Interlinker Hydrogen Bonds Govern CO₂ Adsorption in a Series of Flexible 2D Diacylhydrazone/Isophthalate-Based MOFs: Influence of Metal Center, Linker Substituent, and Activation Temperature

Four new layered flexible metal–organic frameworks (MOFs) containing a diacylhydrazone moiety, namely, guest-filled [Zn₂(iso)₂(tdih)₂]_n (1), [Zn₂(NH₂iso)₂(tdih)₂]_n (2), [Cd₂(iso)₂(tdih)₂]_n (3) and [Cd₂(NH₂iso)₂(tdih)₂]_n (4) were synthesized using terephthalaldehyde di-isonicotinoylhydrazone (tdih) as a linear ditopic linker as well as isophtalate (iso) or 5-aminoisophthalate (NH₂iso) as angular colinkers. The MOFs with hexacoordinated cadmium centers feature two-dimensional pore systems as compared to the MOFs with pentacoordinated zinc centers showing either zero-dimensional or mixed zero-/one-dimensional voids, as evidenced by single-crystal X-ray diffraction. In contrast to the frameworks based on isophtalates which do not show any significant gas uptakes, introduction of amino-substituted linker enables CO₂ adsorption. Gently activated aminoisophthalate-based frameworks, that is, guest-exchanged in methanol and heated to 100 °C, show reversible gated CO₂ adsorptions at 195 K, whereas the increase of activation temperature to 150 °C or more leads to one-step isotherms and lower adsorption capacities. X-ray diffraction and IR spectroscopy reveal significant structural differences in interlayer hydrogen bonding upon activation of materials at higher temperatures. The work emphasizes the role of hydrogen bonds in crystal engineering of layered materials and the importance of activation conditions in such systems.