Ethylene oligomerization promoted by nickel-based catalysts with silicon-bridged diphosphine amine ligands

Article abstract of DOI:10.1007/s11243-018-0276-7

A series of nickel complexes [Ni(L1)Br₂] (C1), [Ni(L2)Br₂] (C2) and [Ni(L3)Br₂] (C3) (L1 = N-isopropyl-N-(((diphenylphosphanyl)methyl)dimethylsilyl)-1,1-diphenylphosphanamine, L2 = N-cyclopentyl-N-(((diphenylphosphanyl)methyl)dimethylsilyl)-1,1-diphenylphosphanamine, L3 = N-(2,6-diisopropylphenyl)-N-(((diphenylphosphanyl)methyl)dimethylsilyl)-1,1-diphenylphosphanamine) were synthesized and characterized by elemental analysis, mass spectrometry, spectroscopy and single-crystal X-ray diffraction. L1, L2 and L3 each act as bidentate ligands. Upon activation with ethylaluminum dichloride, these complexes produce efficient catalytic systems for selective dimerization of ethylene to 1-butene, with catalytic activities of 3.45 × 10⁷?g/(mol_Ni·h) and 95.6% butene selectivity containing 87.6% 1-butene fraction.

Full text of DOI:10.1007/s11243-018-0276-7

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0276-7
Ethylene oligomerization promoted by nickel‑based catalysts
with silicon‑bridged diphosphine amine ligands
1
2
1
1
1
1
Wei Wei · Buwei Yu · Fakhre Alam · Yongwang Huang · Shaoling Cheng · Tao Jiang
Received: 8 August 2018 / Accepted: 19 September 2018
©
Springer Nature Switzerland AG 2018
Abstract
A series of nickel complexes [Ni(L1)Br ] (C1), [Ni(L2)Br ] (C2) and [Ni(L3)Br ] (C3) (L1 = N-isopropyl-
2
2
2
N-(((diphenylphosphanyl)methyl)dimethylsilyl)-1,1-diphenylphosphanamine, L2=N-cyclopentyl-N-(((diphenylphosphanyl)
methyl)dimethylsilyl)-1,1-diphenylphosphanamine, L3 = N-(2,6-diisopropylphenyl)-N-(((diphenylphosphanyl)methyl)
dimethylsilyl)-1,1-diphenylphosphanamine) were synthesized and characterized by elemental analysis, mass spectrometry,
spectroscopy and single-crystal X-ray diꢀraction. L1, L2 and L3 each act as bidentate ligands. Upon activation with ethyla-
luminum dichloride, these complexes produce eꢁcient catalytic systems for selective dimerization of ethylene to 1-butene,
7
with catalytic activities of 3.45×10 g/(mol ·h) and 95.6% butene selectivity containing 87.6% 1-butene fraction.
Ni
Introduction
was reported as 1.6 million metric tons in 2011 and is esti-
mated to reach up to 3.5 million metric tons in 2025 [2].
The low rate of production and rapid increase in demand
have stimulated research into nickel-based catalysts to meet
these challenges.
Ethylene oligomerization is an important method for the
production of α-oleꢂns [1]. In the past two decades, nickel
complexes have emerged as transition metal catalysts for
ethylene oligomerization and have attracted wide attention
in both academic and industrial research [2–6]. The tradi-
tional nickel catalysts mainly produce a wide Schulz–Flory
The structure of the ligand plays a crucial role in control-
ling the catalytic performance of a metal complex, espe-
cially the product selectivity in ethylene oligomerization. In
order to develop more eꢁcient catalytic systems, extensive
research has been focused on the design of new ligands.
Nickel complexes with diꢀerent donor ligands, including
NN, NO, NP and PO donors, have been investigated for
selective ethylene oligomerization [3]. The chelating NO
donor (imino/amino)phenol nickel(II) complexes reported
by Ngcobo et al. [4] demonstrated that the activity and selec-
tivity of these complexes for 1-butene show opposite trends
to each other, such that increasing selectivity can dimin-
ish activity, and vice versa. Campora and coworkers [8]
demonstrated that the product selectivity toward 1-butene
of nickel(II) complexes with NP donor phosphinito-imine
ligands was also inꢃuenced by the steric bulk and showed
different selectivities when different substituents were
attached to the P atom. Recently, we reported nickel(II)
complexes based on PNSiP ligands, which proved to be
eꢁcient for the catalytic production of 1-butene [9]. Based
on our previous ꢂndings, we aimed to design and investigate
nickel(II) complexes with silicon-bridged diphosphinoam-
ine ligands (PNSiCP) for selective ethylene oligomerization
of 1-butene. The eꢀects of the reaction conditions on the
(
S–F) distribution which contains a small proportion of
each α-oleꢂn fraction, particularly 1-butene. The low selec-
tivity for 1-butene has been mainly attributed to the high
isomerization rate of nickel catalysts during ethylene oli-
gomerization [7]. 1-Butene is used as a co-monomer in the
production of high-density polyethylene (HDPE) and linear
low-density polyethylene (LLDPE). Therefore, the produc-
tion of 1-butene directly inꢃuences the market for both
HDPE and LLDPE. The global annual demand for 1-butene
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-018-0276-7) contains
supplementary material, which is available to authorized users.
