2-{2,6-Bis[bis(4-fluorophenyl)methyl]-4-chlorophenylimino} -3-aryliminobutylnickel(II) bromide complexes: Synthesis, characterization, and investigation of their catalytic behavior

Article abstract of DOI:10.1016/j.apcata.2014.01.034

The series of 2-{2,6-bis[di(4-fluorophenyl)methyl]-4-chlorophenylimino}-3- aryliminobutane derivatives (L1-L5) and their nickel(II) dibromide complexes (Ni1-Ni5) were synthesized, and all organic compounds were fully characterized by the Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy and by elemental analysis, while the nickel complexes were characterized by FT-IR spectroscopy, elemental analysis, as well as by single-crystal X-ray diffraction for two representative examples, namely Ni1 and Ni4. A distorted tetrahedral geometry was observed for these four-coordinate nickel complexes. Upon the activation with either Methylaluminoxane or modified methylaluminoxane as co-catalyst, all nickel complex precatalysts showed very high activity toward ethylene polymerization with activities of up to 10 ⁷ g(PE)·mol^-1(Ni)·h^-1, and afforded highly branched polyethylene with a bimodal distribution.

Full text of DOI:10.1016/j.apcata.2014.01.034

Applied Catalysis A: General 475 (2014) 195–202
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
2-{2,6-Bis[bis(4-fluorophenyl)methyl]-4-chlorophenylimino}-3-
aryliminobutylnickel(II) bromide complexes: Synthesis,
characterization, and investigation of their catalytic behavior
Qingyun Liua,b, Wenjuan Zhangb, Dedong Jiab, Xiang Haob,
Carl Redshawc,∗, Wen-Hua Sunb
a College of Chemistry and Environmental Engineering, Yangtze University, Hubei, Jingzhou 434023, China
b Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China
c Department of Chemistry, University of Hull, Hull HU6 7RX, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The series of 2-{2,6-bis[di(4-fluorophenyl)methyl]-4-chlorophenylimino}-3-aryliminobutane deriva-
tives (L1–L5) and their nickel(II) dibromide complexes (Ni1–Ni5) were synthesized, and all organic
compounds were fully characterized by the Fourier transform infrared (FT-IR) and nuclear magnetic res-
onance (NMR) spectroscopy and by elemental analysis, while the nickel complexes were characterized by
FT-IR spectroscopy, elemental analysis, as well as by single-crystal X-ray diffraction for two representative
examples, namely Ni1 and Ni4. A distorted tetrahedral geometry was observed for these four-coordinate
nickel complexes. Upon the activation with either Methylaluminoxane or modified methylaluminoxane
as co-catalyst, all nickel complex precatalysts showed very high activity toward ethylene polymeriza-
tion with activities of up to 107 g(PE)·mol−1(Ni)·h−1, and afforded highly branched polyethylene with a
bimodal distribution.
Received 16 November 2013
Received in revised form 13 January 2014
Accepted 16 January 2014
Available online 24 January 2014
Keywords:
␣-Diiminonickel complex
bulky unsymmetrical 2,3-diiminobutane
ethylene polymerization
highly branched polyethylene
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
precatalysts, potentially having meso- and rac-stereo isomers, can
produce polyethylene with broad polydispersity or a bimodal
The discovery of ␣-diimine nickel(II) and Pd(II) complexes as
highly active precatalysts resurrected interest in late-transition
metal based systems for possible polyolefin production [1–3].
Extensive modification of the complex precatalysts and explo-
ration of their catalytic behavior as well as the properties of
the resulting polyethylene has been well documented by several
review articles [4–8] and more recent papers on iminopyridyl-
nickel pre-catalysts have also appeared [9–11]. The steric bulk of
the modified ligands can play an important role in determining
the catalytic behavior of their nickel complexes. For example, the
use of less bulky ligands can transform their nickel precatalysts in
such a way that results in more oligomers due to enhanced chain
transfer versus chain propagation [3,12,13]; whereas the use of
bulkier ligands aids the formation of highly branched polyethyl-
ene [12]. In addition, unsymmetrical ␣–diiminonickel(II) complex
distribution [12]. Interestingly, benzhydryl-substituted unsym-
metrical acenaphthyl-diiminonickel complexes not only exhibited
very high activity toward ethylene polymerization, but also pro-
duced highly branched polyethylene with a unimodal feature
[14–16]. Subsequently, butane-based unsymmetrical diiminon-
ickel complexes were shown to produce polyethylene with broad
polydispersity [17], while benzhydryl-substituted butane-based
symmetrical diiminonickel complexes were confirmed to possess
high activity toward ethylene polymerization [18]. Furthermore,
the use of a fluoro-substituent has been recognized to have
a positive influence on the catalytic behavior of their metal
complexes, and can result in living polymerization [19] and/or
better thermal stability [20]. Furthermore, our previous expe-
rience has revealed a number of positive effects using iron
complex precatalysts bearing this type of ligand set in ethyl-
ene polymerization [21,22]. In order to extend the scope of
butane-based unsymmetrical diiminonickel complexes available
[17], the synthesis of 2-{2,6-bis[di(4-fluorophenyl)methyl]-4-
chlorophenylimino}-3-aryliminobutane derivatives (L1–L5) and
∗
Corresponding author: Tel.:+44 1482 465219; fax: +44 1482 466410.
