Highly active and trans-1,4 specific polymerization of 1,3-butadiene catalyzed by 2-pyrazolyl substituted 1,10-phenanthroline ligated iron (II) complexes

Article abstract of DOI:10.1016/j.polymer.2013.07.021

A new family of iron (II) complexes (2a-h) bearing tridentate 2-pyrazolyl substituted 1,10-phenanthroline ligands were successfully prepared and characterized by IR spectroscopy, elemental analysis, and mass spectra. Complexes 2a-f and 2h were further confirmed by the X-ray crystallographic analysis. 2a, 2b, 2e, and 2f adopted distorted trigonal bipyramidal configuration. 2c displayed a distorted octahedron formed by six coordinated nitrogen atoms of the two ligands. Linked by two bridged chloride atoms, complex 2d was a centrosymmetric dimmer, and complex 2h adopted a six-coordinate distorted octahedral geometry due to the coordination of two solvent molecules. These complexes activated by alkylaluminum were examined in butadiene polymerization. In combination with AlⁱBu₃, complexes 2a-c exhibited high catalytic activity (73.5%-94.3%) at 20 C, whereas other complexes exhibited much lower activity. Interestingly, the activity and selectivity of the complexes increased as increasing polymerization temperature. In particular, 2b and 2c displayed both high activity (99% and 80%, respectively) and trans-1,4 selectivity (95.6% and 96.2%, respectively) at 60 C. The trans-1,4 selectivity of 2b varied as alkylaluminum used as a cocatalyst, in the following order: AlⁱBu₃ > AlOct₃ > AlEt₃ > AlMe₃, whereas much lower trans-1,4 selectivity was observed in the cases of using MAO and MMAO.

Full text of DOI:10.1016/j.polymer.2013.07.021

Polymer 54 (2013) 5174e5181
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Polymer
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Highly active and trans-1,4 specific polymerization of 1,3-butadiene
catalyzed by 2-pyrazolyl substituted 1,10-phenanthroline ligated iron
(II) complexes
,
,
Baolin Wang a b, Jifu Bi a, Chunyu Zhang a, Quanquan Dai a, Chenxi Bai a, Xuequan Zhang a
**
*
,
,
a
Yanming Hu c , Liansheng Jiang
a Research Center of High Performance Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street,
Changchun 130022, PR China
b University of Chinese Academy of Sciences, Beijing 100049, PR China
c State Key Laboratory of Fine Chemicals, Department of Polymer Science and Engineering, School of Chemical Engineering, Dalian University of Technology,
Dalian 116012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new family of iron (II) complexes (2aeh) bearing tridentate 2-pyrazolyl substituted 1,10-phenanthroline
ligands were successfully prepared and characterized by IR spectroscopy, elemental analysis, and mass
spectra. Complexes 2aef and 2h were further confirmed by the X-ray crystallographic analysis. 2a, 2b, 2e,
and 2f adopted distorted trigonal bipyramidal configuration. 2c displayed a distorted octahedron formed
by six coordinated nitrogen atoms of the two ligands. Linked by two bridged chloride atoms, complex 2d
was a centrosymmetric dimmer, and complex 2h adopted a six-coordinate distorted octahedral geometry
due to the coordination of two solvent molecules. These complexes activated by alkylaluminum were
examined in butadiene polymerization. In combination with AliBu3, complexes 2aec exhibited high cat-
alytic activity (73.5%e94.3%) at 20 ꢀC, whereas other complexes exhibited much lower activity. Interest-
ingly, the activity and selectivity of the complexes increased as increasing polymerization temperature. In
particular, 2b and 2c displayed both high activity (99% and 80%, respectively) and trans-1,4 selectivity
(95.6% and 96.2%, respectively) at 60 ꢀC. The trans-1,4 selectivity of 2b varied as alkylaluminum used as a
cocatalyst, in the following order: AliBu3 > AlOct3 > AlEt3 > AlMe3, whereas much lower trans-1,4
selectivity was observed in the cases of using MAO and MMAO.
Received 20 April 2013
Received in revised form
25 June 2013
Accepted 2 July 2013
Available online 12 July 2013
Keywords:
1,3-Butadiene
Iron (II) catalysts
Phenanthroline ligand
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
“green” tire [18,19]. It is known that Fe-, Ti-, and V-based cata-
lysts can initiate trans-1,4 polymerization of 1,3-butadiene [20e22].
Since the importance of regioselective and stereoselective
polymerization of 1,3-dienes [1e3], numerous catalysts based on
transition metals (Ti, Co, and Ni, etc.) and rare earth metal have
been developed to polymerize 1,3-dienes in a selective fashion [4e
17]. Considerable attention has been devoted to the synthesis of cis-
1,4 polydienes, which are important materials used for tires and
other elastomers. Recently, trans-1,4 polybutadiene and poly-
isoprene have attracted research interest due to their excellent
dynamic properties such as excellent anti-fatigue properties, low
rolling resistance, low heat buildup, good strength, and low abra-
sion loss, and such polymers can be used for producing long-life
Referring to the VIIIB transition metal-based catalysts, Co- and Ni-
based catalysts have been commercially used worldwide for pro-
ducing cis-1,4 polybutadiene in large capacity, while iron-based
catalyst, suffering from low activity or poor selectivity as well as
relatively low molecular weight of the resultant polymer, is still a
tough issue in practical utilization [23,24]. Many efforts have been
made to improve the catalytic performance of iron-based catalysts
by the addition of electron donors. For instance, Fe(acac)3/AliBu3/
nitrogen-containing compound (1,10-phenanthroline and 2,20-
bipyridine) catalysts are effective for butadiene polymerization and
give equivalent cis-1,4 and 1,2 polybutadiene [25]. The catalysts
with dialkylphosphite or phosphate as a donor exhibit high
activity and afford syndiotactic-1,2 polybutadiene [26e28]. More-
over, living behaviors for butadiene polymerization of iron(III)
2-ethylhexanoate/AliBu3/diethyl phosphite have been found in our
previous work [29].
* Corresponding author. Tel./fax: þ86 431 8526 2303.
** Corresponding author. Tel./fax: þ86 411 84986101.
E-mail addresses: xqzhang@ciac.jl.cn (X. Zhang), ymhu@dlut.edu.cn (Y. Hu).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.07.021

B. Wang et al. / Polymer 54 (2013) 5174e5181
5175
Although great progresses have been made by extensive re-
searches in the development of well-defined late transition metal-
based catalysts for olefin polymerization [30e36], and though
comparably less work reported, synthesis of those catalysts for
regioselective and stereoselective polymerization of 1,3-dienes is of
increased interest. Similar to the commercialized ZieglereNatta
catalyst, well-defined iron complexes reported for 1,3-diene poly-
merization are comparably few [6,37e41]. For instance, Fe(bipyr-
idine)2Cl2 and Fe(1,10-phenanthroline)2Cl2 activated by MAO are
effective for the polymerization of butadiene and isoprene, high
selectivity (1,2-selectivity for butadiene: 91%; 3,4-selectivity: 93%)
can be achieved at low temperature of ꢁ78 ꢀC [6]. FeCl3(2,20:60,200-
terpyridine)/MMAO catalyst exhibits high activity and affords
trans-1,4 polybutadiene [40]. The same as the most well-defined
late transition metal-based catalysts, these iron complexes also
encountered low polymerization performances above room tem-
perature, thus, the development of new catalysts with desired
performances is always worth anticipating.
area detector, using graphite monochromated Mo K radiation
ꢀ
(l
¼ 0.71073 A). The determination of crystal class and unit cell
parameters was carried out by the SMART program package. The
raw frame data were processed using SAINT and SADABS to yield
the reflection data file. The structures were solved by using
SHELXTL program. Refinement was performed on F2 anisotropically
for all non-hydrogen atoms by the full-matrix least-squares
method. The hydrogen atoms were placed at the calculated posi-
tions and were included in the structure calculation without further
refinement of the parameters.
