Voltammetric, potentiometric and spectrophotometric studies of some hydrazones and their metal complexes in ethanolic-aqueous buffered solutions

Article abstract of DOI:10.1007/s00706-005-0447-z

The electrochemical behavior of some hydrazones derived from 6-chloro-2-hydrazinopyridine in the Britton-Robinson universal buffer of pH 2-11 containing 35% ethanol was investigated at the mercury electrode using dc-polarography, controlled-potential coulometry, and cyclic voltammetry techniques. The examined hydrazones were reduced in solutions of pH < 9 in a single 4-electron diffusion-controlled irreversible step corresponding to both the saturation of -N=C< double bond and cleavage of the -HN-NH- single bond of the hydrazone molecule via the consumption of two electrons for each center. Whereas the starting compound, 6-chloro-2-hydrazinopyridine, was reduced in a single 2-electron diffusion-controlled irreversible step corresponding to cleavage of its -NH-NH₂ single bond. The mechanistic pathway of the electrode reaction of the studied compounds was elucidated and discussed. The pK _a values of the examined hydrazones and the stoichiometry of their complexes in solution with some transition metal ions were determined spectrophotometrically. The dissociation constants and the thermodynamic parameters of the investigated hydrazones, and the stability constants of their metal complexes in solution were determined potentiometrically. Springer-Verlag 2006.

Full text of DOI:10.1007/s00706-005-0447-z

Monatshefte f €u r Chemie 137, 285–299 (2006)
DOI 10.1007/s00706-005-0447-z
Voltammetric, Potentiometric and
Spectrophotometric Studies of Some
Hydrazones and Their Metal Complexes
in Ethanolic-Aqueous Buffered Solutions
ꢀ
Hanaa S. El-Desoky, and Mona A. El-Attar
Mohammed M. Ghoneim , Ibrahim S. El-Hallag, Kamal Y. El-Baradie,
Chemistry Department, Faculty of Science, Tanta University, 31527 Tanta, Egypt
Received May 20, 2005; accepted June 15, 2005
Published online February 3, 2006 # Springer-Verlag 2006
Summary. The electrochemical behavior of some hydrazones derived from 6-chloro-2-hydrazinopyr-
idine in the Britton-Robinson universal buffer of pH 2–11 containing 35% ethanol was investigated at
the mercury electrode using dc-polarography, controlled-potential coulometry, and cyclic voltammetry
techniques. The examined hydrazones were reduced in solutions of pH<9 in a single 4-electron
diffusion-controlled irreversible step corresponding to both the saturation of –N¼C< double bond
and cleavage of the –HN–NH– single bond of the hydrazone molecule via the consumption of two
electrons for each center. Whereas the starting compound, 6-chloro-2-hydrazinopyridine, was reduced
in a single 2-electron diffusion-controlled irreversible step corresponding to cleavage of its –NH–NH2
single bond. The mechanistic pathway of the electrode reaction of the studied compounds was
elucidated and discussed. The pK values of the examined hydrazones and the stoichiometry of their
a
complexes in solution with some transition metal ions were determined spectrophotometrically. The
dissociation constants and the thermodynamic parameters of the investigated hydrazones, and the
stability constants of their metal complexes in solution were determined potentiometrically.
Keywords. Hydrazones; 6-Chloro-2-hydrazinopyridine; Polarography; Coulometry; Cyclic
voltammetry.
Introduction
Hydrazones and their metal complexes are of current interest for their physico-
chemical properties and applications in many important chemical processes that
include sensors, non-linear optics, medicine, and others [1–7]. Here we report the
electrochemical behavior of seven hydrazones derived from 6-chloro-2-hydra-
zinopyridine and their electrode reaction pathway at the mercury electrode in
ꢀ Corresponding author. E-mail: mmghoneim@usa.net

