2-iminopyridine nickel(II) complexes bearing electron-withdrawing groups in the ligand core: Synthesis, characterization, ethylene oligo- and polymerization behavior

Article abstract of DOI:10.1016/j.jorganchem.2016.08.031

A series of novel 2-[(arylimino)alkyl]pyridine derivatives, containing electron-withdrawing groups (F, Cl, Br, CF₃, NO₂, CN) in the aryl rings, have been synthesized and characterized by¹H,¹³C and¹⁹F NMR spectroscopy. The corresponding nickel(II) complexes (24 complexes) have been prepared in good yields. The nature of the Ni complexes has been established by elemental analysis, paramagnetic¹H NMR and X-ray diffraction studies. Those complexes, activated with MAO, have been found to be highly active in the oligomerization and polymerization of ethylene (up to 9600?kg (mol of Ni)^?1?h^?1?bar^?1), affording highly branched low-MW PE or ethylene oligomers, predominantly butenes and hexenes. The dependence of content and ratio of ethylene oligomers as well as of the polymer characteristics (molecular weight, degree of branching) on the nature of the substituents in the aryl(imino) and pyridine rings has been analyzed systematically. (E)-N-(1-(pyridin-2-yl)ethylidene)-2-(trifluoromethyl)aniline Ni complex supported on silica-alumina affords polyethylene with higher molecular weights, broader molecular weight distribution, and lower level of branching than the homogeneous catalyst.

Full text of DOI:10.1016/j.jorganchem.2016.08.031

Accepted Manuscript
2-Iminopyridine nickel(II) complexes bearing electron-withdrawing groups in the ligand
core: Synthesis, characterization, ethylene oligo- and polymerization behavior
Artem A. Antonov, Nina V. Semikolenova, Evgenii P. Talsi, Mikhail A. Matsko,
Vladimir A. Zakharov, Konstantin P. Bryliakov
PII:
S0022-328X(16)30372-2
10.1016/j.jorganchem.2016.08.031
JOM 19610
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 14 April 2016
Revised Date: 24 August 2016
Accepted Date: 31 August 2016
Please cite this article as: A.A. Antonov, N.V. Semikolenova, E.P. Talsi, M.A. Matsko, V.A. Zakharov,
K.P. Bryliakov, 2-Iminopyridine nickel(II) complexes bearing electron-withdrawing groups in the ligand
core: Synthesis, characterization, ethylene oligo- and polymerization behavior, Journal of Organometallic
Chemistry (2016), doi: 10.1016/j.jorganchem.2016.08.031.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
2-Iminopyridine Nickel(II) Complexes Bearing Electron-Withdrawing
Groups in the Ligand Core: Synthesis, Characterization, Ethylene Oligo-
and Polymerization Behavior
Artem A. Antonov,a,b* Nina V. Semikolenova,b Evgenii P. Talsi,a,b Mikhail A. Matsko,b
Vladimir A. Zakharov,a,b and Konstantin P. Bryliakova,b*
aNovosibirsk State University, Ul. Pirogova 2, Novosibirsk 630090, Russian Federation
bBoreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russian Federation
Fax: +7 383 326 9578
Email: antonov@catalysis.ru, bryliako@catalysis.ru
Keywords: 2-iminopyridine, nickel, ethylene, polymerization, oligomerization,
methylaluminoxane.
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1. Introduction
The history of post-metallocene Ni-based olefin polymerization catalysts dates back to the
Brookhart’s milestone discovery of nickel(II) α-diimine complexes; those complexes,
activated with methylaluminoxane (MAO), displayed very high activity in the polymerization
of ethylene and α-olefins [1]. An important catalytic feature the Brookhart’s catalysts has
been the formation of branched polyethylene via the «chain-walking» mechanism [2,3]. In
recent years, complexes of nickel(II) with bi- and tridentate N-donor ligands have attracted
great academic and practical attention as highly active and versatile catalysts of olefin
polymerization and oligomerization [4-12], providing the access to novel polymeric
functional materials [13]. Lately, close attention has been paid to Ni-based catalysts, capable
of affording highly- or even hyperbranched polyolefins during polymerization [14,15-17].
Such polymers represent a challenging class of polyethylene materials with unprecedented
chain architectures and unique physical properties. Possible application areas of such
materials have been delineated, including e.g. lubricant additives or drug delivery [4,18].
Nickel(II) complexes with bidentate iminopyridine ligands are known to carry out
ethylene polymerization, affording branched low-MW polyethylene. For the first time such
catalysts (containing 2,6-dialkylphenylimino moieties) were reported by Leskelä and co-
workers [19-21]. Later, catalytic behaviors of various homogeneous single-site catalysts based
on 2-iminopyridine nickel complexes with electron-donating substituents in ethylene
polymerization were described [22-30]. In addition, a few examples of bridged dinuclear [31-
35], dendritic [36-38] and MOF-supported [39] Ni(II) complexes containing 2-iminopyridine
moieties were synthesized and tested towards ethylene reactivity. Structurally similar 2-
iminoquinolyl nickel complexes displayed high activity towards ethylene oligomerization
[40,41].
