Control of Channel Size for Selective Guest Inclusion with Inlaid Anionic Building Blocks in a Porous Cationic Metal–Organic Host Framework

Abstract

Step by step: By attaching anionic building blocks of variable bulk to a cationic metal-organic framework, stepwise channel-size adjustment of the resulting porous three-dimensional host framework is achieved (see picture). This method is a new and viable approach for materials with predesigned nanopores for application in molecular recognition and selective guest inclusion.Through the introduction of perfluorocarboxylates as counteranions that line the inner surface of each channel in a host cationic metal-organic open framework, stepwise channel-size control has been realized, resulting in wide guest compatibility for [{Ag(L)(CF(3)CO(2))}(6)6 G](infinity) (1 supersetG; G=guest, L=ligand), selective guest recognition for [{Ag(L)(C(2)F(5)CO(2))}(6)4 G'](infinity) (2 supersetG'), and a lack of inclusion behavior for [{Ag(L)(C(3)F(7)CO(2))}(6)](infinity) (3; G and G' represent the same or different guest molecules). The cationic frameworks in 1-3 are constructed from the linkage of hexameric inorganic-organic hybrid macrocycles through multiple argentophilic interaction plus pi-pi interactions between pyridyl rings and carbonyl-carbonyl interactions, to which corresponding counteranions are attached. With different anions as intrachannel arms, similar frameworks in complexes 1 supersetG, 2 supersetG', and 3 exhibit percentages of guest-accessible voids of approximately 30-35, 25, and 18 % for 1-3, respectively. The highly flexible framework 1 in 1 supersetG contains stretchable channels with up to 21.7 % effective-volume change of the solvent-accessible void for inclusion of various guest species in the series of solvates 1 a-m. The pair of complexes 1 supersetG and [Ag(L)(CF(3)CO(2))](infinity) (4), and likewise the pair 2 supersetG' and [Ag(L)(C(2)F(5)CO(2))](infinity) (5), are interconvertible through distinct controllable processes.