Supramolecular coordination cages as crystalline sponges through a symmetry mismatch strategy.

Abstract

The crystalline sponge method enables single-crystal X-ray diffraction analysis of guests absorbed within single-crystalline porous materials. However, its application with large or highly polar guests remains challenging. In this study, we addressed some of these limitations using palladium-based octahedron-shaped M₆L₄ (T_d) coordination cages as crystalline sponges. The key to facilitate the crystallization of the cage is the addition of large aromatic polysulfonates ('sticker' anions); the symmetry mismatch between the cage and the sticker (D_2h) results in a low-symmetry space group (P $\overline{1}$ ), preventing guest disorder and leading to the formation of guest-accessible channels in the crystal. Guests can be encapsulated either before or after cage crystallization. The size and host-guest properties of the cavity enable analysis of a broad range of compounds, including water-soluble molecules, large amphiphilic molecules (molecular weight of ~1,200) and molecular aggregates. We have demonstrated the versatility of the cage-sticker strategy through its application to a triaugmented triangular-prism-shaped M₉L₆ cage, extending the guest scope to medium-sized pharmaceutical molecules.