.31 (t, J = 7.6 Hz, 3H). 13C NMR (100 MHz, DMSO-d6): δ 206.6, 154.1, 151.5, 142.0, 141.8, 138.3, 134.3, 133.8, 133.6, 130.2, 129.6, 126.1, 124.5, 121.9, 112.1, 111.5, 111.2, 108.3, 106.1, 48.0, 22.4, 19.5, 10.4. Elemental analysis: calcd for C48H46FeN10I2: C 53.75, H 4.32, N 13.06; found C 53.68, H 4.36, N 13.13. HRMS (MALDI) calcd for C48H46FeN10I2 [M−I]+ 945.2290, found 945.2276. Iron Complex 1g. 1-(6-(1H-Pyrazol-1-yl)pyridin-2-yl)-3-methyl1H-benzo[d]imidazol-3-ium iodide (242.1 mg, 0.598 mmol) and FeI2 (93.6 mg, 0.301 mmol) were successively placed into a 25 mL Schlenk tube and dissolved in 2 mL of DMSO under a nitrogen atmosphere. The mixtures were subsequently heated to 110 °C for 24 h. The solvent was concentrated under vacuum and the products were isolated by flash chromatography (DCM/MeOH = 10:1, Rf = 0.38). Compound 1g was isolated as a brown, air-stable, and moisture-stable powder (106 mg, 39%). 1H NMR (400 MHz, DMSO-d6): δ 9.38 (d, J = 2.4 Hz, 1H), 8.78 (d, J = 8.0 Hz, 1H), 8.68 (t, J = 8.4 Hz, 1H), 8.57 (d, J = 8.4 Hz, 1H), 8.46 (d, J = 8.4 Hz, 1H), 7.52−7.47 (m, 2H), 7.44−7.40 (m, 2H), 6.72 (dd, J = 3.2, 2.0 Hz, 1H), 2.81 (s, 3H). 13C NMR (100 MHz, DMSO-d6): δ 209.9, 154.3, 151.2, 144.1, 141.8, 137.2, 132.2, 130.9, 124.4, 123.9, 111.8, 111.6, 110.6, 109.0, 106.6, 31.60. Elemental analysis: calcd for C32H26FeN10I2: C 44.68, H 3.05, N 16.28; found C 44.75, H 3.08, N 16.32. HRMS (MALDI) calcd for C32H26FeN10I2 [M−I]+ 733.0731, found 733.0721. Iron Complex 1h. Iron complex 1g (433.4 mg, 0.503 mmol) and KPF6 (140.6 mg, 0.76 mmol) were successively placed into a 100 mL flask and dissolved in 20 mL of H2O. The mixtures were subsequently stirred at room temperature for 48 h. The raw products were filtered from the reaction solution. The desired products were isolated by flash chromatography (DCM/MeOH = 15:1, Rf = 0.36). Compound 1h was isolated as a brown, air-stable, and moisture-stable powder (144 mg, 51%). 1H NMR (400 MHz, DMSO-d6): δ 9.33 (d, J = 2.4 Hz,

