Full text of DOI:10.1016/j.molliq.2018.05.037

Journal of Molecular Liquids 264 (2018) 58–65
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Selective colorimetric sensing of nickel (II) ions using
2
-hydroxy-5-nitrobenzaldehyde-4-hydroxybenzoylhydrazone ligand:
Spectroscopic and DFT insights
Ismail Abdulazeez, Chanbasha Basheer ⁎, Abdulaziz A. Al-Saadi ⁎
Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
The abnormal levels of nickel (II) ions in body fluids have been linked to various ailments, and their ubiquitous
environmental occurrence could pose a potential hazard. Hence, there is a pressing need to develop selective
and miniaturized analytical techniques to efficiently monitor the level of nickel (II). In the present study, 2-
hydroxy-5-nitrobenzaldehyde-4-hydroxybenzoylhydrazone (HNHBH) was synthesized and investigated as a
potential colorimetric sensor for nickel (II) ions. A distinct color change was observed due to complex formation
between HNHBH and nickel (II) ions. Both the free ligand and resulting complexes were characterized by UV–Vis,
Received 16 February 2018
Received in revised form 3 May 2018
Accepted 8 May 2018
Available online 12 May 2018
1
infrared and H NMR spectroscopies. A theoretical DFT method has been implemented to understand the nature
of complexation based on the calculated binding energies and frontier molecular orbitals. The computational ap-
proach used predicts that HNHBH adopts an imidic acid tautomeric form when it coordinates with the metal ion
center. The selectivity of HNHBH towards nickel ions was evaluated by taking into account a number of factors,
such as solvent, pH and EDTA effects.
©
2018 Elsevier B.V. All rights reserved.
1
. Introduction
without the use of instrumentation is of a special interest as it is less
labour-intensive compared to classical instrumental techniques
[11,12] and has the potential to be developed into simple, convenient
and cost-effective test kits [13].
The impact of nickel ions coupled with their ubiquitous environ-
mental occurrence from anthropogenic and natural sources makes
monitoring their level with the use of feasible means an important
task [1,2]. Nickel compounds, except metallic nickel, are deemed carci-
nogenic to humans [1]. It has been proposed that nickel acts as a cofac-
tor for some enzymes, such as urease [3,4], but the role of nickel in
human physiology remains poorly understood. No consensus regarding
whether nickel is an essential trace element for humans has been
reached yet [1,3,4].
Trace analysis of nickel is often carried out through various methods
such as atomic absorption spectroscopy [5,6] and inductively coupled
plasma-mass spectroscopy [7]. These methods provide a good sensitiv-
ity and accuracy but can be costly and inconvenient due to the infra-
structure and sample pre-treatment required, or when large numbers
of samples need to be analyzed. Simpler alternatives that offer an ac-
ceptable sensitivity at a lower cost such as ion selective electrodes
A number of aroyl hydrazones were shown to function as colorimet-
ric sensors for some metal ions, and their facile synthesis, tuneable elec-
tronic and steric properties and efficient chelating capability have been
explored [14–19]. Sumita et al. reported the synthesis and spectroscopic
studies of Co(III) aroyl-hydrazone complexes with a characteristic oxi-
dative and reductive electrochemistry [14]. Anthracene-based aroyl
hydrazones were also synthesized and shown to readily form com-
plexes with Co(III) and Ni(II) ions [16]. Aroyl hydrazones of 2-
phenylindole-3-carbaldehyde for the inhibition of the growth of cancer
cells were also synthesized [18]. Metal complexes of aroyl hydrazones
are reported to exhibit interesting biological and pharmaceutical activ-
ities and are of interest in several research fields including selective
metal extraction and spectroscopic determination [19,20]. Hence this
work aimed to investigate the potential application of 2-hydroxy-5-
nitrobenzaldehyde-4-hydroxybenzoylhydrazone (HNHBH) which is
an aroyl hydrazones derivative as a promising colorimetric sensor for
nickel (II). Computational modelling tool has been employed to under-
stand the complexation nature of nickel ions with HNHBH in terms of
binding energies, structural parameters and atomic charges. Compari-
son between computational and experimental results provides deep in-
sights into the chemistry of the possibly formed complexes [21].
[
[
8,9], UV–Visible absorption and fluorescent emission chemosensors
10] are being investigated. The colorimetric sensing of metal ions
⁎
Corresponding authors.
