Electrochemical properties of N'-ferrocenylmethyl-N'-phenylbenzohydrazide in aqueous and organic mediums

We carried out a detailed study of the kinetics of oxidation of N'-ferrocenylmethyl-N'-phenylbenzohydrazide (FcX) to ferrocenium ion (FcX + ) in aqueous and organic mediums. This study using cyclic (CV) and rotating disk electrode (RDE) voltammetry showed that the FcX/FcX + redox couple is reversible. The N'-ferrocenylmethyl-N'-phenylbenzohydrazide and ferrocenium ion diffusion coefficients (D) were calculated from these results. In addition, the electron transfer rate constant and the exchange current density for the oxidation of ferrocene were determined. A comparison of the kinetic data obtained from the two electrochemical techniques appears to show that the data from the RDE experiments are more reliable because they are collected under strict mass transport control.


INTRODUCTION
Many studies and analysis by electrochemical methods were effected on the oxidoreducing properties of ferrocene. In general, the cathodic behavior ferrocene usual in organic media such as dichorométhane, acetonitrile and DMF can be described by a reversible reduction in an electron, leading to ion Ferrocerium [1][2]. In the present work the oxidation of ferrocene, Fe(C 5 H 5 ) 2 , to the ferrocenium cation, Fe(C 5 H 5 ) 2 + , was examined in the solvents dichloromethane solution containing tetrabutylammonium tetrafluoroborate, and aqueous solution containing sulfuric acid using the technique of cyclic voltammetry [3][4]. The results indicated that redox reactions of ferrocene/ ferricenium couple were a reversible process of diffusion-controlled single electron transfer in both studied solutions. One of the ferrocene derivatives the compound N'-Ferrocenylmethyl-N'-henylbenzohydrazide are very important electron-transfer systems for molecular electronics owing to its characteristic redox behaviors [5,6], and they could also be expected to play a key role of an electron chemical probe of the electron-transferprocess in biological molecules [7,8]. It is well known that N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide easily undergoes one electron oxidation to form ferrocenium cation in a reversible manner [9,10] Figure 1. Thus, we investigated the electrochemical N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide behaviors in aqueous media.

1. Instrumentation and software
Cyclic voltammetric measurements were performed using PGZ301 potentiostat (radiometer analytical SAS) and a voltammetric cell with a volumetric capacity of 25ml containing a glassy carbon electrode (GCE) working electrode (radiometer analytical SAS), a Pt wire counter electrode, and an Hg/Hg 2 Cl 2 reference electrode (3.0M KCl). Solutions were deoxygenated with high purity nitrogen for 3 min prior to each experiment. Data acquisitions were accomplished with a Pentium IV (CPU 3.0 GHz and RAM 1 Gb) microcomputer using VoltaMaster software version 7.08 (radiometer analytical SAS). Graphs plot and calculus were carried out using OriginLab software version 2.0 (Integral Software, France).

Chemicals
Electrochemical characterization was carried out on a potentiostat type voltalab 40 of radiometer, with a three-stand electrode cell. Cyclic voltammetric experiments were performed in deoxygenated CH 2 Cl 2 and aqueous ethanol solutions of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide with respectively 10-1 M of Bu 4 NBF 4 and H 2 SO 4 as supporting electrolyte and N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide concentration of 10 -3 M. The three electrodes used were glassy carbon disk as the working electrode, saturated calomel electrode as a reference electrode, and Pt wire as an auxiliary electrode. The working electrode was polished with 0.05 μm alumina slurry for 1-2 minutes, and then rinsed with double-distilled and deionized water. This cleaning process is done before each cyclic voltammetry experiment.
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1. Electrochemical measurement on a fixed electrode
The synthesized compound in previous work [5]. Cyclic voltammograms of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide at glassy carbon electrode were performed at concentration of 10 -3 M of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide in deoxygenated dichloromethane and in aqueous ethanol solutions with respectively 10 -1 M of Bu 4 NBF 4 and H 2 SO 4 as supporting electrolyte, each solution was scanned at scan rate equal to 0.05, 0.10, 0.30 and 0.50 V·s -1 .
The resultant CV curves and the electrochemical parameters are shown respectively in Figure 2 and Table 1. The anodic and the cathodic peak heights as function of the square root of the scanning rate for glassy carbon electrode in different medium are shown in Figure 3. The obtained linear relation ship indicates clear diffusion character.
As it can be seen from figures 3, the ratio of the anodic and cathodic current peak heights is close to one for both solutions; this indicates the reversible character of the oxidation of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide in both studied medium.

2. Electrochemical measurement on rotating disk electrode
Rotating disk electrode is a hydrodynamic electrode technique which utilizes convection as the mode of mass transport as opposed to CV which is governed by diffusion. Convection is more efficient and is not diffusion limited with the result that the analytical data is more reproducible and precise.
Thus a comparison of the kinetic parameters obtained from CV and RDE experiments is informative to elucidate the role of mass transport on electrode reaction kinetics. Figure 4 A shows RDE voltammograms for N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide at a series of rotation rates.
It is evident from the data that the current generated by the RDE method is much larger than that generated under diffusion control. The much larger current obtained using RDE reflects the efficiency of this method. Also notice that there is significant increase in anodic current while the amount of cathodic current is negligible, essentially making the cyclic voltammogram anodic. This is due to the vast The diffusion current limit, the current half-wave and half-wave potential are calculated at different rotation speed of the electrode, Table 2.

Calculation of diffusion coefficient
The Levich equation predicts the current observed at a rotating disk electrode and shows that the current is proportional to the square root of rotation speed. The equation is: Where D ox : diffusion coefficient of the oxidant is expressed in cm 2 .s -1 ω: rotational speed of the electrode (rad s -1 ) γ: kinematic viscosity in cm 2 . s -1 Kinematic viscosity: is the ratio of the viscosity on the density, we have for dichloromethane: vi On another hand the limited current is given by, = Where as: n, number of electrons F: is the Faraday (9.65·10 4 C/mol) A: is the area of the working electrode (cm 2 ). D: is the coefficient diffusion (cm 2 ·s -1 ) C: is the concentration (mol/cm 3 ), in our case is equal to10 -3 mol/l Replacing equations 2 and 3 in 4 gives,

International Letters of Chemistry, Physics and Astronomy Vol. 10
For a rotating rate of the working electrode equal to 400 t/min., the coefficient diffusion of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide in dichlormethane is. = 13.2 × 10 −6 cm 2 · s −1 The coefficient diffusion of ferrocene in aqueous ethanol is calculated as above. Table 2 summarize the obtained values.

CONCLUSION
Voltammetry analysis on a fixed electrode of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide in aqueous and organic solutions indicates that the electrochemical reaction of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide in both studied solutions is a diffusion controlled process, namely, electrochemical process and,show that the electron withdrawing N'-Phenylbenzohydrazide group introduced to the ferrocene influences the redox potential of the iron centre. This is may be due to the non-insulating effect of methylene between the N'-Phenylbenzohydrazide group and the cyclopentadienyl ring of ferrocene. In addition ΔEp for the ferrocene in CH 2 Cl 2 is grater than ΔEp in aq. ethanol, this difference can be attributed to the difference in diffusion coefficient between ferrocene in each medium which is a major contributor. However there is a minor contributor which is related to the difference in the solution resistance of the two electrochemical medium.