Chromium complexes bearing pyrazolyl-imine-phenoxy/pyrrolide ligands: Synthesis, characterization, and use in ethylene oligomerization

Article abstract of DOI:10.1016/j.catcom.2016.08.007

Chromium(III) complexes {NNZ}CrCl₂(THF) (2a-d) (NNZ = pyrazolyl-imine-phenoxy/pyrrolide) have been synthesized and characterized by elemental analysis and X-ray diffraction analysis. Upon activation with methylaluminoxane (MAO), chromium precatalysts 2a-b showed moderate activity in ethylene oligomerization (TOF = 10.9 and 14.5 × 10³ (mol ethylene)(mol Cr)^{? 1}·h^{? 1} at 80 °C, respectively), producing mostly oligomers (78.4–85.7 wt% of total products). On the other hand, under identical oligomerization conditions, 2c-d/MAO behaved as a polymerization catalysts generating predominantly polyethylene (76.6 and 86.1 wt% of the total amount of products, respectively). Under optimized conditions, precatalyst 2a led to TOF = 71,500 mol(C₂H₄)?(mol(Cr)^{? 1} h^{? 1} and 95.6 wt% of oligomers.

Full text of DOI:10.1016/j.catcom.2016.08.007

Catalysis Communications 86 (2016) 77–81
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Catalysis Communications
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Short communication
Chromium complexes bearing pyrazolyl-imine-phenoxy/pyrrolide
ligands: Synthesis, characterization, and use in ethylene oligomerization
Adão L. Bergamo, Halana K. Da Cas, Raoni S. Rambo, Cristiane S. Schwalm, Adriana C.A. Casagrande,
Rafael Stieler, Osvaldo L. Casagrande Jr. ⁎
Laboratory of Molecular Catalysis, Institute of Chemistry, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Porto Alegre, RS 91501-970, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 June 2016
Received in revised form 3 August 2016
Accepted 4 August 2016
Available online 05 August 2016
Chromium(III) complexes {NNZ}CrCl (THF) (2a-d) (NNZ = pyrazolyl-imine-phenoxy/pyrrolide) have been syn-
thesized and characterized by elemental analysis and X-ray diffraction analysis. Upon activation with
2
methylaluminoxane (MAO), chromium precatalysts 2a-b showed moderate activity in ethylene oligomerization
3
−1 −1
(
TOF = 10.9 and 14.5 × 10 (mol ethylene)(mol Cr) ·h at 80 °C, respectively), producing mostly oligomers
(78.4–85.7 wt% of total products). On the other hand, under identical oligomerization conditions, 2c-d/MAO
behaved as a polymerization catalysts generating predominantly polyethylene (76.6 and 86.1 wt% of
Keywords:
the total amount of products, respectively). Under optimized conditions, precatalyst 2a led to TOF =
Pyrazolyl-imine, anionic tridentate ligands
Chromium catalysts
1
−1
71,500 mol(C H
2 4
)⋅(mol(Cr)−
h
and 95.6 wt% of oligomers.
Ethylene oligomerization
Molecular structure
© 2016 Elsevier B.V. All rights reserved.
Ethylene polymerization
1
. Introduction
The oligomerization of ethylene is one of the most important indus-
the synthesis of chromium complexes bearing sterically crowded
naphthoxy-imine ligands; upon activation with MAO, high catalytic
activities of up to 23,730 kg·mol ·h , and high selectivity for linear
α-olefins could be achieved [8].
−
1
−1
trial processes to obtain linear α-olefins (LAOs), which are important
commodity chemical for preparing detergents, lubricants, plasticizers,
and oil field chemicals, or used as co-monomers for the production of
linear low-density polyethylene (LLDPE) [1]. Among all the transition-
metal-based oligomerization catalysts, chromium catalysts have re-
ceived special attention since they provide both selective and nonselec-
tive ethylene oligomerization [2]. In this context, a number of selective
homogeneous catalysts have been used in commercially viable tri-
and tetramerization catalytic systems (Phillips [3], Mitsubishi [4], BP
Chemicals [5], Sasol [6]). In addition, a considerable amount of research
effort has been dedicated to nonselective ethylene oligomerization with
the aim of improving the understanding of this industrially relevant cat-
alytic process [7–8].
