Zn-salen complexes with multiple hydrogen bonding donor and protic ammonium bromide: Bifunctional catalysts for CO2 fixation with epoxides at atmospheric pressure

Article abstract of DOI:10.1016/j.molcata.2016.04.018

Bifunctional Zn-salen complexes with multiple hydrogen bonding donors and protic ammonium bromides were developed as efficient catalysts for the formation of 4-phenyl-1,3-dioxolan-2-one from styrene oxide and CO₂. Owing to synergistic effects o

Full text of DOI:10.1016/j.molcata.2016.04.018

Accepted Manuscript
Title: Zn-salen complexes with multiple hydrogen bonding
donor and protic ammonium bromide: bifunctional catalysts
for CO2 fixation with epoxides at atmospheric pressure
Author: Xian-Dong Lang Yi-Chen Yu Liang-Nian He
PII:
S1381-1169(16)30149-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.04.018
MOLCAA 9862
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
19-3-2016
18-4-2016
18-4-2016
Please cite this article as: Xian-Dong Lang, Yi-Chen Yu, Liang-Nian He, Zn-salen
complexes with multiple hydrogen bonding donor and protic ammonium bromide:
bifunctional catalysts for CO2 fixation with epoxides at atmospheric pressure, Journal
of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.04.018
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Zn-salen complexes with multiple hydrogen bonding donor
and protic ammonium bromide: bifunctional catalysts for
CO2 fixation with epoxides at atmospheric pressure
Xian-Dong Lang,a Yi-Chen Yu,a Liang-Nian He*a,b
a State Key Laboratory and Institute of Elemento-Organic Chemistry
Nankai University, Tianjin 300071, China
b Collaborative Innovation Center of Chemical Science and Engineering
Tianjin 300071, China
*Corresponding author: Liangnian He
E-mail address: heln@nankai.edu.cn
Telephone: +86-022-23503878
Fax: +86-022-23503878

Graphical abstract
Research highlights
► Bifunctional Zn-salen complex was decorated by multiple hydrogen bonding donors.
► The reaction was performed under mild reaction conditions.
► High values of TON and TOF (up to 49288 and 39473 h−1) were obtained.
► A variety of cyclic carbonates were obtained in high yields.

Abstract
Bifunctional Zn-salen complexes with multiple hydrogen bonding donors and
protic ammonium bromides were developed as efficient catalysts for the formation
of 4-phenyl-1,3-dioxolan-2-one from styrene oxide and CO2. Owing to synergistic
effects of multiple sites on epoxide activation (Lewis acidic zinc, phenolic
hydroxyl group and protonated tertiary ammonium able to activate epoxide and
bromide anion acting as a nucleophile), the easily-synthesized Zn-salen complexes
showed outstanding performance at mild reaction conditions (CO2 0.1 MPa, 100
oC) using only 0.3 mol% catalyst amount. Besides, this protocol is applicable to a
variety of epoxides including internal epoxide, giving the corresponding cyclic
carbonates in good to excellent yields. Notably, 200 mmol-scale experiment was
successfully conducted and TON of 49288±13 and TOF of 39473±8 h−1 were
achieved.
Keywords: Carbon dioxide fixation; Cyclic carbonate; Homogeneous catalysis;
Zn-salen complex; Hydrogen bonding donors

1. Introduction
Carbon dioxide is not only one of the greenhouse gases, but also can be
regarded as a sustainable feedstock in organic synthesis due to the favourable
characteristics of CO2 such as low cost, nontoxicity, nonflammability, easy
accessibility and so on [1-3]. Therefore, efficient transformation of CO2 into
value-added chemicals and fuels through constructing C-N bond, C-O bond and
C-C bond can meet the requirements of green chemistry and sustainable
development [4-7]. One of the promising and most studied methodologies in
this context is the formation of cyclic carbonates via the 100%
atom-economical cycloaddition of epoxides with CO2, due to extensive
applications of cyclic carbonates as economically competitive products, such as
aprotic high-boiling polar solvents, electrolytes for lithium-ion batteries,
precursors of polymeric materials, and fine chemicals [8-10]. Therefore, great
progresses have been witnessed on development of various catalytic systems
for this coupling reaction including metal-based catalysts e.g. metal-organic
frameworks (MOFs) [11-12], metal-salen complexe [13-23], metalloporphyrin
[24-26], metal chloride [27], polyoxometalate [28], metal oxide [29-30] and
metal-free catalysts containing hydroxyl [31-35], carboxyl [36-38] and so on
[39-40], which activate the epoxide through hydrogen bonding interaction.
Generally, metal catalysts exhibit superior performance compared to metal-free
counterparts. In particular, metal-salen complexes have undoubtedly attracted
much attention presumably owing to the intrinsic features with salen-ligand
such as facile preparation, strong complexation ability with almost all metal
ions and structural diversity and tunability [41-49].
From the mechanistic viewpoint, epoxide activation by Lewis acidic
species or via hydrogen bonding interaction and subsequent ring-opening by
nucleophilic attack of an halide anion e.g. bromide are two crucial steps [50]. In
order to improve catalytic efficiency, incorporation of ionic moieties into the
framework of salen ligand seems to be an advisable strategy as far as the

