CTAB-assisted synthesis of reduced graphene oxide supported Pd nanoparticles(Pd@rGO) as a sustainable heterogeneous catalyst for C-2 arylation of indoles with arylboronic acids.

A set of Pd nanoclusters embedded in rGO, referred to as Pd@rGO (viz. Pd@rGO 0.16 , Pd@rGO 0.32, Pd@rGO 0.48, Pd@rGO 1 ), where subscripts indicate the mmol of cetyltrimethylammonium bromide (CTAB) were synthesized using water as a solvent by simply varying the amount of CTAB that was used to control both morphology and size. TEM analysis indicated that the average particle sizes of Pd@rGO 0.16 and Pd@rGO 0.32 fall in the range of 4.5-5.0 nm and 20–25 nm, respectively. On the other hand, particles were found to be agglomerated in Pd@rGO 0.48 and Pd@rGO 1 . The Pd@rGO 0.16 composite was exhaustively characterized by TEM, SEM-EDAX, powder X-ray diffraction, XPS, and ICP-AES measurements. ICP-AES analysis of Pd@rGO 0.16 indicated that 0.01g of Pd@rGO 0.16 contains 0.09 mol % Pd. The catalytic potentiality of these NPs was investigated for direct C(sp 2 )-H bond activation of various indoles with aryl boronic acids. Among the four composites, Pd@rGO 0.16 exhibited the best activity for the abovementioned organic transformation. Different indoles with varying electronic groups underwent coupling with aryl boronic acids giving up to 86% product yield. It was retrievable for up to �ve consecutive catalytic cycles without compromising its catalytic activity.


Introduction
C-H bond activation reactions have emerged as a potent method for functionalizing organic molecules (Moncea et al. 2018).The class of nitrogen-containing heterocycle compounds known as arylated indole moieties possess a variety of bene cial properties.These properties include antibacterial activity, anti-cancer properties, anti-in ammatory effects, cytotoxicity, and antioxidant properties.The wide range of biological and pharmaceutical applications of arylated indole moieties make them an intriguing subject of study (Nandi et al. 2016, Devaraj et al. 2016, Sandtorv 2015).
There are several classical methods that have been documented for synthesizing functionalized indoles, which are well-known up to present day (Bheeter et al. 2016, Vila et al. 2019).The synthesis of indoles with selective arylation at the C3 and C2 positions has been achieved through methods that are guided by directing groups, reagents, and ligands (Leitch et al. 2017, Li et al. 2010, Kalepu et al. 2019).
Researchers across the world have reported C-2 arylation of indole with various coupling partners such as aryl halides, aryl boronic acid, and benzoic acid, etc using transition metal-based catalytic systems viz Rh, Pd, Ru, Co, Ir, Cu, and Fe (Sandtorv et Gao et al. 2017).Shi and colleagues successfully performed the aforementioned reaction by utilizing Pd(OAc) 2 as a catalyst and conducting the reaction in an oxygen atmosphere within acetic acid (Yang et al. 2008).Cheng and co-workers have reported aryltri uoroborate salts as coupling partners in the presence of Pd(OAc) 2 and Cu(OAc) 2 in the air (Zhao et al., 2008).Despite the signi cant advantages, most of the protocols are still suffering from several issues such as harsh reaction conditions, expensive metal catalysts, multi-step catalyst synthetic procedures, acidic solvents, high reaction temperature, long reaction time, limited functional group tolerance, as well as a narrow substrate scope and homogenous catalytic systems in many cases causing di culty in catalyst recovery and reuse (Bhattacharjee et al. 2020, Figueiras et al. 2022, Saya et al. 2019, Yun et al. 2020).Therefore, the direct C-H arylation of indoles continues to be a crucial challenge, and opportunities for developing mild and e cient protocols are abundant.The activation of C-H bonds in N-methoxy-1H-indole-1-carboxamides was achieved by Cui and co-workers using aryl boronic acids as reagents.They obtained two different products when Cu(OAc) 2 was employed as oxidant 2-arylated indoles was obtained exclusively.However, when Ag 2 O was introduced as an oxidant, the tetracycle was formed mostly (Zheng et al. 2014).This gives insight into the role of oxidants in the reaction (Yeung et al. 2011).To date, very less reports are available for direct C-H arylation of indoles using heterogeneous Pd catalysts.A mesoporous metal-organic framework was utilized by Rong Cao and co-workers as a host material to synthesize Pd nanoparticles and applied them for C-2 arylation, but the use of O 2 and an acidic medium makes it less viable to the environment (Huang et al. 2013).The research group led by Henri Doucet found that Pd/C is a suitable heterogeneous catalyst for the direct arylation of (poly) uorobenzene at 150 0 C and observed low yield even after 16 h reaction time (Mao et al. 2019).Utpal Bora and co-investigators reported that Pd@C, when employed as a heterogeneous catalyst, can be used repeatedly for the direct arylation process (Bhattacharjee et al. 2020).The application of reduced graphene oxide-supported palladium nanoparticles, designed to speci c size and shape, could be a solution for boosting the e ciency of the C-H arylation of indoles when compared to other catalysts.Graphene oxide is a highly sought after material for catalyst heterogenization due to its numerous desirable properties.It boasts a large surface area, edge reactivity, and superior thermal, mechanical, and electrical properties.Furthermore, its unique 2-D structure featuring oxygen-containing functional groups, such as hydroxyl, epoxy, carboxyl, and carbonyl, enables metal to bind to it more easily, making it a prime location for crystal growth ( 1017) The interaction between GO and metal also results in the formation of new micropores at their interface, which greatly increases the availability of reactants to the active sites (Wang et al. 2010, Dreyer et al. 2010, Jahan et al. 2010, Rezaei et al. 2016).In addition to these, the synergism between two components in GO@M composite (M = metal) via internal electron transfer process certainly improve the catalytic activity (Zubir et al. 2015).
For this study, we set out to explore the synthesis of reduced graphene oxide-supported Pd nanoparticles (Pd@rGO) by varying the molar concentration of CTAB to tune the size and shape of the NPs.The catalytic performances of these NPs have been delved into the C-2 functionalization of indoles through the use of aryl boronic acids.

