Highly cis-1,4 selective polymerization of 1,3-butadiene with Co(II) complexes bearing N-aryl-phenanthrene-o-iminoquinones

Article abstract of DOI:10.1016/j.poly.2019.05.020

Several chlorine-bridging dinuclear Co(II) complexes bearing the Schiff base ligands N-aryl-phenanthrene-o-iminoquinones (Ar-PICoCl₂) were synthesized and characterized by elemental analyses, infrared analyses and X-ray crystallography. The X-r

Full text of DOI:10.1016/j.poly.2019.05.020

Accepted Manuscript
Highly cis-1,4 Selective Polymerization of 1,3-Butadiene with Co(II) Com-
plexes Bearing N-aryl-Phenanthrene-o-iminoquinones
Bo Gao, Dongni Li, Qian Duan
PII:
S0277-5387(19)30350-X
https://doi.org/10.1016/j.poly.2019.05.020
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Please cite this article as: B. Gao, D. Li, Q. Duan, Highly cis-1,4 Selective Polymerization of 1,3-Butadiene with
Co(II) Complexes Bearing N-aryl-Phenanthrene-o-iminoquinones, Polyhedron (2019), doi: https://doi.org/10.1016/
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Polyhedron XXX (2019) 00–00
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Polyhedron
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Highly cis-1,4 Selective Polymerization of 1,3-Butadiene with Co(II)
Complexes Bearing N-aryl-Phenanthrene-o-iminoquinones
Bo Gao,a Dongni Li,*a, b and Qian Duan*a, c
a School of Materials Science and Engineering, Changchun University of Science and Technology, 7989 Weixing Road, Changchun 130022, China.
bDepartment of Blood Transfusion, China–Japan Union Hospital of Jilin University, Jilin University, 126 Xiantai Street, Changchun, 130033, China.
c Engineering Research Center of Optoelectronic Functional Materials, Ministry of Education, Changchun 130022, China
a r t i c l e i n f o
a b s t r a c t
Several chlorine-bridging dinuclear Co(II) complexes bearing the Schiff base ligands N-aryl-phenanthrene-o-iminoquinones (Ar-
PICoCl2) were synthesized and characterized by elemental analyses, infrared analyses and X-ray crystallography. The X-ray
crystallography analysis of Co(II) complex 2 revealed that the geometry around the cobalt centers is twisted quadrilateral
configuration. Upon activation with MAO, these complexes showed good activities (82.5%-98.3%) for 1,3-butadiene
polymerization affording predominantly cis-1,4 polybutadiene (up to 96.2%). GPC showed that the polybutadiene have relatively
narrow PDI (Mw/Mn = 1.20－2.21).
Article history:
Received xxxxxxxxxxxxxxxx
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Keywords:
Cobalt
© 2019 Elsevier Ltd. All rights reserved.
Complexes
Schiff base
Polymerization
Cis-1,4 -
polybutadiene
1. Introduction
Polybutadiene is one of the polydiene and is the most commonly used
synthetic rubber. [1-3] The microstructure of the polymer plays an important
role in determining performance. The polymerization process of conjugated
dienes has been known for a long time, but a real breakthrough has been
achieved by stereospecific polymerization of conjugated dienes and transition
metal-based coordination catalysts. [4] A homogeneous Ziegler-Natta catalyst
system consisting of various transition metal complexes, such as Ti,[5, 6]
V,[7, 8] Fe,[9, 10] Co,[11-15] Ni,[16] Cr[17-20], and the co-activators
aluminum alkyls or the aluminum alkyl chloride have been extensively
investigated. Cobalt complexes have attracted attention due to their unique
properties as diene polymerization catalysts. Ricci found that the CoCl2
(PR3)2-MAO(R=alkyl) system obtained polymers of different structures
from butadiene (cis-1, 4; cis-1, 4/1, 2 mixed; predominantly 1, 2)
depending on the type of phosphine ligand, this meant that the ligand had
a large influence on the chemical selectivity of the polymerization.[21] He
has also reported on the synthesis of CoCl2(PiPrPh2)2 and its behavior in
the polymerization of butadiene,[22] when used in association with
MAO[23], it was found to be extremely active and stereospecific, giving a
highly syndiotactic 1, 2-polybutadiene. Other papers concerning the
polymerization of 1,3-butadiene with catalysts based on cobalt complexes
with nitrogen ligands have also been reported,[24-25] but an in-depth
investigation on the influence of the ligand nature on chemo- and
stereoselectivity has not been carried out. Schiff base ligands were
important ligands and were widely used in stabilizing metal complexes.