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So far, researchers have mostly focused on 2-iminopyridine nickel catalysts bearing
electron-donating groups (EDG) in the aryl rings. It would thus be intriguing to examine the
effect of electron-withdrawing groups (EWG) on the catalytic behavior of 2-iminopyridine
nickel complexes, keeping in mind that the introduction of the EWGs had been reported to
enhance the catalytic activity of analogous iron, cobalt [42-49] and nickel [50] complexes for
ethylene oligo- and polymerization, and for polymerization of norbornene.
Herein, the synthesis and characterization of a series of 2-iminopyridine ligands
bearing various electron-withdrawing groups (F, Cl, Br, CF3, NO2, CN), and of the
corresponding nickel(II) complexes, are reported. The influence of both electronic and steric
properties of the ligand on the catalytic behavior of those complexes in ethylene
polymerization and oligomerization is discussed, using methylaluminoxane (MAO) as
cocatalyst. Furthermore, supported nickel catalysts have demonstrated good activity in
ethylene polymerization also with Et2AlCl, modified MAO (MMAO), and Al(iBu)3 as
cocatalysts.
2.
Experimental Section
2.1. General Experimental Details
All manipulations with air- and/or moisture-sensitive compounds were performed under an
atmosphere of argon using glove box, break-sealed or standard Schlenk techniques. Toluene
was dried over molecular sieves (4 Å) and distilled over sodium under argon. THF was stored
over NaOH and distilled over sodium under argon before use. CH3CN was stored over
molecular sieves (4 Å) under an atmosphere of argon. Dichloromethane used for the synthesis
of Ni(II) complexes was stored over Na2SO4 and distilled over P2O5 under argon before use.
Ethylene glycol dimethyl ether (DME) was dried over sodium and distilled under argon.
Other solvents were used as received.
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2-quinolinecarboxaldehyde, 2-acetylpyridine, 2-benzoylpyridine and 2-formylpyridine
derivatives were purchased from Acros, Aldrich, Alfa Aesar and ABCR. Substituted anilines
were purchased from Acros, Aldrich, Alfa Aesar, Lancaster Synthesis and used as received.
4,6-dibromo-2-(trifluoromethyl)aniline was synthesized as reported [51]. 2-(2’-
methoxyphenyl)aniline was prepared as described [52]. Anhydrous NiCl2 and NiBr2 were
prepared by heating commercially available NiCl2·6H2O and NiBr2·6H2O at 140 °C in vacuo
for 4.5 h and stored in glove box. Nickel(II) bromide dimethoxyethane adduct (DME)NiBr2
was prepared by stirring of NiBr2 in hot ethylene glycol dimethyl ether for 48 h under argon
followed by filtration and drying in vacuo as reported [53]. MAO/SiO2 used for the ligand
synthesis (8-12 wt.-% of Al) was prepared as described [54].
Silica, modified with alumina prepared according to published procedure [55] was
used as a support for catalyst preparation. Supported nickel catalyst was prepared as follows:
the weighed amount of support was stirred with calculated amount of nickel complex solution
in CH3CN (1 ꢀmol Ni/mL) during 0.5 h at room temperature to provide the desired nickel
content in the catalyst. The liquid fraction was decanted, solid catalyst was washed with
CH3CN and dried under vacuum. The nickel content in the prepared catalysts was determined
by AES-ICP.
Analytical thin layer chromatography (TLC) was performed on Merck silica gel 60
1
13
1
F254 plates. H and C NMR spectra of 2-iminopyridine ligands and H NMR spectra of
corresponding nickel(II) complexes were measured either on a Bruker Avance-400 MHz
NMR spectrometer (at 400.130 and 100.613 MHz, respectively) or on a Bruker DPX-250
MHz NMR spectrometer (at 250.130 and 62.90 MHz) using 5 mm o.d. glass NMR tubes. 19F
NMR specta of ligands were measured on a Bruker DPX-250 MHz NMR spectrometer at
1
13
235.32 MHz. H and C NMR spectra of branched polyethylene were recorded on a Bruker
Avance-400 MHz NMR spectrometer in o-dichlorobenzene at 99 °C using 10 mm o.d. glass
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1
13
NMR tubes. H and C NMR spectra of oligomeric mixtures were recorded on a Bruker
DPX-250 MHz NMR spectrometer in toluene at room temperature.
Elemental analyses were performed using a CHNS Euro EA3000 elemental analyzer
from EuroVector, the presented data being the average of three runs.
Methylalumoxane was purchased from Crompton as a toluene solution (total Al
content 1.8 M, Al as AlMe3 ~0.5 M). For polymerization experiments, stock toluene solution
with 0.5 M Al content was prepared. MMAO was purchased from AKZO as a heptane
solution (total Al content 7.1 wt %). Commercial samples of Al(i-Bu)3 and Et2AlCl were
purchased from Aldrich and used as heptane solutions.
2.2. 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 cocatalyst (1 mmol Al) in toluene or heptane. After
setting up the polymerization 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 recording of the ethylene
consumption.
2.3. Polymers MM and MWD Measurements
GPC measurements were performed using a PL-220 High Temperature Chromatograph
equipped with set of PLgel Olexis columns. Run conditions used: 160 °C; flow rate 1
cm3/min; 1,2,4-trichlorobenzene was used as a solvent. Conventional calibration was made
using PS standards and PE standards with narrow MWD.
