Reversible One-electron Transfer Research Articles

Electrochemical properties of ‘figure-eight’-cyclooctapyrroles (octaphyrins) and their metal complexes

In this paper the first electrochemical results obtained on ‘figure-eight’-cyclooctapyrrolic macrocycles (octaphyrins) 1 to 3, and on some of their mono- and bi-nuclear transition metal complexes, i.e. compounds 5–8 and 9–16, respectively, are reported. All these species undergo five distinct redox steps. The two reductions and the first two oxidations are reversible one-electron transfers. The third oxidation is a global two electron reaction except in the Co(II) complexes. In the latter, this step corresponds to the Co(II)→Co(III) oxidation. The free base octaphyrins display electrochemical gaps (Eox1−Ered1) from 1.43 to 0.85 V, lower than any previously observed gap in porphyrins and porphycenes, and—at further variance with these tetrapyrrolic molecules—this gap changes with the coordinated metal. Another special feature of binuclear metallo-octaphyrins is an intense and sharp absorption band exhibited at 700 nm and beyond in their two electron reduced form.

Di- and trinuclear Pd(II) and Pt(II) complexes containing bridging 2,7-disubstituted naphthyridines

Dinuclear complex cations of the type cis-[M 2L 2(bpy) 2] 2+ ( 1, 2, M=Pd, L=onp −, monp −; 5, 6, M=Pt, L=onp −, monp −) contain μ-1κ N 1:2κ N 8 bridging monoanionic naphthyridine ligands (Honp=1,8-naphthyridin-2-one, Hmonp=7-methyl-1,8-naphthyridin-2-one) in a head/tail arrangement and exhibit short M⋯M distances in the range 2.794–2.804 Å. The analogous reaction of [MCl 2(bpy)] with the dibasic 2,7-disubstituted naphthyridine H 2donp (1,8-naphthyridin-2,7-dione) in the presence of LiOH leads to the trinuclear complexes cis-[M 3(donp) 2(bpy) 3] 2+ ( 3) (M=Pd) and 9 (M=Pt) with similar short M⋯M distances (2.801–2.807 Å). A head/tail arrangement is once again adopted by the μ 3-1κ O 2:2κ N 1:3κ N 8 bridging donp 2− dianionic ligands. Blue–green coloured 9 exhibits reversible one-electron transfer waves at E 1/2 0.65 and 1.30 V (in CH 3CN versus Ag/AgCl) accompanied by a colour change from blue–green (λ max=725 nm) to violet (λ max=695, 540 nm) and corresponding to oxidation to [Pt 3(donp) 2(bpy) 3] 3+ and [Pt 3(donp) 2(bpy) 3] 4+. 9 can be reduced to green coloured [Pt 3(donp) 2(bpy) 3] + at E 1/2=−0.82 V. The structures of the above compounds and [Pt(mbznnp)(bpy)(H 2O)]ClO 4 ( 4, Hmbznnp=2-benzylamino-7-methyl-1,8-naphthyridine), [Pt(donp)(bpy)] ( 7) and [Pt 2Cl 2(donp)(bpy) 2] ( 8) were established by X-ray structural analysis.

Electrochemical oxidation in different media of bis(p-phenylene crown ether) as a symmetrical molecule bearing two identical redox centers

The oxidation of the long chain polyether p-disubstituted benzene derivative (BO5O5) and the related macrocyclic paracyclophane (BBO5O5) have been investigated by cyclic voltammetry at different concentrations (10−3–3×10−2 M) and potential scan rates (0.05–250 V s−1). At about 10−3 M in CH3CN or CH2Cl2 containing a perchlorate salt as the supporting electrolyte, BO5O5 was oxidized reversibly into moderately stable radical cations. The formal potential E°′ corresponding to this first oxidation step was found to be shifted towards more positive values when the cation was changed from Bu4N+ to Na+, and this variation may be ascribed to the coordination of the alkali cation by the polyether chains. At higher concentrations of the reactant and in a thoroughly dried medium, no deposit of a polymeric material onto the electrode surface was electrogenerated. At low scan rates and under the same conditions, the oxidation of BBO5O5 gave rise to two closely spaced, irreversible processes. The radical cations involved in each step were thought to be highly reactive towards nucleophilic species present in solution. At high scan rates, a single reversible one-electron transfer was observed, the formal potential of which varied with the nature of the electrolyte cation in the same way as for BO5O5. The heterogeneous charge-transfer rate constants estimated for BO5O5 and BBO5O5 were compared with the values predicted by the Marcus theory. As for BO5O5, attempts to electropolymerize BBO5O5 were unsuccessful.

