Electrochemical activation of CO2 by a di-Schiff base of N, N′-bis(2-hydroxy-1-naphthaldehyde)-m-phenylenediimine

Article abstract of DOI:10.1016/j.poly.2018.08.039

Electrochemical behavior of a di-Schiff base, N, N′-bis(2-hydroxy-1-naphthaldehyde)-m-phenylenediimine, NMPD, was investigated using cyclic voltammetry, chronoamperometry and controlled-potential coulometry in acetonitrile solution containing tetrabutylammonium perchlorate, as supporting electrolyte. The effect of different parameters such as potential scan rates, and time window of the chosen electrochemical method was examined in studying the electrochemical behavior of NMPD. According to the results of experiments, an EC₂CE mechanism was proposed to electrochemical reduction of NMPD on the glassy carbon electrode surface. Furthermore, NMPD is an efficient electrocatalyst in reduction of CO₂. This electrocatalytic activity is confirmed by 700 mV decreases in overpotential of CO₂ reduction, significant increase in cathodic current of NMPD and also the spectrum characterization of the main product of the electrolysis process which is oxalic acid.

Full text of DOI:10.1016/j.poly.2018.08.039

Accepted Manuscript
Electrochemical activation of CO2 by a di-Schiff base of N, N'-bis(2-hydroxy-1-
naphthaldehyde)-m-phenylenediimine
Kobra Ghobadi, Hamid R. Zare, Alireza Gorji, Ali Benvidi
PII:
S0277-5387(18)30505-9
https://doi.org/10.1016/j.poly.2018.08.039
POLY 13362
DOI:
Reference:
Polyhedron
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29 July 2018
10 August 2018
Please cite this article as: K. Ghobadi, H.R. Zare, A. Gorji, A. Benvidi, Electrochemical activation of CO2 by a di-
Schiff base of N, N'-bis(2-hydroxy-1-naphthaldehyde)-m-phenylenediimine, Polyhedron (2018), doi: https://
doi.org/10.1016/j.poly.2018.08.039
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Electrochemical activation of CO2 by a di-Schiff base of N, N'-bis(2-hydroxy-
1-naphthaldehyde)-m-phenylenediimine
Kobra Ghobadi, Hamid R. Zare*, Alireza Gorji, Ali Benvidi
Department of Chemistry, Faculty of Science, Yazd University, Yazd, 89195-741, Iran
Fax number: +98 3538210991
Telephone number: +98 35 31232669
E-mail address: hrzare@yazd.ac.ir
1

Abstract
Electrochemical behavior of a di-Schiff base, N, N'-bis(2-hydroxy-1-naphthaldehyde)-m-
phenylenediimine, NMPD, was investigated using cyclic voltammetry, chronoamperometry
and controlled-potential coulometry in acetonitrile solution containing tetrabutylammonium
perchlorate, as supporting electrolyte. The effect of different parameters such as potential scan
rates, and time window of the chosen electrochemical method was examined in studying the
electrochemical behavior of NMPD. According to the results of experiments, an EC2CE
mechanism was proposed to electrochemical reduction of NMPD on the glassy carbon
electrode surface. Furthermore, NMPD is an efficient electrocatalyst in reduction of CO2. This
electrocatalytic activity is confirmed by 700 mV decreases in overpotential of CO2 reduction,
significant increase in cathodic current of NMPD and also the spectrum characterization of the
main product of the electrolysis process which is oxalic acid.
Keywords: Di-Schiff base; Dimerization; Electrochemical behavior; Electrocatalyst; CO2
reduction
2

