Synthesis, characterization and ethylene oligomerization behavior of 2-(chloro-substituted-1H-benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridylnickel dihalides

Article abstract of DOI:10.1016/j.ica.2011.01.054

A series of 2-(chloro-substituted-1H-benzoimidazol-2-yl)-6-(1- aryliminoethyl)pyridines (L1-L6) was synthesized and fully characterized. They reacted with NiCl₂ or (DME)NiBr₂ to form the corresponding N^N^N tridentate nickel dichlorides (C1-C6) or dibromides (C7-C12). All nickel complexes were characterized by elemental analysis and infrared spectroscopy, while the X-ray diffraction study reveals the distorted octahedral geometry of representative nickel dichloride complex C3 and nickel dibromide complex C7. All nickel complexes, activated with either Et₃Al₂Cl ₃ or Et₂AlCl, showed good activities in ethylene oligomerization, and notably remarkably high selectivity of α-olefins was achieved in the presence of Et₃Al₂Cl₃. The chloro-substituted ligands enhanced the activities of complexes in ethylene polymerization.

Full text of DOI:10.1016/j.ica.2011.01.054

Inorganica Chimica Acta 370 (2011) 156–163
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Synthesis, characterization and ethylene oligomerization behavior of
2-(chloro-substituted-1H-benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridylnickel
dihalides
Xia Chen a, , Liping Zhang a,b, Jiangang Yu b, Xiang Hao b, Hao Liu b, Wen-Hua Sun b,
⇑
⇑
a School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China
b Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2-(chloro-substituted-1H-benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridines (L1–L6) was
synthesized and fully characterized. They reacted with NiCl2 or (DME)NiBr2 to form the corresponding
N^N^N tridentate nickel dichlorides (C1–C6) or dibromides (C7–C12). All nickel complexes were charac-
terized by elemental analysis and infrared spectroscopy, while the X-ray diffraction study reveals the
distorted octahedral geometry of representative nickel dichloride complex C3 and nickel dibromide
complex C7. All nickel complexes, activated with either Et3Al2Cl3 or Et2AlCl, showed good activities in
ethylene oligomerization, and notably remarkably high selectivity of a-olefins was achieved in the pres-
ence of Et3Al2Cl3. The chloro-substituted ligands enhanced the activities of complexes in ethylene
polymerization.
Received 23 September 2010
Received in revised form 11 January 2011
Accepted 17 January 2011
Available online 28 January 2011
Keywords:
2-Benzoimidazolyl-6-iminopyridine
Nickel complex
Ethylene oligomerization
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
gands of electron-withdrawing substituents possessed better
catalytic activities [40–43]. With this in our mind, the series of
The consumption of
a
-olefins annually is about 6 million tons
6-(chloro-substituted-1H-benzoimidazol-2-yl)-2-iminopyridines
(Scheme 1) was deemed a worthy target (Scheme 1), and the cat-
alytic behavior of corresponding nickel complexes are interesting
and compared with their analogs [33,34,37–39].
[1]. In the preparation by ethylene oligomerization, one of most
important processes employs neutral Ni(II) complexes bearing
bidentate monoanionic ligands [2–4], a system that has been com-
mercialized since the 1980s [5,6]. Recently, cationic diimino-nickel
complexes have been found to be active as procatalysts in ethylene
polymerization and oligomerization [7]. Extensive investigations
for new procatalysts of various late-transition metal complexes
have been conducted [8], and nickel complexes bearing bidentate
[9–19] or tridentate ligands [20–24] have been the focus of much
research. Beyond our efforts for iron procatalysts in ethylene reac-
tivity [25,26], some nickel procatalysts were also investigated and
possessed good activities [27–31]. More recently, the metal com-
plexes ligated by 2-(benzimidazol-2-yl)-6-(1-aryliminoethyl)pyri-
dines have been explored [32–38], and the metal complexes
ligated by 2-(1-alkylbenzimidazol-2-yl)-6-(1-aryl-iminoethyl)pyr-
idines (model A, Scheme 1) [32–35] possessed lower activity than
their analogs bearing 2-(1H-benzimidazol-2-yl)-6-(1-arylimino-
ethyl)pyridines (model B, Scheme 1) [36–38]. Extensive work
was conducted on substituting the phenyl group of the benzimi-
dazoles, and nickel complexes bearing 6-(methyl-substituted-1H-
benzoimidazol-2-yl)-2-iminopyridines were studied in ethylene
oligomerization (model C, Scheme 1) [39]. According to our previ-
ous observations, late-transition metal procatalysts containing li-
Employing the previous synthetic procedure [36,44], the
6-(chloro-substituted-1H-benzoimidazol-2-yl)-2-acetylpyridine is
synthesized with two isomers in the 3:2 ratio of the major
6-(6-chloro-1H-benzoimidazol-2-yl)-2-acetylpyridine and the
minor 6-(5-chloro-1H-benzoimidazol-2-yl)-2-acetylpyridine. Fur-
ther condensation reactions with anilines produced the corre-
sponding ligands, which when reacted with nickel halides
formed the title complexes. All organic compounds and nickel
complexes were fully characterized, including the single-crystal
X-ray diffraction analysis of complexes C3 and C7. In the presence
of Et2AlCl or Et3Al2Cl3 as cocatalyst, all nickel procatalysts showed
good activities in ethylene oligomerization. The influences on their
catalytic activities and the selectivity of producing
a-olefins have
been investigated in detail according to the nature of ligands, the
Al/Ni molar ratio and reaction temperature. Herein, the syntheses
and characterization of the title complexes are reported with their
catalytic performance in ethylene oligomerization.
2. Experimental
⇑
All experimental manipulations were performed at nitrogen
atmosphere using standard Schlenk techniques. Toluene was
Corresponding authors. Tel.: +86 10 62557955; fax: +86 10 62618239.
E-mail addresses: chenxia@sxu.edu.cn (X. Chen), whsun@iccas.ac.cn (W.-H. Sun).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.01.054

X. Chen et al. / Inorganica Chimica Acta 370 (2011) 156–163
157
8.1, 1H, Ph), 2.84 (s, 3H, CH3). dC (100 MHz, CDCl3, TMS): 199.1,
156.2, 152.1, 148.6, 146.9, 138.0, 129.8, 128.1, 123.4, 122.7, 112.1,
25.9, 17.4. 6-(5-Chloro-1H-benzoimidazol-2-yl)pyridine-2-acetyl-
pyridine: dH (400 MHz, CDCl3, TMS): 10.40 (s, 1H, NH), 8.57 (d, J
7.7, 1H, Py), 8.12 (d, J 7.7, 1H, Py), 8.02 (t, J 7.7, 1H, Py), 7.84 (s, 1H,
Ph), 7.50 (d, J 8.5, 1H, Ph), 7.31 (t, J 8.1, 1H, Ph), 2.84 (s, 3H, CH3). dC
(100 MHz, CDCl3, TMS): 199.1, 166.5, 153.2, 151.7, 148.6, 145.3,
138.0, 128.4, 128.1, 125.4, 123.8, 122.7, 111.5, 18.0.
