Stereospecific polymerization of 1,3-butadiene catalyzed by cobalt complexes bearing N-containing diphosphine PNP ligands

Article abstract of DOI:10.1016/j.jorganchem.2012.05.051

A series of cobalt complexes bearing N-containing diphosphine PNP ligands has been synthesized and characterized. The nature of the ligand structure affects the binding of the ligand to the cobalt center and determines the coordination geometry of the cobalt complexes. All the complexes have been employed to catalyze the polymerization of 1,3-butadiene, in combination with methylaluminoxane (MAO) or ethylaluminum sesquichloride (EASC) as the cocatalyst. Both the nature of the ligand and the type of cocatalyst had a remarkable influence on the polymerization activity, microstructure and molecular weight of the resulting polymers. The [Co]/MAO catalytic systems resulted in relatively lower conversions of butadiene and cis-1,4 contents in the polymers than the corresponding [Co]/EASC catalytic systems. Upon activation with EASC, the polymerization behaviors of the catalytic systems were also affected by the reaction parameters.

Full text of DOI:10.1016/j.jorganchem.2012.05.051

Journal of Organometallic Chemistry 716 (2012) 55e61
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Stereospecific polymerization of 1,3-butadiene catalyzed by cobalt complexes
bearing N-containing diphosphine PNP ligands
,
a,
Lin Chen a, Pengfei Ai a, Jianming Gu b, Suyun Jie a , Bo-Geng Li
*
*
a State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China
b Department of Chemistry, Zhejiang University, Hangzhou 310027, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of cobalt complexes bearing N-containing diphosphine PNP ligands has been synthesized and
characterized. The nature of the ligand structure affects the binding of the ligand to the cobalt center and
determines the coordination geometry of the cobalt complexes. All the complexes have been employed
to catalyze the polymerization of 1,3-butadiene, in combination with methylaluminoxane (MAO) or
ethylaluminum sesquichloride (EASC) as the cocatalyst. Both the nature of the ligand and the type of
cocatalyst had a remarkable influence on the polymerization activity, microstructure and molecular
weight of the resulting polymers. The [Co]/MAO catalytic systems resulted in relatively lower conversions
of butadiene and cis-1,4 contents in the polymers than the corresponding [Co]/EASC catalytic systems.
Upon activation with EASC, the polymerization behaviors of the catalytic systems were also affected by
the reaction parameters.
Received 20 April 2012
Received in revised form
24 May 2012
Accepted 30 May 2012
Keywords:
Cobalt
Phosphine
1,3-Butadiene
Polymerization
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
organometallic single-site catalysts bearing chelating organic
ligands, mainly based on rare earth metals and firsterow transition
It is well known that the polymerization of 1,3-butadiene (BD)
leads to the formation of polybutadienes (PBDs) with different
microstructures, including cis-1,4-, trans-1,4-, syndiotactic 1,2- and
isotactic 1,2-PBD via 1,4- or 1,2-insertions, which endow PBDs with
extensive and diverse properties and applications [1]. For the
industrial production of PBDs, the ZieglereNatta catalysts based on
titanium [2], nickel [3,4], cobalt [5,6] or neodymium [7] with
halides, carboxylates, acetylacetonates or alcoholates, are currently
employed in combination with various organoaluminums. Among
those catalysts, the addition of phosphines to certain cobalt-based
catalytic systems could alter the regio- and/or stereospecificity of
the active species from cis-1,4-specific to syndiotactic 1,2-specific
[8e17]. For example, the halides and carboxylates of cobalt are
stereoselective to high cis-1,4-PBD upon activation with methyl-
aluminoxane (MAO) [18e22]. However, the cobalt phosphine
complexes of the type CoCl2(PRPh2)2 (R ¼ methyl, ethyl, n-propyl,
isopropyl, and cyclohexyl) have been reported to be highly active
and stereoselective in the polymerization of butadiene in combi-
nation with MAO, giving 1,2-vinyl polymers (1,2-vinyl content: ca.
80%) with a syndiotacticity degree depending on the phosphine
ligand bound to the metal center [10e17].
metals, have been focused on [1,23e27]. Among those cobalt-based
catalysts, the addition of triphenylphosphine to the chelate cobalt
catalytic systems containing bis(benzimidazolyl)amine ligands led
to great increase in the 1,2-vinyl content, when activated with MAO
[28,29]. However, the addition of triphenylphosphine to the cata-
lytic systems consisting of cobalt complexes with 3-
aryliminomethyl-2-hydroxybenzaldehyde ligands and ethyl-
aluminum sesquichloride (EASC) didn’t change the polymer
microstructure [30]. This demonstrated that the type of cocatalyt
affected importantly not only the catalytic activity but also the
microstructure of polymers whether the phosphine-containing or
phosphine-free catalytic systems. In order to further prove this
point, a series of chelate cobalt complexes bearing nitrogen-
containing phosphine PNP ligands, which combine two soft (P)
and one hard (N) donor atoms to give flexible coordination modes,
has been synthesized and investigated in the polymerization of
butadiene with the cocatalyst MAO or EASC in this paper.
