Cluster degradation

Cluster-expansion and cluster-degradation reactions are a feature of many polyhedral borane species. Examples of cluster-expansion are " ... 

Cluster degradation has already been mentioned in connection with the unsymmetrical cleavage reaction (p. 165) and other examples are ... 

Conversely, cluster degradation reactions lead to more open structures, e.g. ... 

Controlled cluster degradation is more difficult than controlled expansion, due to the problem of preventing total degradation. For this reason few new clusters have been obtained as yet by this method. However, the mixed carbido iron clusters show possible future applications in providing a convenient entry via a cluster of an inexpensive metal which is partially degra-dated at the end of a synthesis sequence. 

Deprotonation reactions are not so facile. Such bases as OH and NR3 have to be used with caution as they can lead to cluster degradation via attack of the base on the carbonyl ligands. Inkrott and Shore (94) reported a deprotonation method using KH, and we (80) have successfully employed this procedure to deprotonate FeRu3H2(CO)13, Eq. (83). 

As with oxidation, the reduction of HNCC can be complicated by redox condensation reactions and by cluster degradation induced by free carbon monoxide. The latter is most commonly observed when reduction is effected... 

The ejection of CO and subsequent cluster degradation may be avoided if nucleophiles such as OH , OR , H , and R are used instead. These species oxidize CO to COj simultaneously with cluster reduction. It is believed that the mechanism for this reaction in HNCC is similar to that known for mononuclear complexes [Eqs. (28) and (29)] 418). 

It should be clear from the discussion of Section 2 that all of these clusters can be unstable with respect to cluster degradation in the presence of Lewis bases with the formation of saturated fragment species (see Boron Hydrides and Polynuclear Organometallic Cluster Complexes). Thus, in the presence of an excess of phosphine, the reaction of [HFe3(CO)9(/u-H)2BH] changes from that shown in equation (10) to that shown in equation (12). The mononuclear boron and iron products demonstrate that both transition metal and main group element vertices are removed by base and the cluster is degraded. ... 

The oxidative addition may be coupled with reductive elimination of Hj, resulting in cluster degradation ... 

The chemistry developed from T3T-I9 mutants is summarized in Fig. 4, and the main conclusions are as follows (1) Tyr-19 excludes H2O from the cluster binding pocket (2) degradation is not mediated by reactive oxygen species and (3) cluster degradation arises through hydrolysis of the oxidized cluster. The influence of the charged residues or increased solvent access on the of the cluster is discussed in Section IV,C. 

Our preliminary experiments on whole cells show that the molecular basis of the FNR inactivation in aerobic medimn, namely the [4Fe-4S] + cluster degradation via a [2Fe-2S] intermediate, previously deduced from studies of purified FNR, is valid also in vivo. It is not yet clear whether the 4Fe 2Fe cluster conversion observed in vitro as well as in vivo puts the protein in a ready state from which the 4Fe cluster can be rebuilt once the bacterial environment becomes anaerobic again. Conceivably, the [2Fe-2S] cluster may be just an intermediate in the pathway of complete aerobic degradation of the 4Fe-4S cluster. These are questions which need to be addressed by future studies. 

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