Integrating Hydrogen Production and Transfer Hydrogenation with Selenite Promoted Electrooxidation of α-Nitrotoluenes to E-Nitroethenes

Article abstract of DOI:10.1002/anie.202108666

Developing an electrochemical carbon-added reaction with accelerated kinetics to replace the low-value and sluggish oxygen evolution reaction (OER) is markedly significant to pure hydrogen production. Regulating the critical steps to precisely design electrode materials to selectively synthesize targeted compounds is highly desirable. Here, inspired by the surfaced adsorbed SeO_x^2? promoting OER, NiSe is demonstrated to be an efficient anode enabling α-nitrotoluene electrooxidation to E-nitroethene with up to 99 % E selectivity, 89 % Faradaic efficiency, and the reaction rate of 0.25 mmol cm^?2 h^?1 via inhibiting side reactions for energy-saving hydrogen generation. The high performance can be associated with its in situ formed NiOOH surface layer and absorbed SeO_x^2? via Se leaching-oxidation during electrooxidation, and the preferential adsorption of two -NO₂ groups of intermediate on NiOOH. A self-coupling of α-carbon radicals and subsequent elimination of a nitrite molecule pathway is proposed. Wide substrate scope, scale-up synthesis of E-nitroethene, and paired productions of E-nitroethene and hydrogen or N-protected aminoarenes over a bifunctional NiSe electrode highlight the promising potential. Gold also displays a similar promoting effect for α-nitrotoluene transformation like SeO_x^2?, rationalizing the strategy of designing materials to suppress side reactions.

Full text of DOI:10.1002/anie.202108666

A Journal of the Gesellschaft Deutscher Chemiker
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International Edition
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Accepted Article
Title: Integrating Hydrogen Production and Transfer Hydrogenation
with Selenite Promoted Electrooxidation of α-Nitrotoluenes to E-
Nitroethenes
Authors: Xiaodan Chong, Cuibo Liu, Changhong Wang, Rong Yang,
and Bin Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202108666
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10.1002/anie.202108666
Angewandte Chemie International Edition
RESEARCH ARTICLE
Integrating Hydrogen Production and Transfer Hydrogenation
with Selenite Promoted Electrooxidation of α-Nitrotoluenes to E-
Nitroethenes
Xiaodan Chong,+,[a] Cuibo Liu,+,[a] Changhong Wang,[a] Rong Yang,[a] and Bin Zhang*,[a],[b]
[a]
X. Chong, Dr. C. Liu, Dr. C. Wang, R. Yang, Prof. B. Zhang
Institute of Molecular Plus, Department of Chemistry, School of Science, Tianjin University
Tianjin 300072, China
E-mail: bzhang@tju.edu.cn
[b]
[+]
Frontiers Science Center for Synthetic Biology, (Ministry of Education), Collaborative Innovation Center of Chemical Science and Engineering, Tianjin
University, Tianjin 300072, China
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Developing an electrochemical carbon-added reaction
with accelerated kinetics to replace the low-value and sluggish
oxygen evolution reaction (OER) is markedly significant to pure
hydrogen production. Regulating the critical steps to precisely design
electrode materials to selectively synthesize targeted compounds is
Thus, exploring a diversified more readily oxidized reaction
replacing OER to produce carbon-added product with a facile
purification procedure, in particular using a bifunctional electrode,
is highly desirable.
The electrochemical transformation has provided a milder
and more efficient method to access the organic products under
additional reductant/oxidant-free conditions compared to
conventional thermocatalytic processes.[4] As for selective
electrooxidation in an aqueous solution, the key challenge is the
unruly control on competing OER and other side reactions.[5]
Additionally, Ni-based materials are prevailing anodes in
electrochemical OER and organic oxidation reactionsbecause of
their low cost, easy preparation, and high reaction efficiency.[6] In
situ formed hypervalent Niδ+ species are now identified as the
active phase driving the oxidation reactions.[6,7] But, the anode is
mainly screened through a trial-and-error approach. And, the
origin of the reaction activity and/or selectivity controlled by
electrodes has been rarely explored, restricting the development
of high-performance electrode materials. In this regard, unveiling
the underlying mechanism and regulating the critical step of an
organic oxidation reaction to initiate the rational design of Ni-
based electrodes is urgently needed.
