Making CâC Bonds with Gold: Identification of ... - ACS Publications

Feature Article pubs.acs.org/JPCC

Making C−C Bonds with Gold: Identification of Selective Gold Sites for Homo- and Cross-Coupling Reactions between Iodobenzene and Alkynes Mercedes Boronat,† Diego Combita,† Patricia Concepción,† Avelino Corma,*,† Hermenegildo García,† Raquel Juárez,† Siris Laursen,† and Juan de Dios López-Castro‡ †

Instituto de Tecnología Química, Universidad Politécnica de Valencia − Consejo Superior de Investigaciones Científicas, Av .de los Naranjos, s/n, 46022 Valencia, Spain ‡ Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, Facultad de Ciencias, Universidad de Cádiz, Campus Rio San Pedro, 11510 Puerto Real, Cádiz, Spain S Supporting Information *

ABSTRACT: The nature of the active sites involved in the gold catalyzed Sonogashira cross-coupling reaction between iodobenzene and phenylacetylene, and in the competitive homocoupling reactions, has been investigated by means of DFT calculations, kinetic measurements, and synthesis of catalysts with different gold surface species. Several catalyst models have been theoretically investigated to simulate gold nanoparticles of different size either isolated, supported on inert materials, or supported on CeO2. The mechanistic studies show that IB dissociation occurs on low coordinated Au0 atoms present in small gold nanoparticles, either isolated or supported, while PA is preferentially adsorbed and activated on Auδ+ species existing at the metal−support interface. When this occurs, the activation energy of the rate-determining step of the Sonogashira reaction, which has been found experimentally to be the bimolecular coupling, is minimized. The product distribution obtained with Au/CeO2 catalysts containing different ratios of Au0/Auδ+ sites confirms the positive role played by cationic gold in the Sonogashira cross-coupling reaction. Importantly, only metallic Au0 atoms present in gold nanoparticles are required to perform the homocoupling of iodobenzene.

1. INTRODUCTION The Sonogashira reaction between aryl halides and alkynes1,2 to produce difunctionalized alkynes is a major route to obtain compounds used as intermediates in the synthesis of polymers, natural products, and bioactive compounds. Palladium complexes are highly active and selective homogeneous catalysts for C−C bond forming reactions, and in particular for the Sonogashira reaction.3−7 In the search for alternative heterogeneous solid catalysts, it was reported that gold nanoparticles supported on nanocrystalline CeO2 are active and highly selective toward the Sonogashira product, and it was proposed that cationic gold species present in gold nanoparticles supported on CeO2 were key to catalyzing the Sonogashira reaction.8 There was some controversy about the ability of AuI complexes in solution to activate aryl iodide by oxidative addition, following the mechanism accepted for the Pd catalyzed Sonogashira reaction,9,10 and it was claimed that the reported activity of gold is actually due to the presence of palladium impurities.11 However, this controversy has been resolved12,13 and it has been clearly demonstrated by different groups that small gold clusters in solution14 or in the gas phase,15 gold nanoparticles supported on different metal oxides,8,12,16 and even extended pure gold surfaces17 are able to activate and dissociate the C−I bond in iodobenzene. Theoretical work has established that, contrary to what occurs with Pd catalysts in the homogeneous phase, IB activation and © 2012 American Chemical Society

dissociation on heterogeneous gold catalysts does not involve a formal oxidative addition, but it is energetically possible on small gold nanoparticles due to the dispersion of the charge among all the gold atoms in the nanoparticle.12 On the other hand, alkyne deprotonation to form the reactive alkynyl fragment requires the presence of a base in all reaction media.1,8−10 In the Sonogashira reaction between aryl halides and alkynes, other competitive processes such as aryl halide homocoupling yielding biaryl, and alkyne homocoupling, can also occur on the gold surface, with a negative impact on the final selectivity toward the desired cross-coupling product (see Scheme 1). Thus, when iodobenzene (IB) and phenylacetylene (PA) react over an extended Au(111) surface, the selectivity to the desired cross-coupling diphenylacetylene product (DPA) is only ∼20% as compared to 40% of biphenyl (BP).17 Gold nanoparticles supported on different metal oxides have also been tested in this reaction, and it has been found that Au/CeO2 and Au/ La2O3 are considerably more selective toward the crosscoupling product (>85% selectivity to DPA) than Au/SiO2 and Au/TiO2, which preferentially catalyze the homocoupling of iodobenzene (38% DPA vs 62% BP).8,12,14,16 However, the Received: July 19, 2012 Revised: September 27, 2012 Published: October 2, 2012 24855

dx.doi.org/10.1021/jp3071585 | J. Phys. Chem. C 2012, 116, 24855−24867

The Journal of Physical Chemistry C

Feature Article

Scheme 1. Elementary Steps in the Mechanism of Sonogashira Cross-Coupling (Blue Arrows) and Competitive Homo-Coupling of IB (Red Arrows) and PA (Green Arrows)

basis set with a kinetic energy cutoff of 500 eV in the case of pure Au models and of 400 eV for the Au/CeO2 catalyst models, since preliminary tests showed that these are the values at which adsorption energies converge on each system. The effect of the core electrons in the valence density was taken into account by means of the projected augmented wave (PAW) formalism.22 Integration in the reciprocal space was carried out at the Γ k-point of the Brillouin zone. Transition states were located using either the nudged-elastic-band (NEB)23 or the DIMER algorithm,24,25 and transition states were characterized by pertinent frequency analysis calculations. Charge distributions were estimated using the theory of atoms in molecules (AIM) of Bader using the algorithm developed by Henkelman.26,27 In the case of modeling CeO2, the Hubbard repulsion term (U) with a value of 5 eV was applied to the cerium atoms in all calculations.28 Electronic PW91 energies were further corrected with a dispersion term calculated using the D3 method over the PW91 optimized geometries.29,30 The Au(111) extended surface was modeled by a (5 × 5) supercell slab containing 100 atoms in four layers, and during the geometry optimizations, the positions of the adsorbates and of the gold atoms of the two uppermost layers were allowed to fully relax. The neutral Au nanoparticle was modeled by a Au38 cluster having a typical cuboctahedral shape and 1 nm diameter. It was placed in a 20 × 20 × 20 Å3 cubic box, large enough so as to avoid interactions between periodically repeated nanoparticles or adsorbates, and during the geometry optimizations, only the positions of the adsorbates were allowed to fully relax. Finally, a model for the Au/CeO2 catalyst was produced by supporting a two atomic layer thick Au nanorod containing 20 Au atoms over a partially oxidized CeO2(111) surface. The stoichiometric CeO2(111) surface was modeled by a (4 × 3) supercell slab containing 48 O atoms and 24 Ce atoms, and then six additional oxygen atoms were added between the Au nanorod and the CeO2 surface Ce4+ sites. This system was found to be energetically favorable with respect to a Au/CeO2 and O2 reference, and was motivated by the fact that many Au catalysts are commonly produced by depositing Au(OH)x on an oxide surface, which decomposes to metallic and cationic gold sites, and theoretical studies that have shown that the interface can be oxidized and be thermodynamically stable.31−33 2.2. Catalyst Preparation. Nanoparticulated ceria12 has a BET suface area of 115 m2/g. Catalyst A. Au supported on nanoparticulated ceria was prepared using a solution of 2123 mg of HAuCl4·3H2O in 2 L of deionized water that was brought to pH 10 by addition of a solution of NaOH 0.2 M. Once the pH value was stable, the