2. Results and discussion
2.1. Synthesis and characterization of PNP ligands and their cobalt
complexes
In the interest of better control over catalytic activity, molecular
weight and microstructure of the resulting polymers, well-defined
Bis[2-(diphenylphosphino)ethyl]amine
(L1,
PNHP)
was
prepared as an air-stable bis[2-(diphenylphosphino)ethyl]amine
hydrochloride (L1$HCl, PNHP$HCl) by the reaction of bis(2-
* Corresponding authors. Tel.: þ86 571 87951515; fax: þ86 571 87951612.
E-mail addresses: jiesy@zju.edu.cn (S. Jie), bgli@zju.edu.cn (B.-G. Li).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.05.051

56
L. Chen et al. / Journal of Organometallic Chemistry 716 (2012) 55e61
chloroethyl)amine hydrochloride with the in-situ formed potas-
sium diphenylphosphide from diphenylphosphine and potassium
tert-butoxide in dry THF according to the literature method
[31e33]. This ligand is a typical representative of N-containing
diphosphine PNP ligands where an amine nitrogen atom is doubly
connected via highly flexible ethylenic chains to two diphenyl-
phosphine ends. The ligand L1$HCl reacted directly with one equiv.
of CoCl2 in degassed absolute ethanol at room temperature to yield
a green powder, which was confirmed to be complex 1a by the
single crystal X-ray diffraction analysis (Fig. 1). However, the reac-
tion of ligand L1, obtained by the treatment of L1$HCl with sodium
hydroxide, with one equiv. of CoCl2 under the same conditions
afforded the corresponding complex 1b as a purple powder
(Scheme 1). Although it was not successful to grow crystals suitable
for a crystallographic analysis, we assigned 1b to be a tetrahedral
geometry with the cobalt center coordinated by two chlorides and
the two phosphorus atoms from the ligand, as reported in the
literature [34]. This ligand was usually used to form chromium
complexes as the efficient catalyst precursors for the selective tri-
merization of ethylene [35e37] and its nickel and palladium
complexes have been also reported [33,38,39]. Since the complexes
1a and 1b have low solubility in ethanol, the products directly
precipitated from the reaction solutions, which made for conve-
nient separation and good yields.
In the solid state of complex 1a, one chloride of HCl molecule
from the ligand PNHP$HCl is unexpectedly coordinated to the
cobalt center, which makes the formation of six-coordinated
complex displaying a distorted octahedral geometry as shown in
Fig. 1. It has been reported that the addition of a third chloride, from
either hydrochloric acid or ammonium chloride, to tetrahedral
cobalt(II) complex with an aminodiphosphine ligand, resulted in
the transfer of the third chloride to the cobalt center, even if the
coordination geometry around the cobalt center was still tetrahe-
dronal with PNP acting as a monodentate ligand through one
phosphorus atom [40]. In the molecular structure of 1a, PNP atoms
of the tridentate ligand are coordinated in a meridional manner, in
which the P1, P2, Cl1, and Cl3 atoms could be located in an equa-
torial plane and the axial bonds nearly form a straight angle
(N1eCo1eCl2, 179.1(2)ꢀ). All the bond angles in the equatorial
plane are very close to a right angle in the range from 88.24(1)ꢀ to
90.74(1)ꢀ. The Co1eN1 bond length (1.997(7) Å) is short enough to
form a strong coordination bond. The Co1eCl1 (from HCl) bond was
slightly shorter than the Co1eCl2 and Co1eCl3 (both from CoCl2)
bonds. The slight difference of the two CoeP bonds indicates the
asymmetry of the whole molecule without a mirror plane.
H
N
Cl
H2
N
CoCl2
Ph2P
Cl
PPh2
Ph2P
PPh2
Co
EtOH, r.t.
L1·HCl
H
Cl
Cl
1a
NaOH
EtOH-CH2Cl2
H
N
H
CoCl2
N
Ph2P
PPh2
Ph2P
PPh2
EtOH, r.t.
