Cubanes structure

Figure 10.12 (a) Prismane structure of [Ge6 CH(SiMej)2)6l (for clarity only the ipso C atoms of the R groups are shown), (b) The tetracycio structure of [GegBugBrj] with only the ipso C atoms of the Bu groups shown, (c) The cubane structure of [Geg(CMeEt2>g] again with only the ipso C atoms of the t-hexyl groups shown. 

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... 

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).

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. 

Zinc carbamate complexes are well known, and the structural types and stabilities can be compared with thiocarbamates and dithiocarbamates which are discussed in Sections 6.8.11.1.3 and 6.8.7.1.4482 Carbamates of zinc can be formed from the reaction of carbon dioxide with alkylzinc alkyl amides and further reaction with alkylzinc can give a distorted cubane structure.483 The tetrameric diethylcarbamate species initially formed can also be used to produce monomeric or dimeric carbamate structures in reaction with amines tetramethylethylenediamine forms a monomer [(Me2NCH2)2Zn(02CN(C2H5)2)2] with an octahedral zinc center and pyridine forms a dimer[CsH5NZn2Me(02CN(C2H5)2)3] with tetrahedral zinc centers.484... 

Investigations on the thermal decomposition of 32 revealed the continuous loss of CO with concomitant formation of the di- and tetranuelear Tc(I) clusters [TcX(CO)4]2 (62) and [TcX(CO)3]4 (63) respectively. The latter is assumed to have cubane structure comparable to [Tc(OHXCO)3]4 [76,77,78]. The thermal decomposition reactions are depicted in Scheme 13. 

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. 

An interesting oxycarbonyl cluster has been isolated in the reaction of 0s04 with CO under pressure. This was an intermediate in the preparation of the Os3(CO)i2. The X-ray analysis has established this as a cubane structure, with an oxygen bridging the four faces of the osmium tetrahedron. The Os-Os distance is 3.20 A and implies no bonding between the osmium centers. This molecule is of obvious interest as a potential model in the studies of carbon monoxide interaction with metal oxides and also metal surfaces, when the formation of metal oxides occurs (200). 

The interconnection of two phosphanide substituents by sterically demanding RR Si groups allows the preparation of cage compounds [38], The two-fold lithia-tion of bis(phosphanyl)(alkyl)arylsilane and the dimerization of this bis(phos-phanido)silane leads to the formation of 19 with a strongly distorted Li4P4 hetero-cubane structure according to Eq. (5). 

Furthennore, resonance Raman studies of aconitase by Johnson et al. (53) demonstrated homologous spectra for both inactive and active aconitase. This suggests similar vibrational modes and thus similar core structures for the two forms. Finally, a cubane structure for the [3Fe-4S] cluster is supported by recent protein crystallographic studies of inactive aconitase by Robbins and Stout (54). (Recent results from Jensen s group (55) on the redetermination of the crystal structure of the Azotobacter ferredoxin I clearly show that the 3Fe cluster does not have a [3Fe-3S] ring structure, as originally determined (37), but has a [3Fe-4S] cubane structure.)... 

Use of the tripodal ligand Saltren creates a tetranuclear complex [Mn402(Sal-tren)2][MnCl4] 2CH3CN (172) having a fused open cubane structure without a carboxylate group. At 300 K the value of the effective magnetic moment for (172) is 5.1 /Ub, which decreases to 2.0 /iB at 4.2 K. The redox properties of (172) in propylene carbonate have been studied by cyclic voltammetry. Two oxidation responses are observed at = —0.32V and 0.43 V vs. SCE. 

Perhaps one of the most elegant object lessons demonstrating the importance of careful compositional analysis comes from the work by Beinert et al. (1983) on the 3Fe cluster. Even with structural analysis by numerous sophisticated spectroscopic methods and by X-ray diffraction, there was substantial controversy about whether the cluster was a modified cubane structure with four inorganic sulfurs and three ligands or a more open chair structure with three sulfurs and six ligands. The analysis of aconitase indicated four sulfurs and resolved the question in favor of the modified cubane. Further X-ray diffraction studies confirmed the cubane 3Fe 4S cluster for aconitase and ferredoxins from A. vinelandii and Desulfovibrio pgas (Stout et al., 1988 Stout 1988 Kissinger et al., 1989). 

FIGURE 5. Cubane structure of the tetrameric compounds RMgOR. The tetramers are thermodynamically favoured when the alkyl groups are small and the solvent is only weakly co-ordinating... 

FIGURE 4. Tetrameric cubane structure of 68 with hydrogens omitted... 

FIGURE 56. Proposed hetero-cubane structures for EtZnCl and EtZnBr... 

The formation of tetrameric aggregates with a hetero-cubane structure is a structural motif that is observed for several alkylzinc alkoxides in the solid state (Figure 80). In these structures the alkoxy group is -bonded with its oxygen atom to three zinc atoms. The structural elucidation of [MeZnOMe]4 (152) represented the first example of such a structural motif in organozinc alkoxide chemistry . Later, similar structures were observed for alkylzinc alkoxides containing other alkoxy groups (153, = f-... 

R = halomethyl, R = benzyloxy ) bound to zinc. Also, compounds containing a diorganoboryl oxide (157) or a triorganosiloxy group (158) form tetrameric aggregates with a hetero-cubane structure. 

More complicated structures of alkylzinc alkoxides have been observed in the solid state with the stoichiometry R eZn7(OR )8. These compounds have a hetero-di-cubane structure (two hetero-cubanes sharing a corner), and have been observed for Me6Zn7(OMe)g (159)240,241 Et6Zu7(OMe)8 (160)242 (ICH2)6Zu7(OMe)8 (161)2 (Figure 80). 

FIGURE 80. Alkylzinc alkoxides having a hetero-cubane structure... 

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