Nickel metallodendrimers as catalyst precursors in the tandem oligomerization of ethylene and Friedel-Crafts alkylation of its olefinic products Metallodendrimers Special Issue

Article abstract of DOI:10.1016/j.ica.2013.06.016

The synthesis and catalytic activity of nickel metallodendrimers (C1-C6) are reported. The metallodendrimers were synthesized by the coordination of the metal precursors, Ni(OAc)₂ and Ni(DME)Br₂, to the periphery of salicylaldimine and iminopyridyl modified dendritic ligands, DL1-DL6, based on the generation 1 to generation 3 diaminobutane polypropylene imine (DAB-PPI) dendrimers. The metallodendrimers were tested in the oligomerization of ethylene and were found to be active catalysts. When using this catalyst system, it was found that a tandem process occurred in which ethylene oligomerization was followed by Friedel-Crafts alkylation of the reaction solvent, toluene. The latter process is mediated by the aluminium co-catalyst, EtAlCl₂.

Full text of DOI:10.1016/j.ica.2013.06.016

Inorganica Chimica Acta 409 (2014) 96–105
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Nickel metallodendrimers as catalyst precursors in the tandem
oligomerization of ethylene and Friedel–Crafts alkylation of its olefinic
products
⇑
Rehana Malgas-Enus, Selwyn F. Mapolie
Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 21 June 2013
The synthesis and catalytic activity of nickel metallodendrimers (C1–C6) are reported. The metalloden-
drimers were synthesized by the coordination of the metal precursors, Ni(OAc)2 and Ni(DME)Br2, to
the periphery of salicylaldimine and iminopyridyl modified dendritic ligands, DL1–DL6, based on the
generation 1 to generation 3 diaminobutane polypropylene imine (DAB-PPI) dendrimers. The metallo-
dendrimers were tested in the oligomerization of ethylene and were found to be active catalysts. When
using this catalyst system, it was found that a tandem process occurred in which ethylene oligomeriza-
tion was followed by Friedel–Crafts alkylation of the reaction solvent, toluene. The latter process is med-
iated by the aluminium co-catalyst, EtAlCl2.
Metallodendrimers Special Issue
Keywords:
Nickel metallodendrimer catalysts
Ethylene oligomerization
Friedel–Crafts alkylation
Ethylene aluminium dichloride
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
incorporate metal ions into their infrastructure either as cores,
branching centres, arm connectors, termini or by metal incorpora-
a
-Olefins can undergo many transformations depending on the
tion at specific loci within the preassembled dendritic framework,
are termed metallodendrimers [12,13]. Metallodendrimers have
applications in a large number of areas such as drug delivery,
molecular electronics, sensors and catalysis.
reagents, the catalyst and the reaction conditions employed. Some
of these transformations include hydrogenation, cracking, cycliza-
tion, isomerization, oligomerization and polymerization reactions
[1]. Ethylene oligomers are the primary source of
a
-olefins, which
We previously reported the synthesis of dendritic nickel
complexes derived from generation 1 (G1) and generation 2 (G2)
dendrimeric salicylaldimine ligands based poly(propyleneimine)
dendrimer scaffolds of the type, DAB-(NH2)n (n = 4 or 8, DAB =
diaminobutane). These complexes are referred to as C1 and C2
respectively (Fig. 1). The complexes evaluated were previously
found to be active in norbornene polymerization, yielding poly-
mers with moderate to high molecular weights and low polydis-
persity indices [14–15].
In this paper we report the evaluation of some nickel metallo-
dendrimer complexes based on dendrimeric Schiff base ligands
as catalyst precursors in the oligomerization of ethylene. The diam-
inobutane polypropylene imine (DAB-PPI) dendrimer range was
used as dendritic scaffolds. The G1–G3 amino functionalized PPI
dendrimers were modified via Schiff base condensation reactions
to produce salicylaldimine as well as iminopyridyl functionalized
dendrimers. These functionalized dendrimers were then com-
plexed to nickel salts to form dendritic complexes.
find application in the preparation of detergents, synthetic lubri-
cants, plasticizers, alcohols and some copolymers. Therefore this
process is of great interest to industrial and academic researchers
in the field. Major commercial processes for ethylene oligomeriza-
tion using homogeneous catalysts have been developed by compa-
nies like Shell (SHOP) and IFP (Dimersol and Alfabutol) [2–8].
Dendrimers are hyperbranched macromolecules that are mono-
disperse in nature and are characterized by a high density of
peripheral groups. They possess three distinguishing architectural
components, mainly (a) an initiator core, (b) an interior layer or
branching points and (c) an exterior (surface groups or periphery).
The very first synthesis of dendrimers was described by Vögtle
et al. in 1978 who used the term ‘‘cascade macromolecules’’ [9].
This was followed closely by the parallel and independent develop-
ment of the divergent, macromolecular synthesis of ‘‘starburst
polymers’’ by Tomalia et al. in 1985, first using the term
‘‘dendrimer’’ and describing in great detail the preparation of
poly(amidoamine) (PAMAM) dendrimers [10]. In the same year a
communication by Newkome et al. reported the synthesis of arbo-
rols (a synonym for dendrimers) [11]. Dendrimer molecules that
2. Experimental
2.1. General
⇑
Corresponding author. Address: Inorganic Chemistry Building, Bosman Street,
Stellenbosch, 7602, Western Cape, South Africa. Tel.: +27 218082722.
Toluene was dried by refluxing over sodium/benzophenone.
