Metal complexes bearing 2-(imidazol-2-yl)phenol ligands: synthesis, characterization and catalytic performance in the fixation of carbon dioxide with epoxides

Article abstract of DOI:10.1039/c5ra08237d

A series of metal complexes bearing 2-(imidazol-2-yl)phenol ligands (Zn, Cu, Ni, Co, Pb) were synthesized and their structures were characterized by IR, NMR, elemental analysis and X-ray. The catalytic activities of all complexes for the coupling reaction of CO₂ and epoxide were then detected. The activity influence factors, such as temperature, time, pressure, substituents of ligands and metal centre, were systematically investigated. All these complexes were efficient to catalyze the coupling of CO₂ and epoxide to generate cyclic carbonate in perfect yields (>90%) and selectivities (>99%) under optimized conditions of (2 MPa, 5 h, 110 °C) without any organic solvents. A 99.7% yield and >99% selectivity for propylene carbonate (PC) were obtained with C₇/n-Bu₄NI as catalyst system under the optimized conditions. The catalysts were also proved to be applicable to other terminal epoxides. It is worthy noted that the Pb(ii) complex was firstly used to catalyze the coupling reaction of epoxides with carbon dioxide. Moreover, these metal catalysts were recyclable with only minor losses in catalytic activity after simple separation. Finally, a plausible mechanism was given.

Full text of DOI:10.1039/c5ra08237d

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ARTICLE TYPE
Metal Complexes Bearing 2-(Imidazol-2-yl)phenol Ligands: Synthesis,
Characterization and Catalytic Performance in the Fixation of Carbon
Dioxide with Epoxides
Jing Penga,b, HaiꢀJian Yang*,b Zidong Weia, CunꢀYue Guo*,c
5
Received (in XXX, XXX) Xth XXXXXXXXX 2015, Accepted Xth XXXXXXXXX 2015
DOI: 10.1039/b000000x
A series of metal complexes bearing 2ꢀ(imidazolꢀ2ꢀyl)phenol ligands (Zn, Cu, Ni, Co, Pb) were
synthesized and their structures were characterized by IR, NMR, elemental analysis and Xꢀray. The
catalytical activities of all complexes for the coupling reaction of CO2 and epoxide were then detected.
10 The activity influence factors, such as temperature, time, pressure, substituents of ligands and metal
centre, were systematically investigated. All these complexes were efficient to catalyze the coupling of
CO2 and epoxide to generate cyclic carbonate in perfect yields (>90%) and selectivities (>99%) under
optimized conditions of (2 MPa, 5 h, 110 °C) without any organic solvents. A 99.7% yield and >99%
selectivity for propylene carbonate (PC) were obtained with C7/nꢀBu4NI as catalyst system under the
15 optimized conditions. The catalysts were also proved to be applicable to other terminal epoxides. It is
worthy noted that the Pb(II) complex was firstly used to catalyze the coupling reaction of epoxides with
carbon dioxide. Moreover, these metal catalysts were recyclable with only minor losses in catalytic
activity after simple separation. Finally, a plausible mechanism was given.
from long reaction time39,40 and the need for toxic coꢀsolvent.41ꢀ43
Therefore, developing stable, efficient and solventless catalytic
system for the synthesis of cyclic carbonate is still highly
required. Herein, we reported the synthesis and characterization
50 of a series of metal complexes bearing 2ꢀ(imidazolꢀ2ꢀyl)phenol
ligands, which were proven to be efficient and recyclable
catalysts for the reaction of various epoxides with CO2 to
selectively yield cyclic carbonates under mild conditions without
any organic solvents. The catalytic performance has been
55 systematically investigated, and the optimization of the catalytic
system to produce cyclic carbonates with maximal yields was
performed. The recyclability of the catalysts was also been
detected. Furthermore, a proposed mechanism was given. To the
best of our knowledge, the Pb(II) chelate complex was firstly
60 used to catalyze the coupling reaction of epoxides with carbon
dioxide in good catalytic activity and high selectivity.
1.
Introduction
20 As an alternative, sustainable feedstock for the chemical industry,
the conversion of CO2 into useful chemicals has become a public
concern due to the global warming and severe energy crisis.1ꢀ4
The coupling reaction of CO2 with epoxides to produce either
polycarbonates5ꢀ7 or cyclic carbonates8ꢀ11 is considered to be one
25 of the promising routes for CO2 utilization. The cyclic
carbonates, especially fiveꢀmembered cyclic carbonates, are one
kind of CO2 fixation products and widely used as monomer for
polymer synthesis, aprotic solvents, electrolytes for lithiumꢀion
batteries, and as intermediates in the manufacture of fine
30 chemicals.12ꢀ14 This approach is 100% atom economical and
benefits from eliminating phosgene as a reagent.
Recently, numerous homogeneous and heterogeneous catalysts
have been used for the synthesis of the cyclic carbonates.
Homogeneous catalysts include salen, porphyrin, phthalocyanine
35 and other complexes of the main group and transition metals,15ꢀ18
2.
Experimental section
quaternary
ammonium
salts,19
ionic
liquids,20,21
,
2.1.
Chemicals and analytical methods
polyoxometalates22, Lewis acids or bases23ꢀ25 and so on. Some
heterogeneous catalysts26 also have been explored for the
cycloaddition of CO2 to epoxides, such as metal oxides,27
40 immobilized complexes or ionic liquids,28ꢀ33 titanosilicates,34 and
zeolites.35 There are also few examples of the use of the metal–
organic frameworks (MOFs).36ꢀ38 Among these catalyst systems,
metal complexes have been of significant interest due to their
easy synthesis and excellent stability against moisture and air.39ꢀ43
45 Nevertheless, although the advances are significant, most suffer
The detailed information of the materials used in present work is
65 listed in Table S1 (see supporting information). The epoxides
were distilled from CaH2.
NMR spectra were recorded on a Bruker Alꢀ400 MHz
instrument using TMS as an internal standard. IR spectra were
recorded on a PerkinꢀElmer 2000 FTꢀIR spectrometer. Elemental
70 analysis was conducted on a PE 2400 series II CHNS/O
elemental analyzer. Melting point was obtained from Xꢀ4ꢀtype
digital microꢀmelting point apparatus. Xꢀray diffraction studies
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were performed on a BrukerꢀAPEX diffractometer equipped with
a CCD area detector, MoKaꢀradiation (λ= 0.71073 Å), and a
graphite monochromator. Elemental analyses were performed on
3321, 3069, 1450, 1367, 1259, 1077, 697. 1H NMR (400 MHz,
DMSO) δ 8.34 (d, J = 2.3 Hz, 1H), 7.79 (d, J = 2.3 Hz, 1H), 7.54
(d, J = 7.0 Hz, 5H), 7.44 (s, 7H). 13C NMR (101 MHz, DMSO) δ
a Flash EA 1112 microanalyzer. IR spectra were recorded on a 45 155.88, 146.49, 137.37, 134.63, 134.28, 129.97, 129.91, 128.12,
5
Nicolet 6700 FTꢀIR spectrometer as KBr discs in the range of
4000ꢀ600 cmꢀ1.