2-
highly desirable. Here, inspired by the surfaced adsorbed SeOx
promoting OER, NiSe is demonstrated to be an efficient anode
enabling α-nitrotoluene electrooxidation to E-nitroethene with up to
99% E selectivity, 89% Faradic efficiency, and the reaction rate of
0.25 mmol cm-2 h-1 via inhibiting side reactions for energy-saving
hydrogen generation. The high performance can be associated with
its in situ formed NiOOH surface layer and absorbed SeOx2- via Se
leaching-oxidation during electrooxidation, and the preferential
adsorption of two –NO2 groups of intermediate on NiOOH. A self-
coupling of α-carbon radicals and subsequent elimination of a nitrite
molecule pathway is proposed. Wide substrate scope, scale-up
synthesis of E-nitroethene, and paired productions of E-nitroethene
and hydrogen or N-protected aminoarenes over a bifunctional NiSe
electrode highlight the promising potential. Gold also displays a
similar promoting effect for α-nitrotoluene transformation like SeOx2-
,
rationalizing the strategy of designing materials to suppress side
reactions.
The conjugated nitroalkenes with
E configuration are
important synthons in organic synthesis.[8] Nevertheless, the
dominant base-assisted Henry condensation reactions of
aldehydes or ketones with nitroalkanes, and organoselenide-
participated nitration of activated alkenes usually suffer from low
yield and/or poor selectivity of E-nitroethenes, using expensive
reagents, and harsh reaction conditions (e.g., -78 oC).[9]
Therefore, it is urgent to develop an electrochemical method to
selectively synthesize E-nitroethenes, especially via a carbon-
added upgrading manner from a single nitroalkane substrate,
over a cost-effective electrode.
Here, stimulated by the facts that tuning hydroxyl binding
energy enabled selective electrochemical oxidation of ethylene
to ethylene glycol[10] and surface-absorbed chalcogenate of Ni-
electrode boosted OER,[11] we speculate and validate NiSe
nanorod arrays (NAs) is a promising electrode to promote
aqueous electrooxidation of α-nitrotoluene to E-nitroethene with
high conversion yield and excellent selectivity. NiSe experiences
a surface-reconstruction to generate SeOx2- adsorbing on in situ
formed NiOOH surface to tune the OH bonding energy during
anodic oxidation processes (Figure 1). The E-nitroethene
product can easily precipitate from the electrolyte, simplifying the
purification process. Furthermore, integrating the cathodic
Introduction
Electrochemical oxygen evolution reaction (OER) is a significant
half-reaction in electrocatalytic hydrogen evolution reaction
(HER), CO2 reduction, aqueous organic electrosynthesis,
nitrogen fixation, and metal-air batteries. At present, the kinetic-
sluggish and low-value OER severely impedes its development
of green hydrogen.[1] Therefore, thermodynamically more
favorable eletrooxidation of organics has been recently
developed as a viable alternative to replacing OER for both
generating fine chemicals and boosting H2 generation with lower
energy input.[2] Despite impressive advances, current chemicals-
assisted water splitting still mainly relies on directly anodic
dehydrogenation or oxidation of amine, alcohol, and thioether.[3]
Electrochemical upgrading from a simple substrate to a carbon-
added complex molecule, especially with a defined configuration,
has not yet been touched. Furthermore, most of the reported
anodic reactions involve multi-electron oxidation steps, leading
to lower reaction rates for synthesizing oxidized products. And,
most of the anodic products are soluble in electrolytes, requiring
considerable manpower input for time-consuming separations.