2
86
M. M. Ghoneim et al.
ethanolic-aqueous buffered solutions of pH 2–11. The pK values, dissociation
a
constants, and the thermodynamic parameters of the examined hydrazones, and
the stoichiometry of their complexes in solution with some transition metal ions
and stability constants of these metal complexes were determined.
Results and Discussion
DC-Polarograhic Studies
Polarograms of the examined hydrazones (L1–L7) in the Britton-Robinson (B-R)
universal buffer of pH<9 containing 35% (v=v) ethanol exhibited a single 4-elec-
tron irreversible cathodic wave (e.g. Fig. 1), whereas those of the starting com-
pound 6-chloro-2-hydrazinopyridine (L0) showed a single 2-electron irreversible
cathodic wave. The limiting current (i ) of the studied hydrazones was pH-inde-
l
pendent in solutions of pH<9 (Fig. 2). At higher pH values the limiting current
decreases on the increase of pH with the formation of a large minimum at more
negative potentials which is a behavior usually observed in alkaline media for
hydrazones [8–12]. This behavior was attributed to the protonation that occurs as
a heterogeneous process at the electrode surface, where the amount of adsorbed
conjugate base decreases with increasing negative potential and the limiting cur-
rent is controlled by the rate of protonation [9–13].
Logarithmic analysis of the irreversible polarographic waves [14] of the inves-
tigated compounds by plotting Ed.e versus log ði=i ꢁ iÞ at various pH values dis-
l
played straight lines with slope values S ¼ 55 ꢁ 84 mV (S =mV ¼ 59=ꢀn ), from
1
1
a
which values of ꢀn and the symmetry coefficient ꢀ at the pH values<8 were
a
ꢁ
4
Fig. 1. Dc-polarograms of 1ꢂ10 M solution of compound (L1) in the Britton-Robinson universal
buffer of various pH values: (a) 2.0, (b) 3.6, (c) 4.2, (d) 5.2, (e) 6.4, (f) 8.6, (g) 9.2, (h) 10.2, and (i) 11.2

Hydrazones and Their Metal Complexes
287
Fig. 2. Limiting current (i ) vs. pH plots for 1ꢂ10ꢁ M solutions of the hydrazones L3 (a) and L1 (b)
4
l
in the B-R universal buffer
estimated (Table 1). The most probable values (0.3–0.5) of the symmetry coeffi-
cient ꢀ revealed that the number of electrons n involved in the rate-determining
a
step (the slowest step of the electrode reaction) of the reduction process equals two.
The half-wave potential (E1=2) of the polarographic waves of the examined
compounds shifted to more negative values with the increase of pH indicating
the involvement of protons in the electrode reaction and that the proton-transfer
reaction preceds the electrode process properly [15]. The E1=2 – pH plots of the
reduction waves of the examined compounds were straight lines with slope values
dE1=2=dpH ¼ S ¼ 57–89 mV, {S =mV ¼ (59=ꢀn ) P}. The number of protons P
2
2
a
participating in the rate-determining step of the reduction process was calculated
from Eq. (1) [14] which was expressed in the form of Eq. (2) [14, 16] and was
found to equal one (Table 1).
dE1=2=dpH=mV ¼ ð59=ꢀn ÞP
ð1Þ
ð2Þ
a
P ¼ ðdE1=2=dpHÞ=ð59=ꢀn Þ ¼ S =S
a
2
1
The obtained data indicated that the rate-determining step of the reduction
processes of the examined compounds involved the transfer of one proton
(
P ¼ 1) and two electrons (n ¼ 2), i.e. the ratio P=n ¼ 0.5.
a
a
Controlled-Potential Electrolysis and TLC Studies
The number of electrons n transferred in the overall electrode reduction of each
of the investigated hydrazones was determined by means of controlled-potential

