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.