Lanthanide Complexes Supported by a Trizinc Crown Ether as Catalysts for Alternating Copolymerization of Epoxide and CO2: Telomerization Controlled by Carboxylate Anions

Article abstract of DOI:10.1002/anie.201709218

A new family of heterometallic catalysts based on trimetalated macrocyclic tris(salen) ligands and rare-earth metals was prepared and structurally characterized. The LaZn₃ system containing anionic ligands such as acetate plays a critical role in catalyzing the alternating copolymerization of cyclohexene oxide (CHO) and CO₂ with a high proportion of carbonate linkages. Among the lanthanide metals, the CeZn₃ system exhibits high catalytic activity with a turnover frequency (TOF) of over 370 h^?1. NMR analysis of the complex and end-group analysis of the polymer suggest that the acetate ligands are rapidly exchanged, not only among coordinated acetates, but also between coordinated acetates and added carboxylate anions. These unique properties make this the first example of telomerization for the copolymerization of CHO and CO₂.

Full text of DOI:10.1002/anie.201709218

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Accepted Article
Title: Lanthanide Complexes Supported by a Trizinc-crown Ether as
Catalysts for Alternating Copolymerization of Epoxide and CO₂:
Telomerization Controlled by Carboxylate Anions
Authors: Haruki Nagae, Ryota Aoki, Shin-nosuke Akutagawa, Julian
Kleemann, Risa Tagawa, Tobias Schindler, Gyeongshin Choi,
Thomas P. Spaniol, Hayato Tsurugi, Jun Okuda, and Kazushi
Mashima
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709218
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Angewandte Chemie International Edition
10.1002/anie.201709218
COMMUNICATION
Lanthanide Complexes Supported by a Trizinc-crown Ether as
Catalysts for Alternating Copolymerization of Epoxide and CO
Telomerization Controlled by Carboxylate Anions
2
:
[
a]
[a]
[a]
[b]
[a]
Haruki Nagae, Ryota Aoki, Shin-nosuke Akutagawa, Julian Kleemann, Risa Tagawa, Tobias
[
b]
[a]
[b]
[a]
[b]
Schindler, Gyeongshin Choi, Thomas P. Spaniol, Hayato Tsurugi, Jun Okuda,* and Kazushi
Mashima*[a]
Abstract:
trimetallated macrocyclic tris(salen) ligands and rare earth metal was
prepared and structurally characterized. The LaZn system
A
new family of heterometallic catalysts based on
such as magnesium, particularly mixed metal systems of group 2
metals, as well as rare-earth metal polyhydrides and alkyls and
amidos have been introduced as catalysts for the
3
containing anionic ligands such as acetate play a critical role in
catalyzing alternating copolymerization of cyclohexene oxide (CHO)
copolymerization of cyclohexene oxide (CHO) and CO
2
. The
highest turnover frequency (TOF) based on these group 2 and
-1 [13k]
and CO
2
with a high carbonate linkage. Among lanthanide metals,
rare-earth metal complexes is 750 h .
Recently, Williams
-
the CeZn
1
3
system exhibits high catalytic activity with TOF over 370 h
successfully isolated a rare example of a heterodinuclear complex
having a magnesium atom and a zinc atom, which exhibited
.
NMR analysis of the complex and end-group analysis of the
polymer suggested that the acetate ligands are rapidly exchanged,
not only among coordinated acetates, but also between coordinated
acetates and added carboxylate anions. Such unique properties
provide the first example of telomerization for copolymerization of
higher activity toward alternative copolymerization of CHO and
CO
2
than either magnesium or zinc homodinuclear complexes.[
14]
Metal complexes supported by macrocycles made of salen
units recently attracted considerable attention in a wide range of
CHO and CO
2
by adding excess amounts of ammonium carboxylates
applications, including metal ion recognition, magnetism,
supramolecular assemblies, and nano-machines.[
15]
One of the
to control the molecular weight of the polymer.
