Thermodynamics of substituted rhodanine III: Potentiometric and spectrophotometric studies of complexes of some transition metals with 3-phenylsulfonamidorhodanine

Article abstract of DOI:10.1007/PL00010137

The dissociation constants of 3-phenylsulfonamidorhodanine (PSR) were determined potentiometrically in 0.1 M KCl and 20% (v/v) ethanol-water. The stepwise stability constants of the complexes of Mn²⁺ Co²⁺, Ni²⁺, Cu^2+<

Full text of DOI:10.1007/PL00010137

Monatshefte fuÈr Chemie 129, 1259±1265 (1998)
Thermodynamics of Substituted Rhodanine III:
Potentiometric and Spectrophotometric Studies
of Complexes of Some Transition Metals with
3-Phenylsulfonamidorhodanine
Ashraf A. El-Bindary1;Ã, Mohammed M. Ghoneim2, Adel Z. El-Sonbati1,
and Sahar A. Barakat1
1
Chemistry Department, Faculty of Science, Mansoura University, Demiatta, Egypt
2
Chemistry Department, Faculty of Science, Tanta University, Egypt
Summary. The dissociation constants of 3-phenylsulfonamidorhodanine (PSR) were determined
potentiometrically in 0.1 M KCl and 20% (v/v) ethanol-water. The stepwise stability constants of the
complexes of Mn2‡; Co2‡; Ni2‡; Cu2‡; Zn2‡; La3‡; Ce3‡; Gd3‡; UO22‡, and Th4‡ with PSR were
determined. The stabilities of the complexes were found as follows: Th4‡ > UO22‡ > Gd3‡ > Ce3‡
> La3‡ > Mn2‡ < Co2‡ < Ni2‡ < Cu2‡ > Zn2‡. The thermodynamic parameters for PSR and its
complexes were evaluated and discussed. The dissociation process is non-spontaneous, endothermic,
and entropically unfavourable. The formation of the complexes was found to be spontaneous,
exothermic or endothermic (depending on the metal), and entropically favourable. The
stoichiometries of the complexes were determined spectrophotometrically and indicate the formation
of 1:1 and 1:2 (metal:ligand) complexes.
Keywords. 3-Phenylsulfonamidorhodanine; Dissociation; Stability constants; Thermodynamic
parameters.
Thermodynamik von substituiertem Rhodanin III:
Potentiometrische und spektrophotometrische Untersuchungen der Komplexe
È
einiger Ubergangsmetalle mit 3-Phenylsulfonamidorhodanin
Zusammenfassung. Die Dissoziationskonstanten von 3-Phenylsulfonamidorhodanin (PSR) in 0.1 M
KCl und 60% (v/v) Ethanol-Wasser wurden potentiometrisch ermittelt. Die stufenweisen
2‡
3‡
Stabilitatskonstanten seiner Komplexe mit Mn
È
; Co2‡; Ni ; Zn2‡; La3‡; Ce ; Gd3‡; UO2‡ und
2‡
È
4‡
Th4‡ wurden bestimmt. Die Stabilitatsreihenfolge lautet Th > UO2‡ > Gd3‡ > La3‡ > Mn2‡
< Co2‡ < Ni2‡ < Cu2‡ > Zn2‡. Die thermodynamischen Parameter von PSR und seinen
È
Komplexen werden diskutiert. Der Dissoziationsprozeû verlauft nicht-spontan, endotherm und
entropisch unguÈnstig. Die Komplexbildung ist spontan, exo- oder endotherm je nach Metall und
entropisch beguÈnstigt. Die Zusammensetzung der Komplexe wurde spektrophotometrisch bestimmt;
es entstehen 1:1- und 1:2-Komplexe (Metall:Ligand).
Ã
Corresponding author

