Ligand and Metal Exchanges between [(AuAg)₄₄(SR)₃₀]^4– Clusters via Bimolecular Superatomic Collision

Reported herein is a study of intercluster exchanges of ligands and/or metals between [(AuAg)₄₄(SR)₃₀]^4- nanoclusters via a combination of electrospray ionization mass spectrometry, X-ray crystallography, and UV-vis spectroscopy. The results revealed that ligand exchange reactions are instantaneous, giving rise to the standard Gaussian distribution indicative of a random exchange or substitution process. In sharp contrast, metal exchange reactions are slow (hours to days), resulting in fractal-like patterns. The intercluster Au-Ag exchange occurs in the innermost icosahedral core of the cluster. These reactions, when performed with a permeable membrane separating the two reactants, provided strong evidence that both exchanges require bimolecular collisions resembling conventional <i>S</i>_<i>N</i>2-type reactions between two superatomic clusters. This notion was further corroborated by the unexpected observation of "<i>half</i>" Au and "<i>half</i>" Ag peaks in the mass spectra, which may be taken as circumstantial evidence for the formation of a transient dimer [(AuAg)₄₄(SR)₃₀]₂^8- upon intercluster collisions in solution. Finally, the effects of experimental conditions, such as solvent and temperature, were also investigated.