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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), [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 Å2 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.

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to aniline.8 Furthermore, Chen et al. demonstrated the application of Pd encapsulated UiO-67 MOF for reduction of nitrobenzene.9 Recently, Mukherjee and co-workers have demonstrated the application of Au-doped Zn(II)-MOFs for reduction of nitrophenols.10 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 CO2 reduction, H2 evolution, and so on.11 However, visible-light-assisted reduction of nitroaromatics by MOFs has been rarely studied. In this regard, porphyrin-based molecular complexes12 and MOFs containing metallo-porphyrin SBUs have attracted considerable attention due to their diverse applications in light-harvesting13 and visible-light-assisted photocatalysis,14 including photocatalytic redox reactions.15 Motivated by these studies, we have investigated the synthesis and visible-light photocatalytic

INTRODUCTION 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 national security and environmental safety.1 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.2 The conventional synthesis of amines following the Bechamp3 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.4 In this regard, photocatalytic reduction of nitroaromatics employing visible-light photocatalysts has attracted considerable attention. Metal−organic frameworks (MOFs) or porous coordination polymers (PCPs) have received growing interest as suitable candidates for carrying out heterogeneous catalysis due to their high surface areas, tunable pore size, and functionality.5 Extensive research efforts are being carried out on development of luminescent MOFs for sensing of nitroaromatics.6 However, the application of MOFs as heterogeneous catalysts for reduction of nitroaromatics has been scarcely studied.7 In this context, Matsuoka and co-workers have reported Pt loaded TiNH2−BDC MOF for photocatalytic reduction of nitrobenzene © XXXX American Chemical Society

Received: February 3, 2016

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DOI: 10.1021/acs.inorgchem.6b00296 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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 SAINT17 was used for integration of diffraction profiles, and absorption correction was made with SADABS18 program. The structure was solved by SIR 9219 and refined by full matrix least-square method using SHELXL-201320 and WinGX system, Ver 2013.3.21 All the non-hydrogen atoms were located from the difference Fourier map and refined anisotropically. The disordered DMF molecules were treated with the SQUEEZE option of PLATON22 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

application of porphyrin MOF containing 3d metal ion for reduction of nitroaromatics without the assistance of noble metal cocatalysts. Herein, we report construction of a Ni(II)-MOF, [Ni3(NiHTCPP)2(μ2-H2O)2(H2O)4(DMF)2]·2DMF (MOF1), constituted by Ni3 cluster and [Ni-HTCPP]3− metalloligand (where [Ni-HTCPP]3− represents [5,10,15,20-tetrakis(4-benzoate) 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 {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 Å2 running along the crystallographic a-axis. Compound 1 shows an intense absorption band in the visible region at 421 nm due to ligand based charge transfer transitions (Figure S1). Visible-light-assisted photocatalytic investigation of MOF1 for reduction of nitrobenzene using NaBH4 has hydrogen source at 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.

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Table 1. Crystallographic Data for MOF1

EXPERIMENTAL SECTION

Materials. All the chemicals employed were commercially available and used without further purification. Ni(NO3)2·6H2O, NiCl2·6H2O, 4-nitrotoluene, 1-chloro-4-nitrobenzene, 1-bromo-4-nitrobenzene, 1, 3-dinitrobenzene, 2, 6-dinitrotoluene were obtained from SigmaAldrich 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− H4TCPP metallo-ligand and H4TCPP ligand were synthesized by following the previously reported procedure.16 Measurements. Thermogravimetric analysis (TGA) was carried out using a Mettler Toledo thermogravimetric analyzer in a nitrogen atmosphere (flow rate = 50 mL min−1) in the temperature range of 25−900 °C (heating rate = 10 °C min−1). Powder X-ray diffraction (XRD) patterns of the compounds were recorded by using Cu Kα radiation (λ = 1.542 Å; 40 kV, 20 mA) with PANalytical’s X’PERT PRO diffractometer. 1H NMR spectra were recorded on a JEOL 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 into absorbance (KM units) using Kubelka−Munk function. N2 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. X-ray Crystallography. Single-crystal X-ray structural data of MOF1 was collected on a Bruker D8 Venture PHOTON 100 CMOS

parameters

MOF1

formula fw cryst syst space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Z Dc/g cm−3 μ(Mo Kα)/mm−1 F(000) cryst size/mm T (K) λ(Mo Kα) (Å) θ range/° reflns collected independent reflns R(int) GOF on F2 data [I > 2σ(I)] R1 [I ≥ 2σ(I)]a wR2 [I ≥ 2σ(I)]b

C108H86N12O26Ni5 2261.37 triclinic P1̅ 10.1116(4) 16.0288(7) 17.8163(7) 92.999(2) 102.940(2) 101.055(2) 2748.3(2) 1 1.278 0.911 1086 0.20 × 0.26 × 0.32 293 0.71073 1.8 to 28.3 75000 13568 0.069 1.048 9531 0.0712 0.2279

a R1 = Σ ∥ Fo | − | Fc ∥/Σ | Fo |. bwR2 = [Σ[w(Fo2 − Fc2)2]/ Σw(Fo2)2]1/2

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. Synthesis of MOF1. A mixture of [Ni(NO3)2]·6H2O (14.5 mg, 0.05 mmol), H4TCPP ligand (16.0 mg, 0.02 mmol), and DMF (3 mL) 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 H4TCPP ligand. Photocatalytic Reduction of Nitroaromatics. In a typical reaction, 0.2 mmol of substrate, 2 mol % of as-synthesized MOF1, and 19 mg (0.5 mmol) of NaBH4 were dispersed in 5 mL of ethanol in a 10 mL pyrex glass reactor sealed with a rubber stopper and exposed to B