127.70, 122.95, 118.37, 116.64. Anal. Calcd for C21H14N2OBr2:
C, 53.62; H, 2.98; N, 5.96%. Found: C, 53.65; H, 3.01; N, 5.97%.
L4 (2,4ꢀdiiodoꢀ6ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenol):
2.2.
The synthesis of ligands L1–L9 followed the general procedure
shown in Scheme 1.44
mixture of salicylaldehyde or
Synthesis of ligands L1–L9
o
48% yield. Mp. 203 – 205 C. Selected IR peaks (KBr, cmꢀ1): ν
1
50 3331, 3064, 1443, 1378, 1257, 1001, 697. H NMR (400 MHz,
A
DMSO) δ 8.45 (d, J = 2.0 Hz, 1H), 8.02 (d, J = 2.0 Hz, 1H), 7.54
(d, J = 7.1 Hz, 5H), 7.46 – 7.41 (m, 7H). 13C NMR (101 MHz,
DMSO) δ 163.91, 147.89, 145.66, 137.49, 133.28, 129.37, 129.15,
128.73, 127. 38, 121.57, 99.99, 99.84. Anal. Calcd for
55 C21H14N2OI2: C, 44.68; H, 2.48; N, 4.96%. Found: C, 44.64; H,
2.50; N, 4.94%.
10 corresponding salicylaldehyde derivatives (9.51 mmol), benzil
(9.51 mmol), and ammonium acetate (20 equivalent) were
refluxed for 2 h in glacial acetic acid (30 mL) under N2 to form a
precipitate. An excess of deꢀionised water was added to complete
the precipitation. The crude product was collected by filtration,
15 washed with water, and dried by suction. The resulting solid was
dissolved in CH2Cl2 and dried over MgSO4. The solution was
filtered and the solvent was evaporated from the filtrate to
produce a powder. After recrystallization from CH2Cl2–pentane,
a crystalline solid was obtained.
L5
(4ꢀmethylꢀ2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenol):
o
61% yield. Mp. 190 – 192 C. Selected IR peaks (KBr, cmꢀ1): ν
1
3280, 3035, 1505, 1378, 1249, 1073, 695. H NMR (400 MHz,
60 DMSO) δ 12.97 (s, 1H), 12.67 (s, 1H), 7.89 (d, J = 1.4 Hz, 1H),
7.58 – 7.23 (m, 10H), 7.09 (m, 1H), 6.88 (d, J = 8.3 Hz, 1H), 2.30
(s, 3H). 13C NMR (101 MHz, DMSO) δ 155.06, 146.45, 134.70,
131.17, 130.79, 129.17, 128.95, 128.70, 127.81, 125.52, 117.07,
112.98, 20.69. Anal. Calcd for C22H18N2O: C, 80.98; H, 5.52; N,
65 8.59%. Found: C, 80.95; H, 5.49; N, 8.58%.
R3
R2
R1
O
OH
R3
H
N
O
R1
R2
CHO
CH3COOH
+
+
CH3COONH4
OH
N
L6
(4ꢀchloroꢀ2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenol):
L1ꢀL9
o
49% yield. Mp. 198 – 200 C. Selected IR peaks (KBr, cmꢀ1): ν
R1
R2
R3
L1
H
H
H
71
L2
Cl
H
Cl
70 74
L3
Br
H
L4
I
H
I
L5
H
H
CH3 Cl
61 49
L6
H
H
L7
H
H
L8
H
H
L9
H
OCH3
H
1
3224, 2370, 1489, 1374, 1254, 1077, 696. H NMR (400 MHz,
Br
Br
NO2
Yield(%)
48
37 56
91
DMSO) δ 13.14 (s, 1H), 13.05 (s, 1H), 8.18 (d, J = 2.6 Hz, 1H),
70 7.59 – 7.43 (m, 7H), 7.33 (m, 4H), 7.02 (d, J = 8.8 Hz, 1H). 13C
NMR (101 MHz, DMSO) δ 153.14, 147.07, 133.61, 130.25,
129.71, 129.53, 128.96, 128.83, 127.75, 127.40, 120.11, 117.93.
Anal. Calcd for C21H15N2OCl: C, 72.73; H, 4.33; N, 8.08%.
Found: C, 72.77; H, 4.35; N, 8.08%.
Ph
R3
R2
R1 Ph
R2
R1
NH
Ethanol
H
N
N
O
N
R3
M
Metal salt
C1ꢀC9 Zn(CH3COO)2—2H2
OH
N
O
R3
O
HN
Ph
C10
C11
C12
C13
Cu(CH3COO)2—H2O
Ph
R1
R2
Pb(CH3COO)2—3H2
NiCl2—6H2O
O
L1ꢀL9
C1ꢀC13
CoCl2—6H2
O
75
L7
(4ꢀbromoꢀ2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenol):
M
R1
R2
R3
C1
Zn
H
H
H
C2
Zn
Cl
H
Cl
99
C3
Zn
Br
H
Br
98
C4
Zn
I
H
I
C5
Zn
H
H
CH3
82
C6
Zn
H
H
Cl
93
C7
Zn
H
H
Br
86
C8
Zn
H
H
NO2
93
C9
Zn
H
OCH3
H
88
C10
Cu
H
H
H
C11
Pb
H
H
H
C12 C13
37% yield. Mp. 181 – 183 C. Selected IR peaks (KBr, cmꢀ1): ν
Ni
H
Co
H
o
1
H
H
3207, 2370, 1485, 1369, 1253, 1075, 696. H NMR (400 MHz,
H
H
Yield(%)
96
92
99
49
12
26
20
DMSO) δ 13.14 (s, 1H), 13.08 (s, 1H), 8.31 (d, J = 2.4 Hz, 1H),
7.53 (s, 5H), 7.46 – 7.34 (m, 5H), 6.97 (d, J = 8.8 Hz, 2H). 13C
Scheme 1 Synthesis of ligands L1ꢀL9 and complexes C1ꢀC13. The
molecular structures of ligands are only different by substituents.
