2-(1-(Arylimino)ethyl)-8-arylimino-5,6,7-trihydroquinolylcobalt dichloride: Synthesis and polyethylene wax formation

Article abstract of DOI:10.1016/j.apcata.2012.09.011

A series of 2-(1-(arylimino)ethyl)-8-arylimino-5,6,7-trihydroquinolylcobalt dichloride (aryl = 2,6-R¹-4-R²C₆H₂) pre-catalysts were synthesized and structurally characterized by FT-IR and elemental analyses. The molecular structures of Co1 (R¹ = Me, R ² = H), Co2 (R¹ = Et, R² = H) and Co5 (R ¹ = Et, R² = Me) were determined by single-crystal X-ray diffraction analysis, and confirmed Co1 as a distorted trigonal bipyramidal geometry at the metal, whilst in Co2 and Co5 the metal is square-pyramidal. Upon treatment with either MAO or MMAO, all cobalt pre-catalysts exhibited highest activities at 60 °C for ethylene polymerization. The resultant polyethylenes, under optimization reaction parameters, possessed low molecular weight (waxes) and narrow polydispersity.

Full text of DOI:10.1016/j.apcata.2012.09.011

Applied Catalysis A: General 447–448 (2012) 67–73
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
2-(1-(Arylimino)ethyl)-8-arylimino-5,6,7-trihydroquinolylcobalt dichloride:
Synthesis and polyethylene wax formation
Wen-Hua Suna,∗, Shaoliang Konga,b, Wenbin Chaic, Takeshi Shionod, Carl Redshawe,1
,
Xinquan Huc, Cunyue Guob, Xiang Haoa
a Key Laboratory of Engineering Plastics, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
c College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, China
d Department of Applied Chemistry, Hiroshima University, Higashi-Hiroshima 739-8527, Japan
e School of Chemistry, University of East Anglia, Norwich NR4 7TJ, UK
a r t i c l e i n f o
a b s t r a c t
A series of 2-(1-(arylimino)ethyl)-8-arylimino-5,6,7-trihydroquinolylcobalt dichloride (aryl = 2,6-R1-4-
R2C6H2) pre-catalysts were synthesized and structurally characterized by FT-IR and elemental analyses.
The molecular structures of Co1 (R1 = Me, R2 = H), Co2 (R1 = Et, R2 = H) and Co5 (R1 = Et, R2 = Me) were
determined by single-crystal X-ray diffraction analysis, and confirmed Co1 as a distorted trigonal bipyra-
midal geometry at the metal, whilst in Co2 and Co5 the metal is square-pyramidal. Upon treatment with
either MAO or MMAO, all cobalt pre-catalysts exhibited highest activities at 60 ◦C for ethylene polymer-
ization. The resultant polyethylenes, under optimization reaction parameters, possessed low molecular
weight (waxes) and narrow polydispersity.
Article history:
Received 10 August 2012
Received in revised form 1 September 2012
Accepted 4 September 2012
Available online 15 September 2012
Keywords:
2,8-Bis(imino)5,6,7-trihydroquinolylcobalt
dichloride
Cobalt pre-catalyst
© 2012 Elsevier B.V. All rights reserved.
Thermal-stable pre-catalyst
Single-site active species
Polyethylene wax
1. Introduction
developed some efficient complex pre-catalysts using new triden-
tate ligands such as 2-imino-1,10-phenanthrolines [25,26],
Bis(arylimino)pyridyl metal (M = Fe2+ or Co2+
)
complexes
2-benzimidazolyl-6-(1-(arylimino)ethyl)pyridines
[27–30],
(A in Scheme 1) have received much attention due to high
catalytic activity for the conversion of ethylene either to high-
density polyethylene or to ␣-olefins [1–5]. In light of the ease
of preparation and the use of low-cost metals with minimal
environmental impact, numerous types of related complex
pre-catalysts have been designed and extensively investigated
[6–11]. A number of reviews have highlighted the influence on
the catalytic activities and the properties of the resultant poly-
ethylene of fine tuning the size, nature and regio-chemistry of
the substituents on the ligand frameworks [6–15]. In particular,
the facile modification of the bis(imino)pyridyline framework
has been explored by changing the substituents at the N-
bound aryl groups [16–22] as well as the substituents linked
to the imino groups [23,24]. In other studies, our group has
2-quinox-alinyl-6-imino-pyridines [31], 2-(benzoxazolyl)-
1,10-phenanthrolines [32], 2-(pyrid-2-yl)-1,10-phenanthrolines
[33], 2-(benzothiazol-2-yl)-6-iminopyridines [34], 2-(pyrid-2-
yl)-6-imino-pyridines [35], 2,8-bis(imino)quinolines [36], and
8-(benzimidazol-2-yl)-2-iminoquinolines [37]. Industrially (BP),
the scaled-up bis(imino)pyridyliron dichloride system was utilized
for high-density linear polyethylene production, however, critically
at elevated temperatures, there were problems associated with
deactivation and the formation of polyethylenes of low molecular
weight [1–3,8]. Recently, the use of bulky anilines has led to both
enhanced activities and thermo-stabilities for bis(imino)pyridyl
metal (Fe or Co) complex pre-catalysts [19–22], and attractively
produces polyethylenes with narrow polydispersity. Importantly,
the formation of linear polyethylene waxes is industrially relevant
for accessing functionalized polymers with high value [21,22].
