Electrochemical investigations of some newly synthesized arylazapyrazole derivatives

Article abstract of DOI:10.1007/s00706-016-1768-9

Abstract: Three derivatives of arylazapyrazole ((E)-4-(phenyldiazenyl)-3,5-dimethyl-1H-pyrazole, (E)-4-[(4-fluorophenyl)diazenyl]-3,5-dimethyl-1H-pyrazole, and (E)-4-[(4-iodophenyl)diazenyl]-3,5-dimethyl-1H-pyrazole) were synthesized, characterized, and further investigated for their electrochemical behavior at glassy carbon electrode using cyclic voltammetry. All compounds were reduced following E_i C_i mechanism giving single cathodic peak in potential range 0 to ?2?V vs. Ag/AgCl. The plots of log i_P vs. log ν showed that the electrode process is mixed adsorption-diffusion controlled. The kinetic parameters such as transfer coefficient (αn), diffusion coefficients (D_o), and standard heterogeneous rate constants (k_s) were determined from the electrochemical data. The values of D_o were determined and found greater for the smallest among the three compounds. k_s values were calculated by Laviron formalism which lies in the order of 10^?2?s^?1. Temperature and pH effects were studied and thermodynamic parameters such as change in free energy of activation (ΔG^#), apparent activation energy (E_a), enthalpy (ΔH^#), and entropy (ΔS^#) of activation were determined. Negative values of E_a, ΔH^#, and ΔS^# imply that the electrode process needs lesser over potential with temperature rise, pre-adsorption of the analyte onto the electrode surface and the activated complex has a more organized structure than the reactants, respectively. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-016-1768-9

Monatsh Chem
DOI 10.1007/s00706-016-1768-9
ORIGINAL PAPER
Electrochemical investigations of some newly synthesized
arylazapyrazole derivatives
1
1
1
1
Nasima Arshad
•
Ikramullah
•
Muhammad Aamir
•
Muhammad Sher
Received: 20 May 2015 / Accepted: 28 April 2016
Ó Springer-Verlag Wien 2016
Abstract Three derivatives of arylazapyrazole ((E)-4-
phenyldiazenyl)-3,5-dimethyl-1H-pyrazole, (E)-4-[(4-flu-
orophenyl)diazenyl]-3,5-dimethyl-1H-pyrazole, and (E)-4-
(4-iodophenyl)diazenyl]-3,5-dimethyl-1H-pyrazole) were
Graphical abstract
(
[
synthesized, characterized, and further investigated for
their electrochemical behavior at glassy carbon electrode
using cyclic voltammetry. All compounds were reduced
following E C mechanism giving single cathodic peak in
i
i
potential range 0 to -2 V vs. Ag/AgCl. The plots of log iP
vs. log m showed that the electrode process is mixed
adsorption-diffusion controlled. The kinetic parameters
such as transfer coefficient (an), diffusion coefficients (Do),
Keywords Arylazapyrazole Á Cyclic voltammetry Á
and standard heterogeneous rate constants (k ) were
s
Temperature and pH effect Á Kinetics Á Thermodynamics
determined from the electrochemical data. The values of
D were determined and found greater for the smallest
o
among the three compounds. k values were calculated by
s
-
2 -1
s
Laviron formalism which lies in the order of 10
.
Introduction
Temperature and pH effects were studied and thermody-
namic parameters such as change in free energy of
#
Pyrazoles are 5-membered heterocyclic compounds with
two nitrogen atoms in 1,2-positions. Due to their
tremendous biological activities, pyrazole derivatives are
well established in literature. They have long been studied
as an important class of heterocyclic compounds and still
continue to attract considerable interest due to their broad
range of pharmacological and industrial applications.
Being so composed and having pharmacological effects
on humans, they are classified as alkaloids, although they
are rare in nature. This rare class of alkaloids has wide-
spread potential biological activities such as antimicrobial
activation (DG ), apparent activation energy (E ), enthalpy
a
#
#
(
DH ), and entropy (DS ) of activation were determined.