[17, 18, 26-38] To date, only a few metal complexes with phenanthrene-o-
arylene (PI) ligands have been demonstrated.[17,27,31]
Scheme 1. Synthesis route of Co complexes.
In our previous studies, we reported a monoanionic Cr(III) complex produced
by reduction of PIQ ligands with CrCl2. It exhibits moderate activity in the
polymerization of diene cis-1,4 units.[17] In other our recent studies, we
demonstrated that the reaction of PIQ ligands with magnesium produces a free
radical anion- or amide-phenol-type magnesium complex depending on the
ratio of reactants. [27, 31] To the best of our knowledge, cobalt complexes
containing this ligand have not been explored. In the chemical process of
extending PIQ ligands, we report the synthesis and characterization of
cobalt(II) complexes based on N-aryl-phenanthrene-o-imidazolium ligands.
When activated by MAO, these complexes showed good activity in the
polymerization of butadiene to give cis 1,4-polybutadiene.
2.Experimental
2.1. General Considerations
All manipulations involving air and moisture-sensitive compounds were
carried out under an atmosphere of dried and purified nitrogen using standard
Schlenk and vacuum-line techniques. Toluene and hexane were dried over
sodium metal and distilled under nitrogen. Elemental analyses were performed
on a Varian EL microanalyzer; infrared spectra were recorded as KBr disks
with a Nicolet Avatar 360. NMR spectra were performed on Bruker AV
300MHz apparatu at room temperature in CDCl3 solution for ligands and
polymers. The molecular weight and molecular weight distribution of the
polymers were measured by TOSOH HLC 8220 GPC at 40 °C using THF as
eluent against polystyrene standards. The UV-Vis spectra of the samples were
recorded over different irradiation time intervals by using a Thermo Scientific
Evolution 220 spectrophotometer. Aniline, 2,6-dimethylaniline, 2,6-
⇑
Corresponding authors. Phone: +86 043185583105, Fax: +86 043185583105; Email address:
ldndddiana@jlu.edu.cn (D. N. Li); duanqian88@hotmail.com(Q. Duan).
https://doi.org/XXXXXX
XXXXXXXXX-XXXXXX/© 2019 Elsevier Ltd. All rights reserved.

diethylaniline and 2,6-diisopropylaniline were bought from Aldrich
Chemical Co. and used without further purification. The ligands N-phenyl-
molecular structure is shown in Fig. 1, the selected bond distances and angles are
listed in Table 1, and the crystallographic data are summarized in Table S1 in
supporting information. From the Fig. 1, we can see that it is a Cl-bridged
dinuclear complex, in which the surrounding environment of two Co atoms can be
phenanthrene-o-iminoquinones
(N-C6H5-PI),
N-dimethylphenyl-
phenanthrene-o-iminoquinones (N-2,6-Me2C6H3-PI), N-diethylphenyl-
phenanthrene-o-iminoquinones (N-2,6-Et2C6H3-PI), N-diisopropylphenyl-
phenanthrene-o-iminoquinones (N-2,6-iPr2C6H3-PI), and were synthesized
according to the literatures.[17, 18, 27, 31]
described as
a distorted square pyramidal configuration with ligands of
monoimine and Cl atoms.The amount of distortion in a five coordinate metal
complex can be quantified using the geometric calculation, τ = (β-α)/60.[40-42]
As shown in Fig. 2, α (c-e) and β (b-d) are the two largest angles at the metal,
where a lies along the z axis and is not associated with either of the two largest
interligand angles. By convention β is the most obtuse angle. The τ value ranges
