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 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.Effects of electroCite this: RSC Adv., 2018, 8, 5362
Received 22nd December 2017
Accepted 24th January 2018
DOI: 10.1039/c7ra13588b
rsc.li/rsc-advances
aDepartment of Chemistry, University of
Campus, Auckland Park 2006, Johannesburg
bDepartment of Chemistry, University of
cobuah@ug.edu.gh
† Electronic supplementary informa
10.1039/c7ra13588b
5362 | RSC Adv., 2018, 8, 5362–5371chemical properties of
ferrocenylpyrazolylnickel(II) and palladium(II)
compounds on their catalytic activities in ethylene
oligomerisation reactions†
Collins Obuah, *ab Michael K. Ainoosonab and James Darkwaa
Palladium complexes of ferrocenylpyrazolylpyridine and ferrocenylpyrazolylamine were synthesised and
screened as pre-catalysts (1–4) for olefin polymerisation. The pre-catalysts 1–4 on activation with
EtAlCl2 in the presence of ethylene with chlorobenzene or hexane as solvent were highly active with 1
being the most active, with an activity of 360 kg mol Pd
1 h
1. The major product from the reaction was
1-butene and high carbon content oligomers. The molecular weight (m/z) of the high carbon content
oligomers is as high as 623.0. When toluene is used as solvent, the products obtained were ethyltoluene
and butyltoluene and 1-butene. The electronic properties of the ligands (L1–L7) and complexes (1–10)
were determined by cyclic voltammetry (CV) and molecular modelling. The CV results show that the
ferrocenyl is easily oxidized upon the introduction of pyrazolyl derivatives, the process is quasi-
reversible. However, complexation of the ligands with palladium or nickel results in difficulty in oxidizing
the ferrocenyl moiety. This is an indication of the electrophilic nature of both the palladium and nickel
centres. The mechanism of the oxidation was observed to be diffusion-controlled and is independent of
scan rate. Molecular modelling experiments show that nickel and palladium complexes have lower
HOMO–LUMO gaps and high global descriptors, an indication of a highly electrophilic metal centre. A
plot of the electrophilicity indices of the pre-catalysts against yield of the oligomers show a linear
correlation, an indication that the electrophilicity of the metal centre plays an important role in the
activity of these pre-catalysts.1. Introduction
Ethylene is the cheapest and most widely produced organic
compound in the world,1,2 and is also an abundant feedstock for
the manufacturing of value-added products including linear a-
olens (LAOs), a class of commodity chemicals that are used in
industries such as synthetic polymers, detergents, plasticizers,
and lubricants, and have a global demand of over four million
tons per year.3 The synthesis of these commodity chemicals
involves the use of suitable catalysts. The catalyst has to be ne-
tuned for high activities. Fine-tuning of the catalyst involves the
control of the electrons in the catalytic system through ligand
design.
The synthesis of functionalised ferrocene is of great impor-
tance in the eld of organometallic chemistry,4 this can be
attributed to the unique properties of ferrocene and howJohannesburg, Auckland Park Kingsway
, South Africa
Ghana, Legon, Accra, Ghana. E-mail:
tion (ESI) available. See DOI:functional groups attached changes the chemistry of ferrocene.
Due to the robustness of ferrocene, many applications for its
use exist, including its use as redox-active component in homo-
and heterometallic transition metal complexes.4 Ferrocene and
its derivatives have been used as molecular sensors,5 in energy
transfer processes6 and as catalysts in various chemical reac-
tions7 because of their reversible electrochemical nature. The
redox reaction between the ferrocene and ferrocenium ion (FeII/
FeIII) is a fast and reversible one electron transfer process. This
is one of the vital properties of ferrocene and its derivatives.
Scientists have taken advantage of this property to focus on the
electronic and communication properties in these compounds8
through electrochemical studies using techniques such as cyclic
voltammetry (CV).9,4b CV is a very useful electroanalytical tech-
nique for compounds containing transition metal centres that
may take on several different oxidation states. The CV experi-
ment using the ferrocene as proxy can provide important
information about the electron density of transition metal
centres in a compound as well as the compound's stability
under the experimental conditions employed.
Computational modelling, on the other hand, is another
technique that can be used to predict the electronic nature ofThis journal is © The Royal Society of Chemistry 2018
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Open Access Article. Published on 31 January 2018. Downloaded on 01/02/2018 00:46:14. 
 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.compounds. Recent developments in the area of olen oligo-
merisation and polymerisation catalysis using group 10 transi-
tionmetal catalysts, mainly nickel(II) and palladium(II) catalysts,
have shown that the electronic nature of the catalysts is
crucial.10 To obtain the desirable catalysts requires not only
synthetic skills but also an understanding of all kinds of factors
inuencing the fundamental steps of olen oligomerisation
and polymerisation reactions.11 There are numerous articles
dealing with polymerisation catalysts of late-transition metals
with N^O or N^N ligand systems. The more established N^N
catalysts have been the subjects of numerous mechanistic
studies based on experimental and theoretical techniques.12
The catalytic activity of the transition metal complex is relies
more on the electronic conguration of the catalyst in the
ground state, especially the charge density. Möhring et al.13 have
shown that the electronic effect could contribute as much as
80% of the change in the polymerisation activities. This is by
studying the inuence of steric and electronic effect of ligands
on the catalytic activity of (CpR)2ZrCl2/ethylaluminoxane. Guo
et al. have also shown that metallocene catalyst's activities
increase with the reducing charge density14 while the catalytic
activities of a-diimine nickel(II) complexes increase with
increase charge density.15 Other report using Brookhart's type
catalysts have demonstrated through theoretical mechanistic
studies using quantum mechanics the importance of the
conformation of the substituted on the catalysts and the elec-
trophilicity of the central metal toward the activity of the cata-
lysts.16 A part from theoretical and experimental evidence, other
analytical methods to determine the electronic properties of
catalysts have not been extensively investigated.