2
88
M. M. Ghoneim et al.
Table 1. Dc-polarographic results of the investigated hydrazones in the ethanolic-aqueous B-R
universal buffer
ꢀ
pH
ꢁE1=2
S1
S2
p (S =S )
2
1
P=na
V
mV
mV
2
.0
1.0
0.5
0.5
Compound L0
4
6
.0
.5
1.00
1.28
55
71
57
89
87
75
70
77
72
66
1.0
0.8
2.1
1.3
1.4
1.6
1.7
1.5
1.6
1.8
1.0
0.7
0.7
0.8
0.8
0.8
0.8
0.9
Compound L1
2
6
.0
.4
0.78
1.17
56
58
1.6
1.5
0.3
0.3
0.4
0.4
0.4
0.4
0.4
Compound L2
2
6
.0
.6
0.90
1.30
62
74
1.4
1.3
Compound L3
1
6
.8
.2
0.84
1.19
67
64
1.1
1.2
Compound L4
2
7
.0
.7
0.98
1.40
67
65
1.0
1.1
Compound L5
2
6
.0
.2
0.85
1.18
60
66
1.3
1.2
Compound L6
2
6
.0
.7
0.65
0.99
74
65
1.0
1.1
Compound L7
1
6
.7
.8
0.75
1.10
80
84
0.8
0.8
electrolysis of various concentrations of the reactant in buffered solutions of
pH values <9 at a mercury pool working electrode and was found to equal four
per hydrazone molecule (L1–L7), whereas it was two electrons in case of their
starting compound (L0). The transfer of four electrons may be attributed to
both the reductive saturation of the –N¼C< double bond (E ) and cleavage of
1
the –HN–NH– single bond (E ) via the consumption of two electrons for each
2
center (E ¼ E ¼ E1=2). Azo-test for the completely electrolyzed solutions of the
1
2
examined hydrazones (L1–L7) in the B-R universal buffer of pH values<9, at a
potential corresponding to the plateau of their polarographic waves revealed the
cleavage of the –HN–NH– single bond with the formation of 2-amino-6-chlor-
opyridine (iii, Scheme 2). Thin layer chromatographic (TLC) analysis of the
extracted product of the hydrazones (L1–L7) confirmed the presence of two
products in their electrolyzed solutions. The UV-spectra of the completely elec-
trolyzed solutions of the examined hydrazones (L1–L7) indicated the disap-

Hydrazones and Their Metal Complexes
289
ꢁ
4
Fig. 3. UV-spectra in an ethanolic-aqueous B-R universal buffer of pH ¼ 5.3 for: 1ꢂ10 M hy-
ꢁ
4
drazone (L1) solution before (1) and after (2) complete electrolysis, and 1ꢂ10 M solution of the
starting compound L0 (3)
ꢀ
Fig. 3, curve 2) compared to that of the analyte before electrolysis (Fig. 3,
pearance of the band due to the n ! ꢁ transition of the –HN–NH– single bond
(
curve 1, ꢂ ¼ 343.5 nm). This confirms the reductive cleavage of the –HN–NH–
single bond (Scheme 2). In case of the starting compound (L0), the transfer of two
electrons may be attributed to the reductive cleavage of the –HN–NH single
2
bond leading to formation of 2-amino-6-chloropyridine and the release of ammo-
nia (Scheme 1). The latter was confirmed by adding Nessler’s reagent to the
completely electrolyzed solution of compound L0 where a reddish brown preci-
pitate was formed [17].
Cyclic Voltammetric Studies
Cyclic voltammograms of the studied hydrazones in the B-R universal buffer of
pH values<9 at various scan rates (20–500 mV=s) exhibited a single irreversible
cathodic peak (Fig. 4). The peak potentials (E ) of the examined compounds
p
shifted to more negative values with the increase of scan rate. The extent of the
cathodic shift of the peak potential (E ) and the difference between peak potential
p
and the half-peak (E –Ep=2) as a function of the scan rate (Table 2) confirmed the
p
irreversible nature of the reduction process of the investigated hydrazones [18].
ꢃ
The peak potential (E ) varies with scan rate according to Eq. (3) [19] where k is
p
f ;h

