Potentiometric, thermodynamics and theoretical calculations of some rhodanine derivatives

Article abstract of DOI:10.1016/j.molliq.2016.01.084

A series of 5-(4′-alkylphenylazo)-3-phenylamino-2-thioxothiazolidin-4-one (HL_n) have been prepared and characterized by elemental analysis, ¹H NMR and IR spectra. X-ray diffraction pattern of ligand (HL₂) was studied. The geometrical structures of these ligands are carried out by HF method with 3-21G basis set. Proton-ligand dissociation constant of ligands (HL_n) and their metal-ligand stability constants of Mn²⁺, Co²⁺, Ni²⁺ and Cu²⁺ have been determined potentiometrically in 0.1 M KCl and 30% (v/v) DMSO-water mixture at 298, 308 and 318 K. The stability constants of the formed complexes increase with the order: Mn²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺. The effect of temperature was studied and the corresponding thermodynamic parameters (ΔG, ΔH and ΔS) were derived and discussed. The dissociation process is non-spontaneous, endothermic and entropically unfavorable. The formation of the metal complexes has been found to be spontaneous, endothermic and entropically favorable. Proton-ligand dissociation constant (pK^H) of the ligands increases according to the following order p-(OCH₃ > H > NO₂) as expected from Hammett's constant (σ^R).

Full text of DOI:10.1016/j.molliq.2016.01.084

Journal of Molecular Liquids 216 (2016) 821–829
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Potentiometric, thermodynamics and theoretical calculations of some
rhodanine derivatives
⁎
A.Z. El-Sonbati, M.A. Diab, A.F. Shoair, A.A. El-Bindary , A.M. Barakat
Chemistry Department, Faculty of Science, Damietta University, Damietta 34517, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 5-(4′-alkylphenylazo)-3-phenylamino-2-thioxothiazolidin-4-one (HLn) have been prepared and
characterized by elemental analysis, 1H NMR and IR spectra. X-ray diffraction pattern of ligand (HL2) was studied.
The geometrical structures of these ligands are carried out by HF method with 3-21G basis set. Proton-ligand dis-
sociation constant of ligands (HLn) and their metal–ligand stability constants of Mn2+, Co2+, Ni2+ and Cu2+ have
been determined potentiometrically in 0.1 M KCl and 30% (v/v) DMSO–water mixture at 298, 308 and 318 K. The
stability constants of the formed complexes increase with the order: Mn2+ b Co2+ b Ni2+ b Cu2+. The effect of
temperature was studied and the corresponding thermodynamic parameters (ΔG, ΔH and ΔS) were derived
and discussed. The dissociation process is non-spontaneous, endothermic and entropically unfavorable. The for-
mation of the metal complexes has been found to be spontaneous, endothermic and entropically favorable.
Proton–ligand dissociation constant (pKH) of the ligands increases according to the following order p-
(OCH3 N H N NO2) as expected from Hammett's constant (σR).
Received 30 October 2015
Received in revised form 14 December 2015
Accepted 17 January 2016
Available online xxxx
Keywords:
Rhodanines
Potentiometry
Thermodynamics
Hammett's constant
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Furthermore, the corresponding thermodynamic functions of dissociation
and stability constants were evaluated and discussed.
Azo compounds based on rhodanine usually react as chelating li-
gands with transition metal ions by bonding through the oxygen and
hydrazinic nitrogen atoms [1,2]. These compounds were synthesized
as potential medicinal preparation [3] and can also be used as analytical
reagents [4]. Many binary complexes of transition and inner transition
metals have been studied potentiometrically [5,6]. There has been
growing interest in studying azo compounds especially heterocyclic
compounds and their transition metal complexes because of their
wide importance in industrial fields [7] such as, dying of textile fibers
[8], biological studies [9] and high technology area [10]. Metal com-
plexes of rhodanine have been extensively studied because rhodanine
possesses good synthetic flexibility, selectivity and sensitivity towards
the central metal atom [11,12].
In continuation of our previous work [13–15], we report herein the
dissociation constant of 5-(4′-methoxyphenylazo)-3-phenylamino-2-
thioxothiazolidin −4-one (HL1), 5-(4′-phenylazo)-3-phenylamino-
2-thioxothiazolidin-4-one (HL2) and 5-(4′-nitrophenylazo)-3-
phenylamino-2-thioxothiazolidin-4-one (HL3). The stability
constants of these ligand complexes with Mn2+, Co2+, Ni2+ and Cu2+
were measured by potentiometric studies at different temperatures.
2. Experimental
2.1. Preparation of the ligands
All the compounds and solvents used are pure grade chemicals from
BDH Sigma-Aldrich.
