Electrochemical reduction of l-(meso-tetraphenylporphyrin)-pyridinium cations

1-(M-meso-tetraphenylporphyrin)-4-R-pyridinium cations MTPP-β-(4-R-Py⁺) (M = H₂, Zn^II, Ni^II, Cu^II or Pd^II, R = H, phenyl or pyridine), formed by a pyridinium cation bearing a porphyrin, metalated or not, at the N-position, were synthesised electrochemically and studied by polarography, stationary and cyclic voltammetry, UV–Vis spectroelectrochemistry and exhaustive coulometry. The pyridinium and the porphyrin rings are both electroactive species in the range of potentials investigated. The results allowed us to discriminate the distinct sites of the different charge transfers and to propose a mechanism for the primary electrochemical reduction step which corresponds to a one-electron transfer to the pyridinium cation. The redox behaviour of the pyridinium in these systems depends on the nature of the core of the porphyrin (free-base or metalated). The differences observed between the free-base and the metalated porphyrin systems were explained by differences in the distribution of the electronic density on the porphyrin ligand in the two types of molecules. In the metalated porphyrin–pyridinium systems, the measured reduction potentials of the pyridinium cation were correlated with the electronegativity ${\chi }_{\mathrm{M}}^{\mathrm{P}}$ of the metal in the porphyrin core. The obtained correlation revealed the occurrence of strong coulombic interactions between the metal in the porphyrin and the pyridinium. The chemical reactivity of the electrochemically generated pyridinium radical depends on the substitution at the 4-position of the ring. The generation of a dimeric species following the first electron transfer to the pyridinium cation was detected by cyclic voltammetry.     

Amperometric study of immuno-ion-selective electrode responses using biotin-aza-crown-6 ionophore

The liquid—liquid electrochemistry of N-biotin-1-monoaza-crown-6 (BMAZC6) for the assisted transfer of potassium has been studied across the 1,2-dichloroethane—water interface by cyclic voltammetry. A model of the mechanism of the assisted K⁺ transfer by BMAZC6 is proposed. The influence of avidin in the aqueous phase on the assisted transfer of potassium is discussed when BMAZC6 is present in either the aqueous or the 1,2-dichloroethane phase. The mechanism of the interaction of avidin with the BMAZC6 is discussed. This assisted ion-transfer method is useful for the analytical determination of avidin down to 5 × 10^?7 M by differential pulse voltammetry.     

Evidence of the withdrawing effect of a pyridinium N-cation on the electrochemical reduction of gold (III) tetraphenylporphyrin

1-(Au^III-meso-tetraphenylporphyrin)-4-pyridinium dication Au^IIITPP⁺-β-Py⁺, formed by a pyridinium cation bearing a charged gold porphyrin at the N-position, has been synthetised and studied by stationary voltammetry, cyclic voltammetry, UV–Vis spectroelectrochemistry and exhaustive coulometry. The pyridinium cation and the porphyrin ring are both electroactive species on the investigated range of potentials. The obtained results allowed to discriminate the sites of the different charge transfers and to propose a mechanism for the three first electrochemical reduction processes involved. The gold porphyrin reduces before the reduction of the pyridinium cation whose signal is intercalated between two reduction steps of the porphyrin. A transient dimeric gold porphyrin is detected. The withdrawing effect of the pyridinium cation on the reduction potentials of the porphyrin is discussed and quantified.     

SnO reduction in lithium cells: study by X-ray absorption, 119Sn Mössbauer spectroscopy and X-ray diffraction

We studied the reduction mechanism of SnO in lithium cells by X-ray absorption near OK and SnL_I edge spectroscopy, ¹¹⁹Sn Mössbauer spectroscopy and X-ray diffraction. The reduction mechanism is complex, involving mixed valence intermediate compounds. In the interval 0≤Li/Sn≤2 the main reaction corresponds to a partial reduction of Sn^II, which is partially reversible, regenerating the tin oxide during the charge. For Li/Sn greater than two, LiSn bonds are formed, but SnO interactions are still present. The charge within this interval also partially regenerates tin oxide, but the reversibility does not extend to Li/Sn lower than two. At low voltage and very large depth of discharge, the formation of LiSn alloys and Li₂O clearly takes place, with negligible SnO interactions.     

