Preferential Location of Coinage Metal Dopants (M = Ag or Cu) in [Au_{25–<i>x</i>}M_<i>x</i>(SC₂H₄Ph)₁₈]⁻ (<i>x</i> ∼ 1) As Determined by Extended X‑ray Absorption Fine Structure and Density Functional Theory Calculations

The preferential locations of Ag and Cu atoms in the initial stage of doping into [Au₂₅(SC₂H₄Ph)₁₈]⁻ were studied by X-ray absorption spectroscopy and density functional theory computations. The extended X-ray absorption fine structure (EXAFS) spectra of [Au_23.8Ag_1.2(SC₂H₄Ph)₁₈]⁻ at the Ag K-edge were reproduced using a model structure in which the Ag dopant occupied a surface site in the icosahedral Au₁₃ core that was computationally the most stable site. In contrast, the Cu K-edge EXAFS spectra of [Au_23.6Cu_1.4(SC₂H₄Ph)₁₈]⁻ indicated that the Cu dopant was preferentially located at the oligomer site that was computationally less stable than the surface site. This discrepancy between the Cu location experimentally determined and that theoretically predicted was explained in terms of variations in the stability of the Cu dopant at the two sites against aerobic oxidation. These results demonstrate that the mixing patterns of bimetallic clusters are determined not only by the thermodynamic stability but also by the durability of the mixed structure under synthetic and storage conditions