Methanol was dried by refluxing over magnesium turnings and
E-mail addresses: smapolie@sun.ac.za, smapolie@me.com (S.F. Mapolie).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.06.016

R. Malgas-Enus, S.F. Mapolie / Inorganica Chimica Acta 409 (2014) 96–105
97
O
O
N
N
N
N
O
O
Ni
N
Ni
N
C1
O
O
N
N
Ni
Ni
O
O
N
N
N
N
N
N
N
N
N
N
O
O
Ni
O
N
Ni
N
O
C2
Fig. 1. Structures of DAB G1 and G2 salicylaldimine nickel metallodendrimers, C1 and C2 [14].
iodine. Dichloromethane was dried by refluxing over phosphorus
pentoxide. The DAB dendrimer range was obtained from Symo-
chem, Netherlands, and used without any further purification. Sal-
icylaldehyde, pyridine-2-carboxyaldehyde, Ni(OAc)2ꢀXH2O, NiBr2,
dimethoxyethane (DME) and EtAlCl2 (25% w/v solution in toluene),
were purchased from Sigma Aldrich and used without any further
purification. Ethylene (99.9%) was obtained from Afrox. Infrared
spectra were recorded on a Nicolet Avatar 330 FT-IR spectropho-
tometer, using an ATR accessory with a ZnSe crystal. 1H NMR
(300 MHz and 400 MHz) or 13C NMR (75 MHz) spectra were re-
corded on a Varian VNMRS 300 MHz and a Varian Unity Inova
400 MHz spectrometer, using tetramethylsilane as an internal
standard. Magnetic susceptibility was measured with a Sherwood
Scientific MK-1 magnetic susceptibility balance at 298 K. Magnetic
data were corrected for the magnetization of diamagnetic contri-
butions, which were estimated from Pascal constants. UV–Vis
absorption spectroscopy was conducted using a GBC UV–Vis 920
spectrometer at room temperature. TGA data were determined
with a Q500 Thermogravimetric analyzer, in which the TGA scans
were recorded at 10 °C/min under a nitrogen atmosphere from 30
to 590 °C. Melting points were recorded on a Stuart Scientific Melt-
ing Point apparatus SMP3, and are uncorrected. ESI Mass spectra
were recorded on a Waters API Q-TOF Ultima instrument in V-
mode. The source temperature was 100 °C and the desolvation
temperature was 350 °C. The capillary voltage used was 3.5 kV.
Microanalyses were done at the University of Cape Town. All work
involving moisture and air sensitive compounds was carried out
using standard Schlenk techniques. The GC analysis was done
using a Varian CP-3800 gas chromatograph equipped with a HP
PONA column (50 m ꢁ 0.2 mm ꢁ 0.5
l
m). The internal standard
used for GC analysis was o-Xylene. MALDI-TOF spectra were
recorded on a Voyager-DE FTR spectrometer.
2.2. Synthesis
Ligands and complexes DL1, DL2, DL4, C1, C2, and C4 were syn-
thesized according to literature procedures [14,15].
2.2.1. Synthesis of ligand DL3, DAB-PPI G3 salicylaldimine modified
dendrimer
DAB-G3 dendrimer (0.53 g, 0.31 mmol) was added to dry tolu-
ene (15 ml), in a Schlenk tube, under nitrogen. Salicylaldehyde
(0.54 ml, 5 mmol) was added to the solution. The mixture was al-
lowed to stir under reflux, for 96 h. The solvent was evaporated
on a rotary evaporator, and a yellow oil was obtained. Dichloro-
methane (20 ml) was added to the residue and the product was
washed with water (5 ꢁ 30 ml). The dichloromethane layer was
dried over potassium carbonate after which the latter was filtered
off. The filtrate was evaporated to produce a yellow oil. Yield: 80%.
IR (cm-1): (C–O) 1277 (s). 1H
m(O–H) 2941 (m); m(C@N) 1629 (s); m
NMR in CDCl3 (d ppm): 1.37 (–NCH2CH2 DAB core), 1.54 (CH2 alkyl),
1.78 (CH2 alkyl), 2.36 (CH2 tert amine), 2.40 (CH2 tert amine), 2.48
(CH2 tert amine), 3.56 (HC@NCH2), 8.27 (N@CH imine), 6.82–7.56
(aryl). 13C NMR in CDCl3 (d ppm): 24.6; 28.7; 31.2; 51.7; 52.5;
57.7; 117.2; 118.6; 131.4; 132.3; 161.5; 165.1. Anal. Calc. for
C
200H272N30O16: C, 71.38; H, 7.86; N, 11.30. Found: C, 71.19; H,
7.85; N, 11.13%. MS (MALDI-TOF) (C200H272N30O16) [M]+ at m/
z = 3352 m/z. ESI: [M]2+at m/z = 1676.5.

98
R. Malgas-Enus, S.F. Mapolie / Inorganica Chimica Acta 409 (2014) 96–105
2.2.2. Synthesis of ligand DL5, DAB-PPI G2 iminopyridyl modified
dendrimer
colour change from orange to brown, and all reagents dissolved
to give a clear brown solution. The reaction mixture was refluxed
for 24 h. The solvent was evaporated to give a brown solid residue.
Dichloromethane (10 ml) was added to the residue and then tritu-
rated, after which a brown solid precipitates. The precipitate was
filtered to give C5 as a brown solid which was washed several
times with CH2Cl2. Yield = 80%. m.p.: 225–231 °C. IR (cmꢂ1):
DAB-G2 dendrimer (0.523 g, 0.676 mmol) was added to dry tol-
uene (20 ml) in a Schlenk tube, under nitrogen. 2-Pyridine carb-
oxyaldehyde (0.51 ml, 0.541 mmol) was added to the solution.
The mixture was allowed to stir for 48 h at room temperature.
