Liquid-liquid extraction of metal ions, DFT and TD-DFT analysis of some 1,2,4-triazole Schiff Bases with high selectivity for Pb(II) and Fe(II)

Article abstract of DOI:10.1016/j.molstruc.2016.02.046

Liquid-liquid extraction of metal ions using some 1,2,4-triazole Schiff base derivatives as new extractants was studied. Fe²⁺, Zn²⁺, Cu²⁺, Co²⁺, Cd²⁺ and Pb²⁺ were extracted from the aqueou

Full text of DOI:10.1016/j.molstruc.2016.02.046

Accepted Manuscript
Liquid-liquid extraction of metal ions, DFT and TD-DFT analysis of some 1,2,4-triazole
Schiff Bases with high selectivity for Pb(II) and Fe(II)
Mohamed Khoutoul, Morad Lamsayah, Fawzia F. Al-blewi, Nadjet Rezki, Mohamed
Reda Aouad, Messali Mouslim, Rachid Touzani
PII:
S0022-2860(16)30139-9
10.1016/j.molstruc.2016.02.046
MOLSTR 22255
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 10 November 2015
Revised Date: 11 February 2016
Accepted Date: 11 February 2016
Please cite this article as: M. Khoutoul, M. Lamsayah, F.F. Al-blewi, N. Rezki, M.R. Aouad, M. Mouslim,
R. Touzani, Liquid-liquid extraction of metal ions, DFT and TD-DFT analysis of some 1,2,4-triazole Schiff
Bases with high selectivity for Pb(II) and Fe(II), Journal of Molecular Structure (2016), doi: 10.1016/
j.molstruc.2016.02.046.
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ACCEPTED MANUSCRIPT
100
80
60
40
20
0
y=1.10078 x + 5.90273
R2=0.99089
Fe2+
Cu2+
Pb2+
Cd2+
1.6
1.5
1.4
L1
L2
L3
L4
L5
L6
L7
L8
L9
L10
L11
-4.1
-4.0
-3.9
Log L
Ligands
100
80
60
40
20
0
y=0.96257x + 5.5503
R2=0.99712
1.8
1.7
1.6
Fe(II)/L11
Pb(II)/L1
0
1
2
3
4
5
6
7
8
9
-4.1
-4.0
-3.9
Log L
pH

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Liquid-liquid extraction of metal ions, DFT and TD-DFT analysis of some
1,2,4-triazole Schiff Bases with high selectivity for Pb(II) and Fe(II)
Mohamed Khoutoul1, Morad Lamsayah1, Fawzia F. Al-blewi2, Nadjet Rezki2,3, Mohamed Reda
Aouad2,3, Messali Mouslim2 and Rachid Touzani1,4*
1LCAE-URAC18, COSTE, Faculté des Sciences, Université Mohammed Premier, BP524, Oujda 60000,
Morocco.
2Department of Chemistry, Faculty of Sciences, Taibah University, Al-Madinah Al-Munawarah 30002, Saudi
Arabia.
3Laboratoire de Chimie&Electrochimie des Complexes Métalliques (LCECM) USTO-MB, Department of
Chemistry, Faculty of Sciences, University of Sciences and Technology Mohamed Boudiaf, B.p. 1505 El
M`nouar, Oran 31000, Algeria.
4Faculté Pluridisciplinaire de Nador, Université Mohammed Premier, BP300, Selouane, Nador62700,
Morocco.
Abstract
Liquid-liquid extraction of metal ions using some 1,2,4-triazole Schiff base derivatives as
new extractants was studied. Fe2+, Zn2+, Cu2+, Co2+, Cd2+ and Pb2+ were extracted from the
aqueous phase into the organic phase and the extractability for each metal ion was
determined by atomic absorption. Interestingly, a competitive extraction was also
investigated and then examined at different pH in order to explore the effect of the different
substituent groups on metal extraction. Accordingly, high selectivity towards Fe2+ (90.1%)
and Pb2+ (94.3%) provided respectively by the presence of electron withdrawing group and
electron donor group was attained. In addition, geometry optimizations of the ground and
excited-states of the ligands in order to get better insight into the geometry and the electronic
structure were carried out by means of DFT and TD-DFT calculations.
Keywords: 1,2,4-triazoles; Schiff base; DFT; Liquid-Liquid extraction; metal cations.
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1. Introduction
1,2,4-Triazoles are class of nitrogen heterocyclic compounds which attract considerable
attention in organic chemistry owing to their pharmacological and medicinal properties such
as anticonvulsant, [1] antimicrobial, [2] antihypertensive, [3] analgesic, [4] antiviral, [5]
antioxidant, [6] anti-inflammatory, [7] and anticancer [8,9] properties. Moreover, Schiff
bases incorporating 1,2,4-triazole scaffold are well established in the literature as important
active compounds in medicinal [10] and coordination chemistry [11]. Liquid–liquid
extraction which is one of the most important and promising techniques for metals separation
has recently attracted a great deal of attention for selective recovery of metal ions from
industrial wastes [12-14]. However, the success of this recovery process is still limited by the
extraction selectivity for the targeted metal using a specific extractant. Actually, despite the
large number of extractants that have been investigated and developed to efficiently separate
metal ions, such as phosphoric acid, α-hydroxy oximes, β-diketones, amines and substituted
pyridines [15-21], the design of selective extractants for the separation of heavy metals
particularly bivalent metal cations has remained a great challenge in wastewater treatment.