Covalent modification of a glassy carbon surface by electrochemical oxidation of r-aminobenzene sulfonic acid in aqueous solution

R-aminobenzene sulfonic acid (r-ABSA) was covalently modified on a glassy carbon electrode (GCE) by electrochemical oxidation in 0.1 M KCl aqueous solution. The presence of an r-ABSA monolayer on the GCE was proven by X-ray photoelectron spectroscopy (XPS). Electron transfer to Fe(CN)₆^3? was investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) on the modified electrodes in solutions of various pHs. Changes in the solution pH value result in the variation of the terminal group (sulfonic acid group) charge state, based on which its surface pK_a values are estimated. The r-ABSA monolayer film on GCE has good stability and can be used as a charge-rich precursor to assemble oppositely charged species by layer-by-layer electrostatic interaction. For example, multilayer films of anionic Fe(III) tetrakis(p-sulfonatophenyl) porphyrin (FeTSPP) and cationic polymer diazo-resins (DAR) can be obtained on the r-ABSA/GCE based on electrostatic and covalently attached interaction and the resulting modified electrodes have good electrochemical response and stability.     

Voltammetric and EQCM investigation of conductive films formed by electroreduction of Ru(L)(CO)2Cl2 compounds

Electroreduction of trans(Cl) and cis(Cl)Ru(L)(CO)₂Cl₂ (L=bpy=2,2′-bipyridine, dmbpy=4,4′-dimethyl-2,2′-bipyridine) in CH₃CN and DMSO resulted in electrode-adhering films and the reactions were studied by voltammetry and the electrochemical quartz crystal microbalance. These methods were complemented by hydrodynamic rotating ring-disc and quartz crystal impedance methods. The deposition was strongly influenced by the type of solvent, polarisation mode and electrolyte stirring, but for all compounds, rapid charge transfer was found. For monomers with trans(Cl)Ru(bpy)(CO)₂Cl₂, the two-electron reduction and Ru⁰Ru⁰ polymerisation process suggested in the literature was confirmed. Some adsorption of CH₃CN was also indicated and for potentiostatic deposition in static solution the impedance measurements indicated increased film viscoelasticity. Depositions with cis(Cl)Ru(bpy)(CO)₂Cl₂ were more complex and the rate declined gradually in static solution. In stirred solution a straightforward polymer formation took place. For trans(Cl)Ru(dmbpy)(CO)₂Cl₂ the literature mechanism could not be confirmed and large deviations were observed during the early stages of deposition. The trans(Cl) dimer, [Ru(bpy)(CO)₂Cl]₂ displayed a lower current efficiency during film formation than the monomer and no simple deposition mechanism could be suggested. For the trans(Cl)[Ru(dmbpy)(CO)₂Cl]₂ dimer a straightforward deposition was found. The films could be removed by electrooxidation and mass/charge data show that the films break up in large fragments. The reduction and deposition process for trans(Cl) monomers (L=bpy, dmbpy) was light sensitive and the mass of films deposited during cyclic voltammetry (CV) was doubled when the electrode was irradiated.     

Current–potential curves for liquid ∣ liquid microinterfaces with no added supporting electrolyte in the water phase

Equations describing steady-state voltammograms have been derived for ion transfer at liquid ∣ liquid microinterfaces with no supporting electrolyte in the aqueous phase. The effect of the absence of any added supporting electrolyte on limiting current, half-wave potential and shape of the current–potential curves is derived theoretically and verified experimentally. Ion-transfer voltammetry across a water ∣ nitrobenzene microinterface with no supporting electrolyte in the aqueous phase has been employed for the determination of standard Gibbs energies of ion partition (in kJ mol^?1) of relatively hydrophilic ions such as Tl⁺ (19.7), K⁺ (22.4), Ag⁺ (26.0), Na⁺ (31.5), Li⁺ (35.5) and H⁺ (34.4). The data obtained agree well with those obtained previously by voltammetry and extraction measurements. With respect to the paper of Quinn et al. (J. Electroanal. Chem. 460 (1999) 149), it is assumed that measurements without a supporting electrolyte in the aqueous phase are useful in systems with a relatively high polarity of the organic phase, e.g. the system water ∣ nitrobenzene. The decisive prerequisite is that the ions of interest in the aqueous phase are transferred sufficiently before the hydrophobic ion of the reverse sign of the charge number in the organic phase.     

