Synthesis, characterization and butadiene polymerization studies of cobalt(II) complexes bearing bisiminopyridine ligand

Article abstract of DOI:10.1016/j.jorganchem.2011.01.015

A series of 2,6-bis(imino)pyridyl Co(II) complexes of the general formulas [2,6-(ArNCMe)₂C₅H₃N]CoCl₂ (Ar = -C₆H₅, 3a; 2-MeC₆H₄, 3b; 2-EtC ₆H₄, 3c

Full text of DOI:10.1016/j.jorganchem.2011.01.015

Journal of Organometallic Chemistry 696 (2011) 1584e1590
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Journal of Organometallic Chemistry
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Synthesis, characterization and butadiene polymerization studies of cobalt(II)
complexes bearing bisiminopyridine ligand
,
,
,
,
a
Dirong Gong a b, Baolin Wang a b, Hongguang Cai a b, Xuequan Zhang a , Liansheng Jiang
*
a Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, PR China
b Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR 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 2,6-bis(imino)pyridyl Co(II) complexes of the general formulas [2,6-(ArN]CMe)2C5H3N]CoCl2
(Ar ¼ eC6H5, 3a; 2-MeC6H4, 3b; 2-EtC6H4, 3c; 2-iPrC6H4, 3d; 2,6-iPr2C6H3, 3e; 4-iPrC6H4, 3f; 4-FC6H4, 3g; 4-
CF3C6H4, 3h; 2-FC6H4, 3i; 2,6-F2C6H3, 3j; 2-Me-4-FC6H3, 3k and 2,6-Me2-4-FC6H2, 3l) and [2,6-(ArN]
CH)2C5H3N]CoCl2 (Ar ¼ eC6H5, 3m; 2-EtC6H4, 3n and 4-iPrC6H4, 3o) have been synthesized and charac-
terized. The structures of new complexes 3a, 3fe3h and 3me3o are further confirmed by X-ray crystal-
lography. All complexes adopt distorted trigonal bipyramidal configuration with the equatorial plane
formed by the pyridyl nitrogen atoms and the two chlorine atoms. In the complexes 3m and 3o, three
aromatic rings are essentially coplanar, which is in sharp contrast to the other complexes, where three rings
are almost orthogonal to each other. With methylaluminoxane (MAO) as cocatalyst in toluene at room
temperature, the complexes show moderate to high conversion (42e99%) in butadiene polymerization,
producing polybutadiene with tunable cis-1,4 structure (77.5e97%) and controllable molecular weight and
molecular weight distribution. The catalytic activity, selectivity as well as the molecular weight and
molecular weight distribution of the resultant polymer are found to be dependent on the size and nature of
substituents on iminoaryl rings and their positions located. By deliberately tuning the ligand structure,
more efficient catalyst in terms of high activity and high selectivity can be obtained.
Received 4 October 2010
Received in revised form
7 January 2011
Accepted 11 January 2011
Keywords:
Cobalt(II) catalysts
2,6-Bis(imino)pyridine
Butadiene
Polybutadiene
Selectivity
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
versatile cobalt-based catalysts are much highlighted for initiating
regio- and/or stereoselective polymerization of butadiene, espe-
The past decades have experienced the rapid development of
the catalysts for regio- and/or stereoselective polymerization of
butadiene, because the resultant polybutadienes have been found
extensive and diverse applications in rubber industry. Thus,
numerous catalyst systems have been invented in past decades,
especially those based on transition metals, which under the
presence of aluminum alkyls, aluminum chlorides, MAO or borates,
provide polybutadiene in high cis-1,4 [1e11], trans-1,4 [12e16] or
1,2 configuration [17e23] used as important synthetic rubbers and
thermal plastic elastomers. Numerous metal systems, including
lithium/sodium, rare earth metal and transition metal recipes have
been explored, particular attentions have been paid to the late
transition metals such as iron-, nickel- and cobalt-based catalysts,
due to the high activity and high selectivity in some systems,
particularly the remarkable functional-group tolerance, rendering
them competitive in industrial application. Among the systems,
cially those for high cis-1,4 selectivity, because the resulted polymer
is the most important elastomer used for tires and other elastic
materials. As a result, a large number of complexes [1,24e34] have
been designed to initiate cis-1,4 selective polymerization, such as
CoCl2/MAO [33], CoCl2(PR3)2/AlEt2Cl [24] and Co(salen)2/MAO [29],
while some of them are either heterogeneous, leading to slow
initiation and gel formation [25e27], or suffering from compara-
tively low cis-1,4 selectivity [31]. Meanwhile, for some of cobalt
catalyst recipes [24,30,33,34], the active species are generated in
situ by mixing the components (cobalt compound, donor and
activator), therefore, little is known about the structure of
precursor and catalystepolymer correlation, hindering mechanism
study and catalyst design for screening more efficient candidates.
Thus, investigation of new cobalt catalyst that is homogeneous,
well-defined, straightforward and high yielding in synthesis, and
providing highly cis-1,4 oriented catalysis has still been one of the
most fascinating and challenging subjects.
Tridentate ligands such as N,N,N [35e43], N,X,N (X ¼ C, O, S)
[44e50] and X,N,X (X ¼O, P, S) [51e54] ligated complexes have
been booming in olefin polymerization over the past decade for
* Corresponding author. Tel.: þ86 431 8526 2303; fax: þ86 431 8526 2307.
E-mail address: xqzhang@ciac.jl.cn (X. Zhang).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.01.015

D. Gong et al. / Journal of Organometallic Chemistry 696 (2011) 1584e1590
1585
Scheme 1. Syntheses of complexes (3ae3o).
their crucial roles in promoting catalytic activity, improving selec-
tivity as well as controlling molecular weight of products. The
impressive catalytic performance observed in 2,6-bis(imino)pyridyl
ligated complexes, especially the remarkable activities of iron and
cobalt derivatives for ethylene polymerization, indicates the
significant scope for incorporating such kind of ligand into late
transition metal-based catalysts. Recently, Chirik et al. [55e57]
revealed how this ligand system is capable of accepting and
donating negative charges, thereby, influencing the redox potential
of the coordinated metals. More significantly, a single site active
iron-alkyl species bearing bisiminopyridine has been firstly isolated
and characterized, which even in the absence of MAO, displayed
comparable catalytic properties as compared to the corresponding
FeCl2(bisiminopyridine)/MAO systems [55]. A surprising finding of
such ligand set in olefin polymerization may also be extrapolated to
the area of conjugated diene polymerization. Reports on transition
metals bearing such kind of ligands for butadiene polymerization,
however, have only started to emerge in the literature [1,14,15,58].