*
Tao Jiang
jiangtao@tust.edu.cn
1
Tianjin Marine Chemical Technology Engineering
Center, College of Chemical Engineering and Material
Science, Tianjin University of Science and Technology,
Tianjin 300457, China
2
PetroChina Daqing Chemical Engineering Research Center,
Daqing 163714, China
Vol.:(0
1
123456789)
3

Transition Metal Chemistry
activity and product selectivity of these catalysts were inves-
tigated. Our results are reported in this paper.
HP-5 GC capillary column (length 60 m; diameter 0.32 mm;
ꢂlm thickness 0.25 μm).
Experimental
Synthesis of organic precursors
Materials and methods
The proligands L1–L3 were prepared as shown in Scheme 1.
A solution of CH MgI in Et O (20 mL) was added dropwise
3
2
All the proligands and their complexes were synthesized
under a nitrogen atmosphere, in a glovebox or in oven-dried
ꢃasks using standard Schlenk techniques. Bis(phenyl)phos-
phorus chloride, diphenylphosphine, dichlorodimethylsilane,
at −35 °C to a solution of chlorodipenylphosphane (29.8 g,
135.06 mmol) in Et O (100 mL) at − 35 °C under N and
2
2
the mixture was stirred for overnight. After ꢂltration, the
organic phase was washed with distilled water (3×10 mL),
separated and dried with anhydrous Na SO . Methyldiphe-
n-butyllithium (2.4 mol/L in n-hexane) and NiBr (DME)
2
2
4
were purchased from Aldrich and were used as received.
Lithium diphenylphosphine was prepared using the method
reported by Peterson [10]. Lithium (diphenylphosphanyl)
nylphosphine (20 g, 99.90 mmol) was obtained as a colorless
1
liquid by reduced pressure distillation in 74% yield. H NMR
(400 MHz, CDCl , 298 K): δ 1.6 (d, 3H, CH ), 7.3–7.7 (m,
3
3
31
(
alkyl/aryl)amide and chlorodimethyl ((diphenylphosphino)
10H, Ph). P NMR (162 MHz, CDCl , 298 K): δ −26.8 (s).
3
methyl) silane were prepared using the method reported
by Zhang [11, 12]. Ethylaluminum dichloride (EADC,
A solution of n-BuLi (21.5 mL, 2.4 mol/L in n-hexane,
51.6 mmol) in n-hexane (30 mL) was added dropwise to a
solution of methyldiphenylphosphine (10 g, 49.95 mmol)
and TMEDA (6.1 g, 25 mL) in n-hexane (15 mL) at below
0 °C. The reaction mixture was then stirred for 60 h under
0
.9 mol/L in heptane), diethylaluminum chloride (DEAC,
.0 mol/L in hexane), methylaluminoxane (MAO, 1.4 mol/L
2
in toluene) and triethylaluminum (TEA) were purchased
from Albemarle Corp (USA). Dried methylaluminoxane
N at room temperature. The yellow precipitate was ꢂltered
2
(
DMAO) was prepared by pumping oꢀ all the volatile com-
oꢀ and washed with n-hexane (3×20 mL) to give (diphe-
nylphosphino)methyllithium-TMEDA (16 g, 49.6 mmol,
99% yield).
pounds from MAO at 40 °C for 6 h. Methylcyclohexane,
dichloromethane, triethylamine and n-hexane were dried,
degassed and puriꢂed by distillation prior to use. Polym-
erization-grade ethylene was purchased from Tianjin Sum-
mit Specialty Gases (China). NMR spectra were obtained
A solution of (diphenylphosphino)methyllithium-TMEDA
complex (16 g, 49.6 mmol) in n-hexane (50 mL) was added
to a stirred solution of dichlorodimethylsilane (12.8 g,
99.17 mmol) in n-hexane (25 mL) at −35 °C. The mixture
was stirred overnight at room temperature. The precipitated
LiCl was ꢂltered out, and a yellowish oily semisolid product
was obtained which still contained TMEDA. Distillation in
a vacuum gave a pure colorless oil of ((chlorodimethylsi-
III
with a Bruker AVANCE 400 M spectrometer (400 MHz
1
31
13
for H, 162 MHz for P and 100 MHz for C) at 298 K
in non-aqueous solvent with tetramethylsilane (TMS) as
an internal reference. Elemental analyses were obtained
using an Elemental Vario EL analyzer. Mass spectra were
recorded using a MALDI-TOF mass spectrometer at the Chi-
nese Academy of Sciences (Shanghai Institute of organic
chemistry). GC was performed using an Agilent 7890A GC
chromatograph with a ꢃame ionization detector (FID) and an
1
lyl)methyl)diphenylphosphane (8 g) in 57% yield. H NMR
(400 MHz, C D , 298 K): δ 0.00 (s, 6H, SiMe ), 1.38 (s, 2H,
6
6
2
3
1
CH ), 6.87–7.21 (m, 10H, Ph). P NMR (162 MHz, C D ,
2
6
6
298 K): δ −24.45 (s).
Scheme 1 Synthesis and structures of the ligands. L1 R=isopropyl L2 R=cyclopentyl L3 R=2,6-diisopropylphenyl
1
3

Transition Metal Chemistry
Synthesis of L1
6.01 mmol) was used to obtain lithium (diphenylphos-
phanyl)(cyclopentyl)amide (L2b) (1.24 g, 4.50 mmol) in
75% yield. Similarly, L2 (0.295 g, 0.56 mmol, 56% yield)
was collected as a white solid, by reacting L2b (0.275 g,
1.00 mmol) and ((chlorodimethylsilyl)methyl)diphe-
A solution of isopropylamine (1.9 mL, 22.22 mmol) in tetrahy-
drofuran (10 mL) was added at −35 °C to a solution of Et N
3
(
4.6 mL, 3.37 g, 33.33 mmol) in tetrahydrofuran (10 mL).