E-mail address: c.redshaw@hull.ac.uk (C. Redshaw).
http://dx.doi.org/10.1016/j.apcata.2014.01.034
0926-860X/© 2014 Elsevier B.V. All rights reserved.

196
Q. Liu et al. / Applied Catalysis A: General 475 (2014) 195–202
2.2.1. Synthesis of nickel complexes (Ni1–Ni5)
According to our previous procedure [17], the complexes
Ni1–Ni5 were prepared by the reaction of (DME)NiBr2 with the
corresponding ligand (L1–L5) in dichloromethane. The proce-
dure for Ni1 is as follows. The ligand L1 (0.1 g, 0.14 mmol) and
(DME)NiBr2 (0.05 g, 0.16 mmol) were added to a Schlenk tube
together with 10 ml of dichloromethane. The reaction mixture was
stirred for 12 h at room temperature, and diethyl ether (10 ml) was
added to precipitate the complex. The precipitate was washed with
diethyl ether and dried under vacuum to afford a brick red powder
of Ni1 in 87.8% (0.12 g) yield. FT-IR (KBr, cm−1): 3058 (w), 2168 (w),
1899 (w), 1602 (m), 1506 (s), 1436 (m), 1377 (m), 1225 (s), 1158
(s), 1098 (m), 1012 (m), 983 (w), 907 (w), 832 (s), 793 (m), 721 (m),
668 (m). Analytical Calculated (Anal. Calcd.) for C44H35Br2ClF4N2Ni
(921.71): C, 57.34; H, 3.83; N, 3.04. Found: C, 56.91; H, 4.16; N, 2.93.
Data for Ni2. Yield: 85.0% (0.11 g), brick red powder. FT-IR (KBr,
cm−1): 3052 (w), 2043 (w), 1601 (m), 1577 (m), 1504 (s), 1437 (m),
1408 (m), 1377 (s), 1298 (w), 1212 (s), 1156 (s), 1096 (m), 1038 (w),
1012 (w), 995 (m), 903 (m), 831 (s), 764 (m), 668 (m). Anal. Calcd.
for C46H39Br2ClF4N2Ni (949.76): C, 58.17; H, 4.14; N, 2.95. Found:
C, 58.11; H, 4.53; N, 2.91.
p-TsOH
O
O
+
N
N
O
CH2Cl2
reflux
NH2
Toluene
reflux
(DME)NiBr2
CH2Cl2
N
N Ar
N Ar
Ni
Br
Ni1-Ni5
Ln
Br
L1 - L5
L1 L2 L3 L4 L5
Ni1 Ni2 Ni3 Ni4 Ni5
Nin
R1
Me Et iPr Me Et
R2
H
H
H
Me Me
FPh
PhF
Ar
as
as
R1
R1
FPh
PhF
R2
Cl
Data for Ni3. Yield: 92.9% (0.12 g), brick red powder. FT-IR (KBr,
cm−1): 3052 (w), 2967 (m), 2166 (w), 1600 (m), 1578 (w), 1505 (s),
1439 (m), 1413 (w), 1378 (s), 1204 (m), 1158 (s), 1097 (m), 1054
(w), 1008 (w), 911 (m), 836 (s), 794 (m), 735 (m), 669 (m). Anal.
Calcd. for C48H43Br2ClF4N2Ni (977.82): C, 58.96; H, 4.43; N, 2.86.
Found: C, 58.57; H, 4.48; N, 2.93.
Data for Ni4. Yield: 86.6% (0.11 g), brick red powder. FT-IR (KBr,
cm−1): 3043 (w), 2910 (w), 2168 (w), 2043 (w), 1599 (m), 1576 (m),
1504 (s), 1437 (m), 1408 (m), 1377 (s), 1297 (w), 1219 (s), 1156 (s),
1096 (m), 1011 (m), 983 (w), 907 (m), 831 (s), 713 (w), 668 (m).
