Ethylene oligomerization catalyzed by a novel iron complex containing fluoro and methyl substituents

Article abstract of DOI:10.1016/j.molcata.2004.05.014

A novel 2,6-bis(imino)pyridyl iron complex, [2,6-(2-F-4-CH ₃C₆H₃N=CCH₃)₂C ₅H₃N]FeCl₂·H₂O (3) has been synthesized by the reaction of the corresponding bis(imino)pyridyl ligand with FeCl₂·4H₂O in tetrahydrofuran (THF). The activity of complex 3 reaches to 10⁷ g/mol Fe h in ethylene oligomerization, and the products are mostly (>90%) 1-butene, 1-hexene and 1-octene. As reaction temperature increases, the activities decrease rapidly and the product distributions shift to the low-molecular-weight part. The product distributions are almost unchanged when the Al/Fe molar ratio increases from 420 to 2500, but the catalytic activities increase rapidly at first and then decrease. Other three complexes {2,6-(2-X-4-Y-6-ZC₆H₃N=CCH ₃)₂C₅H₃N}FeCl₂· nH₂O (X = CH₃, Y = CH₃, Z = H, n = 0 (1), X = F, Y = H, Z = F, n = 1 (2)) and {Fe[2,6-(2-FC₆H₃N=CCH ₃)₂C₅H₃N]₂} ²⁺[FeCl₄]^2-·H₂O (4) have been also synthesized for comparison and used for ethylene oligomerization. The ortho and para substituents on aniline have performed effects on oligomerization activities and distributions of oligomers for 2,6-bis(imino)pyridyl iron complexes.

Full text of DOI:10.1016/j.molcata.2004.05.014

Journal of Molecular Catalysis A: Chemical 219 (2004) 249–254
Ethylene oligomerization catalyzed by a novel iron complex
containing fluoro and methyl substituents
Zhicheng Zhanga,b, Junfeng Zoua,b, Nannan Cuia,b, Yucai Kea, Youliang Hua,∗
a
State Key Laboratory of Engineering Plastics, Institute of Chemistry, Center for Molecular Science, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, PR China
Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
b
Received 5 February 2004; received in revised form 17 May 2004; accepted 18 May 2004
Available online 7 July 2004
Abstract
=
A novel 2,6-bis(imino)pyridyl iron complex, [2,6-(2-F-4-CH3C6H3N CCH3)2C5H3N]FeCl2·H2O (3) has been synthesized by the reaction
of the corresponding bis(imino)pyridyl ligand with FeCl2·4H2O in tetrahydrofuran (THF). The activity of complex 3 reaches to 107 g/mol
Fe h in ethylene oligomerization, and the products are mostly (>90%) 1-butene, 1-hexene and 1-octene. As reaction temperature increases,
the activities decrease rapidly and the product distributions shift to the low-molecular-weight part. The product distributions are almost
unchanged when the Al/Fe molar ratio increases from 420 to 2500, but the catalytic activities increase rapidly at first and then decrease.
=
Other three complexes {2,6-(2-X-4-Y-6-ZC6H3N CCH3)2C5H3N}FeCl2·nH2O (X = CH3, Y = CH3, Z = H, n = 0 (1), X = F, Y = H, Z
= F, n = 1 (2)) and {Fe[2,6-(2-FC6H3N CCH3)2C5H3N]2}2+[FeCl4]2−·H2O (4) have been also synthesized for comparison and used for
=
ethylene oligomerization. The ortho and para substituents on aniline have performed effects on oligomerization activities and distributions
of oligomers for 2,6-bis(imino)pyridyl iron complexes.
© 2004 Elsevier B.V. All rights reserved.
Keywords: 2,6-Bis(imino)pyridyl iron complex; Ethylene oligomerization; ␣-Olefin; Oligomers distribution
1. Introduction
other complexes [6–9] were synthesized and reported for
oligomerization of ethylene, and most of their ligands con-
tain alkyls on the ortho position of aryl rings. But the dis-
tributions of oligomers are very wide because of the large
steric hindrance of alkyl substituents.
