Synthesis, crystal structure, and characterization of two Cu(II) and Ni(II) complexes of a tetradentate N2O2 Schiff base ligand and their application in fabrication of a hydrazine electrochemical sensor

Article abstract of DOI:10.1016/j.ica.2020.119537

Two new Cu(II) and Ni(II) complexes of a tetradentate N₂O₂ Schiff base ligand (H₂L = N,N'-bis-(4-hydroxysalicylidene)-ethylenediamine) were synthesized. The structures of the H₂L ligand and its complexes (1 and 2) were unambiguously determined by elemental analysis, FT-IR, ¹H NMR, and UV–Vis spectroscopy. The crystal structures of the complexes were also determined by X-ray crystallography technique. In addition, the complexes were tested as a modifier for a glassy carbon electrode (GCE) to prepare an electrochemical sensor for detection of hydrazine in real samples. The prepared sensor with Ni(II) complex showed admirable sensitivity and selectivity for determination of hydrazine at 0.55 V without any interference of other common substrates. A dynamic linear range of 0.5–150 μM with a detection limit (LOD) of 166.66 nM and sensitivity of 0.1174 μA μM^?1 cm^?2 was achieved. The obtained LOD was much lower than that specified in the U.S. Environmental Protection Agency (EPA) regulations. The prepared sensor was also successfully applied to detect hydrazine in Tehran's Jajrood river water as a real sample.

Full text of DOI:10.1016/j.ica.2020.119537

InorganicaChimicaActa506(2020)119537
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Synthesis, crystal structure, and characterization of two Cu(II) and Ni(II)
complexes of a tetradentate N2O2 Schiff base ligand and their application in
fabrication of a hydrazine electrochemical sensor
Tahereh Hosseinzadeh Sanatkara, Alireza Khorshidia,⁎, Esmail Sohoulib, Jan Janczakc
a Department of Chemistry, Faculty of Sciences, University of Guilan, 41335-1914 Rasht, Guilan, Iran
b Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Iran
c Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 1410, 50-950 Wrocław, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords:
Schiff base
Tetradentate ligand
Electrochemical sensor
Hydrazine
Two new Cu(II) and Ni(II) complexes of a tetradentate N2O2 Schiff base ligand (H2L = N,N'-bis-(4-hydro-
xysalicylidene)-ethylenediamine) were synthesized. The structures of the H2L ligand and its complexes (1 and 2)
were unambiguously determined by elemental analysis, FT-IR, 1H NMR, and UV–Vis spectroscopy. The crystal
structures of the complexes were also determined by X-ray crystallography technique. In addition, the complexes
were tested as a modifier for a glassy carbon electrode (GCE) to prepare an electrochemical sensor for detection
of hydrazine in real samples. The prepared sensor with Ni(II) complex showed admirable sensitivity and se-
lectivity for determination of hydrazine at 0.55 V without any interference of other common substrates. A
Glassy carbon electrode
dynamic linear range of 0.5–150 µM with
a detection limit (LOD) of 166.66 nM and sensitivity of
0.1174 μA µM−1 cm−2 was achieved. The obtained LOD was much lower than that specified in the U.S.
Environmental Protection Agency (EPA) regulations. The prepared sensor was also successfully applied to detect
hydrazine in Tehran’s Jajrood river water as a real sample.
1. Introduction
[12].
Our motivation in this study was to prepare a tetradentate N2O2
Schiff bases are unique ligands that play an essential role in the
development of coordination chemistry. They are easily synthesized
and can be readily modified both electronically and sterically [1]. Schiff
bases form an outstanding class of ligands due to their unique proper-
ties such as stability in different conditions, diversity of donor sites,
synthetic flexibility, and formation of a wide range of complexes in
various coordination geometries. Among them, salen-type Schiff base
ligands are definitely appealing to coordination chemists. These are
tetradentate ligands with four donor sites (N2O2), and their complexes
have received much attention due to a wide range of applications such
as catalysis [2], electrochemistry [3], bioscience [4], optics [5], host-
guest chemistry [6], and molecular recognition [7]. As shown in
Scheme 1, the salen ligand provides an ideal environment for equatorial
coordination of transition metals, leaving two axial sites open for co-
ordination of extra ligands or substrates. So far, Cu(II) and Ni(II)
complexes of the salen ligand have been used as catalysts in many re-
actions such as oxidation of alcohols [8], epoxidation of olefins [9],
alkylation [10], arylation [11], and many other industrial reactions
Schiff base ligand (H2L, Scheme 1) from 2,4-dihydroxybenzaldehyde
and ethylenediamine. To investigate the coordination modes of this
ligand, two new mononuclear Cu(II) and Ni(II) complexes (1 and 2)
were synthesized and the structures of the ligand and its complexes
were determined by appropriate techniques. Single crystal X-ray ana-
lysis confirmed the tetradentate binding mode of the H2L ligand. To
study their potential application in detection of hazardous materials,
the Ni(II) complex was used as a modifier for a glassy carbon electrode
(GCE) to prepare an electrochemical sensor for detection of hydrazine.
Hydrazine is widely used in many industries. This compound is highly
toxic and is classified as a possible carcinogen with a threshold limit
value (TLV) of 10 ppb (312.06 nM) according to the U.S. Environmental
Protection Agency (EPA) regulations [13]. Thus, hydrazine detection is
of great importance from both environmental and human health
viewpoints. Incorporation of the Ni(II) complex into the glassy carbon
electrode (GCE) resulted in significant improvements in detection of
hydrazine in real samples.
⁎ Corresponding author.
E-mail address: khorshidi@guilan.ac.ir (A. Khorshidi).
https://doi.org/10.1016/j.ica.2020.119537
Received 5 January 2020; Received in revised form 15 February 2020; Accepted 16 February 2020
Availableonline18February2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

T. Hosseinzadeh Sanatkar, et al.
InorganicaChimicaActa506(2020)119537
Scheme 1. Chemical structure of the H2L ligand.
