2-Benzoxazolyl-6-[l-(arylimino)ethyl]pyridyliron(II) chlorides as ethylene oligomerization catalysts

Article abstract of DOI:10.1002/ejic.200900491

A series of iron(II) dichloride complexes (Fe1-Fe7) ligated by 2-(2-benzoxazolyl)-6-[l-(arylimino)ethyl]pyridines was synthesized and characterized [aryl = 2,6-R¹₂C₆H₃; R¹ = Me (1), Et (2), iPr (3), C

Full text of DOI:10.1002/ejic.200900491

FULL PAPER
DOI: 10.1002/ejic.200900491
2-Benzoxazolyl-6-[1-(arylimino)ethyl]pyridyliron(II) Chlorides as Ethylene
Oligomerization Catalysts
Rong Gao,[a] Yan Li,[a] Fosong Wang,*[a] Wen-Hua Sun,*[a] and Manfred Bochmann[b]
Keywords: Iron / N ligands / Oligomerization / Polymerization
A series of iron(II) dichloride complexes (Fe1–Fe7) ligated by
2-(2-benzoxazolyl)-6-[1-(arylimino)ethyl]pyridines was syn-
thesized and characterized [aryl = 2,6-R12C6H3; R1 = Me (1),
Et (2), iPr (3), Cl (4), Br (5); 2,6-Me2C6H2-4-R2, R2 = Me (6),
Br (7)]. The molecular structures of Fe1, Fe3, and Fe5 were
determined by the single-crystal X-ray diffraction. Com-
plexes Fe1 and Fe3 both display distorted trigonal-bipyrami-
dal geometries, whereas complex Fe5 is a distorted square
pyramid. Upon activation with modified methylaluminoxane,
all iron complexes showed moderate to good activities [up to
ca. 106 g(product)ϫ(mol Fe)–1 h–1 bar–1] for the oligomeriza-
tion and polymerization of ethylene, with high selectivity for
vinyl-terminated oligomers or polyethylene waxes.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
well as bimetallic complexes[21] were found to have good to
high activities towards ethylene oligomerization and poly-
merization. However, N,N,N-tridentate ligands for iron
complexes do not necessarily lead to good activities; for ex-
ample, bis(imino)bipyridine[22] (Figure 1, complex G) or bis-
(imino)phenanthroline complexes[23] (complex H) showed
only low to moderate activity for ethylene polymerization.
Part of the reason for this is thought to be the free imino
group, which is likely to compete with ethylene for a coordi-
nation site in the active species.
For practical applications, iron catalyst show two advan-
tageous features: high selectivity in forming vinyl-termin-
ated products and good activity for linear α-olefins from C4
to C28. Highly active catalytic systems are of great interest
for the preparation of α-olefins,[24] and this is the basis of a
joint venture between PetroChina and DuPont in the iron-
catalyzed oligomerization of ethylene.[25] Finding oligomer-
ization catalysts with improved efficiency continues there-
fore to be of great importance.
Introduction
The discovery of 2,6-diiminopyridyliron(II) complexes as
highly active catalysts by the groups of Bennett,[1a] Brook-
hart,[1b] and Gibson[2] was a milestone in the development
of ethylene polymerization and oligomerization catalysts.
Most research has involved modifications in the substituent
pattern of the 2,6-diiminopyridine skeleton or variation of
the reaction conditions.[3–9] Bianchini et al.[10] described
complexes with unsymmetrical ligands in which one of the
arylimino groups was replaced by an alkyl or aryl substitu-
ent. Several modified FeII complexes are very active and
present high activity in ethylene polymerization as well as
oligomerization, although in most cases ligand modifica-
tion did not lead to competitive catalytic activities.[11] These
results have been summarized in review articles.[12]
We recently reported several alternative iron complexes
as catalysts for ethylene oligomerization and polymeriza-
tion.[13] Iron complexes bearing 2-ester-6-iminopyridines
showed moderate activities,[14] whereas complexes ligated by
2-imino-1,10-phenanthrolines (Figure 1, A),[15] 2-benzimid-
azolyl-6-iminopyridines (B),[16] 2-benzimidazolyl-1,10-
phenanthroline (C),[17] 2-quinoxalinyl-6-iminopyridines
(D),[18] N-[(pyridin-2-yl)methylene]quinolin-8-amines (E),[19]
and 2-oxazoline/benzoxazole-1,10-phenanthrolines[20] (F) as
One approach to ligand design has been to replace the
imino groups by heterocycles. For example, Nomura made
complexes of 2,6-bis(2-oxazolinyl)pyridines that showed
moderate polymerization activity.[26] Iron complexes of 2,6-
bis(2-benzimidazolyl)pyridines gave no improvement; how-
ever, their chromium(III) analogues showed good activity
towards ethylene oligomerization and polymerization.[27]
Replacing one of the imino groups in 2,6-diiminopyr-
idine[1,2] resulted in 2-(benzimidazolyl)-6-iminopyridines B,
which formed iron(II) complexes with high ethylene oligo-
[a] Key Laboratory of Engineering Plastics and Beijing National
Laboratory for Molecular Science, Institute of Chemistry, Chi-
nese Academy of Sciences,
Beijing 100190, P. R. China
Fax: +86-10-62618239
merization
activities.[16]
2-(2-Benzoxazolyl)-6-[1-(aryl-
E-mail: whsun@iccas.ac.cn
imino)ethyl]pyridines, which have previously been shown to
give nickel complexes with high ethylene oligomerization
activities,[28] could also be of interest in iron catalysts. We
describe here the synthesis and characterization of new
[b] School of Chemical Sciences and Pharmacy, University of East
Anglia,
Norwich, NR4 7TJ, United Kingdom
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200900491.
