2-(R-1H-Benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridyliron(II) dichlorides: Synthesis, characterization and the ethylene oligomerization behavior

Article abstract of DOI:10.1016/j.ica.2011.09.035

Iron(II) dichloride complexes bearing 2-(methyl-substituted 1H-benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridines (Fe1-Fe6) or 2-(chloro-substituted 1H-benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridines (Fe7-Fe12) were synthesized and characterized by FT-IR

Full text of DOI:10.1016/j.ica.2011.09.035

Inorganica Chimica Acta 379 (2011) 70–75
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2-(R-1H-Benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridyliron(II) dichlorides:
Synthesis, characterization and the ethylene oligomerization behavior
b
b,
Liping Zhang a,b, Xiaohua Hou a,b, Jiangang Yu b, Xia Chen a, , Xiang Hao , Wen-Hua Sun
⇑
⇑
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:
Received 21 May 2011
Received in revised form 7 September 2011
Accepted 16 September 2011
Available online 29 September 2011
Iron(II) dichloride complexes bearing 2-(methyl-substituted 1H-benzoimidazol-2-yl)-6-(1-aryliminoeth-
yl)pyridines (Fe1–Fe6) or 2-(chloro-substituted 1H-benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridines
(Fe7–Fe12) were synthesized and characterized by FT-IR and elemental analysis. Single crystal X-ray
crystallographic analyses revealed that complexes Fe2 and Fe3 possessed a distorted square-pyramidal
geometry at iron. Upon activation with either MAO or MMAO, all iron pro-catalysts showed good activ-
ities toward ethylene oligomerization with high selectivity for a-olefins and high K values. The influence
of the reaction conditions and the nature of the ligands on the catalytic performance of these iron com-
plexes were investigated.
Keywords:
2-Benzimidazol-2-yl-6-iminopyridine
Iron complex
Ó 2011 Elsevier B.V. All rights reserved.
Ethylene oligomerization
1. Introduction
[43,44] showed interesting results in ethylene oligomerization
[45–47]. As a consequence, the title iron complexes were synthe-
Over the past decade or so, iron pro-catalysts for ethylene acti-
vation have attracted increased attention, both in academic and
industrial research [1–8], primarily due to the emergence of highly
efficient systems containing the 2,6-diiminopyridyl ligand set
[9–11]. Subsequently, the number of research papers in the area
mushroomed, with much work done on varying the 2,6-diimino-
pyridine substituents and using various reaction conditions
[12–16], as well as synthesizing unsymmetrical 2,6-diiminopyri-
dines [17–22]. Indeed, numerous iron complexes have been ex-
plored with various alternate tridentate ligand sets such as
N^N^O [23], N^P^N [24], P^N^N [25,26], and most importantly
alternative N^N^N models [27–42]; the latter have also been the fo-
cus of our investigations [31–42]. Significant advantages have been
exhibited by iron pro-catalysts, such as high efficiency, price
(cheap) and environmental-friendly iron precursors. Moreover,
the ability to form a wide variety of vinyl-type products as indicated
by the oligomers and polymers that have obtained to-date. Re-
cently, iron pro-catalysts bearing 2-(benzimidazole)-6-(1-arylimi-
noethyl)pyridine derivatives were shown to exhibit high activities
in ethylene oligomerization [37–39]; ligand modification was con-
ducted at the 1-substituent position of the imidazole ring within
the benzimidazole. Furthermore, 2-(benzimidazole)-6-(1-arylimi-
noethyl)pyridines were modified with substituents at the benzo-
ring of the benzimidazole, and the nickel pro-catalysts thereof
sized and characterized. When activated with either methylalumi-
noxane (MAO) or modified methylaluminoxane (MMAO), all
iron pro-catalysts showed high catalytic activity in ethylene
oligomerization.