Recently, researches have been directed towards Schiff-base derivatives as the main
extractant candidates. In this context, we have previously investigated the liquid-liquid
extraction of metals using nitrogen-donor pyrazole ligands and studied their ability to extract
metals from aqueous solutions [22-26].
In view of these above, we report herein the synthesis of some Schiff base derived from 4-
amino-5-methyl-1,2,4-triazol-3-thione, the study of theirliquid-liquid extraction of metals
ions and experimental (UV-Vis) and theoretical calculations DFT and TD-DFT at B3LYP
level [27] have been carried out aiming to investigate the structure geometries and to
determine the nature of electronic transitions and their dependences.
2. Experimental section
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2.1. Synthesis of 4-amino-5-methyl-2,4-dihydro-1,2,4-triazole-3-thione (1)
The starting material 4-amino-5-methyl-2,4-dihydro-1,2,4-triazole-3-thione (1) was prepared
according to the reported literature [28]. A mixture of thiocarbohydrazide (5 g) in glacial
acetic acid (15 mL) was refluxed in ethanol for 4 hr. The solution was cooled and a white
solid appeared. The obtained precipitate was filtered and recrystallized from water to afford
the desired product. Yield 85%, m.p. 153 °C, IR (KBr, ν, cm-1): 3213-3367 (NH, NH2), 1687
(C=N), 1299 (C=S); 1H NMR (DMSO-d6, δ ppm):2.36 (3H, s, CH3), 5.65 (2H, s, NH2),14.34
(1H, s, SH triazole); 13C NMR (DMSO-d6, δ ppm): 10.56 (CH3), 161.83, 167.06 (C=N).
2.2. General procedure for the synthesis of ligandsL1-L11
A mixture of 4-amino-5-methyl-2,4-dihydro-1,2,4-triazole-3-thione (1) (10 mmol) and the
appropriate benzaldehyde derivatives 2-12 (10 mmol) was refluxed in ethanol for 6 hours
with the presence of catalytic amount of hydrochloric acid (conc. HCl). After cooling, the
resulting precipitate was filtered, and recrystallized from ethanol to obtain the desired Schiff
Bases (ligands) L1-L11 in good yield (80-90%) (Figure1).
Fig. 1. Chemical structures of studied ligands L1-L11
2.3. Characterization of ligandsL1-L11
All synthesized ligands have been characterized using IR, 1H NMR techniques (Table. 1).
Table. 1. Spectroscopic data of ligandsL1-L11
2.4. Liquid-liquid extraction tests
A solution of 7.10-5M of ligand in CH2Cl2 (20 mL) was stirred for 2h with 20 mL of an
aqueous solution of metal salt (7.10-5M). Liquid-liquid extraction experiments were carried
out at room temperature and pH neutral [26]. All experiments were carried out in an
atmosphere of dry argon using schlenck techniques. Solvents were distilled and degassed.
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The aqueous phase was separated and analysed by atomic absorption spectrometry with an
air-acetylene flame. Single element standards were prepared for these metals from Fluka
1000 mg/L stock solutions. The measurements are carried out using standard conditions for
calibration. All standards were made acidic to avoid metal hydrolysis and to match content in
the sample using nitric acid 2%.
The organic phase was back-extracted with HCl, H2SO4 and HNO3 solution (1M).
The metal ion concentrations in the organic phases were calculated from the mass balance
between the aqueous and organic phases.
The extractability or the extraction efficiency (E%) and distribution ratio (D) of the metal
were calculated from Equation (1) and (2), respectively.
(1)
(2)
Where [M]aq,init represents the initial concentration of metal ion in the aqueous phase. [M]aq,eq
and [M]org,eq are the total concentrations of metal ion in the aqueous and organic phases,
respectively, at equilibrium.
Moreover, the complexation mechanism between metals and ligands was investigated by
slope analysis.
2.5. Theoretical calculations
All calculations were performed using the GAUSSIAN 09 program package [29] with the aid
of the GaussView visualization program [30]. The ground state geometries of L1 and L11were
fully optimized using the hybrid B3LYP functional methods [27] in combination with the 6-
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31G (d,p) basis set. The excited state properties of the optimized ground state geometries
were calculated in both the gas phase and with dichloromethane solvent by using the time-
dependent density functional formalism (TD-DFT) and the pople 6-31G+(d,p) basis set.
