Cobalt-Β-diimine complexes for ethylene oligomerization

Article abstract of DOI:10.1016/j.catcom.2017.08.002

This work describes the synthesis of cobalt-β-diimine complexes as well as their application as ethylene oligomerization catalyst precursors. The ligands differ from each other by substituents on the aromatic rings. The catalytic precursors were tested in

Full text of DOI:10.1016/j.catcom.2017.08.002

Accepted Manuscript
Cobalt-β-diimine complexes for ethylene oligomerization
Marcela Caovilla, Daniel Thiele, Roberto F. de Souza, José R.
Gregório, Katia Bernardo-Gusmão
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DOI:
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doi: 10.1016/j.catcom.2017.08.002
CATCOM 5151
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Cobalt-β-diimine complexes for ethylene oligomerization
Marcela Caovillaa,b, Daniel Thielea,c,1, Roberto F. de Souzaa,1, José R.
Gregórioa, Katia Bernardo-Gusmãoa*
a Instituto de Química, Universidade Federal do Rio Grande do Sul – Av. Bento
Gonçalves, 9500, Agronomia, P.O. Box 15003, CEP 91501-970, Porto
Alegre/RS, Brazil
b Braskem S. A., III Polo Petroquímico, Via Oeste Lote, 5 Passo Raso, Triunfo,
Brazil
c
Instituto de Química, Universidade Federal de Alagoas - Av. Lourival Melo
Mota, s/n, Tabuleiro dos Martins, CEP 57072-970, Maceió/AL, Brazil.
1 In memoriam
* corresponding author
e-mail address: katiabg@iq.ufrgs.br (K. Bernardo-Gusmão)

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Abstract
This work describes the synthesis of cobalt-β-diimine complexes as well as their
application as ethylene oligomerization catalyst precursors. The ligands differ
from each other by substituents on the aromatic rings. The catalytic precursors
were tested in ethylene oligomerization, using EASC (Ethylaluminium
sesquichloride, Et2Al2Cl3) as co-catalyst, and toluene as solvent and displayed
with TOF in the range of 0.8 to 34 s-1. The catalytic systems were selective in
the C4-C6 fraction, attaining 100% selectivity in C4. It was observed that the
ligands nature strongly influence the catalytic activity.
Keywords: oligomerization, ethylene, -diimine, Schiff bases, cobalt

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1. Introduction
The activity and selectivity of transition metal catalysts are dependent on
the electronic nature and the steric environment of the central metal. So, the
appropriated ligand choice is important for the catalyst performance and allows
to drive the reaction to obtention of the desired products [1]. The use of a -
diimine as supporting ligand systems in both main and transition metals
coordination chemistry has recently attracted considerable attention [2-5].
These ligands have several attractive features, including the possible tunability
in both the Ar and R groups, which has a direct influence on the steric
environment and the electron demand of the bound metal [6].
-Diimine ligands became more popular since the discovery by Feldman
and co-workers of the ‘‘Brookhart type’’ nickel and palladium polymerization
catalysts [7]. They synthesized the first cationic -diimine palladium complex by
the reaction between the so-called ‘‘-iminoamine’’ and [Pd(MeCN)4][BF4]2.
Studies on transition metal complexes with β-diimine ligand have greatly
increased in the last few years, being 2007 the year with the largest number of
publications on this field. These studies were initiated by Brookhart group with
nickel and palladium catalysts [8] and developed simultaneously by Brookhart
[9] and Gibson [10] with cobalt and iron complexes.
Since 2002 a lot of works on β-diimine complexes synthesis with several
transition metals were reported in the literature. Some studies present metal
and ligand similarities with the complexes synthesized in this work [6, 11-12].
Concerning the synthesis of N,N-bidentate cobalt(II) complexes there
are, in the literature, the works reviewed by Sun and Redshaw [13] and other
studies conducted by Sun [14].
Regarding to the studies that have employed specifically cobalt(II)-β-
diimine complex as catalyst precursors, we found in the literature only one
paper, published by Nagashima [1], which reports results obtained for ethylene
polymerization.
Although the literature doesn’t present results for oligomerization using β-
diimine complexes, some studies show oligomerization reaction conditions
similar to those used in our catalytic tests specially the works from Sun [14-17].
The work that is the most similar to our study was published by Qian [18] in

