Flexible and Rigid Amine‐Functionalized Microporous Frameworks Based on Different Secondary Building Units: Supramolecular Isomerism, Selective CO 2 Capture, and Catalysis

We report the synthesis, structural characterization, and porous properties of two isomeric supramolecular complexes of ([Cd(NH 2 bdc)(bphz) 0.5 ]⋅DMF⋅H 2 O} n (NH 2 bdc=2‐aminobenzenedicarboxylic acid, bphz=1,2‐bis(4‐pyridylmethylene)hydrazine) composed of a mixed‐ligand system. The first isomer, with a paddle‐wheel‐type Cd 2 (COO) 4 secondary building unit (SBU), is flexible in nature, whereas the other isomer has a rigid framework based on a μ‐oxo‐bridged Cd 2 (μ‐OCO) 2 SBU. Both frameworks are two‐fold interpenetrated and the pore surface is decorated with pendant −NH 2 and NN functional groups. Both the frameworks are nonporous to N 2 , revealed by the type II adsorption profiles. However, at 195 K, the first isomer shows an unusual double‐step hysteretic CO 2 adsorption profile, whereas the second isomer shows a typical type I CO 2 profile. Moreover, at 195 K, both frameworks show excellent selectivity for CO 2 among other gases (N 2 , O 2 , H 2 , and Ar), which has been correlated to the specific interaction of CO 2 with the −NH 2 and NN functionalized pore surface. DFT calculations for the oxo‐bridged isomer unveiled that the −NH 2 group is the primary binding site for CO 2 . The high heat of CO 2 adsorption (Δ H ads =37.7 kJ mol −1 ) in the oxo‐bridged isomer is realized by NH 2 ⋅⋅⋅CO 2 /aromatic π⋅⋅⋅CO 2 and cooperative CO 2 ⋅⋅⋅CO 2 interactions. Further, postsynthetic modification of the −NH 2 group into −NHCOCH 3 in the second isomer leads to a reduced CO 2 uptake with lower binding energy, which establishes the critical role of the −NH 2 group for CO 2 capture. The presence of basic −NH 2 sites in the oxo‐bridged isomer was further exploited for efficient catalytic activity in a Knoevenagel condensation reaction.