In addition, metal complex pre-catalysts bearing 1,8-diimino-
2,3,4,5,6,7-hexahydroacridines (B in Scheme 1), reported by Kim
group, in the case of iron afforded polyethylene waxes and cobalt
polyethylene waxes with lower molecular weight [38]. Inspired
by these results and following on from our nickel pre-catalysts
containing 8-imino-5,6,7-trihydroquinoline derivatives [39–43],
∗
Corresponding author at: Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China. Tel.: +86 10 62557955; fax: +86 10 62618239.
E-mail address: whsun@iccas.ac.cn (W.-H. Sun).
Present address: Department of Chemistry, University of Hull, Hull HU6 7RX,
1
UK.
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.09.011

68
W.-H. Sun et al. / Applied Catalysis A: General 447–448 (2012) 67–73
N
N
N
N
N
N
N
HN
N
N
N
N
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
M
M
L1 - L5
L1' - L5'
X
X
Cl Cl
M = Fe or Co; X = Cl or Br
M = Fe or Co
A
B
CoCl2
EtOH
R1
Ar as
R1
R2
H
H
H
L1 Co1 Me
L2 Co2 Et
L3 Co3 i-Pr
R1
N
N
N
N
N
N
N
N
N
L4 Co4 Me Me
L5 Co5 Et Me
Ar
Ar
Ar
Ar
Fe
Cl Cl
Co
Cl Cl
This work
Ar
Ar
Co
R2
Cl Cl
C
Scheme 2. Synthesis of the cobalt(II) complexes (Co1–Co5).
Scheme 1. The 2,6-bis(imino)pyridylmetal(II) dihalide.
dichloride in ethanol formed the corresponding complexes, 2,8-
bis(imino)-5,6,7-trihydroquinolylcobalt(II) dichloride (Co1–Co5)
in good yields (Scheme 2).
2-(1-(arylimino)ethyl)-8-arylimino-5,6,7-trihydroquinolines were
designed and were used to access highly efficient iron complex pre-
catalysts (C in Scheme 1) [44]. Herein, the title cobalt complexes,
2-(1-(arylimino)ethyl)-8-arylimino-5,6,7-trihydroquinolylcobalt
dichloride (D in Scheme 1) have been prepared and their ethylene
polymerization behavior explored. When activated with either
MAO or MMAO, all title cobalt complexes exhibited good thermal
stability and high activity during ethylene polymerization, produc-
ing polyethylene waxes with narrow polydispersity (as low as 1.5).
The synthesis and characterization of the title cobalt complexes
are reported, and the impact of the polymerization parameters
and the nature of the ligands on the catalytic activities and on the
properties of polyethylene products are investigated.
2.2.2. Synthesis of 2-(1-(2,6-dimethylphenylimino)ethyl)-8-(2,6-
dimethylphenylimino)-5,6,7-trihydroquinoline cobalt(II)
dichloride (Co1)
The
ligand
2-(1-(2,6-dimethylphenylimino)ethyl)-8-(2,6-
dimethylphenylimino)-5,6,7-trihydroquinoline (L1) and N-(2,6-
dimethylphenyl)-2-(1-(2,6-dimethylphenylimino)ethyl)-5,6-dih-
ydroquinolin-8-amine (L1ꢀ) (0.12 g, 0.30 mmol) and CoCl2 (0.04 g,
0.30 mmol) were mixed in degassed ethanol (5 mL) and stirred for
24 h at room temperature. Then Et2O was added to the reaction
mixture to precipitate the green solid. After being collected by fil-
tering, the precipitate was washed with diethyl ether (3 mL × 5 mL)
and dried under vacuum to give the product as a green powder
(0.10 g, 62.5%) for complex Co1. Anal. Calcd. For C27H29N3CoCl2:
C, 61.72, H, 5.56, N, 8.00; Found: C, 61.54, H, 5.56, N, 7.82. FT-IR
(Diamond; cm−1): 2914, 2165, 2030, 1624, 1585, 1468, 1429, 1371,
1264, 1236, 1198, 1098, 1039, 834, 767.