Anal. Calcd. for C45H37Br2ClF4N2Ni (935.74): C, 57.76; H, 3.99; N,
2.99. Found: C, 57.78; H, 4.36; N, 2.87.
Scheme 1. Synthesis of diiminobutanes L1–L5 and nickel complexes Ni1–Ni5.
the corresponding nickel(II) (Ni1–Ni5) complexes has now been
carried out, and investigations into of the catalytic behavior of
the nickel complexes have been conducted. Upon activation with
either Methylaluminoxane (MAO) or modified methylaluminoxane
(MMAO), all nickel complexes performed with very high activity
toward ethylene polymerization, producing polyethylene with a
bimodal distribution.
2. Experiments
Data for Ni5. Yield: 92.5% (0.12 g), brick red powder. FT-IR (KBr,
cm−1): 2967 (m), 2168 (m), 2043 (w), 1600 (m), 1576 (w), 1504 (s),
1438 (w), 1414 (w), 1379 (m), 1335 (w), 1301 (w), 1221 (s), 1157
(s), 1096 (m), 1011 (w), 984 (w), 911 (m), 832 (s), 721 (m), 669 (m).
Anal. Calcd. for C47H41Br2ClF4N2Ni (963.79): C, 58.57; H, 4.29; N,
2.91. Found: C, 58.77; H, 4.23; N, 2.89.
2.1. General
All manipulations of air- and moisture-sensitive compounds
were carried out under an atmosphere of nitrogen using standard
Schlenk techniques. Toluene was dried by refluxing with sodium
and distilled under nitrogen prior to use. MAO, 1.46 M solution in
toluene) and MMAO, 1.93 M in heptane, 3 A, were purchased from
Akzo Nobel Corp. High-purity ethylene was purchased from Beijing
Yanshan Petrochemical Co. and used as received. 1H and 13C NMR
spectra were recorded on a Bruker DMX 400 MHz instrument at
ambient temperature using tetramethylsilane(TMS) as an internal
standard. IR spectra were recorded on a Perkin-Elmer System 2000
FT-IR spectrometer. Elemental analyses were carried out using a
Flash EA 1112 microanalyzer. Molecular weights (Mw) and molec-
ular weight distribution (Mw/Mn) of polyethylene were determined
by a PL-GPC220 at 150 ◦C, with 1,2,4-trichlorobenzene as the sol-
vent. Differential scanning calorimetric (DSC) traces and melting
points of polyethylene were obtained from the second scanning
run on Perkin-Elmer DSC-7 at a heating rate of 10 ◦C/min. 13C
NMR spectra of polymer were recorded on a Bruker DMX-300 MHZ
instrument at 135 ◦C in deuterated 1,2-dichlorobenzene with TMS
as an internal standard.
2.2.2. X-Ray crystallographic studies
Single crystals of complexes Ni1 and Ni4 suitable for X-ray
diffraction were grown by slow diffusion of diethyl ether into the
respective dichloromethane solution. Data collection of Ni and Ni4
was performed with on a Rigaku R-AXIS Rapid IP diffractome-
ter with graphite-monochromated Mo K␣ radiation (␭ = 0.71073 Å)
at 173(2) K. Intensities were corrected for Lorentz and polariza-
tion effects and empirical absorption. The structures were solved
by direct methods and refined by full-matrix least squares on F2.
All hydrogen atoms were placed in calculated positions. Struc-
ture solution and refinement were performed using the SHELXL-97
package [23]. Crystal data and processing parameters for complexes
Ni1 and Ni4 are summarized in Table 1.
2.3. General procedure for ethylene polymerization
2.2. Syntheses and characterization
2.3.1. Ethylene polymerization at 1 atmosphere (atm) ethylene
pressure
The organic compounds to be used as the ligands were pre-
pared according to the modified literature procedure [17], and the
resulting diimino compounds were reacted with nickel bromide in
dichloromethane to afford the corresponding nickel complexes; the
synthetic procedure is illustrated in Scheme 1. The detailed proto-
col of the synthesis of diimino compounds (L1–L5) is available in
the Supplementary information.
The precatalyst Ni1 was dissolved in toluene in a Schlenk tube,
and then the reaction solution was stirred with a magnetic stir bar at
1 atm of ethylene at the required reaction temperature. The require
amount of cocatalyst was added by a syringe. After the requisite
time, the reaction solution was quenched with 10% hydrochlo-
ric acid in ethanol. The precipitated polymer was collected by

Q. Liu et al. / Applied Catalysis A: General 475 (2014) 195–202
197
Table 1
Table 2
Crystal data and structure refinement for Ni1 and Ni4.
Selected bond lengths (Å) and angle (◦) for complexes Ni1 and Ni4.
