Synthesis, characterization and use of imidazole and methyl-pyrazole based pyridine ligands as extractants for nickel(II) and copper(II)

Article abstract of DOI:10.1016/j.ica.2019.02.020

This work consists of the synthesis of seven pyridine based ligands, 2-(3-butyl-1H-pyrazol-5-yl)-pyridine (1), 2-[3-(tert-butyl)-1H-pyrazol-5-yl]-pyridine (2), 2-(3-octyl-1H-pyrazol-5-yl)-pyridine (3), 2-(1-octyl-1H-imidazol-2-yl)pyridine (4), 2-[(1H-pyrazol-1-yl)methyl]pyridine (5), 2-[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]pyridine (6) and 2-[(3-methyl-1H-pyrazol-1-yl)methyl]pyridine (7) which have been made before. We modified some routes and obtained improved yields in most cases. The ligands were fully characterised using ¹H and ¹³C NMR, infrared (IR) spectroscopy, mass spectrometry (ms), elemental analysis (EA) and melting point where appropriate. We managed to grow single crystals of the 2-[(1H-pyrazol-1-yl)methyl]pyridine copper(II) perchlorate complex and solved the structure using single crystal X-ray diffraction. Coordination around the copper(II) centre is pseudo square planar, having two ligands coordinated to the metal ion. Indications are that the extraction stoichiometry using this ligand for copper(II) is also 2:1, L:M. This indicates how these ligands are capable of coordinating to metal ions. All the ligands were then used for the extraction of Ni(II) with and without a synergist, viz., sodium p-dodecylbenzenesulfonate (SDBS). The solvent extraction of only Ni(II) in the absence of synergist produced values from 3% to 15% and values between 69% and 79% with synergist. The competitive extraction of metal ions from a base metal ion mix (Co²⁺, Ni²⁺, Zn²⁺, Cu²⁺, Pb²⁺ and Cd²⁺) produced extraction values of 15%?87% extraction of different metal ions with synergist. Finally, time dependent extraction studies with Ni(II) showed that after 4 h equilibrium was established and in some cases even after 1 h equilibrium had been reached.

Full text of DOI:10.1016/j.ica.2019.02.020

Accepted Manuscript
Research paper
Synthesis, characterization and use of imidazole and methyl-pyrazole based
pyridine ligands as extractants for nickel(II) and copper(II)
Brendan H. Pearce, Hezron F.O. Ogutu, Waheed Saban, Robert C. Luckay
PII:
S0020-1693(18)31435-X
https://doi.org/10.1016/j.ica.2019.02.020
ICA 18783
DOI:
Reference:
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Inorganica Chimica Acta
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Revised Date:
Accepted Date:
17 September 2018
18 February 2019
18 February 2019
Please cite this article as: B.H. Pearce, H.F.O. Ogutu, W. Saban, R.C. Luckay, Synthesis, characterization and use
of imidazole and methyl-pyrazole based pyridine ligands as extractants for nickel(II) and copper(II), Inorganica
Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.02.020
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Synthesis, characterization and use of imidazole and
methyl-pyrazole based pyridine ligands as extractants for
nickel(II) and copper(II)
Brendan H. Pearce, Hezron F. O. Ogutu, Waheed Saban and Robert C.
Luckay
⃰
Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag
X1, Matieland, Stellenbosch, 7602, South Africa. Tel: +27 21 808 3333; email:
rcluckay@sun.ac.za
Abstract
This work consists of the synthesis of seven pyridine based ligands, 2-(3-butyl-1H-pyrazol-5-yl)-
pyridine (1), 2-[3-(tert-butyl)-1H-pyrazol-5-yl]-pyridine (2), 2-(3-octyl-1H-pyrazol-5-yl)-pyridine (3),
2
1
-(1-octyl-1H-imidazol-2-yl)pyridine (4), 2-[(1H-pyrazol-1-yl)methyl]pyridine (5), 2-[(3,5-dimethyl-
H-pyrazol-1-yl)methyl]pyridine (6) and 2-[(3-methyl-1H-pyrazol-1-yl)methyl]pyridine (7) which
have been made before. We modified some routes and obtained improved yields in most cases. The
1
13
ligands were fully characterised using H and C NMR, infrared (IR) spectroscopy, mass spectrometry
ms), elemental analysis (EA) and melting point where appropriate. We managed to grow single
(
crystals of the 2-[(1H-pyrazol-1-yl)methyl]pyridine copper(II) perchlorate complex and solved the
structure using single crystal X-ray diffraction. Coordination around the copper(II) centre is pseudo
square planar, having two ligands coordinated to the metal ion. Indications are that the extraction
stoichiometry using this ligand for copper(II) is also 2:1, L:M. This indicates how these ligands are
capable of coordinating to metal ions. All the ligands were then used for the extraction of Ni(II) with
and without a synergist, viz., sodium p-dodecylbenzenesulfonate (SDBS). The solvent extraction of
only Ni(II) in the absence of synergist produced values from 3% – 15% and values between 69% -
2
+
2+
7
9% with synergist. The competitive extraction of metal ions from a base metal ion mix (Co , Ni ,
2
+
2+
2+
2+
Zn , Cu , Pb and Cd ) produced extraction values of 15% - 87% extraction of different metal ions
with synergist. Finally, time dependent extraction studies with Ni(II) showed that after 4 hours
equilibrium was established and in some cases even after 1 hour equilibrium had been reached.
Keywords: N-heterocyclic ligands, nickel(II), copper(II), solvent extraction, synergism.
Introduction
The application of amine extractants in nickel solvent extraction systems are generally well-known.
These systems would typically include industrial extractants such as Alamine 336, Alamine 308 and
1

Alamine 300, which are all tertiary alkylamine extractants [1]. Aromatic amine extractants, on the
other hand, have been studied to a much lesser extent because of their inability to be exclusively
soluble in the organic phase. This obstacle has hindered research in this field and can be overcome
with the addition of hydrophobic moieties (e.g., alkyl chains) to the aromatic amine extractants,
resulting in ligands which are highly soluble in the organic phase. However, additional time, money
and effort to design these ligands is worthy if high selectivity for a particular metal ion is achieved.
Over several years, [2-5] publications appeared in which chemists reported the use of imidazole,
pyrazole and pyridine ligands for the separation of nickel, cobalt and copper from several base metal
ions. The imidazole ligands showed excellent selectivity towards Cu(II), and once these ligands were
modified, they were selective towards Ni(II) and Co(II) as well. They used the imidazole, pyrazole and
pyridine ligands independently and never attempted to combine these moieties into a single extractant,
thereby forfeiting the opportunity of using bidentate aromatic amine ligands in the selective extraction
of base metal ions. These precursor extractants can also coordinate with metal ions in slightly acidic
a
media due to their mild pK values. See structures of these aromatic precursors in Pearce et. al.[8].
Due to the pKa values of pyridine (5.31), imidazole (7.31) and pyrazole (2.78) it was felt that a good
pH to carry out the extraction work would be approximately 5.0. This is weakly acidic and at these pH
values, amines should not be protonated, allowing for facile complexation to metal ions.
There have been many bidentate aromatic amine ligands synthesised over the past few years, but very
few of them were used for solvent extraction. There have been three publications related to solvent
extraction on this, one by Tshentu et al.[6] , one by Tasker et. al. [7] and our recent work [8] which
investigates the use of pyridine pyrazole ligands as extractants in the solvent extraction of Ni(II) and
Cu(II). Because of this, we set out to investigate further pyridine based ligands, pyridine imidazole
and pyridine methyl pyrazole ligands with regard to their extraction of nickel(II) and copper(II). As
indicated above, there is not much in the literature regarding their selectivity for the metal ion Ni(II).
Most previous solvent extraction work was carried out with one extractant which coordinates to the
metal ion via an inner sphere coordination mechanism. However, since 1958, Blake et. al. [9] noticed
an increased extraction with the use of two extractants. He termed this synergistic solvent extraction.
The term “synergism” comes from the Greek word “synergos” meaning working together. It refers to
the interaction between two or more “things” when the combined effect is greater than if you added
the “things” on their own [10]. This is also the case in synergistic solvent extraction (SSX). We used
the synergist, sodium p-dodecylbenzenesulfonate (SDBS) which has a long alkyl tail, an aromatic
body and a sulfonate head, which is quite similar to the synergist dinonyl naphthalene sulfonic acid
(DNNSA) used by others. [6, 7]
Based on our previous work on the pyrazole ligands [8], we set out to synthesise further heterocyclic
bidentate N-donor ligands having varying alkyl substituents and the imidazole creating a 5-membered
chelate ring on coordination to a metal ion and the methyl pyrazole ligands creating a 6-membered
2