The azo dye ligands (HLn) were prepared previously [15] by cou-
pling of 3-phenylamino-2-thioxothiazolidin-4-one with aniline and
its p-derivatives (Fig. 1). A stoichiometric amount of aniline or its
p-derivatives (0.01 mol) in 25 ml of hydrochloric acid (0.01 mol)
was added dropwise to a solution of sodium nitrite (0.01 mol) in
20 ml of water at −5 °C. The formed diazonium chloride was consec-
utively coupled with an alkaline solution of 3-phenylamino-2-
thioxothiazolidin-4-one (0.01 mol) [16,17]. The colored precipitate,
which was formed immediately, was filtered through a sintered
glass crucible, washed several times with water and ethanol then
dried in a vacuum desiccator over anhydrous CaCl2.
The resulting formed ligands are:
HL1 = 5-(4′-Methoxyphenylazo)-3-phenylamino-2-thioxothiazol-
idin-4-one;
HL2 = 5-(phenylazo)-3-phenylamino-2-thioxothiazolidin-4-one; and
HL3 = 5-(4′-nitrophenylazo)-3-phenylamino-2-thioxothiazolidin-
4-one.
⁎
Corresponding author.
E-mail address: abindary@yahoo.com (A.A. El-Bindary).
http://dx.doi.org/10.1016/j.molliq.2016.01.084
0167-7322/© 2016 Elsevier B.V. All rights reserved.

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A.Z. El-Sonbati et al. / Journal of Molecular Liquids 216 (2016) 821–829
(iii) 5 cm3 0.001 M HCl + 5 cm3 l M KCl + 10 cm3 DMSO + 5 cm3
0.001 M ligand + 10 cm3 0.0001 M metal chloride.
For each mixture, the volume was made up to 50 cm3 with bidistilled
water before the titration. These titrations were repeated for tempera-
tures of 308 K and 318 K. The temperature was controlled to within
0.05 K by circulating thermostated water (Neslab 2 RTE 220) through
the outer jacket of the vessel. The pH measurements were carried out
using VWR Scientific Instruments Model 8000 pH-meter accurate to
0.01 units. All titrations have been carried out between pH 4.0–11.0
and under nitrogen atmosphere.
3. Results and discussion
3.1. IR spectra
The infrared spectra of ligands exhibit band at ~1230 cm−1. This
band can be attributed to intramolecular hydrogen-bonded-NH
group, [21]. Furthermore, all ligands exhibit a strong band at
1727–1754 cm−1, this is due to C_O [2]. The discussed infrared fea-
tures beside the band appearing at 1435–1520 cm−1 can guide to
assume the presence of C_N structure through resonating phe-
nomena [22]. Such class of compounds are with different types of
hydrogen bonding [23–25].
Fig. 1. The structure of 5-(4′-alkylphenylazo)-3-phenylamino-2-thioxothiazolidin-4-
one (HLn).
1. H-bonding of the type NH⋯O between the −NH (hydrazone) group
and C_O group.
2. Intermolecular hydrogen bonding of the NH⋯O type of one molecule
to another one.
2.2. Measurements
The spectra of ligands do not show absorption characteristic of the
N_N functions owing to the formation of the hydrazone and show
no characteristic absorption assignable to NH2 function. This con-
firms the formation of the azo compounds.
Elemental microanalyses of the separated ligands for C, H, and N
were determined on Automatic Analyzer CHNS Vario ELIII,
Germany. The 1H NMR spectra was obtained by Bruker WP
300 MHz using DMSO-d6 as a solvent containing TMS as the internal
standard. IR spectra (KBr disks, 4000–400 cm−1) were measured by
a Jasco-4100 spectrophotometer. The pH measurements were car-
ried out using VWR Scientific Instruments Model 8000 pH-meter ac-
curate to 0.01 units. The pH-meter readings in the non-aqueous
medium were corrected [18]. The electrode system was calibrated
according to the method of Irving et al. [19]. The temperature was
controlled to within 0.05 K by circulating thermostated water
(Neslab 2 RTE 220) through the outer jacket of the vessel. X-ray dif-
fraction analysis of the ligand powder form was recorded on X-ray
diffractometer in the range of diffraction angle 2θ° = 4–70°. This
analysis was carried out using CuKα radiation (λ = 1.540598 Å).
The applied voltage and the tube current are 40 kV and 30 mA, re-
spectively. The calculations of geometry optimization were per-
formed using Perkin Elmer ChemBio 3D software by HF method
with 3-21G basis set. Geometry optimization option was employed
to obtain the most stable structure.
The strong band observed at ~1111–1123 cm−1 and 823–882 cm−1
in the ligands assigned to region may be assigned to υ(N–N) and υ(CS)
vibration modes [25].