1. Introduction
Schiff bases are one of the most important organic compounds which have gained a lot
of applications as mediator of electron transfer [1]. The biological activities of these
compounds like anticancer [2], antioxidant [3], anti-inflammatory [4], antimicrobial [5],
anticonvulsant [6], antitubercular [7] have been also proved. Meanwhile, tetradentate OH-
substituted aromatic Schiff bases with delocalized π-systems have remarkable photochromic
properties [8] and are used as organic light emitting diodes (OLED) [9, 10]. Also, these
compounds have important part in development of coordination chemistry because they can
easily form stable complexes with most transition metals in various oxidation states [11]. Schiff
base transition metal complexes have been widely used as the catalyst in a large number of
heterogeneous and homogeneous reactions [11, 12]. Electrocatalytic reduction of carbon
dioxide by some of these complexes is one example of their catalytic activities [13-18]. Carbon
dioxide with some properties such as availability, non-toxic and inexpensive can be a useful
raw material to synthesis a variety of organic compounds [19]. In thermodynamic and kinetic
view, CO2 is an inert molecule without any tendency to participate in chemical reactions [20].
Then the development of methods to activation of the inert CO2 molecule like electrochemical
techniques is a challenge for chemists. One methodology to transform CO2 into useful
chemicals is using of electricity as an external physical energy that decreases Gibbs free energy
of the reactions include CO2 [20]. But large overpotential is required for CO2 reduction in
electrochemical techniques. It is well-known that electrolysis in high negative potentials, may
leads to reduction of other components like solvent, supporting electrolyte and other impurities
in the solution. Then use of catalysts to overcome this problem, by mediate electron transfer,
is necessary [21, 22]. For this purpose, catalysis of CO2 reduction by metal complexes has been
also widely reported [23]. Despite the widespread use of complexes as catalyst to this purpose,
3

only a few papers are available on the catalysis by organic compounds [24-26]. Under
consideration that synthesis of metal complexes is an expensive and difficult process and also
they have less solubility in most organic solvents, we decided to study new simple and low-
cost catalysts. In electrocarboxylation of some imine derivatives with phenyl substituent,
formation of oxalic acid beside amino acids has been reported [27]. Furthermore, in
electrocatalytic reduction of CO2 in the presence of some complexes, involving Schiff bases as
the ligand, more increase at the catalytic current has been observed for reduction peak of ligand
[28].
Hence, we interested to study process involved in electrochemical activation of CO2 by
a di-Schiff base, N, N'-bis(2-hydroxy-1-naphthaldehyde)-m-phenylenediimine (NMPD).
Furthermore, in order to propose a mechanism for the overall process, the electrochemical
behavior of the di-Schiff base, NMPD, was also investigated. This present study extends our
earlier investigations on the electrocatalytic reduction of CO2 in the presence of different
electron transfer mediators [29-33]. These studies are useful for designing more efficient
catalysts with the purpose of electrocatalytic reduction of CO2.
2. Experimental
2.1. Chemicals and apparatus
All reagents were analytical grade and were purchased from Merck Company. Carbon
dioxide (CO2) and Argon (Ar) gases had a purity of 99.995%. N, N'-bis(2-hydroxy-1-
naphthaldehyde)-m-phenylenediimine, NMPD, was prepared as reported in literatures [34-35].
Acetonitrile and tetrabutylammonium perchlorate (TBAP) were used as solvent and supporting
electrolyte respectively. To elimination of trace water, the acetonitrile solvent was more
purified by molecular sieve 5A. All the experiments were carried out inside a Faraday cage at
room temperature. Cyclic voltammetry and chronoamperometry were performed using an
4

