Influence of the ligand structure on the properties of bidentate salicylaldimine nickel(II) complexes in ethylene oligomerization

Article abstract of DOI:10.1007/s11243-021-00447-7

Three nickel(II) complexes (C1–C3) bearing diamine-bridged 4-hydroxysalicylaldehyde ligands (L1–L3) were successfully synthesized, and all the compounds were characterized by physicochemical and spectroscopic methods. The influence of the oligomerization parameters on the catalytic properties of complex C2 was systematically investigated. The results showed that oligomerization parameters played an important role in the catalytic properties and the catalytic activity was 19.90 × 10⁴?g/(mol·Ni·h) and the selectivity of C₈₊ olefins was 60.25% when the precatalyst dosage was 5?μmol, the Al/Ni molar ratio was 500, the temperature was 25?°C, the reaction time was 30?min and the pressure of ethylene was 0.7?MPa. Complexes C1–C3 with different lengths of the bridged group were evaluated for ethylene oligomerization, and the results showed that the length of the alkyl chain in the ligand had little influence on the catalytic properties. Complex C4 based on ethanediamine-bridged salicylaldimine and C5 based on the hyperbranched salicylaldimine in our previous work were also investigated to study the influence of the ligand structure on the catalytic properties. The catalytic activity [31.80 × 10⁴?g/(mol·Ni·h)] and the content of the low-carbon oligomers (70.16%) for complex C4 were higher than complex C2 with hydroxyl substituent in benzene ring. The catalytic activity and the content of the low carbon oligomers for complex C5 were far higher than other four complexes.

Full text of DOI:10.1007/s11243-021-00447-7

Transition Metal Chemistry (2021) 46:307–314
https://doi.org/10.1007/s11243-021-00447-7
Infuence of the ligand structure on the properties of bidentate
salicylaldimine nickel(II) complexes in ethylene oligomerization
Yang Li1 · Lijun Guo1 · Feng Li1 · Jin Huang1,2 · Dan Li1 · Na Zhang1 · Cuiqin Li1
Received: 2 October 2020 / Accepted: 4 January 2021 / Published online: 2 February 2021
© The Author(s), under exclusive licence to Springer Nature Switzerland AG part of Springer Nature 2021
Abstract
Three nickel(II) complexes (C1–C3) bearing diamine-bridged 4-hydroxysalicylaldehyde ligands (L1–L3) were success-
fully synthesized, and all the compounds were characterized by physicochemical and spectroscopic methods. The infuence
of the oligomerization parameters on the catalytic properties of complex C2 was systematically investigated. The results
showed that oligomerization parameters played an important role in the catalytic properties and the catalytic activity was
19.90×104 g/(mol·Ni·h) and the selectivity of C8+ olefns was 60.25% when the precatalyst dosage was 5 μmol, the Al/Ni
molar ratio was 500, the temperature was 25 °C, the reaction time was 30 min and the pressure of ethylene was 0.7 MPa.
Complexes C1–C3 with diferent lengths of the bridged group were evaluated for ethylene oligomerization, and the results
showed that the length of the alkyl chain in the ligand had little infuence on the catalytic properties. Complex C4 based on
ethanediamine-bridged salicylaldimine and C5 based on the hyperbranched salicylaldimine in our previous work were also
investigated to study the infuence of the ligand structure on the catalytic properties. The catalytic activity [31.80×104 g/
(mol·Ni·h)] and the content of the low-carbon oligomers (70.16%) for complex C4 were higher than complex C2 with
hydroxyl substituent in benzene ring. The catalytic activity and the content of the low carbon oligomers for complex C5
were far higher than other four complexes.
Introduction
late-transition metal complexes, particularly the nickel com-
plexes, as precatalysts over the last 2 decades [4, 5]. Saad
et al. [6] synthesized frst-generation and second-generation
pyridylimine-terminated nickel dihalide metallodendrimers,
and these metallodendrimers were applied to the ethylene
oligomerization. The results showed that the nickel com-
plexes exhibited the promising catalytic activities and the
polymerization products contained low-density polyethylene
with the branching structure and high-density polyethylene
with the linear chain structures.