origin of the enhanced selectivity of Au/CeO2 and Au/La2O3 is not clear yet. It was initially proposed that cationic AuI sites present in Au/CeO2 are required to perform the Sonogashira reaction with high selectivity.8 On the other hand, it has been shown that an Au/La2O3 catalyst containing highly dispersed cationic AuI and AuIII species but no metallic Au0 was inert, indicating that cationic gold cannot be the only site responsible for catalytic activity. Meanwhile, metallic Au0 nanoparticles supported on acid (γ-alumina) or basic (BaO) oxides were active but less selective than Au/CeO2.16 Since the catalytic activity of gold nanoparticles supported on metal oxides depends on several factors, such as particle size and morphology, the oxidation state of gold, electron density transfer from or to the support, and presence of special active sites at the metal−support interface, it is not always easy to analyze separately the role played by each of these aspects. In order to produce a unified mechanism and to explain the performance in the Sonogashira reaction of the different gold catalysts reported in the literature that will allow selective solid gold catalysts to be designed and synthesized, it is key to identify the active centers responsible for each one of the elementary steps involved in the cross-coupling and the competing homocoupling reactions. With this objective, in this work, we have investigated first, by means of DFT calculations on several catalyst models containing neutral and cationic gold sites with different coordination numbers, the elementary steps involved in the mechanism of the homo- and cross-coupling reactions between IB and PA, i.e., reactant adsorption and dissociation, and surface coupling steps (see Scheme 1). Once the gold active species for each elementary step have been theoretically identified, a series of Au/CeO2 catalysts with different concentrations of cationic and metallic sites have been synthesized, characterized, and tested for crosscoupling and homocoupling reactions. By combining DFT calculations with kinetic studies and catalyst synthesis, it is shown that, while IB homocoupling only requires neutral Au0 centers, the selectivity toward the Sonogashira product is promoted if the catalyst contains both Au0 and Auδ+ sites.

2. EXPERIMENTAL SECTION 2.1. Models and Methods. The mechanisms of the Sonogashira reaction between IB and PA, and the competitive homocoupling reactions, were investigated by means of periodic density functional theory, using the Perdew−Wang (PW91) exchange-correlation functional within the generalized gradient approach (GGA)18,19 as implemented in the VASP code.20,21 The valence density was expanded in a plane wave 24856



Feature Article

Binding energies (BE) were referenced to the Ce3d v1 peak of the ceria support set at 882.178 eV. IR Spectroscopy. FTIR spectra were obtained on a Thermo “Nexus” spectrometer equipped with a DTGS detector. An IR cell allowing in situ treatments in controlled atmospheres and temperatures from −176 to 500 °C was connected to a vacuum system with gas dosing facility. Prior to CO adsorption experiments, the sample was evacuated at 25 °C in a vacuum (10−6 mbar) for 1 h. After activation, the samples were cooled down to −176 °C under dynamic vacuum conditions followed by CO dosing at increasing pressure (0.4−8.5 mbar). IR spectra were recorded after each dosage. Raman Spectroscopy. Raman spectra were recorded using a 785 nm laser excitation on a Renishaw Raman Spectrometer (“in via”) equipped with a CCD detector. The laser power on the sample was 25 mW, and a total of 20 acquisitions were taken for each spectra. In some cases, analyses on different sample positions were recorded (spectral resolution ∼1 μm). TEM. TEM images were taken using normal bright-field and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The images were acquired with a JEOL 2010 field emission gun transmission electron microscope operated at 200 kV. EDXRF. The gold content measurements were performed using a Panalytical Minipal 4 EDXRF spectrometer equipped with a rhodium anode tube operated at 30 kV and 300 mA, using an Ag filter in air atmosphere. 2.4. Reaction Procedure. Iodobenzene and phenylacetylene used in this study are commercially available from Sigma-Aldrich with purities higher than 95%, and were used without further purification. n-Dodecane was used as an internal standard to determine conversions and selectivity by GC. Catalytic experiments were performed in reinforced glass reactors. In a typical experiment, 510 mg of each reactant (iodobenzene and phenylacetylene) were dissolved in DMF in a 5 mL graduated flask. A 2 mL portion of this solution and 212 mg of sodium carbonate (2 equiv with respect to iodobenzene) were placed into the reactor, that was then sealed and heated in a silicon bath until a temperature of 120 °C was reached. Then, at time t = 0 s, 197 mg of the 5% Au/CeO2 catalyst (0.05 equiv of Au with respect to iodobenzene) was added. The reactors were sealed again and deeply introduced into the silicone bath preheated at 150 °C. During the experiment, the reaction mixtures were magnetically stirred at 700 rpm. The course of the reaction was followed by analyzing aliquots taken at different times during the reaction. At the final times, the catalyst particles were removed from the solution by filtration. The reaction samples were analyzed by GC/MS methods for identification purposes, using an Agilent 6890N chromatograph coupled with an Agilent 5973 mass selective detector. The chromatograph was equipped with a capillary column (30 m length; 0.25 mm I.D.; 0.25 μm film) with 5% phenylmethylpolisiloxane as the stationary phase. For quantification purposes, an Agilent 7890A chromatograph equipped with FID detector and a capillary column (30 m length; 0.32 mm I.D.; 0.25 μm film) with 5% phenyl methylpolisiloxane as the stationary phase was used. Only experiments with mass balances ≥95% were considered. 2.5. Kinetic Study. The kinetic experiments were carried out by keeping constant the initial concentration of one of the reactants, IB or PA ([A]0 = 1 M) while varying the initial concentration of the other reactant ([B]0 = 0.25, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 M) and with a catalyst concentration

solution was added to a slurry containing nanoparticulated CeO2 (20 g). After adjusting the pH to 10 with 0.2 M NaOH, the slurry was left under vigorous stirring for 18 h at room temperature. The Au/CeO2 solid was then filtered and exhaustively washed with distilled water until no traces of chlorides were detected by the AgNO3 test. This is an important treatment, since traces of Cl− remain strongly bonded to gold and are highly detrimental to the overall activity in some reaction systems. In order to obtain the active state of the catalyst, the solid was reduced with 1-phenylethanol at 160 °C for 1 h (mass proportion 1:10 catalyst:1-phenylethanol). Finally, the active Au/CeO2 solid was filtered and exhaustively washed with distilled water and was dried at 120 °C in air for 12 h. The total Au content of the final catalyst was 4.99%, determined by FRX. Catalyst B. A dispersion of colloidal Au in nanoparticulated CeO2 was prepared by combining an Au colloid produced via the Turkevich method34,35 with nanoparticulated ceria in aqueous media. Briefly, 10 mL of 2% weight sodium citrate in H2O was added to 100 mL of 1 mM HAuCl4·3H2O in deionized H2O under vigorous boiling (100 °C). The solution was kept at this temperature for approximately 30 min. The solution passed through a purple color and finally turned red, indicating the formation of colloidal particles. The particles were analyzed as-prepared via TEM, and were found to have a mean diameter of 10 nm. To form the dispersion, the Au colloidal solution was mixed with the appropriate amount of CeO2 to produce a 5 wt % Au/CeO2 dispersion. The H2O was removed via rotavap, and the final powder was calcined under flowing N2 for 4 h at 400 °C. Catalyst C. A new Au/CeO2 preparation method was developed here with the aim of producing higher concentrations of cationic Au on the CeO2 surface. This was achieved by depositing Au(OH)x on the CeO2 surface with a short solution aging time to prevent precursor decomposition to metallic Au nanoparticles. First, a 30 wt % HAuCl4·H2O solution was prepared in 5 wt % HCl in H2O. The dissolution of HAuClx in HCl promotes the transformation of AuClx to Au(OH)x markedly and improves overall reproducibility of the method. In approximately 400 mL of H2O at 70 °C, 2000 mg of CeO2 was mixed with the appropriate amount of HAuCl4/ HCl/H2O solution to produce a nominal loading of 5 wt % Au. The pH of the solution was adjusted to pH 7 with 0.2 M NaOH and maintained for the duration of the preparation. After approximately 45 min, the solution was cooled in an ice bath and centrifuged three times. After each centrifuge run, the supernatant was replaced with clean deionized H2O and the catalyst redispersed with agitation. Finally, the catalyst was filtered until dry, further dried overnight at 120 °C in flowing dry air, and then calcined at 175 °C to facilitate the transformation of Au(OH)x to AuOx, the catalytically active species. The as-prepared catalyst has the color of the nanoparticulated CeO2 before drying. After drying, the catalyst takes on a grayish purple color. 2.3. Catalyst Characterization. XPS. X-ray photoelectron spectroscopy (XPS) measurements were performed with a SPECS spectrometer equipped with a 150 MCD-9 Phoibos multichannel detector and using a non-monochromatic Mg Kα (1253.6 eV) X-ray source. Spectra were recorded using an analyzer pass energy of 30 V, using an X-ray power of 50 W, and under an operating pressure of 10−9 mbar. Spectra treatment has been performed using the CASA software. 24857



