Oxidative ring cleavage of 4-(4-R-phenyl)-1,2,4-triazolidine-3,5-diones: Electrochemical behavior and kinetic study

Article abstract of DOI:10.1149/2.111206jes

Cyclic voltammetry results as a diagnostic technique for electrochemical oxidation of 4-(4-R-phenyl)-1,2,4-triazolidine-3,5-diones (1-5) are reported and discussed. The results indicate that the electrochemically generated 4-(4-R-phenyl)-4H-1,2,4-triazole-3,5-diones (1ox-5ox) are unstable and participate in oxidative ring cleavage. In this study, the effect of different parameters such as pH, 4-phenylurazole concentration, solvent, temperature, substitute effect and time window of chosen electrochemical method have been studied. Also, the transfer coefficient, α, exchange current density, J₀, the formal potential, E^0′ and diffusion coefficient, D, of 4-phenylurazole (1) have been calculated. In addition, the observed homogeneous rate constants of oxidative ring cleavage of 4-phenylurazole derivatives were estimated by comparing the experimental cyclic voltammetric responses with digital simulated results.

Full text of DOI:10.1149/2.111206jes

F174
Journal of The Electrochemical Society, 159 (6) F174-F180 (2012)
0013-4651/2012/159(6)/F174/7/$28.00 © The Electrochemical Society
Oxidative Ring Cleavage of
4-(4-R-phenyl)-1,2,4-triazolidine-3,5-diones: Electrochemical
Behavior and Kinetic Study
F. Varmaghani,a D. Nematollahi,a,z and S. Mallakpourb
aFaculty of Chemistry, Bu-Ali-Sina University, Hamedan 5178-38683, Iran
bOrganic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology,
Isfahan 84156-83111, Iran
Cyclic voltammetry results as a diagnostic technique for electrochemical oxidation of 4-(4-R-phenyl)-1,2,4-triazolidine-3,5-diones
(1-5) are reported and discussed. The results indicate that the electrochemically generated 4-(4-R-phenyl)-4H-1,2,4-triazole-3,5-
diones (1ox-5ox) are unstable and participate in oxidative ring cleavage. In this study, the effect of different parameters such as
pH, 4-phenylurazole concentration, solvent, temperature, substitute effect and time window of chosen electrochemical method have
been studied. Also, the transfer coefficient, α, exchange current density, J0, the formal potential, E0’ and diffusion coefficient, D,
of 4-phenylurazole (1) have been calculated. In addition, the observed homogeneous rate constants of oxidative ring cleavage of
4-phenylurazole derivatives were estimated by comparing the experimental cyclic voltammetric responses with digital simulated
results.
© 2012 The Electrochemical Society. [DOI: 10.1149/2.111206jes] All rights reserved.
Manuscript submitted December 19, 2011; revised manuscript received March 26, 2012. Published April 30, 2012.
Electrochemistry provides very versatile means for the electrosyn-
4-(4-R-phenyl)-1,2,4-triazolidine-3,5-diones (1-5) were estimated by
comparing the experimental cyclic voltammetric responses with dig-
ital simulated results.
