Metal core bonding motifs of monodisperse icosahedral Au₁₃ and larger Au monolayer-protected clusters as revealed by x-ray absorption spectroscopy and transmission electron microscopy

The atomic metal core structures of the subnanometer clusters Au ₁₃[PPh₃]₄S(CH₂)₁₁CH ₃]₂Cl₂ (1) and Au₁₃-[PPh ₃]₄[S(CH₂)₁₁CH₃] ₄ (2) were characterized using advanced methods of electron microscopy and X-ray absorption spectroscopy. The number of gold atoms in the cores of these two clusters was determined quantitatively using high-angle annular dark field scanning transmission electron microscopy. Multiple- scattering-path analyses of extended X-ray absorption fine structure (EXAFS) spectra suggest that the Au metal cores of each of these complexes adopt an icosahedral structure with a relaxation of the icosahedral strain. Data from microscopy and spectroscopy studies extended to larger thiolate-protected gold clusters showing a broader distribution in nanoparticle core sizes (183 ± 116 Au atoms) reveal a bulklike fee structure. These results further support a model for the monolayer-protected clusters (MPCs) in which the thiolate ligands bond preferentially at 3-fold atomic sites on the nanoparticle surface, establishing an average composition for the MPC of Au ₁₈₀[S(CH₂)₁₁CH₃]₄₀. Results from EXAFS measurements of a gold(I) dodecanethiolate polymer are presented that offer an alternative explanation for observations in previous reports that were interpreted as indicating Au MPC structures consisting of a Au core, Au₂S shell, and thiolate monolayer.