Highly active and cis-1,4 selective polymerization of 1,3-butadiene catalyzed by cobalt(II) complexes bearing α-diimine ligands

Article abstract of DOI:10.1016/S1872-2067(12)60625-1

A series of cobalt(II) complexes bearing α-diimine ligands were synthesized and characterized by elemental and spectroscopic analysis. These complexes had the general formulas [ArN=C(Me)-(Me)C=NAr]CoCl₂ (Ar = C₆H₅, 3a; 4-M

Full text of DOI:10.1016/S1872-2067(12)60625-1

Chinese Journal of Catalysis 34 (2013) 1560–1569
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Highly active and cis‐1,4 selective polymerization of 1,3‐butadiene
catalyzed by cobalt(II) complexes bearing α‐diimine ligands
Xiangyu Jiaa,b, Heng Liua,b, Yanming Huc,#, Quanquan Daia, Jifu Bia, Chenxi Baia, Xuequan Zhanga,*
a Research Center of High Performance Synthetic Rubber, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,
Jilin, China
b University of Chinese Academy of Sciences, Beijing 100049, China
c Department of Polymer Science and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, Liaoning, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A series of cobalt(II) complexes bearing α‐diimine ligands were synthesized and characterized by
elemental and spectroscopic analysis. These complexes had the general formulas
[ArN=C(Me)–(Me)C=NAr]CoCl2 (Ar = C6H5, 3a; 4‐MeC6H4, 3b; 4‐MeOC6H4, 3c; 4‐FC6H4, 3d; 4‐ClC6H4,
3e; 2‐MeC6H4, 3f; 2‐EtC6H4, 3g; 2‐iPrC6H4, 3h; 2,4,6‐Me3C6H2, 3i; 2,6‐Et2C6H3, 3j; 2,6‐iPrC6H3, 3k).
2,6‐Bis[(2,6‐diisopropylphenylimino)ethyl]pyridine CoCl2 (4a) was also synthesized for compari‐
son. The structures of complexes 3i, 3k, and 4a were further analyzed by X‐ray crystallography.
When the Co(II) complexes were activated with ethylaluminum sesquichloride, they exhibited high
catalytic activity for 1,3‐butadiene polymerization. The polymers produced have high cis‐1,4 stere‐
oregularity (up to 98.0%) and high molecular weights (Mn = 1  104–1  105). The substituent ligand
affected both catalytic activity and stereoselectivity through an electronic effect while steric hin‐
drance by the substituent was not important. The effects of the polymerization conditions, such as
polymerization time, temperature, different alkylaluminum compounds used as cocatalyst, and
[Al]/[Co] molar ratio, on polymerization behavior were investigated.
Received 11 March 2013
Accepted 10 May 2013
Published 20 August 2013
Keywords:
Cobalt
α‐Diimine
1,3‐Butadiene
Polybutadiene
Polymerization
© 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
plays an important role in controlling stereoselectivity [33,34].
Cis‐1,4 selective polymerization of 1,3‐butadiene is important
The stereoselective polymerization of conjugated dienes,
especially 1,3‐butadiene and isoprene, by transition metal cat‐
alysts for producing high performance synthetic rubber is of
substantial interest [1–7]. Polybutadiene is an important poly‐
mer with extensive and diverse applications that depend on its
microstructure [8]. Numerous catalysts based on transition
metals (Ti, Ni, Co, etc.) and rare earth metals have been devel‐
oped to selectively produce cis‐1,4‐ [9–21], trans‐1,4‐ [22–25],
and 1,2‐polybutadienes [26–32]. The addition of Lewis bases
because the product, cis‐1,4‐polybutadiene, is a major raw ma‐
terial in the tire industry. Industrially, high cis‐1,4‐polybutadi‐
ene is obtained by utilizing conventional Ziegler‐Natta catalysts
based on transition metals or rare earth metals, such as
Ni(naph)2/BF3·Et2O/AlEt3 [35], CoCl2/AlEt2Cl [36], TiCl4/I2/
Al(i‐Bu)3 [37], or Nd(vers)3/Al(i‐Bu)2H/Al2Et3Cl3 [38].
Among the above catalyst systems, Co‐based catalysts have
attracted special attention because they can produce polybuta‐
dienes with tunable structures, such as cis‐1,4‐ and syndiotactic
*Corresponding author. Tel: +86‐431‐85262303; Fax: +86‐431‐85262307; E‐mail: xqzhang@ciac.jl.cn
# Corresponding author. Tel: +86‐411‐84986101; Fax: +86‐411‐84986102; E‐mail: ymhu@dlut.edu.cn
This work was supported by the National Key Technology R&D Program of China (2007BAE14B01‐06) and the Fund for Creative Research Groups of
the National Natural Science Foundation of China (50621302).
DOI: 10.1016/S1872‐2067(12)60625‐1 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 34, No. 8, August 2013

Xiangyu Jia et al. / Chinese Journal of Catalysis 34 (2013) 1560–1569
1,2‐isomers, which are determined by the structure of the lig‐
polymerization temperature on the polymerization of 1,3‐bu‐
and on the Co metal center. Various Co catalysts have been
designed to polymerize 1,3‐butadiene selectively. For example,
cobalt halides and carboxylates activated with methylalumi‐
noxane (MAO) can give high cis‐1,4‐polybutadiene, and
CoCl2/MAO catalysts with alkylphosphine adducts are effective
for 1,2‐selective polymerization of 1,3‐butadiene [39–41]. Alt‐
hough these catalysts can produce polybutadienes with high
stereoregularity, their active sites are not uniform, and the mo‐
lecular weight distribution of the polybutadienes is broad. Also,
it has not been clarified how the structure of the catalyst pre‐
cursors affect the polymerization behavior. Recent studies have
focused on well defined homogeneous single site organometal‐
lic catalysts. These studies are primarily an empirical search for
understanding the catalyst structure that determines catalytic
activity and stereoselectivity. For example, four‐coordinated
(salen)Co(II) ([ONNO]2–) [42] and bis(salicylaldiminate)Co(II)
tadiene were investigated in detail.
2. Experimental
2.1. General considerations and materials
All manipulations were carried out under N2 atmosphere
using standard Schlenk techniques. Fourier‐transform infrared
spectroscopy (FT‐IR) was performed on a Bruker Vertex‐70
1
FTIR spectrophotometer. H and 13C NMR spectra were rec‐
orded on a Varian Unity‐400 spectrometer with CDCl3 at room
temperature and tetramethylsilane as the internal standard.
Elemental analyses were recorded on an elemental Vario EL
spectrometer. The proportions of cis‐1,4‐, trans‐1,4‐ and
1,2‐units of the polymer were determined from the IR, 1H NMR,
and 13C NMR spectra [53,54]. The molecular weight (Mn) and
molecular weight distribution (Mw/Mn) of the 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 × 2, HMW 2 THF), and a Waters 2414
refractive index detector. Tetrahydrofuran (1.0 ml/min) was
used as the eluent.