Linear low-molecular-weight ␣-olefins are used primar-
ily as comonomers for the production of linear low-density
polyethylene (LLDPE), plasticizers and synthetic lubricants.
Since Ziegler’s original work on AlR3 catalysis of ethylene
oligomerization, there has been considerable sustained in-
terest in developing new oligomerization catalysts. In 1998,
Small and Brookhart [1–3] and Britovsek et al. [4,5] reported
iron and cobalt complexes with mono-alkyl-substituted
bis(imino)pyridyl ligands. Compared with traditional
Ziegler-Natta catalysts of early transition metals, the novel
late transition metal catalysts possess advantages not only in
catalytic activity and selectivity, but also in their potential for
tolerating heteroatom functionalities. These iron and cobalt
catalysts exhibit high activity in ethylene oligomerization,
and the oligomers consist of >95% linear ␣-olefins. Some
In order to obtain the resulting oligomers with nar-
row distribution, Bluhm et al. [10] reported some com-
plexes without ortho substituents at aryl rings and used for
oligomerization of ethylene. The distribution of oligomers
obtained is much narrower (mainly C4–C10), but cat-
alytic activities (TOF < 2.5 × 104 h−1) and selectivity
for ␣-olefins (<88%) are lower because of too small
steric hindrance. Recently, a series of halogen-substituted
2,6-bis(imino)pyridyl iron and cobalt complexes were re-
ported by Chen et al. [11,12]. Their result shows that the
oligomer distributions obtained from the complexes are
much narrower when the ortho position of aryl rings are
halogen than those are alkyls. Meanwhile, alkyl in the meta
or para position of aryl rings could improve the catalytic
activities evidently as reported in literatures [4–6]. In order
to find an oligomerization catalyst with both high activity
∗
Corresponding author. Tel.: +86 10 6256 2815;
fax: +86 10-6255 4061.
E-mail address: huyl@iccas.ac.cn (Y. Hu).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.05.014

250
Z. Zhang et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 249–254
=
and narrow oligomer distributions, a novel iron (II) com-
7.07 (t, Aryl), 6.70 (d, Aryl), 2.36 (s, N C–CH3), 2.14
=
plex [2,6-(2-F-4-CH3C6H3N CCH3)2C5H3N]FeCl2·H2O
(s, Aryl-CH3); Elemental analysis: Calc. (%): C, 80.90;
H, 6.79; N, 12.31; Found (%): C, 80.75; H, 6.73; N,
12.42.
(3) will be reported in the present paper, on which there
are a methyl substituted on para and a fluoro substituted on
ortho position of aryl rings respectively. {Fe[2,6-(2-FC6H3
Ligand 2 (C21H15F4N3) was synthesized by the method
reported in reference [11]. A solution of 2,6-diacetylpyridine
(0.49 g, 3 mmol), 2,6-difluoroaniline (0.9 g, 7 mmol),
silica-alumina catalyst support (0.3 g), and molecular sieves
4A (1.0 g) in toluene (10 mL) was stirred at 30–40 ◦C for
24 h. Then the reaction mixture was filtered, and the molec-
ular sieves were washed with toluene several times. The
toluene of the combined filtrates was removed in vacuum.
Some anhydrous methanol was added to the residue. A
yellow solid was filtered off to give Ligand 2 in 62% yield.
N CCH3)2C5H3N]2} [FeCl4]2−·H2O (4) and the other
2+
=
=
two complexes ({2,6-(2,4-(CH3)2C6H3N CCH3)2C5H3N}
=
FeCl2 (1) and ({2,6-(2,6-F2C6H3N CCH3)2C5H3N} FeCl2
(2) are synthesized for comparison. The relation between
the structures of the complexes and their activity and selec-
tivity is also discussed.
