Nickel-based ethylene oligomerization catalysts supported by PNSiP ligands

Article abstract of DOI:10.1080/10426507.2018.1424157

A series of nickel (II) complexes bearing silicon bridged diphosphines ligands (PNSiP) have been synthesized and characterized. All nickel precatalysts, activated with ethylaluminum dichloride (EtAlCl₂), exhibited moderate to high activities for ethylene dimerization to butylene. The in situ nickel precatalysts formed by mixing N-cyclopentyl-N-((diphenylphosphanyl)dimethylsilyl)-1,1-diphenylphosphanamine (L2) with NiBr₂(DME) showed high catalytic activity (2.40 × 10⁸ g/(mol_Ni·h)) and high product selectivity (88.6%) towards butene using methylcyclohexane as solvent at 1.0?MPa ethylene pressure and 45°C temperature, no polyethylene(PE) was observed. Ligand backbone tuning of PNSiP-based catalytic systems help in precise understanding of steric bulk variation effects on catalytic performance.

Full text of DOI:10.1080/10426507.2018.1424157

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Nickel-based ethylene oligomerization catalysts
supported by PNSiP ligands
Yongwang Huang, Le Zhang, Wei Wei, Fakhre Alam & Tao Jiang
To cite this article: Yongwang Huang, Le Zhang, Wei Wei, Fakhre Alam & Tao Jiang (2018):
Nickel-based ethylene oligomerization catalysts supported by PNSiP ligands, Phosphorus, Sulfur,
and Silicon and the Related Elements, DOI: 10.1080/10426507.2018.1424157
To link to this article: https://doi.org/10.1080/10426507.2018.1424157
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Date: 09 January 2018, At: 05:20

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Nickel-based ethylene oligomerization catalysts
supported by PNSiP ligands
Yongwang Huang†, Le Zhang†‡, Wei Wei†, Fakhre Alam†, Tao Jiang†
†Tianjin Marine Chemical Technology Engineering Center, College of Chemical Engineering and
Material Science, Tianjin University of Science & Technology, Tianjin 300457, China
‡Handan Key Laboratory of Organic small molecule materials, Handan University, Handan
056005, China
Abstract: A series of nickel (II) complexes bearing silicon bridged diphosphines
ligands (PNSiP) have been synthesized and characterized. All nickel precatalysts,
activated with ethylaluminum dichloride (EtAlCl2), exhibited moderate to high
activities for ethylene dimerization to butylene. The in situ nickel precatalysts formed
by mixing
N-cyclopentyl-N-((diphenylphosphanyl)dimethylsilyl)-1,1-diphenylphosphanamine
(L2) with NiBr2(DME) showed high catalytic activity (2.40×108 g/(molNi·h)) and high
product selectivity (88.6%) towards butene using methylcyclohexane as solvent at
Yongwang Huang•Le Zhang•Wei Wei• Fakhre Alam •Tao Jiang (
)

College of Chemical Engineering and Material Science, Tianjin University of Science & Technology, Tianjin 300457, China
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1.0MPa ethylene pressure and 45C temperature, no polyethylene(PE) was observed.
Ligand backbone tuning of PNSiP-based catalytic systems help in precise
understanding of steric bulk variation effects on catalytic performance.
Keywords: ethylene dimerization; butene; nickel complexes; ethylaluminum dichloride
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1. Introduction
Worldwide butene availability, especially as crude mixture, is enormous. 1-Butene is mainly
used as a comonomer in the production of polyethylene, polypropylene, butene oxide
and butanone.[1] 2-Butene is mainly used in the production of gasoline, butadiene and butanone.
