Cu2O Pseudomorphic Conversion to Cu Crystals for Diverse

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Cu2O Pseudomorphic Conversion to Cu Crystals for Diverse Nitroarene Reduction Mahesh Madasu, Chi-Fu Hsia, Sourav Rej, and Michael H. Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02537 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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ACS Sustainable Chemistry & Engineering

Cu2O Pseudomorphic Conversion to Cu Crystals for Diverse Nitroarene Reduction Mahesh Madasu, Chi-Fu Hsia, Sourav Rej, and Michael H. Huang* Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, 101, Section 2, Kuang Fu Road, Hsinchu 30013, Taiwan E-mail: [email protected]

ABSTRACT:

Cu2O cubes, octahedra, and rhombic dodecahedra can be

pseudomorphically converted to Cu crystals of the corresponding morphologies through the addition of ammonia borane.

Nitroarene can be completely reduced

during the compositional transformation with four equivalents of ammonia borane at 30 ºC in 25 min.

All the obtained polyhedral Cu crystals can give 100% nitroaniline

conversion to p-phenylenediamine exclusively, but commercial Cu2O powder shows a comparatively lower 4-bromonitrobenzene conversion and yields a mixture of products.

Use of sodium borohydride as a reducing agent resulted in the formation

of deformed Cu particles and a low nitroaniline conversion percentage.

Cu2O cubes

cannot be converted to Cu particles with the addition of hydrazine, and nitroaniline conversion did not occur.

Nitro group reduction is successful with high yields for

diverse nitroarene molecules giving only a single product starting from a solution of 1

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the nitroarene compound, Cu2O cubes and ammonia borane. KEYWORDS: copper nanocrystals, cuprous oxide, facet effects, nitroarene reduction, pseudomorphic conversion

INTRODUCTION Formation of copper particles exposing different crystal planes should be highly desirable for the examination of facet effects on catalytic activity.

This is because

copper is a cheap material and it also has surface plasmon resonance (SPR) properties. However, direct synthesis of Cu nanocrystals with cubic, octahedral, and rhombic dodecahedral structures is still not possible using the same reaction conditions in both organic and aqueous solutions.1‒8

In contrast, cuprous oxide crystals with these

particle shapes can be easily synthesized in aqueous solution using practically the same reaction conditions.9,10 Cu2O cubes, octahedra, and rhombic dodecahedra can undergo a heat treatment under CO/N2 stream for 2 h to obtain Cu crystals of the corresponding shapes, but the Cu surfaces are rough and contain many pits.11 Recently we have reported pseudomorphic transformation of polyhedral Cu2O crystals to Cu crystals maintaining the same particle morphologies through addition of ammonia borane in ethanol.12

Ethanolysis of ammonia borane generates hydrogen

gas, which is directly used as the hydrogen source for alkyne semihydrogenation 2


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reaction on the Cu crystal surface.

Although Cu2O crystals after treating with other

reducing agents including hydrazine (N2H4) and sodium borohydride (NaBH4) showed inferior alkyne semihydrogenation performance, the solution color and the composition of the resulting particles was not analyzed to understand why ammonia borane was the best reducing agent to achieve pseudomorphic conversion. Since hydrogen atoms formed from the decomposition of ammonia borane should be present on the resulting Cu surface to catalyze the alkyne semihydrogenation reaction, naturally one can conceive using these hydrogen atoms for nitroarene reduction reaction.

