Shigeru Aoyagui

Polarographic studies on bipyridine complexes: I. Correlation between reduction potentials of iron(II), ruthenium(II) and osmium(II) complexes and those of free ligands

Summary A linear relationship between the reduction half-wave potential of a transition metal complex and of its free ligand is predicted, when it is assumed that the electron added to the complex in electrochemical reduction occupies a molecular orbital mainly composed of a ligand π*-orbital. This relationship is found to hold for the complexes ML32+, ML3+ and ML3 (M=iron, ruthenium, osmium; L=2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, 5,5′-dimethyl-2,2′-bipyridine). When the central metal is iron, the linear relationship is extended to the terpyridine complexes, i.e. Fe(terpy)22+ and Fe(terpy)2+, where terpy is 2,2′,2″-terpyridine. The assumed electronic configuration is supported spectroscopically in two cases. However, the relation does not hold in the cases where the added electron occupies a metal t2g-orbital. This finding suggests that the electronic configuration of a transition metal complex in a low oxidation state be determined polarographically.

A correlation between the half-wave potential and the micelle-solubilization equilibrium of ferrocene in cationic micellar solutions

Abstract The reversible half-wave potential of the ferricinium/ferrocene redox couple in aqueous 0.2 M Li 2 SO 4 , (0.165±0.005) V vs. SCE, is determined directly by cyclic voltammetry of ferrocene, in spite of its poor solubility. The reversible half-wave potentials of this couple in cationic micellar solutions of alkyltrimethylammonium bromide are independent of the ferrocene concentration, showing that the pseudo-phase model or the Poisson distribution is applicable to these micellar solutions. The relation between the half-wave potentials in the water and the micellar solution is formulated on the basis of the micelle-solubilization equilibrium. The solubility data and the above half-wave potential data of ferrocene together offer corroborating evidence for the validity of this relation. This fact means that the voltammetric technique can be applied to a complicated micellar system to yield a clear-cut conclusion, and can be used in conjunction with the solubility measurement to determine the formal standard potential of a substance which is insoluble in water but can be solubilized in micellar solution.

Electron-transfer rate constants for redox systems of Fe(III)/Fe(II) complexes with 2,2′-bipyridine and/or cyanide ion as measured by the galvanostatic double pulse method

Abstract The formal standard rate constants for the redox systems, Fe(bipy) 3 3+ /Fe(bipy) 3 2+ ,Fe(bipy) 2 (CN) 2 − /Fe(bipy) 2 (CN) 2 , Fe(bipy)(CN) 4 − /Fe(bipy)(CN) 4 2− and Fe(CN) 6 3− /Fe(CN) 6 4− (bipy=2,2′-bipyridine), in aqueous solution and N,N-dimethylformamide solution are measured with the aid of the galvanostatic double pulse method. The standard rate constant decreases as the number of the coordinated 2,2′-bipyridine decreases. It is in accordance with the trend in the homogeneous rate constants for these systems and is interpreted on the basis of the extension of ligand π-orbitals. This finding may be evidence for the mechanistic similarity of the electrochemical electron-transfer reaction of a redox system to the corresponding homonuclear electron-exchange reaction occurring in solution phase. An empirical relation between rate constants for both kinds of reactions is discussed. It is noted that the maximum electrochemical rate constant is limited at a value much smaller than the one theoretically allowed.

The half-wave potential and homogeneous electron-transfer rate constant in sodium dodecyl sulphate micellar solution