2.3. Synthesis and characterization of iron (II) complexes
The tridentate 2-pyrazolyl substituted 1,10-phenanthroline li-
gands (1aeh) were prepared in good yields by the reaction of 2-
chloro-1,10-phenanthroline and substituted pyrazoles using
similar procedure in the reported literature [45], and the corre-
sponding iron (II) complexes (2aeh) were readily prepared via the
reaction of FeCl2$4H2O with the corresponding ligands in THF so-
lution at room temperature.
In the present study, we reported the synthesis of iron (II)
complexes bearing tridentate 2-pyrazolyl substituted 1,10-
phenanthroline ligands. Highly active and trans-1,4 selective for
butadiene polymerization was achieved by modification of the
ligand structure. Polymerization behaviors at varying polymeriza-
tion conditions, especially polymerization temperature, were
investigated.
2.3.1. 2-(Pyrazol-1-yl)-1,10-phenanthroline FeCl2 (2a)
A mixture of 1a (0.3 g, 1.22 mmol) and FeCl2$4H2O (0.24 g,
1.22 mmol) was stirred in THF (6 mL) for 6 h at room temperature.
The precipitated reddish-brown product was collected by filtration
and washed with cold THF (3 ꢂ 5 mL). The desired product (0.39 g,
85.6%) was obtained after dried in vacuum at 40 ꢀC for 24 h. IR (KBr,
cmꢁ1): 3052, 2928, 1609, 1588, 1530, 1505, 1467, 1424, 1396, 1359,
1347, 1154, 1036, 953, 851, 732, 644. Anal. Calcd. for C15H10N4FeCl2:
C, 48.30; H, 2.70; N, 15.02. Found: C, 48.51; H, 2.78; N, 15.24. ESI-MS
for C15H10N4FeCl2 (relative ratio): (m/z) ([MeCl]þ 335).
2. Experimental
2.1. General considerations
3-Phenyl-1H-pyrazole and 3-(4-chlorophenyl)-1H-pyrazole
were purchased from Aldrich. 5-Methyl-3-(trifluoromethyl)-1H-
pyrazole and 3,5-dimethyl-1H-pyrazole were purchased from
TCI Co. Other substituted pyrazoles and FeCl2$4H2O were ob-
tained from Alfa and used without further purification. 2-Chloro-
1,10-phenanthroline was purchased from J&K. MAO was available
from Akzo Noble. Toluene and tetrahydrofuran (THF) were
freshly distilled in the presence of sodium and benzophenone.
N,N-Dimethylformamide (DMF) was purified by vacuum distil-
lation under N2 atmosphere. Polymerization grade 1,3-butadiene
was supplied from Jinzhou Petrochemical Corporation and pu-
2.3.2. 2-(3-Methylpyrazol-1-yl)-1,10-phenanthroline FeCl2 (2b)
The procedure as above using 1b and FeCl2$4H2O gave 2b as a
red powder in 78.4% yield. IR (KBr, cmꢁ1): 3061, 2928, 1608, 1585,
1508, 1470, 1432, 1413, 1393, 1348, 1249, 1235, 1054, 961, 876, 793,
733, 643. Anal. Calcd. for C16H12N4 FeCl2: C, 49.65; H, 3.12; N, 14.43.
Found: C, 49.85; H, 3.33; N, 14.58. ESI-MS for C16H12N4FeCl2 (rela-
tive ratio): (m/z) ([MeCl]þ 349).
2.3.3. [Fe2Cl6$H2O][{2-(3-Phenylpyrazol-1-yl)-1,10-
phenanthroline}2Fe] (2c)
ꢀ
rified by passing through four columns packed with 4 A molec-
ular sieves and KOH. 1H NMR (400 MHz) and 13C NMR (100 MHz)
were measured on a Varian Unity spectrometer in CDCl3 at room
temperature. Elemental analyses were recorded on an elemental
Vario EL spectrometer. Mass spectrometric detection was carried
out on Xevo TQ mass spectrometer (Waters Corp, Milford, MA,
USA) with a electrospray ionization (ESI) source (Changchun
Center of Mass Spectrometry). IR spectra were performed on
BRUKE Vertex-70 FTIR spectrophotometer. The proportion of 1,2,
cis-1,4 and trans-1,4 units of polymer was determined by IR
spectra, 1H NMR and 13C NMR [42e44]. The molecular weights
(Mn) and molecular weight distributions (Mw/Mn) of polymer
were measured at 30 ꢀC by gel permeation chromatography
(GPC) equipped with a Waters 515 HPLC pump, four columns
(HMW 7 THF, HMW 6E THF ꢂ 2, HMW 2 THF) and a Waters 2414
refractive index detector. THF was used as an eluant at a flow
rate of 1.0 mL/min. The molecular weight of polymer was
calculated using the polystyrene calibration.
The procedure as above using 1c and FeCl2$4H2O gave 2c as a
reddish-brown powder in 86.5% yield. IR (KBr, cmꢁ1): 3057, 2924,
1606, 1591, 1534, 1504, 1457, 1364, 1288, 1265, 1042, 961, 867, 753,
736, 684. Anal. Calcd. for C42H28Cl6Fe3N8O: C, 48.56; H, 2.72; N,
10.79. Found: C, 48.68; H, 2.98; N, 10.91. ESI-MS for
C
42H28Cl6Fe3N8O (relative ratio): (m/z) ([Me6Cle2Fe]þ 700).
2.3.4. [2-(3-(4-Cholorophenyl)pyrazol-1-yl)-1,10-phenanthroline
FeCl( -Cl)]2 (2d)
m
The procedure as above using 1d and FeCl2$4H2O gave 2d as a
reddish-brown powder in 68.5% yield. IR (KBr, cmꢁ1): 3052, 2923,
1607, 1591,1506, 1458, 1425, 1358, 1288, 1265, 1224,1160, 1049, 963,
850, 837, 761, 736, 629. Anal. Calcd. for C21H13ClN4FeCl2: C, 52.29; H,
2.72; N, 11.62. Found: C, 52.35; H, 2.83; N, 11.51. ESI-MS for
C
21H13ClN4FeCl2 (relative ratio): ([MeCl]þ 445).