Table 1
Crystal data and structure refinement for complexes 1 and 2.
Parameter
1
2
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
C16H14CuN2O4·CH3CN·H2O
420.90
100(1)
Monoclinic
P21/c
C16H14N2NiO4·H2O
375.02
100(1)
Monoclinic
C2/c
10.2793 (3)
19.5904 (4)
9.6778 (3)
90
114.252 (4)
90
1776.88 (10)
4
1.573
19.5243 (8)
18.6443 (7)
10.1928 (4)
90
10.1928 (4)
90
3628.6 (3)
8
1.373
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
−3
Dx (Mg m
μ (mm
F (0 0 0)
)
−1
)
1.26
868
1.10
1552
Crystal size (mm)
θ range for data
collection (°)
0.28 × 0.24 × 0.21
3.1–29.1
0.23 × 0.18 × 0.11
3.0–29.0
Fig. 1. View of the molecular structure of (a) complex 1 and (b) complex 2. The
solvent molecules (water and acetonitrile in 1 and water in 2) have been
omitted for clarity. Displacement ellipsoids are drawn at the 50% probability
level.
Index ranges
–13 ≤ h ≤ 14
–26 ≤ k ≤ 26
–13 ≤ l ≤ 13
43,416
–26 ≤ h ≤ 26
–24 ≤ k ≤ 24
–12 ≤ l ≤ 13
31,822
Reflections collected
Independent
Table 2
4658 (0.044)
4678 (0.044)
Selected bond lengths (Å) and angles (°) for the complexes 1 and 2.
reflections (Rint
Reflections with
I > 2σ(I)
)
4054
3727
1 (M = Cu)
2 (M = Ni)
Data/restraints/
parameters
4658/0/250
4678/0/222
Bond lengths (Å)
M-N1
1.9374 (13)
1.8460 (17)
Goodness-of-fit on F2
Final R indexes
[I > 2σ (I)]
1.00
1.03
M-N2
M-O1
M-O4
1.9376 (12)
1.9284 (10)
1.9035 (10)
1.8475 (17)
1.8504 (14)
1.8676 (14)
R1 = 0.0274, wR2 = 0.0633
R1 = 0.0373,
wR2 = 0.0875
R1 = 0.0538,
wR2 = 0.0939
0.340/–0.275
Final R indexes [all
data]
R1 = 0.0360, wR2 = 0.0673
0.396/–0.283
Bond angles (°)
N1-M-N2
N1-M-O1
N1-M-O4
N2-M-O1
N2-M-O4
O1-M-O4
84.29 (5)
93.70 (5)
167.23 (5)
170.66 (5)
93.98 (5)
89.96 (4)
85.88 (8)
95.14 (7)
179.14 (7)
177.43 (7)
94.27 (7)
84.74 (6)
Largest difference in
peak/hole (e. Å
−3
)
2. Experimental
Table 3
2.1. Materials and instruments
Hydrogen-bond geometry (Å, °) in the crystal structure of 1 and 2.
D—H⋯Aa
D—H H⋯A D⋯A
D—H⋯A Symmetry
All of the chemicals including 2,4-dihydroxybenzaldehyde, ethyle-
nediamine, and hydrazine were purchased from Merck (http://www.
1
merck.com)
or
Sigma-Aldrich
(http://www.sigmaaldrich.com)
O1W—H1W1⋯N3 0.85
2.15
1.90
1.80
2.988 (2) 169
2.735 (2) 176
2.630 (2) 168
Companies. The reagents were of analytical grade and used without
further purification.
Elemental analyses were performed on a Heraeus CHN‐O‐Rapid
elemental analyzer. FT-IR spectra were recorded on a Bruker Alpha
spectrophotometer using KBr pellets over the range of 4000–400 cm‐1.
O2—H2A⋯O1i
O3—H3A⋯O1W
2
0.84
0.84
x, −y + 1/2, z + 1/2
x, −y + 1, z − 1/2
O2—H2A⋯O5
0.84
0.84
1.86
1.82
2.675 (3) 162
2.653 (2) 175
O3—H3A⋯O4i
UV–Vis spectra were recorded on
a
Rayleigh UV–1800 spectro-
a
D: Donor, A: Acceptor
photometer. 1H NMR spectra were recorded in DMSO‑d6 using tetra-
methylsilane as internal standard on a Bruker Avance III 300 MHz
spectrometer at 298 K.
Switzerland) driven by the Nova 2.1.2 software. A conventional three-
electrode setup consisting of a modified or unmodified GCE as a
working electrode, an Ag/AgCl (3.00 M KCl) reference electrode and a
Pt wire auxiliary electrode (Azar Electrode, Iran) were used. Some
All of the electrochemical measurements were carried out by a po-
tentiostat/galvanostat, µ-Autolab type II/FRA2 (Eco Chemie B. V,
2

T. Hosseinzadeh Sanatkar, et al.
InorganicaChimicaActa506(2020)119537
Fig. 2. Arrangement of (a) complex 1 and (b) complex 2. Dashed lines represent the OeH⋯O hydrogen bonds that stabilized the structural architecture.
electrochemical techniques such as cyclic voltammetry (CV), electro-
chemical impedance spectroscopy (EIS) and amperometry techniques
were used in the experiments. The EIS experiment was performed at a
frequency range of 0.01 Hz to 50 kHz and a 1:1 mixture of 5 mM [Fe
(CN)6]3@−/4− and 0.1 M KCl was utilized as the redox probe. 0.1 M
NaOH was used as an electrolyte in the experiments. A pH/mV meter
(Metrohm-691 pH-meter, Switzerland) was used for pH measurements.