B
A
R
N
H
N
N
N
N
N
N
N
N
N
M
M
Ar
Ar
X
X
X
X
X
X
C
2.1.2. Synthesis of 2-(chloro-substituted-1H-benzoimidazol-2-yl)-6-
(1-aryliminoethyl)pyridines (L1–L6)
H
H
N
N
N
N
2.1.2.1. 6-[1-(2,6-Dimethylphenylimino)ethyl]-2-(6-chloro-1H-ben-
zoimidazol-2-yl)pyridine and 6-[1-(2,6-dimethylphenylimio)ethyl]-
2-(5-chloro-1H-benzoimidazol-2-yl)pyridine (L1). A solution of
precursor compounds (0.82 g, 3.0 mmol), 2,6-dimethylaniline
(0.54 g, 4.50 mmol), and a catalyticamountof p-toluenesulfonic acid
dissolved in toluene (25 mL) was refluxed for 24 h. After solvent
evaporation, the crude product was purified by column chromatog-
raphy on basic Al2O3 with petroleum ether/ethyl acetate (v/v) 8:1 as
eluent to afford the product as a light yellow powder in 81.3%
(0.91 g) yield and with the ratio of 3:2. For combined isomers solids:
Mp: 173–174 °C. IR (KBr, cmꢁ1): 3454w, 3083m, 2968w, 1665vs,
1571m, 1463s, 1433s, 1314s, 1226s, 824s, 789s, 761m. Anal. Calc.
for C22H19ClN4 (374.87): C, 70.49; H, 5.11; N, 14.95. Found: C,
70.27; H, 5.19; N, 14.68%. 6-[1-(2,6-Dimethylphenylimino)ethyl]-
2-(6-chloro-1H-benzoimidazol-2-yl)pyridine: dH (400 MHz, CDCl3,
TMS): 10.54 (s, 1H, NH), 8.50 (d, J 7.8, 1H, Py), 8.45 (d, J 7.8, 1H, Py),
8.00 (t, J 7.8, 1H, Py), 7.77 (d, J 8.6, 1H, Ph), 7.56 (s, 1H, Ph), 7.28–
7.30 (m, 1H, Ph), 7.10 (d, J 7.4, 2H, Ph), 6.97 (t, J 7.5, 1H, Ph), 2.29 (s,
3H, CH3), 2.06 (s, 6H, CH3). dC (100 MHz, CDCl3, TMS): 166.3, 155.9,
151.6, 148.5, 147.0, 143.2, 137.9, 134.5, 129.7, 128.1, 125.4, 123.7,
123.4, 122.8, 122.6, 121.0, 119.9, 111.4, 18.0, 16.7. (6-[1-(2,6-Dim-
ethylphenylimino)ethyl]-2-(5-chloro-1H-benzoimidazol-2-yl)pyri-
dine: dH (400 MHz, CDCl3, TMS): 10.57 (s, 1H, NH), 8.50 (d, J 7.8, 1H,
Py), 8.45 (d, J 7.8, 1H, Py), 8.00 (t, J 7.8, 1H, Py), 7.77 (d, J 8.6, 1H, Ph),
7.84 (s, 1H, Ph), 7.48 (d, J 8.6, 1H, Ph), 7.28–7.30 (m, 1H, Ph), 7.10 (d, J
7.4, 2H, Ph), 6.97 (t, J 7.5, 1H, Ph), 2.29(s, 3H, CH3), 2.06 (s, 6H, CH3). dC
(100 MHz, CDCl3, TMS): 166.3, 155.9, 151.9, 148.5, 147.0, 145.4,
137.9, 132.5, 128.5, 128.1, 125.4, 124.6, 123.4, 122.9, 122.6, 121.0,
119.9, 112.1, 18.0, 16.7.
N
N
Cl
Ni
Ar
Ni
Ar
X
X
This work
Scheme 1. Metal complexes ligated by 6-(benzoimidazol-2-yl)-2-iminopyridines.
refluxed over sodium-benzophenone and distilled under nitrogen
prior to use. Diethylaluminum chloride (Et2AlCl, 1.7 M in toluene)
and ethylaluminum sesquichloride (Et3Al2Cl3, 1.7 M in toluene)
was purchased from Ablemarle Chemicals. 1H and 13C NMR spectra
were recorded on a Bruker DMX 400 MHz instrument at ambient
temperature using TMS as an internal standard. IR spectra were re-
corded on a Perkin–Elmer System 2000 FT-IR spectrometer. Ele-
mental analysis was carried out using a Flash EA 1112 micro-
analyzer. GC analysis was performed with a Varian CP-3800 gas
chromatograph equipped with a flame ionization detector and a
30 m (0.2 mm i.d., 0.25 lm film thickness) CP-Sil 5 CB column.
The yield of oligomers was calculated by referencing with the mass
of the solvent on the basis of the prerequisite that the mass of each
fraction was approximately proportional to its integrated areas in
the GC trace. Selectivity for the linear
(amount of linear -olefin of all fractions)/(total amount of oligo-
a-olefin was defined as
a
mer products) in percentage.
2.1. Preparation of the organic compounds and nickel complexes
2.1.1. Preparation of 6-(chloro-substituted-1H-benzoimidazol-2-yl)-
2-acetylpyridine
2.1.2.2.
6-[1-(2,6-Diethylphenylimino)ethyl]-2-(6-chloro-1H-ben-
According to the synthetic procedure of 6-(1H-benzoimidazol-2-
yl)-2-acetylpyridine [36], a mixture of diethyl 2,6-bis(b-keto-car-
boxylate)pyridine [44] (9.27 g, 0.030 mol), 1.5 equivalent molar
4-chlorinebenzene-1,2-diamine (5.49 g, 0.045 mol), and a catalytic
amount of p-toluenesulfonic acid in the 80 mL solution of toluene
and isopropyl alcohol (3:1, v/v) was refluxed for 14 h. A mixture of
20 mL of acetic acid and 5 mL of concentrated HCl was added and
continued to reflux for additional 6 h. The resultant solution was
cooledto roomtemperature, andthena 20% KOHsolutionwas added
to neutralize the solution to the pH value between 9 and 10. The
aqueous phase was extracted with ethyl acetate (20 mL ꢀ 3), all or-
ganic extracts were combined and dried over anhydrous Na2SO4.
After drying and purification by column chromatography (alumina
column 3/1 petroleum ether/ethyl acetate), the product is obtained
as white solids (2.19 g, 27.0% isolated yield) with the ratio of 3:2
for 6-(6-chloro-1H-benzoimidazol-2-yl)-2-acetylpyridine and 6-
(5-chloro-1H-benzoimidazol-2-yl)-2-acetylpyridine. For combined
isomers solids: Mp: 186–188 °C. IR (KBr, cmꢁ1): 3398s, 3081w,
3004w, 1692vs, 1454s, 1410s, 1301s, 1207s, 1054s, 922s, 824m.
Anal. Calc. for C14H10ClN3O (271.70): C, 61.89; H, 3.71; N, 15.47.
Found: C, 62.00; H, 3.75; N, 15.32%. 6-(6-Chloro-1H-ben-
zoimidazol-2-yl)pyridine-2-acetylpyridine: dH (400 MHz, CDCl3,
TMS): 10.37 (s, 1H, NH), 8.57 (d, J 7.7, 1H, Py), 8.12 (d, J 7.7, 1H, Py),
8.02 (t, J 7.7, 1H, Py), 7.77 (d, J 8.5, 1H, Ph), 7.58 (s, 1H, Ph), 7.31 (t, J
zoimidazol-2-yl)pyridine and 6-[1-(2,6-diethylphenylimino)ethyl]-2-
(5-chloro-1H-benzoimidazol-2-yl)pyridine (L2). Using the same
procedure as for thesynthesis of L1, L2 was obtained as a lightyellow
powder in 82.3% (1.0 g) yield and with the ratio of 3:2. For combined
isomers solids: Mp: 114–117 °C. IR (KBr, cmꢁ1): 3423w, 3062w,
2964m, 1646vs, 1569m, 1459s, 1416s, 1300s, 925m, 807m,
738m. Anal. Calc. for C24H23ClN4 (402.92): C, 71.54; H, 5.75;
N, 13.91. Found: C, 71.25; H, 5.91; N, 13.61%. 6-[1-(2,6-Diethylphe-
nylimino)ethyl]-2-(6-chloro-1H-benzoimidazol-2-yl)pyridine: dH
(400 MHz, CDCl3, TMS): 10.48 (s, 1H, NH), 8.50(d, J 7.7, 1H, Py),
8.45 (d, J 7.8, 1H, Py), 7.99 (t, J 7.8, 1H, Py), 7.77 (d, J 8.6, 1H, Ph),
7.55 (s, 1H, Ph), 7.28–7.30 (m, 1H, Ph), 7.13–7.15 (m, 2H, Ph),
7.06(t, J 7.3, 1H, Ph), 2.34–2.47(m, 4H, CH2), 2.30 (s, 3H, CH3), 1.15
(t, J 7.4, 6H, CH3). dC (100 MHz, CDCl3, TMS): 166.1, 156.1, 151.7,
147.5, 146.9, 142.9, 137.9, 134.6, 131.1, 128.4, 126.1, 123.6, 122.7,
122.5, 120.7, 111.5, 24.6, 16.8, 13.8. 6-[1-(2,6-Diethylphe-
nylimino)ethyl]-2-(5-chloro-1H-benzoimidazol-2-yl) pyridine: dH
(400 MHz, CDCl3, TMS): 10.50 (s, 1H, NH), 8.50(d, J 7.7, 1H, Py),
8.45 (d, J 7.8, 1H, Py), 7.99 (t, J 7.8, 1H, Py), 7.84(s, 1H, Ph), 7.47 (d, J
8.5, 1H, Ph), 7.28–7.30 (m, 1H, Ph), 7.13–7.15 (m, 2H, Ph), 7.06 (t, J
7.3, 1H, Ph), 2.34–2.47(m, 4H, CH2), 2.30 (s, 3H, CH3), 1.15 (t, J 7.4,
6H, CH3). dC (100 MHz, CDCl3, TMS): 166.1, 156.1, 152.1, 147.5,
146.9, 137.9, 132.6, 131.1, 129.7, 126.1, 124.5, 123.6, 122.6, 119.6,
112.2, 24.6, 16.8, 13.8.

