Geometrical structure, molecular docking, potentiometric and thermodynamic studies of 3-aminophenol azodye and its metal complexes

Article abstract of DOI:10.1016/j.molliq.2015.06.010

The proton-ligand dissociation constants of 4-(2,3-dimethyl-1-phenylpyrazol-5-one azo)-3-aminophenol (HL) and its metal stability constants with Mn(II), Co(II), Ni(II) and Cu(II) ions have been determined using potentiometric studies. The molecular struct

Full text of DOI:10.1016/j.molliq.2015.06.010

Journal of Molecular Liquids 209 (2015) 625–634
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Geometrical structure, molecular docking, potentiometric and
thermodynamic studies of 3-aminophenol azodye and its
metal complexes
b
a
b
a,1
A.Z. El-Sonbati a, , G.G. Mohamed , A.A. El-Bindary , W.M.I. Hassan , A.K. Elkholy
⁎
a
Chemistry Department, Faculty of Science, Damietta University, Damietta 34517, Egypt
Chemistry Department, Faculty of Science, Cairo University, Giza 12613, Egypt
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The proton–ligand dissociation constants of 4-(2,3-dimethyl-1-phenylpyrazol-5-one azo)-3-aminophenol (HL)
and its metal stability constants with Mn(II), Co(II), Ni(II) and Cu(II) ions have been determined using potentio-
metric studies. The molecular structure of the ligand is optimized theoretically and the quantum chemical param-
eters are calculated. The proton–ligand dissociation constants of HL and its metal stability constants with Mn(II),
Co(II), Ni(II) and Cu(II) have been determined potentiometrically. The potentiometric studies were carried out in
0.1 M KCl and 20% (by volume) DMF–water mixture. At constant temperature the stability constants of the
formed complexes decrease in the order of Cu(II) N Ni(II) N Co(II) N Mn(II). The effect of temperature was studied
at 298, 308 and 318 K and the corresponding thermodynamic parameters (ΔG, ΔH and ΔS) were derived and
discussed. The dissociation process is non-spontaneous, endothermic and entropically unfavorable. The forma-
tion of the metal complexes has been found to be spontaneous, endothermic and entropically favorable. Molec-
ular docking was used to predict the binding between azodye ligand and the receptor of prostate cancer mutant
2q2k-Hormon and receptor of breast cancer mutant 3hb5-Oxidoreductase.
Received 14 April 2015
Received in revised form 31 May 2015
Accepted 2 June 2015
Available online xxxx
Keywords:
3-Aminophenol azodye
Potentiometry study
Stability constants
Thermodynamics parameters
Quantum chemical parameters
Molecular docking
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
used to predict protein–ligand [8,9] and to screen large libraries for mol-
ecules that will modulate the activity of a biological receptor.
Azo compounds containing heterocyclic moieties have drawn the at-
tention of many researchers [1–5]. It has been well established that the
use of heterocyclic amines with oxygen as the π-excessive hetero atom
as diazo component has a marked bathochromic effect compared to
analogous dyes derived from benzenoid compounds [6]. Azo derivatives
containing antipyrine moiety have many advantages including color de-
pending effect as an intrinsic property leading to better dye ability. The
color of these azo derivatives depends on the nature of both the diazo
and the coupling components. Majority of the azo compounds are de-
rived from the coupling of diazotized heterocyclic amines with aromatic
hydroxyl and amino compounds. The position of azo and hydroxyl
groups in these molecules brings into play the azo-hydrazone equilibri-
um [7]. The use of protein–ligand docking has become a standard meth-
od in potentiometric studies. The protein groups surrounding the ligand
can highly influence the local pH, so that a different protonation could
be favored in the bound state. To account for this effect, the ideal case
would be to use multiple protonations in the docking and have the algo-
rithm automatically pick the correct state. Molecular docking is widely
In this paper, the potentiometric studies are used to determine the
dissociation constants of 4-(2,3-dimethyl-1-phenylpyrazol-5-one
azo)-3-aminophenol ligand (HL) and the stability constants of its com-
plexes with some divalent transition metal ions such as Mn(II), Co(II),
Ni(II) and Cu(II) at different temperatures. The molecular structure of
the investigated ligand (HL) is studied and quantum chemical parame-
ters are calculated. Moreover, the corresponding thermodynamic func-
tions are calculated and discussed.