Solvent effects were included by employing the Polarizable Continuum Model (PCM)[31], as
implemented in Gaussian09. The fractional contributions of various groups to each molecular
orbital were calculated by using Gauss Sum [32] and Chemissian was used for spectrum
analysis [33].
3. Results and discussions
The resulting spectroscopic data were in agreement with that previously reported in the
literature [34-39] (see Table. 1). Thus, the IR spectra of all Schiff BasesL1-L11showed
characteristic absorption bands at 1630-1660 and 3200-3356 cm−1 assigned for the C=N and
N–H groups, respectively. Moreover, the1HNMR spectral results, obtained for all Schiff
Bases in DMSO-d6, together with the hydrogen assignments are presented in the Table. 1.
All ligands showedtwo characteristic singlets at δH 14.40–14.60 and 9.60-10.20 ppm
assigned to the NH and CH=N imine protons, respectively.
3.1. Individual extraction of metal ions with the ligands L1-11
To examine the coordination capacity of these ligands against each metal, we proceeded to
extract the metals individually from aqueous media. Obtained extractabilities are presented in
Table. 2.
Table. 2. Extraction efficiencies of metal cations using L1-L11.
The Table.2 display a remarkable and high extraction efficiency for both Fe2+ and Pb2+,
while this ligands L1-L11 shown a medium and low efficiencies with Cu2+, Cd2+, Zn2+ and
Co2+.
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The extractabilities of Fe2+ by these ligands vary between 7.5% (L7) and 96.3% (L4). In fact,
the presence of a methoxyormethyl group in para position of the phenyl gives a high value of
extraction efficiency; 92.6% (L1) and 91.7% (L2) respectively.Interestingly, the displacement
of the chloro group from the ortho position (L3)to the meta position (L4) has allowed higher
extractability to be reached; 96.3% instead of 42.1%, whilst 85.3% can be attained when the
substitution in the para position (L5).Ligands bearing a bromo group display a low
extractability in the case of meta position (7.5% for L7), a medium efficiency in the case of
ortho position (53.7% for L6). The extraction efficiency reach 92.9%when the presence of
fluoro group in para position (L8). Interestingly, the existence of nitro group has significantly
improvedthe extraction of Fe2+. Actually, the extraction efficiencyhas not ceased increasing
by passing from the position para (91.6%) for L10to the meta position (93.6%) for L9to
finally reachits maximum value of 95.7%with the ligand L11bi-substituted in ortho position
by a nitro group and a chloro group in meta position.
In general, the extractability of Pb2+by these ligands is good and varies from 37.9% (L9) to
98.9% (L4),the substitution of a methyl group by a methoxy group in para position on the
phenyl ring has improved the extraction efficiency, from 93.6% for L2 to 97.3% for
L1respectively.The existence of a chloro group in meta (L4; 98.9%) or in para (L5; 98.1%)
position has perfectly improved the efficiency when comparing with ortho (L3; 90.1%)
position. With the bromo substituent groups, a notable amelioration in extraction efficiency
was observed when passing from ortho(65.7% for L6) to meta(79.3% for L7) position.
Ligand L8 which has bearing a fluoro group in para position show a very good extractability
against Pb2+ (96.7%). In the case of nitro substituent group, the extraction was improved
while changing position on the phenyl ring from meta (37.9% for L9) to para (72.4% for L10)
and reached 87.2% (L11) with ortho position in presence of chloro group in meta position.
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For the extraction of Cu2+, the tested ligands do not show a good efficiency and the values
remain between 25% and 63.1%(L9).
In general, the liquid-liquid extraction of Zn2+, Co2+ and Cd2+ remains low. For Zn2+ and
Co2+these ligands show an identical behavior but the extraction efficiencies remain very low
and do not exceed 7.2% in the best case. The ligands do not show an important
efficiencytoward Cd2+ and the maximum value is 49.8% for L10.
According to the results obtained by these ligands, it seems that some molecules have trend
to coordinate with Fe2+, other with Pb2+ and most of cases with both of metals. This can be
attributed to the different nature of each ligand as well asits capacity to complex the various
metals, the nature of metal may also be animportant factor.The phenyl ring which is directly
attached to an imine function which involvea sp2 nitrogen and which forms with the sulphur
a coordination site to receive metals is strongly influenced by the nature of the substituent
group and its position. For these ligands, there are various substituents into different
positions. Therefore, the extraction efficiencyisaffected by electronic effects of substituents
at ortho, meta and parapositions on the phenyl ring, each substituent group has a
specificeffect on the coordination site in way that may increase or decrease the coordination
capacity of the ligands.
3.2. Selective extraction of divalent metal cations
To understand the behavior of these ligands toward metal cations, we proceeded to
investigate the selectivity of these ligands through a competitive extraction. For this purpose,
we have extracted metal cations from a solution containing the four metals that have
previously shown certain affinity vis-a-vis these ligands (Fe2+, Pb2+, Cu2+ and Cd2+). The
obtained results of this study are displayed in Fig. 2.