Feature Article

to better approach the experimental situation where gold nanoparticles are not isolated but supported on a metal oxide material, a model for a Au/CeO2 catalyst was generated by placing a gold nanorod on a partially oxidized CeO2(111) surface, so that both Au0 and Auδ+ species are present in the system. The total charge on gold in the Au/CeO2 model unit cell is positive and high (4.15 e), and besides neutral Au0 atoms with net charges oscillating between −0.1 and 0.1, there are also positive Auδ+ species interacting with O atoms of the CeO2 surface at the particle perimeter, and other less accessible Au atoms at the bottom layer of the nanorod directly bonded to two or three O atoms of the CeO2 surface bearing positive charges as large as 0.65 e (see Table 1). For each elementary step, the geometry of the reactant, transition state, and reaction product was optimized at different sites on each catalytst model, and the DFT and dispersion corrected DFT-D calculated adsorption, activation, and reaction energies schematized in Figure 2 are summarized in

corresponding in all cases to 0.05 M Au. The reaction procedure is exactly as described above, and again, the course of the reaction was followed by GS, by analyzing aliquots taken at different reaction times.

3. RESULTS AND DISCUSSION 3.1. Theoretical Study of the Reaction Mechanism on Isolated Au Nanoparticles. The elementary steps involved in the mechanism of the Sonogashira reaction between iodobenzene and phenylacetylene (IB dissociation, PA deprotonation, and bimolecular coupling or C−C bond forming step) were investigated over four different gold catalyst models containing neutral and cationic Au atoms with different coordination numbers (see Figure 1). An extended Au(111)

Figure 1. Optimized structures of the gold catalyst models used in the theoretical study: (a) Au(111) slab containing highly coordinated neutral Au0 atoms, (b) Au38 cuboctahedral nanoparticle containing low coordinated neutral Au0 atoms, (c) partially oxidized Au38 nanoparticle containing low coordinated metallic Au0 and cationic Auδ+ sites, and (d) Au/CeO2 model containing neutral Au0 atoms on top of the rod and cationic Auδ+ sites at the metal−support interface. Au, O, and Ce atoms are depicted in yellow, red, and blue, respectively.

Figure 2. Schematic representation of energy profiles for IB and PA dissociation (left) and for the bimolecular coupling step (right) in the gold catalyzed Sonogashira cross-coupling reaction.

Table 2. Optimized geometries of all species involved in the Sonogashira reaction on an isolated Au38 nanoparticle and on an extended Au(111) surface are depicted in Figure 3 and Figure S1 in the Supporting Information, respectively. IB and PA adsorb on low coordinated neutral Au0 atoms at corner or edge sites of small Au38 nanoparticles, with optimized Au−I and Au−C distances of 2.77 and 2.19 Å, respectively (see Figure 3), while the interaction of reactants with a perfect Au(111) surface is geometrically different, and the molecules lay flat parallel to the surface at distances larger than 3 Å (see Figure S1, Supporting Information). IB and PA adsorption energies calculated at the PW91 level are ∼ −7 kcal/mol on the Au(111) surface and somewhat larger on the Au38 cluster. Inclusion of dispersion corrections stabilizes all adsorption complexes by at least 10 kcal/mol, this effect being larger on the Au(111) surface because in this system all atoms of IB and PA molecules are between 3 and 4 Å from the surface, in the region where dispersion forces are more important. IB dissociation occurs through a transition state in which the I atom and the phenyl fragment are interacting with two neighboring gold atoms. After dissociation, the phenyl fragment is placed on top of a gold atom, while the I atom becomes strongly chemisorbed on the catalyst, occupying a 3-fold hollow position on the Au(111) model and a bridge position between two Au atoms on the Au38 nanoparticle (see Figure 3 and Figure S1 in the Supporting Information). The activation

surface model was used to simulate the behavior of gold nanoparticles with diameters larger than ∼5 nm, that usually contain terraces exposing the most stable Au(111) facet and gold atoms directly coordinated to other nine gold atoms (Figure 1a). Small gold nanoparticles of ∼1 nm diameter containing low coordinated metallic Au0 atoms were modeled using a Au38 cuboctahedral cluster (Figure 1b). The possible presence of cationic gold species was initially modeled by means of a Au38O2 cluster having one Au atom directly bonded to two O atoms adopting an oxide-like structure and four Au atoms directly bonded to one O atom (Figure 1c).36 In this model, the net atomic charge on the former Au atom is 0.48 e, similar to those present in bulk Au2O oxide (see Table 1), while the net atomic charge on the four Au atoms bonded to only one O atom is 0.20 e, suggesting Auδ+ species. Finally, in order Table 1. Calculated Bader Charges on Selected Au Atoms in Different Gold Catalyst Models model

Au0

Au(111), Au38 Au2O, Au2O3 Au38O2 Au/CeO2

∼0 ∼0 ∼0

Auδ+

Au+

Au3+

0.47 0.48 0.65

1.19

0.20 0.33

24858



Feature Article

Table 2. Adsorption, Activation, and Reaction Energies (in kcal/mol) Calculated at the DFT and Dispersion Corrected DFT-D Levels for the Three Elementary Steps in the Sonogashira Reaction over Neutral Au0 and Cationic Auδ+ Sites on Different Gold Catalyst Models model

Au(111)

site level

a

Au DFT

Au38

0

Au DFT-D

DFT

Au/CeO2

0

Au DFT-D

DFT

δ+

Au DFT-D

Eads IB

−7.0

−25.9

−16.6

−27.3

−1.9

−11.4

Eact IB

15.7

11.8

11.3

6.2

20.9

16.0

ΔE IB Eads PA Eact PA ΔE PA Eact BP Eact DPA

−19.4 −7.2

−24.2 −21.6

8.9 31.8 30.0

27.5 34.2 29.1

−23.2 −11.9 39.7 20.4 35.0 28.0

−27.9 −21.9 40.5 21.8 31.9 26.4

−9.5 −2.5 5.4 −16.7

−12.1 −8.9 6.7 −16.2

12.7

11.4

Auδ+

0

DFT

DFT-D

−7.4 −6.7a 30.1 14.6a −2.7 −1.1a

−21.7 −17.4a 25.0 13.9a −8.4 −8.4a

38.8 37.3

38.2 35.8

DFT

DFT-D

−2.8 8.7 −49.1

−8.2 5.9 −51.4

24.0

24.1

Values in italics obtained on a Au22/CeO2 model.

