Reversible Half-wave Potentials Research Articles

Syntheses and reactivities of polymethylated ferrocenes: The effect of methylation upon electrophilic addition

A series of ferrocene derivatives with three, four, five, six and seven methyl group substituents has been prepared and characterized by multinuclear NMR, high resolution mass spectrometry and elemental analyses. Chemical oxidation of heptamethylferrocene with silver tetrafluoroborate produced paramagnetic heptamethylferricenium tetrafluorborate and the structure of one isomer of [C5Me4HFeC5Me3H2][BF4] was determined by single crystal X-ray diffraction. Electrochemical studies of the entire series of iron sandwich compounds from ferrocene to decamethylferrocene reveal a highly linear shift of the reversible half-wave potential (E1/2) for the FeIII/FeII couple such that each additional methyl group lowers E1/2 by an average of 54 mV. A new triple-decker complex has been synthesized by the addition of a ruthenium electrophile, [Cp*Ru(NCMe)3][PF6], (Cp* = C5Me5) to heptamethylferrocene, Me7FcH. While previous work established that reactions between [Cp*Ru(NCMe)3][PF6] and ferrocene derivatives with eight or more methyl group substituents readily form bimetallic complexes, reactions with ferrocene derivatives with six or fewer methyl groups brought about ring abstraction and afforded only monometallic products. Thus it may be concluded that there is a “tipping point” in the series of polymethylated ferrocenes such that the electron donating ability and/or steric influence of seven methyl groups is just sufficient to stabilize a ruthenium-iron tripledecker complex.

Steady state voltammetry of charge transfer processes with nonunity electrode reaction orders

Relevant electrochemical processes present rate equations with non-unity reaction orders. For such reactions, an analytical treatment is developed for the stationary voltammetric response and the surface concentrations at the rotating disc electrode and at microelectrodes of any shape, as frequently/preferably employed to reveal their electrode kinetics. The expressions deduced are applicable when one or both redox species are initially present and cover any degree of reversibility. From the general solution obtained, the limit cases of fully reversible and irreversible reactions are derived. By linearizing the current–potential response of the former, the reaction orders can be determined, as well as the value of the reversible half-wave potential that strikingly depends on whether one or both species are initially present in contrast with the case of unity orders. For irreversible processes, the linearization of the complete current–potential response is also possible and it enables us to extract the reaction order and the half-wave potential. An expression for the latter is given as a function of the heterogeneous rate constant and the mass transport coefficient. Additional protocols of kinetic analysis are discussed on the basis of Tafel and Koutecký-Levich-like plots.

CHRONOVOLTAMMETRIC STUDY OF ANTIRADICAL PROPERTIES OF FLAVONOIDS IN REACTION WITH REACTIVE OXYGEN SPECIES

Using the chronovoltammetric method it was shown that on a mercury film electrode in an acidic medium the reduction of oxygen to hydrogen peroxide in the presence of flavonoids is realized as a reversible electrode process followed by a first-order limiting chemical reaction with the participation of the primary product of the electrode reaction – the hydroperoxyl radical. For this type of reaction, in the case of reduction processes, the peak of potential shifts relative to the reversible half-wave potential in the direction of positive potentials. This shift is the larger then the higher the rate constant of the subsequent chemical reaction and the lower the sweep speed of the polarization voltage. To calculate the constants, the half-wave potential of oxygen electroreduction is determined without the addition of a flavonoid. Its value equals to –0.27 V and can be used both in acidic and alkaline media, since it does not change on mercury electrodes in the pH range of 1–9. The reaction rate constants of flavonoids with hydroperoxyl are a characteristic of their antiradical activity. The flavonone groups are the most reactive towards the radical compounds – morin and myricetin. For all studied compounds, the number of electrons participating in the limiting stage varies from 0.96 to 1. This indicates the occurrence of an elementary chemical act of a flavonoid with a radical and the absence of an effect on the kinetics of the investigated reaction of the products of the conversion of flavonoids. The applicability of the presented method for calculating the rate constants to reactions involving active oxygen forms is confirmed by the presence of a high correlation between the data obtained and the reaction rate constants of flavonoids with 2,2′-diphenyl-1-picrylhydrazyl radical.

Synthesis and Application of a Perfluorinated Ammoniumyl Radical Cation as a Very Strong Deelectronator.