#
#
Negative values of E , DH , and DS imply that the elec-
a
trode process needs lesser over potential with temperature
rise, pre-adsorption of the analyte onto the electrode sur-
face and the activated complex has a more organized
structure than the reactants, respectively.
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-016-1768-9) contains supplementary
material, which is available to authorized users.
[
1], antitubercular [2], and anti-inflammatory [3]. The
pyrazole moiety makes up the core structure of numerous
biologically active compounds. Thus, some representa-
tives of this heterocycle exhibit anti-viral/anti-tumor,
antibacterial, analgesic, fungistatic, and anti-hyper-
glycemic activity [3].
&
Nasima Arshad
nasimaa2006@yahoo.com
1
Department of Chemistry, Allama Iqbal Open University,
Islamabad, Pakistan
123

N. Arshad et al.
Among variety of pyrazole derivatives such as pyrazolic
are non-steroidal anti-inflammatory drugs, pyrazolines are
antibacterial and the aza-substituted pyrazole enhance flu-
orescence and are used as important synthetic dyes for food
stuffs and cosmetics [4, 5]. The electrochemical studies of
pyrazole derivatives are often informative about their redox
behavior inside living systems [6] and in their utilization in
electro-synthesis of commercially important light sensitive
materials [5]. Although pyrazoles are among the most
thoroughly studied compounds, but there has been very
limited study of the aza-substituted pyrazoles as electro-
chemically active compounds.
and showed no redox activity in the potential range of
2 to -2 V. The compound solutions were then buffered
with Britton Robinson buffer (pH 2) and showed
electro activity in the potential range of 0 to -2 V as
azo moiety of the compounds get protonated in the
presence of buffer. The sample solutions of compounds
APZ-1, APZ-2, and APZ-3 having pH 2 were run at
scan rate of 20 mV/s and at 298 K. Voltammograms
showed one cathodic peak characteristic of each com-
pound in the forward scan with no anodic peak in the
backward scan which points to the irreversible elec-
trode process (Fig. S1, Supplementary Material). The
cathodic peak potential ranges from -0.336 V to
-0.450 V vs. Ag/AgCl with the order APZ-2 \ APZ-
1 \ APZ-3. This makes it clear that the p-substitution
of benzene ring in aza-pyrazole derivatives has a
strong effect on the reduction potentials of their azo
moiety, –N=N–. It either makes the reduction easier or
difficult depending upon the substituent attached. The
electron withdrawing groups such as fluoro group (-F)
make the reduction easier and shift Ep towards less
negative values whereas the electron donating groups
In view of biological and industrial importance, present
research is focused to synthesize, characterize, and to inves-
tigate electrochemical behavior of some new aza-pyrazole
derivatives: (E)-4-(phenyldiazenyl)-3,5-dimethyl-1H-pyrazole
(APZ-1), (E)-4-[(4-fluorophenyl)diazenyl]-3,5-dimethyl-1H-
pyrazole (APZ-2), and (E)-4-[(4-iodophenyl)diazenyl]-3,5-
dimethyl-1H-pyrazole (APZ-3) (Fig. 1). Through electro-
chemical investigations of compounds using the technique of
cyclic voltammetry, an extensive studies on both kinetic and
thermodynamic aspects was carried out which is the main
rationale of present studies. A plausible electrochemical
mechanistic path has also been suggested.
make the reduction difficult and shift the E towards
p
more negative values [7].
Voltammograms were recorded at different scan rates
(Fig. 2; only shown for compounds at pH 2 and at 298 K),
Result and discussion
and the electrochemical parameters E (peak potential), i
p
p
(peak current), and E –E were tabulated as Table S1.
p p/2
Redox activity of the compounds
The peak currents increase whereas the peak potential
shifts to more negative values with increasing scan rates
which further confirm the irreversible nature of the elec-
trode process [7].
First the sample solutions of the compounds APZ-1,
APZ-2, and APZ-3 prepared in DMF solvent were run
Fig. 1 Structures of
synthesized arylazapyrazole
derivatives
1
23

Electrochemical investigations of some newly synthesized arylazapyrazole derivatives
the process is diffusion controlled; if greater than 0.5
adsorption processes take place while for lesser than 0.5
both adsorption and diffusion processes are taken into
account [9]. The slope values obtained for all the com-
pound were found to be lesser that 0.5 which further
verified a mixed diffusion-adsorption trend in redox
processes (Fig. 3).