E-mail addresses: cbasheer@kfupm.edu.sa, (C. Basheer), asaadi@kfupm.edu.sa
(A.A. Al-Saadi).
https://doi.org/10.1016/j.molliq.2018.05.037
167-7322/© 2018 Elsevier B.V. All rights reserved.
0

I. Abdulazeez et al. / Journal of Molecular Liquids 264 (2018) 58–65
59
2
. Experimental
2
.1. Chemicals and materials
Metal salts used include nickel (II) acetylacetonate, copper (II) ace-
tate and zinc (II) acetate (all of them were analytically pure and com-
mercially purchased). All solvents used were of HPLC grade and used
without any further purification.
The HNHBH ligand was synthesized following a previously reported
method [22,23] and then tested with the salts of nickel (II) copper (II)
and zinc (II) ions. Briefly, 0.30 g (2 mmol) of 4-hydroxyphenylhydrazide
and 0.33 g (2 mmol) of 2-hydroxy-5-nitrophenyl aldehyde were
dissolved in 25 mL MeOH and stirred for 3 h. The resulting product was
2
filtered and washed three times with MeOH and Et O and dried over
fused CaCl
2
.
Fig. 1. Color changes observed on adding HNHBH ligand to Ni(acac)
2
, Cu(OAc)
2
and Zn
2
2
.2. Methodology
(OAc) solutions. The most intense change was noticed in the case of nickel (II) solution.
2
.2.1. Preliminary experiments
Solutions of HNHBH with tetrahydrofuran (THF) (2 × 10 M) were
−4
0.5 M NaOH, and vice versa. A colorimetric titration of the ligand (2
−3
−4
−3
prepared. Each metal salt in acetonitrile (ACN) (2 × 10 M) was pre-
pared and sonicated for 10 min. 10 μL of each metal salt solution was
added to 1 mL of HNHBH ligand solutions in clean glass vials. While
the mixture was shaken, any color changes were noted. After, another
2
× 10 M) and Ni(acac) (2 × 10 M) in a 50% THF mixed aqueous so-
lution with the addition of aqueous NaOH (0.5 M) and aqueous HCl
(0.5 M) was carried out.
1
0-μL quantity of the metal salt solution was added, and the process
was repeated. The experiment was also carried out with the metal
salts dissolved in THF.
2.2.3.4. Ligand interference effect. 50% THF mixed aqueous solutions of
tetrasodium ethylenediaminetetraacetate (Na
centrations were prepared. 50% THF mixed aqueous solutions of various
metal salts, Mg(OAc) , Ca(OAc) , Ni(acac) , Cu(OAc) , Zn(OAc) and Pd
(2 × 10 M) were added in 10-μL aliquots to 0.5 mL of a Na
4
-EDTA) at various con-
2
.2.2. Characterization of ligand-metal complexes
2
2
2
2
2
A 0.5-mL quantity of a ligand solution in THF (2 × 10−4 M) was di-
(OAc)
EDTA solution (2 × 10
noted. Mixed aqueous solutions of 0.5 mL of the HNHBH ligand (2
−3
2
4
-
−
4
luted with 2.5 mL of THF in a quartz cell and then scanned from 200
M) in a glass vial, and any changes were
−3
to 700 nm. The relevant metal salt solution in THF (2 × 10 M) was
added to the ligand solution in the quartz cell in 10-μL aliquots. The so-
lution was mixed with a dropper and the spectra were recorded after
each addition of the metal salt solution. The infrared spectroscopy was
collected using KBr pellets containing a small amount of the analyte.
−4
−3
× 10 M) and 10 μL of a metal salt (2 × 10 M) in a glass vial were
treated with 1–10 μL of Na
−
2
4
-EDTA solutions (2 × 10
M and 2
M) and Ni
−
4
−4
× 10
(acac)
dition of Na
M). A colorimetric titration of ligand (2 × 10
−3
2
(2 × 10 M) in a 50% THF mixed aqueous solution with the ad-
−
1
−2
Pellets were prepared and scanned from 4000 to 400 cm . For the
4
-EDTA (2 × 10 M) and aqueous HCl (0.5 M) was carried
1
HNMR spectroscopy, equimolar solutions of the ligand and metal
out.
−3
6
salts were prepared in DMSO d (2 × 10 M). The HNHBH ligand was
analyzed directly. The ligand-metal complexes were prepared by
mixing equal volumes of the ligand and metal salt solutions. Purity of
the ligand and metal-ligand complexes was verified using the energy-
dispersive X-ray spectroscopy (EDX).
a.
