Trans-1,4 selective polymerization of 1,3-butadiene with symmetry pincer chromium complexes activated by MMAO

Article abstract of DOI:10.1016/j.jorganchem.2014.05.018

Tridentate chromium complexes (Cr1-Cr7) incorporated with symmetrical pincer ligand bis(arylimino)pyridine and bis(pyrzaolyl)pyridine have been synthesized and characterized by elemental analyis, FT-IR as well as ESI-MS. X-ray diffraction reveals solids-s

Full text of DOI:10.1016/j.jorganchem.2014.05.018

Journal of Organometallic Chemistry 766 (2014) 79e85
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Trans-1,4 selective polymerization of 1,3-butadiene with symmetry
pincer chromium complexes activated by MMAO
,
,
Dirong Gong a *, Xiaoyu Jia c d, Baolin Wang e, Xuequan Zhang e, Kuo-Wei Huang b
a School of Material Science and Chemical Engineering, Ningbo University, Ningbo 315211, People's Republic of China
b King Abdullah University of Science and Technology, Division of Physicals and Life Sciences and Engineering and KAUST Catalysis Center,
Thuwal 23955-6900, Saudi Arabia
c Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, No. 1799, Jimei Road, Xiamen,
Fujian 361021, People's Republic of China
d Ningbo Urban Environment Observation and Research Station-NUEORS, Chinese Academy of Sciences, Ningbo, Zhejiang 315830, People's Republic of China
e Research Center of High Performance Synthetic Rubber, Changchun Institute of, Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,
People's, Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Tridentate chromium complexes (Cr1eCr7) incorporated with symmetrical pincer ligand bis(arylimino)
pyridine and bis(pyrzaolyl)pyridine have been synthesized and characterized by elemental analyis, FT-IR
as well as ESI-MS. X-ray diffraction reveals solids-state structures of Cr2, Cr4 and Cr6 all adopt pseudo-
octahedral coordination environment with respect to metal center. All complexes have been tested in
stereoregulated polymerization of butadiene under various polymerization conditions. The trans-1,4 and
cis-1,4 enchainment of resultant polymer are found to be dependent on the structure of ligand and
amount of activator used. Under the optimized condition, free ortho-substitutes Cr catalysts Cr1, Cr3, Cr4
and Cr6 are capable of initiating high trans-1,4 selectivity (trans-1,4: 89.2%e92.0%) with good polymer
yields (71.5%e78.0%), while counterparts with ortho-positioned alkyl groups Cr2, Cr5 and Cr7 display
mixed selectivities with moderate polymer yields. The sterical effect of ligand and amount of MMAO on
the catalytic performance, in particular, the stereoselectivity and polymer yield, has been also elucidated
by conjugated diene polymerization mechanism.
Received 25 March 2014
Received in revised form
11 May 2014
Accepted 17 May 2014
Available online 27 May 2014
Keywords:
Chromium catalyst
Pincer ligand
Butadiene
Trans-1,4 polybutadiene
© 2014 Elsevier B.V. All rights reserved.
Introduction
conspicuously less, which might be the fact that 1,3-conjugated
dienes kinetically prefer
h h
4-cis to 2-trans coordination to
The stereoselective polymerization of conjugated dienes pro-
moted by transition and rare earth metal complexes is an exten-
sively studied topic and challenging field in both academic and
industrial environments for the relevance of these materials as
synthetic rubbers. Trans-1,4 regulated polybutadiene has attracted
renewed interest over the past decade, as such polymer possesses
excellent dynamic properties, including excellent anti-fatigue
properties, low rolling resistance, low heat buildup, good strength
and low abrasion loss [1]. They can also be copolymerized with
ethylene [2] and styrene [3] to afford high valued copolymers or
be blended with elastomers to prepare materials with unique
mechanical properties. Contrast to vast exploration of cis-1,4 and
1,2 selective catalysts, study on trans-1,4 selective catalysis are
active metal center, ultimately leading to cis-1,4 and 1,2-
stereoregulated growing polymer chain [4e6].
Early research on trans-1,4-polybutadiene dates back to the in-
fancy of ZieglereNatta type of transition metal recipes like tita-
nium, vanadium halides activated by aluminum alkyls which
produce mixtures of polymers containing variable amounts of
trans-1,4-polybutadiene depending on type of halogen [5]. Rare-
earth metal compounds such as lanthanocene aluminates [7,8],
lanthanide bis(allyl)s [9], lanthanide dialkyl [10e12] and neodym-
ium alkoxides or aryl oxides [13,14] which need activation of
borates or organo magnesium and aluminum, have been known for
catalyzing trans-1,4 homo- and copolymerization of butadiene.
