Nickel(II) complexes bearing pyrazolylimine ligands: Synthesis, characterization, and catalytic properties for ethylene oligomerization

Article abstract of DOI:10.1016/j.ica.2008.03.068

Four phenyl-substituted pyrazolylimine ligands 2-(C₃HN₂Me₂-3,5)(C(Ph){double bond, long}N(4-R₂C₆H₂(R₁)₂-2,6)) (L1: R₁ = ⁱPr, R₂ = H; L

Full text of DOI:10.1016/j.ica.2008.03.068

Inorganica Chimica Acta 362 (2009) 166–172
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Nickel(II) complexes bearing pyrazolylimine ligands: Synthesis,
characterization, and catalytic properties for ethylene oligomerization
*
Yuan-yuan Wang, Shang-an Lin, Fang-ming Zhu , Hai-yang Gao, Qing Wu
Institute of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four phenyl-substituted pyrazolylimine ligands 2-(C3HN2Me2-3,5)(C(Ph)@N(4-R2C6H2(R1)2-2,6)) (L1:
R1 = iPr, R2 = H; L2: R1 = H, R2 = NO2; L3: R1 = R2 = H; L4: R1 = H, R2 = OCH3) were synthesized. The influences
of steric bulk and electronic effect of pyrazolylimine ligands on the structures of their corresponding nickel
complexes were investigated. Ligands with more bulky and electron withdrawing substituents on N-phe-
nyl ring produced four-coordinate nickel complexes (2-(C3HN2Me2-3,5))(C(Ph)@(4-R2C6H2(R1)2-2,6)NiBr2
(1, R1 = iPr, R2 = H; 2, R1 = H, R2 = NO2)), whereas the ligands with less bulky and electron donating substit-
uents on N-phenyl ring formed bis-pyrazolylimine dinickel tetrahalides (bis-2-(C3HN2Me2-3,5))
(C(Ph)@N(4-R2C6H2 (R1)2-2,6)Ni2Br4 (3, R1 = R2 = H; 4, R1 = H, R2 = OCH3)) and six-coordinate nickel diha-
lides (bis-2-(C3HN2Me2-3,5))(C(Ph)@N(4-R2C6H2(R1)2-2,6) NiBr2 (5, R1 = R2 = H; 6, R1 = H, R2 = OCH3)).
The solid-state structures of complexes 1, 4 and 5 have been confirmed by X-ray single-crystal analyses.
Activated by methylaluminoxane (MAO), complexes 1, 2, 5 and 6 showed moderate to high activity for eth-
ylene oligomerization, and complex 5 revealed the highest activity up to 8.96 Â 105 g oligomer/(mol Ni Á h).
The proportions of resultant oligomers were mainly C4–C8 and a little C10–C14 determined by gas chroma-
tography/mass spectrometry.
Received 30 October 2007
Accepted 14 March 2008
Available online 20 March 2008
Keywords:
Pyrazolylimine nickel complexes
X-ray single-crystal analyses
Ethylene oligomerization
Methylaluminoxane
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
a-diimines and pyridines, whose nickel or palladium complexes
were used for ethylene oligomerization.
In recent years, there has been considerable interest in the
development of the oligomerization and polymerization of olefins
based on the late transition metal catalysts [1–8]. The most famous
example of these catalysts is the SHOP-type complexes which
showed high activity and selectivity for the conversion of ethylene
to linear a-olefins [9–20]. Since Brookhart and co-workers reported
that nickel complexes bearing diimine ligands were effective for
ethylene oligomerization and polymerization, the interest in
designing new catalysts of late transition metal complexes with
various nitrogen ligands was in an increasing exploration [21].
These ligands have been including diimine [21–32], bipyridine
[33], unsymmetric pyridinylimine [34–39], as well as alkylnitrile
[40]. There were also a few reports about late metal complexes
bearing pyrazole and pyrazolyl ligands for ethylene oligomeriza-
tion and polymerization [41–45].
Herein, we imagined to select and design a kind of late metal
catalysts possessing the merits of pyrazoles being weaker r-donors
and a-diimines bearing steric hindrances. In the present investiga-
tion, various phenyl-substituted pyrazolylimine nickel complexes
1–6 including 2-(C3HN2Me2-3,5)(C(Ph)@N(4-R2C6H2(R1)2-2,6))
NiBr2 (1, R1 = iPr, R2 = H; 2, R1 = H, R2 = NO2), bis-2-(C3HN2Me2-
3,5)(C(Ph)@N(4-R2C6H2(R1)2-2,6)Ni2Br4 (3, R1 = R2 = H; 4, R1 = H,
R2 = OCH3)) and bis-2-(C3HN2Me2-3,5)(C(Ph)@N(4-R2C6H2(R1)2-
2,6))NiBr2 (5, R1 = R2 = H; 6, R1 = H, R2 = OCH3) were synthesized
and characterized. These complexes were used as catalysts for
ethylene oligomerization in the presence of MAO.