158
X. Chen et al. / Inorganica Chimica Acta 370 (2011) 156–163
2.1.2.3. 6-[1-(2,6-Diisopropylphenylimino)ethyl]-2-(6-chloro-1H-ben-
zoimidazol-2-yl)pyridine and 6-[1-(2,6-diisopropylphenylimino)ethyl]
-2-(5-chloro-1H-benzoimidazol-2-yl)pyridine (L3). Using the same
procedures for the synthesis of L1, L3 was obtained as a light yellow
powder in 87.4% (1.13 g) yield and with the ratio of 3:2. For combined
isomers solids: Mp: 135–139 °C. IR (KBr, cmꢁ1): 3449w, 3064w,
1646vs, 1583s, 1569s, 1533w, 1424m, 1316s, 1157m, 1057m, 928s,
812m, 736m. Anal. Calc. for C26H27ClN4 (430.97): C 72.46, H 6.31, N
13.00. Found: C, 72.61; H, 6.59; N, 12.75%. 6-[1-(2,6-Diisopropylphe-
137.9, 134.7, 132.7, 131.0, 129.7, 126.8, 123.7, 122.7, 122.5, 120.6,
24.8, 21.0, 16.7, 13.9. 6-[1-(2,6-Diethyl-4-methylphenylimi-
no)ethyl]-2-(5-chloro-1H-benzoimidazol-2-yl)
pyridine:
dH
(400 MHz, CDCl3, TMS): 10.54 (s, 1H, NH), 8.49 (d, J 7.5, 1H, Py), 8.42
(d, J 7.9, 1H, Py), 7.98 (t, J 7.8, 1H, Py), 7.84 (s, 1H, Ph), 7.47 (d, J 8.5,
1H, Ph), 7.26–7.29 (m, 1H, Ph), 6.95 (s, 2H, Ph), 2.35–2.39 (m, 7H,
CH2, CH3), 2.30 (s, 3H, CH3), 1.14 (t, 6H, J 7.4, CH3). dC (100 MHz, CDCl3,
TMS): 166.3, 156.3, 152.2, 146.8, 145.3, 145.0, 137.9, 134.7, 132.7,
131.0, 128.4, 125.2, 126.8, 124.5, 122.8, 122.6, 119.6, 24.8, 21.0,
16.7, 13.9.
nylimino)ethyl]-2-(6-chloro-1H-benzoimidazol-2-yl)pyridine:
dH
(400 MHz, CDCl3, TMS): 10.51 (s, 1H, NH), 8.50 (d, J 7.6, 1H, Py), 8.45
(d, J 7.8, 1H, Py), 8.00 (t, J 7.7, 1H, Py), 7.77 (d, J 8.6, 1H, Ph), 7.55 (s,
1H, Ph), 7.28–7.30 (m, 1H, Ph), 7.18–7.20 (m, 2H, Ph), 7.13–7.15 (m,
1H, Ph), 2.72–2.78 (m, 2H, CH), 2.32 (s, 3H, CH3), 1.17 (d, J 6.6, 12H,
CH3). dC (100 MHz, CDCl3, TMS): 166.1, 156.0, 151.7, 146.9, 146.2,
143.0, 137.9, 135.7, 134.6, 129.7, 124.6, 123.9, 123.1, 122.7, 122.7,
122.5, 119.7, 111.4, 28.3, 23.2, 22.8. 6-[1-(2,6-Diisopropylphenylimi-
no)ethyl]-2-(5-chloro-1H-benzoimidazol-2-yl)pyridine: dH (400
MHz, CDCl3, TMS): 10.53 (s, 1H, NH), 8.50 (d, J 7.6, 1H, Py), 8.45 (d, J
7.8, 1H, Py), 8.00 (t, J 7.7, 1H, Py), 7.84 (s, 1H, Ph), 7.47 (d, J 8.5, 1H,
Ph), 7.28–7.30 (m, 1H, Ph), 7.18–7.20 (m, 2H, Ph), 7.13–7.15 (m, 1H,
Ph), 2.72–2.78 (m, 2H, CH), 2.32 (s, 3H, CH3), 1.17 (d, J 6.6, 12H,
CH3). dC (100 MHz, CDCl3, TMS): 166.1, 156.0, 152.0, 146.8, 146.2,
145.2, 137.9, 135.7, 132.5, 128.4, 123.9, 123.7, 123.1, 122.8, 122.6,
120.7, 112.2, 28.3, 23.2, 22.8.
2.1.2.6.
zoimidazol-2-yl)pyridine and 6-[1-(2,6-dichlorophenylimino)ethyl]-
2-(6-chloro-1H-benzoimidazol-2-yl)pyridine (L6). Using similar
6-[1-(2,6-Dichlorophenylimino)ethyl]-2-(6-chloro-1H-ben-
a
procedure to that for the synthesis of L1, with silicic acid tetraethyl
ester employed as solvent instead of toluene, L6 was obtained as a
white powder in 27.3% yield and with the ratio of 3:2. For com-
bined isomers solids: Mp: 228–230 °C. IR (KBr, cmꢁ1): 3402m,
3082w, 1648vs, 1568m, 1460m, 1308s, 1222s, 925m, 789m,
738s. Anal. Calc. for C20H13N4Cl3 (415.7): C, 57.79; H, 3.15; N,
13.48. Found: C, 57.88; H, 3.19; N, 13.11%. 6-[1-(2,6-Dichlorophe-
nylimino)ethyl]-2-(6-chloro-1H-benzoimidazol-2-yl)pyridine: dH
(400 MHz, CDCl3, TMS): 10.34 (s, 1H, NH), 8.53 (d, J 7.7, 1H, Py),
8.47 (d, J 7.8, 1H, Py), 8.02 (t, J 7.9, 1H, Py), 7.77 (d, J 8.9, 1H, Ph),
7.57 (s, 1H, Ph), 7.39 (d, J 8.0, 2H, Ph), 7.17–7.30 (m, 2H, Ph),
7.03 (t, J 7.9, 1H, Ph), 2.42 (s, 3H, CH3). dC (100 MHz, CDCl3, TMS):
170.4, 155.2, 151.2, 146.9, 145.3, 143.1, 138.0, 134.2, 129.7,
128.3, 124.6, 124.4, 123.6, 123.2, 119.9, 111.3, 17.8. 6-[1-
(2,6-Dichlorophenylimino)ethyl]-2-(5-chloro-1H-benzoimidazol-
2-yl)pyridine: dH (400 MHz, CDCl3, TMS): 10.37 (s, 1H, NH), 8.53 (d,
J 7.7, 1H, Py), 8.47 (d, J 7.8, 1H, Py), 8.02 (t, J 7.9, 1H, Py), 7.85 (s, 1H,
Ph), 7.49 (d, J 8.9, 1H, Ph), 7.39 (d, J 8.0, 2H, Ph), 7.17–7.30 (m, 2H,
Ph), 7.03 (t, J 7.9, 1H, Ph), 2.42 (s, 3H, CH3). dC (100 MHz, CDCl3,
TMS): 170.4, 155.2, 151.6, 146.9, 145.3, 143.1, 138.1, 132.3,
128.4, 128.3, 124.6, 124.4, 123.3, 123.2, 121.0, 112.0, 17.8.