We have reported 2,6-bis(imino)pyridyl ligands ligated transition
complexes for selectivity-controlled polymerization of butadiene
[59], finding the selectivity can be tuned within a wide range from
high cis-1,4 (97.0%) to high trans-1,4 (95.4%) just by altering the
exerting a positive impact on polymerization activity. More
importantly, high regio- and/or stereoselectivity can also be ach-
ieved, due to the specified coordination mode from the rigidity of
the pincer framework around the metal center [59]. In this report,
we present coordination chemistry of new cobalt dichlorides sup-
ported by 2,6-bis(iminoaryl)pyridyl ligands and their catalytic
behaviors in butadiene polymerization activated by MAO. The
substituents on the ligand backbones of the complexes varying
from alkyl-free to bulky groups, from electron-donating alkyl
groups to electron-withdrawing groups, provide an insight into
both electronic and steric effects on their butadiene polymerization
performances, which, in turn, renders more powerful catalyst
accessible by designing ligand structure.
2. Results and discussions
2.1. Synthesis and characterization of ligands and complexes
A general synthetic route for cobalt complexes used in this study
is shown in Scheme 1. The reaction of CoCl2 with 1 equiv of 2,6-bis
(imino)pyridines in THF at room temperature affords cobalt(II)
complexes with high yields. The obtained complexes are identified
by FTIR and elemental analysis. The IR of the free ligands show that
the C]N stretching frequencies appear at 1642e1624 cmꢀ1, while
in complexes, that shifts toward lower frequencies by 5e19 cmꢀ1
and the intensities are greatly reduced, indicating the occurrence of
metal center. We assume that, the large p-system, featuring all 2,6-
bis(imino)pyridyl ligands, to accommodate negative charge has
been believed to increase the Lewis-acidity of coordinated metal
centers, facilitating interaction with butadiene molecule, therefore,
Table 1
Crystal data and structure refinements of complexes 3fe3h and 3me3o.
3f
3g
3h
3m
3n
3o
Formula
C27H31N3CoCl2
527.38
C21H17F2N3CoCl2
479.22
Triclinic
C23H17F6N3CoCl2
579.23
Monoclinic
P21/n
13.189(5)
15.525(5)
14.798(5)
90.000(5)
111.709(5)
90.000(5)
2815.1(17)
4
C38H30N6Co2Cl4
830.36
Monoclinic
P2(1)/n
16.8014(14)
13.5413(12)
18.4366(16)
90
97.624(2)
90
4157.5(6)
8
1.509
1.222
1924
C23H23N3CoCl2
471.27
Monoclinic
C2/c
11.9187(11)
8.9839(8)
19.9946(18)
90
92.9290(10)
90
2138.2(3)
4
1.464
1.067
972
C25H27N3CoCl2
499.33
Monoclinic
P2(1)/n
12.2538(9)
9.3736(7)
20.8322(15)
90
94.0020(10)
90
2387.0(3)
4
1.389
0.960
1036
Molecular weight
Crystal system
Space group
Orthorhombic
P2(1)2(1)2(1)
12.3543(18)
14.073(2)
15.343(2)
90.000
90
90
2667.5(7)
4
1.313
P-1
ꢀ
a (A)
8.9684(5)
12.7985(7)
22.9214(12)
101.2960(10)
91.6320(10)
95.5380(10)
2564.9(2)
2
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
3
ꢀ
V (A )
Z
Dcalcd (Mg/m3)
Absorp coeff (mmꢀ1
F(000)
1.571
1.222
1224
1.567
1.048
1332
)
0.863
1100
Crystal size (mm)
0.29 ꢂ 0.17 ꢂ 0.09
1.96e25.03
13,981
4723 (Rint ¼ 0.0361)
4723/0/304
1.010
0.13 ꢂ 0.10 ꢂ 0.08
1.63e25.68
14,055
9500 (Rint ¼ 0.0129)
9500/0/604
1.026
0.18 ꢂ 0.12 ꢂ 0.09
1.77e26.06
15,468
5571 (Rint ¼ 0.0266)
5571/0/343
1.047
0.25 ꢂ 0.14 ꢂ 0.08
1.54e25.08
21,191
7339 (Rint ¼ 0.0678)
7339/2/497
0.992
0.24 ꢂ 0.15 ꢂ 0.08
2.04e26.03
5825
2110 (Rint ¼ 0.0264)
2110/0/143
0.963
0.22 ꢂ 0.11 ꢂ 0.11
1.87e26.03
12,959
4679 (Rint ¼ 0.0257)
4679/0/284
1.027
q
Range (deg)
No. of reflns collected
No. of indep reflns
No. of data/restraint/params
GOF on F2
R1(I > 2
s(I))
0.0386
0.0344
0.0479
0.0601
0.0364
0.0364
wR2
0.0786
0.0828
0.1280
0.1475
0.0886
0.0866

1586
D. Gong et al. / Journal of Organometallic Chemistry 696 (2011) 1584e1590
Table 2
ꢁ
ꢀ
The selected bond length (A) and angles ( ) for complexes 3fe3h and 3me3o.
3f
3g
3h
2.035(3)
2.201(3)
2.195(3)
3m
2.019(4)
2.269(4)
2.230(5)
3n
2.033(3)
2.226(2)
2.226(2)
3o
ꢀ
Bond distance (A)
Co1eN1
Co1eN2
Co1eN3
Co1eCl1
Co1eCl2
2.036(2)
2.191(2)
2.205(3)
2.2668(10)
2.2439(10)
2.0317(18)
2.1947(19)
2.2085(19)
2.2608(7)
2.2816(6)
2.0142(19)
2.2686(18)
2.2752(18)
2.2574(6)
2.2618(6)
2.2566(11)
2.2552(12)
2.2631(16)
2.2692(16)
2.2617(6)
2.2617(6)
Bond angle (ꢁ)
N1eCo1eN3
N1eCo1eN2
N3eCo1eN2
N1eCo1eCl2
N3eCo1eCl2
N2eCo1eCl2
N1eCo1eCl1
N3eCo1eCl1
N2eCo1eCl1
Cl2eCo1eCl1
75.32(10)
75.49(9)
149.97(10)
127.51(8)
97.11(8)
95.01(7)
111.33(8)
95.63(8)
74.98(7)
76.13(7)
151.09(7)
119.49(5)
96.26(5)
98.84(5)
120.55(5)
93.17(5)
100.47(5)
119.63(3)
75.38(10)
75.37(10)
150.67(10)
123.01(8)
96.45(8)
97.55(8)
120.01(8)
95.63(8)
75.85(17)
75.57(17)
151.41(16)
119.76(13)
96.67(12)
97.95(12)
118.66(13)
98.34(12)
94.71(12)
121.57(6)
75.55(5)
75.55(5)
151.10(11)
118.967(19)
94.06(5)
99.84(5)
118.967(19)
99.84(5)
75.52(7)
75.73(7)
151.18(7)
121.18(5)
99.95(5)
96.58(5)
120.94(5)
95.97(5)
96.94(5)
117.87(3)
101.38(7)
121.14(4)
100.75(8)
116.89(4)
94.06(5)
122.07(4)
the coordination reaction between the imino nitrogen atoms and
the metal ions. The elemental analysis results reveal that the
components of all complexes are in accord with the formula CoCl2L.
Single crystals of complexes 3a, 3fe3h and 3me3o suitable for
X-ray diffraction analysis are also obtained.