Over the past decade, efforts to expand variety of ligand families for
homogeneous catalysts able to polymerize butadiene have resulted
in cascade exploration of trans-1,4 selective systems, notably, those
based on various tridentated ligands for supporting Fe, V, Ti, and Cr
complexes. The tridentate N,N,N-donor ligands (Chart 1) such as
* Corresponding author.
E-mail address: gongdirong@nbu.edu.cn (D. Gong).
http://dx.doi.org/10.1016/j.jorganchem.2014.05.018
0022-328X/© 2014 Elsevier B.V. All rights reserved.

80
D. Gong et al. / Journal of Organometallic Chemistry 766 (2014) 79e85
iron and cobalt complexes are trans-1,4 catalysis in polymerization
of butadiene under proper conditions [17,30,31]. As continued
work, herein, we focus on bisiminopyridine and bis(pyrazolyl)
pyridine as auxiliary ligand for chromium complexes in selective
polymerization of butadiene. The polymerization conditions and
ligand structure effects, particularly, the ligand framework and the
incorporated substitutes, on the selectivity and activity will be
discussed by conjugated diene polymerization mechanism, and this
might give hint on designing desirable catalyst.
Results and discussion
Syntheses and characterization and complexes
Ligands and the corresponding complexes were prepared in
moderate to good yields via various modified procedures [17,32].
The reaction of CrCl3(THF)3 with 1.0 equiv. of the appropriate ligand
in THF at room temperature under argon atmosphere afforded the
corresponding complexes Cr1eCr7 (Chart 2), which were isolated
as stable green solids. All products have been confirmed by FT-IR,
elemental analysis and mass spectroscopy. In addition, single
crystals of Cr2, Cr4 and Cr6 were obtained by slowly diffusion of
ethyl ether to their corresponding methanol solution at room
temperature, and their solid structures were further studied by
single crystal X-ray diffraction. The crystal data and structure re-
finements were compiled in Table 1.
Chart 1. The reported tridentated N,N,N pincer ligand in butadiene polymerization.
Single-crystal X-ray analysis reveals that complexes Cr2 (Fig. 1),
Cr4 (Fig. 2), and Cr6 (Fig. 3) all adopt a distort octahedral geometry
in a meridional manner with metal center chelated by tridentate
ligand through two nitrogen atoms and a pyridine nitrogen atom,
where the equatorial plane consists of theses three nitrogen atoms,
and one chlorine atom. The complexes Cr2 and Cr4 display
approximate Cs symmetry about a plane consisted of three chlorine
groups and the pyridyl nitrogen atom, while Cr6 has C2 symmetry.
The equatorial planes of Cr2 and Cr4 form dihedron angles of
65.67ꢀ, 77.18ꢀ (for Cr2) and 81.08ꢀ, 67.67ꢀ (for Cr4) with two pyri-
dine rings, while equatorial plane is almost coplanar to two pyr-
azoles in Cr6. The bond distance of CreN (pyridine) in all three
complexes are longer than those of CreN(imino, pyrazole), with
CreN (pyridine) length decreasing in the order of Cr6 > Cr2 > Cr4.
The unsymmetrical CreN(imino, pyrazole) bond are also varied
with the ligand type and substitutes, following the decreasing
trend of Cr2 > Cr4 > Cr6. The bond distance between Cr atom and
trans-positioned chloride are longer than those of CreCl (cis-posi-
tioned), suggesting that subtle inequality of three chlorides. The
N1eCreN3 angle in complex Cr2 is 151.69(13)ꢀ, slightly smaller
than 154.4(2)ꢀ and 153.92(6)ꢀ found in the corresponding Cr4 and
2,6-bisiminopyridine [15e17], terpyridine [18], 2,6-bis(oxazoline)
pyridine [19], bis(benzimidazolyl)amine [20e23] and 2-pyrazol-
1,10-phenroline [24] have been well described for promoting good
trans-1,4 selectivity. This may be related to the multi-dentate co-
ordination ability of ligands, which are known for their special
capability not only of stabilizing active species but inducing
h
2-trans coordination of incoming monomer to metal center [4],
however, the reason why this kind of ligand set can promote such
unique trans-1,4 selectivity remains to be elucidated. Recently, a
new type of titanium catalyst precursors incorporated with a
chelated ligand (OSSO) having two phenolate units linked through
a 1,
u
-dithiaalkanediyl bridge S(CH2)nS (n ¼ 2 and 3) efficiently
catalyze trans-1,4 homo-polymerization and copolymerization
with styrene and ethylene, giving copolymers with unprecedented
microstructural architectures [2,3].