1
A solution of chlorodipenylphosphane (2.0 mL, 2.45 g,
1.11 mmol) in tetrahydrofuran (10 mL) was then added. The
nylphosphane (0.293 g, 1.00 mmol). H NMR (400 MHz,
1
C D , 298 K): δ 0.27 (s, 6H, SiMe ), 1.16–1.63 (m, 8H,
6
6
2
mixture was stirred overnight at room temperature. The pre-
cipitated quaternary ammonium salt was ꢂltered out, and the
pale yellow ꢂltrate was concentrated to give a yellow residue,
which was recrystallized from hexane to give N-isopropyl-
CH ), 1.81 (s, 2H, Si-CH -P), 3.58–3.67 (m, 1H, CH),
2
2
3
1
7.01–7.67 (m, 20H, Ph). P NMR (162 MHz, C D ,
6
6
298 K): δ − 22.61 to − 22.58 (d, C–P), 46.60 (br, N–P).
1
3
C NMR (100 MHz, C D , 298 K): δ 1.05, 2.91, 2.95,
6
6
1
,1-diphenylphosphanamine (L1a) (1.37 g, 5.64 mmol) in 51%
yield. A solution of n-BuLi (2.4 mL, 2.4 mol/L in n-hexane,
.64 mmol) was added dropwise at −35 °C to a solution of
2.99, 3.04, 17.50, 17.91, 22.68, 23.80, 34.13, 34.17, 61.32,
61.36, 128.06, 128.12, 128.17, 128.20, 128.27, 131.97,
132.17, 132.57, 132.77, 140.41, 140.59, 141.60, 141.76.
5
L1a (1.37 g, 5.64 mmol) in n-hexane (10 mL). The mixture
was stirred at room temperature for 5 h. After ꢂltering and
washing with n-hexane (2×10 mL) and drying under vacuum,
the solid lithium (diphenylphosphanyl)(isopropyl)amide (L1b)
Synthesis of L3
(0.7256 g, 2.91 mmol, 51% yield) was obtained.
L3 was prepared by the same method as described for
A solution of L1b (0.7256 g, 2.91 mmol) in n-hexane
5 mL) at − 35 °C was added dropwise to a solution of
(chlorodimethylsilyl)methyl)diphenylphosphane (0.8521 g,
L1. 2,6-Diisopropylamine (4.2 mL, 22.22 mmol), Et N
3
(
(
(4.6 mL, 3.37 g, 33.33 mmol) and chlorodipenylphosphane
(2 mL, 2.45 g, 11.11 mmol) were used to prepare the inter-
mediate: N-2,6-Diisopropylphenyl-1,1-diphenylphos-
phanamine (L3a) (2.29 g, 6.34 mmol) in 57% yield. Next,
n-BuLi (2.64 mL, 2.4 mol/L in n-hexane, 6.34 mmol) was
used to obtain lithium (2,6-diisopropylphenyl)(diphe-
nylphosphanyl)amide (L3b) (2.98 g, 8.11 mmol) in 73%
yield. Solid L3 (0.278 g, 0.45 mmol, 45% yield) was col-
lected, from the reaction of L3b (0.37 g, 1.00 mmol) with
((chlorodimethylsilyl)methyl)diphenylphosphane (0.293 g,
2
.91 mmol) in n-hexane (10 mL) under stirring. The mixture
was stirred overnight at room temperature and then ꢂltered.
The ꢂltrate was concentrated under vacuum, followed by
washing with n-hexane (3×10 mL), and then dried again in a
vacuum. The white solid ligand L1 was collected after recrys-
tallization from hexane at −35 °C and drying in a vacuum
1
(
71% yield). H NMR (400 MHz, C D , 298 K): δ 0.15 (s, 6H,
6
6
SiMe ), 0.96–0.98 (d, 6H, CH ), 1.54 (s, 2H, CH ), 3.19–3.26
2
3
2
31
1
(
m, 1H, CH), 7.02–7.48 (m, 20H, Ph). P NMR (162 MHz,
1.00 mmol). H NMR (400 MHz, C D , 298 K): δ 0.30
6
6
1
3
C D , 298 K): δ-24.49 (s, C–P), 34.80 (s, N–P). C NMR
(s, 6H, SiMe ), 0.65–0.66 (d, 6H, CHMe ), 1.15–1.17
6
6
2
2
(
100 MHz, C D , 298 K): δ 0.18, 0.22, 15.50, 15.82, 23.42,
(d, 6H, CHMe ), 1.89 (s, 2H, CH ), 3.48–3.55 (m, 2H,
2 2
6
6
3
1
23.49, 45.81, 46.14, 125.73, 125.78, 125.91, 125.98, 126.13,
128.71, 128.91, 128.99, 129.71, 129.90, 129.99, 130.11,
130.22, 130.31, 130.42, 131.97, 132.09, 132.22, 137.91,
138.06, 141.05, 141.19.