Ni1
Ni4
Complexes
Ni1
Ni4
Empirical formula
Fw
C44 H35Br2ClF4N2Ni
921.72
C45 H37Br2ClF4N2Ni
935.75
Bond lengths (Å)
Ni(1)-N(1)
2.026(7)
2.004(7)
2.038(7)
2.004(9)
T/K
173(2)
173(2)
Ni(1)-N(2)
␭/Å
0.71073
Monoclinic
Cc
24.466(5)
11.099(2)
16.588(3)
90
121.07(3)
90
3858.5(13)
4
0.71073
Monoclinic
Cc
24.183(5)
11.066(2)
16.655(3)
90
117.48(3)
90
3954.0(14)
4
Ni(1)-Br(1)
Ni(1)-Br(2)
N(1)-C(2)
N(1)-C(5)
N(2)-C(1)
N(2)-C(13)
C(1)-C(2)
2.3276(14)
2.3383(15)
1.301(10)
1.421(11)
1.282(11)
1.459(11)
1.500(11)
2.3307(17)
2.3328(16)
1.291(14)
1.444(13)
1.279(13)
1.447(14)
1.468(15)
Cryst. syst.
Space group
a/Å
b/Å
c/Å
˛ (o)
ˇ (o)
ꢀ (o)
V (Å3)
Z
Bond angles (◦)
N(1)-Ni(1)-N(2)
N(1)-Ni(1)-Br(1)
N(1)-Ni(1)-Br(2)
N(2)-Ni(1)-Br(1)
N(2)-Ni(1)-Br(2)
Br(1)-Ni(1)-Br(2)
80.9(3)
80.8(3)
112.9(2)
111.7(2)
118.7(2)
107.7(2)
118.74(6)
111.2(3)
114.1(3)
108.0(3)
116.7(2)
119.64(6)
D calcd. (gcm−3
)
1.587
2.698
1856
1.572
2.634
1888
ꢁ/mm−1
F(000)
Cryst. size/mm
0.24 × 0.21 × 0.05
2.79–27.48
-31 ≤ h ≤ 31
-14≤ k ≤ 14
-21≤ l ≤ 20
0.19 × 0.16 × 0.03
1.90–27.50
-31≤ h ≤ 31
-14 ≤k ≤ 14
-21 ≤ l ≤ 21
13010
␪ range (◦)
Limiting indices
diffraction analysis were obtained, and the crystal structures are
shown in Figs. 1 and 2; selected bond lengths and bond angles are
tabulated in Table 2.
No. of reflections collected 14124
Similar to our previous report on ␣-diimino nickel analogues
[17], both structures of the complexes Ni1 and Ni4 revealed a dis-
torted tetrahedral geometry at the nickel center, for which the
atoms of N1, N2, and Br2 comprised the basal plane, and the atom
Br1 occupied the apical position. The aryl rings of the ␣-diimine
are almost perpendicular to the coordination plane formed by N1-
C2-C1-N1. In the structure of complex Ni1 (shown in Fig. 1), the
dihedral angle of the coordination plane to the plane formed by
C13, C14, C18 and to the plane formed by C5, C6, C10 are 86.91◦ and
88.97◦, respectively, while the respective values in Ni4 are 86.60◦
and 88.56◦, which are slightly larger than for the analog without
the fluoro-substituent [17]. The bond lengths of Ni-N [2.026(7)
No. unique reflections
[R(int)]
Completeness to ꢂ (%)
Absorption correction
8072 (0.0604)
7865 (0.0438)
99.4%
none
98.9%
none
7865/2/496
1.089
R1= 0.0868
wR2= 0.2263
R1= 0.1005
wR2= 0.2553
1.045 and -0.964
Data/restraints/parameters 8072/2/487
Goodness of fit on F2
1.057
Final R indices[I > 2ꢃ(I)]
R1= 0.0823
wR2= 0.2050
R1= 0.0890
wR2= 0.2131
0.917 and -0.659
R indices (all data)
Largest diff. peak and hole
(e Å−3
)
filtration, washed with ethanol several times, and dried in a vacuum
at 60 ◦C until constant weight.
´
´
˚
˚
and 2.004(7) A for Ni1 (2.038(7) and 2.003(9) A for Ni4] are much
longer than those reported for the nickel analogs minus the fluoro-
´
ꢀ
˚
substituent [2.012(4), 1.999(4) A for Ni4 , 2.002(5) and 1.989(6)
2.3.2. Ethylene polymerization at 10/5 atm ethylene pressure
The polymerization at 10 atm of ethylene pressure was per-
formed in a 250 mL stainless steel autoclave equipped with
a mechanical stirrer, a temperature controller, and gas ballast
through a solenoid clave for continuous feeding of ethylene at
constant pressure. First, 30 mL of toluene was injected into the
autoclave, which has previously been filled with ethylene. When
the temperature required was reached, another 20 mL toluene con-
taining the dissolved complex was added, plus the required amount
of co-catalyst (MAO or MMAO), and the remaining toluene were
successively added using a syringe. The reaction mixture was inten-
sively stirred for the desired time under the corresponding pressure
of ethylene throughout the entire experiment. The reaction was ter-
minated and analyzed using the same method as above for ethylene
polymerization at ambient pressure.
