Characterization and properties of non-heme iron peroxo complexes

Iron-peroxo Fe(III)O-2 and hydroperoxo Fe(III)OOH systems are important int ermediates between the initial Fe(II)-dioxygen adduct and the "activated" f orm of the catalytic site in many mono-iron biomolecules. To the same perox idic level correspond, in diiron enzymes, bridged peroxo Fe(III)-O-O-Fe(III ) intermediates. This review is concerned with the preparation and spectros copic 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 p resent a eta(1)-coordination mode for the hydroperoxo group. These Fe(III)O OH units have given clear signatures in UV-vis, resonance Raman and mass sp ectrometry. 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 characterist ics of the EPR g-tensor. The electronic properties of the Fe(III)(eta(1)-OO H) known so far, point toward a strong Fe-O bond and a weal; O-O bond, in t otal agreement with the reactivity scheme implying a cleavage of the O-O bo nd to lead formally to a Fe(V)O unit. Alkylperoxo systems are also included in this review. Fe(III)-peroxo systems Fe(III)O-2 have been prepared and d escribed. They contain high-spin Fe(III) and those identified seem to be of the eta(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)O-2 units in cat alysis is unclear. "Complementary" systems, such as Fe(III)(eta(1)-OO) or F e(III)(eta(2)-OOH) have been evoked in publications but not identified spec troscopically. These systems certainly deserve to be actively looked for. F inally, bridged peroxo Fe(III)-O-O-Fe(III) systems have been characterized and most remarkably even crystallized and studied by X-ray diffraction in t hree cases. Their configuration was found cis-planar or cis-gauche. Here th e main question is how enzymes and possibly models ran go from the Fe(III)- O-O-Fe(III) state to the di-mu-oxo Fe(IV)O2Fe(IV) active state?