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Fabrication of Polythiophene Supported Ag@Fe3O4 Nanoclusters and their Utilization as Photocatalyst in Dehydrogenative Coupling Reactions Radhika Chopra, Manoj Kumar, and Vandana Bhalla ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04891 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Sustainable Chemistry & Engineering

Fabrication of Polythiophene Supported Ag@Fe3O4 Nanoclusters and their Utilization as Photocatalyst in Dehydrogenative Coupling Reactions Radhika Chopra, Manoj Kumar and Vandana Bhalla* Department of Chemistry, UGC-Centre for Advanced Studies-II, Guru Nanak Dev University, Amritsar-143005, Punjab (India) Email: [email protected] ABSTRACT: Fluorescent assemblies of pentacenequinone derivative 3 bearing 3-thienyl groups served as nanoreactors for the generation of Ag@Fe3O4 nanoclusters (NCs). This sensing/reduction process was accompanied by oxidation of assemblies of pentacenequinone derivative to polymeric species 4. The as prepared Ag@Fe3O4 NCs serve as recyclable photocatalyst for the construction of indazole derivatives under mild conditions (visible light irradiation, room temperature and aerial conditions). The present investigation emphasizes the role of pentacenequinone based supramolecular assemblies as nanoreactors for generation of Ag@Fe3O4 NCs and as stabilizers as well as oxidants in the C-H to C-N bond transformation. KEYWORDS: Pentacenequinone, Ag@Fe3O4 nanoclusters, Assemblies, Nanoreactors, Indazole Derivatives. INTRODUCTION Catalytic alteration of ubiquitous C-H bonds into the beneficial C-N bonds is an attractive synthetic protocol for preparation of indazole derivatives.1 Indazoles are important heterocyclic skeltons due to their wide range of biological and pharmaceutical activities.2 They are important building blocks for preparation of anti-cancer, anti-microbial, anti-inflammatory and anti-HIV drug candidates.3 Conventional approaches for the synthesis of indazole derivatives involve oxidative annulation of hydrazones or o-halo substituted hydrazones under palladium, nickel and copper catalyzed conditions.4-8 All of these approaches involve high amount of catalyst loading (5-10 mol%) and presence of various additives under extreme temperature conditions which make these reactions ecounfriendly. Further these methods necessitate the presence of stoichiometric amount of oxidant or 1 ACS Paragon Plus Environment

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oxygen atmosphere. Additionally, purification of target molecules requires separation of catalyst from the reaction mixture which is a tedious multistep process. Thus, it would be very interesting if a catalyst is developed which is economical, easy to separate and carries out C-H to C-N bond transformation efficiently and in ecofriendly manner. Our research interests are the development of new catalysts/photocatalysts composed of metal nanocomposites and supramolecular assemblies for synthesizing derivatives having material and pharmaceutical applications.9 In extension of our endeavors in this field, we were then motivated to develop an effective, recyclable, efficient and oxidant free catalyst for the construction of indazoles. Keeping in mind the limitations of already reported catalytic systems, our focus was then to choose an economic and recyclable metallic species as the centre of the system. In addition, to get rid of thermal heating, we focussed on the development of a photocatalyst for transforming C-H bond to C-N bond. Earlier very economic and efficient iron centered catalyst has been used to synthesize indazoles.10 Though the reaction progressed well in the presence of iron salt but the reaction conditions required heating at high temperature (110⁰C) which actually decreased the economic advantage of the strategy. Further, reaction worked well only in the presence of oxygen atmosphere which necessitates the need of special apparatus/conditions which is again a disadvantage of this strategy. Moreover, the system was not recyclable. This study inspired us to develop an iron based photocatalytic system, however, the main hurdle in the development of iron based photocatalysts is their rapid photocorrosion under prolonged light irradiation.11-12 We envisioned that bimetallic nanocomposites stabilized by supramolecular assemblies and prepared by mixing plasmonic metal species such as silver and catalytically active metal centre such as iron may have sufficient potential to overcome the limitations of previously reported systems. We preferred silver over gold due to its relatively low cost and excellent qualities in terms of plasmonic abilities. 13 To eliminate the

