Catalytic behavior tuning via structural modifications of silylated-diphosphine Ni(II) complexes for ethylene selective dimerization

Article abstract of DOI:10.1002/aoc.5722

The methylene spacers and an uncoordinated diphenylphosphine moiety in the scaffold of the CH₃Si(CH₂)_n(PPh₂)₃ and Si(CH₂PPh₂)₄-type silylated diphosphine Ni(II) comple

Full text of DOI:10.1002/aoc.5722

Received: 10 February 2020
DOI: 10.1002/aoc.5722
Revised: 20 March 2020
Accepted: 25 March 2020
F U L L P A P E R
Catalytic behavior tuning via structural modifications of
silylated-diphosphine Ni(II) complexes for ethylene
selective dimerization
Jiadong Wang | Fakhre Alam | Qiqi Chang | Yanhui Chen | Tao Jiang
Tianjin Marine Chemical Technology
The methylene spacers and an uncoordinated diphenylphosphine moiety in
the scaffold of the CH3Si(CH2)n(PPh2)3 and Si(CH2PPh2)4-type silylated
diphosphine Ni(II) complex systems have a marked impact on their catalytic
performance in selective ethylene dimerization. Ni(II)-based precatalyst 1,
bearing two methylene spacers in its framework, exhibited the highest catalytic
activity of 1.29 × 108 g (molNi)-1 h-1, while precatalyst 3, with three methylene
spacers, affords the highest product selectivity (88%) toward the C4 fraction.
Crystallographic investigations revealed that the precatalyst 3 adopts the
mononuclear bidentate binding mode and the steric constraints of its
uncoordinated diphenylphosphine moiety may successfully tailor the
catalytic environment of the catalyst. The precatalyst 4 may form a dinuclear
complex and exhibits high catalytic activity by changing the ligand/Ni molar
ratio. The high C4 selectivity of precatalyst 3 has been rationalized by density
functional theory (DFT) calculations and found to be consistent with the
experimental results. The study also revealed that designing new systems of
Ni(II)-based complexes and their systematic modifications may further provide
potential and industrially viable catalyst systems for selective ethylene
oligomerization.
Engineering Center, College of Chemical
Engineering and Material Science, Tianjin
University of Science and Technology,
Tianjin, 300457, China
Correspondence
Tao Jiang, Tianjin Marine Chemical
Technology Engineering Center, College
of Chemical Engineering and Material
Science, Tianjin University of Science and
Technology. Tianjin 300457, China.
Email: jiangtao@tust.edu.cn
Funding information
Natural Science Foundation of Tianjin
City, Grant/Award Numbers:
14JCYBJC23100, 15JCYBJC48100;
National Key Research and Development
Program of China, Grant/Award Number:
2017YFB0306700
K E Y W O R D S
1-butene, DFT calculations, ethylene dimerization, nickel, silylated-diphosphine ligands
1 | INTRODUCTION
The intractability and multiplicity of active sites
observed in heterogeneous catalysts may often compli-
The interest in ethylene dimerization toward butenes
(predominantly 1-butene) is accounted for its main use as
a comonomer for the production of linear low-density
polyethylene (LLDPE) and high-density polyethylene
(HDPE). By 2025, the annual demand of 1-butene is spec-
ulated to be 3.5 million metric tons and therefore it is
considered an important commodity from an industrial
perspective.[1–4]
cate activity and selectivity tuning via structural modifi-
cations. To govern the catalytic performance of catalysts,
researchers therefore mostly rely on homogeneous cataly-
sis.[5] The AlphaButol process, which is used for the
industrial-scale production of 1-butene via selective ethyl-
ene dimerization, is an excellent example of homoge-
neous catalysis. In this process the titanium tetrabutoxide
catalyst in combination with triethylaluminum cocatalyst
offers production of 7 million tons per annum,[6–8] but to
further improve the catalytic performance other metal-
Jiadong Wang and Fakhre Alam contributed equally.
Appl Organomet Chem. 2020;e5722.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 12
https://doi.org/10.1002/aoc.5722

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WANG ET AL.
based catalysts (e.g., Ni, Co, and Pd) have also been
extensively investigated.[1,9–11]
precatalysts 1–4 of the target L1–L4 ligands were effi-
ciently activated for ethylene dimerization, providing the
highest catalytic activity of 1.29 × 108 g (molNi)-1 h-1 and
88% C4 selectivity under experimental conditions. The
results were systematically compared with the reported
precatalysts 5[21] and rationalized. Moreover, the high
selectivity of complex 3 was elucidated using density
functional theory (DFT) calculations and found to be
consistent with the experimental results.
Since the 1970s, the cationic nickel (Ni) complexes
have been reported for the remarkable selectivity of
butylene and therefore they are one of the major protago-
nists for selective ethylene dimerization. In this regard,
Ni(II) complexes based on bi- and tridentate ligands are
promising
candidates
for
selective
ethylene
dimerization/polymerization because of their potential to
generate oligomers with various levels of branching.