The crystallographic data of complexes 3fe3h and 3me3o
together with the collection and refinement parameters are
summarized in Tables 1 and 2 (3a was previously reported [59]),
respectively. The molecular structures of 3g, 3m and 3n are shown
in Figs. 1e3, respectively.
Cl(2) in 3f, 3g and 3h are 121.14ꢁ, 119.63ꢁ and 116.89ꢁ, respectively,
obviously larger than those observed in ortho-substituted
complexes (2-butyl (103.67ꢁ) [35]; 2-isopropyl (116.50ꢁ) [37]; 2,4-
difluoro (103.28ꢁ) [41] and 2,6-diisopropyl (116.43ꢁ) [37]), also, the
angles of N(2)eCo(1)eN(3) in three complexes are larger than
those of ortho-positioned counterparts. Nevertheless, clear corre-
lation between the substituents of ligand and angles/bonds of
complexes cannot be concluded. As described previously, the
planes of the phenyl rings are essentially orthogonal to the coor-
dination plane of the pyridyl plane. The two imino C]N bonds have
typical double bond character with C]N bond lengths from 1.268
The conformers of 3f, 3g and 3h are isomorphous [59e62], all
having approximate Cs symmetry about a plane containing the
cobalt, two chlorines, and pyridyl nitrogen atom. The deviations of
the metal ion from the plane formed by the coordinated nitrogen
ꢀ
(4) to 1.293(3) A.
Compound 3m crystallizes with two independent molecules (Co
(1) in Fig. 1) per asymmetric unit, which differ in the orientation of
the phenyl groups with respect to the plane formed by three
nitrogen atoms [Co(1)eN(1)eC(1)eC(2) 33.37ꢁ, Co(1)eN(2)eC
(14)eC(15) 0ꢁ and Co(2)eN(4)eC(20)eC(21) 32.12ꢁ, Co(2)eN(5)eC
(33)eC(34) 24.50ꢁ]. Due to the fact that the bond distances and the
other angles are similar in the two independent molecules, refer-
ence can be made to the molecule identified by Co(1). The molec-
ular structures of Co(1) and complex 3o are isomorphous, both
adopting approximate Cs symmetry about a plane containing the
cobalt atom, the two chlorine atoms, and the pyridyl nitrogen atom.
The deviations of the metal from the plane formed by the coordi-
ꢀ
atoms in 3f, 3g and 3h are 0.182, 0.016 and 0.056 A, respectively.
The geometry around the cobalt atom can also be described as
a distorted trigonal bipyramidal with the equatorial plane formed
by the pyridyl nitrogen atoms, the two chlorine atoms, and the two
axial CoeN bonds. The CoeN(pyridyl) bond distances of complexes
ꢀ
are about 2.032e2.035 A in these para-substituted complexes,
shorter than those of ortho-substituted complexes (2-isopropyl
ꢀ
ꢀ
(2.051 A) [37]; 2,4-difluoro (2.045 A) [41] and 2,6-diisopropyl
ꢀ
(2.0496 A) [37]). While those of CoeN(imino) are between 2.191
ꢀ
and 2.215 A, comparable to the analogues (2-isopropyl [37], 2,4-
ꢀ
difluoro [41], and 2,6-diisopropyl [37]). The angles of Cl(1)eCo(1)e
nated nitrogen atoms in Co(1) and 3o are 0.044 and 0.056 A,
respectively. The geometry at the cobalt center can also be better
Fig. 1. ORTEP view of complex 3g, drawn at 35% of probability. Hydrogen atoms and
Fig. 2. ORTEP view of complex 3m (molecule Co(1)), drawn at 35% of probability.
one methylene chloride molecule were omitted for clarity.
Hydrogen atoms and methylene chloride molecule were omitted for clarity.

D. Gong et al. / Journal of Organometallic Chemistry 696 (2011) 1584e1590
1587
Table 3
Polymerization results with cobalt catalysts.
Entrya Complex Yield (%) Mn (ꢂ10ꢀ4
)
Mw/Mn Microstructure (%)
cis-1,4 trans-1,4 1,2
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
CoCl2
76
66
62
60
42
63
95
99
93
96
87
71
61
57
58
24
4.6
14.2
15.8
20.1
11.4
4.4
7.7
8.1
8.4
9.1
14.7
19.4
5.1
14.5
4.6
4.2
2.5
2.4
2.0
2.5
4.6
3.8
3.6
3.7
3.8
2.4
2.0
4.8
2.4
4.4
4.3
79.9
92.9
97.0
95.9
93.9
79.8
83.5
84.9
84.2
89.6
94.1
96.4
77.5
94.7
77.6
95.1
17.0
6.2
2.5
3.4
5.1
17.2
14.1
13.2
13.0
8.3
4.4
3.0
21.4
2.5
19.7
0.8
3.1
0.9
0.5
0.7
1.0
3.0
2.4
1.9
2.8
2.1
1.5
0.6
1.1
2.8
2.7
2.1
9
10
11
12
13
14
15
16
Fig. 3. ORTEP view of complex 3n, drawn at 35% of probability. Hydrogen atoms were
omitted for clarity.
2.1
a
Polymerization conditions: precursor 10 mmol; butadiene 30 mmol; MAO(Al)
2.0 mmol; toluene 12 mL; time 4 h; temperature 20 ꢁC.
described as distorted trigonal bipyramidal, with the equatorial
plane formed by the pyridyl nitrogen atoms and the two chlorine
ligands, and the two axial CoeN bonds. Three equatorial angles are
very close to each other in Co(1) [Cl(1)eCo(1)eCl(2) 121.57(6)ꢁ; N
(3)eCo(1)eCl(2) 119.76(13)ꢁ and N(3)eCo(1)eCl(1) 118.66(13)ꢁ],
but different in the case of 3o [Cl(1)eCo(1)eCl(2) 117.87(3)ꢁ; N(3)e
Co(1)eCl(2) 99.95(5)ꢁ and N(3)eCo(1)eCl(1) 95.97(5)ꢁ]. The dihe-
dral angle between two phenyl rings in Co(1) is 33.37ꢁ, much larger
than that (14.92ꢁ) observed in 3o. It should be mentioned that one
phenyl ring in both complexes is almost coplanar with the three
nitrogens formed plane, while the other ring forms dihedral angles
of 33.37ꢁ(in Co(1)) and 14.92ꢁ(in 3o) with the plane, largely
different from 3n, and also in sharp contrast to the reported cobalt
analogues where two imino rings are both almost perpendicular to
the plane formed by three nitrogen atoms [37,59e62]. The CoeN
examining steric and electronic effects on catalytic activity and
properties of resulting polymers. The polymerization results,
together with that of CoCl2/MAO as reference are summarized in
Table 3 under the conditions ([Bd]/[Co] ¼ 3000, [MAO]/[Co] ¼ 200,
T ¼ 20 ꢁC, time 4 h).