80 NMR (101 MHz, DMSO) δ 154.28, 147.95, 134.77, 133.82,
133.34, 129.61, 129.07, 128.93, 127.81, 120.89, 119.02, 116.28.
Anal. Calcd for C21H15N2OBr: C, 64.85; H, 3.84; N, 7.16%.
Found: C, 64.85; H, 3.85; N, 7.19%.
L1 (2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenol): 71% yield.
Mp. 202 – 204 C. Selected IR peaks (KBr, cmꢀ1): ν 3208, 3058,
o
25 1598, 1539, 1135, 1074, 695. 1H NMR (400 MHz, DMSO) δ
13.04 (s, 1H), 12.96 (s, 1H), 8.05 (d, J = 7.6 Hz, 1H), 7.59 – 7.37
(m, 7H), 7.37 – 7.19 (m, 4H), 6.97 (m, 2H). 13C NMR (101 MHz,
DMSO) δ 157.18, 146.33, 134.64, 130.76, 128.98, 128.80, 127.78,
127.54, 127.27, 119.33, 117.30, 113.36. Anal. Calcd for
30 C21H16N2O: C, 80.77; H, 5.13; N, 8.97%. Found: C, 81.70; H,
5.15; N, 8.95%.
L8 (4ꢀnitroꢀ2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenol): 56%
85 yield. Mp. 250 – 252 C. Selected IR peaks (KBr, cmꢀ1): ν 3295,
o
2367, 1486, 1334, 1296, 1130, 696. 1H NMR (400 MHz, DMSO)
δ 14.12 – 13.82 (m, 2H), 9.16 (d, J = 2.7 Hz, 1H), 8.18 (m, 1H),
7.61 – 7.49 (m, 4H), 7.42 (m, 6H), 7.19 (d, J = 9.1 Hz, 1H). 13C
NMR (101 MHz, DMSO) δ 158.32, 144.51, 138.79, 130.03,
90 129.55, 129.31, 128.47, 128.22, 124.92, 123.76, 118.23, 99.99.
Anal. Calcd for C21H15N3O2: C, 73.90; H, 4.40; N, 12.32%.
Found: C, 73.86; H, 4.42; N, 12.34%.
L2 (2,4ꢀdichloroꢀ6ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenol):
70% yield. Mp. 209 – 210 C. Selected IR peaks (KBr, cmꢀ1): ν
3315, 3063, 1463, 1373, 1262, 1079, 696. H NMR (400 MHz,
35 DMSO) δ 8.18 (d, J = 2.5 Hz, 2H), 7.56 (m, 8H), 7.44 (s, 4H).
13C NMR (101 MHz, DMSO) δ 156.13, 146.57, 134.24, 131.37,
129.20, 129.11, 127.95, 127.88, 127.31, 125.63, 125.45, 118.29.
Anal. Calcd for C21H14N2OCl2: C, 66.14; H, 3.67; N, 7.35%.
Found: C, 66.17; H, 3.67; N, 7.34%.
o
1
L9 (5ꢀmethoxyꢀ2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenol):
91% yield. Mp. 220 – 221 C. Selected IR peaks (KBr, cmꢀ1): ν
o
1
95 3225, 3061, 2939, 1604, 1445, 1268, 1201, 1075, 696. H NMR
(400 MHz, DMSO) δ 13.11 (s, 1H), 12.84 (s, 1H), 7.95 (d, J = 8.4
Hz, 1H), 7.48 (m, 7H), 7.31 (m, 3H), 6.60 – 6.52 (m, 2H), 3.79 (s,
3H). 13C NMR (101 MHz, DMSO) δ 162.03, 156.19, 146.97,
132.92, 129.37, 129.13, 128.91, 128.32, 127.46, 110.98, 108.47,
40
L3 (2,4ꢀdibromoꢀ6ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenol):
74% yield. Mp. 193 – 195 C. Selected IR peaks (KBr, cmꢀ1): ν
o
2
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102.95, 54.73. Anal. Calcd for C22H18N2O2: C, 77.19; H, 5.26; N,
8.19%. Found: C, 77.15; H, 5.30; N, 8.16%.
peaks (KBr, cmꢀ1): ν 3054, 1598, 1478, 1379, 1306, 1242, 696.
60 1H NMR (400 MHz, DMSO) δ 12.77 (s, 2H), 7.87 (d, J = 2.9 Hz,
2H), 7.55 – 7.21 (m, 10H), 7.20 – 6.97 (m, 8H), 6.88 (t, J = 7.9
Hz, 4H), 6.74 (d, J = 9.0 Hz, 2H). 13C NMR (101 MHz, DMSO) δ
152.37, 135.72, 132.04, 130.26, 129.78, 129.53, 128.98, 128.64,
128.30, 128.16, 120.31, 118.89, 56.49. Anal. Calcd for
65 C42H28N4O2Cl2Zn: C, 66.67; H, 3.70; N, 7.41%. Found: C, 66.70;
H, 3.72; N, 7.45%.
2.3.
Synthesis of complexes C1–C13
The synthesis of complexes C1–C13 followed the general
procedure shown in Scheme 1.44 Ligands L1–L9 (12.50 mmol)
and corresponding metal salts (6.30 mmol) were dissolved in 50
mL of ethanol and refluxed at 70 oC for 3 h. The resulting
precipitate was filtered, washed with a few milliliters of ethanol,
and dried to obtain complexes C1–C13.
C1 (bis(2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenoxy)zinc) ：
96% yield. Mp. 336 – 338 oC. Selected IR peaks (KBr, cmꢀ1): ν
3414, 3057, 1604, 1534, 1481, 1312, 1260, 697. 1H NMR (400
MHz, DMSO) δ 12.62 (s, 2H), 7.76 (d, J = 6.3 Hz, 2H), 7.32 (m,
10H), 7.16 (t, J = 7.8 Hz, 2H), 7.12 – 6.92 (m, 6H), 6.84 (t, J =
15 7.7 Hz, 4H), 6.75 (d, J = 8.3 Hz, 2H), 6.55 (t, J = 6.8 Hz, 2H).
13C NMR (101 MHz, DMSO) δ 156.10, 136.93, 133.24, 130.79,
129.76, 129.48, 128.92, 128.74, 127.68, 121.86, 119.05, 116.65,
58.23. Anal. Calcd for C42H30N4O2Zn•COOCH3: C, 70.78; H,
4.42; N, 7.51%. Found: C, 70.75; H, 4.43; N, 7.54%.
