Lewis acid-base bifunctional aluminum-salen catalysts: synthesis of cyclic carbonates from carbon dioxide and epoxides

Article abstract of DOI:10.1039/c5ra24596f

Two Lewis acid-base bifunctional monometallic aluminum-salen complexes were prepared based on a new type of salen ligand with two N-methylhomopiperazine moieties at the 3,3′-position. The Al(salen) complexes proved to be efficient and recyclable homogeneous catalysts towards the organic solvent-free synthesis of cyclic carbonates from epoxides and CO₂ in the absence of a co-catalyst, in which >90% yield of cyclic carbonate could be obtained under relatively mild conditions. The catalysts can be easily recovered and reused five times without significant loss of activity and selectivity. Furthermore, the Lewis acid-base cooperative activation mechanism by the bifunctional Al(salen) complexes was proposed according to experimental data.

Full text of DOI:10.1039/c5ra24596f

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DOI: 10.1039/C5RA24596F
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Lewis acid-base bifunctional aluminum-salen catalysts: synthesis
of cyclic carbonates from carbon dioxide and epoxides†
Received 00th January 20xx,
Accepted 00th January 20xx
Yanwei Ren, Ou Jiang, Hang Zeng, Qiuping Mao and Huanfeng Jiang*
Two Lewis acid-base bifunctional monometallic aluminum-salen complexes were prepared based on a new type of salen
ligands pending two N-methylhomopiperazine moieties at 3,3’-position. The Al(salen) complexes proved to be efficient
and recyclable homogeneous catalysts towards the organic solvent-free synthesis of cyclic carbonates from epoxides and
CO2 in the absence of a co-catalyst, in which >90% yield of cyclic carbonate could be obtained under relatively mild
conditions. The catalysts can be easily recovered and five times reused without significant loss of activity and selectivity.
Furthermore, the Lewis acid-base cooperative activation mechanisms by the bifunctional Al(salen) complexes was
proposed according to experimental data.
DOI: 10.1039/x0xx00000x
www.rsc.org/
O
O
O
Introduction
bifunctional catalyst
R
+
CO2
R
O
Carbon dioxide (CO2) is one of the greenhouse effect gases but
it has been attracting much attention as a nontoxic,
nonflammable, highly abundant and renewable C1 feedstock
for organic synthesis in recent years.1 One of the most
promising reactions for the easy and economical chemical
fixation of CO2 is the synthesis of cyclic carbonates via the 100%
atom-economical cycloaddition of epoxides with CO2, since
cyclic carbonates products are wildly used as electrolytes in
lithium-ion batteries, fine chemical intermediates, and aprotic
high-boiling polar solvents.2 The driving force for this process is
provided by the release of the ring strain energy in the three-
membered ring of the epoxide to afford the more stable five-
membered cyclic product, even though CO2 is such a kinetically
and thermodynamically stable molecule. The use of catalysts,
expecially metallosalen-based binary or bifunctional
electrophile-nucleophile systems,3 significantly facilitates this
transformation under relatively mild conditions. Among them,
ionic bifunctionality (as shown in Scheme 1a, a Lewis acid
(metal center) and a halide anion X- (nucleophile) are required)
has proven to be highly useful in various cases to create more
powerful catalyst mediators. On the other hand, for the first
time, we recently reported a new type of bifunctional single-
component M-salphen (salphen = N, N’-bis(salicyladehyde-o-
phenylenediamine)) catalysts system (Scheme 1b) that
involves tertiary amine moiety as Lewis base and metal center
as Lewis acid within one structure.4 The catalysis data support
the synergistic effect of the Lewis acidic site and tertiary amine
nucleophile resulting in markedly improved catalytic behaviour
(a)
(b)
R
R
X
N
halide ion
tertiary
amine
O
O
M
M
Lewis acid
Lewis acid
Scheme 1 Cooperative activation of epoxide with bifunctional catalysts.
compared with a system that lacks a Lewis base. More
importantly, this structural design increases the stability of
catalysts compared with the ionic bifunctional systems,
therefore enhances their recycling performance which are also
decisive criteria for possible large-scale application. These
results stimulate us to exploit the more efficient and easy-
recycling bifunctional non-ionic salen catalysts towards the
fixation of CO2 for cyclic carbonates under mild conditions.
