Mechanistic aspects of the copolymerization of CO2 with epoxides using a thermally stable single-site cobalt(III) catalyst

Article abstract of DOI:10.1021/ja9033999

The mechanism of the copolymerization of CO₂ and epoxides to afford the corresponding polycarbonates catalyzed by a highly active and thermally stable cobalt(III) complex with 1,5,7-triabicyclo[4,4,0] dec-5-ene (designated as TBD, a sterically

Full text of DOI:10.1021/ja9033999

Published on Web 07/22/2009
Mechanistic Aspects of the Copolymerization of CO2 with
Epoxides Using a Thermally Stable Single-Site Cobalt(III)
Catalyst
Wei-Min Ren, Zhong-Wen Liu, Ye-Qian Wen, Rong Zhang, and Xiao-Bing Lu*
State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, Dalian 116012, China
Received May 6, 2009; E-mail: lxb-1999@163.com
Abstract: The mechanism of the copolymerization of CO2 and epoxides to afford the corresponding
polycarbonates catalyzed by a highly active and thermally stable cobalt(III) complex with 1,5,7-
triabicyclo[4,4,0] dec-5-ene (designated as TBD, a sterically hindered organic base) anchored on the ligand
framework has been studied by means of electrospray ionization mass spectrometry (ESI-MS) and Fourier
transform infrared spectroscopy (FTIR). The single-site, cobalt-based catalyst exhibited excellent activity
and selectivity for polymer formation during CO2/propylene oxide (PO) copolymerization even at temperatures
up to 100 °C and high [epoxide]/[catalyst] ratios, and/or low CO2 pressures. The anchored TBD on the
ligand framework plays an important role in maintaining thermal stability and high activity of the catalyst.
ESI-MS and FTIR studies, in combination with some control experiments, confirmed the formation of the
carboxylate intermediate with regard to the anchored TBD on the catalyst ligand framework. This analysis
demonstrated that the formed carboxylate intermediate helped to stabilize the active Co(III) species against
decomposition to inactive Co(II) by reversibly intramolecular Co-O bond formation and dissociation. Previous
studies of binary catalyst systems based on Co(III)-Salen complexes did not address the role of these
nucleophilic cocatalysts in stabilizing active Co(III) species during the copolymerization. The present study
provides a new mechanistic understanding of these binary catalyst systems in which alternating chain-
growth and dissociation of propagating carboxylate species derived from the nucleophilic axial anion and
the nucleophilic cocatalyst take turns at both sides of the Co(III)-Salen center. This significantly increases
the reaction rate and also helps to stabilize the active SalenCo(III) against decomposition to inactive
SalenCo(II) even at low CO2 pressures and/or relatively high temperatures.
temperature was observed in the resulting polycarbonates.12
Prior to these studies of (salen)CoX catalysts, Coates and co-
Introduction
The catalytic transformation of carbon dioxide into biode-
gradable polycarbonates by the alternating copolymerization
with epoxides1 has attracted much attention in the last decades
due to the economic and environmental benefits arising from
the utilization of renewable source and the growing concern on
the greenhouse effect. Numerous catalyst systems have been
developed for this transformation.2-7 Prominent among these
are the binary or bifunctional catalyst systems based on
cobalt-Salen complexes. These systems provides for efficient
CO2/epoxides coupling reactions even under mild conditions
and in some cases allow for the regio- and/or stereoselective
polymerization,8-11 in which the enhanced glass transition
workers were the first to report the use of chiral SalcyCoX (salcy
) N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanedi-
amine) complexes with a nucleophilic axial X group alone as
catalysts for CO2/propylene oxide (PO) copolymerization. This
approach produced poly(propylene carbonate) (PPC) with >99%
selectivity and with 90-99% carbonate linkages at a CO2
pressure of 55 bar and ambient temperature.13
Although simple SalenCo(III)X complexes alone could
catalyze the copolymerization of CO2 and epoxides to yield the
corresponding polycarbonates with high selectivity at very high
(3) (a) Darensbourg, D. J.; Holtcamp, M. W. Macromolecules 1995, 28,
7577–7579. (b) Darensbourg, D. J.; Holtcamp, M. W.; Struck, G. E.;
Zimmer, M. S.; Niezgoda, S. A.; Rainey, P.; Roberston, J. B.; Draper,
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(1) For recent reviews on CO2/epoxides copolymerization, see: (a) Coates,
G. W.; Moore, D. R. Angew. Chem., Int. Ed. 2004, 43, 6618–6639.
(b) Darensbourg, D. J.; Mackiewicz, R. M.; Phelps, A. L.; Billodeaux,
D. R. Acc. Chem. Res. 2004, 37, 836–844. (c) Sugimoto, H.; Inoue,
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A R T I C L E S
Ren et al.
CO2 pressure (up to 5.5 MPa), these cobalt complexes were
proved to be inactive under low CO2 pressures or at elevated
temperature (∼50 °C) and unexpectedly reduced to Co(II)
derivatives as red solid precipitates, which were ineffective in
producing copolymer. The addition of a nucleophilic cocatalyst
such as quaternary ammonium halides or sterically hindered
strong organic bases could significantly improve the activity
even at low CO2 pressures.8-10 The nucleophilic cocatalysts
were demonstrated to function as an initiator, as well as
tentatively thought to stabilize the active SalenCo(III) against
decomposition to SalenCo(II).8,9 In our opinion, the latter role
may be more important in maintaining high activity of these
Co(III) complexes. Except for this experimental observation,
we did not have any direct evidence concerning the role of these
nucleophilic cocatalysts in stabilizing active Co(III) species
during the copolymerization of CO2 and epoxides.
