Efficient Solvent-Free Synthesis of Cyclic Carbonates from the Cycloaddition of Carbon Dioxide and Epoxides Catalyzed by New Imidazolinium Functionalized Metal Complexes Under 0.1?MPa

Article abstract of DOI:10.1007/s10562-020-03163-6

Abstract: A series of imidazolinium functionalized metal complexes were prepared as efficient and recyclable homogeneous catalysts for the synthesis of cyclic carbonates via the coupling reaction of carbon dioxide and epoxides in the absence of co-catalysts and solvent. Due to the synergistic effect between the metal center as Lewis acid and the imidazolinium groups with halide anions as Lewis base, these catalysts exhibited excellent catalytic capability. Particularly, high turnover frequency (TOF) values of (1174, 1240 and 1262?h^?1) for bifunctional catalysts (2c, 2i and 2j, respectively) were achieved via adjusting reaction parameters. These catalysts also showed excellent catalytic performances under atmospheric pressure. Recyclability test showed that this kind of bifunctional catalyst can be easily recovered and reused for at least five times without dramatic loss of activity and selectivity. The optimum reaction conditions were also screened as (120?°C, 1.0?MPa CO₂ and 2?h). The catalyst could also be applied to a variety of substrates and be used for a long time with high catalytic activities. We also performed the Kinetic studies, which suggested that the activity of catalysts following the order I^? > Br^? > Cl^?. Moreover, a plausible reaction mechanism by imidazolinium functionalized metal complexes was proposed. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-020-03163-6

Catalysis Letters
https://doi.org/10.1007/s10562-020-03163-6
Efcient Solvent‑Free Synthesis of Cyclic Carbonates
from the Cycloaddition of Carbon Dioxide and Epoxides Catalyzed
by New Imidazolinium Functionalized Metal Complexes Under 0.1 MPa
Qian Zhang1 · Peng Yu1 · Bo Lei1 · Hai‑Jian Yang1 · Juncheng Hu1 · Lihua Wang1 · Tengfei Zhou1 · Qingqing Jiang1 ·
Junjiang Zhu2 · Buxing Han3
Received: 15 January 2020 / Accepted: 23 February 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
A series of imidazolinium functionalized metal complexes were prepared as efcient and recyclable homogeneous catalysts
for the synthesis of cyclic carbonates via the coupling reaction of carbon dioxide and epoxides in the absence of co-catalysts
and solvent. Due to the synergistic efect between the metal center as Lewis acid and the imidazolinium groups with halide
anions as Lewis base, these catalysts exhibited excellent catalytic capability. Particularly, high turnover frequency (TOF)
values of (1174, 1240 and 1262 h−1) for bifunctional catalysts (2c, 2i and 2j, respectively) were achieved via adjusting reac-
tion parameters. These catalysts also showed excellent catalytic performances under atmospheric pressure. Recyclability
test showed that this kind of bifunctional catalyst can be easily recovered and reused for at least fve times without dramatic
loss of activity and selectivity. The optimum reaction conditions were also screened as (120 °C, 1.0 MPa CO2 and 2 h). The
catalyst could also be applied to a variety of substrates and be used for a long time with high catalytic activities. We also
performed the Kinetic studies, which suggested that the activity of catalysts following the order I− >Br− >Cl−. Moreover, a
plausible reaction mechanism by imidazolinium functionalized metal complexes was proposed.
Graphic Abstract
2a: M= Zn, R1= H, X=Cl
2b: M= Zn, R1= Cl, X=Cl
2c: M= Zn, R1= NO2, X=Cl
2d: M= Zn, R1= C4H9, X=Cl
2e: M= Co, R1= H, X=Cl
2f: M= Pb, R1= H, X=Cl
2g: M= Ni, R1= H, X=Cl
2h: M= Cu, R1= H, X=Cl
2i: M= Zn, R1= NO2, X=Br
2j: M= Zn, R1= NO2, X=I
R1
C4H9
N
X
N
N
O
O
M
N
N
X
N
C4H9
CO2
R
O
R1
O
O
R
O
facil synthesis high activity with high TOF
low cost and wide substrate scope bifunctional
solvent-free
active at 0.1MPa
Keywords Carbon dioxide · Cyclic carbonate · Atmospheric pressure · Bifunctional metal complexes · Activation energy ·
Selectivity
Peng Yu is a co-frst author.
1 Introduction
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-020-03163-6) contains
Nowadays, there are lots of fossil fuels abused in many
supplementary material, which is available to authorized users.
ways. Because of this, a series of problems have been
come up. The carbon dioxide emission is one of the most
Extended author information available on the last page of the article
Vol.:(0123456789)
1 3

Q. Zhang et al.
important problems. A large amount of carbon dioxide
emissions will cause greenhouse efect. However, carbon
dioxide (CO2) is also an economical, nontoxic, recycla-
ble and inexhaustible C1 source and has attracted much
attention from many scientists [1–4]. As a 100% atom-eco-
nomical reaction, the coupling reaction between epoxides
and carbon dioxide is an attractive way to obtain cyclic
carbonates [5–8], which could be used as lithium batteries’
electrolyte, fuel additives, aprotic polar solvents, chemical
intermediates, and so on [9–13]. In summary, the cou-
pling reaction of carbon dioxide with epoxides shows good
applications in industry [14].
conditions. Furthermore, the possible reaction mechanism
was discussed.
2 Experimental
2.1 Chemicals and Analyzing Method
The information of materials are listed in Table S1. 1H NMR
spectra were recorded on a Bruker Al-400 MHz instrument
using trimethylsilane (TMS) as an internal standard to iden-
tify the yield and selectivity of products. All the known com-
pounds were identifed by comparison of their physical and
spectral data with those in previous reports. The epoxides
were distilled from CaH2 before using, and all other com-
pounds were used as received.
