Pi-Stacking Geometry Directed Supramolecular Secondary Building Units Shaping Hydrogen-Bonded Frameworks for Intensive NH3 Adsorption

Abstract

Exploiting supramolecular secondary building units (SSBUs) for developing porous crystalline materials represents an exciting breakthrough that extends the boundaries of reticular chemistry. However, shaping polynuclear clusters sustained by non-covalent interactions for the assembly of hydrogen-bonded frameworks remains a critical challenge. This study presents a novel strategy to stabilize SSBUs by tuning the π-stacking geometry of conjugated building blocks, facilitating the creation of hydrogen-bonded frameworks with tailored architectures for demanding gas separation. Specifically, parallel-displaced π–π stackings of aromatic heterocycles bearing carboxyls promote the formation of SSBUs bridged by ammonium cations [NH₄⁺]₈[COO⁻]₈ (SSBU-4), enabling the assembly of hydrogen-bonded frameworks with permanent porosity and structural diversity influenced by the solvent effect. Comparatively, the non-heterocyclic building units exhibit geometrically- or energetically-unfavorable π stackings, resulting in fragile frameworks that collapse after removing disordered guests. Significantly, the heterocycle conjugated frameworks contain abundant open Brønsted acid N−H sites within pore channels, demonstrating remarkable NH₃ adsorption ability among diverse industrial gases with a high capacity (275.7 mL/g, at 273 K, 100 kPa) as compared to reported porous molecular crystals.