Catalysis and kinetics for alternating copolymerization of carbon dioxide with epoxides using dinuclear nickel catalysts of pyrazolyl based diamine-bisphenolate ligands

Article abstract of DOI:10.1016/j.polymer.2020.122553

The synthesis of structurally well-characterized dinuclear nickel complexes bearing a new type of pyrazolyl based diamine-bisphenolate ligand was reported for the first time. Catalysis towards nickel-catalyzed copolymerization of CO₂ with cyclohexene oxide (CHO) by these newly prepared bimetallic complexes was systematically investigated. Particularly, dinickel complex 4 supporting 2-nitrophenoxide coligands was demonstrated to effectively catalyze CO₂-copolymerization of CHO, 4-vinyl-1,2-cyclohexene oxide or cyclopentene oxide, generating the corresponding CO₂-based polycarbonates with >99% carbonate linkages under optimal conditions. Kinetic studies of CO₂/CHO copolymerization revealed that such copolymerization catalysis is first-order in dinickel complex 4 and second-order in CHO concentration.

Full text of DOI:10.1016/j.polymer.2020.122553

Polymer 200 (2020) 122553
Contents lists available at ScienceDirect
Polymer
journal homepage: http://www.elsevier.com/locate/polymer
Catalysis and kinetics for alternating copolymerization of carbon dioxide
with epoxides using dinuclear nickel catalysts of pyrazolyl based
diamine-bisphenolate ligands
*
Yu-Chia Su, Chi-Tien Chen, Bao-Tsan Ko
Department of Chemistry, National Chung Hsing University, Taichung, 402, Taiwan
A R T I C L E I N F O
A B S T R A C T
Keywords:
The synthesis of structurally well-characterized dinuclear nickel complexes bearing a new type of pyrazolyl based
diamine-bisphenolate ligand was reported for the first time. Catalysis towards nickel-catalyzed copolymerization
Dinickel catalyst
CO
2
-Copolymerization
of CO
tigated. Particularly, dinickel complex 4 supporting 2-nitrophenoxide coligands was demonstrated to effectively
catalyze CO -copolymerization of CHO, 4-vinyl-1,2-cyclohexene oxide or cyclopentene oxide, generating the
corresponding CO -based polycarbonates with >99% carbonate linkages under optimal conditions. Kinetic
studies of CO /CHO copolymerization revealed that such copolymerization catalysis is first-order in dinickel
complex 4 and second-order in CHO concentration.
2
with cyclohexene oxide (CHO) by these newly prepared bimetallic complexes was systematically inves-
Catalysis
2
2
2
1
. Introduction
catalysis behavior of such copolymerization, there has been an
increasing interest in the investigation of its kinetic studies. Previous
studies have focused on the kinetics of CO /CHO copolymerization by
Driven by the requirements to reduce green-house gas emissions in
2
chemical processes, the transformation of carbon dioxide (CO
commercially viable fine-chemicals is a growing field of research to
date. Of CO utilization investigations, fixing CO to produce high value-
2
) into
well-defined bimetallic catalysts containing initiating groups, enabling
to propose the plausible mechanism for their copolymerization catalysis
[21–23]. Based on these kinetic data, such metal complex-catalyzed
2
2
added polycarbonates has received considerable attention over the past
few decades [1–4]. Consequently, all sorts of discrete catalysts including
aluminum, cobalt, chromium, magnesium, and zinc complexes based on
2
CO -copolymerization of CHO was found to be the first-order kinetics
in both catalyst and epoxide monomer.
In a previous work, our group reported a new class of bimetallic Ni
and Co complexes supported by nitrogen-heterocycle containing
diamine-bisphenolate ligand frameworks [24]. These studies displayed
2
diverse ancillary ligands were reported for CO -copolymization of ep-
oxides [5–9]. Among the homogeneous catalysis of this kind, increasing
attention has been recently paid to the development of bimetallic
that
benzotriazole-containing salan-type derivatives were demonstrated to
efficiently copolymerize CO and CHO with satisfactory activities, high
diacetate
Ni(II)/Ni(II)
complexes
coordinated
on
2
complex-catalyzed CO /epoxides copolymerization because of its good
catalytic efficiency with satisfactory turnover frequency (TOF) [10]. For
instance, Williams and coworkers have developed a series of impressive
one-component bimetallic catalysts; these homodinuclear Zn(II), Mg(II),
Co(II/III) and Fe(III) macrocyclic complexes were demonstrated to be
2
selectivities and good controllability of molecular weights under
optimal conditions. With the ongoing research interest, we extended our
study introducing a pyrazolyl (Pz) moiety as the N-heterocycle group in
the ancillary ligand and anticipated that modifications of different
N-donor groups in the ligand might lead to structural variations and
significant influences on the catalytic efficiency. In this study, we
developed the new examples of dinickel complexes of Pz based
active and selective for 1 atm copolymerization of CO
oxide (CHO) to generate poly(cyclohexene carbonate)s (PCHCs)
11–16]. Importantly, Lu’s group has also performed extensive work in
catalytic conversion of CO with a wide range of epoxides by dinuclear
2
and cyclohexene
[
2
catalyst systems comprising of Co(III) or Cr(III) salen-type complexes to
salan-type ligands as effective catalysts for the CO
2
-copolymerization of
various polycarbonates [17–20]. In order to deeply understand the
epoxides into valuable CO -based polycarbonates. Kinetic studies for
2
*
Corresponding author.
