Flexible and Rigid Amine-Functionalized Microporous Frameworks Based on Different Secondary Building Units: Supramolecular Isomerism, Selective CO2Capture, and Catalysis

Abstract

We report the synthesis, structural characterization, and porous properties of two isomeric supramolecular complexes of ([Cd(NH2bdc)(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 Cd2(COO)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 −NH2 group 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.