Visible-Light-Assisted Photocatalytic Reduction of Nitroaromatics by Recyclable Ni(II)-Porphyrin Metal-Organic Framework (MOF) at RT

Article abstract of DOI:10.1021/acs.inorgchem.6b00296

A microporous Ni(II)-porphyrin metal-organic framework (MOF), [Ni₃(Ni-HTCPP)₂(μ₂-H₂O)₂(H₂O)₄(DMF)₂]·2DMF, (MOF1) (where, Ni-HTCPP = 5,10,15,20-tetrakis(4-benzoate) porphyrinato-Ni(II)) has been synthesized by the solvothermal route. Single-crystal X-ray diffraction study of 1 reveals a 2D network structure constituted by Ni₃ cluster and [Ni-HTCPP]^3- metalloligand having (3, 6)-connected binodal net with {4³}2{4⁶·6⁶·8³}-kgd net topology. The 2D layers are further stacked together through π-π interactions between the porphyrin linkers to generate a 3D supramolecular framework which houses 1D channels with dimension of ～5.0 × 9.0 ?² running along the crystallographic a-axis. Visible-light-assisted photocatalytic investigation of MOF1 for heterogeneous reduction of various nitroaromatics at room temperature resulted in the corresponding amines with high yield and selectivity. On the contrary, the Ni(II)-centered porphyrin tetracarboxylic acid [Ni-H₄TCPP] metalloligand does not show the photocatalytic activity under similar conditions. The remarkably high catalytic performance of MOF1 over [Ni-H₄TCPP] metalloligand has been attributed due to cooperative catalysis involving the Ni-centered porphyrin secendary building units (SBUs) and the Ni₃-oxo node. Further, the MOF1 was recycled and reused up to three cycles without any significant loss of catalytic activity as well as structural rigidity. To the best of our knowledge, MOF1 represents the first example of MOF based on 3d metal ion exhibiting visible-light-assisted reduction of nitroaromatics under mild conditions without the assistance of noble metal cocatalysts.

Full text of DOI:10.1021/acs.inorgchem.6b00296

Article
pubs.acs.org/IC
Visible-Light-Assisted Photocatalytic Reduction of Nitroaromatics by
Recyclable Ni(II)-Porphyrin Metal−Organic Framework (MOF) at RT
M. S. Deenadayalan, Nayuesh Sharma, Praveen Kumar Verma, and C. M. Nagaraja*
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India
*
S Supporting Information
ABSTRACT: A microporous Ni(II)-porphyrin metal−organic framework (MOF),
[
5
Ni (Ni-HTCPP) (μ -H O) (H O) (DMF) ]·2DMF, (MOF1) (where, Ni-HTCPP =
3 2 2 2 2 2 4 2
,10,15,20-tetrakis(4-benzoate) porphyrinato-Ni(II)) has been synthesized by the
solvothermal route. Single-crystal X-ray diffraction study of 1 reveals a 2D network
3
−
structure constituted by Ni cluster and [Ni-HTCPP] metalloligand having (3, 6)-
3
3
6
6
3
connected binodal net with {4 }2{4 ·6 ·8 }-kgd net topology. The 2D layers are further
stacked together through π−π interactions between the porphyrin linkers to generate a
3
D supramolecular framework which houses 1D channels with dimension of ∼5.0 × 9.0
2
Å
running along the crystallographic a-axis. Visible-light-assisted photocatalytic
investigation of MOF1 for heterogeneous reduction of various nitroaromatics at room
temperature resulted in the corresponding amines with high yield and selectivity. On the
contrary, the Ni(II)-centered porphyrin tetracarboxylic acid [Ni−H TCPP] metal-
4
loligand does not show the photocatalytic activity under similar conditions. The
remarkably high catalytic performance of MOF1 over [Ni−H TCPP] metalloligand has been attributed due to cooperative
4
catalysis involving the Ni-centered porphyrin secendary building units (SBUs) and the Ni -oxo node. Further, the MOF1 was
3
recycled and reused up to three cycles without any significant loss of catalytic activity as well as structural rigidity. To the best of
our knowledge, MOF1 represents the first example of MOF based on 3d metal ion exhibiting visible-light-assisted reduction of
nitroaromatics under mild conditions without the assistance of noble metal cocatalysts.
