Isomers among the carbon sulfides C4S6—synthesis and crystal structures of α,α-C4S6, α,β-C4S6, and of a second polymorph of the diiodine adduct α,β-C4S6·I2

Abstract

The reaction of the α and β forms of C₃S₅²⁻ with thiophosgene yields two isomeric carbon sulfides α,α-C₄S₆ and α,β-C₄S₆, respectively. The crystal structures of both compounds could be determined for the first time. Both structures are made up of almost planar molecules. The α,α-isomer (1,3-dithiolo-(4,5-d)-1,3-dithol-2,5-dithione) is D_2h-symmetric, while the α,β-isomer is approximately C_s-symmetric. In the molecules of both isomers the two different C₃S₅ units are retained without significant alterations of structural parameters. α,α-C₄S₆ is unstable with respect to α,β-C₄S₆. The molecular rearrangement can be induced by a short thermal treatment at 150 °C. Significant differences are found in the mass spectra fragmentation patterns. Only α,β-C₄S₆ shows an intense signal for C₃S₂⁺ and is therefore a potential source for the synthesis of carbon subsulfide via flash vacuum pyrolysis. Only α,β-C₄S₆ forms a stable adduct with I₂. α,β-C₄S₆·I₂ was already known (F. L. Lu, K. M. Keshavarz-K, G. Srdanov, R. H. Jacobson and F. Wudl, J. Org. Chem., 1989, 54, 2165, ), but a second polymorph is formed on crystallisation from a different solvent. The two polymorphic forms do not show differences in the structures of the individual molecules but show a different packing pattern. α,β-C₄S₆·I₂ is remarkably thermally stable. Thermal analysis shows that I₂ cleavage occurs in that temperature region above 200 °C when C–S bonds are broken and CS₂ and I₂ are simultaneously liberated. Performed at 270 °C thermolysis of α,β-C₄S₆·I₂ yields under cleavage of I₂ and CS₂ a black polymeric carbon sulfide (CS)_x which is probably a mixture of graphitic carbon and unidentified amorphous polymeric carbon sulfides.