2. Experimental
1
1H NMR(CDCl3): δ = 8.47 (d, 2H, Py-m-H), 7.93 (t, H,
2.1. Materials
Py-p-H), 7.07 (t, 4H, Aryl), 6.99 (d, 2H, Aryl+), 2.46 (s, 6H,
=
N C–CH3). EI mass spectrum: m/z 385 [M ]. Elemental
2,6-Diacetylpyridine, 2-fluoro-4-methylaniline, 2,4-di-
methylaniline, 2,6-difluoroaniline and 2-fluoroaniline were
purchased from Acros. Polymerization grade ethylene was
obtained from Yanshan Petro-chemical Company Sinopec
China. Methylaluminoxane (MAO) solution (1.4 mol/L) of
toluene was purchased from Albemarle Corp. Toluene, THF
and ether were distilled from solidum/benophenone and
degassed. All other chemicals were obtained commercially
and used without further purification.
analysis: Calc. (%): C, 65.45;H, 3.92; N, 10.90; Found (%):
=
C, 65.61; H, 4.02; N, 10.78.IR (KBr): 1636 (νC N), 1576,
1465, 1369, 1277, 1237, 1215, 1126, 1032, 1002, 827, 789,
759 cm−1
.
Ligand 3 (C23H21F2N3) was synthesized following the
above procedure. 2,6-Diacetylpyridine (0.49 g, 3 mmol),
2-fluoro-4-methylaniline (0.84 g, 7 mmol), silica-alumina
catalyst support (0.3 g), and molecular sieves 4A (2.0 g)
were used. Ligand 3 was obtained as a yellow crystal in
1
65% yield. H NMR(CDCl3): δ = 8.26 (d, 2H, Py-m-H),
8.05 (t, 1H, Py-p-H), 6.88 (d, 2H, Aryl), 6.85 (d, 2H, Aryl),
2.2. Synthesize and characterization of catalysts
6.80 (s, 2H, Aryl), 2.82 (d, 6H, Aryl-CH3), 2.29 (s, 6H,
+
=
N C–CH3). EI mass spectrum: m/z 377 [M ]. Elemental
The structures of ligands and complexes 1–4 were shown
in Scheme 1.
analysis: Calc. (%): C, 73.19; H, 5.61; N, 11.13; Found
=
(%): C, 73.37; H, 5.66; N, 10.89. IR (KBr): 1645 (νC N),
Ligand 1 (C25H27N3) was synthesized according to
the method reported in literature [1]. H NMR (CDCl3):
δ = 8.42 (d, Py-m-H), 7.91 (t, Py-p-H), 7.24 (m, Aryl),
1
1570, 1499, 1423, 1364, 1324, 1270, 1242, 1212, 1119,
1080, 940, 872, 826, 778, 724 cm−1
.
Scheme 1.

Z. Zhang et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 249–254
251
Ligand 4 (C21H17F2N3) was synthesized following the
above procedure. 2,6-diacetylpyridine (0.49 g, 3 mmol),
2-fluoroaniline (0.92 g, 7 mmol), silica-alumina catalyst
support (0.3 g), and molecular sieves 4 A (2.0 g) were used.
Ligand 4 was obtained as a yellow crystal in 55% yield.
1H NMR(CDCl3): δ = 8.22 (d, 2H, Py-m-H), 8.03 (t, 1H,
Py-p-H), 7.26 (t, 2H, Aryl), 6.98 (t, 2H, Aryl), 6.80 (d, 2H,
GCT-MS (Micromass UK) and BIFLFX III (Bruker) spec-
trometers, respectively. The distribution of oligomers was
determined by GC–MS analysis using an HP-5890 appara-
tus with an HP-1 capillary column (30 m × 0.25 mm) and
an HP-5971 mass spectrometer. The column temperature
started with 35 ◦C (10 min), heated at 10 ◦C/min to 220 ◦C
and kept at 220 ◦C for 10 min.
