Effects of backbone substituent and intra-ligand hydrogen bonding interaction on ethylene polymerizations with Α-diimine nickel catalysts

Article abstract of DOI:10.1016/j.jcat.2019.05.026

The design of transition metal catalysts and the development of new catalyst design strategies for olefin polymerization have received substantial attention in the field of polyolefin research. In this paper, we initially reported backbone substituent eff

Full text of DOI:10.1016/j.jcat.2019.05.026

Journal of Catalysis 375 (2019) 113–123
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Journal of Catalysis
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Effects of backbone substituent and intra-ligand hydrogen bonding
interaction on ethylene polymerizations with
a
-diimine nickel catalysts
⇑
Liu Zhong, Cheng Du, Guangfu Liao, Heng Liao, Handou Zheng, Qing Wu, Haiyang Gao
School of Materials Science and Engineering, PCFM Lab, GD HPPC Lab, Sun Yat-sen University, Guangzhou 510275, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The design of transition metal catalysts and the development of new catalyst design strategies for olefin
polymerization have received substantial attention in the field of polyolefin research. In this paper, we
Received 14 March 2019
Revised 16 May 2019
Accepted 22 May 2019
initially reported backbone substituent effects on ethylene polymerizations with
lysts and intra-ligand hydrogen bonding interactions operating ethylene polymerizations. A series of
-diimine nickel catalysts with electron-donating/withdrawing groups (OMe, H, Cl, Br, and I) on the
a-diimine nickel cata-
a
dibenzobarrelene backbone were synthesized and employed in ethylene polymerization. Halogen sub-
stituents simultaneously showed steric and electronic effects on ethylene polymerization, and catalyst
5 with diiodo (I) substituent showed the highest activity and produced the highest molecular weight
polyethylene. The intra-ligand hydrogen bonding interactions (CAHÁÁÁOMe) were initially observed in
Keywords:
Nickel catalyst
Substituent effect
Hydrogen bonding
Ethylene polymerization
Living polymerization
the methoxy-substituted dibenzobarrelene a-diimine nickel complex, and the weak noncovalent interac-
tions enhanced the catalyst thermal stability and living fashion of ethylene polymerization at high tem-
peratures up to 80 °C.
Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
amount of effort has been made to sterically modify the ortho-
aryl substituents of
the -diimine nickel and palladium catalysts bearing dibenzhydryl
(CH(Ph)2) groups recently reported by Long, Sun, and Chen groups
[36-41]. The electronic modification of aniline moieties of
a-diimine ligands. A representative sample is
Olefin polymerization catalysts offer an effective means to con-
trol microstructural variations in the molecular architecture of
polyolefins [1-4]. The design of new transition metal catalysts
and the modification of the reported catalysts not only allow the
fine-tuning of the chemical environment around the metal center,
but also advance the development of new polyolefin materials
[1,5-15]. Two common approaches, including steric and electronic
effects, are usually applied to modify transition metal catalysts to
enhance catalytic performance and tune the architecture of poly-
a
a-
diimine nickel and palladium catalysts can also perturb the elec-
tronic environment around the metal center, thereby having a sig-
nificant influence on olefin polymerizations [18-21,44-48].
Commonly, the increased positive charge of the metal center by
the introduction of electron-withdrawing substituents is favorable
for olefin coordination and insertion, thereby improving catalytic
activity. On the other hand, the introduction of electron-
withdrawing groups causes poor catalyst stability and easy deacti-
vation of the catalyst, thus leading to declining activity. Therefore,
the design and development of robust nickel and palladium cata-
lysts by electronic modification is highly challenging because of
the competition between these two factors [18-20].
olefins. For example, the a-diimine nickel and palladium catalysts
discovered by Brookhart and coworkers have received intensive
attention as a staple of late-transition metal catalysts [16-35]. Over
the past decades, numerous studies have focused on steric and
electronic modifications of the aniline moieties of the
a-diimine
catalyst to improve catalytic performance, such as thermostability,
activity, and tolerance towards polar monomers [36-55].
In comparison with modifications of aniline moieties, modifica-
tions of the ligand backbone are far less intensive and extensive,
especially electronic modification of the ligand backbone [43,56-
63]. Our work has addressed a promising steric effect of the ligand
The demand of bulk ortho-aryl substituents is a crucial factor for
blocking axial space and suppressing of chain transfer and catalyst
deactivation, which contributes to thermostable catalysts and high
molecular weight products [1,10-15]. Therefore, a tremendous
backbone on the a-diimine-Ni/Pd-catalyzed (co)polymerization of
ethylene and polar monomers [57-60]. However, the electronic
effect of the ligand backbone in catalyst design has not been sys-
tematically studied. Few investigations of the electronic effect of
⇑
Corresponding author.
E-mail address: gaohy@mail.sysu.edu.cn (H. Gao).
https://doi.org/10.1016/j.jcat.2019.05.026
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

114
L. Zhong et al. / Journal of Catalysis 375 (2019) 113–123
ligand backbone (4-fluorophenyl and methoxy-substituted ace-
naphthyl) on ethylene polymerization with -diimine Ni/Pd cata-
lysts have been reported [18,24,56]. It remains a great challenge
nickel and palladium catalysts bearing nitrogen-containing second
coordination spheres for (co)polymerizations of ethylene. Facile
metal-nitrogen interactions greatly reduced polymer branching
densities in both nickel and palladium catalysts. Higher stability,
greater tolerance towards polar groups, and a wider range of polar
a
to modify the ligand backbone from electronic effect because the
installation of electron-withdrawing groups on the
a-diimine
ligand backbone improves the difficulty of catalyst synthesis.