most widely used macrocyclic multidentate ligands is the
tris(salen)” ligand that is composed of three salen moieties,
“
Since 1969, when Inoue et al. discovered a heterogeneous
although catalytic applications of such trimetalated macrocyclic
tris(salene) complexes have never been reported. Herein, we
report a new family of heterometallic catalysts comprised of a
macrocyclic tris(Zn-salen) unit and a group 3 metal, among which
catalyst of ZnEt
CO
2
/H
2
O for the copolymerization of epoxides with
to give alternating copolymers,[ several heterogeneous and
1]
2
homogeneous catalyst systems have been intensively
investigated toward developing a promising process to convert
CO into biodegradable polymers. In addition to heterogeneous
2
catalysts based on zinc,[2] double metal cyanides,[3] and rare-earth
metal-based ternary catalysts,[4] there are homogeneous
catalysts based on various metals, especially zinc, chromium, and
cobalt, with significant activities.[5] Coates reported that zinc β-
diiminate complexes are loosely associated into dimers during
copolymerization,[6] leading to the deliberate preparation of
various bimetallic zinc catalysts.[7] Such a bimetallic mechanism,
III
-1
the Ce Zn complex exhibits high activity with a TOF over 370 h
for alternating copolymerization of CHO and CO . Furthermore,
2
we provide the first example of the telomerization of
polycarbonate using ammonium salts as chain transfer reagents
to control the molecular weight while retaining the catalytic activity
Various end-functionalized polycarbonates are selectively
isolated in this catalytic system, and the key feature of this unique
polycarbonate formation is the rapid ligand exchange of external
carboxylate anions and coordinating polycarbonate anions on a
though homogeneous, became
heterogeneous zinc dicarboxylates,
a
useful alternative for
leading to the rapid
precisely designed mixed-metal scaffold.
[
8]
L1] (1La), where L is the hexa-anion
1
A complex [LaZn
3
(OAc)
3
[
9]
[10]
development of mononuclear chromium- and cobalt -salen
catalysts. Moreover, cooperation in dinuclear Cr[11] and Co[12]
salen complexes ultimately enhanced the catalytic activity
compared with their corresponding monomeric analogs.
Recent interest has been focused on less toxic metals;[13]
however, few well-defined dinuclear complexes of group 2 metals
of the macrocyclic tris(salen)-based ligand, was prepared by the
template reaction of 1,4-diformyl-2,3-dihyroxylbenzene with 2,2-
dimethyl-1,3-propanediamine in the presence of 1.0 equiv of
O (Scheme 1).[15e]
Zn(OAc) ·2H O and 1/3 equiv of La(OAc) ·xH
2 2 3 2
To identify complexes that copolymerize CHO and CO
screened several diamines (1,3-diaminopropane,
2
, we first
1,2-
diaminoethane, (1R,2R)-1,2-diphenylethylenediamine, and 1,2-
bis(aminooxy)ethane) to prepare macrocyclic complexes.
Complex 1La exhibited better solvability and the highest catalytic
activity with high carbonate linkage. Thus, we focused our efforts
on characterizing 1La and its derivatives (Scheme S1 and Table
S1).
[
a]
b]
Dr. H. Nagae, R. Aoki, S. Akutagawa, R. Tagawa, Dr. G. Choi, Prof.
Dr. H. Tsurugi, Prof. Dr. K. Mashima
Department of Chemistry, Graduate School of Engineering Science
Osaka University
Toyonaka, Osaka 560-8531, Japan
E-mail: mashima@chem.es.osaka-u.ac.jp
J. Kleemann, Tobias Schindler, Dr. T. P. Spaniol, Prof. Dr. J. Okuda
Institute of Inorganic Chemistry
[
RWTH Aachen University
Landoltweg 1, D-52062 Aachen, Germany
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
10.1002/anie.201709218
COMMUNICATION
Scheme 1. Template Synthesis of 1La
.
patterns of 2La and 3La were quite similar to those of the acetate
complex 1La except for the singlet due to the acetate ligands.
