Independent LUMO Reactivity in Mesoionic N-Heterocyclic Thiones: Synthesis of a Stable Radical Anion.

Mesoionic compounds, with positive and negative charges, are expected to have dual-site highest occupied molecular orbital (HOMO, donor) and lowest unoccupied molecular orbital (LUMO, acceptor) reactivity. Herein, we report a novel class of air-stable mesoionic N-heterocyclic thiones (mNHTs) synthesized from abnormal N-heterocyclic carbenes (aNHCs). DFT studies revealed a highly polarized exocyclic thione moiety and computed Fukui function analysis suggests the dual-site HOMO/LUMO reactivity of mNHTs predicting donor property at the negatively charged 'S' center while acceptor property at the cationic imidazole ring. The independent LUMO reactivity of the mNHT was realized by its chemical reduction to an elusive radical anion, which was characterized by a single crystal X-raydiffraction study. Further, we explore the reactivity of radical anion for the activation of SO₂ gas, C-Br bonds of aryl bromide and photocatalytic functionalization of C-X (X = F, Br) bonds. This work unlocks the independent LUMO reactivity of a mesoionic compound.