2.4. X-ray Measurements
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Crystals of Ni1·2DMF suitable for X-ray structural determination were grown from N,N-
o
dimethylformamide upon slow diffusion of Et2O at +4 C. Single-crystal diffraction data for
Ni1·2DMF were collected at 130 K on an automated Agilent Xcalibur diffractometer
equipped with a CCD AtlasS2 detector (MoKα, graphite monochromator, ω-scans).
Integration, absorption correction, and determination of unit cell parameters were performed
using the CrysAlisPro program package [56]. The structures were solved by a direct method
and refined by the full-matrix least squares technique in the anisotropic approximation (except
hydrogen atoms) using the SHELX-2013 software package [57]. Positions of hydrogen atoms
of organic ligands were calculated geometrically and refined in the riding model.
3. Results and Discussion
3.1. Synthesis and Characterization of 2-(Imino)pyridine Ligands and of Nickel(II)
Complexes
2-Iminopyridine ligands were synthesized according to modified literature procedure [58] via
Schiff-base condensation of the corresponding substituted 2-pyridinecarboxaldehyde (or 2-
quinolinecarboxaldehyde, 2-acetylpyridine, 2-benzoylpyridine) with aniline derivatives
containing EWG in the presence of catalysts (Brønsted or Lewis acids). The general method
for the preparation of the 2-iminopyridine ligands and of the corresponding nickel(II)
complexes is illustrated in Scheme 1.
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Scheme 1. Synthesis of 2-iminopyridine ligands and of Ni(II) complexes.a
NH2
R4
R5
R3
A
B
R2
method or
R1
N
R3
R2
+
R1
N
N
O
R6
R4
R6
R1
Me
Me
Me
Me
Me
Me
Br
R2
R3
F
R4
H
F
R5
H
H
H
H
H
H
H
H
H
H
H
H
tBu
H
H
H
H
H
H
H
H
H
H
H
H
R6
H
H
F
X
R5
H
H
Cl
Cl
Cl
Cl
Br
Cl
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
1
2
3
4
F
NiCl2 or NiBr2*DME
H
F
F
C
D
method or
H
CF3
CF3
CF3
F
H
F
H
H
H
H
H
H
H
H
H
H
H
H
H
F
H
5a
5b
6
H
F
H
F
R2
R3
Br
H
CF3
F
H
F
7
R1
N
Ni
4-F-Ph
H
8
N
X
H
CF3
iPr
H
iPr
H
H
Cl
H
F
9
4-F-Ph
4-F-Ph
4-F-Ph
4-F-Ph
4-F-Ph
4-F-Ph
4-F-Ph
4-F-Ph
3-CN-Ph
4-CN-Ph
H
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
X
R4
R6
H
2-(o-MePh)Ph
tBu
R5
H
Ni1-Ni8
Ni10-Ni24
and
H
Cl
H
CF3
H
CF3
H
CF3
H
H
H
H
H
F
H
CF3
NO2
H
H
H
H
Br
H
H
N
Ni
Br
Ni9
CF3
H
CF3
N
H
CF3
Br
H
CF3
H
H
CF3
H
H
CF3
Br
H
H
H
CH3
Ph
CF3
H
CF3
aMethod A: trifluoroacetic acid, benzene, reflux, MS 4Å. Method B: p-toluenesulfonic acid or
o
MAO/SiO2, toluene, 50-85 C or reflux, MS 4Å, inert atmosphere. Method C: NiCl2, dry
CH3CN, room temperature. Method D: (DME)NiBr2, dry THF or CH2Cl2, room temperature.
It is well-known that the reaction of 2-pyridinecarboxaldehyde with anilines
containing electron-donating (alkyl) substituents proceeds readily in ethanol without catalysts
[19]. However, the presence of electron-acceptors requires more severe conditions: high
temperature, strong acid as catalyst, careful moisture removal and prolonged reaction times.
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For instance, in the case of L1, L3, L4, L6 with small substituens at the 6th position of the
pyridine ring (R1=Me, Br) and EWG’s in phenyl ring refluxing the precursors in benzene with
trifluoroacetic acid was sufficient to obtain the ligands with good yields (from 75 to 88 %)
and purity. At the same time, when the 6th position of pyridine ring contained bulky phenyl
group, it was more effective to carry out the reaction in dry toluene in the presence of p-
toluenesulfonic acid (yields up to 92 %). Using methods A and B, ligands L1-L24 have been
synthesized in good yields and were characterized by 1H, 13C and 19F NMR spectroscopy (SI).
The 2-iminopyridine derivatives readily reacted with one equivalent of nickel halide
(NiCl2 or (DME)NiBr2) according to methods C or D (Scheme 1) to form the corresponding
complexes. It is worth noting that nickel chloride compounds were easily obtained in dry
acetonitrile while nickel bromide complexes were to be obtained in less polar THF or
dichloromethane. All complexes (except the poorly soluble Ni5b and Ni6) exhibited
1
paramagnetic H NMR spectra, indicative of a high-spin (S = 1) d8 configuration of the
central atom (SI). Elemental analysis data confirmed the empirical formula LNiX2 (X = Cl,
Br) for all synthesized complexes.
