Oligomerization of ethylene using tridentate nickel catalysts bearing ether-pyrazol ligands with pendant O- and S-donor groups

Article abstract of DOI:10.1016/j.catcom.2011.09.006

A series of tridentate pyrazol ligands with pendant O- and S-donor groups (L1-L5) and their corresponding nickel(II) chloride complexes (1-5) were synthesized and fully characterized. All nickel precatalysts, activated with methylaluminoxane (MAO), exhibited high activities for ethylene oligomerization [TOF = 17.1-75.8 × 10³ mol(ethylene)(mol(Ni))^{- 1} h^{- 1})] with good selectivities for 1-butene produced (62.9 - 81.3%). The catalytic performance was substantially affected by the ligand environment regarding the pendant oxygen- and sulfur-donor groups, and the substituents on the phenol group. Activation of nickel precatalyst 1 with ethylaluminum sesquichloride (Et₃Al₂Cl₃, EASC) instead of MAO produced a significantly better catalyst system than 1/MAO (TOF = 227,000 vs. 30,900 (mol C₂H₄)·(mol Ni^{- 1} h ^{- 1}); however, the 1-butene selectivity was drastically reduced, attaining only 28% with a concomitant production of larger amounts of internal butenes (51.4%) and hexenes (28.2%). Under optimized conditions ([Ni] = 10μmol, 30°C, oligomerization time = 20 min, 40 bar ethylene, [Al]/[Ni] = 250), precatalyst 1 led to TOF = 75,800 (mol C₂H₄) ·(mol Ni^{- 1} h^{- 1}) and 76.1% selectivity for 1-butene.

Full text of DOI:10.1016/j.catcom.2011.09.006

Catalysis Communications 16 (2011) 245–249
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Oligomerization of ethylene using tridentate nickel catalysts bearing ether-pyrazol
ligands with pendant O- and S-donor groups
⁎
Ana H.D.P.S. Ulbrich, Adão L. Bergamo, Osvaldo de L. Casagrande Jr.
Brazilian Advanced Center For Olefin Catalysts - Laboratory of Molecular Catalysis, Instituto de Química, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, 9500, RS,
90501–970, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2011
Received in revised form 26 August 2011
Accepted 10 September 2011
Available online 17 September 2011
A series of tridentate pyrazol ligands with pendant O- and S-donor groups (L1-L5) and their corresponding nickel
(II) chloride complexes (1–5) were synthesized and fully characterized. All nickel precatalysts, activated with
methylaluminoxane (MAO), exhibited high activities for ethylene oligomerization [TOF=17.1–75.8×103 mol
(ethylene)(mol(Ni))−1 h−1)] with good selectivities for 1-butene produced (62.9 – 81.3%). The catalytic perfor-
mance was substantially affected by the ligand environment regarding the pendant oxygen- and sulfur-donor
groups, and the substituents on the phenol group. Activation of nickel precatalyst 1 with ethylaluminum ses-
quichloride (Et3Al2Cl3, EASC) instead of MAO produced a significantly better catalyst system than 1/MAO
(TOF=227,000 vs. 30,900 (mol C2H4)⋅(mol Ni−1 h−1); however, the 1-butene selectivity was drastically re-
duced, attaining only 28% with a concomitant production of larger amounts of internal butenes (51.4%) and hex-
enes (28.2%). Under optimized conditions ([Ni]=10μmol, 30°C, oligomerization time=20 min, 40 bar ethylene,
[Al]/[Ni]=250), precatalyst 1 led to TOF=75,800 (mol C2H4)⋅(mol Ni−1 h−1) and 76.1% selectivity for 1-
butene.