The Lewis acid of aluminum has already been successfully
exploited to provide catalyst systems for the cycloaddition of
CO2 and epoxides.3c-e, 5 Of these reports, the μ-oxo-bimetallic
Al(salen) complexes have been developed by North and co-
workers, which provided a highly-active system under room
temperature and atmospheric pressure conditions. However,
in this case the introduction of highly polar ionic groups to
these complexes often leads to the decreased solubility of
catalyst in epoxide substrate; in turn, relatively low catalytic
activity was obtained compared to the binary catalyst
system.5f-i More recently, Lu studied a bifunctional Al(salen)
framework in conjunction with intramolecular quaternary
ammonium salts as co-catalysts that can efficiently catalyze
the regioselective ring opening of three-membered
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou, 510640, China.
E-mail: Jianghf@scut.edu.cn.
† Electronic Supplementary Information (ESI) available: [Copies of relevant NMR,
MS and IR spectra for ligands and complexes]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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heterocyclic compounds (epoxides or N-substituted aziridines) the range of 4000-400 cm−1. Mass spectraVie(wMASrt)iclewOnelirnee
DOI: 10.1039/C5RA24596F
at various temperatures in coupling reactions with CO2, measured on a Bruker HCT-plus mass spectrometer. Gas
affording the corresponding five-membered cyclic products chromatography (GC) spectrometry was recorded on Agilent
with complete configuration retention at the methane 7820A system that was equipped with a 0.25 mm × 30 m DB-
carbon.3c, Ji recently also reported a series of monometallic Al WAX capillary column (or Supelco-β-DEX 225 chiral column)
(salen) complexes prepared by covalent linkage of the and the FID detector.
imidazolium-based ionic liquid moieties containing various
polyether chains with the salen ligands, which presented
Synthetic procedures
Synthesis of salen Ligands H2LS1 and H2LS2. The anhydrous
excellent “CO2 capture” capability in the context of organic
ethanol solution (20 mL) of ethylenediamine (5 mmol, 0.3 g)
carbonates formation.3d Furthermore, to date, the highest
was added dropwise into the solution of 5-bromo-2-hydroxy-3-
activity, with a turnover frequency of up to 36000 h-1, was also
(4-methyl-homopiperazine-1-ylmethyl)-benzaldehyde
(10
realized by a binary ionic catalytic system consisting of an
aluminum-aminotriphenolate complex and quaternary
ammonium salt, and this catalyst system exhibited broad
substrate scope and functional group tolerance in the
formation of cyclic carbonates.5c
mmol) in anhydrous ethanol (20 mL) in a 1:2 molar ratio at
room temperature. The resulting mixture was refluxed for
another 12 h; following this, the solvent was evaporated to
dryness and the salen ligand H2LS1 was obtained as an orange
powder, used without further purification. Yield: 96%. 1H NMR
(CDCl3), δ=8.34(2H), 7.50(2H), 7.34(2H), 3.94(4H), 3.70(4H),
2.79-2.81(8H), 2.69-2.72(8H), 2.40(6H), 1.86(4H). 13C
NMR(CDCl3), δ=164.4, 158.3, 135.1, 131.6, 128.9, 120.2, 110.2,
59.7, 58.2, 57.6, 56.9, 55.9, 54.5, 54.2, 46.9, 27.0, 18.5. MS
(MALDI+) for C30H42Br2N6O2: m/z, calcd (M + H+): 679.5, found:
679.2.
Based on above fact that aluminum represents the stronger
Lewis acidity for the ring-opening of the epoxide5 and our
previous work towards the CO2 fixation over bifunctional
single-component M(salen) catalysts,4 herein, we report two
new bifunctional Al (salen) complexes (Scheme 2, achiral 1a
and chiral 1b) consisting of tertiary amine moiety as Lewis base
and metal center as Lewis acid within one molecule. The
tertiary amine moieties in these two catalysts could not only
increase the solubility of CO2 in reaction liquid phase due to
the large affinity of aliphatic amines towards CO2, but also
provide an inherent nucleophile in the reaction of CO2 with
epoxides under solvent-free and additive-free conditions.
Notably, these catalysts are easily recycled and can be
conveniently reused, which is an important aspect in the
development of practical processes.