Electrogenerated chemiluminescence of sterically hindered porphyrins in aqueous media

A sterically hindered water-soluble porphyrin, tetrakis(3-sulfonatomesityl)porphyrin (H₂TSMP), could form stable radical cation in aqueous media after electrochemical one electron oxidation. The anodic oxidation of H₂TSMP in the presence of tripropylamine or C₂O₄^2? as a coreactant in aqueous solution produces electrogenerated chemiluminescence (ECL) with maxima at 640 and 700 nm. The same emission spectrum of ECL and fluorescence indicates that the ECL emission is from the singlet state of H₂TSMP. The annihilation reactions of ZnTSMP⁺and ZnTSMP^?, which are generated electrochemically, in CH₃CN+H₂O (1:1) mixed solution results in an emission which is identical to the photoluminescence. Protection of the active sites against the nucleophilic attack of water or OH^? by sterical hinderance is a successful strategy in designing new ECL-active compounds in aqueous media. Both ECL reaction mechanisms are proposed.     

Electrochemical impedance spectroscopy study on the solvent extraction mechanism of Ni(II) at the water | 1,2-dichloroethane interface

The charge transfer mechanism at the water | 1,2-dichloroethane (DCE) interface in solvent extraction of Ni(II) was investigated by electrochemical impedance spectroscopy (EIS) and spectrophotometry. The charge transfer species were identified by spectrophotometry using an optical fiber system. In the experimental I–E curves, the diffusion limiting current, which is related to the extraction rate, increases with increasing Ni(II) concentration in the aqueous phase and 8-hydroxyquinoline (HQ) concentration in the organic phase. The kinetic parameters were determined by numerical simulation for the experimental results of EIS. It was found that the solvent extraction rate was accelerated by the imposed potential and that the interfacial capacitance was a function of the amount of the intermediate.     

Characterization of lapachol in artificial organic-film membrane: Application for the trans-membrane transport of Mg

Species transfer across a liquid|liquid interface is studied by means of a thin film-modified electrode using cyclic voltammetry and square-wave voltammetry. The thin film-modified electrode consists of an edge plane pyrolytic graphite electrode (EPG) covered with a thin film of a water-immiscible electro-inactive organic solvent (nitrobenzene) containing a neutral redox probe and/or a suitable electrolyte. For this study we used, as redox probe in the organic phase, 2-hydroxy-3-isopropenyl-1,4-naphthoquinone also known as lapachol (Q) and an appropriate electrolyte. The redox transformations of Q at the graphite electrode/organic (EG|NB) interface was coupled to an ion-transfer reaction from aqueous to organic phase. The proton transfer at the nitrobenzene/water (NB|W) interface is essential for the electrochemical conversion of Q within the membrane. The voltammograms obtained are influenced by the pH of the aqueous phase. Q has two reduction systems due to the redox transformation of its two tautomeric forms resulting from the migration of a proton between the hydroxyl group in position 2 and the carbonyl group in position 4. The electrochemical mechanism consist of 2e⁻/2H⁺ exchange to form the separate redox compounds H₂Q. The experiments conducted reveal the ability of both tautomers to form 1:1 complexes with Mg²⁺ when this cation is present in the aqueous phase.     

