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MISHRA, P. Chandra and M. M. Shaikh, RSC Adv., 2016, DOI: 10.1039/C6RA12920J.
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ARTICLE
This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1
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a. Akbar Mohammad, Veenu Mishra, Prakash Chandra and Shaikh M. Mobin
Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Indore 452020,
M.P.,India
E-mail: [email protected]
b. Shaikh M. Mobin
Centre for Material Science and Engineering,
Indian Institute of Technology Indore, Simrol, Indore 452020, M.P., India.
E-mail:[email protected]
Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here].
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Reduction of selective polyaromatic nitrotriptycene via
azoxytriptycene intermediate under ambient conditions using
cobalt/cobalt oxide nanocomposite (CoNC)
Akbar Mohammad,a Veenu Mishra,a Prakash Chandraa and Shaikh M. Mobin*a,b
Cobalt-based nanocomposite (CoNC) has been prepared from recently reported single source molecular precursor (SSMP)
[CoII(hep-H)(H2O)4]SO4 (A) (hep-H= 2-(2-hydroxylethyl) pyridine). The resulting nanocomposite material was characterized
by using various physicochemical techniques such as XRD, SEM, EDAX, TEM and XPS spectroscopy. X-ray diffraction pattern
shows weakly crystalline nature of the catalyst. This was also confirmed by SAED pattern obtained from HR-TEM. XPS
analysis reveals the formation of metallic cobalt and cobalt oxide (CoO) nanocomposite.CoNC was employed for the facile
catalytic hydrogenation of 2-nitrotriptycene (M1) and 2,6,14-trinitrotriptycene (M2) as model substrates under
atmospheric reaction conditions, which otherwise takes place either by Raney-Ni or Pd/C or SnCl2/HCl catalyst under
drastic conditions. The mechanistic pathway reveals that the reduction of M1 proceeds via the intermediacy of azoxy
triptycene (III) and N-hydroxylamine triptycene (IV).
Introduction
Catalysis has revolutionized today’s chemical
manufacturing. One such manufacturing process is reduction
of organic molecules by catalytic hydrogenation. Last few
decades witness an enormous progress in the field of catalytic
hydrogenation of aliphatic/aromatic nitro compounds. In this
process the focused was on improving the conversion yields by
varying the i) reaction conditions, ii) catalyst from metal to
non-metal, and iii) solvents.1 The catalytic transformation of R-
NO2 to R-NH2 undergoes via the formation of versatile
intermediates, aryl hydroxylamine, azoxy and azo compounds.2
These products and intermediates are potentially used in
biologically active natural products, dyes, pigments, polymers,
pharmaceuticals, and agro-chemicals.3 In general, the catalytic
hydrogenation involved the use of noble or expensive metal
catalyst such as Au, Pd, Pt, Rh, Ru or Raney Ni.4 Generally, the
use of Raney Ni in industrial process believes to be effective
due to the fact that no side product except water is formed.
However, the key disadvantages of Raney Ni in catalytic
hydrogenation are tedious conditions, use of highly flammable
molecular hydrogen, high pressure and expensiveness. On the
other hand, supported catalysts for the reduction of
nitroarenes with high intrinsic activity suffers from non-
selective hydrogenation leading to undesired by-products and
difficult purification.5Therefore there is a demand for more
selective, faster and appropriate method for the reduction of
R-NO2 to R-NH2.
In order to control the high cost and unavailability
associated with these metal catalysts, much attention has
been paid to an economical and robust catalyst as an alternate
by using nanoparticles of abundant metal such as Co, Fe, Cu or
Ni.6 The Co nanoparticles have gained much interest due to its
magnetic nature and wide applications in catalysis, dye
adsorption, MRI, biotechnology and data storage.1f,6a,7The
construction and employment of magnetic nanoparticles
(MNPs) guaranteed non-toxic, easily accessible and smooth
reproducibility.8 Additionally, the movement and specificity of
nano-catalysts can be manipulated by their shell
modifications.9Not only metallic cobalt but cobalt oxides have
extensive applications in high density magnetic recording,
sensors and heterogeneous catalysis. Co and cobalt oxide have
electronic configuration dn (n=5 to 9) active for catalytic
reduction of nitro compounds to amines.10 The oxides with dn
configuration induce reduction by relying electron form the
BH4- to the nitro-compounds once they get adsorbed on the
metal surface. According to the previous literature reports
surface positive charge of the dn elements having p-type
semiconductor properties facilitate interaction between metal
surface and donor BH4- ions.10 b
To the best of our knowledge transformation of M1 and
M2 to their respective amines by cobalt based nanocomposite
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(CoNC) have not been investigated under atmospheric reaction
conditions using NaBH4 as reducing agent.