Materials
All reagents were obtained from commercial suppliers unless stated otherwise and used as received.
FT-IR spectra were recorded using a Shimadzu IR prestige-21 spectrophotometer in KBr pellet form (400-4000 cm − 1 ).UV-vis spectra (200-800 nm) were taken using a Jasco V-750 spectrometer.PXRD patterns were recorded with a Bruker D8 Advance X-ray diffractometer.TEM micrographs were obtained using a JEM 2100 instrument at SAIF, Cochin.SEM-EDX analysis was performed using a Jeol 6390A device.X-ray photoelectron spectra were obtained at SAIF, IIT Rookee.ICP-AES spectra were received from SAIF, IIT Bombay using an ARCOS simultaneous ICP spectrometer.1H NMR spectra were recorded in CDCl 3 with TMS as the internal standard, using a Bruker Ascend 500 MHz spectrometer.

Methods
Synthesis of graphene oxide (GO) from graphite Graphene oxide was synthesized using the Hummer method (Sultana et al. 2020).5g of graphite powder and 2.5g of NaNO3 were mixed with 120 mL of H 2 SO 4 in a 500 mL round bottom ask and stirred at 0-50°C for 30 minutes in an ice bath.15g of KMnO 4 was slowly added to the mixture, maintaining the reaction temperature below 20°C.The ice bath was then removed and the mixture was stirred overnight until it turned a hasty brown color.150 mL of distilled water was slowly added to the reaction mixture, keeping the temperature at 98°C for a day, followed by the slow addition of 50 mL of 30% H 2 O 2 , which turned the mixture bright yellow.The product was puri ed by washing with 5% HCl and deionized water, dried in a vacuum oven at room temperature, and obtained as a powder.

Synthesis Of Pd@rgo Nanoparticles
The synthesis of Pd@rGO NPs was performed using a straightforward method (Xuehong et al. 2011).0.03 g (0.16 mmol) of PdCl 2 was rst dissolved in 100 mL of distilled water at room temperature, then HCl was added while stirring until a clear reddish-brown solution was obtained.The pH of the solution was adjusted to 7 by adding NaOH solution (0.5mol/L).
Next, 0.060 g (0.16 mmol) of CTAB was added to 30 mL of water, stirred at 50 0 C for 10 minutes, and then the PdCl 2 solution was added to it, causing the color to change to orange.A NaBH 4 solution was prepared by dissolving 0.006 g (0.16 mmol) in 5 mL of water, which was then added dropwise to the mixture, causing the color to immediately turn dark brown.
The solution was stirred for 1 hour at 80 0 C and then 500 mg of graphene oxide (GO) was added to it and stirred for another 3 hours.The mixture was allowed to cool and stirred overnight.The nal product was collected after centrifugation and washed with acetone, ethanol, and water, then dried at 50 0 C in a vacuum oven for 10 hours.This material was designated as Pd@rGO0.16.

