Synthesis of cationic cobalt(II) complexes and their efficiency as catalysts for the polymerization of 1,3-butadiene

Article abstract of DOI:10.1016/j.ica.2014.01.012

¹ A series of novel cationic cobalt(II) compounds accompanied by various counter ions ([BF₄]^-, [PF₆] ^-, [SbF₆]^-) and N-bearing ligands (1,10-phenanthroline, bipyridine, benzimidazole, and imidazole) were synthesized. Neutral compounds, CoCl₂-nLigands, were prepared as reference compounds. All the compounds were characterized using IR spectral analysis and elemental analysis and some of the compounds were further characterized by single-crystal X-ray diffraction analysis. The synthesized compounds were evaluated as precursors for butadiene polymerization in conjunction with ethylaluminum sesquichloride as a cocatalyst, and the polymerization mechanism was proposed to involve anionic coordination polymerization. The active centers of the catalysts were cationic in nature, and the catalytic activities were affected by the electronegativity of the counter ion.

Full text of DOI:10.1016/j.ica.2014.01.012

Inorganica Chimica Acta 414 (2014) 8–14
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis of cationic cobalt(II) complexes and their efficiency as cata-
lysts for the polymerization of 1,3-butadiene
Li Liu a,b, Xu Zhang a,b, Heng Liu , Xuequan Zhang , Guangping Sun , Hexin Zhang b,
b
b
a,
⇑
⇑
a
Materials Science and Engineering, Jilin University, Changchun 130022, PR China
Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
b
a b s t r a c t 1
a r t i c l e i n f o
À
À
À
6
Article history:
4 6
A series of novel cationic cobalt(II) compounds accompanied by various counter ions ([BF ] , [PF ] , [SbF ] )
Received 2 November 2013
Received in revised form 3 January 2014
Accepted 6 January 2014
and N-bearing ligands (1,10-phenanthroline, bipyridine, benzimidazole, and imidazole) were synthesized.
Neutral compounds, CoCl -nLigands, were prepared as reference compounds. All the compounds were
2
characterized using IR spectral analysis and elemental analysis and some of the compounds were further
characterized by single-crystal X-ray diffraction analysis. The synthesized compounds were evaluated as
precursors for butadiene polymerization in conjunction with ethylaluminum sesquichloride as a cocatalyst,
and the polymerization mechanism was proposed to involve anionic coordination polymerization. The
active centers of the catalysts were cationic in nature, and the catalytic activities were affected by the
electronegativity of the counter ion.
Available online 25 January 2014
Keywords:
Cobalt
Catalyst
1
,3-Butadiene
Polybutadiene
Ó 2014 Elsevier B.V. All rights reserved.
Polymerization mechanism
1
. Introduction
Regio- and/or stereo-selective polybutadiene have been exten-
increases the coordination and insertion rates of the olefin mono-
mer during the polymerization process. In our previous research
[30], a titanium-based Ziegler–Natta catalyst with a cationic active
metal center was prepared via modification of the commercial Zie-
gler–Natta catalyst, and the catalytic activity was found to drasti-
cally increased after the modification.
Herein, we report for the first time, highly active cationic cobal-
t(II) catalysts. The effects of the catalyst structure (e.g., counter
ions, ligands) and reaction conditions on the polymerization were
studied. Furthermore, a plausible catalysis mechanism was put for-
ward to explain the differences between the traditional and cat-
ionic cobalt(II) catalysts.
sively and diversely applied throughout the rubber industry since
the discovery of Ziegler–Natta-type catalyst systems [1–9]. Hence,
investigation of butadiene polymerization, structures of catalyst
precursors and catalyst-polymer correlations has served as a
fascinating and challenging subject over the past decades and
has promoted studies of catalyst design and catalysis mechanisms.
Recently, late transition metal catalysts have initiated a completely
new catalytic domain and have attracted a significant amount of
interest due to their simple synthesis, stability and negligible envi-
ronmental impact [10–21]. However, some late transition metal
catalysts suffer from low activity, comparatively low cis-1,4 selec-
tivity, and uncertain catalytic mechanism. Therefore, efficient
catalysts that can produce polybutadiene with a high cis-1,4
content are highly desirable.