8
INTRODUCTION
to aniline. Furthermore, Chen et al. demonstrated the
application of Pd encapsulated UiO-67 MOF for reduction of
■
Nitroaromatics are common ingredients of explosive materials,
and hence, their rapid detection and catalytic conversion to less
dangerous products is an important process for the sake of
9
nitrobenzene. Recently, Mukherjee and co-workers have
demonstrated the application of Au-doped Zn(II)-MOFs for
10
1
reduction of nitrophenols. The above reports of MOF-based
catalysts for the reduction of nitroaromatics use either drastic
conditions or noble metal (Pd, Pt, or Au) cocatalysts. On the
other hand, there are no reports of MOFs based on 3d metal
ion known for photocatalytic reduction of nitroaromatics under
mild conditions. In addition, photocatalysts which operate by
visible light absorption are highly desirable for the utilization of
the abundant sunlight compared to those functioning under UV
light, which constitutes only 3−5% of the solar energy reaching
the earth. In this regard, the effectiveness of MOF-based
photocatalysts which operate under UV or a visible light source
have been reported for photocatalytic CO reduction, H
evolution, and so on. However, visible-light-assisted reduction
of nitroaromatics by MOFs has been rarely studied. In this
regard, porphyrin-based molecular complexes and MOFs
containing metallo-porphyrin SBUs have attracted considerable
attention due to their diverse applications in light-harvesting
and visible-light-assisted photocatalysis, including photo-
national security and environmental safety. On the other hand,
functionalized aminobenzenes are important intermediates for
the synthesis of dyes, fine chemicals, agrochemicals, and
pharmaceuticals, and they can be easily obtained by the
reduction of nitroarenes. The conventional synthesis of amines
following the Bechamp reduction process and others produces
large amounts of environmentally toxic waste and is associated
with selectivity issues. Therefore, development of green
catalytic routes for selective hydrogenation of nitroaromatics
into their corresponding amines is highly desirable for industrial
applications of these compounds. In this regard, photocatalytic
reduction of nitroaromatics employing visible-light photo-
2
3
4
2
2
11
catalysts has attracted considerable attention.
Metal−organic frameworks (MOFs) or porous coordination
polymers (PCPs) have received growing interest as suitable
12
candidates for carrying out heterogeneous catalysis due to thei5r
13
high surface areas, tunable pore size, and functionality.
14
Extensive research efforts are being carried out on development
of luminescent MOFs for sensing of nitroaromatics. However,
the application of MOFs as heterogeneous catalysts for
reduction of nitroaromatics has been scarcely studied. In this
15
6
catalytic redox reactions. Motivated by these studies, we
have investigated the synthesis and visible-light photocatalytic
7
context, Matsuoka and co-workers have reported Pt loaded Ti-
Received: February 3, 2016
NH −BDC MOF for photocatalytic reduction of nitrobenzene
2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b00296
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Inorganic Chemistry
Article
application of porphyrin MOF containing 3d metal ion for
reduction of nitroaromatics without the assistance of noble
metal cocatalysts.
diffractometer equipped with a INCOATEC microfocus source and
graphite monochromated Mo Kα radiation (λ = 0.71073 Å) operating
at 50 kV and 30 mA. The program SAINT was used for integration
1
7
of diffraction profiles, and absorption correction was made with
Herein, we report construction of a Ni(II)-MOF, [Ni (Ni-
3
18
19
SADABS program. The structure was solved by SIR 92 and refined
HTCPP) (μ -H O) (H O) (DMF) ]·2DMF (MOF1), consti-
20
2
2
2
2
2
4
2
by full matrix least-square method using SHELXL-2013 and WinGX
system, Ver 2013.3. All the non-hydrogen atoms were located from
3−
tuted by Ni cluster and [Ni-HTCPP] metalloligand (where
Ni-HTCPP] represents [5,10,15,20-tetrakis(4-benzoate)
21
3
3
−
[
the difference Fourier map and refined anisotropically. The disordered
porphyrinato-Ni(II)] with one protonated carboxylate group).
