Preparation, structure and ethylene polymerization/oligomerization behavior of 2-carbethoxy-6-iminopyridine-based transition metal complexes

Article abstract of DOI:10.1016/j.poly.2006.06.015

The synthesis, characterization and ethylene polymerization/oligomerization of a series of transition metal complexes with the formula {2-carbethoxy-6-[1-[(2,6-dimethylphenyl)imino]-ethyl]pyridine}_nMCl₂ [M = Mn(II) (8), Fe(II) (9), Co(II) (10), Cu(II) (11), Zn(II) (12), n = 1; Ni(II) (13), n = 2]; {2-carbethoxy-6-[1-[(2,4,6- trimethylphenyl)imino]ethyl]pyridine}MCl₂ [M = Fe(II) (14), Co(II) (15), Ni(II) (16)]; {2-carbethoxy-6-[1-[(2,6-diethylphenyl)-imino]ethyl]pyridine}MCl₂ [M = Fe(II) (17), Co(II) (18), Ni(II) (19)] have been reported. Systematic studies are focused on the relationship between the catalytic activity of these complexes for ethylene polymerization/oligomerization and the electronic property of the central metal, the ligand structure, as well as the reaction conditions. The Fe(II), Co(II), Ni(II) catalyst systems afford moderate activities ranging from 0.35 × 10⁴ to 5.9 × 10⁴ g/mol-Metal h atm, and the most active catalysts are those with two smaller substituents in the ortho-positions of the N-aryl ring. X-ray crystallographic analysis of complex 12 reveals a four-coordinated distorted tetrahedral geometry, while complex 18 indicates a five-coordinated distorted trigonal bipyramid, in which the tridentate ligand coordinates via weak bonding between the carbonyl oxygen of the ester group and cobalt.

Full text of DOI:10.1016/j.poly.2006.06.015

Polyhedron 25 (2006) 3289–3298
www.elsevier.com/locate/poly
Preparation, structure and ethylene
polymerization/oligomerization behavior of
2-carbethoxy-6-iminopyridine-based transition metal complexes
*
Bi-Yun Su, Jian-She Zhao
Shaanxi Key Laboratory of Physico-Inorganic Chemistry, Department of Chemistry, Northwest University, Xi’an, 710069, China
Received 25 February 2006; accepted 4 June 2006
Available online 30 June 2006
Abstract
The synthesis, characterization and ethylene polymerization/oligomerization of a series of transition metal complexes with the for-
mula {2-carbethoxy-6-[1-[(2,6-dimethylphenyl)imino]-ethyl]pyridine}nMCl2 [M = Mn(II) (8), Fe(II) (9), Co(II) (10), Cu(II) (11), Zn(II)
(12), n = 1; Ni(II) (13), n = 2]; {2-carbethoxy-6-[1-[(2,4,6- trimethylphenyl)imino]ethyl]pyridine}MCl2 [M = Fe(II) (14), Co(II) (15),
Ni(II) (16)]; {2-carbethoxy-6-[1-[(2,6-diethylphenyl)-imino]ethyl]pyridine}MCl2 [M = Fe(II) (17), Co(II) (18), Ni(II) (19)] have been
reported. Systematic studies are focused on the relationship between the catalytic activity of these complexes for ethylene polymeriza-
tion/oligomerization and the electronic property of the central metal, the ligand structure, as well as the reaction conditions. The Fe(II),
Co(II), Ni(II) catalyst systems afford moderate activities ranging from 0.35 · 104 to 5.9 · 104 g/mol-Metal h atm, and the most active
catalysts are those with two smaller substituents in the ortho-positions of the N-aryl ring. X-ray crystallographic analysis of complex
12 reveals a four-coordinated distorted tetrahedral geometry, while complex 18 indicates a five-coordinated distorted trigonal bipyramid,
in which the tridentate ligand coordinates via weak bonding between the carbonyl oxygen of the ester group and cobalt.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Transition metal; 2-Carbethoxy-6-iminopyridine; Synthesis; Ethylene polymerization/oligomerization catalyst; Crystal structure
1. Introduction
lar the discovery of exceptionally active catalysts (between
3.7 · 106 and 2.1 · 107 g (mol of Fe)À1 hÀ1 barÀ1) based on
iron and cobalt which convert both ethylene and a-olefins
to high molar mass polymers, have been reported by the
groups of Brookhart [8–10] and Gibson [11–15] indepen-
dently in 1998. Following this discovery, a considerable
amount of effort has been dedicated to investigate the nat-
ure of the active species [16–18] and to design new catalysts
based on these types of ligands [19–27]. However, to date
no other donor combinations have matched the high activ-
ities obtained with bis(imino)pyridyl ligands by Brookhart
and Gibson.
With a view to obtaining a great understanding of
structure–activity relationships in the bis(imino)pyridyl-
transition metal catalyst system, we have targeted alterna-
tive bis(imino)pyridyl ligands. A relatively straightforward
modification to the ligand frame is to alter one of the imine
Transition metal-catalyzed olefin polymerization is cur-
rently a subject of intense academic and industrial interest.
Since Ziegler-Natta’s original work on AlR3 catalysis of
ethylene oligomerization, there has been considerable sus-
tained interest in developing new oligomerization active
catalysts, but most attention has been focused on group
4B transition metal catalyst systems such as metallocene
analogues [1], hybrid ‘‘half-metallocene’’ analogues [2],
non-bridged ‘‘half-metallocene’’ analogues [3–5], and oth-
ers [6,7]. Recently, significant late transition metal catalyst
systems incorporating bis(imino)pyridyl ligands, in particu-
*
Corresponding author. Tel.: +86 029 8837 3118; fax: +86 029 8830
3798.
E-mail address: jszhao@nwu.edu.cn (J.-S. Zhao).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.06.015

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B.-Y. Su, J.-S. Zhao / Polyhedron 25 (2006) 3289–3298
groups. In this paper, we introduce a series of new mono-
iminopyridyl ligands and their transition metal complexes.