Cyclic voltammetric measurements were made on ML33+/2+, ML2(CN)2+/0, ML(CN)4−/2−, M(CN)63−/4−, Fe(H2O)63+/2+ and methylviologen2+/+ in aqueous solutions with and without sodium dodecyl sulphate (SDS), where M=Fe, Ru and Os and L=2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine, 5,5′-dimethyl-2,2′-bipyridine, 1,10-phenanthroline and 5-chloro-1,10-phenanthroline. At a low scan rate every voltammogram exhibited a one-electron reversible step. In most cases an addition of SDS caused a peak current depression and a half-wave potential shift towards a positive potential, which suggested that the microenvironmental change due to micelle solubilization might be operative. On the other hand, no changes were observed in the voltammograms for Fe(CN)63−/4−, Fe(bpy)(CN)4−/2− and Fe(H2O)63+/2+. A qualitative analysis of the results of the measurements of the rate constants kt for the electron transfer between OsL33+ and Fe(H2O)62+ in micellar solutions pointed to the predominance of the reaction path in which OsL33+ in the micelle reacted directly with Fe(H2O)62+ in the water. A straight line was obtained when log kt was plotted against the standard free energy of reaction calculated from the half-wave potential, including the data obtained in the absence of SDS. This shows that a linear free energy relationship is obeyed and the environmental change in micelle solubilization is reflected in the free energy change of the reactants.

Polarographic studies on bipyridine complexes: III. Polarography of tris(2,2′-bipyridine) complexes of chromium(I), chromium(0), vanadium(0), titanium(0) and molybdenum(0)

Abstract Polarograms and cyclic voltammograms for tris(2,2′-bipyridine) complexes of V(0), Cr(0), Cr(I), Ti(0) and Mo(0) in N,N-dimethylformamide are reported. The reversible half-wave potentials for the following redox systems in lower oxidation states are determined: Cr(−I)/Cr(−II), Cr(−II)/Cr(−III), V(I)/V(0), V(0)/V(−I), V(−I)/V(−II), V(−II)/V(−III), Ti(0)/Ti(−I), Ti(−I)/Ti(−II), Mo(−I)/Mo(−II) and Mo(−II)/Mo-(−III). On the basis of the half-wave-potential shift caused by the methyl substitution of ligands, it is concluded that each excess electron of the reductant species of the redox systems, V(bipy)3−/V(bipy)32−, Cr(bipy)3/Cr(bipy)3−, Cr(bipy)3−/Cr(bipy)32− and Cr(bipy)32−/Cr(bipy)33− (bipy=2,2′-bipyridine), occupies a ligand π*-orbital and that of the V(bipy)32+/V(bipy)3+ and V(bipy)3+/V(bipy)3 systems a metal t2g-orbital. The apparent π-character of the excess electron of the redox systems Cr(bipy)3+/Cr(bipy)3 and V(bipy)3/V(bipy)3− is discussed. It is pointed out that the relative electron affinities of trisbipyridine complexes can be determined from the half-wave potential data. The lowest π*-orbitals of V(bipy)3−, Cr(bipy)3 and Fe(bidy)32+ become higher in this order. This suggests that the electrostatic interaction between a π*-electron and the residual charge on the central metal ion predominantly accounts for the observed π*-level shift.

Electrode kinetics of solvated electrons in hexamethylphosphoric triamide

Summary Measurements of the kinetic parameters of the electron transfer process were carried out on solvated electrons in hexamethylphosphoric triamide. The reaction was thought to be simple with the electron transfer process identical with the overall electrode reaction. Measurements were made in the absence of oxygen and moisture. Platinum electrodes were used. The electrode reaction was proved to be a simple one-electron transfer reaction not followed by any chemical reaction. It was polarographically quasi-reversible. The Nernstian plot of the e− (Pt)/esolv− system was linear and its slope was in agreement with the theoretical value for n=1. The standard electrode potential of this system was −3.441 V vs. Ag/AgClO4 (0.1 M) at 5° C. The kinetic parameters were measured by the potential step method. The exchange current density was 7.2×10−4 A cm−2 for 9 mM; the transfer coefficient 0.15–0.20 and the observed heat of activation 4.2 kcal mol−1. The latter is unexpectedly small considering the small value of the exchange current density, and this may suggest some anomaly in this reaction. The diffusion coefficient of solvated electrons and the activation energy of diffusion were estimated to be 10−7 cm2 s−1 and 10–15 kcal mol−1 respectively. The results were discussed in comparison with those in liquid ammonia.

