Stability of the Two Au-S Binding Modes in Au(25)(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)(SCH(2)CH(2)Ph)(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 Au(25)S(12) 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.