Modulation of the Dynamics of a Two-Dimensional Interweaving Metal-Organic Framework through Induced Hydrogen Bonding

Inducing, understanding, and controlling the flexibility in metal-organic frameworks (MOFs) are of utmost interest due to the potential applications of dynamic materials in gas-related technologies. Herein, we report the synthesis of two isostructural two-dimensional (2D) interweaving zinc(II) MOFs, TMU-27 [Zn(bpipa)(bdc)] and TMU-27-NH₂ [Zn(bpipa)(NH₂-bdc)], based on N,N'-bis-4-pyridyl-isophthalamide (bpipa) and 1,4-benzenedicarboxylate (bdc) or 2-amino-1,4-benzenedicarboxylate (NH₂-bdc), respectively. These frameworks differ only by the substitution at the meta-position of their respective bdc groups: an H atom in TMU-27 vs an NH₂ group in TMU-27-NH₂. This difference strongly influences their respective responses to external stimuli, since we observed that the structure of TMU-27 changed due to desolvation and adsorption, whereas TMU-27-NH₂ remained rigid. Using single-crystal X-ray diffraction and CO₂-sorption measurements, we discovered that upon CO₂ sorption, TMU-27 undergoes a transition from a closed-pore phase to an open-pore phase. In contrast, we attributed the rigidification in TMU-27-NH₂ to intermolecular hydrogen bonding between interweaving layers, namely, between the H atoms from the bdc-amino groups and the O atoms from the bpipa-amide groups within these layers. Additionally, by using scanning electron microscopy to monitor the CO₂ adsorption and desorption in TMU-27, we were able to establish a correlation between the crystal size of this MOF and its transformation pressure.