Probing the Structure and Charge State of Glutathione-Capped Au₂₅(SG)₁₈ Clusters by NMR and Mass Spectrometry

Despite the recent crystallographic determination of the crystal structure of Au₂₅(SCH₂CH₂Ph)₁₈ clusters, the question−whether all thiolate-capped, 25-atom gold clusters adopt the same structure, regardless of the types of thiols (e.g., long-chain alkylthiols, aromatic thiols, or other functionalized ones)−still remains unanswered. To crystallize long-chain or bulky ligand (e.g., glutathione)-capped Au₂₅(SR)₁₈ clusters has proven to be difficult due to the major amorphousness caused by such ligands; therefore, one needs to seek other strategies to probe the structural information of such gold clusters. Herein, we report a strategy to probe the Au₂₅ core structure and surface thiolate ligand distribution by means of NMR in combination with mass spectrometry. We use glutathione-capped Au₂₅(SG)₁₈ clusters as an example to demonstrate the utility of this strategy. One-dimensional (1D) and two-dimensional (2D) correlation NMR spectroscopic investigation of Au₂₅(SG)₁₈ reveals fine spectral features that explicitly indicate two types of surface binding modes of thiolates, which is consistent with the ligand distribution in the Au₂₅(SCH₂CH₂Ph)₁₈ cluster. Laser desorption ionization (LDI) mass spectrometry analysis shows that Au₂₅(SG)₁₈ exhibits an identical ionization and core fragmentation pattern with phenylethylthiolate-capped Au₂₅ clusters. The charge state of the native Au₂₅(SG)₁₈ clusters was determined to be −1 by comparing their optical spectrum with those of [Au₂₅(SCH₂CH₂Ph)₁₈]^q of different charge states (q = −1, 0). Taken together, our results led to the conclusion that glutathione-capped Au₂₅(SG)₁₈ clusters indeed adopt the same structure as that of Au₂₅(SCH₂CH₂Ph)₁₈. This conclusion is also valid for other types of thiolate-capped Au₂₅ clusters, including hexyl- and dodecylthiolates. Interestingly, the chiral optical responses (e.g., circular dichroism (CD) signals in the visible wavelength region) from the Au₂₅(SG)₁₈ clusters seem to be imparted by the chiral glutathione ligands because no similar CD signals were observed in Au₂₅(SCH₂CH₂Ph)₁₈.