Synthesis and characterization of para-nitro substituted 2,6-bis(phenylimino)pyridyl Fe(II) and Co(II) complexes and their ethylene polymerization properties

Article abstract of DOI:10.1016/j.jorganchem.2009.07.027

Four iron(II) and cobalt(II) complexes ligated by 2,6-bis(4-nitro-2,6-R₂-phenylimino)pyridines, LMCl₂ (1: R = Me, M = Fe; 2: R = iPr, M = Fe; 3: R = Me, M = Co; 4: R = iPr, M = Co) have been synthesized and fully characterized, and t

Full text of DOI:10.1016/j.jorganchem.2009.07.027

Journal of Organometallic Chemistry 694 (2009) 3793–3799
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and characterization of para-nitro substituted 2,6-bis(phenylimino)
pyridyl Fe(II) and Co(II) complexes and their ethylene polymerization properties
a
a
Zerong Long a,c, Biao Wu a,b, , Peiju Yang , Gang Li , Yanyan Liu a,c, Xiao-Juan Yang a,b
*
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
b State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
c Graduate School of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four iron(II) and cobalt(II) complexes ligated by 2,6-bis(4-nitro-2,6-R2-phenylimino)pyridines, LMCl2
(1: R = Me, M = Fe; 2: R = iPr, M = Fe; 3: R = Me, M = Co; 4: R = iPr, M = Co) have been synthesized and fully
characterized, and their catalytic ethylene polymerization properties have been investigated. Among
these complexes, the iron(II) pre-catalyst bearing the ortho-isopropyl groups (complex 2) exhibited
higher activities and produced higher molecular weight polymers than the other complexes in the pres-
ence of methylaluminoxane (MAO). A comparison of 2 with the reference non-nitro-substituted catalyst
(2,6-bis(2,6-diisopropylphenylimino)pyridyl)FeCl2 (FeCat 5) revealed a modest increase of the catalytic
activity and longer lifetime upon substitution of the para-positions with nitro groups (activity up to
6.0 Â 103 kg molÀ1 hÀ1 barÀ1 for 2 and 4.8 Â 103 kg molÀ1 hÀ1 barÀ1 for 5), converting ethylene to highly
linear polyethylenes with a unimodal molecular weight distribution around 456.4 kg molÀ1. However,
the iron(II) pre-catalyst 1 on changing from ortho-isopropyl to methyl groups displayed much lower
activities (over an order of magnitude) than 2 under mild conditions. As expected, the cobalt analogues
showed relatively low polymerization activities.
Received 17 November 2008
Received in revised form 8 July 2009
Accepted 14 July 2009
Available online 17 July 2009
Keywords:
Iron(II)
Cobalt(II)
2,6-bis(4-nitro-2,6-R2-
phenylimino)pyridines
Ethylene polymerization
Electronic effect
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
merization activities with an excellent selectivity of
(>95%) [2,8].
a-olefins
Since Brookhart [1] and Gibson [2] independently reported that
2,6-bis(phenylimino)pyridyl Fe(II) and Co(II) dihalide were highly
effective catalysts for ethylene polymerization on treatment with
methylaluminoxane (MAO), a great deal of interest has focused
on the relationship between structure and activity of such catalysts
[3–7]. It is generally thought that the steric and electronic effects of
the ligand play a crucial role in the catalytic performance and poly-
mer properties. The steric bulk of the ortho-substituents on the aryl
rings, being close to the active site, is particularly important in eth-
ylene polymerization/oligomerization. Studies have shown that a
moderate increase of the size of the ortho-alkyl groups seemed to
facilitate an increase of the productivity and the polymer molecu-
lar weight for the 2,6-bis(phenylimino)pyridyl iron(II) catalysts
under mild reaction conditions [1–4]. When the iron(II) complex
was substituted by a single alkyl group on one ortho-position, the
catalyst showed either a rather low polymerization productivity
and low polymer molecular weight (Mw) or high ethylene oligo-
The catalytic activities of 2,6-bis(phenylimino)pyridyl iron and
cobalt complexes can also be remarkably influenced by the elec-
tronic effect of the ligand substituents. It has been reported that
iron or cobalt complexes with electronegative substituents (F, Cl,
Br etc.) on both or single ortho-positions of the aryl rings displayed
high activities for ethylene polymerization and/or oligomerization
[5,6,9,10]. Introduction of electron-withdrawing groups (e.g. Br, F,
CF3 etc.) to the para-positions resulted in an increase of polymeri-
zation or oligomerization activities for such iron catalytic systems
treated with MAO/MMAO, while steric effect appeared to be more
dominant for cobalt catalysts [6,11–15]. Recently, Gibson and
coworkers [16] reported a series of para-ferrocene-substituted
bis(phenylimino)pyridyl iron(II) complexes which exhibited
a
modest increase of activity compared with the non-ferrocenyl iron
catalysts due to the higher electrophilicity induced by the para-
groups. There are also reports of palladium
a-diimine complexes
and iron(II) bis(imino)pyridyl complexes with strongly electron-
withdrawing nitro groups. The former showed less activities and
produced lower Mw polymers, whereas the latter (iron catalysts
incorporating two para-nitro substituents) displayed moderate
thermal stabilities, longer lifetimes and lower oligomerization
activities compared with the non-nitro-substituted systems
[17,18]. In view of these results, it would be an effective approach
* Corresponding author. Address: State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou 730000, China. Tel./fax: +86 931 4968286.