The solvent was evaporated on a rotary evaporator and a dark
brown oil was obtained. Dichloromethane (20 ml) was added to
the oil and the product was washed with water (5 ꢁ 30 ml). The
dichloromethane layer was dried over potassium carbonate after
which the latter was filtered off. The filtrate was evaporated to give
m
(C@N) at 1640 (s); m(C@C) pyr at 1597 (s) and m(C@N) pyr at
1568 (s). ESI–MS (C88H120Br16N22Ni8), found [M+Na]4+at m/
z = 814, calcd. 3234 g/mol.
a brown oil. Yield: 85%. IR (cmꢂ1):
m(C@N) aliphatic 1647 (s);
2.2.7. Synthesis of DAB-PPI G3 iminopyridyl nickel metallodendrimer,
C6
m(C@N) pyr 1586 (s);
m
(C@C) pyr 1562 (m). 1H NMR in CDCl3 (d
ppm): 1.36 (–NCH2CH2 DAB core), 1.57 (CH2 alkyl), 1.84 (CH2 alkyl),
2.41 (CH2 tert amine), 2.52 (CH2 tert amine), 3.58 (HC@NCH2), 8.36
(N@CH imine), 7.28–7.96 (CH aryl), 8.61 (CH pyridine). 13C NMR in
CDCl3 (d ppm): 24.6; 28.3; 51.7; 59.5; 121.2; 124.5; 128.9; 136.4;
149.3; 154.5; 161.8. Anal. Calc. for C88H120N22: C, 69.62; H, 7.93;
N, 20.13. Found: C, 69.61; H, 8.13; N, 20.58%. ESI-MS
(C88H120N22) [M+H]+at m/z = 1487.
Ni(DME)Br2 (0.31 g, 1.0 mmol) was added to a solution of DL6
(0.20 g, 0.062 mmol) in dry ethanol (20 ml). There’s an immediate
colour change from orange to brown, and all reagents dissolved to
give a clear brown solution. The reaction mixture was refluxed for
24 h. The solvent was evaporated to give a brown solid residue.
Dichloromethane (10 ml) was added to the residue and then tritu-
rated, after which a brown solid precipitates. The precipitate was
filtered to give C6 as a brown solid. Yield = 78%. m.p.: 201–
204 °C. IR (cmꢂ1):
m(C@N) 1642 (s); m(C@C) pyr at 1598 (s) and
2.2.3. Synthesis of ligand DL6, DAB-PPI G3 iminopyridyl modified
dendrimer
The same synthetic approach was used to prepare the genera-
tion 3 DAB iminopyridyl analogue DL6, by using the generation 3
DAB dendrimer with 16 mol equivalents of 2-pyridine carboxyal-
dehyde to yield a brown oil as a product. Yield: 80%. IR (cmꢂ1):
m
(C@N) pyr at 1571 (s). ESI–MS (C184H256Br32N46Ni16), found
[M+Na]5+ at m/z = 1326, calcd 6688 g/mol.
2.3. Ethylene oligomerization
m
(C@N) aliphatic 1648 (s); m(C@N) pyr 1587 (s); m(C@C) pyr 1567
The catalyst amounts were varied to ensure a constant amount
of Ni in the system at 5 lmol. EtAlCl2 was employed as a co-cata-
lyst. The reaction time was one hour at room temperature. The eth-
ylene pressure was maintained at 5 atm. and the solvent employed
was toluene (50 ml). Acidic ethanol (10 ml) was used to quench the
reaction. The Al:Ni ratios ranged from 1000:1 to 5000:1.
(m). 1H NMR in CDCl3 (d ppm): 1.38 (–NCH2CH2 DAB core), 1.59
(CH2 alkyl), 2.14 (CH2 tert amine), 2.40 (CH2 tert amine), 2.52 (CH2
tert amine), 3.67 (HC@NCH2), 8.37 (N@CH imine), 7.29–7.97 (CH
aryl), 8.62 (CH pyridine). 13C NMR in CDCl3 (d ppm): 24.3; 28.2;
30.9; 51.7; 59.5; 121.2; 124.6; 136.5; 149.3; 154.6; 161.7. Anal.
Calc. for C184H256N46: C, 67.41; H, 7.86; N, 19.39. Found: C, 67.02;
H, 7.28; N, 20.60%. ESI-MS (C184H256N46) [M]5+at m/z = 622.16.
2.3.1. Typical ethylene oligomerization procedure
The ethylene oligomerization reactions were carried out in a
300 ml stainless steel autoclave equipped with an overhead stirrer
and internal cooling coil. The autoclave was loaded in a nitrogen-
purged glove box. The appropriate amount of catalyst correspond-
2.2.4. Synthesis of DAB-PPI G3 salicylaldimine nickel
metallodendrimer, C3
To a solution of the DAB-G3 salicylaldimine ligand, DL3 (1.36 g,
0.41 mmol) in ethanol (20 ml) was added nickel acetate tetra hy-
drate (0.82 g, 3.20 mmol) and the reaction mixture was allowed
to stir under reflux for 24 hours, forming a green precipitate. The
precipitate was filtered off by vacuum filtration and washed exten-
sively with ethanol to give a green solid. Dichloromethane (15 ml)
was added to the solid, and the solution was then filtered. The fil-
trate was evaporated to give C3 as a green solid. Yield = 75%. m.p.:
ing to 5 lmol of nickel was suspended in dry toluene (50 ml) in a
stainless steel PARR reactor. The required amount of EtAlCl2 was
added to the solution using a glass syringe. The reactor was sealed
and removed from the glove-box. The reactor was flushed with
ethylene 3 times. The ethylene pressure was set at 5 atm. and
maintained at this pressure throughout the oligomerization proce-
dure. The reaction was conducted at room temperature for 1 h. The
oligomerization was stopped by quenching the reaction mixture
with 10 ml of acidic ethanol (ethanol:HCl, 95:5). A GC aliquot
was taken after which the solvent from the remaining sample
was evaporated via rotary evaporation and dried on the vacuum
line for 24 h. Any remaining non-volatile product was weighed
and subjected to analysis by MALDI-TOF spectrometry.
