Nickel catalysts based on phenyl ether-pyrazol ligands: Synthesis, XPS study, and use in ethylene oligomerization

Article abstract of DOI:10.1016/j.apcata.2012.12.031

A series of nickel(II) complexes bearing phenyl ether-pyrazol ligands [NiCl₂L] (Ni1: L = 3,5-dimethyl-1-(3-phenoxypropyl)-1H-pyrazole; Ni2: L = 1-(3-phenoxypropyl)-3-phenyl-1H-pyrazole; Ni3: L = 3,5-di-tert-butyl-1-(3- phenoxypropyl)-1H-pyrazole; Ni4: L = 1-(3-phenoxypropyl)-1H-pyrazole) were synthesized and fully characterized by elemental analysis, high-resolution mass spectrometry (HRMS) and X-ray photoelectron spectroscopy (XPS). X-ray photoelectron data illustrates that the probability of (co)existing dimeric species increases in the following order: Ni4 < Ni1 < Ni2 < Ni3. All nickel precatalysts, activated with methylaluminoxane (MAO), exhibited moderate to high activities for ethylene oligomerization [TOF = 18.4-45.7 × 10 ³ mol(ethylene)(mol(Ni))^-1 h^-1)] with good selectivities for 1-butene produced (62.6-80.7%). The ligand environment regarding the substituents on the pyrazolyl unit as well as the reaction parameters influence the catalytic performance and selectivity toward production of 1-butene. When activated with ethylaluminum sesquichloride (Et ₃Al₂Cl₃, EASC), Ni1 displayed low catalytic activity (TOF = 9300 (mol C₂H₄)·(mol Ni ^-1 h^-1); however, the 1-butene selectivity was increased, attaining 92.5%. The use of triphenylphosphine (PPh₃) as auxiliary ligand afforded highly active catalyst system [TOF = 118.3 × 10 ³ mol(ethylene)(mol(Ni))^-1 h^-1)] with poor selectivity for production of 1-butene (13.7%).

Full text of DOI:10.1016/j.apcata.2012.12.031

Accepted Manuscript
Title: Nickel Catalysts Based on Phenyl Ether-Pyrazol
Ligands: Synthesis, XPS study, and use in Ethylene
Oligomerization
Authors: Ana H.D.P.S. Ulbrich, Roberta R. Campedelli, Jorge
ˆ
˜
L. Sonego Milani, Joao H.Z. dos Santos, Osvaldo de L.
Casagrande Jr.
PII:
DOI:
Reference:
S0926-860X(12)00808-3
doi:10.1016/j.apcata.2012.12.031
APCATA 14032
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
9-9-2012
13-12-2012
23-12-2012
Please cite this article as: A.H.D.P.S. Ulbrich, R.R. Campedelli, J.L.S. Milani, J.H.Z.
Santos, O.L. Casagrande Jr., Nickel Catalysts Based on Phenyl Ether-Pyrazol Ligands:
Synthesis, XPS study, and use in Ethylene Oligomerization, Applied Catalysis A,
General (2010), doi:10.1016/j.apcata.2012.12.031
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*Graphical Abstract (for review)
Page 1 of 24

*Highlights (for review)
Research highlights
> Nickel complexes supported by pheny ether-pyrazol ligands
> The probability of (co)existing dimeric species increases in the following order:
Ni4 < Ni1 < Ni2 < Ni3.
> Nickel precatalysts exhibit high activities in ethylene oligomerization
> Selectivities for 1-butene produced varying from 57 to 92.5 %
Page 2 of 24

*Manuscript
Nickel Catalysts Based on Phenyl Ether-Pyrazol
Ligands: Synthesis, XPS study, and use in Ethylene
Oligomerization
Ana H. D. P. S. Ulbrich, Roberta R. Campedelli, Jorge L. Sônego Milani, João H. Z. dos Santos,
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).