Co
L1
Cl
Cl
1b
Scheme 1. Synthesis of complexes 1a and 1b.
reaction of 2,6-bis(chloromethyl)pyridine and the in-situ formed
potassium diphenylphosphide from diphenylphosphine and
potassium tert-butoxide in dry THF afforded the formation of 2,6-
bis((diphenylphosphino)methyl)pyridine (L2) in good yield, in
which the PPh2 substituents are connected to the central pyridine
backbone by CH2 groups. Complex 2 was readily prepared as
a greenish blue powder by the reaction of ligand L2 with CoCl2 in
absolute ethanol (Scheme 2), forming a trigonal-bipyramidal
coordination geometry with two 5-membered chelate rings,
which was applied as catalyst precursors for the polymerization of
ethylene after activation with MAO [42], but much less active than
the bis(imino)pyridyl catalysts (NNN ligands) [43,44].
The
Schiff-base
diphenylphosphinobenzenaldehyde
condensation
reaction
(2-PCHO)
of
2-
2-
with
aminophenyldiphenylphosphine (2-PNH2) in the presence of
a catalytic amount of toluene-p-sulfonic acid in refluxing benzene
yielded the ligand, 2-(diphenylphosphino)-N-[2-(diphenylphos-
phino)benzylidene]aniline (L3, PN]CHP) (Scheme 3), which has
been used for binding to a range of metals [45e47]. The reduction
of C]N double bond in the ligand L3 with the excess amount of
sodium borohydride in methanol resulted in the formation of
ligand L4 with amino group. These two ligands can be visualized as
two triphenylphosphine moieties tethered together in the ortho
positions by an imino or amino linker. The corresponding cobalt
complex 3 and 4 with this type of ligands acting as a bidentate
ligand through two phosphorus atoms binding to the cobalt center
were obtained as blue or green powders. Complex 3 was reported
with less interest for ethylene activity because of its inactivity on
treatment with Al-containing cocatalyst (MAO or AlEt3) [48]. All the
synthesized cobalt complexes are air-stable in the solid state, but in
solution they are sensitive to traces of water and oxygen.
2,6-Bis(chloromethyl)pyridine was prepared by the quantitative
transformation of 2,6-bis(hydroxymethyl)pyridine reacted with the
stoichiometric quantity of thionyl chloride in dry THF [41]. The
2.2. Butadiene polymerization catalyzed by cobalt complexes
2.2.1. MAO as cocatalyst
The CoCl2(PR1R22)2/MAO (R1, R2 ¼ Me, Et, n-Pr, i-Pr, t-Bu, Cy, Ph)
and CoCl2[R2P(CH2)nPR2]/MAO (R ¼ Me, Et, Ph; n ¼ 1,2) catalytic
Fig. 1. Molecular structure of complex 1a. Hydrogen atoms (apart from H1 and H1A)
and solvent molecules have been omitted for clarity.
Scheme 2. Synthesis of ligand L2 and complex 2.

L. Chen et al. / Journal of Organometallic Chemistry 716 (2012) 55e61
57
complexes had moderate molecular weight (82,000e127,000 g/
mol) with relatively narrow molecular weight distributions
(2.35e3.57) and the lowest molecular weight along with the
broadest molecular weight distribution was observed for complex 5
(entry 6 in Table 1).
2.2.2. EASC as cocatalyst
In order to gain higher catalytic activities and cis-1,4 content,
ethylaluminum sesquichloride (EASC) was attempted as cocatalyst
since it has been proved to be more effective for the chelate cobalt
complexes in the polymerization of butadiene [30,50e54]. With the
1a/EASC catalytic system, reaction parameters were varied to
investigate their effects on the catalytic activities and properties of
the resulting polymers and to optimize the polymerization condi-
tions. The Al/Co molar ratio had a significant influence on the cata-
lytic activity and molecular weight. The Al/Co molar ratio of 20
afforded the 60.1% conversion of butadiene (entry 1 in Table 2). The
conversion of butadiene and molecular weight of the resulting
polymers increased graduallyalong with the increase of Al/Co molar
ratio from 20 to 80 (entries 1e3 in Table 2) and a complete conver-
sion of butadiene was obtained under the Al/Co molar ratio of 80 in
1 h. When the Al/Co molar ratio was further increased to 160, the
monomer could be completely transformed to polymer as well, but
the molecular weight of obtained polymer decreased obviously,
from which we could infer that the chain transfer to aluminum
reacted on the molecular weight in the current catalytic system.
However, the Al/Co molar ratio slightly affected the molecular
weightdistribution in therangeof2.09e2.57 andthe microstructure
of the resulting polymers didn’t change apparently, having the cis-
1,4 content in the range of 95.6%e96.7% along with small amounts of
trans-1,4-PBD (1.2%e2.6%) and 1,2-PBD (1.8%e2.5%).
