Polymerization of 1,3-butadiene catalyzed by pincer cobalt(II) complexes derived from 2-(1-arylimino)-6-(pyrazol-1-yl)pyridine ligands

Article abstract of DOI:10.1016/j.apcata.2013.04.026

A new class of air stable and structurally well-defined cobalt complexes with unsymmetrical pincer type ligands ([2-(ArNCMe)-6-(Py)C₅H ₃N]CoCl₂) (Ar = C₆H₅, Py = pyrazol-1-yl, 5a; Ar = 2,4,6-Me₃C₆H₂, Py = pyrazol-1-yl, 5b; Ar = 2,6-ⁱPr₂C₆H₃, Py = pyrazol-1-yl, 5c; Ar = C₆H₅, Py = 3,5-Me ₂pyrazol-1-yl, 5d; Ar = 2,4,6-Me₃C₆H ₂, Py = 3,5-Me₂pyrazol-1-yl, 5e; Ar = 2,6- ⁱPr₂C₆H₃, Py = 3,5-Me ₂pyrazol-1-yl, 5f; Ar = 2,6-ⁱPr₂C ₆H₃, Py = 3,5-ⁱPr₂pyrazol-1-yl, 5g and [2-(OCMe)-6-(3,5-diphenylpyrazol-1-yl)C₅H₃N]CoCl ₂ 5h) were prepared and the molecular structures of 5a, 5c and 5f were determined by single crystal X-ray crystallography. Upon activation by methylaluminoxane (MAO) in toluene at room temperature, all complexes initiate polymerization of 1,3-butadiene (polymer yields: 65-99%), affording polybutadiene with excellent cis-1,4 regularity (97.5-98.7%). The polymer yields and properties in terms of molecular weight and distribution are well controlled by the substituents on iminoaryl rings and pyrazole rings. Selectivity switch from cis-1,4 to syndio-1,2 was also achievable by adding phosphine as microstructure regulator.

Full text of DOI:10.1016/j.apcata.2013.04.026

Applied Catalysis A: General 464–465 (2013) 35–42
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Applied Catalysis A: General
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Polymerization of 1,3-butadiene catalyzed by pincer cobalt(II)
complexes derived from 2-(1-arylimino)-6-(pyrazol-1-yl)pyridine
ligands
Dirong Gonga,b, Weiguo Jiaa, Tao Chena, Kuo-Wei Huanga,∗
a King Abdullah University of Science and Technology (KAUST), Division of Physical Sciences and Engineering and KAUST Catalysis Center, Thuwal
23955-6900, Saudi Arabia
b Department of Polymer Science and Engineering, Ningbo University, Ningbo, 315211, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of air stable and structurally well-defined cobalt complexes with unsymmetrical pincer
type ligands ([2-(ArN CMe)-6-(Py)C5H3N]CoCl2) (Ar = C6H5, Py = pyrazol-1-yl, 5a; Ar = 2,4,6-Me3C6H2,
Py = pyrazol-1-yl, 5b; Ar = 2,6-iPr2C6H3, Py = pyrazol-1-yl, 5c; Ar = C6H5, Py = 3,5-Me2pyrazol-1-
yl, 5d; Ar = 2,4,6-Me3C6H2, Py = 3,5-Me2pyrazol-1-yl, 5e; Ar = 2,6-iPr2C6H3, Py = 3,5-Me2pyrazol-1-
yl, 5f; Ar = 2,6-iPr2C6H3, Py = 3,5-iPr2pyrazol-1-yl, 5g and [2-(O CMe)-6-(3,5-diphenylpyrazol-1-
yl)C5H3N]CoCl2 5h) were prepared and the molecular structures of 5a, 5c and 5f were determined by
single crystal X-ray crystallography. Upon activation by methylaluminoxane (MAO) in toluene at room
temperature, all complexes initiate polymerization of 1,3-butadiene (polymer yields: 65–99%), afford-
ing polybutadiene with excellent cis-1,4 regularity (97.5–98.7%). The polymer yields and properties in
terms of molecular weight and distribution are well controlled by the substituents on iminoaryl rings
and pyrazole rings. Selectivity switch from cis-1,4 to syndio-1,2 was also achievable by adding phosphine
as microstructure regulator.
Received 23 November 2012
Received in revised form 24 February 2013
Accepted 17 April 2013
Available online xxx
Keywords:
Cobalt(II) complexes
2-(1-Arylimino)-6-(pyrazol-1-yl)pyridine
1,3-Butadiene
Selectivity
Polybutadiene
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
precursors to prepare the cis-1,4 polymer [33]. Ziegler–Natta cat-
alysts of titanium [34–38], nickel [39–41], cobalt [17,33,42–51]
Polymerization of 1,3-butadiene is an important process in
the polymer industry as the resultant polymers possess versatile
properties for a wide range of applications [1–3]. As these prop-
erties depend mainly on the microstructures generated during
the polymerization, much effort has been devoted to designing
highly regio- and/or stereoselective catalyst systems, to prepare
polybutadienes with precisely tailor-made microstructures. The
development of homogeneous Ziegler–Natta catalysts has real-
ized regio- and stereospecific polymerization of 1,3-butadiene to
produce cis-1,4- [4–11], trans-1,4- [9,12–16] and syndiotactic 1,2-
polybutadienes [17–31]. Among them, cis-1,4-polybutadiene is one
of the most important products since it is employed as one of
the major materials in the tire and automotive industries [32].
Cis-1,4-polybutadiene can also be copolymerized or blended with
other compounds to prepare materials with improved mechan-
ical properties, such as Styrene-Butadiene Rubber (SBR), High
Impact PolyStyrene (HIPS), Acrylonitrile-Butadiene-Styrene (ABS)
or Hydroxyl Terminated PolyButadiene (HTPB). As a result, transi-
tion metal complexes have been considerably explored as catalyst
and neodymium-based catalysts [2,8,11,52–55] activated with var-
ious organoaluminum reagents have been used in the production
of cis-1,4-polybutadiene, with the neodymium-based ones being
the main catalysts used for the production on the industrial scale.