chelate ring on coordination to a metal ion. Hence, seven pyridine based ligands were synthesised.
These ligands are 2-(3-butyl-1H-pyrazol-5-yl)-pyridine (1), 2-[3-(tert-butyl)-1H-pyrazol-5-yl]-
pyridine (2), 2-(3-octyl-1H-pyrazol-5-yl)-pyridine (3), 2-(1-octyl-1H-imidazol-2-yl)pyridine (4), 2-
[
(1H-pyrazol-1-yl)methyl]pyridine (5), 2-[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]pyridine (6) and 2-
(3-methyl-1H-pyrazol-1-yl)methyl]pyridine (7). All are known in the literature and figure 1 depicts
[
1
13
their structures. The ligands would then be fully characterised using H and C NMR, infrared (IR)
spectroscopy, mass spectrometry (MS) and elemental analysis (EA) and melting point determination
where appropriate. We then carried out a solvent extraction study by (i) firstly, determining the
optimum synergist concentration. (ii) studied the solvent extraction of only Ni(II) in the presence and
absence of synergist. (iii) studied the competitive extraction of base metal ions from a base metal ion
2
+
2+
2+
2+
2+
2+
mix (Co , Ni , Zn , Cu , Pb and Cd ). (iv) carry out equilibration studies with Ni(II) and finally
v) grow single crystals of the 2-[(1H-pyrazol-1-yl)methyl]pyridine copper(II) perchlorate complex
(
and solve its structure.
R2
5
: R = R = H
1 2
R
1
2
: R = H
6: R = R = methyl
7
1 2
: R = methyl / H
1
R = H / methyl
2
: R = methyl
N
N
3: R = butyl
4: R = octyl
N
N
N
N
R1
Figure 1. Ligands synthesized and discussed in this work
Results and Discussion
Synthesis
The synthesis of all ligands followed well-established literature methods. We modified some methods,
where the order of addition of some reagents was changed. In some cases we used a different solvent
and also modified the reaction conditions. A short path Kugelrohr distillation apparatus was used in
some cases to remove low boiling substances instead of the classic recrystallization technique. All of
this was slightly more time consuming, but resulted in hugely improved yields.
1
13
All H and C NMR spectra corresponded well with the synthesised ligands. IR spectra displayed the
correct signals for all the ligands and mass spectrometry and elemental microanalyses furthermore
confirmed the synthesis of these ligands.
1
13
Ligand L7’s H and C NMR spectra were initially difficult to interpret without assuming that there
was a large quantity of side product in the reaction mixture. However, once running the mass spectrum
and getting elemental analyses which corresponded extremely well, we realised there were isomeric
3

forms of the product present. This is because there are two tautomeric forms in the deprotonated
precursor. Please see further details and all spectra in the supplementary material, sections S1-S8.
9 9 3 2 4 2
Crystal and molecular structure of [Cu(C H N ) ](ClO ) complex
Suitable x-ray quality crystals of the complex were grown using the diffusion method. The asymmetric
unit cell contains [Cu(C ](ClO , with two of 2-[(1H-pyrazol-1-yl)methyl]pyridine (5) bound
to the copper(II) ion in a distorted square planar fashion with two perchlorate counterions. The
complex crystallised in a monoclinic space group P2 /c. The crystallographic data and structural
refinement details are summarised in Table 1. The molecular structure of [Cu(C ](ClO with
9
H
9
N
3
)
2
4 2
)
1
9
H
9
N
3
)
2
4 2
)
the appropriate atom numbering scheme is shown in Figure 2, whilst selected bond lengths and angles
are shown in Tables 2 and 3 respectively. Cu-N bond lengths are in the range 1.979 Å – 2.028 Å
which are within the normal bond lengths for copper(II) to nitrogen [11,13]. The geometry around the
metal ion is distorted square planar which is a normal coordination mode for most low spin metal ions
such as Cu(II) [11]. However, the structure shows a high degree of distortion of the complex which
can be viewed to be oriented in an octahedral manner with the two perchlorate molecules in the axial
positions [12]. See figure S13 in supplementary material. The perchlorate oxygen atoms (O2 and O5)
show weak interactions with the Cu(II) centre and are however far beyond the coordination bond
distance [13]. The Cu1-O2 and Cu1-O5 distances are 3.780(2) and 2.444(2) Å, respectively.
The structure also shows marked disordered ellipsoids with puckering of the pyridine ring [14,15],
however Figure 2 only displays one particular position. The high distortion of the ligand is due to the
rigidity of the molecule around the central Cu- metal.
The packing diagram along the a- and c-axes is shown in Figure 3. This shows how the perchlorate
molecules are aligned between the complexes and how these layers lie on top of each other.
9 9 3 2 4 2
Figure 2. Ortep diagram of the [Cu(C H N ) ](ClO ) complex with 50% thermal ellipsoids
(picture generated by olex2)
4

9 9 3 2 4 2
Table 1. Crystallographic data and structure refinement of the [Cu(C H N ) ](ClO ) complex
Empirical formula
Formula weight
6 6 2 2.67
C H Cl0.67Cu0.33N O
193.55
Temperature/K
100(2)
Crystal system/ Space group
monoclinic (P2
15.009(3)
9.6377(18)
15.487(3)
90
1
/c)
a/Å
b/Å
c/Å
α/°
β/°
97.301(4)
90
γ/°
3
Volume/Å
2222.1(7)
12
Z
3
ρcalcg/cm
1.736
-
1
μ/mm
1.283
F(000)
1180.0
3
Crystal size/mm
0.3 × 0.2 × 0.1
4.99 to 52.092
2
Θ range for data collection/°
Index ranges
-18 ≤ h ≤ 18, -11 ≤ k ≤ 11, -19 ≤ l ≤ 19
Reflections collected
35868
Independent reflections
Data/restraints/parameters
4329 [Rint = 0.0358, Rsigma = 0.0201]
4329/0/312
1.044
2
Goodness-of-fit on F
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole / e Å
R
R
1
= 0.0841, wR
= 0.0948, wR
2
2
= 0.2207
= 0.2317
1
-
3
2.32/-1.64
5

