Chromium(III) complexes ligated by 2-(1-isopropyl-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridines: Synthesis, characterization and their ethylene oligomerization and polymerization

Article abstract of DOI:10.1016/j.jorganchem.2007.11.049

A series of chromium(III) complexes bearing 2-(1-isopropyl-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridines were synthesized and characterized by IR spectroscopic and elemental analysis. The X-ray crystallographic analysis revealed a distorted octahedral

Full text of DOI:10.1016/j.jorganchem.2007.11.049

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 750–762
www.elsevier.com/locate/jorganchem
Chromium(III) complexes ligated by 2-(1-isopropyl-2-benzimidazolyl)-
6-(1-(arylimino)ethyl)pyridines: Synthesis, characterization and their
ethylene oligomerization and polymerization
Yanjun Chen a,b, Weiwei Zuo b, Peng Hao b, Shu Zhang b, Kun Gao a, Wen-Hua Sun a,b,
*
a State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
b Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, China
Received 14 September 2007; received in revised form 8 November 2007; accepted 27 November 2007
Available online 4 December 2007
Abstract
A series of chromium(III) complexes bearing 2-(1-isopropyl-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridines were synthesized and
characterized by IR spectroscopic and elemental analysis. The X-ray crystallographic analysis revealed a distorted octahedral geometry
of the chromium complexes. When activated by Et2AlCl, MAO or MMAO, these chromium complexes exhibited catalytic activities for
ethylene oligomerization and polymerization; while the good to high activities (up to 3.95 ꢀ 106 g molꢁ1 (Cr) hꢁ1) were observed in the
catalytic systems with MMAO. Therefore, various reaction parameters of the catalytic system with MMAO were investigated in detail.
The steric and electronic effects of ligands affected the catalytic activities and the distribution of the products predominantly. Interest-
ingly, sometimes their distributions of oligomers did not resemble the rules of Schulz-Flory or Poisson due to the hexenes produced in
low yield.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Chromium complexes; 2-(Benzimidazolyl)-6-iminopyridine; Ethylene oligomerization; Polymerization
1. Introduction
Chromium-based catalysts have played important roles
in commercial polymerization [8] and selective trimeriza-
tion of ethylene [9,10], which is also an ongoing scale-up
process for 1-hexene in Chinese SINOPEC Company.
The rapid progress has recently been focused on synthesiz-
ing new chromium complexes and investigating their cata-
lytic behaviors toward ethylene reactivity, the numerous
chromium complexes have been explored with numerous
ligands in several chelate models such as N^O [11,12],
N^N [13–15], N^N^N [16,17], P^P^P or P^N^P [18–22],
S^N^S [21], C^N^C [23], N^S^N [24], N^N^N^N [25]
along with half-metallocene [26]. In order to understand
the active species and alternative intermediates of chro-
mium complexes, some chromium complexes containing
O^N^N [27] and N^N^N [28] ligands were investigated in
our group as the extensive researches of late-transition
metal analogues as catalysts in ethylene reactivity [29]. In
Among all of the synthetic polymers, polyolefins have
the largest production volumes [1]. The growing demand
for high performance polyolefins has inspired extensively
industrial and academic researches in developing new cata-
lysts for polyolefins [2]. Metallocene catalysts, based upon
metal complexes, have shown some advantages of produc-
ing polyolefins with unique properties [3–6]. Recently com-
mon metal complexes as catalysts for olefin polymerization
and oligomerization have been attractive of both academic
and industrial considerations [6,7].
*
Corresponding author. Present address: Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, China. Tel.: +86 10
62557955; fax: +86 10 62618239.
E-mail address: whsun@iccas.ac.cn (W.-H. Sun).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.11.049

Y. Chen et al. / Journal of Organometallic Chemistry 693 (2008) 750–762
751
general, the chromium complexes coordinated with
N^N^N ligands performed good to high catalytic activities
toward ethylene oligomerization and polymerization [28],
moreover, their oligomers are highly selected in a-olefins
formations even their polyethylenes with vinyl-type charac-
teristics. It is necessary to use MAO or MMAO in those
chromium catalytic systems; and the good point is that
the process of MAO is well commercialized in PetroChina
Company. Indeed, the process for ethylene oligomerization
is nowadays under consideration by using chromium cata-
lysts in China. However, the technical problems are still
remained with patent issue such as 2,6-bis(2-benzimidazol-
yl)pyridylchromium trichlorides by two individual groups
[27,28a] and scale-up in synthesizing suitable ligands as
chromium(III) complexes ligated by 2-imino-1,10-
phenanthrolines [28b]. Recently 2-(benzimidazole)-6-
(1-arylimino-ethyl)pyridines were synthesized for their
late-transition-metal complexes as highly active catalysts
toward ethylene oligomerization and polymerization [30],
in further extensive research, their chromium analogues
were prepared. Our detail investigation revealed that the
titled chromium complexes gave good catalytic activities
toward ethylene oligomerization and polymerization.
Uncommonly, sometimes the distributions of oligomers
did not resemble the rules of Schulz-Flory or Poisson, espe-
cially for an extraordinary observation of low yield of hex-
enes. Herein, the syntheses, characterizations of these
chromium complexes were reported, and their catalytic
properties for ethylene oligomerization and polymerization
were investigated under various reaction conditions.
propyl group in current work as 2-(1-isopropyl-2-benzimi-
dazolyl)-6-acetylpyridine instead of previous one, 2-(1-
methyl-2-benzimidazolyl)-6-acetylpyridine
[30a].
The
2-(1-isopropyl-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyr-
idines (L1–L4 and L6) were easily prepared in satisfactory
yields (51–80%) through the condensation reactions of
2-(1-isopropyl-2-benzimidazolyl)-6-acetylpyridine with the
appropriate substituted anilines in the presence of p-tolu-
enesulfonic acid as the catalyst. The analogue L5 with chlo-
ride substituents on the phenyl ring, however, was
synthesized with adding ethyl silicate as water absorber.
All the synthesized organic compounds were well charac-
1
terized and confirmed by elemental analysis, H and 13C
NMR and IR. The IR spectra of the N^N^N ligands
showed that the C@N stretching frequencies appeared in
the range of 1643–1658 cmꢁ1 with the typical absorption.
The chromium complexes C1–C6 were prepared by the
stoichiometric reaction of CrCl3(THF)3 with the corre-
sponding ligands L1–L6 in dichloromethane at room tem-
perature (Scheme 1). The obtained complexes were
precipitated from the reaction solution after adding diethyl
ether and separated as green powders in proper yields (52–
82%).
These complexes could be soluble in dichloromethane,
THF and DMF at room temperature. Compared to the
IR spectra of the free ligands with C@N stretching frequen-
cies, the C@N stretching vibrations in complexes C1–C6
were shifted to lower frequencies in the range of 1588–
1593 cmꢁ1, indicating an effective coordination interaction
between the imino nitrogen atom and the chromium center.
The identity of the complexes C1–C6 was established on
the basis of element analysis and X-ray diffraction studies
for complexes C2 and C3.
2. Results and discussion
2.1. Synthesis and characterization of 2-(1-isopropyl-2-
benzimidazolyl)-6-(1-(arylimino)ethyl)pyridines and their
chromium complexes
Crystals of complex C2 suitable for X-ray diffraction
experiment were grown from its N,N-dimethylformamide
(DMF) solutions layered with diethyl ether, in which the
slow diffusion of diethyl ether happened to result a satu-
rated solution. The molecular structure of complex C2 is
shown in Fig. 1, and selected bond lengths and angles are
listed in Table 1.
The crystal structure of C2 confirms its geometry as a
slightly distorted octahedron with a tridentate N^N^N
ligand and three chlorides. The N–Cr–N angles are
In preparation of 2-(1-isopropyl-2-benzimidazolyl)-6-(1-
(arylimino)ethyl)pyridines and their chromium complexes
(Scheme 1), the important substance is 2-(2-benzimidazol-
yl)-6-acetylpyridine, which was made from 2-(carboeth-
oxy)-6-(2-benzimidazolyl)pyridine according to our
reported procedure [30a]. The difference is having an iso-
H
H+, H2O
i-PrI
EtO
Na, EtOAc
N
EtO
N
N
N
N
N
K2CO3
reflux, 6h
toluene,reflux
O
N
O
N
O
N
R1
R1
R2
NH2
N
N
R1
N
R1
N
CrCl3(THF)3
CH2Cl2
N
N
N
N
Cr
Cl Cl
Cl
R2
R1
R2
R1
Ln
Cn
n
1
2
3
4
5
Cl
H
6
Me
Me
R1
R2
Me Et i-Pr
F
H
H
H H
Scheme 1. Synthesis of 2-(1-isopropyl-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridines and their chromium complexes.

