Nickel(II) complexes bearing pyrazolylpyridines: Synthesis, structures and ethylene oligomerization reactions

Article abstract of DOI:10.1002/aoc.3402

Reactions of 2-bromo-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridine (L1) and 2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)pyridine (L2) with NiCl₂ and NiBr₂ led to the formation of their respective metal complexes [NiCl₂(L1)] (1), [NiBr₂(L1)] (2) and [NiBr₂(L2)] (3) in moderate to high yields. The complexes were characterized using elemental analyses, mass spectrometry and single-crystal X-ray diffraction for 2. The solid-state structure of 2 confirmed the bidentate coordination mode of L1 and formation of a monometallic compound. Activation of the nickel(II) pre-catalysts with methylaluminoxane afforded active catalysts in the ethylene oligomerization reaction to produce mainly butenes (84-86%). In contrast, activation of nickel(II) pre-catalyst 2 with ethylaluminium dichloride resulted in partial Friedel-Crafts alkylation of the toluene solvent by the preformed oligomers. Complex structure, nature of co-catalyst employed, type of solvent and reaction conditions influenced the catalytic behaviour of these pre-catalysts.

Full text of DOI:10.1002/aoc.3402

Full paper
Received: 8 September 2015
Revised: 2 October 2015
Accepted: 6 October 2015
Published online in Wiley Online Library: 16 November 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3402
Nickel(II) complexes bearing
pyrazolylpyridines: synthesis, structures
and ethylene oligomerization reactions
George S. Nyamato, Mohd. Gulfam Alam, Stephen O. Ojwach*
and Matthew P. Akerman
Reactions of 2-bromo-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridine (L1) and 2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)pyridine (L2) with
NiCl2 and NiBr2 led to the formation of their respective metal complexes [NiCl2(L1)] (1), [NiBr2(L1)] (2) and [NiBr2(L2)] (3) in
moderate to high yields. The complexes were characterized using elemental analyses, mass spectrometry and single-
crystal X-ray diffraction for 2. The solid-state structure of 2 confirmed the bidentate coordination mode of L1 and formation
of a monometallic compound. Activation of the nickel(II) pre-catalysts with methylaluminoxane afforded active catalysts in
the ethylene oligomerization reaction to produce mainly butenes (84–86%). In contrast, activation of nickel(II) pre-catalyst 2
with ethylaluminium dichloride resulted in partial Friedel–Crafts alkylation of the toluene solvent by the preformed oligo-
mers. Complex structure, nature of co-catalyst employed, type of solvent and reaction conditions influenced the catalytic be-
haviour of these pre-catalysts. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: nickel(II) complex; ethylene; oligomerization; co-catalyst; solvent
alkylation of the toluene solvent used by the pre-formed
Introduction
oligomers.[9] Modification of complexes of types I and II by
Ethylene oligomerization towards the production of linear α-olefins
has witnessed significant growth over past decades[1] due to the
applications of oligomer products in the synthesis of plasticizers,
lubricants and detergents, and as co-monomers in the synthesis of
high-density polyethylene and linear low-density polyethylene.[2]
The global consumption of co-monomer grade linear α-olefins in
the range C4–C8 is the largest and fastest-growing application.[1]
These linear α-olefins are mainly produced in three full-range pro-
cesses by Ethyl Corporation (INEOS), Gulf (Chevron Philips Chemical
Company) and the nickel-based Shell higher olefin process (SHOP).[3]
However, as a consequence of growing market demand for linear
α-olefins in the range C4–C10 rather than in the C12+ range,[4] inten-
