Alternating Copolymerization of CO2and Cyclohexene Oxide Catalyzed by Cobalt-Lanthanide Mixed Multinuclear Complexes

Article abstract of DOI:10.1021/acs.inorgchem.0c01156

Heteromultimetallic complexes consisting of three Co(II) ions and one lanthanide ion were synthesized and applied to the alternating copolymerization of CO2 and cyclohexene oxide. Unlike the conventional cobalt(III) salen complexes, the high thermal stabi

Full text of DOI:10.1021/acs.inorgchem.0c01156

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Alternating Copolymerization of CO2 and Cyclohexene Oxide
Catalyzed by Cobalt−Lanthanide Mixed Multinuclear Complexes
Hiroki Asaba, Takanori Iwasaki, Masahiro Hatazawa, Jingyuan Deng, Haruki Nagae, Kazushi Mashima,*
and Kyoko Nozaki*
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ABSTRACT: Heteromultimetallic complexes consisting of three Co(II) ions and one lanthanide ion were synthesized and applied
to the alternating copolymerization of CO2 and cyclohexene oxide. Unlike the conventional cobalt(III) salen complexes, the high
thermal stability of the present catalyst allowed us to reach a turnover number of 13000, one of the highest values ever reported for
multimetallic systems. The chain propagation was first-order to the catalyst, suggesting a cooperative behavior of the metal centers.
opolymerization of CO2 and epoxides is a promising
The increased Lewis acidity of the metal centers can activate
1
Ctransformation for CO into polycarbonates, which are the electrophilic substrates more efficiently; however, the
2
expected to be biodegradable2 and biocompatible materials.3
nucleophilicity of the propagating terminal gets decreased
Since the first discovery of Zn-catalyzed CO2−epoxide
copolymerization,4 homogeneous catalysts containing Zn,5
Co,6 Cr,7 Al,8 lanthanides (Ln),9 etc., have been developed10,11
and the mechanistic insights addressed. Since the bimetallic
ring opening of epoxides was proposed as the key step for
initiation by Coates in 2005,12 a bimetallic mechanism is often
suggested where the intermolecular reaction between the
propagating terminal bound to one metal and the epoxide on
another metal center takes place to propagate the polycar-
bonate chain (Scheme 1a).13
when the metal center is electron-deficient. Therefore, in
comparison to homobimetallic catalysts, heterobimetallic
systems15−20 consisting of hard Lewis acid and soft Lewis
acid metals are the more promising catalyst design for the
independent activation of both electrophile and nucleophile
(Scheme 1c).17−19 Indeed, heterobimetallic and heteromulti-
metallic complexes have recently been employed for the
copolymerization of CO2 and epoxides, but the combinations
of metal elements are limited to a Zn-based combination with
Fe,15 Ti,16 group 1 and 2 metals,17 group 13 metals,18 or Ln
metals,19 until most recently.20 Williams et al. reported a
Mg(II)/Co(II) bimetallic system for the copolymerization of
CO2 and CHO with a turnover frequency (TOF) of >12000
h−1, which is better than those of homobimetallic and Zn-
based heterobimetallic catalysts supported by the same
ligand.21
Scheme 1. Copolymerization of CO2 and Epoxides and
Possible Reaction Mechanisms for the Ring Opening of
Epoxides
Herein, we report the copolymerization of CO2 and CHO
using a combination of Co and Ln metals as heteromultime-
tallic catalysts. The catalysts used herein contain three Co(II)
ions and one Ln ion supported by a cyclic multidentate ligand.
This catalytic system differs from the conventional cobalt(III)
salen complexes, which suffer from lower thermal stability.22
After optimization of the Ln metals, the high thermal stability
of the catalyst allowed us to reach a turnover number (TON)
of 13000, one of the highest values ever reported for
multimetallic systems.
Until today, various kinds of homobimetallic complexes have
been developed for the copolymerization of CO2 and
epoxides.14 These homobimetallic complexes exhibited higher
catalytic activity for the copolymerization of CO2 and
cyclohexene oxide (CHO) or propylene oxide than the
corresponding mononuclear complexes even at low catalyst
loading because such homobimetallic catalysts can activate the
electrophilic substrates, CO2 and epoxides, at the neighboring
position to the nucleophilic chain end to promote propagation
in an intramolecular fashion (Scheme 1b).
