Toward Total Synthesis of Thiolate-Protected Metal Nanoclusters

Total synthesis, where desired organic- and/or biomolecules could be produced from simple precursors at atomic precision and with known step-by-step reactions, has prompted centuries-lasting bloom of organic chemistry since its conceptualization in 1828 (Wöhler synthesis of urea). Such expressive science is also highly desirable in nanoscience, since it represents a decisive step toward atom-by-atom customization of nanomaterials for basic and applied research. Although total synthesis chemistry is less established in nanoscience, recent years have witnessed seminal advances and increasing research efforts devoted into this field. In this Account, we discuss recent progress on introducing and developing total synthesis routes and mechanisms for atomically precise metal nanoclusters (NCs). Due to their molecular-like formula and properties (e.g., HOMO-LUMO transition, strong luminescence and stereochemical activity), atomically precise metal NCs could be regarded as "molecular metals", holding potential applications in various practical sectors such as biomedicine, energy, catalysis, and many others. More importantly, the molecular-like properties of metal NCs are sensitively dictated by their size and composition, suggesting total synthesis of them as an indispensable basis for reliably realizing their practical applications. Atomically precise thiolate-protected Au, Ag and their alloy NCs are employed as model NCs to exemplify design strategies and governing principles in total synthesis of inorganic nanoparticles. This Account starts with a brief summary of total synthesis methodologies of atomically precise metal NCs. Following the methodological summary is a detailed discussion on the mechanisms governing these synthetic strategies, which is the main focus of this Account. Based on unprecedented precision (at atomic resolution) and ease (ensured by size-dependent properties) of tracking clusters' size/structure changes, mechanisms driving growth (e.g., reduction growth and seeded growth) and functionalization (e.g., alloying reaction and ligand exchange) of metal NCs have been explored at molecular level. With definitive step-by-step reaction routes, two-electron (2 e^-) reduction (driving the growth reactions) and surface motif exchange (SME, prompting alloying and ligand exchange reactions) are discussed in depth and details. In addition to those sub- and/or individual-cluster level understandings, the self-assembly chemistry delivering high orderliness and enhanced materials performance in NC assemblies/supercrystals is also deciphered. This Account is then concluded with our perspectives toward potential development of cluster chemistry. Advances in total synthesis chemistry of metal NCs could not only serve as guidelines for future synthetic practice of NCs, but also provide molecular-level clues for many pending fundamental puzzles in nanochemistry, including nucleation growth, alloying chemistry, surface engineering and evolution of metamaterials.