Moreover, the systematic modifications in their ligand
scaffold can alter electronic and steric properties, which
in turn alters the catalytic environment of the catalysts
and provides the desired products.[12,13] For instance, the
bidentate NˆP-based Ni(II) catalysts when activated with
diethylaluminum chloride (DEAC) in chlorobenzene cat-
alyzed ethylene dimerization with high catalytic activity
and C4 selectivity.[14] Furthermore, nickel complexes
based on phosphine and pyridine ligands when activated
2 | EXPERIMENTAL
2.1 | General comments
All procedures were carried out in oven-dried flasks
(Gongyi Shi Yu Hua Instrument, China) under a nitrogen
atmosphere using a nitrogen-filled glovebox (Mikrouna,
China) or the standard Schlenk technique. DEAC
(2.0 mol/L in hexane) was purchased from Albemarle
Corporation (USA) and used as received, and all other
reagents were purchased from Aldrich (shanghai Aladdin
Bio-Chem Technology, China) and used as received.
Polymerization-grade ethylene was purchased from Tian-
jin Summit Specialty Gases (China). Anhydrous solvents
were obtained using a multicolumn purification system.
NMR spectra were recorded by Bruker AscendIII-400
(Bruker, Switzerland) (400 MHz for 1H, 162 MHz for 31P,
and 101 MHz for 13C in CDCl3) at 300 K. Elemental ana-
lyses were performed using an Elemental Vario EL ana-
lyzer (Elementar, Germany). Mass spectra were recorded
using a MALDI-TOF mass spectrometer (Bruker Daltonics,
Germany). X-ray data for the crystal structure determina-
tion of complex 3 were collected with a Bruker SMART
APEX CCD diffractometer (Bruker, Germany) at 113.15 K
using the ω-scan technique (X-ray data for details of the
analysis methods see the Supporting Information).
with
tetrakis(3,5-bis
(trifluoromethyl)phenyl)borate
(NaBAF) afforded considerable activity for ethylene olig-
omerization and polymerization.[15] Moreover, the nickel
complexes of amidine/phosphine oxide and α-imino-
ketone ligands exhibited high thermal stability and con-
siderable catalytic activity toward homopolymerization
and copolymerization even in the absence of
cocatalyst.[16,17] Similarly, tridentate pyrazolyl-based
nickel complexes on activation with Methylaluminoxane
(MAO) in toluene offered high dimerization activity and
excellent C4 selectivity.[18] Tridentate PˆNˆN- and PˆNˆP-
based Ni(II) complexes when activated with MAO or
AlEtCl2 also show considerable activity for dimerization
reaction.[19] Our group recently established that the
Ni(II) complexes of silicon-bridged ligands can offer
excellent catalytic performance for selective ethylene
dimerization (Scheme 1). The Ni(II) complexes based on
PCSiP (A),[20] silicon-bridged PNP (B),[21] and silylated-
PNP (C)[22] ligands have been shown to be efficient
dimerization catalysts that can provide as much as
2.40 × 108 g (molNi)-1 h-1 high catalytic activity (in the
case of B) and more than 97% C4 selectivity (in the case
of A).
2.2 | Ligand synthesis: General procedure
The L1–L6 ligands were prepared via a simple salt
metathesis protocol. Treatment of n-butyllithium with
diphenylphosphane at −80ꢀC provided the lithium-
diphenylphosphide salt. Ligands L1 and L2 were
obtained by reacting the corresponding chlorosilyl
Considering the unique attributes of silicon-based
ligands and their active role in catalytic performance, we
report a new class of silylated-diphosphine ligands for
selective ethylene dimerization. The Ni(II)-based
SCHEME 1 Silicon-bridged ligands A–C in
Ni(II) complexes reported for ethylene
dimerization

WANG ET AL.
3 of 12
reagents with lithium-diphenylphosphide in n-hexane
at room temperature. Ligands L3 and L4 were prepared
by modifying the previously reported methods.[23,24]
2.4 | Ethylene oligomerization: General
procedure
The relevant chlorosilyl reagents were treated with
All the catalytic runs of ethylene oligomerization were
performed in a pre-dried (dried at 105ꢀC for 2 hr) 150-ml
glass autoclave equipped with a magnetic stirring bar.
After repeated evacuation and high purity nitrogen purg-
ing, the reactor was charged with a preweighed amount
of cocatalyst and methylcyclohexane (20 ml) and stirred
for 1 min under an inert atmosphere. Subsequently, the
catalyst solution (in 1 ml methylcyclohexane) was added,
pressurized with ethylene (1.0 MPa), and the reaction
was run for the desired time. All the reaction conditions
(ethylene pressure, temperature, and stirring speed) were
maintained throughout the run. On completion of the
run, the reactor was depressurized and cooled to 0ꢀC
(using an ice bath), and the reaction was quenched with
acidified methanol (50 ml). In the presence of heptane
(an internal standard), the liquid oligomeric fraction was
analyzed with an Agilent 7890A GC chromatograph
(Agilent Technologies, USA) with a flame ionization
detector and an HP-5 GC capillary column.
[25]
Ph2PCH2Li·(CH3)2NCH2CH2N(CH3)2
complex to
provide the L3 and L4 ligands. Ligands L5 and L6 were
prepared according to the method established by our
group.[20] The preparation methods for L1–L4 and the
structures of L5 and L6 are illustrated in Scheme 2
(for detailed synthesis procedures of L1–L6 see the
Supporting Information).
2.3 | Complex formation: General
procedure
Ligands L1–L6 in CH2Cl2, when treated with
Dibromonickel,1,2-dimethoxyethane (NiBr2(DME)) at room
temperature, afforded the corresponding Ni(II)-based com-
plexes 1–6 in high yield. The synthesis route for complexes
1–6 is illustrated in Scheme 3 (for detailed synthesis proce-
dures of complexes 1–6 see the Supporting Information).