The butadiene polymerizations were also carried out over the
temperature range from 0 to 90 ꢀC at an Al/Co molar ratio of 80 with
the 1a/EASC catalytic system (entries 3 and 5e8 in Table 2). Similar
to other chelate cobalt-based catalysts [30,50e53], the butadiene
polymerization behavior with the 1a/EASC catalytic system was
very sensitive to reaction temperature (Figs. 2 and 3). The 29.0%
conversion of butadiene, highest cis-1,4 content of 98.6% and
highest molecular weight were obtained at 0 ꢀC. The highest
catalytic activity was gained at 25 ꢀC with the complete conversion
of butadiene. However, the catalytic activity decreased remarkably
as the temperature was enhanced. The 1a/EASC catalytic system
was relatively stable at 50 ꢀC and the conversion of butadiene was
85.2%. But the conversion dropped sharply to 48.2% at 70 ꢀC and
20.1% at 90 ꢀC, which was in consistent with the olefin polymeri-
zation catalyzed by the late-transition metal complexes and was
generally explained due to the deactivation of active species at
higher temperatures [55e57]. In general, the variation of reaction
temperature results in the important change of polymer micro-
structure. The content of cis-1,4-PBDs in the polymers decreased
gradually along with the elevation of reaction temperature, but the
trend of trans-1,4 content was opposite. When the temperature was
increased from 0 ꢀC to 70 ꢀC, the cis-1,4 content in the polymers
decreased from 98.6% to 92.0%. Nevertheless, for the current
system, the PBDs produced at 50 ꢀC and 70 ꢀC had similar molecular
weights and microstructures (Figs. 2 and 3). The polymerization at
90 ꢀC yielded the 13.8% content of trans-1,4 isomer and the 9.4%
content of 1,2 isomer; and the cis-1,4 content reduced to 76.8%. The
molecular weights of PBDs were also decreased from 283,000 to
23,000 g/mol as the temperature was increased from 0 ꢀC to 90 ꢀC,
but the molecular weight distributions (1.65e2.27) of PBDs didn’t
change greatly.
Scheme 3. Synthesis of ligands L3, L4 and complexes 3, 4.
systems were reported to be very active for the polymerization of
butadiene [25]; the former yielded the mixture of cis-1,4-PBD and
1,2-PBD depending on the bulkiness of R1 and R2 groups on the
phosphorous atom, whereas the latter bearing bidentate PP ligands
mainly produced cis-1,4-PBD with the cis-1,4 content over 94% [49].
Therefore, the solution polymerizations of butadiene catalyzed by
complexes 1a, 1b, and 2e4 bearing N-containing diphosphine PNP
ligands [CoCl2(PPh3)2 (5) as a reference catalyst] were firstly carried
out upon activation with MAO and the results are shown in Table 1.
The chelate complexes 1a, 1b, and 2e4/MAO systems were found to
be much less active than complex 5 with free PPh3 ligands and the
corresponding conversions of butadiene ranged from 12.7% to
46.1%, while the conversion of 5/MAO system reached 94.2% under
the same reaction conditions. This indicated that the chelate cata-
lyst structures were more unfavorable than the nonchelate ones for
the current PNP cobalt complexes/MAO catalytic systems in buta-
diene polymerization. It is interesting to notice that much higher
conversion of butadiene was obtained when the imino group in
ligand L3 was reduced to amino group (ligand L4), comparing
complex 3 (conversion: 12.7%) with complex 4 (conversion: 46.1%)
although the nitrogen atoms were not coordinated to the cobalt
center (entries 4 and 5 in Table 1). Similar to the results reported in
the literature [16,17], the 5/MAO system produced much more
amount of 1,2-PBD (1,2-vinyl content: 76.9%) than cis-1,4-PBD (cis-
1,4 content: 20.6%) because the nature of PPh3 as a strong s-donor
makes it more preferable to gain a high content of 1,2-PBD (entry 6
in Table 1). However, for the chelate complexes 1a, 1b, and 2e4/
MAO systems, the cis-1,4-PBD as the main product ranging from
75.7% to 88.8% was obtained along with a relatively higher content
(9.0%e20.6%) of 1,2-PBD formed. Furthermore, among the above
cobalt complexes with PNP ligands, the four-coordinated 1b, 3 and
4 yielded slightly lower cis-1,4 and higher 1,2 contents than the
other five or six-coordinated 1a and 2. The PBDs produced by all the
Table 1
Polymerization of 1,3-butadiene with complexes 1a, 1b, 2e5/MAO.a
Entry Cat. Conv. (%) Mnb (104 g/mol) Mw/Mn Microstructurec (mol%)
b
cis-1,4- trans-1,4- 1,2-
1
2
3
4
5
6
1a
1b
2
3
4
18.7
25.4
17.7
12.7
46.1
94.2
12.7
10.4
9.04
12.7
9.94
8.20
2.37
2.76
2.35
2.45
2.78
3.57
83.1
80.3
88.8
75.7
78.9
20.6
2.8
2.8
2.2
3.7
1.8
2.5
14.1
16.9
9.0
20.6
19.3
76.9
5
a
Polymerization conditions: precatalyst: 5.0 mmol, cocat.: MAO, [Al]/[Co]: 1000,
solvent: toluene, total volume: 10 mL, [BD]/[Co]: 2000, reaction temperature: 25 ꢀC,
reaction time: 2 h.