solvent was concentrated under vacuum and the desired products were isolated by flash chromatography. The resulting iron catalysts were stable in air for 7 days. Iron Complex 1a. 1-(6-(1H-Indazol-1-yl)pyridin-2-yl)-3-methyl1H-benzo[d]imidazol-3-ium iodide (272.7 mg, 0.601 mmol) and FeI2 (93.3 mg, 0.301 mmol) were successively put into a 25 mL Schlenk tube and dissolved in 2 mL of DMSO under a nitrogen atmosphere. The mixtures were subsequently heated to 110 °C for 24 h. The solvent was concentrated under vacuum, and the products were isolated by flash chromatography (dichloromethane/methanol (DCM/MeOH) = 5:1, Rf = 0.41 (where Rf is the retardation factor)). Compound 1a was isolated as a brown, air-stable, and moisture-stable powder (103 mg, 34%). 1H NMR (400 MHz, DMSO-d6): δ 8.79 (d, J = 4.0 Hz, 1H), 8.68 (t, J = 4.8 Hz, 3H), 8.60 (d, J = 7.8 Hz, 1H), 8.12 (s, 1H), 7.76 (t, J = 7.6 Hz, 1H), 7.68 (d, J = 8.0 Hz, 1H), 7.51 (d, J = 6.8 Hz, 2H), 7.42 (dd, J = 17.2, 8.0 Hz, 2H), 2.88 (s, 3H). 13C NMR (100 MHz, DMSO-d6): δ 209.0, 154.2, 151.6, 141.9, 141.6, 138.4, 137.2, 131.0, 130.0, 126.2, 124.4, 124.3, 124.0, 121.8, 111.7, 111.6, 110.7, 108.0, 106.3, 31.7. Elemental analysis: calcd for C40H30FeN10I2: C 50.02, H 3.15, N 14.58; found C 49.96, H 3.18, N 14.49. Highresolution mass spectroscopy (HRMS) (matrix-assisted laser desorption/ionization (MALDI)) calcd for C40H30FeN10I2 [M−I]+ 833.1044, found 833.1056. Iron Complex 1b. 1-(6-(1H-Indazol-1-yl)pyridin-2-yl)-3-propyl1H-benzo[d]imidazol-3-ium iodide (289.1 mg, 0.60 mmol), and FeI2 (93.2 mg, 0.301 mmol) were successively put into a 25 mL Schlenk tube and dissolved in 2 mL of DMSO under a nitrogen atmosphere. The mixtures were subsequently heated to 110 °C for 24 h. The solvent was concentrated under vacuum, and the products were isolated by flash chromatography (DCM/MeOH = 10:1, Rf = 0.47). Compound 1b was isolated as a brown, air-stable, and moisture-stable powder (163 mg, 51%). 1H NMR (400 MHz, DMSO-d6): δ 8.86 (d, J = 4.9 Hz, 1H), 8.73 (s, 2H), 8.66 (s, 2H), 8.07 (s, 1H), 7.77 (t, J = 8.0 Hz, 1H), 7.70 (d, J = 7.6 Hz, 1H), 7.62 (d, J = 8.0 Hz, 1H), 7.56 (t, J = 8.0 Hz, 1H), 7.50−7.38 (m, 2H), 3.20−3.13 (m, 2H), 0.67 (d, J = 7.2 Hz, 2H), 0.32 (t, J = 7.2 Hz, 3H); 13C NMR (100 MHz, DMSO-d6): δ 208.3, 154.1, 151.5, 142.3, 142.0, 138.3, 135.9, 131.2, 130.3, 126.1, 124.8, 124.5, 124.4, 121.9, 111.9, 111.5, 111.1, 108.4, 106.4, 47.7, 22.8, 10.8. Elemental analysis: calcd for C44H38FeN10I2: C 51.99, H 3.77, N 13.78; found C 51.85, H 3.73, N 13.67. HRMS (MALDI) calcd for C44H38FeN10I2 [M−I]+ 889.1662, found 889.1657. Iron Complex 1c. 1-(6-(1H-Indazol-1-yl)pyridin-2-yl)-3-propyl-1Hbenzo[d]imidazol-3-ium bromide (260.2 mg, 0.598 mmol) and FeBr2 (65.3 mg, 0.302 mmol) were successively placed into a 25 mL Schlenk tube and dissolved in 2 mL of DMSO under a nitrogen atmosphere. The mixtures were subsequently heated to 110 °C for 24 h. The solvent was concentrated under vacuum and the products were isolated by flash chromatography (DCM/MeOH = 10:1, Rf = 0.55). Compound 1c was isolated as a brown, air-stable, and moisture-stable powder (125 mg, 43%). 1H NMR (400 MHz, DMSO-d6): δ 8.87 (t, J = 4.4 Hz, 1H), 8.73 (d, J = 4.8 Hz, 2H), 8.66 (dd, J = 8.8, 5.6 Hz, 2H), 8.06 (s, 1H), 7.76 (t, J = 8.0 Hz, 1H), 7.69 (d, J = 8.0 Hz, 1H), 7.62 (d, J = 7.8 Hz, 1H), 7.55 (t, J = 8.0 Hz, 1H), 7.43 (dt, J = 12.4, 8.0 Hz, 2H), 3.16 (d, J = 5.2 Hz, 2H), 0.67 (d, J = 7.2 Hz, 1H), 0.31 (t, J = 7.2 Hz, 3H); 13C NMR (100 MHz, DMSO-d6): δ 208.1, 154.3, 151.7, 142.4, 142.1, 138.2, 136.0, 131.2, 130.4, 126.0, 124.9, 124.5, 124.4, 121.9, 111.9, 111.5, 111.1, 108.4, 106.4, 47.9, 22.4, 10.4. Elemental analysis: calcd for C44H38FeN10Br2: C 57.29, H 4.15, N 15.18; found C 57.17, H 4.18, N 15.23. HRMS (MALDI) calcd for C44H38FeN10Br2 [M−Br]+ 841.1811, found 841.1802. Iron Complex 1d. 1-(6-(1H-Indazol-1-yl)pyridin-2-yl)-3-propyl1H-benzo[d]imidazol-3-ium chloride (234.9 mg, 0.603 mmol) and FeCl2 (38.1 mg, 0.299 mmol) were successively placed into a 25 mL Schlenk tube and dissolved in 2 mL of DMSO under a nitrogen atmosphere. The mixtures were subsequently heated to 110 °C for 24 h. The solvent was concentrated under vacuum, and the products were isolated by flash chromatography (DCM/MeOH = 10:1, Rf = 0.46). Compound 1d was isolated as a brown, air-stable, and moisture-stable powder (69 mg, 26%). 1H NMR (400 MHz, DMSO-d6): δ 8.94 (d, J = 8.0 Hz, 1H), 8.80 (d, J = 8.0 Hz, 1H), 8.72 (dd, J = 16.8, 8.8 Hz, 3H), 9066