5
C7
(bis(4ꢀbromoꢀ2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀ
yl)phenoxy)zinc): 86% yield. Mp. 383 – 385 oC. Selected IR
peaks (KBr, cmꢀ1): ν 3054, 2370, 1598, 1478, 1376, 1306, 1242,
10
1
70 697. H NMR (400 MHz, DMSO) δ 12.78 (s, 2H), 7.98 (s, 2H),
7.59 – 7.16 (m, 12H), 7.04 (m, 6H), 6.89 (t, J = 7.7 Hz, 4H), 6.70
(d, J = 9.1 Hz, 2H). 13C NMR (101 MHz, DMSO) δ 154.93,
137.63, 134.68, 133.27, 133.19, 128.99, 128.67, 128.31, 128.17,
120.97, 118.54, 104.20, 55.94. Anal. Calcd for C42H28N4O2Br2Zn:
75 C, 59.64; H, 3.31; N, 6.63%. Found: C, 59.61; H, 3.34; N, 6.62%.
C8
(bis(4ꢀnitroꢀ2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀ
yl)phenoxy)zinc): 93% yield. Mp. 399 – 401 oC. Selected IR
peaks (KBr, cmꢀ1): ν 3245, 1604, 1563, 1484, 1306, 1133, 693.
1H NMR (400 MHz, DMSO) δ 13.35 (s, 2H), 8.94 (s, 2H), 8.07 (s,
80 2H), 7.35 (d, J = 18.8 Hz, 10H), 6.98 (d, J = 20.9 Hz, 6H), 6.93 (d,
4H), 6.84 – 6.79 (m, 2H). 13C NMR (101 MHz, DMSO) δ 162.04,
141.77, 136.76, 133.12, 129.73, 129.34, 128.74, 127.89, 123.86,
122.75, 119.84, 117.43, 59.88. Anal. Calcd for C42H28N6O6Zn: C,
64.86; H, 3.60; N, 10.81%. Found: C, 64.84; H, 3.61; N, 10.86%.
20
C2
(bis(2,4ꢀdichloroꢀ6ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀ
yl)phenoxy)zinc): 99% yield. Mp. 340 – 342 oC. Selected IR
peaks (KBr, cmꢀ1): ν 3414, 3180, 1599, 1529, 1467, 1392, 1247,
698. 1H NMR (400 MHz, DMSO) δ 12.98 (s, 2H), 10.29 (d, 4H),
7.29 – 7.89 (m, 14H), 6.86 – 7.06 (m, 6H). 13C NMR (101 MHz,
25 DMSO) δ 154.63, 135.86, 133.81, 131.02, 129.74, 129.29, 128.85,
128.82, 127.39, 126.83, 126.21, 121.43, 53.97. Anal. Calcd for
C42H26N4O2Cl4Zn•COOCH3: C, 59.73; H, 3.28; N, 6.33%. Found:
C, 59.71; H, 3.30; N, 6.34%.
85
C9
(bis(5ꢀmethoxyꢀ2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀ
yl)phenoxy)zinc): 88% yield. Mp. 356 – 358 oC. Selected IR
peaks (KBr, cmꢀ1): ν 3226, 3053, 1606, 1546, 1447, 1322, 1155,
1
C3
(bis(2,4ꢀdibromoꢀ6ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀ
696. H NMR (400 MHz, DMSO) δ 12.43 (s, 2H), 7.69 (d, J =
30 yl)phenoxy)zinc): 98% yield. Mp. 335 – 337 oC. Selected IR
8.8 Hz, 2H), 7.31 (m, 10H), 7.04 (m, 6H), 6.88 (t, J = 7.6 Hz, 4H),
peaks (KBr, cmꢀ1): ν 3412, 3063, 1596, 1525, 1462, 1389, 1249,
90 6.27 (d, J = 2.7 Hz, 2H), 6.23 – 6.15 (m, 2H), 3.75 (s, 6H). 13C
NMR (101 MHz, DMSO) δ 168.40, 162.47, 148.89, 133.58,
129.69, 128.95, 128.69, 128.32, 128.08, 127.49, 126.23, 106.74,
102.30, 55.13. Anal. Calcd for C44H34N4O4Zn: C, 70.68; H, 4.55;
N, 7.50%. Found: C, 70.66; H, 4.54; N, 7.51%.
1
699. H NMR (400 MHz, DMSO) δ 13.00 (s, 2H), 8.03 (s, 2H),
7.67 (s, 2H), 7.34 (d, J = 10.4 Hz, 10H), 7.02 (d, J = 7.0 Hz, 6H),
6.89 (s, 4H). 13C NMR (101 MHz, DMSO) δ 158.25, 136.14,
35 135.98, 133.72, 133.31, 129.63, 129.28, 128.90, 127.48, 122.79,
118.37, 116.62, 58.23. Anal. Calcd for C42H26N4O2Br4Zn: C,
50.25; H, 2.59; N, 5.58%. Found: C, 50.29; H, 2.54; N, 5.59%.
95
C10 (bis(2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenoxy)copper):
o
99% yield. Mp. 226 – 228 C. Selected IR peaks (KBr, cmꢀ1): ν
C4
(bis(2,4ꢀdiiodoꢀ6ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀ
3410, 3057, 1604, 1540, 1480, 1315, 1142, 696. A resolvable
NMR spectrum could not be measured owing to
paramagnetism.45 Anal. Calcd for C42H30N4O2Cu•CH3CH2OH: C,
yl)phenoxy)zinc): 92% yield. Mp. 280 – 281 oC. Selected IR
40 peaks (KBr, cmꢀ1): ν 3418, 1594, 1505, 1455, 1390, 1251, 696.
1H NMR (400 MHz, DMSO) δ 12.97 (s, 2H), 8.12 (s, 2H), 7.93 (s,
2H), 7.34 (d, J = 6.2 Hz, 10H), 7.01 (d, J = 6.7 Hz, 6H), 6.87 (s,
4H). 13C NMR (101 MHz, DMSO) δ 163.23, 145.86, 136.94,
137.63, 133.28, 129.49, 129.42, 128.84, 127.63, 127.49, 89.81,
45 89.58, 60.16. Anal. Calcd for C42H26N4O2I4Zn•COOCH3: C,
42.24; H, 2.32; N, 4.48%. Found: C, 42.25; H, 2.34; N, 4.50%.
100 72.18; H, 4.92; N, 7.66%. Found: C, 72.17; H, 4.89; N, 7.68%.
C11 (bis(2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenoxy)lead):
o
49% yield. Mp. 234 – 236 C. Selected IR peaks (KBr, cmꢀ1): ν
3418, 3060, 1600, 1528, 1482, 1245, 1139, 699. A resolvable
NMR spectrum could not be measured owing to
105 paramagnetism.46 Anal. Calcd for C21H15ON2Pb•2COOCH3: C,
47.17; H, 3.30; N, 4.40%. Found: C, 47.18; H, 3.32; N, 4.41%.
