Bridged Double-Cubane Clusters

Progress on the first four items is described in the following sections. 

Interest in clusters of this type arises primarily from the possibility of examining interactions between component subclusters as dependent on their separation. In proteins magnetic interactions between [4Fe-4S] + (5 = i) clusters are often signaled by the appearance of additional features 

4-Me and 6-Me and, particularly, 5-H make very clear the presence of a structure not encountered previously. 

Electron-transfer coupling across bridges of variable length increases as the separation of subcluster centroids decreases. The order of coupling is 41 S 40e 40b 40a, 40c, and 40d. The effect becomes detectable at 11 A and reaches a maximum at 7.5 A, or less if the bridge angle is overestimated. With reference to Fig. 28, it is seen that subcluster sepa- 

The cluster set 40 is merely one of a number with different bridging groups that can be conceived. An interesting question, but one difficult to explore experimentally because of the values of. com for comproportionation reaction 16 (Fig. 27), is that of electron-transfer rates between sub- 

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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 5.2 Representative sulfide-bridged double cubane clusters.

Nitrogenase catalyzes the ATP-driven eight-electron and eight-proton reduction of N2 to 2 NH3 and H2 and offers the most complex and fascinating example of Fe-S cluster mediated electron transfer. The electron transport chain involves transfer of electrons from the subunit-bridging [4Fe-4S] center on the homodimeric Fe-protein, to the subunit-bridging double-cubane [8Fe-7S] P-cluster (see... 

Hydroxide-promoted core conversions of molybdenum-iron-sulfur edge-bridged double cubanes Oxygen-ligated topological PN clusters... 

In solution, the heterometallic clusters display some specific reactivity but no evidence of dinitrogen activation. Reduced bridged double cubanes... 

Scheme 5.6 Formation of all-sulfide P -type clusters via edge-bridged double cubanes.

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