The development of single-site Cr catalysts has also renewed
interest for exploring stereoselective 1,3-conjugated diene cata-
lysts. Collectively, the selectivity of homogeneous chromium cata-
lysts is closely related to catalyst formula, in particular, the ligand
framework and nature of dentated atoms, for example, chro-
miumdichloride compounds supported by bidentated phosphines
ligands linked with alkyl chain (P,P-ligand) have been demon-
strated as a syndio-1,2-selective catalyst [25,26]. Gibson and co-
workers [21,22] explored bis(benzimidazolyl)amine (N,N,N-
ligand) as an auxiliary ligand for chromium(III) trichloride, which
results in a high active and trans-1,4 selective (99%) catalyst when
activated with MMAO. The tridentated terpyridine (N,N,N) [27] and
1,3-bisiminobenzene (NCN) ligand incorporated complexes devel-
oped by Nakayama [28] and Mu [29], respectively, are also able to
initiate trans-1,4 selective polymerization of butadiene with good
activity. Despite various efficient catalysts have been explored, the
ligand effect on catalytic performance have not yet been well un-
derstood, in particular, how the stereoselectivity is induced as well
as the high activity is promoted by the ligand backbone and it's
substitute has not yet been elucidated.
In the previous work, we described the metal dependent poly-
merization behaviors of 2,6-bis[(iminophenyl)methyl]pyridine
chelated transition metal (Cr, Fe, Co and Ni) and found chromium,
Chart 2. The structures of tridentated N,N,N pincer ligand ligated chromium com-
plexes in this study.

D. Gong et al. / Journal of Organometallic Chemistry 766 (2014) 79e85
81
Table 1
Crystal data and structure refinements of complexes Cr2, Cr4 and Cr6.
Cr2
Cr4
Cr6
Formula
C31H39N3Cl3Cr
C23.5H24N3CrCl4O0.5 C14H16N6Cl3CrO
Molecular weight 612.03
550.26
442.68
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Orthorhombic
Fdd2
25.8302(17)
34.342(2)
14.1398(10)
90.00
90.00
90.00
12543.0(15)
16
1.296
Monoclinic
C2/c
Triclinic
P-1
27.341(5)
15.509(5)
16.159(5)
90.000(5)
124.768(5)
90.000(5)
5629(3)
8
9.2046(6)
10.5251(8)
10.5630(7)
90.5570(10)
114.9810(10)
97.5500(10)
917.13(11)
2
a
b
g
(deg)
(deg)
(deg)
V (Å3)
Z
Fig. 2. ORTEP drawing of complex Cr4 with thermal ellipsoids at 30% probability level.
Hydrogen atoms have been omitted for clarity.
Dcalcd (Mg/m3)
Absorp coeff
1.299
0.803
1.603
1.076
0.645
(mmꢁ1
)
F(000)
7700
2256
450
Crystal size(mm) 0.21 ꢂ 0.21 ꢂ 0.15 0.15 ꢂ 0.12 ꢂ 0.10 0.28 ꢂ 0.22 ꢂ 0.17
facilitates monomer access and coordination. The polymer ob-
tained has monotonously decreased selectivity with [Al]/[Cr]
increasing, for example, the trans-1,4 structure is around 89% at
[Al]/[Cr] ¼ 100e300, decreases to 87.5%, 82.5% at 400 and 500,
respectively, and this is may be related to overuse of MMAO,
making trans-1,4 enchainment in more errors. Worth noting is
further reducing to 45.6% and 35.5% when [Al]/[Cr] ¼ 800 and 1000,
respectively, this can be reasonably ascribed to increased percent-
age of cis-1,4 coordination as one more vacant available resulted
from cleavage of one CreN bond, which render cis-1,4 more likely
(Chart 3, path a). The increasing of [Al]/[Cr] does not have a regular
effect on polymer molecular weights and PDI value. Upon activation
with MAO, AlEt3 and AlBu3, Precatalyst Cr1 is also found to be active
for butadiene polymerization, and polymer yield follows the order
of MMAO > MAO > AlBu3 > AlEt3 under comparable conditions. The
trans-1,4 selectivity is influenced as well, and decreases in the
Q
range (deg)
1.75e26.02
2.59e26.18
2.48e26.07
No. of reflns
collected
No. of indep
reflns
7032
14,706
5057
3747
5402
3571 (Rint ¼ 0.01)
(Rint ¼ 0.0255)
(Rint ¼ 0.0748)
No. of data/
restraint/
params
3747/1/358
5402/0/249
3571/0/228
GOF on F2
1.023
1.167
1.026
R1(I > 2sigma(I)) 0.0367
wR2 0.0914
0.0829
0.2405
0.0274
0.0722
Cr6, indicating an opener space around the metal center, and this
could be a factor influencing catalytic selectivity and activity.