2
90
M. M. Ghoneim et al.
Fig. 4. Cyclic voltammograms of 5ꢂ10ꢁ M of hydrazone (L1) at different scan rates in an etha-
5
nolic-aqueous B-R universal buffer of pH 2
Table 2. Electrochemical parameters extracted from cyclic voltammetric data of the investigated
hydrazones in the ethanolic-aqueous B-R universal buffer
ꢀ
0
5
D ꢂ10
kf;h
Compound
pH
Ep vs ln ꢃ
Ep–Ep=2
2
cm =s
cm=s
ꢁ
6
8
L1
L2
L3
L4
L5
L6
L7
2.0
0.2
0.2
0.2
0.2
2
2
2ꢂ10
ꢁ
7
.4
2.0
.5
2.3
.0
2.2
.3
2.2
.9
1.7
.2
1.7
.7
2ꢂ10
ꢁ
7
0.3
0.4
0.3
0.3
5
4
4ꢂ10
ꢁ
10
7
2ꢂ10
ꢁ
6
0.3
0.4
0.3
0.3
5
2
1ꢂ10
ꢁ
10
7
7ꢂ10
ꢁ
7
9
0.2
0.3
0.3
0.2
8
5
9ꢂ10
ꢁ
7
4ꢂ10
ꢁ
7
0.3
0.4
0.3
0.4
3
1
1ꢂ10
ꢁ
10
7
1ꢂ10
ꢁ
5
7
0.3
0.3
0.3
0.3
1
9
3ꢂ10
ꢁ
7
3ꢂ10
ꢁ
7
0.3
0.4
0.3
0.4
2
1
7ꢂ10
ꢁ
10
7
1ꢂ10
the standard heterogeneous rate constant and the other terms have their usual
meanings.
ꢃ
1=2
E ¼ ꢁ1:14ðRT=ꢀn FÞ þ ðRT=ꢀn FÞ ln k =D ꢁ RT=2ꢀn F ln ðꢀn ꢃÞ ð3Þ
p
a
a
f;h
a
a

Hydrazones and Their Metal Complexes
291
Values of ꢀn and the cathodic symmetry coefficient ꢀ at various pH values
a
were calculated from slope values of the obtained linear plots of E versus ln ꢃ
p
(
[
Table 2). Values of the symmetry coefficient ꢀ were also determined using Eq. (4)
18] and were found to be less than 0.5 when n ¼ 2 (Table 2) which confirmed the
a
irreversible nature of the electrode process of the investigated hydrazones.
Ep ꢁ Ep=2 ¼ 1:857ðRT=ꢀn FÞ
ð4Þ
The peak current of an irreversible electrode process (ipirr) was expressed by
a
Eq. (5) [18] where C is the bulk concentration of reactant species and the other
0
terms have their usual meanings.
ipirr ¼ 2:99ꢂ10 nðꢀn Þ AD1=2C0ꢃ
5
1=2
1=2
ð5Þ
The diffusion-controlled reduction process of the examined hydrazones was
a
1
=2
identified from the linearity of the ipirr versus ꢃ plots with slight deviation from
the origin [20]. The plots of E versus pH for the investigated hydrazones were
p
straight lines with slope values of 66–90 mV=pH which agree well with those
obtained from dc-polarograhic measurements.
0
The diffusion coefficients D of the examined hydrazones were calculated
ꢃ
using Eq. (5) and then the heterogeneous rate constant (k ) of their irreversible
electrode reactions was calculated (Table 2) from cyclic voltammetric data using
f;h
ꢃ
Eq. (3). It seems that the higher the pH values the lower is the value of (k )
denoting a more difficult electron transfer.
f ;h
The Electrode Reaction
Reduction pathway of 6-chloro-2-hydrazinopyridine: This compound was reduced
at the mercury electrode surface in the B-R universal buffer of pH<9 through
a single 2-electron irreversible step corresponding to the reductive cleavage of
the –HN–NH single bond with the release of ammonia as shown in Scheme 1.
2
The released ammonia was confirmed by Nessler’s reagent test [17].
Reduction pathway of hydrazones: The examined hydrazones L1–L7 were
reduced at the mercury surface in the B-R universal buffer of pH<9 through a
single 4-electron irreversible step corresponding to both the saturation of –N¼C<
double bond and cleavage of the –HN–NH– single bond as shown in Scheme 2.
Spectrophotometric Studies
pK values: The absorbance-wavelength spectra of the hydrazones L1–L7 in the
a
B-R universal buffer of pH values 2–11 containing 35% (v=v) ethanol were recorded
within the wavelength range 200–600 nm. Compounds L1 and L2 showed a single
band at ꢂmax ¼ 346 and 340 nm, respectively, and the absorbance of each is pH-
independent indicating that the species of the compound in the acidic and alkaline
solutions are similar. Whereas the absorption spectra of compounds L3–L7 exhib-
ited two bands with the appearance of an obvious isosbestic point. The first band
may be due to the non-ionized form whereas the second band that developed with
the increase of pH may be due to the ionized species. The observed isosbestic point
indicates an acid-base equilibrium. The spectra of compounds L6 and L7 in acidic
solutions of pH<5 exhibited absorption bands at ꢂmax ¼ 385 and 380 nm, respec-