1
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10.1002/anie.202108666
Angewandte Chemie International Edition
RESEARCH ARTICLE
hydrogen production or transfer hydrogenation of nitroarenes
with the anodic transformation of α-nitrotoluene to E-nitroethene
are accomplished at a low cell voltage in a NiSe||NiSe two-
electrode alkaline electrolyzer.
Indeed, the promoting effect of surface SeOx2- on 1a
electrooxidation is validated by using Ni(OH)2 as the anode in
1.0 M KOH with the addition of selenites (SeO32-). A remarkable
enhancement of current density (Figure 2d) and an obvious
increase in 2a conversion yield (Con.) confirm the promotion
effect of SeO32- (Figure 2e).
e-
e-
Surface reconstruction
Thirdly, aqueous electrooxidation can lead to in situ
formation of a layer of hypervalent Niδ+ oxyhydroxide on the
surface of Ni-based electrodes, and subsequent Niδ+/Ni2+ redox
couple drives the oxidation reactions.[6,7] Our recent study
reveals that NiSe2 surface can reconstruct to form a NiOOH
layer under anodic conditions, and in situ produce dissolved and
adsorbed SeOx2- on as-formed nickel oxyhdroxide layer to boost
OER. Additionally, direct growth of nickel selenide
electrocatalyst on conductive Ni foam to build a 3D self-
supported electrode can avoid extra polymer binders, enhance
their electrocatalytic performance, and promote the practical use
of the cheap electrode.
(H2O)
Soluble reactants
NiSe NAs
H*
NiSe
2-
SeOx
NiOOH
(H2)
Precipitated products
SeOx2-@NiOOH@NiSe
Surface SeOx2- and -NO2 adsorption promotion
99% E selectivity and 89% FE of E-nitroethene Easy isolation of E-nitroethene
Integrating H2 production or transfer hydrogenation with E-nitroethene synthesis
Thus, these above consideration factors suggest self-
supported nickel selenide as a promising anode for converting
1a to 2a in an alkaline medium with high efficiency and
selectivity to replace OER.
Figure 1. Integrating hydrogen production or one-pot transfer hydrogenation
/N-protection of nitroarenes with selenite promoted electrooxidation of α-
nitrotoluenes to E-nitroethenes over
a bifunctional NiSe nanorod arrays
electrode.
a
b
d
Hypothesized Mechanism for α-Nitrotoluene Electrooxidation to E-Nitroethene
Results and Discussion
Hypothesized mechanism and theoretical calculations
promote the electrode design. Aqueous selective
electrooxidation of α-nitrotoluene (1a) to E-nitroethene (2a) is
explored as a model to replace OER due to its easier oxidation
property (Scheme S1) and faster reaction kinetics of α-carbon
anion generating from deprotonation of α-nitrotoluene under an
alkaline condition than that of OH-.[8] Additionally, 1a is well
dissolved in alkaline solution, whereas 2a as a solid is easy to
precipitate for purification (Figure S1). Moreover, aqueous
anodic oxidation of α-nitrotoluene involves the generation and
adsorption of active oxygen species for OER and other by-
products (e.g., aldehyde and acid), and the co-production of
nitroethene with E- and Z-configurations. So, dictating the
reaction pathway towards one targeted compound to suppress
possible side reactions is proposed to design optimal catalytic
materials.
c
Eads = -0.90 eV
Eads = -0.59 eV
H
C
N
Ni
O
H
C
N
Ni
O
e
Ni(OH)2 with 0.4 mmol of 1a
2a
3a
Ni(OH)2 + SeO32- with 0.4 mmol of 1a
Firstly, we analyze the possible reaction mechanism of
aqueous 1a-to-2a electrooxidation (Figure 2a). We hypothesize
both α-carbon radical (II) and hydroxyl radical (IV), one well-
known intermediate for OER,[12] will be generated during the
anodic oxidation processes via losing an electron. A self-
coupling of II forms intermediate (III), and then eliminating a
molecule of nitrite (HNO2) leads to the formation of both Z- and
E-nitroethene due to their very similar Gibbs free energies
(Figure S2). We thus propose the preferential adsorption of two
–NO2 groups of III on electrode surface can dominate the E-
isomer product via subsequently eliminating a HNO2 molecule.
