Synthesis of bis(N-arylcarboximidoylchloride)pyridine cobalt(ii) complexes and their catalytic behavior for 1,3-butadiene polymerization

Article abstract of DOI:10.1039/c3dt51403j

A new family of bis(N-arylcarboximidoylchloride)pyridine cobalt(ii) complexes with the general formula [2,6-(ArNCCl)₂C₅H ₃N]CoCl₂ (Ar = 2,4,6-Me₃C₆H ₂, 4a; 2,6-ⁱPr

Full text of DOI:10.1039/c3dt51403j

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Synthesis of bis(N-arylcarboximidoylchloride)pyridine
cobalt(II) complexes and their catalytic behavior for
1,3-butadiene polymerization†
Cite this: Dalton Trans., 2013, 42, 13723
Heng Liu,a,b Xiangyu Jia,a,b Feng Wang,a,b Quanquan Dai,a Baolin Wang,a Jifu Bi,a
Chunyu Zhang,a Liping Zhao,a Chenxi Bai,a Yanming Hu*c and Xuequan Zhang*a
A new family of bis(N-arylcarboximidoylchloride)pyridine cobalt(II) complexes with the general formula
[2,6-(ArNvCCl)2C5H3N]CoCl2 (Ar = 2,4,6-Me3C6H2, 4a; 2,6-iPr2C6H3, 4b; 2,6-Me2C6H3, 4c; C6H5, 4d; 4-Cl-
2,6-Me2C6H2, 4e) and a typical Brookhart–Gibson-type reference complex [2,6-(2,4,6-Me3C6H2NvCMe)2-
C5H3N]CoCl2 (5a) were synthesized and characterized. Determined by X-ray crystallographic analysis,
complexes 4a, 4c–e, and 5a adopted a trigonal bipyramidal configuration, and 4b adopted a distorted
square pyramidal geometry. In combination with ethylaluminum sesquichloride (EASC), all the complexes
were highly active towards 1,3-butadiene polymerization, affording polybutadiene with predominant cis-
1,4 content (up to 96%). 4a with chlorine atoms at the imine groups exhibited higher catalytic activity
than did 5a, indicating that the incorporation of chlorine atoms into the ligand improves the activity. The
activity of the complexes in 1,3-butadiene polymerization was in the order of 4a > 4c ∼ 4e ∼ 4b > 4d,
which is consistent with the trend of spatial opening degree around the metal center in the complexes
as revealed by crystallographic data. Screening polymerization conditions proved that EASC was the most
efficient among the cocatalysts examined.
Received 29th May 2013,
Accepted 15th July 2013
DOI: 10.1039/c3dt51403j
www.rsc.org/dalton
complexes through modifying the parent ligand framework
have been designed and explored in the field of olefin
Introduction
The stereospecific polymerization of 1,3-dienes has been high- polymerization.8 Much work is also in progress on aspects of
lighted in both academic and industrial domains in the past 1,3-diene polymerization. For instance, bis(benzimidazolyl)-
few decades due to the myriad of applications of polydienes.1 amine cobalt(II) dichloride/MAO is an efficient system to afford
The endeavor to design novel metal complexes makes available polybutadiene with high cis-1,4 stereoregularity.9 2-Arylimino-
numerous precursors based on transition metals (Ti,2 Fe,3 Ni,4 6-(alcohol)pyridine nickel(II)10 and bis(oxazolinyl)pyridine
Co,5 etc.) and rare earth metals.6 Among them, Co-based cata- chromium(III)11 complexes are reported to be highly cis-1,4 or
lysts become increasingly attention-grabbing because poly- trans-1,4 selective in 1,3-butadiene polymerization. We found
butadienes with versatile microstructures7 can be afforded, that cobalt and iron complexes supported by bis(arylimino)-
including cis-1,4 and syndiotactic 1,2- moieties, depending on pyridine ligands afforded polybutadienes having high cis-1,4
catalyst formulation.
or trans-1,4 contents depending on the metal centre and the
Since the discovery of bis(arylimino) pyridine Co(II) com- ligand environment.12
plexes as effective catalysts for the ethylene polymerization
As mentioned above, Brookhart–Gibson-type bis(arylimino)-
and oligomerization, a variety of well-defined transition metal pyridine and its derivatives are well-known ligands for tran-
sition metal complexes due to the ease of tuning their catalytic
performance by modifying the ligand backbone.13 The nature
aChangchun Institute of Applied Chemistry, Chinese Academy of Sciences,
of substituents of the ligand has a great influence on the cata-
5625 Renmin Street, Changchun 130022, PR China. E-mail: xqzhang@ciac.jl.cn;
lytic behavior of the complex in olefin and 1,3-diene polymeri-
Fax: +86 8526 2307; Tel: +86 431 8526 2303
bGraduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
cDepartment of Polymer Science and Engineering, School of Chemical Engineering,
Dalian University of Technology, Dalian 116024, PR China.
zation. Generally speaking, three modification sites are
available on the ligand skeleton of the typical Brookhart–
Gibson-type catalyst (Scheme 1). For instance, the transition-
metal complexes ligated by bis(arylimino)pyridine with chlor-
ine, t-butyl, and nitro groups at para-position of the pyridyl
ring (site 1) exhibit rather different polymerization behaviors.14
E-mail: ymhu@dlut.edu.cn; Fax: +86 411 8498 6101; Tel: +86 431 8526 2303
†Electronic supplementary information (ESI) available. CCDC 872215, 903515,
903516, 903517, 941866 and 941867. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c3dt51403j
This journal is © The Royal Society of Chemistry 2013
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Results and discussion
Synthesis and characterization of the ligands and
corresponding cobalt(II) complexes
The ligands 3a–e containing chlorine atoms at the imine
carbon atom and different substituents at the ortho- and/or
para-position of the phenyl ring were prepared with high
purity in good yields according to Scheme 2. Complexes of
4a–e and 5a were prepared by the treatment of the corres-
ponding ligands with 1.0 equivalent of anhydrous CoCl2 in dry
Scheme 1 Three main modification sites on the typical bis(arylimino)pyridine
ligand framework.
