The non-innocent role of graphene in the formation/immobilization of ultra- small gold nanoparticles functionalized with N- heterocyclic carbene ligands

Ultra-small gold nanoparticles (AuNPs) are obtained by treatment of well-defined gold complexes with reduced graphene oxide (rGO) without any auxiliary reducing agent. The AuNPs are functionalized with N-heterocyclic carbene (NHC) ligands, which allow controlling both the size and morphology, avoiding aggregation. The AuNPs are directly immobilized on the surface of graphene yielding hybrid materials composed of metal nanoparticles and a carbonaceous support. The catalytic properties of these AuNPs immobilized onto graphene have been tested in the hydration of alkynes. The AuNPs showed superior


Introduction
Metal nanoparticles (MNPs) have attracted considerable attention in many research areas and particularly in the field of catalysis. [1][2][3][4][5][6] However, they suffer from the Oswald ripening phenomenon a common feature for metal nanoparticles (MNPs) employed in catalytic applications. To tackle this problem, different stabilizing agents such as surfactants, polymers or ligands have been used as coating agents. [7][8][9][10][11][12] Unfortunately, the presence of these coating agents may block the mass transport preventing direct contact between the reagents and the catalytic active sites. [13][14][15][16][17][18] Therefore, the design of efficient catalysts based on MNPs requires the use of labile coating agents in combination with supports to enhance stability. [19][20][21][22][23][24] In the last years, many efforts have been devoted to develop AuNPs functionalized with different types of ligands. However, examples dealing with the combination of the ligands with any supports are scarce. [25][26][27][28][29][30][31][32][33][34] Graphene among others, has emerged as a promising support, since combines a high surface area with a strong interaction with MNPs enabling the preparation of catalytic hybrid materials, which display improved activity, selectivity and stability. [35][36][37][38][39] In this context, we recently described a direct approach for the immobilization of well-defined organometallic complexes onto the surface of graphene ( Figure 1). [40][41][42] The forces that maintain the complexes onto the surface of graphene are -stacking interactions between polyaromatic groups and the p cloud electron density of graphene. These non-covalent interactions are particularly strong when pyrene is used as a polyaromatic group. This methodology allows us to obtain a homogeneous distribution of well-defined complexes over the entire graphene surface and not only at the edges/defects or at the functional groups. The hybrid materials displayed interesting catalytic properties in the dehydrogenation of alcohols [43] and primary amines [44], in the coupling of silanes with alcohols [45,46] and in the intramolecular hydroamination of alkynes. [47] Interestingly, these hybrid materials show increased catalytic activity compared to the parent molecular complexes. This enhancement of catalytic properties is not common in the heterogeneization of molecular complexes suggesting that graphene plays an important role in the catalytic process. Additionally, these hybrid catalytic materials were recycled up to ten times without loss of activity. The properties of the organometallic complexes and the support are preserved during immobilization because of the mild conditions used in the procedure, which allows a rationale design of heterogenized molecular catalysts.
Herein we now report, a synthetic methodology for the preparation of a hybrid catalyst comprised of AuNPs functionalized with NHC ligands supported onto graphene ( Figure 1). The hybrid catalyst is obtained from the direct reaction of NHC-Au complexes and graphene without any reducing agent. Its preparation reveals the non-innocent role of graphene on the formation/immobilization of AuNPs. This new platform efficiently catalyze the hydration of alkynes [48][49][50][51][52] which is an environmentally friendly methodology for obtaining carbonyl derivatives by the addition of water. [53][54][55][56][57] In addition, the hybrid material shows high activity at low catalysts loadings, does not require additives and is highly recyclable.

