Highly efficient synthesis of cyclic carbonates from epoxides catalyzed by salen aluminum complexes with built-in "cO2 capture" capability under mild conditions

Article abstract of DOI:10.1039/c3gc42388c

A series of monometallic salen aluminum complexes were prepared by covalent linkage of the imidazolium-based ionic liquid moieties containing various polyether chains with the salen ligand at the two sides of the 5,5′-position. The salen aluminum 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. The catalysts presented excellent "CO₂ capture" capability due to the molecules containing polyether chains and the metal aluminum center, in which >90% yield of cyclic carbonate could be obtained under mild conditions. The catalysts can be easily recovered and six times reused without significant loss of activity and selectivity. Moreover, based on experimental and previous work, the "CO₂ capture and activation" cycloaddition reaction mechanisms by monometallic or bimetallic salen aluminum complexes were both proposed.

Full text of DOI:10.1039/c3gc42388c

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PAPER
Highly efficient synthesis of cyclic carbonates
from epoxides catalyzed by salen aluminum
complexes with built-in “CO2 capture” capability
under mild conditions
Cite this: Green Chem., 2014, 16,
1496
Rongchang Luo, Xiantai Zhou, Shaoyun Chen, Yang Li, Lei Zhou and Hongbing Ji*
A series of monometallic salen aluminum complexes were prepared by covalent linkage of the imidazo-
lium-based ionic liquid moieties containing various polyether chains with the salen ligand at the two sides
of the 5,5’-position. The salen aluminum 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. The catalysts presented excellent “CO2 capture” capability due to the molecules
containing polyether chains and the metal aluminum center, in which >90% yield of cyclic carbonate
could be obtained under mild conditions. The catalysts can be easily recovered and six times reused
without significant loss of activity and selectivity. Moreover, based on experimental and previous work, the
“CO2 capture and activation” cycloaddition reaction mechanisms by monometallic or bimetallic salen
aluminum complexes were both proposed.
Received 21st November 2013,
Accepted 5th December 2013
DOI: 10.1039/c3gc42388c
www.rsc.org/greenchem
electronic properties conveniently by changing metal centers.9b
Many different binary salen catalysts for the coupling reaction
Introduction
Carbon dioxide is one of the greenhouse effect gases but it has of CO2 and epoxides have been reported, in which a co-catalyst
been attracting much attention as an inexpensive, nontoxic, is required for the ring opening of the epoxide. With respect to
nonflammable, bio-renewable and highly abundant single the binary salen catalyst systems, multi-functional metallo-
carbon atom (C1 resource) building block for organic synthesis salen complexes presented higher catalytic activity and selecti-
in recent years.1 Many procedures have been developed vity due to the two or three catalytic sites in one catalyst
towards the easy and economical chemical fixation of CO2.2 molecule. It is not only unnecessary to use a co-catalyst, but
Among these, the synthesis of five-membered cyclic carbonates also easy to explore the mechanism and reuse the catalyst.11 In
via the 100% atom-economical cycloaddition of epoxides with general, on the basis of the motif towards activating epoxide
CO2 is one of the most promising ways because cyclic carbon- and CO2, a Lewis acid (metal center) and an anion X− (nucleo-
ate products are widely used as aprotic high-boiling polar sol- phile) are required to build this kind of catalyst. Fig. 1 shows a
vents, electrolytes for lithium-ion batteries, precursors of catalytic motif for the double activation of an epoxide with this
polymeric materials, and fine chemical intermediates.3 There- type of compound.
fore, various catalytic systems, including alkali metal halides,4
Aluminum, as a nontoxic, readily available, environmental
quaternary ammonium5 or quaternary phosphonium salts,6 benign and earth-abundant metal, is a better choice, because
ionic liquids,7 metalloporphyrins8 or metallosalen complexes,9
have been developed to promote this transformation so far.
Based on our previous work towards the biomimetic dioxy-
gen activation10 and CO2 fixation8a over metalloporphyrin cata-
lysts, metal salen complexes as another kind of enzyme-like
catalyst have drawn our attention due to their unique nature,
such as easy synthesis procedures, modulating the steric and
School of Chemistry and Chemical Engineering, Key Laboratory of Low-Carbon
Chemistry & Energy Conservation of Guangdong Province, Sun Yat-sen University,
Guangzhou 510275, P.R. China. E-mail: jihb@mail.sysu.edu.cn;
Fax: +86 20 84113654; Tel: +86 20 84113658
Fig. 1 Cooperative activation of epoxide with bi-functional catalyst.
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the stronger Lewis acidity is important for the ring-opening of catalysts, can be well resolved. Notably, these catalysts are
the epoxide.12 Most recently, a series of mononuclear bi-func- easily recycled and can be conveniently reused, which is an
tional salen aluminum complexes containing quaternary important aspect in the development of practical coupling pro-
ammonium salts13 or quaternary phosphonium salts14 teth- cesses. In addition, it was found that the total length of the
ered into the frame of the salen ligand in the context of polyether chain in the range of low molecular weights has a
organic cyclic carbonate formation under mild conditions certain influence on the catalytic performances of the novel
have been reported. Even the μ-oxo-bimetallic salen alu- complexes.