752
Y. Chen et al. / Journal of Organometallic Chemistry 693 (2008) 750–762
Fig. 1. Molecular structure of complex C2. Thermal ellipsoids are shown at 30% probability; hydrogen atoms and solvent have been omitted for clarity.
Table 1
˚
(N3–C6 (1.326(5) A) and imino linkage (N1–C16
˚
Selected bond lengths (A) and angles (°) for complexes C2 and C3
˚
(1.284(5) A)) retained after the coordination. The two che-
C2
C3
lating rings Cr1–N3–C6–C5–N2 and Cr1–N1–C16–C1–N2
were almost in the same plane with the dihedral angle of
1.0°. The planes of benzimidazole ring and pyridine are
almost coplanar with the dihedral angle defined at 5.6°,
while the substituted phenyl group is situated approxi-
mately perpendicular to the chelating ring(Cr1–N1–C16–
C1–N2) with the dihedral angle of 98.5°. Nonetheless, the
phenyl plane lies almost perpendicular to the pyridyl ring
(97.1°). It is worthy to mention that the bond lengths
between the chromium and the mutually trans-disposed
chlorine atoms are different. The bond length of Cr1–Cl1
Bond lengths
Cr(1)–N(1)
Cr(1)–N(2)
Cr(1)–N(3)
Cr(1)–Cl(1)
Cr(1)–Cl(2)
Cr(1)–Cl(3)
N(1)–C(16)
N(1)–C(18)
N(2)–C(1)
N(2)–C(5)
N(3)–C(6)
N(3)–C(7)
2.126(3)
2.138(4)
2.009(3)
2.051(3)
2.3308(1)
2.2875(1)
2.3159(1)
1.284(5)
1.470(5)
1.326(5)
1.358(5)
1.326(5)
1.370(5)
2.019(4)
2.039(4)
2.3248(2)
2.2965(2)
2.3151(2)
1.293(6)
1.476(6)
1.340(6)
1.343(6)
1.332(6)
1.381(6)
˚
(2.3159(1) A) is shorter than the bond length of Cr1–Cl2
˚
˚
(2.3308(1) A), while the bond length of Cr1–Cl3
Bond angles
N(2)–Cr(1)–N(3)
N(2)–Cr(1)–N(1)
N(3)–Cr(1)–N(1)
N(2)–Cr(1)–Cl(3)
N(3)–Cr(1)–Cl(3)
N(1)–Cr(1)–Cl(3)
N(2)–Cr(1)–Cl(2)
N(3)–Cr(1)–Cl(2)
N(1)–Cr(1)–Cl(2)
Cl(3)–Cr(1)–Cl(2)
N(2)–Cr(1)–Cl(1)
77.36(1)
77.37 (1)
154.73(1)
88.44(1)
87.26(1)
91.60(1)
176.94(1)
105.42(9)
99.85(9)
92.93(5)
86.27(1)
76.92(2)
77.34(2)
154.26(2)
90.46(1)
86.30 (1)
93.75 (1)
177.17(1)
105.91(1)
99.83(1)
89.77(6)
87.34(1)
(2.2875(1) A) is the shortest of these three Cr–Cl bonds.
In addition, the bond angles of Cl(3)–Cr(1)–Cl(1), Cl(3)–
Cr(1)–Cl(2), Cl(2)–Cr(1)–Cl(1) are 171.48(5)°, 92.93(5)°,
92.68(5)°, also indicating that the three chlorides are not
equally situated.
Similarly, the complex C3 (Fig. 2) also shows a slightly
distorted octahedron geometry. The selected bond dis-
tances and angles were collected in Table 1. The imino link-
˚
age (N1–C16 (1.293(6) A)) and the imidazole linkage (N4–
˚
C6 (1.332(6) A) retain the typical double-bond character
after the coordination. The substituted phenyl group is also
situated nearly perpendicular to the chelating ring (Cr1–
N1–C16–C1–N2) with the dihedral angle of 96.6°. How-
ever, comparing with complex C2, some differences were
observed in the structure of complex C3. The dihedral
angles between two chelating rings (Cr1–N3–C6–C5–N2
and Cr1–N1–C16–C1–N2) and between benzimidazole
ring and pyridyl ring are respectively, 2.2° and 7.9°, which
are bigger than those of complex C2. In addition, the Cr–N
77.36(1)°, 77.36(1)°, 154.73(1)°, which are bigger than those
angles found in iron(II) [30a], cobalt(II) [30a] and nickel(II)
analogues [30b]. The Cr–N(pyridine) bond distance
˚
˚
(2.009(3) A (Cr1–N2)) is about 0.04 A shorter than that
˚
of the Cr–N(imidazole) bond (2.051(3) A (Cr1–N3)), and
˚
the Cr–N(imino) bond distance (2.126(3) A (Cr1–N1)) is
the longest among the length of the Cr–N bond. In addi-
tion, the double-bond characters of the imidazole linkage