The perfluorinated dihydrophenazine derivative (perfluoro‐5,10‐bis(perfluorophenyl)‐5,10‐dihydrophenazine) (“phenazineF”) can be easily transformed to a stable and weighable radical cation salt by deelectronation (i.e. oxidation) with Ag[Al(ORF)4]/ Br2 mixtures (RF=C(CF3)3). As an innocent deelectronator it has a strong and fully reversible half‐wave potential versus Fc+/Fc in the coordinating solvent MeCN (E°′=1.21 V), but also in almost non‐coordinating oDFB (=1,2‐F2C6H4; E°′=1.29 V). It allows for the deelectronation of [FeIIICp*2]+ to [FeIV(CO)Cp*2]2+ and [FeIV(CN‐tBu)Cp*2]2+ in common laboratory solvents and is compatible with good σ‐donor ligands, such as L=trispyrazolylmethane, to generate novel [M(L)x]n+ complex salts from the respective elemental metals.

Synthesis and the Effect of Anions on the Spectroscopy and Electrochemistry of Mono(dimethyl sulfoxide)-Ligated Cobalt Corroles.

A new series of cobalt A3-triarylcorroles were synthesized and the compounds examined as to their electrochemical and spectroscopic properties in CH2Cl2 or dimethyl sulfoxide (DMSO) containing 10 different anions added to the solution in the form of tetrabutylammonium salts. The investigated anions were PF6-, BF4-, HSO4-, ClO4-, Br-, I-, Cl-, OAc-, F-, OTs-, and CN-, all but three of which were found to facilitate reduction of the cobalt corrole in dilute CH2Cl2 solutions, as determined by a combination of UV-vis spectroscopy and spectroelectrochemistry. The synthesized corroles are represented as (Ar)3CorCo(DMSO), where Ar is a meso-phenyl group containing one of 10 different electron-donating or -withdrawing substituents. The axial DMSO ligand was found to dissociate in dilute (10-5 M) CH2Cl2 solutions, but this was not the case at the higher electrochemical concentration of 10-3 M, where the investigated corroles exhibit a rich redox reactivity, undergoing up to five reversible one-electron-transfer reactions under the different solution conditions. The reversible half-wave potentials for generation of the singly oxidized corroles varied by over 1.0 V with a change in the electron-donating or -withdrawing meso-phenyl substituents and type of anion added to the solution, ranging from E1/2 = 0.83 V in one extreme to -0.42 V in the other. Much smaller shifts in the potentials (on the order of ∼210 mV) were observed for the reversible first reduction as a function of changes of the anion and/or corrole substituents, with the only exception being in the case of CN-, where the E1/2 values in CH2Cl2 ranged from +0.08 V in solutions containing 0.1 M TBAClO4 to >-1.8 V upon the addition of CN-.

A comparative study of the cathodic behaviour of EuCl3 in two imidazolium chloride ionic liquids, the 1-butyl-3-methylimidazolium (C4mimCl) and the 1-ethyl-3-methylimidazolium (C2mimCl), on a glassy carbon electrode

This work presents a comparative study of the electrochemical behaviour, on a glassy carbon electrode, of dissolved EuCl3 in two chlorobasic ionic liquids, the 1-ethyl-3-methylimidazolium chloride, C2mimCl, and the 1-buthyl-3-methylimidazolium chloride, C4mimCl, over the temperature ranges 363–398K and 343–363K respectively. In both media, the electro-reduction of EuCl63− takes place via only one electrochemical step EuCl63−/EuCl64−. The electrochemical system EuCl64−/Eu(0) has not been observed within the electrochemical window of both ILs, due to the prior reduction of the respective imidazolium cation from the solvents, which inhibits the electro-extraction of Eu(0) from the media on the GC electrode.The paper describes how several electrochemical techniques (e.g. cyclic voltammetry (CV), convolutive potential sweep voltammetry (CPSV), steady state voltammetry (SSV), chronopotentiometry (CP) and chronoamperometry (CA)) have been used experimentally: i) verifying assumptions used in their theoretical analysis, and ii) ensuring reproducible conditions at the electrode/electrolyte interface, with the aim to obtain high-precision measurements of the diffusion coefficient of EuCl63−. In order to know if there are discrepancies between the obtained data, a two-way analysis of variance, ANOVA, has been carried out. The analysis has shown that with a 95% confidence level there are no significant differences between the diffusion coefficients obtained by the different techniques. On the other hand, the diffusion coefficient of EuCl63− increases with the temperature following the Arrhenius law, being the activation energy for diffusion 40.4±2.6 and 60.2±1.8kJmol−1 in C2mimCl and C4mimCl respectively. The dimensionless Schmidt numbers, defined as the ratio between solvent viscosity and solute diffusivity (Sc=ν/D), have also been calculated to characterise the solute global mass transport through its environment.On the GC electrode, the electro-reduction of EuCl63− to EuCl64− is a quasi-reversible process. Accurate values of the kinetic parameters (i.e. the intrinsic rate constant of charge transfer, k0, and the charge transfer coefficient, α), as well as the reversible half wave potential, Er1/2, have been obtained for the first time in the mentioned ionic liquids, by simulation of the cyclic voltammograms and logarithmic analysis of the voltammograms and convoluted curves.