Catalytic reduction of nitroarene compounds

remains attractive because substituted anilines are important intermediates for pharmaceuticals, dyes, and agricultural chemicals, so there is a persistent interest in using various metal catalysts for nitroarene reduction.13–15

Various metal catalysts

including ZrO2-supported Au particles,13 light-activated Au–Cu alloy nanoparticles@ZrO2,14 Ag nanoparticles/TiO2,15 FeBr2/FeCl2-based hydrosilylation,16 and FeOx-supported Pt single atoms have been used to catalyze nitroarene reduction with improved product selectivity.17 reduction.18

Cu colloids were also used for 4-nitrophenol

We have also employed polyhedral Au and Au–Ag core–shell

nanocrystals for nitroarene reduction in the presence of NaBH4 acting as a reducing agent.19,20

In addition, ultrasmall CoPd alloy nanoparticles on reduced graphene 3



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oxide,21 NiPd/CuNi alloy nanoparticles deposited on a graphene surface,22,23 Pd@MIL-101,24 Au nanoparticles on TiO2,25 and polyhedral Au–Cu and Au–Pd core–shell nanocrystals were also used as catalysts for hydrogenation of nitro and nitrile compounds.6,26 Carbonyl group reduction is also possible using the CoPd particles in the presence of ammonia borane.21 However, it is clear from the above examples that careful catalyst design involving the use of ultrasmall metal nanoparticles is usually necessary.

It is important to emphasize that exquisite

catalyst design is needed to avoid aromatic azo and hydroxylamine intermeidate formation, unless such intermediate species are the focus of the study.27,28

In

contrast, the possibility of using Cu crystals for clean nitroarene reduction starting from polyhedral Cu2O crystals has not been demonstrated before. In this study, we have used ammonia borane for pseudomorphic conversion of Cu2O cubes, octahedra, and rhombic dodecahedra to Cu crystals of the corresponding morphologies and simultaneous nitroarene reduction in ethanol at 30 ºC. The superiority of NH3BH3 over NaBH4 and N2H4 as reducing agents has been determined. Remarkably, all Cu2O particle shapes showed 100% nitroarene reduction conversion and 100% product selectivity.

The Cu crystals can catalyze a wide variety of

nitroarene compounds, including those bearing halogen substituents, showing they are highly active, selective, and green catalysts. 4


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RESULTS AND DISCUSSION The Cu2O cubes, octahedra, and rhombic dodecahedra were synthesized following our reported procedures (see the Supporting Information for the synthetic conditions).9,10

Basically, an aqueous mixture of SDS surfactant, CuCl2, NaOH, and

NH2OH·HCl reductant was prepared at 31 ºC and left undisturbed for 1–2 h to collect the crystals.

Figure 1a–c shows scanning electron microscopy (SEM) images of the

synthesized Cu2O cubes, octahedra, and rhombic dodecahedra with sharp faces and good size uniformity. particles.

Figure S1 provides size distribution histograms of these

The average cube edge length is 192 nm.

For octahedra and rhombic

dodecahedra, their average opposite corner and face distances are ~ 410 and 250 nm, respectively.

The Cu2O crystals (4 mg) were added to a solution containing

nitroaniline (0.1 mmol), ammonia borane (0.4 mmol) and 3 mL of ethanol and stirred at 30 oC for 25 min. crystals.

After 3 min, these Cu2O crystals have been converted to Cu

To obtain Cu crystals without concurrent nitroarene reduction, it is not

necessary to add nitroaniline.

The generated copper crystals were isolated from the

reaction mixture for further characterization.

Figure 1d–l presents SEM and

transmission electron microscopy (TEM) images of the resulting Cu crystals after 6 min into the nitroaniline reduction reaction.

The Cu particles maintain the original

5



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Cu2O shapes and sizes, confirming the success of this simple pseudomorphic transformation process.

Small particles on some crystals are observable due to the

presence of organic residue, which can be avoided by not adding nitroaniline. Similar to the previous observations, the obtained selected-area electron diffraction (SAED) patterns indicate formation of nearly single-crystalline Cu cubes and octahedra, whereas Cu rhombic dodecahedra have a more polycrystalline interior structure (Figure 1m–o).12

The diffraction pattern matches with that of metallic

copper. X-ray diffraction (XRD) patterns of all Cu2O and Cu samples were taken to determine their compositions (Figure 2).

All Cu2O cubes, octahedra, and rhombic

dodecahedra gave the expected XRD patterns of Cu2O.