Electrodeposition of Al–Ni intermetallic compounds from aluminum chloride-N-(n-butyl)pyridinium chloride room temperature molten salt

Electrodeposition of aluminum–nickel intermetallic compounds (particularly Ni₃Al) has been carried out onto platinum and mild steel cathodes from a 2:1 (mole ratio) aluminum(III) chloride-N-(n-butyl)pyridinium chloride (BPC) molten bath saturated with nickel(II) chloride at room temperature. A single phase of Al–Ni alloy is difficult to obtain by controlled-potential and controlled-current methods; however, it can be obtained by pulse current plating. The electrodeposition of nickel from an AlCl₃–BPC–NiCl₂ (6.14:3.07:0.09 mole ratio) molten bath occurs via an instantaneous nucleation mechanism in the very initial stage of the crystal growth. The deposition reaction mechanisms of nickel in this molten bath are revealed by electrochemical analysis. The experimental Tafel slope of 42 mV dec^?1 and the calculated transfer coefficient $\text{α}\text{→}{}_{\mathrm{<mtext>c</mtext>}}$of 1.5 suggest that the rate determining step is a charge transfer reaction of an adsorbed bare monovalent cation to the metallic state. The effect of the cycle regime on the electrodeposition of Al–Ni alloys has been investigated. The current efficiency for the deposition of alloys is about 99%.     

Transfer and adsorption of 1-pyrene sulfonate at the water ∣ 1,2-dichloroethane interface studied by potential modulated fluorescence spectroscopy

The transfer mechanism of 1-pyrene sulfonate anion (PSA^?) across the polarized water ∣ 1,2-dichloroethane (DCE) interface was investigated by a combination of electrochemical techniques and potential modulated fluorescence (PMF) spectroscopy under total internal-reflection. The dependence of the cyclic voltammogram and the ac voltammogram on the PSA^? concentration showed an apparent reversible ion transfer process. However, the PMF responses exhibited a complex dependence on the PSA^? concentration, revealing adsorption and dimerization processes taking place at the interface. Analysis of the dynamic spectroelectrochemical responses suggests that PSA^? is adsorbed at the interface prior to the transfer step. Upon transferring to the organic phase, PSA^? appears to accumulate at the interface undergoing a dimerization reaction.     

Scanning electrochemical microscopy (SECM): Study of the formation and reduction of oxides on platinum electrode surfaces in Na2SO4 solution (pH = 7)

The formation and reduction of oxides on polycrystalline platinum were studied in a neutral solution with a scanning electrochemical microscope (SECM). Experiments were carried out with tip—substrate voltammetry where the faradaic current flowing to the tip is recorded while cycling the potential of the substrate, and with tip—substrate chronoamperometry where the faradaic tip current is recorded against time following the application of a potential step to the substrate. The tip current was made pH sensitive by holding the tip potential in a region where a pH dependent reaction occurs. Hydrogen evolution was used to probe pH decreases, oxygen evolution was used for the detection of pH increases and Pt oxide formation was used to detect both pH increases and decreases. The results showed that oxide formation occurs in two stages, each involving the transfer of electrons and the release of protons into the solution. During the first stage the release of H⁺ precedes the transfer of electrons. while in the second stage H⁺ release and electron transfer proceed simultaneously. Results are analysed in terms of the formation of PtOH during stage I and PtO during stage 2. However stage 2 behaves differently under slow potential changes and the release of protons lags behind the transfer of electrons. This is interpreted as the result of a place-exchange mechanism from PtOH to HOPt prior to stage 2, followed by the oxidation of HOPt to OPt during stage 2. Similarly, Oxide reduction was found to occur in two stages, each involving the transfer of electrons and the consumption of protons. During the first stage, the consumption of H⁺ precedes the transfer of electrons. The results suggest that during the transfer of electrons, protons diffuse from the outer layer of the oxide (OPt) into the inner layer to form HOPt. For the second stage results are analysed in terms of a place exchange mechanism from HOPt to PtOH running in parallel with the consumption of H⁺ followed by a surface reduction from PtOH₂⁺ to Pt metal.     