The preliminary density functional theory (DFT) calculations
support the favorable adsorption of III with two –NO2 groups on
NiOOH (Figures 2b,c).
2a
1a
3a
2-
E / V vs. Hg/HgO
Ni(OH)2
Ni(OH)2 + SeO3
Figure 2. (a) Hypothesized mechanism for α-nitrotoluene electrooxidation to
E-nitroethene. (b), (c) Calculated adsorption energies of III with one and two –
NO2 groups adsorption on (0112) facet of NiOOH. (d) LSV curves of a Ni(OH)2
anode at a scan rate of 5 mVꢀs−1 in 1.0ꢀM KOH in the presence of 0.4mmol of
1a with and without 0.1 M SeO32-. (e) Comparisons of 2a and 3a conversion
2-
yields over a Ni(OH)2 anode with and without 0.1 M SeO3 in 1.0ꢀM KOH at
0.55 V vs. Hg/HgO.
Secondly, the cross-coupling of II and IV with the elimination
of HNO2 will produce aldehyde and/or acid by-products (Figures
2a and S3). Thus, one promising method for suppressing by-
products is optimizing surface structure to alter the bonding
energy of IV and facilitate IV conversion to subsequent
intermediates or gaseous O2. Such a process can be promoted
by surface adsorbed SeOx2-, as reported by our previous work.[11]
Material characterizations and performances.
Self-supported NiSe nanorod arrays (NAs) are synthesized and
used as the anode (details see the Supporting Information). X-
ray diffraction (XRD) pattern (Figure 3a), scanning electron
microscopy (SEM, Figure S4a), and transmission electron
microscopy (TEM, Figure 2b) images, and X-ray photoelectron
2
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10.1002/anie.202108666
Angewandte Chemie International Edition
RESEARCH ARTICLE
spectroscopy (XPS) spectra (Figures S4b-d) suggest the
successful preparation of NiSe NAs.
electricity, water, and KOH for converting 1a to 2a (Table S2).
We have compared the performances of our NiSe electrode with
the other electrodes for 1a electrooxidation under other identical
reaction conditions (Figure S6). However, inferior efficiencies
are observed, demonstrating the high activity and selectivity of
our NiSe anode in this electrochemical transformation of 1a to
2a. Additionally, Figure 2f demonstrates the good performance
durability of NiSe at 0.55 V.
b
a
8 nm
Ni
NiSe
0.270 nm
NiSe (101)
2 nm
10 nm
JCPDS 75-0610
Electrochemical in situ spectra and mechanistic studies. A
series of quasi-in situ experiments and DFT calculations are
utilized to unveil the high-performance origin of 1a
electrooxidation to 2a in 1.0 M KOH over the NiSe anode. Firstly,
time-dependent Raman spectra demonstrate the gradual
weakening and disappearance of NiOOH[3d,11,13] peaks after
adding 1a (Figure 4a), suggesting the Ni3+/Ni2+ redox mediated
1a oxidation. Additionally, potential-dependent Raman spectra
further illustrates the presence of surface oxidized selenite
(SeO32-, ~808 cm-1) and selenate (SeO42-, ~835 cm-1) via Se
leaching-oxidation,[14] and in situ formed hypervalent Ni3+ acting
as the actual active phase for driving 1a electrooxidation
(Figures 4a,b). Furthermore, potential-dependent Raman tests
reveal the appearance of the C＝C stretching vibration peaks of
the aryl ring and ethenyl group (1596 cm-1 and 1647 cm-1)[15] and
the characteristic hydrogen motions of the ethenyl group at 1321
2θ / degree
E / V vs. RHE
E / V vs. RHE
c
d
Without1a
2a
3a
With 0.4 mmol of 1a
2a
1a
E / V vs. Hg/HgO
E / V vs. Hg/HgO
e
f
Con.
Sel.