The macrocyclic tris(N2O2) hexaoxime ligand L6H invented
by Nabeshima23 was used for our study. The treatment of L6H
Received: April 22, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c01156
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

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with 3 equiv of Co(OAc)2 and 1 equiv of La(OAc)3 afforded
tetranuclear Co3/La complex LLaCo3(OAc)3(H2O)4(MeOH)·
H2O (1La, where MeOH = methanol and H2O = water) in 99%
yield (Scheme 2). Although complex 1La was inactive in the
crystallography and are essentially the same as that of 1La
(Figures S1−S3).
The Co−O bond lengths of salen-type coordinating sites in
1La suggest the formal oxidation states of Co and La to be
Co(II) and La(III), respectively.24 Similar structural behaviors
were observed for 1Ce, 1Sm, and 1Eu.
Scheme 2. Synthesis of Co3/Ln Complexes
Next, we investigated the copolymerization of CO2 and
CHO using 1La. The initial assessment to evaluate the reaction
conditions revealed that copolymerization by 1La was sensitive
to H2O contamination; the higher H2O concentration results
in the lower TON and molecular weight (Mn) as well as poor
reproducibility (Table S2). Therefore, we conducted further
investigations using well-dried CHO under an inert atmos-
phere to prevent the pernicious influence of H2O on the
catalytic activity and reproducibility. The copolymerization of
CO2 and CHO was examined, and the results are summarized
in Table 1 (see Tables S3−S9 for details). In the presence of
0.005 mol % 1La (substrate−catalyst ratio: S/C = 20000), the
copolymerization proceeded selectively to give almost-
completely alternating polycarbonate with >99% carbonate
linkage (an ether linkage was not detected by 1H NMR
analysis; Figure S4), where the concomitant formation of cyclic
carbonate was negligible (99% selectivity for the copolymer).
The TON of 1La reached 11000 to afford bimodal
polycarbonate with Mn = 66000 and 30900 (for analysis of
the initiation terminal, see the Supporting Information), where
polydispersity index (PDI) of each bimodal polycarbonate was
1.04 (entry 1).
NMR analysis because of its paramagnetic nature, the
formation of complex 1La was confirmed by electrospray
ionization mass spectrometry and elemental analysis. The
molecular structure of 1La was unambiguously identified by X-
ray crystallography, as shown in Figure 1. The molecular
Cooperative catalysis in the heteromultinuclear complex 1La
was supported by the following control experiments using the
corresponding mononuclear cobalt(II) salen complexes 2 and
3 (Chart 1). Complex 2 without any additives or oxidant
exhibited no catalytic activity under the conditions, and only a
trace amount of copolymer was obtained (entry 2). Similarly,
copolymerization using La(OAc)3 as the sole catalyst afforded
only a trace amount of copolymer (entry 3). Even the
combination of La(OAc)3 with complex 2 or its analogous
complex 3 bearing methoxy groups did not show any catalytic
activity (entries 4 and 5). These results clearly indicate that the
coexistence of La and Co ions in complex 1La is essential for its
catalytic activity. Among the carboxylate ligands tested, the
acetate complex 1La gave the best results, and the Co3/La
complex having electron-deficient trifluoroacetate 4La or
electron-rich pivalate 5La resulted in lower TON and Mn
values (entries 6 and 7) in contrast to previously reported
catalysts, which allow one to introduce various carboxylate
moieties into polycarbonates.17c,19a This is probably due to the
low efficiency of the initiation step for these carboxylates.
Among the examined Ln ions, the Nd complex 1Nd exhibited
the highest TON of 13000 with the highest Mn of 114000 and
53600 (entries 8−13). The TON of 13000 is one of the
highest values ever reported for multimetallic systems, and the
TOF of >1600 h−1 after 8 h is a better activity for multimetallic
systems with a few exceptions.14a,d,n,17c,21 In addition, the
observed TON and TOF of 1Nd are higher than those of the
Zn3/Ce catalytic system.19a On the other hand, 1Ce and 1Pr
were less effective catalysts with TONs of 6800 and 8600,
respectively (entries 8 and 9). The complexes containing Sm,
Eu, and Gd resulted in poor yields of polycarbonate (entries
5−7). When the copolymerization was conducted at low CO2
pressures of 1.0 and 0.5 MPa, the CO2 incorporation ratios
decreased to 74% and 40%, respectively.