Figure 3. Packing of the Cu-L5 complex showing the perchlorate ions enclosed between the
layers (diagram generated by mercury).
Table
Atom
2:
Selected
bond
lengths
Table
Atom Atom Atom
Angle/˚
88.5(2)
178.6(2)
90.6(2)
92.6(2)
88.5(2)
166.2(2)
110.6(5)
112.3(5)
Atom
N6
Length/Å
2.011(5)
1.997(5)
2.028(5)
1.979(5)
1.350(8)
1.364(7)
3:
N6
N1
N1
N3
N3
N3
N5
N2
Cu1
Cu1
Cu1
Cu1
Cu1
Cu1
C15
C6
N4
N6
N4
N6
N1
N4
C14
C5
Cu1
Cu1
Cu1
Cu1
N5
Selecte
d bond
angles
N1
N4
N3
C16
N6
N5
Solvent Extraction
6

From the literature[16-21] it was clear that there were numerous examples of pyrazolyl pyridine
ligands and imidazole pyridine ligands forming 1:1 complexes with Ni(II), Cu(II), Cd(II), Zn(II), etc.
or in some cases 2:1 complexes with these metal ions. Our own crystal structure produced a complex
of L5 with Cu(II) in the ratio of 2:1. Since all these ligands are bidentate ligands, it is not unusual to
expect 1:1, 2:1 or 3:1 ligand:metal ion coordination. When there is 1:1 or 2:1 ligand to metal ion
coordination, sometimes an anion would also be coordinated to the metal ion. It has also been shown
by du Preez et. al.[1, 22, 23] that imidazole extractants show high formation constants with later 3d-
transition metal ions, thereby producing large extraction percentages and thereby interacting with
these metal ions in acidic media.
We carried out each individual solvent-extraction experiment at least twice to ensure that experimental
error is minimised and that data is reliable and reproducible. Bearing this in mind, a ligand
concentration of 0.01 M would ensure that if we considered maximal 3:1 ligand:metal ion
coordination (although 2:1 and 1:1 ligand to metal ion coordination could also take place), the metal
ion concentrations would be in the region of 25 ppm – 66 ppm. This would lead to batch experiments
which we carried out where concentrations of metal ions are not too low for analysis on the ICP-OES
system. If concentrations are too low, this would resultantly incorporate large instrumental error.
We carried out a study to determine the best synergist concentration for use with these ligands under
the present experimental conditions and as previously reported [8]. We looked at synergist to ligand
ratios of 1:1; 5:1; 10:1; 15:1 and 20:1 and found that synergist:ligand ratio of 5:1 worked the best.This
ratio was then used throughout.
We searched the literature [24-27], finding that SDBS is well-known as a surfactant which has an
amphiphilic nature, i.e., it is both hydrophilic (due to its sulfonate head) and hydrophobic (due to its
long alkyl tail). This leads to the fact that SDBS can form micelles under a specific set of conditions,
one of which is the critical micelle concentration (CMC): the minimum concentration needed for the
1 2
formation of micelles to occur. CMC is 1.5 ± 0.05 mM and CMC (which is the critical micelle
concentration for the formation of cylindrical rods, vesicles or nanotubes) is 7.0 ± 0.05 mM [28].
These are values in aqueous media, however, we dissolved the ligand and synergist in the organic
phase chloroform and there is an interaction between the ligand donor atoms and synergist polar
region. However, the concentration of synergist (surfactant) which we used is beyond CMC
1
and
CMC , so it is likely that reverse micelles are formed in the organic phase which greatly assists with
2
the separation of Ni(II)[29,30]. The mechanism of action of extraction when ligand and synergist are
combined is currently being studied by several researchers [41-46].
It is also known that liquid-liquid (aqueous-organic) systems encourage the formation of reverse
micelles in the less polar organic phase, particularly when the synergist (surfactant) is dissolved in the
organic phase. This means that the aliphatic chains are pointed outward while the sulfonate heads are
pointing inward [29].
7

Osseo-Asare and Keeney [29] reported a dramatic increase in nickel(II) extraction with didodecyl
naphthalene sulfonic acid (DDNSA) at the critical micelle concentration and showed that the
percentage extraction only increases past this point, albeit very slight. We, on the other hand, observed
a sharp rise in extraction from a 1:1 to 1:5 ratio and then witnessed a steady decline in the extraction of
nickel(II) from a 1:10 to 1:20 ratio. As yet, we could not fully explain this interesting phenomenon,
although we theorise that the high concentration of sulfonate synergist may be thermodynamically
more stable in forming colloids/micelles and somehow linked to the ligand with improved
coordination to the nickel(II) ions. High sulfonate concentrations also make it difficult to distinguish
between the effects of micelles and microemulsions [29].
For the experimental procedure of the solvent extraction experiments, please see experimental section.
3
.3.1 Determining the better synergist concentration for nickel(II) extractions
To further prove that the better synergist concentration was 0.05 M, we performed experiments similar
to that in Pearce et. al. [8], only this time in the presence of a ligand as well. Two ligands were chosen
at random, one
from each of
Percentage extraction of nickel(II) using 2-(1-octyl-1H-imidazol-
2
-yl)pyridine (4) and varying concentrations of synergist
1
00
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
7
6
6
6
No Synergist
.01M Synergist
0.05M Synergist
5
8
0
53
42
0
0
0
.10M Synergist
.15M Synergist
.20M Synergist
the
main
1
5
ligand classes:
2
-(1-octyl-1H-
Percentage extraction of nickel(II) using 2-[(1H-pyrazol-1-
Increasing concentration of synergist (M)
imidazol-2-
yl)methyl]pyridine (5) and varying concentrations of synergist
yl)pyridine (4)
1
00
7
9
80
60
40
20
0
71
No synergist
5
9
5
7
0.01M Synergist
4
8
0
0
.05M Synergist
.10M Synergist
0.15M Synergist
.20M Synergist
5
0
Increasing concentration of synergist (M)
Figure
4:
8

