Gas-Phase Structures of [Au21(SR)14]− and [Au17(SR)10]− with Eight Electrons: Can They Support an Icosahedral Au13 Core?

A prototypical thiolate (RS)-protected gold cluster [Au25(SR)18]− has high stability due to specific geometric and electronic structures: an icosahedral (Ih) Au13 core with a closed electronic shell containing eight electrons is completely protected by six units of Au2(SR)3. Nevertheless, collisional excitation of [Au25(SR)18]− in a vacuum induces the sequential release of Au4(SR)4 to form [Au21(SR)14]− and [Au17(SR)10]− both containing eight electrons. To answer a naive question of whether these fragments bear an Ih Au13(8e) core, the geometrical structures of [Au21(SC3H7)14]− and [Au17(SC3H7)10]− in the gas phase were examined by the combination of anion photoelectron spectroscopy and density functional theory (DFT) calculation of simplified models of [Au21(SCH3)14]− and [Au17(SCH3)10]−. We concluded that [Au21(SC3H7)14]− retains a slightly distorted Ih Au13(8e) core, while [Au17(SC3H7)10]− has an amorphous Au13 core composed of triangular Au3, tetrahedral Au4, and prolate Au7 units. DFT calculations on putative species [Au19(SCH3)12]− and [Au18(SCH3)11]− suggested that the Ih Au13(8e) core undergoes dramatic structural deformation due to mechanical stress from μ2 ligation of only one RS.