After pseudomorphic

conversion to Cu crystals, XRD pattern of Cu was obtained for all samples, confirming complete compositional transformation was achieved. compositional change is visually detectable.

This

The initially yellow, orange, and light

brown colors of Cu2O cube, rhombic dodecahedron, and octahedron solutions have turned into a dark brown color (Figure 2).

Please note the Cu crystal solution does

not look quite reddish brown because yellowish nitroaniline is also present in the solution.12

The discernable Cu2O solution color variation emerges from the

existence of facet-dependent optical properties of semiconductor nanocrystals, 6


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showing band gap is tunable with particle size and the exposed crystal faces.10,29 Figure 3 shows the solution color changes at various time points during the pseudomorphic transformation and nitroaniline reduction processes.

The initially

light brown Cu2O cube solution has become dark brown within 10 sec after the introduction of ammonia borane and nitroaniline, showing the extremely rapid rate of the pseudomorphic conversion reaction.

After 3 min, the solution has turned

transparent as Cu crystals precipitated out from the solution. the solution color.

Nitroaniline takes over

The solution turned nearly colorless after 25 min due to the

complete conversion of nitroaniline to benzene 1,4-diamine (see data below). Since ammonia borane is used as a reducing agent for Cu2O conversion to Cu and a source of hydrogen atoms for nitroaniline reduction reaction, one can expect the amount of ammonia borane needed for complete nitroaniline reduction should be higher than the stoichiometric amount.

Table S1 and Figure S2 reveal that 4

equivalents of ammonia borane relative to the amount of nitroaniline are required for 100% nitroaniline conversion on the basis of 1H-NMR spectroscopic data.

Here 0.1

mmol of nitroarene, 0.4 mmol of NH3BH3, and 4 mg of Cu2O nanocube catalyst were mixed at 30 ºC for 25 min to complete the nitroaniline reduction reaction.

Next we

used the synthesized Cu2O octahedra and rhombic dodecahedra to carry out the same nitroaniline reduction reaction.

Remarkably, Table 1 indicates that all particle shapes 7



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give 100% nitroaniline conversion to benzene-1,4-diamine (see Figure S3 for 1

H-NMR spectra using Cu2O octahedra and rhombic dodecahedra as the catalysts).

The results show these Cu2O crystals are exceptional catalysts for this reaction without generation of intermediate compounds. The Cu2O crystals maintain their particle shapes after 25 min of the reaction (Figure S4).

XRD pattern taken on the

used Cu2O cubes shows preservation of the particle composition as copper (Figure S5). To confirm the catalytic reaction is heterogeneous in nature, leaching experiments have been carried out.

Initially, Cu2O cubes (4 mg) and 0.4 mmol of

ammonia borane in 3 mL of ethanol were stirred for 25 min. the nanocrystals were separated from the ethanol solution.

After centrifugation, To this ethanol solution

was added 0.1 mmol of 4-aminonitrobenzene and 0.4 mmol of ammonia borane and stirred at 30 oC for 25 min.

After completion of the reaction, the solvent was

removed and the crude 1H-NMR spectrum was taken. observed.

No product peaks were

In another experiment, Cu2O nanocubes (4 mg) and 3 mL of ethanol were

first stirred at 30 oC for 25 min.

Again we separated the Cu2O nanocrystals from the

ethanol, and 0.1 mmol of 4-aminonitrobenzene and 0.4 mmol of ammonia borane were added.

After isolation of product, the crude 1H-NMR spectrum gave only the

starting material.

These experiments indicate that leaching of copper ions did not 8


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happen, and the catalytic reaction occurs on the Cu crystal surface.

In our previous

study using ammonia borane to form polyhedral Cu crystals, the collected upper solution after particle centrifugation showed a copper concentration of only 0.039 mg/L using the inductively coupled plasma mass spectroscopy (ICP-MS) analysis, a level too low to have any catalytic activity.12 When Cu2O cubes were used for 4-bromonitrobenzene reduction, again 100% 4-bromoaniline was obtained.

