الملخص الإنجليزي
A Schiff base ligand (Benzalaniline) was prepared by reacting aniline with benzaldehyde. Its Fe(III) complex was synthesized. The Schiff base ligand and its complex were characterized by elemental analysis, 'H and 'C NMR, infrared and UV-Visible spectra. The analytical data and infrared spectra show that Fe to N ratio is 1:3 (1 benzalaniline and 2 anilines) and that Fe to water ratio is also 1:3, yielding an overall coordination number of 6 for the metal atom. The proposed suggestion for such result was that the Schiff base ligand undergoes hydrolysis during complexation process to yield both benzaldehyde and aniline which can be coordinated to the metal as well. Kinetic studies using single mixing stopped flow spectrophotometry were carried out to investigate the reduction of Fe(III)-Schiff base complex by Lcysteine hydrochloride. Kinetic data were best fitted with a sum of three exponential equation. The two first rate constants (kı ~ 70 s' and k2 ~ 10 s') were found to be independent of the reactants concentrations. These two rates were explained by an inner-sphere mechanism in which water ligand molecules are exchanged with cysteine-HCI molecules. The third rate constant (kz) was shown to be first-order concentration dependent with respect to both cysteine-HCI and Fe(III)-Schiff base complex and was associated to the oxidation reaction of cysteine-HCl by the iron complex. Finally, the effect of pH on the oxidation reaction rate constant was investigated. kz was found to increase from 0.23 s' at neutral pH to 0.50 s ' at pH = 8.0 and remains practically constant as the pH is varied from 8.0 to 9.0. This effect was related to the presence of different species of cysteine-HCl at different pH, due to its diprotic acid nature (pkia = 1.96 and pk2a = 10.28) and to its acidic sulfhydryl side chain (pksu = 8.33). It was suggested that the increase of kz with pH is due to the anionic H2NCH(CH2SH)COO species whose concentration is pH sensitive. On the contrary, the invariance of kz with pH was related to the presence of predominant anionic species H2NCH(CH2S)COO and H3NCH(CH2S )COO whose concentrations seem to be pH insensitive.