2.1.2.4. 6-[1-(2,4,6-Trimethylphenylimino)ethyl]-2-(6-chloro-1H-ben-
zoimidaz ol-2-yl)pyridine and 6-[1-(2,4,6-trimethylphenylimi-
no)ethyl]-2-(5-chlo ro-1H-benzoimidazol-2-yl)pyridine (L4). Using
the same procedure as for the synthesis of L1, L4 was obtained
as a light yellow powder in 66.7% (0.78 g) yield and with the ratio
of 3:2. For combined isomers solids: Mp: 200 °C. IR (KBr, cmꢁ1):
3460m, 2918m, 1647vs, 1568s, 1465m, 1428m, 1324s, 1125m,
854s, 824m, 753s. Anal. Calc. for C23H21ClN4 (388.89): C, 71.03;
H, 5.44; N, 14.41. Found: C, 71.09; H, 5.50; N, 14.31%. 6-[1-(2,4,6-
Trimethyl-phenylimino)ethyl]-2-(6-chloro-1H-benzoimidazol-2-yl)
pyrindine: dH (400 MHz, CDCl3, TMS): 10.60 (s, 1H, NH), 8.49 (d, J 7.7,
1H, Py), 8.41 (d, J 7.9, 1H, Py), 7.98 (t, J 7.8, 1H, Py), 7.76 (d, J 8.7, 1H,
Ph), 7.55 (s, 1H, Ph), 7.28–7.30 (m, 1H, Ph), 6.91(s, 2H, Ph), 2.30
(s, 3H, CH3), 2.28 (s, 3H, CH3), 2.02 (s, 6H, CH3). dC (100 MHz,
CDCl3, TMS): 166.6, 156.2, 151.7, 147.0, 146.0, 143.1, 137.9, 132.6,
128.5, 125.2, 123.7, 122.8, 122.6, 119.8, 111.5, 20.9, 17.9, 16.6.
6-[1-(2,4,6-Trimethylphenylimino)ethyl]-2-(5-chloro-1H-benzoi-
midazol-2-yl)pyridine: dH (400 MHz, CDCl3, TMS): 10.63 (s, 1H,
NH), 8.49 (d, J 7.7, 1H, Py), 8.41 (d, J 7.9, 1H, Py), 7.98 (t, J 7.8,
1H, Py), 7.84(s, 1H, Ph), 7.47(d, J 8.4, 1H, Ph), 7.28–7.30 (m, 1H,
Ph), 6.91(s, 2H, Ph), 2.30 (s, 3H, CH3), 2.28 (s, 3H, CH3), 2.02 (s,
6H, CH3). dC (100 MHz, CDCl3, TMS): 166.6, 156.2, 152.1, 146.9,
146.0, 145.3, 134.6, 132.6, 129.7, 128.7, 125.2, 124.6, 122.9,
122.6, 120.9, 112.3, 20.9, 17.9, 16.6.
2.1.3. Synthesis of tridentate nickel complexes C1–C12
Complexes C1–C6 were prepared by the same synthetic proce-
dures and obtained as a yellow powder. The synthetic procedure of
nickel dichloride complex C1 can be described as follows: to a mix-
ture of ligand L1 (0.131 g, 0.35 mmol) and NiCl2ꢂ6H2O (0.083 g,
0.35 mmol) was added freshly distilled ethanol (5 mL) at room
temperature. The solution turned yellow immediately. The reac-
tion mixture was stirred for 10 h, and the precipitate was collected
by filtration and washed with diethyl ether, followed by drying in
vacuum. The desired complex was obtained as a yellow powder in
good yield (81.2%, 0.14 g). IR (KBr, cmꢁ1): 3269m, 2968w, 1596vs,
1482m, 1465m, 1416s, 1315s, 1212s, 1051s, 805s, 771m. Anal.
Calc. for C22H19Cl3N4Ni (504.47): C, 52.38; H, 3.80; N, 11.11. Found:
C, 52.63; H, 3.57; N, 11.35%.
Data for C2 Yield: 78.3% (0.15 g). IR (KBr, cmꢁ1): 3087w, 2979m,
1594vs, 1462m, 1407s, 1317s, 1054s, 928s, 816m, 799m, 748m.
Anal. Calc. for C24H23Cl3NiN4(532.52): C, 54.13; H, 4.35; N, 10.52.
Found: C, 53.97; H, 4.17; N, 10.72%.
2.1.2.5. 6-[1-(2,6-Diethyl-4-methylphenylimino)ethyl]-2-(6-ch-
loro-1H-benzoimidazol-2-yl)pyridine and 6-[1-(2,6-diethyl-4-met-
hylphenylimino)ethyl]-2-(5-chloro-1H-benzoimidazol-2-yl)pyridine
(L5). Using the same procedure as for the synthesis of L1, L5 was ob-
tainedasalightyellowpowderin67.3% yieldandwiththeratioof3:2.
Forcombinedisomerssolids:Mp: 159–160 °C. IR(KBr,cmꢁ1):3448w,
2965m, 1645vs, 1569m, 1461s, 1415s, 1300s, 1124m, 925m, 859s,
738m. Anal. Calc. for C25H25ClN4 (416.95): C, 72.02; H, 6.04; N,
13.44. Found: C, 72.09; H, 6.21; N, 13.15%. 6-[1-(2,6-Diethyl-4-meth-
Data for C3 Yield: 81.7% (0.16 g). IR (KBr, cmꢁ1): 3406m, 3021w,
2961s, 1597vs, 1462m, 1434m, 1318s, 1057m, 824s, 792s, 751s.
Anal. Calc. for C26H27Cl3NiN4 (560.57): C, 55.71; H, 4.85; N, 9.99.
Found: C, 55.82; H, 4.61; N, 10.21%.
Data for C4 Yield: 83.2% (0.15 g). IR (KBr, cmꢁ1): 3308m, 2981w,
1595vs, 1483m, 1415s, 1316s, 1218s, 933m, 815s, 784m. Anal. Calc.
for C23H21Cl3NiN4 (518.49): C, 53.28; H, 4.08; N, 10.81. Found: C,
53.47; H, 4.10; N, 10.62%.
ylphenylimino)ethyl]-2-(6-chloro-1H-benzoimidazol-2-yl)
pyri-
dine: dH (400 MHz, CDCl3, TMS): 10.51(s, 1H, NH), 8.49 (d, J 7.5, 1H,
Py), 8.42 (d, J 7.9, 1H, Py), 7.98 (t, J 7.8 Hz, 1H, Py), 7.77 (d, J 8.6, 1H,
Ph), 7.55(s, 1H, Ph), 7.26–7.29 (m, 1H, Ph), 6.95 (s, 2H, Ph), 2.35–
2.39 (m, 7H, CH2, CH3), 2.30 (s, 3H, CH3), 1.14 (t, J 7.4, 6H, CH3). dC
(100 MHz, CDCl3, TMS): 166.3, 156.3, 151.8, 146.9, 145.0, 142.9,
Data for C5 Yield: 80.7% (0.15 g). IR (KBr, cmꢁ1): 3398s, 2968m,
1599vs, 1487w, 1413s, 1315s, 858s, 813s, 749m. Anal. Calc. for
C25H25Cl3NiN4 (546.55): C, 54.94; H, 4.61; N, 10.25. Found: C,
55.04; H, 4.28; N, 10.35%.

