2-Benzoxazolyl-6-(1-(arylimino)ethyl)pyridyl cobalt (II) chlorides: A temperature switch catalyst in oligomerization and polymerization of ethylene

Article abstract of DOI:10.1016/j.molcata.2009.05.021

A series of cobalt (II) complexes ligated by 2-(2-benzoxazolyl)-6-(1-(arylimino)ethyl)pyridines was synthesized and characterized by FT-IR spectroscopic and elemental analysis. The molecular structures of Co1 (Ar = 2,6-Me₂C₆H₃

Full text of DOI:10.1016/j.molcata.2009.05.021

Journal of Molecular Catalysis A: Chemical 309 (2009) 166–171
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
2-Benzoxazolyl-6-(1-(arylimino)ethyl)pyridyl cobalt (II) chlorides: A temperature
switch catalyst in oligomerization and polymerization of ethylene
Rong Gaoa, Kefeng Wanga, Yan Lia, Fosong Wanga, Wen-Hua Suna,∗, Carl Redshawb, Manfred Bochmannb
a Key laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b 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
Article history:
A series of cobalt (II) complexes ligated by 2-(2-benzoxazolyl)-6-(1-(arylimino)ethyl)pyridines was syn-
thesized and characterized by FT-IR spectroscopic and elemental analysis. The molecular structures of
Co1 (Ar = 2,6-Me2C6H3), Co2 (Ar = 2,6-Et2C6H3) and Co7 (Ar = 2,6-Br2C6H3) were determined by single-
crystal X-ray diffraction. Compounds Co1 and Co2 display a distorted trigonal-bipyramidal geometry,
whereas complex Co7 is a distorted square-pyramid. Upon activation with methylaluminoxane (MAO),
all cobalt complexes showed reactivity towards ethylene. There was however an unexpected temperature
dependence; whereas, at room temperature oligomerization was observed, giving 1-alkenes with high
selectivities, at elevated temperatures polyethylene was produced. This unusual catalytic behavior of the
new complexes was investigated in detail under various reaction parameters.
Received 28 February 2009
Received in revised form 25 May 2009
Accepted 27 May 2009
Available online 6 June 2009
Keywords:
Cobalt complexes
Temperature switch catalyst
Ethylene oligomerization
Ethylene polymerization
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
unusual temperature dependence, whereby at room temperature
ethylene oligomerization took place, whereas at elevated temper-
Iron and cobalt complexes based on bis(arylimino)pyridines are
known as highly active catalysts for the polymerization of ethylene
[1–3], and such systems are currently the focus of much research
[4–10]. Bianchini, Erker and others [11–17] have investigated
unsymmetrical systems in which one of the arylimino groups was
replaced by an alkyl or aryl substituent. Complexes of this type have
recently been reviewed [18–25]. Cobalt catalysts have attracted
less attention than their iron counterparts because in general they
show lower activities. On the other hand, cobalt complexes of 2,9-
bis(1-(arylimino)ethyl)phenanthrolines [26] and 2-methyl-2,4-
bis(6-iminopyridin-2-yl)-1H-1,5-benzodiazepines [27,28] showed
higher activities than their iron analogues. In line with the behav-
ior of most polymerization catalysts, the molecular weights of the
polyethylenes produced with these Co and Fe catalysts decrease
with increasing reaction temperatures.
atures polyethylenes were produced. We report here the syntheses
and characterization of these cobalt complexes and their catalytic
behavior in ethylene oligomerization and polymerization.
2. Experimental
2.1. General considerations
All manipulations of air- and moisture-sensitive compounds
were performed under
a nitrogen atmosphere using stan-
dard Schlenk techniques. The 2-(2-benzoxazolyl)-6-(1-(arylimino)
ethyl)pyridines (L1–L7) were prepared according to our reported
procedure [34]. Toluene was refluxed over sodium-benzophenone
and distilled under argon prior to use. Methylaluminoxane (MAO,
a 1.46 M solution in toluene) and modified methylaluminoxane
(MMAO, 1.93 M in heptane, 3A) were purchased from Akzo Nobel
Corporation. Diethylaluminum chloride (Et2AlCl, 1.7 M in toluene)
was purchased from Acros Chemicals. Other reagents were pur-
chased from Aldrich or Acros Chemicals. 1H and 13C NMR spectra
were recorded on a Bruker DMX 300 MHz instrument at ambi-
ent temperature using TMS as an internal standard. FT-IR spectra
were recorded on a PerkinElmer System 2000 FT-IR spectrometer.
