Metal-Sulfur Clusters

Let us consider briefly but systematically the redox properties of a few homonuclear metal-sulfur clusters in order of increasing metal atom number. 

There exist a number of homo metal-sulfur clusters possessing an M3S core (M = Fe, Co, Ni, Mo, W n = l l)la, but the most represented assemblies are M3S2 and M3S4, respectively. The latter is present in a few biological functions as an Fe3S4 unit2 (see also Chapter 12, Section 3). 

The formal oxidation state of the central Mo atom is + 6, whereas that of the two outer Mo atoms is 0. Such a dianion undergoes in MeCN solution a one-electron reduction ( 2-/3- = —1.72 V vs. SCE), which is only partially chemically reversible, and an ill-defined irreversible oxidation (Ep = + 0.14 V).7a 

It has been assumed that the reduction process is centred on the central, high-valent, Mo atom, whereas the oxidation process is centred on the outer, low-valent, Mo atoms. 

Similar redox behaviour is exhibited by the isostructural trianion [Fe3(/i-S)4(SPh)4]3-. a 8 

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Because of possible catalytic and biological relevance of metal-sulfur clusters, several such compounds of cobalt have been prepared. The action of H2S or M2S (M = alkali metal) on a non-aqueous solution of a convenient cobalt compound (often containing, or in the presence of, a phosphine) is a typical route. Diamagnetic [Co6Ss(PR3)6] (R = Et, Ph) comprise an octahedral array of metal atoms (Co-Co in the range 281.7 to 289.4pm), all faces capped by atoms,and show facile redox behaviour... 

Metal polysulfido complexes have attracted much interest not only from the viewpoint of fundamental chemistry but also because of their potential for applications. Various types of metal polysulfido complexes have been reported as shown in Fig. 1. The diversity of the structures results from the nature of sulfur atoms which can adopt a variety of coordination environments (mainly two- and three-coordination) and form catenated structures with various chain lengths. On the other hand, transition metal polysulfides have attracted interest as catalysts and intermediates in enzymatic processes and in catalytic reactions of industrial importance such as the desulfurization of oil and coal. In addition, there has been much interest in the use of metal polysulfido complexes as precursors for metal-sulfur clusters. The chemistry of metal polysulfido complexes has been studied extensively, and many reviews have been published [1-10]. 

Fig. 10. The putative transition-state complex formed between the Fe protein MgADP AlFj and the MoFe protein. For simplicity only one a/3 pair of subunits of the MoFe protein is shown. The polypeptides are indicated by ribbon diagrams and the metal-sulfur clusters and MgADP AlFi" by space-filling models (MOLSCRIPT (196)). The figure indicates the spatial relationship between the metal-sulfur clusters of the two proteins in the complex.

In the presence of the product ethylene during turnover the MoFe protein exhibits an EPR signal, with g values at 2.12, 1.998, and 1.987 129), which has been demonstrated to arise from FeMoco in an S = z spin state 130). However, direct interaction of the ethylene with the metal-sulfur cluster was not demonstrated. 

Some metal-sulfur clusters have been used in anhydrous organic solvents as C02 electrocatalysts. They lead mainly to HCOO-,183-185 except when LiBF4 is used as electrolyte, where oxalate formation is observed.186,187... 

The interest for these ligands will be probably negatively affected by the hydrolytic sensitivity of silthianes, but the cited complexes promise to be useful precursors for metal sulfur clusters. 

Metal-Sulfur Clusters as the Model for the Active Sites of Metalloenzymes... 

Figure 2 Structures of the actrive sites of metalloenzymes containing metal-sulfur cluster units, (a) Fe only hydrogenase, H-cluster (Hoxfarm) (b) Sulfite reductase (c) NiFe carbon monoxide dehydrogenase, C-cluster and (d) NiFe carbon monoxide dehydrogenase, A-cluster, which functions as acetyl-CoA synthase...

Well-Defined Metal-Sulfur Clusters in Metalloenzymes and Syntheses of Their Structural Models... 

On the other hand, such approaches to the metalloenzymes described above in Figures 1 and 2 are still under way. Thus, the model clusters reproducing precisely their complex metal-sulfur assemblies in the native form have not yet been isolated. In this section, the studies aiming at the syntheses of the model compounds of two clusters in nitrogenase, FeMo cofactor and P-cluster, will be surveyed. The choice of these clusters as the representatives of the metal-sulfur clusters in metalloenzymes arises from the fact that these are the largest and most complicated metal-sulfur clusters known at present among those observed in natural enzymes. 

Most common nitrogenase is composed of two proteins, Mo-Fe protein and Fe protein, both of which contain metal-sulfur clusters. The former protein... 