When the reactants were approached to the cationic gold species, it was found that neither IB nor PA adsorb on the Au atom with larger charge density. The interaction of IB with the Auδ+ sites is weaker than with neutral Au0 atoms, and the activation barrier for its dissociation is about 10 kcal/mol larger than on Au0 sites (see Table 2), indicating that cationic gold will not favorably compete with Au0 for IB dissociation. On the other hand, PA adsorption on a Auδ+ atom is relatively strong, with a calcculated adsorption energy of −9.0 kcal/mol at the DFT-D level, and the deprotonation step in which the PA proton is transferred to one of the oxide-like O atoms (see Figure 4) has a very low barrier, 6.7 kcal/mol at the DFT-D level. However, the most interesting result on the Au38O2 nanoparticle is the low activation energy required for the surface reaction when it occurs between one phenyl fragment attached to a Au0 atom, where IB preferentially adsorbs and dissociates, and one phenylacetylenyl fragment bonded to an Auδ+ site, where PA adsorbs and is deprotonated with the lowest activation energy (see Figure 4 and Table 2). Indeed, in this case, the coupling step is still the rate-determining step, but the calculated activation energy (12.7 and 11.4 kcal/mol at the DFT and DFT-D levels, respectively) is much lower than when both reactants are adsorbed on Au0 (28 and 26.7 kcal/mol at the DFT and DFT-D levels, respectively). The optimized geometry and charge distribution in the reactant complex depicted in Figure 4 are equivalent to that obtained on the clean Au38 nanoparticle (Figure 3). Each organic fragment is directly attached to one Au atom, and bears a net negative charge of −0.20 e in the case of phenyl and −0.38 e in the case of phenylacetylenyl. In the transition state, the phenyl fragment is neutral and the negative charge on the phenylacetylenyl fragment decreases to −0.23 e. Despite the similar charge distribution, the optimized geometries of the transition states for the coupling step on the Au38 and Au38O2 models are slightly different. In this last case, the phenylacetylenyl fragment is not bonded to one corner Au0 atom, but it is bridged between the Auδ+ atom and the corner Au0 atom attached to the phenyl fragment, facilitating the formation of the new C−C bond. For comparison, the bimolecular coupling between one phenyl fragment adsorbed on Auδ+ and one phenylacetylenyl fragment adsorbed on Au0 was also explored. The calculated activation energy is higher, 19.2 and 17.4 kcal/mol at the DFT

energy for IB dissociation is lower on the Au38 cluster used to simulate smaller nanoparticles, and inclusion of dispersion corrections decreases the calculated values by ∼5 kcal/mol. The energy barrier necessary to break the C−H bond in PA on the Au38 model is high, 40 kcal/mol, and the process is clearly endothermic (see Table 2). However, as mentioned in the Introduction, when the Sonogashira coupling is carried out under practical conditions in solution, alkyne deprotonation is assisted by a base.1,8−10 Thus, as a first approximation, a carbonate CO32− fragment was introduced in the calculations (see Figure 3), and the barrier for PA deprotonation on the Au38 cluster decreased to 8.9 and 7.8 kcal/mol at the DFT and DFT-D levels, respectively, indicating that the presence of a base strongly enhances PA deprotonation. The possibility that I atoms generated in the dissociation of IB could facilitate the deprotonation of PA was also considered, but the process was found to be endothermic by more than 30 kcal/mol on both the Au38 and Au(111) models. Finally, the activation energies for the homocoupling of phenyl fragments yielding BP and for the cross-coupling step producing DPA were calculated on both the Au38 and Au(111) models (see Table 2), and it was found that both reactions involve similar barriers, although smaller particles slightly favor the desired Sonogashira coupling. These results indicate that, in the presence of a base able to facilitate PA deprotonation, the rate of formation of BP and DPA on gold catalysts containing only metallic Au0 sites should be quite similar and high selectivity to the Sonogashira reaction cannot be expected. In agreement with these findings, selectivities to the crosscoupling DPA product of ∼40% have been reported on Au/ SiO2 and Au/TiO2 catalysts in the presence of K2CO3.14,16 Nevertheless, selectivities toward the cross-coupling DPA product larger than 85% have been reported on gold nanoparticles supported on CeO2,8,16 and it was proposed8 that cationic gold was key to performing the Sonogashira reaction. The possible presence of cationic gold species has been initially modeled here by means of a Au38O2 nanoparticle previously described,18 which contains one gold atom that could be assimilated to AuI and four gold atoms bearing a partial positive density of charge, denoted as Auδ+ species (see Figure 1c and Table 1). 24859



Feature Article

Figure 4. Optimized geometries of the species involved in the Sonogashira reaction on a partially oxidized Au38 nanoparticle containing low coordinated neutral Au0 atoms and cationic Au+ and Auδ+ sites. (a) PA deprotonation and (b) bimolecular surface crosscoupling step. Au, O, C, and H atoms are depicted in yellow, red, orange, and white, respectively.

activation energies for IB dissociation are around 25−30 kcal/ mol (see Table 2). The reason for these large activation barriers is the higher coordination of the Au atoms in the nanorod model as compared with the isolated Au38 nanoparticle, that results in a weaker interaction and activation of IB. To check whether the presence of low coordinated Au atoms is key for IB dissociation, a Au22/CeO2 model generated by adding two Au atoms on top of the rod in the Au/CeO2 model was used to investigate this process. Each of these two Au atoms is directly bonded to five other Au atoms. The optimized structures obtained on the Au22/CeO2 model are more similar to those described on the Au38 isolated particle, with an optimized Au−I distance of 3.04 Å, and the calculated activation barrier decreases to ∼14 kcal/mol (see Table 2, values in italics), indicating that IB dissociation is favored on neutral low coordinated Au0 atoms at corner sites. PA adsorption and deprotonation at Au0 sites present in the Au/CeO2 model was not theoretically investigated in this work, because the results obtained using the isolated Au38 nanoparticle show that the presence of a base is necessary to activate PA when it is adsorbed on neutral Au0 atoms. Therefore, only the reaction with PA initially adsorbed on an Auδ+ atom at the metal−support interface was considered. The calculated adsorption energy is −8.2 kcal/mol at the DFT-D level, and the optimized C−Au distance of 2.19 Å (see Figure 5) reflects a non-negligible interaction similar to that found on the neutral Au38 and partially oxidized Au38O2 isolated nanoparticles. Moreover, the activation barrier of 8.7 and 5.9 kcal/mol at the DFT and DFT-D levels, respectively, indicates that the process is energetically feasible. After deprotonation, the phenylacetylenyl fragment remains adsorbed on the Auδ+ atom, but it is also interacting with two neutral Au0 atoms at the top edge of the nanorod. The most stable initial configuration of the reactants for the bimolecular coupling step is depicted in Figure 5, with the

Figure 3. Optimized geometries of the species involved in the Sonogashira reaction on an isolated Au38 nanoparticle containing low coordinated neutral Au0 atoms. (a) IB dissociation, (b) PA deprotonation, (c) bimolecular surface cross-coupling step yielding DPA, and (d) bimolecular surface homocoupling step yielding BP. Au, O, C, and H atoms are depicted in yellow, red, orange, and white, respectively.

and DFT-D levels, respectively, and although the phenyl fragment in the transition state is also bridged between the Auδ+ atom and the corner Au0 atom attached to the phenylacetylenyl fragment, the optimized CPh−Auδ+ distance is quite long (2.75 Å), reflecting a weaker interaction. 3.2. Theoretical Study of the Reaction Mechanism on Au Nanoparticles Supported on CeO2. In order to better simulate the reported experimental information, a model for a Au/CeO2 catalyst was generated by placing a gold nanorod on a partially oxidized CeO2(111) surface, so that both Au0 and Auδ+ species are present in the system (see Figure 1 and Table 1). IB adsorption and dissociation was investigated on this Au/ CeO2 model, and the catalytic performance observed was similar to that previously described for the Au(111) model. The optimized geometries depicted in Figure 5 indicate that the molecule lies flat above the gold nanorod, with an optimized Au−I distance of 3.67 Å. The calculated adsorption energies are only −7.4 kcal/mol at the DFT level and considerably higher, −21.7 kcal/mol, at the DFT-D level, while the calculated 24860