for each image was used for data collection. One image was used as a
standard after every 40 images for monitoring of the crystal stability
and data collection, and no correction on the relative intensity variation
was necessary. Integration, scaling of the reflections, correction for
Lorenz and polarization effects and absorption corrections were per-
formed using the CrysAlis Red program [14]. The structures were
solved by direct methods using SHELXT [15] and refined using SHEXL-
2018 program [15]. The hydrogen atoms were introduced in their
geometrical positions and refined with isotropic displacement para-
meters. It was possible to localize and refine both solvent CH3CN and
H2O molecules in crystal 1, whereas for crystal 2 was possible to lo-
calize only the water molecule as a solvent. The Checkcif Platon Raport
of the cif file show additional solvent accessible VOID in the structure of
the crystal 2. The correct modelling of the disordered molecules was not
2.2. Crystal structure determination
Single-crystal X-ray data collection for the complexes 1 and 2 were
performed using graphite-monochromated Mo-Kα (λ = 0.71073 Å)
radiation on a four-circle к geometry KUMA KM-4 diffractometer with a
2D CCD area detector at 100 K. The ω-scan technique with Δω = 1.0°
3

T. Hosseinzadeh Sanatkar, et al.
InorganicaChimicaActa506(2020)119537
Fig. 3. Projection along c-axis of molecular packing with space filling of (a) complex 1 and (b) complex 2 showing the channels along c-axis.
Fig. 4. UV–Vis spectra of (A) H2L, (B) complex 1 and (C) complex 2 (20 µM in DMSO) at room temperature. Insets show the absorption of each complex at higher
wavelengths.
possible and we performed a “squeeze” treatment to remove the scat-
tering contribution of these molecules, which could not to be satisfac-
tory modelled. The final difference Fourier maps showed no peaks of
chemical significance. The data collection parameters, crystallographic
data, and final agreement parameters are collected in Table 1. The
structures were visualized by using the DIAMOND 3.0 [16]. The data
were deposited in Cambridge Crystallographic Data Center with de-
position number CCDC 1,954,841 for C16H14CuN2O4·CH3CN·H2O (1),
and 1,954,842 for C16H14N2NiO4·H2O (2).
1.0 mmol). The yellow color of the solution was gradually faded with
simultaneous precipitation of a purple microcrystalline compound. The
reaction mixture was stirred for 6 h at room temperature. The crude
product was then filtered off, washed with methanol and recrystallized
from methanol/acetonitrile. The solvent was partially evaporated, and
after three days several dark blue single crystals suitable for X-ray
single crystal analysis were collected. X-ray single crystal analysis
showed that the complex crystallizes as a solvate with water and
acetonityle molecules, Cu(C16H14N2O4).(CH3CN).(H2O). Yield: 0.324 g
(77% based on copper). IR (KBr pellet, cm‐1): 3440 (br, νOH), 1621 (vs,
ν
C=N), 1224 (s, νPhO), 520 (w, νCu-O), 427 (w, νCu-N). UV–Vis (DMSO):
λmax (nm) (ε, M−1 cm−1): 290 (39900), 348 (20600). Anal. Calc. for
18H19CuN3O5 (MW = 420.9 gmol−1): C, 51.36; N, 9.98; Cu, 15.10; O,
19.01 and H, 4.55%. Found: C, 51.14; N, 9.90, Cu, 15.22O, 19.28 and H
4.46%.
2.3. Synthesis
C
2.3.1. Synthesis of H2L
H2L was synthesized by condensation of ethylenediamine with 2,4-
dihydroxybenzaldehyde in methanol. Briefly, ethylenediamine
(0.13 mL, 2.0 mmol) was added dropwise to a solution of 2,4-dihy-
droxybenzaldehyde (0.55 g, 4.0 mmol) in 25 mL of methanol at room
temperature. The reaction mixture was then refluxed for 3 h with
continuous stirring. After cooling to room temperature, the yellow
precipitate was filtered and washed with methanol. Yield: 88%. 1H
NMR (300 MHz DMSO‑d6, ppm); δ: 13.58 (br, 2H7), 9.80 (br, 2H5), 8.37
(s, 2H2), 7.18 (d, J = 8.4 Hz, 2H3), 6.28 (dd, J = 8.4 and 2.4 Hz, 2H4),
6.27 (d, J = 2.4 Hz, 2H6), 3.79 (s, 4H1). IR (KBr pellet, cm‐1): 3416 (br,
2.3.3. Synthesis of complex 2
A methanolic solution (10 mL) of Ni(OAc)2·4H2O (0.25 g, 1.0 mmol)
was added dropwise to a methanolic solution (5 mL) of H2L (0.30 g,
1 mmol). The yellow color of the solution was gradually faded with
simultaneous precipitation of a dark orange microcrystalline com-
pound. After stirring for 6 h at room temperature, the crude product
was filtered off, and recrystallized from methanol. After four days
several orange single crystals suitable for X-ray single crystal analysis
were collected. Yield: 0.30 g (81% based on Ni(II)). IR (KBr pellet,
cm‐1): 3414 (br, νOH), 1620 (vs, νC=N), 1229 (s, νPhO), 568 (w, νNi-O),
434 (w, νNi-N). UV–Vis (DMSO): λmax (nm) (ε, M−1 cm−1): 270 (46200),
310 (16900), 392 (8400), 438 (4100). Anal. Calc. for C16H16N2NiO5
(MW = 375.02 g mol−1): C, 51.24; N, 7.48, Ni, 15.65; O, 21.33 and H,
4.30%;. Found: C, 51.40; N, 7.38; Ni, 15.52; O, 21.40 and H, 4.30%.
ν
OH), 1639 (vs, νC=N), 1234 (s, νPhO). UV–Vis (DMSO): λmax (nm) (ε,
M−1 cm−1): ~280 (25750), 308 (21100), 390 (2100). Anal. Calc. for
16H16N2O4 (MW = 300.31 g mol−1): C, 63.99; H, 5.37; N, 9.33.
