Silane-bridged diphosphine ligand for palladium-catalyzed ethylene oligomerization

Article abstract of DOI:10.1002/aoc.4014

In this study, bis(diphenylphosphinemethyl)dimethyl silane (L1) and its palladium(II) halide complex, L1/PdCl₂ (C1), were synthesized and characterized. Single-crystal X-ray analysis of the complex revealed bidentate coordination at the Pd cent

Full text of DOI:10.1002/aoc.4014

Received: 17 April 2017
DOI: 10.1002/aoc.4014
Revised: 3 July 2017
Accepted: 4 July 2017
F U L L P A P E R
Silane‐bridged diphosphine ligand for palladium‐catalyzed
ethylene oligomerization
YongWang Huang | Wei Wei | XueJiao Meng | Le Zhang | YanHui Chen | Tao Jiang
Tianjin Marine Chemical Technology
In this study, bis(diphenylphosphinemethyl)dimethyl silane (L1) and its
palladium(II) halide complex, L1/PdCl2 (C1), were synthesized and
characterized. Single‐crystal X‐ray analysis of the complex revealed bidentate
coordination at the Pd center. In combination with methylaluminoxane
(MAO) as co‐catalyst, C1 exhibited excellent catalytic activity and selectivity
for ethylene dimerization toward butene. The maximum catalytic activity
obtained from the C1/MAO system for ethylene dimerization to yield butenes
was 7.33 × 105 g/(molPd·h). The selectivity toward butene remained stable and
high (> 96%) over the various conditions.
Engineering Center, Chemical
Engineering and Material Science, Tianjin
University of Science & Technology,
Tianjin 300457, China
Correspondence
Tao Jiang, College of Chemical
Engineering and Material Science, Tianjin
University of Science & Technology,
Tianjin 300457, China.
Email: jiangtao@tust.edu.cn
Funding information
KEYWORDS
National Key Research and Development
Program of China, Grant/Award Number:
2017YFB0306700; Natural Science Foun-
dation of Tianjin City, Grant/Award
Number: 14JCYBJC23100,
butene, ethylene dimerization, methylaluminoxane, palladium complex
15JCYBJC48100 and 16JCZDJC31600
1 | INTRODUCTION
complex with phosphine‐sulfonate and phosphine‐sulfon-
amide ligands,[11–15] these phosphine‐sulfonamide ligands
Linear α‐olefins are industrially significant olefins used
commonly as co‐monomers, plasticizers, synthetic lubri-
cants and detergents. Oligomerization of ethylene is cur-
rently the most advanced industrial technology for the
preparation of linear α‐olefins. Many catalysts have been
tested for the production of linear α‐olefins through the
oligomerization of ethylene. For example, late‐transition
metal catalysts, particular nickel and palladium com-
plexes, have been developed during the last decade,[1–6]
to take advantage of their lower oxophilicity.[7,8] They
can display high selectivity and excellent catalytic activity.
Pyrazole units in mixed‐nitrogen donor ligands have also
become more widespread after their introduction, by Tsuji
et al. in 1999, as catalysts for ethylene oligomerization.[9]
Darkwa and co‐workers performed subsequent studies
using simple palladium complexes of pyrazole or mixed
donors with pyrazole, obtaining excellent catalytic
were used for catalytic ethylene oligomerization. Specifi-
cally, when the substituent on the phosphorus atom is
phenyl, 1‐butene is the only product with an activity of
7.28 × 104 g/(molPd·h). By contrast, analogous phos-
phine‐sulfonate palladium complexes are highly active
[7.50 × 106 g/(molPd·h)] in ethylene polymerization. This
dramatic difference may be attributed to the relatively
weak electron‐donating ability of the sulfonamide group
in comparison with the sulfonate and phosphine monox-
ide groups. This leads to an electron‐deficient palladium
center, which makes the palladium − lutidine coordina-
tion tighter. The electron‐deficient feature of the palla-
dium center may promote β‐hydride elimination during
chain propagation and result in the formation of lower
oligomers.[16,17]
For
ethylene
oligomerization,
diphosphine ligand systems were the most suitable for
catalysis.[18–20] Nevertheless, research into ethylene oligo-
merization using bidentate Pd‐based catalysts bearing
P^P‐type ligands has been limited. Therefore, we have
chosen to focus our efforts on the development of a
activities [1.77
×
106 g/(molPd·h)] toward ethylene
polymerization.[10] Recently, Chen and co‐workers
reported the synthesis of a series of palladium (II)
Appl Organometal Chem. 2017;e4014.
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Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4014