EG&G PARSTAT 2273 equipped with a Power Suite software in a conventional three-
electrode electrochemical cell containing a glassy carbon electrode (GCE), with 2 mm
diameter, as a working electrode and a Pt wire as the counter electrode. All the potentials in
the text were reported with respect to the Ag/AgCl/KCl (sat’d) reference electrode. Controlled
potential coulometry was carried out using a SAMA 500 electroanalyzer system in a divided
glass cell equipped with a gas inlet and outlet with Ag/AgCl/KCl (sat’d) as the reference
electrode, a graphite rod as the cathode and a platinum plate (ca. 3 cm2) as the anode. Fourier
transform infrared (FTIR) spectrum analysis was performed on an EQUINOX55 spectrometer.
All measurements were performed at room temperature.
2.2. Electrolysis procedure
All the electrolysis experiments have been conducted at controlled potential mode in
50.0 mL ACN containing 0.1 M TBAP. Prior to every experiment, definite concentration of
NMPD in such solution was prepared and Ar gas was bubbled for 20 min. During the
electrolysis, consumption of NMPD was followed by thin layer chromatography, TLC, tests of
aliquots withdrawn from the electrolysis solution. The spots were distinguished by irradiation
of an ultraviolet light. When the spot corresponding to NMPD disappeared from the TLC plate,
the electrolysis was stopped. To obtain the number of electrons transferred at the first (Fig. 1)
and the second peak potential of NMPD (Fig. 2), constant potential of -1.5 V and -2.0 V were
applied to the working electrode respectively and Ar gas was continuously bubbling into
solution during the electrolysis. The products could not be separated due to the polymerization
during the evaporation of solvent. At the second set of experiments, the constant potential of -
1.5 V was applied to the working electrode, but CO2 was continuously bubbling into the
solution. At the end of electrolysis, the solution was concentrated to 5.0 mL and then 10.0 mL
of 0.5 M NaOH was added by stirring for 30 minutes. After filtration, 10.0 mL of 0.5 M HCl
5

was added to the solution, and it was stirred for 30 minutes at room-temperature. Then, liquid-
liquid extraction was carried out with chloroform. Aqueous and organic phases were separated
and the solvents were evaporated under vacuum. A polymeric brown material was obtained for
the organic phase which could not be identified but the residue of aqueous phase was then
dissolved in 50 mL diethyl ether. After evaporation of diethyl ether, the residue was identified
by FTIR spectrum (Fig. S1, Supporting Information). The IR characteristics of the electrolysis
product of CO2 in the presence of NMPD was determined as follows: 1128 and 1255 cm-1
(C−O), 3484 cm-1 (O−H), 1694 cm-1 (C=O) (Fig. S1, Supporting Information). Based on these
spectral characteristics, oxalic acid is a possible product. The results of electrolysis showed
current efficiency of 57% for formation of oxalic acid.
3. Results and discussion
3.1. Cyclic voltammetric studies of NMPD
All experiments have been conducted in acetonitrile solution containing 0.1 M TBAP,
as the supporting electrolyte. The cyclic voltammogram of 2.0 mM NMPD in such solution at
potential range of -0.9 V to -1.8 V versus the reference electrode has been shown in Fig. 1A.
At this range of potential, the voltammogram only consists of one irreversible peak with
reduction potential (Ep) of -1.5 V. This peak is related to the reduction of one of the imine
bonds, azomethine groups [27, 36, 37]. The irreversible reduction peak (Fig. 1A) was analyzed
from the kinetic point of view of the heterogeneous electron transfer. The value of │Ep – Ep/2│
for the voltammogram of Fig. 1A is about 60 mV. To obtain the number of electron transferred
(n) at the process that is characterized by this peak, the controlled potential coulometry was
carried out. The obtained result indicates that n 0.92. This result shows that the electrode
=
reaction includes a one-electron process. Fig. 1B shows cyclic voltammograms of the solution
containing 1.0 mM NMPD in different potential scan rates. The variation of the cathodic peak
6

