(Ferrocenylpyrazolyl)nickel(II)-catalysed ethylene oligomerisation

Article abstract of DOI:10.1039/c5cy02055g

Compounds L1-L6 (3-ferrocenylpyrazole (L1), 3-ferrocenyl-5-methylpyrazole (L2), 3-ferrocenylpyrazolyl-methylenepyridine (L3), 3-ferrocenyl-5-pyrazolyl-methylenepyridine (L4), 3-ferrocenylpyrazolyl-ethylamine (L5) and 3-ferrocenyl-5-pyrazolyl-ethylamine (L6)) reacted with [NiBr₂(DME)] or [NiCl₂·6H₂O] to give the mononuclear nickel complexes [NiBr₂(κ¹-L1)₂] (1), [NiBr₂(κ¹-L2)₂] (2), [NiBr₂(κ²-L3)] (3), [NiBr₂(κ²-L4)] (4), [NiBr₂(κ²-L5)] (5), [NiBr₂(κ²-L6)] (6), [NiCl₂(κ²-L3)] (7) and [NiCl₂(κ²-L4)] (8). Because these nickel complexes are paramagnetic, they were characterised by a combination of IR spectroscopy, mass spectrometry, elemental analysis and, in selected cases, single crystal X-ray crystallography. Activation of complexes 1-8 with EtAlCl₂ in chlorobenzene produced active species that catalysed ethylene oligomerization to butenes and C₁₆-C₆₄ olefins, showing a non-Schulz-Flory distribution of products. Complexes 2 and 3 were the most active (1989 kg of ethylene oligomer per mol of Ni per h and 1776 kg of ethylene oligomer per mol of Ni per h, respectively) and, in toluene, produced isomers of butene and small amounts of butyltoluenes via Friedel-Crafts alkylation of toluene by the butenes.

Full text of DOI:10.1039/c5cy02055g

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Page 1 of 32
Catalysis Science & Technology
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(Ferrocenylpyrazolyl)nickel(II) catalysed ethylene oligomerisaDtiOoI:n10.1039/C5CY02055G
Collins Obuah,a Johan H. L. Jordaanb and James Darkwaa*
aDepartment of Chemistry, University of Johannesburg, Auckland Park Kingsway Campus,
Auckland Park 2006, South Africa.
bFocus Area Chemical Resource Beneficiation, Catalysis and Synthesis group North-West University,
Potchefstroom, 2531, South Africa.
Abstract
Compounds L1-L6 (3-ferrocenylpyrazole (L1), 3-ferrocenyl-5-methylpyrazole (L2), 3-
ferrocenylpyrazolyl-methylenepyridine (L3), 3-ferrocenyl-5-pyrazolyl-methylenepyridine
(L4), 3-ferrocenylpyrazolyl-ethylamine (L5) 3-ferrocenyl-5-pyrazolyl-ethylamine (L6))
reacted with [NiBr2(DME)] or [NiCl2.6H2O] to give the mononuclear nickel complexes
[NiBr2(1-L1)2] (1), [NiBr2(1-L2)2] (2), [NiBr2(2-L3)] (3), [NiBr2(2-L4)] (4), [NiBr2(2-
L5)] (5), [NiBr2(2-L6)] (6), [NiCl2(2-L3)] (7) and [NiCl2(2-L4)] (8). Because these nickel
complexes are paramagnetic they were characterised by a combination of IR spectroscopy,
mass spectrometry, elemental analysis and in selected cases by single crystal X-ray
crystallography. Activation of complexes 1-8 with EtAlCl2 in chlorobenzene produced active
species that catalysed ethylene oligomerization to butenes and C16-C64 olefins; showing a non-
Schulz-Flory distribution of products. Complexes 2 and 3 were the most active (1 989 kg of
ethylene oligomer.mol.-1(Ni).h-1 and 1 776 kg of ethylene oligomer.mol.-1(Ni).h-1 respectively)
and in toluene produced isomers of butene and small amounts of butyltoluenes via Friedel-
Crafts alkylation of toluene by the butenes.
Introduction
Nickel complexes bearing bidentate ligands are known to exhibit higher catalytic activities for
ethylene oligomerisation and polymerisation reactions than those containing tridentate
1

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ligands.1 As a result progress in using bidentate nickel complexes to catalyse ethylene reactions
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DOI: 10.1039/C5CY02055G
has been rapid in recent years. Majority of bidentate ligands for single-site nickel complexes
are made from homo- or hetero- donor ligands such as P^P, P^N, P^O, N^O and N^N and these
complexes tend to catalyse the production of branched polymers.2 The branching in the
polymers are mainly the result of β-hydride elimination reactions during the polymerisation
process leading to methyl branches and “chain walking” that form longer branches such as
propyl and amyl.3 This has led to increased interest in both academia and industry to further
study new nickel(II) catalysts, especially modifications of bidentate N^N ligands. As a result
of these modifications, various nitrogen donor ligands such as α-diimine derivatives,4 1,10-
phenanthroline derivatives5 and quinolone derivatives,6 and recently pyrazoles7 have been
studied as ligands in nickel catalysed ethylene oligomerisation and polymerisation reactions.