Ethylene oligomerization or dimerization can produce high purity 1-butene.[2] Since the pioneering
work of Keim and co-workers,[3] nickel(II)-based precatalysts played important function in the
development of alkene oligomerization process. Due to the commercial success, nickel-catalyzed
ethylene oligomerization was studied in detail at the fundamental level.[4-9] Nickel precatalysts
based on bi- and tridentate ligands with various combinations of donor atoms (N, P, O, and S) have
been extensively studied. Bi-dentate ligands (N^N, N^O, N^P, P^O and P^P) based nickel
complexes gained much interest in their synthesis. Extensive research on ethylene polymerization
using bi-dentate Ni-based precatalysts bearing P^P type ligation has been reported.[10] Nickel
precatalysts of symmetric carbon-bridge diphosphines ligands show high activity toward
oligomer/polymer products.[11-13] In addition, the groups of Le Floch [14] and Matt[15], reported
ethylene dimerization (butene>90%) from nickel precatalysts based on xanthene-calixarene
associated with methylaluminoxane (MAO) activator. Symmetrical diphosphinoamine
Ph2P-NR-PPh2 nickel complexes [with R = Ph, CH2-C6H5, CH2-(C4H3O), CH2-(C4H3S),
CH2-(C5H4N), CH2-CH2-(C4H3S)] can catalyze ethylene oligomerization to light olefins with
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moderate activity when activated by MAO.[10, 16] High active neutral nickel(II ) complexes with
non-symmetrical P,O ligands, are successfully used in the Shell Higher Olefin Process (SHOP)
[17-19] to produce linear -olefins. Recently, new varieties of pincer ligands with non-symmetrical
backbones and/or ligating groups, have been reported and their application in transition metal
complexes has been explored in a variety of catalytic transformations.[20] This approach proved
beneficial in a variety of ways, such as the use of a single weakly coordinating moiety, capable of
dissociating and thereby creating a vacant coordination site to increase catalyst activity. Silyl
groups, as large electropositive substituents, have a substantial effect in the complex structure and
can hence control the stereochemistry of the reaction. Our group reported the synthesis of a series
of Ni (II) complexes with diphosphine ligand (PCSiP),[21] and achieved a high activity and
selectivity towards ethylene dimerization to butene. In this paper, we have designed and
synthesized a series of novel silicon-bridged diphosphine ligands (PNSiP). The results showed that
it has greatly improved catalytic activity from 3.12106 g/(molNi·h) to 2.40×108 g/(molNi·h).
2. Results and discussion
The target PNSiP ligands L1–L3 were prepared by convenient salt metathesis (Figure 1).
Deprotonation by n-butyllithium and subsequent dropwise addition of chlorodiphenylphosphine
converted primary amines to N,P-based units. Diphenylphosphanide was obtained from the
diphenylphosphine and butyllithium treatment and was added to dichlorodimethylsilane to
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produce Si,P-based units. The designed ligands were obtained by reacting the N,P-based units with
n-butyllithium, followed by dropwise addition of Si, P-based units. These ligands were
coordinated with NiBr2(DME) to produce precatalysts C1–C3 as brown powders (Figure 2).
Figure 1 Structures of complex L1, L2 and L3
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2.1 Effect of precatalyst loading on catalytic activity and product selectivity
Table 1 displays the influence of catalyst loading on catalytic activity and product selectivity.
C2 was firstly synthesized for the ethylene oligomerization test. Reducing the catalyst dosage
from 2.4μmol to 0.1μmol, the catalytic activity and 1-butene selectivity increased significantly. It
showed that precatalyst with large quantity was unable to activate completely and to form active
species, which is consistent with the spectral study results reported by Do.[22] When the catalyst
loading is low, the ethylene can contact faster with the catalytic active center to generate
catalytically active species. Moreover when the catalyst loading is too high, the diffusion velocity
of ethylene molecules into solvent is slow, the ethylene can not coordinate with nickel complex in
time, which reduce the catalytic activity of C2 catalyst. The proportion of 1-butene in butene
fraction decreases rapidly with increasing catalyst dosage. These results indicate that 1-butene is
the primary product during ethylene oligomerization. The rising and uncontrollable temperature
caused by the exothermicity of increasing amount of nickel precatalyst speed up the 1-butene
isomerization. [23]
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Table 1 Effect of catalyst loading on catalytic activity and product selectivity
Product selectivity (%)
Catalyst
Activity
Catalyst
mass(μmol) [107 g/(molNi·h)]
=
=
=
C4
1-C4 trans-C4 cis-C4
C6
1-C6
≥1-C8
Nd
2.4
1.2
0.4
0.1
0.7
1.8
6.3
9.4
76.2
7.2
61.7
56.3
53.2
36.8
31.1
30.0
26.9
28.8
23.8
15.9
19.8
16.2
1.3
3.1
4.0
8.0
84.1 13.7
80.2 19.8
83.8 34.4
Nd
C2
Nd
Nd
Reaction conditions: n(Al)/n(Ni)=500; solvent, 20 mL (methylcyclohexane); time, 30 min; T,
45C; EADC used as co-catalyst.
