2-Ethyl-ketimino-1,10-phenanthroline iron(II) complexes as highly active catalysts for ethylene oligomerization

Article abstract of DOI:10.1016/j.molcata.2010.01.009

A series of 2-(1-aryliminopropyl)-1,10-phenanthrolines (L1-L7) and their corresponding iron(II) chlorides (Fe1-Fe7) are synthesized and characterized by elemental and spectroscopic analyses. The molecular structures of 2-[1-(2,6-diisopropy1phenylimino)propyl]-1,10-phenanthroline (L3) and {2-[1-(2,6-diethylphenylimino)propyl]-1,10-phenanthroline}·FeCl₂ (Fe2) are determined by the single crystal X-ray diffraction. All iron complexes, activated with methylaluminoxane (MAO), exhibit high activities in ethylene oligomerization with high selectivity for α-olefins produced. Compared with other 2-imino-phenanthrolines, the title complexes show better thermal stability, and their oligomerization products are potentially more useful due to lower contents of butenes and waxes observed.

Full text of DOI:10.1016/j.molcata.2010.01.009

Journal of Molecular Catalysis A: Chemical 320 (2010) 92–96
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
2-Ethyl-ketimino-1,10-phenanthroline iron(II) complexes as highly active
catalysts for ethylene oligomerizationଝ
Min Zhanga, Wenjuan Zhanga, Tianpengfei Xiaoa, Jun-Feng Xianga, Xiang Haoa, Wen-Hua Suna,b,∗
a Key laboratory of Engineering Plastics and Beijing Natural Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2-(1-aryliminopropyl)-1,10-phenanthrolines (L1–L7) and their corresponding iron(II) chlo-
rides (Fe1–Fe7) are synthesized and characterized by elemental and spectroscopic analyses. The
molecular structures of 2-[1-(2,6-diisopropy1phenylimino)propyl]-1,10-phenanthroline (L3) and {2-[1-
(2,6-diethylphenylimino)propyl]-1,10-phenanthroline}·FeCl2 (Fe2) are determined by the single crystal
X-ray diffraction. All iron complexes, activated with methylaluminoxane (MAO), exhibit high activities
in ethylene oligomerization with high selectivity for ␣-olefins produced. Compared with other 2-imino-
phenanthrolines, the title complexes show better thermal stability, and their oligomerization products
are potentially more useful due to lower contents of butenes and waxes observed.
Received 17 November 2009
Received in revised form 15 January 2010
Accepted 19 January 2010
Available online 25 January 2010
Keywords:
2-Ethyl-ketimino-1,10-phenanthroline
Iron(II) complexes
© 2010 Elsevier B.V. All rights reserved.
Ethylene oligomerization
1. Introduction
bis(imino)pyridine iron catalysts has been initiated. In review arti-
cles [5], most researchers have focused on iron complexes bearing
␣-Olefins are major industrial substances extensively used for
preparing detergents, lubricants, plasticizers and oil field chemi-
cals, and as monomers for copolymerization. The annual demand
of ␣-olefins is more than 6 million tons with growing rate around
5% per year. The industrial processes include the full range pro-
cess such as the original Ziegler process [1] and the shell higher
olefin process (SHOP) [2], and chromium-based on-purpose pro-
cess for 1-hexene [3]. Beyond the recent progress of on-purpose
process towards trimerization or tetramerization of ethylene by
newly designed Cr (III) complexes [2c,4], there are numerous
papers of N∧N∧N tridentate iron(II) complexes showing high activ-
ities in ethylene polymerization and oligomerization, which are
reviewed in relative articles [5]. Regarding the scientific advan-
tages such as high linearity and high selectivity of ␣-olefins
produced, and considering of the most abundant transition metal
in the earth and non-toxic and sustainable metal, iron catalytic
systems are highly promising for the commercial application
in ethylene oligomerization on the base of their high catalytic
activities. In view of this, a multi-million dollars joint-adventure
between DuPont and PetroChina on ethylene oligomerization using
bis(imino)pyridines. We have also been involved in the research
of the bis(imino)pyridine iron dihalides for a number of years. A
few of our results are documented [6], serious problems are found
within our experiences of the bis(imino)pyridine iron catalytic
systems: their catalytic activities would be decreased along with
increasing reaction temperatures. Regarding ethylene oligomeriza-
tion, the polyethylene waxes formed contain some polymers with
high molecular weights, which would cause trouble in the com-
mercial process. To target alternative catalyst models, new N∧N∧N
tridentate ligands and their late-transition metal complexes have
been designed with promising properties [7]. The polymerization
reaction is a highly exothermal reaction; therefore, an impor-
tant property of an applicable catalyst should be to maintain high
activity during the ethylene polymerization at elevated tempera-
ture. To the best of our knowledge, there is only one model of a
cobalt catalyst reported for improved ethylene polymerization at
increased reaction temperature [8]. In general, ethylene oligomer-
ization by late-transition metal catalysts is promising [7], however,
it is important that the polyethylene waxes obtained should be
without fractions of high molecular weights.
