Synthesis, characterization and 1,3-butadiene polymerization behaviors of cobalt complexes bearing 2-pyrazolyl-substituted 1,10-phenanthroline ligands

Article abstract of DOI:10.1002/aoc.2972

A series of cobalt(II) complexes containing tridentate 2-pyrazolyl- substituted 1,10-phenanthroline ligands (L) with the general formula [LCoCl ₂] have been successfully synthesized and fully identified by IR spectroscopy, elemental analysis and mass spectroscopy. Cobalt complexes Co4-Co8 were further confirmed by X-ray crystallographic analysis, and all the complexes adopted distorted trigonal pyramid geometries around the cobalt center. In combination with methylaluminoxane, the complexes exhibit high cis-1,4-selectivity for 1,3-butadiene polymerization. The catalytic activities of the complexes mainly depend on the nature of the substituent and its position at the pyrazolyl ring of the ligand. Complexes having a bulkier substituent on the pyrazolyl ring of the ligand show lower catalytic activity and the incorporation of electron-withdrawing substituent enhances the activity. Polymerization behaviors were almost not affected with varying [Al]/[Co] ratio, but both activity and the cis-1,4 content decrease slightly as polymerization temperature increasing. Copyright 2013 John Wiley & Sons, Ltd. A new family of Co(II) complexes bearing tridentate 2-pyrazolyl substituted 1,10-phenanthroline ligands were prepared, characterized and investigated as catalyst precursor for butadiene polymerization. Highly cis-1,4 selective butadiene polymerization was achieved. Copyright

Full text of DOI:10.1002/aoc.2972

Full Paper
Received: 4 December 2012
Revised: 31 December 2012
Accepted: 2 January 2013
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2972
Synthesis, characterization and 1,3-butadiene
polymerization behaviors of cobalt complexes
bearing 2-pyrazolyl-substituted
1,10-phenanthroline ligands
Baolin Wanga,b, Dirong Gonga, Jifu Bia, Quanquan Daia, Chunyu Zhanga,
Yanming Huc*, Xuequan Zhanga* and Liansheng Jianga
A series of cobalt(II) complexes containing tridentate 2-pyrazolyl-substituted 1,10-phenanthroline ligands (L) with the
general formula [LCoCl2] have been successfully synthesized and fully identified by IR spectroscopy, elemental analysis and
mass spectroscopy. Cobalt complexes Co4–Co8 were further confirmed by X-ray crystallographic analysis, and all the
complexes adopted distorted trigonal pyramid geometries around the cobalt center. In combination with methylaluminoxane,
the complexes exhibit high cis-1,4-selectivity for 1,3-butadiene polymerization. The catalytic activities of the complexes
mainly depend on the nature of the substituent and its position at the pyrazolyl ring of the ligand. Complexes having a bulkier
substituent on the pyrazolyl ring of the ligand show lower catalytic activity and the incorporation of electron-withdrawing
substituent enhances the activity. Polymerization behaviors were almost not affected with varying [Al]/[Co] ratio, but both activity
and the cis-1,4 content decrease slightly as polymerization temperature increasing. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: cobalt(II) complex; 1,10-phenanthroline; 1,3-butadiene
tridentate ligands, such as NNO,[25,26] NSN[27] and NNN,[28–30]
have been widely employed as precursors for olefin polymerization. Al-
Introduction
The regio- and stereoselective polymerization of 1,3-butadiene is
a subject of intense interest because of the extensive and diverse
practical applications of polybutadienes.[1] The properties of
the polymer mainly depend on their microstructures, including
cis-1,4, trans-1,4, and 1,2 isomers, generated during polymeriza-
tion. Therefore, numerous catalysts based on transition metals
(Ti, Ni, Co, etc.)[2–6] and rare earth metal have been developed
to polymerize 1,3-butadiene in a selective fashion.[7–11] Among
them, cobalt-based catalysts have attracted special attention
because polymers with versatile microstructures can be produced
depending on the catalyst formulation.[12] For instance, cobalt
halides and carboxylates activated with methylaluminoxane
though the attractive progress of such catalysts in olefin polymeriza-
tion has led to an acceleration of research efforts in 1,3-butadiene po-
lymerization, their extension for 1,3-butadiene polymerization has still
been limited until now. Endo et al. reported (salen)Co(II) complexes
for cis-1,4 selective polymerization and found that the introduction of
a t-butyl group into the ligand can increase the activity and cis-1,4
selectivity simultaneously.[15] Cariou et al. reported that bis
(benzimidazole)-supported Co(II) complexes exhibited high cis-1,4
selectivity towards 1,3-butadiene polymerization.[31,32] Appukuttan
et al. prepared a series of Co(II) complexes supported by bis(benzimi-
dazolyl)amine and 2,6-bis(benzimidazolyl)pyridine, and found
(MAO) produce high cis-1,4-polybutadienes.[13–15] In addition,
[16,17]
external donors such as CS2 and PPh3
have been widely
*
Correspondence to: Xuequan Zhang, Research Center of High Performance Syn-
thetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, 5625 Renmin Street, Changchun 130022, People’s Republic of China.