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2.2 Effect of reaction temperature on catalytic activity and product selectivity
From Table 2 we found that the catalytic activity of C1, C2 and C3 initially increased upon
increasing the reaction temperature, reached maximum at 45C and then decreased significantly
with further increase in temperature. It indicates that reaction temperature is an important
parameter that affects the catalytic activity of C1, C2 and C3 complexes. The decrease in activity
attributed to the lower concentration of ethylen in methylcyclohexane at high temperature.[24] On
the other hand, elevated temperature increases the deactivation rate of active sites which also
lowers the catalytic activity of the complex. In most cases, a steady increase in 1-butene selectivity
was observed with a decrease of temperature from 60 C to 30 C, suggesting that the selectivity of
the complexes was substantially affected with temperature. These results indicate that 1-butene is
the initial product of ethylene oligomerization and that its catalyzed isomerization to cis/trans
2-butene becomes prominent when the reaction temperature increases. The catalytic activity and
1-butene selectivity were also profoundly affected by the ligand environment. Higher catalytic and
lower 1-butene selectivity of C1 and C2 compared to C3 also made confirm the existence of a
general correlation between catalytic performance and N atom substituent. On the other hand,
butene selectivity remains unaffected, showing its independent selectivity from N atom
substitution. The effect of the steric hindrance of ortho-position substituent on the catalytic activity
of (a-diimine)nickel(II) complexes is evident. The different activities obtained using diphosphines
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with C1, C2 and C3 suggest that ligand rigidity and/or steric hindrance play an important role in
catalyst performance.
Table2 Effect of reaction temperature on catalytic activity and product selectivity
Activity
Product selectivity (%)
T
Catalyst
[107
g/(molNi·h)]
8.6
=
=
=
(C)
C4
1-C4 trans-C4 cis-C4
C6
1-C6
7.9
≥1-C8
60
45
30
60
45
30
15
60
45
30
83.5
86.2
67.5
81.9
83.8
83.7
84.6
86.7
86.3
86.9
30.1
30.4
67.6
34.9
34.4
34.1
32.9
52.4
54.1
58.2
36.6
37.7
19.6
37.4
36.8
36.7
35.9
25.9
26.5
23.5
33.3
31.9
12.9
33.9
28.8
29.2
31.2
21.7
19.4
18.3
16.5
13.8
32.5
18.1
16.2
16.3
15.4
13.3
13.7
13.1
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
9.2
10.1
12.0
9.9
C1
2.6
9.2
9.4
8.0
C2
C3
9.0
7.4
1.6
6.5
3.2
11.3
10.2
6.9
6.6
2.4
Reaction conditions: n(Cat), 0.1μmol; n(Al)/n(Ni), 500; solvent, 20 mL (methylcyclohexane); P,
1.0 MPa; time, 30 min; EADC used as co-catalyst.
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2.3 Effect of Al/Ni molar ratio on catalytic activity and product selectivity
Table 3 displays the influence of Al/Ni molar ratio on the catalytic activity and product
selectivity. Upon increasing Al/Ni molar ratio from 100 to 700, the catalytic activity increased
significantly, but selectivity toward butene decreased. This trend can be attributed to less dosage of
EADC, in which some of its amount is consumed and the remaining amount is not enough to form
catalytic active species with nickel and other catalyst components. Therefore, an optimal amount
of EDAC is needed to activate catalyst effectively.