minium complexes and their bi-functionalized compounds
have been reported by North and co-workers, which provided a
highly-active system under room temperature and atmospheric
Results and discussion
pressure conditions.9d–g However, in this case the introduction
of highly polar ionic groups to these complexes often leads to Recently, chiral salen manganese complexes functionalized by
the decreased solubility of catalyst in epoxide substrate; in polyether chain-modified imidazolium IL were first reported
turn, relatively low catalytic activity was obtained compared to
and acted as inherent phase-transfer catalysts in the enantio-
the binary catalyst system. Meanwhile, distillation was necess- selective epoxidation of unfunctionalized olefins with aqueous
ary to recover the catalyst in each reaction cycle. Therefore, NaOCl as an oxidant.23 However, the long polyether chain pre-
towards the fixation of CO2 cyclic carbonates under mild con-
pared by auto-polymerization of explosive ethylene oxide pos-
ditions, the efficient and easy-recycling multifunctional salen sesses a broad molecular weight distribution and the hydroxyl
catalyst is still desired.15
at the end of the chain increased the uncertainty of the catalyst
spatial structure. Therefore, polyether-based IL functionalized
Polyethylene glycol (PEG) or polyethylene glycol mono-
methyl ether (mPEG), as an inexpensive, non-volatile and salen aluminum complexes were synthesized by a similar
environmentally benign reagent, could be regarded as a CO2- method using commercial mPEG with known molecular
philic material through interaction of CO2 with the oxygen
weight as the raw material. Meanwhile, the salen ligand was
atoms of the ether linkages of PEG or mPEG. More impor- produced as the cheaper and readily-available salicyclaldehyde
tantly, the “CO2-expansion” effect could lead to changes in the and ethylenediamine, which is beneficial for industrial
physical properties of the liquid phase mixture including
lowered viscosity and increased gas–liquid diffusion rates.16
Addition of PEG or mPEG into the IL could enhance the rates
of absorption of CO2 significantly by decreasing the viscosity
of the absorption system.17 PEG-functionalized basic ionic
liquids (ILs) have been proved to be highly efficient and stable
catalysts for the cycloaddition reaction of CO2 to epoxides.18,19
The “CO2-expansion” effect of PEG or mPEG and the excel-
lent reusability of the IL-functionalized salen complex20 encou-
rage us to prepare imidazolium IL containing polyethylene
oxide (PEO) chain grafted salen aluminum complexes and
catalyze the cycloaddition reaction of CO2 with epoxide.21 We
envisage that the cooperative action of an anion X− (nucleo-
phile) and a metal center M (Lewis acid) of a catalyst promotes
ring-opening of the epoxide, and polyether-based imidazolium
IL units possess a built-in “CO2 capture” capability.22 The
introduction of polyether chains can effectively solve CO2 gas
mass transfer in the liquid phase reaction system. Further-
more, the solubility properties of the PEG or mPEG with low
molecular weight, that is, it is soluble in epoxide or product,
but can be precipitated with ether, potentially endow the novel
complexes with the feature of solvent-regulated separation.
Herein, novel mononuclear salen aluminum complexes
(denoted as PISA) were synthesized by covalent linkage of the
imidazolium-IL moieties containing various PEO chains with
the salen ligand at the two sides of 5,5′-position. It has been
proved that these catalysts could embody an inherent “CO2
capture” capability in the coupling reaction of CO2 to epoxide
under solvent-free and additive-free conditions. Thus, the
enhanced solubility of the catalysts and the problem of gas–
production.
Preparation of PISA
The synthesis route for the PISA is outlined in Scheme 1. At
first, N-(polyoxyethylene methyl ether) imidazole
different numbers of polymerized ethylene oxide units (n = 7,
11, 16), was provided by combination between sodium
B with
liquid phase mass transfer, even the separation of the Scheme 1 Synthesis of the catalysts of PISA.
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imidazole and chlorine-substituted mPEG A, and then B directly
reacted with salicyclaldehyde to afford mPEG-based IL-substi-
tuted salicyclaldehyde C. The successive condensation between
the aldehyde (–CHO) group of the compound C and the amino
(–NH2) groups of ethylenediamine was used to form the salen
ligand PISL. Treatment of the PISL with diethyl aluminum
chloride (0.9 M solution in toluene) under nitrogen gave the dia-
nionic complex PISA. The newly-synthesized catalysts are light
yellow solid powders at room temperature. Moreover, it is found
that the PISA is miscible in some organic solvents, e.g., ethanol,
water and DMF, but can be precipitated with other organic sol-
vents, e.g., n-hexane, ether and ester. It is suggested that PISA
should be an easily recoverable catalyst for the cycloaddition
reaction by simple phase separation techniques via changing
the solvents.
Chart 1 The structures of IL, SA, ISA1-4, PISZ-350 and PISC-350.
Obviously, no reaction occurred when SA was used as the
catalyst even after 24 h (Table 1, entry 1).24 In addition,
0.5 mol% of the conventional IL could catalyze the cyclo-
addition reaction, but the yield of allyl glycidyl carbonate
(AGC) was very low (Table 1, entry 2) under 1.0 MPa CO2
pressure at 100 °C for 2.5 h under solvent-free conditions.25
Catalytic performances
Firstly, the activity of various homogeneous monometallic However, together with the equivalent mole ratio it represents
catalysts such as PISA-350 in the cycloaddition reaction of an efficient binary catalyst system for the production of AGC
epoxides with CO2 was explored using allyl glycidyl ether (AGE) (Table 1, entry 3). This observation encouraged us to evaluate
as the model substrate under mild conditions in a semi-batch the activity of various single component bi-functional catalysts
operation (CO2 was continuously supplied to the reactor) and in detail. As expected, catalyst ISA-1 presented moderate yield
the results are shown in Table 1. To investigate the built-in in the absence of a co-catalyst in the coupling reaction of CO2
stronger “CO2 capture” capability originating from the PEO to AGE (Table 1, entry 5). But it was less active than the catalyst
moiety, the traditional IL (1-benzyl-3-methylimidazoliumchlo- PISA-350 with 96% yield of AGC under the above conditions
ride, denoted as IL), the neat complex ([N,N′-bis(salicylidene)- (Table 1, entry 9). It is worth noting that both of the catalysts
ethylene diaminato] aluminum chloride, denoted as SA) and presented excellent selectivity (>98%) towards cyclic carbon-
the simple IL-functionalized salen aluminum complex ates. There is no other product such as polycarbonate as con-
(denoted as ISA1–4) were also prepared for comparison firmed by FT-IR, 1H NMR, 13C NMR and GC-MS. The
(Chart 1).
remarkable enhancement of reaction rates using catalyst
Table 1 Results of the cycloaddition reaction of AGE with CO2 catalyzed by various catalystsa
Entry
Catalyst
T/°C
p(CO2) /Mpa
Time/h
Conv.b/%
Yieldb/%
TOFc/h−1
1
2
3
4
5
6
7
8
SA
IL
100
100
100
100
100
100
100
100
100
100
100
80
1
1
1
1
1
1
1
1
1
1
1
1
24
n.d.