Research Article

ACS Sustainable Chemistry & Engineering 1H), 8.79−8.63 (m, 2H), 8.56 (d, J = 8.0 Hz, 1H), 8.41 (d, J = 8.0 Hz, 1H), 7.58−7.34 (m, 4H), 6.73 (s, 1H), 2.82 (s, 3H). 13C NMR (100 MHz, DMSO-d6): δ 209.9, 154.4, 151.2, 144.2, 141.8, 137.2, 132.1, 131.0, 124.4, 124.0, 111.8, 111.6, 110.6, 109.0, 106.5, 31.5. 19F NMR (376 MHz, DMSO-d6): δ −69.17, −71.06. Elemental analysis: calcd for C32H26FeN10P2F12: C 42.88, H 2.92, N 15.63; found C 42.81, H 2.87, N 15.69. HRMS (MALDI) calcd for C32H26FeN10P2F12 [M− PF6]+ 751.1328, found 751.1321. Iron Complex 1i. 1-(6-(1H-Pyrazol-1-yl)pyridin-2-yl)-3-propyl-1Hbenzo[d]imidazol-3-ium iodide (259.6 mg, 0.601 mmol) and FeI2 (93.5 mg, 0.301 mmol) were successively placed into a 25 mL Schlenk tube and dissolved in 2 mL of DMSO under a nitrogen atmosphere. The mixtures were subsequently heated to 110 °C for 24 h. The solvent was concentrated under vacuum and the products were isolated by flash chromatography (DCM/MeOH = 5:1, Rf = 0.38). Compound 1i was isolated as a brown, air-stable, and moisture-stable powder (182 mg, 63%). 1H NMR (400 MHz, DMSO-d6) δ 9.40 (d, J = 3.2 Hz, 1H), 8.85 (d, J = 8.0 Hz, 1H), 8.74 (t, J = 8.2 Hz, 1H), 8.63 (d, J = 8.4 Hz, 1H), 8.50 (d, J = 8.0 Hz, 1H), 7.62 (d, J = 8.0 Hz, 1H), 7.52 (t, J = 7.6 Hz, 1H), 7.43 (t, J = 8.0 Hz, 1H), 7.41−7.39 (m, 1H), 6.74 (t, J = 2.4 Hz, 1H), 3.25−2.94 (m, 2H), 0.69 (dt, J = 7.2 Hz, 2H), 0.33 (t, J = 7.2 Hz, 3H). 13C NMR (100 MHz, DMSO-d6): δ 209.1, 154.3, 151.1, 144.5, 142.2, 135.9, 132.2, 131.2, 124.7, 124.4, 112.2, 111.9, 111.0, 109.4, 106.5, 47.8, 22.5, 10.4. Elemental analysis: calcd for C36H34FeN10I2: C 47.18, H 3.74, N 15.28; found C, 47.25, H 3.79, N 15.36. HRMS (MALDI) calcd for C36H34FeN10I2 [M−I]+ 789.1353, found 789.1351. Iron Complex 1j. 1-(6-(1H-Pyrazol-1-yl)pyridin-2-yl)-3-propyl-1Hbenzo[d]imidazol-3-ium bromide (231.4 mg, 0.601 mmol) and FeBr2 (65.8 mg, 0.301 mmol) were successively placed into a 25 mL Schlenk tube and dissolved in 2 mL of DMSO under a nitrogen atmosphere. The mixtures were subsequently heated to 110 °C for 24 h. The solvent was concentrated under vacuum and the products were isolated by flash chromatography (DCM/MeOH = 10:1, Rf = 0.33). Compound 1j was isolated as a brown, air-stable, and moisture-stable powder (104 mg, 40%). 1H NMR (400 MHz, DMSO-d6): δ 9.42 (s, 1H), 8.84 (d, J = 8.4 Hz, 1H), 8.74 (t, J = 8.0 Hz, 1H), 8.63 (d, J = 8.4 Hz, 1H), 8.50 (d, J = 7.6 Hz, 1H), 7.62 (d, J = 8.4 Hz, 1H), 7.53 (t, J = 8.0 Hz, 1H), 7.44 (t, J = 8.0 Hz 1H), 7.40 (d, J = 1.6 Hz, 1H), 6.81− 6.68 (m, 1H), 3.26−2.94 (m, 2H), 0.70 (d, J = 6.9 Hz, 2H), 0.34 (t, J = 7.2 Hz, 3H). 13C NMR (100 MHz, DMSO-d6): δ 209.1, 154.3, 151.1, 144.5, 142.2, 135.9, 132.3, 131.2, 124.7, 124.4, 112.17, 111.9, 111.0, 109.4, 106.5, 48.0, 22.6, 10.4. Elemental analysis: calcd for C36H34FeN10Br2: C 52.58, H 4.17, N 17.03; found C 52.47, H 4.13, N 17.09. HRMS (MALDI) calcd for C36H34FeN10Br2 [M−Br]+ 741.1496, found 741.1488. Iron Complex 1k. Iron complex 1i (460.7 mg, 0.502 mmol) and KPF6 (140.5 mg, 0.76 mmol) were successively placed into a 100 mL flask and dissolved in 20 mL of H2O. The mixtures were subsequently stirred at room temperature for 48 h. The raw products were filtered from the reaction solution. The desired products were isolated by flash chromatography (DCM/MeOH = 15:1, Rf = 0.46). Compound 1k was isolated as a brown, air-stable, and moisture-stable powder (102 mg, 34%). 1H NMR (400 MHz, DMSO-d6): δ 9.37 (d, J = 2.8 Hz, 1H), 8.83 (d, J = 8.4 Hz, 1H), 8.74 (t, J = 8.4 Hz, 1H), 8.62 (d, J = 8.3 Hz, 1H), 8.50−8.40 (m, 1H), 7.60 (d, J = 8.0 Hz, 1H), 7.54 (t, J = 7.8 Hz, 1H), 7.45 (t, J = 7.8 Hz, 1H), 7.39 (t, J = 1.6 Hz, 1H), 6.75 (t, J = 2.4 Hz, 1H) 3.26−2.95 (m, 2H), 0.77−0.67 (m, 2H), 0.35 (t, J = 7.4 Hz, 3H). 13C NMR (100 MHz, DMSO-d6): δ 209.1, 154.4, 151.2, 144.5, 142.1, 136.0, 132.2, 131.2, 124.8, 124.4, 112.2, 111.8, 111.0, 109.4, 106.5, 47.8, 22.5, 10.4. 19F NMR (376 MHz, DMSO-d6): δ −69.16, −71.05. Elemental analysis: calcd for C36H34FeN10P2F12: C 45.39, H 3.60, N 14.71; found C 45.28, H 3.56, N 14.64. HRMS (MALDI) calcd for C36H34FeN10P2F12 [M−PF6]+ 807.1948, found 807.1941. General Procedure for the Synthesis of Cyclic Carbonates. Epoxides (10.0 mmol), catalyst 1i (0.03 mmol), and TBAB (0.30 mmol) were successively placed into a 25 mL stainless steel reactor that was equipped with a magnetic stirrer. The reactor was pressurized with CO2 to 0.5 MPa and reacted under ambient temperature for 24 h.