This is not the case for commercial Cu2O powder.

Under the same reaction conditions, commercial Cu2O powder gave a conversion percentage of just 83.8%, and the products contain 60.3% of 4-bromoaniline and 39.7% of N-(4-bromophenyl)hydroxylamine.

After the reaction, 1H-NMR spectrum

shows a mixture of 4-bromonitrobenzene and the two products (Figure S6).

The

significant byproduct formation illustrates the importance of using polyhedral Cu crystals for this reaction. A standard mechanistic pathway has been drawn to demonstrate the catalytic conversion of nitrobenzene to aniline over the Cu (110) crystal plane (Figure S7).30 Firstly, the dissociation of B–H bonds from ammonia borane on the Cu surface leads to the formation of Cu–H species.

These species catalyze the reduction of

nitrobenzene into the corresponding N-hydroxylaniline via consecutive reaction intermediates.

N-hydroxylaniline then gets reduced to aniline on the Cu surface 9



under the standard reaction condition.

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Apparently the polyhedral Cu surfaces

facilitate adsorption of N-hydroxylaniline for further reduction to aniline.

We have

also performed recycling experiments using the Cu cubes for two additional cycles of 4-nitroaniline reduction reaction.

1

H-NMR spectra shows 100% conversion to

benzene-1,4-diamine for the first recycling experiment, and 88% conversion for the second recycling run giving benzene-1,4-diamine as the only product (Figure S8). SEM images show good preservation of the Cu cubes after the first recycling run, but some organic compounds appear to coat the particle surfaces after the second recycling experiment (Figure S9).

The results suggest these Cu crystals are

recyclable catalysts, but a reduced activity can happen after the second cycle. In addition to using ammonia borane to pseudomorphically convert Cu2O crystals to Cu particles and catalyze nitroarene reduction, we also evaluated using hydrazine (N2H4) and NaBH4 as reducing agents to form polyhedral Cu crystals. Previously we have shown these reagents gave low diphenylacetylene (DPA) conversions in the presence of Cu2O crystals possibly due to structural deterioration, but no further compositional analysis was performed.12

Figure S10 presents

photographs of the Cu2O solutions after nitroaniline reduction using different reducing agents and after centrifugation to collect the precipitate.

While the solution

has turned brown with the addition of NaBH4, no color change was observed when 10


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hydrazine was introduced, and the solution remained orange or light brown.

After

centrifugation, black precipitate was obtained in samples using ammonia borane and NaBH4 as reducing agents, but orange precipitate was collected for the hydrazine sample.

It is also interesting to note that a yellow solution appears after

centrifugation in samples containing NaBH4 and hydrazine, yet the ammonia borane sample shows no trace of yellowish hue. observations.

Further analysis can understand these

Figure 4 gives SEM images of the resultant catalyst after nitroaniline

reduction using NaBH4, N2H4, and NH3BH3 as the reducing agents.

The original

Cu2O cubes have been destroyed beyond recognition after adding NaBH4, while the cubic particle shape is preserved in the presence of N2H4.

XRD patterns of these

particles reveal conversion of Cu2O to Cu after the introduction of ammonia borane and sodium borohydride, but no reduction occurs upon addition of hydrazine (Figure 5).

Clearly only in the presence of ammonia borane can Cu2O be pseudomorphically

converted to Cu.

Under the standard conditions, there is no nitroaniline conversion

in a solution containing Cu2O cubes and hydrazine, because hydrazine cannot reduce Cu2O to Cu (see Table S2 and Figure S11 for 1H-NMR spectrum).

This result also

implies that hydrazine is ineffective at nitro group reduction, since Cu2O crystals can catalyze this reaction in the presence of NaBH4.31,32

The resulting Cu particles after

treating Cu2O cubes with NaBH4 gave a nitroaniline conversion yield of just 46.8% 11



(Table S2).