Y. Chen et al. / Journal of Organometallic Chemistry 693 (2008) 750–762
753
Fig. 2. Molecular structure of complex C3. Thermal ellipsoids are shown at 30% probability; hydrogen atoms and solvent have been omitted for clarity.
(imino) bond length of C3 is obviously longer than that of
complex C2. This effect is due to the relatively bulkier
groups at the ortho positions of the phenyl ring in C3.
same conditions. A similar trend could be observed when
ethylene pressure was increased to 10 atm, and noteworthy,
the polymerization activity obtained by C1/MMAO system
was nearly 20 times greater than that generated by C1/
MAO system. So the further investigations were carried
out with MMAO as cocatalyst. What’s more, from the
results in Table 2, it can be found that the main product
was polymer, which indicated that chain termination might
be suppressed by the propagation process for this chro-
mium complex. This is inconsistent with our previous
observations in 2,6-bis(2-benzimidazolyl)pyridylchromium
trichlorides and 2-carbethoxy-6-iminopyridine chromium
trichlorides [27,28a], where oligomers dominated the poly-
merization products. In addition, long chain olefins were
produced and the percentage of these longer chain olefins
(PC10) could achieve 49.9%. Surprisingly, the yield of hex-
enes was very low. Take the C1/MMAO with ratio of 1000
as the example (entry 2 in Table 3), the content of obtained
C6 (4.4 wt%) was much less than that of C4 (26.6 wt%) and
C8 (19.1 wt%). Repeated trying and experiments under
other conditions or with other complexes revealed the same
results. This observation was different from those found in
2.2. Ethylene oligomerization and polymerization
2.2.1. Cocatalyst selection
The effects of various cocatalysts on the productivity for
ethylene activation were first studied. The chromium(III)
complex C1 for catalytic behaviors was evaluated in the
presence of different cocatalysts such as methylaluminox-
ane (MAO), modified methylaluminoxane (MMAO), and
Et2AlCl at both 1 atm and 10 atm; and the results are col-
lected in Table 2. At 1 atm of ethylene pressure, the cata-
lytic system with Et2AlCl produced butenes in lower
oligomerization activities and a trace of polymer. When
activated by MAO, the catalyst showed increased activity,
giving both oligomers and polymers in the ratio of about
1:2. The activities for both oligomerization and polymeri-
zation were further increased when MMAO was employed
as cocatalyst. In addition, olefins with much longer chain
were produced compared to C1/MAO system under the
Table 2
Catalytic system of C1 with various cocatalystsa
Entry
P/atm
Cocatalyst
Al/Cr
Oligomer
Activityd
Polymer
Activityd
Distributione
1b
2
1
1
1
10
10
10
Et2AlCl
MAO
MMAO
Et2AlCl
MAO
200
0.36
0.55
0.93
0.88
2.26
27.0
C4
trace
1.20
3.70
2.0
6.76
114.5
1000
1000
200
1000
1000
C4–C12
C4–C12
C4
C4–C16
C4–C16
3
4c
5
6
MMAO
a
Reaction conditions: 5 lmol of catalyst; 20 °C.
60 min, 30 ml of toluene for entries 1–3.
30 min, 100 ml of toluene for entries 4–6.
b
c
d
e
In units of 104 g (mol of Cr)ꢁ1 hꢁ1
Determined by GC and GC–MS.
.

754
Y. Chen et al. / Journal of Organometallic Chemistry 693 (2008) 750–762
Table 3
Oligomerization and polymerizatiom with C1/MMAO at 1 atm ethylenea
Entry Al/
Cr
Tb
tc
Productivityd Oligomers
(wt%)
PE
(wt%)
Tem (°C)
Oligomer distribution (%)f
(°C)
(min)
1
2
3
4
C4/
C
C6/
C
C8/
C
PC10
/
a-Olefin
(%)
P
P
P
P
C
1
2
3
4
5
6
7
8
9
10
11
12
a
500
20
60
60
60
60
60
60
60
60
15
30
90
120
0.22
0.47
0.51
0.67
1.31
0.35
0.28
0.06
0.38
0.46
0.39
0.30
24.0
20.8
24.9
25.2
12.2
63.8
65.2
P99
46.4
25.2
25.6
26.6
76.0
79.2
75.1
74.8
87.8
36.2
34.8
61.0
53.6
74.8
74.4
73.4
–
92.0 106.6 130.0 10.0
3.7
4.4
4.1
3.3
3.9
4.4
19.5
8.0
3.6
4.9
5.0
4.2
13.0
19.1
9.3
73.3
49.9
58.7
66.3
58.9
56.4
18.2
10.6
71.4
46.0
54.7
62.9
92.1
92.6
93.5
90.8
87.8
89.9
83.9
80.2
94.7
96.8
91.8
91.9
1000 20
1500 20
2000 20
3000 20
1500 40
1500 60
1500 80
1500 20
1500 20
1500 20
1500 20
72.8 86.9 104.8 125.3 26.6
68.9 86.3 103.0 126.0 27.9
71.5 85.6 102.5 126.4 23.7
6.7
7.7
–
93.2 104.7 123.7 29.5
70.0
74.0
nd
–
–
101.9 126.9 25.2
101.9 127.7 48.2
47.4
14.0
14.1
34.0
9.1
75.2 91.7 103.8 125.0 15.9
74.6 90.6 103.4 126.4 25.9
23.2
15.3
11.9
70.9
–
–
102.9 127.0 25.0
83.8 104.3 127.7 21.0
Reaction conditions: 5 lmol of catalyst; 30 ml of toluene.
Reaction temperature.
b
c
Reaction time.
d
In units of 105 g (mol of Cr)ꢁ1 hꢁ1
Determined by DSC.
.
e
f
Determined by GC.
other chromium complexes [17,31,32], and the reason was
hitherto unclear.
of oligomers and the selectivity for a-olefins were not
strongly affected by the Al/Cr ratio, which can be observed
in Fig. 3. At the ratio of Al/Cr 1500, both the productivity
and the selectivity for a-olefins reached satisfied results.
Therefore, the further experiments were performed with
the molar ratio of Al/Cr 1500. DSC analysis revealed the
presence of several compositions of the obtained polymers,
and the DSC curves included a main peak at around 125 °C
and broad shoulders ranged from 40 to 110 °C. Unusually
two or three small peaks could be found from the wide
shoulders. Under different Al/Cr molar ratios, the profile
of DSC curves were similar except for the various shoulder
peaks. These phenomena demonstrated polyethylenes
2.2.2. Ethylene oligomerization and polymerization at
ambient pressure
2.2.2.1. Effects of the molar ratio of Al/Cr and reaction
temperature. The amount of cocatalyst was found to have
significant influences on the catalytic behaviors of C1/
MMAO system. Increasing the Al/Cr molar ratio from
500 to 3000 led to an enhancement of the productivity
(entries 1–5 in Table 3). When the ratio of Al/Cr is 3000,
the catalytic system displayed high productivity up to
1.31 ꢀ 105 g (mol of Cr)ꢁ1 hꢁ1. However, the distribution
Fig. 3. Oligomer distribution obtained with C1 at different Al/Cr ratios at 1 atm.