´
ꢀ
˚
A for Ni5 ]; this is ascribed to fluoro-incorporation leading to an
electron-deficient nickel center. According to the data in Table 2,
the complexes Ni1 and Ni4 have similar structural features, and
therefore the complex Ni4 is not further discussed.
3.2. Ethylene polymerization
Using Ni2 as precatalysts, the ethylene polymerization was eval-
uated in the presence of different co-catalysts such as MAO, MMAO,
and Et2AlCl. The cocatalysts MAO or MMAO showed high effi-
ciency in the ethylene polymerization, which is consistent with the
observation found for other nickel precatalysts [17], and therefore
investigations were conducted by employing either MAO or MMAO
as cocatalyst.
3. Results and Discussion
3.2.1. Ethylene polymerization by the Ni1-Ni5/MAO systems
In the presence of MAO, precatalyst Ni2 was employed for opti-
mizing the ethylene polymerization conditions and the results
are collected in Table 3. At 20 ◦C, increasing the Al/Ni ratio
from 1000 to 3000 led to little change of polymerization activ-
ity, which range from 3.84 to 4.53 × 106g mol−1(Ni) h−1 (runs
1–5, Table 3), while the highest activity was achieved at a
molar ratio of Al/Ni 2000:1; this is similar to that found for
the analog Ni2ꢀ/MAO [14]. In contrast, the molecular weights
of the obtained polyethylenes decreased from 10.7 to 6.73 × 105
g·mol−1 along with higher Al/Ni ratios, indicating that more
chain transfer to aluminum occurs at higher concentrations of
cocatalyst. The effect of temperature on the activity was also
3.1. Synthesis, characterization of diiminobutanes and nickel(II)
complexes
The synthetic procedure was carried out as previously reported
[17], and all organic compounds (L1–L5) were fully charac-
terized by 1H/13C NMR and FT-IR spectroscopy as well as by
elemental analysis. Treatment of the compounds L1–L5 with one
equivalent of (DME)NiBr2 afforded the respective nickel bromide
complexes (Ni1–Ni5) (Scheme 1), which were characterized by
FT-IR spectroscopy and elemental analysis. To further confirm
their structures, single crystals of Ni and Ni4 suitable for X-ray

198
Q. Liu et al. / Applied Catalysis A: General 475 (2014) 195–202
Fig. 1. ORTEP drawing of complex Ni1 with thermal ellipsoids at 30% probability level. Hydrogen atoms have been omitted for clarity.
from 8.19 to 0.93 × 106g mol−1 (Ni) h−1 (runs 3, 6–9, Table 3),
reflecting the poor stability of the nickel intermediate at higher
temperatures. As anticipated, the molecular weight of the resul-
tant polyethylene (PE) decreased from 8.76 to 3.81 × 105 g·mol−1
on increasing the temperature from 20 ◦C to 60 ◦C. At the same
investigated at the molar ratio of Al/Ni of 2000:1. On elevat-
ing the temperature from 20 ◦C to 30 ◦C, the polymerization
activity dramatically increased from 4.53 to 8.19 × 106g mol−1
(Ni) h−1, while further increasing the temperature from 30 ◦C
to 60 ◦C led to
a sharp decrease of polymerization activity
Fig. 2. ORTEP drawing of complex Ni4 with thermal ellipsoids at 30% probability level. Hydrogen atoms have been omitted for clarity.

Q. Liu et al. / Applied Catalysis A: General 475 (2014) 195–202
199
Table 3
Catalytic Results of Ethylene Polymerization with Ni1–Ni5/MAOa
c,d
d
run
Cat.
T/◦C
t/min
Al/Ni
Activityb
Mw
Mw/Mn
Tme(◦C)
Branchesf
1
2
3
4
5
6
7
8
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni1
Ni3
Ni4
Ni5
20
20
20
20
20
30
40
50
60
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
5
10
20
60
30
30
30
30
30
30
1000
1500
2000
2500
3000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
3.84
3.92
4.53
3.87
3.84
8.19
2.79
1.06
0.93
11.5
9.84
8.72
6.14
2.61
0.83
4.95
3.81
4.89
4.57
10.7
9.39
8.76
6.73
–
5.1
2.3
2.6
3.0
–
3.6
2.4
9.7
97.8
101.8
92.5
72.2
–
43
49
48
63
–
6.20
5.91
4.14
3.81
5.26
4.71
6.66
4.92
5.44
3.01
7.64
8.18
6.34
6.59
53.9
44.9
38.8
30.6
58.8
53.2
47.1
51.5
58.7
31.2
88.8
46.5
52.9
56.8
58
86
89
92
59
82
78
69
71
102
77
46
80
60
9
7.0
10
11
12
13
14g
15h
16
17
18
19
11.9
14.7
17.5
13.3
23.5
1.8
38.3
51.4
17.9
37.8
a
Conditions: 2 ␮mol Ni; 10 atm of ethylene; 30 min; total volume 100 ml.
b
c
d
e
f
106g mol-1 (Ni) h-1
.
