Post-Assembly Modification of Homochiral Titanium-Organic Cages for Recognition and Separation of Molecular Isomers

A chiral metal-organic cage (MOC) was extended and fixed into a porous framework using a post-assembly modification strategy, which made it easier to study the host-guest chemistry of the solid-state MOC using a single-crystal diffraction technique. Anionic Ti₄ L₆ (L=embonate) cage can be used as a 4-connecting crystal engineering tecton, and its optical resolution was achieved, thus homochiral ΔΔΔΔ- and ΛΛΛΛ-[Ti₄ L₆ ] cages were obtained. Accordingly, a pair of homochiral cage-based microporous frameworks (PTC-236(Δ) and PTC-236(Λ)) were easily prepared by a post-assembly reaction. PTC-236 has rich recognition sites provided by the Ti₄ L₆ moieties, chiral channels and high framework stability, affording a single-crystal-to-single-crystal transformation for guest structure analyses. Thus it was successfully utilized for the recognition and separation of isomeric molecules. This study provides a new approach for the orderly combination of well-defined MOCs into functional porous frameworks.