The discovery of cationic Ni(II) and Pd(II) precatalysts by
Brookhart triggered a true exploration of the late-transition
metal precatalysts for olefn polymerization [1, 2]. Com-
pared to early-transition metal precatalysts, the nickel(II)
precatalysts had a higher tolerance toward the polar rea-
gents and ofered the potential to yield polymers with dif-
ferent microstructures and properties [3]. Great progress
had been made in the area of ethylene polymerization with
At the beginning of the century, the discovery of salicy-
laldimine catalysts synthesized by Grubbs was another
breakthrough facilitating forays into neutral nickel catalysts
[7, 8]. More and more investigations have focused on the
improvement in the catalytic properties through modifca-
tion of the precatalyst structure [9, 10]. For the past few
years, our group has been absorbed in the research focused
on late-transition metal precatalysts based on salicylaldimine
Schif base ligands with various structures. Chen and her
co-workers synthesized a series of nickel complexes with
3-substituted salicylaldehyde-imine ligands based on ethyl-
enediamine and explored the infuence of the electronic and
steric efect of the ligands on catalytic properties of nickel
Supplementary material The online version contains
supplementary material available at (https://doi.org/10.1007/
s11243-021-00447-7).
* Cuiqin Li
dqpilicuiqin@126.com
1
Heilongjiang Provincial Key Laboratory of Oil
and Gas Chemical Technology, College of Chemistry
and Engineering, Northeast Petroleum University,
Daqing 163318, Heilongjiang, China
2
Heilongjiang Provincial Key Laboratory of Oilfeld Applied
Chemistry and Technology, Daqing Normal University,
Daqing 163712, Heilongjiang, China
Vol.:(0123456789)
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Transition Metal Chemistry (2021) 46:307–314
complexes in ethylene oligomerization [11]. Compared with
the complexes with the tert-butyl group and the methoxy
group, the nickel complex with methyl group performed
the highest activity of 1.63× 106 g/(mol·Ni·h) in ethylene
oligomerization and showed the specifc selectivity toward
C8–C12 olefn. Wang and co-workers synthesized a series of
nickel complexes based on hyperbranched salicylaldimine
ligands and explored the relationship between the structure
of salicylaldimine ligands and the catalytic activity [12].
The results showed that the catalytic activity decreased with
the increase in molecular cavity. The complexes with the
smaller molecular cavity displayed a higher catalytic activity
[5.80×105 g/(mol·Ni·h)] and a high carbon olefn content
(16.25%).
the nickel complex with 2 mL of HCl, 6 mL of HNO3 and
5 mL of HF, adding the mixture to Tefon autoclaves and
subsequently using a digester for 24 h at 120 °C. After
cooling to room temperature, the samples were diluted to
50 mL with deionized water. GC analyses of oligomers
were performed on a Fuli GC9720 equipped with a fame
ionization detector (FID) and a 50 μm (0.2 mm i.d., 0.5 μm
flm thickness) HP-PONA column. PXRD analysis was
performed using X-ray difraction of Japan (CuK alpha
radiation, tube voltage 40 kV, tube current 30 mA, scan
rate 10°/min, scanning scope 2θ = 10°–80°).
Synthesis of series of diamine‑bridged
4‑hydroxysalicylaldehyde ligands (L1–L3)
A majority of the investigations on nickel precatalysts
have focused on the benzene ring of salicylaldiminato
ligands because of the facility for introducing various sub-
stituents to enhance the activity and to control the polymer
microstructure. There have been a relatively small number of
studies about the relationship between the alkyl chain length
of the ligand backbone and the catalytic properties [13–15].
Based on the previous works, three nickel complexes based
on diamine-bridged 4-hydroxysalicylaldehyde Schif base
ligands with the same substituent group of the benzene ring
but the diferent bridged groups were synthesized to study
the structure–activity relationship of these nickel complexes
in ethylene oligomerization. The efects of the ligands with
diferent steric and electronic groups on the catalytic proper-
ties were explored.