Various substituents such as fluorineamine,15 meta-fluorinated
aryls,16 ferrocenyl groups,17 and other aniline analogs18 have
been introduced into the iminophenyl ring of the ligand (site
3); it was found that the steric and electronic properties of
these aniline analogs had a significant influence on olefin
polymerization. Interestingly, the incorporation of electron-
withdrawing substituents into site 3 enhances the activity of
the complexes for olefin polymerization.19 In contrast, studies
regarding site 2 are fairly limited.20 Previous reports described
the synthesis of a series of transition metal complexes ligated
by bis(arylimino)pyridine bearing electron-donating groups
(–OR, –SR, –NR2, etc.) at site 2 and found that the nature of the
heteroatom plays an important role in governing the catalytic
activity in ethylene polymerization. Thus, it is reasonable to
speculate that the incorporation of electron-withdrawing groups
will also exert a large effect on the catalytic activity. In the
present study, the electron-withdrawing chlorine atom was intro-
duced into site 2; due to its proximity to CvN, it is expected that
the electropositive character of the cobalt centre will be
enhanced; therefore, a higher catalyst activity will be attained.
In the following content, we deal with the synthesis of bis-
(N-arylcarboximidoylchloride)pyridine cobalt(II) complexes and
their catalytic behavior for 1,3-butadiene polymerization along
with the typical Brookhart–Gibson-type ketimine cobalt(II)
complex for comparison. Furthermore, the influences of the
substituent of the ligand and polymerization conditions on
the polymerization of 1,3-butadiene were investigated in detail.
1
THF. The ligands and complexes were identified by H NMR,
13C NMR, IR, and elemental analysis.
In the IR spectra, absorption bands of the imine CvN in
complexes 4a–e appear at 1614–1627 cm−1
, which are
obviously red shifted in comparison with the corresponding
ligand (1650–1660 cm−1), indicating that the nitrogen atoms of
imines coordinated to the central metal as donors. The struc-
tures of 4a–e and 5a were further confirmed by a single X-ray
crystallographic study; selected bond lengths and angles are
summarized in Table 2. The molecular structures are shown in
Fig. 1 and 4.
Crystals of 4a and 5a suitable for X-ray structure determi-
nation were obtained by slow diffusion of pentane into their
CH2Cl2 solutions, and the molecular structures are shown in
Fig. 1. Both 4a and 5a are closer to Cs symmetry about a plane
containing cobalt, two chlorines, and a pyridyl nitrogen atom.
The coordination geometries at the cobalt metal center can be
best described as distorted trigonal bipyramidal with the equa-
torial plane formed by the pyridyl nitrogen atom and the two
chlorine atoms, and the other two Co–N(CvN) bonds positioned
in the axial direction. In both complexes: (1) the Co–N(CvN)
bond is clearly longer than that of the Co–N(pyridyl) bond; (2)
the cobalt atom deviates slightly from the N3 plane, 0.125 Å for
4a and 0.064 Å for 5a, respectively; (3) the pyridine ring is
inclined slightly with respect to the N3 plane, ca. 4.1° in 4a
and ca. 5.6° in 5a, respectively; (4) the planes of the phenyl
Scheme 2 Synthesis of complexes 4a–4e and 5a.
13724 | Dalton Trans., 2013, 42, 13723–13732
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Fig. 1 ORTEP view of complex 4a and 5a, drawn at 35% of probability; hydro-
gen atoms and methylene chloride are omitted for clarity.
Fig. 3 Explanative drawings that 4a (right) exhibits an enhanced electropositi-
vity and more spatial opening degree than 5a (left). (The size of plus sign “+”
stands for the intensity of electropositivity at the Co centre; the size of pink
○
circle “ ” stands for the space of the coordination sphere for 1,3-butadiene;
“P” stands for the polybutadienyl chain.)
to the N3 plane only by 1.27°, suggesting that the N3 structure
and the pyridine are nearly coplanar. The deviation of the
cobalt atom from the N3 plane was 0.549 Å, which is clearly
larger than that of 4a. The spanned angle between the two
phenyl rings in 4b is 99.67°, and the angles of Co(1)–N(1)–
C(14) and Co(1)–N(3)–C(8) are 125.96(13)° and 125.80(13)°,
respectively.
Fig. 2 The spanned angles between two phenyl rings in 5a (left) and 4a
(right).
Solid structures of 4c, 4d, and 4e are isomorphous, having
approximate Cs symmetry about a plane formed by the cobalt
atom, the two chlorine groups, and the pyridyl nitrogen atom.
The coordination geometries around the cobalt atoms can be
best described as distorted trigonal bipyramidal with the equa-
torial planes formed by the two chlorine atoms, the pyridyl
nitrogen atom, and the other two axial Co–N(CvN) bonds.
