Revealing the Monosubstituted Thiol Exchange Mechanism of Atomically Precise CdAu₂₄(SR)₁₈ Nanoclusters

Ligand exchange, as an efficient post-synthesis strategy, has become a versatile tool for the modification and functionalization of atomically precise gold nanoclusters (Au NCs). However, the underlying ligand exchange mechanism remains obscure. In this study, using the atomically precise CdAu₂₄(CHT)₁₈ NC (CHT = cyclohexanethiol) as a prototype model, we performed ab initio molecular dynamics (AIMD) simulations to reveal the PET-for-CHT (PET = phenylethanethiol) exchange mechanism to form the monosubstituted CdAu₂₄(CHT)₁₇PET product. The simulation results demonstrate that the ligand exchange process follows an S_N2-like concerted mechanism rather than an S_N1-like stepwise pathway. The steric orientation of CHT ligands exerts negligible influence on the reaction, and the ligand exchange exhibits distinct site selectivity determined by the intrinsic energy hierarchy of ligand-binding sites. In particular, the special site and the o-special site have the lowest reaction barrier (∼1.30 eV), confirming the preferential exchange at these two sites observed in the experiments. This study elucidates the ligand exchange mechanism of atomically precise Cd-doped gold NCs and uncovers the intrinsic origin of site-specific monosubstitution reactions. These findings provide a comprehensive theoretical basis for surface ligand engineering of metal NCs, which would promote the future in-depth investigation of ligand exchange reactions in the related nanocluster systems.