Single-crystal X-ray structure determination reveals that MOF1
exhibits a 2D network structure having (3, 6)-connected
binodal net with {4 }2{4 ·6 ·8 }-kgd net topology. The 2D
layers are further stacked together through π−π interactions
between the porphyrin linkers to generate a 3D supramolecular
DMF molecules were treated with the SQUEEZE option of
2
2
PLATON multipurpose crystallographic software. All the hydrogen
atoms were fixed by HFIX and placed in ideal positions and included
in the refinement process using riding model with isotropic thermal
parameters. All the crystallographic and structure refinement data of
MOF1 are summarized in Table 1. Selected bond lengths and angles
3
6
6
3
framework which houses 1D channels with dimension of ∼5.0
2
Table 1. Crystallographic Data for MOF1
×
9.0 Å running along the crystallographic a-axis. Compound
1
shows an intense absorption band in the visible region at 421
parameters
formula
MOF1
nm due to ligand based charge transfer transitions (Figure S1).
C108H N O Ni
86
12 26
5
Visible-light-assisted photocatalytic investigation of MOF1 for
fw
2261.37
reduction of nitrobenzene using NaBH has hydrogen source at
4
cryst syst
space group
a/Å
triclinic
room temperature resulted aniline with high yield (>99%). The
scope of this reaction was further extended to the reduction of
other substituted mono- and dinitroaromatics. To the best of
our knowledge, compound 1 constitutes the first example of
MOF based on 3d metal ion exhibiting visible-light-assisted
photocatalytic reduction of nitroaromatics without the use of
noble metal cocatalysts.
P1
̅
10.1116(4)
16.0288(7)
17.8163(7)
92.999(2)
102.940(2)
101.055(2)
2748.3(2)
1
b/Å
c/Å
α/°
β/°
γ/°
V/Å3
EXPERIMENTAL SECTION
Materials. All the chemicals employed were commercially available
and used without further purification. Ni(NO ) ·6H O, NiCl ·6H O,
■
Z
−3
D /g cm
1.278
c
3
2
2
2
2
−1
μ(Mo Kα)/mm
F(000)
0.911
4
3
-nitrotoluene, 1-chloro-4-nitrobenzene, 1-bromo-4-nitrobenzene, 1,
-dinitrobenzene, 2, 6-dinitrotoluene were obtained from Sigma-
1086
cryst size/mm
T (K)
0.20 × 0.26 × 0.32
293
Aldrich chemical Co. Pyrrole, methyl 4-formyl benzoate, propionic
acid, and 1-nitronaphthalene were obtained from TCI Chemicals.
Nitrobenzene, N,N-dimethylformamide (DMF), methanol, and
tetrahydrofuran (THF) were obtained from Spectrochem. Ni−
H TCPP metallo-ligand and H TCPP ligand were synthesized by
λ(Mo Kα) (Å)
θ range/°
reflns collected
0.71073
1.8 to 28.3
75000
4
4
16
following the previously reported procedure.