After the reaction, the reactor was cooled to room temperature, and excess CO2 was carefully vented off. The reaction mixtures were added to brine (15 mL) and extracted three times with dichloromethane (3 × 15 mL). The solvent was removed under reduced pressure, and the products were isolated by flash chromatography. 4-Methyl-1,3-dioxolan-2-one (3a).75 Purification by flash chromatography (petroleum ether/EtOAc = 2:1) gave a colorless oil (938.3 mg, 92%). 1H NMR (400 MHz, CDCl3): δ 4.79−4.73 (m, 1H), 4.48− 4.43 (m, 1H), 3.94−3.89 (m, 1H), 1.36−1.34 (m, 3H). 13C NMR (100 MHz, CDCl3): δ 155.16, 73.76, 70.68, 19.03. 4-Ethyl-1,3-dioxolan-2-one (3b).39 Purification by flash chromatography (petroleum ether/EtOAc = 2:1) gave a yellow oil (986.6 mg, 85%). 1H NMR (400 MHz, CDCl3): δ 4.67−4.59 (m, 1H), 4.51−4.46 (m, 1H), 4.06−4.01 (m, 1H), 1.78−1.65 (m, 2H), 0.99−0.94 (m, 3H). 13 C NMR (100 MHz, CDCl3): δ 155.21, 78.11, 69.06, 26.79, 8.38. 4-Hexyl-1,3-dioxolan-2-one (3c).29 Purification by flash chromatography (petroleum ether/EtOAc = 5:1) gave a colorless oil (1.4963 g, 87%). 1H NMR (400 MHz, CDCl3): δ 4.73−4.66 (m, 1H), 4.52 (t, J = 8.0 Hz, 1H), 4.06 (dd, J = 8.4 Hz, J = 7.2 Hz, 1H), 1.82−1.74 (m, 1H), 1.71−1.63 (m, 1H), 1.49−1.28 (m, 8H), 0.87 (t, J = 7.2 Hz, 3H). 13 C NMR (100 MHz, CDCl3): δ 155.15, 77.15, 69.43, 33.76, 31.46, 28.73, 24.26, 22.39, 13.91. 4-Butyl-1,3-dioxolan-2-one (3d).75 Purification by flash chromatography (petroleum ether/EtOAc = 5:1) gave a colorless oil (1.2104 g, 84%). 1H NMR (400 MHz, CDCl3): δ 4.73−4.66 (m, 1H), 4.54− 4.50 (m, 1H), 4.08−4.04 (m, 1H), 1.83−1.64 (m, 2H), 1.47−1.27 (m, 4H), 0.93−0.89 (m, 3H). 13C NMR (101 MHz, CDCl3): δ 155.2, 77.2, 69.5, 33.6, 26.5, 22.3, 13.8. 4-(Chloromethyl)-1,3-dioxolan-2-one (3e).75 Purification by flash chromatography (petroleum ether/EtOAc = 2:1) gave a yellow oil (1.2428 g, 91%). 1H NMR (400 MHz, CDCl3): δ 5.02−4.96 (m, 1H), 4.61(t, J = 8.0 Hz, 1H), 4.42 (dd, J = 8.8 Hz, J = 5.6 Hz, 1H), 3.83− 3.73 (m, 2H). 13C NMR (100 MHz, CDCl3): δ 154.54, 74.54, 66.99, 44.20. 4-(Chloromethyl)-4-methyl-1,3-dioxolan-2-one (3f).88 Purification by flash chromatography (petroleum ether/EtOAc = 2:1) gave a yellow oil (1.3205 g, 88%). 1H NMR (400 MHz, CDCl3): δ 4.54 (d, J = 8.4 Hz, 1H), 4.11 (d, J = 8.4 Hz, 1H), 3.77 (d, J = 12.4 Hz, 1H), 3.53 (d, J = 12.4 Hz, 1H), 1.45 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 155.48, 84.04, 71.50, 65.80, 21.29. 4-Phenyl-1,3-dioxolan-2-one (3g).75 Purification by flash chromatography (petroleum ether/EtOAc = 10:1) gave a yellow solid (1.2467 g, 76%), melting point (MP) = 50.0−51.4 °C. 1H NMR (400 MHz, CDCl3): δ 7.48−7.43 (m, 3H), 7.38−7.36 (m, 2H), 5.69 (t, J = 8.0 Hz, 1H), 4.81 (t, J = 8.0 Hz, 1H), 4.34 (t, J = 8.0 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 154.95, 135.87, 129.72, 129.23, 125.94, 78.05, 71.21. 4-(Phenoxymethyl)-1,3-dioxolan-2-one (3h).39 Purification by flash chromatography (petroleum ether/EtOAc = 2:1) gave a white solid (1.6883 g, 87%), MP = 100.9−101.2 °C. 1H NMR (400 MHz, CDCl3): δ 7.33 (t, J = 8.0 Hz, 2H), 7.04 (t, J = 8.0 Hz, 1H), 6.94 (d, J = 8.0 Hz, 2H), 5.08−5.02 (m, 1H), 4.64 (t, J = 8.4 Hz, 1H), 4.56 (dd, J = 8.8 Hz, J = 6.0 Hz, 1H), 4.26 (dd, J = 10.4 Hz, J = 4.4 Hz, 1H), 4.17 (dd, J = 10.8 Hz, J = 3.6 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 157.77, 154.71, 129.71, 121.99, 114.63, 74.18, 66.90, 66.25. 4-(((9H-Carbazol-4-yl)oxy)methyl)-1,3-dioxolan-2-one (3i). Purification by flash chromatography (DCM) gave a white solid (1.3874 g, 49%), MP = 215.7−216.2 °C. 1H NMR (400 MHz, DMSO-d6): δ 11.33 (s, 1H), 8.13 (d, J = 8.0 Hz, 1H), 7.48 (d, J = 8.0 Hz, 1H), 7.38− 7.31 (m, 2H), 7.16−7.13 (m, 2H), 6.72 (d, J = 8.0 Hz, 1H), 5.35 (s, 1H), 4.78 (t, J = 8.4 Hz, 1H), 4.65−4.41 (m, 3H). 13C NMR (100 MHz, DMSO-d6): δ 155.51, 154.54, 141.64, 139.44, 126.89, 125.22, 122.55, 121.88, 119.21, 111.85, 110.96, 105.04, 100.98, 75.41, 67.82, 66.67. HRMS (electrospray ionization (ESI)) calcd for C16H13NO4 [M−H]+ 282.0769, found 282.0774. (S)-4-((Benzyloxy)methyl)-1,3-dioxolan-2-one ((S)-3j).39 Purification by flash chromatography (petroleum ether/EtOAc = 2:1) gave a white solid (2.0384 g, 98%), MP = 87.3−89.6 °C. 1H NMR (400 MHz, CDCl3) δ 7.40−7.30 (m, 5H), 4.84−4.78 (m, 1H), 4.64−4.55 (m, 2H), 4.49−4.43(m, 1H), 4.39−4.34 (m, 1H), 3.74−3.69 (m, 1H), 9067


Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Iron Catalysts in This Work

■

3.63−3.58 (m, 1H). 13C NMR (100 MHz, CDCl3): δ 155.14, 137.27, 128.57, 128.03, 127.73, 75.22, 73.60, 68.94, 66.30. Hexahydrobenzo[d][1,3]dioxol-2-one (3k).75 Purification by flash chromatography (petroleum ether/EtOAc = 5:1) gave a colorless oil (1.3357 g, 94%). 1H NMR (400 MHz, CDCl3): δ 4.67−4.62 (m, 2H), 1.88−1.74 (m, 4H), 1.57−1.48 (m, 2H), 1.41−1.31 (m, 2H). 13C NMR (100 MHz, CDCl3): δ 155.38, 75.78, 26.62, 19.03. 5-Vinylhexahydrobenzo[d][1,3]dioxol-2-one (3l).75 Purification by flash chromatography (petroleum ether/EtOAc = 5:1) gave a colorless oil (1.4282 g, 85%). 1H NMR (400 MHz, CDCl3): δ 5.75−5.65 (m, 2H), 5.05−4.94 (m, 4H), 4.78−4.61 (m, 4H), 2.29−2.09 (m, 5H), 2.02−1.96 (m, 1H), 1.79−1.52 (m, 5H), 1.34−1.11 (m, 3H). 13C NMR (100 MHz, mixture of diastereoisomers, CDCl3): δ 155.15, 155.12, 141.07, 140.96, 114.16, 113.86, 76.01, 75.65, 75.60, 75.11, 36.24, 33.85, 33.50, 31.59, 26.63, 25.72, 25.60, 25.00. 4,4-Dimethyl-1,3-dioxolan-2-one (3m).75 Purification by flash chromatography (petroleum ether/EtOAc = 5:1) gave a colorless oil (963.6 mg, 83%). 1H NMR (400 MHz, CDCl3): δ 4.13 (s, 2H), 1.50 (s, 6H). 13C NMR (100 MHz, CDCl3): δ 154.64, 81.81, 75.37, 25.95. 1,3-Dioxan-2-one (3n).75 Purification by flash chromatography (petroleum ether/EtOAc = 5:1) gave a colorless oil (684.9 mg, 67%). 1 H NMR (400 MHz, CDCl3): δ 4.40−4.32 (m, 4H), 2.11−2.00 (m, 2H). 13C NMR (100 MHz, CDCl3): δ 148.62, 68.09, 21.62. Typical Procedure for the Recycling Experiments. PO (586.8 mg, 10.1 mmol), the catalyst 1i (27.5 mg, 0.029 mmol), and TBAB (97.6 mg, 0.29 mmol) were successively placed into a 25 mL stainless steel reactor equipped with a magnetic stirrer. The reactor was pressurized with CO2 to 0.5 MPa and reacted under ambient temperature for 24 h. After the reaction, the reactor was cooled to room temperature, and the remaining CO2 was removed slowly. The catalyst could easily be precipitated from the PC product upon addition of ethyl acetate (5 mL). After filtration, the recycling catalyst is directly used without further purification process, and another batch of propylene oxide (PO) (586.6 mg, 10.1 mmol) and TBAB (98.3 mg, 0.30 mmol) was added and reused for subsequent cycles under the same reaction conditions. The volatile organic-phase-containing product was analyzed using GC with biphenyl as the internal standard. Preparation of the Crystalline Material. The crystalline material was prepared via a layer-to-layer diffusion method. Iron complex 1i was added into the tetrahydrofuran (THF) solution and layered with n-hexane. After three days, a crystal suitable for single-crystal X-ray diffraction was obtained. Crystallographic data of the structures were deposited in the Cambridge Crystallographic Database Centre (No. CCDC 1511481 for iron complex 1i). Analytical Methods. The NMR spectra were recorded on a Bruker Avance III HD 400 spectrometer using tetramethyl silane (TMS) as an internal standard (400 MHz for 1H NMR and 100 MHz for 13C NMR). Elemental analysis was performed on a Elementar Analysensysteme GmbH vario EL cube. Mass spectroscopy (MS) data were collected on a Bruker ultrafleXtreme mass spectrometer. Singlecrystal structure determination was conducted on a Bruker Smart APEX II diffractometer equipped with an APEX II CCD detector. Gas chromatogrpahy (GC) analyses were performed on a Shimadzu Model GC-2014 system that was equipped with a packed column (Model GDX-301, 2 m × 4 mm), using a flame ionization detector.

RESULTS AND DISCUSSION The unsymmetrical pyridine-bridged pincer-type imidazolium salts were initially prepared according to our reported method.88 Subsequent reaction with iron(II) salts under a nitrogen atmosphere afforded iron(II) bis-CNN pincer complexes 1a−1k (see Scheme 1). The previous studies using Fe catalysis in this area were mostly focused on the Fe(III) complexes. The Fe(II) complexes applied in the cycloaddition of CO2 are rarely explored and have been reported only by Rieger et al.76 Our prepared Fe(II) complexes were stable to air, and to moisture, even for 7 days under an air atmosphere. Since the Fe(II) complexes are all stable to air, no glovebox or inert-atmosphere techniques were required to use these complexes for cycloaddition reactions of CO2 with epoxides. The molecular structure of iron(II) catalyst 1i was confirmed by X-ray crystallography (Figure 1). The complex cation [Fe(CNN)2]2+ features an approximate octahedral Fe(II) center with two noncoordinating iodine anions.

Figure 1. X-ray structure of Fe complex 1i.