2. Experimental
2.1. Materials and methods
Metallocenes are known to show much higher catalytic activity
toward olefin polymerization as compared to the Zielger–Natta
catalyst [22–24]. A plausible explanation for this difference is the
formation of a cationic active metal center in the metallocene cat-
alyst activated with methylaluminoxane or borates [25–29], which
Cobalt carbonate (CoCO
orophosphoric acid (HPF6), and fluoroantimonic acid (HSbF
3
), tetrafluoroboric acid (HBF
4
), hexaflu-
) were
6
purchased from Alfa Aesar, China. 1,10-phenanthroline (Phen),
bipyridine (Bipy), benzimidazole (BZI), and imidazole (Im) were
supplied by J&K, China. Ethylaluminum sesquichloride (EASC)
was purchased from Akzo Nobel, USA. Polymerization-grade buta-
diene was supplied by Jinzhou Petrochemical Corporation and
purified by passing through four columns packed with molecular
sieves and solid potassium hydroxide. Toluene was distilled by
heating to reflux in the presence of sodium and benzophenone.
Other chemicals were used as received. Infrared (IR) spectra were
⇑
Corresponding authors. Tel.: +86 431 85262889; fax: +86 431 85262274 (H.X.
Zhang).
E-mail addresses: sungp@jlu.edu.cn (G. Sun), polyhx@ciac.ac.cn (H. Zhang).
1
Phen, 1,10-phenanthroline; Bipy, bipyridine; BZI, benzimidazole; Im, imidazole;
EASC, ethylaluminum sesquichloride; GPC, gel permeation chromatography.
http://dx.doi.org/10.1016/j.ica.2014.01.012
0
020-1693/Ó 2014 Elsevier B.V. All rights reserved.

L. Liu et al. / Inorganica Chimica Acta 414 (2014) 8–14
9
obtained on a Bruker Vertex-70 FT-IR spectrophotometer. The
weight-average molecular weights (M ) and polydispersity (M
) of the resulting polymers were measured at 30 °C by gel
permeation chromatography (GPC), using chromatographic
frame data were processed by SAINT and SADABS to yield the reflection
data file. The structures were determined using the SHELXTL pro-
gram. Refinement was performed on F anisotropically for all
w
w
/
2
M
n
a
non-hydrogen atoms by the full-matrix least-squares method.
system equipped with a Waters 515 HPLC pump, four columns,
and a Waters 2414 refractive index detector. THF was used as
the eluent at a flow rate of 1.0 mL/min. Crystals for X-ray diffrac-
tion were obtained as described in the Results and discussion
section. Data collection for compound 1b was perfrormed at
2.2. Syntheses and characterization of compounds
A general synthetic route to cobalt(II) compounds is shown in
Scheme 1. The typical synthetic procedure for 1a is as follows: Co-
balt(II) tetrafluoroborate hexahydrate (0.5 g, 1.47 mmol), prepared
by mixing excess CoCO with HBF in H O at ambient temperature,
2
2 °C, and data for the other compounds were collected at
À88.5 °C on a Bruker SMART APEX diffractometer with a CCD area
detector. Determination of the crystal class and unit cell parame-
ters was carried out using the SMART program package. The raw
3
4
2
was mixed with Phen (0.79 g, 4.39 mmol) in methanol (20 mL) to
afford a yellow transparent solution. Compound 1a was obtained
Scheme 1. Syntheses of cobalt(II) compounds.
Table 1
Crystal data and structure refinements of compounds 1a, 1b, and 4a.