H
N
O
NO2
HO
N
NO2
2
.2.3. Effect of parameters
N
N
HO
HO
2
.2.3.1. Metal interference effect. Interference solutions of calcium, zinc,
magnesium, palladium and copper were prepared by adding 200 μL of
−
3
each metal salt solution in THF (2 × 10
M) to a microcentrifuge
OH
OH
tube and manually shaking the mixture for 3 min. This solution was
used for colorimetric titrations with the HNHBH ligand.
Amide form
Imidic acid form
2
.2.3.2. Solvent effect. To investigate the effect of other organic solvents,
−3
solutions of metal salts in ACN (2 × 10 M) were used for colorimetric
titrations. To investigate the effect of mixed aqueous solvents, solutions
of metal salts were prepared in 50% THF mixed aqueous solutions (2
b.
H
H
−
3
-
×
10
was used to dilute the HNHBH ligand at 25% (w/v), 50% (w/v) and
5% (w/v), and the ligand and metal salts in mixed aqueous solutions
were tested following the procedure described above. A colorimetric ti-
M) from THF and nano-pure water. This solvent mixture
O
N
NO
2
O
N
-
N
NO2
N
7
+
HO
HO
−4
−3
tration of HNHBH ligand (2 × 10 M) and Ni(acac)
2
(2 × 10 M) in a
+
5
0% THF mixed aqueous solution was also performed.
OH
OH
2
×
.2.3.3. pH effect. Mixed aqueous solutions of 0.5 mL of the ligand (2
−
4
−3
10
M) and 10 μL of the metal salts (2 × 10
M) in glass vials
Fig. 2. (a) Amide-imidic acid tautomerism of HNHBH ligand, and (b) Possible resonance
were treated with 1–10 μL of aqueous 0.5 M HCl followed by aqueous
structures of HNHBH ligand as an amide tautomer.

60
I. Abdulazeez et al. / Journal of Molecular Liquids 264 (2018) 58–65
Fig. 3. FT-IR spectra of the free HNHBH ligand (upper) and Ni-HNHBH complex (lower).
2
.3. Computation
metal ions of Ni, Cu and Zn were calculated using the following
equation:
Geometry optimization and electronic structure study of the investi-
gated ligand were carried out using the density functional theory (DFT)
approach employing the B3LYP hybrid functional with the 6-31G* and
ꢀ
ꢁ
2þ
Binding Energy ðBEÞ ¼ Ecomplex– EM þ 2 ꢀ EHNHBH
ð1Þ
6
-311 + G* basis sets [24,25]. Structural geometry optimizations of
where Ecomplex is the total energy of the ligand-metal complex, E2 and
EHNHBH are the total energies of the free metal ions and free ligand, re-
spectively. All calculations were carried out using the Gaussian 09
suite [26].
+
the HNHBH ligand and formed complexes were carried out to the min-
ima on the potential energy surfaces without constrains. The relative
stability and vibrational frequencies of HNHBH were also computed.
The optimized binding energies (BE) of the ligand towards three
M
Table 1
Selected characteristic infrared absorption frequencies of the amides (free) and imidic acids (coordinated to the metal ion) forms.
Amides
Imidic acids
Wavenumber, ν/cm−
1
Assignment
Wavenumber, ν/cm−1
Assignment
3
1
300–3280 (m)
680–1640 (vs)
N \\ H stretch
C_O stretch
3500–3000 (s)
1675–1600 (m)
O \\ H stretch
C_N
(
Amide I band)
N \\ H bend
Amide II band)
1560–1530 (vs)
1615–1580
C_N \\ N_C
(
1
6
310–1290 (m)
30–570 (s)
C \\ N stretch
N \\ C_O bend
1160–1120 (s)
1440–1260
C \\ OH stretch
C \\ OH in-plane bend
(m-s; br)
6
5
15–535 (s)
20–430 (m-s)
C_O out-of-plane bend
C \\ C_O bend
700–600
(m-s; br)
C \\ OH out-of-plane deformation
Notes: s = strong; m = medium; br = broad.