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2001, in which the authors presented ethylene oligomerization catalyzed by
Co(II)-α-diimine complexes. There is also a repport from Gomes [19] concerning
the polymerization of ethylene catalized by Co(II)-α-diimine complexes.
As far as we are concerned, the present results on the synthesis of
homogeneous Co(II)-β-diimine complexes and their use in ethylene
oligomerization reaction are unprecedented.
2. Experimental
2.1 Chemicals
All experiments and distillations were performed under an atmosphere of
argon using standard Schlenk tube techniques. The solvents were
deoxygenated by refluxing over appropriate drying agents (toluene, hexane,
ethyl ether and benzene on sodium/benzophenone; dichloromethane and
acetonitrile on phosphorous pentoxide) under argon and distilled immediately
prior to use. Methanol, ethanol, were used without further purification. Celite
was activated in an oven at 100 C for 2 h. 2,6-dimethylaniline and 2,4,6-
trimethylaniline (Aldrich) were distilled prior to use. 4-methoxyaniline (Aldrich)
and 2,4-pentanodione (Aldrich) were used without further purification. Sodium
carbonate (Vetec) and thionyl chloride (Fluka) were used without further
purification. p-Toluenesulfonic acid (Vetec) was distilled on benzene by Dean
Stark technique. Cobalt(II) chloride (Vetec) was previously dried according to
methodology proposed by Pray [20]. EASC (Ethylaluminium sesquichloride,
Et2Al2Cl3) was supplied by Akzo Nobel and used with previous dilution in dry
toluene (10% v/v).
2.2 Equipments
Elemental analyses were carried out in a Perkin Elmer M CHN/O
1
Analyser, Model 2400. The H NMR spectra were performed in an Inova 300
MHz Spectrometer, samples were dissolved in deuterated chloroform.
Chromatographic analyses were performed on a Varian 3400CX Gas
Chromatograph, equipped with a Petrocol DH capillary column (methylsilicone,
100 m long, i.d. 0.25 mm, and film thickness of 0.5 μm). Analysis conditions: 36
°C (15 minutes) then heating rate of 5° C/minute until 250° C. The products
were quantified using isooctane (0.43 mmol) as internal standard.

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2.3 β-diimine ligands synthesis
Insert Fig. 1
Figure 1. Proposed structure of β-diimine ligands.
The synthesis and characterization of ligands, shown in figure 1, L1, 2-
(2,6-dimethylphenyl)amine-4-(2,6-dimethylphenyl)imine-2-pentene and L2, 2-
(2,4,6-trimethylphenyl)-amine-4-(2,4,6-trimethylphenyl)imine-2-pentene
were
previously described [2]. L3, 2-(4-methoxyphenyl)amine-4-(4-
methoxyphenyl)imine-2-pentene, was synthesized according to Budzelaar [21]
using p-toluenesulfonic acid as catalyst. In a typical reaction, in a 250 mL flask,
two equivalents of the required 4-methoxy-aniline (8.79 g, 71.4 mmol) were
added to 2,4-pentanodione (3.5 mL, 34 mmol) and p-toluenesulfonic acid (5.8 g,
34 mmol) in benzene. The suspension was refluxed for 24 h, and the water
formed was removed using a Dean-Stark. After, the solvent was removed under
reduced pressure, and to the solid formed were added 60 mL of ethyl ether and
80 mL of a satured solution of sodium carbonate. The aqueous phase was
extracted with ethyl ether (L3). The solvent was evaporated under vacuum, and
the solid was recrystallized from methanol, yielding 7.18 g (68%).
1
Characterization: H NMR (δ, ppm) 12.15 (s, 1H, N-H), 6.85 (s, 4H, m-
ArH), 4.85 (s, 1H, β-CH), 2.25 (6H, s, p-CH3Ar), 2.11 (s, 12H, o-CH3Ar), 1.68 (s,
6H, α- CH3). IR: C=N 1504 cm-1. CHN (C19H22N2O2): Calculated C = 82.59% H
= 9.04% N = 8.37%; Experimental: C = 82.68% H = 9.40 % N = 8.37%.
2.4 Cobalt(II)--diimine complexes synthesis
Insert Fig. 2
Figure 2. Proposed structure of cobalt--diimine complexes.
The proposed structures of the complexes are represented in figure 2
(based on reference [1]).
The methodology for the synthesis of the complexes, C1, dichlorido-
1,5,4-bis(2,6-dimethylphenyl)pentanediiminecobalt(II),
C2,
dichlorido-1,5-