Under nitrogen atmosphere, 4-hydroxysalicylaldehyde
(0.56 g, 4 mmol) and ethanediamine (0.12 g, 2 mmol) were
dissolved in 20 mL ethanol, respectively. The 4-hydrox-
ysalicylaldehyde solution was added to the ethanediamine
solution drop by drop. The reaction mixture was refuxed
for 6 h, and a light-yellow precipitate was obtained. Then,
the precipitate was separated by fltration and washed with
ethanol. The solid was dried in vacuum at 80 °C for 24 h
to obtain the ethylenediamine-bridged 4-hydroxysalicyla-
ldehyde imine ligand (L1) based on ethylenediamine as
a light-yellow solid, and the yield was 94%. Anal. Calcd.
for C16H16N2O4 (300): C 64.00, H 5.33, N 9.34. Found: C
64.05, H 5.41, N 9.37. IR data (KBr cm−1): ν(C=N) 1642
and ν(C–O) 1236. 1H NMR (400 MHz, CDCl3) δ 8.15 (2H,
–CH=N–), 6.07–7.06 (6H, Ph-H), 4.89 (4H, Ph-OH), 3.84
(4H, –CH2–N=C–).
Experimental section
The 4-hydroxysalicylaldehyde imine ligand (L2) based
on butanediamine was prepared with the same synthetic
method as L1 using 4-hydroxysalicylaldehyde (0.56 g,
4 mmol) and butanediamine (0.18 g, 2 mmol) as materi-
als. Light-yellow solid was obtained and the yield was
93%. Anal. Calcd. for C18H20N2O4 (328): C 65.85, H
6.09, N 8.54. Found: C 65.81, H 6.05, N 8.57. IR data
Materials and methods
All reactions were carried out under an atmosphere of nitro-
gen using standard Schlenk techniques. Solvent was dried
and distilled under nitrogen prior to use. All the raw materi-
als were purchased from Tianjin Kemiou Chemical Reagent
Co. Ltd. and used without further purifcation. Methylalu-
minoxane (MAO, 10 wt% in toluene) was purchased from
Sigma-Aldrich (China) and stored in N2 atmosphere.
1
(KBr cm−1): ν(C=N) 1646 and ν(C–O) 1226. H NMR
(400 MHz, CDCl3) δ 8.12 (2H, –CH=N–), 6.12–7.07 (6H,
Ph-H), 4.92 (4H, Ph-OH), 3.33 (4H, –CH2–N=C–), 1.75
(4H, –CH2–C–N=C–).
FTIR spectra were recorded on Bruker Vector 22 spec-
trophotometer in KBr pellets from 4000 to 500 cm−1. H
The 4-hydroxysalicylaldehyde ligand (L3) based on hex-
amethylenediamine was prepared with the same synthetic
method as L1 using 4-hydroxysalicylaldehyde (0.56 g,
4 mmol) and hexamethylenediamine (0.23 g, 2 mmol) as
materials. Light-yellow solid was obtained and the yield was
92%. Anal. Calcd. for C20H24N2O4 (356): C 67.42, H 6.74,
N 7.87. Found: C 67.51, H 6.79, N 7.817. IR data (KBr
cm−1): ν(C=N) 1643 and ν(C–O) 1232. 1H NMR (400 MHz,
CDCl3) δ 8.08 (2H, –CH=N–), 6.11–7.04 (6H, Ph-H), 4.88
1
NMR spectra were recorded with Bruker-400 MHz NMR
in CDCl3 with tetramethylsilane (TMS) as an internal
reference. TG analyses were carried out using a thermal
gravimetric analysis instrument (Diamond TG/DTA Per-
kin-Elmer SII) with a fow rate of 20 mL/min and the range
of 25–850 °C with the heating rate of 10 °C/min. The
element analysis of the nickel complexes was calculated
using ICPS-7510 spectrometer. The detail process was as
follows: the samples were prepared by treating 15 mg of
1 3

Transition Metal Chemistry (2021) 46:307–314
309
(s, 4H, O–H), 3.58 (4H, –CH2–N=C–), 1.54–1.39 (4H,
–CH2–C–N=C–), 1.16 (4H, –CH2–C–C–N=C–).