E-mail address: btko@dragon.nchu.edu.tw (B.-T. Ko).
https://doi.org/10.1016/j.polymer.2020.122553
Received 27 February 2020; Received in revised form 28 April 2020; Accepted 2 May 2020
Available online 18 May 2020
0
032-3861/© 2020 Published by Elsevier Ltd.

Y.-C. Su et al.
Polymer 200 (2020) 122553
copolymerization of CO
also studied detailedly.
2
with CHO by the best catalyst in this work were
conformations with two six-coordinate Ni atoms having distorted octa-
hedral environments, which are similar to those dinickel diacetate
complexes incorporated by benzotriazole-containing salan-type ligands
[
24]. Their selected bond distances and angles between nickel atoms and
2
. Results and discussion
ligands are outlined in Table S1, and the Ni-including bond lengths are
all within the normal ranges compared with the earlier reported dinickel
complexes coordinated by nitrogen-heterocycle containing salen-type
derivatives [28–30]. The solid-state structure (Fig. 1) of 4 is also a
dinuclear nickel species constructed by a ligand scaffold of the
Pz-supported salan-type derivative and two different bonding types of
NP groups, resulting in two significantly inequivalent environments of
Ni atoms. The Ni(1) atom is hexa-coordinated by two μ2-O(phenoxy)
atoms and two N(amine) atoms of a salan-type ligand as well as two
oxygen atoms from a chelating NP moiety, adopting a distorted octa-
hedral geometry with the average distances between the Ni(1) and O
2
.1. Synthesis and crystal structure
The pyrazolyl (Pz) substituted phenol (MePzMePhOH) derivative was
0
0
prepared through a two-step synthetic pathway from 2 -hydroxy-5 -
methylacetophenone as the starting material by according to the pre-
vious literature procedures (Scheme S1(a), SI) [25–27]. As shown in
Me Me
Scheme S1(b) of SI, the one-pot Mannich condensation of Pz PhOH
0
with the mixtures of N,N -dimethylethylenediamine and para-
formaldehyde in refluxing ethanol solution was further executed to
furnish the multidentate diamine-bis(pyrazolyl phenol) derivative,
(
phenoxy of L) and N(amine) being 1.999(2) Å and 2.085(3) Å,
respectively. In contrast, the Ni(2) atom assumes a penta-coordinated
and distorted square pyramidal coordination sphere (
¼ 0.058) that
is bonded by two N(Pz) atoms, two -O(phenoxy) atoms from the
2
namely Pz-containing salan-type ligand precursor (L-H ) in this study.
This proligand was isolated as crystalline solids and fully characterized
by microanalyses as well as NMR spectroscopy. The synthetic routes of
bimetallic nickel complexes 1–4 coordinated on such Pz-containing
salan-type ligand were illustrated in Scheme 1. In the presence of
τ
μ
2
ancillary ligand and one O atom of monodentate NP fragment with
average bond distances of Ni(2)–O(phenoxy of L) ¼ 1.997(2) Å and Ni
(2)–N(Pz) ¼ 2.046(3) Å. It was noted that the NP-involving bond dis-
tance of Ni(1)-O(3) was found to be 2.004(2) Å which is slightly larger
than that (1.992(2) Å) of Ni(2)-O(5), possibly due to the six-coordinated
coordination environment on Ni(1) atom. The structural studies of
compounds 1–4 suggested that the Pz-containing salan-type ligand was
capable of stabilizing the formation of bimetallic nickel complexes that
excess triethylamine (NEt
3
), treatment of L-H
O] followed by the addition of trifluoroacetic acid or 2,3,4,5,
-pentafluorobenzoic acid (5.0 M equiv.) in MeOH solution under reflux
gave the dicarboxylate dinickel complexes [L Ni (O CCF ] (1) and
Ni (O CC ] (2) in good yield. However, utilizing the similar
2
with 2.0 equivalent [Ni
.
(
4 2 2
ClO ) 6H
6
2
2
2
3 2
)
[
L
2
2
2
6 5 2
F )
synthetic procedure of 4-(trifluoromethyl)benzoic acid as the coligand
precursor in refluxing MeOH solution afforded the water-solvated
2
could serve as catalysts for CO -copolymerization of epoxides.
binickel dicarboxylate complex [L
2
Ni
2
(O
2
CC
6
H
4
-4-CF
3
)
2
(H
2
O)] (3).
