Eco-friendly hydrogenation of aryl azides to primary amines on graphene oxide-decorated bimetallic Rh-Pt nanoparticles (RhPt@GO NPs)

Article abstract of DOI:10.1016/j.catcom.2018.12.018

Graphene oxide-supported rhodium/platinum (Rh-Pt) nanoparticles (Rh-Pt@GO NPs) are a promising catalyst for the reduction of aryl azides to primary amines. Rh-Pt@GO NPs have been produced by the microwave-assisted sonochemical method. This catalyst showed superior catalytic activity to afford quantitative yields of primary amines under an extremely mild condition in the presence of sodium borohydride. The synthesized novel catalyst was easily recovered by centrifugation and reused without loss of its catalytic activity. The spectroscopic analyses of the RhPt@GO NPs indicated the formation of highly crystalline, monodisperse, and colloidally stable RhPt@GO NPs.

Full text of DOI:10.1016/j.catcom.2018.12.018

Accepted Manuscript
Eco-friendly hydrogenation of aryl azides to primary amines on
graphene oxide-decorated bimetallic Rh-Pt nanoparticles
(RhPt@GO NPs)
Benan Kilbas, Belguzar Yasemin Kara
PII:
S1566-7367(18)30591-0
DOI:
https://doi.org/10.1016/j.catcom.2018.12.018
CATCOM 5581
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
15 October 2018
23 December 2018
26 December 2018
Please cite this article as: Benan Kilbas, Belguzar Yasemin Kara , Eco-friendly
hydrogenation of aryl azides to primary amines on graphene oxide-decorated bimetallic
Rh-Pt nanoparticles (RhPt@GO NPs). Catcom (2018), https://doi.org/10.1016/
j.catcom.2018.12.018
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ACCEPTED MANUSCRIPT
Eco-friendly hydrogenation of aryl azides to primary amines on graphene
oxide-decorated bimetallic Rh-Pt nanoparticles (RhPt@GO NPs)
a
b
Benan Kilbas* , Belguzar Yasemin Kara
a
Moltek A. S. Gebze Organize Sanayi, 41400 Gebze, Kocaeli, Turkey.
b
Department of Chemistry, Faculty of Sciences, Duzce University, 81620 Duzce, Turkey.
Corresponding Authors: benan.kilbas@moltek.com.tr
Abstract
Graphene oxide-supported rhodium/platinum (Rh-Pt) nanoparticles (Rh-Pt@GO NPs) are a promising
catalyst for the reduction of aryl azides to primary amines. Rh-Pt@GO NPs have been produced by the
microwave-assisted sonochemical method. This catalyst showed superior catalytic activity to afford
quantitative yields of primary amines under an extremely mild condition in the presence of sodium
borohydride. The synthesized novel catalyst was easily recovered by centrifugation and reused without
loss of its catalytic activity. The spectroscopic analyses of the RhPt@GO NPs indicated the formation
of highly crystalline, monodisperse, and colloidally stable RhPt@GO NPs.
Key words: RhPt@GO NPs, Heterogeneous catalyst, Hydrogenation, Sodium borohydride, Azides
1
.Introduction
The chemistry of azides has attracted considerable interest from scientists because they are
important key compounds for the production of biologically significant molecules such as nucleosides,
carbohydrates, and heterocycles [1-4]. They are extensively used for the formation of peptides [5],
1