SCHEME 2 Synthesis of
silylated-diphosphine L1–L4
ligands, and L5 and L6 structures

4 of 12
WANG ET AL.
SCHEME 3 Synthesis of
Ni(II) complexes 1–6 based on ligands
L1–L6
3 | RESULTS AND DISCUSSION
6, the catalytic activities of complexes 1 and 2 improved by
three and one orders of magnitude, respectively (Table 1,
entries 1 and 2). Insertion of a methylene spacer in the
structure of complex 1 afforded complex 3, which exhibited
lower activity than complexes 1 and 2, but higher than
complexes 5 and 6 by two and one orders of magnitude,
respectively (Table 1, entry 3). Modification of the structure
of complex 3 by the addition of a diphenylphosphine moi-
ety afforded complex 4, which on oligomerization again
showed a decrease in catalytic activity by an order of mag-
nitude compared to precatalyst 3 (Table 1, entry 4).
Among the silylated-diphosphine Ni(II) complexes 1–
4, the precatalyst 1 exhibited the highest catalytic activity
under identical conditions (Table 1, entry 1). Structural
modification of precatalyst 1 by the abstraction (as in 2)
or insertion (as in 3) of a methylene spacer or addition of
To elucidate the role of ligand structure in controlling the
catalytic
behavior
of
silylated-diphosphine-based
Ni(II) complexes, precatalysts 1–6 were activated with
DEAC in methylcyclohexane solvent and scrutinized for
ethylene oligomerization (Table 1). On evaluation, pre-
catalysts 5–6 show similar results in terms of catalytic
activity and C4 selectivity as reported previously (Table 1,
entries 5–6).[21] Substitution of one methyl group with a
diphenylphosphine moiety at Si in L5 and L6 respectively
afforded L1 and L2 (Scheme 2), which on reaction with
Ni(II) precursors afforded the corresponding complexes
1 and 2 (Scheme 3). This modification may lead to
remarkable changes in the catalytic performance of com-
plexes 1 and 2. In the comparison with complexes 5 and
TABLE 1
Ethylene dimerization with silylated-diphosphine based complexes 1–6a
Product selectivity (%)
b
=
b
=
b
=
+b
Entry Catalyst Activity [107g (molNi
)
-1 h-1] C4
1-C4
trans-C4 cis-C4 C6
1-C6
C8
1-C8
0.94
0.26 <0.10
C10
1
2
3
4
5
6
1
2
3
4
5
6
12.90
6.00
2.29
0.31
0.02
0.14
59.73
9.67
28.59
22.89
8.64
21.47
19.65
7.36
33.47
19.28
11.68
29.99
1.36
1.81
2.50
2.78
5.63
2.00
1.17
78.72 36.18
88.32 72.32
65.67 54.56
95.08 47.74
95.11 22.95
<0.10 <0.10 <0.10
4.34 0.87 <0.10
6.54
4.57
33.23
31.01
19.03
46.04
4.92 21.80
4.89 7.56
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
aGeneral conditions: n (catalyst), 0.1 μmol; pressure, 1.0 MPa; solvent, methylcyclohexane (20 ml); reaction time, 30 min; T, 45ꢀC; n (Al)/n
(Ni), 500 equiv; DEAC used as cocatalyst.
bwt% of liquid products (oligomers).

WANG ET AL.
5 of 12
a diphenylphosphine moiety (as in 4) may tailor the cata-
lytic environment of the catalyst and consequently atten-
uate the catalytic activity in each case (Table 1, entries
2–4).[26] Under identical conditions, the activities of com-
plexes 1–4 decrease in the order 1 > 2 > 3 > 4 (Table 1,
entries 1–4).
In contrast to catalytic activity, the product selectivity
of the C4 fraction exhibited an opposite trend for com-
plexes
1–3.
Structural
modification
by
abstraction/insertion of a methylene spacer in precatalyst
1 increased selectivity from 59% to 88% (Table 1, entries
1–3). The absence of a methylene spacer between the
coordinated phosphorus and Si (as in 2) or its presence
between the uncoordinated phosphorus and Si (as in 3)
may play a vital role in ethylene dimerization and
enhance C4 selectivity. Further modification of 3 by addi-
tion of a diphenylphosphine moiety (as in 4) may steri-
cally hinder the catalytic environment and lowered C4
selectivity from 88% to 65% (Table 1, entry 4).[14] These
results suggest that complexes 1–4 are flexible toward
modification, and their catalytic activity and product
selectivity can be easily tailored by a slight modification
in the ligand structure.
FIGURE 1 Molecular structure of the Ni(II) complex 3.
