Fluorine substitution effect in bis(imino)pyridine cobalt complex in propylene oligomerization

Article abstract of DOI:10.1016/j.cattod.2017.04.005

Two bis(imino)pyridine cobalt complexes have been synthesized, the dichlorido-(2,6-bis[1-(phenylimine)ethyl]pyridine)cobalt(II) ([Co-NNNPhH]) and the dichlorido-(2,6-bis[1-(2-trifluoromethyl-4-fluorophenylimine)ethyl]pyridine)cobalt(II) ([Co-NNNPhF]) and used as catalyst precursor in association with methylaluminoxane (MAO) as cocatalyst in propylene oligomerization. The presence of a fluorine group in [Co-NNNPhF] have been investigated and compared to those of [Co-NNNPhH] regarding the catalytic properties in propylene oligomerization and the structure of the complexes. The correlation of the catalytic properties of the cobalt complexes and their structures have been evidenced.

Full text of DOI:10.1016/j.cattod.2017.04.005

Catalysis Today xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Fluorine substitution effect in bis(imino)pyridine cobalt complex in
propylene oligomerization
Anna Paula Rodrigues Ehlert, Eduardo Miraglia Carvalho, Daniel Thiele1, Cristiano Favero,
Isabel Vicente, Katia Bernardo-Gusmão, Rafael Stieler, Roberto Fernando de Souza1,
Michèle Oberson de Souza⁎
Institute of Chemistry/Federal University of Rio Grande do Sul - UFRGS, Porto Alegre, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
Bis(imino)pyridine cobalt complex
Propylene
Two bis(imino)pyridine cobalt complexes have been synthesized, the dichlorido-(2,6-bis[1-(phenylimine)ethyl]
pyridine)cobalt(II) ([Co-NNNPhH]) and the dichlorido-(2,6-bis[1-(2-trifluoromethyl-4-fluorophenylimine)ethyl]
pyridine)cobalt(II) ([Co-NNNPhF]) and used as catalyst precursor in association with methylaluminoxane
(MAO) as cocatalyst in propylene oligomerization. The presence of a fluorine group in [Co-NNNPhF] have been
investigated and compared to those of [Co-NNNPhH] regarding the catalytic properties in propylene
oligomerization and the structure of the complexes. The correlation of the catalytic properties of the cobalt
complexes and their structures have been evidenced.
Oligomerization
1. Introduction
ethylene oligomerization [4–6]. Thiele et al. have reported that the
presence of fluorinated groups (F and/or CF3) as substituents in bis
Light olefins oligomerization is of industrial interest since the
corresponding products are widely used intermediates for the synthesis
of gasoline additives, polymers, detergents, and others [1]. Among
oligomers, the production of 1-hexene (trimer of ethylene or dimer of
propylene) represents a specific interest since it is efficiently used as co-
monomer for the production of ethylene-based co-polymers.
The employment of organometallic catalytic systems enables,
through the use of suitable ligands, selectivity tuning of catalytic
reactions [2,3]. Propylene being a dissymmetric molecule, its possible
dimers, which can be isomerized, span a wide range of products since
olefin coordination to the catalytic metal center can either occur by the
carbon 1 (C1) or 2 (C2) of propylene double bond (see Fig. 1, M-H being
a metal hydride, typically the catalytically active species in Ziegler-
Natta systems).
Thus, based on the mechanism depicted in Fig. 1, identification of
the produced dimers gives information concerning the insertion mode
of the two propylene molecules.
Organometallic metal complexes having bis(imino)pyridine ligands
are known for their efficiency in oligomerization/polymerization of
light olefins and have been the subject of numerous studies. In
particular, bis(imino)pyridine cobalt complexes associated to alkylalu-
minium compounds as co-catalysts have shown high activity in
(imino)pyridine ligands was responsible for the selective formation in
1-butene. Among these fluorinated complexes, the dichorido-(2,6-bis[1-
(2-trifluoromethyl-4-fluorophenilimine)ethyl]pyridine)cobalt(II) pre-
cursor exhibited the highest 1-butene selectivities, with an activity of
15300 h−1 and a selectivity of 95.6% in 1-butene ([Al]/[Co] = 600
and 30 °C) [6].
