INFLUENCE OF THE DISTAL PARA SUBSTITUENT THROUGH NH-S HYDROGEN-BONDS ON THE POSITIVE SHIFT OF THE REDUCTION POTENTIALS OF [FE4S4(Z-CYS-GLY-NHC6H4-P-X)4]2- (X = H, OME, F, CL, CN) COMPLEXES

[Fe4S4(Z-cys-Gly-NHC6H4-p-X)4]2- (X = H, OMe, F, Cl, CN) complexes were prepared as models of P. aerogenes ferredoxin, whose redox potential is controlled by NH---S hydrogen bonds. Their reduction potentials became more positive in CH2Cl2 at 298 K as the electron-withdrawing tendency of the substituent increased. The most positive potential was observed at -0.80 V vs SCE for the 3-/2- couple of [Fe4S4(Z-cys-Gly-NHC6H4-p-CN)4]2- (1), in which all of the amide NH of an anilide residue participated in intramolecular NH---S hydrogen bonds probably with a cysteinyl sulfur atom. The reduction potential of 1 controlled by the electronic property of its ligand was -1.13 V vs SCE.