2. Materials and methods
2.1. Preparation of the ligand
4-(2,3-Dimethyl-1-phenylpyrazol-5-one azo)-3-aminophenol li-
gand (HL) was prepared previously [2–5,10] by coupling an equimolar
amount of 1-phenyl-2,3-dimethyl-4-amino pyrazol-5-one and 3-
aminophenol as shown in Scheme 1. In a typical preparation, 25 ml
of distilled water containing 0.01 mol hydrochloric acid was added to
1-phenyl-2,3-dimethyl-4-amino pyrazol-5-one (0.01 mol). To the
resulting mixture stirred and cooled to 0 °C, a solution of 0.01 mol sodi-
um nitrite in 20 ml of water was added dropwise. The formed diazoni-
um chloride was consecutively coupled with an alkaline solution of
⁎
Corresponding author.
E-mail address: elsonbatisch@yahoo.com (A.Z. El-Sonbati).
Abstracted from her Ph.D. Thesis.
1
http://dx.doi.org/10.1016/j.molliq.2015.06.010
0167-7322/© 2015 Elsevier B.V. All rights reserved.

626
A.Z. El-Sonbati et al. / Journal of Molecular Liquids 209 (2015) 625–634
Scheme 1. The formation mechanism of the azo ligand (HL).
0.01 mol 3-aminophenol, in 10 ml of pyridine. Immediately, the deep
purple precipitate formed was filtered through sintered glass crucible,
washed several times by distilled water. The crude product was purified
by recrystallization from hot ethanol, (yield ~68%) then dried in a vacu-
um desiccator over anhydrous P2O5. The structure of the formed ligand
(HL) was established by IR, 1H NMR and X-ray spectroscopies.
2.2. Potentiometric studies
A ligand solution (0.01 M) was prepared by dissolving an accurately
weighed amount of the solid in DMF. Metal ion solutions (0.001 M)
were prepared from metal chloride salts in bidistilled water and stan-
dardized with EDTA [11]. Solutions of 0.005 M HCl and 1 M KCl were
Fig. 1. 1H NMR spectrum of HL ligand

A.Z. El-Sonbati et al. / Journal of Molecular Liquids 209 (2015) 625–634
627
CuKα radiation (λ = 1.54056 Å). The applied voltage and the tube cur-
rent are 40 kV and 25 mA, respectively.
The molecular structure of the investigated compound was opti-
mized initially with the PM3 semiempirical method so as to speed
up the calculations. The resulting optimized structures were fully
re-optimized using ab initio Hartree–Fock (HF) [16] with 6-31G
basis set. The molecules were built with the Gauss View 3.09 and op-
timized using Gaussian 03W program [17]. The corresponding ge-
ometries were optimized without any geometry constraints for full
geometry optimizations. Frequency calculation was executed suc-
cessfully, and no imaginary frequency was found, indicating minimal
energy structures.
In the study simulates the actual docking process in which the
ligand-protein interaction energies are calculated using a Docking Serv-
er [18]. The MMFF94 Force field was for used energy minimization of li-
gand molecule using a Docking Server. Gasteiger partial charges were
added to the ligand atoms. Non-polar hydrogen atoms were merged,
and rotatable bonds were defined. Docking calculations were carried
out on 2q2k-Hormone and 3hb5-Oxidoreductase protein models.
Essential hydrogen atoms, Kollman united atom type charges, and
solvation parameters were added with the aid of AutoDock tools [19].
Affinity (grid) maps of ×× Å grid points and 0.375 Å spacing were gen-
erated using the Autogrid program [20]. Auto Dock parameter set- and
distance-dependent dielectric functions were used in the calculation
of the van der Waals and the electrostatic terms, respectively.
The pH measurements were carried out using a VWR Scientific
Instruments Model 8000 pH-meter accurate to 0.01 units. The pH-
meter readings in the non-aqueous medium were corrected [21]. The
electrode system was calibrated according to the method of Irving
et al. [22]. Titrations were performed in a double walled glass cell in
an inert atmosphere (nitrogen) at an ionic strength of 0.1 M KCl. Poten-
tiometric measurements were carried out at different temperatures.
The temperature was controlled to within 0.05 K by circulating
thermostated water (Neslab 2 RTE 220) through the outer jacket of
the vessel.
