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Bifunctional cobalt Salen complex a highly selective catalyst for the coupling of CO2 and epoxides under mild conditions.

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Received: 17 November 2010
Revised: 16 December 2010
Accepted: 30 December 2010
Published online in Wiley Online Library: 20 April 2011
( DOI 10.1002/aoc.1781
Bifunctional cobalt Salen complex: a highly
selective catalyst for the coupling of CO2
and epoxides under mild conditions
Hongchun Li and Yongsheng Niu∗
A bifunctional cobalt Salen complex containing a Lewis acid metal center and two covalent bonded Lewis bases on the ligand
was designed and used for the coupling of CO2 and epoxides under mild conditions. The complex exhibited excellent activity
(turnover frequency = 673/h) and selectivity (no less than 97%) for polymer formation in the copolymerization of propylene
oxide (PO) and CO2 at an appropriate combination of all variables. High molecular weight of 110 200 and yield 99% were
achieved at a higher [PO]/[complex] ratio of 6000 : 1. The complex also worked satisfactorily for the terpolymerization of CO2 ,
PO and cyclohexene oxide (CHO), without formation of cyclic carbonate or ether linkages to give the polycarbonate. High
c 2011
cyclohexene carbonate unit content in the CO2 /PO/CHO terpolymers resulted in enhanced thermal stability. Copyright John Wiley & Sons, Ltd.
Keywords: cobalt catalysts; carbon dioxide; copolymerization; polycarbonates; terpolymerization
The copolymerization of carbon dioxide (CO2 ) with epoxides has
been intensively studied in recent decades as one of the most
promising processes for fixation of CO2 .[1] After the pioneering
work of Inoue and co-workers in the late 1960s,[2] a wide variety of
metal catalysts have been developed to promote this process.[3 – 14]
For metal complex catalysts, cobalt Schiff base complexes in
conjunction with Lewis base or organic salts as co-catalysts
have been of significant interest.[7,8,11 – 19] The most prominent
advantages of the complexes are easy synthesis and good stability
against moisture and air.[1,6] On the other hand, these systems are
suitable for controlling the molecular weight and the distribution
of the produced polycarbonates as well as regulating selective
formation of the alternating copolymer.[16,18 – 21] The binary
catalyst system is not highly active at high monomer/catalyst
mole ratios, which prevents it from being commercially viable.[15]
Single cobalt Schiff base complexes show a moderate catalytic
activity only at high pressure (up to 5.5 MPa).[7,13] However, these
complexes are inactive at low CO2 pressures (less than 1.5 MPa).[7,9]
A Salen cobaltate complex {acetate-[[2,2 -][(1,2-cyclohexanediyl) bis(nitrilomethylidyne)-bis[4-(methylene-N-piperidino)-6(1,1-dimethylethyl)phenolato]]-[N,N ,O,O ]cobalt(III)} containing a
piperidinium unit as a pendant group was designed to provide a
new unique cobalt Salen complex-based catalyst for the propylene oxide (PO)/CO2 copolymerization to enhance the selectivity
for polycarbonate over cyclic carbonate at high conversions.[21,22]
On the other hand, the Salen ligand structure plays an important role in catalyst activity and regioselectivity.[7,13] Coates and
coworkers reported that the groups at the para and ortho positions play an important role in controlling the order of the catalytic
activity and regioselectivity.[18]
Considering the Salen ligand structure and the function of the
Lewis base in epoxide and CO2 copolymerization, a bifunctional
cobalt Salen complex containing a Lewis acidic cobalt center and
Appl. Organometal. Chem. 2011, 25, 424–428
two Lewis base units in one molecule was designed and used for
the coupling of CO2 and epoxides, shown in Fig. 1.