Duchateau and co-workers have shown that activation of Cr(III)
complex bearing dipyrrole-based ancillary ligand with MAO in
methylcyclohexane led to a highly active oligomerization system with
production of a statistical distribution of oligomers [14]. Gibson et al.
showed that Cr(III) complexes based on tridentate bulky ortho-
triptycenyl substituted salicylaldimines containing pendant pyridyl
methyl or quinolyl donors are thermally robust catalysts which
oligomerize ethylene to give predominantly linear α-olefins [15]. We
recently communicated on tridentate pyrrolide-imino-amine/ether
complexes of Cr(III), which the steric effects in the pyrrole moiety
play a pronounced role in controlling the oligomer/PE ratio while tuning
of electronic effects, using either a phenylamine or a phenoxy unit, has a
higher impact on the catalytic activity [16].
Over the past seventeen years, several classes of pyrazolyl-based li-
gands have been used to generate efficient catalysts for oligo- and poly-
merization of ethylene [17–19]. As an extension of our studies focused
on the synthesis and use of pyrazolyl-based metal complexes in ethyl-
ene oligomerization, we present here the synthesis and characterization
of chromium (III) complexes bearing pyrazolyl-imine-phenoxy/
pyrrolide ligands, and their application in ethylene oligomerization.
Our aims were also to evaluate the role of the ligand, and the experi-
mental parameters on the activity and on the product distribution.
In the course of the past decade, several well-defined chromium
catalysts bearing NˆNˆN [9], PˆNˆP [10], SˆNˆS [11], PˆN [12],and NˆNˆP
[
13], ligands have been reported and their ethylene oligomerization
behavior has been investigated. While, the vast majority of these
ligands are used in the neutral form, just a few examples of anionic
ligands have been utilized in the synthesis of active chromium
species (Chart 1). [8] [14–16] For instance, Carpentier et al. described
⁎
Corresponding author.
E-mail address: osvaldo.casagrande@ufrgs.br (O.L. Casagrande).
http://dx.doi.org/10.1016/j.catcom.2016.08.007
566-7367/© 2016 Elsevier B.V. All rights reserved.
1

78
A.L. Bergamo et al. / Catalysis Communications 86 (2016) 77–81
Chart 1. Examples of chromium complexes based on anionic ligands applied in ethylene oligo- and polymerization.
2
. Results and discussion
(HC_N), with the corresponding 13C NMR resonances for the carbons
of the imine moieties at ca. d 168–160 ppm. Single crystals of 1a suitable
for crystal X-ray diffraction analysis were obtained by slow evaporation
from pentane solution. The molecular geometry and atom-labeling
scheme is shown in Fig. 1.
2
.1. Synthesis of chromium complexes bearing pyrazolyl-imine-phenoxy/
pyrrolide ligands
The monoproteo pyrazolyl-imine-phenoxy/pyrrolide proligands
were readily synthesized by Schiff base condensations between the
pyrazolyl amines and the corresponding aldehyde. The identity of this
The molecular structure of 1a shows that the geometry around the
C_N bond is essentially co-planar, with phenoxy unit trans to the
pyrazolyl moiety. Intramolecular hydrogen bonding occurs between
the phenol proton and the nitrogen of the nitrogen of the imine group.
The distance between N(1)…H-O(1) (1.847 Å) is comparable to that
1
13
class of proligands was established by elemental analysis, H and
C
NMR spectroscopy, and by an X-ray diffraction study for 1a.
1
The H NMR spectra of 1a-d in CDCl
3
at room temperature exhibit
one in {3,5-dimethyl-1-pyrazolyl-C
(OH)C } [20].
2 4
H -(N_CH)-2,4-tertbutyl-2-
resonances in the region 8.05–7.61 ppm assigned to the imine proton
6 2
H
Fig. 1. Molecular structure of 1a (thermal ellipsoids drawn at 50% probability level).

A.L. Bergamo et al. / Catalysis Communications 86 (2016) 77–81
79
2
Scheme 1. Synthesis of {NNZ}CrCl (THF) complexes.