Keywords:
Ni(II) catalysts
Tridentate ligands
Oligomerization
1-butene
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
By changing from the six-membered chelate ring (A in Fig. 1) to the
five-membered chelate ring (B in Fig. 1) of the bis(pyrazol) ligand, the
The oligomerization of ethylene is one of the most important
industrial processes to obtain linear α-olefins (LAOs) [1–5]. These sub-
strates have been extensively used for preparing detergents, lubricants,
plasticizers, and oil field chemicals or as monomers for copolymers,
etc.[6–7] Among classes of catalysts used for the production of α-olefins,
nickel complexes containing P,P-,[8–11] P,N-,[12–24] P,O-,[25–32]
N,N-,[33–42] or N,O-[43–50] as bidentate chelating ligands are the
most frequently studied. More recently, nickel complexes bearing
tridentate ligands have attracted much interest owing to their good to
excellent performance towards the production of α-olefins [51–69]. In
comparison with bidentate complexes, the scope of tridentate nickel
complexes are still at the exploratory stage; moreover, the affective fac-
tors of the ligands on their catalytic behaviors are still arguable.
We have previously reported on the use of 5- and 6-membered-ring
nickel complexes based on tridentate pyrazolyl ligands [70–73] as highly
selective and active systems for ethylene oligomerization when activated
with alkylaluminum (MAO or AlEt2Cl). The nickel complexes based on
tridentate nitrogen- or oxygen-bridged bis(pyrazolyl) ligands (A in
Fig. 1) proved to be highly selective and efficient precatalysts for ethyl-
ene dimerization in the presence of methylaluminoxane (MAO) [71].
Substitution of the central nitrogen- or oxygen-donor by a sulfur atom
promoted higher catalytic activities.
activity increases, probably due to the formation of a more rigid and sta-
ble structure imposed by this class of ligands. Activation of these nickel
complexes with diethylaluminumchloride (DEAC) instead of MAO pro-
duced, in general, much more active systems [TOFs varying from 33,900
to 91,000(mol C2H4)⋅(mol Ni−1 h−1)], along with lower selectivity for
1-butene (72.0–81.3%) and concomitant production of larger amounts
of butenes (18.7–25.7%) [72]. This observation, which is in striking con-
trast with the aforementioned marked difference with the correspond-
ing MAO-activated systems, could reflect better stabilization of the
active species with DEAC than with MAO, possibly thanks to the chlo-
rine atom.
Herein, we wish to report the synthesis and characterization of sev-
eral new nickel(II) complexes bearing tridentate ether-pyrazoyl ligands
with pendant oxygen- and sulfur-donor groups (C in Fig. 1), as well as
their catalytic behaviors for ethylene oligomerization. We also discuss
the effect of substituents and reaction conditions on their activity and
selectivity towards the production of 1-butene.
2. Experimental section
2.1. General procedures
All manipulations were performed using a standard vacuum line
and Schlenk techniques under a purified argon atmosphere. All sol-
vents were distilled from the appropriate drying agents under argon
⁎
Corresponding author. Tel.: +55 51 3308 6300; fax: +55 51 3308 7304.
E-mail address: osvaldo.casagrande@ufrgs.br (O.L. Casagrande).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.09.006

246
A.H.D.P.S. Ulbrich et al. / Catalysis Communications 16 (2011) 245–249
10 μmol) was injected into the reactor under a stream of ethylene
and then the reactor was immediately pressurized. Ethylene was con-
tinuously fed in order to maintain the desired ethylene pressure. After
20 min, the reaction was stopped by cooling the system to −60 °C
and depressurizing. An exact amount of cyclohexane was introduced
(as an internal standard) and the mixture was analyzed by quantita-
tive GLC.
3. Results and discussion
The tridentate ether-pyrazol ligands (L1–L5) were prepared by re-
action of chloroethoxy-ethyl pyrazole with O- or S-donor aryl groups
in DMF/THF in moderate to good yields (41–88%) (Scheme 1). The
identity of this class of ligands was established by IR, ESI-HRMS and
NMR spectroscopy.
Fig. 1. Models of nickel catalysts based on tridentate pyrazolyl ligands.