X-ray quality single crystals for complex Ni1 have been obtained by crystallization
o
from DMF upon slow diffusion of Et2O at +4 C (Experimental section), and its X-ray
structure has been determined. Complex Ni1 co-crystallized with two molecules of DMF per
nickel, forming an octahedral coordination environment (Figure 1). The structural data for
Ni1 can be found in the SI (Tables S1 and S2).
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Figure 1. Molecular structures of complex Ni1·2DMF (thermal ellipsoids at the 50%
probability level). Hydrogen atoms omitted for clarity.
3.2. Ethylene Oligomerization over 2-(Imino)Pyridine Nickel Complexes
Nickel(II) complexes Ni1-Ni9 were systematically tested as catalysts for ethylene
oligomerization upon the activation with MAO under optimized reaction conditions. All of
them displayed high activity (up to 9600 kg (mol of Ni)-1 h-1 bar-1). In most cases, the major
products were dimers of ethylene (with moderate selectivity for α-C4), with minor amount of
trimers (various isomers of hexenes). The oligomerization results are summarized in Table 1;
detailed oligomer distribution of the C4-C6 fractions are provided separately in Table 2.
The major focus of this work has been the effect of the substituents’ nature on the
activity and selectivity of the nickel(II) complexes in ethylene oligomerization and
polymerization. Note that the correct comparison of the catalytic properties of nickel
complexes can only be performed by changing the position or nature of substituents in one
fragment of the organic ligand. So, we have first studied the ethylene oligomerization over 2-
iminopyridine nickel complexes with the same substituent at the 6th position of the pyridine
ring, but bearing different EWG’s in the aniline moieties.
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Table 1. Oligomerization of ethylene with Ni1-Ni9/MAO a
Oligomer
distribution, % c
Activity
Complex
b
ΣC4/ ΣC6/ ΣC8+/
ΣC
97.9
100
95.6
90.1
90.2
100
ΣC
2.1
-
ΣC
7400
1500
6200
9200
9600
1300
5200
4600
7600
-
-
Ni1
Ni2
4.4
9.9
9.8
-
-
Ni3
-
Ni4
-
Ni5b
-
Ni6
Ni7 d
Ni8
89.6 10.4
55.5 34.6
-
9.9
-
90.4
9.6
Ni9
a
Reaction conditions: toluene, 50 ml; Ni complex, 2 ꢀmol; cocatalyst MAO (Al/Ni=500);
reaction time 15 min; P(C2H4) 2 bar; T = 35 oC. b In kilograms of oligomers·(mol Ni)-1·h-1·bar-
1. c Determined by 13C NMR. d Reaction time 30 min.
Table 2. Oligomer distribution a
C4 fraction, %
C6 fraction, %
E-
2-
Comple
x
Z-
E-
Z-
E-
Butene-
Ethyl- Hexene
Butene-
1
Hexene Hexene Hexene
Butene- Butene-
1
-1
-2
-2
-3
2
2
34.5
77.8
15.1
43.1
50.6
71.9
66.8
28.9
56.2
22.3
12.9
23.7
18.8
16.2
10.1
8.5
41.1
9.3
2.1
-
-
-
-
-
-
Ni1
Ni2
-
-
-
56.8
28.2
23.4
18.0
14.3
15.3
4.4
2.7
3
-
-
-
-
Ni3
2.3
2.8
-
1.8
2.2
-
2.6
1.1
-
0.5
0.7
-
Ni4
Ni5b b
-
Ni6
2.7
2
2.8
16.8
3.5
2.6
7.9
2.1
1.2
5.7
1.3
1.1
2.2
0.6
Ni7
Ni8 b
11.3
14.5
19.7
2.1
Ni9
Determined by 13C NMR. Traces of 3-methyl-pentene-1 ( < 0.2 %) were also
detected using precatalysts Ni5b and Ni8. Possible mechanisms of formation of 2-ethyl-
butene-1 and 3-methyl-pentene-1 are presented in the SI.
a
b
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It has been found that the introduction of trifluoromethyl group at the ortho-position of
the phenyl ring resulted in increased activity of the complexes in the oligomerization of
ethylene (Table 1, cf. data for complexes Ni1 and Ni4, Ni2 and Ni5b, Ni6 and Ni7
respectively). A similar trend was also documented for complex Ni9 with quinoline unit:
according to the literature, analogous nickel complex with iPr-substituted ortho-positions of
the aniline ring exhibited much lower activity than Ni9 (2.6*105 versus 7.6*106 g (mol Ni)-1 h-
1
bar-1) [25]. The effect of introduction of additional EWG's at the ortho and para-positions
was not uniform: while activity of the Ni5b (containing 2-CF3-6-F-substituted aryl ring) was
slightly higher than for the Ni4 (2-CF3-substituted), in the case of complexes Ni1/Ni2,
additional fluorination resulted in a dramatic drop of catalytic activity.