Quantum chemical modeling of tri-Mn-substituted W-based Keggin polyoxoanions

Using Density Functional Theory (DFT) calculations, we studied the electrochemistry of polyoxometalates (POMs), specifically the redox properties of Mn in tri-Mn-substituted W-based Keggin ions. For direct comparison with recent cyclic voltammetry results [J. Friedl et al. Electrochim. Acta, 141 (2014) 357], we estimated the reversible half-wave potentials of proton- and cation-coupled electron transfer for Mn(IV/III) and Mn(III/II), respectively. The calculated reversible potentials agree well with experiment, reproducing the trend with pH for Mn(IV/III). For adequate DFT energies, it is crucial to apply a reliable description of the electrolyte environment of the POM, accounting also for their rather high charges, up to −7 e. To this end, we included the Li+ counterions, required for charge neutralization, directly in the quantum chemical models which were embedded in a polarizable continuum. We explored various arrangements of the Li+ ions around the POMs and their effect on both structural parameters and electrochemical properties of the POMs. Hybrid functionals (TPSSh, B3LYP, PBE0) overestimate the experimental reduction potentials: the larger the exact-exchange contribution, the larger the resulting reduction potential. The best agreement with experiment is achieved with the PBE approach, likely due to fortuitous error cancellation. The results of the present work indicate that a more sophisticated (atomistic) representation of the electrolyte environment will be beneficial for predicting redox potentials in better agreement with experiment.

Electrochemical behavior of copper species in the 1-buthyl-3-methyl-imidazolium chloride (BMIMCl) ionic liquid on a Pt electrode

The electrochemical behavior of Cu(I) and Cu(II) species in the ionic liquid 1-buthyl-3-methylimidazolium chloride, BMIMCl, on a platinum electrode in the interval of 333–363K, has been investigated. Experiments performed in a controlled N2 atmosphere cell show that: (i) Cu(I) can be oxidized to Cu(II) and reduced to Cu(0), and (ii) Cu(II) can be reduced to Cu(I) by copper metal according to a comproportionation reaction whose constant has been calculated.The diffusion coefficients of Cu(I) and Cu(II) have been obtained by different electrochemical techniques. The dimensionless Schmidt numbers (Sc=ν/D) were also calculated to characterize the solute global mass transport through its environment. The kinetic parameters for the quasi-reversible electrochemical exchange Cu(II)/Cu(I) (i.e. the intrinsic rate constant of charge transfer, k0, and the charge transfer coefficient, α), as well as the reversible half wave potential, Er1/2, have been obtained for the first time in the mentioned ionic liquid, by simulation of the cyclic voltammograms and logarithmic analysis of the convoluted curves.The apparent standard potential of the electrochemical system Cu(I)/Cu(0) has been determined by potentiometry. Founding that nucleation and crystal growth plays an important role in the electrodeposition of metallic Cu onto the Pt surface. Experimental current–time transients followed the theoretical models based on instantaneous nucleation with three-dimensional growth of the nuclei at the studied temperatures. Electrodeposits of copper were obtained by potentiostatic electrolysis and analyzed by scanning electron microscopy with EDAX.