2.3.5. 2-(3,5-Dimethylpyrazol-1-yl)-1,10-phenanthroline FeCl2 (2e)
The procedure as above using 1e and FeCl2$4H2O gave 2e as a
reddish-brown powder in 91.8% yield. IR (KBr, cmꢁ1): 3054, 2926,
1609, 1584, 1564, 1510, 1458, 1429, 1405, 1384, 1364, 1343, 1131,
1069, 986, 851, 734, 645. Anal. Calcd. for C17H14N4FeCl2: C, 51.00; H,
3.53; N, 14.00. Found: C, 51.18; H, 3.69; N, 14.15. ESI-MS for
2.2. X-Ray structure determinations
Crystals suitable for X-ray analyses were obtained as described
in the Experimental section. Data collections were performed
at ꢁ88.5 ꢀC on a Bruker SMART APEX diffractometer with a CCD
C
17H14N4FeCl2 (relative ratio): (m/z) ([MeCl]þ 363).

5176
B. Wang et al. / Polymer 54 (2013) 5174e5181
2.3.6. 2-(3,5-Diisopropylpyrazol-1-yl)-1,10-phenanthroline FeCl2
(2f)
The procedure as above using 1f and FeCl2$4H2O gave 2f as a
reddish-brown powder in 88.2% yield. IR (KBr, cmꢁ1): 3055, 2935,
2871, 1607, 1581, 1559, 1508, 1463, 1428, 1405, 1384, 1365, 1331,
1061,1029, 851, 734, 651. Anal. Calcd. for C21H22N4FeCl2: C, 55.26; H,
4.86; N, 12.28. Found: C, 55.41; H, 5.01; N, 12.39. ESI-MS for
C
21H22N4FeCl2 (relative ratio): (m/z) ([MeCl]þ 419).
2.3.7. 2-(3,5-Diphenylpyrazol-1-yl)-1,10-phenanthroline FeCl2 (2g)
The procedure as above using 1g and FeCl2$4H2O gave 2g as a
reddish-brown powder in 87.3% yield. IR (KBr, cmꢁ1): 3064, 2930,
1607, 1591, 1537, 1512, 1428, 1415, 1391, 1331, 1054, 965, 845, 835,
730, 644. Anal. Calcd. for C27H18N4FeCl2: C, 61.83; H, 3.46; N, 10.69.
Found: C, 61.75; H, 3.57; N, 10.58. ESI-MS for C27H18N4FeCl2 (rela-
tive ratio): (m/z) ([MeCl]þ 487).
Scheme 1. Synthesis of iron (II) complexes.
coplanar with the plane (N(1)eN(2)eN(3)). The bond length of
ꢀ
Fe(1)eN(3) in complex 2b is 2.240(2) A, which is slightly shorter
ꢀ
than that in complex 2a (2.295(4) A), indicating that electron
2.3.8. [2-(3-Methyl-5-(trifluoromethyl)pyrazol-1-yl)-1,10-
phenanthroline FeCl(CH3OH)2]Cl (2h)
donating methyl group at 3-position of the pyrazole ring enhanced
the electronic density of the coordination bond Fe(1)eN(3). Similar
to those of 2a and 2b, the deviation of the iron atom from the plane
The procedure as above using 1h and FeCl2$4H2O gave 2h as a
reddish-brown powder in 75.8% yield. IR (KBr, cmꢁ1): 3052, 2932,
1623,1588,1511,1502,1486,1429,1402,1370,1341,1238,1152,1135,
978, 853, 737, 646. Anal. Calcd. for C19H17Cl2F3FeN4O2: C, 44.13; H,
3.31; N, 10.83. Found: C, 44.88; H, 2.58; N, 12.44. ESI-MS for
ꢀ
(N(1)eN(2)eN(3)) in 2f is 0.173 A, which is longer than that in 2e
ꢀ
(0.021 A). The bond length of Fe(1)eN(2) in complexes 2a and 2b is
ꢀ
ꢀ
2.111(4) A and 2.115(2) A respectively, slightly shorter than their
nickel analogs [46].
C
19H19Cl2F3FeN4O2 (relative ratio): (m/z) ([MeCl]þ 481).
Crystal of complex 2c suitable for X-ray diffraction analysis was
also grown from its DMF solution layered with diethyl ether. The
structure is proved to be ion pair comprised of [FeL2]2þ and
[FeCl3H2OFeCl3]2ꢁ (Fig. 3), and similar structure is also found in
bis(imino)pyridine ligated iron complex [47]. The geometry around
the iron center can be described as a distorted octahedron formed
by the six coordinated nitrogen atoms of the two ligands. The bond
2.4. Procedure for butadiene polymerization
Solution polymerization of 1,3-butadiene was carried out in a
glass reactor (20 mL) connected to a rubber plug. A certain amount
of iron (II) catalyst precursor and 1,3-butadiene/toluene solution
were added into a glass reactor under dried nitrogen atmosphere.
The polymerization started by injecting the prescribed amount of
cocatalyst. After polymerization in water bath at a given tempera-
ture for a given time, the mixture was poured into a large amount of
ethanol containing 2,6-di-t-butyl-4-methylphenol (1.0 wt.%) to
precipitate the polymer. The polymer was washed with ethanol
repeatedly and dried in vacuum at 40 ꢀC to constant weight.
Polymer yield was calculated by gravimetry.
ꢀ
ꢀ
lengths of Fe(1)eN(1) (2.2200(16) A), Fe(1)eN(2) (2.1094(14) A)
ꢀ
and Fe(1)eN(3) (2.2310(16) A) are slightly shorter than those in
complexes 2a and 2b, indicating the relatively tighter coordination
between the Fe(1) atom and the two ligands. The dihedral angle of
the pyrazole ring and the phenanthroline plane is 14.24ꢀ, which is
considerably larger than that in complexes 2a (3.69ꢀ) and 2b
(1.20ꢀ). In addition, there is one molecule of water in the lattice,
which is coordinated with the two ligand-free iron atoms.
Single crystal of complex 2d (Fig. 4), obtained from its DMF
solution layered with diethyl ether, is a centrosymmetric chloride-
bridged dimmer, which is similar to 2-(benzimidazol-2-yl)-
1,10-phenanthrolines ligated nickel complex [48]. Two iron (II)
atoms in the dimmer exhibit distorted octahedral geometry, and
each metal center is coordinated with a tridentate 2-pyrazolyl
substituted 1,10-phenanthroline ligand, two bridged chloride
atoms (Cl(1) and Cl(1A)), and one terminal chloride atom (Cl(2) or
3. Results and discussions
3.1. Synthesis and characterization of iron (II) complexes
The tridentate 2-pyrazolyl substituted 1,10-phenanthroline li-
gands (1aeh) were synthesized using similar procedure in the re-
ported literature, and their corresponding iron (II) complexes 2aeh
were synthesized by the treatment of FeCl2$4H2O with 1 equivalent
of the corresponding ligand in THF at room temperature in high
yields (Scheme 1). All the complexes were characterized by
elemental analysis, IR spectroscopy as well as electrospray ioniza-
tion mass spectroscopy (ESI), and the results were consistent with
the chemical formula. Fortunately, all the iron (II) complexes except
2g were further confirmed by X-ray diffraction analysis. The crys-
tallographic data and the collection refinement parameters are
summarized in Table 1.