Weak adsorption of anions on gold: measurement of partial charge transfer using Fast Fourier Transform electrochemical impedance spectroscopy

Using electrochemical impedance spectroscopy (EIS) based on the Fast Fourier Transform (FFT) method, we have investigated the adsorption properties of F^?, NO₃^? and CH₃COO^? on Au in aqueous electrolytes. The results show that these anions adsorb on Au in considerably small amounts. The adsorption reactions are associated with partial charge transfer across the interface. We present a theoretical framework to analyze these data. Certain previously reported impedance features of simple adsorption reactions are found as special cases of the present analysis. By combining theory and experiment, we determine the double layer capacitance, adsorption capacitance, and the amount of partial charge transfer as functions of the applied voltage. The theoretical framework of the present work can be applied to both cases of weak and strong adsorption, and can be extended to describe coadsorption of different anions.     

Spectroelectrochemical investigations on the reduction of thin films of hexadecafluorophthalocyaninatozinc (F16PcZn)

Redox potentials were determined for solutions and thin films of hexadecafluorophthalocyaninatozinc (F₁₆PcZn). A value of ?0.6 V versus SCE was determined for the first reduction to the radical anion of F₁₆PcZn^? in N,N′-dimethylformamide (DMF) and ?0.9 V versus SCE for the second reduction to F₁₆PcZn^2?. Both potentials were shifted about 0.4 V towards positive potentials when compared to the unsubstituted phthalocyaninatozinc (PcZn) caused by the stabilization of the π-system by the electron-withdrawing fluorine atoms in the ligand. Vapor-deposited thin films of F₁₆PcZn on indiumtinoxide (ITO) were reduced in contact with aqueous electrolytes of pH 5.5 to avoid H⁺-reduction in acidic regimes. A chemically reversible reduction accompanied by cation intercalation was found. The kinetics of the reaction were studied in detail by cyclic voltammetry under variation of the intercalating ionic species, the film thickness and sweep rate. From the charge uptake as well as from the dependence of peak current densities on the square root of the sweep rate it was found that the reaction rate is limited by the diffusion of intercalating cations. Optical absorption spectra were collected in situ. An irreversible structural change was observed during initial reduction, also in accordance with a strongly irreversible initial CV before the reversible behavior was obtained. The presence of the dianionic F₁₆PcZn^2? was detected even under conditions of an average charge uptake of less than 1 electron/molecule. From the analysis of peak current densities according to the Randles–Sev?ik equation as well as the observed charge flow dependent on film thickness and chronoamperometric characterization of the reaction a diffusion constant D_i for K⁺ in F₁₆PcZn in the range from 1.6×10^?12 to 8.0×10^?12 cm² V^?1 s^?1 was calculated.     

Preparation of a novel clay/metal complex hybrid film and its catalytic oxidation to chiral 1,1′-binaphthol

An ITO electrode modified with a hybrid film of chiral metal complex (Λ-[Os(phen)₃]²⁺) and a clay (montmorillonite) has been prepared for the purpose of chiral sensing. As a first step, a floating monolayer of amphiphilic Os(II) complex, [Os(phen)₂(dC18bpy)](ClO₄)₂ (phen=1,10-phenanthroline, dC18bpy=4,4^′-dioctadecyl-2,2^′-bipyridyl), was formed on an aqueous dispersion of sodium montmorillonite. The monolayer acted as an organic part for the hybridization of clay particles in an aqueous phase. The hybrid film of clay and amphiphilic metal complex was transferred onto an indium tin oxide (ITO) substrate by the vertical dipping method. The next step was to immerse the electrode in chloroform, during which the amphiphilic Os(II) complex was removed from the clay surface. Thereafter the electrode was immersed in an aqueous solution of 0.5 mM Λ-[Os(phen)₃](ClO₄)₂ and rinsed with water. Cyclic voltammetric measurements were performed at each step of the above procedures. When the observed curves were simulated on the basis of a double-layered modified electrode, the electron transfer rate constant (k₁) for Λ-[Os(phen)₃]²⁺/Λ-[Os(phen)₃]³⁺ was determined to be 0.25 s^?1. This Os^II/Os^III redox couple was found to mediate the electrochemical oxidation of chiral 1,1^′-2-binaphthol in a stereoselective way: i.e., the S-isomer was oxidized at a 1.4 times higher rate than the R-isomer.     

