Multivalent Crown Ether Receptors Enable Allosteric Regulation of Anion Exchange in an Fe₄ L₆ Tetrahedron

We report a strategy for regulating the rate of internally bound anion exchange within an Fe₄ L₆ metal-organic tetrahedron through external coordination of tripodal tris(alkylammonium) cations. The cage features three flexible 18-crown-6 receptors at each of its Fe^II vertices, facilitating strong tritopic interactions with tris(ammonium) cations to "cap" the vertices of the tetrahedron. This capping mechanism restricts the flexibility of the cage framework, thereby reducing the rate of anion exchange within its central cavity by 20-fold. Thus, we demonstrate the first use of an externally bound multivalent effector to allosterically control internal guest binding in a molecular cage.