Copolymerization of epoxides with carbon dioxide catalyzed by iron-corrole complexes: Synthesis of a crystalline copolymer

Article abstract of DOI:10.1021/ja4028633

Iron-corrole complexes were found to copolymerize epoxides with CO ₂. The first iron-catalyzed propylene oxide/CO₂ copolymerization has been accomplished. Moreover, the glycidyl phenyl ether (GPE)/CO₂ copolymerization with this catalyst provided a crystalline material as a result of the isotactic poly(GPE) moiety.

Full text of DOI:10.1021/ja4028633

Communication
pubs.acs.org/JACS
Copolymerization of Epoxides with Carbon Dioxide Catalyzed by
Iron−Corrole Complexes: Synthesis of a Crystalline Copolymer
Koji Nakano,† Kazuki Kobayashi,‡ Takahiro Ohkawara,‡ Hideyuki Imoto,‡ and Kyoko Nozaki*,‡
†Department of Organic and Polymer Materials Chemistry, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho,
Koganei, Tokyo 184-8588, Japan
‡Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
S
* Supporting Information
ether linkages, while the CHO/CO2 copolymers were rich in
carbonate linkage.
ABSTRACT: Iron−corrole complexes were found to
copolymerize epoxides with CO2. The first iron-catalyzed
propylene oxide/CO2 copolymerization has been accom-
plished. Moreover, the glycidyl phenyl ether (GPE)/CO2
copolymerization with this catalyst provided a crystalline
material as a result of the isotactic poly(GPE) moiety.
Iron−corrole complexes 1−3 used in this work are shown in
Figure 1.15 In complexes 1 and 2, the iron center in the formal
ron is one of the most ideal metals as catalysts for chemical
Itransformations since it is abundant on the earth, cost-
effective, and environmentally benign.1 Indeed, iron catalysts
have been widely used to date as Lewis acid and oxidation
catalysts. Recently, in terms of sustainable chemistry, iron
catalysts have emerged as promising alternatives to more
expensive catalysts in modern metal-catalyzed reactions such as
cross-coupling reactions and hydrogenations catalyzed by
platinum-group metals.2
Carbon dioxide is also abundant on the earth and is an
attractive carbon source for materials synthesis.3 The
alternating copolymerization of epoxides with CO2, which
was first reported by Inoue and co-workers in 1960s,4 has been
considered as one of the most promising processes.5 Previously
reported active catalysts have typically been based on zinc,6
aluminum,7 chromium,8 cobalt,9 and M(IV) (M = Ti, Ge).10
Iron-containing complexes have also been reported as active
catalysts, but the examples are still limited.11 Recently, a
dinuclear iron complex12 and a mononuclear iron complex13
were shown to catalyze epoxide/CO2 copolymerization, but the
reported examples are still limited to cyclohexene oxide
(CHO). To the best of our knowledge, no example of an
iron catalyst that mediates the copolymerization of CO2 with
other epoxides has ever been reported.
Figure 1. Structures of iron−corrole complexes 1−3 and reference Co
complex 4 used in this work.
+4 oxidation state is ligated by a trianionic, tetradentate corrole
ligand and a monoanionic axial ligand.16 Such a structure is
identical to that of the metal(IV) complexes that we previously
applied to the PO/CO2 copolymerization.10 In addition to
complexes 1 and 2, iron(III)−corrole complex 3 without any
monoanionic axial ligand was also investigated as a catalyst for
the copolymerization.17
The copolymerization of PO with CO2 ([PO]/1 = 4000
([PO]/Fe = 2000), PCO = 2.0 MPa) was carried out at 60 °C
2
for 1 h using complex 1a in the presence of [Ph3PNPPh3]
Cl ([PPN]Cl) as an additive (0.5 equiv relative to Fe; Table 1,
entry 1). The copolymer was obtained with high catalytic
activity [turnover frequency (TOF) = 1004 (mol of PO
incorporated into the copolymer)·(mol of Fe)−1·h−1] without
the concomitant production of propylene carbonate (PC). On
1
the basis of H NMR analysis, the obtained copolymer was
The polycarbonates prepared by epoxide/CO2 copolymer-
ization are known to be amorphous, except for perfectly
stereocontrolled poly(CHO-alt-CO2) and poly(epichloro-
hydrin-alt-CO2), which exhibited crystallinity.14 Here we
present iron complexes with a corrole ligand as the first
examples of iron complexes applicable to not only CHO/CO2
but also propylene oxide (PO)/CO2 copolymerization.
