Synthesis of nitrocarbazole compounds and their electrocatalytic oxidation of alcohol

Article abstract of DOI:10.1016/S1872-2067(15)61047-6

Three compounds with nitrocarbazole frameworks were synthesized and their electrochemical reversibility as organic electrocatalysts was studied by cyclic voltammetry. The electrochemical reversibility and oxidation-reduction potential of the compounds were greatly affected by their substituents. The oxidation-reduction potential of the compound with an electron-donating group was negative, while that of the compound with an electron-withdrawing group on the carbazole framework was positive. The electrocatalytic oxidation activities of the nitrocarbazole compounds were investigated through cyclic voltammetry and controlled potential electrolysis at room temperature. The electrocatalysts showed excellent selectivity for p-methoxybenzyl alcohol, converting it to the corresponding aldehyde through electro-oxidation with just 2.5 mol% of the electrocatalysts presented. The electrocatalysts maintained their excellent electroredox activity following recycling.

Full text of DOI:10.1016/S1872-2067(15)61047-6

Chinese Journal of Catalysis 37 (2016) 533–538
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Synthesis of nitrocarbazole compounds and their electrocatalytic
oxidation of alcohol
Yinghong Zhu, Jianqing Zhang, Ziying Chen, Anlun Zhang, Chunan Ma*
State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology,
Hangzhou 310032, Zhejiang, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Three compounds with nitrocarbazole frameworks were synthesized and their electrochemical
reversibility as organic electrocatalysts was studied by cyclic voltammetry. The electrochemical
reversibility and oxidation‐reduction potential of the compounds were greatly affected by their
substituents. The oxidation‐reduction potential of the compound with an electron‐donating group
was negative, while that of the compound with an electron‐withdrawing group on the carbazole
framework was positive. The electrocatalytic oxidation activities of the nitrocarbazole compounds
were investigated through cyclic voltammetry and controlled potential electrolysis at room tem‐
perature. The electrocatalysts showed excellent selectivity for p‐methoxybenzyl alcohol, converting
it to the corresponding aldehyde through electro‐oxidation with just 2.5 mol% of the electrocata‐
lysts presented. The electrocatalysts maintained their excellent electroredox activity following re‐
cycling.
Received 1 December 2015
Accepted 27 January 2016
Published 5 April 2016
Keywords:
Nitro‐carbazole compounds
Organic electro‐catalyst
Indirect electro‐oxidation
Selective electro‐oxidation
Redox reversibility
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
searchers have also devoted effort to the study of organic cata‐
lysts. For example, the organic sulfonate [6], immobilized
The catalytic oxidation of small organic molecules is an im‐
portant research area in the field of catalysis. Selective oxida‐
tion of alcohols to the corresponding aldehydes and ketones is
important in the synthesis of fine chemicals and organic inter‐
mediates [1,2]. Many catalysts with high selectivity, stability
and low cost have been developed to improve the conversion
and selectivity in the oxidation of alcohols to the corresponding
aldehydes. For instance, gold‐copper bimetallic catalysts [3],
and nitrogen‐doped carbon nanosphere‐supported palladium
catalysts [4] have been prepared and used in the catalytic oxi‐
dation of benzyl alcohol. Jia et al. [5] reported the selective oxi‐
dation of benzyl alcohol to benzaldehyde using alkali‐treated
ZSM‐5 zeolite as a catalyst and H2O2 as an oxidant. Many re‐
2,2,6,6‐tetramethylpiperidine‐N‐oxyl (TEMPO) [7] and Lewis
acid‐activated TEMPO have been studied as catalysts for the
oxidation of alcohol. These approaches are limited by the use of
either noble metals or high temperature.
Organic electrocatalysis is recognized as an environmentally
compatible methodology because it avoids using toxic and
dangerous oxidizing or reducing reagents, and most of the re‐
actions can be carried out under mild conditions [9,10]. How‐
ever, in direct electro‐oxidation synthesis, the working elec‐
trode is easily passivated by the formation of a polymer film on
the electrode surface, which can sharply decrease current effi‐
ciency. Indirect electro‐oxidation with an electron transfer me‐
diator is a good way to avoid passivation of the electrode
* Corresponding author. Tel/Fax: +86‐571‐88320830; E‐mail: science@zjut.edu.cn
This work was supported by the Special Program for the National Basic Research Program of China (973 Program, 2012CB722604).