Fig. 2. X-ray diffraction pattern of HL powder form.
also prepared in bidistilled water. A carbonate-free NaOH solution in
20% (by volume) DMF–water mixture was used as titrant and standard-
ized against oxalic acid.
The apparatus, general conditions and methods of calculation were
the same as in the previous work [2,3,12–15]. The following mixtures
(i)–(iii) were prepared and titrated potentiometrically at 298 K against
standard 0.02 M NaOH in a 20% (by volume) DMF–water mixture:
i) 5 cm3 0.005 M HCl + 5 cm3 1 M KCl + 10 cm3 DMF.
ii) 5 cm3 0.005 M HCl + 5 cm3 1 M KCl + 5 cm3 0.01 M
ligand + 5 cm3 DMF.
iii) 5 cm3 0.005 M HCl + 5 cm3 l M KCl + 5 cm3 0.01 M
ligand + 10 cm3 0.001 M metal chloride + 5 cm3 DMF.
For each mixture, the volume was made up to 50 cm3 with bidistilled
water before the titration. These titrations were repeated for the tem-
peratures of 308 and 318 K. All titrations have been carried out between
pH 3.0–12.0 and under nitrogen atmosphere.
3. Results and discussion
2.3. Measurements
3.1. Characterization of the ligand (HL)
All the compounds and solvents used were purchased from Aldrich
and Sigma and used as received without further purification. The IR
spectra were recorded as KBr discs using a Perkin-Elmer 1340 spectro-
photometer. The 1H NMR spectrum was obtained with a JEOL FX90
Fourier transform spectrometer with DMSO-d6 as the solvent and TMS
as an internal reference. X-ray diffraction analysis of the ligand (HL)
powder form was recorded on an X-ray diffractometer in the range of
diffraction angle 2θ° = 5–60°. This analysis was carried out using
3.1.1. IR spectra
The infrared spectrum of HL ligand gives interesting results and con-
clusions. The ligand gives a broad absorption band located at 3210 cm−1
assigned to υ(OH). The strong band at 1620 cm−1 is due to carbonyl
stretching vibration mode. The frequency for the N_N stretching vibra-
tion was located at 1490 cm−1. The region between 1219 and
1323 cm−1 is due C–OH stretching, C–OH in plane or out of plane bend-
ing and out-of-plane C–OH bending vibrations.
Fig. 3. The optimized structure of HL ligand within numbering of atoms.

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A.Z. El-Sonbati et al. / Journal of Molecular Liquids 209 (2015) 625–634
3.1.2. 1H NMR spectra
structure of the HL ligand is presented in Fig. 3. Selected geometric pa-
rameters bond lengths and bond angles of HL ligand are tabulated in
Tables 1 and 2. The HOMO and LUMO are shown in Fig. 4. Quantum
chemical parameters of the ligand are obtained from calculations such
as energies of the highest occupied molecular orbital (EHOMO) and the
lowest unoccupied molecular orbital (ELUMO) as listed in Table 3. Addi-
tional parameters such as HOMO–LUMO energy gap, ΔE, absolute
electronegativities, χ, chemical potentials, Pi, absolute hardness, η, abso-
lute softness, σ, global electrophilicity, ω, global softness, S, and addi-
tional electronic charge, ΔNmax are calculated using the following
Eqs. (1)–(8) [3,10,12]:
The 1H NMR spectrum of the ligand was recorded in DMSO-d6 at
room temperature which supports the occurrence of the form depicted
in Fig. 1. The _C–CH3 and–N–CH3 protons were observed as singlet at δ
2.47 and 2.60 ppm, respectively. In the aromatic region, a few doublets
and in few cases some overlapping doublets/multiplets are observed in
the range of δ 6.10–7.6 ppm. Another singlet corresponding to one pro-
ton for compound is observed in the range of δ ~9.59 and 13.03 ppm.
This signal disappeared when a D2O exchange experiment was carried
out. It can be assigned to OH and NH (Scheme 1). The absence of –CH
(3.85 ppm) proton signal of the ligand (HL) moiety indicated the exis-
tence of the ligand in the azo-enol form. According to El-Sonbati et al.
[10], hydrogen bonding leads to a large deshielding of the protons.
ΔE ¼ ELUMO−EHOMO
ð1Þ
ð2Þ
3.1.3. X-ray diffraction
−ðEHOMO þ ELUMO
Þ
χ ¼
The X-ray diffraction, XRD, and pattern of the ligand (HL) is shown in
Fig. 2. The XRD pattern of the ligand (HL) has sharp diffraction peaks at
around 2θ = 10–20° and 2θ = 25–30°. This indicates that HL is a mix-
ture of crystalline and amorphous phases.
