Potentiometric studies on the complexation reactions of N-(2,2-[1-(3-Aminophenyl)ethylidene]hydrazino-2-oxoethyl)benzamide with Ni 2+, Cu2+, and Cd2+ ions in aqueous dioxane and micellar media

Article abstract of DOI:10.1021/je900583g

N-(2,2-[1-(3-Aminophenyl)ethylidene] hydrazino-2-oxoethyl)benzamide, aehb, was prepared by condensation of N-benzoyl glycine hydrazide with 3-aminoacetophenone and characterized on the basis of elemental and spectral data. Potentiometric studies of the Ni(II), Cu(II), and Cd(II) complexes of aehb were carried out at a constant ionic strength (I, mol ·dm^-3 ) 0.10 M KNO₃) at different temperatures, T = (290, 300, and 310) K, in aqueous dioxane (40 %) and in nonionic and anionic micellar media. The ligand accepted one proton in the pH range of 5.0 to 11.0 and reacted with the Ni ²⁺, Cu²⁺, and Cd²⁺ ions in all media to form only 1:1 metal-ligand complexes. The protonation constant of aehb and the stability constants of the complexes had been evaluated from the pH metric titration curves employing Irving and Rossotti's pH metric titration technique. The stability constant of the complexes was in the following order: Cu(II) > Ni(II) > Cd(II). Thermodynamic parameters associated with the protonation and complexation reactions of aehb were also calculated. The values of ΔG and ΔH were negative for all of the systems in all of the media, which suggested that all of the reactions were exothermic and enthalpy-driven. The presence of surfactants in the media resulted in a decrease of the protonation constant of the ligand and stability constants of the complexes due to the compartmentalizing action of the micelles on the reacting species, and the values in different media followed this trend: aqueous > Triton X-100 (Tx-100) > Labs.

Full text of DOI:10.1021/je900583g

1166
J. Chem. Eng. Data 2010, 55, 1166–1172
Potentiometric Studies on the Complexation Reactions of
N-(2,2-[1-(3-Aminophenyl)ethylidene]hydrazino-2-oxoethyl)benzamide with Ni2+,
Cu2+, and Cd2+ Ions in Aqueous Dioxane and Micellar Media
Promila Devi Thangjam and Lonibala Rajkumari*
Department of Chemistry, Manipur University, Canchipur 795 003, Manipur, India
N-(2,2-[1-(3-Aminophenyl)ethylidene] hydrazino-2-oxoethyl)benzamide, aehb, was prepared by condensation
of N-benzoyl glycine hydrazide with 3-aminoacetophenone and characterized on the basis of elemental and
spectral data. Potentiometric studies of the Ni(II), Cu(II), and Cd(II) complexes of aehb were carried out at
a constant ionic strength (I, mol · dm-3 ) 0.10 M KNO3) at different temperatures, T ) (290, 300, and 310)
K, in aqueous dioxane (40 %) and in nonionic and anionic micellar media. The ligand accepted one proton
in the pH range of 5.0 to 11.0 and reacted with the Ni2+, Cu2+, and Cd2+ ions in all media to form only 1:1
metal-ligand complexes. The protonation constant of aehb and the stability constants of the complexes had
been evaluated from the pH metric titration curves employing Irving and Rossotti’s pH metric titration
technique. The stability constant of the complexes was in the following order: Cu(II) > Ni(II) > Cd(II).
Thermodynamic parameters associated with the protonation and complexation reactions of aehb were also
calculated. The values of ∆G and ∆H were negative for all of the systems in all of the media, which suggested
that all of the reactions were exothermic and enthalpy-driven. The presence of surfactants in the media
resulted in a decrease of the protonation constant of the ligand and stability constants of the complexes due
to the compartmentalizing action of the micelles on the reacting species, and the values in different media
followed this trend: aqueous > Triton X-100 (Tx-100) > Labs.