Feature Article

Finally, the coupling of two phenyl fragments on the Au/ CeO2 model yielding BP was investigated. The optimized geometry of reactants and transition state is similar to that obtained on the isolated Au38 nanoparticle, and the calculated activation barrier is slightly larger, 38 kcal/mol, suggesting that gold nanoparticles supported on CeO2 should be intrinsically more selective to DPA and DPDA than when isolated or supported on inert carriers. From a theoretical point of view, it can be concluded that the Sonogashira reaction can occur on large and small gold nanoparticles, that both IB and PA can be adsorbed and activated on neutral Au0 atoms of the surface, and that IB homocoupling yielding BP strongly competes with crosscoupling, thus lowering the selectivity toward the Sonogashira product. However, if Auδ+ species are present, PA is preferentially activated on these sites, and then, the coupling between one IB molecule dissociated on Au0 and one PA activated on Auδ+ requires a much lower activation energy. These results allow it to be predicted that, if Auδ+ sites are also introduced in a gold catalyst containing mostly Au0, the rate of the Sonogashira cross-coupling and therefore the selectivity to DPA should increase. Finally, taking into account the strong adsorption of I on gold, the possibility of catalyst poisoning by the I atoms generated by IB dissociation was considered, and the interaction energies Eint were calculated according to E int = E(I−Au) − E(Au) − 1/2E(I 2)

where E(I−Au) is the total energy of I atom adsorbed on the catalyst model, E(Au) is the total energy of the catalyst model, and E(I2) is the total energy of I2 molecule in the gas phase. In all cases, the I atoms are strongly adsorbed on the gold surface, occupying either 3-fold hollow sites in Au(111) facets of neutral gold models, with a calculated Eint value of ∼ −20 kcal/ mol, or bridge positions between two edge Au atoms as depicted in Figure 3a, with calculated Eint values larger than −30 kcal/mol. 3.3. Kinetic Study. From the theoretical results presented above, the following elementary steps can be considered for the Sonogashira cross-coupling reaction:

Figure 5. Optimized geometries of the species involved in the Sonogashira reaction on a Au/CeO2 model containing neutral Au0 atoms and cationic Auδ+ sites at the metal−support interface. (a) IB dissociation, (b) PA deprotonation, and (c) bimolecular surface coupling step at Au0 and Auδ+ sites. Au, O, Ce, C, and H atoms are depicted in yellow, red, blue, orange, and white, respectively.

phenyl fragment attached to a neutral Au0 atom and the phenylacetylenyl fragment occupying a bridge position between a Au0 and a Auδ+ species. From this initial configuration, two different transition states lead to DPA weakly interacting with different points at the catalyst surface. In the first one, the coupling step occurs on two neutral Au0 atoms, with an optimized geometry similar to that found on the Au38 nanoparticle and with a calculated activation energy of 36−37 kcal/mol (see Table 2). This high barrier is not reflecting a less stable transition state in relation to that found on the Au38 model but is mainly related to the higher stability of the reactant complex described above. However, a second transition state involving a lower barrier of 24 kcal/mol was localized that connected the same initial reactants with adsorbed DPA. In this transition state, the phenylacetylenyl fragment remains attached to the Auδ+ species, while the phenyl fragment moves from its initial position to form the new C−C bond. It should be remarked that both transition states were characterized by frequency calculations and it was checked that they lead to the same initial reactant structure. Thus, the results found with a Au/CeO2 catalyst model containing Au0 and Auδ+ atoms are in line with those obtained using isolated nanoparticles containing the same type of Au0 and Auδ+ sites.

KB

(1)

IB + L ↔ IB−L kB

IB−L + K 2CO3 → B−L + KI + KCO3− KA

(3)

PA + L → PA−L kA

PA + L + KCO3− → A−L + KHCO3 kS

B−L + A−L → DPA + 2L KA ′

PA + L′ ⎯→ ⎯ PA−L′ kA ′

PA + L′ + KCO3− → A−L′ + KHCO3 k S′

(2)

B−L + A−L′ → DPA + L + L′

(4) (5) (6) (7) (8)

In the first elementary step, described by eq 1, IB adsorbs on a surface Au0 atom, denoted as L, and then dissociates according to eq 2 into a phenyl fragment that remains adsorbed on the Au0 site (B−L) and a iodine atom that could be removed from the surface as KI by means of the base (K2CO3) introduced in large excess in the reaction media (though some iodine might 24861



Feature Article

remain adsorbed on the most stable edge sites and might accumulate on the gold surface with time). Meanwhile, PA can adsorb (eq 3) and be activated by the same Au0 sites (L), assisted by the base that abstracts a proton, and leaving a phenylacetylenyl fragment adsorbed on the surface (A−L), according to eq 4. Finally, the phenyl and phenylacetylenyl fragments adsorbed on Au0 (sites L) react to give the DPA product, leaving the two active sites free to start a new catalytic cycle (eq 5). According to the theoretical study, PA can also adsorb and be activated on cationic Auδ+ sites, denoted as L′, as described by eqs 6 and 7 and, in the last step of the mechanism, the phenyl fragments adsorbed on Au0 sites (B−L) and the phenylacetylenyl residues bonded to Auδ+ (A−L′) sites react to give the DPA product, leaving the two active sites free to start a new catalytic cycle (eq 8). The theoretical study concluded that the rate-determining step of the sequence is the surface reaction between the two organic fragments, and that iodine atoms generated by IB dissociation can remain strongly adsorbed on the gold surface, blocking Au0 sites. To determine which is the rate-determining step of the global process, a series of kinetic experiments were designed where the initial concentration of PA was kept constant while varying the initial concentration of IB and vice versa. Initial reaction rates were obtained for the Sonogashira cross-coupling yielding DPA, as well as for the homocoupling of IB and PA, yielding biphenyl (BP) and diphenyldiacetylene (DPDA), respectively (see Figure 6). Simultaneously, in order to check if some iodine is accumulated on the catalyst surface during the reaction, even in the presence of the base, two Au/ CeO2 samples were analyzed by Raman spectroscopy before and after 30 min of reaction time. As shown in Figure 7, only the peak at 465 cm−1 associated to the CeO2 support is observed in the as-prepared reference Au/CeO2 catalyst, while a Raman band at 158 cm−1 corresponding to the Au−I vibration37 is also observed in the samples that had been used in the kinetic experiments described above, demonstrating that at least some iodine remains adsorbed on the gold surface after reaction. The large influence of adsorbed iodine on the catalyst performance is clearly seen in Figure 8, that shows IB conversion and yield of the different homo- and cross-coupling products obtained in a series of experiments in which increasing amounts of I2 (between 0 and 24% of initial IB concentration) were added to the reaction mixture. Adding 4% of I2 decreased IB conversion from 12% to less than 5%, with BP and DPA yields decreasing from 3.6 and 6.5% to 1.3 and 1.2%, respectively. When larger amounts of I2 were added, conversion was further reduced to ∼2%, and the only products observed were DPA and DPDA. This behavior agrees with the theoretical proposal that iodine atoms can block the Au0 sites where IB adsorbs and dissociates but not the cationic Auδ+ sites where PA is activated. Moreover, the almost constant yield of DPDA product regardless of the amount of I2 added suggests that PA homocoupling might only involve cationic Auδ+ species as active sites. Taking into account all the theoretical and experimental information described so far, and according to the kinetic models developed by Hougen−Watson/Langmuir−Hinshelwood for reaction mechanisms in heterogeneous catalysis, the following equations were derived to describe the kinetics of the reactions between IB and PA on Au/CeO2 catalysts. If the rate-determining step in this sequence was IB dissociation, the initial reaction rate (r0) for the Sonogashira reaction would be described by eq 9:

Figure 6. Initial reaction rate r0 (in mmol/min) for the Sonogashira cross-coupling (blue circles), IB homocoupling (red squares), and PA homocoupling (green triangles) as a function of initial [PA] at constant [IB]0 (top) and as a function of initial [IB] at constant [PA]0 (bottom), and fitting of the Sonogashira cross-coupling r0 values to eq 13 (solid line).