2
92
M. M. Ghoneim et al.
Scheme 1
Scheme 2
tively, whereas at higher pH values these bands were blue shifted to ꢂmax ¼ 341 and
30 nm, respectively (Fig. 5). This behavior can be explained on the basis that
3
protonation takes place in acidic solutions on the nitrogen of the unsubstituted
pyridine ring which enhances the charge transfer from the hydrazone moiety. How-
ever, with the increase of pH this effect disappears, thus the absorption bands were
blue shifted. The pK values of the investigated compounds were estimated from
a
the typical Z or S-shaped absorbance-pH curves at two different wavelengths using
the half-height [21], the limiting absorbance [22], and the modified limiting absor-
bance [23, 24] methods. The obtained pK values are reported in Table 3.
a

Hydrazones and Their Metal Complexes
293
ꢁ
5
Fig. 5. Absorption spectra of 4ꢂ10 M solution of hydrazone L6 in the ethanolic-aqueous B-R
universal buffer of various pH values: (1) 1.8, (2) 3.6, (3) 4.0, (4) 4.2, (5) 4.5, (6) 4.7, (7) 5.1, and (8) 6.3
Table 3. The ionization constants (pK ) of the investigated hydrazones in the ethanolic-aqueous B-R
a
universal buffer using the half-height (1), limiting-absorbance (2), and modified limiting-absorbance
(3) methods
Compound
ꢂ
pKa
nm
(
1)
(2)
(3)
Mean value
L0
L3
304
337
3.4
9.4
9.6
3.1
10.3
9.3
3.5
9.4
9.6
3.3
9.7
9.5
3
70
289
37
370
06
341
85
330
80
L4
L5
L6
L7
10.1
10.4
10.4
10.7
10.1
10.4
10.2
10.5
3
10.5
10.4
10.9
10.2
10.5
10.3
10.6
10.3
4
4.3
4.2
3.9
4.4
4.4
4.3
4.2
4.3
3
4.9
4.7
4.6
4.8
5.0
4.7
4.9
4.7
3
Stoichiometry of hydrazone-metal complexes: The complexes formed in solu-
tion for the investigated hydrazones L3–L7 with the metal ions Mn(II), Co(II),
Cu(II), Zn(II), and Cd(II) were studied. The stoichiometry of the formed com-
plexes was determined by the molar ratio [25] and continuous variation [26–29]
methods. The results indicated formation of complexes of stoichiometric ratios 1:1
and 1:2 (M:L) (Scheme 3).
Potentiometric Studies
Dissociation constant: The average number of protons, n , associated with the
A
reagent molecule at different pH values was calculated from the titration curves

2
94
M. M. Ghoneim et al.
Scheme 3
Fig. 6. Potentiometric titration curves using 0.02 M NaOH ethanolic-aqueous (40% v=v) solution at
3
3
3
3
2
5
5
93 K for the solutions: (a) 5 cm 0.1 M HCl þ 5 cm 1 M KCl þ 20 cm H O þ 20 cm EtOH; (b)
2
3 3 3 3 3
ꢁ3
cm 0.1 M HCl þ 5 cm 1 M KCl þ 5 cm 1ꢂ10 M hydrazone L4 þ 20 cm H O þ 15 cm EtOH;
2
3
3
3
ꢁ3
3
3
cm 0.1 M HCl þ 5 cm 1 M KCl þ 5 cm 1ꢂ10 M hydrazone L4 þ 10 cm H O þ 15 cm
2
3
ꢁ4
2þ
2þ
2þ
2þ
2þ
2þ
EtOH þ 10 cm 1ꢂ10 M (c) Mn ; (d) Co ; (e) Ni ; (f) Cu ; (g) Zn ; and (h) Cd
of hydrochloric acid in the absence and presence of hydrazone compound (Fig. 6).
As shown in Fig. 6, the metal- titration curves (curves c–h) are well separated from
that of free ligand (curve b), along the axis of added volume of NaOH solution,
þ
which attributed to liberation of H ions as a result of the complexation process.
Thus, the formation curves (n versus pH) for the proton-hydrazone systems were
A
constructed and found to extend between 0 and 2 on the n scale. This means that
A
there are two dissociable protons one may be from the enolized –HN–N– moiety
and the second from the phenolic –OH. The proton-ligand dissociation constants of
the examined hydrazones in a mixture of 40% (v=v) ethanol-water and 0.1 M KCl
at 293, 303, and 313 K were calculated [30] and reported in Table 4. From values of

