Highly active and stereospecific polymerization of 1,3-butadiene catalyzed by dinuclear cobalt(ii) complexes bearing 3-aryliminomethyl-2- hydroxybenzaldehydes

Article abstract of DOI:10.1039/c1dt11073j

A series of cobalt(ii) complexes bearing 3-aryliminomethyl-2- hydroxybenzaldehydes (tridentate [NOO] ligands) was prepared and characterized by FT-IR and elemental analysis along with single-crystal X-ray diffraction. The X-ray diffraction analysis revealed that a dinuclear centrosymmetrical structure formed, in which each cobalt atom is surrounded by two bridged ligands and two acetate groups as a distorted octahedron. These dinuclear cobalt complexes displayed high catalytic activities for the polymerization of 1,3-butdiene on activation with organoaluminum cocatalysts to yield cis-1,4-polybutadiene with high selectivity. Ethylaluminum sesquichloride (EASC) was found to be the most efficient cocatalyst resulting in high conversion of butadiene and cis-1,4 content in the polymers with moderate molecular weight. The high catalytic activity and stereoselectivity could be achieved in a wide range of reaction conditions. All the dinuclear cobalt complexes (C1-C6) yielded predominantly cis-1,4-polybutadienes (> 96%) with negligible amounts of trans-1,4 (< 2.4%) and 1,2-vinyl (< 1.5%) products under the Al/Co molar ratio of 80 at 25 °C. The ligand modification by varying the substituents at the 4-position of phenol and on the imino-N aryl ring showed slight influence on the catalytic activity and microstructure of the resulting polymers.

Full text of DOI:10.1039/c1dt11073j

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PAPER
Highly active and stereospecific polymerization of 1,3-butadiene catalyzed by
dinuclear cobalt(II) complexes bearing 3-aryliminomethyl-2-hydroxyben-
zaldehydes†
Suyun Jie,* Pengfei Ai and Bo-Geng Li
Received 8th June 2011, Accepted 9th August 2011
DOI: 10.1039/c1dt11073j
A series of cobalt(II) complexes bearing 3-aryliminomethyl-2-hydroxybenzaldehydes (tridentate [NOO]
ligands) was prepared and characterized by FT-IR and elemental analysis along with single-crystal
X-ray diffraction. The X-ray diffraction analysis revealed that a dinuclear centrosymmetrical structure
formed, in which each cobalt atom is surrounded by two bridged ligands and two acetate groups as a
distorted octahedron. These dinuclear cobalt complexes displayed high catalytic activities for the
polymerization of 1,3-butdiene on activation with organoaluminum cocatalysts to yield
cis-1,4-polybutadiene with high selectivity. Ethylaluminum sesquichloride (EASC) was found to be the
most efficient cocatalyst resulting in high conversion of butadiene and cis-1,4 content in the polymers
with moderate molecular weight. The high catalytic activity and stereoselectivity could be achieved in a
wide range of reaction conditions. All the dinuclear cobalt complexes (C1–C6) yielded predominantly
cis-1,4-polybutadienes (> 96%) with negligible amounts of trans-1,4 (< 2.4%) and 1,2-vinyl (< 1.5%)
products under the Al/Co molar ratio of 80 at 25 ◦C. The ligand modification by varying the
substituents at the 4-position of phenol and on the imino-N aryl ring showed slight influence on the
catalytic activity and microstructure of the resulting polymers.
combined with alkylphosphines and pyridyl adducts produced
Introduction
predominantly 1,2-PBD.7
The metal-catalyzed stereospecific polymerization of 1,3-
In order to gain better control over catalytic activity, molecular
butadiene is of considerable interest from both an academic
weight and more importantly stereoselectivity of the polymer-
and industrial point of view.1 Among the different isomeric
polybutadienes, cis-1,4-polybutadiene (PBD) has gained much
ization, academic and industrial research has focused on well-
defined organometallic single-site catalysts, mainly based on rare
earth metals and first-row transition metals.1c,1d,8 The discovery
of 2,6-bis(imino)pyridine ligands for iron and cobalt catalysts in
the field of olefin polymerization9 has extended a large number of
tridentate ligands based on either derivatives of bis(imino)pyridine
or designing new organic compounds especially consisting of
heterocycles.10 Although their complexes of transition metals
were extensively studied for the oligomerization and/or poly-
merization of olefins, the cobalt-based complexes bearing these
ligands as catalysts in the polymerization of butadiene are
few in number. For example, four-coordinated (Salen)cobalt(II)
(A, Chart 1) or bis(salicylaldiminate)cobalt(II) complexes
(B, Chart 1) achieved high activity and high cis-1,4 selectivity
in the polymerization of butadiene in combination with MAO or
EASC.11 Tridentate ligands (especially [NNN]) have also featured
in the cobalt-based catalyst systems for the polymerization of
butadiene, mainly producing high cis-1,4-PBDs upon activa-
tion with MAO or EASC, such as bis(imino)pyridine ligands
(C, Chart 1),12 bis(benzimidazolyl)amine ligands (D, Chart 1),13
bis(benzimidazolyl)pyridine ligands (E, Chart 1).14
industrial importance, especially for tire production, due to its
natural-rubber-like properties. The large-scale technical synthesis
of cis-1,4-polybutadiene is currently carried out by solution poly-
merization with organometallic complex catalysts of the Ziegler–
Natta type containing TiC14/I2/Al(i-Bu)3,2 CoCl2/AlEt2Cl,1b,3
Ni(OOCR)2/BF3·OEt2/AlEt3,4 or Nd(OOCR)3/Et3Al2Cl3/Al(i-
Bu)2H5 in aromatic or aliphatic hydrocarbon solvents at 50–
70 ◦C. The cobalt-based catalysts play a very important role in the
butadiene polymerization, depending on the reaction conditions
and catalyst formulation, since they can produce polymers with
different microstructures, including cis-1,4-PBD and syndiotactic
1,2-PBD, which are the only two kinds of polybutadienes produced
on an industrial scale.1a,1d The halides and carboxylates of cobalt
are stereoselective to high cis-1,4-PBD when activated with
methylaluminoxane (MAO).6 However, the cobalt halides when
State Key Laboratory of Chemical Engineering, Department of Chemical
and Biological Engineering, Zhejiang University, Hangzhou, 310027, China.
