Assembly of Tetrahedral Units in the Atomic-Resolved Structural Evolution of Gold Nanostructures

Materials achieve complexity from simple building blocks, yet directly capturing the atomic pathways of their dynamic assembly has remained a long-standing challenge. Here, we employ atomically precise gold nanoclusters as a model system to resolve a tetrahedral assembly toward [Au₂₅(SR)₁₈]^- (SR = thiolate). In this pathway, tetrabutylammonium acts as a supramolecular director, selectively stabilizing metastable intermediates and steering Au(I)-SR reduction along a dominant trajectory. High-throughput robotic synthesis combined with tandem in situ spectroscopy uncovers a growth sequence: Au(I)-SR → [Au₁₅(SR)₁₃]⁰ → [Au₂₀(SR)₁₆]⁰ → [Au₂₂(SR)₁₆]⁰ → [Au₂₃(SR)₁₆]^- → [Au₂₅(SR)₁₈]^-. The cluster core grows through the stepwise incorporation of Au₄ tetrahedral units from one to four corner-sharing tetrahedrons, culminating in a spontaneous rearrangement into a symmetric Au₁₃. These results reveal atomic-level evidence that discrete Au₄ units direct cluster growth and fuse into higher-order building blocks, resolving a long-sought question in nanomaterial formation and establishing smart, data-driven synthesis as a powerful paradigm for decoding complex reaction networks.