Gold nanoparticle decorated reduced graphene oxide sheets with high catalytic activity for Ullmann homocoupling

Abstract

A Au nanoparticle-reduced graphene oxide nanocomposite (Au NPs–RGO) was used as an effective and reusable heterogeneous catalyst for the Ullmann homocoupling of aryl iodides with short reaction times and good yields. For the first time, aryl bromides bearing electron-withdrawing groups were coupled in the presence of a catalyst giving moderate yields.

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Supported Au nanoparticles (Au NPs) and their catalytic applications in heterogeneous organic reactions have attracted considerable attention in recent years. Aldehyde–alkyne–amine coupling (A3-coupling),¹ oxidation of alcohols,² secondary amines³ and carbon monoxide,⁴ hydrogenation of olefins,⁵ Suzuki⁶ and Sonogashira⁷ reactions, formation of nitrogen containing compounds,⁸ etc. are some of the most important applications of supported Au NP catalysts. The catalytic performance of Au NPs–support strongly depends on the size and shape of the Au NPs, the nature of the support, and the Au NPs-support interface interaction.⁹ Supporting carriers may function by dispersing and fixing the Au NPs. Supports such as oxides (CeO₂,¹⁰ SiO₂,¹¹ Al₂O₃,¹² MgO¹³ and TiO₂ (ref. 14)), mixed oxides (Mg–Al–O¹⁵ and Ga–Al–O¹⁶), polymers (PVP¹⁷ and PS derivatives¹⁸) and ordered mesoporous carbon¹⁹ have been used as supports for Au NPs. Among these, it has been reported that the π-interaction of aromatic rings with gold nanoparticles leads to nanoparticle stabilization and improves the catalyst performance in a variety of gold-catalyzed reactions.^9,18,20

Graphene is a two-dimensional sheet of sp² bonded carbon atoms, which can be viewed as an extra-large polycyclic aromatic molecule.²¹ Recently, graphene has been used as the support for metal and metal oxide that is mainly due to its large surface area, excellent electrical and thermal conductivity, low price, high chemical inertness, ease of modification and strong interactions with metal clusters.²² However, only a few reports have involved the application of graphene based nanocomposites as heterogeneous catalysts in organic transformations.²³ We therefore reasoned that graphene as a polycyclic aromatic molecule is a potential candidate as both a support and a stabilizer of Au NPs for chemical transformations.

Aryl–aryl bond formation is one of the most important tools of modern organic synthesis. These bonds are very often found in commercial dyes, natural products and organic conductors or semiconductors as well as being the backbone of some ligands used for asymmetric catalysis. The Ullmann type reaction, which is predominately catalyzed by copper, nickel and palladium catalysts, is a powerful and convenient synthetic method in organic chemistry for the aryl–aryl bond formation.²⁴ However, only a few studies have involved the application of free or supported Au NPs as catalysts in the Ullmann type reaction of aryl iodides.^20,25–27 Aryl bromides have not been used in gold catalyzed Ullmann homocoupling because the oxidative addition is more difficult relative to that of aryl iodides.

Herein, we report a novel protocol for the Ullmann homocoupling of aryl halides catalysed by Au NPs–RGO. Additionally, the effects of solvent polarity, base, and temperature on yield, and recycling potential of the catalyst have all been assessed.

Graphene oxide (GO) was prepared using a modified Hummers method,²⁸ and subsequent reduction using NaBH₄.²⁹ Gold nanoparticles were deposited on the surface of the reduced graphene oxide (RGO) through spontaneous chemical reduction of HAuCl₄ by RGO³⁰ (Scheme 1). The mechanism of the spontaneous reductive deposition process that generates Au NPs on the RGO sheets likely involves a galvanic displacement and redox reaction because of the relative potential difference between RGO and HAuCl₄ (ref. 30) (Scheme 1). In addition, a similar catalyst was prepared by an alternative route involving the physical mixture of Au NPs from the Turkevich–Frens method³¹ with RGO. This would enable a direct comparison of the two methodologies upon the catalytic activity.

Scheme 1 Schematic route for preparation of Au NPs–RGO.

The Au NPs–RGO nanocomposite was characterized by FT-IR, XRD, TEM, Raman, XPS and EDS measurements. The electronic properties of the nanocomposite were probed by X-ray photoelectron spectroscopy (XPS). The XPS spectrum of Au 4f core for the region of the Au NPs–RGO level displays main peaks at 83.8 and 87.4 eV, which corresponds to the binding energies of Au 4f_7/2 and Au 4f_5/2, respectively, and was shifted to the lower binding energy compared with the characteristic peaks for metallic Au⁰ at 84.0 and 87.7 eV (Fig. 1). The negative shift (0.3 eV) arises from the electron transfer from the graphene sheet to the Au NPs.³² The XPS spectrum of the Au 4f core for the region of the physical mixture of the Au NPs and RGO level displays its main peaks at 83.9 and 87.6 eV, which correspond to the binding energies of Au 4f_7/2 and Au 4f_5/2, respectively, and were shifted to the higher binding energies compared with the characteristic peaks for Au NPs–RGO. Sodium citrate used as the reducing and capping agent for synthesis of Au NPs in Turkevich–Frens method inhibits the electron transfer from the graphene sheet to Au NPs, which causes a low shift (0.1 eV).