During past decades, many catalyst systems, such as
metal–organic frameworks (MOF) [15, 16], porphyrins [17,
18], ionic liquids [19–22], organocatalysts [23–30], metal
compounds [31–39], immobilized catalysts [40–43], and
so forth, have been reported to improve the coupling reac-
tion of epoxides and CO2. Owing to relatively high catalytic
activity and no co-catalyst requirements, some bifunctional
metal catalysts containing two catalytic sites in one catalyst
molecule have attracted much interests [44–50]. But some
reported bifunctional catalysts still have some weakness such
like solvent demand and low stability [48–50]. Therefore,
developing solvent-free, efcient and stable catalyst is still
of importance and challenge [51–53]. Herein, we reported
the synthesis and characterization of a series of mononu-
clear bi-functional metal complexes bearing ligands func-
tionalized by various imidazolinium cation moieties, which
were proven to be efcient and recyclable catalysts for the
reaction of various epoxides with CO2 to generate cyclic
carbonates selectively under solvent-free and additive-free
2.2 Synthesis of Imidazolinium Modifed
Salicyclaldehyde and Imidazolinium
Functionalized Metal Complexes
The synthesis of imidazolinium modifed salicyclaldehyde
followed the general procedure shown in Scheme 1. First of
all, 5-Chloromethylsalicylaldehyde was synthesized accord-
ing to the published procedures [44, 47, 54]. Salicylaldehyde
(15.25 g, 13 mL), paraformaldehyde (6.75 g), and 75 mL
concentrated hydrochloric acid (hydrobromic acid for 1c and
hydriodic acid for 1d) were stirred in a 250 mL fask under
the protection of nitrogen for three days at room tempera-
ture. The resulting solids were washed with 0.5% NaHCO3
solution and dried under vacuum. White needle-like crystals
CHO
OH
N
CHO
OH
CHO
OH
N
R2
Tetrahydrofuran
(HCHO)n
N
XH2C
X
N
HX
X=Cl,Br,I
R2
1a: R2= C4H9, X=Cl
1b: R2=C4H9, X=Br
1c: R2=C4H9, X=I
R1
R1
R2
N
H2N
Ethanol, Melt salt
X
N
2a: M= Zn, R1= H, R2= C4H9, X=Cl
2b: M= Zn, R1= Cl, R2= C4H9, X=Cl
2c: M= Zn, R1= NO2, R2= C4H9, X=Cl
2d: M= Zn, R1= C4H9, R2= C4H9, X=Cl
2e: M= Co, R1= H, R2= C4H9, X=Cl
2f: M= Pb, R1= H, R2= C4H9, X=Cl
2g: M= Ni, R1= H, R2= C4H9, X=Cl
2h: M= Cu, R1= H, R2= C4H9, X=Cl
2i: M= Zn, R1= NO2, R2= C4H9, X=Br
2j: M= Zn, R1= NO2, R2= C4H9, X=I
O
N
N
O
M
N
X
N
R2
R1
Scheme 1. Synthesis of bifunctional catalysts 2a-2j
1 3

Efcient Solvent‑Free Synthesis of Cyclic Carbonates from the Cycloaddition of Carbon Dioxide…
were obtained by recrystallization with petroleum ether. The
obtained compound was used directly for the following pro-
cedure: 1-Butylimidazole (10 mmol) dissolved in anhydrous
tetrahydrofuran (10 mL) was added into the stirring tetrahy-
drofuran solution of 5-halomethylsalicylaldehyde (10 mmol,
1.7 g) drop by drop under nitrogen atmosphere. The reaction
sustained for 2 h at room temperature to give yellow sticky
liquid. After the stratifcation of liquid, the upper layer of
liquid was removed, and the lower layer liquid was dried in
vacuum. Finally, the product was obtained after recrystal-
lization from methanol-tetrahydrofuran system.
1H), 5.41 (d, J = 28.0 Hz, 2H), 4.17 (td, J = 7.2, 3.1 Hz,
2H), 1.88–1.57 (m, 2H), 1.22 (dd, J = 14.2, 6.6 Hz, 2H),
0.87 (t, J = 7.4 Hz, 3H); 13C NMR (101 MHz, DMSO) δ
197.20, 161.49, 137.14, 136.37, 129.56, 126.14, 123.13,
123.01, 122.76, 118.48, 51.57, 49.19, 47.45, 40.65, 40.44,
40.23, 40.03, 39.82, 39.61, 39.40, 31.73, 19.26, 19.22,
13.76; Selected IR peaks (KBr, cm−1): ν 3070, 1660,
1620, 1560, 1450, 1280, 1160, 760, 633; Anal. Calcd for
C15H19IN2O2Zn: C, 46.65; H, 4.96; N, 7.25%. Found: C,
46.69; H, 4.99; N, 7.20%.
The synthesis of imidazolinium functionalized metal
complexes 2 also followed the general procedure shown in
Scheme 2.The anhydrous ethanol solution (20 mL) of ani-
line derivatives (10 mmol) were added dropwise into the
anhydrous ethanol (50 mL) solvent of imidazolinium-based
ionic liquid modifed salicylaldehyde (10 mmol) in the molar
ratio of 1:1 and refuxed at 60℃ for 3 h. After that, a little
excess of corresponding metal salts (5.1 mmol) dissolved in
50 mL of anhydrous ethanol were added into the reaction
mixture. The resulting mixture was refuxed for another 6 h.
The resulting precipitate was fltered, washed with ethanol
for three times, and dried under vacuum to obtain complex.
The structures of these complexes were fully confrmed,
however, resolvable NMR spectra of complexes 2e-2 h could
not be measured owing to paramagnetism [15].
1a Yield: 83%; 1H NMR (400 MHz, DMSO) δ 11.41 (s,
1H), 10.28 (s, 1H), 9.60 (s, 1H), 7.88 (d, J = 9.4 Hz, 2H),
7.73 (d, J=2.3 Hz, 1H), 7.64 (dd, J=8.6, 2.3 Hz, 1H), 7.27
(d, J = 8.5 Hz, 1H), 5.41 (s, 2H), 4.18 (t, J= 7.2 Hz, 2H),
1.86–1.60 (m, 2H), 1.21 (dd, J=15.0, 7.4 Hz, 2H), 0.84 (t,
J = 7.4 Hz, 3H); 13C NMR (101 MHz, DMSO) δ 190.42,
161.95, 136.86, 136.53, 128.79, 126.02, 123.22, 122.95,
122.79, 118.70, 56.41, 51.53, 49.09, 40.37, 40.16, 39.95,
39.74, 39.53, 31.74, 19.25, 18.98, 13.69; Selected IR peaks
(KBr, cm−1): ν 2960, 1680, 1613, 1562, 1441, 1249, 1161,
761, 634; Anal. Calcd for C15H19ClN2O2Zn: C, 61.12; H,
6.50; N, 9.50%. Found: C, 61.19; H, 6.47; N, 9.56%.