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need of oxygen atmosphere or external oxidant during the reaction, we planned to utilize the supramolecular assemblies having potential to act as an internal oxidant. Thus, we synthesized compound 3 in which pentacenequinone scaffold was appended with 3-thienyl moieties at 2, 9/2, 10 positions. We envisaged that pentacenequinone based assemblies will enhance the affinity of assemblies toward soft metal ions and will assist the generation of metal nanoparticles (NPs). Second, we expected that due to the oxidant nature of quinone moiety,

14-15

the need of an external oxidant or

oxygen atmosphere will be eliminated. Interestingly, in mixed aqueous media, compound 3 formed supramolecular assemblies which exhibited high affinity towards Ag+ and Fe3+ ions.16-18 During this process of binding, Ag+ and Fe3+ ions were reduced to Ag0 and Fe3O4 NPs, respectively and supramolecular assemblies of the organic component 3 were oxidized to polymeric derivative 4. As of now, there is no report in the literature where supramolecular assemblies of pentacenequinone scaffold 3 appended with 3-thienyl groups at 2, 9/2, 10 positions have been developed which exhibit high affinity for Ag+ and Fe3+ ions. In comparison to the reported strategies,

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this wet chemical

technique is a rapid and convenient protocol for the generation of polythiophene 4 supported Ag@Fe3O4 nanocomposites (Table S1 and Scheme S3). Interestingly, as prepared polythiophene 4 supported Ag@Fe3O4 NCs exhibited high photocatalytic efficiency in dehydrogenative coupling for the preparation of indazoles. Further, these metal oxide nanoclusters exhibited enhanced catalytic activity due to their small size22 and the reaction conditions did not necessitate the presence of an external oxidant or oxygen atmosphere. The in situ generated polythiophene species 4 acts as an internal oxidant in this photocatalytic transformation. This is the first report where quinone based assemblies in combination with Ag@Fe3O4 NCs as catalytic centre have been utilized as internal oxidant for carrying out C-H to C-N bond transformation. This catalytic system does not require thermal heating at elevated temperature and support of additives to synthesize indazoles, thus avoiding



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the formation of side products. On the whole, the catalytic performance of the polythiophene 4 supported Ag@Fe3O4 NCs is superior to other reported catalysts for dehydrogenative coupling (Table S2). RESULTS AND DISCUSSION:

Scheme 1 Preparation of pentacenequinone compound 3.

The reaction of dibromopentacenequinone 123 with 3-thienylboronic acid 2 afforded compound 3 in 60% yield via Suzuki-Miyaura protocol (Scheme 1 and S1). The structure of compound 3 was elucidated on the basis of spectroscopic as well as analytical data (PS45-S46). The photophysical properties of compound 3 were studied in mixed aqueous media. The solution of compound 3 in dry THF showed the presence of three bands at 322, 370 and 422 nm in UV-vis spectrum (Figure S1). The addition of water fraction upto 50% to the above solution results in decrease in intensity with red shift of all the bands (Figure S2).24 Thereafter, we performed the UV-vis studies of compound 3 in H2O/THF (1/1) solvent mixture by varying the temperature. The blue shift in UV-vis spectra on increasing the temperature upto 70⁰C confirms the existence of J-like assemblies (Figure S3).25 The fluorescence spectrum of compound 3 exhibits a broad emission band at 470 nm in THF (Фf = 0.12). The addition of H2O fractions (upto 10%) to the THF solution results in the red shift of emission band from 470 to 510 nm with enhancement of emission intensity (Фf = 0.68) (Figure S4). Further, increasing the water fraction upto 50%, the emission intensity decreased (Фf = 0.32) and further red shift from 510 to 535 nm in the emission band was observed. The TEM image of the above solution of compound 3 indicated the presence of irregular shaped assemblies (Figure S5)


