The Pivotal Radical Intermediate [Au₂₁(SR)₁₅]⁺ in the Ligand-Exchange-Induced Size-Reduction of [Au₂₃(SR)₁₆]^- to Au₁₆(SR)₁₂

The atomic precision of sub-nanometer-sized metal nanoclusters makes it possible to elucidate the kinetics of metal nanomaterials from the molecular level. Herein, the size reduction of an atomically precise [Au-23(CHT)(16)](-) (HCHT = cyclohexanethiol) cluster upon ligand exchange with HSAdm (1-adamantanethiol) has been reported. During the 16 h conversion of [Au-23(CHT)(16)](-) to Au-16(SR)(12), the neutral 6e Au-21(SR)(15), and its 1e-reduction state, i.e. the 5e, cationic radical, [Au-21(SR)(15)](+), are active intermediates to account for the formation of thermodynamically stable Au-16 products. The combination of spectroscopic monitoring (with UV-vis and ESI-MS) and DFT calculations indicates the preferential size-reduction on the corner Au atoms on the core surface and the terminal Au atoms on longer AunSn+1 staples. This study provides a reassessment on the electronic state of the Au-21 structure and highlights the single electron transfer processes in cluster systems and thus the importance of the EPR analysis on the mechanistic issues.