Catalytic ethylene oligomerization on cobalt(II) bis(imino)pyridine complexes bearing electron-withdrawing groups

Article abstract of DOI:10.1016/j.jorganchem.2019.02.002

A series of novel bis(imino)pyridine cobalt(II) chlorides, LCoCl₂, with the bis(imino)pyridine ligands bearing one or several electron-withdrawing substituents (F, Cl, Br, CF₃) at the aniline moieties, have been prepared and characterized. In the presence of methylalumoxane (MAO), these complexes have demonstrated high ethylene oligo- and polymerization activity (up to 1.8·10⁷ g products?(mol Co)^?1 h^?1 bar^?1), affording products ranging from 1-butene and Z,E-2-butenes to strictly linear low-molecular-weight (M_n ～ 300…700) polyethylene. The dependence of the reaction outcome on the ligand structure is discussed.

Full text of DOI:10.1016/j.jorganchem.2019.02.002

Accepted Manuscript
Catalytic ethylene oligomerization on cobalt(II) bis(imino)pyridine complexes bearing
electron-withdrawing groups
Artem A. Antonov, Nina V. Semikolenova, Evgenii P. Talsi, Konstantin P. Bryliakov
PII:
S0022-328X(18)31008-8
DOI:
https://doi.org/10.1016/j.jorganchem.2019.02.002
JOM 20704
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 14 December 2018
Revised Date: 24 January 2019
Accepted Date: 1 February 2019
Please cite this article as: A.A. Antonov, N.V. Semikolenova, E.P. Talsi, K.P. Bryliakov, Catalytic
ethylene oligomerization on cobalt(II) bis(imino)pyridine complexes bearing electron-withdrawing groups,
Journal of Organometallic Chemistry (2019), doi: https://doi.org/10.1016/j.jorganchem.2019.02.002.
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ACCEPTED MANUSCRIPT
Catalytic ethylene oligomerization on cobalt(II) bis(imino)pyridine complexes
bearing electron-withdrawing groups
Artem A. Antonov,a,b Nina V. Semikolenova,a Evgenii P. Talsi,a,b Konstantin P. Bryliakov,a,b*
a Boreskov Institute of Catalysis, pr. Lavrentieva 5, Novosibirsk 630090, Russian Federation
b Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russian Federation
Fax: +7 383 330 8056, E-mail: bryliako@catalysis.ru
_______________________________________________________________________
This work is dedicated to Prof. Armando J. L. Pombeiro for his prominent contribution to modern
organometallic chemistry and on the occasion of his 70th birthday.
_______________________________________________________________________
ABSTRACT
A series of novel bis(imino)pyridine cobalt(II) chlorides, LCoCl2, with the bis(imino)pyridine
ligands bearing one or several electron-withdrawing substituents (F, Cl, Br, CF3) at the aniline
moieties, have been prepared and characterized. In the presence of methylalumoxane (MAO), these
complexes have demonstrated high ethylene oligo- and polymerization activity (up to 1.8·107 g
products·(mol Co)-1 h-1 bar-1), affording products ranging from 1-butene and Z,E-2-butenes to
strictly linear low-molecular-weight (Mn ~ 300…700) polyethylene. The dependence of the reaction
outcome on the ligand structure is discussed.
Keywords: Ethylene; bis(imino)pyridine; cobalt; electron-withdrawing; oligomerization;
polymerization
1

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Highlights
• Novel cobalt(II) bis(imino)pyridine complexes bearing electron-withdrawing substituents
• High ethylene oligomerization/polymerization activities in the presence of MAO
• Ligand structure-catalytic behavior relationships
Graphical Abstract
2