The reaction of (DME)NiCl2 with 1.1 equiv of the appropriate tri-
dentate ligand (L1–L5) in THF at room temperature afforded the
corresponding complexes [NiCl2L] (1–5) (Scheme 2), which were
isolated as yellow solids in high yields (typically 81–90%). These nick-
el complexes show moderate solubility in ethanol or methanol at
room temperature. Due to the paramagnetic nature of these com-
pounds, 1H NMR spectra featured very broad resonances and proved
uninformative. The identity of 1–5 was established on the basis of
elemental analysis and ESI-HRMS (which indicated the formation of
[M–Cl]+ ions).
before use. NiCl2⋅6H2O (Aldrich) was used as received. 1-(2-(2-
chloroethoxy)ethyl)-3,5-dimethyl-1H-pyrazole [74], and (DME)
NiCl2 [75], (DME=1,2-dimethoxyethane) were prepared by follow-
ing literature procedures. Ethylene (White Martins Co.) and argon
were deoxygenated and dried through BTS columns (BASF) and ac-
tivated molecular sieves (3A) prior to use. PMAO-IP (Akzo Nobel,
12.9 wt.% Al solution in toluene) was used as received. EASC (Akzo
Nobel) was used with the previous dilution (2.1 wt.% Al solution in
toluene). Elemental analyses were performed by the Analytical Cen-
tral Service of the Institute of Chemistry-UFRGS (Brazil) and are the
average of two independent determinations. Infrared spectra were
performed on a FT-IR Bruker Alpha Spectrometer. 1H and 13 C{1H}
NMR spectra were recorded on a Varian Inova 300 spectrometer op-
erating at 25 °C. Chemical shifts are reported in ppm vs. SiMe4 and
were determined by reference to the residual solvent peaks. Infrared
spectra were performed on a Bruker Alpha-P Spectrometer. High-
resolution mass spectra of tridentate ligands (L1–L5) and Ni(II)
complexes (1–5) were obtained by ESI in the positive mode in
CH3CN or CH3OH solutions using a Micromass Waters® Q-Tof spec-
trometer. Quantitative gas chromatographic analysis of ethylene
oligomerization products was performed on a Varian 3400CX instru-
ment with a Petrocol HD capillary column (methyl silicone, 100 m
length, 0.25 mm i.d. and film thickness of 0.5 μm) operating at
36 °C for 15 min followed by heating at 5 °C min− 1 until 250 °C; cy-
clohexane was used as the internal standard.
The oligomerization catalytic testing was carried out for each of
the nickel precatalysts. The conditions and results of the experiments
are reported in Table 1. More details are described in the Experimen-
tal Section. When activated with MAO, all catalytic systems proved to
be active for dimerization of ethylene with turnover frequencies
(TOFs) varying from 19,400 to 35,800 (mol C2H4)⋅(mol Ni−1 h−1).
As shown in Table 1 (entries 1–4), the ligand environment regarding
the substituents on the phenol group influenced the catalytic perfor-
mance of the nickel precatalysts on ethylene oligomerization, as can
be better visualized in Fig. 2. Precatalyst 1, when activated with 250
equiv of MAO, was found to give reasonable activity [TOF=30,900
(mol C2H4)⋅(mol Ni−1 h−1)] along with good selectivity towards 1-
butene (65.7%). Increasing the steric hindrance on the pyrazolyl unit
by substitution of the methyl group by a phenyl ring at the 3-
position (precatalyst 2) caused a decrease in activity by a factor of
1.65. Similarly, the presence of a bulky tert-butyl group in the ortho-
position of the phenol ring (precatalyst 3) also showed lower activity
[TOF=20,100 (mol C2H4)⋅(mol Ni−1 h−1)] as compared to the cata-
lytic performance displayed by 1. The abovementioned observations
are the direct consequence of the effect of the size of the R substitu-
ents toward the insertion of monomers; bulkier substituents offer
maximum resistance to monomers coming into the active metal cen-
ter. On the other hand, the use of a less bulky O-aryl pendant group
(precatalyst 4) generated a catalytic system that was more active
than that one using complex 1 [TOF=35,800 (mol C2H4)⋅(mol
Ni−1 h−1)]. Similar trends have been observed for ethylene oligo-
merization with MAO-activated nickel complexes bearing tridentate
ligands [60,76–78].