Toluene was refluxed over sodium‐benzophenone and dis‐
tilled under N2 prior to use. MAO (1.5 mol/L in toluene) was
purchased from Akzo Nobel Corp. Diethylaluminium chloride
(DEAC, 0.9 mol/L in toluene) was purchased from Alfa Aesar
and used without further purification. EASC was purchased
from Sigma‐Aldrich Co. and diluted into toluene solution (1.16
mol/L) before use. Polymerization grade 1,3‐butadiene was
purified by passing it through KOH and molecular sieve col‐
umns. CoCl2, 2,3‐butanedione, 2,6‐diacetylpyridine, and various
anilines were purchased from Alfa Aesar and used without
further purification.
([NO]– ) [43] complexes activated with MAO or ethylaluminum
2
sesquichloride (EASC) exhibited both high catalytic activity and
cis‐1,4 selectivity for 1,3‐butadiene polymerization. Some
three‐coordinated catalysts, such as bis(imino)pyridine Co(II)
[44], bis(benzimidazolyl)amine Co(II) [34], bis(benzimidazolyl)
pyridine Co(II) [45,46], bis(thiazolinyl)pyridine Co(II) [47],
2‐arylimino‐6‐(alcohol)pyridine/2‐arylamino‐6‐(alcohol) pyri‐
dine Co(II) [48], and 3‐aryliminomethyl‐2‐hydroxybenzalde‐
hyde Co(II) [49] complexes, activated with alkylaluminum co‐
catalysts also displayed high catalytic activity and high cis‐1,4
selectivity. In our previous study [44], we found that the incor‐
poration of electron‐withdrawing groups into the imino ligands
enhanced both the catalytic activity and selectivity of a
bis(imino)pyridine Co(II)/MAO catalyst for 1,3‐butadiene
polymerization. Steric hindrance by the substituent on the lig‐
and also influenced the polymerization behavior to some ex‐
tent.
In 1995, a new type of catalyst based on Ni and Pd com‐
plexes bearing α‐diimine ligands was discovered by Brookhart
and coworkers [50] for producing high molecular weight poly‐
ethylene and linear α‐olefins. These bidentate ligands are very
rigid, and it is possible to easily change their backbone and
change the electronic effect at the metal center. In ethylene
polymerization, when the steric hindrance of the aryl groups
attached to the imino nitrogens is reduced, the product compo‐
sition is shifted from high molecular weight polymer towards
linear oligomers [51,52]. However, [NN] bidentate Co(II) com‐
plexes for 1,3‐butadiene polymerization have not been report‐
ed yet. Compared with the [NNN] tridentate ligand, the
α‐diimine ligand gives the metal complex a more open coordi‐
nated environment due to less steric hindrance. Thus, it is ex‐
pected that these Co complexes would display interesting
1,3‐butadiene polymerization behavior, which may be different
from that of the [NNN] tridentate complexes.
2.2. Synthesis and characterization of the ligands and Co(II)
complexes
Ligands 2a–2k, 2,6‐bis[(2,6‐diisopropylphenylimino)ethyl]
pyridine, and the corresponding complexes 3a–3k and 4a were
synthesized according to the literature [11,36,48–50,55–57].
For synthesis of ligand 2a, a reaction mixture of 2,3‐butane‐
dione (2.0 g, 0.023 mol), aniline (4.33 g, 0.047 mol), a few drops
of formic acid and methanol (20 ml) was refluxed for 12 h and
cooled to room temperature. A yellow crystal‐like solid precip‐
itated after several hours. The desired product (4.2 g, 76.5%)
was identified by FT‐IR, 1H NMR, and 13C NMR spectra and el‐
emental analysis. 1H NMR (400 MHz, CDCl3, δ): 7.368 (t, 4H, J =
7.6 Hz, Haryl), 7.112 (t, 2H, J = 7.6 Hz, Haryl), 6.798 (d, 4H, J = 7.6
Hz, Haryl), 2.150 (s, 6H, –C(CH3)=N–). 13C NMR (100 MHz, CDCl3,
δ): 168.22 (N=C), 150.93 (CAr–N), 128.95, 123.78, 118.71, 15.34
(N=CCH3). IR (KBr, cm–1): 1635 (νC=N), 1480, 1445, 1355, 1207,
1116, 1071, 809, 762, 695. Anal. Calcd for C16H16N2: C 81.32, H
6.82, N 11.86; Found: C 81.53, H 6.75, N 11.81.
In this study, in order to understand how the ligand affects
catalytic behavior, we report the synthesis and characterization
of (α‐diimine)Co(II) complexes and used 2,6‐bis[(2,6‐diiso‐
propylphenylimino)ethyl]pyridine Co(II) dichloride for com‐
parison. The influences of the ligand structure, polymerization
time, alkylaluminum compound, [Al]/[Co] molar ratio, and
For synthesis of ligand 2b, a procedure similar to 2a was
adopted. 2,3‐Butanedione (2.0 g, 0.023 mol), 4‐methylaniline
1
(5.0 g, 0.047 mol), yield 5.06 g (83.2%). H NMR (400 MHz,

Xiangyu Jia et al. / Chinese Journal of Catalysis 34 (2013) 1560–1569
CDCl3, δ): 7.175 (d, 4H, J = 7.6 Hz, Haryl), 6.70 (d, 4H, J = 8.0 Hz,
aryl), 2.347 (s, 6H, –CH3), 2.148 (s, 6H, –C(CH3)=N–). 13C NMR
1242, 1116, 1050, 821, 752. Anal. Calcd for C20H24N2: C 82.15, H
8.27, N 9.58; Found: C 82.47, H 8.41, N, 9.53.
H
(100 MHz, CDCl3, δ): 168.29 (N=C), 148.35 (CAr–N), 133.21,
129.45, 118.84, 20.82 (Ar–CH3), 15.29 (N=CCH3). IR (KBr,
cm–1): 1628 (νC=N), 1501, 1431, 1362, 1204, 1116, 1040, 810,
746, 709. Anal. Calcd for C18H20N2: C 81.78, H 7.63, N 10.60;
Found: C 81.54, H 7.88, N 10.56.
For synthesis of ligand 2h, a procedure similar to 2a was
adopted. 2,3‐Butanedione (2.0 g, 0.023 mol), 2‐isopropylaniline
(6.28 g, 0.046 mol), yield 6.67 g (90.5%). H NMR (400 MHz,
1
CDCl3, δ): 7.327 (d, 2H, J = 7.2 Hz, Haryl), 7.182 (t, 2H, J = 7.2 Hz,
H
H
aryl), 7.112 (t, 2H, J = 7.2 Hz, Haryl), 6.616 (d, 2H, J = 7.6 Hz,
aryl), 2.912–3.015 (sept, 2H, –CH(CH3)2), 2.159 (s, 6H,
For synthesis of ligand 2c, a procedure similar to 2a was
adopted. 2,3‐Butanedione (2.0 g, 0.0232 mol), 4‐methoxyani‐
–C(CH3)=N–), 1.209 (d, 12H, J = 6.8 Hz, –CH(CH3)2). 13C NMR
(100 MHz, CDCl3, δ): 167.55 (N=C), 148.26 (CAr–N), 137.75,
126.05, 125.64, 124.27, 117.86, 28.48 (–CH(CH3)2), 22.68
(–CH(CH3)2), 15.63 (N=CCH3). IR (KBr, cm–1): 1633 (νC=N), 1482,
1446, 1357, 1281, 1115, 1032, 813, 752. Anal. Calcd for
C22H28N2: C 82.45, H 8.81, N 8.74; Found: C 82.54, H 8.92, N
8.67.