The extent of progress with these N^N ligands is seen in the several review articles on this
subject.8 However, the utilisation of these newly developed nickel(II) catalysts in industrial
applications is yet to be realised due to a number of critical issues concerning both the nature
of the catalytic systems and the usefulness of the resulting products that still remain. To
overcome these shortcomings, nickel catalysts bearing ligands with bulky substituents such as
benzhydryl-groups have been explored to produce thermally stable catalysts that have high
catalytic activities and also to understand the effect of such bulky substituents.9
Our contribution to enhancing catalyst stability and high activity include using bulky ferrocenyl
group(s) to make pyrazolyl ligands to prepare nickel precatalysts. Herein we report novel nickel
catalysts that oligomerises ethylene to butenes but skip the usually observed Schultz-Flory
product distribution to form C16+ olefins.
2

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Catalysis Science & Technology
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Results and discussion
Synthesis and characterisation of nickel complexes
Compounds L1-L6 were prepared as reported in literature10 and reacted with the nickel
precursors [NiBr2(DME)] and [NiCl2.6H2O] to produce eight new (ferrocenylpyrazolyl)nickel
complexes (Schemes 1 and 2). Complexes 1 and 2 were prepared by reacting L1 or L2 and
[NiBr2(DME)] in a 2:1 ratio while the reaction of [NiBr2(DME)] or [NiCl2.H2O] in a 1:1 ratio
with ligands L3-L6 gave complexes 3-8. Complexes 1 and 2 are light orange solids, while 3-
8 are light brown solids. Complexes 1-4, 7 and 8 are air and moisture stable and were stored
without special precautions, but 5 and 6 are extremely moisture sensitive and had to be stored
in a glovebox. All these nickel complexes are soluble in most organic solvents but insoluble in
diethyl ether and hexane.
Because complexes 1-8 are paramagnetic they were characterised by IR spectroscopy, mass
spectrometry, magnetic moment, micro analysis and for 3a and 4a by single crystal X-ray
crystallography.
3

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Scheme 1: Synthesis of (ferrocenylpyrazolyl)nickel(II) bromide complexes
4

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Catalysis Science & Technology
Scheme 2: Synthesis of (ferrocenylpyrazolyl)nickel(II) chloride complexes
5

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IR spectroscopic data for complexes 1-8 showed the presence of N-containing functional groups.
Complexes 1 and 2 contain a secondary amine where the N-H stretching vibrations showed a shift
to higher frequencies from 3078 cm-1 (LI) and 3118 cm-1 (L2) for the respective ligands to between
3244 cm-1 (1) and 3257 cm-1 (2) for the respective complexes. Complexes 5 and 6 that have primary
amine functional groups showed N-H stretching vibration shifting to lower frequencies of 3111
cm-1 (5) and 3193 cm-1 (6) in the complexes from 3313 cm-1 (L5) and 3324 cm-1 (L6). Their
bending vibration also shifted from 1555 cm-1 (L5) and 1585 cm-1 (L6) to 1616 cm-1 (5) and 1626
cm-1 (6). Complexes 3, 4, 7 and 8 showed υ(C=N) bands for the corresponding ligands between
1620 cm-1 and 1628 cm-1. These suggest that in all complexes the nickel atom is coordinated to the
respective ligands.11 Mass spectrometry of the complexes showed various fragments of their
proposed structures (Schemes 1 and 2). For instance, the mass spectrum of complex 7 (Figure S1)
showed a molecular ion at m/z = 493.9389 corresponding to [M+Na]+ and other fragments with
m/z values of 437.9782, 402.0028 and 366.0674 which represents [M-Cl]+, [M-2Cl]+ and [M-
NiCl2+Na]+ respectively, supporting the proposed molecular structures in Schemes 1 and 2.
Magnetic susceptibility measurement at room temperature on complexes 1-8 gave µeff in the range
3.34 to 3.94 BM, typical of tetrahedral nickel complexes that have two unpaired electrons.12-15
Single crystal X-ray structures of 3a and 4a confirmed the tetrahedral nature of these nickel
complexes. In addition to the above techniques used to characterised 1-8, elemental analyses were
consistent with the proposed molecular structures in Schemes 1 and 2 and also confirmed the
analytical purity of these complexes. Crystallographic and structural refinement data can be found
in Table S1; while molecular structures are in Figures 1 and 2, each of which has selected bond
lengths and angles in the figure caption. Bond angles for 3a and 4a are in the range 93.72(14)o to
111.35(3)o. The Ni-N bonds in both complexes are in the range 2.000-2.015(18) Å for Ni-N(py)
6

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and 1.999-2.004(18) Å for Ni-N(pz) and similar to Ni-N bond lengths of 2.005-2.121(2) Å reported
in the literature.16 The cyclopentadienyl rings in 3a and 4a are eclipsed and have dihedral angles
of 3.080(1)o and 1.907(4)o respectively.
Figure 1: Molecular structure of 3a, drawn with 50% probability ellipsoids. Selected bond lengths
[Å] and bond angles [o]: Br1-Ni1, 2.3715(4); Br2-Ni1, 2.3614(4); Ni1-N1, 2.000(18); Ni1-N3,
2.004(18); N1-C5, 1.352(3); N2-N3, 1.362(3); N2-C6, 1.458(3); N1-Ni1-N3, 94.20(7); N1-Ni1-
Br2, 104.24(5); N3-Ni1-Br2, 129.17(5); N1-Ni1-Br1, 106.88(5); N3-Ni1-Br1, 102.68(5); Br2-
Ni1-Br1, 115.837(17); N2-C6-C5, 113.59(18).