Figure 7. Raman spectra of nanoparticulated CeO2, of AuI impregnated on CeO2, and of Au/CeO2 catalyst as prepared and after reaction.

r0 =

kBKBL0[IB]0 (1 + KA[PA]0 + KI[IB]0 2 )

(9)

where the KI[IB]20 term was introduced to take into account the negative effect of iodine atoms that, according to the interaction energies summarized in Table 1, adsorb more strongly at bridge positions blocking each iodine atom two Au0 active sites. If PA activation was the rate-determining step, then the initial reaction rate (r0) for the Sonogashira reaction would fit eq 10 if both reactants were activated on the same Au0 sites: 24862



Feature Article

The fitting of the experimental initial reaction rates plotted in Figure 6 to the expressions obtained by keeping constant either [IB]0 or [PA]0 in eqs 9, 12, and 13 (see Supporting Information) is summarized in Table 3. It can be seen there that the rate-determining step is not IB dissociation (negative value for b parameter at constant [IB]0) but the bimolecular surface coupling step. On the other hand, while the two models consideredonly one type (L) or two types (L and L′) of active sites are involved in the cross-coupling stepdescribe correctly the catalyst performace at constant [IB]0, the initial rates at constant [PA]0 are better reproduced when it is assumed that IB and PA adsorb and dissociate on different active sites (negative value for the c parameter in eq 12b), although the possibility of both reactants being activated at the same (L) sites cannot be absolutely ruled out. The kinetic study demonstrates, in agreement with theory, that on Au/CeO2 catalysts IB dissociates on neutral Au0 sites (L), PA can be activated on both neutral Au0 (L) and cationic Auδ+ sites (L′), and the bimolecular surface reaction is the ratedetermining step. However, while kinetic experiments seem to support the theoretical prediction that the presence of Au atoms with a positive density of charge (Auδ+) together with Au0 strongly favors the Sonogashira reaction by helping to activate PA and decreasing the activation energy of the coupling step, this was not unambiguously confirmed. This point will be further considered in the next section. 3.4. Synthesis, Characterization, and Catalytic Performance of Au/CeO2 Catalysts with Different Ratios of Au0/Auδ+ Sites. In order to unambiguously demonstrate the key role played by cationic gold sites in the selectivity toward the cross-coupling DPA product, a series of Au/CeO2 catalysts with a different ratio of Au0/Auδ+ centers were synthesized (see the Experimental Section) and characterized by HAADFSTEM, XPS, and IR spectroscopy of CO adsorption. According to the HAADF-STEM images shown in Figure 9, catalyst A used in the kinetic study and catalyst C obtained with a different synthesis procedure have a similar average particle size, 1.8 ± 0.5 and 2.3 ± 0.5 nm, respectively. However, catalyst B obtained by dispersion of 10 nm colloidal gold nanoparticles with nanoparticulated CeO2 resulted in agglomeration of the gold nanoparticles, with a final average gold particle size of ∼50 nm. The electronic properties of the supported gold nanoparticles were analyzed by XPS spectroscopy, and the results are shown in Figure 10. The oxidation state of gold was determined using the binding energy (BE) of the Au 4f7/2 level, with a BE of 84.0, 84.6−85.1, and 85.9−86.3 eV being associated with Au0, Au+, and Au3+, respectively.38 The relative amount of the different gold oxidation states on each catalyst was determined by spectra deconvolution using the CASA software. Catalyst B

Figure 8. IB conversion and yield of BP, DPA, and DPDA as a function of amount of added I2.

r0 =

kAKAL0[PA]0 (1 + KB[IB]0 + KI[IB]0 2 )

(10)

and r0 would fit eq 11 if PA was preferentially activated on cationic Auδ+ sites (L′), where IB adsorption does not compete: r0 = kA′KA′L0′[PA]0

(11)

Taking into account eqs 10 and 11, it is clear that, when the initial concetration of IB is kept constant, a linear relationship between the initial reaction rate and PA concentration should be obtained if PA dissociation was the controlling step. However, the experimental values ploted in Figure 7, top, do not fit this model. Therefore, it can already be concluded that PA dissociation is not the rate-determining step of the Sonogashira reaction. If the slowest step in the global reaction scheme was the surface reaction between adsorbed phenyl and phenylacetylenyl fragments, B−L and A−L, then the initial reaction rate would be described by eq 12 when considering that IB and PA adsorb and dissociate on the same sites L (Au0): r0 =

k SkBKBkAKA[IB]0 [PA]0 [L]0 2 (1 + KA[PA]0 + KB[IB]0 + KI[IB]0 2 )2

(12)

On the other hand, the initial reaction rate would be described by eq 13 when considering that IB adsorbs and dissociates on L sites and PA is preferentially adsorbed and activated on L′ sites: ⎞ ⎛ kBKB[IB]0 [L]0 ⎟ r0 = k S′⎜ 2 ⎝ 1 + KB[IB]0 + KA[PA]0 + KI[IB]0 ⎠ ⎛ kA′KA′[PA]0 [L′]0 ⎞ ⎜ ⎟ ⎝ 1 + KA′[PA]0 ⎠

(13)

Table 3. Fitting of the Experimental Data Plotted in Figure 6 to the Equations Obtained for the Different Mechanistic Models Described in the Supporting Information

a

controlling step

conditions

eqa

IB dissociation IB dissociation surface reaction surface reaction surface reaction surface reaction

[IB]0 = 1 M [PA]0 = 1 M [IB]0 = 1 M [PA]0 = 1 M [IB]0 = 1 M [PA]0 = 1 M

9a 9b 12a 12b 13a 13b

standard deviation 8.6 3.1 1.0 1.6 1.01 1.5

× × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3

a

b

c

d

53.749 5.290 3.807 1.462 7.780 1.567

−6.965 23.535 2.054 4.108 4.217 14.354

−0.023 1.863 15.181

1.000

Equations fully described in the Supporting Information. 24863



Feature Article

Figure 9. HAADF-STEM images and average particle size of catalysts A (top), B (center), and C (bottom).

and 2125 cm−1 provides an estimation of the Au0/Auδ+ ratio. It is clear in Figure 11 that CO does not adsorb on catalyst B, probably due to the large particle size and therefore little amount of low coordinated Au0 and metal−support interface Auδ+ sites able to bind CO. On the other hand, the presence of Au+, Auδ+, and Au0 species in catalysts A and C is confirmed, and the relative intensity of the 2100 and 2125 cm−1 IR bands indicates that sample A contains more metallic gold nanoparticles with coordinatively unsaturated Au0 atoms, while sample C contains more cationic Au+ sites and a similar amount of neutral Au0 and slightly positive Auδ+ sites at the metal− support interface. If selectivity to DPA is indeed governed by the Au0/Auδ+ ratio, catalyst C should be more selective than catalyst A in the Sonogashira cross-coupling reaction.

consisting of large gold nanoparticles only contains metallic Au0, while cationic Au+ sites are observed on catalysts A (∼11%) and C (∼25%). Finally, the three Au/CeO2 samples were characterized by IR spectroscopy of CO adsorption, since the stretching vibrational mode of CO is highly sensitive toward the nature of the particular site at which it adsorbs. Thus, the peaks at 2175 and 2154 cm−1 (see Figure 11) correspond to CO adsorbed on Au3+ and Au+ sites, respectively.39 In addition, the IR band at 2125 cm−1 has been assigned to Auδ+ sites present in Au−O− support linkages at the metal−support interface, while the broad IR band centered at ∼2100 cm−1 is associated to low coordinated metallic Au0 surface sites.40 Although IR spectra are not quantitative, the relative intensity of the bands at 2100 24864



Feature Article

Figure 11. IR spectra of CO adsorption on the three Au/CeO2 catalysts: A (blue line, top), B (green line, center), and C (red line, bottom).

