Sulfur iron polynuclear complexes

The recent discovery that a class of electron mediating proteins, the ferredoxins (Sec. IVE2), characteristically contain two or more iron atoms bridged by sulfur atoms has stimulated interest in polynuclear iron-sulfur complexes. It seems appropriate therefore to review here what little is known about such species. 

A large number of studies devoted to metal-sulfur centers are motivated by the occurrence of such arrangements at the active site of various metalloenzymes [1-13]. Mononuclear complexes with Mo=0 func-tion(s) and possessing sulfur ligands in their coordination sphere have been extensively investigated since they can be seen as models of the active site of enzymes such as nitrate- and DM SO reductases or sulfite- and xanthine oxidases [1-4]. On the other hand, a large variety of mono-, di-, and polynuclear Mo—S centers have been synthesized in order to produce functional models of the Mo-nitrogenase since the exact nature (mono-, di- or polynuclear) of the metal center, where N2 interacts within the iron-molybdenum cofactor (FeMo—co) of the enzyme is still unknown [4-8]. 

The diamagnetic behavior of the diiron and tetrairon complexes, despite the presence of formally d1 and/or d9 iron centers, indicates very strong coupling between the individual paramagnetic centers all theoretical treatments of polynuclear iron-sulfur-nitrosyl complexes to date have been based on the assumption of diamagnetism in even-electron species and have employed molecular orbital methods at various levels of approximation. 

T. Waters, X.-B. Wang and L.-S. Wang, Coord. Chem. Rev., 2007, 251, 474. Review describing the use of electrospray to transfer negatively charged transition metal complexes to the gas phase for photoelectron spectroscopy. Examples discussed include square planar and octahedral halide complexes, metal-metal bonded species, transition metal bis(diihiolene) complexes, and mononuclear and polynuclear iron-sulfur clusters. 

This topic has been reviewed by Ingledew (55). The major components of the respiratory chain for T. ferrooxidans are a cytochrome oxidase of the Ci type, cytochromes c, and the blue copper protein rusticyanin. Initial electron transfer from Fe(II) to a cellular component takes place at the outer surface of the plasma membrane in the periplasmic space. The rate of electron transfer from Fe(II) to rusticyanin is too slow for rusticyanin to serve as the initial electron acceptor. Several proposals have been made for the primary site of iron oxidation. Ingledew (56) has suggested that the Fe(II) is oxidized by Fe(III) boimd to the cell wall the electron then moves rapidly through the polynuclear Fe(III) complex to rusticyanin or an alternative electron acceptor. Other proposals for the initial electron acceptor include a three-iron-sulfur cluster present in a membrane-bound Fe(II) oxidoreductase (39, 88), a 63,000 molecular weight Fe(II)-oxidizing enzyme isolated from T. ferrooxidans (40), and an acid-stable cytochrome c present in crude extracts of T. ferrooxidans (14). 

Iron-iron bonds exist in carbonyl and nitrosyl derivatives, but these are not considered in this section. Properties consistent with Fe-Fe interactions in other systems are dominated by sulfur donor ligands. The presence of Fe-Fe bonds is not unambiguous, with sulfur bridges invariably accompanying short metal-metal separations. Synthetic routes to polynuclear iron-sulfur complexes are presented for completeness without implying the cluster bonding scheme. 

Chalcogenides Solid-state Chemistry Copper Enzymes in Denitrification Copper Hemocyanin/Tyrosinase Models Copper Proteins Oxidases Copper Proteins with Dinuclear Active Sites Copper Proteins with Type 1 Sites Copper Proteins with Type 2 Sites Iron-Sulfur Models of Protein Active Sites Iron-Sulfur Proteins Nickel Enzymes Cofactors Nickel Models of Protein Active Sites Polynuclear Organometallic Cluster Complexes. 

The most recent additions to the family of polynuclear iron-sulfur cluster compounds are those having a hexanuclear six-iron core. Two types have been isolated, exemplified by [Fc5S9(SR)2] " (49) (R = Et, Bu or and [Fe iSj (PEtj) ] (50). The complexes (49) have a delocalized... 

Metal thiolate complexes will reduce elemental sulfur or red selenium via the oxidative elimination of RSSR. In a similar manner, metal selenolate complexes ean be used to reduce elemental selenium. The resulting E ligands favor the formation of polynuclear cluster complexes, due to the greater tendency of E (vs. RE ) to form bridging interactions between metal centers. Originally developed in the synthesis of [Fe4Se4(SPh)4], this method has been well utilized in the synthesis of a number of iron thiolate/sulfide clusters, as well as their selenide counterparts (Equation (5)). More recently, sulfur- and selenium-containing lanthanide clusters (see Section 7.2.5.5) have been synthesized by the displacement of ER from Ln(ER)3 ... 

---