7

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Figure 2: Molecular structure of 4a, drawn with 50% probability ellipsoids. Selected bond lengths
[Å] and bond angles [o]: Br1-Ni1, 2.3728(8); Br2-Ni1, 2.3637(7); Ni1-N1, 2.015(4); Ni1-N3,
1.999(3); N1-C5, 1.347(5); N2-N3, 1.372(5); N2-C6, 1.452(5); N1-Ni1-N3, 93.72(14); N1-Ni1-
Br2, 106.09(10); N3-Ni1-Br2, 133.62(10); N1-Ni1-Br1, 107.67(10); N3-Ni1-Br1, 101.29(10);
Br2-Ni1-Br1, 111.35(3); N2-C6-C5, 112.4(3).
Ethylene oligomerisation reactions using precatalysts 1-8
Precatalysts 1-8 were activated with EtAlCl2 as co-catalyst and investigated for their activities
towards oligomerisation or polymerisation of ethylene in chlorobenzene. Optimum Al:Ni ratio and
catalyst loading were established with precatalyst 2. On adding the co-catalyst the temperatures of
o
o
o
the reactions increased rapidly from 25 C to between 50 C and 60 C for about 20 min before
8

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dropping to the set temperature, usually within 10 min. A similar temperature increase has been
reported by Darkwa and co-workers with other (pyrazolylmethylenepyridine)nickel(II) and
(pyrazolylmethylenebenzene)nickel(II) catalysed ethylene reactions where temperature increase
o
o
were even higher, rising to between 80 C and 110 C.15a,17 Catalytic activity increased steadily
from a Al:Ni ratio of 100:1 (348 kg.molNi-1.h-1) to a ratio of 400:1 (1 989 kg of ethylene
oligomer.mol.-1(Ni).h-1) before decreasing to 1 356 kg of ethylene oligomer.mol.-1(Ni).h-1 at Al:Ni
ratio of 500:1 (Table 1). Amount of precatalyst used in a catalytic reaction has influence on the
activity of the active species and the products obtained, so the influence of catalysts loading on the
activity of the ethylene oligomerisation was studied using precatalyst 2. We observed an increase
in catalyst activity from a catalyst loading of 6 µmol to 10 µmol but activity decreased at 11 µmol
(Table 1: entries 4, 6-8).
Table 1: Determination of optimum Al:Ni ratio and precatalyst loading
Entry Al:Ni
Precatalyst
loading
(μmol)
aYield
(g)
Activity
(kg of ethylene
oligomer.mol.-
1(Ni).h-1)
bOligomer molecular
weight (Da)
1
2
3
4
5
6
7
8
100:1
200:1
300:1
400:1
500:1
400:1
400:1
400:1
10
10
10
10
10
6
3.5
7.9
348
378.0
481.0
567.0
611.0
521.0
456.0
561.0
610.0
786
15.1
19.9
13.6
6.9
1 506
1 989
1 356
1 151
1 315
1 788
8
10.5
19.7
11
Conditions: 6 mL of chlorobenzene; co-catalyst = EtAlCl2; ethylene = 10 bar; precatalyst = 2;
o
a
temperature 25 C; time 1 h; determined by mass difference of 6.0 mL chlorobenzene (6.66 g)
and mass of final solution; bdetermined by APCI.
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The rest of the nickel complexes were tested as catalyst for the ethylene reaction described earlier
using Al:Ni ratio 400:1 and 10 μmol catalyst loading. Precatalyst 2 produced the most active
catalyst (Table 2). In all these reactions (Tables 1 and 2) GC analyses of the products showed four
product peaks in the chromatogram (Figure 3). These product peaks were identified as 1-butene,
cis-2-butene, trans-2-butene and 2-methylpropene (Figure 3); indicating that the ethylene
dimerised to 1-butene, which further transformed into the remaining C4 products. The formation
of the cis- and trans- 2-butenes suggest that there is “chain-walking” of the 1-butene to form the
internal butenes in a similar manner reported by Brookhart and co-workers.19 Furthermore, the
formation of 2-methyl-propene suggests that there is cleavage of the C-C bond of a coordinated
internal butene to the nickel centre, which then undergoes rapid recombination as proposed by
Jacobson et. al.20 The selectivity of these catalysts towards 2-methylpropene is unique and seems
to be at the expense of 1-butene (Table 2); hence the small amount of 1-butene in the product mix.
A similar observation has been made by Braunstein and co-workers when they used (pyridyl-
triazole)nickel(II) and EtAlCl2 co-catalyst to catalyse ethylene dimerization to butenes. The
selectivity of these (pyridyl-triazole)nickel(II) catalysts to 1-butene was only 10%,21 contrary to
typical 1-butene selectivity of > 80% for most catalysts.22 There was no evidence of the formation
of hexenes and octenes with precatalysts 1-8 unlike (pyrazolylethylpyridylamine)nickel(II) and
(pyrazolylethylpyrroleamine)nickel(II) complexes that produced 1-butene and 1-hexene when the
solvent is chlorobenzene and EtAlCl2 co-catalyst.17b There was also no evidence from the GC
analysis (Figure S2) of olefins between C10 and C14 and Friedel-Crafts alkylation of chlorobenzene
by the butene oligomers from any of the catalytic reactions with ethylene and precatalysts 1-8.