2.2. General oligomerization procedure
Ethylene oligomerization reactions were performed in a 100 mL
double-walled stainless Parr reactor equipped with mechanical stir-
ring, internal temperature control and continuous feed of ethylene.
The Parr reactor was dried in an oven at 120 °C for 5 h prior to each
run, then placed under vacuum for 30 min. A typical reaction was per-
formed by introducing toluene (30 mL) and the proper amount of
cocatalyst (MAO or EASC) into the reactor under an ethylene atmo-
sphere. After 20 min, the toluene catalyst solution (10 mL, [Ni]=
Scheme 1. Synthesis of ether-pyrazolyl ligands with pendant O- and S-donor groups.

A.H.D.P.S. Ulbrich et al. / Catalysis Communications 16 (2011) 245–249
247
Scheme 2. Synthesis of Ni(II) catalysts bearing ether-pyrazolyl ligands with pendant O- and S-donor groups.
For all nickel complexes bearing an O-donor pendant group (1–4),
the selectivity for butenes and especially 1-butene was high, attaining
65.7–76.4% of the total amount of olefins produced in the oligomeri-
zation reactions. As shown in Table 1, the selectivities for 1-butene
afforded by precatalysts 1–4 were similar, indicating that the intro-
duction of steric hindrance on the pyrazol/phenol groups had little in-
fluence on the product distribution as already observed for ethylene
oligomerization with MAO-activated nickel complexes bearing com-
parable tridentate ligands [70–71]. In all cases, minimal amounts
of hexenes (6.4–8.7%) were detected with poor selectivity for 1-
hexene (1.25–1.84%).
Replacing the phenol with a thiophenol group afforded precatalyst
5 which exhibited lower activity [TOF=19,600 (mol C2H4)⋅(mol
Ni−1 h−1)] as compared to the analog precatalyst 4 [TOF=35,800
(mol C2H4)⋅(mol Ni−1 h−1)]; however, a slight improvement in 1-
butene selectivity was found using 5 [78.1% (5) vs. 70.2% (4)]. Overall,
the presence of a pendant O- or S-donor group on one side of the
ether-pyrazol ligand determines a dramatic effect on the activity of
precatalysts (1–5) as compared to the similar nickel(II) complexes
bearing oxygen-bridged bis(pyrazolyl) ligands [71]. For instance, pre-
catalyst 4 [TOF=35,800 (mol C2H4)⋅(mol Ni−1 h−1)] is 5.0 times
more active than NiCl2{bis[2-(3,5-dimethylpyrazolyl)ethyl)]ether}
[TOF=7,100 (mol C2H4)⋅(mol Ni−1 h−1)] under identical reaction
conditions (MAO; 30°C; [Al]/[Ni]=250; 20 atm of ethylene, time=
20 min).
temperature [10 °C, TOF=34,700 (mol C2H4)⋅(mol Ni−1 h−1], and el-
evating the reaction temperature to 50 °C led to a noticeable reduction
in activity [TOF=17,100 (mol C2H4)⋅(mol Ni−1 h−1)], which may be
attributable to the decomposition of the active catalytic sites and
lower ethylene solubility at higher temperatures. However, it should
be pointed out that even at a higher temperature, the 1/MAO system
was much more active than NiCl2{bis[2-(3,5-dimethylpyrazolyl)
ethyl)]ether} [TOF=7,100 (mol C2H4)⋅(mol Ni−1 h−1)][71], indicat-
ing that the presence of the pendant group affords a thermally robust
catalyst system. At 10 °C, the catalytic system displayed the best selec-
tivity for 1-butene (81.3%). An increase in temperature (30 °C) led to
reduced 1-butene selectivity (65.3%) which remained at the same
level at 50 °C (69.9%).