DOI: 10.1016/S1872‐2067(15)61047‐6 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 37, No. 4, April 2016

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Yinghong Zhu et al. / Chinese Journal of Catalysis 37 (2016) 533–538
[11,12]. Triarylamines have received extensive attention from
researchers as a new type of electro‐redox mediator because of
their broad range of redox potentials. The oxidation potential of
triarylamines can become positive after introduction of an
electron‐withdrawing group [13–18]. Compared with the tri‐
arylamine framework, the carbazole framework has better
planarity and the substituents have a greater effect on the re‐
dox properties [19]. Carbazole compounds have been widely
used as fluorescent materials [20], but have not been employed
as an electro‐redox mediator in electrocatalytic oxidation, es‐
pecially carbazole compounds with nitro groups. Based on the
intrinsic properties of the carbazole framework and strong
electron‐withdrawing nature of nitro groups, in this paper,
nitrocarbazole compounds were synthesized and used as or‐
ganic electrocatalysts in the mediated electro‐oxidation of al‐
cohols at room temperature.
3a–3c, a 100‐mL two‐necked flask equipped with a reflux
condenser was charged with a carbazole compound with a
–OCH3, H or Br substituent (15.3 mmol), K2CO3 (76.52 mmol),
p‐nitrofluorobenzene (61.4 mmol), and DMF (80 mL). Each
reaction mixture was heated under reflux for 12 h, cooled, and
then poured into water (500 mL). Each precipitate was filtered,
dried, and recrystallized.
3a, orange powder, yield 66%. 1H NMR (500 MHz,
DMSO‐d6): δ 8.47 (d, J = 8.6 Hz, 2H), 7.93 (d, J = 8.6 Hz, 2H), 7.87
(d, 2H), 7.52 (d, J = 8.9 Hz, 2H), 7.08 (d, J = 9.0 Hz, 2H), 3.90 (s,
6H).
1
3b, bright yellow powder, yield 66%. H NMR (500 MHz,
CDCl3): δ 8.48 (d, J = 9.0 Hz, 2H), 8.14 (d, J = 7.8 Hz, 2H), 7.8 (d, J
= 9.0 Hz, 2H), 7.49 (d, J = 8.4 Hz, 2H), 7.46–7.43 (m, 2H),
7.35–7.33 (m, 2H).
1
3c, bright yellow powder, yield 81%. H NMR (500 MHz,
CDCl3): δ 8.57–8.47 (m, 2H), 8.23 (d, J = 1.9 Hz, 2H), 7.79–7.71
(m, 2H), 7.57 (d, J= 8.7, 2.0 Hz, 2H), 7.35 (d, J = 8.7 Hz, 2H).
4‐(3,6‐Dimethoxy‐9H‐carbazol‐9‐yl)aniline (4a), 3a (0.41
mmol) was dissolved in ethanol (30 mL), and then SnCl2·2H2O
(2.80 mmol) was added. The reaction mixture was heated un‐
der reflux for 15 h and then cooled to room temperature. Sub‐
sequently, the reaction mixture was adjusted to basic pH with
saturated NaHCO3, and then extracted with CH2Cl2. The organic
layer was washed with brine, dried over Na2SO4, filtered, and
concentrated. The residue was purified by silica‐gel column
chromatography to afford an orange powder in a yield of 75%.
1H NMR (500 MHz, CDCl3): δ 7.57 (d, J = 2.4 Hz, 2H), 7.33–7.23
(m, 4H), 7.09–7.01 (m, 2H), 6.90–6.79 (m, 2H), 3.96 (s, 6H),
3.85 (s, 2H).
4‐(9H‐Carbazol‐9‐yl)aniline (4b), compound 3b (0.09 mol),
10% Pd/C (0.103 g) and ethanol (20 mL) were added to a
50‐mL round‐bottom flask equipped with a stirring bar. The
reaction mixture was heated under reflux. Hydrazine monohy‐
drate (3 mL) was added slowly to the mixture, and then the
solution was stirred under reflux for 10 h. The solution was
cooled to room temperature, filtered to remove Pd/C, and then
concentrated to afford a light brown viscous liquid in a yield of
80%. 1H NMR (500 MHz, CDCl3): δ 8.15 (d, J = 8.0 Hz, 2H), 7.41
(t, J = 7.5 Hz, 2H), 7.34–7.26 (m, 6H), 6.88– 6.87 (m, 2H), 3.83 (s,
2H).