Page 12 of 27

And NMR spectrum shows a significant amount of unreacted

4-nitroaniline after the reaction (Figure S12).

Because of the unreacted

4-nitroaniline, a yellow solution appears after separating the particles. Clearly only the use of ammonia borane can give 100% 4-nitroaniline conversion through pseudomorphic transformation to polyhedral Cu crystals. To show the versatility of these Cu crystals, the nitro group reduction reaction has been performed using diverse nitroarene molecules bearing various substituent groups.

Table 2 lists the reagents, the products, and the product yields.

In all the

13 cases, full conversions were achieved as confirmed by 1H- and 13C-NMR data (Figure S14–S25).

Mass spectroscopic data are also available for

benzene-1,4-diamine (Figure S14). for halogen substituents.

The catalytic process shows excellent tolerance

Product yields are generally high, particularly for

nitroaniline and 4-nitrobenzaldehyde at 99%.

However, 4-nitrobenzaldehyde is

converted to (4-aminophenyl)methanol (entry 10), showing an aldehyde substituent also gets reduced.

Interestingly, the ester group is intact, and 4-nitrophenyl acetate is

converted to 4-aminophenyl acetate (entry 13).

It is clear that one can start with

easily synthesized Cu2O cubes to catalyze a broad range of nitroarene reduction reaction using ammonia borane as the reducing agent.

12


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CONCLUSIONS Cu2O cubes, octahedra, and rhombic dodecahedra were pseudomorphically converted to Cu crystals of the corresponding shapes in the presence of ammonia borane at 30 ºC in about 3 min.

The added nitroaniline during this compositional

transformation was directly reduced to benzene-1,4-diamine with 100% conversion and 99% yield in 25 min.

All Cu2O shapes can perform this reaction with complete

nitroaniline conversion, but commercial Cu2O powder shows a relatively low 4-bromonitrobenzene conversion percentage and produces a mixture of products. Using hydrazine as a reducing agent cannot cause pseudomorphic conversion and nitroarene reduction.

Adding sodium borohydride to Cu2O cubes leads to deformed

Cu particles with a low nitroarene conversion yield.

The converted Cu cubes can

catalyze a broad scope of nitroarene substrates giving generally excellent product yields.

Direct pseudomorphic conversion of conveniently prepared polyhedral Cu2O

particles to Cu crystals is a very attractive approach to yield highly active catalysts for direct nitroarene reduction.

EXPERIMENTAL SECTION Chemicals.

Anhydrous copper (II) chloride (CuCl2; 97%) and hydroxylamine

hydrochloride (NH2OH·HCl; 99%) were purchased from Aldrich. 13


Sodium


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hydroxide (98.2%) and sodium dodecyl sulfate (SDS; 100%) were acquired from Mallinckrodt.

Diphenylacetylene (DPA; 98%), ammonia borane (H3NBH3; 97%),

anhydrous ethanol (99.5%), and Cu2O (99.99%) were purchased from Aldrich. chemicals were used as received without further purification.

All

Ultrapure distilled and

deionized water (18.3 MΩ) was used for all solution preparations.

Commercially

available reagents were used for the nitroarene reduction reactions. Nitroarene Reduction.

Nitroarene compound (0.1 mmol), ammonia borane

(0.4 mmol, or 0.0123 g), and Cu2O nanocrystals (4 mg) were added to 3 mL of ethanol solution and stirred at 30 ºC for 25 min.

After completion of the reaction,

the reaction mixture was centrifuged at 6000 rpm for 3 min.

After centrifugation,

the solution was carefully removed from the copper nanocrystals. was removed from the product using a rotavapor.

Organic solvent

After high vacuum, 1H- and

13

C-NMR spectra were collected. Characterization.

SEM images of the samples were obtained using a JEOL

JSM-7000F electron microscope.

TEM characterization was performed on a JEOL

JEM-2100 microscope with an operating voltage of 200 kV.