† Corresponding author. Tel.: +55 51 3308 6300; Fax: +55 51 3308 7304
E-mail address: osvaldo.casagrande@ufrgs.br
1
Page 3 of 24

ABSTRACT
A series of nickel(II) complexes bearing phenyl ether-pyrazol ligands [NiCl2L] (Ni1: L= 3,5-
dimethyl-1-(3-phenoxypropyl)-1H-pyrazole; Ni2: L= 1-(3-phenoxypropyl)-3-phenyl-1H-pyrazole; Ni3:
L = 3,5-di-tert-butyl-1-(3-phenoxypropyl)-1H-pyrazole; Ni4: L = 1-(3-phenoxypropyl)-1H-pyrazole)
were synthesized and fully characterized by elemental analysis, high-resolution mass spectrometry
(HRMS) and X-ray photoelectron spectroscopy (XPS). X-ray photoelectron data illustrates that the
probability of (co)existing dimeric species increases in the following order: Ni4 < Ni1 < Ni2 < Ni3. All
nickel precatalysts, activated with methylaluminoxane (MAO), exhibited moderate to high activities for
ethylene oligomerization [TOF = 18.4-45.7 x 103 mol(ethylene)(mol(Ni))−1 h−1)] with good selectivities
for 1-butene produced (62.6 – 80.7%). The ligand environment regarding the substituents on the
pyrazolyl unit as well as the reaction parameters influence the catalytic performance and selectivity
toward production of 1-butene. When activated with ethylaluminum sesquichloride (Et3Al2Cl3, EASC),
Ni1 displayed low catalytic activity (TOF = 9,300 (mol C2H4)(mol Ni−1 h−1); however, the 1-butene
selectivity was increased, attaining 92.5%. The use of triphenylphosphine (PPh3) as auxiliary ligand
afforded highly active catalyst system [TOF = 118.3 x 103 mol(ethylene)(mol(Ni))−1 h−1)] with poor
selectivity for production of 1-butene (13.7%).
Keywords: Nickel(II) complexes; Phenyl ether-pyrazol ligands; Ethylene oligomerization; 1-butene;
XPS studies
2
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1. Introduction
The oligomerization of ethylene is one of the most important industrial processes to obtain linear -
olefins (LAOs).[1-5] These substrates 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 -olefins, nickel complexes containing P,P-,[8-12] P,N-,[13-31]
P,O-,[32-44] N,N-,[45-61] or N,O-[62-68] as bidentate chelating ligands are the most frequently studied.
Specially, anionic bidentate N,O ligands such as the imidazole-alcoholate, [69] iminocarboxamide,[70]
and 2-(alkylideamino)benzoate ligands[71] have been successfully applied in the synthesis of nickel(II)
oligomerization catalysts. On the other hand, just a few examples of Ni(II) complexes based on neutral
N,O-type ligands have been reported in the literature.[72-73] For instance, Braunstein et al. have
recently reported a nickel complexes bearing oxazoline alcohols/pyridine alcohols bidentate ligands that
show high activities in ethylene oligomerization [TOF up to 187,000 (mol C2H4)(mol Ni−1 h−1)] giving
mostly C4 olefins and 1-butene. Over the past few years, pyrazolyl-based ligand metal complexes have
attracted attention as efficient catalysts for oligo- and polymerization of ethylene.[74-78] Our group has
also been interested in exploring the potential applications of pyrazolyl-based ligands in the
oligomerization catalysis field. We have thus previously communicated new classes of NiII[79-83] and
CrIII[84] complexes based on tridentate pyrazolyl ligands, which act as highly selective and highly active
precatalysts for ethylene oligomerization when activated with alkylaluminum (MAO or AlEt2Cl).
Herein, we wish to report the synthesis and characterization of several new nickel(II) complexes bearing
bidentate phenoxi-pyrazoyl ligands, as well as their catalytic behavior for ethylene oligomerization. We
also discuss the effect of substituents and reaction conditions on their activity and selectivity towards the
production of 1-butene.
3
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2. Experimental
2.1. General Procedures
All manipulations involving air- and/or water-sensitive compounds were carried out in an MBraun
glovebox or under dry nitrogen using standard Schlenk techniques. Solvents were dried from the
appropriate drying agents under argon before use. 3-bromopropyl phenyl ether (Aldrich) and
NiCl26H2O (Aldrich) were used as received. (DME)NiCl2 (DME = 1,2-dimethoxyethane),[85] 3,5-di-
tert-butyl-1H-pyrazole and 3-phenyl-1H-pyrazole were prepared by following literature procedure.[86]
Ethylene (White Martins Co.) and argon were deoxygenated and dried through BTS columns (BASF)
and activated molecular sieves 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
1
toluene). Infrared spectra were performed on a FT-IR Bruker Alpha Spectrometer. H and 13C{1H}
NMR spectra were recorded on a Varian Inova 300 spectrometer operating at 25°C. Chemical shifts are
reported in ppm vs. SiMe4 and were determined by reference to the residual solvent peaks. High-
resolution mass spectra of bidentate ligands (L1-L4) and Ni(II) complexes (Ni1-Ni4) were obtained by
ESI in the positive mode in CH3CN or CH3OH solutions using a Micromass Waters® Q-Tof
spectrometer. Quantitative gas chromatographic analysis of ethylene oligomerization products was
performed on a Varian 3400CX instrument with a Petrocol HD capillary column (methyl silicone, 100m
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; cyclohexane was used as the internal standard.