2. Experimental
2.1. General procedures
Darkwa and his colleagues reported substituted pyrazoles nick-
el and palladium complexes as ethylene polymerization catalysts
[43,44]. They revealed that in these catalyst systems monomer
insertion had to be faster than agostic interactions in order to pre-
vent ‘‘chain walking” since pyrazoles were weaker r-donors than
All manipulations were performed under nitrogen atmosphere
using glove box and Schlenk techniques. Polymerization-grade
ethylene and extra-pure-grade nitrogen were further purified be-
fore feeding into the reactor by passing them through a DC-IB
gas purification instrument. Toluene and hexane were refluxed
over metallic sodium for 24 h, and CH2Cl2 was refluxed over P2O5
for 8 h, and then they were distilled under nitrogen atmosphere
* Corresponding author. Tel.: +86 020 84113250; fax: +86 020 84112245.
E-mail address: ceszfm@mail.sysu.edu.cn (F.-m. Zhu).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.03.068

Y.-y. Wang et al. / Inorganica Chimica Acta 362 (2009) 166–172
167
before use. 2,6-Diisopropylaniline, aniline and 3,5-dimethylpyra-
zole were purchased from Aldrich. 2,6-Diisopropylaniline and ani-
line were distilled under reduced pressure before use. The other
reagents were purchased and used as received. 1H NMR spectra
were recorded on a Varian Mercury-Plus 300 MHz NMR spectrom-
eter and referenced versus TMS as standard. Elemental analyses
were determined with a Vario EL Series Elemental Analyzer from
Elementar. The GC–MS data were recorded with a Finnigan Voy-
ager GC-8000 Top Series GC–MS System with DB-5MS GC column
and the GC spectra were recorded with Varian CP3800 Series GC
System with a HP-5MS GC column. MS-FAB spectra were obtained
with a VG ZAB-HS scan instrument, using m-nitrobenzylalcohol as
matrix.
7.39 (m, 3H, Ar-H); 7.20–7.08 (m, 2H, Ar-H); 6.81–6.71 (m, 3H,
Ar-H); 5.80 (s, 1H, Pz-H); 2.28 (s, 3H, Pz-CH3); 1.78 (s, 3H, Pz-CH3).
3.4. 2-(C3HN2Me2-3,5)(C(Ph)@N(C6H4OCH3-4)) (L4)
L4 was prepared according to the method describes for L1 using
(C6H4OCH3-4)NH((C6H5)C@O) (3.41 g, 15.0 mmol) and 3,5-dimeth-
ylpyrazole (1.44 g, 15.0 mmol). The resulting mixture was purified
by column chromatography on silica gel using petroleum ether/
ethyl acetate (5/1) as eluent, and removed the solvent to afford
L4 as yellow ropy substance in 58.6% yield (2.69 g, 8.8 mmol).
FAB+-MS: m/z: 304, 305, 306, 307, [M+]; 226, 227, 228, [M+Àph];
210, 211, 212, [M+ÀC5H7N2]. Anal. Calc. for C19H19N3O: C, 74.73;
H, 6.27; N, 13.76. Found: C, 74.43; H, 6.52; N, 13.70%. 1H NMR
(300 MHz, CDCl3, ppm): d: 7.65–7.62 (d, 2H, Ar-H); 7.48–7.37 (m,
3H, Ar-H); 6.78–6.67 (m, 4H, Ar-H); 5.84 (s, 1H, Pz-H); 3.78 (s,
3H, OCH3); 2.31 (s, 3H, Pz-CH3); 1.77 (s, 3H, Pz-CH3).
3. Preparation
i
3.1. 2-(C3HN2Me2-3,5)(C(Ph)@N(C6H3 Pr2-2,6)) (L1)
3.5. 2-(C3HN2Me2-3,5)(C(Ph)@N(4-R2C6H2(R1)2-2,6))NiBr2
(1, R1 = iPr, R2 = H; 2, R1 = H, R2 = NO2)
To a solution of 2,6-diisopropylaniline (2.0 ml, 10.6 mmol) in
THF (50 ml) and triethylamine (2.0 ml, 14.6 mmol) were added
benzoyl chloride (1.2 ml, 10.6 mmol). The reaction mixture was
stirred for 12 h at room temperature. After the filtration of
(C2H5)3N Á HCl and the evaporation of THF, the white solid
2.0 mmol of (DME)NiBr2 was added under nitrogen atmosphere
to the 2.0 mmol of L1 or L2 which was dissolved in 40 ml dry
CH2Cl2. The mixture was stirred at room temperature for 18 h.