E-mail: jiesy@zju.edu.cn; Fax: +86 571 87951612; Tel: +86 571 87952631
† CCDC reference number 827834. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt11073j
With the view of exploring new suitable chelate cobalt(II)
complexes effective for the stereospecific polymerization of
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solubility in ethanol. All the complexes were isolated as orange
or reddish orange air-stable powders in good yields and high
purities by adding diethyl ether to the concentrated ethanol
solutions. The structures of these complexes were determined
by FT-IR spectra and elemental analysis. It was unexpected that
all these cobalt complexes possessed dinuclear centrosymmetrical
structure with bridged phenolate ligands and acetate groups, as
was established by single-crystal X-ray diffraction (see below).
In the IR spectra, the stretching vibration bands of C N double
bonds of these cobalt complexes (1618–1621 cm-1) shifted to lower
wave number and the peak intensity greatly reduced, as compared
to the corresponding ligands (1624–1630 cm-1), indicating the
coordination interaction between the imino-N atom and the metal
center; and so did the carbonyl groups from 1672–1686 cm-1
(free ligands) shifting to 1645–1653 cm-1 (cobalt complexes).
The elemental analysis results revealed that the components
of these complexes were in accordance with the formula of
(m-L)2Co2(m-OAc)2. The molecular structure of complex C2 was
further confirmed by the single-crystal X-ray diffraction analysis
(Fig. 1).
Chart
1,3-butadiene
1
Cobalt(II)-based catalysts for the cis-polymerization of
butadiene, we chose 3-aryliminomethyl-2-hydroxybenzaldehydes
for cobalt(II) complexes, two of which have been used as asym-
metric bidentate [NO] ligands for neutral arylnickel(II) phosphine
complexes, showing high activities for ethylene oligomerization in
conjunction with MAO.15 Herein we are reporting the synthesis
and characterization of dinuclear cobalt(II) complexes bearing
3-aryliminomethyl-2-hydroxybenzaldehydes as tridentate [NOO]
ligands and their application in the stereospecific polymerization
of 1,3-butadiene to produce high contents of cis-1,4-PBD by using
low amounts of EASC as cocatalyst.
Results and discussion
Synthesis and characterization
The starting materials, 2,6-diformylphenols, were prepared
from the corresponding 4-substituted phenols via the Dull
reaction.16 The titled 3-aryliminomethyl-2-hydroxybenzaldehyde
ligands L1H–L6H were then synthesized as yellow or orange
solids in moderate yields through the Schiff-base condensation
reaction of 4-substituted 2,6-diformylphenols with one equivalent
of appropriate aniline in the presence of a catalytic amount of
toluene-p-sulfonic acid (p-TsOH) in refluxing ethanol (Scheme 1).
All the ligands were identified on the basis of FT-IR, elemental
1
analysis, H and 13CNMR spectra, and the results of L1H and
L2H were compared with the reference data.15
Fig. 1 ORTEP drawing of complex C2 with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
Crystal structure of complex C2
Single crystals of complex C2 suitable for X-ray diffraction
analysis were obtained by slow diffusion of n-hexane into its
dichloromethane solution. Analysis of X-ray crystallography
revealed that complex C2 displays a dinuclear centrosymmetrical
structure as shown in Fig. 1, in which each cobalt metal center is
coordinated to the [N,O] atoms of one ligand, the [O,O] atoms
of the other ligand, and two oxygen atoms from two bridged
acetate groups forming a distorted octahedron. The phenoxy-O
atoms of two ligands bridge the two cobalt atoms in such a way
that a planar Co2(m-O)2 core forms and the two m-acetate groups
symmetrically locate in either side of this plane, respectively. The
dihedral angles between Co2(m-O)2 plane and phenoxy plane, N-
aryl ring are 153.3◦ and 72.0◦, respectively, while the phenoxy
plane and N-aryl ring are nearly perpendicular with the dihedral
angle of 96.8◦.
Scheme 1 Synthesis of ligands L1H–L6H and cobalt complexes C1–C6.
The cobalt complexes C1–C6 were readily prepared by com-
bining ethanol solutions of Co(OAc)2·4H2O and one equivalent
of the corresponding ligand at room temperature (Schemes 1).