Powder XRD patterns

were recorded on a Shimadzu XRD-6000 diffractometer with Cu Kα radiation. 1

H-NMR (600 MHz) and 13C-NMR (150 MHz) spectra were recorded with a Bruker

DMX spectrometer. 14


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Cu2O synthesis procedures, particle size histograms, XRD pattern, SEM images, NMR spectra, TEM images, and the reaction mechanism scheme.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Michael H. Huang: 0000-0002-5648-4345 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by Ministry of Science and Technology in Taiwan (MOST 104-2119-M-007-013-MY3 and 105-2633-M-007-003). 15



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21



1 µm

1 µm

bb

1 µm

Figure 1.

1 µm

1 µm

200 nm

ii

f ff

1 µm

µm 11 µm

) (

(

200nm n 200 nm 200

200 nm

1 µm

ne

(111)

(200)

50 nm

[011]

o

lll

l

)

[001]

kk d

1 µm

1 µm

mb

200 nm 50 nm

h h h

1 µm 1 µm

µm 11 µm

jj a

g

ee

cc c

1 µm

g

dd

aa

Page 22 of 27

o

(311)

(200)

(220)

(111)

200 nm

(a‒f) SEM images of Cu2O cubes, octahedra, and rhombic dodecahedra

(a‒c) before and (d‒f) after 6 min into the nitroaniline reduction reaction.

(g‒i)

Large-area and (j‒l) high-magnification TEM images of the resulting Cu crystals taken after 6 min of the reduction reaction. these Cu crystals.

(m‒o) Corresponding SAED patterns of

Viewing zone axes are also indicated.

22


Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2.

(a) XRD patterns of Cu2O cubes, rhombic dodecahedra, and octahedra

and the corresponding Cu crystals formed after pseudomorphic transformation (6 min of reaction).

(b) Solutions of the Cu2O cubes, rhombic dodecahedra, and octahedra.

(c) Solutions of the resulting Cu cubes, rhombic dodecahedra, and octahedra.

Figure 3.

Changes in the solution color from the original Cu2O cubes to various

time points during the nitroaniline reduction process.

Cu cubes have been formed

after 3 min into the reaction. 23



Table 1.

Page 24 of 27

Nitroaniline reduction conditions and the conversion yields.

Entry

Cu2O nanocrystals

Temp (oC)

EtOH

1

(4 mg) cubes

/Time (min) 30 (25 min)

(mL) 3

yield (%) 100

2

octahedra

30 (25 min)

3

100

3

rhombic dodecahedra

30 (25 min)

3

100

Figure 4.

1

H-NMR conversion

SEM images of the resultant catalyst after nitroaniline reduction using (a)

NaBH4, (b) N2H4, and (c) NH3BH3 as the reducing agents. for the reaction.

All scale bar equal to 1 µm.

24


Cu2O cubes were used

Page 25 of 27

NH3BH3

NaBH4

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N2H4

Cu2O cubes

(111)

(111) (200)

(200)

Data base (220)

(311)

(220)

(110)

(311)

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

2θ (degrees) Figure 5.

XRD patterns of the products after treating Cu2O cubes with different

reducing agents.

Standard XRD patterns of Cu2O (red) and Cu (black) are also

provided.

25



Table 2.

Page 26 of 27

Nitroarene reduction reactions performed using diverse nitroarene

molecules and their conversion yields.

Reaction conditions: 0.1 mmol of nitroarene,

0.4 mmol of NH3BH3, and 4 mg of Cu2O cube catalyst at 30 ºC for 25 min. entry

reagent

product

yield (%)

1

99

2

97

3

96

4

98

5

92

6

95

7

92

8

85

Cl

9

NH2

80

HO

10

99

11

98

12

93

13

82 26


Page 27 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic Synopsis Cu2O crystals are pseudomorphically converted to Cu crystals of the corresponding morphologies through the addition of ammonia borane for complete nitroarene reduction reaction.

27


Cu2O Pseudomorphic Conversion to Cu Crystals for Diverse

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