Y. Chen et al. / Journal of Organometallic Chemistry 693 (2008) 750–762
755
containing different components had broad molecular
weight distributions, which were concurrent with the
results observed for chromium(III) complexes supported
by bis(2-pyridylmethyl)amines [33]. Such phenomena of
multiple endothermic peaks in polyolefin have also
observed in our previous research [28] and literatures [34–
38]. The broad molecular weight distribution of polymers
was in agreement with that of oligomers.
effect of reaction time on catalytic activity was also studied
using the C1/MMAO system with an Al/Cr molar ratio of
1500 at 20 °C. When the reaction time was increased from
15 min to 120 min, the catalytic productivity initially
increased and then decreased, with the optimum activity
being at reaction time of 60 min (entries 3, and 9–12, Table
3), indicating that there may be an induction period at the
initial stage of reaction and the active species of C1 were
deactivated with the prolonged reaction time. All the
DSC curves gave one obvious peak and a broad shoulder.
Increasing the reaction time from 15 to 120 min, the main
peak on the DSC curves of the resultant polyethylene
increased from 125.0 to 127.7 °C.
As the ethylene oligomerization and polymerization are
highly exothermic reactions, the reaction temperature signif-
icantly affects the catalytic activity. To understand this influ-
ence, the catalytic system of C1 with 1500 equiv. of MMAO
at 1 atm of ethylene was investigated with changing reaction
temperature. Elevation of the reaction temperature from 20
to 80 °C resulted in a sharp decrease of catalytic activity,
which can be explained as the decomposition of some active
species and lower ethylene solubility at higher temperature
[39]. At the same time, increasing the temperature results in
a decrease of PE proportion (entries 3 and 6–8, Table 3),
which was due to an increase in the rate of the b-hydrogen
elimination relative to the rate of propagation at higher tem-
perature [40]. In addition, increasing the temperature from
20 to 80 °C led to a rapid decrease of a-olefin selectivity from
93.5% to 80.2%. This can be attributed to the faster chain
transfer or isomerization at high temperature. It was note-
worthy that the oligomer distribution showed an unprece-
dented small amount of C6. This is similar to the reported
observation [41], in which the chromium system showed
much lower activity towards ethylene trimerization. The
reaction temperature had an obvious effect on the thermal
property of the resultant polyethylenes. DSC curves of poly-
ethylenes produced at different temperatures were similar
with one obvious peak and a broad shoulder at low temper-
ature. However, elevating the temperature from 20 to 60 °C
has increased the melting points (judging from the main peak
of DSC curves) of the polyethylenes (entries 3 and 6–8,
Table 3). In general, the suitable active sites were formed at
temperature 20 °C for the high activity and good selectivity
of a-olefins, therefore, more investigations were carried out
at 20 °C.
2.2.2.3. Effect of the ligand environments. The data listed
in Table 4 (entries 1–6) showed that the structure of the
ligands considerably affected the catalytic behaviors of
the complexes. It is evident to find that the steric hindrance
of ligands affected the ethylene activities of chromium com-
plexes. Comparing C3 (with i-Pr), C2 (with Et) and C1
(with Me) (entries 1, 2 and 3, Table 4), an increase of the
bulkiness of the groups in the ortho positions of the ani-
lines renders a less active catalyst. Complex C1 bearing
2,6-dimethyl substituents on the imino-N aryl ring, showed
the highest activity at 0.51 ꢀ 105 g (mol of Cr)ꢁ1 hꢁ1. This
could be explained that in this catalytic system with
MMAO, the reaction speed mostly depended on the inser-
tion of ethylene at the active species, so a more bulky group
caused slower insertion reaction and consequently lower
activity. Meanwhile, with more bulkiness, the content of
oligomers is obviously increased, which might be attributed
that the catalysts with less steric bulkiness facilitate the
reaction of propagation not the b-hydrogen elimination
reaction. The electronic effect of ligands on the activities
of chromium complexes was also observed. Complexes
C4 (R1 = F) and C5 (R1 = Cl) bearing halogen groups
exhibited relatively higher activities with better selectivity
of a-olefin than the corresponding complexes bearing alkyl
groups at the ortho-positions of the imino-N aryl ring. In
addition, products obtained by C4/MMAO systems com-
prised of more proportions of oligomers than those by
alkyl-substituted systems. The improved catalytic activity
2.2.2.2. Effect of the reaction time. The catalyst lifetime is
one significant factor in industrial considerations. The
Table 4
Ethylene reactivity with C1–C6/MMAO at ambient pressurea
Entry
Complex
Productivityb Oligomers PE
Tcm (°C)
Oligomer distribution (%)d
P
P
P
P
(wt%)
(wt%)
1
2
3
4
C4/
C
C6/
C
C8/
C
PC10
/
C
a-Olefin (%)
1
2
3
4
5
6
a
C1
C2
C3
C4
C5
C6
0.51
0.46
0.19
2.10
1.47
2.03
24.9
27.3
55.3
82.5
36.5
11.5
75.1
72.7
44. 7
17.5
63.5
88.5
68.9 86.3 103.0 126.0 27.9
69.8 85.6 101.9 127.0 34.0
67.9 88.6 102.1 125.4 31.8
4.1
5.5
9.3
19.2
17.2
5.8
58.7
41.3
47.4
6.7
59.4
74.6
93.5
93.2
89.4
99.6
97.5
96.8
3.6
5.9
66.7
64.9
65.2
–
–
–
102.7 123.4 81.6
124.6 15.3
102.8 125.7 11.6
–
11.1
4.2
14.2
9.6
Reaction conditions: 5 lmol of catalyst; Al/Cr = 1500; 20 °C; 60 min; 30 ml of toluene.
b
c
In units of 105 g (mol of Cr)ꢁ1 hꢁ1
Determined by DSC.
.
d
Determined by GC and GC–MS.

756
Y. Chen et al. / Journal of Organometallic Chemistry 693 (2008) 750–762
was attributed to the more electrophilic nature of the chro-
mium center after the introduction of the electron-with-
drawing substituents on the ortho-positions of phenyl
group. Comparing C4 and C5, the fluoro-substituted com-
plex C4 showed much higher activity than C5 and pro-
duced more oligomers in the final products, which can be
explained as the combined result of the electronic and steric
effects. Therefore, less bulky ligands with electron-with-
drawing substituents would be helpful in increasing the cat-
alytic activity of the tridentate chromium complexes with
the MMAO as the activator. All these chromium
complexes especially complexes C4 and C5 bearing elec-
tron-withdrawing substituents displayed the high a-olefin
selectivity for ethylene oligomerization at 1 atm. The distri-
bution of oligomers produced by complex C4 resembles
Schulz-Flory rules with K = 0.73, where K represented
the probability of chain propagation (K = rate of propaga-
tion/((rate of propagation) + (rate of chain trans-
fer)) = (moles of Cn+2)/(moles of Cn)) [42]. However, the
distribution of oligomers, which contained a small portion
of C6 by complex C1–C3, C5 and C6, did not resemble
neither Schulz-Flory rules or Poisson distribution (entries
1–3, 5 and 6 in Table 4). The oligomer distributions
achieved by C2, C3 and C5 were shown in Fig. 4.
In addition, the introduction of a methyl group in the
para-position has exerted obvious influences on the cata-
lytic behaviors. For example, complex C6 exhibited much
higher activity than the corresponding complexes C1 and
produced more polymers in the final products compared
with other systems. Furthermore, complex C6 bearing
2,4,6-trimethyl substituents on the imino-N aryl ring exhib-
ited exceptionally higher activity than complexes C2 and
C3. This may be caused by the changed net charge on
the transition metal due to the presence of the methyl
group in the para-position. This phenomenon is in agree-
ment with the results observed for the chromium(III) cata-
lysts with bis(imino)pyridyl ligands [43].
These findings indicated that the ligand environments had
dramatic effect on the ethylene oligomerization and poly-
merization. Less bulky ligands with electron-withdrawing
substituents can be helpful in enhancing the catalytic activity
of the tridentate chromium complexes. In addition, both
products of oligomers and polyethylenes were obtained in
current catalytic systems at ambient pressure, while chro-
mium complexes with bis(imino)pyridines favorably per-
formed ethylene polymerization in combination with
MMAO [34,43], and [2,6-bis(2-benzimidazolyl)pyridyl]
chromium complexes gave oligomers as main products
[28a]. This may be attributed to the geometry of the com-
plexes. In the molecular structures of bis(imino)pyridyl chro-
mium(III) complexes, the planes of the phenyl rings are
oriented essentially orthogonal to the pyridyl plane [34,43],
while benzimidazole rings are almost coplanar with pyridyl
rings in [2,6-bis(2-benzimidazolyl)pyridyl]chromium com-
plexes [28a]. Comparing with imino groups in chromium
complexes, the introduction of benzimidazole groups copla-
nar with pyridyl rings favor b-hydrogen elimination reaction
during ethylene activation, while the existence of imino
groups favor chain propagation reaction. So the titled com-
plexes which contain both benzimidazole groups and imino
groups produce both polymers and oligomers.
All the DSC curves obtained by C1–C6 showed one
main peak around 125 °C and a broad shoulder at lower
temperature ranges, which indicated that all samples had
broad distribution of molecular weights. These phenomena
were commonly observed among complexes containing
pyridyl ligands [44]. Under the same condition, the ligand
environment showed only little effect on the thermal prop-
erties of the resulting polymer. In addition, the Tm of the
polymers obtained by complexes C4 (R1 = F) and C5
Fig. 4. Oligomer distribution obtained in entries 2, 3 and 5 in Table 4.

