FeS clusters

Several iron sulfide nitrosyl compounds are known. These have stmctures that in some cases are formally related to the FeS clusters by replacement of thiolate by NO. The compounds include the anions [Fe2S2(NO)4] and [Fe4S2(NO)2] (Roussin s red and black salts, respectively) and the neutral compounds [Fe2S2(NO)4] and [Fe4S4(NO)4]. Roussin s black salt has found use as a NO releasing vasodilator. 

A. N. Andriotis, N. Lathiotakis and M. Menon, Magnetic properties of Ni and Fe clusters A tight binding molecular optimization study , Chem. Phys. Lett, (in press (1996)). ... 

The biologically uncommon Ni center associated with FeS clusters is a powerful and unique catalytic unity. In this chapter we have reviewed the structural and mechanistic aspects of three NiFeS centers the active site of hydrogenase and Clusters A and C of CODH/ACS. In the former, the association of a Ni center with the most unusual FeCOCN2 unit is a fascinating one. Model chemists, spectroscopists, and crystallographers have joined efforts to try and elucidate the reaction mechanism. Although a consensus is being slowly reached, the exact roles of the different active site components have not yet been fully established. Ni appears to be the catalytic center proper, whereas the unusual Fe center may be specially suited to bind a by-... 

Mossbauer spectroscopy has been used to characterize the iron clusters in fuscoredoxin isolated from D. desulfuricans (133). The authors explained why the iron nuclearity was incorrectly determined, and studied the protein in three different oxidation states fully oxidized, one-electron reduced, and two-electron reduced. The error made in determining the iron cluster nuclearity was caused by the assumption that in the as-purified fuscoredoxin, cluster 2 is in a pure S = state. This assumption was proven to be false and unnecessary. In fact, the observation of four resolved, equal intensity (8% of total Fe absorption) spectral components associated with the S = i species in the as-purified protein is consistent with cluster 2 being a tetranuclear Fe cluster. The 4x8 = 32% Fe absorption for the four components indicates that only 64% of clusters 2 are in the S = state (the total Fe absorption for cluster 2 is 50% of the total Fe absorption). The remaining clusters 2 are in a different oxidation state, the spectrum of which is unresolved from that of cluster 1. 

Another SBU with open metal sites is the tri-p-oxo carboxylate cluster (see Section 4.2.2 and Figure 4.2). The tri-p-oxo Fe " clusters in MIL-100 are able to catalyze Friedel-Crafts benzylation reactions [44]. The tri-p-oxo Cr " clusters of MIL-101 are active for the cyanosilylation of benzaldehyde. This reaction is a popular test reaction in the MOF Hterature as a probe for catalytic activity an example has already been given above for [Cu3(BTC)2] [15]. In fact, the very first demonstration of the catalytic potential of MOFs had aheady been given in 1994 for a two-dimensional Cd bipyridine lattice that catalyzes the cyanosilylation of aldehydes [56]. A continuation of this work in 2004 for reactions with imines showed that the hydrophobic surroundings of the framework enhance the reaction in comparison with homogeneous Cd(pyridine) complexes [57]. The activity of MIL-lOl(Cr) is much higher than that of the Cd lattices, but in subsequent reaction rans the activity decreases [58]. A MOF with two different types of open Mn sites with pores of 7 and 10 A catalyzes the cyanosilylation of aromatic aldehydes and ketones with a remarkable reactant shape selectivity. This MOF also catalyzes the more demanding Mukaiyama-aldol reaction [59]. 

In recent years, several model complexes have been synthesized and studied to understand the properties of these complexes, for example, the influence of S- or N-ligands or NO-releasing abilities [119]. It is not always easy to determine the electronic character of the NO-ligands in nitrosyliron complexes thus, forms of NO [120], neutral NO, or NO [121] have been postulated depending on each complex. Similarly, it is difficult to determine the oxidation state of Fe therefore, these complexes are categorized in the Enemark-Feltham notation [122], where the number of rf-electrons of Fe is indicated. In studies on the nitrosylation pathway of thiolate complexes, Liaw et al. could show that the nitrosylation of complexes [Fe(SR)4] (R = Ph, Et) led to the formation of air- and light-sensitive mono-nitrosyl complexes [Fe(NO)(SR)3] in which tetrathiolate iron(+3) complexes were reduced to Fe(+2) under formation of (SR)2. Further nitrosylation by NO yields the dinitrosyl complexes [(SR)2Fe(NO)2], while nitrosylation by NO forms the neutral complex [Fe(NO)2(SR)2] and subsequently Roussin s red ester [Fe2(p-SR)2(NO)4] under reductive elimination forming (SR)2. Thus, nitrosylation of biomimetic oxidized- and reduced-form rubredoxin was mimicked [121]. Lip-pard et al. showed that dinuclear Fe-clusters are susceptible to disassembly in the presence of NO [123]. 

Figure 4. Fe cluster ionization thresholds as a function of cluster size, as determined by photoionization yield measurements using tunable UV/VUV laser radiation.

Figure 5b. Variation in the magnetic properties of metal clusters are investigated by measuring the depletion of a highly collimated cluster beam by an inhomogeneous magnetic field. Fe clusters and their oxides (FexO and Fex02) at several applied fields. The uniform depletion of Fe clusters indicates that their magnetic moments increase approximately linearly with number of atoms, as would be anticipated for incipient ferromagnetic iron. Unexpected, however, is the much larger depletion of iron oxide clusters.

Figure 2.13 STM images of Fe clusters on Al203/Ni3AI(1 1 1) taken for different sample bias voltages. The image size is 48 nm x 48 nm. The labeled particles were used for the evaluation of the particle height and diameter [44].

Figure 2.14 Apparent cluster height (a) and diameter (b) as a function of bias voltage. Data were obtained from the Fe clusters shown in Figure 2.13 [44],...

Figure 5.23 Ring of 48 tip-generated Fe clusters on Au(1 1 1) in l-butyl-3-methyl-imidazolium BF4 + approximately 50mM FeCl3. (Reproduced with permission from Ref. [95].)...

Phosphino-metallaborate complexes in Section 12.13.6.7 can be applied to N2 fixation and modeling systems for electron-transfer processes occurring in biological systems Fe(l)-Fe(m)/Fe(m)-Fe(l), such as the reducing FeS clusters of certain metaloenzymes <2003JA322>. 

FIGURE 13.2 An EPR-monitored redox titration of an Fe-O-Fe cluster with three stable oxidation states. The dinuclear iron center (= +210 mV and = +50 mV) in Pyrococcus furio-sus ferritin was titrated in the presence of a mediator mix. The fit is based on Equation 13.14. (Data from Tatur and Hagen 2005.)... 

The 8-Fe proteins contain two four-iron-sulfur cubes which are separated (center-to-center) by about 12 A. The EPR spectra from the fully reduced proteins are typical for a 4-Fe center with intercluster spin-spin coupling. An ENDOR study on the 8-Fe ferredo-xin from C. pasteurianum in the fully reduced state has been reported by Anderson et al.273. Since the 57Fe-56Fe difference spectra are only poorly resolved, analysis of the ENDOR data turned out to be difficult. All eight iron atoms are assumed to have similar AFe tensors with principal values Afe = 25 MHz, AFe = 29 MHz and AFe = 33 MHz. These coupling parameters suggest that the reducing electrons are delocalized over the four irons of each of the two nearly identical 4-Fe clusters. 

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