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 [Au23(CHT)16]−(HCHT = cyclohexanethiol) cluster upon ligand exchange with HSAdm (1-adamantanethiol) has been reported. During the 16 h conversion of [Au23(CHT)16]−to Au16(SR)12, the neutral 6e Au21(SR)15, and its 1e-reduction state, i.e. the 5e, cationic radical, [Au21(SR)15]+, are active intermediates to account for the formation of thermodynamically stable Au16products. 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+1staples. This study provides a reassessment on the electronic state of the Au21structure and highlights the single electron transfer processes in cluster systems and thus the importance of the EPR analysis on the mechanistic issues.