currents, (Ip), versus the square root of potential scan rate, v1/2, has been shown in (Fig. 1C).
The linearly dependence of these parameters demonstrates that the electrode reaction of NMPD
is diffusion controlled [38]. To obtain more information about the NMPD reduction, variation
of the peak potential, Ep, versus scan rate, v, was examined. Fig. 1D shows the plot of Ep versus
log v. This figure shows shift of Ep to more negative values by increasing of log v. Based on
this result and the difference value between Ep and Ep/2 (Fig. 1A), it can be concluded that
NMPD reduction mechanism is a reversible electrochemical reaction that is followed by a
chemical reaction. To confirm the reversibility of the electrochemical reaction, the number of
electrons involve in the reduction process was also determined by means of Eq. (1a). This
equation describes the variations of the Ip value versus v1/2 for a reversible diffusion controlled
process:
Ip = 2.69 × 105 n3/2 A D1/2 C v1/2
(1a)
In above equation A, C and D are the electrode surface area (cm2), concentration (mol
cm-3) and diffusion coefficient (cm2 s-1) of NMPD in acetonitrile solution respectively. The
experimental value of D was obtained as it will be described at the section 3.2. According to
the described method, the number of electrons corresponding to reduction peak of Fig. 1A was
calculated as 0.86 that is nearly in agreement with that (n=0.92) obtained by coulometry
method. Then, it can be concluded again that the electrochemical reduction of NMPD include
a reversible one-electron process that leads to formation of the corresponding radical-anion,
NMPD•−. Due to the lack of proton donor, one-electron reduction has been usually reported for
the first step of organic compounds reduction in aprotic solvents, such as acetonitrile [38-40].
The next step of the process would be the second electron transfer or dimerization which
depends on some parameters such as the molecular structure, electron donor ability of the
substituents on the carbon of azomethine group (carbon 1 or 1' in Scheme 1.), and proton donor
ability of the medium [39, 40]. In NMPD, the negative charge of the electrogenerated radical
7

anion, NMPD•−, can be distributed by an intra-molecular hydrogen bonding via OH substitutes
(Scheme 1) [37, 41-43]. But, naphthyl groups, as the electron donating groups, on the both
sides of NMPD prevent from the second electron transfer to NMPD•−. Then, it is logical to
conclude that dimerization is the most favor process for stabilization of NMPD•− as shown in
Scheme 1 and also reactions (1) and (2) [42, 43]. Dimerization is the most likely route for the
radical anions generated by electrochemical reduction of unsaturated compounds like olefins
or imines in non-aqueous medium [44-46].
NMPD + e
2NMPD•−
NMPD•−
(E)
(1)
2−
(NMPD)2
(C2) (2)
Fig. 2 shows the voltammogram of NMPD at the potential range of -1.1 V to -2.2 V.
The voltammogram is characterized with a single reduction peak (peak I) and a quasi-reversible
redox couple (peaks II/III). CPC showed that 1.45 electron equal per mole was consumed when
the reduction potential of peak II (-2.0 V) was applied to the working electrode. Considering
consumption of one electron at the reduction potential of peak I, the number of electrons that
is related to the quasi-reversible couple II/III is equal to 0.5 per each mole NMPD. As seen in
Fig. 2, the intensity of peak I is also higher than that in the peak II. The concentration of the
dimerization product,
2 ‒ , is half of NMPD concentration at the electrode surface.
(NMPD)
2
Therefore, it is expected the reduction current of peak II would be half of that for peak I (Fig.
2). This observation provides more evidence that the peak II correspond to the reduction of
2 ‒
dimerization product. It is well-known that
with two negative charges and electron
(NMPD)
2
donor groups, like hydroxyl naphthyl, is not reducible in the form of dimeric dianion.
2 ‒
Therefore, reduction of
is only possible if this dimer rearranges to form a neutral
(NMPD)
2
specie. Because of the fact that ACN is not a suitable source of hydrogen ions, it seems that the
best way for the rearrangement is elimination of water because it can supply the necessary
hydronium ions for neutralization. Considering these issues, a mechanism was proposed for
8

