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(NH(2)bdc)(bphz)(0.5)]DMFH2O}(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 -oxo-bridged Cd-2(-OCO)(2) SBU. Both frameworks are two-fold interpenetrated and the pore surface is decorated with pendant -NH2 and NN functional groups. Both the frameworks are nonporous to N-2, revealed by the typeII adsorption profiles. However, at 195K, the first isomer shows an unusual double-step hysteretic CO2 adsorption profile, whereas the second isomer shows a typical typeI CO2 profile. Moreover, at 195K, both frameworks show excellent selectivity for CO2 among other gases (N-2, O-2, H-2, and Ar), which has been correlated to the specific interaction of CO2 with the -NH2 and NN 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 (H-ads=37.7kJmol(-1)) in the oxo-bridged isomer is realized by NH2CO2/aromatic CO2 and cooperative CO2CO2 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.