1260
A. A. El-Bindary et al.
Introduction
Although stability constants of transition metal complexes have been studied
extensively, no investigation including 3-phenylsulfonamidorhodanine (PSR) and
its substituted derivatives as ligands has been reported. In continuation of our
earlier work [1±4] on metal complexes in solution and in view of the importance of
these ligands [5±7], we report herein the dissociation constants of PSR and the
stability constants for its complexes with Mn2‡; Co2‡; Ni2‡; Cu2‡; Zn2‡; La3‡;
Ce3‡; Gd3‡; UO22‡ and Th4‡ at different temperatures. The corresponding thermo-
dynamic parameters are derived and discussed. Moreover, the stoichiometries of
the complexes are determined spectrophotometrically.
Experimental
Preparation of the ligand
3-Phenylsulfonamidorhodanine (PSR) was prepared by adding equimolar amounts of trithiocarbo-
diglycolic acid [8] to a solution of phenylsulfonyl hydrazide in bidistilled water with constant stirring
on a steam bath for 6 hours. The white precipitate was ®ltered off, washed with hot bidistilled water,
and recrystallized from absolute ethanol (m.p.: 140ꢀC, molecular weight: 288.36). Its purity was
checked by elemental analysis (found: C 37.60, H 3.0, N 9.90; calcd.: C 37.48, H 2.8, N 9.72), IR
1
and H NMR spectra.
Reagents and materials
Metal ion solutions (0.001 M) were prepared from AnalaR metal chlorides in bidistilled water and
standardized with EDTA [9]. The ligand solution (0.01 M) was prepared by dissolving the accurate
weight of the solid in ethanol (AnalaR). A solution of 1 M KCl was also prepared in bidistilled water.
A carbonate-free sodium hydroxide solution in 20% (v/v) ethanol-water was used as titrant and
standardized against AnalaR oxalic acid.
Apparatus and procedures
Apparatus, general conditions, and methods of calculation were the same as in previous investi-
gations [1±3]. The following mixtures were prepared and titrated potentiometrically at 303 K against
standard 0.02 M NaOH in 20% (v/v) ethanol-water:
1) 5 ml 0.005 M HCl ‡ 5 ml 1 M KCl ‡ 10 ml EtOH
2) 5 ml 0.005 M HCl ‡ 5 ml 1 M KCl ‡ 5 ml EtOH ‡ 5 ml 0.01 M ligand
3) 5 ml 0.005 M HCl ‡ 5 ml 1 M KCl ‡ 5 ml EtOH ‡ 5 ml 0.01 M ligand ‡ 10 ml 0.001 M metal salt