Except for complex C4, the resulting cobalt complexes had good
10976 | Dalton Trans., 2011, 40, 10975–10982
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◦
˚
Table 1 Selected bond lengths [A] and angles [ ] of complex C2
of butadiene were firstly investigated with complex C2 and
the results are summarized in Table 2. The C2/MAO catalytic
system showed relatively lower conversion of butadiene (25.4%
in 30 min, entry 1 in Table 2) under the Al/Co molar ratio of
100 with 5.0 mmol of precatalyst load. When both precatalyst and
butadiene concentrations increased, much higher conversions of
butadiene were obtained (entries 2 and 3 in Table 2); however, the
polybutadienes produced showed similar molecular weights and
microstructures with high contents of the cis-1,4 isomer (ca. 94%).
The conversion of butadiene with the C2/AlEt2Cl system reached
96.4% in 60 min and the cis-1,4 content (92.8%) and molecular
weight of the polymers were relatively low when compared to those
obtained by other cocatalysts (entry 4 in Table 2). Ethylaluminum
sesquichloride (EASC) was found to afford high catalytic activity
andthe highest cis-1,4 content in the resulting polymers by using 40
equivalents of EASC to precatalyst. The higher molecular weight
was also achieved by the C2/EASC system (entry 5 in Table 2).
Therefore, EASC was employed for the following investigations
under various reaction parameters.
With the C2/EASC catalytic system, a series of reaction
parameters such as Al/Co molar ratio, reaction temperature,
and reaction time were varied in order to investigate the effects
of polymerization conditions on the catalytic activities and
polymer microstructure. Although no conspicuous variation of
microstructure was observed according to the Al/Co molar ratio,
the conversion of butadiene changed greatly. The conversion of
butadiene increased considerably as the increase of Al/Co molar
ratio from 10 to 80 (entries 1–5 in Table 3) and a complete
consumption of butadiene was observed under the Al/Co molar
ratio of 80, as usually observed in transition metal catalyzed olefin
polymerizations in combined with alkylaluminum cocatalysts.
However, the influence of Al/Co molar ratio on the molecular
weight and molecular weight distribution of polymers didn’t
exhibit an obvious regularity, from which we could infer that the
chain transfer to aluminum was not as significant for butadiene
polymerization as in the case of olefin polymerization.20
Bond lengths
Bond angles
Co–O1
2.078(5)
2.096(5)
2.095(6)
2.068(6)
2.048(6)
2.085(6)
1.274(9)
1.236(10)
1.268(9)
1.269(10)
2.888(2)
O1–Co–O1A
O1–Co–O2A
O1–Co–O4A
O1–Co–O3
O1–Co–N1
O3–Co–N1
O3–Co–O1A
O3–Co–O2A
O3–Co–O4A
O1A–Co–N1
O1A–Co–O2A
O1A–Co–O4A
O2A–Co–N1
O2A–Co–O4A
O4A–Co–N1
O3–C19–O4
92.4(2)
171.5(3)
83.7(2)
85.2(2)
85.4(2)
93.2(2)
82.0(2)
101.5(2)
162.2(2)
174.9(2)
83.5(2)
84.6(2)
99.2(2)
88.5(3)
99.8(2)
125.3(8)
Co–O1A
Co–O2A
Co–O3
Co–O4A
Co–N1
C8–N1
C1–O2
C19–O3
C19–O4
Co ◊ ◊ ◊ CoA
Around each cobalt center, an equatorial plane is composed by
O1, O2A, O3, and O4A atoms and the two axial bonds nearly form
a straight angle (O1A–Co–N1, 174.9(2)◦). All the bond angles
in the equatorial plane are close to a right angle (O1–Co–O4A,
83.7(2)◦; O1–◦Co–O3, 85.2(2)◦; O3–Co–O2A, 101.5(2)◦; O2A–Co–
˚
O4A, 88.5(3) ) and the central cobalt atom deviates by 0.1666 A
from this plane (Table 1). The Co–O3 and Co–O4A bond lengths
between cobalt atom and two acetate ligands from both sides of
˚
˚
Co2(m-O)2 core are 2.068(6) A and 2.048(6) A, respectively. Both
Co–N1 and Co–O1(A) bond lengths between cobalt atom and
phenolate ligand are slightly longer than those in the neutral
arylnickel(II) phosphine complexes with the same ligand,15 or
those in the 2,6-bis(imino)phenoxy cobalt complexes.17 The imino
C8–N1 bond length (1.274(9) A) is consistent with the typical
characters of C N double bond although it is relatively shorter
than those in the literature.15,17 The carbonyl C1–O2 bond length
˚
˚
is 1.236(10) A and slightly longer than the non-coordinated C
bond (1.212(4) A in arylnickel(II) phosphine complexes). The
two C–O distances in the electron-delocalized acetate groups are
O
15
˚
The butadiene polymerization behavior with the C2/EASC
catalytic system was highly sensitive to reaction temperature. The
catalytic system was nearly inactive at 0 ◦C and the optimum
activity was gained at 25 ◦C with the complete conversion of
butadiene (entries 5 and 6 in Table 3). The C2/EASC catalytic
system showed remarkable thermal stability affording the 81.8%
conversion of butadiene even at 90 ◦C (entries 7–9 in Table 3),
whereas the decrease in polymerization rate at the elevated reaction
temperature was common in olefin polymerization catalyzed by
˚
˚
almost equal (1.268(9) A and 1.269(10) A). The Co–Co distance of
18
˚
2.888(2) A is longer than those found in some dicobalt complexes
and carbonyl cobalt compounds.19
Solution polymerization of butadiene
The solution polymerization of 1,3-butadiene was carried out
in toluene under various reaction conditions. The effects of
different type and amount of cocatalysts on the polymerization
Table 2 Effects of different cocatalysts on the polymerization of 1,3-butadiene with complex C2a
Microstructuree (mol%)
c
d
d
Entry
Cocat.
