Zwitterionic Design Principle of Nickel(II) Catalysts for Carbonylative Polymerization of Cyclic Ethers

Article abstract of DOI:10.1002/anie.201808507

Zwitterionic structure is necessary for Ni^II complexes to catalyze carbonylative polymerization (COP) of cyclic ethers. The cationic charge at the Ni^II center imparts sufficient electrophilicity to the Ni–acyl bond for it to react wi

Full text of DOI:10.1002/anie.201808507

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Accepted Article
Title: Zwitterionic Design Principle of Ni(II) Catalysts for Carbonylative
Polymerization of Cyclic Ethers
Authors: Li Jia, Yiwei Dai, Shiyu He, Bangan Peng, Laura A Crandall,
Briana R Schrage, and Christopher J Ziegler
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Zwitterionic Design Principle of Ni(II) Catalysts for Carbonylative
Polymerization of Cyclic Ethers
Yiwei Dai,[a] Shiyu He, Bangan Peng, Laura A. Crandall, Briana R. Schrage,[b] Christopher J.
[a]
[a]
[b]
[
b]
[a],[b]
Ziegler, and Li Jia*
Dedication: This paper is dedicated to Professor Richard A. Andersen on the occasion of his 75th birthday.
Abstract: Zwitterionic structure is necessary for Ni(II) complexes to
catalyze carbonylative polymerization (COP) of cyclic ethers. The
cationic charge at the Ni(II) center imparts sufficient electrophilicity to
the Ni-acyl bond for it to react with cyclic ethers to give an acyl-cyclic
ether oxonium intermediate, while the ligand-centered anionic charge
ensures that the resultant oxonium cation is ion-paired with the Ni(0)
nucleophile. The current catalysts give non-alternating copolymers of
carbon monoxide and cyclic ethers and are the most effective when
both ethylene oxide and tetrahydrofuran are present as the cyclic
ether monomers.
3
another ligand invariably (perhaps with the exception of PF )
retards the reactivity of the acyl group toward nucleophiles [11e].
The problem is particularly impairing for the epoxide COP since
4
even acyl-Co(CO) is barely reactive enough.
After testing an exhaustive list of organic acyl-transfer co-
catalysts without any appreciable gain in either the productivity of
the polymerization or the molecular weight of the product, we
decided to explore other transition metals. What metal-acyl
complexes are more electrophilic than acyl-Co(CO) ? One can
4
arrive at a reasonable estimate by assuming a rough correlation
between the electrophilicity of the metal-acyl bond and the acidity
of the corresponding metal hydride. The documented acidities of
a wide range of metal hydrides [17,18] suggest that acyl-Co(CO)
4
Transition metal-catalyzed carbonylative polymerization (COP)
utilizes CO as a comonomer to produce a variety of polymers,
including polyketones, polyamides, and polyesters [1]. Since it
is likely the most electrophilic among neutral metal-acyl
complexes and only cationic Ni(II) and Pd(II) acyl complexes are
possibly more electrophilic than acyl-Co(CO) .
4
2
can be easily obtained by CO reduction, CO is a raw materials
However, we extensively screened cationic Ni(II) and Pd(II)
complexes at the onset of our pursuit for the COP of cyclic ethers
even before we turned our attention to cobalt. The results were
disappointing without exception. Typically, when epoxides were
subjected to the cationic Ni(II)- or Pd(II)-acetyl compounds under
high-pressure CO, oligoethers with an acetate ester end group
were obtained, but not poly- or oligoesters. The same type of
reactivity was also observed for tetrahydrofuran (THF) by
Arndtsen and cleverly taken advantage of for the synthesis of
poly(norbornene-block-THF) using cationic Pd complexes [19].
with low carbon footprint. Among the products, polyketone is
photodegradable, and polyesters are hydrolytically degradable.
These features make COP an attractive platform for producing
sustainable polymers for a variety of applications [2]. Among the
COPs, CO-olefin copolymerization is well-studied [3-8], while the
COPs of heteroatom-containing comonomers (for example,
epoxides, aziridines, imines, etc.) only experienced a modest
amount of research interest [9-16].