C12 (bis(2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenoxy)nickel):
C5
(bis(4ꢀmethylꢀ2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀ
yl)phenoxy)zinc): 82% yield. Mp. 352 – 354 oC. Selected IR
peaks (KBr, cmꢀ1): ν 3052, 2920, 1598, 1494, 1444, 1305, 1242,
50 696. 1H NMR (400 MHz, DMSO) δ 12.57 (s, 2H), 7.98 – 7.82 (m,
2H), 7.60 (s, 2H), 7.30 (m, 12H), 7.00 (m, 6H), 6.93 – 6.74 (m,
4H), 2.27 (d, J = 20.4 Hz, 4H), 1.84 (s, 2H). 13C NMR (101 MHz,
DMSO) δ 152.91, 135.22, 133.34, 131.96, 131.63, 130.12, 129.62,
129.43, 128.69, 127.67, 118.74, 116.83, 56.29, 25.31. Anal.
55 Calcd for C44H34N4O2Zn: C, 73.85; H, 4.76; N, 7.83%. Found: C,
73.81; H, 4.77; N, 7.86%.
o
12% yield (0.03 g). Mp. 360 – 363 C. Selected IR peaks (KBr,
cmꢀ1): ν 3424, 3062, 1604, 1533, 1483, 1253, 1142, 696. A
110 resolvable NMR spectrum could not be measured owing to
paramagnetism.47 Anal. Calcd for C42H30N4O2Ni•Cl•CH3CH2OH:
C, 69.27; H, 4.72; N, 7.35%. Found: C, 69.27; H, 4.70; N, 7.37%.
C13 (bis(2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀyl)phenoxy)cobalt):
o
26% yield. Mp. 362 – 365 C. Selected IR peaks (KBr, cmꢀ1): ν
115 3412, 3060, 1604, 1533, 1482, 1243, 1143, 696. A resolvable
NMR spectrum could not be measured owing to
C6
(bis(4ꢀchloroꢀ2ꢀ(4,5ꢀdiphenylꢀ1Hꢀimidazolꢀ2ꢀ
yl)phenoxy)zinc): 93% yield. Mp. 362 – 364 oC. Selected IR
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paramagnetism.48 Anal. Calcd for C42H30N4O2Co•Cl•CH3CH2OH:
C, 69.25; H, 4.72; N, 7.34%. Found: C, 69.23; H, 4.73; N, 7.31%.
50 The activity of various additives was tested using the reaction of
PO and CO2 to produce PC, and the results are summarized in
Table 3. Both the catalyst C1 and the coꢀcatalyst nꢀBu4NI could
catalyze the cycloaddition alone, but the yields of PC were very
low (entries 1, 2). As expected, the addition of moderate amounts
55 of coꢀcatalyst could greatly improve the yield of PC, TON value
and TOF value (entry 3 vs 1, 2), especially for the quaternary
ammonium salts. As for the quaternary ammonium salts (nꢀBu4NI,
nꢀBu4NBr, and nꢀBu4NCl), the effect of halide anions Iꢀ, Brꢀ, and
Clꢀ on the catalytic activity(entries 3ꢀ5) showed that the catalytic
60 activity improved with the increase of leaving ability (Iꢀ > Brꢀ >
Clꢀ).53 Under the same condition, however, the activity decreased
with other coꢀcatalysts like Et4NBr, PPh3, DMAP and KI (entries
6ꢀ9). Therefore, nꢀBu4NI was selected as the coꢀcatalyst to study
the effect of reaction conditions on the coupling reaction in the
65 presence of catalyst C1.
2.4.
General procedure for the coupling reaction of
epoxides and CO2
5
A typical procedure for the coupling reaction of CO2 and epoxide
was as following (Scheme 2): 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
10 solvent. The reactor vessel was sealed and immersed into a oil
bath at the desired temperature under stirring. Then, the CO2 was
pressurized into the reactor to the given pressure and the reaction
started. After the given time, the reaction was stopped and the
vessel was then cooled quickly by placing in an ice water and the
15 pressure was released slowly. The result mixture was transferred
to a 50 ml round bottom flask. The unreacted propylene oxide
was removed in vacuo. The yield was calculated either by taking
the weight of the result product or by comparing the ratio of the
Table 1 Crystallographic data for complexes C1 and C5
Identification code
Empirical formula
Formula weight (g molꢀ1
Crystal size (mm3)
Crystal system
Space group
Unit cell dimensions
a (Å)
C1—3CH3OH
C5—2CH3OH
C44H34N4O2Zn—C2H8O2
780.21
C42H30N4O2Zn—C3H12O3
784.20
)
0.12 × 0.10 × 0.10
Triclinic
0.12 × 0.10 × 0.10
Monoclinic
1
product to substrate peak areas obtained by H NMR analysis.
P
1
C2/c
20 The selectivity of cyclic carbonate was determined by GC/MS
(HP6890/5973). For all the experiments with different catalysts,
no byproduct was detected.
12.366(3)
12.634(3)
14.695(4)
67.273(3)
67.681(4)
75.098(4)
1942.4(8)
2
28.552(3)
13.6389(12)
19.5849(18)
90
b (Å)
c (Å)
α
β
γ
(°)
(°)
(°)
94.9510(10)
90
C
C
C
C
C
C
C
C
C
C
C
C
C
: R = H, R = H, R = H, M = Zn
1 2 3
1
Ph
NH
: R = Cl, R = H, R = Cl, M = Zn
2
1
2
3
R
R
: R = Br, R = H, R = Br, M = Zn
Volume (Å3)
1
Ph
2
7598.3(12)
8
3
1
2
3
: R = I, R = H, R = I, M = Zn
4
1
2
3
Z
R
3
O
N
: R = H, R = H, R = CH , M = Zn
5
1
2
3
3
Dcalc (g/cm3)
M
1.341
1.364
: R = H, R = H, R = Cl, M = Zn
6
1
2
3
R
O
N
3
: R = H, R = H, R = Br, M = Zn
7
1
2
3
0.684
0.697
σ
(mmꢀ1
)
HN
: R = H, R = H, R = NO , M = Zn
Ph
8
1
2
3
2
R
1
R
2
36849
7972
Collected refl.