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Inorganic Chemistry 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 Ni3 cluster occupies an inversion center with distorted octahedral geometry and coordinated by 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 Ni(II) ion (Ni2) of Ni3 cluster is also in a distorted octahedral geometry with NiO6 chromophore satisfied by two carboxylate 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 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 molecule forming a Ni3O16 cluster (Figure S3). Out of four carboxylate groups of [Ni-HTCPP]3− metalloligand, two are coordinating to each Ni2 and Ni3 in a monodentate μ1-O fashion, the third carboxylate group bridges Ni2 and Ni3 through μ1, μ1-OO-fashion, and the fourth one is protonated (H4) (Figure 2 and S4). If we consider the Ni3-oxo cluster as 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 Å2 running along crystallographic a-axis (Figure 3, S5). Topological analysis by TOPOS24 suggests the presence of (3, 6)-connected binodal net and the overall structure has {43}2{46·66·83}-kgd topology (Figure 3b). The structure possesses a solvent accessible void volume of ∼27.3% (2748.3 Å3) per unit cell volume calculated using PLATON22 after the removal of guest DMF molecules. N2 Adsorption Studies and SEM Analysis. As mentioned before, single-crystal structural determination of MOF1 revealed that the compound is microporous with a pore diameter of ∼5.0 × 9.0 Å2. For further confirmation of microporosity of 1, N2 adsorption measurements were carried out at 77 K. Prior to the adsorption measurements the sample

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RESULTS AND DISCUSSION Synthesis and Crystal Structure of [Ni3(Ni-HTCPP)2(μ2H2O)2(H2O)4(DMF)2]·2DMF (MOF1). Solvothermal treatment of Ni(NO3)2·6H2O and H4TCPP ligand in a mixed solvent (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 centrosymmetric, P1̅ space group and exhibits a 2D network structure, and it is isostructural to the previously reported MOF.23 The asymmetric unit consists of one and half Ni(II) ions, one [Ni-HTCPP]3− metalloligand, one coordinated DMF, two coordinated water molecules and one bridging water molecule including two guest DMF molecules (Figure 1). The

Figure 1. ORTEP drawing of the asymmetric unit for the [Ni3(NiHTCPP)2(μ2-H2O)2(H2O)4(DMF)2]·2(DMF) MOF1; ellipsoids are displayed at the 50% probability level. Hydrogen atoms and guest DMF molecules are omitted for clarity.

Figure 2. (a) View of the coordination mode of [Ni-HTCPP]3− metalloligand and 6-connected Ni3-oxo node. (b) Two-dimensional network structure of MOF1. C


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Inorganic Chemistry

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.

(∼0.13 g) was activated at 353 K under vacuum (18 mTorr) for 20 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 diffusion through narrow pore apertures.25 The estimated BET surface area of MOF1 was found to be 80.4 m2 g−1. 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.,

Table 2. Photocatalytic Reduction of Various Nitroaromatics Catalyzed by MOF1

Scheme 1. Various Conditions Screened for the Reduction of Nitrobenzene

a Conversion determined by GC-MS. bTOF: mmol of substrate/mmol of catalyst × time. c5% of aniline and 95% of 4-chloroaniline were formed. d8% of aniline and 92% of 4-bromoaniline were formed.

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 light irradiation, MOF1 as catalyst and NaBH4 as hydrogen source, the nitrobenzene was reduced to aniline with almost 100% conversion within 1.5 h (Scheme 1d, Table 2). Further,

the catalytic activity of the Ni(II)-centered porphyrin tetracarboxylic acid (Ni−H4TCPP) metalloligand and the metal-free porphyrin tetra carboxylic acid (H4TCCP) ligand 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 D


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Inorganic Chemistry 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 presence of sacrificial electron donor, NaBH4, which is not favorable in the case of [Ni−H4TCPP] molecular complex, as depicted in the Figure 4. In support of this hypothesis, the

Scheme 2. Plausible Mechanistic Pathway Involved in the Reduction of Nitrobenzene by MOF1

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/bromonitrobenzenes, 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-nitronaphthalene 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. Furthermore, 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). Two possible mechanistic pathways have been proposed for the reduction of nitrobenzene and substituted analogues by Haber (Scheme. S1).28 To investigate the mechanistic pathway for the reduction of nitroaromatics, several controlled experiments 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

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 NaBH4 acting as sacrificial electron donor (Scheme 2).26 As shown in Scheme 2 the HOMO and LUMO orbital energies of [Ni−H4TCPP] 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 from the plot of (αhν)2 vs photon energy (hν) following the Tauc relationship.27 After the optimization of reaction conditions, the scope of the reaction was extended to the reduction of more challenging (structurally and electronically different) nitroaromatics (Table 2). Nitroaromatics with electron-donor (−CH3) and electronacceptor (−Cl and −Br) as well as dinitroaromatics were tested E


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Inorganic Chemistry

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

10 20 60 80 90

80 65 18 5 -

13 15 >99

8 5 -

25 61 75 -

20 10 -

nitrobenzene to aniline using MOF1 as visible-light photocatalyst. 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 reduction 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 (2p1/2) spectrum shown in Figure S10a divulges the presence of +2 state at the binding energy of 873.8 eV.29 While the fitted peak at binding energy of 880.0 eV can be ascribed to shakeup satellite.30 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

of porphyrin-based MOFs which exhibit high chemical stability have been reported in the literature.15 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− 250 °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).

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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 photocatalyst 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.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00296. Detailed experimental procedures, tables of crystallographic data, experimental and simulated PXRD patterns F


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Inorganic Chemistry

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for the compounds, TGA, UV−vis, fluorescence, and XPS plots (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Website: http://www.iitrpr.ac. in/chemistry/nagaraja. Notes

The authors declare no competing financial interest.

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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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