that reaction behavior observed at surfactant interfaces is
expected to be more representative of many biological reactions
than reactions studied in dilute aqueous solutions.11 Protonation
constants of ciprofloxacin and formation constants of Fe(III)
complexes were greatly influenced12 by the presence of sodium
dodecyl sulfate and cetyl trimethylammonium bromide (CTAB),
and the effects of surface active substances could be used in
modeling studies of iron based antianemia drugs or multivitamin
formulations and ciprofloxacin interactions in vivo. It has also
been reported13-15 that metal complexes show better micelli-
zation and biocidal properties than the parent compounds. Shin
et al.16,17 evaluated the values of formation constants of Cu(II)
and Cd(II) complexes of SCN- in both aqueous and micellar
phases. The study suggested that the anionic complexes
[M(SCN)3]- may form ion pairs in micelles to keep electric
neutrality and be stabilized in the hydrophobic environment of
micelles. The behavior of metal complex formation in surfactant
micellar solutions has been demonstrated in aqueous solution18
and employed in removing heavy metals from soils, under acidic
and alkaline conditions, through direct complexation followed
by solubilization.
1. Introduction
Metal complexes of Schiff bases have played a central role
in the development of coordination chemistry. Development of
the field of bioinorganic chemistry has also led to an increased
interest in Schiff base complexes, since it has been recognized
that many of these complexes may serve as models for
biologically important species especially for metalloenzymes,1
and have a variety of applications including biological, clinical,
analytical, and industrial ones in addition to their important roles
in catalysis and organic synthesis.2-7 It is also well-known that
some drugs exhibit increased activity when administered as
metal complexes.8 To understand the complex formation ability
of the ligands and the activities of the complexes, it is necessary
to have detailed knowledge about the thermodynamic and
solution equilibria involved in the reactions. The extent to which
a ligand binds to a metal ion is normally expressed in terms of
the stability constant, and information about the concentration
of a metal complex in an equilibrium mixture can be predicted
on the basis of the formation constants in solution. A survey of
the literature reveals that most of the reported studies on the
formation and stability constants of the complexes of Schiff
bases are carried out in aqueous or mixed aqueous organic
solvent media. Although there are reports9,10 that the presence
of micelles in the systems affects many chemical reactions, not
enough attention has been given to understand the effect of
micelles on metal complexation reactions. Since micelles
represent a multiphase system where a species may be distrib-
uted in both the bulk aqueous phase and on the micelle’s surface,
a study of metal/Schiff base complexation reactions in micellar
media would assume critical significance in view of the fact
Thus, in an attempt to quantify the influence of the micelles
on the protonation and metal complex formation, we have
measured the acid dissociation constant of a new Schiff base,
N-(2,2-[1-(3-aminophenyl)ethylidene]hydrazino-2-oxoethyl)ben-
zamide (aehb, Figure 1) and the formation constants of Ni(II),
Cu(II), and Cd(II) complexes in aqueous dioxane, nonionic
Triton X-100 (Tx-100), and anionic Labs micellar media at a
constant ionic strength and at three different temperatures. We
report herein the equilibrium and stability constants of the
complexes and thermodynamic parameters associated with the
protonation and complexation reactions calculated based on
* Corresponding author. E-mail: lonirk@yahoo.co.uk. Fax: 0385-2435145.
10.1021/je900583g  2010 American Chemical Society
Published on Web 11/17/2009

Journal of Chemical & Engineering Data, Vol. 55, No. 3, 2010 1167
310) K. All of the titrations were stopped when the pH readings
became unstable because of the hydrolysis of the metal ions.
In all cases, no calculations were performed beyond the
precipitation point. Therefore, the hydroxyl species likely to be
formed beyond this point could not be studied.
2.3. Calculations. The equations of Irving and Rossotti21,22
were used to determine the protonation constant of the ligand
and the stability constants of the complexes. The average number
of protons associated with the ligand (njH) at various pH meter
readings was determined from the acid and ligand titration
curves employing eq 1 of Irving and Rossotti.
Figure 1. Structure of aehb.
the formation constants. The effects of the nonionic and anionic
micelles on the protonation of the ligand and formation of the
metal complexes have also been reported.
