From Sequential Molecular Adsorption on Atomically Precise Ag₂₉ Nanoclusters to Aggregates of Soot-Like Particles

Adsorption of molecules onto the surface of atomically precise, ligand-protected metal nanoclusters (NCs) is a complex and, unlike ligand exchange, relatively unexplored aspect of NC chemistry. It has significant implications for catalysis as well as for particle nucleation and growth, e.g., in the context of atmospheric chemistry. Here, we investigate the solution-phase adsorption of dithiol molecules, 2,2'-[1,4-Phenylenebis(methylidynenitrilo)]bis[benzenethiol], (abbreviated as R1S₂H₂) on atomically precise [Ag₂₉(BDT)₁₂]^3- NCs (where BDT is 1,3-benzenedithiol). Using electrospray ionization mass spectrometry (ESI MS), high-resolution transmission electron microscopy (HRTEM), and dynamic light scattering (DLS) to probe solution composition after mixing, we resolve rapid adsorption of multiple R1S₂H₂ secondary ligands onto individual NCs. This is also associated with the formation of NC dimers (detectable by ESI MS) and eventually larger aggregates. Time-dependent measurements at high mass resolution show increasing hydrogen atom loss from the adsorbate modified NCs (and dimers), indicating a transition from a pure physisorptive to a partially chemisorptive state. Molecular docking simulations and density functional theory calculations were used to elucidate the multilayered, soot-particle-like structures of the NC adducts and to establish binding energies for secondary ligand adsorption which govern their assembly and aggregation. These observations provide valuable insights into the complex supramolecular interactions which can occur within the outer shell of NCs exposed to secondary ligands. The observation of the dynamic interplay between physisorptive and chemisorptive phenomena upon aging of the reaction mixture provides important mechanistic insights into the aggregation and growth of nanoscale particles starting from NCs.