E-mail address: wubiao@lzb.ac.cn (B. Wu).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.07.027

3794
Z. Long et al. / Journal of Organometallic Chemistry 694 (2009) 3793–3799
to design bis(imino)pyridyl ligands with appropriate ortho-steric
bulk and electron-withdrawing para-nitro-substituents for the
purpose to achieve extended lifetimes and high activities for ethyl-
ene polymerization.
In the present work, we have designed two 2,6-bis(2,6-R2-phe-
nylimino)pyridyl ligands (R = CH3 or iPr) bearing strongly elec-
tron-withdrawing para-nitro substituents for the purpose of
evaluating the electronic effect on the polymerization behavior of
bis(imino)pyridine complexes. Herein we report the synthesis and
characterization of the corresponding (2,6-bis(4-nitro-2,6-R2-phe-
nylimino)pyridyl)-FeCl2 and –CoCl2 complexes (1–4), as well as de-
tailed studies of the effect of modification in ligand architecture (i.e.
the steric and electronic properties) on the polymerization activi-
ties and the resultant polyolefins.
Fig. 1. Molecular structure of L1; hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (°): N4–C16 1.271(2), N2–C9 1.279(2), N4–C16–C15
116.7(2), N2–C9–C11 116.3(2), O3–N5–O4 123.5(2), O2–N3–O1 123.3(2).
for the ligands and at 1614–1634 cmÀ1 for the complexes, indicat-
ing the coordination of the nitrogen atoms of phenylimino to the
metal ions [3]. Complexes 2 and 4 were further characterized by
X-ray crystallography.
The crystals of complex 2 were grown by slow diffusion of Et2O
into its THF-methanol solution under nitrogen atmosphere at room
temperature. Complex 2 (Fig. 2) shows an approximate Cs symme-
try about the plane defined by the iron atom, the two Cl atoms, and
the pyridyl nitrogen atom. The geometry around the iron center
2. Results and discussion
2.1. Synthesis
Ligands L1 and L2 were prepared by the condensation of 2.2
equivalents of the corresponding nitroaniline with one equivalent
of 2,6-diacetylpyridine (Scheme 1). When toluene was used as sol-
vent according to traditional methods, a poor yield (6–10%) was
obtained, perhaps due to the low basicity of nitroaniline [17]. Fur-
thermore, prolonging reaction time (up to refluxing for 3 days) in
cooperation with the use of molecular sieves to extract water could
not improve the yield. Finally, we used tetraethyl silicate as solvent
and water absorbent, and 2,6-bis(4-nitro-2,6-R2-phenylimino)pyr-
idines (L1 and L2, Scheme 1) were isolated in acceptable yields (33–
55%).
can be described as a distorted square pyramid (s value 0.45)
[19] with a tridentate N^N^N ligand and two chlorides. The iron
atom deviates ca. 0.54 Å from the plane of coordinated nitrogen
atoms (N1, N2 and N3). The central Fe1–N1(Py) bond (2.093(4)
Å) is significantly shorter than Fe1–N(imino) (2.250(4) and
2.251(4) Å). The Fe–Cl bonds (2.241(2) and 2.296(2) Å) in 2 are
shorter than those (2.266(2) and 2.311(2) Å) in the known complex
5 [3], while the Fe–N bond lengths are slightly longer compared
with those in 5 (2.088(4), 2.238(4) and 2.250(4) Å).
Complex 4 was crystallized by slow evaporation of its CH2Cl2-
toluene solution at room temperature. The cobalt complex 4
(Fig. 3) is isomorphous to its iron analogue 2 with similar geomet-
ric characteristics. The deviation of the cobalt atom from the plane
of its coordination nitrogen atoms is 0.55 Å. As in 2, complex 4 has
a five-coordinate, distorted square-pyramidal geometry around the
Single crystals of L1 suitable for X-ray diffraction were obtained
in dimethyl sulfoxide solution. The molecular structure of L1
(Fig. 1) reveals that the phenylimino groups are stretched away
from the central pyridyl ring to release the steric constraints and
are in the (E, E) conformation with typical C@N double bonds
(1.279(2) and 1.271(2) Å) [6]. The two phenyl rings are nearly per-
pendicular to the pyridyl ring (dihedral angles 91.2° and 91.9°),
while the nitro groups are approximately coplanar with the corre-
sponding dimethylphenyl moiety (dihedral angle 4.0° and 12.5°,
respectively).