For synthesis of ligand 2i, a procedure similar to 2a was
adopted. 2,3‐Butanedione (2.0 g, 0.023 mol), 2,4,6‐trimethyl‐
aniline (6.28 g, 0.046 mol), yield 7.13 g (96.8%). 1H NMR (400
MHz, CDCl3, δ): 6.886 (s, 4H, Haryl), 2.284 (s, 6H, –C(CH3)=N–),
2.024 (s, 6H, –CH3), 1.995 (s, 12H, –CH3). 13C NMR (100 MHz,
CDCl3, δ): 168.30 (N=C), 145.86 (CAr–N), 132.35, 128.54, 124.49,
20.68, 17.67, 15.73 (N=CCH3). IR (KBr, cm–1): 1637 (νC=N), 1478,
1433, 1360, 1211, 1120, 1035, 856, 790, 726. Anal. Calcd for
C22H28N2: C 82.45, H 8.81, N 8.74; Found: C 82.56, H 8.53, N
8.78.
1
line (5.72 g, 0.0464 mol), yield 5.15 g (75.6%). H NMR (400
MHz, CDCl3, δ): 6.930 (d, 4H, J = 8.4 Hz, Haryl), 6.775 (d, 4H, J =
8.4 Hz, Haryl), 3.818 (s, 6H, –OCH3), 2.176 (s, 6H, –C(CH3)=N–).
13C NMR (100 MHz, CDCl3, δ): 168.39 (N=C), 156.37 (CAr–N),
144.01, 120.50, 114.17, 55.40 (Ar–OCH3), 15.32 (N=CCH3). IR
(KBr, cm–1): 1633 (νC=N), 1501, 1440, 1356, 1205, 1116, 1035,
842, 756, 710. Anal. Calcd for C18H20N2O2: C 72.95, H 6.80, N
9.45; Found: C 73.06, H 6.71, N 9.47.
For synthesis of ligand 2d, a procedure similar to 2a was
adopted. 2,3‐Butanedione (2.0 g, 0.0232 mol), 4‐fluoroaniline
(5.16 g, 0.0464 mol), yield 6.02 g (96.2%). 1H NMR (400 MHz,
CDCl3, δ): 7.068 (t, 4H, J = 8.8 Hz, Haryl), 6.736–6.770 (m, 4H,
H
aryl), 2.146 (s, 6H, –C(CH3)=N–). 13C NMR (100 MHz, CDCl3, δ):
168.81 (N=C), 160.87 (d, JCF = 242 Hz), 146.72 (CAr–N), 120.31,
115.78, 15.29 (N=CCH3). IR (KBr, cm–1): 1649 (νC=N), 1497,
1431, 1356, 1198, 1123, 1093, 852, 758, 708. Anal. Calcd for
C16H14F2N2: C 70.58, H 5.18, N 10.29; Found: C 70.46, H 5.23, N
10.33.
For synthesis of ligand 2j, a procedure similar to 2a was
adopted. 2,3‐Butanedione (2.0 g, 0.023 mol), 2,6‐diethylaniline
1
For synthesis of ligand 2e, a procedure similar to 2a was
adopted. 2,3‐Butanedione (2.0 g, 0.023 mol), 4‐chloroaniline
(6.93 g, 0.046 mol), yield 7.66 g (95.5%). H NMR (400 MHz,
CDCl3, δ): 7.116 (d, 4H, J = 7.6 Hz, Haryl), 7.024 (t, 2H, J = 6.8 Hz,
Haryl), 2.30–2.447 (nonuple, 8H, –CH2CH3), 2.053 (s, 6H,
1
(5.93 g, 0.046 mol), yield 4.56 g (64.9%). H NMR (400 MHz,
CDCl3, δ): 7.346 (d, 4H, J = 8.4 Hz, Haryl), 6.734 (d, 4H, J = 8.4 Hz,
–C(CH3)=N–), 1.158 (t, 12H, J = 7.6 Hz, –CH2CH3). 13C NMR (100
MHz, CDCl3, δ): 167.93 (N=C), 147.39 (CAr–N), 130.48, 126.06,
123.48, 24.68, 16.13 (N=CCH3), 13.60. IR (KBr, cm–1): 1643
(νC=N), 1448, 1362, 1188, 1036, 872, 800, 763. Anal. Calcd for
C24H32N2: C 82.71, H 9.25, N 8.04; Found: C 82.67, H 9.17, N
8.06.
H
aryl), 2.129 (s, 6H, –C(CH3)=N–). 13C NMR (100 MHz, CDCl3, δ):
168.66 (N=C), 149.20, 132.46, 129.05, 120.17, 15.35 (N=CCH3).
IR (KBr, cm–1): 1634 (νC=N), 1480, 1431, 1360, 1205, 1121,
1086, 845. Anal. Calcd for C16H14Cl2N2: C 62.97, H 4.62, N 9.18;
Found: C 63.06, H 4.71, N 9.32.
For synthesis of ligand 2f, a procedure similar to 2a was
adopted. 2,3‐Butanedione (2.0 g, 0.023 mol), 2‐methylaniline
(5.0 g, 0.047 mol), yield 5.08 g (83.6%). H NMR (400 MHz,
For synthesis of ligand 2k, a procedure similar to 2a was
adopted. 2,3‐Butanedione (2.0 g, 0.023 mol), 2,6‐diisopropy‐
laniline (8.24 g, 0.046 mol), yield 6.82 g (73.3%). 1H NMR (400
MHz, CDCl3, δ): 7.177 (d, 4H, J = 7.2 Hz, Haryl), 7.096 (t, 2H, J =
6.8 Hz, Haryl), 2.663–2.766 (sept, 4H, –CH(CH3)2), 2.068 (s, 6H,
–C(CH3)=N–), 1.186 (t, 24H, J = 8.0 Hz, –CH(CH3)2). 13C NMR
(100 MHz, CDCl3, δ): 168.15 (N=C), 146.16 (CAr–N), 135.02,
123.72, 122.97, 28.48, 22.67, 16.54 (N=CCH3). IR (KBr, cm–1):
1639 (νC=N), 1438, 1363, 1181, 1160, 1041, 792, 763. Anal.
Calcd for C28H40N2: C 83.11, H 9.96, N 6.92; Found: C 83.06, H
10.13, N 6.98.