In terms of selectivity (see Table 2), an increase of steric hindrance at the ortho-
position of the aniline ring led to the formation of both butene and hexene isomers (Ni4,
Ni5b, Ni7), whereas complexes with ortho-fluorine substituents yielded only the C4 fraction
(Ni1-Ni3, Ni6). Almost all catalytic experiments (Table 2) demonstrated that α-olefins
prevailed in the obtained products. A remarkable exception is Ni3 containing an additional
fluorine atom at the para-position of the aniline: in that case, the major product of
oligomerization was trans-2-butene. One can conclude that the introduction of EWG at the
para-position promotes the formation of internal olefins, apparently via sequential β-H
elimination, 2,1-reinsertion and termination via β-H elimination [59,60], see Scheme 2.
Secondly, the influence of substituent’s nature at the 6th position of pyridine ring
towards the catalytic properties of nickel complexes has been examined. The catalytic activity
decreased in the order R1 = 4-F-Ph > Me > Br for the complexes Ni8, Ni2, Ni6, respectively
(Table 1). A similar trend was observed for complexes Ni4, Ni9, Ni7: the activity decreased
in the order R1 = Me > Quinolyl > Br (Table 1). Thus, the reduction of the activity may be
correlated with the basicity of the pyridine moiety [61] and attributed to the electronic effect
of substituent at the 6th position of pyridine ring. The introduction of the Me substituent with
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positive inductive effect (+I) increased the basicity of pyridine ring and gave more active
catalysts Ni4, Ni2. In contrast, the presence of electron withdrawing bromine atom (-I effect)
led to less active catalysts Ni7, Ni6, respectively.
Unlike the catalytic activity, the α-C4 selectivity increased in the order Ni4 < Ni9 <
Ni7 (the overall C4 selectivity remaining about 90 % for all three complexes). The
compositions of the oligomeric mixtures obtained over Ni2 and Ni6 were more or less similar,
the distinctive feature of catalyst Ni2 being the prevalence of Z-2-butene over E-2-butene.
This unusual cis-selectivity may stem from a proper interplay between the rate of
β-H-
elimination in the intermediate IINi2 (Scheme 2) and that of interconversion between IINi2 and
IIINi2. The introduction of bulky 4-fluorophenyl substituent at the 6th position (Ni8)
dramatically enhanced the hexene concentration in the reaction products, most likely owing to
the higher steric restrictions to
β-H-elimination in the corresponding intermediate INi8
(Scheme 2), which favors an additional enchainment.
Scheme 2. Proposed scheme for the formation of 1-butene, Z-2-butene and E-2-butene. (N,N)
stands for the bidentate N,N-donor ligand.
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3.3. Ethylene Polymerization over 2-(Imino)Pyridine Nickel Complexes
The catalytic properties of precatalysts Ni10-Ni24 have been evaluated employing MAO as
cocatalyst. These complexes displayed moderate to high ethylene polymerization activity (up
to 6600 kg PE/(mol cat)-1 h-1 bar-1), yielding low-MW branched polyethylene. The results are
summarized in Table 3; the data on the nature and content of branches are provided separately
in Table 4.
Table 3. Polymerization of ethylene over Ni10-Ni24/MAO a
Precatalyst, PE yield,
Activity
с
c,d
Complex
Mn
Nav
b
ꢀmol
g
Ni10 e
Ni11
Ni12 e
Ni13 e
Ni14
Ni15
Ni16 e
Ni17
Ni18
Ni19
Ni20
Ni21
Ni22
Ni23
Ni24
1.99
2.00
2.00
2.00
2.49
2.07
2.19
2.24
2.49
2.00
2.00
2.26
2.01
2.57
2.44
traces
traces
0.35
0.2
-
N/Df N/D
N/D N/D
N/D N/D
N/D N/D
300 21.6
310 22.2
N/D N/D
300 21.6
450 31.9
360 26.0
340 23.9
320 22.9
360 25.8
460g 32.6
460g 32.6
-
350
200
1.27
5.72
1.1
510
2800
1000
1700
3000
3200
5800
3500
6600
3000
4200
3.9
7.6
6.3
11.5
7.9
13.2
7.7
10.2
a
Reaction conditions: toluene, 50 ml; cocatalyst MAO (Al/Ni=500); reaction time 30 min;
o
b
c
P(C2H4) 2 bar; T = 35 C. In kilograms of PE·(mol Ni)-1·h-1·bar-1. Mn and Nav were
1
d
determined by H NMR spectroscopy [15,62]. Nav – average number of C in the polymer
e
f
g
chain. Reaction time 15 min. Not determined. The GPC-measured PDI values of PE
obtained over Ni23 and Ni24 were nearly identical (Mw/Mn = 1.7).
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Table 4. Branch data content for polyethylenes obtained with Ni14-Ni24 a
Branches per 1000 C
Σ branches
per 1000 C
Complex
Sec-
Methyl Ethyl Propyl Butyl
Amyl
Long
Butyl
42.0
38.7
40.4
10.7
10.0
9.0
42.4
28.8
32.1
30.9
29.3
8.5
32.3
22.3
22.2
11.6
5.6
59.1
52.1
49.4
229.0
186.7
168.1
Ni14
Ni15
Ni17
6.5
NOb
5.5
2.1
2.7
3.3
7.3
5.9
44.6
8.0
19.3
20.9
13.7
39.1
145.6
Ni18
30.4
11.8
4.9
6.5
3.8
0.1
0.5
3.3
2.2
18.7
7.0
21.5
12.9
1.4
15.9
4.1
2.7
5.1
7.3
6.4
47.5
62.7
71.9
56.7
45.0
55.3
146.0
104.4
85.9
Ni19
Ni20
Ni21
Ni22
Ni23
Ni24
2.2
12.9
38.7
37.1
7.8
14.8
9.0
101.1
124.8
127.4
14.3
12.1
8.4
aBranch content was determined by 13C NMR as reported [63,64]. bNot observed.
