Correlating the Crystal Structure of A Thiol-Protected Au₂₅ Cluster and Optical Properties

TLDR

Determining the crystal structure of thiol‑protected Au clusters has been a longstanding challenge, and the highly symmetric icosahedral Au25(SR)18 structure reported here contradicts prior DFT predictions and the empirical “cluster‑of‑clusters” rule. The study reports an unusual single‑crystal structure of a 25‑gold‑atom Au25(SR)18 cluster protected by eighteen phenylethanethiol ligands. The Au25(SR)18 cluster consists of a centered icosahedral Au13 core capped by twelve gold atoms arranged in six pairs around three perpendicular 2‑fold axes, with eighteen phenylethanethiol ligands binding exclusively in a bridging mode. The study highlights the critical role of ligand–gold core interactions and shows that the Au25(SR)18 cluster displays multiple molecular‑like absorption bands that agree well with TD‑DFT calculations.

Abstract

The total structure determination of thiol-protected Au clusters has long been a major issue in cluster research. Herein, we report an unusual single crystal structure of a 25-gold-atom cluster (1.27 nm diameter, surface-to-surface distance) protected by eighteen phenylethanethiol ligands. The Au25 cluster features a centered icosahedral Au13 core capped by twelve gold atoms that are situated in six pairs around the three mutually perpendicular 2-fold axes of the icosahedron. The thiolate ligands bind to the Au25 core in an exclusive bridging mode. This highly symmetric structure is distinctly different from recent predictions of density functional theory, and it also violates the empirical golden rule—"cluster of clusters", which would predict a biicosahedral structure via vertex sharing of two icosahedral M13 building blocks as previously established in various 25-atom metal clusters protected by phosphine ligands. These results point to the importance of the ligand−gold core interactions. The Au25(SR)18 clusters exhibit multiple molecular-like absorption bands, and we find the results are in good correspondence with time-dependent density functional theory calculations for the observed structure.