Palladium decorated on a new dendritic complex with nitrogen ligation grafted to graphene oxide: fabrication, characterization, and catalytic application

Immobilized Pd nanoparticles on a new ligand, namely, tris(pentaethylene-pentamine)triazine supported on graphene oxide (Pdnp-TPEPTA(L)-GO) was introduced as a novel and robust heterogeneous catalyst for use in C–C bond formation reaction. The Pdnp-TPEPTA(L)-GO catalyst was synthesized by complexation of Pd with TPEPTA as a ligand with high N-ligation sites that were supported on graphene oxide through 3-chloropropyltrimethoxysilane. The prepared catalyst was characterized using some microscopic and spectroscopic techniques. The TPEPTA(L)-GO substrate is a 2D heterogeneous catalyst with a high specific surface area and a large amount of N-ligation sites. The Pdnp-TPEPTA(L)-GO catalyst used in the C–C bond formation reaction between aryl or heteroaryl and phenylboronic acid derivatives was applied towards the synthesis of biaryl units in high isolated yields. Notably, a series of competing experiments were performed to establish the selectivity trends of the presented method. Also, this catalyst system was reusable at least six times without a significant decrease in its catalytic activity.


Experimental
Optimization study of the reaction of TCT and PEHA Table 1S, Optimization of the reaction conditions for the preparation of TPEPTA (Ligand)  Into a canonical flask (250 mL), a mixture of TCT (9 mmol, 1.66 g) and PEHA (30 mmol, 6.96 g) was stirred in DMF (25 mL) and 0.1 mL of Et3N at 80 C for 12 h under N2 gas as an inert atmosphere.After completion of the reaction, the content of the flask allowed to cool down to room temperature.The precipitate (dark red-orange) was filtered over sinter glass (G-4) and washed thoroughly with aqueous solution of NaHCO3 (10%) and large amount of warm distilled water.The obtained solid product (TPEPTA) was dried at 80 C for 24 h under vacuum conditions (Isolated yields: 68%, 4.7 g).

Determination of chlorine atom on GO using Mohr's method
The Mohr's method was used to determine the amount of chlorine atom of 3-chloropropylsilyl group supported on GO.At first, 100 mg of 3-chloropropylsilyl group supported on GO was dispersed in distilled water (10 mL) and 10.0 mL of NaHCO3 (0.005 M) was added to the solution of functionalized GO.Then, the reaction mixture was sonicated for 2 min under ultrasound irradiation and subsequently the reaction mixture was stirred for 1 h at room temperature.Next, the reaction mixture was filtered over sinter glass (G-4) and thoroughly washed several times with distilled water.Therefore, the filtrate solution has sodium chloride as well as excess amount of sodium hydrogen carbonate.At this time, we could determine the amount of chlorine ions in solution using Mohr's method.The pH of the sample solutions should be between 6.5 and 10 (the pH was checked).This method determines the chloride ion concentration of obtained solution by titration with silver nitrate (AgNO3).As the silver nitrate solution is slowly added, a precipitate of silver chloride forms.The end point of the titration occurs when all the chloride ions are precipitated.Then additional silver ions react with the chromate ions of the indicator, potassium chromate (K2CrO4), to form a red-brown precipitate of silver chromate.For Mohr's method, the following solutions are needed.Silver nitrate solution: (0.1 mol L -1 ) If possible, dry 5.0 g of AgNO3 for 2 h at 100°C and allow cooling.Accurately weigh about 4.25 g of solid AgNO3 and dissolve it in 25.0 mL of distilled water in a conical flask.Store the solution in a brown bottle.Potassium chromate indicator solution (approximately 0.25 mol L -1 ) Dissolving 1.0 g of K2CrO4 in 20.0 mL of distilled water.In according to Mohr's method, the total of chlorine ion in this solution was 0.95 mmol per 100 g of functionalized graphene.Therefore, we obtained the total density of chloropropylsilyl groups on GO was approximately 0.95 mmol g -1 .