(VL - VA)(N + E0)
2. Experimental Section
njH ) Y -
(1)
(V0 + VA)TL0
2.1. Materials and Solutions. N-Benzoyl glycine, 3-ami-
noacetophenone, 1,4-dioxane, KOH, KNO3, and metal salts
MCl2 ·nH2O (M ) Ni, Cu, and Cd) were purchased from Merck.
The surfactants used in this study (Tx-100 and Labs) were from
Sigma-Aldrich and used as obtained without any purification.
All of the other chemicals were of AnalaR grade. The solutions
used in the potentiometric titrations were prepared in double-
distilled water. The KOH solution was standardized19 with
standard oxalic acid solution (0.05 M), and the standard alkali
solution was again used for the standardization of HNO3. The
metal salt solutions were also standardized.19 Since the ligand
was not soluble in water, the stock solution of the ligand was
prepared in 40 % aqueous dioxane solution. The critical micellar
concentrations (CMCs) of Tx-100 and Labs are (0.24 and 1.6)
mmol, respectively. Therefore, the concentration of the solutions
of the nonionic surfactant Tx-100 and anionic surfactant Labs
used for preparing the reaction mixtures for the potentiometric
titrations in micellar media was 5 mmol each, well above the
CMCs of the surfactants, so that the surfactant molecules would
aggregate to form micelles.
2.2. Apparatus and Procedure: Potentiometric Titration. A
digital pH meter, Eutech Cyberscan pH 1100, with a glass
calomel electrode was employed for the potentiometric titrations
at three different temperatures [(290, 300, and 310) K]. The
desired temperature for the titrations was maintained using a
thermostat model (D8-G Haake Mess-Techinik). The pH meter
was standardized before each titration with standard buffer
solutions of pH 4.00, 7.00, and 9.00 obtained from Eutech
Instruments, Singapore.
For evaluating the protonation constant of the ligand and the
formation constants of the complexes in aqueous dioxane media,
the following three sets of reaction mixtures, (a) 2.5 mL of
HNO3 (0.001 M) + 5 mL of 0.1 M KNO3, (b) 2.5 mL of HNO3
(0.001 M) + 5 mL of 0.1 M KNO3 + 1.5 mL of aehb (0.0006
M), and (c) 2.5 mL of HNO3 (0.001 M) + 5 mL of 0.1 M KNO3
+ 1.5 mL of aehb (0.0006 M) + 0.75 mL of MCl2 ·nH2O
(0.0003 M), were prepared. Low concentrations of metal ions
were used to preclude the possibility of the formation of
polynuclear complexes,20 while the metal-ligand ratio was
maintained at 1:2 molar concentrations. The volume of each
set was made up to 25 mL with 40 % (v/v) aqueous dioxane
mixture. The ionic strength of the solutions was maintained at
0.1 M using KNO3 as the background electrolyte. The reaction
mixtures were potentiometrically titrated against standard 0.02
M KOH solution at 290 K. For the titration in micellar media,
1.20 mL (5 mmol) of Tx-100 and 2.25 mL (5 mmol) of Labs
were added separately in each set of the above reaction mixtures
before making up the volume to 25 mL, and the reaction
solutions were also potentiometrically titrated against the
standard alkali solution at 290 K. These titrations in both
aqueous dioxane and micellar media were repeated at (300 and
where Y is the number of dissociable protons present in the
ligand. VL and VA are the volumes of KOH of concentration N
(0.02 M) consumed by solutions (a) and (b), respectively, for
the same pH reading, and (VL - VA) measures the displacement
of the ligand curve with respect to the acid curve. V0 is the
initial volume of the reaction mixture (25 mL), and E0 and TL0
are the resultant concentrations of nitric acid (0.001 M) and
the ligand (0.0006 M) in the reaction mixture, respectively. The
protonation curves for the proton-ligand systems were obtained
by plotting njH versus pH, and the protonation constants have
been evaluated from the protonation curves at half njH values
using Bjerrum’s half integral method.23
nj, the average number of ligands attached per metal ion, and
pL, the free ligand exponent, were calculated from the experi-
mental titration curves using eqs 2 and 3.