=
Aryl), 6.67 (d, 2H, Aryl), 2.42 (s, 6H, N C–CH3). EI mass
spectrum: m/z 349 [M+]. Elemental analysis: Calc. (%): C,
72.19; H, 4.90; N, 12.03; Found (%): C, 71.98; H, 4.97; N,
3. Results and discussion
=
11.97. IR (KBr): 1634 (νC N), 1601, 1575, 1482, 1365,
1237, 1194, 1123, 1102, 1079, 1031, 842, 825, 780, 762,
2,6-Diacetylpyridinebis(2,6-difluoroanil) (L2), 2,6-di-
acetylpyridinebis(2-fluoro-4-methylanil) (L3) and 2,6-di-
acetylpyridinebis(2-fluoroanil) (L4) were synthesized in
good yield by condensation of 2,6-diacetylpyridine with the
corresponding aniline using silica-alumina catalyst support
as the catalyst and molecular sieves as the water adsorbent
[11]. 2,6-Diacetylpyridinebis(2,4-dimethylanil) (L1) was
743 cm−1
.
Complexes 1–4 were synthesized according to literature
[12].
=
Complex 3: [2,6-(2-F-4-CH3C6H3N CCH3)2C5H3N]
FeCl2·H2O. Elemental analysis: Calc. (%): C, 52.90; H,
4.44; N, 8.05; Found (%): C, 53.33; H, 4.35; N, 7.97.
TOF mass spectrum: m/z 810 [M+]. IR (KBr): 3481, 1623
1
synthesized for comparison. Elemental analysis, H NMR,
=
(νC N), 1584, 1502, 1424, 1377, 1323, 1267, 1223, 1113,
IR and mass spectrometry were used for characterization of
ligands. Iron complexes of these ligands were synthesized
by dissolving the ligands in tetrahydrofuran (THF) (Scheme
1), followed by the addition of 1.1 equiv of FeCl2·4H2O.
The complexes were characterized with elemental analy-
sis, IR and mass spectrometry. The IR spectra of the free
1031, 943, 842, 808, 729 cm−1
.
2+
=
Complex 4: {Fe[2,6-(2-FC6H3N CCH3)2C5H3N]2}
[FeCl4]2−·H2O. Elemental analysis: Calc. (%): C, 52.04;
H, 3.64; N, 8.67; Found (%): C, 52.22; H, 3.83; N, 8.17.
TOF mass spectrum: m/z 754 [M+]. IR (KBr): 3448, 1624
=
(νC N), 1584, 1529, 1487, 1402, 1322, 1246, 1104, 807,
=
ligands shows that the C N stretching frequencies appear
764 cm−1
.
at 1634–1645 cm−1. In complexes 2–4, the C N stretching
=
vibrations shift toward lower frequencies around 1624 cm−1
and were greatly reduced in intensity, which indicated the
coordination interaction between the imino nitrogen atoms
and the metal ions. Elemental analysis results also show
good accordance with corresponding ligands and complexes.
In addition, Chen et al. [11] reported that the complexes
would be formed in two structures depending on the solvent
selected. The structure of FeLCl2 would be formed when
THF was used and an ion pair structure would appear in
strong polar solvent such as CHCl3 and CH3CN. The TOF
mass spectrum shows that complexes 2 and 3 have the same
ion pair structure as complex 4, even when they are synthe-
sized in THF. This could be attributed to the transformation
of complex (Scheme 2) happening during the process of
characterization by mass spectrometer, in which some strong
polar solvents is used. The structure of complex 1, whose
ortho substituents of aryl rings is methyl, could be charac-
terized with a mass spectrum in close accord with FeLCl2.
So the transformation only happened in the complexes with
very small steric hindrance, such as F and H ortho substi-
tuted aryl rings.
2.3. Oligomerization of ethylene at atmosphere pressure
A 250 mL dried three-necked flask with a stir ring bar was
purged with dry nitrogen two to three and then ethylene once.
Then, 50 mL of toluene and a prescribed amount of MAO
were injected in it and the mixture was magnetically stirred
at different temperatures. The ethylene monomer was con-
tinuously fed in and its pressure was maintained at 0.1 MPa
by an electromagnetic valve, and 2 min later, oligomeriza-
tion was started by adding a catalyst. The reaction was ter-
minated by the addition of wt.10% acidified ethanol after
30 min and catalytic activities were computed by pressure
changes in the buffer storage. The samples used for GC-MS
were prepared in the following procedure: 3 mL water was
injected into the reactor to terminate the reaction, and then
the resulting product was stocked in the close reactor for
more than 2 h at 0 ◦C before subjected to GC–MS analysis,
which could prevent the evaporation of C4 and C6. The dis-
tribution of oligomers and selectivity for ␣-olefins could be
obtained with high reliability.