thesis, mechanistic and kinetic studies.1–8 Among electrochemical
methods, it is known that cyclic voltammetry is a powerful technique
for investigation of electrochemical reactions that are coupled with
chemical reactions.9,10 Urazole as an important biochemical reagent
and a prebiotic compound acting as the precursor to uracil has been
used in studies of the origin of life.11,12 Effectiveness of urazole as
an herbicide against weeds, production of anti-tumor drugs,13,14 the
stabilizer in milk, and insecticides have been confirmed. Some urazole
derivatives were found to be potent cytotoxic agents in murrain and
human cancer cell lines and also reduce DNA synthesis considerably
with moderate reduction in RNA synthesis.15 The comparison between
electrochemical and bioactive properties with some evidence of rela-
tionship, bears a great significance.16 The propensity of a molecule to
donate or accept an electron in solution is measured by its standard
redox potential and knowledge of standard redox potential is funda-
mental to understanding the chemical and biological electron-transfer
reactions.17,18 Furthermore, numerous fundamental and harmful pro-
cesses in living cells, such as cytochrome P450 catalyzed oxidation
in liver microsomes, are governed and stimulated by redox reaction.19
The oxidation of urazole derivatives has previously been studied in
different media.20–24 It is reported that, the high reactivity of these com-
pounds is due to the simultaneous presence of both carbonyl and azo
moieties.24 Also, the oxidative cleavage pathway which leading to the
various products was discussed.24 In addition, in our previous paper,
it is discovered that the stability of electrogenerated 4-R-4H-1,2,4-
triazole-3,5-dione (1ox-5ox) depended on the media conditions.25 So
investigation the effect of different parameters on the stability of elec-
trogenerated 4-R-4H-1,2,4-triazole-3,5-dione (1ox-5ox) is useful to
have a success chemical or electrochemical synthesis via oxidation of
urazoles andseems tobeinterestingfor all workers whodeal with these
compounds. Therefore, inthis work, theanodicoxidativeringcleavage
of 4-(4-R-phenyl)-1,2,4-triazolidine-3,5-diones (1-5) has been stud-
ied in aqueous solution and the effect of different parameters such
as pH, 4-phenylurazole concentration, solvent, temperature, substi-
tute effect and time window of chosen electrochemical method have
been taken in order to more understanding about instability of 4-R-
4H-1,2,4-triazole-3,5-dione (1ox-5ox). Also, the transfer coefficient,
α, exchange current density, J0, formal potential, E0’, diffusion coeffi-
cient, D, and some thermodynamic parameters for oxidation-reduction
reaction of 4-phenylurazole (1) have been calculated. In addition, the
observed homogeneous rate constants of oxidative ring cleavage of
Experimental
Apparatus and reagents.— Cyclic voltammetry and chronoamper-
ometry were performed using an Autolab model PGSTAT 30 poten-
tiostat/galvanostat. The working electrode used in the voltammetry
experiments was a glassy carbon disk (1.8 mm diameter) and a plat-
inum wire was used as the counter electrode. The working electrode
potentials were measured vs. Ag/AgCl. More details are described in
our previous paper.26 Urazole derivatives were synthesis according to
previously reported producers.27,28 Digital simulation was performed
using the DIGIELCH simulation software version 2.0.29 All solu-
tions in electroreduction studies were deoxygenated by bubbling pure
nitrogen.
The peak current ratios (IpC/IpA) were determined using the fol-
lowing equation.
Ipc/Ipa = (Ipc)0/Ipa + 0.485(Isp)0/Ipa + 0.086
where (Ipc)0 and (Isp)0 are cathodic peak current and “switching
potential” current respect to the zero current, respectively. Ipc and Ipa
have their usual meanings.
Results and Discussion
The effect of potential scan rate.— The electrochemical study of
4-phenylurazole (1) in the aqueous phosphate buffer (c = 0.2 M, pH
2.0), at a glassy carbon electrode has been performed using cyclic
voltammetry at different scan rates (Fig. 1).
As shown in Fig. 1 in low scan rate (100 mV/s), the cyclic voltam-
mogram exibits the feature of an irreversible electron-transfer process
with an anodic peak (A1) at 0.70 V vesus Ag/AgCl which corresponds
to the transformation of 4-phenylurazole (1) to 4-phenyl-4H-1,2,4-
triazole-3,5-dione (1ox) (Scheme 1).25 It is seen that, proportionally
to the augmentation of potential sweep rate, the peak current ratio
(IpC1/IpA1) increases. The ratio of IpC1/IpA1 and the anodic peak current
function, IpA1/v1/2, versus the potential scan rate are also shown in
Fig. 1. It is observed that, proportional to the increasing of the poten-
tial sweep rate IpC1/IpA1 increases gradually. In the cyclic voltammetric
method, the potential scan rate is an effective experimental parameter
which can be used to control τ, a measure of the period during which
a stable electroactive species can communicate with the electrode. By
increasing the scan rate this characteristic time is limmited compared
zE-mail: nemat@basu.ac.ir
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Journal of The Electrochemical Society, 159 (6) F174-F180 (2012)
F175
I/c (mA/mM)
A1
IpA1/v1/2
IpC1/IpA1
0.9
17
12
7
15
11
7
0.86
0.78
0.7
b
e
IpC1/IpA1
A1
0.7
0.8
0.7
a
a
0.5
50
I/μA
250
450
v/mV s-1
0.6
0.2
0.4
0.6
0.8
2
[4-Phenylurazol]/mM
3
-3
-8
C1
-1
-5
0.4
0.5
0.6
E/V vs. Ag/AgCl
0.7
0.8
C1
0.4
0.5
0.6
0.7
0.8
0.9
Figure 1. Cyclic voltammograms of 0.5 mM 4-phenylurazole (1) in phosphate
buffer (c = 0.2 M, pH = 2.0) solution at different scan rates at a glassy carbon
electrode. Scan rates from (a) to (e) are: 100, 200, 300, 400 and 500 mV/s.