A typical procedure for the C-2 arylation of indoles with arylboronic acids
To an oven-dried 25 mL round-bottomed ask containing 0.6 mmol indole, 1.2 mmol aryl boronic acid, 1.2 mmol sodium acetate, 0.010g (0.09 mmol) Pd/ rGO was added in 5 mL DMSO and allowed to stir in the presence of air at 120 0 C for 3 to 12 h.The reaction's advancement was kept track of through TLC (Thin Layer Chromatography) by using hexane as the eluent at various time intervals.Upon completion of the reaction, monitored by GC-MS, the catalyst was separated from the reaction mixture through simple ltration.The ltrate was diluted with 25mL of EtOAc and treated with a saturated NaHCO 3 aqueous solution (2x15mL).The organic layer was dried using anhydrous Na 2 SO 4 , ltered, and evaporated under decreased pressure.The desired product was obtained through puri cation of the residue by silica gel column chromatography with an eluent of n-hexane: EtOAc (9:1).The used catalyst was washed successively with water and ethyl acetate, then centrifuged.It was dried under vacuum in preparation for the recycling test, which involved multiple consecutive runs.

Results And Discussion
The shape, size, and activity of nanoparticles depend on various parameters such as the nature and amount of surfactant, reducing agent, the concentration of precursors, nature of solvents, reaction time, etc., and can be tailored by varying these parameters.We have used a simple and comparatively green procedure to synthesize the different nanoparticles by simply changing the concentration of CTAB in an aqueous solution.TEM analysis showed that the use of 0.16 mmol of CTAB in 30 mL of water resulted in spherical NPs with an average particle size of 4.5 to 5 nm.On the other hand, using 0.32 mmol of CTAB in 30 mL water produced a mixture of spherical and rod-like NPs with an average particle size of 20 to 25 nm.It was discouraging to observe that 0.48 and 1.0 mmol of CTAB produced NPs in totally agglomerated form.
In an aqueous solution, CTAB generates CTA + and Br − ions.The CTA + ions then interact with [PdBr4] 2− ions formed in situ, forming complex-surfactant [CTA + ] 2 [PdBr 4 ] 2− through electrostatic interaction, which then assembles into micelles.This surfactant, formed upon mixing PdCl 2 and CTAB solution, may be the actual precursor in the reduction process.The nal morphology of the Pd particles was determined by the speci c micellar structure of [CTA] 2 [PdBr 4 ] and stabilized by CTAB coordination to the surface of the Pd particles after reduction.As the concentration of CTAB increases, the interaction between the [CTA + ] 2 [PdBr 4 ] 2− entities becomes stronger, resulting in the formation of spherical to network-like Pd nanostructures, and eventually, agglomerates (Xuehong et al. 2011).

Morphological characterization
The morphological and surface characterization of Pd@rGO composites with the different molar concentrations of CTAB was performed by using UV-vis absorption, PXRD, TEM, SEM-EDAX, and XPS techniques.

UV-vis study
The UV-vis absorption spectrum of GO showed absorption at 235 and 300 nm could be assigned to π-π* and n-π* transition of C = C and C = O bonds, respectively.The reduction of GO was con rmed upon the formation of the Pd@GO composite by disappearance of bands at 235 and 300 nm, along with the observation of a new band at 270 nm (Fig. 1).Moreover, the absence of any peak above ~ 415 nm a rmed the formation of Pd NPs.

