Charge-assisted soft supramolecular porous frameworks: effect of external stimuli on structural transformation and adsorption properties

Abstract

Four novel supramolecular frameworks {(H₂–bpee)₄[Fe(CN)₆]₂·9H₂O} (1), {(H₂–bpee)(H₃O)₂[Fe(CN)₆]·2H₂O} (2), {[(H₂-bpee)(H-bpee)]₂[Fe(CN)₆]_2·5H₂O} (3), and {[(H_1.5–bpee)₂][Fe(CN)₆]·6H₂O}, (4) [bpee = 1,2-bis(4–pyridyl)ethylene] have been synthesized by the reaction of bpee and K₄[Fe(CN)₆] (for 1 and 2) or K₃[Fe(CN)₆] (for 3 and 4) in aqueous medium. The supramolecular structures are constructed by cooperative noncovalent hydrogen bonding and π···π interactions between protonated bpee, [Fe(CN)₆]^3/4–, and crystalline water molecules. All the supramolecular soft hosts transformed to two different states depending upon the dehydration condition, as observed from the PXRD patterns. When the compounds were activated under vacuum at 373 K (1′–4′), they exhibit different adsorption behavior with different adsorbates like H₂O, MeOH, and EtOH compared to when they were activated under high vacuum at room temperature for 24 h (1″–4″). Desolvated compounds 1′–4′ show gradual uptake of water, while multistep water adsorption profiles were observed with 1″–4″, suggesting a difference in hydrophilicity in these two sets of compounds. Further, 1′–4′ and 1″–4″ unveil type I and double step MeOH (at 293 K) adsorption profiles, respectively, which have been correlated to guest-induced structural transformation in the desolvated soft frameworks obtained at two different activation conditions.