Direct electrochemical reduction of hemin in imidazolium-based ionic liquids

The direct electrochemical reduction of hemin, protoporphyrin(IX) iron(III) chloride, ligated with strong or weak heterocyclic bases, was investigated in the ionic liquids (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([omim][PF₆]), using cyclic voltammetry and chronocoulometry. Hemin complexed with N-methylimidazole (NMI) or with pyridine had E_1/2 values slightly (4–59 mV) more positive in IL (without electrolyte) than in methanol (1.0 M electrolyte) using a gold electrode. NMI-ligated hemin had a lower E_1/2 than pyridine-ligated hemin in either IL, consistent with the stronger electron donor characteristic of NMI. [Bmim][PF₆] solutions consistently yielded E_1/2 values 30 mV more negative than [omim][PF₆] solutions. The diffusion coefficients D_o of hemin in the IL ranged between 1.50 and 2.80×10⁻⁷ cm² s⁻¹, while the heterogeneous electron-transfer rate constants k_s ranged between 3.7 and 14.3×10⁻³ cm s⁻¹. Cyclic voltammetry of hemin adsorbed to a gold surface through 4,4′-bispyridyl disulfide (AT4) linkages showed a large positive shift in the oxidation wave, indicating that adsorption stabilizes the reduced hemin state. The surface concentration Γ_o of the adsorbed hemin was determined to be 1.21×10⁻¹⁰ mol cm⁻², indicating the presence of one or more complete monolayers of hemin. These findings suggest that while hemin is electrochemically active in IL, its behavior is modified by the ligand field strength and surface adsorption phenomena.     

气体在不同离子液体中电化学响应灵敏度的研究

不同气体在不同离子液体中的电化学响应存在差异.本文以氢气、氧气作为目标研究气体,采用循环伏安法和计时安培电流法研究对比了基于两种不同离子液体[Bmpy][NTf2]和[Bmim][NTf2]的电化学传感器中气体的电化学行为,并就气体电化学信号的响应时间及灵敏度等电化学行为进行了讨论分析.结果表明:在[Bmpy][NTf...     

Applications of ionic liquids in electrochemical sensors

Ionic liquids (ILs) are molten salts with the melting point close to or below room temperature. They are composed of two asymmetrical ions of opposite charges that only loosely fit together (usually bulky organic cations and smaller anions). The good solvating properties, high conductivity, non-volatility, low toxicity, large electrochemical window (i.e. the electrochemical potential range over which the electrolyte is neither reduced nor oxidized on electrodes) and good electrochemical stability, make ILs suitable for many applications. Recently, novel ion selective sensors, gas sensors and biosensors based on ILs have been developed. IL gels were found to have good biocompatibility with enzymes, proteins and even living cells. Besides a brief discussion of the properties of ILs and their general applications based on these properties, this review focuses on the application of ILs in electroanalytical sensors.     

Cation electrochemical stability in chloroaluminate ionic liquids

The electrochemical stability of 10 organic cations, which can be used in ionic liquids (IL), was investigated as solutes in acetonitrile (ACN). The stability of three of the salts, BenMe2EtNCl (salt III), 1-butyl-2-methyl pyrrolidium chloride (salt VI), and its structural isomer, BuMe2ProNCl (salt VII), were also compared in chloroaluminate ILs. The chloroaluminate ILs of salts VI and VII are investigated for the first time. The NaCl-neutralized ILs of salts VI and VII have melting points of 43.2 and 3.7 degrees C, respectively. The benzyl-substituted cation, salt III, was more easily reduced in ACN or as the neutral chloroaluminate IL than the alkyl-substituted cation, salt VII, due to the better leaving ability of the benzyl group. Mass spectroscopy measurements before and after electrolysis on the benzyl-substituted solutions confirmed that reduction involves the loss of an alkyl group. In ACN, salt VI was found to be the most difficult to reduce (1 mA/cm2 at -2.09 V) due to its cyclic structure. However, in the chloroaluminate IL, the pyrrolidinium cation was more easily reduced than salt III or its isomer, salt VII, resulting in an insoluble black deposit. This is consistent with the mass spectrometry data, which do not show formation of low-molecular-weight products, as in the reduction of salts III and VII. The IL of salt VII was the most stable in the presence of sodium. Sodium ions could be reduced and reoxidized with a maximum Coulombic efficiency of 94.1% versus 87.2% for salt VI. Reduction of the pyrrolidinium cation produces insoluble products, most likely through opening of the cyclic ring, and an inferior medium for sodium ion reduction compared to the benzyl- and butyl-substituted cations, even though reduction of the cation occurs at a more negative potential in acetonitrile.     