For 1, the resulting solution was concentrated and then hexane
was added to precipitate the product which was washed with
20 ml of hexane and dried in vacuum. As for 2, owing to its poor
solubility in CH2Cl2, the resulting solid was filtrated and the prod-
uct was washed with 20 ml CH2Cl2 and dried in vacuum. 2-
i
((C6H3 Pr2-2,6)NH((C6H5)C@O)) was obtained. The thionyl chloride
(1.1 ml, 15.1 mmol) was added to amide (2.81 g, 10.0 mmol), and
the reaction mixture was heated to reflux for 2 h. After the removal
of the redundant thionyl chloride under reduced pressure, toluene
(50 ml), triethylamine (1.7 ml, 12.2 mmol) and 3,5-dimethylpyra-
zole (0.96 g, 10.0 mmol) were added to the reaction system. After
the addition was completed, the mixture was heated to reflux for
12 h. The (C2H5)3N Á HCl was removed by filtration, toluene was
evaporated and the resulting mixture was recrystallized from hex-
ane to afford L1 as light yellow crystals in 80.5% yield (2.89 g,
8.1 mmol). FAB+-MS: m/z: 358, 359, 360, 361, [M+]; 280, 281, 282,
[M+ÀPh]; 263, 264, 265, [M+ÀC5H7N2]. Anal. Calc. for C24H29N3: C,
80.18; H, 8.13; N, 11.69. Found: C, 79.96; H, 8.12; N, 11.53%. 1H
NMR (300 MHz, CDCl3, ppm): d: 7.22–7.00 (m, 8H, Ar-H); 6.05 (s,
1H, Pz-H); 2.99–2.89 (m, 2H, CH); 2.61 (s, 3H, Pz-CH3); 2.23 (s,
3H, Pz-CH3); 1.13–1.11 (d, 6H, CH3); 0.89–0.84 (d, 6H, CH3).
i
(C3HN2Me2-3,5)(C(Ph)@N(C6H3 Pr2-2,6))NiBr2 (1), purple. Yield,
78.6%. FAB+-MS: m/z: 496, 497, 498, 499, 500, [M+ÀBr]; 414, 415,
416, 417, 418, 419, [MÀ2Br]2+; 264, 265, [L1-C5H7N2]+. Anal. Calc.
for C24H29N3NiBr2: C, 49.87; H, 5.06; N, 7.27. Found: C, 49.74; H,
5.36; N, 7.10%. 2-(C3HN2Me2-3,5)(C(Ph)@N(C6H4NO2-4))NiBr2 (2),
yellow. Yield, 63.7%. FAB+-MS: m/z: 457, 458, 459, 460, 461,
[M+ÀBr]; 377, 378, 379, 380, [MÀ2Br]2+; 225, 226, [L2-C5H7N2]+.
Anal. Calc. for C18H16N4O2NiBr2: C, 40.12; H, 2.99; N, 10.40. Found:
C, 40.19; H, 3.22; N, 9.84%.
3.6. Bis-2-(C3HN2Me2-3,5)(C(Ph)=N(4-R2C6H2(R1)2-2,6))Ni2Br4
(3, R1 = R2 = H; 4, R1 = H, R2 = OCH3)
3.2. 2-(C3HN2Me2-3,5)(C(Ph)@N(C6H4NO2-4)) (L2)
2.0 mmol of (DME)NiBr2 was added under nitrogen atmosphere
to the 2.0 mmol of L3 or L4 which was dissolved in 40 ml dry
CH2Cl2. After the mixture was stirred at room temperature for
18 h, the resulting solution was concentrated and then hexane
was added to precipitate the product which was washed with
20 ml of hexane and dried in vacuum. Bis-2-(C3HN2Me2-
3,5)(C(Ph)@N(C6H5))Ni2Br4 (3), green. Yield, 74.9%. FAB+-MS: m/z:
686, 688, 690, [M+ÀNiBr3]; 608, 610, [MÀNiBr4]2+; 411, 412, 413,
414, 415, [M+ÀNiBr3-L3]; 331, 332, 333, 334, 335, [MÀNiBr4-
L3]2+; 180, 181, [L3-C5H7N2]+. Anal. Calc. for C36H34N6Ni2Br4: C,
43.78; H, 3.47; N, 8.51. Found: C, 43.94; H, 3.35; N, 8.62%. Bis-2-
(C3HN2Me2-3,5)(C(Ph)@N(C6H4OCH3-4))Ni2Br4 (4), green. Yield,
79.6%. FAB+-MS: m/z: 746, 748, [M+ÀNiBr3]; 667, 668, 669,
[MÀNiBr4]2+; 442, 443, 444, 445, 446, [M+ÀNiBr3-L4]; 362, 363,
364, 365, [MÀNiBr4-L4]2+; 210, 211, [L4-C5H7N2]+. Anal. Calc. for
C38H38N6O2Ni2Br4: C, 43.56; H, 3.66; N, 8.02. Found: C, 43.08; H,
3.92; N, 7.84%.