Fig. 1 XPS spectra of the Au 4f core level region for Au NPs–RGO and the physical mixture of Au NPs and RGO nanocomposites.

Fig. 2 shows the Raman spectra of GO, RGO, Au NPs–RGO and the physical mixture of Au NPs and RGO. In the Raman spectra of GO, RGO and Au NPs–RGO two similar fundamental peaks about 1308 and 1590 cm⁻¹ were observed, which correspond to the disorder induced by the D and G bands of the carbon atoms, respectively. The I_(D)/I_(G) intensity ratio of RGO (1.48) was slightly higher than that of the GO (1.40). This result may reflect the reduction in size of the graphene.³³ The I_(D)/I_(G) intensity ratio of Au NPs–RGO (1.77) and the physical mixture of Au NPs and RGO (1.49) was much larger than that of the RGO. Such an enhancement has also been observed for metal nanoparticle composites of GO, indicating a probable chemical interaction or bond between the metal nanoparticles and graphene.³⁴ The I_(D)/I_(G) intensity ratio for the physical mixture of Au NPs and RGO was lower than that of Au NPs–RGO, which indicates the presence of a weak chemical interaction or bond between the metal nanoparticles and graphene. Additionally, after conjugation of the Au NPs onto the RGO sheets, the intensity of the Raman signals of the nanocomposite was enhanced, which was due to the surface enhanced Raman spectroscopy (SERS) of Au NPs. SERS can be via an electromagnetic enhancement (excitation of localized surface plasmons involving a physical interaction) or chemical enhancement (formation of charge-transfer complexes involving chemical interaction) with enhancement factors of ∼10¹² and ∼10 to 100, respectively. The low enhancement factor for the Au NPs–RGO nanocomposite indicates the presence of a chemical interaction or bond between Au NPs and RGO.³⁵ The enhancement factor for the physical mixture of Au NPs with RGO was lower than that of Au NPs–RGO, which indicates the presence of a week chemical interaction or bond between the Au NPs and RGO.

Fig. 2 Raman spectra of GO, RGO, Au NPs–RGO and physical mixture of Au NPs and RGO nanocomposites.

The morphology of GO and the Au NPs–RGO nanocomposite was determined by transmission electron microscopy (TEM). A TEM image of the GO displayed a crumpled and layer-like structure (Fig. 3a). As Fig. 3b shows, the surfaces of graphene are covered by distributed Au NPs with an average size of 10–20 nm. Additionally, Au nanoparticles are not found outside of the RGO sheets. The chemical composition of the Au NPs–RGO nanocomposite was determined using energy dispersive spectroscopy (EDS) analysis. The EDS analysis in Fig. 3c clearly shows the presence of gold in the composite and the sample consists mainly of carbon with an insignificant amount of oxygen probably due to the presence of some unreduced oxygen functional groups. Additionally, the absence of the chlorine in the EDS analysis of the Au NPs–RGO composite corroborates the reduction of HAuCl₄ by RGO.

Fig. 3 (a) TEM image of GO (b) TEM image of Au NPs–RGO nanocomposite (c) EDS result for Au NPs–RGO nanocomposite.

The catalytic activity of the Au NPs–RGO catalyst was then tested in the Ullmann homocoupling reaction. To optimize the reaction conditions, iodobenzene was selected as a model substrate in the presence of various bases and solvents (Table 1). By using DMSO as the solvent, the reaction was tested employing various bases such as K₃PO₄, Cs₂CO₃, K₂CO₃, and KOH (entries 1–4). A superior yield was obtained when K₃PO₄ was used as the base (entry 1). Then, different solvents were screened in the model reaction using K₃PO₄ as the base. It was found that the reaction using NMP after 6 h resulted in a higher yield. To optimize the reaction temperature, we also performed several experiments at 90 °C, 100 °C and 110 °C in the presence of K₃PO₄ in NMP using 1.0 mol% Au NPs–RGO (Table 1, entries 5, 9 and 10). As can be seen from Table 1, the most suitable reaction temperature was 100 °C (entry 5). The effect of the Au NPs–RGO loading was also investigated under the optimum reaction conditions (Table 1, entries 5, 11 and 12). It was found that 1 mol% Au was sufficient to push this reaction forward (entry 5). Additionally, the catalytic activity of the physical mixture of Au NPs with the RGO catalyst was tested in the Ullmann homocoupling reaction and afforded a relatively poor yield of the corresponding coupled product (Table 1, entry 13).