2. Experimental
2.1. General procedures
All work involving air- and/or moisture-sensitive compounds
was performed using standard Schlenk techniques under a nitrogen
atmosphere. The 2,8-dimino-5,6,7-trihydroquinoline derivatives
used as ligands were prepared by using the previously reported
procedure [44]. Toluene was refluxed over sodium benzophe-
none and distilled under nitrogen prior to use. Methylaluminoxane
(MAO, 1.46 M solution in toluene) and modified methylaluminox-
ane (MMAO, 1.93 M in heptane, 3A) were purchased from the
Akzo Nobel Corp. High-purity ethylene was purchased from Bei-
jing Yansan Petrochemical Co. and used as received. Other reagents
were purchased from Aldrich, Acros, or local suppliers. NMR spectra
were recorded on a Bruker DMX 400 MHz instrument at ambient
temperature using TMS as an internal standard; ı values are given in
ppm and J values in Hz. IR spectra were recorded on a Perkin-Elmer
System 2000 FT-IR spectrometer. Elemental analysis was carried
out using a Flash EA 1112 micro-analyzer. Molecular weights and
molecular weight distribution of polyethylene were determined by
PL-GPC220 at 150 ◦C, with 1,2,4-tri-chlorobenzene as the solvent.
The melting points of polyethylene were measured from the sec-
ond scanning run on a Perkin-Elmer TA-Q2000 differential scanning
calorimetry (DSC) analyzer under a nitrogen atmosphere.
2.2.3. Synthesis of 2-(1-(2,6-diethylphenylimino)ethyl)-8-(2,6-
diethylphenylimino)-5,6,7-trihydroquinoline cobalt(II) dichloride
(Co2)
Using the above procedure, Co2 was isolated as a brown powder
in 71.7% yield. Anal. Calcd. For C31H37N3CoCl2: C, 64.03, H, 6.41, N,
7.23; Found: C, 63.74, H, 6.74, N, 6.08. FT-IR (Diamond; cm−1): 2968,
2938, 2874, 2360, 2031, 2166, 1621, 1584, 1446, 1373, 1244, 1190,
1108, 869, 809, 779.
2.2.4. Synthesis of 2-(1-(2,6-diisopropylphenylimino)ethyl)-8-
(2,6-diisopropylphenylimino)-5,6,7-trihydroquinoline cobalt(II)
dichloride (Co3)
Using the above procedure, Co3 was isolated as a brown powder
in 64.5% yield. Anal. Calcd. For C35H45N3CoCl2: C, 65.93, H, 7.11, N,
6.59; Found: C, 65.72, H, 6.77, N, 6.83. FT-IR (Diamond; cm−1): 2965,
2865, 2166, 2031, 1617, 1584, 1458, 1369, 1320, 1245, 1186, 1106,
1044, 929, 802, 775.
2.2.5. Synthesis of 2-(1-(2,4,6-trimethylphenylimino)ethyl)-8-
(2,4,6-trimethylphenylimino)-5,6,7-trihydroquinoline cobalt(II)
dichloride (Co4)
2.2. Syntheses and characterization
Using the above procedure, Co4 was isolated as a green powder
in 81.7% yield. Anal. Calcd. For C29H33N3CoCl2: C, 62.94, H, 6.01,
N, 7.59; Found: C, 62.62, H, 6.18, N, 7.42. FT-IR (Diamond; cm−1):
2951, 2913, 2329, 2166, 2030, 1625, 1581, 1476, 1431, 1371, 1264,
1238, 1212, 1152, 1116, 1034, 852, 760.
2.2.1. Synthesis and characterization of the cobalt complexes
Co1–Co5
The stoichiometric reactions of the mixture of isomeric 2,8-
dimino-5,6,7-trihydroquinoline derivatives (ligand) and cobalt

W.-H. Sun et al. / Applied Catalysis A: General 447–448 (2012) 67–73
69
2.2.6. Synthesis of
2-(1-(2,6-diethyl-4-methylphenylimino)ethyl)-8-(2,6-diethyl-4-
methylphenylimino)-5,6,7-trihydroquinoline cobalt(II) dichloride
(Co5)
Using the above procedure, Co5 was isolated as a brown powder
in 74.5% yield. Anal. Calcd. For C33H41N3CoCl2: C, 65.03, H, 6.78, N,
6.89; Found: C, 64.79, H, 7.05, N, 6.41. FT-IR (Diamond; cm−1): 2965,
2929, 2868, 2361, 2167, 2032, 1618, 1577, 1456, 1372, 1244, 1207,
1142, 1036, 858, 828, 794.
2.3. Ethylene polymerization
2.3.1. Ethylene polymerization under 10/5 atm ethylene
A 300 mL stainless steel autoclave, equipped with a mechanical
stirrer and a temperature controller, was employed for the reac-
tion. Firstly, 30 mL toluene (freshly distilled) was injected to the
clave which is full of ethylene. When the temperature required
was reached, another 20 mL toluene was added to dissolved the
complex (3 ␮mol), the required amount of co-catalyst (MAO and
MMAO), the residual toluene were added by syringe successively.