199-201 °C. IR (cmꢂ1):
Calc. for C200H256N30Ni8O16: C, 59.10; H, 6.42; N, 10.14. Found: C,
m(C@N) 1629 (s) and m(C–O) 1344 (s). Anal.
59.84; H, 7.73; N, 9.41%. MALDI-TOF MS (C200H256N30Ni8O16
)
[M+H]3+ at m/z = 1269.
2.2.5. Synthesis of G1 Ni(DME)Br2 using triethylorthoformate as a
dehydrating agent
This was prepared as previously reported in the literature [16].
NiBr2 (0.50 g, 2.30 mmol), a yellow solid, was added to a mix-
ture of ethanol (20 ml) and triethylorthoformate (1.2 ml) and re-
fluxed for 3 h to give a clear green solution. The solvent was then
concentrated to 1 ml, after which excess dimethoxyethane
(30 ml) was added. The mixture was allowed to crystallize and
then filtered to give a bright orange solid. Yield = 84%. IR (cmꢂ1):
3. Results and discussion
3.1. Synthesis of DAB polypropylene imine based Schiff-base ligands
3.1.1. Modification and characterization of the generation 1–3 DAB-
PPI salicylaldimine ligands DL1–DL3
m
(C–H) aliphatic at 2935 (s) and
m
(C–O) at 1049 (s).
We have previously reported the synthesis of ligands DL1 and
DL2 via Schiff base condensation of the G1 and G2 DAB-PPI dendri-
mers respectively with salicylaldehyde [14]. The same synthetic
approach was used to prepare the generation 3 DAB salicylaldi-
mine analogue DL3 (Scheme 1). Due to the greater number of sur-
face groups, the G3 ligand DL3 needed longer reaction time than its
2.2.6. Synthesis of DAB-PPI G2 iminopyridyl nickel metallodendrimer,
C5
Ni(DME)Br2 (0.33 g, 1.10 mmol) was added to a solution of DL5
(0.20 g, 0.14 mmol) in dry ethanol (15 ml). There’s an immediate

R. Malgas-Enus, S.F. Mapolie / Inorganica Chimica Acta 409 (2014) 96–105
99
lower generation analogues, DL1 and DL2, to ensure complete
reaction of all the peripheral amino groups with the aldehydes.
The progress of the reaction was monitored by FTIR using the
but sharp singlet in the 1H NMR spectrum of DL3 at 9.91 ppm.
When complexing this ligand to nickel, the pure product is ob-
tained without the salicylaldehyde impurity.
m
(C@N) band. This band is clearly visible in the FTIR spectra of all
three ligands in the region 1629–1632 cmꢂ1. 1H and 13C NMR spec-
troscopy was also used to monitor the formation of the imine
group. For example in the 1H NMR spectra of the ligands, the signal
for the imine proton occurs around d 8.32–8.37 ppm while in the
case of the 13C NMR, the peak for the imine carbon is observed
around 161 ppm for all three generations of dendrimer ligands.
This thus confirms the successful surface modification of the
DAB-PPI dendrimer with salicylaldimine units. The G3 dendritic li-
gand DL3 has several empty cavities that can potentially trap unre-
acted salicylaldehyde molecules. This is indeed what we observe.
Several attempts to remove the excess aldehyde from the products
proved to be unsuccessful. Solvent extraction did not work since
the salicylaldehyde and the dendritic ligand have similar solubility
properties. The presence of the salicylaldehyde entrapped within
the dendrimer cavities was confirmed by the presence of a weak
3.1.2. Preparation and characterization of the generation 1–3 DAB
Iminopyridyl ligands DL4–DL6
The synthesis of the iminopyridyl modified dendrimer ligands
DL4 and DL5 have previously been reported by Smith et al. [17]
and Mketo [18]. The generation 3 iminopyridyl analogue, DL6,
was synthesized in a similar manner to its lower generation ana-
logues i.e. via the Schiff base condensation of the DAB-G3 dendri-
mer with pyridine-2-carboxyaldehyde. The lower generation
ligands require only 48 h for complete reaction, whereas the prep-
aration of the generation 3 iminopyridyl ligand is performed over
at least 72 h due to the large number of surface groups. All three
ligands were isolated as dark brown oils. FTIR spectroscopy shows
the
the aldehyde with the amino groups on the periphery of the DAB-
dendrimer. The (C@N) bands associated with the pyridine ring are
m
(C@N) band around 1645 cmꢂ1 which confirms the reaction of
m
H N
2
NH
2
H N
2
NH
2
NH
2
N
H N
2
N
N
N
NH
2
N
N
H
N
2
N
N
N
N
N
N
NH
2
H
N
2
N
NH
2
N
H N
2
H N
2
NH
2
H N
2
NH
2
HO
O
HO
OH
OH
N
HO
N
N
N
OH
HO
N
N
N
N
HO
N
N
OH
N
N
N
N
N
N
N
N
N
N
N
HO
N
OH
N
N
N
N
OH
HO
N
N
N
N
OH
HO
HO
OH
Scheme 1. Synthesis of the DAB G3 salicylaldimine modified dendrimer, DL3.

100
R. Malgas-Enus, S.F. Mapolie / Inorganica Chimica Acta 409 (2014) 96–105
also observed in the product spectrum and are around 1586 cmꢂ1
.
In the 1H NMR spectra of the ligands, the signal for the imine pro-
ton occurs around 8.37 ppm and the 13C NMR spectra, show a peak
assigned to the carbon atom of the imine of ligands DL4–DL6 in the
range d 161.4–161.8 for the G1–G3 iminopyridyl ligands. Mass
spectrometry analysis for DL4 and DL5 show a singly charged
molecular ion peak at m/z = 672.80 and m/z = 1487.0 respectively.