Hydrogen abstraction from ionic liquids by benzophenone triplet excited states

The activation energy for hydrogen abstraction from imidazolium-based ionic liquids is significantly higher than that observed in conventional solvents.     

Oscillating electrochemical reaction in copper-containing imidazolium ionic liquids

An example of an electrochemical oscillator in ionic liquids is presented. Solutions of the ionic liquid 1-ethyl-3-methylimidazolium chloride, [C(2)mim]Cl, which contain both Cu(+) and Cu(2+) ions, show current oscillations during potentiostatic polarization. The oscillations were analyzed by the Quartz Crystal Microbalance (QCM) technique and by Electrochemical Impedance Spectroscopy (EIS). The electrochemical oscillations are of the N-NDR-type, because the low frequency end of the impedance spectrum has negative real impedances. The oscillating current leads to an oscillating growth speed of a metallic copper layer. Besides the presence of both Cu(+) and Cu(2+), the presence of chloride is a necessary, yet not a sufficient, condition for the occurrence of current oscillations. Oscillating currents were also observed for the ionic liquids 1-butyl-3-methylimidazolium chloride and 1-butyl-2,3-dimethylimidazolium chloride, but not for tributyltetradecylphosphonium chloride and N-butylpyridinium chloride.     

离子液体中2,4,6-三溴苯酚的电化学还原脱卤

<正>室温离子液体的许多独特的性质为其在化学分离、电化学传感器、电池等领域展示了广阔的应用前景[1-3]。室温离子液体作为有机反应介质,可以克服水溶液电化学窗口较窄的缺点,还可避免或减少造成二次污染的大量有机溶剂[4]。利用离子液体的导电性,可将电化学反应中的     

Rapid proton diffusion in hydroxyl functionalized imidazolium ionic liquids

There is considerable interest in using ionic liquids(ILs) as protic electrolytes. However, the reported proton transfer rate in ILs is quite slow. In this study, we report functionalizing imidazolium ILs with alcohol hydroxyls, aiming at constructing hydrogen bonding networks in the electrolyte, can stimulate fast proton hopping transfer. For demonstration, the diffusion of proton and Cl. in 1-(3-hydroxypropyl)-3-methylimidazolium tetrafluoroboride(C_3OHmimBF_4) were studied using cyclic voltammetry and potentiostatic method at 30 °C. The diffusion coefficient of proton is about one order of magnitude higher than that of Cl. in the same electrolyte, and about 5 times that of proton in the non-hydydroxyl 1-(butyl)-3-methylimidazolium tetrafluoroboride(BmimBF_4) when normalized to the diffusion coefficients of Cl. in respective ILs. In the meantime, 1H NMR spectra revealed a strong hydrogen bonding interaction between proton and C_3OHmimBF_4 which is absent between proton and BmimBF_4, thus the significantly higher diffusion coefficient of proton in C_3OHmimBF_4 may suggest the formation of effective hydrogen bonding networks, enabling rapid proton hopping via the Grotthuss mechanism.     

Recent advances in the use of ionic liquids for electrochemical sensing

Ionic Liquids are salts that are liquid at (or just above) room temperature. They possess several advantageous properties (e.g. high intrinsic conductivity, wide electrochemical windows, low volatility, high thermal stability and good solvating ability), which make them ideal as non-volatile electrolytes in electrochemical sensors. This mini-review article describes the recent uses of ionic liquids in electrochemical sensing applications (covering the last 3 years) in the context of voltammetric sensing at solid/liquid, liquid/liquid interfaces and carbon paste electrodes, as well as their use in gas sensing, ion-selective electrodes, and for detecting biological molecules, explosives and chemical warfare agents. A comment on the future direction and challenges in this field is also presented.     