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Catalysis Science & Technology
Table 2: Ethylene oligomerisation data using precatalysts 1-8, EtAlCl2 as co-catalyst
Entry Precatalyst Pressure aYield
Activity (kg of
ethylene
Temperature Time
bSelectivity
cOligomer
(bar)
(g)
(oC)
(min) for 1-butene molecular weight
oligomer.mol.-
(%)
(Da)
1(Ni).h-1)
1
2
3
4
10
10
10
10
12.2
19.9
17.8
10.2
1 221
25
25
25
25
60
60
60
60
8
10
9
551.0
611.0
581.0
546.0
1
1 989
1 776
1 023
2
3
4
5
6
7
8
7
5
6
7
8
10
10
10
10
6.7
5.8
8.2
7.1
671
578
823
710
25
25
25
25
60
60
60
60
6
5
6
6
486.0
477.0
492.0
461.0
9
10
5
20
12.2
22.2
1 221
2 220
25
25
60
60
8
10
399.0
887.0
2
2
2
11
12
13
14
15
10
10
10
10
10
10.2
6.3
1 023
634
40
50
60
25
25
60
60
9
8
481.0
310.0
210.0
679.0
788.0
2
2
2
2
4.1
412
60
8
21.3
24.1
1 066
803
120
180
9
10
Conditions: 6.0 mL of solvent; Chlorobenzene, catalyst loadings = 10 µmol, Al:Ni = 400:1; adetermined by mass difference of 6.0 mL
chlorobenzene (6.66 g) and mass of final solution; bdetermined by GC; cdetermined by APCI.
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Figure 3: Expanded GC showing the four isomers of butene formed in the ethylene oligomerisation reaction
12

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When the chlorobenzene was evaporated from the product mix, oily liquids were obtained.
Attempts to determine the molecular weights of these oils by GPC were unsuccessful as their
molecular weights were lower than 1 000 Da. Atmospheric pressure chemical ionization (APCI)
was therefore used to determine the molecular weight of these oils. A typical APCI mass spectrum
is Figure 4, and with this technique molecular weights of the oily products were found to be
between 210.0 and 887.0 Da, which shows that these were compounds that have C16-C64 carbon
atoms. An expanded form of the spectrum showed isotopic distributions (Figure 4 insert) and
differences between two sets of isotopic of 14.0 units that corresponds to a methylene group
fragmentation. Further analysis of the oils by 1H NMR spectroscopy showed olefinic end groups
signals between 4.75 ppm and 5.48 ppm (Figure S3). These are vinylene (-CH2CH=CH2) and
vinylidene (-CHC(R)=CH2) end groups that results when β-hydride elimination is the dominant
chain-transfer pathway23 and that chain transfer to alkyl-Al is negligible.24
We have also used the nomenclature by Usami25 and Mclain.26 to assigning the peaks in the
13C{1H} NMR spectra of the oils (Figure S4). From the 13C{1H} NMR spectrum (Figure S4) the
major peaks are butyl branching (at 14.20 ppm, 22.90 ppm and 27.40 ppm), and some amount of
longer chain branching that has a signal at 29.50 ppm. The presence of the later chain could explain
why the oils are very fluid. Combining the mass spectral data and the 1H NMR spectral end group
analyses allowed us to conclude that the oily products are indeed a mixture of oligomers that have
C16 to C64 carbon atoms. It is interesting to note that ethylene oligomerisation products have no C6
to C14 components; however it is not clear why there are no C6 to C14 olefins in the product mix;
although it is possible that formation of C16 to C64 olefins could be from further reactions of butenes
and other lower carbon content olefins in the oligomerisation reaction to the oily olefins. Examples
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of oily products from ethylene oligomerisation catalysed by N,N-iminopyridine nickel(II)
complexes supported on dendritic framework have been reported by Flores27a and Mapolie27b. Both
reactions produce olefins with up to C34 atoms; and while olefinic products from the Flores
catalysts follow Schulz-Flory distribution, the Mapolie catalysts give a mixture of alkyltoluenes
and C22+ olefins. As such it is difficult to firmly establish that the Mapolie catalysts oligomerise
ethylene in a Schulz-Flory manner, but the MALDI-TOF of their oily product show olefin carbon
content above C60. Nevertheless, the both Flores and Mapolie catalysts produce high carbon
content olefinic oils similar to our catalysts.
14

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Catalysis Science & Technology
Intens.