Powder X-ray Diffraction study
The XRD patterns of GO, bare rGO, and Pd@rGO are depicted in Fig. 2. As depicted in Fig. 2(a) characteristic sharp peak at 2θ value of 11.2 0 could be assigned to the diffraction from the (001) basal plane of GO (Krishna et al 2017).The XRD pattern of bare rGO showed two peaks at 2θ values of 26.1 0 and 42.12 0 could be assigned to (002) and (102) planes, respectively (Fig. 2).After the formation of the rGO@Pd composite the x-ray diffraction pattern shows a series of diffraction peaks at 2θ = 40.1 0 , 46.6 0 , 68.21 0 , and 82.10 0 attributed to the (111), ( 200), (220), and (222) crystal planes of Pd NPs along with peaks at 2θ values of 26.60 and 42.25 0 arises due to the diffraction from the (002) and (102) planes of rGO (JCPDS 05-0681) ( Ma et al. 2017, Saikia et al. 2016).This enumerated successful immobilization of Pd onto rGO sheets.The XRD pattern of rGO@Pd agrees with the results obtained from the HR-TEM analysis (Fig. 3).

HR-TEM analysis
The TEM micrographs unambiguously showed the occurrence of Pd NPs over graphene oxide sheets.
The morphology and size of the Pd NPs change with the change in CTAB concentration.As revealed in Figs.3(a) and (b), the average particle sizes of Pd@rGO 0.16 and Pd@rGO 0.32 fall in the range of 4.5-5.0nm and 20-25 nm, respectively.On the other hand, particles were found to be agglomerated in the case of Pd@rGO 0.48 and Pd@rGO 1 .It was interesting to observe that 0.16 mmol of CTAB in 30 mL water produced spherical NPs, while 0.32 mmol of CTAB in 30 mL water gave spherical and rod-like NPs.However, the application of 0.48 and 1.0 mmol of CTAB produced NPs in an agglomerated form.The SAED pattern of Pd@rGO 0.16 (Fig. 3a) showed concentric diffraction rings resulting from re ection of (111), (200), (220), and (311) planes corresponding to 2.312 Ǻ, 1.891 Ǻ, 1.336 Ǻ, 1.171 Ǻ of d spacing values and Pd@rGO 0.48 also showed concentric diffraction rings due to the re ection of (111), ( 200

SEM-EDAX and ICP-AES study
Figure 5(a) demonstrated the SEM image of Pd@rGO 0.16 .In the SEM micrograph, rGO sheets appeared clearly which were randomly aggregated with distinct edges, folded, and wrinkled surfaces.The EDAX analysis con rms the presence of C, O, and Pd in proportions (Fig. 5b).

XPS study
To uncover the surface composition and oxidation state of palladium in the composite material Pd/rGO 0.16 its XPS study was carried out [Figure 6(a)].The high-resolution XPS of the composite exhibited two peaks at 335 eV and 340 eV attributed to Pd 3d 5/2 and Pd 3d 3/2 spin-orbit components of Pd(0) (Yang et al. 2014).The full XPS survey scan of Pd/rGO 0.16 nanocomposite with atomic percentages has been incorporated in (Figure S1).The C1s core level XPS of Pd/rGO 0.16 [Figure 6