It is of special interest that the iron complexes ligated
by 2-imino-1,10-phenanthrolines (model A, Rꢀ = H, Me or Ph,
Chart 1) have high activities for ethylene oligomerization [7a,d].
Their catalytic activities are generally good for being scaled-
up, but it would be better to reduce amounts of by-products
such as polyethylene waxes or butenes. Checking the previ-
ous data carefully [7a,d], the variation of the substituent on
the imino-C of 2-imino-1,10-phenanthroline ligands results in
ଝ
Dedicated to Prof. Dr. Kazuyuki Tatsumi on the occasion of his 60th birthday.
Corresponding author at: Institute of Chemistry, Chinese Academy of Sciences,
∗
No. 2, Beiyijie, Zhongguancun, Haidian District, Beijing 100190, China.
Tel.: +86 10 62557955; fax: +86 10 62618239.
E-mail address: whsun@iccas.ac.cn (W.-H. Sun).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.01.009

M. Zhang et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 92–96
93
Chart 1. Modified catalyst models.
changing their catalytic performance with the active order
of aldimine < phenyl-ketimine < methyl-ketimine. Therefore, the
transition metal complex model with ethyl-ketimine group is wor-
thy of investigation (this work, Chart 1). The title complexes indeed
perform high catalytic activities towards ethylene oligomeriza-
tion, which is comparable to the analogues of methyl-ketimine.
In addition, their oligomers distributions are narrower with lower
content of butenes among oligomer products and less amount
of polyethylene waxes compared with those of their analogues.
Herein we report in detail the synthesis and characterization of the
title iron complexes and their catalytic behaviors towards ethylene
oligomerization.
Scheme 1. Synthesis procedure.
other 2-imino-9-phenyl-1,10-phenanthroline ligand in which the
aryl ring on the imino nitrogen atom is approximately perpendicu-
lar to the plane of the phenanthroline moiety with a dihedral angle
of 92.1◦ [7e], the corresponding dihedral angle in ligand L3 is much
wider (115.3◦).
2. Results and discussions
2.1. Syntheses and characterization of ligands and iron complexes
In the structure of complex Fe2 (Fig. 3), the iron atom slightly
deviates by 0.0396 Å from the triangular plane of N2, Cl1 and Cl2
with equatorial angle ranges between 105.84(1)◦ and 143.85(1)◦.
This equatorial plane is nearly perpendicular to the phenanthrolinyl
plane, with a dihedral angle of 89.8◦. The dihedral angle between
the phenyl ring and the phenanthrolinyl plane is 79.6◦, which is
very close to that of 2-acetyl-1,10-phenanthroline(2,6-diethylanil)
iron(II) dichloride complex (79.8◦, model A, Rꢀ = Me, R = Et) [7a].
The Fe–N(2)(phenanthrolinyl) bond (2.112(4) Å) is shorter by
about 0.16 Å than the Fe–N(1)(phenanthrolinyl) (2.279(4) Å)
and Fe–N(3)(imino) (2.269(4) Å) bonds, which is similar to the
2,6-bis(imino)pyridyl iron(II) complexes [9] and 2-imino-1,10-
phenanthrolinyl iron(II) complexes [7a]. The two Fe–Cl bond
lengths show a slight difference between the Fe–Cl(2) (2.2949(2) Å)
and Fe–Cl(1) (2.3080(2) Å). The imino N(3)–C(13) bond length is
1.295(7) Å, with the typical character of a C N double bond.