These catalysts offer high activity and selectivity (cis-1,4 > 97%) to
provide polymers with desirable material properties such as excel-
lent abrasion and cracking resistance and high tensile strength
of the vulcanizates [6]. Their ill-defined and multi-site nature,
nevertheless, causes relatively broad molecular weight distribu-
tions, and thus complicates the mechanistic study and hinder
catalyst design and optimization. In order to gain better control
over molecular weight and polymer compositions, academic and
industrial research has focused on well-defined single-site cata-
lysts in recent years, mainly based on rare earth metals and the first
row transition metals. Various well-defined Ln C bond containing
complexes, such as lanthanide-based metallocene alkyl complexes,
lanthanocene aluminates, alkyl bridged lanthanide carboxylates,
and lanthanide alkyl complexes supported by versatile pincer lig-
ands, are extensively investigated and developed [1,2,5,56–59].
With these active systems, excellent controls over the molecular
weights as well as the microstructures of polymers are facilely
achieved, though industrial applications have not yet been demon-
strated. Meanwhile, late transition metal catalysts, such as iron,
∗
Corresponding author. Tel.: +966 2 8080328; fax: +966 2 8081025.
E-mail address: hkw@kaust.edu.sa (K.-W. Huang).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.04.026

36
D. Gong et al. / Applied Catalysis A: General 464–465 (2013) 35–42
nickel and cobalt, have been received increased attention due to
their good functional group tolerance, decreased air and moisture
sensitivity and lower cost and environmental impact. In particular,
cobalt-based catalysts have been demonstrated for high regio- and
stereoselective polymerization of 1,3-butadiene [60].
The carboxylates (or acetylacetonate) [61,62] and halides of
cobalt [17,31,33,42–45,47,49] are the major classes of catalysts
for producing cis-1,4-polybutadiene when activated by methyla-
luminoxane (MAO) or aluminum chlorides. Simple cobalt salts
CoCl2, activated with organoaluminum compounds, are able
to generate heterogeneous active species to polymerize 1,3-
butadiene with cis-specific living polymerization. Interestingly,
in the polymerization process, the selectivity can be switched
and led to the formation of desired cis-1,4-1,2 block copoly-
mer by addition of PPh3 [17,47,49]. Homogeneous systems such
as Co(acac)3 [61] and Co(Salen)2 [42,63] in combination with
MAO or ethyl aluminum sesquichloride (EAS) were also reported
to catalyze cis-regiospecific polymerization of 1,3-butadiene.
More recently, research efforts in this field have been gradu-
ally shifted on the design of well-defined, homogeneous and
high active CoCl2 complexes supported with various triden-
tate ligands, such as bis(imino)pyridine, bis(imidazolyl)amine and
bis(imidazolyl)pyridine, and the high cis-1,4 selectivity and activity
have been maintained [9,16,33,44,64–68]. Despite these achieve-
ments, further exploration of well-defined homogeneous catalyst
systems with high cis-1,4 selectivity and thermo-, air and mois-
ture stability has remained a fascinating and challenging subject.
We are interested in pincer ligated complexes in a general formula
of [(NNL)M] because their coordination geometries and electronic
properties (and thus the reactivity of the metal center) could be
modified by tuning the structures of the arms and substituents [60].
When the pyridyl-supported pyrazolyl-imine system is adopted,
the large ␲-system is believed to increase the Lewis-acidity of the
metal center to facilitate the interactions between the metal and the
monomer which may enhance the activity [69]. Herein, we describe
the synthesis and characterization of a new class of cobalt com-
plexes with pyridyl-supported pyrazolyl-imine ligands and their
applications in the 1,3-butadiene polymerization.
Fig. 1. ORTEP drawing of cationic complex 5a with thermal ellipsoids at the 30%
probability level. Hydrogen atoms have been omitted for clarity.
˚
the Co1 N2 (iminoaryl) bond (2.190 A), respectively. The Co1 Cl2
˚
˚
bond is 0.078 A longer than the Co1 Cl1 bond (2.310 vs. 2.232 A).
˚
The imino N(2) C(6) bond length is 1.284 A, with the typical char-
acter of a C N double bond.
The unit cell of complex 5f contains two independent molecules
(Fig. 3), with only few differences in the coordination geometry of
the central metals. Interestingly, the distance between two planes
˚
of pyrazole rings is approx. 3.4 A, suggesting certain degree of ␲-␲
stacking stabilization in the solid state. The geometrical param-
eters are comparable to those in 5c, but the N2 Co1 N3 and
N6 Co1 N7 angles of 5f are slightly larger than the N2 Co1 N3
angle in 5c (144.0 and 142.2 vs. 139.2, respectively). The imino N
bond lengths of 1.285 and 1.282 A indicate the C N double bond
character.
C
2. Results and discussion
˚
2.1. Synthesis and characterization of complexes
The cobalt complexes 5a–5g were obtained by reacting CoCl2
with one equivalent of corresponding ligands at room tempera-
ture (Scheme 1). These complexes were characterized by IR, and
elemental analysis, as well as single crystal X-ray analysis of the
selected compounds. The C N (arylimino) stretching bands of
free ligands at 1649–1631 cm−1 shift toward lower frequencies by
7–23 cm−1 when coordinated to cobalt. The structures of 5a, 5c and
5f were further confirmed by X-ray crystallography.
The molecular structure of 5a shows it is an ion-paired complex
[L2Co]2+[CoCl4]2− (Fig. 1). The cobalt center in the cation of 5a is
six-coordinate, with Co N (imino nitrogen and pyrazole nitrogen)
˚
bonds ranging from 2.004 to 2.140 A. These bonds are shorter by
˚
0.04–0.07 A than those observed in complexes 5c and 5f. The geom-
etry of 5c shows a distorted trigonal bipyramidal structure with
the N1 Cl1 Cl2 equatorial plane (Fig. 2). The cobalt atom slightly
◦
˚
lies 0.0078 A out of the plane with three equatorial angles of 92.0
(N1 Co1 Cl2), 115.9◦ (Cl2 Co1 Cl1) and 151.8◦ (N1 Co1 Cl1).
The equatorial plane is almost perpendicular to both of the pyrazole
and pyridine planes, with dihedral angles of 88.7◦ and 89.6◦, respec-
tively. The dihedral angle between the pyridine and the iminoary
plane is 88.9◦. The Co1 N1 (pyridine) bond is 0.087 A shorter than
˚
Fig. 2. ORTEP drawing of 5c with thermal ellipsoids at the 30% probability level.
Hydrogen atoms have been omitted for clarity.
˚
˚
the Co1 N3 (pyrazole) bond (2.164 A) and 0.113 A shorter than

D. Gong et al. / Applied Catalysis A: General 464–465 (2013) 35–42
37
Scheme 1. Synthesis of ligands and the corresponding cobalt complexes.