In view of the importance of electronic properties on cata-
lysts on activity, this report deals with two simple analytical
methods (electrochemical and computational modelling) to
study the electronics of ferrocenylpyrazolyl nickel(II) and palla-
dium(II) complexes. The ferrocene in the complexes is to service
as a probe for the electrochemical studies using CV to indirectly
determine the electrophilicity of the metal centre of the catalyst
and relate it to the activity of the catalyst towards ethylene oli-
gomerisation and polymerisation reactions.
2. Experimental section
2.1 Materials and instruments
Compounds were synthesised using standard Schlenk tech-
niques under N2. Organic solvents such as N,N-dimethyl form-
amide (DMF) were dried over CaH2. Hexane over Na lamps while
tetrahydrofuran (THF) was dried over Na with benzophenone as
indicator. Compounds L1–L7 and 1–10 were synthesised
according to literature procedure.17 Tetra-n-butylammonium-
tetrauoroborate ([tBuN][BF4]) was purchased from Sigma
Aldrich and used as received. All electrochemical experiments
were performed using Autolab potentiostat PGSTAT 302 (Eco
Chemie, Utrecht, The Netherlands) driven by the general
purpose Electrochemical System data processing soware
(GPES, version 4.9). Infrared (IR) spectra of complexes were
recorded on a Bruker Tensor 27 equipped with a diamond ATR.
Elemental analyses were performed on a Vario elementar IIIThis journal is © The Royal Society of Chemistry 2018microcube CHNS analyzer. The ESI-MS spectra were recorded
on a Waters API Qualtro micro spectrophotometer. NMR
spectra were recorded on a Bruker 400 MHz instrument (1H at
400 MHz and 13C{1H} at 100 MHz). The chemical shis are re-
ported in d (ppm) and referenced to the residual proton and
carbon signals 7.24 ppm and 77.0 ppm respectively of CDCl3
NMR solvent.
2.2 Catalysis
2.2.1 General procedure for ethylene oligomerisation
reactions. In a typical experiment, a 500 mL steel autoclave was
heated under vacuum for 2 h at 160 C and allowed to cool to the
required temperature. The appropriate solution of the catalyst
precursor for each reaction was rst introduced into the reactor
and the reaction mixture saturated with ethylene at 1 bar for
5 min and ethylene vented off. The co-catalyst, EtAlCl2 was
added to the reaction mixture via a cannula. The reactor was
then saturated with ethylene to the desired pressure and stirred
at the required temperature and time. The ethylene pressure
was maintained throughout the reaction. The reaction was
stopped by turning off ethylene feed and connecting the reactor
to two traps cooled with liquid nitrogen, followed by venting off
the reactor. The reactor contents were isolated and quenched
with 10% HCl in methanol, and the product sampled for gas-
chromatography (GC) and gas-chromatography-mass spec-
trometry (GC-MS) analyses. The solvent from the remaining
product was removed in vacuo and the mass of the product
determined.
2.3 Electrochemical methods
The electrochemical cell consists of a three-electrode cell, which
involved a glassy carbon (geometric area ¼ 0.071 cm2) as
a working electrode, Ag/AgCl (3 M KCl) reference electrode, and
platinum wire as the counter electrode. The measurement was
performed using a concentration of 1.0 mM samples, in the
presence of 0.1 M tetra-butyl ammonium tetrauoroborate
([tBuN][BF4]) (supporting electrolyte) in dry DMF under argon
atmosphere at room temperature. The glassy carbon electrode
was polished on a Buehler-felt pad using alumina (0.05 mm) and
then washed with millipore water, sonicated for 5 min in mil-
lipore water and nally rinsed with pH 9.2 buffer solution. This
was done to ensure that there are no contaminants on the
electrode surface before the electrochemical measurements.
Argon was bubbled through the solution before recording the
cyclic voltammogram to ensure an oxygen free solution.
2.4 Molecular modelling method
Modelling programs Spartan 08 V1.2.0 and Spartan 10.1.10 (ref.
18) were used for all calculations at the DFT B3LYP level of
theory. The B3LYP functional level of theory is a Hartree–Fock
DFT hybrid. However, the LACVP* basis set is a relativistic
effective core-potential or pseudo potential that describes the
atoms H–Ar and transition metals with the 6-31G* basis. The
LANL2DZ basis set was used to model heavier atoms which
make use of all-electron valence double zeta basis set D95V,
done by Dunning, for rst row elements19 and the Los AlamosRSC Adv., 2018, 8, 5362–5371 | 5363
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Scheme 1 Oligomerisation of ethylene catalysed by pre-catalyst 1–4
with EtAlCl2.
Open Access Article. Published on 31 January 2018. Downloaded on 01/02/2018 00:46:14. 
 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.ECP plus double zeta basis set for the atoms Na–La, Hf–Bi20
done by Wadt and Hay. Spartan's graphical model builder and
minimised which interact with the SYBYL force eld21were used
to construct the starting geometries of the molecular systems.