Partial thermal ellipsoids drawn at 50% probability. The hydrogen
atoms are omitted for clarity. Selected bond lengths (Å): Br1–Ni1
2.3729(16); Br2–Ni1 2.3368(10); Ni1–P1 2.2882(16); Ni1–P2
2.2904(17). Selected bond angles (ꢀ): Br2–Ni1–Br1 126.16(4); P1–
Ni1–Br1 94.75(5); P1–Ni1–Br2 119.67(5); P1–Ni1–P2 99.39(6); P2–
Ni1–Br1 102.14(6); P2–Ni1–Br2 110.40(5)
As mentioned earlier, alteration of the ligand scaf-
fold remarkably induced the catalytic activity and
product selectivity of complexes 1–4. To elucidate the
role of ligand structure in controlling catalytic perfor-
mance and to establish the coordination chemistry of
silylated-diphosphine Ni(II) complexes, efforts were
made to prepare single crystals of these complexes. X-
ray quality single crystals of complex 3 were obtained
by slow diffusion of n-hexane into a CH2Cl2 solution
of 3 at room temperature. The molecular structure of
Triphenylphosphine (PPh3) has been extensively used
as an auxiliary ligand to enhance the catalytic perfor-
mance of ethylene dimerization catalysts. This moiety
may be coordinated with the complex and contribute
toward the catalyst's stability and consequent catalytic
activity and product selectivity.[27,28] For instance, adding
10 equiv PPh3 at 30 atm ethylene pressure enhances the
catalytic activity of N–O-based Ni catalysts by an order of
magnitude.[29] To probe the role of the uncoordinated
diphenylphosphine moiety in controlling the catalytic
performance, we speculated that changing the ligand/Ni
molar ratio may increase the ligand–Ni interaction and
hence increase the chance of coordination of the
uncoordinated diphenylphosphine moiety with the
complex
Figure 1.
3
[Ni(Br)2(k2-P, P-L3)] is illustrated in
Complex 3 adopts a mononuclear k2-P, P bidentate
binding mode in which two phosphorus and two
bromide atoms coordinated around the central Ni atom
in a distorted tetrahedral geometry while the third
phosphorus atom (P3) remains uncoordinated. By
comparing the structural parameters of complex 3 with
the previously reported Me2Si(CH2PPh2)2/NiBr2 complex
5 with the structure [Ni(Br)2(k2-P, P-L5)][21] suggested
that the uncoordinated diphenylphosphine moiety
may successfully tune the catalytic environment of the
complex 3 based system. The high activity of catalyst
3, in comparison to catalyst 5, is due to the flexible
nature of the uncoordinated diphenylphosphine
moiety, which may enable complex 3 to exhibit a stable
configuration and consequently deliver high catalytic
activity. The important differences in the structural
parameters between complexes 3 and 5 are illustrated in
Table 2.
TABLE 2
Comparison of the bond angles of complexes
3 and 5
Bond angles (ꢀ)
Br1–Ni1–Br2
P1–Ni1–Br1
P1–Ni1–Br2
P2–Ni–Br1
Complex 3
126.16(4)
Complex 5 [25]
127.28(5)
94.75(5)
99.93(7)
119.67(5)
111.58(8)
102.14(6)
98.32(7)
P2–Ni–Br2
110.40(5)
115.94(7)
C
Ph–P2–Ni1
115.80(average)
113.65(average)

6 of 12
WANG ET AL.
complex. This may stabilize the catalyst and consequently
enhance catalytic performance by forming tridentate
(in case of 1–3)[18] or dinuclear (in case of 4) com-
plexes.[30] For this purpose, the 1–4 complexes were scru-
tinized at different ligand/Ni ratios for ethylene
oligomerization and the results are shown in Table 3.
The catalytic activity of complexes 1–3 decreases on
changing the ligand/Ni ratio from 1:1 to 1:1.5 (Table 3,
entries 1–6), which suggests that these complexes may
fail to coordinate in a tridentate fashion. The decrease in
activity could be attributed to the excess amount of Ni
precursor, which may disturb the catalytic systems of
complexes 1–3. The excess amount of Ni precursor may
cause inadequate separation between the Ni cations and
counterions in the reaction medium and consequently
contribute to low catalytic activity.[31] In contrast to com-
plexes 1–3, the catalytic activity of complex 4 increased
by an order of magnitude when the ligand/Ni ratio was
changed from 1:1 to 1:2 (Table 3, entries 7 and 8).
Although not definitive, we speculate that this increase
in the catalytic activity may be attributed to the forma-
tion of a stable complex by the coordination of two mole-
cules of NiBr2 with one molecule of L4 in a dinuclear
fashion, as seen in Scheme 4.
SCHEME 4 Proposed Ni(II)-based dinuclear complex L4
To evaluate the thermal stability of the silylated-
diphosphine based Ni(II) catalysts, ethylene oligomeriza-
tion of complexes 1–4 was performed over a broad range
of reaction temperature (Table 4). The catalytic activity of
complexes 1–4 increased with increasing reaction tem-
perature, reached a maximum value at 45ꢀC (Table 4,
entries 3, 8, 13, and 18). The increase in catalytic activity
at 45ꢀC could be attributed to the high rate of oligomeri-
zation due to high ethylene solubility in the reaction
medium and to the high thermal stability of the cata-
lysts.[18] A further increase in reaction temperature
resulted in a marked decrease in catalytic activity for pre-
catalysts 1 and 2 and the lowest activities were obtained
at the highest temperature (Table 4, entries 5 and 10).
This decrease in activity could be attributed to low ethyl-
ene solubility in the reaction medium. Moreover, high
temperatures may deactivate the reaction center of pre-
catalysts 1 and 2 and consequently offer low activity.[23]
Above 45ꢀC, the catalytic activity of complex 3 remains
almost constant while a very small change was observed
in the activity of complex 4, which suggests that the pres-
ence of three and four methylene spacers, respectively, in
complexes 3 and 4 may offer extra thermal stability to
these complexes and therefore provide almost constant
activity at high temperature (Table 4, entries 15 and 20).