structure-property correlation is concerned [51-52], so that an alternative
process not only avoids decomposition of cocatalyst for example, tetrabutyl
ammonium bromide TBAB via retro-Menschutkin reaction [13], but also
facilitates the product separation and catalyst reusability. In this aspect, Jiang
and co-workers have developed a Lewis acid-base bifunctional Zn-salen
complex through covalent linkage of the tertiary amine i.e.
N-methylhomopiperazine [16], which can be used as efficient and recoverable
catalyst for this cycloaddition. In this system, the epoxide is likely activated by
the Lewis acidic zinc center, and N-methylhomopiperazine species presumably
attacks at the less-hindered carbon atom of the epoxides. Additionally, the
ligands can be modified by quaternary ammonium group, producing
bifunctional monometallic [17] and bimetallic [13,61,63] Al-salen complexes,
reported by Lu and North, respectively. For the thus synthesized bifunctional
monometallic Al-salen complex, a high TON of 116230 was achieved at 2.5
MPa CO2 with the reaction time of 48 h. Besides, oxazolidinones were also
obtained in high yields with complete configuration retention. The bimetallic
Al-salen complex can be supported on silica to afford an equally efficient and
heterogeneous bifunctional catalyst, leading to facilitate the catalyst recovery
and product separation [19]. Furthermore, the silica particle size plays an
important role in the catalytic activity. Lee and co-workers have developed the
bifunctional Co-salen complex modified with quaternary ammonium salt as
efficient and thermally robust catalyst for the copolymerization of propylene
oxide with CO2. High TOF (up to 3500 h-1) and TON (up to 14 500) were
attained [23].
The introduction of halide anion can also be operated via the alkylation at
the pyridyl-N atom, producing bifunctional alkylated Zn(salpyr) complexes. In
this aspect, imidazolium moieties containing polyethylene oxide (PEO) chains
can also be embodied in to endow the synthesized bifunctional catalyst with
“CO2-philic” capability [15]. Most recently, Jing et al. have developed an
efficient protocol for asymmetric coupling reaction of CO2 and epoxides [18],

using a series of chiral SalenCo(III)Y catalysts supported on the alkyl
imidazole ionic liquids at room temperature and 1.2 MPa CO2. The influence of
−
axial anion Y on catalytic activity followed the order of OAc− > CF3CO2 >
−
−
CCl3CO2 > OTs−, whereas on enantioselectivity OTs− > OAc− > CCl3CO2 >
−
CF3CO2 .
Our group has developed bifunctional cobalt-salen complex containing a
Lewis acidic metal and two quaternary phosphonium salt units anchored on the
salen ligand. This is an excellent catalyst for the synthesis of cyclic carbonates
from CO2 and epoxides without the utilization of additional organic solvents
under relatively elevated CO2 pressure (4 MPa) [14]. As a kind of nontoxic,
easily available metal with strong Lewis acidity, Zn has been a well exploited
metal for the cycloaddition of CO2 with epoxides [53-55]. In view of the
cooperative effect of both the epoxides activation by Lewis acidic species
together with hydrogen bonding interaction, and nucleophilic ability of the
bromide anion, a series of bifunctional Zn-salen complexes with multiple
hydrogen bonding donor and protic ammonium bromide were synthesized and
evaluated the activity for the cycloaddition between epoxides and CO2, as
depicted in Scheme 1a. Because of the synergistic effects of multiple sites on
epoxide activation illustrated in Scheme 1b (Lewis acidic zinc, phenolic
hydroxyl group and protonated tertiary ammonium able to activate epoxide and
bromide anion acting as nucleophile), Zn(OH-salC2NH2Am) showed
outstanding catalytic performance at 0.1 MPa CO2.
<Scheme 1 here>
2. Experimental
2.1. Materials and methods
CO2 with a purity of 99.999 % was commercially available. Propylene
oxide was obtained from Alfa Aesar, while other epoxides, hydrobromic acid,
pyridine,
N,N-diethylethylenediamine,
salicylaldehyde,