L2 was prepared according to the method describes for L1 using
(C6H4NO2-4)NH((C6H5)C@O) (3.63 g, 15.0 mmol) and 3,5-dimeth-
ylpyrazole (1.44 g, 15.0 mmol). The resulting mixture was purified
by column chromatography on silica gel using petroleum ether/
ethyl acetate (5/1) as eluent, and recrystallization was attempted
from ethanol to afford L2 as yellow crystals in 62.3% yield
(2.99 g, 9.4 mmol). FAB+-MS: m/z: 320, 321, 322, [M+]; 242, 243,
244 [M+ÀPh]; 225, 226, [M+ÀC5H7N2]. Anal. Calc. for C18H16N4O2:
C, 67.49; H, 5.03; N, 17.49. Found: C, 67.38; H, 5.05; N, 17.46%.
1H NMR (300 MHz, CDCl3, ppm): d: 8.08–8.06 (m, 4H, Ar-H);
7.70–7.65 (m, 1H, Ar-H); 6.85–6.83 (m, 4H, Ar-H); 5.88 (s, 1H,
Pz-H); 2.48 (s, 3H, Pz-CH3); 1.89 (s, 3H, Pz-CH3).
3.3. 2-(C3HN2Me2-3,5)(C(Ph)@N(C6H5)) (L3)
L3 was prepared according to the method describes for L1 using
(C6H5)NH((C6H5)C@O) (1.97 g, 10.0 mmol) and 3,5-dimethylpyra-
zole (0.96 g, 10.0 mmol). The products were recrystallized from
hexane to afford L3 as light yellow crystals in 77.8% yield (2.14 g,
7.8 mmol). FAB+-MS: m/z: 274, 275, 276, [M+]; 196, 197, 198,
[M+ÀPh]; 179, 180, 181, [M+ÀC5H7N2]. Anal. Calc. for C18H17N3: C,
78.52; H, 6.22; N, 15.26. Found: C, 78.22; H, 6.25; N, 15.23%. 1H
NMR (300 MHz, CDCl3, ppm): d: 7.67–7.64 (d, 2H, Ar-H); 7.50–
3.7. Bis-2-(C3HN2Me2-3,5)(C(Ph)@N(4-R2C6H2(R1)2-2,6))NiBr2
(5, R1 = R2 = H; 6, R1 = H, R2 = OCH3)
2.0 mmol of (DME)NiBr2 was added under nitrogen atmosphere
to the 4.0 mmol of L3 or L4 which was dissolved in 60 ml dry
CH2Cl2. After the mixture was stirred at room temperature for

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Y.-y. Wang et al. / Inorganica Chimica Acta 362 (2009) 166–172
18 h, the resulting solution was concentrated and then hexane was
added to precipitate the product which was washed with 40 ml of
hexane and dried in vacuum. Bis-2-(C3HN2Me2-3,5)(C(Ph)@
N(C6H5))NiBr2 (5), green. Yield, 96.2%. FAB+-MS: m/z: 687, 689,
691, [M+ÀBr]; 608, 610, [MÀ2Br]2+; 411, 412, 413, 414, 415,
[M+ÀBr-L3]; 331, 332, 333, 334, 335, [MÀ2Br-L3]2+; 180, 181,
[L3-C5H7N2]+. Anal. Calc. for C36H34N6NiBr2: C, 56.21; H, 4.46; N,
10.93. Found: C, 56.42; H, 4.28; N, 10.62%. Bis-2-(C3HN2Me2-
3,5)(C(Ph)@N(C6H4OCH3-4))NiBr2 (6), green. Yield: 98.0%. FAB+-
MS: m/z: 746, 748, [M+ÀBr]; 668, [MÀ2Br]2+; 441, 442, 443, 444,
445, [M+ÀBr-L4]; 362, 363, 364, 365, [MÀ2Br-L4]2+; 210, 211,
[L4-C5H7N2]+. Anal. Calc. for C38H38N6O2NiBr2: C, 55.04; H, 4.62;
N, 10.13. Found: C, 55.40; H, 4.63; N, 9.73%.
under 0.05 Mpa of ethylene for the desired time. The flask was
cooled to À20 °C and the quenched with concentrated HCl in eth-
anol (10 ml, HCl/ethanol, 5:95, v/v). About 2 ml organic solution
was dried by anhydrous Na2SO4 for GC–MS analysis.
3.9. X-ray structure determination
The crystals were mounted on a glass fiber using the oil drop
scan method, date obtained with the x–2h scan mode were
collected on a Bruker SMART 1000 CCD diffractometer with
graphite-monochromated MoKa radiation (k = 0.71073 Å). The
structures were solved by direct methods, while further refine-
ment with full-matrix least squares on F2 was obtained with the
SHELXTL program package. All non-hydrogen atoms were refined
anistotropically. Hydrogen atoms were introduced in calculated
positions with the displacement factor of the host carbon atoms.