(VM - VL)(N + E0)
(V0 + VA)TM0njH
nj )
(2)
and
n)j
n
1
ꢀ H
∑
n
(
)
antilogpH
TL0 - njTM0
V0 + VM
n)0
pL ) log10
·
(3)
[
]
V0
where N, E0, VL, V0, and VA have the same meaning as in eq 1.
VM is the volume of alkali added to solution c to attain the pH
reading as that of VA and VL, and T0M is the metal ion
concentration (0.0003 M) in the reaction mixture. ꢀnH is the
overall proton-ligand stability constant. The metal-ligand
stability constants have been determined from the metal-ligand
formation curves obtained by plotting the values of nj against
pL.
2.4. Preparation and Characterization of the Ligand. N-
Benzoyl glycine hydrazide was prepared from N-benzoyl glycine
as previously reported.24 N-(2,2-[1-(3-Aminophenyl) ethylidene]
hydrazino-2-oxoethyl) benzamide, aehb, was prepared by re-
fluxing ethanolic solutions of N-benzoyl glycine hydrazide (0.02
M, 1.0 g in 30 mL) and 3-aminoacetophenone (0.02 M, 0.7 g
in 10 mL) for 4 h. The white precipitate obtained on slow
cooling of the reaction mixture was filtered and washed
repeatedly with ethanol. It was then recrystallized from hot
ethanol and dried at room temperature (% yield ) 60 %; mp of
(486 to 488) K; M+ peak at 311 as the base peak in the mass
spectrum of the ligand; see Figure 2).
Elemental Analysis: Found % (calcd %) for C17H18N4O2.
C, 65.60 (65.80); H, 5.80 (5.80); N, 17.89 (18.06); N2H4, 10.40
(10.32).

1168 Journal of Chemical & Engineering Data, Vol. 55, No. 3, 2010
Figure 2. Mass spectrum of aehb.
Figure 3. Acid and ligand potentiometric titration curves at 300 K and I )
0.10 mol·dm-3 KNO3 in different media.
Figure 4. Potentiometric titration curves of M(II)-aehb complexes at 300
K and I ) 0.10 mol ·dm-3 KNO3 in different media.
IR (ν, cm-1). 1688 (amide I), 1577 (amide II), 1329 (amide
III) of hydrazidic moiety, 1634 (amide I), 1552 (amide II), 1311
(amide III) of benzamide moiety, 1597 (CN), 995 (NN).
1H NMR (δ). 10.74, 10.92 (d, NHCO), 8.98, 9.14 (d,
C6H5CONH), 2.50 (d, NC-CH3), 4.77 (s, CH2), 5.44 (s, NH2),
6.93 to 8.22 (m, ring protons).
ligand titration curves in Tx-100 and Labs micellar media in
the initial stage were shifted to a higher pH as compared to
those in aqueous dioxane media. This indicates that the presence
of the surfactants caused an increase in the basicity of the ligand.
However, as the ligand protonated, there was a decrease in the
basicity of the ligand, and the drop in the basicity was
comparatively more in the presence of Labs.
13C NMR (ppm). 171.07 (s, > CO hydrazide), 166.82, 165.93
(d, > CO benzamide), 41.37 (s, CH2), 148.55 (s, NC), 13.57,
14.24 (d, CH3), 111.45 to 138.81 (9s, ring carbons).
At 300 K, the protonation of the ligand occurred over the
pH range of 5.0 to 11.0 in aqueous dioxane, 5.5 to 10.5 in Tx-
100, and 6.5 to 9.8 in Labs micellar media. A single inflection
point observed in the pH range of 5.0 to 11.5 in the ligand curves
in all of the media showed the neutralization of only one proton.