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Fig. 2.Competitive extraction E(%) of metal ions using L1-L11
From the obtained results of the competitive extraction, the ligands unveil good efficiencies
to extract selectively Fe2+ and Pb2+. The ligands L1 and L2 show an excellent selectivity
towards Pb2+ while L3 and L7 display a remarkable efficiency but some interference was
shown with Fe2+ and Cu2+. So at equilibrium these ligands complex the lead (II) after a
competitive liquid-liquid extraction. The most efficient ligands that extract selectively Pb2+
with efficiencies higher than 90% are L1 (94.3%) and L2 (91.2%). Iron (II) is selectively
extracted by L9, L10, and L11, the best one is L11 with reported efficiency of 90.1%.
When extracting from a mixture of metals L4, L5, L6, and L8 do not show any trend to
complex selectively one cation, so a competition between Fe2+ and Pb2+ has displayed.
Certain ligands present an important affinity towards Pb2+, especially with the molecules
bearing an electron-donor substituent, others with the electron-withdrawing group show a
good affinity towards Fe2+. Therefore, the electronic effect of substituent does not only affect
the extraction efficiency of the ligand, but also its selectivity by the fact that molecules with
electron-donor group extract Pb2+ selectively and molecules with electron-withdrawing group
extract Fe2+ selectively. The substitution on the phenyl by a halogen group (Br, Cl and F) and
the changing of the position do not improve sufficiently the selectivity
3.3. Determination of complexation mechanism
To characterize the extraction ability, the dependence of the distribution ratioDof thecation
between the two phases on the ligand concentrationwas examined.
If the general extraction equilibrium is given by Eq.(3)
M2+
+ x[L](org)↔[MLx](org)(3)
(aq)
The extraction equilibrium constant Kex is given by Eq. (4)
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(4)
Moreover, the distribution ratio of the metal cation between the organic and aqueousphases is
defined as
(2)
By introducing D into (4) and taking logarithm of both sides, (5)isobtained:
(5)
Under these assumptions, a plot of logD versus log [L]should be linear and its slope should
be equal to the number of ligand molecules per metal cation in the extracted species (Figure.
3).
Fig. 3. Plot of log D versus log L for Pb2+ of ligand L1
The obtained logD versus logL plot displayed in Figures 3 and 4 are displayed as 1:1 in ratio
ofmetal-ligand for Fe2+ with L11 and for Pb2+ with L1.
Fe2+, x = 1.10078 (L11)
The logarithmic extraction constant log Kex corresponding to (5) is calculated:
logKex = 5.90273
for Fe2+with L11 log Kex= 5.5503 for Pb2+ with L1
3.4. Effect of the pH medium on metal extraction
; Pb2+, x = 0.96257 (L1)
,
The aqueous phase pH is one of the determining factors for metals extraction by ligands. To
examine the behavior of these ligands in different pH values and to determine the optimum
pH, we have carried out a competitive liquid-liquid extraction of four metals (Fe2+, Pb2+,
Cu2+ and Cd2+) at different pH.The extraction properties of Fe2+by L11 and of Pb2+ by the
ligand L1 were investigated over the pH range 1 to 8.
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For the both ligands L1 with Pb2+ and L11 with Fe2+, an identical behavior has been shown. In
4-8 pH range, the efficiency decreased slowly with pH decreasing, which can be explained
by a stable coordination. Under pH 4 the efficiencies decreases seriously and become
neglected when the pH attained a value of pH 1, this can be explained by a rapid and total
protonation of coordination sites. Above pH 8 and because of hydrolysis of Fe2+ and Pb2+, we
cannot distinguish between hydrolyzed or extracted cation. It seems that these ligands resist
to
the
protonation
and
remain
effective
in
acidic
medium.
There is an increase in Fe2+ and Pb2+during extraction to organic phase as pHincreases.When
log D-pHgraph is examined, there are two pH range (1-4 and 4-8), which can be attributed to
different formof protonation or deprotonation.It can be said that there is two structural
conformations when forming metal complex at different pH.
3.5. Back extraction of metal ions extracted with L1 and L11
In back-extraction, metal ions are extracted from the metal ligand complex using a stripping
liquid immiscible with the organic solvent used. As the extraction of Fe2+ and Pb2+ by L11
and L1 respectively decreases at acidic pH, we chose aqueous acid as stripping solution for
the back-extraction. All back-extraction tests were carried out in 10 minutes; the results are
summarized in Table. 3.
Table. 3. Back-extraction of metal ions extracted with L11 and L1.
According to the results presented inTable 3, a complete stripping of Fe2+ from the organic
phase containing Fe-L11 complex was observed using 1 M HNO3 (97%) while a recovery
percentage of 95% and 91% were respectively achieved in the presence of 1 M H2SO4 and 1
M HCl. Pb2+ was effectively stripped from the organic phase, a maximum back-extraction
percentage of metal may be achieved using 1 M HNO3 (99%) and 1 M H2SO4 (94%), high
recovery percentage of 89% was also obtained with 1 M HCl. It was found that the used
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acids as back-extraction reagents were extremely effective; the ligands could be recycled and
reused after stripping the metal ions from the organic phase and the kinetic of back-extraction
was found to be fast.