Similarly, the dinickel diphenolate analogue [L
2
Ni (NP)
2
2
] (4) could be
generated in 84% yield by employing 2-nitrophenol (NP–H) as the col-
igand precursor under the synthetic method having ligand pre-
cursor/metal salt/phenol derivative ratio of 1:2:5 on the addition of
2
2.2. Copolymerization of carbon dioxide (CO ) with epoxides catalyzed
by bimetallic nickel complexes 1–4
excess NEt
3
in refluxing methanolic solution. Air-stable crystalline solids
The newly synthesized dinickel complexes 1–4 were performed as
single-component catalysts for CO -copolymerization of CHO, and
were obtained for complexes 1–4 and all of them are fully characterized
by elemental analysis and single crystal X-ray crystallography.
X-ray structural analysis indicates that crystal structures (Figs. S1-S3
in SI) of complexes 1–3 display bimetallic Ni(II)/Ni(II) dicarboxylate
2
Table 1 summarizes their representative results. Ni complex 4 was first
applied to catalyze such copolymerization under the conditions of a
2
0.0625% catalyst loading and an initial CO pressure of 300 psi. As can
.
Scheme 1. Synthetic pathways for dinuclear nickel complexes 1–4. (i) Ni(ClO
4 2 2
) 6H O (2.0 equiv)/trifluoroacetic acid or 2,3,4,5,6-pentafluorobenzoic acid (5.0
.
3 4 2 2 3
equiv)/NEt (15.0 equiv), MeOH, reflux, 48 h, 81% for 1 and 72% for 2. (ii) Ni(ClO ) 6H O (2.0 equiv)/4-(trifluoromethyl)benzoic acid (5.0 equiv)/NEt (15.0
.
equiv), MeOH, reflux, 48 h, 77%. (iii) Ni(ClO
4
)
2
6H
2
3
O (2.0 equiv)/2-nitrophenol (5.0 equiv)/NEt (15.0 equiv), MeOH, reflux, 48 h, 84%.
2

Y.-C. Su et al.
Polymer 200 (2020) 122553
Fig. 1. ORTEP drawing of complex 4 with probability
ellipsoids drawn at level 50%. Hydrogen atoms are
omitted for clarity. Selected bond lengths/Å and an-
gles/deg: Ni(1)-O(1) 2.003(2), Ni(1)-O(2) 1.995(2),
Ni(1)-O(3) 2.004(2), Ni(1)-O(4) 2.174(2), Ni(1)-N(5)
2
.076(3), Ni(1)-N(6) 2.093(3), Ni(2)-O(1) 2.025(2),
Ni(2)-O(2) 1.969(2), Ni(2)-O(5) 1.992(2), Ni(2)-N(1)
.047(3), Ni(2)-N(3) 2.044(3), O(2)-Ni(1)-O(1) 77.58
9), O(2)-Ni(1)-O(3) 167.88(9), O(1)-Ni(1)-O(3)
9.13(9), O(2)-Ni(1)-N(5) 100.24(10), O(1)-Ni(1)-N
2
(
9
(
(
(
(
5) 89.84(10), O(3)-Ni(1)-N(5) 91.35(10), O(2)-Ni
1)-N(6) 90.56(10), O(1)-Ni(1)-N(6) 167.21(10), O
3)-Ni(1)-N(6) 93.45(10), N(5)-Ni(1)-N(6) 87.57
10), O(2)-Ni(1)-O(4) 88.38(9), O(1)-Ni(1)-O(4)
8
1
4.69(9), O(3)-Ni(1)-O(4) 79.67(9), N(5)-Ni(1)-O(4)
68.58(10), N(6)-Ni(1)-O(4) 99.91(10), O(2)-Ni(2)-
O(5) 92.30(9), O(2)-Ni(2)-O(1) 77.64(9), O(5)-Ni
(
(
(
2)-O(1) 159.83(9), O(2)-Ni(2)-N(3) 86.52(10), O
5)-Ni(2)-N(3) 86.65(10), O(1)-Ni(2)-N(3) 109.81
10), O(2)-Ni(2)-N(1) 156.37(10), O(5)-Ni(2)-N(1)
1
01.74(10), O(1)-Ni(2)-N(1) 82.96(10), N(3)-Ni(2)-
N(1) 112.94(11), Ni(1) … Ni(2) 3.095(2).
Table 1
Copolymerization of CO
2
and epoxides catalyzed by dinickel complexes 1–4.
a
b
c
e
Entry
Catalyst (mol%)
Time
% epoxide
% Copolymer (%CO
3
linkages)
TON
TOF
n
M (Đ)
a
À 1d
/
h
Conv.