Enabling new electrochemical methods with redox-active ionic liquids

Redox-active ionic liquids are obtained by tethering an electroactive center to either the cation, the anion, or both ions in an ionic liquid. While such phases were studied back in the 1990s for their particular electrochemical behavior, they are currently under increased scrutiny for applications in electrochemical systems. This contribution identifies the redox-active ionic liquids’ most important aspects to promote their development. This article provides a review of their features and offers several design guidelines. In addition, it offers an overview of the key properties which enhance their suitability as electrolytes in electrochemical systems, with particular attention paid to lithium-ion batteries and electrochromic devices.     

Describing the unsuspected advantage of redox ionic liquids applied to electrochemical energy storage

Ionic liquids are a class of solvents widely studied in the literature for various applications. As a subclass of ionic liquids, redox ionic liquids can endow charge exchange properties (electrons transfer) to these electrolytes for electrochemical energy storage. In this review article, we propose to study this family of ionic liquids and suggest a chronological classification. We introduce five generations of redox ionic liquids with different basic compounds such as polyethylene glycol, ferrocene, different linker lengths, TFSI anion, and biredox ionic liquids. The versatility of the redox ionic liquids synthesis will be discussed as well as the fundamental and applied aspects of their use as electrolytes, which have high charge densities. The impact of the redox ionic liquids on the electrochemical mechanisms will be described. We also present how the redox shuttle effect, detrimental to supercapacitors, can be prevented while it can be used to improve lithium-ion batteries.     

Highly selective electrochemical fluorination of organic compounds in ionic liquids

Recent studies on electrochemical partial fluorination in ionic liquid fluoride salts are reviewed. At first, historical background and some problems of electrochemical fluorination in organic solvents are briefly mentioned. Solvent-free electrochemical fluorinations in ionic liquids are explained as follows. Ultrasonication was found to improve both the yield and current efficiency for electrochemical fluorination of α-phenylthioacetate, which is mainly attributable to marked mass transport promotion of the substrate and the suppression of anode passivation. Highly regioselective and efficient fluorination of cyclic ethers, lactones, and cyclic carbonate was achieved in Et₄NF·4HF and Et₃N·5HF. Selective fluorination of hardly oxidizable phthalide was realized using a combination of imidazolium and fluoride ionic liquids. The unique effect of imidazolium ionic liquids on electrochemical fluorodesulfurization of 3-phenylthiophthalide was explained. Reuse of ionic liquids for electrochemical fluorination is also possible.     

Hydrogen-transfer reduction of aromatic ketones in basic ionic liquids

Abstract  

Several achiral and chiral basic ionic liquids (ILs) were prepared and tested as the medium for Ru-catalyzed hydrogen-transfer reduction of different aromatic ketones. Hydrogen-transfer reduction of ketones proceeded well in achiral basic ILs using chiral catalysts. The interesting observation was made that raising the reaction temperature did not have a negative effect on enantioselectivity of the reaction. On the other hand no reaction was observed in chiral ILs.     

Asymmetric reduction of aromatic ketones in pyridinium-based ionic liquids

The asymmetric reduction of aromatic ketones has been studied in pyridinium-based room temperature ionic liquids, namely, 1-ethyl-pyridinium tetrafluoroborate, [EtPy]⁺[BF₄]⁻ and 1-ethyl-pyridinium trifluoroacetate, [EtPy]⁺[CF₃COO]⁻. Ionic liquids were employed as solvents, while (R)-BINOL and (R)-BINOL-Br were used as chiral promoters. The effects of solvent, reaction time, temperature, catalyst loading and substituents were investigated. The reduction could be easily carried out in both ionic liquids with lower catalyst loading. 1-Ethyl-pyridinium tetrafluoroborate was recycled and reused efficiently.     

Hydrogen transfer reduction of different ketones in ionic liquids

Ionic liquids were tested as the reaction media for hydrogen transfer reduction of substituted acetophenones and some other ketones with the [RuCl(TsDPEN)]₂ complex as the catalyst. Reactions were going well and faster than in common solvents. Corresponding alcohols had high ees in the case of aryl alkyl ketones, but just medium ees were reached in the case of dialkyl or unsaturated ketones. An interesting phenomenon was observed, namely that rise of the reaction temperature did not have negative influence on the ee of the reaction product. Correspondence: Štefan Toma, Faculty of Natural Science, Comenius University Bratislava, SK-84215 Bratislava, Slovakia.     