3.6. DFT analysis
To investigate the structure geometries, we performed full geometry optimization of L1 and
L11 in the gas phase using DFT/B3LYP level calculations. As illustrated in Fig. 4, the
optimized structures are planar and the imine function constitutes a bridge between the two
rings of the molecule. We also notice that the cavities formed between C=S and sp2 Nitrogen
in both molecules have similar size and the arrangement of the triazole and the phenyl ring
minimizes the steric hindrance and thus enabling the ligands structures to have stable
geometries.
Additionally to the geometry optimization, density functional theory has been found to be
successful in providing insights into the chemical reactivity and stability, the analysis of the
frontier molecular orbitals is a crucial way to obtain information on reactivity of molecules
and to study electron excitation from the highest occupied molecular orbital (HOMO) to the
lowest unoccupied molecular orbital (LUMO). The LUMO represents the ability of the
molecule to capture an electron while the HOMO represents the ability to donate an electron.
The difference between the HOMO and the LUMO energy values is the HOMO–LUMO
energy gap, which explains the eventual charge transfer interaction taking within the
molecule.
Molecules with a small energy gap are generally associated with a high chemical reactivity,
low kinetic stability and are also considered as soft molecules [40, 41] while those with large
energy gap have higher stability and are termed as hard molecules because they oppose
charge transfer and changes in their electron density and distribution.
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In addition, the energy of HOMO is directly related to the ionization potential (IP = -EHOMO
)
and the energy of LUMO is related to the electron affinity (EA= -ELUMO) [42]. By using these
values, interesting properties such as global hardness (η), global softness
(σ),electronegativity (χ), chemical potential (ꢀ) and electrophilicity index (ω) may be
calculated.
The hardness of a species is a qualitative indication of how polarizable it is, in another
wayhow much its electron cloud is distorted in an electric field. The hardness and softness
were suggested in literature [43-45]. Electronegativity is the tendency of molecules to attract
electrons [46]. Chemical potential denotes the affinity of an electron to flee and is defined as
the first derivative of the total energy with respect to thenumber of electrons in a molecule
[40], chemical potential is simply the negative of electronegativity value; ꢀ= -χ. The
capability of a substance to acceptelectrons is quantified as electrophilicity index and it was
proposed as ameasure of the electrophilic power of a molecule [47].The calculated energy
values and some propertiesofL1 -L11 are presentedin Table. 4.
Table. 4. Quantum chemical parameters forL1-L11 obtained by using B3LYP/6-31G(d,p).
Based on the results reported in Table. 4, a comparison between L1 and L11, show a marked
difference in their HOMO, LUMO and gap energies. The presence of the nitro function on
the phenyl ring in L11, which reduce the value of the HOMO conversely to L1 that contains
the methoxy group. Therefore, the withdrawing group on the phenyl ring decreases the donor
character of the molecule while the donor group increases this character. Furthermore, the
reported energies gap of L1and L11are respectively, 3.9603 eV and 2.9435eV, which
meansthat L1 is more stable than L11. Thus, the presence of themethoxy improves the
stability of the molecule while the presence of the nitro group allows the molecule to be more
reactive.
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The values of the electronegativity, chemical potential and electrophilicity index confirm the
electron donor character of L1 whilst the values of chemical hardness and softness explain
the reactivity and the easier excitation of L11 compared to L1that displays higher stability and
opposes charge transfer.
In Fig. 4, we also presented the MOs frontiers shapes of the studied ligands. In all molecules,
the occupied orbitals are mainly localized on the triazole fragment and the imine function
while the unoccupied orbitals have their major contribution from the phenyl ring. These
results suggest that the transitions involve charge transfer from the triazole moiety to the
phenyl ring.
Fig. 4. Shapes of MOs frontiers ofL1-L11 obtained by DFT (B3LYP/6-31G(d,p)) method.
3.7. Experimental and theoretical investigation of the electronic properties
According to the results reported in Table. 5. We can notice that the third transition is the
most important electronic transition for L1 owing to its oscillator strength (0.485 and 0.453
(DCM)). This transition corresponds to the raise of an electron from the occupied molecular
orbital (HOMO-2) to the lowest unoccupied molecular orbital (LUMO) (HOMO-2⟶
LUMO). The second electronic transition has also an important oscillator strength in the gas
phase (0.241) and greater value in DCM (0.426). This transition presents a π ⟶ π* character
and correspond to the transition from the high occupied molecular orbital (HOMO) to the
lowest uncopied molecular orbital (LUMO). The other transitions display a weak value of
oscillator strength and has also a π ⟶ π* character. Using the same analysis, one can notice
that the most interesting transitions for L11in the gas phase is the second, the fourth and the
sixth owing to their high f values. The second transition corresponds to the raise of an
electron from HOMO to LUMO in gas phase as well as in DCM with respectively an
oscillator strength of 0.082 and 0.09. The fourth corresponds to the transition from HOMO to
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L+1 with an oscillator strength in the gas phase of 0.062 and in DCM (0.087). The sixth
transition is the most important transition with an oscillator strengthof 0.146 and 0.162
respectively for gas phase and DCM. The last transition mainly corresponds to a transition
from H-2toLUMO (88%). The other transitions have negligible oscillator strength and are
considered as weakly allowed transitions.