In comparison of CoCl2/MAO system (entry 16, Table 3), bisi-
minopyridine incorporated CoCl2 complexes (entry 1e15, Table 3)
generally exhibit much higher activity and tunable cis-1,4 selec-
tivity by varying the substituents on the ligand set, implying ligand
dissociation from active species does not take place during poly-
merization, and do play an important role in controlling both
catalytic activity and polymer properties.
As the results shown in Table 3 demonstrate the ligand struc-
ture, particularly, the substituent at ortho-positions of iminoaryl
moiety of ligand considerably influences catalytic activity, selec-
tivity and molecular weight of the obtained polymers. The case is
also hold for the complexes discussed in the ethylene polymeri-
zation [38].
ꢀ
(pyridyl) bond distances in both complexes are (2.019 A in Co(1),
ꢀ
2.0142 A in 3o) shorter than those of 2,6-bis((iminoaryl)ethyl)
ꢀ
pyridine complexes and 3n (2.022e2.045 A), while the CoeN
ꢀ
(imino) (2.230e2.2752 A) distances are a little longer than those of
ꢀ
analogues (2.204e2.230 A) [59e62].
Co-based ketimine complexes in general reveal that an increase
of steric bulk at the ortho-position leads to a decreased activity, an
increased cis-1,4 selectivity as well as increased molecular weight
and narrowed molecular weight distribution of the resultant poly-
mer. Complex 3a, owning the least steric hindrance around the Co
atom, is the most active catalyst among the alkyl-substituted
complexes (3ae3e), polymerizing butadiene to afford poly-
butadiene with mainly cis-1,4 structure about 79.9%. The obtained
polybutadiene possesses a low molecular weight of 4.6 ꢂ 104 and
a broad molecular weight distribution about 4.2, indicating poly-
merization may be initiated by multiple active species having
different selectivity. Complex 3b containing methyl groups at 2-
positions of each aryl ring (entry 2, Table 3) displays a conversion of
66%, less productive than the benchmark complex 3a (76%, entry 1,
Table 3). These results may be relevant to spatial opening degree
around the cobalt center and can be attributed to the decreased
chain propagation rate resulting from a slowdown in coordination
as well as insertion rate of butadiene molecule to cobalt center
resulted from the crowded space around the active site. In the
meantime, with increment of steric hindrance near the metal
The structure of the complex 3n is observed to be disordered
with “up” or “down” orientation of the iminoaryl rings, similar to
iron analogue [41]. The conformation of the complexes displays C2v
symmetry about a plane containing the cobalt atom, the two
chloro groups, and the pyridyl nitrogen atom. As shown in Fig. 3,
the geometry at the metal center in complex 3n can also be better
described as a distorted trigonal bipyramidal with the equatorial
plane formed by the pyridyl nitrogen atoms, the two chloro
ligands, and the two axial CoeN bonds. The equatorial angles range
between 118.967(19)ꢁ and 122.07(4)ꢁ, and the two axial CoeN
bonds subtend an angle of 151.10(11)ꢁ. The two mono-ethyl phenyl
rings form the equal dihedral angle of 58.10ꢁ with the plane
formed by three nitrogen atoms, and the angle between two
phenyls is about 75.10ꢁ. The CoeN bond distances here are
between those of 2,6-bis((iminoaryl)ethyl)pyridine and 2,6-bis
(iminoaryl)pyridine complexes 3o and Co(1). The two imino C]N
bonds in 3n have typical double bond character with C]N bond
ꢀ
lengths of 1.272(3) A.
2.2. Butadiene polymerization behaviors of complexes
activated by MAO
center, b-hydride elimination is suppressed, bringing about
a remarkable jump of Mn (14.2 ꢂ104) and a narrowed molecular
weight distribution (Mw/Mn ¼ 2.5) [35e37]. More interestingly, the
cis-1,4 selectivity rises to 92.9%. Presumably, the more congested
environment favors to produce higher selective active species
[2,22,24]. Within our expectation, altering the substituent from
Co-based complexes carrying a variety of ortho- or para-
substituted ligands, including alkyl groups as well as electron-
withdrawing groups are employed in butadiene polymerization for

1588
D. Gong et al. / Journal of Organometallic Chemistry 696 (2011) 1584e1590
methyl (3b) to a more bulky ethyl (3c) or isopropyl group (3d) at one
ortho-position in each ring, gives rise to further reduced yields (3b,
66%; 3c, 62%; 3d, 60%) and appreciably increased cis-1,4 contents
(3b, 92.9%; 3c, 97%; 3d, 95.9%) concurrent with higher molecular
weight (3b, 14.2 ꢂ104; 3c, 15.8 ꢂ 104; 3d, 20.1 ꢂ104) and more
narrowed molecular weight distribution. As for complex 3e, where
both ortho-positions of each ring are positioned with isopropyl
groups, the space around the Co center becomes the most con-
gested, and accordingly, the complex shows the lowest activity,
merely 42% in conversion. However, Mn of polymer produced by
complex 3e shows an exception, drops from 20.1 ꢂ104 (3d) to
a rather low value about 11.4 ꢂ104 (3e). This is most likely due to the
seriously blocked insertion of butadiene molecule, leading to
a substantially decrease in chain propagation rate, favoring the
chain transfer over chain propagation reaction, despite the chain
transfer is suppressed simultaneously, but with a less extent. In the
meanwhile, cis-1,4 content somewhat decreases as well, most likely
that slower insertion rate caused by bulky isopropyl group favors
configuration rearrangement of the active site from anti- to syn-allyl
intermediate. Thus, the alkyl bulkiness influences the catalytic
activity in the trend of 2-H (3a) > 2-methyl (3b) > 2-ethyl (3c) > 2-
isopropyl (3d) > 2,6-diisopropyl (3e), while the effects on cis-1,4
selectivity is more complicated, which is in the order of 2-ethyl
(3c) > 2-isopropyl (3d) > 2,6-diisopropyl (3e) > 2-methyl (3b) > 2-
H (3a).
For comparison, the effects of substituents at the para-position
are also investigated with the corresponding complexes 3f, 3g and
3h carrying isopropyl, fluoro and trifluoromethyl, respectively.