Hydrazones and Their Metal Complexes
295
Table 4. Dissociation constants and thermodynamic parameters for the dissociation process of the
investigated hydrazones L3 and L4 in a mixture of 40% (v=v) ethanol-water and 0.1 M KCl at different
temperatures
Temperature
K
pK1H
pK2H
DG1
kJmol
DG
DH1
kJmol
DH2
DS1
Jmol
DS
K
2
2
ꢁ
1
ꢁ1
ꢁ1 ꢁ1
Compound L3
2
3
3
93
03
13
5.4
5.1
4.7
10.6
9.9
30.3
29.6
28.2
59.5
57.4
57.5
60.9
87.0
140.0
304.4
95.1
9.6
Compound L4
2
3
3
93
03
13
5.9
4.9
4.5
11.0
10.0
9.5
33.1
28.4
27.0
61.7
58.0
56.9
121.8
130.5
236.1
H
dissociation constants pK of free ligands L3 and L4 estimated at 293, 303, and
313 K the thermodynamic parameters DG, DH, and DS of the dissociation process
were calculated (Table 4) using Eqs. (6)–(8) [31–33], where, R is the gas constant,
H
pK is the dissociation constant of the ligand and T is the temperature (in K).
H
DG ¼ ꢁ2:303 RT log K ¼ 2:303 RT pK
2:303 RT log K ¼ DH ꢁ TDS
ð6Þ
ð7Þ
ꢁ
DS
:303R
DH
1
log K ¼ 2
ꢁ
ꢄ
2:303R T
ð8Þ
1
Values of DH were estimated from plotting log K vs. T while those of the
entropy change (DS) were estimated from the intercept of Eq. (8). From the ob-
tained results (Table 4) the following information can be concluded: (i) values of
H
1
H
2
the stepwise dissociation constants pK and pK of both the compounds L3 and
L4 decreased with the increase of temperature revealing that the acidity of ligands
increases with increasing temperature; (ii) the positive values of DG indicated that
the dissociation process of the ligand is thermodynamically unfavorable [34, 35];
(
iii) the positive values of DH of the dissociation process of the ligand indicated
its endothermic nature and that the process enhances with rising temperature;
and (iv) the positive values of DS confirmed that the dissociation process is
entropically favorable.
Stability constant of hydrazone-metal complexes: The formation curves for the
metal complexes of the two investigated hydrazones L3 and L4 with the metal ions
2
þ
2þ
2þ
2þ
2þ
2þ
3þ
3þ
3þ
3þ
4þ
2þ
Mn , Co , Ni , Cu , Zn , Cd , Cr , Fe , La , Ce , Th , and UO
2
were obtained by plotting the average number ( ꢀn ) of ligands attached per metal ion
versus the free ligand exponent (pL). By analysis of the formation curves, the
successive stability constants log K and log K of the studied metal complexes
1
2
ML and ML , respectively, were determined (Table 5) using different computa-
2
tional methods [36, 37]. The obtained results can be summarized as follows:
i) the maximum ꢀn value of all the studied metal complexes was found to equal ffi 2,
(