3.8. Ethylene oligomerization procedure
The oligomerization with 0.05 Mpa of ethylene was carried out
as follows. In a typical procedure, the appropriate MAO solid was
added into a 50 ml flask. The flask was back-filled twice with eth-
ylene, and then 19ml freshly distilled toluene was added via syr-
inge under ethylene atmosphere at the desired polymerization
temperature. The resulting mixture was stirred for a further
10 min, and the precursor catalyst solution in 1 ml toluene was in-
jected via a syringe. The reaction solution was vigorously stirred
4. Results and discussion
4.1. Syntheses of pyrazolylimine ligands and nickel(II) complexes
Four phenyl-substituted pyrazolylimine ligands L1–L4 were
synthesized (Scheme 1) according to the reported literature [46].
In the first step, the amide formation occurred rapidly at room
R1
R1
R1
O
Cl
PhCOCl
SOCl2
reflux
R2
NH2
R2
N C
H
R2
N C
Et3N, THF, RT
R1
CH3
R1
R1
NH
N
CH3
L1 R1=i-Pr, R2=H
L2 R1=H, R2=NO2
L3 R1=R2=H
Ph
N
R1
H3C
Et3N, Toluene, reflux
N
N
R2
H3C
L4 R1=H, R2=OCH3
R1
Scheme 1. Synthesis of pyrazolylimine ligands L1–L4.
CH3
(DME)NiBr2
CH3
Ph
N
R1
Ph
N
R1
N
N
N
N
1 R1=i-Pr, R2=H
2 R1=H, R2=NO2
R2
H3C
0.5(DME)NiBr2
R2
H3C
Ni
Br
R
1
Br
R1
R2
R1
Ph
H3C
N
N
Ni
N
Br
Br
R
1Br
(DME)NiBr2
H3C
3 R1=R2=H
4 R1=H, R2=OCH3
Ni
R1
Br
CH3
N
CH3
N
R2
Ph
N
R1
N
Ph
R1
N
N
R2
H3C
H3C
R1
CH3
R1
Ph
R2
N
N
N
0.5(DME)NiBr2
H3C
R1
R1
CH3
5 R1=R2=H
6 R1=H, R2=OCH3
Br Ni Br
N
N
N
R2
Ph
R1
CH3
Scheme 2. Synthesis of pyrazolylimine nickel complexes 1–6.

Y.-y. Wang et al. / Inorganica Chimica Acta 362 (2009) 166–172
169
temperature via the reaction of corresponding substituted anilines
(or aniline) and benzoyl chloride. In the second step, the interme-
diate benzimidolyl chlorides were prepared from the correspond-
ing benzamides by treatment with thionyl chloride. In the last
step, the syntheses of pyrazolylimines were based on the reaction
of benzimidolyl chlorides with 3,5-dimethylpyrazole in the pres-
ence of triethylamine. The pure pyrazolylimines could be obtained
after purification in yield 58–80%.
A general synthetic route for pyrazolylimine nickel(II) com-
plexes 1–6 was shown in Scheme 2. We attempted to synthesize
mono-pyrazolylimine nickel complexes and bis-pyrazolylimine
nickel complexes by the way of using different ratios of pyrazolyli-
mines with (DME)NiBr2. Nickel dibromide complexes 1 and 2 were
prepared under moderate conditions by the reaction of (DME)NiBr2
with both 1.0 and 2.0 equiv. of L1 with bulky 2,6-diisopropyl
substituent or L2 with electron withdrawing 4-nitro substituent
on N-phenyl ring in CH2Cl2 at room temperature in yield 78.6%
for complex 1, and 63.7% for complex 2, respectively. When (DME)-
NiBr2 was in reaction with 1.0 equiv. of L3 with less steric bulk or
L4 with electron donating 4-methoxy substituent on N-phenyl
ring, it was suggested that the resultant complexes 3 and 4 should
be mono-pyrazolylimine dibromide based on the characterization
of element analyses at first. However, the structures of the ob-
tained complexes 3 and 4 were not similar to complexes 1 and 2
based on the characterization of single-crystal X-ray diffraction
and MS-FAB analysis. When L3 or L4/(DME)NiBr2 molar ratios were
2.0, pure bis-pyrazolylimine nickel complexes 5 and 6 were
obtained in high yield more than 95%.
Fig. 2. ORTEP diagram for complex 4. Hydrogen atoms were omitted for clarity.
