Fe centers

The stmcture of Pmssian Blue and its analogues consists of a three-dimensional polymeric network of Fe —CN—Fe linkages. Single-crystal x-ray and neutron diffraction studies of insoluble Pmssian Blue estabUsh that the stmcture is based on a rock salt-like face-centered cubic (fee) arrangement with Fe centers occupying one type of site and [Fe(CN)3] units randomly occupying three-quarters of the complementary sites (5). The cyanides bridge the two types of sites. The vacant [Fe(CN)3] sites are occupied by some of the water molecules. Other waters are zeoHtic, ie, interstitial, and occupy the centers of octants of the unit cell. The stmcture contains three different iron coordination environments, Fe C, Fe N, and Fe N4(H20), in a 3 1 3 ratio. 

All the complexes consist of several subunits (Table 2) complex I has a flavin mononucleotide (FMN) prosthetic group and complex II a flavin adenine dinucleotide (FAD) prosthetic group. Complexes I, II, and III contain iron-sulphur (FeS) centers. These centers contain either two, three, or four Fe atoms linked to the sulphydryl groups of peptide cysteine residues and they also contain acid-labile sulphur atoms. Each center can accept or donate reversibly a single electron. 

Bertini I, Ciurli S, Luchinat C (1995) The Electronic Structure of FeS Centers in Proteins and Models. A Contribution to the Understanding of Their Electron Transfer Properties. 

E. Interaction of the Active Site with Other FeS Centers... 

The atomic temperature factors obtained after crystallographic refinement are significantly higher for cys530 than for the other site cysteine residues. This is also true when the Ni ion is compared to the Fe center. This may reflect conformational disorder due to the fact that the crystals are made of a mixture of different Ni states (40% Ni-A, 10% Ni-B, and 50% of an EPR-silent species) (52). 

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-... 

Although the crystal structure of CODH or CODH/ACS has not yet been solved, a great deal of work has been done on these enzymes and plausible catalytic mechanisms have been proposed. Concerted action between the Ni ion and one of the Fe centers of a 4Fe-4S cluster are thought to elicit the formation of CO2 from CO. But perhaps the most extraordinary reaction is the one catalyzed by Cluster A the insertion of CO to a Ni-CHs complex. Through the two reactions catalyzed by CODH/ACS, the highly toxic, CO is not only removed, but is used as a source of carbon and electrons. 

The proposed mechanism of H2 evolution by a model of [FeFeJ-hydrogenases based upon DFT calculations [204-206] and a hybrid quanmm mechanical and molecular mechanical (QM/MM) investigation is summarized in Scheme 63 [207]. Complex I is converted into II by both protonation and reduction. Migration of the proton on the N atom to the Fe center in II produces the hydride complex III, and then protonation affords IV. In the next step, two pathways are conceivable. One is that the molecular hydrogen complex VI is synthesized by proton transfer and subsequent reduction (Path a). The other proposed by De Gioia, Ryde, and coworkers [207] is that the reduction of IV affords VI via the terminal hydride complex V (Path b). Dehydrogenation from VI regenerates I. 

Abstract The unique and readily tunable electronic and spatial characteristics of ferrocenes have been widely exploited in the field of asymmetric catalysis. The ferrocene moiety is not just an innocent steric element to create a three-dimensional chiral catalyst enviromnent. Instead, the Fe center can influence the catalytic process by electronic interaction with the catalytic site, if the latter is directly coimected to the sandwich core. Of increasing importance are also half sandwich complexes in which Fe is acting as a mild Lewis acid. Like ferrocene, half sandwich complexes are often relatively robust and readily accessible. This chapter highlights recent applications of ferrocene and half sandwich complexes in which the Fe center is essential for catalytic applications. 

As a consequence of the molecular orbital interactions, ferrocene adopts an axially symmetrical sandwich structure with two parallel Cp ligands with a distance of 3.32 A (eclipsed conformation) and ten identical Fe-C distances of 2.06 A as well as ten identical C-C distances of 1.43 A [12]. Deviation of the parallel Cp arrangement results in a loss of binding energy owing to a less efficient orbital overlap [8]. All ten C-H bonds are slightly tilted toward the Fe center, as judged from neutron-diffraction studies [13]. 

As mentioned above, ferrocene is amenable to electrophilic substitution reactions and acts like a typical activated electron-rich aromatic system such as anisole, with the limitation that the electrophile must not be a strong oxidizing agent, which would lead to the formation of ferrocenium cations instead. Formation of the CT-complex intermediate 2 usually occurs by exo-attack of the electrophile (from the direction remote to the Fe center. Fig. 3) [14], but in certain cases can also proceed by precoordination of the electrophile to the Fe center (endo attack) [15]. 

Due to the pronounced electron donating character of ferrocene, ot-ferrocenyl carbocations 3 possess a remarkable stability and can therefore be isolated as salts [16]. They can also be described by a fulvene-type resonance structure 3 (Fig. 4) in which the Fe center and the ot-center are significantly shifted toward each other as revealed by crystal stmcture analysis, indicating a bonding interaction [17]. 

X-ray crystal stmcture analysis of 10 and 11 has revealed a close contact between Fe and the Li ion of ca. 2.5 A. Moreover, the Li ion resides close to the olefin units. Stabilization of the electron-rich Fe center by the 7t-accepting character of the olefins is evidenced by the significant elongation of the C=C bonds to ca. 

X-ray crystallographic analysis of Fe[0Si(0 Bu)3]3THF revealed a distorted tetrahedral geometry (toward a trigonal pyramid) at the Fe center. Related alkyl siloxide complexes of Fe(III) with dimeric structures, [Fe(OSiMe3)3]2 and [Fe(OSiEt3)3]2, have been reported by Schmidbaur and Richter [98]. 

In summary. SRPAC is a powerful tool that yields information about both rotation rates and rotation mechanisms. This method can be extended to molecules other than FC, provided they possess a finite EFG at the Fe center. 

HF/STO-3G Amber - for R2met only). In all models the QM part consists of the two Fe centers and the first shell ligands of four formates, two imidazoles, and a few oxo, hydroxo, and/or aquo groups (see Figure 2-4). 

Reaction of the Ni11 thiolate species [Ni(L)] (L = /V,/V -diethyl-7V,7V -bis(2-mercaptoethyl)-l,3-propanediamine) with the tetraiodo cluster anion [Fe4S4I4]2 yields [Ni(L)(Fe4S4I2)(L)Ni] (793).1984 It incorporates a dithiolate bridge between Ni and Fe centers with a Ni—Fe distance of 2.827(1) A and exhibits a quasi-reversible oxidation wave at 1/2 = +0.15V (vs. SCE). The corresponding monosubstituted cluster anion [Ni(L)Fe4S4I3] (794) was also reported.1985... 

---