105g mol−1
.
determined by GPC.
determined by DSC.
/1000 carbons, determined by FT-IR [26].
5 atm of ethylene.
1 atm ethylene.
g
h
time, higher temperatures also led to a higher branch density for
the obtained PE, namely from 48 to 92/1000 carbon. These results
can be explained in terms of deactivation of the metal species
[1,2,24] and increased chain walking [5,25] at higher temperature.
In addition, lower Tm values for the PE were also observed at higher
temperature, which agreed with the trend observed for the branch
density.
Fig. 3 shows the gel permeation chromatography (GPC) traces
of the PE obtained at different temperatures and it reveals that the
PE obtained above 20 ◦C possessed a bimodal distribution and that
the fraction for the low molecular weight increased on increas-
ing the temperature. The peaks of the traces also shifted to lower
molecular weights, which is similar to the situation observed for
Ni2ꢀ/MAO [17], which is consistent with the formation of more
polyethylene of lower molecular weights due to enhanced chain
transfer occurring at higher reaction temperatures.
Table 3). The reason here is the competition between ethylene cap-
ture and “chain walking”, which always exists in the polymerization
process, with the chain walking being favored at lower pressure
leading to higher branching in the polyethylene and the chain prop-
agation favored at higher pressure leading to higher activity and
higher molecular weight polymers. On prolonging the reaction time
from 5 to 60 min at 30 ◦C, the catalytic activities gradually decreased
from 11.46 to 6.14 × 106 g·mol−1(Ni) h−1, suggesting that the cat-
alytic species remained active over 60 min (run 10–13, Table 3). All
the polyethylene obtained possessed a bimodal distribution.
Under the optimized conditions (Al/Ni = 2000, temp. = 30 ◦C),
the ethylene polymerization catalysis of all the nickel complexes
Ni1–Ni5 was investigated. In general, all exhibited good activity
for ethylene polymerization, affording a bimodal distribution for
the polyethylene (shown in Fig. 4) akin to the results using unsym-
metrical butene ␣–diimine complexes, but somewhat different to
the unimodal distribution observed for acenaphthylene ␣–diimine
nickel complexes [14–16]; the different conformation of the
In addition, the ethylene pressure also had an impact on the cat-
alytic performance. For example, increasing the ethylene pressure
from 1 to 10 atm led to a huge increase in the activity from 0.83
to 8.91 × 106 g·mol−1(Ni) h−1 and a decrease in the branch density
from 102 to 58/1000 carbon for the obtained PE (runs 6, 14, 15 in
Ni1
Ni2
Ni3
Ni4
Ni5
20 oC
30 oC
40 oC
50 oC
60 oC
3
4
5
6
7
LogM
w
2
3
4
5
6
7
Fig. 3. GPC curves of the polyethylene obtained by Ni2/MAO at different tempera-
Fig. 4. GPC curves of polyethylene obtained by Ni1–Ni5/MAO (runs 9, 13–16 in
tures (runs 3, 6–9 in Table 3).
Table 3).

200
Q. Liu et al. / Applied Catalysis A: General 475 (2014) 195–202
9
8
7
6
5
4
3
2
1
0
Al/Ni=1000
Ni1 Ni2 Ni3 Ni4 Ni5
Ni1' Ni2' Ni3' Ni4' Ni5'
Ni2
Al/Ni=1500
Al/Ni=2000
Al/Ni=2500
Al/Ni=3000
R1
R2
Me Et iPr Me Et
H
H
H
Me Me
Ni1
Ni1'
Ni4
Ni5
Ni2'
Ni3
Ni4'
Ni5'
Ni3'
Fig. 5. Comparison of polymerization activities by the Ni1–Ni5/MAO with nonfluo-
rinated analogs Ni1ꢀ–Ni5ꢀ/MAO [17].
2
3
4
5
6
7
LogM
w
coordinated ring that favors the generation of two kinds of active
species was thought to be the reason.
Fig. 6. GPC curves of the polyethylene obtained by Ni2/MMAO at different Al/Ni
ratios (runs 1–5 in Table 4).