Cobalt was the metal of choice to catalyze the COPs of
+
epoxides, aziridines, imines, etc [11-16]. Acyl-Co(CO)
4
is likely
Apparently, the initial attack by the cyclic ether on the acetyl-Ni(II)
+
ubiquitously involved as the catalytically active species. The
mechanisms for the enchainment of the polar co-monomers have
much commonalities among themselves but are different from the
coordinative-insertion mechanism of olefin enchainment. The
electrophilic acyl group is the reactive site for the nucleophilic co-
monomers instead of the Co center. The COPs of cyclic amines
and imines, which are good nucleophiles, are relatively fast, give
polyamides in high yields, and often display living characters [11,
and acetyl-Pd(II) bond indeed happens, but the resulting neutral
Ni(0) and Pd(0) intermediate are separated from the acyloxonium
cation in solution (Scheme 1). The acyloxonium cation then goes
on by itself to initiate the cationic oligomerization of epoxide or
polymerization of THF.
1
6]. Cyclic ethers, primarily epoxides, require the presence of a
pyridine derivative as the co-catalyst to undergo COP [11d, 12-
4]. The pyridine co-catalyst activates the acyl-Co bond just as in
1
organic acyl-transfer reactions but unfortunately also cleaves the
ester bonds in the polyester product [11d]. Continuous progress
in this area has been hampered by the fact that acyl-Co(CO)
cannot be readily modified. Substitution of a CO ligand with
4
Scheme 1. Difference between Cationic and Zwitterionic M-acyl Species (M =
Ni/Pd) in Reaction with Cyclic Ether.
[
[
a]
b]
Y. Dai, S. He, B. Peng, and Prof. Li Jia
Department of Polymer Science
The University of Akron
Akron, OH, 44325-3909, USA
E-mail: ljia@uakron.edu
L. A. Crandall, B. R. Schrage, Prof. C. J. Ziegler
Department of Chemistry
The University of Akron
In retrospect, the existence of the anionic metal nucleophile and
the cationic chain end as an ion pair apparently is a key to the
success of the charge-neutral acyl-Co(CO)
Ni(II)-acyl complexes possess the critical features of both their
cationic analogs and acyl-Co(CO) namely, the high
4
catalyst. Zwitterionic
Akron, OH 44325, USA
4
,
electrophilicity at the acyl site and the overall charge neutrality.
One can expect that after nucleophilic displacement by a cyclic
ether from the acyl site (Scheme 1), the Ni(0) species carrying an
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
anionic phosphine ligand [20] would remain ion-paired with the
acyl-oxonium. The probability for addition of the anionic Ni(0)
nucleophile to the activated cyclic ether should be vastly improved
in comparison to a neutral Ni(0) nucleophile.
Our group has explored the above concept in the last several
years [21]. The previous zwitterionic Ni(II) catalysts suffered from
insufficient stability under CO and were inactive for the COP of
cyclic ethers. With our newly developed ligands that feature ortho-
methoxy auxiliaries to stabilize the coordination, we demonstrate
here the validity of the zwitterionic design principle of Ni(II)
catalysts for the COP of cyclic ethers.
Zwitterionic Ni(II) acetyls, 1a and 1b, are synthesized by
reaction of the lithium salt of the corresponding Ni(0) anion with
+
-
3 4
Me O BF under CO (eq 1). Both 1a and 1b are stable in aromatic
solvents at room temperature and are isolated and completely
characterized (see Supporting Information). They are the only
detectable species under CO (1 atm) in solution by 1H and 31
P
NMR spectroscopy but are in equilibrium presumably with the
methyl complexes, 1a-CO and 1b-CO, under nitrogen. Since 1a-
CO and 1b-CO are absent under CO, the relatively broad
1
linewidths of the o-methoxy H NMR signal in 1a and 1b (9.6 an
8.4 Hz at half height, respectively) under CO are likely attributable
to weak dynamic interactions between the o-methoxy groups and
Ni. Also worth noting is that we were unable to isolate or observe
the methyl or acetyl compounds with the previous generations of
anionic o,o’-diphosphinobenzene ligands devoid of the o-methoxy
auxiliaries.
