Characterization and Properties of Non-Heme Iron Peroxo Complexes

Iron-peroxo Fe(III)O2 and hydroperoxo Fe(III)OOH systems are important intermediates between the initial Fe(II)-dioxygen adduct and the “activated” form of the catalytic site in many mono-iron biomolecules. To the same peroxidic level correspond, in diiron enzymes, bridged peroxo Fe(III)-O-O-Fe(III) intermediates. This review is concerned with the preparation and spectroscopic characterization of such intermediates in non-heme chemical systems, the properties of the natural systems being quoted as references. Although none have been crystallized, it seems very likely that Fe(III)OOH systems present a η 1 - coordination mode for the hydroperoxo group. These Fe(III)OOH units have given clear signatures in UV-vis, resonance Raman and mass spectrometry. By EPR it was found that in Fe(III)OOH, the Fe(III) is low-spin (S = 1/2) and we propose here a simple rationalization of the characteristics of the EPR g-tensor. The electronic properties of the Fe(III)(η1-OOH) known so far, point toward a strong Fe-O bond and a weak O-O bond, in total agreement with the reactivity scheme implying a cleavage of the O-O bond to lead formally to a Fe(V)O unit. Alkylperoxo systems are also included in this review. Fe(III)-peroxo systems Fe(III)O2 have been prepared and described. They contain high-spin Fe(III) and those identified seem to be of the η 2 type. The Fe-O bond is weaker and the O-O one is stronger than in the Fe(III)OOH systems. The implication of these Fe(III)O2 units in catalysis is unclear. “Complementary” systems, such as Fe(III)(η 1-OO) or Fe(III)(η 2-OOH) have been evoked in publications but not identified spectroscopically. These systems certainly deserve to be actively looked for.