Table. 5. Experimental (λExp) and calculated (λTh) wavelengths, energy in eV (E), oscillator
strength multiplied by 100 (f * 100) and their percentage contributions for L1 and L11.
To obtain information about the nature of the electronic transitions,the absorption spectra of
the ligands L1 and L11were recorded in dichloromethane solution of a concentration about
7.10-5 M at room temperature. As it can be seen an example for L11 in Fig. 5, the ligands
display three principal peaks at 345 nm, 283 nm and 262 nm for L1, at 357 nm, 283 nm and
250 nm for L11. For L1, The transition which corresponds to the raise of an electron from
HOMO to LUMO orbital appears in the gas phase at 359 nm and a shift to 348 nm in DCM
was observed. This blue shift may be ascribed to the solvent effect and the pure transition π
⟶ π*. The experimental maximum absorption at this range was observed at 345 nm and may
be attributed to the π-conjugation in the triazol system.On the other hand, we notice a red
shift from 293 nm (gas phase) to 301 nm (DCM), these transitions correspond to HOMO-2
⟶ LUMO whilst the experimental maximum absorption at this range was observed at 283
nm and may be attributed to π-conjugation in phenyl system.
Fig. 5. Experimental and theoretical electronic spectra of L11
In the case of L11, a blue shift was observed due to the solvent effect and the pure transition
HOMO ⟶ LUMO which corresponds to π ⟶ π*, this transition appears at 520 nm in gas
phase and is shifted to 510 nm in DCM while the experimental absorption appears at 357
nm.Additionally, we remark two transitions (HOMO ⟶L+1 and H-2 ⟶LUMO) at higher
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energies in the case of gas phase which correspond to 379 nm and 326 nm respectively.
Thesetransitions was combined in the case of DCM, the higher in terms of oscillator strength
was H-2 ⟶LUMO which appears at 346 nm, from HOMO toL+1 correspond to 360 nm.
The experimental values was obtained at 283 nm and 250 nm, may be ascribed to the effect
of the nitro group on the π-conjugation in phenyl system.
For complex Fe-L11, an increase in absorption intensity was observed in the short wavelength
region that can be attributed to the hyperchromic effect in which the metal may participate.
For complex Pb-L1, unless a week perturbation in the transitions at short wavelength region,
no clear effect was observed. After the analysis of electronic transitions, we can conclude
that the maximum absorption for both ligands (L1and L11) at the longest wavelength region
corresponds to the HOMO ⟶ LUMO transition which has a π ⟶ π* nature and primarily
caused by the intramolecular charge transfer from the triazole to the phenyl ring. Whilst the
maximum absorption in the short wavelength region represent the effect of substituent group
on the phenyl.
4. Conclusion
The studiedligands were found to be efficient for the liquid-liquid extraction of metal cations.
Fe2+ and Pb2+ were selectively extracted with these triazole derivatives and the studies of pH
effect confirm the performance of such type of ligands in selective extraction as well as in
protonation resistance.These results have conclusively shown that the formation of
coordination complexes for these ligands is highly dependent on the substituent nature; high
selectivity towards Fe2+ is provided by electron withdrawing group whilst high selectivity
against Pb2+ is ensured by the presence of electron donor group. Interestingly, it was found
that the back-extraction was extremely effective by using acidic solutions.The present work
has also investigated the ground and excited state geometries using DFT and TDDFT
method. From the results of our calculation, we suggest that the presence of the methoxy
15

ACCEPTED MANUSCRIPT
group improves the stability of the molecule while the presence of the nitro group allows the
molecule to be more reactive. On the other hand, The electronic transitions at the longest
wavelengths region correspond to HOMO ⟶ LUMO transition which has a π ⟶ π* and are
primarily related to the intramolecular charge transfer from the triazole to the phenyl
ring.Moreover, the maximum absorption in the short wavelength region is represented by
different transitions which elucidate the effect of substituent group. It was also shown that
the calculated electronic transitions resulted from TD-DFT (B3LYP/6-31+G(d,p))
calculations are in an acceptable agreement with experimental data.