2.2. Synthesis of the Phenyl Ether-Pyrazolyl ligands
3,5-dimethyl-1-(3-phenoxypropyl)-1H-pyrazole (L1): A solution of 3,5-dimethyl-1H-pyrazole (0.30 g,
3.2 mmol) in DMF/THF (v/v = 1:1, 12 mL) was added to a suspension of NaH (0.15 g, 6.4 mmol) in
DMF/THF (v/v = 2:1, 15 mL) and stirred at 60°C for 2 h. Then, the resulting solution was added under
stirring to a solution of 3-bromopropyl phenyl ether (0.70 g, 3.2 mmol) in DMF (7 mL). The mixture
was allowed to stir for 24 h at 60 °C, cooled, and treated cautiously with H2O (5 mL) to decompose
4
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excess NaH. The solvents were then evaporated under reduced pressure. The residue was extracted with
ethyl acetate (3 x 15 mL), washed with H2O (2 x 15 mL). The organic phase was dried over MgSO4 and
filtered, before the solvent was evaporated under reduced pressure. After workup and purification by
chromatographic column on silica gel (hexane/ethyl acetate, 90:10), L1 was obtained as a colorless oil
(0.47 g, 64%). 1H NMR (CDCl3, 298K, 300 MHz):  2.18 (3H, s), 2.26 (3H, s), 2.30 (2H, m, 3JHH = 6.2
3
3
3
Hz), 3.88 (2H, t, JHH = 5.8 Hz), 4.18 (2H, t, JHH = 6.6 Hz), 5.77 (1H, s), 6.88 (2H, d, JHH = 7.6 Hz),
3
3
13
6.95 (1H, t, JHH = 7.3 Hz), 7.29 (2H, t, JHH = 7.4 Hz). C NMR (CDCl3, 298K, 300MHz): 10.54,
13.29, 29.62, 44.44, 63.76, 104.39, 114.11, 120.51, 129.20, 138.91, 147.23, 158.33. IV (ATR): 3062
(w), 3042 (w), 2946 (m), 2930 (m), 2874 (m), 1600 (s), 1586 (s), 1552 (s), 1496 (s), 1470 (s), 1423 (m),
1387 (m), 1355 (f), 1298 (m), 1241 (s), 1172 (s), 1152 (w), 1130 (w), 1079 (m), 1054 (s), 1037 (m),
1021 (m), 997 (w), 978 (w), 908 (s), 882 (w), 778 (m), 752 (s), 728 (s), 690 (s), 643 (m), 597 (m), 510
(m). Anal. calcd. for C14H18N2O: C, 73.01; H, 7.88; N, 12.16. Found: C, 72.54; H, 8.58; N, 12.09.
1-(3-phenoxypropyl)-3-phenyl-1H-pyrazole (L2): This product was prepared by following a procedure
similar to what was described above for L1, starting from 3-phenyl-1H-pyrazole (0.70 g, 4.5 mmol),
NaH (0.22 g, 9.0 mmol) and 3-bromopropyl phenyl ether (0.96 g, 4.5 mmol). After workup and
purification by chromatographic column on silica gel (hexane/ethyl acetate, 90:10), L2 was obtained as
1
3
a colorless oil (0.75 g, 60%). H NMR (CDCl3, 298K, 300 MHz):  2.32 (2H, m, JHH = 6.3 Hz), 3.87
3
3
3
3
(2H, t, JHH = 5.8 Hz), 4.32 (2H, t, JHH = 6.7 Hz), 6.48 (1H, d, JHH = 2.2 Hz), 6.88 (2H, d, JHH = 7.8
3
3
13
Hz), 6.93 (1H, t, JHH = 7.4 Hz), 7.34 (6H, m), 7.81 (2H, d, JHH = 7.0 Hz). C NMR (CDCl3, 298K,
300MHz): 29.56, 48.63, 63.95, 102.35, 114.29, 120.71, 125.40, 127.39, 128.45, 129.35, 130.99, 133.45,
151.53, 158.51. Anal. calcd. for C18H18N2O: C, 77.67; H, 6.52; N, 10.06. Found: C, 77.01; H, 6.35; N,
10.01.