4.2. Single-crystal X-ray structures of complexes 1, 4 and 5
Single crystals of complexes 1, 4 and 5 suitable for single-crystal
X-ray diffraction study were grown from CH2Cl2/hexane solutions
under nitrogen atmosphere at room temperature. The single-crys-
tal X-ray structures of complexes 1, 4 and 5 were obtained, and the
ORTEP diagrams for these three compounds are included in Figs. 1–
3, respectively. The crystal data, together with the data collection
and structure refinement parameters are presented in Table 2,
and the selected bond lengths and angles were summarized in
Table 1.
Complex 1 adopts a distorted tetrahedral geometry structure in
which the central Ni atom binds to two bromine atoms and one
bidentate pyrazolylimine ligand. In the predicted structure, the
Fig. 3. ORTEP diagram for complex 5. Hydrogen atoms were omitted for clarity.
five-membered chelate ring is substantially flat and appears not
to be significantly strained. The narrow N1–Ni–N3 angle
80.35(13)° results from the relatively small bite size of the ligand.
The 2,6-diisopropyl-phenyl ring of the imine side arm adopts a po-
sition almost perpendicular to the metal coordination plane with
the 2,6-diisopropyl-phenyl-coordination plane angle 84.68(19)°,
and thus shields the metal center from one side. Moreover, the
phenyl group on the C6 atom swings out of the coordination plane
forming a dihedral angle of 67.53 (19)°.
Complex 4 is crystallized in a dimeric structure. The Ni1 atom
adopts a distorted octahedral geometry with two bidentate ligands
and two bromines in which two pyrazole-N atoms are situated
in trans positions (N6–Ni1–N4, 163.68(18)°) and two imine-N
atoms (N1–Ni1–N2, 93.13(16)°) and two Br atoms(Br2–Ni1–Br1,
88.24(3)°) are cis to each other. However, the Ni2 atom binds to four
bromine atoms forming a distorted tetrahedral geometry structure.
It is not similar to the dimeric nickel or palladium complexes bear-
ing pyridinylimine [35,37] and diimine ligands [47], in which each
Fig. 1. ORTEP diagram for complex 1. Hydrogen atoms were omitted for clarity.

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Y.-y. Wang et al. / Inorganica Chimica Acta 362 (2009) 166–172
Table 1
Selected bond lengths and bond angles for complexes 1, 4 and 5
1
4
5
Bond lengths (Å)
Ni–Br(1)
Ni–N(1)
Ni–N(3)
Ni–Br(2)
Bond lengths (Å)
Ni(1)–N(1)
Ni(1)–N(6)
Ni(1)–N(2)
Ni(1)–N(4)
Ni(1)–Br(2)
Ni(1)–Br(1)
Ni(2)–Br(4)
Ni(2)–Br(3)
Ni(2)–Br(2)
Ni(2)–Br(1)
Bond lengths (Å)
Ni(1)–N(3)
Ni(1)–N(3)A
Ni(1)–O(1 W)
Ni(1)–O(1W)A
Ni(1)–N(1)
2.3599(7)
1.967(3)
2.032(3)
2.3145(7)
2.065(5)
2.066(5)
2.074(5)
2.086(5)
2.5857(9)
2.5969(9)
2.3576(9)
2.3667(9)
2.4221(10)
2.4233(10)
2.069(4)
2.069(4)
2.076(3)
2.076(3)
2.091(4)
2.091(4)
Ni(1)–N(1)A
Bond angles (°)
N(1)–Ni–N(3)
N(1)–Ni–Br(2)
N(3)–Ni–Br(2)
N(1)–Ni–Br(1)
N(3)–Ni–Br(1)
Br(2)–Ni–Br(1)
Bond angles (°)
N(1)–Ni(1)–N(6)
N(1)–Ni(1)–N(2)
N(6)–Ni(1)–N(2)
N(1)–Ni(1)–N(4)
N(6)–Ni(1)–N(4)
N(2)–Ni(1)–N(4)
Br(2)–Ni(1)–Br(1)
Br(4)–Ni(2)–Br(3)
Br(2)–Ni(2)–Br(1)
Bond angles (°)
N(3)–Ni(1)–N(3) A
N(3)–Ni(1)–N(1)
N(3)–Ni(1)–O(1W)
N(1)–Ni(1)–N(1)A
N(3)A–Ni(1)–O(1W)
N(3)A–Ni(1)–N(1)
O(1W)–Ni(1)–N(1)
O(1W)A–Ni(1)–N(1)
O(1W)–Ni(1)–O(1W)A
80.35(13)
110.68(10)
124.88(10)
101.08(10)
107.95(10)
121.20(3)
91.99(18)
93.13(16)
78.21(18)
77.32(18)
163.68(18)
89.96(18)
88.24(3)
94.4(2)
76.45(15)
172.28(14)
167.2(2)
89.73(14)
94.78(15)
96.74(16)
92.52(16)
86.9(2)
118.28(3)
96.25(3)
Table 2
Crystallographic data for complexes 1, 4 and 5
1
4
5
Empirical formula
Fw
T (K)
C24H29N3NiBr2
578.01
293(2)
C38H38N6O2Ni2Br4
1047.75
173(2)
C36H44N6NiBr3O5
939.18