2.2. Butadiene polymerization behaviors of complexes activated
by MAO
polymer decreased first, and then increased with increasing of the
MAO/Co molar ratio (entries 2, 6–9).
2.2.1. Polymerization conditions optimization
2.2.2. Ligand environment effect
The influences of the MAO/Co molar ratio and the reaction tem-
peratures on the 1,3-butadiene polymerizations were investigated
in detail with complex 5c as the catalyst precursor (Table 1). An
obvious increase in the production of polymer was observed when
elevating the polymerization temperatures from 0 to 40 ◦C (entries
1–3). However, a further increase from 40 ◦C to 80 ◦C resulted in
decreased productivities, presumably due to the deactivation of the
active centers (entries 4 and 5) [45]. At the same time, the higher
polymerization temperature also led to a higher PDI (entries 1–5),
suggesting an increase in the ˇ-hydrogen elimination rate relative
to the propagation rate at a higher temperature. Analysis on the
microstructures of the resulting polymers revealed that increasing
the temperature led to a slight decrease in the cis-1,4 selectivity,
consistent with the literature observations [2]. For example, the
selectivity for cis-1,4 decreased from 98.7% at 0 ◦C to 97.2% at 80 ◦C.
A higher MAO/Co molar ratio generally led to a higher monomer
conversion. When the ratio of MAO/Co was 600, the catalytic sys-
tem reached the highest monomer conversion of up to 83%. The
selectivity of the resulting polymer structure, however, was not
strongly affected by the MAO/Co ratio (entries 2, 6–9). The PDI of
The successful characterization of the molecular structures of
these Co complexes allowed us to interrogate the influence of the
ligand structures on the catalytic performances of the metal center
(entries 2 in Tables 1 and 2). Complex 5a demonstrated the highest
total conversion of butadiene of 94%. The resultant polymer pos-
sesses predominantly cis-1,4 enchainment, with approx. 2.5% of
1,2 and negligible trans-1,4 structures. A broad but single GPC peak
indicated that multiple sites were involved in the polymerization
processes, presumably due to the presence of at least two types
2−
of active species derived from the naked CoCl4
and the ligated
2+
CoL2 ions (L: 4a) by intra- and/or intermolecular ligand redistri-
bution during activation process. Trimethyl-substituted complex
5b clearly suppressed the monomer conversion, suggesting that
bulky groups near the metal center may prevent the access of
the monomer to the active site, and lead to the decrease of the
polymerization activity. Interestingly, the cis-1,4 selectivity wit-
nesses a slightly beneficial effect, an increase from 97.1% to 97.7%,
which is an indication that the crowded environment around the
metal center is favorable to the cis-1,4 selectivity in the current
catalytic system. These observations could be rationalized by the
fact that the bulky substituents near the active site could reduce
the enchainment error during the polymerization. The molecular
weight of the polymer obtained was doubled, suggesting that the
chain transfer reactions were effectively suppressed in this system.
As expected from this trend, the highest selectivity and the lowest
conversion were observed for the most crowded (the space around
the metal is most crowded) complex 5c among the 5a–c triad. While
increasing the steric bulk of the ligand could result in an increase
in the molecular weight, a slightly smaller molecular weight, cal-
culated to be 169,000, was achieved with 5c, compared to that
obtained from 5b. This result might be caused by the more severely
suppressed chain propagation rate, with a lesser suppressed chain
transfer. The substituents on the pyrazole moiety also affected the
catalyst performance. The similar trends were observed for the
group 5d–5f, with one exception that 5e gave the highest monomer
conversion. Although the exact reason is not yet clear, the synergic
effect from methyl groups in the pyrazoyl and iminoaryl moieties
may play a role. The highest selectivity of 98.7% was achieved with
5f at a little expense of conversion. Complex 5g, possessing the
bulkiest environment around the metal center, also showed a high
Fig. 3. ORTEP drawing of 5f with thermal ellipsoids at the 30% probability level.
Hydrogen atoms have been omitted for clarity.

38
D. Gong et al. / Applied Catalysis A: General 464–465 (2013) 35–42
Table 1
Effect of MAO/Co molar ratio and temperature on butadiene polymerization using 5c.
Entry
MAO/Co
Temp. (◦C)
Time (min)
Yield. (%)
Mn (×10−4
)
PDI
Microstructure (%)
Cis-1,4
Trans-1,4
1,2
1
2
3
4
5
6
7
8
9
400
400
400
400
400
100
200
600
1000
0
25
40
60
80
25
25
25
25
180
120
120
120
120
120
120
120
120
42
72
66
41
11
44
62
83
73
7.6
16.9
13.0
6.1
2.1
9.9
14.9
15.3
11.5
1.4
1.6
2.1
2.3
2.8
2.5
2.1
1.7
2.0
98.7
98.0
97.6
97.2
97.2
97.3
97.8
97.8
97.8
0.2
0.4
0.2
0.2
0.3
0.3
0.2
0.2
0.2
1.1
1.6
2.2
2.6
2.5
2.4
2.0
2.0
2.0
Polymerization conditions: precursor, 10.0 ␮mol; butadiene, 30.0 mmol; toluene, 5.0 mL.
Table 2
Butadiene polymerization catalyzed by various cobalt complexes.
Entry
Comp.
Yield (%)
Mn (×10−4
)
PDI
Microstructure (%)
Cis-1,4
Trans-1,4
1,2
10
11
12
13
14
15
16
5a
5b
5d
5e
5f
94
75
72
80
65
61
99
7.9
18.3
7.6
19.1
11.9
12.0
21.5
4.2
1.3
1.7
1.6
1.9
1.5
1.6
97.1
97.7
97.6
98.0
98.7
98.0
97.1
0.4
0.3
0.2
0.2
0.3
0.2
0.4
2.5
2.0
2.2
1.8
1.0
1.8
2.5
5g
5h
Polymerization conditions: precursor, 10.0 ␮mol; butadiene, 30.0 mmol; MAO (Al), 3.0 mmol; toluene, 5.0 mL; time, 4 h; temperature, 25 ◦C.