Decreasing the ligand/Ni molar ratio tends to
increase the C4 selectivity for complexes 1–4 (Table 3,
entries 1, 3, 5, and 7). These results suggest that an excess
amount of free metal precursor may promote C4 selectiv-
ity. To probe the role of excess Ni precursor, a test run for
free NiBr2(DME) (without coordination of the silylated-
diphosphine ligand) was conducted. This showed that an
excess amount of Ni precursor attenuated the activity
while enhancing C4 selectivity (Table 3, entry 9).
TABLE 3
Evaluation of the effect of ligand/Ni molar ratios for complexes 1–4 on ethylene dimerizationa
Product selectivity (%)
Activity [107g
L/Ni (molNi
-1 h-1]
b
=
b
=
b
=
+b
Entry Catalyst
)
C4
88.40 66.39
59.73 9.67
1-C4
trans-C4 cis-C4 C6
1-C6
C8
<0.10 <0.10 <0.10
5.63 0.94 1.17
<0.10 <0.10 <0.10
2.00 0.26 <0.10
1-C8
C10
1
2
3
4
5
6
7
8
9
1
1:1.5
1:1
3.45
12.90
3.37
6.00
1.93
2.29
2.48
0.31
0.18
10.17
28.59
18.30
22.89
7.12
11.84
21.47
17.02
19.65
9.39
11.60 2.95
33.47 1.36
15.26 2.94
19.28 1.81
10.41 2.85
11.68 2.50
19.99 3.38
29.99 2.78
17.83 0.12
2
1:1.5
1:1
84.74 49.42
78.72 36.18
89.59 73.07
88.32 72.32
80.01 49.13
65.67 54.56
82.17 75.53
3
1:1.5
1:1
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
8.64
7.36
4
1:2
16.12
6.54
14.76
4.57
1:1
4.34
0.87 <0.10
NiBr2DME)
-
2.81
3.84
<0.10 <0.10 <0.10
aGeneral conditions: n (catalyst), 0.1 μmol; pressure, 1.0 MPa; solvent, methylcyclohexane (20 ml); reaction time, 30 min; T, 45ꢀC; n (Al)/n
(Ni), 500 equiv; DEAC used as cocatalyst.
bwt% of liquid products (oligomers).

WANG ET AL.
7 of 12
TABLE 4
Evaluation of the effect of reaction temperature on ethylene dimerizationa
Product selectivity (%)
T
Activity [107g
b
=
b
=
b
=
+b
Entry Catalyst (ꢀC)
(molNi
)
-1 h-1]
C4
1-C4
trans-C4 cis-C4 C6
1-C6
C8
1-C8
C10
1
1
2
3
4
15
30
45
60
75
15
30
45
60
75
15
30
45
60
75
15
30
45
60
75
1.13
12.10
12.90
8.77
2.63
1.93
3.78
6.00
5.35
0.20
0.33
0.82
2.31
2.27
2.31
0.09
0.23
0.31
0.22
0.11
74.25 24.58
65.33 16.43
33.14
30.54
28.59
27.42
23.39
18.22
22.14
22.89
26.72
26.60
6.22
16.53
18.36
21.47
21.88
25.36
12.68
13.96
19.65
21.80
20.22
6.56
25.75 1.68
29.32 1.52
33.47 1.36
36.23 1.39
36.30 1.31
10.17 1.30
14.67 1.68
19.28 1.81
19.71 2.11
20.58 2.42
8.88 1.97
9.46 2.21
11.68 2.50
13.22 2.78
13.59 2.91
28.59 2.22
29.07 2.54
29.99 2.78
30.16 2.65
30.60 1.41
<0.10 <0.10 <0.10
2
4.41
5.63
5.35
6.92
1.14
1.82
2.00
2.92
7.38
0.92
0.94
0.73
0.68
0.94
1.17
1.00
1.78
3
59.73
57.42
55.00
9.67
8.12
6.25
4
5
6
88.68 57.78
83.51 47.42
78.72 36.18
77.37 28.84
72.04 25.23
91.12 78.34
90.54 75.80
88.32 72.32
86.78 66.66
85.88 63.50
71.41 61.52
66.72 56.44
65.68 54.56
65.68 53.98
64.45 52.08
0.13 <0.10
0.21 <0.10
0.26 <0.10
0.32 <0.10
0.67 <0.10
7
8
9
10
11
12
13
14
15
16
17
18
19
20
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
7.66
7.08
8.64
7.36
10.12
12.24
6.01
10.00
10.14
3.88
0.53
0.18 <0.10
<0.10 <0.10 <0.10
6.22
4.06
4.21
4.34
4.52
4.95
0.98 <0.10
0.87 <0.10
0.96 <0.10
0.41 <0.10
6.54
4.57
6.67
5.03
6.85
5.52
aGeneral conditions: n (catalyst), 0.1 μmol; pressure, 1.0 MPa; solvent, methylcyclohexane (20 ml); reaction time, 30 min; n (Al)/n (Ni), 500
equiv; DEAC used as cocatalyst.
bwt% of liquid products (oligomers).