X-ray crystallographic study confirms that 1a adopts square
planar conformation (Figure 1). The distances between Ni and the
O atoms in the o-methoxy groups (O(3)-Ni(1) = 3.021(8) and O(4)-
Ni(1) = 3.531(8) Å) are substantially greater than the sum of the
Ni and O atomic radii. However, one of the o-methoxy groups
appears to point toward the Ni center, suggesting some weak
interaction. Single crystals of 1b-CO were obtained by diffusion of
Figure 1. X-Ray structures of 1a (top) and 1b-CO (bottom) with 35% probability
ellipsoids. Hydrogen atoms have been omitted for clarity. Selected bond lengths
in 1a: C(1)-Ni(1) = 1.785(7), C(2)-Ni(1) = 1.926(8), C(1)-O(1) = 1.120(10), C(2)-
O(2) = 1.236(13), P(1)-Ni(1) = 2.2252(19), P(2)-Ni(1) = 2.2039(16) Å. Selected
bond lengths in 1b-CO: C(1)-Ni(1) = 1.971(3), C(2)-Ni(1) =1.780(3), C(2)-O(1)
pentane into the solution of 1b under N
2
. The crystal structure of
1b-CO reveals a similar bonding situation (Figure 1). The O-Ni
distances (O(2)-Ni(1) = 2.977(3) and O(3)-Ni(1) = 3.593(3) Å) are
out of the bonding range, but one of the o-methoxy groups points
toward the Ni center.
=
1.119(3), P(1)-Ni(1) = 2.1817(7), P(2)-Ni(1) = 2.1994(7) Å.
The reactions of neat cyclic ethers with CO (300 – 900 psi) were
screened in the presence of 1a as the catalyst. For ethylene oxide
(
EO), no catalytic reaction occurred. For oxetane (OT) and THF,
a very small amount of oligomer was obtained in each case. The
product primarily consisted of ether repeat units but also ester
linkages as the result of CO incorporation (eqs 2 and 3). In both
cases, an acetyl end group, which apparently originates from 1a,
was observable in the 1H NMR spectra of the product (see
Supporting Information).
d
c
e
b
f
a
g
Interestingly, when the polymerization was performed in the
presence of both EO and THF (entry 1 in Table 1), a substantial
amount of viscous product was obtained. 1- and 2-Dimensional
NMR spectroscopic studies and GPC analysis were carried out to
1
Figure 2. H NMR spectra of the products of the COP of EO and THF (top) and
the COP of EO and THF-d
on the basis of chemical shifts and H- H COSY (see Supporting Information).
8
(bottom) in CDCl . The peak assignments are made
3
1 1
1
characterize the product (see Supporting Information). H NMR
shows that the product is composed of ester and ether units with
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COMMUNICATION
Table 1. Summary of EO-THF COP catalyzed by 1a and 1b.[
a]
[
b]
[c,d]
[d]
Entry Catalyst
Temperature CO Pressure
°C)
Reaction
(Atmosphere) Time (h)
EO (mL)
CO : EO : THF ratio
M
w
PDI
Yield
(g)
(
in the product (1 : x : y)
(g/mol)
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1b
1b
25
25
25
25
25
0
20
20
60
20
20
20
20
20
16
16
16
3
0.5
1
1
1
1
1
0.5
1
1 : 2.3 : 6.7
1 : 5.2 : 7.4
1 : 5.2 : 8.0
1 : 3.1 : 6.9
1 : 2.6 : 4.5
1 : 4.5 : 14.2
1 : 3.2 : 8.0
1 : 4.9 : 7.2
21,200
16,500
15,800
13,500
16,600
34,500
11,800
12,600
2.01
1.97
2.00
2.01
2.08
2.35
1.85
1.85
0.84
1.09
0.93
0.32
0.69
0.28
1.08
1.21
6
16
16
16
25
25
1
[
a] Reaction conditions: 15 mg of catalyst in 1 mL of THF. [b] Determined by H NMR integration. [c] Weight-average molecular weight. [d] Determined by GPC
relative to polystyrene standards.
a CO : EO : THF stoichiometry of 1 : 2.3 : 6.7 (Figure 2). The
The drastic difference in productivity between the COP of EO
and THF together and the COPs of EO or THF alone can be
satisfactorily accounted for by a mechanistic scenario (Scheme
2) fully compatible with the aforementioned working hypothesis
(Scheme 1) for zwitterionic Ni(II)-acyl catalysts. A priori, EO is
appreciably less nucleophilic than THF [26]. As such, EO cannot
react with 1a+CO, but THF can to give b. The THF ring in b is
not sufficiently activated by the acyl group to undergo rapid ring-
opening reaction but can rapidly exchange with EO to form c.