Complexes of Substituted Rhodanine III
1261
For each mixture, the volume was made up to 50 ml with bidistilled water before the titration. These
titrations were repeated for temperatures of 308 and 318 K. Temperature was kept constant within
Æ0.05 K using an ultrathermostate (Gallenkamp thermo stirrer 85). The pH-meter readings in 20%
(v/v) ethanol-water are corrected according to van Uitert and Hass [10].
Results and Discussion
Proton-ligand stability constants
The average number of protons associated with PSR at different pH values, ꢀnA, was
calculated from the titration curves of acid in the absence and presence of PSR.
Thus, the formation curves (ꢀnA vs. pH) for the proton-ligand systems were
constructed and found to extend between 0 and 2 on the ꢀnA scale. This means that
PSR has two dissociable protons (the enolized hydrogen ion of the carbonyl oxygen
in the rhodanine moiety and the sulfonamide proton). Different computational
methods [11] were applied to evaluate the stepwise dissociation constants. The
average values obtained are listed in Table 1.
Table 1. Potentiometric thermodynamic functions for the dissociation of PSR in 20% (v/v) ethanol-water and 0.1 M KCl
at different temperatures
T(K)
Dissociation constant a ÁG (kJ Á molꢁ1
)
ÁH (kJ Á molꢁ1
)
ÁS (J Á molꢁ1Kꢁ1
ꢁÁS1 ꢁÁS2
)
pK1H
pK2H
ÁG1
ÁG2
ÁGꢀ
ÁH1
ÁH2
ÁHꢀ
ꢁÁSꢀ
303
308
318
4.42
4.36
4.26
9.1
25.64 52.79 78.43
25.71 53.19 78.9
25.94 53.89 79.83
9.02
8.85
20.30 31.78 52.08 18.00 69.5Æ1.7 87.5Æ1.7
a
Æ(0.03 ꢁ 0.05)
Table 2. Stepwise stability constants for ML and ML2 complexes of PSR in 20% (v/v) ethanol-water and 0.1 M KCl at different
temperatures
303 K
308 K
318 K
logK1 logK2 logKꢀ logK1/logK2 logK1 logK2 logKꢀ logK1/logK2 logK1 logK2 logKꢀ logK1/logK2
Mn2‡
Co2‡
Ni2‡
Cu2‡
Zn2‡
La3‡
Ce3‡
Gd3‡
UO22‡
7.00 5.00
4.60 4.35
6.57 3.44
9.88 6.08
7.51 5.00
7.27 6.26
7.47 7.00
7.28 6.95
8.43 7.39
12.00 1.40
8.95 1.06
10.01 1.91
15.96 1.60
12.51 1.50
13.53 1.16
14.47 1.07
14.23 1.05
15.82 1.14
19.94 1.13
6.53 4.93
5.36 5.09
6.80 4.77
10.03 7.46
7.61 5.74
7.60 6.36
7.42 6.91
7.22 6.9
11.46 1.32
10.45 1.05
11.57 1.43
17.49 1.34
13.35 1.30
13.96 1.19
14.33 1.07
14.12 1.05
16.95 1.16
19.77 1.10
6.05 4.86
7.00 5.83
7.20 6.10
10.30 8.32
7.71 6.47
7.92 7.00
7.36 6.82
7.15 6.85
9.75 8.32
10.44 9.66
10.91 1.24
12.83 1.20
13.30 1.18
18.62 1.24
14.18 1.19
14.92 1.13
14.18 1.08
14.00 1.04
18.07 1.17
20.10 1.08
9.09 7.86
10.37 9.4
Th4‡ 10.3
9.14

1262
A. A. El-Bindary et al.
Metal-ligand stability constants
The formation curves for the metal complexes were obtained by plotting the
average number of ligands attached per metal ion (ꢀn) vs. the free ligand exponent
(pL) according to Irving and Rossotti [12]. These curves were analyzed, and the
successive stability constants were determined using different computational
methods [13, 14]. The values of stability constants (logK1, logK2 and logꢀ) as well
as the ratio log K1/logK2 are given in Table 2. The following general remarks can
be pointed out:
(i) The maximum ꢀn values in all cases were found to be ' 2, revealing that both
ML and ML2 types of complexes are formed in solution.
(ii) No precipitate was observed in the titration vessel, indicating that the
possibility of formation of metal hydroxide can be excluded.
(iii) For all complexes, the vacant sites of the metal ions move freely available for
the binding of a ®rst ligand than for a second one.
(iv) The order of stability constants of the metal complexes of PSR was found to
be Th4‡ > UO22‡ > Gd3‡ > Ce3‡ > La3‡ > Mn2‡ < Co2‡ < Ni2‡ < Cu2‡
> Zn2‡. The sequence of stability ꢂMn2‡ < Co2‡ < Ni2‡ < Cu2‡ > Zn2‡†
of the complexes of PSR are in agreement with that found by Irving and
Williams [15, 16]. The order re¯ects the changes in the heat of complex
formation across the series and arise from a combination of the in¯uence of
both the polarizing ability of the metal ion [17] and crystal ®eld stabilization
energies [18]. The greater stability of Cu2‡ complexes result from the well
known Jahn-Teller effect. The stability constant of the divalent oxygenated
cation complex ꢂUO22‡† is higher than that of the other divalent cation
complexes (Table 2). This may be attributed to the bonded oxygen atoms
which may increase the electrostatic attraction between the metal ion and the
coordinated ligands and overcome any steric hindrance offered by the oxygen
of the oxygenated cation [19]. The higher values of the stability constants of
Th4‡ complexes compared to those of the other divalent metal complexes is
expected on the basis of charge and ionization potential of Th4‡.
Effect of temperature
The dissociation constants (pK1H and pK2H) for PSR as well as the stability constants
of its complexes with Mn2‡; Co2‡; Ni2‡; Cu2‡; Zn2‡; La3‡; Ce3‡; Gd3‡; UO22‡, and
Th4‡ have been evaluated at 303, 308, and 318 K and are given in Tables 1 and 2.
The slope of the plot of (pKH or logK vs. T1) was utilized to evaluate the enthalpy
change for the dissociation or complexation process, respectively. From the free
energy change and ÁH values one can deduce the entropy changes (ÁS) using Eqs.
(1) and (2).
ÁG ˆ ꢁ2:303RTlogK ˆ 2:303 RT Á pK
ꢂ1†
ꢂ2†
ÁH ꢁ ÁG
ÁS ˆ
T