General procedures
Anhydrous solvents were dried using a solvent purification system. 4-hydroxy-1,3-(2,6diisopropylphenyl)imidazolium chloride [58] and bromomethyl pyrene [59] were prepared according to reported procedures. Nuclear magnetic resonance (NMR) spectra were recorded on Bruker spectrometers operating at 300 or 400 MHz ( 1 H NMR) and 75 or 100 MHz ( 13 C{ 1 H} NMR), respectively, and referenced to SiMe4 ( in ppm and J in Hertz). NMR spectra were recorded at room temperature with the appropriate deuterated solvent. Elemental analysis were carried out in a TruSpec Micro Series. Electrospray Mass Spectra (ESI-MS) were recorded on a MicroMass Quatro LC instrument. MeOH was used as mobile phase and nitrogen was used as the drying and nebulizing gas. Scanning electron microscopy images (SEM), were taken with a field emission gun scanning electron microscope model JEOL 7001F. High-resolution images of transmission electron microscopy (HRTEM) and high-angle annular dark-field (HAADF-STEM) images of the samples were obtained using a Jem-2100 LaB6 (JEOL) transmission electron microscope coupled with an INCA Energy TEM 200 (Oxford) energy dispersive X-Ray spectrometer (EDX) operating at 200 kV. Samples were prepared by drying a droplet of a MeOH dispersion on a carboncoated copper grid. X-ray photoelectron spectra (XPS) were acquired on a Kratos AXIS ultra DLD spectrometer with a monochromatic Al Kα X-ray source (1486.6 eV) using a pass energy of 20 eV. To provide a precise energy calibration, the XPS binding energies were referenced to the C1s peak at 284.6 eV. GC analyses were obtained on a shimadzu GC-2010 apparatus equipped with a FID detector, and using a Teknokroma column (TRB-5MS, 30 m x 0.25 mm x 0,25 m). UV-vis spectra were acquired on a Varian Cary 50 spectrophotometer.

General procedure for the hydration of alkynes
In a Pyrex ® tube and under air, the corresponding alkyne (1 eq.) water (2 eq.), catalyst (2 or 2-rGO), and Methanol (concentration of alkyne 0.15 M) were mixed. The reaction was stirred at 50 ºC in an oil bath.
Conversion of alkyne into the corresponding ketone was monitored by GC-FID using anisole as an internal standard. When the reaction was completed the solvent was removed and the yield of the isolated product analyzed by NMR.

Synthesis of HAuCl4-rGO-NPs
A suspension of rGO (90 mg) in CH2Cl2 (65 mL) was immersed in an ultrasounds bath for 30 min. Then, HAuCl4·3H2O (22.5 mg, 0.06 mmol) was added to the suspension and was stirred at room temperature for 48 h. The black solid was isolated by filtration and washed with 100 mL of CH2Cl2 affording the hybrid material HAuCl4-rGO-NPs. The exact amount of supported complex was determined by ICP-MS analysis. The results accounted for a 0.028 mg Au/100 mg rGO of gold in the hybrid material HAuCl4-rGO-NPs.

Synthesis and characterisation
We have previously immobilized the gold complex 1 onto reduced graphene oxide (rGO) generating the hybrid material 1-rGO (Figure 1). [47] The procedure consisted in the exfoliation of rGO by ultrasounds (30 min) and stirring in the presence of the molecular gold complex (10 h). Microscopy analysis by HRTEM revealed the homogeneous distribution of gold and the absence of metal nanoparticles. A comparative high-resolution XPS analysis of 1 and 1-rGO showed that the core-level peaks of gold, nitrogen and chloride appear at the same binding energies, indicating the presence of the molecular complex onto the graphene surface. Considering these results, we sought to immobilize complex 2 on the surface of reduced graphene oxide using the same methodology (Scheme 1). Synthesis of 2 is carried out by a two-step process starting from the imidazolium salt L1. This imidazolium salt contains the pyrenetag at the backbone of the azol ring instead of at the nitrogen. This allows the design of NHC ligands containing bulky protecting groups such as the 2,6-diisopropylphenyl (Dipp). The metalation of L1 under basic conditions produces the Au(I) intermediate complex i1 that has been completely characterized including the single X-ray crystal structure (See SI for details). Treatment of i1 with a silver salt allows the introduction of a labile OTfligand that is very convenient in the design of homogeneous catalysts. [55] The immobilization of complex 2 on the surface of graphene gave results completely different to complex 1. The gold complex 2 is stable in the solid state and in solution but decomposes in the presence of rGO forming gold nanoparticles (2-rGO-NPs) that are directly immobilized on the surface of graphene. These AuNPs are produced under mild reaction conditions which prevent side reactions and alteration of the support. The AuNPs on the surface of graphene are characterized by transmission microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and UV/vis spectroscopy. HRTEM analysis confirms the presence of ultrasmall and spherical shape AuNPs (Figure 2a-b). The particle size histogram reveals a distribution with an average diameter of 3.2 ± 1.5 nm (n = 450) (Figure 2c). We have not observed an increase in the size of AuNPs for prolonged reaction times indicating that graphene controls the growing of NPs. Thus, reduced graphene oxide acts as a stabilizer controlling the size and shape of these AuNPs. We have previously evidenced that graphene stabilizes PdNPs and also that control the size and  Based on these precedents, we decided to investigate the synthesis of AuNPs anchored onto graphene in order to get a better insight of the formation procedure. The synthesis of AuNPs functionalized with NHC ligands anchored on rGO is general and works under different reaction conditions. For instance, using methanol or ethanol yields similar AuNPs/rGO hybrid materials. For comparative purposes and in order to evaluate the ligand effect in the design of nanoparticles, we synthesized AuNPs lacking of NHC ligands using the commercially available HAuCl4. Addition of a solution of HAuCl4 to an exfoliated suspension of rGO leads to the formation of AuNPs immobilized on the surface of graphene (HAuCl4-rGO-NPs).
Characterization by HRTEM confirms the formation of larger AuNPs compared to 2-rGO-NPs. The histogram shows a wide particle size distribution with an average diameter of 12.4 ± 4.2 nm (n = 74) ( Figure 3). The AuNPs synthesised from HAuCl4 lacking of NHC ligands (HAuCl4-rGO-NPs) showed an increased average size of 9.2 nm vs. 2-rGO-NPs, indicating that the presence of NHC ligands has an important impact in the size of the nanoparticle. The combination of gold complexes containing NHC ligands with graphene is a convenient procedure for the synthesis of hybrid catalytic materials containing ultrasmall AuNPs.