The deviations of the Co atoms from N3 planes in 4c, 4d, and
4e are 0.010, 0.026, and 0.048 Å, respectively, indicating a
nearly coplanar structure. Two phenyl rings in complexes 4c
and 4e are oriented nearly orthogonal to the plane of the N3
plane, 88.79° and 86.52° in 4c, 82.89° and 78.54° in 4e,
respectively; however, for 4d, the two dihedral angles are
rings are oriented essentially orthogonally to the plane of the
N3 plane, 88.86° and 82.00° for 4a, 87.50° and 88.72° for 5a,
respectively. On the other hand, the bond lengths of CvN in
4a [C(1)–N(1): 1.250(8)Å and C(7)–N(3): 1.270(7) Å] are shorter
than those in 5a [1.280(5) Å and 1.289(5) Å, respectively], and
the bond distances of Co(1)–N(1) and Co(1)–N(3) in 4a
[2.303(5) and 2.405(4) Å, respectively] are considerably longer
than those in 5a [2.263(4) and 2.229(4) Å, respectively], indicat-
ing an enhanced electropositivity of the cobalt(II) center in
4a.21 Introducing chlorine atoms into the ligand, the spanned
angle between the two phenyl rings in 4a is 105.39°, which is
much larger than that in 5a (93.96°) (Fig. 2). Meanwhile, the
angles of Co(1)–N(1)–C(14) [129.1(4)° (4a) vs. 126.2(3)° (5a)]
and Co(1)–N(3)–C(8) [131.8(4)° (4a) vs. 126.2(3)° (5a)] become
larger in 4a. These results suggest that 4a exhibits a more open,
i.e. a sterically less congested coordination sphere around the
metal centre (Fig. 3), allowing easier accessibility for 1,3-buta-
diene molecules.
The molecular structures of 4b, 4c, 4d and 4e are shown in
Fig. 4. 4b has an approximate Cs molecular symmetry about
the plane containing the cobalt atom, and its geometry at the
metal center can be best described as distorted square pyrami-
dal with Cl(1) as the apical position. The four basal atoms
(three nitrogen atoms and one chlorine atom Cl(2)) are nearly
coplanar with the Cl(2) atom deviating only 0.120 Å out of the
N3 plane. The two Co–Cl bond lengths are almost the same
(2.2592 Å and 2.2209 Å, respectively). The Co–N(pyridyl) bond
distance [2.0662(17) Å] is clearly shorter than that of Co–N
(CvN) [2.2645(68) Å]. The two CvN bonds have a typical
double-bond character with a bond length of 1.263(5) Å and
Fig. 4 ORTEP view of complexes 4b(1), 4c(2), 4d(3), and 4e(4), drawn at 35%
1.251(5) Å, respectively. The pyridine ring inclines with respect of probability; hydrogen atoms and methylene chloride are omitted for clarity.
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89.53° and 55.52°, the latter one is much smaller than the summarized in Table 3. Complex 4a bearing 2,4,6-trimethyl
others. groups on the phenyl ring of the ligand exhibited the highest
Variation of the substituents on the phenyl rings of the activity; the conversion reached 97% in 20 minutes. It is clear
ligand leads to significant environmental differences (mainly from Fig. 5 that the catalyst activity of 4a is obviously higher
spatial opening degree) around the metal centre (Table 1). than that of its chlorine-free counterpart 5a during the whole
Complex 4a bearing 2,4,6-trimethyl groups possessed the polymerization process. This result might be attributable to
largest C(14)–N(1)–Co(1) and C(8)–N(3)–Co(1) bond angles the higher electropositivity and a more open spatial structure22
(129.1(4) and 131.8(4)°, respectively), giving the 1,3-butadiene of the metal center in 4a as revealed by the crystallographic
monomer the best accessibility to the cobalt centre. Changing data, which facilitates the monomer coordination. Complexes
the ortho-position substituents from isopropyl to hydrogen, an 4b and 4c having isopropyl and methyl substituents at the 2,6-
order of 4c > 4b > 4d was observed. Introduction of a chlorine position of the phenyl ring of the ligand are found to be more
atom to the para-position of the phenyl ring results in a active than the unsubstituted analogue 4d. 4e bearing methyl
reduction of C(14)–N(1)–Co(1) and C(8)–N(3)–Co(1) bond groups and chlorine atom at the 2,6-position and 4-position of
angles when comparing with 4c (126.0(3)° vs. 129.44(12)°; the phenyl ring exhibited nearly the same activity as those of
126.7(3)° vs. 127.20(13)°). The smallest C(14)–N(1)–Co(1) and 4b and 4c due to their similar spatial structure (for 4b, 4c, and
C(8)–N(3)–Co(1) bond angles (123.8(2)° and 122.7(2)°, respect-
ively) are found in the case of 4d affording the worst 1,3-buta-
diene accessibility to the cobalt centre.
Table 3 1,3-Butadiene polymerization with Co(II) complexes bearing different
ligand environments
1,3-Butadiene polymerization behavior of the cobalt(II)
complexes
Microstructurec (%)
Yield
Runa Complex (%)
Mn × 10−4 b Mw/Mn Cis-1,4- Trans-1,4- 1,2-
b
The complexes activated with EASC were examined for the
polymerization of 1,3-butadiene, and the results are
1
2
3
4
5
6
7
4a
5a
4b
4c
4d
4e
CoCl2
97.0 14.5
92.0 14.5
93.1 13.6
93.7 17.8
71.8 31.1
93.6 15.2
3.1
2.9
3.0
2.6
2.7
3.2
—
93.1
93.9
92.0
94.5
96.0
94.4
97.6
3.6
3.0
4.0
2.7
1.6
2.7
1.4
3.3
3.1
4.0
2.8
2.4
2.8
1.0
Table 1 Different spanned angles around the metal centre
Complex
C(14)–N(1)–Co(1) (°)
C(8)–N(3)–Co(1) (°)
3.1
—
4a
4b
4c
4d
4e
129.1(4)
131.8(4)
a Polymerization conditions: at 20 °C for 20 min; precursor 12.3 μmol,
125.80(13)
129.44(12)
123.8(2)
125.96(14)
127.20(13)
122.7(2)
1,3-butadiene 37 mmol, [Al]/[Co]
= 50, EASC, toluene 20 mL.
b Determined by GPC eluted with THF (polystyrenes as standards).
c Determined by IR and NMR.