Inset: Variation of the peak currents ratio (IpC1/IpA1) (curve a) and anodic peak
current function, IpA1/v1/2 (curve b) versus scan rate, T = 25 1◦C.
E/V vs. Ag/AgCl
Figure 2. Normalized cyclic voltammograms (I/c) of 4-phenylurazole (1) in
phosphate buffer (c = 0.2 M, pH = 2.0) solution at various concentrations (c
= 0.25, 0.5 and 0.75 mM), at a glassy carbon electrode. Scan rate: 200 mV/s,
T = 25 1◦C.
to t, which t is the charactristic lifetime of a coupled chemical reac-
tion. This behavior is fully confirmed by plotting the variation IpA/v1/2
vesus the potential scan rate. Under the above conditions, the plots of
IpA/v1/2 vesus the potential scan rate is a characteristic shape typical
of an ErCi mechanism established by Nicholson and Shain.30
donor number of mixture. Water has DN = 33, the addition of amount
of AN with DN = 14.1 decreases the donor number of mixture and
abstraction of proton.31
On the basis of these results, it is appeared that hydroxyl ion
is the main driving force of the coupled chemical reaction in the
EC mechanism. It is suggested that the observed instability of 1ox
corresponds to the nucleophilic attack of hydroxyl ion of the solution.
Fig. 4 shows the effect of pH solution on the voltammetric behavior
of 1 at a glassy carbon electrode surface in AN/buffer solution in a pH
range of 3.0–7.0.
It was found that the potential for peak A1 (EpA1) shifted to the
negative potentials by increasing pH. This is expected because of the
participation of proton(s) in the oxidation reaction of 1 to 4-phenyl-
4H-1,2,4-triazole-3,5-dione (1ox).
The effect of concentration of 4-phenylurazole (1).— Electrochem-
ical oxidation of 4-phenylurazole (1) has been studied in various con-
centrations. Fig. 2 shows normalized cyclic voltammograms of 1 at
various concentrations. Normalization was performed by dividing the
current on concentration (I/c vs. μA/mM). As can be seen in Fig. 2 the
peak current ratio (IpC1/IpA1) is constant and independent to concentra-
tion of l. This confirms EC mechanism for electrochemical oxidation
of 4-phenylurazole and rejects a second order reaction such as dimer-
ization of l and oxidation form of l (1ox).
The anodic peak potential (EpA1), is given by:
The effect of solvent and pH.— Fig. 3 shows the cyclic voltam-
mograms of l in 0.1 M HCl containing various amount of acetonitrile
(AN). It is observed that proportional to increasing of AN percent in
mixture, the IpC1/IpA1 ratio increases. In fact, proportional to increas-
ing of AN percent, the rate of chemical reaction in EC mechanism
decreases. This behavior can be depended on the water amount or hy-
droxide ion concentration. Actually, the acidity and basicity of solvent
have a significant influence on the following chemical reaction. This
behavior is related to decrease in solvent basicity arising decrease in
Epꢀ A = EpA(pH=0) − (2.303 mRT/2F)pH
[1]
8
6
A1
IpC1/IpA1
0.9
0.6
0.3
4
0
25
%AN
50
75
O
O
2
-2e- -2H+
pH < 5.3
N
N
NH
NH
(1)
N
N
0
O
O
O
-2
-4
-6
1
1ox
C1
Increasing AN percent
O
-2e- -H+
N
N
NH
N
0.45
0.55
0.65
0.75
0.85
(2)
N
N
pH > 5.3
E vs. Ag/AgCl
1An-
O
O
1ox
Figure 3. Cyclic voltammograms of 0.5 mM 4-phenylurazole (1) in 0.1 M HCl
containing various percents of acetonitrile (AN) at a glassy carbon electrode.