2. Results and discussion
2.1. Synthesis and characterization of the title complexes
The ligands, as prepared by our previous procedure, possessed
two isomers in a 3:2 M ratio, viz. 2-(7-methyl-1H-benzoimidazol-
2-yl)-6-(1-aryliminoethyl)pyridines (major) and 2-(4-methyl
-1H-benzoimidazol-2-yl)-6-(1-aryliminoethyl)pyridines
(minor,
L1–L6) [43], and 2-(6-chloro-1H-benzoimidazol-2-yl)-6-(1-arylimi-
noethyl)pyridines (major) and 2-(5-chloro-1H-benzoimidazol
-2-yl)-6-(1-aryliminoethyl)pyridines (minor, L7–L12) [44]. The
stoichiometric reaction of the respective 2-(R-1H-benzoimidazol-
2-yl)-6-(1-aryliminoethyl)pyridines and FeCl2Á4H2O in ethanol pre-
cipitated
a
blue powder of the corresponding 2-(R-1H-ben-
dichlorides
zoimidazol-2-yl)-6-(1-aryliminoethyl)pyridyliron
(Fe1–Fe12) in good to high yield (60–92%) at room temperature
(Scheme 1).
All iron complexes were air-stable in the solid state, and ele-
mental analytical data confirmed their formula; they slowly turn
from blue to yellow in solution on exposure to air (oxidation of
Fe2+). Compared with the IR spectra of the free ligands, for which
the C@N stretching frequencies are in the range of 1639–
⇑
Corresponding authors. Tel.: +86 10 62557955; fax: +86 10 62618239 (W.-H.
Sun).
E-mail address: 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.2011.09.035

L. Zhang et al. / Inorganica Chimica Acta 379 (2011) 70–75
71
the 2,6-diethylphenyl group. A similar geometry was also observed
for related iron(II) complexes [37–39]. Selected bond lengths and
angles are listed in Table 1.
H
N
R1
N
FeCl2
EtOH
With similar structural features to Fe2, the molecular structure
of complex Fe3 is shown in Fig. 2. According to Table 1, the bond
lengths of Fe–N and Fe–Cl are close to those of Fe2. Slightly differ-
ent to Fe2, the basal plane (N2, N3, N4, Cl1) in Fe3 is more flat with
less deviation (0.201 Å) of Cl1 to the plane (N2, N3, N4), whilst the
iron atom deviates from the plane (N2, N3, N4) by 0.371 Å to the
opposite side. The two nitrogen atoms (N2 and N4) occupy the ax-
ial sites with the bond angle of 143.34(9)° for N2–Fe1–N4. The
dihedral angles between the equatorial plane (which formed by
N3, Cl1 and Cl2) with the phenyl ring and the benzimidazole plane
are 52.4° and 86.2°, respectively. Similar to the structure of Fe2,
longer Fe–N bonds than observed in nickel complexes bearing
the same ligands are observed in Fe3. Selected bond lengths and
angles are listed in Table 1.
N
N
R2
R1
R
N
H
N
R1
N
Fe
N
Cl
Cl
R2
R1
R
R1
R2
R = Me
R = Cl
Me Et iPr Me
Et
Cl
H
Fe6
H
H
H
Me Me
Fe1 Fe2 Fe3 Fe4 Fe5
Fe7 Fe8 Fe9 Fe10 Fe11 Fe12
Scheme 1. Synthesis of iron complexes.
2.2. Ethylene oligomerization
Based on the series of ligands used, the complexes were classi-
fied into two groups, namely Fe1–Fe6 and Fe7–Fe12. Complexes
Fe1 and Fe7 were selected as the representative pre-catalysts and
were investigated for determining the optimum reaction conditions
at 30 atm of ethylene using the co-catalyst methylaluminoxane
(MAO) or modified methylaluminoxane (MMAO). The predominant
products isolated were oligomers in the range C4–C28, with very
1650 cmÀ1, the C@N stretching vibrations of the complexes Fe1–
Fe12 were shifted toward lower frequencies (1591–1600 cmÀ1
)
and with reduced intensity of the absorptions, indicating effective
coordination between the imino group and the iron center. To con-
firm their unambiguous molecular structures, single crystals of the
representative complexes Fe2 and Fe3 were obtained by slow dif-
fusion of diethyl ether into methanol solutions under a nitrogen
atmosphere. It is necessary to mention that there are present
two isomers of the iron complex in the solid state, which are con-
sistent with two isomers of the ligand. The major isomers of both
iron complexes C2 and C3 have the coordinated ligands with the
methyl group (linked to the carbon C4) on benzimidazole ring far
to iron atom; meanwhile the minor isomers of them have the
ligands with the methyl group (linked to the carbon C1) on the
benzimidazole ring close to iron atom.