1
CDCl3, δ): 7.175–7.251 (quintet, 4H, Haryl), 7.033 (t, 2H, J = 7.6
Hz, Haryl), 6.657 (d, 2H, J = 8.0 Hz, Haryl), 2.119 (s, 12H,
–C(CH3)=N–, –CH3). 13C NMR (100 MHz, CDCl3, δ): 167.65
(N=C), 149.46 (CAr–N), 130.36, 126.32, 123.43, 117.58, 17.69
(Ar–CH3), 15.49 (N=CCH3). IR (KBr, cm–1): 1640 (νC=N), 1484,
1452, 1355, 1216, 1118, 1042, 855, 782, 717. Anal. Calcd for
C18H20N2: C 81.78, H 7.63, N 10.60; Found: C 81.53, H 7.71, N
10.77.
For synthesis of ligand 2g, a procedure similar to 2a was
adopted. 2,3‐Butanedione (2.0 g, 0.023 mol), 2‐ethylaniline
(5.63 g, 0.047 mol), yield 6.23 g (92.7%). H NMR (400 MHz,
CDCl3, δ): 7.272–7.290 (d, 2H, J = 8 7.2 Hz, Haryl), 7.197 (t, 2H, J =
7.6 Hz, Haryl), 7.077 (t, 2H, J = 7.6 Hz, Haryl), 6.639 (d, 2H, J = 7.6
Hz, Haryl), 2.464–2.520 (q, 4H, –CH2CH3), 2.141 (s, 6H,
–C(CH3)=N–), 1.160 (t, 6H, J = 7.2 Hz, –CH2CH3). 13C NMR (100
MHz, CDCl3, δ): 167.59 (N=C), 148.90 (CAr–N), 133.06, 128.62,
126.27, 124.09, 117.71, 24.70 (–CH2CH3), 15.52 (N=CCH3),
13.97 (–CH2CH3). IR (KBr, cm–1): 1640 (νC=N), 1481, 1460, 1361,
For synthesis of 2,6‐bis[(2,6‐diisopropylphenylimino)ethyl]
pyridine, a procedure similar to 2a was adopted. 2,6‐Diacetyl‐
pyridine (1.5 g, 0.009 mol), 2,6‐diisopropylaniline (3.46 ml,
0.018 mol), yield 3.26 g (75.2%). 1H NMR (400 MHz, CDCl3, δ):
8.502 (d, 2H, J = 8Hz, HPy), 7.934 (t, 1H, J = 8Hz, HPy),
7.091–7.168 (m, 6H, Haryl), 2.783 (sept, 4H, J = 5.6 Hz,
–CH(CH3)2), 2.268 (s, 6H, –C(CH3)=N–), 1.182 (d, 24H,
–CH(CH3)2). 13C NMR (100 MHz, CDCl3, δ): 166.85, 155.10,
146.46, 136.80, 135.72, 123.52, 122.95, 122.14, 28.25, 22.84,
17.07. IR (KBr, cm–1): 1644 (νC=N), 1454, 1254, 1122, 766. Anal.
1

Xiangyu Jia et al. / Chinese Journal of Catalysis 34 (2013) 1560–1569
Calcd for C33H43N3: C 82.28, H 9.00, N 8.72; Found: C 82.16, H
9.23, N 8.63.
For synthesis of complex 3a, a mixture of 2a (0.25 g, 0.001
For synthesis of complex 3j, a procedure similar to 3a was
adopted using 2j and CoCl2, which gave 3j as a green powder in
72.5% yield (0.35 g). IR (KBr, cm–1): 1641 (νC=N), 1449, 1361,
1189, 1038, 870, 803, 765. Anal. Calcd for C24H32Cl2CoN2: C
60.26, H 6.74, N 5.86; Found: C 60.55, H 6.92, N 5.61.
For synthesis of complex 3k, a procedure similar to 3a was
adopted using 2k and CoCl2, which gave 3k as a green powder
in 81.6% yield (0.44 g). IR (KBr, cm–1): 1638 (νC=N), 1443, 1364,
1211, 1184, 1043, 788, 762. Anal. Calcd for C28H40Cl2CoN2: C
62.92, H 7.54, N 5.24; Found: C 63.18, H 7.76, N 5.03.
For synthesis of complex 4a, a procedure similar to 3a was
adopted using 2,6‐bis[(2,6‐diisopropylphenylimino)ethyl]pyri‐
dine and CoCl2, which gave 4a as a brown powder in 93.7%
yield (0.57 g). IR (KBr, cm–1): 1617, 1467, 1264, 1104, 780.
Anal. Calcd for C33H43Cl2CoN3: C 64.81, H 7.09, N 6.87; Found: C
65.17, H 7.44, N 6.63.
mol) and anhydrous CoCl2 (0.137 g, 0.001 mol) were added to a
flask containing THF (10 ml). The mixture was stirred at room
temperature for 12 h, during which a green suspension was
formed. Diethyl ether was added and the precipitate was col‐
lected by filtration and washed with 4 × 5 ml heptane. The
product (0.21g, 54.3%) was obtained after drying in vacuo at
40 °C. IR (KBr, cm–1): 1644 (νC=N), 1486, 1449, 1363, 1244,
1143, 1074, 846, 814, 765. Anal. Calcd for C16H16Cl2CoN2: C
52.48, H 4.40, N 7.65; Found: C 52.62, H 4.77, N 7.56.
For synthesis of complex 3b, a procedure similar to 3a was
adopted using 2b and CoCl2, which gave 3b as a green powder
in 72.6% yield (0.31 g). IR (KBr, cm–1): 1644 (νC=N), 1509, 1421,
1373, 1246, 1144, 1037, 839, 781, 713. Anal. Calcd for
C18H20Cl2CoN2: C 54.84, H 5.11, N 7.11; Found: C 54.95, H 5.17,
N 6.94.
2.3. 1,3‐Butadiene polymerization
For synthesis of complex 3c, a procedure similar to 3a was
adopted using 2c and CoCl2, which gave 3c as a green powder
in 56.1% yield (0.24 g). IR (KBr, cm–1): 1638 (νC=N), 1507, 1442,
1374, 1171, 1143, 1037, 875, 785, 719. Anal. Calcd for
C18H20Cl2CoN2O2: C 50.73, H 4.73, N 6.57; Found: C 51.06, H
4.82, N 6.51.
For synthesis of complex 3d, a procedure similar to 3a was
adopted using 2d and CoCl2, which gave 3d as a green powder
in 82.4% yield (0.33 g). IR (KBr, cm–1): 1644 (νC=N), 1500, 1423,
1375, 1215, 1142, 1091, 869, 785, 712. Anal. Calcd for
C16H14Cl2CoF2N2: C 47.79, H 3.51, N 6.97; Found: C 48.01, H
3.82, N 6.82.
A typical procedure for the polymerization is as follows. A
toluene solution of 1,3‐butadiene (1 g, 1.85 × 10–2 mol) was
added to a moisture‐free ampoule bottle preloaded with com‐
plex 3a (3.4 mg, 9.25 × 10–6 mol). Then, EASC (1.16 × 10–3
mol/ml, 0.2 ml) was injected to initiate the polymerization at
20 °C. After 30 min, methanol was added to the system to
quench the polymerization. The mixture was poured into a
large quantity of methanol containing 2,6‐di‐tertbutyl‐4‐
methylphenol (1.0%) as a stabilizer. After filtering and drying
under vacuum at 40 °C, polybutadiene was obtained (0.918 g,
91.8%).