2
96
M. M. Ghoneim et al.
Table 5. Stability constants of the hydrazone L4-metal complex in a mixture of 40% (v=v) ethanol-
water and 0.1 M KCl at different temperatures
2
93 K
303 K
313 K
nþ
M
log K1
log K2
log K1
log K2
log K1
log K2
Mn2
Co2
þ
þ
þ
þ
þ
þ
þ
þ
10.1
10.3
10.5
10.9
10.6
10.4
11.0
11.9
11.3
11.5
12.3
11.9
8.5
8.5
9.8
10.1
10.4
10.7
10.6
10.2
10.9
11.7
11.2
11.4
11.85
11.80
8.2
8.4
9.7
10.0
10.3
10.6
10.5
10.1
10.9
11.0
10.9
11.0
11.14
11.03
8.0
8.1
8.8
9.1
8.9
8.6
9.2
9.7
9.6
9.6
10.2
9.8
Ni2
Cu2
Zn2
Cd2
Cr3
Fe3
La3
Ce3
Th4
9.0
8.9
9.3
9.2
9.2
8.9
8.8
8.7
9.4
9.3
10.3
9.9
10.3
9.8
þ
þ
þ
10.0
10.8
10.6
9.9
10.7
10.3
2
2
þ
UO
revealing the formation of two types of metal complexes, ML and ML , in solu-
2
tion. No precipitate was observed in the titration vessel, thus the formation of
metal hydroxide is excluded; (ii) values of log K were always found higher
1
than those of log K . This may be due to the fact that the vacant sites of the
2
metal ions are easier available for binding of the first ligand molecule than for
the second one [38, 39]; and (iii) the order of stability constants of metal com-
plexes of compound L3 and L4 with the transition metal ions was found as:
3
þ
3þ
2þ
2þ
2þ
2þ
2þ
2þ
Cr >Fe >Cu >Zn >Ni >Cd >Co >Mn which agrees will with
that reported by Irving and Williams [40, 41].
For actinides and lanthanides, the stability of the formed complexes increases
4
þ
2þ
3þ
3þ
in the order: Th >UO >Ce >La . A similar order was previously reported
[
plexes as a result of increasing both the oxidation state and the ionization potential.
2
42–45]. The Th complexes are more stable than those of the other metal com-
4
þ
2
þ
The stability constant of the divalent oxygenated cation complex (UO ) has a
2
higher value than that of the other divalent cation complexes, which may be attrib-
uted to the bonded oxygen atoms that increases the electrostatic attraction between
the metal ion and the coordinated ligand and overcomes any steric hindrance
offered by the oxygen of the oxygenated cations [46, 47].
Experimental
Preparation of the Investigated Hydrazones
The examined hydrazones L1–L7 were synthesized [48] by the condensation of 6-chloro-2-hydrazi-
nopyridine (L0) with 2-thiophen-aldehyde (L1), methyl-2-thienylketone (L2), salicylaldehyde (L3),
2
-hydroxy-acetophenon (L4), 2-hydroxy-1-naphthaldehyde (L5), ꢀ-formyl-pyridine (L6), or ꢀ-acet-
ylpyridine (L7) in 1:1 molar ratio in absolute ethanol. The prepared hydrazones were purified by
1
recrystallization. H NMR, IR, and elemental analysis confirmed the structures of the prepared hydra-
zones (Scheme 4).

Hydrazones and Their Metal Complexes
297
Scheme 4
Solutions
ꢁ
3
Solutions (1ꢂ10 M) of each of the examined hydrazones were obtained by dissolving the accurate
ꢁ
4
weight of the solid compound in appropriate volumes of ethanol. Solutions of 1ꢂ10 M of each of the
metal salts (MnCl ꢄ4H O, CoCl ꢄ6H O, NiCl ꢄ6H O, CuCl ꢄ2H O, ZnCl ꢄ6H O, CdCl ꢄ1H O,
2
2
2
2
2
2
2
2
2
2
2
2
CrCl ꢄ6H O, FeCl , La(NO ) ꢄ6H O, Ce(NO ) ꢄ6H O, Th(NO ) , and UO (Ac) ꢄ2H O) were prepared
3
2
3
3 3
2
3 3
2
3 4
2
2
2
by dissolving the accurate weight of the metal salt in appropriate volumes of deionized water. Solutions
of 0.1 M HCl, 1 M KCl and 0.1 M NaOH were also prepared in deionized water.
A series of the Britton-Robinson universal buffer [49] of pH 2–11 containing 35% (v=v) ethanol
was prepared and used as a supporting electrolyte. A pH-meter (Crison, Barcelona, Spain) with com-
bined glass and calomel (KCl ) electrodes was used for the pH measurements.
s
Nessler’s reagent (K (HgI ) in dilute sodium hydroxide) was used for testing the formation of
2
4
ammonia during the controlled-potential electrolysis of the investigated hydrazones. On mixing the
ꢃ
electrolyzed solution with Nessler’s reagent while warming to about 40 C the presence of ammonia
gives a reddish brown coloration and a yellow or orange-red to brown precipitate of basic mercury(II)
amido-iodide (HgOꢄHg(NH )I), depending on the amount of ammonia [17].
2
Instrumentation and Procedures
Dc-polarogramic measurements were recorded using a pen recording Polarography Model 4001
(Sargent–Welch, USA). A polarographic cell described by Meites [14] was used. The characteristics
of the dropping mercury electrode (DME) were: m ¼ 1.07mg=s, t ¼ 3.3 s, at a mercury height¼ 60cm
in 0.1M KCl at open circuit.
A computer controlled Electrochemical Analyzer Model 273-PAR (Princeton Applied Research
PAR, Princeton, NJ, USA) and the electrode assembly Model 303A, incorporated with an electro-
2
chemical cell comprising of a hanging mercury drop electrode (HMDE) of area 0.026 cm as a working
electrode, an Ag=AgCl=KCl as a reference electrode, and a platinum wire as a counter electrode were
s
used for cyclic voltammetric measurements.
A potentiostat=galvanostat Model 173-PAR incorporated with a digital coulometer model 179-PAR
was used for controlled-potential electrolysis of the investigated hydrazones at a mercury pool as a
working electrode. A saturated calomel electrode as a reference electrode and a platinum gauze