126.0(3)
126.7(3)
Table 2 Selected bond lengths (Å) and angles (°) of complexes 4a–e and 5a
4a
5a
4b
4c
4d
4e
Bond lengths (Å)
Co(1)–N(1)
Co(1)–N(2)
Co(1)–N(3)
Co(1)–Cl(1)
Co(1)–Cl(2)
N(1)–C(1)
N(1)–C(14)
N(2)–C(6)
2.303(5)
2.033(5)
2.405(4)
2.2457(17)
2.2286(17)
1.250(8)
1.452(7)
1.343(7)
1.348(7)
1.270(7)
1.421(7)
2.263(4)
2.042(4)
2.229(4)
2.2586(15)
2.2762(14)
1.280(5)
1.443(6)
1.355(5)
1.344(6)
1.289(5)
1.450(6)
2.2469(18)
2.0662(17)
2.2822(18)
2.2209(7)
2.2592(7)
1.257(3)
1.453(3)
1.342(3)
1.329(3)
1.260(3)
2.3182 (16)
2.0562 (16)
2.3001 (17)
2.2473 (6)
2.2477 (6)
1.262 (2)
1.450 (2)
1.341 (2)
1.343 (3)
1.263 (3)
2.258(3)
2.051(3)
2.215(3)
2.2917(14)
2.2308(14)
1.251(5)
1.434(4)
1.339(5)
1.354(5)
1.263(5)
1.435(5)
2.251 (3)
2.042 (3)
2.302 (3)
2.2522 (12)
2.2264 (12)
1.259 (5)
1.443 (5)
1.342 (5)
1.341 (5)
1.264 (5)
1.443 (5)
N(2)–C(2)
N(3)–C(7)
N(3)–C(8)
1.449(3)
1.446 (3)
Bond angles (°)
N(1)–Co(1)–N(2)
N(1)–Co(1)–Cl(1)
N(1)–Co(1)–Cl(2)
N(2)–Co(1)–N(3)
N(2)–Co(1)–Cl(1)
N(2)–Co(1)–Cl(2)
N(3)–Co(1)–Cl(1)
N(3)–Co(1)–Cl(2)
N(1)–Co(1)–N(3)
Co(1)–N(1)–C(14)
Co(1)–N(3)–C(8)
C(1)–N(1)–C(14)
C(7)–N(3)–C(8)
76.03(18)
97.79(13)
98.98(13)
73.93(17)
109.28(14)
127.10(14)
96.10(11)
95.60(12)
149.64(17)
129.1(4)
74.59(15)
99.79(10)
95.29(10)
75.70(16)
119.76(11)
95.29(10)
96.89(10)
99.20(10)
150.19(14)
126.2(3)
74.39(6)
97.58(5)
100.43(5)
73.44(7)
147.06(6)
93.36(5)
74.92 (6)
99.16 (4)
98.03 (4)
74.72 (6)
114.39 (5)
130.81 (5)
96.98 (4)
75.04(11)
96.45(9)
98.01(9)
75.54(12)
101.84(10)
142.73(10)
98.67(9)
99.59(9)
149.08(11)
123.8(2)
122.7(2)
124.6(3)
124.3(3)
75.85 (13)
97.21 (9)
100.57 (9)
74.61 (13)
103.65 (10)
136.11 (10)
97.29 (9)
95.19 (9)
149.41 (12)
126.0 (3)
126.7 (3)
122.4 (4)
122.4 (4)
98.21(5)
101.57(5)
141.75(7)
125.80(13)
125.96(14)
122.24(19)
122.18(19)
98.57 (5)
149.36 (6)
129.44 (12)
127.20 (13)
129.44 (12)
121.62 (17)
131.8(4)
120.7(5)
119.9(5)
126.2(3)
119.3(4)
119.4(4)
13726 | Dalton Trans., 2013, 42, 13723–13732
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afforded polybutadiene with a high cis-1,4 content (97.6%) but
in a much lower yield (3.1%).
With the complex 4a/EASC catalyst system, the effects of
reaction parameters such as the polymerization temperature,
the [Al]/[Co] ratio, and the cocatalyst type on the catalytic
activities and polymer microstructures were further examined
(Table 4). Similar to most transition metal-based catalysts, the
Co complexes also suffered from the decay of catalytic activity
at elevated polymerization temperature.10,25 In the present
study, the catalyst gave the polymer in a low yield (14.8%) at
0 °C. The optimum activity (97%) was gained at 20 °C, and
further increasing the temperature resulted in a decreased cata-
lytic activity. It is worth noting that the present catalyst exhibi-
ted a high activity affording 70% yield even at 80 °C, indicating
the high thermal stability of the active species. The Mn of the
polymer decreased as the temperature increased, and mean-
while, the Mw/Mn increased from 1.6 to 5.2, which might be
due to the facilitated chain transfer reaction at the elevated
temperature. GPC curves obtained at different temperatures
are shown in Fig. S1 in ESI,† from which it was clearly
observed that the peaks shifted towards the low molecular
weight fraction along with a gradual broadening molecular
weight distribution as the temperature increased. Fig. 6 shows
the variation of microstructures of the polybutadiene obtained
at different temperatures. The polymer obtained at 0 °C had a
high cis-1,4 content of 98.5% and negligible 1,2- and trans-1,4-
contents. The trans-1,4 and 1,2- contents of the polymer gradu-
ally increased at the expense of cis-1,4 selectivity with the polymer-
ization temperature increasing. The polymer obtained at 80 °C
is composed of 76.8% cis-1,4, 12.5% trans-1,4, and 10.7% 1,2-
contents. Fig. 7 provides the NMR spectra obtained in runs 9
and 12, from which we could clearly observe an increase in
trans-1,4 and 1,2- contents in run 12 when compared with run
9. In principle, the terminal π-allylic unit of the polymer chain
can be coordinated in the anti- or syn- conformation to the
metal centre, the anti- isomer gives rise to cis-1,4 and 1,2-
polymerization, while the syn- isomer causes trans-1,4- and 1,2-
polybutadiene formation. At lower temperature, most of the
Fig. 5 Plots of the relationship between polymer yield and polymerization
time with 4a and 5a as precatalysts.