The reduction of l-cystine hydrochloride at lead using static and rotating disc electrodes

The reduction of the disulphide, l-cystine hydrochloride to the l-cysteine hydrochloride thiol, in 0.1 mol dm^?3 HCl at 298 K, has been studied at pre-treated, circular, 0.50 cm² lead disc electrodes using steady state linear sweep voltammetry, non-steady state voltammetry and controlled potential coulometry. The diffusion coefficient for l-cystine hydrochloride was approximately 4.8 × 10^?10 m² s^?1 from the three techniques. Reduction of the disulphide was irreversible and hydrogen evolution occurred as a competitive reaction at approximately ?1.35 V vs. SCE. Analysis of the mixed control kinetics, using a Koutecky–Levich approach, allowed the relative roles of charge transfer and mass transport to be resolved. Anomalously high Tafel slopes, of typically ?183 mV, were observed due to disulphide adsorption. The charge transfer kinetics are consistent with the first electron gain being rate determining while reaction orders are +1 with respect to both the disulphide and proton concentrations. The mechanism of l-cystine hydrochloride reduction has been critically discussed.     

Speciation of a water-soluble chromium porphyrin by spectral and electrochemical methods

A water-soluble chromium porphyrin, [Cr^IIITF₄TMAP]⁵⁺ (chromium(III) meso-tetrakis(2,3,5,6-tetrafluoro-N,N,N-trimethyl-4-anilinium)yl) porphyrin) was synthesized and the spectral and electrochemical properties were examined. In aqueous solution, [Cr^IIITF₄TMAP]⁵⁺ exhibits two pK_as at 7.04 and 10.07. The cyclic voltammogram showed that a reversible reduction wave appeared at ?0.84 V in pH 5.0 buffer solution, which is assigned as the Cr^III/II redox couple. The oxidation formal potentials were obtained by spectroelectrochemistry. With suitable control of the oxidation potentials and solution conditions, the high valent oxo-Cr^IV and oxo-Cr^V porphyrins were obtained. The ligand trans to the oxygen atom is either H₂O or OH^?, depending on the pH of the solution. In this paper, oxo-Cr^VTF ₄TMAP is thus the first reported water-soluble Cr^V porphyrin that can be generated electrochemically and chemically. Electrogenerated oxo-Cr^V porphyrin species catalyzed the oxidation of cyclopent-2-ene-1-acetic acid to cyclopent-2-ene-4-one-1-acetic acid in the presence of dioxygen and returned to oxo-Cr^IV porphyrin, which is not reactive toward the substrate.     

Electrochemical characterization and electrocatalysis of high valent manganese meso-tetrakis(N-methyl-2-pyridyl)porphyrin

Electrochemical oxidation of water-soluble manganese(III) meso-tetrakis(N-methyl-2-pyridyl)porphyrin (Mn^III(2-TMPyP)) generates stable Mn^IV and Mn^V porphyrins. Speciation of various oxidation states of the porphyrin are characterized by spectroelectrochemical methods. The acid dissociation constants (pK_as) for Mn^III(2-TMPyP)(H₂O)₂ are 9.6 and 10.7, respectively. Spectroelectrochemical results of the one-electron oxidation of Mn^III(2-TMPyP) exhibit different forms of oxomanganese(IV) porphyrin, depending on the pH of the solution and the applied potential. The pK_a for OMn^IV(2-TMPyP)(H₂O) is 10.5. The axial oxygen atom ligated to the Mn(IV) center is protonated in acidic solution (pK_a 3.4). Further one-electron oxidation generates dioxomanganese(V) porphyrin, (O)₂Mn^V(2-TMPyP), which is stable in alkaline solution at room temperature. No oxidation wave is observed in the cyclic voltammograms, indicating the slow heterogeneous electron transfer rate of these oxidation reactions. The electrogenerated dioxomanganese(V) porphyrin exhibits higher reactivity toward olefin oxidation than oxomanganese(IV) porphyrin in basic solutions.