Polythiophene-Encapsulated Bimetallic Au-Fe3O4 Nano-Hybrid

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Polythiophene-Encapsulated Bimetallic Au-Fe3O4 Nano-Hybrid Materials: A Potential Tandem Photocatalytic System for Nondirected C(sp2)−H Activation for the Synthesis of Quinoline Carboxylates Mandeep Kaur, Subhamay Pramanik, Manoj Kumar, and Vandana Bhalla* Department of Chemistry, UGC Sponsored Centre for Advanced Studies-II, Guru Nanak Dev University, Amritsar 143005, Punjab, India S Supporting Information *

ABSTRACT: Hetero-oligophenylene derivative 3 appended with thiophene moieties has been designed and synthesized which undergoes aggregation to form J-type fluorescent aggregates in in H2O/THF (7/3) media. These aggregates served as reactors for the preparation of bimetallic Au-Fe3O4 NPs. During the reduction process, aggregates of derivative 3 were oxidized to the polythiophene species 4. Interestingly, the polythiophene species 4, having a fibrous morphology, served as a shape- and morphology-directed template for assembly of bimetallic Au-Fe 3 O 4 NPs in a flower-like arrangement. Furthermore, polythiophene-encapsulated bimetallic 4:AuFe3O4 nanohybrid materials served as an efficient and recyclable catalytic system for C(sp2)−H bond activation of unprotected electron-rich anilines for the construction of synthetically versatile quinoline carboxylates via C−H activation, carbonylation, and subsequent annulation under mild and eco-friendly conditions (aqueous media, room temperature, visible-light irradiation, and aerial conditions). KEYWORDS: hetero-oligophenylene, bimetallic Au-Fe3O4 nanoflower, polythiophene, C(sp2)−H activation, photocatalyst, quinoline carboxylates

1. INTRODUCTION Quinoline carboxylates are valuable synthons for the preparation of various derivatives having applications in supramolecular and materials chemistry.1 These derivatives are also known for their antimalarial, antimicrobial, anti-HIV, anti-inflammatory, anti-TB, anticancer, antibiotic, and antihypertensive activities.2 The versatile applications of quinoline carboxylates provide sufficient impetus for the development of new approaches for convenient synthesis of these derivatives.3 Among various approaches, transition-metal-catalyzed C−H functionalization has emerged as a powerful synthetic strategy for the preparation of quinoline derivatives.4 This strategy is still growing, and in the recent past, various catalytic systems have been developed for the indirect synthesis of quinoline derivatives from protected anilines and alkynes.5−7 Unfortunately, the literature related to the direct synthesis of quinoline 3-carboxylates from readily available unprotected aniline is scarce.8 Our research interest focuses on the development of catalytic/photocatalytic supramolecular ensembles of fluorescent materials and metal nanoparticles for carrying out various carbon−carbon and carbon−heteroatom bond formation reactions under mild and eco-friendly conditions. In a continuation of our efforts in this direction, we planned to develop an efficient, recyclable catalytic system for the © 2017 American Chemical Society

preparation of quinoline carboxylates from readily available substrates: i.e., unprotected anilines. Very recently, [Rh(cod)Cl]2-catalyzed reactions between unprotected anilines and electron-deficient alkynes in the presence of a CO source have been reported for the development of quinoline derivatives.9 However, this approach suffers from limitations such as utilization of costly and nonrecyclable Rh catalysts, prolonged heating at high temperature, and requirement of an inert atmosphere, which essentially decreased the environmental and economic advantages of the strategy. Above all, the recyclability of the system was not explored. To enhance the environmental advantages of C−H functionalization reactions, we planned to develop a recyclable photocatalytic system which could harvest solar radiation to synthesize quinoline derivatives from unsubstituted aniline under mild conditions. Recently, we reported aggregates of perylene bisimide (PBI) derivative stabilized Fe3O4 NPs as an efficient, recyclable catalytic system for the preparation of quinoline derivatives through one-pot multicomponent reactions among benzaldehyde, aniline derivatives, and phenylacetylene via C−H activation under Received: September 19, 2016 Revised: January 19, 2017 Published: January 31, 2017 2007

