Cubane clusters

An exception to the above types of structure is the cubane cluster in (Aglpiperidine)4 [43],... 

Fig. 3. Ground state spin (S) and valence delocalization schemes for the known oxidation states of [Fe3S4] clusters. Discrete [Fe3S4] clusters have not been observed in siny protein, but they have been identified as fragments in heterometallic cubane clusters. Reduction of the [Fe3S4]+ cluster by three electrons, to yield a putative aU-ferrous cluster, occurs with the concomitant addition of three protons. Key S , grey Fe +, black Fe +, white Fe, white with central black dot.

Cluster 1 is a conventional [4Fe-4S] cubane cluster bound near the N-terminus of the molecule as shown in Fig. 13. Within the cluster the Fe-S bonds range from 2.26 to 2.39 A. The cluster is linked to the protein by four cysteine residues with Fe-S distances ranging from 2.21 to 2.35 A, but the distribution of the cysteine residues along the polypeptide chain contrasts markedly with that found, for example, in the ferredoxins as indicated in Section II,B,4 [also see, for example, 41) and references therein]. In the Fepr protein all four cysteine residues (Cys 3, 6, 15, and 21) originate from the N-terminus of the molecule, and the fold of the polypeptide chain in this region is such that it wraps itself tightly around the cluster, yet keeps it near the surface of the molecule. In such a position the cluster is ideally placed to participate in one-electron transfer reactions with other molecules. 

Fig. 10. An alternative view of the Fepr molecule showing that cluster 1, a [4Fe-4S] cubane cluster, is located toward the outside of the molecule and therefore in a position to participate in one-electron transfer interactions with other appropriate molecules.

Fig. 13. Cluster 1 is a [4Fe-4S] cubane cluster located at the N-terminus of the Fepr molecule and close to the first long helix (residues 24-50 inclusive). The cluster is bound to the protein by four cysteine residues Cys 3, Cys 6, Cys 15, and Cys 21. The distribution of these cysteine residues contrasts mEnkedly with that found in the ferre-doxins.

FIGURE 7.4 Example of a spin-spin interaction spectrum. The complex spectrum is from two adjacent cubane clusters in the enzyme FeFe-hydrogenase from Desulfovibrio vulgaris. The lOOx blowup is to show the extended spectral field range resulting from interaction. 

H4C4BR)Rh(H4C4BR)Rh(H4C4BR) (R-Ph, Me) reaction with I2 — cubane clusters 24... 

CODHs catalyse the oxidation of carbon monoxide in a reversible, two-electron process. They are homodimeric enzymes with five metal clusters, two C-clusters that catalyse the oxidation of CO to C02 and three typical [Fe4S4] cubane clusters (Figure 15.4). 

Electroreductive fixation of CO2 into formate can be efficiently performed by using Fe4S4 cubane clusters (276) bearing a 36-membered methylene backbone in a DMF-Bu4NBF4-(Pt/Hg) system... 

Scheme 106 Fe4S4 cubane cluster for the cathodic fixation of carbon dioxide.

The electrochemical results described above indicate that unlike in the cases of other cobalt-catalyzed oxidation processes where the Co /Co redox couple is invariably involved [19b,38], in the present case where cubane clusters of the general formula Co4(p3-0)4( J,-02-CR)4(L)4 are to be employed as catalysts for the air/02 or TBHP oxidation of alkylaromatics, alcohols, etc., we have a catalytic system wherein the oxidation states of cobalt cycle between +3 and +4. The kinetic inertness of Co(lll) coupled with the inadequately explored reactivity of Co(lV) thus make the catalysts based on C04O4 cubanes quite interesting [36]. We shall now discuss the resulting materials prepared by supporting the cubane-like cobalt(lll)-oxo clusters discussed above in this section by following the chemical route in which the carboxylate anion derived from CMS-CH2CH2CO2H binds the in situ or preformed cobalt(III)-oxo tetramers at elevated temperatures. 

---