Copolymerization of CO2 and cyclohexene oxide using a lysine-based (salen)CrIIICl catalyst

Article abstract of DOI:10.1039/b821184a

A new, natural lysine-based (salen)Cr^IIICl ((lys-salen)Cr ^IIICl) complex was prepared and its catalytic activity for the copolymerization of CO₂ and cyclohexene oxide (CHO) was described in the presence of PPNCl (PPN⁺

Full text of DOI:10.1039/b821184a

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Copolymerization of CO2 and cyclohexene oxide using a lysine-based
(salen)CrIIICl catalyst†
Liping Guo,a Congmin Wang,a Wenjia Zhao,a Haoran Li,*a Weilin Sunb and Zhiquan Shenb
Received 26th November 2008, Accepted 1st May 2009
First published as an Advance Article on the web 28th May 2009
DOI: 10.1039/b821184a
A new, natural lysine-based (salen)CrIIICl ((lys-salen)CrIIICl) complex was prepared and its catalytic
activity for the copolymerization of CO2 and cyclohexene oxide (CHO) was described in the presence of
PPNCl (PPN+ = bis(triphenylphosphoranylidene)ammonium) as cocatalyst. The influence of the
reaction time, operating temperature and the molar ratio of the catalyst components on the
copolymerization was investigated in detail. The results showed that the (lys-salen)CrIIICl, synthesized
from non-ortho-diamine, could effectively catalyze the alternating copolymerization (carbonate
linkages = 94.6–99.0%). The selectivity was >95%, and was less sensitive to the temperature and the
molar ratio of catalyst components, compared to that of the copolymerization catalyzed by traditional
salen–metal complexes. The ESI-MS analyses of oligomer and (lys-salen)CrIIICl indicated that a
possible chain-transfer reaction had taken place, which might be induced by the water coordinating to
the central metal ion.
been investigated in detail. The results showed that the steric
Introduction
and electronic structure of salicylaldehyde and cocatalyst played
A series of investigations have been performed in the area of
an important role in the copolymerization. Up to now, di-
CO2 utilization, since CO2 is an abundant, economical and
biorenewable resource. One of the most promising reactions in
amines, as the framework for H2salen ligands, were only fo-
cused on some ortho-diamines, such as 1,2-cyclohexenediamine,
this area is the copolymerization of CO2 with epoxides announced
1,2-phenylenediamine, 1,2-ethylenediamine and some derivatives
in 1960’s by Inoue et al.1 to prepare polycarbonates, which have
displayed great potential as biodegradable polymeric materials.2
Among the numerous metal-based catalyst systems,2 the salen–
metal complex (Fig. 1) system is of special interest.3 Using
a binary catalyst system of a salen–metal complex as catalyst
in conjunction with ionic organic ammonium salt or Lewis
base as cocatalyst could markedly increase turnover frequency
(TOF) and gave the alternating copolymer with high selectivity.4,5
Recently, the effects of different slicylaldehyde-modified H2salen
ligands 6,7 and various cocatalysts8 on the copolymerization have
of 1,2-ethylenediamine.3,6 So far as we know, the copolymerization
catalyzed by a salen–metal complex obtained from non-ortho-
diamine was absent. This prompted us to verify whether the
salen–metal complex synthesized from non-ortho-diamines such
as amino acids could effectively catalyze the copolymerization of
CO2 and epoxides.
It is well known that natural amino acids, as renewable
natural sources, are highly economic and scientific materials for
the chemistry industry. What is more, natural amino acids are
attractive sources of chirality. A lot of metal complexes with
Schiff base ligands composed of one salicylaldehyde and one
natural amino acid have been synthesized and applied.9 However,
few studies on metal complexes with salen ligands composed
of two salicylaldehydes and one amino acid were reported.
This has inspired us to determine whether we could develop a
new lysine-based salen–metal catalyst for the copolymerization
of CO2 and epoxides. Herein we report the synthesis of a
natural lysine-based (salen)CrIIICl complex 4, [(lys-salen)CrIIICl],
and investigate its catalytic activity in the copolymerization of
1,2-cyclohexene oxide (CHO) and CO2 (Scheme 1) for the first
time.
Fig.