SalenCoX in conjunction with a quaternary ammonium salt for
CO2/epoxides copolymerization, Lee et al. reported an elegant
design of catalyst system 1. This system contains a Lewis acidic
metal center and quaternary ammonium salt units in a molecule
that has the highest activity for polycarbonoate synthesis even
at high temperatures and high [epoxide]/[catalyst] ratios.15 The
high catalytic activity and polymer selectivity at high temper-
atures was thought to result from the chain-growing carbonate
unit attracted to the metal center through Coulombic interaction
between the quaternary ammonium cation anchored on the
ligand framework and the chain-growing anion. Further sub-
stitution of a small methyl group for the bulky tert-butyl group
on the 3-position of the aromatic rings significantly enhanced
activity, and thereby resulting in the discovery of the fastest
catalyst to date.15b Prior to this study, Nozaki and co-workers
utilized a Salcy-type cobaltate complex 2 with a piperidinium
end-capping arm as catalyst for selectively synthesizing aliphatic
polycarbonates from CO2 and terminal epoxides.16 The high
polymer selectivity was achieved based on the proposal that
the piperidinium arm controls the formation of cyclic carbonates
by protonating the anionic propagating species when they
dissociate from the cobalt center. Notably, complete consump-
tion of the epoxide was accomplished with high copolymer
selectivity, which allowed for the production of block terpoly-
mers by stepwise addition of two different aliphatic epoxides.
Although this cobalt complex was active at 60 °C, there was a
concomitant production of cyclic carbonates in the final products
(up to 10-40%).15,16
Since the coupling reaction of CO2 with epoxides is exo-
thermic, the development of a thermally robust catalyst is
industrially desirable. Five-membered cyclic carbonates are more
thermodynamically stable than the corresponding polycarbon-
ates. In addition, the difference in the energies of activation for
cyclic carbonate versus copolymer formation in the terminal
epoxides such as PO is very small.14 As a result, the concomitant
production of cyclic carbonate easily occurs in the coupling
reaction carried out at elevated temperatures. Recently, based
on the mechanistic understanding of binary catalyst systems of
(5) (a) Super, M.; Berluche, E.; Costello, C.; Beckman, E. Macromolecules
1997, 30, 368–372. (b) Super, M.; Beckman, E. J. Macromol. Symp.
1998, 127, 89–108. (c) Ree, M.; Bae, J. Y.; Jung, J. H.; Shin, T. J. J.
Polym. Sci., Part A: Polym. Chem. 1999, 37, 1863–1876. (d) Nozaki,
K.; Nakano, K.; Hiyama, T. J. Am. Chem. Soc. 1999, 121, 11008–
11009. (e) Nakano, K.; Nozaki, K.; Hiyama, T. J. Am. Chem. Soc.
2003, 125, 5501–5510. (f) Eberhardt, R.; Allmendinger, M.; Zintl,
M.; Troll, C.; Luinstra, G. A.; Riger, B. Macromol. Chem. Phys. 2004,
205, 42–47. (g) Lee, B. Y.; Kwon, H. Y.; Lee, S. Y.; Na, S. J.; Han,
S.-i.; Yun, H.; Lee, H.; Park, Y.-W. J. Am. Chem. Soc. 2005, 127,
3031–3037. (h) Kro¨ger, M.; Folli, C.; Walter, O.; Do¨ring, M. AdV.
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Reung, P. T. R.; Williams, C. K. Angew. Chem., Int. Ed. 2009, 48,
931–933.
We have reported that the addition of a nucleophilic cocatalyst
could significantly improve the activity of Co(III)-Salen
complexes even at low CO2 pressures and have confirmed its
initiator role during CO2/epoxides copolymerization.8 Although
these studies resulted in the discovery of some highly active
catalyst systems,15 no attempt was made to investigate the
mechanistic properties of these nucleophilic cocatalysts for
stabilizing active Co(III) species during the copolymerization.
Herein, we report a highly active and thermally stable cobalt-
based catalyst with 1,5,7-triabicyclo[4,4,0] dec-5-ene (designated
as TBD, a sterically hindered organic base) anchored on the
ligand framework. This report will provide details of mechanistic
studies on the role of the anchored TBD in maintaining high
activity and thermal stability of the catalyst during the copo-
lymerization of CO2 and epoxides. These mechanistic studies
are beneficial to elucidating the propagating polymer-chain anion
with regard to nucleophilic cocatalysts in previously much-
studied binary catalyst systems for stabilizing active Co(III)
(6) (a) Chen, X. H.; Shen, Z. Q.; Zhang, Y. F. Macromolecules 1991, 24,
5305–5308. (b) Hsu, T. J.; Tan, C. S. Macromolecules 1997, 30, 3147–
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X. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 5050–5056.
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Mechanistic Aspects of the Copolymerization of CO2 with Epoxides
A R T I C L E S
Table 1. CO2/Propylene Oxide Copolymerizationa
entry
catalyst
PO/catalyst molar ratio
temp (°C)
pressure (MPa)
time (h)
TOFb (h-1
)
selectivityc (% polymer)
Mnd(kg/mol)
PDId (Mw/Mn)
1
2
3
4
5
6
7
8
3a
3a
3a
3a
3a
3b
3c
3b
3b
3b
3b
3b
1
5000
10000
20000
5000
5000
10000
5000
10000
10000
10000
25000
10000
25000
25000
10000
10000
10000
10000
25
25
25
25
25
25
25
50
80
90
90
100
90
80
80
25
25
25
1.5
1.5
1.5
0.6
0.1
1.5
1.5
1.5
2.0
2.0
2.0
2.5
2.0
2.0
2.0
1.5
1.5
1.5
5.0
6.0
6.0
5.0
5.0
4.0
6.0
2.0
0.5
0.25
1.0
0.25
1.0
3.0
0.5
6.0
6.0
6.0
432
410
425
428
265
612
13
1923
6290
8235
7139
10882
3500
370
3863
25
>99
>99
>99
>99
>99
>99
70
>99
98
98
97
97
90
60
95
64
0
85
112.4
100.8
114.6
121.6
73.1
134.6
-
148.7
101.2
85.4
112.3
60.2
80.0
-
1.09
1.05
1.06
1.08
1.10
1.09
-
1.12
1.18
1.15
1.18
1.23
1.32
-
9
10
11
12
13e
14e
15
16
17
18
2
4
5
6
62.3
-
-
1.29
-
-
7
41
7
7.8
1.20
a The reaction was performed in neat propylene oxide (PO) (14 mL, 200 mmol) in 75 mL autoclave. Carbonate linkages of the resulted
polycarbonates are >99% based on 1H NMR spectroscopy. b Turnover frequency of PO to products (polycarbonate and cyclic carbonate). c Determined
using H NMR spectroscopy. d Determined by gel permeation chromatography in THF, calibrated with polystyrene standards. e Data from ref 15a.