Determination of Pd nanoparticles using AAS
The content of Pd nanoparticles on TPEPTA(L)-GO was measured according to the previous reported articles [1,2].A Perkin-Elmer 3110 model atomic absorption spectrometer equipped with an air-acetylene burner and a palladium hollow cathode lamp operated at 20 mA was used for the determination of palladium without background correction.The operating conditions were as follows: wave length: 244.8 nm; band width: 0.2 nm, flow rate of acetylene and air were 2 and 41 min -1 , respectively.The total amount of Pd element on the prepared catalyst was estimated 28 wt%.

Determination of TPEPTA ligand using EDX analysis
The amount of TPEPTA ligand grafted on to GO was calculated through the following equation, using the nitrogen content of TPEPTA from EDX analysis: Where Wt is the weight percent of the element measured, X is the theoretical weight percent of the element in the molecule and Y is the theoretical molecular weight of the molecule.TPEPTA has carbon, hydrogen, and nitrogen contents 51.28, 10.49, and 38.07 wt% with 772.16 g mol -1 , respectively.Based on this equation, the amount of TPEPTA ligand detected from the nitrogen content is 1.43 mmol g -1 [3].

The extent of TPEPTA ligand per C atoms in graphene layers using TGA analysis
For calculation the extent of functionalization of TPEPTA ligand per C atoms in graphene layeres, weight loss values were employed together with the molecular weight of TPEPTA and the following Equation was used.Where X stands for the number of carbon atoms in the graphene layeres sample per each ligand group, R (%) is the residual mass at 600 C in the TGA plot, L (%) is the weight loss between 197-239 C, and Mw is the molecular weight of the TPEPTA ligand.In according the following equation, the extent of the TPEPTA ligand per C atoms in the graphene layers was measured which one ligand per ninety six carbon atoms [4].

Synthesis of compound P21
Approach (a): Into a canonical flask (50 mL), a mixture of 2,4-dimethylphenol (6.0 mmol), 4phenylbenzaldehyde (2.0 mmol), and RGO-SO3H (40 mg) were stirred under solvent-free conditions at 100 C for 2.5 h.The progress of the reaction was followed by thin layer chromatography (TLC) (n-hexane: ethyl acetate; 10:4).After completion of the reaction confirmed by TLC, the reaction mixture was cooled down to room temperature and 15 mL of acetone (35 mL) was added.The RGO-SO3H was filtered under reduced pressure using vacuum pump over sinter glass grade 4. The solution was recovered by evaporation on a rotary evaporator.After that, the solid materials were washed with n-hexane (5 mL) and deionized water, successively, to afford the pure product P21.The desired compound P21 was kept in an oven at 80  C for 12 h.(isolated yield: 81%, 661.8 mg).

Synthesis of compound P22
Compound P22 was synthesized using different approaches from g to j conditions.

Pathway (g):
In a round-bottom flask, 3.3 mmol of 4-aminoacetophenone and 80 mg of RGO-SO3H were stirred in 5.0 mL of toluene as solvent for 16 h under reflux conditions.When the reaction was completed, the flask was cooled to room temperature using cool water.At this time, the product was solidified and the remained solvent was decanted.Then, the hot methanol or ethyl acetate (10 mL) was added to the flask and the catalyst was filtered over sinter-glass grade 4 using vacuum pump.The solution under the sinter-glass was slowly heated during 2 h (approximately 40 C).The crude product was recrystallized in ethanol to obtain the pure products.The pure products were dried and stored at 70 C for 24 h (isolated yield: 62%, 217.8 mg).The RGO-SO3H was prepared according to the previous work [5].