16
84
41
57
75
11
<3
96
12
78
41
10
76
43
30
94
5
n.d.
15
82
40
56
74
11
2
95
10
76
40
9
—
12
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
4
SA/IL (1 : 1)
PISL-350
ISA-1
ISA-2
ISA-3
65.6
32
44.8
59.2
8.8
1.6
76
8
60.8
20
4.5
60
33.6
4.7
15.7
<0.1
ISA-4
9
PISA-350
PISZ-350
PISC-350
PISA-350
PISA-350
PISA-350
PISA-350
ISA-1
10
11
12
13
14
15
16
17
18
60
1
4
100
100
100
100
30
0.5
0.1
0.1
0.1
0.1
2.5
2.5
12
12
72
75
42
28
92
<3
PISA-350
PISA-350
a Reaction conditions: 10 mL stainless-steel autoclave, AGE (6 mmol), catalyst (0.03 mmol). b Determined by GC using biphenyl as the internal
standard. c Turnover frequency (TOF): mole of synthesized AGC per mole of catalyst per hour.
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which is attributed to the weak interaction between the CO2
molecule and the PEO chain. Moreover, bulky ILs, having
longer distance between the cation and the anion, may be con-
sidered to have higher anion activation ability. Therefore, they
are more effective in nucleophilic attack of the anion (X−) to
the epoxide ring of AGE due to the weak electrostatic inter-
action. It follows that the total length of the polyether chain in
the range of low molecular weights directly affects the catalytic
activity of catalysts in the coupling reactions.
Fig. 2 Possible forms of “CO2 capture”.
Fig. 3 also shows the effect of reaction time on the catalytic
performance of 0.5 mol% of the catalyst PISA with different
lengths of polyether chain in the cycloaddition reaction of AGE
with CO2 at 1.0 MPa CO2 pressure and 100 °C within 2.5 h. It
clearly suggests that differences of reaction rates between
various catalysts were observed. At relatively low pressure, the
difference in solubility of CO2 in PEG with different low mole-
cular weights is negligible.19 Thus, when the numbers of ethyl-
ene oxide units of the polyether chain increased from 3 to 11,
appropriate increased yields of AGC were obtained, which indi-
cated that the length of polyether chain within the catalyst has
a certain effect on the coupling reaction. The PISA-550 with
eleven EO units presented the highest catalytic activity.
However, when the EO numbers further increased to 16, a
slightly decreased yield of AGC was observed. The subtle drop
probably derives from the increased mass-transport
limitation.18a
In consideration of a catalytic motif for the double acti-
vation of an epoxide in the coupling reaction system, it is
found that the metal center, the imidazolium cation and
halogen ions (X−) were all important to obtain the high
activity, which were indispensable. For instance, when directly
using the IL-functionalized ligand PISL-350 as a catalyst, it
shows moderate activity (Table 1, entry 4) due to the lack of a
metal center. Hence, the catalytic activity of a catalyst is closely
related with the metal center; a substantial number of studies
reported that catalysts containing a zinc ion or a cobalt ion
were the most attractive for CO2 coupling with epoxide owing
to their high activity. However, the catalyst PISZ-350 bearing a
zinc cation has very low activity in cycloaddition reactions
(Table 1, entry 10), which could be because Zn2+ owns weaker
Lewis acidity than Al3+.8b Additionally, using cobalt instead of
aluminium as a metal center, the catalyst PISC-350 exhibited
higher activity with respect to the catalyst PISZ-350 under iden-
tical conditions (Table 1, entry 11). Finally, it was noteworthy
that the activities of ISA-3 with quaternary ammonium-based
ILs and ISA-4 with pyridinium-based ILs were both obviously
inferior to that of ISA-1 (see Fig. 3), which shows that the incor-
poration of the imidazolium group within the salen ligand is
beneficial for catalytic performance. It shows that both the
metal center and the imidazolium cation are critical for the
rate-determined step in the coupling reaction, which could
activate and ring-open epoxides.
PISA-350 under mild conditions could be attributed to the
“CO2-expansion” effect of PEO chains.19,26 Since CO2 is an
electron acceptor and the oxyethylene (EO) group is an electron
donor, the Lewis acid–base interaction between CO2 and EO
enhances the dissolution of CO2 in the epoxide substrates (see
Fig. 2). The weak interaction between electron-donating func-
tional groups and CO2 was detected by the in situ FT-IR tech-
nique reported by Kazarian and co-workers.27 Moreover, a
remarkable difference between ISA-1 and PISA-350 under
1 atm CO2 pressure could also be observed. More than 92%
yield of AGC with PISA-350 as a catalyst was obtained at 100 °C
after 12 h (Table 1, entry 17), whereas the catalyst ISA-1
resulted in an extremely low yield (Table 1, entry 16).
In order to better understand the stronger “CO2 capture”
capability originating from polyether chains, we firstly investi-
gated the catalytic activity of the simple IL-functionalized cata-
lyst ISA-2 with longer alkyl side chains without an oxygen atom
on the imidazolium ring for comparison; when the length of
the alkyl chain is increased from methyl to n-octyl group, the
AGE conversion increased since the solubility of the catalyst in
epoxide AGE increased with the increasing hydrophobic alkyl
chain length (see Fig. 3). However, ISA-2 exhibited lower
activity with respect to PISA-150 bearing three EO units with a
similar hydrophobic alkyl chain length. This result suggests
that the presence of the oxygen atoms within the PEO chains
was crucial to the “CO2 capture” capability of the catalyst,
Furthermore, Table 1 also shows the effect of CO2 pressure
on the reactivity of PISA-350 at 100 °C after 2.5 h. The conver-
sion of AGE decreased as CO2 pressure decreased from 1 to
0.5 MPa, further dropping to 0.1 MPa (Table 1, entries 14–15
Fig. 3 Effect of reaction time on the catalytic performance of various
catalysts. Reaction conditions: 10 mL stainless-steel autoclave, AGE
(6 mmol), catalyst (0.03 mmol), CO2 pressure (1.0 MPa), reaction temp-
erature (100 °C).