I. Abdulazeez et al. / Journal of Molecular Liquids 264 (2018) 58–65
61
3
2
2
1
1
0
0
.0000
.5000
.0000
.5000
.0000
.5000
on adding Ni(acac)
nations of the ligand with Ni(acac)
change, hence they have been investigated further. The ligand and
ligand-metal complexes of interest were characterized using spectro-
scopic and theoretical techniques. Infrared, HNMR and EDX techniques
2
, Cu(OAc)
2
and Zn(OAc)
2
salts (Fig. 1). The combi-
2
showed the most significant color
0uL Ni(acac)2
0uL Ni(acac)2
40uL Ni(acac)2
0uL Ni(acac)2
80uL Ni(acac)2
1
2
were used to elucidate the chemical structure and check the purity of
the free ligand and ligand-metal complexes, while UV–Vis spectroscopy
was used to shed light on the nature of binding. Ligand HNHBH is ex-
pected to exhibit amide-imidic acid tautomerism (Fig. 2). It has been re-
ported that similar hydrazonic ligands tend to exist in the amide form
but coordinate with the imidic acid tautomeric form in a ligand-metal
complex [19,27–29]. The IR spectra (Fig. 3) of the ligand help in identi-
fying possible tautomeric forms by the presence or absence of charac-
teristic bands associated with the hydroxyl, carbonyl, amide and
imidic acid groups, for which some of the observed frequencies were
assigned as listed in Table 1. The absorption infrared band at
6
.0000
2
20.0
320.0
420.0
520.0
Wavelength ( / nm)
2
Fig. 4. Absorbance against wavelength (λ/nm) for ligand/THF with Ni(acac) /THF. Some
spectra were omitted for the sake of clarity.
−
1
1
654 cm
in the amide form of HNHBH was assigned to the C_O
3
. Results and discussions
stretching. The NH stretching band which is characteristic of the
−1
amide form can be clearly observed in the 3300 cm range. Moreover,
−1
3
.1. Characterization of the ligand and ligand-metal complexes
the medium intensity broad band at 3209 cm can be attributed to the
intramolecular hydrogen bonding between the hydroxyl group hydro-
gen and hydrazone group nitrogen atoms. The three most deshielded
protons in the 1HNMR spectrum of the ligand at 12.47, 12.10 and
The
solutions
of
2-hydroxy-5-nitrobenzaldehyde-4-
hydroxybenzoylhydrazone (HNHBH) were observed to change color
Fig. 5. Optimized structures, relative stability, energy barriers and OH stretching frequencies for the amide forms of HNHBH (a) with and (b) without the intramolecular hydrogen bonding.
2
+
2+
2+
Ni(HNHBH)
2
Cu(HNHBH)
2
Zn(HNHBH)
2
Fig. 6. Optimized metal-ligand complexes with Ni2+, Cu2+ and Zn2+ ions adopting an octahedral coordination environment.

62
I. Abdulazeez et al. / Journal of Molecular Liquids 264 (2018) 58–65
0
0
0
0
0
0
0
0
.7000
.6000
.5000
.4000
.3000
.2000
.1000
concerned here about the frequency difference rather than the absolute
values which could deviate from the corresponding experimental
values due to the difference between the gas and solid phases. The
−1
0uL Mixture
C_O stretching band (1654 cm ) in the free ligand amide spectrum
20uL Mixture
60uL Mixture
80uL Mixture
120uL Mixture
−1
has also disappeared, while the new band at 1615 cm can be attrib-
uted to the C_N\\N_C stretching vibration supporting the formation
1
of imidic acid tautomer after complexation. The H NMR spectrum of
the ligand-Ni complex (Fig. S2) shows three new peaks observed at
160uL Mixture
10.45, 8.59 and 5.68 ppm. The 5.68 ppm peak can be assigned to protons
from the acetylacetonate anion, the peak at 8.59 ppm was assigned as an
aromatic proton, while the 10.45 ppm peak could be linked to the amide
proton which has become after complexation more deshielded due to
the absence of the hydrogen bonding. The EDX spectrum of the complex
confirmed the presence of the metal ions which were absent in the
spectrum of the pure ligand (Fig. S3). The overall shift and subsequent
appearance of new peaks upon the coordination of the ligands with
the metal ion are considered a sign of the tautomeric exchange that al-
lows complexation between the ligand molecules and the metal ions.
The conformational properties of the tautomeric forms of HNHBH
have been further investigated theoretically to get some insights on
the energetics and stability of the ligand before and after complexation.