Complexes 1–4 were used in the oligomerization of ethy-
lene in order to investigate the effect of steric bulk and elec-
tronic effect on the catalytic properties. It is very difficult
2.4. Measurements
1H NMR spectra were recorded on a Bruker DMX
(300 M Hz) spectrometer. Elemental analyses were obtained
using Carlo Erba 1106 and ST02 apparatus. IR spectra were
recorded using a Perkin-Elmer system 2000 FT-IR spec-
trometer. EI and TOF mass spectra were carried out with
Scheme 2.

252
Z. Zhang et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 249–254
Table 1
Effect of temperature on oligomerization result of complexes 1–4
Run
Complex
Temperature (◦C)
Yield (g)
Activity (106 g/molFe·h)
␣-Olefin (%)
␣
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1
1
1
1
1
2
2
2
2
3
3
3
3
3
4
20
30
40
50
60
40
50
60
70
30
40
50
60
70
30
8.15
7.75
6.55
5.25
4.93
5.20
3.29
2.46
0.89
9.15
7.55
5.90
3.08
1.17
Inactive
16.3
15.5
13.1
10.5
9.85
10.4
6.57
3.83
1.78
18.3
15.1
11.8
6.15
2.33
97
98
99
99
99
96
97
98
99
95
95
96
98
98
0.87
0.83
0.78
0.71
0.69
0.64
0.52
0.48
0.45
0.42
0.39
0.36
0.29
0.32
Reaction conditions: Fe = 1 ␮mol; ethylene pressure = 0.1 MPa; reaction time = 30 min; 50 mL toluene as solvent; MAO as cocatalyst, Al/Fe = 1500.
20
to compare results obtained by different authors since reac-
tion conditions are usually different. The typical results of
Complex 1
Complex 2
Complex 3
18
16
14
12
10
8
oligomerization by complexes 1–4 under different tempera-
tures are summarized in Table 1. It is clear that the catalytic
activity and selectivity for ␣-olefins formation catalyzed by
complex 2 are much higher than those reported by Qian et al.
[11]. It is well known that the evaporation of C4 and C6 at
room temperature makes it very difficult to accurately mea-
sure the content and distribution of oligomers with GC–MS.
We keep the reactor closed at 0 ◦C until the products were
measured which could prevent the evaporation of C4 and
C6 and make the results more reliable. This could account
for the difference of results between ours and those reported
in literature. These complexes exhibit high activity to ethy-
lene oligomerization and high selectivity for ␣-olefins ex-
cept complex 4, as shown in Table 1. Very interesting by the
complex 3 exhibits high activity to ethylene oligomeriza-
tion, but complex 4, where there is no substituent on other
aryl positions, shows no product detected. This could be at-
tributed to the ion pair structure of complex 4 [12].
The effect of temperature on activity of complexes 1–3
and the distribution of oligomers obtained from complex 3
are given in Figs. 1 and 2, respectively. The catalytic activ-
ities and product distributions of complexes 1, 2 and 3 for
ethylene oligomerization are strongly affected by reaction
temperature. As temperature increases, the activity of com-
plex 3 decreases from 1.83 × 107 g/mol Fe h (30 ◦C) to 2.33
× 106 g/mol Fe h (70 ◦C). The distribution of oligomers ob-
tained follows Schulz–Flory rules, which could be charac-
terized by a constant α, where α represents the probability
of chain propagation [α = rate of propagation/(rate of prop-
agation + rate of chain transfer) = mol of Cn+2/mol of Cn].
The α value can be determined by the molar ratio of C12
and C14. The α value of complex 3 decreases from 0.39 to
0.29 as reaction temperature increases from 30 ◦C to 70 ◦C.