For synthesis of complex 3e, a procedure similar to 3a was
adopted using 2e and CoCl2, which gave 3e as a green powder
in 93.7% yield (0.41 g). IR (KBr, cm–1): 1648 (νC=N), 1483, 1421,
1377, 1237, 1143, 1008, 864. Anal. Calcd for C16H14Cl4CoN2: C
44.17, H 3.24, N 6.44; Found: C 44.29, H 3.38, N 6.35.
2.4. X‐ray crystallography
Single crystals of 3i, 3k, and 4a suitable for X‐ray diffraction
(XRD) were obtained by the slow diffusion of diethyl ether into
the THF solution. Data collection was performed at –88 °C on a
Bruker SMART APEX diffractometer with a CCD area detector
using graphite monochromated Mo Kα radiation (λ = 0.071073
nm). The determination of the crystal class and unit cell pa‐
rameters 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 using
the SHELXTL program. Refinement was performed on F2 ani‐
sotropically for all non‐hydrogen atoms by the full matrix least
squares method. The hydrogen atoms were placed at the cal‐
culated positions and were included in the structure calculation
without further refinement of the parameters. The crystallo‐
graphic and refinement data are summarized in Table 1.
For synthesis of complex 3f, a procedure similar to 3a was
adopted using 2f and CoCl2, which gave 3f as a green powder in
76.2% yield (0.30 g). IR (KBr, cm–1): 1637 (νC=N), 1487, 1460,
1373, 1233, 1113, 1047, 866, 813, 712. Anal. Calcd for C18H20‐
Cl2CoN2: C 54.84, H 5.11, N 7.11; Found: C 55.17, H 5.27, N 7.04.
For synthesis of complex 3g, a procedure similar to 3a was
adopted using 2g and CoCl2, which gave 3g as a green powder
in 86.4% yield (0.36 g). IR (KBr, cm–1): 1639 (νC=N), 1482, 1459,
1362, 1243, 1117, 1051, 821, 751. Anal. Calcd for C20H24Cl2Co‐
N2: C 56.89, H 5.73, N 6.63; Found: C 57.12, H 5.88, N 6.47.
For synthesis of complex 3h, a procedure similar to 3a was
adopted using 2h and CoCl2, which gave 3h as a green powder
in 92.6% yield (0.42 g). IR (KBr, cm–1): 1635 (νC=N), 1481, 1444,
1359, 1280, 1116, 1031, 812, 751. Anal. Calcd for
C22H28Cl2CoN2: C 58.68, H 6.27, N 6.22; Found: C 58.96, H 6.60,
N 6.00.
3. Results and discussion
For synthesis of complex 3i, a procedure similar to 3a was
adopted using 2i and CoCl2, which gave 3i as a green powder in
88.9% yield (0.40 g). IR (KBr, cm–1): 1635 (νC=N), 1477, 1437,
1359, 1210, 1122, 1038, 857, 791, 727. Anal. Calcd for
C26H36Cl2CoN2O: C 59.78, H 6.95, N 5.36; Found: C 59.95, H 7.14,
N 5.17.
3.1. Synthesis and characterization of the ligands and Co(II)
complexes
The α‐diimine ligands 2a–2k were synthesized in high
yields by the condensation reaction of 2,3‐butadione with the
corresponding aniline derivatives in the presence of a catalytic

Xiangyu Jia et al. / Chinese Journal of Catalysis 34 (2013) 1560–1569
3.2. X‐ray structures of 3i, 3k, and 4a
Table 1
The molecular structures of 3i, 3k, and 4a are shown in Figs.
1–3. Complex 3i crystallized in the monoclinic space group
P21/c. The Co atom is in a deformed tetrahedron comprising
two Cl atoms and two N atoms of the ligand. The dihedral angle
formed by the planes defined by Cl1–Co1–Cl2 and N1–Co1–N2
is 86.73°. The Cl1–Co1–Cl2 angle is 114.55(3)°, and the
N1–Co1–N2 angle is 79.55(8)°. The Co–Cl bond distances are
0.22140(8) nm and 0.22199(8) nm, and the Co–N bonds are
0.20444(19) nm and 0.20455(19) nm. The [Co1–N1–C1–C2–N2]
plane is nearly planar (maximum deviation from least squares
plane is 0.0044 nm). The angle between the two phenyl groups
is 19.48°, and these groups are perpendicular to the central
plane [Co–N1–C1–C2–N2] with angles of 89.07° and 88.05°,
respectively.
Complex 3k displayed a deformed tetrahedral geometry
around the Co center with the equatorial plane defined by Co
and two Cl atoms. It crystallizes in the orthorhombic space
group Pnma. The deformed tetrahedral site around the Co atom
is characterized by the following distances and angles: Co1–Cl1,
0.22037(10) nm; Co1–Cl2, 0.22159(10) nm; Co1–N1,
0.20560(18) nm; Co1–NlA, 0.20560(18) nm; Cl1–Co1–Cl2,
111.67(4)°; N1–Co1–N1A, 79.34(10)°. Compared with 3i, the
plane containing the Co atom is not well fitted by the least‐
squares planes calculated, with a maximum deviation from the
plane of 0.0094 nm. The angle between the two phenyl groups
is 15.07°, and these groups are nearly perpendicular to the
central plane [Co1–N1–C2–C2A–N1A] with angles of 84.1°.
Complex 4a crystallized in the triclinic space group P1. The
deviation of the Co atom from the N3 plane 0.0507 nm has an
approximate Cs molecular symmetry about a plane bisecting
the central pyridine ring and containing the metal atom and
two halogen atoms. The Co–N(pyridyl) bond distance in 4a is
0.2051(2) nm, which is almost the same as the Co–N bonds in
3i and 3k. Both of the two Co–N(imino) bond distances are
0.2206(2) nm, obviously larger than those of 3i and 3k. The
angle of Cl1–Co–Cl2 is 116.53(3)°. The distances of Co–Cl bonds
(0.22589(8) and 0.22951(8) nm) are longer than those of 3i
(0.22140(8) and 0.22199(8) nm) and 3k (0.2159 and 0.22031
nm).
Crystal data and data collection parameters for the cobalt complexes.
Complex
Formula
Molecular weight
Wavelength (nm)
Crystal system
Space group
a (nm)
b (nm)
c (nm)
α (°)
β (°)
γ (°)
V (nm3)
Z
Dcalcd (mg/m3)
Absorption coefficient
(mm–1)
F (000)
Crystal size (mm)
3i·THF
3k
4a·H2O
C26H36Cl2CoN2O C28H40Cl2CoN2 C33H45Cl2CoN3O
522.40
0.071073
534.45
0.071073
629.55
0.071073
triclinic
P1
monoclinic orthorhombic
P21/c Pnma
1.30523 (9) 1.25882 (9) 0.86539 (8)
1.43998 (10) 2.13733 (16) 0.98630 (9)
1.44271 (10) 1.04173 (8) 2.08347 (18)
90.00
91.059 (1)
90.00
2.7111 (3)
4
90
90
90
84.147 (1)
88.312 (1)
65.634 (1)
1.6113 (3)
2
2.8028 (4)
4
1.267
1.280
1.298
0.85
0.82
0.728
1100
0.31 × 0.25
× 0.14
2.8–25.4
16613
1132
0.28 × 0.19
× 0.10
2.2–21.9
15802
666
0.31 × 0.18
× 0.11
2.6–26.0
8579
θ range (°)
No. of reflcns collected
No. of independent reflcns
No. of data/restraints
/params
Goodness‐of‐fit on F2
R1 (1> 2sigma(1))
wR2
5325
2559
6201
5325/0/297 2559/0/159 6201/0/371
(Rint = 0.034) (Rint = 0.054) (Rint = 0.015)
1.050
0.043
0.108
1.054
0.038
0.091
1.099
0.045
0.125
amount of formic acid (Scheme 1). All the ligands were identi‐
fied by FT‐IR, elemental analysis, and H NMR and 13C NMR.
