Ubiquitous 8 and 29 kDa Gold:Alkanethiolate Cluster Compounds: Mass-Spectrometric Determination of Molecular Formulas and Structural Implications

摘要

Since the first report of chemical preparation of thiolate-protected gold (Au:SR) clusters by Brust et al., the synthesis has reached a stage where Au_n(SR)_m can be isolated with a well-defined formula. Elucidation of the structures of a series of stable Au_n(SR)_m is of fundamental importance to understand both the aufbau principle and the origin of the unique physicochemical properties of these compounds. Alkanethiolate-protected gold (Au:SC_n) clusters are ideal systems to address these challenges because the sequence of preferred core sizes has been established and their stabilities are mostly governed by those of the Au-S core moieties. Whetten and others have shown that Au:SC_n clusters, with core masses of ca. 5, 8, 14, 22, and 29 kDa, can be isolated as stable compounds on the basis of the mass distributions of Au_xS_y~+ fragment ions in the laser-desorption ionization (LDI) mass spectra. Of these, only the 5, 8, and 29 kDa clusters can survive the harsh conditions of etching by free thiols, whereas the others are degraded by core etching.~(2f,6b,7 This indicates that the former three clusters are thermodynamically and chemically stable, whereas the others correspond to metastable species trapped by kinetic stabilization. The smallest Au:SC_5 (5 kDa) clusters, originally believed to have a Au28-based core,~(2g,9) have recently been reassigned to [Au_25(SC_6)_(18)]~z (z = 1-, 0, 1+) using nondestructive electrospray ionization mass spectrometry (ESI-MS).