2. Experimental
2.1. Preparation of the organic electrocatalysts [21–25]
The route used to synthesize carbazole compounds with
different substituents is shown in Scheme 1.
3,6‐Dibromo‐9H‐carbazole (1), a solution of N‐bromosuc‐
cinimide (0.21 mol) in DMF (80 mL) was slowly added to a
solution of carbazole (0.1 mol) in DMF (20 mL) in an ice bath.
After reaction for 30 min, the mixture was poured into ice wa‐
ter (1 L), and the crude product was collected by filtration to
give a blue powder. Recrystallization from EtOH/H2O afforded
blue crystals with a yield of 68%. 1H NMR (500 MHz, CDCl3): δ
8.14 (d, J = 1.9 Hz, 2H), 8.12 (s, 1H), 7.53 (d, J = 8.6, 1.9 Hz, 2H),
7.32 (d, J = 8.6 Hz, 2H).
3,6‐Dimethoxycarbazole (2), in a 100‐mL three‐necked
flask, a solution of sodium methoxide (0.1 mol) in methanol (15
mL) was stirred at room temperature for 30 min. CuI (0.02
mol), 1 (0.005 mol), and DMF (17 mL) were added and then the
mixture was heated under reflux for 8 h under N2 atmosphere.
The solution was filtered while hot to remove CuI and the fil‐
trate was poured into stirred water (1 L). The resulting precip‐
itate was collected by filtration, washed thoroughly with water,
and dried to afford a black powder in a yield of 88%. 1H NMR
(500 MHz, CDCl3): δ 3.95 (s, 6H, –OCH3), 7.00 (d, J = 8.4 Hz, 2H),
7.28 (d, J = 8.7 Hz, 2H), 7.53 (s, 2H), 7.79 (s, 1H, –NH).
4‐(3,6‐Dibromo‐9H‐carbazol‐9‐yl)aniline (4c), compound
4c was synthesized according the method used to prepare 4a
Scheme 1. Synthesis of the organic electrocatalysts.

Yinghong Zhu et al. / Chinese Journal of Catalysis 37 (2016) 533–538
535
with 3c instead of 3a to afford a white powder in a yield of
92%. 1H NMR (500 MHz, CD3CN): δ 8.34 (d, J = 2.0 Hz, 2H), 7.55
(d, J = 9.0, 2.0 Hz, 2H), 7.25–7.23 (m, 4H), 6.92–6.90 (m, 2H),
4.74 (–NH2, 2H).
3b
0.02
0.01
0.00
-0.01
3c
3a
2.2. Electrochemical measurements
Cyclic voltammetry (CV) was carried on a CHI 660B elec‐
trochemical workstation (Shanghai CH Instrument Company,
China) equipped with a three‐electrode system. All experi‐
ments were performed in an undivided electrochemical cell.
Glassy carbon (3‐mm diameter) served as the working elec‐
trode, a platinum sheet (1 × 2 cm) as the counter electrode, and
Ag/AgNO3 as the reference electrode. The electrolyte consisted
of 0.2 mmol/L LiClO4 in CH3CN/CH2Cl2 (4:1 by volume). The
concentration of the organic electrocatalyst was 1 mmol/L. The
concentration of p‐methoxybenzyl alcohol (p‐MBzOH) was
varied from 3 to 20 mmol. The concentration of other
substrates was 5 mmol. The scan rate was 50 mV/s. All the
measurements were performed at room temperature and the
CV curves shown are the third stable circle.
Controlled potential electrolysis was performed in an undi‐
vided electrochemical cell equipped with a graphite rod (5‐mm
diameter) as the working electrode, platinum sheet (1 × 2 cm)
as the counter electrode, and Ag/AgNO3 as the reference elec‐
trode. The electrolyte consisted of 0.2 mmol/L LiClO4 in
CH3CN/CH2Cl2 (4:1 by volume). The concentration of the or‐
ganic electrocatalyst was 1 mmol/L. The concentration of
p‐MBzOH in the controlled potential electrolysis measurements
was varied from 20 to 200 mmol. The electrolysis process was
performed for 6 h at room temperature. The major products
were detected using GC–MS (Thermo Fisher). The yield of the
oxidation products was determined by GC (Agilent 7890A) and
calculated by the area normalization method. The organic elec‐
trocatalyst was recycled by extracting the organic phase of the
reaction liquid, concentration of the organic phase and purifi‐
cation by silica‐gel column chromatography.