C
Found: C, 63.87; H, 5.45; N, 9.41%.
2.3.2. Synthesis of complex 1
A methanolic solution (10 mL) of Cu(OAc)2·H2O (0.20 g, 1.0 mmol)
was added dropwise to a methanolic solution (5 mL) of H2L (0.30 g,
4

T. Hosseinzadeh Sanatkar, et al.
InorganicaChimicaActa506(2020)119537
Fig. 5. 1H NMR spectrum of the H2L salen ligand in DMSO‑d6.
2.4. Modification of the GCE with the Ni(II) complex
and N2-M-O4 angles of 93.98° and 94.27° for the complexes 1 and 2,
respectively. The N1-C8-C9-N2-M atoms form five-membered chelate
rings with the N1-M-N2 angles of 84.29° and 85.88° for the complexes 1
and 2, which are smaller than their analogues in the six-membered
rings [1]. The Cu2+ cation is deviated by ~0.157 Å from the average
plane defined by N2O2 of the chelate L2− ligand, whereas the deviation
of Ni2+ cation from this plane is only 0.039 Å. The whole complex
molecule (1 and 2, without H atoms) is almost planar. The C atoms of
both CH2 groups, however, are displaced up and down the average
plane through the all atoms of the L2− excluding both C atoms of the
methylene groups. The displacement of C8 and C9 atoms form the
average plane of the other atoms of L2− is 0.58 and −0.25 Å for the Cu-
complex, and −0.22 and 0.47 Å for the Ni-complex, respectively.
Any atoms of solvent molecules or adjacent units do not coordinate
to the metal (Cu, Ni) center at the axial sites, whereas the H2O and
A bare GCE was polished on an emery paper with some alumina
powder (0.05 µm). Then, the GCE was carefully sonicated in a bath of
deionized water and ethanol (1:1) to remove any absorbed particles.
Finally, the washed GCE was dried at room temperature. In the next
step, 1 mg of the Ni(II) complex was dispersed in 1 mL of ethanol
(containing 10 μL of nafion 0.2 wt%) for 1 h, and then 2 µL of the
obtained dispersion was dropped onto the cleaned GCE surface. The
GCE modified with the Ni(II) complex, denoted as the Ni(II) complex/
GCE, was air dried before use.
3. Results and discussion
3.1. Description of the crystal structures
CH3CN solvents in crystal
1 are hydrogen bonded together via
O1W—H1W1···N3 and further linked via OeH⋯O with the hydroxyl
group of CuL complex, and in crystal 2 the water molecule acts as ac-
ceptor in the OeH···O with hydroxyl group of NiL complex (Table 3).
Arrangement of molecules in the crystal 1 and 2 together with the
hydrogen bonding interactions is illustrated in Fig. 2a, b, respectively.
The discrete CuL in 1 and NiL in 2 chelates interact mainly by van der
Waals forces and further are connected by hydrogen bonds through
solvent molecules.
As can be seen from the Fig. 3, the packing of molecules in the unit
cell is less compact in 2 than in 1, because crystal 2 contains channels
available to the solvent. The additional solvent molecules (CH3OH) may
have been partially removed from the channels because the crystals
were allowed to dry at ambient temperature prior to the X-ray analysis.
The complexes 1 and 2 were crystallized in the monoclinic crystal
system with P21/c and C2/c space groups, respectively (Table 1). The
molecular structures of 1 and 2 are illustrated in Fig. 1. The selected
bond lengths and angles are presented in Table 2. In both of the com-
plexes, the deprotonated ligand L2− is tetradentate and the central
cation (Cu or Ni) is coordinated through four donor atoms to form the
coordination complexes with one five-membered and two six-mem-
bered chelate rings (6–5-6). Also, the central metal ions have square-
planar geometries with a slight distortion. Sum of the different angles
around the metal in the complexes 1 (361.93°) and 2 (360.03°) confirms
their slightly distorted square-planar geometries [17].
The O1-C1-C6-C7-N1-M and N2-C10-C11-C16-O4-M atoms form six-
membered chelate rings with the O1-M-N1 angles of 93.70° and 95.14°,
5

T. Hosseinzadeh Sanatkar, et al.
InorganicaChimicaActa506(2020)119537
Fig. 6. (a, b) SEM images of GCE and Ni(II) complex/GCE, (c) EDX pattern of Ni(II) complex/GCE, and (d) Mapping images of Ni(II) complex/GCE.
Therefore, these solvent molecules are highly disordered in channels
with partial occupation, and as was mentioned in the experimental part,
the correct modelling of the disordered molecules was not possible and
therefore the scattering contribution of these molecules was removed
by a SQUEEZE procedure. Presence of channels in the crystal structure
of 2 (as a MOF) can be useful for the study of the absorption capacity of
various gases or other pollutant molecules.
(II) complex (2) were 568 and 434 cm−1, respectively.