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HUANG ET AL.
diphosphine ligand‐supported catalytic system. Silyl
groups, as a large electropositive substituent, have a sub-
stantial effect on the complex structure, hence they can
control the stereochemistry of the reaction.[21] Therefore,
our group has synthesized novel silicon‐bridged
diphosphine ligands and characterized Cr pre‐catalyst
supported by it, and achieved a high activity and selectiv-
ity towards 1‐octene.[20] However, the Pd complexes of the
silicon‐bridged diphosphine ligands have never been
reported for ethylene polymerization or oligomerization.
Herein, we report a detailed study on the use of palladium
complexes based on silicon‐bridged diphosphine ligands
in ethylene oligomerization.
A solution of diphenylphosphine lithium base (1.94
g, 10.1 mmol) and bis(chloromethyl)dimethylsilane
(0.790 g, 5.00 mmol) in n‐hexane (50 ml) was stirred
for 3 h at −30°C using ice ethanol. The mixture was
warmed to room temperature, and then the solution
was filtered and concentrated. The residue was taken
up into hot n‐hexane, filtered to remove insoluble mate-
rial, and concentrated to afford a yellow oily substance.
Recrystallization (n‐hexane) gave a yellowish crystalline
1
product (1.21 g, 52%). H NMR (400 MHz, CDCl3): δ =
7.51–7.34 (m, 8H), 7.34–7.15 (m, 12H), 1.27 [s,
(Ph2PCH2)2Si(CH3)2, 4H], −0.19 [s, (Ph2PCH2)2Si(CH3)2,
6H] ppm. 31P NMR (162 MHz, C6D6): δ = –22.75 (s)
ppm. 13C NMR (101 MHz, C6D6): δ = 141.02, 140.88,
132.62, 132.42, 128.37, 128.25, 128.21, 128.18, 14.55,
14.50, 14.25, 14.21, −0.68, −0.73, −0.78.
2 | EXPERIMENTAL
2.1 | Materials and methods
2.3 | Metal complexes (C1)
Bis(phenyl)phosphorus chloride, diphenylphosphine,
potassium hydride, bis(chloromethyl)dimethylsilane,
n‐butyllithium (2.4 M in n‐hexane) and PdCl2(COD)
were purchased from Aldrich and used as received.
Polymerization‐grade ethylene was obtained from
L1 (0.149 g, 0.327 mmol) and [PdCl2(COD)] (0.932 g,
0.327 mmol) were dissolved in CH2Cl2 (20 ml) to give an
orange solution that was stirred for 3 h. Concentration
and recrystallization gave an orange powder (0.181 g,
87%). Anal. calcd for C28H30Cl2PdP2Si: C, 53.03; H, 4.73.
Found: C, 53.05; H, 4.77. MALDI‐TOF‐MS: m/z 598.0
([M–Cl]+) (Figure 1).
Tianjin
Summit
Specialty
Gases
(China).
Methylaluminoxane (MAO; 1.4 M in toluene), toluene,
dichloromethane and n‐hexane were dried and degassed
1
prior to use. H, 13C and 31P NMR spectra of the prod-
ucts were recorded at 25 °C using a Fourier 400 NMR
spectrometer (Bruker, Billerica, MA, USA) with
tetramethylsilane as an internal reference. All intensity
data for X‐ray structural determination were collected
using a Bruker SMART CCD diffractometer and graphite
monochromated Mo Kα radiation (λ = 0.71073 Å). The
structures were resolved through direct methods and
refined through a full‐matrix least‐squares method on F2.
Hydrogen atoms were considered in calculated positions.
Elemental analyses were performed using an Elemental
Vario EL analyzer. Mass spectra were recorded using a
MALDI‐TOF mass spectrometer. Gas chromatography
(GC) was performed using a Agilent 7890A GC system.
2.4 | Molecular structure of C1
The molecular structure of complex C1 was determined
using X‐ray diffraction (see the Supporting Information)
(Figure 2). The crystallographic data and structural refine-
ment parameters are presented in Table 1. Selected bond
lengths and bond angles of the complex are listed in the
captions of Figure 2. The crystal structure of the complex
confirmed a doubly coordinated palladium metal center,
which adopted an almost ideal planar geometry.
2.2 | Bis(diphenylphosphine)
methyldimethylsilane (L1)
n‐BuLi (2.4 M) in hexane (41.6 ml, 99.8 mmol) was
added dropwise to a solution of diphenylphosphine
(18.6 g, 0.100 mol) in THF (200 ml) at −80°C. The mix-
ture was stirred for 1 h at −80°C, and then for 1 h at
room temperature. The solvent was evaporated under
vacuum, and n‐hexane (100 ml) was added simulta-
neously, to form a fully mixed dispersion. After filtra-
tion, the filtrate was evaporated to afford
diphenylphosphine lithium base (18.8 g, 98.6%).
FIGURE 1 Synthesis of the diphosphine palladium pre‐catalyst C1