Based on these results, the present study aims at investigating the
catalytic properties (activity and selectivity) of the 2,6-bis[1-(2-tri-
fluoromethyl-4-fluorphenilimina)ethyl]pyridine
precursor,
[Co-
NNNPhF], in propylene oligomerization for the selective production
of 1-hexene through propylene dimerization. This would diminish the
probability to obtain branched oligomers as is the case for ethylene
trimers. For comparison, a study have also been conducted using the
dichorido-(2,6-bis[1-(phenylimine)ethyl]pyridine)cobalt(II) precursor,
named [Co-NNNPhH]. The structures of the two complexes, [Co-
NNNPhH] and [Co-NNNPhF] are respectively shown Fig. 2a and b.
The propylene oligomerization has been conducted using methylalu-
minoxane (MAO) as cocatalyst. The influence of the co-catalyst/catalyst
precursor ratio (Al/Co) and the temperature on catalytic properties
have been also evaluated (Al/Co = 100, 300 and 600; T = 10 and
30 °C). Additionally, the solid state structure of [Co-NNNPhF] has been
determined through X-ray diffraction studies and compared to those of
⁎
Corresponding author.
E-mail address: michele.souza@ufrgs.br (M.O. de Souza).
In memoriam.
1
http://dx.doi.org/10.1016/j.cattod.2017.04.005
Received 14 January 2017; Received in revised form 20 March 2017; Accepted 4 April 2017
0920-5861/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Ehlert, A.P.R., Catalysis Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.04.005

A.P.R. Ehlert et al.
Catalysis Today xxx (xxxx) xxx–xxx
Fig. 1. Dimerization of propylene mechanism.
[Co-NNNPhH] already published [7]. The differences between the two
structures are discussed with regard to their catalytic performance.
A Bruker CCD X8 Kappa APEX II diffractometer outfitted with a
graphite monochromator and Mo-Kα radiation (λ = 0.71073 Å) was
used to collect the X-ray data for the structural analysis of [Co-
NNNPhF]∙CH2Cl2. A combination of ϕ and ω scans was carried out to
obtain at least a unique data set. The crystal structure was solved using
direct methods in the SHELXS program [9]. The final structure was
refined using SHELXL [9], where the remaining atoms were located
from difference Fourier synthesis. Anisotropic displacement parameters
were applied to all non-hydrogen atoms, followed by full-matrix least-
squares refinement based on F2. All hydrogen atoms were placed in
ideal positions and refined as riding atoms with relative isotropic
displacement parameters.
2. Material and methods
2.1. Reagents
Cobalt (II) chloride (Aldrich), formic acid (Aldrich, 88%), methanol
(Aldrich, 99.8%), aniline (Aldrich, 99%), 2,6-diacetylpyridine (Aldrich,
99%), tetrahydrofuran (Aldrich, ≥99.9%), 4-fluoro-2-(trifluoromethyl)
aniline (Aldrich, 99%), toluene (Aldrich, ≥99.8%), dichloromethane
(Tedia Brazil), methylaluminoxane (Aldrich, 7 wt% aluminum in
toluene) were used as received.
2.4. Propylene oligomerization reactions
2.2. Cobalt complexes synthesis
All reagents have been transferred to the reactor under argon
atmosphere using standard Schlenk techniques. The reactions were
conducted in a 450 mL stainless steel PARR reactor, using 20 μmol of
cobalt precursor and a chosen amount of MAO as cocatalyst with a Al/
Co ratio of 100; 300 or 600, a propylene pressure of 6 atm (total
pressure) and toluene (60 mL) as solvent. The reactions were conducted
at 10 or 30 °C during 30 min.
The reaction products were analyzed by gas chromatography using
the co-injection method for the identification of the products and an
internal standard (isooctane) for their quantification.
All the syntheses have been conducted under argon atmosphere
using standard Schlenk techniques. The complexes [Co-NNNPhH] and
[Co-NNNPhF] were prepared by reacting CoCl2 [8] with 2,6-bis[1-
(phenylimine)ethyl]pyridine and 2,6-bis[1-(2-trifluoromethyl-4-fluoro-
phenylimine)ethyl]pyridine, respectively, in THF. The solution was
stirred at room temperature for 24 h. The ligands were characterized
through Nuclear Magnetic Resonance (1H and 13C NMR) analysis and
Infrared spectroscopy (IR).