The addition of a metal ion to the free ligand solution shifted
the buffer region of the ligand to a lower pH value as observed
in the curves a to i in Figure 4. This was an indication that
complex formation was proceeded by releasing protons from
the base. A large decrease in pH for the metal titration curves
relative to the ligand curve might be attributed to strong
metal-ligand interaction. The metal-ligand curves were well-
separated from the ligand curves. However, the pH ranges at
which the separation of the complex curves from the ligand
curves occurred were different in different media. The curve
containing the Cu2+ ion started separate from the ligand curve
at pH of 5.5, 5.0, and 5.7 and showed a wide divergence at pH
7.0, 6.6, and 6.5 in aqueous dioxane, Tx-100, and Labs media,
respectively, indicating that the complexes were formed in these
pH ranges in the respective media. Similarly, the formation of
the Ni(II) complex occurred in the pH ranges of 6.2 to 8.8, 6.4
to 8.8, and 6.6 to 8.6, while Cd(II) complexes were formed at
pH 6.2 to 8.7, 6.4 to 8.7, and 7.8 to 8.4 in aqueous, Tx-100,
and Labs media, respectively. The acid-ligand and metal-ligand
2.5. Physical Measurements. Hydrazine was determined
volumetrically19 by titrating against KIO3 after subjecting the
compound to acid hydrolysis for 4 h. C, N, and H were
microanalyzed using a Perkin-Elmer C, H, and N analyzer model
240C. An IR spectrum was obtained using a Shimadzu Fourier
transform infrared (FTIR) spectrophotometer 8400S in KBr
medium,whilethemassspectrumwasrecordedonaJEOLSX102/
1
DA6000 mass spectrophotometer and the H and 13C NMR
spectra on a Jeol AL 300 FT NMR spectrometer.
3. Results and Discussion
3.1. Titration CurWes. The ligand was potentiometrically
titrated with standard KOH at I ) 0.1 mol ·dm-3 KNO3 and at
three different temperatures [(290, 300, and 310) K] in aqueous
dioxane and in Tx-100 and Labs micellar media. The poten-
tiometric titration curves at 300 K in different media are given
in Figures 3 and 4, where Figure 3 represents the acid and the
ligand titration curves, and the metal-ligand titration curves
for Ni(II), Cu(II), and Cd(II) complexes in aqueous dioxane and
in Tx-100 and Labs micellar media are exhibited in Figure 4.
Figure 3 shows that at a constant temperature, for the same
volume of alkali added to the reaction solution, the acid and

Journal of Chemical & Engineering Data, Vol. 55, No. 3, 2010 1169
Scheme 1
titration curves at (290 and 310) K showed similar features,
though the pH ranges at which the protonation of the ligand
and the formation of the complexes occurred were found to vary.
The presence of the surfactants in the media caused a shift in
the complex curves toward a higher pH value. These observa-
tions would suggest stronger proton-ligand and metal-ligand
interactions in aqueous dioxane media as compared to those in
surfactant media.
3.2. Protonation Constant. The protonation curves for the
ligand at three different temperatures, (290, 300, and 310) K,
in aqueous dioxane and in Tx-100 and Labs micellar media were
obtained by plotting njH versus pH, and Figure 5 represents the
protonation curves for the ligand at 300 K in different media.
The curves extended from 0.1 to 1.0 on the njH scale, showing
that only one proton dissociated from the ligand in all media.
The dissociation constants (log KHaehb) of the ligand at different
temperatures and in different media were evaluated from the
curves using Bjerrum’s half integral method23 and are collected
in Table 1. In all media, the values of log KHaehb were observed
to decrease with an increase in temperature, showing that the
dissociation of the ligand was exothermic and favorable at lower
temperatures. The values so obtained were in good agreement
with the reported data.22
Hydrazones are reported25-27 to bond to metal ions through the
carbonyl oxygen and hydrazinic nitrogen/nitrogen of the imino
group in the aqueous medium where there is a conjugate chelate
ring formed by ligand enolization in the complexes. Thus, aehb
was also expected to exhibit keto-enol tautomerism, and the amido
proton could be dissociated through enolization. As shown by
Figure 2, two dissociable amido protons are present in the ligand.
However, pH metric studies indicated the dissociation of only one
proton from the base. Studies23,28-31 on the metal complexes of
hydrazones derived from the N-benzoyl glycine hydrazide showed
that of the two amide groups, only the hydrazidic group would
deprotonate and be involved in the complexation with metal ions,
while the benzamide group would not participate in complex
formation. Therefore, for aehb, too, only the amido proton of the
hydrazidic group would be dissociated through enolization, and
the keto-enol tautomerism exhibited by the ligand might be as
shown in Scheme 1.