In addition to steric and electronic modification approaches,
weak noncovalent interactions are also employed in modulating
catalytic olefin polymerization [27,28,64-84]. The typical hydrogen
bonding interaction between hydrogen (H) and an electronegative
atom such as fluorine (F) has been observed between the ligand
and growing polymer chain [64-71]. A series of phenoxy-imine
titanium complexes (Fig. 1A) with ortho-fluoro aryls reported by
Fujita and Coates showed living character for ethylene and propy-
lene polymerizations, and the hydrogen bonding interaction
between the ortho-F and the b-H of the growing polymer chain
(CAH. . .F) could inhibit chain transfer and had been well accepted
as an explanation for living polymerizations of ethylene and propy-
lene [64,65,76,77]. In addition, the weak noncovalent interaction
between the metal and the heteroatom of the ligand was also
observed (Fig. 1B) [27,28]. For fluoro-substituted cyclophane-
monomers were realized in the a-diimine palladium system [80].
Installing a polyethylene glycol moiety was also used to facilitate
the (co)polymerizations of ethylene (Fig. 1C) [82-84]. The introduc-
tion of the polyethylene glycol unit on phosphine sulfonate-based
palladium and nickel catalysts significantly enhanced the (co)poly-
merizations of ethylene and polar monomers because of the
ligand-comonomer interaction [83]. In principle, there is a fourth
type of weak noncovalent interaction within different parts of a
ligand (intra-ligand), but no intra-ligand interaction has been
reported in olefin polymerization (Fig. 1D) to the best of our
knowledge.
Inspired by the significant enhancement of the dibenzobarre-
lene backbone on a-diimine Ni/Pd-catalyzed ethylene polymeriza-
tion [59,60], we sequentially paid attention to the substituent
effects of the dibenzobarrelene backbone on ethylene polymeriza-
tion. In this contribution, a series of a-diimine nickel catalysts with
based
14-electron alkyl intermediates, thereby suppressing chain trans-
fer processes involving b-H elimination [27]. second
coordination-sphere strategy was developed to modulate olefin
(co)polymerizations [80,81]. Chen reported a series of -diimine
a
-diimine palladium catalysts, a Pd. . .F interaction stabilized
electron-donating/withdrawing groups (OMe, H, Cl, Br, and I) on
the dibenzobarrelene backbone were designed and employed in
ethylene polymerization. Strikingly, the steric and electronic
effects of the dihalo substituent were shown to simultaneously
impact ethylene polymerization. The unprecedented intra-ligand
interactions (CAHÁÁÁOMe) were initially observed in the methoxy-
A
a
substituted dibenzobarrelene a-diimine nickel complex (Fig. 1D),
and these weak noncovalent interactions enhanced the catalyst
thermal stability and living fashion of ethylene polymerization at
high temperatures up to 80 °C.
2. Results and discussion
We previously reported the synthesis and characterization of
dibenzobarrelene
disubstituted dibenzobarrelene
a
-diimine nickel complex 2 [59]. Herein, 1,5-
-diimine nickel complexes with
a
electron-donating methoxy (OMe) and electron-withdrawing halo-
gen groups were originally designed on the basis of electronic
effects (Scheme 1). These substituents were installed on the
anthracene from 1,5-disubstituted-anthracene-9,10-dione as raw
materials according to reported methods (Scheme S1 in supporting
information) [85-89]. 1,5-Disubstituted-9,10-dihydro-9,10-etha
noanthracene-11,12-dione compounds were prepared by Diels-
Alder addition of 1,5-disubstituted anthracene with vinylene car-
bonate, hydrolysis, and Swern oxidation reactions using modified
literature procedures [90,91]. Corresponding
were obtained by condensation reactions in high yields. The target
-diimine nickel complexes were synthesized by the reaction of
corresponding -diimine ligands and (DME)NiBr2 (DME:
dimethoxyethane) in CH2Cl2. An attempt to synthesize -diimine
a-diimine ligands
a
a
a
nickel complex containing difluoro (F) substituent on the dibenzo-
barrelene backbone was unsuccessful because the strong electron-
withdrawing effect of fluoro groups caused easy decomposition of
the nickel complex [92]. In fact, nickel complexes 3 (X = Cl) and 4
(X = Br) also showed relatively poor stability in solution, and the
solution color gradually turned light yellow from red beyond
24 h under a nitrogen atmosphere.
Single crystals of two nickel complexes 1 and 5 suitable for X-
ray diffraction analysis were obtained by slow diffusion of the
nickel complex solution in CH2Cl2. Similar to the typical structure
of a-diimine nickel complexes, two nickel complexes display a dis-
torted tetrahedral geometry (Figs. 2 and 3). The NANiAN bite angle
of the nickel complex is a crucial factor, which is closely related to
the ethylene polymerization property. Interestingly, the steric
Fig. 1. The proposed weak noncovalent interactions in catalytic olefin
polymerizations.

L. Zhong et al. / Journal of Catalysis 375 (2019) 113–123
115
Scheme 1. Synthetic routes of a-diimine nickel complexes 1–5.
Fig. 2. Crystal structure of a-diimine nickel complex 1 (X = OMe) with thermal ellipsoids of 30% probability (front and side views).
Fig. 3. Crystal structure of a-diimine nickel complex 5 (X = I) with thermal ellipsoids of 30% probability (front and side views).
effect of substituents has an influence on the NANiAN bite angle of
the nickel complex (Table 1). Nickel complex 2 without sub-
stituents has a bite angle of 78.51° [59], while nickel complexes
1 and 5 have larger bite angles of 81.45° and 82.1°, respectively.
Increasing the N–Ni–N bite angle of the nickel complex by instal-
ling 1,5-disubstituents can be attributed to a similar well-known
Thorpe-Ingold effect between two nitrogen donor atoms [93].
Besides, the introduction of substituents influences the NANi bond

116
L. Zhong et al. / Journal of Catalysis 375 (2019) 113–123
Table 1
stability by prohibiting free rotation of the N–aryl bonds [52]. As an
alternative approach, the intramolecular hydrogen bonding inter-
action can restrict rotation of the N–aryl bonds and lock the
Crystal parameters regard to the substituent effect of nickel complexes.
