Electrochemical Reduction of Oligo-nitrocalix[4]Arenes - Molecules with Multiple Redox Centers, Different Conformations and Variable Shape

Skeleton of calixarene is a stable supramolecular frame which contains several aromatic units (in this contribution four) connected by methylene („calixarenes“) or sulphur („thiacalixarenes“) bridges. Calix[4]arenes can exist in four defined conformations: cone-, partial cone-, 1,2- and 1,3-alternating. Their possible interconversion is controlled by the size of the substituents on the upper or the lower rim, or by other interactions (e.g. hydrogen bonds). These compounds having the shape of a cavity are able to exhibit host-guest interaction depending on the substitution [1] therefore they are attractive for analytical and supramolecular chemists. Our project aims to sensor application.  

Calixarenes generally are not electrochemically reducible. Suitable substitution on the upper and/or lower rim, eventually structure modification or introduction of heteroatoms lead to unusual and surprising properties of this system. Easily reducible nitro group was proved to be a suitable substituent (redox probe) enabling electrochemical investigations [2]. Due to the presence of multiple nitro groups, electrochemical investigation can give information about the mutual electron interactions of redox centres and extent of electron delocalization within the molecule [3]. In the case of tetranitro derivative, the in-situ EPR spectroelectrochemical measurements revealed the presence of a stable tetraanion tetraradical as the reduction intermediate [4].   

One of many principal questions is, whether (and how) the electrochemical response reflects also the actual shape of the molecule and its conformation, alternatively the position of substituents in case of isomers.

For this study several series of oligo-nitro- and oligo-nitroso calix[4]arenes were synthesized and electrochemically investigated on mercury electrodes in aprotic solvents, differing in a) conformation; b) number and position of nitro- or nitroso substitution; c) presence/absence of thia-bridges; d) various substitution on the "lower rim". The most important result is the ability of experimental electrochemical data to identify various "space arrangement"  and extent of flexibility/rigidity of this family of compounds. It was demonstrated that electrochemical methods (polarography + cyclic voltammetry) can be used as an independent and powerful tool for investigation of the correlation between the  redox properties of calixarene molecules and their actual shape under influence of several structural factors.

Acknowledgement:  This work was supported by the GACR grant No. 13-21704S and institutional support RVO: 61388955.

References:

[1]   a) Gutsche, C.D.: Calixarenes Revisited (Monographs in Supramolecular Chemistry).

The Royal Society of Chemistry, Cambridge 1998.

        b) Gutsche, C.D.: Calixarenes, An Introduction (2nd Edition). The Royal Society of

Chemistry, Cambridge 2008.

[2]   Alan Liška, Jiří Ludvík. Chem. Listy 104, (2010) 23-26.

[3]   Alan Liška, Pavel Vojtíšek, Albert J. Fry, Jiří Ludvík: J.Org.Chem 78 (2013) 10651 – 10656.

[4]   Alan Liška, Marco Rosenkranz, Jiří Klíma, Lothar Dunsch, Pavel Lhoták, Jiří Ludvík:

Electrochimica Acta 140 (2014) 572 – 578.