3,5-di-tert-butyl-1-(3-phenoxypropyl)-1H-pyrazole (L3): This product was prepared by following a
procedure similar to what was described above for L1, starting from 3,5-di-tert-butyl-1H-pyrazole (0.81
5
Page 7 of 24

g, 4.5 mmol), NaH (0.22 g, 9.0 mmol) and 3-bromopropyl phenyl ether (0.96 g, 4.5 mmol). After
workup and purification by chromatographic column on silica gel (hexane/ethyl acetate, 90:10), L3 was
1
obtained as a colorless oil (1.10 g, 75%). H NMR (CDCl3, 298K, 300 MHz):  1.28 (9H, s), 1.35 (9H,
3
3
3
s), 2.37 (2H, m, JHH = 6.3 Hz), 4.03 (2H, t, JHH = 5.9 Hz), 4.32 (2H, t, JHH = 7.2 Hz), 5.78 (1H, s),
6.87-6.94 (3H, m), 7.09 (2H, t, 3JHH = 7.9 Hz). 13C NMR (CDCl3, 298K, 300MHz): 30.19, 30.45, 30.92,
31.16, 31.88, 47.36, 64.90, 99.04, 114.32, 120.58, 129.36, 150.95, 158.70, 158.78. IR data (neat, cm-1):
3104 (w), 3061 (w), 3039 (w), 2945 (m), 2876 (m), 1703 (w), 1599 (s), 1586 (s), 1494 (s), 1470 (s),
1454 (m), 1395 (s), 1358 (w), 1338 (w), 1289 (s), 1239 (s), 1172 (s), 1153 (w), 1080 (s), 1039 (s), 996
(w), 965 (s), 918 (m), 882 (m), 831 (m), 816 (m), 791 (w), 752 (s), 690 (s), 653 (m), 617 (s), 596 (m),
510 (s). Anal. calcd. for C20H30N2O: C, 76.39; H, 9.62; N, 8.91. Found: C, 76.80; H, 10.43; N, 8.72.
1-(3-phenoxypropyl)-1H-pyrazole (L4): This product was prepared by following a procedure similar to
what was described above for L1, starting from 1H-pyrazole (1.01 g, 14.7 mmol), NaH (0.70 g, 29.4
mmol) and 3-bromopropyl phenyl ether (3.46 g, 16.1 mmol). After workup and purification by
chromatographic column on silica gel (hexane/ethyl acetate, 90:10), L4 was obtained as a colorless oil
(2.23 g, 75%). 1H NMR (CDCl3, 298K, 300 MHz):  2.31 (2H, q, 3JHH = 6.3 Hz), 3.87 (2H, t, 3JHH = 5.8
3
3
3
Hz), 4.33 (2H, t, JHH = 6.6 Hz), 6.20 (1H, t, JHH = 2.0 Hz), 6.85 (1H, d, JHH = 7.8 Hz), 6.93 (1H, t,
3JHH = 6.9 Hz) 7.27 (2H, t, 3JHH = 7.7 Hz), 7.34 (1H, d, 3JHH = 2.2 Hz), 7.51 (1H, d, 3JHH = 1.5 Hz). 13C
NMR (CDCl3, 298K, 300MHz): 29.77, 48.29, 63.85, 105.00, 114.19, 120.63, 129.25, 129.35, 139.25,
158.42. Anal. calcd. for C12H14N2O: C, 71.26; H, 6.98; N, 13.85. Found: C, 70.90; H, 6.42; N, 13.97.
2.3. Synthesis of the Ni(II) Complexes
NiCl2{3,5-dimethyl-1-(3-phenoxypropyl)-1H-pyrazole} (Ni1): To a solution of (DME)NiCl2 (0.22 g,
0.99 mmol) in dichloromethane (10 mL) was added a solution of 3,5-dimethyl-1-(3-phenoxypropyl)-1H-
pyrazole (0.20 g, 1.1 mmol) in dichloromethane (10 mL) and the resulting solution was stirred for 24 h
at room temperature. The solvent was removed, and the resulting yellow solid residue was washed with
6
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Et2O (3 x 10 mL). Complex Ni1 was obtained as a yellow solid (0.27 g, 76%). Anal. calcd. for
C28H36N4Cl4Ni2O2: C, 46.72; H, 5.04; N, 7.78. Found: C, 46.13; H, 4.87; N, 7.44. ESI-HRMS (CH3OH,
m/z): 323.0468 [M-Cl]+ (Calc. for C14H18ClN2NiO, 323.0461).
NiCl2{1-(3-phenoxypropyl)-3-phenyl-1H-pyrazole}(Ni2): This compound was prepared according to
the method described for Ni1 using (DME)NiCl2 (0.18 g, 0.83 mmol) and 1-(3-phenoxypropyl)-3-
phenyl-1H-pyrazole (0.25 g, 0.92 mmol). Complex Ni2 was obtained as a yellow solid (0.27 g, 81%).
Anal. calcd. for C18H18N2Cl2NiO: C, 53.00; H, 4.45; N, 6.87. Found: C, 52.87; H, 4.21; N, 6.61. ESI-
HRMS (CH3OH, m/z): 371.0473 [M-Cl]+ (Calc. for C18H18ClN2NiO, 371.0461).