Ethylene oligomerization
Ethylene oligomerization was carried out in a 100-ml stain-
less steel autoclave equipped with a magnetic stirrer and
ethylene pressure control system. The reactor was charged
with ethylene three times under vacuum. Under ethylene
atmosphere, the appropriate amounts of solvent, cocatalyst
and precatalyst precursor were injected into the reactor with
a syringe, respectively. Ethylene must be supplied continu-
ously in order to hold the desired stable pressure. The mix-
ture was vigorously stirred for the desired time under the
appropriate pressure of ethylene throughout the experiment.
After the reaction, the reaction mixture was cooled in an
ice-water bath and the gas mixture was collected with gas
bags. The liquid mixture was quenched by 10% hydrochlo-
ric acid in ethanol. The qualitative analysis of gas mixture
and the liquid mixture was identifed by GC. The chromato-
graphic analyses were performed on a Fuli GC9720 instru-
ment equipped with HP-PONA capillary column (60 m long,
0.25 mm ID, 0.5 μm flm thickness). The analysis conditions
were 50 °C for 5 min, heated at 10 °C /min until the tem-
perature reached 140 °C, then heated at 5 °C /min until the
temperature reached 240 °C and maintained at the tempera-
ture for 5 min. The oligomers were identifed by co-injecting
n-hexane as the internal standard for quantifcation.
Synthesis of series of bidentate salicylaldimine
nickel complexes (C1–C3)
L1 (0.30 g, 1 mmol) and nickel acetate tetrahydrate (0.25 g,
1 mmol) were dissolved in 40 mL ethanol under nitrogen
atmosphere. The reaction mixture was stirred for 12 h at
78 °C under nitrogen atmosphere and the khaki solid
formed. The olive khaki powder was collected by fltration
and washed with ethanol. The solid was dried in vacuum at
80 °C for 24 h to obtain the 4-hydroxysalicylaldehyde imine
nickel (C1) based on ethanediamine, and the yield was 83%.
FTIR data (KBr cm−1): ν(C=N) 1621 and ν(C–O) 1232.
Anal. calcd for C16H14O4N2Ni (357): C 53.78, H 3.92, N
7.84. Found: C 53.52, H 3.74, N 7.87. Anal. by ICP (atomic
%): Ni 16.53 (Calcd.), 16.65 (Found). MS (m/z): 299.1
[M-Ni]+, 179.1 [M-Ni-C6H6O2]+.
The 4-hydroxysalicylaldehyde imine nickel (C2) based
on butanediamine was prepared using the same synthetic
method as C1 with L2 (0.33 g, 1 mmol) and nickel ace-
tate tetrahydrate (0.25 g, 1 mmol) as materials. The khaki
powder was obtained and the yield was 84%. IR data (KBr
cm−1): ν(C=N) 1624 and ν(C–O) 1222. Anal. calcd for
C18H18O4N2Ni (385): C 56.11, H 4.68, N 7.27. Found: C
56.03, H 4.61, N 7.19. Anal. by ICP (atomic %): Ni 15.32
(Calcd.), 15.43 (Found). MS (m/z): 348.3 [M-2OH]+, 325.3
[M-Ni]+, 292.2 [M-Ni-2OH]+.
Results and discussion
The 4-hydroxysalicylaldehyde imine nickel (C3) based
on hexamethylenediamine was prepared using the same
synthetic method as C1 with L3 (0.36 g, 1 mmol) and
nickel acetate tetrahydrate (0.25 g, 1 mmol) as materials.
The khaki powder was obtained and the yield was 84%. IR
data (KBr cm−1): ν(C=N) 1606 and ν(C–O) 1224. Anal.
calcd for C20H22O4N2Ni (413): C 58.11, H 5.33, N 6.78.
Found: C 58.03, H 5.25, N 6.70 Anal. by ICP (atomic %):
Ni 14.29 (Calcd.), 14.36 (Found). MS (m/z): 353.1 [M-Ni]+,
320.6[M-Ni-2OH]+.
Synthesis and characterization of bidentate nickel
complexes
A series of diamine-bridged 4-hydroxysalicylaldehyde
ligands and their nickel complexes were synthesized with
the method described in the literature [16], and the products
were all formed in high yield. The synthesis routes of the
ligands and their nickel complexes are shown in Scheme 1.
