REACTIVITY OF SUPEROXIDE TOWARD IRON(II) COMPLEXES WITH PENTADENTATE AND HEXADENTATE LIGANDS DERIVED FROM CYCLONONANE

The reactivity of superoxide toward two iron(II) complexes, one with t he pentadentate ligand N,N'-di-2-picolyl-4,7-diaza-1-oxacyclononane) ( DPC) and the other with the hexadentate ligand N,N',N''-tripicolyl-1,4 ,7-triazacyclononane) (TPC), has been examined using a range of electr ochemical, spectroelectrochemical, and spectroscopic techniques. Repet itive cyclic voltammograms of both complexes indicate the formation of electrically conductive polymers on the electrode surface at negative potentials under aerobic conditions. Presumably, this electropolymeri zation is initiated by a hypervalent iron species obtained as a final product of the electroreduction of dioxygen in the presence of the com plexes. Under nonaqueous conditions, both complexes react with superox ide giving pseudo-first-order rate constants: Fe(DPC)2+, 100 +/- 10 s- 1, and Fe(TPC)2+, 335 +/- 30 s-1. The results of stopped-flow and pote ntial=step spectroelectrochemical measurements were consistent with fo rmation of an iron(III)-peroxo adduct as an intermediate in the reacti on. The final product of the reaction of Fe(TPC)2+ with superoxide app ears to be the iron(III) complex as indicated by UV-visible and ESR sp ectroscopy, but for Fe(DPC)2+ the product is a modified iron(III) comp lex that is ESR silent.