The other two complexes (1 and 2) exhibit the same trend
as described in Table 1 and Fig. 1. Meanwhile, complex 1
6
4
2
20
30
40
50
60
70
Temp /oC
Fig. 1. Effect of temperature on activity of complexes 1–3. Reaction
conditions: ethylene pressure = 0.1 MPa; 50 mL toluene as solvent; MAO
as cocatalyst, Al/Fe = 1500.
60
55
50
30oC
45
50oC
40
60oC
70oC
35
30
25
20
15
10
5
0
C4
C6
C8
C10
C12
C14
C16
C18
C20
α
Carbon number of -olefin
Fig. 2. Effect of temperature on distribution of oligomers obtained from
complex 3. Reaction conditions: ethylene pressure = 0.1 MPa; 50 mL
toluene as solvent; MAO as cocatalyst, Al/Fe = 1500.

Z. Zhang et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 249–254
253
Table 2
60
Effect of Al/Fe ratio on oligomerization result of complex 2 and complex
Al/Fe=420
Al/Fe=750
Al/Fe=1500
Al/Fe=2500
3
50
40
30
20
10
0
Run Complex Al/Fe Yield Activity
␣-Olefin
␣
(g)
(106 g/mol Fe h) (%)
1
2
3
4
5
6
7
8
9
2
2
2
2
2
3
3
3
3
300
500
1000
3000
5000
420
750
1000
2500
1.03
2.59
1.93
1.69
1.74
2.07
3.51
3.72
2.23
2.06
5.18
3.85
3.38
3.48
4.13
7.01
7.44
4.45
98
98
98
99
99
98
99
99
99
0.51
0.47
0.52
0.51
0.48
0.26
0.30
0.29
0.26
Reaction conditions: Fe = 1 ␮mol; ethylene pressure = 0.1 MPa; reaction
time = 30 min; reaction temperature = 60 ◦C; 50 mL toluene as solvent;
MAO as cocatalyst.
C4
C6
C8
C10
C12
C14
C16
C18
C20
Carbon number of α-olefin
Fig. 4. Effect of Al/Fe molar ratio on product distribution obtained from
complex 3. Reaction conditions: ethylene pressure = 0.1 MPa; 50 mL
toluene as solvent; MAO as cocatalyst; reaction temperature = 60 ◦C.
exhibits higher catalytic activities than complexes 2 and 3
at 60 ◦C and 70 ◦C. Larger steric bulk could provide more
protection for central metal from reaction with trimethyla-
luminium (TMA) in commercial MAO, which would deac-
tivate the active species and decrease the catalytic activities
dramatically as reported in literature [5]. All the results in-
dicate that although enhanced temperatures are expected to
result in overall higher propagation and transfer rates, the
rate of chain transfer increases more than the rate of prop-
agation, which could result in more low-molecular-weight
products. In addition, a decrease in ethylene solubility and
higher rates of catalyst deactivation at higher temperatures
may result in a decline of catalytic activity and productiv-
ity. Therefore, a combination of above effects is likely to
account for the obtained temperature dependence of the ac-
tivity and products distribution.
increases from 300 to 500, and then decreases gradually
when the Al/Fe ratio increases from 500 to 3000. Dissimi-
larly, the activity of complex 3 is increased to a maximum
when the Al/Fe ratio is 1000 and then decreases rapidly with
an increase in the Al/Fe ratio. In other words, less MAO is
needed for complex 3 to reach the highest catalytic activity
than complex 2. But the product distributions are almost
unchanged with different Al/Fe ratios as shown in Fig. 4,
and the α values of complexes 2 and 3 are around 0.50 and
0.30, respectively. More active species are formed when
more MAO is introduced to the reaction system, which con-
duce to the increase of activities, but too much MAO could
make the active species deactivate because of the reaction
of active species with TMA. This indicates that the reaction
between TMA and active species is easier to happen in
complex 3 than that in complex 2, which could be attributed
to the different protection provided by ortho substituents in
complexes 2 and 3 as described in literature [5].
