Guest-Facilitated Heteroleptic Assembly of Helical Anionocages Enables Reversible Chirality Modulation

We report a novel strategy for reversible modulation of the supramolecular chirality based on guest-facilitated heteroleptic assembly of helical anionocages. Two triple-stranded helical anionocages including a chiral cage 1 (A2L13) and a crown ether functionalized achiral cage 2 (A2L23) were synthesized by anion coordination of bis-monourea-based ligands and PhPO32-. Both cages exhibited favorable binding with tetraethylammonium TEA+ and cobaltocenium Cob+ (endo-guest, bound in the cavity). Additionally, cage 2 could reversibly release and recapture the guests through binding the exo-guest potassium ions (K+) in the crown ethers and subsequent removal of the K+ by [2,2,2]-cryptand. The circular dichroism (CD) spectrum of cage 1 was not significantly affected by guest encapsulation or mixing with the "empty" cage 2. However, in the presence of both cage 2 and an endo-guest/exo-guest, the Cotton effects were reversed at 391 nm and significantly enhanced at 310 nm. This observation was attributed to the guest-facilitated formation of heteroleptic cages that enabled effective chirality transfer from the chiral to the achiral ligands. The CD changes induced by K+ could be fully reversed by removing it with [2,2,2]-cryptand. Sequential addition and removal of K+ allowed reversible modulation of the chirality for at least 10 cycles without significant attenuation. Guest-facilitated heteroleptic assembly of helical anionocages has proved effective for chirality modulation of a supramolecular system, which was induced by effective chirality transfer from the chiral to achiral ligands. While both the endo-guest (bound in the cavity) and exo-guest (K+, bound in the crown ethers) are effective for chirality modulation, only that induced by K+ could be reversibly switched.+image