CH), 7.00–7.62 (m, 23H, Ph). P NMR (162 MHz, C D ,
6 6
298 K): δ − 21.21 to − 21.18 (d, C–P), 52.89 – 52.92 (d,
1
3
N–P). C NMR (100 MHz, C D , 298 K): δ 2.52, 2.57,
6
6
2.61, 3.71, 3.86, 17.67, 20.25, 22.71, 24.15, 25.18, 28.56,
24.40, 124.42, 126.17, 126.18, 128.08, 128.24, 128.31,
129.01, 132.61, 132.81, 134.67, 134.92, 139.18, 139.41,
41.50, 141.66, 142.71, 142.73, 147.65, 147.67.
1
Synthesis of L2
1
The proligand L2 was prepared by the same method as
described for L1. Cyclopentylamine (2.2 mL, 1.89 g,
2
2.22 mmol), Et N (4.6 mL, 3.37 g, 33.33 mmol) and
Synthesis of the complexes
3
chlorodipenylphosphane (2 mL, 2.45 g, 11.11 mmol) were
used to prepare the intermediate compound N-cyclopentyl-
The nickel complexes were prepared as displayed in
scheme 2.
1
5
,1-diphenylphosphanamine (L2a) (1.62 g, 6.01 mmol) in
4% yield. Then, n-BuLi (2.5 mL, 2.4 mol/L in n-hexane,
Scheme 2 Synthesis and
structures of the precatalyst.
C1 R=isopropyl C2 R=cyclo-
pentyl C3 R=2,6-diisopropyl-
phenyl
1
3

Transition Metal Chemistry
Preparation of complex C1
was analyzed with an Agilent 7890A GC system, using
n-heptane as an internal standard.
A solution of L1 (0.0525 g, 0.105 mmol) in dichlorometh-
ane (10 mL) was added to a dichloromethane dispersion
Crystal structures of complexes C2 and C3
(
10 mL) of NiBr (DME) (0.0310 g, 0.1 mmol). The reac-
2
tion mixture rapidly became dark brown and was stirred
for 36 h at room temperature. The solvent was then evapo-
rated under vacuum, and then the dark brown solid resi-
due was washed three times with n-hexane. The solid was
collected by ꢂltration, and then dried in vacuum to obtain
a ginger-colored nickel complex (C1). Yield 0.050 g
Single crystals of complexes C2 and C3 suitable for X-ray
studies were obtained by two-phase diꢀusion of n-hexane/
dichloromethane. Crystallographic data were collected on a
Bruker SMART APEX CCD diꢀractometer with graphite-
monochromated Mo Kα radiation (λ=0.71073 Å) using the
ω-scan technique. The determination of crystal class and
unit cell parameters was carried out by the SMART program
package. The raw frame data were processed using SAINT
and SADABS [13] to yield the reꢃection data ꢂle. The
structures were solved using the SHELXTL [14] program.
(
70%). Anal. Calcd for C H Br NNiP Si: C, 50.17; H,
30 35 2 2
4
.91; N, 1.95. Found: C, 50.27; H, 5.03; N, 1.89. MALDI-
8
1
+
TOF–MS: m/z = [M- Br] = 637.0.
2
Reꢂnement was performed on F anisotropically for all the
Preparation of complex C2
non-hydrogen atoms by full-matrix least-squares methods
[
14]. The crystallographic data and structural reꢂnement
Complex C2 was prepared using the same synthetic
approach as complex C1, but with L2 (0.0552 g,
parameters for C2 and C3 are summarized in Table 1. One
dichloromethane molecule was found within the crystal
structure of complex C2.
0
.105 mmol) and NiBr (DME) (0.0310 g, 0.100 mmol)
2
as starting materials. Yield 0.060 g (81%). Anal. Calcd
for C H Br NNiP Si: C, 51.65; H, 5.01; N, 1.88.
3
2
37
2
2
Found: C, 51.20; H, 4.85; N, 1.76. MALDI-TOF–MS:
Results and discussion
8
1
+
m/z = [M- Br] = 662.9.
The molecular structures and selected bond parameters for
complexes C2 and C3 are given in Figs. 1 and 2. Complex
C2 crystallizes in the triclinic space group P-1, and complex
C3 crystallizes in the monoclinic space group P2(1)/c. The
nickel atom is four coordinated with a distorted square pla-
nar geometry. The coordination environment of the nickel
atom for complex C3 is very similar to that of complex C2.
After activation with EADC, the precatalysts C1-C3 were
tested for selective ethylene oligomerization and showed
good catalytic activity toward 1-butene and 1-hexene as
well as good selectivity toward 1-butene. The insertion of
Preparation of complex C3
Complex C3 was prepared using the same synthetic approach
as for complex C1, but with L3 (0.0650 g, 0.105 mmol) and
NiBr (DME) (0.0310 g, 0.100 mmol) as starting materials.
2
Yield 0.0694 g (83%). Anal. Calcd for C H Br NNiP Si:
3
9
45
2
2
C, 56.01; H, 5.42; N, 1.67. Found: C, 55.19; H, 5.27; N,
8
1
+
1
.58. MALDI-TOF–MS: m/z=[M- Br] =755.0.
1
-butene into the cationic nickel species can produce cis/
General procedure for ethylene oligomerization
trans isomers, which can decrease the selectivity of the reac-
tion considerably. It was demonstrated that the 2,1-inser-
tion of 1-butene followed by the β-hydride elimination pro-
duces trans-2-butene, while if C , C rotation occurs after
Ethylene oligomerization reactions were carried out in a 150
mL glass autoclave ꢂtted with a magnetic stirrer and tem-
perature controller. The reactor was dried for 2 h at 105 °C
prior to use and then vacuumed to cool to room temperature,
then charged with high-purity nitrogen. After repeating the
vacuuming and nitrogen ꢂlling operation three times, the
nitrogen was replaced with ethylene. Methylcyclohexane
α
β
the 2,1-insertion followed by β-hydride elimination, the
product will be cis-2-butene (Scheme 3) [15–17]. Similarly,
1-hexene is formed by a chain growth mechanism, while
its isomers 2-hexene and 3-hexene are formed as a result of
Ni-catalyzed isomerization in the same way as demonstrated
for 1-butene isomers [18].