In these reactions, the 4-hydroxysalicylaldehyde and nickel
acetate tetrahydrate were used in excess to obtain the ligands
and nickel complexes with high yield. The unreacted reac-
tants were removed by diethyl ether, and recrystallization
Scheme 1: Synthesis route of
diamine-bridged Schif ligands
and their nickel complexes
1 3

310
Transition Metal Chemistry (2021) 46:307–314
was an essential step in order to improve the product purity.
The nickel complexes (C1–C3) with the four coordinate
square planar structure they were all khaki powders [17]. In
order to verify the purity of the ligands and the complexes,
the element content of all the complexes was measured with
an element analyzer and ICPS-7510 spectrometer and the
results were listed in Experimental section. The measured
values of the elements were consistent with the theory chem-
ical structure for the ligands (L1–L3) and the corresponding
complexes (C1–C3). Moreover, the series of ligands and
their corresponding complexes were also characterized by
various analytical techniques.
from C1, which might be caused by the bending and winding
of fexible alkyl chains in backbones. With increase in the
fexibility for the molecule of the complexes, the crystal-
linity decreased [20]. The PXRD patterns of the complexes
[Fig. S3 (b)] showed that C1 had the strong difraction peaks
of nickel appear at 2θ=19° and 21°, respectively, which cor-
responded to the crystal planes (JCPDS 89-5881) of (111)
and (JCPDS 89-7101) (200) of Ni–O. The nickel complexes
had showed strong difraction peaks of nickel at 2θ = 34°
and 60°. These difraction peaks corresponded to the crys-
tal planes (JCPDS 78-0643) of (111) and (200) of Ni–O.
Moreover, the PXRD patterns of the ligands were diferent
from those of the corresponding complexes, which indicated
that the complexes were formed by the coordination reaction
between the ligands and nickel acetate tetrahydrate.
Series of 4-hydroxysalicylaldehyde imine ligands
(L1–L3) based on diamine were characterized by FTIR, 1H
NMR and PXRD. IR spectra of the ligands L1–L3 and the
complexes C1–C3 are shown in Fig. S1. For the ligands
[Fig. S1 (a)], the absorption peaks around 1645 cm−1 were
assigned to the stretching vibration of C=N and the peaks
at around 1230 cm−1 were associated with the stretching
vibration of C–O, which indicated the successful Schif base
reaction between diamine and 4-hydroxysalicylaldehyde
[18]. Compared with the FTIR spectra of the ligands, the
ν (C=N) band in the IR spectra of complexes C1–C3 [Fig.
S1(b)] shifted from around 1645 cm−1 to 1620 cm−1, and the
ν(C–O) band shifted from around 1230 cm−1 to 1225 cm−1.
Moreover, the absorption peaks at 620 cm−1 were attrib-
uted to the stretching vibration of N-Ni in the IR spectra
of complexes C1–C3. These results indicated an efective
coordination interaction between the metal and the nitrogen
and oxygen donor sites [3].
The MS spectra of the complexes C1–C3 are shown in
Fig. 1. The molecular ion peaks [M-1]+ of complex C1,
complex C2 and complex C3 were at 356.0, 384.2 and
412.1, respectively. The other detail peaks of MS spectra
for series of 4-hydroxysalicylaldehyde imine nickel based on
diamine were summarized in the synthesis section.
The TG curves of the three nickel complexes from 30 to
900 °C are shown in Fig. 2. The TG curve displayed that
the thermal decomposition of the complexes occurred in
three stages and the three complexes had the same trend of
weightlessness. The frst weight loss occurred in the range of
30–150 °C, and the loss values of the three complexes were
7.39%, 9.02% and 5.41%, respectively, which was attributed
to the emitting of the surface and inherent moisture [21]. The
second weight loss occurred in the range of 150–450 °C, and
the loss values of C1–C3 were 27.31%, 42.05% and 31.37%,
respectively. This signifcant weight loss corresponded to
step-by-step oxidation and decomposition of the alkyl chain
in the ligand [3]. The third weight loss occurred in the range
1H NMR spectra of ligands L1–L3 showed the azome-
thine proton (–CH=N) resonance as a sharp singlet was
at around 8.15 ppm (Fig. S2), which indicated series of
diamines had reacted with 4-hydroxysalicylaldehyde to
form the Schif base ligands [19]. Signals for the protons of
methylenes of the alkyl chains were found in the range of
1.49–3.32 ppm, the protons of hydroxyl occurred at around
4.9 ppm and proton peaks from 7.07 to 6.12 ppm were
assigned to the benzene rings. The appearance of the above
characteristic proton peaks indicated that the structures of
ligand were consistent with the theoretical structure.