The structure of complex Fe2 (Fig. 1) is best described as dis-
torted square-pyramidal, with the basal plane composed of N2,
N3, N4, and Cl1. The iron atom deviates from the plane (N2, N3
and N4) by 0.157 Å, whereas Cl1 deviates by 0.999 Å on the opposite
side. There are slight differences between Fe–Cl1 (2.2634(11) Å) and
Fe–Cl2 (2.3774(11) Å) due to the apical elongation in the square-
pyramidal geometry. The Fe–N3 (pyridyl) (2.153(3) Å) bond is
shorter (by about 0.09 Å) than the Fe–N4 (imino) (2.240(3) Å) bond
high selectivity observed for linear
a-olefins. Interestingly, the
most suitable co-catalyst for Fe1–Fe6 was MMAO, whilst MAO
was preferable for Fe7–Fe12. The distribution of oligomers ob-
tained in all cases resembled the Schulz–Flory distribution, which
is characterized by the constant K, where K represents the probabil-
ity of chain propagation (K = rate of propagation/((rate of propaga-
tion) + (rate of chain transfer) = (moles of Cn + 2)/(moles of Cn))
[45–49] and the K values are determined by the molar ratio of the
C14 and C12 fractions. The results are summarized in Tables 2
and 3, respectively.
2.2.1. Ethylene oligomerization by Fe1–Fe6
As shown in Table 2 (entries 1–2 in Table 2), pro-catalyst Fe1
performed with higher activity using MMAO than with MAO,
therefore further detailed investigations of Fe1 with MMAO were
carried out by changing the Al/Fe molar ratios (entries 2–7 in Table
and only
a little shorter than the Fe–N2 (benzimidazole)
(2.173(3) Å). The equatorial plane (which formed by N3, Cl1 and
Cl2) is almost perpendicular to the benzimidazole ring with the
dihedral angle of 84.6°, and has a dihedral angle of 134.4° with
Table 1
Selected bond lengths (Å) and angles (°) for complexes Fe2 and Fe3.
Fe2
Fe3
Bond length (Å)
Fe–N2
Fe–N3
Fe–N4
Fe–Cl1
Fe–Cl2
N1–C8
N2–C8
N4–C14
2.173(3)
2.153(3)
2.240(3)
2.2634(11)
2.3774(11)
1.343(4)
1.331(5)
1.284(5)
2.183(3)
2.147(3)
2.244(2)
2.2696(12)
2.3936(12)
1.346(4)
1.325(4)
1.287(4)
Bond angle (°)
N2–Fe–N3
N2–Fe–N4
N3–Fe–N4
N2–Fe–Cl1
N3–Fe–Cl1
N4–Fe–Cl1
Cl–Fe–Cl2
N2–Fe–Cl2
N3–Fe–Cl2
N4–Fe–Cl2
73.87(11)
145.58(11)
72.22(11)
102.93(9)
145.07(8)
101.39(8)
114.65(5)
93.34(8)
73.79(10)
143.34(9)
72.39(9)
105.58(8)
155.09(8)
99.22(7)
106.98(4)
96.01(8)
97.78(8)
102.12(7)
100.27(8)
98.26(8)
Fig. 1. ORTEP molecular structure of Fe2. Thermal ellipsoids are shown at 30%
probability. Hydrogen atoms and solvent molecules have been omitted for clarity.

72
L. Zhang et al. / Inorganica Chimica Acta 379 (2011) 70–75
30 °C. Iron pro-catalysts are required to be activated with a suitable
amount of co-catalyst; a lower Al/Fe ratio would mean the MMAO
present would be consumed by impurities in the solvent, whilst
higher Al/Fe molar ratios would increase the amount of isobutyl
groups from MMAO and hinder the insertion reaction of ethylene
at the active species [37,48]. At room temperature, a lower catalytic
activity (entry 8 in Table 2) was observed than that at 30 °C (entry 5
in Table 2), indicating the enhanced thermal stability of the active
species present. However, deactivation was evident at higher reac-
tion temperatures (entries 9–11 in Table 2), indicating unstable ac-
tive species or partially lower solubility of the ethylene in solution.
