Catalysis clusters

In what may be an example of tme cluster catalysis, [HRU3 (CO) ] shows good catalytic activity and high regioselectivity using propylene as substrate (24,25). Solvent, CO partial pressure, and temperature are important variables. In monoglyme, at 80°C and starting partial pressures for C H, ... 

Research into cluster catalysis has been driven by both intrinsic interest and utilitarian potential. Catalysis involving "very mixed -metal clusters is of particular interest as many established heterogeneously catalyzed processes couple mid and late transition metals (e.g., hydrodesulfurization and petroleum reforming). Attempts to model catalytic transformations arc summarized in Section II.F.I., while the use of "very mixed -metal clusters as homogeneous and heterogeneous catalysis precursors are discussed in Sections I1.F.2. and I1.F.3., respectively. The general area of mixed-metal cluster catalysis has been summarized in excellent reviews by Braunstein and Rose while the tabulated results are intended to be comprehensive in scope, the discussion below focuses on the more recent results. 

Laine and co-workers have studied the mechanism involved in rhodium-catalysed benzaldehyde hydrogenation, using [Rh6(CO)i6] as catalyst precursor. Following kinetic arguments, the authors proposed cluster catalysis with a limiting step corresponding to the break of metal-metal bond and/or isomerisation of the cluster formation [22]. 

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. 

NMR studies involving para-hydrogen has recently been introduced as a powerful tool to obtain direct evidence for cluster catalysis (vide infra). 

This chapter reviews the literature involving well-defined molecular metal clusters as hydrogenation catalysts or catalyst precursors, with particular emphasis being placed on those systems that are likely to involve only or predominantly cluster intermediates throughout the hydrogenation cycle. The mechanisms in cases where cluster catalysis is strongly supported by experimental evidence are discussed in more detail. 

Phosphine-substituted complexes have shown promise for cluster catalysis, especially in the case of chelating ligands, because of the added stability that might help avoid cluster fragmentation. Bergounhou et al. reported a detailed study of the hy-... 

The first term represents the classic unicyclic rhodium catalysis, while the second indicates a hydride attack on an acyl species. These spectroscopic and kinetic results strongly suggested the presence of bimetallic catalytic binuclear elimination as the origin of synergism of both metals rather than cluster catalysis. This detailed evidence for such a catalytic mechanism, and its implications for selectivity and nonlinear catalytic activity illustrate the important mechanistic knowledge that can be revealed by this powerful in situ spectroscopic technique. 

Metal cluster compounds simulate surface species produced by the interaction of molecules with metal surfaces (Muetterties et al, 1979) and this is of value in understanding heterogeneous catalysis. The development of selective catalysts for the C, chemical industry employing CO (and possibly CO2) as the raw material has resulted in major efforts in metal cluster research. Criteria have been developed to distinguish between cluster catalysis and mononuclear catalysis. Typical of the catalysts investigated hitherto are [Ir4(CO),2. <(PPh3)J where Ph = phenyl and X = 1, 2 or 3. 

It should be kept in mind that it is often difficult to ascertain what the active catalyst is. There are, in fact, very few proven catalytic mechanisms where the structure of the actual catalyst is known with certainty. There are, to the author s knowledge, few, and possibly no, proven useful homogeneous catalytic processes that involve metal dimers in the catalytic cycle. In an excellent review on cluster catalysis, Muetterties lists the following processes as metal-cluster-catalyzed reactions (see Table 2). This review also lists the important classes of metalloenzyme clusters however, these compounds do not normally involve metal-metal bonds per se but are usually bridged by chalcogens, such as sulfur or oxygen donors. 

The HDN of aliphatic amines (equation 25) is relevant to the HDN of indoles (equation 26), pyridine (equation 27), and quinoline (equation 28) because these heterocycles are first hydrogenated to the aliphatic amines. A general mechanism proposed for the HDN of aliphatic amines is based on metal cluster catalysis of the transalkylation reaction in equation (33) and on metal complex catalyzed exchange of deuterimn for hydrogen in tertiary aliphatic amines (equation 34). There are other examples of amine activation in metal complexes. 

When the total concentration in ruthenium was decreased, the catalytic activity fell off indicating that cluster catalysis was occurring (46). Moreover, Laine has found a synergistic effect between ruthenium and iron. Indeed, whereas the turnover frequencies displayed by [Fe3(CO),2] and [Ru3(CO),2] for the hydroformylation of pent-l-ene were, respectively, 38 and 40 hour , a 1 1 mixture of [Fe3(CO),2] and [Ru3(CO),2] gave a value of 230 hour . 

Judai K, Abbet S, Worz AS, Heiz U, Henry CR (2004) Low-temperature cluster catalysis. J Am Chem Soc 126 2732... 

During the compilation of this book, an important member and friend of the catalysis and surface science community, J. Mike White (University of Texas, Austin and Pacific Northwest National Laboratory) suddenly passed away. Mike was to author two chapters in this book, Photocatalysis at Adsorbate-Single Crystal Metal and Metal Oxide Interfaces and Supported Early Transition Metal Oxide Clusters Structural and Catalytic Properties. It is truly unfortunate that we will not be able to read his chapters they would have been first rate. I thank Professor Hicham Idriss for including a section on photocatalysis on TiO single crystals in Chap. 7 and Professors Gunther Rupprechter and Simon Penner for then-chapter on metal oxide cluster catalysis (Chap. 17). 

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