4
a
1a
1b
Formula
C
27
H
23
C
l2CoN
4
C
38
H
29.5
B
2
CoF
N
8 6
31 31 2 8 8
C H B CoF N
Molecular weight
Wavelength (Å)
Crystal system
Space group
Color of crystal
a (Å)
533.32
0.71073
triclinic
P1ꢀ
yellow
10.7724
10.8378
10.9523
85.6420
84.4350
78.6430
1245.60
2
810.73
0.71073
triclinic
P1ꢀ
purple
11.1961
12.9138
14.3453
81.2415
75.9587
68.5142
1867.7
2
1496.38
0.71073
triclinic
P1ꢀ
yellow
10.3908
10.6832
19.3985
87.055
75.488
71.841
1980.01
2
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
3
V (Å )
Z
D
calc. (mg/m3)
1.442
0.926
548.0
1.442
0.538
825.0
1.255
0.502
764.0
À1
Absorption coefficient (mm
F(000)
)
Crystal size (mm)
h (°)
No. of reflections collected
0.28 Â 0.2 Â 0.12
1.87–26.04
6826
0.29 Â 0.16 Â 0.10
1.47–25.35
9766
0.24 Â 0.18 Â 0.10
1.08–26.19
11218
No. of independent reflections
No. of data/restraints/params
Goodness-of-fit (GOF) on F
4806
4806/0/308
1.054
6739
6739/12/534
1.007
7833
7833/0/452
1.087
2
R
1
(1 > 2
r(1))
0.0460
0.0666
0.0690
wR
2
0.1217
0.2030
0.1612

1
0
L. Liu et al. / Inorganica Chimica Acta 414 (2014) 8–14
Table 2
Selected bond distances (Å) and angles ( ) of cobalt compounds 1a, 1b, and 4a.
o
1
a
1b
4a
Co1–N1
Co1–N6
Co1–N4
Co1–N5
Co1–N2
Co1–N3
2.137 (3)
2.137 (3)
2.140 (3)
2.142 (3)
2.142(3)
2.150(3)
Co1–N4
Co1–N8
Co1–N2
Co1–N6
1.985 (2)
1.994 (2)
1.991 (2)
2.003 (2)
Co1–N2
Co1–N1
Co1–N4
Co1–N3
Co1–Cl1
Co1–Cl2
2.142 (3)
2.159 (2)
2.161 (2)
2.194 (2)
2.3915 (7)
2.4213 (7)
77.55 (10)
164.99 (8)
92.35 (10)
91.25 (8)
84.83 (8)
76.64 (8)
94.43 (6)
169.93 (8)
94.26 (6)
89.32 (5)
96.43 (6)
88.92 (7)
94.42 (6)
168.83 (6)
98.12 (3)
N1–Co1–N6
N1–Co1–N4
N6–Co1–N4
N1–Co1–N5
N6–Co1–N5
N4–Co1–N5
N1–Co1–N2
N6–Co1–N2
N4–Co1–N2
N5–Co1–N2
N1–Co1–N3
N6–Co1–N3
N4–Co1–N3
N5–Co1–N3
N2–Co1–N3
96.19(13)
165.80 (13)
95.25 (13)
95.54 (13)
78.13 (14)
95.02 (13)
78.36 (14)
94.68 (13)
92.35 (13)
170.12 (13)
92.17 (13)
167.84 (13)
77.95 (13)
92.30 (13)
95.66 (13)
N4–Co1–N8
N4–Co1–N2
N8–Co1–N2
N4–Co1–N6
N8–Co1–N6
N2–Co1–N6
107.75 (9)
106.96 (10)
114.82 (9)
113.37 (9)
105.57 (9)
108.55 (9)
N2–Co1–N1
N2–Co1–N4
N1–Co1–N4
N2–Co1–N3
N1–Co1–N3
N4–Co1–N3
N2–Co1–Cl1
N1–Co1–Cl1
N4–Co1–Cl1
N3–Co1–Cl1
N2–Co1–Cl2
N1–Co1–Cl2
N4–Co1–Cl2
N3–Co1–Cl2
Cl1–Co1–Cl2
Fig. 1. X-ray structure of 1a. Hydrogen atoms were omitted for clarity. Displace-
Fig. 2. X-ray structure of 1b. Hydrogen atoms were omitted for clarity. Displace-
ment ellipsoids are drawn at the 50% probability level.
ment ellipsoids are drawn at the 50% probability level.
by extracting and subsequently removing the solvent in vacuo. The
neutral compounds were also prepared for comparison. All prod-
ucts were dried at 50°C in a vacuum drying oven and obtained as
air-stable powders in high yields (62.8–90.7%).
for C36
H, 2.32; N, 7.83%.
