Electrochemical reduction of hydrocarbons in organic solvents and ionic liquids: the story of stilbenes and polyaromatic compounds.
2017-02-06T05:50:35Z (GMT) by
Historically, electrochemical reduction of organic compounds was performed in a protic mixture of dioxin-water using the traditional polarography with a dropping mercury electrode. In 1952, acetonitrile, an aprotic solvent, was first employed by Wawzonek and then was adopted by electrochemists as the default solvent for electrochemical studies. Being a prototype for organic compounds, the electrochemical reduction of stilbene was investigated in acetonitrile. Although an electrochemically reversible process generating the stilbene radical anion can be obtained in acetonitrile, the second process was irreversible. However, despite this success and its great advantages, the drawback of such an organic solvent system is associated with the presence of adventitious or solvent or electrolyte protons that may interfere with the electrochemical reduction. In addition, earlier studies in organic solvents failed to detect the variation between the trans- and cis stilbene isomers. An ultimate goal of this thesis is to reduce organic compounds via the addition of multi electrons and to stabilise the reduced species under ambient conditions. This required investigating a set of organic solvents and ionic liquids that may meet the desired characteristics. This thesis showed that trans- and cis-stilbene initially undergo a chemically reversible one-electron reduction to their radical anions at very negative potentials, but with the trans-form being more readily reduced than the cis-form in MeCN or IL. Furthermore, while the trans reduction to the radical anion is electrochemically reversible, the cis process is more complex. Further reduction is irreversible (in most solvents) with the cis radical anion being more readily reduced to its dianion than the trans¬-form. This irreversibility is due to proton abstraction from the solvent, electrolyte or adventitious water. The thesis also exposed an additional property for the boronium cation based ionic liquid, (ethyl dimethyl ammonio) (trimethyl ammonio) dihydroborate bis (trifluoro methyl sulfonyl) amide, ILB; its anion lacks a proton and none of the cation protons is readily available. In ILB a new form of room temperature voltammetric behaviour of stilbene is unveiled; two consecutive chemically reversible reduction processes for trans- and cis-stilbene are detected with the trans form being more highly favoured upon reduction. Accordingly, the boronium based ionic liquid provides an opportunity to generate stable multi-electron reduced organic compounds at very negative potential for the first time at room temperature. After achieving encouraging outcomes with stilbene, this thesis examined and compared the electrochemical reduction of a series of organic molecules in MeCN or IL. Introduction of a substituent into stilbene shifts the first and the second reduction processes of both the trans- or the cis isomer to either less or more negative potentials. The substituent type (functional group) or position (ortho or para) can have a dramatically different effect on either isomer reduction. The electrochemical reduction of conjugated olefins and polyaromatic helicenes was investigated. In brief, olefins were successfully and reversibly reduced via addition of two or more electrons. The important findings of this work clearly establish that ionic liquids may provide better alternative of the traditional protic or organic solvents for the electro-reduction of organic molecules by eliminating the source of protons. However, irreversible reduction of polyaromatic hydrocarbons arises from dimerisation of the dianion rather than its protonation. Therefore, in this situation ILB could not induce reversible reduction of polyaromatic compounds. Another condition that can permit multi-electron reduction of stilbene is the incorporation of TCNQ with stilbene to form a charge-transfer complex. Electrochemical reduction of TCNQ is reversible in most organic solvents. Cyclic voltammograms of the chemically-synthesized STB-TCNQ charge transfer complex revealed four reversible reduction processes. This behaviour implies that the association and interaction between the complex components (STB and TCNQ) and the intra molecular interactions help to stabilise the generated stilbene dianion, by preventing it from reacting with adventitious proton. In summary, the major conclusion from this thesis is then ILB could constitute a nouvel step towards the development and adoption of a third generation solvent system for organic electrochemistry with the potential to stabilise multi-electron reduced species for water-sensitive compounds.