The selectivities and catalyst activity were monitored versus time
in the ethylene oligomerization reaction promoted by the 1/MAO
catalytic system (Fig. 3). The oligomerization activity progressively
decreased when the reaction was prolonged from 5 to 20 min and fi-
nally to 40 min, which indicates that the catalyst lifetime is relatively
short. On the other hand, the reaction time causes a minimum impact
on the selectivity for 1-butene, varying from 76.2% (5 min) to 65.1%
(40 min). These results thus indicate that isomerization of butenes
is a minor pathway in these reactions and that the observed selectivi-
ties directly reflect the dimerization abilities of the catalyst.
Increasing the ethylene pressure resulted in much higher TOF
values, most likely as an effect of increased ethylene concentration
in solution. For instance, the activity of the 1/MAO system was signif-
icantly enhanced from 10 to 40 bar, giving a maximal TOF of 75,800
(mol C2H4)⋅(mol Ni−1 h−1) (entry 12). Better selectivity for 1-
butene was also observed at higher pressure (compare entries
Based on these preliminary results, precatalyst 1 was selected for
further optimization, investigating the influence of temperature, oligo-
merization time, ethylene pressure, [Al]/[Ni] ratio, and cocatalyst type.
The precatalyst 1 showed higher oligomerization activity at lower
Table 1
Ethylene oligomerization with catalyst systems1-5.a
Entry
Cat
[Al]/[Ni]
T (°C)
Time (min)
PC2H4 (bar)
Olig.(g)
TOFb(103⋅h−1
)
Selectivity (%)c
C4 (α-C4)
C4 (cis)
C4 (trans)
C6 (α-C6)
1
2
3
4
5
6
7
8
1
2
3
4
5
1
1
1
1
1
1
1
1
1
1
1
250
250
250
250
250
250
250
250
250
250
250
250
100
500
100
50
30
30
30
30
30
10
50
30
30
30
30
30
30
30
30
30
20
20
20
20
20
20
20
5
10
40
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
10
40
20
20
20
20
3.1
1.8
1.7
3.3
1.8
3.2
1.6
1.5
2.0
4.9
1.8
7.1
0.5
3.7
20.8
10.3
30.9
19.4
20.1
35.8
19.6
34.7
17.1
64.1
44.4
26.4
19.0
75.8
6.1
91.3 (72)
93.2 (82)
92.5 (80)
93.6 (75)
95.2 (82)
94.5 (86)
92.0 (76)
94.5 (81)
93.8 (76)
92.7 (71)
89.9 (70)
93.9 (81)
93.1 (89)
91.3 (72)
71.8 (28)
83.2 (52)
16.5
11.8
12.3
15.5
11.2
8.7
14.1
12.2
14.7
17.7
17.4
12.0
6.4
8.9
6.0
6.4
8.2
5.5
4.6
7.8
6.2
7.8
9.4
9.5
6.3
3.4
8.8
20.8
16.8
8.7 (20)
6.8 (27)
7.5 (21)
6.4 (26)
4.8 (40)
5.5 (27)
8.0 (23)
5.5 (27)
6.2 (26)
7.3 (22)
10.1 (20)
6.1 (26)
6.9 (25)
8.7 (20)
8.7 (20)
16.8 (13)
9
10
11
12
13
14
15d
16e
39.9
227.0
211.0
16.3
30.6
23.1
a
Reaction Conditions: [Ni]=10 μmol; toluene=40 mL; MAO as cocatalyst (250 equiv). The results shown are representative of at least duplicated experiments.
Mol of ethylene converted (mol of Ni)−1·h−1 as determined by quantitative GLC.
Cn, amount of olefin with n carbon atoms in oligomers; α-Cn, amount of terminal alkene in the Cn fraction, as determined by quantitative GLC.
EASC as cocatalyst (100 equiv).
b
c
d
e
[Ni]=5 μmol and EASC as cocatalyst (50 equiv).

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A.H.D.P.S. Ulbrich et al. / Catalysis Communications 16 (2011) 245–249
80
[TOF=211,000 (mol C2H4)⋅(mol Ni−1 h−1, entry 16), which was
very close to the test using higher EASC loading (100 equiv,
TOF=227,000 (mol C2H4)⋅(mol Ni−1 h−1), entry 15). At the same
time, decreasing the amount of EASC from 100 to 50 equiv led to im-
proved selectivity for 1-butene (28.0 to 52%) with production of
lower amounts of 2-butenes (51.4 to 39.9%) and hexenes (28.2 to
16.8%). The possibility of using small amounts of cocatalyst is obvi-
ously an interesting feature of this catalyst system.