Y. Chen et al. / Journal of Organometallic Chemistry 693 (2008) 750–762
757
(R1 = Cl) bearing halogen groups were lower than those
obtained by their analogues bearing alkyl groups at
ortho-positions of the imino-N aryl ring (entries 4 and 5,
Table 5). IR spectra recorded using a KBr disk in the range
of 4000–400 cmꢁ1 revealed only four obvious peaks at
2920, 2850, 1464, and 720 cmꢁ1, suggesting the linear struc-
ture of the obtained polyethylenes. This is in line with what
we observed in our previous models [28a]. The oligomer
distribution behavior and DSC analysis indicate that the
obtained polymers have a very broad molecular weight
distribution.
the catalytic behavior of all the complexes including
activities, product distributions and polymer properties.
Comparing with the catalytic behavior at ambient pres-
sure (Table 4), the higher catalytic activities could be
obtained with increasing the ethylene pressure and the
catalytic activities for both oligomerization and polymer-
ization increased by nearly one order of magnitude at
10 atm of ethylene. However, the increase of ethylene
pressure did not significantly change the weight ratios
between oligomers and polymers of the products com-
pared with the results obtained at 1 atm.
2.2.3. Ethylene oligomerization and polymerization at
10 atm of ethylene
2.2.3.1. Effect of the ethylene pressure. The results of
polymerization performed under 10 atm pressure employ-
ing C1–C6 are summarized in Table 5. The data indi-
cated that the ethylene pressure significantly affected
2.2.3.2. Effect of the ligand environment. Upon activation
with modified methylaluminoxane (MMAO), all the chro-
mium complexes exhibited good catalytic activities for olig-
omerization and polymerization at 10 atm of ethylene. The
detailed results are listed in Table 5 (entries 1–6).
The ligand environment has exerted similar influences on
Table 5
Ethylene reactivity with C1–C6/MMAO at 10 atma
Entry
Complex
Productivityb
Oligomers
(wt%)
PE
(wt%)
Tcm (°C)
Oligomer distribution (%)d
P
P
P
P
1
2
3
4
C4/
C
C6/
C
C8/
C
PC10
/
C
a-Olefin
1
2
3
4
5
6
a
C1
C2
C3
C4
C5
C6
1.42
0.57
0.48
3.95
1.49
1.70
23.9
35.0
46.1
88.4
66.2
22.3
76.1
65.0
53.9
11.6
33.8
77.7
69.6
68.9
71.3
65.2
61.8
60.9
89.2
88.7
89.4
–
–
89.1
101.7
102.2
102.5
98.1
–
121.7
125.7
124.0
119.5
120.7
120.7
14.1
22.2
38.6
81.0
21.9
9.8
9.5
18.8
18.4
9.3
9.0
16.9
14.4
3.5
67.4
42.1
28.6
6.2
44.8
67.6
96.1
97.5
97.5
99.2
98.7
100
18.4
8.2
14.9
14.4
100.7
Reaction conditions: 5 lmol of catalyst; Al/Cr = 1500; 20 °C; 60 min; 30 ml of toluene.
b
c
In units of 106 g (mol of Cr)ꢁ1 hꢁ1
Determined by DSC.
.
d
Determined by GC and GC–MS.
Fig. 5. Oligomer distribution obtained in entries 2, 3, 5 in Table 5.

758
Y. Chen et al. / Journal of Organometallic Chemistry 693 (2008) 750–762
catalytic behaviors as those found at 1 atm of ethylene
pressure. For example, the introduction of electron-with-
drawing halogen groups on the imino-N aryl ring resulted
in increased catalytic activities, higher a-olefin selectivity
together with higher oligomer fraction in the polymeriza-
tion products. In addition, for the three alkyl-substituted
complexes (C1–C3), greater bulky substituents at the
ortho-positions of the imino-N aryl ring led to decrease
of catalytic activity and lower proportion of polymers.
At ambient pressure, complex C6 made an exception for
ethylene oligomerization and polymerization. Similarly, at
10 atm of ethylene, complex C6 showed higher productivity
with much more polymers than the corresponding alkyl-
substituted analogues (entry 6, Table 5).
marked influence on the activity. Less bulky ligand with
electron-withdrawing substituent produce an positive influ-
ence on increasing the catalytic activity of the tridentate
chromium complexes with the MMAO as the activator.
In addition, complexes bearing electron-withdrawing sub-
stituents produced much more oligomers than polymers
in the products at 10 atm of ethylene. Higher ethylene
pressure favors increased catalytic activities and higher
a-olefin selectivity. In some cases, the polymers have been
obtained and confirmed to be vinyl-terminal long-chain
alkenes. Finally, we can conclude that these tridentate
chromium complexes could be promising catalysts with
technological application in ethylene oligomerization and
polymerization.
In addition, the distribution of olefin oligomers obtained
by C1–C5 deviated more from the Schulz-Flory rule, and
the oligomer distributions achieved by complexesC2, C3
and C5 at 10 atm were shown in Fig. 5. In addition, GC
analysis of the oligomers indicated that the selectivity for
linear a-olefins in the range of 96.1–99.2% are a little higher
than those found at 1 atm. Polymers obtained by complex
C1 at 10 atm are also mainly linear-olefins through the
characterization by the IR spectrum.
At 10 atm, the effects of the ligand environments on
the Tm accorded with the results at 1 atm. However,
the main Tm peak of the resultant polymers obtained
at 10 atm ranged from 119.5 to 120.7 °C, which are
lower than those of polyethylenes (range from 123.4 to
127.0 °C) at 1 atm.
4. Experimental
4.1. General considerations
All manipulations of air- and moisture-sensitive com-
pounds were carried out under a nitrogen atmosphere
using standard Schlenk techniques. Toluene was refluxed
over sodium-benzophenone and distilled under nitrogen
prior to use. Methylaluminoxane (MAO) was purchased
from Albemarle as a 1.4 M solution in toluene. Modified
methylaluminoxane (MMAO, 1.93 M in heptane) was
purchased from Akzo Corp. Diethylaluminum chloride
(Et2AlCl, 1.7 M in hexane) was purchased from Acros
Chemicals. All other reagents were purchased from
Aldrich or Acros Chemicals; the boiling range of light
petroleum is 60–90 °C and the type of silica gel used is
3. Conclusion
1
200–300 mesh. H and 13C NMR spectra were recorded
Inspired by the success of 2,6-bis(2-benzimidazolyl)pyr-
idyl chromium trichlorides [28a] and 2-(1-methyl-2-benzim-
idazolyl)-6-(1-(arylimino)ethyl)pyridyl iron and cobalt
complexes [30] as highly active catalysts toward ethylene
activation, a family of octahedral chromium(III) complexes
ligated by the 2-(1-isopropyl-2-benzimidazolyl)-6-(1-(aryli-
mino)ethyl)pyridines were prepared and characterized.
Upon activation with modified methylalumoxane
(MMAO), these tridentate chromium complexes afforded
moderate to high activity for ethylene oligomerization
and polymerization with broad oligomer distributions from
C4 to C36 and some complexes got special distributions of
oligomers. In most cases, polyethylene was the main prod-
uct and higher carbon number olefins usually accounted
for the majority of the oligomers. The obtained polymers
were confirmed to be vinyl-terminal long-chain alkenes.
Reaction conditions including cocatalyst, temperature, eth-
ylene pressure and reaction time were found to have signif-
icant influences on the catalytic behaviors. Increasing the
Al/Cr molar ratio led to an enhancement of the productiv-
ity, however, exerted little influences on the a-olefin selec-
tivity and the product compositions. Elevation of the
reaction temperature resulted in the decreased activity,
reduced polymer content and less a-olefin selectivity. The
electronic nature of the aromatic substituents has also a
on a Bruker DMX 300 MHz instrument at ambient tem-
perature using TMS as an internal standard. IR spectra
were recorded on a Perkin–Elmer System 2000 FT-IR
spectrometer. Elemental analysis was carried out using
an HPMOD 1106 microanalyzer. GC analysis was per-
formed 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 col-
umn. The yield of oligomers was calculated by referenc-
ing to the mass of the solvent on the basis of the
prerequisite that the mass of each fraction was approxi-
mately proportional to its integrated area in the GC
trace. Selectivity for the linear a-olefin was defined as
(amount of linear a-olefin of all fractions)/(total amount
of oligomer products) in percent. Melting points of the
polymers were obtained on a Perkin–Elmer DSC-7
instrument in the standard DSC run mode. The instru-
ment was initially calibrated for the melting point of
an indium standard at a heating rate of 10 °C/min.
The polymer sample was first equilibrated at 0 °C and
then heated to 160 °C at a rate of 10 °C/min to remove
thermal history. The sample was then cooled to 0 °C at a
rate of 10 °C/min. A second heating cycle was used for
collecting DSC thermogram data at a ramping rate of
10 °C/min.

