The electrophilic substitution reaction of the dithionitronium cation [SNS]+ with benzene

The compound [SNS]+([SNS][AsF6]) reacts with benzene in liquid sulfur dioxide to give orange-, blue-, and then purple-colored solutions. The assignment of the orange color to a molecule-ion charge-transfer complex [C6H6·SNS]+ is supported by the linear dependence of the ionization potential of the arenes (C6H6, C6HMe5, C6H5But, C6HMe5) and the energy of the charge-transfer absorption of freshly prepared arene-[SNS][AsF6] mixtures in liquid SO2 solution. Variable-temperature multinuclear NMR studies of the reactions of [SNS][AsF6] and [SNS][Sb2F11] with benzene are consistent with the blue color being due to a sulfur protonated substitution product [C6H5(S2N)H]+, providing the first example of a CH electrophilic substitution reaction of SNS+. The geometries calculated at the RHF/6-31G' level for [C6H5(SNS)H]+, the isomeric [C6H5NSSH]+, and [C6H5N(S)SH]+, together with NMR data, support [C6H5(SNS)H]+(i.e., suggest S, not N, is attached to the ring) as the structure of the cation. The electrophilic aromatic substitution reaction of [SNS]+ and benzene is also supported by NMR studies of [SNS][AsF6] and other arenes (e.g., C6HMe5) in SO2 solution. The UV-visible spectrum of [SNS]+ ([SNS][AsF6]) in liquid SO2 is reported, and the absorption ( lamda = 406 nm, epsilon = 80) responsible for the yellow color is assigned to the [SNS]+ HOMO-LUMO transition. Evidence is also presented for the formation of a molecule-ion charge-transfer complex between 5-methyl-1,3,2,4-dithiadiazolium and hexamethylbenzene in liquid SO2, the first dithiadiazolium charge-transfer complex.Key words: UV-visible, charge transfer, dithionitronium, benzene, electrophilic substitution.