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Abstract
We report the synthesis of iron (II) complexes with Schiff bases derived from a series of ligands (L = 2-(pyridylethyl) imino-3-butanone oxime (HDPE) and 2-(pyridylmethyl) imino-3-butanone oxime (HDPM). The reaction of [FeII(OH2)4 Cl2] in aqueous suspension with L (stoichiometry 1:2) yields after addition of NaClO4 deep dark red crystals of [FeII(LH)2](ClO4)2. The complexes have electronic spectra dominated by an intense metal-to-ligand charge-transfer (MLCT) transition in the visible region of the spectrum. Their redox potential dependence on pH was extensively studied. The results from the electrochemistry, the potentiometric pKa values and the elemental analysis unequivocally confirm that the iron complex with HDPE has been isolated as [FeII(HDPE)(DPE)] ClO4. At pH <4 a single reversible one-electron, two-proton transfer was observed for [FeIII(L)2]+ – [FeII(LH)2]2+ (E°298 = 0.883 V for L = HDPM and E°298 = 0.770 V for L = HDPE, vs. NHE). In the pH range of 4.5 < pH < 7.0 a single reversible one-electron, one-proton couple was observed for [FeIII(L)2]+ -[FeII(LH)(L)]+ (E°298 = 0.646 V for L = HDPM and E°298 = 0.578 V for L = HDPE, vs. NHE). At pH>7 a single reversible one-electron couple [FeIII(L)2]+-[FeII(L)2] was observed (E°298 = 0.246 V for L = HDPM and E°298 = 0.186 V for L = HDPE, vs. NHE). The protonation constants have been determined by potentiometric titration and electrochemical analysis of the iron (II) species. The pKa values using both techniques were in close agreement. From the potentiometric titration data, the acid-dissociation constants pKa1 and pKa2 were found to be 4.37 and 6.38 for [FeII(HDPM)2]2+ and pKa to be 6.82 for [FeII)(HDPE)(DPE)]+. Electrochemical analysis gives acid-dissociation constants pKa1 and pKa2 of 4.30 and 6.,80 for [FeII(HDPM)2]2+ and pKa of 6.77 for [FeII(HDPE)(DPE)]+.
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