from 0 (perfectly square pyramidal) to 1 (perfectly trigonal bipyramidal). In this
case, the τ value is 0.353, suggesting that the geometry of the cobalt centre in 2 is
square pyramid. In complex 2, the Co atoms and the phenanthrene ring are almost
in the same plane (N(1)-Co(1)-O(1)-C(14) has an angle of -18.01(16) ° in the
ligand. The ring phase is vertical. The bond length of Co-Cl bridge is (2.3266(7)–
2.3911(6) Å), while the Co–Cl termial bond length is 2.2498(5)–2.2500(8) Å,
which is slightly shorter than the bond length of Co-Cl bridge. This is close to the
Co-Cl bond length in the complexes, [Py(Bm-R)2]CoCl2 reported by Appukuttan
[43] [Co(1)–Cl(2): 2.283(4) Å, Co(1)–Cl(1): 2.324(3) Å], trident Co complexes
reported by Gong [44] [Co(1)–Cl(1): 2.2680(6) Å, Co(1)–Cl(2): 2.2613 (6) Å],
Co2(PymPz)2Cl4 ,reported by Jana [45] [Co(1) – Cl(1): 2.2825(5) Å],
[(bpma)Co(μ – Cl)Cl]2 reported by Park [46] [Co(1) – Cl(1): 2.2886(2) Å],
hemilabile ligand supported cobalt(II) complex reported by Chen [47] [Co(1)–
Cl(1): 2.2656(9) Å], diisopropylphenyl(pyridyl)imine derived chloro-bridged
dimeric Co(II) complex reported by Murugavel [48] [Co(1)–Cl(1): 2.399(1) Å]
and μ-Cl dicobalt complexes reported by Gao [49] [Co−Cl(2): 2.2761(6) Å]. The
angle of N–Co–O is 75.49(7)°. Unlike the BIP Cr complex [17], the intermediate
complex has a C–C bond length of (1.513(3) Å), a C–N bond length of (1.290(3)
Å), and a C–O bond length of (1.228(3). Å). By carefully analyzing these bond
length data, it is not difficult to find that the PI ligands in the Co complex 2 does
not have the same free radical anion state as the IP ligand in the Cr complex[17],
but a simple coordination function, and Co
2.2 Synthesis of Co(II) complexes (Ar-PI)CoCl(μ-Cl)2ClCo(Ar-PI) 1-4
General process: ligand L1-L4 (1.0 mmol) and anhydrous CoCl2 (0.130 g,
1.0 mmol) were dissolved in 30 mL of dry THF under nitrogen atmosphere.
The mixture was stirred at 60 °C for 2 h, concentrated to 5 mL, and added to
20 mL of hexane. The suspension solution was filtered over a sifting funnel,
and the filter cake was washed twice with 3 mL of dry diethyl ether and dried
in vacuo to give a dark brown powder, yield: 76-87 % .
2.2.1 Co(II) complex
1 yield: 0.314 g, 76%. Anal. Calcd for
C40H26Cl4Co2N2O2: C, 58.14; H, 3.17; N, 3.39; Found: C, 58.16; H, 3.19; N,
3.42 . IR (KBr) ν (cm–1) 3488w, 3181w, 3082w, 1686m, 1660w, 1630m,
1591m, 1555m, 1449m, 1390w, 1363w, 1300w, 1248w, 1231w, 1161w,
1125w, 1020w, 944w, 894w, 773s, 713w, 677w, 617w, 535w, 492w, 436w.
2.2.2 Co(II) complex 2 yield: 0.271 g, 87 %. The crystals of Co(II) complex
2 were cultured in a mixed solvent of hexane and THF, black. Anal. Calcd for
C44H34Cl4Co2N2O2 : C, 59.89; H, 3.88; N, 3.17 ; Found: C, 59.90; H, 3.89; N,
3.18; IR (KBr) ν (cm–1) 3381w, 3070w, 2976w, 2921w, 1677w, 1651m,
1594s, 1549s, 1487w, 1452s, 1386m, 1345s, 1310w, 1300w, 1262w, 1243w,
1172w, 1125w, 1094w, 1039w, 955w, 878w, 796w, 786s, 772s, 722w, 671w,
603w, 565w, 433w.
2.2.3 Co(II) complex
3 yield: 0.366 g, 78%. Anal. Calcd for
C48H42Cl4Co2N2O2 : C, 61.43; H, 4.51; N, 2.98; Found: C, 61.39; H, 4.53; N,
2.97; IR (KBr) ν (cm–1) 3452m, 3067m, 2854w, 2800w, 1675w, 1645s, 1589s,
1448s, 1483w, 1383m, 1318s, 1303m,
C(1)- N(1)
C(1)- C(14)
1.290(3)
Cl(2)- Co(1)
Cl(3)- Co(1)
2.3266(7)
2.2500(8)
2.0694(19)
2.1472(16)
111.6(2)
1290w, 1259w, 1246w, 1187w, 1135w,
1067w, 1018w, 968m, 760s, 718m,
659w, 582w, 563m, 519m, 504m.
is Co (II).
