Stable N-Heterocyclic Carbene-Derived α-Diimine Radical Cations with Anionic Nitrogen Character

Abstract

Nitrogen-centered radicals possessing substantial anionic character have remained highly challenging to isolate because of their intrinsic electronic instability in the absence of metal coordination. Herein, we report a family of stable N-heterocyclic carbene (NHC)-derived α-diimine radical cations that constitute a rare class of persistent organic radicals featuring pronounced anionic nitrogen-centered radical character. Combined crystallographic, EPR, electrochemical, spectroscopic, and computational studies reveal an unusual electronic structure in which the molecular framework is formally cationic whereas both the unpaired electron and substantial negative charge are predominantly localized on the α-diimine nitrogen atoms. These radicals exhibit remarkable stability, undergo selective proton- and hydrogen-atom-transfer reactions, and participate in fully reversible, stepwise interconversion among dicationic, radical cationic, and neutral redox states without degradation of the molecular framework. Two-electron reduction induces reversible C–N bond formation to generate either an azetidinimine or a zwitterionic structure, with their relative stability governed by remote electronic substituents. Variable-temperature spectroscopy, UV–Vis analysis, and DFT calculations reveal dynamic solution-state equilibria that can be thermodynamically tuned through peripheral electronic modulation. These findings establish a new design principle for persistent nitrogen-centered radicals and reveal how redox state, charge distribution, spin localization, and reversible covalent bond reorganization can be coupled within a metal-free molecular framework.