Based on the data obtained for Ni8 (Table 1) we have proposed that the introduction of
bulkier substituents at the o-position of the aniline ring instead of the fluorine atoms should
lead to an increase in molecular weight of the reaction products and allow us to obtain
branched polyethylene. Therefore, we first synthesized complexes Ni10-Ni13 with 2-
iminopyridine ligands containing bulky electron-donating and electron-withdrawing
substituents in the aniline ring and p-fluorophenyl substituent at the 6th position of the
pyridine ring. The resulting complexes showed moderate activity of about 100 kg PE (mol
Ni)-1 h-1 bar-1, which is consistent with previous data for the complexes with alkyl substituents
in the aniline ring and phenyl-substituted 6th position of the pyridine ring [27,29].
Keeping in mind that the introduction of trifluoromethyl group into the aniline moiety
substantially increases the activity towards olefin polymerization of different types of post-
metallocene precatalysts [42-50], further studies have been focused on 2-iminopyridine
nickel(II) complexes containing CF3 group at the ortho-position of the aniline ring (Ni14-
Ni24). Such a structural change positively affected the catalytic activity (Table 3), which was
tentatively ascribed to steric (CF3 group is rather bulky substituent compared with a fluorine
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ACCEPTED MANUSCRIPT
atom), and electronic factors (increased electrophilicity of the metal atom). In addition, we
would expect that the fluorine atoms of CF3 group may provide additional stabilization to the
active site of the catalyst and suppress the chain transfer/termination reactions through the
interactions of fluorine atoms with metal centers, as previously reported for metallocene
polymerization catalysts [65-69].
The analysis of the influence of the substituents at the 6th position of the pyridine ring
on the catalytic properties of Ni14-Ni24 in ethylene polymerization shows that the
introduction of substituted phenyl group reduces the activity of these complexes (comparing
data for Ni14, Ni18, Ni19 and Ni20, Ni15 and Ni21, respectively, see Table 3). This activity
reduction may be attributed to negative in-plane steric effect of pyridine fragment as reported
for analogous pyridine-amine nickel(II) complexes [70]. As to polymer properties, there was
no direct correlation between the Mn and the nature of R1. The number-average molecular
weights of the polyethylenes obtained using Ni14 and Ni15 (R1 = 4-FPh, Mn = 300 and 310)
were lower than molecular weights of the polymers formed over Ni20 and Ni21 (R1 = H, Mn =
340 and 320). However, in the case of Ni18, the introduction of a CN group (R1 = 3-CNPh)
resulted in the formation of polyethylene with slightly higher Mn (Mn = 450). At the same
time, the overall number of branches increased in the order Ni14 > Ni18, Ni19 > Ni20, in line
with increasing the in-plane steric bulk of the R1 substituent.
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Bsec-butyl
δB1-n
Asec-butyl
1B1
brB1
βB1
αB2
βB2
1B2
αBn
αB1 γB1
2B2
4Bn
3Bn
γBn
3Bn
4Bn
2Bn
βB1
1Bn
2Bn
αB1
1Bn
brB1
1B1
Bsec-butyl
1B2
10
αB3-n
Asec-butyl
βB2-n
1B3
2B4
35
40
30
20
25
15
ppm
13
Figure 2. C NMR spectrum of branched PE produced over homogeneous catalyst Ni23 +
MAO (Table 3, Table 4).
In addition, the electronic and steric effects of substituents in aniline ring were
evaluated. In the case of complexes Ni14-Ni17, the catalytic activity increased in the
following order: Ni14 < Ni16 < Ni17 < Ni15, demonstrating that the introduction of
additional EWG’s into the ortho- and para- positions improves the catalytic properties of the
complexes; Ni15 with 2,6-disubstituted aromatic ring displayed the highest activity in the
above series (Table 3). One can also can see that the activity increased when R6 = F (Ni16) is
substituted by R6 = NO2 (Ni17). In contrast to Ni15, the introduction of an additional F atom
reduced the activity (Ni20 vs. Ni21, Table 3). At the same time, the replacement of hydrogen
atoms at the R4 and R6 positions by bromine atoms led to a highly reactive catalyst Ni22 (its
activity value 6600 kg PE/(mol cat)-1 h-1 bar-1 is among the highest reported for ethylene
polymerization on 2-iminopyridine nickel catalysts [9,10]).
Considering the effect of substituents in the phenyl ring on the properties of the
resulting PE, it should be noted that in the series Ni14-Ni17, the Mn slightly increased when
hydrogen at the ortho position of the aniline moiety was substituted by a fluorine atom (Ni15).
Meanwhile, the introduction of electron-withdrawing substituents at the para-position did not
change the molecular weight of the polymer. Interestingly, the introduction of additional
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ACCEPTED MANUSCRIPT
substituents reduced the number of branches per 1000 carbon atoms in all cases and at the
same time favored the formation of long branches (Ni14-Ni18 and Ni20-Ni22 in Table 4).