The influence of the structure of the substituents bearing a pyridine ring on the catalytic performance was investigated using the cycloaddition of propylene oxide (PO, 2a) to CO2 as the model reaction under ambient temperature with 0.5 MPa CO2 pressure and solvent-free conditions (Table 1). The catalysts of the pyridine ring bearing the pyrazolyl group showed higher relatively catalytic activity than those bearing the indazolyl group (Table 1, entries 1 vs 7; entries 2 vs 9; entries 3 vs 10). The influence of alkyl chain lengths on the catalytic performance was evaluated. The catalytic activity of imidazolium salts with a long alkyl chain is stronger than those with a shorter alkyl chain (Table 1, entries 2 vs 1; entries 9 vs 7). The effect of the counteranions on the catalytic performance was also investigated. The activity order of anion is I− > Br− > Cl− > PF6− (see Table 1, entries 2 vs 3 vs 4 vs 5). Control experiments in the absence of complex 1i or a co-catalyst revealed that cooperation between the iron complex and the co9068


Research Article

ACS Sustainable Chemistry & Engineering Table 1. Optimization of PC Synthesisa

entry

catalyst

Co-catalystb (mol %)

CO2 pressure (MPa)

yieldc (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

1a 1b 1c 1d 1e 1f 1g 1h 1i 1j 1k 1i none 1i 1i 1i 1i 1i 1i 1i 1i 1i

TBAB (1.5) TBAB (1.5) TBAB (1.5) TBAB (1.5) TBAB (1.5) TBAB (1.5) TBAB (1.5) TBAB (1.5) TBAB (1.5) TBAB (1.5) TBAB (1.5) none TBAB (3.0) TBAI (1.5) TBAC (1.5) TBASO4H (1.5) TBAO (1.5) TOAB (1.5) PPNCl (1.5) TBAB (3.0) TBAB (3.0) TBAB (3.0)

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.5 0.1

21 62 35 24 18 55 53 31 83 69 36 0 trace 57 35 69 72 33 66 99 96 65

(see Table 1, entries 9 vs 20). When the CO2 pressure was decreased from 1.0 MPa to 0.1 MPa, a moderate yield of 65% of PC was obtained under ambient temperatures (see Table 1, entries 20−22). Under such optimized reaction conditions, the substrate scope was then investigated, and the results are shown in Table 2. The protocol can tolerate the presence of various functional groups for the terminal epoxides and give desired products 3a− 3h in 78%−99% yield at ambient temperature and 0.5 MPa CO2 under solvent-free conditions. Substrate 3i required increasing the reaction temperature to 45 °C, because of its high melting point; this gave a yield of 49%. Internal epoxides are challenging substrates, because of their high hindrance effect and low selectivity. These were also evaluated in this catalytic system. Elevated reaction temperature and increased CO2 pressure were required to achieve the high conversion of internal epoxides. Note that, when cyclohexene oxide was used, the cyclic carbonate product was obtained with 96% selectivity and 94% yield (3j). This high selectivity for cyclohexene oxide is surprising, because North63 reported that the control of selectivity is difficult for the cycloaddition of cyclohexene oxide with the Cr(salophen)/TBAB system, in which the cyclohexene oxide is prone to polymerize and form poly(cyclohexene carbonate). Our catalytic systems tolerate useful functional groups such as exo-cyclic double bonds. This affords the targeted products with 85% yield and 93% selectivity (3k). However, the configuration of 3k is difficult to control and a diastereomeric ratio (d.r.) value of 66:34 was obtained for the carbonate. This result agrees with the report by Kleij and co-workers.76 The challenging substrate 2,2-dimethyloxirane was also converted to the targeted product (3l) with 83% yield. Oxetanes are often more challenging substrates, because of their lower ring strain, compared to the epoxides. We were pleased to find that the increasing reaction temperature and high CO2 pressure conditions gave access to six-membered organic carbonates (3m) in 67% yields. We investigated the efficiency of the catalysts by reducing the amounts of iron complex 1b and 1i, and we were delighted to find that considerable conversions could still be achieved that resulted in very high initial TOF values (up to 2167 and 2733 h−1, respectively), in the presence of complex 1b or 1i (0.03 mol %) (see Table 3, entries 1 and 2). Importantly, the transformation did not require the TBAB co-catalyst under high-temperature conditions. Very recently, Capacchione et al.81 reported a catalytic system based on bimetallic Fe(III) complexes combined with TBAB. This catalyzed cycloaddition of CO2 to PO at an unprecedented (for Fe) TOF values of 5200 h−1 under 120 °C and 2.0 MPa of CO2. To test the catalytic efficiency of our catalysts, we heated the reaction system to 120 °C and reduced the catalyst loading from 0.03 to 0.01 mol %; TOF values up to 7900 h−1 were achieved (see Table 3, entry 3). The use of enantiopure terminal epoxides as substrates gave us the opportunity to gain insight into the reaction mechanism. The enantiomerically pure terminal epoxides (S)-2n was converted into the corresponding cyclic carbonates with retention of the configuration (see Table 4, entry 1). The result suggests that (S)-2n underwent a ring-opening attack at the methylene carbon (less hindered carbon atom), because ring opening at the methine carbon (more hindered carbon atom) would cause racemization of the chiral center. It is wellknown that terminal epoxides with electron-donating sub-

a

Conditions: PO (10.0 mmol), catalyst (0.030 mol, 0.3 mol %), cocatalyst (as indicated in this table), no solvent, 25 °C, 24 h. b Abbreviations: TBAB, tetrabutylammonium bromide; TBAI, tetrabutylammonium iodide; TBAC, tetrabutylammonium chloride; TBASO4H, tetrabutylammonium hydrogen sulfate; TBAO, tetrabutylammonium acetate; TOAB, tetraoctylammonium bromide; and PPNCl, bis(triphenylphosphoranylidene) ammonium chloride. cDetermined by GC using biphenyl as an internal standard.

