Cubanes metallation

There are some small differences between the metallic boxes used in Cuba and Campeche. On figures 1 and 2 it can be observed that Cuban metallic boxes are relatively more closed than the one used at Campeche PCGM corrosion station. 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

Figure 13.24 Structure of the cubane-like mixed metal-metal cluster complex [Sb4- Co(CO)3l4].

It contains a Hg atom bridging two Co atoms. Similarly, [Tj -Cp(CO)3MoMoHg]4 forms by reduction of [Mo(CO)3Cp-Tj ]2 by Na/Hg in the presence of 2-chlorobutene and THF. The central metal atoms are arranged in a cubane structure... 

The elucidation of the crystal structures of two high-spin EPR proteins has shown that the proposals for novel Fe-S clusters are not without substance. Two, rather than one novel Fe-S cluster, were shown to be present in nitrogenase, the key enzyme in the biotic fixation of molecular nitrogen 4, 5). Thus the FeMoco-cofactor comprises two metal clusters of composition [4Fe-3S] and [lMo-3Fe-3S] bridged by three inorganic sulfur atoms, and this is some 14 A distant from the P-cluster, which is essentially two [4Fe-4S] cubane moieties sharing a corner. The elucidation of the crystal structure of the Fepr protein (6) provides the second example of a high-spin EPR protein that contains yet another unprecedented Fe-S cluster. 

Fig. 6. A schematic view of the [3Fe-4S] Emd [4Fe-4S] cores, as versatile structures. The absence of one site leads to the formation of a [3Fe-4S] core. The cubane structure can incorporate different metals (in proteins, M = Fe, Co, Zn, Cd, Ni, Tl, Cs), and S, N, O may be coordinating atoms from hgands (Li). The versatihty csm be extended to higher coordination number at the iron site and a water molecule can even be a ligand, exchangeable with substrate (as in the case of aconitase (,87)). The most characteristic binding motifs are schematically indicated, for different situations proteins accommodating [3Fe-4S], [4Fe-4S], [3Fe-4S] + [4Fe-4S], and [4Fe-4S] -I- [4Fe-4S] clusters. A disulfide bridge may replace a cluster site (see text).

This supramolecular approach has been used as a way to facilitate the crystallization of cuboidal aqua ions and to capture single intermediates present in complex solution mixtures. For example, [M3Q4Clx(H20)9.x] (Q = S, Se) supramolecular adducts have been isolated for x =1-5 from aqueous HCl solutions by varying the acid concentrations where the aggregates contain in each case only one of all possible isomers [46]. In addition to the hydrogen bonds, other kinds of interactions, namely C1---H20, C1---C1 and Q---Q contacts, are also involved in the network propagation. This supramolecular approach has also been efficiently employed for the crystallization of a large number of aqua complexes within the [M3M Q4] cubane-type family, where M is a transition or post-transition metal [47]. 

Herberhold reported activation of water with Re2(CO)io. The reaction proceeded at 200°C to give a tetranuclear complex [Re(CO)3( X -OH)]4 (23) in quantitative yield, and evolution of dihydrogen and CO was observed (Eq. 6.13) [21]. Complex 23 has a pseudo-cubane structure without metal-metal bonds in which the Re(CO)3 groups are linked by triply-bridging OH ligands. Also in this case, no presumed intermediate hydrido(hydroxo) species was detected. 

With other transition metal sulfides the mixed vanadium-silver cubane-like cluster, (Et4N)2[Ag2V2S4 S2CN(OC4H8)2 2(SPh)2] (166)1092 and [ y-C5Me5)WS3 2Ag3 (PPh3)3]N03 (167), have been reported.1093... 

The colorless tetramer Ccu(py)l] is fairly stable only in non- or weakly coordination solvents such as benzene, CH Cl, or acetone. At room temperature in solution this copper complex shows an intense red photoluminescence ( 0.04 a f nax = nm (54). The emitting state is a metal-centered 3d 4s excited state which is strongly modified by Cu(I)-Cu(I) interaction in the tetramer. This consists of a (Cul) cubane core. 

Only a small number of heavier group 1 metal primary amides has been structurally characterized. Table 2 lists six compounds, three containing sodium and three containing cesium. The structures characterized contain the well-established M2N2 ring motif to build dimers 280 and 281, a tetrameric cubane 282, and polymers 283-285. The... 

Amino acid is one of the most important biological ligands. Researches on the coordination of metal-amino acid complexes will help us better understand the complicated behavior of the active site in a metal enzyme. Up to now many Ln-amino acid complexes [50] and 1 1 or 1 2 transition metal-amino acid complexes [51] with the structural motifs of mononuclear entity or chain have been synthesized. Recently, a series of polynuclear lanthanide clusters with amino acid as a ligand were reported (most of them display a Ln404-cubane structural motif) [52]. It is also well known that amino acids are useful ligands for the construction of polynuclear copper clusters [53-56], Several studies on polynuclear transition metal clusters with amino acids as ligands, such as [C03] [57,58], [Co2Pt2] [59], [Zn6] [60], and [Fe ] [61] were also reported. 

---