293(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
orthorhombic
P2(1)2(1)2(1)
10.1271(10)
15.1013(16)
15.8589(17)
90
orthorhombic
Pna2(1)
22.3373(17)
20.1266(15)
18.3954(13)
90
monoclinic
I2/a
17.600(4)
10.537(2)
23.031(7)
90
a (°)
b (°)
90
90
2425.3(4)
4
1.583
90
90
93.201(4)
90
4264.3(19)
2
1.447
3.310
c (°)
V (Å3)
8270.1(11)
4
1.677
4.814
4144
Z
Dcalc (Mg/m3)
Absorption coefficient (mmÀ1
F(000)
)
4.110
1168
1860
Crystal size (mm)
h range (°)
Reflections collected
Unique reflections
Completeness to h (%)
Data/restraints/parameters
Goodness-of-fit on F2
0.31 Â 0.20 Â 0.10
1.86–27.06
14708
0.48 Â 0.20 Â 0.11
1.36–27.05
48447
0.39 Â 0.27 Â 0.19
1.77–26.08
9914
5309
17601
4172
99.9 (27.06)
5309/0/271
1.017
99.7 (27.05)
17601/1/937
0.985
98.5 (26.08)
4172/0/242
1.018
Final R indices [I > 2r(I)]: R1, wR2
R indices (all data)
R1 = 0.0362, wR2 = 0.0739
R1 = 0.0596, wR2 = 0.0859
0.451 and À0.410
R1 = 0.0394, wR2 = 0.0788
R1 = 0.0838, wR2 = 0.0959
0.785 and À0.508
R1 = 0.0452, wR2 = 0.1271
R1 = 0.0756, wR2 = 0.1447
0.751 and À0.375
Largest difference in peak and hole (e ÅÀ3
)
Table 3
metal atom is coordinated to two ligand nitrogen atoms as
two bridging halogen atoms connect to two metal centers. The
effect of Ni2 atom contributes to further structural distortion lead-
ing to unequal nickel–nitrogen and nickel–bromine bond
lengths: Ni(1)–N(1) 2.065(5) Å, Ni(1)–N(2) 2.074(5) Å; Ni(1)–N(4)
2.086(5) Å, Ni(1)–N(6) 2.066(5) Å; Ni(1)–Br(1) 2.5969(9) Å, Ni(1)–
Br(2) 2.5857(9) Å. In the predicted structure, the 4-methoxyphenyl
group on the N1 swings out of the plane formed by the pyrazolyli-
mine chelate ring, forming a dihedral angle of 75.3(3)°. However,
the dihedral angle of 4-methoxyphenyl group on the N2 and the
chelate ring is 83.6(3)°.
Ethylene oligomerization with complexes 1, 2, 5 and 6/MAO catalytic systems and the
distributions of oligomersa
Entry
Complex
Activityb
Distribution of oligomers (%)
C4
C6
C8
C10–C14
1
2
3
4
1
2
5
6
0.82
3.17
4.56
2.80
12.9
32.2
41.8
28.2
22.7
50.3
47.1
43.0
49.8
14.7
9.2
14.6
2.8
1.9
24.8
4.0
a
General conditions: solvent toluene, total volume 20 ml, 5 lmol nickel complex,
molAl/molNi 400, temperature 20 °C, reaction time 1 h, 0.05 MPa ethylene.
b
105 g oligomer/(mol Ni Á h).
The single crystal structure of complex 5 bearing two pyra-
zolylimine ligands is crystallized with two molecules of water
which substituted bromine atoms coordinated to nickel atom mak-
ing this a six coordinate octahedral complex, having molecular C2
symmetry. Similar to complex 4, the two pyrazole-N atoms are
situated in trans positions (N1–Ni1–NlA, 167.2(2)°), while the

Y.-y. Wang et al. / Inorganica Chimica Acta 362 (2009) 166–172
171
Table 4
Influences of temperature and Al/Ni molar ratio on ethylene oligomerization with complexes 2, 5/MAO catalytic systems and the distributions of oligomersa
Entry
Complex
Tp (°C)
Al/Ni
Activityb
Distribution of oligomers (%)
C4
C6
C8
1.1
14.7
13.5
5.6
1.9
15.5
2.2
1.9
2.4
9.2
8.9
C10–C14
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
2
2
2
2
2
2
2
2
5
5
5
5
5
5
5
5
0
20
40
60
40
40
40
40
0
20
40
60
20
20
20
20
400
400
400
400
200
600
800
1000
400
400
400
400
200
600
800
1000
0.72
3.17
3.60
3.27
1.83
7.03
6.76
6.20
1.80
4.56
4.27
2.36
1.92
8.96
6.07
5.46
81.5
32.2
16.8
42.5
34.7
27.4
33.3
26.8
58.4
41.8
45.5
55.7
53.0
28.1
17.0
35.1
16.5
50.3
63.7
47.6
63.1
51.4
63.6
70.9
38.8
47.1
43.1
40.2
32.2
50.7
28.4
53.7
0.9
2.8
6.0
4.3
0.3
5.7
0.9
0.4
0.4
1.9
2.5
0.5
1.7
2.9
1.8
2.1
3.6
13.1
18.3
52.8
9.1
a
General conditions: solvent toluene, total volume 20 ml, 5 lmol nickel complex, reaction time 1 h, 0.05 MPa ethylene.