1a
2a
a
b
NiOOH with 1a
474
NiSe with 1a
833
474
initial
5 s
10 s
556
833
807
0.65 V
557
835
Cycles
Time / min
0.60 V
0.60 V
0.55 V
0.55 V
0.50 V
Figure 3. (a) XRD pattern, (b) TEM and high-resolution TEM (HRTEM) images
of NiSe NAs. (c) LSV curves of NiSe anode at a scan rate of 5 mVꢀs−1 in 1.0ꢀM
KOH with and without 0.4mmol of 1a. (d) Potential- and (e) time-dependent
conversions of 1a to 2a in 1.0ꢀM KOH. (f) Cycle-dependent 1a con. and 2a sel.
at 0.55 V vs. Hg/HgO.
x10
0.50 V
0.45 V
0.45 V
808
0.40 V
0.30 V
0.40 V
0.30 V
807
Raman shift / cm-1
NiSe
Raman shift / cm-1
Raman shift / cm-1
d
The electrochemical measurements are performed in a 1.0ꢀM
KOH. All the potentials are referred to Hg/HgO except for
additional notes. The linear sweep voltammetry (LSV) curve
without 1a displays an obvious peak centered at 0.50 V
belonging to the oxidation of NiSe (Figure 3c).[3d,11] An
appreciable increase in current density from 0.50 V after adding
1a, which is ascribed to the oxidation of 1a. This result indicates
the lower energy barrier of 1a oxidation than that of OER.
Additionally, the cyclic voltammetry (CV) curve also reveals a
pair of redox peaks corresponding to the oxidation/reduction of
the NiSe on glass carbon electrode. When adding 1a into the
electrolyte, a big oxidation peak appears at about 0.63 V
corresponding to 1a oxidation (Figure S5a). However, the
oxidation peak of NiSe is not observed clearly, which may be
due to the peaks overlapping with the 1a oxidation (Figure S5b).
Then, we investigate the conversion of 2a at different potentials
at full consumption of 1a (Figure 3d). The highest con. of 2a can
be delivered at 0.45 V with the Faradic efficiency (FE) of 89%.
However, the current density is lower. When the potentials are
more positive than 0.6 V, the OER increases greatly and more
benzoic acid 3a by-product is generated. Whereas, at 0.55 V, 2a
can be obtained with a relatively large current density and high
con. of 94%. Time-dependent transformation at 0.55 V shows
the electrooxidation of 1a can be finished within 2 h and the
calculated reaction rate is 0.25 mmol cm-2 h-1. The consumption
rate of 1a and productivity of 2a at different potentials are listed
in Table S1. Control experiments confirm the necessity of
c
Without electricity
Without electricity
Reaction solution
-
-
With electricity
With electricity
Reaction solution with NO2 and NO3
-
-
NO2
NO3
Time / min
Magnetic field / G
e
f
Eads = -1.85 eV
(0001)
Eads = -1.60 eV
(01-12)
Ni
O
H
C
N
Figure 4. (a) Time-dependent Raman spectra of NiOOH with 1a and potential-
dependent Raman spectra of NiSe without 1a collected at 0.50 V in 1.0ꢀM
KOH. (b) Potential-dependent Raman spectra of NiSe collected in 1.0ꢀM KOH
with 1a. (c) EPR trapping for α-carbon radical. (d) –NO2 detection by IC after
1a electrooxidation. (e) Adsorption energies of α-carbon and –NO2 on NiOOH.
(f) Proposed mechanism.
