Clusters as catalysts

A metal cluster can be considered as a polynuclear compound which contains at least one metal-metal bond. A better definition of cluster catalysis is a reaction in which at least one site of the cluster molecule is mechanistically necessary. Theoretically, homogeneous clusters should be capable of multiple-site catalysis. Many heterogeneous catalytic reactions require multiple-site catalysis and for these reasons discrete molecular metal clusters are often proposed as models of metal surfaces in the processes of chemisorption and catalysis. The use of carbonyl clusters as catalysts for hydrogenation reactions has been the subject of a number of papers, an important question actually being whether the cluster itself is the species responsible for the hydrogenation. Often the cluster is recovered from the catalytic reaction, or is the only species spectroscopically observed under catalytic conditions. These data have been taken as evidence for cluster catalysis. 

Homogeneous catalysts have now been reported for hydrogenation of carbon monoxide, a combustion product of coal (see Section VI,B). More effective catalysts will undoubtedly be discovered in the near future. Polynuclear or, at least, binuclear sites are favored for reduction of the triple bond in carbon monoxide (see Section VI,B), and this together with the popular parallelism to heterogeneous systems, has renewed interest in metal clusters as catalysts (see Section VI). A nickel cluster is the first catalyst reported for mild (and selective) hydrogenation of the triple bond in isocyanide (see Section VI,A). The use of carbon monoxide and water as an alternative hydrogen source is reattracting interest (see Section VI,C). 

Vargaftik, M.N. et al., Giant Pd clusters as catalysts of oxidative reactions of olefins and alcohols, J. 

Synthesis of Methylformate by C02 Reduction with Molecular Hydrogen in the Presence of Methanol Using Ruthenium Clusters as Catalysts... 

It was thought that the formation of inactive cobalt clusters such as Co4(CO)i2, formed by dimerisation of the remaining cobalt carbonyl species after release of the cyclopentenone product, were responsible for the shutdown of the catalytic cycle when dicobalt octacarbonyl was employed.51 Krafft and co-workers were able to show that Co4(CO)12 can actually be exploited as a catalytic species in the PK reaction and were able to obtain excellent yields if cyclohexylamine was introduced as an additive alongside the metal cluster.57,58 The use of metal clusters as catalysts for the reaction has been extended to involve mixed metal clusters.59... 

Reetz MT, Breinbauer R, Wedemann P, Binger P (1998) Nanostructured nickel-clusters as catalysts in [3+2] cycloaddition reactions. Tetrahedron 54 1233... 

Reetz MT, Lohmer G (1996) Propylene carbonate stabilized nanostructured palladium clusters as catalysts in Heck reactions. Chem Commun (Camb) 16 1921... 

The carbonylation of methanol to give acetic acid, according to Eq.(l), based on the catalyst [Rh(CO)2I2], is a major industrial process (Monsanto acetic acid process). However, ruthenium clusters as catalysts seem to favor the insertion of carbon monoxide into the O-H and not into the C-O bond, according to Eq.(2). Ru3(CO)12 in basic solution converts methanol to methyl formate with 90% selectivity (400-450 bar CO,... 

In this report we will discuss only transition metal carbonyl clusters as catalysts, or catalyst precursors. However, the reader must be aware that the family of compounds able to catalyze reactions 1-3 is mainly composed of mononuclear carbonyl derivatives. Moreover, even when the starting compound is a cluster, the true active species is often mononuclear (or at least of lower nuclearity) and only little, indirect evidence supports the participation of the cluster in the main catalytic cycle. 

The few studies made with catalytic precursors containing phosphine ligands have revealed that the presence of such ligands in the clusters generally results in slower catalytic rates and sometimes in ligand degradation via P-C or C-H bond-activation processes. Current data do not, therefore, warrant recommendation of the use of phosphine-substituted clusters as catalyst precursors for hydrogenation reactions. 

Beller et al. have shown for the first time that palladium colloids are effective catalysts for the olefination of aryl bromides (Heck reaction). Reetz et al. have studied Suzuki and Heck reactions catalyzed by preformed palladium clusters and palladium/nickel bimetallic clusters and further progress was achieved by Reetz and Lohmert using propylene carbonate stabilized nanostructured palladium clusters as catalysts in Heck reactions. In addition, the use of nanostructured titanium clusters in McMurry-type coupling reactions has been demonstrated by Reetz et... 

The hydrogenation of arenes is industrially important, but so far is dominated by the use of heterogeneous catalysts. Ionic liquids offer, in principle, the chance to use a liquid-liquid biphasic system where the homogeneous catalyst is immobilized and therefore recyclable. Dyson et al. applied ruthenium clusters as catalyst for... 

The involvement of metallacycles has been proposed for the [3+2] cycloaddition of meth-ylenecyclopropanes with alkenes to produce methylenecyclopentanes.l Oxidative cyclization of a methylenecyclopropane and an electron-deficient alkene produces a spi-rocyclic metallacyclopentane 86. Cyclopropane ring opening followed by reductive elimination affords the observed methylenecyclopentane products 87 and 88 (Scheme 65). Another report describes the novel use of nanostructured nickel clusters as catalysts. ]... 

In recent years homogeneous systems containing organometallic compounds, especially metal carbonyl clusters as catalysts have been investigated very actively with the aim of selectivity improvement and mechanism elucidation. 

Thus the answer to the first question posed in the introduction is clear no totally new catalytic activities or selectivities have been observed as a consequence of the use of molecular metal clusters as catalyst precursors. Indeed, in general the catalytic activity of metal clusters does appear to be rather low in reactions for which comparisons with mononuclear counterparts have been possible, e.g., alkene hydroformylation. 

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