Low-temperature ethylene oligomerization provides
the highest C4 selectivity for complexes 1–4, which
suggests that low temperature may increase the rate of
dimerization and therefore be favorable for the forma-
tion of the C4 fraction (Table 4, entries 1, 6, 11, and
16).[32] Highest temperature screening provides the
lowest C4 selectivity, it enhances the formation of C6
products at the expense of the C4 fraction (Table 4,
entries 5, 10, 15, and 20). High temperatures may pro-
mote chain propagation instead of the dimerization
reaction.
Changing the cocatalyst molar ratio in the oligomeri-
zation reaction can tune the catalytic environment and
therefore influence the catalytic performance.[20,22] Prob-
ing the role of DEAC by changing its molar ratio
suggested that the concentration of cocatalyst signifi-
cantly influences the catalytic activity and C4 selectivity
(Table 5). The precatalysts 1, 2, and 4 offer high catalytic
activity at 500 equiv DEAC (Table 5, entries 2, 6, 14),
while precatalyst 3 exhibited the highest activity at
700 equiv DEAC (Table 5, entry 11). Any increase
(Table 5, entries 3, 4, 7, 8, 12, 15, and 16) or decrease
(Table 5, entries 1, 5, 9, 10, and 13) in the concentration
of DEAC may lead to a decrease in catalytic activity. A
low concentration of DEAC may unable to completely
activate the catalytic system[20,33] while a higher concen-
tration may cause over-reduction and deactivate the cata-
lytic system.[34] The highest catalytic activity of
1.29 × 108 g (molNi)-1 h-1 was obtained by precatalyst 1 at
500 equiv DEAC (Table 5, entry 2).
The selectivity of the C4 product tends to decrease
with increasing molar ratio of cocatalyst and the highest
C4 selectivities were obtained at the lowest molar ratio of
DEAC (Table 5, entries 1, 5, 9, and 13). Increasing the
cocatalyst molar ratio promoted C6 selectivity at the
expense of C4 product, which suggests that the high
molar ratio of DEAC may favor the formation of the C6
fraction.
The precatalysts 1 and 3 were further selected to
probe the role of catalyst loading on ethylene oligomeri-
zation. Table 6 shows that on increasing catalyst loading
from 0.1 to 0.6 μmol, the catalytic activity significantly

8 of 12
WANG ET AL.
TABLE 5
Evaluation of the effect of Al/Ni molar ratios of complexes 1–4 on ethylene dimerizationa
Activity
[107g (molNi
h-1]
Product selectivity (%)
-1
n(Al)
/
)
b
=
b
=
b
=
+b
Entry Catalyst n(Ni)
C4
1-C4
trans-C4 cis-C4 C6
1-C6
C8
1-C8
C10
1
1
2
3
4
300
500
700
1000
300
500
700
1000
300
500
700
1000
300
500
700
1000
3.22
12.90
11.80
10.10
0.23
6.00
5.91
5.86
0.11
2.29
3.85
2.54
0.04
0.31
0.27
0.20
64.16 15.35
59.73 9.67
58.26 8.43
56.50 8.21
86.60 51.34
78.72 36.18
76.70 33.27
72.38 23.20
89.13 75.57
88.32 72.32
86.94 59.12
86.25 44.97
70.44 58.34
65.67 54.56
65.75 55.14
64.09 53.98
32.17
28.59
27.16
23.06
16.30
22.89
23.53
28.75
7.45
16.65
21.47
22.67
25.23
18.96
19.65
19.90
20.43
6.11
30.96 1.32
33.47 1.36
34.34 1.24
34.86 1.11
13.40 1.93
19.28 1.81
21.89 1.60
26.16 1.40
10.87 2.52
11.68 2.50
13.06 2.12
13.74 1.48
27.44 4.38
29.99 2.78
31.69 2.16
33.29 1.82
4.88
5.63
5.17
6.22
1.30
0.94
0.85
0.81
<0.10
1.17
2
3
2.23
4
2.42
5
<0.10 <0.10 <0.10
6
2.00
1.16
0.62
0.26
0.21
0.12
<0.10
0.25
7
8
0.84
9
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
10
11
12
13
14
15
16
8.64
7.36
14.98
20.45
7.34
12.84
19.19
4.76
2.12
4.34
2.56
2.62
0.97
0.87
0.68
0.45
<0.10
<0.10
<0.10
<0.10
6.54
4.57
6.24
4.37
5.92
4.19
aGeneral conditions: n (catalyst), 0.1 μmol; pressure, 1.0 MPa; solvent, methylcyclohexane (20 ml); reaction time, 30 min; T, 45ꢀC; DEAC
used as cocatalyst.
bwt% of liquid products (oligomers).
