Organic counteranion co-assembly strategy for the formation of γ-cyclodextrin-containing hybrid frameworks

A class of γ-cyclodextrin-containing hybrid frameworks (CD-HFs) has been synthesized, employing γ-cyclodextrin (γ-CD) as the primary building blocks, along with 4-methoxysalicylate (4-MS–) anions as the secondary building blocks. CD-HFs are constructed through the synergistic exploitation of coordinative, electrostatic, and dispersive forces. The syntheses have been carried out using an organic counteranion co-assembly strategy, which allows for the introduction of 4-MS–, in place of inorganic OH–, into the cationic γ-CD-containing metal–organic frameworks (CD-MOFs). Although the packing arrangement of the γ-CD tori in the solid-state superstructure of CD-HFs is identical to that of the previously reported CD-MOFs, CD-HFs crystallize with lower symmetry and in the cuboid space group P43212—when compared to CD-MOF-1, which has the cubic unit cell of I432 space group—on account of the chiral packing of the 4-MS– anions in the CD-HF superstructures. Importantly, CD-HFs have ultramicroporous apertures associated with the pore channels, a significant deviation from CD-MOF-1, as a consequence of the contribution from the 4-MS– anions, which serve as supramolecular baffles. In gas adsorption–desorption experiments, CD-HF-1 exhibits a Brunauer–Emmett–Teller (BET) surface area of 306 m2 g–1 for CO2 at 195 K, yet does not uptake N2 at 77 K, confirming the difference in porosity between CD-HF-1 and CD-MOF-1. Furthermore, the 4-MS– anions in CD-HF-1 can be exchanged with OH– anions, leading to an irreversible single-crystal to single-crystal transformation, with rearrangement of coordinated metal ions. Reversible transformations were also observed in CD-MOF-1 when OH– ions were exchanged for 4-MS– anions, with the space group changing from I432 to R32. This organic counteranion co-assembly strategy opens up new routes for the construction of hybrid frameworks, which are inaccessible by existing de novo MOF assembly methodologies