cis-1,4-selectivity of 98.0%. Unfortunately, several attempts in the
preparation of cobalt pre-catalysts with 3d-based imine ligands,
which are anticipated for higher selectivity, were not successful,
presumably due to the effects of steric congestion. Complex 5h was
thus prepared instead for the examination of polymerization activ-
ity. Very interestingly, while a lower cis-1,4 selectivity of 97.1% was
obtained, quantitative monomer conversion and a high molecular
weight were achieved. However, the investigation of ligand effects
of complexes derived from analogous ligands, 3a–3c, could not be
conducted due to the inaccessibility of their corresponding cobalt
complexes. A plausible active species for these catalysts precursors
was shown in Fig. 4
regio- and/or stereospecificity of the active species. The incorpo-
ration of phosphine as auxiliary ligands to the Co catalyst systems
has been demonstrated to afford higher monomer conversions with
changes of their selectivity from cis-1,4-specific to syndiotactic 1,2-
specific [29,31,49,70–72]. As reflected from data in Table 3, both
the stereospecificity and monomer conversion can be regulated by
the amount and the property of the added phosphine. The addi-
tion of 0.25–1.0 equiv. of PPh3 resulted in the gradually increase
of the monomer conversion from 11.3% to 81.1%. More impor-
tantly, the 1,2 regio-selectivity was also enhanced from 2.4% to
83.3%, with high syndiotacticity (rr > 60%). 13CNMR analysis has
shown that the 1,2 unit was incorporated randomly, rather than
producing a blocky microstructure with the 1,4 unit, different from
the literature systems [31,33]. This characterization was also sup-
ported by the amorphous polymer appearance and the absence
of a melting point even at a higher 1,2-enchainmet (entry 20) or
syndiotacticity (entry 21). Various 1,2 or 1,4 regio- and stereose-
lectivities are found, depending on the phosphine structure at 1.0
P/Co molar ratio. Definite trend of catalytic performance in terms
of monomer conversion and selectivity with respect to the phos-
phine structure fails to be concluded, indicating that synergic steric
and electronic influences based on the Tolman rule should be con-
sidered [73], as neither steric nor electronic factor can clarify the
complicated performances. Collectively, PPh3 proved to be the best
choice regarding the monomer conversion and 1,2 regio- and stere-
oselectivity. A possible role of PPh3 could be its coordination to the
cobalt core, where it served as a protector of active species. A similar
effect was also observed in the nickel catalyzed ethylene polymer-
ization or oligomerization systems [74,75]. The widely accepted
mechanism for producing the cis-1,4-polybutadiene structure pro-
ceeds first with the cis-1,4 coordination of butadiene to the cobalt
center followed by the insertion into the terminal C Co bond of
the cobalt allylic polymer chain. Such a process is favored in the
NNNCoCl2 system in absence of additive. By adding phosphine, the
monomer insertion mode is shifted, leading to the generation of
the 1,2-structure.
2.2.3. Selectivity regulator
Addition of Lewis bases plays
a very important role in
the control of activity as well as the stereospecificity in the
polymerization of butadiene [29,31,49,70–72]. Among these Co cat-
alysts, the addition of phosphine was examined for altering the
Fig. 4. The proposed active species.

D. Gong et al. / Applied Catalysis A: General 464–465 (2013) 35–42
39
Table 3
Effect of phosphine on polymerization behaviors with 5c/MAO/phosphine catalyst systems.
Entry
P
P/Co
Yield. (%)
Mn (×10−4
)
PDI
Microstructure (%)
1,4
1,2
rr (rrrr)
mr
mm
17
18
19
20
21
22
23
24
25
26
27
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
TFP
TTMP
TNP
TCP
0
11.3
26.3
58.7
59.0
66.6
81.1
57.1
45.2
44.3
50.9
60.7
11.1
9.4
8.7
7.9
8.3
8.9
6.0
2.7
9.6
7.8
12.3
2.7
2.1
2.1
2.5
2.3
2.4
2.2
2.4
4.5
2.6
2.1
97.6
67.5
58.0
55.1
44.0
16.7
36.6
86.3
87.5
62.1
79.1
–
–
–
0.10
0.25
0.50
0.75
1.0
1.0
1.0
1.0
1.0
1.0
61.2 (26.4)
61.1 (23.3)
69.8 (17.6)
61.6 (41.3)
64.2 (25.7)
60.0 (29.9)
44.3 (39.0)
47.0 (22.4)
51.9 (20.9)
52.9 (20.8)
36.7
36.2
29.0
37.0
35.6
31.1
55.2
36.1
42.6
42.4
2.1
2.7
1.2
1.4
2.0
9.0
0.5
16.9
6.5
4.7
TMP
Polymerization conditions: precursor, 10 ␮mol; butadiene, 30 mmol; MAO, 3.0 mmol; toluene, 12 mL; time, 1 h; temperature, 25 ◦C. PPh3: triphenylphosphine; TFP: tri(2-
fluorophenyl) phosphine; TTMP: tri(2,4,6-trimethylphenyl)phosphine; TNP: tri(1-naphthyl)phosphine; TCP: tricyclohexylphosphine; TMP: tri-o-toylphosphine.
3. Conclusion
class and unit cell parameters was carried out by the SMART pro-
gram package. The raw frame data were processed using SAINT
and SADABS to yield the reflection data file. Space groups were
determined using XPREP implemented in APEX2. The structures
were solved by using SHELXTL-97 (direct method). Refinement
was performed by the full-matrix least-squares method on F2. The
hydrogen atoms were added at the calculated positions.
In summary, a serial of new cobalt complexes carrying unsym-
metrical 2-(1-arylimino)-6-(pyrazol-1-yl) pyridine were prepared
and fully characterized. The complex 5a with the smallest-sized
ligand was isolated as an ion-pair in the form of [L2Co]2+[CoCl4]2−
,
while complex 5f showed two slightly different geometries in the
crystal unit cell. All the complexes converted butadiene selectively
to cis-1,4-polybutadiene with moderate to high monomer con-
version upon activation with MAO. Studies on the ligand effects
revealed that bulky groups were crucial for the higher level of cis-
1,4 selectivity and in general for the higher molecular weight of
resulting polymer, accompanied by certain degree of attenuation of
the catalytic activity. These observed correlations of catalytic prop-
erties and ligand structures may facilitate the development of new
catalysts with improved selectivity and activity. The addition of
phosphine not only enhanced the monomer conversion but tuned
the polymer enchainment from cis-1,4 to syndio-1,2 structures,
broadening the potential application of this catalytic system.
CCDC reference numbers 849,283, 849,284 and 849,285 are for
complexes 5a, 5c and 5f, respectively.