DL6 however shows a multiply charged (+5) parent ion [M]5+ at
m/z 622.12. Microanalyses of the ligands DL5 and DL6 suggest sol-
vent (CH2Cl2) entrapment within the cavities of the dendrimers.
This was confirmed by 1H NMR spectroscopy which shows a peak
at 5.37 ppm assigned to the methylene chloride protons. Attempts
to completely remove the entrapped solvent molecules were not
successful.
Fig. 3. UV–Vis spectra of the G1 DAB salicylaldimine ligand, DL1, and nickel
metallodendrimer complex, C1.
3.2. Synthesis of DAB polypropylene imine based nickel
metallodendrimers
m
(C–O) stretching frequencies from around 1277 cmꢂ1 to
1324 cmꢂ1 when comparing the ligand and complex spectra. These
shifts are due to the coordination of the ligand to the metal via
both the nitrogen and the oxygen donor sites. These effects are also
observed in the FTIR spectra of other salicylaldimine-type nickel
complexes reported in the literature [19–21].
3.2.1. Synthesis and characterization of the generation 1–3 DAB
salicylaldimine nickel metallodendrimers, C1–C3
We have previously reported the synthesis of the G1 and G2
nickel salicylaldimine complexes, C1 and C2 by reacting the
salicylaldimine modified dendrimer ligands DL1 and DL2 with
Ni(OAc)2ꢀ4H2O [14]. We subsequently synthesized the generation
3 salicylaldimine nickel complex, C3, using the same synthetic
procedure as that employed for C1 and C2, using the generation
3 salicylaldimine ligand, DL3. Fig. 2 shows the structure of the
Generation 3 salicylaldimine nickel metallodendrimer, C3. All
three complexes were isolated as green solids, with yields ranging
from 75% to 85%. All complexes were characterized by FTIR spec-
troscopy, magnetic susceptibility measurements, mass spectrome-
try and elemental analysis. From the IR spectra, we observe a shift
All the complexes are paramagnetic thus no NMR analysis was
done. The magnetic moments of these nickel salicylaldimine
metallodendrimers range from 2.95 BM for the G1 complex salicyl-
aldimine, C1, to 3.38 BM for the G3 pyridine–imine complex, C6.
Tetrahedral nickel(II) complexes commonly have a magnetic mo-
ment of 3.1–4.0 BM as reported in literature. [22–23]. The ESI mass
spectra for C1 and C2 show peaks that correspond to [M+H]+ at m/
z 848 and m/z 1833 respectively. The mass spectrum for C3 shows
a triply charged peak molecular ion peak at m/z 1269. The melting
points for C1–C3, decrease when going from the G1 complex to the
in the
m
(C@N) band from around 1645 cmꢂ1 to 1630 cmꢂ1 and the
O
O
O
Ni
O
Ni
N
N
N
N
O
O
N
N
N
N
Ni
Ni
O
N
O
N
N
N
N
N
N
N
N
N
N
N
N
O
N
O
Ni
Ni
N
N
N
N
O
O
N
N
N
N
Ni
O
Ni
O
O
O
Fig. 2. Structure of DAB G3 salicylaldimine nickel metallodendrimer, C3.

R. Malgas-Enus, S.F. Mapolie / Inorganica Chimica Acta 409 (2014) 96–105
101
G3 complex, indicating that the G1 complex, C1, is the most ther-
mally stable of the three salicylaldimine complexes. The UV–Vis
spectra of the DAB G1 salicylaldimine ligand, DL1, and the DAB
G1 salicylaldimine nickel complex, C1, in a dichloromethane solu-
tion are shown in Fig. 3. The G2 and G3 analogues of these ligands
(DL2 and DL3) and complexes (C2 and C3) gave similar UV–Vis
spectra, hence only the G1 spectra are discussed.
From the IR spectra of the complexes, we observe a shift in the
m
(C@N) stretching frequency from around 1647–1648 cmꢂ1 to
1639–1641 cmꢂ1 when comparing spectra of the ligands and com-
plexes. The
m(C@N) stretching frequency of the pyridine ring
shifted from around 1586–1587 cmꢂ1 to around 1596–1598 cmꢂ1
for complexes, C4–C6. The magnetic moments of these nickel imi-
nopyridyl metallodendrimers also gives some information regard-
ing the geometry of the complexes. The magnetic moments of the
iminopyridyl complexes are in a similar range as that of the salicyl-
aldimine complexes, ranging from 3.23 BM for the G1 complex, C4,
to 3.38 BM for the G3 complex, C6. It is believed that these com-
plexes therefore have a tetrahedral geometry, although the mag-
netic moments of our complexes are slightly lower than what is
expected for conventional tetrahedral nickel complexes. The melt-
ing points for C4–C6 also decrease when going from the G1 com-
plex to the G3 complex, again indicating that the G3 complex,
C6, has the lowest thermal stability. On the whole, the iminopyr-
idyl complexes are less thermally stable than the corresponding
salicylaldimine complexes, C1–C3. The ESI mass spectrum for C4
shows a molecular ion peak at m/z 1592 which can be assigned
to an adduct of the parent ion containing two sodium ions. The
mass spectra of the G2 and G3 complexes, C5 and C6, show multi-
ply charged ions. This is not unusual since high molecular weight
complexes commonly tend to form multiply charged ions. The
spectrum for C5 shows a quadruply charged molecular ion peak
as a Na+ adduct at m/z = 814. The MS for C6 shows a peak at m/
z = 1326 which can be assigned to a molecular ion carrying a +5
charge and detected as sodium adduct. Due to the highly hygro-
scopic nature of the iminopyridyl complexes, we were unable to
obtain satisfactory microanalysis results.