Electrochemical behaviour of terbium in the eutectic LiCl-KCl in Cd liquid electrodes.- Evaluation of the thermochemical properties of the TbCdx intermetallic compounds

The electrode reaction of Tb(III)/Tb couple in the eutectic LiCl-KCl, at Cd liquid electrodes (i.e a Cd pool and a Cd coated W electrodes) was investigated in the temperature range of 673–823K. In both electrodes, the electrochemical reduction of Tb(III) was observed at less cathodic potential values than at the surface of an inert W electrode, due to the decrease of Tb activity in the metal phase.Cyclic voltammetry, using a Cd bulk electrode, suggest a quasi-reversible behaviour of the system Tb(III)/Tb(dissolvedinliquid Cd), and the values of the kinetic parameters, k0 and α, as well as the reversible half wave potential, Er1/2, have been obtained. The differences between the equilibrium potential adopted by a Tb electrode and the Er1/2 observed with the same Tb(III) solution at the Cd pool electrode, were used to calculate approximate values of the excess Gibbs energy change of Tb in liquid metal, and hence the activity coefficient of Tb in Cd.The formation of intermetallic compounds was also studied. Electromotive force, emf, measurements for five intermetallic compounds in two-phase coexisting states were carried out using a Cd coated tungsten electrode. The activities and relative partial molar Gibbs energies of Tb were obtained for TbCd6, TbCd45/11, TbCd3, TbCd2 and TbCd. The formation energy of each intermetallic compound, and the global formation constants were also calculated. The linear dependence of the Gibbs free energies with temperature yields to the enthalpies and entropies of formation of the five intemetallic compounds.Analysis of the samples after electrolysis runs by scanning electron microscopy (SEM) with energy dispersive X-ray allowed the identification of TbCd6, TbCd3, TbCd2 and TbCd.

Electrochemically Driven Intramolecular Oxidative Aromatic Coupling as a Pathway toward π-Extended Porphyrins

A Ni(II) complex of a π-extended porphyrin bearing three mesityl substituents and one electron-rich naphthalene moiety has been prepared via electrochemical oxidation. It was proven that the whole oxidative process starts from electrochemical generation of a radical-cation on the porphyrin core. Electrochemistry and spectroelectrochemistry of both a naphthalenyl-substituted porphyrin and a porphyrin with a fused naphthalenyl group on the π-ring system provide clear distinction between metal- and ring-centered processes. The redox reactivity of the naphthalenyl-substituted metalloporphyrin in nonaqueous media is presented while outlining the most important structural factors which influence the reversible half-wave potentials for oxidation and reduction of this complex and the following chemical reactions which lead to an extended π-system.

Sophisticated Design of PVC Membrane Ion-Selective Electrodes Based on the Mixed Potential Theory

The mixed potential (MP) theory was successfully utilized to design an ionophore-based polyvinyl chloride (PVC) membrane K(+) ion-selective electrode (ISE). Prior to the application of the MP theory, the transfer of K(+) and interfering ions (Na(+), Li(+), and H(+)) facilitated by bis(benzo-15-crown-5) (BB15C5) or dibenzo-18-crown-6 (DB18C6) at a micro PVC membrane/water interface was studied by ion-transfer voltammetry (ITV). The reversible half-wave potentials were then obtained for the facilitated transfer of the ions. Using such voltammetric data and the literature data about diffusion coefficients of ions, we could well-predict the potential responses of the BB15C5- or DB18C6-based K(+) ISE, as the function of the concentrations of primary and interfering ions, and also of the counterion for K(+) [e.g., tetrakis(4-chlorophenyl)borate] added to the membrane. Thus, the MP theory has been proven to be useful to optimize the membrane composition for a higher ion selectivity and a lower detection limit. It has also been found that the leaching of ions from an inner solution is too small to affect the detection limit, at least for the designed PVC membrane ISE.

Cathodic behaviour and oxoacidity reactions of samarium (III) in two molten chlorides with different acidity properties: The eutectic LiCl–KCl and the equimolar CaCl2–NaCl melt