3,5-Bis(1,1-dimethylethyl)-2-hydroxy-benzaldehyde were purchased from
Aladdin and used as received. 2,3,4-Trihydroxybenzaldehyde and
1
2-bromoethylamine hydrobromide were obtained from Bide Pharmatech. H
NMR spectra were recorded on Bruker 400 MHz spectrometer using CDCl3 or
DMSO-d6 as solvent referenced to CHCl3 (7.26 ppm) or DMSO-d6 (2.50 ppm).
13C NMR spectra were recorded at 100.6 MHz in CDCl3 (or DMSO-d6) using
CDCl3 (77.0 ppm) (or DMSO-d6, 39.5 ppm) as an internal reference. GC
analysis were performed on a Shimadzu GC-2014 equipped with a capillary
column (RTX-50 30 m × 0.25 μm) using a flame ionization detector. High
resolution mass spectrometry was conducted using a Varian 7.0 T FTICR-MS
by ESI technique. Infrared (IR) spectra were recorded on a Bruker Tensor 27
FT-IR spectrophotometer with KBr pellets. In situ FT-IR was collected on a
Mettler Toledo React IR ic10, Sicomp (Silicon) ATR probe, using ic IR
analysis system. The probe is placed in the middle of the reaction mixture,
which is constantly stirred by magnetic bar, and the spectra are collected in situ
during the whole process. All the products were characterized and identified by
comparison with those reported in the literature [14,35,39].
2.2. Typical procedure for the preparation of bifunctional Zn-salen complexes
The four bifunctional Zn-salen complexes listed in Scheme 1 were
synthesized according to the reported procedure with a minor modification
[69-72]. Take the synthesis of Zn(OH-salC2NH2Am) as an example which is
shown in Scheme 2, an solution of N,N-diethylethylenediamine hydrobromide
[C2NH2am][Br] (2 mmol, 0.3942 g) in 5 mL ethanol was added dropwise to a
solution of 2,3,4-trihydroxybenzenaldehyde (2.2 mmol, 0.3388 g). After stirring
at 70 oC overnight, Zn(OAc)2·H2O (2 mmol, 0.432 g) was added to the flask in
portions. And immediately, large amount of yellow solid formed, which was
o
then filtrated and dried under vacuum at 70 C for one day. For the preparative
procedure and characteristic data of 1-(2-aminoethyl) pyridinium bromide
[C2NH2py][Br]
and
N,N-diethylethylenediamine
hydrobromide

[C2NH2am][Br], see Supporting Information.
<Scheme 2 here>
2.3. Characterization data of the salen ligand
1-(2-((2-hydroxybenzylidene)amino)ethyl)pyridin-1-ium
bromide:
Yellow viscous liquid; 1H NMR (400 MHz, DMSO-d6) δ 12.41 (s, 1H), 9.23 (s,
1H), 8.64 (t, 1H), 8.55 (s, 1H), 8.20 (s, 2H), 7.38-7.40 (d, 1H), 7.31 (t, 1H),
7.84-6.86 (t, 2H), 5.07 (s, 2H), 4.20 (s, 2H) ppm; 13C NMR (100.6 MHz,
DMSO-d6) δ 168.07, 159.81, 145.93, 144.96, 132.71, 131.63, 127.92, 118.82,
118.50, 116.34, 60.65, 58.09 ppm; IR (KBr) ν/cm-1 3442, 3388, 3050, 1630,
1489, 1278, 1026, 1006, 783, 685
found 305.0295 (M-H)-.
;
HRMS calcd for [C14H14BrN2O]- 305.0292,
N,N-diethyl-2-((2-hydroxybenzylidene)amino)ethanaminium bromide: Yellow
1
viscous liquid; H NMR (400 MHz, DMSO-d6) δ 8.75 (s, 1H), 7.54 (d, 1H), 7.39 (t,
13
1H), 6.94 (t, 1H), 4.01 (t, 2H), 3.47 (t, 2H), 3.22 (t, 4H), 1.24 (t, 6H) ppm; C NMR
(100.6 MHz, DMSO-d6) δ 168.19, 160.05, 136.33, 131.93, 118.93, 116.44, 80.28,
50.89, 47.09, 8.45 ppm; IR (KBr) ν/cm-1 3443, 2980, 2939, 2652, 2485, 2246, 1631,
1461, 1276, 1226, 1026, 1002, 824, 763; HRMS calcd for [C13H20BrN2O]-
299.0764, found 299.0760 (M-H)-.
N,N-diethyl-2-((3,5-ditert-butyl-2-hydroxybenzylidene)amino) ethanaminium
1
bromide: Yellow viscous liquid; H NMR (400 MHz, DMSO-d6) δ 13.43 (s, 1H),
8.67 (s, 1H), 7.29 (d, 2H), 3.34 (t, 8H), 1.38 (d, 8H), 1.27 (t, 18H) ppm; 13C NMR
(100.6 MHz, DMSO-d6) δ 170.33, 157.64, 140.36, 136.12, 127.17, 118.22, 53.33,
51.63, 47.61, 35.05. 34.34, 31.75, 29.70, 8.97 ppm; IR (KBr) ν/cm-1 2959, 2645, 2487,
1628, 1470, 1438, 1392, 1359, 1270, 1237, 1024, 821, 714; LC-MS calcd for
[C21H38BrN2O]+ 413.2, found 413.4 (M-H)+.
N,N-diethyl-2-((2,3,4-trihydroxybenzylidene)amino) ethanaminium bromide:
1
Yellow viscous liquid; H NMR (400 MHz, DMSO-d6) δ 9.86 (d, 1H), 8.53 (s, 1H),
6.37-7.10 (m, 2H), 3.19 (d, 10H), 2.51 (d, 1H), 1.24 (t, 6H) ppm; 13C NMR (100.6
MHz, DMSO-d6) δ 168.19, 160.05, 136.33, 131.93, 118.93, 116.44, 80.28, 50.89,