Till dates 2-nitrotriptycene (M1) and 2,6,14-
trinitrotriptycene (M2) are transformed to their respective
amines only by using either Raney Ni or Pd/C or SnCl2/HCl
catalyst under drastic conditions (Scheme 1).11
Conditions: For M1; Raney Ni, N2H4/THF, N2, 60ᵒC, 90 min. For M2;
Raney Ni, N2H4/THF, N2, 60ᵒC, 6-16h or Pd/C,N2H4/EtOH, Reflux, 12h or
SnCl2.2H2O, HCl/ H2O/ EtOH, 100ᵒC, 24h.
Scheme 1 Reduction of M1 and M2 to their respective amines (1 and 2) using
Raney Ni, Pd/C, SnCl2 reported so for.11
Results and Discussion
The present work reports the synthesis and
characterization of novel cobalt/cobalt oxide nanocomposite
(CoNC). The CoNC was derived from recently reported
monomeric Co(II) complex, [CoII(hep-H)(H2O)4]SO4 (A) (Scheme
2) as a single source molecular precursor (SSMP) by a facile
method in aqueous media in the presence of NaBH4 at room
temperature (Scheme 2).12
Scheme 2 General scheme for the synthesis of molecular
precursor and CoNC.
Physicochemical characterization of CoNC was performed
by PXRD, TEM, SEM, EDAX, and XPS.13The crystallinity and
phase purity of the material was determied using powder X-
ray diffraction. X-ray diffraction pattern shows weakly
crystalline to amorphus nature of the catalyst (Fig. S1).
Fig. 1 TEM images of CoNCs; (A) 100 nm magnification; (B) 5 nm magnification
(C) SAED pattern.
TEM analysis shows that the primary CoNC are less than 5
nm in diameter. These primary nanoparticles aggregated to
form secondary particles with capsule like structure having size
100-150 nm (Fig. 1A and B). Selected area electron diffraction
SAED performed on samples of nanopartiles aggregrate
obtained from HR-TEM showed a concentric and quite diffused
ring pattern indicating weakly crystalline nature of the material
(Fig. 1C). The capsule like structure was further confirmed by
SEM images as shown in Fig.S2 a-e.
The newly developed CoNC was employed as a catalyst
towards the hydrogenation of 2-nitrotriptycene (M1) and
2,6,14-trinitrotriptycene (M2) in presence of NaBH4 under
ambient conditions (Scheme 3). M1 and M2 were prepared by
following the previously reported methods.11a Catalytic
conversion of M1 was established to proceed via the
azoxytriptycene intermediate (III) (see later). No reaction was
detected in the absence of hydrogen source or CoNC. The
hydrogenation process was monitored by 1H NMR at regular
intervals of 10 mins and 5 mins for M1 and M2, respectively at
room temperature (Fig.S3 and S4). Further, the evaluation of
solvent on the hydrogenation process established that
dichloromethane (DCM) (Table 1, entry 1, Table S1) was the
best choice, leading to 95% conversion of M1 and M2 to 1 and
2 at 60 and 30 min respectively. The superior catalytic activity
of DCM as compared to ether and ethylacetate was due to its
higher polarity and solubility of the reactant in the following
order DCM>ethyl acetate> ether. To our surprise, moderate
conversion of M1/M2 to 1/2 was also observed in water
because of lower solubility of the nitrotriptycene played a
detrimental role in product selectivity, inspite of higher
polarity of water molecule.
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Scheme 3 Nirotriptycene reduction catalysed by NaBH4 and CoNC.
Table 1. Effect of solvent on catalytic performance of CoNC for reduction of M1
and M2.
The comparison relating to the reaction conditions between
the reported Raney-Ni or Pd/C or SnCl2 and CoNC as the
catalysts for the hydrogenation of M1/M2 (Table 2) revealed
the superiority of the latter.
Table 2. Superiority of CoNC over other reported catalyst.
