Transition metals, cluster production

The microscopic understanding of tire chemical reactivity of surfaces is of fundamental interest in chemical physics and important for heterogeneous catalysis. Cluster science provides a new approach for tire study of tire microscopic mechanisms of surface chemical reactivity [48]. Surfaces of small clusters possess a very rich variation of chemisoriDtion sites and are ideal models for bulk surfaces. Chemical reactivity of many transition-metal clusters has been investigated [49]. Transition-metal clusters are produced using laser vaporization, and tire chemical reactivity studies are carried out typically in a flow tube reactor in which tire clusters interact witli a reactant gas at a given temperature and pressure for a fixed period of time. Reaction products are measured at various pressures or temperatures and reaction rates are derived. It has been found tliat tire reactivity of small transition-metal clusters witli simple molecules such as H2 and NH can vary dramatically witli cluster size and stmcture [48, 49, M and 52]. 

The reactions of some transition metal cluster ions have been described in a review by Parent and Anderson (201). The review covered reactions reported up to 1992 and so the reactions reported here are generally later than 1992. A recent review by Knickelbein (202) discusses the reactions of cation clusters of iron, cobalt, nickel, copper, silver, niobium, and tungsten with small molecules such as H2 and D2. Some of the reactions in Knickelbein s review are included in the following tables of reactions (Tables IV and V). Table IV gives examples of the reactions of transition metal cluster ions and includes the vaporization source, experimental apparatus, the reactants, and the observed product ions. A few examples from these tables will be selected for further discussion. 

Scheme 3.2-29. Hexaalkyl-2,3,4,5-tetracarba-n/do-hexabo-ranes(6) 56a (R -R5 = Me, Et) react in boiling BBr3 by Et/Br exchange, selectively at the 6-position. The products 56b can be used to combine carboranes with transition metal clusters as shown in the case of 56c.

It has been found in the meantime that reaction (1) is generalizable (752), and that oxidative additions of this type occur for such widely differing substrates H2Y as ethylene, benzene 130), cyclic olefins, alkyl and aryl phosphines, aniline 337, 406), and H2S 130), ail of which give the same product structure with a triply-bridging Y ligand. The stability of these third-row transition metal clusters has stiU prevented catalytic reactions of these species, but it is likely that similar ones are involved in olefin and acetylene reactions catalyzed by other metal complexes. 

Several factors affect the nature of the products in a reaction between a transition metal cluster and an alkyne or alkene. In this section, the various synthetic routes to alkyne or alkene-substituted clusters will be presented, and these will be used to analyze the changes in reactivity of the cluster systems when one or more of the important reaction parameters is altered. In order to simplify the discussion, tri-, tetra-, and higher nuclearity clusters will be treated separately. Finally, in this section, there is a brief description of the chemistry of alkylidyne-substituted clusters since synthetic routes to alkyne-containing complexes may involve these species. 

Transition Metal Clusters. Reactions of Lewis bases with metal clusters may yield either mononuclear or polynuclear products. Substitution reactions on Fe3(CO)i2 represent the features that may be seen. Reaction with L at 50 °C leads to substituted metal clusters, but reaction at 80 °C produces substituted mononuclear fragments ... 

The past two decades have seen much excitement in the area of transition metal cluster chemistry, which was stimulated, at least to a large extent, by the catalytic perspectives of these species. Twenty years of intensive research have demonstrated the versatile catalytic potential of clusters. However, it seems that clusters are not very likely to replace soon well-established conventional catalysts in large-scale industrial production. The challenge in cluster catalysis appears to reside in the search for novel catalytic reactions and for highly selective catalytic processes, making use of the unique polymetallic coordination sites in soluble organometallic molecules. 

Schubert B., Gocke E., Schollhom R., AlonsoVante N. and Tributsch H. (1996), In situ X-ray-electrochemical studies on the origin of H2O2 production during oxygen reduction at transition metal cluster materials , Electrochim. Acta 41, 1471-1478. 

However, a totally different catalysis is observed when the protons are not neutralized, so that transition metal clusters and Bronsted sites co-exist in the same catalyst. For such bifimctional catalysts, for instance Pd/HNaY or Pd/HY, ring opening is a minor side reaction, but ring-enlargement becomes the major reaction pathway, with benzene and cyclohexane as the predominant reaction products [28. 29]. Apparently, a carbenium ion has been formed from MCP, it is isomerized via the fused cyclopropane ring to the cyclohexylcarbocation, as depicted in scheme 9 ... 

At the same time cis Smalley and students at Rice University, Houston Texas, developed the laser vaporization method for production of clusters [84], a similar set-up was built at Exxon s Research Laboratory, New Jersey, USA, by the group of Kaldor and Cox [102,103]. They studied in particular transition metal clusters but also produced clusters of carbon containing up to more than hundreds of atoms as shown in the mass spectrum in Fig. 12. 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

Oxidation of ketones to carboxylic acids. This transition metal cluster compound catalyzes the oxidation of CO to COj by molecular oxygen. When acetone is used as solvent, acetic acid is identified as another product of oxidation. When cyclohexanone is used as solvent, adipic acid is formed in high yield. 

Ever since transition metal clusters have been discussed as catalysts, [4] there have been many attempts to develop catalytically active polynuclear complexes in which the metal centers interact during the formation of the target molecule ( cooperativity ) to control activity and selectivity of the catalytic process. In this way, a highly selective hydroformylation catalyst for propene was found in the cluster anion [HRu3(CO)n] (linear to branched product ration of butyraldehyde... 

In the transition metal cluster arena, this type of phenyl bridge is also well established, the first example (Fig. 8) being characterized more than 25 years ago by Nyholm et They obtained product Os3(//->/ -Ph)( 3-PhPC6H4)-... 

We have found that the main group metal and metalloid reductants mocury, bismuth, and antimony are highly effective " in reducing WCIe or M0CI5 at surprisingly lower temperatures than commonly used in the solid-state synthesis of early transition metal cluster halides. BorosUicate ampules can be substituted for the more expensive and less easily sealed quartz ampules at these lower temperatures, and the metals and metalloids are not as impacted by oxide coatings that inhibit sohd-state reactions with more active metals. These lower temperatures may allow access to kinetic products, such as trinuclear clusters, instead of thermodynamic products. 

Product ion scan, where Ql is set to aUow only the target precursor ion mass to enter the cell, while Q2 scans to measure all the product ions formed in the cell, including controUed cluster ion analysis. An example of this is the use of NH3 gas to create cluster ions of an analyte such as titanium. By allowing only Ti through Ql, only titanium cluster complex ions are formed in the cell, and not other potentially interfering transition metal cluster ions as with a traditional coUision/reaction cell. 

---