1
species, and provide insight as to how these cobalt-based catalyst
systems operate during the copolymerization.
framework, could effectively catalyze CO2/PO copolymerization
to selectively afford the corresponding copolymer with more
than 99% carbonate linkages at a high [epoxide]/[catalyst] ratio
of 5000 under a CO2 pressure of 1.5 MPa and at ambient
temperature (Table 1, entry 1). The catalyst activity is about 6
times that of our previously reported binary catalyst system of
the complex 8a in conjunction with 7-methyl-1,5,7-triazabicy-
clo[4.4.0] dec-5-ene (MTBD) under the same conditions in
which the initiator role of MTBD was confirmed by electrospray
ionization mass spectrometry (ESI-MS).8b With further increase
in the molar ratio of epoxide to catalyst, no obvious change in
copolymerizaiton rate and polymer selectivity was observed in
this reaction (entries 2 and 3). The isolated polymers have
narrow molecular-weight distributions less than 1.1, but the Mn
values are not close to the expected values, which indicate the
existence of chain transfer during the reaction.17 Indeed,
although every effort has been made to keep the copolymeri-
zation reaction anhydrous, we were concerned that trace
quantities of water might be present which could have a
deleterious effect on the reaction rate.2g,18 Furthermore, in the
presence of an organic solvent such as 1,2-dimethoxyethane,
quantitative conversion of the epoxide was achieved with high
polymer selectivity by a prolonged reaction time. Although the
Mn values are not close to the expected values due to chain
transfer caused by trace quantities of water in the copolymer-
ization system, the chain transfer does not result in chain
termination and the dissociated chain can repropagate when
Results and Discussion
The unsymmetrical Schiff-base ligands in the complexes 3-6
were synthesized by the reaction of the corresponding functional
salicyaldehydes with the condensation product of 1,2-diami-
nocyclohexane mono(hydrogen chloride) and 3,5-di-tert-butyl-
2-hydroxybenzaldehyde (detailed synthesis procedure, see Sup-
porting Information). Since these cobalt complexes 3-8 are
easily dissolved in neat epoxides surveyed, the catalyzed
coupling of CO2 and epoxides does not require any organic
cosolvent. Only in some cases for complete conversion of
epoxides, the addition of an organic solvent such as 1,2-
dimethoxyethane is necessary for effective diffusion of the
reactants.
(17) Although PO was refluxed over a mixture of KOH/CaH2 for more
than 10 h, the water content in the epoxide was in the range of 20-
25 ppm, which was determined by means of Karl-Fischer coulometry.
In most experiments, the resulted copolymers exhibited bimodal
distribution, but with a prolonged reaction time, the proportion of the
higher-molecular-weight copolymers decreased and the PDIs became
narrower.
Our initial studies showed that 3a, a cobalt-based complex
with an anchored sterically hindered TBD on its ligand
(18) Darensbourg, D. J.; Mackiewicz, R. M. J. Am. Chem. Soc. 2005, 127,
14026–14038.
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J. AM. CHEM. SOC. VOL. 131, NO. 32, 2009 11511

A R T I C L E S
Ren et al.
Scheme 1. The Copolymerization of CO2 and Cyclohexene Oxide
(CHO) Catalyzed by the Complex 3a at Various Conditionsa
activated on a catalyst center. The polymer molecular weight
is linearly proportional to the conversion, and its distribution is
less than 1.1. This is consistent with an immortal polymerization
(a living polymerization that involves rapid and reversible chain
transfer), which allows for the production of block terpolymers
by stepwise addition of two different epoxides.2c,16 Noteworthy
successes were realized for the synthesis of various block
terpolymers without sacrificing polymer selectivity, which will
be published elsewhere.
a All polymerizations were performed in neat epoxide with [CHO]/[Co]
) 5000. Turnover frequency: moles of CHO consumed per mole of metal
per hour. Carbonate linkages of the resulted polycarbonates are >99% based
We were gratified to discover that the catalyst system can
operate at high efficiency with CO2 at atmospheric pressure,
with a turnover frequency (TOF) up to 265 h-1 at 25 °C.
Significantly, the variation of CO2 pressure does not lead to an
observable decrease in selectivity for PPC formation (entries 4
and 5). This is distinct from the previously reported catalyst
systems using zinc or chromium complexes,7,19 in which an
increase in the pressure of CO2 is beneficial for suppressing
cyclic carbonate formation and effectively increases the selectiv-
ity for polymer while moderately decreasing the catalyst activity
for PPC formation. The nucleophilicity of the axial anion of
cobalt(III) complexes has a significant influence on CO2/PO
copolymerization (entries 2, 6, and 7). A change in the axial
anion from NO3- to more nucleophilic CH3COO- increases the
TOF from 410 to 612 h-1 at a [PO]/[catalyst] ratio of 10000 at
1
on H NMR spectroscopy.
system with the use of 3a as catalyst at various temperatures.