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1. Introduction
The milestone discoveries of Brookhart [1] and of Gibson [2] established the bis(imino)pyridine
cobalt complexes as highly active post-metallocene catalysts of ethylene polymerization, affording
highly linear polyethylene (PE) with narrow molecular weight distribution (MWD), characteristic
of “single-site” catalysts [3-8]. The success of the bis(imino)pyridine ligand framework inspired
further ligand modifications, retaining the N,N,N-donor chelating core, aimed at tuning the catalytic
properties of cobalt complexes, as well as improving their thermal stability [9,10].
In the last years, we exploited a synthetic approach to single-site catalysts, relying on the
introduction of electron-withdrawing substituents into the ligand structures, which was found to
enhance the catalytic reactivities of nickel(II) complexes toward norbornene polymerization [11,12]
and ethylene oligomerization [13], and provided a powerful tool to manipulate the relative rates of
olefin enchainment and chain termination. Currently, we have been interested in designing highly-
active single-site catalysts for selective ethylene dimerization, the crucial first step for further
production of commodity plastics and value-added chemicals [14-20], as well as for the synthesis of
strictly linear polyethylene with low molecular weight (PE waxes with narrow MWD), the
precursors to food-grade paraffins, used as components of food coating compositions [21-24]. In
this work, we have focused on bis(imino)pyridine Co based catalysts, owing to their high ethylene
reactivity, sometimes matching that of their Fe counterparts, but leading to oligo- and polymeric
products with lower molecular weight [1-10]. Another attractive feature of Co bis(imino)pyridine
based catalysts is the absence of chain transfer to co-catalyst, chain termination operating
exclusively by
β-hydride transfer, thus ensuring stability of chain transfer rate over the
polymerization course and hence narrow molecular-weight distribution (Mw/Mn down to 1.4)
[8,9,25].
Herewith, we report designing a series of cobalt(II) bis(imino)pyridine complexes, bearing
one or more electron-withdrawing substituents, their catalytic activities in ethylene
dimerization/oligomerization/polymerization, and analyze the substituents effects on their catalytic
behavior.
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2. Experimental
General experimental details, preparation and characterization of bis(imino)pyridine ligands with
electron-withdrawing substituents and the corresponding cobalt(II) complexes are reported in the
Supporting Information.
2.1. Ethylene Oligomerization/Polymerization Procedure
The reaction was performed in a steel 0.3 L autoclave. The catalyst was introduced into the
autoclave in a vacuum-sealed glass ampoule. The reactor was evacuated at 80 °C, cooled to 20 °C
and charged with the solution of MAO (1 mmol Al) in toluene. After setting up the temperature and
ethylene pressure, the reaction was started by breaking the ampoule with the complex. During the
reaction, ethylene pressure, stirring speed and temperature were maintained constant. The
experimental unit was equipped with an automatic computer-controlled system for ethylene feed
and the ethylene consumption recording. The catalytic activity values were calculated based on the
ethylene consumption.
2.2. Analysis of Oligomeric and Polymeric Products
After the catalytic experiment ended, an aliquot of the oligomerization products in toluene (0.6 ml)
was quickly taken by a gastight syringe through a technical hole of the reactor lid, and was
immediately transferred to the 5 mm o.d. glass NMR tube, capped with a septum stopper. The
13
1
13
relative content of ethylene oligomers was determined by C NMR. H and C NMR spectra of
oligomeric mixtures were recorded on a Bruker DPX 250 MHz NMR spectrometer (at 250.130 and
62.90 MHz) at room temperature.
With regard to the polyethylene analysis, typically, 50-100 mg of dry polymer were
1
13
dissolved in 3 ml of 1,2-dichlorobenzene at 140-150 °C. The H and C NMR spectra of linear
polyethylene were recorded on a Bruker Avance-400 MHz NMR spectrometer (at 400.130 and
100.613 MHz, respectively) in 1,2-dichlorobenzene at 100 °C using 10 mm o.d. glass NMR tubes.
4