Ni complex Substituent X NANiAN bite angle/° NANi bond length/Å
perpendicular-coordination plane conformation of the
nickel complex, thereby being anticipated to enhance the thermal
stability of the catalyst [52,59].
a-diimine
1
2
5
OMe
H
I
81.45 (7)
78.51 (8)
82.1 (2)
2.0280(18)/2.0545(18)
2.133(2)/2.0923(19)
2.049(6)/2.059(6)
To address the substituent effects of the dibenzobarrelene back-
bone on ethylene polymerization,
a-diimine nickel complexes 1
and 3–5 were activated by Et2AlCl in a temperature range from
20 to 80 °C (Table 2). An atmospheric ethylene pressure of
1.2 atm was selected because encapsulation of the nickel species
into the precipitated high-molecular-weight polymer produced at
high ethylene pressure impacts the evaluation reliability. Ethylene
polymerizations with catalyst 2 (X = H) were previously reported
by our group and are herein listed in Table 2 as comparisons [59].
Under the same polymerization temperatures, catalyst 1 with
electron-donating methoxy groups showed lower activities than
catalyst 2 without a substituent. However, catalysts 3–5 with
electron-withdrawing halogen groups were more active than cata-
lyst 2. The electronic effect of substituents also had the same influ-
ence on the molecular weight of the obtained polyethylenes. These
results were ascribed to the decreased positive charge of the nickel
center by introduction of electron-donating substituents on the
dibenzobarrelene backbone. The low positive charge of the metal
center was unfavorable for olefin coordination and insertion,
thereby reducing the chain growth rate and decreasing the cat-
alytic activity as well as the polymer molecular weight. Although
the electronic effect impacted polymerization activity and polymer
molecular weight, all of the obtained polyethylenes had narrow
polydispersity and catalysts 1–5 showed a living fashion. This
result indicated that the chain transfer reaction was hardly
affected by the electronic effect of the substituents and the steric
effect predominantly determined the chain transfer reaction.
It is well known that the electronegativity order of halogen
groups is Cl > Br > I. However, the introduction of halogen groups
on the dibenzobarrelene backbone led to a contrary activity trend
(3(Cl) < 4(Br) < 5(I)), which was not reasonably interpreted by the
electronic effect mentioned above. Diiodo substituted catalyst 5
showed the highest activity and produced the highest molecular
weight polyethylene although iodo substituent had the lowest
electronegativity. This unprecedented halogen effect of the diben-
length of nickel complexes (Table 1), which is associated with com-
plex stability. The NANi bond lengths of diiodo substituted com-
plex 5 (2.049/2.059 Å) are shorter than those of complex 2
without substituents (2.133/2.0923 Å), which further supports
the steric effect of the substituents. Complex 1 with electron-
donating methoxy groups has the shortest NANi bond length
(2.0280/2.0545 Å), which is reasonably attributed to the electronic
effect of the substituents. This result indicates that the introduc-
tion of electron-donating groups on the ligand backbone can
enhance the stability of the nickel complex, which is consistent
with the above observation for the stability of nickel complex in
solution.
More strikingly, the introduction of methoxy groups on the
dibenzobarrelene backbone leads to noncovalent hydrogen bond-
ing interactions. The distances between the oxygen atoms of meth-
oxy groups and the hydrogen atoms of the isopropyl CH(CH3)
groups are 2.6581 Å for O1. . .H11A and 2.9315 Å for O2ÁÁÁH42A
,
strongly indicating the existence of two intramolecular interac-
tions by OÁÁÁH hydrogen bonds in nickel complex 1 (Fig. 2). An
attempt to prove the existence of these hydrogen bonds in solution
of a
-diimine nickel complex 1 by 1H NMR evidence was impractical
due to its paramagnetism. Fortunately, single crystal of ligand L1
were also obtained and ORTEP diagram was given in Fig. 4. Nonco-
valent hydrogen bonding interactions are also present in ligand L1
on the basis of the distances between oxygen atoms and hydrogen
atoms of the isopropyl CH(CH3) groups of 2.8855 Å and 3.0866 Å.
1H NMR analysis of ligands showed that the methyl protons of
the halogenated ligands (L3-L5, X = Cl, Br, I) (1.39–1.20, 1.17–
1.04, and 1.08–0.95 ppm) shifted to the downfield relative to those
of ligand L2 (X = H) (1.05 and 1.02 ppm) because of the electron-
withdrawing effect. However, the methyl protons of the ligand
L1 with the electron-donating methoxy groups also shifted to the
downfield (1.21–0.96 ppm). This contrast observation was safely
attributed to hydrogen bonding interactions between oxygen
atoms and hydrogen atoms of the isopropyl groups, strongly sup-
porting that these hydrogen bonds existed in both solution and
solid. Previously, the rigid framework of the cyclophane-based
nickel complex designed by Guan was reported to enhance thermal
zobarrelene backbone on
a-diimine-nickel-catalyzed ethylene
polymerization was a result of the steric effect. The iodo sub-
stituent had the largest atomic size among the three halogens,
thereby showing the bulkiest steric hindrance. Increasing the steric
bulk on the dibenzobarrelene backbone enhanced the catalytic
activity of the nickel catalyst [59], and the atomic size of halogen
Fig. 4. Crystal structure of ligand L1 (X = MeO) with thermal ellipsoids of 30% probability (front and side views).

