Reactivity of a novel Fe(III) carboxymethylenecyclam complex towards dioxygen and superoxide

Abstract

The synthesis and physico-chemical characterization of a novel Fe(III) low-spin (V/Δ = 0.648), macrocyclic complex, [FeIIIL-(Cl-)]Cl having the pentadentate ligand N-carboxymethylene-1,4,8,11-tetraazacyclotetradecane, AcOH[14]aneN4, is reported. It has been shown that the complex undergoes a reversible one-electron reduction (E1 2 = -0.19±0.01 V versus NHE in DMSO/Bu4NBF4 solution) forming the Fe(II) complex which reacts rapidly with dioxygen, if present. Under aerobic conditions, the electrochemical response of [FeIIIL(Cl-)]+ indicates the production of an Fe(III) μ-peroxo dimer. On the basis of stopped-flow kinetics analysis in aqueous solution, it has been demonstrated that the Fe(III) complex accelerates dismutation of the superoxide radical, k = 3.0±1.0×107 M-1 s-1; 21.0 °C; pH = 8.1. Moreover, ESR studies on potassium superoxide in DMSO show that the radical is rapidly removed by [FeIIIL(Cl-)]+. In the course of this reaction, a new ferric low-spin form of the Fe(III) complex with lower g anisotropy (V/Δ = 0.332; gx = 2.331, gy = 2.192, gz = 1.989) has been found. © 1995.