independent reflns
13568
Measurements. Thermogravimetric analysis (TGA) was carried
R(int)
0.069
out using a Mettler Toledo thermogravimetric analyzer in a nitrogen
GOF on F2
1.048
−1
atmosphere (flow rate = 50 mL min ) in the temperature range of
data [I > 2σ(I)]
9531
−1
2
(
5−900 °C (heating rate = 10 °C min ). Powder X-ray diffraction
a
R1 [I ≥ 2σ(I)]
wR [I ≥ 2σ(I)]
2
0.0712
XRD) patterns of the compounds were recorded by using Cu Kα
b
0.2279
radiation (λ = 1.542 Å; 40 kV, 20 mA) with PANalytical’s X’PERT
1
a
b
2
2 2
c
PRO diffractometer. H NMR spectra were recorded on a JEOL
R = Σ ∥ F | − | F ∥/Σ | Fo |. wR = [Σ[w(F − F ) ]/
1
o
c
2
o
2
o
2 1/2
spectrometer at 400 MHz in deuterated chloroform. GC-MS analyses
were done on Shimadzu, GCMS-QP2010 Ultra Gas chromatograph
mass spectrometer. Diffuse reflectance spectra were measured with
respect to BaSO4 reference using an integrating sphere detector
equipped in a Shimadzu UV−visible spectrophotometer and converted
Σw(F ) ]
are given in Table S1. The selected hydrogen bond interactions
involving the free carboxylic acid group and the coordinated water
molecules are summarized in Table S2. CCDC1439596 (for 1)
contains the supplementary crystallographic data for this paper. This
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
into absorbance (KM units) using Kubelka−Munk function. N
2
adsorption measurements were carried out using Quantachrome
Quadrasorb-SI surface area analyzer with high purity (99.995%) N2
gas. XPS measurements were recorded on X-ray Photoelectron
Spectrometer (Omicron) using Al Kα radiation. The data were
acquired with pass energy of 100 eV, for the survey scans and 25 eV
for the core-levels, with a resolution of 0.1 eV at 90% of the peak
height. The charging effect were corrected by taking C(1s) core-level
peak at 284.5 eV binding energy reference. After doing Shirley
background corrections, Gaussian deconvolution for Ni (2p) and
O(1s) peaks was performed to probe different electronic states of Ni.
Fluorescence spectra were recorded at room temperature on a Perkin
Elmer LS55 fluorescence spectrophotometer. Scanning electron
microscopy (SEM) images were recorded using JEOL JSM-6610LV
SEM facility.
Synthesis of MOF1. A mixture of [Ni(NO ) ]·6H O (14.5 mg,
0.05 mmol), H TCPP ligand (16.0 mg, 0.02 mmol), and DMF (3 mL)
3
2
2
4
was stirred for 15 min in a Teflon vial. Subsequently, water (2 mL) was
added dropwise, and stirring was continued for another 15 min. The
vial was sealed into a stainless steel autoclave which was kept in a
preheated oven at 100 °C for 3 days. After the vial was cooled to room
temperature, red-colored rod-like crystals were obtained in 68% yield
based on H TCPP ligand.
4
Photocatalytic Reduction of Nitroaromatics. In a typical
reaction, 0.2 mmol of substrate, 2 mol % of as-synthesized MOF1, and
X-ray Crystallography. Single-crystal X-ray structural data of
MOF1 was collected on a Bruker D8 Venture PHOTON 100 CMOS
19 mg (0.5 mmol) of NaBH were dispersed in 5 mL of ethanol in a
10 mL pyrex glass reactor sealed with a rubber stopper and exposed to
4
B
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Article
visible light from commercially available three white LED lamps (15 W
each) with stirring at room temperature. The temperature of the
mixture was kept constant by circulating water through the outer
jacket of the reactor. The progress of the reactions was monitored by
GC-MS analysis of the aliquots taken at certain time intervals.
central Ni(II) atom (Ni3) of Ni cluster occupies an inversion
center with distorted octahedral geometry and coordinated by
3
four carboxylate oxygen (O8, O8a, O5 and O5a; a = −x, −y,
−
z) atoms placed in the equatorial plane. The axial positions
are occupied by oxygen (O9 and O9a; a = −x, −y, −z) atoms
of two bridging water molecules (Figure S3). The terminal
RESULTS AND DISCUSSION
Synthesis and Crystal Structure of [Ni (Ni-HTCPP) (μ -
■
Ni(II) ion (Ni2) of Ni cluster is also in a distorted octahedral
3
3
2
2
geometry with NiO chromophore satisfied by two carboxylate
6
H O) (H O) (DMF) ]·2DMF (MOF1). Solvothermal treatment
2
2
2
4
2
oxygen (O1 and O7) atoms and an oxygen (O12) atom of a
coordinated DMF molecule and three oxygen (O9, O10, and
O11) atoms from a bridging water molecule and two terminal
water molecules, respectively. The Ni2−O and Ni3−O bond
lengths are in the range of 2.006(3)−2.094(3) Å and
of Ni(NO ) ·6H O and H TCPP ligand in a mixed solvent
3
2
2
4
(
DMF/Water) at 100 °C yielded red-colored rod-like crystals
of MOF1. The phase purity of the as-prepared sample was
confirmed by X-ray powder diffraction analysis (Figure S2).