3.2. 1H NMR spectra
The 1H NMR spectra of all the ligands were recorded in DMSO-d6 at
room temperature using TMS as the standard. The broad signal exhibit-
ed by the ligands can be assigned to intramolecular hydrogen bonded
A ligand solution (0.001 M) was prepared by dissolving an accu-
rately weighted amount of the solid in DMSO (Analar). Metal ion so-
lutions (0.0001 M) were prepared from Analar metal chlorides in
bidistilled water and standardized with EDTA [20]. Solutions of
0.001 M HCl and 1 M KCl were also prepared in bidistilled water. A
carbonate-free sodium hydroxide solution in 30% (v/v) DMSO–
water mixture was used as titrant and standardized against oxalic
acid (Analar).
The apparatus, general conditions and methods of calculation were
the same as in previous work [13–15]. The following mixtures (i)–(iii)
were prepared and titrated potentiometrically at 298 K against standard
0.002 M NaOH in a 30% (v/v) DMSO–water mixture:
(i) 5 cm3 0.001 M HCl + 5 cm3 1 M KCl + 15 cm3 DMSO;
(ii) 5 cm3 0.001 M HCl + 5 cm3 1 M KC l + 10 cm3 DMSO + 5 cm3
0.00 l M ligand; and
Fig. 2. XRD pattern for the ligand (HL2).

A.Z. El-Sonbati et al. / Journal of Molecular Liquids 216 (2016) 821–829
823
proton of NH (hydrazone) at δ ~11.5 ppm and NH (3-phenylamine) at δ
~9.0 ppm was not affected by dilution and disappear in the presence of
D2O. Aromatic ring protons appear at δ ~6.5–8.0 ppm [26]. The 13C NMR
of ligands exhibited signals at 170–172 ppm corresponding to the car-
bon of (C_N) groups, signals appears at 160–163 ppm corresponding
to (C_O) groups and signals around ~200 ppm corresponding to the
carbon of (C_S). Ligands have different chemical shift due to aromatic
ring substituent.
The XRD of (HL2) shows many diffraction peaks which indicate
the polycrystalline phase. The average crystallite size (ξ) can be cal-
culated from the XRD pattern according to the Debye–Scherrer equa-
tion [27,28]:
Kλ
ξ ¼
:
ð1Þ
β
1=2 cosθ
3.3. X-ray diffraction analysis
The equation uses the reference peak width at angle (θ), where λ is
wavelength of X-ray radiation (1.541874 Å), K is constant taken as 0.95
for organic compounds [2] and β1/2 is the width at half maximum of the
reference diffraction peak measured in radians. The dislocation density,
δ, is the number of dislocation lines per unit area of the crystal. The value
X-ray diffraction (XRD) pattern of ligand (HL2) in powder forms is
shown in Fig. 2. The XRD pattern ligand (HL2) shows many peaks and
this behavior indicates that the polycrystalline phase.
Fig. 3. The calculated molecular structures of the ligands (HLn).

824
A.Z. El-Sonbati et al. / Journal of Molecular Liquids 216 (2016) 821–829
of δ is related to the average particle diameter (ξ) by the relation
[27,28]:
and is mainly described by one electron excitation from the highest oc-
cupied molecular orbital to the lowest unoccupied molecular orbital.
Quantum chemical parameters of the ligands are obtained from calcula-
1
ξ2
tions such as energies of the highest occupied molecular orbital (EHOMO
)
δ ¼
:
ð2Þ
and the lowest unoccupied molecular orbital (ELUMO) as listed in Table 5.
Additional 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 [29,30]. Recently, the
energy gap between HOMO and LUMO has been used to prove the activ-
ity and stability of the compounds. The values of (ΔE) for HL1, HL2, HL3
ligands were found to be 0.236, 0.121 and 2.266 eV, respectively. The
calculations indicated that the HL2 is a more stable form than the
other ligands [32]. The relation between Hammett's substitution coeffi-
cients (σR) vs. energy gap (ΔE) is shown in Fig. 5.
The value of ξ is calculated and found to be 76.5 nm. The value of δ is
1.71 × 10−4 nm−2
.
3.4. Molecular structure of the ligand
The optimized structures, bond length and bond angles of the li-
gands (HLn) are presented in Fig. 3 and Table 1–3. The computed net
charges on active centers for the ligands are found that the most nega-
tive charges in O(13), N(6) and S(14) for enol-hydrazo (Table 4). Both
the highest occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) are the main orbital that take part in chemical
stability. The HOMO represents the ability to donate an electron, LUMO
as an electron acceptor represents the ability to obtain an electron. The
HOMO and LUMO energy for ligands calculated by HF method with 3-
21G basis set is presented in Fig. 4. This electronic absorption corre-
sponds to the transition from the ground state to the first excited state
3.5. Potentiometric studies
The average number of the protons associated with ligands (HLn) at
different pH values, nA, was calculated from the titration curves of the
acid in the absence and presence of ligands (HLn).