179.1
299.1
120.2
356.0
C1
100
150
200
250
300
350
400
450
348.3
In order to obtain the crystal structure of series of the
synthesized ligands and the corresponding complexes, the
single crystals were cultivated by volatilization method, dif-
fusion method and temperature diference method, respec-
tively. However, crystals suitable for single-crystal X-ray
crystallography could not be obtained. The PXRD patterns
of the ligands L1–L3 and the nickel complexes C1–C3
showed that they all had a certain crystal structure (Fig. S3).
The PXRD patterns of the ligands showed that L2 and L3
had the similar crystal structures and were diferent from L1.
The PXRD patterns of the complexes showed that C2 and
C3 also had the similar crystal structures and were diferent
240.1
292.2
325.6
192.2
384.2
400
C2
100
101.1
150
200
250
300
350
450
179.1
412.1
320.6
280.2
353.3
C3
100
150
200
250
300
350
400
450
m/z
Fig. 1 MS spectra of series of nickel complexes
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Transition Metal Chemistry (2021) 46:307–314
311
previous works. Because complexes C1–C3 have similar
chemical structures and the length of the bridged group
for complex C2 was between complex C1 and complex
C3, complex C2 systematically investigated the changes
in catalytic properties under diferent condition param-
eters in ethylene oligomerization, and the results are
listed in Table 1. With increase in the precatalyst dosage
at the range of 1–10 μmol, the catalytic activity increased
(Entries 1–4 in Table 1), and the catalytic activity was
up to the highest of 15.50 × 104 g/(mol·Ni·h). This might
be because more catalysts could be activated at the Al/
Ni molar ratio of 500. However, with increase in the pre-
catalyst dosage, the selectivities of C8 and C10+ olefns
increased frstly and reached up to 63.94% when the pre-
catalyst dosage was 5 μmol. With further increase in the
precatalyst dosage, the selectivity of C8 and C10+ olefns
decreased. Excess catalyst might hinder the contact of
active species with ethylene, decreasing the selectivity of
the high olefns [22]. Based on the catalytic activity and
the selectivity of C8 and C10+ olefns, the optimum pre-
catalyst dosage was 5 μmol.
C1
C2
C3
100
80
60
40
20
49.36%
45.90%
27.41%
200
400
600
800
Temperature(°C)
Fig. 2 TG curve of series of nickel complexes
of 450 °C to 890 °C, which corresponded to the decomposi-
tion of benzene ring in the ligand, and the weight loss val-
ues for three complexes were 15.94%, 21.52% and 17.32%,
respectively. And there was no further weight loss when the
temperature was above 890 °C. The results of the thermal
analysis revealed that the three nickel complexes were stable
up to around 150 °C.