ꢀ
ꢀ
Cl(2A)). The Fe(1) atom deviates by 0.132 A and 0.004 A from the
planes of Cl(1)eN(2)eCl(2) and N(1)eN(2)eN(3), respectively. The
coordination plane formed by N(1), N(2), and N(3) is nearly
perpendicular to the ꢀFe(1)eCl(1)eFe(1A)eCl(1A) plane with a
ꢀ
dihedral angle of 88.61 . The Fe(1)eN(3) bond (2.391(3) A) is longer
ꢀ
ꢀ
than those of Fe(1)eN(1) (2.224(3) A) and Fe(1)eN(2) (2.152(3) A).
ꢀ
The bridged Fe(1)eCl(1A) bond (2.5450(10) A) is longer than those
ꢀ
of bridged Fe(1)eCl(1) (2.4617(9) A) and terminal Fe(1)eCl(2)
ꢀ
(2.4021(10) A).
Single crystals of complexes 2a, 2b, 2e and 2f suitable for X-ray
diffraction analysis were obtained from their DMF solutions layered
with diethyl ether. Analysis of X-ray crystallography revealed that
these four complexes all adopted distorted trigonal bipyramidal
geometry around the iron center with the equatorial plane formed
by one phenanthroline nitrogen atom (N(2)) and two chlorines
(Cl(1) and Cl(2)). The deviation of the iron (II) atom from the plane
Crystal of complex 2h (Fig. 5) was grown from its methanol
solution layered with diethyl ether. Surprisingly, two methanol
molecules coordinate with the iron (II) atom, and one chlorine atom
(Cl(2)) is replaced by one methanol molecule and acts as a coun-
terion relatively far from the iron atom Fe(1) with an Fe(1)$$$Cl(2)
ꢀ
distance of 4.482 A, which is remarkably shorter than that of the
nickel complex [48]. The geometry around the iron atom can be
described as a distorted octahedron with the equatorial plane
formed by three coordination nitrogen atoms (N(1), N(2), and N(3)),
ꢀ
(N(1)eN(2)eN(3)) in complexes 2a and 2b (Figs. 1 and 2) is 0.049 A
ꢀ
and 0.110 A, respectively, suggesting that the iron (II) atom is nearly

B. Wang et al. / Polymer 54 (2013) 5174e5181
5177
Table 1
Crystal data and structure refinements of iron (II) complexes.
2a
2b
2c
2d
2e
2f
2h
Formula
C18H17N5O
FeCl2
C16H14N4
FeCl2
C42H28N8O
Fe3Cl6
C42H26N8
Fe2Cl6
C20H21N5O
FeCl2
C21H22N4
FeCl2
C19H17F3N4O2
FeCl2
Molecular weight
Crystal system
Space group
446.12
Monoclinic
P21/c
9.6511(12)
26.104(3)
7.7843(10)
90.00
105.309(2)
90.00
1891.5(4)
4
1.567
1.10
912.0
389.06
Triclinic
P-1
1040.97
Monoclinic
P21/n
12.7625(8)
7.9545(5)
21.0257(12)
90.00
103.5730(10)
90.00
2074.9(2)
2
967.11
Triclinic
P-1
474.17
Monoclinic
P21/c
11.4172(16)
23.417(3)
8.0274(11)
90.00
101.759(2)
90.00
2101.1(5)
4
1.499
0.99
976.0
457.18
Monoclinic
P21/c
9.680(3)
8.125(2)
26.240(7)
90.00
99.698(4)
90.00
2034.3(9)
4
1.493
1.02
1083
517.12
Monoclinic
P21/n
9.6454(11)
19.976(2)
11.4416(13)
90.00
101.759(2)
90.00
93.840(2)
4
1.562
0.98
1048.0
ꢀ
a (A)
8.1214(18)
9.049(2)
10.707(2)
94.414(4)
97.127(4)
93.092(4)
776.9(3)
2
8.5384(8)
11.0582(11)
11.1403(11)
97.768(2)
107.421(2)
102.726(2)
955.83(16)
1
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(deg)
(deg)
(deg)
3
ꢀ
V (A )
Z
Dcalcd (mg/m3)
Absorp coeff (mmꢁ1
F(000)
1.663
1.32
396
1.663
1.32
1235
1.680
1.22
638.0
)
Crystal size (mm)
0.21 ꢂ 0.17
0.21 ꢂ 0.13 ꢂ 0.08
0.27 ꢂ 0.15
0.25 ꢂ 0.21 ꢂ 0.18
0.23 ꢂ 0.19 ꢂ 0.16
0.24 ꢂ 0.18 ꢂ 0.11
0.21 ꢂ 0.16 ꢂ 0.14
ꢂ 0.15
ꢂ 0.08
q
Range (deg)
1.56e26.06
1.92e26.03
4181
1.71e26.03
12,743
1.93e26.08
5156
1.74e26.07
12,616
2.13e26.03
11,963
2.04e26.06
13,426
No. of reflns collected 12,010
No. of indep reflns
3739
(Rint ¼ 0.0458)
2958 (Rint ¼ 0.0115) 4079
3719 (Rint ¼ 0.0211) 4132 (Rint ¼ 0.0512) 3984 (Rint ¼ 0.0560) 4331 (Rint ¼ 0.0274)
(Rint ¼ 0.0207)
No. of data/restraint/ 3739/0/250
params
2958/0/209
4079/0/272
3719/0/262
4132/0/270
3984/0/257
4331/0/283
GOF on F2
R1(I > 2sigma(I))
wR2
1.033
0.0675
0.1780
1.047
1.068
0.906
1.044
1.062
1.062
0.0371
0.1014
0.0281
0.0735
0.0473
0.1361
0.0613
0.1500
0.0617
0.1625
0.0414
0.1142
one chlorine ligand (Cl(1)), and two axial FeeO bonds. The Fe(1)e
N(3) bond length is 2.273(2) A and longer than that in complex 2e
3.2. Butadiene polymerization
ꢀ
(2.187(3)), which can be ascribed to the strong electron-
withdrawing effect of trifluoromethyl group at 3-position of the
pyrazolyl ring on the coordinated N(3). The deviation of the Fe(1)
and Cl(1) from the plane (N(1)eN(2)eN(3)) is 0.026 A and 0.084 A,
respectively, indicating that these two atoms are coplanar with the
coordinated plane (N(1)eN(2)eN(3)). The iron atom and the
mutually trans-disposed oxygen atoms are almost in the same line
with an angle of 170.27(7)ꢀ (O(1)eFe(1)eO(2)). Meanwhile, Fe(1),
N(2), and Cl(1) are almost in the same line with an angle of
172.50(6)ꢀ (N(2)eFe(1)eCl(1)).