2.3. Characterization of cobalt complexes
The catalytic activities were calculated in terms of turnover
frequency (TOF), selectivity in dimer and trimer (SC6 and SC9), in linear
hexene (Sn-H) and in 1-hexene
Solution-state NMR spectra were recorded on a Varian 300 operat-
ing at 75 MHz. Samples of 75 mg were solubilized in deuterated
chloroform (CDCl3) placed in 5 mm diameter NMR tubes.
(S1-H) using Eqs. (1)–(3).
Single crystals of [Co-NNNPhF]∙CH2Cl2 suitable for X-ray diffraction
studies were grown at room temperature by layering hexane into a
concentrated dichloromethane solution of [Co-NNNPhF].
mol of consumed propylene
TOF (h−1) =
(1)
mol Cobalt × time
Fig. 2. Cobalt complexes: a) Co-NNNPhH and b) Co-NNNPhF.
2

A.P.R. Ehlert et al.
Catalysis Today xxx (xxxx) xxx–xxx
Table 1
It is important to note that: i) for all the reactions employing [Co-
NNNPhF] the dimers are exclusively linear (Sn−H= 100%) and ii) the
relative amount of 1-hexene is ≥ 50%. The catalytic activity for [Co-
NNNPhF] was found to be dependent on the Al/Co ratio with a higher
activity with a Al/Co ratio of 300 (T = 10 and 30 °C). Comparing runs 7
to 10; 8–11 and 9–12, a decrease in the catalytic activity is observed
upon increasing the reaction temperature, which is certainly due to the
deactivation of the catalytic species as observed with catalyst precursor
[Co-NNNPhH]. Such a deactivation, observed for both precursors, can
be related to a non-efficient heat dissipation of the equipment used in
this study. When the reactions were conducted at 50 °C, no activity was
observed confirming that the cobalt complexes herein employed are
thermally sensitive.
Taking in account the runs conducted with the highest (Al/Co) ratio
(equal to 600), i.e. runs 3 and 6 (conducted with [Co-NNNPhH]) and
runs 9 and 12 (conducted with [Co-NNNPhF]), it seems that a large
quantity of cocatalyst is detrimental to catalytic activity. Studies
performed with nickel [10,11] and cobalt [4,6] complexes as precursors
and alkylaluminium compounds as cocatalyts (Ziegler-Natta systems)
have already shown such behaviours.
The high activity and selectivity values achieved with catalytic
systems formed from [Co-NNNPhF] highlight the potential of such
complexes. For instance, run 8 (at 10 °C) corresponds to a TOF activity
of 47300 h−1 with 1-hexene selectivity of 60% and run 11 (at 30 °C) to
activity of 2570 h−1 with 1-hexene selectivity of 70%. Comparison of
the selectivity data observed for these two experiments confirms that
the cobalt complexes do not readily mediate olefin isomerization, since
the increase of catalytic activity does not come along with a decrease of
linear and terminal olefins production as usually observed for oligo-
merization Ziegler-Natta catalytic systems [12].
Comparison of runs 1- 6 (employing [Co-NNNPhH]) with runs 7–12
(employing [Co-NNNPhF]) shows that the presence of fluorinated
groups in the ligand of the diimine cobalt complex significantly
influences the catalytic performance, i.e., activity and 1-hexene selec-
tivity. Fig. 3 illustrates the 1-hexene selectivity data for runs conducted
with [Co-NNNPhH] and [Co-NNNPhF] with Al/Co ratios of 100 and
300, and temperatures of 10 and 30 °C.
Catalytic performance of precursors [Co-NNNPhH] and [Co-NNNPhF] associated to MAO.
Run
*Precursor
T
(°C)
Al/Co
TOF
SC6 (%)
Sn-H (%)
S1-H (%)
(h−1
)
1
2
3
4
5
6
10
10
10
30
30
30
100
300
600
100
300
600
135
35
0
50
45
0
100
100
0
100
100
0
100
91
0
100
100
0
20
25
0
30
15
0
[Co-NNNPhH]
7
8
9
10
11
12
10
10
10
30
30
30
100
300
600
100
300
600
1510
47300
705
1510
2570
380
84
90
88
83
93
88
100
100
100
100
100
100
70
60
65
65
70
50
[Co-NNNPhF]
*20 μmol of Cobalt complex associated to MAO;
t = 30 min.