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and the DLS analysis of the same solution of compound 3 showed the formation of nanoaggregates having average size of 100 nm (Figure S6). The presence of thiophene groups motivated us to determine the binding affinity of assemblies of compound 3 for different metallic species (Fe3+, Ag+, Hg2+, Co2+, Fe2+, Li+, Mg2+, Ca2+, Cd2+, Ni2+, Pb2+, Na+, K+, Zn2+, Al3+, Cu2+ and Pd2+ as their nitrate/chloride/perchlorate salts). The absorption studies of compound 3 were performed in H2O/THF (1/1) solvent mixture in the presence of different metallic species. Addition of Ag+ ions (100 equiv. in aliquots) to H2O/THF (1/1) solvent system resulted in the appearance of a band at 438 nm (SPR band of AgNPs) (Figure S7). The time dependent UV-vis studies indicated the gradual enhancement in the intensity of SPR band. On the basis of these studies, the rate constant of 8.18 × 10-5 s-1 was calculated for the generation of AgNPs (Figure S8 and PS16).26-27 On the other hand, the absorption spectra got broadened upon addition of aqueous solution of FeCl3 (20 equiv. in aliquots) to the solution of compound 3 (Figure S9), thus, suggesting the generation of Fe3O4. Further, the colorless solution turned black during this process. From these studies, we may conclude that assemblies of compound 3 have potential to serve as reactors for the generation of AgNPs and Fe3O4 NPs. However, the absorption behavior of assemblies of compound 3 remains unchanged in the presence of other metal ions under similar conditions (Figure S10 and S11). We believe that specific interactions between 3-thienyl groups of compound 3 and Ag+/Fe3+ ions are responsible for the high selectivity of compound 3.15-16 For the preparation of nanocomposites, first of all, we prepared the AgNPs using the aggregates of derivative 3 and AgNO3. Thereafter, we added the AgNPs (dispersed in water) and Fe3+ ions (0.1M) simultaneously in the ratio of 1:1 to the solution of compound 3 in H2O/THF (1/1). The TEM studies of the resulting solution suggested the formation of Ag-Fe3O4 hybrid nanocomposites (Figure S12A). Interestingly, when the ratio of AgNPs and Fe3+ ions was switched to



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1:2, the TEM image of resulting solution showed the generation of Ag@Fe3O4 core-shell nanoclusters (Figure S12B). These studies suggest that morphology of nanocomposites can be tuned by varying the composition of AgNPs and Fe3+ ions. Since, core-shell nanocomposites are known to facilitate the plasmonic energy transfer processes more in comparison to hybrid nanoparticles, 28-29 thus, we focussed our attention on the development and characterization of Ag@Fe3O4 NCs (1:2) generated by utilizing assemblies of compound 3. On addition of Fe3+ ions, the SPR band of AgNPs at 438 nm shifted to 415 nm (Figure S13). This blue shifting of SPR band may be attributed to lowering of dielectric constant of solvent mixture (H2O-THF) in the presence of Fe3O4 NCs.30 These absorption studies support the generation of Ag@Fe3O4 NCs. During the process of generation of Ag@Fe3O4 NCs, the colorless solution turned light brown and ultimately blackish brown (Figure S14). The intensity of absorbance band at 415 nm corresponding to Ag@Fe3O4 NCs increased with time (Figure S15). The rate constant of 4.09×10-4 s-1 was calculated for the Ag@Fe3O4 NCs (Figure S16 and PS21). Next, emission behavior of compound 3 was studied in the presence of solution of AgNPs and FeCl3. The emission band at 535 nm was quenched on addition of AgNPs and Fe3+ ions (40 equiv. in aliquots) to the mixed aqueous solution of compound 3 (Figure S17). To examine the in situ generation of Ag@Fe3O4 NCs, we also performed the cyclic voltammetric studies. The cyclic voltammogram of compound 3 (15 µL THF to dissolve) in H2O/CH3CN (1/1) shows reversible redox wave (-0.19 eV) (Figure S18 and Table S3). On addition of AgNPs and Fe3+ ions, the redox wave showed positive oxidative potential of 0.22 eV. These studies support the role of assemblies of compound 3 as reductants in the preparation process of Ag@Fe3O4 NCs.