This monoiminopyridyl-transition metal catalyst system
should be an interesting contrast to the previous system
of iron and cobalt catalysts containing the 2,6-bis-
(imino)pyridine ligand [8–15]. We have explored the ethyl-
ene polymerization and oligomerization activity of these
monoiminopyridyl-transition metal complexes in the pres-
ence of the cocatalyst methylaluminoxane (MAO) to clar-
ify the reasons for this difference. We have also studied
the influences of the metal center, the ligand architecture
and the reaction conditions on the catalyst activity.
2.1.2. Synthesis of the ligands 5–7
2-Carbethoxy-6-{1-[(2,6-dimethylphenyl)imino]ethyl}-
pyridine (5), 2-carbethoxy-6-{1-[(2,4,6-trimethylphenyl)-
imino]ethyl}pyridine (6), 2-carbethoxy-6-{1-[(2,6-diethyl-
phenyl)-imino]ethyl}pyridine (7) were synthesized by
condensation of 2-carbethoxy-6-acetylpyridine (4) and an
excess of the corresponding substituted anilines using
microwave irradiation (Scheme 2). After purification by
column chromatography, the highly pure products were
obtained in moderate yields (17.0–62.5%). Attempts to pre-
pare 5–7 by catalytic condensation of 4 with the corre-
sponding aryl aniline in refluxing ethanol and in the
presence of acetic acid [31] were unsuccessful. We also
found that the use of toluene as solvent, p-toluenesulfonic
acid as catalyst and a Dean–Stark apparatus to remove
the water formed in the reaction [32] afforded 5–7 in very
low yields (about 10–15%), even after a long refluxing per-
iod (up to 3 days), and led to many byproducts. Compared
with the two pathways mentioned above, microwave-
assisted condensation seems to be much better in terms
of improved reaction rate, simple work-up procedure and
formation of cleaner products in preparing 5–7, and more-
over, it is a green method for chemical synthesis. The
ligands 5–7 were characterized by element analysis, 1H
NMR and IR.
2. Results and discussion
2.1. Synthesis aspects
2.1.1. Synthesis of the precursor 4
2-Carbethoxy-6-acetylpyridine (4) was prepared from
2,6-dimethylpyridine (1) (Scheme 1). Initially 2,6-dimethyl-
pyridine (1) was converted to pyridine-2,6-dicarboxylic
acid (2) according to the method reported in the literature
[28,29]. Esterification of 2 was done by a modified proce-
dure with a better yield (83%) than that of the reported
method [30], resulting in 2,6-dicarbethoxypyridine (3),
which was purified by a simplified method. 3 was dissolved
in freshly distilled EtOAc, then treated with dry EtONa
powder and an excess of concentrated HCl to afford com-
pound 2-acetyl-6-carbethoxypyridine (4). The structure of
2.1.3. Synthesis of the complexes 8–19
The transition metal complexes 8–13 (Schemes 3 and 4)
were prepared by the reactions of ligand 5 with stoichiom-
etric amounts of Mn(II), Fe(II), Co(II), Cu(II), Zn(II) and
Ni(II) chlorides in ethanol at room temperature. The
1
the new compound 4 was proven by element analysis, H
NMR, IR and MS.
Scheme 1.
Scheme 2.

B.-Y. Su, J.-S. Zhao / Polyhedron 25 (2006) 3289–3298
3291
Scheme 3.
Scheme 4.
complexes 14–19 (Scheme 5) were synthesized by the
2.2. The molecular structures of complexes 12 and 18
selected metal Fe(II), Co(II), Ni(II) chlorides and the cor-
responding ligands 6 and 7 under the same experimental
conditions as above. All the complexes were isolated as
air-stable solids in high purity and in different yields. They
were characterized by elemental analysis and IR spectros-
copy. The elemental analysis results suggested that the
components of complexes 8–12 and 14–19 were consistent
with the formula MLCl2, and complex 13 with the formula
ML2Cl2 Æ H2O. The IR spectra of the free ligands 5, 6 and 7
showed that the C@N stretching frequencies appeared at
1644, 1643 and 1640 cmÀ1, respectively, and C@O vibra-
tions at 1741, 1716 and 1742 cmÀ1, respectively. In com-
plexes 8–19, the C@N stretching vibrations shifted
toward lower frequencies ranging from 1621 to
1636 cmÀ1, and were greatly reduced in intensity, which
indicated coordination between the imino nitrogen atom
and the metal ion. The C@O stretching vibrations also
shifted toward lower frequencies ranging from 1665 to
1725 cmÀ1, and were reduced in intensity, which suggested
weak coordination between the carbonyl oxygen of the
ester group and the metal ions.
Complexes 12 and 18 were further characterized by X-
ray crystallography. Yellow crystals of 12 suitable for X-
ray diffraction analysis were grown from a THF solution
at room temperature, while green single crystals of complex
18 were obtained from a solution of CoCl2 Æ 6H2O in EtOH
top layered with a solution of ligand 7 in Et2O. The crystal
structures of 12 and 18 are shown in Figs. 1 and 2, respec-
tively, whereas selected bond lengths and angles are listed
in Table 1. Fig. 1 indicates that in complex 12 the center
Zn atom is coordinated in a distorted-tetrahedral environ-
ment by the pyridyl nitrogen atom, imino nitrogen atom
and two chlorine atoms. The dimethyl-substituted phenyl
ring is oriented nearly perpendicular to the coordination
plane, and the dihedral angle is 89.50ꢁ. One THF molecule
is observed in the crystal lattice of 12. It should be pointed
out that the oxygen atom in the carbonyl group is orien-
tated toward the Zn center in 12, and the distance between
˚
oxygen and Zn is 2.581 A, which is longer than the normal
Zn–O coordination bond length. The imino linkage N(1)–
C(10) and linkage O(1)–C(16) have distinctive double-bond
Scheme 5.

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B.-Y. Su, J.-S. Zhao / Polyhedron 25 (2006) 3289–3298
character, with C–N and C–O distances of 1.275(7) and
˚
1.206(7) A, respectively. A perusal of the bond angles
around the metal center of 12 shows that its coordination
geometry is greatly distorted from an idealized tetrahe-
dron, which may be attributed to the effect of the carbonyl
oxygen atom on the coordination environment.