(
20 mL) was then injected into the reactor, followed by the
co-catalyst. After stirring for 1 min at the required reaction
temperature, the precatalyst solution was transferred into
the reactor. The autoclave was immediately pressurized
with 1.0 MPa of ethylene. The reaction was then run for the
required time. The reactor was then quenched by cooling to
Eꢀects of co‑catalyst on ethylene oligomerization
The eꢀects of EADC, DEAC, DMAO/AlEt and MAO as
3
diꢀerent co-catalysts on the catalytic performance of precat-
0
°C in an ice bath, and depressurized. The organic phase
alyst C3 were investigated in methylcyclohexane or toluene
1
3

Transition Metal Chemistry
Table 1 Crystallographic
determination parameters for
C2 and C3
Complex
C2
C3
Empirical formula
Formula weight
C H Br Cl NNiP Si
C H Br NNiP Si
3
3
39
2
2
2
39 45
2
2
829.11
836.31
Temperature
Wavelength
Crystal system, space group
Unit cell dimensions
113(2) K
0.71073 Å
Triclinic, P-1
a=11.625(2) Å; α=95.49(3)°
b=12.702(3) Å; β=108.20(3)°
c=13.119(3) Å; γ=98.35(3)°
113(2) K
0.71073 Å
Monoclinic, P2(1)/c
a=10.944(2) Å; α=90°
b=18.370(4) Å; β=97.78(3)°
c=17.803(4) Å; γ=90°
3546.23(12) Å
4, 1.565 Mg/m
3
3
Volume
1800.2(7) Å
3
3
Z, calculated density
Absorption coeꢁcient
F(000)
2, 1.530 Mg/m
3.053 mm−
840
1
2.955 mm
1708
−1
Theta range for data collection
Limiting indices
1.639° to 27.863°
−15≤h≤15
1.60° to 28.09°
−14≤h≤13
−23≤k≤23
−
−
16≤k≤16
17≤l≤17
−23≤l≤23
Reꢃections collected/unique
Completeness to theta (°)
Reꢂnement method
21,496/8516 [R(int)=0.0673]
99.9%
Full-matrix least-squares on F
8516/0/381
35,955/8592 [R(int)=0.0908]
99.3%
2
Full-matrix least-squares on F2
8592/0/421
Data/restraints/parameters
2
Goodness of ꢂt on F
0.961
0.990
Final R indices [I>2sigma(I)]
R indices (all data)
Largest diꢀ. peak and hole
R1=0.0505, wR2=0.1124
R1=0.0790, wR2=0.1263
0.829 and −1.142 e Å−
R1=0.0566, wR2=0.1222
R1=0.0814, wR2=0.1352
0.858 and −1.329 e Å
3
−3
solvents. No activity was observed when MAO or DMAO/
Eꢀects of N‑substituent on ethylene oligomerization
AlEt was used for activation (Table 2, entries 3 and 4),
3
which could reꢃect a better stabilization of the active spe-
cies with EADC and DEAC, possibly due to the chlorine
atom [19]. Other activators like EADC and DEAC can suc-
cessfully activate C3 precatalyst and show good selectivity
toward 1-butene (Table 2, entries 1 and 2). Furthermore, in
comparison with DEAC, the co-catalyst EADC showed bet-
ter selectivity toward butene (Table 2, entry 1). We therefore
selected EADC for further screening experiments.
To explore the inꢃuence of the N-substituent on the activ-
ity and selectivity of these catalytic systems, three catalysts
were synthesized. The catalyst with the most bulky N-sub-
stituent (C3) showed highest selectivity toward 1-butene
compared to the other two catalysts with smaller N-substit-
uents (Table 4). On the other hand, the bulky substituent in
C3 may hinder coordination of ethylene to the metal center,
resulting in low catalytic activity.
Eꢀect of C2 catalyst loading on ethylene
oligomerization
Eꢀects of reaction temperature on ethylene
oligomerization
The mass loading of the catalyst had a considerable inꢃu-
ence on the catalytic performance. We found that upon
decreasing the amount of precatalyst from 2.4 to 0.1 μmol,
the catalytic activity and product selectivity were both sig-
Our results showed that C1 and C2 displayed high selec-
tivity at 45 °C toward 1-butene (Table 5, entries 2 and 5),
while C3 showed the highest selectivity (95.6%) at 30 °C.
The catalysts C1 and C2 showed high activities at the rela-
tively low temperature of 30 °C (Table 5, entries 1 and 4).
Lower activity at higher temperatures might be attributed to
low ethylene solubility and/or catalyst decomposition [20].
Moreover, the low temperature could promote the forma-
tion of active species for the formation of α-oleꢂns [21].
Compared to C1 and C2, catalyst C3 with a more bulky
niꢂcantly increased (Table 3). A very high catalytic activity
6
(
10.1×10 g/(mol ·h)) and good product selectivity (85.7%)
Ni
were obtained toward 1-butene when the catalyst loading
was reduced to 0.1 μmol (Table 3, entry 3), showing that a
small amount of precatalyst could eꢀectively suppress the
formation of non-selective oligomer species [12].