NiCl2{3,5-di-tert-butyl-1-(3-phenoxypropyl)-1H-pyrazole }(Ni3): This compound was prepared
according to the method described for Ni1 using (DME)NiCl2 (0.19 g, 0.89 mmol) and 3,5-di-tert-butyl-
1-(3-phenoxypropyl)-1H-pyrazole (0.31 g, 0.98 mmol). Complex Ni3 was obtained as a yellow solid
(0.31 g, 79%). Anal. calcd. for C16H22N2Cl2NiO: C, 49.53; H, 5.72; N, 7.22. Found: C, 49.16; H, 5.13;
N, 6.81. ESI-HRMS (CH3OH, m/z): 407.1415 [M-Cl]+ (Calc. for C20H30ClN2NiO, 407.1400).
NiCl2{1-(3-phenoxypropyl)-1H-pyrazole} (Ni4): This compound was prepared according to the method
described for 1 using (DME)NiCl2 (0.53 g, 2.40 mmol) 1-(3-phenoxypropyl)-1H-pyrazole (0.53 g, 2.65
mmol). Complex Ni4 was obtained as a yellow solid (0.59 g, 74%). Anal. calcd. for C24H28N4Cl4Ni2O2:
C, 43.43; H, 4.25; N, 8.44. Found: C, 42.92; H, 3.87; N, 7.88. ESI-HRMS (CH3OH, m/z): 295.0152
[M-Cl]+ (Calc. for C12H14ClN2NiO, 295.0148 g).
2.4 General oligomerization procedure
Ethylene oligomerization reactions were performed in a 100 mL double-walled stainless Parr
reactor equipped with mechanical stirring, 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, and then placed under vacuum
for 30 min. A typical reaction was performed by introducing toluene (30 mL) and the proper amount of
7
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co-catalyst (MAO or EASC) into the reactor under an ethylene atmosphere. After 20 min, the toluene
catalyst solution (10 mL, [Ni] = 10 μmol) was injected into the reactor under a stream of ethylene and
then the reactor was immediately pressurized. Ethylene was continuously 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 quantitative GLC.
2.5. X-ray photoelectron spectroscopy (XPS)
X-ray photoelectron spectroscopy was performed in an Omicron-SPHERA station using Al K 
radiation (1486.6 eV). The anode was operated at 225W (15 kV, 15 mA). Survey spectra were recorded
with a 50 eV pass energy. O 1s, Cl 2p, and Ni 2p regions were recorded with a higher resolution (pass
energy of 20 eV). The detection angle of the photoelectrons with respect to the sample surface (take-off
angle) was fixed at 53° for all measurements. The C 1s signal from adventitious carbon at 285 eV was
used as an internal energy reference. All spectra were fitted assuming a Shirley background. Lines were
fitted by 70% Gaussian + 30% Lorentzian functions with set values of full width at half maximum for
each line
3. Results and Discussion
3.1 Synthesis and Characterization of the Phenyl Ether-Pyrazol ligands and Nickel Complexes
The phenyl ether-pyrazol ligands (L1-L4) were prepared by reaction of 3-bromopropyl phenyl ether
with sodium pyrazolate in DMF/THF in moderate to good yields (60-75%) as shown in Scheme 1. The
identity of this class of ligand was established by IR, elemental analysis, and NMR spectroscopy.
8
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Scheme 1
Reacting (DME)NiCl2 (DME = 1,2-dimethoxyethane) with 1.1 equiv of the appropriate phenyl
ether-pyrazolyl ligand (L1-L4) in THF at room temperature leads to the corresponding nickel
complexes (Ni1–Ni4), which were isolated as yellow solids in high yields (typically 74–81%)(Scheme
2).
Scheme 2
These nickel complexes were initially characterized by elemental analysis, and mass spectrometry.
High-resolution mass spectrometry (HRMS) with the electrospray ionization method afforded spectra
where the main peak corresponds to [Ni(L)Cl]+ ions (L = L1-L4). The informative NMR data were not
obtained due to the paramagnetic nature of these nickel complexes. In addition, no good-quality single
crystals were obtained in order to obtain the structural characterization of these nickel complexes. Thus,
these results prompted us to explore the structural nature of these nickel species using X-ray
photoelectron spectroscopy (XPS). Table 1 shows the binding energies (BE) extracted from the high-
resolution spectra from the following regions: Ni 2p, O 1s, and Cl 2p
9
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Table 1.
BE of investigated nickel complexes in the Ni 2p, O 1s and Cl 2p regions.
Complex
Binding Energy (eV)
O (1s)
Ni (2p)
857.53
856.63
855.16
856.00
Cl (2p)
201.63
200.63
199.36
199.90
533.43
Ni1
Ni2
Ni3
Ni4
532.73
531.16
532.00
The lowest BE energy value found for Ni3 indicates highest electron density in its coordination
sphere compared to the others nickel compounds (Ni1-Ni2, Ni4). It seems that the Ni metal center is
affected by the presence of the tert-Bu group, which its donor character drives electron density up to Ni
atom. In the high resolution Cl 2p region, the relative low BE (in comparison to the other nickel
complexes) suggests that such Cl ligand may be terminal. Furthermore, the potential steric effect played
by the groups may suggest that such complex is monomeric. In addition, the narrow full width at half
maximum (FHWM) of Cl 2p peak (1.63 eV) may also suggest homogeneity in the nature of such
species, i.e., mostly in a monomeric form.