Figures 2(a) and 2(b) show the solid state structures of 1La and
2La, respectively. Complex 1La has two kinds of acetate ligands:
two of the three acetates form a bridge between a zinc atom and
1
a lanthanum atom, and the other is  -mode bound to zinc. In the
2
case of 2La, the lanthanum atom has two  -nitrate ligands, one
nitrate exists as an anion, and each zinc atom has one methanol
as a ligand.
Complex 1La was characterized by spectroscopic methods and
1
combustion analysis. The H NMR spectrum in methanol-d
4
at
3
0 °C displayed five singlets at  8.31 (s, 6H, N=CH), 6.62 (s, 6H,
Ar), 3.81 (s, 12H, =NCH -), 1.81 (s, 9H, CH CO ), and 1.08 (s,
8H, CH C(CH CH ), suggesting a C symmetric structure in
2
3
2
1
2
3
)
2
2
3
solution. Three acetate anions were observed as a magnetically
equivalent singlet at room temperature, and the fluxional
intramolecular exchange of the three acetate ligands was rapid
even at –90 °C (Figure S1)[16a,17], though the different coordination
modes of the three acetates were clarified by a single crystal
structure determination of 1La (vide infra).
Another notable feature of 1La was the rapid exchange
between the coordinated acetates and outer sphere acetate
anions: the singlet signal at 1.85 due to the three acetate ligands
shifted to  1.88 (s, 12H, OCOCH
3
) upon adding 1 equiv of
n
[
Bu
Bu
4
N][OAc] (1.90), and, furthermore, increasing amounts of
N][OAc] up to 10 equiv induced the saturation of chemical
n
[
4
shift at  1.89 (Figure S2). On the basis of the Job-Plot, 1La and
n
[
4 4
Bu N][OAc] formed a 1:1 anionic complex in methanol-d , and
-1
the binding constant was estimated to be 373 M (Figures S3 and
S4).[18,19]
Under saturated conditions, the intramolecular
exchange was still too rapid because the singlet signal at  1.88
was not coalesced even at –90 °C (Figures S5). Accordingly, we
proposed an equilibrium mixture of possible structures in Figure
Figure 2. Molecular structures of 1La (a) and 2La (b). All hydrogen atoms and
solvent molecule are omitted for clarity.
1
.[16b,17]
With three lanthanum complexes having different anions, 1La
La, and 3La, in hand, we conducted the copolymerization of CHO
with CO and the results are shown in Table 1. The nature of the
,
2
2
anionic ligand was critical to achieve superior catalytic
performance. Among the lanthanum complexes tested, acetate
complex 1La was the best catalyst for copolymerization in terms of
the high catalytic activity and perfect carbonate linkage (>99%).
In sharp contrast, nitrate complex 2La resulted in only a trace
amount of the polymer, and triflate complex 3La showed high
catalytic activity for polyether formation with only 1% carbonate
linkage.
1
Table 1. Catalytic activity and selectivity of LaZn
3
X
3
L .
Figure 1. Intramolecular and intermolecular rapid exchange of acetate anions.
Such a rapid exchange between inner acetates and outer
acetates prompted us to compare the effects of different anions.
Thus, we prepared nitrate and triflate analogs by treating
TOF[a] /h-1
Entry
Cat.
Carbonate
Cyclic
carbonate /%
linkage[ /%
b]
[c]
La(NO
3 3 2 2 3 2 2
) ·6H O with Zn(OAc) and La(OTf) ·xH O with Zn(OAc)
1
2
1
La
La
230
> 99
-
< 1
-
as the sources of the lanthanum and zinc ion. The corresponding
1
1
complexes [LaZn
3 3 2 3 3 2
(NO ) L ](NO ) (2La) and [LaZn (OTf) (OAc)L ]
2
trace
(
3
La) were obtained in moderate yields. The 1H NMR spectral
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Angewandte Chemie International Edition
10.1002/anie.201709218
COMMUNICATION
n
larger amounts of [ Bu
4
N][OAc] (2.0, 5.0, and 10.0 equiv to 1Ce),
3
3
La
413
1
< 1
the molecular weight of the polycarbonate gradually decreased
(
Entries 2—4). The catalytic activity was reduced by 20.0 equiv
[
a] Turn Over Frequency = mole [CHO] consumed per mole catalyst per
1
n
hour. [b] Determined by the relative integrals of the H NMR resonances at
δ = 3.45 (polyether) and δ = 4.65 ppm (polycarbonate). [c] Determined by
1
of [ Bu
4
N][OAc] and the oligomer was afforded (Entry 5). These
1
polymers were characterized by H NMR spectroscopic analysis
and MALDI-TOF-MS, and determined to be bimodal due to two
kinds of polymers, one of which had two hydroxyl groups at both
terminals (HO-/HO-terminated polymer) due to water
contamination,[13c] and the other of which had one acetate
H NMR spectroscopy.