The 2-(1-aryliminopropyl)-1,10-phenanthrolines (2-ethyl-keti-
mino-1,10-phenanthrolines, L1–L7) are routinely synthesized from
the condensation reaction of 2-propionyl-1,10-phenanthrolines
with corresponding anilines. Because of the difference in the reac-
tive nature between ketones and alkyl- or halogen-substituted
anilines, various solvents or the water absorption reagent of
tetraethyl silicate were employed in order to improve the yields of
imino products. All compounds were characterized by the analysis
of FT-IR and 1H and 13C NMR spectra as well as elemental analysis.
The iron complexes Fe1–Fe7 are routinely synthesized by mix-
ing the corresponding ligands with one equivalent of FeCl2·4H2O
in THF at room temperature under nitrogen. The resulting com-
plexes are precipitated from the reaction solution and separated as
blue or purple solids. All the complexes were characterized by FT-IR
spectra and elemental analysis. Compared with the corresponding
ligands (C N absorption, 1625–1645 cm−1), the stretching vibra-
tion bands of C N of these iron(II) complexes in the IR spectra
(1604–1608 cm−1) apparently shift to lower wave number along
with their peak intensities greatly reduced, indicating the coordi-
nation interaction of nitrogen with metal center. Their 1H NMR
spectra were acquired in N2 atmosphere at 20 ◦C on a 600 MHz
instrument and Fig. 1 showed the 1H NMR spectra of Fe1 and Fe3
(all 1H NMR spectra of iron complexes were collected in supporting
information). All protons chemical shifts significantly different
from those of the corresponding protons in the free ligands, which
is consistent with the paramagnetic compound. On the basis of
integration and proximity to the paramagnetic center, some peaks
could be assigned. The protons of phenanthroline could be assigned
as Ha–Hg (shown in Fig. 1). The differences between complexes are
caused by the different distance between the aryl substituents and
the iron center on the NMR timescale (Scheme 1).
2.2. Catalytic behavior toward ethylene reactivity
Oligomerizations are carried out in toluene under different
reaction conditions, and the resulting mixtures are evaluated
for soluble oligomers via GC and insoluble waxes via a filtra-
tion/gravimetric method (Table 1). Inspired by the highest activities
observed with 2-imino-phenanthrolinyl iron(II) complexes at 40 ◦C
and 10 atm ethylene [7a], the complex Fe1 has been used in select-
ing suitable ratios of MAO at 40 ◦C and 10 atm ethylene. Adapting
Al/Fe molar ratios within the range of 1000–2000 (entries 1–3,
Table 1), the optimum activities for both oligomers and waxes
are observed at the Al/Fe ratio 1500, in which it is observed
their activities of 1.39 × 107 g mol−1(Fe) h−1 for oligomers and
1.03 × 106 g mol−1(Fe) h−1 for wax, respectively (entry 3, Table 1).
There is a slight decrease of selectivity for ␣-olefins with the
enhancement of cocatalyst concentration (entries 4 and 5, Table 1).
However, there is no significant difference of the chain propagation,
which is represented with the constant K, and K = rate of propa-
gation/((rate of propagation) + (rate of chain transfer)) = (moles of
Cn+2)/(moles of Cn), the K values are determined by the molar ratio
of C12 and C14 fractions [10,11].
The molecular structures of representative ligand L3 and iron
complex Fe2 are further confirmed by the single-crystal X-ray
diffraction analysis. Molecular structures of ligand L3 and complex
Fe2 are shown in Figs. 2 and 3, respectively, along with selected
bond lengths and angles. Similar to the 2-imino-9-phenyl-1,10-
phenanthroline ligand [7e], the imino nitrogen atom in L3 display
a trans-format to the phenanthroline nitrogen atoms with a typi-
cal C N double bond length of 1.275(3) Å (Fig. 2). However, unlike
An increase of the reaction temperature to 50 ◦C results in better
activities for ethylene oligomerization and polymerization (entry 6

94
M. Zhang et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 92–96
Fig. 1. 1H NMR spectra of complexes Fe1 and Fe3 in CDCl3 at 293 K.
1.39 × 107 g mol−1(Fe) h−1 for ethylene oligomerization (entry 9,
Table 1).
vs. entry 3, Table 1); however, the further increase of the reaction
temperature to 60 ◦C shows a negative result for ethylene oligomer-
ization but positive result for ethylene polymerization (entry 7 vs.
entry 3, Table 1). Therefore, the optimum reaction condition is
assumed with reaction temperature as 50 ◦C, which is higher than
the optimal temperature of 40 ◦C for the 2-methyl-ketimino-1,10-
phenanthroline iron(II) complexes, indicating a better thermal
stability.