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imines [6], tetrazoles [7], triazoles [8], nitriles [9], and amines [10]. Numerous synthetic routes have
been published in literature for the reduction of azides to amines, for example, NiCl
2
-Zn [11], FeCl -
3
Zn [12], NaBH /phase-transfer catalyst [13], Sm/I [14], and tellurium metal [15]. However, these
4
2
designed catalytic systems require harsh reaction conditions to employ hydrogenation of azides.
During the years, scientific efforts have been undertaken on the use of heterogeneous catalysts for
azide reduction process because they are accessible, reusable, and recyclable, and they are easily
separable from the reaction medium [16-18]. Recently, the design of bimetallic nanocatalysts
experience advanced interest for transfer hydrogenation reaction because of the cooperative interaction
regarding better selectivity, activity, and stability [19-23].
In this paper, we report the production of highly efficient and monodisperse bimetallic RhPt@GO
NPs for the reduction of aryl azides to primary amines for the first time. Sodium borohydride
contributed to azide reduction as a potential hydrogen source in minimum amount of water. Various
azide derivatives were easily converted to primary amines by an eco-friendly reaction condition where
water is used as a green solvent.
2
.Experimental
2
.1 Catalyst Preparation
The previously published catalytic system was modified [24]. Briefly, 0.25 mmol PtCl
4
, 0.25 mmol
RhCl , and 0.25 mmol oleylamine (OA) were vigorously stirred in 25 mL of ethylene glycol (EG)
3
solution, and the pH value of the mixture was set to 12.0 using NaOH–EG mixture. Then, the mixture
was placed at the center of a microwave oven (1200 W) for consecutive heating time of 60 s. In this
reaction, EG has been used for the reduction of PtCl
4
and RhCl . The observation of a brownish–black
3
color indicates the formation of Rh-Pt nanoparticles. After reaction, filtration process was applied, and
the crude product was washed with acetone and deionized water for several times. OA was used to
stabilize metal nanoparticles against agglomeration. The prepared NPs were furnished in a 1:1 ratio
2

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with graphene oxide using an ultrasonic sonicator. Finally, the solid Rh-Pt@GO NPs were dried under
vacuum at room temperature.
2
.2 General procedure for the catalytic reduction of aromatic azido compounds
To a mixture of 1 ml of H
2
O and 2 mg of catalyst was added 0.25 mmol of aromatic azido
was added to the reaction media, and the
compound at room temperature. Next, 0.25 mmol of NaBH
4
vessel was closed. The reaction then proceeded with unequivocal stirring and checked by TLC. Most
of the reactions were completed within 3-60 min. After completion of the reaction, the catalyst was
removed by centrifugation at 6000 rpm and washed several times with ethanol and water. At that
point, the prepared catalyst was exposed to dry for further use. The solvent was removed under
vacuum atmosphere.
3
. Results and discussion
The as-synthesized RhPt@GO bimetallic nanoparticles were analyzed by XRD, TEM, HRTEM,
and XPS techniques. Fig. 1 indicates the comparison of XRD patterns of the monometallic Pt@GO
and the bimetallic RhPt@GO nanoparticles. Four diffraction peaks have been observed for the
monometallic and bimetallic nanoparticles that are related to the face-centered cubic crystalline form
and assigned to the (111), (200), (220), and (311) planes. For the monometallic Pt@GO and bimetallic
RhPt@GO NPs, the shift in peak positions confirmed the alloy formation of Pt and Rh. Furthermore,
the average crystalline size of RhPt@GO NPs was determined from the related XRD patterns using
Scherrer equation, and it was determined to be approximately 3.64 ± 0.34 nm as a crystalline particle
size [25-31].
3

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Fig. 1. XRD pattern of Rh-Pt@GO NPs
Particle size distribution has been analyzed by TEM and HR-TEM as illustrated in Fig. S4 (see ESI
page S4). The TEM image of RhPt@GO indicates that fine particles are homogeneously distributed on
graphene oxide. Moreover, HR-TEM measurement for monodisperse RhPt@GO has been utilized to
analyze the atomic lattice fringes in Fig. S4a [32]. A particle size histogram was employed for a
distribution test of 100 particles, and it was observed that the distribution is Gaussian, and the most
reasonable size of the particles was approximately 4.12 ± 0.45 nm (Fig. S4c). Particle size identified
by TEM results coordinates well with XRD results. Fig. S4b and d indicates the electron energy loss
spectroscopy mapping image and line profile scanned on the arrow shown in HRTEM for platinum
and rhodium elements of RhPt@GO NPs, and it is apparent that both rhodium and platinum exist as a
whole region of the designed catalyst that verified the alloy formation of the nanocatalyst.
The Pt 4f and Rh 3d regions in XPS spectra of the RhPt@GO are illustrated in Fig. S4e-f. As
shown in Fig. S4f, for the Rh-Pt@GO bimetallic nanoparticles, the Pt 4f spectrum appears as doublet
that comprises a low-energy band (Pt 4f5/2) at 70.8 eV and a high-energy band (Pt 4f7/2) at 74.3 eV, and
the Rh 3d spectrum appears as doublet that comprises a high-energy band (Rh 3d3/2) at 313.4 eV and a
low-energy band (Rh 3d5/2) at 307.1 eV, thus demonstrating the existence of mainly metallic platinum
and rhodium rather than oxygen-containing compounds. The peaks of Rh (III) at 309.3 eV, Pt (II) at
4