2
ELUMO−EHOMO
η ¼
ð3Þ
2
3.2. Geometrical structure
1
σ ¼
η
ð4Þ
ð5Þ
ð6Þ
Geometrical structures and electronic properties of the investigated
compounds and their protonated forms were calculated by optimizing
their bond lengths, bond angles and dihedral angles. The optimized
Pi ¼ −χ
1
S ¼
2η
Table 1
Selected geometric bond lengths of the ligand (HL).
Bond lengths (Å)
Table 2
Selected geometric bond angles of the ligand (HL).
R(N1,N2)
R(N1,C5)
R(N1,C34)
R(N2,C3)
R(N2,C23)
R(C3,C4)
R(C3,O6)
R(C4,C5)
R(C4,N7)
R(C5,C38)
R(N7,N8)
1.4107
1.3869
1.4702
1.4168
1.421
1.4659
1.2151
1.3742
1.3845
1.4919
1.275
1.3843
1.4306
1.4094
1.3935
1.3366
1.3978
1.0843
1.4148
1.3826
1.3761
1.0841
1.0839
0.9948
1.0077
1.0079
1.3984
1.3985
1.3915
1.0809
1.3945
1.084
Bond angles (°)
A(N2,N1,C5)
107.3827
114.0398
119.348
A(N2,C23,C28)
A(C24,C23,C28)
A(C23,C24,C25)
A(C23,C24,H29)
A(C25,C24,H29)
A(C24,C25,C26)
A(C24,C25,H30)
A(C26,C25,H30)
A(C25,C26,C27)
A(C25,C26,H31)
A(C27,C26,H31)
A(C26,C27,C28)
A(C26,C27,H32)
A(C28,C27,H32)
A(C23,C28,C27)
A(C23,C28,H33)
A(C27,C28,H33)
A(N1,C34,H35)
A(N1,C34,H36)
A(N1,C34,H37)
A(H35,C34,H36)
A(H35,C34,H37)
A(H36,C34,H37)
A(C5,C38,H39)
A(C5,C38,H40)
A(C5,C38,H41)
A(H39,C38,H40)
A(H39,C38,H41)
A(H40,C38,H41)
121.1253
A(N2,N1,C34)
A(C5,N1,C34)
A(N1,N2,C3)
A(N1,N2,C23)
A(C3,N2,C23)
A(N2,C3,C4)
A(N2,C3,O6)
A(C4,C3,O6)
A(C3,C4,C5)
A(C3,C4,N7)
120.1385
119.4982
119.5256
120.9663
120.6918
119.2412
120.0653
119.5084
120.2561
120.2351
120.4333
120.1601
119.4005
119.7167
119.7168
120.5581
111.1379
109.1582
108.7584
109.3433
109.8082
108.5901
111.1438
111.143
109.5938
119.1681
123.8443
104.3904
124.9891
130.5904
108.4509
118.1529
133.314
R(N8,C9)
R(C9,C10)
R(C9,C14)
R(C10,C11)
R(C10,O15)
R(C11,C12)
R(C11,H20)
R(C12,C13)
R(C12,N17)
R(C13,C14)
R(C13,H21)
R(C14,H22)
R(O15,H16)
R(N17,H18)
R(N17,H19)
R(C23,C24)
R(C23,C28)
R(C24,C25)
R(C24,H29)
R(C25,C26)
R(C25,H30)
R(C26,C27)
R(C26,H31)
R(C27,C28)
R(C27,H32)
R(C28,H33)
R(C34,H35)
R(C34,H36)
R(C34,H37)
R(C38,H39)
R(C38,H40)
R(C38,H41)
A(C5,C4,N7)
A(N1,C5,C4)
109.8282
119.9292
130.2421
117.9326
115.9037
125.4225
116.4365
118.1407
119.5431
122.1014
118.3555
121.1588
117.9236
120.9171
119.53
A(N1,C5,C38)
A(C4,C5,C38)
A(C4,N7,N8)
A(N7,N8,C9)
A(N8,C9,C10)
A(N8,C9,C14)
A(C10,C9,C14)
A(C9,C10,C11)
A(C9,C10,O15)
A(C11,C10,O15)
A(C10,C11,C12)
A(C10,C11,H20)
A(C12,C11,H20)
A(C11,C12,C13)
A(C11,C12,N17)
A(C13,C12,N17)
A(C12,C13,C14)
A(C12,C13,H21)
A(C14,C13,H21)
A(C9,C14,C13)
A(C9,C14,H22)
A(C13,C14,H22)
A(C10,O15,H16)
A(C12,N17,H18)
A(C12,N17,H19)
A(H18,N17,H19)
A(N2,C23,C24)
109.2703
108.3535
106.9915
109.8451
120.455
119.973
1.3933
1.0838
1.3929
1.0842
1.0829
1.096
1.0886
1.0884
1.0961
1.0923
1.0875
119.5561
119.8419
120.6013
122.071
117.3025
120.6265
107.3641
117.4924
117.5924
114.2606
118.7342

A.Z. El-Sonbati et al. / Journal of Molecular Liquids 209 (2015) 625–634
629
HOMO
LUMO
Fig. 4. HOMO and LUMO molecular orbitals of ligand (HL).