Effect of pH on the redox process
Voltammograms for the compounds were recorded at three
different pH values (2, 6, and 9) to see the effect of pH on
the redox process (Fig. S1) and the electrochemical
parameters were tabulated as Table 1. The reduction peak
shifted to more negative potential with increasing pH val-
ues (Table 1), which showed that the reduction process
become difficult as solution’s pH increases. Since pre-
protonation is required for the investigated compounds to
determine their redox behavior and at high pH values,
compound solution becomes poorer in protons hence
making the reduction more difficult as pH of the solution
increases.
Effect of temperature on the electrode process
Voltammetric responses were taken for all the compounds
at three different temperatures (298, 303, and 308 K;
Fig. S2, Supplementary Material). A slight positive shift
(towards less negative potential) in the cathodic peak
potential (E ) and increase in peak current was observed as
p
the temperature increased (Table 1), which implies that the
reduction is easier with increasing temperature. This shift
may correspond to decrease in overpotential with increase
in temperature and the rise in peak current with tempera-
ture showed linear dependency of peak current with
temperature.
Determination of standard heterogeneous rate
constants
Fig. 2 Cyclic voltammograms of compounds at different scan rates
298 K, pH 2)
(
A linear relationship between the peak potential (E ) and
p
the logarithm of the scan rate for an irreversible electrode
Determination of surface phenomena
process is described by Laviron equation [8, 10, 11].
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
A linear behavior was observed for the increase of
reduction peak current at low scan rates, but at scan rates
higher than 140 mV/s, a non-linearity was observed in
the voltammorams. This trend may be attributed to
mixed diffusion-adsorption control of the redox process
2:303RT
anF
RTks
2:303RT
anF
Ep ¼ Eo þ
log
þ
log m
anF
ð1Þ
(V) is cathodic peak potential, E is formal
o
where E
potential (V), a is the cathodic electron transfer coefficient,
p
[
8]. The values of logarithm of peak current (log i ) were
p
-
1
plotted vs. log m for all the compounds. The value of
n is the electron transfer number, k
(s ) is heterogeneous
s
slope of log i vs. log m plots has been reported as 0.5 if
rate constant, T is the temperature (K), R the gas constant
p
123

N. Arshad et al.
p
Fig. 3 log i vs. log m plots of
compounds at 298 K and pH 2
Table 1 Electrochemical parameters of the compounds at different pH and temperatures
-1
Compound
APZ-1
pH
2
m/V s
T/K
i
p
/A
E
p
/-V
E
p
–Ep/2/-V
a
-5
-5
-5
-5
-5
-5
-6
-6
-6
-5
-5
-5
-5
-5
-5
-6
-6
-6
-6
-5
-6
-5
-5
-5
-6
-6
-6
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
298
2.41 9 10
2.84 9 10
2.75 9 10
1.56 9 10
1.97 9 10
2.04 9 10
6.40 9 10
6.74 9 10
7.57 9 10
1.52 9 10
1.58 9 10
1.88 9 10
1.32 9 10
1.43 9 10
1.43 9 10
8.08 9 10
9.78 9 10
10.2 9 10
9.92 9 10
1.04 9 10
9.42 9 10
1.09 9 10
1.12 9 10
1.83 9 10
3.87 9 10
4.42 9 10
6.46 9 10
0.379
0.379
0.316
0.705
0.678
0.678
1.031
1.031
0.995
0.336
0.345
0.327
0.657
0.648
0.648
0.882
0.882
0.882
0.403
0.403
0.376
0.698
0.689
0.680
1.011
0.993
0.984
0.045
0.046
0.056
0.073
0.074
0.075
0.070
0.078
0.079
0.035
0.040
0.042
0.052
0.053
0.054
0.065
0.067
0.068
0.043
0.046
0.047
0.062
0.064
0.065
0.054
0.064
0.065
1.06
3
3
03
08
1.04
0.917
0.653
0.644
0.636
0.681
0.611
0.604
1.362
1.192
1.136
0.911
0.900
0.883
0.733
0.712
0.701
1.109
1.036
1.015
0.769
0.745
0.733
0.833
0.745
0.733
6
9
2
6
9
2
6
9
298
3
3
03
08
298
3
3
03
08
APZ-2
298
3
3
03
08
298
3
3
03
08
298
3
3
03
08
APZ-3
298
3
3
03
08
298
3
3
03
08
298
3
3
03
08
-
1
-1
8.314 J K mol ), F the Faraday constant (96485 C
(
Table 2). E for Eq. (1) was determined from the intercept
o
-
1
-1
mol ) and m is a scan rate (V s ).