L. Zhong et al. / Journal of Catalysis 375 (2019) 113–123
117
Table 2
Ethylene polymerization results using nickel catalysts 1–5.a
c
Entry
Ni
X
T(°C)
Yield (mg)
Activityb
Mn (kg/mol)
PDIc
BDd(/1000C)
1
2
3
4
5
6
7
8
1
1
1
1
1
2
2
2
2
2
3
3
3
3
3
4
4
4
4
4
5
5
5
5
5
OMe
OMe
OMe
OMe
OMe
H
H
H
H
H
20
35
50
65
80
20
35
50
65
80
20
35
50
65
80
20
35
50
65
80
20
35
50
65
80
111
147
212
193
166
182
248
281
261
trace
219
262
283
274
205
228
282
349
312
199
239
401
560
423
238
111
147
212
193
166
182
248
281
261
–
219
262
283
274
205
228
282
349
312
199
239
401
560
423
238
101
112
149
130
124
110
136
156
131
–
147
153
190
176
146
192
208
261
219
151
238
262
308
259
170
1.03
1.03
1.04
1.04
1.03
1.20
1.16
1.09
1.07
–
1.11
1.09
1.03
1.04
1.08
1.18
1.11
1.06
1.05
1.11
1.17
1.16
1.07
1.09
1.10
91
93
96
98
100
84
87
93
97
–
87
94
98
100
102
87
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Cl
Cl
Cl
Cl
Cl
Br
Br
Br
Br
Br
I
I
I
I
I
95
101
102
107
89
98
104
105
107
a
b
c
Polymerization conditions: 2 lmol of nickel, Et2AlCl as cocatalyst, Al/Ni = 600, 1.2 atm ethylene pressure, 30 min, 20 mL toluene and 1 mL CH2Cl2.
Activity in kg PE/(mol NiÁh).
Mn and PDI were determined by gel permeation chromatography (GPC) in 1,2,4-trichlorobenzene at 150 °C.
d
Branching density, branches per 1000 carbon atoms determined by 1H NMR spectroscopy.
was consistent with the activity order of the dihalo substituted cat-
alysts. The crystal structure data mentioned above showed that
diiodo substituted nickel complex 5 had a larger NANiAN bite
angle, suggesting that the nickel center was deeply surrounded
by more crowded space.
merizations. For
a
-diimine nickel catalysts, the utilization of N-aryl
moieties containing steric bulk has been reported to increase their
thermal stability by restricting rotation of the N-aryl bonds [1,10-
15]. The substituent effect of the dibenzobarrelene backbone on
the thermal stability of the catalyst was evaluated in a temperature
range from 20 to 80 °C under atmospheric pressure. As shown in
Table 2, all nickel catalysts showed the same temperature influ-
ence on polymerization activity and exhibited the highest activity
at 50 °C. A further increase in temperature from 50 °C led to a
decrease in catalytic activity. At a high temperature of 80 °C, disub-
stituted catalysts 1 and 3–5 still showed moderate activities, while
catalyst 2 without substituents produced traces of polymer. This
result indicated that the introduction of groups with either
electron-donating or electron-withdrawing effects on the dibenzo-
To further verify the steric effect of the halogen groups, we pur-
sued a quantitative calculation of the space-filling capabilities of
nickel centers using Cavallo’s SambVca 2.0 program [94]. Topo-
graphic steric maps were used to visualize catalytic pockets around
the nickel metal center to show the effect of backbone substituents
on the steric hindrance. As shown in Fig. 5, the buried volume of
four nickel complexes calculated by X-ray single crystal data con-
sistently increased with an increase in steric bulk on the ligand
backbone [95,96]. Nickel complex 2 without a substituent had
the smallest buried volume of 45.3, and the introduction of meth-
oxy groups improved the buried volume of the nickel complex (1:
46.6). In combination with the activity order (2 > 1), the electronic
effect of methoxy groups dominated the catalytic activity for ethy-
lene polymerization. Nickel complex 2a with 2,6-di-tert-butyl
groups reported by our groups was also used for comparison
[59]. Indeed, the introduction of 2,6-di-tert-butyl groups on the
dibenzobarrelene backbone improved the buried volume of the
nickel complex (2a: 47.6). This calculation result was in good
agreement with reported experimental results of ethylene poly-
merization and demonstrated the calculation reliability of the
space-filling capabilities [59]. Diiodo-substituted nickel complex
5 had the largest buried volume of 49.6, indicating a steric effect
of the iodo substituent and crowded catalytic pockets around the
nickel metal center. The iodo substituent showed bulkier steric
hindrance around the nickel center than the tert-butyl group,
which might be a result of different substituent positions. 1,5-
Disubstituents placed on the dibenzobarrelene backbone were
more closed to the aniline moieties than 2,6-disubstituents,
thereby perturbating steric effect of aniline moieties.
barrelene backbone enhanced the thermal stability of the
a-
diimine nickel catalysts. Usually, it was reported that the installa-
tion of electron-donating groups improved catalyst stability
[18,19]; enhanced catalyst stability by the introduction of
electron-withdrawing halogen groups was herein ascribed to the
steric effect on the basis of the buried volume analysis mentioned
above.
Additionally, it was noteworthy that the declining percentage of
activity was dependent on the steric effect of the substituents,
which was closely related to the intramolecular interactions by
OÁÁÁH hydrogen bonds. With increased temperatures from 50 to
80 °C, the activity of catalyst 1 with electron-donating methoxy
groups decreased slightly while the activity of catalyst 5 with
electron-withdrawing diiodo substituent dropped sharply. The
activity of catalyst 1 decreased only by 9% at 65 °C and by 14% at
80 °C from the highest level, whereas the activity of catalyst 5
dropped by 25% at 65 °C and by 44% at 80 °C. This result indicated
that catalyst 1 (X = OMe) showed better thermal tolerance than
catalyst 5 (X = I). The molecular weight and polydispersity of
polyethylenes obtained at high temperatures further supported
this claim. With increasing temperature from 50 to 80 °C, the
molecular weight of the polyethylenes produced by catalyst 1
The development of thermally stable olefin polymerization cat-
alysts is of significant interest for industrial gas-phase olefin poly-

118
L. Zhong et al. / Journal of Catalysis 375 (2019) 113–123
Fig. 5. Topographical steric maps and percent buried volume (%Vbur) of a-diimine nickel complexes bearing various substituents on the ligand backbone.
decreased slightly, and the polydispersity (PDI) was nearly invari-
able and narrow (PDI = ꢀ1.04). In contrast, a remarkable decline in
polymer molecular weight was observed for catalyst 5, and the
polydispersity became slightly broad (PDI = ꢀ1.10) with increasing
temperature.