E-mail: xqzhang@ciac.jl.cn; Yanming Hu, Department of Polymer Science and
Engineering, School of Chemical Engineering, Dalian University of Technology,
Dalian 116012, People’s Republic of China. E-mail: ymhu@dlut.edu.cn
documented as effective regulators in the catalytic systems,
[18]
Co(acac)3/AlEt3/CS2
and Co(acac)2/AlEt3/H2O/PPh3,[19,20] for
1,2-selective polymerization of 1,3-butadiene.
Catalytic activity and chemo- and stereoselectivity of the
catalyst systems mainly depend on the steric and electronic
nature of the active site. To gain a deeper understanding of
how the ligand structure affects the activity and specificity
of the catalyst, recent studies have focused on well-defined
homogeneous single-site catalysts based on transition metal
complexes. Based on the pioneering work of the highly active
cobalt(II) and iron(II) complexes supported by 2,6-bis(arylimino)
pyridine ligands for ethylene polymerization/oligomerization,[21–24]
various well-defined late transition metal complexes bearing
a Research Center of High Performance Synthetic Rubber, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s
Republic of China
b University of Chinese Academy of Sciences, Beijing 100049, People’s
Republic of China
c
Department of Polymer Science and Engineering, School of Chemical Engineering,
Dalian University of Technology, Dalian 116012, People’s Republic of China
Appl. Organometal. Chem. 2013, 27, 245–252
Copyright © 2013 John Wiley & Sons, Ltd.

B. Wang et al.
high cis-1,4 selectivity for 1,3-butadiene polymerization with the
assistance of ethylaluminum sesquichloride (EASC).[33,34] Jie and
co-workers[35–37] found that Co(II) complexes ligated by PNP, NNO
and NO ligands are effective for producing high cis-1,4 polybutadienes.
In our previous study,[38,39] it was found that the incorporation of an
electron-withdrawing group into the imino group of the ligand
can enhance both activity and selectivity of bis(imino)pyridine
cobalt(II)/MAO catalyst for the polymerization of 1,3-butadiene. As
mentioned above, it could be concluded that both the steric and
electronic properties of substituents adjacent to the metal center
influence the polymerization behaviors.
1,10-Phenanthroline and its derivatives are well-known ligands
for late transition metal complexes because of its ease in tuning
their steric and electronic properties by modifying the parent
ligand.[40–46] However, the metal complexes supported by such
ligands as catalyst precursors for 1,3-butadiene polymerization
are scarce. In the present study, in order to study suitable
new chelating cobalt complexes for the selective polymerization
of 1,3-butadiene, we reported the synthesis and characterization
of a new family of cobalt complexes bearing tridentate 2-pyrazolyl
substituted 1,10-phenanthroline ligands and their catalytic beha-
viors for 1,3-butadiene polymerization. Furthermore, the influences
of the substituent of the ligand and polymerization conditions on
the polymerization of 1,3-butadiene were investigated in detail.
detector, using graphite monochromated Mo K radiation
(l = 0.71073 Å). The determination of crystal class and unit cell
parameters was carried out using the SMART program package.
The raw frame data were processed using SAINT and SADABS
to yield the reflection data file. The structures were solved
using the 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 positions and were included in the structure calcula-
tion without further refinement of the parameters. CCDC
887490–887494 for complexes 2d–h contain supplementary
crystallographic data for this paper and can be obtained free of
charge from www.ccdc.ac.uk/data-requesst/cif.
Polymerization Procedure
Solution polymerization of 1,3-butadiene was carried out in a
glass reactor connected to a rubber plug. A certain amount of
cobalt complex and 1,3-butadiene/toluene solution was added
to a glass reactor under dried nitrogen atmosphere. Polymeriza-
tion started by injecting the prescribed amount of co-catalyst.
After polymerization in a water bath at a given temperature for
the 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 under vacuum at 40ꢀC to constant weight.
Polymer yield was calculated by gravimetry.
Experimental
General Considerations
Syntheses and Characterization of Ligands and Complexes
3-Phenyl-1H-pyrazole and 3-(4-fluorophenyl)-1H-pyrazole were
purchased from Aldrich. 3,5-Dimethyl-1H-pyrazole and 5-methyl-
3-(trifluoromethyl)-1H-pyrazole were purchased from TCI Co.