Tetra(Benzimidazole)cobalt(II) tetrafluoroborate (1b): Purple
powder, 62.8% yield. IR (KBr, cm ): 1623(w), 1597(w), 1493(w),
6 2
H24CoF12N Sb : C, 40.37; H, 2.26; N, 7.85. Found: C, 40.45;
À1
Tri(1,10-phenanthroline)cobalt(II) tetrafluoroborate (1a): Yellow
powder, 90.7% yield. IR (KBr, cm ): 1624(m), 1581(w), 1518(s),
1420(s), 1302(m), 1270(w), 1250(s), 1083(s), 741(s), 614(m),
521(w). Anal. Calc. for C28H B CoF N : C, 47.97; H, 2.88; N,
20 2 8 8
À1
1
5
1
495(w), 1425(s), 1340(w), 1304(w), 1062(s), 847(s), 724(s),
15.98. Found: C, 47.94; H, 2.30; N, 15.95%.
21(w). Anal. Calc. for C36 CoF : C, 55.92; H, 3.13; N,
H
24
B
2
8
N
6
Tri(Bipyridine)cobalt(II) tetrafluoroborate (1c): Yellow powder,
À1
0.87. Found: C, 55.87; H, 3.37; N, 10.95%.
79.2% yield. IR (KBr, cm ): 1637(w), 1598(s), 1575(m), 1492(m),
Tri(1,10-phenanthroline)cobalt(II) hexafluorophosphate (2a): Yel-
low powder, 87.2% yield. IR (KBr, cm ): 1624(w), 1581(w),
1474(s), 1442(s), 1315(s), 1284(w), 1062(s), 774(s), 737(m),
521(w). Anal. Calc. for C30H B CoF N : C, 51.39; H, 3.45; N,
24 2 8 6
À1
1
7
4
517(s), 1427(s), 1346(w), 1142(m), 1104(m), 861(m), 847(s),
25(w), 482(w), 463(w). Anal. Calc. for C,
8.61; H, 2.72; N, 9.45. Found: C, 48.65; H, 2.80; N, 9.43%.
11.99. Found: C, 51.33; H, 3.54; N, 11.21%.
C
36
H24CoF12
N
6 2
P :
Tetra(Imidazole)cobalt(II) tetrafluoroborate (1d): Pink powder,
À1
85.6% yield. IR (KBr, cm ): 1589(s), 1536(w), 1430(m), 1330(w),
Tri(1,10-phenanthroline)cobalt(II) fluoroantimonic (3a): Yellow
powder, 82.3% yield. IR (KBr, cm ): 2364(w), 1624(w), 1518(s),
1298(w), 1256(w), 1070(s), 770(s), 664, 621(s), 521(w). Anal. Calc.
À1
for C28
20 2 8 4
H B CoF N : C, 28.78; H, 2.42; N, 22.37. Found: C, 28.81; H,
1
491(w), 1426(s), 1103(w), 1084(w), 846(m), 726(m). Anal. Calc.
2.39; N, 22.43%.

L. Liu et al. / Inorganica Chimica Acta 414 (2014) 8–14
11
rubber septum. Then a solution of butadiene in toluene (20 mL,
.1 g/mL) was injected into the ampoule. After that, EASC
0.37 mL, 0.5 mol/L) was added to the ampoule to initiate the poly-
0
(
merization. The polymerization was carried out at 25 °C for 40 min
and quenched by adding ethanol containing 2,6-di-tert-butyl-4-
methylphenol (1.0% v/v) as a stabilizer. The product was dried
overnight in a vacuum oven (yield: 0.642 g, 32.1%).