4.3. Syntheses and characterizations of ligands and complexes
The representative procedure for the ligand synthesis was modi-
fied based on the literature. To a solution of ethyl 6-bromopyridine-
2-carboxylate (6.90 g, 30 mmol) in dioxane (150 mL) under argon
atmosphere, potassium tert-butoxide (3.69 g, 33 mmol) and pyr-
azole (2.16 g, 31.5 mmol) were added. The resulting mixture was
heated at reflux overnight, then cooled to room temperature and
quenched by a few drops of water. The solvent was removed and the
residue was extracted with ethyl acetate (3× 15 mL) to afford the
crude product, which was further purified by column chromatog-
raphy on silica gel using petroleum ether/ethyl acetate (v/v = 10:1)
to give a yellow powder (2a, 4.50 g, 70% yield). Compounds 2b and
2c were prepared using the same procedure.
2d. Under argon atmosphere, CuI (0.38 g, 2 mmol), proline
(0.46 g, 4 mmol) and K2CO3 (2.76 g, 20 mmol) were added to
a dimethyl sulfoxide (15 mL) solution of 6-bromopyridine-2-
carboxylate (4.60 g, 20 mmol) and 3,5-diphenylprazole (4.44 g,
20 mmol). The formed mixture was stirred at 80 ◦C overnight.
Water was added after cooling down to room temperature, and the
resulting mixture was extracted with ethyl acetate (3× 20 mL). The
organic layers were combined and the solvent was removed to give
the crude product, which was purified by column chromatography
(3.76 g, 83% yield).
3a. To an ethanol solution of 2a (4.30 g, 20 mmol), fresh dis-
tilled ethyl acetate (7.04 g, 80 mmol) and sodium (2.3 g, 0.1 mol)
were added and the reaction mixture was heated at reflux for
5 h. It was then allowed to cool down before 25 mL of concen-
trated hydrochloric acid was added. The suspension was again
heated at reflux with intermittent shaking for 4 h. The mixture was
cooled to room temperature, and the organic layer was removed.
The aqueous phase was neutralized with sodium carbonate and
extracted with ethyl acetate (3× 15 mL). The organic layers were
combined and dried over anhydrous sodium carbonate. Compound
3a was obtained after further purification by column chromatog-
raphy using petroleum ether/ethyl acetate (v/v = 15:1) (3.24 g, 82%
4. Experimental
4.1. General procedure and materials
CoCl2 and anilines were purchased from Alfa Aesar. 6-Bromo-2-
pyridinecarboxylic acid, MAO, pyrazole, 3,5-dimethylpyrazole and
butadiene (25 wt% in toluene) were available from Aldrich. Phos-
phines were purchased either from Strem or Aldrich. Dioxane was
dried over CaH2. Toluene and tetrahydrofuran (THF) were dried
with Na/benzophenone and distilled prior to use. 1H and 13C spec-
tral were recorded on a Bruker AV400 spectrometer at 25 ◦C with
CDCl3 as solvent (400 MHz for 1H NMR, 100 MHz for 13C NMR). Ele-
mental analyses were conducted by Flash 2000-Thermo Scientific
CHNO Analyzer. HRMS was performed on Thermo ScientificTM
–
LTQ Velos Orbitrap MS using acidified methanol or acetonitrile. IR
spectra were performed on JASCO V-670 spectrophotometer. The
microstructures of polymers were determined by 13C NMR and 1
H
NMR. The number average molecular weights (Mn) and polymer
dispersion index (PDI) were measured by VISCOTEK GPC1000 with
TDA305 (Triple Detector Array) as detector at 35 ◦C using THF as
eluent (flow rate: 1.0 mL/min) against polystyrene standards.
4.2. X-ray structure determinations
X-ray diffraction data were collected by a Bruker SMART APEX
diffractometer with a CCD area detector, using graphite monochro-
mated Mo K radiation (ꢀ = 0.71073 A). The determination of crystal
˚

40
D. Gong et al. / Applied Catalysis A: General 464–465 (2013) 35–42
yield). The same procedure was adopted for the synthesis of com-
pounds 3b–3d.
4.3.6. 6-(3,5-Dimethylpyrazol-1-yl)-2-acetylpyridine (3b)
Obtained as a yellow solid (4.33 g, 87% yield); 1H NMR (400 MHz,
CDCl3, ppm): 8.11 (d, 1H, J = 7.8 Hz, Hpyri), 7.86–7.92 (m, 2H, Hpyri),
6.03 (s, 1H, Hpyra), 2.74 (s, 3H, CH3), 2.69 (s, 3H, C(O)CH3), 2.30
(s, 3H, CH3). 13C NMR (100 MHz, CDCl3, ppm): 199.43, 152.70,
151.20, 150.37, 141.62, 139.22, 118.97, 118.27, 109.73, 26.24, 15.21,
13.65.
4a. To a methanol (25 mL) solution of 3a (748 mg, 4 mmol),
excess aniline (1.82 g, 20 mmol) and three drops of acetic acid were
added. The mixture was refluxed overnight, and all volatile was
removed under vacuum. The resulting residue was dissolved in
methanol (8.0 mL) and recrystallized at −30 ◦C to give 4a (833 mg,
80%). The same procedure was adopted for the synthesis of com-
pounds 4b–4f.
5a. A blue suspension of CoCl2 (63 mg, 2 mmol) in THF was added
to a solution of the corresponding ligand (4a, 524 mg, 2 mmol) in
10 mL of THF. The reaction mixture was stirred at room temperature
overnight, forming a brown precipitate, which was filtered, washed
with diethyl ether and hexane. Recrystallization was conducted by
slow diffusion of diethyl ether into its saturated dichloromethane
solution to give 5a (511 mg, 87%). The same procedure was adopted
for the synthesis of all cobalt complexes.
4.3.7. 6-(3,5-Diisopropylpyrazol-1-yl)-2-acetylpyridine (3c)
Obtained as a yellow oil (4.22 g, 61% yield); 1H NMR (400 MHz,
CDCl3, ppm): 8.12 (d, 1H, J = 7.6 Hz, Hpyri), 7.87 (m, 2H, Hpyri), 6.13 (s,
1H, Hpyra), 4.07 (hepta, 1H, J = 6.8 Hz, CH(CH3)2), 3.03 (hepta, 1H,
J = 6.8 Hz, CH(CH3)2), 2.67 (s, 3H, CH3), 1.30 (d, 12H, J = 6.8 Hz,
CH(CH3)2). 13C NMR (100 MHz, CDCl3, ppm): 199.45, 160.49,
153.19, 152.90, 151.31, 139.12, 120.24, 118.46, 102.78, 28.09, 26.74,
26.21, 23.06, 22.60.