2-Chloro-1,10-phenanthroline was purchased from J&K. Other
substituted pyrazoles and anhydrous CoCl2 were obtained from
Alfa and used without further purification unless otherwise
mentioned. MAO was available from Akzo Noble. Toluene and
THF were freshly distilled in the presence of sodium and benzophe-
none. DMF was purified by vacuum distillation under N2 atmo-
sphere. Polymerization-grade butadiene was supplied by Jinzhou
Petrochemical Corporation and purified by passing through columns
packed with 4 Å molecular sieves and KOH.
The tridentate 2-pyrazolyl-substituted 1,10-phenanthroline ligands
(L1–L8) were synthesized by the reaction of 2-chloro-1,10-phenan-
throline and the corresponding pyrazoles using a similar procedure
to 2,6-bis(N-pyrazolyl)pyridine,[49] and the corresponding cobalt
complexes (Co1–Co8) were readily prepared via the reaction of
anhydrous CoCl2 with the corresponding ligands in THF solution
at room temperature.
2-(Pyrazol-1-yl)-1,10-phenanthroline Cobalt(II) chloride (Co1)
A mixture of L1 (0.4 g, 1.63 mmol) and anhydrous CoCl2 (0.21 g,
1.63 mmol) was stirred in THF (8 ml) for 12 h at room tempera-
ture. The blue product precipitated was collected by filtration
and washed with 3 Â 5 ml cold THF. The desired product (0.56
g, 91.8 %) was obtained after drying under vacuum at 40ꢀC for
12 h. IR (KBr, cmÀ1): 3056, 2927, 1621, 1588, 1531, 1506, 1468,
1437, 1397, 1361, 1348, 1151, 1035, 951, 877, 851, 733, 646. Anal.
Calcd for C15H10N4CoCl2: C, 47.90; H, 2.68; N, 14.90; Cl, 18.85.
Found: C, 48.45; H, 2.89; N, 15.07; Cl, 18.98. ESI-MS for
C15H10N4CoCl2 (relative ratio): (m/z) ([M À Cl]+ 342).
1H NMR (400 MHz) was measured on a Varian Unity spectrom-
eter in CDCl3 at room temperature. Elemental analyses were
recorded on an elemental Vario EL spectrometer. IR spectra were
performed on a Bruker Vertex-70 FT-IR spectrophotometer. Mass
spectrometric detection was carried out on a Xevo TQ 151 mass
spectrometer (Waters Corp., Milford, MA, USA) with a 152 electro-
spray ionization (ESI) source. The proportion of 1,2, cis-1,4 and
1
trans-1,4 units of polymer were determined by IR and H NMR
spectra.[47,48] The molecular weights (Mn) and molecular weight
distributions (Mw/Mn) of polymer were measured at 30ꢀC by gel
permeation chromatography (GPC) utilizing a Waters 515 HPLC
pump, four columns (HMW 7 THF, HMW 6E THF Â 2, HMW 2
THF) and a Waters 2414 refractive index detector. THF was used
as eluent at a flow rate of 1.0 ml minÀ1. The values of Mn and
Mw/Mn were calculated using polystyrene calibration.
2-(3-Methylpyrazol-1-yl)-1,10-phenanthroline Cobalt(II) chloride (Co2)
This complex was prepared by the same method as for Co1 using
L2 instead of L1 to give Co2 as a blue powder in 87.8% yield. IR
(KBr, cmÀ1): 3064, 2929, 1621, 1586, 1546, 1512, 1471, 1457, 1393,
1348, 1330, 1052, 958, 876, 852, 790, 733, 645. Anal. Calcd for
C16H12N4CoCl2: C, 49.26; H, 3.10; N, 14.36; Cl, 18.17. Found: C,
50.05; H, 3.28; N, 14.63; Cl, 18.36. ESI-MS for C16H12N4CoCl2 (rela-
tive ratio): (m/z) ([M À Cl]+ 356).
X-Ray Structure Determinations
2-(3-Phenylpyrazol-1-yl)-1,10-phenanthroline Cobalt(II) chloride (Co3)
Crystals suitable for X-ray analyses were obtained as described in
the Experimental section. Data collections were performed at
À88.5ꢀC on a Bruker SMART APEX diffractometer with a CCD area
This complex was prepared by the same method as for Co1 using
L3 instead of L1 to give Co3 as a blue powder in 76.2% yield. IR
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Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2013, 27, 245–252

1,3-Butadiene polymerization initiated by new cobalt(II) catalysts
(KBr, cmÀ1): 3059, 2927, 1609, 1588, 1537, 1514, 1470, 1454, 1431,
1392, 1367, 1331, 1157, 1039, 961, 878, 851, 768, 732, 687, 647.