The electronic spectra of the ligand and its complexes (H2L, 1 and 2)
were recorded at room temperature (20 µM in DMSO, Fig. 4). The H2L
ligand showed its characteristic absorption bands at 280, 308, and
390 nm. The high intensity bands at 280 and 308 nm are attributable to
the π → π* and n → π* transitions of the aromatic rings and the azo-
methine groups, respectively. Also, the broad band at about 390 nm is
attributable to the intraligand charge-transfer. Upon formation of the
complexes, notably these bands were shifted to some extent, suggesting
that the nitrogen atom of the azomethine group is coordinated to the
metal ion. The distinct bands at 348 and 310 nm in the electronic
spectra of the complexes 1 and 2 are arising from the n → π* transitions
of the azomethine groups [19]. Moreover, high intensity bands at 290
and 270 nm (for 1 and 2, respectively) are associated with the aromatic
π → π* intraligand transitions [20]. The spectrum of the complex 2
showed a band at about 392 nm which may be assigned to the MLCT
transition from the metal-centered HOMO to the ligand centered LUMO
3.2. Spectroscopic characterizations
In the FT-IR spectrum of the H2L ligand, characteristic bands of the
(OeH) and (C]N)imine functional groups were observed at 3416 and
1639 cm−1, respectively. Likewise, in the FT-IR spectra of the com-
plexes 1 and 2, the broad peaks of the (OeH) moieties were observed at
3440 and 3414 cm−1, respectively. After complexation with Cu(II) and
Ni(II), however, the (C]N) stretching vibrations of 1 and 2 were ob-
served at 1621 and 1620 cm−1, which is a direct indication of the in-
volvement of the azomethine nitrogen in coordination [18]. In addi-
tion, the ν(Ph-O) was shifted to lower frequencies in both complexes,
which further confirms the complexation of Cu(II) and Ni(II). Moreover,
the FT-IR spectrum of complex 1 showed the ν(CueO) and ν(CueN) at
520 and 427 cm−1, respectively. The corresponding values for the Ni
(C]N(π*)) [21]. The Ni(II) complex is diamagnetic and the band
1
around 440 nm could be assigned to A1g
→
1B1g transition, consistent
with other square-planar Ni(II) complexes [22]. Both complexes
showed the anticipated d-d transitions at higher wavelengths (insets of
Fig. 4), attributable to their square planar geometries [23].
6

T. Hosseinzadeh Sanatkar, et al.
InorganicaChimicaActa506(2020)119537
Fig. 7. The recorded (a) CVs and (b) Nyquist plot of the GCE and Ni(II) complex/GCE in a 1:1 mixture of 5 mM [Fe(CN)6] 3−/[Fe(CN)6] 4− and 0.1 M KCl at a scan
rate of 50 mV s−1 in the frequency range of 0.01 Hz to 50 kHz and (c) the circuit model for Ni(II) complex/GCE.
Fig. 8. (a) CVs of the Ni(II) complex/GCE and GCE in 0.1 M NaOH in the absence and presence of 100 µM hydrazine. Inset: the CVs of the bare GCE in the absence
and presence of 100 µM hydrazine with higher magnification and (b) CVs of the Cu(II) complex/GCE in 0.1 M NaOH in presence and the absence of 100 µM
hydrazine.
1H NMR spectra of the H2L ligand was recorded at 298 K in
DMSO‑d6 (Fig. 5). The broad signals at 13.58 and 9.80 ppm can be
readily attributed to the phenolic OH7 and OH5 moieties. The aromatic
protons of the salen ligand resonated in the range of 6.18–7.18 ppm,
and unambiguously assigned according to their coupling constants (see
experimental). The expected singlet for the proton of the imine moiety
(H2) was observed at 8.37 ppm. The aliphatic protons (H1) were also
observed as a singlet at 3.79 ppm.
smooth, and free from any particles. On the other hand, non-uniform
dispersion of particles of different sizes was observed on the surface of
the modified electrode. Therefore, the SEM images show that the
electrode has been modified successfully. Also, the EDX and MAP
analyses reveal the presence of nitrogen, carbon, oxygen, and nickel
atoms on the electrode’s surface, which further confirms that the elec-
trode was coated by the nickel complex appropriately (Fig. 6c, d).
In order to study the potential applications of the synthesized
complexes, the Ni(II) complex was employed as a modifier for a glassy
carbon electrode (GCE) to prepare an electrochemical sensor for de-
tection of hydrazine. To study the properties of the modified electrode,
cyclic voltammograms of the GCE and Ni(II) complex/GCE in a 1:1
mixture of 5 mM [Fe(CN)6]3−/4− and 0.1 M KCl as the redox probe
were recorded. As seen in Fig. 7a, the GCE shows a pair of reversible
3.3. Electrochemical studies
SEM, EDX and MAP analyses were used to compare morphologies of
the modified and unmodified electrodes. The SEM images are shown in
Fig. 6a, b. The SEM image of the unmodified electrode is uniform,
7

T. Hosseinzadeh Sanatkar, et al.
InorganicaChimicaActa506(2020)119537
Fig. 9. (a) CVs recorded for the different amounts of Ni(II) complex in 0.1 M NaOH in presence of 100 µM hydrazine, and b) dependence of the Ip to the amount of
the Ni(II) complex.
Fig. 10. (a) CVs of the Ni(II) complex/GCE in different scan rates of 10, 20, 30, 50, 70, 90, 120 and 150 mV s−1 in a solution consisting of 0.1 M NaOH and 100 µM
hydrazine, (b) plot of the peak current vs square root of the scan rate, (c) anodic peak potential vs the logarithm of the scan rate and (d) variation of the scan rate
normalized current (Ip/v1/2) with scan rate.
redox peaks with a peak-to-peak difference (ΔEp) of 140 mV, while the
Ni(II) complex/GCE represents better reversibility with a higher current
and lower ΔEp value of 90 mV. EIS studies were used to investigate the
changes in the electrochemical behavior of the modified electrode and
the value of the electron transfer resistance (Rct) by monitoring of the
semicircular diameter of the curves [24,25]. To do so, the Nyquist
curves of the GCE and the Ni(II) complex/GCE in the range of 0.01 Hz
to 50 kHz were recorded (Fig. 7b). As expected, the semicircular dia-
meter of the Ni(II) complex/GCE is smaller than that of the bare GCE,
representing a decrease in the Rct value and an increase in the kinetics
8

T. Hosseinzadeh Sanatkar, et al.
InorganicaChimicaActa506(2020)119537
Fig. 11. (a) Amperometric response of the Ni(II) complex/GCE in presence of different concentrations of hydrazine including 0.5, 1, 5, 10, 15, 25, 30, 40, 50, 70, 90,
100 and 150 µM at 0.55 V in 0.1 M NaOH and (b) the relationship between the current response and hydrazine concentration.
