Chromatographic Isolation of “Missing” Au55 Clusters Protected by Alkanethiolates

We report on the first synthesis of alkanethiolate-protected Au55 (11 kDa), which has been a “missing” counterpart of Schmid's Au55(PR3)12Cl6. Au:SCx clusters (x = 12, 18) were prepared by the reaction of alkanethiol (CxSH) with polymer-stabilized Au clusters (∼1.3 nm) and subsequently incubated in neat CxSH. The resulting clusters were successfully fractionated by recycling gel permeation chromatography into Au∼38:SCx and Au∼55:SCx and identified by laser-desorption ionization mass spectrometry. The Au∼55:SCx clusters exhibited structured optical spectra, suggesting molecular-like properties. The thiolate monolayers were found to be liquid-like on the basis of the IR spectrum and the monolayer thickness, which was estimated from the hydrodynamic diameter.