Pathway (h):
The protection of 4-aminoacetophenoe was conducted according to the reported work [6].After this step, the protected 4-aminoacetophenoe was used in cyclotrimerization reaction to obtain protected compound P22 based on previous part.For de-protection step, tertbutyl carbamates were cleaved under anhydrous acidic conditions (total yield: 45%, 158.1 mg).): 1,3,5-tribromobenzene (TBB) (1.0 mmol), 4-aminophenyl boronic acid (1.2 mmol), Pdnp-TPEPTA(L)-GO (30 mg), K2CO3 (2.0 mmol), and DMF: H2O (2:1) (6.0 mL) were placed in a 25 mL round-bottom-flask equipped with a condenser and magnetic stirring bar and heated at 80 C for 1 h.The progress of the reaction was monitored using TLC until the 100% conversion of TBB was confirmed.After completion of the reaction, 5.0 mL of hot water and 5.0 mL of ethyl acetate were added to the reaction mixture.The Pdnp-TPEPTA(L)-GO catalyst was separated under reduced pressure using vacuum pomp over sinter-glass grade-4.The organic solution was evaporated on rotary evaporator and the crude product P22 was obtained.After that, the solid materials were recrystalized in ethanol, successively, to afford the pure product P22.The desired compound P22 was kept in an oven at 80  C for 12 h.(Yield: 89%, 312.7 mg).Pathway (j): 4-bromoaniline (3.3 mmol), 1,3,5-phenyltriboronic acid (1.0 mmol), Pdnp-TPEPTA(L)-GO (60 mg), K2CO3 (2.0 mmol), and DMF: H2O (2:1) (6.0 mL) were placed in a 25 mL round-bottom-flask equipped with a condenser and magnetic stirring bar and heated at 80 C for 1.5 h.The progress of the reaction was monitored using TLC until the 100% conversion of 4bromoaniline was confirmed.After completion of the reaction, 5.0 mL of hot water and 5.0 mL of ethyl acetate were added to the reaction mixture.The Pdnp-TPEPTA(L)-GO catalyst was separated under reduced pressure using vacuum pomp over sinter-glass grade-4.The organic solution was evaporated on rotary evaporator and the crude product P22 was obtained.After that, the solid materials were recrystalized in ethanol, successively, to afford the pure product P22.

Pathway (i
The desired compound P22 was kept in an oven at 80  C for 12 h.(Yield: 80%, 281.1 mg).