While the two tautomeric forms were predicted to exhibit comparable
stabilities, the relative stability between the amide form with the intra-
molecular hydrogen bond and the imidic acid form without that sort of
bonding was calculated to be as high as 12 kcal/mol (Fig. 5). The transi-
tion state on the potential energy scan (PES) resulting from rotating the
hydroxyl proton outward of the hydrazonic nitrogen corresponds to a
noticeably high rotational energy barrier of about 30 kcal/mol. It is pre-
dicted that upon introducing the metal ions, the hydroxyl group is
forced to rotate outwards in order to allow for the complexation be-
tween the HNHBH ligand and the metal ion to take place in a 2:1
ratio, as optimized at the B3LYP/6-311 + G* level of theory and pre-
sented in Fig. 6. This is also evidenced in the IR spectra which show a
blue-shifted strong band in the region of OH absorption upon addition
of the metal ion compared to the case of the free ligand.
.0000
2
20.0
320.0
420.0
520.0
Fig. 7. Absorbance against wavelength (λ/nm) for hydrazone ligand/THF with interference
solutions in THF. Some spectra were omitted for the sake of clarity.
1
0.24 ppm were assigned to the two phenol and one amide protons, re-
spectively, as shown in Fig. S1. A summary of the NMR analysis of the li-
gand showing detailed peak assignments is given in Table S1. The UV–
Vis spectrum of the ligand complexation (Fig. 4) also suggests that the
free ligand exists as an amide tautomer. The ligand in THF has an ab-
sorption maximum at 295 nm (ε295 = 39,700 cm
der peak at 335 nm (ε335 = 21,800 cm
the overlapping absorption from the two delocalized electron systems
in the amide tautomer of the ligand (Fig. 2b). In the colorimetric titra-
2
tion of the ligand with Ni(acac) in THF (Fig. 4), two isosbestic points
were observed at 308 and 349 nm, and the complex showed a new,
broad absorption band at approximately 410 nm. The new absorption
band could be attributed as a charge transfer band. The peak in the re-
−1
−1
M
) with a shoul-
). These are likely due to
−1
−1
M
2
gion of 295 nm was not deemed significant as Ni(acac) absorbs at
−1
2
95 nm (ε = 16,650 cm−1
M
).
The infrared spectrum of the ligand-Ni complex (Fig. 3) indicates no-
ticeable changes in the spectral region above 3000 cm . This region
mainly describes the nature of the N\\H and O\\H bonds. The strong,
−
1
−
1
broad infrared band appeared at 3433 cm explains the presence of a
non-hydrogen bonded OH stretching vibration. It suggests that the in-
tramolecular hydrogen bonding found in the amide form no longer ex-
ists upon ligand complexation. The frequency shift in the O\\H
stretching vibrations from the amide to imidic acid forms is supported
with calculated DFT frequencies (Fig. 5). The νOH stretching vibration
3.2. Effect of parameters
3.2.1. Metal interference effects
The interference solutions used contained six metal ions, namely
Mg, Ca, Ni, Cu, Zn and Pd, each of which is of a concentration of approx-
−
1
−4
was calculated to be 3385 cm when it is intramolecularly bonded,
imately 3 × 10 M. The ligand was observed to be very sensitive to
while it was calculated at 3654 cm− in the imidic acid form. We are
1
metal ions. When treated with interference solutions, a solution of
Fig. 8. Frontier orbitals of HNHBH depicting the HOMO and LUMO orbital energies before (a) and after complexation with (b) Ni2+, (c) Cu2+ and (d) Zn2+ metal ions.

I. Abdulazeez et al. / Journal of Molecular Liquids 264 (2018) 58–65
63
Table 2
285 nm and from 349 to 347 nm, and the charge transfer band shifted
from 410 to 405 nm. Such a small hypsochromic shift could be due to
the addition of ACN which is more polar than THF. The reason for the
dark yellowish-green formation could be due to the ACN solvent stabi-
lizing non-bonding orbitals through hydrogen bonding, leading to an in-
crease in the amount of energy required to excite an electron originating
from a non-bonding orbital.
Moreover, the complexation in water has been investigated. Since
HNHBH is hardly soluble in water, a 50:50 ratio of water and THF was
used. The ligand in a 50% THF mixed aqueous solution had an absorption
maxima at 296 nm and a shoulder peak at 335 nm, which is similar to
the case of the ligand in THF. The spectrum of the ligand-Ni complex
in a 50% THF mixed aqueous solution was also similar to that of the
ligand-Ni complex in THF (Fig. 4), with a charge transfer band at
400 nm and two isosbestic points at 293 and 347 nm. The slight
hypsochromic shift observed was similar to the case of ACN, since
water is also a more polar solvent compared to THF.