Furthermore, the copolymerization was applied to glycidyl
phenyl ether (GPE), which provided highly crystalline
copolymers. Among the obtained copolymers, the PO/CO2
and GPE/CO2 copolymers consisted of both carbonate and
poly(PC-co-PO), in which the amount of carbonate linkages
was rather minor (17%) because of consecutive insertion of PO
without CO2 incorporation. 13C NMR analysis suggested that
carbonate linkages were distributed over the polymer chain in a
random manner rather than existing as a block. The catalytic
performance was susceptible to the 1a/[PPN]Cl molar ratio.
The copolymerization did not proceed in the absence of [PPN]
Cl (entry 2). Increasing the amount of [PPN]Cl resulted in a
Received: March 21, 2013
Published: May 28, 2013
© 2013 American Chemical Society
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Communication
a
Table 1. Copolymerization of Propylene Oxide with Carbon Dioxide Using Fe−Corrole Complexes 1−3
P
CO2
Fe
T
yield of copolymer
copolymer/
TOF for
copolymer
carbonate-linkage
Mn
e
b
c
c
d
c
e
entry complex equiv of [PPN]Cl
(MPa) (°C)
+ PC (%)
PC
content (%)
(g·mol−1
)
Mw/Mn
1
2
3
4
5
6
7
8
9
1a
0.50
0
2.0
2.0
2.0
2.0
2.0
2.0
0.50
5.0
2.0
2.0
2.0
60
60
60
60
40
80
60
60
60
60
60
51
0.1
26
3
>99/<1
>99/<1
>99/<1
34/66
1004
2
17
−
21
62
23
11
6
29000
−
18000
1.26
−
1.20
0.76
1.0
525
19
−
−
f
0.50
0.50
0.50
0.50
0.50
0.50
0.50
7
>99/<1
>99/<1
>99/<1
>99/<1
>99/<1
>99/<1
>99/<1
74
8000
51000
38000
5700
12000
30000
1.11
1.32
1.38
1.17
1.15
1.21
1.18
65
59
11
8
1314
1158
229
197
701
1209
29
24
18
g
10
11
2
3
35
60
19
39000
a
b
c
Conditions: PO (1.0 mL, 14.3 mmol) and Fe complex [(7.0−7.2) × 10−3 mmol of Fe] in a 50 mL autoclave for 1 h. Relative to Fe. Determined
d
by 1H NMR analysis of the crude product using phenanthrene as an internal standard. TOF = [(mol of carbonate repeating unit) + (mol of ether
e
f
g
repeating unit)]·(mol of Fe center)−1·h−1. Determined by size-exclusion chromatography (SEC) using a polystyrene (PS) standard. 2 h. PO (2.5
mL, 35.8 mmol), 2 h.
a
Table 2. Copolymerization of Glycidyl Phenyl Ether with Carbon Dioxide Using Fe−Corrole Complex 1a
P
CO2
time
(h)
conv. of
GPE (%)
copolymer
yield (g)
copolymer
carbonate-linkage
Mn
Tg/Tc/T
h m
(°C)
T
d5 i
de
,
d
f
g
g
entry cat. (MPa)
TOF
content (%)
m/r
(kg·mol−1
)
Mw/Mn
(°C)
d
j
j
j
j
j
1
2
3
4
5
6
1a
1a
1a
1a
4
2.0
2.0
5.0
0
1
49
20
>99
>99
>99
59
0.174
401
41
21
83
26
17
9
11
−
−
−
−
−
1.06
1.05
1.06
0.57
1.07
74/26
79/21
67/33
13.7
7.01
14.4
8.7
13/92/183
11/79/180
9/99/186
344
317
394
210
347
95
22
5.1
24
none
95
15.4
13.0
2.8
b
c
k
l
l
2.0
2.0
24
−
1.2
46/− /−
15/88/172
1a
116
>99
10
>99/<1
41
a
b
Conditions: GPE (1.1 mL, 7.4 mmol) and 1a (1.8 × 10−3 mmol) in a 50 mL autoclave at 60 °C. Conditions: GPE (1.1 mL, 7.4 mmol) and
c
d
complex 4 (3.7 × 10−3 mmol) in a 50 mL autoclave at 15 °C. (S)-GPE was used. Determined by H NMR analysis of the crude product using
1
e
f
phenanthrene as an internal standard. TOF = [(mol of carbonate repeating unit) + (mol of ether repeating unit)]·(mol of Fe center)−1·h−1. Meso/
g
h
racemo ratio in the ether diad, as determined by 13C NMR analysis. Determined by SEC using a PS standard. Glass transition temperature/
i
j
crystallization temperature (cooling process)/melting temperature as determined by DSC. Decomposition temperature of 5% weight loss. Not
determined because of difficulty in separating the monomer from the copolymer. Not determined because of the low content of ether linkages. Not
k
l
observed.
higher content of carbonate linkages (up to 62%). However, it
diminished the catalytic activity and the copolymer selectivity,
giving a large amount of PC (entries 3 and 4). The
polymerization temperature and CO2 pressure also affected
the catalytic performance: increasing the polymerization
temperature resulted in lower carbonate-linkage content and
higher catalytic activity (entries 1, 5, and 6), while higher CO2
pressure gave higher carbonate-linkage content and lower
catalytic activity (entries 1, 7, and 8).
similar to those of complex 1a [see the Supporting Information
(SI)]. In contrast, iron(III) complexes with 5,10,15,20-
tetraarylporphyrin ligands (aryl = C6F5, Ph) gave only cyclic
carbonate. Accordingly, the combination of iron with a corrole
ligand was crucial for the copolymerization.