On Heterometallic Ferrocenyl Compounds of Gold and Platinum

The syntheses, properties, and chemical shifts of the heterometal gold(I) complexes FcNAuċP(C6H4Cl-4)3 (1), FcN′[AuċPPh3]2 (2), and Fc′[AuċPPh3]2 (3) are reported. The electrochemical behaviour of complexes 1 and 3 is compared with that of the related platinum derivatives (FcN)2Pt (4) and FcNPtClċPPh3 (5). Cyclovoltammetric measurements confirm reversible one-electron transfers..

A Fullerene/Lipid Electrode Device: Reversible Electron Transfer Reaction of C60 Embedded in a Cast Film of an Artificial Ammonium Lipid on an Electrode in Aqueous Solution.

Two reversible one-electron transfers are observed for an electrode device made from C60 and an artificial lipid (see schematic drawing). Cyclic voltammetric studies reveal that the redox couples are unchanged even after 50 cycles, thus indicating that the C60 radical monoanion and the C60 dianion generated in aqueous solution are very stable.

A Fullerene/Lipid Electrode Device: Reversible Electron Transfer Reaction of C60 Embedded in a Cast Film of an Artificial Ammonium Lipid on an Electrode in Aqueous Solution

Two reversible one-electron transfers are observed for an electrode device made from C60 and an artificial lipid (see schematic drawing). Cyclic voltammetric studies reveal that the redox couples are unchanged even after 50 cycles, thus indicating that the C60 radical monoanion and the C60 dianion generated in aqueous solution are very stable.

Spectroscopic and electrochemical evaluation of a perfluorosulfonated ionomer and its gel as preconcentrating media for [ReI(DMPE)3]+, where DMPE = 1,2-bis(dimethylphosphino)ethane.

The interaction of [ReI(DMPE)3]+, where DMPE = 1,2-bis(dimethylphosphino)ethane, a nonradioactive analogue of a heart imaging agent, with Nafion gel, which is Nafion plasticized with tri-n-butyl phosphate, has been evaluated spectroscopically and electrochemically. Thin-layer spectroelectrochemistry on the rhenium compound yields a linear Nernst plot with an n value of 0.99 and E degree' of 0.049 V vs Ag/AgCl. The electrochemistry is consistent with a reversible one-electron transfer between the mono- and dicationic forms of the complex. The UV-visible spectrum of electrogenerated [ReII(DMPE)3]2+ is identical to that obtained by air oxidation of [ReI(DMPE)3]+. Thin, free-standing films of Nafion gel and Nafion that were sufficiently clear to record visible spectra were cast. Spectroscopic measurement of the partitioning of [ReI-(DMPE)3]+ from aqueous solution into these films shows a more rapid uptake of the complex by the Nafion gel. Preconcentration factors into Nafion gel and Nafion were 350 and 50, respectively, after 4 h of soaking. Cyclic voltammetry of 1.0 x 10(-4)-1.0 x 10(-7) M (ReI(DMPE)3]+ in 0.15 M supporting electrolyte aqueous solution at bare gold and spectroscopic graphite electrodes suggests that the complex adsorbs to these electrodes. By comparison, the well-defined cyclic voltammograms at Nafion gel-modified electrodes exhibit diffusion-controlled behavior. The formal reduction potential at Nafion gel-modified electrodes is shifted positively compared to bare electrodes. A current enhancement of approximately 4 was observed at Nafion gel-modified spectroscopic graphite over a bare electrode. A calibration plot of peak current for differential pulse voltammetry vs concentration at Nafion gel-modified spectroscopic graphite was linear in the 10(-7)-10(-5) M concentration range, with a detectable signal down into the 10(-9) M range.