Co(II) center with a
s value of 0.44. The Co–N(Py) bond (2.045(3) Å)
in complex 4 is slightly shorter than that in (2,6-bis(2,6-diisopro-
pylphenylimino)pyridyl)CoCl2 (2.051(3) Å), yet the average Co–
N(imino) bond length is comparable to that of the non-nitro Co(II)
complex (Co–N, 2.211 Å). The Cl1–Co1–Cl2 bond angle (113.8(1)°)
is slightly smaller than that in the latter compound (116.5°) [3].
The complexes 1–4 were prepared by reaction of equimolar
FeCl2Á4H2O or CoCl2Á6H2O with the corresponding ligand in fleshly
distilled THF or dichloromethane at room temperature, and were
isolated as air-stable solids in moderate yield. They were charac-
terized by elemental analysis, ESI–MS spectrometry and IR spec-
troscopy, which confirmed the composition LMCl2 of the
complexes. The IR spectra showed C@N stretching at 1648 cmÀ1
2.2. Ethylene polymerization
The complexes 1–4 were used as pre-catalysts for ethylene
polymerization. Upon treatment with MAO, all the complexes are
active toward ethylene polymerization. Results of the polymeriza-
tion reactions are listed in Tables 1 and 2 and Figs. 4 and 5.
NH2
R1
R1
R1
R1
N
N
(1)
N
+
N
R2
R1
R1
R2
O
O
R2
L
(2)
R1
R2
NO2 Fe
i-Pr NO2 Fe
M
R2
R1
CH3
1
2
3
R1
N
R1
CH3 NO2
i-Pr NO2
L1
L2
L3
M
N
N
CH3
i-Pr
i-Pr
NO2 Co
NO2 Co
Cl Cl
i-Pr
H
R2
R1
R1
R2
4
5
1--5
Fe
H
Scheme 1. Synthesis of the ligands and complexes. Reagents and conditions: (1) p-toluenesulfonic acid, tetraethyl silicate; (2) FeCl2Á4H2O or CoCl2Á6H2O, THF.

Z. Long et al. / Journal of Organometallic Chemistry 694 (2009) 3793–3799
3795
Table 2
Ethylene polymerization with complexes 2 and 5 in the presence of MAO.a
Run
Pre-catalyst
T (°C)
t (min)
PE (g)
Activityb
4
9
2
5
2
5
2
5
2
5
2
5
2
5
2
5
2
5
2
5
2
5
0
0
10
10
10
10
10
10
10
10
10
10
5
2.00
1.61
1.73
1.43
1.70
1.30
1.24
1.20
0.73
0.50
0.98
0.96
2.15
1.70
2.20
1.90
2.25
2.10
2.40
2.20
6.0
4.8
5.2
4.3
5.1
3.9
3.7
3.6
2.2
1.5
5.9
5.8
4.3
3.4
2.2
1.9
1.5
1.4
1.2
1.1
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
15
15
30
30
45
45
60
60
0
0
0
0
0
Fig. 2. Molecular structure of 2. Hydrogen atoms and the crystalline methanol
molecule are omitted for clarity. Selected bond lengths (Å) and angles (°): Fe1–N1
2.093(4), Fe1–N2 2.250(4), Fe1–N3 2.251(4), Fe1–Cl1 2.241(2), Fe1–Cl2 2.296(2),
C2–N2 1.280(6), C8–N3 1.285(6), N1–Fe1–N2 73.0(2), N1–Fe1–N3 72.6(2), N2–Fe1–
N3 140.3(2), N1–Fe1–Cl1 150.6(1), N1–Fe1–Cl2 94.8(1), N2–Fe1–Cl2 105.0(1), N2–
Fe1–Cl1 100.2(1), N3–Fe1–Cl1 100.0(1), N3–Fe1–Cl2 97.1(1), Cl1–Fe1–Cl2 114.5(1).
5
15
15
30
30
45
45
60
60
0
0
0
0
0
a
Polymerization conditions: 2 lmol of pre-catalyst; 30 mL of toluene; ethylene
of 1 bar; 2000 equiv of MAO.
b
Polymers (103 kg molÀ1 hÀ1 barÀ1).
Fig. 3. Molecular structure of 4. Hydrogen atoms and the crystalline CH2Cl2
molecule are omitted for clarity. Selected bond lengths (Å) and angles (°): Co1–N1
2.045(3), Co1–N2 2.208(3), Co1–N3 2.215(3), Co1–Cl1 2.217(2), Co1–Cl2 2.273(1),
C2–N2 1.281(5), C8–N3 1.279(5), N1–Co1–N2 73.9(1), N1–Co1–N3 73.7(1), N2–
Co1–N3 141.3(1), N1–Co1–Cl1 153.5(1), N1–Co1–Cl2 92.5(1), N2–Co1–Cl2 96.2(1),
N2–Co1–Cl1 99.3(1), N3–Co1–Cl1 100.3(1), N3–Co1–Cl2 105.8(1), Cl1–Co1–Cl2
113.8(1).