1
(R,R)-(salen)CrIIICl for the copolymerization of CO2 and
epoxide.
aDepartment of Chemistry, Zhejiang University, Hangzhou, 310027, People’s
Republic of China. E-mail: lihr@zju.edu.cn; Fax: (86)-571-8795-1895;
Tel: (86)-571-8795-2424
bDepartment of Polymer Science and Engineering, Zhejiang University,
Hangzhou, 310027, People’s Republic of China
† Electronic supplementary information (ESI) available: Experimental
spectra. See DOI: 10.1039/b821184a
Scheme
1 Copolymerization of CO2 and CHO catalyzed by
(lys-salen)CrIIICl.
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Scheme 2 Synthesis of (lys-salen)CrIIICl.
=
in the IR spectrum both indicated the formation of C N
bonds.
Results and discussion
Synthesis and characterization of the (lys-salen)CrIIICl
Finally, the H2salen ligand 3 was metalated by CrCl2 and
oxidized in air to form the desired (lys-salen)CrIIICl 4. Complex
4 was characterized by IR spectroscopy, ESI-MS and elemental
analyses. Upon binding 3 to the metal center, there was a
The synthesis of (lys-salen)CrIIICl 4 started with commercially
available (S)-lysine monohydrochloride (Scheme 2). Firstly, the
(S)-lysine monohydrochloride was esterified by thionyl chloride
and methanol10 to obtain lysine methyl ester dihydrochloride
2 in 95% yield. The product was characterized by 1H NMR
spectroscopy, and the signal at 3.74 ppm corresponded to the
methoxyl group.
Complex 2 was treated with sodium carbonate,11 and then
3,5-di-tert-butylsalicylaldehyde was added to afford the lysine-
based H2salen ligand 3. After recrystallization from methanol,
the ligand was obtained in 45% yield as a bright yellow
solid. It was characterized by IR and NMR spectroscopies
and elemental analysis. The signals at 8.38 and 8.34 ppm in
the 1H NMR spectrum, and the absorbance at 1632 cm-1
=
remarkable shift of the n(C N) vibration to the frequency at
1617 cm-1. The ESI-MS measurement showed that the m/z was
660.3, which corresponded to [(C37H54N2O4Cr)·(H2O)]+ (= [(4 -
Cl)·(H2O)]+). It indicated that one molecule of water per molecule
of 4 was strongly coordinating to the central metal ion.
Catalytic copolymerization of CO2 and CHO
The copolymerization of CO2 and CHO was carried out in
neat CHO at 4.5 MPa of CO2 with the (lys-salen)CrIIICl-PPNCl
(PPN+=bis(triphenylphosphoranylidene)ammonium) as catalyst
system. As shown in Table 1, the (lys-salen)CrIIICl, combined with
Table 1 Copolymerization of CO2 and CHO by the (lys-salen)CrIIICl–PPNCl systema
Entry Time/h Temp/◦C
Equiv of cocatalyst
Yield/%
TOFb/h-1
Selectivity/% PCHCc
% carbonatec
Mnd/g mol-1
Mw/Mn
d
1
2
3
4
5
6
7
8
9
10
11
12
13
4
12
24
32
18
18
18
18
18
24
24
24
24
80
80
80
80
60
70
80
90
100
80
80
80
80
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
0.5
31
67
87
89
44
72
81
89
99
54
75
86
81
76.3
55.9
36.4
27.9
24.5
40.2
44.9
49.3
55.0
22.3
31.1
35.8
33.6
99.9
98.8
99.1
99.0
100
94.6
98.1
98.6
99.0
95.9
98.2
98.9
98.7
98.8
98.0
98.8
97.8
97.8
895
3430
7661
9138
1876
2136
7695
5862
6607
7369
9865
5984
3515
1.83
1.25
1.32
1.39
1.64
1.75
1.20
1.22
1.27
1.15
1.16
1.14
1.22
100
99.4
97.6
95.0
100
99.6
98.8
97.9
1
3
5
a Copolymerizations run in neat cyclohexene oxide (CHO) (14.7 g, 150 mmol; [CHO] : [Cr]=1000 : 1) at 4.5 MPa of CO2. b Measured in moles of CHO
consumed (mol Cr)-1 h-1. c Estimated based on 1H NMR. d Estimated by gel permeation chromatography in THF at 30 ◦C on the basis of polystyrene
standard.
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PPNCl, could effectively catalyze the alternating copolymerization
with high selectivity. All isolated copolymers appeared narrow
molecular weigh distributions, ranging from 1.14 to 1.83, along
with high carbon dioxide incorporation (greater than 94%), and
only trace quantities of cyclic carbonate, as a byproduct, were
produced.