catalyst was required for a reaction transformation (see Table 1, entries 12 and 13). A screening of various co-catalysts, such as TBASO4H, TBAO, TOAB, PPNCl, TBAI, TBAC, and TBAB, in combination with iron complex 1i was performed (see Table 1, entries 9 and 14−19). The combination of complex 1i and TBAB showed the highest catalytic activity (see Table 1, entry 9). Maginn and co-workers have proved that the most significant effect on CO2 solubility in imidazolium salts relies on the nature of the anion.89 Based on our experimental results and previous literature,89 we hypothesized that iron iodide complex 1i may not only play the role of catalyst, but also assist the solubility of CO2 in the reaction system. We inferred that an iron complex bearing I− as a counteranion possesses a higher CO2 solubility than those with Br− and Cl−. It well-known that Cl− has the highest nucleophilicity, whereas I− has the strongest leaving ability.57 Bromide is a good nucleophile and also a good leaving group, so that TBAB with bromide can effectively ring open the epoxide.63,81 The results suggest that the synergistic effects between iodide in iron complex 1i and bromide in TBAB are responsible for the high efficiency observed in the reaction. An improvement in the catalytic activity was observed by increasing the amount of TBAB from 1.5 mol % to 3.0 mol % 9069


Research Article

ACS Sustainable Chemistry & Engineering Table 2. Scope of Substratesa

Conditions: epoxides (10 mmol), 1i (27.5 mg, 0.029 mmol), TBAB (97.6 mg, 0.29 mmol), no solvent, 25 °C, 0.5 MPa CO2, 24 h. Isolated yield. At 45 °C. cAt 80 °C, 1.0 MPa CO2. dThe d.r. values were determined by 1H and 13C NMR spectroscopy.

a

b

Table 3. Catalytic Activity of Catalystsa

entry catalyst 1 2 3c

1b 1i 1i

temperature, T (°C)

yieldb (%)

turnover frequency, TOF (h−1)

100 100 120

65 82 79

2167 2733 7900

counteranion bearing iron complexes on the ratio of cis- and trans-cyclic carbonate products from the conversion of cis-2,3epoxybutane was examined using iron complexes 1b−1d (see Table 5). The results reveal that the retention of configuration in the cyclic carbonate product appears to have a strong dependence on the leaving ability of the counteranion. The leaving ability order of the anion is I− > Br− > Cl−. When iron complex 1d is used as a catalyst, the reaction proceeds exclusively through the SN2 pathway, because chloride is a poor leaving group, resulting in an overall retention of configuration (see Table 5, entry 3). In contrast, the use of iron complex 1b results in some loss of stereoselectivity from the original configuration of the starting epoxides, because of the iodides being a strong leaving group (see Table 5, entry 1). The loss of configuration suggests that the reaction underwent partially an SN1 pathway because a single attack at the chiral carbon atom would cause the inversion of configuration. This stereochemical divergence relating to two accessible catalytic pathways (SN1 and SN2) has been well-described by Werner and co-workers.84 In order to isolate the carbene CO2 adducts, we tried to convert 1i in the absence of the epoxide with CO2 under 100 °C for 30 min. However, we cannot find the desired intermediates. Next, the reaction of 1i with 2a, in the absence of CO2 under 100 °C for 30 min, was performed; however, the attempt to separate the two intermediates using column chromatography failed, because of the instability of active species. To obtain a clue about how the intermediates are formed in the reaction procedure, matrix-assisted laser desorption/ionization time-of-flight high-resolution mass spectrometry (MALDI-TOF-HRMS) methods were used to investigate the transformation. A MALDI-TOF-HRMS measurement of a mixture of 2a (10 mmol) and 1i (0.003 mmol) in

a

Conditions: PO (10.0 mmol), catalyst (0.0030 mmol, 0.03 mol %), 1.0 MPa CO2, no solvent, 100 °C, 1 h. bDetermined by GC using biphenyl as an internal standard. cIron complex 1i (0.0010 mmol, 0.01 mol %), 120 °C.

stituents, such as (S)-2n, favor attack at the methylene carbon (less-hindered carbon atom), while those electron-withdrawing substituents undergo attack at the methine carbon (morehindered carbon atom).57 To study the electronic effects in the epoxide, further two enantiopure terminal epoxides with electron-withdrawing substituents, such as (S)-2g and (R)-2g, were used as substrates to investigate the reaction mechanism. As shown in Table 4, partial racemization occurring in the reaction suggests that (S)-2g and (R)-2g undergo partially an SN1 reaction pathway through the attack at the methine carbon and, hence, an inversion of the configuration occurs. This is consistent with results reported by Kleij and Bo.90 The stereochemistry obtained from the conversion of either pure cis- or trans-2,3-epoxybutane also revealed some mechanistic insights into reaction pathways. The effect of the 9070


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ACS Sustainable Chemistry & Engineering Table 4. Investigation of the Reaction Mechanism by Enantiopure Terminal Epoxidesa

Conditions: epoxide (10.0 mmol), 1i (0.030 mmol), 0.5 MPa CO2, no co-catalyst or solvent, 80 °C, 1 h. bIsolated yield. cThe term “ee” denotes the enantiomeric excess of the resulting cyclic carbonates, which was determined by chiral HPLC (see the Supporting Information for details). a

Table 5. Variation of Iron Complexes 1b−1d during the Conversion of cis-2,3-Epoxybutane into cis- and trans-Cyclic Carbonatea

entry

catalyst

cis-3o (%)b

trans-3o (%)b

total yield (%)c

1 2 3

1b 1c 1d

55 68 >99

45 32 trace

63 53 46

a

Conditions: cis-2o (10.0 mmol), catalyst (0.0030 mmol, 0.03 mol %), 0.5 MPa CO2, no co-catalyst or solvent, 80 °C, 1 h. bDetermined by 1 H NMR (CDCl3) from the raw reaction mixture. cIsolated yield.

