Electrochemical Reduction and Intramolecular Electron Communication of Nitro Substituted Thiacalix[4]arenes
Abstract
This contribution is focused on the question, what is the influence of the sulphur bridging units in thiacalix[4]arenes on their electrochemical response. For this reason, two series of oligonitro thiacalix[4]arenes were investigated, where the "classic" methylene groups are replaced by the sulphur bridge.
The reduction pattern of p-tetranitro-1,3-alt-thiacalix[4]arenes (2+2+12 electrons) resembles that of the p-tetranitro-cone-calix[4]arenes and refers to the presence of two different couples of non-interacting nitro groups in the molecule. In contrast, m-oligonitro-1,3-alt-thiacalix[4]arenes exhibit a mutual intramolecular communication between multiple redox centers (nitro groups) due to the combination of two circumstances: nitro groups in the m-positions towards the lower rim are in fact in para- and/or ortho position towards the thia-bridges enabling thus 1,2- and 1,4-quinoid resonance structure, and larger sulphur bridging atom forms two adjacent C-S bonds, which are able to exhibit a hyperconjugative effect.
In any case, the intramolecular electron communication takes place via the skeleton, not through space. Consequently, the substituent (inductive, mesomeric) and charge effects are observable, additive and partly quantifiable.
Classic polarography proved to be the optimal technique for such investigations.