Homogeneous Cluster Catalyzed Reactions

The number of publications related to catalytic reactions using molecular clusters has increased considerably over the last five years. In 1977, the first review [6] dealing with this aspect of catalysis contained only 50 

Regarding the first question, it is important to mention the works of Union Carbide related to the synthesis of ethylene glycol from syn-gas with Rh or Ru complexes under drastic conditions of pressure (above 500 atm.). With rhodium complexes it has been established that an anionic cluster [Rh5(CO)i5] was present in the reaction medium under catalytic conditions. 

The same kind of observation was made recently by D. Dombeck [31] of Union Carbide for the same reaction using ruthenium clusters. In the case of ruthenium there is, under catalytic conditions, evidence for the presence of [HRu3(CO)ii] HRu(CO)4 and Ru(CO)3lJ. An almost complete catalytic cycle has been established by Dombeck. It appears that the hydrido anionic cluster [HRu3(CO)ii] or HRu(CO)7 make a nucleophilic attack at CO coordinated to the mononuclear carbonyl Ru(II) complex to give a formyl species. The reaction, here, would obey a very complex mechanism involving both mononuclear and polynuclear species. This phenomenon seems to be a general rule in many reactions involving CO. 

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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 advantage of a uniform size must be qualified for colloids, having a size distribution per se which, however, can be relatively narrow in some cases. In principle the solubility of ligand-protected clusters and colloids makes their use as homogeneous catalysts possible. But as it has turned out, cluster solutions tend to decompose during catalytic processes. The isolation of an unchanged cluster material after a homogeneously catalyzed reaction is indicated in only a very... 

Hydridic cluster anions are also important in the homogeneous WGSR cata-lysis.t The mechanism of WGSR reactions occurring on Ru3(CO)i2 with an acid cocatalyst (CF3COOH) has been fully elucidated.The radical anion [Fe3(CO)ii], obtained in the phase-transfer-catalyzed reaction of Fe3(CO)i2 with OH , catalyzes the reduction of nitrobenzene to aniline. 

Although mononuclear metal carbonyls are purportedly less effective as catalysts for this process when compared with metal carbonyl clusters (14,15), investigations of these systems will provide for a better understanding of the fundamental steps in the homogeneous metal-catalyzed water gas shift reaction. Therefore, the primary objective of this work was to examine (i) the reversible nature of the reaction of hydroxide ion with Cr(CO)e, along with the concomitant formation of /A-H[Cr-(CO)5]2" and CO2 and (ii) the ligand substitution reactions of /x-H[Cr-(CO)5]2" with CO, both thermally and photochemically (Scheme 1). 

Another SBU with open metal sites is the tri-p-oxo carboxylate cluster (see Section 4.2.2 and Figure 4.2). The tri-p-oxo Fe " clusters in MIL-100 are able to catalyze Friedel-Crafts benzylation reactions [44]. The tri-p-oxo Cr " clusters of MIL-101 are active for the cyanosilylation of benzaldehyde. This reaction is a popular test reaction in the MOF Hterature as a probe for catalytic activity an example has already been given above for [Cu3(BTC)2] [15]. In fact, the very first demonstration of the catalytic potential of MOFs had aheady been given in 1994 for a two-dimensional Cd bipyridine lattice that catalyzes the cyanosilylation of aldehydes [56]. A continuation of this work in 2004 for reactions with imines showed that the hydrophobic surroundings of the framework enhance the reaction in comparison with homogeneous Cd(pyridine) complexes [57]. The activity of MIL-lOl(Cr) is much higher than that of the Cd lattices, but in subsequent reaction rans the activity decreases [58]. A MOF with two different types of open Mn sites with pores of 7 and 10 A catalyzes the cyanosilylation of aromatic aldehydes and ketones with a remarkable reactant shape selectivity. This MOF also catalyzes the more demanding Mukaiyama-aldol reaction [59]. 

The main issue of the book is application of nanosized particles in both homogeneous and heterogeneous catalysis. A variety of reactions catalyzed by metal colloids or supported nanosized metals is discussed. The most intriguing reaction seems to be ethane hydrogenolysis catalyzed by Pt clusters on porous carrier and studied by G. A. Somorjai and his group. Another challenging observation by this group is shape isomerization of Pt metal particles affected by the addition of silver ions. 

There has been considerable recent research interest in the activation of carbon monoxide en route to more complex organic molecules. Among the various reactions that have been investigated and/or newly discovered, the transition metal catalyzed reduction of CO to hydrocarbons (Fischer-Tropsch synthesis) has enjoyed particular attention (l- ). Whereas most of the successful efforts in this area have been directed toward the development of heterogeneous catalysts, there are relatively few homogeneous systems. Among these, two are based on clusters (10,11) and others are stoichiometric in metal (12-17). In this report we detail the synthesis and catalytic chemistry of polystyrene ( ) supported... 

Saito and coworkers134 reported on the homogeneous reverse water-gas shift reaction catalyzed by Ru3(CO)i2. Conditions employed were 20 ml of N-methyl-2-pyrrolidone solution 0.2 mmol Ru3(CO)i2 1 mmol bis(triphenylphosphine)immi-nium chloride and C02-H2 1 3 under 80 kg/cm2 at 160 °C. The major products were CO (15.1 mmol), H20 (21.6 mmol), and methanol (0.8 mmol). As no formic acid was detected, and because the authors only detected Ru cluster anion species H3Ru4(CO)i2, H2Ru4(CO)i22, and HRu3(CO)n, they concluded that the mechanism did not involve formic acid as an intermediate. Rather, they proposed that the mechanism proceeds by dehydrogenation of a metal hydride, C02 addition, and electrophilic attack from the proton to yield H20, as outlined in Scheme 48. 

Clusters, as possible catalytic reactors, are perfectly dispersed in solutions. They are thus suitable systems for observing, under quasi-homogeneous conditions by time-resolved techniques, the kinetics of catalyzed electron transfer, which would be inaccessible on a solid catalyst. It was demonstrated that the reaction of radiation-induced free radicals COT and (CH3)2COH catalyzed by metal clusters started by the storage of electrons on clusters as charge pools and that electrons were then transferred pairwise to water-producing molecular hydrogen [22,75]. 

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