Iron(III) complexes of bis (benzimidazol-2-yl) methyl) thiophene-2,5-dicarboxamide: Synthesis, spectral and oxidation of o-phenylenediamine

Iron(III) complexes of a potentially pentadentate ligand N2, N5-bis ((1H-benzo [d] imidazol-2-yl) methyl) thiophene-2,5-dicarboxamide are synthesized with an exogenous anion X = Cl−, NO3-. Mössbauer and EPR spectroscopy indicates axially distorted complexes. These complexes were utilized for the oxidation of o-phenylenediamine to 2,3-diaminophenazine in presence of H2O2. The initial rate of reaction is dependent on the concentration of o-phenylenediamine as well as the iron(III) complex. Rates of reaction were found to be at least five times higher for the Cl− bound complex. The effect of an added anion like acetate, azide and citrate is found to inhibit the rate of reaction. This suggests that one of the factors affecting the rate determining step is the binding of these anions on a vacant site at the iron(III) centre. The oxidation of o-phenylenediamine to 2,3-diaminophenazine is reminiscent of the functioning of horse radish peroxidase.

Graphical abstractNew iron(III) complexes have been utilized as catalysts for the oxidation of o-phenylenediamine to 2,3-diaminophenazine in the presence H2O2. The effect of added exogenous anion like acetate, azide and citrate, is found to inhibit the rate of reaction suggesting that one of the factors affecting the rate determining step is the loss of bound Cl−, NO3− at the iron(III) centre, creating a vacant site and the concomitant binding of exogenous anions or o-phenylenediamine.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Iron(III) complexes of a bisbenzimidazolyl diamide ligand. ► EPR, Mössbauer indicate axially distorted complexes. ► Complexes catalyze the oxidation of o-phenylenediamine. ► Effect of added anion on the rate of reaction. ► Implication of Fe(III) in inner sphere electron transfer.