2
98
M. M. Ghoneim et al.
immersed in a bridge tube as a counter electrode were used. The potential selected was adjusted to a
value equal to the E1=2 of the polarographic wave of reactant plus 0.1 V or at the beginning of the
limiting current of the polarographic wave. The number of electrons (n) transferred per hydrazone
molecule was determined using Faraday’s relation: N ¼ Q=nF (where N is the number of moles of
substance being electrolyzed) and was found to equal four in case of compounds L1–L7, whereas it
was two in case of compound L0. Thin layer chromatographic (TLC) analysis was used for character-
ization of the extracted electrolysis products of the investigated hydrazones L1–L7.
Electronic absorption spectra of the hydrazones under consideration in the ethanolic-aqueous B-R
universal buffer of pH values 2–11 were recorded at room temperature within the wavelength range
2
00–600 nm using a Shimadzu UV-visible spectrophotometer Model 160A.
Potentiometric measurements were performed using a pH-meter Model HI8014 (Hanna Instru-
ments, Italy) accurate to Æ0.01 pH units. The electrode was standardized before and checked after
each titration with buffer solutions produced by FISHER (New Jersey, USA). All the titration mea-
surements were carried out at different temperatures by circulating water from an Ultra-thermostat
(JULABO F10, Seelbach, Germany) through the annular space of a double-walled Pyrex titration cell
of 50 ml capacity. A standard solution of 0.02 M NaOH (40% v=v ethanol-water) was added from the
3
micro burette (5cm ) and the contents of the titration vessel were stirred using a magnetic stirrer
(Sargent-Welch, USA). The following mixtures were prepared and titrated potentiometrically at dif-
ferent temperatures (293, 303, and 313 K) against ethanolic-aqueous (40% v=v) 0.02 M NaOH solution.
3
For each mixture, the volume was made up to 50cm using bidistilled water before the titration:
3
3
3
3
(
i) 5 cm 0.1 M HCl þ 5 cm 1 M KCl þ 20cm H O þ 20 cm EtOH
2
3 3 3
ꢁ3
3
3
(
ii) 5 cm 0.1 M HCl þ 5 cm 1 M KCl þ 5 cm 1ꢂ10 M hydrazone þ 20cm H O þ 15cm EtOH
2
3 3 3 3
ꢁ3
ꢁ4
(
iii) 5 cm 0.1 M HCl þ 5 cm 1 M KCl þ 5 cm 1ꢂ10 M hydrazone þ 10cm 1ꢂ10 M metal ion
2þ
2þ
3
2þ
2þ
2þ
2þ
3þ
3þ
3þ
3þ
4þ
2þ
3
(
Mn , Co , Ni , Cu , Zn , Cd , Cr , Fe , La , Ce , Th , and UO ) þ 10cm
2
H O þ 15cm EtOH
2
The obtained potentiometric titration curves were analyzed using a constructed basic language – PC
program.
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