Table 4
Comparison of the different electrochemical techniques for determination of hydrazine.
Modified Electrode
Technique
Sensitivity (μA µM−1 cm−2
)
LOD (µM)
Linear range (µM)
Refs.
Pd/CNF/GCE
Mn7O135H2O/α-MnO2/GCE
CoHCF-rGO/GCE
CV
CV
0.0012
0.1096
–
–
–
0.1722
–
0.0162
0.1
0.51
0.1174
2.9
2.1
0.069
3.07
0.1
0.5
1
11
10–4000
3–2800
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
Amperometry
Amperometry
LSV
Amperometry
Amperometry
CV
Amperometry
Amperometry
Amperometry
0.25–100
25–1000
0.5–1000
2.5–500
0–230
13.2–197
0.5–50
0.8–200
0.5–150
AuNP-GPE
AuNPs/poly(BCP)/CNT/GCE
Nano-Au/porous-TiO2/GCE
GCE-CoPc-(CoTPP)4
FePc-linked-MPyr-SAM
ZnO/SWCNT/GCE
0.17
0.25
0.166
[38]
[39]
This study
ZnO/Nf/Au/GCE
Ni(II) complex/GCE
of electron transfer on the electrode surface (Fig. 7c). These findings
represent that the Ni(II) complex may be a good choice in the electrode
modification process. Both CV and EIS techniques corroborate that the
modification process was successfully done.
To investigate the electrochemical behavior of the modified elec-
trode on the electrocatalysis of hydrazine, the CV technique was ap-
plied (Fig. 8). In the absence of hydrazine, the Ni(II) complex/GCE
showed a couple of well-defined Ni(II)/Ni(III) redox peaks in the po-
tential range of 0.2–0.8 V in 0.1 M NaOH, while there were not any
redox peaks for the bare GCE. Upon addition of 100 µM hydrazine to
the electrolyte solution, the Ni(II) complex/GCE indicated a significant
increase in the anodic peak current at the potential of +0.55 V and a
decrease in the cathodic peak current, whereas, the bare GCE indicated
no sharp signal. This behavior may be attributed to the oxidation of Ni
(II) to Ni(III), and reduction of Ni(III) to Ni(II) over the surface of Ni(II)
complex/GCE through the reaction between Ni species and hydrazine
[26,27]. The results obtained in this study are in agreement with the
previous reports on electrochemical behavior of hydrazine on the sur-
face of the modified electrodes [26,28,29].
Electrochemical behavior of hydrazine was also investigated at the
surface of the copper complex-modified electrode. Fig. 8b shows the
cyclic voltammogram of the copper modified electrode in 0.1 M NaOH
in presence and in the absence of hydrazine. In the absence of hy-
drazine, an oxidation-reduction response was observed for the copper
complex, which can be related to the oxidation of Cu to Cu(I) (or Cu(II))
and reduction of the product to Cu. In presence of hydrazine, the redox
potential of the copper complex was also shifted, but the difference was
noticeable when compared with the nickel complex. This suggests the
nickel complex as a better modifier.
Fig. 12. Amperometric current response of the Ni(II) complex/GCE against
10 µM hydrazine and 10 mM FeSO4, FeCl2, NaCl, CuNO3, and H2O2 in 0.1 M
NaOH at 0.55 V and 150 s−1
.
Table 5
Determination of hydrazine in the Tehran’s Jajrood river water.
Real sample
Added (μM)
Found (μM)
Recovery (%)
Tehran’s Jajrood river water
25
50
100
24.1
51.3
98.7
96.40
102.60
98.70
Ni(II) complex → Ni(III) complex + e−
9

T. Hosseinzadeh Sanatkar, et al.
InorganicaChimicaActa506(2020)119537
Ni(III) complex + N2H4 → Ni(III)N2H4 complex
The amount of the modifier used was also investigated. The results
showed that the modifier amount has no significant effect on the
electrochemical behavior of hydrazine. Therefore, 2 µL of the modifier
was selected as the optimum amount of the nickel complex for mod-
ification of the glassy carbon electrode (Fig. 9).
To further study the electrocatalysis of hydrazine onto the Ni(II)
complex/GCE surface, the scan rate effect was investigated by re-
cording the CVs of the modified electrode in the range of
10–150 mV s−1. As shown in Fig. 10a, the anodic peak potentials were
gradually shifted to more positive potentials upon increasing the scan
rate. This behavior confirms the kinetic limitation of the electro-
chemical reaction onto the modified electrode surface. It was also ob-
served that the anodic peak current of the hydrazine was linearly
changed with the square root of the scan rate (Fig. 10b). These findings
may be ascribed to the fact that the reaction is controlled by mass
transfer, and the electrocatalytic process is controlled by diffusion of
the hydrazine from the solution bulk to the modified electrode surface.
Furthermore, the slope of the Tafel line can be calculated from Eq. (1)
by considering the plot of the peak potential versus the logarithm of the
scan rate (Fig. 10c).
sharp amperometric signal was obtained by injecting 10 µM hydrazine
into the electrolyte solution, none of the other compounds produced a
significant signal even at a 103-fold concentration. Thus, the Ni(II)
complex/GCE can be introduced as a specific sensor for detection of
hydrazine in solution.
Finally, the practicality of the Ni(II)-modified electrod was eval-
uated towards determination of hydrazine in three different real sam-
ples from Tehran’s Jajrood river. The hydrazine concentration was
determined using the standard addition technique. As listed in Table 5,
the recovery values are in the range of 96.40–102.60%, corroborating
that the sensor has fantastic potential in detection of hydrazine in real
samples such as river water.
