Mechanism
for C–I Bond Dissociation in Iodoethane,
Iodobenzene, and Iodoethene for the C–C Cross Coupling Reactions
over Gold Clusters
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Abstract
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<sub>3</sub>, Au<sub>4</sub>, Au<sub>14</sub>, and Au<sub>20</sub> 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 π electrons instead of the lone pair on iodine which
fine-tunes the bond dissociation process