sive efforts have been devoted towards finding more selective routes
that can direct product distribution towards shorter α-olefins.[1]
Since the discovery of α-diimine nickel(II) complexes as highly ef-
ficient pre-catalysts for ethylene polymerization,[5] nitrogen-donor
nickel(II) complexes have continued to attract much interest as eth-
ylene oligomerization catalysts. Much effort has particularly been
devoted to the modification of known ligands or to the design
and development of new ligand systems with a view to enhancing
both activity and selectivity.[6] We have previously reported the use
of pyrazolyl late transition metal catalysts in ethylene oligomeriza-
tion and polymerization reactions.[7–11] The results from those
studies, which are also consistent with other recent reports,[12–15]
indicate that catalytic properties greatly depend on reaction condi-
tions, aluminium co-catalysts and ligand architecture. For example,
(pyrazol-1-ylmethyl)pyridine nickel complexes I and II (Scheme 1)
catalyse ethylene oligomerization to C4, C6 and C8 alkenes upon
activation with ethylaluminium dichloride (EtAlCl2), followed by
introducing a chloromethyl group in the pyridine ring, to give com-
plexes III, results in the production of mainly C4 and C6 oligomers
upon activation with EtAlCl2. Even though these reactions also lead
to Friedel–Crafts alkylation, the absence of any C8 oligomer
fractions demonstrates the pivotal role played by ligand design in
the development of ethylene oligomerization catalysts.[16]
Herein, we report the syntheses and characterization of
some nickel(II) complexes bearing bidentate and tridentate
(pyrazolyl)pyridine ligands (types IV and V) as well as their cata-
lytic performance in ethylene oligomerization. The influences of
catalyst structure, co-catalyst/complex ratio, time and pressure
on the ethylene oligomerization reactions have been studied
and are herein discussed.
Experimental
Materials and general considerations
All synthetic manipulations were performed under a nitrogen at-
mosphere using standard Schlenk techniques. All solvents were of
*
Correspondence to: Stephen O. Ojwach, School of Chemistry and Physics,
University of KwaZulu-Natal, Scottsville, South Africa. E-mail: ojwach@ukzn.ac.za
School of Chemistry and Physics, University of KwaZulu-Natal, Scottsville, South
Africa
Appl. Organometal. Chem. 2016, 30, 89–94
Copyright © 2015 John Wiley & Sons, Ltd.

G. S. Nyamato et al.
dissolved. To this solution was added 2.0g (8.44 mmol) of 2,6-
dibromopyridine and the resulting mixture was stirred for 96 h.
The solvent was removed in vacuo, and water was added resulting
in the formation of a light brown precipitate. The compound was
then filtered and purified by column chromatography on silica gel
using hexane to dichloromethane in the ratio of 1:1 to afford a
white solid.
Yield: 1.29g (57%).1H NMR (CDC13, δ, ppm): 2.32 (s, 6H, CH3, pz),
2.60 (s, 6H, CH3, pz), 6.01 (s, 2H, pz), 7.69 (d, 2H, 3,5-py-H,
3JHH = 8.0Hz), 7.87 (dd, 1H, 4-py-H, 3JHH = 8.0Hz). 13C NMR (CDC13,
δ, ppm): 13.41 (3-pz-CH3), 14.14 (5-pz-CH3), 109.01 (4-pz-C), 113.55
(3,5-py-C), 140.43 (4-py-C), 141.28 (5-pz-C), 150.08 (2,6-py-C),
151.51 (3-pz-C). ESI-MS: m/z (%) 290 [(M + Na)+]. Anal. Calcd for
C15H17N5 (%): C, 67.39; H, 6.41; N, 26.20. Found (%): C, 67.37; H,
5.98; N, 26.31.
Scheme 1. (Pyrazolyl)pyridine Ni(II) complexes.
Synthesis of [{2-bromo-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridine}NiCl2] (1)
analytical grade and were dried and distilled prior to use.
Dibromopyridine, 3,5-dimethylpyrazole, diethylene glycol dimethyl
ether, EtAlCl2 (1.0M in hexane), methylaluminoxane (MAO; 10 wt%
in toluene), and metal salts were obtained from Sigma-Aldrich and
used as received.