Electrochemical and in-situ UV-visible-near-IR and FTIR spectroelectrochemical characterisation of the mixed-valence heteropolyanion PMo12O40n− (n=4, 5, 6, 7) in aprotic media

Abstract The mixed-valence heteropolyanion PMo12O40n− (where n=4, 5, 6, 7) in aprotic media was investigated by cyclic voltammetry and in-situ FTIR and UV-visible-near-IR spectroelectrochemical methods. A new optically transparent thin layer cell with adjustable optical path length was designed and characterised by thin layer cyclic voltammetry and double step chronoamperometry using ferrocene as a test redox system. The results indicate that this cell has small ohmic drop and good spectral response. The cell was used for both in-situ FTIR and UV-visible-near-IR spectroelectrochemical measurements. The cyclic voltammetric results indicate that the heteropolyanion PMo12O403− undergoes two reversible one-electron transfer reductions (first and second redox waves) and two quasi-reversible one-electron transfer reductions (third and fourth redox waves). The mixed-valence heteropolyanion PMo12O40n− (where n=4, 5, 6, 7) was formed after electroreduction. In-situ FTIR and UV-visible-near-IR spectroelectrochemical preliminary results indicate that the electronic structure of electrogenerated mixed-valence species PMo12O404− corresponds to class II system in Robin and Day’s classification of mixed-valence compounds. In-situ FTIR spectroelectrochemical studies also suggest that the bond energy of the MoOd double bond and Mo–Ob–Mo and MoOc–Mo bridge bonds was reduced after reduction of the original compound.

Synthesis and spectral and electrochemical properties of 2,3,9,10,16,17,23,24-octabutylthiophthalocyaninatozinc(II)

The title Zn(II) complex was synthesized, and its UV-Vis electronic absorption spectrum and cyclic voltammograms in N, N-dimethylformamide were measured. The Q and B bands appear around 704 and 370 nm, respectively. The two redox couples indicating reversible one-electron transfer processes appear in the reduction side of the cyclic voltammogram. Adsorption on an electrode surface and irreversible oxidation with decomposition of the complex molecule were confirmed by a controlled-potential electrolysis experiment. The Q and B bands, reduction potentials and oxidation potential of the present complex shift to longer wavelength, positive direction and negative direction, respectively, compared with those of 2,3,9,10,16,17,23,24-octabutoxyphthalocyaninatozinc(II).

Siloxane and Organosilicon Dimers, Monomers, and Polymers with Amide-Linked Ferrocenyl Moieties. Synthesis, Characterization, and Redox Properties

Polymers [ -S~(CH~)~(CHZ)~N HC(O)(~~-C~~)F~( ~;~~-C~H)~O-~~ (4), and [-C&C(O)NHCH2CH2(q5-C5&)Fe($-Cg&)CH2CH2NHC(O)C6I-L+Si(CH3)2-]n (S), in which the amide-linked ferrocenyl moieties are part of the main polymer chain, have been prepared via solution and interfacial polycondensations. As an aid to the spectroscopic and electrochemical characterization, the corresponding dimeric model compounds [ { $CsHs}Fe{115-Cs&C(0)NH(CH~)3Si(CH3)2}l~-0 (I), and [{T~-C~H~}F~{~~~-C~&CHZCH~NHC(O)(C&)}IZS~M~Z (2) were synthesized. Electrochemical measurements show that in 1 and 2 the oxidation wave represents a twoelectron process, as expected for independent reversible one-electron transfer, at the same potential, of the two ferrocenyl moieties. The cationic dimetallic species [ 12+][PF6-]2 and [22+][PF6-]2 have been generated and characterized via infrared spectroelectrochemistry. A redox-active monomer, { T,W~HS}F~{ vs-C$I&(0)NH(CH2)3SiCH3(0CH2CH3)2} (3), has been successfully attached to Pt electrodes and to silica surfaces, via siloxane bond formation. In addition, a series of poly(methylsi1oxanes) 6-11, containing pendant ferrocenyl moieties attached to the polymer backbone through amide linkages, with varying degrees of ferrocene substitution, were prepared and characterized. Solution electrochemical studies showed that all the ferrocenyl redox centers present in the polymers are electrochemically independent, and that neutral 6,7, and 10 undergo oxidative precipitation, yielding polymer films on the platinum electrode surfaces. The electrochemistry of electrodes modified with electroactive films of ferrocene-containing polysiloxanes was studied by cyclic voltammetry. Amperometric glucose sensing electrodes based on a ferrocene-containing poly(methylsi1oxane) have been prepared.

Mercury-electroplated platinum electrodes and microelectrodes for sonoelectrochemistry

The preparation of mercury-electroplated platinum disc electrodes of both micrometre and conventional dimensions from an aqueous solution of mercury(I) nitrate and potassium cyanide is described. The mercury film electrodes so produced led to very reproducible voltammetric results, as observed by the cathodic reduction of fluorescein to semifluorescein at high pH, both under silent conditions and under ultrasonic irradiation. The electroreduction in the absence of ultrasound is a simple reversible one-electron transfer process (Tafel gradient ∼ 65 mV decade −1). On ultrasonic irradiation an increase in the mass transport-limited current was observed in excess of that expected for a simple one-electron transfer. Possible mechanisms for the electro-reduction in ultrasound are postulated.