Fig. 2 shows that the carbonyl oxygen atom in 18 coor-
dinates with Co(II). The coordination geometry around the
central cobalt can be best described as distorted trigonal
bipyramid, with the pyridyl nitrogen atom and the two
chlorine atoms forming the equatorial plane. The deviation
˚
of the cobalt atom from the equatorial plane is 0.173 A.
The phenyl ring is oriented essentially orthogonal
(88.11ꢁ) to the coordination plane containing the ligand
backbone and the cobalt atom. It is noticeable that the
˚
˚
Co–O(1) bond length (2.374(2) A) is shorter than reported
Co–O distances (2.460, 2.610 A) [33], which indicates a
strong coordination between the cobalt and carbonyl
oxygen atom in 18. As previously observed [12,13], the
˚
Fig. 1. Molecular diagram of {2-carbethoxy-6-[1-[(2,6-dimethylphenyl)-
imino]ethyl]pyridine}-ZnCl2 Æ THF (12).
Co–N(pyridyl) bond (2.036(3) A) is shorter than the
˚
Co–N(imino) bond (2.156(3) A) in 18.
Fig. 2. Molecular diagram of {2-carbethoxy-6-[1-[(2,6-diethylphenyl)imino]ethyl]pyridine}-CoCl2 (18).
Table 1
˚
Selected bond lengths (A) and angles (ꢁ) for complexes 12 and 18
Complex 12
Complex 18
Zn–N(2)
Zn–N(1)
Zn–Cl(2)
Zn–Cl(1)
2.084(5)
2.157(5)
2.194(2)
2.226(2)
N(1)–C(10)
O(1)–C(16)
C(10)–C(11)
C(15)–C(16)
1.275(7)
1.206(7)
1.496(8)
1.493(8)
Co–N(1)
Co–N(2)
Co–Cl(1)
Co–Cl(2)
2.156(3)
2.036(3)
2.234(1)
2.242(1)
Co–O(1)
2.374(2)
1.214(3)
1.308(4)
1.275(4)
O(1)–C(13)
O(2)–C(13)
N(1)–C(7)
N(2)–Zn–N(1)
N(2)–Zn–Cl(2)
N(1)–Zn–Cl(2)
N(2)–Zn–Cl(1)
N(1)–Zn–Cl(1)
76.7(2)
120.8 (1)
107.8(1)
113.0(1)
107.8(1)
Cl(2)–Zn–Cl(1)
C(10)–N(1)–Zn
C(6)–N(1)–Zn
C(11)–N(2)–Zn
C(15)–N(2)–Zn
120.3(8)
N(2)–Co–N(1)
N(2)–Co–Cl(1)
N(1)–Co–Cl(1)
N(2)–Co–Cl(2)
N(1)–Co–Cl(2)
76.6(1)
125.9(9)
101.7(8)
108.6(8)
104.5(8)
Cl(1)–Co–Cl(2)
N(2)–Co–O(1)
N(1)–Co–O(1)
Cl(1)–Co–O(1)
Cl(2)–Co–O(1)
123.5(4)
115.1(4)
124.1(4)
116.4(4)
124.7(4)
73.3(1)
149.2(9)
90.8(7)
91.2(7)

B.-Y. Su, J.-S. Zhao / Polyhedron 25 (2006) 3289–3298
3293
2.3. Application of complexes 8–19 in the polymerization/
oligomerization of ethylene
7.00E+04
6.00E+04
5.00E+04
4.00E+04
3.00E+04
2.00E+04
1.00E+04
0.00E+00
Curve 1
To probe the catalytic nature of these complexes for the
ethylene polymerization/oligomerization, we carried out
series of experiments over a range of active metal centers,
ligand modifications, as well as a variety of polymeriza-
tion/oligomerization reaction conditions in which the
reaction temperature, time and Al/Metal ratio were sys-
tematically altered. The results are indicated in Table 2,
Figs. 3–6, respectively.
Curve 2
Curve 3
Fe
Co
Ni
2.3.1. Effect of the active metal center on the polymerization
and oligomerization of ethylene
Center metal ions of catalysts
The nature of the center metal greatly influences the cat-
alyst activity. The activities of the transition metal Mn(II),
Fe(II), Co(II), Cu(II), Zn(II), Ni(II) complexes 8–13 bear-
ing the same ligand 5 were tested at 1.0 atm of ethylene at
15.0–23.5 ꢁC for 1 h, employing 1000 equiv. of MAO as
cocatalyst (Table 2). Fe complex 9 shows considerable
activity for the ethylene polymerization and moderate
activity for ethylene oligomerization (Run 2), which implies
that in this system the rate of chain propagation is in excess
of the rate of chain transfer. While the ethylene oligomer-
ization activities indicated by Co complex 10 and Ni com-
plex 13 (Runs 3, 6) imply the reverse rate order, mainly
producing butenes and hexenes and small amounts of
higher oligomers. From Table 2, it can be concluded that
the Mn, Cu, Zn complexes 8, 11, 12 are inactive for the eth-
ylene polymerization/oligomerization (Runs 1, 4, 5), which
is in agreement with the results observed from the
bis(imino)pyridine-based transition metal analogues. The
above results show that the different inherent electronic
properties of the transition metals play a significant role
in the catalyst activity, possibly leading to differences in
the catalytic activity and product distribution.
Fig. 3. Dependence of the activity on ligands 5 (curve 1 j), 6 (curve 2 r),
7 (curve 3 m) for the Fe, Co, Ni-based catalysts.
1.90E+04
1.60E+04
1.30E+04
1.00E+04
7.00E+03
4.00E+03
-10
10
30
50
70
90
Temperature(ºC)
Fig. 4. Dependence of the activity on the reaction temperature for the
catalyst 10.
and product distribution of ethyl polymerization and olig-
omerization. The effect of these modifications on the oligo-
mer distribution is not as remarkable as the effect on the
catalytic activity, and no precise conclusion can be reached.