Catalytic properties
The catalytic performance of gold complexes and gold nanoparticles were evaluated in the hydration of alkynes. Optimization of the reaction conditions was made using 4-octyne as model substrate (SI, S11).
The results showed that the additives (AgOTf) and the support (rGO) are not catalytic active species in the hydration of alkynes. The gold complex i1 was completely inactive and the HAuCl4 displayed modest conversion (18%) implying a limited catalytic activity. The best catalysts in the hydration of 4-octyne were the gold complex 2 and the AuNPs 2-rGO-NPs. Full conversions were obtained in short reaction times and low catalyst loadings. Interestingly the gold nanoparticles without capping ligands (HAuCl4-rGO-NPs) were completely inactive in the hydration of 4-octyne. Most probably the lack of catalytic activity is due to a large particle size distribution and the absence of stabilizing NHC ligands. Catalysts 2 and 2-rGO-NPs were both competent in the hydration of a variety of alkynes suggesting a wide reaction scope (Figure 4a). We made a comparison of the catalytic activity of the best catalytic systems (2 and 2-rGO-NPs) for different substrates (SI, S11). The experimental results indicate that, while catalyst 2-rGO-NPs is active even at very low catalyst loadings (0.02 mol%), the molecular complex 2, requires a tenfold higher catalyst loading to reach similar outcomes. As an example, the reaction monitoring profile in the hydration of 4-octyne afforded quantitative yield after 60 min using a catalyst 2-rGO-NPs (0.02 mol%) but required 150 min in the case of catalyst 2 using a loading of 0.25 mol% (Figure 4b). This observation is general and better catalytic outcomes were observed using catalyst 2-rGO-NPs for a variety of substrates. In parallel, poisoning experiments were used to determine the nature of the active catalytic species derived from catalyst 2. The hydration reaction of phenyl acetylene was carried out in the presence of poly(4-vinylpyridine) as a scavenger of gold molecular complexes and the Hg drop test was used as a scavenger of AuNPs (SI, S11.2). [62][63][64][65] The presence of Hg has no effect in the reaction but it is inhibited by the addition of poly(4-vinylpyridine). These experiments suggest that the catalytic active species derived from 2 is molecular in nature. In the case of catalysts 2-rGO-NPs, we performed a Maitlis hot filtration test using the hydration of 4-octyne (SI, S11.3). After 10 min (GC conversion 38%), half of the reaction mixture was filtered off through celite at 50 ºC. The filtrate was maintained for 300 min under identical conditions, but the GC analysis indicated that no further hydration occurred (GC conversion 53%, TOF = 240 h -1 ). On the contrary, the remaining mixture achieved full conversion in less than 50 min (TOF = 4360 h -1 ). Analysis of the apparent reaction rates and TOF values of the filtrate and the solid catalyst confirms the heterogeneous nature of 2-rGO-NPs and suggests that the AuNPs are strongly attached to the support. The direct synthesis and immobilization of AuNPs onto graphene lead to a more efficient catalytic system in terms of activity and catalysts loading. The stability and recycling properties of the hybrid catalytic material 2-rGO-NPs were evaluated in the hydration of alkynes using 4-octyne as model substrate. In a typical run were added 4-octyne, MeOH, H2O (2 Eq.) and catalyst 2-rGO-NPs (0.05 mol %) and the mixture was heated at 50 ⁰C for the appropriate time. The reaction progress was monitored by GC until full conversion. After each run, the catalyst 2-rGO-NPs was removed from the solution by decantation washed with methanol, dried with pentane and reused. The hybrid material 2-rGO-NPs was reused up to 5 times without a significant decrease in activity observing similar apparent rate constants in the reaction time profile (Figure 5a). Then, there is a gradual decrease in activity but quantitative yields are obtained in less than 200 min (runs 6 to 8). After run 8, the apparent rate constants decrease gradually. However, still quantitative yields are achieved in less than 400 min. In order to elucidate this deactivation process, we performed an analysis by HRTEM microscopy and XPS at the end of the recycling experiment and analyzed the gold amount by ICP/MS. The HRTEM images before and after the recycling experiments show similar properties for the morphology of reduced graphene oxide, indicating that the support is not altered during extended operation. A high magnification HRTEM micrograph confirms the crystallinity of AuNPs after the 10 th catalytic run (Figure 5c). More interesting is that the size and morphology of AuNPs supported on the surface of graphene remain almost intact after ten catalytic runs ( Figure 5). Additionally, the XPS spectrum of the Au4f region at the end of the recycling experiment shows the same doublet peak at 88.6 and 84.9 eV to these observed for the hybrid catalytic material 2-rGO-NPs before being used (Figure 5e). To rationalize a possible deactivation mechanism caused by leaching the amount of gold was analyzed by ICP/MS in the filtrate of each run and in the hybrid catalyst after the 10 th run. The gold amount in the filtrates corresponding to runs 1 to 5 revealed insignificant gold content. In the following runs, there is a gradual increase of gold content that correlates well with the decrease in catalytic activity. This result was confirmed by analyzing the gold amount of the hybrid catalyst after the recycling experiment. After digestion of the solid, ICP/MS analysis revealed that 50 wt % of the initial gold content is lost by leaching. This study supports that (1) the Ostwald ripening is negligible, (2) there is a strong interaction between the AuNPs and the surface of rGO (3) the rGO favors the stabilization of nanoparticles and (4) deactivation mechanism is caused by leaching of AuNPs. These results point out that 2-rGO-NPs is a robust and recyclable catalytic hybrid material.