The reaction mixture was intensively stirred for the desired time
under the corresponding pressure of ethylene through the entire
experiment. After the reaction was carried out for the required
period, the reaction solution was quenched with 10% hydrochloric
acid/ethanol. The precipitated polymer was collected by filtration,
washed with ethanol and water, and dried in a vacuum at 60 ◦C
until constant weight.
Fig. 1. Molecular structure of complex Co1 with thermal ellipsoids at 30% probabil-
ity. Hydrogen atoms have been omitted for clarity.
and Co5 were further confirmed by the single crystal X-ray diffrac-
tion.
3.2. X-ray crystallographic studies
To confirm their unambiguous molecular structures, single crys-
tals of complexes Co1, Co2 and Co5 were obtained by slow diffusion
of diethyl ether into a mixed solution of dichloromethane and
ethanol, and the solid-state structures were determined by single-
crystal X-ray diffraction studies. The coordination geometry at the
cobalt center is quite flexible in this family of complexes, vary-
ing from trigonal-bipyramidal to square-pyramidal, with various
degrees of distortion from the idealized geometries, depending on
the substitution pattern of the N-aryl groups [46]. The molecular
structures of these cobalt complexes are shown in Figs. 1–3, and
their selected bond lengths and angles are tabulated in Table 2.
In complex Co1 (Fig. 1), the coordinating environment of the
metal is a distorted trigonal bipyramidal geometry, in which the
atoms N1, N2 and N3 form the basal plane, the Cl atoms occu-
2.3.2. Ethylene polymerization under 1 atm ethylene
The pre-catalyst Co4 was dissolved in toluene using standard
Schlenk techniques, and the reaction solution was stirred with
a magnetic stir bar under ambient ethylene atmosphere. The
required amount of co-catalyst (MAO) was added by a syringe.
After the reaction was carried out for the required period, the reac-
tion solution was quenched with 10% hydrochloric acid/ethanol.
The precipitated polymer was collected by filtration, washed with
ethanol and water, and dried in a vacuum at 60 ◦C until constant
weight.
˚
pied the apical position, and the Co atom is placed 0.086 A above
2.4. X-ray crystallographic studies
the basal plane. The two Nimino-aryl planes are nearly perpendic-
ular to the coordinated basal plane (N1 N2 N3), indicating the
dihedral angle 87.50◦ for phenyl plane (C20 C25) and 87.23◦ for
phenyl plane (C12 C17); and the four methyl-substituents of the
two aryl groups block the axial sites at the metal center, which is
Single-crystal X-ray diffraction studies for Co1, Co2 and Co5
were carried out on a Rigaku RAXIS Rapid IP diffract meter
˚
with graphite-monochromatic Mo K␣ radiation (ꢀ = 0.71073 A).
Cell parameters were obtained by global refinement of the pos-
itions of all collected reflections. Intensities were corrected for
Lorentz and polarization effects and empirical absorption. The
structures were solved by direct methods and refined by full-
matrix least-squares on F2. All non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were placed in calculated pos-
itions. Structure solution and refinement were performed by using
the SHELXL-97 package [45]. Crystal data and processing parame-
ters for Co1, Co2 and Co5 are summarized in Table 1.
3. Results and discussion
3.1. Synthesis and characterization of the cobalt complexes
Co1–Co5
Using the same procedure of synthesizing their iron analogs
[44] (Scheme 2), the title cobalt(II) complexes, 2,8-bis(imino)-5,6,7-
trihydroquinolylcobalt(II) dichlorides (Co1–Co5), were readily
obtained in good yields. Elemental analyses for all of the complexes
were consistent with the proposed formulae and, in their IR spec-
tra, a band in the range 1580–1590 cm−1 can be ascribed to the
stretching vibration of C N. The molecular structures of Co1, Co2
Fig. 2. Molecular structure of complex Co2 with thermal ellipsoids at 30% probabil-
ity. Hydrogen atoms have been omitted for clarity.

70
W.-H. Sun et al. / Applied Catalysis A: General 447–448 (2012) 67–73
Table 1
Crystal data and structure refinement for Co1, Co2 and Co5.