Butadiene polymerization behaviors of complexes
sequence
of
MAO(89.6%) > MMAO(89.0%) > AlEt3(88.5%) > AliBu3(85.3%). The
reduced molecular weight of polymer observed from Table 2 (run 3,
10 vs 8, 9) indicates chain transfer reaction is more likely in MAO
and MMAO activated system.
The catalytic performances of Cr catalyst in butadiene poly-
merization were investigated under various experimental condi-
tions, and the results were summarized in Table 2.
The optimal polymerization conditions were obtained for a
representative precatalyst Cr1 by varying [Al]/[Cr] mole ratio from
100:1 to 1000:1 (Fig. 4). The results show polymer yield varies with
MMAO amount, going up with increased amount of MMAO,
achieving a plateau appearing around 78.3% when the ratio of [Al]/
[Cr] is 300, and then levels off from 300 to 500, probably due to
decay of active center resulted from overuse of cocatalyst. Inter-
estingly, the polymer yield increases with [Al]/[Cr] amount
increasing from 500 to 1000, and this could be explained by the fact
that opening space resulted from cleavage of CreN bond, which
Upon reaching efficient cocatalyst MMAO, temperature depen-
dence study was also performed with representative precatalyst
Cr1. As it demonstrates in Table 2, when polymerization tempera-
ture goes up from 0 to 50 ꢀC, the polymer yield obviously increases,
reaching a peak and then decreases a little, implying number of
active species might decline at higher temperature, however, the
trans-1,4 selectivity is increased with going up of temperature,
which can be explained on diene mechanism that more thermal
Fig. 1. ORTEP drawing of complex Cr2 with thermal ellipsoids at 30% probability level.
Fig. 3. ORTEP drawing of complex Cr6 with thermal ellipsoids at 30% probability level.
Hydrogen atoms have been omitted for clarity.
Hydrogen atoms have been omitted for clarity.

82
D. Gong et al. / Journal of Organometallic Chemistry 766 (2014) 79e85
Table 2
high trans-1,4 selectivity, as coordination mode is influenced by
bulky groups. The selectivity of precatalysts Cr1, Cr3, Cr4 and Cr6,
are unanimously higher than those of precatalysts Cr2, Cr5 and Cr7
with an ortho-alkyl substituent under the comparable conditions,
and the more bulky the substitute is, the less the trans-1,4 selec-
tivity is, and such effect on the activity of precatalyst has been also
found in the reported literature [31,33,34]. Interestingly, adding
more cocatalyst to Cr2, Cr5 and Cr7 systems, for example, the [Al]/
[Cr] is increased from 300 to 500, the polymer yields all increase
with increasing of cis-1,4 at expense of trans-1,4 selectivity,
implying one CreN could be cleaved which makes monomer more
accessible and cis-1,4 coordination mode more feasible.
GPC analysis on the polymers from all the catalysts reveals that
molecular weight distribution is basically unimodal, being char-
acteristic for homogeneous catalysis, and remarkably dependent on
the structure of ligand, with the higher molecular weight poly-
butadiene being produced by precatalysts Cr2, Cr5 and Cr7, prob-
ably due to the relatively large steric hindrance of ligands
incorporated in the catalyst disfavoring chain-transfer reaction. The
structures of polymers was analyzed by NMR, typically, H NMR and
13C NMR of sample from run 9 in Table 2 were shown in Fig. 5 and
Fig. 6, respectively.
Butadiene polymerization results by Cr precatalysts under various conditions.