This work presents a study on the chemical and electrochemical properties of Sm(III) solutions in two molten chloride mixtures with different acidity properties: (i) the eutectic LiCl–KCl, in the temperature range 673–823K and (ii) the equimolar CaCl2–NaCl melt at 823K. In both media and on a W inert electrode, the electro-reduction of Sm(III) takes place via one step Sm(III)/Sm(II). The second system Sm(II)/Sm(0) has not been observed within the electrochemical windows, because of the prior reduction of Li(I) and Na(I) from the solvent, which inhibits the electro-extraction of Sm species from the salts on the inert substrate.On the W electrode, the electro-reduction of Sm(III) to Sm(II) takes place in a quasi-reversible electrochemical mode. Accurate values of the reversible half wave potential, the intrinsic rate constant of charge transfer, k0, and the charge transfer coefficient, α, have been calculated for the first time in both molten chlorides, by simulation of the cyclic voltammograms and logarithmic analysis of the convoluted curves. The diffusion coefficient of Sm(III) has been also calculated by different electrochemical techniques, avoiding the meniscus effect by modification of the immersion dept of the working electrode in stages. The values of the diffusion coefficient indicate that Sm(III) diffuses slower in the equimolar CaCl2–NaCl than in the eutectic LiCl–KCl. This behaviour can be explained by the different viscosities of both media, which significantly reduces the mobility of Sm(III) in the CaCl2–NaCl melt.The identification of the SmO compounds that are stable in the melts as well as the determination of their solubility products were carried out by potentiometric titration using an yttria stabilised zirconia membrane electrode (YSZME). The results indicated that SmOCl is a solid stable compound in the studied melts and that Sm2O3 is a strong oxobase leading to the formation of SmOCl. The best chlorination conditions have been extracted from the comparison of the E-pO2− diagram of samarium and that of some gaseous chlorinating mixtures previously reported.

On the meaning of the diffusion layer thickness for slow electrode reactions

A key concept underpinning electrochemical science is that of the diffusion layer - the zone of depletion around an electrode accompanying electrolysis. The size of this zone can be found either from the simulated or measured concentration profiles (yielding the 'true' diffusion layer thickness) or, in the case of the Nernst ('linear') diffusion layer by extrapolating the concentration gradient at the electrode surface to the distance at which the concentration takes its bulk value. The latter concept is very well developed in the case of fast (so-called reversible) electrode processes, however the study of the linear diffusion layer has received scant attention in the case of slow charge transfer processes, despite its study being of great interest in the analysis of the influence of different experimental variables which determine the electrochemical response. Analytical explicit solutions for the concentration profiles, surface concentrations and real and linear diffusion layers corresponding to the application of a potential step to a slow charge transfer process are presented. From these expressions the dependence of the diffusion layer thickness on the potential, pulse time, heterogeneous rate constant and ratio of bulk concentrations of electroactive species and of diffusion coefficients is quantified. A profound influence of the reversibility degree of the charge transfer on the diffusion layer thickness is clear, showing that for non-reversible processes the real and linear diffusion layers reveal a minimum thickness which coincides with the equilibrium potential of the redox couple in the former case and with the reversible half-wave potential in the latter one.

Thermodynamic driving force effects in the oxygen reduction catalyzed by a metal-free porphyrin

Cyclic voltammetry and the stopped-flow kinetic measurements are used to investigate the thermodynamic driving force effects in the molecular oxygen reduction with the methyl (Me)-substituted ferrocenes MenFc (n=0, 2, 4, 6, 8, 10) at the polarized water|1,2-dichloroethane (DCE) interface and in the homogeneous DCE phase, which is catalyzed by 5,10,15,20-meso-tetraphenylporphyrin (H2TPP) dissolved in DCE. Ferrocene derivatives are characterized by their reversible half-wave potentials E1/2rev which are inferred from cyclic voltammetric data, and by the coresponding formal potentials E′0(SHE,o) referenced to the standard hydrogen electrode in DCE. It is shown that the reduction of the activated oxygen with MenFc is the rate-determining step in both electrochemical and chemical catalytic cycles. Plots of the logarithm of the rate constant of the catalyzed, non-catalyzed and electrocatalyzed oxygen reduction vs. the reversible half-wave potential for the MenFc+/MenFc redox couples in DCE have approximately the same slopes. Their values correspond to the Brønsted coefficient of about 1/2, which is characteristic for reactions with the symmetric activation barrier. A comparison of rates of the catalyzed and non-catalyzed oxygen reduction points to the inhibitory effect of water in the former case. Observed difference between the rates of the catalyzed and electrocatalyzed oxygen reduction was ascribed to the inhibitory effect of the present counteranion.