/h
f
1
4(0.0625)
4(0.0625)
4(0.0625)
4(0.0625)
1(0.0625)
2(0.0625)
3(0.0625)
4(0.0625)
4(0.0625)
4(0.0625)
4(0.0625)
4(0.0625)
4(0.125)
6
10
26
49
76
54
63
67
34
52
67
92
40
27
71
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
98(>99)
160
27
4100(1.14)
9900(1.10)
19100(1.23)
25400(1.16)
7800(1.23)
15000(1.23)
14600(1.23)
9400(1.20)
16900(1.18)
21100(1.14)
38700(1.28)
20100(1.50)
4700(1.66)
22200(1.21)
g
2
6
416
69
h
3
6
784
131
203
144
168
179
272
277
268
61
4
5
6
7
8
9
6
1216
864
6
6
1008
1072
544
6
2
3
832
1
1
1
1
1
0
1
2
3
4
4
1072
1472
640
24
6
i
98(>99)
107
9
g, j
k
24
4
88(>99)
216
A(0.0625)
>99(>99)
1136
284
be seen from entries 1–4 of Table 1, complex 4 was active in copoly-
displayed in entries 1–4 of Table S2. The reason why the dinickel cata-
lyst incorporated by the NP coligands performed the best efficiency may
be attributed to Ni complex 4 possessing an unsaturated Ni(2) atom
(Fig. 1) that might serve as a more effective active site to activate
merizing 1600 equiv. of CHO with CO
2
while the copolymerization
�
temperature was set from 100 to 140 C. Temperature effect also found
�
that the same copolymerization at 140 C was able to give the higher
productivity (CHO conversion ¼ 76%; TON >1200) and could generate
the alternating poly(cyclohexene carbonate)s (PCHCs, Fig. S4, SI) with
excellent copolymerization selectivity (>99% copolymer and >99%
carbonate linkages). Following the optimized conditions for the suc-
2
monomers during CO /CHO copolymerization. The effect of different
carboxylate coligands on the catalytic efficiency (entries 5–7 of Table 1
& entries 1–3 of Table S2) also suggested that the better electron-
withdrawing group on the carboxylic carbon make the poor nucleo-
philicity of the carboxyl group to proceed ring-opening of CHO, leading
to the worse catalytic activity for complex 1 among these examinations.
Consequently, complex 4 was chosen to further investigate the
�
cessful preparation of PCHC mediated by 4 at 140 C, complexes 1–3
were also conducted to evaluate their catalytic efficiency as shown in
entries 5–7 of Table 1. These catalysis data clearly displayed that the
activity of bimetallic Ni complexes containing diverse coligand moieties
controlled character on CO
2
/CHO copolymerization. Three extra par-
/[4]
has the order of 4 (NP) > 3 (O
CCF ). A same trend also appears in the Ni-catalyzed CO
copolymerization by complexes 1–4 at the lower conversion as
2
CC
H
6 4
3
-4-CF ) > 2 (O
CC
2 6
F
5
) > 1
allel experiments by the same catalyst with [CHO]
0
0
¼ 1600/1 at
(
O
2
3
2
/CHO
different copolymerization durations were carried out. It can be seen
from entries 8–10 of Table 1 that a nearly linear relationship (Fig. S5)
3

Y.-C. Su et al.
Polymer 200 (2020) 122553
between the observed molecular weights M
n
versus the CHO conversion
2-cyclohexene oxide (VCHO) or cyclopentene oxide (CPO) as the
as well as narrow Đs (<1.25) of the afforded PCHCs were found. These
results demonstrated satisfactory controllability of PCHC molecular
weights by NP-ligated dinickel 4. Note that 4 was also shown to give a
high CHO conversion up to 92% with preserving the copolymerization
monomer substrate to copolymerize CO
ingly, NP-ligated Ni complex 4 was found to be active in catalyzing the
CO -coupling reaction of VCHO or CPO as exhibited in entries 12 and 13
2
was also examined. Gratify-
2
of Table 1. For instance, copolymerization with a VCHO-to-catalyst ratio
of 1600:1 by 4 realized a comparable activity (entry 12 vs 4) under
identical conditions and enabled the formation of alternatively poly-
carbonate involving the vinyl group in the polymer backbone (Fig. S7).
In comparison, pyrazolyl-ligated bimetallic Ni catalyst 4 reveals the
similar activity to that of benzotriazole-bonded dinickel complex
n
selectivity and to obtain high-molecular-weight PCHC (M > 38,000 g/
mol) on extending the copolymerization duration to 24 h (entry 11,
Table 1). Bimodal molecular weight distributions (Fig. S6) were
observed for all produced PCHCs by dinickel catalyst 4, which is
consistent with the previous observations [28–30]. This may be resulted
from the occurrence of both initiation of NP moiety for the polymeric
growth and chain transfer reactions by the contamination from protic
species in the monomer/catalyst. The matrix-assisted laser desorptio-
n/ionization time-of-flight (MALDI-TOF) mass spectrum of a small mo-
lecular weight sample (Table 1, entry 1) that was displayed in Fig. 2
C8MEA
�
[(
DiBTP)Ni
2
(OAc)
2
(H
2
O)] (A) under the conditions of 140
C
and pCO0
¼ 300 psi for 4 h (TOF: 268 vs. 284 h ) [24].