TABLE 6
Evaluation of the effect of catalyst loading on ethylene dimerizationa
Product selectivity (%)
n (cat)
Entry Catalyst (μmol)
Activity [107g
b
=
b
=
b
=
+b
(molNi
)
-1 h-1]
C4
1-C4
trans-C4 cis-C4 C6
1-C6
C8
1-C8
C10
1
2
3
4
5
6
1
0.1
0.2
0.6
0.1
0.2
0.6
12.9
59.73
58.13
55.21
9.67
8.58
7.11
28.59
28.32
27.23
8.64
21.47
21.23
20.87
7.36
33.47 1.36
34.07 1.22
35.69 0.80
11.68 2.50
12.56 3.40
14.42 4.94
5.63
6.32
7.36
0.94
0.86
0.81
1.17
1.48
1.74
10.40
4.22
3
2.29
88.32 72.32
86.32 71.11
83.18 69.08
<0.10 <0.10 <0.10
1.86
8.16
7.05
1.12
2.40
0.54 <0.10
0.89 <0.10
1.69
7.95
6.15
aGeneral conditions: pressure, 1.0 MPa; solvent, methylcyclohexane (20 ml); reaction time, 30 min; T, 45ꢀC; n (Al)/n (Ni), 500 equiv; DEAC
used as cocatalyst.
bwt% of liquid products (oligomers).
decreased. The decrease in catalytic activity could be
explained by the fact that an excess amount of catalyst
loading might interfere with the catalytic environment of
the active species, resulting in the attenuation of the cata-
lytic activity.[35] Moreover, high catalyst loading may also
limit the ethylene concentration and consequently dimin-
ish the activity.[36]
In contrast to catalytic activity, high catalyst load-
ing has no remarkable influence on C4 selectivity.
Increasing the catalyst mass loading resulted in only
a slight decrease in C4 selectivity (Table 6, entries
3 and 6).
Furthermore, the effect of reaction time on catalytic
performance was investigated by conducting the reac-
tions over a broad range of reaction times from 15 to
75 min (Table 7). The precatalysts 1–4 exhibited high
activity during the first 15 min of the reaction, suggesting
that these precatalysts completely activated at an early
stage of the reaction initiation (Table 7, entries 1, 6,
11, and 16).[35] Increasing the reaction time decreased the

WANG ET AL.
9 of 12
TABLE 7
Evaluation of the effect of reaction time on ethylene dimerizationa
Product selectivity (%)
Time
Entry Catalyst (min)
Activity [107g
b
=
b
=
b
=
+b
(molNi
)
-1 h-1]
C4
1-C4
trans-C4 cis-C4 C6
1-C6
C8
1-C8
C10
1
1
2
3
4
15
30
45
60
75
15
30
45
60
75
15
30
45
60
75
15
30
45
60
75
18.50
12.90
8.47
6.67
5.12
6.74
6.00
4.12
2.03
1.56
4.77
2.29
0.72
0.29
0.12
0.42
0.31
0.18
0.04
0.02
58.56
59.73
8.05
9.67
28.72
28.59
28.45
28.23
27.82
24.56
22.89
23.43
23.92
22.83
9.05
21.79
21.47
21.42
20.49
20.17
20.38
19.65
19.13
17.68
15.10
7.73
34.53 1.51
33.47 1.36
32.60 1.30
31.77 1.23
31.71 1.23
19.79 1.98
19.28 1.81
18.81 1.61
18.08 1.45
16.75 1.35
12.20 3.02
11.68 2.50
10.36 1.74
9.88 1.64
8.55 1.43
30.76 3.45
29.99 2.78
28.97 2.32
28.85 2.11
28.48 1.81
5.68
5.63
5.44
5.32
5.21
2.15
2.00
1.98
1.81
1.74
0.96
0.94
0.88
0.83
0.70
1.23
1.17
1.00
1.03
0.95
2
3
60.96 11.09
61.88 13.16
62.13 16.14
78.06 33.12
78.72 36.18
79.21 36.65
80.11 38.51
81.51 43.58
87.80 71.02
88.32 72.32
89.64 75.36
90.12 76.23
91.45 77.98
64.38 52.55
65.67 54.56
67.22 56.55
68.37 58.87
69.29 60.25
4
5
6
0.32 <0.10
0.26 <0.10
0.19 <0.10
0.18 <0.10
0.16 <0.10
7
8
9
10
11
12
13
14
15
16
17
18
19
20
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
<0.10 <0.10 <0.10
8.64
7.36
7.66
6.62
7.34
6.55
6.98
6.49
6.70
5.13
4.86
4.34
3.81
2.78
2.23
1.17 <0.10
0.87 <0.10
0.68 <0.10
0.62 <0.10
0.58 <0.10
6.54
4.57
6.43
4.24
5.40
4.10
5.20
3.24
aGeneral conditions: n (catalyst), 0.1 μmol; pressure, 1.0 MPa; solvent, methylcyclohexane (20 ml); T, 45ꢀC; n (Al)/n (Ni), 500 equiv; DEAC
used as cocatalyst.
bwt% of liquid products (oligomers).