In the Ni20-Ni22 series, the polymer obtained using Ni21 (R4 = F) displayed
somewhat lower Mn, while the molecular weight of the PE formed on Ni22 (R4, R6 = Br)
increased as compared with Ni20. Such unusual behavior may be explained by prevalent
electronic effect of small fluorine substituent. By contrast, larger atomic radius of bromine
suggests that a steric effect may be responsible for increased molecular weight in the case of
Ni22. The content of branches for Ni21 and Ni22 was also smaller than for Ni20.
Finally, we have studied the effect of substituent at the imine carbon atom on the
catalytic properties of nickel complexes and the structure of the polymer product. The
replacement of the hydrogen atom (Ni20, R2 = H) by methyl (Ni23) and phenyl (Ni24) group
led to a slight decrease in the activity. However, both the molecular weight and the number of
branches substantially increased (Tables 3 and 4). Interestingly, the molecular weight and the
number of branches of polymers obtained using Ni23 and Ni24 were very close. One could
expect that the introduction of bulky substituent at the imine carbon exerts additional
constraints to the rotation of the aniline ring around the C-N bond. We speculate that this may
cause some distortion of the complex geometry, which may result in increased steric
hindrance at the axial coordination cites of Ni. Previously, such an improvement of the axial
sites protection increased the molecular weight of the polymers in the case of similar
iminopyridine nickel complexes [28,30].
3.4. Ethylene Polymerization over Supported 2-(Imino)Pyridine Nickel Complex
To evaluate the possibility of immobilization of 2-iminopyridine nickel(II) catalysts on an
inorganic support, Ni23 was grafted on silica modified with alumina (SiO2(Al)) and the
resulting heterogeneous catalyst was tested in ethylene polymerization using different
cocatalysts (MAO, MMAO, Et2AlCl, Al(iBu)3). The polymerization activities and polymer
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characteristics are summarized in Table 5, and the data on the degree and nature of branching
are provided in Table 6.
It was found that the activity of supported complex Ni23 in the presence of Et2AlCl
reaches the value of 473 kg PE (mol of Ni)-1 h-1 bar-1 (Table 5, entry 4) at 50 °C, which is
twice higher than the activity of homogeneous Ni23 under the same experimental conditions
(Table 5, entry 2). Such difference reflects the fact that homogeneous Ni23 is less stable at
this temperature and tends to deactivate faster than the supported catalyst. At the same time,
Ni23 in the presence of MAO at 35 °C displays three times higher activity (Table 5, entry 1)
than the activity of Ni23/SiO2(Al) (Table 5, entry 3). Homogeneous system Ni23 + MAO also
exhibits higher initial activity, while the deactivation rates for Ni23 + MAO and
Ni23/SiO2(Al) + MAO differ significantly (ninefold decrease of ethylene consumption rate
within 1 h in the case of Ni23+MAO vs. only threefold decrease in the case of Ni23/SiO2(Al)
+ MAO, see Figure 3).
50
40
30
20
10
0
Ni23+MAO
Ni23/SiO2(Al)+MAO
20
30
40
50
60
10
Time, min
Figure 3. Kinetic profile of ethylene polymerization over homogeneous catalyst Ni23 + MAO
(table 5, entry 1) and supported catalyst Ni23/SiO2(Al) + MAO (table 5, entry 3).
The use of MMAO and Al(i-Bu)3 reduced the catalytic activity and broadened the
molecular weight distribution (see Figure 4). We also note that SiO2(Al)-supported Ni23
catalyst bearing ortho-trifluoromethyl group at the aniline moiety is more than one order of
magnitude more active in the presence of Al(i-Bu)3 as cocatalyst than its ortho-iPr substituted
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ACCEPTED MANUSCRIPT
counterpart (120 vs. 7.6 kg PE (mol Ni)-1 h-1 bar-1 cf. entry 6 of Table 5 and data reported
earlier [22]) under similar reaction conditions.
Ni23+Et2AlCl
1.0
Ni23/SiO2(Al)+Et2AlCl
Ni23/SiO2(Al)+MMAO
0.5
Ni23/SiO2(Al)+Al(i-Bu)3
0.0
1
2
3
4
5
6
log M
Figure 4. The influence of the cocatalysts nature on molecular weight and MWD of PE
produced over homogeneous catalyst Ni23 + Et2AlCl (table 5, entry 2) and supported catalyst
Ni23/SiO2(Al) with different cocatalysts (table 5, entry 4-6).