The electronic spectra of the salicylaldimine ligands in the solu-
tion phase show three absorption bands at 330, 260 and 225 nm,
involving
p?
p⁄ transitions. Similar absorption bands were ob-
served by Fan et al. [22] who reported the synthesis of salen-type
Schiff base ligands. The UV–Vis spectra of the salicylaldimine Ni
complexes show two bands at 240 nm and 380 nm which is as-
signed to the transition of n?p⁄of the imine ligand and d–d tran-
sitions in the metal, respectively. The latter is very weak but
resembles what is normally seen for other salen type complexes
[23].
3.2.2. Synthesis and characterization of the generation 1–3 DAB
iminopyridyl nickel metallodendrimer complexes, C4–C6
The G1–G3 DAB iminopyridyl nickel complexes, C4–C6, were
synthesized by reacting the iminopyridyl modified dendritic li-
gands, DL4–DL6, with Ni(DME)Br2 as a metal precursor. We re-
cently reported the synthesis of the generation 1 iminopyridyl
nickel metallodendrimer, C4[15]. The generation 2 (C5) and gener-
ation 3 (C6) (Fig. 4) iminopyridyl nickel metallodendrimers are no-
vel complexes. The reaction of ligands, DL4–DL6 with Ni(DME)Br2,
resulted in brown solids being isolated as products with yields
ranging from 78% to 85%. The iminopyridyl complexes are all fairly
hygroscopic and consequently have to be stored under an inert
atmosphere and handled in a nitrogen-purged glove-box. This is
in contrast to the salicylaldimine complexes which are quite air
stable in the solid state, as well as in solution. Another major dif-
ference between the salicylaldimine complexes C1–C3 and the imi-
nopyridyl complexes C4–C6, is that the latter complexes have
double the amount of nickel centres than the former for each cor-
responding dendrimer generation. Thus for example the G1 imino-
pyridyl complex, C4, has four nickel centres whereas the G1
salicylaldimine complex, C1, has only 2 nickel centres.
3.3. Ethylene oligomerization studies
3.3.1. Catalytic activity and selectivity of nickel metallodendrimers,
C1–C6, at a constant Al/Ni ratio of 2000:1
Catalysts C1–C6 were evaluated in the oligomerization of
ethylene, keeping the metal loading as well as the co-catalyst:
metal ratio constant. The results are shown in Table 1. The Turn-
Over-Frequency (TOF) values range from 629 to 2582 kg
N
[M]
N
[M]
N
N
[M]
N
[M]
N
N
N
N
N
N
[M]
[M]
N
N
N
N
N
N
N
N
N
N
[M]
N
[M]
N
N
N
N
N
[M]
N
[M]
N
N
N
N
N
N
[M]
N
[M]
N
N
N
N
N
N
[M]
N
N
[M]
[M]
N
[M]
N
N
[M] = NiBr2
Fig. 4. Structure of DAB G3 iminopyridyl nickel metallodendrimer, C6.

102
R. Malgas-Enus, S.F. Mapolie / Inorganica Chimica Acta 409 (2014) 96–105
Table 1
Table 2
TOFa of catalysts (C1–C6) at 2000:1 Al/Ni ratio.
Selectivity of catalysts, C1–C6 at 2000:1 Al/Ni ratio.
a
a
+
Dendrimer
generation
TOF valuesa
Catalyst
%C4
% C6
% C8
% C11
% C13
% C14
% C16
% C22
C1
C2
C3
C4
C5
C6
24.2
16.8
–
55.6
–
5.1
–
–
6.4
–
–
4.2
–
–
0
–
31.6
42.4
46.0
18.0
17.3
21.5
–
–
3.5
–
–
11.8
6.8
17.8
–
5.2
–
8.4
7.5
14.0
–
1.5
–
14.7
26.5
18.7
20.0
76.0
74.5
DAB Ni Salicylaldimine
catalysts
DAB NiBr2 Iminopyridyl
catalysts
1
2
3
2417.2 (C1)
2018.0 (C2)
2582.0 (C3)
2274.8 (C4)
629.0 (C5)
971.6 (C6)
4
–
–
a
a
TOF = Kg of oligomer per mol of Ni per hour.
Friedel–Crafts alkylation products.
product molꢂ1Ni hꢂ1. The highest activity is observed for the
generation 3 salicylaldimine complex, C3. When comparing the
salicylaldimine catalysts (C1–C3) to the iminopyridyl catalysts
(C4–C6), it is evident that the salicylaldimine complexes generally
exhibit higher activity for all generations of dendrimers evaluated
(Fig. 5). This could be related to the higher stability of the salicyl-
aldimine complexes.
The selectivity of catalysts C1–C6 is shown in Table 2. The gen-
eration 1 catalysts (C1 and C4) tend to form mostly short chain
oligomers (C4–C16). This is also the case for the other Ni salicylaldi-
mine catalysts (C2–C3). The higher generation G2 and G3 Ni
pyridine–imine catalysts (C5 and C6) on the other hand form pre-
dominantly long chain oligomers (C22+). Thus the pyridine–imine
catalysts seem to be favouring chain propagation over chain termi-
nation processes. This could be due to the high degree of conges-
tion on the dendrimer periphery of C5 and C6 which could play a
role in the retardation of chain termination by sterically preventing
b-hydride transfer, thus leading to longer chain products. These
products of longer chain length were analyzed by MALDI-TOF mass
spectrometry. It was found that the length of these oligomers typ-
ically ranged from C22–C60, with the average chain length being
around C44. A typical MALDI-TOF mass spectrum showing the dis-
tribution of oligomers formed in one of the reactions can be seen in
Fig. 6.
short chain alkenes (C4–C6) formed during the ethylene oligomer-
ization reaction. Thus for example the C11 products observed are
isomers of butyl toluene and the C13 product isomers of hexyl tol-
uene. The distribution of the oligomerization and the Friedel–
Crafts alkylation products obtained using the different catalysts
are shown in Table 2. No ethyl toluenes are observed indicating
that direct Friedel–Crafts alkylation of toluene by ethylene does
not take place under the reaction conditions employed. The Fri-
edel–Crafts reaction was proven independent by carrying out a test
reaction in which 1-hexene was reacted with EtAlCl2 in the ab-
sence of any metal catalyst. This produced hexyl toluenes as the
sole products.