The careful monitoring of the reduction of M1 by CoNC via
thin layer chromatography (TLC) facilitated the isolation and
structural characterization of the intermediate azoxytriptycene
(III) which in turn extended the mechanistic outlook of the
transformation process.
Mechanism for nitro reduction is similar to the proposed
mechanism for Cu nanoparticles as reported earlier.14aLike
other metals cobalt is a good conductor of electrical
conductivity facilitating easy electron transfer from adsorbed
species on its surface to another. This property makes CoNC an
excellent catalyst for nitro reduction. Probable mechanism for
the reduction of nitro compounds by NaBH4 and CoNC has
been discussed.
XPS analysis was performed to obtain the elemental
composition and chemical and electronic state of CoNC (Fig. 2,
S5 E-H). All the binding energies were calibrated using the C 1s
peak at 284.6 eV. Formation of metallic Co nanoparticle was
confirmed by the presence of peak at 778 eV. Presence of
peaks at 781.6 eV, 793.2 eV and 787 eV was due to 2p3/2, 2p1/2
and a satellite peak respectively confirms presence Co2+
oxidation state. The aforementioned results show that
complex A is reduced to metallic Co and its oxides CoO or
Co2O3. Presence of peak at 532.6 eV in both fresh and spent
catalyst confirmed the presence of O1s (Fig. S5 G-H).14b,c
Reduction of complex A with NaBH4 results in the formation of
CoNC. Metallic Co nanoparticle present in CoNC catalyzed
nitrotriptycene reduction according to classical Langmuir-
Hinshelwood model and is depicted in Chart 1A. BH4− and 2-
nitrotriptycene sequentially get adsorbed on the CoNC surface.
The adsorption of the nitrotryptecene is followed by transfer
of surface-hydrogen species and electrons (reduction) from
BH4− to 2-nitrotriptycene on its surface to furnish 2-
aminotripticyene.
Fig. 2 XPS spectra; (A) fresh CoNC; (B) Co present in CoNC; (C) spent CoNC; (D)
Co present in spent CoNC.
During the catalytic hydrogenation process, the formation
of Co2O3 or Co3O4 composite via partial oxidation of metallic
Co nanoparticles in the alkaline media was also observed. XPS
analysis of the spent catalyst did not show appearance of any
peak at 778 eV confirming that there was oxidation of metallic
Co nanoparticles to Co2+ during the reaction. Oxidation of
cobalt nanoparticles was further confirmed by SEM-EDAX
analysis. EDAX of fresh and spent catalyst show presence of
Co: O atomic ratio 65.59: 34.51 and 35.08: 64.08 respectively
further confirm oxidation of metallic cobalt and cobalt oxide to
their higher oxidation state (Fig.S2 d-e and S6 c-d). This results
metal oxide nanocomposite having p-type semiconductor
properties. Plausible mechanism for reduction of M1 to 1 on
Entry Solvent
Catalyst
(Mol%)
(CoNC)
1 2
Yield
(%) Time(min)
Yield
(%) Time(min)
1. DCM 5 95 60 95 30
2. EtOAc 5 73 70 82 70
3. Ether 5 61 70 90 70
4. Water 5 30 240 66 180
Substrate Conditions Catalyst Yield
(%) Ref.
M1
N2H4, THF, 60°C,
N2, 1.5h Raney Ni 75 11a
NaBH4, DCM, RT,
Air, 60 min CoNC 95
This
work
M2
N2H4, THF,
60°C,N2,
6-16h
Raney Ni >90 11b-d
N2H4, EtOH,
Reflux,12h Pd/C 88-90 11e-g
HCl,
H2O/EtOH,100°C
24h
SnCl2.2H2O 57 11h
NaBH4, DCM, RT,
Air,30 min CoNC 95
This
work
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(Scheme 4 and Table S1) using a wide variety of solvents such
as DCM, EtOAc, Ether, water and MeOH.
Scheme 4 Reduction of other polyaromatic nitro compounds.