8000
207.2133
193.1976
6000
4000
2000
0
14.0157
209.1950
205.1968
195.1827
197.1694
191.1812
189.1664
211.1784
213.1855
203.1809
190
195
200
205
210
215
Figure 4: A typical APCI spectrum of the oligomer synthesised using precatalyst 2. Insert is an expansion of the a portion of
the spectrum showing fragmentation pattern and isotopic distribution
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The effect of the ligand systems on the catalyst activity
There are three ligand types, namely mono-dentate ferrocenylpyrazolyl (L1 and L2), bidentate
ferrocenylpyrazolyl-pyridine (L3 and L4) and bidentate ferrocenylpyrazolyl-amine (L5 and L6)
that were used to prepare the nickel precatalysts. Of the nickel complexes that have mono- dentate
ligands, precatalyst 2 was the most active whilst the nickel complexes that have bi-dentate ligands
precatalysts with pyridine groups (3 and 4) were more active than precatalysts with amine group
(5 and 6). Overall the most active was precatalyst 2. The observed differences in catalytic activity
could come from the electrophilicity differences at the nickel centre the ligands have on the
catalysts. This is supported by the σ-donor abilities of the nitrogen donor atoms in the ligands
which increases in the order pyrazolyl < amine < pyridyl. This explains the higher catalytic activity
of precatalyst 2.
It appears that the type of halide has effect on catalyst activity. Comparing the activities of
precatalysts 3, 4, 7 and 8 (Table 2: entries 3, 4, 6 and 7), the dibromide precatalysts are more active
compared to the dichloride precatalysts. This could be due to the higher solubility of the dibromide
complexes compared to the dichloride analogues. Similar observations have been made for
(pyrazolyliminophosphorane)-nickel(II) precatalysts28 and 2-quinoxalinyl-6-iminopyridine
nickel(II) precatalysts29 where bromide complexes were more active than the chloride analogues
in the oligomerisation of ethylene. Generally, these complexes upon activation catalysed ethylene
to butenes and higher ethylene oligomers between C16 and C64. We attribute the selectivity of these
complexes to the presence of the ferrocenyl unit in the ligands due to the fact a similar ligand
system without ferrocenyl substituents reported by us15a produce oligomers in the C4-C8 range;
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although the actual role of the ferrocenyl unit in the current ethylene catalytic reaction is not clear
to us.
Effect of pressure, time, temperature on the product obtained with 2
Pressure
Kinetic experiments reported in literature show that increasing ethylene pressure results in
increased activity and molecular weight of oligomers produced.30 Therefore, it was not surprising
that increasing the pressure of ethylene from 5 bar to 10 bar resulted in increase in both activity
and molecular weights of the ethylene oligomers (Table 2: entries 2 and 9). The activity and
molecular weight at 10 bar showed nearly a twofold increase compared to 5 bar. However, a further
increase of pressure to 20 bar gave activity of 2 220 kg of ethylene oligomer.mol.-1(Ni).h-1, which
is a smaller increase than when pressure was increased by 5 bar (Table 2: entries 2 and 10). This
could be due to saturation of the reaction mixture by ethylene.31 Also, pressure increase from 5 bar
to 20 bar had very small influence on the selectivity of 1-butene which marginally increased from
8% to 10%. Because of the smaller increase of activity at 20 bar, time and temperature variations
were performed at 10 bar.
Time
When the ethylene reactions were run with precatalysts 2 at different times namely 60 min, 120
min and 180 min (Table 2: entries 2, 14 and 15), there was increase in the amounts of ethylene
oligomers formed and the molecular weight of the oligomers. However, the catalyst activity
gradually decreased from 1 989 kg of ethylene oligomer.mol.-1(Ni).h-1 at 60 min to 803 kg of
ethylene oligomer.mol.-1(Ni).h-1 at 180 min. This could be due to catalyst deactivation or mass
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transfer limitation of the ethylene to the catalyst. Selectivity toward the formation of 1-butene was
also not influenced by increase in time.
Temperature
The effect of temperature on the ethylene reactions was investigated by varying temperature from
o
o
25 C to 60 C with 2 (Table 2: entries 2 and 4-6). Activity and molecular weight decreased
o
significantly from 1 989 kg of ethylene oligomer.mol.-1(Ni).h-1 and 611.0 at 25 C to 412 kg of
ethylene oligomer.mol.-1(Ni).h-1 and 210.0 at 60 oC respectively (Table 2: entries 2, 11-13). This
observation is similar to α-diimine nickel32 or 2-iminopyridine nickel33 catalysts reported in
literature; and the decrease could be due to decomposition or deactivation of the active species and
fast termination of the growing chain at high temperature. We were also able to establish through
o
the temperature variation experiments that 25 C is the optimum temperature for this precatalyst
and that increase in temperature beyond 25 oC did not influence the activity and selectivity towards
1-butene by much.
Effect of solvent on products using precatalysts 2 and 3
When reaction with precatalysts 2 and 3 were performed in toluene at optimum conditions of Al:Ni
= 400, 1 h, precatalyst loading of 10 μmol and 25 oC temperature, the products obtained from these
reactions were butenes and butyltoluenes (the butyltoluenes is as a result of Friedel-Crafts
alkylation of toluene). Typical GC and GC-MS for the reaction in toluene are shown in Figures
S5-S7. Here too GC also shows four signals for butene, which was also observed when
chlorobenzene was used as reaction solvent. However, a larger amount of 1-butene was obtained
in toluene than in chlorobenzene (Table 3). Furthermore, the major butene product in the toluene
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reaction is trans-2-butene (Figure S6). The selectivity towards 1-butene improved significantly in
toluene compared to the reaction in chlorobenzene. The selectivity for 1-butene was as high as
25% (Table 3). These results suggest that the type of solvent plays a role in the isomerisation
process, and is in agreement with results recently reported by Nyamato et al.34 that used
(chloromethylpyrazolylmethylpyridine)nickel(II) precatalysts to produce 1-butene is solvent
dependent.