1b Yield:73%; 1H NMR (400 MHz, DMSO) δ 10.94 (s,
1H), 10.27 (s, 1H), 9.41 (s, 1H), 7.85 (dd, J =4.6, 1.4 Hz,
2H), 7.75 (d, J=2.3 Hz, 1H), 7.64 (dd, J=8.6, 2.3 Hz, 1H),
7.08 (d, J = 8.5 Hz, 1H), 5.39 (s, 2H), 4.17 (t, J = 7.2 Hz,
2H), 1.85–1.63 (m, 2H), 1.22 (dd, J=15.0, 7.5 Hz, 2H), 0.86
(t, J=7.4 Hz, 3H); 13C NMR (101 MHz, DMSO) δ 190.83,
161.54, 137.04, 136.45, 129.35, 126.23, 123.25, 122.92,
122.81, 118.53, 51.55, 49.14, 40.65, 40.44, 40.23, 40.02,
39.82, 39.61, 39.40, 31.73, 19.26, 13.73; Selected IR peaks
(KBr, cm−1): ν 3060, 1674, 1614, 1563, 1443, 1249, 1160,
762, 633; Anal. Calcd for C15H19BrN2O2Zn: C, 53.11; H,
5.65; N, 8.26%. Found: C, 53.16; H, 5.68; N, 8.22%.
1
2a Yield: 86%; Melt point: 198–199 °C; H NMR
(400 MHz, DMSO) δ 9.20 (s, 2H), 8.47 (s, 2H), 7.79 (d,
J=8.7 Hz, 4H), 7.62 (d, J=7.8 Hz, 4H), 7.46 (dd, J=15.6,
8.5 Hz, 6H), 7.32 (d, J= 8.1 Hz, 4H), 6.65 (d, J= 8.8 Hz,
2H), 5.24 (s, 4H), 4.16 (t, J = 7.2 Hz, 4H), 1.81–1.69 (m,
4H), 1.25 (d, J=7.5 Hz, 4H), 0.89 (t, J=7.3 Hz, 6H); 13
C
NMR (101 MHz, DMSO) δ 173.93, 167.94, 138.63, 129.68,
129.23, 127.64, 127.06, 122.12, 121.25, 121.00, 118.00,
49.24, 47.15, 32.62, 22.18, 19.47, 13.73; Selected IR peaks
(KBr, cm−1): ν 3061, 1617, 1532, 1488, 1469, 675, 530;
Anal. Calcd for C42H46Cl2N6O2Zn: C, 62.81; H, 5.77; N,
10.46%. Found: C, 62.85; H, 5.80; N, 10.41%.
1
1c Yield:70%; H NMR (400 MHz, DMSO) δ 11.13
(d, J=139.2 Hz, 1H), 10.16 (d, J=67.7 Hz, 1H), 9.28 (d,
J = 14.5 Hz, 1H), 7.88–7.79 (m, 2H), 7.79–7.71 (m, 1H),
7.62 (dd, J=8.6, 2.4 Hz, 1H), 7.09 (dd, J=44.1, 11.9 Hz,
1
2b Yield: 82%; Melt point: 216–218 °C; H NMR
(400 MHz, DMSO) δ 9.19 (s, 2H), 8.48 (s, 2H), 7.78 (d,
2a: M= Zn, R1= H, R2= C4H9, X=Cl
2b: M= Zn, R1= Cl, R2= C4H9, X=Cl
2c: M= Zn, R1= NO2, R2= C4H9, X=Cl
2d: M= Zn, R1= C4H9, R2= C4H9, X=Cl
2e: M= Co, R1= H, R2= C4H9, X=Cl
2f: M= Pb, R1= H, R2= C4H9, X=Cl
2g: M= Ni, R1= H, R2= C4H9, X=Cl
2h: M= Cu, R1= H, R2= C4H9, X=Cl
2i: M= Zn, R1= NO2, R2= C4H9, X=Br
2j: M= Zn, R1= NO2, R2= C4H9, X=I
R1
R2
N
X
N
O
N
N
O
M
N
X
N
R2
R
R1
O
O
R
CO2
O
O
Scheme 2. Cycloaddition of PO and CO2 to give PC catalyzed by the metal catalysts 2a-2j
1 3

Q. Zhang et al.
J = 11.6 Hz, 4H), 7.64 (d, J = 8.6 Hz, 4H), 7.57–7.43 (m,
6H), 7.34 (d, J=9.0 Hz, 2H), 6.65 (d, J=8.6 Hz, 2H), 5.24
(s, 4H), 4.16 (t, J = 7.1 Hz, 4H), 1.86–1.72 (m, 4H), 1.25
(dd, J = 14.8, 7.6 Hz, 4H), 0.89 (t, J = 7.4 Hz, 6H); 13C
NMR (101 MHz, DMSO) δ 171.55, 168.94, 138.48, 136.08,
129.74, 128.95, 126.83, 124.08, 123.11, 122.63, 121.27,
115.80, 114.18, 49.24, 47.22, 32.55, 31.73, 19.42, 19.25,
13.72; Selected IR peaks (KBr, cm−1): ν 3047, 1619, 1534,
1490, 1467, 698, 519; Anal. Calcd for C42H44Cl4N6O2Zn:
C, 57.85; H, 5.09; N, 9.64%. Found: C, 57.89; H, 5.12; N,
9.62%.
2 h Yield: 86%; Melt point: 200–202 °C; Selected IR peaks
(KBr, cm−1): ν 3059, 1619, 1545, 1487, 1448, 692, 519; Anal.
Calcd for C42H46Cl2N6O2Cu: C, 62.95; H, 5.79; N, 10.49%.