Table 1
The selected geometric parameters for HL1 ligand.
Bond lengths (Å)
Bond angles (°)
Bond angles (°)
C(24)–H(38)
C(24)–H(37)
C(24)–H(36)
C(21)–H(35)
C(20)–H(34)
C(18)–H(33)
C(17)–H(32)
O(13)–H(31)
C(11)–H(30)
C(10)–H(29)
C(9)–H(28)
C(8)–H(27)
C(7)–H(26)
N(6)–H(25)
C(17)–C(22)
C(21)–C(22)
C(20)–C(21)
C(19)–C(20)
C(18)–C(19)
C(17)–C(18)
C(7)–C(12)
C(11)–C(12)
C(10)–C(11)
C(9)–C(10)
C(8)–C(9)
C(7)–C(8)
C(3)–S(4)
C(5)–S(4)
C(1)–C(5)
N(2)–C(1)
C(3)–N(2)
C(22)–N(16)
N(15)–N(16)
C(12)–N(6)
N(2)–N(6)
C(19)–O(23)
C(5)–N(15)
C(3)–S(14)
C(1)–O(13)
O(23)–C(24)
1.113
1.113
1.113
1.105
1.102
1.104
1.103
0.972
1.104
1.103
1.103
1.103
1.103
1.051
1.346
1.346
1.343
1.347
1.347
1.342
1.345
1.345
1.342
1.341
1.341
1.342
1.789
1.483
1.356
1.277
1.268
1.268
1.252
1.273
1.356
1.374
1.268
1.574
1.365
1.409
H(38)–C(24)–H(37)
H(38)–C(24)–H(36)
H(38)–C(24)–O(23)
H(37)–C(24)–H(36)
H(37)–C(24)–O(23)
H(36)–C(24)–O(23)
C(19)–O(23)–C(24)
H(34)–C(20)–C(21)
H(34)–C(20)–C(19)
C(21)–C(20)–C(19)
C(20)–C(19)–C(18)
C(20)–C(19)–O(23)
C(18)–C(19)–O(23)
H(33)–C(18)–C(19)
H(33)–C(18)–C(17)
C(19)–C(18)–C(17)
H(35)–C(21)–C(22)
H(35)–C(21)–C(20)
C(22)–C(21)–C(20)
H(32)–C(17)–C(22)
H(32)–C(17)–C(18)
C(22)–C(17)–C(18)
C(17)–C(22)–C(21)
C(17)–C(22)–N(16)
C(21)–C(22)–N(16)
C(22)–N(16)–N(15)
H(29)–C(10)–C(11)
H(29)–C(10)–C(9)
C(11)–C(10)–C(9)
H(28)–C(9)–C(10)
H(28)–C(9)–C(8)
111.882
108.173
110.348
108.179
110.345
107.776
118.76
S(4)–C(3)–S(14)
N(2)–C(3)–S(14)
N(16)–N(15)–C(5)
C(3)–S(4)–C(5)
H(31)–O(13)–C(1)
C(1)–N(2)–C(3)
C(1)–N(2)–N(6)
C(3)–N(2)–N(6)
S(4)–C(5)–C(1)
S(4)–C(5)–N(15)
C(1)–C(5)–N(15)
C(5)–C(1)–N(2)
C(5)–C(1)–O(13)
N(2)–C(1)–O(13)
C(12)–N(6)–N(2)
S(4)–C(3)–N(2)
125.048
129.265
119.236
93.394
110.086
113.994
122.379
123.512
110.776
131.817
117.407
116.149
123.089
120.762
123.161
105.687
117
120.978
122.022
115.614
125.576
118.81
118.442
118.618
122.94
119.981
118.307
121.712
121.887
117.273
120.84
116.872
125.996
117.133
119.467
120.044
119.885
120.071
120.156
120.151
119.693
119.852
120.016
120.132
120.718
118.659
120.623
121.116
118.34
C(10)–C(9)–C(8)
H(27)–C(8)–C(9)
H(27)–C(8)–C(7)
C(9)–C(8)–C(7)
H(30)–C(11)–C(12)
H(30)–C(11)–C(10)
C(12)–C(11)–C(10)
H(26)–C(7)–C(12)
H(26)–C(7)–C(8)
C(12)–C(7)–C(8)
C(7)–C(12)–C(11)
C(7)–C(12)–N(6)
C(11)–C(12)–N(6)
H(25)–N(6)–C(12)
H(25)–N(6)–N(2)
120.544
118.937
122.902
118.161
114.959
116.329

A.Z. El-Sonbati et al. / Journal of Molecular Liquids 216 (2016) 821–829
825
Table 2
The selected geometric parameters for HL2 ligand.