Run Cr
Cocatalyst [Al]/[Cr] Yield Microstructure (%)
Mn
PDI
(%)
(104)
Trans-1,4 Cis-1,4 1,2
1
2
3
4
5
6
7
8
Cr1 MMAO
100
200
300
400
500
800
1000
200a
200b
300
100
100
300
800
300
300
300
800
300
300
800
52.0 89.3
75.0 89.5
78.3 89.0
64.2 87.5
52.2 82.5
65.8 45.6
72.1 35.5
41.2 86.2
31.5 92.0
66.9 89.6
39.2 85.3
48.7 88.5
21.0 45.6
45.2 23.5
76.5 89.2
81.5 89.4
33.2 55.3
46.9 20.9
71.9 91.2
45.8 56.2
45.3 24.9
1.9
2.5
3.6
3.2
7.1
39.5
45.0
9.3
3.3
2.1
5.1
3.5
35.4
66.7
6.5
9.8 1.2
2.6
2.1
1.9
2.4
1.9
2.6
2.8
1.8
2.6
2.5
1.8
1.9
2.3
3.2
2.6
2.4
3.0
3.8
2.7
2.8
3.2
Cr1 MMAO
Cr1 MMAO
Cr1 MMAO
Cr1 MMAO
Cr1 MMAO
Cr1 MMAO
Cr1 MMAO
Cr1 MMAO
Cr1 MAO
9.0 1.9
7.4 2.1
9.3 2.3
10.4 2.1
15.0 4.9
19.5 5.2
4.5 2.5
4.7 1.0
8.3 0.9
9.6 3.5
8.0 3.7
19.0 0.7
9.8 3.1
4.3 1.1
4.0 1.8
11.0 1.3
10.2 2.8
5.9 1.5
10.0 1.1
10.6 2.6
9
10
11
12
13
14
15
16
17
18
19
20
21
Cr1 AlEt3
Cr1 AliBu3
Cr2 MMAO
Cr2 MMAO
Cr3 MMAO
Cr4 MMAO
Cr5 MMAO
Cr5 MMAO
Cr6 MMAO
Cr7 MMAO
Cr7 MMAO
6.6
33.7
68.9
2.9
36.8
65.5
Polymerization conditions: toluene, 20 mL; butadiene, 0.1 mol; Cr, 0.05 mmol; re-
a
action temperature, room temperature, at 0 ꢀC, b at 50 ꢀC.
Mechanism consideration
The mechanism of diene polymerization by transition metal
catalysts such as titanium, cobalt and nickel which were able to
initiate trans-1,4 polymerization have been well developed previ-
ously by Tobisch [35]. In our previous reports involving bisimino-
pyridine ligated iron, cobalt and nickel catalysts, we have discussed
the possible reaction pathways involved in cis-1,4, trans-1,4 and 1,2
formation regulated by tridentated ligand [4,17,30]. Polymerization
performance herein indicates the similar reaction mode is induced
by tridentated ligand in the present catalyst system (Chart 3). In
presence of cocatalysts, active alkyl Cr center is produced by
alkylation and reduction, and vacant site opening preferable for
monomer trans-h2 coordination could subsequently generate. This
coordinated monomer inserts into the Cr-alkyl bond forming allyl
coordinated terminal group which takes preferably a thermal stable
syn-conformation, affording trans-stereoregulated growing poly-
mer chain. When monomer insertion is less favored, the isomeri-
zation arrangement of syn-allyl Cr species could result in an anti-
allyl configuration which leads to a kinetic favored cis-1,4 product.
The polymerization results obtained from Cr1, Cr3, Cr4 and Cr6
indicate isomerization is suppressed by using the tridentate ligand.
stable syn-allylic intermediate is produced under a relative higher
temperature.
Under similar conditions, catalytic performance of Cr2eCr7
were compared, and polymerization results show that polymer
yields from these complexes is notably influenced by size of sub-
stituents placed at ortho-position. It can be clearly seen that the
absence of substitute on ortho-position of ligand is helpful for high
catalytic activity. Free ortho-substitute precatalysts Cr1, Cr3, Cr4
and Cr6, regardless of the ligand backbone, all show higher active
towards butadiene polymerization, and those with ortho-alkyls
such as methyl, isopropyl on bisiminopyridine or methyl on pyr-
azole are obviously less active (Table 2, run 11e12, 15e16 and
18e19). When the ortho-substituent is replaced by more bulky
isopropyl group in case of complex Cr2, a remarkable decay of
catalytic activity is observed (Table 2, run 13e14). This might be
attributed to more crowded metal center, which significantly
blocks orientation and coordination of incoming butadiene to
metal center, thus influences insertion rate. The size of alkyl sub-
stituents in ortho-position of ligands seems to be also important for
Conclusion
Chromium complexes chelated with tridentated pincer ligand
have been developed and examined in butadiene polymerization.
Effect of ligand on catalytic selectivities was studies, and the results
shown spatial opening at ortho-position on the ligand of catalyst
Cr1, Cr3, Cr4 and Cr6 are favorable for monomer access and trans-
1,4 coordination, resulting in high activity and high trans-1,4
selectivity, which is, to some extent, controlled by MMAO amount.