Under the optimum condition (Al/Fe molar ratio of 200 and 30 °C),
iron pro-catalysts Fe2–Fe6 were investigated (entries 12–16 in
Table 2).
Fig. 2. ORTEP Molecular structure of Fe3. Thermal ellipsoids are shown at 30%
probability. Hydrogen atoms and solvent molecules have been omitted for clarity.
The nature of the ligands was found to affect the catalytic
behavior of the iron pro-catalysts Fe1–Fe6 (entries 5 and 12–16
in Table 2), and variation of the R1 and R2 substituents resulted
in changes in catalytic performance. Regarding the steric hindrance
of the R1 group, pro-catalysts having a less bulky R1 group led to an
2) and the reaction temperature (entries 5 and 8–11 in Table 2). The
optimum activity was observed at the Al/Fe molar ratio of 200 at
Table 2
Ethylene oligomerization with Fe1–Fe6.a
Entry
Complex
Co-cat.
Al/Fe
T (°C)
K
Activityb
Oligomer distributionc
C4/ C6/
9.6
R
C
R
C
C8/
R
C
PC10/R
C
a-Olefin (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe2
Fe3
Fe4
Fe5
Fe6
MAO
1000
1000
500
250
200
150
100
200
200
200
200
200
200
200
200
200
30
30
30
30
30
30
30
20
40
60
80
30
30
30
30
30
0.92
0.95
0.67
0.63
0.69
0.67
0.83
0.76
0.70
0.72
0.90
0.52
0.61
0.66
0.63
0.52
0.7
1.8
2.5
6.0
18.6
9.6
3.2
11.3
5.0
4.2
3.4
8.0
4.4
30.3
11.9
1.8
44.5
49.5
50.4
38.4
40.2
40.0
74.8
45.1
39.6
34.4
51.1
54.2
61.9
41.0
40.3
39.1
29.3
6.3
8.9
13.4
13.3
7.5
16.6
31.6
22.4
24.3
21.4
29.5
17.1
25.2
25.8
30.0
20.3
11.5
9.6
93.7
95.2
95.0
98.0
98.2
98.8
97.5
96.3
96.6
97.5
96.8
97.5
98.0
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
12.6
18.3
23.9
25.1
23.0
5.2
18.3
21.4
22.0
19.0
24.2
21.7
28.8
24.8
25.8
2.9
11.4
13.2
13.2
9.6
10.1
6.8
14.9
13.7
13.3
15.3
21.2
21.8
>99
>99
98.1
a
b
c
Conditions: 5 lmol Fe, 30 min, 30 atm ethylene; 100 mL toluene.
105 g molÀ1(Fe) hÀ1
Determined by GC.
.
Table 3
Ethylene oligomerization with Fe7–Fe12.a
Entry
Complex
Co-cat.
Al/Fe
T (°C)
K
Activityb
Oligomer distributionc
C4/ C6/
R
C
R
C
C8/
R
C
PC10/R
C
a-Olefin (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Fe7
Fe7
Fe7
Fe7
Fe7
Fe7
Fe7
Fe7
Fe7
Fe7
Fe7
Fe8
Fe9
Fe10
Fe11
Fe12
MMAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
1000
1000
500
350
250
200
150
250
250
250
250
250
250
250
250
250
30
30
30
30
30
30
30
20
40
60
80
30
30
30
30
30
0.56
0.75
0.72
0.71
0.71
0.72
0.77
0.65
0.69
0.67
0.75
0.76
0.79
0.72
0.63
0.52
0.6
4.1
9.6
13.2
28.2
7.5
41.1
34.2
33.6
34.4
32.2
36.8
28.3
30.9
33.6
35.5
28.7
33.5
38.5
39.3
34.4
49.2
21.1
20.6
20.9
21.6
22.3
20.4
13.0
20.7
22.8
24.1
28.3
20.8
22.0
20.9
28.6
17.1
10.3
11.2
12.5
12.8
13.7
11.9
10.4
12.4
13.2
12.3
14.9
12.0
11.9
11.6
15.1
9.0
27.5
34.0
33.0
31.2
31.8
30.9
48.3
36.0
30.4
28.1
28.1
33.7
27.6
28.2
21.9
24.7
95.5
96.3
98.0
98.2
98.6
99.0
98.4
98.8
98.5
98.2
97.8
>99
>99
98.0
>99
97.3
2.7
25.3
24.1
7.5
5.1
8.0
6.7
12.9
7.5
5.6
a
b
c
Conditions: 5 lmol Fe, 30 min, 30 atm ethylene, 100 mL toluene.
