Identification of Intermediate Au22(SR)4(SR')14 Cluster on Ligand-Induced Transformation of Au25(SR)18 Nanocluster.

We report the ligand-exchange-induced transformation from an icosahedral Au25(SR)18 cluster (where SR = 2-phenylethanethiol (PET)) to a bitetrahedral Au22(SR)4(SR')14 cluster (where SR' = 4-tert-butylbenzenethiol (TBBT)). This partial exchange of the ligands was achieved by controlling the concentration of the incoming TBBT ligand. Being a bulky and aromatic ligand, TBBT can efficiently distort the atomic structure of the Au25PET18 cluster, resulting in Au22(PET)4(TBBT)14, which is highly stable and considered to be an intermediate with a bitetrahedral structure. Time-dependent mass spectrometry and optical spectroscopy revealed the dissociation of the parent cluster and gave a deep insight on the ligand-exchange mechanism. Theoretical calculations and extended X-ray absorption fine structure studies confirm the formation of the Au22 structure. Identifying the atomic structure of the intermediate species opens a new avenue to study the transformation of one cluster to another.