0.6
0.8
1.0
1.2
1.4
E/V (vs Ag/AgNO3)
Fig. 1. Cyclic voltammograms of nitrocarbazole‐based redox catalysts
with different substituents.
pounds tended to form stable ammonium cations after losing
an electron, and the reduction increased the delocalization of
the conjugated compound. The oxidation potentials (Eox), re‐
duction potentials (Ere) and differences between them (∆E) of
the carbazole‐framework compounds are listed in Table 1.
3.2. Electrochemical oxidation of an alcohol
The CVs of p‐MBzOH with and without 3c are depicted in
Fig. 3. The oxidation peak potential for the electro‐oxidation of
p‐MBzOH exhibited a 64‐mV negative shift from 1.350 to 1.286
V in the presence of 3c. The energy consumption of the oxida‐
tion process was effectively decreased because it could be car‐
ried out at a relatively lower potential compared with the elec‐
tro‐oxidation process in absence of 3c. Simultaneously, the
oxidation peak current increased markedly. These results show
that 3c has excellent catalytic performance in the elec‐
tro‐oxidation of p‐MBzOH.
Figure 4 presents the CV curves of different concentrations
of p‐MBzOH in the presence of 1 mmol/L of 3c. The oxidation
current increased obviously with increasing substrate concen‐
tration, indicating that relatively high concentrations of
3. Results and discussion
3.1. Electrochemical properties of the organic electrocatalysts
0.16
The CV curves of the nitrocarbazole‐based redox catalysts
with different substituents are shown in Fig. 1. The oxida‐
tion‐reduction potentials of the catalyst with an elec‐
tron‐donating group (–OCH3) was negative, while they became
positive after introducing an electron‐withdrawing group (–Br)
onto the carbazole framework. Compound 3c has a positive
oxidation potential of 1.282 V versus Ag/AgNO3. Meanwhile, the
introduction of a –OCH3 or –Br substituent led to better oxida‐
tion‐reduction reversibility than that of the compound with H.
The CVs of the substituted carbazole compounds indicate that
they are suitable for use as mediators in electrochemical oxida‐
tion reactions. However, the electrochemical oxida‐
tion‐reduction reversibility of the carbazole compounds disap‐
peared when –NO2 was replaced by –NH2 (Fig. 2). A reasonable
explanation for this may be that the aminocarbazole com‐
0.12
4b
0.08
4c
0.04
4a
0.00
-0.04
-0.5
0.0
0.5
1.0
1.5
2.0
E/V (vs Ag/AgNO3)
Fig. 2. Cyclic voltammograms of aminocarbazole‐based redox catalysts
with different substituents.

536
Yinghong Zhu et al. / Chinese Journal of Catalysis 37 (2016) 533–538
0.8
Table 1
Oxidation potentials Eox, reduction potentials Ere and difference be‐
tween the oxidation and reduction potentials ∆E of the carbazole‐based
compounds.
(5)
(4)
0.6
0.4
0.2
0.0
(1) 3 mmol/L
(2) 5 mmol/L
(3) 10 mmol/L
(4) 15 mmol/L
(5) 20 mmol/L
R2
(3)
N
R1
R1
(2)
(1)
Carbazoles
R1
R2
Compound
ΔE/mV
Eox,1/V
Ere,1/V
0.644
0.905
1.203
—
OMe
H
Br
OMe
H
Br
NO2
NO2
NO2
NH2
NH2
NH2
3a
3b
3c
4a
4b
4c
0.717
1.051
1.282
—
73
146
79
—
—
0.0
0.4
0.8
1.2
1.6
E/V (vs Ag/AgNO3)
1.086
1.709
—
—
—
Fig. 4. Cyclic voltammograms of different concentrations of p‐MBzOH in
the presence of electrocatalyst 3c (1 mmol/L).
p‐MBzOH could be electrochemically oxidized and the electrol‐
ysis efficiency was improved by 3c. To expand the applicability
of 3c to different substrates, the electrocatalytic performance of
the nitrocarbazole compound toward benzyl alcohol with dif‐
ferent para‐substituents was investigated, as shown in Fig. 5.