Percentage extraction of nickel(II) using 2-(1-octyl-1H-imidazol-2-yl)pyridine (4) and varying
concentrations of synergist (SDBS). [L] = 0.01 M and pH = 5.03. (SDBS) and L are in chloroform
and Ni(II) is in aqueous solution.
Percentage extraction of nickel(II) using imidazolyl ligands 1–4
1
00
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
7
7
77
76
Ligand 1
Figure
5:
7
3
Ligand 1 + synergist
Ligand 2
Percentage
extraction of
nickel(II)
Ligand 2 + synergist
Ligand 3
Ligand 3 + synergist
Ligand 4
using 2-[(1H-
pyrazol-1-
1
5
7
6
8
Ligand 4 + synergist
yl)methyl]pyri
dine (5) and
Ligands (+ synergist)
varying concentrations of synergist (SDBS). [L] = 0.01 M and pH = 5.03. (SDBS) and L are in
chloroform and Ni(II) is in aqueous solution.
and 2-[(1H-pyrazol-1-yl)methyl]pyridine (5). These ligands (0.01 M) in conjunction
with SDBS (varying concentrations) were used in the extraction of nickel(II)
(
Figures 4 and 5). From Figures 4 and 5 we could see without doubt that the better
synergist concentration was in fact 0.05 M under the present experimental
conditions. The first sign of synergistic effects was observed from Figures 4 and 5.
Figure 6: A comparison of the percentage extraction of nickel(II) using 2-(1H-imidazol-2-yl)pyridine
(
1), 2-(1-methyl-1H-imidazol-2-yl)pyridine (2), 2-(1-butyl-1H-imidazol-2-yl)pyridine (3) and 2-(1-
2
+
octyl-1H-imidazol-2-yl)pyridine (4). [SDBS] = 0.05 M. [L] = 0.01 M. [Ni ] = 0.01 M in aqueous
solution and pH = 5.03. Ligand and synergist are dissolved in chloroform.
Without any synergist added, 2-(1-octyl-1H-imidazol-2-yl)pyridine (4) extracted 15% nickel(II) while
2
-[(1H-pyrazol-1-yl)methyl]pyridine (5) extracted 5%. When these extraction values along with the
optimum extraction value of the synergist (55 ± 0.9%, Figure S12 in SI) were subtracted from their
individual maximum extraction values (76 ± 0.1 and 79 ± 0.6%, Figures 4 and 5), we observed
synergistic gains of 6 (± 0.9) and 19 (± 0.9)% respectively.
9

Once again, in both cases, we observed a steep increase in the extraction of nickel(II) from no
synergist (ligand only) to a 1:5 (M:S) ratio and a steady decline from a 1:10 to 1:20 ratio. The
involvement of the synergist proved to be essential to this study, in view of the fact that a lack of
extraction was observed in its absence. This could possibly be rationalised on the basis that nitrate ions
do not readily phase-transfer cationic complexes from the aqueous to the organic phase due to high
hydration energies offered by nitrate ions [31].
3
.3.2 Solvent extraction of nickel(II)
The extraction of nickel(II) by means of imidazolyl pyridine ligands (1–4) yielded good results in
terms of synergistic extractions (Figure 6). All four ligands (1–4), in conjunction with SDBS (at
optimum concentration: 0.05 M), extracted nickel(II) in the low to mid-70% range and gained 10 (±
0.2%), 16 (± 0.8%), 14 (± 0.3%) and 6 (± 0.2)% respectively for their synergistic interactions with
SDBS. Tshentu et al. [6] reported extraction results for nickel(II) in the mid-90% range using 2-(1-
heptyl-1H-imidazol-2-yl)pyridine, 2-(1-octyl-1H-imidazol-2-yl)pyridine and 2-(1-decyl-1H-imidazol-
2
-yl)pyridine in conjunction with DNNSA. In light of their results it would appear as though our
extraction results don’t compare as well, but it has to be noted that Tshentu et al. [6] used an extractant
to metal ratio of 25:1 and a synergist to metal ratio of 15:1. This means that their extraction system has
far greater extractive power, hence their excellent results. We used a different synergist to Tshentu et.
al., hence it becomes more difficult to make a direct comparison.
From Figure 6, in the absence of synergist, we noted that ligand 4 extracted slightly more nickel(II)
than ligands 1–3 and can most likely be attributed to the presence of the octyl moiety, making it more
soluble in the organic phase and less soluble in the aqueous phase. Generally, it would appear as
though the following extraction trend holds true (no synergist): ligand 4 > ligand 3 > ligand 2 ≈ ligand
1
.
Nickel(II) extraction using methylpyrazolyl ligands (5–7/7') yielded similar results to the above
mentioned imidazolyl ligands (1–4) (Figure 7). Very poor extraction results were observed for ligands
5
, 6 and 7/7' without the addition of synergist. This was somewhat surprising, since we expected an
increase in chelate ring size (smaller bite-size) to effect an increase in their selectivity towards smaller
divalent metal ions, such as nickel(II) [32]. Upon the addition of SDBS, however, we observed a
dramatic increase in nickel(II) extraction with synergistic gains of 18 (± 0.6%), 10 (± 0.4%) and 16 (±
0.4)% for ligands 5, 6 and 7/7' respectively.
1
0

From the data (Figure 7) we observed the following synergistic extraction trend: ligand 5 > ligand
7/7’ > ligand
Percentage extraction of nickel(II) using methylpyrazolyl ligands
5
–7/7'
1
00
90
80
70
60
50
40
30
20
10
0
7
9
7
4
6
9
Ligand 5
Ligand 5 + synergist
Ligand 6
Ligand 6 + synergist
ligand 7/7'
Ligand 7/7' + synergist
5
4
3
Ligands (+ synergist)
6
.
This
possibly indicates that an increase in methyl groups on the pyrazole moiety leads to a
Figure 7: A comparison of the percentage extraction of nickel(II) using 2-[(1H-pyrazol-1-
yl)methyl]pyridine (5), 2-[(3,5-dimethyl-1H-pyrazol-1-yl)methyl]pyridine (6), 2-[(3-methyl-1H-
pyrazol-1-yl]methyl)pyridine / 2-[(5-methyl-1H-pyrazol-1-yl)methyl]pyridine (7/7’). [SDBS] = 0.05
2
+
M. [L] = 0.01 M. [Ni ] = 0.01 M in water and pH = 5.03. Ligand and synergist are dissolved in
chloroform.
slight decrease in nickel(II) extraction. The methyl group closest to the chelating nitrogen atom may
hinder access to that nitrogen, resulting in slightly poorer extraction results for ligands 6 and 7/7’. For
ligand 5, however, the absence of alkyl groups on the pyrazole moiety seems to play a significant role
in its superior nickel(II) extraction results.
Due to lack of studies on the extraction of nickel(II) using similar ligands, no meaningful comparison
to literature could be made.
3
.3.3 Time-dependent extraction study of nickel(II)
1
1

The extraction process is a dynamic one, meaning that it takes various extraction systems different
times to reach equilibrium at 25 (± 2) °C. It is essential that extraction processes are at equilibrium and
not in the process of reaching equilibrium when samples are analysed using ICP. When ICP analyses
are performed on samples that have not yet reached equilibrium, the results would be inaccurate and
misinterpreted.
Figure 8: Percentage extraction of nickel(II) over a 24-hour period using 2-(1-octyl-1H-imidazol-2-
yl)pyridine
(
4)
and
Percentage extraction of nickel(II) over a 24-hour period using
imidazolyl ligand 4 and SDBS
SDBS.
1
00
[SDBS]
=
90
80
70
60
50
40
30
20
10
0
0
0
.05 M. [L] =
.01 M.
2
+
[Ni ] = 0.01
M in aqueous
solution and
pH = 5.03.
Ligand and
synergist are
0
2
4
6
8
10
12
14
16
18
20
22
24
Time (h)
dissolved in chloroform.
1
2

