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 ∼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:SC ₁₈H₃₇ compound was assigned to be Au₅₅(SC _nH_2n+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: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 Au₅₄(SC₁₈H₃₇) ₃₀ and Au₅₅(SC₁₈H₃₇)₃₁. On the basis of our present understanding of the structures of other stable Au:SR clusters, we proposed that Au₅₄(SC₁₈H ₃₇)₃₀ and Au₅₅(SC₁₈H ₃₇)₃₁ have a structural motif comprising a Au₃₇ 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.