Obviously, as compared with 3a, the isopropyl group affects activity
negatively, while the microstructure and molecular weight of
resultant polymer change little. Significant improvement of yield,
up to 95% and 99%, is clearly found in the complexes 3g and 3h,
respectively, and the cis-1,4 selectivity is benefited as well,
increasing from 79.9% (3a) to 83.5% (3g) and 84.9% (3h), respec-
tively. This indicates that electron-withdrawing groups can
promote activity and enhance selectivity simultaneously. Due to
the high electronegativity of fluoro-containing substituents, the
electron density at the cationic cobalt center is significantly
reduced, and the increased Lewis acidic character facilitates the
coordination of butadiene molecule, causing much increased chain
propagation rate. The more positive cobalt center also increases the
probability of diene coordination through two double bonds (cis-h4
coordination mode) rather than that through coordination with
one double bond (trans-h2 coordination mode), ensuing more cis-
1,4 inserted unit in the produced polybutadiene [27]. The electron
negative groups can also serve to raise polymer molecular weight
and regulate molecular weight distribution, suggesting remote
electronic withdrawing substituents retard chain transfer reaction
to some extent.
substituent is not bulky enough to achieve much higher cis-1,4
selectivity, thus, the corresponding position is stuffed with methyl
group for higher selectivity, along with a withdrawing group F at
para-position for promoting catalytic performance (complexes 3k
and 3l). Higher activities up to 71% (3l) and 87% (3k), respectively,
than the alkyl-substituted analogues 3ae3e are observed, and
more importantly, the cis-1,4 selectivity is increased to 94.1% for 3k
and 96.4% for 3l within expectation. We have demonstrated the
strategy for increasing the activity and selectivity simultaneously,
though some of the desired ligands are inaccessible because the
more appropriate substituted aniline, 2,6-di(trifluoromethyl)
aniline or 2-trifluoromethyl-6-isopropylaniline is not commercially
available.
In olefin catalysts, replacing a ketimine (R1 ¼ CH3) with an
aldimine (R1 ¼ H) usually leads to difference in molecular weight,
and molecular weight distribution, particularly the productivity,
usually, those based on ketimine ligand approximately are an order
of magnitude more active than catalysts based on the aldimine
ligand [37]. Herein, three cobalt complexes (3me3o) by using the
formyl instead of the acetyl pyridine complex are also described.
The activities found in theses complexes are only marginally lower
than those of the corresponding acetyl pyridine complexes, and the
molecular weight and distribution of produced polymers are also
comparable to those from acetyl pyridine analogues. Similar to
ketimine systems, those without substituents (3m and 3o) show
a higher activity and a lower cis-1,4 selectivity than the 2-ethyl
complex (3n).
3. Conclusion
Bis(arylimino)pyridine cobalt complexes containing alkyl or
fluorine substituents on arylimino rings were described. Activated
with MAO, these catalysts actively polymerize butadiene to give
polybutadiene with tunable cis-1,4 structure depending on the
nature and location of substituent used. Steric bulky group near the
cobalt center are beneficial for cis-1,4 selectivity, while electron-
withdrawing substituents are, irrespective of their positions,
favorable to catalytic activity. By introducing steric bulky groups at
ortho-position, together with electron-negative groups on the
iminoaryl rings, high activity (87%) and cis-1,4 selectivity (94.1%)
can be achieved simultaneously.
4. Experimental
4.1. General considerations
Anhydrous CoCl2 and anilines were purchased from Alfa Aesar.
2,6-Pyridinedimethanol and oxalyl chloride were purchased from
Aldrich. MAO was available from AkzoNoble. Toluene and Et3N
were distilled over CaH2 prior to use. Toluene and tetrahydrofuran
(THF) were freshly distilled in the presence of sodium and benzo-
phenone. Dimethylsulfoxide was purified by vacuum distillation
under N2 atmosphere. Polymerization grade butadiene was
supplied from Jinzhou Petrochemical Corporation and purified by
As discussed above, with bulky groups at ortho-position bene-
ficial to cis-1,4 selectivity and electronic withdrawing groups
propitious to catalytic activity, introduction of the bulkiness at
ortho-position, and electronic withdrawing groups regardless of
the position are expected to acquire more efficient catalysts.
Complexes 3i and 3j, bearing one and two F atoms at ortho-posi-
tions, respectively (entries 9 and 10, Table 3), display high activity
about 93% and 96%, respectively, providing polymer with reason-
able cis-1,4 content and molecular weight, in proportion to the
bulkiness near the cobalt center. On the other hand, the overall
trend implies that extra fluorine atoms at the ortho-position of
ligand framework can enhance monomer turnover rate, cis-1,4
selectivity as well as molecular weight more effectively than its
para-analogue. In this respect, an ortho-F appears to have a more
complex and potent influence than a para-F group, and this effect
may be beyond purely electronic in nature. Clearly, ortho-F
ꢀ
passing through columns packed with 4 A molecular sieves and
KOH. 2,6-Pyridinedicarboxaldehyde was prepared according to the
published methods [63]. 1H NMR (400 MHz) was recorded on
a Varian Unity spectrometer in CDCl3 at room temperature.
Elemental analyses were recorded on an elemental Vario EL spec-
trometer. IR spectra were performed on BRUKE Vertex-70 FIR
spectrophotometer. The proportion of 1,2, cis-1,4 and trans-1,4 units
of polymer was determined by IR spectra and 1H NMR [64e66].
The molecular weights (Mn) and molecular weight distributions
(Mw/Mn) of polymer were measured at 30 ꢁC by gel permeation
chromatography (GPC) equipped with a Waters 515 HPLC pump,

D. Gong et al. / Journal of Organometallic Chemistry 696 (2011) 1584e1590
1589
four columns (HMW 7 THF, HMW 6E THF ꢂ 2, HMW 2 THF) and
a Waters 2414 refractive index detector. THF was used as eluent at
a flow rate of 1.0 mL/min. The molecular weight of polymer was
calculated using the polystyrene calibration.
2973, 2921, 1635 (nC]N), 1592, 1569, 1428, 1361, 1274, 1202, 1150,
1121, 868, 820, 777. Anal. Calcd for C25H25F2N3: C, 74.05; H, 6.21; N,
10.36. Found: C, 73.24; H, 6.19; N, 10.34.
4.3.6. 2,6-Bis(phenylimino)pyridine (2m)
4.2. X-ray structure determinations
Yield: 67.4%. 1H NMR (400 MHz, CDCl3,
d, ppm): 8.69 (s, 2H,
eCH]Ne), 8.30 (d, 2H, J ¼ 8.0 Hz, Hpyr), 7.97 (t, 1H, J ¼ 7.9 Hz, Hpyr),
7.46e7.42 (m, 4H, Haryl), 7.34e7.27 (m, 6H, Haryl). IR (KBr, cmꢀ1):
1628 (nC]N), 1591, 1484, 1336, 1202, 1073, 855, 816, 759, 737. Anal.
Calcd. for C19H15N3: C, 79.98; H, 5.30; N, 14.73. Found: C, 79.84; H,
5.49; N, 14.89.
Crystals for X-ray analyses were obtained as described in the
experimental section. Data collections were performed at ꢀ88.5 ꢁC
on a Bruker SMART APEX diffractometer with a CCD area detector,
ꢀ
using graphite monochromated Mo K
a
radiation (
l
¼ 0.71073 A).
The determination of crystal class and unit cell parameters was
carried out by the SMART program package. The raw frame data
were processed using SAINT and SADABS to yield the reflection data
file. The structures were solved by using 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 refinement of
the parameters.