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FULL PAPER
Figure 1. Ligands and complexes as models for iron catalysts.
iron(II) complexes with ligands bearing one heterocyclic Table 1. Selected bond lengths [Å] and angles [°] for complexes Fe1,
Fe3, and Fe5.
substituent, as well as their behavior in ethylene oligomer-
ization and polymerization.
Fe1
Fe3
Fe5
Fe1–N1
Fe1–N2
2.133(3)
2.223(3)
2.142(4)
2.258(4)
2.128(4)
2.209(3)
Fe1–N3
Fe1–Cl1
Fe1–Cl2
N2–C6
2.215(3)
2.285(4)
2.228(4)
Results and Discussion
2.2524(12)
2.3243(12)
1.297(4)
2.3072(14)
2.2709(14)
1.298(6)
2.3547(14)
2.2634(14)
1.291(5)
Syntheses and Characterization of the Iron Complexes
O1–C6
N3–C12
1.357(4)
1.283(4)
1.356(5)
1.280(5)
1.360(5)
1.295(6)
The 2-(2-benzoxazolyl)-6-[1-(arylimino)ethyl]pyridines
used as ligands were prepared according to our reported
procedure.[28] On treating these ligands with ferrous dichlo-
ride in ethanol at room temperature for 10 h the product
complexes precipitated as blue powders in high yield
(Scheme 1). The complexes are air stable in the solid state.
The FTIR spectra show C=N stretching vibrations in the
range of 1613–1618 cm–1 compared to 1644–1651 cm–1 for
the free ligands. The reduced intensity of this band com-
pared to the free ligand is indicative of a coordinated C=N
group. This was confirmed by the molecular structures of
complexes Fe1, Fe3, and Fe5.
N1–Fe1–N2
N2–Fe1–N3
N1–Fe1–N3
N1–Fe1–Cl1
N2–Fe1–Cl1
N3–Fe1–Cl1
N1–Fe1–Cl2
N2–Fe1–Cl2
N3–Fe1–Cl2
Cl1–Fe1–Cl2
73.64(11)
144.56(11)
72.63(11)
140.95(8)
97.13(8)
102.15(8)
100.45(8)
91.36(8)
73.91(14)
146.23(13)
72.56(13)
113.30(10)
96.01(10)
100.69(10)
134.04(10)
94.53(11)
105.86(10)
112.06(5)
74.17(13)
143.46(13)
72.48(13)
94.28(10)
91.34(10)
105.57(10)
153.27(10)
102.92(10)
100.13(10)
112.42(5)
105.11(8)
117.90(5)
In the structure of complex Fe1 (Figure 2), the pyridine
nitrogen atom (N1) and two chlorine atoms lie approxi-
mately in the equatorial plane. The iron atom deviates by
0.0382 Å from the equatorial plane; the equatorial angles
range from 140.95(8) to 100.45(8)°. The equatorial plane
and the trans nitrogen atoms N2 and N3 form angles of
87.1 and 86.0°, respectively. The 2,6-dimethylphenyl substit-
uent is oriented almost orthogonally (92.5°) to the N3–N1–
N2 plane. The two Fe–Cl bond lengths are significantly dif-
ferent, with the Fe1–Cl1 bond being 0.072 Å shorter than
the Fe1–Cl2 bond. This is usually seen in apically elongated
Scheme 1. Synthesis of ferrous complexes.