Elemental analyses were carried out using an HPMOD 1106 micro-
analyzer. GC analyses were performed with a Varian CP-3800 gas
chromatograph equipped with a flame ionization detector and a
Since good activities were found for metal complexes bear-
ing 2-(benzimidazolyl)-6-(1-(arylimino)ethyl)pyridines [29–32],
we decided to explore related 2-(2-benzoxazolyl)-6-(1-(arylimino)
ethyl)pyridine ligands. Nickel complexes of these ligands show
good to high catalytic activities for ethylene oligomerization
[33,34]. On activation by MAO these cobalt catalysts showed an
∗
Corresponding author. Tel.: +86 10 62557955; fax: +86 10 62618239.
E-mail address: whsun@iccas.ac.cn (W.-H. Sun).
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.05.021

R. Gao et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 166–171
167
Table 1
Crystal data and structure refinement details for Co1, Co2 and Co7.
Co1
Co2
Co7·CH3OH
Formula
Formula weight
T (K)
C
44H38Cl4Co2N6O2
C24H23Cl2CoN3O
499.28
173(2)
C20H13 Br2Cl2CoN3O·CH3OH
904.16
173(2)
633.03
173(2)
Wavelength (Å)
Crystal system
Space group
a (Å)
0.71073
Triclinic
P1
0.71073
Monoclinic
P21/c
8.7814(18)
15.155(3)
17.494(4)
90
99.94(3)
90
2293.1(8)
4
0.71073
Monoclinic
P21/n
10.858(2)
15.337(3)
13.744(2)
90
101.06(3)
90
2246.4(8)
4
9.3733(19)
9.5328(19)
11.674(2)
92.97(3)
99.60(3)
95.30(3)
1021.7(3)
2
b (Å)
c (Å)
˛ (◦)
ˇ (◦)
ꢀ (◦)
V (Å3)
Z
Dcalc (Mg m−3
)
1.469
1.118
444
1.446
1.003
1028
1.777
4.575
1172
ꢁ (mm−1
F(0 0 0)
)
Crystal size (mm)
0.24 × 0.24 × 0.08
0.25 × 0.25 × 0.19
0.30 × 0.18 × 0.18
ꢂ range (◦)
1.77–27.38
2.35–27.48
2.01–27.44
Limiting indices
−12 ≤ h ≤ 12
−11 ≤ k ≤ 12
−15 ≤ l ≤ 15
−11 ≤ h ≤ 11
−19 ≤ k ≤ 19
−22 ≤ l ≤ 22
−14 ≤ h ≤ 14
−19 ≤ k ≤ 19
−17 ≤ l ≤ 17
No. of refinement collected
No. of unique refinement
Completeness to ꢂ (%)
Absorption correction
No. of parameters
8390
4609
10074
5263
9818
5124
99.5 (ꢂ = 27.38◦)
Multi-scan
262
99.9 (ꢂ = 27.48◦)
Multi-scan
281
99.8 (ꢂ = 27.44◦)
Multi-scan
284
Goodness of fit on F2
1.225
1.221
1.218
Final R indices (I > 2ꢃ(I))
R1 = 0.0569
R1 = 0.0578
R1 = 0.0618
wR2 = 0.1296
wR2 = 0.1002
wR2 = 0.0844
R indices (all data)
R1 = 0.0746
R1 = 0.0747
R1 = 0.0911
wR2 = 0.1450
wR2 = 0.1052
wR2 = 0.0902
Largest difference peak and hole (e Å−3
)
0.614, −0.577
0.506, −0.465
0.546, −0.524
30 m (0.25 mm i.d., 0.25 ␮m film thickness) CP-Sil 5 CB column. The
yield of oligomers was calculated by referencing to the mass of the
solvent on the basis of the prerequisite that the mass of each fraction
was approximately proportional to its integrated areas in the GC
trace. Selectivity for linear 1-alkenes was defined as (amount of lin-
ear 1-alkenes of all fractions)/(total amount of oligomer products)
in percent.