47.09, 8.45 ppm; IR (KBr) ν/cm-1 3444, 2985, 2250, 2125, 1636, 1507, 1454, 1350,
1278, 1233, 1024, 825, 764; LC-MS calcd for [C13H22BrN2O3]+ 333.1, found
333.3 (M+H)+.
2.4. Characterization data of the Zn-salen complexes [69-72]
Zn(salC2NH2Py): yellow powder; IR (KBr) ν/cm-1 3052, 1616, 1535, 1448,
1322, 1185, 1152, 758, 682; HRMS calcd for C28H28BrN2O4Zn (M-Br)+
595.0708, found 595.0711.
Zn[salC2NH2Am]: white powder; IR (KBr) ν/cm-1 2978, 2621, 2484, 1625,
1546, 1468, 1441, 1413, 1291, 1195, 1154, 751; HRMS calcd for
C26H40N4O2Zn (M-2Br)+ 494.2638, found 494.2634.
Zn(t
-Bu-salC2NH2Am): yellow powder; IR (KBr) ν/cm-1 2954, 1632, 1560,
1438, 1254, 1159, 1047, 908, 936, 791, 683; HRMS calcd for C42H72BrN4O2Zn
(M-Br)+ 807.4125, found 807.4128.
Zn(OH-salC2NH2Am): brown powder; IR (KBr) ν/cm-1 3524, 3306, 2983,
1582, 1437, 1277, 1101, 790; HRMS calcd for C26H40BrN4O6Zn (M-Br)+
647.1417, found 647.1418.
2.5. Cycloaddition procedure for the reaction of epoxides with CO2
In a 20 mL Schlenk flask, Zn(OH-salC2NH2Am) (0.3 mol%, 11 mg), epoxide
(5 mmol) were added. The reaction mixture was stirred at 100 °C for the desired time
under an atmosphere of CO2 (99.999%, balloon). After the reaction, the mixture was
cooled to room temperature, and added biphenyl (3 mmol, 0.4626 g) as internal
standard for GC yield determination. For gram-scale operations, in a typical
experiment, Zn(OH-salC2NH2Am) (0.03 mol%, 1.1 mg) and epichlorohydrin were
introduced into the inner glass tube successively. Afterwards, the mixed gas pressure
o
was adjusted to 3 MPa at 120 C by charging CO2 three times. The reaction mixture
o
was stirred for desired time at 120 C. When the reaction finished, the reactor was
cooled in ice-water, and CO2 was ejected slowly. And then 1,1,2,2-tetrachloroethane
was used as internal standard for 1H NMR yield determination.

