Redox-Active Inverse Crowns for Small Molecule Activation

Abstract

The discovery of crown ether host-guest systems initiated the multidisciplinary field of supramolecular chemistry. Whereas cyclic crown ethers selectively bind metal cations in their center, the lesser known inverse crowns are rings of metal cations that efficiently bind anionic entities. In this role, inverse crowns enabled spectacular multiple deprotonation reactions, often with unusual selectivity. Self-assembly of the inverse crown around the multiply charged carbanion during the deprotonation reaction is the driving force for this reactivity. Here, we report on the synthesis of a pre-assembled inverse crown consisting of Na+ cations and a redox-active Mg0 center. We demonstrate its bifunctionality by reducing N2O and subsequent encapsulation of O2ˉ. Calculations reveal that this essentially barrier-free process involves a rare N2O2ˉ dianion, embedded in the inverse crown. The inverse crown can adapt itself for binding larger anions like N2O22ˉ, by extending the ring of metals cations. These redox-active inverse crowns combine the advantages of a strong reducing agent with anion stabilizing properties provided by the ring of metal cations, leading to high reactivity and selectivity.