Metal Nanoparticles Confronted with Foreign Ligands: Mere Ligand Exchange or Further Structural Transformation?

Ligand exchange reactions have attracted extensive interest due to their utility in tailoring the properties and applications of metal nanoparticles, but these techniques are not well understood due to the unclearness of metal nanoparticles at an atomic level. The emerging ultrasmall metal nanoparticles (metal nanoclusters) provide excellent opportunities for investigating this challenging issue. Herein, it is revealed that structurally similar Au25 (SC2 H4 Ph)18 , Au24 Cd(SC2 H4 Ph)18, and Au24 Hg(SC2 H4 Ph)18 transform to Au24 (SCH2 Ph-t Bu)20 , Au24 Cd(SCH2 Ph-t Bu)18 and ≈5 nm gold nanoparticles after reacting with HSCH2 Ph-t Bu, respectively. Based on this and some other facts, it is suggested that the intrinsic stability is one key factor that determines the ligand exchange direction, whether by mere ligand exchange or further structure transformation (including aggregation). Single crystal X-ray crystallography further reveals that the partial surface ligand configuration reverses after ligand exchange, providing structure evidence for the unimolecular nucleophilic substitution (SN 1)-like mechanism.