Typically procedures for competing experiments (CE)
Procedure for CE-1: Into a canonical flask (50 mL), a mixture of phenylboronic acid (PBA) (1.2 mmol), 4-methoxyphenylboronic acid (4-OMe-PBA) (1.2 mmol), bromobenzene (1.0 mmol), K2CO3 (1.0 mmol), TPEPTA(L)-GO (30 mg), and DMF: H2O (2:1) (6.0 mL) were placed in a 25 mL round-bottom-flask equipped with a condenser and magnetic stirring bar and heated at 80 C for 1 h.The progress of the reaction was monitored using TLC until the 100% conversion of bromobenzene was confirmed.After completion of the reaction, 5.0 mL of hot water and 5.0 mL of ethyl acetate were added to the reaction mixture.The Pdnp-TPEPTA(L)-GO catalyst was separated under reduced pressure using vacuum pomp over sinter-glass grade-4.The organic solution was evaporated on rotary evaporator and the crude product 5a was obtained.
The crude product 5a was purified by column chromatography (n-hexane/ethyl acetate) to obtain the desired purity in 75% isolated yield.
Procedure for CE-2: Into a canonical flask (50 mL), PBA (1.2 mmol), 4-OMe-PBA (1.2 mmol), bromobenzene (1.0 mmol), K2CO3 (2.0 mmol), TPEPTA(L)-GO (30 mg), and DMF: H2O (2:1) (6.0 mL) were placed in a 25 mL round-bottom-flask equipped with a condenser and magnetic stirring bar and heated at 80 C for 1 h.The progress of the reaction was monitored using TLC until the 100% conversion of bromobenzene was confirmed.After completion of the reaction, 5.0 mL of hot water and 5.0 mL of ethyl acetate or ethanol were added to the reaction mixture.
The Pdnp-TPEPTA(L)-GO catalyst was separated under reduced pressure using vacuum pomp over sinter-glass grade-4.Into the organic solution, 10.0 mL of CH2Cl2 was added.After separation of dichloromethane layer from aqueous layer, the aqueous phases were extracted with dichloromethane (2  10 mL) again.The combined organic layers were then dried over anhydrous Na2SO4, filtered and concentrated in vacuum to yield the mixture of crude products 6a and 6b.The crude products 6a and 6b were purified by column chromatography (nhexane/ethyl acetate) to obtain the desired purity of 6a and 6b in 69% and 20% isolated yield, respectively.
Procedure for CE-3: Into a canonical flask (50 mL), PBA (1.2 mmol), 4-OMe-PBA (1.2 mmol), bromobenzene (1.0 mmol), K2CO3 (4.0 mmol), TPEPTA(L)-GO (30 mg), and DMF: H2O (2:1) (6.0 mL) were placed in a 25 mL round-bottom-flask equipped with a condenser and magnetic stirring bar and heated at 80 C for 1 h.The progress of the reaction was monitored using TLC until the 100% conversion of bromobenzene was confirmed.After completion of the reaction, 5.0 mL of hot water and 5.0 mL of ethyl acetate or ethanol were added to the reaction mixture.
The Pdnp-TPEPTA(L)-GO catalyst was separated under reduced pressure using vacuum pomp over sinter-glass grade-4.Into the organic solution, 10.0 mL of CH2Cl2 was added.After separation of dichloromethane layer from aqueous layer, the aqueous phases were extracted with dichloromethane (2  10 mL) again.The combined organic layers were then dried over anhydrous Na2SO4, filtered and concentrated in vacuum to yield the mixture of crude products 7a and 7b.The crude products 7a and 7b were purified by column chromatography (nhexane/ethyl acetate) to obtain the desired purity of 7a and 7b in 61% and 47% isolated yield, respectively.
Procedure for CE-4: Into a canonical flask (50 mL), 4-bromobenzaldehyde (1.0 mmol), 3bromobenzaldehyde (1.0 mmol), PBA (1.2 mmol), K2CO3 (2.0 mmol), TPEPTA(L)-GO (30 mg), and DMF: H2O (2:1) (6.0 mL) were placed in a 25 mL round-bottom-flask equipped with a condenser and magnetic stirring bar and heated at 80 C for 1 h.The progress of the reaction was monitored using TLC until the highly conversion of 3-bromobenzaldehyde and 4bromobenzaldehyde was confirmed.After completion of the reaction, 5.0 mL of hot water and SI-12 5.0 mL of ethyl acetate or ethanol were added to the reaction mixture.The Pdnp-TPEPTA(L)-GO catalyst was separated under reduced pressure using vacuum pomp over sinter-glass grade-4.
Into the organic solution, 10.0 mL of CH2Cl2 was added.After separation of dichloromethane layer from aqueous layer, the aqueous phases were extracted with dichloromethane (2  10 mL) again.The combined organic layers were then dried over anhydrous Na2SO4, filtered and concentrated in vacuum to yield the mixture of crude products 8a and 8b.The crude products 8a and 8b were purified by column chromatography (nhexane/ethyl acetate) to obtain the desired purity of 8a and 8b in 70% and 15% isolated yield, respectively.
Procedure for CE-5: Into a canonical flask (50 mL), 4-bromobenzaldehyde (1.0 mmol), 3bromobenzaldehyde (1.0 mmol), 4-OMe-PBA (1.2 mmol), K2CO3 (2.0 mmol), TPEPTA(L)-GO (30 mg), and DMF: H2O (2:1) (6.0 mL) were placed in a 25 mL round-bottom-flask equipped with a condenser and magnetic stirring bar and heated at 80 C for 1 h.The progress of the reaction was monitored using TLC until the highly conversion of 3-bromobenzaldehyde and 4bromobenzaldehyde was confirmed.After completion of the reaction, 5.0 mL of hot water and 5.0 mL of ethyl acetate or ethanol were added to the reaction mixture.The Pdnp-TPEPTA(L)-GO catalyst was separated under reduced pressure using vacuum pomp over sinter-glass grade-4.The organic solution was evaporated on rotary evaporator and the crude product 9a was obtained.
The crude product 9a was purified by column chromatography (n-hexane/ethyl acetate) to obtain the desired purity in 82% isolated yield.