Ni(III)N2H4 complex + 4OH− → Ni(II) complex + N2 + 4H2O + 3e−
4. Conclusion
A tetradentate N2O2 Schiff base ligand (N,N'-bis-(4-hydroxy-salicy-
lidene)-ethylenediamine, H2L) was synthesized by condensation of 2,4-
dihydroxybenzaldehyde and ethylenediamine. The ligand was utilized
for preparation of two new mononuclear Cu(II) and Ni(II) complexes.
All of the products were characterized by appropriate techniques. The
Ni(II) complex was used to fabricate a Ni(II) complex/GCE as an elec-
trochemical sensor for determination of hydrazine. The sensor pre-
sented unique features such as low LOD, broad dynamic linear range,
specificity, high stability, and operational repeatability compared to the
other reported hydrazine sensors. These findings may extend the po-
tential applications of the synthesized Schiff-base complexes in various
fields such as electrochemistry.
2.3 RT
Ep = (b log ϑ)/2 + constant b =
(1 − α)nα F
(1)
where, all symbols have the normal meanings in the electrochemical
studies. Accordingly, the slope of the Tafel line was estimated to be
0.0593 assuming a single electron transfer for the rate-determining step
with the diffusion coefficient (α) equal to 0.58. The plot of the sweep
rate normalized current (Ip/ν1/2) versus sweep rate is an ECCat catalytic
process (Fig. 10d).
CRediT authorship contribution statement
Tahereh Hosseinzadeh Sanatkar: Methodology, Investigation,
Writing
- original draft. Alireza Khorshidi: Conceptualization,
In order to explore the analytical performance of the modified
electrode as a hydrazine sensor, different concentrations of hydrazine
were injected into the electrolyte solution and the amperometric signals
at the applied potential of 0.55 V were recorded. As can be seen in
Fig. 11a, the sensor offers an excellent applicability for detection of
hydrazine in the range of 500 nM to 150 µM. Accordingly, there is a
linear relationship between the amperometric signal and the con-
Methodology, Supervision. Esmail Sohouli: Electrochemistry. Jan
Janczak: Crystallography.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
centration of hydrazine under the regression equation of
I
(µA) 0.9886
=
0.1174 [hydrazine] (µM) – 0.0724, with R2
=
(Fig. 11b). The limit of detection (LOD) and sensitivity values of the
strategy were calculated to be 166.6 nM and 0.1174 (μA µM−1 cm−2).
This fantastic low LOD value and wide dynamic linear range reflects
the sensor‘s admirable potential in detection of hydrazine in compar-
ison to the other reported methods in the literature (Table 4). These
satisfactory results are attributed to the embedded interface sensing
based on the Ni(II) complex as a modifier, and indicate its great ap-
plicability in the electrochemistry studies.
Another attractive feature of this strategy is its high operational
stability for hydrazine detection, so that the signal current of the sensor
does not decrease significantly after 50 repetitive cycles in the elec-
trolyte solution (data not shown). Furthermore, to study the reprodu-
cibility of the results, five separate Ni(II) complex/GCEs were used to
measure hydrazine in a known 100 µM solution. The relative standard
deviation (RSD) of the measurements was found to be 2.15%, which
indicates that the sensor reproducibility is satisfactory (data not
shown). Also, the repeatability of the results was checked by using the
above mentioned solution for six times and a relative standard devia-
tion (RSD) of 1.85% was obtained which indicates the good repeat-
ability of the method for analysis of hydrazine (data not shown). An
important issue in sensor studies, is to determine specificity which is a
vital factor to evaluate the assay protocol.
Acknowledgement
We gratefully acknowledge financial supports of the Research
Council of University of Guilan.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2020.119537.
References
[1] B. Liu, J. Chai, S. Feng, B. Yang, Spectrochim. Acta A 140 (2015) 437–443, https://
doi.org/10.1016/j.saa.2015.01.012.
[2] M. Sedighipoor, A.H. Kianfar, G. Mohammadnezhad, H. Görls, W. Plass, Inorg.
Chim. Acta 476 (2018) 20–26, https://doi.org/10.1016/j.ica.2018.02.007.
[3] E. Deunf, E. Zaborova, S. Guieu, Y. Blériot, J.N. Verpeaux, O. Buriez, M. Sollogoub,
C. Amatore, Eur. J. Inorg. Chem. 29 (2010) 4720–4727, https://doi.org/10.1002/
ejic.200901208.
[4] N. Zhang, Y. Fan, G. Huang, D. Buac, C. Bi, Y. Ma, X. Wang, Z. Zhang, X. Zhang,
Q.P. Dou, Inorganica Chim. Acta 466 (2017) 478–485, https://doi.org/10.1016/j.
ica.2017.07.006.
[5] J. Cisterna, V. Dorcet, C. Manzur, I. Ledoux-Rak, J.R. Hamon, D. Carrillo,
Inorganica Chim. Acta 430 (2015) 82–90, https://doi.org/10.1016/j.ica.2015.02.
030.
[6] L. Martínez-Rodríguez, N.A. Bandeira, C. Bo, A.W. Kleij, Chem.: Eur. J. 21 (2015)
7144–7150, https://doi.org/10.1002/chem.201500333.
[7] K.S. Kashyap, A. Kumar, S.K. Hira, S. Dey, Inorganica Chim. Acta 498 (2019)
To study this factor and to minimize the false-positive responses,
several chemicals such as FeSO4, FeCl2, NaCl, CuNO3 and H2O2 were
tested under the optimized sensor working conditions (Fig. 12). While a
10

T. Hosseinzadeh Sanatkar, et al.
InorganicaChimicaActa506(2020)119537
119157–119163, https://doi.org/10.1016/j.ica.2019.119157.
[8] K.C. Weerasiri, A.E. Gorden, Tetrahedron 70 (2014) 7962–7968, https://doi.org/
10.1016/j.tet.2014.08.050.
org/10.1016/j.molcata.2011.11.017.
