Isolation and Reactivity of Carbene-Stabilized Carbon Disulfide Radical Anions

The reaction of CAAC-CS2 betaine (1; CAAC = cyclic(alkyl)(amino)carbene) and alkali metal reductants under ambient conditions yields carbene-stabilized carbon disulfide radical anions as crystalline alkali metal salts. The radicals 3–5 form multinuclear clusters featuring diverse metal sulfide and disulfide interactions, which promote unusual reductive coupling and cyclization of adjacent CS2 units to C2S3 heterocycles (6). The addition of crown ethers to 3–5 sequesters the alkali cations and facilitates disulfide cleavage to yield stable [CAAC-CS2]·– monomers (7 and 8). Calculated natural atomic spin populations suggest that the spin densities in the clustered and monomeric species are comparable and evenly distributed between the CAAC and CS2 subunits. Subsequent reductions afford [CAAC-CS2]2– dianions (9–12), which can be reoxidized to radicals by comproportionation reactions with 1. The radicals are, in turn, oxidized to betaine 1 through salt elimination reactions with transition metals. Cyclic voltammograms of 1 feature reversible 1/1·–/12– couples with a small separation between the events (ΔΔG = 11.1 kcal mol–1). All isolated compounds were characterized by a combination of electron paramagnetic resonance spectroscopy, heteronuclear NMR spectroscopy, infrared spectroscopy, and single-crystal X-ray diffraction. Insights into their electronic structure are supported by density functional theory calculations.