Binuclear iron

Dobias B (1984) Surfactant Adsorption on Minerals Related to Flotation. 56 91-147 Doi K, Antanaitis BC, Aisen P (1988) The Binuclear Iron Centres of Uteroferrin and the Purple Acid Phosphatases. 70 1-26 Domcke W, see Bradshaw AM (1975) 24 133-170 Dophin D, see Morgan B (1987) 64 115-204... 

Despite several decades of studies devoted to the characterization of Fe-ZSM-5 zeolite materials, the nature of the active sites in N20 direct decomposition (Fe species nuclearity, coordination, etc.) is still a matter of debate [1], The difficulty in understanding the Fe-ZSM-5 reactivity justifies a quantum chemical approach. Apart from mononuclear models which have been extensively investigated [2-5], there are very few results on binuclear iron sites in Fe-ZSM-5 [6-8], These DFT studies are essentially devoted to the investigation of oxygen-bridged binuclear iron structures [Fe-0-Fe]2+, while [FeII(p-0)(p-0H)FeII]+ di-iron core species have been proposed to be the active species from spectroscopic results [9]. We thus performed DFT based calculations to study the reactivity of these species exchanged in ZSM-5 zeolite and considered the whole nitrous oxide catalytic decomposition cycle [10],... 

Figure 1 Binuclear iron core inserted over a) Z" and b) Z"0h clusters used to represent the 5MR portion of the ZSM-5 zeolite framework. T site numbers follow the ZSM-5 nomenclature. Pink balls are Al, grey balls are Si, blue balls are Fe, red balls are O and white balls are H.

Table 2. Geometric parameters of binuclear iron core cluster models. Oz corresponds to the oxygen of the zeolite. Within the brackets are indicated the considered configurations (a, b, c or d) of Figures. 2i and 2ii...

A number of binuclear iron complexes have also been isolated (with a neutral base attached to each metal in an axial position). The iron complexes undergo net two-electron redox reactions with dioxygen to yield products containing two identical low-spin Fe(n) metal sites superoxide or peroxide are simultaneously generated. Remarkably, the reaction can be partially reversed by removal of 02 from the system by, for example, flushing with N2 in a mixed aqueous solvent at 0°C. 

This behavior may be compared with that of [Ru(bpy)3]2+ (157) and [Pd(dien)Cl]+ (Section IV.C), which associate with anionic regions of DNA, of A-[Ru(phen)3]2+ which is believed to bind in the minor groove (158), and the helical binuclear iron(II) complex mentioned in Section II.D.2, which is accommodated in the major groove (118). 

J. Sanders-Loehr, in Binuclear Iron proteins, T. M. Loehr, Ed., CVH, New York, 1989, p. 375 and references cited therein. 

Hemerythrin is a respiratory protein isolated from sipun-culids (marine worms). All sipunculids examined have, in the coelomic fluid, erythrocytes loaded with the protein which in most species so far examined is octameric, but sometimes tri-meric (18, 19) and in one instance dimeric and tetrameric (20, 21). From the retractor muscle of Themiste zostericola, the protein has been characterized as a monomer (22). The monomer (23) and the subunits of the trimer (24) and octamer (25) are remarkably similar in tertiary structure, having a M.W. of about 13,500 daltons. Each subunit contains one binuclear iron site. There is no porphyrin ring and the irons are coordinated only to amino acids, some of which, as well as probably an oxy group, form the binding atoms (26). 

We have been investigating the oxidation-reduction reactions of the binuclear iron site in the protein matrix (27-32). The methemerythrin form contains both irons in the +3 oxidation state and can be reduced in two steps (by dithionite ion (27, 31), reduced methylviologen, and photochemically using a riboflavin/EDTA mixture (28)) to the deoxy form in which both irons are in the +2 oxidation state. The intermediate (semi-met), in which one iron is +3 and the other iron +2, has been... 

The structures are known for the octamer from Phascolopsis gouldii and Themiste dyscritum (33, 34). The eight identical subunits are packed as a "square donut" with four subunits in each of two layers. The binuclear iron units are at corners of an almost regular square antiprism with distances of 2.8-3.0 nm between adjacent binuclear iron centers. The disproportionation... 

The binuclear iron complexes (317) have been found to undergo a thermal rearrangement to afford the complexes (318), which were evidently formed via a metathesis between Si—Si and Fe—Fe bonds in (317). A similar rearrangement has been observed in a disilyl-bridged bis(cyclopentadienyl)tetracarbonyldiruthenium complex. Unprecedented and stable in — rf J7 -pentafulvadiene)dimthenium complexes (320) have been prepared" from a two-electron oxidation of frani -l,2-bis(mthenocenyl)ethylenes (319), and dimethyl analogues have been similarly obtained from trans- and cM-l,2-dimethyl-l,2-bis(ruthenocenyl)ethylenes. 

The binuclear iron unit consisting of a (p,-oxo(or hydroxo))bis(p.-carboxylato)diiron core is a potential common structural feature of the active sites of hemerythrin, ribonucleotide reductase, and the purple acid phosphatases. Synthetic complexes having such a binuclear core have recently been prepared their characterization has greatly facilitated the comparison of the active sites of the various proteins. The extent of structural analogy among the different forms of the proteins is discussed in light of their spectroscopic and magnetic properties. It is clear that this binuclear core represents yet another stractural motif with the versatility to participate in different protein functions. 

Table I. Comparison of Structural and Spectroscopic Parameters of Binuclear Iron Complexes...

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 . ... 

The best characterized and thus prototypical of the binuclear iron-oxo proteins is hemerythrin (Hr), an oxygen carrier from some classes of marine invertebrates, such as sipunculids. The protein consists of several identical 13 kDa subunits which contain two iron atoms and reversibly bind one molecule of O2. Myohemerythrins contain a single 13 kDa subunit (25,26,27). 

Both the binuclear iron center and the tyrosyl radical contribute to the visible spectrum of active RRB2. By reducing the radical with hydroxyurea, the contributions of the two chromophores can be separated (Figure 4) (60). The radical has sharp absorption peaks at 390 and 410 nm and a broad absorbance at 600 nm which are also found in 2,4,6-tii(rcrt-butyl)phenoxy radical (61). The iron chromophore has a spectrum very similar to that of metHr(OH). 

The binuclear iron(II) complex (141) has a short C—C bond (1.41 A, 40% double bond character) linking its two halves, and an intense absorption band in the near IR ( 374= 14,000, in... 

The structure and properties of [Fe (H20)6][Fe (tsb)2]2 2H20, where tsb = the terdentate Schiff base derived from salicylideneglycine, have been described. The detailed 2D structures of salts of [ Fe(saleiA 2 Fe(CN) 6] depend on the nature of the cation and the solvent used in their preparation. Binuclear iron(III) complexes have been prepared from Schiff bases derived from 2,6-diformyl-p-cresol and anilines. 

Binuclear iron(II) complexes in which a hydroxide bridge is supported by the dinucleating bis-carboxylate ligand dibenzofuran-4,6-bis(diphenylacetate), (217), have proved useful models for hemerythrin. The nature of the binuclear iron center in hemerythrin itself, and in other metalloproteins, has been reviewed, the binding of O2, NO, N3, and NCS to the iron of hemerythrin discussed, " and the volume profile for hemerythrin reacting with O2 established. Bulky tolyl-substituted carboxylate ligands, both bridging and terminal, and... 

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