Aggregation insensitivity

Finke has reported remarkable catalytic lifetimes for the polyoxoanion- and tetrabutylammonium-stabi-lized transition metal nanoclusters [288-292]. For example in the catalytic hydrogenation of cyclohexene, a common test for structure insensitive reactions, the lr(0) nanocluster [296] showed up to 18,000 total turnovers with turnover frequencies of 3200 h [293]. As many as 190,000 turnovers were reported in the case of the Rh(0) analogue reported recently. Obviously, the polyoxoanion component prevents the precious metal nanoparticles from aggregating so that the active metals exhibit a high surface area [297]. 

All intermediate species produced by the absorption of radiation (electrons, ions, excited states, free radicals, etc.) may be potentially useful for synthesis. However, the most frequently used intermediates are the free radicals. Their yield is high and relatively insensitive to temperature or state of aggregation (Wagner, 1969). 

The accessibility of new techniques such as EXAFS brings researchers a powerful tool for unambiguous determination of the true core metallic framework of such systems. Thus, the relationship between the parent carbonyl precursor, the support and the final metal-supported particles has been studied at the structural atomic level in some cases. This can allow differentiation of the catalytic behavior of supported metal particles with bulk-like properties from that of supported metal clusters, opening the way to understanding the mechanism of metal-catalyzed reactions and extending the concept of sensitive or insensitive structure reactions from metal aggregates to clusters. 

Consequently, all the data in Figure 2 can be explained in terms of the different reactivities of the various aggregates. The insensitivity of the rate of thiocyanate entry to the thiocyanate concentration simply shows that, over the whole concentration range studied, the substrate was always in the form of the ion triplet. The rate dependence on the chloride or bromide concentration represents the change in the distribution of the substrate between the 2 1 and the 3 1 aggregate as the anion concentration is increased. 

The destabilization of the premlcellar aggregates at high water content may give rise to a) separation of liquid water, b) formation of Inverse micelles, or c) separation of a lamellar liquid crystal. Approximate calculations using the Tanford-Nlnham approach gave correct Information for a model system, but the critical ratio appeared too Insensitive to the alcohol/soap ratio to be useful. 

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