Complex 1 was prepared by adding a solution of NiCl2 (0.05 g,
0.39 mmol) in dichloromethane (5ml) to a solution of L1 (0.10 g,
0.39 mmol) in dichloromethane (5ml). The resultant solution was
stirred at room temperature for 24 h. The resultant green solution
was then concentrated to about 5 ml and a green product was pre-
cipitated by addition of diethyl ether to afford complex 1 as a green
solid.
1H NMR and 13C {1H} NMR spectra were recorded with a Bruker
Ultrashield 400 (1H NMR 400 MHz, 13C {1H} NMR 100 MHz) spec-
trometer in CDCl3 solution at room temperature and chemical shifts
were determined relative to internal tetramethylsilane and are
given in ppm. Elemental analyses were performed with a Thermo
Scientific Flash 2000 while mass spectra were recorded with an LC
premier micro-mass spectrometer. Magnetic moments of the com-
plexes were determined using an Evans balance. GC analyses were
performed using a Varian CP-3800 gas chromatograph equipped
with a flame ionization detector and a 30 m (0.2mm inner diameter,
0.25 μm film thickness) CP-Sil 5 CB capillary column. GC-MS analy-
ses were performed using a Shimadzu GCMS-QP2010 fitted with a
quadrupole mass detector.
Yield: 0.11 g (71%). ESI-MS: m/z (%) 346 (M+ À Cl, 100%). Anal.
Calcd for C10H10BrN3NiCl2 (%): C, 31.47; H, 2.64; N, 11.01. Found
(%): C, 31.07; H, 2.86; N, 10.75.
Synthesis of [{2-bromo-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridine}NiBr2] (2)
Complex 2 was prepared following the procedure described for 1.
[NiBr2] (0.09 g, 0.39 mmol) and L1 (0.10 g, 0.39 mmol). Pale green
solid. Yield: 0.15 g (83%). ESI-MS: m/z (%) 469 (M+, 65%); 389
(M+ À Br, 72%). Anal. Calcd for C10H10N3NiBr3Á2H2O (%): C, 23.71;
H, 2.79; N, 8.29. Found (%): C, 23.43; H, 2.33; N, 7.95.
Synthesis of [{2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)pyridine}NiBr2] (3)
Syntheses of ligands and their metal complexes
Complex 3 was prepared following the procedure described for 1.
[NiBr2] (0.10g, 0.46mmol) and L2 (0.12g, 0.46mmol). Sky-blue solid.
Yield: 0.14 g (64%). ESI-MS: m/z (%) 404 (M+ À Br, 100%). Anal. Calcd
for C15H17N5NiBr2 (%): C, 37.08; H, 3.53; N, 14.42. Found (%): C, 36.78;
H, 3.41; N, 13.97.
Synthesis of 2-bromo-6-(3,5-dimethyl-1H-pyrazol-1-yl)pyridine (L1)
A solution of 0.81g of 3,5-dimethylpyrazole (8.44mmol) in 20 ml of
diethylene glycol dimethyl ether (diglyme) was stirred with 0.33 g
of potassium (8.44 mmol) at 60°C until the metal dissolved. To this
solution was added 2.0g (8.44 mmol) of 2,6-dibromopyridine and
the resulting mixture was stirred for 48h. The solvent was removed
in vacuo, and water was added resulting in the formation of a pre-
cipitate, a light brown solid. The compound was then filtered and
purified by column chromatography on silica gel using hexane to
dichloromethane in the ratio of 1:1 to afford a white solid.
Yield: 1.72 g (81%). 1H NMR (CDCl3, δ, ppm): 2.30 (s, 3H, CH3, pz),
2.67 (s, 3H, CH3, pz), 6.01 (s, 1H, pz) 7.29 (d, 1H, 3-py-H, 3JHH = 8.0 Hz),
7.61(dd, 1H, 4-py-H, 3JHH =8.0 Hz), 7.85 (d, 1H, 5-py-H, 3JHH = 8.0 Hz).