Anal. Calcd for C21H14N4CoCl2: C, 55.78; H, 3.12; N, 12.39; Cl,
15.68. Found: C, 55.96; H, 3.44; N, 12.75; Cl, 15.79. ESI-MS for
C21H14N4CoCl2 (relative ratio): (m/z) ([M À Cl]+ 418).
THF at room temperature (Scheme 1). The obtained cobalt
complexes were characterized by elemental analysis, IR spectros-
copy and ESI mass spectroscopy. Fortunately, single crystals of
cobalt complexes Co4–Co8 suitable for X-ray diffraction analysis
were gained by slow diffusion of diethyl ether into their DMF
solutions. The molecular structures of complexes Co1–Co8 are
shown in Figs 1–5 respectively and the selected bond lengths
and angles crystallographic data are listed in Tables 1 and 2.
The coordination geometry around the cobalt atom in com-
plex Co4 can be described as a distorted bipyramid, in which
one nitrogen atom (N2) of 1,10-phenanthroline and two chlorines
(Cl(1) and Cl(2)) form the equatorial plane, with the total value
of three equatorial angles, N(2)ÀCo(1)ÀCl(2) (130.09(11)ꢀ), N
(2)ÀCo(1)ÀCl(1) (114.13(10)ꢀ) and Cl(2)ÀCo(1)ÀCl(1) (115.75(5))
equal to 360ꢀ. The bond lengths of Co(1)ÀN(1) and Co(1)ÀN(2)
are 2.181(3) and 2.033(3) Å, respectively, which are much shorter
than that of Co(1)ÀN(3) (2.302(4) Å). The deviation of the
cobalt atom from the plane N(1)ÀN(2)ÀN(3) is 0.049 Å, indicating
that the four atoms are coplanar. The pyrazole ring twists slightly
from the phenanthroline plane (dihedral angle 3.48ꢀ), and the
dihedral angle between the pyrazolyl ring and para-fluorophenyl
plane is 15.84ꢀ.
2-(3-(4-Fluorophenyl) pyrazol-1-yl)-1,10-phenanthroline Cobalt(II) chloride
(Co4)
This complex was prepared by the same method as for Co1 using
L4 instead of L1 to give Co4 as a blue powder in 95.4% yield. IR
(KBr, cmÀ1): 3071, 2927, 1606, 1589, 1544, 1517, 1450, 1430, 1390,
1333, 1238, 1163, 1049, 963, 856, 735, 646, 628. Anal. Calcd for
C21H13FN4CoCl2: C, 53.64; H, 2.79; N, 11.92; Cl, 15.08. Found: C,
53.50; H, 3.01; N, 12.19; Cl, 15.18. ESI-MS for C21H13FN4CoCl2
(relative ratio): (m/z) ([M À Cl]+ 436).
2-(3,5-Dimethylpyrazol-1-yl)-1,10-phenanthroline Cobalt(II) chloride(Co5)
This complex was prepared by the same method as for Co1 using
L5 instead of L1 to give Co5 as a blue powder in 85.6% yield. IR
(KBr, cmÀ1): 3054, 2927, 2863, 1621, 1586, 1565, 1511, 1498, 1429,
1405, 1384, 1365, 1342, 1130, 1068, 984, 851, 772, 735, 647. Anal.
Calcd for C17H14N4CoCl2: C, 50.52; H, 3.49; N, 13.86; Cl, 17.54.
Found: C, 50.45; H, 3.78; N, 14.26; Cl, 17.72. ESI-MS for
C17H14N4CoCl2 (relative ratio): (m/z) ([M À Cl]+ 370).
The crystal structures of complexes Co5–Co8 having alkyl
substituent at the 3- and 5-position of the pyrazolyl ring of the
ligand are similar to that of Co4, and the coordination geometries
2-(3,5-Diisopropyl pyrazol-1-yl)-1,10-phenanthroline Cobalt(II) chloride(Co6)
This complex was prepared by the same method as for Co1 using
L6 instead of L1 to give Co6 as a blue powder in 76.3% yield. IR
(KBr, cmÀ1): 3057, 2935, 2871, 1584, 1561, 1509, 1498, 1464, 1428,
1384, 1375, 1332, 1061, 1029, 1011, 851, 735, 646. Anal. Calcd for
C21H22N4CoCl2: C, 54.80; H, 4.82; N, 12.17; Cl, 15.41. Found: C,
54.82; H, 5.18; N, 12.56; Cl, 15.59. ESI-MS for C21H22N4CoCl2 (rela-
tive ratio): (m/z) ([M À Cl]+ 426).
2-(3,5-Diphenylpyrazol-1-yl)-1,10-phenanthroline Cobalt(II) chloride(Co7)
This complex was prepared by the same method as for Co1 using
L7 instead of L1 to give Co7 as a blue powder in 89.5% yield. IR
(KBr, cmÀ1): 3057, 2872, 1619, 1586, 1560, 1508, 1497, 1464, 1430,
1397, 1363, 1331, 1151, 1063, 976, 959, 859, 770, 757, 701, 648.