Al/Co
t (min)
Conv (%)
Mcalcd (104 g mol-1)
Mn (104 g mol-1)
Mw/Mn
cis-1,4-
trans-1,4-
1,2-
1
MAO
MAO
MAO
AlEt2Cl
EASC
100
100
100
40
30
60
120
60
60
25.4
63.3
72.4
96.4
90.7
2.75
6.85
7.83
10.4
9.81
15.1
17.5
15.4
9.45
23.6
1.69
1.95
2.05
2.92
2.39
94.9
94.3
93.6
92.3
96.6
3.2
3.5
4.2
5.3
2.2
1.9
2.2
2.2
2.4
1.2
2b
3b
4
5
40
a Polymerization conditions: precatalyst: 5.0 mmol, solvent: 20 mL of toluene, [BD]/[Co]: 2000, reaction temperature: 25 ◦C. MAO: methylaluminoxane;
EASC: ethylaluminum sesquichloride. b precatalyst: 10.0 mmol, solvent: 30 mL of toluene, [BD]/[Co]: 2000. c Mcalcd = 2000 ¥ 54.09 ¥ X (X = conversion).
d Determined by GPC against polystyrene standards and reported uncorrected. e Determined by 1H and 13C NMR spectroscopies.
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Table 3 Effects of various reaction parameters on the polymerization of 1,3-butadiene with complex C2/EASCa
Microstructured (mol%)
T/◦C
t (min)
Conv (%)
Mcalcd (104 g mol-1)
Mn (104 g mol-1)
Mw/Mn
cis-1,4-
trans-1,4-
1,2-
b
c
c
Entry
Al/Co
1
2
3
4
5
6
7
8
10
20
40
60
80
80
80
80
80
80
80
80
80
25
25
25
25
25
0
50
70
90
25
25
25
25
60
60
60
60
60
60
60
60
60
10
20
30
40
12.2
83.2
90.7
96.5
100
trace
97.4
95.1
81.8
63.4
84.8
92.9
96.5
1.32
9.00
9.81
10.4
10.8
—
10.5
10.3
8.85
6.86
9.17
10.0
10.4
29.8
16.7
23.6
17.8
20.5
—
14.3
9.46
10.6
25.8
20.9
23.0
23.9
2.00
2.84
2.39
3.18
2.39
—
3.13
3.84
3.00
1.42
2.16
2.19
2.54
97.1
97.1
96.6
95.1
95.0
—
92.1
85.7
83.6
97.5
96.9
96.6
96.5
0
2.9
1.2
1.2
1.7
1.4
—
1.8
2.8
3.4
1.0
1.1
1.5
1.2
1.7
2.2
3.2
3.6
—
6.1
11.5
13.0
1.5
2.0
1.9
2.3
9
10
11
12
13
b
a Polymerization conditions: precatalyst: 5.0 mmol, cocatalyst: EASC, solvent: 20 mL of toluene, [BD]/[Co]: 2000. Mcalcd = 2000 ¥ 54.09 ¥ X (X =
conversion). c Determined by GPC against polystyrene standards and reported uncorrected. d Determined by 1H and 13C NMR spectroscopies.
late transition metal catalysts.10 The molecular weights of PBDs
decreased and the molecular weight distributions became broader
at the elevated reaction temperature. The most distinguished
feature was the obvious change of polymer microstructure induced
by the variation of reaction temperature. The trans-1,4 content
of PBDs increased gradually along with the elevation of reaction
temperature, producing 13.0% content of trans-1,4 isomer at 90 ◦C
and cis-1,4 content reducing to 83.6%; however, the content of
1,2-inserted isomer had a slight increase from 1.4% to 3.4%. The
formation of increased trans-1,4 content at elevated temperature
was probably ascribed to the facilitated anti-syn isomerization
because the higher reaction temperature might supply the needed
energy requirement.21
The plots of conversion of butadiene and ln([BD]0/[BD]t) vs. the
reaction time initiated by C2/EASC system are depicted in Fig.
2, where [BD]0 and [BD]t are the initial monomer concentration
and the monomer concentration at time t, respectively. A good
linear relationship between ln([BD]0/[BD]t) and reaction time was
observed from Fig. 2, indicating that first-order dependency of
polymerization rate on monomer concentration. The molecular
weights and molecular weight distributions tended to increase with
reaction time up to 10–40 min. The formation of polymers with
similar stereoregularity indicated the identical nature of the active
center.