The percent of AN are 0, 30, 50 and 70% (v/v). Scan rate: 100 mVs−1 and T
= 25 1◦C.
Scheme 1. Electrochemical oxidation of 4-phenylurazole (1) in different
ranges of pH.
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F176
Journal of The Electrochemical Society, 159 (6) F174-F180 (2012)
O
O
11
pH = 1.0
pH = 3.0
pH = 5.0
pH = 7.0
NH
NH
pKa = 5.3
N
9
7
+
H+
N
N
NH
1
O
1An-
O
5
Scheme 2. Acid/base equilibrium of 4-phenylurazole (1).
3
1
decreases. This peak disappears in pHs > 7.0. This data shows that by
increasing pH, 1ox is more unstable and disappears in the time scale
of performed experiments. It is suggested that the observed instability
corresponds to the nucleophilic addition of hydroxyl ion.
-1
-3
-5
The study of the effect of pH solution, solvent, 4-phenylurazole
concentration and the potential scan rate on the cyclic voltammetric
response indicates that the electrochemical oxidation of 1 is not a
simple mechanism and a coupled chemical reaction participate in the
oxidation pathway. Based on the 4-phenylurazole structure, some sub-
sequent chemical reactions such as hydroxylation,32,33 dimerization34
and oxidative ring cleavage26 are expected. Additionally, it cannot
be considered dimerization of 1 and oxidized form of 1 (1ox) as
the follow up chemical reaction. The data obtained from Fig. 2
shows the rate of the subsequent chemical reaction is independent on
4-phenylurazole concentration, so that with increase of concentration,
the values of IpA1/IpC1 remained constant. This observation is adapted
with firs-order or pseudo-first order chemical reactions.
For more details, multi cyclic voltammograms repeated at the
glassy carbon electrode don’t show any alternation on the num-
ber of peaks (Fig. 6). These results reject hydroxylation of electro-
chemically generated 4-phenyl-4H-1,2,4-triazole-3,5-dione (1ox) via
Michael type reaction. Hydroxylation of 4-phenylurazole via Michael
type reaction must show an ECE mechanism with appearing a new
peak in multi-cyclic voltammetry.32,33
0.1
0.3
0.5
E/V vs. Ag/AgCl
0.7
0.9
Figure 4. Cyclic voltammograms of 0.5 mM 4-phenylurazole (1) in buffer/AN
(60/40, v/v) solutions with various pHs and the same ionic strength, at a glassy
carbon electrode. Scan rate = 100 mV s−1 and T = 25 1◦C.
Where m is the number of protons involved in the reaction and
EpA(pH = 0) is the anodic peak potential at pH = 0.0, R, T, and F have
their usual meanings. An EpA-pH diagram is constructed for oxidation
of compound 1 by plotting the EpA values as a function of pH (Fig. 5).
The EpA–pH diagram comprise two linear segments with different
equations and slopes around of pH = 5.3.
In pHs < 5.3:
Epꢀ A = 0.81 − 0.059 pH
In pHs > 5.3:
These voltammetry results accompanied by the previous published
papers,24,26,35 allow us to propose the following mechanism for the
electrochemical oxidation of 1 (Scheme 3). According to the presented
mechanism, it seems that electrochemically generated 4-phenyl-4H-
1,2,4-triazole-3,5-dione (1ox) is a reactive electrophile and nucle-
ophilic addition of hydroxide ion (or water) to carbonyl group led to
oxidative ring cleavage. Oxidative ring cleavage of 1 in the presence
of hydroxyl ion (or water) is a pseudo-first order reaction. The above
experimental results confirm the fact that the probability of dimer-
ization reaction is very low. With increasing pH and decreasing AN
percent, as the hydroxyl ion concentration increased the oxidative ring
cleavage occurs faster, and the IpA1/IpC1 ratio increased parallel to the
increase of pH.
Epꢀ A = 0.65 − 0.029 pH
On the basis of the above mentioned slope, it can be concluded
that the electrode surface reaction occurring at the pH > 5.3 is a two-
electron, one-proton process involving the oxidation of “anionic form”
1 (1An−) to the corresponding 4-phenyl-4H-1,2,4-triazole-3,5-dione
(1ox) (Scheme 1, Eq. 2). Whereas, the electrode surface reaction at pH
< 5.3, corresponds to the two-electron, two-proton process (Scheme 1,
Eq. 1). Also, the pKa obtained in this work for acid/base equilibrium
shown in Scheme 2 is: 5.3.