Herein we mainly give the results on the catalytic activity.
In Fig. 3, the curve 1 (n) was obtained from the activity of
Fe, Co, Ni-based catalysts bearing ligand 5, the curve 2 (r)
2.3.2. Aryl ring ligand modification
A variety of substituted aryl-ring-containing ligands,
including those with steric bulk and ortho- and para-substi-
tuted position modifications in the aryl ring, were
employed to study their influence on the catalytic activity
Table 2
Ethylene polymerization and oligomerization by the MLnCl2-MAO system [L = 5, M = Mn (8), Fe (9), Co (10), Cu (11), Zn (12), Ni (13)]a
Run
Complex
Temperature (ꢁC)
Time (h)
Oligomer distributionc (%)
Activity
(104g/mol-cat h atm)
P
91.50
82.10
P
4.33
15.46
P
C4/
C
C6/
C
CP8
/
C
Linear a-olefin
Oligomerc
Polymer
1
2
3
4
5
6
a
8
9
10b
11
12
13b
23.5
15.0
15.5
15.5
20.5
15.0
1.0
1.0
0.5
1.0
1.0
1.0
no
no
4.17
2.44
>99.0
96.3
0.15
5.93
no
0.16
3.97
3.13
trace
no
no
trace
96.80
98.20
2.11
1.20
1.09
0.60
>99.0
97.6
Reaction conditions: Complex 10 lmol, M/Al = 1/1000, P = 1 atm, solvent: toluene 30 mL, 100 mL autoclave, MAO in toluene (14%).
b
c
Solvent: toluene 50 mL.
P
Determined by GC. C means the total amounts of oligomers.

3294
B.-Y. Su, J.-S. Zhao / Polyhedron 25 (2006) 3289–3298
A study reported by Bianchini and co-workers [34,35]
2.00E+04
1.80E+04
1.60E+04
1.40E+04
1.20E+04
1.00E+04
8.00E+03
suggested that steric groups directly linked to the 2-posi-
tion of pyridine were considered irrelevant in accounting
for the superior activity of thiophenyl-substituted cata-
lysts, but the chain length of the oligomers had been
found to increase with the size of the sulfur-containing
group in the catalyst 2-imino-6-(organyl)pyridine Co(II)
dichloride. Compared with the other heteroatoms, the
sulfur atom was suggested to have a great role in increas-
ing both the activity and the Schulz–Flory parameter a
(mol Cn+2/mol Cn). So the lesser activity indicated by
the carbonyl group substituted in the 2-position of the
pyridine ligand based Co(II) catalyst can be mainly
attributed to the oxygen atom, but the great difference
between the carbonyl group and the thiophenyl ring is
also a reason for the catalytic performance. As yet no
conclusion can be forwarded in the absence of further
studies.
-10
70
10
90
30
110
50
Time(min)
Fig. 5. Dependence of the activity on the reaction time for the catalyst 10.
2.20E+04
1.90E+04
1.60E+04
1.30E+04
1.00E+04
7.00E+03
4.00E+03
2.3.3. Effect of reaction conditions
To study the influence of the catalytic reaction condi-
tions on the productivity of the catalyst system employed,
we performed a series of experiments by employing Co cat-
alyst 10 under different reaction temperatures, reaction
times, as well as the ratio of Al/Metal. The results are
shown in Figs. 4–6.
2.3.3.1. Effect of temperature on catalyst performance. To
examine the role of the temperature variation on catalyst
performance, four experiments were carried out at different
temperatures (0, 30, 45 and 75 ꢁC, respectively) for 60 min
with an Al/Co ratio of 1000. As shown in Fig. 4, the activ-
ity increases slightly when the temperature rises from 0 to
30 ꢁC (from 1.312 · 104 g/mol-Co h atm at 0 ꢁC to
1.50 · 104 g/mol-Co h atm at 30 ꢁC), and then it decreases
as the temperature is increased continually up to 75 ꢁC
(0.593 · 104 g/mol-Co h atm at 75 ꢁC). The lower tempera-
ture is unfavorable probably because it hinders the forma-
tion of the active species, but too high a temperature may
also lead to a decrease of the activity due to a decrease
of ethylene solubility [36,37] in toluene, and the deactiva-
tion of the catalyst.
0
500
1000
Al/Co ratio
1500
2000
Fig. 6. Dependence of the activity on Al/Co ratio for the catalyst 10.
and 3 (m) were obtained from the activity of Fe, Co, Ni-
based catalysts bearing ligands 6 and 7, respectively. As
shown in this figure, curve 1 is above both curve 2 and
curve 3, which indicates that the activity of Fe, Co, Ni-
based catalysts with a methyl group in the 2- and 6-position
of the aryl ring are generally higher than those with a
methyl group in the 2, 4, 6-position and those with an ethyl
group in the 2- and 6-position. It suggests that replacing
the para-aryl proton with a methyl group may result in a
decrease in rate of propagation or b-H transfer (the major
chain-transfer process), thus leading to a lower productiv-
ity in polymer or oligomer production (compare curve 1
with curve 2). On the other hand, bulky ortho-substituted
aryl groups attached to the imine nitrogen also render less
active catalysts (compare curve 1 with curve 3).
The influence of the metal center on catalytic activity
also can be seen from Fig. 3. Comparison of the Fe, Co,
Ni catalysts bearing different ligands indicates the same
trend in activity changes. In general, Co catalysts are more
active than the corresponding Ni catalysts, and Fe catalysts
show the lowest activity in every set of Fe, Co, Ni catalysts.
This result is slightly different from the trend observed for
the bis(imino)pyridine ligand analogues [8,9]. The reason
probably relates to the difference of the structure and elec-
tronic properties between the two types of ligands.
2.3.3.2. Effect of reaction time. The reaction time can have a
great effect on productivity as exemplified for catalyst 10.