[24] M. Roushani, F. Shahdost-fard, Microchim. Acta 185 (2018) 214, https://doi.org/
10.1007/s00604-018-2709-6.
[9] Z. Abbasi, M. Behzad, A. Ghaffari, H.A. Rudbari, G. Bruno, Inorganica Chim. Acta
414 (2014) 78–84, https://doi.org/10.1016/j.ica.2014.01.047.
[10] Y.N. Belokon, R.G. Davies, J.A. Fuentes, M. North, T. Parsons, Tetrahedron Lett. 42
(2001) 8093–8096, https://doi.org/10.1016/S0040-4039(01)01718-X.
[11] A. Gogoi, G. Sarmah, A. Dewan, U. Bora, Tetrahedron Lett. 55 (2014) 31–35,
https://doi.org/10.1016/j.tetlet.2013.10.084.
[25] F. Shahdost-fard, M. Roushani, Sens. Actuators B Chem. 246 (2017) 848–853,
https://doi.org/10.1016/j.snb.2017.02.113.
[26] U.P. Azad, V. Ganesan, Electrochim. Acta 56 (2011) 5766–5770, https://doi.org/
10.1016/j.electacta.2011.04.051.
[27] X. Gu, X. Li, S. Wu, J. Shi, G. Jiang, G. Jiang, S. Tian, RSC Adv. 6 (2016) 8070–8078,
https://doi.org/10.1039/C5RA23809A.
[12] K.P. Bryliakov, Chem. Rev. 117 (2017) 11406–11459, https://doi.org/10.1021/acs.
chemrev.7b00167.
[28] L. Zheng, J.F. Song, Talanta 79 (2009) 319–326, https://doi.org/10.1016/j.talanta.
2009.03.056.
[13] G. Choudhary, H. Hansen, Chemosphere 37 (1998) 801–843, https://doi.org/10.
1016/S0045-6535(98)00088-5.
[14] CrysAlis CCD and CrysAlis Red 1.171.38.41, Rigaku Oxford Diffraction, 2015.
[15] G.M. Sheldrick, Acta Cryst. A 71 (2015) 3–8, https://doi.org/10.1107/
S2053273314026370.
[29] M.R. Majidi, A. Jouyban, K. Asadpour-Zeynali, Electrochim. Acta 52 (2007)
6248–6253, https://doi.org/10.1016/j.electacta.2007.04.019.
[30] H. Zhang, J. Huang, H. Hou, T. You, Electroanalysis 21 (2009) 1869–1874, https://
doi.org/10.1002/elan.200904630.
[31] J. Wu, T. Zhou, Q. Wang, A. Umar, Sens. Actuators B Chem. 224 (2016) 878–884,
[16] K. Brandenburg, H. Putz, DIAMOND Version 3.0, Crystal Impact GbR, Bonn,
Germany, 2006.
[17] P. Bhowmik, M.G. Drew, S. Chattopadhyay, Inorganica Chim. Acta 366 (2011)
62–67, https://doi.org/10.1016/j.ica.2010.10.010.
[18] L.H. Li, X.L. Feng, X.H. Cui, Y.X. Ma, S.Y. Ding, W. Wang, J. Am. Chem. Soc. 139
(2017) 6042–6045, https://doi.org/10.1021/jacs.7b01523.
[19] M. Khorshidifard, H.A. Rudbari, B. Askari, Polyhedron 95 (2015) 1–13, https://doi.
org/10.1016/j.poly.2015.03.041.
[20] A.H. Kianfar, L. Keramat, M. Dostani, M. Shamsipur, M. Roushani, F. Nikpour,
Spectrochim. Acta A 77 (2010) 424–429, https://doi.org/10.1016/j.saa.2010.06.
008.
https://doi.org/10.1016/j.snb.2015.09.016.
[32] X. Luo, J. Pan, K. Pan, Y. Yu, A. Zhong, S. Wei, J. Li, J. Shi, X. Li, J. Electroanal.
Chem. 745 (2015) 80–87, https://doi.org/10.1016/j.jelechem.2015.03.017.
[33] M.A. Aziz, A.N. Kawde, Talanta 115 (2013) 214–221, https://doi.org/10.1016/j.
talanta.2013.04.038.
[34] S. Koçak, B. Aslışen, Sens. Actuators B Chem. 196 (2014) 610–618, https://doi.org/
10.1016/j.snb.2014.02.061.
[35] G. Wang, C. Zhang, X. He, Z. Li, X. Zhang, L. Wang, B. Fang, Electrochim. Acta 55
(2010) 7204–7210, https://doi.org/10.1016/j.electacta.2010.07.053.
[36] K. Ozoemena, Sensors 6 (2006) 874–891, https://doi.org/10.3390/s6080874.
[37] K.I. Ozoemena, T. Nyokong, Talanta 67 (2005) 162–168, https://doi.org/10.1016/
j.talanta.2005.02.030.
[21] R. Klement, F. Stock, H. Elias, Polyhedron 18 (1999) 3617–3628, https://doi.org/
10.1016/S0277-5387(99)00291-0.
[22] C. Natarajan, P. Tharmaraj, R. Murugesan, J. Coord. Chem. 26 (1992) 205–213,
https://doi.org/10.1080/00958979209409214.
[38] K.N. Han, C.A. Li, M.P.N. Bui, X.H. Pham, G.H. Seong, Chem. Commun. 47 (2011)
938–940, https://doi.org/10.1039/C0CC03848B.
[39] Y. Ni, J. Zhu, L. Zhang, J. Hong, CrystEngComm 12:2213-2218. https://doi.org/10.
1039/B923857N.
[23] S. Khare, R. Chokhare, J. Mol. Catal. A: Chem. 353 (2012) 138–147, https://doi.
11