1
3

Transition Metal Chemistry
N-substituent showed the opposite trend toward 1-butene
activity at lower temperatures.
Eꢀects of Al/Ni molar ratios on ethylene
oligomerization
In order to ꢂnd the optimum Al/Ni molar ratio for these
catalysts, we varied the Al/Ni molar ratios from 300 to 700
(
Table 6). Catalysts C1 and C2 showed high selectivity
when the Al/Ni molar ratio was 500. Upon increasing the
molar ratio up to 700, the selectivity decreased, while the
catalytic activity increased (Table 6, entries 1–4). On the
other hand, catalyst C3 showed high selectivity as well as
high activity at a relatively high (700) Al/Ni molar ratio
(
Table 6, entry 7).
Eꢀect of reaction time on ethylene oligomerization
Fig. 1 The molecular structure of complex C2 with 50% ellip-
soids. The hydrogen atoms and dichloromethane solvent mol-
ecule are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Br(1)–Ni(1)=2.3544(10), Br(2)–Ni(1)=2.3567(8),
In order to investigate the eꢀect of reaction time on the cata-
lytic performance, we analyzed the activity and selectivity
at diꢀerent time intervals (10, 20, 30 and 45 min) (Table 7).
The catalytic activity proved to be strongly inꢃuenced by
reaction time, whereas the product selectivity remained
almost unaꢀected. The similar selectivity toward 1-butene
during the course of reaction indicates the existence of a
single active species responsible for 1-butene formation, in
line with previous results reported by Oliveira et al. [22].
The change in catalytic activity suggests decomposition of
the active species during the course of reaction, with the for-
mation of other species which give rise to cis/trans isomers
of 1-butene [23].
Ni(1)–P(2)=2.1497(13),
Ni(1)–P(1)=2.1767(12),
P(1)–
N(1)=1.710(3), P(2)–C(20)=1.808(4), Si(1)–N(1)=1.760(4), Si(1)–
C(20)=1.889(4), N(1)–C(13)=1.511(5); P(2)–Ni(1)–P(1)=94.32(6),
Br(1)–Ni(1)–Br(2)=94.88(4), N(1)–P(1)–Ni(1)=118.71(13), C(20)–
P(2)–Ni(1)=114.57(14), N(1)–Si(1)–C(20)=106.33(18), P(1)–N(1)–
Si(1)=119.0(2), P(2)–C(20)–Si(1)=116.7(2)
Conclusion
Three nickel(II) complexes of silicon-bridged diphosphine
amine ligands were synthesized, characterized and investi-
gated as catalysts for selective ethylene dimerization. Com-
plex C3 proved to be the most eꢁcient catalytic system for
selective ethylene dimerization, producing 88.7% of 1-C
4
6
with 2.05×10 g/(mol ·h) activity under appropriate con-
Ni
ditions, when activated by EADC co-catalyst in methylcy-
clohexane solvent. The steric bulk of the N-substituent was
found to inꢃuence both the activity and selectivity of the
catalysts. Moreover, the reaction conditions such as cata-
lyst loading, co-catalyst type, reaction temperature, Al/Ni
molar ratio and reaction time were also identiꢂed as impor-
tant parameters which inꢃuence the catalytic performance.
Fig. 2 The molecular structure of complex C3 with 50% ellipsoids.
The hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (deg): Br(1)–Ni(1)=2.3330(7), Br(2)–Ni(1)=2.3328(8),
Ni(1)–P(1)=2.1826(11),
Ni(1)–P(2)=2.2045(12),
Si(1)–
N(1)=1.759(3), Si(1)–C(27)=1.868(4), P(1)–N(1)=1.704(3), P(2)–
C(27)=1.806(4), N(1)–C(13)=1.465(5); P(1)–Ni(1)–P(2)=94.64(5),
Br(2)–Ni(1)–Br(1)=91.01(3), N(1)–Si(1)–C(27)=105.65(16), N(1)–
P(1)–Ni(1)=116.35(11), C(27)–P(2)–Ni(1)=114.29(12), P(1)–N(1)–
Si(1)=121.10(18), P(2)–C(27)–Si(1)=118.2(2)
Acknowledgements 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).