Ppropylene = 6 bar; Vtoluene = 20 mL;
mol Ci
mol products
SCi (%) =
× 100 with Ci = 6 or 9
(2)
(3)
moli
molofdimers
Si−H (%) =
× 100withi = nor1
3. Results and discussion
1H and 13C NMR and IR data for bis(imino)pyridine ligands and the
corresponding cobalt complexes were compared to published data [4]
confirming the syntheses of the cobalt precursors [Co-NNNPhH] and
[Co-NNNPhF].
The results of catalytic tests employing [Co-NNNPhH] and [Co-
NNNPhF] as precursors and MAO as co-catalyst are summarized in
Table 1.
The results using [Co-NNNPhH] catalyst precursor (runs 1–2, 4–5)
led in most cases to 100% of dimerization (C6), with dimers being
mainly linear (> 90%). The highest selectivity in 1-hexene (S1-
H = 30%) is obtained with an Al/Co ratio of 300 and a reaction
conducted at 30 °C. In contrast, a higher ratio (Al/Co = 600, runs 3 and
6) led to no catalytic activity. Also, increasing the reaction temperature
yielded lower activities, which is probably due to catalyst deactivation.
When [Co-NNNPhF] is used (Run 7–12) dimers and trimers were
produced (C9 selectivity values being directly drived from SC6 data,
Fig. 3 evidences that, for each catalytic system, varying the Al/Co
ratio or the reaction temperature has no significant influence on 1-
hexene selectivity. In contrast, 1-hexene selectivity is strongly enhanced
with [Co-NNNPhF] precursor, showing the presence of fluorinated
phenyl groups in [Co-NNNPhF] positively impacts catalytic perfor-
mance. In this context, it is interesting to verify the influence of the
fluorinated groups in the structure of the two complexes.
X-ray diffraction analysis of a monocrystal of [Co-NNNPhF] allowed
the determination of its solid state molecular structure. The main
crystallographic data and structure refinement parameters are reported
S
C9 = 100 − SC6). Though trimers are produced as minor components,
the proportion of dimers is higher than 80% in all cases and all catalytic
activities are much higher (see run 7–12: TOF = 380–47300 h−1) than
those with precusor [Co-NNNPhH] (see run 1–6: TOF = 35–135 h−1).
Fig. 3. Selectivity in 1-hexene (1-H) with [Co-NNNPhH] and [Co-NNNPhF] precusors as a function of the MAO/Co ratio (100 and 300) and reaction temperature (10 and 30 °C).
3

A.P.R. Ehlert et al.
Catalysis Today xxx (xxxx) xxx–xxx
Table 2
in Table 2. Selected bond lengths and angles are listed in Table 3.
A molecule of the diimine ligand chelates the central cobalt atom
and two chlorine atoms supplement its coordination sphere. The
molecule of the ligand with the atom of Cl (2) forms the base of the
pyramid, with the Cl (1) atom occupying the apex. The Co-N (pyridine)
distance [2.053(5) Å] in compound [Co-NNNPhF] is longer than that in
[Co-NNNPhH] [2.027 Å] (see Table 3). The Co-N(imino) bond length in
[Co-NNNPhF] [2.164(6) Å] is shorter than that in the reported
compound [Co-NNNPhH] [2.216 Å] [7], which likely reflects the
presence of electrons withdrawing fluorine substituents in [Co-
NNNPhF] rendering the imino donors harder.
Crystallographic data and structure refinement parameters for [Co-NNNPhF].
Molecular Formula
C24H15Cl2CoF8N3∙CH2Cl2
Fw [g mol−1
T [K]
Crystal system
Space group
a [Å]
]
700.13
293(2)
Orthorhombic
Pca21
15.8695(13)
10.1761(9)
17.3243(13)
90
90
90
2797.7(4)
4
1.662
1.069
1396
0.26 × 0.05 × 0.03
3.088 to 28.395
−21 ≤ h ≤ 21; −13 ≤ k ≤ 13;
−23 ≤ l ≤ 23
82878
6988 [0.1600]
99.8
6988/1/361
Gaussian
0.6058 and 0.7457
0.0506
0.1080
0.1027
0.1336
b [Å]
c [Å]
α [°]
β [°]
The crystal structure of [Co-NNNPhH] published by X.-G. Li et al.