of E vs. m plot, when the curve was extrapolated to the
p
The values of na and k in Laviron equation (Eq. (1))
can be determined from the slope and intercept of the linear
vertical (ordinate) axis at m = 0 (Fig. 4, right) [8]. The ks
values evaluated from the intercept using Laviron formu-
lation (Table 2) were found in the order APZ-2 [ APZ-
s
plot of E vs. log m, if the value of E is known (Fig. 4, left;
p
o
1
23

Electrochemical investigations of some newly synthesized arylazapyrazole derivatives
3
[ APZ-1 following almost the same trend as observed in
chemical potential between the interface and bulk of the
solution as the pH is increased. Generally, as the pH
increases the redox process slows down. The expense of
diffused species decreases at the electrode–solution inter-
face. Hence the chemical potential between the interface
and the bulk of the solution decreases which drives the
diffusion process. Consequently, the values of the diffusion
coefficients decrease.
a values given in Table 1.
Among all the compounds APZ-2 showed compara-
tively highest value of charge transfer coefficient and
electron transfer rate. Since kinetic parameters are strongly
dependent on structural properties of the compounds, this
rise may be due to the presence of fluoro group on benzene
which stabilizes the activated complex by helping to
delocalize the negative charge on the benzene ring due to
its inductive effect [12].
Furthermore, the rate is affected by the diffusion of the
analyte towards the electrode as well as the charge transfer
between the analyte and the electrode. If either diffusion
coefficient and rate constant increases or decreases simul-
taneously with temperature, the behavior is attributed to
fully diffusion controlled electrode process. However, the
diffusion coefficient values for all the compounds
increased with increase in temperature, while rate constant
values decreased (Table 2), which showed that the redox
process is not fully diffusion controlled but activation
control may also be involved.
The values of standard heterogeneous rate constants
decreased with increasing temperatures. Since rate depends
upon the surface concentration of the redox species that
adsorbed at electrode surface, increase in temperature may
decrease the surface concentration of the adsorbed species,
hence decreasing the rate. Similarly k decreases with
s
increase in pH values. This decrease may be due to decrease
in pre-protonation of the redox species with increase in pH
values, hence decreasing the electron transfer rate.
Determination of diffusion coefficients
Determination of free energy of activation
The diffusion coefficients for all the compounds were
/2
The free energy changes of activation of the electrode
1
calculated from the slopes of i vs. m plots (Fig. 5), using
process for all the compounds were calculated from the ks
p
to Randles–Sevcik equation [13].
values using the following equation [14], and given in
Table 3.
5
1=2
Ã
1=2 1=2
i ¼ ð2:99 Â 10 ÞnðanÞ AC D
m
o
ð2Þ
where i is peak current (A), n is number of electrons
p
o
#
DG ¼ 5778:89ð5:096 À logk Þ
ð3Þ
s
p
#
transferred, a is transfer coefficient, A is surface area of the
electrode (cm ), C is bulk concentration of the analyte
DG values increases both with increase in temperature and
2
*
o
#
pH (Table 3). The increase in DG values with temperature
-
3
-1
(
mol cm ), m is potential scan rate (V s ), and D is
showed that redox process becomes more difficult with
increase in temperature and may be attributed to the
increasing non-efficiency of charge transfer process with
increasing temperature. This is also confirmed by the
decrease in charge transfer coefficient values (a) with
o
2 -1
diffusion coefficient (cm s ).