All geometries were fully optimised without any symmetry
constraints. Equilibrium geometries were characterised by the
absence of imaginary frequencies, which also gave the HOMO,
LUMO and charge density values. The transition state structures
were with only one imaginary frequency and located without
constrains using fully optimised standard transition state
optimisation procedure in Spartan.
3. Results and discussions
3.1 Catalysis
3.1.1 Ethylene oligomerisation or polymerisation reactions
using pre-catalysts 1–4. The ethylene catalysis of pre-catalysts 5–
10 have been reported in literature by us.17b,c We report here the
extensive catalytic activity of pre-catalysts 1–4.
The ethylene oligomerisation or polymerisation reactions for
pre-catalysts 1–4 were investigated. Pre-catalyst 1 was used as
a basis to investigate the optimum co-catalyst ratio and pre-
catalyst loading needed for the polymerization reaction in
hexane. The co-catalyst used is EtAlCl2, whiles the Al : Pd ratio
was varied from 100 : 1 to 200 : 1. This led to an increase in both
the activity of the catalyst and molecular weights of the poly-
ethylene produced. However, increasing in the ratio above
200 : 1 was followed by a drastic decrease in both activity and
molecular weight. The decrease in molecular weight on
increasing the Al : Pd ratio is an indication of high chain
transfer from the palladium species to the aluminium co-
catalyst and fast chain termination, this in line with what is
reported in the literature.22
The effect of catalyst loading of pre-catalyst 1 was also
investigated. It was observed that increasing the pre-catalyst
loading from 4 mmol to 5 mmol resulted in an increase in
activity from 200 kg mol Pd
1 h
1 to 262 kg mol Pd
1 h
1. A
further increase in pre-catalyst amount to 10 mmol, caused the
activity to drop to as low as 131 kg mol Pd
1 h
1. However, the
molecular weight of the polymer remained unchanged. A
similar observation has been reported by Junges et al.23 for
a tridentate pyrazolyl Cr(III) pre-catalyst. In this system, a pre-
catalyst loading of 10 mmol gave a TOF of 66 200 h
1 but at 30
mmol loading reduces the TOF to 29 000 h
1. Junges et al.
attributed this to the ease at which the pre-catalyst solubilised
in the reaction solvent at low pre-catalyst loading and catalyst
aggregation at high pre-catalyst loading.
The optimum Al : Pd ratio was 200 : 1 and pre-catalyst
loading of 5 mmol was established. Further catalytic investiga-
tions were performed in ethylene oligomerisation and poly-
merisation reactions using EtAlCl2 as co-catalyst (Scheme 1). At
these optimum conditions other reaction parameters such as
ethylene pressure, temperature, solvent and time were
investigated.
The oligomerisation reaction was not exothermic, as there
was no temperature increase during the reaction. All the pre-
catalysts show moderate activities as ethylene oligomerisation5364 | RSC Adv., 2018, 8, 5362–5371catalysts to produce mostly butene and higher chain oligomers.
A typical GC (Fig. S1†) shows the presence of butene, which is 1-
butene but did not show the presence of higher oligomers.
However, the product obtained aer removing the solvent was
a viscous liquid, which was initially suspected to be a low
molecular weight polymer. GPC analyses were performed on
these liquid products, their molecular weights were found to be
lower than 1000 Da. Using Atmospheric pressure chemical
ionization (APCI) a so ionization technique, the molecular
weights observed were between 267.0 and 623.0, an indication
that the viscous liquid products are oligomers of ethylene. This
technique gave an envelope shape spectra, indicating a single
active catalytic species present upon activation with EtAlCl2 co-
catalyst. A typical spectrum of the APCI is shown in Fig. S2.† An
expanded form of the spectrum which shows isotopic distri-
butions is presented in Fig. S3.† The difference between the
isotopic patterns is 14.0157 which correspond to a methylene
group, indicating fragmentation of the oligomer chain.
1H NMR spectrum (Fig. S4†) shows an olenic group that
support the low molecular weight of the ethylene oligomer
produced. The olenic group, appearing at 5.21 ppm is a vinyl-
ene group (E and Z R1CH]CHR2). The peaks appearing
between 0.80 ppm and 2.10 ppm are different protons from CH2
and CH3 groups indicating that the products obtained in the
catalytic reactions are ethylene oligomers.
3.1.2 Effect of catalyst structure on the catalytic activity.
Among the six pre-catalysts investigated pre-catalyst 1 was
found to be the most active (Table 1: entries 1–6). It appears the
nature of the ligands play a signicant role in the catalyst
activity and themolecular weight of the ethylene oligomers. Pre-
catalysts 1 and 2 that have pyrazolyl-pyridyl donor groups were
more active and produced polymers with the highest molecular
weights compared to 3 and 4 that have pyrazolyl-amine donor
groups. For instance, under the same reaction conditions, the
best activity for the pre-catalyst containing pyrazolyl-pyridyl
ligands (1 and 2) was 262 kg mol Pd
1 h
1 with a correspond-
ing molecular weight of 507.0 for 1. The activity for pre-catalyst
with pyrazolyl-amine donors (3 and 4) was 210 kg mol Pd
1 h
1
with 401.0 molecular weight for 3. The high activities of pre-
catalysts 1 and 2 compared to 3 and 4 could be due to the
stable pre-catalysts formed by the strong donor ability of the
pyridyl unit. Also pre-catalysts 1 and 3 that show high activities
compared to 2 and 4. These have no substituent in position 3 of
the pyrazolyl unit. This is an indication that steric factors play
a crucial role in the activity of these pre-catalysts. Due to the
high catalytic activity of pre-catalyst 1, it was used in further
studies on various catalytic reaction parameters.
