Extreme Stabilization and Redox Switching of Organic Anions and Radical Anions by Large-Cavity, CH Hydrogen-Bonding Cyanostar Macrocycles

Encapsulation of unstable guests
is a powerful way to enhance their
stability. The lifetimes of organic anions and their radicals produced
by reduction are typically short on account of reactivity with oxygen
while their larger sizes preclude use of traditional anion receptors.
Here we demonstrate the encapsulation and noncovalent stabilization
of organic radical anions by C–H hydrogen bonding in π-stacked
pairs of cyanostar macrocycles having large cavities. Using electrogenerated
tetrazine radical anions, we observe significant extension of their
lifetimes, facile molecular switching, and extremely large stabilization
energies. The guests form threaded pseudorotaxanes. Complexation extends
the radical lifetimes from 2 h to over 20 days without altering its
electronic structure. Electrochemical studies show tetrazines thread
inside a pair of cyanostar macrocycles following voltage-driven reduction
(+e–) of the tetrazine at −1.00 V and that
the complex disassembles after reoxidation (−e–) at −0.05 V. This reoxidation is shifted 830 mV relative
to the free tetrazine radical indicating it is stabilized by an unexpectedly
large −80 kJ mol–1. The stabilization is
general as shown using a dithiadiazolyl anion. This finding opens
up a new approach to capturing and studying unstable anions and a
radical anions when encapsulated by size-complementary anion receptors.