(NMPD)2 ‒
rearrangement of
to form a benzo-phenanthridine derivative (structure D in
2
Scheme 2) as shown in reaction (3). At the first step, the negative charge on nitrogen atom
transfers on carbon 1 (structure A) by resonance. Then the carbanion 1 attacks to carbonyl
group in naphthyl moiety (carbon 3) that is in equilibrium with enol form. The enol and ketol
tautomerism in naphthaldiimine Schiff bases have been previously reported by some authors
[37-41]. The nucleophilic attack of carbanions to electrophile sites like carbonyl groups to form
6 member cycles are the most favorable kind of cyclization in organic chemistry [47], because
it leads to enhance the stability by delocalization of charge in aromatic system. At the next
steps, the resulted intermediate A abstracts two hydronium ions to form intermediate B. Finally,
by elimination of two water molecules, a benzo-phenanthridine derivative compound (structure
D) is formed [47].
+ 2H + ‒ 2H2O and Rearrangement

(NMPD)2 ‒
→
D
(C)
(3)
2
In view of all these, reduction peak II and oxidation peak III of Fig. 2 can be assigned
to the redox reaction of one of the two azomethine groups on either side of structure D (Scheme
2) according to reaction (4).
D + e
D•−
(E)
(4)
The quasi-reversibility behavior of couple D/D•− (peaks II and III in Fig 2 or Fig. 3)
provides additional evidence for the structure D. This is due to the fact that the anion radical
which is formed via reduction of imine bond in structure D (rings 3 or 3' in Scheme 2), D•−, is
stable because the single electron can be delocalized in the aromatic rings 1, 2, 3 and 4 (or 1',
2', 3' and 4'). These results indicate that D•− is stable even in low scan rates and therefore there
is no any following chemical reaction for reduced form of D, D•−. To more understand about
the electrochemical behavior of couple D/D•−, the influence of time window on variation of the
currents of peaks II and III was examined. Fig. 3 shows cyclic voltammograms of NMPD in
potential range of -1.1 V to -2.2 V in different scan rates. Inset (A) in Fig. 3 shows the variation
9

of the peak potential of peak II, Epc, (plot of 1) and the peak potential of peak III, Epa, (plot of
2) against logarithm scan rates. As it can be seen, the cathodic peak shifts to more negative
values of potential with increasing scan rates while there is no change in the anodic peak. Inset
(B) in Fig. 3 exhibits the ratio of peak currents of peak III to the peak II (Ipa(III)/Ipc(II)). As seen,
by increasing scan rate the ratio of Ipa(III)/Ipc(II) increases. These observations are the
characteristics of an CE mechanism (reactions 3 and 4). At this mechanism, it is expected with
increase of potential scan rate (decrease of time window), the concentration of D to be limited
by the rate of the preceding chemical reaction [38, 48] that is shown in Scheme 2 and reaction
(3). According to the electrochemical results and all the mentioned points at this section, it
seems that EC2CE, is the most likely mechanism for electrochemical behavior of NMPD at
surface of GCE. However, to verify the proposed mechanism, CPC was also done at both
potential of peak I and peak II. But unfortunately, attempt to isolate and identification of the
products was failed because by evaporation of the solvent, the solution was starting to be
viscous and a polymeric brown material was obtained at the end. Therefore, the separation of
products and their identification at this stage was not carried out.
3.2. Chronoamperometric studies of NMPD
The diffusion coefficient of NMPD was also obtained by chronoamperometry method
based on the Cottrell equation and according to the procedure described in Ref. [38]. Fig. 4A
shows the chronoamperograms of different concentration of NMPD at a potential step of -1.7
V. Fig. 4B shows the experimental plot of I versus t−1/2 with the best fits for different
concentrations of NMPD. After that, the slopes of the resulting straight lines were plotted
versus NMPD concentration (inset of Fig. 4B). Considering one electron transfer at the first
step of NMPD reduction and also the slope of straight line in Fig. 4B, inset, the D value of
NMPD in acetonitrile (0.1 M TBAP) solution was obtained as 1.59 × 10-5 cm2 s−1 [38].
10

