Flexible and rigid amine-functionalized microporous frameworks based on different secondary building units: supramolecular isomerism, selective Co₂ capture, and catalysis

Abstract

We report the synthesis, structural characterization, and porous properties of two isomeric supramolecular complexes of ([Cd(NH₂-dc)(bphz)_0.5]⋅DMF⋅H₂O}_n (NH₂-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 Cd₂COO)₄ secondary building unit (SBU), is flexible in nature, whereas the other isomer has a rigid framework based on a μ‐oxo‐bridged Cd₂(μ‐OCO)₂ SBU. Both frameworks are two‐fold interpenetrated and the pore surface is decorated with pendant −NH₂ and =N-N= functional groups. Both the frameworks are nonporous to N₂, revealed by the type II adsorption profiles. However, at 195 K, the first isomer shows an unusual double‐step hysteretic CO₂ adsorption profile, whereas the second isomer shows a typical type I CO₂ profile. Moreover, at 195 K, both frameworks show excellent selectivity for CO₂ among other gases (N₂, O₂, H₂, and Ar), which has been correlated to the specific interaction of CO₂ with the −NH₂ and =N-N= functionalized pore surface. DFT calculations for the oxo‐bridged isomer unveiled that the −NH2 group is the primary binding site for CO₂. The high heat of CO₂ adsorption (ΔHads=37.7 kJ mol−1) in the oxo‐bridged isomer is realized by NH₂⋅⋅⋅CO₂/aromatic π⋅⋅⋅CO₂ and cooperative CO₂⋅⋅⋅CO₂ interactions. Further, postsynthetic modification of the −NH₂group into −NHCOCH₃ in the second isomer leads to a reduced CO₂ uptake with lower binding energy, which establishes the critical role of the −NH₂ group for CO₂ capture. The presence of basic −NH₂ sites in the oxo‐bridged isomer was further exploited for efficient catalytic activity in a Knoevenagel condensation reaction.