Metal clusters isomerization

Several reports dealing with metal-cluster isomerism and possible mechanisms of polyhedral rearrangements have recently been published. Although work in this field was first undertaken by Braunstein in 1991,no further studies have subsequently appeared. Given the increasing interest in cluster isomerism, the purpose of this article is to provide an updated systematic classification to facilitate the study of this area. 

Full catalyst formulations consist of zeolite, metal and a binder, which provides a matrix to contain the metal and zeolite, as well as allowing the composite to be shaped and have strength for handling. The catalyst particle shape, size and porosity can impact the diffusion properties. These can be important in facile reactions such as xylene isomerization, where diffusion of reactants and products may become rate-limiting. The binder properties and chemistry are also key features, as the binder may supply sites for metal clusters and affect coke formation during the process. The binders often used for these catalysts include alumina, silica and mixtures of other refractory oxides. 

It has been reported that the size of the metal cluster frame of Ru6Pt3(CO)2i( X3-H) ( X-H)3 remains on Y-AI2O3 and MgO after its impregnation and decarbonylation under He at 300 °C [65, 66]. The metallic clusters were rather strongly bound to both supports, Y-AI2O3 and MgO. The catalytic behavior of these materials in n-butane hydrogenolysis shows the suppression of the isomerization reaction according with an intimate association of Pt with Ru atoms. 

Similar reactions applied to transition metal-acetylene complexes appear capable of separating the 2 carbon atoms originally linked by the acetylenic triple bond 18). Thermal isomerization of metal-acetylene complexes may achieve the same result, showing how metal clusters can catalyze scrambling reactions of acetylenes, e.g.. 

A number of novel products have been isolated from the reaction of [B Hg]- [31426-87-6] and CoCl and [C HJ in THF (162,163). The predominant product is /< -2-(CpCo)-B4H8 [43061-99-0]. Also obtained are isomeric clusters containing up to four cobalt atoms, eg, (t]5-C5H5Co)4B4H8 [59370-82-0]. Characterization of these clusters indicate an unusual 2n framework electron count having geometries reminiscent of stricdy metallic clusters (11,164). 

Structures of small metal clusters-II Phase transitions and isomerization (with D.G. Vlachos and L.D. Schmidt). J. Chem. Phys. 96, 6891-6901 (1992). 

Mixed metal clusters (clusters containing two different metals) have considerable potential for mechanistic studies. Three separate studies on iron mthenium clusters show the possibilities. Reactions of FeRu2(CO)i2 and Fe2Ru(CO)i2 in comparison to Fe3(CO)i2 and Ru3(CO)i2 show a very interesting activation of the iron center towards CO dissociation by ruthenium centers in the mixed metal-cluster system. Such an activation of the iron center by ruthenium has also been demonstrated for (/r-H)FeRu2(/(r-COMe)(CO)io. The presence of different metal centers for H2FeRu3(CO)i2 allowed unusually detailed interpretation of the isomerization, substitution, and CO exchange reactions. ... 

In butterfly clusters the chemical shift is dictated by the nature of the wing-tip metals. [Ru4N(CO)i2] appears at 519 ppm, while [Fe4N(CO)i2] is located at 619 ppm. In the mixed-metal cluster [FeRu3N(CO)i2] two isomeric forms (32 and 33) exist and interconvert [Eq. (59)] 43). 

A transition metal cluster complex [Ru3H(CO)n] catalyzes isomerization of allylic alcohols to saturated aldehydes [11]. 

Some reactions such as alkene isomerization, alkene hydrogenation, and H2 -I- D2 exchange can be used as sensitive chemical probes of the coordination environment of metal atoms associated with surface-bound metal clusters. Other catalytic reactions such as CO -I- H2 and alkane hydroge-nolysis, which are sensitive to metal ensemble sizes, are applied as a further structural probe. Several attempts have been made to stabilize cluster frameworks in such a way that catalytic activity is maintained. One of the more promising approaches involves the introduction of a capping group into the... 

Isomerization of Olefins and Dienes Catalyzed by Various Transition Metal Clusters... 

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