Interestingly, the addition of trace amounts of PO resulted in
rapid conversion of CHO to the corresponding polycarbonate
even at ambient temperature and low CO2 pressure (Scheme
1). The completely alternating nature (>99% carbonate linkages)
of the resulting polymer was reflected in the presence of the
methine proton peak in the cyclohexene carbonate unit at a
chemical shift of 4.65 ppm and the absence of that peak in the
repeating oxy(1,2-cyclohexene) unit (ether linkage) at 3.45 ppm
1
in the H NMR spectra. Both samples from the initial period
-
and after the reaction were analyzed by ESI-MS. No propagating
polymer species consistent with TBD anchored on the ligand
framework of 3a was found and only one species of m/z 784.5
was clearly observed in the positive-mode ESI tandem mass
spectra of all tested samples. When the complex 3a was first
treated with PO followed by the complete removal of the
residual PO from the 3a/PO mixture system via vacuum, a
species of m/z 784.5 was observed in the mass spectra. The
resulting solid was found to be more efficient for catalyzing
CO2/CHO copolymerization to selectively produce the corre-
sponding polycarbonates with >99% carbonate linkages and high
catalytic activities up to 220 h-1 (TOF) at ambient temperature
and 3079 h-1 at 90 °C. This was achieved without sacrificing
polymer selectivity (Scheme 1), and proved to be the highest
record in this polymerization.21
Advances in polymer chemistry rely not only on revolutionary
catalyst discoveries, but also a detailed analysis of the catalytic
mechanism and its refinement.22 This research can provide
considerable insight concerning catalyst design and modification
and therefore help in the development of commercial applica-
tions. The production of polycarbonates from CO2/epoxides
copolymerization is based upon Inoue’s discovery of the
heterogeneously catalyzed copolymerization of CO2 and PO to
give PPC with a very low reaction rate (TOF: 0.12 h-1).23
Following this discovery, numerous heterogeneous and homo-
geneous catalyst systems have been developed for this trans-
ambient temperature, while substituted non-nucleophilic BF4
for NO3- caused nearly complete loss in catalyst activity (entry
7). The reaction temperature has a strong influence on the rate
(entries 6, 8-12). For example, with 3b as catalyst at a [PO]/
[catalyst] ratio of 10000, an increase in the temperature from
25 to 90 °C resulted in a dramatic increase of TOF from 612 to
8235 h-1. Notably, the polycarbonate selectivity of up to 97%
and an activity more than 10000 h-1 were achieved when the
copolymerization reaction was performed at a high temperature
of 100 °C. It is worth noting here that a decrease in polycar-
bonate molecular weight and a slight increase in polymer
molecular-weight distribution were observed at enhanced reac-
tion temperatures. A similar decrease in polymer molecular
weight was also observed in the binary catalyst systems of
SalenCoX in conjunction with quaternary ammonium halides
or sterically hindered strong organic bases for this reaction.8b
We suspect that, aside from trace water, perhaps the elevated
temperature causes the dissociation of the initiating anion from
the propagating polymer chain and thereby results in the
molecular weight discrepancy.20
Stimulated by our success with these cobalt-based catalyst
systems for the alternating copolymerization of CO2 and PO,
we likewise applied these systems to the copolymerization of
alicyclic cyclohexene oxide (CHO) and CO2. Unfortunately,
neither copolymer nor cyclic carbonate was observed in the
(19) Allen, S. D.; Moore, D. R.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 2002, 124, 14284–14285.
(21) Indeed, only limited catalysts exhibit certain activities for CHO/CO2
copolymerization at room temperature and relatively low CO2 pressure,
probably due to the more sterically hindered, less reactive CHO
monomer. For example, Williams and co-workers recently reported a
novel macrocyclic dizinc catalyst for CHO/CO2 copolymerization, and
an activity of 24 h-1 (TOF) was achieved at 1 atm pressure and 90 °C
(see ref 5j). Also, the catalyst system (3) in Scheme 1 was performed
at 0.1 MPa, and the TOF up to 153 h-1 was obtained at ambient
temperature.
(20) In previous paper, we have confirmed the initiator role of MTBD in
the binary 8a/MTBD catalyst system for CO2/PO copolymerization
by ESI-MS, and further demonstrated the living polymerization
characteristics in the measurable range of the instrument (see ref 8b).
In the ESI-Q-TOF mass spectra in positive ion mode, we observe the
species of [MTBD + H+], [MTBD + PO + H+] and [-OCH(CH3)CH2-
(CO2-alt-PO)n-MTBD+ + H+], but with time, the former two species
gradually disappear and the latter species re-moves to high m/z region.
After no species with regard to MTBD was observed, the reaction
system was heated to 60 °C, and the species of [MTBD + H+],
[MTBD + PO + H+], and [-OCH(CH3)CH2-(CO2-alt-PO)n-MTBD+
+ H+] appeared in the ESI-MS spectra again (see Supporting
Information, Figure S1). This result indicates that the elevated
temperature causes the dissociation of the MTBD initiator from the
propagating polymer-chain.
(22) (a) Raanby, B. Macromolecular Concept and Strategy for Humanity
in Science, Technology and Industry; Springer; New York, 1996; pp
14-21. (b) Stepto, R.; Horie, K.; Kitayama, T.; Abe, A. Pure Appl.
Chem. 2003, 75, 1359–1369.
(23) (a) Inoue, S.; Koinuma, H.; Tsuruta, T. J. Polym. Sci., Polym. Lett.
1969, 7, 287–292. (b) Inoue, S.; Koinuma, H.; Tsuruta, T. Macromol.
Chem. 1969, 130, 210–220.
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Mechanistic Aspects of the Copolymerization of CO2 with Epoxides
A R T I C L E S
Figure 1. (A) ESI-Q-TOF mass spectrum of 3a; (B) representative ESI-Q-TOF mass spectrum of the reaction mixture of PO and CO2 catalyzed by 3a
(3a/PO ) 1/10000, molar ratio) at 25 °C and at 1.5 MPa; and (C) a time profile of the absorption at 1750 cm-1 (corresponding to polycarbonate) of the
reaction mixture diluted by CH2Cl2.
formation.1 Prominent among these systems are discrete zinc
phenoxides, ꢀ-diiminate zinc alkoxides, and binary or bifunc-
tional catalyst systems based on metal-Salen complexes being
the most efficient for the CO2/epoxides coupling reac-
tions.3,4,8,9,15,16 It is worth mentioning that some elegant
mechanism studies contributed significantly to the discovery of
these highly active catalyst systems.19,24-28 As to the binary or
bifunctional catalyst systems based on cobalt-Salen complexes,
the observation that the addition of a nucleophilic cocatalyst
significantly improves the activity of cobalt-Salen complexes
even at low CO2 pressure has remained poorly understood.