The diffusion coefficient values increased with
increasing temperature (Table 2), which according to the
Nernst-Einstein equation may be attributed to the increased
mobility of the protonated species towards the electrode
surface. The diffusion coefficient values decreased with
increasing pH. This trend may arise due to the decrease in
#
increasing temperature (Table 1). The increase in DG
values with pH may be due to lacking of pre-protonation of
the redox species with increasing pH.
p s p o
Fig. 4 Left; E vs. log m plots for an and k determination. Right; E vs. m plots for the calculation of E (298 K, pH 2)
123

N. Arshad et al.
Table 2 Standard heterogeneous rate constants and diffusion coefficients of the compounds calculated at different pH and temperatures
-
/s 9 10
1
-2
2
-1
-6
Compound
APZ-1
pH
2
T/K
an
k
s
o
D /cm s 9 10
298
0.712
0.639
0.576
0.730
0.632
0.754
0.664
0.620
0.518
1.159
0.846
0.745
0.923
0.834
0.985
0.552
0.801
0.773
0.579
0.646
0.611
0.739
0.715
0.773
0.695
0.567
0.536
52.61
46.40
40.86
41.22
37.57
33.78
37.24
25.55
19.55
62.06
58.26
53.67
59.66
50.14
43.17
42.01
35.88
32.82
61.05
49.66
43.96
36.31
34.20
32.81
30.25
24.80
19.83
130.05
274.2
774.6
15.63
66.04
41.14
7.20
3
3
03
08
6
9
2
6
9
2
6
9
298
3
3
03
08
298
3
3
03
08
12.72
22.46
24.69
107.52
156.18
17.15
28.86
19.46
10.41
5.27
APZ-2
298
3
3
03
08
298
3
3
03
08
298
3
3
03
08
8.59
APZ-3
298
19.84
30.91
57.53
7.78
3
3
03
08
298
3
3
03
08
18.05
20.72
13.50
18.60
29.22
298
3
3
03
08
the non-protonated forms of the reducible species required
#
higher activation energy. The increased values of DG
indicated that the electrode reaction becomes more
irreversible.
Determination of apparent energy of activation
If the rate determining step is a charge transfer reaction at
the electrode–solution interface, the apparent activation
energy will be considered a significant kinetic parameter in
an ordinary electrode reaction [15]. The apparent activation
energies were calculated from the slopes of ln k vs. 1/T
s
plots using the Arrhenius equation (Fig. 6).
1
/2
p
Fig. 5 I vs. m plots of compounds at 298 K and pH 2
Ea
lnks ¼ lnA À
ð4Þ
RT
Furthermore, k values decreased on increasing the pH
s
#
(
Table 2), whereas the activation energy (DG ) is increased
Table 3). This behavior can be explained by the fact that
The Arrhenius plots showed a straight line with a positive
slope which results in the negative value of the apparent
activation energy. According to Arrhenius theory, the rate
constants should increase with increase in temperature and
the energy of activation must be positive. Since for all the
(
the protonated or neutral species can readily diffuse
towards the electrode in lower pH media, which in turn
#
decreases the activation energy DG . At higher pH values
1
23

Electrochemical investigations of some newly synthesized arylazapyrazole derivatives
compounds, a slight positive shift in the cathodic peak
preceding adsorption of the analyte onto the electrode
potential (E ) was observed as the temperature increases
p
surface with sufficient negative enthalpy which surmounts
the positive enthalpy of the rate determining redox step of
the electrochemical reaction [16, 17]. Before undergoing
electrochemical reduction, compound solvated in liquid
medium and get adsorbed on the electrode surface
(Eqs. (6), (7)).
(
Table 1), which implies that at higher temperatures,
decrease in overpotential affects the electrode process
which in turn decreases the values of rate constant
(
Table 2). The decrease in rate constants is thus not due to
increase in temperature but due to applying smaller over-
potential. This explains the negative value of the apparent
activation energy [15].