Single crystals of complexes Fe1, Fe3, and Fe5 suitable square-pyramidal (N,N,N)FeCl2 complexes[9b] and might be
for X-ray diffraction analysis were obtained by slow dif- ascribed to a distortion towards such a structure. The mo-
fusion of diethyl ether into methanol solutions under a ni- lecular structure of Fe3 is given in the Supporting Infor-
trogen atmosphere. Complexes Fe1 and Fe3 both show dis- mation.
torted trigonal-bipyramidal geometries, whereas complex
Fe5 is a distorted square pyramid. Selected bond lengths as distorted square-pyramidal, with the basal plane com-
and angles are collected in Table 1. posed of N1, N2, N3, and Cl2. The iron atom deviates from
The structure of complex Fe5 (Figure 3) is best described
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Iron(II) Complexes as Ethylene Oligomerization Catalysts
Me). In Fe3, seven peaks can be assigned to pyridyl protons
(A = Hm, B = Hm, C = Hp), ketimine protons (D = NCMe),
and isopropyl protons (E = CHMe2, F = CHMe2). Two
relatively narrow signals for the iPr groups confirm the hin-
dered rotation of the aryl group. Similar phenomena have
been observed in symmetric 2,6-bis(imino)pyridyliron[6]
complexes and asymmetric (N-{(E)-1-[6-(cyclohexylethani-
midoyl)-2-pyridinyl]ethylidene}-2,6-diisopropylaniline)iron
dichloride.[10a] The only differences between complexes
Fe1–Fe3 and Fe6 are indicated with the chemical shifts of
the protons of the aryl substituents, which are caused by
the different distance between the aryl substituents and the
paramagnetic iron centre on the NMR timescale.
Figure 2. Molecular structure of Fe1. Thermal ellipsoids are shown
at 50% probability level. Hydrogen atoms are omitted for clarity.
this plane by 0.1293 Å, whereas Cl2 deviates by 0.0392 Å
to the opposite side. The two fused rings of Fe1–N2–C6–
C7–N1–Fe1 and Fe1–N1–C11–C12–N3–Fe1 formed
through coordination are coplanar and nearly perpendicu-
lar to the aryl ring linked to the imino group (88.0°). The
Fe–N bond lengths are in the order Fe1–N3 Ͼ Fe1–N2 Ͼ
Fe1–N1. The two Fe–Cl bond lengths are significantly dif-
ferent, similar to the structures of Fe1, Fe3, and its 2-
(benzimidazolyl)-6-[1-(arylimino)ethyl]pyridyliron(II) ana-
logue.[16] The N2–C6 bond length is 1.291(5) Å, which is
shorter than the C6–O1 bond length of 1.360(5) Å, dis-
playing clear C=N character.
Figure 3. Molecular structure of Fe5. Thermal ellipsoids are shown
at 50% probability level. Hydrogen atoms are omitted for clarity.
The Cl1–Fe1–Cl2 angle in Fe1 [117.90(5)°] is much wider
than that in Fe3 [112.06(5)°] and Fe5 [112.42(5)°]. Thus, the
molecular structure of Fe1 is more open, something that
would favor ethylene coordination, in line with the observa-
tion that the catalytic activity of Fe1 is about an order of
magnitude higher than that of Fe3 and Fe5.
1
Figure 4. H NMR spectra of Fe2 and Fe3 in CDCl3 at 298 K.
Although the complexes are paramagnetic, 1H NMR
spectroscopy can be informative. Complexes Fe1–Fe3 and
Fe6, which have better solubility in CDCl3, were charac-
Reactions with Ethylene
Precatalyst Fe1 was studied in ethylene oligomerization
1
1
terized by H NMR spectroscopy. Figure 4 shows the H and polymerization by using different activators [methylalu-
NMR spectra of Fe2 and Fe3. All protons resonate at minoxane (MAO), modified methylaluminoxane (MMAO),
chemical shifts significantly different from the correspond- Et2AlCl, and Et3Al]. Because the activity at ambient ethyl-
ing protons in the free ligands. In the spectrum of Fe2, five ene pressure is low, the influence of the activator was as-
peaks can be assigned clearly, on the basis of integration sessed at 10 atm C2H4 (Table 2). Mixtures of Fe1 with AlEt3
and proximity to the paramagnetic center, to pyridyl pro- or AlClEt2 were inactive. However, with modified MMAO
tons (A = Hm, B = Hm, C = Hp), ketimine protons (D = the activity for ethylene polymerization and oligomerization
NCMe), and methyl protons on the aromatic rings (E = was much higher than that of its 2-(benzimidazolyl)-6-[1-
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(arylimino)ethyl]pyridyliron(II) analogue,[16] with high
The substituents at the 4-position of the aryl ring also
selectivity for α-olefins. The oligomers showed a Schulz– influence the catalytic activity and the K value. For exam-
Flory distribution (Table 3). The probability of chain prop- ple, 2,4,6-trimethyl-substituted complex Fe6 exhibited
agation is given by K, where K = (rate of propagation)/[(rate higher catalytic activity in ethylene oligomerization and po-
of propagation) + (rate of chain transfer)] = (mol of Cn+2)/ lymerization and a larger K value than catalyst Fe7, which
(mol of Cn); the K values were determined by the molar bears a 4-bromo-2,6-dimethyl group.