X. Chen et al. / Inorganica Chimica Acta 370 (2011) 156–163
159
Table 1
Anal. Calc. for C25H25ClBr2NiN4 (635.45): C, 47.25; H, 3.97; N,
8.82. Found: C, 47.38; H, 4.03; N, 8.57%.
Crystal data and structure refinement for [L3NiClꢂCH3OHꢂDMF]Cl (C3) and
[L1NiBrꢂ2CH3OH]Br (C7).
Data for C12 Yield: 70.7% (0.16 g). IR (KBr, cmꢁ1): 3309s, 2936w,
1599vs, 1436s, 1414s, 1316m, 933m, 814m, 785m. Anal. Calc. for
[L3NiClꢂCH3OHꢂDMF]Cl [L1NiBrꢂ2CH3OH]Br
(C3)
(C7)
C20H13Cl3Br2NiN4 (634.2): C, 37.88; H, 2.07; N, 8.83. Found: C,
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C
31H42Cl3N5NiO3
C24H26Br2ClN4NiO2
656.43
173(2)
0.71073
monoclinic
c2/c
30.367(9)
13.586(3)
14.519(3)
90
116.18(4)
90
5376(2)
8, 1.622
3.822
2631
0.20 ꢀ 0.10 ꢀ 0.08
1.49–27.44
ꢁ39 6 h 6 39
ꢁ11 6 k 6 17
ꢁ13 6 l 6 18
21398
38.13; H, 2.27; N, 8.65%.
697.76
173(2)
0.71073
triclinic
2.2. X-ray crystallographic studies
ꢀ
P1
The crystals of [L3NiClꢂCH3OHꢂDMF]Cl (C3) was obtained by
laying diethyl ether on the solution of mixed solvents of methanol
and dimethyl formamide (DMF) at room temperature and
[L1NiBrꢂ2CH3OH]Br (C7) was obtained by laying diethyl ether on
8.8411(18)
14.298(3)
14.727(3)
103.57(3)
99.01(3)
105.72(3)
1693.0(6)
2, 1.367
0.848
732
0.25 ꢀ 0.08 ꢀ 0.05
1.79–27.48
ꢁ11 6 h 6 11
ꢁ18 6 k 6 18
ꢁ19 6 l 6 19
22981
b (Å)
c (Å)
a
(°)
b (°)
a methanol solution at room temperature. With graphite-mono-
0
c
(°)
chromated Mo Ka radiation (k = 0.71073 ÅA) at 173(2) K, cell param-
eters were obtained by global refinement of the positions of all
collected reflections. Intensities were corrected for Lorentz and
polarization effects and empirical absorption. The structures were
solved by direct methods and refined by full-matrix least squares
on F2. All hydrogen atoms were placed in calculated positions.
Structure solution and refinement were performed by using the
SHELXL-97 package [45]. Details of the X-ray structure determina-
tions and refinements are provided in Table 1.
V (Å3)
Z, Dcalc (g cmꢁ3
l
)
(mmꢁ1
)
F(0 0 0)
Crystal size (mm)
h Range (°)
Limiting indices
Reflections collected
Independent reflections
7759 [R(int) = 0.0573]
6129
[R(int) = 0.1212]
317
99.8
2.3. Procedure for ethylene oligomerization
Number of parameters
Completeness to h (%)
Goodness of fit on F2
409
99.9
1.078
1.245
Ethylene oligomerizations at 10 atm ethylene pressures were
carried out in a 500 mL stainless steel autoclave equipped with a
mechanical stirrer and a temperature controller. Toluene, the de-
sired amount of cocatalyst, and a toluene solution of the catalytic
precursor (the total volume was 100 mL) were added to the reactor
in this order under an ethylene atmosphere. When the desired
reaction temperature was reached, the ethylene pressure was in-
creased to 10 atm, and maintained at this level by constant feeding
of ethylene. After desired reaction time, the reaction was stopped
by cooling the reactor on an ice bath before the excess pressure
was released. A small amount of the reaction solution was col-
lected, the reaction was terminated by the addition of 5% aqueous
hydrogen chloride, and then this mixture was analyzed by gas
chromatography (GC) to determine the distribution of oligomers
obtained. The remaining reaction solution was quenched with
HCl-acidified ethanol (5%).
Final R indices [I > 2
r
(I)]
R1 = 0.0761
wR2 = 0.1925
R1 = 0.0834
wR2 = 0.1984
3.773 and ꢁ1.556
R1 = 0.1288
wR2 = 0.2427
R1 = 0.1521
wR2 = 0.2542
1.171 and ꢁ0.869
R indices (all data)
Maximum/minimum
(e Åꢁ3
Dq
[a]
)
Data for C6 Yield: 62.9% (0.12 g). IR (KBr, cmꢁ1): 3369s, 3068w,
1600vs, 1485m, 1436m, 1317s, 1059m, 934m, 814m. Anal. Calc. for
20H13Cl5NiN4 (545.30): C, 44.05; H, 2.40; N, 10.27. Found: C,
C
44.27; H, 2.43; N, 10.03%.
Complexes C7–C12 were synthesized by the reaction of
(DME)ꢂNiBr2 with the corresponding ligands in tetrahydrofuran
(THF). A typical synthetic procedure for nickel dibromide complex
C7 is as follows: to a mixture of ligand L1 (0.131 g, 0.35 mmol) and
(DME)ꢂNiBr2 (0.107 g, 0.35 mmol) was added tetrahydrofuran
(5 mL) at room temperature, and the mixture was stirred for
12 h. The precipitate was collected by filtration which was washed
with ether and dried in vacuo. The complex C7 was obtained as yel-
low powder with isolate yield: 89.3% (0.19 g). IR (KBr, cmꢁ1):
3326s, 3069w, 1592vs, 1485w, 1412s, 1315s, 1126m, 933m,
805s. Anal. Calc. for C22H19ClBr2NiN4 (593.37): C, 44.53; H, 3.23;
N, 9.44. Found: C, 44.51; H, 2.99; N, 9.53%.
3. Results and discussion
3.1. Synthesis and characterization of2-(chloro-substituted-1H-benzo-
imidazol-2-yl)-6-(1-arylimino ethyl)pyridines and their nickel
complexes
The
6-(chloro-substituted-1H-benzoimidazol-2-yl)-2-acetyl-
pyridine was synthesized by the condensation reaction of
4-chloro-1,2-diaminobenzene with 2,6-bis(b-keto-carboxylate)pyri-
dine [39,44] according to our previous procedure [36]. The resul-
tant mixture contained two isomers in the molar ratios of 3:2
with major 6-(6-chloro-1H-benzoimidazol-2-yl)-2-acetylpyridine
and minor 6-(5-chloro-1H-benzoimidazol-2-yl)-2-acetylpyridine
according to the 1H NMR spectrum. Further condensation reaction
of 6-(chloro-1H-benzoimidazol-2-yl)-2-acetylpyridine with aniline
derivatives gave the corresponding compounds, 2-(chloro-1H-ben-
zo-imidazol-2-yl)-6-(1-aryliminoethyl)pyridines (L1–L6), which
also have two isomers with the same molar ratios as the acetylpyr-
idine substance. According to their 1H NMR spectra, there was no
difference for products in the early and late eluting fractions on
column chromatography, which indicated fast isomerization of
the two isomers in each producing mixture. The stoichiometric
Data for C8 Yield: 87.1% (0.19 g). IR (KBr, cmꢁ1): 3360m, 2935s,
1599vs, 1487m, 1413vs, 1316s, 1184w, 868w, 796s. Anal. Calc. for
24H23ClBr2NiN4 (621.42): C, 46.39; H, 3.73; N, 9.02. Found: C,
C
46.65; H, 3.57; N, 9.18%.
Data for C9 Yield: 89.4% (0.20 g). IR (KBr, cmꢁ1): 3378s, 2961m,
1593vs, 1464m, 1321s, 1178w, 814m, 792s. Anal. Calc. for
C
26H27ClBr2NiN4 (649.47): C, 48.08; H, 4.19; N, 8.63. Found: C,
48.25; H, 4.01; N, 8.65%.
