The Reactivity of an Iron(II)/Iron(III) System with Polyamine Ligands in Ethanol: An Electrochemical Study

Abstract

AbstractThe pentadentate ligand picdien [LA; 1,9‐bis(2′‐pyridyl)‐2,5,8‐triazanonane)] and [Fe(DMSO)6](NO3)3 produce the hexacoordinate coordination compound [FeL′B]2+ after reaction in ethanol. This compound shows a change in the oxidation state of FeIII to FeII as well as an increase in the denticity of the starting ligand LA. The new ligand L′B {1,9‐bis(2′‐pyridyl)‐5‐[(ethoxy‐2′‐pyridyl)methyl]‐2,5,8‐triazanon‐1‐ene} has an imine carbon−nitrogen double bond. This study provides evidence for the nature of the intermediate species during the formation of [FeL′B](BPh4)2. A systematic study was performed based on the evolution of the voltammograms observed after the addition of ligand (LA, LB) or base (LiOEt) to FeIII, FeII solutions or stoichiometric metal‐ligand mixtures (LB is L′B without the imine group). The stability of FeLA3+ and FeLA2+ species in acidic media and under nitrogen atmosphere was confirmed. The reactivity of FeLB3+ and FeLA3+ systems was found to be very similar, and the reduction products were found to be FeII species with simultaneous ligand oxidation, which, in both cases, produces an imine double bond, giving rise to an intramolecular redox reaction. The following disproportionation equation describes the general form of this process: 2 FeL3+ ⇄ FeL2+ + FeL′2+ + 2 H+. The oxidation was verified by the double bond formation in LA and LB giving rise to L′A and L′B as can be seen in the general equation: R1−CH2−NH−R2 ⇄ R1−CH=N−R2 + 2 H+ + 2e−. The disproportionation reaction for FeLA3+ and FeLB3+ species depends on the acidity level of the media: this reaction cannot be verified in acidic media, while neutral and slightly basic media favour formation of the FeII species with L′A and L′B. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)