All manipulations involving air- and/or water-sensitive compounds were carried out using standard Schlenk techniques
under dry argon. Propylene oxide (PO) and cyclohexene oxide (CHO) were purchased from Acros company and distilled
under a nitrogen atmosphere from CaH2 prior to use. CO2
(99.9995%) was purchased from Qingdao Institute of Special
Gases and used as received. Diethyl ether was purchased
from Alfa Aesar and distilled from sodium benzophenone.
Methylene chloride, hexane and chloroform were purchased
from Sigma-Aldrich Company and distilled from CaH2 under argon. N,N -[2,2 -bis(nitrilomethylidyne)]bis[4-(methylene-Nmorpholino)-6-tert-butyl phenolato]-1,2-cyclohexanediamine was
prepared according to the literature method.[22b]
and 13 C NMR spectra were recorded on a Varian Inova400 MHz-type (1 H, 400 MHz) and a Bruker 400 MHz-type (13 C,
100 MHz) spectrometer, respectively. Their peak frequencies were
referenced vs an internal standard (Tetramethylsilane) shift at
0 ppm for 1 H NMR and against the solvent, chloroform-d,
Correspondence to: Yongsheng Niu, College of Chemistry and Pharmacy,
Qingdao Agricultural University, Qingdao 266109, China.
College of Chemistry and Pharmacy, Qingdao Agricultural University, Qingdao
266109, China
c 2011 John Wiley & Sons, Ltd.
Copyright The coupling of CO2 and epoxides
Figure 1. Structue of complex 1.
at 77.0 ppm for 13 C NMR. The glass transition temperatures
(Tg ) were determined using a DSC instrument (model Netzsch
204) at a heating rate of 10 ◦ C/min under nitrogen flow of
100 ml/min. The reported Tg values were recorded from the
second scan after first heating and quenching. Gel permeation
chromatography (GPC) measurements were carried out with a
Waters instrument (515 HPLC pump) equipped with a Wyatt
interferometric refractometer. GPC columns were eluted with
THF at 25 ◦ C at 1 ml/min. The molecular weights were calibrated
against polystyrene standards. Element analyses of C, H, N were
determined by the service Elementar Vario EL. The experiment
was performed on a Micromass Q-Tof (Micromass, Wythenshawe,
UK) mass spectrometer equipped with an orthogonal electrospray
source (Z-spray) operated in positive and negative ion mode.
Synthesis of N,N -[2,2 -bis(nitrilomethylidyne)]bis
[4-(methylene-N-morpholino)-6-tert-butyl phenolato]-1,
2-cyclohexanediamino Cobalt(II) (SalenCoII Complex)
The compound was synthesized according to the method
described in the literature[24] with some modifications as
follows. A solution of cobalt acetate tetrahydrate (0.75 g,
3.0 mmol) in methanol (200 ml) was added to a solution
of the ligand, N,N -[2,2 -bis(nitrilomethylidyne)]bis[4-(methyleneN-morpholino)-6-tert-butyl phenolato]-1,2-cyclohexanediamine
(2.07 g, 3.0 mmol), in CH2 Cl2 (20 ml) via a cannula under an atmosphere of argon. A brick-red precipitate was observed before all
of the cobalt acetate solution was added. The residue on the wall
of the reaction flask was rinsed with methanol (20 ml), and the
collected mixture was allowed to stir for further 15 min at room
temperature, and then for 30 min at 0 ◦ C. The solids were collected
by filtration and rinsed with cold (0 ◦ C) methanol (3×50 ml), before
drying at 60 ◦ C in vacuum for 24 h. Yield was 93%.
Synthesis of N,N -[2,2 -bis(nitrilomethylidyne)]bis
[4-(methylene-N-morpholino)-6-tert-butyl phenolato]-1,
2-cyclohexanediamino CobaltIII (2,4-Dinitrophenolato)
(Complex 1)
Appl. Organometal. Chem. 2011, 25, 424–428
Representative Copolymerization Procedure
A 250 ml Parr autoclave was heated to 120 ◦ C under vacuum
for 4 h, and then cooled under vacuum to room temperature.
