From 2 + 2 to 8 + 8 Condensation Products of Diamine and Dialdehyde: Giant Container-Shaped Macrocycles for Multiple Anion Binding

The combination of
2,6-diformylpyridine and trans-1,2-diaminocyclopentane
fragments results in 2 + 2, 3 + 3, 4 + 4,
6 + 6, and 8 + 8 macrocyclic imine condensation products. These imines
can be reduced to the corresponding 2 + 2, 3 + 3, 4 + 4, 6 + 6, and
8 + 8 macrocyclic amines. The X-ray crystal structures of their protonated
derivatives show a rich variety of macrocycle conformations ranging
from a stepped 2 + 2 macrocycle to a multiply folded 8 + 8 macrocycle
of globular shape. These compounds bind anions via hydrogen bonds:
two chloride anions are bound above and below the macrocyclic ring
of the 2 + 2 amine, one chloride anion is bound approximately in the
center of the 3 + 3 macrocycle, and two chloride anions are deeply
buried inside a folded container-shaped 4 + 4 macrocycle, while in
the case of the previously reported 6 + 6 amine four chloride anions
and two solvent molecules are buried inside a container-shaped macrocycle.
Yet another situation was observed for a multiply folded protonated
8 + 8 macrocycle which binds six sulfate anions; two of them are deeply
buried inside the container structure while four anions interact with
the clefts at the surface of the container.