Although the nucleophilic cocatalysts were tentatively thought
to stabilize the active SalenCo(III) species against decomposition
to inactive SalenCo(II), no direct evidence was obtained for
supporting this hypothesis. The current study seeks to understand
how these cobalt-based catalyst systems operate, by investigating
what role the anchored TBD of the complex 3a or 3b plays in
maintaining its thermal stability and high activity in CO2/
epoxides copolymerization.
Electrospray ionization (ESI) is a soft ionization method that
can keep any weakly bound ligand intact in a complex ion. This
property has enabled ESI to become increasingly popular as an
analytical tool in inorganic/organometallic chemistry.29 ESI, in
combination with tandem mass spectrometry (MS/MS), has been
employed to study mechanistic aspects of some homogeneously
catalyzed reactions including Ziegler-Natta polymerization of
olefins.30,31 In the recent report of Chen et al., the binding of
aliphatic epoxides to SalenM(III) cations (where M ) Cr, Co,
Ga, and Al) was studied in the gas phase by electrospray tandem
mass spectroscopy. The results were discussed in terms of the
reactivity of these complexes in the ring-opening polymeriza-
tions of epoxides and copolymerizations with CO2, respec-
tively.32 In the present paper, we also performed ESI-MS in
positive-mode for continuous determination of the transient
cationic species at various time points. In sharp contrast to the
binary 8a/MTBD catalyst system utilizing the complex 3a as
catalyst with a [PO]/[catalyst] ratio of 10000, we did not observe
the propagating polymer species with TBD anchored on the
ligand framework of 3a but found only one species of m/z 784.5
(Figure 1B). It should be noted that continuous Fourier transform
infrared spectroscopy (FTIR) determination of intensity at 1750
cm-1 at various time points demonstrated that the copolymer-
ization reaction promptly occurred after CO2 was charged into
the system (Figure 1C). To our surprise, even after complete
conversion of PO in the system with 1,2-dimethoxyethane as
organic solvent, the species of m/z 784.5 could be clearly
observed in the positive-mode ESI tandem mass spectra. When
the molar ratio of PO to 3a was reduced to 2000, apart from
the species of m/z 784.5, weak signals of the propagating
polymer species [-OCH(CH3)CH2-(CO2-alt-PO)n-(3a)+
-
NO3-]+ with regard to TBD anchored on the ligand framework
of 3a were observed in the positive-mode ESI-MS spectra of
the reaction mixture (see Supporting Information Figure S3).
To further the investigation of the m/z 784.5 species, we
studied the reaction of 3a with PO in the absence of CO2 by
ESI-MS spectroscopy. We detected two species of m/z 784.5
and 726.5. The latter species rapidly disappeared with time. This
suggests that the formation of the species of m/z 784.5 is a fast
reaction (Figure 2A). The species of m/z 784.5 was characterized
by collision-induced dissociation (CID). This analysis did not
result in any scission even via CID experiments up to 15 V,
indicating that m/z 784.5 is highly stable (Figure 2B). These
results demonstrate that the species of m/z 784.5 in Figure 1B
can be ascribed to [-OCH(CH3)CH2-(3a)+ - NO3-]+.
Since the complex 3a did not initiate the homopolymerization
of PO, we can investigate the insertion of CO2 into the Co-O
bond using the in situ FTIR method by monitoring the ν(CO2)
region of the infrared spectra. After only the species of m/z 784.5
was observed in the mass spectra, complete removal of PO from
the 3a/PO mixture system was performed via a vacuum
and the resulting solid was dissolved in anhydrous CH2Cl2.
When the solution was bubbled with CO2, two equivalent
(24) Moore, D. R.; Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 2003, 125, 11911–11924.
(25) Chisholm, M. H.; Zhou, Z. P. J. Am. Chem. Soc. 2004, 126, 11030–
11039.
(26) Darensbourg, D. J.; Moncada, A. I.; Choi, W.; Reibenspies, J. H. J. Am.
Chem. Soc. 2008, 130, 6523–6533.
(27) Luinstra, G. A.; Hass, G. R.; Molnar, F.; Bernhart, V.; Eberhardt, R.;
Rieger, B. B. Chem.-Eur. J. 2005, 11, 6298–6314.
(28) (a) Van Meerendonk, W. J.; Duchateau, R.; Koning, C. E.; Gruter,
G. M. Macromolecules 2005, 38, 7306–7313. (b) Duchateau, R.; Van
Meerendonk, W. J.; Yajjou, L.; Staal, B. B. P.; Koning, C. E.; Gruter,
G. M. Macromolecules 2006, 39, 7900–7908.
(29) (a) Chen, P. Angew. Chem., Int. Ed. 2003, 42, 2832–2847. (b)
Combariza, M. Y.; Fahey, A. M.; Milshteyn, A.; Vachet, R. W. Int.
J. Mass Spectrom. 2005, 244, 109–124. (c) Rao, D. Y.; Li, B.; Zhang,
R.; Wang, H.; Lu, X. B. Inorg. Chem. 2009, 48, 2830–2836.
(30) (a) di Lena, F.; Quintanilla, E.; Chen, P. Chem. Commun. 2005, 5757–
5759. (b) Santos, L. S.; Metzger, J. O. Angew. Chem., Int. Ed. 2006,
45, 977–981.
(31) (a) Plattner, D. A.; Feichtinger, D.; El-Bahraoui, J.; Wiest, O. Int. J.
Mass Spectrom. 2000, 195/196, 351–362. (b) Feichtinger, D.; Plattner,
D. A. Chem.-Eur. J. 2001, 7, 591–599.
(32) (a) Scho¨n, E.; Zhang, X.; Zhou, Z.; Chisholm, M. H.; Chen, P. Inorg.