In the work up of products from the reactions performed in toluene, the solvent was pumped off
1
to afford oily products which were identified by H NMR spectroscopy and GC-MS as Friedel-
Crafts butyltoluenes. Table 3 shows the product distribution at different reaction conditions.
Interestingly very low levels (20%) of Friedel-Crafts alkylation were recorded compared to
previous reports where alkylation levels were as high as 80%.15a,17 Changes in reaction conditions
resulted in changes in product distribution. For example increasing ethylene pressure from 5 bar
to 20 bar for precatalysts 2 and 3 saw high levels of butenes been produced compared to
butyltoluenes (Table 3: entries 1-3 and 7-9). Under the same conditions, precatalyst 2 produced
more butenes and less butyltoluenes than for precatalyst 3. This could be attributed to the type of
active species formed during the activation of the precatalyst upon abstraction of the halides. Gao
and co-workers have shown the influence of temperature on the Friedel-Crafts alkylation process
with nickel precatalysts.35 They reported that Friedel-Crafts alkylation occurs at high temperature.
o
So observing higher alkylation products as high as 30% at 60 C when we increased temperature
from 25 oC to 60 oC compared to 20% at 25 oC for precatalyst 2 was no surprise (Table 3: entries
2, 4-6). For precatalyst 3 alkylation products was about 60% at 60 oC from 40% at 25 oC (Table 3:
entries 8, 10-12). Also alkylation can be dependent on the type of precatalyst used; an observation
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made by Dyer and co-workers,36 where out of the four nickel precatalysts investigated only one of
them resulted in Friedel-Crafts alkylation products. It is therefore worth noting of all the pyrazolyl
nickel precatalysts investigated by us15a,17 the (ferrocenylpyrazolyl)nickel complexes in this report
produced the least alkyltoluene products and could be the reason why higher carbon content olefins
were produced from further incorporation of lower carbon content olefins in the oligomerisation
reactions.
Conclusions
Novel mono-dentate ferrocenylpyrazolyl, bi-dentate ferrocenylpyrazolylmethylpyridine and
ferrocenylpyrazolylethylyamine nickel(II) complexes can be activated with EtAlCl2 to catalyse
ethylene oligomerisation to butene isomers and C16-C64 ethylene oligomers. It is interesting to note
that products formed in these ethylene oligomerisation reactions do not follow a Schulz-Flory
distribution. It begins with the formation of butenes and then skip C6-C14 products but then gets to
C16-C64 olefins. This is unique and represents a rare example where products from nickel catalysed
ethylene oligomerisation reaction do not have C6-C14 but has C 16+. The products from the reactions
were greatly influenced by the solvent used. In chlorobenzene the butene isomer 2-methylpropene
was the major product, whilst in toluene the trans-2-butene isomer was the major product but also
only small amounts of butyl-toluenes were formed.
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Table 3: Ethylene oligomerisation and Friedel-Crafts alkylation reactions data, using precatalysts 2 and 3, with EtAlCl2 as co-catalyst
Entry Catalyst Pressure aYield Activity (kg of Temperature
bOligomer and FC
distributions (%)
bSelectivity
(%)
(bar)
(g)
ethylene
oligomer.mol.-
1(Ni).h-1)
823
(oC)
C4
A
B
C
1-C4
1
2
5
8.23
17.73
21.56
8.70
25
25
25
40
50
60
25
25
25
40
50
60
57.50 21.52 11.36 9.62
10
15
20
20
19
18
13
18
25
20
17
15
2
2
2
2
2
2
3
3
3
3
3
3
10
20
10
10
10
5
1 773
2 156
870
80.10 11.22 5.01
93.03 4.43 1.20
3.67
1.25
3.12
2.13
5.45
-
3
4
79.23 13.09 4.56
78.23 13.52 6.12
72.19 15.13 7.23
54.10 39.90 6.00
61.66 24.53 8.42
71.23 13.45 9.93
5
7.09
609
6
5.14
514
7
4.56
456
8
10
20
10
10
10
12.45
19.24
10.56
8.12
1 245
1 924
1 056
812
3.12
5.39
9
10
11
12
52.45 27.12 10.45 9.98
48.12 31.24 12.12 8.52
40.34 35.78 13.10 10.78
5.67
567
Conditions: 6 mL of toluene; catalyst loading = 10 μmol of precatalyst; monomer = ethylene; co-catalyst = EtAlCl2; Al:Ni = 400:1;
time 1 h. aDetermined by mass difference of 6.0 mL toluene (5.22 g) and final mass of solution. FC = Friedel-Crafts alkylated product.
bDetermined by GC; A = mono-butyltoluene; B = di-butyltoluene; C = tri-butyltoluene.