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(b)

(a)

(c)

Fe3O4

Ag@Fe3O4 Polythiophene layer

5 nm

100 nm (d)

Ag

20 nm (e)

Ag

Fe3O4 0.20 nm

Ag (200) 0.25 nm

Fe3O4 (311)

20 nm

5 nm

Figure 1 (a) and (b) TEM images; (c), (d) and (e) High resolution TEM images of polythiophene nanosheets 4 supported Ag@Fe3O4 nanoclusters.

The TEM images of this solution show the deposition of Ag@Fe3O4 NCs on the surface of nanosheets of polythiophene 4 (Figure 1a-b, S19A). The average size of nanoclusters was determined to be 34 nm (high contrast Ag core of 22 nm and low contrast Fe3O4 shell of 12 nm). The Fe3O4 shell was composed of numerous small magnetite nanocrystals having average size of 2.5 nm (Figure 1c-d). In HR-TEM image, lattice fringes of 0.20 nm and 0.25 nm were assigned to interlayer spacings of the (200) plane of Ag and (311) plane of Fe3O4, respectively (Figure 1e).31-32 Superposition of Ag and Fe3O4 lattices in SAED pattern further confirmed the formation of Ag@Fe3O4 NCs (Figure S19B).32 The DLS studies indicated the existence of two sets of particles of average size of about 43 nm and 120 nm (Figure S20). The powder XRD studies show diffraction pattern corresponding to Fe3O4 NCs and AgNPs (Figure S21).30,33,34 In Raman scattering studies of polythiophene 4 supported Ag@Fe3O4 NCs, a band at 670 cm-1 (vibrational A1g mode of magnetite NCs) was detected (Figure S22).35 The XPS analysis of the polythiophene 4 supported Ag@Fe3O4



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NCs showed peaks at 374.3 (Ag 3d3/2) and 368.3 eV (Ag 3d5/2) (Figure S23).36-37 Further, peaks at 711.6 (Fe 2p3/2) and 724.5 eV (Fe 2p1/2) were detected, which are attributed to Fe2+/Fe3+ species in ferric oxide lattice.38 Besides, peaks at 526.4, 284.2 and 162.4 eV corresponds to O1s, C1s and S2p, respectively, were observed.39-40 Thus, formation of Ag@Fe3O4 NCs and polythiophene species 4 is confirmed. The AAS analysis showed the presence of 45.20 wt% of iron and 19.6 wt% of silver in Ag@Fe3O4 NCs. On the basis of TGA analysis of polythiophene 4 supported Ag@Fe3O4 NCs, the amount of polythiophene in Ag@Fe3O4 NCs was determined to be 28.00 wt% (Figure S24). The ferromagnetic character of polythiophene 4 supported Ag@Fe3O4 NCs was established by hysteresis measurements (Figure S25 and Table S4). The solution of compound 3 having Ag@Fe3O4 NCs was slowly concentrated. The precipitates obtained after 48 hours were isolated, washed thoroughly with CHCl3 and the filtrate was evaporated to obtain the solid residue. The FTIR spectrum of the solid residue did not show stretching (3092 cm-1) and out of plane bending (890 cm-1) peaks of C-Ha bond in the vicinity of „S‟ atom of the 3-thienyl groups (Figure S26).41 However, the absorption peaks in the range of 400-600 cm-1 (Fe-O vibration) and 1028 cm-1 (Ag0) were observed.42-43 In ESI-MS spectrum of the solid component, a peak was observed at m/z 1413.0007 which validated the mass of trimer [C90H44O6S6H]+ (Figure S27). All these spectroscopic data confirms the ortho-Ha-Ha coupling of thiophene rings. In conclusion, in the presence of assemblies of compound 3, Ag+ ions and Fe3+ ions are reduced to form nanoclusters. This reduction event is accompanied by the oxidation of assemblies of compound 3 to polymeric species 4 (Figure S28 and S29). To study the influence of amount of AgNPs and ferric ions on the size and shape of nanocomposites, we mixed AgNPs (dispersed in water) to Fe3+ ions solution in ratio of 2:1 in the