3
. Results and discussion
3.1. Preparation and characterization of compounds
Cationic cobalt(II) compounds were prepared by the reaction of
cobalt tetrafluoroborate with different N-bearing ligands in meth-
anol. The products were isolated as air-stable powders in good
yields and characterized by IR spectra and X-ray analysis. The IR
spectra of the cobalt(II) compounds were recorded in the region
À1
4
000–400 cm region, and tentative assignments were made on
Fig. 3. X-ray structure of 4a. Hydrogen atoms were omitted for clarity. Displace-
ment ellipsoids are drawn at the 50% probability level.
the basis of published data [31–34]. In the IR spectra of the free
ligands, the C@N stretching frequencies appeared at 1561–
À1
Bis(1,10-phenanthroline)cobalt(II) chloride (4a): Yellow powder,
6.9% yield. IR (KBr, cm ): 1624(w), 1584(w), 1516(s), 1425(s),
342(w), 1305(w), 1143(m), 1103(m), 850(m), 725(s), 641(w),
16 4
22(w). Anal. Calc. for C24H Cl2CoN : C, 58.80; H, 3.29; N, 11.43.
1618 cm . In compounds, the C@N stretching vibrations shifted
À1
7
1
4
toward lower frequencies and were greatly reduced in intensity,
which suggested the coordination interaction between the nitro-
gen atoms and the metal ion. IR peaks of other cobalt compounds
were in good agreement with the literature values [35,36]. In order
to establish the coordination mode of the compounds, the struc-
tures of compounds 1a, 1b, and 4a were confirmed by X-ray
diffraction analysis. The crystallographic data of the compounds
and the collection and refinement parameters are summarized
in Tables 1 and 2 and their crystal structures are depicted in
Found: C, 55.84; H, 3.22; N, 11.58%.
Bis(Benzimidazole)cobalt(II) chloride (4b): Purple powder, 80.2%
À1
yield. IR (KBr, cm ): 1622(m), 1597(m), 1492(s), 1460(m),
1
7
3
414(s), 1300(s), 1271(s), 1246(s), 1114(m), 1002(m), 977(m),
42(s), 618(m), 418(s). Anal. Calc. for C15 l2CoN : C, 47.52; H,
.46; N, 14.78. Found: C, 47.80; H, 3.50; N, 14.69%.
H
13
C
4
2
+
Bis(Bipyridine)cobalt(II) chloride (4c): Brown powder, 72.1%
Figs. 1–3. The [Co(Phen)3] cation coordinated by six N atoms of
Phen displayed a distorted octahedral coordination geometry and
À1
yield. IR (KBr, cm ): 1595(s), 1574(m), 1490(m), 1473(s),
1
7
440(s), 1315(s), 1174(m), 1157(m), 1060(m), 1037(s), 1018(m),
85(s), 766(s), 737(s), 650(m), 627(m), 420(m). Anal. Calc. for C20-
was balanced by two [BF ] counter anions, both of which had a
slightly distorted tetrahedral coordination geometry. The mean
4
H
16
C
l2CoN
4
: C, 54.32; H, 3.65; N, 12.67. Found: C, 54.40; H, 3.66; N,
deviation of the cobalt atom from the equatorial plane (N(1)–
N(3)–N(4)–N(6)) was 0.176 Å, and all the bond angles in the equa-
torial plane were in the range of 77.97–96.18° (N(1)–Co–N(3),
92.13°; N(3)–Co–N(4) 77.97°; N(4)–Co–N(6), 95.26°; N(1)–Co–
N(6), 96.18°). The axial Co–N bonds subtended an angle of
170.11° (N(2)–Co–N(5)), and the axial bond lengths of the Co–N
bonds were both 2.142 Å. Compound 4a (Fig. 3) had similar struc-
tural features as the cation of 1a, and the coordination geometry
around the cobalt center was distorted octahedron. The N1, N3,
Cl1, and Cl3 atoms were located in the equatorial plane with a
mean deviation of 0.164 Å, and the equatorial angles N(1)–Co–
N(3), N(3)–Co–Cl(1), Cl(1)–Co–Cl(2), and Cl(2)–Co–N(1) were
84.82°, 89.32°, 98.12°, and 88.93° respectively. As expected, the
1
2.62%.
