Structural Evolution in Transition-Size Gold Nanoclusters: Insights into Nucleation and Growth through the Insertion–Collapse Mechanism

Conspectus As an important research object for studying the formation and evolution of metal condensed matter, metal nanoclusters (NCs) can provide accurate structures for relevant studies, and provide visual images for exploring the metal–metal interaction and packing mode in evolution. The close structural correlation between different NCs and the realizable interstructural transformation process provide a research platform for further understanding of the conversion mechanism of their structural evolution. As NCs are connected at the evolutionary scale metal–organic complex and plasmonic metal nanoparticles, the origin of metal condensed matter can be explored from the initial stage of the formation of NCs, also known as transition-sized NCs. By exploring the structural transformation reaction of transition-size NCs, key information can be obtained regarding nucleation, changes in metal packing modes, and the size-dependent evolution of core–shell geometries and electronic structures. To clarify the geometric and electronic evolution of NCs at the early stage of formation, we synthesized a series of transition-sized Au NCs and determined their precise structures, establishing a structural evolution map for NCs containing fewer than 25 metal atoms. Using a two-phase ligand-exchange method and a phosphine-ligand-induced size-tailoring strategy, we obtained a family of transition-sized Au NCs, including Au13, Au14, Au16, Au18, Au21, Au22, and Au24. Their precise core–shell structures served as model systems for investigating structural evolution. Based on the behavioral characteristics of structural transformation between these Au NCs, we propose the insertion–collapse mechanism for understanding the size growth of transition-size Au NCs. It indicates that the growth of the transition-size NCs is characterized by the insertion of external Au-SR units and the inward collapse of the Au atoms into the kernel. Besides, based on the structural analysis between Au12(SR)12 complex and Au13 NC, the origin mechanism of the kernel in Au NCs was unraveled to be the transition from Au4(SR)4 ring to bi-trigonal Au5 kernel. Taking the Au5 kernel as a starting point, the pattern of kernel size growth in subsequent Au NCs is also clearly summarized as a sequence of Au5→Au6→Au7→Au9→Au10, which is manifested as the gradual increase in the number of Au3 and Au4 building units and kernel configuration converts from planar to three-dimensional types. Using the exact NC structures as theoretical models, we also clarified the evolution of electronic structures in transition-sized Au NCs. The calculations show a regular size-dependent trend characterized by a two-electron growth pattern in valence electron count, a decreasing HOMO–LUMO gap, and a transition in HOMO–LUMO excitation behavior from interband (sp←d) to intraband (sp←sp) transitions. While previous studies have focused mainly on static structures and structure–activity relationships, this work addresses a more fundamental issue: how transition-sized Au NCs nucleate and evolve through stepwise atomic-level growth. Our results on nucleation, kernel growth, structural evolution, and electronic structure changes depict the initial stage of NC formation and provide important models for understanding the origin and growth mechanism of NCs and metallic condensed matter.