Figure 10. XPS spectra of the three Au/CeO2 catalysts: A (top), B (center), and C (bottom).

Figure 12. Product distribution obtained in the reaction of IB with PA on Au/CeO2 catalysts having different amounts of cationic gold sites: (a) catalyst A, (b) catalyst B, (c) catalyst C.

The catalytic performance of the three Au/CeO2 samples in the reaction between IB and PA is shown in Figure 12. As expected from the theoretical and experimental data discussed above, catalyst B, with mainly metallic gold, preferentially activates IB and therefore BP is the most abundant product. When cationic sites are present (samples A and C), DPA becomes the predominant product, and the best overall performance is obtained with catalyst C containing the largest concentration of acessible Auδ+ sites at the metal−support interface.

The gold content of the catalysts after reaction was analyzed by EDXRF (see the Supporting Information), and significant gold leaching was observed in all samples, as had been previously reported.8,13 To check whether the leached gold was active, three new experiments were done in which the reaction was allowed to proceed during 20 min and after that time the solid catalyst was removed from the reaction mixture by hot filtration and the reaction monitored by GC analysis. The results given in Figure S3 in the Supporting Information show that the reactions do not proceed in the absence of the solid 24865



Feature Article

Notes

Au/CeO2 catalyst, and therefore, the following conclusions are valid for this heterogeneously catalyzed process.

The authors declare no competing financial interest. Biographies

4. CONCLUSIONS The elementary steps involved in the mechanism of the homoand cross-coupling reactions between IB and PA have been investigated by means of DFT calculations on several catalyst models containing neutral and cationic gold sites with different coordination numbers. DFT calculations show that IB can be dissociated on metallic Au0 atoms with different coordination numbers, and that PA deprotonation at neutral Au0 sites is also energetically feasible if the assistance of a base (CO32−) is included in the model. When only metallic gold is present, the bimolecular cross-coupling is the rate-determining step with an activation energy comparable to that required for IB homocoupling, thus explaining the selectivity results obtained with Au/SiO2 catalysts that mainly contain Au0 sites. When gold nanoparticles, either isolated or supported on CeO2, contain both Au0 and Auδ+ sites, IB preferentially dissociates on Au0, and PA is activated on both sites. And, what is more important, the coupling between IB dissociated on Au0 and PA on Auδ+ requires a much lower activation energy than if both reactants, i.e., PA and IB, are activated on Au0, with this activation energy being much lower than that necessary for IB homocoupling. This implies that the selectivity toward the Sonogahira cross-coupling product is promoted if the catalyst contains both neutral Au0 and cationic Auδ+ sites; that is, it is governed by the Au0/Auδ+ ratio. DFT calculations also suggest that iodine atoms might remain strongly adsorbed on the gold nanoparticles, and catalyst poisoning by iodine atoms is confirmed by Raman spectroscopy and kinetic experiments. With this information, a kinetic study of the Sonogashira reaction on Au/CeO2 catalysts within the Hougen−Watson−Langmuir−Hinselwood formalism confirms that the rate-determining step is the bimolecular surface coupling reaction, and that, in the preferential reaction pathway for the Sonogashira reaction, IB and PA are activated on different gold sites that can be assimilated to Au0 (IB) and Auδ+ (PA). Finally, the catalytic performance of several Au/ CeO2 catalysts containing different ratios of Au0/Auδ+ sites unequivocally shows that cationic gold plays a key role in the Sonogashira cross-coupling reaction. These conclusions can explain the diversity of selectivity results reported in the literature on different types of gold catalysts, including the 20% selectivity toward DPA obtained on Au(111) crystals,17 the less than 40% selectivity to DPA obtained on Au/SiO2 catalyts,14 and the considerably higher selectivity values found on Au/CeO28 and Au/La2O3,16 on which more cationic gold can be stabilized.

■

Mercedes Boronat obtained her Ph.D. from University of Valencia in 1999 under supervision of Prof. Avelino Corma and Prof. Pedro Viruela. She joined the Institute of Chemical Techonology (ITQ) with a postdoctoral grant (1999−2001) and worked as a Junior Researcher in CEAM Foundation (2001−2003). Since 2007, she is a Tenured Scientist of the Spanish National Research Council (CSIC) at ITQ. Her work focuses on the theoretical study of reaction mechanisms, mostly heterogeneous catalyzed reactions such as hydrocarbon transformations catalyzed by acid zeolites, selective hydrogenations and oxidations catalyzed by metal nanoparticles, and C−C forming reactions catalyzed by metals and metal oxides. She has published more than 50 scientific papers in international journals and has participated in research projects in collaboration with different companies. Diego Cómbita-Merchán received a B.S. in chemical engineering by the Universidad Nacional de Colombia in 2004 and a M.Sc. in Green Chemistry by the Universidad Politécnica de Valencia in 2008. Since 2008, he has been studying for his Ph.D. degree under the guidance of ́ Prof. Avelino Corma at the Instituto de Tecnologiá Quimica (ITQ) in Valencia, Spain. His research focuses on metal-based heterogeneous catalysis, including catalysts synthesis, characterization, and kinetics and reaction mechanisms studies. Patricia Concepción obtained her Ph.D. in 1996 at the Institute of Chemical Technology (ITQ) under the supervision of Prof. Jose Manuel López-Nieto. Between 1997 and 1998, she worked as a postdoc in the Fritz-Haber-Institut der Max Planck Geselschaft Berlin under the supervision of Prof. Karge, and between 1998 and 1999, in the Institut für Physikalische Chemie, LMU, München, under the supervision of Prof. Knözinger. In 1999, she returned to the ITQ, Valencia, Spain, achieving a permanent position as Tenured Scientist of the Spanish National Research Council (CSIC) in 2002. Her research area is focused on catalyst characterization under reaction conditions in order to develop structure−activity relationships. Systems of interest are supported metal nanoparticles, metal oxide catalyst, and zeolites. The main spectroscopic tools used are infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. She is the author of more than 50 publications and 3 patents on different aspects of inorganic, organic, and material chemistry. Avelino Corma studied Chemistry at the Universidad de Valencia (1967−1973) and received his Ph.D. at the Universidad Complutense de Madrid in 1976. He was Postdoctoral in the Department of Chemical Engineering at the Queen’s University (Canada, 1977− 1979). He has been a research professor at the Instituto de Tecnologiá ́ Quimica (UPV-CSIC) at the Universidad Politécnica de Valencia since 1990. His current research field is catalysis, covering aspects of synthesis, characterization, and reactivity in acid−base and redox catalysis. Avelino Corma is coauthor of more than 900 articles and 100 patents on these subjects. Hermenegildo Garciá was made a full Professor at the Technical University of Valencia in 1996 and is a staff member of the Instituto de ́ Tecnologıiá Quıimica, a joint center of the Technical University of Valencia and the Spanish National Research Council. He made postdoctoral stays at the University of Reading with Professor Andrew Gilbert and several sabbatical leaves in the group of Professor J. C. Scaiano at the University of Ottawa. Prof. Garcia has been active in the field of heterogeneous catalysis working with porous catalysts and nanoparticles, has published over 450 papers, and has filed over 25

