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:SC_x clusters (x = 12, 18) were prepared by the reaction of alkanethiol (C_xSH) with polymer-stabilized Au clusters (_∼1.3 nm) and subsequently incubated in neat C_xSH. The resulting clusters were successfully fractionated by recycling gel permeation chromatography into Au_∼38:SC_x and Au_∼55:SC_x and identified by laser-desorption ionization mass spectrometry. The Au_∼55:SC_x 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.