Figure 1. ORTEP drawing of 1La with thermal ellipsoids at the 50%
probability level. Nonprotic H atoms, disordered molecules, and a
solvent molecule are omitted for clarity. Color code: gray, C; red, O;
blue, N; deep blue, La; purple, Co; white, H.
structure of 1La contains a ligand, a Co ion, and a La ion in a
1:3:1 molar ratio, and a Co ion is bound to each salen-type
coordinating site. La is surrounded by six phenoxy moieties
and located at center of the macrocyclic motif. One of three
acetate anions bridges La and Co, and the other two are bound
to either La or Co. The fact that all Co ions exhibit octahedral
geometry and solvent molecules such as MeOH and H2O
coordinate to the axial positions of the Co centers implies that
Co ions in complex 1La can act as Lewis acids. Although three
Co metals are in an ideal octahedral geometry, the molecule
exhibits a saddle-shaped structure. Nine-coordinated La bears
six phenoxy moieties, two acetates, and one solvent molecule,
suggesting that the axial positions of the La center can also act
as Lewis acids.
For various Ln ions, a series of Co3/Ln complexes were
successfully obtained in good-to-excellent yields by the same
procedure. The molecular structures of complexes consisting of
Ce, Sm, and Eu as Ln were determined by X-ray
B
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a
Table 1. Copolymerization of CHO with CO2
b
c
b
,d
b,
e
f
f
entry
catalyst
1La
yield (%)
TON
polymer selectivity (%)
98
CO2 incorporation (%)
>99
Mn
PDI
1
55
11000 600
66000 6000/30900 2000
1.04/1.04
2
3
4
5
6
7
8
9
2
trace
trace
trace
trace
7
3
34
43
La(OAc)3
2/La(OAc)3
3/La(OAc)3
4La
5La
1Ce
1Pr
1300
500
98
87
98
98
98
94
95
97
65
>99
>99
>99
>99
>99
>99
>99
1800
500
1.12
1.44
6800 1800
8600 2600
13000 300
3900 1500
2300 400
4100 2,100
52300 14700/24500 6800
62100 18200/29000 8700
114000 5000/53600 2600
45500 13500/21100 6,300
14500 1300/6800 600
35600 17200/16400 8100
1.04/1.05
1.05/1.06
1.05/1.04
1.05/1.05
1.04/1.06
1.04/1.06
10
11
12
13
1Nd
1Sm
1Eu
1Gd
65
20
12
21
a
CHO (1.7 mL, 17 mmol), catalyst (8.0 × 10−4 mmol), and CO2 (2.0 MPa) in a 50 mL autoclave at 130 °C for 8 h. Average of four runs.
b
c
d
Determined on the basis of 1H NMR spectroscopy of the crude product. (mole of repeating unit)/(mole of catalyst). (mole of repeating unit)/
e
[(mole of repeating unit) + (mole of cyclic carbonate)]. (mole of carbonate linkage)/[(mole of carbonate linkage) + (mole of ether linkage)].
f
Determined by size-exclusion chromatography analysis using a polystyrene standard.
Chart 1. Structure of Mononuclear Co Complexes
The TON and Mn of each catalyst are plotted along with
some parameters of Ln ions including the ionic radii,25 bond
dissociation energy of the Ln−O bond,26 hydrolysis con-
stant,27 and water-exchange rate constant28 (Figure S33).
However, a lower correlation of these parameters to the overall
polymerization results was observed because several pathways
may compete each other in the catalytic cycle (vide infra).
The unique thermal stability of complex 1Nd was confirmed
by the real-time observation of 1Nd-catalyzed copolymerization
by in situ IR analysis (Figure 2a). Absorption of the CO
stretching of polycarbonate at approximately 1750 cm−1
appeared after a short induction period and then continuously
increased until 8 h even at 130 °C. Such a high thermal
stability of 1Nd is due to the thermally stable nature of Co(II)
Figure 2. Time course of in situ IR spectra (a) and double-
in contrast to cobalt(III) salen complexes, which promote the
logarithmic plot of the initial reaction rates against the initial
copolymerization of CO2 and epoxides but undergo a
concentrations of 1Nd (b).
reduction to form a catalytically inactive cobalt(II) salen
complex in the reaction of propylene oxide with CO2 at 30
°C.22
a rate-determining step17b with cooperation of Nd and Co in
the same molecule rather than intermolecular propagation.
We propose the reaction pathway shown in Scheme 3 for the
alternating copolymerization of CO2 and CHO catalyzed by
complex 1Ln. The reaction is triggered by the coordination of
CHO to an oxophilic Ln center via a ligand-exchange reaction
of coordinating solvents and CHO. Then, an acetate anion on
Co attacks CHO in an intramolecular fashion to form the
alkoxy complex B. In the early stage of the reaction, CHO
reacts with not only acetate anions but also H2O, MeOH, and
cyclohexenol14h arising from CHO to form propagating
terminals, which were detected by matrix-assisted laser
desorption/ionization time-of-flight mass spectrometry (Figure
S35) and 13C NMR analyses (Figure S36) of polycarbonate.