Found: C, 62.92; H, 5.82; N, 10.50%.
2i Yield: 76%; Melt point: 224–228 °C; 1H NMR
(400 MHz, DMSO) δ 9.22 (s, 2H), 8.57 (s, 2H), 8.34 (d,
J=8.2 Hz, 4H), 7.80 (d, J=10.5 Hz, 8H), 7.53 (s, 2H), 7.39
(d, J=9.8 Hz, 2H), 6.70 (d, J=8.4 Hz, 2H), 5.26 (s, 4H),
4.16 (t, J=7.2 Hz, 4H), 1.81–1.73 (m, 4H), 1.25 (dd, J=15.0,
7.4 Hz, 4H), 0.89 (t, J=7.4 Hz, 6H); 13C NMR (101 MHz,
DMSO) δ 172.15, 170.10, 136.90, 136.17, 125.28, 123.73,
123.16, 122.75, 119.89, 52.19, 49.17, 40.67, 40.46, 40.40,
40.25, 40.19, 40.05, 39.98, 39.84, 39.77, 39.63, 39.56,
39.42, 31.78, 19.30, 13.74; Selected IR peaks (KBr, cm−1):
ν 3081, 1620, 1578, 1528, 1461, 854, 754; Anal. Calcd for
C42H44Br2N8O6Zn: C, 51.38; H, 4.49; N, 11.42%. Found: C,
51.35; H, 4.51; N, 11.38%.
1
2c Yield: 92%; Melt point: 220–222 °C; H NMR
(400 MHz, DMSO) δ 9.22 (s, 2H), 8.55 (s, 2H), 8.30 (d,
J=7.7 Hz, 4H), 7.79 (d, J=11.3 Hz, 8H), 7.52 (s, 2H), 7.36
(d, J=8.5 Hz, 2H), 6.64 (d, J=7.2 Hz, 2H), 5.25 (s, 4H), 4.17
(t, J=7.2 Hz, 4H), 1.81–1.70 (m, 4H), 1.25 (dd, J=14.6,
7.3 Hz, 4H), 0.89 (t, J=7.3 Hz, 6H); 13C NMR (101 MHz,
DMSO) δ 174.94, 171.33, 168.98, 154.21, 144.76, 137.67,
135.71, 135.07, 125.79, 124.27, 123.26, 122.52, 122.06,
121.67, 118.55, 117.76, 111.77, 51.14, 48.07, 30.71, 18.43,
18.32, 18.23, 18.09, 12.72, 12.67; Selected IR peaks (KBr,
cm−1): ν 3055, 1623, 1515, 1531, 1461, 688, 515; Anal.
Calcd for C42H44Cl2N8O6Zn: C, 56.48; H, 4.97; N, 12.55%.
Found: C, 56.43; H, 5.02; N, 12.56%.
1
2j Yield: 74%; Melt point: 229–231 °C; H NMR
(400 MHz, DMSO) δ 9.23 (s, 2H), 8.43 (s, 2H), 8.28 (s, 4H),
7.90–7.54 (m, 8H), 7.49 (s, 2H), 7.36 (d, J=7.9 Hz, 2H),
6.67 (s, 2H), 5.24 (s, 4H), 4.16 (t, J=7.3 Hz, 4H), 1.83–1.71
(m, 4H), 1.23 (dt, J=14.7, 7.3 Hz, 4H), 0.88 (t, J=7.4 Hz,
6H).13C NMR (101 MHz, DMSO) δ 171.79, 136.26, 125.07,
124.19, 124.07, 123.17, 122.68, 119.99, 52.10, 49.14, 40.59,
40.38, 40.17, 39.96, 39.75, 39.54, 39.34, 31.78, 19.30, 13.79;
Selected IR peaks (KBr, cm−1): ν 3099, 1619, 1578, 1526,
1461, 853, 750; Anal. Calcd for C42H44I2N8O6Zn: C, 46.88;
H, 4.09; N, 10.42%. Found: C, 46.90; H, 4.07; N, 10.44%.
1
2d Yield: 60%; Melt point: 192–194 °C; H NMR
(400 MHz, DMSO) δ 9.51 (d, J = 24.9 Hz, 2H), 8.80 (d,
J=169.6 Hz, 2H), 8.05–6.49 (m, 16H), 5.42 (d, J=54.1 Hz,
4H), 4.20 (s, 4H), 1.81 (d, J =47.7 Hz, 4H), 1.51 (s, 4H),
1.34–1.15 (m, 8H), 1.10 (t, J=7.0 Hz, 4H), 0.85 (dt, J=13.2,
6.8 Hz, 12H); 13C NMR (101 MHz, DMSO) δ 175.39,
171.13, 167.32, 147.16, 141.77, 138.43, 136.22, 129.44,
128.94, 127.18, 123.07, 122.55, 122.23, 119.70, 114.76,
56.69, 52.12, 49.33, 47.19, 34.87, 33.50, 32.74, 31.94, 23.35,
22.25, 19.35, 18.86, 14.11, 14.09, 13.61, 13.60; Selected IR
peaks (KBr, cm−1): ν 3006, 1618, 1559, 1505, 1463, 665,
523; Anal. Calcd for C50H62Cl2N6O2Zn: C, 65.61; H, 6.83;
N, 9.18%. Found: C, 65.66; H, 6.86; N, 9.15%.
2.3 Cycloaddition Reaction and Catalyst Recycling
Typical steps for the coupling reaction of CO2 and epoxide
were as following (Scheme 2). A 50 mL high-pressure stain-
less-steel autoclave was used as reactor. A prescribed amount
of epoxide and catalyst were added into reactor without any
additional solvent and co-catalyst. Then, the reactor was sealed
and specifed amount of carbon dioxide was pressurized into
the reactor. After all these, the autoclave with its contents was
immersed into an oil bath with a fxed temperature and stirred
for a designated period of time. Then, the reactor was cooled
quickly by dipping into an ice-water bath, and the remaining
CO2 was released slowly. The resulting mixture was defned
by 1H NMR on Bruck400 super conductive nuclear magnetic
resonance spectrometer using TMS as internal standard. Sub-
sequently, 20 mL ether was added into the reactor, and the
catalyst was separated as a solid by centrifugation, which
was washed with ether or ethyl acetate three times and dried
under vacuum for the recycling experiment without further
purifcation.