Experimental
General methods
All manipulation of air or moisture sensitive reaction and pu-
rification were carried out under nitrogen atmosphere. Anilines,
pyrazoles, 2,6-dibromopyridine and pyridine-2.6-dicarboxylic acid,
pyridine-2.6-dicarboxylaldehyde were purchased from Alfa Aesar.
Modified methylaluminoxane (MMAO) was purchased from
Fig. 4. Effects of [MMAO]/[Cr] on polymer yields and trans-1,4 incorporation.

D. Gong et al. / Journal of Organometallic Chemistry 766 (2014) 79e85
83
Chart 3. The mechanism pathway of trans-1,4 and cis-1,4 polybutadiene: the effect of MMAO amount on polymer microstructure.
AkzoNoble, MAO, AlEt3 and AliBu3 were commercially available
from Aldrich. All solvents were purified by the standard methods.
Polymerization grade 1,3-butadiene was purified by AlEt3 prior to
use. NMR analysis of ligands and polymers were recorded on a 400
or 500 Hz Bruker spectrometer in CDCl3 as solvent. FT-IR of ligand
and complexes were analyzed by BRUKER Vertex-70 FIR spectro-
photometer. Elemental analyses were calculated on an elemental
Vario EL spectrometer. ICP-MS for Cr analysis was performed at
Fig. 5. H NMR of polybutadiene (sample from run 9 in Table 2).

84
D. Gong et al. / Journal of Organometallic Chemistry 766 (2014) 79e85
Fig. 6. 13C NMR of polybutadiene (sample from run 9 in Table 2).
Agilent 7500cx. Mass spectroscopy of sample dissolved in meth-
anol was performed on Thermo Scientific™-LTQ Velos Orbitrap MS.
Molecular weights (Mn) and molecular weight distributions (PDI) of
polymer were determined at 30 ꢀC by VISCOTEK GPC1000 with
TDA305 (Triple Detector Array) as detector and calibration was
made against polystyrene.
{2,6-Bis[1-(phenylimino)ethyl]pyridine}CrCl3 (Cr3)
Yield: 79.4%. Green powder. IR
(nC]N,
KBr, cmꢁ1): 1614
(1637 cmꢁ1 for free ligand). Anal. Calcd. For C21H19N3CrCl3: C, 53.47;
H, 4.06; N, 8.91. Found: C, 53.20; H, 3.85; N, 8.66. Cr: 11.02%
(Calc.11.02%). MS (ESI): m/z 436.3 [M
ꢁ
Cl]þ (100%), 468.3
[M e Cl þ CH3OH]þ(47%).
X-ray structure determinations
{2,6-Bis[1-(4-methylphenylimino)ethyl]pyridine}CrCl3 (Cr4)
nC]N,
KBr, cmꢁ1): 1615
Yield: 88.6%. Green powder. IR
(
X-ray analysis data were collected on a Bruker SMART APEX
diffractometer with a CCD area detector by using graphite mono-
(1641 cmꢁ1 for free ligand). Anal. Calcd. For C23H23N3CrCl3: C,
55.27; H, 4.64; N, 8.41. Found: C, 55.90; H, 4.79; N, 8.69. Cr: 10.40%
(Calc.10.44%). 464.6 [M ꢁ Cl]þ(100%), 496.6 [M e Cl þ CH3OH]þ(8%),
428.9 [M ꢁ 2Cl]þ(2%).
chromated Mo K radiation (
l
¼ 0.71073 Å). Crystal class and unit
cell parameters were determined by the SMART program package.
Reflection data file was yielded from raw frame data by SAINT and
SADABS and the structures were solved based on SHELXTL pro-
gram. Refinement was processed on F2 anisotropically for all non-
hydrogen atoms by the full-matrix least-squares method.
{2,6-Bis[1-(2,6-dimethylphenylimino)ethyl]pyridine}CrCl3 (Cr5)
Yield: 72.9%. Green powder. IR
(nC]N,
KBr, cmꢁ1): 1621
(1646 cmꢁ1 for free ligand). Anal. Calcd. For C25H27N3CrCl3: C, 56.88;
H, 5.16; N, 7.96. Found: C, 57.03; H, 4.99; N, 7.89. Cr: 9.85% (Calc.
9.75%). 492.3 [M ꢁ Cl]þ(100%), 524.3 [M e Cl þ CH3OH]þ(23%),
457.0 [M ꢁ 2Cl]þ(22%).