The other synthesized cobalt complexes (1b and 2e5) were also
used for butadiene polymerization to examine the influence of
ligand environment on catalytic activity and properties of the
b
Determined by GPC against polystyrene standards and reported uncorrected.
Determined by 1H and 13C NMR spectroscopy.
c

58
L. Chen et al. / Journal of Organometallic Chemistry 716 (2012) 55e61
Table 2
Polymerization of 1,3-butadiene with complexes 1a, 1b, 2e5/EASC.a
b
b
Entry
Cat.
Al/Co
T
(ꢀC)
t
Conv.
(%)
Mn
Mw/Mn
Microstructurec (mol%)
(min)
(105 g/mol)
cis-1,4-
trans-1,4-
1,2-
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1b
2
3
4
5
1a
1b
2
3
4
20
40
80
160
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
25
25
25
25
0
60
60
60
60
60
60
60
60
60
60
60
60
60
30
30
30
30
30
30
60.1
83.6
100
1.76
2.12
2.32
1.60
2.8s
0.90
0.83
0.23
2.33
1.91
3.90
2.40
2.15
1.81
2.33
1.89
2.89
2.30
1.76
2.09
2.27
2.27
2.57
1.65
2.33
1.90
1.70
2.19
2.34
1.84
2.29
2.24
2.68
2.12
2.39
1.99
1.94
2.05
96.2
96.7
96.6
95.6
98.6
93.1
92.0
76.8
96.6
97.1
98.4
98.1
96.9
97.3
97.0
98.0
98.1
98.7
96.9
1.3
1.3
1.2
2.6
0
3.7
4.7
13.8
1.1
1.4
0
2.5
2.0
2.2
1.8
1.4
3.2
3.3
9.4
2.3
1.5
1.6
1.9
1.7
1.8
2.3
1.5
1.9
1.3
1.8
100
29.0
85.2
48.2
20.1
95.3
100
34.5
100
100
98.0
67.0
92.3
22.7
92.5
100
50
70
90
25
25
25
25
25
25
25
25
25
25
25
9
10
11
12
13
14
15
16
17
18
19
0
1.4
0.9
0.7
0.5
0
0
1.3
5
a
Polymerization conditions: precatalyst: 5.0 mmol, cocat.: EASC, solvent: toluene, total volume: 10 mL, [BD]/[Co]: 2000.
b
c
Determined by GPC against polystyrene standards and reported uncorrected.
Determined by 1H and 13C NMR spectroscopy.
resulting polymers. The complete conversions of butadiene were
obtained by the 1a, 2, 4, 5/EASC catalytic systems and the 1b/EASC
system gave a slightly lower conversion of butadiene (95.3%, entry
9 in Table 2). However, complex 3 with imino group was much less
active for butadiene polymerization and the 34.5% conversion of
butadiene was acquired under the same conditions (entry 11 in
Table 2). In contrast with the [Co]/MAO catalytic systems, the PBDs
produced by all the [Co]/EASC catalytic systems had high cis-1,4
contents (96.6%e98.4%) and higher molecular weight
(191,000e390,000 g/mol). The formation of high cis-1,4-PBD with
the CoCl2(PPh3)2 (5)/EASC system proved that the addition of PPh3
to the P-free [Co]/EASC could not change the microstructure of
PBDs or further increase the 1,2-vinyl content [30]. In order to make
a better comparison, the reaction time was shortened from 1 h to
30 min. The high conversions of butadiene (92.3%e100%) were still
maintained by the 1a, 2, 4, 5/EASC catalytic systems in 30 min,
while the conspicuous decrease in monomer conversion was
observed with the 1b, 3/EASC catalytic systems, to 67.0% and 22.7%
in 30 min, respectively. Differently from the MAO systems, the six-
coordinated complex 1a (98.0% in 30 min) produced a higher
conversion of butadiene than the corresponding four-coordinated
1b (67.0% in 30 min) upon activation with EASC as cocatalyst. The
complex 4 with amino group in the ligand was much more active
for butadiene polymerization than the complex 3 with imino group
in either 1 h or 30 min, which probably ascribed to the rigid ligand
structure and bulkiness of L3 restricting the insertion of butadiene
to the active center in more extent. The molecular weights of PBDs
obtained in 30 min were slightly lower than the corresponding
ones in 1 h. However, the variation of reaction time nearly didn’t
affect the molecular weight distributions (1.94e2.68) and the
microstructures (cis-1,4 contents in the range of 96.9%e98.7%) of
the resulting polymers.
