Two-electon-transfer redox systems: Part 6. Two-electron oxidation of hexakis(benzylthio)benzene—a study by electrolysis and cyclic voltammetry

doi:10.1016/S0022-0728(02)00996-8

Journal of Electroanalytical Chemistry

Volume 531, Issue 1, 9 August 2002, Pages 9-18

Dedicated to Professor Hans J. Schäfer on the occasion of his 65th birthday.

https://doi.org/10.1016/S0022-0728(02)00996-8 Get rights and content

Abstract

The anodic oxidation of the oligo-thioether hexakis(benzylthio)benzene in a dichloromethane+NBu₄PF₆ electrolyte is investigated by several complementary techniques: cyclic voltammetry, ESR spectroscopy, and macroscopic electrolysis (including fractional electrolysis in combination with the determination of open circuit potentials, i.e. ‘potentiometric titration’). While ESR spectroscopy proves the formation of a π-delocalized radical cation, the electrochemical techniques indicate further oxidation to a dicationic stage, which undergoes follow-up reactions. Analysis of fractional electrolysis results clearly shows strong potential compression for the two electron transfer steps with a difference of formal potentials ΔE°=23 mV. This value is succesfully used to simulate the cyclic voltammograms of the starting compound.

Introduction

Organic sulfur compounds play an important role in the redox chemistry of biochemical systems. Among many other processes, such as oxidative stress response [2], neuron repair [3], or redox regulation of apoptosis [4] are linked to the presence and action of the cystine/cystein redox couple or gluthatione. The electrochemistry of compounds such as RSH (thiols), RSR (thioethers or sulfides), and RSSR (disulfides), and, in particular anodic oxidation [5], provides model reactions for the biological behavior. The first intermediates in their oxidation process are sulfur radical cations, and the structure and preparation as well as the properties of these species have recently been reviewed [6]. In particular, a variety of follow-up reactions was discussed [6].

Hexakis(benzylthio)benzene (1) is a specific example of an alkyl–aryl sulfide. Although its preparation was described as early as 1989 [7], and the compound is commercially available, nothing is known about its redox properties or electrochemistry. Thioether 1 caught our additional attention as a sulfur analogue of the highly substituted hexakis(dimethylamino)benzene (2), the oxidation of which we have recently investigated by electrochemical means [8], [9]. In particular, 2 exhibited unusually strong potential inversion, as did some of its sterically strained derivatives [10], i.e. the primary oxidation to radical cations is thermodynamically more difficult than the subsequent second oxidation step forming a dication. This potential inversion was attributed to a considerable structural rearrangement during the oxidation process, which shifts the formal potential of the oxidation 2⁺−e⁻⇌2²⁺ below the E° of the process 2−e⁻⇌2⁺.

A comparative view of the oxidation processes of the two hexasubstituted benzenes 1 and 2 may result in additional insight into such highly substituted aromatic redox systems, e.g. as regards a possible second oxidation of a 1⁺ species. We were thus interested in a detailed understanding of the anodic reactions of 1, and present results from cyclic voltammetry (CV) and electrolytic experiments in the present paper.

Finally, as will become obvious during the discussion of the results in the present paper, investigation of 1 is an example for the advantageous use of fractional electrolysis with recording of the open circuit potential of an electrode in the electrolyte. Analysis of such an experiment for two-electron transfer processes is discussed in the accompanying paper [11].

Section snippets

General

All electrochemical experiments were performed with a BAS100B/W workstation (Bioanalytical Systems, West Lafayette, IN, USA). The potential during electrolysis was controlled by a BAS PWR-3 MF-9052 power module. Simulations of cyclic voltammograms were performed with the BAS DigiSim2.1 software.

Electron spin resonance (ESR) spectra were recorded with a BRUKER ESP 300, NMR spectra with a BRUKER AC 250 spectrometer and mass spectra with a Finnigan MAT TSQ 70 for EI, a MAT-711 A spectrometer for

General characterization of hexakis(benzylthio)benzene oxidation

Hexakis(benzylthio)benzene (1) is readily soluble in dichloromethane (CH₂Cl₂) and tetrahydrofuran (THF), but only moderately soluble in acetonitrile and other solvents frequently used in electrochemical experiments. Owing to the better conductivity properties, we used CH₂Cl₂ electrolytes in all electrochemical experiments.

Overall cyclic voltammograms of 1 are shown in Fig. 1 at two different switching potentials (E_λ). A first oxidation appears at E≈+0.8 V, which clearly corresponds to a

Conclusion

The experimental data from fractional electrolyses, their analyses by comparison to theoretical results, and the combination of experiment and simulation in CV demonstrate that hexakis(benzylthio)benzene is oxidized in an overall two-electron process. Various complicating factors (follow-up reactions in electrolyses at long time scales, deactivation of the electrode surface during CV) contribute to the overall complexity of the system. Still, however, the results from the two very different

Acknowledgements

This work was funded in part by the Fonds der Chemischen Industrie, Frankfurt/Main. We acknowledge financial support for the work of MGQ in the Tübingen laboratory by the Consejo Social of the Universidad de Alcalá. We thank Paul Schuler for recording and simulation the ESR spectra.

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^☆: Work of MGQ performed while on leave at the Tübingen laboratory. Part 5, see Ref. [1].

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Two-electon-transfer redox systems: Part 6. Two-electron oxidation of hexakis(benzylthio)benzene—a study by electrolysis and cyclic voltammetry☆

Abstract

Introduction

Section snippets

General

General characterization of hexakis(benzylthio)benzene oxidation

Conclusion

Acknowledgements

Electrochem. Commun.

J. Electroanal. Chem.

J. Electroanal. Chem.

Comput. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

Tetrahedron Lett.

Tetrahedron Lett.

Expericntia

Neurotoxicology

J. Immunol.

Anodic oxidation of sulfur- and selenium-containing compounds

Z. Chem.