Pd/rgo Catalyzed C-2 Arylation Of Indoles With Arylboronic Acids
To investigate the catalytic activity of Pd/rGO for the above-mentioned reaction a set of reactions were carried out between indole and phenylboronic acid under various reaction conditions (Table 1).We have evaluated multiple solvents to identify a suitable one, as the solvent plays a crucial role in product formation.Using MeOH as a solvent with various oxidants such as tert-Butyl hydroperoxide (TBHP), Cu(OAc) 2 .H 2 O, and AgNO 3 at 50 0 C we received low yields of the products (Table 1, entries 2-4) while using Ag(OAc) and NaOAc as oxidant and keeping other conditions same we got 78% and 82% isolated yields (Table 1, entries 5&6).Another two protic solvents EtOH and i-PrOH with NaOAc at 50 0 C gave 80% and 65% product yield (Table 1, entries 7&8), while aprotic solvents such as H 2 O, CH 3 CN, and THF were found unsuitable for the present protocol (Table 1, entries 9-11).On the other hand, between DMSO and DMF, DMSO gave 83% product yield at 50 0 C (Table 1, Entries 12 & 13).Since DMSO served the purpose best we performed the rest of the optimization reactions in DMSO solvent.We have observed that with the increase of temperature up to 120 0 C product yield increases slightly while lowering of temperature to 25 0 C resulted in a 30% yield (Table 1, entries 18 & 25).Moreover, we have carried out a series of reactions with varied catalytic species viz.Pd/rGO 0.16 , Pd/rGO 0.32 , Pd/rGO 0.48 , PdCl 2 ,Pd(OAc) 2 , rGO and found Pd/rGO 0.16 as the best among all (Table 1, entries 19,[22][23].It is noteworthy since in the presence of rGO a trace amount of product formed that's why the Pd nanoparticle was the true catalyst (Table 1, entry 24).After getting the best catalyst we carried out reactions with varied mol% of Pd/rGO 0.16 and observed that while enhancing the amount of Pd/rGO 0.16 from 0.09 mol% to 0.18 mol% and 0.27 mol% although product yield increased by 1%, with lowering the catalyst amount product yield also decreased (Table 1, entries [15][16][17].Again, in the absence of an oxidant and catalyst, the reaction failed to proceed (Table 1, entries 1& 21), and it a rmed the importance of both.As oxidant is an inevitable part of C-2 arylation of indoles, the reaction was conducted with the addition of several oxidants like TBHP, NaOAc, Cu(OAc) 2 , AgNO 3 , Ag(OAc), and NaOAc proved to be the most appropriate oxidant for this reaction (Table 1, entry 14).These results demonstrated that 0.09 mol% of Pd/rGO 0.16 catalyst in 4 mL of DMSO solvent with NaOAc as oxidant at 80 0 C temperature is the best suitable reaction condition to carry out the said organic transformation (Table 1, entry 14).It was fascinating to note that the current protocol did not require the use of any directing groups or ligands to attain C-2 selectivity.a Reaction conditions: Indole (1 mmol), phenylboronic acid (1.2 mmol), catalyst (10 mg, 0.09 mol%), solvent (4 mL), temperature (25-120 0 C), time (6 h), in air.
Pd/rGO 0.16 nanocomposite was synthesized using 0.16 mmol of CTAB; 12mg and 9 mg of Pd /rGO 0.32 and Pd /rGO 0.48 NPs contain 0.09 mol% of Pd, respectively.NR: No reaction After the preliminary insight into the reaction, multiple types of aryl boronic acids and indoles were studied to examine the scope and limitation of the catalyst under the optimized condition.The results are summarized in Table 2.The use of phenylboronic acid with substituents that have both electrondonating and electron-withdrawing properties resulted in good yields of the desired products ( 82-86%) (Table 2, entries 1-4).The results showed no connection between the electronic character of the aryl boronic acid substituents and the reaction yield, as good yields of products were obtained with both electron-donating and electron-withdrawing groups in the phenylboronic acid.Conversely alkyl boronic acid remained ineffective (Table 2, entry 7) and thiophene-3-yl boronic acid and furan-2-yl boronic acid produced moderate yields of the arylated products (Table 2, entries 5&6).
Contrarily, the electronic nature of the indole component signi cantly impacts the C-2 selective direct arylation reaction.The reactivity of indoles with electron-donating groups was higher and resulted in better yields (Table 2, entries 8 &9), compared to indoles with electron-withdrawing substituents.viz.Cl, which furnished about 70-75% of the desired product (Table 2, entries 10&11) and NO 2 did not undergo the transformation (Table 2, entries 12&13).
Catalyst Leaching And Reusability Of The Catalyst Hot ltration test A test was conducted using hot ltration to assess the catalyst's heterogeneity.A reaction mixture was prepared by mixing indole (1mmol), phenylboronic acid (1.2mmol), sodium acetate (41mg, 0.5mmol), Pd/rGO0.16 catalyst (10mg, 0.09 mol%), and DMSO (4mL) in a 25 mL round-bottomed ask, and stirring the mixture at 80°C in air.After 30 min the reaction was stopped and the solid phase was ltered off using a Whatman lter paper (Grade 41).The reaction was allowed to proceed for another one and a half hour without the catalyst this resulted in no increase in the product yield (Figure S2).The analysis of the aqueous phase by ICP-AES showed a low residual level of Pd, less than 0.01 ppm.This suggests the non-leaching of Pd during the catalytic reaction and a rmed the true heterogeneous nature of the catalyst.