Kinetically, the rate of oligomerisation or polymerisation
increases with increase in ethylene pressure.24 The activity ofThis journal is © The Royal Society of Chemistry 2018
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Table 1 Ethylene oligomerisation reaction using pre-catalysts 1–4 and EtAlCl2 as co-catalyst
a
Entry Catalyst Pressure (bar) Yieldd (g) Activity (kg mol Pd
1 h
1) Temperature (C) Time (min) Oligomer weighte (m/z)
3 1 10 1.31 262 25 60 507.0
4 2 10 1.05 210 25 60 425.0
5 3 10 0.98 196 25 60 401.0
6 4 10 0.65 130 25 60 345.0
7 1 5 0.66 132 25 60 301.0
8 1 20 1.41 282 25 60 511.0
9 1 30 1.56 312 25 60 520.0
10 1 10 0.90 180 40 60 302.0
11 1 10 0.61 122 50 60 243.0
12 1 10 0.56 112 60 60 220.0
13 1b 10 1.65 330 25 60 —
14 1c 10 1.81 362 25 60 623.0
15 1 10 1.46 146 25 120 512.0
16 1 10 1.58 105 25 180 519.0
a Conditions: 6.0 mL of hexane. b Toluene used as reaction solvent. c Chlorobenzene used as reaction solvent; catalyst loading ¼ 5 mmol; Al : Pd ¼
200 : 1. d Determined by mass difference of 70.0 mL hexane (45.8 g), toluene (60.9 g) or chlorobenzene (77.7 g) and mass of nal solution.
e Determined by APCI.
Table 2 Cyclic voltammetry data for compounds L1–L7a
Entry Compound E1/2 (mV) DE (mV) Ipc/Ipa
1 Fc 483.0 84.0 0.85
2 L1 335.0 97.0 0.72
3 L2 362.0 101.0 0.75
4 L3 325.0 83.0 0.63
5 L4 329.0 97.0 0.74
6 L5 357.0 86.0 0.72
7 L6 332.0 87.0 0.68
8 L7 633.0 155.0 0.12
a Conditions: solvent ¼ N,N-dimethyl formamide (DMF); supporting
electrolyte ¼ tetra-n-butylammonium-tetrauoroborate [tBuN][BF4];
compound concentration ¼ 1 mM; supporting electrolyte
concentration ¼ 0.1 M. Reference electrode ¼ Ag/AgCl; counter
electrode ¼ platinum wire; working electrode ¼ glassy carbon; Fc ¼
ferrocene, scan rate of 100 mV s
1. Fc ¼ ferrocene.
Open Access Article. Published on 31 January 2018. Downloaded on 01/02/2018 00:46:14. 
 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.pre-catalyst 1 and molecular weight of the ethylene oligomers
produced showed marginal increment with increase in pres-
sure. As an illustration, increasing the pressure from 10 bar to
30 bar saw only 0.10 g and 0.25 g increase from 1.31 g of
products formed and also slight increase in activity (Table 1:
entries 3, 8 and 9). A similar trend was observed for the
molecular weights recorded. Where pressure is increased from
10 bar to 30 bar resulted in molecular weight of 507.0, 511.0 and
520.0 respectively (Table 1: entries 3, 8 and 9). The marginal
increase in both mass and molecular weight as pressure was
increased could be due to catalyst saturation.
Temperature variation affects the activity of the catalysts and
molecular weight of the polymer obtained. For example an
increase in temperature from 25 C to 60 C resulted in
a decrease in activity from 262 kg mol Pd
1 h
1 to 112 kg mol
Pd
1 h
1 (Table 1: entries 3, 10–12). This can be attributed to
decomposition or deactivation of the active species as well as
lower ethylene solubility in the reaction solution at high
temperatures.25 The decrease in molecular weight could be due
to faster chain termination than chain propagation at elevated
temperature.26
The reaction solvent was also varied by using hexane, toluene
and chlorobenzene respectively. An increase in activity is
observed which can be attributed to solubility of the active
species (Table 1: entries 3, 13 and 14). The results show that the
active species is more soluble in a polar solvent, hence the
observed increase in activity in the order hexane < toluene <
chlorobenzene. Butene and liquid polyethylene were obtained
when hexane and chlorobenzene were used as reaction solvent.
The higher molecular weight of 623.0 was obtained for the
oligomer using chlorobenzene, compared to 507.0 obtained for
the reaction in hexane under the similar reaction conditions. In
toluene, the products obtained were butene, ethyltoluene,
mono-butyltoluenes and di-butyltoluenes (Fig. S5†). The alkyl-
toluenes are produced via Friedel–Cras alkylation reaction.27
The oligomerisation reactions performed in toluene resulted in
alkylation of the solvent with the oligomer produced and theThis journal is © The Royal Society of Chemistry 2018ethylene. There was no clear evidence of triple alkylation of
toluene from the GC data (Fig. S5†); suggesting that alkylation is
more facile for butyltoluenes compared to ethyltoluenes.