Single-crystal X-ray structure determination revealed that
MOF1 crystallizes in triclinic crystal system with the
2
.025(3)−2.0988(3) Å, respectively (Table S1). Ni2 and Ni3
ions are bridged by two carboxylate oxygen (O7 and O8) atoms
of porphyrin ligand and an oxygen (O9) atom of a water
centrosymmetric, P1
̅
space group and exhibits a 2D network
structure, and it is isostructural to the previously reported
2
3
molecule forming a Ni O16 cluster (Figure S3). Out of four
3
MOF. The asymmetric unit consists of one and half Ni(II)
3−
carboxylate groups of [Ni-HTCPP] metalloligand, two are
3
−
ions, one [Ni-HTCPP] metalloligand, one coordinated DMF,
two coordinated water molecules and one bridging water
molecule including two guest DMF molecules (Figure 1). The
coordinating to each Ni2 and Ni3 in a monodentate μ -O
1
fashion, the third carboxylate group bridges Ni2 and Ni3
through μ , μ -OO-fashion, and the fourth one is protonated
1
1
(
H ) (Figure 2 and S4). If we consider the Ni -oxo cluster as
4 3
single node, then it is connected to six other nodes through
four [Ni-HTCPP]3 SBUs which are acting as three connecting
node resulting a (3, 6)-connected binodal net (Figure 2a). The
Ni(II) nodes are further extended in two dimensions to
generate a 2D network (Figure 2b). These 2D layers are
stacked with each other through π−π interactions to form a 3D
supramolecular framework which houses 1D channels lined
−
with free carboxylic acid groups with a dimension of ∼5.0 × 9.0
2
Å
running along crystallographic a-axis (Figure 3, S5).
24
Topological analysis by TOPOS suggests the presence of
(
{
3, 6)-connected binodal net and the overall structure has
4 }2{4 ·6 ·8 }-kgd topology (Figure 3b). The structure
3
6
6
3
possesses a solvent accessible void volume of ∼27.3%
3
22
(
2748.3 Å ) per unit cell volume calculated using PLATON
after the removal of guest DMF molecules.
N Adsorption Studies and SEM Analysis. As mentioned
2
before, single-crystal structural determination of MOF1
revealed that the compound is microporous with a pore
Figure 1. ORTEP drawing of the asymmetric unit for the [Ni (Ni-
3
2
diameter of ∼5.0 × 9.0 Å . For further confirmation of
HTCPP) (μ -H O) (H O) (DMF) ]·2(DMF) MOF1; ellipsoids are
2
2
2
2
2
4
2
microporosity of 1, N adsorption measurements were carried
displayed at the 50% probability level. Hydrogen atoms and guest
DMF molecules are omitted for clarity.
2
out at 77 K. Prior to the adsorption measurements the sample
Figure 2. (a) View of the coordination mode of [Ni-HTCPP]3 metalloligand and 6-connected Ni -oxo node. (b) Two-dimensional network
−
3
structure of MOF1.
C
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Inorganic Chemistry
Article
Figure 3. (a) Perspective view of 3D supramolecular framework of MOF1 showing 1D channels along the a-axis. (b) Topological representation of
the 2D network of MOF1.
