The Physical Chemistry of 1,2-Dithiole Compounds-the Question of Aromaticity

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Abstract
The parent compound, the 1,2-dithiolylium ion, can by successive addition of vinylenethio groups give rise to higher polycyclic systems each composed of 1,2-dithiole rings with the sulfur atoms in a row. Until now, however, it has only been possible to extend the system to compounds with 3 (l,6,6aλ4-tri-thiapentalene), 4 and 5 sulfur atoms in a row. The discussion of the structure of these systems has been based on results obtained by use of X-ray crystallography, electron diffraction, 1H NMR-, 13C NMR-, ESR-, Infrared-, microwave-, ESCA-, photo-electron spectroscopy and mass spectrometry. The results from other physico-chemical studies such as photo- and electrochemical studies have also been used in the discussion of bonding and structure. Physico-chemical data from the period 1970-1979 are included. Based on these physico-chemical studies the l,2, dithiolylium system seems to be an aromatic 6-φ-electron system. l,6,6aλ4-trithiapentalenes in all respects behave as bicyclic aromatic loλ-electron systems, which are much alike naphthalene. These carbon-sulfur systems can be modified by replacing one or more sulfur atoms by selenium, tellurium, oxygen or nitrogen. A further modification can be made by substitution of one or more carbon atoms by nitrogen. Very few physico-chemical data are known for such systems. It is for most of these systems impossible to draw any conclusion concerning the electronic structure. In a few cases available data suggest a trithiapentalene-like structure.