Long Chalcogen–Chalcogen Bonds in Electron-Rich Two and Four Center Bonds: Combination of π- and σ‑Aromaticity to a Three-Dimensional σ/π-Aromaticity

Abstract

Quantum chemical calculations were carried out by applying density functional theory to study the two center-three electron (2c-3e) bonds between the sulfur centers of cyclic dithio­ethers. Calculated were the S–S distance, the stabilization energy, and the energy of the σ → σ* transition. The extension of the calculations to two (2c-3e) bonds in one molecule shows that a rearrangement to one σ bond and two lone pairs on sulfur is usually more favorable. Exceptions are [H₂S₂ ⁺]₂, the dimer of the 1,2-dithia-3,5-diazolyl radical (<b>27a</b>), the dimer of the 1,2,4-trithia-3,5-diazolyl radical cation (<b>26a</b> ²⁺), and its Selena congeners and derivatives. In the case of [H₂S₂ ⁺]₂, the (4c-6e) bond between the chalcogen centers is a good description of this dimer. To describe the binding situation in the dimer <b>26a</b> ²⁺ and <b>27a</b>, the concept of a “simple” (4c-6e) bond was extended. Our calculations reveal a strong σ-aromaticity within the plane of the four sulfur centers in addition to a strong π-conjugation within the five-membered rings. The whole phenomenon can best be described as a three-dimensional σ/π-aromaticity within the 14π dimers