2.2.3. (E)-2,6-Diisopropyl-N-(1-(6-(benzo[d]oxazol-2-yl)
pyridin-2-yl)ethylidene)benzenamine CoCl2 (Co3)
Obtained as green powder in 85.8% yield. FT-IR (KBr; cm−1):
3066 (w), 2962 (m), 2927 (w), 1613 (ꢄC N) (m), 1594 (w), 1548
(m), 1448 (m), 1373 (s), 1278 (s), 1193 (w), 1103 (w), 1022 (w), 817
(w), 789 (m), 762 (m), 749 (m). Anal. Calcd. for C26H27N3Cl2CoO
(527.35): C, 59.22; H, 5.16; N, 7.97. Found: C, 59.13; H, 5.07; N, 7.96.
2.2. Synthesis of cobalt complexes
2.2.4. (E)-2,4,6-Trimethyl-N-(1-(6-(benzo[d]oxazol-2-yl)
pyridin-2-yl)ethylidene)benzenamine CoCl2 (Co4)
General procedure. A solution of CoCl2 in ethanol was added
dropwise to a solution of the ligand in ethanol. The reaction mixture
was stirred at room temperature for 12 h. The resulting precipitate
was collected, washed with diethyl ether and dried in vacuum. All
the complexes were prepared in high yield in this manner.
Obtained as green powder in 77.0% yield. FT-IR (KBr; cm−1): 3061
(w), 1610 (ꢄC N) (m), 1591 (w), 1540 (m), 1479 (m), 1447 (m), 1371
(m), 1290 (w), 1275 (s), 1218 (m), 1171 (m), 1101 (w), 1021 (w), 814
(w), 772 (s), 743 (w). Anal. Calcd. for. C23H21Cl2CoN3O (485.27): C,
56.93; H, 4.36; N, 8.66. Found: C, 56.70; H, 4.42; N, 8.68.
2.2.1. (E)-2,6-Dimethyl-N-(1-(6-(benzo[d]oxazol-2-yl)
pyridin-2-yl)ethylidene)benzeneamine CoCl2 (Co1)
2.2.5. (E)-4-Bromo-2,6-dimethyl-N-(1-(6-(benzo[d]oxazol-2-yl)
pyridin-2-yl)ethylidene)benzenamine CoCl2 (Co5)
Obtained as green powder in 90.8% yield. FT-IR (KBr; cm−1):
3070 (w), 1616 (ꢄC N) (m), 1594 (w), 1570 (w), 1545 (m), 1469 (w),
1450 (m), 1375 (s), 1278 (s), 1214 (m), 1173 (w), 1095 (w), 1023 (w),
813 (w), 791 (w), 766 (s), 750 (w). Anal. Calcd. for C22H19 Cl2CoN3O
(471.24): C, 56.07; H, 4.06; N, 8.92. Found: C, 56.23; H, 4.12; N, 8.81.
Obtained as green powder in 78.4% yield. FT-IR (KBr disk, cm−1):
2911 (w), 1618 (ꢄC N) (m), 1595 (w), 1548 (m), 1453 (w), 1374 (m),
1276 (m), 1211 (s), 1023 (m), 995 (w), 865 (w), 772 (m). Anal. Calcd.
for. C22H18 BrCl2CoN3O (550.14): C, 48.03; H, 3.30; N, 7.64. Found:
C, 47.72; H, 3.35; N, 7.62.
2.2.2. (E)-2,6-Diethyl-N-(1-(6-(benzo[d]oxazol-2-yl)
2.2.6. (E)-2,6-Dichloro-N-(1-(6-(benzo[d]oxazol-2-yl)
pyridin-2-yl)ethylidene)benzenamine CoCl2 (Co2)
pyridin-2-yl)ethylidene)benzenamine CoCl2 (Co6)
Obtained as green powder in 91.3% yield. FT-IR (KBr; cm−1): 2971
(m), 1614 (ꢄC N) (m), 1594 (w), 1570 (w), 1546 (m), 1449 (s), 1375 (s),
1276 (s), 1209 (w), 1195 (w), 1173 (w), 1102 (w), 1023 (w), 813 (w),
787 (s), 767 (s), 750 (w). Anal. Calcd. for C24H23Cl2CoN3O (499.30):
C, 57.73; H, 4.64; N, 8.42. Found: C, 57.72; H, 4.79; N, 8.39.