Y. Chen et al. / Journal of Organometallic Chemistry 693 (2008) 750–762
759
4.2. Preparation of ligands
NMR (75 MHz, CDCl3, TMS): d 166.5, 155.5, 150.3,
150.0, 146.4, 143.7, 137.7, 135.8 (2C), 134.6, 126.7, 123.8,
123.2 (2C), 122.9, 122.4, 121.4, 120.7, 113.1, 49.1, 28.5
(2C), 23.3 (2C), 22.9 (2C), 21.6 (2C), 17.4. IR (KBr;
cmꢁ1): 2960, 2868, 1650 (mC@N), 1588, 1570, 1454, 1398,
824, 782, 750. Anal. Calc. for C29H34N4 (438.6): C, 79.41;
H, 7.81; N, 12.77. Found: C, 79.37; H, 7.91; N, 12.48%.
4.2.1. Preparation of (E)-N-(1-(6-(1-isopropyl-1H-
benzo[d]imidazol-2-yl)pyridin-2-yl)ethylidene)-2,6-
dimethylbenzenamine (L1)
2-(1-Isopropyl-2-benzimidazolyl)-6-acetylpyridine (167
mg, 0.6 mmol) and 2,6-dimethylaniline (72.7 mg, 0.6 mmol)
were refluxed in toluene (20 ml) in the presence of p-tolu-
enesulfonic. After solvent evaporation, the crude product
was purified by column chromatography on silica gel with
petroleum ether/ethyl acetate (5/1 v/v) as eluent to afford
the product as a yellow powder in 65% yield. M.p.: 143–
145 °C. 1H NMR (300 MHz, CDCl3, TMS): d 8.47 (d,
1H, J = 7.9 Hz, Py), 8.38 (d, 1H, J = 7.7 Hz, Py), 7.99 (t,
1H, J = 7.9, 7.7, Py), 7.87 (m, 1H, Ph), 7.70 (m, 1H, Ph),
7.33 (m, 2H, Ph), 7.09 (d, 2H, J = 7.5 Hz, Ph), 6.96 (t,
1H, J = 6.9, 7.2 Hz, Ph), 6.06 (m, 1H, CH), 2.20 (s, 3H,
CH3), 2.06 (s, 6H, CH3), 1.75 (d, 6H, J = 6.9 Hz, CH3).
13C NMR (75 MHz, CDCl3, TMS): d 165.2, 154.0, 148.9,
148.6, 147.3, 142.3, 136.3 (2C), 133.25, 126.7 (2C), 125.5,
124.1, 121.9, 121.6, 121.0, 119.9, 119.4, 111.7, 47.7, 20.2
(2C), 16.7 (2C), 15.3. IR (KBr; cmꢁ1): 2972, 2937, 1654
(mC@N), 1589, 1570, 1455, 1398, 1207, 825, 754. Anal. Calc.
for C25H26N4 (382.5): C, 78.50; H, 6.85; N, 14.65. Found:
C, 78.32; H, 6.98; N, 14.76%.
4.2.4. (E)-2,6-Difluoro-N-(1-(6-(1-isopropyl-1H-
benzo[d]imidazol-2-yl)pyri-dine-2-yl)ethylidene)-
benzenamine (L4)
Using the same procedure as for the synthesis of L1, L4
was obtained as a yellow powder in 51% yield. M.p.: 94–
1
95 °C. H NMR (300 MHz, CDCl3, TMS):d 8.48 (d, 1H,
J = 7.9 Hz, Py), 8.39 (d, 1H, J = 7.8 Hz, Py), 7.99 (t, 1H,
J = 7.9, 7.8, Py), 7.88 (m, 1H, Ph), 7.71 (m, 1H, Ph), 7.33
(m, 2H, Ph), 7.08 (m, 2H, Ph), 6.99 (m, 2H, Ph), 6.05 (m,
1H, CH), 2.43 (s, 3H, CH3), 1.75 (d, 6H, J = 6.9 Hz,
CH3). 13C NMR (75 MHz, CDCl3, TMS): d 171.9, 154.7,
151.4, 149.9, 149.8, 143.5, 137.7 (2C), 134.5, 127.2 (2C),
124.3, 122.9, 122.3, 121.9, 120.6, 113.0, 111.8, 111.5, 49.0,
21.4 (2C), 17.7. IR (KBr; cmꢁ1): 2969, 1643 (mC@N), 1588,
1453, 824, 745. Anal. Calc. for C23H20F2N4 (390.4): C,
70.75; H, 5.16; N, 14.35. Found: C, 70.80; H, 5.17; N,
14.41%.
4.2.2. (E)-2,6-Diethyl-N-(1-(6-(1-isopropyl-1H-
benzo[d]imidazol-2-yl)-pyridin-2-
yl)ethylidene)benzenamine (L2)
4.2.5. (E)-2,6-Dichloro-N-(1-(6-(1-isopropyl-1H-
benzo[d]imidazol-2-yl)pyridin-2-yl)ethylidene)-
benzenamine (L5)
Using the same procedure as for the synthesis of L1, L2
was obtained as a yellow powder in 79% yield. M.p.: 130–
132 °C. 1H NMR (300 MHz, CDCl3, TMS): d 8.45 (d, 1H,
J = 7.9 Hz, Py), 8.38 (d, 1H, J = 7.8 Hz, Py), 7.99 (t, 1H,
J = 7.9, 7.8 Hz, Py), 7.85 (m, 1H, Ph), 7.61 (m, 1H, Ph),
7.32 (m, 2H, Ph), 7.28 (d, 2H, J = 7.1 Hz, Ph), 7.07(m,
1H, Ph), 6.10 (m, 1H, CH), 2.40 (m, 4H, CH2), 2.22 (s,
3H, CH3), 1.75 (d, 6H, J = 6.0 Hz, CH3) 1.15 (t, 6H,
J = 7.5, 7.5 Hz, CH3). 13C NMR (75 MHz, CDCl3,
TMS): d 165.0, 154.1, 148.6, 148.5, 146.4, 142.3, 136.3,
133.3, 129.9 (2C), 125.4, 124.7 (2C), 122.2, 121.6, 120.9,
119.9, 119.4, 111.7, 47.7, 23.4 (2C), 20.2 (2C), 15.7, 12.4
(2C). IR (KBr; cmꢁ1): 2970, 2934, 1654 (mC@N), 1587,
1568, 1455, 1397, 1384, 827, 748. Anal. Calc. for
C27H30N4 (410.6): C, 78.99; H, 7.37; N, 13.65. Found: C,
78.74; H, 7.31; N, 13.44%.
2-(1-Isopropyl-2-benzimidazolyl)-6-acetylpyridine (279
mg, 1 mmol) and 2,6-dichlorolaniline (324 mg, 2 mmol)
were refluxed in ethyl silicate (4 ml) in the presence of p-tol-
uenesulfonic. After solvent evaporation, the crude product
was purified by column chromatography on alumina with
petroleum ether/ethyl acetate (10/1 v/v) as eluent to afford
the product as a yellow powder in 5% yield. M.p.: 121–
122 °C. 1H NMR (300 MHz, CDCl3, TMS): d 8.47 (d,
1H, J = 7.4 Hz, Py), 8.41 (d, 1H, J = 7.4 Hz, Py), 8.00 (t,
1H, J = 7.4, 7.4, Py), 7.86 (m, 1H, Ph), 7.70 (m, 1H, Ph),
7.37 (d, 2H, J = 7.9, 7.9 Hz, Ph), 7.30 (m, 2H, Ph), 7.01
(t, 1H, J = 8.3, 7.9 Hz, Ph), 6.04 (m, 1H, CH), 2.33 (s,
3H, CH3), 1.75 (d, 6H, J = 6.9 Hz, CH3). 13C NMR
(75 MHz, CDCl3, TMS): d 170.8, 155.2, 154.6, 150.1,
145.6, 143.6, 137.9 (2C), 134.6, 128.4 (2C), 127.4, 124.7,
124.6, 123.0, 122.4, 122.1, 120.7, 113.1, 49.1, 21.6 (2C),
17.8. IR (KBr; cmꢁ1): 2970, 1658 (mC@N), 1588, 1455,
1434, 1399, 824, 789, 744. Anal. Calc. for C23H20Cl2N4
(423.3): C, 65.25; H, 4.76; N, 13.23. Found: C, 64.93; H,
4.90; N, 12.94%.
4.2.3. (E)-2,6-Diisopropyl-N-(1-(6-(1-isopropyl-1H-
benzo[d]imidazol-2-yl)pyridin-2-
yl)ethylidene)benzenamine (L3)
Using the same procedure as for the synthesis of L1, L3
was obtained as a yellow powder in 80% yield. M.p.: 179–
180 °C. 1H NMR (300 MHz, CDCl3, TMS): d 8.44 (d, 1H,
J = 7.9 Hz, Py), 8.38 (d, 1H, J = 7.3 Hz, Py), 7.99 (t, 1H,
J = 7.9, 7.3 Hz, Py), 7.73 (m, 1H, Ph), 7.62 (m, 1H, Ph),
7.32 (m, 2H, Ph), 7.18 (m, 2H, Ph), 7.14 (m, 1H, Ph),
6.07 (m, 1H, CH), 2.76 (m, 2 H, CH), 1.75 (d, 6H,
J = 7.0 Hz, CH3), 1.16 (d, 12H, J = 6.8 Hz, CH3). 13C
4.2.6. (E)-2,4,6-Trimethyl-N-(1-(6-(1-isopropyl-1H-
benzo[d]imidazol-2-yl)pyridin-2-yl)ethylidene)-
benzenamine (L6)
Using the same procedure as for the synthesis of L1, L6
was obtained as a yellow powder in 71% yield. M.p.: 189–
190 °C. 1H NMR (400 MHz, CDCl3, TMS): d 8.49 (d, 1H,
J = 8.0 Hz, Py), 8.37 (d, 1H, J = 7.8 Hz, Py), 8.00 (t, 1H,