(
2
(
∼0.13 g) was activated at 353 K under vacuum (18 mTorr) for
0 h. The adsorption measurements shows a type II isotherm
Figure S6) with a hysteresis behavior upon desorption which
can be attributed to the framework flexibility and hindered
Table 2. Photocatalytic Reduction of Various Nitroaromatics
Catalyzed by MOF1
25
diffusion through narrow pore apertures. The estimated BET
2
−1
surface area of MOF1 was found to be 80.4 m g . In order to
check the particle size and morphology of the sample we
recorded the SEM images of the as-synthesized sample of
MOF1 (Figure S7). SEM images show rod-shape morphology
with a particle size of 1−10 μm. This observation is also in
accordance the estimated low value of BET surface area as
discussed before.
Photocatalytic Reduction of Nitroaromatics by MOF1.
To test the visible-light photocatalytic application of MOF1 for
the reduction of nitroaromatics into corresponding amines, we
carried out several controlled experiments as shown in Scheme
1. Interestingly, we noticed that all the three components (i.e.,
Scheme 1. Various Conditions Screened for the Reduction of
Nitrobenzene
a
b
Conversion determined by GC-MS. TOF: mmol of substrate/mmol
c
of catalyst × time. 5% of aniline and 95% of 4-chloroaniline were
formed. 8% of aniline and 92% of 4-bromoaniline were formed.
d
visible light, hydrogen source, and catalyst (MOF1)) were
necessary for the reduction of nitroaromatics, and in the
absence of any one of the three components, formation of the
product did not occur. To our delight, in the presence of visible
the catalytic activity of the Ni(II)-centered porphyrin
tetracarboxylic acid (Ni−H TCPP) metalloligand and the
4
metal-free porphyrin tetra carboxylic acid (H TCCP) ligand
4
light irradiation, MOF1 as catalyst and NaBH as hydrogen
were investigated using similar conditions used for MOF1
(Scheme 1e, f). Surprisingly, these reactions did not show the
reduction of nitrobenzene even after 24 h of reaction time. This
4
source, the nitrobenzene was reduced to aniline with almost
100% conversion within 1.5 h (Scheme 1d, Table 2). Further,
D
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Inorganic Chemistry
Article
observation clearly suggests the inability of Ni(II)-centered
porphyrin complex and the metal-free ligand to reduce
nitrobenzene and highlight the importance of MOF1 in
catalyzing the reaction (Table 2). Furthermore, thorough
literature survey revealed that there have been no reports of
Ni(II)-centered porphyrin complexes or MOFs known for
visible-light photocatalytic reduction of nitroaromatics reported
so far. Therefore, the remarkable photocatalytic activity of
MOF1 for the reduction of nitrobenzene to aniline can be
attributed to its 3D supramolecular structure constituted by the
close stacking and organized arrangement of 2D layers due to
π−π interactions resulting in cooperative redox reaction
between the porphyrin and the Ni(II)-oxo connecting node.
Here the porphyrin SBU acts as a photosensitizer delevering
redox equivalents to the Ni-oxo node which serves as a catalyst
for the reduction of electron-deficient nitroaromatics in the
Scheme 2. Plausible Mechanistic Pathway Involved in the
Reduction of Nitrobenzene by MOF1
presence of sacrificial electron donor, NaBH , which is not
4
favorable in the case of [Ni−H TCPP] molecular complex, as
4
depicted in the Figure 4. In support of this hypothesis, the
under optimized reaction conditions. The results showed that
there is not any electronic effect of substituents on the
photocatalytic reduction of nitroaromatics to corresponding
amines (Table 2, entries 2−4). In the case of 4-chloro/bromo-
nitrobenzenes, partial dehalogenation was observed (Table 2,
entries 3 and 4). Remarkably, dinitroaromatics were converted
to the corresponding diamines with high conversion and
selectivity. Furthermore, to examine whether the catalytic
reaction take place inside the pore of the MOF, the reaction
was carried out with a relatively bulky substrate, 1-nitro-
naphthalene which resulted the corresponding amine with only
about 35% conversion even after 24 h (Table 2). The lower
catalytic conversion of 1-nitronaphthalene can be attributed due
to the surface catalysis with the Ni-oxo nodes exposed to the
surface of the MOF and its restricted diffusion into the narrow
pores of MOF1.