100
90
3.00
100
80
cis-1,4
80
60
40
20
2.25
1.50
0.75
0.00
trans-1,4
70
1,2
10
5
0
0
20
40
60
80
100
0
20
40
60
80
100
Temperature (oC)
Temperature (oC)
Fig. 2. The effects of reaction temperature on the conversion (%) of butadiene (-) and
molecular weight of PBDs (=) with the 1a/EASC catalytic system (entries 3 and 5e8 in
Table 2).
Fig. 3. Microstructure (%) of PBDs produced by the 1a/EASC catalytic system at
different temperatures (entries 3 and 5e8 in Table 2).

L. Chen et al. / Journal of Organometallic Chemistry 716 (2012) 55e61
59
Table 3
2-(diphenylphosphino)benzaldehyde (2-PCHO) [58], 2-aminophe-
nyldiphenylphosphine (2-PNH2) [59], 2,6-bis((diphenylphos-
phino)methyl)pyridine (L2) and its cobalt complex (2) [42], and
CoCl2(PPh3)2 (5) [11,12] were prepared according to the literature.
Crystallographic data and refinement parameters for complex 1a$3CH2Cl2.
Formula
Formula weight 862.53
Temp. (K) 293(2)
Wavelength (Å) 0.71073
Crystal system Monoclinic
C31H36Cl9CoNP2 Dcalc (Mg mꢁ3
(mmꢁ1
F(000)
Crystal size(mm)
range (ꢀ)
Limiting indices
)
1.449
1.146
1756
0.48 ꢂ 0.30 ꢂ 0.28
2.99e25.20
ꢁ14 ꢃ h ꢃ 13, ꢁ20
ꢃ k ꢃ 20, ꢁ25 l ꢃ 23
m
)
4.2. Synthesis of ligands (L1, L3, L4)
q
Space group
P2(1)/c
4.2.1. Bis[2-(diphenylphosphino)ethyl]amine (L1)
a (Å)
b (Å)
c (Å)
12.1774(8)
16.9996(10)
19.5028(14)
No. of rflns collected 28,962
No. of unique rflns
No. of obsd
Bis[2-(diphenylphosphino)ethyl]amine
(L1,
PNHP)
was
7095
2523
prepared as an air-stable bis[2-(diphenylphosphino)ethyl]amine
hydrochloride (L1$HCl, PNHP$HCl) by the reaction of bis(2-
chloroethyl)amine hydrochloride with the in-situ formed potas-
sium diphenylphosphide from diphenylphosphine and potassium
tert-butoxide in dry THF according to a literature method [31e33].
FT-IR (KBr disc, cmꢁ1): 3422 (nNeH), 1585, 1480, 1433, 1388, 748,
rflns(I > 2
Completeness to
No. of parameters
s(I))
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
90.00
101.621(3)
90.00
q
(%) 99.6 (
q
¼ 25.20ꢀ)
372
Goodness-of-fit on F2 0.943
Volume (Å3)
3954.5(4)
Final R indices
[I > 2 (I)]
Largest diff peak,
hole (e Åꢁ3
R1 ¼ 0.0939,
wR2 ¼ 0.2374
0.600, ꢁ0.449
s
738, 695, 513. 1H NMR (400 MHz, CDCl3),
NH2Cl), 7.38e7.28 (m, 20 H, PheH), 2.93e2.91 (m, 4 H, CH2P),
2.54e2.50 (m, 4 H, NCH2). 31P {1H} NMR (CDCl3),
(ppm): e20.5 (s).
d (ppm): 9.95 (s, 2 H,
Z
4
)
d
4.2.2. 2-(Diphenylphosphino)-N-[2-(diphenylphosphino)benzylide
ne]benzenamine (L3)
3. Conclusions
The ligand L3 (PN]CHP) was prepared by using a literature
In summary, a series of chelate cobalt complexes bearing N-
containing diphosphine PNP ligands was synthesized and charac-
terized. The cocatalyst EASC was more effective than MAO for the
current cobalt-based catalytic systems to catalyze the polymeriza-
tion of butadiene to give high cis-1,4-polybutadiene with high
selectivity (>96.6%) (even for complex 5, CoCl2(PPh3)2). Upon
activation with EASC, the increase of Al/Co molar ratio enhanced
the catalytic activity, but didn’t change the polymer microstructure.