Complex 1 was dissolved in PO under CO2 atmosphere. The
solution was stirred about 10 min and then injected into the
autoclave equipped with a magnetic stirrer under 0.1 MPa CO2 .
The autoclave was heated to the desired temperature in an oil
bath. Then the system was pressurized to 1.5 MPa CO2 . The mixture
was stirred for the allotted reaction time, then cooled to room
temperature and vented in a fume hood. A small aliquot of the
resultant polymerization mixture was removed from the reactor
for 1 H NMR analysis. The remaining polymerization mixture was
then dissolved in chloroform, quenched with 5% HCl solution
in methanol, and precipitated from methanol. The polymer was
collected and dried in vacuo to constant weight, and the polymer
yield was determined.
Results and Discussion
This research was initiated by testing the potential catalytic
activity of complex 1 in the copolymerization of CO2 and
PO. Complex 1 was prepared by the in situ reaction of 1
equivalent of cobalt acetate tetrahydrate with one equivalent
of ligand N,N -[2,2 -bis(nitrilomethylidyne)]bis[4-(methylene-Nmorpholino)-6-tert-butyl phenolato]-1,2-cyclohexanediamine and
subsequent oxidation in the presence of 1 equivalent of 2,
4-dinitrophenol and oxygen, and the product was sufficiently
dried under reduced pressure. Although the exact structure of
complex 1 has not been confirmed yet owing to the difficulties associated with its isolation and structural characterization,
we recently reported that cobalt 2,4-dinitrophenolate based on
a N,N ,O,O -tetradentate Schiff base ligand framework was a sixcoordinated central cobalt octahedron in the solid.[24] Compared
with the structure, complex 1 should be a stable six-coordinate
c 2011 John Wiley & Sons, Ltd.
To a stirred mixture of SalenCoII complex (1.3 g, 1.5 mmol) in CH2 Cl2
(150 ml), the mixture of 2,4-dinitrophenol (0.276 g, 1.5 mmol, 1
equivalent) and CH2 Cl2 (20 ml) was added. The solution was
stirred under dry oxygen at room temperature for 60 min. The
solvents were removed in vacuo to leave a crude green dark solid
in near-100% yield. The resulted solid was further treated with the
mixture of diethyl ether and hexane, and then dried at 60 ◦ C in
vacuum for 24 h. Yield was 89%. Elemental analysis calcd (%) for
C44 H57 CoN6 O9 :C, 60.54; H, 6.58; N, 9.63. Found: C, 60.45; H, 6.34; N,
9.45. UV–vis: 260, 333, and 413 nm.
H. Li and Y. Niu
Table 1. The coupling of epoxides and CO2 catalyzed by complex 1a
(h−1 )
Mn e
(g mol−1 )
(Mw /Mn )
Tg f
(◦ C)
33 200
12 500
48 600
87 100
110 200
52 300
60 200
68 700
76 500
Reaction conditions: ncomplex 1 = 2.52 × 10−3 mmol, VDME = 1.0 ml, at 25 ◦ C (entries 1 and 3–9) or 60 ◦ C (entry 2) with 1.5 MPa of CO2 .
Based on isolated PPC yield.
Turnover frequency of epoxide to products (polycarbonate and cyclic carbonate).
d Determined on the basis of 1 H NMR spectroscopy of the crude product.
e Determined by GPC.
f Determined by DSC.
Figure 2. 1 H NMR spectrum of PPC from the alternating copolymerization of CO2 /PO sample produced by complex 1 (entry 1 in
Table 1).
central cobalt octahedron. The key to the catalyst design lies in two
Lewis base units as phenolate para substituents on complex 1 to
stabilize the active SalenCoIII 2,4-dinitrophenolate (Fig. 1) species
against decomposition to SalenCoII , which were ineffective in
producing copolymer.