The ethylene polymerization at different time periods ranging
from 15 to 90 min was used to test the lifetime of catalyst 1 at
80 °C. In fact, catalyst 1 showed living nature within 60 min at
80 °C for ethylene polymerization. As shown in Fig. 6, GPC traces
of the polymers produced at different times shifted to the higher
molecular weight region with prolonged polymerization time,
the number-average molecular weight (Mn) grew linearly with
the polymerization time, and the PDI (Mw/Mn) values remained
below 1.10. Therefore, catalyst 1 was a living system for ethylene
polymerization below 80 °C. To the best of our knowledge, the
temperature of 80 °C reached the highest value at which the living
polymerization of ethylene was realized to date [59].
sure of 6 atm, catalyst 1 showed significantly improved catalytic
activity and produced the polyethylene with slightly reduced
branching density, indicating accelerating chain growing rate and
reducing chain walking rate at high polymerization pressures. At
80 °C, catalyst 1 still kept a living manner and produced ultrahigh
molecular weight polyethylene (Mn > 1000 kg/mol) with a narrow
polydispersity of 1.10. A further increase in temperature up to
100 °C led to a decrease in activity and molecular weight. High
temperature polymerization results at different time periods rang-
ing from 15 to 60 min showed that the turnover frequency (TOF) of
catalyst 1 reduced gradually and the molecular weight nonlinearly
increased (Fig. 7) with prolonged polymerization time, suggesting
a nonliving manner at 100 °C in combination with broad PDI values
(PDI = 1.2–1.3). However, catalyst 1 was thermostable and did not
decompose on the basis of increased catalytic efficiency, which was
calculated gravimetrically on the basis of the weight of the pro-
duced polymer and the experimental number-average molecular
weight, to the amount of nickel catalyst used [97-99]. In combina-
tion with increased PDI values, it was safely concluded that ethy-
lene polymerization with catalyst 1 produced a low molecular
weight polymer with broad distribution because the chain transfer
Additional ethylene polymerizations at high temperatures were
performed at a high ethylene pressure of 6 atm because high pres-
sure could minimize the influence of temperature on the ethylene
solubility in the reaction medium (Table 3). At an ethylene pres-
Fig. 6. Plots of Mn (▲) and Mw/Mn (PDI) (j) as a function of reaction time and GPC traces using 1/Et2AlCl at 80 °C (conditions: 1.2 atm, 2 lmol Ni, Al/Ni = 600).

L. Zhong et al. / Journal of Catalysis 375 (2019) 113–123
119
Table 3
Ethylene polymerization results using nickel catalyst 1 at 6 atm.a.
Entry
T(°C)
t(min)
Yield (g)
TOFb
Mn (kg/mol)
PDIc
BDd(/1000C)
Catalytic efficiencyd
1
2
3
4
5
80
30
15
30
45
60
1.27
0.52
0.84
1.16
1.35
45.4
37.1
30.0
27.6
24.1
1106
385
545
599
636
1.10
1.15
1.20
1.26
1.28
94
96
97
97
97
0.57
0.68
0.77
0.97
1.06
100
100
100
100
a
b
c
Polymerization conditions: 2 lmol of nickel, Et2AlCl as cocatalyst, Al/Ni = 600, 6 atm ethylene pressure, 50 mL toluene and 1 mL CH2Cl2.
TOF in kmol E/(mol NiÁh).
Mn and PDI were determined by gel permeation chromatography (GPC) in 1,2,4-trichlorobenzene at 150 °C.
d
Catalytic efficiency was calculated by equation of yield/(2
lmol  Mn).
alyst [52], which provided viable access to enhancing thermal sta-
bility of the catalyst. Unlike previously reported interactions of
polymer-ligand, ligand-metal, and ligand-comonomer modes
[27,28,64-84], our work also addressed
a new ligand-ligand
(intra-ligand) hydrogen bonding interaction operating olefin poly-
merization. Overall, bulky backbone substituents and noncovalent
hydrogen bonding interactions are cooperatively responsible for
the living polymerization of ethylene at 80 °C, the highest temper-
ature reported to date.
The polyethylenes produced by catalysts 1–5 were highly
branched (87–107/1000C) according to 1H NMR analyses
(Fig. S9), which was consistent with previous observations [59].
13C NMR spectroscopies also revealed that all branches were pre-
sent, and the short methyl branch accounted for the largest propor-
tion (Fig. S10), which was a result of the chain walking process
[23]. These branched polyethylenes produced by nickel catalysts
1–5 exhibited very broad melting endotherms with nearly the
same profiles (Fig. S11). Therefore, the substituents with either
electron-donating or electron-withdrawing effects on the dibenzo-
barrelene backbone have almost no effect on the microstructure of
polyethylenes, and the steric demand of the dibenzobarrelene
backbone dominates the chain walking during ethylene polymer-
ization. Also note that the polymer branching density is not sensi-
tive to the polymerization temperature (Table 2), and there is a
mild change of polymer branching density within the tested tem-
Fig. 7. Plots of TOF and Mn as a function of polymerization time using 1 under
100 °C and 6 atm.
took place at 100 °C. A reduction in the TOF at 100 °C could result
from encapsulation of the nickel species into the precipitated high
molecular weight polymer [59].