The effect of MAO on catalytic activity of complexes (2
and 3) and product distribution is given in Table 2, Figs. 3
and 4, respectively. The catalytic activity of the complex
2 is enhanced dramatically as the molar ratio of Al/Fe
The effect of steric hindrance and reaction temperatures
on the product distributions from different complexes (1–3)
are given in Table 3 and Fig. 5. The product distributions
are greatly affected by reaction temperatures and ortho sub-
stituents. When temperature increases from 30 ◦C to 60 ◦C,
7.5
7.0
Complex2
Complex3
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
Table 3
Distributions of oligomers obtained from complexes 1–3 at different
temperatures
Temperature Distribution of oligomers (%)
(◦C)
C4–C10
C12–C20
>C20
1
2
3
1
2
3
1
2
3
30
40
50
60
41.2 71.5 90.0 46.7 26.6 10.0 12.1 1.9 0.0
0
500
1000
1500
2000
2500
3000
42.8 79.3 92.3 46.1 18.9
43.6 84.1 94.2 45.5 14.4
44.4 87.1 96.9 45.6 12.2
7.7 11.1 1.8 0.0
5.8 10.8 1.5 0.0
3.1 10.0 0.7 0.0
Al/Fe
Fig. 3. Effect of Al/Fe molar ratio on catalytic activity of complexes (2
and 3). Reaction conditions: ethylene pressure = 0.1 MPa; 50 mL toluene
as solvent; MAO as cocatalyst; reaction temperature = 60 ◦C.
Reaction conditions: ethylene pressure = 0.1 MPa; 50 mL toluene as sol-
vent; MAO as cocatalyst; Al/Fe = 1500.

254
Z. Zhang et al. / Journal of Molecular Catalysis A: Chemical 219 (2004) 249–254
much higher than that obtained from complexes 1 and 2
50
40
30
20
10
0
because of its small steric hindrance in the ortho position
of aryl rings. The catalytic activity decreases and selectivity
for low-molecular-weight oligomers increases as reaction
temperature increases. The Al/Fe molar ratio displays no
obvious effect on the distribution of products, but the ac-
tivity increases to the maximum and then decreases with
increasing Al/Fe molar ratio. Displacing H with methyl,
complex 4, is inactive for ethylene oligomerization due
to its special ion pair structure. All the results show that
=
Complex 1
Complex 2
Complex 3
[2,6-(2-F-4-CH3C6H3N CCH3)2C5H3N]2FeCl2·H2O
is
an excellent oligomerization catalyst for its high catalytic
activity and narrow oligomer distributions.
C4 C6 C8 C10 C12 C14 C16 C18 C20 C22 C24 C26 C28
α
Carbon number of -olefin
Acknowledgements
Fig. 5. The product distributions from different complexes (1–3). Reaction
conditions: ethylene pressure = 0.1 MPa; 50 mL toluene as solvent; MAO
as cocatalyst; reaction temperature = 60 ◦C.
This project was subsidized by the National Natural Sci-
ence Foundation (NO. 20334030) and Sinopec.
the α values of all complexes decrease. More than 90%
of oligomers are C4–C10, when complex 3 was used for
ethylene oligomerization at 0.1 MPa ethylene pressure. Less
C4–C10 products are detected in oligomers obtained from
complex 1 (about 40%) and complex 2 (about 70%) under
the same reaction conditions. In addition, the ␣ values de-
crease from 0.78 to 0.39 with respect to the three complexes
with different steric bulk. By comparing of the structures
of three complexes, it is not difficult to find that the steric
hindrance of substituents in the ortho position is in the fol-
lowing series: Me > bisF > F, which could account for the
different product distributions of three catalysts as reported
in literature [5].
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4. Conclusions
A novel 2,6-bis(imino)pyridyl iron complex, [2,6-(2-F-4-
=
CH3C6H3N CCH3)2C5H3N] FeCl2·H2O has been synthe-
sized and used for ethylene oligomerization. It exhibits
higher catalytic activity than the other two complexes due
to methyl in the para position of aryl rings. Meanwhile,
more than 95% of the whole product is C4–C10, which is