Bis(Imidazole)cobalt(II) chloride (4d): Blue powder, 85.4% yield.
À1
IR (KBr, cm ): 1539(m), 1487(m), 1327(m), 1256(m), 1107(w),
1
068(s), 940(m), 852(w), 665(s), 614(s). Anal. Calc. for C
6 6 l2-
H C
CoN : C, 27.30; H, 2.29; N, 21.22. Found: C, 27.34; H, 2.31; N,
4
2
1.20%.
2.3. Procedure for butadiene polymerization
All manipulations were carried out under a dry nitrogen atmo-
sphere. Butadiene polymerization was conducted as follows: 1a
was added as a solid to a moisture-free ampoule capped with a
Table 3
Effects of various counter ions and ligands on butadiene polymerization.
Entrya
Catalyst
Yield (%)
Microstructureb
M
w
c
(Â10À4
)
M
w n
/M c
cis-1,4 (%)
trans-1,4 (%)
1,2 (%)
1
2
3
4
5
6
7
8
9
[Co(Phen)
[Co(Phen)
[Co(Phen)
[Co(Phen)
3
3
3
2
][BF
][PF
][SbF
]Cl
4
]
2
99.9
93.9
93.0
88.0
86.6
80.5
95.3
89.0
79.0
65.5
92.9
89.9
93.6
86.4
94.8
84.5
95.2
88.5
94.7
86.6
5.0
7.6
7.6
9.8
3.8
11.5
3.2
8.5
3.9
11.0
2.1
2.5
2.4
3.8
1.4
4.0
1.6
3.0
1.4
2.4
47.9
20.6
26.5
17.3
42.6
21.5
50.0
19.8
57.5
23.4
3.8
4.6
4.0
5.3
3.7
4.3
3.6
3.9
3.6
4.1
6
]
2
6 2
]
2
[Co(Bipy)
[Co(Bipy)
3 4 2
][BF ]
2
]Cl
][BF
]Cl
2
[Co(BZI)
[Co(BZI)
4
4
]
2
2
2
[Co(Im)
[Co(Im)
4
][BF
]Cl
4 2
]
1
0
2
2
a
b
c
Polymerization conditions: [Bd] = 0.1 g/mL, [Bd]/[Co] = 2000, [Al]/[Co] = 50, toluene, 50 °C, 40 min.
Determined by FTIR.
Determined by GPC.

1
2
L. Liu et al. / Inorganica Chimica Acta 414 (2014) 8–14
Table 4
Effects of [Al]/[Co] and [Bd]/[Co] on butadiene polymerization behaviors with [Co(Phen)
3
4 2
][BF ] /EASC.
Entrya
[Al]/[Co]
[Bd]/[Co]
Yield (%)
Microstructureb
M
w
c
(Â10À4
)
M
w n
/M c
cis-1,4 (%)
trans-1,4 (%)
1,2 (%)
1
1
1
1
1
1
1
1
2
3
4
5
6
7
10
10
10
30
50
70
100
1000
2000
3000
2000
2000
2000
2000
49.4
32.1
5.8
49.8
78.0
79.5
84.8
96.7
96.8
94.5
96.6
95.8
95.5
95.5
2.1
1.7
3.4
2.0
2.5
2.6
2.6
1.2
1.5
2.1
1.4
1.7
1.9
1.9
51.5
62.2
63.1
63.6
63.5
64.9
65.3
2.7
2.3
2.3
2.4
3.0
2.8
3.3
a
b
c
Polymerization conditions: [Bd] = 0.1 g/mL, 25 °C, toluene, 40 min.
Determined by FTIR.
Determined by GPC.
Co–Cl bond lengths of 4a (Co–Cl(1) = 2.3915 Å, Co–Cl(1) =
the N atoms of the Phen ligand. These results indicated that the
metal center of the synthesized cobalt compounds was cationic
in nature, whereas that in the traditional cobalt chloride com-
pound was not. We tentatively hypothesized that such compounds
probably catalyzed butadiene polymerization more effectively and
rapidly during the chain initiation and propagation.