At constant temperature, the protonation constants decreased
in the presence of surfactants. The trend in the protonation
constant values in aqueous dioxane and in Tx-100 and Labs
micellar media, aqueous > Tx-100 > Labs, showed that the
affinity of the ligand for protons decreased in presence of the
surfactants in the media.
3.3. Metal-Ligand Stability Constant. The metal-ligand
formation curves at temperatures of (290, 300, and 310) K and
in aqueous dioxane and in Tx-100 and Labs micellar media were
obtained by plotting nj versus pL, calculated using eqs 2 and 3.
The metal-ligand formation curves at 300 K in aqueous dioxane
and in Tx-100 and Labs micellar media are shown in Figure 6.
It has been observed from the figure that the formation curves
of Cu(II) complexes in all three media extended from 0.2 to
1.0, while those of Ni(II) and Cd(II) complexes extended from
0.1 to 0.85 on the nj scale. Thus, the values of nj, 0.1 < nj < 1.5,
indicated the formation of only 1:1 metal-ligand complexes
in all media. The metal-ligand stability constants (log KaMehb
)
of Cu(II), Ni(II), and Cd(II) complexes at temperatures of (290,
300, and 310) K and in aqueous dioxane and in Tx-100 and
Labs micellar media evaluated from these curves are given in
Table 2. There was a gradual decrease in the stability constants
of the complexes in all media with an increase in temperature
showing that the complexation reactions were exothermic in
nature and hence favorable at lower temperature. This behavior
might also be ascribed to the thermal hydrolysis of the metal
complexes.22
Figure 5. Proton-ligand formation curves at 300 K and I ) 0.10 mol ·dm-3
KNO3 in different media.
Table 1. Protonation Constant (log KHaehb) of aehb and Thermodynamic Parameters of the Dissociation Reaction of the Ligand at Different
Temperatures and at I ) 0.10 mol ·dm-3 KNO3 in 40 % (v/v) Aqueous Dioxane and in Tx-100 and Labs Micellar Media
medium
temperature (K)
log KHaehb
-∆G (kJ·mol-1
)
-∆Ha (kJ·mol-1
)
-∆Sa (J·deg-1 ·mol-1
)
aqueous dioxane
290
300
310
290
300
310
290
300
310
10.84 ( 0.02
10.68 ( 0.03
10.24 ( 0.02
10.75 ( 0.02
10.25 ( 0.13
10.05 ( 0.03
10.26 ( 0.03
10.09 ( 0.05
9.82 ( 0.01
60.19 ( 0.01
61.35 ( 0.01
60.78 ( 0.03
59.69 ( 0.02
58.88 ( 0.04
59.65 ( 0.02
56.97 ( 0.05
57.96 ( 0.01
58.29 ( 0.01
51.34 ( 0.02
+33.37 ( 0.05
Tx-100
Labs
61.54 ( 0.04
37.93 ( 0.02
8.87 ( 0.05
+66.77 ( 0.02
a ∆H and ∆S were computed using the linear fit program.

1170 Journal of Chemical & Engineering Data, Vol. 55, No. 3, 2010
Ni(II) > Cd(II). The values of stability constants for the Cu(II)
complex were higher than those for Ni(II) and Cd(II) complexes
in all media. The extra stabilization of the Cu(II) complex could
be attributed to the unique electronic configuration on Cu2+ and
the Jahn-Teller effect. As observed in the protonation curves
of the ligand in different media, Figure 6 also showed that the
presence of anionic and nonionic surfactants in the media caused
a shift in the formation curves of all of the complexes toward
the left.
3.4. Effect of Temperature. The thermodynamic parameters
associated with the protonation of aebh and the formation of
1:1 complexes in the systems (M(II) + aehb) were also studied
at I ) 0.10 mol ·dm-3 KNO3 in different media. The change in
free energy (∆G) was calculated from the formation constant
values (log K) at various temperatures and in different media
using the following equation:
Figure 6. Metal-ligand formation curves of M(II)-aehb chelates at 300
K and I ) 0.10 mol ·dm-3 KNO3 in different media.