Bond lengths (Å)
Bond angles (°)
Bond angles (°)
C(21)–H(34)
C(20)–H(33)
C(19)–H(32)
C(18)–H(31)
C(17)–H(30)
O(13)–H(29)
C(11)–H(28)
C(10)–H(27)
C(9)–H(26)
C(8)–H(25)
C(7)–H(24)
N(6)–H(23)
C(17)–C(22)
C(21)–C(22)
C(20)–C(21)
C(19)–C(20)
C(18)–C(19)
C(17)–C(18)
C(7)–C(12)
C(11)–C(12)
C(10)–C(11)
C(9)–C(10)
C(8)–C(9)
C(7)–C(8)
C(3)–S(4)
C(5)–S(4)
C(1)–C(5)
N(2)–C(1)
C(3)–N(2)
C(22)–N(16)
N(15)–N(16)
C(12)–N(6)
N(2)–N(6)
C(5)–N(15)
C(3)–S(14)
C(1)–O(13)
1.105
1.103
1.103
1.103
1.103
0.972
1.104
1.103
1.103
1.103
1.103
1.051
1.348
1.348
1.342
1.341
1.341
1.343
1.345
1.345
1.342
1.341
1.341
1.342
1.789
1.483
1.356
1.277
1.267
1.268
1.252
1.273
1.356
1.268
1.574
1.365
H(33)–C(20)–C(21)
H(33)–C(20)–C(19)
C(21)–C(20)–C(19)
H(32)–C(19)–C(20)
H(32)–C(19)–C(18)
C(20)–C(19)–C(18)
H(31)–C(18)–C(19)
H(31)–C(18)–C(17)
C(19)–C(18)–C(17)
H(34)–C(21)–C(22)
H(34)–C(21)–C(20)
C(22)–C(21)–C(20)
H(30)–C(17)–C(22)
H(30)–C(17)–C(18)
C(22)–C(17)–C(18)
C(17)–C(22)–C(21)
C(17)–C(22)–N(16)
C(21)–C(22)–N(16)
C(22)–N(16)–N(15)
H(27)–C(10)–C(11)
H(27)–C(10)–C(9)
C(11)–C(10)–C(9)
H(26)–C(9)–C(10)
H(26)–C(9)–C(8)
C(10)–C(9)–C(8)
H(25)–C(8)–C(9)
H(25)–C(8)–C(7)
C(9)–C(8)–C(7)
H(28)–C(11)–C(12)
H(28)–C(11)–C(10)
C(12)–C(11)–C(10)
H(24)–C(7)–C(12)
H(24)–C(7)–C(8)
C(12)–C(7)–C(8)
C(7)–C(12)–C(11)
C(7)–C(12)–N(6)
C(11)–C(12)–N(6)
H(23)–N(6)–C(12)
H(23)–N(6)–N(2)
C(12)–N(6)–N(2)
S(4)–C(3)–N(2)
120.117
119.892
119.99
C(3)–N(2)–N(6)
S(4)–C(5)–C(1)
S(4)–C(5)–N(15)
C(1)–C(5)–N(15)
C(5)–C(1)–N(2)
C(5)–C(1)–O(13)
N(2)–C(1)–O(13)
C(3)–S(4)–C(5)
H(29)–O(13)–C(1)
C(1)–N(2)–C(3)
C(1)–N(2)–N(6)
123.504
110.761
131.837
117.402
116.162
123.084
120.753
93.403
110.072
113.989
122.383
120.288
120.313
119.399
119.649
120.032
120.319
120.365
118.181
121.454
121.936
116.993
121.071
117.766
125.636
116.597
119.606
120.038
119.888
120.073
120.152
120.159
119.689
119.849
120.015
120.136
120.718
118.66
120.621
121.117
118.342
120.54
118.94
122.915
118.145
114.993
116.379
123.209
105.684
125.056
129.26
S(4)–C(3)–S(14)
N(2)–C(3)–S(14)
N(16)–N(15)–C(5)
119.239
Applying Eq. (3):
The formation curves for the metal complexes were obtained by
plotting the average number of ligands attached per metal ion (nA)
vs. the free ligands exponent (pL). The average number of the re-
agent molecules attached per metal ion, n , and free ligands expo-
nent, pL, can be calculated using Eqs. (4) and (5):
ðV1−V2ÞðNꢁ þ EꢁÞ
nA ¼ Y ꢀ
ð3Þ
ðVꢁ−V1ÞTCꢁ
L
ðV3−V2ÞðNꢁ þ EꢁÞ
n ¼
ð4Þ
where Y is the number of available protons in ligands (HLn) (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,
and 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 hydrox-
ide 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. This means that ligands (HLn) have ionizable proton (the
enolized hydrogen ion of the –OH group, pKH as shown in Table 6.