The preparation of H2LS2 was similar to that of H2LS1 except
that (R,R)-1,2-diaminocyclohexane was used in place of
1
ethylenediamine. Yellow powder were obtained in 98%. H
NMR (CDCl3), δ=8.20(2H), 7.47(2H), 7.21(2H), 3.67(4H),
3.31(2H), 2.70-2.82(16H), 2.41(6H), 1.72-1.88(8H), 1.46(2H),
1.26(2H). 13C NMR(CDCl3), δ=163.1, 158.4, 135.0, 131.8, 129.0,
119.9, 110.0, 72.5, 57.6, 56.8, 55.6, 54.4, 54.2, 46.8, 32.9, 26.9,
24.1, 18.4. MS (ESI) for C34H48Br2N6O2: m/z, calcd (M + H+):
733.6, found: 733.2.
Synthesis of catalysts 1a and 1b. Under nitrogen protection
o
and constant stirring at 40 C, to a 100 mL round-bottom flask
Experimental
Materials and methods
containing the above-obtained salen ligands H2LS1 ( or H2LS2) (4
mmol), anhydrous toluene (30 mL) was added via a
hypodermic syringe to dissolve the ligand, and then a little
excess of Et2AlCl (1.0 M solution in toluene, 4.1 mL, 4.1 mmol)
was added slowly. The reaction was highly exothermic and
resulted in a yellow solution and a yellow solid. The resulting
yellow mixture was refluxed for an additional 4 h. after
removal of the solvent under vacuum, the mixture was washed
with ether several times and then dried at 40 oC under vacuum
N-methylhomopiperazine, Propylene oxide (PO), 1,2-
epoxybutane, epichlorohydrin, glycidol, allyl glycidyl ether,
methyl glycidyl ether, syrene oxide and cyclohexene oxide
were purchased from TCI. Diethyl aluminum chloride (Et2AlCl,
1.0 M solution in toluene) were obtained by J&K Scientific Ltd.
Other commercially available chemicals were laboratory grade
reagents from local suppliers. All solvents were purified by
standard
procedures.6
5-bromo-2-hydroxy-3-(4-methyl-
to obtain light yellow powders of 1a or 1d. Yield: 90%. For 1a
:
homopiperazine-1-ylmethyl)-benzaldehyde was synthesized
according to our published procedures.7 Mononuclear
M(salphen) catalysts 2a and 2b were synthesized according to
our recent work.4 Mononuclear Al (salen) catalysts 3a and 3b
were synthesized according to previous literature.8
FT-IR (KBr): 3421, 2948, 1643, 1549, 1469, 1446, 1426, 1388,
1309, 1218, 1134, 1046, 877, 763, 690, 530. 1H NMR (CD3OD),
δ= 8.50(2H), 7.69(2H), 7.54(2H), 3.90-4.08(8H), 3.30(4H), 3.20-
3.36(8H), 3.00-3.15(4H), 2.77(6H), 2.11(4H). 13C NMR(CD3OD),
δ=169.1, 163.9, 139.6, 137.0, 131.2, 129.9, 129.3, 126.3, 122.3,
107.9, 57.2, 56.9, 55.2, 54.7, 52.5, 45.7, 25.7. 27Al NMR(CD3OD),
δ=70.0, 14.6, 7.8. MS (MALDI+): 739.9 [M+H]+. Anal. Calcd. for
C30H40AlBr2ClN6O2: C 48.76, H 5.46, N 11.37; found: C 49.18, H
5.48, N 11.22. For 1b: FT-IR (KBr): 3501, 3001, 2934, 1630,
1598, 1478, 1464, 1427, 1389, 1361, 1302, 1269, 1233, 1201,
Elemental analyses for C, H, and N were carried out using a
1
Vario EL III Elemental Analyzer. H NMR and 13C NMR were
done on a Bruker Model AM-400 (400 MHz) spectrometer. 27Al
NMR spectra were recorded on a Bruker Avance III HD 600M
spectrometer. Infrared (IR) spectra were measured from a KBr
pellets on a Nicolet Model Nexus 470 FT-IR spectrometer in
2 | J. Name., 2012, 00, 1-3
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1
O
ethylenediamine or
1171, 1089, 1031, 974, 936, 866, 798, 769, 704, 558, 446. H
NMR (CD3OD), δ= 8.43(2H), 7.69(2H), 7.20(2H), 3.99(4H),
3.48(2H), 3.25-3.38(8H), 2.80-2.95(8H), 2.31(6H), 1.98-
2.17(8H), 1.50(4H). 13C NMR(CD3OD), δ=165.7, 163.6, 130.2,
129.3, 126.4, 122.2, 108.1, 65.3, 58.0, 57.0, 56.7, 55.2, 52.2,
45.6, 28.4, 25.1, 21.6. 27Al NMR(CD3OD), δ= 70.0, 13.7, 8.8. MS
(MALDI+): 794.3[M+H]+. Anal. Calcd. for C34H48Br2N6O2: C 51.36,
H 6.08, N 10.57; found: C 50.75, H 5.69, N 10.82.