Hydrogen transfer reduction of different ketones in ionic liquids

Ionic liquids were tested as the reaction media for hydrogen transfer reduction of substituted acetophenones and some other ketones with the [RuCl(TsDPEN)]₂ complex as the catalyst. Reactions were going well and faster than in common solvents. Corresponding alcohols had high ees in the case of aryl alkyl ketones, but just medium ees were reached in the case of dialkyl or unsaturated ketones. An interesting phenomenon was observed, namely that rise of the reaction temperature did not have negative influence on the ee of the reaction product.     

Catalysis of the electrochemical oxygen reduction in room-temperature ionic liquids on a pyrolytic graphite electrode by iron-containing superoxide dismutase

Catalysis of the electrochemical oxygen reduction reaction (ORR) on a pyrolytic graphite electrode (PGE) by iron-containing superoxide dismutase (Fe-SOD) is investigated for the first time using cyclic voltammetry and electrochemical impedance spectroscopy. The study is carried out in three room-temperature ionic liquids (RTILs), namely, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄), 1-propyl-3-methylimidazolium tetrafluoroborate (PMIBF₄), and 1-butyl-3-methylimidazolium tetrafluoroborate (EMIBF₄). The results demonstrate that in EMIBF₄, Fe-SOD exhibits the most satisfactory catalysis for ORR, with the standard rate constant of ORR on bare PGE, k _s, increasing from 3.9 to 5.1 times 10⁻³ cm s⁻¹, while in PMIBF₄ and BMIBF₄ containing Fe-SOD k _s increases from 2.6 to 3.6 and from 1.4 to 2.2 times 10⁻³ cm s⁻¹, respectively. In addition to the increased k _s, adding Fe-SOD renders the formal potential of ORR more positive. To accelerate the electron transfer, multi-walled carbon nanotubes (MWCNTs) are employed to modify PGE, consequently, yielding the dramatically increased peak current and k _s. For MWCNTs-modified PGE in EMIBF₄ free of Fe-SOD, k _s increases from 3.9 to ∼7.1 times 10⁻³ cm s⁻¹. The ORR catalysis by Fe-SOD in the presence of Fe-SOD is also evidenced by the formal-potential shift in the positive direction. With MWCNTs accounting for the larger k _s and Fe-SOD being responsible for the formal-potential shift, the catalysis of ORR is satisfactory. Chronocoulmetry experiments proved that some Fe-SOD could be adsorbed on PGE. After analyzing the results, dismutation of superoxide anion O ₂ ⁻ by Fe-SOD is thought to be the main reason for the formal-potential shift. The different polarity of RTILs is probably partly responsible for different k _s obtained in different RTILs. Basing on an earlier proposition, the catalysis of ORR by MWCNTs in RTILs is discussed. Published in Russian in Elektrokhimiya, 2007, Vol. 43, No. 9, pp. 1137–1146. The text was submitted by the authors in English.     

Voltammetric studies on the reduction of polyoxometalate anions in ionic liquids

The electrochemical reduction of tetrabutylammonium salts of isostructural pairs of polyoxometalates [Bu4N]2[M6O19], [Bu4N]4[alpha-SiM12O40], and [Bu4N]4[alpha-S2M18O62] (M = Mo or W) has been investigated at glassy carbon electrodes in dissolved and surface-confined states in ionic liquids and other media. In the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate [BMIM][PF(6)], between two and six reversible one-electron-transfer processes were detected. Detailed studies on the process [alpha-S2W18O62](4-/5-) in a range of ionic liquids, water, and conventional organic solvents (containing 0.1 M electrolyte) suggest that the polarity of the medium plays a key role in the determination of the reversible potential. Reduction processes involving very highly charged [alpha-S2W18O62](8-/9-/10-) species are strongly influenced by the purity of the medium.     

Selective electrochemical fluorination of organic molecules and macromolecules in ionic liquids

This article provides an outline of recent studies on selective electrochemical fluorination in ionic liquid fluoride salts toward green sustainable chemistry. First, a brief historical background of electrochemical fluorination in organic solvents is provided, and some particular problems and unique solvent effects associated with this technique are briefly mentioned. Second, recent progress in selective fluorination and fluorodesulfurization of organic molecules and macromolecules in ionic liquids using direct and indirect electrolysis with recyclable mediators is reviewed.