MALDI Mass Analysis of 11 kDa Gold Clusters Protected by Octadecanethiolate Ligands

We previously reported the isolation of octadecanethiolate-protected Au (Au:SC18H37) clusters having a core mass of similar to 11 kDa from the crude Au:SC18H37 samples obtained in the reaction between octadecanethiol (C18H37SH) and poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized Au clusters. This stable Au:SC18H37 compound was assigned to be Au-55(SCnH2n+1)(32) by destructive mass spectrometry and thermogravimetry (TG) performed in the framework of the classical structure model, in which the thiolates are bound to the surface of a magic Au55 core. In the present study, the molecular formula of the Au:SC18H37 (11 kDa) cluster was reassessed by non-destructive matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and it was confirmed that the Au:SC18H37 (11 kDa) cluster is a mixture of Au-54(SC18H37)(30) and Au-55(SC18H37)(31). On the basis of our present understanding of the structures of other stable Au:SR clusters, we proposed that Au-54(SC18H37)(30) and Au-55(SC18H37)(31) have a structural motif comprising a Au-37 cluster core that is completely protected by -SR-[Au(I)-SR](x) oligomers (x = 1 and 2). The nonformation of the Au:SR (11 kDa) cluster during the conventional chemical reduction of Au(I) SR oligomers is ascribed to the kinetics of Au:SR formation.