Obtained as green powder in 90.8% yield. FT-IR (KBr; cm−1):
3066 ( ), 2962 (w), 2927 (w), 1617 (ꢄC N) (m), 1594 (w), 1545 (m),
1438 (s), 1376 (m), 1279 (s), 1176 (w), 1089 (w), 1023 (w), 812 (w),
771 (s), 748 (w). Anal. Calcd. for C20H13Cl4CoN3O (512.08): C, 46.91;
H, 2.56; N, 8.21. Found: C, 46.73; H, 2.83; N, 8.13.

168
R. Gao et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 166–171
2.2.7. (E)-2,6-Dibromo-N-(1-(6-(benzo[d]oxazol-2-yl)
pyridin-2-yl)ethylidene)benzenamine CoCl2 (Co7)
Obtainedasgreen powderin77.0%yield. FT-IR(KBr; cm−1): 3049
(w), 1616 (ꢄC N) (m), 1594 (w), 1548 (m), 1449 (w), 1431 (s), 1374
(m), 1277 (s), 1229 (w), 1176 (w), 1099 (w), 1022 (w), 810 (w), 783
(m), 771 (s), 730 (m). Anal. Calcd. for C20H13Br2Cl2CoN3O (600.98):
C, 39.97; H, 2.18; N, 6.99. Found: C, 39.65; H, 2.25; N, 6.89.
2.3. Crystal structure determinations
Single-crystals of Co1, Co2 and Co7 suitable for X-ray diffrac-
tion studies were obtained by the slow diffusion of diethyl ether
into their methanol solutions. Single-crystal X-ray diffraction stud-
ies were carried out on a Rigaku RAXIS Rapid IP diffractometer
using graphite monochromated Mo K␣ radiation (ꢅ = 0.71073 Å).
Cell parameters were obtained by global refinement of the positions
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.
All hydrogen atoms were placed in calculated positions. Structure
solution and refinement were performed by using the SHELXL-97
package [35]. Crystal data and processing parameters for Co1, Co2
and Co7 are summarized in Table 1.
Scheme 1. Synthesis of complexes Co1–Co7.
Complexes Co1 and Co2 have essentially identical structures
(for the ORTEP plot of Co2 see the Supporting Information). The
coordination geometry around the cobalt center is a distorted
trigonal bipyramid, in which the pyridine-nitrogen (N(1)) and
two chlorine atoms form the equatorial plane, while the cobalt
atom lies slightly out of the equatorial plane by 0.0356 Å in Co1
and 0.0203 Å in Co2. The axial positions are occupied by the
other two nitrogen atoms N(2) and N(3). The N(2)–Co–N(3) moi-
ety deviates significantly from linearity (Co1: N(2)–Co(1)–N(3)
147.28(1)◦; Co2: 149.66(8)◦). The equatorial planes of these two
complexes are nearly perpendicular to the benzoxazole rings,
with dihedral angles of 88.4◦ in Co1 and 83.7◦ in Co2. The 2,6-
dimethylphenyl substituent lies almost orthogonally to the plane
N(3)–N(1)–N(2) (91.7◦ in Co1 and 95.9◦ in Co2). Similar geometries
are also observed in cobalt (II) complexes of 2-(benzimidazolyl)-6-
iminopyridines [29,32], 2-imino-1,10-phenanthrolines [36,37], and
2-(benzimidazolyl)-1,10-phenanthrolines [38]. The steric effect of
substituents at ortho-positions of the phenyl ring is reflected in the
adjacent C–N bond lengths, i.e. the Co(1)–N(3) bond length of Co1
(2.198(3) Å) is noticeably shorter than that of Co2 (2.259(2) Å).
The structure of Co7 is best described as a distorted square
pyramid. The atoms N(1), N(2), N(3) and Cl(1) are almost copla-
nar, deviating only by 0.01 Å. The cobalt atom deviates from the
N(1)–N(2)–N(3) plane by 0.1448 Å. The benzoxazole is almost
coplanar with the pyridine ring, with a dihedral angle of 7.8◦. The
two Co–Cl bond lengths are substantially different, 2.3240(1) and
2.2523(1) Å, much larger than the bond length difference in Co1
and Co2 (0.02 Å). Clearly the introduction of electron-withdrawing
substituents in the ortho-positions of the phenyl rings has a pro-
nounced effect on the structure, and this is also likely to affect the
catalytic activities.
2.4. Oligomerization or/and polymerization of ethylene
General procedure. Ethylene oligomerizations at 10 atm ethylene
pressures were carried out in a 500 mL stainless steel autoclave
equipped with a mechanical stirrer and a temperature controller.