Table 2. Stability Constants (log KMaehb) of M(II)-aehb Chelates (1:1)
at Different Temperatures and I ) 0.10 mol·dm-3 KNO3 in 40 % (v/v)
Aqueous Dioxane and in Tx-100 and Labs Micellar Media
∆G ) -2.303RT log K
(4)
where R (ideal gas constant) ) 8.314 J ·K-1 ·mol-1; K )
protonation constant of aehb or stability constant of the
complexes; T ) absolute temperature (K).
metal ions
temperature
The enthalpy change (∆H) for the dissociation of the ligand
and complexation reactions in different media was evaluated
from the slope of the plot of log K1H or log K versus 1/T (Figure
7) using the graphical representation of the van’t Hoff eq 5,
and the change in entropy (∆S) could then be calculated using
relationship from eq 6:
medium
(K)
Cu
Ni
Cd
aqueous dioxane
290
300
310
290
300
310
290
300
310
7.41 ( 0.03 5.92 ( 0.00 5.49 ( 0.11
7.32 ( 0.04 5.54 ( 0.01 5.39 ( 0.05
7.25 ( 0.01 5.23 ( 0.01 5.16 ( 0.13
7.35 ( 0.01 5.60 ( 0.01 5.26 ( 0.08
7.16 ( 0.01 5.40 ( 0.01 5.12 ( 0.06
7.03 ( 0.01 5.21 ( 0.04 4.96 ( 0.07
7.30 ( 0.01 5.25 ( 0.02 5.18 ( 0.03
7.07 ( 0.04 5.19 ( 0.03 5.08 ( 0.09
6.97 ( 0.05 4.88 ( 0.01 4.88 ( 0.08
Tx-100
Labs
∆G ) ∆H - T∆S
∆S ) (∆H - ∆G)/T
(5)
(6)
The calculated thermodynamic parameters for the dissociation
of the ligand in both aqueous dioxane and micellar media are
included in Table 1, and those for the complexation reactions
are collected in Table 3.
At constant temperature, the stability constants of the
complexes in aqueous dioxane and in Tx-100 and Labs micellar
media fell in the Irving-Williams order21,22 where Cu(II) >
In the aqueous dioxane media, the ∆G values for both
protonation and complexation reactions were negative, indicating
the spontaneity of the reactions. The tables show that the ∆G
values had no sharp behavior with temperature, indicating the
independent nature of the reactions with respect to temperature.32
Negative ∆H values and positive ∆S values for the protonation
of the ligand suggest that the reaction was both enthalpy and
entropy-driven in aqueous dioxane media. The ∆G and ∆H
values associated with the complexation reactions were negative.
Thus, it is evident that the reactions were spontaneous and
enthalpy-driven and the values followed the order Cu(II) > Ni(II)
> Cd(II), which is consistent with the Irving-Williams series.21,22
Positive ∆S terms for Cu(II) and Cd(II) complexes indicated
that the formation of these complexes was entropy-favored,
while ∆S values were negative for the Ni(II) complex suggesting
a highly solvated metal complex.
Figure 7. Plot of log KMaehb vs 1/T in different media.