1
The α‐diimine Co(II) complexes 3a–3k were prepared by the
reaction of the ligands with anhydrous CoCl2 in THF. All the
complexes were isolated as green air‐stable powder in high
yield and high purity by precipitation from THF and washing
with heptane. In the FT‐IR spectra, the C=N groups of the free
ligands 2a–2k displayed stretching frequencies at 1628–1649
cm–1, while the absorption peaks of the C=N groups in com‐
plexes 3a–3k were shifted to lower or higher frequencies, and
the intensities of the corresponding peaks were greatly re‐
duced. This indicated the occurrence of the coordination reac‐
tion between the α‐diimine nitrogen atoms and the metal ions.
2,6‐Bis[(2,6‐diisopropylphenylimino)ethyl]pyridine and its Co
complex 4a were synthesized according to the literature [44].
The structures of complexes 3i, 3k, and 4a were further stud‐
ied by XRD analysis.
3.3. Solution polymerization of 1,3‐butadiene
Co‐based complexes with a variety of ortho‐ or pa‐
ra‐substituted α‐diimine ligands were employed to examine the
NH2
R1
R1
R1
R1
R1
R2
HCOOH
CH3OH
CoCl2
THF
R3
N
N
R3
+
R3
N
N
R3
O
O
Co
R2
R2
R2
R2
R3
Cl
Cl
2a-2k
3a-3k
a: R1 = R2 = R3 = H
e: R1 = R2 = H, R3 = Cl
f: R1 = R3 = H, R2 = CH3
g: R1 = R3 = H, R2 = C2H5
h: R1 = R3 = H, R2 = iPr
i: R1 = R2 = R3 = CH3
b: R1 = R2 = H, R3 = CH3
c: R1 = R2 = H, R3 = OCH3
d: R1 = R2 = H, R3 = F
j: R1 = R2 = C2H5, R3 = H
k: R1 = R2= iPr, R3 = H
Scheme 1. Synthesis of ligands 2a–2k and Co complexes 3a–3k.

Xiangyu Jia et al. / Chinese Journal of Catalysis 34 (2013) 1560–1569
Table 2
Polymerization of 1,3‐butadiene (BD) using (α‐diimine)Co(II) com‐
plex/EASC catalyst.
Microstructure b (%)
cis‐1,4 trans‐1,4 1,2
a
Entry Complex Yield (%) Mn a (104) Mw/Mn
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
3g
3h
3i
91.8
91.8
32.2
95.7
94.0
91.2
91.2
92.7
91.7
93.2
92.6
76.3
6.3
10.5
10.6
38.1
9.0
8.7
9.8
10.2
8.9
9.0
9.4
8.9
3.99
4.08
1.96
4.11
3.50
3.96
3.65
4.18
3.96
3.87
3.98
2.57
—
95.9
96.0
98.0
95.4
95.8
95.3
95.4
95.5
95.3
94.9
95.3
95.9
—
2.1
2.1
1.1
2.4
2.7
2.3
2.3
2.1
2.3
2.4
2.1
2.4
—
2.0
1.9
0.9
2.2
1.5
2.4
2.3
2.4
2.4
2.7
2.6
1.7
—
Fig. 1. ORTEP view of complex 3i drawn at 35% probability. The hy‐
drogen atoms and one THF molecule are omitted for clarity. Selected
bond distances (nm): Co1–N1 0.20444(19), Co1–N2 0.20455(19),
Co1–Cl1 0.22140(8), Co1–Cl2 0.22199(8). Selected bond angles (°):
N1–Co–N2 79.55(8), Cl1–Co–Cl2 114.55(3), N1–Co1–Cl1 113.06(6),
N2–Co1–Cl1 116.73(6), N1–Co1–Cl2 116.27(6), N2–Co1–Cl2 112.20(6).
9
10
11
12
13
3j
3k
4a
CoCl2
14.4
—
Polymerization conditions: [BD] = 1.85 mol/L, [BD]/[Co] = 2000,
[Al]/[Co] = 50, toluene 10 ml, 30 min, 20 °C.
a Determined by GPC.
b Determined by 1H and 13C NMR.
spectively) gave higher polymer yields (95.7% and 94.0%, re‐
spectively) than unsubstituted 3a (91.8%). 3b with a methyl
group displayed the same catalytic activity as 3a. The lowest
catalytic activity was observed in the case of 3c with an elec‐
tron‐donating group (–OCH3), with the polymer yield of 32.2%.
These results can be explained by that an electron‐donating
group reduces the cationic nature of the metal center, which
consequently weakened the coordination of the monomer to
the active center, while an electron‐withdrawing group reduces
the electron density of the metal center, and the increased
Lewis acidity facilitated the coordination between 1,3‐butadi‐
ene and the Co center, leading to an increased chain propaga‐
tion rate [44,49]. Thus, an electron‐withdrawing substituent in
the ligand of the Co complex gives a higher catalytic activity for
1,3‐butadiene polymerization. On the other hand, a complex
with an electron‐donating group exhibited a higher cis‐1,4 ste‐
reoselectivity than its counterpart with an electron‐withdraw‐
ing group. For example, 3c gave polybutadiene with the highest
cis‐1,4 content of 98.0% (Fig. 4), while 3d displayed the lowest
cis‐1,4 stereoselectivity of 95.4%. This result implies that
1,3‐butadiene was coordinated to the metal center with a high
electronic density by the cis‐η4 mode rather than the trans‐η2
mode [34,49], which would result in the formation of the
cis‐1,4‐isomer. In addition, the complex having an elec‐
tron‐donating group 3c afforded polymer with a high molecu‐
lar weight (Mn = 38.1 × 104) and narrow molecular weight dis‐
tribution (Mw/Mn = 1.96), suggesting that remote electron‐do‐
nating substituents can retard the chain transfer reaction to
some extent [44].
Fig. 2. ORTEP view of complex 3k drawn at 35% probability. The hy‐
drogen atoms are omitted for clarity. Selected bond distances (nm):
Co1–N1 0.20560(18), Co1–N2 0.20560(18), Co1–Cl1 0.22037(10),
Co1–Cl2 0.22159(10). Selected bond angles (°): N1–Co–N2 79.34(10),
Cl1–Co–Cl2 111.67(4), N1–Co1–Cl1 113.68(5), N2–Co1–Cl1 117.51(5),
N1–Co1–Cl2 117.51(5), N2–Co1–Cl2 113.68(5).