ratio of the C12 and C14 fractions.[29]
The influence of reaction parameters on ethylene oligo-
merization and polymerization activities was investigated in
detail for complex Fe2 (Table 4). When the Al/Fe molar ra-
tio was increased from 500 to 1000, both oligomerization
and polymerization activities increased sharply (Table 4,
Entries 1–3), but a further increase in the molar ratio to
1500 resulted in decreased polymerization activity and a
slightly increased oligomerization activity. Increasing the
ratio to 2000 decreased both polymerization and oligomer-
ization. In contrast, the Al/Fe molar ratio had little influ-
ence on the oligomer distribution and K values.
Raising the reaction temperature from 30 to 50 °C re-
sulted in an obvious decrease in productivity and selectivity
for α-olefins (Table 4, Entries 3, 7, and 8), suggesting that
active centers are thermally unstable under these condi-
tions.
In the Fe2/MMAO system, the oligomerization rate was
fairly constant, whereas the polymerization rate was quite
low over the first 20 min (Table 4, Entries 9–11) but then
increased dramatically in the second 20 min period (Table 4,
Entries 3 and 12). With prolonged reaction times, both
oligomerization and polymerization activity decreased
(Table 4, Entry 13).
Table 2. Ethylene oligomerization activity of Fe1 as a function of
the catalyst activator.[a]
Entry
Activator
Al/Fe
Oligomer
activity[b]
Oligomer
Polymer
distribution[c] activity[d]
1
2
3
4
MAO
MMAO
Et2AlCl
Et3Al
1000
1000
200
2.41
6.81
–
C4–C28
C4–C28
–
–
trace
9.70
–
200
–
–
[a] Reaction conditions: Fe1 (5 µmol), ethylene (10 atm), 30 min,
30 °C, toluene (100 mL). [b] In units of 105 g(mol Fe)–1 h–1 bar–1.
[c] Determined by GC. [d] Polymerization activity: 104 gPE(mol
Fe)–1 h–1 bar–1.
The substituents of the imino N-aryl rings strongly influ-
enced the catalytic performance (Table 3). The effects are
rather subtle, and both steric and electronic factors are im-
portant, with the former dominating. Of the 2,6-dialkyl-
substituted complexes Fe1–Fe3 (Table 3, Entries 1–3), com-
plex Fe2, with ethyl groups in the ortho positions, gave the
highest catalytic activity, whereas methyl substituents gave
lower oligomerization activities but a higher K value, as well
as larger amounts of low-molecular-weight wax (Table 3,
Compared with the iron(II) complexes bearing 2-benz-
Entry 1). Changing R1 to isopropyl resulted in decreased imidazole-6-(1-aryliminoethyl)pyridines[16] and 2,6-bis(2-
catalytic activity. Furthermore, the bulkier the R1 group, oxazolin-2-yl)pyridine,[26] the benzoxazolyl(aryliminoethyl)-
the smaller the K value and the lower the amount of poly- pyridine complexes described here present much higher
ethylene produced. The same trend was observed in the 2- catalytic activities. These results also follow conclusions
(benzimidazolyl)-6-[1-(arylimino)ethyl]pyridyliron(II) sys- drawn from computational studies on the relationship of
tem.[16] Compared with complexes Fe1–Fe3, which contain the catalytic activity with the net charge of late-transition-
electron-donating 2,6-substituents, complexes Fe4 and Fe5, metal complexes in that the stronger the electron-donating
bearing halogen groups, exhibited comparable activity and ability of the ligand the lower the net charge on the metal
relatively lower selectivity for α-olefins (Table 3, Entries 4 and the lower the catalytic activity.[12a,30]
and 5). As to the distribution of oligomers, the bulkier the
In most cases, some polyethylene waxes accompanied the
R1 substituents, the higher the content of C4, and the distri- formation of oligomers. According to their FTIR spectra,
bution of oligomers obtained did not strictly follow Schulz– the waxes are mainly linear α-olefins. 1H and 13C NMR
Flory rules (Table 3).
