SYNTHESIS, CHARACTERIZATION AND ELECTROCHEMICAL STUDIES ON IRON(II) COMPLEXES OF 2-(2-PYRIDYLETHYL) IMINO-3-BUTANONE OXIME (HDPE) AND 2-(2-PYRIDYLMETHYL) IMINO-3-BUTANONE OXIME (HDPM)

We report the synthesis of iron (II) complexes with Schiff bases deriv ed from a series of ligands (L = 2-(pyridylethyl) imino-3-butanone oxi me (HDPE) and 2-(pyridylmethyl) imino-3-butanone oxime (HDPM). The rea ction of [Fe-II(OH2)(4) Cl-2] in aqueous suspension with L (stoichiome try 1:2) yields after addition of NaClO4 deep dark red crystals of [Fe -II(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 p H was extensively studied. The results from the electrochemistry, the potentiometric pKa values and the elemental analysis unequivocally con firm that the iron complex with HDPE has been isolated as [Fe-II(HDPE) (DPE)] ClO4. At pH < 4 a single reversible one-electron, two-proton tr ansfer was observed for [Fe-III(L)(2)](+) - [Fe-II(LH)(2)](2+) [Fe-II( LH)(2)](2+) (E(298)(o)' = 0.883 V for L = HDPM and E(298)(o)' = 0.770 V for L = HDPE, vs. NHE). In the pH range of 4.5 < pH < 7.0 a single r eversible one-electron, one-proton couple was observed for [FeIII(L)2] (+) -[FeII(LH)(L)](+) (E(298)(o)' = 0.646 V for L = HDPM and E(298)(o) ' = 0.578 V for L = HDPE, vs. NHE). At pH > 7 a single reversible one- electron couple [Fe-III(L)(2)](+) -[Fe-II(L)(2)] was observed (E(298)( o)' = 0.246 V for L = HDPM and E(298)(o)' = 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 pot entiometric titration data, the acid-dissociation constants pK(a1) and pK(a2), were found to be 4.37 and 6.38 for [Fe-II(HDPM)(2)](2+) and p K(a) to be 6.82 for [Fe-II(HDPE)(DPE)](+). Electrochemical analysis gi ves acid-dissociation constants pKa(1) and pKa(2) of 4.30 and 6.,80 fo r [Fe-II(HDPM)(2)](2+) and pK(a) of 6.77 for [Fe-II(HDPE)(DPE)](+).