Chem. 2004, 43, 7278–7280. (b) Chen, P.; Chisholm, M. H.; Gallucci,
J. C.; Zhang, X. Y.; Zhou, Z. P. Inorg. Chem. 2005, 44, 2588–2595.
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J. AM. CHEM. SOC. VOL. 131, NO. 32, 2009 11513

A R T I C L E S
Ren et al.
Figure 2. (A) ESI-Q-TOF mass spectra of the mixture solution of PO/3a (10000, molar ratio) at 20 °C with various time points after 3a was dissolved into
PO; and (B) collision-induced dissociation (CID) of the species of m/z 784.5 resulted from the system of PO/3a (10000, molar ratio) at 20 °C with various
voltages (diluted with CH2Cl2).
Figure 3. FTIR spectra of (A) the CH2Cl2 solution of the solid product obtained by the complete removal of the residual PO from the 3a/PO mixture (PO/3a
) 10000, molar ratio) solution; (B) after bubbling CO2 through the (A) solution; (C) after bubbling N2 through the (B) solution; (D) after the (C) solution
was heated at 60 °C for 30 min; (E) after bubbling CO2 through the CH2Cl2 solution of the solid product obtained by the complete removal of PO from the
3c/PO mixture (PO/3c ) 10000) solution; (F) after bubbling N2 through the (E) solution.
absorption peaks at 1750 and 1719 cm-1 were found in the FTIR
spectrum (Figure 3B).33 Interestingly, the bubbling of N2 for
removing the free CO2 from the solution resulted in the complete
disappearance of the absorption at 1719 cm-1 but no obvious
change in the absorption at 1750 cm-1 (Figure 3C). If the FTIR
sample was heated at 60 °C, a significant decrease in the
intensity at 1750 cm-1 was also observed (Figure 3D). To
confirm the identifications of the two absorption peaks, we
further performed a similar experiment with the substitution of
3c for 3a. Only one absorption peak at 1719 cm-1, which is
also easily removed by bubbling N2 into the system, was
observed in the spectrum after the addition of CO2 into the
CH2Cl2 solution of the cobalt complex formed from the reaction
of 3c and PO (Figure 3E,F). It is generally known that non-
-
nucleophilic BF4 anion cannot initiate the copolymerization
of CO2 with PO. Therefore, we can conclude that the two
equivalent absorption peaks at 1750 and 1719 cm-1 in the former
system originate from the vibrations of CdO in carboxylate (I)
and (II) (Figure 3), respectively. These results lead us to
conclude that the species of m/z 784.5 in Figure 2B assigned to
- +
[-OCH(CH3)CH2-(3a)+ - NO3 ] predominantly results from
the dissociation of the real species [-OOCOCH(CH3)CH2-(3a)+
- NO3-]+,34 due to its low collision dissociation energy and
the effect of ESI gas of N2. Therefore, we can reasonably assume
(33) Darensbourg et al. have investigated the interaction of CO2 with
4-(dimethylamino)pyridine (DMAP) in the presence of chromium
Salen complex by in situ infrared spectroscopy, and observed several
new bands at 1650, 2097, and 2018 cm-1, in which the two latter
signals were assigned to CO2 insertion into a metal-amide bond. Also
the authors further concluded from some control experiments that
bicarbonate generated by adventitious water was not likely the
cocatalyst in the catalyst systems which consisted of SalenCr(III) and
DMAP for CO2/epoxides copolymerization (See ref 18).
(34) Recently, Luinstra et al. reported the mechanism understanding on
the formation of aliphatic polycarbonates from aliphatic epoxides and
CO2 copolymerization with chromium(III) and aluminum(III) metal-
Salen complexes by a DFT theoretical calculation method, in which
they also proposed the propagating carboxylate easily dissociating from
the metal center during the copolymerization (see ref 27).
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Mechanistic Aspects of the Copolymerization of CO2 with Epoxides
A R T I C L E S
Scheme 2. Mechanistic Understanding on the Role of the Anchored TBD on the Ligand Framework of the Complex 3 (3a or 3b) during
CO2/PO Copolymerization
that the species [-OOCOCH(CH3)CH2-(3a)+ - NO3-]+ with
respect to the anchored TBD on the ligand framework helps to
stabilize the active Co(III) species by reversible intramolecular
Co-O bond formation and dissociation (Scheme 2).
that the anchored TBD on the ligand framework of the complex
5 does not stabilize the active Co(III) species.
Surprisingly, substituted imidazole (a coordination base, 6)
for sterically hindered TBD anchored on the ligand framework
of the complex 3a resulted in nearly complete loss in activity
at ambient temperature (entry 17). As anticipated, in the presence
of 2 equiv of N-methylimidazole, the complex 3a also did not
show any activity for this reaction under the same conditions.
This could be due to the coordination of N-methylimidazole to
the central metal ion and thereby retarding the activation of the
epoxide. These results indicate that the steric repulsion origi-
nating from TBD effectively inhibits its coordination to Lewis
acidic metal ion and benefits the formation of the carboxylate
intermediate with regard to the anchored TBD, which efficiently
stabilizes the active Co(III) species during the copolymerization.
For a comparison purpose, the cobalt complex 7 with two
It is worth noting that the length of the spacer linked ligand
framework and TBD plays an important role in maintaining high
activity. A flexible and sufficiently long spacer is usually a
prerequisite for obtaining excellent catalytic activity at high
[epoxide]/[catalyst] ratio. The activity of the complex 5 with
the methylene spacer is less than 1/10 that of 3a under the same
conditions (Table 1, entry 16), and the corresponding Co(II)
complex was unexpectedly observed after the reaction. When
the molar ratio of [PO]/5 is 10000, we did not detect the species
- +
of m/z 756.5 assigned to [-OCH(CH3)CH2-(5)+ - NO3 ] , but
clearly observed the species of 699.5, which is ascribed to the
corresponding Co(II) complex (Figure 4B). This result indicates
9
J. AM. CHEM. SOC. VOL. 131, NO. 32, 2009 11515

A R T I C L E S
Ren et al.