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Experimental
Materials and general instrumentations
Complexes were synthesised using standard Schlenk techniques under N2. Organic solvents such
as methanol were dried over CaH2, chlorobenzene over CaCl2, Hexane over Na lamps while
tetrahydrofuran (THF) was dried over Na with benzophenone as indicator. [NiBr2(DME)] and
[NiCl2.6H2O] were purchased from Sigma Aldrich and used as received. Compounds L1-L6 were
synthesised following procedures described in literature,10 were used to prepare the nickel(II)
complexes 1-8. Magnetic moments of the nickel complexes were measured using a Magway MSB
MK1 magnetic susceptibility balance. Effective magnetic moments, μeff were calculated as
described in literature.37 GC analyses were performed on a Varian 3900 with 50 m x 0.21 mm with
a CP coated silica pona column. The GC experiments were run at initial temperature of 40 oC and
o
o
then ramped to 250 C at 5 C/min. GC-MS analyses were performed on a Shimadzu GC-MS-
QP2010 fitted with a quadrupole mass detector. Elemental analyses were performed on a Vario
elementar III microcube CHNS analyser at Rhodes University. The mass spectrometry unit at the
University of Stellenbosch performed the ESI-MS spectra on a Waters API Qualtro micro
spectrophotometer. Atmospheric pressure chemical ionization (APCI) was run on Burker
micrOTOF-Q II 10390 at a dry gas flow of 8.0 L/min and dry heater at 200 oC.
Syntheses of nickel complexes
Synthesis of [{bis(3-ferrocenyl-1H-methylpyrazolyl}nickel(II) bromide] (1)
A THF solution (20 mL) of compound L1 (0.09 g, 0.35 mmol) was added to a stirring THF solution
(30 mL) of [NiBr2DME] (0.05 g, 0.17 mmol). The reaction mixture turned dark orange and was
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allowed to stir for 12 h under N2 at room temperature. The solution was then reduced to 10 mL
and hexane was added to precipitate the product as an orange solid. The product was filtered,
washed three times with hexane and dried in vacuo. Yield = 89% (0.11 g). IR (Diamond ATR, cm-
1): 3244 υ(N-H) amine. High resolution ESI-MS m/z (%) = 652.8664 (70%) [M-Br]+. Anal. Calcd.
for C26H34Br2Fe2N4Ni: C, 43.21; H, 3.35; N, 7.75%. Found: C, 42.84; H, 3.75; N, 7.32%. μeff =
3.60 BM.
Complexes 2-6 were prepared using the procedure described for 1.
Synthesis of [{bis(3-ferrocenyl-5-methylpyrazolyl}nickel(II) bromide] (2)
Compound L2 (0.13 g, 0.50 mmol) was reacted with [NiBr2(DME)] (0.08 g, 0.25 mmol) to give
an orange solid. Yield = 92% (0.17 g). IR (Diamond ATR, cm-1): 3257 υ(N-H) amine. High
resolution ESI-MS m/z (%) = 761.1672 (70%) [M+1]+. Anal. Calcd. for C28H38Br2Fe2N4Ni: C,
44.80; H, 3.76; N, 7.46%. Found: C, 44.89; H, 3.93; N, 7.42%. μeff = 3.57 BM.
Synthesis of [{3-ferrocenylpyrazolyl-methylenepyridine}NiBr2] (3)
A mixture of L3a and L3b (0.10 g, 0.29 mmol) was added to [NiBr2DME] (0.09 g, 0.29 mmol) to
afford a light brown solid. Yield = 0.13 g (82%). IR (Diamond ATR, cm-1): 1626 υ(C=N). High
resolution ESI-MS m/z (%) = 481.4412 (80%) [M-Br]+. Anal. Calcd. for C19H17Br2FeN3Ni: C,
40.63; H, 3.05; N, 7.48%. Found: C, 40.58; H, 2.90; N, 7.73%. μeff = 3.51 BM.
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Synthesis of [{3-ferrocenyl-5-methylpyrazolyl-methylenepyridine}NiBr2] (4)
A mixture of L4a and L4b (0.15 g, 0.42 mmol) was added to [NiBr2DME] (0.13 g, 0.42 mmol) to
give light brown solid. Yield = 70% (0.19 g). IR (Diamond ATR, cm-1): 1620 υ(C=N). High
resolution ESI-MS m/z (%) = 494.6214 (50%) [M-Br]+. Anal. Calcd. for C20H19Br2FeN3Ni: C,
41.72; H, 3.33; N, 7.30%. Found: C, 41.57; H, 3.36; N, 6.97%. μeff = 3.94 BM.
Synthesis of [{3-ferrocenylpyrazolyl-ethylamine}NiBr2] (5)
A mixture of L5a and L5b (0.15 g, 0.51 mmol) was added to [NiBr2DME] (0.16 g, 0.51 mmol) to
afford a light brown solid. Yield = 85% (0.22 g). IR (Diamond ATR, cm-1): 3111 υ(N-H) amine
stretching vibration, 1626 υ(N-H) amine bending vibration. High resolution ESI-MS m/z (%) =
536.5312 (50%) [M+Na]+. Anal. Calcd. for C15H17Br2FeN3Ni: C, 35.07; H, 3.34; N, 8.18%.
Found: 35.65; H, 3.71; N, 8.40%. μeff = 3.41 BM.