Darkwa et al., described similar observations when evaluating
their (pyrazol-1-ylmethyl) pyridine-nickel catalysts in the oligo-
merization of ethylene, using EtAlCl2 as a co-catalyst [24]. How-
ever, they reported that their catalysts exclusively produce
Friedel–Crafts products A–C (Scheme 2), whereas in our case even
numbered ethylene oligomers in addition to the Friedel–Crafts
alkylation products are observed in the product stream. This indi-
cates that the Ni/EtAlCl2 active species catalyzes the oligomeriza-
tion of ethylene, while the EtAlCl2 is primarily responsible for the
Friedel–Crafts alkylation and that the two processes occur simulta-
neously. With our dendritic catalyst systems, the oligomerization
reaction appears to proceed faster than the Friedel–Crafts alkyl-
ation reaction, hence the presence of ethylene oligomers in the
product mix.
Closer examination of the products leads to an interesting
observation in which the presence of uneven carbon number com-
pounds was detected in the product mix. For all the catalysts eval-
uated, a C11 product is formed in relatively high amounts and in the
case of DAB G3 salicylaldimine catalyst (C3), a C13 product is also
formed. Further investigation into the nature of these odd-carbon
numbered hydrocarbon products revealed these to be alkylated
benzenes. It was found that these products were the result of a Fri-
edel–Crafts alkylation of the solvent toluene in the presence of
In the case of the reactions reported by Darkwa et al., it seems
the Friedel–Crafts alkylation occurs faster than the oligomerization
process, leading to the oligomeric products being consumed as
soon as they are produced. Therefore no traces of oligomeric prod-
ucts are observed directly in this case.
It should be noted that in our systems, a fair amount of the
butene produced initially subsequently becomes consumed in the
Fig. 5. Catalyst activity of C1–C6 at Al/Ni 2000:1.

R. Malgas-Enus, S.F. Mapolie / Inorganica Chimica Acta 409 (2014) 96–105
103
Voyager Spec #1[B P = 565.6, 14046]
565.5824
1.4E+4
100
90
563.5792
535.5495
537.5465
80
70
60
591.6204
593.6206
507.5102
509.5118
621.6584
619.6544
647.6879
50
40
649.6891
675.7236
677.7267
538.7282
536.5568
30
20
10
506.2424
508.5093
592.6223
620.6457
703.7546
705.7582
733.7999
540.7336
477.2856
481.4820
648.6884
633.6738
579.6036
551.5639
676.7287
663.6980
521.5283
761.8323
756.5602
789.8600
790.8639
704.7648
691.7358
690.7269 719.7825
674.7088 718.7735
970.4026
972.3951
431.8228
444.4452
435.4116
493.4960
550.5664
543.5503
815.8931
752.5679
578.6172
575.5959
617.6562
662.7035
845.9401
829.9138 859.9591
480.4881
467.4276
873.9533
407.3766
406.4354
511.5139
641.7199
640.6744
611.6611
901.9933
956.0811
750.5561
545.5646
785.2493
0
0
1001.0
399.0
519.4
639.8
760.2
880.6
Mass (m /z)
Fig. 6. Representative MALDI-TOF spectrum of long chain oligomer products (C22–C60).
Scheme 2. Friedel–Crafts alkylation products reported by Darkwa et al. [23].
Friedel–Crafts alkylation reaction to produce the C11 product. In
fact, in almost all cases more than half of the butene produced re-
acts with the solvent in a subsequent reaction. In the case of cata-
lysts C3 and C5, all the butene produced is converted as none is
detected in the final product mix. Catalyst C4 interestingly shows
the lowest percentage of butene produced being converted to the
alkylated toluene product. It also seems that when using the lower
generation catalysts (C1 and C4) lower levels of Friedel–Crafts
alkylation occur when compared to the higher generation cata-
lysts, as shown by the higher amounts of butene still present in
the product mix for the former catalysts. Thus it would appear that
the transition metal catalyst has some impact on the process. At
this stage the precise role of the transition metal is not entirely
understood.
Table 3
TOFa of generation 3 DAB nickel catalyst, C3.
Al:Ni
DAB G3 Ni Sal
1000
2000
3000
4000
5000
7000
1796.2
2582.0
2568.4
2714.6
3582.8
2963.2
a
TOF = Kg of oligomer per mol of Ni per hour.
Ni hꢂ1. The optimum Al/Ni ratios for C1 and C2 were previously
found to be 500:1 and 2000:1 respectively [25]. These results fur-
ther support our theory that the tertiary amine units present in
these polypropyleneimine based dendritic systems act as Lewis
base donor sites, and therefore are potential positions for coordi-
nating to the Lewis acidic organoaluminium co-catalyst. The G1
catalyst has only two tertiary amine units within its internal struc-
ture, while the G2 catalyst has six tertiary amine units and the G3
catalyst has fourteen. It is thought that the co-catalyst, EtAlCl2, first
coordinates to these Lewis basic sites before activation of the metal
centre occurs. It is well known that N-donor molecules form ad-
ducts with Lewis acidic Al complexes [26]. Since the G3 catalyst
3.3.2. Catalytic activity and selectivity of the generation 3 DAB nickel
catalyst, C3
Since we’ve previously reported the evaluation of C1 and C2 in
the oligomerization of ethylene [25], at various Al/Ni ratio’s, we
found it apt to also evaluate C3 at different Al/Ni ratio’s. The results
are shown in Table 3. As expected, the DAB generation 3 salicyl-
aldimine catalyst (C3) needed a much higher Al/Ni ratio than its
lower generation counterparts (C1 and C2) to reach optimum
activity at 5000:1, with the TOF reaching up to 3582.8 kg molꢂ1
-

104
R. Malgas-Enus, S.F. Mapolie / Inorganica Chimica Acta 409 (2014) 96–105
Table 4
Selectivity of C3 at various Al/Ni ratios.