DOI: 10.1021/acscatal.6b02681 ACS Catal. 2017, 7, 2007−2021

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ACS Catalysis thermal conditions.10 Amazingly, this catalytic system could be reused up to five cycles without significant loss in the yield of final product. We examined the efficiency of this catalytic system in the reaction among unprotected anilines, benzaldehyde, and alkynes under photocatalytic conditions (vide infra). Unfortunately, the reaction did not proceed and the target compound was not obtained. We believe that limited energy transfer between aggregates of PBI derivatives and Fe3O4 NPs and the inability of Fe3O4 NPs to activate the electron-rich ortho hydrogen of aniline are the possible reasons for the failure of this reaction. To overcome these limitations, we then planned to develop a photocatalytically active bimetallic catalytic system by mixing plasmonic metal species and iron oxide nanoparticles. We envisioned that, in the proposed bimetallic system, plasmonic metal species in combination with supramolecular aggregates will harvest solar energy to carry out the reaction and the Fe3O4 NPs will ensure the recyclability of the system. For plasmonic material, gold is our metallic species of choice due to its benign nature and its known catalytic efficiency in C(sp2)−H functionalization reactions.11 Thus, we designed and synthesized the thiophene-appended heterooligophenylene derivative 3. We expected derivative 3 to form supramolecular aggregates in mixed aqueous media due to the presence of multiple rotors. Furthermore, the presence of thiophene moieties could enhance its affinity toward Au3+ and Fe3+ ions.12 During the binding, supramolecular aggregates of derivative 3 acted as reducing agents for preparation of bimetallic Au-Fe3O4 NPs and themselves became oxidized to polythiophene species which gratifyingly served as shapedirecting nanoreactors for the assembly of bimetallic Au-Fe3O4 NPs in a flower-like arrangement. Recent studies have shown that a nanoflower-like arrangement of a bimetallic nanoarchitecure is appropriate for offering good activity.13 In this context, confinement of metallic species in nanoreactors and their subsequent reduction to form alloy nanoparticles has emerged as a facile strategy for arranging multimetallic nanoparticles in a desired manner.14 However, this method of generation of confined alloy nanoparticles suffers from limitations such as the requirement of long preparation times and annealing at high temperature.15 In comparison, the aggregates of oxidized species 4 served as convenient shapedirecting templates and arranged the bimetallic NPs in a flowershaped arrangement at room temperature in a simple manner in 60 min. The utilization of supramolecular aggregates of hetero-oligophenylene derivatives as nanoreactors in aqueous media for the preparation of bimetallic nanohybrid materials is unprecedented. This wet chemical method for the preparation of polymer-encapsulated bimetallic Au-Fe3O4 nanohybrid material in the manuscript is a fast and convenient approach in comparison to other reported methods (Table S1 in the Supporting Information). Further, the supramolecular nanoreactors based on oxidized polythiophene species in combination with Au-Fe3O4 NPs exhibited remarkable photocatalytic efficiency in reactions between unprotected anilines and electron-deficient alkynes for tandem cabonylation and annulation reaction to furnish the synthetically versatile quinoline carboxylates. As per our information, this is the first report which demonstrates the ability of bimetallic AuFe3O4 NPs in nondirected C(sp2)−H bond activation of unprotected anilines for the efficient synthesis of quinolines under photocatalytic and aerial conditions (Table S2 in the Supporting Information).

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterization. 2.1.1. Synthesis and Characterization of Hetero-Oligophenylene Derivative 3. The Diels−Alder reaction of phenylacetylene (1a) with 2,5diphenyl-3,4-bis(thiophen-2-yl)cyclopenta-2,4-dienone (2a)16 in diphenyl ether at 240 °C furnished the desired heterooligophenylene derivative 3 in 70% yield (Scheme 1). The 1H Scheme 1. Synthesis of Hetero-Oligophenylene Derivative 3

NMR spectrum of compound 3 exhibited a singlet at 7.55 ppm (1H), six doublets of doublets at 7.22, 7.14, 7.07, 6.95, 6.54, and 6.48 ppm (4H, 6H, 1H, 2H, 1H, 1H), two triplets at 6.88 and 6.63 ppm (1H, 1H), and a multiplet at 7.02−6.99 ppm (4H). The ESI-MS mass spectrum of compound 3 exhibited a base peak at 471.1448 corresponding to [M + H]+. 2.1.2. Aggregation and Photophyical Behavior of Derivative 3. The photophysical and aggregation behavior of derivative 3 was studied using UV−vis and fluorescence spectroscopy. The absorption spectrum of compound 3 in THF showed the appearance of an absorption band at 270 nm due to a π−π* transition.17 When the water fractions were increased up to 70% (volume fraction) in the THF solution, the absorption band red-shifted from 270 to 290 nm and its intensity decreased; however, an increase in intensity of the level-of f tail in the visible region was observed (Figure 1A). The UV−vis studies of derivative 3 in H2O/THF (7/3) showed a blue shift of the absorption band from 290 to 280 nm with increasing temperature up to 75 °C, which suggests the formation of J-aggregates18 (Figure S1 in the Supporting Information). The fluorescence spectrum of derivative 3 in H2O/THF (7/ 3) solvent mixture exhibited a broad emission at 350 nm (quantum yield Φ = 0.08) on excitation at 290 nm. Further addition of a water fraction up to 70% to the THF solution resulted in red shift of the emission band from 350 to 360 nm and its intensity decreased (Figure S2 in the Supporting Information). The scanning electron microscopy (SEM) image of compound 3 in H2O/THF (7/3) solution mixture showed the formation of spherical aggregates (Figure 1B,C). A dynamic light scattering (DLS) analysis showed the presence of particles having an average diameter of around 200 nm (Figure S3 in the Supporting Information). Further, we performed 1H NMR studies by varying the concentrations of derivative 3 in CDCl3, and an average upfield shift of 0.04 ppm was observed in the signals corresponding to aromatic protons (Figure S4 in the Supporting Information). Thus, the 1H NMR and absorption studies suggest the formation of J-type assemblies of derivative 3 in aqueous media.18,19 2.1.3. Preparation of Bimetallic Au-Fe3O4 NPs. The presence of thiophene moieties in derivative 3 prompted us to investigate its molecular recognition behavior toward different metal ions such as Ag+, Au3+, Fe2+, Fe3+, Hg2+, Zn2+, Cu2+, Co2+, Ni2+, Pd2+, Cd2+, Ba2+, and Ca2+ as their perchlorate/chloride salts using absorption and emission studies. The absorption and emission studies suggested the 2008

DOI: 10.1021/acscatal.6b02681 ACS Catal. 2017, 7, 2007−2021

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Figure 1. (A) UV−vis spectra showing the change in absorbance of compound 3 (5.0 μM) in H2O/THF mixture (0−70% volume fraction of water in THF). Scanning electron micrographs showing the spherical-shaped aggregates in H2O/THF (7/3) mixture with scale bars of (B) 500 nm and (C) 100 nm.