105 g molÀ1(Fe) hÀ1
Determined by GC.
.

L. Zhang et al. / Inorganica Chimica Acta 379 (2011) 70–75
73
Table 4
Crystal data and structure refinement for Fe2 and Fe3.
Fe2
Fe3ÁCH3OH
Formula
Formula weight
T (K)
C
25H26Cl2FeN4
C28H34Cl2FeN4O
569.34
173(2)
509.25
173(2)
k (Å)
0.71073
0.71073
Crystal system
Space group
a (Å)
orthorhombic
Pbca
triclinic
P1
8.7280(17)
ꢀ
15.341(3)
b (Å)
15.906(3)
9.4894(19)
c (Å)
19.334(4)
17.988(4)
a
(°)
90
89.70(3)
b (°)
90
77.66(3)
c
(°)
90
69.20(3)
1356.4(5)
V (Å3)
4717.8(16)
Z
8
2
Dcalc (g cmÀ3
)
1.434
0.887
1.394
0.781
l
(mmÀ1
)
F(000)
2112
596
Crystal size (mm)
h range (°)
0.22 Â 0.22 Â 0.08
0.13 Â 0.11 Â 0.02
2.11–27.46
2.30–27.47
Limiting indices
À18 6 h 6 19, À20 6 k 6 20, À20 6 l 6 25
À11 6 h 6 11, À12 6 k 6 12, À23 6 l 6 23
Reflections collected
Independent reflections (Rint
Number of parameters
Completeness to h (%)
Goodness-of-fit (GOF) on F2
31141
18403
)
5383 (0.0401)
289
99.7
6214 (0.0549)
363
99.9
0.932
1.102
Final R indices [I > 2
R indices (all data)
Maximum/minimum
r
(I)]
R1 = 0.0695, wR2 = 0.1873
R1 = 0.0719, wR2 = 0.1896
1.079 and À0.858
R1 = 0.0617, wR2 = 0.1431
R1 = 0.0747, wR2 = 0.1431
0.419 and À0.542
D
q
[a] (e ÅÀ3
)
enhanced activity and the order Fe1 > Fe2 > Fe3 (entries 5, 12, 13,
Table 2), and also Fe4 was higher than Fe5 (entries 14 and 15 in
Table 2). Such phenomena were reversed to the previously cata-
lytic observations by their analogues [37,38], indicating the charac-
teristic feature of the current pre-catalysts containing an active
N–H group within the 1H-benzoimidazol-2-yl substituent. More-
over, Fe4 and Fe5 with additional methyl groups exhibited higher
activities than did the corresponding analogues of Fe1 and Fe2.
Such phenomena were in agreement with other catalytic systems
of iron pro-catalysts [36–38] due to their enhanced solubility.
The procatalyst Fe6 (entry 16 in Table 2) containing ligands with
chloro-substituents showed a lower activity, consistent with a
lower solubility of a metal complex ligated by halo-organic com-
pounds [37–39,43,44]. In all cases, the oligomers were mostly
Although both series of iron pro-catalysts herein performed
with high activities for ethylene oligomerization, they did not sur-
pass the high activities associated with other iron pro-catalysts
[31,32,37–39]. Interestingly, the substituents which were modified
were far from the metal center, the synergic catalytic performances
of iron pro-catalysts with different co-catalysts were changed with
the influences of R (methyl- or chloro-) substituents in their
ligands. Promisingly, such modified iron pro-catalysts showed high
selectivity for linear
a-olefins with high K values. Thus these
systems hold further promise for exploration for industrial
applications.