Anal. Calcd for C27H18N4CoCl2: C, 61.38; H, 3.43; N, 10.60; Cl,
13.42. Found: C, 61.74; H, 3.89; N, 10.87; Cl, 13.55. ESI-MS for
C27H18N4CoCl2 (relative ratio): (m/z) ([M À Cl]+ 494).
Scheme 1. Syntheses of cobalt complexes Co1–Co8.
2-(3-Methyl-5-(trifluoromethyl)pyrazol-1-yl)-1,10-phenanthroline Cobalt(II)
chloride (Co8)
This complex was prepared by the same method as for Co1 using
L8 instead of L1 to give Co8 as a blue powder in 78.4% yield. IR
(KBr, cmÀ1): 3053, 2932, 1623, 1587, 1511, 1485, 1431, 1402, 1372,
1343, 1238, 1154, 1135, 1062, 1004, 977, 887, 854, 738, 648. Anal.
Calcd for C17H11F3N4CoCl2: C, 44.57; H, 2.42; N, 12.23; Cl, 15.48.
Found: C, 44.97; H, 2.96; N, 12.71; Cl, 15.65. ESI-MS for
C17H11F3N4CoCl2 (relative ratio): (m/z) ([M À Cl]+ 424).
Results and Discussions
Synthesis and Characterization of the Complexes
Ligands L1–L8 were prepared in good yields by the reaction of
2-chloro-1,10-phenanthroline and the corresponding pyrazoles
according to reported literature. Cobalt complexes Co1–Co8
were obtained in high yields by the treatment of the
corresponding ligands with 1.0 equiv. of anhydrous CoCl2 in
Figure 1. ORTEP view of complex Co4, drawn at 30% of probability.
Hydrogen atoms and one DMF molecule are omitted for clarity.
Appl. Organometal. Chem. 2013, 27, 245–252
Copyright © 2013 John Wiley & Sons, Ltd.
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B. Wang et al.
Figure 2. ORTEP view of complex Co5, drawn at 30% of probability.
Figure 5. ORTEP view of complex Co8, drawn at 30% of probability.
Hydrogen atoms and DMF molecule are omitted for clarity.
Hydrogen atoms are omitted for clarity.
respectively. The bond length of Co(1)ÀN(2) is shorter than that
of Co(1)ÀN(1) and Co(1)ÀN(3), which is in accordance with com-
plex Co4 and nickel analogous.[30] It is notable that substituents
and positions of the ligand do have a certain influence on the
molecular structure. As the substituent at the 3- and 5-positions
of the pyrazolyl ring changes from methyl group (Co5) to bulky
isopropyl (Co6) and phenyl group (Co7), the Co(1)ÀN(3) bond
length increases gradually (Co5: 2.163(3) Å; Co6: 2.1653(18) Å;
Co7: 2.208(6) Å). The longest Co(1)ÀN(3) bond (2.283(2) Å) is found
in complex Co8 (R1 = CF3, R2 = CH3), suggesting that the strong
electron-withdrawing trifluoromethyl group at the 3-position of
the pyrazolyl ring reduces the electron density of coordinated N
(3) atom. In addition, the pyrazole plane twists towards the
phenanthroline plane with the dihedral angle in the follow order:
Co5 (2.31ꢀ) > Co6 (6.24ꢀ) > Co7 (6.39ꢀ) > Co8 (15.98ꢀ).
Figure 3. ORTEP view of complex Co6, drawn at 30% of probability.
Hydrogen atoms are omitted for clarity.
Effects of Substituent on the pyrazolyl Ring of the Catalyst on
1,3-Butadiene Polymerization
Complexes Co1–Co8 activated by MAO were examined for
the polymerization of 1,3-butadiene as summarized in Table 3.
All the complexes gave polybutadienes having predominantly
cis-1,4 contents in the range 91.0–95.1%, while the polymer yields
were appreciably affected by the substituent on the pyrazolyl
ring of the complex ranging from 34% to 94.3%. Complex Co1
without substituent on the pyrazolyl ring (R1 = H, R2 = H) afforded
polymer in 74% yield (Scheme 1). When R2 was changed to
methyl (Co2), phenyl (Co3) and para-fluorophenyl (Co4) on the
pyrazolyl ring, respectively, the activity exhibited in the order
Co2 (methyl) > Co4 (para-fluorophenyl) > Co3 (phenyl). Compre-
hensively, the methyl group (Co2) with hyperconjugation to
the pyrazolyl ring gave the highest activity (yield 94.3%), while
with Co3, though having a stronger conjugation to the pyrazolyl
ring, bulkiness of the phenyl group might inhibit coordination of
monomer, giving rise to the lowest activity (34%), and in Co4,
though still bulky, the electron-withdrawing para-fluorophenyl
group enhanced the Lewis acid nature of metal center and
consequently facilitated the coordination of the monomer,[38]
leading to better activity (51.5%) than that of Co3. Furthermore,
when both R1 and R2 of the pyrazolyl ring were changed to
methyl (Co5), i-propyl (Co6) and phenyl (Co7) respectively, the
catalytic activity of complexes was in the order of iPr > Me > Ph,
which is consistent with the distance of deviation of the cobalt
atom from the equatorial plane [N(1)ÀN(2)ÀN(3)] of these
Figure 4. ORTEP view of complex Co7, drawn at 30% of probability.