Kinetic parameters of trioxalatoferriate/trioxalatoferrate electrode reaction measured by the galvanostatic double-pulse method

Summary The electron transfer reaction trioxalatoferriate/trioxalatoferrate was measured on the dropping mercury electrode by the galvanostatic double-pulse method using an improved controlled-current double-pulse supply. The apparent standard rate constant and the transfer coefficient were obtained as 1.44±0.05 cm s−1 and 0.85±0.05, respectively, at 25°C. These are in fairly good agreement with the corresponding values obtained by De Leeuwe et al. with the faradaic rectification method: 1.16±0.04 cm s−1 and 0.86±0.04. These two methods of measurement can give consistent results in the measurement of the kinetic parameters of fast electrode reactions with high rate constants approaching the upper limit accessible with existing methods. The area of the dropping mercury electrode was adjusted to have a definite value at the moment of application of the double-pulse. A simple set-up to detect the drop fall was arranged for this purpose.

Electrochemical formation of stable ferrocene anion and the formal rate constant of the ferrocene0/− electrode

A reversible cyclic voltammogram for the one-electron reduction of ferrocene in 1,2-dimethoxyethane is recorded under experimental conditions that enable the ferrocene anion to exist for a few minutes. The formal rate constant of the ferrocene0/− electrode, determined by cyclic voltammetry at −45°C, ca. 10−3 cm s−1, is in striking contrast with that of ferrocene+/0, > 10−1 cm s−1. The distortion of the ferrocene molecule caused by reduction may be a reason for this difference in electron-transfer rate.

Galvanostatic double-pulse method with polarographic generation of reactant in situ

Summary Modified procedure in the galvanostatic double-pulse (g.d.p.) method is presented. In this procedure the electrolyte only requires to contain initially either the oxidant or the reductant so that the rates of fast electron transfer reactions on a dropping mercury electrode (DME) may be measured. The redox species lacking in the bulk of electrolyte is generated on the DME surface by the polarographic electrolysis at a controlled potential. A g.d.p. is applied between the DME and the counter electrode immediately after the polarographic electrolysis is interrupted. The real moment of termination of the polarographic electrolysis is detected by generation of a sharp voltage spike from the output of the potentiostat; the spike activates the g.d.p. generator. The duration between the termination of the polarographic electrolysis and the application of the g.d.p is successfully reduced to 0.1 μs; the procedure of the analysis of experimental results may thus substantially be identical with that in the conventional g.d.p. method. A rigorous analysis is made for the modified g.d.p. method and a potential-time characteristic is derived. The assessment of the success of the instrumentation as well as the theoretical treatment is made in measuring the electron transfer rate for the trioxalatoferriate/trioxalatoferrate redox system. The observed formal standard rate constant and the transfer coefficient are 1.15±0.05 cm s −1 and 0.78±0.05, respectively, at 25°C. They agree satisfactorily with the results obtained by the conventional g.d.p. method.

Dye sensitization on SnO2 and Au electrodes chemically modified with Langmuir-Blodgett films of surfactant derivatives of rhodamine B and Ru(II)(bpy)32+ complexes

Abstract Three kinds of surfactant derivatives of rhodamine B and Ru(bpy) 3 2+ complexes with two long alkyl chains were synthesized. Surfaces of optically transparent SnO 2 electrodes (SnO 2 OTEs) and of optically semitransparent vapour-deposited gold film electrodes on quartz (Au OTEs) were coated with (i) a Langmuir-Blodgett (LB) film of the dye surfactant itself, (ii) a mixed film with arachidic acid or its cadmium or calcium salt, or (iii) a dye surfactant LB film with a spacer of arachidic acid or its salt. The thickness of the spacer was controlled by the number of arachidic monolayers. The photoelectrochemical characteristics and some physicochemical properties, i.e. UV-visible absorption and emission spectra, of the LB-film-modified SnO 2 OTEs and Au OTEs are discussed mainly in terms of the distance between the electrode surface and the excited dye moiety.