On the base of the above results, other iron complexes are inves-
tigated at the MAO/Fe ratio 1500 with reaction temperature as 50 ◦C
in 60 min, and all results are collected in Table 1 (entries 12–17).
The activities by complexes Fe1–Fe3 (entries 6, 12 and 13) show
the order as Fe1 (R1 = Me) > Fe2 (R1 = Et) > Fe3 (R1 = iPr), suggest-
ing that steric effects play an important role. The bulky groups
at the ortho-positions of the imino-N aryl ring partly prevent the
access of ethylene to the active species, imaging the same active
sites of their analogues. The bulkier the substituents used, the
smaller are the K values and less amounts of low-molecular-weight
waxes observed. In their analogues with methyl-ketimine [7a],
iron complexes having the ethyl group on the phenyl showed the
best activity. Similar to other 2-imino-1,10-phenanthroline iron(II)
complexes [7a], complexes Fe4 and Fe5 possessing methyl group
on the para-position of the phenyl show much higher activity in
producing polyethylene wax, respectively (entry 14 vs. entry 6,
With
ambient
ethylene
pressure,
its
activity
of
2.80 × 105 g mol−1(Fe) h−1 is observed for ethylene oligomer-
ization (entry 8, Table 1). Relying on the reaction lifetime, the
highest productivity is obtained within 30 min (entry
9 vs.
entries 6 and 10, Table 1), which indicates decaying active sites
along with reaction time, however, some of active sites are still
remaining in reactive species up to 90 min. Regarding other
potential cocatalysts, MMAO is also proved effective (entry 11,
Table 1); a good activity of 6.18 × 106 g mol−1(Fe) h−1 (entry 11,
Table 1) with MMAO is a little lower than that using MAO as

M. Zhang et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 92–96
95
Table 1
Ethylene oligomerization by complexes Fe1–Fe7.a.
Entry
Cat.
Al/Fe
T (◦C)
t (min)
Oligomer distribution (%)b
Wax
␣-O%c
Ao
K
Ap
Tm (◦C)
d
e
C4/ꢀC
C6/ꢀC
≥C8/ꢀC
1
2
3
4
5
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe1
Fe2
Fe3
Fe4
Fe5
Fe6
Fe7
1000
1200
1500
1800
2000
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
1500
40
40
40
40
40
50
60
50
50
50
50
50
50
50
50
50
50
60
60
60
60
60
60
60
60
30
90
30
60
60
60
60
60
60
21.7
34.1
20.2
28.1
42.5
19.9
40.3
12.1
35.2
18.0
34.8
26.1
34.8
22.6
30.2
16.7
15.1
16.5
23.5
18.1
20.8
28.1
17.1
31.4
19.8
27.1
15.3
27.9
20.9
23.4
18.5
23.7
14.2
13.3
61.8
42.4
61.7
51.1
29.4
63.0
28.3
68.1
37.7
78.6
37.3
53.0
41.8
58.9
46.1
69.1
71.6
>97
>96
>97
>95
>91
>95
>82
>79
>96
>94
>94
>95
>93
>94
>94
>81
>80
9.81
9.92
11.7
9.37
8.15
13.9
6.19
0.28
20.9
9.38
6.18
11.8
0.62
0.60
0.63
0.64
0.62
0.70
0.52
–
0.68
0.69
0.60
0.66
0.59
0.60
0.71
0.41
0.44
5.17
7.25
10.3
9.03
8.32
12.1
10.6
Trace
18.4
8.21
2.91
7.14
1.06
56.2
29.3
Trace
Trace
60.2
61.2
69.8
65.3
63.2
59.2
55.6
–
58.1
70.4
65.1
63.5
62.4
58.4
59.2
–
6
7
8f
9
10
11g
12
13
14
15
16
17
8.37
7.99
9.29
7.42
10.4
–
a
Conditions: 2 ␮mol cat., 100 ml toluene, 10 atm ethylene and MAO used except of other indications.
b
c
d
e
f
Determined by GC.
Determined by GC and GC–MS.
Oligomer activity: 106 g mol−1 (Fe) h−1
.