Complexes of Substituted Rhodanine III
1263
Table 3. Thermodynamic functions for ML and ML2 complexes of PSR in 20% (v/v) ethanol-water and 0.1 M KCl (error values
in parentheses)
Mn‡
T (K)
ÁG (kJ Á molꢁ1
)
ÁH (kJ Á molꢁ1
)
ÁS (J Á molꢁ1 Á Kꢁ1
)
ꢁÁG1
ꢁÁG2
ꢁÁGꢀ
ÁH1
ÁH2
ÁHꢀ
ÁS1
ÁS2
ÁSꢀ
Mn2‡
Co2‡
Ni2‡
Cu2‡
Zn2‡
La3‡
Ce3‡
Gd3‡
UO22‡
Th4‡
303
308
318
303
308
318
303
308
318
303
308
318
303
308
318
303
308
318
303
308
318
303
308
318
303
308
318
303
308
318
40.61
38.51
36.84
26.69
31.61
42.62
38.12
40.10
43.84
57.32
59.15
62.71
43.57
44.88
46.94
42.18
44.82
48.22
43.34
43.76
44.81
42.24
42.58
43.53
48.91
53.61
59.37
59.76
61.16
63.57
29.01
29.07
29.59
25.24
30.02
35.50
19.96
28.13
37.14
35.27
43.99
50.66
29.01
33.85
39.39
36.32
37.51
42.62
40.61
40.75
41.53
40.32
40.69
41.71
42.87
46.35
50.66
53.03
55.43
58.82
69.62
67.58
66.43
51.93
61.63
78.12
58.08
68.23
80.98
92.59
103.14
113.37
72.58
78.73
86.33
78.5
ꢁ121.2 ꢁ17.81 ꢁ139.01
37
37
(Æ1)
(Æ1)
306.35
80.42
53.61
25.47
82.33
189.56
338.9
495.91
419.32
338.9
1098
710
1808
(Æ1)
(Æ2)
(Æ2)
391
1186
1577
(Æ3.7)
(Æ3.7)
(Æ3.7)
285.29
187.64
93.82
366
1061
1427
(Æ5.3)
(Æ5.3)
(Æ5.3)
213.11
176.15
288
716
1004
(Æ2)
(Æ2)
(Æ2)
82.33
90.84
83.95
84.51
86.34
82.56
83.27
85.24
91.78
99.96
110.03
112.79
116.59
122.39
412
428
840
(Æ1)
(Æ1.7)
(Æ1.7)
ꢁ13.98 ꢁ22.98 ꢁ36.96 97
(Æ1.5)
58
155
(Æ1.5)
(Æ1.5)
ꢁ16.66 ꢁ12.83 ꢁ29.49 84.5
(Æ0.5)
91
175.5
(Æ0.5)
(Æ0.5)
168.49
118.71
287.20
719
534.5
837.71
(Æ1.7)
(Æ1.5)
(Æ1.5ꢁ1.7)
17.81
66.44
84.25
256
395
651
(Æ1)
(Æ1)
(Æ1)
All thermodynamic parameters of the dissociation process of PSR are given in
Table 1. Inspection of these values reveals that
(i) the stepwise pKH values decrease with increasing temperature, demonstrating
that its acidity increases with increasing temperature;
(ii) ÁH is positive, indicating that dissociation is accompanied by adsorption of
heat and the process is endothermic;
(iii) ÁG is positive, indicating that the dissociation process is not spontaneous;
(iv) ÁS is negative due to increased order as a result of the solvation process.
All thermodynamic parameters of the stepwise stability constants of PSR
complexes are given in Table 3. It is known that the divalent metal ions exist in