At 30 ꢁC and when Al/Co ratio was 1000, four ethylene
polymerization experiments were performed for 15, 30, 60
and 90 min. From Fig. 5 we can see that as the reaction
time increases from 15 to 30 min, the activity increases
from 1.30 · 104 to 1.81 · 104 g/mol-Co h atm. Typically,
30 min later the activity slows down gradually and when
the reaction has been performed for 90 min the activity
reduces to 1.30 · 104 g/mol-Co h atm. These observations
imply that a deactivation of the catalyst occurred about
30 min after initiating. The employed catalyst system has
a relatively longer lifetime compared to other catalyst
systems [38].

B.-Y. Su, J.-S. Zhao / Polyhedron 25 (2006) 3289–3298
3295
2.3.3.3. Effect of cocatalyst MAO. The cocatalyst is also a
significant factor for the oligomerization of ethylene. To
study this effect, three tests were performed for 30 min at
30 ꢁC, in which the ratios of [MAO]/[Co] were systemati-
cally varied from 500 to 1500. As indicated in Fig. 6, the
activity shows a maximum (1.81 · 104 g/mol-Co h atm)
around a [MAO]/[Co] ratio of 1000. The possible reason
is that an optimal amount of MAO as cocatalyst is needed
to effectively activate the catalyst, while an excess amount
of MAO will create too many trialkylaluminum impurities
and may cause the degradation of the catalyst system.
ses were performed with a Kratos AEI MS-50 instrument
using the electron impact (EI) method. The microwave-
assisted reactions were carried out in a domestic oven Midea
PJ 21B-A 800w (21 L). The oligomerization products
obtained were analyzed on a BF 3400 GC instrument which
was equipped with a SE-54 capillary column (25 m ·
0.32 mm) using a temperature program from 40 to 280 ꢁC.
2,6-Dimethylpyridine, 2,6-dimethylaniline, 2,4,6-trimethy-
laniline and 2,6-diethylaniline were purchased from Acros
and used as received. MAO in toluene (14 wt%) was from
the Albemarle Co. (USA). Polymerization grade ethylene
monomer, produced by Beijing Yanshan Petrochemical
Co., was used without further purification.
3. Conclusion
A new compound 2-carbethoxy-6-acetylpyridine (4) was
synthesized, from which a series of aryl substituted 2-carb-
ethoxy-6-iminopyridine ligands (5, 6 and 7) were prepared
by microwave irradiation. Reactions of these ligands 5, 6
and 7 with group 4B translation metal salts in ethanol pro-
vided the corresponding transition metal complexes (8–19).
When MAO was employed as a cocatalyst, the Fe, Co, Ni
complexes exhibited moderate catalytic activity and the
Mn, Cu, Zn complexes were inactive for ethylene polymer-
ization and oligomerization, which parallel the results
observed for catalysts containing the 2,6-bis(imino)pyri-
dine ligand. For the complexes of Fe, Co, Ni, the catalytic
activities depend greatly on the structure and electronic
properties of the ligand. In contrast to the bis(imino)pyri-
dine analogue, the complexes bearing mono(imino)pyri-
dine with two substituents in the ortho-position of the
parent aniline are more active than those with substituents
in both ortho- and para-positions as well as those with lar-
ger steric hindrance in the ortho-positions. Research on the
influence of the carbethoxy group on catalytic activity is
underway. The catalytic activity of the complexes is also
influenced by the amount of cocatalyst employed and other
factors such as reaction temperature and time. The opti-
mum conditions have been chosen through a series of
experiments to exhibit high activity. Comparing the activ-
ity of the catalyst bearing the asymmetrical mono(imino)-
pyridine ligand 5 with the analogous catalyst [8,11] bearing
the symmetrical bis(imino)pyridine ligand, we assume at
this stage that the lesser activity of the former is due to
the influence of the carbethoxy group in the ligand.
4.2. Synthesis of complexes
4.2.1. 2-Carbethoxy-6-acetylpyridine (4)
Sodium (2.10 g, 91 mmol) was dissolved in 60 mL of
pure, dry ethanol, then the excess ethanol was removed
and the obtained white powder was dried in vacuum for
6 h. 11.17 g (50 mmol) of 2,6-dicarbethoxypyridine (3) in
40 mL of freshly distilled EtOAc was added dropwise to
the dry EtONa powder with stirring to afford a yellow mix-
ture, which was refluxed for 12 h, then allowed to stand
overnight. An excess (33 mL) of concentrated HCl was
added dropwise with stirring to the resulting mixture and
refluxed for a period of 7–8 h to complete the reaction.
After water was added to dissolve the NaCl formed in
the reaction, the mixture turned into an orange solution
(100 mL). The aqueous phase was shaken with CHCl3
(4 · 25 mL). The combined extracts were washed with 5%
aqueous Na2CO3 and the aqueous phase was extracted
with CHCl3 (3 · 10 mL) again. The combined organic
phase was dried over anhydrous Na2SO4, filtered and the
solvent was evaporated under reduced pressure. The
obtained mixture was separated by column chromato-
graphy (silica-gel, petroleum ether:EtOAc = 4:1). First
2,6-diacetylpyridine was eluted, followed by 2-acetyl-6-car-
bethoxypyridine (yield: 2.71 g, 28.0%). M.p. 50.0–51.5 ꢁC.
1H NMR (400 MHz, CDCl3): d 8.27 (d, 1H, Py-Hm),
8.20 (d, 1H, Py-Hm), 7.99 (t, 1H, Py-Hp), 4.49 (m, 2H,
CH2), 2.81 (s, 3H, O–C(CH3)), 1.47 (t, 3H, CH2(CH3)).
IR (KBr): mC–O 1726, 1706, mC–O–C 1156 cmÀ1. MS (EI):
m/z 193 (M+). Anal. Calc. for C10H11NO3: C, 62.17; H,
5.74; N, 7.25. Found: C, 62.51; H, 5.76; N, 7.30%.