3.3. Electrochemistry of NMPD in the presence of CO2
Fig. 5, voltammogram (a), shows the voltammetric response of GCE in ACN solution
(TBAP 0.1 M) that is saturated with CO2 at potential range of -1.1 to -2.2 V. This
voltammogram shows that CO2 is not electroactive at potentials up to -2.2 V. Also,
voltammograms of 1.0 mM NMPD solution in the absence (voltammogram b) and the presence
(voltammogram c) of CO2 at potential range of -1.1 V to -1.6 V have been shown in Fig. 5. As
seen, the cathodic peak in voltammogram (b) that is related to the reduction of NMPD (reaction
(1)), significantly increases in the presence of CO2 (voltammogram c). The noticeable increase
of the cathodic peak current of NMPD in the presence of CO2 (voltammogram c) indicates that
NMPD has excellent electrocatalytic activity for reduction of CO2 as shown in reactions of (1)
and (5) [21]. In addition, the peak potential of CO2 reduction shifts about 700 mV to less
negative value in the presence of NMPD.
•−
NMPD•− + CO2 → NMPD + CO2
(5)
The effect of scan rate on electrocatalytic reduction of CO2 in the presence of NMPD
was also investigated (Fig. 6). A plot of normalized current, Ip v-1/2, versus scan rate, v, (inset
of Fig. 6) shows a sharp decrease with raising scan rate. This observation that is the
characteristics of an EC' mechanism is due to the limitation of time for electrocatalytic process
in high scan rates. Electrocatalytic reduction of CO2 by some organic compounds like benzyl
[24, 33] and substituted benzenes [25, 49] have been reported. In our previous paper, it was
•−
shown the product of electrocatalytic reduction of CO2, CO2 , in the presence of NMPD can
be used for carboxylation of pyridine [32]. At the present work, control potential coulometry
of NMPD solution saturated with CO2 was also done at potential of -1.5 V but in the absence
of any electron acceptor such as pyridine. As described in experimental section, the product of
electrolysis was separated and identified by FTIR method. The results revealed that oxalic acid
11

with current efficiency of 57% is the main product of CO2 reduction in the presence of NMPD.
This result proves the homogenous electron transfer from NMPD•− to CO2. Formation of oxalic
acid as a byproduct of benzyl imines reduction in the presence of CO2 have been previously
reported by D. K. Root and et. al [27]. Based on the literature review, formation of amino acids
by electrochemical reduction of compounds involving imine bonds, in the presence of CO2 is
possible [27, 50]. According to the published papers [27, 50], following the first reduction of
the imine bond, the produced radical anion is protonated. Afterward, the second reduction
occurs. The result of the process is formation of carbanion as a nucleophile which can attacks
to CO2 molecule to form an amino acid. Based on a published paper [50], activation of imine
bond is the key step in formation of amino acid by such carboxylation process. They found that
increasing electron accepting ability of the substituents in both side of azomethine group leads
to increase of amino acid yields. As an example, they used of fluorine as a powerful electron
acceptor in molecular structure [50] for increasing the yield. However, in the case of NMPD,
as a di-Schiff base, with a symmetrical structure as well as two hydroxynaphthyl groups as
donating electron, there is a high density of negative charge on the carbon of azomethine group
(Scheme 1, carbon 1 and 1'). Then, it is expected that the second electron transfer to the reduced
form of azomethine group is difficult in both form of radical anion, –HC•–N− or even
protonated form –HC•–NH– (Scheme 1). At this regard, for the product of the first reduction
of NMPD, NMPD•−, can consider two competitive chemical reactions of dimerization and
homogeneous electron transfer from NMPD•− to CO2. The reasonable current efficiency of %57
for formation of oxalic acid reveals higher speed of homogenous electron transfer respect to
the dimerization or other unidentified reactions. It seems that the electronic structure of NMPD
has an important role in electrocatalytic behavior toward CO2 reduction.
4. Conclusion
12