2
.4213 Å) were shorter than the CoÁ Á ÁB bond distances of 1a
(
CoÁ Á ÁB(1) = 7.401 Å, CoÁ Á ÁB(2) = 5.776 Å). Similarly, compound 1b
À 2+
consisted of two [BF
4
]
anions, and the [Co(BZI)
4
]
cation was
coordinated by four N-atoms of BZI. The geometries of the cation
and anion were both confirmed as a slightly distorted tetrahedron.
Steric and electronic effects of the various ligands had significant
influence on the electropositivity of the center metal, which
affected the interaction between the metal and counter ions. The
different interactions between the cation and the anion were re-
flected in the difference in the CoÁ Á ÁB distance. The CoÁ Á ÁB distances
in 1b (CoÁ Á ÁB(1) = 5.299 Å, CoÁ Á ÁB(2) = 5.585 Å) were shorter than
those in 1a (CoÁ Á ÁB(1) = 7.401 Å, CoÁ Á ÁB(2) = 5.776 Å), presumably
due to the stronger coordination between the cobalt center and
3.2. Butadiene polymerization of compounds activated by
alkylaluminium chlorides
As shown in Table 2, butadiene polymerization initiated by the
cobalt(II) compounds was carried out in conjunction with EASC as
a cocatalyst. The catalytic activity of the cationic cobalt(II) com-
pounds in the polymerization of butadiene was higher, as com-
pared to that of the traditional cobalt(II) compound. Furthermore,
the catalytic activity of the compounds was affected by the
corresponding counter ions (Entries 1–4) was in the following
order: [BF
of the polybutadiene prepared using cationic cobalt(II) catalysts
Entries 1–3) was higher as compard to that of the polybutadiene
4 6 6
] > [PF ] > [SbF ] > Cl. Additionally, the cis-1,4 content
(
prepared using the traditional cobalt(II) compound (Entry 4). This
indicated that the catalytic activity and stereo-selectivity of the
cationic compounds with counter ions were superior to that of co-
balt chloride compound in the polymerization of butadiene. In
addition, M of the polymers obtained from the cationic cobalt(II)
w
compounds (Entries 1–3) was higher than that of the polybutadi-
ene prepared using the traditional cobalt(II) compound. The higher
w
M of obtained polybutadiene could be ascribed to the higher
chain propagation rate and/or lower chain transfer rate of the cat-
ionic cobalt(II) compounds. The effect of N-bearing ligands on the
butadiene polymerization was also studied. As shown in Table 3,
w
the cis-1,4 content and M of the polymers prepared using the
Fig. 4. Plots of yield and cis-content against [Al]/[Co] ratio.
cationic cobalt(II) compounds (Entry1, Entry 4–10) were more
Table 5
3 4 2
Effects of Polymerization Temperature and Time on Butadiene Polymerization with [Co(Phen) ][BF ] /EASC.
Entrya
T (°C)
t (min)
Yield (%)
Microstructureb
M
w
c
(Â10À4
)
M
/M c
w n
cis-1,4 (%)
trans-1,4 (%)
1,2 (%)
2
2
2
2
2
2
2
2
2
0
1
2
3
4
5
6
7
8
0
40
40
40
40
40
20
80
120
240
–
–
–
–
–
–
25
50
70
90
25
25
25
25
32.1
80.3
82.5
91.0
13.5
53.7
64.1
78.5
96.8
91.5
87.0
81.0
96.8
95.8
95.2
94.9
1.7
5.0
8.5
13.6
2.0
2.5
3.1
3.1
1.5
3.5
4.4
5.4
1.3
1.7
1.7
1.9
62.2
34.9
29.1
27.0
52.5
67.1
65.2
53.2
2.3
3.9
5.7
7.0
2.3
2.9
2.9
3.4
a
b
c
Polymerization Conditions:[Bd] = 0.1 g/mL, [Bd]/[Co] = 2000, [Al]/[Co] = 10, toluene, 40 min.
Determined by FTIR.
Determined by GPC.