Figure 2. (A) Absorption spectra of derivative 3 (5 μM) in the presence of Au3+ and Fe3+ ions (0−20 equiv) in water/THF (7/3, v/v) media. The inset shows 3-M (M = Au3+, Fe3+) complexation at lower concentrations (0−4 equiv). (B) Shifting of bands at 290 and 330 nm to 295 and 335 nm. (C) Expanded absorption spectra showing the appearance of a band at 545 nm corresponding to Au NPs and a red shift of the band to 570 nm corresponding to bimetallic Au-Fe3O4 NPs.

affinity of aggregates of compound 3 toward Au3+ and Fe3+ ions among the various metal ions tested. Upon addition of Au3+ ions (0−2 equiv) to the H2O/THF (7/3, v/v) solution of derivative 3, a new band at 330 nm appeared and an isosbestic point was observed at 310 nm, which suggested the formation of a 3-Au3+ complex. Further addition of Au3+ ions (2−20 equiv) to the same solution led to a red shift of the band from 330 to 335 nm and the appearance of a plasmon resonance band at 540 nm which suggested the formation of Au NPs (Figure S5 in the Supporting Information). These spectral changes were accompanied by a solution color change from colorless to reddish.20 However, addition of Fe3+ ions (0−20 equiv) to a solution of derivative 3 in H2O/THF (7/3, v/v) resulted in formation of an absorption band at 335 nm and spectrum became broadened in the 500−700 nm range (Figure S6 in the Supporting Information). Further, the color of the solution changed from colorless to black. All of these changes suggested the formation of Fe3O4. The above results suggest that the supramolecular aggregates of derivative 3 served as reactors for the preparation of Au NPs as well as Fe3O4 NPs. The UV−vis spectra of aggregates of derivative 3 in the presence of other metal ions such as Fe2+, Cu2+, Co2+, Zn2+,

Ni2+, Pd2+, Ag+, Hg2+, Cd2+, Ca2+, and Ba2+ (perchlorate/ chloride salts) showed no substantial change in absorption behavior under the same conditions (Figure S7 in the Supporting Information). In the next part of the investigation, we planned to prepare bimetallic Au−Fe3O4 NPs using aggregates of derivative 3 as a common platform. Solutions of Au3+ ions (0−5 equiv) and Fe3+ ions (0−5 equiv) were added simultaneously to a solution of derivative 3 in H2O/THF (7/3, v/v), as shown in Figure 2. The absorption spectrum of this solution showed a gradual enhancement in absorbance at 290 nm and appearance of a new band spectrum at 330 nm. An isosbestic point was also observed at 316 nm, which suggested the generation of 3-Au3+ and 3-Fe3+ complexes, at low concentration (Figure 2A and inset). Further addition of Au3+ and Fe3+ ions (5−20 equiv) resulted in red shifts of the absorption bands from 290 to 295 nm and from 330 to 335 nm, respectively. A new band was also observed at 545 nm which indicated the generation of Au NPs (Figure 2B,C).21 Further, the absorption bands at 335 and 295 nm became broadened, which indicated polymerization of the thiophene-appended hetero-oligophenylene derivative 3 (vide infra).12,22 In the absorption spectrum of the same solution 2009

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ACS Catalysis after it was kept for 60 min the intensity of the absorbance band at 335 nm gradually increased, and the band at 545 nm redshifted to 570 nm. The red shift of the LSPR band of gold NPs suggested an increase in the optical index of the medium surrounding the NPs.23 On the other hand, the intensity of the level-of f tail increased, which suggested the decoration of the Au NPs by Fe3O4 NPs (Figure 2C). Interestingly, the solution changed from colorless to yellow to red and finally reddish brown (Figure 3).21 All of these results demonstrate that the

Figure 4. Fluorescence spectra of derivative 3 (5 μM) in the presence of Au3+ and Fe3+ ions (0−20 equiv) in water/THF (7/3, v/v) media with λex 300 nm.

2.1.4. Preparation and Characterization of Polythiophene Species. To get insight into the oxidation of aggregates of derivative 3, we performed the reaction between derivative 3 in H2O/THF (7/3, v/v) with Au3+ + Fe3+ ions at room temperature (Figure 5). After the complete formation of AuFe3O4 NPs (as indicated by a color change in solution) the solution of aggregates of derivative 3 containing nanoparticles was slowly evaporated. After 2 days, brown precipitates appeared, which were separated by filtration and then washed with CHCl3/THF to obtain bimetallic Au-Fe3O4 NPs. The filtrate was evaporated to acquire the solid residue, and its 1H NMR was recorded. In the 1H NMR spectrum, the peak corresponding to the C−Ha proton at 6.95 ppm was not observed. On the other hand, an upfield shift of broad signals of all of the aromatic protons was observed (Figure S11 and Table S3 in the Supporting Information).25 The ESI-MS spectrum showed peaks at m/z 939.2240 and 1430.3114, which corroborated very well the theoretical exact mass of dimeric [C64H42S4Na]+ and trimeric species [C96H62S6H]+, respectively (Figure S12 in the Supporting Information). The FT-IR spectrum of oxidized species showed the absence of peaks at 3092 and 890 cm−1 corresponding to stretching and out-ofplane bending of the C−Ha bond adjacent to the S atom of the thiophene moiety. All of the above spectroscopic data clearly indicate the formation of polythiophene species 4 through ortho-Ha−Ha coupling of thiophene rings (Figure S13 in the Supporting Information).26 Thus, we conclude that the aggregates of derivative 3 reduced Au3+ ions to Au NPs and Fe3+ ions to Fe2+ ions which were further oxidized to Fe3O4 NPs; however, during the reduction process, the aggregates of derivative 3 were oxidized to polythiophene species 4 (Figure 5). The UV−vis spectra of oxidized polythiophene species exhibited absorption bands at 295 and 335 nm, and the absorption spectrum of oxidized species was similar to the absorption spectrum of derivative 3 in the presence of Au3+ and Fe3+ ions.12 These results support our assumption that the absorbance at 335 nm originated from the oxidized polythiophene species 4 (vide supra) (Figure S14 in the Supporting Information). Interestingly, the oxidized polythiophene species 4 exhibited a broad emission band in the range of 380−500 nm (Figure S15 in the Supporting Information). The fluorescence spectrum of the solution of oxidized species 4 (5 μM) in H2O/THF (7/3) showed an emission band at 405 nm. Upon addition of bare Au-Fe3O4 NPs (5 mol %) to this solution, the