105 g oligomer/(mol Ni Á h).
b
two imine-N atoms (N3–Ni1–N3A, 94.4(2)°) and the two oxygen
atoms (O1W–Ni1–O1WA 86.9(2)°) oriented cis to each other at
the central nickel atom.
5. Conclusions
Phenyl-substituted pyrazolylimine ligands and their nickel(II)
complexes were successfully synthesized and characterized. The
structures of pyrazolyimine nickel complexes depended on the
substituents on N-phenyl ring. The 2,6-diisopropyl- or 4-nitro-phe-
nyl-substituted pyrazolylimine ligands produced four-coordinate
nickel dihalide complexes (1 and 2), and 4-methoxy- or non-
substituted pyrazolylimine ligands formed bis-pyrazolylimine
dinickel tetrahalides (3 and 4) and six-coordinate bis-pyrazolyli-
mine nickel dihalides (5 and 6). Moreover, the structures of 3
and 4a re not similar to the dimeric nickel or palladium complexes
bearing pyridinylimine or diimine ligands. These nickel(II) com-
plexes catalyzed ethylene to produce mainly C4–C8 oligomers in
the presence of MAO. The electronic effect and steric hindrance
of substituents on N-phenyl ring could significantly influence the
catalytic activity and the distributions of oligomers.
4.3. Ethylene oligomerization
On treatment with MAO, the pyrazolylimine nickel complexes
1, 2, 5 and 6 were used as catalysts for ethylene oligomerization.
It is notable that the r-donor of pyrozolylimine ligands are similar
to a-diimines or pyridines. However, compared with a-diimines
nickel complexes, the pyrazolylimine complexes have not enough
steric hindrances towards to the axial positions of the coordinated
plane for synthesis of polyethylene. Table 3 shows the oligomeriza-
tion activity and molecular weight distributions of the oligomers
produced by resultant nickel complexes. Complexes 1, 2, 5 and 6
displayed moderate to high activity in ethylene oligomerization.
The oligomers were mainly C4–C8 and a little C10–C14 detected
by GC–MS analysis. GC–MS analysis confirmed that C4 was mostly
2-butylene; C6 and C8 were chiefly linear inner alkene.
Acknowledgements
Table 3 revealed that the proportion of C8–C14 oligomers pro-
duced by complex 1 was much higher than complex 2 owing to
the steric effect of bulky 2,6-diisopropyl substituent and the elec-
tron withdrawing effect of the 4-nitro substituent. And complex
5 shows higher activity than complex 6 because of the 4-methoxy
substituent’s electron donating property.
The financial supports of the National Natural Science Founda-
tion of China (Contract Grant Number: 20604034) and the
Guangdong Natural Science Foundation (Contract Grant Number:
039184) are gratefully acknowledged.
Appendix A. Supporting information available
The influences of temperature and Al/Ni molar ratio on the
activity as well as the molecular weight distribution of oligomers
obtained by complexes 2 and 5 are summarized in Table 4. The
activities for ethylene oligomerization with 2/MAO and 5/MAO cat-
alytic systems increased following the increase of temperature
below 40 °C (for 2) and 20 °C (for 5). However, the activities
decreased when the temperatures were above 40 °C (for 2) and
20 °C (for 5). This may be ascribed to decomposition of the active
species and lower ethylene solubility at higher temperature
[33,48,49]. Increasing temperature leads to an increase in the pro-
portions of higher oligomers (C8–C14) below 40 °C, and above
40 °C the distributions of oligomers shift to low carbon olefins
(C4–C6) with the increase in temperature. This suggests that
increasing temperature have a heavier impact on the b-H elimina-
tion than on the chain propagation in the oligomerization process.
The highest oligomerization activity of 2/MAO and 5/MAO were
observed when the Al/Ni molar ratios are 600 (2, 7.03 Â 105 g
oligomer/(mol Ni Á h); 5, 8.96 Â 105 g oligomer/(mol Ni Á h)).
CCDC 652631–652633 contain the supplementary crystallo-
graphic data for 1, 4 and 5. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2008.03.068.
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