On the other hand, it is shown that the current of peak C1 is strongly
depended on the pH, so that with increasing pH the current of peak C1
0.8
E p = -0.059pH + 0.81
R2 = 0.9931
15
O
A1
0.7
N
1st Scan
11
N
N
0.6
O
O
2nd Scan
7
NH
NH
E p = -0.029pH + 0.65
N
R2 = 0.9979
0.5
0.4
0.3
O
3
O
NH
N
N
-1
O
0.3
0.5
0.7
0.9
0
2
4
6
8
10
E/V vs. Ag/AgCl
pH
Figure 6. Multi-cyclic voltammogram of 1.0 mM 4-phenylurazole (1) in phos-
phate buffer (c = 0.2 M, pH = 2.0), at a glassy carbon electrode. Scan rate:
100 mV/s. T = 25 1◦C.
Figure 5. Variations of oxidation potential (EpA) of 4-phenylurazole (1) as a
function of pH, T = 20 1◦C.
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Journal of The Electrochemical Society, 159 (6) F174-F180 (2012)
F177
kobs
1.4
I/μA
I/μA
O
O
8
a
a
B
C
A
-2e- -2H+
6
4
5.5
2.5
NH
NH
N
N
N
N
b
(1)
b
0.9
2
0.4
O
O
-0.5
0
1
1ox
-0.1
-3.5
-2
0
20
40
% AN
60
80
0.48
0.6
0.72
0.84
0.48
0.6
0.72
0.84
E/V vs. Ag/AgCl
HO
N
O
E/V vs. Ag/AgCl
OH
O
N
OH
NH
-OH (or H2O)
N
N
N
N
(2)
N
Figure 8. (A): experimental (a) and simulated (b) cyclic voltammograms of
0.5 mM 4-phenylurazole (1) in water/acetonitrile (50/50, v/v) solution con-
taining 0.1 M HCl. Scan rate: 70 mV/s. (B) experimental (a) and simulated
(b) cyclic voltammograms of 0.5 mM 4-phenylurazole (1) in water/acetonitrile
(70/30, v/v) solution containing 0.1 M HCl, at a glassy carbon electrode. Scan
rate: 60 mV/s. and (C) Variation of kobs versus AN percent.
N
O
O
O
1ox
3
2
Final product(s) which
produce slowly in the
time scale of cyclic
voltammetry.24
4-phenylurazole as a function of different percents of AN is shown in
Fig. 8.
To study the effect of substituted group on the rate of oxidative ring
cleavage, electrochemical oxidation of (4-(4-methoxyphenyl)-1,2,4-
triazolidine-3,5-dione (2), 4-(4-chlorophenyl)-1,2,4-triazolidine-3,5-
dione (3), 4-(3,4-dichlorophenyl)-1,2,4-triazolidine-3,5-dione (4) and
4-(4-nitrophenyl)-1,2,4-triazolidine-3,5-dione (5)) (Fig. 9) were also
performed and kobs of these urazole derivatives were calculated by dig-
ital simulation. The calculated values of the rate constant for oxidative
ring cleavage have shown in Table I.
As shown in Table I, the magnitude of homogeneous rate constants
depended on the nature of the substituted group on the phenyl ring.
The presence of electron-donating groups such as methoxy causes a
decrease in kobs. As mentioned, 4-phenyl-4H-1,2,4-triazole-3,5-dione
(1ox) is very reactive as a result of the simultaneous presence of
both carbonyl and azo moieties in the molecule. Therefore, 4-(4-
methoxyphenyl)-4H-1,2,4-triazole-3,5-dione generated at the surface
of electrode is more stable than 1ox, by the presence of methoxy group
as electron-donating group. In contrast, in the case of 3-5 the pres-
ence of chloro, dichloro and nitro groups with electron-withdrawing
characters cause an increase in kobs. Instability of oxidized forms of
3-5 becomes more critical as a result of the presence of electron-
withdrawing groups on the phenyl ring. The first-order rate constants
(kobs) can be related with the Hammett ρ–σ parameters, where the
Hammett equation is:
Scheme 3. Oxidative ring cleavage mechanism of 4-phenylurazole (1).