a
a
a
a
+
Al/Ni
% C4
% C6
% C11
% C13
% C14
% C15
% C16
% C17
% C18
% C22
1000
2000
3000
4000
5000
7000
10.1
–
–
–
–
–
–
–
–
–
–
49.3
46.0
31.5
40.0
38.1
33.8
–
3.5
11.1
4.0
10.2
30.5
14.6
17.8
19 .6
6.8
13.0
2.5
–
–
5.6
–
–
12.9
14.0
8.8
9.6
6.5
–
–
3.9
–
–
–
–
2.1
–
–
13.1
18.7
17.4
39.6
32.2
19.6
–
11.0
2.6
–
–
a
Friedel–Crafts products.
has the most N-donor sites it will need to react with a larger
amount of the aluminium alkyl than its lower generation counter-
parts therefore requiring larger amounts of co-catalyst before the
optimum activity is reached.
that the Friedel–Crafts alkylation only occurs after the formation
of butene and other even carbon number olefins, as a result of eth-
ylene oligomerization by the metallodendritic catalysts. Therefore,
the two processes occur in tandem. First we have the oligomeriza-
tion of the ethylene to even numbered olefins, followed by the Fri-
edel–Crafts alkylation of the toluene by olefins mediated by
EtAlCl2.
In addition, the EtAlCl2 does not mediate the alkylation of tolu-
ene in the presence of only ethylene to produce ethyl benzene.
Hence, the alkylation only occurs with butene and longer chain
olefins. This theory was tested by using 1-hexene as a substrate in-
stead of ethylene. No metallodendritic catalyst was included in the
reaction mixture, only EtAlCl2. Under similar reaction conditions,
the EtAlCl2 converted the 1-hexene to the alkylated toluene prod-
ucts within 10 minutes, with 100% conversion. From GC-MS anal-
ysis, we observed the selective formation of C13 and C19
compounds, which can be assigned to the Friedel–Crafts products
formed via single- and di-alkylation of toluene with the C6 oligo-
mer respectively. We also observed a C25 species which can be as-
signed to tri-alkylated Friedel–Crafts products (Fig. 7).
Table 4 shows the selectivity obtained at the different Al/Ni ra-
tio’s ranging from 1000:1 to 7000:1. The results indicate that irre-
spective of the Al/Ni ratio, the major product remains short chain
oligomers. C4 is only present at an Al/Ni ratio of 1000:1. Uneven
carbon number products C11, C13, C15 and C17 are also present in
the product mixture. This, as discussed earlier, is due to Friedel–
Crafts alkylation of the solvent, toluene. In all cases, except at Al/
Ni ratio of 1000:1, all the butenes formed is converted to C11 Fri-
edel–Crafts alkylation products. Furthermore, all the hexenes
formed is converted to C13 Friedel–Crafts alkylation products.
It is also noted that an increase in the Al/Ni ratio leads to a de-
crease in the production of C4 and subsequently a decrease in the
formation of C11 Friedel–Crafts alkylation products. Since all cata-
lytic reactions formed C11 as a product, we thought it best to run
a blank reaction, containing no metallodendrimer catalyst, with
only EtAlCl2 present under the same reaction conditions. After
one hour, the reaction mixture was worked-up as described earlier,
and no product was observed. This proves that without our den-
dritic catalyst systems, neither ethylene oligomerization nor Fri-
edel–Crafts alkylation of toluene using ethylene occurs. It is clear
It can thus be concluded that even though EtAlCl2 on its own
does not promote the alkylation of toluene in the presence of eth-
ylene, it does mediate the alkylation of toluene with 1-hexene in
the absence of any transition metal catalyst system. Reactions with
Fig. 7. The GC–MS chromatogram indicating F/C alkylation with 1-hexene and toluene.

R. Malgas-Enus, S.F. Mapolie / Inorganica Chimica Acta 409 (2014) 96–105
105
1-pentene and 1-octene were also evaluated and similar results
Acknowledgements
were obtained where the toluene was alkylated to produce pentyl-
and octyl-toluenes respectively. Most of the Friedel–Crafts prod-
ucts obtained are largely mono alkylated toluenes instead of di-
or tri-alkylated products, since C11 (butyl-toluene) and C13 (hex-
yl-toluene) Friedel–Crafts products are observed as major products
rather than C15 (dibutyl-toluenes) and/or C19 (tributyl toluenes)
Friedel–Crafts products. Therefore we can conclude that toluene
undergoes Friedel–Crafts single-alkylation with oligomers of ethyl-
ene. When a different solvent such as hexane, is used under similar
reaction conditions, no Friedel–Crafts alkylation products are ob-
served as expected, only small amounts of short chain oligomers
(C4 and C8) were detected. The catalyst system shows quite low
activity in hexane as solvent with the result that we did not pursue
this any further.
We would like to thank the NRF-DST Centre of Excellence in
catalysis for financial support, the University of Stellenbosch for
performing the GPC analysis.
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4. Conclusion
A range of metallodendritic catalysts were synthesized and
evaluated in ethylene oligomerization reactions. The synthesized
nickel catalysts produced a range of short-medium chain ethylene
oligomers (C4–C18), as well as some longer chained oligomers
when employing EtAlCl2 as a co-catalyst. An interesting observa-
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longer chain oligomers. Changing the solvent to hexane does pre-
vent the Friedel–Crafts alkylation reaction from occurring, how-
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catalyst system.
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