The dinuclear cobalt complexes (C1–C6) with different lig-
and environments were applied in butadiene polymerization for
examining steric and electronic effects on catalytic activity and
properties of resulting polymers (Table 4). When comparing the
complexes bearing the same substituents on the imino-N aryl ring,
complexes C1 and C2 with the electron-drawing Cl group at the
4-position of phenol (R = Cl) had slightly higher conversions
than the corresponding complexes C3, C4 (R = Me) and C5,
C6 (R = t-Bu) with the electron-donating methyl and tert-butyl
groups, respectively; furthermore, the bulkier tert-butyl group also
resulted in a little higher conversion than the methyl group (that
is, conversion orders: C1 > C5 > C3; C2 > C6 > C4). Concerning
the complexes with the same R group at the 4-position of the
phenol, the Cl-substituted complexes C1 and C2 obtained similar
conversion, whereas the molecular weight of resulting polymers
had large difference and the bulkier isopropyl groups on the
imino-N aryl ring led to lower molecular weight (Mn = 155 kg
mol-1) than methyl groups (Mn = 230 kg mol-1). As for the Me-
substituted complexes C3 and C4, the bulkier isopropyl groups
yielded relatively higher conversion and lower molecule weight
of PBDs than methyl groups. The t-Bu-substituted complexes C5
with bulkier isopropyl groups also had higher conversion than C6
with methyl groups; however, they produced the PBDs with the
same molecular weight. The PBDs produced by complexes C1–
C6 had very high cis-1,4 contents (96.4%–98.0%) and relatively
narrow molecular weight distributions (2.19–2.70) albeit with their
different molecular weights.
The microstructure of the resulting polymers was analyzed by
FT-IR and NMR spectra and found to be predominantly cis-
1,4-PBD together with small amounts of trans-1,4 and 1,2-vinyl
isomers by all the cobalt complexes. Characterized by FT-IR
spectra recorded in the range 4000–400 cm-1, the polymers can
be confirmed to be mainly cis-1,4-PBD from the characteristic
vibration absorption bands at 740 cm-1 of C–H bonds.22 The
1H and 13C NMR spectra of PBDs obtained by the C1/EASC
Fig. 2 The plots of conversion (%) of BD (᭡) and ln([BD]0/[BD]t) (ꢀ)
against reaction time for BD polymerization with C2/EASC system.
Polymerization conditions: precatalyst: 5.0 mmol, [Al]/[Co]: 80, solvent:
20 mL of toluene, [BD]/[Co]: 2000, reaction temperature: 25 ◦C.
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Table 4 Polymerization of 1,3-butadiene with complexes C1–C6/EASCa
Microstructured (mol%)
Mcalcd (104 g mol-1)
Mn (104 g mol-1)
Mw/Mn
cis-1,4-
trans-1,4-
1,2-
b
c
c
Entry
Cat.
Conv (%)
1
2
3
4
C1
C2
C3
C4
C5
C6
C1
C1
92.1
92.9
89.5
86.8
90.4
88.6
98.4
98.2
9.96
10.0
9.68
9.39
9.78
9.58
10.6
10.6
15.5
23.0
21.7
24.4
20.0
20.0
15.1
13.4
2.70
2.19
2.42
2.60
2.28
2.50
2.67
2.62
96.4
96.4
97.3
98.0
97.2
97.1
95.8
94.9
2.4
2.1
1.6
0.8
1.7
1.8
2.9
3.0
1.2
1.5
1.1
1.2
1.1
1.1
1.3
2.1
5
6
7e
8f
a Polymerization conditions: precatalyst: 5.0 mmol, cocatalyst: EASC, [Al]/[Co]: 80, solvent: 20 mL of toluene, [BD]/[Co]: 2000, reaction temperature:
25 ◦C; reaction time: 30 min. b Mcalcd = 2000 ¥ 54.09 ¥ X (X = conversion). c Determined by GPC against polystyrene standards and reported uncorrected.
d Determined by 1H and 13C NMR spectroscopies. e Addition of 0.5 equiv. of PPh3/Co. f Addition of 1.0 equiv. of PPh3/Co.
system are shown in Fig. 3 as a representative example, and the
assignments and contents of isomers were determined according
Conclusions
to the literature.23 The 13C NMR spectra further demonstrated
that cis-1,4-PBD was predominant in the polymers, and the single
peaks at d 129.6 and 27.4 ppm corresponded to CH- and -CH2-
carbon atoms in cis-1,4-PBD, respectively. In all the cases at 25 ◦C,
the molecular weight of the resulting PBDs was higher than the
theoretic values, which probably attribute to the rapid propagation
rather than initiation, leading to the reduction of catalyst efficiency
(Mcalcd/Mmeasd).24
The addition of phosphines to butadiene polymerization reac-
tions catalyzed by Ziegler–Natta type of cobalt-based catalytic
systems had been previously shown to increase the 1,2-vinyl
content in the resulting polymers,25,26 which was also proved
by bis(benzimidazole)-based cobalt catalysts.13b,13c Therefore, the
butadiene polymerizations were carried out in the presence of 0.5
and 1.0 equivalent of PPh3/Co by using the C1/EASC system. As
shown in Table 4 (entries 7 and 8), the addition of PPh3 resulted in
the increased conversions of butadiene. The molecular weights
and molecular weight distributions of the resulting polymers
were slightly lower than those obtained in the PPh3-free system,
indicating that PPh3 induced the chain transfer reactions. However,
unexpectedly, the microstructure didn’t show an important change
and 1,2-vinyl content was still present in negligible amounts.
