Diferrous complexes

The dimeric p-oxo diferric complex is then reduced and reacts with H02 to produce a terminal oxo-iron(V) complex which oxidizes hydrocarbons. 

This collection of triply bridged diferric complexes, together with other 0x0-and hydroxo-bridged complexes affords a range of structural and spectroscopic properties that should serve as useful models for comparison with the proteins in... 

There is currently a paucity of structural data for binuclear iron complexes in other oxidation states. Wieghardt et al. have reported the structure of the only triply bridged diferrous complex thus far, [(Me3tacnFe)20H(OAc)2] (7,8). Its structure is closely related to that of the corresponding difertic complex, with Fe-p.-OH bond lengths of 1.99 A and an Fe Fe separation of 3.4 A. The two metal centers are antiferrcMnagnetically coupled with a J value of -13 cm . ... 

There are no reported optical spectra attributed to FeIV in non-haem iron proteins suggested to have ferryl intermediates. However, a low-intensity peak at about 600 nm is observed in a model non-haem ferryl compound generated by the addition of H202to a (p-oxo) diferric complex [130],... 

To account for the reactivity of 19 toward oxygen atom donors, a catalytic cycle was proposed. In this mechanism, the diferrous complex reacts with an oxygen donor to give an adduct with 19 that can either act as the atom transfer species or collapse to a ferry 1 species [Fe4+ = 0] intermediate. The data currently available does not allow for a more detailed description, although spectroscopic characterization and determination of the kinetic competence of the observed intermediate will allow the differentiation of several possible pathways. 

The UV-vis spectrum of 40 is similar to the spectrum reported for 42 with absorptions at 320 nm (eM = 10,000), 360 nm (cM = 8,000), and 500 nm (sh), with no strong absorption bands in the 400-700 nm region typically observed for complexes containing bent Fe-O-Fe moieties (66). The assignment of 40 as a fi-oxo diferric complex with monodentate terminal acetates is consistent with NMR and IR data. 

C. Duboc-Toia, S. Menage, C. Lambeaux, M. Fontecave, p-Oxo diferric complexes as oxidation catalysts with hydrogen peroxide and their potential in asymmetric oxidation. Tetrahedron Lett. 38 (1997) 3727. 

S. Menage, J. M. Vincent, C. Lambeaux, G. Chottard, A. Grand, M. Fontecave, Alkane oxidation catalyzed by i-oxo-bridged diferric complexes A structure/reactivity correlation study, Inorg. Chem. 32 (1993) 4766. 

Use of the chiral iron catalyst 28 for asymmetric sulfide oxidation was recently reported by Fontecave [48]. The complex is a dimer with two iron centers which are connected via a r -oxo bridge. Two bidentate (-)-4,5-pinene-bipyridine hg-ands 29 and one molecule of water are coordinated to each iron atom thus completing the octahedral environment of this diferric complex. Complex 28 can be isolated as a green solid with four perchlorates as counterions. 

It was shown by the magnetic susceptibility measurement and Mossbauer, ESR, and UN-visible spectroscopy that TBA-I shows an antiferromagnetic coupling of the two high-spin Fe centers. For example, the 8, AEq, and J values of TBA-I, diferric complexes, oxidized methane monooxygenase (abbreviated as MMOox), and oxidized ribonucleotide reductase (abbreviated as RRox) are shown in Table 1. The 5 and AEq values for TBA-I are close to those for MMOox, 1, 2, and 4 with the symmetrical iron centers, and different from those for RRox (5 = 0.53 mms, AEqi = 1.65 mms, and 82 =... 

The diferric oxidation state is the best understood form of the Fe-O(R)-Fe unit. (iJL-Oxo)diferric complexes have been known for many years (33), but only recently has intense attention been concentrated on the detailed properties of these complexes as they pertain to corresponding sites in the proteins. 

Both complexes exhibit a low field EPR signal at g = 16, presumably derived from a coupled diferrous system (88, 126). These complexes thus serve as models for deoxyHrNj and MMOred- What structural and electronic factors determine the appearance of an integer spin EPR signal from diferrous complexes is not understood at present, but the availability of such models provides opportunities to explore the effects of the coordination chemistry of the diferrous state on the properties of this signal. 

The Mc3TACN and TPA complexes are thus far the only examples of mixed-valence complexes derived from oxo- and carboxylato-bridged diferric complexes. Their EPR properties are remarkably similar to each other and the / values estimated are close to those found for the semimetHr complexes. Unfortunately, structural information on these models is lack-... 

RRB2 can also be reduced to the diferrous form with dithionite, but only under certain conditions, such as the presence of a mediator like benzyl viologen or a deforming buffer like imidazole (27, 50). No mixed-valence form has thus far been observed. When the diferrous form is exposed to dioxygen, the dinuclear unit is oxidized to the diferric state, the tyrosine radical appears, and active RRB2 is produced. The stoichiometry of this oxidation process remains to be sorted out, since dioxygen is presumably a four-electron oxidant and only three electrons need to be removed to convert a diferrous complex and one tyrosine to a diferric complex and a tyrosyl radical. 

The mechanisms for O, utilizaton by MMO d and RRB2jed are not established. The diferrous complex in either protein presumably serves to bind dioxygen, as in hemerythrin, forming a diferric peroxide complex (Fig. 19). This putative intermediate has been observed in neither the MMO nor the RRB2 cycle. Unlike in oxyHr, the peroxide intermediates of MMO and RRB2 must become activated to perform the required oxidation chemistry. This activation may derive from the ligand environments... 

N. Kitajima, N. Tamura, M. Tanaka, and Y. Moro-oka. Synthesis and molecular structures of diferrous complexes containing a bis(hydroxo) or a hydroxo carboxylato bridge. Inorg. Chem., 31 3342 (1992). 

Representative complexes for our studies are [Fe(TPA)Cl2]C104 (1) and [Fe20(0Ac)(TPA)2](C104)3 (2), whose structures are shown in Figure 2. 1 is a mononuclear six-coordinate complex,while 2 is a ( i-oxo)( 4-acetato)diferric complex with distinct iron sites because of the different orientations of the TPA ligands relative to the bridging groups. A typical reaction consists of a 0.77 M... 

Kitajima, N. Tamura, N. Amagai, H. Fukui, H. Moro-oka, Y. Mizutani. Y. Kitagawa, T. Mathur. R. Heerwegh, K. Reed, C.A. Randall, C.R. Que, L., Jr. Tatsumi, K. Monomeric carboxylate ferrous complexes as models for the dioxygen binding-sites in non-heme iron proteins The reversible formation and characterization of p-peroxo diferric complexes. J. Am. Chem. Soc. 1994,... 

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