3
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10.1002/anie.202108666
Angewandte Chemie International Edition
RESEARCH ARTICLE
cm-1 at above 0.4 V[15] and their intensities go up gradually with
the potential increasing (Figure S7), confirming the converting of
1a to 2a. The adsorption behaviors of 1a over in situ formed
NiOOH are also investigated. After optimizing the most stable
adsorption modes, the α-carbon radical of 1a has prioritized
adsorption of carbon atom on the O site of NiOOH with the
adsorption energy (Eads) of -1.85 eV (Figure 4e), much lower
than the Eads of –NO2 group via two oxygen atoms adsorbing on
Ni sites. The preferential adsorption of α-carbon is favorable to
their subsequent self-coupling to carbon-added intermediate
over the NiOOH surface.
on the reaction efficiency to produce 2i with 85% selectivity. And
the –Cl and –Br groups in the products provide good
opportunities for fabricating more complex molecules (2d-e, 2h,
and 2j). Impressively, one-gram scale synthesis of 2a with 92%
con. and 87% sel. is easily achieved (Figure S9), showing the
potential practicability. All substrates used in Table 1 are
dissolvable in the electrolyte, and the produced E-nitroethenes
can readily precipitate from the reaction solution, reducing the
complexity for purifications. Not that, although the release of
-
NO2 during the electrooxidation of 1a may be not sustainable,
the recent developed electrochemistry provides a reliable way to
-
-
convert nitrate (NO3 ) or nitrite (NO2 ) into useful and safe
Based on the experimental and theoretical results above, a
possible reaction mechanism for selective 1a electrooxidation to
2a over in situ formed NiOOH with surface SeOx2- promotion is
proposed in Figure 4f. First, α-carbon anion I that generated via
deprotonation of 1a in 1.0 M KOH and OH- adsorbed on NiOOH
surface. Then, each anion loses an electron to produce α-carbon
radical II and •OH IV. The formation of II is confirmed by the
electron paramagnetic resonance (EPR) measurements (Figure
4c).[16] Because of the accelerated consumption of •OH by
surface SeOx2- for OER,[12] benzaldehyde and benzoic acid by-
products originated from the potential intermediate V (Figure 2a)
that is formed via cross-coupling of II and IV are intrinsically
inhibited. Thus, a self-coupling of radical II effectively occur to
form intermediate III, which preferentially adsorbs on NiOOH
surface via two –NO2 groups (Figure 2c), and then eliminates a
HNO2 molecule, as confirmed by the ion chromatography (IC)
(Figure 4d), giving rise to 2a. Note that, we cannot detect
intermediate III during the reaction course, which may be due to
its fast conversion to 2a. However, the self-coupled product of α-
carbon radical that generates from 2-phenylacetonitrile under
our electrooxidation conditions is trapped (Scheme 1a),[17]
rationalizing our proposed radical coupling process of II for E-
nitroethene 2a synthesis. Furthermore, subjecting benzaldehyde
and 1a to the standard reaction conditions without electricity
gives only a small amount of 2a even prolonging the reaction
time to 24 h, ruling out 2a coming from the condensation of
benzaldehyde with 1a (Scheme 1b). Finally, the release of 2a
regenerates NiOOH for the next reaction cycle.
ammonia (NH4 ).[18] And, the leakage of Se into the electrolyte
+
via NiSe reconstruction will be treated according to reported
methods to ensure the Se level to meet the emission standards
before disposing.[19] Our previous work also demonstrated that
gold (Au) modification could improve the OER performance via
accelerating the intermediate conversions.[20] Thus, we also test
the possibility of Au in promoting the electrooxidation of 1a to 2a.
Compared with pure Ni(OH)2, both of the current density and 2a
con. are obviously improved when using Au decorated Ni(OH)2
as the anode (Figure S10), suggesting the positive effect of Au
in promoting the transfer of 1a to 2a.
Table 1. Substrate scope of electrooxidation of α-nitrotoluenes to E-
nitroethenes over a NiSe anode.a
aReaction conditions: in the anodic cell containing 1 (0.4ꢀmmol), 1.0 M KOH
(7.0 mL), and NiSe anode, 0.55 V, RT. b0.6 V. Nearly no Z isomers for all
Scheme 1. Control experiments.
cases are observed. Con. refer to the conversion yield of
1 and are
determined by GC. Sel. is the selectivity of 2. Every data are repeated three
a
times and the lowest value for each product is provided.