R
Treatment of 1a-d with 1 equiv of KH and then CrCl
the corresponding chromium complexes {NNZ}CrCl
3
(THF)
3
yielded
based chromium complexes { NNO}CrCl
pyridine) [8,15].
2
(L) (L = acetonitrile, THF,
2
(THF) (2a-d),
which were isolated as yellow and brown solids in moderate to good
yields (66–73%) (Scheme 1). All Cr(III) complexes show moderate solu-
bility in common solvents such as ether, dichloromethane and THF, and
are readily soluble in acetonitrile. The elemental analyses of complexes
2.2. Ethylene oligomerization studies
Pre-catalysts 2a-d were tested in ethylene oligomerization in tolu-
ene at 80 °C, and 20 bar constant ethylene pressure, and using
methylaluminoxane (MAO) as cocatalyst. Representative data are sum-
marized in Table 1. Upon activation with MAO, all the chromium com-
plexes generated active systems affording a nonselective distribution
of α-olefins with turnover frequencies (TOFs) in the range of 6800–
2
a-d are consistent with one molecule of THF occupying chromium's
sixth coordination site.
Single crystals of 3a suitable for X-ray analysis were obtained from
an attempt to grow crystals of 2a from an ether solution at room tem-
perature. The solid state molecular structure of complex 3a features mo-
nomeric molecule with a chromium center in a slightly distorted
octahedral geometry (Fig. 2).
−
1
−1
25,700 mol(ethylene)·mol(Cr) ·h
along with the production of
varied amounts of polymers (14.3–86.1 wt%). Complex 2d was found
to be the most active catalyst as can be seen in Fig. 3.
2
The complex 3a cocrystallizes with Et O in the monoclinic system
3
with the C2/c space group. The chromium center bears κ -chelating
N,N,O-pyrazolyl-imino-phenoxy ligand in a tridentate meridional fash-
ion, with the two chloride ligands occupying mutually trans positions
and the water molecule coordinated trans to the imino nitrogen
donor. The Cl(1)-Cr-Cl(2), N(1)-Cr-O(1) and N(3)-Cr-O(2) angles are
The oligomerization reactions performed using 2a,b/MAO catalytic
systems afforded mostly oligomers (78.4–85.7 wt% of total products) in-
dicating that the presence of the mesityl group at 3- or 5-position of the
pyrazolyl ring has no significant influence on the product distribution.
The similar catalytic performances of these precatalysts may be tenta-
tively associated with the possible disassociation of the pyrazolyl
group from the chromium metal center during the oligomerization
reaction.
1
75.34(3)°, 176.64(8)°, and 176.74(9)°, respectively. The structure fea-
tures Cr\\Ophenolate (1.8925(17) Å) and the Cr\\Nimine (2.017(19) Å)
bond distances similar to the structurally characterized phenoxy-
Fig. 2. Molecular structure of 3a (thermal ellipsoids drawn at 50% probability level).

80
A.L. Bergamo et al. / Catalysis Communications 86 (2016) 77–81
Table 1
a
Ethylene oligomerization with 2a-d/MAO systems.