The elevation of reaction temperature led to the decrease in the
conversion of butadiene, cis-1,4 content and molecular weight. In
combination with MAO, the resultant polymers had relatively
higher 1,2-vinyl content (9.0%e20.6%). Among all the synthesized
cobalt complexes, complex 3 with imino group in the ligand
showed the lowest catalytic activity for both MAO and EASC
systems. One conclusion could be drawn that both the type of
cocatalyst and the nature of the ligand had an important influence
on the catalytic activity and polymer microstructure for the current
N-containing PNP cobalt-based catalysts.
method [45]. FT-IR (KBr disc, cmꢁ1): 1625 (
n
C]N), 1561, 1476, 1466,
1433, 1262, 1093, 1069, 1026, 803, 768, 740, 695, 503, 478. 1H NMR
(400 MHz, CDCl3),
d
(ppm): 8.92 (d, 1 H, J ¼ 4.2 Hz, CH]N),
7.97e7.94 (m, 1 H, AreH), 7.67e7.55 (m, 2 H, AreH), 7.50e7.47 (m,
1 H, AreH), 7.35e7.21 (m, 20 H, PheH), 7.06 (t,1 H, J ¼ 7.2 Hz, AreH),
6.82 (t, 1 H, AreH, J ¼ 6.0 Hz), 6.75 (t, 1 H, AreH, J ¼ 6.0 Hz), 6.46 (t,
1 H, AreH, J ¼ 6.0 Hz).
4.2.3. 2-(diphenylphosphino)-N-[2-(diphenylphosphino)benzyl]
benzenamine (L4)
To a suspension of L3 (0.3169 g, 0.58 mmol) in 40 mL of anhy-
drous methanol was slowly added sodium borohydride (0.0655 g,
1.73 mmol) under nitrogen. The reaction mixture was stirred at
room temperature for 12 h. The distilled water was added and
methanol was removed under reduced pressure. The residual
mixture was then extracted four times with dichloromethane, and
the extracts were combined and dried over sodium sulfate. After
filtration, CH2Cl2 was removed in vacuo and the residue was eluted
on a silica column with petroleum ether and ethyl acetate
(v:v ¼ 5:1) to give L4 (PNHCH2P) as a white solid (yield: 0.1632 g,
51.3%). FT-IR (KBr disc, cmꢁ1): 3422, 3053, 1589, 1484, 1436, 1176,
1119, 1097, 1069, 1027, 948, 745, 725, 694, 549, 517. 1H NMR
4. Experimental section
4.1. General considerations and materials
(400 MHz, CDCl3),
d (ppm): 7.35e7.27 (m, 20 H, PheH), 7.17e7.06
All manipulations of air- or moisture-sensitive compounds were
carried out under an atmosphere of argon using standard Schlenk
techniques. IR spectra were recorded on a PerkineElmer FT-IR 2000
spectrometer by using KBr disks or liquid films for polybutadienes
in the range 4000e400 cmꢁ1 1H NMR, 13C NMR and 31P NMR
spectra were recorded on a Bruker DMX-400 instrument in CDCl3
with TMS as the internal standard. Elemental analysis was per-
formed 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.
(m, 4 H, AreH), 6.81 (t, 1 H, J ¼ 6.0 Hz, AreH), 6.74 (t, 1 H,
J ¼ 6.0 Hz, AreH), 6.57 (t, 1 H, J ¼ 7.2 Hz, AreH), 6.33 (t, 1 H,
J ¼ 6.4 Hz, AreH), 4.46 (s, 2 H, CH2NH). 13C NMR (100 MHz, CDCl3),
d
(ppm): 135.9 (d,1J (PC) ¼ 9.7), 135.5 (d, 1J (PC) ¼ 7.4), 134.3, 134.1,
133.9, 133.8, 133.6, 132.9, 130.6, 129.0, 128.9, 128.8, 128.7, 128.6,
128.5, 127.0, 126.6, 126.5, 117.3, and 110.7 (aromatic-C), 46.0 (d, 3J
(PC) ¼ 26.9, CH2NH). 31P {1H} NMR (CDCl3),
d (ppm): e15.9 (s),
e21.7 (s). Anal. Calcd for C37H31NP2 (551.6): C, 80.57; H, 5.66; N,
2.54. Found: C, 80.74; H, 5.47; N, 2.25.
4.3. Preparation of cobalt(II) complexes
Toluene and tetrahydrofuran were refluxed over sodium-
benzophenone and distilled under nitrogen prior to use. Methyl-
aluminoxane (MAO, 10 wt% solution in toluene) was purchased
from Akzo Nobel Corp. Ethylaluminum sesquichoride (0.4 M in
hexane) was purchased from Acros Chemicals and used directly
without further 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.