The effect of reaction conditions on the copolymerization was
investigated in detail. Firstly, when the reaction time increased
from 4 to 32 h (Table 1, entries 1–4), the conversion of monomer
continued increasing, and the molecular weight of copolymers
increased from 895 to 9138. Meanwhile, the selectivity was not
sensitive to the reaction time and remained at about 99%.
Secondly, higher temperature led to higher y◦ields and greater
molecular weights (Table 1, entries 5–9). At 60 C, the yield was
only 44% and the molecular weight of the copolymer was only
1876 after 18 h. The yield increased significantly to 99% at 100 ◦C,
◦
and the molecular weight was 7695 at 80 C. A previous report
had demonstrated that the increase of the temperature for the
copolymerization of CHO and CO2 resulted in a sharp increase in
cyclic carbonate formation.12 The activation barrier for cyclic car-
bonate formation was higher than that of polycarbonate, therefore
cyclic carbonate tended to form at elevated temperatures.12 For 4–
PPNCl, the selectivity decreased slightly from 100% to 95% when
the temperature increased from 60 to 100 ◦C.
Finally, a slight increase in cyclic carbonate formation and a
decrease in the molecular weight were observed with an increase
in the cocatalyst loading (Table 1, entries 3, 10–13). The molar
ratio of cocatalyst to catalyst also played an important role in
the cyclic carbonate formation8d,13 because excessive anions of
the cocatalyst could displace the growing polymer chain from
the metal center,5 which then led to an increased rate of chain
degradation and the formation of cyclic carbonate. When the ratio
of 4 : PPNCl was 0.5, there was no cyclic carbonate produced.
Increasing the ratio to 5 resulted in a slight (2%) decrease
in selectivity. However, for the copolymerization of CHO and
CO2, catalyzed by the aluminium–salen complex–ammonium salt
system in similar reaction conditions,13 the increase of the ratio
from 0.5 to 5 decreased the selectivity by over 50%. The decrease
in the molecular weight might be caused by accompanying water
of the cocatalyst.14
The GPC curve for the copolymer obtained from the re-
action showed a bimodal molecular weight distribution. This
phenomenon has been reported several times.7a,13,15 Using MS
characterization, in combination with GPC and the corresponding
UV spectra of the copolymer, Sugimoto and coworkers have
demonstrated that a possible chain-transfer reaction with con-
comitant water occurred, resulting in the bimodal distribution of
the obtained copolymer.13 Accordingly, an oligomer produced at
80 ◦C for 1 h catalyzed by 4–PPNCl(1 : 2.25), was measured by
ESI-MS, which was applied to defining the end groups of the
copolymer in the copolymerization of CO2 and epoxides.8d,13,15
Similar to previous reports,13,15 two series of signals having the
same regular intervals of a repeating unit (142) were observed.
They matched copolymer a and copolymer b (Fig. 2), respectively.
Copolymer a had a –OH group at the end, which was probably
introduced by the chain transfer by water,13 and copolymer b
had a terminal –Cl group, which was an initiating group for the
copolymerization of CO2 and CHO. The effect of water on the
copolymerization was investigated. It was seen from Table 2 that
Fig. 2 ESI-MS spectrum of the Na+-ionized polymer obtained with the
catalytic system (CHO : (lys-salen)CrIIICl : PPNCl= 1000 : 1 : 2.25) at 80 ◦C
and a 4.5 MPa CO2 pressure for 1 h, and possible structures of copolymer
a and copolymer b.
there was no obvious change in the TOF when the quantity of
water was increased, while the molecular weight of the copolymer
decreased significantly. In order to remove the concomitant water,
the autoclave reactor containing 4 and PPNCl was heated at 100 ◦C
for 6 h in vacuo and then cooled in a N2 atmosphere before the
copolymerization. However, the copolymer obtained still showed a
bimodal GPC curve. The ESI-MS of 4 indicated that one molecule
of water was coordinating to the central metal ion, and this might
be the source of the water, which resulted in the chain transfer.15
The (R,R)-(salen)CoIII catalysts, which have two chiral centers,
have been proven to be active in enantioselective copolymerization
Table 2 Copolymerization of CO2 and CHO by the 4–PPNCl system
with added watera
d
Entry
Equiv of H2Ob
TOFc/h-1
Mnd/g mol-1)
Mw/Mn
1e
2
3
0
0
5
35.6
36.4
34.2
33.3
10592
7661
2472
1636
1.26
1.32
1.31
1.24
4
40
a Copolymerizations run in neat cyclohexene oxide (CHO) (14.7 g,
150 mmol; [CHO] : [Cr] : [PPNCl] = 1000 : 1 : 2.25) at 4.5 MPa of CO2
for 24 h. b The equiv. of H2O to 4. c Measured in mole of CHO consumed
(mol Cr)-1 h-1. d Estimated by gel permeation chromatography in THF at
30 ◦C on the basis of polystyrene standard. e Before the copolymerization,
the autoclave reactor containing 4 and PPNCl was heated at 100 ◦C for
6 h in vacuo and then cooled in a N2 atmosphere.