3. Conclusion
linear a-olefins with high K values.
Iron(II) dichlorides bearing 2-(methyl substituted 1H-ben-
zoimidazol-2-yl)-6-(1-aryliminoethyl)pyridines (Fe1–Fe6) and
2-(chloro substituted 1H-benzoimidazol-2-yl)-6-(1-aryliminoeth-
yl)pyridines (Fe7–Fe12), were synthesized and characterized. X-
ray crystallographic studies on the iron complexes Fe2 and Fe3
revealed a distorted square-pyramidal geometry at iron. When
activated with MMAO, pro-catalysts Fe1–Fe6 gave high activities
(61.86 Â 106 gÁ(mol Fe)À1 hÀ1) in ethylene oligomerization, whilst
pro-catalysts Fe7–Fe12 showed high activities (2.82 Â 106 gÁ(mol
Fe)À1 hÀ1) upon treatment with MAO. All oligomers produced were
2.2.2. Ethylene activation for the Fe7–Fe12
Employing MAO or MMAO as co-catalyst (entries 1–2 in Table
3), pro-catalyst Fe7 showed a higher activity with MAO than with
MMAO. Changing the molar ratio of MAO to iron (entries 2–7 in
Table 3), the best activity was observed at the Al/Fe molar ratio
of 250 (entry 5 in Table 3). In the temperature range of 20–80 °C
(entries 5, 8–11 in Table 3), the optimum reaction temperature
was found to be 30 °C. Therefore, the pro-catalysts Fe8–Fe12 were
investigated with the Al/Fe molar ratio of 250 at 30 °C (entries
12–16 in Table 3). All iron pro-catalysts exhibited high activities
towards ethylene oligomerization and with high selectivity for lin-
in the range of C4–C28 with very high selectivity for linear a-ole-
fins and high K values.
ear
a-olefins and high K values.
For the influence of the R1 substituents, similar trends for Fe7–
4. Experimental
Fe12 were observed as seen in the series Fe1–Fe6; the catalytic
activities were in the order Fe7 > Fe8 > Fe9 (entries 5, 12 and 13
in Table 3), and Fe10 > Fe11 (entries 14 and 15 in Table 3). How-
ever, the R2 substituent did not show any positive effects. With
additional methyl groups, the catalytic activities were decreased
with the observed order Fe7 > Fe10 and Fe8 > Fe11. Again,
pro-catalyst Fe12 (entry 16 in Table 3) bearing 2,6-dichlorophenyl
groups exhibited a lower activity.
4.1. General considerations
All manipulations of air- and moisture-sensitive compounds
were performed at nitrogen atmosphere using standard Schlenk
techniques. Toluene was refluxed over sodium-benzophenone
and distilled under argon prior to use. Methylaluminoxane
(MAO, a 1.46 M solution in toluene) and modify methylaluminox-

74
L. Zhang et al. / Inorganica Chimica Acta 379 (2011) 70–75
ane (MMAO, 1.93 M in heptane, 3A) were purchased from Akzo
Nobel Corp. Other reagents were purchased from Aldrich or Acros
Chemicals. IR spectra were recorded on a Perkin–Elmer System
2000 FT-IR spectrometer. Elemental analysis was carried out
using an Flash EA 1112 micro-analyzer. GC analysis was per-
formed with a Varian CP-3800 gas chromatograph equipped with
Data for Fe8 are as follows. Yield: 88.7%. IR (KBr; cmÀ1): 3067.8
(m), 2967.5 (m), 1596.5 (vs), 1486.3 (m), 1406.3 (m), 1315.7 (s),
1204.6 (w), 1056.1 (m), 815.0 (s), 767.2 (m). Anal. Calc. for
C24H23Cl3FeN4 (528.0): C, 54.42; H, 4.38; N, 10.58. Found: C,
54.52; H, 4.71; N, 10.27%.
Data for Fe9 are as follows. Yield: 82.1%. IR (KBr; cmÀ1): 3067.8
(m), 2967.5 (m), 1596.5 (vs), 1486.3 (m), 1406.3 (m), 1315.7 (s),
1204.6 (w), 1056.1 (m), 815.0 (s), 767.2 (m). Anal. Calc. for
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 calcu-
lated by referencing with the mass of the solvent on the basis of
the prerequisite that the mass of each fraction was approxi-
mately proportional to its integrated areas in the GC trace.