4e, the average Co–N(CvN)–C bond angle approximated
126.8° and the average difference did not exceed 1.5°). Thus,
the activity of the complexes in the polymerization of 1,3-buta-
diene is in the order of 4a > 4c ∼ 4e ∼ 4b > 4d, which is con-
sistent with the trend in the spatial opening degree of the
ligand in the complex as mentioned above. Similar results
were observed in the cases of Co(II) complexes bearing bis-
(benzimidazolyl)amine ligands; that is, more open spatial struc-
ture of the metal center results in higher catalytic activity.23
The molecular weight and cis-1,4 selectivity significantly
changed with the variation of the ligand environment. Solely
i
changing the ortho-position substituents from Pr- to H-, an
obvious increment in both cis-1,4-content and Mn was
observed. These results may be correlated to the spatial
opening degree around the cobalt metal centre; with an incre-
ment of spatial congestion near the cobalt centre, the chain
termination process could be efficiently suppressed giving rise
to a remarkable jump from 13.6 × 104 (4b) to 31.1 × 104 (4d) in
Mn. As for the stereoselectivity, the cis-1,4 content of poly(1,3-
butadiene) obtained with 4b bearing ortho-isopropyl was 92%,
and slightly increased for 4c bearing less bulky ortho-methyl
groups. The highest cis-1,4 selectivity of 96% was observed in
the case of 4d without substituents. It seems that narrowing
the spatial opening degree favors the formation of cis-1,4
content. Similar results were observed for the alkylphosphine
cobalt(II) complex.24 In the case of CoCl2, the catalyst system
Table 4 1,3-Butadiene polymerization with 4a under different polymerization conditions
Polymerization conditions
Microstructuree (%)
Runa
Alc
Al/Co
T (°C)
Yield (%)
Mn × 10−4
Mw/Mn
Cis-1,4
Trans-1,4
1,2-
d
8
9
50
50
50
50
50
15
80
100
50
50
50
0
20
40
60
80
20
20
20
20
20
20
14.8
97.0
93.7
89.2
70.0
89.0
100
95.0
73.5
2.0
32.2
14.5
7.81
5.93
4.52
12.59
13.08
10.23
13.60
—
1.6
3.1
4.6
4.4
5.2
2.7
2.5
2.9
3.4
—
98.5
93.1
87.7
83.6
76.8
91.7
94.0
93.0
94.9
—
0.8
3.3
6.6
8.6
12.5
6.0
4.3
4.9
2.8
—
0.7
3.6
5.7
7.8
10.7
2.3
1.6
2.1
2.3
—
10
11
12
13
14
15
16
17
18
EASC
MAOb
TEAb
TIBAb
1.3
—
—
—
—
—
a Polymerization conditions: precursor 12.3 μmol; 1,3-butadiene 37 mmol; polymerization time 20 min, toluene 20 mL. b Polymerization time:
4 h. c EASC: ethylaluminum sesquichloride; MAO: methyl aluminoxane; TEA: triethylaluminium; TIBA: triisobutylaluminium. d Determined by
GPC eluted with THF (polystyrene as standards). e Determined by IR and NMR.
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monomers formed cis-1,4 polybutadiene through path 1 obtained at 0 °C performed the lowest Tg (−102.95 °C); increas-
(Scheme 3) due to the hypothesis that the newly formed ing the temperature from 20 to 80 °C, Tg increased mono-
double bonds helped stabilize the active species through tonously from −101.48 °C to −94.74 °C, indicating that the
π-back-bonding and it was more kinetically favored. With the microstructures influenced significantly the Tg of
increasing of temperature, anti–syn isomerization through the polybutadienes.
π–σ-rearrangement was gradually facilitated owing to the
On the other hand, the catalytic activity increased gradually
reason that the syn- isomer was thermodynamically more with an increasing ratio of [Al]/[Co], reaching complete conver-
stable, which led to an increment of trans-1,4 and 1,2- contents sion at [Al]/[Co] = 80. With the variation of the [Al]/[Co] ratio,
(paths 3 and 4). DSC was done to investigate the influence of no conspicuous changes in Mn and MWD of polymers were
microstructure on the glass transition temperature Tg, and the observed, suggesting that EASC was not an efficient chain
data and spectra are provided in Fig. S3.† Polybutadiene transfer agent at 20 °C for the present system.
Since the cocatalyst plays an important role in transition
metal catalyzed polymerization, the polymerization of 1,3-buta-
diene was carried out by employing different cocatalysts.
