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

We previously reported the isolation of octadecanethiolate-protected Au (Au:SC 18 H 37 ) clusters having a core mass of ∼11 kDa from the crude Au:SC 18 H 37 samples obtained in the reaction between octadecanethiol (C 18 H 37 SH) and poly( N -vinyl-2-pyrrolidone) (PVP)-stabilized Au clusters. This stable Au:SC 18 H 37 compound was assigned to be Au 55 (SC n H 2 n +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 Au 55 core. In the present study, the molecular formula of the Au:SC 18 H 37 (11 kDa) cluster was reassessed by non-destructive matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and it was confirmed that the Au:SC 18 H 37 (11 kDa) cluster is a mixture of Au 54 (SC 18 H 37 ) 30 and Au 55 (SC 18 H 37 ) 31 . On the basis of our present understanding of the structures of other stable Au:SR clusters, we proposed that Au 54 (SC 18 H 37 ) 30 and Au 55 (SC 18 H 37 ) 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.