4.3.8. 6-(3,5-Diphenylpyrazol-1-yl)-2-acetylpyridine (3d)
Obtained as a yellow solid (2.59 g, 55% yield); 1H NMR (400 MHz,
CDCl3, ppm): 8.21 (d, 1H, J = 7.8 Hz, Hpyri), 7.86–7.97 (m, 4H,
2HAr, 2Hpyri), 7.48–7.36 (m, 8H, HAr), 6.83 (s, 1H, Hpyra), 1.89 (s,
3H, C(O)CH3). 13C NMR (100 MHz, CDCl3, ppm): 199.41, 152.85,
151.74, 151.12, 145.48, 139.37, 132.61, 132.58, 128.89, 128.78,
128.53, 128.22, 128.10, 126.00, 120.31, 119.16, 107.73, 24.97. Anal.
Calcd. For C22H17N3O1: C, 77.86; H, 5.05; N, 12.38. Found: C, 77.50;
H, 5.29; N, 11.99.
4.3.1. 6-(Pyrazol-1-yl)-ethylpyridine-2-carboxylate (2a)
Obtained as a white solid (4.50 g, 70% yield); 1H NMR (400 MHz,
CDCl3, ppm): 8.62 (s, 1H, Hpyra), 8.14 (d, 1H, J = 7.8 Hz, Hpyri), 8.04
(d, 1H, J = 7.8 Hz, Hpyri), 7.95 (t, 1H, J = 7.8 Hz, Hpyr), 7.73 (s, 1H,
Hpyra), 6.48 (s, 1H, Hpyra), 4.49 (q, 2H, J = 8.8 Hz, CH2CH3), 1.43 (t,
3H, J = 8.8 Hz, CH2CH3). 13C NMR (100 MHz, CDCl3, ppm): 165.11,
154.99, 150.97, 151.52, 143.00, 123.77, 118.89, 113.51, 106.99,
62.00, 13.41.
4.3.9. 2-[1-(Phenylimino)ethyl]-6-(pyrazol-1-yl)-pyridine (4a)
Obtained as a yellow solid (833 mg, 80%); 1H NMR (400 MHz,
CDCl3, ppm): 8.61 (s, 1H, Hpyra), 8.16 (d, 1H, J = 7.8 Hz, Hpyri), 8.07
(d, 1H, J = 7.8 Hz, Hpyri), 7.92 (t, 1H, J = 7.8 Hz, Hpyr), 7.76 (s, 1H, Hpyra),
7.40–7.38 (m, 2H, HAr), 7.12–7.10 (m, 1H, HAr), 7.85–7.83 (m, 2H,
HAr), 6.48 (s, 1H, Hpyra). 13C NMR (100 MHz, CDCl3, ppm): 166.61,
155.21, 151.13, 150.50, 142.17, 139.21, 129.06, 127.00, 123.77,
119.24, 118.86, 113.31, 107.80, 16.28. IR (KBr, cm−1): 1637 (vC N),
Anal. Calcd. For C16H14N4: C, 73.26; H, 5.38; N, 21.36. Found: C,
74.01; H, 5.04; N, 21.69.
4.3.2. 6-(3,5-Dimethylpyrazol-1-yl)-ethylpyridine-2-carboxylate
(2b)
Obtained as a white solid (5.20 g, 85% yield); 1H NMR (400 MHz,
CDCl3, ppm): 8.09 (d, 1H, J = 7.6 Hz, Hpyri), 7.98–7.95 (m, 2H, Hpyri),
6.35 (s, 1H, Hpyra), 4.44 (q, 2H, J = 8.8 Hz, CH2CH3), 2.75 (s, 3H,
CH3), 2.25 (s, 3H, CH3), 1.41 (t, 3H, J = 8.8 Hz, CH2CH3). 13C NMR
(100 MHz, CDCl3, ppm): 165.00, 153.33, 150.40, 146.06, 142.40,
139.13, 121.59, 118.53, 109.57, 61.76, 14.84, 14.31, 13.73.
4.3.3.
6-(3,5-Diisopropylpyrazol-1-yl)-ethylpyridine-2-carboxylate (2c)
Obtained as a colorless oil (3.90 g, 54% yield); 1H NMR (400 MHz,
CDCl3, ppm): 8.07 (m, 1H, 7.8 Hz, Hpyri), 7.98–7.86 (m, 2H, Hpyri),
6.01 (s, 1H, Hpyra), 4.45 (q, 2H, J = 8.4 Hz, CH2CH3), 4.11 (hepta,
1H, J = 8.0 Hz, CH(CH3)2), 3.03 (hepta, 1H, J = 8.0 Hz, CH(CH3)2),
1.40 (t, 3H, J = 8.4 Hz, CH2CH3), 1.25 (d, 12H, CH(CH3)2). 13C NMR
(100 MHz, CDCl3, ppm): 164.55, 152.13, 151.42, 147.13, 143.41,
138.53, 120.99, 117.46, 108.31, 60.55, 26.74, 26.21, 23.06, 22.60,
12.89.
4.3.10. 2-[1-(2,4,6-Trimethylphenylimino)ethyl]-6-(pyrazol-1-
yl)-pyridine
(4b)
Obtained as a yellow solid (1.34 g, 66%); 1H NMR (400 MHz,
CDCl3, ppm): 8.62 (s, 1H, Hpyra), 8.24 (d, 1H, J = 8.0 Hz, Hpyri), 8.07 (d,
1H, J = 8.0 Hz, Hpyri), 7.96 (t, 1H, J = 8.0 Hz, Hpyri), 7.76 (s, 1H, Hpyra),
6.90 (s, 2H, HAr), 6.48 (s, 1H, Hpyra), 2.29 (s, 3H,
N CH3), 2.20 (s, 3H,
Ar CH3), 2.00 (s, 6H, Ar CH3). 13C NMR (100 MHz, CDCl3, ppm):
166.65, 154.95, 151.44, 146.23, 142.14, 139.17, 132.35, 128.59,
126.97, 125.23, 118.67, 113.27, 107.75, 20.75, 17.88, 16.45. IR (KBr,
cm−1): 1649 (vC N), Anal. Calcd. For C19H20N4: C, 74.97; H, 6.62; N,
18.41. Found: C, 74.63; H, 6.91; N, 18.40.
4.3.4. 6-(3,5-Diphenylpyrazol-1-yl)-ethylpyridine-2-carboxylate
(2d)
Obtained as a white solid (3.76 g, 83% yield).1H NMR (400 MHz,
CDCl3, ppm): 8.01 (m, 1H, 7.8 Hz, Hpyri), 7.87–7.96 (m, 4H, 2HAr
,
4.3.11. 2-[1-(2,6-Diisopropylphenylimino)ethyl]-6-(pyrazol-1-
yl)-pyridine
2Hpyri), 7.48–7.36 (m, 8H, HAr), 6.69 (s, 1H, Hpyra), 4.40 (q, 2H,
J = 8.4 Hz, CH2CH3), 1.44 (t, 3H, J = 8.4 Hz, CH2CH3). 13C NMR
(100 MHz, CDCl3, ppm): 165.43, 151.33, 151.10, 145.44, 143.44,
135.52, 133.44, 132.59, 128.99, 128.82, 128.67, 128.21, 128.17,
125.89, 121.44, 117.42, 114.51, 64.22, 11.33.