2e Yield: 91%; Melt point: 226–228 °C; Selected IR
peaks (KBr, cm−1): ν 3061, 1624, 1532, 1491, 1465, 691,
514; Anal. Calcd for C42H46Cl2N6O2Co: C, 63.32; H, 5.82;
N, 10.55%. Found: C, 63.28; H, 5.88; N, 10.52%.
2f Yield: 71%; Melt point: 205–206 °C; Selected IR
peaks (KBr, cm−1): ν 3058, 1619, 1536, 1480, 1442, 691,
518; Anal. Calcd for C42H46Cl2N6O2Pb: C, 53.38; H, 4.91;
N, 8.89%. Found: C, 53.36; H, 4.98; N, 8.93%.
2 g Yield: 26%; Melt point: 222–224 °C; Selected IR
peaks (KBr, cm−1): ν 3050, 1621, 1550, 1492, 1440, 691,
516; Anal. Calcd for C42H46Cl2N6O2Ni: C, 63.34; H, 5.82;
N, 10.55%. Found: C, 63.36 H, 5.84; N, 10.59%.
1 3

Efcient Solvent‑Free Synthesis of Cyclic Carbonates from the Cycloaddition of Carbon Dioxide…
following order: 2c (-NO2) (97.8%)>2b (–Cl) (94.5%)>2a
3 Results and Discussion
(–H) (92.6%)>2d (–C4H9) (85.6%) (Table 1, entries 1–4),
which agrees well with the order of electron-withdrawing
efect: –NO2 >–Cl>–H>–C4H9. It is obvious that electron-
withdrawing efect plays an important role for the catalytic
activities of these complexes [55]. Electron-withdrawing
group could enhence the acidity of the metal center, which
was considered to be able to activate the epoxide via metal-
O bond. The relatively lower activity of 2d could also be
attributed to the steric hindrance, which was harmful to the
coordination of propylene oxide to the metal center.
3.1 Catalytic Performances of Catalysts
Firstly, the activity of various homogeneous monometallic
catalysts in the cycloaddition reaction of epoxides with CO2
was explored which were using propylene oxide (PO) as the
model substrate and the results are shown in Table 1.
The substitution on the aromatic rings of ligand probably
causes geometrical distortion of metal complexes’ space
structure to afect the catalytic properties of catalysts. So, the
efect of substitution on the aromatic rings (aniline moiety)
of complexes was investigated frstly. Although, the results
in Table 1 showed that all the bifunctional zinc catalysts
had good catalytic performances under same conditions,
the catalytic activities of these catalysts exhibited some dif-
ference for diferent substitutions. Under the same condi-
tions, catalytic activities of bifunctional catalysts were in the
In order to fgure out optimal metal center, the cata-
lytic properties of 2a (Zn), 2e (Co), 2f (Pb), 2 g (Ni) and
2 h (Cu) were also investigated (Table 1, entries 1, 5–8).
Except 2 h with Cu center, all the other four metal center
complexes (Zn, Pb, Ni, Co) showed good activity, and the
PC yields were all above 80%. The activity order of cata-
lyst with diferent metal center was 2e (Co) (99.2%) > 2a
(Zn) (92.6%) > 2f (Pb) (91.2%) > 2 g (Ni) (84.7%) > 2 h
(Cu) (66.0%), which indicated that the catalytic activity is
highly dependent on the type of metal center. It is possible
that the high catalytic activities of the cobalt(II)-, zinc(II)-,
lead(II)-, nickel(II)-center complexes and the low activity of
the copper(II)-center complexes are related to the diferent
coordination ability of metals with propylene oxide (PO)
[56–59]. The catalytic activities of bromated and iodized
catalyst (2i and 2j) have also been investigated and both of
them showed excellent yields (>99%) higher than chlorin-
ated catalyst 2a (Table 1, entries 1, 9 and 10). The catalytic
diference could be explained by the leaving ability of halide
ions (I− > Br− > Cl−). Relative high TOF values could be
reached as 1174 (2c), 1240 (2i) and 1262 h−1 (2j), respec-
tively, by ajusting the reaction parameters. The order of TOF
is I− >Br− >Cl−, which matches the leaving ability of halo-
gen (Table 1, entries 11, 12 and 13). Considering that 2i and
2j are difcult to synthesize, 2c was chosen to perform the
following studies.
Table 1 Results of the cycloaddition reaction of PO with CO2 cata-
lyzed by various catalysts
O
O
CO2 (1.0 MPa, 120oC, 2 h)
O
O
1 mol% Catalyst, solvent-free
Entry
Catalyst
Conversion (%)a
TONb
TOF (h−1 c
)
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
92.6
94.5
97.8
85.6
99.2
91.2
84.7
66.0
99.1
99.3
58.7
62.2
63.1
92.6
94.5
97.8
85.6
99.2
91.2
84.7
66.0
99.0
99.0
587
46.3
47.3
48.9
42.8
49.6
45.6
42.4
33.0
49.5
49.5
1174
1240
1262
2 g
2 h
2i
A rough catalytic performance comparison between many
reported bifunctional metal catalysts and our present com-
plexes were also made, which was listed in Table S14. Obvi-
ously, we can achieve the conversion of CO2 efectively by
using our catalysts and these present catalysts are among the
most efcient bifunctional metal catalysts. Moreover, our
bifunctional catalysts have the merits of simple structure
and easy synthesis.