The result of this work shows that, on the surface of glassy carbon electrode, GCE, in
acetonitrile solution, NMPD (N, N'-bis(2-hydroxy-1-naphthaldehyde)-m-phenylenediimine) is
reduced in two steps. The first reduction stage is characterized by a reversible one-electron
reduction process which is followed by dimerization. The second stage of the reduction process
that is considered by a quasi-reversible couple peaks shows the characteristics of a CE
mechanism. Molecular rearrangement of the dimer, that is produced at the first step, is proposed
before the second reduction. According to the proposed rearrangement, a benzo-phenanthridine
derivative is formed by cyclization and elimination of two molecules of water from the dimer.
The proposed rearrangement could well explain the electrochemical behavior of NMPD at
GCE. Furthermore, electrocatalytic activity of NMPD toward reduction of carbon dioxide,
CO2, was shown by 700 mV decrease in overpotential and significant increase in reduction
current. Also, the electrocatalytic effect of NMPD in reduction of CO2 was confirmed by the
spectrum characterization of the main product which is obtained by controlled-potential
coulometry. It has been demonstrated that the electronic structure of the groups which are
located on the either side of the unsaturated C=N bond has the important effect on the
electrocatalytic reduction of CO2 by NMPD. Our present study is only a minor contribution to
the electrocatalytic reduction of CO2 in the presence of a di-Schiff base. The question that
beside oxalic acid, what products are exactly produced and how the molecular structure can
affect the overall process remains open.
13

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16

Legends of Schemes and Figures
Scheme 1. Intra-molecular hydrogen bonding in NMPD as well as dimerization at the first step
of reduction.
2 ‒
Scheme 2. Proposed mechanism for rearrangement of
dimer.
(NMPD)
2
Fig. 1. Cyclic voltammogram of a glassy carbon electrode in an acetonitrile solution (0.1 M
TBAP) containing (A) 2.0 mM NMPD at potential range of -0.1 V to -1.8 V. Potential scan
rate: 50 mV s-1. (B) 1.0 mM NMPD at potential range of -1.4 V to -1.7 V and at different
potential scan rates. The numbers 1–9 correspond to potential scan rates 10, 20, 40, 60, 80, 120,
200, 300 and 450 mV s−1 respectively. (C) Variations of the cathodic peak currents vs. the
square root of potential scan rate. (D) Variation of cathodic peak potential vs. logarithm of scan
rates.
Fig. 2. Cyclic voltammogram of a glassy carbon electrode in an acetonitrile solution (0.1 M
TBAP) containing 1.5 mM NMPD at potential range of -1.1 V to -2.2 V. Potential scan rate:
100 mV s-1.
Fig. 3. Cyclic voltammograms of a glassy carbon electrode in an acetonitrile solution (0.1 M
TBAP) containing 1.5 mM NMPD at potential range of -1.1 V to -2.2 V and at different
potential scan rates. The numbers 1–9 correspond to potential scan rates 20, 35, 50, 100, 150,
200, 300, 500 and 1000 mV s−1 respectively. Inset (A) variation of cathodic peak potential of
peak II (plot of 1) and peak III (plot of 2) vs. logarithm of scan rates. Inset (B) variations the
ratio of the anodic peak currents (peak III) to the cathodic peak currents (peak II) vs. scan rate.
Fig. 4. (A) Chronoamperograms obtained for different concentration of NMPD in acetonitrile
solution (0.1 M TBAP) at glassy carbon electrode and at potential step of -1.7 V vs. the
reference electrode. The numbers 1-4 correspond to NMPD concentration of 0.15, 0.60, 0.70,
17