13C NMR (CDC13, δ, ppm): 13.58 (3-pz-CH3), 14.66 (5-pz-CH3), 109.71
(4-pz-C), 113.56 (3-py-C), 124.26 (4-py-C), 138.87 (5-py-C), 140.24
(6-py-C), 142.28 (5-pz-C), 150.48 (2-py-C), 153.01 (3-pz-C). ESI-MS:
m/z (%) 273 [(M + Na)+, (79Br) 100%], 275 [(M + Na)+, (81Br) 97%].
Anal. Calcd for C10H10BrN3 (%): C, 47.64; H, 4.00; N, 16.67. Found
(%): C, 48.11; H, 3.58; N, 17.02.
X-Ray crystallography
X-ray data for compound 2 were recorded using a Bruker Apex Duo
equipped with an Oxford Instruments Cryojet operating at 100(2) K
and an Incoatec micro-source operating at a power of 30 W. Crystal
and structure refinement data are given in Table 1. The data were
collected using Mo Kα (λ = 0.71073 Å) radiation at a crystal-to-
detector distance of 50 mm. The following conditions were used
for data collection: omega and phi scans with exposures taken at
X-ray power of 30 W and frame widths of 0.50° using APEX2.[17]
The data were reduced with the programme SAINT[17] using outlier
rejection, scan speed scaling as well as standard Lorentz and polar-
ization correction factors. A SADABS semi-empirical multi-scan
absorption correction[17] was applied to the data. Direct methods,
SHELX-2014[18] and WinGX[19] were used to solve all three struc-
tures. All non-hydrogen atoms were located in the difference
density map and refined anisotropically with SHELX-2014.[18] All
hydrogen atoms were included as idealized contributors in the
least squares process. Their positions were calculated using a
standard riding model with C–Haromatic distances of 0.93 Å and
Synthesis of 2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)pyridine (L2)
A solution of 1.62 g of 3,5-dimethylpyrazole (16.85 mmol) in 20 ml
of diethylene glycol dimethyl ether (diglyme) was stirred with
0.67 g of potassium (16.85 mmol) at 120 °C until the metal
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Appl. Organometal. Chem. 2016, 30, 89–94

Ethylene oligomerization catalysed by nickel(II) complexes
Table 1. Crystal data and structure refinement for 2
dichloromethane afforded the corresponding nickel(II) metal
complexes 1–3 in moderate to high yields (Scheme 2).
Parameter
Value
Due to the paramagnetic nature of complexes 1–3, NMR spec-
troscopy was not useful for their structural elucidation. Their
identities were thus established using elemental analyses, mass
spectrometry (Figs S3 and S4) and single-crystal X-ray analysis for
2. Analysis by mass spectrometry indicates that the fragmentation
of these complexes occurs in a similar fashion, characterized by
halide abstraction. For example, the ESI mass spectrum of 1 (Fig. S3)
shows a peak at m/z = 345.92 (M+ À Cl) as the base peak. Since
chlorine is isotopic, 35Cl and 37Cl, an additional peak is also
observed at m/z = 347.92. Elemental analyses data of 1–3 are
consistent with one ligand motif and one metal centre in the
complexes as proposed in Scheme 2 and confirm their purity.
Empirical formula
C
10H12Br3N3NiO
Formula weight
488.67
Temperature (K)
100(2)
Wavelength (Å)
0.71073
Monoclinic
P21/n
Crystal system
Space group
a (Å)
9.4883(3)
7.5257(3)
21.4124(8)
90
b (Å)
c (Å)
α (°)
β (°)
90.546(2)
90
γ (°)
Volume(Å3)
1528.91(10)
4
Molecular structure of complex 2
Z
Density (calculated) (Mg mÀ3
)
2.123
A single crystal of complex 2 suitable for X-ray analysis was
obtained from slow diffusion of n-hexane into a solution of the
complex in dichloromethane at room temperature. The complex
has a monoclinic crystalline system and its space group is P21/n.
The molecular structure and hydrogen bonding of complex 2 are
shown in Figures 1 and 2, respectively, while the crystallographic
data and structure refinement parameters are given in Table 1.