The EO ring in c is highly reactive and undergoes rapid ring-
opening reaction with free THF (less likely with free EO or the
least likely with the Ni(0) counter-anion) to give d. In essence,
THF acts as a co-catalyst for promoting EO enchainment like
number-average molecular weight (M
n
) is 10,600 g/mol relative
to polystyrene standards, and the polydispersity index (PDI) is
~
2.01. 1H NMR DOSY analysis [22, 23] revealed that the
diffusion coefficients individually determined using 1H
resonances of the ester and ether units fall in a narrow range (D
=
0.88~1.17 x 10-10 m2 s-1, see Supporting information), in
agreement with notion that these structures belong to a single
product. Ethanolysis of the product resulted in a substantial
reduction in molecular weight (See Supporting Information). The
experimentally determined M
consistent with the expected M
n
of the ethanolyzed product is
(730 vs 658 g/mol), which is the
n
average molecular weight between two ester units plus the
molecular weight of ethanol. The PDI of the ethanolyzed product
is 3.24, indicating that the distribution of the ester groups is
relatively uniform in the original product before ethanolysis. The
expected ethyl ester end group after ethanolysis is identified by
4
pyridine in the COP of epoxides catalyzed by acyl-Co(CO) [11d];
while EO promotes THF enchainment the same way as it does
when a cationic initiator does not sufficiently activate THF to
initiate THF polymerization [27]. The tertiary oxonium
propagating chain end is sustained for several THF and EO
enchainments (d → e) until nucleophilic addition of the Ni(0)
counteranion to the THF oxonium re-establishes a Ni-C  bond
1
H NMR (See Supporting Information). Thus, all spectroscopic
and chemical evidence unequivocally confirms the structure of
the poly(ether-co-ester) product.
(
e → f). Coordinative CO insertion ensues to regenerate the
When the aforementioned reactions were carried out under
atm of CO under otherwise identical conditions, no polymer or
zwitterioinic Ni-acyl active species at the chain end.
1
oligomer products were obtained. The requirement of high CO
pressure suggests that 5-coordinate species 1a+CO is
catalytically active, not 1a. The proposition is in agreement with
+
the anticipated reactivities of 4- vs 5-coordinate Ni -acyl
compounds, judging from the IR absorption wavenumber of the
acyl C=O stretching vibration (C=O. A high C=O value similar to
that of an organic ester or acyl chlorides indicates high
electrophilicity and high reactivity, and a low C=O value similar to
that of organic amide indicates the opposite [24]. The reported

C=O values of a pair of 4- and 5-coordinate cationic Ni-acetyl
-1
compounds are 1698 and 1733 cm [25], respectively, indicating
that the acetyl group of the 5-coordinate compound is more
electrophilic than that of the 4-coordinate compound. The C=O
-1
value is 1,691 cm for 1a. This suggests that the acetyl group in
Scheme 2. Proposed mechanism of mutually promoted THF and EO
1a is quite unreactive similar to an organic amide.
enchainments.
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COMMUNICATION
The above mechanistic scenario mandates a microstructural
signature; that is, a CO enchainment is followed by an EO
enchainment but rarely by a THF enchainment. To decipher this
catalyst design principle unlocks a new venue of catalyst
development for COPs of heteroatom-containing monomers.
With such novel catalysts, new polymerizations and useful
polymer products will emerge.
specific sequence, we carried out the COP of deuterated THF
1
(
THF-d
8
, 99.5 atom% D) and EO. The H NMR spectrum of the
product is compared to that of the product of regular THF and
EO (Figure 2). The triplet of the methylene group vicinal to the
ester bond remains intact in the H NMR spectrum of the product
Experimental Section
1
from THF-d
8
and EO, but the triplet of the -methylene next to
Synthesis of 1a. The overall synthetic route is outlined in Supporting
Information (Scheme S1). The lithium salt of the corresponding Ni(0)
anion (6a, 500 mg, 0.4 mmol) and trimethyloxonium tetrafluoroborate
the carbonyl group is no longer detectable. This observation
confirms that the ester linkages arise primarily from EO. In other
words, a CO enchainment is indeed followed by an EO
enchainment but rarely by a THF enchainment. The specific
sequence therefore provides convincing evidence for the
mechanism depicted in Scheme 2.
(
200 mg, 1.35 mmol) were dissolved in CH
2
Cl
2
(8 mL). The mixture was
o
stirred under CO at -30 C for 6 h. Hexane (20 mL) was added to the
orange solution. The mixture was stirred to give copious precipitate. The
2 2
solvent mixture of hexane and CH Cl was removed by filtration. The solid
was washed with hexane and extracted with toluene (20 mL) under CO.
