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

A representative thiolate (RS)-protected gold cluster, Au-25(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 Au-13 core fully protected by six staple units of Au-2(SR)(3). On the other hand, we reported in 2006 that an Au-25(SPG)(18) cluster (PGSH = N-(2-mercaptopropionyl)glycine) exhibited an optical absorption spectrum significantly different from that of the conventional Au-25(SR)(18), suggesting the formation of a nonicosahedral Au core. Here, we investigated the structure of Au-25(SPG)(18) by UV-vis spectroscopy, extended X-ray absorption fine structure analysis and density functional theory calculations. Spectroscopic results indicated that Au-25(SPG)(18) has a face-centered cubic (FCC) Au core. We proposed a model structure formulated as Au-15(SPG)(4)[Au-2(SPG)(3)](2)[Au-3(SPG)(4)](2) in which an Au-15(SPG)(4) core with an FCC motif is protected by two types of staples with different lengths, Au-2(SPG)(3) and Au-3(SPG)(4). The formation of an FCC-based Au core is attributed to bulkiness around the alpha-carbon of the PGS ligand.