From [NHC─H]^• to Persistent σ-Complex Radicals: Photoinduced Radical Chemistry of Imidazolium Salts

Imidazolium salts are ubiquitous precursors to N-heterocyclic carbenes (NHCs), yet their radical chemistry remains largely unexplored, and structurally well-defined imidazolium-derived radicals are rare. Here, we show that visible-light excitation of charge-transfer (CT) bands of electron donor-acceptor (EDA) complexes between imidazolium cations and a triarylamine donor induces a single-electron transfer. Variable-temperature EPR spectroscopy reveals that irradiation at 100 K affords neutral [NHC─H]^• radicals, which undergo protonation upon warming to room temperature to give the persistent σ-complex cations [IDippH₂]^•+ and [IXylH₂]^•+. Analysis of hyperfine couplings, in combination with density functional theory (DFT) calculations, establishes a close electronic analogy between [NHC─H₂]^•+ and the classical cyclohexadienyl radical [C₆H₇]^•, highlighting σ-π hyperconjugation as the dominant stabilization motif. These findings identify imidazolium-derived σ-complex radicals as a unique class of carbene-based open-shell species and showcase imidazolium cations as noninnocent, redox-active platforms for controlled radical generation under mild photochemical conditions.