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presence of assemblies of compound 3. The TEM image showed the formation of ellipsoidal shaped core-shell nanoparticles having average size of 100 nm (Figure S30). This study emphasizes the utility of assemblies of compound 3 as shape directing agent. After characterization of polythiophene 4 supported Ag@Fe3O4 NCs, we examined the photophysical behavior of polythiophene species 4. The emission studies of the oxidized species 4 (10 μM) in H2O/THF (1/1) solvent mixture shows the existence of band at 427 nm when excited at 322 nm. Further, the fluorescence intensity was reduced to 72% on addition of bare Ag-Fe3O4 nanocomposites (5 mol%) to the above solution (Figure S31A). Finally, a clear spectral overlap between absorption spectrum of core-shell nanoclusters and emission spectrum of polymeric species 4 indicates the energy transfer between coreshell nanoclusters and polymeric component 4 (Figure S31B). To study the role of polymeric derivative 4 in the generation of Ag@Fe3O4 NCs, we synthesized compound 5 having 2-thienyl group at 2-position of pentacenequinone scaffold (Scheme S2, PS47-S48). In the presence of AgNPs and Fe3+ ions, the absorption spectra of the H2O/THF (1/1) solution of compound 5 pointed to the existence of Ag-Fe3O4 nanocomposites (Figure S32A). The TEM image shows the existence of uniformly dispersed Fe3O4 and AgNPs but the formation of core-shell nanoclusters was not observed (Figure S32B). We conclude that presence of 3thienyl groups at 2, 9/2, 10 positions allow the slow and uniform nucleation of Fe3O4 NCs around Ag core via redox process. This study highlights the influence of topology of 3-thienyl groups on pentacenequinone scaffold in the controlled fabrication of polythiophene 4 supported Ag@Fe3O4 NCs. After complete characterization of nanocomposites, we planned to investigate the catalytic efficiency of polythiophene 4 supported Ag@Fe3O4 NCs (1:2) in C-N bond formation via dehydrogenative coupling for synthesizing indazole derivatives. We began our study with easily available and versatile benzophenone phenylhydrazone 6a as model substrate. First of all, the



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dehydrogenative coupling reaction of 6a was carried out in toluene without employing external oxidant or oxygen atmosphere under thermal conditions by using 0.01 mmol of polythiophene 4 supported Ag@Fe3O4 NCs as catalyst (Table S5, entry 1). The reaction took 10 h to complete and afforded the target product 7a in 76% yield. Encouraged by this result, we accomplished the model reaction in toluene-H2O (1:1) solvent mixture utilizing polythiophene 4 supported Ag@Fe3O4 NCs as catalyst (Table S5, entry 2). The yield of target compound 7a was 75% after 9 h. These outcomes suggest the efficiency of as prepared catalytic system in mixed aqueous media to construct C-N bond via dehydrogenative coupling. Thereafter, we conducted dehydrogenative coupling of 6a using different solvents (THF, EtOH and CH3CN) as reaction media but formation of target product 7a was not observed (Table S5, entries 3, 4 and 5). Then we tried the dehydrogenative coupling of 6a in the presence of DMSO and 1, 4-dioxane by heating the reaction mixture. The product 7a was acquired in 25% and 30% yields, respectively after 24 h under thermal conditions (Table S5, entries 6 and 7). These studies reveal that toluene-H2O is an appropriate solvent mixture to perform dehydrogenative coupling of hydrazones. Therefore, we chose toluene-H2O as the solvent system for carrying out further studies. Next, we focussed on examining the photocatalytic efficiency of the as prepared catalyst in the model reaction. For this, dehydrogenative coupling of 6a was examined in the presence of polythiophene 4 supported Ag@Fe3O4 NCs in toluene-H2O (1:1) solvent mixture using tungsten filament bulb (100 W) as the irradiation source (Table S5, entry 8). To avert the photoheating effect, the reaction flask was submerged in a water vessel. The reaction took 8 h to complete and afforded the target compound 7a in 78% yield. This study clearly demonstrates the photocatalytic efficiency of polythiophene 4 supported Ag@Fe3O4 NCs in C-N bond formation via dehydrogenative coupling.