Kinetic evaluation.— In the evaluating kinetic parameters, the
Tafel plot was drawm (Fig. 7), derived from data of the rising part
of the anodic branch of current-potential curves. This part of cyclic
voltammogram, known as Tafel region, is affected by electron trans-
fer kinetics between substrate (4-phenylurazole) and the electrode
surface. In Tafel equation, log I = log I0 + (1−α)nF η, I0 is exchange
2.30RT
current, η is over voltage, and n, R, T and F have their usual meanings.
The transfer coefficient, α, and exchange current density, J0, evalu-
ated from the slope and intercept of log I-η plot were found to be 0.52
and 3.7×10−3 μA cm−2 respectively, assuming two-electron transfer
process.
Based on an EC mechanism under pseudo-first order conditions,
the observed homogeneous rate constants (kobs) of oxidative ring
cleavage of 4-phenylurazole have been estimated in 0.1 M HCl con-
taining different ratio of AN. The simulation was carried out assuming
semiinfinite one-dimensional diffusion and planar electrode geometry.
The experimental parameters entered for digital simulation consisted
of the following: Estart, Eswitch, Eend, t = 25◦C and analytical concen-
tration of 1. All these parameters were kept constant throughout the
fitting of the digitally simulated voltammogram to the experimental
data. The parameter kobs was allowed to change through the fitting pro-
cesses. The observed oxidative cleavage rate constants (kobs) of 1ox
in various percents of water/acetonitrile are: 1.70 0.126 (100%),
log ki = log k0 + ρσ
[2]
where ki is the rate constant for substituted 4-phenylurazole, k0 is the
rate constant for 4-phenylurazole, σ is a constant characteristic of a
given substituted group,36 and ρ is the slope of the logki—σ graph.
The Hammett plot is shown in Fig. 10.
0.53
0.068 (70/30%), 0.14
0.011 (50/50%) and 0.04
0.008
(30/70%) (Reported standard deviations were obtained for four in-
dependent scan rates). Also, the plot of kobs of the ring cleavage of
Fig. 11, curve a, shows the cyclic voltammogram of 1.0 mM solu-
tion of 4-(4-nitrophenyl)-1,2,4-triazolidine-3,5-dione (5) in the aque-
ous acetate buffer solution (c = 0.2 M, pH 5.0) at a glassy carbon elec-
trod in negative going scan. The voltammogram shows one cathodic
peak (C1) and anodic peak (A2) at −0.55 and 0.20 V versus Ag/AgCl
which correspond to the reduction of 5 to 4-(4-aminophenyl)-1,2,4-
triazolidine-3,5-dione (6) (Scheme 4, Eq. 1) and oxidation of cathod-
ically generated 6 to 4-(4-aminophenyl)-4H-1,2,4-triazole-3,5-dione
0.6
log I = 16.31η - 4.03
R² = 0.990
0.4
0.2
0
HN NH
N
HN NH
HN NH
O
O
N
N
HN NH
N
HN NH
O
N
O
O
O
O
O
O
O
-0.2
0.235
0.245
0.255
0.265
0.275
Cl
η/V vs. Ag/AgCl
H
OCH3
Cl
Cl
NO2
1
2
3
4
5
Figure 7. Tafel plot for the oxidation of 1.0 mM of 4-phenylurazole (1), Data
obtained from the rising part of the anodic branch of cyclic voltammogram,
with scan rate of 50 mV/s, in phosphate buffer (c = 0.2 M, pH = 2.0).
Figure 9. The structure of studied 4-phenylurazoles.
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F178
Journal of The Electrochemical Society, 159 (6) F174-F180 (2012)
Table I. Calculated first-order rate constants (kobs/s−1) for oxidative ring cleavage of 4-phenylurazole derivatives in water/acetonitrile (60:40)
solution containing 0.1 M HCl.
Phenylurazole derivativesa
1
2
3
4
5
kobs/s−1
0.22 7.64×10−3
0.16 2.91×10−3
0.43 0.017
1.03 0.026
2.56 0.111
aStandard deviation of four independent simulations at various scan rates.