Syntheses and characterization of complexes
To a stirred solution of corresponding ligand (2.0 mmol) in THF
(10 mL), a THF solution containing 1.0 equiv. of CrCl3$3THF
(2.0 mmol, 0.345 g in 5 mL THF) was added dropwise. The reaction
mixture was stirred overnight at room temperature, and then a
green precipitate was formed. The green solid was collected by
filtration, washed with diethyl ether (5 mL) and hexane (5 mL),
dried in vacuo at 40 ꢀC overnight.
[2,6-Bis(pyrazole)pyridine]CrCl3 (Cr6)
Yield: 65.5%. Green powder. IR (nC]N, KBr, cmꢁ1): 1620, 1614
(1624, 1619 cmꢁ1 for free ligand). Anal. Calcd. For C11H9N3CrCl3: C,
38.68; H, 2.66; N, 12.30. Found: C, 38.79; H, 2.51; N, 12.59. Cr:
15.22% (Calc.15.02%). MS (ESI): m/z 338.0 [M ꢁ Cl]þ(100%).
[2,6-Bis(3,5-dimethylpyrazole)pyridine]CrCl3 (Cr7)
Yield: 81.5%. Green powder. IR (nC]N, KBr, cmꢁ1): 1619, 1605
(1623, 1609 cmꢁ1 for free ligand). Anal. Calcd. For C15H17N3CrCl3: C,
45.30; H, 4.31; N, 10.57. Found: C, 45.29; H, 4.55; N, 10.61. Cr: 13.07%
(Calc. 13.27%). MS (ESI): m/z 362.3 [M ꢁ Cl]þ(100%).
[2,6-Bis(1-phenylimino)pyridine]CrCl3 (Cr1)
Yield: 75.1%. Green powder. IR (nC]N, KBr): 1620 (1628 cmꢁ1 for
ligand). Anal. Calcd. for C19H15N3CrCl3: C, 51.43; H, 3.41; N, 9.47.
Found: C, 51.01; H, 3.33; N, 9.69%. Cr: 11.71% (Calc.11.99%). MS (ESI):
m/z 408.3, [M ꢁ Cl]þ(100%); 440.3 [M e Cl þ CH3OH]þ(57%); 372.8
[M ꢁ 2Cl]þ(11%).
Procedure for butadiene polymerization
Polymerization was processed in 50 mL glass bottle capped with
rubber. Taking Cr1 as an example, a toluene (20 mL) solution of
butadiene (5.4 g, 0.10 mol) was introduced to the glass bottle pre-
loaded with complex Cr1 (22.2 mg, 0.05 mmol), then MMAO
(50.0 mmol) was injected for initiating polymerization. After 6 h,
the reaction was terminated by acidified methanol (0.5%, 15 mL),
{2,6-Bis[1-(2,6-diisopropylphenylimino)]pyridine)CrCl3 (Cr2)
Yield: 89.1%. Green powder. IR (nC]N, KBr, cmꢁ1): 1619 (1624 cmꢁ1
for free ligand). Anal. Calcd. for C31H39N3CrCl3: C, 60.84; H, 6.42; N,
6.87. Found: C, 59.64; H, 6.65; N, 7.01. Cr: 8.49% (Calc. 8.50%). MS (ESI):
m/z 576.6 [M ꢁ Cl]þ(100%); 608.6 [M e Cl þ CH3OH]þ(13%).

D. Gong et al. / Journal of Organometallic Chemistry 766 (2014) 79e85
85
[13] J. Gromada, L. Pichon, A. Mortreux, F. Leising, J.-F. Carpentier, J. Organomet.
Chem. 683 (2003) 44e55.
[14] D.K. Jenkins, Polymer 26 (1985) 147e151.
and the polymer precipitate was collected by filtration, washed
with methanol (10 mL) and dried at 50 ꢀC to a constant weight.
[15] G. Ricci, S. Italia, L. Porri, Macromol. Chem. Phys. 194 (1994) 1389e1392.
[16] E. Colamarco, S. Milione, C. Cuomo, A. Grassi, Macromol. Rapid Commun. 25
(2004) 450e454.
Acknowledgment
[17] D. Gong, B. Wang, C. Bai, J. Bi, F. Wang, W. Dong, L. Jiang, X. Zhang, Polymer 50
(2009) 6259e6264.
[18] Y. Nakayama, Y. Baba, H. Yasuda, K. Kawakita, N. Ueyama, Macromolecules 36
(2003) 7953e7979.
[19] J.D. Nobbs, A.K. Tomov, R. Cariou, V.C. Gibson, A.J.P. White, G.J.P. Britovsek,
Dalton Trans. 41 (2012) 5949e5964.
Funding from Natural Science Foundation of China (Grant No.