4.3.7. 2,6-Bis(2-ethylphenylimino)pyridine (2n)
Yield: 70.1%. 1H NMR (400 MHz, CDCl3,
d, ppm): 8.61 (s, 2H,
eCH]Ne), 8.33 (d, 2H, J ¼ 8.0 Hz, Hpyr), 7.95 (t, 1H, J ¼ 7.9 Hz, Hpyr),
7.30e7.07 (m, 6H, Haryl), 7.07e7.05 (m, 2H, Haryl), 2.86 (q, 4H,
J ¼ 7.6 Hz, eCH2CH3), 1.22 (t, 6H, J ¼ 7.6 Hz, eCH2CH3). IR (KBr,
cmꢀ1): 2961, 2871, 1624 (nC]N), 1584, 1575, 1485, 1452, 1336, 1073,
857, 809, 754, 735. Anal. Calcd. for C23H23N3: C, 80.90; H, 6.79; N,
12.31. Found: C, 80.79; H, 6.87; N, 12.20.
4.3. Syntheses and characterizations of ligands and complexes
4.3.8. 2,6-Bis(4-isopropylphenylimino)pyridine (2o)
Yield: 75.7%. 1H NMR (400 MHz, CDCl3,
d, ppm): 8.71 (s, 2H,
Ligands (2ae2o) and the corresponding complexes (3ae3o)
were synthesized according to the literatures [35,41], the new
compounds were characterized as the desired products.
eCH]Ne), 8.28 (d, 2H, J ¼ 7.6 Hz, Hpyr), 7.92 (t, 1H, J ¼ 7.6 Hz, Hpyr),
7.29e7.27 (m, 8H, Haryl), 2.96 (s, 2H, 7.6 Hz, eCH(CH3)2),1.92 (d,12H,
7.6 Hz, eCH(CH3)2). IR (KBr, cmꢀ1): 2994, 2914, 1624 (nC]N), 1573,
1504, 1464, 1338, 1201, 958, 820, 738. Anal. Calcd. for C25H27N3: C,
81.26; H, 7.37; N, 11.37. Found: C, 81.29; H, 7.27; N, 11.22.
4.3.1. 2,6-Bis[1-(4-isopropylphenylimino)ethyl]pyridine (2f)
Yield: 79.3%. 1H NMR (400 MHz, CDCl3,
d, ppm): 8.33 (d, 2H,
J ¼ 8.0 Hz, Hpyr), 7.85 (t, 1H, J ¼ 8.0 Hz, Hpyr), 7.24 (d, 4H, J ¼ 8.0 Hz,
Haryl), 6.79 (d, 4H, J ¼ 8.0 Hz, Haryl), 2.93 (hept, 2H, J ¼ 7.6 Hz, eCH
(CH3)2), 2.42 (s, 6H, eN]CeCH3), 1.28 (d, 12H, J ¼ 7.6 Hz, eCH
(CH3)2). IR (KBr, cmꢀ1): 2957, 1642 (nC]N), 1566, 1505, 1362, 1219,
1120, 847, 792. Anal. Calcd. for C27H31N3: C, 81.57; H, 7.86; N, 10.57.
Found: C, 81.34; H, 7.64; N, 10.49.
4.3.9. {2,6-Bis[(4-isopropylphenylimino)ethyl]pyridine}cobalt
dichloride (3f)
Yield: 93.1%. IR (KBr, cmꢀ1): 2960, 2926, 2868, 1627 (nC]N), 1587,
1502, 1465, 1370, 1259, 1230, 1057, 1019, 853, 814, 774. Anal. Calcd.
for C27H31N3CoCl2: C, 61.49; H, 5.92; N, 7.97. Found: C, 61.73; H,
5.72; N, 7.69.
4.3.2. 2,6-Bis[1-(4-fluorophenylimino)ethyl]pyridine (2g)
4.3.10. {2,6-Bis[(4-fluorophenylimino)ethyl]pyridine}cobalt
dichloride (3g)
Yield: 73.3%. 1H NMR (400 MHz, CDCl3,
d, ppm): 8.32 (d, 2H,
J ¼ 7.9 Hz, Hpyr), 7.88 (t,1H, J ¼ 8.0 Hz, Hpyr), 7.10e7.06 (m, 4H, Haryl),
6.83e6.80 (m, 4H, Haryl), 2.41 (s, 6H, eN]CeCH3). IR (KBr, cmꢀ1):
3068, 1648 (nC]N), 1608, 1584, 1506, 1362, 1217, 991, 855, 733, 704.
Anal. Calcd. for C21H17F2N3: C, 72.19; H, 4.90; N, 12.03. Found: C,
72.34; H, 4.79; N, 11.89.
Yield: 91.9%. IR (KBr, cmꢀ1): 2070, 1633 (nC]N), 1598, 1570, 1501,
1363, 1078, 840, 821, 809, 765, 744, 727. Anal. Calcd. for
C21H17F2N3CoCl2: C, 52.63; H, 3.58; N, 8.77. Found: C, 52.33; H, 3.72;
N, 8.69.
4.3.11. {2,6-Bis[(4-trifluoromethylphenylimino)ethyl]pyridine}
cobalt dichloride (3h)
4.3.3. 2,6-Bis[1-(4-trifluoromethylphenylimino)ethyl]pyridine (2h)
Yield: 65.7%. 1H NMR (400 MHz, CDCl3,
d
, ppm): 8.36 (d, 2H,
Yield: 96.3%. IR (KBr, cmꢀ1): 2969, 2919, 1627 (nC]N), 1587, 1505,
1188, 1175, 1150. Anal. Calcd. for C23H17F6N3CoCl2: C, 47.69; H, 2.96;
N, 7.25. Found: C, 47.90; H, 2.92; N, 7.39.
J ¼ 7.6 Hz, Hpyr), 7.91 (t, 1H, J ¼ 7.9 Hz, Hpyr), 7.64 (d, 4H, J ¼ 8.4 Hz,
Haryl), 6.93 (d, 4H, J ¼ 8.0 Hz, Haryl), 2.40 (s, 6H, eN]CeCH3). IR
(KBr, cmꢀ1): 3070, 1633 (nC]N), 1598, 1501, 1410, 1318, 1254, 1208,
821, 809, 765. Anal. Calcd for C23H17F6N3: C, 61.47; H, 3.81; N, 9.35.
Found: C, 61.24; H, 3.89; N, 9.14.
4.3.12. {2,6-Bis[(2-methyl-6-fluorophenylimino)ethyl]pyridine}
cobalt dichloride (3k)
Yield: 89.1%. IR (KBr, cmꢀ1): 1622 (nC]N), 1589, 1477, 1372, 1309,
1263, 1220, 1205, 1026, 861, 818. Anal. Calcd. for C23H21F2N3CoCl2:
C, 54.46; H, 4.17; N, 8.28. Found: C, 55.11; H, 4.82; N, 10.01.
4.3.4. 2,6-Bis(2-methyl-6-fluorophenylimino)pyridine (2k)
Yield: 70.1%. 1H NMR (400 MHz, CDCl3,
d, ppm): 8.39 (d, 2H,
J ¼ 7.8 Hz, Hpyr), 7.89 (t,1H, J ¼ 7.8 Hz, Hpyr), 7.98e7.89 (m, 4H, Haryl),
6.65e6.62 (m, 2H, Haryl), 2.34 (s, 6H, eN]CeCH3), 2.12 (s, 6H,
eCH3). IR (KBr, cmꢀ1): 2973, 2917, 1641 (nC]N), 1605, 1511, 1473,
1367, 1305, 1207, 1125, 1016, 850, 856, 787. Anal. Calcd for
C23H21F2N3: C, 73.19; H, 5.61; N, 11.13. Found: C, 72.84; H, 4.99; N,
11.24.