spectra (Figure 5) of the waxes produced by complex Fe1
Table 3. Polymerization and oligomerization of ethylene with Fe1–Fe7/MMAO.[a]
Entry
Catalyst
R1
K
α-O[b]
Activity
Oligomer[c]
Oligomer distribution [%][e]
[%]
Wax[d]
C4/ΣC
C6/ΣC
C8/ΣC
ΣCՆ10/ΣC
1
2
3
4
5
6
7
Fe1
Fe2
Fe3
Fe4
Fe5
Fe6
Fe7
Me
Et
iPr
Cl
Br
Me
Me
0.75
0.71
0.55
0.65
0.64
0.78
0.77
97.0
96.8
99.0
96.6
92.4
99.0
94.9
6.81
10.20
6.05
7.40
7.72
8.37
1.85
9.70
0.73
0.14
1.54
0.70
10.44
6.68
21.1
30.3
44.2
27.8
49.7
20.3
20.2
19.7
22.4
26.3
23.6
24.2
17.8
17.1
14.6
15.1
13.9
16.0
10.2
14.7
14.6
44.6
32.2
15.6
32.6
15.9
47.2
48.1
[a] Reaction conditions: catalyst (5 µmol), MMAO (1000 equiv.), ethylene (10 atm), 30 °C, 30 min, toluene (100 mL). [b] Percent α-olefin
content determined by GC. [c] Oligomerization activity: 105 g(mol Fe)–1 h–1 bar–1. [d] Polymerization activity: 104 g(mol Fe)–1 h–1 bar–1.
[e] Determined by GC; ΣC denotes the total amount of oligomers.
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Iron(II) Complexes as Ethylene Oligomerization Catalysts
Table 4. Polymerization and oligomerization of ethylene with Fe2/MMAO.[a]
Entry
Al/Fe
t
T
K
α-O[b]
Activity
Oligomer[c]
Oligomer distribution [%][e]
[min]
[°C]
[%]
Wax[d]
C4/ΣC
C6/ΣC
C8/ΣC
ΣCՆ10/ΣC
1
2
3
4
5
6
7
8
500
800
30
30
30
30
30
30
30
30
5
10
20
40
60
30
30
30
30
30
30
40
50
30
30
30
30
30
0.69
0.71
0.71
0.70
0.68
0.53
0.64
–
0.73
0.71
0.71
0.67
0.70
96.9
96.8
96.8
96.6
96.8
96.7
96.6
72.2
98.0
98.3
97.5
98.0
98.3
9.36
9.60
10.20
10.56
10.80
9.86
7.09
0.07
13.00
11.70
11.28
8.10
0.16
0.54
0.73
0.71
0.69
0.53
trace
–
trace
0.08
0.25
0.69
0.54
29.6
29.9
30.3
35.4
36.5
42.4
29.1
100
33.5
33.0
29.0
37.5
27.4
22.4
22.6
22.4
24.4
25.5
22.1
23.6
–
19.4
23.3
22.2
24.8
21.5
14.5
14.8
15.1
14.5
33.5
32.7
32.2
25.7
24.7
23.8
30.7
–
33.6
28.5
32.8
22.6
35.1
1000
1200
1500
2000
1000
1000
1000
1000
1000
1000
1000
13.3
11.7
16.6
–
9
13.5
15.2
16.0
15.1
16.0
10
11
12
13
5.74
[a] Reaction conditions: Fe2 (5 µmol), ethylene (10 atm), 30 min, toluene (100 mL). [b] Percent α-olefin content determined by GC.
[c] Oligomerization activity: 105 g(mol Fe)–1 h–1 bar–1. [d] Polymerization activity: 104 g(mol Fe)–1 h–1 bar–1. [e] Determined by GC.
further confirmed that the wax consisted mainly of linear
α-olefins with vinyl chain ends with a carbon number of
Conclusions
Iron complexes of 2-(benzoxazolyl)-6-[1-(arylimino)-
ethyl]pyridines form a new type of readily accessible oligo-
merization catalysts. Whereas AlEt3 and AlClEt2 proved in-
effective as catalyst activators, activation with MMAO gave
activities of up to 105 g(mol Fe)–1 h–1 bar–1 for ethylene po-
lymerization and 1.3ϫ106 g(mol Fe)–1 h–1 bar–1 for oligo-
merization. α-Olefins were in the range of C4–C28, with high
selectivity for terminal unsaturation. The catalytic activity,
oligomer distribution, and selectivity depend on both the
steric and the electronic properties of the substituents on
the N-aryl rings, with moderately large alkyl groups (i.e.,
ethyl) showing superior properties than methyl or isopro-
pyl. Raising the reaction temperature resulted in an obvious
decrease in productivity and in the selectivity for α-olefins.
about 34.
Experimental Section
General Considerations: All manipulations of air- and moisture-
sensitive compounds were performed under a nitrogen atmosphere
by using standard Schlenk techniques. Toluene was heated at reflux
over sodium–benzophenone and distilled under argon prior to use.
