Unlocking spontaneous chiral resolution in silver clusters through steric and anionic control

Abstract

The spontaneous emergence of chirality from racemic or achiral small molecules and nanoclusters remains a fascinating yet poorly predictable phenomenon. Herein, we report a rational design strategy for achieving spontaneous chiral resolution, as demonstrated through the controlled synthesis of a series of trinuclear silver clusters based on C₃-symmetric tris(2-benzimidazolylmethyl)amine and its derivatives. Our results show that complexes formed by trinuclear clusters bearing sterically demanding ligands and trifluoromethanesulfonate (OTf⁻) favor spontaneous chiral resolution, forming homochiral structures due to helical scaffolds from bidirectional intermolecular π⋯π interactions. In contrast, trinuclear silver clusters bearing ligands with less steric hindrance paired with OTf⁻ lead to social self-sorting, resulting in racemic crystals. On the other hand, only social self-sorting was observed for the pairing of the trinuclear silver clusters with methanesulfonate (OMs⁻). Remarkably, the homochiral structures can be reversibly converted to racemic forms via anion exchange, enabling controllable switching between chiral and centrosymmetric polymorphs. The resulting chiral crystals exhibit optical activity and second-harmonic generation (SHG) responses, underscoring their potential for nonlinear optical applications. This work demonstrates that chiral self-sorting can be directed by systematically tuning intermolecular π⋯π interactions, offering key insights for the rational design of spontaneous resolution systems.