2.2.1. Effect of the ligand environment and metal center
The precatalysts 2 and 5 have the same ortho-isopropyl groups
on the aryl rings of the N^N^N tridentate ligand, and the difference
between them is that the precatalyst 2 bears the strongly electron-
withdrawing para-nitro substituents. Replacing the para-aryl pro-
tons with nitro groups (precatalyst 2) resulted in a modest increase
of the productivity and the molecular weight (Mw) compared to 5,
from 4.8 Â 103 to 6.0 Â 103 kg molÀ1 hÀ1 barÀ1 and from 201.5 to
456.4 kg molÀ1 respectively, and a decrease of the molecular
weight distribution (MWD:Mw/Mn) from 37.8 to 21.0 (Table 1,
runs 4, 9). Moreover, the precatalyst 2 showed a much higher
productivity than the ortho-methyl analogue 1 (6.0 Â 103 kg molÀ1
Fig. 4. 13C NMR spectra (400 MHz, o-C6D4Cl2, 135 °C) of the PEs produced by (A)
complex 1/MAO (run 1), and (B) complex 2/MAO (run 4 in Table 1).
Table 1
Ethylene polymerization with complexes 1–5.a
Run
Catb
Al/M (mol/mol)
t (min)
Yield (g)
Activityc
Mnd kg/mol
Mwd kg/mol
Mw/Mnd
1
2
3
4
5
6
7
8
1
2
2
2
2
2
3
4
5
2
5
2000
1000
1500
2000
2500
3000
2000
2000
2000
2000
2000
10
10
10
10
10
10
10
10
10
30
30
0.14
0.64
1.29
2.00
1.85
1.71
0.20
0.31
1.59
5.39
2.69
0.4
1.9
3.9
6.0
5.5
5.1
0.6
0.9
4.8
1.1
0.5
8.9
277.0
645.9
621.7
456.4
544.0
458.5
206.5
286.7
201.5
781.1
365.8
31.1
23.4
28.7
21.0
10.9
11.1
45.0
24.3
37.8
9.1
27.5
21.6
21.7
49.9
41.4
4.6
11.8
5.3
85.9
163.3
9
10e
11e
2.2
a
b
c
Conditions: ethylene of 1 bar; reaction time: 10 min; reaction temperature: 0 °C; co-catalyst: MAO; solvent: 30 mL toluene.
Pre-catalyst: 2 lmol.
Polymers (103 kg molÀ1 hÀ1 barÀ1).
d
e
Determined by GPC.
Ethylene of 5 bar, reaction temperature: 30–40 °C, 100 mL toluene.

3796
Z. Long et al. / Journal of Organometallic Chemistry 694 (2009) 3793–3799
(Fig. 4A). However, no detectable unsaturated groups for the poly-
2 Al/Fe=1000
2 Al/Fe=1500
2 Al/Fe=2000
2 Al/Fe=2500
2 Al/Fe=3000
5 Al/Fe=2000
0.6
0.5
0.4
0.3
0.2
0.1
0.0
mers produced by the precatalyst 1 were observed, probably due to
the chain transfer to aluminum which resulted in almost com-
pletely saturated polymer chains. The polymers obtained by 1
are highly linear and only one methyl branch per 1000 carbon
atoms can be observed. The PEs produced by the precatalyst 2
are also highly linear with saturated methyl ends and a 1:3 ratio
of unsaturated (1.3 per 1000 C) to saturated end groups, and al-
most no branch is observed (Fig. 4B). These results are consistent
with the polymers produced by the non-nitro substituted iron-
based precatalysts, in which saturated end groups are much more
than unsaturated end groups involving a combination of chain
transfer to aluminum and b-H transfer [3]. The facts that the pre-
catalyst 1 and 2 produced exclusively high molecular weight and
highly linear polyethylenes may suggest that the para-nitro substi-
tuted 2,6-bis(phenylimino)pyridyl ligands could restrain the active
iron center from deactivation and chain transfer reactions.
3
4
5
6
7
logMw
Fig. 5. GPC curves of the polymers prepared with pre-catalyst 2/MAO (Al/Fe = 1000,
1500, 2000, 2500, 3000) and the reference pre-catalyst 5/MAO (Al/Fe = 2000) at
0 °C, 1 bar ethylene for 10 min.
2.2.2. Effects of the reaction conditions
hÀ1 barÀ1 for 2 and 0.4 Â 103 kg molÀ1 hÀ1 barÀ1 for 1) under the
same reaction conditions, and the Mw also dropped from 456.4
to 277.0 kg molÀ1 (Table 1, runs 1, 4). It is likely that both the elec-
tronic effect and the steric bulk of the substituents caused the dif-
ferences of the activities and polymer Mw for these catalytic
systems. With respect to the precatalyst 2 containing a more elec-
tron-withdrawing ligand, the electrophilic character on the center
iron atom is enhanced remarkably, which makes the cationic spe-
cies more active for ethylene insertion. Thus, the precatalyst 2
showed higher activities and produced higher molecular weight
polyethylenes in comparison with the non-nitro substituted 5.