APZsol ꢀ APZads
APZads þ e À! ½ APZadsŠ ðRDS; rate determining stepÞ
ð7Þ
ð6Þ
ꢀ
Determination of the apparent enthalpy change
of the activation
The apparent enthalpy change of activation can thus be
given as
Using the values of apparent activation energy, E , the
a
#
DH ¼ DHads þ DHRDS
ð8Þ
#
apparent enthalpy change of activation DH were calcu-
lated from the following equation [14].
Determination of the entropy change
of the activation
#
DH ¼ E À RT
ð5Þ
a
#
The values calculated for enthalpy change of activation
were negative (Table 3). This may be attributed to the
The entropy changes of activation (DS ) were determined
according to the following relationship [14]:
Table 3 Thermodynamic parameters calculated at different pH and temperatures
#
-1
-1
#
-1
#
-1
Compound
APZ-1
pH
2
T/K
DG /kJ mol
-E
a
/kJ mol
-DH /kJ mol
-DS /kJ mol
298
31.06
31.37
31.69
31.67
31.90
32.17
31.93
32.87
33.54
30.64
30.80
31.01
30.74
31.18
31.56
31.62
32.01
32.24
30.69
31.20
31.51
31.99
32.14
32.44
32.95
33.50
19.27
15.16
49.2
21.75
21.79
21.83
17.64
13.68
17.72
51.68
51.72
51.76
13.54
13.58
13.62
27.15
27.19
27.24
18.54
18.59
18.63
27.59
27.63
27.67
10.20
10.25
10.29
34.65
34.69
34.73
177.21
175.47
173.52
165.49
163.66
162.01
280.56
279.16
276.97
148.29
146.50
144.93
194.29
192.66
190.89
168.37
167.03
165.18
195.57
194.20
192.17
141.61
139.90
138.10
225.17
223.24
221.57
3
3
03
08
6
9
2
6
9
2
6
9
298
3
3
03
08
298
3
3
03
08
APZ-2
298
11.06
24.67
16.54
25.11
7.73
3
3
03
08
298
3
3
03
08
298
3
3
03
08
APZ-3
298
3
3
03
08
298
3
3
03
08
298
32.17
3
3
03
08
123

N. Arshad et al.
Electrochemical mechanism
Generally, reduction of azo molecules proceeds only when
they are pre-protonated [21]. The reversibly protonated
species in bulk of the solution diffuses towards the elec-
trode where it gets reduced irreversibly in a slow rate
determining step (Eq. (7)). This is followed by further
uptake of protons and reduction in fast steps which may
lead to the electrochemical decomposition of the analyte
into aniline and aminopyrazole moieties. The possible
mechanistic path which APZ compounds may follow when
studied for their redox behavior is predicted E C and given
Fig. 6 Plots of k
s
vs. 1/T of compounds at pH 2
i
i
as Scheme 1.
#
#
#
DG ¼ DH À TDS
ð9Þ
#
The negative values of DS were obtained (Table 3), which
showed that there forms an adsorbed ‘‘activated complex’’
on the electrode surface which is more organized than the
initial reactants [18–20]. The decrease in the negative
values of entropy of activation with increasing temperature
shows that the ‘organized structure’ is disturbed with
increasing temperature which results in the decreased val-
ues of charge transfer coefficient and that of ks.
Conclusions
Compounds APZ1, APZ-2, and APZ-3 were synthesized,
characterized, and studied for their electrochemical
behavior. All the compounds were found to be electroac-
tive in pH range from 2 to 9 and showed one cathodic
peak. At low scan rates (up to 140 mV/s) peak current
showed linearly with the square root of scan rate. The
Scheme 1
1
23

Electrochemical investigations of some newly synthesized arylazapyrazole derivatives
slope values of plots of log i vs. log m confirmed mixed
recorded on Agilent technologies (6890 N-GC) and an
P
diffusion-adsorption control trends in the redox processes.
The peak currents increase with temperature while
decreases with pH, whereas the peak potential shifts
towards lesser negative values which showed that the
electrode process requires lesser over potential to take
place as the temperature increased and protons are
involved in the electrode process at high pH. The diffusion
coefficients are increased and the rate constants are
decreased with increasing temperature, while both
decreased with increasing pH. This indicated that mobility
of the redox species are increased with temperature while
decreased with pH whereas the charge transfer process
becomes more difficult with both temperature and pH.