21304050) and sponsored by the K. C. Wong Magna Fund in Ningbo
University, are gratefully acknowledged.
Appendix A. Supplementary material
[20] D. Gong, X. Jia, B. Wang, X. Zhang, L. Jiang, J. Organomet. Chem. 702 (2012)
10e18.
[21] R. Cariou, J.J. Chirinos, V.C. Gibson, G. Jacobsen, A.K. Tomov, M.R.J. Elsegood,
Macromolecules 42 (2009) 1443e1444.
CCDC 965626, 965627 and 965628 contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[22] R. Cariou, J.J. Chirinos, V.C. Gibson, G. Jacobsen, A.K. Tomov, G.J.P. Britovsek,
A.J.P. White, Dalton Trans. 39 (2010) 9039e9045.
[23] V. Appukuttan, L. Zhang, C. Ha, I. Kim, Polymer 50 (2009) 1150e1158.
[24] B. Wang, J. Bi, C. Zhang, Q. Dai, C. Bai, X. Zhang, Y. Hu, L. Jiang, Polymer 54
(2013) 5174e5181.
[25] G. Ricci, A. Forni, A. Boglia, M. Sonzogni, Organometallics 23 (2004)
3727e3732.
References
[26] G. Ricci, A. Boglia, T. Motta, J. Mol. Catal. A Chem. 267 (2007) 102e107.
[27] Y. Nakayama, K. Sogo, Z. Cai, H. Yasuda, T. Shiono, Polym. Int. 60 (2011)
692e697.
[28] J. Zhang, W. Gao, X. Lang, Q. Wu, L. Zhang, Y. Mu, Dalton Trans. 41 (2012)
9639e9645.
[29] Z. Liu, W. Gao, X. Liu, X. Luo, D. Cui, Y. Mu, Organometallics 30 (2011)
752e759.
[30] D. Gong, X. Jia, B. Wang, X. Zhang, L. Jiang, Inorg. Chim. Acta 373 (2011)
47e53.
[1] J.J. Benvenuta-Tapia, J.A. Tenorio-Lopez, R. Herrera-Najera, L. Rios Guerrero,
Polym. Eng. Sci. 49 (2009) 1e10.
[2] S. Milione, C. Cuomo, C. Capacchione, C. Zannoni, A. Grassi, A. Proto, Macro-
molecules 40 (2007) 5638e5643.
[3] C. Capacchione, A. Avagliano, A. Proto, Macromolecules 41 (2008) 4573e4575.
[4] A. Fischbach, R. Anwander, Prog. Polym. Sci. 204 (2006) 155e281.
[5] G. Natta, L. Porri, A. Carbonaro, Makromol. Chem. 77 (1964) 126e138.
[6] L. Porri, A. Giarrusso, G. Ricci, Prog. Polym. Sci. 16 (1991) 405e441.
[7] S. Kaita, M. Yamanaka, A. Horiuchi, Y. Wakatsuki, Macromolecules 39 (2006)
1359e1363.
[8] D. Robert, T.P. Spaniol, J. Okuda, Eur. J. Inorg. Chem. (2008) 2801e2809.
[9] R. Taube, S. Maiwald, J. Sieler, J. Organomet. Chem. 621 (2001) 327e336 and
references therein.
[31] D. Gong, B. Wang, H. Cai, X. Zhang, L. Jiang, J. Organomet. Chem. 696 (2011)
1584e1590.
ꢀ
[32] A.R. Karam, E.L. Catarì, F. Lopez-Linares, G. Agrifoglio, C.L. Albano, A. Dìaz-
ꢀ
Barrios, T.E. Lehmanna, S.V. Pekerar, L.A. Albornoz, R. Atencio, T. Gonzalez,
H.B. Ortega, P. Joskowics, Appl. Catal. A Gen. 280 (2005) 165e174.
[33] D. Gong, W. Jia, T. Chen, K. Huang, Appl. Catal. A Gen. 464e465 (2013) 35e42.
[34] D. Gong, B. Wang, X. Jia, X. Zhang, Dalton Trans. 43 (2014) 4169e4178.
[35] S. Tobisch, Can. J. Chem. e Revue Can. Chim. 87 (2009) 1392e1405.
[10] W. Rong, D. Liu, H. Zuo, Y. Pan, Z. Jian, S. Li, D. Cui, Organometallics 32 (2013)
1166e1175.
[11] D. Wang, S. Li, X. Liu, W. Gao, D. Cui, Organometallics 27 (2008) 6531e6538.
[12] D. Liu, D. Cui, Dalton Trans. 40 (2011) 7755e7761.