Table 1 Screening of the reaction conditions^a

Entry	Solvent	Base	Temp. (°C)	Yield (%)
a Iodobenzene (1.0 mmol), base (3.0 mmol), Au NPs–RGO (1.0 mol% Au), 6 h and solvent (2 ml). GC yield, n-dodecane was used as an internal standard.b Au NPs–RGO (0.5 mol% Au).c Catalyst (1.5 mol% Au).d Physical mixture Au NPs with RGO (1 mol% Au).
1	DMSO	K3PO4	100	94
2	DMSO	Cs2CO3	100	85
3	DMSO	K2CO3	100	67
4	DMSO	KOH	100	39
5	NMP	K3PO4	100	97
6	DMF	K3PO4	100	72
7	PhCH3	K3PO4	100	15
8	CH3CN	K3PO4	Reflux	56
9	NMP	K3PO4	110	97
10	NMP	K3PO4	90	74
11^b	NMP	K3PO4	100	68
12^c	NMP	K3PO4	100	97
13^d	NMP	K3PO4	100	48

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After the best reaction conditions were set, various aryl halides were screened to explore the scope of the Ullmann homocoupling reaction. As shown in Table 2, the aryl iodides bearing electron-donating and electron-withdrawing groups reacted well and gave good yields (Table 2, entries 1–4). The aryl iodides possessing electron-donating groups (p-OMe and p-Me) led to higher yields compared to the aryl iodide possessing an electron-withdrawing group (p-COCH₃) (Table 2, entries 2–4). The hindered 2-iodotoluene substrate converted to the corresponding homocoupling product with a lower yield (Table 2, entry 5). Under the same reaction conditions the homocoupling of bromobenzene and aryl bromide bearing an electron-donating group (p-Me) failed to provide the target product (Table 2, entries 6–7). The aryl bromides bearing electron-withdrawing groups (p-COCH₃ and p-CHO) were coupled in the presence of catalyst giving moderate yields (Table 2, entries 8–9).

Table 2 Au NPs–RGO nanocomposite catalyzed Ullmann homocoupling

Entry	Aryl halide	Product	Yield^a
a Isolated yields.
1			97%
2			94%
3			88%
4			81%
5			80%
6		2a	Trace
7		2c	Trace
8		2d	68%
9			41%

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We proposed the following four-step mechanism for the Ullmann homocoupling reaction of an aryl halide catalyzed by the Au NPs–RGO nanocomposite (Scheme 2). First, the aryl halide was absorbed on the surface of the Au NP on the reduced graphene oxide sheet to form a (ArX) fragment. The C–X bond activation follow by the dissociation of C–X bond by the neutral Au⁰ atoms, gave rise to [Au NP Ar] [Au NP X] fragments.^25,36 The homocoupling of [Au NP Ar]₂ fragments generated the final product. Also, the spontaneous chemical reduction of Au⁺ by RGO regenerated the Au⁰–RGO nanocomposite. We assumed that the electron transfer from graphene sheets to Au nanoparticles caused an increase in the reactivity of the Au NPs–RGO nanocomposite toward the Ullmann homocoupling of aryl bromides containing electron withdrawing groups.

Scheme 2 Proposed mechanism for the Ullmann homocoupling reaction of an aryl halide catalyzed by Au NPs–RGO nanocomposite.

The recyclability of the Au NPs–RGO nanocomposite was also examined by the Ullmann homocoupling of iodobenzene. It was found that the recovery can be successfully achieved in six successive reaction runs (Table 3).

Table 3 Reusability of the Au NPs–RGO nanocomposite in the homocoupling reaction of iodobenzene^a

a Iodobenzene (1.0 mmol), K3PO4 (3.0 mmol), Au NPs–RGO (1.0 mol% Au), 6 h and NMP (2 ml).b GC yield, n-dodecane was used as an internal standard.
Reaction cycle	1st	2nd	3rd	4th	5th	6th
Yield^b (%)	97	94	92	88	85	83

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Table 4, compares the efficiency of the Au NPs–RGO nanocomposite (time, yield, reaction conditions) with the efficiency of other reported heterogeneous gold nanoparticle catalysts in the Ullmann homocoupling reaction of aryl iodides. It is clear from Table 4 that our method is simpler, more efficient, and less time consuming for the Ullmann homocoupling reaction.

Table 4 Comparison of the efficiency of various gold nanoparticle catalysts in the Ullmann homocoupling reaction of aryl iodides

Catalyst	Condition	Yield	Time	Ref.
Au NPs–RGO	K3PO4, NMP, 100 °C	80–97 (%)	6 h	This work
Au@PMO	K3PO4, NMP, 100 °C	80–95 (%)	16 h	20
Au NP	H2O/TBAOH, glucose, 100 °C	45–99 (%)	7–15 h	25
Au25(SR)18/CeO2	K2CO3, DMF, 130 °C	67.5–99.8 (%)	48 h	26
NAP–Mg–Au	K2CO3 DMF, 140 °C	45–92 (%)	48 h	27

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Conclusions

In conclusion, we have demonstrated that the Au NPs–RGO nanocomposite is an effective and reusable heterogeneous catalyst for the Ullmann homocoupling reaction with a short reaction time leading to good yields. The catalyst was recovered by simple filtration and reused for several cycles without a significant loss of catalytic activity. In this paper for the first time, aryl bromides bearing electron-withdrawing groups (p-COCH₃ and p-CHO) were coupled in the presence of catalyst giving moderate yields.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: FT-IR spectra of GO, RGO and Au NPs–RGO, XRD spectra of GO, RGO Au NPs–RGO and a physical mixture of Au NPs with RGO. See DOI: 10.1039/c3ra45518a

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