[7] and that of [Co-NNNPhF] obtained in the present study are
represented in Fig. 4 a) and b) respectively.
γ [°]
V [Å3]
Z
ρcalc [g cm−3
]
The geometric and electronic differences observed in [Co-NNNPhH]
vs. [Co-NNNPhF] and attributed to the presence or absence of fluori-
nated groups on the ligand may be related to the catalytic results. In the
case of [Co-NNNPhF], the much higher activities and 1-hexene
selectivity when compared to [Co-NNNPhH] can be due to: i) the
fluorine electron withdrawing effect accelerates propylene dimerization
as the coordination/insertion paths are promoted by a more electro-
philic Co center; ii) the analysis of the bond angles and the calculation
of the structural parameter suggests a slightly distorted square pyr-
amidal base geometry in which the cobalt is located [13] can direct the
insertion mode of the two propylene molecules (C1-C2, see mechanism
Fig. 1) to form 1-hexene with high selectivity (higher than 60%).
The better stability of the olefin-Co intermediate species in the
presence of fluorinated substituents is also illustrated by the fact that
trimers are formed in larger amount with [Co-NNNPhF] vs. [Co-
NNNPhH] (60 vs. 20% respectively; see Table 1).
μ [mm−1
F(000)
]
Crystal size [mm]
θ range [°]
Limiting indices (h, k, l)
Reflections collected
Reflections unique [Rint
Completeness to θmax [%]
Data/restraints/param.
Absorption correction
Min. and max. transmission
R1 [I > 2σ(I)][a]
]
wR2 [I > 2σ(I)][a]
R1 (all data)[a]
wR2 (all data)[a]
S on F2[a]
1.060
0.552 and −0.610
Largest diff. peak and hole
(e Å−3
)
As defined by the SHELXL program [9].
It reveals that the central atom of Cobalt (II) is pentacoordinated, with a slightly distorted
square pyramidal base coordination geometry (Fig. 4b) [13].
4. Conclusion
This study investigated the effect of fluorinated groups in the
chelating ligand of bis(imino)pyridine cobalt complexes on propylene
oligomerization performance when associated to MAO as co-catalyst.
[Co-NNNPhH] and [Co-NNNPhF] have been synthesized and their
catalytic properties compared. The structure of [Co-NNNPhF] has been
obtained and compared to that of [Co-NNNPhH] [5]. Structural
comparison of the cobalt complexes and their catalytic properties
allowed a structure/catalytic activity correlation. It has been shown
that the cobalt complex [Co-NNNPhF], which contains more electrons
withdrawing fluor-substituted phenyl groups, is highly active in
propylene oligomerization (47000 h−1) and highly selective in 1-
hexene (60%).
Table 3
Selected bond lengths (Å) and angles (°) for [Co-NNNPhF].
Bond lengths
Bond angles
CoeN(1)
CoeN(2)
CoeN(3)
CoeCl(1)
CoeCl(2)
2.053(5)
2.164(6)
2.164(6)
2.300(2)
2.238(2)
N(1)eCoeCl(2)
N(1)eCoeCl(1)
N(3)eCoeN(2)
N(3)eCoeCl(1)
Cl(2)eCoeCl(1)
154.35(17)
96.84(16)
144.9(2)
102.44(16)
108.80(9)
Fig. 4. Spatial representation of the structure of [Co-NNNPhH] and [Co-NNNPhF] obtained from literature [7] and the present study respectively.
4

A.P.R. Ehlert et al.
Catalysis Today xxx (xxxx) xxx–xxx
Materials and Manufacture, in: John Wiley & Sons Inc (Ed.), John Wiley & Sons Inc.,
1980, 2017.
Acknowledgements
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The authors acknowledge UFRGS for infrastructure, CNPq and
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like to dedicate this work to Roberto Fernando de Souza and his PhD
student, Daniel Thiele, who started to study bis(imino)pyridine cobalt
complexes in ethylene oligomerization at the Laboratory of Reactivity
and Catalysis (Institute of Chemistry − UFRGS − Brazil).
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.cattod.2017.04.005.
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