760
Y. Chen et al. / Journal of Organometallic Chemistry 693 (2008) 750–762
J = 8.0, 7.8, Py), 7.87 (m, 1H, Ph), 7.71 (m, 1H, Ph), 7.31
(m, 2H, Ph), 6.91 (s, 2H, Ph), 6.06 (m, 1H, CH), 2.36 (s,
3H, CH3), 2.21 (s, 3H, CH3), 2.06 (s, 6H, CH3), 1.75 (d,
6H, J = 7.0 Hz, CH3). 13C NMR (75 MHz, CDCl3,
TMS): d 167.8, 155.5, 149.9, 150.2, 146.2, 143.6, 137.6
(2C), 132.5, 128.7 (2C), 126.7, 125.3 (2C), 122.9, 122.4,
121.4, 120.7, 113.1, 49.1, 21.5 (2C), 20.9, 18.0 (2C), 16.6.
IR (KBr; cmꢁ1): 2970, 1651 (mC@N), 1570, 1478, 1453,
1399, 824, 769, 750. Anal. Calc. for C26H28N4 (396.5): C,
78.75; H, 7.12; N, 14.13. Found: C, 78.73; H, 7.21; N,
14.40%.
(548.8): C, 50.34; H, 3.67; N, 10.21. Found: C, 50.30; H,
3.62; N, 9.98%.
4.3.5. Complex C5
Analogous to the procedure for C1, ligand L5 (0.085 g,
0.200 mmol) and CrCl3(THF)3 (0.075 g, 0.200 mmol)
reacted to form 0.068 g (0.117 mmol) of a green solid in a
yield of 52.1%. IR (KBr; cmꢁ1): 2976, 1589 (mC@N), 1564,
1503, 1473, 1436, 1408, 1377, 1337, 1281, 1229, 1197,
1162, 1137, 1036, 857, 791, 756. Anal. Calc. for
C23H20Cl5CrN4 (581.7): C, 47.49; H, 3.47; N, 9.63. Found:
C, 47.41; H, 3.96; N, 9.45%.
4.3. Synthesis of complexes (C1–C6)
4.3.6. Complex C6
4.3.1. Complex C1
Analogous to the procedure for C1, ligand L6 (0.238 g,
0.600 mmol) and CrCl3(THF)3 (0.225 g, 0.600 mmol)
reacted to form 0.202 g (0.364 mmol) of a green solid in a
yield of 60.7%. IR (KBr; cmꢁ1): 2970, 1586 (mC@N), 1564,
1505, 1472, 1409, 1374, 1337, 1281, 1222, 1138, 1038,
862, 809, 748. Anal. Calc. for C26H28Cl3CrN4 (554.9): C,
56.28; H, 5.09; N, 10.10. Found: C, 56.15; H, 5.28; N,
9.94%.
Complexes C1–C6 were synthesized by the reaction of
CrCl3(THF)3 with the corresponding ligands in dichloro-
methane. A typical synthetic procedure, for C1, is as fol-
lows. A solution of CrCl3(THF)3 (0.225 g, 0.600 mmol)
and L1 (0.138 g, 0.600 mmol) in dichloromethane was
stirred at room temperature for 12 h, giving a green sus-
pension. The reaction volume was reduced, diethyl ether
was added, and a green solid was obtained, which was
washed repeatedly with diethyl ether and dried under
vacuum. The green powder (0.151 g, 0.279 mmol) was
obtained in a yield of 46.5%. IR (KBr; cmꢁ1): 2979,
1589 (mC@N), 1503, 1472, 1409, 1376, 1336, 1281, 1163,
1136, 1108, 1038, 752. Anal. Calc. for C25H26Cl3CrN4
(540.9): C, 55.52; H, 4.85; N, 10.36. Found: C, 55.32;
H, 4.90; N, 10.26%.
4.4. Procedure for ethylene oligomerization and
polymerization at 1 atm
Ethylene oligomerization and polymerizations were car-
ried out as follows: the catalyst precursor (chromium com-
plex) was dissolved in toluene in a Schlenk tube stirred with
a magnetic stirrer under an ethylene atmosphere (1 atm),
and the reaction temperature was controlled by a water
bath. The reaction was initiated by adding the desired
amount of cocatalyst. After the desired period of time, a
small amount of the reaction solution was collected with
a syringe and was quenched by the addition of 5% hydro-
chloric acid in an ice-water bath in accordance with the
oligomers of C4 and C6 produced. An analysis by gas chro-
matography (GC) was carried out to determine the distri-
bution of oligomers obtained. The remaining solution
was quenched with HCl–acidified ethanol (5%), and the
precipitated polyethylene was collected by filtration,
washed with ethanol, dried under vacuum at 60 °C to con-
stant weight, weighed, and finally characterized.
4.3.2. Complex C2
Analogous to the procedure for C1, ligand L2 (0.246 g,
0.600 mmol) and CrCl3(THF)3 (0.225 g, 0.600 mmol)
reacted to form 0.258 g (0.454 mmol) of a green solid in a
yield of 75.7%. IR (KBr; cmꢁ1): 2968, 1588 (mC@N), 1469,
1408, 1374, 1336, 1280, 1163, 1134, 1038, 855, 792, 752.
Anal. Calc. for C27H30Cl3CrN4 (568.9): C, 57.00; H, 5.32;
N, 9.85. Found: C, 57.69; H, 5.28; N, 9.86%.
4.3.3. Complex C3
Analogous to the procedure for C1, ligand L3 (0.263 g,
0.600 mmol) and CrCl3(THF)3 (0.225 g, 0.600 mmol)
reacted to form 0.213 g (0.356 mmol) of a green solid in a
yield of 59.4%. IR (KBr; cmꢁ1): 2970, 1589 (mC@N), 1564,
1504, 1472, 1409, 1382, 1338, 1280, 1218, 1162, 1133,
1038, 848, 806, 788, 759. Anal. Calc. for C29H34Cl3CrN4
(597.0): C, 58.35; H, 5.74; N, 9.39. Found: C, 58.30; H,
5.58; N, 9.30%.
4.5. Procedure for ethylene oligomerization and
polymerization at higher pressure
Ethylene oligomerization and polymerization were per-
formed in a 250 ml autoclave stainless steel reactor
equipped with a mechanical stirrer and a temperature con-
troller. A 100 ml amount of toluene containing the catalyst
precursor was transferred to the fully dried reactor under a
nitrogen atmosphere. The required amount of cocatalyst
(MAO, MMAO, or Et2AlCl) was then injected into the
reactor via a syringe. At the reaction temperature, the reac-
tor was sealed and pressurized to high ethylene pressure,
and the ethylene pressure was maintained with feeding of
4.3.4. Complex C4
Analogous to the procedure for C1, ligand L4 (0.234 g,
0.600 mmol) and CrCl3(THF)3 (0.225 g, 0.600 mmol)
reacted to form 0.176 g (0.321 mmol) of a green solid in a
yield of 53.5%. IR (KBr; cmꢁ1): 2977, 1591 (mC@N), 1567,
1505, 1474, 1409, 1376, 1338, 1289, 1164, 1109, 1137,
1033, 1008, 785, 757. Anal. Calc. for C23H20Cl3CrF2N4

