Supramolecular Chirality from Hierarchical Self-Assembly of Atomically Precise Silver Nanoclusters Induced by Secondary Metal Coordination

Chiral assembly of metal nanoparticles (NPs) into complex superstructures has been widely studied, but their formation mechanisms still remain mysterious due to the lack of precise structural information from the metal-organic interface to metallic kernel. As "molecular models" of metal NPs, atomically precise metal nanoclusters (NCs) used in the assembly of a macroscale superstructure will provide details of microscopic structure for deep understanding of such highly sophisticated assemblies; however, chiral superstructures have not been realized starting from achiral metal NCs with atomic precision. Herein, we report the supramolecular assembly of a water-soluble silver NC ((NH4)9[Ag9(mba)9], H2mba = 2-mercaptobenzoic acid, abbreviated as Ag9-NCs hereafter) into chiral hydrogels induced by the coordination of secondary metal ions. Single crystal X-ray diffraction reveals the triskelion-like structure of Ag9-NCs with a pseudochiral conformation caused by special arrangement of the peripheral mba2- ligands. The enantioselective orientation of the peripheral carboxyl group facilitates the assembly of Ag9-NCs into nanotubes with a chiral cubic (I*) lattice when coordinating to Ba2+. The nanotubes can further intertwine into one-dimensional chiral nanobraids with a preferred left-handed arrangement. These multiple levels of chirality can be tuned by drying, during which the I* phase is missing but the chiral entanglement of the nanotubes is enhanced. Through the gelation of atomically precise, achiral NCs coordination of secondary metal ions, chiral amplification of superstructures was realized. The origination of the chirality at different length scales was also discussed.