Procedure for CE-6:
A mixture of 4-bromobenzaldehyde (1.0 mmol), 4-bromoanisole (1.0 mmol), 4-OMe-PBA (1.2 mmol), K2CO3 (2.0 mmol), TPEPTA(L)-GO (30 mg), and DMF: H2O (2:1) (6.0 mL) were placed in a 25 mL round-bottom-flask equipped with a condenser and SI-13 magnetic stirring bar and heated at 80 C for 1 h.The progress of the reaction was monitored using TLC until the highly conversion of starting compounds was confirmed.After completion of the reaction, 5.0 mL of hot water and 5.0 mL of ethyl acetate were added to the reaction mixture.The Pdnp-TPEPTA(L)-GO catalyst was separated under reduced pressure using vacuum pomp over sinter-glass grade-4.The organic solution was evaporated on rotary evaporator and the crude product 5a was obtained.The crude product 10a was purified by column chromatography (n-hexane/ethyl acetate) to obtain the desired purity of 10a in 89% isolated yield.The Pdnp-TPEPTA(L)-GO catalyst was prepared according to the following synthetic route shown in Scheme 2 in the main text of the manuscript.

Characterization of reused
Step 1: Synthesis of TPEPTA ligand Into a two-neck round-bottom flask equipped with magnet and condenser, a mixture of TCT (9.0 mmol, 1.66 g), PEHA (30 mmol, 6.96 g), and Et3N (0.1 mL) were added in dry DMF (25 mL) as solvent at 80 C for 12 h under N2 atmosphere.After completion of the reaction, the reaction mixture was cooled down to room temperature and was filtered and washed with aqueous NaHCO3 10% and warm deionized water.It is noteworthy that washing the TPEPTA with warm deionized water is necessary until all unreacted PEHA and Et3N are eliminated.After drying in the oven at 80 C for 24 h, the TPEPTA ligand was obtained as a dark red-orange powder in 68% isolated yield (4.7 g).
Step 2: Preparation of Graphene oxide and functionalization with 3 chloropropyltrimethoxysilane Graphene oxide was produced from raw graphite powder (Merck Company) regarding our previous work [5].First, sodium nitrate (NaNO3) (2.5 g) and natural graphite powder (5.0 g) were mixed with H2SO4 (98%) (115 mL) in a 1000 mL round-bottom flask equipped with a magnetic stirrer and condenser place in an ice won at 0-5 C.The mixture was stirred and potassium permanganate (KMnO4) (15.0 g) was slowly added during 1 h, and the stirring of mixture was followed for 2 h.The mixture was transferred to water won (35 o C) and stirred for SI-15 30 min.Then, distilled water (230 mL) was slowly added into the mixture and the temperature of mixture was about 98 °C and stirred for 15 min.Then, distilled water (700 mL) and hydrogen peroxide (H2O2) (30%) (50 mL) were sequentially added to the mixture until the reaction was concluded.The final materials were filtered and eluted exhaustively with hydrogen chloride (5%) and distilled water for several times.The solution was filtered under reduced pressure by vacuum pump upon sinter-glass (G4).The graphite oxide powder was obtained after drying in vacuum oven at 60 °C for 12 h.The graphite oxide was dispersed in distilled water to make concentration of 0.5 mg mL -1 , and exfoliated by ultrasound (40 W) for 30 min, followed by centrifugation at 3500 rpm for 30 min to delete un-exfoliated graphite oxide.
After preparation of graphene oxide, Into a two-neck round-bottom flask equipped with magnet and condenser under N2 atmosphere, a mixture graphene oxide (1.0 g) and 3chloropropyltrimethoxysilane (5.0 mL) were added in dry toluene (25 mL) as solvent for 6 h under reflux conditions.After completion of the reaction, the reaction mixture was cooled down to room temperature and was filtered over sinter glass grade-4 and washed with dry toluene.The functionalized graphene oxide with 3-chloropropyltrimethoxysilane was stored in desiccator under N2 atmosphere. Step