: R = H, R = OCH , R = H, M = Zn
9
1
2
3
3
Ph
nꢀBu NI
O
O
R
R
+
CO
2
: R = H, R = H, R = H, M = Cu
7490(0.0504)
502
10991(0.0787)
502
Independent refl. (Rint)
10
11
12
13
1
2
3
4
O
: R = H, R = H, R = H, M = Pb
O
1
2
3
Parameters
: R = H, R = H, R = H, M = Ni
1
2
3
0.0779
0.0363
0.1130
1.101
R1[I> 2σ(I)]
wR2 (all data)
: R = H, R = H, R = H, M = Co
1
2
3
0.2323
25 Scheme 2 Cycloaddition of PO and CO2 to give PC and the metal
catalysts C1ꢀC13 used in this study.
1.080
GOF
3.
Results and discussion
Table 2 Selected bond distances (Å) and angles (º) for compounds C1 and
70 C5
3.1. X-ray crystallographic studies
C5—2CH3OH
C1—3CH3OH
The compounds C1 and C5 could be crystallized by slow
30 evaporation of concentrate methanol solution. The formation of
the complexes C1 and C5 was confirmed using single crystal Xꢀ
ray crystallography (Fig. 1), and their crystallographic data,
selected bond lengths and angles were reported in Table 1 and
Bond distances (Å)
Zn(1)ꢀO(2)
1.9375(9)
1.9336(9)
1.9925(11)
1.9801(11)
1.944(2)
1.940(2)
1.972(3)
1.979(3)
Zn(1)ꢀO(1)
Zn(1)ꢀN(1)
Zn(1)ꢀN(3)
Table
2,
respectively.
The
Xꢀray
crystallographic
Bond angles (°)
O(2)ꢀZn(1)ꢀO(1)
O(2)ꢀZn(1)ꢀN(1)
O(1)ꢀZn(1)ꢀN(1)
O(2)ꢀZn(1)ꢀN(3)
O(1)ꢀZn(1)ꢀN(3)
N(1)ꢀZn(1)ꢀN(3)
35 characterizations have shown that each complex contains a zinc
centre with a distorted tetrahedral N2O2ꢀcoordination sphere. The
zinc centre of both complexes C1 and C5 possessed a cisꢀ
arrangement of the phenol Oꢀdonor atoms; i.e. the two 4,5ꢀ
diphenyl imidazole units were accommodated on the same side of
40 the complexes. The two ligands of each complex are related by a
C2ꢀaxis, but the two ligands are structurally inequivalent. Though
the geometry at each zinc centre could be best described as a
distorted tetrahedron in which two chelating ligands were placed
in a similar disposition about the zinc centre, slight differences of
45 the bond length and dihedral angle still occurred (Table 2), which
indicated that in each complex, the orientation of the two ligands
accommodated the normal geometrical preference of the zinc
centre.49ꢀ52
116.94(4)
113.53(4)
96.06(4)
110.21(11)
121.40(11)
96.15(11)
94.63(10)
110.17(11)
124.40(11)
94.59(4)
115.46(4)
121.92(4)
3.2.
Catalytic performances of C1 with various additives
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decreased. It is noted that a further increase in the reaction time
45 led to a slight decrease in PC yield. This may be due to that the
interaction between catalysts and reactant was obstructed by the
formed PC.56 The growing viscosity of the reaction system was
also another negative factor for the activation of CO2 at longer
reaction time. The selectivity of PC stayed above 99% throughout.
50 So, a reaction time of 5 h was required for further research.
3.5.
The effect of reaction temperature
The effect of the temperature on the cycloaddition reaction
catalyzed by C1 in the presence of nꢀBu4NI in the temperature
range of 50ꢀ130 C was shown in Fig. 4. The figure illustrated
Fig. 1 ORTEP representations of the molecular structures of (a) complex
C1 and (b) complex C5. Structural differences are caused by steric effects
of substituentson the imidazole ring.
o
55 that the activity of the catalyst was strongly affected by reaction
temperature.57 The yield of PC increased with increasing
temperature, and reached 90.3% at 110 oC, then the PC yield only
slightly increased with further increase of temperature, indicating
5
Table 3 Coupling of CO2 and PO catalyzed by various componentsa
Entry
Catalyst
C1
nꢀBu4NI
C1/nꢀBu4NBr
C1/nꢀBu4NCl
C1/nꢀBu4NI
C1/Et4NBr
C1/PPh3
Yield (%)b
3.5
TONc
35.0
TOF (hꢀ1)d
7.0
50.8
182.8
84.7
191.6
174.6
11.0
o
1
2
3
4
5
6
7
8
9
that 110 C was the optimal temperature. It is also interesting to
21.7
91.4
42.4
95.8
87.3
5.5
254.0
914.0
423.5
958.0
873.0
55.0
60 note that no byꢀproduct polyether was detected even at very low
temperature, which indicated that the catalyst used in this work
had a good selectivity for the production of PC.58
100
C1/DMAP
C1/KI
12.6
52.1
125.5
521.0
25.1
104.2
95
a Catalyst: 0.214 mmol; coꢀcatalyst: 0.214 mmol; PO: 15 mL, 0.214 mol;
CO2 pressure: 5 MPa; time: 5 h; temperature: 130 oC, the selectivity to
products are all > 99 %. b Isolated yields. c Turnover number for PC
calculated as moles of PC produced per mole of catalyst. d Turnover
10 frequency for PC calculated as mole of PC produced per mole of catalyst
per hour.
90
85
PC Yield (%)
80
75
3.3.
The effect of reaction pressure
Generally, a significant disadvantage associated with CO2 as
reagent or reaction medium in organic synthesis is the potential
15 dangers operated at high pressures.24 Thus, the effect of CO2
pressure on the catalytic activity was studied at 130 oC in
pressure range of 1ꢀ7 MPa, and the results are shown in Fig. 2.
PC yield increased dramatically with increasing pressure in the
range of 1ꢀ2 MPa, then remained almost constant with further
20 increase in pressure. The further CO2 pressure increase beyond 2
MPa apparently favored a slightly increased PC formation, but
the rise is unnecessary. This may be attributed to the phase
behaviour of CO2ꢀPO system which resulted in the effect of
pressure on the concentrations of CO2 and PO.54 There were three
25 phases in the reaction system including the top CO2ꢀrich gas
phase, catalystꢀrich solid phase and the bottom POꢀrich liquid
phase. The reaction mostly took place in the PO–catalyst
interface. The initial increase of CO2 pressure (0–2 MPa) led to
the increased concentration of CO2 in the liquid phase, which is
30 why the PC yield was raised significantly. Nevertheless, the PC
yield no longer increased with the CO2 pressure beyond 2 MPa
up to 7 MPa. This may be due to the higher pressure extracted a
certain amount of PO into the gas phase, and caused the decline
of PO concentration in the vicinity of the catalyst in the liquid
35 phase.55 Besides, the instrument which is operated under high
pressure condition is too expensive to afford in industry
application. Therefore, 2 MPa was chosen as the optimal reaction
pressure in the following catalytic reaction of CO2 and PO to PC.
