Redox-active inverse crowns for small molecule activation

Abstract

Cyclic crown ethers bind metal cations to form host–guest complexes. Lesser-known inverse crowns are rings of metal cations that encapsulate anionic entities, enabling multiple deprotonation reactions, often with unusual selectivity. Self-assembly of a cycle of metal cations around the multiply charged carbanion during the deprotonation reaction is the driving force for this reactivity. Here we report the synthesis of a pre-assembled inverse crown featuring Na⁺ cations and a redox-active Mg⁰ centre. Reduction of N₂O followed by N₂ release and subsequent encapsulation of O²⁻ demonstrates its reduce-and-capture functionality. Calculations reveal that this essentially barrier-free process involves a rare N₂O²⁻ dianion, embedded in the metalla-cycle. The inverse crown can adapt itself for binding larger anions like N₂O₂²⁻ through a self-reorganization process involving ring expansion. The redox-active inverse crown combines the advantages of a strong reducing agent with anion stabilizing properties provided by the ring of metal cations, leading to high reactivity and selectivity.