The separate ligands, together with synergists were dissolved in the organic phase and Ni(II) was the
metal ion in the aqueous phase. Two ligands, one from each of the main classes of ligands, were
Percentage extraction of nickel(II) over a 24-hour period using
methylpyrazolyl ligand 5 and SDBS
1
00
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
chosen
to
determine
whether the
extraction of
0
2
4
6
8
10
12
14
16
18
20
22
24
Time (h)
nickel(II) reached equilibrium within the 24-hour time frame (Figures 8 and 9). These ligands were 2-
1-octyl-1H-imidazol-2-yl)pyridine (4) and 2-[(1H-pyrazol-1-yl)methyl]pyridine (5). From Figures 8
(
and 9 it was clear that equilibrium had been reached for both ligands well before the 24-hour mark. In
fact, ligand 4 reached equilibrium after about one hour while ligand 5 reached equilibrium after
approximately four hours. This agrees with observations made in the laboratory, in that upon the
2
+
addition of the pale blue aqueous phase (Ni -rich) to the pale yellow organic phase (ligand- and
synergist-rich) the organic phase turned bright blue-green almost instantly. This was the first
2
+
indication that the ligand/synergist combination had a high affinity for Ni ions and that equilibrium
would be reached. The percentage extraction of nickel(II) after 24 hours is in good accord with the
extraction results reported earlier. (Figures 6 and 7).
Figure 9: Percentage extraction of nickel(II) over a 24-hour period using 2-[(1H-pyrazol-1-
2
+
yl)methyl]pyridine (5) and SDBS. [SDBS] = 0.05 M. [L] = 0.01 M. [Ni ] = 0.01 M and pH = 5.03.
Similar systems from the literature [33,34] show equilibration times of 5 - 30 minutes, but these
systems do not incorporate a synergist. With the synergist and ligand present most likely requires more
time ( as in our case) for equilibrium to be achieved. We observed similar equilibration times with our
pyridine-pyrazole ligands [8].
1
3

Time-dependent studies are extremely valuable for industry, where time equals money. The faster an
extraction process can occur, the quicker the end-product can be obtained. This would not only save
money, time and resources, but limit waste production as well.
3
.3.4 Competitive extraction studies
2
+
2+
2+
2+
The competitive (or simultaneous) extraction of six different base metal ions (Cd , Co , Cu , Ni ,
2
+
2+
’
Pb and Zn , 1:1:1:1:1:1 ratio) using ligands 1–7 / 7 , were performed to determine the selectivity of
these ligands for any of the aforementioned metal ions. This is a slightly better representation of real-
world conditions and mimics what one would find in industry today, but on a much smaller scale.
These studies were performed both in the absence and presence of the synergist, SDBS, to determine
its impact on the extraction results. Firstly, we performed a competitive extraction experiment using
SDBS in the absence of ligands as in Pearce et. al.[8]. One glaringly obvious fact from Figure 10 [8]
was the
Competitive extraction using SDBS only
1
00
8
7
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
2
9
2
7
2
4
2
1
1
3
Cd
Co
Cu
Ni
Pb
Zn
Metal ion
Figure 10: Competitive extraction of base metal ions in the presence of the synergist, sodium
2
+
dodecylbenzenesulfonate (SDBS). [SDBS] = 0.05 M, [M ] = 0.01 M in aqueous solution and pH =
.07. Synergist is dissolved in chloroform. From Pearce et. al. [8].
5
exceptional selectivity of SDBS towards lead(II) ions. This was not surprising, since it is traditionally
known that oxygen extractants selectively extract lead(II) in high yields, especially sulfonic acids [35].
The competitive extraction results of lead(II) throughout this study was for all purposes solely
’
attributed to the synergist and not ligands 1–7 / 7 . Most other base metal ions were extracted in the
mid-20% range, with the exception of copper(II) extraction at 13 (± 0.8)%.
3.3.4.1 Competitive extraction using imidazolyl ligands
1
4

Photos were taken of the competitive extraction sample tubes for ligands 1–4 (Figure 11). From the
three sample tubes on the left-hand side of each mini photo (a–d), one can clearly see the dark blue
colours present in the aqueous phase (top layer), with the exception of photo d (ligand 4). These
sample tubes indicate the extraction of the base metal ion mix without the addition of SDBS. The
absence of colour from the organic phases are good visual indicators of poor metal ion extractions,
since we expected to see a brightly coloured organic phase upon metal ion coordination and extraction.
This fact was supported by Figures 12 – 15 where we performed competitive extraction studies both
in the presence and absence of SDBS. Figures 12 – 14 show poor extraction results for all metal ions
in the absence of SDBS, with ligand 4 being the exception to the rule. Ligand 4 clearly shows some
selectivity towards nickel(II), but more so towards copper(II) (Figure 15). This follows the trend
proposed by Irving and Williams [36] in 1953, that divalent transition metal ions form stable
complexes in the order Mn < Fe < Co < Ni < Cu > Zn. The presence of the octyl moiety in ligand 4
seems to have an important effect on extraction results. It is proposed that an increase in alkyl length
enhances the solubility of the complex in the organic phase, leading to an increase in extraction. This
is confirmed by Figures 12 – 15 (blue bars) as we observe the following extraction trend: ligand 1 <
ligand 2 < ligand 3 < ligand 4. This trend is not only applicable to copper(II) extraction, but to
nickel(II) extraction as well.
The use of SDBS in conjunction with ligands 1–4 resulted in a colour change of the organic phase
(three sample tubes on right-hand side of each mini photo, a–d). These colour conversions indicate
that metal ions have been transferred from the aqueous phase to the organic phase. As we expected, a
significant increase in metal extraction was observed upon the addition of SDBS (Figures 12 – 15, red
bars). The lead(II) extraction can solely be attributed to the extractive ability of SDBS, as the
extraction values are almost identical to the results displayed in Figure 10. The most significant
synergistic gains were observed for copper(II) extractions using ligands 1, 2 and 3. These yielded
mammoth gains of 32(± 1.0%), 35 (± 0.9%) and 54 (± 0.9)% respectively. No other noteworthy
synergistic gains were made.
a)
b)
d)
c)
1
5

Competitive extraction using ligand 1, both in the
presence and absence of SDBS
1
00
8
6
8
6
4
2
0
0
0
0
0
4
6
3
6
Without synergist
With synergist
2
4
1
7
18
6
7
4
5
3
Figure 11: Varying colours
Cd
Co
Cu
Ni
Pb
Zn
Metal ion
of the aqueous (top layers)
and organic phases (bottom layers) during competitive extraction studies using a) ligand 1, b) ligand
II
II
II
II
II
II
2
, c) ligand 3 and d) ligand 4. The top layer contains the metal ions Co , Ni , Cu , Zn ,Cd and Pb ,
whilst the bottom layer contains the ligand throughout. In each picture, the first three sample tubes
(
left) represent competitive extractions without the use of synergist, while the next three sample tubes
right) represent competitive extractions in conjunction with the synergist, SDBS. This macroscopic
(
picture corresponds to the analytical results in figures 12, 13, 14 and 15 for ligands 1-4 respectively.
From the above figure, it can be seen that ligand 4 on its own is the only one which extracts Cu(II) to
2
+
any extent. [SDBS] = 0.05 M. [L] = 0.01 M. [M ] = 0.01 M in the aqueous phase and pH = 5.07.
Ligand and synergist are dissolved in chloroform.
1
6