Reduction mechanisms of some α-dicarbonyl compounds in basic solutions

Reduction mechanisms of a series of α-dicarbonyl compounds are studied in basic media. The electroreductions of 3,4-hexanedione, methylglyoxal and 1,2-cyclohexanedione are investigated through polarographic, voltammetric and kinetic studies. The effect of pH, reactant concentration and scan rate on the electrochemical parameters is studied. The results indicate that the two first compounds are reduced through CECE mechanisms where the rate-determining step is the second one-electron transfer, while the last compound is reduced through a CEC mechanism, where the rate-determining step is a protonation reaction placed after two reversible one-electron transfers. The influence of adsorption on the reduction of 1,2-cyclohexanedione is also studied. The results obtained here, together with those reported in the literature for other α-dicarbonyl compounds, are discussed and parallel electrochemical behaviour of these compounds is made evident.

An explicit finite difference simulation for chronoamperometry at a disk microelectrode in a channel flow solution

An explicit finite difference simulation is used to describe chronoamperometry at a disk microelectrode in a channel flow solution. All known phenomenologically important processes are represented in the simulation, including the electrode “edge effect” in all three spatial dimensions and the parabolic flow profile of a viscous liquid in a channel. The simulation was achieved with the use of an exponentially expanding spatial grid and a variable time increment to minimise the computational requirements. The simulation was used to study the effects of electrode radius, solution velocity and channel thickness on both the near steady-state limiting current ( I lim nss) and the time taken to achieve the near steady-state condition ( t nss) for a reversible one-electron charge transfer process. It was found that the solution velocity had a dramatic effect on both I lim nss and t nss, with an increase in I lim nss and decrease in t nss being observed with increased solution velocity. The relationships between I lim nss or t nss and solution velocity were found to be non-linear. The results of the simulation compared favourably with experimental data obtained from the limiting current region of near steady-state voltammograms for the reversible oxidation of ferrocene at platinum disk microelectrodes of radii over the range 5–50 μm.

Electrochemical oxidation of niazid and isoniazid at mercury electrodes. Influence of the adsorption of the reaction product on the polarographic and voltammetric curves

The electrochemical oxidation of niazid and isoniazid, at mercury electrodes, was studied by dc and differential pulse polarography and linear-sweep cyclic voltammetry in the pH range 6–13. At pH ⪢ 8.5, the positive scans show a prewave, in addition to the main oxidation wave, which can be suppressed by changing experimental variables such as the concentration, temperature and ethanol content in the medium. In the absence of the prewave, Tafel slopes and reaction orders were obtained at the potentials corresponding to the foot of the polarographic waves. On the basis of polarographic, voltammetric and kinetic results and taking into account the literature data, the oxidation processes were found to be of the ECE type, where the rate-determining step was the release of an H + ion from the intermediate formed after two reversible one-electron transfers. The results obtained for the prewave agree with those expected for a process in which the product is more strongly adsorbed than the reactant. It is also shown that the adsorption follows a Langmuir isotherm for which the Gibbs energy of adsorption is potential dependent.

Preparation of meso-tetra(4-galvinolphenyl)porphyrin—A building block for molecular magnetic materials

The synthesis and oxidative electrochemistry of a free-radical substituted metalloporphyrin are described. The porphyrin is prepared in four steps in good yield. Oxidation at a platinum electrode shows the molecule undergoes five reversible one-electron transfers to a stable pentaradical.

Preparation and spectral and electrochemical properties of the nickel(II), palladium(II) and platinum(II) complexes of 2,3,9,10,16,17,23,24-octapropyltetrapyrazinoporphyrazine

The Ni(II), Pd(II) and Pt(II) complexes with 2,3,9,10,16,17,23,24-octapropyltetrapyrazinoporphyrazinate were prepared. Their absorption spectra and cyclic voltammograms (CV) in CH 2Cl 2 were measured. The two absorption bands corresponding to the Q and B bands of metallophthalocyanines are observed in the 500–660 and 270–400 nm ranges, respectively. The two redox couples indicating reversible one-electron transfer processes appear in the reduction side of the CV. Irreversible oxidation with decomposition of the complex molecule was confirmed by a controlled-potential electrolysis experiment. The spectral and electrochemical properties of these complexes reflect the energy gap between the highest occupied and lowest unoccupied molecular orbitals of the ligand.