As shown in Figure 1, the solid-state structure of complex 2 is
monometallic containing one nickel metal centre and one ligand
Absorption coefficient (mmÀ1
)
9.100
F(000)
936
Theta range for data collection (°)
Reflections collected
2.34 to 26.12
15 766
Completeness to theta = 26.48° (%)
Goodness-of-fit on F2
99.0
1.028
R indices (all data)
Largest diff. peak and hole(e ÅÀ3
R1 = 0.0186, wR2 = 0.0363
0.382 and À0.275
)
Uiso = 1.2 Ueq, C–Hmethylene distances of 0.99 Å and Uiso = 1.2 Ueq
and C–Hmethyl distances of 0.98 Å and Uiso = 1.5 Ueq.
General procedure for ethylene oligomerization reactions
Ethylene oligomerization reactions were carried out in a 400 ml stain-
less steel Parr reactor equipped with a mechanical stirrer, tempera-
ture controller and internal cooling system. In a typical experiment,
the reactor was preheated to 100 °C in vacuo and cooled to room
temperature. The appropriate amount of the catalyst precursor
(10.0 μmol) was transferred into a dry Schlenk tube under nitrogen
and toluene (20 ml) was added using a syringe. The required amount
of co-catalyst (EtAlCl2 or MAO) was then injected into the Schlenk
tube containing the pre-catalyst to form the active catalytic system.
The resultant solution was then transferred via cannula into the reac-
tor. An additional 60 ml of toluene solvent was also transferred via
cannula into the reactor giving a total volume of 80 ml. The reactor
was then flushed three times with ethylene and the desired pressure
and temperature were set and the reaction started. After the reaction
time, the reactor was cooled to À20 °C using a bath of ice–liquid
nitrogen and excess ethylene vented off. The reaction was then
quenched by addition of 10% HCl (5 ml). A portion of the reaction
mixture was sampled for GC and GC-MS analyses to determine the
oligomer by comparison with standard authentic samples. The
amount of products formed was determined by mass difference of
80 ml of toluene (69.60 g) and mass of the final solution.
Scheme 2. Synthesis of pyrazolylpyridine nickel(II) pre-catalysts.
Results and discussion
Synthesis of (pyrazolyl)pyridine nickel(II) complexes
Figure 1. Molecular structure diagram of 2 shown with 50% probability
ellipsoids. Selected bond lengths (Å) and angles (°): Ni(1)–N(1), 1.9937(17);
Ni(1)–N(3), 2.0561(17); Ni(1)–Br(1), 2.4783(3); Ni(1)–Br(2), 2.4797(3); N(1)–Ni
(1)–N(3), 79.62(6); N(1)–Ni(1)–Br(1), 94.06(5); N(3)–Ni(1)–Br(1), 90.08(4); O
(1)–Ni(1)–N(3), 171.18(7); Br(1)–Ni(1)–Br(3), 165.512(13).
L1 and L2 were prepared in 81 and 78% yields, respectivelys by
following literature procedures.[20] Reactions of L1 and L2 with
the appropriate nickel(II) metal salts in a 1:1 mole ratio in
Appl. Organometal. Chem. 2016, 30, 89–94
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G. S. Nyamato et al.
All the complexes are active for ethylene oligomerization reac-
tions to afford moderate catalytic activities with turnover frequen-
cies of between 300 and 1400 kg (mol Ni)À1 À1. A combination
h
of GC and GC-MS analyses confirms that the ethylene oligomeriza-
tion products consist of C4, as the predominant product, C6, C8 and
trace amounts of double alkylated (C2)2-toluenes. Ethylene oligo-
merization reactions accompanied by Friedel–Crafts alkylation of
toluene solvent by the pre-formed oligomers is consistent with
our recent studies.[10] However, it is important to note that, in
this study, only trace amounts of the alkylated toluene products
are observed (1–3%) compared to complete alkylation in other sys-
tems we previously reported.[9–11] This is supported by the forma-
tion of polyethylene (no alkylation reported) using cobalt(II) and
iron(II) complexes of 2,6-bis(pyrazol-1-yl)pyridine and 2,6-bis(3,4,5-
trimethyl-pyrazol-1-yl)pyridine, respectively (analogous ligands to
L2) in combination with MAO as the co-catalyst.[21]
Figure 2. Hydrogen bonding in compound 2. The coordinated water
molecule bridges two adjacent molecules through hydrogen bonding to
the bromide ligands.