4. Experimental
4.2.2. 2-Carbethoxy-6-{1-[(2,6-dimethylphenyl)imino]-
ethyl}pyridine (5)
4.1. General procedure
2-Carbethoxy-6-acetylpyridine (4) (0.580 g, 3.0 mmol)
and an excess of 2,6-dimethylaniline (0.7272 g, 6.0 mmol)
were mixed together in an opened Erlenmeyer flask
(25 mL). The mixture was subjected to microwave for
30 min on the ‘‘High’’ setting (800 W). The crude product
was purified by column chromatography (60 · 2.5 cm
silica-gel, petroleum ether:EtOAc = 3:2). Ligand 5 was
obtained as a highly pure yellow solid in 62.5% yield.
All experiments of air- and/or moisture-sensitive com-
pounds were carried out under a nitrogen atmosphere using
standard Schlenk techniques. Solvents were refluxed over an
appropriate drying agent and distilled prior to use. C, H and
N analyses were performed on a HP-MOD 1106 microana-
lyzer. IR spectra were measured with a Perkin–Elemer
1
FTIR 2000 spectrometer. H NMR spectra were recorded
on a Bruker spectrometer DMX-300. Mass spectral analy-
1
M.p. 58.0–59.5 ꢁC. H NMR (400 MHz, CDCl3): d 8.57

3296
B.-Y. Su, J.-S. Zhao / Polyhedron 25 (2006) 3289–3298
(d, 1H, Py-Hm), 8.19 (d, 1H, Py-Hm), 7.94 (t, 1H, Py-Hp),
7.06 (d, 2H, Ar-Hm), 6.95 (t, 1H, Ar-Hp), 4.50 (m, 2H,
CH2), 2.26 (s, 3H, N-C(Me)), 2.02 (s, 6H, Ar-Me), 1.46
(t, 3H, CH2(CH3)). IR (KBr): mC–O 1741, mC–N 1644,
mC–O–C 1141 cmÀ1. Anal. Calc. for C18H20N2O2: C, 72.95;
H, 6.80; N, 9.45. Found: C, 73.04; H, 6.81; N, 9.26%.
1624 cmÀ1. Anal. Calc. for C18H20Cl2FeN2O2: C, 51.10;
H, 4.76; N, 6.62. Found: C, 51.10; H, 4.83; N, 6.53%.
4.2.7. {2-Carbethoxy-6-[1-[(2,6-dimethylphenyl)imino]-
ethyl]pyridine}CoCl2 (10)
Using an analogous procedure as above, 10 was
obtained as a green solid in 70.7% yield. M.p. >300 ꢁC.
IR (KBr): mC–O 1714, mC–N 1625 cmÀ1. Anal. Calc. for
C18H20Cl2CoN2O2: C, 50.73; H, 4.73; N, 6.57. Found: C,
50.38; H, 4.85; N, 6.27%.
4.2.3. 2-Carbethoxy-6-{1-[(2,4,6-trimethylphenyl)imino]-
ethyl}pyridine (6)
2,4,6-Trimethylaniline (1.08 g, 8.0 mmol) was mixed
with 4 (0.772 g, 4.0 mmol) in a 25 mL Erlenmeyer flask.
The mixture was subjected to microwave for 20 min on
the ‘‘High’’ setting (800 W). The crude product was puri-
fied by column chromatography (60 · 2.5 cm silica-gel,
petroleum ether:EtOAc = 2:1). Ligand 6 was obtained as
4.2.8. {2-Carbethoxy-6-[1-[(2,6-dimethylphenyl)imino]-
ethyl]pyridine}CuCl2 (11)
Using an analogous procedure as above, 11 was
obtained as a yellow solid in 88.6% yield. M.p. >300 ꢁC.
IR (KBr): mC–O 1725, mC–N 1628 cmÀ1. Anal. Calc. for
C18H20Cl2CuN2O2: C, 50.18; H, 4.68; N, 6.50. Found: C,
50.26; H, 4.77; N, 6.35%.
1
yellow solid in 74.1% yield. M.p. 75.0–76.0 ꢁC. H NMR
(400 MHz, CDCl3): d 8.56 (d, 1H, Py-Hm), 8.19 (d, 1H,
Py-Hm), 7.94 (t, 1H, Py-Hp), 6.90 (s, 2H, Ar-Hm), 4.88
(m, 2H, CH2), 2.26 (d, 3H, N-C(Me)), 2.06 (s, 6H, Ar-
Me), 1.47 (t, 3H, CH2(CH3)). IR (KBr): mC–O 1715, mC–N
1643, mC–O–C 1137 cmÀ1. Anal. Calc. for C19H22N2O2: C,
73.52; H, 7.14; N, 9.03. Found: C, 73.31; H, 7.12; N, 8.94%.
4.2.9. {2-Carbethoxy-6-[1-[(2,6-dimethylphenyl)imino]-
ethyl]pyridine}ZnCl2 (12)
Using an analogous procedure as above, 12 was
obtained as a yellow solid in 88.3% yield. M.p. >300 ꢁC.
IR (KBr): mC–O 1723, mC–N 1636 cmÀ1. Anal. Calc. for
C18H20Cl2N2O2Zn: C, 49.97; H, 4.66; N, 6.47. Found: C,
49.68; H, 4.68; N, 6.21%.
4.2.4. 2-Carbethoxy-6-{1-[(2,6-diethylphenyl)imino]-
ethyl}pyridine (7)
2,6-Diethylaniline (1.07 g, 7.2 mmol) was mixed with 4
(0.67 g, 3.5 mmol) in a 25 mL Erlenmeyer flask. The mix-
ture was subjected to microwave for 35 min on the ‘‘High’’
setting (800 W). The crude product was purified by column
chromatography (30 · 1.5 cm silica-gel, n-hexane:dichloro-
methane = 1:2, 40 · 1.5 cm silica-gel, diethyl ether:petro-
leum ether = 2:3). Ligand 7 was obtained as yellow solid
4.2.10. {2-Carbethoxy-6-[1-[(2,6-dimethylphenyl)imino]-
ethyl]pyridine}2NiCl2 (13)
Using an analogous procedure as above, 13 was obtained
as a red solid in 42.1% yield. M.p. >300 ꢁC. IR (KBr): mC–N
1624 cmÀ1. Anal. Calc. for C36H42Cl2N4NiO5: C, 58.40; H,
5.72; N, 7.57. Found: C, 58.72; H, 5.46; N, 7.89%.