C26H27Cl3FeN4 (556.1): C, 55.99; H, 4.88; N, 10.05. Found: C,
55.72; H, 4.93; N, 9.83%.
Data for Fe10 are as follows. Yield: 79.8%. IR (KBr; cmÀ1):
3032.0 (m), 1593.8 (vs), 1480.9 (m), 1406.7 (m), 1318.1 (s),
1215.1 (s), 1019.4 (m), 925.5 (m), 848.7 (s), 809.1 (s), 740.3 (m).
Anal. Calc. for C23H21Cl3FeN4 (514.0): C, 53.57; H, 4.10; N, 10.87.
Found: C, 53.98; H, 4.31; N, 10.55%.
Selectivity for the linear
ear -olefin of all fractions)/(total amount of oligomer products)
in percentage.
a-olefin was defined as (amount of lin-
a
2-(Methyl-substituted 1H-benzoimidazol-2-yl)-6-(1-arylimino-
ethyl)pyridines were prepared according to our previous work
[43], and 2-(chloro-substituted 1H-benzoimidazol-2-yl)-6-(1-
aryliminoethyl)pyridines were prepared according to our previous
work [44].
Data for Fe11 are as follows. Yield: 78.9%. IR (KBr; cmÀ1):
3448.4 (w), 3048.9 (m), 2968.7 (m), 1598.5 (vs), 1460.4 (m),
1410.5 (m), 1318.6 (s), 1146.8 (s), 966.4 (m), 858.6 (s), 740.3 (s).
Anal. Calc. for C25H25Cl3FeN4 (542.1): C, 55.23; H, 4.63; N, 10.30.
Found: C, 55.15; H, 4.77; N, 10.01%.
4.2. Synthesis of tridentate iron complexes Fe1–Fe12
Data for Fe12 are as follows. Yield: 60.2%. IR (KBr; cmÀ1):
3437.9 (m), 3058.2 (m), 1599.7 (vs), 1557.1 (m), 1485.6 (m),
1435.1 (s), 1319.9 (s), 1130.7 (m), 977.6 (m), 863.0 (m), 814.0 (s),
791.4 (m), 659.3 (m). Anal. Calc. for C20H13Cl5FeN4 (539.9): C,
44.28; H, 2.42; N, 10.33. Found: C, 44.40; H, 2.51; N, 10.36%.
The complexes Fe1–Fe12 were synthesized by the reaction of
FeCl2Á4H2O with the corresponding ligands in ethanol. A typical
synthetic procedure for Fe1 can be described as follows: the ligand
L1 (0.12 g, 0.35 mmol) and FeCl2Á4H2O (0.069 g, 0.35 mmol) were
added to a Schlenk tube, followed by the addition of freshly dis-
tilled ethanol (5 mL) with rapid stirring at room temperature.
The solution turned green immediately, and a blue precipitate
was formed. The reaction mixture was stirred for 12 h, and then
the precipitate was washed with diethyl ether twice and dried to
give the pure product as a blue powder in 91.3% yield. IR (KBr;
cmÀ1): 3325.1 (m), 2971.3 (w), 1591.5 (vs), 1472.5 (s), 1206.4 (s),
1041.4 (m), 790.8 (s), 762.6 (s). Anal. Calc. for C23H22Cl2FeN4
(480.1): C, 57.41; H, 4.61; N, 11.64. Found: C, 57.78; H, 4.23; N,
11.38%.
4.3. Procedure for ethylene oligomerization
Ethylene oligomerization was performed in a stainless steel
autoclave (0.5 L capacity) equipped with a gas ballast through a
solenoid clave for continuous feeding of ethylene at constant pres-
sure. A 100 mL amount of toluene containing the catalyst precursor
and the required amount of co-catalyst was transferred into the
fully dried reactor via a syringe under a nitrogen atmosphere. At
the reaction temperature, the reactor was sealed and pressurized
to high ethylene pressure, and the ethylene pressure was main-
tained during feeding of ethylene. After the reaction mixture was
stirred for the desired period, the pressure was released and a
small amount of the reaction solution was collected, which was
then analyzed by gas chromatography (GC) to determine the com-
position and mass distribution of the oligomers obtained. To keep
the reaction temperature constant, the autoclave is equipped with
inert heat exchange tube of water.