Fig. 3. ORTEP view of complex 4a drawn at 35% probability. The hy‐
drogen atoms and one H2O molecule are omitted for clarity. Selected
bond distances (nm): Co1–N1 0.2051(2), Co1–N2 0.2206(2), Co1–N3
0.2206(2), Co1–Cl1 0.22589(8), Co1–Cl2 0.22951(8). Selected bond
angles (°): N1–Co–N2 74.04(8), N1–Co1–N3 74.27(8), N2–Co1–N3
141.13(8), Cl1–Co–Cl2 116.53(3), N1–Co1–Cl1 150.31(6), N2–Co1–Cl1
98.25(6), N1–Co1–Cl2 93.16(6), N2–Co1–Cl2 102.58(6), N3–Co1–Cl1
98.20(6), N3–Co1–Cl2 101.06(6).
In the polymerization of 1,3‐diene with transition metal cat‐
alysts, both the electronic effect and steric hindrance of the
substituent on the ligand influence catalytic activity and the
microstructure of the polymer [42–49]. For example, we re‐
cently found that the 2,6‐bis(imino)pyridyl Co(II) complex with
a bulky isopropyl group at the ortho‐position of the phenyl ring
exhibited a lower catalytic activity for 1,3‐butadiene polymeri‐
zation and gave a polymer with a higher molecular weight and
electronic effect and steric hindrance of the ligand on
1,3‐butadiene polymerization. As shown in Table 2, it is clear
that the ligand structure, particularly the substituent at the
para‐position of the iminoaryl ring, influenced the polymeriza‐
tion behavior of 1,3‐butadiene. For example, complexes 3d and
3e with electron‐withdrawing substituents (R3 = F and Cl, re‐

Xiangyu Jia et al. / Chinese Journal of Catalysis 34 (2013) 1560–1569
2
H
1,3
3
H
1
H
1
2
1
2
2
6.0
5.6
CDCl3
5.2
4.8
34
32
30
28
26
CDCl3
(b)
(a)
10
8
6
4
2
0
140
120
100
80
60
40
20


Fig. 4. NMR spectra of polybutadiene obtained with the 3c/EASC system (entry 3 in Table 2). (a) 1H NMR; (b) 13C NMR.
cis‐1,4 content than did the complexes bearing methyl and
ethyl groups [44]. Jie et al. [49] reported that dinuclear Co(II)
complexes (3‐aryliminomethyl‐2‐hydroxybenzaldehyde Co(II))
with an isopropyl group at the ortho‐position of the phenyl ring
gave a higher polymer yield and lower molecular weight poly‐
mer than the complex having a methyl group. Therefore, the
effect of the steric hindrance of substituents (methyl, ethyl, and
isopropyl at 2‐position (3f–3h) and 2,6‐positions (3i–3k) of
iminoaryl ring) on 1,3‐butadiene polymerization was further
investigated in this study (Table 2, entries 6–11). To our sur‐
prise, it was found that the steric hindrance of the substituents
on the iminoaryl rings of α‐diimine hardly influenced the cata‐
lytic behavior of the complexes. All these complexes (3i–3k)
gave polybutadienes in high yields ranging from 91% to 93%
with cis‐1,4 contents of about 95%. This result may be due to
the similar structural parameters of the cobalt complexes giv‐
ing rise to the similar polymerization behavior. For example,
the crystal structures of the two representative complexes 3i
and 3k have almost the same bond distances for Co1–N1,
Co1–N2, Co1–Cl1, and Co1–Cl2 and the same bond angles for
N1–Co–N2 and Cl1–Co–Cl2. Moreover, both the phenyl groups
of 3i and 3k are nearly perpendicular to the central plane
[Co–N1–C1–C2–N2]. It is worth noting that cobalt complexes
with the α‐diimine ligands displayed a higher catalytic activity
than 2,6‐bis[(2,6‐diisopropylphenylimino)ethyl]pyridine CoCl2
(4a), which is possibly due to their more open coordinated
environment. 1,3‐Butadiene polymerization conducted with
the CoCl2/EASC system under identical reaction conditions
produced polybutadiene in a significantly lower yield, showing
the importance of the ligand at the active center (Table 2, entry
13).
The effect of polymerization time on 1,3‐butadiene
polymerization catalyzed by 3e/EASC ([Al]/[Co] = 50) was
examined at 20 °C. The polymer yield versus polymerization
time is depicted in Fig. 5. The polymer yield sharply increased
during the early period (~15 min) of polymerization, and then
it increased monotonically until 60 min, when it attained the
almost complete conversion of 1,3‐butadiene. The plot of
ln[BD]0/[BD]t with polymerization time showed a good linear
relationship, indicating that the polymerization rate is first
order with respect to the monomer. The polymerization results
are shown in Table 3. The Mn of the produced polymer in‐
creased with polymerization time, while the Mw/Mn increased
from 1.93 to 4.57. The polymers produced had high stereoreg‐
ularity throughout the entire polymerization.
The effects of polymerization conditions on catalytic activity
and polymer microstructure were further examined using the
3e/EASC catalyst. The type of cocatalyst, [Al]/[Co] molar ratio,
and polymerization temperature affected the catalytic behavior
of the catalyst. The results are summarized in Table 4. In com‐
100
6
Table 3
5
4
3
2
1
0
80
60
40
20
0
Effect of polymerization time on 1,3‐butadiene polymerization with the
3e/EASC catalyst.
Microstructureb (%)
cis‐1,4 trans‐1,4 1,2
a
Time
(min)
2
5
10
15
20
30
40
Yield
(%)
Mn
(104)
5.7
5.6
6.4
7.7
8.2
8.7
8.0
7.9
a
Entry
Mw/Mn
1
2
3
4
5
6
7
8
14.1
37.6
63.4
79.5
88.4
94.0
97.4
99.6
1.93
2.82
3.34
3.78
3.84
3.51
4.33
4.57
96.5
96.6
95.8
95.3
95.6
95.0
95.3
94.9
1.9
1.3
2.7
2.3
2.2
2.3
2.4
2.8
1.6
2.1
1.5
2.4
2.2
2.7
2.3
2.3
0
10
20
30
40
50
60
Time (min)
60
Fig. 5. Polymer yield and ln([BD]0/[BD]t) versus polymerization time
for 1,3‐butadiene polymerization catalyzed by 3e/EASC catalyst.
Polymerization conditions: [BD] = 1.85 mol/L, [BD]/[Co] = 2000,
[Al]/[Co] = 50, toluene 10 ml, 20 °C.
Polymerization conditions: [BD] = 1.85 mol/L, [BD]/[Co] = 2000,
[Al]/[Co] = 50, toluene 10 ml, 20 °C.
a Determined by GPC.
b Determined by 1H and 13C NMR.