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vs. entry 9). Lower CO2 pressure could reduce the absorption of 0.75 h under similar conditions (Table 2, entry 5).15 Unfortu-
CO2 in the solution of AGE; in turn, the turnover frequency nately, the internal epoxide, cyclohexene oxide, exhibited lower
(TOF) was decreased.28 In general, the cycloaddition reaction activity even after prolonging the reaction time to 24 h
was more sensitive to temperature for other similar catalytic (Table 2, entry 8), presumably due to the high steric hin-
systems. While the reaction temperature dropped to 80 °C, and drance.29 This steric effect was more likely to hinder the
further dropped to 60 °C, the yield of AGC also decreased nucleophilic attack of the epoxide rather than its coordination
obviously (Table 1, entries 12–13 vs. entry 9). Unfortunately, the to the Lewis acid metal center.30 Especially, for 1,2-epoxy-
coupling reaction could basically not occur under room tempera- octane and 1,2-epoxydodecane with a linear long alkyl chain
ture and atmospheric pressure even for 72 h (Table 1, entry 18).
(Table 2, entries 3–4), the excellent yields were obtained under
To evaluate the application range of the catalytic system, quite mild conditions as a consequence of higher solubility of
various epoxides, such as propylene oxide (PO), 1,2-epoxybu- the catalyst PISA-350 in epoxide substrate, while no activity
tane, 1,2-epoxyoctane, 1,2-epoxydodecane, epichlorohydrin was observed using the catalyst ISA-1 under the same pressure
(ECH), styrene oxide (SO) and cyclohexene oxide (CHO), were and temperature due to the characteristics of insolubility.
used as the substrate for the reaction system using PISA-350 as Therefore, catalysts PISA with PEO chains not only enhanced
a catalyst. As shown in Table 2, most substrates could be the activity but also improved the dissolution of the catalyst
smoothly converted to corresponding cyclic carbonates with under solvent-free condition.
high conversion and excellent selectivity (Table 2, entries 1–8).
Both steric and electronic effects play an important role. The
Recycling experiments
electron-withdrawing nature of the chloromethyl group of epi-
chlorohydrin tends to drive the cycloaddition reaction for only
Based on the concept of “one-phase catalysis and two-phase
separation” 20 and the special solubility of the PISA, the
Table 2 Results of the coupling reaction of CO2 to various epoxide substrates over the PISA-350 and ISA-1a
Entry
1
Epoxide
Productb
Time/h
2
Conv.c /%
99 (50)
Yieldc /%
98 (49)
TOFd/h−1
98 (49)
2
3
4
6
99 (37)
97 (0)
98 (36)
95 (0)
49 (18)
31.7 (0)
4
5
9
98 (0)
97 (0)
21.6 (0)
0.75
97 (74)
90 (68)
240 (181)
6
7
8
2.5
4
96 (57)
94 (33)
50 (24)
95 (56)
90 (31)
45 (22)
76 (44.8)
45 (15.5)
3.8 (1.8)
24
a Reaction conditions: 10 mL stainless-steel autoclave, epoxide (6 mmol), catalyst (0.03 mmol), CO2 pressure (1.0 MPa), reaction temperature
(100 °C), the value in parentheses refers to the catalyst ISA-1. b Product identification via FT-IR, 1H NMR, 13C NMR and GC-MS. c Same as in
Table 1. d Same as in Table 1.
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catalyst bearing PEO chains could be precipitated from the
reaction solution by the addition of ether (Et2O). The upper
organic phase was obtained by simple decantation and the
lower catalyst solid can be reused by adding fresh reaction
substrates.
Experiments were also conducted to examine the recyclabi-
lity and reusability of the PISA-350 catalyst using AGE as the
substrate under the optimal reaction conditions. The results
indicate that the PISA-350 catalyst could be reused for six suc-
cessive runs without any significant loss in its catalytic activity,
and the selectivity still remained at 98% (see Fig. 4), reflecting
high stability of the catalyst. The comparative FT-IR analysis of
the fresh catalyst PISA-350, ISA-1, SA and the recovered
PISA-350 after the 6th reuse in the cycloaddition reaction was
performed, and the results are shown in Fig. 5. 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 CvN and C–O, respectively
(Fig. 5a–d).23 In addition, the stretching vibration ν(C–N) of
the C–N bond in the IL units at around 1491 cm−1 and the
stretching vibration of C–O–C groups in the PEO chain at
1098 cm−1 suggest the intact polyether-based imidazolium IL
moiety on the salen ligand (Fig. 5b vs. 5a, 5c).23
Fig. 6 (I) 27Al NMR spectra of PISA-350 (a), the recovered PISA-350 (b)
and SA (c); (II) 27Al NMR spectra of PISA-350 (partial amplification
figure).
Fig. 7 Thermogravimetric (TG) and differential thermogravimetric
(DTG) results of the catalyst PISA-350.
Moreover, 27Al NMR spectra of complexes SA, PISA-350 and
the recovered catalyst PISA-350 have all been found to display
broad strong resonances around 70 ppm (Fig. 6a–c), which are
usually attributed to five-coordinate aluminum species.13 The
results suggest that the recovered catalyst PISA-350 presented
here does not change the structure of catalytic active sites.
In addition, thermogravimetric analysis (TGA) results (see
Fig. 7) proved that the PISA-350 catalyst could endure about
238 °C with little loss of its weight. The decomposition of the
ionic liquid started from about 465 °C, showing the high
thermal stability.20
Fig. 4 Recyclability and reusability of the catalyst PISA-350 in the
coupling reaction of AEG with CO2. Reaction conditions: 10 mL stain-
less-steel autoclave, AGE (6 mmol), catalyst (0.03 mmol), CO2 pressure
(1.0 MPa), reaction temperature (100 °C), reaction time: 3 h.
Reaction mechanism
Based on the molecular structure of the complex PISA and the
cycloaddition reaction results, a possible mechanism15 of the
monometallic pathway involving two nucleophiles for cyclic
carbonate synthesis was proposed, which is shown in
Scheme 2.