Toluene, the desired amount of cocatalyst, and a toluene solu-
tion of the catalytic precursor (the total volume was 100 mL) were
added to the reactor in this order under an ethylene atmosphere.
When the desired reaction temperature was reached, the ethy-
lene pressure was increased to 10 atm, and maintained at this level
by constant feeding of ethylene. After 30 min, the reaction was
stopped by cooling the reactor on an ice bath before the excess
pressure was released. A small amount of the reaction solution
was collected, the reaction was terminated by the addition of
5% aqueous hydrogen chloride, and then this mixture was ana-
lyzed by gas chromatography (GC) to determine the distribution of
oligomers obtained. The remaining reaction solution was quenched
with HCl-acidified ethanol (5%), and the precipitated polyethylene
was filtered, washed with ethanol, and dried under vacuum at 60 ◦C
to constant weight.
Table 2
Selected bond lengths (Å) and angles (deg) for Co1, Co2 and Co7.
3. Results and discussion
Co1
Co2
Co7
Bond lengths
Co(1)–N(1)
Co(1)–N(2)
Co(1)–N(3)
Co(1)–Cl(1)
Co(1)–Cl(2)
N(2)–C(6)
N(2)–C(40)
O(1)–C(6)
N(3)–C(12)
3.1. Synthesis and characterization of cobalt complexes
(Co1–Co7)
2.080(3)
2.064(2)
2.089(4)
2.218(3)
2.198(3)
2.2833(12)
2.2439(12)
1.300(4)
1.397(4)
1.352(4)
1.285(4)
2.235(2)
2.259(2)
2.2477(1)
2.2671(9)
1.297(3)
1.402(3)
1.350(3)
1.281(3)
2.179(4)
2.194(4)
2.2523(13)
2.3240(13)
1.296(5)
1.397(5)
1.360(5)
1.297(5)
A series of 2-(2-benzoxazolyl)-6-(1-(arylimino)-ethyl)pyridine
(L1–L7) ligands was prepared following previously reported pro-
cedures [34]. The cobalt complexes Co1–Co7 were prepared by
treating CoCl2 with one equiv. of L in ethanol at room temperature
(Scheme 1). The products were collected by filtration, washed with
diethyl ether and dried under vacuum. The complexes are air-stable
powders which were characterized by FT-infrared spectroscopy.
The elemental analyses confirmed the composition LCoCl2. The
Bond angles
N(1)–Co(1)–N(2)
N(1)–Co(1)–N(3)
N(2)–Co(1)–N(3)
N(1)–Co(1)–Cl(1)
N(2)–Co(1)–Cl(1)
N(3)–Co(1)–Cl(1)
Cl(1)–Co(1)–Cl(2)
N(1)–Co(1)–Cl(2)
N(2)–Co(1)–Cl(2)
N(3)–Co(1)–Cl(2)
74.99(11)
74.50(11)
147.28(1)
101.93(9)
92.46(8)
105.16(8)
118.28(5)
139.18(9)
95.46(8)
99.86(8)
75.69(8)
74.19(8)
75.10(14)
73.81(14)
144.46(14)
155.33(1)
100.78(1)
100.27(1)
111.10(5)
93.48(1)
149.66(8)
129.98(7)
95.54(6)
100.92(7)
115.66(3)
114.17(7)
95.43(6)
100.07(7)
C
N stretching frequencies of Co1–Co7 are shifted to lower values
(1613–1616 cm−1) compared to the free ligands.
Single crystals of Co1, Co2 and Co7 were grown from their
methanol solutions layered with diethyl ether. The molecular struc-
tures were determined by X-ray diffraction. Selected bond lengths
and angles are collected in Table 2. Fig. 1 shows the structures of Co1
and Co7; they differ mainly in the orientation of the CoCl2 moiety.
92.88(1)
105.75(1)

R. Gao et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 166–171
169
Table 3
Oligomerization of ethylene with Co1–Co7 at 20 ◦C.