Co1
Co2
Co5
Empirical formula
Formula weight
C27H29Cl2CoN3
525.36
C31H37Cl2CoN3
581.47
C33H41Cl2CoN3
609.52
T (K)
173 (2)
173(2)
173(2)
Wavelength (Å)
0.71073
0.71073
0.71073
Crystal system
Space group
a (Å)
Monoclinic
P2(1)/n
13.246(3)
15.101(3)
14.871(3)
90
106.45(3)
90
2902.2(10)
4
Monoclinic
P2(1)/c
18.550(4)
8.7089(17)
18.108(4)
90
99.89(3)
90
2881.9(10)
4
1.340
Monoclinic
C2/c
43.507(9)
8.1400(16)
18.091(4)
90
104.74(3)
90
6196(2)
b (Å)
c (Å)
˛ (◦)
ˇ (◦)
ꢁ (◦)
V (Å3)
Z
8
Dcalcd. (mg m−3
)
1.223
0.807
1092
1.307
0.753
2568
ꢂ (mm−1
F(0 0 0)
)
0.806
1220
Crystal size (mm)
0.32 × 0.28 × 0.12
1.82–27.54
−17 ≤ h ≤ 17
−19 ≤ k ≤ 19
−19 ≤ l ≤ 18
31,316
6552 (0.0722)
99.7 (ꢃ = 27.54◦)
1.116
0.26 × 0.22 × 0.12
1.11–27.52
−22 ≤ h ≤ 24
11 ≤ k ≤ 11
−23 ≤ l ≤ 13
14,540
6586 (0.0645)
99.1 (ꢃ = 27.52◦)
1.175
0.32 × 0.28 × 0.24
0.97–27.49
−56 ≤ h ≤ 56
−10 ≤ k ≤ 9
−16 ≤ l ≤ 23
22,395
7095 (0.0531)
99.5 (ꢃ = 27.49◦)
1.188
ꢃ range (◦)
Limiting indices
No. of reflections collected
No. unique reflections [R(int)]
Completeness to ꢃ (%)
Goodness of fit on F2
Final R indices [I > 2ꢄ(I)]
R1 = 0.0757
wR2 = 0.2028
R1 = 0.0870
wR2 = 0.2135
0.901 and −0.627
R1 = 0.1128
wR2 = 0.2384
R1 = 0.1382
wR2 = 0.2574
1.157 and −1.079
R1 = 0.0821
wR2 = 0.1097
R1 = 0.0932
wR2 = 0.2031
0.929 and −0.852
R indices (all data)
−3
˚
Largest diffraction peak and hole (e A
)
thought to protect the active site in order to maintain the high activ-
ity. Moreover, the Co Npyridyl bond is stronger than the Co Nimino
which is reflected by the shorter distance of Co Npyridyl (2.040(3)
the bond distances of Co
Npyridyl (2.051(5)–2.047(3) Å) are shorter
,
than the Co Nimino (2.202(5)–2.222(3) Å).
In all the cobalt complexes, the sp3-hybridized C(9) is bonded to
two adjacent sp3 carbon atoms (C(8) and C(10)), in order to attain
a more stable cyclic rings (C(6) C(11)) on the pyridine. The sp3
carbon atoms are always positioned away from the pyridyl plane;
Å) compared to the Co Nimino (2.238(3) and 2.283(3) Å); the dif-
fering lengths of the two Co Nimino bonds are attributed to the
unsymmetrical framework.
˚
The molecular structures complexes Co2 and Co5 are shown
in Figs. 2 and 3. The coordination geometry at the cobalt center is
square-pyramidal, for which the atoms N1, N2, N3 and Cl1 form the
basal plane and the Cl2 atom occupies the apical position, and with
various degrees of distortion from the idealized geometries. In Co2
C(9) is positioned 0.540 A away from the pyridyl plane in Co1, whilst
˚
˚
in Co2, the deviations are 0.265 A for C(9) and 0.461 A for C(10) on
the different sides of the pyridine plane. In Co5, the deviations are
˚
˚
0.310 A for C(9) and 0.363 A for C(10) on the different sides of the
pyridine plane.
˚
˚
the deviations are 0.005 A for the chloride Cl1 and 0.562 A for the
cobalt atom above the basal plane, whilst in Co5 the deviations are
˚
˚
0.252 A for the chloride Cl1 and 0.570 A. Similar to the molecular
structure of Co1, the arene rings at Nimine plane are nearly perpen-
dicular to the basal plane (the dihedral angle are 81.67–81.36◦) and
Table 2
Selected bond lengths (Å) and angles (◦) for Co1, Co2 and Co5.
