Probing the Structure and Charge State of Glutathione-Capped Au<sub>25</sub>(SG)<sub>18</sub> Clusters by NMR and Mass Spectrometry

Despite the recent crystallographic determination of the crystal structure of Au<sub>25</sub>(SCH<sub>2</sub>CH<sub>2</sub>Ph)<sub>18</sub> 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 Au<sub>25</sub>(SR)<sub>18</sub> 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 Au<sub>25</sub> core structure and surface thiolate ligand distribution by means of NMR in combination with mass spectrometry. We use glutathione-capped Au<sub>25</sub>(SG)<sub>18</sub> clusters as an example to demonstrate the utility of this strategy. One-dimensional (1D) and two-dimensional (2D) correlation NMR spectroscopic investigation of Au<sub>25</sub>(SG)<sub>18</sub> reveals fine spectral features that explicitly indicate two types of surface binding modes of thiolates, which is consistent with the ligand distribution in the Au<sub>25</sub>(SCH<sub>2</sub>CH<sub>2</sub>Ph)<sub>18</sub> cluster. Laser desorption ionization (LDI) mass spectrometry analysis shows that Au<sub>25</sub>(SG)<sub>18</sub> exhibits an identical ionization and core fragmentation pattern with phenylethylthiolate-capped Au<sub>25</sub> clusters. The charge state of the native Au<sub>25</sub>(SG)<sub>18</sub> clusters was determined to be −1 by comparing their optical spectrum with those of [Au<sub>25</sub>(SCH<sub>2</sub>CH<sub>2</sub>Ph)<sub>18</sub>]<sup><i>q</i></sup> of different charge states (<i>q</i> = −1, 0). Taken together, our results led to the conclusion that glutathione-capped Au<sub>25</sub>(SG)<sub>18</sub> clusters indeed adopt the same structure as that of Au<sub>25</sub>(SCH<sub>2</sub>CH<sub>2</sub>Ph)<sub>18</sub>. This conclusion is also valid for other types of thiolate-capped Au<sub>25</sub> clusters, including hexyl- and dodecylthiolates. Interestingly, the chiral optical responses (e.g., circular dichroism (CD) signals in the visible wavelength region) from the Au<sub>25</sub>(SG)<sub>18</sub> clusters seem to be imparted by the chiral glutathione ligands because no similar CD signals were observed in Au<sub>25</sub>(SCH<sub>2</sub>CH<sub>2</sub>Ph)<sub>18</sub>.