Stability of the Two Au-S Binding Modes in Au25(SG)18 Nanoclusters Probed by NMR and Optical Spectroscopy

This report presents a detailed investigation on the structural stability of the Au-25(SG)(18) nanoclusters (where, -SG represents glutathionate) with a focus on the oxidation resistance and thermal stability of the Au-S bonds in the cluster. Two types of Au-S binding modes were previously indentified in the crystal structure of phenylethylthiolate-capped Au-25 clusters, and the Au-25(SG)(18) nanoclusters have been confirmed to adopt the same structure as Au-25(SCH2CH2Ph)(18) in our previous works. Herein, NMR in combination with optical spectroscopy revealed some distinct differences in both antioxidation and thermal stability of the two Au-S binding modes in the Au-25(SG)(18) nanocluster, that is, the mode I thiolate ligands (total 12 ligands) exhibit a much higher stability than the mode II ligands (total 6 ligands). Upon the basis of this major observation as well as an abundant [Au25S12](-) species detected in laser desorption ionization (LDI) mass spectrometry analysis of Au-25(SG)(18) clusters, a metastable species similar to Au-25(SG)(12) (i.e., a loss of six type II ligands of -SG) may exist in the processes of oxidation and thermal treatment. Overall, this work provides a deep insight into the intriguing structure of thiolate-capped Au-25 clusters, which will benefit future studies pertaining to the potential applications of such nanoclusters.