Table 3. Thermodynamic Parameters of M(II)-aehb Chelates (1:1) at Different Temperatures and I ) 0.10 mol ·dm-3 KNO3 in 40 % (v/v)
Aqueous Dioxane and in Tx-100 and Labs Micellar Media
-∆G (kJ·mol-1
)
M(II)-aehb
medium
complexes
290 K
300 K
310 K
-∆Ha (kJ·mol-1
)
-∆Sa (J·deg-1 ·mol-1
)
aqueous dioxane
Cu
Ni
Cd
Cu
Ni
Cd
Cu
Ni
41.14 ( 0.04
32.87 ( 0.02
30.48 ( 0.06
40.81 ( 0.03
31.09 ( 0.03
29.21 ( 0.06
40.53 ( 0.01
29.15 ( 0.02
28.76 ( 0.05
42.05 ( 0.02
31.82 ( 0.02
30.96 ( 0.05
41.13 ( 0.01
31.02 ( 0.01
29.41 ( 0.04
40.61 ( 0.02
29.81 ( 0.02
29.18 ( 0.04
43.03 ( 0.01
31.04 ( 0.01
30.63 ( 0.03
41.73 ( 0.01
30.92 ( 0.02
29.44 ( 0.06
41.37 ( 0.02
28.96 ( 0.02
28.96 ( 0.07
13.94 ( 0.02
60.07 ( 0.02
28.30 ( 0.04
27.93 ( 0.01
33.87 ( 0.02
25.98 ( 0.04
28.98 ( 0.02
27.04 ( 0.02
25.77 ( 0.05
+93.70 ( 0.02
94.17 ( 0.03
+8.87 ( 0.04
+44.00 ( 0.02
9.50 ( 0.03
Tx-100
Labs
+11.43 ( 0.04
+38.77 ( 0.01
5.50 ( 0.03
Cd
+11.37 ( 0.05
a ∆H and ∆S were computed using the linear-fit program.

Journal of Chemical & Engineering Data, Vol. 55, No. 3, 2010 1171
solvent molecules. From the observed trend, it would appear
that the ligand was better stabilized in the core of the anionic
micelles.
On the basis of the foregoing discussion, the chelation of the
base with Cu2+, Ni2+, and Cd2+ ions appeared to be through
the enolic oxygen of the hydrazinic carbonyl group forming
the covalent bond and the azomethine nitrogen forming a
coordinate covalent bond, as shown in Figure 8.
Figure 8. Representative structure showing the bonding sites of the ligand.
M ) Cu, Ni, or Cd.
A decrease in log K with an increase in temperature and the
negative values of ∆H for all of the reactions in all of the media
showed exothermic and enthalpy-favored reactions. This result
would suggest the importance of the enthalpy rather than the
entropy factor in both the protonation and the complexation
processes.
Supporting Information Available:
Tables 1, 2, and 3 containing values of nj and pL for Cu(II),
Ni(II), and Cd(II) complexes, respectively, in different media
(aqueous dioxane, Tx-100, and Labs) at 300 K at different pH
values. This material is available free of charge via the Internet at
http://pubs.acs.org.
Tables 1 and 3 show that the thermodynamic parameters
were perturbed by the presence of surfactants in the reacting
systems. Though all of the reactions were spontaneous and
enthalpy-driven in micellar media, a gradual decrease in the
∆G values for the protonation and complexation reactions
in the presence of surfactants, in the order aqueous > Tx-
100 > Labs, was observed, while the ∆H and ∆S values
varied randomly. The ∆S value for the protonation of aehb
was negative in Tx-100 media, showing that the protonation
had an unfavorable change of entropy in the presence of Tx-
100, while it was evident from the negative ∆H and positive
∆S values that the reaction was both enthalpy and entropy-
driven in other media.
3.5. Effect of Micelles on the Stability Constants. Micelles
can incorporate reactant molecules in their hydrophobic core
because of their solubilizing properties. The solubilization
of the species into the micellar pseudophase can affect the
absolute and relative concentrations of the reacting species
and thus affect the reaction equilibria.33 If there was an
increasing partitioning of the ligand in favor of the micellar
pseudophase and the actual complexation reaction was
assumed to take place in the bulk aqueous phase, the
possibility of complex formation would decrease, resulting
in a fall of the formation constant.
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Received for review July 12, 2009. Accepted November 6, 2009. The
authors gratefully acknowledge the financial support from the
Department of Science and Technology (DST), New Delhi under the
Scheme No. SR/SI/PC-39/2003 dated 13/09/2004, Sophisticated
Analytical Instrumentation Facilities (SAIF), Central Drug Research
Institute, Lucknow for mass spectral recording, and Head, Department
of Chemistry, Banaras Hindu University for the NMR spectra of the
ligand.
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