Hydrogen atoms are omitted for clarity.
of these complexes also exhibit distorted trigonal bipyramid
structures. The deviations of the cobalt atom from the equatorial
plane (N(1)ÀN(2)ÀN(3)) are 0.026, 0.150, 0.001 and 0.169 Å,
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Appl. Organometal. Chem. 2013, 27, 245–252

1,3-Butadiene polymerization initiated by new cobalt(II) catalysts
Table 1. Crystal data and structure refinements of complexes Co4–Co8
Co4.DMF
Co5.DMF
Co6
Co7
Co8
Formula
C
24H20FN5O CoCl2
543.28
C20H21N5O CoCl2
477.25
C21H22N4 CoCl2
460.26
C27H18N4 CoCl2
528.28
C17H11F3N4 CoCl2
458.13
Molecular weight
Crystal system
Space group
a (Å)
Triclinic
Monoclinic
P21/c
Monoclinic
P21/c
Triclinic
P-1
Monoclinic
P21/n
P-1
10.1027 (7)
10.3879 (8)
12.4120 (9)
83.8940 (10)
69.4020 (10)
74.5240 (10)
1174.99 (15)
2
11.3856 (8)
23.4281 (17)
8.0127 (6)
90.00
9.6134 (6)
8.2383 (6)
26.0362 (18)
90.00
9.1735 (9)
11.5613 (11)
21.700 (2)
90.158 (2)
90.226 (2)
93.814 (2)
2296.3 (4)
4
11.0731 (13)
9.4321 (11)
17.690 (2)
90.00
b (Å)
c (Å)
a (deg)
b (deg)
g (deg)
V (Å3)
101.7500 (10)
90.00
100.1350 (10)
90.00
108.24ꢀ
90.00
2092.5 (3)
4
2029.8 (2)
4
1754.7 (4)
4
Z
Dcalcd (mg/m3)
Absorp. coeff. (mmÀ1
F(000)
1.536
1.515
1.506
1.528
1.734
)
0.993
1.10
1.12
1.00
1.32
554.0
980.0
948.0
1450
1125
Crystal size (mm)
θ range (ꢀ)
0.22 Â 0.19 Â 0.16
1.75–26.08
6529
0.19 Â 0.13 Â 0.08
1.74–26.06
13 278
0.20 Â 0.18 Â 0.12
1.59–26.03
12 549
0.24 Â 0.21 Â 0.15
1.77–26.04
12 331
0.20 Â 0.16 Â 0.12
1.94–26.03
10 147
No. of reflns collected
No. of indep. reflns
Rint
4581
4 112
3 983
8 856
3 451
0.0229
0.0240
0.0265
0.0372
0.0341
No. of data/restraint/params
GOF on F2
4581/0/309
1.065
4 112/0/266
1.069
3 983/0/257
1.045
8 856/0/613
1.153
3 451/0/245
1.007
R1(I > 2s(I))
0.0578
0.0455
0.0343
0.0918
0.0374
wR2
0.1716
0.1105
0.0885
0.1791
0.0827
Table 2. Selected bond length (Å) and angles (ꢀ) for complexes Co4–Co8
Co4.DMF
Co5.DMF
Co6
Co7
Co8
Bond distance (Å)
Co1ÀN1
2.181(3)
2.193(3)
2.045(2)
2.163(3)
2.2726(9)
2.2748(9)
2.2145(19)
2.0703(18)
2.1653(18)
2.2697(6)
2.2791(6)
2.176(6)
2.039(5)
2.208(6)
2.271(2)
2.264(2)
2.178(2)
2.042(2)
2.283(2)
2.2561(8)
2.2683(8)
Co1ÀN2
2.033(3)
Co1ÀN3
2.302(4)
Co1ÀCl1
2.2626(14)
2.2695(12)
Co1ÀCl2
Bond angle (ꢀ)
N1ÀCo1ÀN3
N1ÀCo1ÀN2
N3ÀCo1ÀN2
N1ÀCo1ÀCl2
N3ÀCo1ÀCl2
N2ÀCo1ÀCl2
N1ÀCo1ÀCl1
N3ÀCo1ÀCl1
N2ÀCo1ÀCl1
Cl2ÀCo1ÀCl1
150.93(13)
77.11(13)
73.87(13)
96.28(10)
100.88(9)
130.09(11)
95.62(10)
97.79(10)
114.13(10)
115.75(5)
151.94(10)
77.75(10)
74.21(9)
96.03(7)
98.40(7)
119.12(7)
94.34(7)
98.99(7)
120.76(7)
120.08(3)
149.01(7)
76.15(7)
73.41(7)
95.32(6)
101.49(5)
133.89(5)
100.10(6)
98.01(5)
115.33(5)
110.77(2)
152.0(2)
150.20(9)
78.37(9)