L. Liu et al. / Inorganica Chimica Acta 414 (2014) 8–14
13
1
7
1
1
2, 14–17). At a [Al]/[Co] molar ratio of 50, the conversion reached
8.0% (Entry 15). Further increase in the [Al]/[Co] molar ratio to
00 produced a slight increase in the catalytic activity (Entries
w
6, 17; Fig. 4). In addition, the M and polydispersity of the resul-
tant polybutadiene were less affected by the cocatalyst concentra-
tion, which indicated that the transfer to the cocatalyst was very
weak.
The effects of polymerization temperature and time on the
microstructure of the polymer were examined and the results are
summarized in Table 5. The yield of the polymer increased observ-
ably with increased temperature (Entries 20–24), while the M
drastically decreased and the distribution became broader
Fig. 5). This result suggested that rate of the chain transfer reac-
tion increased with increasing temperature. In addition, the cis-
,4 content of the polymer decreased with increasing temperature,
w
(
1
whereas the trans-1,4 and 1,2 content increased correspondingly.
As shown in Fig. 6, the polymer yield increased with increasing
polymerization time, while the cis-1,4 content slightly decreased
Fig. 5. Plots of molecular weight and polydispersity against polymerization
temperature.
(
Entries 25–28).
higher than those in the case of the traditional one. The highest
yield was achieved with compound 1a, and hence, this compound
was chosen as the optimal catalyst for further investigations.
Butadiene polymerization was carried out with varying [Al]/
3.3. Mechanism of butadiene polymerization
Cationic cobalt(II) catalysts with counter ions produced polybu-
tadiene in a high yield and cis-1,4 content as compared to the tra-
ditional cobalt(II) catalysts. Compound 1a was used as an example
to explain the catalytic mechanism in the case of the novel cationic
cobalt compounds. Porri et al. demonstrated that the active species
of alkylated cobalt compounds were monovalent in hydrocarbon-
based solvents [37]. The polymerization mechanism in the case
of the traditional cobalt catalyst and cationic cobalt catalysts are
given in Scheme 2. The polymerization occurred via two steps after
the catalyst was activated by the cocatalyst. In the first step, the
butadiene monomer coordinated to the active center, followed by
the migratory insertion of the coordinated monomer into the co-
balt-carbon bond. In the migratory insertion step (second step), a
vacant coordination site was regenerated, which enabled further
chain propagation. Compared to the traditional cobalt catalyst,
the active center of the cationic cobalt catalyst showed very strong
negative electron affinity, which increased the coordination and
insertion rate of the butadiene monomer [38–40]. Additionally,
we hypothesized that the cationic cobalt center (electron deficient)
would lead to an attractive force to the C@C double bond of the
penultimate monomer unit. Therefore, the polybutadiene afforded
by the cationic cobalt(II) catalyst had a higher cis-1,4 content as
compared to polybutadiene prepared by using the traditional
catalyst.
[
Co] molar ratios and catalyst concentrations; the results are
shown in Table 4. The catalytic activity gradually decreased with
higher [Bd]/[Co] ratios, and the maximum yield (49.4%; Entry 11)
was observed at the lowest [Bd]/[Co] ratio (1000). Higher activities
were observed with increased ratios of [Al]/[Co] (10–100; Entries
Fig. 6. Plots of yield and cis-content against polymerization time.
Scheme 2. The proposed polymerization mechanism of (a) traditional cobalt(II) catalyst and (b) cationic cobalt(II) catalyst toward butadiene polymerization.

1
4
L. Liu et al. / Inorganica Chimica Acta 414 (2014) 8–14
[
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4
. Conclusion
A new and effective methodology toward improving the perfor-
mance of a traditional cobalt(II) catalyst was established. A series
of highly active cationic cobalt(II) catalysts were successfully syn-
thesized. The active centers of the catalysts prepared were cationic
in nature, whereas the active center of the traditional catalyst was
not. In addition, the catalytic activity and stereoselectivity of the
cationic compounds were superior to those of cobalt chloride in
the polymerization of butadiene. The mechanism of butadiene
polymerization with the novel cationic cobalt(II) catalyst system
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