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3. Results and Discussion
3.1. Synthesis and Characterization of Bis(imino)pyridine Ligands and Cobalt(II) Complexes
Cobalt(II) bis(imino)pyridine complexes were prepared in good yields (typically 62-95 %) by
mixing the corresponding chelating ligands with CoCl2·2H2O in THF under an argon atmosphere
for 5-24 h (SI). Overall, 13 cobalt complexes (Scheme 1) were synthesized and characterized by
elemental analysis and 1H NMR spectroscopy. All complexes contained cobalt(II) (d7
1
configuration) in high-spin state (S = 3/2) and displayed well informative paramagnetic H NMR
spectra from the chelating ligands, with the paramagnetic shifts ranging from 100 to -65 ppm
(Figure S1, SI). We notice that some cobalt(II) bis(imino)pyridine complexes containing alkyl
[1,2,26-32] , bulky aromatic [33-37], both alkyl and electron-withdrawing [38-43], only electron-
withdrawing [44-49] substituents at the aniline moieties, and supported cobalt(II)
bis(imino)pyridine complexes [50-52] were reported previously by others and were studied as
catalysts of ethylene polymerization or oligomerization in the presence of MAO, MMAO, AlMe3,
B(C6F5)3/AlMe3, B(C6F5)3/AliBu3, AlEt3/[Ph3C][Al(OtBuF)4].
Complex
R1
H
H
Cl
F
H
CF3
H
CF3
H
F
Cl
H
R2
H
H
H
H
Cl
H
F
H
CF3
H
R3
F
Br
F
H
H
H
H
F
F
F
H
H
H
H
R4
H
H
H
H
Cl
H
F
H
H
H
H
H
H
H
R5
H
H
H
H
H
H
H
H
H
F
Cl
H
H
1
2
3
4
5
6
7
8
R5
N
R5
R4
R3
N
Co
N
R4
R3
Cl
Cl
R1
R1
R2
R2
9
10
11
12
13
14
H
Cl
H
Br
Me
H
Me
Scheme 1. Structure of cobalt(II) complexes 1-14.
3.2. Ethylene Oligomerization on Bis(imino)pyridine Cobalt(II) Complexes
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The reactivity of complexes 1-13 toward ethylene was examined, using MAO as activator. For
comparison, the ‘reference’ 2,6-dimethyl substituted complex 14 (Scheme 1) [1] was tested under
the same conditions. The obtained data are summarized in Table 1.
Complexes 6, 8, and 11, bearing either o-CF3 substituent or two o-Cl substituents at each
aromatic ring, demonstrated the highest catalytic activities, affording, predominantly, a mixture of
С4-С12+ oligomers (17-19 wt. %), along with low-molecular-weight polyethylene (81-83 wt. %,
Table 1, entries 6, 8, 11, and Table 2, entries 1-3). In comparison, reference complex 14 showed
somewhat lower activity, yielding higher-molecular-weight linear PE as the only reaction product
(Table 1, entry 14, and Table 2, entry 4).
Table 1. Oligomerization of ethylene in the presence of cobalt complexes 1-14. a
b
Entry
Catalyst
Time, min
Activity
Products
1
2
3
4
5
6
7
8
9
10
11
12
13
14
6
0.3·106
1.3·106
6·106
1-butene
1-butene, Z,E-2-butene
oligomers С4
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
15
15
15
15
15
15
10
15
15
15
15
15
2.5·106
1.8·106
7.1·106
2.9·106
11.0·106
1.4·106
0.6·106
18.2·106
4.7·106
2.3·106
2.4·106
1-butene , Z,E-2-butene
1-butene , Z,E-2-butene
oligomers С4-С12+, linear PE
1-butene , Z,E-2-butene
oligomers С4-С12+, linear PE
1-butene , Z,E-2-butene
1-butene
oligomers С4-С12+, linear PE
1-butene , Z,E-2-butene
1-butene
linear PE
a Conditions: Cobalt complex, 2
µmol, toluene, 50 mL, MAO (Al/Co = 500/1), ethylene pressure 2
bar, Т = 35 оС. b In g products·(mol Co)-1 h-1 bar-1.
In all cases, the ratio of CH3 and CH2=CH polymer end groups was close to 1:1 (Table 2),
thus witnessing that β-hydride transfer is the only chain termination pathway. The polyethylene Mn
reduces when passing from 6 (one o-CF3 substituent) to 8 (o-CF3 substituent and p-F substituent)
and further to 11 (two electron-withdrawing o-Cl substituents). Tentatively, this trend, dictated by
the increasing propensity to
β
-H transfer in the order 6 < 8 < 11, may be ascribed to an increase of
6