Figure 12: Competitive extraction of various base metal ions using 2-(1H-imidazol-2-yl)pyridine (1),
2
+
both in the presence and absence of SDBS. [SDBS] = 0.05 M, [M ] = 0.01 M. [L] = 0.01 M in
aqueous solution and pH = 5.07. Ligand and synergist are dissolved in chloroform.
Figure 13: Competitive extraction of various base metal ions using 2-(1-methyl-1H-imidazol-2-
2
+
yl)pyridine (2), both in the presence and absence of SDBS. [SDBS] = 0.05 M, [M ] = 0.01 M in
aqueous solution
and pH = 5.07. [L] =
.01 M. Ligand and
Competitive extraction using ligand 2, both in the presence
and absence of SDBS
0
1
00
8
6
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
synergist
are
in
dissolved
chloroform.
50
3
4
Without synergist
With synergist
2
4
1
8
18
6
7
6
4
4
2
Cd
Co
Cu
Ni
Pb
Zn
Metal ion
Competitive extraction using ligand 3, both in the presence
and absence of SDBS
1
00
8
6
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
72
3
7
Without synergist
With synergist
20
1
5
15
8
6
5
3
4
4
Cd
Co
Cu
Ni
Pb
Zn
Metal ion
1
7

Figure 14: Competitive extraction of various base metal ions using 2-(1-butyl-1H-imidazol-2-
2
+
yl)pyridine (3), both in the presence and absence of SDBS. [SDBS] = 0.05 M, [M ] = 0.01 M in
aqueous
solution and
Competitive extraction using ligand 4, both in the presence and
pH = 5.07.
absence of SDBS
[L] = 0.01 M.
1
00
8
6
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
Ligand and
synergist are
dissolved in
chloroform.
7
0
4
4
3
7
Without synergist
With synergist
22
1
8
1
6
16
5
4
4
2
Cd
Co
Cu
Ni
Pb
Zn
Metal ion
Figure 15: Competitive extraction of various base metal ions using 2-(1-octyl-1H-imidazol-2-
2
+
yl)pyridine (4), both in the presence and absence of SDBS. [SDBS] = 0.05 M, [M ] = 0.01 M in
aqueous solution and pH = 5.07. [L] = 0.01 M. Ligand and synergist are dissolved in chloroform.
3.3.4.2 Competitive extraction using methylpyrazolyl ligands
From Figure 16 we observed brightly coloured aqueous phases (top layers) that indicated a lack of
metal ion extraction in the absence of SDBS. Figures 16 a and b exhibited blue aqueous phases while
Figure 16 c exhibited green aqueous phases. Initially we thought that ligand 7/7’ had extracted a
particular metal ion in great excess, thereby resulting in a green coloured aqueous phase. This,
however, was not the case once we inspected Figure 19. There was no real evidence to suggest that
the aqueous phase colour was due to metal extraction, therefore we proposed that the colour was due
to the mixed ligand system (7/7’). We theorised that the combination of ligands 7 and 7’, each with its
own unique electronic properties, contributed toward the green colour we observed in Figure 16 c.
Once we inspected the sample tubes that contained SDBS (right-hand side of the mini photos) we once
again observed a colour conversion, signifying metal ion migration from the aqueous phase to the
organic phase. Ligand 5’s organic phase (bottom layer) was a light blue hue, while ligand 6 had a
slightly darker blue hue, indicating that ligand 6 possibly extracted copper(II) in slight excess
compared to ligand 5. This was confirmed upon inspection of Figures 17 and 18, which showed that
ligand 5 had extracted 48 (± 0.5)% copper(II) while ligand 6 extracted 60 (± 0.3)%.
1
8

Substantial synergistic gains were observed for copper(II) extractions. Ligands 5–7/7’ in conjunction
with SDBS exhibited gains of 34 (± 0.5%), 43 (± 0.3%) and 39 (± 0.8)% respectively. These results
can most likely be attributed to the fact that larger chelate ring sizes (in this case a six-membered
chelate ring) preferentially coordinate to smaller metal ions, such as copper(II) [30]. The presence of
methyl moieties on the pyrazole rings (ligands 6 and 7/7’) assist in the extraction of copper(II) and
consequently decreases the percentage extraction of nickel(II). This could be purely coincidental that
an increase in the number of methyl substituents on the pyrazole ring would lead to an increase in
copper(II) extraction due to increased solubility of the copper(II) complex in the organic phase. This
was substantiated after the following copper(II) extraction trend was observed from Figures 17 – 19:
ligand 5 (no methyl substituents) < ligand 7/7’ (one methyl substituent) < ligand 6 (two methyl
substituents). This phenomenon should be further exploited by computational or other means.
a)
b)
c)
Figure 16: Varying colours of the aqueous (top layers) and organic phases (bottom layers) during
competitive extraction studies using a) ligand 5, b) ligand 6 and c) ligand 7/7’. The top layer contains
II
II
II
II
II
II
the metal ions Co , Ni , Cu , Zn ,Cd and Pb , whilst the bottom layer contains the ligand throughout.
In each picture, the first three sample tubes (left) represent competitive extractions without the use of
synergist, while the following three sample tubes (right) represent competitive extractions in
conjunction with the synergist, SDBS. This macroscopic picture corresponds to the analytical results
in figures 17, 18 and 19 for ligands 5-7 respectively. From the above figure, it can be seen that no
2
+
ligand on its own extracts Cu(II) to any extent. [SDBS] = 0.05 M. [L] = 0.01 M. [M ] = 0.01 M in the
aqueous phase and pH = 5.07. Ligand and synergist are dissolved in chloroform.
1
9

Competitive extraction using ligand 5, both in the presence and
absence of SDBS
1
00
8
7
90
80
70
60
50
40
30
20
10
0
4
8
4
0
Without synergist
With synergist
2
4
1
7
17
8
7
5
6
4
Cd
Co
Cu
Ni
Pb
Zn
Metal ion
Figure 17: Competitive extraction of various base metal ions using 2-[(1H-pyrazol-1-
yl)methyl]pyridine (5), both in the presence and absence of SDBS. [SDBS] = 0.05 M, [L] = 0.01 M,
2
+
[
M ] = 0.01
Competitive extraction using ligand 6, both in the presence and
M
in the
absence of SDBS
aqueous phase
1
00
8
7
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
6
0
N
Without synergist
With synergist
2
7
N
2
3
R2
1
9
18
N
6
8
5
5
3
4
R1
Cd
Co
Cu
Ni
Pb
Zn
Metal ion
6
: R = R = methyl
1
2
and pH = 5.07.
Ligand and synergist are dissolved in chloroform.
2
0

