Isolation and Reactivity of Carbene-Stabilized Carbon Disulfide Radical Anions

The reaction of CAAC-CS₂ betaine (<b>1</b>; 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 <b>3-5</b> form multinuclear clusters featuring diverse metal sulfide and disulfide interactions, which promote unusual reductive coupling and cyclization of adjacent CS₂ units to C₂S₃ heterocycles (<b>6</b>). The addition of crown ethers to <b>3-5</b> sequesters the alkali cations and facilitates disulfide cleavage to yield stable [CAAC-CS₂]^·- monomers (<b>7</b> and <b>8</b>). 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 (<b>9-12</b>), which can be reoxidized to radicals by comproportionation reactions with <b>1</b>. The radicals are, in turn, oxidized to betaine <b>1</b> through salt elimination reactions with transition metals. Cyclic voltammograms of <b>1</b> feature reversible <b>1</b>/<b>1</b>^·-/<b>1</b>^2- couples with a small separation between the events (ΔΔ<i>G</i> = 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.