From the previous discussion, it is clear that the present
method can be utilized for the synthesis of diamines, which are
very important building blocks in organic synthesis. Further-
more, the heterogeneity of the catalyst was evaluated to
determine whether the reaction is catalyzed by species in the
solution phase. To address this issue, we carried out a separate
reaction in which MOF1 was separated out after 20 m, and the
conversion of nitrobenzene was found to be ∼32%. At this
stage, the catalyst was separated from the reaction mixture by
filtration, and the reaction was continued with the filtrate for an
additional 1 h; the conversion of nitrobenzene remained almost
unchanged, suggesting the heterogeneity of the catalyst (Figure
5a). Furthermore, the catalyst (MOF1) was recycled and
reused up to three cycles, and the activity of the recovered
catalyst does not decrease significantly with retaining the
original structure (Figure 5b and S2).
Figure 4. Pictorial representation of reduction of nitroaromatics
catalyzed by MOF1.
fluorescence measurement of MOF1 in comparison to the
porphyrin ligand revealed quenching in the emission of MOF1
compared to that of porphyrin ligand due to efficient charge
transfer from porphyrin SBU to the Ni-oxo node (Figure S8).
Hence the overall catalytic mechanism involves transfer of
photoexcited electron from porphyrin to Ni-oxo node of
MOF1 at which the reduction of nitroaromatics take place and
the electron deficiency is simultaneously filled by NaBH acting
4
26
as sacrificial electron donor (Scheme 2). As shown in Scheme
the HOMO and LUMO orbital energies of [Ni−H TCPP]
2
4
and nitrobenzene were calculated by the density functional
theory at the B3LYP/6-31G(d) level. The calculated value of
the energy difference (2.95 ev) between the HOMO−LUMO
of MOF1 is in good agreement with the measured value (2.80
ev) from the UV−vis absorption data (Figure S9) determined
Two possible mechanistic pathways have been proposed for
the reduction of nitrobenzene and substituted analogues by
28
Haber (Scheme. S1). To investigate the mechanistic pathway
for the reduction of nitroaromatics, several controlled experi-
ments were performed. The reduction reaction of two
intermediates of different pathways, phenylhydroxylamine
(direct route) and azobenzene (condensation route) was
carried out under optimized conditions (Scheme S2). The
complete conversion of phenylhydroxylamine and azobenzene
to aniline was observed within 10 min suggesting the possibility
of both direct and condensation pathways for the conversion of
2
from the plot of (αhν) vs photon energy (hν) following the
2
7
Tauc relationship.
After the optimization of reaction conditions, the scope of
the reaction was extended to the reduction of more challenging
(
2
structurally and electronically different) nitroaromatics (Table
). Nitroaromatics with electron-donor (−CH ) and electron-
3
acceptor (−Cl and −Br) as well as dinitroaromatics were tested
E
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Figure 5. Catalytic reduction of nitrobenzene with time. (a) MOF1 was removed by filtration after 20 m of catalytic reaction. (b) Recycling test.
Table 3. Photocatalytic Reduction of Nitrobenzene by MOF1 Analyzed at Different Time Intervals
time (min)
% nitrobenzene
% aniline
% azobenzene
% azoxybenzene
% nitrosobenzene
1
2
6
8
9
0
0
0
0
0
80
65
18
5
-
-
-
-
-
20
10
-
25
61
75
-
13
15
>99
8
5
-
-
-
-
nitrobenzene to aniline using MOF1 as visible-light photo-
catalyst. Moreover, the condensation pathway was further
supported by the observation of azobenzene and azoxybenzene
intermediates during the course of reduction of nitrobenzene
confirmed by GC-MS analysis of aliquots taken at different time
intervals (Table 3). Based on the results mentioned above we
propose that the visible-light-responsive photocatalytic reduc-
tion of nitroaromatics using MOF1 proceeds via both the
possible pathways (Scheme S2).