Figure 4. ESI-Q-TOF mass spectra of (A) the complex 5 in CH2Cl2, and (B) the reaction mixture of PO and CO2 catalyzed by the complex 5 (5/PO )
1/10 000, molar ratio) at 25 °C and 1.5 MPa.
anchored TBD groups was synthesized and tested for catalyzing
the CO2/PO copolymerization (entry 18). Unfortunately, the
complex 7 did not exhibit high activity for this reaction, perhaps
due to the intramolecular actions of the two anchored TBD
carbonate chain with the central metal ion. This would retard
coordination activation of the epoxide and thereby significantly
affecting polymer-chain growth.
activation of an epoxide and its further ring-opening is a key
step during the copolymerization reaction. The ring-opening of
the coordinated epoxide attacked by the carbonate anion of the
dissociated polymer-growth chain derived from the axial anion
results in the dissociation of the carboxylate intermediate with
regard to the TBD anchored on the catalyst ligand framework.
The dissociated carbonate anion derived from the anchored TBD
recoordinates the Lewis acidic metal center, and thereby
facilitates the dissociation of its trans propagating carboxylate
species. More importantly, the recoordination of the carboxyl-
ate intermediate associated with the anchored TBD helps to
stabilize the active Co(III) species against decomposition to
inactive Co(II). Although the imidazole anchored on the ligand
framework in the complex 6 can also stabilize the Co(III) state
efficiently, its strong coordination to the central metal ion
significantly decreases the Lewis acidity of the cobalt ion. This
has a negative effect on the epoxide activation and the polymer-
chain growth trans to the coordinated imidazole. It is should be
noted that the complex 3a did not exhibit any activity in the
coupling reaction of alicyclic cyclohexene oxide (CHO) and
CO2 (Scheme 1), in which the species of 727.7 ascribed to the
corresponding Co(II) complex was clearly observed in the ESI-
MS spectrum of the reaction mixture (Figure 5). When the
reaction was performed for only 2 h, the Salen-Co(III) species
were completely transformed into the corresponding Co(II)
species (Figure 5B). Indeed, the intramolecularly nucleophilic
attack of the TBD anchored on the ligand framework of the
complex 3a at the coordinated CHO on the central metal ion
rarely occurs due to their steric hindrances, and thus, the
anchored TBD has no positive effect on stabilizing the active
Co(III) species. On the contrary, in the presence of trace PO or
the treatment of complex 3a with PO, the copolymerization of
On the basis of the facts described above, a reaction
mechanism is proposed and shown in Scheme 2, in which the
anchored TBD on the ligand framework of 3a or 3b helps to
stabilize the active Co(III) species against decomposition to
inactive Co(II) by reversible intramolecular Co-O bond forma-
tion and dissociation. In the initial stage, the epoxide first
coordinates to the active metal center trans to the axial anion
and is further ring-opened by nucleophilic attack of the anchored
TBD. In the meantime, the coordination of the epoxide and its
further ring-opening also labilize the Co-X bond and thereby
promote the leaving of the axial X anion. Then, CO2 rapidly
inserts into the Co-O bond and another epoxide coordinates
the central metal ion trans to the formed carboxylate species
with regard to the anchored TBD. The ring-opening of the
coordinated epoxide attacked by the nucleohilic X anion is
beneficial to the dissociation of the carboxylate species with
regard to the anchored TBD. The insertion of CO2 to Co-O
bond produces the linear carbonate associated with the axial X
anion, which easily dissociates from the central metal due to
the reversible coordination of the dissociated carboxylate species
with respect to the anchored TBD. The observation that the
polycarbonate products contained more than 99% carbonate
linkages suggests that the insertion of CO2 to Co-O bond is a
fast reaction and that the dissociation of propagating carboxylate
species from the central metal ion as well as the coordination
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11516 J. AM. CHEM. SOC. VOL. 131, NO. 32, 2009

Mechanistic Aspects of the Copolymerization of CO2 with Epoxides
A R T I C L E S
Figure 5. Detailed ESI-Q-TOF mass spectra of the reaction mixture of CHO and CO2 catalyzed by 3a (3a/CHO ) 1/5000, molar ratio) at 25 °C with
various time points: (A) 5 min, and (B) 2 h (diluted with CH2Cl2).
Scheme 3. Mechanistic Understanding of Binary Catalyst Systems
CO2 and CHO run smoothly. This suggests that the formed
Based on SalenCo(III) for CO2/PO Copolymerization
carboxylate intermediate with respect to the anchored TBD helps
to stabilize the active Co(III) species against decomposition to
inactive Co(II) by reversible intramolecular Co-O bond forma-
tion and dissociation during the reaction.
This mechanism also can explain how the addition of a
nucleophilic cocatalyst such as quaternary ammonium halides
or sterically hindered strong organic bases could significantly
improve the activity of Co(III)-Salen complexes. A new
mechanistic understanding concerning binary or bifunctional
catalyst systems based on Co(III)-Salen complexes is shown
in Scheme 3. Alternating chain-growth and dissociation of
propagating carboxylate species derived from the nucleophilic
axial anion and the cocatalyst take turns at both sides of the
Co(III)-Salen center. This significantly increases the reaction
rate and also helps to stabilize the active Co(III) species against
decomposition to inactive Co(II) even at low CO2 pressures and/
or relatively high temperatures. In support of this hypothesis,
some control experiments were performed and the results are
listed in Table 2. As previously reported,8 in the absence of a
nucleophilic cocatalyst, the complex 8a could not effectively
catalyze CO2/PO copolymerization at 1.5 MPa pressure and
predominately decomposed to inactive Co(II) species. The
addition of 1 equiv of nBu4NX (X ) 2,4-dinitrophenoxy anion)
significantly improved the catalytic activity of 8a at ambient
temperature (Table 2, entries 2 and 3). A decrease in the molar
ratio of PO to 8a from 10 000 to 2000 results in a TOF of 90
h-1 increasing to 204 h-1. High cocatalyst loading makes a
contribution to the increase in catalytic activity of 8a, but it
has a negative effect on the selectivity for polymer formation
(entries 4 and 6). It can be tentatively assumed that the
nucleophilic attack of the cocatalyst or the dissociated propagat-
ing carboxylate species at the activating PO by its coordination
to the central metal ion of 8a is similar to a bimolecular process.
