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

Abstract 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-CH3 to two for PIC-NH2, thereby enabling systematic control of structural flexibility. Despite their similar porosities, PIC-CH3 and PIC-NH2 exhibit distinct guest sorption behaviors. Thermodynamic analysis of guest (CO2 and CH3OH) sorption reveals that PIC-NH2 displays a type-I isotherm characteristic of a rigid microporous framework dominated by physisorption, while PIC-CH3 shows a continuous increase in uptake with pressure, indicating guest-induced structural transformation. Complementary kinetic analyses further reveal this contrast: PIC-NH2 follows a single-step sorption process, while PIC-CH3 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.