Data for C10 Yield: 90.0% (0.19 g). IR (KBr, cmꢁ1): 3372s, 3059w,
1593vs, 1485m, 1412s, 1316s, 1218m, 853m, 816s. Anal. Calc. for
C
23H21ClBr2NiN4 (607.39): C, 45.48; H, 3.48; N, 9.22. Found: C,
45.37; H, 3.38; N, 9.25%.
Data for C11 Yield: 81.3% (0.18 g). IR (KBr, cmꢁ1): 3323s, 2937w,
1595vs, 1458s, 1417m, 1415s, 1314s, 1214m, 932m, 813s, 747w.

160
X. Chen et al. / Inorganica Chimica Acta 370 (2011) 156–163
H2N
Cl
H
N
H
N
N
EtO
OEt H2N
N
+
N
O
N
O
N
Cl
O
O
O
O
Major
Minor
Cl
R1
R1
R2
NH2
H
N
R1
N
N
N
Cl
H
H
N
Ni
N
R1
N
R1
N
X
X
X
X
NiCl2 or
(DME)NiBr2
R2
R1
Major
+
N
N
N
N
Cl
H
N
R2
R1
Major
R2
R1
Cl
R1
N
Minor
L1 L2 L3 L4 L5 L6
Me Et iPr Me Et Cl
N
N
Ni
R1
R2
R2
R1
Cl
Minor
H
H
H Me Me H
X = Cl, C1 - C6; X = Br, C7 - C12
Scheme 2. Synthesis of nickel complexes.
reactions with NiCl2ꢂ6H2O in ethanol gave their dichloronickel
complexes (C1–C6), and with (DME)ꢂNiBr2 in tetrahydrofuran
(THF) gave dibromonickel complexes (C7–C12) (Scheme 2). All
nickel dihalide complexes were air-stable yellow solids. Compared
with the IR spectra of the free ligands, the C@N stretching vibra-
tions in complexes C1–C12 were shifted to lower frequencies in
the range 1590–1560 cmꢁ1, indicating an effective coordination
interaction between the imino nitrogen atom and the nickel center.
Crystals of C3 suitable for single-crystal X-ray diffraction were
obtained by slowly diffusing diethyl ether into its solution of
mixed solvents of dimethyl formamide (DMF) and methanol solu-
tions, meanwhile the crystals of C7 suitable for single-crystal X-ray
diffraction were obtained by laying diethyl ether on its methanol
solution. The single-crystal X-ray diffraction study indicated two
isomers of each nickel complex, in which the major isomer
contained its ligands with substituted chlorine far away from the
nickel center (Scheme 2). In our previous paper [39], 2-(7-methyl-
1H-benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridylnickel dihali-
des were only obtained in the reaction of nickel halides with the
isomeric mixture of 2-(7-methyl-1H-benzoimidazol-2-yl)-6-(1-
aryliminoethyl)pyridines and 2-(4-methyl-1H-benzoimidazol-2-
yl)-6-(1-aryliminoethyl)pyridines, which was isomerized due to
the methyl steric hindrance in coordinating with nickel. In current
system, the chloro-substituent of ligands is one more carbon away
from the coordination nitrogen and as a consequence exerts a smal-
ler bulky effect on nickel.
idine ring are 87.2° and 91.3°, respectively. The spherical coordi-
nated cationic nickel center is far from the anionic Cl(1) and is
thus considered non-bonding. The disorder phenomena of Cl(3),
caused with two isomers of the ligand, is reflected with the high
spurious peak (3.773 e Å).
Similar coordination was observed in the molecular structure of
complex C7, shown in Fig. 2. The axial plane (O(1)–Ni(1)–O(2)) is
nearly perpendicular to the equatorial plane which is formed by
N(2), N(3) and N(4) with a dihedral angle of 88.9°. Comparison
with the above [L3NiClꢂCH3OHꢂDMF]Cl, the N–Ni lengths of
[L1NiBrꢂ2CH3OH]Br are a little shorter.
In summary, both complexes possessed
a six-coordinated
geometry at the nickel center with two additional molecules of
coordinated solvents. The likely disorder of chlorine on the phenyl
of the benzimidazole was caused by two isomers of the ligands. All
complexes for elemental analysis and for further catalytic screen-
ing were dried for 4 h under vacuum before experimental
processes.
3.2. Ethylene oligomerization
Though there are two isomers of individual nickel complexes,
the similar catalytic activities are assumed because the chloro-sub-
stituent is far from nickel and has little steric influence.
3.2.1. Ethylene activation in the presence of Et2AlCl
The two isomers of complex C3 show a disorder of the chlorine
atom in its solid state the major position having the chlorine far
away from nickel center. In its structure, there were two coordi-
nated molecules of dimethyl formamide (DMF) and methanol sol-
vents around nickel to form a cationic ion, meanwhile a free
chloride was observed. The molecular structure of [L3NiClꢂ-
CH3OHꢂDMF]Cl is shown in Fig. 1, and the selected bond lengths
When procatalyst C4 was activated with diethylaluminum chlo-
ride (Et2AlCl) at 1 atm ethylene pressure, there was low activity
observed. Increasing the ethylene pressure up to 10 atm, good
activity was achieved. The reaction parameters, such as molar ratio
of Et2AlCl to nickel and the reaction temperature, were varied for
the optimum condition. The catalytic activity improved with
increasing Al/Ni ratio from 300 up to 500 (Entries 1–3, Table 3),
however, lower activity was observed on further increasing the
Al/Ni ratio to 600 or 700 (Entries 4–5, Table 3). Using a reaction
temperature within 30–70 °C (Entries 3, 6–9, Table 3), the highest
activity is obtained at 50 °C (Entry 3, Table 3). Comparison with the
analog procatalysts performing the best activities at ambient reac-
tion temperature [33–38], the current system shows good activity
at a higher temperature 50 °C, which is an important practical is-
sue for oligomerization at higher reaction temperature. The
and angles are collected in Table 2. The nickel atom deviates by
0
0.0442 ÅA from the plane containing N(2), N(3), and N(4). The
0
Cl(2) deviates by 0.2718 ÅA from the plane of N(2), N(3) and N(4).
The basal plane composed by N(2), N(3) and N(4) is almost copla-
nar to the pyridyl ring with a dihedral angle of 4.7°, the pyridyl ring
and the benzimidazole ring are also nearly coplanar with a dihe-
dral angle of 5.2°. The dihedral angles between the phenyl plane
with the benzimidazole ring and of the phenyl plane with the pyr-

X. Chen et al. / Inorganica Chimica Acta 370 (2011) 156–163
161
investigations of nickel complexes were carried out with the opti-
mum condition at Al/Ni ratio 500 at 50 °C within 30 min.
On the bases of different halides, the title procatalysts could be
separated into two groups, dichlorides C1–C6 and dibromides C7–
C12. All chlorides procatalysts C1–C6 (Entries 3, 15–19, Table 3),
and the bromides analogs C7–C12 (Entries 20–25, Table 3) showed
high activities. It was found that the higher catalytic activity was
observed with bromide procatalysts over chloride analogs
[34,37,38], possibly due to solubility of metal bromide in organic
solvents which is usually better than that of chloride analogs. In
this work, the complexes showed better activities, the possible
cause is the significant electronic influence by the chloro-substi-
tuted phenyl group of the benzoimidazoles. In addition, it was con-
firmed the optimum reaction temperature at 50 °C by procatalyst
C10 (Entries 23, 26–27, Table 3).
