Clusters and catalysis

Cluster complexes are being studied in the hope that they might provide novel catalytic systems. In a mononuclear complex potential coordination sites must 

Some very large carbonyl cluster anions of the late transition elements have been prepared by condensation of carbonyl anions with polynuclear carbonyls. The first example of such a reaction was discovered by Hieber in 1965. 

These condensations are most favoured with elements of the Second and Third Transition Series e.g. 

Reactions of this type may be endothermic. While D(M—CO) 160kjmol D(M—M) is only about lOOkJ mol Expulsion of CO is favoured on entropy grounds at higher temperatures. In this way large clusters can be built up  

The structural results suggest that large clusters can be viewed as fragments of bulk metals and that reactions occurring on them might mimic those which take place on metal surfaces. Another view is that the two types of system bear little 

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Presented in part at the Ralph W. Rudolph Memorial Symposium at the ACS Meeting, Kansas City, on September 15th, 1982. For a summary of some of the numerous valuable contributions of the late R.W. Rudolph to Inorganic Chemistry cf. "Borane Clusters, Metal Clusters and Catalysis" in Parry, R.W. Kodama,... 

B. C. Gates, L. Guczi, and H. Kmtzioger, eds.. Metal Clusters in Catalysis, Elsevier Science Pubhshers B.V., Amsterdam, The Netherlands, 1986. 

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. 

Volu me 28 New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22,1986 edited by Y. Murakami, A. lijima and J.W. Ward Volume 29 Metal Clusters in Catalysis... 

M. Kolb, Y. Boudeville. Kinetic model for heterogeneous catalysis Cluster and percolation properties. J Chem Phys 92 3935-3945, 1990. 

Gates BC (1995) Supported metal clusters Synthesis, stracture, and catalysis. Chem Rev 95 511-522... 

B.C.Gates, L.Guczi and H.Knozinger (eds.). Metal Cluster in Catalysis, Elsevier, Amsterdam, 1986. 

Henry CR, Chapon C, Giorgio S, Goyhenex C. 1997. Size effects in heterogeneous catalysis. In Lambert RM, Facchioni G, editors. Chemisorption and Reactivity on Supported Clusters and Thin Films. Dordrecht, Kluwer Academic, pp. 117-152. 

A review22 with 100 references, is given on the chemistry of coordinatively unsaturated tripalladium and triplatinum clusters and on the relation between this chemistry and that which occurs on a Pt surface during chemisorption and catalysis. 

A review23 with 1,160 references describes the chemistry of Pd and Pt complexes and cluster compounds, catalysis by Pd and Pt complexes, and nonstoichiometric binary and ternary compounds. 

Platinum forms a wide variety of clusters and these are of interest as models for metallic platinum surfaces and catalysis on such surfaces. Many of these clusters are based on stacked triangles23,571 or bicapped pentagonal prisms.571 Many smaller clusters are also derived from triplatinum building blocks.16... 

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. 

Lu, P. et al., Polymer-protected Ni/Pd bimetallic nano-clusters preparation, characterization and catalysis for Hydrogenation of Nitrobenzene, J. Phys. Chem. B, 103, 9673, 1999. 

Figure 10.15 CCT STM image (/ = 2.0 nA, V = +2.0 V) and the corresponding STS data acquired for Au clusters of varying sizes on the Ti02(110)-(1 x 1) surface. The STS curve of the bare titania surface is also shown. (Reprinted from Meier, D.C. et al., in Surface Chemistry and Catalysis, A.F. Carley et al., Eds., Kluwer, New York, 2002, pp. 147-189. With permission from Springer Science and Business Media.)...

Aleshin and coworkers (49) have reported the X-ray crystal structure at 2.2-A resolution of a G2-type variant produced by Aspergillus awamori. Meanwhile, an attempt was made to determine the amino acid residues that participate in the substrate binding and catalysis provided by G2 of A. niger (52). The results of the chemical approach indicated that the Asp-176, Glu-179, and Glu-180 form an acidic cluster crucial to the functioning of the enzyme. This conclusion was then tested by site-specific mutagenesis of these amino acid residues, which were replaced, one at a time, with Asn, Gin, and Gin, respectively (53). The substitution at Glu-179 provided an inactive protein. The other two substitutions affected the kinetic parameters but were not of crucial importance to the maintenance of activity. The crystal structure (49) supports the conclusion that Glu-179 functions as the catalytic acid but Asp-17 6 does not appear to be a good candidate for provision of catalytic base. Thus, there still exists considerable uncertainty as to how the disaccharide is accepted into the combining site for hydrolysis. Nevertheless, the kind of scheme presented by Svensson and coworkers (52) almost surely prevails. 

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