Roles of Metal-Ligand Complexes for the Controlled Synthesis of Thiolate-Protected Metal Nanoclusters

Synthesis of atomically uniform nanoparticles is crucial in various fields due to their strong structure-property relationship. In this perspective, metal nanoclusters (NCs) have garnered significant interests as they are potential candidates to be synthesized with perfect uniformity. Metal NCs comprise metal atoms and ligands with a precise number from less than ten to a few hundred, endowing them with size less than 2~3 nm. Previous studies have predicted that such ultrasmall metal NCs can be synthesized with negligible heterogeneity in their number of metal atoms. However, it still remains elusive how the metal NCs can actually acquire high monodispersity in atomic level. It is because the formation mechanism of metal NCs, especially in terms of precursor chemistry, has been less understood. Herein, we investigate the synthetic principle of thiolate-protected metal NCs by controlling the formation of metal-thiolate complexes, actual precursors to metal NCs. Firstly, we reveal that the reaction between Au salts and thiol ligands governs the amount and coordination chemistry of Au-thiolate complexes, which alters their reduction kinetics and the uniformity of the finally produced Au NCs. Secondly, we show that such relationship between complex formation and monodispersity of NCs can be widely applied to the controlled synthesis of various metal NCs composed of Ag, Pt, Pd, or Rh. Finally, we discover that the structure of thiol ligands determines the stability of Au-thiolate complexes, which results in the different amount of NaBH₄ required for the controlled synthesis of Au₂₅(SR)₁₈ NCs. Our findings offer a molecular-level insight into the understanding of synthetic mechanism of metal NCs and general principles for their controlled synthesis.