Figure 3. Color change visible to the naked eye of the solution of aggregates of derivative 3 upon addition of Au3+ and Fe3+ ions (20 equiv) with stirring: (A) colorless in the absence of Au3+ and Fe3+ ions; (B−D) colors in the presence of Au3+ and Fe3+ ions (yellow to reddish and finally to brown after different time intervals).

aggregates of derivative 3 interacted with metal ions and served as reactors for reducing the metal ions into their nano forms and finally bimetallic Au-Fe3O4 nanohybrid materials were generated. To examine the in situ reduction of Au3+ and Fe3+ ions to Au-Fe3O4 NPs, we performed spectroelctrochemical studies of derivative 3 in the presence of Au3+ and Fe3+ ions. The cyclic voltammogram of derivative 3 in H2O/CH3CN (1/1) (15 μL of THF to dissolve) showed a reversible redox wave with a reduction potential of −0.30 eV (Figure S8A in the Supporting Information). In the spectroelctrochemical studies, upon addition of 20 equiv of Au3+ and Fe3+ ions to derivative 3, an absorption band was observed at 370 nm. When the reduction potential (−0.30 eV) was applied, the band at 370 nm completely disappeared, thus indicating the reduction of Au3+ to Au(0) and Fe3+ ions to Fe2+ ions. Further, within 60 min a new band suggesting the formation of Au NPs appeared at 540 nm. Gradually, the band at 540 nm broadened and was redshifted to 560 nm, hence indicating the formation of Au NPs along with Fe3O4 NPs (Figure S8B in the Supporting Information).21 These spectroelectrochemical studies clearly suggest that aggregates of derivative 3 served as reducing agents for the generation of bimetallic Au-Fe3O4 NPs.24 Next, we examined the emission behavior of aggregates of derivative 3 toward various metal ions such as Au3+, Fe3+, Fe2+, Cu2+, Co2+, Zn2+, Ni2+, Pd2+, Ag+, Hg2+, K+, Cd2+, Ba2+, and Mg2+ as their perchlorate/chloride salts. Upon addition of Au3+ and Fe3+ ions (0−20 equiv) to the solution of derivative 3 in a H2O/THF (7/3, v/v) mixture, the emission band at 360 nm gradually red-shifted to 390 nm and its emission intensity decreased by 10-fold (Figure 4). Under the same conditions, no considerable change in emission spectrum was observed in the presence of other metal ions (Figure S9 in the Supporting Information). The detection limit of aggregates of derivative 3 for Au3+ ions was found to be 53 nM (Figure S10 in the Supporting Information). 2010

DOI: 10.1021/acscatal.6b02681 ACS Catal. 2017, 7, 2007−2021

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Figure 5. Probable schematic representation of formation of polythiophene species 4 from derivative 3 during reduction of Au3+-Fe3+ ions and further preparation of polythiophene-protected bimetallic Au-Fe3O4 NPs.

Figure 6. (a) TEM images of polythiophene nanofibers. The inset shows the diameter of nanofibers. (b) Au-Fe3O4 NPs stabilized by polythiophene nanofibers. (c) High-resolution TEM images of polythiophene coated Au-Fe3O4 NPs. (d) TEM image showing the formation of in situ polythiophene-encapsulated bimetallic Au-Fe3O4 nanoparticles prepared by mixing aggregates of derivative 3 and Au3+ and Fe3+ ions (scale 100 nm). (e) Flower-shaped nanohybrid materials composed of dark Au NPs and light Fe3O4 NPs, with the outer layer being polythiophene species (scale 50 nm). (f) HR-TEM image showing a d spacing of 0.23 nm corresponding to the (111) plane of Au NPs and d spacings of 0.25 and 0.48 nm corresponding to the (311) and (111) planes of Fe3O4 NPs, respectively.

2.1.5. Characterization of Polythiophene-Encapsulated Au-Fe3O4 Nanohybrid Materials. The transmission electron microscope (TEM) images of derivative 3 in the presence of Au3+ and Fe3+ ions (20 equiv) revealed the presence of flower like Au-Fe3O4 NPs encapsulated into the network of fibers of polythiophene species 4 (Figure 6a,b). The TEM and HRTEM micrographs indicated the presence of nanofibers of

emission intensity decreased by 61% (Figure S16A in the Supporting Information). Further, a strong overlap between the fluorescence spectrum of polythiophene species 4 and the absorption spectrum of Au-Fe3O4 NPs was observed which suggests the possibility of energy transfer from oxidized species 4 to Au-Fe3O4 NPs (Figure S16B in the Supporting Information). 2011