Commonly, conventional
a-diimine nickel catalysts with
methyl and acenaphthene backbones underwent rapid decomposi-
tions above 60 °C because of the acceleration of N–aryl rotation
perature range (20–80 °C) for dibenzobarrelene based
a-diimine
from
a perpendicular to a square-planar coordination plane
nickel catalysts 1–5. This different observation to the classic
a
-
[8,59]. The conformationally locked cyclophane-based nickel cata-
lyst was shown to be thermally stable for the polymerization of
ethylene at temperatures up to 70 °C [52]. We previously reported
that the steric demand of the dibenzobarrelene backbone could
inhibit rotation of the N–aryl bonds by the repulsive interaction
of the bulky backbone with aniline moieties, thereby enhancing
diimine nickel catalysts is ascribed to enhanced chain walking abil-
ity by the bulky dibenzobarrelene backbone [8,59,100].
3. Conclusions
the thermal stability and living fashion of the
a
-diimine catalyst
In summary, we have reported dibenzobarrelene-based a-
[59,60]. In this work, the introduction of methoxy groups on the
dibenzobarrelene backbone further improved the thermal stability
diimine nickel catalysts with different electron donating/with-
drawing groups (X = OMe, H, Cl, Br, and I) on the ligand backbone
for living ethylene polymerizations. The introduction of electron-
withdrawing halogens enhanced catalytic activity and polymer
molecular weight, while the introduction of electron-donating
methoxy groups led to a decrease in catalytic activity and polymer
molecular weight. Intensive study on substituent effects of diben-
zobarrelene backbone revealed that halogen substituents simulta-
neously showed steric and electronic effects on ethylene
polymerization. Catalyst 5 with diiodo (I) substituent showed the
highest activity and produced the highest molecular weight
polyethylene. This systematic investigation of the electronic mod-
and living fashion of the
a-diimine catalyst, which was largely
attributed to the intramolecular interactions by OÁÁÁH hydrogen
bonds between methoxy and isopropyl groups of nickel complex
1. These intramolecular interactions were believed to still exist
during polymerization process despite of the presence of the alky-
laluminum cocatalyst. The existence of these hydrogen bonds in
solution had been proved on the basis of 1H NMR analysis of
ligands mentioned above. Further control experiments showed
that no reaction of Et2AlCl with ligand L1 in toluene took place
by 1H NMR analysis, supporting that Et2AlCl did not disturb these
intramolecular interactions. The intramolecular hydrogen bonding
interaction inhibited the N–aryl rotation and locked the
ifications of the
a-diimine ligand backbone provided a different
understanding of the reported electronic modifications of aniline
moieties. The intra-ligand hydrogen bonding interactions
(CAHÁÁÁOMe) were initially observed in the methoxy-substituted
perpendicular-coordination plane conformation of the
a-diimine
nickel catalyst at elevated temperatures, thereby keeping steric
blocks at the axial sites and suppressing chain transfer. These weak
noncovalent interactions in this system were different from
reported covalent interactions in the cyclophane-based nickel cat-
dibenzobarrelene a-diimine nickel complex, and the weak nonco-
valent interactions enhanced the catalyst thermal stability and liv-
ing fashion of ethylene polymerization at high temperatures up to

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L. Zhong et al. / Journal of Catalysis 375 (2019) 113–123
80 °C by inhibiting rotation of the N–aryl bonds. These intra-ligand
hydrogen bonding interactions provided a fundamentally new
approach in enhancing catalyst thermostability, complementing
previous strategies of weak noncovalent interactions employed in
modulating catalytic olefin polymerization.
hydrogen atoms were refined anisotropically. Hydrogen atoms
were introduced in the calculated positions with the displacement
factors of the host carbon atoms.
4.5. Ethylene polymerization procedure at atmospheric pressure
In a typical procedure, a round bottom Schlenk flask with stir-
ring bar was heated for 2 h at 150 °C under vacuum and then
cooled to ambient temperature. The flask was pressurized to
1.5 atm of ethylene and vented for three times. The appropriate
Et2AlCl compound as cocatalyst and toluene were added into the
glass reactor under 1.2 atm of ethylene. The system was continu-
ously stirred for 5 min, and then toluene and 1 mL of a solution
of catalyst in CH2Cl2 were added sequentially by syringe to the
well-stirred solution, and the total reaction volume was kept at
21 mL. The ethylene pressure was kept constant at 1.2 atm by con-
tinuous feeding of gaseous ethylene throughout the reaction. The
other reaction temperatures were controlled with an external oil
bath or a cooler in polymerization experiments. The polymeriza-
tions were terminated by the addition of 200 mL of acidic metha-
nol (95:5 ethanol/HCl) after continuously stirring for an
appropriate period. The resulting precipitated polymers were col-
lected and treated by filtration, washed with methanol for several
times, and dried in vacuum at 40 °C to a constant weight.
4. Experimental section
4.1. General procedures
All manipulations involving air- and moisture-sensitive com-
pounds were carried out under an atmosphere of dried and puri-
fied nitrogen with standard vacuum-line, Schlenk, or glovebox
techniques. Pressures are absolute values in the whole
experimental.
4.2. Materials
2,6-diisopropylanilinewas purchased from Aldrich Chemical
and distilled under reduced pressure before use. 1,5-
Dihydroxyanthraquinone,
1,5-dichloroanthraquinone,
1,5-
diaminoanthraquinone, vinylene carbonate, and trifluoroacetic
anhydride were purchased from Energy Chemical and used as
received. Toluene and hexane were refluxed over Na/K alloy before
use, dichloromethane was dried over P2O5 and was distilled under
nitrogen. Et2AlCl (1.0 M in hexane) was purchased from Acros.
Ethylene (99.99%) was purified by passing through Agilent mois-
ture and oxygen traps. Other commercially available reagents were
purchased and used without purification.
4.6. Ethylene polymerization procedure at high pressure
A mechanically stirred 100 mL Parr reactor was heated to
150 °C for 2 h under vacuum and then cooled to room temperature.
