Mechanism for C-I Bond Dissociation in Iodoethane, Iodobenzene, and Iodoethene for the C-C Cross Coupling Reactions over Gold Clusters

The mechanisms for the C-I bond dissociation in iodoethane, iodobenzene, and iodoethene over the subnanometer sized gold clusters were studied using density functional theory. Au-3, Au-4, Au-14, and Au-20 clusters and their cations were used as the models to investigate the effects of the size, geometry, and electronic charge on the catalysis of the reaction. Our calculations predict that both iodoethane and iodobenzene interact with the gold through the lone pair on iodine with the small planar clusters activating the reaction more effectively through the formation of strong adducts and low barrier heights. Clusters with large HOMO-LUMO gaps act as poor catalysts owing to the quantum size effects. Even though the cationic clusters bind to the reactants more strongly due to their enhanced Lewis acidity, they also require higher activation energy. However, the catalysis by cations and 3D clusters has a smaller endergonicity which would be desirable in the subsequent steps in cross/homo coupling reactions. In the case of iodoethene, gold binds through the pi electrons instead of the lone pair on iodine which fine-tunes the bond dissociation process.