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Au-Fe3O4 NPs has been carried out, which showed a nonlinear continuous weight loss above 250 °C; however, no weight loss was observed above 650 °C. From the weight loss, the amount of polythiophene32 coating on Au-Fe3O4 NPs was estimated to be approximately 20.0% (Figure S23 in the Supporting Information). The superparamagnetic behavior of bimetallic Au-Fe3O4 was observed by magnetic hysteresis measurements (Figure S24 and Table S5 in the Supporting Information). 2.1.6. Synthesis of Model Compound 5 and Study of Its Photophysical Behavior with Au3+ and Fe3+ Ions. To examine the role of polythiophene species 4 as a template for preparation of flower-shaped Au-Fe3O4 nanohybrid materials, we prepared model compound 5, bearing only one thiophene unit, by the Diels−Alder reaction of 1b and 2b at 240 °C (Scheme 2). The absorption spectra of derivative 5 in water/

polythiophene 4 coated bimetallic Au-Fe3O4 nanoflower (NFs) (Figure 6b,c). The average size of a nanoflower (Figure 6d) was found to be in the range of 70−80 nm (Figure 6e) (gold nanoparticles, 8−12 nm; Fe3O4 nanoparticles, 5−10 nm). The high-resolution TEM images (Figure 6f) showed a d spacing of 0.23 nm corresponding to the (111) plane of face-centered cubic (fcc) gold and d spacings of 0.25 and 0.48 nm corresponding to the (311) and (111) planes bearing fcc crystal lattice fringes of Fe3O4 NPs.21 The selected area electron diffraction (SAED) pattern showed three distinguishable planes of (311), (511), and (731) indices which suggest the fcc phases of Fe3O4 and (111), (200), (220), (311), and (422) planes corresponding to Au NPs (Figure S17A in the Supporting Information).27 The SAED pattern suggested a superposition of Au and Fe3O4 lattices corresponding to bimetallic Au-Fe3O4 NPs. Energy-dispersive X-ray spectroscopy (EDX) showed the presence of Fe, Au, O, S, and C elements in the nanohybrid materials, which confirms the formation of polythiopheneencapsulated bimetallic Au-Fe3O4 NPs (Figure S17B in the Supporting Information).23 Further, we examined the effect of concentration of Au3+ and Fe3+ ions on the generation of AuFe3O4 NPs. The TEM images of the sample prepared by mixing Au3+ and Fe3+ ions in a 1:2 ratio showed larger core−shell AuFe3O4 NPs where the Au NPs are fully encapsulated by Fe3O4 NPs (Figure S18A in the Supporting Information). On the other hand, upon an increase in the amount of Au3+ by mixing Au3+ and Fe3+ ions in a 2/1 ratio, larger irregularly shaped Au and Fe3O4 NPs were found (Figure S18B in the Supporting Information). The above studies suggest that the supramolecular aggregates of derivative 3 served as shape-, size-, and morphology-directing agents. The DLS studies indicated the presence of two sets of particles having sizes in the ranges of 4−8 and 70−100 nm, respectively (Figure S19 in the Supporting Information). The powder X-ray diffraction (XRD) measurements showed diffraction peaks at 18.24, 30.14, 35.10, 43.16, 53.56, 57.12, and 62.58°, which were assigned to the (111), (220), (311), (400), (422), (511), and (440) planes corresponding to the fcc lattice of Fe3O4 NPs, and diffraction peaks at 37.96, 44.18, 64.34, 77.32, and 81.58° corresponding to the (111), (200), (220), (311), and (422) planes of Au NPs, respectively (Figure S20 in the Supporting Information).28 The Raman scattering spectrum of derivative 3 showed bands at 982.27, 1003.29, 1317.98, 1352.82, 1445.95, 1541.98, and 1603.26 cm−1. In the presence of Au3+ and Fe3+ ions all of the bands became broadened and a new band appeared at 670 cm − 1 corresponding to the A1g vibration mode of Fe3O4 NPs (Figure S21 in the Supporting Information).23 The X-ray photoelectron spectroscopic (XPS) analysis of the polythiophene-coated AuFe3O4 NPs showed the presence of peaks at 84.0 and 87.8 eV corresponding to Au 4f7/2 and Au 4f5/2 peaks of the Au(0) state.29 Further, two peaks were observed at 710.2 and 724.6 eV corresponding to Fe 2p3/2 and Fe 2p1/2, which confirmed the presence of both Fe3+ and Fe2+ species in the Fe3O4 lattice.30 Additionally, peaks were observed at 162.4, 284.2, and 526.4 eV corresponding to S 2p, C 1s, and O 1s of the organic residue, respectively (Figure S22 in the Supporting Information).29,31 All of the above results clearly suggest the existence of polythiophene-encapsulated bimetallic Au-Fe3O4 NPs. The atomic absorption spectrophotometry (AAS) studies show the presence of 39.7 wt % of gold in bimetallic Au-Fe3O4 NPs (Table S4 in the Supporting Information). Thermogravimetric analysis (DT-TGA) of polythiophene-encapsulated bimetallic