Pi2
2η
Table 3
ω ¼
ð7Þ
ð8Þ
The calculated quantum chemical parameters of the ligand (HL).
Parameter
EHOMO (a.u.)
ELUMO (a.u.)
μ (D)
−0.19739
−0.06899
4.529
Pi
η
ΔNmax ¼ −
:
T.E (a.u)
ΔE (a.u.)
χ (a.u.)
−1082.473
0.1284
0.1331
0.0642
15.578
−0.1331
7.7888
The HOMO–LUMO energy gap, ΔE, which is an important stability
index, is applied to develop theoretical models for explaining the struc-
ture and conformation barriers in many molecular systems. Recently,
the energy gap has been used to prove the reactivity and stability of
the compounds [3,12]. The dipole moment, μ, the first derivative of
the energy with respect to an applied electric field, was used to discuss
and rationalize the structure [3,12].
η (a.u.)
σ (a.u.)−1
Pi (a.u.)
S (a.u.)−1
ω (a.u.)
0.1379
2.074
ΔNmax
Table 4
Energy values obtained in docking calculations of ligand (HL) with receptor prostate cancer mutant 2q2k and breast cancer mutant 3hb5.
Receptor
Est. free energy of binding
(kcal/mol)
Est. inhibition constant
(Ki) (μM)
vdW+ bond + desolv energy
(kcal/mol)
Electrostatic energy
(kcal/mol)
Total intercooled energy
(kcal/mol)
Interact
surface
2q2k
3hb5
−2.99
−3.60
6.40
2.28
−4.90
−4.63
+0.07
−0.02
−4.83
−4.65
492.401
506.328

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A.Z. El-Sonbati et al. / Journal of Molecular Liquids 209 (2015) 625–634
Fig. 5. Fig. 5. The ligand (HL) (green in (A) and blue in (B)) in interaction with receptor prostate cancer mutant 2q2k. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article).
3.3. Molecular docking
breast cancer in India is expected to be more than 90,000 in the coming
years and over 50,000 women die each year.
Cancer can be described as the uncontrolled growth of abnormal
cells. Prostate cancer is the most common non-skin malignancy in
men. Except for lung cancer, it is responsible for more deaths than any
other cancer. The American Cancer Society (ACS) estimated 1 man in
6 will be diagnosed with prostate cancer during his lifetime. A little
over 1.8 million men in the United States are survivors of prostate can-
cer [23].
Breast cancer is one of the most recurring worldwide diagnosed and
deadliest cancers next to lung cancer with a high number of mortality
rates among females [24]. At global level, it accounted for more than
1.6 million new cases in 2010. The incidence or prevalence rate of the
Molecular docking is a key tool in computer drug design [25]. The
focus of molecular docking is to simulate the molecular recognition pro-
cess. Molecular docking aims to achieve an optimized conformation for
both the protein and drug with relative orientation between them such
that the free energy of the overall system is minimized.
In this context, we used molecular docking between ligand (HL) and
prostate cancer mutant 2q2k-Hormone and breast cancer (3hb5). The
results showed a possible arrangement between ligand (HL) and recep-
tor (2q2k, 3hb5). The docking study showed a favorable interaction be-
tween ligand (HL) and the receptor (2q2k, 3hb5) and the calculated
energy is listed in Table 4 and Figs. 5(a, b) and 6(a1, b1) for receptors
Fig. 6. The ligand (HL) (green in (a1) and blue in (b1)) in interaction with receptor breast cancer mutant 3hb5. (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article).