3.2.1. Variation of temperature on 1,3-butadiene polymerization
behaviors with 2b/AliBu3
It is known that polymerization temperature affects the speci-
ficity and/or activity of the catalyst in olefin [49,50] and 1,3-diene
polymerization [20,40]. Therefore, the butadiene polymerization
was investigated with 2b/AliBu3 at different temperatures, and the
results are summarized in Table 2. 2b/AliBu3 gave polymer
composed of 38.2% of cis-1,4, 33.1% of trans-1,4, and 28.7% of 1,2
units in the yield of 46.5% at ꢁ20 ꢀC. The polymer yield increased
ꢀ
ꢀ
Fig. 1. Molecular structure of complex 2a with thermal ellipsoids at 30% probability
level. Hydrogen atoms and uncoordinated solvent are omitted for clarity. Selected
Fig. 2. Molecular structure of complex 2b with thermal ellipsoids at 30% probability
ꢀ
bond lengths (A) and angles (ꢀ): Fe(1)eCl(1) 2.2709(15), Fe(1)eCl(2) 2.3244(15),
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (A) and angles ( ):
Fe(1)eN(2) 2.115(2), Fe(1)eN(1) 2.237(2), Fe(1)eN(3) 2.240(2), Fe(1)eCl(1) 2.2582(9),
Fe(1)eCl(2) 2.3459(8), N(2)eFe(1)eN(1) 74.61(8), N(2)eFe(1)eN(3) 72.07(8), N(1)e
Fe(1)eN(3) 146.44(8), N(2)eFe(1)eCl(1) 138.35(7), N(1)eFe(1)eCl(1) 100.38(6), N(3)e
Fe(1)eCl(1) 101.71(6), N(2)eFe (1)eCl(2) 106.82(6), N(1)eFe(1)eCl(2) 95.94(6), N(3)e
Fe(1)eCl(2) 97.33(6), Cl(1)eFe(1)eCl(2) 114.83(4).
ꢀ
ꢀ
Fe(1)eN(1) 2.252(4), Fe(1)eN(2) 2.111(4) Fe(1)eN(3) 2.295(4), N(2)eFe(1)eN(1)
75.40(15), N(2)eFe(1)eCl(1) 121.37(12), N(1)eFe(1)eCl(1) 97.10(11), N(2)eFe(1)eN(3)
71.92(15), N(1)eFe(1)eN(3) 147.28(15), Cl(1)eFe(1)eN(3) 101.20(11), N(2)eFe(1)e
Cl(2) 115.48(11), N(1)eFe(1)eCl(2) 94.76(11), Cl(1)eFe(1)eCl(2) 123.11(6), N(3)e
Fe(1)eCl(2) 97.57(11).

5178
B. Wang et al. / Polymer 54 (2013) 5174e5181
Fig. 4. Molecular structure of complex 2d with thermal ellipsoids at 30% probability
ꢀ
ꢀ
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (A) and angles ( ):
Fe(1)eN(2) 2.152(3), Fe(1)eN(1) 2.224(3), Fe(1)eN(3A) 2.391(3), Fe(1)eCl(2)
2.4021(10), Fe(1)eCl(1) 2.4617(9), Fe(1)eCl(1A) 2.5450(10), N(2)eFe(1)eN(1)
74.70(10), N(2)eFe(1)eN(3A) 70.41(9), N(1)eFe(1)eN(3A) 145.11(10), N(2)eFe(1)e
Cl(2) 98.98(8), N(1)eFe(1)eCl(2) 97.19(8), N(3A)eFe(1)eCl(2) 88.41(7), N(2)eFe(1)e
Cl(1) 164.80(8), N(1)eFe(1)eCl(1) 93.45(7), N(3A)eFe(1)eCl(1) 120.87(7), Cl(2)e
Fe(1)eCl(1) 91.84(3), N(2)eFe(1)eCl(1A) 88.69(8), N(1)eFe(1)eCl(1A) 91.21(8), N(3A)e
Fe(1)eCl(1A) 87.92(7), Cl(2)eFe(1)eCl(1A) 169.86(3), Cl(1)eFe(1)eCl(1A) 81.96(3).
Fig. 3. Molecular structure of complex 2c with thermal ellipsoids at 30% probability
ꢀ
ꢀ
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (A) and angles ( ):
Fe(1)eN(2) 2.1094(14), Fe(1)eN(1) 2.2200(16), Fe(1)eN(3) 2.2310(16), Fe(2)eO(1)
1.7676(7), Fe(2)eCl(3) 2.2170(6), Fe(2)eCl(1) 2.2345(6), Fe(2)eCl(2) 2.2382(6), N(2A)e
Fe(1)eN(2) 177.28(8), N(2)eFe(1)eN(1A) 103.15(6), N(2)eFe(1)eN(1) 75.15(6), N(1A)e
Fe(1)eN(1) 105.17(8), N(2)eFe(1)eN(3A) 72.62(6), N(1)eFe(1)eN(3A) 147.75(5),
N(2A)eFe(1)eN(3) 72.62(6), N(2)eFe(1)eN(3) 109.10(6), N(1)eFe(1)eN(3) 83.31(6),
N(3A)eFe(1)eN(3) 106.24(8), O(1)eFe(2)eCl(3) 110.84(2), O(1)eFe(2)eCl(1) 111.34(6),
Cl(3)eFe(2)eCl(1) 106.25(2), O(1)eFe(2)eCl(2) 109.15(5), Cl(3)eFe(2)eCl(2) 110.44(2),
Cl(1)eFe(2)eCl(2) 108.78(3).
polymerization temperature favors the isomerization from the
anti-form to the syn-form, leading to the increased trans-1,4 units of
the resultant polymer. It is worth noting that there is no cis-1,4 unit
in the polybutadiene obtained at 40 ꢀC and above, suggesting that
the anti-isomers change into syn ones completely. Thus, the present
catalyst system simultaneously exhibited high activity and
with the increment of polymerization temperature. The optimum
activity was achieved at 40 ꢀC with 99% polymer yield, and the yield
kept unchanged when the temperature further increased to 60 ꢀC,
which was quite different from the previous results obtained by
other iron complexes. For instance, the polymer yield obtained
with [2,6-(C6H5N]CMe)2C5H3N]FeCl3/AliBu3 decreased from 99%
to 40% when the temperature increased from 20 ꢀC to 60 ꢀC [20]
and the activity of terpyridine/FeCl3 complex for 1,3-diene poly-
merization also decreased sharply as temperature increased [40].