4.3.1. Preparation of CoCl3(H)(PNHP) (1a)
The ligand, bis[2-(diphenylphosphino)ethyl]amine hydrochlo-
ride (L1$HCl) (0.2161 g, 0.45 mmol) and CoCl2 (0.0570 g,
0.44 mmol) were added in a Schlenk flask that was purged three
times with nitrogen and then charged with 20 mL of degassed
anhydrous ethanol. The reaction mixture was stirred at room
temperature for 3 h. The resulting mixture was concentrated and
diethyl ether was added. Then the precipitate was filtered, washed

60
L. Chen et al. / Journal of Organometallic Chemistry 716 (2012) 55e61
with diethyl ether, and dried under vacuum to give cobalt complex
1a as a green powder (yield: 0.2206 g, 82.7% based on cobalt). FT-IR
(KBr disc, cmꢁ1): 3425, 3052, 2965, 1482, 1458, 1434, 1177, 1100,
998, 742, 695, 510. Anal. Calcd for C28H30Cl3CoNP2 (607.78): C,
55.33; H, 4.98; N, 2.30. Found: C, 55.29; H, 5.13; N, 2.30.
a Rigaku RAXIS Rapid IP diffractometer with graphite mono-
chromated Mo-Ka radiation (
¼ 0.71073 Å). Cell parameters were
l
obtained by global refinement of the positions of all collected reflec-
tions. Intensities were corrected for Lorentz and polarization effects
and empirical absorption. The structures were solved by direct
methods and refined by full-matrix least-squares on F2. All non-
hydrogen atoms, except for the solvent molecules, were refined
with anisotropic atomic displacement parameters. Hydrogen atoms
ere inserted at calculated positions except for the hydrogen (H1A)
boundtoCl1, whichwaslocatedfromadifferencemapandnotfurther
refined. Structure solution and refinement were performed by using
the SHELXL-97 Package [60]. Crystallographic data and processing
parameters for complexes 1a$3CH2Cl2 are summarized in Table 3.
4.3.2. Preparation of CoCl2(PNHP) (1b)
To a solution of bis[2-(diphenylphosphino)ethyl]amine hydro-
chloride (L1$HCl) (0.1535 g, 0.32 mmol) in 20 mL of dichloro-
methane, a solution of NaOH (0.0178 g, 0.45 mmol) in 5 mL of
ethanol was added. The mixture was stirred at room temperature
for 1 h. The organic solvents were removed and the residue was
extracted with CH2Cl2 to give bis[2-(diphenylphosphino)ethyl]
amine (L1). The above ligand (L1) and CoCl2 (0.0415 g, 0.32 mmol)
were mixed in 20 mL of degassed anhydrous ethanol. The reaction
mixture was stirred at room temperature for 3 h. The resulting
mixture was concentrated and diethyl ether was added. Then the
precipitate was filtered, washed with diethyl ether, and dried under
vacuum to give cobalt complex 1b as a reddish violet powder
(yield: 0.1367 g, 74.5% based on ligand). FT-IR (KBr disc, cmꢁ1):
3429, 3297, 3049, 2858, 1483, 1434, 1407, 1100, 1028, 1001, 841, 747,
695, 507. Anal. Calcd for C28H29Cl2CoNP2 (571.32): C, 58.86; H, 5.12;
N, 2.45. Found: C, 59.22; H, 4.95; N, 2.49.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (Grant No. 21006085), the State Key Labora-
tory of Chemical Engineering (Grant No. SKL-ChE-11D03) and
National Basic Research Program of China (973 Program, grant No.
2011CB606001).
Appendix A. Supplementary material
4.3.3. Preparation of CoCl2(PN]CHP) (3)
CCDC 875645 contains the supplementary crystallographic data
for complex 1a. These data can be obtained free of charge from the
Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/
data_request/cif.
Complex 3 was prepared from the ligand L3 and CoCl2 according
to the literature method [48]. FT-IR (KBr disc, cmꢁ1): 3425, 1618
(
nC¼N), 1578, 1560, 1481, 1435, 1311, 1182, 1161, 1097, 766, 748,
695, 500.
4.3.4. Preparation of CoCl2(PNHCH2P) (4)
References
To a solution of ligand L4 (0.1500 g, 0.27 mmol) in degassed
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