In the case of complex Ni2, the donor character of the phenyl group is not enough to guarantee an
electron rich coordination sphere in comparison to that exhibited by Ni3. The BE of Ni (856.63 eV) is
close to that reported to NiCl2 (856.7 eV). [87-89] In such systems, the electron density seems to be
driven by the chloride ligands, which Cl 2p XPS peaks indicates a decrease in electron density (200.63
eV). The comparison to Cl (2p) binding energy reported for NiCl2 (199.4 eV) grounds this
supposition.[87-89] The same rationale can be applied for Ni1, which the Ni metal center seems less
affected by the presence of methyl groups at 3- and 5-positions of the pyrazolyl rings (857.53 eV). In
this case, the electron density is also driven from chloride ligands, which the biding energy is relatively
10
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much higher (201.63 eV) indicating an electron deficiency on the chloride ligands. Furthermore, such
values may determine a bridge chloride configuration, which in turn be possible due to the lower steric
effect played by these methyl groups. The broader FHWM of Cl peaks (1.74 and 1.76 eV) may indicate
a mixture of Cl species.
Finally, in the case of complex Ni4, the lower steric hindrance of the bidentate ligand may affords
the proximity between the Ni metal centers, resulting in the formation of a dimeric species. The
proximity of the nickel metal centers may render easier the electron flux of electrons, which guarantee a
lower cationic character of Ni (856.0 eV) in comparison for that on species Ni1 and Ni2, which in turn
drives lower electron density from the chloride ligands (199.9 eV).
In sum, based on the XPS results, taking into account the BE of Ni (2p) and Cl (2p), and
considering the steric effects played by the ligands coordinated to the Ni center, one can suggest that the
probability of (co)existing dimeric species increases in the following order: Ni4 < Ni1 <Ni2 < Ni3.
3.2 Ethylene Oligomerization Studies
Complexes Ni1-Ni4 have been evaluated in the ethylene oligomerization at 20 bar of ethylene
pressure, and using methylaluminoxane (MAO) (250 equiv) as co-catalyst. Table 2 summarizes the
results of reactions carried out using 10 μmol of precatalyst in 40 mL of a toluene. All nickel complexes
investigated have been found to generate active systems for the production of short-chain olefins in the
C4-C6 range.
The ligand environment regarding the substituents on the pyrazolyl unit influenced the catalytic
performance of the nickel precatalysts on ethylene oligomerization, as can be better visualized in Fig. 1.
Thus, the catalytic system Ni1/MAO was found to give reasonable activity [TOF = 45,700 (mol
C2H4)(mol Ni−1 h−1)] along with good selectivity towards 1-butene (62.6%). The presence of bulkier
ligands in Ni2/Ni3 caused a decrease in activity as compared to the catalytic performance displayed by
Ni1. The abovementioned observations can be associated to the effect of the R substituents size toward
the insertion of monomers; usually bulkier substituents offer maximum resistance to monomers coming
11
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into the active metal center. The lower activity of Ni4 can be associated to the poorer electron-donating
ability of pyrazolyl unit as compared to the substituted ones with alkyls groups (methyl, tert-Butyl). [90]


Fig. 1. Influence of nickel precatalyst on TOF and selectivity for 1-butene (30C, 20 bar, [Al]/[Ni] =
250).
For all nickel complexes (Ni1-Ni4), the selectivity for butenes and especially 1-butene varied from
62.6 to 80.7% of the total amount of olefins produced in the oligomerization reactions. As shown in
Table 2, the selectivities for 1-butene afforded by Ni2-Ni4 were similar, indicating that the introduction
of steric hindrance on the pyrazolyl group had little influence on the product distribution. This
observation is in agreement with the previous ethylene oligomerization results using MAO-activated
nickel complexes bearing pyrazolyl ligands.[79-80] In all cases, minimal amounts of hexenes (4.7-9.2%)
were detected with poor selectivity for 1-hexene (1.4-1.7%).