1
3

Transition Metal Chemistry
Scheme 3 Mechanism of ethyl-
ene dimerization to butenes
Table 2 Eꢀects of co-catalyst
for C3 on catalytic activity and
product selectivity
6
Entry
Co-catalyst
Activity ((10 g/ Product selectivity (%)
(
mol ·h))
Ni
C4
1-C4
trans-C4
cis-C4
C6
1-C6
a
1
2
3
4
EADC
2.48
9.90
No activity
No activity
92.4
87.6
–
78.9
60.2
9.8
17.0
11.4
21.2
7.6
12.4
23.5
11.8
a
DEAC
b
MAO
a,c
DMAO/AlEt3
–
Reaction conditions: n(cat), 0.1 μmol; ethylene pressure, 1 MPa; solvent, 20 mL (a, methylcyclohexane; b,
c
toluene); time, 30 min; T, 45 °C; n(Al)/n(Ni)=500, n(DMAO): n(AlEt )=4: 1; EADC, DEAC, MAO and
3
DMAO/AlEt all used as co-catalyst
3
Table 3 Eꢀect of catalyst
loading for C2 on catalytic
activity and product selectivity
6
Entry
Catalyst load- Activity ((10 g/ Product selectivity (%)
ing (μmol)
(mol ·h))
Ni
C4
1-C4
trans-C4
cis-C4
C6
1-C6
1
2
3
2.4
0.6
0.1
2.82
5.07
10.10
64.1
79.8
85.7
10.2
34.0
61.0
58.2
35.0
19.1
31.5
31.0
19.9
35.9
20.2
14.3
2.6
10.0
16.0
Reaction conditions: ethylene pressure, 1 MPa; solvent, 20 mL (methylcyclohexane); reaction time, 30 min;
T, 45 °C; n(Al)/n(Ni)=500; EADC used as co-catalyst
Table 4 Comparison of the
precatalysts C1–C3 for ethylene
oligomerization
6
Entry
Catalyst
Activity ((10 g/ Product selectivity (%)
(
mol ·h))
Ni
C4
1-C4
trans-C4
cis-C4
C6
1-C6
1
2
3
C1
C2
C3
3.66
10.10
2.48
89.5
85.7
92.4
77.8
61.0
78.9
11.1
19.1
9.8
11.1
19.9
11.4
10.5
14.3
7.6
23.3
16.0
23.5
Reaction conditions: n(cat), 0.1 μmol; ethylene pressure, 1 MPa; solvent, 20 mL (methylcyclohexane);
reaction time, 30 min; T, 45 °C; n(Al)/n(Ni)=500; EADC used as co-catalyst
1
3

Transition Metal Chemistry
Table 5 Eꢀects of reaction
temperature for C1–C3 on
catalytic activity and product
selectivity
6
Entry
Catalyst
C1
T(°C)
Activity ((10
Product selectivity (%)
g/(mol ·h))
Ni
C4
1-C4
trans-C4
cis-C4
C6
1-C6
1
2
3
4
5
6
7
8
9
30
45
60
30
45
60
30
45
60
75
5.50
3.66
4.76
22.80
10.10
16.00
2.05
2.48
7.89
7.03
89.0
89.5
87.9
77.0
85.7
64.8
95.6
92.4
91.6
91.3
51.8
75.8
50.6
41.5
61.0
34.1
87.6
78.9
74.5
62.9
25.5
11.1
26.4
30.7
19.1
34.9
5.3
9.8
13.0
19.8
22.7
11.1
23.1
27.8
19.9
31.0
7.1
11.4
12.5
17.3
11.1
10.5
12.1
23.0
14.3
35.2
4.4
7.6
8.4
8.7
18.8
23.3
17.7
10.5
16.0
10.4
38.2
23.5
23.6
18.4
C2
C3
1
0
Reaction conditions: n(cat), 0.1 μmol; ethylene pressure, 1 MPa; solvent, 20 mL (methylcyclohexane);
reaction time, 30 min; n(Al)/n(Ni)=500; EADC used as co-catalyst
Table 6 Eꢀects of Al/Ni molar
ratios for C1–C3 on catalytic
activity and product selectivity
6
Entry
Catalyst
n(Al)/n(Ni)
Activity ((10
Product selectivity (%)
g/(mol ·h))
Ni
C4
1-C4
trans-C4
cis-C4
C6
1-C6
1
2
3
4
5
6
7
C1
C2
C3
500
700
500
700
300
500
700
3.66
9.33
10.10
34.50
8.85
2.48
8.71
89.5
80.6
85.7
58.6
91.3
92.4
93.4
77.8
48.5
61.0
26.7
66.8
78.9
67.9
11.1
26.0
19.1
39.7
15.4
9.8
11.1
25.5
19.9
33.6
17.8
11.3
15.4
10.5
19.4
14.3
41.4
8.7
7.6
6.6
23.3
12.3
16.0
6.3
19.6
23.5
20.5
16.7
Reaction conditions: n(cat), 0.1 μmol; ethylene pressure, 1 MPa; solvent, 20 mL (methylcyclohexane);
reaction time, 30 min; EADC used as co-catalyst; T, 45 °C
Table 7 Eꢀect of reaction time
for C3 on catalytic activity and
product selectivity
6
Entry
t (min)
Activity ((10 g/ Product selectivity (%)
(
mol ·h))
Ni
C4
1-C4
trans-C4
cis-C4
C6
1-C6
1
2
3
4
45
30
20
10
2.19
2.48
3.99
6.61
92.5
92.4
91.8
93.3
79.2
78.9
83.1
87.1
103
9.8
8.4
6.1
10.6
11.4
8.5
7.5
7.6
8.2
6.7
27.5
23.5
26.5
26.5
6.8
Reaction conditions: n(cat), 0.1 μmol; ethylene pressure, 1 MPa; solvent, 20 mL (methylcyclohexane); T,
5 °C; n(Al)/n(Ni)=500; EADC used as co-catalyst
4
catalytic polymerization and oligomerization of ethylene. Orga-
References
nomet 22(24):4893–4899
4
5
6
.
.
.
Ngcobo M, Ojwach SO (2017) Nickel(II) complexes chelated by
N,N (benzimidazolylmethyl)amine ligands: synthesis and cata-
lytic behavior in tandem ethylene oligomerization and Friedel–
Crafts alkylation reactions. Inorg Chim Acta 467:400–404
Popeney CS, Rheingold AL, Guan Z (2009) Nickel(II) and
palladium(II) polymerization catalysts bearing a ꢃuorinated
cyclophane ligand: stabilization of the reactive intermediate.