A.Z. El-Sonbati et al. / Journal of Molecular Liquids 209 (2015) 625–634
631
Fig. 7. HB plot of interaction between ligand (HL) with receptor a) prostate cancer mutant 2q2k and b) breast cancer mutant 3hb5.
2q2k and 3hb5, respectively. According to the results obtained in this
study, HB plot curve indicates that, the ligand (HL) binds to the two pro-
teins with hydrogen bond interactions and decomposed interaction en-
ergies in kcal/mol exist between the of ligand (HL) and receptors (2q2k
and 3hb5) as shown in Fig. 7. The calculated efficiency is favorable
where Ki values estimated by AutoDock were compared with experi-
mental Ki values, when available, and the Gibbs free energy is negative.
Also, based on this data, we can propose that interaction between the
2q2k and 3hb5 receptors and the ligand (HL) is possible. 2D plot curves
of docking with ligand (HL) are shown in Fig. 8. This interaction could
activate apoptosis in cancer cell energy of interactions with ligand
(HL). Binding energies are most widely used as a mode of measuring
binding affinity of a ligand. Thus, a decrease in binding energy due to
mutation will increase the binding affinity of the azo dye ligand towards
the receptor. The characteristic feature of azodye ligand was represent-
ed in the presence of several active sites available for hydrogen bonding.
This feature gives them the ability to be good binding inhibitors to the
protein and will help to produce augmented inhibitory compounds.
Fig. 8. 2D plot of interaction between ligand (HL) with receptor a) prostate cancer mutant 2q2k and b) breast cancer mutant 3hb5.

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A.Z. El-Sonbati et al. / Journal of Molecular Liquids 209 (2015) 625–634
12
11
10
9
Table 5
Thermodynamic functions for the dissociation of HL in 20% (by volume) DMF–water mix-
ture in the presence of 0.1 M KCl at different temperatures.
Temperature
(K)
Dissociation
Constant
(pKH)
Free energy
Enthalpy
Entropy change
change (ΔG)
change (ΔH)
(−ΔS)
(kJ mol−1
)
(kJ mol−1
)
(J mol−1 K−1
)
298
308
318
9.40
9.23
9.01
53.63
54.43
54.86
35.32
61.46
62.05
61.45
8
HCl
Hl
7
Mn2+
Co2+
Ni2+
where Y is the number of available protons in HL ligand (Y = 1) and V1
and V2 are the volumes of alkali required to reach the same pH on the
titration curve of hydrochloric acid and reagent, respectively, V° is the
initial volume (50 cm3) of the mixture, TC°L is the total concentration
of the reagent, N° is the normality of sodium hydroxide solution and
E° is the initial concentration of the free acid. Thus, the formation curves
(nA vs. pH) for the proton–ligand systems were constructed and found
to extend between 0 and 1 in the nA scale shown in Fig. 10. This
means that azo of 3-aminophenol has one ionizable proton. Different
computational methods were applied to evaluate the dissociation con-
stant [26]. Three replicate titrations were performed and the average
values obtained are listed in Table 5. The completely protonated form
of ligand (HL) has one dissociable proton, that dissociates in the mea-
surable pH range.
The formation curves for the metal complexes were obtained by
plotting the average number of ligand attached per metal ion (nA) vs.
the free ligand exponent (pL), according to Irving and Rossotti [27].
The average number of the reagent molecules attached per metal ion,
nA, and free ligand exponent, pL, can be calculated using Eqs. (10) and
(11):
6
5
Cu2+
4
3
0
1
2
3
4
5
6
7
V, ml added NaOH
Fig. 9. Potentiometeric titration curves of HL ligand and its metal complexes.
The results confirmed also that, the azodye ligand derived from 3-amino
phenol is an efficient inhibitor of prostate cancer mutant 2q2k-
Hormone and breast cancer mutant 3hb5-Oxidoreductase.