Very high conversion and selectivity was obtained for
various polyaromatic nitro compounds in varying solvents
(Supporting information Table S1, entry no. 3-8). The reaction
was observed in all studied solvents with higher conversion
and good selectivity for entry no 7 except water. In case of
entry no 8, MeOH and Ether gave 99% of conversion while
others have either average conversion or poor. Moreover,
other substituted polyaromatics compounds having amino,
methyl and methoxy functionalities (Table S1, entry no. 5-6, 8,
10) show high conversion (75-99%) and selectivity. However,
the product yield for 9 and 10 was slightly lower with 70 and
75 % respectively. However, DCM and MeOH are compatible
for almost all the polyaromatics nitro and other substituted
compounds. High polarity of DCM and MeOH as well as better
solubility of polyaromatic nitro compounds in these solvents as
compared to other solvents was responsible for superior
catalytic performance of CoNC. This facile reduction of
compounds was obtained at RT leading to the formation of
their respective amines with high conversion in the range of
70-99 % (Table S1, Fig.S10-S17).
Conclusions
To summarize, CoNC was prepared by reduction of single
source molecular precursor (SSMP) A using NaBH4 as reducing
agent. CoNC was characterized by using several
physicochemical techniques and used for the reduction of M1.
Powder XRD spectra indicated formation of weakly crystalline
to amorphous material. HRTEM images and SAED pattern
further corroborate the XRD data. Elemental analysis
performed using XPS spectroscopy of CoNC revealedthe
formation of metallic cobalt and cobalt oxide. The superior
catalytic feature of the newly developed CoNC with well define
morphology for the reduction of polyaromatic nitro
compounds including M1 and M2 as compared to the
reported Raney-Ni, Pd/C and SnCl2 based catalysts was
established. Solvent effect was also investigated for M1
because DCM was found to be best solvent for M1 reduction
to 1. Furthermore, the probable outlook of the reaction
pathways was established via the structural characterization of
the azoxytriptycene intermediate, (III). The scope of the
catalytic material (CoNC) was tested for other polyaromatic
nitro compounds. CoNC with magnetic properties can be
considered as a promising synthon for other potential
applications.
Experimental Section Materials: Commercially available starting materials,
CoSO4·7H2O, 2-(2-hydroxyethyl) pyridine (hep-H), sodium
borohydrate (NaBH4), nitro compounds and reagent grade
solvents were used as received. The model substrates (M1 and
M2) are synthesized according to previous reported literature.
All the reagents were of analytical grade and used without
further purification.
Physical characterizations: Powder X-ray diffraction studies
were carried out on Rigaku SmartLab X-ray diffractometer
using CuKα radiation (1.54 Å). FE-SEM attached with EDAX was
done using Supra55 Zeiss Field-Emission Scanning Electron
Microscope. Transmission Electron Microscopy was carried on
FEI Tecnai G2 12 Twin TEM. XPS analysis of fresh and spent
CoNC was performed using X-ray Photoelectron Spectroscopy
(XPS) with Auger Electron Spectroscopy (AES) Module:
Model/Supplier:PHI 5000 Versa Prob II,FEI Inc.1H NMR spectra
were recorded on BrukerAvance(III) 400MHz spectrometer.
Mass analysis was carried on Brucker-Daltonics, micrOTOF-Q II
mass spectrometer. Single-crystal X-ray structural studies were
performed on Supernova Agilent X-ray diffractometer.
X-ray Crystallographic determination:Single crystal X-ray
structural studies of compounds (2 and III) were performed on
a CCD equipped SUPERNOVA diffractometer from Agilent
Technologies with a low-temperature attachment.2 and III
crystals were weakly diffracting in nature, III shows highly
disorder molecule, which were refined with model atoms for
two positions. Data for 2 and III were collected at 293(2)K
using Cu Kα radiation λα = 1.5418 Å. The strategy for the data
collection was evaluated by using the CrysAlisPro CCD
software. The data were collected by the standard 'phi-omega
scan techniques, and were scaled and reduced using
CrysAlisPro RED software. The structures were solved by direct
methods using SHELXS-97 and refined by full matrix least
squares with SHELXL-97, refining on F2.15The positions of all
the atoms were obtained by direct methods. All non-hydrogen
atoms were refined anisotropically. The remaining hydrogen
atoms were placed in geometrically constrained positions and
refined with isotropic temperature factors, generally 1.2 x
Ueqof their parent atoms. All the H-bonding interactions,
mean plane analyses, and molecular drawings were obtained
using the program Diamond (ver. 2.1d). The crystal and
refinement data are summarized in Table S2‡ and hydrogen
bonding parameters are shown in Table S3.