Therefore, the rate is proportional to the concentration of the
propagating species consistent with the cocatalyst and the
nucleophilic axial anion, as well as that of the activating PO
with regard to the complex 8a. On the other hand, high
cocatalyst loading is beneficial to stabilizing the active Co(III)
species at high [epoxide]/[catalyst] ratio and, thus, results in a
high rate in the CO2/epoxides copolymerization. Recently,
combined NMR and quantum chemical studies by Kemper et
al. demonstrated that an equilibrium between the paramagnetic
and diamagnetic complexes could be observed for 8b in THF
(the coordination properties of THF and epoxides are compa-
rable) over a range of temperature, yielding paramagnetic NMR
spectra at increased temperature.35 With regard to the bifunc-
tional catalyst systems (the complexes 1 and 4) containing a
Lewis acidic metal center and quaternary ammonium salt units
(35) Kemper, S.; Hroba´rik, P.; Kaupp, M.; Schlo¨rer, N. E. J. Am. Chem.
Soc. 2009, 131, 4172–4173.
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A R T I C L E S
Ren et al.
Table 2. CO2/PO Copolymerization Catalyzed by the Complex 8aa
entry
cocatalyst (equiv)
PO/8a molar ratio
temp (°C)
time (h)
TOFb (h-1
)
selectivityc (% polymer)
Mnd(kg/mol)
PDId (Mw/Mn)
1e
2
3
4
5
-
10000
10000
2000
10000
10000
10000
25
25
25
25
60
60
10.0
10.0
4.0
6.0
7.0
<5
90
204
560
656
1540
-
-
-
nBu4NX (1)
nBu4NX (1)
nBu4NX (10)
nBu4NX (1)
nBu4NX (10)
99
>99
94
90
63
28.9
30.4
28.8
46.3
-
1.04
1.10
1.13
1.15
-
6
3.5
a The reaction was performed in neat propylene oxide (PO) (14 mL, 200 mmol) in 75 mL autoclave at 1.5 MPa CO2 pressure. b Turnover frequency
c
of PO to products (polycarbonate and cyclic carbonate). Determined by using H NMR spectroscopy. d Determined by gel permeation chromatography
1
in THF, calibrated with polystyrene standards. e Co(II) derivative was predominantly produced.
in a molecule, the interaction between the quaternary ammonium
cation anchored on the ligand framework and the propagating
carbonate anion leads to the dissociated carboxylate species
always hanging around the metal center, and thereby helps to
stabilize the active Co(III) species even at enhanced temperature
and/or high [epoxide]/[catalyst] ratio.15 Therefore, one major
difference between the much-studied binary and bifunctional
catalyst system is the counter cations of the dissociated anions.
In the binary system, the countercation is not linked intramo-
lecularly with the cobalt complex. Thus, the ion pair of the
countercation and the dissociated anion can be far away from
the cobalt center, particularly under highly diluted solution and/
or at high reaction temperatures. In contrast, in the bifunctional
catalyst system, the cobalt complex with quaternary ammonium
moiety can keep the dissociated anion around itself, resulting
in higher stability and catalytic activity. For the present system
involving the complex 3a or 3b, in which the iminium moiety
can also keep the dissociated anion around itself, only one
initiating anion exists in the cobalt complex that is beneficial
to maintaining polymer selectivity and narrow molecular-weight
distribution even at high temperatures (Table 1, entries 9-15).
Notably, it is more efficient that the TBD-linked carboxylate
group stabilizes the active Co(III) species by reversibly in-
tramolecular coordination to the cobalt center.
insight into the mechanistic properties of the CO2/epoxide
copolymerization reaction. We found that the carboxylate
intermediate with regard to the TBD anchored on the catalyst
ligand framework helps to stabilize the active Co(III) species
against decomposition to inactive Co(II) by reversibly intramo-
lecular Co-O bond formation and dissociation. The cobalt
complex with iminium moiety can keep the dissociated anion
around itself, resulting in higher stabilizing ability and catalytic
activity. The sole initiating anion of the cobalt complex is
beneficial to maintaining polymer selectivity and narrow mo-
lecular-weight distribution even under highly diluted solution
and/or at high reaction temperatures. These studies provide a
new mechanistic understanding of our previously studied binary
catalyst systems based on Co(III)-Salen complexes in which
alternating chain-growth and dissociation of propagating car-
boxylate species derived from the nucleophilic axial anion and
the nucleophilic cocatalyst take turns at both sides of the
Co(III)-Salen center. This arrangement significantly increases
the reaction rate and also helps to stabilize the active Co(III)
species against decomposition to inactive Co(II) even at low
CO2 pressures and/or relatively high temperatures.
Acknowledgment. This work was supported by National
Natural Science Foundation of China (NSFC) program (Grant
20634040) and National Basic Research Program of China (973
Program: 2009CB825300). Xiao-Bing Lu gratefully acknowledges
the Outstanding Young Scientist Foundation of NSFC (Grant
20625414).
Conclusion
In conclusion, we have developed a novel single-site, cobalt-
based catalyst with a sterically hindered TBD anchored on the
ligand framework for CO2/epoxides copolymerization. This
catalyst exhibits highly active and selective polymer formation
even at high temperatures (up to 100 °C), high [epoxide]/
[catalyst] ratios, and/or low CO2 pressures. The anchored TBD
on the ligand framework plays an important role in maintaining
thermal stability and high activity of the catalyst. Continuous
monitoring of the reaction mixture at various time points was
achieved by ESI-MS. This analysis in combination with in situ
FTIR studies and some control experiments provided us with
Supporting Information Available: General experimental
procedures, full experimental details on the preparation of all
ligands and their cobalt complexes, detailed ESI-Q-TOF mass
spectra at various time, and characterization of polymers. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JA9033999
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11518 J. AM. CHEM. SOC. VOL. 131, NO. 32, 2009