O
View Article Online
N-methyl homopiperazine
paraformaldehyde
DO(IR:,1R0)-1.1,20-d3ia9mi/nCoc5ycRloAhe2xa4n5e 96F
Br
OH
Br
OH
EtOH, reflux, 12h
EtOH, reflux, 40h
N
N
R
R
N
N
N
O
N
O
Al
AlEt2Cl
toluene, 80 oC, 2 h
Br
OH
HO
Br
Br
Br
Cl
N
N
N
N
Representative procedure for the reaction of CO2 with
epoxides. A 10-mL stainless steel reactor was added with
catalyst (0.1 mmol) and a magnetic stirring bar. Then the
epoxide (10.0 mmol) was added and the steel reactor was
charged under a constant pressure of CO2 (2 MPa) for 1 min
and heated to the desired temperature. After reaction
N
N
N
N
H2LS1: R=1,2-ethanediyl
H2LS2: R=(R, R)-1,2-cyclohexanediyl
1a: R=1,2-ethanediyl
1b: R=(R, R)-1,2-cyclohexanediyl
Scheme 2 Synthesis route of catalysts 1a and 1b.
o
completion, the reactor was cooled down to 0 C and the
R
excess CO2 was released carefully. The reaction mixture was
collected by adding 5 mL of ether and a sample was taken for
GC analysis (using 1,3,5-trimethylbenzene as an internal
standard) to determine yield and selectivity.
N
O
N
O
N
O
N
O
Al
Cl
Μ
Br
Br
Br
Br
tBu
tBu
Ν
Ν
3a: R=1,2-ethanediyl
3b: R=(R, R)-1,2-cyclohexanediyl
For recycling studies of catalyst 1a (or 1b), propylene
carbonate was separated from the catalyst by adding ether.
The catalyst precipitates was recovered by centrifugation.
2a: M=Zn
2b: M=Cu
Ν
Ν
N
N
o
Afterwards, the catalyst was dried at 50 C for 6 h in vacuum.
N
N
By adding the same amount of epoxide (10.0 mmol), the next
catalytic run was started.
N
Cl
N
Cl
Al
O
O
N
O
N
O
Cl
Al
Cl
tBu
tBu
tBu
tBu
4
Results and discussion
Preparation of catalysts
N
O
N
O
Al
Cl
Br
N
Br
N
N
N
Cl
Cl
6
The synthetic route for the Al(salen) catalysts 1a and 1b is
illustrated in Scheme 2. The first key step in the preparation
was accomplished by an aromatic Mannich reaction,
introducing a tertiary amine group (N-methylhomopiperazine)
into 3-position of 5-bromosalicyladehyde, which allowed the
subsequent aldehydic-amide (2:1) condensation with
ethylenediamine or (R,R)-1,2-diaminocyclohexane to form
salen ligands. Treatment of the salen ligand with diethyl
aluminum chloride (1.0 M solution in toluene) under nitrogen
gave light yellow solid powders of 1a or 1d. All attempts to
obtain a single crystal of 1a (or 1d) suitable for X-ray
crystallography were unsuccessful, partly due to the
coordination between Al3+ and salen ligands is rapid and the
inert coordination bonds between Al3+ cation and ligand anion
make ligand-exchange reactions slow, which is unfavorable for
crystal growth.9 These two catalysts are miscible in some
organic solvents, for example, methanol, ethanol and DMF,
but can be precipitated with other organic solvents, for
example, n-hexane, ether and chloroform. It is suggested that
1a or 1d should be an easily recoverable catalyst for the
cycloaddition reaction by simple phase separation techniques
via changing the solvents.
N
N
R
R
5a: R=Me
5b: R= Polyethylene glycol monomethyl ether
Chart 1 The structures of other M(salen) catalysts.