3.4. Potentiometric studies
The interaction of a metal with an electron donor atom of a ligand
(HL) is usually followed by the release of H+ ions. Alkaline potentiomet-
ric titrations are based on the detection of the protons released upon
complexation. The main advantage of this technique, compared to
other methods is that from the titration curves it is possible to follow
complexation continuously as a function of pH and to detect exactly at
which pH complexation takes place. Furthermore, it is possible to calcu-
late the dissociation constants and the stability constants from the po-
tentiometric titration curves as shown in Fig. 9. The average number
of the protons associated with HL ligand at different pH values, nA ,
was calculated from the titration curves of the acid in the absence and
presence of ligand (HL) by applying Eq. (9):
ꢀ
ꢁ
ðV3−V2Þ Nꢁ þ Eꢁ
ꢀ
ꢁ
n ¼
ð10Þ
Vꢁ −V2 ꢂ nA ꢂ TCꢁ
M
and
!
n
n¼J
X
1
βnH
ꢂ
ꢃ
Hþ
Vo þ V3
n¼o
pL ¼ log10
ꢂ
ð11Þ
TCꢁL−n ꢂ TCꢁM
Vo
where TC°M is the total concentration of the metal ion present in the so-
lution, and βHn is the overall proton-reagent stability constant. V1, V2
and V3 are the volumes of alkali required to reach the same pH on the
titration curves of hydrochloric acid, organic ligand and complex, re-
spectively. These curves were analyzed and the successive metal–ligand
stability constants were determined using different computational
methods [28,29]. The values of the stability constants (log K1 and log
K2) are given in Table 6. The following general remarks can be pointed
out:(i) The maximum value of n was ~2 indicating the formation of
1:1 and 1:2 (metal:ligand) complexes only [2,3].(ii) The metal ion solu-
tion used in the present study was very dilute (2 × 10−4 M), hence there
was no possibility of the formation of polynuclear complexes [30,
31].(iii) The metal titration curves were displaced to the right-hand
side of the ligand titration curves along the volume axis, indicating
ꢀ
ꢁ
ðV1−V2Þ Nꢁ þ Eꢁ
ꢀ
ꢁ
nA ¼ Y ꢀ
ð9Þ
Vꢁ −V1 TCꢁL
1.0
0.9
0.8
0.7
0.6
0.5
Table 6
Stepwise stability constants for ML and ML2 complexes of HL in 20% (by volume) DMF–
water mixture in the presence of 0.1 M KCl at different temperatures.
Mn+
298 K
log K1
308 K
log K1
318 K
log K1
log K2
log K2
log K2
5
6
7
8
9
10
Mn(II)
Co(II)
Ni(II)
Cu(II)
5.49
5.57
5.67
5.81
4.41
4.50
4.63
4.70
5.63
5.72
5.80
5.93
4.52
4.64
4.75
4.83
5.74
5.82
5.93
6.01
4.61
4.76
4.85
4.95
pH
Fig. 10. The relation between nA νs. pH for HL ligand.

A.Z. El-Sonbati et al. / Journal of Molecular Liquids 209 (2015) 625–634
633
(a)
(b)
2+
proton release upon complex formation of the metal ion with the ligand.
Cu
The large decrease in pH for the metal titration curves relative to ligand
titration curve points to the formation of strong metal complexes [32,
33].(iv) At constant temperature, the stability of the chelates increases
in the order of Cu(II) N Ni(II) N Co(II) N Mn(II) [2,3]. This order largely
reflects that the stability of Cu(II) complex is considerably larger than
those of other metals of the 3d series. Under the influence of both the
polarizing ability of the metal ion and the ligand field, Cu(II) complex
will receive some extra stabilization due to tetragonal distortion of octa-
hedral symmetry in its complexes. The greater stability of Cu(II) com-
plex is produced by the well known Jahn–Teller effect [34].
4.85
4.80
4.75
4.70
4.65
4.60
4.55
4.50
5.95
5.90
5.85
5.80
5.75
5.70
5.65
5.60
T=308 K
2+
Ni
2+
Co
log K
1
2+
Mn
Stepwise dissociation constants for the HL ligand and the stepwise
stability constants of its complexes with Mn(II), Co(II), Ni(II) and
Cu(II) ions have been calculated at 298, 308 and 318 K. The correspond-
ing thermodynamic parameters (ΔG, ΔH and ΔS) were evaluated.