35
30
25
20
15
60
40
20
0
4. Conclusion
In summary, we have synthesized and characterized a series of
new nickel(II) complexes supported by tridentate ether-pyrazolyl li-
gands bearing pendant oxygen- and sulfur-donor groups. Upon acti-
vation with MAO and EASC cocatalysts, the Ni(II) complexes show
high catalytic activity for ethylene oligomerization. Our results
showed that the nickel precatalysts (1–5) performed at greater activi-
ties compared with similar nickel(II) precatalysts bearing oxygen-
bridged bis(pyrazolyl) ligands. In this case, the presence of the pendant
O- or S-donor group linked to the ether-pyrazolyl moiety generated
more robust catalyst systems. On the other hand, no significant impact
of these pendant groups on the selectivity for 1-butene production was
observed. Both turnover frequency and selectivity were mostly depen-
dent on the nature of the alkylaluminum cocatalyst employed (MAO
or EASC) in the oligomerization reactions, suggesting that these activa-
tors act also as ligands for the nickel center of the active species. The
activity displayed by 1/EASC was several times greater than that
found using 1/MAO under identical reaction conditions; however,
better selectivity towards 1-butene production was achieved using
MAO.
cat 1
cat 2
cat 3
cat 4
cat 5
Fig. 2. Influence of nickel catalyst on TOF and selectivity for 1-butene (30 °C, 20 bar,
[Al]/[Ni]=250).
1/11/12). Increased selectivity with monomer pressure is usually
accounted for by increased dimerization activity, which in turn atten-
uates the effect of parallel isomerization of 1-butene into 2-butenes
(i.e., chain transfer is favored relative to chain isomerization)
[14,20,79]. When activated with 100 equivalents of MAO, precatalyst
1 gave a relatively low activity (TOF=6,100 (mol C2H4)⋅(mol
Ni−1 h−1), entry 13), which was increased upon using 250 equiva-
lents of MAO [TOF=30,900 (mol C2H4)⋅(mol Ni−1 h−1), entry 1]. A
greater loading of MAO (500 equiv) led to an improvement in the cat-
alytic activity with identical oligomer distributions as found using
250 equiv of MAO (compare entries 1 and 14).
Activation of nickel precatalyst 1 with 100 equiv of ethylalumi-
num sesquichloride (Et3Al2Cl3, EASC) instead of MAO produced a sig-
nificantly better catalyst system than 1/MAO [TOF=227,000 vs.
30,900 (mol C2H4)⋅(mol Ni−1 h−1]. This high activity generated sig-
nificant exothermicity, so that the reaction with 1/EASC performed
at an initial temperature of 30 °C (entry 14) rapidly rose to 60–
70 °C. Although the catalyst activity was improved by approximately
seven-fold, the 1-butene selectivity was drastically reduced, attaining
only 28% with a concomitant production of larger amounts of internal
butenes (51.4%) and hexenes (28.2%). The oligomerization reaction
with 1/EASC was better conducted with low catalyst loadings (ca.
5 μmol), and using a lower [Al]/[Ni] molar ratio (50 equiv) at a setting
temperature of 30°C (entry 15); under such conditions, the exother-
micity could be limited to 10–15 °C. In fact, under such oligomeriza-
tion conditions, precatalyst 1 provided to possess high activity
Acknowledgments
This research was financially supported in part by Petrobras S/A
and CNPq (Brazil). A. H. D. P. S. Ulbrich and A. L. Bergamo gratefully
acknowledge CNPq and FAPERGS for fellowships.
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2011.09.006.
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Fig. 3. Monitoring of selectivities and activity in the ethylene oligomerization reaction
promoted by the 1/MAO system ([Al]/[Ni]=250; T=30 °C; 20 atm ethylene).

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