Y. Chen et al. / Journal of Organometallic Chemistry 693 (2008) 750–762
761
Table 6
solved by direct methods and refined by full-matrix least
squares on F2. All non-hydrogen atoms were refined aniso-
tropically. All hydrogen atoms were placed in calculated
positions. Structure solution and refinement were per-
formed by using the SHELXL-97 package [45]. Crystal data
and processing parameters for C2 and C3 are summarized
in Table 6.
Crystal data and refinement details for C2 and C3
C2 ꢂ DMF
C30H37Cl3CrN5O
642.00
Green
C3
Empirical formula
Formula weight
Crystal color
Temperature (K)
Wavelength (A)
C29H34Cl3CrN4
596.95
Green
293(2)
0.71073
Monoclinic
P21/c
8.6022(5)
31.5105(2)
13.5669(7)
90
126.814(3)
90
2944.1(3)
4
1.347
0.686
1244
0.35 ꢀ 0.25 ꢀ 0.20
1.98–28.30
ꢁ11 6 h 6 11
ꢁ41 6 k 6 41
ꢁ17 6 l 6 18
30972
296(2)
˚
0.71073
Monoclinic
P2(1)/n
8.1681(2)
11.801(2)
32.964(7)
90
94.40(3)
90
3168.2(1)
4
1.346
0.645
1340
0.45 ꢀ 0.25 ꢀ 0.04
2.48–25.00
ꢁ9 6 h 6 9
ꢁ14 6 k 6 14
ꢁ39 6 l 6 38
9197
Crystal system
Space group
Acknowledgement
˚
a (A)
˚
b (A)
This work was supported by NSFC No. 20674089 and
MOST No. 2006AA03Z553.
˚
c (A)
a (°)
b (°)
c (°)
Appendix A. Supplementary material
3
˚
Volume (A )
Z
CCDC 660792 and 660793 contain the supplementary
crystallographic data for C2 and C3. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.jorganchem.2007.11.049.
Dcalc (Mg mꢁ3
)
l (mmꢁ1
F(000)
)
Crystal size (mm)
h Range (°)
Limiting indices
Number of reflections
collected
Number of unique
reflections
Rint
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