The preliminary study was extended to investigate the effect of temperature, oligomerization time,
[Al]/[Ni] molar ratio, and co-catalyst type on TOF and oligomers distributions. The precatalyst Ni1
showed slightly lower oligomerization activity at lower temperature [10C, TOF = 38,800 (mol
12
Page 14 of 24

C2H4)(mol Ni−1 h−1] as compared to the result obtained at 30C. This effect can be associated to a
reduced solubility of Ni1 in toluene at 10C generating lower amounts of active species. Elevating the
temperature from 30C to 70C led to a huge reduction in activity [TOF = 10,500 (mol C2H4)(mol Ni−1
h−1)], suggesting that a partial decomposition of the active catalytic sites took place. However, it should
be pointed out that even at a higher temperature (50C), the Ni1/MAO system was much more active
than
nickel
precatalysts
based
on
pyrazoly ligands
such
as
NiCl2{bis[2-(3,5-
dimethylpyrazolyl)ethyl)]ether} [TOF = 7,100 (mol C2H4)(mol Ni−1 h−1)][82], and NiCl2{1-(2-(2-(2,6-
diisopropylphenoxy)ethoxy)ethyl)-3,5-dimethyl-1H-pyrazole}[TOF = 17,100 (mol C2H4)(mol Ni−1
h−1)][83] under identical reaction conditions (30C, toluene, [Al]/[Ni] = 250, 20 bar of ethylene, MAO
as co-catalyst). As expected at 10C, the Ni1/MAO catalytic system displayed the best selectivity for 1-
butene (84.2%). On elevating the temperature from 10 to 30 °C, led to reduced 1-butene selectivity
(62.6%) which remained at the same level at 70C (66.1%).
The selectivities and catalyst activity were monitored versus time in the ethylene oligomerization
reaction promoted by the Ni1/MAO catalytic system. The oligomerization activity progressively
decreased when the reaction was prolonged from 10 to 20 min and finally to 40 min, which indicates
that the catalyst lifetime is relatively short. On the other hand, the reaction time did not significantly
affect the selectivity for 1-butene, varying from 65.0% (10 min) to 57.0% (40 min). These results thus
indicate that isomerization of butenes is a minor pathway in these reactions and that the observed
selectivities directly reflect the dimerization abilities of the catalyst.
The influence of the MAO loading on the catalyst behavior was also studied. When activated with
100 equiv of MAO, precatalyst Ni1 gave a lower activity [TOF = 30,300 (mol C2H4)(mol Ni−1 h−1),
entry 9], which was increased upon using 250 equiv [TOF = 45,700 (mol C2H4)(mol Ni−1 h−1), entry 1]
and even further with 500 equiv (TOF = 65,100 (mol C2H4)(mol Ni−1 h−1), entry10). At the same time,
increasing the amount of MAO from 100 to 500 equiv led to slightly reduced selectivity for the -C4
(72.4 to 62.6 wt%). Thus, the use of lower MAO loading (100 equiv) seems to be more reasonable to get
a moderate active system associated to a more selectivity towards production of 1-butene.
13
Page 15 of 24

Table 2.
Ethylene Oligomerization with Ni1-Ni4 catalytic systems.a
d
Entry Cat.
T
Time Cocat. Olig. TOFb
selectivity (%)c
C4 C4
2-C4
(°C) (min) (equiv)
(g)
(x 103)
C4
C6
(α-C4) (cis)
(trans) (α-C6)
MAO
90.8
18.1
(69)
9.2
(19)
5.4
(27)
5.1
(30)
4.7
(30)
3.3
(34)
10.0
(21)
10.6
(26)
8.4
(21)
10.8
(17)
7.2
(24)
9.3
(19)
3.2
(37)
2.6
(49)
8.6
(16)
1
2
30
30
30
30
10
50
70
30
30
30
30
30
30
30
20
20
20
20
20
20
20
10
40
20
20
20
20
20
4.3
1.6
2.1
1.6
3.6
3.1
1.0
3.5
7.0
2.8
5.9
0.5
0.8
10.7
45.7
18.9
23.5
18.4
38.8
34.5
10.5
76.5
38.4
30.3
65.1
5.5
9.8
1.8
1.8
1.8
1.9
2.0
1.8
1.8
1.9
1.8
1.9
1.8
1.3
1.4
0.6
Ni1
Ni2
Ni3
Ni4
Ni1
Ni1
Ni1
Ni1
Ni1
Ni1
Ni1
Ni1
Ni1
Ni1
(250)
MAO
(250)
MAO
(250)
MAO
(250)
MAO
(250)
MAO
(250)
MAO
(250)
MAO
(250)
MAO
(250)
MAO
(100)
MAO
(500)
EASC
(100)
EASC
(250)
MAO
(250)
94.6
(85)
9.0
5.0
94.9
(85)
3
9.0
5.0
95.3
(84)
4
9.5
5.1
96.7
(87)
5
8.4
4.1
90.0
16.8
(73)
89.4
15.3
(74)
91.6
17.0
(71)
89.2
20.5
(64)
92.8
13.5
(78)
90.7
18.2
(69)
6
9.4
7
8.7
8
9.1
9
11.2
10
11
12
13
14e
7.2
9.8
96.8
(94)
3.2
2.4
97.4
(95)
9.3
2.9
2.1
91.4
28.7
(15)
118.3
48.8
a
Reaction Conditions: toluene = 40 mL, PC2H4 = 20 bar, MAO as co-catalyst (250 equiv). The results shown
b
are representative of at least duplicated experiments. Mol of ethylene converted (mol of Ni)-1·h-1 as
c
determined by quantitative GLC. Cn, amount of olefin with n carbon atoms in oligomers; -Cn, amount of
terminal alkene in the Cn fraction, as determined by quantitative GLC. d [cis-C4]/[trans-C4-trans] molar ratio. e
triphenylphosphine (PPh3) as auxiliary ligand (1 equiv).