2.6. Characterization data of the products
o
1
4-phenyl-1, 3-dioxolan-2-one
(
2a
)
:
White solid; m.p. 53 C; H NMR
3
3
(400MHz, CDCl3) δ 4.32 (t, J(H,H) = 8.4Hz, 1H), 4.78 (t, J(H,H) = 8.4 Hz,
3
3
1H), 5.70 (t, J(H,H) = 8.0 Hz, 1H), 7.35 (d, J(H,H) = 7.6 Hz, 2H), 7.43 (d,
13
3J(H,H) = 6.4 Hz, 3H). C NMR (100.6 MHz, CDCl3) δ 71.29, 78.11, 125.99,
129.34, 129.84, 135.86, 154.97.
1
1, 3-dioxolan-2-one
(
2b
)
:
Colorless solid; H NMR (CDCl3, 400 MHz): δ =
4.51 (s, 4H). 13CNMR (100.6 MHz, CDCl3): δ = 64.7, 155.6.
4-chloromethyl-1, 3-dioxolan-2-one
(
2c) 1H NMR (400MHz, CDCl3) δ 3.72
3
2
3
(dd, J(H,H) = 3.2 Hz, J(H,H) = 12.0 Hz, 1H), 3.77 (dd, J(H,H) = 5.2 Hz,
3
2
2J(H,H) = 12.0 Hz, 1H), 4.37 (dd, J(H,H) = 6.0 Hz, J(H,H) = 8.4 Hz, 1H),
3
13
4.56 (t, J(H,H) = 8.4 Hz, 1H), 4.96 (m, 1H). C NMR (100.6 MHz, CDCl3) δ
43.92, 67.02, 74.43, 154.39.
1
4-methyl-1, 3-dioxolan-2-one
(
2d
)
:
Colorless liquid; H NMR (400 MHz,
CDCl3) δ 1.39 (d, 3J(H,H) = 6.0 Hz, 1H), 3.96 (t, 1H), 4.49(t, 3J(H,H) = 8.4 Hz,
1H), 4.79 (m, 1H). 13C NMR (100.6 MHz, CDCl3) δ 19.28, 70.72, 73.71,
155.16.
4-(hydroxymethyl)-1,3-dioxolan-2-one
(
2e)
:
Yellow liquid; 1H NMR
3
2
(400MHz, CDCl3) δ 2.85 (br, 1H), 3.68 (dd, J(H,H) = 3.2 Hz, J(H,H) = 12.0
3
2
Hz, 1H), 3.97 (dd, J(H,H) = 3.0 Hz, J(H,H) = 11.0 Hz, 1H), 4.46-4.53 (m,
1
2H), 4.80 (m, H), 13C NMR (100.6 MHz, CDCl3) δ 61.75, 65.92, 76.79,
155.58.
4-phenoxymethyl-1, 3-dioxolan-2-one
(
2f). Yellow liquid; 1H NMR
3
2
(400MHz, CDCl3) δ 4.14 (dd, J(H,H)= 4.4 Hz, J(H,H) = 10.8 Hz, 1H), 4.23
2
3
2
(dd, 3J(H,H) = 3.6 Hz, J(H,H) = 10.8 Hz, 1H) 4.54 (dd, J(H,H) = 8.4 Hz, J=
3
3
6.0 Hz, 1H), 4.60 (t, J(H,H) = 8.4 Hz, 1H), 5.02 (m, 1H), 6.90 (d, J(H,H) =
3
8.0 Hz, 2H), 7.00 (t, J(H,H) = 7.4 Hz, 2H), 7.31 (t, 3J(H,H) = 8.0 Hz, 2H); 13C
NMR (100.6 MHz, CDCl3) δ 44.92, 67.02, 74.43, 114.8, 122.2, 129.1, 154.39.