Negative values of change in apparent energy, enthalpy,
and entropy of activation showed that the electrode pro-
cess needs lesser overpotential to occur as the temperature
is increased, a preceding adsorption of the analyte on the
electrode surface and an adsorbed ‘‘activated complex’’ is
formed which is more organized than the initial reactants,
respectively. The electrode reaction is preceded by a
protonation reaction of the analyte and parent pyrazole
redox system followed E C mechanism.
inert mass selective detector 5973 mass spectrometer.
Cyclic voltammetric measurements were carried using
electrochemical system AUTOLAB PGSTAT-302 poten-
tiostat/galvanostat with electrochemical software package
GPES 4.9. (Eco Chemie, Netherland). The sample solu-
tions were taken in Mr. Bob’s electrochemical cell (a single
compartment double layer cell with a three electrode
configuration) attached to temperature control system
(model REX-C410, Zhejiang XinChi Electric Co., China).
2
A glassy carbon electrode with surface area of 0.70 cm
served as the working electrode, a platinum wire and Ag/
AgCl electrode were used as counter and reference elec-
trode, respectively. The working electrode was polished
with powdered a-alumina on a polishing pad, washed with
doubly distilled water and ultrasonicated for 30 s prior to
each experiment. Before every electrochemical assay,
argon gas (99.999 %) was purged within the cell cavity for
least 10–15 min to make the cell environment inert espe-
cially for washing out oxygen from the cell. The pH values
were adjusted with pH meter (model Jenway, 3510). The
milligram moles of the samples were weighed on analytical
balance (model Shimadzu AUW 320).
i
i
Synthesis of aza-substituted pyrazoles
Experimental
Materials
The aryl azapyrazoles were synthesized from aromatic
amines and 1,3-dicarbonyl compounds following a three-
step synthetic route (Scheme 2). The diazotized aromatic
amines were condensed at active methylene position of
acetyl acetone to get (E)-3-(phenyldiazenyl)pentane-2,4-
dione which on cyclization with hydrazine gave the aza-
substituted pyrazoles [7]. The general procedure is given as
Scheme 2.
The commercially available reagents were used as received
from the market. Solvents were dried according to the
standard procedures and redistilled before their use.
Compounds were synthesized according to Scheme 1 given
-
2
in next sections. Stock solutions (1 9 10 M) of all the
synthesized compounds were prepared in N,N- dimethyl-
(E)-4-(Phenyldiazenyl)-3,5-dimethyl-1H-pyrazole
formamde (DMF). For cyclic voltammetric experiments
3
cm of stock solution was taken in Mr. Bob’s cell and
(
APZ-1, C H N )
11 12 2
1
1
Yield: 0.44 g (74 %); m.p.: 232 °C; H NMR (300 MHz,
3
3
added to it 3.5 cm of Britton Robinson buffer and 5.5 cm
of DMF.
DMSO-d ): d = 7.41 (t, 2H, J = 5.6 Hz, ArH), 7.50 (t,
6
2
6
1
1
3
H, J = 8.0 Hz, ArH), 7.71 (d, J = 5.6 Hz, ArH), 2.60 (s,
1
3
H, CH ) ppm; C NMR (75 MHz, CDCl ): d = 152.96,
3
3
Physical methods
40.46, 134.07, 129.65, 129.46, 129.14, 128.85, 122.46,
22.11, 121.36, 11.79 ppm; IR (NaCl): ¯v = 3350 (N–H),
The homogeneity and purity of compounds was confirmed
from thin layer chromatography and the structure was
2
211 (sp CH), 2954 (sp CH), 1617–1458 (C=C), 1502
3
-
C=N) cm ; GC–MS (EI, 70 eV): m/z (%) = 200 (39.1),
1
(
confirmed from spectral analysis. The R values were
f
123 (100), 95 (90.5), 77 (27.7), 64 (18), 41 (19).