Table 3 Effect of Al/Ni molar ratios on catalytic activity and product selectivity
Product selectivity (%)
Activity
n(Al)/n(Ni)
Catalyst
[107 g/(molNi·h)]
=
=
=
C4
1-C4
trans-C4 cis-C4
C6
1-C6
≥1-C8
Nd
Nd
Nd
Nd
Nd
Nd
Nd
Nd
700
500
700
500
300
100
700
500
11.4
9.2
10.8
9.4
82.1
86.2
82.2
83.8
84.1
88.6
85.9
86.3
32.6
30.4
33.2
34.4
41.2
66.8
35.9
54.1
37.8
37.7
39.2
36.8
32.1
16.9
32.4
26.5
29.6
31.9
27.6
28.8
26.7
16.3
31.7
19.4
17.9
13.8
17.8
16.2
15.9
11.4
14.1
13.7
6.7
10.1
7.3
C1
8.0
C2
C3
8.2
10.1
12.3
9.2
1.2
7.6
6.6
10.2
Reaction conditions: n(Cat), 0.1μmol; P, 1.0 MPa; solvent, 20 mL (Methylcyclohexane); T, 45C;
time, 30 min; EADC used as co-catalyst.
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2.4 Effect of the ethylene pressure on catalytic activity and product selectivity
As shown in Table 4, the catalytic activity increased with increasing ethylene pressure, while
selectivity toward butene did not change obviously. Ethylene solubility changed with ethylene
pressure in liquid phase reaction which affected catalytic activity.
Table 4 Effect of ethylene pressure on catalytic activity and product selectivity
Ethylene
Catalyst pressure(M
Pa)
Product selectivity (%)
Activity
[107 g/(molNi·h)]
=
=
=
C4
1-C4
trans-C4 cis-C4
C6
1-C6
≥1-C8
1
9.4
6.4
3.8
1.6
83.8
83.9
83.7
84.6
34.4
34.3
34.1
32.9
36.8
36.7
36.7
35.9
28.8
29.0
29.3
31.2
16.2
16.1
16.3
15.4
8.0
8.1
7.4
6.5
Nd
Nd
Nd
Nd
0.8
C2
0.4
0.1
Reaction conditions: n(Cat), 0.1μmol; n(Al)/n(Ni), 500; solvent, 20 mL (Methylcyclohexane);
time, 30 min; T, 45C; EADC used as co-catalyst.
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2.5 Effect of the reaction time on catalytic activity and product selectivity
Table 5 reveals that the reaction time has a slightly greater effect on the activity of the
catalysts, while the selectivity toward butene remained relatively stable. These results again
suggest that the catalyst can be easily deactivated if treated for long time.
Table 5 Effect of reaction time on catalytic activity and product selectivity
Product selectivity (%)
Reaction
Activity
Catalyst
time (min) [107 g/(molNi·h)]
=
=
=
C4
1-C4
trans-C4 cis-C4
C6
1-C6
≥1-C8
Nd
30
20
10
9.4
83.8
82.3
81.8
34.4
35.7
35.0
36.8
35.1
32.8
28.8
29.2
32.2
16.2
17.7
18.2
8.0
8.5
8.8
13.2
24.0
Nd
C2
Nd
Reaction conditions: n(Cat), 0.1μmol; n(Al)/n(Ni), 500; P, 1.0 MPa; solvent, 20 mL
(Methylcyclohexane); T, 45C; EADC used as co-catalyst.
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3. Experimental
3.1 Materials and methods
Bis(phenyl)phosphorus chloride, diphenylphosphine, dichlorodimethylsilane, n-butyllithium
(2.4 mol/L in n-hexane) and NiBr2(DME) were purchased from Aldrich and were used as received.
Lithium diphenylphosphine was prepared using the method reported by Peterson[25]
.
Polymerization-grade ethylene was obtained from Tianjin Summit Specialty Gases (China).
EADC (1.4 mol/L in toluene) was purchased from Albemarle Corp (USA). Methylcyclohexane,
tetrahydrofuran(THF) and n-hexane were dried and degassed prior to use. 1H, 13C, and 31P NMR
spectra of the products were recorded at 25 °C using a Fourier 400 NMR spectrometer (Bruker,
Billerica, MA) with tetramethylsilane (TMS) as an internal reference. Elemental analysis were
performed using an Elemental Vario EL analyzer. Mass spectra were recorded using a
MALDI-TOF mass spectrometer. Gas chromatography (GC) was performed using Agilent 7890A
GC system.