À 1
2
0
2
Copolymerization conditions: 50.0 mmol CHO, pCO
¼ 300 psi, T ¼
140 C. Based on 1H NMR analysis of the reaction mixture [28–30].
�
a
b
Based on 1H NMR determination of the purified copolymers [28–30].
c
d
supports this deduction. The prime fragment peaks corresponding to [H
TON ¼ number of moles of CHO consumed per mole of catalyst. TOF ¼
þ
e
þ C
6
H
10O þ (C
7
H
10
O
3
)
n
þ C
6
H
4
NO
þ K] [(C
NO
3
þ Na] and [H þ C
6
H
10O þ
TON per hour. Determined by GPC in THF, calibrated using polystyrene
þ
f
�
g
�
h
�
i
(
C
7
H
10
O
3
)
n
þ C
6
H
4
NO
3
7
H
10
O
3
)
n
¼ (À C
6
H
10OC(O)OÀ )
n
,
standards. T ¼ 100 C. T ¼ 120 C. T ¼ 130 C. VCHO as the epoxide.
j
k
C
6
H10O ¼ –C
6
H
10OÀ and C
6
H
4
3
¼ 2-nitrophenoxide] could be
CPO as the epoxide. Dinickel complex A as the catalyst, reference [24].
assigned, definitely indicating that the NP group acted as an initiator to
promote the polymeric chain growth. The other series of signals asso-
2.3. Kinetic studies of CO /CHO copolymerization catalyzed by di-nickel
2
þ
ciating to [H þ C
6
H10O þ (C
7
H
10
O
3
)
n
þ OH þ H] could be attributed to
complex 4
di-hydroxyl end-capped PCHC chains, which may be by virtue of the
happening of chain transfer copolymerization [31–33]. In addition to
Kinetic studies of Ni-catalyzed CO /CHO copolymerization by com-
2
�
CO
2
/CHO copolymerization, utilizing less studied 4-vinyl-1,
plex 4 were carried out at 140 C, and conversion of CHO was
Fig. 2. The MALDI-TOF spectrum of the purified PCHC catalyzed by dinickel complex 4 (Table 1, entry 1).
4

Y.-C. Su et al.
Polymer 200 (2020) 122553
determined with 1H NMR spectrum. To establish reaction order in
monomer and catalyst concentrations, we first investigated the pressure
effect on the catalytic activity by using a catalyst loading of 0.0625 mol
%
for 6 h under different initial CO
00 psi). It was found that their activity almost performed the constant
TOF value from low to high CO
pressures (pCO0
¼ 200 to 400 psi) as
exhibited in Table 2 and Fig. S8. We also observed the same trend in such
2
pressures (200, 250, 300, 350 and
4
2
2
CO
CO
2
2
-copolymerization at the lower conversion of CHO under various
pressures as shown in entries 1–5 of Table S3. These catalysis results
suggested that such high-pressure copolymerization is attributed to a
zero-order on CO
2
pressure. In addition, CHO conversions with time at
/[4] ranges from 1600 to
800) were examined by using a fixed amount of CHO (50.0 mmol) and
various catalyst concentrations ([CHO]
0
0
2
an initial CO
2
pressure of 300 psi as illustrated in Table S4. In all cases,
) versus time (h) are linear, indicating
plots of (1/[CHO]
t
–1/[CHO]
0
copolymerization proceeds with a second-order dependence on CHO
concentration (Fig. 3). Therefore, the rate of copolymerization can be
2
n
written as –d[CHO]/dt ¼ kobs[CHO] , where kobs ¼ k
p p
[4] , and k is the
propagation constant. Furthermore, the observed rate coefficient, kobs
,
Fig. 3. Kinetic plots of (1/[CHO]
t
–1/[CHO]
0
) versus time for CO
2
/CHO copo-
�
lymerization by dinickel complex 4 at 140 C and an initial CO
2
pressure of 300
can be obtained from the slope of the fitted regression line of Fig. 3,
n
psi with various monomer-to-catalyst ratio (1600, 2000, 2400 and 2800).
which was also displayed in Table S5 of SI. Because of kobs ¼ k
p
[4] , it
þ n*ln[4], and the n
can be described as an equation of lnkobs ¼ lnk
p
value will be the reaction order of complex 4. Fig. 4 displays the linear
relationship between lnkobs and ln[4], whose regression equation is
expressed as lnkobs ¼ 5.279 þ 1.118*ln[4]. Based on the fitted regres-
sion analysis, the slope of the regression line equals 1.118, which sug-
gests first-order kinetics in Ni catalyst 4. On the basis of these kinetic
results, the overall rate law can be proposed as –d[CHO]/dt ¼
1
2
0
k
p
[4] [CHO] [CO
in complex 4 and second-order in CHO concentration. Particularly, this
result is different from the Ni-catalyzed CO -copolymerization of CHO
2
] , and such copolymerization catalysis is first-order
2
by a dicarboxylate dinickel salen-type complex that is first-order in both
catalyst and epoxide concentrations [30]. This kinetic analysis also
indicated effective CHO consumption by each nickel atom of dinickel
catalyst 4 during copolymerization catalysis.
