REACTIVITY OF A NOVEL FE(III) CARBOXYMETHYLENECYCLAM COMPLEX TOWARDS DIOXYGEN AND SUPEROXIDE

The synthesis and physico-chemical characterization of a novel Fe(III) low-spin (V/Delta = 0.648), macroyclic complex, [Fe(III)L-(Cl-)]Cl having the pentadentate ligand N-carboxymethylene-1,4,8,11-tetraazacyclotetradecane, AcOH[14]aneN(4), is reported. It has been shown that the complex undergoes a reversible one-electron reduction (E(1/2)= -0.19 +/- 0.01 V versus NHE in DMSO/Bu(4)NBF(4) solution) forming the Fe(II) complex which reacts rapidly with dioxygen, if present. Under aerobic conditions, the electrochemical response of [Fe(III)L(Cl-)](+) indicates the production of an Fe(III) mu-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 x 10(7) M(-1) s(-1); 21.0 degrees C; pH = 8.1. Moreover, ESR studies on potassium superoxide in DMSO show that the radical is rapidly removed by [Fe(III)L(Cl-)](+). In the course of this reaction, a new ferric low-spin form of the Fe(III) complex with lower g anisotropy (V/Delta = 0.332; g(x) = 2331, g(y) = 2.192, g(z) = 1.989) has been found.