The amount of cocatalyst was found to have a great
infuence on the catalytic behavior of C2/MAO catalytic
system. With an increase in the Al/Ni molar ratio from
300 to 900, the catalytic activity was sustainable growth
(Entries 3 and 5–7 in Table 1). The highest activity was
observed at the Al/Ni molar ratio of 900 with the value
of 17.36 × 104 g/(mol·Ni·h). This was because the appro-
priate amount of MAO might eliminate the impurities
and water in solvent and might activate the nickel com-
plex. With increase in the Al/Ni molar ratio, the content
of C8 and C10+ oligomers reduced, which is attributed to
the high chain termination or the high chain transfer to
Infuence of catalytic parameters on the catalytic
properties in ethylene oligomerization
The chemical structure of ligands and the oligomeriza-
tion parameters had an important infuence on the cata-
lytic properties of salicylaldimine nickel complexes in our
Table 1 Ethylene
Entry Precat Dosage (μmol) Al/Ni Press Temp Activity
Oligomer distribution (%)
C4 C6 C8 C10+
oligomerization data for C2
catalyst systems
(MPa) (°C)
(104 g/
(mol·Ni·h))
1
2
3
4
5
6
7
8
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
1
2
5
10
5
5
5
5
5
5
500
500
500
500
300
700
900
500
500
500
500
500
500
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.3
0.7
1.0
0.7
0.7
0.7
25
25
25
25
25
25
25
25
25
25
15
35
45
3.6
8.00
7.06 32.71 20.17 40.06
5.02 34.66 24.18 36.14
10.13 25.93 29.64 34.30
15.01 30.32 27.08 27.62
7.98 27.07 30.12 34.83
21.02 39.66 24.18 15.14
24.57 43.76 20.84 10.83
14.45 30.52 22.12 32.91
11.58 28.17 30.57 29.68
12.15 27.84 31.07 28.94
13.33 23.52 31.30 31.85
18.29 39.26 31.51 10.94
11.80
15.50
5.91
14.50
17.36
7.69
19.90
21.05
10.85
16.68
13.91
9
10
11
12
13
5
5
5
25.09 42.90 25.28
6.73
1 3

312
Transition Metal Chemistry (2021) 46:307–314
alkylaluminum by large excess of MAO [23]. Under the
condition of the Al/Ni molar ratio being 500, complex C2
had the relatively high activity [11.80 × 104 g/(mol·Ni·h)]
with better selectivity for C8 and C10+ oligomers (63.94%).
With increase in the reaction pressure from 0.3 to
1.0 MPa, the catalytic activity was gradually increased
(Entries 3 and 8–10 in Table 1). The ethylene concen-
tration dissolved in the catalytic system increased, which
could cause more ethylene to insert into the quantitative
active center, and the catalytic activity of the precata-
lyst increased [4]. It was worth noting that the ethylene
pressure also signifcantly infuenced the oligomer dis-
tributions. The content of C8 and C10+ olefns increased
frstly and then reduced slightly with the increase in the
ethylene pressure, and the content of C8+ olefn reached
up to a maximum (63.94%) when the ethylene pressure
was 0.5 MPa. This might be due to the fact that the chain
growth rate was greater than the chain termination rate,
and it was more likely to form the high carbon olefns
[11]. When the ethylene pressure was 0.7 MPa, complex
C2 had a relatively high catalytic activity [19.90 × 104 g/
(mol·Ni·h)], which was far higher than the catalytic activ-
ity [11.80 × 104 g/(mol·Ni·h)] at the ethylene pressure of
0.5 MPa. At the same time, complex C2 had relatively
better selectivity of C8 and C10+ oligomers (61.25%) at
the ethylene pressure of 0.7 MPa. Therefore, the ethylene
pressure of 0.7 MPa was the ethylene pressure of the fol-
lowing ethylene oligomerization.
more rapidly than chain growth rate. Complex C2 gave a
high activity [19.90×104 g/(mol·Ni·h)] with better selectiv-
ity for C8 and C10+ oligomers (61.25%) at 25 °C. Therefore,
the reaction temperature of 25 °C was the reaction tempera-
ture of the following ethylene oligomerization.
Infuence of the ligand structure on the catalytic
properties in ethylene oligomerization
Generally, as the length of the alkyl chain in the ligand
increased, the solubility of nickel complexes increased,
which caused the increase in the catalytic activity [23]. In
order to study the infuence of the length of the alkyl chain in
the ligand on the catalytic properties in ethylene oligomeri-
zation, the catalytic properties of complexes C1–C3 were
investigated when the precatalyst dosage was 5 μmol, the
Al/Ni molar ratio was 500, the temperature was 25 °C, the
reaction time was 30 min and the pressure of ethylene was
0.7 MPa. And the results are listed in Table 2 (Entries 1–3).
For complexes C1–C3, the oligomerization products had a
wide distribution, and it was dominated by the products of
trimerization and tetramerization of ethylene (Entries 1–3 in
Table 2). With increase in the length of the alkyl chain in the
ligand backbone, the catalytic activity decreased slightly and
the content of C8+ oligomers increased slightly, which indi-
cated that the length of the alkyl chain in the ligand had little
infuence on the catalytic properties. This was because the
increase in the alkyl chain in the ligands caused the increase
in the solubility of the nickel complexes in the reaction sys-
tem, but the steric hindrance of the longer alkyl chain in the
ligands resulted in the decrease in catalytic activities [11].