L1 unit. Ligand L1 coordinates to the central metal in a bidentate
mode, with two bromide ligands and one water molecule complet-
ing the coordination sphere. Thus, the coordination sphere of the
nickel centre adopts a slightly distorted square pyramidal geome-
try, with N(1) occupying the apical position. The four basal atoms
consist of N(2), Br(1), Br(3) and O(1). Pyridyl and pyrazolyl rings as
well as the methyl groups are on the same molecular plane,
forming a five-membered ring, NiÀN(1)ÀN(3)ÀC(4)ÀN(2), thus
rendering the complex structurally rigid. The hydrogen bonding
in compound 2 (Fig. 2) is quite interesting and relatively strong.
The hydrogen bonding involves a coordinated water molecule
bridging two adjacent molecules through hydrogen bonding to
the bromide ligands. This leads to a one-dimensional hydrogen-
bonded chain collinear with the b-axis.
Effect of catalyst structure on ethylene oligomerization reactions
The identity and denticity of the chelating ligand motif (bidentate
or tridentate) influence the catalytic performance of the nickel
pre-catalysts in ethylene oligomerization reactions (Table 2, entries
1–3). It is evident that catalysts 1 and 2 supported by the bidentate
ligand (L1) are more active than complex 3 bearing the tridentate
ligand (L2). For example, while the tridentate-bound catalyst 3
has an activity of 300 kg (mol Ni)À1 hÀ1 (Table 2, entry 3), the corre-
sponding complex 2 displays higher catalytic activity of 1000 kg
(mol Ni)À1 hÀ1 (Table 2, entry 2). This enhanced performance of
the bidentate system could be due to the greater electrophilicity
of the complexes owing to the presence of the bromide group on
the pyridine ring.[22,23] A similar trend was reported by Svjeda and
Brookhart[24] using α-diimine nickel(II) complexes as ethylene oligo-
merization catalysts. For example, they observed that variation of
methyl to trifluoromethyl group resulted in a two-fold increase in
the turnover frequency of the resultant catalysts. This observation
is in agreement with previous reports on the catalytic performances
Ethylene oligomerization with Ni(II) complexes 1–3
Ethylene oligomerization studies were carried out with nickel(II) pre-
catalysts 1–3 in the presence of MAO or EtAlCl2 as co-catalysts. The
results of the oligomerization reactions are summarized in Table 2.
Table 2. Ethylene oligomerization data for 1–3 using MAO as co-catalyst in toluenea
Entry Cat. Pressure (bar) Time (h) Tmin/Tmax (°C)b Al:Ni Yield (g)c
TON
Product distribution (%)e
(kg (mol Ni)À1 hÀ1
)
d
C4 C6 C8 α-C4 α-C6 α-C8 Alkylated
toluenes
1
1
2
3
2
2
2
2
2
2
2f
2g
10
10
10
10
10
5
1
30/32
30/33
30/31
30/33
30/32
30/33
30/33
30/32
30/33
30/41
30/46
1000
1000
1000
500
8
10
3
800
1000
300
86
84
86
89
85
91
7
8
8
6
8
6
5
7
87
86
87
92
88
81
89
89
82
53
100
57
53
49
58
55
51
56
55
48
59
52
37
38
31
43
39
35
41
39
33
—
—
2
1
2
1
3
1
5
1
4
1
4
400
3
2
5
1
1500
1000
1000
1000
1000
250
8
800
5
2
6
1
5
500
2
1
7
20
10
10
10
10
1
14
4
1400
800
79 11
90
7
3
8
0.5
2
6
3
1
9
15
15
18
750
78 11
27 18
53 47
8
3
10
11
1
1500
1800
—
—
46
—
1
1000
aReaction conditions: nNi = 10 μmol; solvent, toluene, 80 ml; temperature, 30 °C.
