Influence of Interligand Interactions and Core-Charge Distribution on Gold Cluster Stability: Enthalpy Versus Entropy

Understanding the factors underlying the formation, reactivity, and stability of ligated gold clusters is necessary for directing the synthesis of atomically precise clusters with tailored properties. In this study, we employ time- and energy-resolved surface-induced dissociation in conjunction with density functional theory calculations, which provide molecular level insight into ligand–gold cluster interactions and the ligand-exchange process. Ligated gold clusters were prepared in solution by reduction of a gold salt precursor containing triphenylphosphine (PPh3). Subsequent exchange reactions of the preformed clusters with methyldiphenylphosphine (MePPh2) ligands generated a distribution of mixed phosphine gold clusters. The results indicate that MePPh2 exhibits lower binding energy than PPh3, and both ligands are more strongly bound in the mixed-ligand clusters than in any of the pure PPh3-ligated gold clusters studied to date. Along with an increase in the ligand-binding energies, the mixed ligand clusters show large activation entropies which may become a competing factor when determining cluster stability during the ligand-exchange reactions. We propose that changes in the charge distribution of the cluster cores upon ligand removal, in conjunction with ligand–ligand interactions, bring about kinetically driven fragmentation pathways, which may be responsible for the fast ligand-exchange reactions observed in solution.