MAO (1.46  in toluene) and modified MMAO (1.93  in heptane)
were purchased from Akzo Nobel Corp. Diethylaluminum chloride
(Et2AlCl, 1.7  in toluene) was purchased from Acros Chemicals.
Other reagents were purchased from Aldrich or Acros Chemicals.
FTIR spectra were recorded with a Perkin–Elmer System 2000
FTIR spectrometer. Elemental analyses were carried out by using
a Flash EA 1112 microanalyzer. GC analyses were performed with
a Varian CP-3800 gas chromatograph equipped with a flame ion-
ization detector and a 30 m (0.2 mm i.d., 0.25 µm film thickness)
CP-Sil 5 CB column. The yield of oligomers was determined by
GC calculated by referencing to the mass of the solvent assuming
that the mass of each fraction was approximately proportional to
its integrated areas in the GC trace. Selectivity for the linear α-
olefin was defined as (amount of linear α-olefin of all fractions)/
1
(total amount of oligomer products) in percent. H NMR spectra
of the iron complexes were recorded with a Bruker DMX 600 MHz
1
Figure 5. (a) 13C NMR and (b) H NMR spectra of the polyethyl-
ene waxes obtained by Fe1 (cf. Table 3, Entry 1).
instrument at 25 °C with the use of TMS as the internal standard.
Eur. J. Inorg. Chem. 2009, 4149–4156
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1H and 13C NMR spectra of the PE samples were recorded with a
Bruker DMX 400 MHz instrument at 25 °C in 1,2-[D4]dichloro-
benzene with the use of tetramethylsilane as the internal standard.
(w), 788 (s) cm–1. C24H23Cl2FeN3O (496.21): calcd. C 58.09, H
4.67, N 8.47; found C 57.85, H 4.61, N 8.38.
N-{(1E)-1-[6-(1,3-Benzoxazol-2-yl)pyridin-2-yl]ethylidene}-N-(2,6-di-
isopropylphenyl)amine FeCl2 (Fe3): Prepared according to the pro-
cedure outlined for Fe1. Yield: 0.29 g, 91.4%. 1H NMR (600 MHz,
CDCl3): δ = 84.14 (s, 1 H, Py, Hm), 60.94 (s, 1 H, Py, Hm), 35.76
(s, 1 H, Py, Hp), 26.82 (s, 1 H, Ar H), 21.69 (s, 1 H, Ar H), 10.66
(s, 3 H, N=CCH3), 10.37 (s, 2 H, Ar H), 9.56 (s, 1 H, Ar H), 3.71
(s, 1 H, Ar H), –2.62 (s, 6 H, CH3 ϫ2), –5.98 (s, 6 H, CH3 ϫ2),
–9.83 (s, 1 H, Ar H), –11.86 (br. s, 2 H, CHϫ2) ppm. FTIR (KBr
N-{(1E)-1-[6-(1,3-Benzoxazol-2-yl)pyridin-2-yl]ethylidene}-N-(2,6-di-
methylphenyl)amine FeCl2 (Fe1): Schlenk tube was charged with so-
lid N-{(1E)-1-[6-(1,3-benzoxazol-2-yl)pyridin-2-yl]ethylidene}-N-
(2,6-dimethylphenyl)amine (0.20 g, 0.6 mmol) and FeCl2·4H2O
(0.12 g, 0.6 mmol) and purged three times with argon, followed by
the addition of ethanol (10 mL). The reaction mixture was stirred
at room temperature for 10 h. The resulting precipitate was filtered
off, washed twice with diethyl ether, and dried in vacuo to furnish
the pure product as a blue powder. Yield: 0.258 g, 92.0%. 1H NMR
(600 MHz, CDCl3): δ = 83.12 (s, 1 H, Py, Hm), 60.91 (s, 1 H, Py,
Hm), 37.56 (s, 1 H, Py, Hp), 27.33 (s, 1 H, Ar H), 18.55 (s, 3 H,
N=CCH3), 11.35 (s, 2 H, Ar H), 10.63 (s, 6 H, CH3 ϫ2), 6.59 (s, 2
H, Ar H), 3.48 (s, 1 H, Ar), –10.28 (s, 1 H, Ar H) ppm. FTIR
disk): ν = 2962 (s), 1613 (νC=N) (m), 1593 (m), 1548 (m), 1448 (s),
˜
1373 (vs), 1278 (s), 1192 (w), 1103 (w), 817 (w), 789 (m) cm–1.
C26H27Cl2FeN3O (524.26): calcd. C 59.57, H 5.19, N 8.02; found
C 59.18, H 5.15, N 8.00.