72.54(8)
100.12(6)
96.78(6)
112.34(6)
95.18(6)
95.96(6)
124.52(6)
122.98(3)
77.3(2)
74.7(2)
95.94(17)
97.74(17)
119.54(17)
90.52(17)
103.47(16)
120.15(18)
119.92(9)
complexes (0.150 Å (iPr), 0.026 Å (Me) and 0.001 Å (Ph)). Reason-
ably, a longer distance between metal atom and coordinated
plane [N(1)ÀN(2)ÀN(3)] resulted in a more open spatial structure
of the metal center, facilitating monomer coordination to the
metal center, and consequently leading to the higher activity.[38]
As expected, there being a greater distance of the cobalt
atom from the equatorial plane [N(1)ÀN(2)ÀN(3)] than that of
Co5 (0.026 Å) and Co6 (0.150 Å), Co8 (0.169 Å) with an
electron-withdrawing group gave higher activity than that of
both Co5 (R1 = R2 = Me) and Co6 (R1 = R2 = iPr).
Variation of [Al]/[Co] and Temperature on 1,3-Butadiene
Polymerization Behaviors
In general, excess MAO has to be used for obtaining polymer in
high yield for metallocene or late transition metal-based
Appl. Organometal. Chem. 2013, 27, 245–252
Copyright © 2013 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

B. Wang et al.
Table 3. Polymerization of 1,3-butadiene with Co1–Co8 activated by MAO
Runa
Cat.
Yield (%)
Microstructureb (%)
Mn (Â10À4
)
Mw/Mn
c
c
cis-1,4
trans-1,4
1,2
1
2
3
4
5
6
7
8
Co1
Co2
Co3
Co4
Co5
Co6
Co7
Co8
74.0
94.3
33.5
51.5
65.9
72.5
49.2
84.7
91.0
93.9
92.6
90.5
94.7
95.1
91.4
92.3
6.4
3.4
4.5
6.7
3.3
2.7
5.8
5.0
2.6
2.7
2.9
2.8
2.0
2.2
2.8
2.8
2.1
3.6
3.0
3.8
1.9
2.9
2.8
1.9
2.1
6.7
0.97
0.75
2.0
15.2
1.2
1.0
aPolymerization conditions: in toluene at 20ꢀC for 4 h, [Co] = 1.32 mM, [BD]/[Co] = 2000, [Al]/[Co] = 100.
bDetermined by 1H NMR and FT-IR.
cDetermined by GPC (THF, PSt calibration).
Table 4. Effects of [Al]/[Co] on the polymerization of 1,3-butadiene
with Co2/MAO
Table 5. Effects of temperature on the polymerization of 1,3-butadiene
with Co2/MAO
Runa Al/Co Yield (%) Microstructureb (%) Mn (Â10-4) Mw/Mn
Runa T (ꢀC) Yield (%) Microstructureb (%) Mnc (Â10À4)Mw/Mn
c
c
c
cis-1,4 trans-1,4 1,2
cis-1,4 trans-1,4 1,2
11
12
13
14
15
16
25
—
—
—
3.1
3.4
3.2
3.2
3.5
—
2.4
2.7
3.2
3.6
3.1
—
4.2
6.7
6.9
7.1
6.5
—
2.6
3.0
2.8
2.7
2.6
17
18
19
20
21
À20
0
—
—
—
3.2
3.4
3.2
3.7
—
2.2
2.7
5.0
5.7
—
4.2
6.7
5.4
3.8
—
2.7
3.0
2.7
2.8
50
52.3
94.3
95.8
96.7
98.5
94.5
93.9
93.6
93.2
93.4
50.6
93.9
92.5
91.8
94.6
93.9
91.8
90.6
100
150
200
300
20
40
60
aPolymerization conditions: in toluene for 4 h, [Co] = 1.32 mM,
[BD]/[Co] = 2000, [MAO]/[Co] = 100.
bDetermined by H NMR and FTIR.
cDetermined by GPC (THF, PSt calibration).
aPolymerization conditions: in toluene at 20ꢀC for 4 h, [Co] = 1.32
mM, [BD]/[Co] = 2000.
bDetermined by 1H NMR and FTIR.
cDetermined by GPC (THF, PSt calibration).