As shown for other nickel analogs procatalysts [34–38], the less
bulky ligands (varying different substituents of the imino N-aryl
rings) enhance catalyst activities of the corresponding nickel com-
plexes with the activity order as C1 > C2 > C3 (Entries 15–17, Table
3) and C7 > C8 > C9 (Entries 20–22, Table 3). However, the 2,4,6-
trimethyl-substituted procatalysts C4 and C10 showed the highest
catalytic activity in each set of complexes (Entries 3 and 23, Table
3). Regarding nickel analogs [34,37,38], two exceptions are procat-
alysts C6 (Entry 19, Table 3) and C12 (Entry 25, Table 3) bearing the
ligand containing 2,6-dichlorophenyl which showed higher cata-
lytic activities than some of their analogs of ligands having dialkyl-
phenyl (e.g. C6 > C5 > C3 and C12 > C11 > C8 > C9). This
observation is consistent with our simulation results [39], where
there is a turnover point between the catalytic activities and the
net-charge of the metal active species in catalytic system.
Fig. 1. ORTEP structure of [L3NiClꢂCH3OHꢂDMF]Cl (C3). Thermal ellipsoids are shown
at 30% probability level. Hydrogen atoms have been omitted for clarity.
Table 2
Selected bond lengths and angles for complexes C3 and C7.
[L3NiClꢂCH3OHꢂDMF]Cl (C3)
[L1NiBrꢂ2CH3OH]Br (C7)
Bond lengths (Å)
Ni–N2
Ni–N3
Ni–N4
Ni–X
N1–C7
N2–C7
N4–C13
Ni–O1
Ni–O2
2.146(3)
2.026(3)
2.244(3)
2.2926(14)
1.348(5)
1.326(5)
1.282(5)
2.068(3)
2.128(3)
2.127(9)
2.038(9)
2.182(10)
2.4199(18)
1.384(14)
1.321(14)
1.296(14)
2.180(8)
3.2.2. Ethylene activation in the presence of ethylaluminum
sesquichloride (Et3Al2Cl3)
2.126(8)
Bond angles (°)
N2–Ni–N3
N2–Ni–N4
N3–Ni–N4
N2–Ni–X
N3–Ni–X
N4–Ni–X
O1–Ni–O2
In addition, ethylaluminum sesquichloride (Et3Al2Cl3) was used
as the cocatalyst; the ethylene dimerization was predominant with
some trimerization (Table 4). The procatalyst C10 was routinely
studied in finding the optimum condition of Al/Ni molar ratio
and reaction temperature. As observed in using Et2AlCl as cocata-
lyst, the best activity was achieved at 50 °C but using slightly high-
er Al/Ni molar ratio 600 than for the system with Et3Al2Cl3. In the
catalytic systems with Et3Al2Cl3, all procatalysts displayed higher
78.16(13)
153.45(13)
75.34(13)
104.68(10)
174.74(9)
101.87(10)
171.44(12)
78.2(4)
153.7(4)
75.8(4)
102.1(3)
174.1(2)
104.1(3)
170.8(3)
catalytic activities, surprisingly the selectivity of
a-olefin was quite
high (>99%). The high selectivity for -olefin is a quite unique
a
property in nickel catalytic systems, because a high proportion of
inert olefins are obtained due to chain-immigration and b-hydro-
gen elimination on active nickel species. Comparing data by the ti-
tle procatalysts, there is no significant difference between catalytic
systems of chlorides and bromides.
The catalytic behavior of all the nickel procatalysts with
Et3Al2Cl3, as affected by the substituents of the imino N-aryl rings,
is similar as their counterpart systems with Et2AlCl. The activities
were in the order C1 (with substituent of dimethyl) > C2 (with
diethyl) > C3 (with diisopropyl) (Entries 10–12, Table 4) and C7
(with substituent of dimethyl) > C8 (with diethyl) > C9 (with diiso-
propyl) (Entries 16–18, Table 4). In addition, complexes C6 and C12
(Entries 15 and 20, Table 4) bearing the 2,6-dichlorophenyl group
maintained good activity as for the analog complexes containing
2,6-dialkylphenyl.
Fig. 2. ORTEP molecular structure of [L1NiBrꢂ2CH3OH]Br (C7). Thermal ellipsoids are
shown at 30% probability level. Hydrogen atoms have been omitted for clarity.
advantage of maintaining good activity at reasonable reaction tem-
perature is due to the effect of the chlorine substituent on phenyl
group of benzimidazole. The lifetime of the active species was
investigated (Entries 3, 10–14, Table 3), the values of the activities
gradually decreased with prolonged reaction time, indicating no
initial stage required, and the oligomerization terminated after
4. Conclusions
A series of new nickel procatalysts ligated by 2-(chloro-substi-
tuted-1H-benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridines were
synthesized and fully characterized. Using the Et2AlCl as activators,
the catalytic activity of 2.6 ꢀ 106 g molꢁ1 Ni hꢁ1 was reached in
40 min (Entries 12–14, Table 3). As
a consequence, further

162
X. Chen et al. / Inorganica Chimica Acta 370 (2011) 156–163
Table 3
Ethylene catalytic activity with nickel complexes using Et2AlCl.a
Entry
Precatalyst
Al/Ni
T (°C)
t (min)
Activityc
Oligomer distributionb
C4/ C6/
4.0
R
C
R
C
a-Olefin (%)
1
2
3
4
5
6
7
8
C4
C4
C4
C4
C4
C4
C4
C4
C4
C4
C4
C4
C4
C4
C1
C2
C3
C5
C6
C7
C8
C9
C10
C11
C12
C10
C10
300
400
500
600
700
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
500
50
50
50
50
50
30
40
60
70
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
40
60
30
30
30
30
30
30
30
30
30
10
20
40
50
60
30
30
30
30
30
30
30
30
30
30
30
30
30
5.4
20.3
25.9
19.1
13.3
4.5
10.1
9.4
5.8
42.8
34.2
19.8
15.9
13.2
18.2
15.7
12.3
12.5
14.6
13.1
9.3
96.0
91.8
89.2
90.9
92.9
97.1
95.7
90.7
94.4
87.8
88.6
89.4
89.4
89.4
93.4
95.3
92.5
91.6
92.0
92.2
93.6
91.6
90.0
93.3
91.8
98.6
93.6
93.1
60.1
64.4
60.3
73.3
41.2
80.9
72.4
82.3
63.2
65.3
64.1
65.3
65.5
70.6
95.4
70.7
68.2
64.0
74.5
75.1
71.8
65.6
72.8
67.4
61.9
72.7
8.2
10.8
9.1
7.1
2.9
4.3
9.3
5.6
12.2
11.4
10.6
10.6
10.6
6.6
4.7
7.5
8.4
8.0
7.8
6.4
8.4
10.0
6.7
8.2
1.4
6.4
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
8.6
17.1
9.5
12.4
8.8
5.7
a
b
c
Reaction condition: 5
Determined by GC.
lmol; Et2AlCl; 10 atm ethylene; 100 mL toluene.
105 g molꢁ1 Ni hꢁ1
.
Table 4
chloro-modified ligands performed higher activities at elevated
reaction temperature 50 °C, compared with analogs’ procatalysts
with optimum temperature at room temperature [34,37,38].
Ethylene catalytic activity with nickel complexes using Et3Al2Cl3.a
Entry Precatalyst Al/Ni T (°C) Activityc Oligomer distributionb
C4/R
C
C6/
R
C
a-Olefin (%)
Acknowledgments
1
2
3
4
5
6
7
8
C10
C10
C10
C10
C10
C10
C10
C10
C10
C1
C2
C3
C4
C5
C6
C7
C8
C9
200 20
400 20
600 20
800 20
1000 20
600 40
600 50
600 60
600 80
600 50
600 50
600 50
600 50
600 50
600 50
600 50
600 50
600 50
600 50
600 50
7.1
7.7
8.8
7.5
6.7
10.7
13.5
9.4
5.5
7.1
5.2
2.2
8.2
6.5
6.3
6.5
5.8
3.1
8.6
8.2
91.4
90.8
89.7
75.1
88.6
80.4
79.0
80.1
83.1
79.3
77.7
84.9
84.6
83.4
82.5
79.0
84.5
81.7
83.5
84.2
8.6
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
9.2
10.3
24.9
11.4
19.6
21.0
19.9
16.9
20.7
22.3
15.1
15.4
16.6
17.5
21.0
15.5
18.3
16.5
15.8
This work was supported by NSFC No. 21072120, MOST 863
program No. 2009AA033601 and the Foundation for Returned
Overseas Chinese Scholars of Shanxi Province (2009010, X. Chen).