The autoclave was pressurized to 6 atm of ethylene and vented
three times. The autoclave was then charged with solution of
Et2AlCl in toluene under 1.2 atm of ethylene at initialization tem-
perature. The system was maintained by continuously stirring for
5 min, and then 1 mL solution of nickel complex in CH2Cl2 was
charged into the autoclave under 1.2 atm of ethylene. The ethylene
pressure was raised to the specified value. The reaction tempera-
ture was controlled by means of a heater or cooler and found to
be 2 °C as monitored by an internal thermocouple. The reaction
was carried out for a certain time. Polymerization was terminated
by addition of acidic methanol after releasing ethylene pressure.
The resulting precipitated polymers were collected and treated
by filtering, washed with methanol several times, and dried under
vacuum at 40 °C to a constant weight.
4.3. Characterization
The NMR data of organic compounds were obtained on a Bruker
400 MHz instrument in CDCl3 using TMS as a reference at ambient
temperature. The 13C NMR data of polymers were recorded on a
Bruker 500 MHz at 120 °C. Sample solutions of the polymer were
prepared in o-C6H4Cl2/o-C6D4Cl2 (50% v/v) in a 10 mm sample tube,
using 30 ppm for the main chain of PE as a reference. Elemental
analyses were performed on a Vario EL macro analyzer. Mass spec-
tra were obtained using electro- spray ionization (ESI) LCMS-
2010A for ligands and matrix assisted laser desorption ionization
time of flight mass (MALDI-TOF) Bruker ultrafleXtreme for com-
plexes. DSC analyses were conducted with a PerkinElmer DSC-7
system. The DSC curves were recorded as second heating curves
from À100 to 150 °C at a heating rate of 10 °C/min and a cooling
rate of 10 °C/min. GPC analyses of the molecular weights and
molecular weight distributions (PDI = Mw/Mn) of the PE samples
at 150 °C were performed on a PL-GPC 220 high-temperature chro-
matograph equipped with a triple-detection array, including a dif-
ferential refractive-index detector, a two-angle light-scattering
detector, and a four-bridge capillary viscometer. The detection
angles of the LS detector were 15 and 90°, and the laser wavelength
was 658 nm. 1,2,4-Trichlorobenzene (TCB) was used as the eluent
at a flow rate of 1.0 mL/min.
4.7. Synthesis of a-Diimine compounds
L1,
ArAN@C(An)A(An)C@NAAr
(An = 1,5-dimethoxy-
1,5-
dibenzobarrelene,
Ar = 2,6-diisopropylphenyl).
Dimethoxyanthracene was prepared by substitution and reduction
reaction according to the literature procedure (see SI) [85,86]. The
a-dione compound (1,5-dimethoxy-9,10-dihydro-9,10-ethanoan
thracene-11,12-dione) was prepared according to
a previous
method [90]. 1H NMR (CDCl3, 400 MHz), d (ppm): 7.34–7.28 (m,
2H, Ph), 7.09 (s, 1H, Ph), 7.07 (s, 1H, Ph), 6.88 (s, 1H, Ph), 6.86 (s,
1H, Ph), 5.42 (s, 2H, CH), 3.87 (s, 6H, CH3).
4.4. Crystal structure determination
The crystal was mounted on a glass fiber and transferred to a
Bruker CCD platform diffractometer. The X-ray diffraction data
was obtained with the
x-2h scan mode on a Bruker SMART 1000
CCD diffractometer with graphite-monochromated Cu K radiation
a
(k = 1.54178 Å) at 150 K. The structure was solved using direct
methods, and further refinement with full-matrix least squares
on F2 was obtained with the SHELXTL program package. All non-

L. Zhong et al. / Journal of Catalysis 375 (2019) 113–123
121
a-Diimine ligand L1 was synthesized by the reaction of 1,5-di
methoxy-9,10-dihydro-9,10-ethanoanthracene-11,12-dione with
2,6-diisopropylphenylamine in toluene. A typical synthetic proce-
dure for L1 was as follows: 1,5-Dimethoxy-9,10-dihydro-9,10-etha
noanthracene-11,12-dione and 2,6-diisopropylphenylamine were
charged in a round bottom flak with touluene (50 mL). A catalytic
amount of p-toluenesulfonic acid was added and the reaction was
heated to reflux for 24 h in the atmopshere of nitrogen. The result-
ing water was removed as an azeotropic mixture using a Dean-
Stark apparatus. After the reaction finished, the mixture was
cooled to ambient tempetature and the solvent was removed in
vaccum. The product was obtained as yellow crystals in 82% yield
after rectystallization from hot ethanol. 1H NMR (CDCl3, 400 MHz),
d (ppm): 7.24–7.06 (m, 8H, Ph), 6.84–6.76 (d, 2H, Ph), 6.71–6.64 (d,
2H, Ph), 5.36 (s, 2H, CH), 3.68 (s, 6H, CH3), 2.50 (m, 4H, CH), 1.21–
1.13 (d, 6H, CH3),1.12–1.05 (d, 6H, CH3), 1.05–0.96 (t, 12H, CH3).
13C NMR (CDCl3, 100 MHz), d (ppm): 158.32, 154.87, 140.86,
136.62, 136.05, 129.54, 129.21, 128.22, 126.88, 124.59, 123.83,
122.67, 122.51, 119.99, 109.19, 55.15, 44.52, 28.76, 28.38, 23.37,
22.86, 22.33, 22.01. Anal. Calcd for C42H48N2O2: C, 82.31; H, 7.89;
N, 4.57. Found: C, 82.58; H, 7.95; N, 4.60. ESI-MS (m/z): 613 [M
+H]+; 635 [M+Na]+.
L4, Synthesis of ArAN@C(An)A(An)C@NAAr (An = 1,5-dibromo-
dibenzobarrelene,
Ar = 2,6-diisopropylphenyl).