Even for the substrates with high oxidation potentials, 3c still
showed good catalytic oxidation properties.
In an electrolysis system with a volume of 10 mL and keep‐
ing the electrocatalyst concentration at 1 mmol/L, controlled
potential electrolysis was conducted using different concentra‐
tions of p‐MBzOH at the oxidation potential of the electrocata‐
lyst 3c (1.28 V) for 6 h. The yields of 4‐methoxybenzaldehyde
(p‐MBA) obtained with different starting concentrations of
p‐MBzOH are illustrated in Fig. 6. When the concentration of
p‐MBzOH was under 40 mmol/L, it was completely trans‐
formed to the target product p‐MBA after 6 h. The yield of
p‐MBA decreased when the concentration of p‐MBzOH was
above 80 mmol/L. This is attributed to the low stoichiometric
ratio of 3c to the substrate (≤ 1:80) at high substrate concen‐
tration. The controlled potential electrolysis equation of
p‐MBzOH is shown in Eq. 1 and a possible indirect elec‐
tro‐oxidation mechanism for the nitrocarbazole organic cata‐
0.25
(1)
(2)
0.20
0.15
0.10
0.05
0.00
(3)
0.0
0.5
1.0
1.5
2.0
E/V (vs Ag/AgNO3)
Fig. 5. Cyclic voltammograms of different substrates (5 mmol/L) in
presence of 1 mmol/L 3c. (1) p‐MBzOH; (2) p‐tolylmethanol; (3) phe‐
nylmethanol.
lyst is provided in Scheme 2.
3.3. Recycling of the organic electrocatalyst
The organic electrocatalyst 3c was recycled after its first use
in controlled potential electrolysis, and the electrochemical
properties obtained after recycling are depicted in Fig. 7. Eox of
0.10
(4)
0.08
(3)
0.06
100
80
60
40
20
0
0.04
0.02
(2)
(1)
0.00
-0.02
0.0
0.5
1.0
1.5
E/V (vs Ag/AgNO3)
0
40
80
120
160
200
Concentration of p-MBzOH (mmol/L)
Fig. 3. Cyclic voltammograms of p‐methoxybenzyl alcohol (p‐MBzOH)
with and without 3c. (1) blank; (2) 1 mmol/L of 3c; (3) 5 mmol/L of
p‐MBzOH; (4) 1 mmol/L 3c + 5 mmol/L p‐MBzOH.
Fig. 6. Yields of p‐MBA obtained by controlled potential electrolysis of
different concentrations of p‐MBzOH using 3c as a catalyst.

Yinghong Zhu et al. / Chinese Journal of Catalysis 37 (2016) 533–538
537
CH3O
CH2OH
2.5 mol % 3c (Ep = 1.28 V)
(1)
CH3O
CHO
0.2 m /L LiClO4, CH3CN/CH2Cl2
0.2olM
Ep = 1.48 V
(vs Ag/AgNO3)
Scheme 2. Proposed indirect electrolysis mechanism of p‐MBzOH.
the recovered 3c was 1.281 V, Ere was 1.203 V and ∆E was
about 78 mV, indicating that the recycled electrocatalyst 3c still
retains excellent electrochemical reversibility and could be
used again as the mediator in the indirect electro‐oxidation of
alcohols.
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538
Yinghong Zhu et al. / Chinese Journal of Catalysis 37 (2016) 533–538
Graphical Abstract
Chin. J. Catal., 2016, 37: 533–538 doi: 10.1016/S1872‐2067(15)61047‐6
Synthesis of nitrocarbazole compounds and their electrocatalytic oxidation
of alcohol
Yinghong Zhu, Jianqing Zhang, Ziying Chen, Anlun Zhang, Chunan Ma*
Zhejiang University of Technology
The redox potential and electrochemical reversibility were greatly improved by
introducing nitro‐group to the carbazole framework. The nitro‐carbazole com‐
pound showed high activity on the electro‐oxidation of alcohol to corresponding
aldehyde.
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