1
in 17.6% yield. M.p. 84.0–86.0 ꢁC. H NMR (400 MHz,
CDCl3): d 8.48 (d, 1H, Py-Hm), 8.11 (d, 1H, Py-Hm),
7.87 (t, 1H, Py-Hp), 7.30 (d, 2H, Ar-Hm), 6.99 (t, 1H,
Ar-Hp), 4.43 (m, 2H, COOCH2), 2.28 (m, 4H, (CH2)CH3),
2.20 (s, 3H, N-C(Me)), 1.39 (t, 3H, COOCH2(CH3)), 1.05
(t, 6H, CH2(CH3)). IR (KBr): mC–O 1742, mC–N 1640,
mC–O–C 1166 cmÀ1. Anal. Calc. for C20H24N2O2: C, 74.04;
H, 7.46; N, 8.64. Found: C, 74.18; H, 7.70; N, 8.17%.
4.2.11. {2-Carbethoxy-6-[1-[(2,4,6-trimethylphenyl)-
imino]ethyl]pyridine}FeCl2 (14)
Using an analogous procedure as above, under a nitro-
gen atmosphere, 14 was obtained as a blue solid in 40.1%
yield. M.p. >300 ꢁC. IR (KBr): mC–O 1719, mC–N
1589 cmÀ1. Anal. Calc. for C19H22Cl2FeN2O2: C, 52.20;
H, 5.07; N, 6.41. Found: C, 52.17; H, 5.01; N, 6.34%.
4.2.5. {2-Carbethoxy-6-[1-[(2,6-dimethylphenyl)imino]-
ethyl]pyridine}MnCl2 (8)
4.2.12. {2-Carbethoxy-6-[1-[(2,4,6-trimethylphenyl)-
imino]ethyl]pyridine}CoCl2 (15)
A solution of MnCl2 Æ 4H2O (29.7 mg, 0.15 mmol) and 5
(44.4 mg, 0.15 mmol) in ethanol (8 mL) was stirred at room
temperature for 6 h. During this period a yellow solid was
obtained, which was washed repeatedly with diethyl ether
and dried in vacuo. Yield: 26.3 mg (41.5%). M.p.
>300 ꢁC. IR (KBr): mC–O 1697, mC–N 1627 cmÀ1. Anal. Calc.
for C18H20Cl2MnN2O2: C, 51.21; H, 4.77; N, 6.63. Found:
C, 51.33; H, 4.80; N, 6.45%.
Using an analogous procedure as above, 15 was
obtained as a green solid in 48.5% yield. M.p. > 300 ꢁC.
IR (KBr): mC–O 1717, mC–N 1592 cmÀ1. Anal. Calc. for
C19H22Cl2CoN2O2: C, 51.84; H, 5.04; N, 6.36. Found: C,
51.88; H, 5.07; N, 6.20%.
4.2.13. {2-Carbethoxy-6-[1-[(2,4,6-trimethylphenyl)-
imino]ethyl]pyridine}NiCl2 (16)
4.2.6. {2-Carbethoxy-6-[1-[(2,6-dimethylphenyl)imino]-
ethyl]pyridine}FeCl2 (9)
Using an analogous procedure as above, under a
nitrogen atmosphere, 9 was obtained as a blue solid in
30.0% yield. M.p. >300 ꢁC. IR (KBr): mC–O 1712, mC–N
Using an analogous procedure as above, 16 was
obtained as a yellow solid in 50.7% yield. M.p. >300 ꢁC.
IR (KBr): mC–O 1720, mC–N 1592 cmÀ1. Anal. Calc. for
C19H22Cl2N2NiO2: C, 51.87; H, 5.04; N, 6.37. Found: C,
52.09; H, 5.22; N, 6.11%.

B.-Y. Su, J.-S. Zhao / Polyhedron 25 (2006) 3289–3298
3297
Table 3
4.2.14. {2-Carbethoxy-6-[1-[(2,6-diethylphenyl)imino]-
ethyl]pyridine}FeCl2 (17)
Crystal data, data collection and structure refinement for complexes
12 Æ THF and 18
Using an analogous procedure as above, under a
nitrogen atmosphere, 17 was obtained as a blue solid in
23.3% yield. M.p. >300 ꢁC. IR (KBr): mC–O 1665, mC–N
1620 cmÀ1. Anal. Calc. for C20H24Cl2FeN2O2 Æ 0.5H2O:
C, 52.20; H, 5.48; N, 6.09. Found: C, 52.39; H, 5.22; N,
5.99%.
Complex
12
18
Empirical formula
Formula weight
Temperature (K)
Wavelength (A)
Crystal system
Space group
C22H24Cl2N2O3Zn
500.70
296(2)
0.7107(3)
triclinic
C20H24Cl2CoN2O2
454.24
293(2)
˚
0.7107(3)
monoclinic
P2/m
18.500(4)
8.093(2)
14.563(3)
107.49(3)
2079.6(7)
4
ꢀ
P1
˚
a (A)
8.148(2)
9.076(2)
17.039(4)
99.08(4)
1190.4(4)
2
4.2.15. {2-Carbethoxy-6-[1-[(2,6-diethylphenyl)imino]-
ethyl]pyridine}CoCl2 (18)
˚
b (A)
˚
c (A)
Using an analogous procedure as above, 18 was
obtained as a green solid in 64.8% yield. M.p. >300 ꢁC.
IR (KBr): mC–O 1701, mC–N 1625 cmÀ1. Anal. Calc. for
C20H24Cl2CoN2O2: C, 52.88; H, 5.33; N, 6.17. Found: C,
53.31; H, 5.38; N, 6.11%.
b (ꢁ)
Volume (A )
3
˚
Z
Dcalc (g cmÀ3
)
1.397
1.280
1.451
1.099
Absorption
coefficient (mmÀ1
)
F(000)
Crystal color, habit
516
yellow, block
940
green,
irregular block
2.77–27.46
4.2.16. {2-Carbethoxy-6-[1-[(2,6-diethylphenyl)imino]-
ethyl]pyridine}NiCl2 (19)
h Range for data
collection (ꢁ)
2.35–25.10
Using an analogous procedure as above, 19 was
obtained as a pale red solid in 61.0% yield. M.p.