9
10
11d
12d
13d
2j
2c
2i
620
631
2j
Reaction conditions: PO 10 mL (0.143 mol), catalyst 1.43 mmol,
temperature 120 °C, CO2 pressure 1.0 MPa, time 2 h, the selectivity
to products are all>99%
aDetermined by 1H NMR (CDCl3, 400 MHz)
3.2 Catalytic Performances Under Atmospheric
Pressure
bTurnover number for PC calculated as moles of PC produced per
mole of catalyst
cTurnover frequency for PC calculated as mole of PC produced per
mole of catalyst per hour
Chemical fxation of CO2 to cyclic carbonate were also stud-
ied under atmospheric pressure. The cycloaddition reaction
between CO2 and glycidyl isopropyl ether was carried out to
evaluate the catalytic ability of present bifunctional catalysts
dReaction conditions: PO 5 mL (0.0714 mol), catalyst 0.0714 mmol,
temperature 150 °C, CO2 pressure 1.0 MPa, time 0.5 h, the selectivity
to products are all>99%
1 3

Q. Zhang et al.
tert-butyl glycidyl ether, allyl glycidyl ether and glycidyl
isopropyl ether needed 6 h. While, phenyl glycidyl ether
could not be converted totally even after 10 h.
Znꢀ2aꢁ
Coꢀ2eꢁ
Pbꢀ2fꢁ
Niꢀ2gꢁ
Cuꢀ2hꢁ
100
80
60
40
20
0
3.3 Efect of Reaction Parameters
To optimize the catalytic conditions, the efects of reaction
parameters (time, temperature, CO2 pressure, and catalyst
loading) on the catalytic results have been investigated sys-
tematically with 2c as the model catalyst. Firstly, the efect
of the reaction time on the PC yield was studied. Figure 3a
shows that the PC yield increased with reaction time up to a
period of 4 h. It is observed that the cycloaddition reaction
proceeds rapidly within the frst hour, reaching 71.3% of PC
production. Following an increase in time from 1 to 2 h, the
PC yield increased sharply from 71.3 to 97.8% (Fig. 3a).
After 2 h, there was no notable increase of yield, therefore,
the optimum reaction was selected as 2 h.
0
2
4
6
8
10
Time (h)
Fig. 1 Catalytic activity comparision of catalysts with diferent metal
centers under atmosphere pressure. Reaction conditions: glycidyl iso-
propyl ether 5 mL (0.071 mol), catalyst 0.71 mmol, temperature 120
°C, CO2 pressure 0.1 MPa
The results shown in Fig. 3b clearly demonstrate appar-
ently that temperature plays an important role for the PO
conversion. When the catalytic reactions were conducted
at 80, 100, 120, and 140 °C, the PO conversions were 21.7,
68.8, 97.8, and 98.5%, respectively. It is obvious that the
relationship between temperature and conversion could be
attributed to higher reactivity of catalyst at a higher tempera-
ture [60–62]. With the increasing temperature from 80 to
120 °C, the PC yield increased sharply from 21.7 to 97.8%.
However, a little bit better of PC yield from 120 to 140 °C
(97.8 to 98.5%). Based on the concept of green chemistry
that 120 °C was chosen as the optimal temperature.
100
80
60
Butyl Glycidyl Ether
Tert-Butyl Glycidyl Ether
40
Allyl Glycidyl Ether
The efects of the CO2 pressure on the catalytic results
were also investigated. As shown in Fig. 3c, in the low-pres-
sure range (0.5–1.0 MPa), the yield of PC increased (form
43.7 to 97.8%) with rising CO2 pressure. However, a further
increasing in CO2 pressure to 5 MPa resulted in a dramatic
decrease in the PC yield (from 97.8 to 20.9%). In the cata-
lytic reaction system, a higher CO2 pressure can efectively
increase the solubility of CO2 in PO, enabling the reaction
equilibrium shifting towards carbonate formation. However,
when the CO2 pressure reached more than 2 MPa, this high
CO2 pressure would retard the interaction between PO and
the catalysts, resulting in a decrease in the PO conversion.
Thus, the best PC yield could be achieved at 1.0 MPa. Such
phenomenon has been reported by many scientists and could
be explained clearly by the efect of the pressure on the con-
centration of CO2 and epoxide in the two phases (upper gas-
eous phase and bottom liquid phase) in the reaction systems
[63, 64]. Generally, since the catalyst was dispersed in the
liquid phase, so the reaction took place mainly in the bottom
phase. With the increasing pressure higher than 2 MPa, high
CO2 concentration would result in dilution efect, which will
inhibit the contact of PO with the catalyst [65, 66].
Glycidyl Isopropyl Ether
Phenyl Glycidyl Ether
20
0
2
4
6
8
Time (h)
Fig. 2 Cycloaddition of CO2 and various glycidyl ethers under atmos-
phere pressure. Reaction conditions: epoxide (0.071 mol), catalyst 2a
0.71 mmol, temperature 120 °C, CO2 pressure 0.1 MPa
with diferent metal centers under atmospheric pressure
(0.1 MPa CO2). As shown in Fig. 12a (Zn), 2e (Co), 2f (Pb)
and 2 g (Ni) revealed higher catalytic activity than 2 h (Cu)
under 0.1 MPa. Especially, 2e and 2a showed excellent cata-
lytic activity with 100% PC yield in short time (5 h and 6 h
respectively).
Based on the excellent catalytic performance under
atmospheric pressure, catalyst 2a was used for the coupling
reaction of CO2 with various glycidyl ethers (Fig. 2). The
results demonstrated that 2a is applicable to most of glycidyl
ethers. Especially, butyl glycidyl ether could be completely
translated into the cyclic carbonate within only 4 h. And
1 3

Efcient Solvent‑Free Synthesis of Cyclic Carbonates from the Cycloaddition of Carbon Dioxide…
100
(a) 100
(b)
95
90
85
80
75
70
80
60
40
20
80
100
120
140
1
2
3
4
Time (h)
Temperature(°C)
100
(c)
(d)
100
80
60
40
20
0
80
60
40
20
0
1
2
3
4
5
0.0
0.5
1.0
1.5
2.0
Pressure (MPa)
Catalyst (mol%)
Fig. 3 Efects of diferent reaction parameters on the PC synthe-
sis over 2c catalyst. a Efect of reaction time, conditions: PO 10 mL
(0.143 mol), catalyst 1.43 mmol, temperature 120 °C, CO2 pressure
1.0 MPa, b efect of temperature, conditions: PO 10 mL (0.143 mol),
catalyst 1.43 mmol, time 2 h, CO2 pressure 1.0 MPa, c efect of reac-
tion pressure, conditions: PO 10 mL (0.143 mol), catalyst 1.43 mmol,
temperature 120 °C, time 2 h and d efect of catalyst mole percent,
conditions: PO 10 mL , temperature 120 °C, time 2 h, CO2 pressure
1.0 MPa
The dependence of the PC yield and selectivity on cata-
lyst loadings at 120 °C, 1.0 MPa CO2 and 2 h were dem-
onstrated in Fig. 3d. When the molar ratio of catalyst to
epoxide was varied from 0.2 to 1.0 mol%, the PC yield dra-
matically improved from 13.2% to 97.8%. However, there
was no signifcant improvement in the yield of PC for the
reaction conducted with 2 mol% catalyst amount. It could be
concluded that 1 mol% catalyst amount is optimum for this
study and the same catalyst amount was chosen for further
investigations.