Figure
18:
Competitive extraction using ligand 7/7', both in the presence and
Competitive
absence of SDBS
extraction of
1
00
8
7
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
53
Without synergist
With synergist
31
2
5
1
8
19
6
7
5
5
3
1
Cd
Co
Cu
Ni
Pb
Zn
Metal ion
various
base
metal
ions using
2-[(3,5-dimethyl-1H-pyrazol-1-
yl)methyl]pyridine (6), both in the presence and absence of SDBS. [SDBS] = 0.05 M, [L] = 0.01 M,
2
+
[
M ] = 0.01 M in the aqueous phase and pH = 5.07. Ligand and synergist are dissolved in chloroform.
Figure 19: Competitive extraction of various base metal ions using 2-[(3-methyl-1H-pyrazol-1-
yl]methyl)pyridine / 2-[(5-methyl-1H-pyrazol-1-yl)methyl]pyridine (7/7’), both in the presence and
2
+
absence of SDBS. [SDBS] = 0.05 M, [L] = 0.01 M, [M ] = 0.01 M in the aqueous phase and pH =
.07. Ligand and synergist are dissolved in chloroform.
5
Possible mechanistic role of the synergist
There have been several reports recently on the mechanism of action of synergistic solvent extraction
[7, 41-46]. All of these have given more insight into the mechanism and all use model compounds in
crystal structures since it is difficult to get solid state structures of compounds with long alkyl chains.
The type of extractant varies in these cases – it could be a sulfonic acid, a carboxylic acid or a
phosphinic acid. The synergist is also varied in all cases.
The role of the synergist, SDBS, in nickel(II) extraction systems is somewhat unclear and difficult to
unravel. In 1980, Osseo-Asare and Keeney [29] reported the use of didodecyl naphthalene sulfonic
acid (DDNSA) as a synergist in nickel(II) extractions. They claimed that DDNSA formed micelles and
contained large aqueous cores with solubilised nickel(II) [29]. In view of the low aqueous solubility of
all our synthesised ligands (1–7), it is unlikely that ligands 1–7 would be solubilised in the aqueous
core. It is proposed that the ligands are concentrated on the hydrophobic periphery of the sulfonate
micelles, which means that the sulfonate micelles appears to provide a concentrative effect. In other
2
1

words, concentrations of both the ligands and aqueous (solubilised) nickel(II) ions are high in the
micellar region, therefore, reaction rates are also expected to be high [29].
Materials and methods
Chemicals and reagents
All chemicals were bought from Sigma-Aldrich, Merck or Alfa Aesar and used without further
purification.
Nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance (NMR) spectra were recorded at room temperature by using various
Unity
NMR spectrometers. These would include the Varian
Inova 600 Liquid State NMR Spectrometer,
Unity
Varian Inova 400 Liquid State NMR Spectrometer and the Varian VNMRS 300 Liquid State NMR
Spectrometer. All fid/propcar files were processed using MestReNova, Version 6.0.2-5475. All
3
chemical shifts (δ) are given in ppm and are relative to chloroform (CDCl , δ 7.26).
Infrared spectroscopy
All ligands were partially characterised by infrared (IR) spectroscopy. This was done using a Nicolet
−
1
Avatar 330 FT-IR Spectrometer in the mid-IR range (4000–500 cm ).
−
Mass spectrometry
Time of flight (TOF) positive electrospray ionisation (ESI+) mass spectrometry (MS) was carried out
on a Waters Synapt G2 High Resolution Mass Spectrometer, with a cone voltage of 15 V. The ESI
probe was injected into a stream of acetonitrile. All signals were recorded as a mass over charge (m/z)
+
ratio, with the molecular ion typically observed as a (M+H) ion.
Elemental analysis
All elemental analyses were done using a PerkinElmer 2400 Series II CHNS/O Elemental Analyzer
pH determinations
The pH measurements were performed on a Thermo Orion 420A+ pH meter, using standards (pH 4, 7
and 10) to calibrate the system before every set of measurements.
Lab shaker
The organic and aqueous phases were thoroughly mixed during extraction studies using a SPL-MP15
Labcon linear platform shaker.
Inductively coupled plasma
Metal concentrations in the ppm to mid-ppb range were measured on a Thermo ICap 6300 ICP-AES,
while ultra-trace analyses were performed on an Agilent 7900 ICP-MS or Agilent 8800 QQQ ICP-MS.
Solvent extraction procedure and conditions
Solvent extraction method, procedure and conditions are the same as those found in Pearce et. al. [8].
Instrumentation and determination of crystal structure
2
2

A suitable crystal was mounted on a thin glass fiber, while data was collected using a Bruker APEX-II
CCD diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) [37]. Data
reduction was performed using SAINT [37]. A cryostat: Oxford Cryogenics (700 Series Cryostream
Plus) was used to cool the crystal to 100 K. Empirical corrections were performed with SADABS [38].
The structure was solved by direct methods, while the remainder of the atomic positions were found
using difference Fourier methods. All non-hydrogen atoms were refined anisotropically (with
2
appropriate restraints using SIMU and ISOR) by blocked full-matrix least squares calculations on F
using SHELXL-97 [39] within the X-SEED environment [40]. Hydrogen atoms were added to the
structure model on calculated positions and were refined as rigid atoms. Mercury V.3.8 for Windows
was used to generate figures.
9 9 3 2 4 2
Preparation of the crystalline [Cu(C H N ) ](ClO ) complex
To a 100 ml round bottom flask was added 0.0079 g ligand (0.050 mmol) and 0.0092g Cu(ClO
4 2
)
(
0.025 mmol). To this was added 30 ml of methanol and the solution was allowed to stir under reflux
for 3 h. Thereafter the solvent was removed by using the rotary evaporator. The powder was
redissolved in dichloromethane and diethyl ether was allowed to diffuse into the solution. Crystals
were obtained using this vapour diffusion method after 6 days. Elemental microanalysis calculated
C(25.93%); H(2.90%) N(10.08%). Found = C(26.29%) H(3.53%) N(10.13%).
CCDC 1865594 contains the supplementary crystallographic data for this paper. This data can
be obtained free of charge from The Cambridge Crystallographic Data Centre.
Conclusions
This study shows that imidazole and methyl-pyrazolyl pyridine ligands can be synthesised in good
1
13
yields. All the ligands were successfully characterised by means of H and C NMR, IR, MS, EA and
melting point where appropriate.
The role of the synergist, sodium dodecylbenzenesulfonate (SDBS), in this case played an effective
role, and was still higher than the extraction without a synergist. The molecular mechanism by which
the synergist together with the ligand interacts with the metal ion warrants further investigation.
Alkyl chain length played a significant role in ensuring that complexes stayed solubilised in the
organic phase.
The crystal structure showed coordination to copper(II) in a pseudo square planar 4-coordinate
fashion, with distorted square-base bond angles that can be attributed to the steric bulky groups trying
to be orthogonal to each other with maximum separation. The crystal structure shows two ligand
molecules coordinating to one copper(II) ion, denoting a ligand to metal ratio of 2:1, indicating why
extraction results using a 1:1; ratio did not exceed 50% with this ligand.
2
3