Complex 1 efficiently copolymerized CO2 and PO with various
reaction conditions, as shown in Table 1. The turnover frequency
(TOF) of 116/h was attained by running the polymerization under
1.5 MPa CO2 at 25 ◦ C in 1,2-dimethoxyethane (DME) for 10 h.
Poly(propylene carbonate) (PPC) in 58% polymer yield and a small
amount of propylene carbonate (PC) byproducts were obtained
(PPC/PC > 99 : 1, Mn = 33 200, polydispersity indices (PDI) = 1.12;
Table 1, entry 1). The resulting PPC shows nearly perfect alternating
carbonate linkage in the 1 H NMR spectrum (Fig. 2). This is different
from the previously reported (salcy)CoIII X [salcy = N,N -bis(3,5di-tert-butylsalicylidene)-1,2-diaminocyclohexane; X = halide or
carboxylate] complexes as lone catalysts, which were inactive in
the same copolymerization at low CO2 pressures and reduced to
Co(II) derivatives as a red solid precipitate.[13,16] This is attributed
to the Lewis base units on the ligand helping to stabilize the active
SalenCoIII 2,4-dinitrophenolate and prevent decomposition to the
inactive SalenCoII .
The copolymerization of PO with CO2 using the reported
binary catalyst systems (salcy)CoIII X and Lewis base (or organic
salts) decreases the selectivity of PPC/PC at elevated reaction
temperature.[7] A high level of activity together with suppression of
PC formation and high conversion of PPC were accomplished using
complex 1 at 60 ◦ C (TOF = 673/h, PPC/PC = 98 : 1, Mn = 12 500,
PDI = 1.08; Table 1, entry 2). Complex 1 exhibited high thermal
stability. It is essential that the Lewis base units are tethered to the
ligand framework.
The high selectivity of PPC/PC at 25 ◦ C was maintained after
a longer reaction time to attain quantitative formation of the
alternating copolymer. The alternating copolymeriation of CO2
and PO using complex 1 was carried out at 25 ◦ C for 30 h and
afforded the alternating PPC almost quantitatively (PPC/PC = 99,
yield = 99%; Table 1, entry 3). The prolonged reaction did not
lead to an observable decrease in selectivity for PPC formation,
which can be explained by the ‘solvent effect’. A similar solvent
effect was reported for the copolymerization with binary catalyst
systems.[8,23] In order to investigate further the catalyst lifetime,
we should be able to reach the same level of conversion at a higher
[PO]/[complex 1] ratio by running the reaction for a longer time.
Increasing the [PO]/[complex 1] ratio from 20 000 : 1 to 60 000 : 1
did not reduce the catalytic performance of complex 1 (TOF =
120/h; Table 1, entry 5). Therefore, complex 1 was not deactivated
during the polymerization.
The Mn value of the alternating copolymer (48 600) produced by
the copolymerization of CO2 and PO by complex 1 ([PO]/[complex
1] = 20 000:1, yield = 99%; Table 1, entry 3) was much lower
than the theoretical value (205 000) on the assumption that
all the molecules of complex 1 participated in initiation and
propagation; these results are consistent with chain transfer
to water throughout the copolymerization. This works as a
bifunctional initiating group to give a telechelic polymer, as
previously demonstrated by Sugimoto et al.[8] GPC analyses of the
obtained copolymers gave bimodal distribution traces, although
the PDIs remained narrow (PDI < 1.18). Similar results have been
reported before.[8,13,20] To attain higher Mn value of the copolymer,
the copolymerization was tried at a higher [PO]/[complex 1]
ratio of 6000 : 1. With increasing reaction time up to 50 h, the
molecular weight reached 110 200 and the selectivity of PPC/PC
c 2011 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2011, 25, 424–428
The coupling of CO2 and epoxides
Figure 3. 13 C spectrum of the carbonyl region for PPC, (A) 1 h, (B) 10 h and
(C) 50 h.
Figure 4. 1 H NMR spectra of the PO/CHO/CO2 terpolymer.