1
2
3
4
5
6
7
CO2 Pressure (MPa)
Fig. 2 Effect of CO2 pressure on PC yield. Reaction conditions: 0.214
65 mmol C1, 0.214 mmol nꢀBu4NI, 0.214 mol PO, reaction temperature 130
oC, reaction time 5 h, the selectivity to products are all > 99 %.
100
90
80
70
60
PC Yield (%)
50
40
1
2
3
4
5
6
7
Time (h)
Fig. 3 Effect of reaction time on PC yield. Reaction conditions: 0.214
mmol C1, 0.214 mmol nꢀBu4NI, 0.214 mol PO, reaction temperature 130
70 oC, CO2 pressure 2 MPa, the selectivity to products are all > 99 %.
3.4.
The effect of reaction time
40 The PC yield versus reaction time was shown in Fig. 3. The PC
synthesis from PO and CO2 proceeded rapidly, and nearly 80%
PC yield was obtained within the first 3 h at 130 oC. The PC yield
experienced a continuing growth within 5 h, and then gradually
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35 Table 4 Effect of substitution on the aromatic rings on the catalytic
activitya
100
80
60
40
20
0
Entry
1
2
3
4
5
6
7
8
Catalyst
Yield (%)b
90.3
TONc
903.0
940.0
942.0
959.0
893.0
946.0
997.0
941.0
902.0
370.0
TOF (hꢀ1)d
180.6
188.0
188.4
191.8
178.6
189.2
199.4
188.2
180.4
15.4
C1/nꢀBu4NI
C2/nꢀBu4NI
C3/nꢀBu4NI
C4/nꢀBu4NI
C5/nꢀBu4NI
C6/nꢀBu4NI
C7/nꢀBu4NI
C8/nꢀBu4NI
C9/nꢀBu4NI
C8FuIBTP)2Zn
/nꢀBu4NI
94.0
94.2
95.9
89.3
94.6
99.7
94.1
90.2
PC Yield (%)
9
10e
(
37.0
40
60
80
100
120
140
Temperature (oC)
12
13
14
15
C10/nꢀBu4NI
C11/nꢀBu4NI
C12/nꢀBu4NI
C13/nꢀBu4NI
56.2
80.1
81.2
91.1
562.0
801.0
812.0
911.0
112.4
160.2
162.4
182.2
Fig. 4 Effect of reaction temperature on PC yield. Reaction conditions:
0.214 mmol C1, 0.214 mmol nꢀBu4NI, 0.214 mol PO, reaction time 5 h,
CO2 pressure 2 MPa, the selectivity to products are all > 99 %.
a
Catalyst: 0.214 mmol; nꢀBu4NI: 0.214 mmol; PO: 15 mL, 0.214 mol;
CO2 pressure: 2 MPa; time: 5 h; temperature: 110 C, the selectivity to
products are all > 99 %. Isolated yields. Turnover number for PC
40 calculated as moles of PC produced per mole of catalyst. Turnover
o
5
3.6.
Complexes
Effect of substitution on the aromatic rings of
b
c
d
frequency for PC calculated as mole of PC produced per mole of catalyst
per hour. Ref. 62: Catalyst (mol%): 0.1, PO: 5.0 mL, 50 C, 1.0 MPa
initial CO2 pressure, 24 h.
The substitution on the aromatic rings of salicylaldehyde
probably causes geometrical distortion of space structure, and
thus, may affect properties of catalyst. So, the effect of
e
o
3.7.
Effect of central metal on the activity
10 substitution on the aromatic rings of complexes was investigated.
The experimental results in Table 4 showed that although this
series of catalysts had good catalytic performance under optimal
45 In order to make a systematic comparison, the catalytic properties
of C10 (Cu), C11 (Pb), C12 (Ni), and C13 (Co) were also
investigated (Table 4). Except Cu centre, the PC yield catalyzed
by other four metal centre complexes (Zn, Pb, Ni, Co) were all >
80%, and the activity order was C13 (Co) (91.1%) > C1 (Zn)
50 (90.3%) > C12 (Ni) (81.2%) > C11 (Pb) (80.1%) > C10 (Cu)
(56.2%), which indicated that the catalytic activity is highly
dependent on the type of centre metal. It is possible that the high
catalytic activities of the cobalt(II)ꢀ, zinc(II)ꢀ, nickel(II)ꢀ, lead(II)ꢀ
centre complexes and the low activity of the copper(II)ꢀ centre
55 complexes are related to the metal’s different coordination ability
with PO.63,64 It's worthily noted that most Pb or Pb(II) compounds
were used as electrodes for the electrochemical reduction of
CO2.65 The present study is the first report about the usefulness of
lead catalyst for coupling reaction between CO2 and epoxides.
o
conditions (110 C, 5 h, 2 MPa), the catalytic activity of these
catalysts exhibited some difference for different substitutions.
15 Under the same conditions, catalytic activities of 3,5ꢀsubstituted
complexes were in the following order: C4 (ꢀI) (95.9%) > C3 (ꢀBr)
(94.2%) > C2 (ꢀCl) (94.0%) > C1 (ꢀH) (90.3%), which indicated
that the electronꢀwithdrawing effect played a key role in the
decrease of catalytic property of these zinc complexes.59,60 For
20 the 5ꢀsubstituted Zn complexes, the order was: C7 (ꢀBr) (99.7%) >
C6 (ꢀCl) (94.6%)> C8 (ꢀNO2) (94.1%) > C5 (ꢀCH3) (89.3%), while
the order of electronꢀwithdrawing effect was: ꢀNO2 > ꢀCl > ꢀBr >
ꢀCH3. The exception of C5 (ꢀCH3) might be attributed to the steric
hindrance effect of methyl, which was not benefit for the
25 insertion of propylene oxide into the metal centre.61 Ko and coꢀ
workers had reported homologous zinc catalysts bearing
iminebenzotriazole phenoxide ligands,62 which were active
catalyst(TOF: 33.3 hꢀ1) for the cycloaddition of CO2 with
propylene oxide in the presence of nꢀBu4NBr to give propylene
30 carbonate (PC) under mild conditions (table 4, entry 10). Both
Ko’s and present works proved that zinc complexes with
phenoxide ligands possessed excellent catalytic performance for
the coupling reaction of CO2 and epoxide.