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bis(2,4,6-trimethylphenyl)pentanediiminecobalt(II) and C3, dichlorido-1,5-bis(4-
methoxyphenyl)pentanediiminecobalt(II) as follows: in a 250 mL flask, one
equivalent of the appropriated ligand (1.6 mmol) relatively to cobalt chloride
(0.21 g, 1.6 mmol) was added to 30 mL of dry dichloromethane. The
suspensions were stirred at room temperature for 72 h. After, they were filtered
through Schlenk filter tubes and the solids were washed with THF. The solvent
was evaporated under vacuum, and the blue (C1 and C2) and green (C3) solids
recrystallized from cold hexane. Yields, after recrystallization: 650 mg (1.49
mmol, 93%), 528 mg (1.14 mmol, 71%) and 127 mg (0.29 mmol, 18%),
respectively.
C1 Characterization: IR: C=N 1665 cm-1. CHN (C21H26Cl2CoN2):
Calculated C = 57.81% H = 6.01% N = 6.42%; Experimental: C = 57.78% H =
5.77 % N = 6.22%.
1
C2 Characterization: H NMR (CDCl3, 400 MHz): 23.53, 20.86, 17,13,
9.71. Decomposes rapidly yielding increasing ligand peaks. IR: C=N 1558 cm-1.
CHN (C23H30Cl2CoN2.CH2Cl2): Calculated C = 52.48% H = 5.87% N = 5.10%;
Experimental: C = 52.90% H = 6.12 % N = 5.01%.
C3 Characterization: CHN ((C19H22Cl2CoN2O2)3.CH2Cl2): Calculated C =
49.56% H = 4.88% N = 5.98%; Experimental: C = 49.49% H = 4.70 % N =
5.95%.
Elemental analysis are consistent with the proposed constitution of the
complexes. C2 and C3 seem to crystallize with dichloromethane: while C2
seems to crystallize with a molecule of dichloromethane, each three molecules
of C3 seem to crystallize with one molecule of solvent. Unfortunately, no
crystals could be obtained for X-ray diffraction analysis. The presence of CH2Cl2
in a solid structure, although may seem unusual, was already observed by X-
ray diffraction [22] and elemental analysis [19, 22, 23].
2.5. Catalytic tests
Ethylene oligomerization experiments were carried out in a 450 mL Parr
stainless steel autoclave equipped with magnetically driven mechanical stirring,
thermocouple and a pressure gauge. The reaction temperature was kept at 30°
C with a thermostatic bath. In a typical experiment, the reactor was charged with

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a toluene solution containing the alkylaluminium cocatalyst added in amounts
calculated to obtain an aluminium to cobalt molar ratio (Al/Co) of 1000. Then,
the solution was saturated with ethylene and 2 or 6 µmol of the desired catalytic
precursor was added. The pressure was raised up to 10 atm upon addition of
ethylene and the system kept under continuous stirring. For all reactions, after
30 min the reaction was stopped, the mixture cooled to -30 °C and analyzed
immediately by gas chromatography.
3. Results and Discussions
As the goal of this study was to evaluate the effect of ligand substituent in
the catalytic activity, all complexes were investigated under the same
conditions. Reactions were carried out using 2 mol of the C1, C2, and C3. The
choice of reaction conditions was based on the literature for similar systems
based on nickel [2] with two exeptions. First, the catalyst quantities should be
low because high activities of cobalt complexes. Second, Al/Co needs to be
higher for cobalt than that for nickel precursors [24].
Table 1 exhibits the results obtained for ethylene oligomerization.
Table 1. Oligomerization ethylene results obtained using the catalyst precursors
C1-C3.a
Selectivityc
Entry Cat.
TOF (s-1)b
1B
84
76
78
t-2B
9
c-2B
7
C4
100
74
C6
0
1
2
3
C1
C2
C3
21
34
14
12
10
10
26
28
0.8
72
a
Reactional Conditions: 2 mol of catalytic precursor, 10 atm of ethylene, 60
mL of toluene, 30°C, 30 min, EASC, Al/Co = 1000
b
TOF = Turnover Frequency, mol of ethylene converted per mol of Co per
second (s-1)
c
Determined by CG. C4 = total selectivity in butenes; 1-B = 1-butene; t-2B =
trans-2-butene; c-2B = cis-2-butene; C6 = total selectivity in hexenes
(selectivity on individual isomers not determinated). No heavier products were
ever observed.