Appendix A. Supplementary material
9
10
11
12
13
14
15
16
17
18
19
20
CCDC 763875 and 763876 contain the supplementary crystallo-
graphic data for complexes C3 and C7, respectively. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplemen-
tary data associated with this article can be found, in the online
version, at doi:10.1016/j.ica.2011.01.054.
C11
C12
References
[1] J. Skupinska, Chem. Rev. 91 (1991) 613.
[2] W. Keim, F.H. Kowaldt, R. Goddard, C. Krüger, Angew. Chem., Int. Ed. Engl. 17
(1978) 466.
[3] W. Keim, B. Hoffmann, R. Lodewick, M. Peuckert, G.J. Schmitt, Mol. Catal. 6
(1979) 79.
a
Reaction condition:
toluene.
5 lmol; 30 min, Et3Al2Cl3; 10 atm ethylene; 100 mL
b
Determined by GC.
c
106g molꢁ1 Ni hꢁ1
.
[4] W. Keim, Angew. Chem., Int. Ed. Engl. 29 (1990) 235.
[5] C.M. Killian, L.K. Johnson, M. Brookhart, Organometallics 16 (1997) 2005.
[6] W. Keim, A. Behr, B. Limbäcker, C. Krüger, Angew. Chem., Int. Ed. Engl. 22
(1983) 503.
[7] L.K. Johnson, C.M. Killan, M. Brookhart, J. Am. Chem. Soc. 117 (1995) 6414.
[8] S.D. Ittel, L.K. Johnson, M. Brookhart, Chem. Rev. 100 (2000) 1169.
[9] S. Mecking, Angew. Chem., Int. Ed. Engl. 40 (2001) 534.
[10] V.C. Gibson, C. Redshaw, G.A. Solan, Chem. Rev. 107 (2007) 1745.
[11] F. Speiser, P. Braunstein, L. Saussine, Acc. Chem. Res. 38 (2005) 784.
[12] V.C. Gibson, S.K. Spitzmesser, Chem. Rev. 103 (2003) 283.
[13] J. Pietsch, P. Braunstein, Y. Chauvin, New J. Chem. 22 (1998) 467.
[14] A. Kermagoret, P. Braunstein, Dalton Trans. (2008) 822.
ethylene oligomerization. When activated by Et3Al2Cl3, all nickel
procatalysts exhibited higher activities for ethylene oligomeriza-
tion with up to 1.35 ꢀ 107 g molꢁ1 Ni hꢁ1 and high selectivity of
a
-olefins (>99%). In practice, the activities of procatalysts could
be improved by modifying ligands through fine-tuning of substitu-
ents far from the nickel center. The title nickel procatalysts bearing

X. Chen et al. / Inorganica Chimica Acta 370 (2011) 156–163
163
[15] P. Yang, Y. Yang, C. Zhang, X.-J. Yang, H.-M. Hu, Y. Gao, B. Wu, Inorg. Chim. Acta
362 (2009) 89.
[31] R. Gao, M. Zhang, T. Liang, F. Wang, W.-H. Sun, Organometallics 27 (2008)
5641.
[16] C.-Y. Guo, N. Peulecke, K.R. Basvani, M.K. Kinderman, J. Heinicke,
Macromolecules 43 (2010) 1416.
[32] W.-H. Sun, P. Hao, S. Zhang, Q. Shi, W. Zuo, X. Tang, X. Lu, Organometallics 26
(2007) 2720.
[17] J. Heinicke, M. Köhler, N. Peulecke, W. Keim, J. Catal. 225 (2004) 16.
[18] M. Okada, Y. Nakayama, T. Shiono, J. Organomet. Chem. 692 (2007) 5183.
[19] M. Okada, Y. Nakayama, T. Ikeda, T. Shiono, Macromol. Rapid Commun. 27
(2006) 1418.
[33] P. Hao, S. Zhang, W.-H. Sun, Q. Shi, S. Adewuyi, X. Lu, P. Li, Organometallics 26
(2007) 2439.
[34] Y. Chen, P. Hao, W. Zuo, K. Gao, W.-H. Sun, J. Organomet. Chem. 693 (2008)
1829.
[20] F.A. Kunrath, R.F. De Souza, O.L. Casagrande Jr., N.R. Brooks, V.G. Young Jr.,
Organometallics 22 (2003) 4739.
[35] Y. Chen, W. Zuo, P. Hao, S. Zhang, K. Gao, W.-H. Sun, J. Organomet. Chem. 693
(2008) 750.
[21] N. Ajellal, M.C.A. Kuhn, A.D.G. Boff, M. Hörner, C.M. Thomas, J.-F. Carpentier,
O.L. Casagrande Jr., Organometallics 25 (2006) 1213.
[22] S. Al-Benna, M.J. Sarsfield, M. Thornton-Pett, D.L. Ormsby, P.J. Maddox, P. Brés,
M. Bochmann, J. Chem. Soc., Dalton Trans. (2000) 4247.
[23] S.O. Ojwach, I.A. Guzei, L.L. Benade, S.F. Mapolie, J. Darkwa, Organometallics 28
(2009) 2127.
[24] A.P. Armitage, Y.D.M. Champouret, H. Grigoli, Jv.D.A. Pelletier, K. Singh, G.A.
Solan, Eur. J. Inorg. Chem. (2008) 4597.
[25] W.-H. Sun, S. Zhang, W. Zuo, C.R. Chim. 11 (2008) 307.
[26] S. Jie, W.-H. Sun, T. Xiao, Chin. J. Polym. Sci. 28 (2010) 299.
[27] S. Adewuyi, G. Li, S. Zhang, W. Wang, P. Hao, W.-H. Sun, N. Tang, J. Yi, J.
Organomet. Chem. 692 (2007) 3532.
[36] L. Xiao, R. Gao, M. Zhang, Y. Li, X. Cao, W.-H. Sun, Organometallics 28 (2009)
2225.
[37] L. Xiao, M. Zhang, R. Gao, X. Cao, W.-H. Sun, Aust. J. Chem. 63 (2010) 109.
[38] L. Xiao, M. Zhang, W.-H. Sun, Polyhedron 29 (2010) 142.
[39] X. Chen, L. Zhang, T. Liang, X. Hao, W.-H. Sun, C.R. Chim. 13 (2010) 1450.
[40] T. Zhang, D. Guo, S. Jie, W.-H. Sun, T. Li, X. Yang, J. Polym. Sci. Part A: Polym.
Chem. 42 (2004) 4765.
[41] D. Guo, L. Han, T. Zhang, W. Sun, T. Li, X. Yang, Macromol. Theory Simul. 11
(2002) 1006.
[42] Q. Shi, S. Jie, S. Zhang, H. Yang, W.-H. Sun, Macromol. Symp. 260 (2007)
74.
[43] R. Gao, L. Xiao, X. Hao, W.-H. Sun, F. Wang, Dalton Trans. (2008) 5645.
[44] M. Asma, S. Adewuyi, X. Kuang, A. Badshah, W.-H. Sun, Lett. Org. Chem. 5
(2008) 296.
[28] M. Zhang, S. Zhang, P. Hao, S. Jie, W.-H. Sun, P. Li, X. Lu, Eur. J. Inorg. Chem. 24
(2007) 3816.
[29] S. Zhang, W.-H. Sun, X. Kuang, I. Vystorop, J. Yi, J. Organomet. Chem. 692
(2007) 5307.
[45] G.M. Sheldrick, SHELXTL-97, Program for the Refinement of Crystal Structures,
University of Göttingen, Germany, 1997.
[30] W.-H. Sun, K. Wang, K. Wedeking, D. Zhang, S. Zhang, J. Cai, Y. Li,
Organometallics 26 (2007) 4781.