Among the cocatalysts examined, EASC is the most efficient,
affording the polymer in a high yield (97%) in 20 minutes with
high cis-1,4 content (93.1%). Using MAO as a cocatalyst, the
catalytic activities towards 1,3-butadiene polymerization
decreased, and the polymer yield was 73.5% in 4 h. In the
cases of TEA and TIBA, the activities were negligible (2.0% and
1.3% yield, respectively), which means that they are less
effective for activation of the complex than EASC and MAO.
Similar results were observed in the polymerization of
Fig. 6 Changes in the microstructures of PBDs with the variation of reaction
1,3-butadiene with other Co-based catalytic systems.23
temperature.
Fig. 7 NMR spectra of the poly(1,3-butadiene)s obtained by the 4a/EASC system. Run 9: (i) and (ii); run 12: (iii) and (iv).
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Scheme 3 Proposed scheme for 1,3-butadiene polymerization. (Blue lines stand for the newly incorporated monomer.)
chromatography (GPC) equipped with a Waters 515 HPLC
pump, four columns (HMW 7 THF, HMW 6E THF × 2, HMW 2
Conclusions
A series of cobalt(II) complexes supported by bis(N-arylcarbox- THF), and a Waters 2414 refractive index detector. Tetrahydro-
imidoylchloride)pyridine 4a–e were synthesized and fully furan was used as the eluent at a flow rate of 1.0 mL min−1
.
characterized. As determined by X-ray crystallographic analy- The values of Mn and Mw/Mn were determined using poly-
sis, the complexes adopted distorted trigonal bipyramidal and styrene calibration.
square pyramidal configurations. In combination with EASC,
Preparation and characterization of ligands and complexes
all the complexes afforded polybutadienes in high yields ranging
from 72% to 97% with a high cis-1,4 content (up to 96%). Incor- All the bis(N-arylcarboximidoylchloride)pyridine cobalt(II)
poration of chlorine atoms into the imine groups of the ligand dichloride complexes were prepared in the same manner; a
enhanced the catalytic activity of the complexes for 1,3-buta- typical synthesis of complex 4a is described as follows:
diene polymerization. The substituent of the phenyl ring of the
2,6-Pyridinedicarboxylic acid (5.0 g, 30 mmol) and thionyl
ligand affected the activity of the complex, and the activity chloride (30 mL) were added to a flask; the reaction mixture was
decreased in the order of 4a > 4c ∼ 4e ∼ 4b > 4d, which is con- refluxed with stirring for 24 h under a nitrogen atmosphere until
sistent with the trend of spatial opening degree as revealed by a homogenous brown solution was obtained. The remaining
the crystallographic data. With the bulkiness of the substituent unreacted thionyl chloride was removed by distillation under
on the phenyl ring decreasing, the cis-1,4 content of the result- reduced pressure, and a light pink powder of 2,6-pyridine carbo-
ing polybutadiene increased. The catalytic activity of the nyl dichloride was obtained. Yield, 6.1 g (100%). The product was
complex decreased as the polymerization temperature increased, moisture sensitive and used directly in the next procedure.
while the polymer yield remained 70% even at 80 °C, indicating
the high thermal stability of the formed active species.
A solution of 2,6-pyridine carbonyl dichloride (6.1 g,
30 mmol) in 50 mL CH2Cl2 was added dropwise into a solution
of 2,4,6-trimethylaniline (8.11 g, 60 mmol) and triethylamine
(8.36 mL, 60 mmol) in CH2Cl2 (100 mL). The reaction mixture
was refluxed for 6 h under a nitrogen atmosphere, then the
reaction mixture was washed with water and brine, and the
organic layer was dried over anhydrous magnesium sulfate.
Experimental
General considerations
All manipulations were carried out under a nitrogen atmos- After CH2Cl2 was evaporated, the solid was recrystallized from
phere in a glove box. The solvents were refluxed over CaH2 or toluene and dried under vacuum at 50 °C to give the product
1
sodium-benzophenone and distilled prior to use. 1,3-Buta- 2a as a white solid. Yield, 10.95 g (91.0%). H NMR (400 MHz,
diene was obtained from Lanzhou Petrochemical Company. CDCl3, δ, ppm): 9.02 (s, 2H, NH), 8.51 (d, 2H, Pyr-Hm), 8.14 (t,
Other chemicals were commercially available and used 1H, Pyr-Hp), 6.95 (s, 4H, Ar-H), 2.30 (s, 6H, Ar-CpH3), 2.26 (s,
without further purification.
12H, Ar-CoH3). 13C NMR (100 MHz, CDCl3, δ, ppm): 161.6,
1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were 148.8, 139.3, 137.1, 134.8, 130.5, 128.9, 125.5, 20.9, 18.4. FT-IR
recorded using a Varian Unity spectrometer in CDCl3 at (KBr; cm−1): 3005, 2971, 1683, 1661, 1509, 1230, 1160, 1073,
ambient temperature using tetramethylsilane as an internal 1003, 848, 647. Anal. calc. for C25H27N3O2 (401.5): C, 74.79; H,
standard. FTIR spectra were recorded using a BRUKER Vertex- 6.78; N, 10.47. Found: C, 74.73; H, 6.81; N, 10.42.
