Cluster fragmentation, catalysis

Phosphine-substituted complexes have shown promise for cluster catalysis, especially in the case of chelating ligands, because of the added stability that might help avoid cluster fragmentation. Bergounhou et al. reported a detailed study of the hy-... 

Many reactions of metal cluster complexes occurring by electron-transfer catalysis at low temperatures lead to substitutions without fragmentation to monomeric compounds. For example, derivatives of RUj(CO)jj and H Ru fCO) have been prepared using Na PhjCO" as catalyst without detectable byproducts from cluster fragmentation. Successive substitutions on intact clusters often proceed with increasing facility. For example, RUj(CO),2 reacts with less than 0.01 M PBUj at 60 °C to form the trisubstituted complex Ru3(CO) (PBu3)3 as the first observable product. ... 

There are only a few weU-documented examples of catalysis by metal clusters, and not many are to be expected as most metal clusters are fragile and fragment to give metal complexes or aggregate to give metal under reaction conditions (39). However, the metal carbonyl clusters are conceptually important because they form a bridge between catalysts commonly used in solution, ie, transition-metal complexes with single metal atoms, and catalysts commonly used on surfaces, ie, small metal particles or clusters. 

Carboxylate promoters may also be able to coordinate to an Rh(CO)2 fragment, and therefore facilitate a process such as that shown in (45). Reactions of [Rh15(CO)27]3 with H2 in the presence of cesium carboxylates are reported (123) to be consistent with the formation of small equilibrium amounts of Rh(C0)2(02CR) by reaction (45). Carboxylates could therefore be involved in cluster growth or transformation during catalysis. 

In cluster catalysis there is always the possibility that the cluster breaks down and fragments or monomers are the actual catalysts. Os3H2(CO)io [45] in the hydrogenation of alkenes, and Ru4H4(CO)i2 [46] and its phosphine derivatives in the hydrogenation of alkynes, alkenes, and even ketones, seem to catalyze as clusters. The dimer Rh2(OAc)4 [47] is an active hydrogenation calatyst for alkenes. 

In further work of the group of Ervin, the activity of palladium cluster anions in the CO oxidation catalysis was examined and it was found that Pd also efficiently catalyzes the CO combustion reaction. The palladium clusters, however, are reported to exhibit more fragmentation than the platinum clusters, consistent with the weaker meal-metal bond strength in palladium relative to platinum [22]. 

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