Electrochemistry of (Thia)Calix[4]Arenes Bearing Various Redox Probes

Abstract

Calixarene skeleton is a stable supramolecular frame which is, however, electrochemically inactive. Introduction of one or more electroactive substituents to the molecule enable its electrochemical investigation, moreover, the interpretation of series of electrochemical data can reveal the distribution of electrons in the molecule, their mutual communication, shape of the molecule, its dynamic behaviour etc. Suitable substitution on the upper and/or lower rim can influence not only geometry of the molecule, but also its reactivity and coordination ability. Generally, structure modification or introduction of heteroatoms lead to unusual and surprising properties of the whole supramolecule. Thiacalix[4]arenes contain four aromatic units connected by sulphur atoms. This contribution will be focused first on two series of 1,3-alt-conformers bearing one or more nitro groups as electrochemical probes and on other calixarene derivatives bearing various redox probes.. The first series consists of eight para-tetranitro derivatives which resemble each other in their electrochemical response: first two two-electron reversible waves are followed by final step during which a tetrakis(hydroxylamino) derivative is formed. This reduction pattern is analogous to the “standard” methylene-bridged para-tetranitrocalix[4]arenes, where no mutual electron interaction takes place, all nitrogroups are totally independent and a stable tetraanion tetraradical is formed [1,2]. The second series includes four meta-nitro derivatives. Generally, the electroreductive behaviour of them is more complicated (except the first member which bears only one nitrogroup). Presence of two and three nitro groups in the same molecule in position meta towards the lower rim (that means in position ortho- and para- towards the thia bridges), exhibit clear intramolecular communication. [3,4]. This phenomenon was confirmed by quantum chemical calculations. In the third series some newly synthesized derivatives with various redox probes are presented. Acknowledgement: This work was supported by the GACR grant No. 17-18108S and by institutional support RVO: 61388955. The authors thank prof. Pavel Lhoták for granting the compounds. Krunal M. Modi - On leave from Chemistry Department, Gujarat University, Ahmedabad-380009 India