1.513(3)
1.228(3)
1.444(3)
130.5(2)
85.24(5)
75.49(7)
100.67(3)
112.55(15)
0
C(14)- O(1)
Co(1)- N(1)
Table 1. Selected bond length [Å] and
bond angle[°] of complex 2
C(15)- N(1)
Co(1)- O(1)
2.2.4 Co(II) complex 4 yield: 0.294 g,
80%. Anal. Calcd for C52H50Cl4Co2N2O2
: C, 62.79; H, 5.07; N, 2.82; Found: C,
62.70; H, 5.03; N, 2.89; IR (KBr) ν (cm–
1) 3498w, 3167w, 2977s, 2930w, 2858w,
2359w, 1651s, 1599s, 1525m, 1499s,
1454m, 1344m, 1298m, 1260w, 1171w,
1096w, 879w, 784W, 764s, 726m, 671w,
605w, 565w, 512w, 493w.
N(1)-C(1)-C(2)
N(1)-C(1)-C(14)
O(1)-C(14)-C(1)
N(1)-Co(1)-Cl(2)#1
Cl(2)-Co(1)-Cl(2)#1
C(26)-O(2)-C(23)
O(1)- Co(1) Cl(3)
N(1)-Co(1)- O(1)
Cl(3)-Co(1)-Cl(2)#1
C(14)-O(1)-Co(1)
Co(1)#1-Cl(2)-Co(1)-Cl(2)#1
118.1(2)
121.82(6)
89.04(2)
105.5(2)
N(1)-Co(1)-O(1)-C(14) -18.01(16)
2.3 X-ray crystal structural determination of 2
The crystal was mounted on the glass fibers using oil droplets. Data obtained
in the -2 scan mode were collected on a Bruker SMART 1000 CCD
diffractometer with graphite-monochromatic Mo-Kα radiation (λ = 0.71073
Å). A direct method is used to parse the structure, and a further refinement of
the full matrix least squares method on F2 is obtained using the SHELXTL
package [39]. All non-hydrogen atoms are anisotropically refined. The
hydrogen atoms are introduced into the calculation position using the
displacement factor of the host carbon atom.
Fig. 1 Structure of complex 2 (Thermal ellipsoids are drawn at the 30% probability
level). Hydrogen atoms and uncoordinated solvent are omitted for clarity.
2.4 General procedure for butadiene polymerization
In the glove box, toluene (5 mL), butadiene (8 mmol), and MAO (8 mmol)
were added into a 12 mL ampule. Complex 1 (10 μmol, 20 μmol based on Co)
was then added to initiate the polymerization. After 60 min,
methanol was added to the
system
to
quench
the polymerization.
The mixture was poured into a large quantity of methanol containing 2,6-
ditert-butyl-4-methylphenol (1.0 wt%) as a stabilizer to precipitate the white
solids, which were filtered and dried under vacuum at 35 °C for 24 h.
Fig. 2 τ = (β-α)/60.
3. Result and discussion
To further verified that cobalt in complex 2 was Co(II), X-ray photo electron
spectroscopy (XPS) was used to test on it. The XPS pattern of Co 2p in complex
2 showed two peaks at 787 and 804 eV (see Fig. S1), corresponding to the Co
2p3/2 and 2p1/2 peaks of Co(II) species, respectively, which suggested the cobalt
atoms in 2 were indeed in the Co(II) state. [50-52]
The typical absorption spectra for L4 and 4 were recorded in THF. The spectra
are shown in Fig. S2. The absorption spectrum of L4 exhibited three bands at
263, 301 and 370 nm which are assignable to Π-Π* transitions of the benzene
rings, the C=N and the C=O linkage, respectively. Corresponding transitions
undergo a blue shift to 252, 291 and 337 nm for complex 4 as a result of complex
formation.
3.1 Synthesis of the complexes
Reaction of the preformed complexes of ligands (Ar-PI)CoCl(μ-Cl)2ClCo(Ar-
PI) 1-4 (Ar = 2,6-Me2C6H3, 2,6-Et2C6H3, 2,6-iPr2C6H3) with CoCl2 in THF,
after evaporation of the solvent and recrystallization of the residue in
toluene/hexane mixed solution, afforded 1-4 as black solid in good yield
(Scheme 1). These cobalt complexes were characterized by elemental analysis
and infrared characterization. Crystal of 2 suitable for X-ray structure
determination was grown from in a mixed solution of THF and hexane. The

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3.2 Butadiene polymerization
References
All complexes were used to catalyze the polymerization of butadiene.