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of CO2 and CHO.16 However, the study of Darensbourg and co-
workers on the tacticity of copolymers formed by the copoly-
merization of CO2 and CHO catalyzed by chiral (salen)CrIII
catalysts, showed a lack of stereocontrol in the CHO ring-opening
step.4,6 Accordingly, we wondered if the (lys-salen)CrIIICl was
active for enantioselective copolymerization of CO2 and CHO,
since it is unsymmetric and has a chiral center. To determine the
tacticity of the polymer, 13C NMR spectroscopy was employed
and analyzed, following the description of Nozaki and Coates.17
However, chemical shifts observed at 153.9, 153.4, and 153.2 ppm
indicated the production of a largely atactic polymer.
150 mmol) was added dropwise over 30 min. The reaction mixture
quickly became homogeneous and was stirred for 20 min at
0 ◦C. After heating at reflux for 4 h, the solution was concentrated
in vacuo. The crude product was diluted with methanol, the
solution was concentrated again to remove any residual acid, and
then dried under vacuum overnight to afford 2 (4.41 g, 95%) as
a white solid. Melting point: 197.8–198.4 ◦C, 1H NMR (D2O): d
4.08–4.06 (t, 1H), 3.74 (s, 3H), 2.92–2.89 (t, 2H), 1.93–1.85 (m,
2H), 1.63–1.60 (m, 2H), 1.33–1.39 (m, 2H) ppm.
Lysine-based H2salen ligand 3. 2 (1.16 g, 5 mmol) was dis-
solved in 20 mL methanol and stirred together with Na2CO3
(1.06 g, 10 mmol) at room temperature for 30 min.11 After
removing the precipitate by filtration, a solution of 3,5-di-tert-
butylsalicylaldehyde (2.34 g, 10 mmol) in 30 mL methanol was
dropped slowly into the filtrate. Then, the reaction mixture was
heated under reflux for 4 h. After distilling most of the solvent off
and cooling to 0 ◦C, the crude product was collected by filtration.
Recrystallization from methanol afforded the desired compound
3 as a bright yellow solid (1.33 g, 45%). 1H NMR (CDCl3): d 13.87
(s, 1H), 13.30 (s, 1H), 8.38 (s, 1H), 8.34 (s, 1H), 7.42 (d, 1H), 7.38
(d, 1H), 7.12 (d, 1H), 7.08 (d, 1H), 4.00–3.97 (m, 1H), 3.75 (s,
3H), 3.59–3.58 (m, 2H), 2.08–1.92 (m, 2H), 1.77–1.74 (m, 2H),
1.47–1.45 (m, 2H), 1.46 (s, 9H), 1.44 (s, 9H), 1.32 (s, 9H), 1.31 (s,
9H) ppm. 13C NMR (CDCl3): d 172.14, 168.14, 166.12, 158.38,
158.31, 140.52, 140.17, 137.12, 136.90, 127.84, 126.99, 126.62,
126.01, 118.09, 117.88, 71.78, 59.45, 52.56, multiple peaks were
present between 23–36 ppm. IR (KBr, nmax/cm-1):, 2955 and 2866
Conclusions
In summary, we prepared a natural lysine-based (salen)CrIIICl,
and investigated the copolymerization of CO2 and CHO catalyzed
by this catalyst. The results showed that the (lys-salen)CrIIICl,
synthesized from non-ortho-diamine, could effectively catalyze the
alternating copolymerization. The ESI-MS measurement of (lys-
salen)CrIIICl indicated that one molecule of water coordinated
to the central metal ion, responding to the bimodal GPC curve
of copolymers. In addition, the study on the tacticity of the
copolymer showed a lack of stereocontrol in the CHO ring-
opening step catalyzed by (lys-salen)CrIIICl catalyst. In contrast to
the traditional salen–metal catalysts, the (lys-salen)CrIIICl catalyst
was prepared from lysine, which is commercially available and
optically pure. Although the activity of (lys-salen)CrIIICl catalyst
is not better than traditional salen–metal catalysts, it shows some
advantages such as high carbonate linkage, narrow molecular
weight distribution and good selectivity. Further studies are
focused on exploring the reaction mechanism and developing
new amino acid-based salen catalyst systems that exhibit higher
stereoselectivity for epoxide ring-opening.
