Studies On Electrochemical Redox Of Quinonoids Compounds In Ionic Liquids

Ionic liquids are a fascinating class of novel solvents, which are attracting attention as possible "green" alternative to volatile molecular organic solvents to be applied in catalytic and organic reactions and electrochemical and separation processes. Ionic liquids,Supercritical Carbon Dioxide and Aqueous two-phase system are called the new three green solvents.Quinone compounds is a kind of important organic compounds, which widely exists in animals, plants and microorganisms, with biological activity. The Carbonyl group in its structure is the active center of redox reactions. It is a good carrier of electron transfer with favorable electrochemical reactivity.Ionic liquid, as the solvent, its physicochemical properties and application in all aspects were recommended in the first part of the paper, following the research progress on redox mechanism of Quinonoids. Electrochemical behaviors of Benzoquinone and Tetramethyl-p-benzochinon in different ionic liquids like1-butyl-3-methyl imidazolium tetrafluoroborate (BMIMBF4),1-butyl-3-methyl imidazolium hexafluorophosphate (BMIMPF6),1-hexyl-3-methyl imidazolium tetrafluoroborate (HMIMBF4),1-hexyl-3-methyl imidazolium hexafluorophosphate (HMIMPF6) were investigated by the techniques of cyclic voltammetry (CV), in situ FT-IR spectroelectrochemistry (FT-IR), cyclic voltabsorptometry (CVA) and derivative cyclic voltabsorptometry (DCVA). The main achievements were summarized as follows:1. The electrochemical behavior of BQ in ionic liquids(1). Two couples of redox peaks were clearly observed on the condition of semi-infinite diffusion by using cyclic voltammetry (CV), which was similar to a quasi reversible process of BQ with the two-step electrochemical reduction in anhydrous acetonitrile solvent.(2). Constant speed experiments of benzoquinone in ionic liquids were carried out from298.15K to343.15K. Diffusion coefficient and activation energy were acquired by dealing with different cyclic voltammograms. The results show that an increase in the temperature increases the diffusion coefficient, while accompany lower viscosity of ionic liquids. Diffusion coefficien decreases in the order: BMIMBF4> HMIMBF4> BMIMPF6> HMIMPF6, while activation energy follows the opposite order. The viscosity of ionic liquids increases in this order:BMIMBF4<HMIMBF4<BMIMPF6<HMIMPF6.(3). In the thin-layer electrochemical cells, cyclic voltammograms in BMIMBF4and BMIMPF6show the different shap with a larger AE from anhydrous acetonitrile solvent. The concentration changes of relevant redox reactant, intermediate and final product can be tracked simultaneously during electrochemical process based on in situ FT-IR spectroelectrochemistry, which further prove that benzoquinone undergo two successive one-electron reductions, corresponding to the formation of a radical anion intermediate and then the dianion, in ionic liquids. In IR3D map, absorption peaks (BMIMBF4:peak1487cm-1and1233cm-1; BMIMPF6:peak1479cm-1and1241cm-1) standed for BQ2-/BQH2have blue-shift.2. The electrochemical behavior of TMBQ in ionic liquids(1). On the condition of semi-infinite diffusion, a quasi reversible process of TMBQ was obviously showed by two couples of redox peaks. It means that TMBQ experienced the two-step electrochemical reduction process in different ionic liquids.(2). Two couple of redox peaks and an irreversible anodic peak were observed in anhydrous acetonitrile solvent containing tetramethyl-p-benzochinon and electrolyte, with IR absorption peaks of dipolymer and hydrogen bond showing in3D map. Results indicate that TMBQ undergo an irreversible electrochemical process with a dimerization reaction and the formation of hydrogen bonds. We infer that the redox mechanism of TMBQ may be as follows: Reduction process:TMBQ+e→TMBQ-TMBQ+TMBQ-→(TMBQ)2·-(TMBQ)2-+e→(TMBQ)2·(2-) TMBQ·+e→TMBQ2-Oxidation process:(TMBQ)22-→(TMBQ)2·-+e TMBQ22-→TMBQ·-+e (TMBO)2·-→2TMBQ+e TMBQ·-→TMBQ+e Two comproportionations:TMBQ+(TMBQ)2(22-)(?)(TMBQ)2·-TMBQ+TMBQ2-(?)TMBQ(3). In BMIMBF4and BMIMPF6, it only displays two couples of redox peaks on the thin-layer condition in CV under consecutive three scans, which showed a new irreversible anodic peak in anhydrous acetonitrile solvent. Corresponding to cyclic voltammogram, five absorption peaks appeared in IR3D map, which were associated to TMBQ, TMBQ·-, TMBQ2·-, without (TMBQ)2-. The results indicate that TMBQ is reduced to TMBQ-, then it turned to be TMBQ2-at more negative potential. The specific reduction process is listed as follows:TMBQ(?)TMBQ·-(?) TMBQ2-.(4). Due to the possible reason of various anions, the absorption peak1072cm-1which is1333cm-1in BMIMBF4standed for TMBQ2-has a red-shift in IR3D map of TMBQ in BMIMPF6.