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Photocatalyst Polythiophene 4

AgNPs

AgNPs Polythiophene 4

Fe3O4 NCs

6a (1:1)

Fe3O4 NCs

7a

Polythiophene 4 Ag@Fe3O4 Nanocomposites

Ag@Fe3O4 Nanocomposites Polythiophene 4

Figure 2 Schematic illustration of dehydrogenative coupling of 6a in presence of different photocatalysts.

To investigate the role of Ag@Fe3O4 NCs/assemblies of compound 3 in dehydrogenative coupling reactions, we performed a series of experiments (Figure 2). The dehydrogenative coupling of 6a did not yield the desired product under visible irradiations using either polymeric species 4 (Table S6, entry 2) or bare AgNPs (prepared by reported method)

44

as catalyst (Table S6, entry 3). We also

investigated the model reaction using AgNPs and polythiophene species 4 as photocatalyst, but the reaction did not proceed (Table S6, entry 4). Then, we prepared bare Fe3O4 NCs by previously reported method45 and utilized them as photocatalyst in carrying out the same coupling reaction. The target product 7a was acquired in traces (Table S6, entry 5). Even the addition of polythiophene species 4 did not improve the yield of target compound under photocatalytic conditions (Table S6, entry 6). Further we utilized bare Ag@Fe3O4 nanocomposites (prepared by reported method)

21

as

photocatalyst for conducting coupling reaction of 6a. The product 7a was acquired in 20% yield after 24 h (Table S6, entry 7). Very interestingly, the yield of the desired product 7a was enhanced to 62%



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when polythiophene species 4 was added to the above reaction mixture under photocatalytic conditions (Table S6, entry 8). These studies suggest that Fe3O4 NCs shell is the active catalytic centre and it requires the assistance of polythiophene species 4 and AgNPs to work efficiently under photocatalytic conditions. Further, we also studied the dehydrogenative coupling of 6a using 0.01 mmol of compound 5 stabilized Ag-Fe3O4 nanocomposites. The target compound 7a was obtained in 50% yield but longer reaction time (24 h) was required (Table S6, entry 9). This study highlights the utility of polythiophene species 4 in dehydrogenative coupling reactions. Higher catalytic activity of polythiophene 4 supported Ag@Fe3O4 NCs is ascribed to the modification in the electronic structure of Fe3O4 shell by the Ag core.46 We believe that the presence of polythiophene 4 prevented the aggregation of Ag@Fe3O4 NCs, thus enhancing the catalytic efficiency. To understand the role of quinone moiety as internal oxidant in catalytic reaction, we performed the model reaction in the presence of Ag@Fe3O4 NCs and supramolecular assemblies having polythiophene backbone but lacking in the quinone moiety. Recently, we reported the generation of hetero-oligophenylene based polythiophene species 8 during the preparation of AuFe3O4 nanocomposites (Figure S33).9 We utilized this polythiophene species 8 for carrying out the model reaction using bare Ag@Fe3O4 nanocomposites employing visible light source. The required product 7a was obtained in 22% yield (Table S6, entry 10). However, yield was increased to 38% when we added the external oxidant (benzoquinone) to the reaction mixture (Table S6, entry 11). This result support our assumption that polythiophene species 4, itself is acting as an oxidant in the coupling reaction. In order to study the role of shape/size of nanocomposites, we investigated the dehydrogenative coupling of benzophenone phenylhydrazone 6a in toluene-H2O solvent mixture