Time run experiments were performed to establish the life
time of the active species. Increasing the time 60min to 180min
saw only a marginal increase in catalytic activity and molecular
weight (Table 1: entries 15 and 16). Aer 60min catalytic activity
was low towards polymerization. The low activity recorded with
longer reaction time indicates catalyst deactivate with time.
The following sections deal with extensive report on the
determination of the electronic properties of the ligands and
the complexes using CV and computational modelling.3.2 Electrochemical studies
3.2.1 Electrochemical properties of compounds L1–L7. At
a given temperature, the half-wave potential (E1/2) values change
considerably for different solvents. The half-wave potential
shis toward more positive potentials in the order: DMSO <
DMA < NMF < DMF < ACN < PEN < ACE < DCM. This orderRSC Adv., 2018, 8, 5362–5371 | 5365
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Fig. 3 Cyclic voltammogram of compound L2 in DMF, [tBuN][BF ] as
Fig. 1 Ferrocenylpyrazolyl compounds L1–L7. 4
supporting electrolyte, Ag/AgCl as reference electrode, platinum wire
as counter electrode, glassy carbon as working electrode, scan rate of
100 mV s
1, temperature 25 C.
Open Access Article. Published on 31 January 2018. Downloaded on 01/02/2018 00:46:14. 
 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.indicates that the oxidation of ferrocene to the ferrocenium is
more difficult with decreasing polarity.28 The ferrocenium is
more sensitive to interactions with the solvent molecules than
ferrocene. Therefore, the strength of the interactions of the
ferrocenium (which acts as a Lewis electron-pair acceptor) with
solvent molecules (Lewis electron-pair donor) is expected to
affect the oxidation of ferrocene and thus the half-wave poten-
tial.29 Based on the above hypothesis, N,N-dimethyl formamide
(DMF) which is midway through the series was chosen as
solvent for the cyclic voltammetry experiment.
The electrochemical properties of ligands L1–L7 and palla-
dium(II) and nickel(II) complexes 1–10 were investigated by
cyclic voltammetry in DMF with 0.1 M [tBuN][BF4] as supporting
electrolyte. Table 2 summarises the redox potential data for
compounds L1–L7 investigated (Fig. 1).
The cyclic voltammograms of compounds L1 and L2 showed
a quasi-reversible peak corresponding to one electron transfer.
The typical voltammogram for L2 is shown in Fig. 3. FeII–FeIII
redox couples of the ferrocenyl unit of L1 and L2 showed E1/2 at
335 mV (DE ¼ 97 mV, and Ipc/Ipa ¼ 0.72) and 362 mV (DE ¼
101 mV and Ipc/Ipa ¼ 0.75) respectively (Table 2: entries 2 and 3).
The observed redox potentials are in agreement with the elec-
tron donor abilities of the pyrazolyl unit, which donates more
electron density to the ferrocene moiety; and hence favouring
the ease of oxidation of the ferrocenyl unit.30 Modication of L1Fig. 2 Ferrocenylpyrazolyl palladium and nickel complexes 1–10.
5366 | RSC Adv., 2018, 8, 5362–5371and L2 by introducing more electron donating groups such as
methylene pyridine (L3 and L4) and ethylamine (L5 and L6)
show signicant improvement on the oxidation and still show
quasi reversibility of the ferrocenyl unit (Table 2: entries 2, 3 and
4–7). However, L3 and L4 showed the oxidation of ferrocenyl
unit is easier compared L5 and L6. This could be due to the
aromatic nature of the pyridine ring which helps to form
a partial conjugation system with the pyrazolyl unit. Compared
to compounds containing ethylamine group, despite its elec-
tron donating ability, these ligands do not form a conjugated
system; hence this has less oxidative inuence compared to the
methylene pyridine. The inuence of substituent (either
hydrogen or methyl) on the pyrazolyl moiety also appears to play
a role in the oxidation of the ferrocenyl unit. In principle, the
electron donating methyl substituents should enhance the ease
of oxidation compare to hydrogen. Comparing the compounds
which differ by either methyl or hydrogen on the pyrazolyl unit,
only L6 undergoes easy oxidation compared to L5. Compounds
L1, L2, L3 and L4 however, showed otherwise. As expected,
compound L7 which has an electron withdrawing group shows
one electron quasi-reversible oxidation peak (Fig. S6†) of the
ferrocenyl unit with half wave potential of 633.0 mV and Ipc/Ipa
value of 0.12 (Table 2: entry 8).
A plot of current (A) against scan rate (mV s
1) show a linear
relationship for all the compounds discussed above. The linear
relationship suggests that the mechanisms of oxidation of the
ferrocenyl unit in L1–L7 are diffusion controlled. A typical plot
for L2 is shown in Fig. S7.† The diffusion controlled mechanism
indicates that there is a spontaneous transfer of the electro-
active species from regions of higher concentrations to regions
of lower concentrations.
