Electron spin resonance studies of iron(III) complexes of ethylenediaminetetra-acetate and N-(2-hydroxyethyl)ethylenediamine-NN′N′-triacetate in co-ordinating solvents

Abstract

Remarkable solvent dependence of the apparent peak-to-peak linewidth (ΔH_pp) has been observed for the rhombic type of e.s.r. spectra of the high-spin Fe^III(edta)(edta = ethylenediaminetetra-acetate) and Fe^III(hedta)[hedta =N-(2-hydroxyethyl)ethylenediamine-NN′N′-triacetate] complexes in a series of co-ordinating solvents. Moreover, the order of the solvents which gave the characteristic ΔH_pp for each complex was different for the two complex systems: i.e. H₂O dimethyl sulphoxide (dmso) > NN-dimethylformamide (dmf) > CH₃OH for Fe^III(edta) and H₂O > CH₃OH > dmf dmso for Fe^III(hedta). Solvent dependence of the band maxima (λ_max.) corresponding to the d–d transition has been also observed for the same series of iron(III) complex–solvent systems although the λ_max. values obtained were less sensitive to the solvent than the ΔH_pp values. In this case, the solvent dependence of λ_max. for Fe^III(edta) and Fe^III(hedta) was similar: λ_max. was smaller for H₂O and CH₃OH (<600 nm) than for dmf and dmso (>700 nm). The co-ordination behaviour of the solvent molecules to these iron(III) complexes is discussed in relation to both the ΔH_pp and λ_max. values. It is revealed that in the case of rhombic high-spin iron(III) complexes wit such edta and hedta, ΔH_pp values obtained by e.s.r. spectroscopy reflect small changes in the co-ordination circumstances; λ_max. values obtained from electronic spectra did not.