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.