The compounds also show that the half wave potentials are
independent of scan rates. An example is given in Fig. S8,†
which shows half wave potentials are independent of scan rates
for compound L2. This also demonstrates that the diffusion
controlled process occurs quickly, and the rate of reaction is
controlled by rate of transport of the reactants through the
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Table 3 Cyclic voltammetry data for complexes 1–10a
Entry Compound E1/2 (mV) DE (mV) Ipc/Ipa Kc Yield
1 1 456.0 118.0 0.44 — 1.31
2 2 412.0 104.0 0.46 — 1.05
3 3 423.0 120 0.40 — 0.98
4 4 410.0 115 0.48 — 0.65
5 5 425.0 92.0 0.46 589 17.80b
6 6 393.0 95.0 0.49 613 10.20b
7 7 436.0 83.0 0.49 130 6.70b
8 8 433.0 77.0 0.72 170 5.80b
9 9 417.0 81.0 0.50 520 8.20b
10 10 424.0 95.0 0.35 592 7.10b
a Conditions: solvent ¼ N,N-dimethyl formamide (DMF); supporting
electrolyte ¼ tetra-n-butylammonium-tetra uoroborate [t BuN][BF4];
analyte concentration ¼ 1 mM; supporting electrolyte concentration ¼
0.1 M. Reference electrode ¼ Ag/AgCl; counter electrode ¼ platinum
wire; working electrode ¼ glassy carbon, scan rate of 100 mV s
1.
b Published results from ref. 17b.
Open Access Article. Published on 31 January 2018. Downloaded on 01/02/2018 00:46:14. 
 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.3.2.2 Electrochemical properties of complexes 1–10. Palla-
dium complexes 1–4 showed redox potential for the ferrocenyl
unit with higher E1/2 values between 410 and 456 mV (Table 3)
compared to that of the corresponding ligands which are
between 325 and 357 mV. Also, the observed Ipc/Ipa values were
between 0.40 and 0.49, which is an indication of also quasi-
reversible behaviour. This could be due to the dative covalent
bonds which are formed between the palladium and the
nitrogen donor atoms of the ligand. Therefore, electrons are
donated from the ligand to the palladium metal centre, result-
ing in low electron density on the ferrocenyl moiety. Hence, the
palladium complexes showed higher half wave potential for
FeII/FeIII redox couple than their corresponding ligands. A
typical voltammogram for complex 1 is shown in Fig. 4. There is
no evidence from the voltammogram of delocalisation of elec-
trons between the twometal centres. In this voltammogram, the
second wave potential is difficult to determine. This could
suggest aggregation of the complexes or lack of interaction
between the two metal centres.Fig. 4 Cyclic voltammogram of complex 1 in DMF, [tBuN][BF4] as
supporting electrolyte, Ag/AgCl as reference electrode, platinum wire
as counter electrode, glassy carbon as working electrode, scan rate of
100 mV s
1, temperature ¼ 25 C.
This journal is © The Royal Society of Chemistry 2018NiBr2 or NiCl2 complexes of L1–L6 (5–10 (Fig. 2)) also
showed redox oxidation potential of the ferrocenyl unit in the
various ligands. The half wave potentials for these complexes
were between 389 and 436 mV and are higher than corre-
sponding free ligands (Table 3). However, these half wave
potentials are still lower compared to ferrocene (483 mV). The
Ipc/Ipa values also show that the oxidation of these complexes
are quasi-reversible. The voltammograms of the complexes
show delocalisation of electron between the metal centres. A
typical voltammogram showing delocalisation of electron is
Fig. 5. The extent of electron exchange between the iron and
nickel centres could be expressed by the comproportionation
constant (Kc) using the Nernst equation.31
Kc ¼ exp(DE)F/RT
(DE ¼ E2 
 E1, F ¼ Faraday constant, R ¼ gas constant, T ¼
temperature).
To compare the degree of interactions between the metal
centres using their equilibrium constants, the Robin and Day
classication scheme was used.32 The Robin and Day classi-
cation scheme classies molecules into three different classes.
A class I molecule has the least amount of electronic interaction
with a value K 2c < 10 . Class II molecules have moderate elec-
tronic interactions, ranging between 102 and 106. Class III
molecules on the other hand have K 6c > 10 . Class III molecules
have the best electron interactions. The degree of electronic
interaction is directly related to the rate of the electron transfer
i.e. large electronic interaction prefers fast electron transfer
rates, while small electronic interaction depicts slow electron
transfer rates.
The comproportionation constants Kc for complexes 5–10
(Table 3) suggests moderate delocalisation of electrons between
the two metal centres. Complexes 7 and 8 recorded the lowest
comproportionation constants, compared to 5, 6, 9 and 10,
which could be due to the non-aromatic nature of the ligands,
hence reducing the extent of electron transfer through conju-
gation. Complexes 5–10 therefore can be classied under the
class II, according to the Robin and Day classication scheme.Fig. 5 Cyclic voltammogram of complex 6 in DMF, [tBuN][BF4] as
supporting electrolyte, Ag/AgCl as reference electrode, platinum wire
as counter electrode, glassy carbon as working electrode, scan rate of
100 mV s
1, temperature 25 C.
RSC Adv., 2018, 8, 5362–5371 | 5367
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RSC Advances Paper
Open Access Article. Published on 31 January 2018. Downloaded on 01/02/2018 00:46:14. 
 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.The higher the half wave potential the more electrophilic the
metal centres it is associated with. This indicates that the
higher the half wave potential for FeII/FeIII redox couple, the
difficult will it be to oxidise the ferrocenyl unit due to the
withdrawal of electron density by nickel(II) or palladium(II) unit.
In this way, we used ferrocene as a sensor to probe the elec-
trophilic nature of both the nickel and palladium complexes.
We have also correlated the electrophilicity of the nickel or
palladium centre with their catalytic activities.