Hexahydro-benzo[1,3]dioxol-2-one
(
2g) 1H NMR (400MHz, CDCl3) δ 1.36
13
(m, 2H), 1.53 (m, 2H), 1.82 (m, 4H), 4.63 (d, 2H); C NMR (100.6 MHz,
CDCl3) δ 19.13, 26.74, 75.80, 155.42.
3. Results and discussion
Currently, chemical transformations of CO2 under mild conditions
particularly at atmospheric pressure still remain grand challenges [56-60].
Consequently, our exploratory experiments started using styrene oxide (1a) as
the benchmark substrate under 0.1 MPa CO2 to evaluate the catalytic activity of
the bifunctional Zn-salen complexes as shown in Scheme 1a. The results are
summarized in Table 1. Obviously, no reaction occurred in the absence of any
catalyst (entry 1). 4-Phenyl-1,3-dioxolan-2-one (2a) was obtained in only 6%
yield in the presence of Zn(salC2NH2Py) with pyridine-based ionic moiety
(entry 2). It was reported that hydroxyl [31-35], carboxyl [36-38] and amino
[39-40]-containing electrophiles could interact with the epoxide via hydrogen
bonding interaction, thus facilitating the ring opening of the epoxide by
bromide anion. With this idea in mind, two bifunctional Zn-salen complexes
e.g. Zn(salC2NH2Am), Zn(OH-salC2NH2Am) with multiple hydrogen bonding
donors such as protonated tertiary ammonium and phenolic hydroxyl group
were prepared for this purpose. Gratifyingly, the introduction of such hydrogen
bonding donors into the ligand led to the remarkable increase in 2a yield
(entries 3, 4). The bulky alkyl substituents on the aromatic ring could improve
the solubility of Zn(t-Bu-salC2NH2Am) in styrene oxide and thus enhances its
activity [61-64] (entry 3 vs 5). Poor solubility of the complex in the reaction
mixture was observed when polar ionic group was tethered into the salen
ligand, leading to lower activity compared to the binary system [15,24-26,65].
In this context, a suitable solvent is necessary to perform the reaction smoothly.
Among the conventional organic solvents examined, DMF gave the best results
(entries 6-9, 4). To explore the respective role of Lewis acidic zinc and multiple
hydrogen bonding donors, ZnBr2 and ligand of Zn(OH-salC2NH2Am) i.e.

N,N-diethyl-2-((2,3,4-trihydroxybenzylidene)amino) ethanaminium bromide
were utilized as catalyst for the benchmark reaction, affording 37% and 20%
yield of 2a. The results demonstrated the superior of Lewis acidic zinc beyond
multiple hydrogen bonding donors and Zn(OH-salC2NH2Am) beyond ZnBr2
and its corresponding ligand.
<Table 1 here>
Subsequently, the influences of the reaction parameters including reaction
time, temperature and catalyst loading on the reaction were investigated as
listed in Table 2. It can be easily seen that the reaction actually completed
within 16 h, no further increase in 2a yield was observed with prolonging
reaction time (entries 1-4, Table 2). The catalyst loading can be decreased to
0.3 mol% (entry 5 vs 2). To be delighted, Zn(OH-salC2NH2Am) also gave a
good result even with as low as 0.1 mol% amount relative to 1a (entry 6).
Furthermore, the result remained almost unchanged when the temperature was
decreased from 120 oC to 100 oC (entry 5 vs 7). A sharp decline in 2a yield was
observed when the temperature further decreased to 80 oC (entries 5, 7 vs 8). In
addition, DMF was reported to promote the epoxide/CO2 coupling reaction
under the supercritical conditions [66-67]. However, DMF itself was inactive in
o
this study (CO2 0.1 MPa, 100 C, 16 h) and just serves as solvent (entry 9). To
investigate the effect of integrating multiple functional groups such as phenolic
hydroxyl group, protic ammonium bromide in one salen molecule, the catalytic
performance
of
simple
physical
mixture
and
such
as
Zn(salC2NH2Am)/1,2-benzenediol
Zinc(II)
[N,N´-ethylene-bis(salicylidenediamine)] Zn(SAL)/TBAB/1,2- benzenediol
were subsequently evaluated. Lower yield was afforded compared to
Zn(OH-salC2NH2Am, demonstrating the superiority of integrating functional
groups in one molecule (entries 10-11).
<Table 2 here>
Having established the suitable reaction conditions, the generality and
utility of this approach were investigated using various epoxides. As

summarized in Table 3, the terminal epoxides can be efficiently transformed to
the corresponding cyclic carbonates in high yield and excellent selectivity
(entries 1-6, Table 3). This protocol can be applied to the synthesis of various
cyclic carbonates containing electron-withdrawing and electron-donating
substituents in almost quantitative yields, suggesting potential application in
industry. Due to steric hindrance, carboxylation of cyclohexene oxide (CHO)
remains a grand challenge. The target product 2g was formed in only 14%
yield. Fortunately, nice result was obtained under 3 MPa pressure (entry 7).
<Table 3 here>
To exploit the catalytic performance of Zn(OH-salC2NH2Am), 20
gram-scale reactions of epichlorohydrin 1c with CO2 were conducted using
various catalyst/substrate ratios i.e. Zn(OH-salC2NH2Am): 1c. The results are
summarized in Table 4. Relatively high value of TON (26496 and 43906) and
TOF (1019 h-1 and 1688 h-1) were obtained with 51 mmol and 76 mmol 1c
,
respectively (entries 1, 2). To our delight, this bifunctional zinc salen complex
Zn(OH-salC2NH2Am) demonstrated high catalytic activity even with 200 mmol
(18.504 g) scale (entry 4, Table 4). The maximum TON (turnover number) and
TOF (turnover frequency) reached up to 49288
±13 and 39473±
8 h-1,
respectively (entries 3, 4). This is one of the most effective salen complexes
reported to date for the epoxide/CO2 coupling, but shows lower activity
compared with the highly symmetrical and bifunctional porphyrin complexes
developed by Ema and coworkers [24-26].
<Table 4 here>
To monitor the process of gram-scale operation, in situ FT-IR
spectroscopy was conducted at 0.1 MPa CO2 (Figure 1). An induction period
was observed under solvent-free conditions, while disappeared with the
addition of DMF, implying that 2a may act as the solvent in the absence of any
conventional solvent. In the FT-IR spectra, the characteristic peak at 876 cm-1
of the epoxy ring declined gradually, indicating the consumption of 1a
.
Meanwhile, the appearance of adsorption peaks at 1065 cm-1, 1159 cm-1