Scheme 2. Synthesis of Hetero-Oligophenylene Based Model Compound 5

THF (7/3) media in the presence of Au3+ ions and Fe3+ ions led to the appearance of a band at 550 nm, which suggested the generation of Au and Fe3O4 NPs (Figure S25 in the Supporting Information). The TEM images showed the formation of welldispersed Au and Fe3O4 nanoparticles; however, no flowershaped morphology was observed (Figure S26A in the Supporting Information). The DLS studies indicate the formation of larger sized nanoparticles having an average diameter of around 20−30 nm (Figure S26B in the Supporting Information). These results clearly suggest that only the aggregates of oxidized polythiophene species 4 served as a shape-directing template for the generation of bimetallic AuFe3O4 nanohybrid materials in a flower-like arrangement. 2.2. Photocatalytic Efficiency of PolythiopheneEncapsulated Bimetallic Au-Fe3O4 Nanoparticles. The possibility of energy transfer from polythiophene species to AuFe3O4 NPs encouraged us to investigate the catalytic efficiency of polythiophene-encapsulated bimetallic Au-Fe3O4 NPs in the C(sp2)−H activation reaction in the presence of visible light. We planned to synthesize versatile 3-substituted quinolines by reaction between unprotected anilines with electron-deficient alkynes through three steps: i.e., C−H activation, carbonylation, and subsequent annulation. We chose a model reaction between aniline (6a) and methyl propiolate (7a), whereas paraformaldehyde (2.5 mmol) was used as the carbonyl source9,33 (Scheme 3). The reaction was carried out in water using Au-Fe3O4 nanohybrid materials (0.5 mol % with respect to Au) as catalysts under photocatalytic conditions. The reaction was completed in 6 h, and the desired quinoline derivative 8a was acquired in excellent yield (80%) (Table 1, entry 1). We also carried out the model reaction in several solvents such as CH3CN, EtOH, THF, and DMF under photocatalytic conditions. In the presence of all these solvents, the desired product 8a was acquired in lower yield, while under neat conditions only 40% product formation was observed (Table 1, entries 1−6). These studies show that the maximum 2012

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ACS Catalysis Scheme 3. C(sp2)−H Activation Reaction among Aniline (6a, 1 mmol), Methyl Propiolate (7a, 1 mmol), and Paraformaldehyde (2.5 mmol) Catalyzed by PolythiopheneEncapsulated Au-Fe3O4 NPs under Photocatalytic Conditions (Irradiation of 60 W Tungsten Filament Bulb)

Table 2. C(sp2)−H Activation Reactions among Aniline (6a, 1 mmol), Methyl Propiolate (7a, 1 mmol), and Paraformaldehyde (2.5 mmol) under Different Catalytic Conditions entry

control experiment

time (h)

yield (%)

1 2 3 4 5 6 7 8 9 10 11

polythiophene 4:Au-Fe3O4a polythiophene 4:Au-Fe3O4b polythiophene 4b AuCl3 + FeCl3 AuCl3 + FeCl3 + polythiophene 4b bare Au NPsb bare Fe3O4 NPsb bare Au-Fe3O4b bare Au-Fe3O4 + polythiophene 4b bare Au-Fe3O4 + aggregates of derivative 3b derivative 5:Au-Fe3O4b

36 6 24 24 24 24 24 24 6 6 10

32 80 no reaction 15 25 45 280 °C. 1H NMR (500 MHz, CDCl3): δ 7.55 (s, 1H), 7.22 (dd, J = 5 Hz, 4H), 7.14 (d, J = 5 Hz, 6H), 7.07 (dd, J = 5 Hz, 1H), 7.02−6.99 (m, 4H), 6.95 (dd, J = 5 Hz, 2H), 6.68 (t, J = 5 Hz, 1H), 6.63 (t, J = 5 Hz, 1H), 6.54 (dd, J = 5 Hz, 1H), 6.48 (dd, J = 5 Hz, 1H). 13C NMR (125 MHz, CDCl3): δ 142.2, 141.7, 141.5, 141.3, 141.2, 141.0, 140.9, 139.7, 135.7, 133.4, 132.1, 131.0, 129.8, 129.4, 129.3, 129.2, 127.7, 127.6, 127.0, 126.6, 126.5, 126.0, 125.8, 125.6. The ESI-MS of C32H22S2 showed a parent ion peak at m/z 471.1448 corresponding to [M + H]+. FT-IR (KBr): νmax (cm−1) 3092 (C−Ha stretching) and 890 (C−Ha out of plane bending). Anal. Calcd for C32H22S2: C, 81.66; H, 4.71; S, 13.62. Found: C, 81.64; H, 4.72; S, 13.63. The spectroscopic and analytical data confirmed the structure of derivative 3 (Figure S30 in the Supporting Information). 4.4. Synthesis of Hetero-Oligophenylene Derivative 5. A 0.3 g portion (1.8 mmol) of 2-(phenylethynyl)thiophene 2018