Activation of nickel precatalyst Ni1 with 100 equiv of ethylaluminum sesquichloride (Et3Al2Cl3,
EASC) instead of MAO gave a significantly lesser catalyst system than Ni1/MAO [TOF = 5,500 vs.
45,700 (mol C2H4)(mol Ni−1 h−1]. However, although the catalyst activity was reduced by
14
Page 16 of 24

approximately eight-fold, the 1-butene selectivity was substantially increased attaining 91.0% (compare
entries 10 and 12). Higher EASC loading (250 equiv) generated a better catalyst system (TOF = 9,300
(mol C2H4)(mol Ni−1 h−1), entry 12) with slightly improvement on selectivity for 1-butene (91.0 to
92.5%), and production of lower amounts of 2-butenes (5.6 to 5.0%) and hexenes (3.2 to 2.6%).
It is noteworthy to observe that the molar ratio between 2-cis- and 2-trans-butene isomers remains
constant ([cis-C4]/[trans-C4] = 1.9  0.1) for all oligomerization reactions carried out in presence of
MAO (entries 1-11). By replacing MAO with EASC this ratio decreases to 1.3  0.1 which suggests that
the nature of the active species are strictly dependent on the co-catalyst type used in the oligomerization
reaction.
Previous studies on nickel-based catalysts have demonstrated that incorporating PPh3 into catalytic
systems leads to higher activity and longer catalyst lifetime.[82][91-94] With this in mind we decided to
investigate the influence of PPh3 in our nickel catalyst system. Using 1 equiv of Ph3P, the activity of
Ni1/MAO increased dramatically, reaching a TOF of 118,300 (mol C2H4)(mol Ni−1 h−1) (entry 14],
which is 3 times higher than the catalytic system Ni1/MAO in the absence of PPh3 (TOF = 45,700 (mol
C2H4)(mol Ni−1 h−1), entry 1). The presence of this auxiliary ligand in the oligomerization medium
plays no significant impact on the total C4 production with selectivity of 91.4% (compare entries 1 and
14). On the other hand, the PPh3 ligand has a strong impact in the selectivity towards the production of
-olefins, decreasing substantially the amount of 1-butene (13.7%) with a concomitant increase of
internal olefins fractions (cis-C4: 28.7%; trans-C4 fractions: 48.8%). This is consistent with previous
reports [95-98], and could be attributed to a higher isomerization rate. In addition, it was verified that
the molar ratio between 2-cis- and 2-trans-butene changed significantly from 1.8 to 0.6 (compare entry
1 and 14), which strongly suggest that PPh3 is acting as a ligand. Similar trend has been observed for
ethylene oligomerization with MAO-activated nickel complexes bearing pyrazolyl ligands. [82]
15
Page 17 of 24

4. Conclusions
In summary, we have synthesized and characterized a series of new nickel(II) complexes supported
by phenyl ether-pyrazolyl ligands. The XPS results suggest that the probability of (co)existing dimeric
nickel complexes increases in the following order: Ni4 < Ni1 <Ni2 < Ni3. On treatment with MAO as
the co-catalyst, the Ni(II) complexes showed good activities for ethylene oligomerization and high
selectivity for 1-butene. The presence of bulkier ligands in Ni2/Ni3 caused a decrease in activity as
compared to the catalytic performance displayed by Ni1. The use of different co-catalyst (MAO, EASC)
influences the nature of the catalytic species. Thus, the activation of Ni1 with EASC promoted the
formation of very low active catalytic system with higher selectivity for 1-butene. In parallel, the use of
EASC instead of MAO caused a decrease of the molar ratio between 2-cis- and 2-trans-butene isomers
varying from 1.9 to 1.3.
Acknowledgments. This research was financially supported in part by Petrobras S/A and FAPERGS-
PRONEX (Brazil). A. H. D. P. S. Ulbrich, R. R. Campedelli, and J. L. Sônego gratefully acknowledge
CNPq, CAPES, and FAPERGS for fellowships.
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