N-{(1E)-1-[6-(1,3-Benzoxazol-2-yl)pyridin-2-yl]ethylidene}-N-(2,6-
dichlorophenyl)amine FeCl2 (Fe4): Prepared according to the pro-
(KBr): ν = 2919 (w), 1615 (νC=N) (s), 1594 (w), 1548 (w), 1468 (m),
cedure outlined for Fe1. Yield: 0.27 g, 87.6%. FTIR (KBr disk): ν
˜
˜
1450 (s), 1375 (s), 1278 (s), 1212 (m), 1172 (w), 1096 (w), 1018 (w), = 3058 (w), 1615 (νC=N) (m), 1592 (w), 1562 (m), 1438 (s), 1375
812 (w), 789 (m) cm–1. C22H19Cl2FeN3O (468.16): calcd. C 56.44,
H 4.09, N 8.98; found C 56.32, H 4.04, N 8.77.
(s), 1278 (s), 1230 (m), 1174 (w), 813 (m), 784 (vs) cm–1
.
C20H13Cl4FeN3O·0.5H2O (518.00): calcd. C 46.37, H 2.72, N 8.11;
found C 46.41, H 2.84, N 7.90.
N-{(1E)-1-[6-(1,3-Benzoxazol-2-yl)pyridin-2-yl]ethylidene}-N-(2,6-di-
ethylphenyl)amine FeCl2 (Fe2): Prepared according to the procedure
outlined for Fe1. Yield: 0.27 g, 89.3 %. 1H NMR (600 MHz,
CDCl3): δ = 83.41 (s, 1 H, Py, Hm), 60.81 (s, 1 H, Py, Hm), 37.67
(s, 1 H, Py, Hp), 27.27 (s, 1 H, Ar H), 16.24 (s, 3 H, N=CCH3),
11.12 (s, 2 H, Ar H), 10.76 (s, 1 H, Ar H), 9.61 (s, 2 H, CH2), 8.55
(s, 1 H, Ar H), 4.86 (1 H, CH), 3.47 (s, 1 H, Ar H), –2.27 (s, 6 H,
N-{(1E)-1-[6-(1,3-Benzoxazol-2-yl)pyridin-2-yl]ethylidene}-N-(2,6-di-
bromophenyl)amine FeCl2 (Fe5): Prepared according to the pro-
cedure outlined for Fe1. Yield: 0.33 g, 91.0%. FTIR (KBr disk): ν
= 3051 (w), 1616 (νC=N) (m), 1594 (w), 1548 (m), 1449 (m), 1430
(s), 1373 (s), 1277 (s), 1228 (m), 1175 (m), 1102 (w), 1021 (w), 852
(w), 769 (s) cm–1. C20H13Br2Cl2FeN3O (597.90): calcd. C 40.18, H
2.19, N 7.03; found C 39.89, H 2.30, N 6.91.
˜
CH ϫ2), –10.20 (s, 1 H, Ar H) ppm. FTIR (KBr disk): ν = 2967
˜
3
(m), 1614 (νC=N) (s), 1593 (m), 1546 (m), 1449 (s), 1374 (s), 1276 N-{(1E)-1-[6-(1,3-Benzoxazol-2-yl)pyridin-2-yl]ethylidene}-N-mes-
(s), 1208 (w), 1194 (w), 1173 (w), 1102 (w), 1020 (w), 855 (w), 813 itylamine FeCl2 (Fe6): Prepared according to the procedure out-
Table 5. Crystal data and structure refinement for Fe1, Fe3, and Fe5.
Fe1
Fe3
Fe5·CH3OH
Empirical formula
Formula weight
Crystal color
Temperature [K]
Wavelength [Å]
Crystal system
Space group
a [Å]
C22H19Cl2FeN3O
468.15
blue
C26H27Cl2FeN3O
524.26
blue
173(2)
0.71073
monoclinic
P21
8.7478(17)
15.527(3)
9.865(2)
C21H17Br2Cl2FeN3O2
629.95
blue
173(2)
0.71073
monoclinic
P21/n
10.781(2)
15.446(3)
13.822(3)
173(2)
0.71073
triclinic
¯
P1
9.3605(19)
9.6060(19)
11.636(2)
b [Å]
c [Å]
α [°]
β [°]
93.03(3)
99.54(3)
95.34(3)
1024.8(3)
2, 1.517
1.015
90
101.23(3)
90
1314.3(5)
2, 1.325
0.799
90
100.58(3)
90
2262.7(8)
4, 1.849
4.459
γ [°]
V [Å3]
Z, Dcalcd. [gcm–3]
µ [mm–1]
F(000)
480
544
1240
Crystal size [mm]
θ range [°]
Limiting indices
0.29ϫ0.18ϫ0.13
2.13–25.00
–11 Յ h Յ 11
–11 Յ k Յ 11
–13 Յ l Յ 13
6631/3608
0.37ϫ0.26ϫ0.07
2.48–25.00
–10 Յ h Յ 10
–18 Յ k Յ 18
–11 Յ l Յ 11
4623/4623
298
0.34ϫ0.25ϫ0.18
2.00–25.00
–12 Յ h Յ 12
–18 Յ k Յ 18
–16 Յ l Յ 16
7763/3987
285
No. of collected/unique reflections
No. of parameters
262
Completeness to θ [%]
Abs. corr.