Organomet 28(15):4452–4463
1
2
3
.
.
.
Wang J, Zhang N, Li CQ, Shi WG, Lin ZY (2016) Nickel com-
plexes based on hyperbranched salicylaldimine ligands: syn-
thesis, characterization, and catalytic properties for ethylene
oligomerization. J Organomet Chem 822:104–111
Boulens P, Pellier E, Jeanneau E, Reek JNH, Olivier-Bourbigou
H, Breuil P-AR (2015) Self-assembled organometallic nickel
complexes as catalysts for selective dimerization of ethylene
into 1-butene. Organomet 34(7):1139–1142
Speiser F, Braunstein P, Saussine L (2004) New nickel
ethylene oligomerization catalysts bearing bidentate
Chen H-P, Liu Y-H, Peng S-M, Liu S-T (2003) New bulky phos-
phino–pyridine ligands. Palladium and nickel complexes for the
1
3

Transition Metal Chemistry
P,N-phosphinopyridine ligands with diꢀerent substituents α to
17. Sun Y, Chen Y, Mao G, Ning Y, Jiang T (2014) Boron- and
silicon-bridged bis(diphenylphosphino)-type ligands for
chromium-catalyzed ethylene oligomerization. Chin Sci Bull
59(21):2613–2617
phosphorus. Organomet 23(11):2625–2632
7
8
.
.
Lecocq V, Olivier-Bourbigou H (2007) Biphasic Ni-catalyzed
ethylene oligomerization in ionic liquids. Oil Gas Sci Technol
6
2(6):761–773
18. Yang Q-Z, Kermagoret A, Agostinho M, Siri O, Braunstein P
(2006) Nickel complexes with functional zwitterionic N,O-benzo-
quinonemonoimine-type ligands: syntheses, structures, and cata-
lytic oligomerization of ethylene. Organomet 25(23):5518–5527
19. de Oliveira LL, Campedelli RR, Kuhn MCA, Carpentier J-F,
Casagrande OL (2008) Highly selective nickel catalysts for eth-
ylene oligomerization based on tridentate pyrazolyl ligands. J Mol
Catal A: Chem 288(1):58–62
Ortiz de la Tabla L, Matas I, Palma P, Álvarez E, Cámpora J
(
2012) Nickel and palladium complexes with new phosphinito-
imine ligands and their application as ethylene oligomerization
catalysts. Organomet 31(3):1006–1016
9
.
Huang Y, Zhang L, Wei W, Alam F, Jiang T (2018) Nickel-based
ethylene oligomerization catalysts supported by PNSiP ligands.
Phosphorus, Sulfur Silicon Relat Elem 193(6):363–368
1
1
0. Peterson JD (1967) Phosphinomethyllithium compounds IV: an
improved method of preparation and some synthetic applications.
J Organomet Chem 8(2):199–208
20. Zhang N, Wang J, Huo H, Chen L, Shi W, Li C, Wang J (2018)
Iron, cobalt and nickel complexes bearing hyperbranched imi-
nopyridyl ligands: synthesis, characterization and evaluation as
ethylene oligomerization catalysts. Inorg Chim Acta 469:209–216
21. Aid A, Andrei RD, Amokrane S, Cammarano C, Nibou D, Hulea
V (2017) Ni-exchanged cationic clays as novel heterogeneous
catalysts for selective ethylene oligomerization. Appl Clay Sci
146:432–438
1. Zhang L, Meng X, Chen Y, Cao C, Jiang T (2017) Chromium-
based ethylene tetramerization catalysts supported by silicon-
bridged diphosphine ligands: further combination of high activity
and selectivity. ChemCatChem 9(1):76–79
1
1
2. Zhang L, Wei W, Alam F, Chen Y, Jiang T (2017) Eꢁcient chro-
mium-based catalysts for ethylene tri-/tetramerization switched by
silicon-bridged/N,P-based ancillary ligands: a structural, catalytic
and DFT study. Catal Sci Technol 7(21):5011–5018
3. Sheldrick GM (1996) SADABS: program for empirical absorp-
tion correction of area detector data. University of Göttingen,
Göttingen
22. de Oliveira LL, da Silva SM, Casagrande ACA, Stieler R, Casa-
grande OL Jr (2018) Synthesis and characterization of Ni(II)
complexes supported by phenoxy/naphthoxy-imine ligands with
pendant N- and O-donor groups and their use in ethylene oli-
gomerization. Appl Organomet Chem 32(7):e4414
23. Suo H, Zhang Y, Ma Z, Yang W, Flisak Z, Hao X, Hu X, Sun W-H
(2017) 2-Chloro/phenyl-7-arylimino-6,6-dimethylcyclopenta[b]
pyridylnickel chlorides: synthesis, characterization and ethylene
oligomerization. Catal Commun 102:26–30
1
1
4. Sheldrick G (2008) A short history of SHELX. Acta Crystallogr
Sect A 64(1):112–122
5. Dyer PW, Fawcett J, Hanton MJ (2008) Rigid N-phosphino guani-
dine P,N ligands and their use in nickel-catalyzed ethylene oli-
gomerization. Organomet 27(19):5082–5087
1
6. Meng X, Zhang L, Chen Y, Jiang T (2016) Silane-bridged diphos-
phine ligands for nickel-catalyzed ethylene oligomerization. React
Kinet Mech Catal 119(2):481–490
1
3