Heterometal

At potentials above -100 mV, these clusters undergo loss of the heterometEd ion with concomitant formation of [Fe3S4]+ clusters. 

These studies of protein-bound heterometallic cubanes have amply demonstrated that the heterometal site is redox active and able to bind small molecules. Although they have yet to be identified as intrinsic components of any protein or enzyme (except as part of the nitrogenase FeMo cofactor cluster (254)), they are clearly attractive candidates for the active sites of redox enzymes. 

C. Cluster Stmcturrd Interconversions and S3mthesis of Heterometal Clusters Fuscoredoxin (Novel Fe-S Cluster)... 

Fig. 5. The structure of D. gigas Fdll monomer, showing the [3Fe-4S] core, the disulfide bridge, and the tracing of the polypeptide chain. Indicated in the lower part is Cys 11, not bound to the cluster, that is the ligand used for the structural switch from a tri- to a tetranuclear core (also involved in heterometEd cluster formation).

C. Cluster Structural Interconversions and Synthesis of Heterometal Clusters... 

The mimicking of the iron-sulfur clusters by synthetic chemistry has been quite successful over the years. One of the last synthetic clusters to be obtained was the [3Fe-4S] cluster (109, 110). This new synthetic compound was useful for the demonstration of interconversion pathways, as well as for the formation of different heterometal clusters beyond those produced in proteins (111). The [3Fe-4S] core... 

A number of proteins containing [3Fe-4S] centers form cubane-like clustes of the type [M,3Fe-4S] (115-120), which were prepared using this facile [3Fe-4S]/[4Fe-4S] conversion pathway. The [3Fe-4S] core present in D. gigas Fdll was the first precursor used for the synthesis of heterometal cores inside a protein matrix, and the first derivative synthesized was the [Co,3Fe-4S] core (121). Similar synthetic products have also been derived in D. africanus Fdlll (118, 119). 

The redox properties of a series of heterometal clusters were assessed by electrochemical and FPR measurements. The redox potentials of derivatives formed in D. gigas Fdll were measured by direct square wave voltammetry promoted by Mg(II) at a vitreous carbon electrode, and the following values were determined 495, 420,... 

The electrochemical behavior of heterometallic clusters has been reviewed clsewbcre."" The interest in examining clusters stems from their potential to act as "electron sinks " in principle, an aggregate of several metal atoms may be capable of multiple redox state changes. The incorporation of heterometals provides the opportunity to tune the electrochemical response, effects which should be maximized in very mixed"-metal clusters. Few very mixed -metal clusters have been subjected to detailed electrochemical studies the majority of reports deal with cyclic voltammetry only. Table XII contains a summary of electrochemical investigations of "very mixed"-metal clusters. 

For the pyrimidines a range of the binding modes offers the possibility for metal-metal interactions (Fig. 24). Many of these combinations have been confirmed by X-ray crystallography, and a discussion of metal metal interactions has been presented (79). The extent of interaction appears to be dependent on both the electronic configuration and the geometry of the metal. Short intermetallic separations (Pt-Pd = 2.49 A) are observed for f raws-Pt(amine)2 derivatives, particularly those in which the filled Pt dz2 orbital can donate electron density into the heterometal dx2 v2 orbital (Fig. 25) (80). 

Sanders (83) constructed a supramolecular assembly of heterometal-lic porphyrins held together by different types of metal-ligand interaction. The team exploited the different kinetic and thermodynamic properties of the pyridine-zinc, carboxylate-tin, and pyridine-... 

Palladium dimethylsilanedithiolato complex is a precursor for Ti—Pd and Ti—Pd2 heterometal-lic complexes.311 The bis-silyl sulfides (R3Si)2S have been widely used to prepare a variety of metal sulfide clusters, because these reagents exploit the strength of the Si—O and Si—Cl bond to... 

Fig. 3.6-5. Molecular structure of the heterometal cluster [Na4Mg4(Pr 2N)g Fe(C5H3)2 ].

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