A FeIIIO4N2 co-ordination sphere assembled via an enolate–imine–amide ligand. Effect of amide protonation on the redox behaviour and stereochemistry of the iron(III) centre

Abstract

The reaction of anhydrous FeCl₃, acetylacetone benzoylhydrazone (H₂L) and KOH (1 : 2 : 3 mole ratio) in methanol produced an iron(III) complex, [FeL(HL)]1, the crystal structure of which was determined. Each ligand binds through enolate O, imine N and amide O atoms in meridional fashion. In the dianionic L both the enolic OH and the amide protons are dissociated, whereas in the monoanionic HL only the enolic OH is deprotonated. Addition of 1 equivalent of HClO₄ to 1 in MeOH gave [Fe(HL)₂]ClO₄2. Similarly reaction of 1 equivalent of KOH with 1 yielded K[FeL₂]3. All the complexes were characterized by analytical, spectroscopic, electrochemical and magnetic measurements. The metal centre in 1 is redox inactive. However, in cyclic voltammetric experiments 2 displayed Fe^III→ Fe^II reduction at –0.21 V and for 3 an oxidation at 0.40 V (vs. saturated calomel electrode) was observed due to Fe^III→ Fe^IV oxidation. Magnetic moments (at 298 K) of the three complexes reflect a S= 5/2 spin state in each. The ESR spectra (at 298 K) of polycrystalline 1 and 2 are rhombic. On the other hand, an ideal axial spectrum was observed for 3.