Making C–C Bonds with Gold: Identification of Selective Gold Sites for Homo- and Cross-Coupling Reactions between Iodobenzene and Alkynes

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 CeO 2 . The mechanistic studies show that IB dissociation occurs on low coordinated Au 0 atoms present in small gold nanoparticles, either isolated or supported, while PA is preferentially adsorbed and activated on Au δ+ 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/CeO 2 catalysts containing different ratios of Au 0 /Au δ+ sites confirms the positive role played by cationic gold in the Sonogashira cross-coupling reaction. Importantly, only metallic Au 0 atoms present in gold nanoparticles are required to perform the homocoupling of iodobenzene.

Sonogashira cross-coupling on the Au(1 1 1) and Au(1 0 0) facets of gold nanorod catalysts: Experimental and computational investigation

“…Calculated Table 1 The catalytic activity and selectivity of the gold nanoparticles supported on ceria in Sonogashira cross-coupling reactions between p-iodoanisole and phenylacetylene a : adsorption and activation energies are compiled in Table 2. In good agreement with previous studies [19,45], the adsorption process is found to be exothermic by 7.0 and 10.6 kcal/mol on Au(1 0 0) and Au(1 1 1), respectively. Though dispersion corrections are not included in our calculations, we believe that such corrections stabilize IB adsorption on the Au(1 1 1) surface better than on Au(1 0 0), mainly because the planar density of Au(1 1 1) is basically greater than that of Au(1 0 0) (more atoms per area; see the Experimental section).…”

“…It is worth mentioning that the activation energy of the cross-coupling reaction (28.1 kcal/mol, Re 3 ? Im 3 ) is comparable to that required for homo-coupling (27.5 kcal/mol) of phenyl groups on Au(1 1 1) surface, in good agreement with a previous study [19]. For the case of Au(1 0 0), the activation energy of the cross-coupling reaction is 8.3 kcal/mol higher than that for the homo-coupling reaction.…”

“…A previous study by Kanuru et al indicated that the catalytic reaction between aryl halides and alkynes over an extended Au(1 1 1) surface yielded by 20% and 40% of cross-coupling and homo-coupling products, respectively [18]. Recent density functional theory (DFT) studies showed that the activation energy for bimolecular cross-coupling was comparable to that required for IB homo-coupling on Au(1 1 1) surfaces, which explained selectivity results obtained on catalysts that mainly contain neutral gold sites [19]. Experimental and theoretical studies have proposed that if positively charged Au sites (produced by either base or supports such as CeO 2 ) were present as well as neutral gold atoms, the activation energy of the Sonogashira cross-coupling reaction was minimized [19,20].…”

“…According to a recent spectroscopic experiment, Sonogashira cross-coupling is extremely sensitive to the details of catalysts' surface structure. It is shown that IB and PA react on well-developed crystalline Au(1 1 1) facets, but deliberately roughened gold surfaces are totally inert to the reactants [18,19]. Gold nanorods are prepared via a seed-mediated sequential growth process involving the use of 2À3 nm gold seed crystals and their subsequent growth in a series of reaction solutions containing HAuCl 4 , AgNO 3 , and cetyltrimethylammonium bromide (CTAB) [26,30,31].…”

One‐Step Pyrolysis Preparation of 1.1.1 Oriented Gold Nanoplatelets Supported on Graphene and Six Orders of Magnitude Enhancement of the Resulting Catalytic Activity

“…However, according to an existing theory [5,6] control of the crystallographic facet exposed by the NP should be a powerful tool to boost the catalytic activity. Experimental proof of this concept has remained elusive due to the difficulty to prepare MNPs having preferential facet orientation and there are only a few available experimental data supporting the high catalytic activity of morphologically defined Au nanocrystals.…”