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IR νmax (cm−1) 1723 (COOMe); ESI-MS (m/z) 312.1101 [M8k + H]+. Anal. Calcd for C21H13NO2: C, 81.01; H, 4.21; N, 4.50; O, 10.28. Found: C, 81.00; H, 4.19; N, 4.52; O, 10.29. 4.6.9. Compound 8l: yellow solid; mp 112−114 °C; 1H NMR (500 MHz, CDCl3) δ 8.88 (d, J = 10 Hz, 2H), 8.43 (s, 2H), 7.13−7.07 (m, 4H), 6.93−6.81 (m, 4H), 3.64 (s, 6H), 3.49 (s, 6H); 13C NMR (125 MHz, CDCl3) δ 170.2, 154.4, 148.1, 142.7, 129.9, 129.7, 116.7, 115.4, 115.2, 110.5, 102.5, 98.4, 51.2, 50.7; FT-IR νmax (cm−1) 1720 (COOMe); ESI-MS (m/z) 509.1707 [M8l + H]+. Anal. Calcd for C30H24N2O6: C, 70.86; H, 4.76; N, 5.51; O, 18.88. Found: C, 70.86; H, 4.77; N, 5.49; O, 18.89. 4.6.10. Compound 8m: yellow solid; mp 132−134 °C; 1H NMR (500 MHz, CDCl3) δ 8.87 (s, 2H), 8.42 (d, J = 10 Hz, 2H), 8.21 (d, J = 10 Hz, 2H), 7.76 (d, J = 5.0 Hz, 2H), 7.67 (dd, J = 7.5 Hz, 4H), 7.57−7.52 (m, 4H), 7.50−7.44 (m, 6H), 6.96−6.74 (m, 16H), 3.94 (s, 6H, OMe); 13C NMR (125 MHz, CDCl3) δ 165.4, 162.9, 150.3, 149.7, 138.7, 137.8, 135.1, 134.6, 133.4, 132.1, 131.8, 131.2, 130.3, 128.9, 128.5, 124.7, 52.6; FTIR νmax (cm−1) 1723 (COOMe); ESI-MS (m/z) 943.2478 [M8m + K]+. Anal. Calcd for C64H44N2O4: C, 84.93; H, 4.90; N, 3.10; O, 7.07. Found: C, 84.94; H, 4.92; N, 3.10; O, 7.04. 4.6.11. Compound 9b: pale yellow solid; mp 68−70 °C; 1H NMR (300 MHz, CDCl3) δ 9.11 (d, J = 9 Hz, 1H), 8.88 (s, 1H), 8.06 (d, J = 9 Hz, 1H), 7.30 (t, J = 6 Hz, 1H), 7.10 (d, J = 6 Hz, 1H), 4.30 (q, J = 4.5 Hz, 2H, OCH2-CH3), 1.34 (t, J = 7.5 Hz, 3H, OCH2-CH3); 13C NMR (75 MHz, CDCl3) δ 171.7, 141.6, 140.6, 129.5, 122.0, 116.6, 115.3, 101.7, 60.3, 14.7; FTIR νmax (cm−1) 1728 (COOMe); ESI-MS (m/z) 349.9020 [M9b + Na]+. Anal. Calcd for C12H10INO2: C, 44.06; H, 3.08; N, 4.28; O, 9.78. Found: C, 44.05; H, 3.06; N, 4.30; O, 9.79. 4.7. General Procedure of Tandem Photocatalysis for the Synthesis of Quinoline Carboxylates (11a−c). A mixture of aniline (6a, 1 mmol), methyl propiolate (7a, 1 mmol), and substituted benzaldehyde (10a, 1 mmol) were mixed in a 10 mL RBF (Scheme 8). The reaction was carried out in water using Au-Fe3O4 nanohybrid materials (0.5 mol % with respect to Au) as the catalyst under photocatalytic conditions. The reaction was complete in 6 h, and the desired quinoline derivative 8a was obtained in excellent yield (80%). After it was cooled to room temperature, the reaction mixture was extracted with water/ethyl acetate (1/1) (3 × 20 mL). The resultant organic layer was passed through anhydrous Na2SO4 and was concentrated under reduced pressure to acquire the crude sample. The target compounds (11a−c) were purified through recrystallization in a EtOAc/CHCl3 (1/1) mixture. The quinoline derivatives were found in excellent to good yield and confirmed from their spectroscopic and analytical data (page S35 and Figures S49−S51 in the Supporting Information).