(4c)
Obtained as a yellow solid (870 mg, 69%); 1H NMR (400 MHz,
CDCl3, ppm): 8.66 (s, 1H, Hpyra), 8.26 (d, 1H, J = 8.0 Hz, Hpyri), 8.09
(d, 1H, J = 8.0 Hz, Hpyri), 7.98 (t, 1H, J = 8.0 Hz, Hpyri), 7.77 (s, 1H,
Hpyra), 7.20–7.01 (m, 3H, HAr), 6.51 (s, 1H, Hpyra), 2.75 (hepta,
4.3.5. 6-(Pyrazol-1-yl)-2-acetylpyridine (3a)
2H, J = 4.8 Hz, CH(CH3)2), 2.24 (s, 3H, CH3), 1.16 (d, 12H,
N
Obtained as a yellow solid (3.24 g, 82% yield); 1H NMR (400 MHz,
CDCl3, ppm): 8.60 (s, 1H, Hpyra), 8.16 (d, 1H, J = 7.6 Hz, Hpyri),
7.97–7.90 (m, 2H, Hpyri), 7.76 (s, 1H, Hpyra), 6.50 (s, 1H, Hpyra), 2.75
(s, 3H, CH3). 13C NMR (100 MHz, CDCl3, ppm): 199.11, 155.01,
151.09, 152.54, 143.04, 123.88, 118.71, 113.46, 106.45, 26.14.
J = 4.8 Hz, CH(CH3)2). 13C NMR (100 MHz, CDCl3, ppm): 166.28,
154.79, 150.55, 146.31, 142.28, 139.26, 135.79, 132.49, 127.03,
123.40, 123.06, 122.79, 118.72, 113.30, 107.77, 28.33, 23.22, 22.92,
17.21. IR (KBr, cm−1): 1635 (vC N), Anal. Calcd. For C22H26N4: C,
76.27; H, 7.56; N, 16.17. Found: C, 77.01; H, 7.39; N, 16.39.

D. Gong et al. / Applied Catalysis A: General 464–465 (2013) 35–42
41
4.3.12.
4.3.17. 2-[1-(2,4,6-Trimethylphenylimino)ethyl]-6-(pyrazol-1-
2-[1-(Phenylimino)ethyl]-6-(3,5-dimethylpyrazol-1-yl)-pyridine
(4d)
yl)-pyridine cobaltdichloride
(5b)
Obtained as a yellow solid (456 mg, 56%); 1H NMR (400 MHz,
CDCl3, ppm): 8.10 (d, 1H, J = 8.0 Hz, Hpyri), 7.99 (d, 1H, J = 8.0 Hz,
Hpyri), 7.93 (t, 1H, J = 8.0 Hz, Hpyr), 7.39–7.35 (m, 2H, HAr), 7.12 (t, 1H,
J = 6.8 Hz, HAr), 6.83 (d, 1H, J = 6.8 Hz, HAr), 6.02 (s, 1H, Hpyra), 2.72
Obtained as a green solid (433 mg, 92%); IR (KBr, cm−1): 1624
(vC N), Anal. Calcd. For C19H20Cl2CoN4: C, 52.55; H, 4.64; N, 12.90.
Found: C, 52.71; H, 4.71; N, 13.09.
(s, 3H, CH3), 2.32 (s, 3H, CH3), 2.31 (s, 3H,
N
CH3). 13C NMR
4.3.18. 2-[1-(2,6-diisopropylphenylimino)ethyl]-6-(pyrazol-1-
yl)-pyridine cobaltdichloride
(100 MHz, CDCl3, ppm): 164.34, 151.15, 150.02, 141.41, 138.78,
129.03, 123.69, 119.27, 118.03, 116.50, 117.56, 109.29, 16.65, 15.12,
13.69. IR (KBr, cm−1): 1641 (vC N), Anal. Calcd. For C18H18N4: C,
74.46; H, 6.25; N, 19.30. Found: C, 74.09; H, 6.14; N, 19.11.
(5c)
Obtained as a green solid (289 mg, 95%); IR (KBr, cm−1): 1621
(vC N), Anal. Calcd. For C22H26Cl2CoN4: C, 55.48; H, 5.50; N, 11.76.
Found: C, 55.70; H, 5.67; N, 11.49.
4.3.13. 2-[1-(2,4,6-Trimethylphenylimino)ethyl]-6-(3,5-
dimethylpyrazol-1-yl)-pyridine
(4e)
4.3.19.
2-[1-(Phenylimino)ethyl]-6-(3,5-dimethylpyrazol-1-yl)-pyridine
cobaltdichloride (5d)
Obtained as a yellow solid (783 mg, 78%); 1H NMR (400 MHz,
CDCl3, ppm): 8.28 (d, 1H, J = 7.2 Hz, Hpyri), 8.00 (d, 1H, J = 8.0 Hz,
Hpyri), 7.1 (t, 1H, J = 8.0 Hz, Hpyr), 6.89 (s, 2H, HAr), 6.03 (s, 1H, Hpyra),
2.72 (s, 3H, CH3), 2.32 (s, 3H, Ar CH3), 2.29 (s, 3H, CH3), 2.16
(s, 3H, -N = CH3), 2.01 (s, 6H, Ar CH3). 13C NMR (100 MHz, CDCl3,
ppm): 166.41, 157.55, 145.92, 144.36, 140.93, 138.75, 132.46,
129.19, 128.61, 125.13, 120.03, 104.11, 20.74, 17.84, 16.60, 12.20.
