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

Abstract

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 (S_N 1)-like mechanism.

Keywords: ligand exchange; nanoparticles; nucleophilic substitution; reaction direction; stability.