Co1
Co2
Co5
Bond lengths (Å)
Co(1) N(1)
Co(1) N(3)
Co(1) N(2)
Co(1) Cl(1)
Co(1) Cl(2)
N(2) C(11)
N(3) C(2)
2.040(3)
2.051(5)
2.202(5)
2.047(3)
2.238(3)
2.283(3)
2.2636(13)
2.2593(13)
1.277(5)
1.276(5)
2.222(3)
2.214(3)
2.2364(13)
2.3052(13)
1.288(5)
1.283(6)
2.225(5)
2.2397(19)
2.3081(19)
1.292(7)
1.268(7)
Bond angles (◦)
N(1) Co(1) N(3)
N(1) Co(1) N(2)
N(3) Co(1) N(2)
N(1) Co(1) Cl(1)
N(3) Co(1) Cl(1)
N(2) Co(1) Cl(1)
N(1) Co(1) Cl(2)
N(3) Co(1) Cl(2)
N(2) Co(1) Cl(2)
Cl(1) Co(1) Cl(2)
C(11) N(2) Co(1)
C(12) N(2) Co(1)
75.50(13)
74.69(13)
150.01(13)
128.00(10)
98.02(10)
97.71(10)
118.87(10)
99.54(10)
97.50(10)
113.11(5)
113.3(3)
73.84(18)
74.28(18)
141.28(18)
149.65(15)
99.67(13)
96.81(13)
93.95(15)
102.12(13)
101.44(13)
116.36(8)
114.1(4)
73.52(14)
74.72(13)
141.20(13)
155.60(10)
99.24(10)
100.50(9)
90.98(10)
101.35(9)
100.91(9)
113.40(6)
113.7(3)
Fig. 3. Molecular structure of complex Co5 with thermal ellipsoids at 30% probabil-
ity. Hydrogen atoms have been omitted for clarity.
127.7(3)
126.4(4)
127.8(2)

W.-H. Sun et al. / Applied Catalysis A: General 447–448 (2012) 67–73
71
Table 3
Catalytic results of ethylene polymerization with Co1–Co5/MAO.a
T (◦C)
t (min)
P(C2H4) (atm)
Activityb
Tm (◦C)
c
Mw (g mol−1
)
Mw/Mn
d
d
Entry
Pre-cat.
Al/Ni
1
2
3
4
5
6
7
8
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co1
Co2
Co3
Co5
500
750
50
50
50
50
50
30
40
60
70
60
60
60
60
60
60
60
60
60
30
30
30
30
30
30
30
30
30
30
30
5
15
60
30
30
30
30
10
10
10
10
10
10
10
10
10
5
6.19
8.42
9.84
8.86
7.99
4.48
5.10
10.9
10.4
5.46
0.78
19.3
13.5
6.14
10.8
8.92
4.47
10.8
102.2
101.4
99.6
100.9
102.3
100.9
100.3
99.6
98.3
100.5
89.2
100.5
99.8
101.6
99.0
1124
1000
949
1.69
1.48
1.46
1.59
1.55
1.63
1.60
1.54
1.62
1.52
1.40
1.46
1.50
1.59
1.64
1.88
1.99
1.86
1000
1250
1500
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
998
1029
1219
960
910
880
1018
914
891
896
1016
827
9
10
11
12
13
14
15
16
17
18
1
10
10
10
10
10
10
10
117.4
128.9
117.6
1955
10189
2009
a
Conditions: 3 ␮mol of Co; total volume 100 mL.
b
c
106 g of PE (mol of Co)−1 h−1
Determined by DSC.
.
d
Determined by GPC.
3.3. Ethylene polymerization
from 500 to 1500 (entries 1–5 in Table 3). The highest activity of
9.84 × 106 g of PE (mol of Co)−1 h−1 was achieved with the Al/Co
ratio of 1000. The molecular weight of the polymers obtained
revealed an interesting trend, whereby the PDI decreased as the
Al/Co molar ratio was raised from 500 to 1000, and then slightly
increased as the Al/Co molar ratio further raised. This was proba-
bly due to the enhanced chain transfer from cobalt to aluminum
and resulted initially in short-chain polymers [47,48], and then
the presence of more co-catalyst resulted in chain-migration and
termination, and polymers having lower molecular weights. The
resultant polyethylene possessed much lower Mw (about 1000)
and narrower PDI (up to 1.6) than did the corresponding iron sys-
tems [19,24,38,44], suggesting a single-site active species exists in
this ethylene polymerization process. Moreover, the cobalt system
exhibited much higher activity than did the iron analog [44].
The polymerization temperature also affected the catalytic per-
formance (entries 3, 6–9 in Table 3). When the reaction temperature
was raised from 30 ◦C to 60 ◦C with the Al/Co ratio at 1000, the cat-
alytic activity significantly increased from 4.48 to 10.9 × 106 g of
PE (mol of Co)−1 h−1). On further increasing the temperature to
70 ◦C, the activity slightly decreased from 10.9 to 10.4 × 106 g (mol
of Co)−1 h−1 (entries 8–9, Table 3), which suggested that the
active species could withstand high temperatures. On com-
parison with related systems, the polymerization activity for
Methylaluminoxane (MAO) and modified methylaluminoxanes
(MMAO), commonly with 20–25% Al(i-Bu)3, are the most widely
used activators for 2,6-bis-(organylimino)pyridine Fe(II) and Co(II)
dihalide systems. Other activators, such as ethylaluminum chloride
and triethylaluminum, showed lower activity, which is consistent
with previous publications [8,46]. In the case of the correspond-
ing iron pre-catalysts [44], all catalytic systems showed highest
catalytic activities for ethylene polymerization at 50 ◦C. In this
work, complex Co4 was screened under 10 atm of ethylene at ele-
vated temperatures using MAO or MMAO. The type of co-catalyst
employed, the molar ratio of Al/Co, the reaction temperature, the
polymerization time and the nature of the complexes were all found
to have an influence on the observed catalytic activities, as well as
on the Mw and Mw/Mn values of the resultant polyethylenes.