Binding energies of metal ions complexes with HNHBH ligand as calculated at the B3LYP/
6-31G* and B3LYP/6-311 + G* levels of theory.
Metal ion
Binding energy (Kcal/mol)
B3LYP/6-31G*
B3LYP/6-311 + G*
Ni2
Cu
Zn
+
−486.4
−417.4
−399.2
−464.6
−397.2
−378.2
2
+
+
2
HNHBH in THF turned from colourless to pale yellowish-green. The UV–
Vis spectral pattern is similar to that of the ligand-Ni complex in THF
(
Fig. 4). However, the peak wavelength of the charge transfer band
has gradually shifted from 400 to 390 nm upon the addition of the inter-
ference solutions, and three isosbestic points were observed at 244, 344
and 380 nm.
Comparing the spectrum in Fig. 7 with the individual spectra of the
ligand with nickel (II), copper (II) and zinc (II) ions, the charge transfer
band shape suggests that zinc (II) interfered with the formation of the
ligand-Ni complex more than copper (II) did. Hence the formation of
the ligand-Ni complex was influenced by the presence of the zinc (II)
ions and, to less extent, copper (II) ions, but not magnesium (II), calcium
3.2.3. pH effects
The ligand-metal complex is expected to be sensitive to the pH of the
solution, as the ligand is expected to coordinate with the metal ion
through the lone pairs of electrons on the oxygen and nitrogen atoms.
While the addition of a base is expected to deprotonate the oxygen
and nitrogen atoms and hence enhance the ligand ability to bind to
the metal ion, the addition of an acid is expected to protonate the oxy-
gen and nitrogen atoms and thus disrupt the formation of the complex
or even regenerate the ligand in its free form. NaOH was chosen as the
base for the investigation of the pH effect after verifying that a solution
of the ligand did not change color when treated with neutral solution of
sodium chloride. The addition of a few drops of the acid or base to the
solution of the ligand or the ligand-metal complex caused a distinct
color change without a significant change of pH. Furthermore, the
color change was observed to be independent of the composition of
the mixed aqueous solution.
Adding NaOH to a mixed aqueous solution of the ligand and a metal
salt increased the selectivity of the ligand for nickel (II) (Table 3). On a
subsequent addition of HCl to the basic solution, however, the bright
yellow color was retrieved indicating a higher selectivity for nickel
ions in basic conditions. The addition of HCl to a mixed aqueous solution
of the ligand and a metal salt other than nickel (II) resulted with a
colourless solution, and the addition of nickel (II) to the same solution
turned it pale green-yellow.
To further investigate the pH phenomenon, a UV–Vis spectrum was
measured at alkaline conditions (Fig. 9a). The addition of NaOH caused
the charge transfer band of the ligand-Ni complex to broaden, and its
peak value was observed to slightly shift from 400 to 394 nm. The sub-
sequent addition of HCl was tested (Fig. 9b), and a clear back-shift was
observed. However, on further increase of the HCl concentration, no
change in the absorption pattern was observed.
The Mulliken atomic charge distribution of the HNHBH ligand was
calculated (Fig. 10) and used to predict the chemical reactivity at differ-
ent pH values. Atomic charges provide useful insights of relevance to the
electronic nature of the molecule, from which possible bonding, anti-
bonding or non-bonding features could be deduced [30]. For the
(II) or palladium (II) ions.
The nature of interaction between the ligand and the metal ions
was investigated on the basis of calculated frontier molecular or-
bitals (MOs). The charge transfer behaviour of any two reactants de-
pends on the spatial distribution of their frontier orbitals as well as
the energy gap maintained. The frontier orbitals of the HNHBH li-
gand before and after complexation were calculated and depicted
in Fig. 8. The MO interaction map shows that the free ligand LUMO
was primarily confined on the hydrazonic group, which suggests a
more likely binding site available for the metal ion. Upon complexa-
tion with metal ions, a corresponding energy shift in a form of an
overall decrease in the energy gap was expected. The Ni(II) ion com-
plex exhibits the most significant shift and consequently the most
stable complexation among the three metal ions, in agreement
with the trend observed in the color change when a complex
forms. Furthermore, the coordination energies were estimated for
Ni(II), Cu(II) and Zn(II) ions according to Eq. (1). The HNHBH-Ni
(
II) complex was calculated to possess the highest binding energy
as listed in Table 2. The observed selectivity of the ligand towards
nickel compared to the other metal ions could be attributed to the
size that suits the octahedral type of coordination environment
resulting with the relatively least tilted orientation while binding
to the metal ion center (Fig. 6).