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Tables
Table. 1. Spectroscopic data of ligands L1-L11
1H NMR (DMSO-d6, δH ppm)
Ln
R
IR (KBr, ν, cm-1)
4-MeO 3276 (NH), 1698 (C=N), 1306 (C=S)
2.37 (s, 3H, CH3),3.82 (s, 3H,
OCH3), 7.14 (d, 2H, J = 8 Hz, arH),
7.82(d, 2H, J = 8 Hz, arH), 9.70 (s,
1H, H-C=N), 13.74 (s, 1H, NH)
L1
L2
L3
L4
L5
L6
L7
4-Me
2-Cl
3-Cl
4-Cl
2-Br
3-Br
4-F
3280 (NH), 1688 (C=N), 1306 (C=S)
3356 (NH), 1711 (C=N), 1302 (C=S);
3262 (NH), 1690 (C=N), 1274 (C=S)
3258 (NH), 1630 (C=N), 1301 (C=S)
3305 (NH), 1712 (C=N), 1244 (C=S)
3362 (NH), 1745 (C=N), 1309 (C=S)
3262 (NH), 1690 (C=N), 1294 (C=S)
2.39 (s, 3H, CH3), 2.45 (s, 3H,
arCH3), 7.30 (d, 2H, J = 8 Hz, arH),
7.77 (d, 2H, J = 8 Hz, arH), 9.98 (s,
1H, H-C=N), 13.70 (s, 1H, NH)
2.36 (s, 3H, CH3), 7.59 (dd, 2H, J = 4
Hz, J = 8 Hz, arH), 7.79-7.85 (m, 1H,
arH), 8.07-8.12 (m, 1H, arH), 10.74
(s, 1H, H-C=N), 13.71 (s, 1H, NH)
2.34 (s, 3H, CH3), 7.44-7.49 (m, 2H,
arH), 7.80 (d, 1H, J = 8 Hz, arH),
8.14 (s, 1H, arH), 10.16 (s, 1H, H-
C=N), 13.77 (s, 1H, NH)
2.32 (s, 3H, CH3), 7.61 (d, 2H, J = 8
Hz, arH), 7.90 (d, 2H, J = 8 Hz, arH),
10.09 (s, 1H, H-C=N), 13.72 (s, 1H,
NH)
2.38 (s, 3H, CH3), 7.56 (dd, 2H, J = 4
Hz, J = 8 Hz, arH), 7.77-7.82 (m, 1H,
arH), 8.10-8.15 (m, 1H, arH), 10.79
(s, 1H, H-C=N), 13.76 (s, 1H, NH)
2.39 (s, 3H, CH3), 7.50 (dd, 2H, J = 4
Hz, J = 8 Hz, arH), 7.84 (d, 1H, J = 8
Hz, arH), 8.12 (s, 1H, arH), 10.10 (s,
1H, H-C=N), 13.80 (s, 1H, NH)
2.34 (s, 3H, CH3), 7.40 (dd, 2H, arH),
7.99 (dd, 2H, arH), 9.96 (s, 1H, H-
C=N), 14.22 (s, 1H, NH triazole)
L8
L9
3-NO2 3277 (NH), 1723 (C=N), 1324 (C=S)
4-NO2 3262 (NH), 1660 (C=N), 1294 (C=S)
2-NO2- 3200 (NH), 1690 (C=N), 1264 (C=S)
2.41 (s, 3H, CH3), 7.52-7.59 (m, 2H,
arH), 7.87 (d, 1H, J = 8 Hz, arH),
8.16 (s, 1H, arH), 10.17 (s, 1H, H-
C=N), 13.89 (s, 1H, NH)
2.37 (s, 3H, CH3), 7.67 (d, 2H, J = 8
Hz, arH), 7.98 (d, 2H, J = 8 Hz, arH),
10.14 (s, 1H, H-C=N), 13.76 (s, 1H,
NH)
L10
2.43 (s, 3H, CH3), 7.67-7.74 (m, 2H,
arH), 8.13 (s, 1H, arH), 10.79 (s, 1H,
L11

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5-Cl
H-C=N), 13.90 (s, 1H, NH)
Table. 2.Extraction efficiencies of metal cations using L1-L11.
Extractability (E%)
Ligands
L1
Fe(II)
Cu(II)
24.9
25.0
34.8
51.3
43.6
53.7
30.5
33.6
63.1
29.5
53.4
Zn(II)
1.2
2.5
0.5
3.1
2.5
0.9
1.7
2.0
6.7
3.5
2.0
Pb(II)
Co(II)
1.0
0.0
3.0
5.5
0.0
4.1
7.2
2.7
4.9
6.6
6.5
Cd(II)
40.2
37.9
28.4
13.3
10.0
8.9
92.6
91.7
42.1
96.3
85.3
53.7
7.5
97.3
93.6
90.1
98.9
98.1
65.7
79.3
96.7
37.9
72.4
87.2
L2
L3
L4
L5
L6
0.0
L7
1.9
L8
92.9
93.6
91.6
95.7
18.6
49.8
24.0
L9
L10
L11
Table. 3.Back-extraction of metal ions extracted with L11 and L1.