Scheme 1. Proposed mechanism for the copolymerization of CO2 and PO.
Appl. Organometal. Chem. 2011, 25, 424–428
In conclusion, the tetradentate Schiff-base cobalt complex with
two Lewis base units is a novel catalyst for the alternating
copolymerization of CO2 and PO. The copolymerization selectively produced the alternating copolymer without a significant
amount of PC. In addition, the terpolymerization of CO2 /PO/CHO
was also realized with complex 1 without the formation of cyclic
carbonates or ether linkages. The resulting terpolymers had one
adjustable glass-transition temperature depending on cyclohexene carbonate unit content. The high cyclohexene carbonate unit
content in the CO2 /PO/CHO terpolymers resulted in enhanced
thermal stability.
The support by the Young Scientists Fund of the National
Natural Science Foundation of China (grant no. 51003051) and
the High-level Talent Initial Funding for Scientific Research of
Qingdao Agricultural University (grant no. 630924) is gratefully
c 2011 John Wiley & Sons, Ltd.
did not lead to an observable decrease (PPC/PC = 97 : 1; Table 1,
entry 5).
To gain insight into the effect of reaction time in the
region-regularity of PPC, the carbonate regions of the 13 C NMR
spectra of PPC are recorded in Fig. 3. There are three groups of
peaks located at 155.1–155.0, 154.7–154.5 and 154.3–154.0 ppm,
corresponding to head-to-head (HH), tail-to-tail (TT) and headto-tail (HT) structure in the copolymer, respectively.[25] The
copolymerization of CO2 and PO under complex 1 produced
PPC with HT linkages of 95.6% (Fig. 3A) at 1h. When the reaction
time was increased 50 h, HT linkages decreased to 92.3% (Fig. 3B).
Figure 3 shows that a change in the reaction time from 1 to
50 h only caused slight decreases in the enantioselectivity and
microstructure of the resulting polymer.
Scheme 1 shows the proposed mechanism for the alternating
copolymerization of PO and CO2 using complex 1 based on
our copolymerization result. The CO2 monomer might be first
inserted favorably to the Co-O chemical bond of the Schiff
base cobalt complex to yield carbonate complex, followed by
the insertion of PO monomer to produce alkoxide complex.
Subsequent alternating enchainment of PO and CO2 afforded the
alternating polycarbonate. A similar mechanism was proposed by
Coates and co-workers using β-diiminate zinc complexes in the
copolymerization of CO2 and CHO.[17,26]
Thermal properties of the PPC were determined by DSC and
TGA techniques. Glass transition temperature (Tg ) was detected
to be 42 ◦ C in the DSC run under a nitrogen atmosphere, and
thermal degradation temperature appeared to be 317 ◦ C in
the Thermogravimetric (TG) run under a nitrogen atmosphere.
Both results demonstrate good thermal properties for the
produced PPC.
We were interested in the possibility of exploring complex 1
for the terpolymerization of CO2 with PO and CHO. Complex 1
efficiently terpolymerized CO2 , PO and CHO with various feed
ratios of PO and CHO (Table 1, entries 6–8). No signals were
observed at 3–4 ppm in the 1 HNMR spectra of the polymers
(Fig. 4), which indicated strictly alternating placement of CO2 after
PO or CHO with no ether linkages.[27] In the 1 H NMR of terpolymer
the resonances in the regions of 5.01 ppm of the two peaks can be
assigned to CH in propylene carbonate unit and the resonances
in the 4.68 ppm should be from CH in cyclohexene carbonate
unit (Fig. 4).[20] Additionally, terpolymerization by changing the
PO/CHO mole ratio gave different contents of cyclohexene
carbonate linkages and propylene carbonate linkages in the
terpolymer. Notably, the Tg was adjustable between 40 and 118 ◦ C
by controlling the relative proportions of PO and CHO.
H. Li and Y. Niu
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