It is noteworthy that these complexes showed much higher
activity than did their analogs without F incorporation (Fig. 5),
the reason is attributed to the ability of the electron withdraw-
ing fluoro-substituent present in the ligands of the precatalysts
Ni1–Ni5, resulting in a higher net charge at the nickel core in
comparison to the nonfluorinated analogs Ni1ꢀ–Ni5ꢀ. Indeed, com-
putational studies have revealed that higher catalytic activities are
achieved with the enhanced net charges associated with the late-
transition metal complex precatalysts [27,28]. Thus, the complex
precatalysts Ni1–Ni5 exhibited much higher activity when com-
pared with their nonfluorinated analogs Ni1ꢀ–Ni5ꢀ [17] (Fig. 5),
thereby illustrating the remarkable influence of the ligand fluo-
rine atoms on the activity. In addition, the ligand environment had
similar effects on the polymerization behavior to that of previ-
ous unsymmetrical ␣-diimine nickel complexes. According to the
data in Table 2, Ni3 (R1 = iPr) exhibited the lowest activity of all
these catalyst systems, however it produced polyethylene with
the highest molecular weight (run 14 in Table 3). It is thought
that the introduction of the sterically bulky substitutents (iPr)
at the ortho-positions of the phenyl ring would block the axial
sites at the metal center, and can then suppress the chain trans-
fer processes rather than chain propagation [29]. Given that the F
position is far removed from the nickel center, we proposed that the
electron withdrawing group reduces the electron density at the
nickel center which leads to the higher observed activity (rather
than any steric effect). In addition, the molecular weights of
polyethylenes formed by Ni1–Ni5 are generally smaller than those
by the nonfluorinated Ni1ꢀ–Ni5ꢀ systems [17], besides Ni3 and its
analog Ni3ꢀ containing the bulkier iPr substitutent at the ortho-
position of the N-aryl.
3.2.2. Ethylene polymerization with the Ni1–Ni5/MMAO systems
The above results revealed that the catalytic systems
Ni1–Ni5/MAO exhibited higher activity than did their analogs
Ni1ꢀ–Ni5ꢀ/MAO [17], and produced polymer with
a
bimodal
Table 4
Catalytic Results of Ethylene Polymerization with Ni1–Ni5/MMAOa.
c,d
d
run
cat.(R1 R2)
T/◦C
t/min
Al/Ni
activityb
Mw
Mw/Mn
Tme(◦C)
branchesf
1
2
3
4
5
6
7
8
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni1
Ni3
Ni4
Ni5
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
20
20
20
20
20
30
40
50
60
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
5
1000
1500
2000
2500
3000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
3.45
3.75
4.07
3.56
2.51
5.42
3.81
2.64
2.15
4.56
4.34
5.19
4.92
12.36
6.99
6.17
4.03
3.31
0.75
7.00
7.35
6.28
5.65
6.15
6.27
6.24
5.20
3.92
7.05
6.77
7.73
7.07
5.66
4.70
5.78
4.68
5.12
3.21
34.2
35.2
52.0
38.4
30.9
3.6
85.5
84.2
93.1
102.3
103.1
59.2
49.7
46.7
42.6
67.5
91.5
73.5
56.8
59.9
61.8
61.7
70.7
52.2
30.1
40
59
46
55
47
69
72
81
6.1
12.6
13.6
30.1
64.7
33.7
53.9
14.0
29.3
18.3
31.9
30.4
2.2
9
98, 220g
57
51
60
68
62
85
50
10
11
12
13
14
15
16
17
18h
19i
10
20
60
30
30
60
65
62
a
Conditions: 2 ␮mol of Ni; 30 min; total volume 100 ml.
106g mol−1 (Ni) h−1
b
c
d
e
f
.
105g mol−1
.
determined by GPC.
determined by DSC.
determined by FT-IR [26].
measured by 13C NMR spectroscopy.
5 atm of ethylene.
g
h
i
1 atm of ethylene.

Q. Liu et al. / Applied Catalysis A: General 475 (2014) 195–202
201
Fig. 7. 13C NMR spectrum of polyethylene by Ni2/MMAO at 60 ◦C (run 9, Table 4).
distribution. In the presence of MMAO, these precatalysts exhibited
high activities for ethylene polymerization (Table 4).
over 5–10 min (run 14–15, Table 4) suggesting that the active
species suffered from severe deactivation at reaction times of over
5 min. When the reaction time was prolonged to 60 min, the activity
remained fairly constant. However, more polyethylenes of lower
molecular weights were obtained on prolonging the time; it is
tentatively proposed that the active species present produced the
short chain polymers due to difficulties encountered during chain
propagation caused by the close presence of the formed polyeth-
ylene (and access to less ethylene). The influence of pressure on
the ethylene polymerization was also explored. Here also higher
molecular weight polymers were obtained at higher ethylene pres-
sure, similar to the result obtained using the Ni-MAO system.
