Synthesis of High Yield Au-21(SR)(15) Nanoclusters

In recent years, Au nanoclusters have attracted much attention as new nanomaterials, which contain several to two hundred Au atoms and are protected by ligands. The structures and properties of Au nanoclusters are usually sensitive to the particle size due to quantum confinement effect. Au nanoclusters have been applied to different fields, such as optical properties, catalysis, and biology. There are two common methods for the synthesis of atomically precise Au nanoclusters: "size focusing" and "ligand exchange". Although a series of Au nanocluster have been obtained via "size focusing" and "ligand exchange", obtaining high yield of such Au nanoclusters is a challenge. Au-21(S-Adam)(15) was previously synthesized via etching Au-18 nanoclusters with excess thiols, and its crystal structure was determined by X-ray diffraction crystallography; however, the yield of Au nanoclusters was low. In this study, we prepared Au-21(S-Adam)(15) in high yield via conversion of Au-23(S-Adam)(16) to Au-21(S-Adam)(15). Firstly, Au-23(S-Adam)(16) nanoclusters were synthesized using adamantanethiols(HS-Adam) as the protecting ligand and HAuCl4 as the gold resource in ethyl acetate solvent. Au-23(S-Adam)(16) were further etched with excess thiols at room temperature. After reacting for 30 min, highly pure Au-21(S-Adam)(15), with high yield of similar to 20% based on HAuCl4 precursor, were successfully prepared. Au-23(S-Adam)(16) and Au-21(S-Adam)(15) were characterized by electrospray ionization (ESI), UV-Vis absorption spectroscopy, matrix-assisted laser desorption ionization (MALDI) mass spectrometry, and thermogravimetric analysis (TGA). ESI-MS and UV-Vis spectra confirm the high purity of the Au-23(S-Adam)(16). After conversion, UV-Vis spectra show the absorption peaks of Au-21(S-Adam)(15) at 700, 540, 435 and 380 nm. The MALDI-MS of Au-21(S-Adam)(15) shows several peaks at 6502, 6471, 6106, 5411, and 5048, assigned to Au-21(S-Adam)(14S), Au-21(S-Adam)(14), Au-20(S-Adam)(13), Au-19(S-Adam)(10), and Au-18(S-Adam)(9), respectively. The fragments of Au nanoclusters were produced by the strong laser intensity, which easily removes carbon tails from HS-Adam. Thermogravimetric analysis (TGA) was also performed to check the purity of Au-21(S-Adam)(15) nanoclusters. The TGA curve shows a weight loss of 42% (expected value, 38%). UV-Vis absorption spectroscopy was performed to track the conversion of Au-23(S-Adam)(16) to Au-21(S-Adam)(15). It was found that Au-23(S-Adam)(16) can convert to Au-21(S-Adam)(15) with a conversion efficiency of up to 97%, using excess thiols at room temperature within 30 min. In general, we successfully synthesized highly pure Au-21(S-Adam)(15) nanoclusters, with high yield of similar to 20% based on HAuCl4, by etching Au-23(S-Adam)(16) with excess thiols at room temperature