>300 ꢁC. IR (KBr): mC–O 1691, mC–N 1623 cmÀ1. Anal. Calc.
for C20H26Cl2N2NiO3: C, 50.89; H, 5.55; N, 5.93. Found:
C, 50.71; H, 5.21; N, 5.78%.
Index ranges
À9 6 h 6 8,
À23 6 h 6 22,
À10 6 k 6 0,
0 6 l 6 18
À10 6 k 6 9,
À18 6 l 6 20
6169/4182 (0.0468)
Reflections
4592/4592 (0.0000)
collected/unique (Rint
Completeness (%)
Maximum and
minimum transmission
Refinement method
)
98.5 (to h = 25.10ꢁ)
0.9352 and 0.7229
96.1 (to h = 27.46ꢁ)
1.1848 and 0.8675
4.3. General procedure for polymerization and
oligomerization of ethylene
full-matrix
full-matrix
least-squares on F2
4182/0/276
least-squares on F2
4592/0/248
10 lmol of the transition metal complex was charged
into a Schlenk tube. Air was eliminated from the Schlenk
tube by three vacuum/nitrogen steps. Then the desired vol-
ume of MAO/toluene solution (14%) was added. Immedi-
ately the color of the solution changed. The solution was
then stirred at 600 rpm for the time set and the active cat-
alytic species in toluene solution was available.
The polymerization and oligomerization reactions were
carried out in a 100 mL three-neck flask. After evacuation
and flushing with nitrogen three times, then with ethylene
two times, the flask was charged with 50 mL of toluene
and magnetically stirred under ambient ethylene atmo-
sphere. When the desired reaction temperature was estab-
lished, the solution of the catalyst species was injected
into the reactor. Stirring was carried out at 1200 rpm. After
the times set for the experiment, the reaction solution was
quickly cooled down to À20 ꢁC and the reaction quenched
with acidic ethanol solution. The precipitated polymer was
collected and washed for several times with ethanol and
then dried under vacuum at 50 ꢁC for 24 h. The oligomer
solution was dried over anhydrous Na2SO4, an aliquot of
this solution was analyzed by GC (Gas Chromatograph).
Data/restraints/
parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
1.065
0.674
R1 = 0.0650,
wR2 = 0.1451
R1 = 0.1229,
wR2 = 0.1683
0.430 and À0.389
R1 = 0.0431,
wR2 = 0.0771
R1 = 0.1097,
wR2 = 0.0865
0.537 and À0.568
R indices (all data)
Largest difference in
peak and hole (e AÀ3
)
was used to determine the unit-cell parameters. A SADABS
absorption correction was applied. The structure was
solved by direct methods and refined by full-matrix least-
squares techniques with anisotropic thermal parameters
for non-hydrogen atoms. Hydrogen atoms were placed at
calculation positions and were included in the structure cal-
culation without further refinement of the parameters. All
calculations were performed using the SHELXS-97 program.
Crystal data and structure refinement for complexes 12 and
18 are summarized in Table 3.
Acknowledgments
We are grateful to the Program for Changjiang Scholars
and Innovative Research Team in University, PCSIRT
(IRT0559), the Natural Science Foundation of China
(No. 20371039), National Basic Research Program (973
Program) (2003CB214600), the Key Laboratory Research
and Establishment Program of Department of Education
4.4. X-ray analysis of 12 and 18
Data collection for complexes 12 and 18 was performed
at 293(2) and 296(2) K, respectively, on a Bruker SMART
1000 diffractometer with graphite-monochromated Mo Ka
˚
radiation (k = 0.71073 A). The Smart program package

3298
B.-Y. Su, J.-S. Zhao / Polyhedron 25 (2006) 3289–3298
[13] G.J.P. Britovsek, S. Mastroianni, G.A. Solar, S.P.D. Baugh, C.
Redshaw, V.C. Gibson, A.J.P. White, D.J. Williams, M.R.J. Else-
good, Chem. Eur. J. 6 (2000) 2221.
[14] G.J.P. Britovsek, V.C. Gibson, B.S. Kimberley, S. Mastroianni, C.
Redshaw, G.A. Solar, A.J.P. White, D.J. Williams, J. Chem. Soc.,
Dalton Trans. (2001) 1639.
of Shaanxi Province (No. 03JS006) and the Nature Science
Foundation of Department of Education of Shaanxi Prov-
ince (No. 04JK143) for financial support. We also thank
for the Institute of Chemistry, Chinese Academy of Science
for providing the visiting scholarship to J.-S. Zhao.
[15] G.J.P. Britovsek, V.C. Gibson, S. Mastroianni, D.C.H. Oakes, C.
Redshaw, G.A. Solar, A.J.P. White, D.J. Williams, Eur. J. Inorg.
Chem. (2001) 431.
Appendix A. Supplementary material
[16] E.A.H. Griffiths, G.J.P. Britovsek, V.C. Gibson, I.R. Gould, Chem.
Commun. (1999) 1333.
[17] E.L. Dias, M. Brookhart, P.S. White, Organometallics 19 (2000)
4995.
[18] V.C. Gibson, M.J. Humpries, K.P. Tellmann, D.F. Wass, A.J.P.
White, D.J. Williams, Chem. Commun. (2001) 2252.
[19] B. C¸ etinkaya, E. C¸ etinkaya, M. Brookhart, P.S. White, J. Mol. Catal.
A 142 (1999) 101.
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic Data
Centre, CCDC No. 298285 for complex 12, CCDC No.
250904 for complex 18. Copies of these information may
be obtained free of charge from: The Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (fax: +44
1223 336 033; e-mail: depodit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk). Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.poly.2006.06.015.
[20] B. de Bruin, E. Bill, E. Bothe, T. Weyhermuller, K. Weighardt, Inorg.
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W.A. Herrmann, Organometallics 21 (2002) 74.
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