1,2-Epoxyhexane, isobutyl oxide, cyclohexene oxide (CHO)
and bicyclic oxygenanes, were used as the substrate to react
with CO2 using 2c as catalyst. As shown in Table 2, no mat-
ter having electron-withdrawing or electron-donating sub-
stituents, most of substrates, including diepoxides, could be
smoothly converted to corresponding cyclic carbonates with
high conversion and excellent selectivity within 2 h. Unfor-
tunately, isobutyl oxide and cyclohexene oxide, exhibited
lower activity even after prolonging the reaction time to 10 h
(Table 2, entry 7–8), which was presumably due to the high
steric hindrance [67–71].
3.4 Catalytic Activity of Various Epoxides
The cycloaddition reactions of all above mentioned
epoxides with CO2 catalyzed by 2c were also studied under
atmospheric pressure (Table 2). Most of the reactions have
been carried out at 120 °C except for 1,2-epoxyhexane
(bp. 117 °C) and glycol diglycidyl ether (bp. 112 °C).
To survey the scope of substrates, other commercially
available epoxides, such as propylene oxide (PO), epichlo-
rohydrin (ECH), styrene oxide (SO), glycidol ethers,
1 3

Q. Zhang et al.
Table 2 Cycloaddition between CO2 and various epoxides catalyzed by 2c catalyst
O
O
O
O
CO2
+
R
R
1 mol% 2c, solvent-free
Condition Aa
Time (h) Yield (%)c TONd TOF (h−1 e
Condition Bb
Time (h) Yield (%)c TONd TOF (h−1 e
Entry Epoxide
Product
)
)
O
O
1
2
97.8
97.8
48.9
–
–
–
–
O
O
O
2
2
2
2
98.0
93.7
91.5
98.0
93.7
91.5
49.0
46.9
45.8
24
24
6
polymer
99.9
-
–
Cl
O
Cl
O
O
O
O
O
3
99.9
97.8
4.2
16.3
O
4
97.8
O
O
O
O
O
O
95.3f
98.5
–
95.3
98.5
–
4.0
16.4
–
O
O
5
6
7
6
92.4
96.3
65.6
92.4
96.3
65.6
15.4
16.1
6.6
24
6
O
O
O
O
6
O
O
O
O
O
O
10
–
O
O
O
8
10
48.2
48.2
4.8
24
12.9
12.9
0.5
O
O
O
O
O
O
9
2
2
92.8
90.1
92.8
90.1
46.4
45.1
6
6
98.8
95.9
98.8
95.9
16.5
16.0
O
O
O
O
10
O
O
O
O
O
11
12
2
2
98.7
98.1
98.7
98.1
49.4
49.1
6
99.0
99.0
94.5
16.5
3.9
O
O
O
O
O
O
O
94.5f
O
24
O
O
O
O
O
O
O
O
O
O
O
O
O
13
2
97.9
97.9
49.0
24
90.4
90.4
3.8
O
O
O
O
O
O
O
O
aReaction conditions: epoxide 10 mL (0.143 mol), Catalyst 1.43 mmol, temperature 120 °C, CO2 pressure 1.0 MPa, the selectivity to products
are all>99%
bReaction conditions: CO2 pressure 0.1 MPa and others same to condition A, the selectivity to products are all>99%
cDetermined by 1H NMR (CDCl3, 400 MHz)
dTurnover number for PC calculated as moles of PC produced per mole of catalyst
eTurnover frequency for PC calculated as mole of PC produced per mole of catalyst per hour
fReaction temperature 100 °C
Considering about their lower boiling points, their
cycloaddition reactions were conducted at 100 °C. The
boiling points of PO (34 °C) and isobutyl oxide (56 °C)
are much lower than 100 °C and not detected. All of the
substrates could be smoothly converted to corresponding
cyclic carbonates with high yield at atmospheric pressure
except for cyclohexene oxide, which could not achieve
good conversion to cyclic carbonate because of high struc-
ture steric hindrance (Table 2, entries 8). In particular, the
epoxides containing two epoxy groups also gave out excel-
lent yield (>90%) within 24 h under atmospheric pressure
(Table 2, entries 9–10). It is particularly worthy of note
that alternating copolymer was obtained for the reaction
of epichlorohydrin with carbon dioxide as found by D.J.
1 3

Efcient Solvent‑Free Synthesis of Cyclic Carbonates from the Cycloaddition of Carbon Dioxide…
Darensbourgand co-workers [57]. This interesting fnding
100
will be studied further.
80
3.5 Life Span of Catalysts
60
As shown in Fig. 2, butyl glycidyl ether could be completely
translated into corresponding cyclic carbonate with only 4 h.
In order to test life span of these catalysts, we added another
40
5 mL butyl glycidyl ether to the reaction system for the sec-
ond time. As shown in Fig. 4, the yield of cyclic carbonate
20
reached to 78.6% after 4 h, and a 97.6% yield was obtained
more 2 h later. At this time, we added another 5 mL substrate
0
for the third and fourth time, obtained 87.1% and 60.8% pro-
1
2
3
4
5
duction after 8 h and 10 h, respectively. It is demonstrated
that the catalyst could catalyze epoxide consecutively, but
needed enough time to reach a high conversion.