Table 5. Ethylene polymerization catalyzed by homogeneous Ni23 and supported
Ni23/SiO2(Al) a
с
с
d
d
Entry Catalyst precursor Cocatalyst
T, oC PE yield, g Activity b Mn
Mw
PDI c Mn
Nav
1f
2
3g
MAO
Et2AlCl
MAO
35
50
19.2
6.1
960
244
315
473
240
120
41
360
200
936
360
2.6
1.8
22
790
430
56
Ni23
30.4
98.2
59.4
60.6
Ni23
Ni23/SiO2(Al)
Ni23/SiO2(Al)
Ni23/SiO2(Al)
Ni23/SiO2(Al)
Ni23/SiO2(Al)
Ni23/SiO2(Al)
35
14.5
14.5
6.0
590 12980
320 3000
1390
830
4
Et2AlCl
MMAO
Al(i-Bu)3
Al(i-Bu)3
Al(i-Bu)3
50
9.4
22
5
50
500 11000
900 23000
850
6
7e
50
3.5
26
2450 175.1
80
0.3
370
2300
6.2
-
-
8
80-50
3.3
104
640 29000
45
1060
75.7
a Reaction conditions: heptane, 150 ml; Ni complex, 1.1 – 4.6 ꢀmol; Al/Ni=500; reaction time
1 h; P(C2H4) 10 bar.
b Kg PE·(mol Ni)-1·h-1·bar-1. с Determined by gel permeation chromatography (GPC).
1
d Nav – average number of C in the polymer chain, estimated by H NMR spectroscopy. The
GPC-measured Mn values were substantially lower than the NMR-measured Mn data, which is
typical for highly-branched polyethylenes [15].
e Reaction time 40 min.
f In toluene, 50 ml. g In toluene, 150 ml.
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Table 6. Branch content data for polyethylenes obtained in entries 1-6 and 8 of Table 5 a
Branches per 1000 C
Σ branches
per 1000 C
Entry
Complex
Sec-
Methyl
Ethyl
Propyl
Butyl
Amyl
Long
Butyl
Ni23
1
2
3
4
5
6
8
19.3
40.1
11.6
16.3
22.0
13.6
18.0
2.5
3.3
1.9
2.3
2.1
1.4
2.2
0.7
4.9
0.7
0.14
0.7
0
0.9
12.1
0
2.9
13.5
1.0
3.0
3.0
1.0
3.3
1.0
3.7
0
24.0
47.7
12.2
19.6
20.5
7.9
51.3
125.3
27.4
42.2
51.1
23.9
42.2
Ni23
Ni23/SiO2(Al)
Ni23/SiO2(Al)
Ni23/SiO2(Al)
Ni23/SiO2(Al)
Ni23/SiO2(Al)
0.2
0.8
0
0.6
2.1
0
0
0.3
1.4
17.0
a Branch content was determined by 13C NMR as reported [63,64].
The polymeric material generated by homogeneous Ni23 is a low-MW, highly
branched polyethylene. The Ni23/SiO2(Al) catalyst, in contrast to homogeneous Ni23,
produces polymer with higher molecular weight (Mw = 2300 – 29000 g mol-1), broad
molecular weight distribution (Mw/Mn = 6.2 - 45) and substantially lower degree of branching.
The GPC data for PE samples obtained with Ni23 + Et2AlCl and Ni23/SiO2(Al) + Et2AlCl are
presented in Figure 4. In contrast to the single-site behavior of homogeneous Ni23 (Mw/Mn =
1.8), the Ni23/SiO2(Al) + Et2AlCl exhibits multisite catalytic behavior (Mw/Mn = 9.4).
4. Conclusions
In this work, a series of novel 2-(imino)pyridine ligands containing electron-withdrawing
groups and their nickel(II) dihalide complexes (24 complexes) have been prepared and
characterized. Depending on the ligand structure, all nickel complexes in the presence of
MAO displayed moderate to high ethylene oligomerization or polymerization activities (up to
6600 kg (mol of Ni)-1 h-1 bar-1, which is the highest reported for ethylene polymerization on 2-
(imino)pyridine nickel complexes), affording mixtures of ethylene oligomers or highly
branched low-MW polyethylene, with 86 to 229 branches / 1000 C. The effects of steric bulk
of the substituent at the 6th position of the pyridine ring and the presence of ortho-substituents
at the aniline ring on the catalytic activity and the properties of the resulting products have
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been analyzed systematically: the molecular weight of the polymers increase with decreasing
electron-donating ability of the substituent at the pyridine ring, while the presence of fluorine-
containing EWG’s at the aniline moiety enhances the catalytic activity and reduces the degree
of branching. Grafting 2-(imino)pyridine nickel complex onto silica modified with alumina
has yielded a moderately active heterogeneous ethylene polymerization catalyst (with MAO,
MMAO, Et2AlCl, Al(i-Bu)3 as cocatalysts), demonstrating stable catalytic activity during at
least 1 hour of polymerization. The PE obtained with the supported catalyst has higher
molecular weight, broader molecular weight distribution, and drastically lower degree of
branching than the PE obtained with the corresponding homogeneous system.
Acknowledgements
The GPC and NMR measurements were supported by the Russian Academy of Sciences and
Federal Agency of Scientific Organizations (project V.44.2.1). The ligands and complexes
were synthesized with the financial support from the Russian Foundation for Basic Research
(grant 16-53-53014).
Appendix A. Supplementary data
CCDC 1470367 contain supplementary crystallographic data for this paper (complex
Ni1·2DMF). This data can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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ACCEPTED MANUSCRIPT
2-iminopyridine nickel complexes containing electron-withdrawing groups.
Activation with methylaluminoxane (MAO) and Et2AlCl.
Catalytic ethylene polymerization and oligomerization.
Highly branched low-molecular-weight polyethylenes and ethylene oligomers.