Metal carbonyl cluster catalysts

The vast majority of transition metal clusters contain carbonyl ligands, which have been shown in many cases to be fluxional on the metal skeleton of the cluster (40,41). Therefore, the most obvious reactions to be catalyzed by such clusters should be those involving carbon monoxide. In fact, catalytic carbonylations are frequently encountered with transition metal carbonyl cluster catalysts, but very often the carbonylation step is followed by a consecutive step, e.g., a hydrogenation step, to give an overall hydroformylation. Simple carbonylation reactions have nevertheless been observed for various structures. 

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. 

The relationship between metal carbonyl clusters and supported metal catalysts. J. Evans, Chem. Soc. Rev., 1981,10,159-180 (94). 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

The idea (50, 5/) of dual coordination of CO implies the presence of two coordination centers in a Fischer-Tropsch catalyst system, i.e., a carbonyl carbon coordinating center, Ma, and a carbonyl oxygen coordinating center, M6 (14). It is this concept which has led at least two groups to examine transition metal carbonyl cluster compounds as homogeneous Fischer-Tropsch catalysts. 

Cyclopropanation.2 This metal carbonyl cluster is an effective catalyst for cyclo-propanation of alkenes with ethyl diazoacetate. Minor by-products are diethyl maleate and fumarate, but products of allylic C -H insertion are not formed. The yield of the cyclopropane can be increased if the ethyl diazoacetate is added slowly over a period of 6 hours to the olefin and catalyst. Under these conditions yields of eyclopropanes are 85 -90%. 

Recent work by Ford et al. demonstrates that a variety of metal carbonyl clusters are active catalysts for the water-gas shift under the same reaction conditions used with the ruthenium cluster (104a). In particular, the mixed metal compound H2FeRu3(CO)13 forms a catalyst system much more active than would be expected from the activities of the iron or ruthenium systems alone. The source of the synergetic behavior of the iron/ruthenium mixtures is under investigation. The ruthenium and ruthenium/iron systems are also active when piperidine is used as the base, and in solutions made acidic with H2S04 as well. Whether there are strong mechanistic similarities between the acidic and basic systems remains to be determined. 

Exclusive formation of silylstyrenes 76 is achieved when the reactions of styrene and 4-substituted styrenes with HSiEt3 are catalyzed by Fe3(CO)i2 or Fe2(CO)9100. Other iron-triad metal carbonyl clusters, Ru3(CO)i2 and Os3(CO)i2, are also highly active catalysts, but a trace amount of hydrosilylation product 77 is detected in the Ru-catalyzed reactions and the Os-catalyzed reactions are accompanied by 3-12% of 77 (equation 31)100. Mononuclear iron carbonyl, Fe(CO)5, is found to be inactive in this reaction100. 

One of the conclusions of this study is that the first ligand substitution proceeds with a much higher coulombic efficiency than the second one, simply because of the increasing bulk at the metal center [317], This trend was known from previous studies of carbonyl substitution by phosphanes in transition-metal carbonyl clusters catalyzed by benzophenone radical anion [318, 319]. In such a case, selective ETC catalysts may be designed as a function of the required exergonicity of the desired initiation step [317]. 

The family of 19-electron complexes [Fe Cp(/7 -arene)] has also been investigated as catalysts for the substitution of one, two and three carbonyls in metal-carbonyl clusters as shown below. 

Propylene Hydroformylation over Various Metal Carbonyl Clusters Impregnated on Metal Oxides Compared with That over Conventional Rhodium Supported Catalyst"... 

Fig. 24. Structures of mixed metal carbonyl clusters that are potential catalysts eflective in CO hydrogenation to produce C and Cj alcohols.

K. Lazar, Z. Schay and L. Guezi, Direct evidence for the conelation between surface carbon and carbon monoxide + hydrogen selectivity on iron and iion-mtbenium catalysts prepared form metal carbonyl clusters, 1. Mol. Catal. 17(2-3) (1982) 205-218. 

Alternatively, surface-mediated synthesis involves the immobilization of a mononuclear metal complex (similar to the ones described in Sect. 19.2) and the subsequent treatment in CO at different temperatures to form a supported metal carbonyl cluster, for example, [HIr (CO)jJ formed from Ir(CO)2(acac) on MgO and [Rhj(CO),5] formed from Rh(CO)2(acac) on MgO and y-Al Oj [45 7]. Synthesis of metal carbonyl clusters on oxide supports apparently often involves OH groups or water on the support surface analogous chemistry occurs in solution [13]. The synthesis of molecular catalysts from a mononuclear metal complex is likely to occur with a yield less than that associated with simple adsorption of a preformed metal cluster, and so the latter precursors are preferred, except when they do not fit into the pores of the support (e.g., a zeolite). 

Ugo R, Dossi C, Psaro R (1996) Molecular metal carbonyl clusters and volatile organometallic compounds for tailored mono and bimetallic heterogeneous catalysts. 1 Mol Catal A 107 13... 

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