99.9
multiscan
1.204
R1 = 0.0529, wR2 = 0.1099
R1 = 0.0667, wR2 = 0.1147
0.525 and –0.482
99.9
multiscan
0.923
R1 = 0.0456, wR2 = 0.0925
R1 = 0.0586, wR2 = 0.0956
0.385 and –0.288
99.8
multiscan
1.110
R1 = 0.0450, wR2 = 0.0773
R1 = 0.0660, wR2 = 0.0825
0.633 and –0.601
Goodness of fit on F2
Final R indices [I Ͼ2σ (I)]
R indices (all data)
Largest diff peak and hole [eÅ–3]
4154
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Eur. J. Inorg. Chem. 2009, 4149–4156

Iron(II) Complexes as Ethylene Oligomerization Catalysts
1
lined for Fe1. Yield: 0.25 g, 86.0%. H NMR (600 MHz, CDCl3):
δ = 83.29 (s, 1 H, Py, Hm), 60.84 (s, 1 H, Py, Hm), 38.88 (s, 1 H,
Py, Hp), 27.26 (s, 1 H, Ar H), 19.48 (s, 3 H, N=CCH3), 16.54 (s, 3
H, CH3), 11.30 (s, 1 H, Ar H), 10.80 (s, 3 H, CH3), 10.69 (s, 5 H,
CH3 and Ar H), 4.11 (s, 1 H, CH), 3.48 (s, 1 H, Ar H) ppm. FTIR
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(KBr disk): ν = 2912 (w), 1614 (νC=N) (m), 1591 (w), 1541 (m),
˜
1478 (m), 1448 (m), 1371 (m), 1276 (s), 1217 (s), 1171 (m), 1104 (w),
863 (m), 813 (m), 771 (s) cm–1. C23H21Cl2FeN3O (482.18): calcd. C
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the procedure outlined for Fe1. Yield: 0.30 g, 90.0%. FTIR (KBr
disk): ν = 2911 (w), 1618 (νC=N) (m), 1594 (w), 1549 (m), 1450 (m),
˜
1374 (s), 1276 (s), 1210 (s), 1103 (w), 1019 (w), 865 (w), 772 (m)
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General Procedure for Ethylene Reactions: Ethylene oligomerization
and polymerization were performed in a 500-mL stainless-steel au-
toclave equipped with a mechanical stirrer and a temperature con-
troller. Toluene, the desired amount of modified MMAO, and a
toluene solution of the catalyst precursor was added to the reactor
in this order under an ethylene atmosphere; the total volume was
100 mL. When the desired reaction temperature was reached, ethyl-
ene (10 atm) was introduced to start the reaction, and the ethylene
pressure was maintained by a constant feed of ethylene. After
30 min, the pressure was released and a small amount of the reac-
tion solution was collected, which was then analyzed by gas
chromatography (GC) to determine the composition and mass dis-
tribution of oligomers obtained. Because the acid concentration of
the HCl-acidified ethanol (5%) was low and because we tested the
sample as quickly as possible, acid-catalyzed C=C bond isomeriza-
tion was considered negligible. Then, the remaining reaction mix-
ture was quenched with HCl-acidified ethanol (5%), and the pre-
cipitated polyethylene was filtered, washed with ethanol, and dried
under vacuum at 60 °C to constant weight.
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X-ray Crystallography: Single-crystals of Fe1, Fe3, and Fe5 suitable
for X-ray diffraction were obtained by slow diffusion of diethyl
ether into their methanol solutions. Data were collected with a Ri-
gaku RAXIS Rapid IP diffractometer with graphite monochro-
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This work was supported by the National Natural Science Founda-
tion of China (grant No. 20674089) and the Royal Society/National
Natural Science Foundation of China (project IJP 2007/R3).
Eur. J. Inorg. Chem. 2009, 4149–4156
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Received: June 1, 2009
Published Online: August 10, 2009
4156
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Eur. J. Inorg. Chem. 2009, 4149–4156