Complex C4 based on ethanediamine-bridged salicyla-
ldimine [25] and complex C5 based on the hyperbranched
salicylaldimine were evaluated [26] (Fig. 3) as precatalysts
in ethylene oligomerization to study the infuence of the
ligand chemical structure on the catalytic properties. Com-
pared with complex C1, complex C4 had a similar structure
with diferent substituents. There was a hydroxyl group at
the fourth position that replaced the hydrogen on the benzene
ring in complex C1. The participation of hydroxyl group led
to a larger steric hindrance and the high electron density. It
was the primary reason that C1 had a lower catalyst activity
The reaction temperature signifcantly afects not only the
catalytic activity, but also the stability of the active species
in the reaction system. The infuence of the reaction tem-
perature on the catalytic properties of complex C2 is listed
in Table 1 (Entries 9 and 11–13), and complex C2 exhib-
ited the highest catalytic activity at 25 °C over the range
of 15–45 °C. This was because the high temperature was
benefcial for the acceleration of the overall chain growth
rate and the termination rate to make the increase in the
catalytic activity. However, the high temperature might lead
to the deactivation of the cocatalyst, and the catalytic activ-
ity decreased [24]. The selectivity of C8 and C10+ oligom-
ers decreased with the increase in the reaction temperature,
which indicated that the chain termination rate increased
Table 2 Catalytic properties of
Entry
Precat
Activity
Oligomer distribution (%)
series of nickel(II) complexes
with diferent structures of the
ligand
104 g/(mol·Ni·h)
C4
C6
C8
C10+
Polymer
1
2
3
4
5
C1
C2
C3
C4
C5
20.12
19.90
18.69
31.8
12.76
11.58
11.26
41.54
87.61
30.12
28.17
26.23
28.62
8.37
28.96
30.57
31.62
12.17
2.55
28.16
29.68
30.87
12.31
1.47
–
–
–
5.36
–
112.5
1 3

Transition Metal Chemistry (2021) 46:307–314
313
Fig. 3 Structures of nickel(II)
complexes used for ethylene
oligomerization in our previous
works
(a) Complex C4
(b) Complex C5
than C4. The formation of the high carbon oligomer was
favored by the low steric hindrance, and polyethylene was
found in the products of C4 [11]. Compared with the other
complexes, the catalytic activity and the selectivity of low
carbon olefn for catalyst C5 based on the hyperbranched
salicylaldimine ligand were much higher, which were attrib-
uted to the better solubility in the reaction system. However,
the product selectivity was afected by steric and electron
structure of the active center, which caused a lower chain
growth rate than the chain termination rate, and the main
oligomer was C4 olefn (72.19%).
Ethylene oligomerization occurred via a plausible chain
growth and termination mechanism. β-H elimination of the
corresponding intermediate was critical for the formation
of α-olefns [27]. The proposed mechanism of ethylene oli-
gomerization (Scheme 2) included the initiation step: the
methyl cation (B) formed by the reaction between diamine
4-hydroxysalicylaldehyde metal complex A and MAO and
the complex C formed by interacting with an ethylene mol-
ecule into complex B. A second ethylene molecule is migra-
tory insertion the metal-alkyl and formation of a new alkyl
complex D. Ethylene could bind to this species giving the
Scheme 2: Reaction mechanism for ethylene oligomerization by diamine-bridged salicylaldehyde nickel complexes
1 3

314
Transition Metal Chemistry (2021) 46:307–314
complex E, and further insertion of ethylene lead to chain
growth and the formation of linear olefns (Path 1). Alterna-
tively, the 1-alkene hydride complex G was obtained by the
β-H elimination of complex D. Further coordination between
complex G and ethylene could lead to give H (Path 2), with
chain transfer; a chain was terminated with the formation
of a terminal olefn and a new chain was initiated [28, 29].
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Acknowledgements This work was supported by the Heilongjiang
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