From [NHC─H] ^• to Persistent <i>σ</i> ‐Complex Radicals: Photoinduced Radical Chemistry of Imidazolium Salts

Abstract

ABSTRACT 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 [IDippH 2 ] •+ and [IXylH 2 ] •+ . Analysis of hyperfine couplings, in combination with density functional theory (DFT) calculations, establishes a close electronic analogy between [NHC─H 2 ] •+ and the classical cyclohexadienyl radical [C 6 H 7 ] • , 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.