Entry
Catalyst
Cocatalyst
Al/Co
Product mass (g)
Productivitya
Oligomer distribution (%)b
ꢀ
ꢀ
C4/
C
␣-C4
C6/
C
1
2
3
4
5
6
7
8
9
Co1
Co1
Co1
Co2
Co3
Co4
Co5
Co6
Co7
MAO
MMAO
Et2AlCl
MAO
MAO
MAO
MAO
MAO
MAO
1000
1000
200
1000
1000
1000
1000
1000
1000
0.92
0.29
–
0.23
0.17
0.12
0.28
1.81
1.88
3.70
1.17
–
0.91
0.68
0.50
1.12
7.24
7.52
100
100
–
100%c
77.2
–
0
0
–
0
1.2
0
3.3
0.6
0
100
99.4
99.0
97.1
93.2
100%c
100%c
98.8
100
96.7
99.4
100
Reaction conditions: 5 ␮mol Co; 10 atm ethylene; 30 min; 20 ◦C; 100 mL toluene.
a
In units of 105 g (mol Co)−1 h−1
.
ꢀ
b
c
Determined by GC;
C denotes the total amounts of oligomers.
No butene isomerization products could be detected.
3.2. Reactions with ethylene
With methylaluminoxane (MAO) as activator under 10 atm of
ethylene pressure, all the cobalt (II) complexes displayed moderate
activities for the oligomerization of ethylene, with high selectivities
for 1-alkenes (Table 3). Although MMAO or Et2AlCl could also be
used as cocatalyst, they give lower activities (entries 2, 3 in Table 3),
hence further investigations were carried out using MAO as the
cocatalyst.
As we had found previously with cobalt complexes of
2-(benzimidazolyl)-6-iminopyridines [29,32] and other lig-
ands [28,37,38], Co1–Co7 only produced ethylene dimers and
trimers without polymeric by-products at 20 ◦C. The N-aryl
substituents significantly affected the catalytic activities, and
the ethylene oligomerization activities decreased in the order
Co7 ≈ Co6 > Co1 > Co5 > Co2 > Co3 ≈ Co4. In general, electron-
withdrawing substituents enhance catalyst activities, as do less
bulky substituents. A possible explanation is that in late-transition
metal catalytic systems, both bulkier and more electron-rich
ligands weaken the interactions between the metal center and
the incoming monomer and thereby retard the rate of ethylene
insertion. The observed activity trends appear to be a superposition
of electronic and steric factors. It is difficult to rationalize the steric
influence of ligands; however, the electronic effects have been
simulated and explained with the aid of computational studies
that suggest a relationship of catalytic activity with the net charge
of late-transition metal complexes [39–42].
The reaction parameters also affect the observed catalytic activ-
ities. For example, when the Al/Co molar ratio was increased from
200 to 1500, the activities of Co7/MAO initially increased and then
decreased, with the optimum activity observed at an Al/Co molar
ratio of 1000 (entry 4 in Table 4).
Fig. 1. Molecular structure of Co1 (A) and Co7 (B) with thermal ellipsoids drawn at
the 30% probability level. Hydrogen atoms have been omitted for clarity.
Table 4
Influence of the Al/Co ratio on the oligomerization of ethylene with Co7/MAO.
Entry
Al/Co
T (◦C)
Product mass (g)
Activitya
Oligomer distribution (%)b
ꢀ
ꢀ
Oligomer
Polymer
C4/
C
␣-C4
C6/
C
1
2
3
4
5
6
7
8
9
200
500
800
1000
1200
1500
1000
1000
1000
20
20
20
20
20
20
30
40
50
0.86
1.42
1.74
1.88
1.86
0.70
0.64
0.22
0.16
3.45
5.66
6.98
7.52
7.43
2.78
2.55
0.14
0.05
0
0
0
0
0
0
0
0.72
0.61
98.6
99.5
99.6
100%c
99.5
1.4
0.5
0.4
0
0.4
0.3
0.6
5.2
0
99.8
100
100%c
99.9
99.6
99.7
99.4
94.8
99.7
98.8
100%c
100%c
100
Conditions: 5 ␮mol Co7; 10 atm ethylene; 30 min; 100 mL toluene.
a
In units of 105 g (mol Co)−1 h−1
Determined by GC.
No butene isomerization products could be detected.
.
b
c

170
R. Gao et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 166–171
Table 5
Polymerization of ethylene with Co1–Co7/MAO at 40 ◦C.