bInitial temperature was 30 °C; Tmin and Tmax are the lowest and highest temperatures attained during the reaction period.
cDetermined by mass difference of 80 ml of toluene (69.60 g) and mass of final solution.
dTurnover number; kg of oligomer per mol (Ni) per hour.
eDetermined by GC.
fCo-catalyst: EtAlCl2.
gSolvent: chlorobenzene.
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Ethylene oligomerization catalysed by nickel(II) complexes
of bidentate systems compared to tridentate systems.[9,22] Another
plausible reason for the higher catalytic activities of the bidentate
systems could be steric factors. From the ligand shielding
concept,[25] L1 is less sterically demanding compared to the
tridentate ligand L2. Thus complexes 1 and 2 (containing L1) are
not much shielded and allow easier ethylene monomer coordina-
tion, hence the observed higher catalytic activities.[7,9] On the other
hand, the greater shielding of nickel(II) in 3 owing to the bulkier
ligand L2 could hinder substrate coordination leading to the
observed reduced catalytic activities.
It is also observed that the dibromide complex is more active than
its dichloride counterpart (Table 2, entry 1 versus 2). The lower cata-
lytic activity of the dichloride catalyst compared to its corresponding
dibromide analogue is in agreement with other previous reports[26–30]
and has been attributed to enhanced solubility of the dibromide
complexes in toluene compared to the dichloride complexes.
We observe that all the complexes show greater selectivity to-
wards butenes. As evident from Table 2, the selectivity for 1-butene
afforded by pre-catalysts 1–3 is comparable (84–86%), indicating
that the ligand architecture has little influence on the product
distribution. Moreover, minimal amounts of hexenes (7–8%) and
octenes (5–7%) are detected with lower selectivities for 1-hexene
(49–57%) and 1-octene (31–38%). This is rather unusual as one
would expect the bulkier complex 3 to favour chain propagation
and thus lead to higher oligomer fractions (C6 and C8) compared
to complexes 1 and 2.[31] These results allude to faster chain termi-
nation during the oligomerization process, a factor largely pro-
moted by high ethylene insertion barriers.[32]
α-olefins is also observed at higher pressure (Table 2, entries 2, 6
and 7) which may be attributed to chain transfer being favoured
over chain isomerization.[38]
The effect of co-catalyst/pre-catalyst ratio on the catalytic perfor-
mance of complex 2 was studied by varying the Al:Ni ratio from 500
to 1500. We observe an increase in catalytic activity from 400 to
1000 kg (mol Ni)À1 hÀ1 for Al:Ni ratios of 500 and 1000, respectively.
Beyond this ratio of 1000, the activity decreases to 800 kg (mol
Ni)À1 hÀ1 at an Al:Ni ratio of 1500, thereby giving an optimum ratio
of 1000 (Table 2, entries 2, 4 and 5). The decrease in catalytic activity
at higher co-catalyst loading could be attributed to catalyst deacti-
vation due to an increase in trialkylaluminium impurities.[35,39] In
terms of selectivity, lower MAO concentrations favour the forma-
tion of α-olefins. For example, the amount of α-C4 decreases from
92 to 86% at Al:Ni ratios of 500 and 1000, respectively.