Oligomer distribution (wt.%)c
TOFb (10 ·h−1)
3
C
(α-C
C
(α-C
C
C
10 (α-C10
)
C
12+
Olig. (wt.%)
PE (wt.%)
Total product (mg)
Entry
Cat
[Al/Cr]
4
4
)
6
6
)
8
8
(α-C )
1
2
3
4
2a
2b
2c
2d
2d
2a
2a
2a
2a
2a
2a
2a
300
300
300
300
300
500
1000
1500
500
500
500
500
14.5
10.9
6.80
25.7
6.50
36.7
71.5
72.9
32.9
19.2
35.5
21.3
20.5 (81.1)
17.5 (81.4)
20.3 (86.5)
5.1 (96.0)
20.9 (92.4)
18.9 (90.9)
20.1 (88.2)
9.6 (98.7)
18.2 (92.4)
17.2 (90.3)
19.6 (79.1)
12.7 (93.1)
14.4 (85.5)
17.8 (94.6)
17.8 (95.4)
17.5 (95.4)
17.0 (92.8)
17.6 (89.5)
17.3 (90.0)
17.7 (96.6)
14.1 (91.8)
13.9 (89.6)
13.3 (85.6)
13.2 (97.9)
13.4 (98.4)
14.4 (95.1)
14.7 (94.0)
14.9 (94.0)
14.4 (92.2)
14.2 (88.9)
13.7 (91.1)
14.9 (95.3)
26.3
32.5
26.7
59.4
55.7
31.1
32.4
36.2
35.8
29.9
30.2
34.3
85.7
78.4
13.9
23.4
31.5
92.1
95.6
99.4
98.3
98.5
97.6
89.2
14.3
21.6
86.1
76.6
68.5
7.90
4.40
0.60
1.70
1.50
2.40
10.8
1440
970
3460
7450
1460
2790
5230
5140
2350
1370
850
d
5
6.6 (95.0)
9.9 (98.0)
6
7
8
17.2 (85.9)
16.4 (88.9)
13.9 (88.9)
15.5 (83.3)
19.7 (79.2)
19.9 (73.7)
15.0 (89.8)
19.5 (95.5)
18.7 (96.3)
17.5 (96.3)
17.3 (93.8)
18.6 (90.9)
18.9 (89.6)
18.1 (97.6)
e
9
1
1
1
0f
1
2
g
h
3340
a
Reaction conditions unless specified otherwise: toluene = 40 mL, [Cr] = 10 μmol, oligomerization time = 15 min, P(ethylene) = 20 bar (kept constant), T = 80 °C, MAO. The results
shown are representative of at least duplicated experiments, yielding reproducible results within ± 5%.
b
c
Mol of ethylene converted (mol of Cr)−1·h as determined by quantitative GLC.
, percentage of olefin with n carbon atoms in oligomers; α-C , percentage of terminal alkene in the C
00 equiv of MAO + 100 equiv of TMA.
T = 100 °C.
T = 120 °C.
−1
C
2
n
n
n
fraction, as determined by quantitative GLC.
d
e
f
g
h
Time = 5 min.
Time = 30 min.
Quite different results have been found for similar chromium com-
pyrrolide-imine complexes bearing amine/ether pendant groups
where the oligomer distribution is centered on lighter α-olefins frac-
tions with considerable amount of 1-butene (61.6–65.7 wt%) [16].
This distinct result indicates that such a minor difference in the ligand
structure has a significant impact on the direction of the catalytic
process.
Particularly for 2a-c, the K values (0.65–0.69) calculated from a plot
of log [mol%] vs carbon number (see Fig. S1) indicate that the distribu-
tion deviate little from Schulz-Flory behavior. In this case, we speculate
that these precatalysts operate by a linear chain growth alkene insertion
mechanism (Cossee mechanism) [21]. On the other hand, the higher K
value (1.04) calculated for 2d suggests that this precatalyst operates
via a different chain growth mechanism involving the formation of
metallacycles. [22].
In an attempt to decrease the amount of heavier α-olefins fractions
and polymer, an oligomerization reaction was performed using a larger
amount of TMA. However, the activation of 2d with MAO(200 equiv)/
TMA(100 equiv) resulted in a similar distribution of lighter α-olefins
fractions with a slightly decrease of C12+ and polymer along with a sub-
stantial decreasing of the TOF (compare entries 4 and 5).
plexes [Cr{L-C
,5-dimethyl-pyrazol, 3,5-diphenyl-pyrazol) complexes where upon acti-
vation with EtAlCl led to the exclusive formation of polymerization sys-
2 4 6 2 2
H -(N_CH)-3,5-tert-butyl-2-(OH)C H }Cl ] (L = pyrazol,
3
2
tems with activities in the range of 34–122 kg of PE/mol[Cr]⋅h with
production of high-density polyethylene [17]. These contrasting results
might be associate to the type of cocatalyst used to generate the active
catalytic species.
The replacement of the phenoxy by naphthoxy or pyrrolide moiety
promotes a significant impact on the activity and product distribution.
Thus, the activation of 2c,d with MAO lead to a primarily polymerization
systems with activities of 1191 and 2282 kg of PE/mol[Cr]⋅h, respective-
ly, and the amount of oligomers attained only 13.9–23.4 wt% of the total
of products. We assume that this contrasting behavior can likely be as-
sociated to the steric and/or electronic influence of these groups on
the nature of the active species and the oxidation state of chromium
metal center as already observed in similar chromium complexes bear-
ing monoanionic pyrrolide–imino-amine/ether tridentate ligands [16].