X-ray Photoelectron Spectroscopy Studies. In order to
probe the electronic state of the Ni and also to rule out the
possibility of formation of Ni(O) metal nanoparticles during
the catalysis, XPS measurement was performed on the recycled
sample of MOF1 after three catalytic cycles. To quantify the
presence of different states in Ni, the core level spectra of
Ni(2p) and O(1s) were deconvoluted into different related
Gaussian components (Figure S10). The Gaussian fitting of Ni
of porphyrin-based MOFs which exhibit high chemical stability
15
have been reported in the literature.
Thermal Stability of MOF1. Thermogravimetric analysis
of MOF1 shows an initial weight loss of ∼11.8% in the
temperature regime 25−155 °C corresponding to the loss of
two guest DMF molecules (ca. 12 wt %) (Figure S11). The
second weight loss of ∼9.6% in the temperature range of 155−
2
50 °C corresponds to th eloss of one coordinated DMF and
two coordinated water molecules (ca. 9.2 wt %), and the
desolvated framework is stable up to 400 °C. Above 400 °C,
the compound decomposes with loss of framework structure
(
Figure S11).
CONCLUSIONS
■
In conclusion, a microporous, Ni(II)-porphyrin-based 3D
supramolecular MOF has been synthesized by solvothermal
route, and its application as visible-light-responsive photo-
catalyst for heterogeneous reduction of nitroaromatics at room
temperature has been demonstrated. Remarkably, MOF1
represents the first example of MOF based on 3d metal ion
exhibiting visible-light-assisted photocatalytic reduction of
nitroaromatics under mild conditions without the assistance
of noble metal cocatalysts. The catalyst can be recycled up to
three times without any significant loss of catalytic activity as
well as structural rigidity.
(
+
2p ) spectrum shown in Figure S10a divulges the presence of
2 state at the binding energy of 873.8 eV. While the fitted
1/2
29
peak at binding energy of 880.0 eV can be ascribed to shakeup
3
0
satellite. For further confirmation, we have deconvoluted
O(1s) core level peak which revealed the presence of three
different peaks attributed to the Ni−O bond (∼528.8 eV), C−
O (∼533.3 eV), and O−O bond (∼531.0 eV), which could be
arising due to contamination by atmospheric oxygen (Figure
S10b). Overall, the deconvolution of Ni(2p) and O(1s) core
level spectra manifests the presence of Ni in +2 state, and no
additional peak was found corresponding to Ni(O), which rules
out the formation of nickel nanoparticles during the catalysis.
The exceptional stability of MOF1 under catalytic condition
could be arising due to its rigid 3D supramolecular arrangement
originating by close stacking of 2D layers through π−π
interactions between the porphyrin SBUs. Similarly, examples
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00296.
Detailed experimental procedures, tables of crystallo-
graphic data, experimental and simulated PXRD patterns
F
DOI: 10.1021/acs.inorgchem.6b00296
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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XPS plots (PDF)
Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
E-mail: cmnraja@iitrpr.ac.in. Website: http://www.iitrpr.ac.
■
*
in/chemistry/nagaraja.
Notes
The authors declare no competing financial interest.
(m) Nasalevich, M. A.; Goesten, M. G.; Savenije, T. J.; Kapteijn, F.;
Gascon, J. Chem. Commun. 2013, 49, 10575. (n) Sun, X.; Yu, Q.;
Zhang, F.; Wei, J.; Yang, P. Catal. Sci. Technol. 2016 DOI: 10.1039/
c5cy01716e.
ACKNOWLEDGMENTS
■
C.M.N. gratefully acknowledges the financial support from the
Department of Science and Technology (DST), Government
of India (Fast Track Proposal). The authors thank Prof. Shiva
Prasad for the XPS data and Dr. Praveen Kumar for the help
with data analysis. M.S.D. and N.S. are thankful to the Council
of Scientific and Industrial Research (CSIR), Government of
India for the JRF.
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DOI: 10.1021/acs.inorgchem.6b00296
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