In summary, a series of dinuclear cobalt(II) complexes bearing 3-
aryliminomethyl-2-hydroxybenzaldehyde ligands was synthesized
and characterized. Single crystal X-ray analysis revealed that
a dinuclear centrosymmetrical structure formed, in which each
cobalt atom is surrounded by two bridged ligands and acetate
groups. Upon activation with EASC, all the complexes could
catalyze the polymerization of 1,3-butdiene with high activity
to give predominantly cis-1,4-polybutadiene with high selectivity.
The increase of Al/Co molar ratio enhanced the catalytic activity
but only slightly decreased the cis-1,4 content in the polymers.
The catalytic system was rather thermally stable at 25–90 ◦C.
However, the reaction temperature could alter the microstructure
of resulting polybutadienes importantly. The ligand environment
was not found to significantly influence the catalytic activity and
the microstructure of resulting polymers. The addition of PPh3
could improve the conversion of butadiene in some extent, but it
didn’t greatly increase the 1,2-vinyl content of polymers produced
by the current system.
Experimental
All manipulations of air- or moisture-sensitive compounds were
carried out under an atmosphere of argon using standard Schlenk
Fig. 3 NMR spectra of the polybutadiene obtained by C1/EASC system (entry 1 in Table 4). (a) 1H NMR; (b) 13C NMR; *, CDCl3.
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techniques. IR spectra were recorded on a Perkin–Elmer FT-
IR 2000 spectrometer by using KBr disks or liquid films for
Ar–H), 6.94 (s, 2 H, Ar–H), 2.36 (s, 3 H, Ar–CH3), 2.31 (s, 3 H,
Ar–CH3), 2.19 (s, 3 H, Ar–CH3). 13C NMR (125 MHz, CDCl3), d
(ppm): 189.4 (CH O), 166.0 (CH N), 162.4 (aromatic-C–OH),
144.9, 138.5, 135.0, 132.1, 129.1, 128.2, 128.1, 124.0, and 119.9
(aromatic-C), 19.8, 20.1, and 18.4 (Ar–CH3). Anal. Calcd. for
C18H19NO2 (281.35): C, 76.84; H, 6.81; N, 4.98. Found: C, 76.48;
H, 6.82; N, 4.84.
1
polybutadienes in the range 4000–400 cm-1. H NMR and 13C
NMR spectra were recorded on a Bruker DMX-400 instrument
in CDCl3 with TMS as the internal standard. Elemental analysis
was performed on a Flash EA1112 microanalyzer. The molecular
weight and molecular weight distribution of polybutadienes were
measured by GPC using Waters 2414 series system in THF at
25 ◦C calibrated with polystyrene standards.
Synthesis of 3-(2,6-diisopropylphenylimino)methyl-2-hydroxy-5-
Toluene was refluxed over sodium-benzophenone and distilled
under nitrogen prior to use. Methylaluminoxane (MAO, 1.46 M
in toluene) was purchased from Akzo Nobel Corp. Diethyl-
aluminium chloride (0.9 M in toluene), ethylaluminum sesqui-
choride (0.4 M in hexane) and all the anilines were pur-
chased from Acros Chemicals and used directly without fur-
ther purification. Polymerization grade butadiene was purified
by passing it through columns of KOH and molecular sieves.
All other chemicals were obtained commercially and used
without further purification unless otherwise stated. The com-
pounds, 4-methyl-2,6-diformylphenol and 4-chloro-2,6-difor-
mylphenol, 4-tert-butyl-2,6-diformylphenol,16,27 and the ligands,
3-(2,6-Diisopropylphenylimino)methyl-5-chloro-2-hydroxybenz-
aldehyde (L1H) and 3-(2,4,6-Trimethylphenylimino)methyl-5-
chloro-2-hydroxybenzaldehyde (L2H)15 were prepared according
to the literature.
tert-butylbenzaldehyde (L5H)
In a manner similar to that described for L3H, the ligand L5H
was prepared as a yellow solid in 55% yield. FT-IR (KBr disk,
cm-1): 2962, 2928, 2869, 1686 (nC O), 1627 (nC N), 1596, 1585,
1463, 1398, 1364, 1307, 1264, 1236, 1223, 1180, 1099, 993, 863,
755, 726, 578, 498. 1H NMR (400 MHz, CDCl3), d (ppm): 13.94
(s, 1 H, Ar–OH), 10.60 (s, 1 H, CH O), 8.38 (s, 1 H, CH N),
8.02 (s, 1 H, Ar–H), 7.62 (s, 1 H, Ar–H), 7.21 (s, 3 H, Ar–H), 2.99
(sept, 2 H, J = 6.8 Hz, CH(CH3)2), 1.37 (s, 9 H, C(CH3)3), 1.20
(d, 12 H, J = 6.8 Hz, CH(CH3)2). 13C NMR (125 MHz, CDCl3), d
(ppm): 189.5 (CH O), 166.3 (CH N), 162.2 (aromatic-C–OH),
145.4, 141.9, 138.6, 135.1, 129.1, 125.8, 123.8, 123.3, and 119.5
(aromatic-C), 34.2 (C(CH3)3), 31.2 (C(CH3)3), 28.2 (CH(CH3)2),
23.5 (CH(CH3)2). Anal. Calcd. for C24H31NO2 (365.51): C, 78.86;
H, 8.55; N, 3.83. Found: C, 78.73; H, 8.51; N, 3.84.
