Acid- and Base-Stable Porous Organic Cages: Shape Persistence and pH Stability via Post-synthetic “Tying” of a Flexible Amine Cage

Imine cage molecules
can be reduced to amines to improve their
chemical stability, but this introduces molecular flexibility. Hence,
amine cages tend not to exhibit permanent solid-state porosity. We
report a synthetic strategy to achieve shape persistence in amine
cages by tying the cage vertices with carbonyls such as formaldehyde.
Shape persistence is predicted by conformer stability calculations,
providing a design basis for the strategy. The tied cages show enhanced
porosity and unprecedented chemical stability toward acidic and basic
conditions (pH 1.7–12.3), where many other porous crystalline
solids would fail.