At first, the anion of ionic liquid units in the catalyst PISA
(Cl−) coordinates to the metal in the remaining trans axial
position, thus generating a six-coordinated intermediate (I).13
As shown in Fig. 6, 27Al NMR spectra of the complex PISA-350
was found to display broad strong resonances at 69.4 ppm and
a new weak signal around 12.61 ppm (see Fig. 6a), while the
neat complex SA which lacks the attached imidazolium-based
Fig. 5 FT-IR spectra of PISA-350 (a), the recovered PISA-350 after the
6th reuse in the cycloaddition reaction (b), ISA-1 (c) and SA (d).
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purified by standard procedures.31 5-Chloromethylsalicylalde-
hyde was synthesized according to published procedures.11a,32
FT-IR spectra were obtained as potassium bromide pellets
with a resolution of 4 cm−1 and 32 scans in the range
400–4000 cm−1 using a Bruker spectrophotometer. 1H NMR,
13C NMR and 27Al NMR spectra were recorded on a Bruker
Avance III 400 M spectrometer, using TMS, 1,4-dioxane and
standard aqueous aluminum solution (1000 µg mL−1 certified
atomic absorption standard solution) as a calibration reagent,
respectively. The thermogravimetric and differential thermo-
gravimetric (TG–DTG) curves were obtained on a NETZSCH
STA 449C thermal analyzer. Samples were heated from room
temperature up to 700 °C under flowing air using alumina
sample holders. The sample weight was ca. 10 mg and the
heating rate was 10 K min−1. Thin layer chromatography (TLC)
was conducted on glass plates coated with silica gel GF254. The
conversions and yields of cyclic carbonate products were
Scheme 2 A plausible mechanism for the coupling reaction of epoxide
with CO2 catalyzed by PISA or ISA-1.
measured by
a GC2010 gas chromatograph (Shimadzu)
ionic liquid units on the salen ligand exhibits a single reson-
ance at 70.53 ppm (see Fig. 6c). 27Al resonance in complexes
PISA or ISA-1 is assigned to the five-coordinate aluminum
species and the weaker and narrow up-field signal to a six-
coordinate species. These observations were also reported by
Liu and Darensbourg.13,14 Subsequently, the coordination of
the nucleophile (Cl−) serves to labilize the other metal–chorine
bond, favoring coordination and nucleophilic attack of the
epoxide (II) activated by the metal center, followed by the ring
opening. Meanwhile, the polyether-based IL units capture and
activate the CO2 molecule. Next, the formed alkoxide species
(III) acts in turn as a nucleophile that attacks CO2 to form a
metal carbonate species (IV). Certainly, the imidazolium
cations could also stabilize the metal alkoxide bond (V)
through charge interactions. The subsequent ring-closure
forms a relevant cyclic carbonate. In the meantime, the catalyst
is regenerated. This mechanism insists that overall the role of
the catalyst involves initial activation of the epoxide and stabil-
ization of the ring-opened and carbonate intermediates
formed during the reaction, which is important to reduce the
reaction time and the pressure of the reaction.
equipped with the capillary column (Rtx-5, 30 m × 0.32 mm ×
0.25 µm) and the FID detector.
Preparation of the polyether-based IL functionalized salen Al
complex (PISA)
The preparation of PISA is outlined in Scheme 1.
Synthesis of chlorine-substituted poly(ethylene glycol)mono-
methyl ether (A).33 To a 500 mL three-neck round bottom
flask were added polyethylene glycol monomethyl ether
(100 mmol, MW = 350, 550, 750), pyridine (15.82 g, 200 mmol)
and dry toluene (200 mL) under a nitrogen atmosphere. The
mixed solution was heated slowly to 80 °C and then thionyl
chloride (23.79 g, 200 mmol) was added dropwise for 3 h. The
mixture was stirred vigorously for an additional 48 h under
reflux. After cooling to room temperature, a small amount of
H2O was added to quench the reaction, and the lower red salts
were extracted with toluene three times, which merged into
the upper organic phase. Afterwards, the pale-yellow organic
phase concentrated and the residue was dissolved in CH2Cl2
(50 mL), washed with H2O (3 × 50 mL), and then dried over
anhydrous Na2SO4. The solvent was evaporated, dried in vacuo
to give a pale-yellow liquid A. Yield: 90%. A−350: FT-IR (KBr),
γmax/cm−1: 2874, 1457, 1353, 1300, 1250, 1200, 1112, 947, 851,
745, 664, 534; 1H NMR (CDCl3/TMS, 400 MHz), δH ppm:
3.67–3.70(t, 2H, J = 12 Hz, Cl–CH2–CH2), 3.55–3.60(m, 24H,
(OC2H4)6–O), 3.47–3.49(t, 2H, J = 8 Hz, Cl–CH2–CH2), 3.31(s,
Experimental
Reagents and methods
Polyethylene glycol monomethyl ether (mPEG, MW = 350, 550, 3H, O–CH3).
750) was purchased from Alfa Aesar Chemical Reagent Co. Ltd.
Synthesis of N-(polyoxyethylene methyl ether)imidazole
Propylene oxide (PO), epichlorohydrin (ECH), allyl glycidyl (B).34 Ca. 21% sodium ethoxide ethanol solution (16.2 g,
ether (AGE), styrene oxide (SO), cyclohexene oxide (CHO), 50 mmol) was added dropwise into the anhydrous ethanol
sodium ethoxide (ca. 21% in ethanol), tri-n-butylamine, solution (100 mL) of imidazole (3.4 g, 50 mmol) under stirring.