Reusability test
Reusability is a signi cant aspect of a catalyst in terms of catalyst sustainability as well as cost.In the current protocol, Pd@rGO 0.16 was recycled for ve consecutive runs with a loss of ~ 7% isolated yield of the product (Fig. 7a).To conduct the experiment, we took indole (1mmol) and phenylboronic acid (1.2 mmol) as coupling partners, NaOAc ( 41 mg, 0.5 mmol) and 10 mg (0.09 mol% of Pd) of Pd@rGO 0.16 catalyst in DMSO.After completion of the reaction, the catalyst was separated by centrifugal precipitation, washed, dried, and reused in a new coupling reaction.A probable reason for the yield drop might be due to the partial saturation of the pore size of the catalytic surface after repeated runs or the physical loss of the catalyst during the recovery process. [42]TEM and SEM images of the reused catalyst (after the 5th cycle) indicated retention of its structure (Fig. 7b and Fig. 7c).
We The formation of Pd(II) species was evident from the high-resolution XPS analysis of reused catalyst which clearly showed peaks for Pd(II) species along with Pd(0) ( Fig. 8).The peaks at binding energies of 335.5 eV and 340.7 eV indicate the presence of Pd(0) species.Another two peaks at binding energies of 337.9 eV and 342.9 eV correspond to the Pd(II) species (Bhattacharjee et al. 2020).The Pd(II) is attributed to the generation of Pd(OAc) 2 during catalysis as shown in scheme 1.
The current protocol appears to be more e cient in terms of low Pd loading, less time-consuming compared to most of the reported methods, retrievable at least for ve cycles without any apparent leaching, and more importantly, does not require any directing groups or ligands to achieve C-2 selectivity.Table 3 highlights the improved catalytic activity and reaction conditions as well, of the present protocol compared to that of similar Pd-based heterogeneous catalytic systems reported so far.

Conclusion
In summary, we successfully developed a sustainable, environment-friendly, operationally simple, and economic protocol for direct C-2 selective arylation reaction of indoles using a low-loaded palladium catalyst i.e.Pd/rGO 0.16 (10 mg of Pd/rGO 0.16 contains 0.09 mol% of Pd).The size and shape of the catalyst were tuned by using 0.16 mmol of CTAB as a stabilizer.Another three samples of Pd/rGO (Pd/rGO 0.32 Pd/rGO 0.48 and Pd/rGO 1 ) were also prepared by simply varying the amount of CTAB to 0.32 mmol, 0.48 mmol, and 1 mmol.However, it was revealed that with the increase of CTAB concentration NP's size, shape changes, and agglomeration occurred which led to low catalytic e ciency.The presence of 0.09 mol% of Pd in 10 mg of the catalyst conferred excellent catalytic activities with a wide functional group tolerance to the composite.It is truly heterogeneous in nature and reused up to ve cycles with the retention of catalytic performance.

Declarations
Table 2 Table 2 is available in the Supplementary Files section.

Scheme 1
Scheme 1 is available in the Supplementary Files section.
(b)] showed C1s binding energy at 284.68 eV, 286 eV, 289 eV attributed to C-C, C-O, and O-C = O respectively.A clear shifting of Pd 3d 5/2 , Pd 3d 3/2, and C1s peaks for C-O and O-C = O functional groups to lower binding energy was noticed.This occurrence may be caused by the creation of a Pd-O-C type surface structure or linked to the presence of small particles with a high number of defective sites and low coordination with oxygen (Sultana et al.2020).
have proposed a probable mechanism for the C-2 arylation of indoles with aryl boronic acids based on literature reports in scheme 1(Bhattacharjee et al. 2020, Lin et al. 2017, Feng et al. 2017).In the very beginning it is expected that active Pd(II) species (B) is generated by the oxidant NaOAc from Pd(0) and deposited on rGO.Aryl boronic acid is activated by the Pd (II) species forming intermediate C. In the presence of strong π-nucleophile indole, electrophilic palladation takes place at the most nucleophilic center (C-3) on the indole nucleus to form an intermediate species D, which undergoes 1,2-migration and forms E and subsequent deprotonation forms intermediate F. The reductive elimination of F produces 2arylated indole with the regeneration of Pd(0) reabsorbed on the rGO surface to complete the catalytic cycle.

Table 1
Optimization of the reaction conditions for the C-2 arylation of indole by phenylboronic acid a

Table 3
Comparison of catalytic activity of the present catalyst for C-2 arylation of indoles with other similar reported systems.