4.3.13. {2,6-Bis[(2,6-dimethyl-4-fliuorophenylimino)ethyl]pyridine}
cobalt dichloride (3l)
Yield: 90.6%. IR (KBr, cmꢀ1): 1630 (nC]N), 1588, 1375, 1267, 1217,
1149, 1004, 865, 836, 714. Anal. Calcd. for C25H25F2N3CoCl2: C,
56.09; H, 4.71; N, 7.85. Found: C, 55.99; H, 4.81; N, 8.01.
4.3.5. 2,6-Bis(2,6-methyl-4-fluorophenylimino)pyridine (2l)
4.3.14. [2,6-Bis(phenylimino)pyridine]cobalt dichloride (3m)
Yield: 97.8%. IR (KBr, cmꢀ1): 1621 (nC]N), 1584, 1488, 1454, 1160,
1081, 958, 765, 743. Anal. Calcd. for C19H15N3CoCl2: C, 54.96; H,
3.64; N, 10.12. Found: C, 55.20; H, 3.82; N, 10.01.
Yield: 75.7%. 1H NMR (400 MHz, CDCl3,
d, ppm): 8.46 (d, 2H,
J ¼ 7.6 Hz, Hpyr), 7.92 (t, 1H, J ¼ 7.9 Hz, Hpyr), 6.80 (d, 4H, J ¼ 10.2 Hz,
Haryl), 2.23 (s, 6H, eN]CeCH3), 2.04 (s, 12H, eCH3). IR (KBr, cmꢀ1):

1590
D. Gong et al. / Journal of Organometallic Chemistry 696 (2011) 1584e1590
[18] G. Ricci, A. Forni, A. Boglia, M. Sonzogni, Organometallics 23 (2004) 3727.
[19] Y.-M. Hu, W.-M. Dong, L.-S. Jiang, X.-Q. Zhang, Y. Guo, L.-H. Cao, Chin. J. Catal.
8 (2005) 664.
[20] D.-R. Gong, W.-M. Dong, J.-C. Hu, X.-Q. Zhang, L.-S. Jiang, Polymer50(2009)2826.
[21] J. Lu, Y.-M. Hu, X.-Q. Zhang, J.-F. Bi, W.-M. Dong, L.-S. Jiang, B.-T. Huang, J. Appl.
Polym. Sci. 100 (2006) 4265.
[22] Y. Jang, P.S. Kim, H. Lee, Macromolecules 35 (2002) 1477.
[23] D.-R. Gong, W.-M. Dong, Y.-M. Hu, J.-F. Bi, X.-Q. Zhang, L.-S. Jiang, Polymer 50
(2009) 5980.
[24] Y.C. Jang, P.S. Kim, H.Y. Jeong, H. Lee, J. Mol. Catal., A: Chem. 206 (2003) 29.
[25] D.C.D. Nath, T. Shiono, T. Ikeda, Appl. Catal., A: Gen. 238 (2003) 193.
[26] D.C.D. Nath, T. Shiono, T. Ikeda, Macromol. Chem. Phys. 204 (2003) 2017.
[27] D.C.D. Nath, T. Shiono, T. Ikeda, Macromol. Chem. Phys. 203 (2002) 756.
[28] K. Endo, N. Hatakeyama, J. Polym. Sci., Part A: Polym. Chem. 39 (2001) 2793.
[29] D. Chandran, C.H. Kwak, C.S. Ha, I. Kim, Catal. Today 131 (2008) 505.
[30] P. Cass, K. Pratt, T. Mann, B. Laslett, E. Rizzardo, R. Burford, J. Polym. Sci., Part
A: Polym. Chem. 38 (2000) 1083.
4.3.15. [2,6-Bis(2-ethylphenylimino)pyridine]cobalt dichloride (3n)
Yield: 95.5%. IR (KBr, cmꢀ1): 2963, 2931, 1616 (nC]N), 1585, 1484,
1452, 1293, 1222, 1044, 1027, 961, 791, 760, 744. Anal. Calcd. for
C23H23N3CoCl2: C, 58.62; H, 4.92; N, 8.92. Found: C, 58.50; H, 4.79;
N, 9.06.
4.3.16. [2,6-Bis(4-isopropylphenylimino)pyridine]cobalt dichloride (3o)
Yield: 89.6%. IR (KBr, cmꢀ1): 2957, 2913, 1595 (nC]N), 1548, 1508,
1466, 1177, 1062, 1027, 822. Anal. Calcd. for C25H27N3CoCl2: C, 60.13;
H, 5.45; N, 8.42. Found: C, 59.73; H, 5.73; N, 8.60.
4.4. Procedure for butadiene polymerization
[31] G. Ricci, A. Forni, A. Boglia, T. Motta, J. Mol. Catal., A: Chem. 226 (2005) 235.
[32] K. Endo, T. Kitagawa, K. Nakatani, J. Polym. Sci., Part A: Polym. Chem. 44
(2006) 4088.
Atypical procedure for the polymerizationwasasfollows (Table 2,
complex 3a): a toluene solution of butadiene (1.62 g, 30 mmol) was
added to moisture free ampoule bottle preloaded with complex 3a
[33] M. Gippn, Ind. Eng. Chem. Prod. Res. Dev. 1 (1962) 32.
[34] C. Longgivae, R. Castelli, G.F. Groce, Chim. Ind. 43 (1961) 625.
[35] B.L. Small, M. Brookhart, A.M.A. Bennett, J. Am. Chem. Soc. 120 (1998) 4049.
[36] B.L. Small, M. Brookhart, J. Am. Chem. Soc. 120 (1998) 7143.
[37] G.J.P. Britovsek, M. Bruce, V.C. Gibson, B.S. Kimberley, P.J. Maddox,
S. Mastroianni, S.J. McTavish, C. Redshaw, G.A. Solan, S. Strömberg,
A.J.P. White, D.J. Williams, J. Am. Chem. Soc. 121 (1999) 8728.
[38] V.C. Gibson, C. Redshaw, G.A. Solan, Chem. Rev. 107 (2007) 1745.
[39] W.-J. Zhang, W.-H. Sun, S. Zhang, J.-X. Hou, H.-B. Song, Organometallics 25
(2006) 1961.
(4.2 mg, 10 mmol). MAO (Al, 2.0 mmol) was then injected to initiate
the polymerization at 20 ꢁC. After 4 h, methanol was added to the
system to quench the polymerization. The mixture was poured into
a large quantity of methanol containing 2,6-di-tert-butyl-4-methyl-
phenol (1.0 wt-%) as a stabilizer. Filtered and dried under vacuum at
40 ꢁC, polybutadiene was resulted at a constant weight (1.23 g, 76%).
