PNNL

Abstract

Understanding the formation and stability of ligated gold clusters is necessary to direct the scalable solution-phase synthesis of atomically-precise clusters with predetermined properties. Gas-phase studies provide molecular-level insight into ligandgold cluster interactions that control ligand exchange and growth/etching reactions in solution. Here we report a joint experimental and theoretical study of a series of eight gold atom, mixed phosphine, di-cationic clusters using time- and energy-resolved surface induced dissociation (SID) experiments and high-level calculations. Gold clusters were prepared by reduction of a gold salt precursor containing triphenylphosphine (PPh3). Subsequent ligand exchange reactions with methyldiphenylphosphine (MePPh2) generated a distribution of mixed-phosphine gold clusters. Experimentally-derived ligand binding energies and activation entropies for the mixed-ligand gold clusters were compared with ligand removal energies obtained from theoretical calculations. The results show that the exchange ligand, MePPh2, has a lower binding energy to the gold core than PPh3. In addition, both ligands are more strongly bound to the mixed-ligand clusters than to any of the pure PPh3-ligated gold clusters. Larger ligand binding energies in the mixed-ligand clusters are associated with large activation entropies, which become a competing factor determining cluster stability. This study reveals, for the first time, that ligand-ligand Van der Waals interactions contribute substantially to gold cluster stability and may be partially responsible for the large ligand dissociation entropies observed in SID experiments. We propose that ligand-ligand interactions in conjunction with changes in the charge distribution of the gold cores upon ligand removal bring about kinetically-driven fragmentation pathways, which may be responsible for the fast ligand exchange reactions observed in solution. These findings have broad implications for understanding what factors determine the stability of ligated metal clusters as certain ligands may have similar enthalpic and entropic components contributing to overall cluster stability.