The fact that the obtained polymer showed a narrow PDI
supports this initiation mechanism. The coordination of CO2
On the basis of our preliminary kinetic studies using in situ
IR at different catalyst loadings from 0.32 to 0.55 mM,
copolymerization was proven to be close to first-order in the
catalyst, implying a monomolecular mechanism. The evaluated
initial reaction rates (Figure S34 and Table S10) are plotted
against the catalyst loading (Figure 2b). From the slope of the
plot, a reaction order of 0.94 in the catalyst concentration was
obtained. The relatively small reaction order is in sharp
contrast to the reported reaction orders of 1.6129 and 1.5730 of
the cobalt salen/n-Bu4NX binary catalyst concentration in the
copolymerization of CO2 and epoxides, where the intermo-
lecular addition of a carbonate nucleophile to epoxide on Co
has been proposed as the rate-determining step. The value of
0.94 obtained in this study implies a monomolecular
mechanism at the ring-opening reaction of epoxide, which is
C
https://dx.doi.org/10.1021/acs.inorgchem.0c01156
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contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Scheme 3. Proposed Reaction Pathway with 1Ln
AUTHOR INFORMATION
■
Corresponding Authors
Kazushi Mashima − Department of Chemistry, Graduate School
of Engineering Science, Osaka University, Toyonaka, Osaka
560-8531, Japan; orcid.org/0000-0002-1316-2995;
Email: mashima@chem.es.osaka-u.ac.jp
Kyoko Nozaki − Department of Chemistry and Biotechnology,
Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8656, Japan; orcid.org/0000-0002-0321-5299;
Email: nozaki@chembio.t.u-tokyo.ac.jp
Authors
Hiroki Asaba − Department of Chemistry and Biotechnology,
Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8656, Japan
Takanori Iwasaki − Department of Chemistry and
Biotechnology, Graduate School of Engineering, The University
of Tokyo, Tokyo 113-8656, Japan; orcid.org/0000-0002-
6663-3826
Masahiro Hatazawa − Department of Chemistry and
Biotechnology, Graduate School of Engineering, The University
of Tokyo, Tokyo 113-8656, Japan
Jingyuan Deng − Department of Chemistry and Biotechnology,
Graduate School of Engineering, The University of Tokyo,
Tokyo 113-8656, Japan
Haruki Nagae − Department of Chemistry, Graduate School of
Engineering Science, Osaka University, Toyonaka, Osaka 560-
8531, Japan; orcid.org/0000-0002-5589-7375
to the Co center of complex B forms complex C. The alkoxy
moiety on Ln migrates to CO2 on Co to form the carbonate
complex D. Once CHO coordinates to the Ln center, the
carbonate chain end on Co in complex E again attacks CHO
on Ln to elongate the polycarbonate chain via this chain-
shuttling process. The observed reaction order, first-order to
catalyst (vide supra), is in good agreement with this
intramolecular propagation rather than the intermolecular
propagation proposed in mononuclear cobalt(III) salen
complexes, in combination with additional cations such as
bis(triphenylphosphoranylidene)ammonium.29 Chain-transfer
reaction may occur via the competitive coordination of the
other polymer chain (P′OH) to the vacant site of complex D
to form complex F, followed by protonolysis of the propagating
terminal in F to exchange the propagating polymer. Because F
is a resting state, rapid exchange from F to E should accelerate
the polymerization (Figure S33).
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01156
Notes
The authors declare no competing financial interest.
In conclusion, we herein reported the synthesis of
heteromultinuclear complexes containing three Co ions and
one Ln ion supported by a cyclic ligand and the
copolymerization of CO2 and CHO using these hetero-
multinuclear complexes as catalysts. Among the complexes
tested, the Co3/Nd complex exhibited the highest TON of
13000. Control experiments and kinetic studies implied that
the present heteromultinuclear complexes activate both
nucleophilic propagating terminal and electrophilic monomers,
CO2 and CHO, to couple in an intramolecular fashion.
ACKNOWLEDGMENTS
■
This work was financially supported by JSPS KAKENHI
Grants JP15H05796 and JP15H05808 in Precisely Designed
Catalysts with Customized Scaffolding. A part of this work was
conducted at the Advanced Characterization Nanotechnology
Platform of the University of Tokyo, supported by the
Nanotechnology Platform Project by MEXT, Japan (Grant
JPMXP09A19UT0061).
REFERENCES
■
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01156.
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