and 0.80 mM respectively. (B) Plots of I vs. t−1/2 obtained from shown chronoamperograms.
Inset: plot of straight lines slops in (B) vs. the NMPD concentration.
Fig. 5. Cyclic voltammograms of a glassy carbon electrode in (a) an acetonitrile solution (0.1
M TBAP) saturated with CO2 at potential range of -1.1 V to -2.2 V, (b) an acetonitrile solution
(0.1 M TBAP) at potential range of -1.1 V to -1.6 V containing 1.0 mM NMPD. (c) as (b) after
bubbling of CO2. Potential scan rate: 15 mV s-1.
Fig. 6. Cyclic voltammograms of a glassy carbon electrode in acetonitrile solution (0.1 M
TBAP) containing 1.0 mM NMPD and saturated with CO2 at different potential scan rates. The
numbers of 1–6 correspond to 5, 15, 30, 60, 100 and 200 mV s−1 respectively. Inset: plot of
normalized current, Ip v-1/2, vs. scan rate, v.
18

Scheme 1.
NMPD
NMPD
OH
N
OH
N
2
1
1'
2e
+
.-
O
OH
H
2
N
N
(NMPD)22-
O
H
OH
N
OH
N
N
H
N
O
19

Scheme 2.
OH
R=
OH
R
N
N
R
HO
N
N
H
H
O
(NMPD)22-
R
N
H
H
O
R
N
N
N
H
H
O
R
N
H
H
O
1
R
N
N
3
A
B
C
2
N
+2H+
H
H
OH
R
N
N
R
H
H
H
N
HO
H
N
-2H2O
H
R
N
N
1'
H
R
N
1
N
R
NH
R
N
HN
N
D
R
2'
4'
NH
1'
3'
R
N
4
HN
2
3
1
N
20

Fig. 1.
5
A
C
5
-5
y = -0.843x - 0.3713
R² = 0.9996
-5
Ep/2
-15
-25
-15
-25
Ep
-1.9
-1.4
-0.9
-0.4
0
6
12
18
24
E vs.Ag/AgCl/KCl(sat’d) / V
v1/2 / (mV s-1)1/2
5
-5
B
-1.56
-1.53
-1.5
D
1
-15
-25
9
1
2
3
-1.8
-1.6
-1.4
log v (v / mV s-1)
E vs.Ag/AgCl/KCl(sat’d) / V
21

Fig. 2.
5
III
-5
-15
-25
II
I
-2.4
-1.7
E vs. (Ag / Ag+, 0.01 M) / V
-1
22

Fig. 3.
10
-2.2
-2.0
-1.8
A
1
III
1
2
1
2
3
4
log v (v / mV s-1)
-25
2.5
1.5
0.5
B
II
9
0
400 800 1200
v / mV s-1
-60
-4.0
-3.0
-2.0
-1.0
E vs. Ag/AgCl/KCl(sat’d) / V
23

Fig. 4.
20
15
10
5
A
4
1
0
-5
0
5
10
15
t / s
B
9
4
4
13
y = 6.7823x + 0.9775
R² = 0.9959
7
1
0
0.6
1.2
1
[NMPD] / mM
-1
0.3
0.6
0.9
t-1/2/ s-1/2
24

Fig. 5.
2
a
b
c
-5
-12
-2.4
-1.7
-1
E vs. Ag/AgCl/KCl(sat’d) / V
25

Fig. 6.
0
1
6
-3.2
-2.4
-1.6
-11
-22
-33
0
250
v / mV s-1
-1.0
E vs. Ag/AgCl/KCl(sat’d) / V
-1.8
-1.4
-0.6
Graphical abstract - Pictogram:
5
D•− ¾ D + e
NMPD + e → NMPD•−
2NMPD•− → (NMPD)2
-10
2−
Rearrangement
D + e ¾ D•−
2−
(NMPD)2
D
(+ 2H+ - 2H2O)
-25
-2.4
-1.7
-1
-0.3
E vs. (Ag / Ag+, 0.01 M) / V
26

Graphical abstract - Synopsis:
An EC2CE mechanism is proposed to electrochemical reduction of N, N'-bis(2-hydroxy-1-
naphthaldehyde))-m-phenylenediimine, NMPD. Also, the results indicate that NMPD has an effective
electrocatalytic activity in reduction of CO2.
27