70 FTIR spectrometer. Elemental analysis was performed
2a (5 g, 12.46 mmol), phosphorous pentachloride (PCl5,
using an elemental Vario EL spectrophotometer. The pro- 5.19 g, 24.92 mmol), and 200 mL CH2Cl2 were added into a
portion of cis-1,4 and trans-1,4 units of the polymer was deter- flask. After reflux for 3 h with stirring, the volatiles were
mined by FTIR spectra, 1H NMR, and 13C NMR. The molecular removed under vacuum, and the remains were extracted with
weights (Mn) and molecular weight distributions (Mw/Mn) of hexane. The hexane solution was kept at −40 °C and yellow
polymers were measured at 30 °C by gel permeation crystals were obtained. After washing with hexane and drying
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under vacuum at 50 °C, product 3a was obtained. Yield, 2.44 g cm−1): 3075, 2973, 1627, 1587, 1468, 1172, 1081, 955, 842, 775,
1
(44.6%). H NMR (400 MHz, CDCl3, δ, ppm): 8.49 (d, 2H, Pyr- 734. Anal. calc. for C23H21Cl4CoN3 (540.18): C, 51.14; H, 3.92;
Hm), 8.01 (t, 1H, Pyr-Ho), 6.94 (s, 4H, Ar-H), 2.32 (s, 6H, Ar- N, 7.78. Found: C, 51.19; H, 3.98; N, 7.81.
CpH3), 2.10 (s, 12H, Ar-CoH3). 13C NMR (100 MHz, CDCl3, δ,
[N,N′-Bisphenylpyridine-2,6-carboximidoyl dichloride] CoCl2
ppm): 151.2, 145.2, 143.4, 137.6, 134.0, 128.6, 125.8, 125.6, (4d). Yield, 0.21 g (44.5%). FT-IR (KBr; cm−1): 3067, 1627,
20.8, 17.8. FT-IR(KBr; cm−1): 302, 2969, 1656, 1508, 1471, 1202, 1586, 1488, 1189, 1086, 845, 776, 699. Anal. calc. for
1139, 898, 850, 611, 551. Anal. calc. for C25H25Cl2N3 (439.4): C, C19H13Cl4CoN3 (484.07): C, 47.14; H, 2.71; N, 8.68. Found: C,
68.49; H, 5.75; N, 9.59. Found: C, 68.41; H, 5.72; N, 9.55.
47.11; H, 2.70; N, 8.62.
A solution of 3a (0.44 g, 1.0 mmol) in 5 mL THF was added
[N,N′-Bis(4-chloro-2,6-dimethylphenyl)pyridine-2,6-carboximi-
dropwise to a suspension of anhydrous CoCl2 (0.13 g, 1 mmol) doyl dichloride] CoCl2 (4e). Yield, 0.53 g (86.9%). FT-IR (KBr;
in 3 mL THF, the reaction mixture was stirred for 6 h and a cm−1): 3084, 2955, 1625, 1587, 1469, 1177, 1080, 950, 861, 799,
green suspension was formed. The precipitate was filtered and 630. Anal. calc. for C23H19Cl6CoN3 (609.07): C, 45.36; H, 3.14;
dried under vacuum at 40 °C and the desired product 4a was N, 6.90. Found: C, 45.31; H, 3.11; N, 6.88.
obtained. Yield, 0.40 g (70.2%). A single crystal of 4a was
2,6-Bis[1-(2,4,6-trimethylphenylimino)ethyl]pyridine CoCl2
obtained by layering n-pentane on their dichloromethane solu- (5a). 2,6-Diacetylpyridine (0.98 g, 6 mmol) and 2,4,6-trimethyl-
tions at room temperature. FT-IR (KBr; cm−1): 3064, 2919, aniline (1.69 g, 12.5 mmol) were added to a flask containing
1626, 1540, 1483, 1327, 1140, 1078, 853, 818. Anal. calc. for ethanol (40 mL) and formic acid (0.2 mL). The reaction
C25H25Cl4CoN3 (568.2): C, 52.84; H, 4.43; N, 7.39. Found: C, mixture was stirred for 48 h at room temperature. A saturated
52.90; H, 4.51; N, 7.37.
solution was obtained by evaporating ethanol. The solution
N,N′-Bis(2,6-diisopropylphenyl)pyridine-2,6-carboximidoyl was kept at −40 °C for 48 h, and yellow crystals were obtained.
dichloride (3b). Yield, 4.88 g (75%). 1H NMR (400 MHz, The crystals were washed with cold methanol and dried under
CDCl3, δ, ppm): 8.52 (d, 2H, Pyr-Hm), 8.03 (t, 1H, Pyr-Ho), vacuum at 40 °C. Yield, 2.18 g (91.7%). 1H NMR (400 MHz,
7.20–7.26 (m, 6H, Ar-H), 2.82 (hept, 4H, –CH(CH3)2), 1.19 (d, CDCl3, δ, ppm): 8.47 (d, 2H, Pyr-Hm), 7.90 (t, 1H, Pyr-Ho), 6.90
24H, –CH(CH3)2). FT-IR(KBr; cm−1): 3070, 2960, 1650, 1443, (s, 4H, Ar-H), 2.30 (s, 6H, Ar-CpH3), 2.0 (s, 12H, Ar-CoH3). FT-IR
1180, 910(m), 794, 757, 624. Anal. calc. for C31H37Cl2N3 (522.6): (KBr; cm−1): 3005, 2974, 1637, 1569, 1479, 1364, 1214, 1118,
C, 71.25; H, 7.14; N, 8.04. Found: C, 71.20; H, 7.13; N, 8.09.
