Allosterically Regulated Guest Binding Determines Framework Symmetry for an FeII4L4 Cage

Abstract Self‐assembly of a flexible tritopic aniline and 3‐substituted 2‐formylpyridine subcomponents around iron(II) templates gave rise to a low‐spin Fe II 4 L 4 capsule, whereas a high‐spin Fe II 3 L 2 sandwich species formed when a sterically hindered 6‐methyl‐2‐formylpyridine was used. The Fe II 4 L 4 cage adopted a new structure type with S 4 symmetry, having two mer ‐ Δ and two mer ‐ Ʌ metal vertices, as confirmed by NMR and X‐ray crystallographic analysis. The flexibility of the face‐capping ligand endows the resulting Fe II 4 L 4 framework with conformational plasticity, enabling it to adapt structurally from S 4 to T or C 3 symmetry upon guest binding. The cage also displayed negative allosteric cooperativity in simultaneously binding different guests within its cavity and at the apertures between its faces.