For the other factor, the ligand steric bulk, it is obvious that simply
changing the size of the aryl ortho-substituents allows for control
over both the catalyst activity and the polymer molecular weight
[11], as has been reported for the non-nitro substituted iron
catalytic system on changing from 2,6-diisopropyl (1.1 Â 103 kg
molÀ1 hÀ1 barÀ1, 203.0 kg molÀ1) to 2,6-dimethyl groups (0.6 Â
103 kg molÀ1 hÀ1 barÀ1, 29.0 kg molÀ1) upon activation with MAO
[2]. This is in line with the behavior of the nitro-substituted deriv-
atives in the present work, where the precatalyst 2 has a higher
polymer activity and higher Mw than 1 for the distinction of their
steric protection toward the active center.
The effects of the concentration of MAO, reaction time and tem-
perature, ethylene pressure, and especially the ligand architecture
on the polymerization activities and product properties of pre-cat-
alyst 2 have been investigated. To examine the role of the MAO in
the polymerization, the molar ratio of Al/Fe was varied from 1000
to 3000. As shown in Table 1, the productivity of 2 increased with
increasing Al/Fe ratio, reaching a maximum at Al/Fe = 2000, and
then decreased slightly with more MAO (Al/Fe up to 3000) at
0 °C (Table 1, runs 2–6). The molecular weight (Mw) and MWD
of the products showed a decreasing trend with increasing Al/Fe
ratio. Fig. 5 shows the GPC traces of a series of polymerization
tests of precatalysts 2 and 5. When the ratio of Al/Fe is 1000, a
broad unimodal molecular weight distribution is observed, with
an Mpk at 500 kg molÀ1 for precatalyst 2. A similar broad unimodal
molecular weight distribution was also found at other Al/Fe ratios
for precatalyst 2, and the value of Mpk decreased with increasing
Al/Fe ratio (1500, 400; 2000, 280; 2500, 250 and 3000,
125 kg molÀ1). However, with 2000 equiv of MAO a broad distribu-
tion is observed with an Mpk at 10 kg molÀ1 and a broader shoulder
in the range 100–600 kg molÀ1 for the non-nitro substituted 5. A
bimodal molecular weight distribution has been commonly ob-
served for iron catalysts with increasing MAO concentrations [9];
however, the unimodal molecular weight distribution has been
obtained in the para-nitro substituted precatalyst 2 as well as in
In the cases of the cobalt complexes 3 and 4, the ortho-isopropyl
derivative 4 also showed a slightly higher activity (0.9 Â 103 kg
molÀ1 hÀ1 barÀ1
)
and polymer Mw (286.7 kg molÀ1
)
than the
bis(phenylimino)pyridineÁFeCl2 with
x-allyloxy substituents on
methyl derivative 3 (0.6 Â 103 kg molÀ1 hÀ1 barÀ1, 206.5 kg molÀ1),
although their activities are much lower than those of the corre-
sponding iron analogues. It has been reported that non-nitro
substituted cobalt catalytic system displayed a slight increase in
polymer activity and a decrease in Mw on changing from ortho-iso-
the 4-position of the pyridyl ring for less chain transfer reactions
[20].
The polymerization time and reaction temperature were also
considered in evaluating the activity of pre-catalyst 2 (Table 2).
At 1 bar of ethylene, the high catalytic activity of the complex 2
was found to persist for up to 10 min, followed by a laggard atten-
uation. Although the initial activity of the FeCat 5 was rather close
to that of complex 2, a rapid deactivation was observed over 5 min.
Hence, the pre-catalyst 2 displayed a much higher average poly-
merization activity than 5 (Table 2, runs 4, 9, 20–29). The high
polymer activity of the precatalyst 2 might be attributed to the
electron-withdrawing nature of the nitro substituents, which re-
sults in a more electrophilic iron center in the complex, and thus
increases the corresponding activity in ethylene polymerization.
On the other hand, although nitro-substituted 2 has slightly longer
lifetime than the non-nitro substituted 5, deactivation soon oc-
curred with more than 10 min. The reason might be that side reac-
tions involving reduction of the nitro group have taken place in a
prolonged reaction time, resulting in the loss of productivity. As
proved by IR spectroscopy, the strong stretching of the nitro group
in the precatalysts weakened significantly after interaction with an
excess of MAO for 15 min. Similar deactivations were also ob-
propyl (0.7 Â 103 kg molÀ1 hÀ1 barÀ1
, ) to ortho-
43.0 kg molÀ1
methyl groups (0.9 Â 103 kg molÀ1 hÀ1 barÀ1, 3.7 kg molÀ1) with
treatment of MMAO [1], which is slightly different from the ni-
tro-substituted derivatives upon activation with MAO in this work
(Table 1, runs 7 and 8). For the cobalt system, introduction of para-
nitro groups on the aryl rings had no evident effect on the polymer
activities but remarkably improved the polymer mass. On the
other hand, the polymers produced by the cobalt(II) complexes
have lower molecular weights and slightly broader molecular
weight distributions (Mw/Mn) compared with the iron analogues.