Catalytic performances
Firstly, the activity of various homogeneous monometallic
catalysts were tested at 100 C and 2 MPa of CO2 pressure
o
using the cycloaddition reaction of propylene oxide (PO) with
CO2 with 1 mol% catalyst loading, because our recently
reported bifunctional single-component Zn(salphen) complex
(Chart 1, 2a) presented good yield for this reaction under
above conditions (Table 1, entry 1).4 The yield and selectivity
were determined by using GC and 1H NMR, and in all examined
cases, the selectivity for five-membered cyclic propylene
carbonate (PC) was higher than 99%. We delightfully found
that PC could be obtained in excellent yield in the presence of
Al(salen) catalysts 1a and 1b (Table 1, entries 2 and 3), as
expected that aluminum has stronger Lewis acidity for the
ring-opening of the epoxide. However, so far we have not
achieved ee value based on chiral GC analysis (Supelco-β-DEX
225 chiral column) when chiral catalyst 1b was used for the
synthesis of optically active cyclic carbonates. This is in
accordance with that the high enantioselectivity is very hard to
achieve at high temperatures in asymmetric catalysis.
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Table 1 Results of the cycloaddition reaction of PO with CO2 in the presence
View Article Online
DOI: 10.1039/C5RA24596F
of various catalysts.a
100
80
60
40
20
0
Entry
1
Catalyst
2a
Co-catalyst
none
Yieldb/%
90
2
1a
none
96
3
1b
none
95
4
3a
none
<5
5
3b
none
<5
6
7
8
9
none
3a
3b
N-methylhomopiperazine
N-methylhomopiperazine
N-methylhomopiperazine
Et3N
<5
85
85
86
3a
0
1
2
3
4
5
6
10
3a
NBu4Br
85
Reaction time (h)
aGeneral reaction conditions: 10 mL stainless-steel autoclave, PO (10 mmol),
catalyst (0.1 mmol), CO2 pressure (2 MPa). bThe yields were determined by GC.
Fig. 1 The influence of the reaction time on the PC yield. Reaction conditions:
10 mmol of PO with 1 mol% 1a, 2 MPa of CO2, 100 oC.
For a better understanding of the role of the tertiary amine
moieties in N-methylhomopiperazine, the control complexes
3a and 3b without N-methylhomopiperazine were prepared
100
80
60
40
20
0
for comparison (Chart 1). If only 3a
,
3b or N-
methylhomopiperazine was used as catalyst alone, no reaction
occurred (Table 1, entries 4-6), illustrating that the coexistence
of organic base and Al(salen) is essential to promote this
reaction. On the contrary, the binary catalyst system of 3a (or
3b) and N-methylhomopiperazine in a molar ratio of 1/2 gave
PC in 85% yields under the same conditions (Table 1, entries 7
and 8), obviously lower than that of 1a (or 1b). These
phenomena were also observed in M(salphen) (Chart 1, 2a and
2b) catalytic systems4 and reported by Liu and Darensbourg in
an aluminum-salen complex bearing appended quaternary
20
40
60
80
100
120
Reaction temperature (oC)
Fig. 2 Effect of the reaction temperature on the PC yield. Reaction conditions:
10 mmol of PO with 1 mol% 1a, 2 MPa of CO2, 4 h.
ammonium salt substituent (Chart 1, 4
).3e In addition, in the
presence of 2 mol% of other usual nucleophiles such as Et3N
and NBu4Br, the activity of the formed binary catalytic systems
for this reaction still remained (Table 1, entries 9 and 10), but
that it is very difficult to achieve high enantioselectivity in this
reaction because it requires a higher reaction temperature
o
was less than those of 1a and 1b. These results unambiguously (>80 C). Furthermore, the effect of CO2 pressure on the PC
yield was also investigated. When the CO2 pressure decreased
from 2 to 1 MPa, the yield of PC drastically decreased, and
account for that the Lewis base and Lewis acid sites in 1a (or
1b) work together to enhance the catalytic activity.