Conclusions
Decomposition of a well-defined gold complex leads to AuNPs functionalized with NHC ligands anchored onto the surface of graphene. The AuNPs are produced under mild reaction conditions without any external reducing agent, highlighting the non-innocent role of graphene. The hybrid material composed of AuNPs and graphene as support is an efficient catalyst for hydration of alkynes. Interestingly, the AuNPs immobilized onto graphene show superior catalytic activity vs. the parent molecular complexes in terms of activity and stability. The catalytic system is recycled up to five times without significant degradation. Then, there is a gradual decrease in activity but quantitative yields are still obtained if the reaction time is increased. Studies by ICP/MS show that the deactivation mechanism is governed by gradual leaching of active species. Microscopy analysis after the catalytic experiments reveals that AuNPs maintain their integrity in terms of size and morphology. These experiments suggest that graphene plays an important role in the stabilization of AuNPs during catalysis. The presence of NHC ligands covering the AuNPs in combination with graphene represents a convenient approach to develop practical hybrid materials with catalytic properties. Future work will focus on a detail analysis of the interaction between the AuNPs and the support and its impact on catalytic activity.

Supporting information
Experimental details, complete characterization of metal complexes and hybrid materials, crystallographic data and reaction monitoring profiles. CCDC 1897736 contains the supplementary crystallographic data