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around 72.3 eV, and Pt (IV) at around 74.1 eV in Fig. S4 (e) and (f) may be formed by the surface
oxidation and/or chemisorption of atmospheric oxygen during the preparation process [25-33].
After completion of the catalyst system and characterization, the catalytic activity of RhPt@GO
NPs for hydrogenation of aryl azides to primary amines was examined. As depicted in Table 1,
optimization of the reduction reaction was attempted on the basis of the amount of catalyst/hydrogen
source, solvent system, and time interval. The objective was to determine the conditions for the exact
amine formation at room temperature. In literature, the yield of amine formation was successfully
increased when tetrahydrofuran and methanol were utilized as the solvent system [34]. Our goal was
also to accomplish the reduction of azides in a short time where water was only used as an eco-
friendly solvent without any significant loss of yield. Finally, 2.0 mg of the catalyst and 1.0 mmol of
hydrogen source demonstrated excellent performance for the conversion of azides into amines with
only 1.0 ml of water (Table 1, entry 9).
a
Table 1 Optimization experiments for the reduction of 4-chloro azidobenzene to 4-chloro aniline
5

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b
Catalyst
mg)
NaBH
4
Time
(min)
90
Yield
Entry
Solvent
(
(eq.)
(%)
1
2
3
4
5
6
7
8
9
EtOH
4
4
4
4
4
2
1
2
2
-
3
3
3
3
3
3
3
2
1
1
15
CH
2
Cl
2
300
40
No reaction
95
MeOH
MeOH/H
2
O (1:1)
10
>99
H
H
H
H
2
2
2
2
O
O
O
O
6
>99
6
>99
50
>99
7
>99
H
2
O
8
>99
trace
10
H
2
O
360
a
Reaction conditions: 4-chloro azidobenzene (0.25 mmol), RhPt@GO NPs (10.8 wt% metal content), and room
b
temperature. GC yield.
Table 2 indicates that all of azido benzenes were successfully reduced to primary amines in the
presence of RhPt@GO NPs through hydrolysis of sodium borohydride. The aniline derivatives were
quantitatively obtained within 3-60 min at room temperature with a minimum amount of solvent.
Azidobenzene (1), 1-azido-2-chlorobenzene (3), 1-azido-4-chlorobenzene (5), 1-azido-2-
bromobenzene (7), and 1-azido-4-bromobenzene (9) were successfully reduced to aniline (2), 2-
chloroaniline (4), 4-chloroaniline (6), 2-bromoaniline (8), and 4-bromoaniline (10) with the yields
higher than 99% within 8 min (Table 2, entries 1-5, respectively). Surprisingly, 1-azido-4-iodobenzene
1
(
11) was barely converted to 4-iodoaniline (12) after 1 h (Table 2, entry 6). H NMR spectrum of 4-
iodoaniline compound indicates the formation of side product (10%), which confirms the reduction of
both iodine and azido groups. The plausible reason is a competitive reaction between dehalogenation
6

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of iodine and reduction of the azide group. As reported before, C-I bond strength is not quite strong,
and it can be easily broken to afford dehalogenated product in the presence of a heterogeneous
catalyst. For example, dehalogenation of iodobenzene was carried out in methanol after 1.5 h in the
presence of excess ammonia borane (NH
effect, the reaction was continued for 4 h with excess amount of hydrogen source (1 mmol NaBH
GC analyses confirmed the other side product formed by both iodine and azido group reduction. o-, m-
and p-Azido toluene compounds (13, 15, 17) were reduced faster than halogenated aromatic azido
3
BH
3
) and RuPd@GO NPs [35]. To approve the reasonable
4
).
,
compounds because of the electron-donating methyl groups. The corresponding compounds were
converted to aniline derivatives (14, 16, 18) with high reaction efficiency and quantitative yield (Table
2, entries 7-9). Similar results were observed for the reduction of o-, m-, and p-azido anisole
compounds (19, 21, 23). They were easily reduced to primary amines with high yields in short time
(Table 2, entries 10-12) because of the electron-releasing ability of the methoxy group. 1-Azido-2-
(trifluoromethyl)benzene (25) and 1-azido-4-(trifluoromethyl)benzene (27) were also converted to 2-
(trifluoromethyl)aniline (26) and 4-(trifluoro methyl)aniline (28) with high yields (Table 2, entries 13,
14). However, the completion of the reduction process was observed as longer time than expected. The
possible reason is not only the electron-withdrawing effect and repulsion by the negatively charged
fluorines with the surface but also the steric hindrance of the trifluoromethyl group.
Recently, bimetallic nanocatalysts have been preferred more than monometallic nanocatalysts
because of the diverse electronic structure, cooperative effect, and new catalytic sites [19-23]. In the
light of literature data and our previous studies, we compared the activity of bimetallic and
monometallic catalytic systems for the reduction of azides. As illustrated in Table 3, the reduction
process was completed in a shorter time with high yield, while RhPt@GO NPs were used as the
catalyst under the same conditions.
7