Xiangyu Jia et al. / Chinese Journal of Catalysis 34 (2013) 1560–1569
decrease at even higher temperatures, which is common in late
Table 4
transition metal catalyzed polymerization, the present catalytic
system showed a remarkably high catalytic activity indicated
by the high polymer yield of 96.0% at 70 °C and even a high
yield of 86.7% at 90 °C. The Mn of polybutadienes decreased
from 19.5 × 104 to 2.5 × 104 as the temperature increased from
0 to 90 °C, along with Mw/Mn broadening from 2.42 to 7.59.
These changes are due to the enhancement of chain transfer
reactions with increased polymerization temperature. The
microstructure of the polymers was also substantially affected
by the polymerization temperature. The cis‐1,4 content de‐
creased as the polymerization temperature increased, while the
trans‐1,4 and 1,2 contents increased. The increased trans‐1,4
content can be attributed to facilitated anti‐syn isomerization at
higher temperature [49], while the increase of the 1,2 content
is probably due to that the new coming monomer inserts at the
C3 of the η3‐allyl group formed at the active site more easily at
higher temperature, leading to 1,2‐isomer formation [60].
1,3‐Butadiene polymerization behavior with complex 3e activated with
aluminum cocatalyst.
Microstructureb (%)
cis‐1,4 trans‐1,4 1,2
a
[Al]/
T
Yield Mn Mw/
Entry Cocatalyst
a
[Co] (°C) (%) (104) Mn
1
2
3
4
5
6
7
8
MAO
MAO
MAO
DEAC
EASC
EASC
EASC
EASC
EASC
EASC
EASC
EASC
EASC
EASC
100 20 53.0 11.5 1.70 96.9
150 20 73.0 10.4 1.72 96.3
1.7 1.4
2.0 1.7
2.0 1.8
8.2 5.8
1.8 2.9
2.5 2.3
1.2 2.8
2.1 2.6
1.9 2.4
0.9 0.8
2.7 1.5
6.6 5.2
11.7 8.6
15.2 7.5
200 20 74.2
50 20 75.0
10 20 89.2
20 20 91.3
30 20 92.8
70 20 100
100 20 100
9.6 1.74 96.2
2.2 2.86 86.0
5.6 4.02 95.3
8.1 4.53 95.2
6.6 4.56 96.0
6.9 4.74 95.3
8.7 4.12 95.7
9
10
11
12
13
14
50
0
68.7 19.5 2.42 98.3
50 20 94.0
50 50 100
50 70 96.0
50 90 86.7
8.7 3.51 95.8
4.2 6.47 88.2
2.7 7.18 79.7
2.5 7.59 77.3
Polymerization conditions: [BD] = 1.85 mol/L, [BD]/[Co] = 2000, tolu‐
ene 10 ml, 30 min.
a Determined by GPC.
b Determined by 1H and 13C NMR.
4. Conclusions
A series of Co(II) complexes with α‐diimine (2a–2k) ligands
were synthesized and characterized. The representative com‐
plexes 3i and 3k adopted a deformed tetrahedron configura‐
tion comprising two Cl atoms and two N atoms of the ligand,
which was shown by single crystal X‐ray analysis. After activa‐
tion by EASC, all the complexes 3a–3k gave polybutadienes
with high cis‐1,4 contents (up to 98%) in high polymer yields.
Complexes 3c and 3d with electron‐drawing groups (F and Cl,
respectively) on the iminoaryl ring exhibited a higher catalytic
activity but lower stereoselectivity than 3b and 3c with elec‐
tron‐contributing groups (methyl and methoxyl, respectively).
The steric hindrance of the substituents on the iminoaryl rings
did not influence catalytic activity, or the molecular weight,
molecular weight distribution, and microstructure of the poly‐
mers. The catalytic activity of 3e activated with different alkyl‐
aluminum cocatalysts was in the order EASC > DEAC > MAO. An
increase of the [Al]/[Co] ratio enhanced catalytic activity, while
the cis‐1,4 content of the polymer remained almost unchanged.
The microstructure of the polymers was considerably affected
by the polymerization temperature, and the cis‐1,4 content
decreased as the polymerization temperature increased, while
the trans‐1,4 and 1,2 contents increased.
bination with MAO ([Al]/[Co] = 100–200), 3e gave polybutadi‐
enes having a high cis‐1,4 content (more than 96%), high mo‐
lecular weight (~105), and relatively narrow molecular weight
distribution (Mw/Mn ~1.7) in a moderate polymer yield
(53.0%–74.0%). 3e/DEAC exhibited a slightly higher catalytic
activity (75.0%), while the polymer produced had a lower
cis‐1,4 content of 86.0%. 3e/EASC afforded polymer with the
highest yield of 94.0% and the relatively high cis‐1,4 content of
95.8%. It is worth noting that even at the relatively low
[Al]/[Co] ratio of 10, the polymer yield was 89.2%, which is
much higher than that of the 2,6‐bis(benzimidazol‐2‐yl)‐pyri‐
dine Co(II)/EASC catalyst whose polymer yields were 8.1% and
38.1% even at [Al]/[Co] ratios of 50 and 100, respectively [46].
The polymer yield increased with the [Al]/[Co] ratio, and al‐
most complete conversion was obtained at the [Al]/[Co] ratio
of 70. The influence of the [Al]/[Co] ratio on the molecular
weight and molecular weight distribution of the polymers did
not exhibit an obvious pattern, indicating that the chain trans‐
fer to alkylaluminum was not as significant as in the case of
olefin polymerization [58,59]. The cis‐1,4 contents of the poly‐
mers were more than 95% in the [Al]/[Co] ratio range exam‐
ined.
The polymerization temperature considerably influenced
catalytic activity and the microstructure of the polymers. In‐
terestingly, the 3e/EASC catalyst gave the polymer yield of
68.7% at the relatively low temperature of 0 °C, which is much
higher than those of other Co‐based catalysts reported. For
instance, the 2,6‐bis(benzimidazolyl)pyridines Co(II) and
3‐aryliminomethyl‐2‐hydroxybenzaldehydes Co(II) catalyst
systems only produce trace amounts of polymer [46,49], and
the Co complex bearing an imino‐pyridyl alcohol ligand gave a
low polymer yield of 39.3% at 0 °C [48]. The catalytic activity
increased with the polymerization temperature. The polymer
yield was 100% when the polymerization temperature was
increased to 50 °C. Although the polymerization rate would
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Graphical Abstract
Chin. J. Catal., 2013, 34: 1560–1569 doi: 10.1016/S1872‐2067(12)60625‐1
Highly active and cis‐1,4 selective polymerization of 1,3‐butadiene catalyzed by cobalt(II) complexes bearing α‐diimine ligands
Xiangyu Jia, Heng Liu, Yanming Hu*, Quanquan Dai, Jifu Bi, Chenxi Bai, Xuequan Zhang*
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences; University of Chinese Academy of Sciences;
Dalian University of Technology
A series of cobalt(II) complexes ligated by α‐diimine have been synthesized and characterized. The catalyst system [α‐diimine]CoCl2/
EASC shows high catalytic activity and cis‐1,4 selectivity (up to 98%) for 1,3‐butadiene polymerization.

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