Copolymerization conditions: 0.0625 mol % catalyst, 50.0 mmol
�
a
1
CHO, T ¼ 140 C. Based on H NMR analysis of the reaction mixture
b
1
[
[
28–30]. Based on H NMR determination of the purified copolymers
c
28–30]. TON ¼ number of moles of CHO consumed per mole of
d
catalyst. TOF ¼ TON per hour.
3
. Conclusions
Fig. 4. A linear plot of lnkobs versus ln[4] exhibiting the first order dependence
In summary, we reported for the first time the use of Pz-containing
on dinickel catalyst concentration. All polymerizations were carried out at 140
�
2
C and an initial CO pressure of 300 psi.
salan-type ligand to stabilize the formation of four new bimetallic
nickel complexes that contain diverse carboxylate or nitrophenolate
derivatives as coligands. These structurally well-defined dinickel com-
plexes were efficient catalysts for alternating copolymerization of CO
and CHO with excellent selectivity. Particularly, NP-supported Ni
2
Table 2
Copolymerization of CO
2
and CHO catalyzed by using dinickel complex 4 under
complex 4 was able to catalyze such CO
2
-copolymerization with a
À 1
different initial CO pressures.
2
maximum TOF up to 277 h under the optimal conditions. Kinetic
studies of CO
rate law of –d[CHO]/dt ¼ k
in dinickel catalyst 4 and second-order in CHO concentration.
2
/CHO copolymerization by 4 enabled us to propose the
1
2
p
[4] [CHO] , and such catalysis is first-order
4
. Experimental
Safety Note. Caution! Perchlorate salts of nickel complexes with
organic ligands are potentially explosive. Even small amount of these
compounds should be handled with great caution.
a
c
Entry
Pressure
psi
Time
/h
%
% Copolymer (%
TON
TOF
b
À 1d
/
CHO
carbonate)
/h
a
Conv.
4
.1. General conditions
1
2
3
4
5
200
250
300
350
400
6
6
6
6
6
71
74
77
76
76
>99(>99)
>99(>99)
>99(>99)
>99(>99)
>99(>99)
1136
1184
1232
1216
1216
189
197
205
203
203
0
0
2 -Hydroxy-5 -methylacetophenone,
N,N-dimethylformamide
dimethyl acetal, hydrazine monohydrate, sodium hydride (NaH),
0
methyl iodide, N,N -dimethyl-1,2-ethylenediamine, paraformaldehyde
5

Y.-C. Su et al.
Polymer 200 (2020) 122553
(
POM), trifluoroacetic acid, 2,3,4,5,6-pentafluorobenzoic acid, 4-(tri-
CH
151.28, 131.17, 129.74, 128.22, 126.55, 116.76, 116.49, 102.10 (Ar-CH
or Pz-CH), 38.70 (Pz-CH ), 20.63 (Ar-CH ).
3
), 2.31 (s, 3H, Ar-CH
3
). 13C NMR (400 MHz, CDCl
3
): δ 153.63,
fluoromethyl)benzoic acid, 2-nitrophenol, nickel(II) perchlorate hexa-
hydrate, triethylamine, ethanol, methanol, dichloromethane,
3
3
tetrahydrofuran and carbon dioxide (CO
used without further purification. Cyclohexene oxide (CHO), 4-vinyl-
,2-cyclohexene oxide (VCHO) and cyclopentene oxide (CPO) were
2
, 99.95%) were purchased and
4.1.3. Synthesis of Pz-containing salan-type ligand precursor (L-H
complexes
2
) and
1
purified by distillation over calcium hydride (CaH2) at least four times
prior to use. Deuterated solvents were dried over 4 Å molecular sieves.