4-methylpyridine and diethyl aluminum chloride (Et2AlCl, The obtained mixture was refluxed for 8 h. After the com-
0.9 M solution in toluene) were obtained by J&K Scientific Ltd. pletion of the reaction, chlorine-substituted poly(ethylene
1,2-Epoxybutane, 1,2-epoxyoctane and 1,2-epoxydodecane were glycol) monomethyl ether A (50 mmol) was dissolved in anhy-
used as received from TCI. Other commercially available drous ethanol (50 mL) and added dropwise into the above
chemicals were laboratory grade reagents from local suppliers. solution; the resulting mixture was stirred under reflux for
CO2 was purified by passing through a column packed with 4A another 24 h before allowing to cool to room temperature. The
molecular sieves before use (99.99%). All of the solvents were residue was filtered and the filtrate was evaporated in vacuo,
1502 | Green Chem., 2014, 16, 1496–1506
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and then washed with Et2O (3 × 20 mL) to give an orange-red under vacuum, the mixture was washed with ether several
viscous liquid B, used without further purification. Yield: 82%. times and then dried at 40 °C under vacuum to obtain light
B–350: FT-IR (KBr), γmax/cm−1: 3114, 2873, 1457, 1351, 1325, yellow powders of PISA. Yield: 90%. For PISA-350: FT-IR (KBr),
1
1298, 1251, 1200, 1109, 950, 847, 753, 666, 618, 537; H NMR γmax/cm−1: 3420, 3135, 2924, 1639, 1557, 1489, 1456, 1396,
1
(CDCl3/TMS, 400 MHz), δH ppm: 7.62 (s, 1H, ring CH–NvC), 1348, 1311, 1250, 1098, 949, 839, 761, 668, 641, 421; H NMR
7.48 (s, 1H, ring N–CHvCH), 7.01(s, 1H, ring N–CHvCH), (DMSO-d6, 400 MHz), δH ppm: 9.41(s, 2H, ring NCH), 8.50(s,
4.02–4.05(t, 2H, J = 12 Hz, N–CH2), 3.66–3.68(t, 2H, J = 8 Hz, 2H, CHvN), 7.80–7.84(m, 4H, ring NCH), 7.42–7.59(m, 4H,
N–CH2CH2), 3.52–3.58(m, 24H, (OC2H4)6–O), 3.31(s, 3H, O– ring ArH), 6.87–6.89(m, 2H, ring ArH), 5.37(s, 4H, Ph–CH2–
CH3). 13C NMR (CDCl3/TMS, 100.4 MHz), δC ppm: 136.47,
N
ring), 4.37–4.39(t, 4H, J = 8 Hz, N–CH2–CH2), 3.86–3.88(m, 4H,
128.00, 120.83, 70.89, 69.53, 69.47, 68.77, 65.60, 57.97, 46.06.
N–CH2CH2–N), 3.78–3.80(t, 4H, Hz, N–CH2–CH2),
J = 8
Synthesis of polyether-based IL modified salicyclaldehyde 3.36–3.45(m, 48H, (OC2H4)6–O), 3.32(s, 6H, O–CH3); 13C NMR
(C). N-(Polyoxyethylene methyl ether)imidazole B (30 mmol) in (DMSO-d6, 100.4 MHz), δC ppm: 168.50, 166.53, 138.85, 138.16,
dry toluene (100 mL) was added dropwise into the stirring 135.16, 130.14, 124.99, 123.86, 123.33, 121.22, 73.11, 71.61,
toluene solution of 5-chloromethylsalicylaldehyde (30 mmol, 69.92, 59.88, 54.93, 50.74, 38.41; 27Al NMR (DMSO-d6,
5.1 g) under a nitrogen atmosphere. The reaction was heated 104.3 MHz), δAl ppm: 69.4, 12.61; For PISA-550: FT-IR (KBr),
to reflux for 48 h. After cooling, the solvent was evaporated γmax/cm−1: 3421, 2909, 1636, 1549, 1496, 1397, 1346, 1282,
in vacuo and the lower viscous liquid was washed three times 1233, 1088, 1033, 952, 832, 757, 665, 630, 418; 1H NMR
with dry benzene (3 × 20 mL) and ether (3 × 50 mL), respect- (DMSO-d6, 400 MHz), δH ppm: 9.42(s, 2H, ring NCH), 8.49(s,
ively. The solvent was removed to obtain compound C as the 2H, CHvN), 7.80–7.85(s, 4H, ring NCH), 7.42–7.59(m, 4H, ring
orange-red viscous liquids. Yield: 60%. C-350: FT-IR (KBr), ArH), 6.82–6.94(m, 2H, ring ArH), 5.38(s, 4H, Ph–CH2–Nring),
γmax/cm−1: 3415, 3074, 2874, 1657, 1615, 1592, 1561, 1488, 4.38–4.39(t, 4H, N–CH2–CH2), 3.85–3.89(m, 4H, N–CH2CH2–N),
1448, 1352, 1284, 1250, 1215, 1150, 1107, 931, 845, 770, 677, 3.78–3.80(t, 4H, J = 8 Hz, N–CH2–CH2), 3.42–3.56(m, 80H,
1
632, 516, 458; H NMR (D2O, 400 MHz), δH ppm: 9.85(s, 1H, (OC2H4)6–O), 3.23(s, 6H, O–CH3); 13C NMR (DMSO-d6,
ring NCH), 9.75(s, 1H, CHvO), 8.65–8.89(s, 2H, ring NCH), 100.4 MHz), δC ppm: 168.65, 165.92, 138.92, 136.93, 136.65,
7.65(s, 1H, ring ArH), 6.87–6.98(s, 2H, ring ArH), 5.32(s, 2H, 135.11, 130.46, 126.92, 123.33, 121.12, 73.12, 71.62, 71.42,
PhCH2–N), 4.35–4.37(m, 2H, N–CH2–CH2), 3.83–3.85(m, 2H, 57.84, 54.88, 50.65, 39.23; 27Al NMR (DMSO-d6, 104.3 MHz), δAl
N–CH2–CH2), 3.52–3.64(m, 24H, (OC2H4)6–O), 3.31(s, 3H, O– ppm: 69.36, 12.50; For PISA-750: FT-IR (KBr), γmax/cm−1: 3408,
CH3). 13C NMR (D2O, 100.4 MHz), δC ppm: 196.39, 160.29, 2870, 1639, 1549, 1492, 1448, 1395, 1349, 1310, 1250, 1098,
137.43, 133.43, 125.54, 122.71, 122.25, 121.14, 118.16, 70.98, 949, 839, 761, 635, 526, 492, 420; 1H NMR (DMSO-d6,
69.65, 69.56, 69.43, 68.28, 58.05, 52.02, 51.91, 49.31.
