NHC-Stabilized Au₁₀ Nanoclusters and Their Conversion to Au₂₅ Nanoclusters

Herein, we describe the synthesis of a toroidal Au₁₀ cluster stabilized by N-heterocyclic carbene and halide ligands via reduction of the corresponding NHC-Au-X complexes (X = Cl, Br, I). The significant effect of the halide ligands on the formation, stability, and further conversions of these clusters is presented. While solutions of the chloride derivatives of Au₁₀ show no change even upon heating, the bromide derivative readily undergoes conversion to form a biicosahedral Au₂₅ cluster at room temperature. For the iodide derivative, the formation of a significant amount of Au₂₅ was observed even upon the reduction of NHC-Au-I. The isolated bromide derivative of the Au₂₅ cluster displays a relatively high (ca. 15%) photoluminescence quantum yield, attributed to the high rigidity of the cluster, which is enforced by multiple CH-π interactions within the molecular structure. Density functional theory computations are used to characterize the electronic structure and optical absorption of the Au₁₀ cluster. ¹³C-Labeling is employed to assist with characterization of the products and to observe their conversions by NMR spectroscopy.