(asymmetrical C–O vibration) and 1810 cm-1 (C=O stretching vibration) were
suggestive of the formation of 4-phenyl-1,3-dioxolan-2-one 2a
.
<Figure 1 here>
Based on experimental results in this study and previous reports [31,39,52,68], a
tentative mechanism involving bromide anion as nucleophile, multiple hydrogen
bonding donors and Lewis acidic center is illustrated in Scheme 3. Firstly, the epoxide
is activated by the Lewis acidic center (i.e. zinc), protonated tertiary ammonium and
phenolic hydroxyl group through hydrogen bonding interaction, thus resulting in
accelerating the ring-opening of the epoxide. In deed, the experimental results (entry 2
vs 3, entry 3 vs 4 and entry 6 vs 11, Table 1) indicate the key role of catalytic sites i.e.
protonated tertiary ammonium, phenolic hydroxyl group and Lewis acidic species,
respectively. Besides, the superiority of integrating functional groups in one molecule
is also demonstrated (entry 7 vs 11-12, Table 2). All the above mentioned
experimental results validate the cooperative effects of the three different catalytic
sites (Lewis acidic center, protonated tertiary ammonium and phenolic hydroxyl
group). Simultaneously, the nucleophilic attack of bromide anion at the less sterically
hindered β-carbon atom of the epoxide affords the ring-opened intermediate B.
Subsequently, nucleophilic attack of the intermediate B at CO2 produces the acyclic
metal carbonate species C. Finally, intramolecular ring-closure of C furnishes the
cyclic carbonate as target product.
<Scheme 3 here>
4. Conclusions
As a kind of low toxic, readily available metal, Zn has been well exploited
in the cycloaddition of epoxides and CO2. Based on “Lewis acid-base
cooperative catalysis” principle, a series of single-component bifunctional
salen-Zn complexes bearing various functional groups were tested the catalytic
activity for epoxides/CO2 coupling. Originating from co-existence of Lewis
acidic zinc center, hydrogen bonding donors e.g. phenolic hydroxyl group,
protonated tertiary ammonium in combination of nucleophilic species i.e.

bromide anion in one molecule, Zn(OH-salC2NH2Am) showed high catalytic
activity for the formation of cyclic carbonates from epoxides and CO2, which
promoted the reactions at 0.1 MPa CO2. High TON and TOF value (49288
±13
and 39473
±
8 h−1, respectively) were also reached. This process represents an
alternative approach for greener chemical fixation of CO2 to afford valuable
compounds with potential industrial applications.
Acknowledgements
We are grateful to the National Natural Science Foundation of China, the
Specialized Research Fund for the Doctoral Program of Higher Education
(20130031110013), the MOE Innovation Team (IRT13022) of China, the
“111” Project of Ministry of Education of China (project No. B06005), and the
Foundation of State Key Laboratory of Coal Conversion (Grant No.
J16-17-902-2) for financial support.

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15 min
1159
1065
0.20
0.15
0.10
0.05
75 min
135 min
375 min
415 min
876
1810
1800
1200
1000
800
Wavenumbers (cm-1
)
Fig. 1. (a) Results of in situ IR spectroscopy of SO/CO2 coupling collected every 1 min; (b)
Changes of adsorption peaks during SO/CO2 coupling. Reaction conditions: Styrene oxide (30
mmol, 3.6270 g), Zn(OH-salC2NH2Am) (66 mg, 0.3 mol%), DMF (1 mL), 100 °C, CO2 (0.1
MPa).