These results may be due to the catalyst decomposition with the
increase of temperature. Thus, the present catalyst has relatively
high thermal tolerance, probably resulting from the strong electron
donation of the tridentate ligand to the metal center, and endowing
the FeeN coordination bonds with heat resistance as a conse-
quence. In addition, the molecular weight of the resultant polymer
decreased with the polymerization temperature increased, indi-
cating the enhanced chain transfer reaction at the elevated tem-
perature. Quite interestingly, the trans-1,4 content of the resulting
polymer significantly increased as increasing polymerization tem-
perature and reached 95.6% at 60 ꢀC. This may be explained by the
presently accepted mechanism for the formation of cis-1,4 and
trans-1,4 isomers of polydienes with transition metal catalysts
Fig. 5. Molecular structure of complex 2h with thermal ellipsoids at 30% probability
ꢀ
ꢀ
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (A) and angles ( ):
Fe(1)eO(1) 2.1539(18), Fe(1)eN(2) 2.156(2), Fe(1)eO(2) 2.1579(18), Fe(1)eN(1)
2.198(2), Fe(1)eN(3) 2.273(2), Fe(1)eCl(1) 2.3354(8), O(1)eFe(1)eN(2) 86.16(7), O(1)e
Fe(1)eO(2) 170.27(7), N(2)eFe(1)eO(2) 86.08(7), O(1)eFe(1)eN(1) 93.00(8), N(2)e
Fe(1)eN(1) 75.20(8), O(2)eFe(1)eN(1) 90.70(8), O(1)eFe(1)eN(3) 85.75(8), N(2)e
Fe(1)eN(3) 70.38(7), O(2)eFe(1)eN(3) 86.11(7), N(1)eFe(1)eN(3) 145.56(8), O(1)e
Fe(1)eCl(1) 93.54(5), N(2)eFe(1)eCl(1) 172.50(6), O(2)eFe(1)eCl(1) 94.91(5), N(1)e
Fe(1)eCl(1) 97.35(6), N(3)eFe(1)eCl(1) 117.09(6).
[1,51e54], that is, anti- and syn-
and trans-1,4 units, respectively, and antiesyn isomerization occurs
via rearrangement. As the syn-
3-allyl intermediate is
thermodynamically more stable than the anti one, the increment of
h
3-allyl intermediate lead to cis-1,4
p
es
e
p
h

B. Wang et al. / Polymer 54 (2013) 5174e5181
5179
Table 2
Butadiene polymerization behaviors of 2b/AliBu3.a
Run T (ꢀC) Yield (%) Microstructureb (mol%)
Cis-1,4 Trans-1,4 1,2
Mn ꢂ 10ꢁ4 Mw/Mn
c
c
1
2
3
4
5
ꢁ20
0
20
40
60
46.5
93.6
94.3
99
38.2
30.1
21.2
0
33.1
45.3
58.0
91.3
95.6
28.7 10.3
3.5
2.9
3.0
3.0
1.9
24.6
20.8
8.7
7.5
8.2
4.5
1.8
99
0
4.4
a
Polymerization conditions: precatalyst: 0.01 mmol; [Al]/[Fe] ¼ 40; toluene:
10 mL; 1,3-butadiene: 0.02 mol; time: 4 h.
b
Determined by FTIR and 1H NMR.
Determined by GPC (THF, PSt calibration).
c
stereospecificity at high temperature. The 1H and 13C NMR spectra
of the polybutadiene (sample from Table 2, run 5) are shown in
Figs. 6 and 7, respectively. The 13C NMR spectrum displays two
sharp main peaks at 129.5 and 32.2 ppm assigning to trans-1,4 unit,
weak signals assigning to the 1,2 unit, and no visible signals
assigning to cis-1,4 unit, indicating high trans-1,4 stereospecificity
of the catalyst.
Fig. 7. 13C NMR spectrum of the polybutadiene obtained with complex 2b/AliBu3
(sample from Table 2, run 5; in CDCl3).
3.2.2. Variation of alkylaluminum and Al/Fe molar ratio on
syn-conformation. The formation of an allylic terminal group made
C1 and C3 (Scheme 2) positions reactive for the next insertion
which determines the chemoselectivity of the polymerization
process. The alkyl group of the alkylaluminums surrounding the
active species plays an important role in tuning C1 insertion and C3
insertion. The bulkier of the alkyl group, the more crowed of the
environment of the active species. Thus C1 insertion was more
favorable than C3 insertion and high trans-1,4 selectivity was ach-
ieved. In the cases of MAO and MMAO as cocatalysts, which pre-
pared by controlled hydrolysis of AlMe3 and a mixture of AlMe3 and
AliBu3, respectively, the catalysts afforded polybutadienes with
higher cis-1,4 contents than that obtained with AliBu3-based cata-
lyst. This might be due to the remained free AlMe3 in MAO and
MMAO as well as their different structures from alkylaluminum.
butadiene polymerization behaviors using precursor 2b
Table 3 showed butadiene polymerization data using 2b catalyst
activated with different alkylaluminums, respectively. As expected,
alkylaluminums different in alkyl group behave differently in cat-
alytic activity as well as selectivity [6,26,55]. AliBu3 as a cocatalyst
showed the highest activity and then followed AlOct3, probably due
to the better solubility of the activated complex than that by the
other alkylaluminums. Interestingly, the catalytic selectivity was
closely dependent on the bulkiness of alkyl group of alkylalumi-
nums, as the bulkiness of alkyl group of alkylaluminums increased
in the order of AlMe3 < AlEt3 < AlOct3 < AliBu3, the corresponding
trans-1,4 stereospecificity increased monotonously. Based on the
previously proposed iron active species [56,57], the alkylation re-
action of the iron complex with AlR3 generated a bimetallic inter-
mediate (Scheme 2). As the center iron was surrounded by
coordinated bulk ligand and alkylaluminum, the butadiene
monomer coordinating to the center iron would favor in a trans-h2
mode through only one of the double bonds and insert into the
3.2.3. Butadiene polymerization with complexes 2aeh
Complexes 2aeh in combination with AliBu3 were tested for
butadiene polymerization at 20 ꢀC and 60 ꢀC respectively, and the
results were summarized in Table 4. The coordination environment
of the complexes showed significant influence on their activities.
Complexes 2a and 2b exhibited high activity for butadiene poly-
merization, while 2c and 2d with bulky phenyl and para-chlor-
ophenyl substituents at 3-position of the pyrazolyl ring of the
ligand showed lower activities (79.2% and 16.8% at 60 ꢀC,
growing chain forming
h
3-allyl coordinated terminal group in a
Table 3
Influences of cocatalysts on butadiene polymerization behaviors using precursor
2b.a
Al/Fe Yield (%) Microstructureb (mol%) Mn ꢂ 10ꢁ4 Mw/Mn
c
c
Run Al
Cis-1,4 Trans-1,4 1,2
1
2
3
4
5
6
7
8
AliBu3
20 95.1
40 99
60 99
40 58.2
40 95.3
40 96.2
100 91.6
0
0
0
15.2
6.2
0
26.2
24.3
95.8
95.6
95.3
77.9
86.9
92.4
47.1
50.6
4.2 3.6
4.4 1.8
4.7 1.3
6.9 4.2
6.9 2.7
7.6 2.9
26.7 8.2
25.1 9.1
2.5
2.4
2.3
2.7
2.8
2.4
2.1
1.9
AliBu3
AliBu3
AlMe3
AlEt3
AlOct3
MAO
MMAO 100 92.1
a
Polymerization conditions: precatalyst: 0.01 mmol; temperature: 60 ꢀC;
toluene: 10 mL; 1,3-butadiene: 0.02 mol; time: 4 h.
b
Fig. 6. 1H NMR spectrum of the polybutadiene obtained with complex 2b/AliBu3
(sample from Table 2, run 5; in CDCl3).
