Thiolate-protected Ag32 clusters: mass spectral studies of composition and insights into the Ag–thiolate structure from NMR

Clusters composed of a 32 silver atom core, protected with thiolates of glutathione (GSH) and N-(2-mercaptopropionyl)glycine (MPGH), were synthesized by a solid-state route in milligram scale. They do not exhibit surface plasmon resonance unlike their larger sized nanoparticle analogues but show molecule-like features in absorption and luminescence spectra, falling in the visible window. The compositions Ag₃₂SG₁₉ (SG: thiolate of glutathione) and Ag₃₂MPG₁₉ (MPG: thiolate of MPGH) were identified from electrospray ionization mass spectrometry (ESI MS). Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) was not successful for -SG protected clusters as reported before, but for Ag₃₂MPG₁₉ a peak at 6.1 kDa was seen at a threshold laser intensity. This peak shifted to low mass region with increasing laser intensity due to systematic losses of Ag₂S. Further confirmation of the composition Ag₃₂SG₁₉ was made using various studies such as XPS and EDAX. One-dimensional (1D) and two-dimensional (2D) NMR spectroscopic investigations of Ag₃₂SG₁₉ provided interesting spectral features which indicated the dominant -[SR-Ag-SR]- structural motif. This structural motif as the predominant entity is found for the first time in silver clusters.