3.2 Synthesis of ligands
3.2.1 Synthesis of N-(2,6-diisopropylphenyl)-N-((diphenylphosphanyl)dimethylsilyl)-1,1-
diphenylphosphanamine (L1)
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1. n-BuLi (4.2 mL, 2.4 mol/L in n-hexane, 9.96 mmol) was added dropwise to a solution of
2,6-diisopropylaniline (1.791 g, 10.00 mmol) in n-hexane (20 mL) at -20°C. The mixture was
stirred over night at room temperature, followed by addition of chlorodiphenylphosphine (2.207
g, 10.00 mmol). The precipitated LiCl was filtered and the faint yellow solution was
concentrated to give yellow residue, which was recrystallized in n-hexane to give N-(2, 6-
diisopropylpenyl)-1-dipenylphosphanamine (L1a) (Table 6) (2.207g, 6.10mmol) in 68% yield.
2. n-BuLi (4.2 mL, 9.96 mmol, 2.4 mol/L in n-hexane) was added dropwise to n-hexane
solution of diphenylphosphine (1.900 g, 10.00 mmol) in n-hexane (10 mL) at -20°C. The mixture
was stirred over night at room temperature. After filtration and washing twice with n-hexane (2
mL), lithium diphenylphosphanide (L1b) (Table 6) (1.928 g, 10.00 mmol) was obtained in
nearly 100% yield after drying in a vacuum. The above product was added by multiple-low-dose
to a solution of dichlorodimethylsilane (2.581 g, 20.00 mmol) in n-hexane (20 mL) at -20°C
followed by stirring at room temperature for 8h. After filtration, the volatiles were removed in
vacuum, giving yellow oily product. The colorless oil (chlorodimethylsilyl)diphenylphosphine
(L1c) (Table 6) was obtained by vacuum distillation (155 °C~160 °C, 10 mmHg, 1.087 g, 3.90
mmol, 39% yield).
3.n-BuLi (2.8 mL, 2.4 mol/L in n-hexane, 6.80 mmol) was added dropwise to n-hexane
solution of above L1a. The mixture was stirred at room temperature for 5h. After filtration and
washing twice with n-hexane (2 mL), lithium
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(2,6-diisopropylphenyl)(diphenylphosphanyl)amide (L1d) (Table 6) (2.447 g, 6.10 mmol) was
obtained in approximately 100% yield after drying in a vacuum. A solution of L1c (0.224 g, 1.00
mmol) in toluene (20 mL) was added dropwise to a solution of L1d (0.368 g, 1.00 mmol) in
toluene (5 mL) at -20 °C. After stirring overnight and filtration, a yellow oil was obtained by
removing the volatiles in vacuum. Recrystallization from n-hexane produced ligand L1 (0.411 g,
0.68 mmol) of 68% yield. 1H NMR (400 MHz, C6D6) δ 7.98 (t, J = 7.5 Hz, 4H), 7.58 (dd, J = 10.4,
4.9Hz, 4H), 7.14–6.96 (m, 15H), 3.58–3.46 (m, 2H), 1.11 (d, J =6.7 Hz, 6H), 0.43 (d, J = 6.7 Hz,
6H), 0.41 (d, J = 2.0 Hz, 6H). 31P NMR (162 MHz, C6D6) δ 52.2 (d), −43.2 (d, J = 25.9 Hz). 13C
NMR (101 MHz, C6D6) δ 148.4, 142.5, 142.4, 139.1, 138.9, 137.7, 137.7, 137.6, 137.6, 136.2,
136.2, 136.1, 136.0, 136.0, 135.9, 135.9, 129.7, 128.9, 128.9, 128.9, 126.6, 125.2, 29.4, 29.4, 26.7,
26.6, 24.1, 2.8, 2.7, 2.6. Anal. Calcd for C38H43NP2Si: C, 75.59; H, 7.18; N, 2.32. Found: C, 75.55;
H, 7.16; N, 2.28.