IR (KBr, cm−1): 1644 (vC N), Anal. Calcd. For C21H24N4: C, 75.87; H,
7.28; N, 16.85. Found: C, 76.31; H, 7.05; N, 17.11.
Obtained as a green solid (312 mg, 89%); IR (KBr, cm−1): 1622
(vC N), Anal. Calcd. For C18H18Cl2CoN4: C, 51.45; H, 4.32; N, 13.33.
Found: C, 51.34; H, 4.56; N, 13.49.
4.3.20. 2-[1-(2,4,6-Trimethylphenylimino)ethyl]-6-(3,5-
dimethylpyrazol-1-yl)-pyridine cobaltdichloride
(5e)
Obtained as a green solid (239 mg, 95%); IR (KBr, cm−1): 1629
(vC N), Anal. Calcd. For C21H24Cl2CoN4: C, 54.56; H, 5.23; N, 12.12.
Found: C, 54.45; H, 5.45; N, 12.09.
4.3.14. 2-[1-(2,6-Diisopropylphenylimino)ethyl]-6-(3,5-
dimethylpyrazol-1-yl)-pyridine
4.3.21. 2-[1-(2,6-Diisopropylphenylimino)ethyl]-6-(3,5-
dimethylpyrazol-1-yl)-pyridine cobaltdichloride
(5f)
(4f)
Obtained as a yellow solid (909 mg, 49%); 1H NMR (400 MHz,
CDCl3, ppm): 8.31 (d, 1H, J = 8.0 Hz, Hpyri), 8.03 (d, 1H, J = 8.0 Hz,
Obtained as a green solid (322 mg, 93%); IR (KBr, cm−1): 1623
(vC N), Anal. Calcd. For C24H30Cl2CoN4: C, 57.15; H, 6.00; N, 11.11.
Found: C, 57.01; H, 5.81; N, 11.09.
H
pyri), 7.92 (t, 1H, J = 8.0 Hz, Hpyri), 7.25–7.11 (m, 3H, HAr), 6.03
(s, 1H, Hpyra), 2.76 (hepta, 2H, J = 4.8 Hz, CH(CH3)2), 2.74 (s, 3H,
CH3), 2.32 (s, 3H, CH3), 2.20 (s, 3H, CH3), 1.24 (d, 12H,
N
J = 4.8 Hz, CH(CH3)2). 13C NMR (100 MHz, CDCl3, ppm): 152.60,
150.06, 141.48, 138.97, 136.28, 124.11, 123.14, 122.53, 118.42,
116.86, 109.41, 28.39, 23.27, 22.93, 18.34, 17.46, 15.15, 13.66. IR
(KBr, cm−1): 1631 (vC N), Anal. Calcd. For C24H30N4: C, 76.97; H,
8.07; N, 14.96. Found: C, 77.11; H, 7.92; N, 15.10.
4.3.22. 2-[1-(2,6-Diisopropylphenylimino)ethyl]-6-(3,5-
diisopropylpyrazol-1-yl)-pyridine cobaltdichloride
(5g)
Obtained as a green solid (387 mg, 83%); IR (KBr, cm−1): 1625
(vC N), Anal. Calcd. For C28H38Cl2CoN4: C, 60.0; H, 6.83; N, 10.00.
Found: C, 59.81; H, 6.81; N, 10.23.
4.3.23. 2-Acetyl-6-(3,5-diisopropylpyrazol-1-yl)-pyridine
cobaltdichloride (5h)
4.3.15. 2-[1-(2,6-Diisopropylphenylimino)ethyl]-6-(3,5-
diisopropylpyrazol-1-yl)-pyridine
Obtained as a blue solid (405 mg, 86%); IR (KBr, cm−1): 1710
(vC=O), Anal. Calcd. For C22H17Cl2CoN4: C, 56.55; H, 3.67; N, 11.99.
Found: C, 56.41; H, 3.80; N, 12.13.
(4g)
Obtained as a yellow solid (602 mg, 36%); 1H NMR (400 MHz,
CDCl3, ppm): 8.33 (d, 1H, J = 8.0 Hz, Hpyri), 7.66 (d, 1H, J = 8.0 Hz,
H
pyri), 7.62 (t, 1H, J = 8.0 Hz, Hpyri), 7.17–7.12 (m, 3H, HAr), 5.89
(s, 1H, Hpyra), 2.98 (hepta, 2H, J = 4.8 Hz, CH(CH3)2), 2.70 (hepta,
2H, J = 4.8 Hz, CH(CH3)2), 2.19 (s, 3H, CH3), 1.18 (d, 12H,
4.3.24. Representative polymerization procedure
N
In a glovebox, complex 5c (4.6 mg, 10 ␮mol (entry 13, Table 2))
was added into a moisture free ampule bottle. MAO (Al, 3.0 mmol)
was then injected, followed by adding a toluene solution of 1,3-
butadiene (1.62 g, 30.0 mmol). After a designated time, methanol
was injected into the system to quench the reaction. The mixture
was poured into 60 ml of methanol containing hydrochloric acid
(1.0 mL) to precipitate the polymer. The precipitated polymer was
filtered, washed with methanol, and dried under vacuum at 40 ◦C
to a constant weight (1.13 g, 70%).
J = 4.8 Hz, CH(CH3)2), 1.13 (d, 12H, J = 4.8 Hz, CH(CH3)2). 13C NMR
(100 MHz, CDCl3, ppm): 165.93, 157.42, 154.96, 146.13, 140.99,
138.78, 135.65, 129.20, 123.81, 123.403, 120.104, 98.42, 28.28,
26.86, 23.21, 22.28, 22.59, 17.296. IR (KBr, cm−1): 1631 (vC N), Anal.
Calcd. For C28H38N4: C, 78.10; H, 8.89; N, 13.01. Found: C, 77.89; H,
8.71; N, 12.83.
4.3.16. 2-[1-(Phenylimino)ethyl]-6-(pyrazol-1-yl)-pyridine
cobaltdichloride (5a)
Acknowledgement
Obtained as a brown solid (511 mg, 87%); IR (KBr, cm−1): 1627
(vC N), Anal. Calcd. For C16H14Cl2CoN4: C, 49.00; H, 3.60; N, 14.29.
Found: C, 48.71; H, 3.69; N, 14.69.
We are grateful for the generous financial support from King
Abdullah University of Science and Technology.

42
D. Gong et al. / Applied Catalysis A: General 464–465 (2013) 35–42
Appendix A. Supplementary data
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2013.04.026.
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