Double-Cubane Clusters

The compounds 55-60 have all been prepared, albeit in low yields (8-26%), from the reaction of Sn(NMe2)2 with the appropriate primary amine, under conditions which produce cubanes for non-heterocyclic containing amines. Thus, the formation of the double-cubane structural motif presumably arises from the nature of the heterocyclic substituents on the imido nitrogen centers. Inspection of the solid-state structures of these compounds revealed the presence of short intramolecular heterocyclic N- -Sn contacts, ranging from 2.812(4) to 3.188(6) A, which may play a role in the formation of the double cubanes. The likely formation of double-cubane clusters, which do not contain heterocyclic imido substituents, by metathetical reactions of two equivalents of 53 or 54 with main-group or transition-metal tetrahalides has not yet been investigated. 

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Hauser, C., Bill, E., and Holm, R. H. (2002) Single- and double-cubane clusters in the multiple oxidation states [VFe3S4]3+ 2+11+], Inorganic Chemistry 41, 1615-1624... 

The structures of some eight Mo-Fe-S double cubane clusters of the types shown in Fig. 5 have now been determined by single crystal X-ray diffraction studies. Known structures include one example of 1 (R = Et71,76 ), four examples of 2 (R = Et76, U0) R = Ph109 ... 

Fig. 5. Schematic of the structurally characterized MoFe3S4 double cubane clusters prepared to date...

A fascinating recent development in imidotin-cluster chemistry involves the isolation of a series of double cubanes, which contain an Sn7(tt3-NR)8 core. Wright and coworkers have demonstrated that when pyridinyl or pyrimidinyl groups are present on the imido nitrogen centers, the unusual double-cubane clusters 55-60 are obtained (Scheme 2.2.11), rather than the [Sn()a3-NR)]4 cubanes. These clusters are comprised of two interlocked [Sn( a3-NR)]4 cubanes, which share one tin vertex. The central tin center is formally in the +4 oxidation state, so that these double cubanes may be viewed as involving the coordination of two [Sn3()a3-NR)4] anions, such as those present in the clusters 53 and 54, to a central Sn+" cation. The presence of both Sn(II) and Sn(IV) centers was verified by Sn NMR spectroscopy. The deposition of tin metal was observed during the syntheses of 55-60, suggesting that... 

Reaction of the edge-bridged double cubane cluster [V2Fe6(Tp)2-Sg(PEt3)4] with hydrosulfide affords the clusters [V2Fe6(Tp)2S9-(SH)2]3- 4-.31... 

Figure 27. Summary of information for two intramolecular redox centers, including the potential separation for two independent and identical centers (Eq. 14), the electron-transfer series 15 and comproportionation reaction 16 for double-cubane clusters, and the compro-portionation constant (Eq. 17).

Figure 28. Depiction of the cisoid and transoid conformations of double-cubane cluster 40a, including interatomic and centroid-centroid distances.

The model compounds do not bind or reduce N2 or other nitrogenase However, some iron-vanadium-sulfur clusters will reduce hydrazine to ammonia. In a systematic evaluation of the metal M in double cubane clusters [M2Fe6Sg(SEt)9] (M = V, Nb, Mo, W, Re), it was found that the reduction potential is lower, and the Fe Mossbauer isomer shift higher, for Mo relative to However, the Fe isomer shifts are similar between MoFe and VFe proteins, suggesting that protein interactions (especially hydrogen bonding) are likely to play an important role in determining the electron density at the iron atoms in the FeMoco and... 

Figure 5.2 Representative sulfide-bridged double cubane clusters.

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