Redox properties of C₆S₈^n- and C₃S₅^n- (n = 0, 1, 2): Stable radicals and unusual structural properties for C-S-S-C bonds

The new anionic carbon sulfides C₆S₁₀^2- and C₁₂S₁₆^2- are described and crystallographically characterized. The C₁₂S₁₆^2- anion consists of two C₆S₈ units connected by an exceptionally long (2.157(12) Å) S-S bond. In solution, C₁₂S₁₆^2- exists in equilibrium with the radical C₆S₈^-. The equilibrium constant for radical formation (293 K, THF) is 1.2 × 10^-4 M, as determined by optical spectroscopy at varying concentrations. Radical formation occurs through scission of the S-S bond. On the basis of variable temperature EPR spectra, the thermodynamic parameters of this process are ΔH = +51.5 ± 0.5 kJ mol^-1 and ΔS = +110 ± 3 J mol^-1 K^-1. C₆S₁₀^2- is an oxidation product of C₃S₅^2- and consists of two C₃S₅ units connected by an S-S bond. The S-S bond length (2.135(4) Å) is long, and the CS-SC torsion angle is unusually acute (52.1°), which is attributed to an attractive interaction between C₃S₂ rings. The oxidation of (Me₄N)₂C₃S₅ occurs at -0.90 V vs Fc⁺/Fc in MeCN, being further oxidized at -0.22 V. The similarity of the cyclic voltammogram of (Me₄N)₂C₆S₁₀ to that of (Me₄N)₂C₃S₅ indicates that C₆S₁₀^2- is the initial oxidation product of C₃S₅^2-.