Ionic clusters as high-connectivity secondary building units for crystalline porous organic salts

The solid-state assembly of crystalline porous materials is dictated by a balance between primary node-linker interactions and other weaker intermolecular forces. Metal-organic frameworks (MOFs) exploit highly directional and rigid coordination bonds that outweigh competing packing effects. This allows high-connectivity secondary building units (SBUs) to be assembled into a diverse range of architectures. By contrast, molecular crystals comprise weaker intermolecular interactions, making it challenging to realize porous packings with the highly connected reticular topologies achieved in MOFs. Here we introduce Coulombic aggregation as a means to generate primary SBU motifs. By suppressing intermolecular dispersion interactions, ammonium halide ion pairs aggregate into discrete multinuclear clusters that function as non-metal SBUs. Coupled with multitopic three-dimensional linkers, these ionic clusters create extended non-metal organic frameworks with diverse topologies, large voids and polar channels. This study extends reticular chemistry from directional chemical bonds to 'soft' ionic interactions, opening pathways to new porous materials and principles of solid-state assembly.