This could be attributed to the different inherent electronic proper-
ties of Fe and Co that led to distinction in chain propagation and
chain transfer rates of the two systems [3].
The polyethylenes (PEs) produced by catalysts 1 and 2 were
analyzed by 13C NMR spectroscopy (Fig. 4), which revealed that
the precatalyst 1 yielded PEs with small amounts of methyl ends
(d = 14.0 ppm) and isopropyl chain ends (d = 24.4 and 28.2 ppm)

Z. Long et al. / Journal of Organometallic Chemistry 694 (2009) 3793–3799
3797
served in related nitro-substituted bis(phenylimino)pyridyl iron(II)
complexes applied in the oligomerization of ethylene [17].
chased from Albemarle Corp. (USA). All other chemicals were pur-
chased from commercial resources and used without further
purification. 2,6-Dimethyl-4-nitroaniline, 2,6-diisopropyl-4-nitro-
aniline [21,22] and 2,6-diacetylpyridine(2,6-diisopropylanil)FeCl2
were prepared by established procedures [3].
The effect of reaction temperature on the catalytic activity is
remarkable since polymerization is an exothermic process. A series
of experiments were carried out to determine the effect of temper-
ature variation on the catalyst performance. Table 2 shows the
polymerization results at 0, 15, 30, 45 and 60 °C, respectively,
employing pre-catalysts 2 and 5 at 1 bar. A significant reduction
of the productivity was obtained at elevated temperatures for both
precatalysts 2 and 5, which is probably due to potential decompo-
sition of the pre-catalysts 2 and 5 at higher temperature.
4.2. Synthesis of the ligands
4.2.1. 2,6-Dimethyl-4-nitroanil-2,6-diactylpyridine (L1)
2,6-Dimethyl-4-nitroaniline (730.7 mg, 4.4 mmol) was added to
a solution of 2,6-diactylpyridine (326.3 mg, 2.0 mmol) with a cata-
lytic amount of p-toluenesulfonic acid (20.0 mg, 0.1 mmol) in tet-
raethyl silicate (1 mL)/toluene (20 mL) under nitrogen
atmosphere. The mixture was refluxed for 3 days under nitrogen
until no reactant 2,6-diactylpyridine remained. The solvents were
evaporated under reduced pressure and the yellow solid was sub-
sequently purified by column chromatography on silica gel with
light petroleum ether/EtOAc (5:1) as the eluent. The yellow solid
thus obtained was recrystallized by dichloromethane/light petro-
leum ether (1:5) to give a pale-yellow solid (301.7 mg, 33%). Mp:
Polymerization pressure is another significant factor that influ-
ences the activities of the catalysts and molecular weight distribu-
tion of the polymers [3]. At 5 bar ethylene pressure, the complex 2
showed lower polymerization activities and narrower molecular
weight distributions (Table 1, run 10) than those observed at
1 bar ethylene pressure due to the increase of both rate constants
of chain propagation and b-H transfer at higher pressure, which
were responsible for high molecular weights and narrower molec-
ular weight distributions [3]. Although the catalytic activity of the
complex 2 seemed to precede over complex 5, lower molecular
299–300 °C. IR (KBr, cmÀ1): 2912, 1648 (s,
mC@N, imine), 1588,
weight and narrower distribution polymers (Mw: 365.8 kg molÀ1
;
1505, 1328, 1214, 1098, 900, 775, 748, 711, 625. 1H NMR
(400 MHz, CDCl3, ppm): 8.50 (d, J = 16 Hz, 2H, Py-Hm), 8.06 (s,
4H, Ar-H), 7.91 (t, 1H, Py-Hp), 2.28 (s, 6H, N@C-CH3), 2.14 (s,
12H, Ar-CH3). 13C NMR (100.6 MHz, CDCl3, ppm): 167.3 (C@N),
154.7 (Py-Co), 154.3 (Ar-N@C), 143.5 (Ar-C-NO2), 137.3 (Ar-C-
Me), 126.6 (Py-Cp), 123.5 (Py-Cm), 123.0 (Ar-Cm), 18.0 (C@N-CH3),
17.1 (Ar-CH3). ESI–MS: m/z 460.5 [M+H]+. Anal. Calc. for
C25H25N5O4: N, 15.24; C, 65.35; H, 5.48. Found: N, 15.18; C,
65.15; H, 5.35%.
MWD: 2.2) were observed for 5 under same reaction conditions
(Table 1, runs 10 and 11). It is obvious that the polymerization
properties are strongly dependent on the electronic feature of the
ligand.
3. Conclusions
Four iron(II) and cobalt(II) complexes ligated by 2,6-bis(4-nitro-
2,6-R2-phenylimino)pyridines have been synthesized, which are
active ethylene polymerization catalysts in the presence of MAO.
