MECHANISTIC STUDIES OF THE FORMATION AND DECAY OF DIIRON(III) PEROXO COMPLEXES IN THE REACTION OF DIIRON(II) PRECURSORS WITH DIOXYGEN

Mechanistic studies of the reactions of three analogous alkoxo-bridged diiron(II) complexes with O-2 have been carried out. The compounds, w hich differ primarily in the steric accessibility of dioxygen to the d iiron(lI) center, form metastable mu-peroxo intermediates when studied at low temperature. At ambient temperatures, these intermediates deca y to form (mu-oxo)polyiron(III) products. The effect of ligand steric constraints on the O-2 reactivity was investigated. When access to the diiron center was unimpeded, the reaction was first-order with respec t to both [Fe-2(II)] and [O-2] and the activation parameters for O-2 a ddition were similar to those for O-2 reacting with the dioxygen trans port protein hemerythrin, When the binding site was occluded, however, reduced order with respect to [O-2] was observed and a two-step mecha nism was required to explain the kinetic results. Decay of all three p eroxide intermediates involves a bimolecular event, implying the forma tion of tetranuclear species in the transition state.