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utilizing polythiophene 4 supported ellipsoidal shaped Ag@Fe3O4 NCs of average size 100 nm [mixing AgNPs and Fe3+ ions (2:1) in presence of assemblies of compound 3] under photocatalytic conditions. The compound 7a was acquired in 67% yield (Table S6, entry 12). This study suggests that size and shape of the nanocomposites influences their catalytic efficiency. The BET surface area of polythiophene 4 supported Ag@Fe3O4 NCs (1:2) was calculated to be 48.18 m2/ g which was higher than all the other photocatalysts mentioned in Table S6. Thus, excellent photocatalytic activity of polythiophene 4 supported Ag@Fe3O4 NCs (1:2) is attributed to their large surface area and small size. Table 1 Dehydrogenative coupling of phenylhydrazones bearing different halides utilizing polythiophene 4 supported Ag@Fe3O4 NCs as photocatalyst.

S. No. 1. 2. 3. 4.

R1 -H (6a, 7a) -F(6b, 7b) -Cl (6c, 7c) -Br (6d, 7d)

Yield 78% 64% 66% 70%

Time 8h 10h 9h 9h

Further, we evaluated the substrate scope of the as prepared photocatalyst by carrying out dehydrogenative coupling of benzophenone phenylhydrazones substituted with different halides under established optimal conditions (Table 1, PS35-S38). Higher yield was obtained in case of phenylhydrazone bearing bromo group (Table 1, entry 4). We further applied this protocol to various benzophenone phenylhydrazone derivatives bearing different electron rich and electron poor groups using polythiophene 4 supported Ag@Fe3O4 NCs (1:2) as a photocatalyst in toluene-H2O (1:1) solvent mixture (Table 2, PS39-S44). The yields of target compounds were moderate in case of reactions



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involving phenylhydrazones substituted with electron-poor groups (Table 2, entries 1 and 2) while the reactions of the phenylhydrazones having electron-rich groups furnished the desired products in 7590% yields (Table 2, entries 3-5). Interestingly, when phenylhydrazone 6j bearing methoxy group was employed as coupling partner, regioselectivity was observed (Table 2, entry 6). Thus, efficiency of dehydrogenative coupling reaction is dependent on the electronic nature of phenylhydrazones. In all the cases, photocatalyst was separated magnetically and pure products were obtained by recrystallization. Table 2 Scope of dehydrogenative coupling of phenylhydrazones utilizing polythiophene 4 supported Ag@Fe3O4 NCs as photocatalyst.

S. No. R1 R2 1. -F (6e, 7e) -F (6e, 7e) 2. -Cl (6f, 7f) -Cl (6f, 7f) 3. -CH3 (6g, 7g) -CH3 (6g, 7g) 4. -OCH3 (6h, 7h) -OCH3 (6h, 7h) 5. -H (6i, 7i) -CH3 (6i, 7i) 6. -H (6j, 7j) -OCH3(6j, 7j) Further, recyclability of photocatalytic system was checked by

Yield Time 65% 10h 68% 9h 85% 7.5h 90% 7h 75% 7h 84% 7h subjecting the model substrate 6a to

dehydrogenative coupling in the presence of polythiophene 4 supported Ag@Fe3O4 NCs (1:2). After completion of reaction, the photocatalyst was retrieved magnetically (Figure S34) and was recycled under the optimal conditions for next catalytic run. Upto eight consecutive runs, the complete transformation of 6a to 7a occurred. Thus, activity of the Ag@Fe3O4 NCs (1:2) supported by polythiophene 4 was maintained during these eight runs (Figure S35). This photocatalytic system was