Because acetate complex 1La showed superior catalytic
performance with high carbonate linkage (>99%), we examined
variations of 1Ln using lanthanide elements for co-polymerization,
and the results are summarized in Table 2. Complexes of larger
lanthanides, such as La, Ce, Pr, and Nd, resulted in higher
catalytic activities compared with middle or late lanthanide
elements with very high carbonate linkage and a narrow
molecular weight distribution retained. This suggests that living
polymerization proceeded even though these complexes have
three acetate coordination sites that act as initiation sites.
Complex 1Ce showed high catalytic activity with high carbonate
terminal group and one hydroxy terminal group (AcO-/HO-
terminated polymer).[
[nBu
19]
In the case of 10.0 equiv of
t
t
4
N][OCO Bu], we obtained
a
mixture of BuCOO-/HO-
terminated and HO-/HO-terminated polymers along with a trace
amount of AcO-/HO-terminated polymer (Entry 6).
Moreover, the addition of various ammonium benzoates
afforded
ArCOO-/HO-terminated
polymers
with
the
corresponding benzoate moiety as one terminal group and a
hydroxyl group as the other terminal group, along with
contamination of the AcO-/HO-terminated polymer (Entries 7—
2
linkage even under lower CO pressure (Entries 3 and 4). As a
result, we selected complex 1Ce as the optimal catalyst precursor.
The crystal structure of 1Ce is shown in Figure S35.[19]
1
0), suggesting that almost all acetate ligands were readily
replaced by the added ammonium benzoates. To the best of our
knowledge, this is the first example of the telomerization of
polycarbonate by adding excess amounts of carboxylate anions,
Table 2. Catalytic activity and selectivity of 1Ln
.
2 2 3
although additive effects of H O, HNR , HSR, and HOSiR were
investigated,[20] and additive effects of ammonium salts were
observed for catalyst systems of salen-type metal complexes for
copolymerization to improve catalytic activity and control
selectivity, as well as the formation of cyclic carbonate from
epoxides and CO
2
.[10b-i,21]
TOF[a] /h-1
Carbonate
M
n
[c]
/g·mol-1
PDI[c][d]
Entry
Ln
linkage[ /%
b]
Table 3. Telomerization to give polycarbonates.
1
2
La
Ce
Ce
Ce
Pr
230
370
330
300
310
320
290
250
200
100
99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
98
10000
14000
14000
15000
14000
18000
21000
11000
11000
6600
1.2
1.3
1.2
1.2
1.3
1.2
1.2
1.2
1.2
1.2
3[e]
4[e]
5
Entry
R
[nBu
equiv to 1Ce
4
N][OAc]
TOF[a]
/h-1
[b]
M
n
/g·mol-1
PDI[b][c]
(
)
6
Nd
Sm
Eu
Gd
Dy
1
2
CH
CH
CH
CH
CH
3
3
3
3
3
1.0
2.0
5.0
360
300
290
350
180
250
270
310
300
390
11000
9300
6900
4300
1800
4000
5500
6500
4800
5400
1.3
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
7
3
8
4
10.0
20.0
10.0
10.0
10.0
10.0
10.0
5
9
6
tBu
7
Ph
10
8
4-ClC
6 4
H
9
4-BrC
6
H
4
[
a] Turn Over Frequency = mole [CHO] consumed per mole catalyst per
1
hour. [b] Determined by the relative integrals of the H NMR resonances at
δ = 3.45 (polyether) and δ = 4.65 ppm (polycarbonate). [c] Determined by
GPC in THF, using polystyrene standards as the calibrant. [d] Bimodal
10
4-vinylC
6
H
4
[
[
a] Turn Over Frequency = mole [CHO] consumed per mole catalyst per hour.
b] Determined by GPC in THF, using polystyrene standards as the calibrant.
molecular weight distribution in GPC was observed. [e] The pressure of CO
was 0.6 MPa. [e] The pressure of CO was 0.3 MPa.