determined using aluminum pre-coated silica gel plates
Kiesel 60 F254 from Merck (Germany). Melting points of
the compounds were measured in open capillaries using
Stuart melting point apparatus (SMP3). The IR spectra
were recorded on FTS 3000 MX, Bio-Rad Merlin (Excal-
(E)-4-[(4-Fluorophenyl)diazenyl]-3,5-dimethyl-1H-pyra-
zole (APZ-2, C H FN )
2
1
1
11
1
Yield: 0.39 g (65 %); m.p.: 236 °C; H NMR (300 MHz,
DMSO-d ): d = 10.99 (s, N–H), 7.51 (d, 2H, J = 8.7 Hz,
6
1
ibur Model) spectrophotometer. The H NMR spectra were
ArH), 7.86 (d, 2H, J = 8.4 Hz, ArH), 2.60 (s, 6H, CH )
3
1
3
run on a Bruker 300 MHz NMR spectrometer in DMSO-d6
ppm; C NMR (75 MHz, CDCl ): d = 152.85, 141.26,
3
solution using TMS as an internal standard. EIMS were
138.50, 134.64, 123.81, 96.26, 12.34 ppm; IR (NaCl):
123

N. Arshad et al.
Scheme 2
2
v = 3212 (sp CH), 2920 (sp CH), 1730, 1591 (N=N),
3
¯
2] was run and smooth scans both on forward and
reverse directions showed electro inactivity as well as
purity of solutions used in voltammetic assay within
desired potential range of 0 to -2 V. Cyclic voltammo-
grams were recorded separately for all the compounds to
-
492–1421 (C=C), 1007 (C–F) cm ; GC–MS (EI, 70 eV):
1
1
m/z (%) = 218 (100), 123 (83), 95 (47), 64 (6.5), 41 (10).
(
E)-4-[(4-Iodophenyl)diazenyl]-3,5-dimethyl-1H-pyrazole
(
APZ-3, C H IN )
1
1
11
2
3
determine their redox behavior. Stock solution (1 cm ,
1
Yield: 0.36 g (72 %); m.p.: 213 °C; H NMR (300 MHz,
-
9 10 M) of compound prepared in DMF is mixed in
2
1
3
DMSO-d ): d = 12.6 (s, N–H), 7.23 (d, 2H, J = 7.8 Hz,
6
3
3
.5 cm of Britton Robinson buffer and 5.5 cm of DMF.
ArH), 7.70 (d, 2H, J = 8.4 Hz, ArH), 2.60 (s, 6H, CH )
3
The pH of the sample solution at this composition was 2.
Similarly sample solutions were prepared with pH 6 and
1
3
ppm; C NMR (75 MHz, CDCl ): d = 139.22, 138.47,
3
1
1
1
7
38.26, 138.26, 137.87, 137.46, 121.44, 119.66, 117.35,
9
by adjusting the pH of the buffer with 0.1 N NaOH
2
3.80 ppm; IR (NaCl):¯v = 3200 (sp CH), 2954 (sp CH),
3
solution. The sample solution was poured into the cell
and deoxygenated with argon. The experiment was con-
ducted at three different temperatures (298, 303, and
-
561 (C=N), 1580–1476 (C=C) cm ; GC–MS (EI,
1
0 eV): m/z (%) = 326 (100), 203 (6), 123 (69.8), 95
(
24.7), 76 (7.6), 41 (8.1).
308 K) and at pH, 2, 6, and 9 using glassy carbon sur-
face as working electrode. Voltammetric responses were
recorded at various scan rates i.e., 20, 40, 60, 80, 100,
Electrochemical procedure
1
20, and 140 mV/s.
The synthesized compounds were electrochemically
studied in DMF solvent buffered with Britton Robinson
buffer (BRB) using cyclic voltammetic technique. First
blank medium [DMF (65 %) ? BRB (35 %) having pH
Acknowledgments This work is supported by Department of
Chemistry, Faculty of Science, Allama Iqbal Open University
Islamabad, Pakistan.
1
23

Electrochemical investigations of some newly synthesized arylazapyrazole derivatives
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