The completely protonated form of the ligands (HLn) has dissociable
proton, that dissociates in the measurable pH range. The deproton-
ation of the o-hydroxy group most probably results in the formation
of stable intramolecular H– bonding with the nitrogen of the azo
group. Such an interaction decreases the dissociation process of li-
gands (HLn), i.e. increases the pKH value [33,34].
ðVꢁ−V2Þ ꢂ nA ꢂ TCꢁ
M
and
!
n
n¼J
X
1
βnH
ꢀ
ꢁ
Hþ
V
ꢁ þ V3
n¼o
pL ¼ log10
ꢂ
ð5Þ
TCꢁL−n ꢂ TCꢁ
Vꢁ
M
where TC°M is the total concentration of the metal ion present in the
solution, 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,
respectively. These curves were analyzed and the successive metal–
ligand stability constants were determined [35]. The values of the

826
A.Z. El-Sonbati et al. / Journal of Molecular Liquids 216 (2016) 821–829
Table 3
The selected geometric parameters for HL3 ligand.
Bond lengths (Å)
Bond angles (°)
Bond angles (°)
C(21)–H(36)
C(20)–H(35)
C(18)–H(34)
C(17)–H(33)
O(13)–H(32)
C(11)–H(31)
C(10)–H(30)
C(9)–H(29)
C(8)–H(28)
C(7)–H(27)
N(6)–H(26)
C(17)–C(22)
C(21)–C(22)
C(20)–C(21)
C(19)–C(20)
C(18)–C(19)
C(17)–C(18)
C(7)–C(12)
C(11)–C(12)
C(10)–C(11)
C(9)–C(10)
C(8)–C(9)
C(7)–C(8)
C(3)–S(4)
C(5)–S(4)
C(1)–C(5)
N(2)–C(1)
C(3)–N(2)
C(22)–N(16)
N(15)–N(16)
C(12)–N(6)
N(2)–N(6)
C(19)–N(23)
C(5)–N(15)
C(3)–S(14)
C(1)–O(13)
N(23)–O(25)
N(23)–O(24)
1.105
1.104
1.104
1.103
0.971
1.104
1.103
1.103
1.103
1.103
1.051
1.347
1.346
1.343
1.347
1.347
1.343
1.345
1.345
1.342
1.341
1.341
1.342
1.789
1.483
1.356
1.277
1.267
1.268
1.252
1.273
1.356
1.258
1.267
1.574
1.365
1.314
1.314
C(19)–N(23)–O(25)
C(19)–N(23)–O(24)
O(25)–N(23)–O(24)
H(35)–C(20)–C(21)
H(35)–C(20)–C(19)
C(21)–C(20)–C(19)
C(20)–C(19)–C(18)
C(20)–C(19)–N(23)
C(18)–C(19)–N(23)
H(34)–C(18)–C(19)
H(34)–C(18)–C(17)
C(19)–C(18)–C(17)
H(36)–C(21)–C(22)
H(36)–C(21)–C(20)
C(22)–C(21)–C(20)
H(33)–C(17)–C(22)
H(33)–C(17)–C(18)
C(22)–C(17)–C(18)
C(17)–C(22)–C(21)
C(17)–C(22)–N(16)
C(21)–C(22)–N(16)
C(22)–N(16)–N(15)
H(30)–C(10)–C(11)
H(30)–C(10)–C(9)
C(11)–C(10)–C(9)
H(29)–C(9)–C(10)
H(29)–C(9)–C(8)
C(10)–C(9)–C(8)
H(28)–C(8)–C(9)
H(28)–C(8)–C(7)
C(9)–C(8)–C(7)
H(31)–C(11)–C(12)
H(31)–C(11)–C(10)
C(12)–C(11)–C(10)
H(27)–C(7)–C(12)
H(27)–C(7)–C(8)
C(12)–C(7)–C(8)
C(7)–C(12)–C(11)
C(7)–C(12)–N(6)
C(11)–C(12)–N(6)
H(26)–N(6)–C(12)
H(26)–N(6)–N(2)
C(12)–N(6)–N(2)
S(4)–C(3)–N(2)
122.725
123.053
114.222
117.285
121.2
H(32)–O(13)–C(1)
C(1)–N(2)–C(3)
C(1)–N(2)–N(6)
C(3)–N(2)–N(6)
S(4)–C(5)–C(1)
S(4)–C(5)–N(15)
C(1)–C(5)–N(15)
C(5)–C(1)–N(2)
C(5)–C(1)–O(13)
N(2)–C(1)–O(13)
N(2)–C(3)–S(14)
N(16)–N(15)–C(5)
C(3)–S(4)–C(5)
110.324
113.81
122.557
123.55
110.719
132.306
116.974
116.292
123.128
120.577
129.156
119.526
93.389
121.515
116.754
121.799
121.448
121.112
117.071
121.817
120.036
118.362
121.602
121.769
116.955
121.276
117.037
125.749
117.215
119.05
120.038
119.889
120.073
120.151
120.154
119.695
119.856
120.013
120.132
120.717
118.669
120.613
121.098
118.365
120.538
118.948
122.855
118.196
114.845
116.199
122.928
105.788
125.056
S(4)–C(3)–S(14)
stability constants (log K1 and log K2) are given in Table 7. The fol-
lowing general remarks can be pointed out:
(iv) For the same ligand at constant temperature, the stability of the
chelates increases in the order: Mn2+ b Co2+ b Ni2+ b Cu2+
[40,41]. This order largely reflects that the stability of Cu2+ com-
plexes 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 [42] Cu2+ will receive some extra
stabilization due to tetragonal distortion of octahedral symmetry
in its complexes. The greater stability of Cu2+ complexes is pro-
duced by the well known Jahn–Teller effect [43].
