Catalytic action of the iron(II) complexes of 8-methyl-1,4-bis(2-pyridylmethyl)-1,4,8-triazacycloundecane and 1-methyl-5,9-bis(2-pyridylmethyl)-1,5,9-triazacyclododecane

Abstract

The kinetics and mechanism of the catalatic decomposition of hydrogen peroxide by the iron(II) complexes of the macrocyclic ligands 8-methyl-1,4-bis(2-pyridylmethyl)-1,4,8-triazacycloundecane (L¹) and 1-methyl-5,9-bis(2-pyridylmethyl)-1,5,9-triazacyclododecane (L²) have been investigated at neutral pH (in phosphate buffer). The oxidation of two equivalents of iron(II) to iron(III) by one equivalent of hydrogen peroxide was observed by spectrophotometric titration. The catalysed decomposition of hydrogen peroxide by iron(III) complex was observed and studied. During the catalytic process a reactive intermediate was scavenged by 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS), but not by bromide ion, indicating that the strongly oxidative intermediate is not the hydroxyl radical, and supporting the alternative possibility, a high valent iron–oxo species. Stopped-flow and steady-state kinetic experiments provide further evidence for the detailed mechanism suggested.