The dissociation constants (pKa) for HL, as well as the stability con-
stants of its complexes with Mn(II), Co(II), Ni(II) and Cu(II) ions have
been evaluated at 298, 308 and 318 K and are given in Tables 5 and 6,
respectively. The enthalpy (ΔH) for the dissociation and complexation
log K
2
25
26
27
28
29
Atomic number
Fig. 11. The relation between stability constants (log K1 and log K2) and atomic number of
metal complexes.
process was calculated from the slope of the plot pKa or log K vs. 1
/
T
using the graphical representation of van't Hoff Eqs. (12) and (13):
All the thermodynamic parameters of stepwise stability constants
for the complexes of ligand (HL) are recorded in Table 7. It is known
that the divalent metal ions exist in solution as octahedral hydrated spe-
cies [3,35] and the obtained values ΔH and ΔS can then be considered as
the sum of two contributions: (a) release of H2O molecules and
(b) metal–ligand bond formation. Examination of these values shows
that:
ΔG ¼ −2:303 RT log K ¼ ΔH–TΔS
ð12Þ
or
ꢄ
ꢅꢄ ꢅ
−ΔH
2:303R
1
T
ΔS
log K ¼
þ
2:303R
ð13Þ
where R is the gas constant = 8.314 J·mol−1·K−1, K is the dissociation
constant for the ligand stability and T is the temperature (K).
The values of free energy change (ΔG) and enthalpy (ΔH), can be
deduced the entropy (ΔS) using the well known relationships (12)
and (14):
(i) The stability constants (log K1 and log K2) for the rhodanine
azodye complexes increase with increasing temperature [3,35].
(ii) The stability constants of Mn(II), Co(II), Ni(II) and Cu(II) com-
plexes were increased with increasing atomic number in the
order of Cu(II) N Ni(II) N Co(II) N Mn(II) at constant temperature
as shown in Fig. 11.
(iii) The negative values of ΔG for the complex formation suggest a
spontaneous nature of such process [2].
(iv) The positive values of ΔH mean that the complex formation pro-
cesses is endothermic and favored at higher temperature.
(v) The positive values of ΔS confirm that the complex formation
processes are entropically favorable [2,3].
ΔS ¼ ðΔH–ΔGÞ=T:
ð14Þ
The thermodynamic parameters of the dissociation process of HL are
recorded in Table 5. From these results the following points can be
made:
(i) The pKa values decrease with increasing temperature, i.e. the
acidity of ligand increases [2,3].
(ii) Positive values of ΔH indicate that dissociation is accompanied
by absorption of heat and the process is endothermic.
(iii) Large positive values of ΔG indicate that the dissociation process
is not spontaneous [2,35].
(iv) Negative values of ΔS are due to increased order as the result of
the solvation processes.
4. Conclusion
The proton–ligand dissociation constant of 4-(2,3-dimethyl-1-
phenylpyrazol-5-one azo)-3-aminophenol (HL) and metal–ligand
stability constants of its complexes with metal ions (Mn(II), Co(II),
Ni(II) and Cu(II)) at different temperatures were determined. At con-
stant temperature the stability constants of the formed complexes de-
crease in the order of Cu(II), Ni(II), Co(II) and Mn(II). The dissociation
process is non-spontaneous, endothermic and entropically unfavorable.
The formation of the metal complexes has been found to be spontane-
ous, endothermic and entropically favorable. The values of stability con-
stants (log K1 and log K2) of rhodanine azodye complexes increase with
increasing temperature.
Table 7
Thermodynamic functions for ML and ML2 complexes of HL in 20% (by volume) DMF–wa-
ter mixture and 0.1 M KCl.
M(II)
T (K)
Free energy change
Enthalpy change
Entropy change
(J mol−1 K−1
(kJ mol−1
)
(kJ mol−1
)
)
−ΔG1
−ΔG2
ΔH1
ΔH2
ΔS1
ΔS2
Mn(II)
Co(II)
Ni(II)
Cu(II)
298
308
318
298
308
318
298
308
318
298
308
318
31.32
33.20
34.95
31.78
33.73
35.44
32.35
34.21
36.11
34.29
36.27
38.91
25.16
26.66
28.07
25.67
27.36
28.98
26.42
28.01
29.53
33.16
34.97
36.59
22.70
18.16
181.29
181.51
181.29
182.89
183.29
182.89
187.69
187.61
187.69
176.07
176.77
179.51
145.37
145.49
145.37
165.35
165.46
165.35
155.67
155.79
155.66
187.36
187.18
186.41
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22.72
23.58
18.181
23.60
19.97
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