N,N′-Bis(2,6-dimethylphenyl)pyridine-2,6-carboximidoyl 7.86; N, 10.57. Found: C, 81.64; H, 7.95; N, 10.66.
dichloride (3c). Yield, 3.12 g (61%). 1H NMR (400 MHz,
2,6-Bis[1-(2,4,6-trimethylimino)ethyl]pyridine
854, 820, 561. Anal. calc. for C27H31N3 (397.6): C, 81.57; H,
(0.40
g,
CDCl3, δ, ppm):8.51 (d, 2H, Pyr-Hm), 8.02 (t, 1H, Pyr-Ho), 1.0 mmol) and CoCl2 (0.13 g, 1.0 mmol) were added to a flask
7.02–7.13 (m, 6H, Ar-H), 2.14 (s, 24H, –CH3). 13C NMR containing 5 mL THF. The reaction mixture was stirred for 4 h
(100 MHz, CDCl3, δ, ppm): 150.9, 145.8, 145.3, 135.0, 128.2, and a green suspension was formed. 20 mL Et2O was added to
127.7, 127.1, 124.5, 17.8. FT-IR(KBr; cm−1): 3024, 2977, 1655, the suspension and a green powder was obtained by filtration.
1595, 1471, 1282, 1189, 1086, 993, 775, 689, 495. Anal. calc. for The product was dried under vacuum at 40 °C. Yield, 0.49 g
C23H21Cl2N3 (410.3): C, 67.32; H, 5.16; N, 10.24. Found: C, (92.4%). A single crystal of 5a was obtained by layering n-pentane
67.29; H, 5.18; N, 10.25.
on their dichloromethane solutions at room temperature. FT-IR
N,N′-Bisphenylpyridine-2,6-carboximidoyl dichloride (KBr; cm−1): 3031, 2975, 1636, 1583, 1478, 1369, 1262, 1223,
1
(3d). Yield, 0.22 g (5%). H NMR (400 MHz, CDCl3, δ, ppm): 1064, 860, 570. Anal. calc. for C27H31Cl2CoN3 (526.4): C, 61.49; H,
8.40 (d, 2H, Pyr-Hm), 7.98 (t, 1H, Pyr-Ho), 7.10–7.50 (m, 10H, 5.92; N, 7.97. Found: C, 61.45; H, 5.88; N, 7.89.
Ar-H). FT-IR (KBr; cm−1): 3062, 1660, 1552, 1446, 1203, 915,
886, 757, 699, 630. Anal. calc. for C19H13Cl2N3 (354.3): C,
General procedure for 1,3-butadiene solution polymerization
64.42; H, 3.70; N, 11.86. Found: C, 64.32; H, 3.79; N, 11.89.
A typical procedure for the polymerization is as follows (entry
N,N′-Bis(4-chloro-2,6-dimethylphenyl)pyridine-2,6-carboximi- 1 in Table 3): a toluene solution of 1,3-butadiene (20 mL,
doyl dichloride (3e). Yield, 3.76 g (63%). 1H NMR (400 MHz, 0.1 g mL−1) was added to a moisture free ampule preloaded
CDCl3, δ, ppm): 8.50 (d, 2H, Pyr-Hm), 8.03 (t, 1H, Pyr-Ho), 7.11 with complex 4a (7 mg, 12.3 μmol), and then EASC
(s, 4H, Ar-H), 2.11 (s, 12H, -CH3). 13C NMR (100 MHz, CDCl3, (0.62 mmol) was injected to initiate the polymerization at
δ, ppm): 150.8, 145.2, 144.1, 137.6, 129.4, 127.8, 127.7, 125.6, 20 °C. After 20 min, methanol was added to quench the
17.6. FT-IR (KBr; cm−1): 3075, 2983, 1652, 1469, 1442, 1192, polymerization. The mixture was poured into a large amount
986, 914, 828, 805, 616. Anal. calc. for C23H19Cl4N3 (479.3): C, of methanol containing 2,6-di-tert-butyl-4-methylphenol
57.64; H, 4.00; N, 8.75. Found: C, 57.59; H, 4.05; N, 8.78.
(1.0 wt%) as a stabilizer. The precipitated polymer was washed
[N,N′-Bis(2,6-diisopropylphenyl)pyridine-2,6-carboximidoyl with methanol, and then dried under vacuum at 40 °C to con-
dichloride] CoCl2 (4b). Yield, 0.56 g (85.3%). FT-IR (KBr; stant weight. The polymer yield was determined by gravimetry.
cm−1): 3071, 2959, 1614, 1578, 1465, 1382, 1164, 1083, 801,
X-ray crystallographic studies
765. Anal. calc. for C31H37Cl4CoN3 (652.4): C, 57.07; H, 5.72; N,
6.44. Found: C, 57.12; H, 5.73; N 6.38.
Crystals for X-ray analyses were obtained as described in the
[N,N′-Bis(2,6-dimethylphenyl)pyridine-2,6-carboximidoyl Experimental section. Data collections were performed at low
dichloride] CoCl2 (4c). Yield, 0.42 g (77.8%). FT-IR (KBr; or room temperature on a Bruker SMART APEX diffractometer
13730 | Dalton Trans., 2013, 42, 13723–13732
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with a CCD area detector using graphite monochromated MoK
radiation (λ = 0.71073 Å). The determination of crystal class
and unit cell parameters was carried out using the SMART
program package. The raw frame data were processed using
SAINT and SADABS to yield the reflection data file. The struc-
tures were solved using the SHELXTL program. Refinement
was performed on F2 anisotropically for all non-hydrogen
atoms by the full-matrix least-squares method. The hydrogen
atoms were placed at the calculated positions and were
included in the structure calculation without further refine-
ment of the parameters. Details of X-ray structure determi-
nations and refinements are provided in Table S1 in the ESI.†
CCDC 872215, 903515 to 903517, 941866 to 941867.
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Acknowledgements
The authors appreciate financial support from the National
Science
and
Technology
Infrastructure
Program
(2007BAE14B01–06), the Fund for Creative Research Groups
(50621302) and the National Natural Scientific Foundation of
China (no. 20974016).
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