Electrochemical properties of poly- ortho-aminophenol modified electrodes in aqueous acid solutions

Poly- ortho-aminophenol (PolyoAP) modified electrodes were obtained from the electro-oxidation of the corresponding monomer on platinum and glassy carbon electrodes in aqueous solution. The electrochemical behaviour of the film was studied by cyclic voltammetry under various experimental conditions. At low scan rates ( v<100 mV s −1) the film shows thin-layer behaviour, while at higher rates semi-infinite diffusion behaviour is observed. In the domain of the thin-layer cell, the thermodynamic properties of the anodic and cathodic peaks suggest non-ideal behaviour with interaction between active sites. Several methods were applied to measure the interaction parameters. Their values are negative, involving a repulsive energy of interaction. The effects of the pH on the film response are very important, being optimal at pH 3. At pH > 7, there is practically no response. However, the film is not destroyed and its response is recovered at pH 1. The effect of temperature on the peak current was also studied. The anomalies observed may be ascribed to the effect of temperature on the interaction parameters. On the other hand, kinetic effects were observed on the cathodic response of the film. Thus, E pc shifts with the sweep rate. This behaviour can be explained by assuming a chemical reaction coupled with the charge transfer. This kinetic effect depends on the temperature, and E pc shifts with T to more positive values, as expected. A redox mechanism of the polyoAP-modified film is proposed which involves addition-elimination of protons coupled to reversible one-electron transfer. The electrocatalytic properties of the polyoAP film electrode were tested for several typical substrates.

Dirhenium complexes derived from the ligand 2-mercaptoquinoline (2-mqH). Structure of [Re2Cl3(.mu.-dppm)2(2-mq)]PF6 (dppm = Ph2PCH2PPh2)

The triply bonded dirhenium complexes Re 2 X 4 (dppm) 2 (X=Cl,Br; dppm=Ph 2 PCH 2 PPh 2 ) react with 2-mercaptoquinoline (2-mqH) to afford the adducts Re 2 X 4 (dppm) 2 (2-mqH), in which the 2-mqH ligand (in its zwitterionic form) is probably bound in a monodentate fashion through its deprotonated thiol donor. These complexes undergo a reversible one-electron transfer to give [Re 2 X 4 (dppm) 2 (2-mqH)] + (isolable as their PF 6 − salts). This oxidation is followed by the slow elimination of HX to give paramagnetic [Re 2 X 3 (μ-dppm) 2 (2-mp)]PF 6 . A single-crystal X-ray structure determination of the chloride complex has been carried out at 20 o C. Crystal data are as follows: monoclinic space group P2 1 /c

Study of the reduction mechanism of isonicotinamide on mercury electrodes

Isonicotinic amide was studied by dc and DP polarography and linear sweep cyclic voltammetry in the pH range 3–13. Tafel slopes and reaction orders were obtained at potentials corresponding to the foot of the polarographic wave. Polarographic, voltammetric and kinetic data show that the process consists essentially of two reversible one-electron transfers followed by a protonation reaction, which is the rate-determining step (rds). The proton donors present in the solution, including the H + ion, are involved in this reaction. In the pH range 6–9, the simultaneous occurrence of two processes taking place at close reduction potentials causes a characteristic variation in the limiting current. At pH > 11 the main protonating species taking part in the rds is the H 2O molecule.

Study on the electrode process of brilliant cresyl blue by optically transparent thin-layer spectroelectrochemistry

The electrode process of Brilliant Cresyl Blue (BCB) at a platinum gauze optically transparent thin-layer electrode has been investigated by using UV-vis and electron spin resonance (ESR) spectroscopy. The formal potential ( E o') and electron transfer number ( n) of BCB have been determined by optically transparent thin-layer spectroelectrochemistry, and n, by thin-layer cyclic voltammetry and ESR spectroelectrochemistry also. The results reveal a reversible one-electron transfer process at the platinum gauze electrode, on which both the oxidized and reduced forms of BCB were strongly absorbed. The triplet state of oxidized BCB and the single electron free radical of reduced BCB were first observed by “in situ” measurements of ESR spectroelectrochemistry. The mechanism for the electrode process of BCB was investigated initially.