1,5-
Dibromoanthracene was prepared according to the reported liter-
ature (see SI) [88]. 1,5-Dibromo-9,10-dihydro-9,10-ethanoanthra
cene-11,12-dione was isolated as orange solids. 1H NMR (CDCl3,
400 MHz), d (ppm): 7.62 (d, 1H, Ph), 7.60 (d, 1H, Ph), 7.49 (s, 1H,
Ph), 7.47 (s, 1H, Ph),7.31–7.25 (m, 2H, Ph), 5.51 (s, 2H, CH).
a
-Diimine ligand L4 was obtained as yellow crystals in 80%
yield. 1H NMR (CDCl3, 400 MHz), d (ppm): 7.42–7.33 (d, 2H, Ph),
7.25–7.13 (m, 8H, Ph), 7.10–7.03 (t, 2H, Ph), 5.37 (s, 2H, CH), 2.43
(m, 4H, CH), 1.32–1.22 (d, 6H, CH3), 1.16–1.08 (d, 6H, CH3), 1.08–
0.95 (dd, 12H, CH3). 13C NMR (CDCl3, 100 MHz), d (ppm): 140.41,
138.63, 136.59, 135.05, 131.47, 128.79, 124.97, 124.07, 123.18,
122.53, 120.50, 50.84, 29.16, 28.58, 23.31, 22.58, 22.00, 21.88. Anal.
Calcd for C40H42N2Br2: C, 67.61; H, 5.96; N, 3.94. Found: C, 67.89;
H, 6.19; N, 3.73. ESI-MS (m/z): 711 [M+H]+; 733.1 [M+Na]+.
L3, Synthesis of ArAN@C(An)A(An)C@NAAr (An = 1,5-dichloro-
dibenzobarrelene,
Dichloroanthracene was prepared according to the reported litera-
ture (see SI) [87]. Following the above procedure, the -dione com-
Ar = 2,6-diisopropylphenyl).
1,5-
a
pound (1,5-di-chloro-9,10-dihydro-9,10-ethanoanthracene-11,12-
dione) was isolated as orange solids. 1H NMR (CDCl3, 400 MHz), d
(ppm): 7.49–7.31 (m, 6H, Ph), 5.50 (s, 2H, CH).
L5, Synthesis of ArAN@C(An)A(An)C@NAAr (An = 1,5-diiodo-
5
dibenzobarrelene,
Ar = 2,6-diisopropylphenyl).
1,5-
Diiodoanthracene was prepared according to the literature (see
SI) [89]. 1,5-Di-iodo-9,10-dihydro-9,10-ethanoanthracene-11,12-d
ione was isolated as orange solids. 1H NMR (CDCl3, 400 MHz), d
(ppm): 7.86 (d, 1H, Ph), 7.84 (d, 1H, Ph), 7.50 (d, 1H, Ph), 7.48 (d,
1H, Ph), 7.14–7.08 (t, 2H, Ph), 5.37 (s, 2H, CH).
a
-Diimine ligand L3 was obtained as yellow crystals in 90%
yield. 1H NMR (CDCl3, 400 MHz), d (ppm): 7.25–7.09 (m, 12H,
Ph), 5.40 (s, 2H, CH), 2.45 (m, 4H, CH), 1.30–1.20 (d, 6H, CH3),
1.17–1.08 (d, 6H, CH3), 1.07–0.95 (t, 12H, CH3). 13C NMR (CDCl3,
100 MHz),
d (ppm): 156.17, 145.92, 140.26, 136.72, 136.50,
135.23, 131.07, 128.58, 128.30, 124.31, 122.98, 122.54, 48.13,
29.07, 28.52, 23.31, 22.59, 22.01, 21.79. Anal. Calcd for C40H42N2-
Cl2: C, 77.28; H, 6.81; N, 4.51. Found: C, 77.56; H, 6.99; N, 4.80.
ESI-MS (m/z): 622 [M+H]+; 643 [M+Na]+.

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L. Zhong et al. / Journal of Catalysis 375 (2019) 113–123
a
-Diimine ligand L5 was obtained as yellow crystals in 85%
H
42Br4I2N2Ni: C, 49.96; H, 4.14; N, 2.74; Found: C, 50.42; H, 4.30;
yield. 1H NMR (CDCl3, 400 MHz), d (ppm): 7.64–7.55 (d, 2H, Ph),
7.25–7.12 (m, 8H, Ph), 6.95–6.86 (t, 2H, Ph), 5.27 (s, 2H, CH), 2.47
(m, 4H, CH), 1.39–1.28 (d, 6H, CH3), 1.13–1.04 (dd, 12H, CH3),
1.03–0.96 (d, 6H, CH3). 13C NMR (CDCl3, 100 MHz), d (ppm):
139.84, 138.80, 138.58, 138.47, 137.73, 137.09, 135.01, 129.96,
129.09, 126.70, 125.87, 125.21, 124.33, 122.47, 95.48, 65.42,
64.23, 55.78, 55.16, 29.20, 28.96, 28.46, 28.25, 24.14, 23.41,
23.01, 22.56, 22.39, 22.23, 21.80. Anal. Calcd for C40H42N2I2: C,
59.71; H, 5.26; N, 3.48. Found: C, 59.90; H, 5.57; N, 3.70. ESI-MS
(m/z): 805.5 [M+H]+; 827.5 [M+Na]+.
N, 2.89. MALDI-TOF (m/z): 1024.991 [M+H]+; 984.189 [M-Br+K]+;
+
805.365 [ligand+H]
.
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgements
This work was supported by grants from National Natural
Science Foundation of China (Project 21674130, 51873234) and
the Fundamental Research Funds for the Central Universities
(17lgjc02).
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2019.05.026.
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5. Synthesis of
a-diimine nickel complexes
5.1. Synthesis of 1
Ligand L1 (612 mg, 1 mmol) and (DME)NiBr2 (308 mg, 1 mmol)
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H
48Br2N2O2Ni: C, 60.68; H, 5.82; N, 3.37; Found: C, 61.13; H, 6.29;
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Following the above described procedure, the reaction of (DME)
NiBr2 and L4 gave complex 4 in 67% yield.
4,
(ArAN@C(An)A(An)C@NAAr)NiBr2
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H
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5.4. Synthesis of 5
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