The iron pre-catalyst 2 displayed moderately increased catalytic
activities relative to the non-nitro substituted analogue 5, which
was ascribed to the strongly electron-withdrawing para-nitro
groups that can increase the Lewis acidic character of the iron cat-
ionic center. The ortho steric effect in such iron catalytic systems
also played a significant role in controlling the catalyst activity
and polymer mass, as complex 1 with ortho-methyl substituents
on the aryl rings showed much lower polymerization activities
than the ortho-isopropyl analogue 2. The iron pre-catalysts 1/
MAO and 2/MAO produced linear, high molecular weight polymers
with a rather low embranchment, while the cobalt precatalysts 3
and 4 showed low activities and low Mw products by treating with
MAO.
4.2.2. 2,6-Diisopropyl-4-nitroanil-2,6-diactylpyridine (L2)
2,6-Diisopropyl-4-nitroaniline (978.0 mg, 4.4 mmol) was added
to a solution of 2,6-diactylprydine (326.3 mg, 2.0 mmol) with a cat-
alytic amount of p-toluenesulfonic acid (20.0 mg, 0.1 mmol) in tet-
raethyl silicate (5 mL) under nitrogen atmosphere. The mixture
was stirred at 125 °C for 12 h. The solvent was evaporated under
reduced pressure, and the residue was subsequently purified by
column chromatography (silica gel; eluting reagent light petro-
leum/EtOAc (4:1)) to yield a yellow solid (628.9 mg, 55%). Mp:
>300 °C. IR (KBr, cmÀ1): 2911, 1648 (s,
mC@N), 1588, 1504, 1328,
1213,1097, 900, 775, 748, 625. 1H NMR (400 MHz, CDCl3, ppm):
8.49 (d, J = 10.8 Hz, 2H, Py–Hm), 8.08 (s, 4H, Ar–H), 8.00 (t, 1H,
Py–Hp), 2.79 (m, 4H, CH(CH3)2), 2.30 (s, 6H, N@C–CH3), 1.21 (m,
24H, CH(CH3)2). 13C NMR (100.6 MHz, CDCl3, ppm): 167.0 (C@N),
154.3 (Py–Co), 152.4 (Ar–C-N@C), 144.6 (Ar-C-NO2), 137.4 (Ar-C-
Me), 137.2 (Py-Cp), 123.0 (Py-Cm), 119.3 (Ar-Cm), 29.7 (C@N-CH3),
23.7 (CH(CH3)2), 17.6 (CH(CH3)2). ESI–MS: m/z 572.6 [M+H]+. Anal.
Calc. for C33H41N5O4: N, 12.25; C, 69.33; H, 7.23. Found: N, 12.00; C,
69.33; H, 7.17.
4. Experimental
4.1. General
All synthetic manipulations were carried out under a nitrogen
atmosphere using standard Schlenk and cannula techniques. The
NMR data of the polyethylenes were obtained on a Varian Unity-
400 MHz spectrometer at 135 °C, with o-C6D4Cl2 as solvent. The
NMR spectra of the ligands were recorded on a Mercury plus-400
spectrometer at ambient temperature in CDCl3. Elementary analy-
ses were performed on a VarioEL instrument from Elementar Anal-
ysensysteme GmbH. IR spectra were measured with an HP5890II
GC/NEXUS870. ESI-MS measurements were carried out with a
Waters ZQ-4000 instrument (Waters, Manchester, UK). Weight-
average (Mw), molecular weight distribution (Mw/Mn) were mea-
sured by a Waters gel permeation chromatograph Alliance GPCV
2000 at 150 °C using 1,2,4-trichlorobenzene as the eluent. Solvents
were refluxed over an appropriate drying agent and distilled under
nitrogen prior to use. MAO (10% solution in toluene) was pur-
4.3. Synthesis of the complexes
1: A mixture of FeCl2Á4H2O (38.8 mg, 0.20 mmol) with L1
(89.6 mg, 0.2 mmol) in freshly distilled THF (5 mL) was stirred at
room temperature for 24 h. The dark precipitate formed was fil-
tered and washed with THF (2 Â 2 mL) and freshly distilled diethyl
ether (2 Â 5 mL). After dried in vacuum, the iron(II) complex 1 was
obtained as a gray powder (yield: 78.2 mg, 67%). Mp: >300 °C. IR
(KBr, cmÀ1): 3090, 2962, 2917, 2852, 1634 (w,
mC@N), 1588, 1515,
1468, 1437, 1340, 1260, 1217, 1102, 891, 816, 779, 745, 454. Anal.
Calc. for C25H25Cl2FeN5O4: N, 11.95; C, 51.22; H, 4.30. Found: N,
11.64; C, 50.81; H, 4.02%. ESI–MS: m/z 551.5 [M–Cl]+, 258.1 [M–
2Cl]2+
.
2: In a similar manner to that described for 1, the iron complex
2 was prepared as a dark cyan powder (60.1 mg, 43%). Mp: >300 °C.

3798
Z. Long et al. / Journal of Organometallic Chemistry 694 (2009) 3793–3799
Table 3
Crystal data and structure refinement for L1, 2, and 4.