HUANG ET AL.
3 of 5
FIGURE 2 The molecular structure of C1 determined by X‐ray diffraction (showing 50% thermal ellipsoids). The hydrogen atoms and
solvent molecule are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd(1)–P(1) = 2.2736, P(1)–C(1) = 1.822(1), Si(1)–
C(4) = 1.858, Si(1)–C(8) = 1.896, P(1)–C(8) = 1.816(1); P(1)–Pd(1)–Cl(1) = 88.13, P(1)–Pd(1)–P(1) = 94.63, Cl(1)–Pd(1)–P(1) = 176.97, Cl(1)–
Pd(1)–Cl(1) = 89.13, Pd(1)–P(1)–C(1) = 115.94, C(4)–Si(1)–C(8) = 114.98(4), C(4)–Si(1)–C(4) = 109.78, C(13)–P(1)–Ni(1) = 110.4(3), P(1)–
C(1)–C(5) = 121.67(9), P(1)–C(8)–Si(1) = 117.08, C(14)–Si(1)–C(16) = 113.5(4), Pd(1)–P(1)–C(8) = 116.44, Si(1)–C(8)–P(1) = 117.08
TABLE 1 Crystallographic determination parameters for C1
2.5 | General oligomerization procedure
Identification code
Empirical formula
Formula weight
shelxl
A 140‐ml transparent glass reactor was heated in a high‐
temperature drying oven for 2 h at 105 °C prior to use.
A magnet was placed inside the reactor, which was then
heated in an oil bath to the temperature required for the
oligomerization. The reactor was equipped with a thermo-
couple, a pressure meter and needle valves for injections.
The reactor was evacuated and then charged with high‐
purity N2; this process was repeated three times. The
above operation was then repeated using ethylene. A solu-
tion of the catalyst precursor (C1) and co‐catalyst (MAO)
was introduced into the reactor, and then 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 oligo-
merization were cooled in ice ethanol, and the reaction
quenched by slow addition of acidic ethanol. Once
venting was complete, the products of oligomerization
were analyzed by an Agilent 7890A GC. The heptane
was used as the internal standard.
C28H30Cl2PdP2Si
633.85
Temperature
113 K
Wavelength
0.71073 Å
Crystal system, space group
Unit cell dimensions
orthorhombic, p b c n
a = 11.0961(11) Å; α = 90°
b = 16.4202(16) Å; β = 90°
c = 14.7632(15) Å; γ = 90°
2689.9(5) Å3
4, 1.565 Mg m−3
1.069 mm−1
1288
Volume
Z, Calculated density
Absorption coefficient
F(000)
Theta range for data collection
Limiting indices
3.3–27.5°
−14 ≤ h ≤ 14, −21 ≤ k ≤ 20,
−19 ≤ l ≤ 18
Reflections collected/unique
Completeness to theta = 25.25
Refinement method
32 214/3082 [R(int) = 0.0283]
98.8%
Full‐matrix least‐squares on
F2
3 | RESULTS AND DISCUSSION
Data/restraints/parameters
Goodness‐of‐fit on F2
3082/0/156
0.842
3.1 | Effect of reaction temperature on
oligomerization of ethylene
Final R indices [I > 2sigma(I)]
R indices (all data)
R1 = 0.0181, wR2 = 0.0894
R1 = 0.0194, wR2 = 0.0914
0.384 and −0.886 e·Å−3
The reaction temperature had a dramatic effect on the cat-
alytic behavior of the C1/toluene/MAO system (Table 2).
Largest diff. peak and hole

