Site-selective anion recognition of an interlocked dimer
Interlocked dimer 2, which is composed of two physically interlocked monomers 1, has three cavities (cavity A × 2 and cavity B × 1) and can encapsulate three anions, such as NO3– and BF4–. There are six possible encapsulating patters, A–F; the two (A and F) have the same kind of the anion and the others (B–E) have both NO3– and BF4– at the same time with different ratios and with different positions. Anion competition experiments showed that in addition to F, which encapsulates three NO3– ions, C, in which the NO3– and BF4– ions were captured in cavities A and cavity B, respectively, was selectively formed. Detailed investigations have revealed that B–E were formed by the dimerization, but three of the four were subjected to the anion exchange to converge into C and F. The selective formation can be explained by the fact that NO3– is a better anion template than BF4– as well as the molecular structure of the interlocked dimer; cavity A is surrounded by the four bridging ligands and can be accessed by free anions, whereas no space available for the anon exchange remains around cavity B because this cavity is surrounded by the eight bridging ligands. Therefore, the BF4– ions in cavities A were expelled by free NO3–, but the BF4– ion in cavity B was not, resulting in the selection of C and F. We have found that the volume of the cavity influenced the anion recognition. New interlocked dimer 3, which has smaller cavities than those of 2, captured three NO3– ions to form F, whereas only a small amount of an interlocked dimer that contains both NO3– and BF4– was formed.