catalytic activity and a long time run may offer low cata-
lytic activity for these catalyst systems (Table 7, entries
5, 10, 15, and 20). This decline in activity is more pro-
nounced for precatalysts 1, 3, and 4, for which the activity
dropped by an order of magnitude in each case (Table 7,
entries 3–5, 13–15, and 19–20). The decrease in catalytic
activity over a long time run could be attributed to the
partial deactivation of the catalyst active species during
the oligomerization reaction.[35]
In contrast to catalytic activity, the product selectivity
of C4 is slightly affected by reaction time and remains
almost stable. The precatalysts 1–4 may have the ability
to dimerize ethylene over a long reaction time.[35]
Commercially applied efficient ethylene dimeriza-
tion catalysts have been known for many years and
are considered to be a mature industry. However, its
catalytic mechanism has received great interest after
the newly developed aryloxy-based Ti catalysts and
continuously under investigation.[37–40] Some studies
have suggested that high dimerization selectivity could
be attributed to a metallacycle mechanism in which
oxidative coupling of ethylene with an active metal
forms a metallacyclopentane and releases the final
product (1-butene) via β-hydride elimination.[33,37,39]
Alternatively, the product distribution in these systems,
which arises due to the high rate of chain transfer rel-
ative to chain propagation, could be rationalized on
the basis of the Cossee mechanism.[38,41] Recent experi-
mental and theoretical investigations support the
Cossee mechanism for ethylene dimerization while
strongly disfavoring the involvement of the metal-
lacycle mechanism.[42,43]
To rationalize the C4 selectivity of silylated-
diphosphine based Ni(II) systems, precatalyst 3 (the best
system in terms of selectivity) was selected for DFT
calculations.[42–45] Many theoretical investigations have
suggested that DFT is a powerful tool for studying the
mechanistic aspects of the ethylene dimerization reac-
tion. DFT calculations can be effectively used to find the
mechanistic pathway (whether the reaction follows the
Cossee or metallacycle mechanism), the role of the ligand
in catalytic transformation, and the origin of selectivity in

10 of 12
WANG ET AL.
FIGURE 2 Relative Gibbs free energy profile for ethylene dimerization (via the Cossee mechanism) catalyzed by precatalyst 3
dimerization systems.[44,45] Figure 2 shows the relative
Gibbs free energy profile of Cossee-based ethylene dimer-
ization catalyzed by precatalyst 3. At the beginning, the
migratory insertion of the first ethylene molecule into the
Ni–hydride bond of complex 1 via TS1/2 needs only
0.4 kcal/mol of energy to form intermediate 2. The sec-
ond ethylene coordination (2.6 kcal/mol of energy for the
formation of 3) and subsequent insertion (13.0 kcal/mol
of energy to pass through TS3/4) collectively require
15.6 kcal/mol of energy to yield intermediate 4. Interme-
diate 4 then opens up two possible pathways: a dimer-
ization pathway (path 1) and a trimerization pathway
(path 2). Following path 1, intermediate 4 can pass
through TS4/1 with an energy barrier of 31.3 kcal/mol
of energy and release 1-butene via β-hydride elimination.
Considering path 2, the third ethylene coordination
(4.7 kcal/mol of energy to form intermediate 5) and sub-
sequent insertion (12.1 kcal/mol of energy to pass
through TS5/6) collectively require 16.8 kcal/mol of
energy to form intermediate 6. Intermediate 6 can pass
through the TS6/1 transition state with an energy barrier
of 31.2 kcal/mol of energy and release 1-hexene via β-
hydride elimination. The energy barriers for the β-
hydride elimination in path 1 (31.3 kcal/mol) and path
2 (31.2 kcal/mol) are almost same, but path 2 needs
16.8 kcal/mol of extra energy (for ethylene coordination
and subsequent insertion) compared to path 1 to release
1-hexene. Thus, instead of following path 2, intermediate
4 can readily liberate 1-butene via β-hydride elimination.
Moreover, focusing on the mechanism of the β-hydride
process. Its rate also depends on the ethylene con-
certation and proceeds only in the presence of ethylene.
On the contrary, for electron-richmetal-based catalysts
(Ni in our case),[46–48] a concerted transfer of the β-
hydrogen in the alkyl chain to the metal-coordinated
ethylene takes place and provides a facile route for the
liberation of 1-butene.
4 | CONCLUSIONS
We have reported Ni(II) precatalysts with CH3Si(CH2)n
(PPh2)3 and Si(CH2PPh2)4-type silylated diphosphine
ligands, which uon activation with DEAC offer active
and selective ethylene dimerization systems. We
observed that the methylene spacers and
a dip-
henylphosphine moiety in the ligand scaffold strongly
influence the catalytic performance of these catalysts.
Precatalyst 1, with two methylene spacers in its struc-
ture, exhibited high catalytic activity, while precatalyst
3, with three methylene spacers, afforded high C4
(88%) selectivity. Structural modifications in the ligand
framework effectively tune the catalytic activity and
product selectivity. The highest catalytic activity of
1.29 × 108 g (molNi)-1 h-1 was obtained with complex
1 using 0.1 μmol of catalyst loading at 45ꢀC and
1.0 MPa of ethylene, and this high activity was attrib-
uted to the catalyst stability and high ethylene solubil-
ity under these conditions. The results for complexes
1–4 were systematically compared with the reported
precatalysts 5 and 6 and rationalized. Single crystal
transfer may provide
a
reasonable conjecture for
1-butene formation.
For electron-deficientmetal-
analysis of precatalyst
3
revealed that the
basedcatalysts,[41,44] 1-butene formation requires high
energy because of a stepwise β-hydride transfer to ethyl-
ene, which is an endergonic and nonspontaneous
uncoordinated diphenylphosphine moiety in the com-
plex structure effectively tunes the structural parame-
ters, which may tailor the catalytic environment of the

WANG ET AL.
11 of 12
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The influence of reaction conditions (ligand/Ni molar
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SUPPORTING INFORMATION
Synthesis procedures, NMR characterizations, computa-
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How to cite this article: Wang J, Alam F,
Chang Q, Chen Y, Jiang T. Catalytic behavior
tuning via structural modifications of silylated-
diphosphine Ni(II) complexes for ethylene selective
dimerization. Appl Organomet Chem. 2020;e5722.
https://doi.org/10.1002/aoc.5722
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.