1264
A. A. El-Bindary et al.
solution as octahedrally hydrated species, and the values of ÁH and ÁS can then
be considered as a sum of two contributions: release of H2O molecules and metal-
ligand bond formation.
It has been suggested [20] that ions in aqueous solution order the water
molecules around them. During complex formation between oppositely charged
ions (L2ꢁ and M n‡), a breakdown of this metal-water arrangement will occur
resulting in positive entropy and enthalpy changes. The following statements can
be made:
(i) The stepwise stability constants (logK1 and logK2) for PSR complexes
decrease with increasing temperature for Mn2‡; Ce3‡, and Gd3‡, whereas with
Co2‡; Ni2‡; Cu2‡; Zn2‡; La3‡; UO22‡, and Th4‡ the reverse takes place, i.e. its
stability constants increase with increasing temperature.
(ii) The negative values of ÁG for the complexation process of Mn2‡; Co2‡;
Ni2‡; Cu2‡; Zn2‡; La3‡; Ce3‡; Gd3‡; UO22‡, and Th4‡ with PSR suggests a
spontaneous nature.
(iii) The ÁH values are negative for Mn2‡; Ce3‡, and Gd3‡, but positive for
Co2‡; Ni2‡; Cu2‡; Zn2‡; La3‡; UO22‡, and Th4‡. This means that these
processes are exothermic and favourable at lower temperatures for Mn2‡;
Ce3‡, and Gd3‡ complexes, whereas in the case of Co2‡; Ni2‡; Cu2‡; Zn2‡;
La3‡; UO22‡, and Th4‡ complexes the processes are endothermic and favour-
able at higher temperature.
(iv) The positive values of ÁS for the complexation process of PSR with
Mn2‡; Co2‡; Ni2‡; Cu2‡; Zn2‡; La3‡; Ce3‡; Gd3‡; UO22‡, and Th4‡ indicate
that the arrangement of the solvent around the metal ion is lost when the
complex is formed.
Stoichiometries
The stoichiometries of the complexes of PSR with Mn2‡; Co2‡; Ni2‡; Cu2‡; Zn2‡;
La3‡; Ce3‡; and UO22‡ result from spectrophotometric measurements in the range
of 200±400 nm at the optimum pH values. These studies were carried out both
according to the molar ratio [21] and continuous variation [22] methods. Both
methods indicated the formation of 1:1 and 1:2 (metal:ligand) complexes (Table 4).
Table 4. Stoichiometry of Mn‡-PSR complexes in 20% (v/v) ethanol-water by molar ratio (MR) and
continuous variation (CV) methods at 298 K
pH
ꢁmax (nm)
MR
CV
Mn2‡
Co2‡
Ni2‡
Cu2‡
Zn2‡
La3‡
Ce3‡
UO22‡
8
9
9
7
8
9
7
6
380
280
450
420
380
380
380
380
1:1
1:1
1:1
1:1
1:1
1:1
1:1
±
1:2
1:2
1:2
1:2
±
1:1
1:1
±
±
±
1:2
1:2
±
±
1:1
1:1
1:1
±
1:2
1:2
1:2
1:2
±
1:2

Complexes of Substituted Rhodanine III
1265
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Received March 11, 1998. Accepted (revised) May 29, 1998