Cycloaddition reactions of dichlorodithionitronium hexafluoroarsenate(V) with alkynes and alkenes; the preparation and X-ray crystal structure of 1,3-dichloro-1,3,2-dithiazolidinium hexafluoroarsenate(V)

The dichlorodithionitronium cation, ClSNSCl + , undergoes symmetry-allowed, concerted, cycloaddition reactions with alkynes and alkenes in liquid sulphur dioxide. With ethyne and propyne it gives corresponding 1,3,2-dithiazolium salts in quantitative yield. Cycloaddition with alkenes yields (hitherto unknown) 1,3-dichloro-1,3,2-dithiazolidinium cations. The reactions are sensitive to substituents, and only the cycloaddition with ethene occurs quantitatively on a preparative scale. Cycloadditions with propyne and E- and Z-2-butene have been observed in solution. The preparation, characterization, and X-ray crystal structure of the product of the reaction of CISNSCl + with ethene, 1,3-dichloro-1,3,2-dithiazolidinium hexafluoroarsenate(V), are reported. Crystals of 1,3-dichloro-1,3,2-dithiazolidinium hexafluoroarsenate(V) are monoclinic, of space group P2 1 /c, a = 6.3161(6) Å, b = 17.1724(23) Å, c = 19.1558(18) Å, and β = 98.143(8)°; Z = 4 and R = 0.061. The 1,3-dichloro-1,3,2-dithiazolidinium cation consists of a five-membered ring, axially substituted at the sulphurs by chlorine (average bond lengths: SN 1.603(8) Å, SCl 2.025(4) Å, SC 1.822(10) Å, and CC 1.504(13) Å). Two crystallographically unique cations are linked by weak S..N contacts into pseudo-dimers. These data are used, together with structural comparisons with related SNS-containing cations and the sulphoxides and the 14 N chemical shift, to show that the bonding in the SNS region of the cation is best represented by the valence canonicals and , such that the sulphur and nitrogen atoms respectively carry absolute positive and negative charges. This picture is also supported by the fluoride contacts to the cation from the AsF 6 − anion, which are exclusively to sulphur. Nuclear magnetic resonance data were consistent with the retention of the solid state structure of in solution. Keywords: sulphur-nitrogen chloride, sulphur-nitrogen cations, 1,3-dichloro-1,3,2-dithiazolidinium, cycloaddition reactions, X-ray crystallography.