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13
overall cobalt electrophilicity in the same order. C NMR analysis of the oligomeric fractions,
obtained in the presence of complexes 6, 8, 11, witnessed the presence of linear oligomers (Table
3). The share of 1-butene decreases in the order 6 > 8 > 11.
Table 2. 13C NMR characterization of PE obtained on complexes 6, 8, 11, 14.
a
Catalyst
m PE (g)
Mn
End-groups (per PE molecule)a
PE/Oligomers
(wt. %)
Entry
CH3
CH2=CH
1
2
3
4
1.28
2.07
3.32
2.66
690
390
300
1.1
1.1
1.0
1.1
0.9
0.9
1.0
0.9
17.5/82.5
19.3/80.7
17.7/82.3
100/0
6
8
11
14
1300
a Determined by 13C NMR.
Table 3. Analysis of oligomeric products obtained on catalysts 6, 8, 11.
Entry
Catalyst
1-Butene
(mol. %)
29.5
1-Hexene
(mol. %)
22.0
1-Octene
(mol. %)
14.5
1-Decene
(mol. %)
10.0
1-Dodecene+ a
(mol. %)
24.0
1
2
3
6
8
11
22.0
16.0
21.0
20.5
15.5
17.5
12.0
14.5
29.5
31.5
a C12 and higher oligomers, soluble in toluene.
High activity was demonstrated by complexes 5, 7, 9, 12, bearing one or several electron-
withdrawing m-substituents (Table 1), and by 3, 4, 13 (Table 1), having one o-halogen substituent.
Complexes 1 and 2, with p-halogen substituents, and 10, with o,p-F substituted ligands (Table 1),
were less active. In all cases, the only reaction products were C4 oligomers, predominantly 1-butene
(Table 4). Catalyst 13 was most selective for 1-butene (Table 4, entry 10); other complexes without
o-substituents (1, 2), one o-halogen substituent (3, 4) or with m,p-substituents (1, 2, 5, 9) also
showed high 1-butene selectivities (82-88.5 %) (Table 4, entries 1-5,7).
On the contrary, m-substituted complexes 7 and 12 afforded higher share of 2-butenes, with
E-2-butene prevailing over Z-2-butene (Table 4, entries 6 and 9). This result shows that in case of
less sterically demanding complexes 7 and 12, 2,1-reinsertion of the initially formed 1-butene, with
subsequent cis-trans conformational rearrangement and
β-H elimination (Scheme 2), is
substantially facilitated. At the same time, given the thermodynamic parameters of the geometrical
7

ACCEPTED MANUSCRIPT
isomerization of Z-2-butene to E-2-butene [53], the calculated equilibrium constant (corresponding
to E-2-butene/Z-2-butene molar ratio) at +35 °C should have value of 3.0…3.1. The experimentally
observed [E]/[Z] ratios, not exceeding 2.2 (Table 4), do not approach the predicted equilibrium
value, which points to kinetic control of the proportion of Z- and E-2-butenes formed in our catalyst
systems.
Table 4. Composition of C4 fractions, obtained on cobalt complexes 1, 2, 3, 4, 5, 7, 9, 10, 12, 13. a
Entry
Catalyst
1-Butene (mol. %) Z-2-Butene (mol. %) E-2-Butene (mol. %)
1
2
3
4
5
6
7
8
9
n.d. b
88.5
87.0
82.0
85.0
71.0
85.0
n.d. b
58.5
≥98
n.d. b
6.5
n.d. b
5.0
1
2
3
4
5
7
9
10
12
13
9.5
11.5
6.0
10.0
6.5
n.d. b
13.0
≈2
3.5
6.5
9.0
19.0
8.5
n.d. b
28.5
-
10
a Results of 13C NMR analysis of the reaction mixture. b Not determined: concentration of 1-butene
and 2-butenes was insufficient for quantitative 13C NMR measurements.
Scheme 2. Proposed scheme of 1-butene, Z-2-butene, and E-2-butene formation in the presence of
Co complexes.
8

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4. Conclusions
A family of cobalt(II) bis(imino)pyridine complexes, LCoCl2, bearing electron-withdrawing
substituents (F, Cl, Br, CF3) at the aniline moieties, were prepared and found to exhibit high
ethylene oligomerization/polymerization activities (up to 1.8·107 g products*(mol Co)-1 h-1 bar-1) in
the presence of MAO. The highest catalytic activities were demonstrated by cobalt complexes with
electron-withdrawing o-substituents (Cl, CF3). Depending on the nature and number of the electron
withdrawing groups, the process selectivity varies from dimerization (with the selectivity to 1-
butene up to 98 %) to oligomerization (C4–C12+), with accompanying formation of strictly linear
low-molecular-weight polyethylene (the share of PE approaching 19 wt. %). One of the complexes,
12, bearing one m-Cl substituent at the aryl ring, catalyzed ethylene dimerization to a mixture of 1-
butene and 2-butenes (58:42), reflecting significant contribution of double bond migration process.
Analysis of polyethylene end groups gives evidence for
termination pathway.
β-hydride transfer as the only chain
Acknowledgements
Financial support of this work by the Ministry of Higher Education and Science of the Russian
Federation, unique project identifier RFMEFI60717X0169, is acknowledged.
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• Novel cobalt(II) bis(imino)pyridine complexes bearing electron-withdrawing substituents
• High ethylene oligomerization/polymerization activities in the presence of MAO
• Ligand structure-catalytic behavior relationships