We are currently using a shorter chain synergist mimic so as to try and establish the molecular
mechanism by which these metal ions are extracted.
Acknowledgements
We wish to thank the National Research Foundation (NRF) of South Africa, grant number 92777 and
Stellenbosch University for financial support of this work.
References
[1] J. G. H. Du Preez, Sol. Extr. Ion Exch. (2000), 18, 679-701.
[2] A. P. Gray, T. B. O'Dell, Nature, (1958), 181, 634-635.
[3] F. Vidal, J. Org. Chem. (1959), 24, 680-683.
[4] Y. Marcus, A. S. Kertes, Ion Exchange and Solvent Extraction of Metal Complexes; Wiley-
Interscience: London; New York, (1969), p 1037.
[
[
[
5] J. G. H. Du Preez, S. B. Schanknecht, D. P. Shillington, Sol. Extr. Ion Exch. (1987), 5, 789-809.
6] A. I. Okewole, N. P. Magwa, Z. R. Tshentu, Hydrometallurgy, (2012), 121-124, 81-89.
7] J. W. Roebuck, P. J. Bailey, E. D. Doidge, A. J. Fischmann, M. R. Healy, G. S. Nichol, N.
O’Toole, M. Pelser, T. Sassi, K. C. Sole and P. A. Tasker, Sol. Extr. Ion Exch., (2018),
https://doi.org/10.1080/07366299.2018.1506558.
[8] B. H. Pearce, H. F. Ogutu and R. C. Luckay, Eur. J. Inorg. Chem., (2017), 1189-1201.
[9] C. Blake, C. Baes, K. Brown, Ind. Eng. Chem. (1958), 50(12), 1763-1767.
[10] A. E. Sunier, J. Chem. Edu.,. (1972), 49(12), 805-807.
[11] N.J. Parmar, H.A. Barad, B.R. Pansuriya, R.A. Patel, J. Coord. Chem., (2011), 64, 688–698.
[12] I. Van de Voorde, L. Pinoy, E. Courtijn, F. Verpoort, Sol. Extr. Ion Exch. 24 (2006) 893–914.
[13] I. Gonul, M. Kose, G. Ceyhan, S. Serin, Inorganica Chim. Acta. 453 (2016) 522–530.
[14] D. Cremer, J.A. Pople, J. Am. Chem. Soc. 97 (1975) 1354–1358. doi:10.1021/ja00839a011.
[15] D. Cremer, Acta Crystallogr. B 40 (1984) 498–500. doi:10.1107/S0108768184002548.
[16] G. Ndayambaje, K. Laatikainen, M. Laatikainen, E. Beukes, O. Fatoba, N. van der Walt, L.
Petrik, T. Sainio, Chem. Eng. J. (2016), 284, 1106-1116.
[
17] M. N. Magubane, G. S. Nyamato, S. O. Ojwach, O. Q. Munro, RSC Adv. (2016), 6, 65205-65221.
18] G. S. Nyamato, M. G. Alam, S. O. Ojwach, M. P. Akerman, Appl. Organomet. Chem. (2016), 30,
[
8
9-94.
2
4

[19] S. O. Ojwach, I. A. Guzei, L. L. Benade, S. F. Mapolie, J. Darkwa, Organometallics, (2009), 28,
2
127-2133.
[
20] S. O. Ojwach, G. S. Nyamato, B. Omondi, J. Darkwa, Inorg. Chim. Acta, (2012), 392, 141-147.
21] S. O. Ojwach, T. T. Okemwa, N. W. Attandoh, B. Omondi, Dalton Trans. (2013), 42, 10735-
[
10745.
[
[
[
22] J. G. H. Du Preez, N. Sumter, C. Mattheüs, S. Ravindran, B. J. Van Brecht, Solvent Extr. Ion
Exch. (1997), 15, 1007-1021.
23] J. G. H. Du Preez, J. Postma, S. Ravindran, B. J. Van Brecht, Sol. Extr. Ion Exch. (1997), 15, 79-
9
6.
24] X. L. Zhang, D. J. F. Taylor, R. K. Thomas, J. Penfold, J., J. Colloid Interface Sci. (2011), 356,
56-664.
6
[25] A. Mohsenpour, H. Eisazadeh, Polym. -Plast. Technol. Eng. (2015), 54, 57-60.
[26] S. Chabba, S. Kumar, V. K. Aswal, T. S. Kang, R. K. Mahajan, Colloids Surf. A Physicochem.
Eng. Asp. (2015), 472, 9-20.
[
[
[
[
[
[
27] S. K. Hait, P. R. Majhi , A. Blume and S. P. Moulik, J. Phys. Chem. B, (2003), 107, 3650 - 3658.
28] D. Yu, F. Huang and H. Xu, Analytical Methods, (2012), 4, 47 - 49.
29] K. Osseo-Asare, M. E. Keeney, Sep. Sci. Technol. (1980), 15, 999-1011.
30] K. Osseo-Asare, M. E. Keeney, Metall. Trans. B., (1980), 11B, 63-67.
31] K. A. Allen, J. Phys. Chem. (1956), 60, 943-946.
32] A. E. Martell, R. D. Hancock, Metal Complexes in Aqueous Solutions; Modern Inorganic
Chemistry; Plenum Press: New York, (1996), p 253.
[
[
[
[
33] H. Watarai and K. Satoh, Langmuir, (1994), 10,3913 – 3915.
34] H. Watarai, L. Cunningham and H. Freiser, Anal. Chem., (1982), 54, 2390 – 2392.
35] S. Fassbender, D. Dreisinger, Z. Wu, US Patent 20140131220 A1, (2014).
36] H. Irving, R. J. P. Williams, J. Chem. Soc. Dalton, (1953), 3192-3210.
[
[
37] R. H. Blessing, Acta Crystallogr., (1995), A51, 33.
38] SADABS, Version 2.05; Bruker AXS Inc., Madison, WI, (2002)
.
[39] G. M. Sheldrick, Acta Crystall. A-Crys (2008), 64, 112-122.
[
40] L. J. Barbour, J. Supramol. Chem. 2001), , 189-191.
(
1
[41] L. H. Delmau, T. J. Lefranc, P. V. Bonnesen, J. C. Bryan, D. J. Presley, and B. A. Moyer, Sol.
Extr. Ion Exch., (2005), 23, 23 – 57.
[42] F. Hu, H. Hu, Y. Luo, Y. Wang, J. Yang, and J. Hu, Hydrometallurgy, (2018), 181, 240 – 247.
2
5

[43] S. Zhu, H. Hu, J. Hu, J. Li, F. Hu, and Y. Wang, Y. J. Mol. Struc., (2017), 1144, 191 – 198.
[44] F. Hu, H. Hu, J. Hu, S. Zhu, J. Yang, and Y. Wang, J. Mol. Liquids, (2017), 248, 1050 – 1058.
[45] F. Hu, H. Hu, J. Yang, Y. Luo, Y. Wang, and J. Hu, Sep. Pur. Tech., (2018), 204, 141 – 148.
[46] K. R. Barnard, G. L. Nealon, M. I. Ogden, and B. W. Skelton, Sol. Extr. Ion Exch., (2010), 28,
7
78 – 792.
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6

Synergistic, normal and antagonistic effect
Synergism
Antagonism
Normal
100
90
80
70
60
50
40
30
20
10
0
70
50
40
30
30
2
0
20
Zn
1
0
Cu
Pb
Metal ions
2
7

Highlights
This publication displays the following highlights:
(
(
i)
High Nickel(II) extraction was obtained with these ligands in combination with a
synergist, dodecyl benzene sulfonate
When doing competitive extraction using the metal ions Co(II), Ni(II), Cu(II),
Zn(II), Cd(II) and Pb(II), Cu(II) has the highest extraction followed by Ni(II) in
combination with the same synergist. One ligand on its own shows high extraction
of Cu(II)
ii)
(iii) Kinetic studies shows that after at most 4 hours, extraction equilibrium is
established
2
8