With the same ligand system, there is a direct correlation
between the electrophilicity of the metal centre and yield. For
example Table 3, entries 1–8, higher yields are associated with
lower E1/2 values. In addition, Table 3, entries 5 and 9, with
different halide substituents but the same metal and ligand
environment, a similar relationship is observed. The nickel
complexes generally have higher yields compared to palladium
complexes and it is associated with lower E1/2 values. For
example, in Table 3, entries 1 and 9, the nickel complex shows
a higher yield compared to the palladium analogue. However,
a similar observation was not made for entries 2 and 10. Other
factors in addition to the electrophilicity of the metal centre are
responsible for the high activities of the complexes. Other
factors, such as solubility and stability of the active species can
also play roles in the activities of these catalysts.
To further investigate the effect of the electrophilicity of the
metal centre on activity and yield, molecular modeling was
carried out.3.3 Molecular modeling of ligands and complexes
3.3.1 Electronic properties of ligands L1–L6. To investigate
the electronic properties of these ligands; 3HOMO, 3LUMO, and
global descriptors such as chemical potential (m), chemical
hardness (h) and electrophilicity (u) were calculated according
to the method used by Thanikaivelan et al.33 These descriptors
are important electronic features used to describe stability,
reactivity and other related properties of molecules.34 These
descriptors were determined from HOMO and LUMO energies
using the equations:
¼ 3LUMO þ 3HOMOm
2
¼ 3LUMO 
 3HOMOh
2
m2
u ¼
2h
Generally, the HOMO of a ligand provides information about
electron donating ability while the LUMO gives the electron
accepting capability. Therefore, to understand the nature of the
donor–acceptor behaviour the shape of the HOMO and LUMO
can be used.
Fig. S9† shows that the HOMO of both L1 and L4 are local-
ized on both the pyrazolyl nitrogen and the iron of the5368 | RSC Adv., 2018, 8, 5362–5371ferrocenyl unit. The localization of the HOMO on the pyrazolyl
nitrogen atom is in agreement with the formation of s-bond
through that nitrogen atom to a metal. The HOMO of the iron
metal is an indication of a possible transfer of the electron
density to the pyrazolyl unit during the complex formation. The
LUMO also shows localization on the pyridyl unit, which is an
indication of potential p-acceptor behaviours of this arm of the
ligand. However, L1 shows localization in the whole ligand
system also suggesting that this ligand hasp-acceptor potential.
The strength of p-acceptor potential is determined by the more
negative value of 3LUMO. Table 4 shows the values for 3LUMO for
ligands L1–L6. The values show that the ligands containing
pyridyl unit have more electron accepting capabilities
compared to the rest (Table 4: entries 3 and 4). This is followed
by the aliphatic amines (Table 4: entries 5 and 6) then the
pyrazolyl ligands (Table 4: entries 1 and 2).
To conrm further the p-accepting nature of these ligands,
the electrophilicity index of each ligand was determined (Table
4). The higher the electrophilicity index the more electrophilic
the compound. The data shows that the pyridyl ligands are
more electrophilic than the ligands containing aliphatic
amines, which are in turn more electrophilic than to the pyr-
azolyl ligands. Furthermore the ligands with hydrogen at the 3
position of the pyrazolyl unit are more electrophilic compared
to the methyl substituted ligands.
Determination of the electronic properties of the complexes 1–8.
The activity of metal pre-catalyst for olen oligomerisation or
polymerisation reactions, depend strongly on the nature of the
ligands surrounding the metal. The nature of ligands in fact
inuences the electrophilicity of the metal centre. A highly
electrophilic metal pre-catalyst, is expected to be highly active
because the olen is electron rich and therefore can readily
coordinate to a more electrophilic metal centre.
HOMO–LUMO gap, global descriptors (chemical hardness
(h) and electrophilicity (u)) and charge density are the param-
eters considered in predicting the electrophilicity of the metal
centres of these complexes.
3.3.2 HOMO–LUMO gap. The HOMO–LUMO gap of a metal
complex shows the direction of ow of electrons. Since the
HOMOs are predominantly occupied by d-orbital electrons,
smaller HOMO–LUMO gap will facilitate p-back donation of
electrons from the metal to the ligand system and hence the
metal centre will be electrophilic. The calculations performed
on complexes 1–8 (Table 4; entries 7–14) show that the gaps for
the palladium complexes are larger compared to the nickel
complexes. This suggests that the nickel centres are more
electrophilic compared to the palladium. Among the palladium
complexes 1 (Table 4: entry 7) has the least gap while for the
nickel complex 5 recorded the smallest gap (Table 4: entry 11).
The results correspond to the experimental results where 1 and
5 were the most active among their respective complexes.
Typical diagrams of the HOMO–LUMO gaps of complexes 2 and
6 are shown in Fig. S10 and S11.† It is therefore expected that
the nickel complexes would be more active compared to the
palladium complexes for olen oligomerisation or polymerisa-
tion reactions. This is conrmed by experimental observations.