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HUANG ET AL.
TABLE 2 Effect of reaction temperature on catalytic activity and product selectivity
Product selectivity (%)
Activity
Entry
T (°C)
[105 g/(molPd·h)]
C4
1‐C4
trans‐C4
cis‐C4
C6
1‐C6
C8
=
=
1
30
3.99
96.22
25.47
10.90
63.63
2.89
19.25
0.89
2
3
4
5
45
60
75
90
6.28
3.21
2.68
2.34
96.72
98.72
98.86
25.36
28.04
28.59
28.56
12.73
10.25
10.57
10.39
61.91
61.71
60.84
61.06
2.69
1.28
1.14
0
17.92
0.59
100
0
0
0
100
0
100
Reaction conditions: c(Cat), 4.8 μmol; n(Al)/n(Pd), 500; P, 1.0 MPa; solvent, 20 ml (toluene); time, 30 min; MAO used as co‐catalyst.
The maximum activity of 6.28 × 105 g/(molPd·h) was
obtained at 45 °C. The activity of the catalyst decreased
gradually thereafter upon increasing the temperature,
suggesting that the propagating centers were slightly
unstable at higher temperature,[22] along with the lower
solubility of ethylene in toluene and the higher deactiva-
tion rate of the active sites at higher temperatures.[23,24]
Although the catalytic activity was highly dependent on
the reaction temperature, the products obtained at differ-
ent temperatures revealed similar selectivities, with the C4
products being obtained in each case at over 96%. The
selectivity increased upon increasing the temperature,
with only the C4 product obtained when the reaction
was performed at 90 °C.
Here, we varied the Al/Pd ratio from 100:1 to 1000:1.
The results in Table 3 reveal that, irrespective of the Al/
Pd ratio, the catalytic activity was sensitive to the
concentration of the co‐catalyst, with the major products
remaining short‐chain olefins (C4, C6, C8). For the C1/tol-
uene/MAO system, the catalytic activity increased gradu-
ally upon increasing the Al/Pd ratio, with extremely high
activity [7.33 × 105 g/(molPd·h)] measured when the Al/
Pd ratio was equal to 1000. We suspect that, initially, the
MAO was consumed in eliminating residual oxygen and
water from the reaction system. Upon increasing the Al/
Pd ratio, the amount of MAO was sufficient to form
catalytic active species, hence, the activity increased. The
selectivity toward butene remained relatively stable,
remaining greater than 96%.[25,26]
3.2 | Effect of Al/Pd molar ratio on
oligomerization of ethylene
3.3 | Effect of the reaction time on
catalytic properties
Next, we examined the influence of the ratio of co‐catalyst
to catalyst on ethylene oligomerization. The Al/Pd ratio
can have a remarkable effect on the catalytic activity.
To examine the time‐dependence of the activity and
selectivity toward butene, we varied the reaction time
TABLE 3 Effect of Al/Pd molar ratio on catalytic activity and product selectivity
Product selectivity (%)
Activity
Entry
n(Al)/n(Pd)
[105 g/(molPd·h)]
C4
1‐C4
trans‐C4
cis‐C4
C6
1‐C6
C8
=
=
1
300
5.63
96.86
27.29
9.97
62.73
2.54
19.07
17.92
19.08
19.23
19.71
0.60
2
3
4
5
500
700
6.28
6.87
7.02
7.33
96.72
96.90
96.83
97.21
25.36
24.72
24.79
25.11
12.73
13.67
13.14
13.15
61.91
61.61
62.07
61.73
2.69
2.49
2.53
2.46
0.59
0.61
0.64
0.33
900
1000
Reaction conditions: c(Cat), 4.8 μmol; P, 1.0 MPa; T, 45 °C; solvent, 20 ml (toluene); time, 30 min; MAO used as co‐catalyst.
TABLE 4 Effect of reaction time on catalytic activity and product selectivity
Product selectivity (%)
Activity
Entry
Time (min)
[105 g/(molPd·h)]
C4
1‐C4
trans‐C4
cis‐C4
C6
1‐C6
C8
=
=
1
15
4.47
97.65
27.04
11.64
61.32
1.84
18.69
0.51
2
3
4
30
45
60
6.28
5.91
5.52
96.72
97.53
97.41
25.36
26.83
26.26
12.73
11.25
12.45
61.91
61.92
61.29
2.69
1.94
2.03
17.92
18.61
19.41
0.59
0.53
0.56
Reaction conditions: c(Cat), 4.8 μmol/l; T, 45 °C; n(Al)/n(Pd), 700; P, 1 MPa; solvent, 20 ml (toluene); MAO used as co‐catalyst.

HUANG ET AL.
5 of 5
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on the selectivity toward butene. The optimum time
was 30 min. Thus, the catalyst possessed low stability
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We prepared and characterized a bidentate Pd‐based cata-
lytic system featuring a P^P‐type ligand that we evaluated
as a catalyst for ethylene oligomerization in the presence
of MAO as co‐catalyst. We optimized various reaction
conditions (temperature, Al/Pd ratio, reaction time) to
achieve high activity and selectivity. The palladium
complex exhibited excellent catalytic activity, with
relatively consistent selectivity toward butene. The
C1/MAO system produced a maximum catalytic activity
of 7.33 × 105 g/(molPd·h) and selectivity under the
optimized conditions. In addition to butene as the major
product, some byproducts (C6, C8) were also produced.
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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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Tao Jiang http://orcid.org/0000-0002-9854-7228
SUPPORTING INFORMATION
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How to cite this article: Huang YW, Wei W,
Meng XJ, Zhang L, Chen YH, Jiang T. Silane‐
bridged diphosphine ligand for palladium‐catalyzed
ethylene oligomerization. Appl Organometal Chem.
2017;e4014. https://doi.org/10.1002/aoc.4014
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