To a flask containing MePzMePhOH (3.76 g, 20.0 mmol), N,N0-
dimethyl-1,2-ethylenediamine (2.42 g, 27.5 mmol) and para-
formaldehyde (1.65 g, 55.0 mmol), 10 mL of ethanol was added. The
solution was heated under reflux for 24 h and then cooled to room
temperature. The residue was extracted with deionized water and
1
13
H NMR and C NMR spectra were recorded on Varian Mercury-400
400 and 100 MHz) spectrometer with chemical shifts given in parts
(
per million from the peak of internal TMS. Microanalyses were per-
formed using a Heraeus CHN–O-RAPID instrument. Mass analyses were
2 2 4
CH Cl six times, and the organic layer was dried over MgSO . The
þ
performed by employing positive electron spray ionization (ESI )
volatile components were removed in vacuum. The residue was
recrystallized from methanol, and the resulting precipitate was collected
by filtration and dried under vacuum to obtain white solids. Yield: 2.93 g
technique on a Thermo Finnigan TSQ Quantum mass spectrometer for
2 2
new synthesized complexes upon dissolving in CH Cl solvent. Gel
permeation chromatography (GPC) measurements were performed on a
Jasco PU-2080 plus system equipped with a RI-2031 detector using THF
(60%). Anal. Calc. for C28
H
36
N
6
O
2
: C, 68.83; H, 7.43; N, 17.20%. Found:
): δ 11.11 (s, 2H,
1
C, 69.02; H, 7.02; N, 17.55%. H NMR (400 MHz, CDCl
3
(
HPLC grade) as an eluent. The chromatographic column was Phe-
nomenex Phenogel 5 103 Å and the calibration curve utilized to
calculate M (GPC) was produced from polystyrene standards. The cali-
OH), 7.45 (d, J ¼ 1.8 Hz, 2H, Ar–H), 7.34 (d, J ¼ 2.2 Hz, 2H, Pz-H), 6.86
(d, J ¼ 1.7 Hz, 2H, Ar–H), 6.74 (d, J ¼ 2.3 Hz, 2H, Pz-H), 3.92 (s, 6H, Pz-
μ
n
CH
3
), 3.65 (s, 4H, ArCH
2
N), 2.69 (s, 4H, NCH
2
CH
2
N), 2.26 (s, 6H,
): δ 152.59,
1
3
bration curve was constructed by ten polystyrene standards, in which
their molecular weights range from 1580 to 288000. The GPC results
were calculated using the Scientific Information Service Corporation
CH
2
NCH ), 2.25 (s, 6H, Ar-CH ). C NMR (400 MHz, CDCl
3
3
3
149.98, 130.64, 129.66, 127.71, 126.66, 123.63, 118.52, 104.91 (Ar-CH
and Pz-CH), 59.20 (Ar-CH N), 54.60 (NCH CH N), 42.00 (CH NCH ),
38.89 (Pz-CH ), 20.53 (Ar-CH ).
2
2
2
2
3
(
SISC) chromatography data solution 3.1 edition.
3
3
4
.1.1. Synthesis of 4-methyl-2-(1H-pyrazol-3-yl)phenol (a)
2 2 2 3 2
(1) Synthesis of complex [L Ni (O CCF ) ]
0 0
.0 g (20.0 mmol) of 2 -hydroxy-5 -methylacetophenone and 14.0
3
mL (104.9 mmol) of N,N-dimethylformamide dimethyl acetal were
introduced in a round-bottomed flask. The reaction mixture was
A solution of nickel(II) perchlorate hexahydrate (0.73 g, 2.0 mmol)
in methanol (10 mL) was added to a solution of proligand L-H (0.49 g,
2
�
refluxed at 75 C for 3 h in an atmosphere of dry nitrogen using standard
1.0 mmol) in methanol (10 mL) and triethylamine (2.10 mL, 15.0
mmol). The green mixture was heated under reflux for 24 h and then
cooled to ambient temperature. To this solution, five equivalent of tri-
fluoroacetic acid (0.38 mL, 5.0 mmol) dissolved in methanol (10 mL)
was added. The resulting mixture was heated under reflux for 24 h and
then cooled to ambient temperature. The volatile components were
Schlenk line, and then cooled to ambient temperature. The solution was
removed under vacuum to give a yellow-brown powder. The solution of
crude product in 30 mL ethanol was added slowly with hydrazine
monohydrate (1.50 g, 30.0 mmol) under reflux for 3 h and then cooled
to ambient temperature. The volatile components were removed in
vacuum and the final residue was washed with hexane, and the resultant
solids were collected by filtration and dried under vacuum to give yel-
removed in vacuo. The residue was diluted with CH
with a NaCl-saturated solution. The organic layers were dried over
MgSO and concentrated by vacuum evaporation. The final residue was
2 2
Cl and extracted
low solids. Yield: 3.31 g (95%). Anal. Calc. for C10
H
10
N
2
O: C, 68.95; H,
4
1
5
.79; N, 16.08%. Found: C, 69.01; H, 6.01; N, 15.95%. H NMR (400
washed with hexane, and the resulting precipitate was collected by
filtration and dried under vacuum to give green solids. Yield: 0.67 g
MHz, CDCl
3
): δ 10.64 (s, 1H, OH), 10.15 (s, 1H, NH), 7.61 (d, J ¼ 2.5 Hz,
1
H, Pz-H), 7.38 (d, J ¼ 1.4 Hz, 1H, Ar–H), 7.02 (dd, J ¼ 8.3, 1.8 Hz, 1H,
34 6 6 2 6
(81%). Anal. Calc. for C32H F N Ni O : C, 46.31; H, 4.13; N, 10.13%.