[40] C. Bianchini, G. Mantovani, A. Meli, F. Migliacci, F. Laschi, Organometallics 22
(2003) 2545.
Acknowledgment
[41] Y.-F. Chen, R.-F. Chen, C.-T. Qian, X.-C. Dong, J. Sun, Organometallics 22 (2003)
4312.
[42] D.M. Dawson, D.A. Walker, M. Thornton-Pett, M. Bochmann, J. Chem. Soc.,
Dalton Trans. (2000) 459.
[43] D. Takeuchi, R. Masuura, S. Park, K. Osakada, J. Am. Chem. Soc. 129 (2007) 7002.
[44] L.-Y. Wang, W.-H. Sun, L.-Q. Han, Z.-L. Li, Y.-L. Hu, C.-X. He, C.-H. Yan,
J. Organomet. Chem. 650 (2002) 59.
[45] J.M. Vila, M. Gayoso, T. Pereira, T.M. Lopez, A. Fernández, J.M. Ortigueira,
J. Organomet. Chem. 506 (1996) 165.
Authors appreciate financial supports from National Science and
Technology Infrastructure Program (2007BAE14B01-06) and The
Fund for Creative Research Groups (50621302).
Appendix A. Supplementary material
[46] S. Nückel, P. Burger, Organometallics 19 (2000) 3305.
[47] W.J. Hoogervorst, C.J. Elsevier, M. Lutz, A.L. Spek, Organometallics 20 (2001)
4437.
[48] J.S. Fossey, C.J. Richards, Organometallics 21 (2002) 5259.
[49] W.J. Hoogervorst, A.L. Koster, M. Lutz, A.L. Spek, C.J. Elsevier, Organometallics
23 (2004) 1161.
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2011.01.
015.
References
[50] G.J.P. Britovsek, V.C. Gibson, O.D. Hoarau, S.K. Spitzmesser, A.J.P. White,
D.J. Williams, Inorg. Chem. 42 (2003) 3454.
[51] S. Al-Benna, M.J. Sarsfield, M. Thornton-Pett, D. Ormsby, P.J. Maddox, P. Bres,
M. Bochmann, J. Chem. Soc., Dalton Trans. (2000) 4247.
[52] J. Zhang, M. Gandelman, D. Herrman, G. Leitus, L.J.W. Shimon, Y. Ben-David,
D. Milstein, Inorg. Chim. Acta 359 (2006) 1955.
[53] J.-Q. Wu, L. Pan, Y.-S. Li, S. Liu, Y. Li, Organometallics 28 (2009) 1817.
[54] W.-P. Ye, J. Zhan, L. Pan, N.-H. Hu, Y.-S. Li, Organometallics 27 (2008) 3642.
[55] M.W. Bouwkamp, E. Lobkovsky, P.J. Chirik, J. Am. Chem. Soc. 127 (2005) 9660.
[56] S.C. Bart, M.W. Bouwkamp, F. Neese, K. Wieghardt, E. Lobkovsky, P.J. Chirik,
J. Am. Chem. Soc. 128 (2006) 13901.
[57] A.M. Tondreau, J.M. Darmon, B.M. Wile, S.K. Floyd, E. Lobkovsky, P.J. Chirik,
Organometallics 28 (2009) 3928.
[58] J.S. Kim, C. Ha, I. Kim, e-Polymers (2006) 027.
[59] D.-R. Gong, B.-L. Wang, C.-X. Bai, J.-F. Bi, F. Wang, W.-M. Dong, X.-Q. Zhang, L.-
S. Jiang, Polymer 50 (2009) 6259.
[60] J.-X. Chen, Y.-B. Huang, Z.-S. Li, Z.-C. Zhang, C.-X. Wei, T.-Y. Lan, W.-J. Zhang,
J. Mol. Catal., A: Chem. 259 (2006) 133.
[1] V. Appukuttan, L. Zhang, C. Ha, I. Kim, Polymer 50 (2009) 1150.
[2] W. Gao, D.-M. Cui, J. Am. Chem. Soc. 130 (2008) 4984.
[3] A. Miyazawa, T. Kase, K. Hashimoto, Macromolecules 37 (2004) 8840.
[4] S. Kaita, Z.-M. Hou, Y. Wakatsuki, Macromolecules 32 (1999) 9078.
[5] L.-X. Zhang, T. Suzuki, Y. Luo, M. Nishiura, Z.-M. Hou, Angew. Chem., Int. Ed. 46
(2007) 1909.
[6] S. Arndt, K. Beckerle, P.M. Zeimentz, T.P. Spaniol, J. Okuda, Angew. Chem., Int.
Ed. 44 (2005) 7473.
[7] S. Kaita, M. Yamanaka, A.C. Horiuchi, Y. Wakatsuki, Macromolecules 39 (2006)
1359.
[8] S. Kaita, Z.-M. Hou, Y. Wakatsuki, Macromolecules 34 (2001) 1539.
[9] A. Fischbach, C. Meermann, G. Eickerling, W. Scherer, R. Anwander, Macro-
molecules 39 (2006) 6811.
[10] G.M. Meppelder, H.T. Fan, T.P. Spaniol, J. Okuda, Inorg. Chem. 48 (2009) 7378.
[11] W.J. Evans, D.G. Giarikos, J.W. Ziller, Organometallics 20 (2001) 5751.
[12] D. Wang, S.-H. Li, X.-L. Liu, W. Gao, Organometallics 27 (2008) 6531.
[13] E. Colamarco, S. Milione, C. Cuomo, A. Grassi, Macromol. Rapid Commun. 25
(2004) 450.
[61] K.P. Tellmann, V.C. Gibson, A.J.P. White, D.J. Williams, Organometallics 24
(2005) 280.
[62] C. Bianchini, G. Mantovani, A. Meli, F. Migliacci, F. Zanobini, F. Laschi,
A. Sommazzi, Eur. J. Inorg. Chem. (2003) 1620.
[14] Y. Nakayama, Y. Baba, H. Yasuda, K. Kawakita, N. Ueyama, Macromolecules 36
(2003) 7953.
[63] R.G. Hicks, B.D. Koivisto, M.T. Lemaire, Org. Lett. 6 (2004) 1887.
[64] K.F. Elgert, G. Quack, B. Stutzel, Polymer 15 (1974) 612.
[65] K.F. Elgert, G. Quack, B. Stutzel, Polymer 15 (1974) 816.
[66] K.F. Elgert, G. Quack, B. Stutzel, Polymer 16 (1975) 154.
[15] R. Cariou, J. Chirinos, V.C. Gibson, G. Jacobsen, A.K. Tomov, M.R.J. Elsegood,
Macromolecules 42 (2009) 1443.
[16] L.-H. Cao, W.-M. Dong, L.-S. Jiang, X.-Q. Zhang, Polymer 48 (2007) 2475.
[17] G. Ricci, M. Battistella, Macromolecules 34 (2001) 5766.