Determined by FTIR and 1H NMR.
c
Determined by GPC (THF, PSt calibration).

5180
B. Wang et al. / Polymer 54 (2013) 5174e5181
Scheme 2. Proposed mechanism for butadiene polymerization by 2b/AlR3.
respectively) than 2a and 2b did, which might be explained by that
the crowded metal centers (ion pair and chloride-bridged dimmer,
respectively) as revealed by X-ray analysis hindered the monomer
coordination to the metal center. Compared with complexes 2aec,
2eeh having methyl, isopropyl, and phenyl substituents as well as
methyl and trifluoromethyl, respectively, that with substituents at
3- and 5-postions of the pyrazole ring exhibited much lower ac-
tivity at 20 ꢀC. These results indicate that not only the bulkiness but
also the position of the substituent plays an important role in the
catalytic activity of the complex. On the other hand, the polymer-
ization temperature showed no significant effect on their activities
of complexes except 2f, the activities of 2aed at 60 ꢀC were slightly
higher than those at 20 ꢀC, while the activities of 2e, 2g, and 2h
slightly decreased with the temperature increasing. In contrast, the
activity of 2f having isopropyl groups at 3- and 5-postions of the
pyrazole ring remarkably increased from 19.5% to 93.4% as the
temperature increased from 20 ꢀC to 60 ꢀC.
Table 4
Polymerization of 1,3-butadiene with 2aeh/AliBu3.a
Run Cat. T (ꢀC) Yield (%) Microstructureb (mol%)
Cis-1,4 Trans-1,4 1,2
Mn ꢂ 10ꢁ4 Mw/Mn
c
c
1
2
3
4
5
6
7
8
2a 20
2a 60
2b 20
2b 60
2c 20
2c 60
2d 20
2d 60
2e 20
2e 60
2f 20
2f 60
2g 20
2g 60
2h 20
2h 60
87.3
91.6
94.3
99
24.2
15.5
21.2
e
58.0
62.0
58.0
95.6
57.4
96.2
5.0
24.6
11.3
41.4
12.3
73.3
7.8
17.8
22.5
20.8
4.4
20.1
3.8
6.6
2.1
8.2
1.8
7.3
2.5
2.5
2.8
3.0
1.9
2.8
2.9
3.3
1.9
2.8
2.4
2.9
2.6
2.9
2.3
2.3
2.7
These complexes in combination with AliBu3 were not stereo-
specific at 20 ꢀC for butadiene polymerization, affording poly-
butadienes with mixtures of cis-1,4, trans-1,4, and 1,2 units. All
complexes had the same trend in selectivity regarding to polymer-
ization temperature, that is, higher polymerization temperature led
to the higher trans-1,4 selectivity as commonly observed in 1,3-
diene polymerization with transition metal and rare earth cata-
lysts. These results might be attributed to the antiesyn isomeriza-
tion at elevated temperature, and the thermodynamically more
stable syn-isomer leads to the formation of trans-1,4 unit [58e60].
For instance, complexes 2aec showed medium trans-1,4 selectivity
(ca. 58%) at 20 ꢀC, while at 60 ꢀC the trans-1,4 selectivity of 2b and 2c
having methyl and phenyl group at 3-position of the pyrazole ring
sharply increased to 95.6% and 96.2%, respectively, however, only a
slight increase of trans-1,4 selectivity (62.0%) in the case of 2a
without substituent in the pyrazole ring was observed. The results
73.5
79.2
14.2
16.8
16.2
11.7
19.5
93.4
22.7
22.5
27.6
14.5
22.4
e
50.3
41.6
49.1
34.5
26.7
13.5
56.3
39.4
36.4
23.5
44.7 10.3
33.8
39.6
24.1
61.0
13.2
8.8
5.7
2.3
9.3
2.4
9
10
11
12
13
14
15
16
35.9 27.5
29.4
32.9
57.3
31.2
30.7
19.2
9.8
2.5
5.1
a
Polymerization conditions: precatalyst: 0.01 mmol; [Al]/[Fe] ¼ 40; toluene:
10 mL; 1,3-butadiene: 0.02 mol; time: 4 h.
b
Determined by FTIR and 1H NMR.
Determined by GPC (THF, PSt calibration).
c

B. Wang et al. / Polymer 54 (2013) 5174e5181
5181
[11] Appukuttan V, Zhang L, Ha CS, Kim I. Polymer 2009;50(5):1150e8.
indicate that the bulky substituent of the ligand and high poly-
[12] Chen L, Ai PF, Gu JM, Jie SY, Li BG. J Organomet Chem 2012;716:55e61.
[13] Ai PF, Chen L, Guo YT, Jie SY, Li BG. J Organomet Chem 2012;705:51e8.
[14] Jie SY, Ai PF, Li BG. Dalton Trans 2011;40(41):10975e82.
[15] Wang BL, Gong DR, Bi JF, Dai QQ, Zhang CY, Hu YM, et al. Appl Organomet
Chem 2013;27(4):245e52.
[16] Gao W, Cui DM. J Am Chem Soc 2008;130(14):4984e91.
[17] Hou ZM, Wakatsuki Y. Coord Chem Rev 2002;231(1e2):1e22.
[18] Liu DT, Cui DM. Dalton Trans 2011;40(30):7755e61.
[19] Benvenuta-Tapia JJ, Tenorio-Lopez JA, Herrera-Najera R, Rios-Guerrero L.
Polym Eng Sci 2009;49(1):1e10.
merization temperature act synergistically in promoting trans-1,4
selectivity, the former favors
h
2-trans monomer coordination mode
giving syn
h
3-allyl active site, and the latter accelerates antiesyn
isomerization. Similar trend in selectivity was observed in the cases
of 2eeh having substituents at both 3- and 5-positions of the pyr-
azole ring, the trans-1,4 contents of the polybutadienes obtained at
60 ꢀC (29.4%e73.3%) were higher than those at 20 ꢀC (7.8%e32.9%).
[20] Gong DR, Jia XY, Wang BL, Wang F, Zhang CY, Zhang XQ, et al. Inorg Chim Acta
2011;373(1):47e53.
4. Conclusions
[21] Milione S, Cuomo C, Capacchione C, Zannoni C, Grassi A, Proto A. Macro-
molecules 2007;40(16):5638e43.
[22] Colamarco E, Milione S, Cuomo C, Grassi A. Macromol Rapid Commun
2004;25(2):450e4.
[23] Hsu WL, Halasa AF. Rubber Chem Technol 1994;67(5):865e70.
[24] Swift HE, Bozik JE, Wu CY. J Catal 1970;17(3):331e40.
[25] Zhang ZY, Zhang HJ, Ma HM, Wu Y. J Mol Catal 1982;17(1):65e76.
