An Au₂₅(SR)₁₈ Cluster with a Face-Centered Cubic Core

Abstract

A representative thiolate (RS)-protected gold cluster, Au₂₅(SR)₁₈, shows a fingerprint-like characteristic spectral profile regardless of the R-groups, reflecting the common motif of the structural backbone made of Au and S: an icosahedral Au₁₃ core fully protected by six staple units of Au₂(SR)₃. On the other hand, we reported in 2006 that an Au₂₅(SPG)₁₈ cluster (PGSH = <i>N</i>-(2-mercaptopropionyl)­glycine) exhibited an optical absorption spectrum significantly different from that of the conventional Au₂₅(SR)₁₈, suggesting the formation of a nonicosahedral Au core. Here, we investigated the structure of Au₂₅(SPG)₁₈ by UV–vis spectroscopy, extended X-ray absorption fine structure analysis and density functional theory calculations. Spectroscopic results indicated that Au₂₅(SPG)₁₈ has a face-centered cubic (FCC) Au core. We proposed a model structure formulated as Au₁₅(SPG)₄[Au₂(SPG)₃]₂[Au₃(SPG)₄]₂ in which an Au₁₅(SPG)₄ core with an FCC motif is protected by two types of staples with different lengths, Au₂(SPG)₃ and Au₃(SPG)₄. The formation of an FCC-based Au core is attributed to bulkiness around the α-carbon of the PGS ligand