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a
Table 2 RhPt@GO NP-assisted hydrogenation of various R-N
3
compounds
b
b
Time
Yield
Time
(min
Yield
Entry
Azide
Amine
Entry
Azide
Amine
(
%)
(
min)
(%)
1
>99
>99
>99
8
8
>99
6
5
3
1
3
5
2
1
5
16
2
8
9
>99
17
18
4
3
8
10
>99
6
1
9
20
4
>99
8
11
>99
4
7
8
2
1
22
5
>99
8
12
>99
3
9
10
23
24
8

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6
90
60
13
>99
40
1
1
12
25
26
7
>99
5
14
>99
25
1
3
14
28
2
7
a
Reaction conditions: azide (0.25 mmol), NaBH4 (0.25 mmol), and catalyst (2 mg, 10.8 wt%
b
metal content) were used in 1 ml of water at room temperature. GC yield.
Table 3 Comparison test of monometallic and bimetallic systems for the reduction of 4-
a
chloro azidobenzene to 4-chloro aniline
RhPt@GO
Pt@GO
Entry
Substrate
Product
Time Yield Time Yield
min) (%) (min) (%)
(
9

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1
8
>99
20
90
5
6
a
Reaction conditions: 0.25 mmol substrate,0.25 mmol NaBH4, 0.07 mmol % catalyst,1.0 mL of
water at room temperature.
a
Table 4 Reusability test of RhPt@GO NPs
1
st
Yield
%)
5th
b
b
Entry
Substrate
Product
Time
(min)
Yield
(%)
Time
(min)
(
1
>99
>99
8
8
97
98
13
12
2
a
Reaction conditions: substrate (0.25 mmol), NaBH4 (0.25 mmol), RhPt@GO NPs (2 mg, 10.8 wt% metal
b
content). GC yield.
The reusability test of the RhPt@GO NPs was attempted. The catalyst was used five times and was
able to be reused without loss of activity as illustrated in Table 4. There was no significant loss of
10

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rhodium and platinum (0.2 and 0.7 ppm leaching to a solution, respectively) after five cycles of
reusability experiment analyzed by ICP-OES. The designed catalyst maintained high activity with
only a slight increase in reaction time.
4
. Conclusion
In this paper, a novel, eco-friendly, and recoverable synthetic process has been described for the
reduction of aryl azides by the assistance of highly monodisperse RhPt@GO NPs. This catalytic
system exhibited better catalytic activity than that reported in previously published articles (see ESI
Table S1, page S7) for the reduction of corresponding aryl azides because of low particle size, high
2
chemical surface area (67.58 m /g), and % metal (0) contents of the catalytic system. It is concluded
that it will gain a new perspective in literature in both areas of application of heterogeneous catalysts
and reduction of aryl azides.
Acknowledgments
Financial support was received from Duzce University Research Fund (grant no. 2015.26.04.371).
Appendix A. Supplementary data
Supplementary material (full experimental protocols, equipment, characterization of GO,
EDX/SEM images, NMR spectra, and spectral data for the primary amines) is available in the online
version of this article at http://
11

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Highlights





RhPt@GO NPs catalyzed hydrogenation of aryl azides has been well established.
RhPt@GO NPs are recoverable, reusable and stable.
Water was only used as an eco-friendly solvent.
Reducing reagent (NaBH
4
) is stable, cheap and water-soluble.
The novel process is safe, practical and economic.
16