400 MHz), δH ppm: 8.95(s, 2H, ring NCH), 8.52(s, 2H, CHvN),
Synthesis of the polyether-based IL functionalized salen 7.91–7.93(s, 4H, ring NCH), 7.39–7.58(m, 4H, ring ArH),
ligand (PISL). The anhydrous ethanol solution (20 mL) of ethyl- 6.86–6.92(m, 2H, ring ArH), 5.43(s, 4H, Ph–CH2–Nring),
enediamine (5 mmol, 0.3 g) was added dropwise into the solu- 4.37–4.41(t, 4H, N–CH2–CH2), 3.91–3.95(m, 4H, N–CH2CH2–N),
tion of polyether-based ionic liquid modified salicylaldehyde C 3.76–3.85(t, 4H, N–CH2–CH2), 3.42–3.51(m, 120H, (OC2H4)6–
(10 mmol) in anhydrous ethanol (50 mL) in a 1 : 2 molar ratio O), 3.23(s, 6H, O–CH3); 13C NMR (DMSO-d6, 100.4 MHz), δC
at reflux. The resulting mixture was refluxed for another 8 h; ppm: 167.21, 165.20, 136.86, 136.47, 134.49, 128.91, 123.61,
following this, the solvent was evaporated to dryness and the 123.03, 122.50, 119.90, 71.75, 70.25, 70.18, 68.57, 58.52, 53.55,
polyether-based ionic liquid functionalized salen ligand PISL 51.79, 36.87; 27Al NMR (DMSO-d6, 104.3 MHz), δAl ppm: 69.28,
was obtained as a light yellow viscous liquid, used without 12.42.
further purification. Yield: 96%. PISL-350: FT-IR (KBr), γmax
/
Synthesis of 1-benzyl-3-methylimidazoliumchloride. A solu-
cm−1: 3422, 2876, 1635, 1591, 1560, 1497, 1450, 1350, 1285, tion of benzyl chloride (0.1 mol, 12.68 g) in toluene (200 mL)
1
1234, 1103, 943, 839, 759, 727, 667, 637, 518; H NMR (D2O, was mixed with a solution of 4-methylpyridine (0.1 mmol,
400 MHz), δH ppm: 9.98(s, 2H, ring NCH), 8.41(s, 2H, CHvN), 9.3 g) and refluxed overnight. After cooling to room tempera-
7.89(s, 2H, ring NCH), 7.79(s, 2H, ring NCH), 7.00–7.20(m, 6H, ture, the yellow viscous liquids were collected under vacuum
ring ArH), 5.10(s, 4H, Ph–CH2–Nring), 4.18–4.20(t, 4H, J = 8 Hz, following washing with ether several times. 1H NMR (CDCl3/
N–CH2–CH2), 3.79–3.82(m, 4H, N–CH2CH2–N), 3.57–3.64(m, TMS, 400 MHz), δH ppm: 10.49 (s, 1H, ring NCH), 7.72(s, 1H,
48H, (OC2H4)6–O), 3.32(s, 6H, O–CH3).
ring NCH), 7.56(s, 2H, ring NCH), 7.49–7.51(s, 2H, ring ArH),
Synthesis of PISA. Under nitrogen protection and constant 7.33–7.34(s, 3H, ring ArH), 5.58(s, 2H, CH2), 4.04(s, 3H, CH3);
stirring at 40 °C, to a 100 mL round-bottom flask containing 13C NMR (CDCl3/TMS, 400 MHz), δC ppm: 137.14, 133.30,
the above-obtained salen ligand D (5 mmol), anhydrous 129.15, 129.13, 128.70, 123.82, 121.95, 57.24, 52.86, 36.36,
chloroform (50 mL) was added via a hypodermic syringe to dis- 18.33.
solve the ligand, and then a little excess of Et2AlCl (0.9 M solu-
Synthesis of the neat salen Al complex (SA).24b FT-IR (KBr),
tion in toluene, 5.7 mL, 5.1 mmol) was added slowly. The γmax/cm−1: 2987, 1641, 1605, 1550, 1474, 1453, 1400, 1340,
reaction was highly exothermic and resulted in a yellow solu- 1297, 1242, 1206, 1153, 1129, 1094, 1052, 1033, 1004, 987, 957,
tion and a pale yellow solid. The resulting yellow mixture was 908, 858, 814, 759, 635, 592, 555, 486, 464; 1H NMR (DMSO-d6,
refluxed for an additional 12 h. After removal of the solvent 400 MHz), δH ppm: 8.53(s, 2H, CHvN), 7.35–7.41(s, 4H, ring
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ArH), 6.85–6.87(d, 2H, J = 8 Hz, ring ArH), 6.71–6.75(t, 2H, J = 3.80–3.84 (dd, 1H, CH2–Cl), 3.72–3.76 (dd, 1H, CH2–Cl);
16 Hz, ring ArH); 13C NMR (DMSO-d6, 100.4 MHz), δC ppm: 13C NMR (CDCl3, 100.4 MHz), δC ppm: 154.35 (CvO), 74.38
167.59, 164.96, 135.50, 134.60, 121.65, 120.05, 116.38, 53.53; (ring CH), 67.00 (ring CH2), 43.86 (CH2–Cl); 4-allyl-1,3-dioxo-
1
27Al NMR (DMSO-d6, 104.3 MHz), δAl ppm: 70.53.
