Diiron complexes, reactions with dioxygen

Tolman, W. B., Liu, S., Bentsen, J. G., and Lippard, S. J., 1991, Models of the reduced forms of polyiron-oxo proteins An asymmetric, triply carboxylate bridged diiron(ll) complex and its reactions with dioxygen, J. Am. Chem. Soc. 113 1529164. 

Feig, A. L. Becker, M. Schindler, S. van Eldik, R. Lippard, S. J. Mechanistic studies of the formation and decay of diiron(III) peroxo complexes in the reaction of diiron(II) precursors with dioxygen. Inorg. Chem. 1996, 35 (9), 2590-2601. 

Fig. 4. Substrate first binds to the complete system containing all three protein components. Addition of NADH next effects two-electron reduction of the hydroxylase from the oxidized Fe(III)Fe(III) to the fully reduced Fe(II)Fe(II) form, bypassing the inactive Fe(II)Fe(III) state. The fully reduced hydroxylase then reacts with dioxygen in a two-electron step to form the first known intermediate, a diiron(III) peroxo complex. The possibility that this species itself is sufficiently activated to carry out the hydroxylation reaction for some substrates cannot be ruled out. The peroxo intermediate is then converted to Q as shown in Fig. 3. Substrate reacts with Q, and product is released with concomitant formation of the diiron(III) form of the hydroxylase, which enters another cycle in the catalysis.

The nonheme diiron centers in proteins and model complexes with 0,N-donors can reach a number of oxidation states spanning from FenFen to FeIVFeIV The diiron(II) state is reactive with dioxygen yielding different products depending on the nature of ligands and reaction conditions (Figure 4.21). Outer-sphere electron transfer may occur for coordinatively saturated and sterically impeded complexes with sufficiently low redox potential.17... 

FIGURE 4.21 Reactions of diiron(II) complexes with dioxygen and further transformations of diiron-dioxygen adducts.17... 

Feig, A. L. Masschelein, A. Bakac, A. Lippard, S. J. Kinetic studies of reactions of dioxygen with carboxylate-bridged diiron(II) complexes leading to the formation of ( l-oxo)diiron(III) complexes. J. Am. Chem. Soc. 1997, 119 (2), 334-342. 

Reaction Chemistry of Diiron(II) Complexes with Dioxygen... 

Dioxygen diiron complexes have been synthesized by mimicking the probable intermediate structures in methane monooxygenase oxygenations. Examples are summarized in the recent review. One is the Fc2(i7-0)2 diamond core type and the other is the Fe2(tJ-02) peroxo type. Recently, an unusual Fe"-X-Fe (tl -02) species has been suggested in the reaction of O2 with carboxylate-bridged diiron(II,II) paddlewheel complexes, but its role in the oxygenation has not been clarified. ... 

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