=
=
(CH), 1632 (C N), 1597 and 1470 (C C, Ar). Anal. calcd for
C37H56N2O4: C, 74.96; H, 9.52; N, 4.73. Found: C, 74.92; H, 9.55;
N, 4.73.
(lys-salen)CrIIICl 4. Under a nitrogen atmosphere, CrCl2
(0.27 g, 2.2 mmol) was added to a solution of 3 (1.18 g, 2 mmol)
in 40 mL THF. The reaction mixture was stirred under nitrogen
for 24 h and then in air for an additional 24 h. The reaction
mixture was poured into 200 mL ethyl acetate and washed with
aqueous saturated NH4Cl (3 ¥ 100 mL) and brine (3 ¥ 100 mL).
The organic layer was dried with Na2SO4, and solvent was removed
Experimental
Reagents and methods
THF was dried by distillation over sodium–benzophenone and
under vacuum, yielding 4 (0.85 g, 63%) as a dark brown power. IR
˚
stored over 4 A molecular sieves under dry nitrogen. Cyclohexene
-1
=
(KBr, nmax/cm ): 2960, 2904 and 2869 (CH), 1617 (C N), 1533
oxide was freshly distilled over calcium hydride before use.
Other reagents and starting materials were all used as received
unless otherwise stated. The NMR spectra were recorded with
a 500 MHz Bruker spectrometer in CDCl3, D2O and calibrated
with tetramethylsilane (TMS) as the internal reference. Infrared
spectra measurements were performed on a Nicolet 470 FT-IR
spectrometer using KBr optics. ESI-MS analyses were performed
on an Esquire3000plus mass spectrometer. Elemental analyses
were measured by EA-1112 elemental analyser. The relative
molecular weights were measured at 30 ◦C in THF with a
flow rate of 1.0mL min-1, using the polystyrene standards, on
Waters1525/2414/717 GPC (waters 1525 HPLC pump, 2414 dif-
ferential refraction detector, and 717 plus autosampler) equipped
with three series-connected Styragel HR3-HR2-HR1 columns.
=
and 1458 (C C, Ar). Anal. calcd for C37H54N2O4CrCl·H2O: C,
63.82; H, 8.11; N, 4.02. Found: C, 64.13; H, 8.31; N, 3.98. MS
(ESI+, m/z): 660.3 (calcd for [(C37H54N2O4Cr)·(H2O)]+ (= [(4 -
Cl)·(H2O)]+) 660.3).
Copolymerization of cyclohexene oxide and CO2 catalyzed by
(lys-salen)CrIIICl
In a typical experiment, a 100 mL stainless steel autoclave was
charged with◦CHO, catalyst 4 and PPNCl. The autoclave was
heated to 80 C and held at 4.5 MPa for 24 h, and then quickly
cooled to room temperature, and the residual CO2 vented in a fume
hood. A 1H NMR spectrum was immediately taken to obtain the
ratio of polycarbonate to cyclic carbonate and the percentage of
carbonate linkage in the copolymer, where the amount of ether
linkages was determined by integrating the peaks corresponding
to the methane proton of polyether at ca. 3.5 ppm, cyclic carbonate
at ca. 4.0 ppm and polycarbonate at ca. 4.6 ppm. A 13C NMR
spectrum was also taken to determine the tacticity of the polymer
Synthesis of (lys-salen)CrIIICl
10
(S)-lysine methyl ester dihydrochloride 2.
(S)-lysine mono-
hydrochloride (3.65 g, 20 mmol) was partly dissolved in 150 mL
methanol and cooled to 0 ◦C, then thionyl chloride (10.7 mL,
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formed. The polymer was extracted from a dichloromethane
solution and dried under vacuum at 100 ◦C overnight. The isolated
polycarbonate was weighted to calculate the yields. Molecular
weight was determined through GPC in tetrahydrofuran (THF)
(5 wt) and was calculated relative to polystyrene.
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Acknowledgements
Financial support from the Natural Science Foundation of China
(No. 20704035, 20773109 and 20806065) is greatly appreciated.
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