C12H11NO2: C, 71.63; H, 5.51; N, 6.96; O, 15.90. Found: C, 71.64; H, 5.52; N, 6.98; O, 15.90. 4.6.2. Compound 8c: colorless solid; mp 90−92 °C; 1H NMR (500 MHz, CDCl3) δ 9.31 (s, 1H), 8.77 (d, J = 5 Hz, 1H), 7.74 (dd, J = 10 Hz, 1H), 7.15 (dd, J = 10 Hz, 1H), 6.25 (dd, J = 15 Hz, 1H), 4.03 (s, 3H, OMe); 13C NMR (125 MHz, CDCl3) δ 172.6, 160.6, 149.0, 143.2, 141.6, 138.8, 138.3, 133.6, 131.3, 130.9, 118.5, 51.9; FT-IR νmax (cm−1) 1730 (COOMe), 3350 (OH); ESI-MS (m/z): 226.0441 [M8c + Na]+. Anal. Calcd for C11H9NO3: C, 65.02; H, 4.46; N, 6.89; O, 23.62. Found: C, 65.00; H, 4.47; N, 6.88; O, 23.64. 4.6.3. Compound 8d: colorless solid; mp 128−130 °C; 1H NMR (300 MHz, CDCl3) δ 9.00 (d, J = 9.0 Hz, 1H), 8.16 (s, 1H), 7.46 (d, J = 6.0 Hz, 1H), 7.40−7.36 (m, 2H), 3.67 (s, 3H, OMe), 3.15 (broad s, 1H, SH); 13C NMR (75 MHz, CDCl3) δ 171.4, 147.5, 140.3, 134.4, 129.2, 121.6, 117.6, 116.3, 115.0, 101.4, 60.0; FT-IR νmax (cm−1) 1723 (COOMe), 2560 (SH); ESI-MS (m/z) 220.0426 [M8d + H]+. Anal. Calcd for C11H9NO2S: C, 60.26; H, 4.14; N, 6.39; O, 14.59; S, 14.62. Found: C, 60.26; H, 4.16; N, 6.38; O, 14.59; S, 14.61. 4.6.4. Compound 8f: colorless solid; mp 170−172 °C; 1H NMR (300 MHz, CDCl3) δ 9.33 (d, J = 12.0 Hz, 1H), 8.91 (s, 1H), 7.74 (d, J = 15.0 Hz, 1H), 7.45 (s, 1H), 3.84 (s, 3H, OMe), 2.23 (s, 3H, Me), 2.20 (s, 3H, Me); 13C NMR (75 MHz, CDCl3) δ 171.7, 148.1, 142.6, 139.2, 134.6, 129.9, 129.6, 129.5, 129.1, 127.8, 124.5, 122.0, 120.8, 51.3, 18.6; FT-IR νmax (cm−1): 1732 (COOMe); ESI-MS (m/z) 216.1017 [M8f + H]+. Anal. Calcd for C13H13NO2: C, 72.54; H, 6.09; N, 6.51; O, 14.87. Found: C, 72.55; H, 6.07; N, 6.50; O, 14.90. 4.6.5. Compound 8g: colorless solid; mp: 120−122 °C; 1H NMR (300 MHz, CDCl3) δ 9.54 (d, J = 9.0 Hz, 1H), 8.61 (s, 1H), 7.92 (d, J = 9.0 Hz, 1H), 7.57 (d, J = 6.0 Hz, 1H), 6.84 (d, J = 6 Hz, 1H), 3.79 (s, 3H, OMe); 13C NMR (75 MHz, CDCl3) δ 171.1, 152.6, 148.0, 138.3, 136.5, 130.31, 128.9, 122.6, 56.3; FT-IR νmax (cm−1) 1730 (COOMe); ESI-MS (m/ z) 313.9673 [M8g + H]+. Anal. Calcd for C11H8INO2: C, 42.20; H, 2.58; N, 4.47; O, 10.22. Found: C, 42.22; H, 2.59; N, 4.46; O, 10.23. 4.6.6. Compound 8h: colorless solid; mp 152−154 °C; 1H NMR (300 MHz, CDCl3) δ 9.54 (d, J = 12.0 Hz, 1H), 8.53 (s, 1H), 7.92 (d, J = 12.0 Hz, 1H), 7.57 (d, J = 9.0 Hz, 1H), 6.84 (d, J = 9.0 Hz, 1H), 3.79 (s, 3H, OMe); 13C NMR (125 MHz, CDCl3) δ 171.8, 159.1, 150.7, 140.7, 140.2, 132.4, 116.8, 114.2, 60.6; FT-IR νmax (cm−1) 1725 (COOMe); ESI-MS (m/z) 265.9814 [M8h + H]+. Anal. Calcd for C11H8BrNO2: C, 49.65; H, 3.03; N, 5.26; O, 12.03. Found: C, 49.62; H, 3.05; N, 5.27; O, 12.03. 4.6.7. Compound 8j: colorless solid; mp 170−172 °C; 1H NMR (300 MHz, CDCl3) δ 9.56 (d, J = 9.0 Hz, 1H), 8.59 (s, 1H), 7.93 (d, J = 9.0 Hz, 1H), 7.23 (s, 1H), 6.99 (d, J = 6.0 Hz, 1H), 3.79 (s, 3H, OMe); 13C NMR (125 MHz, CDCl3) δ 165.8, 154.5, 152.7, 138.5, 136.7, 134.5, 131.1, 130.5, 129.9, 128.7, 126.2, 122.7, 122.0, 121.8, 56.4; FT-IR νmax (cm−1) 1715 (COOMe); ESI-MS (m/z) 244.0136 [M8j + Na]+. Anal. Calcd for C11H8ClNO2: C, 59.61; H, 3.64; N, 6.32; O, 14.44. Found: C, 59.63; H, 3.63; N, 6.35; O, 14.40. 4.6.8. Compound 8k: yellow solid; mp 107−109 °C; 1H NMR (500 MHz, CDCl3) δ 9.68 (d, J = 10.0 Hz, 1H), 8.77 (s, 1H), 8.22 (dd, J = 5.0 Hz, 1H), 8.16 (t, J = 10.0 Hz, 2H), 8.10− 8.03 (m, 2H), 7.86 (d, J = 10.0 Hz, 1H), 7.73 (d, J = 15 Hz, 1H), 7.54 (d, J = 15.0 Hz, 1H), 3.72 (s, 3H, OMe); 13C NMR (125 MHz, CDCl3) δ 169.4, 148.2, 143.3, 142.8, 141.1, 139.4, 132.5, 130.0, 129.7, 129.3, 125.1, 124.7, 122.7, 122.2, 51.3; FT-

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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/acscatal.6b02681. 1 H and 13C NMR, ESI-MS, and FT-IR spectra of compounds 3, 4, 5, 8a−m, 9a−d and 11a−c, UV−vis and fluorescence studies, SEM and TEM images, powder XRD, Raman analysis, DLS, TGA, and XPS studies, and comparison of data in the present paper with those of previous reports (PDF) 2019

DOI: 10.1021/acscatal.6b02681 ACS Catal. 2017, 7, 2007−2021

Research Article

ACS Catalysis

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AUTHOR INFORMATION

Corresponding Author

*E-mail for V.B.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.B. is grateful to the SERB, New Delhi, India (ref no. EMR/ 2014/000149), for financial support. We are also grateful to the UGC (New Delhi) for the “University with Potential for Excellence” (UPE) project. We are thankful to DST for Fund for Improvement of S&T Infrastructure (FIST). Mandeep Kaur and Subhamay Pramanik are grateful to the UGC (New Delhi) for a Junior Research Fellowship (JRF) and Senior Research Fellowship (SRF).

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DOI: 10.1021/acscatal.6b02681 ACS Catal. 2017, 7, 2007−2021