2
2
[c] Bimodal molecular weight distribution in GPC was observed.
We propose
copolymerization of CHO and CO
based on the above controlled experiments and dynamic behavior
a
reaction mechanism of alternative
Additive effects of ammonium carboxylates.
2
catalyzed by complex 1Ln
We investigated the telomerization of polycarbonate by adding
excess amounts of carboxylate anions based on the observed
fluxional behavior of acetates in an intramolecular and
of the acetate ligands (Scheme 2).
As an initiation of the
1
polymerization, the  -acetate bound to the zinc atom attacks
CHO activated by the proximate lanthanide element to generate
the Ln-alkoxide species. At the propagation stage, the alkoxide
intermolecular manner (Table 3). Alternative copolymerization of
n
CHO and CO
2
in the presence of [ Bu
4
N][OAc] (1.0 equiv to 1Ce
)
gave a lower molecular weight polymer (Entry 1). By adding
2
bound to the lanthanide element reacts with CO activated by the
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Angewandte Chemie International Edition
10.1002/anie.201709218
COMMUNICATION
zinc atom, where the Zn(II) center in macrocycles favors binding
grateful to Prof. Yoshito Tobe and Dr. Kazukuni Tahara for their
cooperation on measuring MALDI-TOF-MS. We appreciate Dr.
Yang Yi, Dr. Naohiro Kirai, Dr. Teruhiko Saito and Dr. Yusuke Kita
for their continuous interest as well as fruitful comments.
[6,22]
with CO
2
,
resulting in the formation of carbonate bound to the
zinc center, which further attacks the activated CHO on the
lanthanide element. Overall, the alternating polymerization
2
proceeded in a living manner, even under low CO pressure. After
the propagation reaction, chain transfer reaction with added
ammonium carboxylates proceeded to regenerate the  -
carboxylate initiator along with the corresponding polymer with an
Keywords: Macrocycles • Lanthanide • Zinc • Copolymerization
1
•
Telomerization
ammonium alkoxide terminal group as a key telomerization step.
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Acknowledgements
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This work was supported by JSPS KAKENHI JP15H05808 in a
Grant-in-Aid for Scientific Research on Innovative Areas,
Precisely Designed Catalysts with Customized Scaffolding,
JP16H06934 in a Grant-in-Aid for Research Activity Start-up of
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a) Figure 1 shoews three possible neutral species where two of
three acetates bridge between a zinc atom and a lanthanum atom,
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1
and uud-isomer were indistinctive, thus we described only uuu-
isomer. b) The structure of anionic complex, which we proposed, a
1
zinc atom has two  -acetate ligands both of up and down side such
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[
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A
new family of homogeneous
Haruki Nagae, Ryota Aoki, Shin-nosuke
Akutagawa, Julian Kleemann, Risa
Tagawa, Tobias Schindler, Gyeongshin
Choi, Thomas P- Spaniol, Hayato
Tsurugi, Jun Okuda, and Kazushi
Mashima
heterometallic complexes based on
trizincated macrocycle trisaloph and
rare earth metals showed high
catalytic activity for the alternating
copolymerization of cyclohexene
2
oxide and CO with high proportion of
Lanthanide Complexes supported by
a Trizinc-crown Ether as catalysts for
carbonate repeat units. Carboxylate
anion of ammonium salts acted as
initiator to proceed telomerization of
Alternating
Epoxide and CO
Controlled by Carboxylate Anions
Copolymerization
of
2
:
Telomerization
polycarbonate
which
have
corresponding carboxylates as the
initiation group.
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