The influence of reaction time on the PC yield is shown in Fig. even increasing this reaction time to 12 h, only about 40% of
o
PC was obtained. The reaction could basically not occur under
room temperature and atmospheric pressure even for 72 h. It
is known that lower CO2 pressure could reduce the
1. The reaction was conducted at 100 C and 2 MPa of CO2
pressure. The yield of PC increased with increasing time at the
beginning and approached about 100% after a reaction time of
6 h. Fig. 2 shows the effect of the temperature on the solubilization effect of CO2 in the solution of PO; in turn, the
yield was decreased.10
cycloaddition reaction catalyzed by 1a at 2 MPa of CO2
pressure in the temperature range of 25-120 oC, with a
Table 2 summarizes the catalytic performances of recent
o
reaction time of 4 h. At a temperature higher than 100 C, reported one-component Al(salen) catalysts on cycloaddition
reaction of CO2 to PO. In general, since different reaction
conditions such as catalysts loading, CO2 pressure, reaction
temperature and time were used, it is hard to critically
nearly all of the PO could be converted into PC. The yield of PC
decreased obviously with decreasing the temperature below
o
100 C. The figure illustrates that the yield was only 67% at 80
oC with a reaction time of 4 h, indicating that temperature has evaluate and compare the efficacy of these mononuclear
great influence on this reaction, especially under 100 oC,
Al(salen) complexes. Nevertheless, the aluminum-salen
catalysts reported upon herein are competitive with most
lower temperature, using chiral 1b as the catalyst, we also catalysts currently available, including transition and main
although aluminium ion exhibits stronger Lewis acidity. Under
group metals as well as organo-catalysts. The extensive
summary of catalysts and their activities for this process were
reported in recent reviews.2a,11
investigated the asymmetric catalytic performance of the
reaction of racemic PO with CO2. Unfortunately, only about 5%
ee of PC at 60 oC (1-2% ee at 80 oC) was achieved. We believe
4 | J. Name., 2012, 00, 1-3
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Table 3 Reactions of other epoxides with CO2 catalyzed by 1a and 1b.a
Table 2 Comparison with other one-component Al(salen) catalysts on
View Article Online
cycloaddition reaction of CO2 with PO.a
DOI: 10.1039/C5RA24596F
Entry
1
Epoxide
Catalyst
Time/h
4
Yieldb/%
95
O
1a
Cat. [PO]/[Cat.] CO2 Pres./MPa Temp./°C Time/h Yield/% Ref.
1a
4
100
2000
200
200
25000
2.0
3.0
1
1
2.5
100
120
100
100
120
4
5
2.5
2.5
4
96.0
74.3
49.0
98.0
80.0
This work
2
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
1a
1b
4
94
99
99
99
98
92
95
94
95
90
92
45
42
3e
3d
3d
3c
5a
5b
6
O
3
1
Cl
4
1
a The structures of catalysts 4, 5a,5b and 6 are depicted in Chart 1.
O
5
1
Having established that 1a and 1b are highly active
catalysts for PC formation, various epoxides, such as 1,2-
epoxybutane, epichlorohydrin, glycidol, allyl glycidyl ether,
methyl glycidyl ether, syrene oxide and cyclohexene oxide
were used as the substrate for the reaction system. As shown
in Table 3, the two Al(salen) catalysts can tolerate
functionalities such as alkene, alkyl halide, alcohol, ether, and
substituted aryl groups. Both electron-withdrawing and
electron-donating terminal epoxides can be converted to the
corresponding cyclic carbonates with high conversion and
excellent selectivity. The very good conversions (for only 1h) of
epichlorohydrin and glycidol (Table 3, entries 3-6) can be
explained by the electron-withdrawing substituents.12 These
substituents result in facilitated nucleophilic attack of the N-
methylhomopiperazine during the ring opening of the epoxide.
Unfortunately, the internal epoxide, cyclohexene oxide, which
is known to undergo cycloaddition with CO2 poorly, is
converted to the corresponding cyclic carbonate with a yield of
45 and 42 % by 1a and 1b (Table 3, entries 11 and 12) even
after prolonging the reaction time to 12 h, respectively,
presumably due to the high steric hindrance.12a
HO
6
1
O
7
4
O
8
4
O
9
4
O
10
11
12
13
14
4
O
4
Ph
4
12
12
O
aReaction conditions: epoxide (10 mmol), catalyst (0.1 mmol), 2 MPa of
CO2, 100 °C. bThe yields were determined by GC.