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also examined for its reusability by monitoring dehydrogenative coupling of model substrate 6a. In first run, complete transformation of 6a to 7a was observed. Afterwards, the same solution was subjected to next cycle. Thus, polythiophene 4 supported Ag@Fe3O4 NCs (1:2) was found to be reusable upto eight times for the synthesis of indazoles via dehydrogenative coupling. To explore the mechanism of this transformation, the dehydrogenative coupling of 6a was carried out in the presence of TEMPO under the optimized conditions. In presence of 1.0 equiv. of TEMPO, the yield of coupling product 7a was 46% (Table S7, entry 2). However, the yield of desired product decreased to 15% when the substrate 7a was subjected to dehydrogenative coupling utilizing 2.0 equiv. of TEMPO (Table S7, entry 3). Meanwhile, formation of 6a-TEMPO adduct was detected by HRMS studies (Figure S36). This result indicates the involvement of radical species in this catalytic transformation. Based on the above experimental results, a plausible mechanism for visible light promoted dehydrogenative coupling of benzophenone phenyl hydrazones is proposed. Visible light empowered photoexcited electrons in the polythiophene 4 supported Ag@Fe3O4 NCs generate superoxide anions by catalyzing the decomposition of dissolved O2 (Scheme S4).47-48 These superoxide anions abstract proton from reactant 6a to afford anionic intermediate A, which through SET process affords radical B, thus, regenerating the catalyst.49 Then intermediate B undergoes radical addition to the aryl ring, followed by oxidation, deprotonation and elimination of H2O2 to furnish the desired product 7a.50 Conclusion:

In summary, fluorescent aggregates of derivative 3 having 3-thienyl groups at 2, 9/2, 10 positions serve as nanoreactors for the generation of Ag@Fe3O4 NCs in mixed aqueous media. Interestingly, as prepared polythiophene 4 supported Ag@Fe3O4 NCs serve as recyclable photocatalyst for the 15 ACS Paragon Plus Environment


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construction of indazoles via dehydrogenative coupling of phenylhydrazones. In this photocatalytic system, Ag core acts as visible light antenna, magnetic Fe3O4 shell plays the role of active catalytic centre and sheet like polythiophene species 4 serve as a support for the catalytic nanocomposites as well as an oxidant for carrying out dehydrogenative coupling of derivatives of benzophenone phenylhydrazones. Further, magnetic Fe3O4 shell enables the catalytic system to be recycled (upto eight times) just by exerting an external magnetic field.

ASSOCIATED CONTENT Supporting information “Characterization data, optical properties of compounds 3 and 5, NMR data of dehydrogenative coupling products, 7a-7j and tables showing comparison of photocatalytic/catalytic activity of present system over other catalytic systems reported in literature. This material is available free of charge via the Internet at http://pubs.acs.org. ” AUTHOR INFORMATION *Corresponding Author: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS “V.B. is thankful to Science and Engineering Research Board (SERB), New Delhi (ref no.EMR/2014/000149) for financial support. R.C. is thankful to UGC (New Delhi) for a Senior Research Fellowship (SRF). We are also thankful to UGC (New Delhi) for the "University with Potential for Excellence" (UPE) project.”


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Table of Contents Polythiophene supported Ag@Fe3O4 nanoclusters

Dehydrogenative coupling

H2O-Toluene

= Ag0 Fe3O4 = Fe3O4 nanoclusters = Polythiophene nanosheets

Ag

Recyclable Photocatalytic Oxidant free

Indazole compounds

20 nm

Synopsis Polythiophene supported Ag@Fe3O4 nanoclusters were characterized and evaluated for the first time in application of C-H to C-N bond transformation.


Fabrication of Polythiophene-Supported Ag ... - ACS Publications

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