Calculation of Diffusion Coefficients for Oxidation of ferrocene derivative synthesized at two different electrodes using Rotating Disk Electrode (RDE)

The electrochemical behavior of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide synthesized is studied by Rotating Disk Electrode (RDE) Voltammetry to study the kinetics of oxidation and the effect of hydrazide group on ferrocene in organic medium. Thus, two different electrodes (Pt and Gc) were used in ordre to determine this latter. According to the ferrocene taken as a witness the hydrazide group related to the ferrocene made oxydation more difficult. This ferrocenic derivative showed an electrochemical stability, a reversible electrochemical system and an electronic attractor effect of these substitutional ferrocene groups. Finally, we calculated some electrochemical parameters which were: the diffusion coefficients (D), the layer thickness in addition to the electron transfer rate.


INTRODUCTION
Ferrocene is a useful reference material for a lot of ferrocene derivatives it demonstrates good solubility, invariant redox potentials and excellent chemical and electrochemical reversibility in organic electrolytes [1]. The reversibility of the (Fc/Fc + ) redox couple was established from polarographic studies [2] soon after the discovery of this organo-iron compound in 1951 by Kealy and Pauson [3]. Previous studies [4] of the electrochemistry of ferrocene and some of his derivatives in various solvents revealed a reversible one-electron process. One of the ferrocene derivatives the compound N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide (3) are very important electron-transfer systems for molecular electronics owing to its characteristic redox behaviors [5], and they could also be expected to play a key role of an electron chemical probe of the electron-transfer process in biological molecules [6]. 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.
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.
In kinetic studies of electrode processes, uniformity of the concentration gradients along the electrode surface and quantitative information concerning these gradients are necessary. In many instances the rotating disk electrode technique provides an effective means for realizing these requirements. Furthermore this technique allows the surface concentration of reactants and products to be varied in a controlled manner through changes in the rotation rate and hence can be used to determine the reaction orders through the dependence of the current on the rotation rate without the necessity of varying the bulk concentrations.

1. Chemicals
All chemicals were of reagent grade and were used without further purification. Solvents were purified according to standard methods [7]. All reactions were conducted under nitrogen. Solutions were dried over anhydrous magnesium sulphate and evaporated under reduced pressure using a rotary evaporator. The electrolyte salt tetrabutylammonium tetrafluoroborate Bu 4 NBF 4 (Fluka, electrochemical grade 99 % purity) was dried for 1 h at 105 °C before use. CH 2 Cl 2 (Sigma-Aldrich, 99.9 % purity) was dried over molecular sieves before use. Argon plunging tube bottle was provided by ENGI (Enterprise nationale des gaz industriels). All the freshly prepared solutions were degassed under argon gas flow before experiments.

Instrument
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 solution of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide with respectively 10 -1 M of Bu 4 NBF 4 as supporting electrolyte and N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide concentration of 10 -3 M. The three electrodes used were glassy carbon and Platinium disk as the working electrodes, 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.

2. Synthesis N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide
N'-Phenylbenzohydrazide was added to a well stirred solution of (Ferrocenylmethyl) trimethylammonium iodide in sodium-dried toluene. The resulting suspension was heated under reflux for 6 h. It was then allowed to cool to room temperature and filtered. The filtrate was washed with water to remove any trace of unchanged quaternary ammonium salt. It was then dried and evaporated. The residue was recrystallized from ethanol to give N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide as yellow-orange needles.  The methylene group of the salt N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide is characterised by its down orientation on the dept. spectrum, Figure 3.
In present work, we report electrochemical behavior of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide in organic medium on a classy carbon and platinum electrodes. Electrochemical behavior of FcX and FcX + couple in both solutions was investigated by RDE.
The N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide was synthesized according to literature procedures [7]. Figure 5, a shows RDE Polarogrammes for ferrocene and 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 that was obtained using RDE, reflects the efficiency of this method. International Letters of Chemistry, Physics and Astronomy Vol. 14 Figure 5 (A, B). Polarogramme of 1 mM Ferrocene and 100 mM Bu 4 NBF 4 in CH 2 Cl 2 (A) at glassy carbon working electrode (B) at platinium working electrode, Pt counter electrode, and CSE reference electrode at 0.50 V·s -1 (Rotating rate 400, 600, 800, 1000 rpm).
The diffusion current limit, the current half-wave and half-wave potential are calculated at different rotation speed of the two electrodes, Table 1.

4. 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:

5. Applications
For a rotating rate of the working electrode equal to 400 t/min., the coefficient diffusion of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide in dichlormethane is. The coefficient diffusion of ferrocene in aqueous ethanol is calculated as above. Table 2 summarize the obtained values. Table 2. Diffusion coefficients of compound calculated from polarogramme of Figure 5.

CONCLUSION
Voltammetry analysis on a RDE of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide in an organic solution indicates that the electrochemical reaction of N'-Ferrocenylmethyl-N'-Phenylbenzohydrazide in the studied solution is a diffusion controlled process using two different electrodes. The layer thickness at ethe (GC) electrode was thicker than at the (Pt). The same was observed in the coefficient diffusion.
This ferrocenic derivative showed an electrochemical stability, a reversible electrochemical system and an electronic attractor effect of these substitutional ferrocene groups.