Allosterically Regulated Guest Binding Determines Framework Symmetry for an FeII 4L4 Cage

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. Metal‐organic cages are an important class of synthetic hosts, which are able to mimic the structural adaptability and allosteric regulation of naturally occurring systems. A low‐symmetry Fe II 4 L 4 tetrahedral cage reported here was able to adapt its geometry from S 4 to C 3 or T symmetry upon guest binding; multiple binding sites allowed the host to simultaneously bind distinct guests with negative allostery.