Chiral Self‐Sorting, Spontaneous Resolution, and Hierarchical Self‐Assembly in Metal–Organic Cages

Abstract

The ability to collectively program chiral recognition and the hierarchical self‐assembly of molecular and supramolecular building blocks into complex higher‐order superstructures is a significant goal in supramolecular chemistry. Metal–organic cages are excellent model systems to examine chiral self‐sorting and build hierarchical self‐assembly. Herein, details on how limiting the conformational flexibility and incorporating hydrogen bonding functional groups in the ligands can influence chiral self‐sorting and hierarchical self‐assembly of metal–organic cages are reported. The urea‐functionalized axially chiral bis‐pyridyl ligands afford high‐fidelity in chiral self‐sorting in Pd2L4 cages, when they have fewer conformations. Ligand L1, with more conformations, affords mixture of heterochiral and homochiral cages (≈70:30). Among them, the heterochiral cage adopts unusual twisted conformation and self‐assembles into 2D sheets, linked by anion coordination between urea and nitrate. Ligand L2, with fewer conformations, affords homochiral cages via high‐fidelity chiral self‐sorting. The choice of counter anions influences further self‐sorting in the solid state: racemate with PF6− and spontaneously resolves conglomerate with BF4−. Urea‐BF4 hydrogen bonding directs hierarchical self‐assembly of the Pd2L4 metal–organic cages into super‐cubic networks. The study introduces a new approach in hierarchical self‐assembly of metal–organic cages into higher‐order networks aided by hydrogen bonding anion coordination with functional ligands.