Isolation and Reactivity of Carbene-Stabilized Carbon Disulfide Radical Anions.

Abstract

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^·-/1^2- 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.