[26] Lu J, Hu YM, Zhang XQ, Bi JF, Dong WM, Jiang LS, et al. J Appl Polym Sci
2006;100(5):4265e9.
[27] Hu YM, Dong WM, Jiang LS, Zhang XQ, Yu G, Cao LH. Chin J Catal 2004;25(8):
664e8.
[28] Gong DR, Dong WM, Hu YM, Bi JF, Zhang XQ, Jiang LS. Polymer 2009;50(25):
5980e6.
[29] Gong DR, Dong WM, Hu JC, Zhang XQ, Jiang LS. Polymer 2009;50(13):2826e9.
[30] Ittel SD, Johnson LK, Brookhart M. Chem Rev 2000;100(4):1169e203.
[31] Gibson VC, Redshaw C, Solan GA. Chem Rev 2007;107(5):1745e76.
[32] Gibson VC, Spitzmesser SK. Chem Rev 2003;103(1):283e315.
[33] Cao XP, He F, Zhao WZ, Cai ZG, Hao X, Shiono T, et al. Polymer 2012;53(9):
1870e80.
[34] Zhao WZ, Yu JG, Song SJ, Yang WH, Liu H, Hao X, et al. Polymer 2012;53(1):
130e7.
[35] Wang KF, Gao R, Hao X, Sun WH. Catal Commun 2009;10(13):1730e3.
[36] Zhang M, Wang KF, Sun WH. Dalton Trans 2009;(32):6354e63.
[37] Gong DR, Jia XY, Wang BL, Zhang XQ, Jiang LS. J Organomet Chem 2012;702:
10e8.
[38] Gong DR, Wang BL, Bai CX, Bi JF, Wang F, Dong WM, et al. Polymer
2009;50(26):6259e64.
[39] Bazzini C, Giarrusso A, Porri L. Macromol Rapid Commun 2002;23(15):922e7.
[40] Nakayama Y, Baba Y, Yasuda H, Kawakita K, Ueyama N. Macromolecules
2003;36(21):7953e8.
[41] Raynaud J, Wu JY, Ritter T. Angew Chem Int Ed 2012;51(47):11805e8.
[42] Elgert KF, Quack G, Stutzel B. Polymer 1975;16(3):154e6.
[43] Elgert KF, Quack G, Stutzel B. Polymer 1974;15(9):612e3.
[44] Mochel VD. J Polym Sci A Polym Chem 1972;10(4):1009e18.
[45] Jameson DL, Goldsby KA. J Org Chem 1990;55(17):4992e4.
[46] Yang Y, Yang PJ, Zhang C, Li G, Yang XJ, Wu B, et al. J Mol Catal A Chem
2008;296(1e2):9e17.
[47] Chen YF, Qian CT, Sun J. Organometallics 2003;22(6):1231e6.
[48] Zhang M, Zhang S, Hao P, Jie S, Sui WH, Li PZ, et al. Eur J Inorg Chem
2007;(24):3816e26.
A
series of iron (II) complexes supported by tridentate
2-pyrazolyl substituted 1,10-phenanthroline ligands were synthe-
sized and characterized. X-Ray diffraction analysis revealed that
complexes 2a, 2b, 2e, and 2f had similar distorted square pyramidal
geometries around the metal center. Comparably, complex 2c dis-
played a distorted octahedron formed by six coordinated nitrogen
atoms of the two ligands. Complex 2d crystallized in a centro-
symmetric dimmer linked by two bridged chloride atoms, and
complex 2h adopted a six-coordinate distorted octahedral geom-
etry due to the coordination of two solvent molecules. The bulki-
ness and position of the substituent at the pyrazole ring of the
ligand influenced the catalytic activity and selectivity. Differenti-
ating from the most late-transition metal-based catalyst which
behaviors decreased activity and stereoselectivity as temperature
increasing, 2b and 2c displayed enhanced activity and significantly
increased trans-1,4 selectivity at elevated temperature. Using
alkylaluminum as a cocatalyst, both the activity and trans-1,4
selectivity of 2b increased with increasing the bulkiness of the alkyl
group of alkylaluminum in the following order: AliBu3 > AlOct3
> AlEt3 > AlMe3. In addition, a mechanism for forming trans-1,4
polybutadiene was also proposed.
Acknowledgments
The authors gratefully acknowledge financial support from the
R&D department of PetroChina (Grant 2011B-2803).
References
[1] Thiele SKH, Wilson DR. J Macromol Sci Polym Rev 2003;C43(4):581e628.
[2] Porri L, Giarrusso A, Ricci G. Prog Polym Sci 1991;16(2e3):405e41.
[3] Ricci G, Sommazzi A, Masi F, Ricci M, Boglia A, Leone G. Coord Chem Rev
2010;254(5e6):661e76.
[4] Zhang JS, Gao W, Lang XM, Wu QL, Zhang L, Mu Y. Dalton Trans 2012;41(32):
9639e45.
[5] Gao B, Luo XY, Gao W, Huang L, Gao SM, Liu XM, et al. Dalton Trans
2012;41(9):2755e63.
[6] Ricci G, Morganti D, Sommazzi A, Santi R, Masi F. J Mol Catal A Chem
2003;204:287e93.
[7] Nobbs JD, Tomov AK, Cariou R, Gibson VC, White AJP, Britovsek GJP. Dalton
Trans 2012;41(19):5949e64.
[8] Cariou R, Chirinos JJ, Gibson VC, Jacobsen G, Tomov AK, Britovsek GJP, et al.
Dalton Trans 2010;39(38):9039e45.
[9] Cariou R, Chirinos J, Gibson VC, Jacobsen G, Tomov AK, Elsegood MRJ. Mac-
romolecules 2009;42(5):1443e4.
[10] Appukuttan V, Zhang L, Ha JY, Chandran D, Bahuleyan BK, Ha CS, et al. J Mol
Catal A Chem 2010;325(1e2):84e90.
[49] Bianchini C, Giambastiani G, Luconi L, Meli A. Coord Chem Rev 2010;254(5e
6):431e55.
[50] Bianchini C, Giambastiani G, Rios IG, Mantovani G, Meli A, Segarra AM. Coord
Chem Rev 2006;250(11e12):1391e418.
[51] Tobisch S, Taube R. Organometallics 1999;18(25):5204e18.
[52] Tobisch S, Taube R. Chem Eur J 2001;7(17):3681e95.
[53] Tobisch S. Acc Chem Res 2002;35(2):96e104.
[54] Tobisch S. Chem Eur J 2002;8(20):4756e66.
[55] Friebe L, Nuyken O, Obrecht W. Adv Polym Sci 2006;204:1e154.
[56] Bryliakov KP, Semikolenova NV, Zakharov VA, Talsi EP. Organometallics
2004;23(22):5375e8.
[57] Bryliakov KP, Talsi EP, Semikolenova NV, Zakharov VA. Organometallics
2009;28(11):3225e32.
[58] Tobisch S. Can J Chem 2009;87(10):1392e405.
[59] Tobisch S. J Mol Struct 2006;771(1e3):171e9.
[60] Tobisch S. Adv Organomet Chem 2003;49:167e224.