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Variations on the imino substituents resulted in marked changes in the
catalytic activity of each complex. The correlation between catalytic activity and
substituents can be explained by the metal atom net charge correlation –
MANCC. Sun and Yang [24] applied this correlation to study the catalytic
activity of iron(II) bis(imino)pyridyl and iron(II) bis(imino)pyrimidyl complexes in
ethylene oligomerization reactions. The authors concluded that the catalytic
activity was more focused on the catalyst electronic configuration, especially in
the electron density charge on the central metal atom rather than in the
structural configuration. They proposed the existence of a turning point for the
net charge versus the activity curve. Their results indicate an un-monotonous
relation of the net charge and the activity for iron(II) catalysts. This seems to
agree with what was observed in this study. The highest electron density of the
metal center induces higher catalytic activity (for catalyst precursors that follow
the oligomerization mechanism of metal-hydride). Since methoxy is a electron-
withdrawing group, it should be expected complex C3 to display the weakest
catalytic activity, which indeed happened [25]. Following this reasoning, it was
also expected that C2 complex (containing three eletron donating methyl groups
in each aromatic ring) should have catalytic activities higher than C1 complex,
which contains two substituents in each aromatic ring, what was really
observed. Thus, the catalytic activity increases in the order: R = methoxy (C3) <
2,6-dimethylphenyl (C1) < 2,4,6-trimethylphenyl (C2). This effects caused on
the activity by the substituents can be justified taking into account other factors
such as bond distances and angles, dihedral angle and steric hindrance, in
addition to full charge on the metal center. There isn’t a general model that
explains such behavior. In most of the cases what is found is a set of factors in
which each case presents its own features [26]. Within this context, it is
suggested that the catalytic activity of any complex can be affected by the
mobility of the aromatic rings of the β-diimine ligands, which hinders the access
of the substrate to the metal center. It can be suggested that the presence of
methyl groups in the aromatic rings of the complexes decrease the mobility of
the ligand and facilitate the access to the active center of the catalyst precursor,
allowing oligomerization. Figure 3 shows that the observations made with
respect to steric hindrance may be further explored.

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Insert Fig. 3
Figure 3. Chem3D representation for C1, C2 and C3 complexes.
Under the conditions studied and by EASC activation, the complexes
showed high catalytic activity in ethylene oligomerization providing high
selectivity on low molecular weight -olefins and a higher prevalence of 1-
butene. The highest selectivity for -olefins can be justified by the low
isomerizating capacity presented by cobalt [27]. This result is in agreement with
those reported in the literature for analogous cobalt complexes, which also
exhibit selectivity for the obtention of light -olefins. [15-18]. Complex C1
produced only the C4 fraction, complex C2 showed the highest activity of all
complexes studied whereas complex C3 showed the lowest activity, the
selectivity of complexes C2 and C3 remaining the same.
Comparing the results obtained in this study with others that have
employed cobalt complexes in oligomerization reactions, we can observe that
the activity values attained with Co(II)-β-diimine complexes are much higher
than those previously reported in the literature [15-18]. Besides that, we
observed that comparing Co(II)-β-diimine complexes with Ni(II)-β-diimine
complexes activity in similar conditions (10 atm, 10 °C, toluene, EASC) cobalt
complex presented higher activity (TOF = 34 s-1) than nickel ones (TOF = 2.7 s-
1) [2].
4. Conclusions
Co--diimine complexes displayed very high activities (up to 34 s-1),
which is a result ten times higher than those reported in the literature. The
complexes were also selective for the obtention of the C4-C6 fraction, reaching
100% of C4 selectivity. It was observed that the different substituents present in
the aromatic rings of β-diimine ligand strongly influence the activity of each
catalyst precursor.
Acknowledgements

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The
authors
thank
to
CAPES,
CNPq,
FAPERGS,
and
PROPESQ/UFRGS for financial support and grants. This work was also
supported by PRONEX/CNPq/FAPERGS-10/0050-6.
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Figure 1

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Figure 2

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Figure 3

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Graphic Abstract

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Highlights
 A new family of Co--diimine complexes was synthetized.
 Co--diimine complexes displayed very high activities in
ethylene oligomerization.
 Complexes produced mainly the C4-C6 fraction.
 Ligands strongly influence the activity of each catalytic
precursor.