(6ox) (Scheme 4, Eq. 2).4 The cathodic peak C2 is related to the
reduction of 6ox to 6. Fig. 11, curve b, shows the cyclic voltammetry
of 5 in positive going scan. The voltammogram shows one anodic peak
(A0) at 0.50 V vs. Ag/AgCl which correspond to the transformation of
5 to 4-(4-nitrophenyl)-4H-1,2,4-triazole-3,5-dione with an irreversible
featur. In contrast to 5, which shows an irreversible featur in low scan
rate, the peak current ratio (IpC2/IpA2) of 6ox is near unity and can be
considered as slow oxidative cleavage rate constant and a criterion
for the stability of 6ox produced at the surface of electrode under the
exprimental conditions. The stability of 6ox is related to the presence
of electron-donating group of –NH2 on the phenyl ring.4
0.6
H
N
log k i = 1.13σ - 0.58
NH
O
0.2
-0.2
-0.6
-1
N
O
H
N
O2N
NH
O
H
N
N
O
N
O
NH
O
H
N
H
N
NH
NH
Cl
O
O
Cl
N
O
N
O
H
H3CO
Cl
Chronoamperometry.— Chronoamperometry measurement of
4-phenylurazole (1) was done by setting the working electrode poten-
tial at 0.70 V and used for the measurement of the diffusion coefficient,
D, of 4-phenylurazole (Fig. 12A).
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
σ
The Shoup and Szabo theory model37 has been used in order
to determine diffusion coefficient of 4-phenylurazole. The fitting of
Figure 10. Hammett plot for studied 4-phenylurazoles.
41
A
A2
31
a
1
-4
a
b
c
21
11
1
C2
I/μA
A0
9
7
b
-9
0
5
10
15
20
5
t / s
3
9
1
y = 18.34x + 0.601
R2 = 0.9998
-14
-19
-1
0.05
B
8
7
6
5
4
3
0.25
0.45
0.65
y = 15.72x + 0.521
R2 = 0.9991
E/V vs. Ag/AgCl
C1
-0.45
-0.65
-0.25
-0.05
0.15
0.35
y = 12.59x + 0.443
R2 = 0.9999
Slop/μAs1/2
E/V vs. Ag/AgCl
c
Figure 11. Cyclic voltammograms of mM 4-(4-nitrophenyl)-1,2,4-
1
b
a
triazolidine-3,5-dione (5) at a glassy carbon electrode, in acetate buffer solution
y = 5.75x + 6.922
R2 = 0.9976
18
(c = 0.2 M, pH 5.0). Scan rate: 50 mVs−1; T = 25 1◦C.
16
14
12
0.95
1.45
1.95
O
O
[4-Phenylurazol]/mM
NH
NH
+6e +6H
NH
NH
(1)
(2)
+
2 H O
N
H N
N
O N
in negative going-scan
0.2
0.3
0.4
-1/2 / s-1/2
0.5
0.6
0.7
O
O
6
5
6
O
t
O
NH
NH
Figure 12. (A) Chronoamperometry responses at glassy carbon electrode in
phosphate buffer solution (c = 0.2 M, pH 2) at potential step 0.7 V for different
concentrations of 4-phenylurazole (1). Concentrations from a to c are: 1, 1.5 and
2 mM, respectively. (B) Plots of I versus t−1/2 obtained for chronoamperograms
shown in (A). Inset shows the plot of the slope of straight lines against the
4-phenylurazole concentrations.
-2e -2H
N
N
H N
N
H N
N
in positive going-scan
6ox
O
O
Scheme 4. Electrochemical reduction of 5 and oxidation of cathodically gen-
erated 6.
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Journal of The Electrochemical Society, 159 (6) F174-F180 (2012)
F179
64
63
62
61
60
59
58
57
56
Table II. Experimental thermodynamic parameters of
4-phenylurazole oxidation-reduction reaction in 20–45◦C.
lnK = 22000T-1 - 11.94
R2 = 0.9999
T (◦C)
20
0.797
30
0.792
35
0.790
40
0.787
45
0.784
E0’(V)a
lnK
ꢀG( molK
63.13
− 153.8
60.66
− 152.8
59.53
− 152.4
58.35
− 151.8
57.22
− 151.3
K J
)
a Calculated by ploting of EpA-pH at each temprature and by assuming
that the E0 ≈ E1/2 = EpA − 28.5/n mV.
chronoamperogram in this method is expressed by Eq. 3 to 5.