Back-extraction (%) of :
Back-extraction reagent
1 M HCl
Fe(II) extracted by L11
Pb(II) extracted by L1
91
97
95
89
99
94
1 M HNO3
1 M H2SO4

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Table. 4. Quantum chemical parameters forL1-L11 obtained by using B3LYP/6-31G(d,p).
Property (eV)
Ligands
EHOMO
ELUMO
ꢀE
χ
η
σ
ꢁ
ω
-5.5231 -1.5628 3.9603 3.5429 -1.9802 -0.5050 -3.5429 -3.1695
-5.6219 -1.7116 3.9103 3.6667 -1.9551 -0.5115 -3.6667 -3.4384
-5.7394 -2.0139 3.7255 3.8767 -1.8628 -0.5368 -3.8767 -4.0340
-5.8290 -2.0471 3.7818 3.9380 -1.8909 -0.5288 -3.9380 -4.1007
-5.8042 -2.0204 3.7837 3.9123 -1.8919 -0.5286 -3.9123 -4.0453
-5.7438 -1.9766 3.7671 3.8602 -1.8836 -0.5309 -3.8602 -3.9556
-5.8232 -2.0417 3.7816 3.9325 -1.8908 -0.5289 -3.9325 -4.0894
-5.7356 -1.8460 3.8896 3.7908 -1.9448 -0.5142 -3.7908 -3.6946
-5.9871 -2.6409 3.3462 4.3140 -1.6731 -0.5977 -4.3140 -5.5616
-6.0684 -2.9603 3.1081 4.5144 -1.5540 -0.6435 -4.5144 -6.5569
-5.9596 -3.0161 2.9435 4.4878 -1.4717 -0.6795 -4.4878 -6.8425
L1
L2
L3
L4
L5
L6
L7
L8
L9
L10
L11

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Table. 5. Experimental (λExp) and calculated (λTh) wavelengths, energy in eV (E), oscillator
strength multiplied by 100 (f * 100) and their percentage contributions for L1 and L11.
E (eV) λExp(nm) λTh(nm) f *100
Major contribs
3.225
3.454
4.228
4.456
4.587
4.714
385
359
293
278
270
263
0.03
24.17
48.53
1.25
H-1->LUMO (97%)
HOMO->LUMO (98%)
345
283
262
H-2->LUMO (92%)
H-3->LUMO (18%), HOMO->L+1 (75%)
H-1->L+1 (12%), H-1->L+2 (80%)
H-1->L+1 (87%), H-1->L+2 (12%)
0.02
0.03
3.461
3.565
4.125
4.539
4.764
4.955
358
348
301
273
260
250
0.04
42.59
45.32
0.27
H-1->LUMO (97%)
HOMO->LUMO (98%)
345
283
262
H-2->LUMO (94%)
H-3->LUMO (48%), HOMO->L+1 (42%)
H-1->L+2 (87%)
0.03
0.15 H-3->LUMO (30%), H-2->L+1 (10%), HOMO->L+1 (53%)
2.213
2.385
3.139
3.271
3.542
3.808
560
520
395
379
350
326
0.01
8.18
0.01
6.20
0.00
14.64
H-1->LUMO (98%)
HOMO->LUMO (100%)
H-1->L+1 (96%)
357
283
250
HOMO->L+1 (99%)
H-4->LUMO (87%)
H-3->LUMO (14%), H-2->LUMO (79%)
2.384
2.430
3.368
3.449
3.583
3.648
520
510
368
360
346
340
0.01
9.01
0.02
8.74
16.20
0.00
H-1->LUMO (98%)
HOMO->LUMO (100%)
H-1->L+1 (95%)
357
283
250
HOMO->L+1 (98%)
H-2->LUMO (88%)
H-4->LUMO (82%)

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Figures
Fig. 1. Chemical structures of studied ligands L1-L11

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100
80
60
40
20
0
Fe2+
Cu2+
Pb2+
Cd2+
L1
L2
L3
L4
L5
L6
L7
L8
L9
L10 L11
Ligands
Fig. 2.Competitive extraction E(%) of metal ions using L1-L11
y=0.96257x + 5.5503
R2=0.99712
1.8
1.7
1.6
-4.1
-4.0
-3.9
Log L
Fig. 3. Plot of log D versus log L for Pb2+ of ligand L1

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Fig. 4.Shapes of MOs frontiers ofL1-L11 obtained by DFT (B3LYP/6-31G(d,p)) method.

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Fig. 5. Experimental and theoretical electronic spectra of L11

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Highlights
•
•
•
•
•
Liquid-liquid extraction of metal ions using eleven ligands 1,2,4-triazole Schiff base.
Determinationof the extractability for each metal ions.
Investigation of competitive extraction at different pH values.
High selectivity towards Fe2+ (90.1%) and Pb2+ (94.3%).
Use of DFT and TD-DFT for electronic structure elucidations.