Understanding the Growth Mechanism of Thiol-Conjugated Au₂₅ Cluster

The synthesis of ligand-conjugated gold nanoclusters has attracted significant attention due to its ability to achieve precise control over cluster size selectivity. Among these, Au₂₅(SR)₁₈ ^-, where R represents an alkyl group, is one of the earliest being synthesized with a very high yield, although its growth mechanism is yet to be fully understood. Using density functional theory, we present the results of a theoretical investigation on the growth process of Au₂₅(SR)₁₈ ^-, beginning from Au₁₃(SR)₁₂ ^-. Our findings indicate that the sulfur atoms in the core structure of Au₁₃(SR)₁₂ ^- preferentially bond with Au-thiol monomers. Monomers attached to two adjacent triangular faces form a staple motif of the gold-sulfur chain, releasing a single linear thiol radical. These reactions occur along the six mutually perpendicular ridges of the Au₁₃ core. The remaining eight triangular faces, linked with linear alkyl parts, cannot bind additional Au-thiol monomers, stopping cluster growth. Furthermore, the capping gold-sulfur chains play a protective role for the core, facilitating the stable formation of the Au₂₅(SR)₁₈ ^-cluster, as confirmed experimentally.