Thermodynamic aspects of the electron transfer across the interface between water and a hydrophobic redox-active ionic liquid

Ion and electron transfer reactions at the interface between a redox-active ionic liquid (IL), (ferrocenylmethyl)dodecyldimethylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (FcMDDATFPB), and water were investigated by cyclic voltammetry (CV) using the FcMDDATFPB membrane cell. The redox-active IL was shown to exhibit the same ion selectivity as the structurally similar but redox-inactive tridodecylmethylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. Ion transfer potentials evaluated from CV measurements allowed establishing the scale of absolute potential differences for the interfacial oxidation of FcMDDA + with the aqueous redox species including IrCl 6 2−, Fe(CN) 6 3−, Mo(CN) 8 3−, ICl 2 − or IBr 2 −. In order to rationalize experimental data, the theory of CV was developed for an electron transfer at one membrane interface that is coupled with the ion transfer at the other membrane interface. Electron transfer mechanism was supported by the linear correlation between the reversible half-wave potentials of the electron transfer across the water IL interface and the standard electrode potentials for the aqueous redox couples. A notable difference between the formal electron transfer potentials for the water IL and water 1,2-dichloroethane interfaces pointed to strong ion–ion interactions within IL affecting the ion activity coefficient term.

Electrochemical and spectrophotometrical investigation of the electron-accepting strength of organic superelectrophiles: X-ray structure of their charge transfer complexes with tetrathiafulvalene

Nitro Benzoxadiazoles (benzofurazans), benzoxadiazoles-N-oxide (benzofuroxans) and benzothiadiazoles are ranked amongst the strongest electrophiles known to date. In the past twenty years, their propensity to act as electron organic acceptors has been less studied. In this paper, we report on the study of their electrochemical behavior and on the structural characterization of charge transfer complexes (CTC) deriving from their interaction with tetrathiafulvalene (TTF) derivatives, both in solution and in the solid state. The first half wave reduction potentials (E(1/2)(I)) associated with a reversible monoelectronic transfer process of a large set of nitro substituted benzoxadiazoles (benzofurazans), benzoxadiazoles-N-oxide (benzofuroxans) and benzothiadiazoles have been determined through a detailed electrochemical approach in acetonitrile with a microelectrode network using the ferrocene as an internal reference potential in this electrochemical study. Determination of the electron affinity (EA(CT)) of this series of substituted electrodeficient heteroaromatics as well as their LUMO energy was performed using the Charge Transfer Spectroscopic (CTS) method in solution and by DFT calculations, respectively. The use of the correlation EA(CT) versus the reversible half wave potential (E(1/2)(I)) appears to be a useful tool to estimate readily the E(1/2)(I) or EA(CT) values when they cannot be experimentally determined. The diffusion coefficient of these electrophiles has, for the first time, been determined in acetonitrile. These air stable electrodeficient heteroaromatics have been explored as potential new organic acceptors in the formation of charge transfer (CT) complexes with TTF derivatives. Crystallographic data of two CT complexes with TTF (especially the C-C and C-S bond lengths of the TTF moieties) indicate that these complexes exhibit weak electron delocalization and that both molecules remain neutral. Their resulting levels of charge transfer were probed using UV-visible, IR spectroscopy and by DFT calculations.

Reduction of indium(III) at dropping mercury electrode in the presence of pyridine in aqueous and ethanol–water media

The reduction of indium(III) at dropping mercury electrode in aqueous as well as in 25% ethanol–water media in the presence of pyridine has been studied at a constant ionic strength (0.1 M KNO3) and at 30 and 40 °C. The reduction is diffusion-controlled but the electrode process is quasi-reversible in both media. The reversible half-wave potential values,E1/2r, have been obtained by Gelling’s method. The plot ofE1/2r versus pyridine concentration is a straight line and the number of ligands,j, was determined from the slope. This shows the formation of a single complex. The stability constant has been determined by Lingane’s method. In(III) forms one complex species with composition 1:1, [In(py)]3+. The values of thermodynamic parameters ΔG, ΔHand ΔSof the complexation reaction have also been determined at 30 °C.

Electrochemical and Spectroscopic Characterization of a Series of Mixed-Ligand Diruthenium Compounds