L1
2ÁCH3OH
4ÁCH2Cl2
Formula
Fw
C25H25N5O4
459.50
C34H45Cl2FeN5O5
730.50
C34H43Cl4CoN5O4
786.46
Crystal system
Space group
monoclinic
P2(1)/c
monoclinic
P2(1)/n
monoclinic
P2(1)/n
a (Å)
b (Å)
c (Å)
b (°)
8.5098(5)
17.060(1)
16.460(1)
91.896(3)
2388.3(3)
4
9.0052(6)
30.556(2)
14.855(1)
94.910(3)
4072.7(4)
4
9.005(2)
28.401(6)
15.018(3)
93.57(3)
3833.3(13)
4
V (Å)
Z
Dcalc/g cmÀ3
1.278
1.191
1.363
crystal size (mm)
h range (°)
Reflns collected/unique
Data/restraints/parameters
Absorption coefficient (mmÀ1
F(0 0 0)
0.29 Â 0.17 Â 0.10
1.72–26.77
14001/5069 [Rint = 0.0378]
5069/0/307
0.089
0.30 Â 0.12 Â 0.04
1.92–25.06
21095/7199 [Rint = 0.0664]
7199/1/424
0.543
0.45 Â 0.15 Â 0.14
1.43–25.06
12231/6674 [Rint = 0.0319]
6674/2/433
0.769
)
968
1536
1636
R1; wR2 [(I) > 2
R1; wR2 (all data)
GOF
r
(I)]
0.0511; 0.1239
0.0900; 0.1469
1.019
0.0755; 0.2181
0.1348; 0.2628
1.040
0.0645; 0.1803
0.0736; 0.1920
1.120
IR (KBr, cmÀ1): 3084, 2965, 2927, 2869, 1614 (w,
m
C@N), 1582, 1525,
nated with acidified ethanol. The solid polyethylene was filtered,
washed with ethanol, and dried in vacuum oven at 60 °C overnight.
The polymer was characterized by GPC and 13C NMR.
1462, 1438, 1348, 1325, 1264, 1217, 1102, 1032, 939, 891, 779,
454. Anal. Calc. for C33H41Cl2FeN5O4?CH3OH: N, 9.59; C, 55.90; H,
6.21. Found: N, 9.89; C, 55.43; H, 6.02%. ESI–MS: m/z 664.5 [M–
Cl]+, 314.2 [M–2Cl]2+
.
4.5.2. Ethylene polymerization at elevated pressure
3: In a similar manner to that described for 1, the cobalt com-
plex 3 was prepared as a green–yellow powder (83.7 mg, 71%).
The ethylene polymerization at elevated pressure was per-
formed in a stainless autoclave (500 mL). The complex (2 lmol)
Mp: >300 °C. IR (KBr, cmÀ1): 2956, 2920, 2852, 1626 (w,
m
C@N),
and the required amount of MAO were dissolved in 100 mL of
fleshly distilled toluene under nitrogen atmosphere, and the solu-
tion was subsequently transferred to the reactor via a syringe. The
reaction mixture was stirred for the desired time under corre-
sponding pressure of ethylene. The reaction was terminated and
analyzed by using the same method as described above for the
reaction with 1 bar ethylene.
1589, 1519, 1469, 1340, 1261, 1216, 1101, 1028, 940, 893, 815,
779, 745, 454. Anal. Calc. for C25H25Cl2CoN5O4Á0.5THF: N, 11.20;
C, 51.85; H, 4.67. Found: N, 10.77; C, 51.69; H, 4.58%. ESI–MS: m/
z 553.5 [M–Cl]+, 259.3 [M–2Cl]2+
.
4: In a similar manner to that described for 1, the cobalt com-
plex 4 was prepared as a yellow powder (65.9 mg, 47%). Mp:
>300 °C. IR (KBr, cmÀ1): 2966, 2922, 2852, 1625 (w,
mC@N), 1586,
1521, 1463, 1349, 1326, 1262, 1212, 1099, 1075, 1025, 900, 739,
448. Anal. Calc. for C33H41Cl2CoN5O4: N, 9.98; C, 56.50; H, 5.89.
Found: N, 9.55; C, 56.07; H, 5.67%. ESI–MS: m/z 665.7 [M–Cl]+,
Acknowledgements
This work was supported by the ‘‘Bairen Jihua” project of the
Chinese Academy of Sciences.
315.6 [M–2Cl]2+
.
4.4. X-ray crystallography
Appendix A. Supplementary material
Diffraction data for crystals of L1, 2 and 4 were collected on a
Bruker SMART APEX II diffractometer with graphite-monochro-
CCDC 726170 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.jorganchem.2009.07.027.
mated Mo Ka radiation (k = 0.71073 Å) at 293(2) K. Intensities
were corrected for empirical absorption [23]. The structures were
solved by direct methods and refined by full-matrix least squares
on F2. All non-hydrogen atoms were refined anisotropically. All
hydrogen atoms were placed in calculated positions. Structure
solutions were performed by using the SHELXL-97 package [24].
The crystal data collections and refinement details for ligand L1
and complexes 2 and 4 are summarized in Table 3.
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