The copolymerization of CHO with CO2 was also
successfully accomplished using the iron−corrole complexes
under the standard conditions for PO/CO2 copolymerization
(CHO/Fe/[PPN]Cl = 2000/1.0/0.50, PCO = 2.0 MPa; see the
2
Other iron−corrole complexes also copolymerized PO with
CO2. Monomeric complex 2 with chloride as a monoanionic
axial ligand demonstrated slightly lower catalytic activity than
complex 1a to give the copolymer enriched with ether linkages
(entry 10). Complex 3 without any monoanionic axial ligand
also copolymerized PO with CO2, exhibiting a high catalytic
activity comparable to that of complex 1a (entry 11). These
complexes exhibited dependences on the reaction conditions
SI). Complexes 1a, 1b, 2, and 3 all gave the copolymer
selectively without any concomitant production of cyclic
cyclohexene carbonate (CHC). The resulting copolymers
possessed high carbonate-linkage content (>90%). The highest
catalytic activity (TOF = 109 h−1) detected with 3 in 3 h was
higher than that of the previously reported dinuclear iron(III)
complex (highest TOF = 54 h−1).18 The 13C NMR spectrum
showed that the resulting copolymer was almost atactic.19
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Communication
Finally, we focused on GPE as a comonomer. The
copolymerization of CO2 with glycidyl ethers has been less
developed than those with PO and CHO, although it was
shown in the early studies that glycidyl ethers can be used as a
comonomer in the copolymerization. Recently, several glycidyl
ethers, such as allyl-, phenyl-, and methoxyethyl-substituted
ones, have been copolymerized with CO2 to produce functional
polycarbonates.20 Here we report the formation of a crystalline
GPE/CO2 copolymer with an ether-rich structure. Representa-
tive results are summarized in Table 2. Cyclic carbonate was
not detected in any entries. Under the same conditions as entry
1 of Table 1 for PO/CO2, a GPE/CO2 copolymer with both
carbonate and ether linkages in the main chain was obtained
(carbonate/ether = 9/91; Table 2, entry 1). The TOF of 401
h−1 was much lower than that for PO (1004 h−1). When the
reaction time was increased to 49 h−1, GPE was completely
consumed, affording a copolymer with a carbonate-linkage
content of 11% (entry 2). Elevating the CO2 pressure to 5.0
MPa gave a copolymer with a higher carbonate-linkage content
of 22% (entry 3).
homopolymer from racemic GPE (190 °C) was higher than
that from enantiomerically pure GPE (176 °C).21a
The iron−corrole complexes in combination with [PPN]Cl
were also applicable to homopolymerization of PO (see the SI).
In particular, iron(III) complex 3 demonstrated high activity
(TOF of up to 685 h−1), providing a polymer with Mn = 95
kg·mol−1 and Mw/Mn = 1.22 in 2 h at 25 °C. The 13C NMR
spectrum showed that the obtained poly(PO) possessed a
regioregular and slightly iso-enriched structure (see the SI).22
In conclusion, iron−corrole complexes were found to
copolymerize epoxides with CO2, providing the first example
of iron-catalyzed PO/CO2 and GPE/CO2 copolymerization. In
particular, the GPE/CO2 copolymers were found to be
crystalline materials because of the existence of isotactic
poly(GPE) units.
ASSOCIATED CONTENT
* Supporting Information
■
S
Full experimental procedures for the preparation of Fe−corrole
complexes and the polymerizations; results of PO/CO2 and
CHO/CO2 copolymerizations and PO homopolymerization;
and NMR spectra, DSC curves, and TGA thermograms of the
copolymers. This material is available free of charge via the
Internet at http://pubs.acs.org.
The products thus obtained were further characterized by
differential scanning calorimetry (DSC) and 13C NMR analysis.
The data for Table 2, entry 3 are shown in Figure 2 as a
AUTHOR INFORMATION
Corresponding Author
nozaki@chembio.t.u-tokyo.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Funding Program for World-
Leading Innovative R&D on Science and Technology from the
Japan Society for the Promotion of Science (JSPS).
Figure 2. (left) Methylene region of the 13C NMR spectrum and
(right) DSC curve of the copolymer from Table 2, entry 3.
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