100
80
60
40
20
0
Recycling capability
Besides the catalyst activity, the recycling capability is also
decisive criteria for possible large-scale application. Based on
the concept of “one-phase catalysis and two-phase
separation”13 and the special solubility of 1a and 1b, the
catalyst could be precipitated from the reaction solution by
the addition of ether. The upper organic phase was obtained
by simple centrifugation and the lower catalyst solid can be
reused by adding fresh reaction substrates. Experiments were
1
2
3
4
5
Run times
Fig. 3 Recyclability of 1a for the reaction of PO with CO2. Reaction conditions:
epoxide (10 mmol), catalyst (0.1 mmol), 2 MPa of CO2, 100 °C, 4 h.
The comparative FT-IR analysis of the fresh catalyst 1a with the
recovered 1a after the 5th reuse in the cycloaddition reaction
was performed, and the results are shown in Fig. 4 (Fig. S16 for
1b, Supporting Information). All the FT-IR spectra show
characteristic vibration bands at around 1640 cm-1 and 1550
cm-1, which are associated with the stretching vibration modes
of C=N and C–O, respectively.3d, 14 The stretching vibration of
tertiary amine groups at 1050 cm-1 suggests the intact
methylhomopiperazine-based moieties on the ligands.7
Moreover, 27Al NMR spectra of 1a and the recovered 1a have
all been found to display broad strong resonances around 70
ppm (see Fig. 5, Fig. S14 for 1b, Supporting Information), which
are attributed to the five-coordinate aluminum species. These
results indicate that the recovered catalysts structure did not
change after catalytic reaction.
conducted to examine the recyclability of catalyst 1a (or 1b
)
using PO as the substrate under the optimal reaction
conditions. The results indicate that 1a could be reused for five
successive runs without any significant loss in catalytic activity,
and the selectivity still remained at 98% (see Fig. 3, Fig. S15 for
1b, Supporting Information). In addition, these catalysts
exhibited stability to moisture. For example, when 1a was
immersed in distilled water overnight at ambient temperature,
following drying, it maintained catalytic activity with a yield of
95% for the production of PC under the optimal reaction
conditions. In this regard, it should also be noted that no
special precautions were taken to purify either PO or CO2.
These considerations are of utmost importance when utilizing
impure sources of CO2 as that obtained from power plant flue
gases or other CO2 producers.
This journal is © The Royal Society of Chemistry 20xx
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ARTICLE
Journal Name
View Article Online
DOI: 10.1039/C5RA24596F
N
R
O
O
R
(a)
(b)
Al
Cl
N
(III)
R
N
N
N
O
O
C
Al
Cl
Al
Cl
Al
Cl
O
1050
(I)
N
(II)
N
1550
1500
N
1640
(IV)
O
R
O
2000
1000
500
O
-1
O
O
Wavenumbers (cm )
N
O
R
Al
Cl
Fig. 4 FT-IR spectra of 1a (a) and the recovered 1a (b) after the 5th reuse in
the reaction of PO with CO2.
(V)
N
Scheme 3 A plausible mechanism for the reaction of epoxide with CO2
catalyzed by 1a or 1b.
5.00E+008
70.0
4.00E+008
coordinate complexes and exhibited enhanced catalytic
activity for the reaction of epoxides and CO2.
3.00E+008
Scheme 3 outlines the proposed reaction pathway. Firstly,
2.00E+008
7.8
14.6
the tertiary amine
N
atom of the pending N-
(a)
1.00E+008
(b)
methylhomopiperazine coordinates to the metal in the
remaining trans axial position, thus generating a six-
coordinated intermediate (II). Secondly, the reaction is
initiated through the subtitution coordination by the oxygen
atom of the epoxide and meanwhile, favoring the tertiary
amine nucleophilic attack of the less-hinder carbon atom of
epoxide (III) activated by the metal center. Next, the formed
alkoxide species (IV) acts in turn as a nucleophile that attacks
0.00E+000
(c)
-1.00E+008
120
100
80
60
40
20
0
-20
-40
ppm
Fig. 5 27Al NMR spectra of 1a (a), the recovered 1a (b) and 3a (c).
