The crystalline radical anion salt of a free N-heterocyclic carbene

Radical ions of free N-heterocyclic carbenes (NHCs) have remained short-lived, mostly elusive, intermediates so far. We probed the square schemes for proton-coupled electron transfers of representative redox-active free NHCs. We found that NHC radical anions NHC˙^- and/or the conjugated acid NHC·H˙ could be persistent. In the case of a benzoquinone-based NHC, both radical forms were isolated as stable crystalline compounds, characterized, including by single-crystal X-ray diffraction studies, and were engaged in the straightforward divergent syntheses of NHC-based radicals.