Synthesis of 3-(2,6-diisopropylphenylimino)methyl-2-hydroxy-5-
methylbenzaldehyde (L3H)
Synthesis of 3-(2,4,6-trimethylphenylimino)methyl-2-hydroxy-5-
tert-butylbenzaldehyde (L6H)
To a solution of 4-methyl-2,6-diformylphenol (0.328 g, 2.0 mmol)
and a few drops of glacial acetic acid in absolute ethanol (20 mL),
2,6-diisopropylaniline (0.337 g, 1.9 mmol) in absolute ethanol
(10 mL) was added dropwise. The reaction mixture was stirred
at 50 ◦C for 24 h. After the evaporation of ethanol, the resulting
mixture was purified by column chromatography on silica gel using
petroleum ether/ethyl acetate (40/1) as an eluent. The ligand L3H
was obtained as a yellow solid in 48% yield (0.312 g). FT-IR (KBr
disk, cm-1): 2961, 2922, 2865, 1680 (nC O), 1630 (nC N), 1590, 1465,
1402, 1361, 1306, 1233, 1180, 1027, 969, 861, 794, 756, 740, 684,
In a manner similar to that described for L3H, the ligand L6H
was prepared as a yellow solid in 60% yield. FT-IR (KBr disk,
cm-1): 2955, 2915, 2866, 1676 (nC O), 1628 (nC N), 1591, 1464,
1407, 1365, 1308, 1264, 1228, 1203, 1145, 1122, 1034, 995, 852,
1
844, 794, 758, 636, 589, 580. H NMR (400 MHz, CDCl3), d
(ppm): 14.09 (s, 1 H, Ar–OH), 10.60 (s, 1 H, CH O), 8.40 (s, 1
H, CH N), 8.00 (s, 1 H, Ar–H), 7.59 (s, 1 H, Ar–H), 6.94 (s,
2 H, Ar–H), 2.31 (s, 3 H, Ar–CH3), 2.20 (s, 6 H, Ar–CH3), 1.36
(s, 9 H, C(CH3)3). 13C NMR (100 MHz, CDCl3), d (ppm): 189.5
(CH O), 166.3 (CH N), 162.4 (aromatic-C–OH), 144.9, 141.7,
135.0, 134.9, 129.1, 128.7, 128.2, 123.8, and 119.6 (aromatic-C),
34.2 (C(CH3)3), 31.2 (C(CH3)3), 20.8 (Ar–CH3), 18.4 (Ar–CH3).
Anal. Calcd. for C21H25NO2 (323.43): C, 77.98; H, 7.79; N, 4.33.
Found: C, 79.68; H, 7.81; N, 4.38.
1
582, 463. H NMR (400 MHz, CDCl3), d (ppm): 13.82 (s, 1 H,
Ar–OH), 10.57 (s, 1 H, CH O), 8.33 (s,1 H, CH N), 7.79 (s, 1
H, Ar–H), 7.43 (s, 1 H, Ar–H), 7.21 (s, 3 H, Ar–H), 2.98 (sept, 2
H, J = 6.8 Hz, CH(CH3)2), 2.38 (s, 3 H, Ar–CH3), 1.20 (d, 12 H,
J = 6.8 Hz, CH(CH3)2). 13C NMR (125 MHz, CDCl3), d (ppm):
189.4 (CH O), 165.9 (CH N), 162.2 (aromatic-C–OH), 145.6,
138.6, 132.4, 128.3, 125.8, 124.0, 123.4, and 119.8 (aromatic-C),
28.2 (CH(CH3)2), 23.5 (CH(CH3)2), 20.2 (Ar–CH3). Anal. Calcd.
for C21H25NO2 (323.43): C, 77.98; H, 7.79; N, 4.33. Found: C,
78.42; H, 7.49; N, 4.58.
Synthesis of cobalt(II) complexes C1–C6
To a stirred solution of the ligand (0.5 mmol) at room temperature,
an ethanol solution containing one equiv. of Co(OAc)2·4H2O
(0.5 mmol) was added. The reaction mixture was stirred at room
temperature for 6 h. Thereafter the solution was concentrated to
ca. 3 mL and diethyl ether was added. The resulting precipitate
was filtered, washed with diethyl ether and dried in vacuo to afford
an orange or reddish orange powder. All the cobalt complexes
were prepared in good yield in this manner.
Complex C1. Obtained as an orange powder in 85% yield. FT-
IR (KBr disk, cm-1): 2966, 2925, 2867, 1652, 1620, 1479, 1534,
1643, 1441, 1410, 1384, 1335, 1320, 1218, 1178, 1032, 786, 777,
716, 662. Anal. Calcd. For C44H48Cl2Co2N2O8 (921.63): C, 57.34;
H, 5.25; N, 3.04. Found: C, 56.99; H, 5.30; N, 2.98.