Reaction mechanism
CO2 to form a metal carbonate species (V). The subsequent
At first, N-methylhomopiperazine moieties in 1a (or 1b) adopt
a ship-shape conformation locating above and below the salen
plane, similar to our previous reported crystal structure of
Cu(salphen) (chart 1, 2b),4 which can also be proved by the 27Al
NMR spectrum of 1a. Because 27Al NMR is sensitive to the
symmetry, chemical environment, and coordination number
around the metal center, this spectroscopic probe should be
useful in assessing differences in coordination types of
aluminum complexes.15 As shown in Fig. 4, 27Al NMR spectra of
1a was found to display broad strong resonances at 70.0 ppm
and two weak signal around 14.6 and 7.8 ppm, which are
assigned to the five-coordinate aluminum species and two
different six-coordinate species (one perhaps is N-
methylhomopiperazine coordinated species, and another is
solvent-coordinated species), respectively, since an upfield
chemical shift is attributed to an increase in aluminum’s
coordination number.16 While in the spectrum of neat complex
3a that lacks the pending N-methylhomopiperazine, only one
weak signal is in the range 0- 15 ppm, which is assigned to the
solvent-coordinated aluminum species. In recent studies, Liu
and Ji also observed six-coordinate Al(salen) complexes
tethered onium salt groups in one structure by 27Al NMR
spectra.3d, 3e These species existed in equilibrium with the five-
ring-closure forms cyclic carbonate, and in the meantime, the
catalyst is regenerated. In one word, the Lewis base and Lewis
acid work together to open the epoxy ring and then react with
CO2 to give the corresponding cyclic carbonates via a ring-
opening and recycling process in one-component catalyst.
Previous reports also suggested the similar requirement of
both Lewis base activation of CO2 and Lewis acid activation of
epoxide in the binary catalyst systems.3n It should be also
noted that the “CO2 sorption” capability originating from
tertiary amine moieties in our catalysts could not be ignored,
because the larger affinity of aliphatic amines towards CO2 has
been confirmed in reported literatures.17
Conclusions
In summary, two Lewis acid-based bifunctional catalysts
pending N-methylhomopiperazine moieties have been
prepared and examined as one-component catalysts for the
cycloaddition of epoxides and CO2 to provide cyclic carbonates.
These two catalysts were significantly more efficient than their
binary analogues under relatively mild conditions.
Furthermore, the catalysts are thermally and moisture stable,
and can be facilely separated and recycled five times without
significant loss in catalytic activity. Combined with other
6 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
M. North, P. Villuendas and C. Young, TetraVhieewdArortincleLOentlitn.e,
advantages of Al(salen) complexes (low cost, environmental
benign and ease of synthesis), these catalysts thus show high
potential in the field of CO2 fixation.
2012, 53, 2736-2740; (g) J. Melendez, M. North, P. Villuendas
DOI: 10.1039/C5RA24596F
and C. Young, Dalton Trans., 2011, 40, 3885-3902; (h) M.
North and R. Pasquale, Angew. Chem., Int. Ed., 2009, 48
,
2946-2948; (i) J. Melendez, M. North and P. Villuendas, Chem.
Commun., 2009, 2577-2579.
W. L. E. Armarego and C. L. L. Chai, Purification of Laboratory
Chemicals, Elsevier Science, USA, 5thedn, 2003.
6
7
8
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (No. 21372087, 21490572 and 21420102003), and
Postdoctoral Science Foundation of China (2015M582379) for
the financial support.
Y.-W. Ren, J.-X. Lu, B.-W. Cai, D.-B Shi, H-F.Jiang, J. Chen, D.
Zheng and B. Liu. Dalton Trans., 2011, 40, 1372-1381.
(a) K. Y. Hwang, H. Kim, Y. S. Lee, M. H. Lee and Y. Do, Chem.
Eur. J., 2009, 15, 6478-6487; (b) A. G. Doyle and E. N.
Jacobsen, Angew. Chem., Int. Ed., 2007, 46, 3701-3705.
Z.-R. Jiang, H.-W. Wang, Y.-L. Hu, J.-L Lu, and H.-L. Jiang.
9
ChemSusChem, 2015, 8, 878-885.
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DOI: 10.1039/C5RA24596F
R
N
O
N
O
Al
Br
Br
Cl
R=1,2-ethanediyl
N
N
(R,R)-1,2-cyclohexanediyl
N
N
Lewis acid-base
bifunctional catalyst
O
O
O
R
CO2
+
R
O
100 °C, 2 MPa CO2
Two Lewis acid-base bifunctional Al(salen) complexes proved to be efficient and recyclable
one-component catalysts for CO2 fixation.