GC-MS detection for2a’
﹢
Integrating H2 production with selective electrooxidation of
α-nitrotoluene. NiSe is also
a promising candidate for
2a’ MS (EI)
m/z 232.1
17.883
Formula of 2a’: C15H11
N
electrochemical hydrogen evolution reaction (HER) from water
splitting due to its highly efficient and stable properties.[21]
Therefore, we assemble an electrolyzer in a two-electrode
configuration using NiSe NAs as both anode and cathode to
achieve simultaneous H2 and E-nitroethene production. NiSe
NAs couple needs a cell voltage of 1.36 V to afford 10 mA cm-2,
332 mV lower than that of conventional overall water splitting
(Figure 5a). To quantify the products at both sides, paired
electrolysis at 1.8 V is conducted to produce H2 with nearly 100%
FE and 2a with 88% selectivity (Figure 5b).
The theoretical value: 232.2
b
﹢
17.818.0
200
220 240
Time/min
m/z
The methodology universality. The general applicability of this
electrooxidation of α-nitrotoluenes to E-nitroethenes is examined
(reaction setup see Figure S8). A series of α-nitrotoluenes
bearing either electron-donating or electron-withdrawing groups
on aryl ring can be transformed into the corresponding E-
nitroalkenes with high conversion yields and good to high
selectivity under our standard reaction conditions (Table 1, 2a-j).
Methyl-substituent at the ortho-position has nearly no influence
Integrating
transfer
hydrogenation
with
selective
electrooxidation of α-nitrotoluene. Developing efficient and
low-cost H2 storage and transport systems is more important for
the future H2 economy. Electrocatalytic transfer hydrogenation
that directly uses the active atomic hydrogen (H*) from water
4
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10.1002/anie.202108666
Angewandte Chemie International Edition
RESEARCH ARTICLE
Table 2. One-pot electrochemical transfer hydrogenation of nitroarenes with
tandem amino group protections to synthesize N-protected aminoarenes over
a NiSe NAs cathode.a
benchmark current density of 20 mA cm−2 after adding 0.4 mmol
of 1a and 0.5 mmol of 4a into the anodic and cathodic cell
(Figure 5c), respectively. This paired strategy can lead to the
simultaneous synthesis of 2a (87% sel.) and 5a (91% sel.) at 2.0
V vs. counter electrode within 4.0 h, which are difficult to be
achieved by the thermal and photochemical methods (Figure 5d).
Conclusion
In summary, we demonstrate an electrooxidation of α-
nitrotoluene to carbon-added E-nitroethene with easy filtration
purification to replace OER for boosting the HER with lower
energy input. The in situ formed hypervalent Niδ+ species,
aReaction conditions: Nitro substrates (0.5 mmol), (BOC)2O (2.0 mmol), NiSe
(working area: 1.0 cm2), 1.0 M KOH (25 mL), RT, -1.1 V vs. Hg/HgO. bBnCl
(2.0 mmol). c-0.7 V vs. Hg/HgO. Every data are repeated three times and the
lowest con. for each product is provided.