the absence of CO2 under 100 °C for 30 min displayed two peaks at m/z = 485.9877 and 304.1563, respectively, which pertains to the existence of a new Fe(II) complex (I) and a carbene (II) (see Figure 2). This result indicated that the Fe(II) complex falls apart in the presence of the epoxide and then produces an active Fe(II) complex and a carbene. In our developed system, the lower temperature reactions necessitate external nucleophiles such as TBAB for transformation, whereas at high reaction temperatures, no external nucleophiles need to be added. This co-catalyst-free phenomenon is different from that observed previously for iron(III) thioether-triphenolate complexes,81,82 iron(III) amino-bis(phenolate) complexes,83 and aluminum amino triphenolate complexes,91 in which a co-catalyst is required, even under high reaction temperature conditions. The results suggested that the two noncoordinating I− anions of the iron complexes may act as the requisite nucleophile under high-temperature conditions. The octahedral Fe(II) center indicates that no vacant site is available for the activation of the epoxide. Analysis of HRMS in Figure 2 implies that the Fe(II) complex falls apart in the reaction system and would then produce a Lewis acidic [Fe(II)]X2 (LA) and a carbene (NHC); carbenes, as proved by Lu28 and Werner,29 are efficient catalysts for the coupling between CO2 and epoxides. The above discussions suggest that the Fe(II) complexes probably act as reservoirs of two active species including LA

Figure 2. MALDI-TOF-HRMS experiment of intermediates I and II.

and NHC. Herein, we propose a possible reaction pathway based on the experimental results (see Scheme 2). This pathway is based on the concept of synergistic activation of the epoxide and CO2 by the LA and NHC resulting from the dissociation of octahedral Fe(II) complexes, respectively. First, the epoxide is activated through hydrogen bonding of the Fe atom bearing LA. Simultaneously, the halide anion initiates the ring opening of the epoxide through the nucleophilic attack on the epoxide to generate an intermediate of the alkoxide.39 The nucleophilic carbene would activate CO2 to form an imidazolium carboxylate; this step is well-recognized.28,29 A transfer carboxylation process of CO2 to the alkoxide and subsequent intramolecular nucleophilic substitution regenerates two catalytic active species LA and NHC, and simultaneously releases the cyclic carbonate. In the SN2 pathway, the double inversion occurs at the chiral carbon atom and thus leads to an overall retention of configuration (see Scheme 2, left). In the SN1 pathway, a carbenium ion is formed due to the halide anion possessing a strong leaving ability such as I− and Br−, and then the resultant sp2-hybridized carbon center undergoes an SN1 ring closure, and, hence, some loss of stereoselectivity from the 9071


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ACS Sustainable Chemistry & Engineering Scheme 2. Proposed Mechanism for the Iron Complex

Figure 4. Recycling experiments. Reaction conditions: PO (586.6 mg, 10.1 mmol), catalyst 1i (27.5 mg, 0.029 mol), CO2 (0.5 MPa), TBAB (97.6 mg, 0.29 mmol), no solvent, 25 °C, 24 h.

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CONCLUSIONS In conclusion, we describe a new and easily accessible bis-CNN pincer complex based on inexpensive and Earth-abundant iron. This material is a highly active and recyclable catalyst for the synthesis of cyclic carbonates. The iron complex 1i, in combination with tetrabutylammonium bromide (TBAB), can catalyze the reaction of terminal epoxides to CO2 at room temperature and 0.5 MPa CO2 to form cyclic carbonates. In the absence of TBAB, iron complex 1i shows high catalytic activity for PO with turnover frequency (TOF) values as high as 7900 h−1 at 120 °C under 1.0 MPa CO2. To the best of our knowledge, these are the highest TOF values known for an iron-based catalyst. The substrate scope was evaluated, and the catalyst system displayed a broad substrate scope, including alkyl, aryl, and functionalized terminal epoxides, as well as internal epoxides and oxetanes, to give the corresponding cyclic carbonates in good yields and excellent selectivity. Moreover, the iron catalyst can be easily recycled six times without significant loss of activity by simple filtration. Mechanical studies reveal that the stereochemical divergence in the cyclic carbonate product has a distinct dependence both on the nature of the counteranion of catalysts and on the electronic nature of the terminal epoxides. The knowledge obtained through these studies provides insight into the rational development of highly diastereoselective catalysts.

original configuration of the starting epoxides occurs (see Scheme 2, right). In addition to the catalytic activity, the recyclability and stability potential also play important roles in possible industrial applications. The homogeneous catalyst used in cycloaddition was separated through distillation under reduced pressure,31,37 extraction,92 and centrifugation.93 Therefore, catalyst 1i could evaluate its reusability to seek a simple and efficient protocol for catalyst recycling. In each cycle, the catalyst could easily be precipitated from the PC product phase via the addition of ethyl acetate (Figure 3). After filtration, the recycling catalyst is

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01990. NMR spectra of the iron complexes and prepared cyclic carbonates (PDF) Crystallographic data of the iron complex 1i (CIF) HPLC spectra for (S)-2n, (S)-2g, and (R)-2g (PDF)

Figure 3. Recycling of iron catalyst. Reaction conditions: PO (586.6 mg, 10.1 mmol), catalyst 1i (27.5 mg, 0.029 mol), CO2 (0.5 MPa), TBAB (97.6 mg, 0.29 mmol), no solvent, 25 °C, 24 h.

directly used without further purification processes, and another batch of PO and TBAB was added and reused for subsequent recycles. Figure 4 shows no decrease in PC yield after six runs, and this indicated high stability and reusability for catalyst 1i. The recycled catalyst 1i was characterized by NMR analysis, as shown in Figure S2 in the Supporting Information for details). There was no difference in the 1H NMR spectra between the fresh and used catalysts, which confirmed the good structural stability of this catalyst.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: (+0086)-0993-205-7277. Fax: (+0086)-0993-205-7270. E-mail addresses: [email protected], [email protected] (N. Liu). 9072


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ACS Sustainable Chemistry & Engineering *Tel.: (+0086)-0993-205-7277. Fax: (+0086)-0993-205-7270. E-mail address: [email protected] (B. Dai).

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ORCID

Ning Liu: 0000-0001-7299-0400 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

Support was provided by the National Natural Science Foundation of China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the support from the National Natural Science Foundation of China (Grant Nos. U1603103, 21466033), and the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT_15R46), and Yangtze River scholar research project of Shihezi University (Grant No. CJXZ201601). We thank Dr. Dengtai Chen (Shihezi University) for X-ray structure analysis.

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Iron(II) Bis-CNN Pincer Complex-Catalyzed Cyclic Carbonate

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