Synthesis of 3-(2,4,6-trimethylphenylimino)methyl-2-hydroxy-5-
methylbenzaldehyde (L4H)
In a manner similar to that described for L3H, the ligand L4H was
prepared as a yellow solid in 56% yield. FT-IR (KBr disk, cm-1):
2965, 2915, 2856, 1672 (nC O), 1625 (nC N), 1592, 1466, 1386, 1307,
1
1261, 1202, 1142, 1092, 1025, 967, 858, 803, 625, 586. H NMR
(400 MHz, CDCl3), d (ppm): 14.02 (s, 1 H, Ar–OH), 10.56 (s, 1 H,
CH O), 8.36 (s,1 H, CH N), 7.76 (s, 1 H, Ar–H), 7.40 (s, 1 H,
10980 | Dalton Trans., 2011, 40, 10975–10982
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Complex C2. Obtained as an orange powder in 78% yield. FT-
IR (KBr disk, cm-1): 2973, 2919, 1653, 1621, 1565, 1531, 1481,
1444, 1421, 1200, 1144, 1028, 860, 786, 775, 665. Anal. Calcd. For
C38H36Cl2Co2N2O8 (837.47): C, 54.50; H, 4.33; N, 3.34. Found: C,
54.09; H, 4.51; N, 3.21.
Complex C3. Obtained as an orange powder in 67% yield. FT-IR
(KBr disk, cm-1): 2965, 2924, 2865, 1647, 1618, 1600, 1588, 1572,
1533, 1465, 1409, 1360, 1334, 1229, 1178, 1049, 997, 876, 829, 802,
775, 717, 662. Anal. Calcd. For C46H54Co2N2O8 (880.80): C, 62.73;
H, 6.18; N, 3.18. Found: C, 62.45; H, 6.24; N, 3.03.
reaction was carried out by vigorous stirring of the reaction
mixture at the various temperature. After the polymerization, the
resulting solution was poured into a large amount of acidified
ethanol (5% v/v solution of HCl) containing 2,6-di-tert-butyl-
4-methylphenol as a stabilizer. The precipitated polymers were
filtered, washed with ethanol and dried under vacuum at 50 ◦C
overnight.
Acknowledgements
Complex C4. Obtained as an orange powder in 75% yield. FT-
IR (KBr disk, cm-1): 2974, 2952, 2917, 1645, 1620, 1599, 1567,
1536, 1483, 1454, 1410, 1203, 1145, 1051, 998, 852, 777, 712, 664.
Anal. Calcd. For C40H42Co2N2O8 (796.64): 60.31; H, 5.31; N, 3.52.
Found: C, 59.93; H, 5.58; N, 3.17.
This work was supported by the National Natural Science Founda-
tion of China (Grant No. 21006085) and the State Key Laboratory
of Chemical Engineering (Grant No. SKL-ChE-11D03).
Complex C5. Obtained as an orange powder in 75% yield. FT-
IR (KBr disk, cm-1): 2963, 2866, 1649, 1618, 1585, 1572, 1532,
1440, 1415, 1362, 1335, 1238, 1222, 1177, 1042, 1020, 839, 776,
665. Anal. Calcd. For C52H66Co2N2O8 (964.96): C, 64.72; H, 6.89;
N, 2.90. Found: C, 64.57; H, 6.93; N, 2.84.
Complex C6. Obtained as a reddish orange powder in 77% yield.
FT-IR (KBr disk, cm-1): 2956, 2918, 2868, 1646, 1618, 1594, 1571,
1530, 1480, 1452, 1411, 1361, 1244, 1223, 1199, 1146, 1045, 1019,
852, 781, 715, 664. Anal. Calcd. For C46H54Co2N2O8 (880.80): C,
62.73; H, 6.18; N, 3.18. Found: C, 62.56; H, 6.23; N, 3.20.
Notes and references
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˚
graphite monochromated Mo-Ka radiation (l = 0.71073 A). Cell
parameters were obtained by global refinement of the positions of
all collected reflections. Intensities were corrected for Lorentz and
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solved by direct methods and refined by full-matrix least-squares
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Crystal data for C2. C38H36Cl2Co2N2O8, Mw = 837.45 g mol-1,
T = 293(2)K, tetragonal crystal system, space group I4(1)/a,
˚
˚
˚
a = 23.979(3) A, b = 23.979(3) A, c = 13.130(2) A, a = b =
◦
3
-3
˚
g = 90 , V = 7549.8(16) A , Z = 8, Dc = 1.474 Mg m , m =
1.074 mm-1, F(000) = 3440, crystal size = 0.04 ¥ 0.04 ¥ 0.02 mm3,
15500 reflections collected, 3330 unique which were used in all
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-3
˚
wR2 = 0.1857. Largest diff. peak and hole 0.350 and -0.509 e A .
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Polymerization of 1,3-butadiene
Solution polymerizations of 1,3-butadiene in toluene were carried
out in a sealed glass reactor (100 mL) with a rubber septum and a
connection to a vacuum system. The reactor was charged with the
desired amounts of precatalyst and cocatalyst solutions, followed
by the addition of PPh3 when required. The mixture was stirred
for 2 min at the desired temperature and followed by the addition
of a solution of 1,3-butadiene in toluene. The polymerization
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10982 | Dalton Trans., 2011, 40, 10975–10982
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©