Catalytic Run
Fig. 5 Recyclability of catalyst 2c in the coupling reaction of PO
with CO2.Reaction conditions: PO 10 mL (0.143 mol), catalyst 1.43
mmol, temperature 120 °C, CO2 pressure 1.0 MPa, time 2 h (> 99%
PC selectivity is maintained)
3.6 Catalyst Recycling
As well-known, the stability and reusability of a catalyst
are two signifcant factors for industrial application [72–75].
Present bifunctional catalyst was also proven to be recycla-
ble using PO as the substrate under the optimal reaction
conditions (120 °C, 2 h, 1 MPa). Catalyst 2c could be pre-
cipitated from the reaction solution by the addition of ether
(Et2O). After simple fltration and drying and vacuum, the
catalyst could be reused by adding fresh reaction substrates.
The results indicate that the catalyst could be reused for fve
successive runs without any signifcant loss in its catalytic
activity and selectivity (Fig. 5), which also refects high sta-
bility of the catalyst. In addition, the FT-IR spectra of fresh
and reused catalyst have been compared in Fig. S14 (please
refer to the supporting information). There are similar peaks
between the fresh and 5th-run catalyst, except for the peak
located at 1790 cm−1 which is the stretching vibration peak
of carbonyl group in cyclic carbonate. The results confrmed
that the structure of catalyst was maintained after reused.
3.7 Kinetic Studies
According to our previous research and other work already pub-
lished [39, 74, 76, 77], the kinetic studies of catalysts 2c, 2i and
2j were carried out for the cycloaddition reaction with CO2 and
butyl glycidyl ether at 0.1 MPa. The calculation shows that the
apparent activation energies Ea of 2c, 2i and 2j are 95.4, 91.2
and 58.1 kJ/mol, respectively (Fig. 6). The Ea decreases with
the increase of the leaving ability of halogen (I−>Br−>Cl−).
2c
Ea
2i
95.4kJ/mol
100
80
60
40
20
0
O
2j
91.2kJ/mol
R
58.1kJ/mol
O
O
O
R
0
5
10
15
20
25
30
Time (h)
Fig. 6 The activation energies for the cycloaddition of CO2 and butyl
Fig. 4 Life span of catalysts
glycidyl ether
1 3

Q. Zhang et al.
And the sequence of Ea, Ea(2c)>Ea(2i)>Ea(2j), agrees well
4 Conclusion
with the catalytic activities in Table 1 (entries 3, 9 and 10).
In summary, a series of easily accessible single-component
and imidazolinium functionalized metal complexes were
synthesized and used as active catalysts for the coupling
reaction of CO2 and epoxides to selectively generate cyclic
carbonate under cocatalyst-free and solvent-free conditions.
Particularly, high TOF values up to 1174, 1240 and 1262 h−1
were obtained. The optimal catalytical conditions were
screened as (120 °C, 2 h, 1 MPa, 1 mol% catalyst amount)
after systematically investigation. These catalysts have been
proven to be workable for the synthesis of various cyclic
carbonates even under 0.1 MPa, and it also could be reused
for at least 5 times without apparently activity loss. Present
catalysts are environmental friendliness, facile synthesis and
low-cost bifunctional metal complexes, which would display
appealing candidate for reliable and efcient catalytic con-
version of CO2 into useful organic chemicals.
3.8 Proposed Mechanism of the Coupling Reaction
Considering the catalyst structure and the results of coupling
reaction, a possible mechanism was proposed (Scheme 3),
which is similar with other reported literatures [78–81]. The
epoxide ring of PO was activated by metal centre via coor-
dination, meanwhile, the nucleophilic attack of X− in the
units of imidazolinium-based ionic liquid on the ligand led
to epoxide ring-opening and formation of a metal-bound
alkoxide. Next, CO2 inserted into the metal–alkoxide bond
to form a metal alkylcarbonate. And then the production of
cyclic carbonate was formed via a backbiting pathway [82,
83]. Meanwhile, the catalyst is regenerated. This mechanism
indicated that the presence of a nucleophilic group and a
metal centre were both necessary for the reaction.
M
O
O
+
X-
N
O
N
R
O
M
+
O
X-
N
N
M
R
+
X
O
N
N
M
O
R
O
+
R
R
O
N
N
R1
X
CO2
N
+
M
N
O
X-
R2
N
M
X-
N
R2
+
N
O
X-
N
+
N
N
R
R1
Scheme 3. Proposed mechanism for cyclic carbonate formation
1 3

Efcient Solvent‑Free Synthesis of Cyclic Carbonates from the Cycloaddition of Carbon Dioxide…
30. Alves M, Grignard B, Mereau R, Jerome C, Tassaing T, Detrem-
Acknowledgements We acknowledge gratefully the fnancial support
from National Natural Science Foundation of China (No. 51073175),
Natural Science Foundation of Hubei province, P. R China (No.
2016CKB704) and the Fundamental Research Funds for the Central
Universities, South-Central University for Nationalities (CZT19002).
Authors are also grateful to the fnancial support of Beijing National
Laboratory for Molecular Sciences (BNLMS201827).
bleur C (2017) Catal Sci Technol 7:2651–2684
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Afliations
Qian Zhang1 · Peng Yu1 · Bo Lei1 · Hai‑Jian Yang1 · Juncheng Hu1 · Lihua Wang1 · Tengfei Zhou1 · Qingqing Jiang1 ·
Junjiang Zhu2 · Buxing Han3
* Hai-Jian Yang
Central University for Nationalities, Wuhan 430074,
People’s Republic of China
yanghaijian@vip.sina.com
2
3
* Junjiang Zhu
Hubei Key Laboratory of Biomass Fibers and Eco-Dyeing
& Finishing, College of Chemistry and Chemical
Engineering, Wuhan Textile University, Wuhan 430200,
People’s Republic of China
ciaczjj@163.com
1
Key Laboratory of Catalysis and Energy Materials
Chemistry of Ministry of Education & Hubei Key
Laboratory of Catalysis and Materials Science, Key
Laboratory of Analytical Chemistry of the State Ethnic
Afairs Commission, National Demonstration Center
for Experimental Ethnopharmacology Education,
College of Chemistry and Materials Science, South-
Beijing National Laboratory for Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100049, People’s Republic of China
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