3.3.1. Ethylene polymerization results using the Co1-Co5/MAO
systems
All the cobalt pre-catalysts were investigated using MAO as the
co-catalyst and the polymerization results are collected in Table 3.
Based on previous work [44], Co4 was initially screened under
10 atm ethylene at 50 ◦C with MAO. The influence of the molar
ratio of Al/Co was investigated by increasing the Al/Co molar ratio
Table 4
Catalytic results of ethylene polymerization with Co1–Co5/MMAO.a
c
d
d
Entry
Pre-cat.
Al/Ni
T (◦C)
Activityb
Tm (◦C)
Mw (g mol−1
)
Mw/Mn
1
2
3
4
5
6
7
8
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co4
Co1
Co2
Co3
Co5
1000
1250
1500
1750
1500
1500
1500
1500
1500
1500
1500
1500
50
50
50
50
30
40
60
70
60
60
60
60
6.24
6.70
7.68
7.37
3.93
5.79
8.81
3.28
5.89
3.81
2.27
5.21
102.0
101.6
99.5
100.8
100.4
99.4
1031
1064
1056
1088
–
927
951
950
996
1.46
1.48
1.59
1.45
–
1.56
1.47
1.46
1.48
1.88
3.15
1.90
98.7
100.3
100.4
118.4
129.1
119.0
9
10
11
12
2162
8096
2052
a
Conditions: 3 ␮mol of Co; 30 min; 10 atm of ethylene; total volume 100 mL.
b
c
106 g of PE (mol of Co)−1 h−1
Determined by DSC.
.
d
Determined by GPC.

72
W.-H. Sun et al. / Applied Catalysis A: General 447–448 (2012) 67–73
1,8-diimino-2,3,4,5,6,7-hexahydroacridine iron/cobalt complexes
was found to decrease on raising the temperature [38], whereas
in the case of 2,8-bis(arylimino)-5,6,7-trihydroquinolyl complexes,
the cobalt complex exhibited higher thermal stability (cf the iron
complex) [44], and the resulting polyethylene possessed lower
molecular weights and narrow PDI. From the GPC data, it was evi-
dent that the Mw of the resultant polyethylene dropped and the PDI
became narrower as the reaction temperature was increased from
30 ◦C to 70 ◦C, consistent with the DSC data and the behavior of the
analogous bis(imino)pyridine pre-catalysts.
The Co4/MAO system was also studied under increasing ethyl-
ene pressure from 1 to 10 atm (entries 10, 11 and 3 in Table 3) at
60 ◦C, and it was found that the activity quickly rose from 0.78 to
10.9 × 106 g of PE (mol of Co)−1 h−1. The polymerization activity
was monitored at different time intervals in the ethylene poly-
merization promoted by Co4/MAO systems (entries 3 and 12–14
in Table 3). On prolonging the catalytic time from 5 min to 60 min,
more polymers were obtained, whilst the overall activity declined.
The highest activity of 1.93 × 107 g of PE (mol of Co)−1 h−1 was
observed within 5 min (entry 12 in Table 3); for extended times
(entries 8 and 12–14 in Table 3), the catalytic activities were signif-
icantly decreased because of the partial deactivation of the active
species and mass transfer problems.
N
N
N
N
N
N
Ar
Ar Ar
Ar
M
M
Cl
Fe
Cl
Co
Cl
Fe
Cl
Co
5
1.48×10 2.57×105 1.52×105 1.60×103
Mw(g/mol)
PDI
10.7
Ref 3
144.5
Ref 3
83.5
Ref 3
2.6
Ref 3
N
N
N
N
N
N
Ar
Ar Ar
Ar
M
M
Cl
Cl
Cl
Fe
Cl
Fe
Co
Co
910
1.5
current
4.61×104
6.4
2.27×104
7.2
Mw(g/mol)
930
1.5
PDI
Ref 38
Ref 38
Ref 44
The effects of the ligand nature on the catalytic behavior were
conducted using the optimum reaction conditions of Al/Co 1000 at
60 ◦C, and all the cobalt complexes exhibited high catalytic activity
(entries 3, 15–18 in Table 3). Different to the iron pre-catalysts, the
activities here declined in the order 2,4,6-tri(Me) > 2,6-di(Me) = 2,6-
di(Et)-4-Me > 2,6-di(Et) > 2,6-(i-Pr), consistent with observations in
the literature [1–3,49,50] that less bulkiness of the ortho sub-
stituents and the methyl substituent in the para position are
beneficial for better activity. At the same time, the molecular weight
Ar as 2,4,6-trimethylpenyl group
Scheme 3. Information illustration of obtained polyethylenes.
