Electrochemical oxidation of a hexasulfonated calix[6]arene

doi:10.1016/S0022-0728(01)00451-X

Journal of Electroanalytical Chemistry

Volume 508, Issues 1–2, 27 July 2001, Pages 81-88

https://doi.org/10.1016/S0022-0728(01)00451-X Get rights and content

Abstract

The water-soluble calixarene p-hexasulfonato-calix[6]arene 1 was subjected to voltammetric studies of its oxidation. At glassy carbon or platinum electrodes in aqueous electrolyte solutions, oxidation was observed at E>0.7 V versus SCE in pH 2 solution. On glassy carbon, a stable derivatisation of the electrode surface was inferred from the perturbation in the electrochemistry of the anionic probe species Fe(CN)₆^3−/4− while the electrochemistry of the cationic probe Ru(NH₃)₆^3+/2+ was unchanged. Investigation of the electrochemistry of 1 as a function of pH (2<pH<10) showed the expected dependence on the acid–base chemistry of 1 for the derivatisation of the electrode surface. On platinum electrodes, the electrochemical derivatisation was less stable and less obvious than on carbon, indicating a weakly adsorbed modifying layer.

Introduction

Calixarenes have received much attention as the basis for molecular and ionic recognition because of their conformational and structural flexibility [1], [1](a), [1](b), [2], [3]. Thus, for example, calix[4]arenes derivatised at the upper (para-phenolic position) may be locked into a cone conformation; calixarenes derivatised at the lower (phenolic oxygen position) may be chemically selective in the species that they bind. They have thus been investigated as the basis for recognition processes for applications including catalysis [4], separations [5] and sensors [6].

Shinkai et al. [7] introduced the water-soluble sulfonated calixarenes as a means of applying these compounds in aqueous-phase applications. One possible application was as uranophiles for the extraction of uranium from seawater [8]. Studies on the binding of other ions have also been reported, e.g. ferrocene and cobalticinium derivatives [9] and ammonium cations [10]. These compounds can also be further derivatised to introduce an additional binding functionality [11]. Recently, the p-hexasulfonated calix[6]arene 1 has been employed as the counter-ion in electropolymerised polypyrrole films [12]. These polypyrrole/hexasulfonato-calix[6]arene films were shown to be capable of binding uranium(VI) [13]. More recently, sulfonated calixarenes were doped into chemically prepared polyaniline materials [14]. These latter investigations may lead sulfonated calixarenes into the area of electrochemical sensors, since electropolymerised films of conducting polymers are widely employed for such applications.

The electropolymerisation of polypyrrole involves application of positive potentials which result in the oxidation of the pyrrole monomers to pyrrole radical cations in solution [15]. Given that the sulfonated calixarenes are phenols, and phenols are well-known electroactive reagents [16], we set out to examine the electrochemical behaviour of such compounds. In particular we were interested to see if this sulfonated calix[6]arene 1 was electroactive in the potential region in which polypyrrole is electropolymerised. In such a case, a simple doping of the compound as the charge-balancing counter-ion in polypyrrole may be too simple a mechanism for incorporation of the calixarene derivative in such films. The electrochemical behaviour of calix[4]arene ester [17] and amide [18] derivatives has recently been demonstrated in non-aqueous media. The results of our present study reveal that the water-soluble sulfonated calixarenes are also electrochemically active. Our results also show that with the appropriate electrode material, electrochemical functionalisation of the surface with calixarenes can occur. As far as we are aware, this is the first report of the electrochemical behaviour of water-soluble sulfonated calixarenes. The compound studied, p-hexasulfonato-calix[6]arene 1, is shown in the structure below.

Section snippets

Experimental

All electrochemical experiments were carried out with standard potentiostatic instruments, either manually or computer-operated, with three-electrode, single compartment cells. The working electrodes were either glassy carbon (GC) discs (3 mm diameter) or platinum discs (2 mm diameter). The counter electrode was a Pt wire and the reference electrode was a saturated calomel electrode (SCE).

Aqueous solutions were prepared with purified water from a UHQ-PS system, resistivity ≥18 MΩ cm (Elga Ltd,

Oxidation of 1

The repetitive cyclic voltammograms (CVs) of a solution of 1 (1.7×10⁻³ M) in 0.1 M KNO₃ are shown in Fig. 1(a). There was an increase of current in the potential region 0.7–1.0 V due to oxidation. On repeated cycling in the potential region 0.0–1.0 V, the current at potentials greater than ca. 0.7 V gradually decreased towards a steady value but did not decrease to zero. This decrease was expected where the electroactive species undergoing oxidation was depleted in concentration at the

Conclusions

We have shown here the oxidation of calix[6]arene hexasulfonate at GC and Pt electrodes. Although the electrochemical reaction is non-passivating, it is believed that the reaction involves the one-electron oxidation of one or more of the phenolic sites present in this compound. The unavailability of the meta- and para-positions for reaction with the phenolate-type radicals produced by such an oxidation explains the non-passivation of the electrode, in comparison to the events, which occur with

Acknowledgements

We thank the University of Salford for a studentship (A.P.) and the EU ERASMUS programme for support towards the visits of N.M.-O. and S.O. to Salford.

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Electrochemical oxidation of a hexasulfonated calix[6]arene

Abstract

Introduction

Section snippets

Experimental

Oxidation of 1

Conclusions

Acknowledgements

Anal. Lett.

Thin Solid Films

Macromolecules

Electrochem. Commun.

Electrochem. Commun.

New J. Chem.

Anal. Chem.

Calixarenes

Calixarenes Revisted

Calixrenes

Angew. Chem., Int. Ed. Engl.

Tetrahedron Lett.

Electroanalysis