Gated, Selective Anion Exchange in Functionalized Self-Assembled Cage Complexes

Graphical Abstract

Appending functional groups to the exterior of Zn₄L₄ self-assembled cages allows gated control of anion binding. The external groups act as a “doorstop”, preventing ligand rotation and slowing the guest exchange rate. Bulky groups affect both anion egress and ingress, introducing an extra layer of selectivity to the exchange. Small changes can have large effects: affinities for anions as similar as PF₆⁻ and SbF₆⁻ can vary by as much as 400-fold between identically sized cavities.

Abstract

Appending functional groups to the exterior of Zn₄L₄ self-assembled cages allows gated control of anion binding. While the unfunctionalized cages contain aryl groups in the ligand that can freely rotate, attaching inert functional groups creates a “doorstop”, preventing rotation and slowing the guest exchange rate, even though the interiors of the host cavities are identically structured. The effects on anion exchange are subtle and depend on multiple factors, including anion size, the nature of the leaving anion, and the electron-withdrawing ability and steric bulk of the pendant groups. Multiple exchange mechanisms occur, and the nature of the external groups controls associative and dissociative exchange processes: these bulky groups affect both anion egress and ingress, introducing an extra layer of selectivity to the exchange. Small changes can have large effects: affinities for anions as similar as PF₆⁻ and SbF₆⁻ can vary by as much as 400-fold between identically sized cavities.