For further details about the progress in the syntheses of the structural models of the clusters in MoFe protein, please see the other recent reviews,33 together with those dealing with the reactions of metal-sulfur clusters relating to the nitrogen-fixation chemistry.34... 

A huge number of transition metal-sulfur clusters have been synthesized,36 most of which have been obtained based on the self-assembly methods. On the other hand, to construct the cluster cores with the desired metal-sulfur compositions and connecting schemes, rational pathways leading to the high-yield syntheses of tailored metal-sulfur clusters have recently been explored. Fragment condensations have been demonstrated to be the powerful methods to obtain such clusters numerously,37 some examples of which are shown below. 

Preparation of Metal-Sulfur Clusters from Dinuclear Precursors... 

As the dinuclear metal-sulfur complexes that can exhibit excellent ability to serve as the precursors to the tailored metal-sulfur clusters, those with the M2S4 cores are known to be used for synthesizing cubane-type and relating sulfido clusters. These include the reactions that start from the complexes having Mo( t-SH)2(jt-S)2Mo,38 M(p-S2)(p-S)2M (M = V,39 Mo40), Ru(p-S2)2Ru,41 and MS(p-S)2MS (M = Mo, W,42 Re,43 Ti44) cores, which are summarized in Figure 10. 

As the preceding discussion of nitrogenase metal-sulfur clusters indicate, analysis of complex bioinorganic systems requires the use of multiple analytical techniques and the cooperative exchange of data and ideas of many researchers. The descriptions in this chapter have attempted to give students some idea of the scope and complexity of instrumental techniques available to the bioinorganic chemist. It has not been intended to be either comprehensive or theoretical in presentation. Students are encouraged to acquaint themselves further with the theory and practice of instrumental techniques, especially those that are important to their particular research interests. 

The mechanism and sequence of events that control delivery of protons and electrons to the FeMo cofactor during substrate reduction is not well understood in its particulars.8 It is believed that conformational change in MoFe-protein is necessary for electron transfer from the P-cluster to the M center (FeMoco) and that ATP hydrolysis and P release occurring on the Fe-protein drive the process. Hypothetically, P-clusters provide a reservoir of reducing equivalents that are transferred to substrate bound at FeMoco. Electrons are transferred one at a time from Fe-protein but the P-cluster and M center have electron buffering capacity, allowing successive two-electron transfers to, and protonations of, bound substrates.8 Neither component protein will reduce any substrate in the absence of its catalytic partner. Also, apoprotein (with any or all metal-sulfur clusters removed) will not reduce dinitrogen. 

The so-called midpoint potential, Em, of protein-bound [Fe-S] clusters controls both the kinetics and thermodynamics of their reactions. Em may depend on the protein chain s polarity in the vicinity of the metal-sulfur cluster and also upon the bulk solvent accessibility at the site. It is known that nucleotide binding to nitrogenase s Fe-protein, for instance, results in a lowering of the redox potential of its [4Fe-4S] cluster by over 100 mV. This is thought to be essential for electron transfer to MoFe-protein for substrate reduction.11 3... 

To successfully describe the structure and function of nitrogenase, it is important to understand the behavior of the metal-sulfur clusters that are a vital part of this complex enzyme. Metal-sulfur clusters are many, varied, and usually involved in redox processes carried out by the protein in which they constitute prosthetic centers. They may be characterized by the number of iron ions in the prosthetic center that is, rubredoxin (Rd) contains one Fe ion, ferredoxins (Fd) contain two or four Fe ions, and aconitase contains three Fe ions.7 In reference 18, Lippard and Berg present a more detailed description of iron-sulfur clusters only the [Fe4S4] cluster typical of that found in nitrogenase s Fe-protein is discussed in some detail here. The P-cluster and M center of MoFe-protein, which are more complex metal-sulfur complexes, are discussed in Sections 6.5.2. and 6.5.3. 

The fourth state with [Fe4S4]° shown in Table 6.1 was recently described as the most reduced form possible for the Fe-protein s [Fe4S4] cluster.16 Usually, only two oxidation states for a given metal-sulfur cluster are stable. Therefore a stable [Fe4S4]° state in Fe-protein s iron-sulfur cluster (as appears likely from experimental evidence presented in reference 16) would be unique because the cluster would then have three stable oxidation states, [Fe4S4]2+/1+/0. It appears also that the all-ferrous state is only stable in the protein-bound cluster and not for model... 

Core extrusion studies—removal of the iron-sulfur cluster intact from the enzyme surroundings—have been carried out and the iron-cluster types in proteins identified through the process shown in equation 6.10.18 DMS0/H20 is the protein unfolding solvent for this process. By this method, Fe-protein and MoFe-protein metal-sulfur clusters have been removed from the holoenzyme for separate analysis by many instrumental techniques. 

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