Aggregate data is listed in Table 2. These catalysts did not exhibit butadiene
polymerization activity when AlR3 was used as a cocatalyst. However, when
these complexes were activated by MAO, they exhibited moderate to high
activity (82.5%-98.3%) at room temperature and the resulting unit was a cis-
1,4 structure-based polybutadiene with relatively narrow PDI (Mw/Mn = 1.20
－2.21, representative GPC profile of polybutadiene was listed in Fig. S4).
The catalytic activity decreased as the volume of the ligand substituent
increased, and the molecular weight of the polymer increased significantly as
the volume of the ligand increased. The reason for the analysis may be that
when MAO was used as a cocatalyst, a complex having a small steric
hindrance had a high activation efficiency, and thus the activity was
relatively high, and the molecular weight of the polymer was low. We can
see that in entries 1-4, the selectivity of the catalyst increased mildly as the
volume of the ligand’s substituent increased. When the catalyst was selected
to be complex 4/MAO, the selectivity was the highest, which was 96.2% (see
Fig. S3, Table 2, entry 4). The volume of the ligand had little effect on
selectivity (92.1%-96.2%). These results obtained by the Co(II) complexes
are close to the previous work which was reported by Appukuttan.[39] The
initial concentration of the cobalt catalyst is reduced by half and a longer
reaction time is required (Table 2, entry 5). Kinetic study using 4/MAO
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Table 2. Polymerization of butadiene under various conditions a
69 (4), (2016) 656.
b
microstructurec (%)c
a
Unless otherwise specified, the
entry Cat.
T
Al/Co time Conv.
Mn
PDI
[30] Z. Wen, D. Li, J. Qi, X. Chen, Y. Jiang, L.
Chen, B. Gao, Y. Cui, Q. Duan, Colloid Polym.
polymerization reactions: toluene (12 mL),
cat (20 μmol based on Co) butadiene 8
℃
min
%
(×10-4)
cis-1,4 trans-1, 4 1, 2
Sci. 293 (12), (2015)3449.
mmol,
Monomer/Co
=
400,
1
2
3
4
5
6
1
2
25 400
25 400
25 400
25 400
60
60
60
60
93.8
90.3
87.2
82.5
1.70
2.24
4.09
7.63
9.02
9.77
2.21
1.51
1.35
1.20
1.32
1.29
92.1
94.9
95.3
96.2
95.8
82.3
5.9
3.3
2.0
1.8
1.5
1.3
1.3
5.7
[31] Y. Liu, Z. Chen, L. Liu, Y. Peng, X.
Hong, B. Yang, H. Liu, H. Liang C. Orvig,
Dalton Trans., 0 (48), (2009) 10813.
[32] B. Gao, R. Duan, X. Pang, X. Li, Z.
Qu, Z. Tang, X. Zhuang, and X. S. Chen,
Organometallics, 32 (19), (2013) 5435.
[33] B. Gao, D. Li, X. Li, R. L. Duan, X.
Pang, Y. Cui, Q. Duan and X. S. Chen,
MAO/Monomer = 1. b Determined by gel
permeation chromatography (GPC) with
respect to
a polystyrene standard. c
3
3.2
Determined by 13C NMR spectrum and IR.
4
4d
2.5
d cat. (10 μmol based on Co).
25 400 360 88.9
50 400 60 98.3
2.9
12.0
4.Conclusion
4
In this work, a series of Co(II)
complexes based on different substituted
monoimine ligands were synthesized. All the
characterized by elemental analysis and infrared characterization, and a
complex was characterized by single crystal structure and XPS. Under
the action of MAO, these complexes catalyzed the polymerization of
butadiene to exhibit different catalytic activities, and obtained
predominantly cis-1,4-polybutadiene.
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Appendix A. Supplementary data
CCDC 1893440 contains the supplementary crystallographic data
for
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
2
.
These data can be obtained free of charge via
from the

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Graphical Abstract
A series of cobalt(II) complexes bearing N-aryl-Phenanthrene-o-iminoquinones have been
prepared and characterized. The catalyst system (Ar-PI)CoCl(μ-Cl)2ClCo(Ar-PI)/MAO shows
good catalytic activity and cis‐ 1,4 selectivity (up
to 96.2%) for 1,3‐ butadiene
polymerization.