(i) The maximum value ofn was ~2 indicating the formation of 1:1
and 1:2 (metal:ligand) complexes [36].
(ii) The metal ion solution used in the present study was very diluted
(2 × 10−5 M), hence there was no possibility of formation of
polynuclear complexes [37,38].
(iii) The metal titration curves were displaced to the right-hand side
of the ligand titration curves along the volume axis, indicating
proton release upon complex formation of the metal ion with
the ligand. The large decrease in pH for the metal titration curves
relative to ligand titration curves points to the formation of
strong metal complexes [39].
The dissociation constant (pKH) for ligands (HLn) as well as the
stability constants of their complexes with Mn2+, Co2+, Ni2+ and
Cu2+ have been evaluated at 298, 308 and 318 K, and given in
Tables 6 and 7, respectively. The enthalpy (ΔH) for the dissociation
and complexation process was calculated from the slope of the plot
pKH or log K vs. 1/T using the graphical representation of van't Hoff
Eqs. (6) and (7):
Table 4
Net charges on active centers of the ligands (HLn).
ΔG ¼ −2:303 RT logK ¼ ΔH–TΔS
ð6Þ
Atom
Charges
HL1
HL2
0.1382
−0.511
−0.38
HL3
0.1382
−0.511
−0.38
or
O(13)
N(6)
S(14)
0.1382
−0.511
−0.38
log K ¼ ð−ΔH=2:303 RÞð1=TÞ þ ðΔS=2:303 RÞ:
ð7Þ

A.Z. El-Sonbati et al. / Journal of Molecular Liquids 216 (2016) 821–829
827
Fig. 4. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the ligands (HLn).
Table 5
The calculated quantum chemical parameters for the ligands (HLn).
Comp.
EHOMO
(eV)
ELUMO
(eV)
ΔE
(eV)
χ
(eV)
η
(eV)
σ
Pi
(eV)
S
Ω
(eV)
ΔNmax
(eV)−1
(eV)−1
HL1
HL2
HL3
−2.26
−2.37
−4.515
−2.024
−2.249
−2.249
0.236
0.121
2.266
2.142
2.309
3.382
0.118
0.060
1.133
8.475
16.529
0.883
−2.142
−2.309
−3.382
4.237
8.264
0.441
19.441
44.081
5.047
18.152
38.173
2.985

828
A.Z. El-Sonbati et al. / Journal of Molecular Liquids 216 (2016) 821–829
Table 7
Stepwise stability constants for ML complexes of ligands (HLn) in 30% (by volume)
DMSO–water mixture and 0.1 M KCl at different temperatures.
Comp.
298 K
log K1
308 K
log K1
318 K
log K1
Mn+
log K2
log K2
log K2
HL1
Mn2+
Co2+
Ni2+
Cu2+
Mn2+
Co2+
Ni2+
Cu2+
Mn2+
Co2+
Ni2+
Cu2+
6.16
6.27
6.38
6.58
6.27
6.40
6.52
6.73
6.49
6.62
6.75
6.89
4.11
4.20
4.32
4.45
4.30
4.41
4.54
4.63
4.52
4.64
4.79
4.93
6.28
6.40
6.50
6.69
6.40
6.53
6.65
6.85
6.61
6.75
6.87
7.00
4.22
4.32
4.43
4.58
4.43
4.52
4.66
4.75
4.64
4.78
4.90
5.05
6.39
6.52
6.63
6.82
6.52
6.66
6.77
6.99
6.74
6.88
7.00
7.13
4.35
4.45
4.55
4.78
4.55
4.65
4.80
4.88
4.75
4.90
5.03
5.18
HL2
HL3
Fig. 5. The relation between Hammett's substitution coefficients (σR) vs. energy gap (ΔE)
of the ligands (HLn).
(iii) The ΔH values are positive, meaning that these processes are en-
dothermic and favorable at higher temperature.
(iv) The ΔS values for the ligand complexes are positive, confirming
that the complex formation is entropically favorable [13].