surface adsorbed SeOx2-
, and preferential adsorption of
intermediates on NiOOH are crucial factors to endow NiSe NAs
as an efficient anode for α-nitrotoluene electrooxidation to E-
nitroethene with up to 99% selectivity, 89% Faradic efficiency,
and the reaction rate of 0.25 mmol cm-2 h-1 in 1.0 M KOH. This
hybrid water electrolysis can produce H2 with nearly 100% FE
and saving about 100 mV cell voltage for achieving a 10 mA cm-
electrolysis has recently been developed as an appealing
strategy for green and efficient synthesis of hydrogenated
products for no requirement of flammable H2 gas and other hard-
to-handle organic donors.[22] Additionally, Ni-based materials are
widely applied in the hydrogenation of CO2 and some organics,
such as –NO2, –C≡C, –C=C, etc. However, the use of NiSe for
the transfer hydrogenation of organic compounds by using water
as a hydrogen source has not been reported. NiSe is also
demonstrated to be an effective cathode for electrochemical
transfer hydrogenation of nitroarenes with subsequent one-pot
amino group protections by di-tert-butyl pyrocarbonate ((BOC)2O)
or benzyl chloride (BnCl) to synthesize N-protected aminoarenes
in 1.0 M KOH aqueous solution. This tandem reaction strategy
delivers a variety of t-Butyl oxy carbonyl (Boc) and benzyl (Bn)
protected aryl amines with good yields (Table 2), demonstrating
good sustainability and step-economy for the synthesis of N-
protected aminoarenes. Moreover, a NiSe||NiSe two-electrode
electrolyzer is also assembled for both 1a oxidation and 4a
hydrogenation/protection. Compared with the overall water
splitting reaction, nearly 539 mV voltage is saved to achieve a
2
benchmark current density in a NiSe||NiSe two-electrode
electrolyzer. In situ and ex situ experiments unveil the surface
formed actual catalytic NiOOH layer and adsorbed SeOx2- in
promoting E-nitroethenes formation via suppressing Z-isomer
and other by-products. DFT calculations support the
preponderant adsorption of two –NO2 groups of intermediate
formed via self-coupling of two α-carbon radicals, leading to a
high
E
configuration selectivity via releasing
a
HNO2.
Additionally, NiSe is also an efficient cathode to produce various
N-protected aryl amines via one-pot nitro-reduction/animo-
protection sequences. Wide substrate scope, scale-up synthesis,
and paired synthesis of E-nitroalkene and N-protected
aminoarene at low cell voltage demonstrate great promise.
Moreover, Au also has the same role as SeOx2- in facilitating the
electrooxidation of α-nitrotoluenes to E-nitroethenes via
suppressing the side reactions. Our work not only provides a
new alternative for future energy-saving H2 production, but also
offers a paradigm for synthesizing low-cost nanostructured
electrodes with the aid of surface-adsorbate to improve the
efficiency and selectivity of organic transformations in aqueous
media.
a
b
FE
Con.
Sel.
H2O
H2
2a
1a
332 mV
Without 1a
With 1a
Acknowledgements
E / V
H2
1a
2a
Sel.
The authors are grateful to the National Natural Science
Foundation of China (Nos. 21871206 and 22001192).
c
d
Con.
Without1a and 4a
With 1a and 4a
Conflict of interest
539 mV
The authors declare no conflict of interest.
Keywords: electrocatalysis
•
self-reconstruction
•
E-
E / V
1a 2a
4a 5a
nitroethenes • enhanced selectivity • selenite promotion
Figure 5. (a) LSV curves and graphical illustrations for a NiSe catalyst couple
in 1.0 M KOH with and without 0.4 mmol of 1a. Scan rate: 5 mV s-1. (b) FE of
H2, and 1a con. and 2a sel. (c)LSV curves and graphical illustration for a NiSe
catalyst couple in 1.0 M KOH with and without 0.4 mmol of 1a and 0.5 mmol of
4a. Scan rate: 5 mV s-1. (d) 1a and 4a con., and 2a and 5a sel.
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RESEARCH ARTICLE
Entry for the Table of Contents
e-
e-
Surface reconstruction
NiSe NAs
(H2O)
Soluble reactants
H*
NiSe
2-
SeOx
NiOOH
(H2)
Precipitated products
SeOx2-@NiOOH@NiSe
Surface SeOx2- and -NO2 adsorption promotion
99% E selectivity and 89% FE of E-nitroethene Easy isolation of E-nitroethene
Integrating H2 production or transfer hydrogenation with E-nitroethene synthesis
Selective electrooxidation of α-nitrotoluenes to carbon-added E-nitroethenes with wide substrate scope, high selectivity, and Faradic
efficiency is integrated with boosting H2 production or transfer hydrogenation over a bifunctional NiSe electrode. In situ formed
hypervalent Niδ+, surface adsorbed SeOx2-, and –NO2 adsorption of intermediate promote aqueous electrooxidation of α-nitrotoluene
to E-nitroethene.
7
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