Table 6 structure of L1a-d
L1a
L1b
L1c
L1d
Structure
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3.2.2 Synthesis of N-cyclopentyl-N-((diphenylphosphanyl)dimethylsilyl)-1,1-
diphenylphosphanamine (L2)
Similarly for L1a, the preparation of N-cyclopentyl-1,1-diphenylphosphanamine (L2a) was
achieved by using cyclopentanamine (0.852 g, 10.00 mmol), n-BuLi (4.2 mL, 9.96 mmol, 2.4
mol/L in n-hexane) and chlorodiphenylphosphine (2.207 g, 10.00 mmol). Vacuum distillation
gave a colorless oil L2a (150 °C ~160 °C, 2.725 g, 10.11 mmol, 50% yield). The preparation of
L2b was conducted by the same method as described for L1b with n-BuLi (2.2 mL, 5.22 mmol,
2.4 mol/L in n-hexane) and L2a (0.276 g, 1.00 mmol) and achieved in nearly 100% yield (1.436
g, 5.22 mmol). 0.322 g L2 (1.34 mmol, 63% yield) was obtained, as for L1, by using L2b (0.276
g, 1.00 mmol) and L1d (0.279 g, 1.00 mmol). 1H NMR (400 MHz, C6D6) δ 7.56 (t, 4H), 7.14 (t,
4H), 7.05 (m, 12H), 3.69–3.78 (m, 1H), 1.78 (m, 2H), 1.44 (m, 4H), 1.23 (m, 2H), 0.53 (s, 6H).
31P NMR (162 MHz, C6D6) δ 51.9 (br), −53.7 (d, J = 53.8 Hz). 13C NMR (101 MHz, C6D6) δ
140.9, 140.9, 140.7, 140.7, 137.3, 137.3, 137.2, 137.1, 135.5, 135.4, 135.3, 133.1, 132.9, 128.9,
128.8, 128.8, 128.7, 128.6, 128.3, 128.2,128.1, 63.1, 34.6, 34.6, 32.3, 23.8, 3.2, 3.1, 3.0. Anal.
Calcd for C31H35NP2Si: C, 72.77; H, 6.90; N, 2.74. Found: C, 72.72; H, 7.11; N, 2.52.
3.2.3 Synthesis of N-((diphenylphosphanyl)dimethylsilyl)-N-isopropyl-1,
1-diphenylphosphanamine (L3)
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N-((diphenylphosphanyl)dimethylsilyl)-N-isopropyl-1,1-diphenylphosphanamine (L3):
N-isopropyl-1,1-diphenylphosphanamine (L3a) was synthesized by the method as L1a using
isopropylamine (0.591 g, 10.00 mmol), n-BuLi (4.2 mL, 9.96 mmol, 2.4 mol/L in n-hexane) and
chlorodiphenylphosphane (2.206 g, 10.00 mmol). Colorless oil L3a was obtained by vacuum
distillation at 10 mmHg (140 °C ~150 °C, 1.314 g, 5.40 mmol, 54% yield). Similarly to L1,
0.325 g L3 (0.65 mmol, 65% yield) was obtained by using L3a (0.243 g, 1.00 mmol), n-BuLi
(0.4 mL, 0.96 mmol, 2.4 mol/L in n-hexane) and L1d (0.279 g, 1.00 mmol). 1H NMR (400 MHz,
C6D6) δ 7.73–7.65 (m, 4H), 7.57–7.52 (m, 4H), 7.16–7.00 (m, 12H), 3.75 (m, J = 13.4, 6.7 Hz,
1H), 1.08 (d, J = 6.7 Hz, 6H), 0.48 (m, 6H). 31P NMR (162 MHz, C6D6) δ 50.4 (br), −53.0 (d, J =
54.7 Hz). 13C NMR (101 MHz, C6D6) δ 140.8, 140.7, 140.6, 140.5, 137.3, 137.3, 137.1, 137.1,
135.6, 133.3, 133.1, 128.9, 128.8, 128.7, 128.7, 128.6, 128.3, 128.1, 32.3, 25.9, 25.8, 25.8, 3.5,
3.4, 3.3. Anal. Calcd for C29H33NP2Si: C, 71.73; H, 6.85; N, 2.88. Found: C, 71.72; H, 6.83; N,
2.75.