Data for Fe2 are as follows. Yield: 86.4%. IR (KBr; cmÀ1): 3396.3
(m), 3051.0 (m), 2970.4 (m), 1594.9 (vs), 1448.3 (s), 1378.0 (s),
1318.5 (s), 1202.0 (s), 792.2 (s), 751.9 (s). Anal. Calc. for
C25H26Cl2FeN4 (508.1): C, 58.96; H, 5.15; N, 11.00. Found: C, 58.69;
H, 5.20; N, 10.75%.
Data for Fe3 are as follows. Yield: 88.3%. IR (KBr; cmÀ1): 3484.8
(s), 3058.4 (m), 2966.0 (m), 1591.0 (vs), 1472.0 (s), 1444.9 (s),
1319.5 (s), 1202.7 (s), 816.6 (m), 766.7 (s), 746.1 (s). Anal. Calc.
for C27H30Cl2FeN4 (536.1): C, 60.35; H, 5.63; N, 10.43. Found: C,
60.37; H, 5.59; N, 10.08%.
4.4. X-ray crystallographic studies
Data for Fe4 are as follows. Yield: 83.4%. IR (KBr; cmÀ1): 3449.7
(m), 3082.6 (w), 1592.3 (vs), 1476.3 (s), 1320.9 (s), 1213.5 (s),
854.5 (s), 788.9 (s), 746.8 (s). Anal. Calc. for C24H24Cl2FeN4
(494.2): C, 58.21; H, 4.88; N, 11.31. Found: C, 58.33; H, 5.01; N,
11.17%.
Single-crystals of Fe2 and Fe3 suitable for X-ray diffraction were
obtained by slow diffusion of diethyl ether into methanol solution.
Data were collected with a Rigaku RAXIS Rapid IP diffractometer
with graphite monochromated MoK
a radiation (k = 0.71073 Å).
Data for Fe5 are as follows. Yield: 81.0%. IR (KBr; cmÀ1): 3423.9
(m), 3051.9 (m), 2966.3 (m), 1600.9 (vs), 1568.6 (m), 1460.2 (s),
1417.5 (m), 1318.3 (s), 1213.8 (s), 858.4 (m), 786.2 (m), 746.2 (s).
Anal. Calc. for C26H28Cl2FeN4 (522.1): C, 59.68; H, 5.39; N, 10.71.
Found: C, 59.33; H, 5.43; N, 10.50%.
Cell parameters 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 non-hydrogen atoms were refined anisotropically.
All hydrogen atoms were placed in calculated positions. Structure
solution and refinement were performed by using the SHELXL-97
package [50]. Crystal data and processing parameters for Fe2 and
Fe3 are summarized in Table 4.
Data for Fe6 are as follows. Yield: 77.6%. IR (KBr; cmÀ1): 3418.4
(w), 3058.2 (m), 1600.9 (vs), 1478.4 (m), 1436.3 (s), 1318.9 (s),
1043.9 (m), 978.4 (m), 773.6 (m), 791.4 (m). Anal. Calc. for
C21H16Cl4FeN4 (519.9): C, 48.32; H, 3.09; N, 10.73. Found: C,
48.39; H, 2.89; N, 10.50%.
Data for Fe7 are as follows. Yield: 90.3%. IR (KBr; cmÀ1): 3343.9
(m), 3058.2 (m), 1597.0 (vs), 1476.3 (m), 1423.8 (m), 1315.3 (s),
1210.7 (m), 923.5 (m), 809.7 (m), 745.6 (s). Anal. Calc. for
Acknowledgments
C
22H19Cl3FeN4 (500.0): C, 52.68; H, 3.82; N, 11.17. Found: C,
This work is supported by MOST 863 Program No. 2009AA0
33601 and NSFC No. 21072120.
52.49; H, 3.55; N, 11.25%.

L. Zhang et al. / Inorganica Chimica Acta 379 (2011) 70–75
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