Entry
Catalyst
Product mass (g)
Activitya
Oligomer
Oligomer distribution (%)b
PE mol. weightc
d
Polymer
C4/ꢆ C
C6/ꢆ C
Mw
Mw/Mn
1
3
5
7
9
Co1
Co2
Co3
Co4
Co5
Co6
Co7
1.41
0.92
0.55
0.26
0.48
0.27
0.22
0
0
0
0
0
1.07
0.14
5.63
3.70
2.21
1.04
1.91
Trace
0.72
0
0
0
0
0
0
0
0
0
0
17.0
33.8
17.5
–
20.9
–
12
4.3
3.3
–
3.8
–
10
11
28.7
94.8
16.0
5.2
31.4
4.6
Conditions: 5 ␮mol Co; 1000 equiv. of MAO; 10 atm ethylene; 30 min; 40 ◦C; 100 mL toluene.
a
In units of 105 g (mol Co)−1 h−1
Determined by GC.
.
b
c
Determined by GPC.
d
In units of 104 g mol−1
.
Rather unexpectedly, increasing the reaction temperature led
The most likely reason for this is more facile ligand transfer to Al
and/or complex decomposition under such conditions.
to the formation of polyethylene (Table 5). At 40 ◦C under 10 atm
of ethylene, the catalysts Co1–Co5 solely produced polyethylenes,
whereas Co6 and Co7 at 40 ◦C gave variable amounts of oligomers
as well as polymers. The polyethylenes have moderate molecu-
lar weights in the order of (17–34) × 104 g mol−1. In all cases the
polydispersities were rather broader than expected for single-site
behavior.
4. Conclusion
The cobalt complexes bearing the unsymmetrical ligand 2-
(benzoxazolyl)-6-(1-(arylimino)ethyl)pyridine give improved cat-
alytic activities compared with the analogous 2-(benzimidazolyl)-
6-iminopyridine complexes [29,32]. An ethylene oligomerization
productivity of up to 7.5 × 105 g (mol Co)−1 h−1 was found, with
good selectivity for ␣-olefins. The activities increased with decreas-
ing electron-donor strength of the ligands. However, increasing the
reaction temperature to 40 ◦C led unexpectedly to the formation of
polyethylene, with about the same order of productivity. This cat-
alytic system is thus behaving as a temperature-switch catalyst for
the oligomerization and polymerization of ethylene.
Increasing the ethylene pressure from 10 to 30 atm over the tem-
perature range of 40–60 ◦C led to an increase in catalytic activities
(compare Table 6 with Table 5), and with Co6 and Co7 especially,
the polymer molecular weight also increase significantly (Table 6,
entries 9 and 10).
The origin of the change in catalytic behavior, i.e. formation
of oligomers below 40 ◦C and of polymers above this threshold
temperature, is as yet unknown. One possibility was the alkyla-
tion of the ligand framework. It did not prove possible to recover
the ligand from the actual polymerization reactions. However,
the following experiment was employed to check stabilities of
the complex in the presence of MAO. A mixture of Co3 with
MAO (Al/Co = 1000:1) was stirred at 60 ◦C for 30 min, followed
by hydrolysis. From this mixture, the ligand 2,6-diisopropyl-N-
(1-(6-(benzoxazol-2-yl)pyridin-2-yl)ethylidene)benzenamine was
recovered in 76%, thus confirming that the ligand did not change on
treatment with MAO or AlMe3.
Supporting information available
All data for molecular structures of Co1, Co2, and Co7 are pro-
vided in detail as supplementary material. Crystallographic data
for the structural analysis have been deposited with the Cambridge
Crystallographic Data Center, CCDC Nos. 721467 for Co1, 721468
for Co2, 721469 for Co7. Copy of this information is available free of
charge from The Director, CCDC, 12 Union Road, Cambridge CB21EZ,
UK (fax: +44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk or
www: http://www.ccdc.cam.ac.uk).
The catalytic activity of these cobalt systems is temperature
dependent. The best activities were observed at 40 ◦C. The activ-
ities decreased with increasing reaction temperature even under
30 atm of ethylene, and at 70 ◦C precursor Co1 had little activity.
Acknowledgments
This work was supported by NSFC No. 20674089 and the Royal
Society/NSFC (project IJP 2007/R3). We thank Dr. Steve Holding
(RAPRA Technology, Shrewsbury, UK) for GPC measurements.
Table 6
Polymerization of ethylene with Co1–Co7/MAO at 30 atm.
Entry
Catalyst
T (◦C)
Product
mass (g)
Activitya
PE mol. weightb
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Mw
Mw/Mn
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Co7
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