Preparation of single source molecular precursor: Co(II)
complex, [CoII(hep-H)(H2O)4]SO4 (A), has been synthesized
using the previously reported procedure.12
Procedure for the synthesis of CoNC: In the typical synthesis
of CoNC, 1mmol of the precursor was dissolved in 20 ml of
distilled water and stir for 5 min. Aqueous solution of NaBH4
was added to reaction mixture drop wise till the evolved
hydrogen discontinued. The color change from orange to black
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was observed as NaBH4solution added. Finally the product was
washed repeatedly with water and ethanol to remove
unreacted materials.
Procedure for catalytic reduction: CoNC (5 mol%) was taken
in glass vial, followed by the addition of 5ml of solvent and
0.1mmol of nitro compound and aqueous NaBH4 (10
equivalent), reaction was magnetically stirred at RT in air for
the desired time duration. The progress of the reaction was
monitored by TLC. After completion of the reaction, the
product was isolated by normal workup procedures.
Acknowledgements
The authors are grateful to CSIR, New Delhi and IIT Indore for funding. We are also grateful to Dr. Sampak Samanta for his support and Advanced Imaging Center, for extending the TEM facility and Advanced Center for Materials Science (ACMS) for XPS, IIT Kanpur. A. M. and V. M. are grateful to MHRD, Government of India for research fellowship and SIC, IIT Indore for providing the characterization facility. V. M. also thanks CSIR, New Delhi for senior research fellowship.
Notes and references
‡Crystal data for 2: C21H19Cl2N3, M = 384.29, monoclinic P21 /n, Z= 4, T= 293(2) K, F(000)= 800, a= 12.084(6)Å, b= 9.0350(12)Å, c= 17.401(6)Å, α = γ= 90°, β= 100.28(3)°, V = 1869.3(11) A3 , Dc = 1.366 mg m−3 , μ(Cu Kα) = 3.188 mm−1 , size = 0.330 x 0.260 x 0.210 mm3 , GOF = 1.076, reflections collected/unique, 3830 / 1132 [R(int) = 0.0352] R1 [I > 2σ(I)] = R1 = 0.0812, wR2 = 0.2345, R indices (all data) ) R1 = 0.0850, wR2 = 0.2400. CCDC no. 1422762. Crystal data for III: C40H26N2O, M = 550.63, triclinic, Pī, Z = 1, T = 293(2) K, F(000) = 576, a =10.6315(7) Å, b=11.8390(9) Å, c=12.1481(8) Å, α = 87.279(6)°, β = 68.228(6)°, γ = 86.725(6)°, V = 1417.08(18) A3 , Dc = 1.290 mg m−3 , μ(Cu Kα) = 0.602 mm−1 , size = 0.210 x 0.170 x 0.130 mm3,GOF = 1.027, reflections collected/unique, 9377 / 5357 [R(int) = 0.0223] R1 [I > 2σ(I)] = R1 = 0.0527, wR2 = 0.1382 , R indices (all data) R1 = 0.0699, wR2 = 0.1545. CCDC no. 1422763. Supporting content:Experimental details, characterization images, spectral data (Fig. S10-S17) and video for catalyst separation attached and crystallographic details can be found in supporting information. 1 (a) N. Yan, C. Xiao and Y. Kou, Coord. Chem. Rev.,2010, 254,
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Journal Name ARTICLE
This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7
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View Article OnlineDOI: 10.1039/C6RA12920J
Reduction of selective polyaromatic nitrotriptycene via azoxytriptycene
intermediate under ambient conditions using cobalt/cobalt oxide
nanocomposite (CoNC)
Selectively targeted polyaromatic 2-nitrotriptycene (M1) and 2,6,14-trinitrotriptycene (M2) were
chosen as model substrates for demonstrating catalytic hydrogenation at ambient conditions
using cobalt/cobalt oxide based nanocomposite (CoNC) as catalytic material. Conventionally
Raney Ni or Pd/C or SnCl2/HCl is used as catalyst under drastic conditions. CoNC can be used
as an alternative catalyst with superior catalytic performance at ambient conditions. Interestingly,
mechanistic path is confirmed by single crystal X-ray structure of azoxytriptycene, (III).
Page 8 of 8RSC Advances
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View Article OnlineDOI: 10.1039/C6RA12920J