In our previous studies, we showed the crucial contribution of
the nature of the solvent to ethylene oligomerization product
compositions.[10] In the present work, activation of nickel pre-catalyst
2 with 250 equiv. of EtAlCl2 instead of MAO results in the formation
of more active species to afford activities as high 1500 kg
(mol Ni)À1 hÀ1 (Table 2, entries 2 and 10). Despite this marked im-
provement in catalytic activity, the formation of ethylene oligomers
suffers from the predominant formation of Friedel–Crafts alkylated
products (46%). It is important to note, however, that unlike in our
earlier report[10] where all the preformed C4 and C6 oligomers were
subsequently alkylated to toluene, catalyst 2 partially promotes
Friedel–Crafts alkylation of the toluene solvent used. Similar
observations have also been made for (3,5-dimethylpyrazol-1-yl)
methylbenzene nickel dihalide complexes[11] and nickel(II)
complexes based on formazane derivatives.[40] Consistent with
recent studies,[10,37,41] ethylene oligomerization reactions performed
in chlorobenzene (Table 2, entry 11) result in the formation of mainly
oligomers without any Friedel–Crafts alkylation products. In addition,
Effect of reaction conditions on ethylene oligomerization reactions
Based on the preliminary results of ethylene oligomerization reac-
tions using complexes 1–3, complex 2 was used to optimize the re-
action conditions. We thus investigated the influence of reaction
time, ethylene pressure, Al:Ni ratio and co-catalyst type. Increasing
the reaction time from 0.5 to 1 h leads to enhanced activity from
800 to 1000 kg (mol Ni)À1 hÀ1 (Table 2, entries 2 and 8). However,
further increase of reaction time to 2 h leads to a decrease in cata-
lytic activity to 750 kg (mol Ni)À1 hÀ1 (Table 2, entry 9). This trend
may be associated with an activation process between 0.5 and
1 h (generating the active species) and deactivation of the active
species between 1 and 2 h.[33,34] We also observe that reaction time
influences the product distribution. For instance, increasing reac-
tion time from 0.5 to 2 h is characterized by an increase of the C6
fraction from 6 to 11% (Table 2, entries 4 versus 9). This is followed
by a subsequent decrease in the amount of C4 from 90 to 78%. A
similar trend has also been observed for hydrazone nickel(II)
complexes[35] where there is a decrease of the amount of C4 from
92.2% at 10min to 86.7% at 40 min while the amount of C6
increases in the course of this time from 7 to 13.1%. Furthermore,
increasing the reaction time from 0.5 to 2 h reduces the selectivity
towards α-alkenes, with, for example, α-C4 declining from 89 to
82% (Table 2, entries 8 and 9). This points to increased chain
isomerization with time and indicates that the α-olefins are first
formed before undergoing isomerization to afford the correspond-
ing internal isomers.
enhanced catalytic activity (1800 kg (mol Ni)À1 À1) in chloroben-
h
zene is observed which may be attributed to improved solubility of
the pre-catalyst in chlorobenzene compared to toluene. These
findings further underscore the role of solvent in controlling the
catalytic activity and product distribution in these ethylene oligomer-
ization reactions.
Conclusions
Nickel(II) complexes bearing (pyrazolyl)pyridine ligands were syn-
thesized and structurally characterized. The complexes form active
catalysts for ethylene oligomerization when activated with MAO
and EtAlCl2 co-catalysts. While activation with MAO afforded mainly
1-butene oligomers, use of EtAlCl2 co-catalyst resulted in partial
alkylation of the preformed oligomers to produce alkyltoluenes.
Both the catalytic activity and product selectivity were mostly
dependent on the nature of alkylaluminium co-catalyst employed,
nature of solvent and structure of complex.
Acknowledgments
The authors thank the University of KwaZulu-Natal for financial
support.
The catalytic activity of 2 was also examined at different ethylene
pressures. It is observed that an increase in ethylene pressure signif-
icantly increases the catalytic activity. For instance, an increase in
ethylene pressure from 5 to 20 bar results in an increased activity
from 500 to 1400 kg (mol Ni)À1 hÀ1 (Table 2, entries 2, 6 and 7). Con-
sistent with literature reports,[24,36,37] improved selectivity towards
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Appl. Organometal. Chem. 2016, 30, 89–94