lan-2-one: H NMR (CDCl3/TMS, 400 MHz), δH ppm: 5.83–5.94
Synthesis of the simple IL-functionalized salen Al complex (1H, CH2vCH), 5.19–5.31 (2H, CH2vCH), 4.84–4.87 (1H, ring
(ISA1–4). Following a similar procedure to that for the above CH2vCH), 4.50–4.54 (1H, J = 16 Hz, ring CH2–CH), 4.38–4.42
newly-synthesized complex (PISA), simple IL-functionalized (1H, J = 16 Hz, ring CH2–CH), 4.01–4.10 (2H, CH2–CHv),
salen Al complexes (denoted as ISA1–4) were also prepared 3.69–3.73 (1H, CH2–CH), 3.60–3.64 (1H, CH2–CH); 13C NMR
using 1-methylimidazole 1-octylimidazole, tri-n-butylamine (CDCl3, 100.4 MHz), δC ppm: 155.06 (CvO), 133.71
and 4-methylpyridine, instead of B respectively. ISA-1: FT-IR (CH2vCH), 117.79 (CH2v), 75.17 (CH2–CH), 72.51 (ring CH),
(KBr), γmax/cm−1: 3417, 3095, 1642, 1552, 1489, 1397, 1310, 68.86 (CH2–CHv), 66.27 (ring CH2); 4-phenyl-1,3-dioxolan-2-
1225, 1164, 1054, 853, 758, 671, 620; 1H NMR (DMSO-d6, one: 1H NMR (CDCl3/TMS, 400 MHz), δH ppm: 7.28–7.38(m,
400 MHz), δH ppm: 9.40(s, 2H, ring NCH), 8.51(s, 2H, CHvN), 4H, ring ArH), 5.58–5.62 (t, 1H, J = 16 Hz, PhCHO), 4.71–4.75
7.75–7.82(s, 4H, ring NCH), 7.47–7.53(s, 4H, ring ArH), (t, 1H, J = 16 Hz, OCH2), 4.26–4.30 (t, 1H, J = 16 Hz, OCH2);
6.88–6.90(s, 2H, ring ArH), 5.36(s, 4H, Ph–CH2–Nring), 3.88(s, 13C NMR (CDCl3, 100.4 MHz), δC ppm: 153.75 (CvO),
6H, N–CH3). 3.85–3.86 (m, 4H, N–CH2CH2–N); 13C NMR 134.78 (Ph), 128.72 (Ph), 128.23 (Ph), 124.83 (Ph), 76.95 (Ph),
(DMSO-d6, 100.4 MHz), δC ppm: 167.19, 165.15, 137.07, 135.83, 70.13 (CH2).
128.67, 125.78, 124.38, 122.59, 119.72, 118.75, 53.55, 51.59,
36.33; 27Al NMR (DMSO-d6, 104.3 MHz), δAl ppm: 70.38, 12.61.
Cycloaddition procedure for the reaction of epoxides with
CO2. Epoxide (6 mmol), catalyst (0.03 mmol) and biphenyl
Conclusions
(0.6 mmol, internal standard for GC analysis) were added into Several salen Al complexes functionalized by polyether-based
a 10 mL stainless-steel autoclave equipped with a magnetic imidazolium ILs have been first prepared and act as catalysts
stirrer, which had been previously dried at 100 °C for 2.5 h for the cycloaddition reaction of CO2 and various epoxides to
under vacuum. After the reaction mixture was rapidly heated provide cyclic carbonate under quite mild conditions. A
to the desired temperature (100 °C), the autoclave was pressur- remarkable enhancement of the reaction rates was observed
ized with CO2 from a reservoir tank to maintain a constant over the one-component tri-functional catalysts at low CO2
pressure (1.0 MPa). After stirring at ca. 200 rpm for the desig- pressure due to the built-in “CO2 capture” capability originat-
nated reaction time, the autoclave was cooled quickly to −5 °C ing from the polyether chains. Among them, the catalyst of
and the remaining CO2 was slowly released and absorbed in a PISA-550 showed the best catalytic activity, which afforded
small amount of ethyl acetate or ether. Subsequently, 20 mL cyclic carbonate in good yield (90–98%) with high selectivity
ether was added into the reactor, and the catalyst was separ- for various epoxides under mild conditions. Furthermore, the
ated as a solid by centrifugation, which was washed with Et2O synthesis catalyst PISA could be facilely separated and recycled
or EA three times and dried under vacuum for the recycling six times with only a slight loss in its catalytic activity by
experiment without further purification. Each catalytic reac- control of the solvent.
tion was repeated three times to secure reproducibility. The
purity and structure of products were also confirmed by FT-IR,
1HNMR,
13CNMR
spectra
GCMS-QP2010) analysis.
and
GC-MS
(Shimadzu
Acknowledgements
NMR characterizations of the typical cyclic carbonate pro- This work was supported by the National Natural Science
ducts were as follows: 4-methyl-1,3-dioxolan-2-one: 1H NMR Foundation of China (no. 21376278, 21176267 and 21036009).
(CDCl3/TMS, 400 MHz), δH ppm: 4.77–4.86 (m, 1H, ring CH– The authors are also thankful to Prof. Donghong Yin and
CH3), 4.49–4.53 (t, 1H, J = 16 Hz, ring CH2), 1.41–1.43 (d, 1H, Dr Rong Tan (Hunan Normal University) for providing help
J = 8 Hz, CH3); 13C NMR (CDCl3, 100.4 MHz), δC ppm: 155.15 with the synthesis method.
(CvO), 73.70 (ring CH–CH3), 70.72 (ring CH2), 19.31(CH3);
4-hexyl-1,3-dioxolan-2-one: 1H NMR (CDCl3/TMS, 400 MHz), δH
ppm: 4.68–4.75 (m, 1H, ring CH), 4.51–4.55 (t, 1H, J = 16 Hz,
ring CH2), 4.05–4.09 (t, 1H, J = 16 Hz, ring CH2), 1.77–1.84 (m,
Notes and references
1H, CH2CH2CH), 1.64–1.72 (m, 1H, CH2CH2CH), 1.30–1.49 (m,
8H, CH3 (CH2)4), 0.88–0.91 (t, 3H, J = 12 Hz, CH3); 13C NMR
(CDCl3, 100.4 MHz), δC ppm: 155.13(CvO), 77.10 (ring
CH–CH2), 69.43 (ring CH2CH), 33.88 (CH2CH2), 31.51
(CH2CH2), 28.80 (CH2CH2), 24.33 (CH2CH2), 22.46 (CH2CH2),
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4.59–4.63 (t, 1H, ring CH2), 4.40–4.44 (dd, 1H, ring CH2),
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