Polymer activity: 105 g mol−1 (Fe) h−1
30 ml toluene, 1 atm ethylene.
MMAO as cocatalyst.
.
g
eration of the potential application of a catalyst. As noted, ␣-olefins
except of butene-1 are targeted in the commercial oligomerization
process because of butene-1 separated as by-product in the natural
oil refinery process, and polyethylene wax is also less interested due
to its low value. Compared with the analogues containing aldimine,
phenyl-ketimine or methyl-ketimine [7a,e], the title complexes
show the advantages of better thermo-stability with narrower dis-
tribution of oligomers and less butenes. Comparing the most active
catalytic systems by complex Fe1 with that by complex 2a (model
A, Rꢀ = Me, R = Et) [7a], the best catalytic performance is observed
at 50 ◦C for complex Fe1, higher than 2a (40 ◦C), along with better
selectivity for ␣-olefins (C6–C16/ꢀC) and less content of butenes
(entries 2–4, Table 2). It would be easier for the industrial process
with the temperature of 10◦ higher because of exothermic reaction
of ethylene oligomerization. Lower content of butenes in oligomers
by using complex Fe1 in comparison with that by 2a (30.8% and
33.2%) [7a] will be favorable for value-adding olefins (entry 2 vs.
entries 3 and 4, Table 2), particularly low amount of butenes as
12.1% at ambient ethylene pressure (entry 1, Table 2). Their distri-
Fig. 2. Molecular structure of ligand L3 and selected bond lengths [Å] and bond
angles [◦]: thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms
have been omitted for clarity.
entry 15 vs. entry 12, Table 1). Moreover, complexes Fe6 and Fe7
bearing electron-withdrawing halogen groups show good activity
for ethylene oligomerization, with a trace amount of polyethylene
wax produced.
The distribution of oligomers, the portions of ␣-olefins and the
amount of polyethylene waxes are important factors for the consid-
Fig. 3. Molecular structure of complex Fe2 and selected bond lengths [Å] and bond
angles [◦]. Thermal ellipsoids are shown at the 30% probability level. Hydrogen atoms
have been omitted for clarity.
Fig. 4. The distribution comparison of oligomers regarding Table 2.

96
M. Zhang et al. / Journal of Molecular Catalysis A: Chemical 320 (2010) 92–96
Table 2
Oligomer distributions by complex Fe1 and literature 2a.a.
Entry
Cat.
Al/Fe
T (◦C)
Oligomersb
c
Ao
Distributions
C4/ꢀC
C6–C16/ꢀC
≥C16/ꢀC
␣-O%
K
1d
2
3
Fe1
Fe1
2a
1500
1500
1000
1000
50
50
50
40
0.28
13.9
25.5
49.1
C4–C18
C4–C24
C4–C28
C4–C28
12.1
19.9
30.8
33.2
86.2
71.8
61.9
58.0
1.7
8.3
7.3
8.8
>79
>95
>93
>94
–
0.70
0.60
0.62
4
2a
a
Condition: 2 ␮mol cat., MAO as cocatalyst, 100 ml toluene, 10 atm ethylene.
b
c
Determined by GC and GC–MS.
106 g mol−1 (Fe) h−1
.
d
1 atm ethylene, 30 ml toluene.
butions of oligomers were further illustrated in Fig. 4. Though it is
a fact of higher ratio of MAO and a little lower activity, the catalytic
systems using the title complexes perform high activity in ethylene
oligomerization with competitive distribution of oligomers.
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3. Conclusions
The 2-(1-aryliminopropyl)-1,10-phenanthrolines and their cor-
responding iron(II) complexes have been easily synthesized
and characterized. Upon treatment with MAO or MMAO,
these iron(II) complexes show high catalytic activities up to
1.39 × 107 g mol−1(Fe) h−1 for ethylene oligomerization. Compared
with previous analogues [7a,e], the title complexes with a ethyl
substituent on the imino-C exhibit a little lower activity for ethy-
lene oligmerization, but show better thermal stability (10◦ higher
in reaction temperature) and have higher content for ␣-olefins
(C6–C16) and less contents of butanes. In addition, the 2-propionyl-
1,10-phenanthroline is commercial available, therefore, the title
complexes could be easily prepared and more promising for their
consideration in commercial application.
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This work was supported by NSFC no. 20674089.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.01.009.
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