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

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 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/CeO₂ catalysts containing different ratios of Au⁰/Au^δ+ 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