Schemes
Scheme 1 (a) Bifunctional Zn-salen complexes in this study; (b) Synergistic effects of
multiple sites of Zn(OH-salC2NH2Am).

Scheme 2. Typical procedure for the preparation of bifunctional Zn(OH-salC2NH2Am).

Scheme 3. Proposed mechanism for the Zn(OH-salC2NH2Am)-catalyzed epoxide/CO2
coupling.

Tables
Table 1 Catalyst screening and solvent effecta.
Entry
Catalyst
-
Solvent
Yieldb (%)
1
2
-
0
Zn(salC2NH2Py)
-
-
6
3
Zn(salC2NH2Am)
38
64
45
90
85
60
52
37
4
Zn(OH-salC2NH2Am)
Zn(t-Bu-salC2NH2Am)
Zn(OH-salC2NH2Am)
Zn(OH-salC2NH2Am)
Zn(OH-salC2NH2Am)
Zn(OH-salC2NH2Am)
ZnBr2
-
5
-
6
DMF
DMSO
Toluene
CH3CN
DMF
7
8
9
10
N,N-diethyl-2-((2,3,4-trihydroxybenzylidene)amino)
ethanaminium bromide
11c
DMF
20
a
Unless otherwise specifically notified, all the reactions were carried out with styrene oxide
(0.6025 g, 5 mmol); catalyst (0.5 mol%); solvent (2 mL); CO2 (0.1 MPa); 120 oC; 12 h.
b Determined by GC using biphenyl as internal standard.
c 1 mol% catalyst was used.

Table 2 Effects of reaction parameters on styrene oxide/CO2 couplinga.
Loading
Entry
Catalyst
T (oC)
t (h)
Yieldb (%)
(mol%)
0.5
0.5
0.5
0.5
0.3
0.1
0.3
0.3
-
1
2
3
4
5
6
7
8
9
Zn(OH-salC2NH2Am)
Zn(OH-salC2NH2Am)
Zn(OH-salC2NH2Am)
Zn(OH-salC2NH2Am)
Zn(OH-salC2NH2Am)
Zn(OH-salC2NH2Am)
Zn(OH-salC2NH2Am)
Zn(OH-salC2NH2Am)
-
120
120
120
120
120
120
100
80
12
16
20
24
16
16
16
16
16
90
96
97
98
92
74
92
85
0
100
10c
Zn(salC2NH2Am)/1,2-benzenediol
Zn(SAL)/TBAB/1,2-benzenediol
0.3
0.3
100
100
16
16
81
74
11d
a Reaction conditions: Styrene oxide (0.6025 g, 5 mmol); CO2 (0.1 MPa).
b Determined by GC using biphenyl as internal standard.
c Zn(salC2NH2Am) (10.5 mg, 0.3 mol%); 1,2-benzenediol (3.5 mg, 0.6 mol%).
d Zn(SAL) (5 mg, 0.3 mol%); 1,2-benzenediol (3.5 mg, 0.6 mol%); TBAB(10 mg, 0.6 mol%).

Table 3 Cycloaddition of CO2 with different epoxides catalyzed by Zn(OH-salC2NH2Am)a.
Entry
1
Substrate
Product
Yieldb (%)
94
1a
2a
2
3
4
5
6
97
96
97
95
94
1b
2b
1c
2c
1d
1e
1f
2d
2e
2f
14
92c
7
1g
2g
a All the reactions were carried out with the epoxide (5 mmol); Zn(OH-salC2NH2Am) (11 mg, 0.3
mol%); CO2 (0.1 MPa); 100 oC; 16 h.
b Determined by 1H NMR (CDCl3) using 1,3,5-trimethoxybenzene as internal standard.
c 3 MPa.

Table 4 The gram-scale synthesis of 4-chloromethyl-1, 3-dioxolan-2-one from epichlorohydrin and
a
CO2 .
1c
Entry
t (h)
2c Yieldb (%)
TONc
TOFd
(mmol)
51
1
2
3
4
26
26
1
78
87
59
37
26496
43906
1019
1688
76
100
200
39473±8
49288±13
39473±8
7
7041±2
a
o
All the reactions were carried out with Zn(OH-salC2NH2Am) (1.1 mg); CO2 (3 MPa); 120 C; DMF
(1 mL).
b Determined by 1HNMR (CDCl3) using 1,1,2,2-tetrachloroethane as internal standard.
c TON (turnover number): mole of chloropropene carbonate/mole of Zn(OH-salC2NH2Am).
d TOF (turnover frequency): mole of chloropropene carbonate/mole of Zn(OH-salC2NH2Am)·t (hour).