3.2.2. Solvent effects
The effect of polar solvents (acetonitrile and water) on the color
change has been also explored. Fig. S4 shows the UV–Vis spectrum of
the ligand-Ni complex in acetonitrile (ACN). The spectrum appears sim-
ilar to that of the complex in pure THF (Fig. 4). However, in the ACN sol-
vent, the selectivity of the ligand for nickel (II) was slightly improved
and yielded a darker yellowish-green solution than the case for the
THF solvent. Interestingly, the isosbestic points moved from 308 to
Table 3
Color observations for HNHBH ligand in a 50:50 THF and water mixture with individual metal salts and mixtures of metal salts at acidic and basic conditions.
Metal salt
Mg(OAc)
2
Ca(OAc)
2
Ni(acac)
2
Cu(OAc)
2
Zn(OAc)
2
Pd(OAc)
2
Mixed metal
salts
Mixed metal salts
without Ni(acac)
2
with Ni(acac)
2
pH 7
Very, very pale
green-yellow
Very, very pale
green-yellow
Pale
Very pale
Very, very pale
green-yellow
Extremely pale
green-yellow
Greenish-yellow Very, very pale
green-yellow
green-yellow green-yellow
Addition NaOH Pale green-yellow Pale green-yellow Bright
Pale
Pale green-yellow Pale green-yellow Bright yellow
Pale green-yellow
of
yellow
Pale
green-yellow
Colourless
HCl
Colourless
Colourless
Colourless
Colourless
Pale
Colourless
green-yellow
green-yellow

64
I. Abdulazeez et al. / Journal of Molecular Liquids 264 (2018) 58–65
HNHBH complex, remarkably negatively charged oxygen atoms of the
hydroxyl and carbonyl groups were calculated. This is in agreement
with the experimental observations in which the addition of a strong
acid would result in the discoloration of the complex due to the compe-
tent protonation of the carbonyl and hydroxyl oxygen atoms. A subse-
quent addition of a base turned the solution bright yellow as this
would deprotonate the binding sites and hence availed them to coordi-
nate with the metal ions.
3.2.4. EDTA effects
The influence of EDTA on the ligand-metal complex formation was
also investigated. A mixture of metal salts was first introduced to a
0% THF aqueous solution of HNHBH. The aqueous solution is 1 M of
Na -EDTA. The result of the absorption study is shown in Fig. S5. At
5
4
the acidic condition, a colourless solution was observed. While
with the basic solution, a bright yellow color was observed (which is
similar to the case without the addition of EDTA). At the basic condition,
EDTA alone caused a pale yellow color to occur without the presence of
HNHBH. These observations clearly indicate that at the basic
condition there is a competition between HNHBH and EDTA in coordi-
nating with nickel (II) ions, with more preference for the ligand over
EDTA.
A UV–Vis absorption test was conducted for a solution with only the
ligand and EDTA in order to find out if any sort of specific interaction
takes place between HNHBH and EDTA. The results presented in
Fig. S6 show the effect of EDTA on HNHBH. It was noted that two new
peaks emerged at 369 and 416 nm, and two isosbestic points were
observed at 269 and 344 nm. Such a change was found to be reversible
on addition of HCl (as shown in Fig. S7). This suggests that at an
acidic condition, the ligand is no longer bound to nickel (II) ions. As
such, the use of HCl offers the possibility of regenerating free ligands
from the complex, and this has a potential application in the develop-
ment of a reusable sensor. A summary of the UV–Vis data of the free
ligand, ligand-Ni complex and affecting parameters is presented in
Table S2.
Fig. 9. Graphs represent the absorbance against wavelength (λ/nm) for the hydrazone
ligand with Ni(acac) in a 50% THF mixed aqueous solution upon (a) the addition of
NaOH; and (b) upon consecutive additions of NaOH and HC. Some spectra were omitted
for the sake of clarity.
2
Fig. 10. Mulliken atomic charges of the HNHBH ligand complex with Ni2+ ion as calculated at the B3LYP/6-311 + G* level. The arrows indicate potential sites for possible protonation.

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