with the systems employing MAO (Table 3), the catalytic system
using activator MMAO produced polyethylenes with slightly higher
molecular weights, but a little lower activity. In general, all cobalt
catalytic systems produced polyethylene waxes, which are in the
desiring region of commercial polyethylene waxes with molecular
weights 450–3000 (Polywax from Baker Petrolite); the resul-
tant polyethylenes showed lower molecular weights than most
polyethylenes obtained from other bis(arylimino)pyridyl metal
pre-catalyst systems [3,38,44]. Focusing on the molecular weights
and polydispersity index (PDI) of polyethylenes obtained at individ-
ual optimum polymerization conditions, a comparison is illustrated
in Scheme 3. The representative examples shown suggest there is
potential for the current catalytic system to be used for accessing
polyethylene waxes. Furthermore, the current cobalt catalytic sys-
tem does not require the use of hydrogen, which contrasts with the
commercial metallocene or Ziegler-Natta processes for polyethyl-
ene wax production where hydrogen is required to controlling the
molecular weight and polydispersity of the polyethylenes obtained.
In addition, the catalytic system has been examined in detail to
confirm no oligomers produced.
of resultant polyethylene increased from 827 to 10,189 g mol−1
,
when the R1 group changed from methyl to isopropyl, which
might be explained by the bulk of the ligands protecting the active
species and preventing chain transfer, thereby affording higher
Mw. Within most model complex pre-catalysts, the iron catalytic
systems performed higher activities than their cobalt analogs did
[7,46]; however, the current cobalt pre-catalysts showed higher
activities than their iron analogs [44], more recognizably, the cobalt
systems required much less amount of MAO in maintaining high
activities.
3.3.2. Ethylene polymerization results by Co1-Co5/MMAO
systems
MMAO was also used as a co-catalyst (Table 4), and on varia-
tion of the Al/Co ratios from 1000 to 1750 at 50 ◦C (entries 1–4 in
Table 4), the activities were found to peak at an optimum ratio of
1500 (entry 3 in Table 4). The resulting polyolefins exhibited sim-
ilar Mw, whilst the PDI of a number of the polyethylene products
became slightly broader. This indicated that there was more chain
transfer to the aluminum center in the catalytic process containing
MMAO. On comparison with the Co4/MAO system, the Co4/MMAO
system required a higher Al/Co ratio to obtain the optimum activity.
With the molar ratio of Al/Co (1500) fixed, the influence of the
reaction temperature was investigated (entries 3, 5–8 in Table 4).
Similar to the Co4/MAO system, on elevating the reaction tempera-
ture from 30 ◦C to 60 ◦C, the activity of Co4/MMAO system increased
quickly, but further temperature increases resulted in the activity
decreasing dramatically. On increasing the reaction temperature,
the obtained polyethylenes exhibited similar molecular weights
and narrow PDIs, indicating a situation more closely resembling
single-site catalysis.
4. Conclusion
In the present work, a series of 2-(1-(arylimino)ethyl)-8-
arylimino-5,6,7-trihydroquinolyl cobalt dichloride (aryl = 2,6-R1-
4-R2C6H2) were synthesized. When employing MAO and MMAO
as the co-catalyst, all the title cobalt pre-catalysts exhibited high
activities in ethylene polymerization. The optimum activity was
found to be 60 ◦C, which suggested such systems have reasonable
thermal stability: the highest activity was 1.93 × 107 g of PE (mol
of Co)−1 h−1 within 5 min and 1.09 × 107 g of PE (mol of Co)−1 h−1
within 30 min. Most importantly, the resultant polyethylenes pos-
sessed low Mw (ranging from 900 to 10,000 g mol−1) and narrow
PDIs (about 1.6), suggestive of single-site active species. Such
obtained polyethylene waxes are in high demand for industrial
consideration, and therefore this kind of cobalt pre-catalysts has
potential for the production of polyethylene wax in industry.
Employing the optimum conditions of Al/Co 1500 at 60 ◦C
over 30 min, all the cobalt pre-catalysts were evaluated for eth-
ylene polymerization (entries 3, 9–12 in Table 4). On comparison

W.-H. Sun et al. / Applied Catalysis A: General 447–448 (2012) 67–73
73
Supporting information available
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