1
catalysts, and it is true for cobalt-based catalyst in 1,3-butadiene
polymerization. Currently, the complex Co2 activated by
MAO reached high-yield 1,3-butadiene polymerization only at
[Al]/[Co] = 100 – much lower than that of reported Co complex
(Table 4).[31,32] With the variation of [Al]/[Co] ratio, stereoregular-
ity of the polymer was maintained almost consistent with cis-1,4
content around 94%, while molecular weight and molecular
weight distribution of the resultant polymer only fluctuated
slightly.
Scheme 2. Proposed mechanism of C1 insertion and C3 insertion.
As with most late transition metal-based catalysts, the Co
complexes also suffered from the decay of catalytic activity as
polymerization temperature increased.[34,50] In the case of Co2
activated by MAO, although polymerization yield also decreased
as temperature increased, promisingly, the decrease was only
from 93.9% to 91.8% when the temperature increased from
20ꢀC to 60ꢀC (Table 5), implying improvement of the thermal
stability of the Co-based catalyst in the present work. With the
increment of temperature, cis-1,4 content of the resultant
polymer decreased simultaneously, with slightly increased
trans-1,4 content and obviously increased 1,2 content. In
principle, coordinated monomer can insert into the p–Z3-allyl in
two ways (Scheme 2); insertion to less steric C1of p–Z3-allyl is
dynamically favored, resulting in the formation of 1,4 units, while
the more bulky C3 of p–Z3-allyl is thermodynamically favored,
leading to the formation of 1,2 structure. Thus, comprehensively,
much increased 1,2 content with increasing temperature was
explainable according to Scheme 2.
Effects of Co-catalyst Type on 1,3-Butadiene Polymerization
by Co2
With complex Co2 as a precursor, the effects of co-catalyst
type on butadiene polymerization behavior were investigated
(Table 6). All the co-catalysts examined displayed high activities,
and the polymer yield reached 93.5% or above. Evidently, MAO
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Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2013, 27, 245–252

1,3-Butadiene polymerization initiated by new cobalt(II) catalysts
Table 6. Effects of co-catalyst type on the polymerization of 1,3-butadiene with Co2
Runa
Co-catalyst
[Al]/[Co]
Yield (% )
Microstructureb (%)
Mn (Â10-4)
Mw/Mn
c
c
Cis-1,4
Trans-1,4
1,2
2.4
22
23
24
25
26
27
28
29
MAO
50
52.3
94.3
91.2
93.5
93.7
94.5
92.8
93.9
94.5
93.9
93.4
93.5
83.0
85.1
84.8
85.3
3.1
3.4
4.4
4.5
8.8
8.2
7.8
7.9
4.2
6.7
6.7
6.4
1.8
2.4
2.1
2.3
2.6
3.0
2.8
2.9
4.7
5.1
4.3
4.5
MAO
100
50
2.7
2.2
2.3
8.2
6.7
7.4
6.8
Al2Et3Cl3
Al2Et3Cl3
AlEt2Cl
AlEt2Cl
AliBu2Cl
AliBu2Cl
100
50
100
50
100
aPolymerization conditions: in toluene at 20ꢀC for 4 h, [Co] = 1.32 mM, [BD]/[Co] = 2000.
bDetermined by 1H NMR and FTIR.
cDetermined by GPC (THF, PSt calibration).
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is more favorable in producing high cis-1,4 polymer (cis-1,4:
93.9%, [Al]/[Co] = 100), slightly higher than that of EASC (cis-1,4:
93.5%, [Al]/[Co] = 100) and remarkably higher than that of
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Conclusion
A series of new cobalt(II) complexes supported by tridentate
2-pyrazolyl-substituted 1,10-phenanthroline ligands have been
synthesized and fully characterized. Determined by X-ray crystallo-
graphic analysis, complexes Co1–Co8 all exhibit distorted trigonal
pyramid geometry around the cobalt atom. Activated by MAO,
the complexes afford polybutadiene with mainly cis-1,4 content
(up to 95.1%) in yields ranging from 33.5% to 94.3%. The catalytic
activity varies with the variation of the steric and electronic nature
of the substituent on the pyrazole ring of the ligand. Differentiating
from the most transition metal-based catalysts, with increasing
polymerization temperature, the activity of the catalyst decreases
only slightly, indicating the catalyst system features much superior
thermal tolerance to its analogues.
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The authors appreciate financial support from National Science
and Technology Infrastructure Program (2007BAE14B01-06) and
the Fund for Creative Research Groups (50621302).
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