Synthesis of [{3-ferrocenyl-5-methylpyrazolyl-ethylamine}NiBr2] (6)
A mixture of L6a and L6b (0.11 g, 0.34 mmol) was added to [NiBr2DME] (0.10 g 0.34 mmol) to
obtain light brown solid. Yield = 82% (0.15 g). IR (Diamond ATR, cm-1): 3193 υ(N-H) amine
stretching vibration, 1616 υ(N-H) amine bending vibration. High resolution ESI MS m/z (%) =
447.6330 (30%) [M-Br]+. Anal. Calcd. for C16H19Br2FeN3Ni: C, 36.42; H, 3.63; N, 7.96%. Found:
C, 36.80; H, 3.71; N, 8.21%. μeff = 3.36 BM.
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Synthesis of [{3-ferrocenylpyrazolyl-methylenepyridine}NiCl2] (7)
A MeOH solution (20 mL) of a mixture of L3a and L3b (0.12 g, 0.35 mmol) was added to a
stirring MeOH solution (30 mL) of NiCl2.6H2O (0.08 g, 0.35 mmol). The reaction mixture turned
orange and was allowed to stir for 12 h under N2 at room temperature. The solution was evaporated
and the product recrystallized from CH2Cl2 and hexane to afford a light brown solid. The product
was isolated by filtration, washed three times with hexane and dried in vacuo. Yield = 71% (0.12
g). IR (Diamond ATR, cm-1): 1628 υ(C=N). High resolution ESI-MS m/z (%) = 493.9389 (10%)
[M+Na]+, 437.9782 (70%) [M-Cl]+. Anal. Calcd. for C19H17Cl2FeN3Ni: C, 48.27; H, 3.62; N,
8.89%. Found: C, 48.48; H, 3.86; N, 8.65%. μeff = 3.34 BM.
Complex 8 was prepared using the procedure described for 7.
Synthesis of [{3-ferrocenyl-5-methylpyrazolyl-methylenepyridine}NiCl2](8)
A mixture of L4a and L4b (0.15 g, 0.42 mmol) in MeOH was added to NiCl2.6H2O (0.10 g 0.42
mmol) to give light brown solid. Yield = 73% (0.15 g). IR (Diamond ATR, cm-1): 1621 υ(C=N).
High resolution ESI-MS m/z (%) = 487.2471 (70%) [M+1]+. Anal Calcd. for
C20H19Cl2FeN3Ni.CH2Cl2: C, 44.11; H, 3.70; N, 7.35%. Found: C, 43.99, H, 4.16; N, 7.11%. μeff
= 3.87 BM.
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Molecular structure determination by single crystal X-ray analysis
Single-crystal suitable for X-ray diffraction analysis for compounds 3a, and 4a were grown and
used to determine the molecular structures for the respective compounds. The procedure for
compound 3a is used to describe the general experimental method adopted in X-ray structural
determination of all the compounds.
Crystal data was collected on a Bruker APEXII diffractometer with Mo Kα (λ = 0.71073 Å)
radiation and diffractometer to crystal distance of 4.00 cm. The initial cell matrix was obtained
from three series of scans at different starting angles. Each series consisted of 12 frames collected
at intervals of 0.5º in a 6º range about with an exposure time of 10 s per frame. The reflections
were successfully indexed by an automated indexing routine built in the APEXII program suite.
The data were collected using the full sphere data collection routine to survey the reciprocal space
to the extent of a full sphere to a resolution of 0.75 Å. Data were harvested by collecting 2982
frames at intervals of 0.5º scans in ω and φ with exposure times of 10 s per frame.38 A successful
solution by the direct methods of SHELXS 2013 provided all non-hydrogen atoms from the E-
map. All non-hydrogen atoms were refined with anisotropic displacement coefficients. All
hydrogen atoms were included in the structure factor calculation at idealized positions and were
allowed to ride on the neighbouring atoms with relative isotropic displacement coefficients.39
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General procedure for ethylene oligomerisation reactions
In a typical reaction, a stainless autoclave (35 mL) containing catalyst a precursor (10 µmol) and
stirrer, was transferred the appreciate solvent (6 mL) under nitrogen atmosphere. Ethylaluminum
dichloride (EtAlCl2), in hexanes, was added to the reaction mixture. The autoclave was then placed
on an Eyela chem-station and pressurised with ethylene to the required pressure and maintained at
this pressure throughout the reaction. The reaction was stirred at 1 200 rpm at various temperatures,
pressures and time. After the reaction, the excess ethylene was vented off and reaction quenched
with 10% HCl in methanol, and the product sampled for GC and GC-MS analyses.
General characterisation of oligomers
Olefin oligomerisation products were characterised using GC. The instruments were calibrated
with standard samples, which were used to establish the retention times for various authentic
oligomer (C6-C20) compositions. The oligomer components in the oligomerisation reactions were
established by comparing the retention times of products of the oligomerisation runs to that of
standard samples. This helped in establishing the identity of the oligomer products. The
butyltoluenes were identified by GC-MS. The molecular weight of the high carbon content
ethylene oligomers were determined by atmospheric pressure chemical ionization (APCI)
technique.
Supporting Information
Electronic Supporting Information (ESI): This material is available free of charge.
Crystallographic data has been deposited with the Cambridge Crystallographic Data Centre with
CCDC 1062600 (3a) and 1062601 (4a). Copies of this information may be obtained free of charge
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from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336063; e-
mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk.
Acknowledgments
We acknowledge the DST-NRF Centre of Excellence in Catalysis (c*change), the National
Research Foundation of South Africa and the University of Johannesburg for financial support for
this project.
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