ASSOCIATED CONTENT

S Supporting Information *

Optimized structures involved in the Sonogashira reaction over the Au(111) surface and in the base-assisted PA deprotonation on an isolated Au38 nanoparticle; full derivation of the equations used in the kinetic study; results of the hot filtration test. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: 34 96 387 7809. E-mail: [email protected]. 24866



Feature Article

(12) Corma, A.; Juárez, R.; Boronat, M.; Sánchez, F.; Iglesias, M.; García, H. Chem. Commun. 2010, 47, 1446−1448. (13) Crabtree, R. H. Chem. Rev. 2012, 112, 1536−1554. (14) Kyriakou, G.; Beaumont, S. K.; Humphrey, S. M.; Antonetti, C.; Lambert, R. M. ChemCatChem 2010, 2, 1444−1449. (15) Robinson, P. S. D.; Khairallah, G. N.; da Silva, G.; Lioe, H.; O’Hair, R. A. J. Angew. Chem., Int. Ed. 2012, 51, 3812−3817. (16) Beaumont, S. K.; Kyriakou, G.; Lambert, R. M. J. Am. Chem. Soc. 2010, 132, 12246−12248. (17) Kanuru, V. K.; Kyriakou, G.; Beaumont, S. K.; Papageorgiou, A. C.; Watson, D. J.; Lambert, R. M. J. Am. Chem. Soc. 2010, 132, 8081− 8086. (18) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671− 6687. (19) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244−13249. (20) Kresse, G.; Furthmüller, J. Phys. Rev. B 1996, 54, 11169−11186. (21) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558−561. (22) Blöchl, P. E. Phys. Rev. B 1994, 50, 17953−17979. (23) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. J. Chem. Phys. 2000, 113, 9901−9904. (24) Heyden, A.; Bell, A. T.; Keil, F. J. J. Chem. Phys. 2005, 123, 224101−224114. (25) Henkelman, G.; Jónsson, H. J. Chem. Phys. 1999, 111, 7010− 7022. (26) Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. J. Comput. Chem. 2007, 28, 899−908. (27) Henkelman, G.; Arnaldsson, A.; Jónsson, H. Comput. Mater. Sci. 2006, 36, 354−360. (28) Loschen, C.; Carrasco, J.; Neyman, K. M.; Illas, F. Phys. Rev. B 2007, 75, 035115−035122. (29) Grimme, S.; Jonsson, H. J. Chem. Phys. 2010, 132, 154104− 154122. (30) http://toc.uni-muenster.de/DFTD3. (31) Laursen, S.; Linic, S. Phys. Chem. Chem. Phys. 2009, 11, 11006− 11012. (32) Laursen, S.; Linic, S. J. Phys. Chem. C 2009, 113, 6689−6693. (33) Park, E. D.; Lee, J. S. J. Catal. 1999, 186, 1−11. (34) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55−75. (35) Kimling, J.; Maier, M.; Okenve, B.; Kotaidis, V.; Ballot, H.; Plech, A. J. Phys. Chem. B 2006, 110, 15700−15707. (36) Alves, L.; Ballesteros, B.; Boronat, M.; Cabrero-Antonino, J. R.; Concepción, P.; Corma, A.; correa-Duarte, M. A.; Mendoza, E. J. Am. Chem. Soc. 2011, 133, 10251−10261. (37) Loo, B. H. J. Phys. Chem. 1982, 86, 433−437. (38) Knecht, J.; Fischer, R.; Overhof, H.; Hensel, F. J. Chem. Soc., Chem. Commun. 1978, 21, 905−906. (39) Binet, C.; Daturi, M.; Lavalley, J.-C. Catal. Today 1999, 50, 207−225. (40) Boronat, M.; Concepción, P.; Corma, A. J. Phys. Chem. C 2009, 113, 16772−16784.

patents, two of them in industrial exploitation. Prof. Garcia is Doctor Honoris Causa from the University of Bucharest and the recipient of the 2011 Janssen-Cilag award given by the Spanish Royal Society of Chemistry and the 2008 Alpha Gold of the Spanish Society of Glass and Ceramics. Raquel Juárez Marin studied Chemistry at the University of Valencia (1998−2005) and obtained her doctorate at Polytechnic University of Valencia in 2011. She worked under the guidance of Professor Hermenegildo Garcia at the Institute of Chemical Technology (UPVCSIC). In this period, she worked on heterogeneous catalysis and catalytic chemical processes in the fields of green chemistry and fine chemistry. She published articles in international reviews and was author of several patents of industrial application. Currently, she is developing her career works in the Institute of Chemical Technology under the supervision of Professor Avelino Corma. Siris Laursen studied Chemical Engineering at the University of Colorado, Boulder (2000−2004), and received his PhD at the University of Michigan, Ann Arbor, in 2009 under the direction of Prof. Suljo Linic. He performed his postdoctoral research at the Instituto de Tecnologia Quimica at the Universidad Politecnica de Valencia from 2010−2012. In 2012, he joined the faculty at the University of Tennessee, Knoxville. His current research focuses on the conversion of biomass to fuels and chemicals, photo- and electrocatalytic processes, selective C−H, C−O, C−N, and C−C activation, and the atom-up design of advanced catalytic materials. Dr. Juan de Dios López-Castro received his doctorate in chemistry in 2011 from the University of Cádiz working within the “Structure and Chemistry of Nanomaterials” Group. His expertise focuses on the application of Advanced Electron Microscopy techniques in the investigation of nanoparticle-based materials, particularly solid catalysts and bionanoparticles.

■

ACKNOWLEDGMENTS Financial support by the Spanish MICINN (CONSOLIDER Ingenio 2010-MULTICAT) and Generalitat Valenciana (PROMETEO project 2088/130) is gratefully acknowledged. R.J. and D.C. thank the Spanish MICINN for postgraduate scolarships, and S.L. thanks ITQ for a postdoctoral fellowship. We thank Red Española de Supercomputación (RES) and Centre de Càlcul de la Universitat de València for computational resources and technical assistance.

■

REFERENCES

(1) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16, 4467−4470. (2) Sonogashira, K. J. Organomet. Chem. 2001, 635, 46−49. (3) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed. 2000, 39, 2632−2657. (4) Astruc, D. Inorg. Chem. 2007, 46, 1884−1894. (5) Chinchilla, R.; Najera, C. Chem. Rev. 2007, 107, 874−922. (6) Amatore, C.; Jutand, A.; Le Duc, G. Chem.Eur. J. 2011, 17, 2492−2503. (7) Amatore, C.; Catellani, M.; Deledda, S.; Jutand, A.; Motti, E. Organometallics 2008, 27, 4549−4554. (8) González-Arellano, C.; Abad, A.; Corma, A.; García, H.; Iglesias, M.; Sánchez, F. Angew. Chem., Int. Ed. 2007, 46, 1536−1538. (9) García-Melchor, M.; Pacheco, M. C.; Nájera, C.; Lledós, A.; Ujaque, G. ACS Catal. 2012, 2, 135−144. (10) Sikk, L.; Tammiku-Taul, J.; Burk, P. Organometallics 2011, 30, 5656−5664. (11) Lauterbach, T.; Livendahl, M.; Rosellon, A.; Espinet, P.; Echevarren, A. M. Org. Lett. 2010, 12, 3006−3009. 24867


Making CâC Bonds with Gold: Identification of ... - ACS Publications

Recommend Documents

Making CâC Bonds with Gold: Identification of ... - ACS Publications