3: synthesis of Pdnp-TPEPTA(L)-GO catalyst
Into a two-neck round-bottom flask equipped (250 mL) with magnet and condenser under N2 atmosphere, a mixture of TPEPTA ligand (3.0 mmol, 2.3 g), 3-chloropropylsilyl supported on graphene oxide (1.0 g), and Et3N (0.2 mL) in dry DMF (50 mL), was added and stirred for 12 h at 80 C.Then, the mixture was filtered and washed with aqueous NaHCO3 10%, hot deionized water, and 10 mL of DMF (2  5 mL).After drying in a vacuum oven at 120 °C overnight, the TPEPTA(L)-GO was obtained as a dark solid.At the final step, into a two-neck round-bottom flask equipped (100 mL) with magnet and condenser under N2 atmosphere, TPEPTA(L)-GO (2.0 g), Pd(OAc)2 (0.2 g, 0.9 mmol), and SDS (3.0 mmol) in absolute ethanol (25 mL) at 40 C for 24 h was added and stirred.After the appropriate time, the dark solid Pdnp-TPEPTA(L)-GO catalyst was filtered and washed with deionized water and absolute ethanol and dried at 100 C overnight.To a solution of 4-iodophenol or 4-bromophenol or 4-chlorophenol (1.0 mmol), phenyl boronic acid (1.2 mmol, 0.146 g), Pdnp-TPEPTA(L)-GO (30 mg), K2CO3 (2.0 mmol, 0.276 g), and DMF: H2O (2:1) (6.0 mL) were placed in a 25 mL round-bottom-flask equipped with a condenser and magnetic stirring bar and heated at 80 C.The progress of the reaction was monitored using TLC.After completion of the reaction, 5.0 mL of hot water and 5.0 mL of ethyl acetate or ethanol were added to the reaction mixture.The Pdnp-TPEPTA(L)-GO catalyst was separated under reduced pressure using vacuum pomp over sinter-glass grade-4.The organic solution was evaporated on rotary evaporator and the crude compound P4 was obtained.The crude product was purified by column chromatography, eluting with hexane/EtOAc, to afford a pure product of compound P4.(X: I; isolated yield after column chromatography: 96%, 163 mg), (X: Br; isolated yield after column chromatography: 96%, 163 mg), (X: Cl; isolated yield after column chromatography: 91%, 154 mg).
IR (KBr), ν (cm -1 ): 3033, 1567, 1477.To a solution of 1-bromo-4-nitrobenzene (1.0 mmol, 0.202 g), phenyl boronic acid (1.2 mmol, 0.146 g), Pdnp-TPEPTA(L)-GO (30 mg), K2CO3 (2.0 mmol, 0.276 g), and DMF: H2O (2:1) (6.0 mL) were placed in a 25 mL roundbottom-flask equipped with a condenser and magnetic stirring bar and heated at 80 C.The progress of the reaction was monitored using TLC.After completion of the reaction, 5.0 mL of hot water and 5.0 mL of ethyl acetate were added to the reaction mixture.The Pdnp-TPEPTA(L)-GO catalyst was separated under reduced pressure using vacuum pomp over sinter-glass grade-4.The organic solution was evaporated on rotary evaporator and the crude compound P6 was obtained.The crude product was purified by column chromatography, eluting with hexane/EtOAc, to afford a pure product of compound P6.(X: Br; isolated yield after column chromatography: 89%, 177 mg).The progress of the reaction was monitored using TLC.After completion of the reaction, 5.0 mL of hot water and 5.0 mL of ethyl acetate or ethanol were added to the reaction mixture.The Pdnp-TPEPTA(L)-GO catalyst was separated under reduced pressure using vacuum pomp over sinter-glass grade-4.The organic solution was evaporated on rotary evaporator and the crude compound P7 was obtained.The crude product was purified by column chromatography, eluting with hexane/EtOAc, to afford a pure product of compound P7.(X: Br; isolated yield after column chromatography: 88%, 174 mg).
IR (KBr), ν (cm mL of ethyl acetate or ethanol were added to the reaction mixture.The Pdnp-TPEPTA(L)-GO catalyst was separated under reduced pressure using vacuum pomp over sinter-glass grade-4.The organic solution was evaporated on rotary evaporator and the crude compound P10 was obtained.The crude product was purified by column chromatography, eluting with hexane/EtOAc, to afford a pure product of compound P10.(X: I; isolated yield after column chromatography: 96%, 175 mg), (X: Br; isolated yield after column chromatography: 95%, 173 mg).

Table 2S .
Synthesis of 8 products that start using aryl chlorides at 120 C for comparison with the same reactions at 80 C