Hexane (50 mL) was layered on top of the toluene solution. The mixture
To assess the limits of the current catalysts and to further
interrogate the mechanism, we carried out the polymerization
under varied conditions (Table 1). For the reactions catalyzed by
o
was kept at -45 C for several days to give a yellow crystalline precipitate
accompanied by a small amount of orange oil. The solution was removed
by filtration. The crude product was washed with benzene (7 mL) quickly
to remove the orange oil. The yield of the resultant yellow crystalline
product was 56% (216 mg).
1a, an increase in the EO amount in the feed resulted in a
decrease in the molecular weight and an increase in the number
of ether units (i.e., x + y) between two ester linkages in addition
to the expected increase in EO incorporation in the product
(
entries 1 vs 2). Increasing the CO pressure from 300 psi to 900
General Polymerization Procedure. Catalyst 1a or 1b (15 mg) was
psi did not result in any marked difference in the composition of
the product (entries 2 vs 3). This is expected according to the
mechanism depicted in Schemes 2 since coordinative CO
insertion takes place at the metal center while EO and THF
enchainments are initiated by the acyl group and proceed via the
oxonium intermediate. They do not directly compete again each
other. Shortening the reactions time decreased the yield (entries
added to an autoclave in the glove box under nitrogen. The nitrogen
atmosphere in the autoclave was replaced with CO (1 atm) on a Schlenk
line. The cyclic ether monomer(s) was cooled to -78 oC and injected to
the autoclave via a syringe under a gentle flow of CO (1 atm). The
pressure of CO was increased. The reaction was magnetically stirred for
a period of time at a set temperature as specified in Table 1. The pressure
was released. Caution! The autoclave was opened in a well-ventilated
hood and allowed to sit in air for at least 1 h with continuing stir. This is to
2, 4, and 5). Lowering the reaction temperature from 25 °C to 0
°C doubled the molecular weight but also significantly lowered
allow oxidative decomposition of any Ni(CO)
4
that might have
the yield. The amount of THF incorporated into the product
markedly increased. When 1b was used as the catalyst, the
yields increased modestly, but the molecular weights of the
products decreased somewhat in comparison to the reactions
catalyzed by 1a under identical conditions (entries 1 vs 3 and 2
vs 4). Overall, the differences between 1a and 1b are small. All
poly(ether-co-ester)s produced by 1 a and 1b are viscous liquids.
adventitiously formed. The solution was then transferred into a vial, and
the volatile fraction was removed under vacuum. The product was
obtained as a viscous liquid.
Acknowledgements
Finally, a direct cationic analog of 1a and 1b with the same
bis(phosphino)benzene lignad framework was synthesized and
tested for the copolymerization of EO and THF under high-
pressure CO (see Supporting Information). Only polyether was
obtained. The amount of ester structure in the product is dubious
at best. This unequivocally confirms that the zwitterionic
structure is necessary.
The research is supported by the National Science Foundation
(
CHE-1266442). CJZ acknowledges The University of Akron for
support of this research. LJ thanks Dr. Eite Drent for helpful
discussions and suggestions. We thank Dr. Jessi A. Baughman
for his assistance with the diffusion NMR experiments.
In summary, we have provided the initial examples of
zwitterionic Ni(II) compound-catalyzed COP of cyclic ethers and
hence validated the zwitterionic design principle of Ni(II)
catalysts. The polymerization catalyzed by 1a and 1b is non-
alternating with respect to CO and cyclic ether enchainments.
The COP of EO and THF together is far more effective than the
COPs of EO, OT, or THF alone. The current poly(ester-co-ether)
products are water-soluble and have glass transition
temperatures well below room temperature. While potential
applications can be envisioned based on these properties, for
example, as the soft segment of thermoplastic elastomers and
hydrogels, the value of the present work is that the zwitterionic
Keywords: Carbonylation • Polymerization • Catalysis
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Angewandte Chemie International Edition
10.1002/anie.201808507
COMMUNICATION
This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201808507
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
The necessity for the Ni(0) nucleophile
and the cyclic ether oxonium to exists
as an ion-pair requires the initial Ni(II)
catalyst to be a zwitterion.
Yiwei Dai, Shiyu He, Bangan Peng,
Laura A. Crandall, Briana R. Schrage,
Christopher J. Ziegler, and Li Jia*
Zwitterionic
Ion-paired
in solution
Page No. – Page No.
Zwitterionic Design Principle of Ni(II)
Catalysts for Carbonylative
Polymerization of Cyclic Ethers
vs
Cationic
Separated
in solution
This article is protected by copyright. All rights reserved.