3.3 Synthesis of metal complexes
All complexes (Figure 2) were synthesized under dry N2 atmosphere in oven-dried flasks
using standard Schlenk techniques. We tried to grow all the complexes into single crystals, but
failed.
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Figure 2 Structures of complex C1, C2 and C3
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3.3.1 L1/NiBr2 complex
A tetrahydrofuran solution (20 mL) of L1 (0.127 g, 0.21 mmol) was added to a
tetrahydrofuran dispersion liquid (30 mL) of [NiBr2DME] (0.062 g, 0.2 mmol). The reaction
mixture was turned dark brown immediately and was allowed to stir for 24 h. The solvent was
evaporated using a cold trap and vacuum pump and then the solid residue was washed three times
with n-hexane. Drying in vacuo provided a dark brown powder. Yield: 0.157 g (85%). Anal. Calcd
for C38H43Br2NNiP2Si: C, 55.50; H, 5.27; N, 1.70. Found: C, 55.37; H, 5.13; N, 1.48.
3.3.2 L2/ NiBr2 complex
L2 (0.107 g, 0.21 mmol) and [NiBr2(DME)] (0.062 g, 0.2 mmol) were dispersed in
tetrahydrofuran (20 mL), the mixture was allowed to stir for 24 h. After evaporation and
purification, a brown powder was obtained. Yield: 0.137 g (84%). Anal. Calcd for
C31H35Br2NNiP2Si: C, 50.99; H, 4.83; N, 1.92. Found: C, 50.65; H, 5.11; N, 1.57.
3.3.3 L3/ NiBr2 complex
L3 (0.102 g, 0.21 mmol) and [NiBr2(DME)] (0.062 g, 0.2 mmol) were dispersed in
tetrahydrofuran (20 mL), the mixture was allowed to stir for 24 h. After evaporation and
purification a brown powder was obtained. Yield: 0.129 g (81%). Anal. Calcd for
C29H33Br2NNiP2Si: C, 49.47; H, 4.72; N, 1.99. Found: C, 49.36; H, 4.58; N, 1.64.
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3.4 General oligomerization procedure
A 140-mL transparent glass reactor was heated in a high temperature drying oven for 2 h at
105 C prior to use. Subsequently a magneton was placed inside the reactor, and then the reactor
was heated in an oil bath at required tempereture for the oligomerization reaction. The reactor was
equipped with a thermocouple, a pressure meter, and needle valves for injections. The reactor was
vacuumed and then charged with high-purity N2, this process was repeated three times. Then
repeated the above operation with ethylene. After that Methylcyclohexane solution of the catalyst
precursor and the co-catalyst EADC were introduced into the reactor, consequently the reactor was
charged with 1.0 MPa of ethylene. The reaction was run for the required time (generally 30 min)
and then the ethylene feed was turned off. The products of oligomerization were cooled an in ice
ethanol bath and the reaction was quenched by slow addition of acidic ethanol (2 mL). Once
venting was complete, the products of oligomerization were analyzed by an Agilent 7890A gas
chromatography. The heptane was used as the internal standard.
4. Conclusion
We synthesized and characterized three new nickel complexes based on silicon bridged
PNSiP ligands and evaluated for ethylene oligomerization upon EADC activation. All nickel
complexes catalyzed ethylene oligomerization to butene and hexene. The ligand structure with
different N atom substituents was an important parameter to influence the catalytic activity. The
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catalytic activity and product selectivity increased with decreasing catalyst mass, which shows that
the presence of a large amount of catalyst tends to prevent the precatalyst from complete activation
into active species. With an increase in temperature, ethylene solubility decreases in
methylcyclohexane, and the deactivation rate of active sites increases which results in lower
catalytic activity and product selectivity. Furthermore, ethylene pressure also greatly influences
the catalytic activity. We found that our systems showed good catalytic performance at optimum
conditions (45°C, n(Al)/n(Ni) =500, 10 min).
Acknowledgments
This study was supported by the National Key Research and Development Program of China
(2017YFB0306700). The Natural Science Foundation of Tianjin City (14JCYBJC23100,
15JCYBJC48100 and 16JCZDJC31600).
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