From the ΔG and ΔH values one can deduce the entropy ΔS using
the well known relationships 6 and 8:
An inspection of the results in Table 6 reveals that the pKH values
of (HL2) and its substituted derivatives are influenced by the induc-
tive or mesomeric effect of the substituents. The p-OCH3 derivatives
ΔS ¼ ðΔH−ΔGÞ=T:
ð8Þ
All thermodynamic parameters of the dissociation process of ligands
(HLn) are recorded in Table 6. From these results the following conclu-
sions can be made:
(i) The pKH values decrease with increasing temperature, i.e. the
acidity of the ligand increases [14].
(ii) A positive value of ΔH indicates that the process is endothermic.
(iii) A large positive value of ΔG indicates that the dissociation pro-
cess is not spontaneous [44].
Table 8
Thermodynamic functions for the stability constants of ML complexes in 30% (by volume)
DMSO–water mixture and 0.1 M KCl at 298 K.
Comp.
Mn+
T/K
Gibbs energy
Enthalpy
change
Entropy change
(J mol−1 K−1
(kJ mol−1
)
)
(kJ mol−1
)
(iv) A negative value of ΔS is obtained due to the increased order as a
result of the solvation process.
−ΔG1
−ΔG2
−ΔH1
−ΔH2
21.75
ΔS1
ΔS2
HL1
Mn2+
Co2+
Ni2+
Cu2+
Mn2+
Co2+
Ni2+
Cu2+
Mn2+
Co2+
Ni2+
Cu2+
298
308
318
298
308
318
298
308
318
298
308
318
298
308
318
298
308
318
298
308
318
298
308
318
298
308
318
298
308
318
298
308
318
298
308
318
35.15
37.04
38.91
35.78
37.74
40.67
36.40
38.33
40.37
37.54
39.45
41.53
35.78
37.74
39.70
36.52
38.51
40.55
37.20
39.22
41.22
38.40
40.40
42.56
37.03
38.98
41.04
37.77
39.81
41.89
38.51
40.51
42.62
39.31
41.28
43.41
23.45
24.89
26.49
23.96
25.48
27.10
24.65
26.13
27.70
25.39
27.01
29.10
24.54
26.13
27.70
25.16
26.66
28.31
25.90
27.48
29.23
26.42
28.01
29.71
25.79
27.36
28.92
26.48
28.19
29.84
27.33
28.90
30.63
28.13
29.78
31.54
20.87
187.98
188.00
187.98
196.17
196.19
196.17
198.21
198.04
198.21
198.96
198.70
198.97
196.17
196.19
196.17
201.67
201.59
201.67
200.96
200.97
200.96
207.92
207.65
207.93
200.32
200.14
200.32
205.88
205.80
205.88
205.29
205.12
205.30
204.90
204.63
204.90
151.67
151.41
151.67
156.47
156.30
156.47
152.68
152.51
152.68
185.40
184.64
185.42
158.45
158.47
158.45
157.41
157.15
157.42
165.99
165.72
165.99
164.70
164.53
164.71
156.57
156.60
156.57
168.04
168.14
168.03
164.69
164.43
164.69
170.45
170.27
170.45
All the thermodynamic parameters of the stepwise stability con-
stants of complexes are recorded in Table 8 and the obtained values of
ΔH and ΔS can then be considered as the sum of two contributions:
(a) release of H2O molecules, and (b) metal–ligand bond formation. Ex-
amination of these values shows that:
37.13
22.66
21.75
22.68
23.58
22.68
23.56
22.66
23.58
22.66
21.75
22.66
20.85
29.86
22.68
21.75
23.56
22.66
20.87
23.60
21.75
22.66
(i) The stability constants (log K1 and log K2) for HLn complexes in-
crease with increasing temperature, i.e. its stability constants in-
crease with increasing temperature [14].
(ii) The negative value of ΔG for the complexation process suggests
the spontaneous nature of such processes.
HL2
Table 6
Thermodynamic functions for the dissociation constants of ligands (HLn) in 30% (by vol-
ume) DMSO–water mixture and 0.1 M KCl at different temperatures.
Compound Temperature Dissociation Gibbs
Enthalpy Entropy
change change
kJ mol−1 kJ mol−1 J mol−1 K−1
(K)
constant
energy
HL3
pKH
ΔG
ΔH
−ΔS
HL1
HL2
HL3
298
308
318
298
308
318
298
308
318
8.67
8.53
8.37
8.27
8.11
7.98
7.88
7.75
7.64
49.47
50.30
50.96
47.19
47.83
48.59
44.96
45.70
46.52
27.80
72.72
73.07
72.84
67.82
67.70
67.96
77.77
77.66
77.78
26.98
21.79

A.Z. El-Sonbati et al. / Journal of Molecular Liquids 216 (2016) 821–829
829
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