60 3.8.
Catalyst recycling
As wellꢀknown, the stability and reusability of a catalyst are two
key factors for practical application in industry. Catalyst C1 was
chosen to evaluate the recyclability, and then a series of catalytic
cycles for the coupling reaction of CO2 with PO was carried out
o
65 under the optimized reaction conditions (110 C, 5 h, 2 MPa). In
each cycle, the catalyst C1 could be easily separated from the
product by the addition of ethanol, subsequent filtration and then
used for the next run directly. The results in Fig. 5 exhibited that
the catalyst C1 could be reusable for at least 5 times without
70 significant loss of activity, while the selectivity of the product
remained the same.
6
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a neutral M–N2O2 species as the starting point in the catalytic
cycle.40 The ligands of the catalysts comprise
N2O2
a
35 coordination pocket into which a wide variety of metal ions can
be easily accommodated and that function as the catalytic center.
Various substituents can be easily introduced in the aromatic
rings to allow, for example, control over the approach of a
substrate by bulky groups or variation of the Lewis acidity of the
40 metal center through electronꢀwithdrawing/donating groups. The
insertion of appropriate substituents on the phenyl ring can also
be employed to anchor the salen scaffold to a solid support, thus
allowing for the preparation of heterogeneous catalysts.75 The
epoxide ring of PO was activated by M–N2O2 species, and then
45 the epoxide ring was attacked by the anion of coꢀcatalyst such as
nꢀBu4NI, leading to epoxide ringꢀopening and formation of a
metalꢀbound alkoxide. At the same time, CO2 inserted into the
metal–alkoxide bond to form a metal alkylcarbonate, and then the
production of cyclic carbonate was formed via a backbiting
80
60
40
20
0
1
2
3
4
5
Catalytic Run
Fig. 5 Catalyst recycling with 0.214 mmol C1/0.214 mmol nꢀBu4NI/0.214 50 pathway.76 This mechanism indicated that the presence of a
mol PO at 110 oC, 2 MPa CO2, and 5 h (>99 % PC selectivity is
nucleophilic group, either a nucleophilic axial anion or an added
maintained).
coꢀcatalyst, and a metal centre were both necessary for the
reaction, that is why the PC yield was very low when catalyzed
by the metal complex C1 or coꢀcatalyst nꢀBu4NI alone (Table 4,
55 entries 1 and 2).
5
3.9.
Applicability of substrates
The catalytical performance of C1/nꢀBu4NI for the cycloaddition
of CO2 with other epoxides were also studied at 110 oC, 5 h and 2
MPa without any organic solvent, and the results are summarized
in Table 5. The catalyst was applicable to a variety of terminal
10 epoxides to produce the corresponding cyclic carbonates with
excellent yields, with the exception of isobutylene oxide (entry 5)
and cyclohexene oxide (entry 6), which was probably due to their
higher steric hindrance.66,67 This steric effect was more likely to
hinder the nucleophilic attack of the epoxide rather than its
15 coordination to the Lewis acid metal centre.68,69
Bu4NX
O
O
O
N
N
O
M
epoxide
activation
X
O
N
O
M
O
N
N
O
NBu4
N
O
M
Table 5 Cycloaddition between CO2 and various epoxides catalyzed by
C1 in the presence of nꢀBu4NIa
O
metal alkoxide
cyclization
O
CO2
Entry
1
Epoxide
Product
Yield (%)b TONc TOF (hꢀ1)d
activation
O
O
98.1
981.0
196.2
CO2
Cl
O
Cl
X
N
O
O
N
O
O
O
O
O
2
93.8
938.0
187.6
carbonate
liberation
M
O
O
NBu4
C
O
O
O
O
O
3
4
93.1
93.4
931.0
934.0
186.2
186.8
M= centre metal
X= halide
metal alkylcarbonate
O
O
Scheme 3 Proposed mechanism for cyclic carbonate formation.
O
O
O
O
O
O
O
O
5
6
19.3
42.6
193.0
426.0
38.6
85.2
Conclusions
O
In summary, a series of easily accessible metal complexes
60 bearing 2ꢀ(imidazolꢀ2ꢀyl)phenol ligands were synthesized and
characterized. Systematical investigation demonstrated that all
the complexes were active and versatile catalysts for the coupling
reaction of CO2 and epoxides to selectively generate cyclic
carbonate without any organic solvents. The effect of reaction
65 conditions (time, temperature and pressure), substitution on the
aromatic rings of ligands and central metal on the catalytic
activity were investigated, and the optimal catalytical conditions
O
O
O
O
a
C1: 0.214 mmol; nꢀBu4NI: 0.214 mmol; epoxide: 0.214 mol; CO2
pressure: 2 MPa; time: 5 h; temperature: 110 oC, the selectivity to
b
c
20 products are all > 99 %.
carbonates calculated as moles of carbonate produced per mole of catalyst.
Turnover frequency for carbonates calculated as mole of carbonate
produced per mole of catalyst per hour.
Isolated yields. Turnover number for
d
3.10.
Proposed mechanism of the coupling reaction
o
were screened as 110 C, 5 h, 2 MPa. The catalyst also could be
25 The metalꢀcatalyzed coupling reaction of CO2 and epoxides is
reused several times with only minor loss in the catalyst activity.
70 This catalyst system was also suitable for the production of cyclic
carbonates from CO2 and epoxides. These characteristics made
them ideal catalysts in terms of potential industrial application in
chemical CO2 fixation.
generally thought to occur via
a
coordinationꢀinsertion
mechanism.70 Taking into account the diverse mechanisms found
in the literature for the coupling of epoxide and CO271ꢀ74 and DFT
studies involving in particular zinc salphen,69
a general
30 mechanism can be illustrated for the M–N2O2/(nꢀBu)4NX
catalytic system in Scheme 3. Considering that the reports on
ionic metal salens of the type [N2O2MꢀX][NR4] have been scarce,
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We are grateful to National Natural Science Foundation of China
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Abbreviations
5
CO2, carbon dioxide; PO, propylene oxide; PC, propylene
carbonate; MOFs, metal–organic frameworks; CaH2, calcium
hydride; TMS, trimethylsilane; CH2Cl2, dichloromethane; MgSO4,
magnesium sulfate anhydrous; DMSO, dimethylsulfoxide;
DMAP, 4ꢀdimethylaminopyridine; KI, potassium iodide; PPh3,
10 triphenylphosphine; nꢀBu4NBr, tetrabutylammonium bromide; nꢀ
Bu4NCl, tetrabutylammonium chloride; nꢀBu4NI,
tetrabutylammonium iodide.
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