The electrochemistry and spectroelectrochemistry of a novel series of mixed-ligand diruthenium compounds were examined. The investigated compounds having the formula Ru(2)(CH(3)CO(2))(x)(Fap)(4-x)Cl where x = 1-3 and Fap is 2-(2-fluoroanilino)pyridinate anion were made from the reaction of Ru(2)(CH(3)CO(2))(4)Cl with 2-(2-fluoroanilino)pyridine (HFap) in refluxing methanol. The previously characterized Ru(2)(Fap)(4)Cl as well as the three newly isolated compounds represented as Ru(2)(CH(3)CO(2))(Fap)(3)Cl (1), Ru(2)(CH(3)CO(2))(2)(Fap)(2)Cl (2), and Ru(2)(CH(3)CO(2))(3)(Fap)Cl (3) possess three unpaired electrons with a Ru(2)(5+) dimetal core. Complexes 1 and 2 have well-defined Ru(2)(5+/4+) and Ru(2)(5+/6+) redox couples in CH(2)Cl(2), but 3 exhibits a more complicated electrochemical behavior due to equilibria involving association or dissociation of the anionic chloride axial ligand on the initial and oxidized or reduced forms of the compound. The E(1/2) values for the Ru(2)(5+/4+) and Ru(2)(5+/6+) processes vary linearly with the number of CH(3)CO(2)(-) bridging ligands on Ru(2)(CH(3)CO(2))(x)(Fap)(4-x)Cl and plots of reversible half-wave potentials vs the number of acetate groups follow linear free energy relationships with the largest substituent effect being observed for the oxidation. The major UV-visible band of the examined compounds in their neutral Ru(2)(5+) form is located between 550 and 800 nm in CH(2)Cl(2) and also varies linearly with the number of CH(3)CO(2)(-) ligands on Ru(2)(CH(3)CO(2))(x)(Fap)(4-x)Cl. The electronic spectra of the singly oxidized and singly reduced forms of each diruthenium species were characterized by UV-visible spectroelectrochemistry in CH(2)Cl(2).

Potential-Dependent Adsorption of Amphoteric Rhodamine Dyes at the Oil/Water Interface as Studied by Potential-Modulated Fluorescence Spectroscopy

Ion transfer and adsorption of amphoteric rhodamines, that is, Rhodamine B (RB), Rhodamine 19 (R19), and Rhodamine 110 (R110), and a cationic rhodamine, Rhodamine 123 (R123), at a polarized 1,2-dichloroethane/water (DCE/W) interface, were studied by means of cyclic voltammetry and potential-modulated fluorescence (PMF) spectroscopy. For all rhodamines, a well-defined voltammetric wave was obtained and the pH dependence of the reversible half-wave potential (i.e., midpoint potential) was investigated to prepare the ionic partition diagram. Theoretical considerations of the diagrams showed that the voltammetric waves obtained for the amphoteric rhodamines were not due to a simple transfer of the protonated form (R+) but due to the transfer of H+ facilitated by the amphoteric form (R±) in DCE (and partly in W for R110): H+(W) + R±(DCE or W) → R+(DCE). In PMF spectroscopy, the PMF signal due to the adsorption of R+ at the interface could be obtained for RB, R19, and R123, only when the Galvani potential difference across the interface ( φ) was lower than −0.14 V, suggesting a significant role of φ in the interfacial adsorption of the rhodamines. The PMF spectrum obtained for the rhodamines under these conditions suggested that the xanthene ring of the adsorbed species should be located in the DCE phase. Furthermore, the dependence of PMF on the angle of polarization of the excitation beam suggested that the longitudinal axis of the xanthene ring should tilt only by 20−25° with respect to the interface.

Quantitative analysis of the structure–hydrophobicity relationship for di- and tripeptides based on voltammetric measurements with an oil/water interface

The transfer of 18 di- and 27 tripeptides with un-ionizable amino acid side chains at a nitrobenzene/water (NB/W) interface was studied by cyclic voltammetry. The reversible half-wave potential (E(r)(1/2)), i.e., the midpoint potential could be accurately determined at pH 2 for both the facilitated and non-facilitated transfers, respectively, in the presence and absence of dibenzo-18-crown-6 (DB18C6) in NB. A multiple linear regression analysis was then performed for the E(r)(1/2) using the 'corrected' Dubois steric parameter for amino acid side chain substitutents. The result shows that the hydrophobicity of the peptides is governed not only by the intrinsic hydrophobicity of the peptide backbone and side chains, but also by the steric effects of side chain substituents. For the non-facilitated transfer of peptides, the steric effect of a bulky side chain is more significant at the N-terminus than at the C-terminus (and central for tripeptides). The more bulky the side chain at the N-terminus, the less hydrophobic the peptide becomes due to inhibition of the solvation of a terminal -NH(3)(+) group by organic solvents. For the facilitated transfer by DB18C6, however, the steric effect of a bulky side chain is the most significant at the central position of a tripeptide. A MOPAC calculation of optimized structures of DB18C6-peptide complexes has also shown that there is a notable steric hindrance between the central side chain and the benzene rings of DB18C6, which would reduce the 'apparent' hydrophobicity or transferability of the tripeptide.