Ar–H), 6.92 (d, J ¼ 8.3 Hz, 1H, Ar–H), 6.70 (d, J ¼ 2.5 Hz, 1H, Pz-H),
Found: C, 46.01; H, 3.95; N, 10.51%. ESI-MS (m/z): 715.3 [100%, (M À
1
3
þ
2
.34 (s, 3H, Ar-CH
29.98, 129.59, 128.77, 126.96, 116.69, 116.38, 101.82 (Ar-CH or Pz-
).
3
). C NMR (400 MHz, CDCl
3
): δ 153.17, 151.41,
O
2
CCF
3
) ].
1
CH), 20.62 (Pz-CH
3
2 2 2 6 5 2
(2) Synthesis of complex [L Ni (O CC F ) ]
4
.1.2. Synthesis of 4-methyl-2-(1-methyl-1H-pyrazol-3-yl)phenol
This complex was prepared as described above for 1, using 2,3,4,5,6-
Me Me
(
Pz PhOH)
pentafluorobenzoic acid (1.06 g, 5.0 mmol) as the coligand precursor.
A solution of a (5.23 g, 30.0 mmol) in 50 mL dry THF was added
Workup generated 2 as green solids. Yield: 0.74 g (72%). Anal. Calc. for
�
slowly with NaH (2.40 g of 60%, 60.0 mmol) at 0 C in an atmosphere of
dry nitrogen using standard Schlenk line. The reaction mixture was
warmed up to room temperature and stirred for another 3 h until
hydrogen evolution ceased. To this solution, 1.87 mL of methyl iodide
C
42
H
34
F
10
N
6
Ni
2
O
6
: C, 49.16; H, 3.34; N, 8.19%. Found: C, 49.01; H,
þ
3.67; N, 8.24%. ESI-MS (m/z): 813.3 [100%, (M À O
2
CC
6 5
F ) ].
(3) Synthesis of complex [L Ni (O CC -4-CF (H
2
2
2
6
H
4
3
)
2
2
O)]
(
30.0 mmol) dissolved in 60 mL of dry THF was added dropwise and
resulting mixture was stirred for 12 h at room temperature. The solvent
was removed in vacuum and 100 mL of a 1 M solution of NaHCO were
This complex was prepared as described above for 1, using 4-(tri-
fluoromethyl)benzoic acid (0.95 g, 5.0 mmol) as the coligand precursor.
Workup produced 3 as green solids. Yield: 0.78 g (77%). Anal. Calc. for
3
added to the residue. The aqueous solution was extracted three times
with 20 mL of dichloromethane, and the organic layer was dried over
C
44
H
43
F
6
6
N Ni
2
O
8
: C, 52.06; H, 4.27; N, 8.28%. Found: C, 52.51; H, 4.01;
þ
MgSO
4
and filtered. All volatiles were removed under reduced pressure
N, 8.01%. ESI-MS (m/z): 791.3 [100%, (M À (O
CC
2 6
4
H -4-CF
3
)) ].
to yield pale yellow solids. The crude product was purified by flash
column chromatography on silica gel (diethyl ether: hexane ¼ 1:8)
(4) Synthesis of complex [L Ni (NP)
2
2
2
]
12 2
afforded white solids. Yield: (4.12 g, 73%). Anal. Calc. for C11H N O: C,
1
7
0.19; H, 6.43; N, 14.88%. Found: C, 69.98; H, 6.24; N, 14.82%. H NMR
400 MHz, CDCl
): δ10.60 (s, 1H, OH), 7.35 (d, J ¼ 2.3 Hz, 1H, Pz-H),
.34 (d, J ¼ 1.4 Hz, 1H, Ar–H), 7.00 (dd, J ¼ 8.3, 1.7 Hz, 1H, Ar–H), 6.91
This complex was prepared as described above for 1, using 2-nitro-
(
3
phenol (0.70 g, 5.0 mmol) as the coligand precursor. Workup afforded
7
4 as brown solids. Yield: 0.74 g (84%). Anal. Calc. for C40
42 8 2 8
H N Ni O : C,
(
d, J ¼ 8.3 Hz, 1H, Ar–H), 6.58 (d, J ¼ 2.4 Hz, 1H, Pz-H), 3.90 (s, 3H, Pz-
54.58; H, 4.81; N, 12.73%. Found: C, 54.85; H, 4.53; N, 12.88%.
6

Y.-C. Su et al.
Polymer 200 (2020) 122553
4
.1.4. Copolymerization of CO
dinickel complex 4
A representative procedure for the copolymerization of CO
epoxides by dinuclear nickel complex 4 was exemplified by using CHO
as the epoxide monomer (Table 1, entry 4). Complex [L Ni (NP) ] (4)
27.5 mg, 0.03125 mmol) was dissolved in 50.0 mmol of neat CHO
under a dry nitrogen atmosphere. The solution was added to the 100-mL
autoclave with magnetic stirrer under CO atmosphere. CO was then
2
with CHO, VCHO or CPO catalyzed by the
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