With regards to product distribution within the liquid fraction, chro-
+
mium complexes 2a-c produced oligomers ranging from C
4
to C12 with a
similar distribution for α-olefins as presented in Fig. 4. On the other
hand, 2d showed an oligomer distribution centered on heavier α-ole-
fins fractions. Contrasting results were found for related chromium
The DSC curves of the polymers produced by 2a and 2c showed only
one melting temperature peak at 128.9 and 135.9 °C, respectively sug-
gesting mostly the formation of high-density polyethylene (HDPE)
(
Figs. S6 and S8). Conversely, the polymers produced by 2b and 2d
displayed multiples melting temperatures in the range of 71.3–123.4 °
C (Figs. S7 and S9) suggesting the production of linear oligoethylenes
with low molecular weight [8,15].
To obtain further insights into the catalytic performance of this class
of complexes, oligomerization experiments were carried out with 2a
under different oligomerization conditions (entries 6–11). Activation
of 2a with other Lewis acid cocatalysts, including EASC and TMA pro-
duced neither oligomers nor polymer in significant amounts, presum-
ably due to the low Lewis acidity of these compounds.
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
2
2
1
1
5
0
5
0
Increasing the [Al]/[Cr] molar ratio from 300 to 1000 equiv caused an
increase in the activity by a factor of 5.0. However, the use of higher
MAO loading (1500 equiv) did not significantly affect the TOF
−
1
−1
[
72,900 mol(ethylene)·mol(Cr) ·h ]. Thus, the optimum [Al]/[Cr]
molar ratio was found to be 1000. At same time, the use of higher
MAO loading resulted in a slightly decrease in the lighter α-olefins
such as 1-butene (16.6 to 12.3 wt%) and 1-hexene (19.3 to 16.8 wt%),
5
2a
2b
2c
2d
+
and an increase of heavier fraction C12 (26.3 to 36.2 wt%). Furthermore,
Fig. 3. Influence of the nature of chromium precatalyst 2a-d on TOF and selectivity for
oligomers (T = 80 °C, 20 bar, time = 15 min, [Al]/[Cr] = 300).
a higher amount of MAO led almost exclusively to the production of

A.L. Bergamo et al. / Catalysis Communications 86 (2016) 77–81
81
5
0
pyrazolyl-imine-naphthoxy/pyrrolide ligands (2c,d) led to a primarily
polymerization systems. Moreover, in the most cases the ligand struc-
ture does not affect the selectivity towards production of α-olefins,
which follows Schulz-Flory rules. Oligomerization studies involving 2a
showed that the use of higher MAO loading positively affected the cat-
alytic performance of this chromium precatalyst resulting in almost ex-
clusively production of oligomers.
α
α
α
α
-C4
-C6
-C8
-C10
40
C12+
30
20
Acknowledgements
This work was supported in part by the Petrobras S/A
(50.0060209.10.9). A. L. Bergamo, and H. K. Da Cas gratefully
acknowledge CNPq for fellowships.
10
0
Appendix A. Supplementary data
2
a
2b
2c
2d
Fig. 4. Oligomer distribution of 2a-d/MAO oligomerization systems (80 °C, 20 bar,
Al/[Cr] = 300).
Crystal data and structure refinement for 1a and 3a are summarized
in Table S1. Supplementary Fig. S1 shows a plot of the log(mol%) of olig-
omer produced by 2a-d/MAO oligomerization systems versus carbon
number. Figs. S2–S4 represent the typical GLC analyses of oligomeriza-
tion mixtures. Figs S5–S9 show the DSC traces of polymers. Supplemen-
tary data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.catcom.2016.08.007.
[
oligomers (99.4 wt% of total products, entry 7). This result possibly
arises from the presence of TMA in the medium that promotes the for-
mation of larger amount of Cr(II)species, which have been assigned to
be responsible for the ethylene nonselective oligomerization process
[23].
The optimal operating temperature for this class of precatalysts
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