Three different complexes, heme-Cu ([(L-6)(FeCuI)-Cu-II](+) (1), L-6 = a binucleating ligand having a heme and covalently tethered copper binding tris(2-pyridyl)methylamine tetradentate moiety), heme complex ((L-6)Fe-II (4), (with "empty" tethered chelate)), and the "'parent" iron-porphyrinate ((F8TPP)Fe-II (5), F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate) were employed for the electrocatalytic reduction of O-2. Complexes 1 and 4 reduce O-2 to water (four-electron reduction) with good efficiency (74 and 59%, respectively), but complex 5 exhibited only an similar to 20% efficiency, thus primarily the two-electron reduction to hydrogen peroxide occurred. The results of the present electrochemical O-2-reduction studies and the previous studies have elucidated the O-2-binding nature of these three complexes, indicating the formation of quite stable Fe-III-(O-2(2-))-Cu-II (peroxo) or Fe-III-O-2(-) (superoxo) species. In line with the thinking of other researchers, the fact that both 1 and 4 can well stabilize Fe-III-O-2(-) superoxo species may suggest that the formation of the latter is a key to the pathway favoring four-electron reduction of dioxygen to water. (C) 2003 Elsevier Ltd. All rights reserved.