Programming Structural Flexibility for Tunable Gas Sorption via Hydrogen Bonding in Polyoxometalate-Based Porous...

Porous ionic crystals (PICs) are porous molecular solids assembled through nondirectional Coulomb interactions, whose inherent structural flexibility during guest sorption and desorption creates adaptive pore environments that lead to unconventional gas sorption and separation selectivity. Although such flexibility is expected to be reflected in both thermodynamics and kinetic aspects of guest sorption, these two viewpoints have rarely been examined together. Herein, we apply a crystal engineering approach based on noncovalent intermolecular interactions to systematically tune the flexibility of PICs constructed from triol-ligand-modified (RC-(CH₂OH)₃, R = CH₃, CH₂OH, NH₂) Anderson-type polyoxometalate (POM) anions as molecular building blocks. The number of hydrogen bonds formed per POM with adjacent ionic components is precisely controlled, ranging from zero for PIC-CH ₃ to two for PIC-NH ₂ , thereby enabling systematic control of structural flexibility. Despite their similar porosities, PIC-CH ₃ and PIC-NH ₂ exhibit distinct guest sorption behaviors. Thermodynamic analysis of guest (CO₂ and CH₃OH) sorption reveals that PIC-NH ₂ displays a type-I isotherm characteristic of a rigid microporous framework dominated by physisorption, while PIC-CH ₃ shows a continuous increase in uptake with pressure, indicating guest-induced structural transformation. Complementary kinetic analyses further reveal this contrast: PIC-NH ₂ follows a single-step sorption process, while PIC-CH ₃ exhibits multistep kinetics involving both guest diffusion and framework expansion. By combining thermodynamic and kinetic perspectives, we establish a hydrogen-bonding-guided crystal engineering strategy for programming structural flexibility to achieve tunable gas sorption in POM-based PICs.