4D0t
a2
τ =
[3]
[4]
T-1 / K-1
Figure 13. Variation of lnK versus 1/T.
−1/2
f(t) = 0.7854 + 0.8862τ−1/2 + 0.2146e−0.7823τ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
I
D1 =
[5]
4nFCaf(t)
Conclusions
Where D is diffusion coefficient, t, time, a is the constant number of
0.09, C is concentration of 4-phenylurazole and n and F have their
usual meanings. To determine diffusion coefficient, an assumption
amount of D (D0) is taken in Eq. 3 and τ, f(t) and D1 is obtained
from Eq. 3 to 5, respectively. Again, D1 is taken in Eq. 3 and D2 is
resulted. This cyclic process is continued until the difference between
two final D be negligible. A mean value of 1.60 × 10−5 cm2/s was
calculated for the diffusion coefficient. Also, the diffusion coefficient,
D, is determined by Cottrell equation. For an electroactive material
with a diffusion coefficient, D, the current for an electrochemical
reaction is described by Cottrell equation.
The results of this work show that 4-phenylurazole (1) is oxidized
to 4-phenyl-4H-1,2,4-triazole-3,5-dione (1ox). Our results indicate
that in water, the electrochemically generated 1ox is unstable as a re-
sult of simultaneous presence of both carbonyl and azo moieties in the
molecule. Also, we studied the effect of 4-phenylurazole concentra-
tion, solvent and pH on the kinetic of ring cleavage of 1ox. According
to our results, the observed rate constant of ring cleavage of 1ox, is
depended on the hydroxyl ion concentration and DN of the solvent. On
the other hand, the pH dependence of 4-phenylurazole was found to be
−59 mV/pH, in the pH range from 1.0 to 5.3 and −29 mV/pH, in the
pH range from 5.3 to 9.0, which are close to the anticipated Nerstian
dependence of −59.0 and −29.5 mV/pH, respectively. The diffusion
coefficient of 1 is calculated using chronoamperometric results. In the
evaluating kinetic parameters the transfer coefficient, α, and exchange
current density, J0, calculated from the slope and intercept of log I-
η plot, were found to be 0.52 and 3.7×10−3 μA cm−2, respectively.
In addition, we examined the kinetics of oxidative ring cleavage by
digital simulation of cyclic voltammograms. Also, the effect of the
temperature on the redox chemistry of 1 was studied in order to the
calculation of some thermodynamic parameters, ꢀH, ꢀS and ꢀG.
nF AD1/2Cb
I =
[6]
π1/2t1/2
Where D and Cb are the diffusion coefficient and the bulk concen-
tration, respectively. From the slop of I-t−1/2, the diffusion coefficient
can be obtained. Fig. 12B shows the experimental plot of I versus t−1/2
along with the best fits for different concentrations of 4-phenylurazole.
A mean value of 1.53×10−5 cm2s−1 was calculated for the diffusion
coefficient of I-t−1/2 plots of different 4-phenylurazole concentrations.
The determined D by this method is in agreement with calculated D
from Shoup and Szabo theory model.
Acknowledgments
We thank Dr. M. Rudolph for his cyclic voltammogram digital
simulation software (DigiElch SB). We acknowledge the Bu-Ali Sina
University Research Council and Center of Excellence in Develop-
ment of Chemical Methods (CEDCM) for their support of this work.
Further partial financial support from National Elite Foundation (NEF)
and Center of Excellency in Sensors and Green Chemistry Research
Isfahan University of Technology is also gratefully acknowledged.
Calculation of thermodynamic parameters.— The effect of the
temperature on the redox chemistry of 1 was studied in order to the
calculation of some thermodynamic parameters, ꢀH, ꢀS and ꢀG.
The oxidation potential (EpA) of a redox reaction is related to the
temperature via the Eq. 1. As is clear, by increasing the temperature,
EpA shifts to the negative potentials. The thermodynamic parameters
can be calculated according to the equations of 7–9.
nFE0
ln K =
[7]
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RT
ꢁ
ꢂ
ꢁ
ꢂ
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