The palladium complex 1 when activated with EtAlCl2 in theThis journal is © The Royal Society of Chemistry 2018
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Table 4 Data of global descriptors; chemical potential (m), chemical hardness (h) and electrophilicity (u) using B3LYP level of theory and LACVP*
basis set for L1–L6 and 1–8
Entry Compound 3HOMO (eV) 3LUMO (eV) m (eV) h (eV) u (eV) Yield (g)
1 L1 
5.15 
0.17 
2.66 2.49 1.42
2 L2 
4.97 0.03 
2.47 2.50 1.22
3 L3 
5.17 
0.71 
2.94 2.23 1.94
4 L4 
5.12 
0.77 
2.95 2.18 1.99
5 L5 
5.14 
0.65 
2.92 2.23 1.91
6 L6 
5.09 
0.69 
2.89 2.20 1.89
7 1 
5.33 
2.24 
3.79 1.55 4.63 1.31
8 2 
5.45 
2.14 
3.80 1.66 4.34 1.05
9 3 
5.28 
2.14 
3.71 1.57 4.38 0.98
10 4 
5.87 
2.12 
3.99 1.88 4.23 0.65
11 5 
5.25 
3.47 
4.36 0.89 10.68 17.80a
12 6 
5.23 
3.39 
4.31 0.92 10.10 10.20a
13 7 
5.30 
3.23 
4.27 1.04 8.77 6.70a
14 8 
5.41 
3.21 
4.31 1.10 8.44 5.80a
a Published result in ref. 17b.
Open Access Article. Published on 31 January 2018. Downloaded on 01/02/2018 00:46:14. 
 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.presence of ethylene shows an activity of 362 kg mol Pd
1 h
1
whereas the nickel complex 5 shows activity of 1776 kg mol Ni
1
h
1 under similar conditions.17b3.4 Determination of the electronic properties of 1–8 using
global descriptors
The global descriptors chemical hardness (h) and electrophi-
licity (u) were used to predict the electronic properties of 1–8.
3.4.1 Chemical hardness (h). The chemical hardness (h) of
a complex is an electronic quantity which is used to characterise
the relative stability and the resistance to changes in the
number of electrons.35 The higher the value for the chemical
hardness the more stable and less reactive. The data in Table 4
entries 11–14 show that the nickel complexes have chemical
hardness values between 0.81 and 1.65 eV which make them
more reactive compare to the palladium complexes with values
between 1.55 and 1.95 eV (Table 4; entries 7–10). In order to nd
a correlation between the calculated chemical hardness of the
complexes and observed catalytic yield, a graph of yield against
calculated chemical hardness was plotted (Fig. 6 and 7). The
graph for the palladium complexes (Fig. 6) shows a near linearFig. 6 A graph of chemical hardness against yield of oligomers
produced using complexes 1–4.
This journal is © The Royal Society of Chemistry 2018t with R2 value of 0.7867, indicating a correlation between
these two parameters. The best yield was obtained when the
chemical hardness was minimum (1.55 eV) and the least yield
was found at 1.88 eV. Similar observation was made for the
nickel complexes (Fig. 7) where the R2 value is 0.7654 was
recorded.
3.4.2 Electrophilicity index (u). A quantitative classica-
tion of the global electrophilic nature of the complex can be
determined by the electrophilicity index. The higher the value of
u, the more electrophilic the metal centre in the complex. Table
4 show that the nickel complexes are more electrophilic
compared to the palladium complexes. To relate the catalytic
yields of the complexes to electrophilicity index, a graph of
electrophilicity index against yield were plotted as shown
Fig. S12 and S13† for palladium and nickel complexes
respectively.
3.4.3 Charge density. The charge density of an atom or
element shows the density of electrons located around the
atom. The more positive the charge density the more electro-
philic the atom or element is. The molecular modelling of the
charge density for complexes 1–8 are presented in Table S2†
respectively and the typical images of the charge densities for 1
and 6 drawn with isovalue of 1.0 are shown in Fig. S14 and S15.†Fig. 7 A graph of chemical hardness against yield of oligomers
produced using complexes 5–8.
RSC Adv., 2018, 8, 5362–5371 | 5369
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Open Access Article. Published on 31 January 2018. Downloaded on 01/02/2018 00:46:14. 
 This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.The electron density values for both the palladium and nickel
complexes are in agreement with the prediction made with the
HOMO–LUMO gap. The palladium centres are less positive
compared to the nickel centres.
4. Conclusion
The palladium complexes were used as pre-catalysts for the
oligomerisation of ethylene upon activation with EtAlCl2 to 1-
butene and higher carbon content oligomers, typically with 20
to 52 carbon atoms in the oligomer. The palladium pre-catalysts
showed high activities in chlorobenzene for ethylene reactions
than in hexane. In toluene, in addition to butene, ethyltoluenes
and butyltoluenes were produced. The cyclic voltammetry
studies show that ligands L1–L6 which have electron donating
groups aid the ease of oxidation of the ferrocenyl moiety, while
for L7 which has electron withdrawing group, oxidation of the
ferrocenyl unit is difficult.
Cyclic voltammetry results for the nickel and palladium
complexes show it is more difficult to oxidise the ferrocenyl unit
in these ligands, indicating the electrophilic nature of both
metals. A direct correlation between the electrophilicity of the
metal is associated with a lower E1/2 and yield within a similar
ligand system.
Molecular modelling of the palladium and nickel complexes
show singlet state species and these complexes are stable
compared to their corresponding ligands, which is expected.
Calculations of the electronic properties of the complexes show
that electrophilicity of the metal centre correlate with the
activity of the pre-catalysts, an indication of the role electro-
philicity plays in the activity of the pre-catalysts. Molecular
modelling show that the higher the electrophilicity of the metal
centre the high the activity of the catalyst.
This study shows that CV and molecular modelling studies
could be used as simple tool to predict activity of a catalyst.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We acknowledge the National Research Foundation (South
Africa) and the University of Johannesburg for nancial support
for this project.
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