In a similar manner, MMAO was also explored with Ni2 to
ascertain the optimum conditions, and the results are tabulated in
Table 4. Employing the same procedures as for the catalytic system
Ni2/MAO, the influence of the ratio Al/Ni and reaction temperature
on the activities of the Ni2/MMAO system indicated that the opti-
mum conditions were 20 ◦C and Al/Ni 2000 (run 6, Table 4), which
is consistent with Ni2/MAO. However, in contrast to the MAO sys-
tem above, the polyethylene obtained using MMAO as cocatalyst
revealed a bimodal molecular weight distribution when the Al/Ni
ratio was varied from 1000 to 3000 (as shown in Fig. 6). With regard
to the influence of the reaction temperature on the PE microstruc-
ture (run 3, 6–9, Table 4), more branches and lower molecular
weight polyethylene were obtained on increasing the temperature.
The lower molecular weight at high temperature was attributed to
a faster ␤–hydride elimination rate [12].
4. Conclusions
A series of unsymmetrical ␣-diimines nickel(II) complexes
(Ni1–Ni5) containing the di(fluorinated benzylhydryl) motif was
synthesized and characterized by 1H/13C NMR spectroscopy and
elemental analysis. When employing either MAO or MMAO as
cocatalyst, these complexes exhibited high activity, reaching as
high as 107 gPE (mol of Ni)−1h−1 for ethylene polymerization, pro-
ducing a bimodal distribution of polymers. The current ligands
bearing electron withdrawing fluoro-substituents are capable of
inducing net charges in these nickel complexes, and the resulting
performance revealed higher activities for ethylene polymeriza-
tion than observed for their nonfluorinated analogs [17]. These new
complexes exhibited much higher activity when using MAO as the
cocatalyst. Different to the geometry-constrained acenaphthene-
based unsymmetrical ␣-diimines nickel (II) complexes [14–16],
the complexes herein produce polyethylene with a bimodal dis-
tribution with either MAO or MMAO as cocatalyst, indicating the
potential rotation of the single bond between two imino-groups
to generate two active species. Additionally, the polyethylene
obtained exhibited a high degree of branching, which was found
to vary with temperature.
In order to assess the branching type in the obtained PE samples,
representative polyethylene prepared using Ni2-MMAO at 60 ◦C
(run 9, Table 4) was measured by 13C NMR spectroscopy (Fig. 7).
According to the method described in the literature [30], it was
calculated that polyethylene with 220 branches/1000 carbons was
obtained. Highly branched polyethylenes have been also obtained
by analogous precatalysts [17] and by aryliminopyridylnickel pre-
catalysts [8–11]. These precatalysts potentially provide alternative
processes for solely conducting ethylene polymerization targeting
branched polyethylene, which are more commonly produced by
the copolymerization of ethylene with ␣-olefins.
On employing the optimum conditions (Al/Ni = 2000, 30 ◦C over
30 min), the Ni1–Ni5/MMAO systems were evaluated for ethyl-
ene polymerization (run 6, 10–13, Table 4). Different to previous
observations [17], it was found here that a bimodal distribution
for the polyethylene was obtained using Ni1, Ni3–Ni5/MMAO,
and indicated that more than one kind of active species was
operating during the polymerization process when MMAO was
used as the cocatalyst. In addition, they showed higher activ-
ity for ethylene polymerization than did their corresponding
analogs Ni1ꢀ–Ni5ꢀ/MMAO [17]; moreover, the fluorinated sys-
tems produced polyethylenes with lower molecular weights. These
phenomena are consistent to the above catalytic systems with
cocatalyst MAO.
Acknowledgement
This work is supported by NSFC Nos. 21374123 and U1362204.
The RSC and EPSRC are thanked for the awarded of a travel grant
(to CR).
Catalyst lifetimes were also investigated (run 6, 14–15, Table 4),
and it was observed that the catalytic activities sharply decreased

202
Q. Liu et al. / Applied Catalysis A: General 475 (2014) 195–202
[15] H. Liu, W. Zhao, J. Yu, W. Yang, X. Hao, C. Redshaw, L. Chen, W.-H. Sun, Cata. Sci.
Appendix A. Supplementary data
Technol. 2 (2012) 415–422.
[16] S. Kong, C. Guo, W. Yang, L. Wang, W.-H. Sun, J. Organomet. Chem 725 (2013)
37–45.
[17] D. Jia, W. Zhang, W. Liu, L. Wang, C. Redshaw, W.-H. Sun, Cata. Sci. Technol. 3
(2013) 2737–2745.
[18] J.L. Rhinehart, L.A. Brown, B.K. Long, J. Am. Chem. Soc 135 (2013) 16316–
16319.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2014.01.034.
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