Diiron model complexes

A method that has recently been introduced to the study of diiron model complexes involves self-assembly reactions using bulky carboxylate ligands (Scheme 6). The steric bulk is employed about the carboxylate group to control nuclearity. In this approach, simple nonchelating N-donor ligands are employed as ancillary ligands. This method... 

Recently, the structure and spectral properties of a well-known diiron Fe model complex ( ji-SCH2CH2CH2S)Fe2(CO)6 were reinvestigated by Darensbourg and coworkers (Lyon et al. 1999). Also the Fe2(CO)4(CN)2 derivative was prepared and investigated. The v(CO) and v(CN) bands of the latter complex best fitted those found for the reduced D. vulgaris [Fe] hydrogenase and hence the possibility of an Fe -Fe pair in the reduced enzyme was suggested by the authors. 

Diiron(II) complexes of the type (p-E)(p-pdt)[Fe(CO)2(PMe3)]2 + (E = H or SMe) as seen in Fig. 1 were examined as potential struc-tural/spectroscopic models of the [Fe]H2ase active site, using PMe3 as a substitute for the reactive cyanide ligands (24-26). 

The BIDPhE model complex provides us with a unique opportunity to explore the reactivity of a phenoxyl radical-diiron(III) moiety with a variety of inhibitors of the R2 active site. Several studies have been carried out in which the R2 protein was treated with different radical inhibitors (52-54). The mechanism of inhibition, the oxidation products formed from the inhibitors and the possible concomitant reactivity of the diiron(III) center with these agents are all subjects of current investigation. The reactivity of the BIDPhE model complex may be more easily studied than that of R2, given the relative ease of handling a small molecule compared to a protein. 

A similarity in the visible spectra of some /i-oxo-di-/i-acetato Mn(III) dinuclear complexes to the spectrum of Mn catalase has been noted (168-174). Reaction of hydrotris(l-pyrazolyl)borate, [HB(pz)3], or 1,4,7-triazacyclononane (TACN) with Mn(0Ac)3 2H20 results in the formation of the dinuclear complexes [Mn2in0(0Ac)2(HB(pz)3)2] (168) and [Mn/1 0(0Ac)2(TACN)2]2+ (169). Selected structural parameters are given in Table VII. The dimanganese cores of the complexes are essentially identical to those of some /i-oxo-di-/i-carboxylato diiron(III) complexes (177,178), which have been shown to be excellent structural models of the diiron site in methemerythrin (Fig. 8) (179). 

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. 

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... 

Peroxo-diiron(III) complexes can undergo not only redox but also ligand substitution reactions. Liberation of H202 was observed in the reactions with phenols and carboxylic acids leading also to the respective phenolate or carboxylate iron(III) complexes.86 Hydrolysis of a peroxo-diiron(III) complex results in an oxo-diiron(III) species and hydrogen peroxide. Such reaction is responsible for the autoxidation of hemerythrin, but is very slow for the native protein due to hydrophobic shielding of the active site (Section 4.2.3).20 The hydrolysis of iron(III) peroxides is reversible, and the reverse reaction, the formation of peroxo intermediates from H202 and the (di)iron(III), is often referred to as peroxide shunt and is much better studied for model complexes. 

Structural perspective. In all cases, some effective symmetry element, either a twofold axis or a plane, relates the two halves of the molecule to give two essentially equivalent iron sites. Thus the inequivalence of the iron sites in metHr and metHrX is not modeled by these complexes. The Fe-Ooxo bond falls in the range of 1.77-1.80 A, not unlike those found in (p,-oxo)diiron(III) complexes that have no additional bridges (33). Where the three groups of the tridentate ligand are identical, the ligand trans to... 

Mossbauer spectra of native RRB2 reveals two quadrupole doublets of equal intensity with 8 values of 0.5 mm s", which are typical of high-spin ferric centers and AEq values of 1.65 and 2.45 mm s" (26). These large quadrupole splittings are expected of (p,-oxo)diiron(III) complexes. The appearance of two iron centers with distinct AEq values was difficult to rationalize in the RRB2 model proposed by Reichard and co-workers... 

The NMR spectra of native RRB2 show a feature at 24 ppm, which disappears upon D2O exchange (120). The observed shift and solvent exchange behavior is as expected for N-H groups of imidazoles coordinated to (ix-oxo)diiron(III) complexes. Methemerythrin exhibits such a peak at 24 ppm (35), and a variety of model complexes show features associated with imidazole N-H in the 14-20 ppm region (72, 85, 96). When compared to those found for mononuclear high-spin Fe(III)-imidazole complexes at... 

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