Hydrogen activating ruthenium

In 1977 Ford and co-workers showed that Ru3(CO)12 in the presence of a ca. fiftyfold excess of KOH catalyzes the shift reaction at 100°C/1 bar CO (79). The effectiveness of the system increased markedly as temperature was increased (rate of hydrogen formation approximately quadrupled on raising the temperature from 100° to 110°C), and over a 30-day period catalyst turnovers of 150 and 3 were found for Ru3(CO)12 and KOH, respectively. Neither methane nor methanol was detected in the reaction products. Although the nature of the active ruthenium species could not be unambiguously established, infrared data indicated that it is not Ru3(CO)12, and the complexity of the infrared spectrum in the... 

There is much current excitement and activity in the field of homogeneous hydrogenation using ruthenium catalysts. This is reflected in the recent, explosive increase in the number of research publications in this area, now rivaling those for rhodium catalysts (Fig. 3.1). Meanwhile, the price of rhodium metal has risen dramatically, becoming about ten times that of ruthenium, on a molar basis. The number of reports on the use of osmium catalysts has remained low, partly because of the higher price of osmium compounds - about ten times that of ruthenium - and partly because the activity of osmium catalysts is often lower. 

The broad range of alkenes undergoing asymmetric hydrogenation using ruthenium-based systems as catalysts has attracted the attention of chemists engaged in the synthesis of chiral biologically active natural products (Scheme 13)[60] and other pharmaceuticals (Scheme 14)[61]. a, (3-Unsaturated phosphoric acids and esters have also proved to be suitable substrates for Ru(II)-catalysed asymmetric hydrogenation [62]. 

F-T Catalysts The patent literature is replete with recipes for the production of F-T catalysts, with most formulations being based on iron, cobalt, or ruthenium, typically with the addition of some pro-moter(s). Nickel is sometimes listed as a F-T catalyst, but nickel has too much hydrogenation activity and produces mainly methane. In practice, because of the cost of ruthenium, commercial plants use either cobalt-based or iron-based catalysts. Cobalt is usually deposited on a refractory oxide support, such as alumina, silica, titania, or zirconia. Iron is typically not supported and may be prepared by precipitation. 

The role of the solvent is crucial in some cases. For example, in the case of hydrogen activation it was actively participating in the heterolytic cleavage of the hydrogen in both the ruthenium and gold catalysts in water and ethanol, respectively. It must be noted that the so-called heterolytic activation of the hydrogen may be... 

The water-soluble Ru(II) complex [Ru(i76-C6H6)(CH3CN)3](BF4)2 catalyzed the biphasic hydrogenation of alkenes and ketones with retention of the catalyst in the aqueous phase (87). However, the ruthenium complex moved to the organic phase when benzaldehyde was hydrogenated. In a benzene-D20 system, H-D exchange was observed between H2 and D20. Both monohydridic pathway and a dihydridic pathway are possible for hydrogen activation, and these two different catalytic cycles influence the yield and product distribution. 

Heterolytic cleavage. This leads to formation of a metal hydride with release of a proton (equation 1). The formal oxidation state of the metal does not change. This mode of hydrogen activation is common in hydrogenation by complexes of ruthenium(II). 

In N,N-dimethylacetamide solution the reduction by hydrogen of ruthenium(HI) chloride is claimed to produce ruthenium(I) complexes which hydrogenate ethylene, maleic and fumaric adds. The complexes are thought to be dimeric but their precise structures are unknown. Interestingly, these d1 ruthenium(I) complexes are believed to activate hydrogen by oxidative addition whereas heterolytic cleavage of hydrogen occurs with most ruthenium catalysts (equation 19). 

Reduced ruthenium catalysts stored in air are usually oxidized on the surface and must be activated by prereduction with hydrogen for 1-2 h before use for hydrogenations at a low temperature and pressure. In contrast for platinum and palladium catalysts, organic as well as inorganic acids strongly poison the ruthenium catalyzed hydrogenation. Thus acetic acid should not be added or used as solvent for the hydrogenations over ruthenium, particularly under mild conditions. 

Methanol homologation catalyzed by ruthenium has been studied by Braca etal. [86, 89, 90]. Catalyst systems such as Ru(acac)3/Nal and Ru(C0)4lj/NaI have been shown to be active. In contrast to cobalt catalysts, no reaction occurs in the absence of 1" and a proton supplier is needed. As can be taken from Table XI, the reaction is higidy selective to C -products and no higlter products are formed. Due to the high hydrogenation activity of ruthenium, however, methane and ethane arc formed as side products in considerable amounts as well as dimethyl ether. Thus, the overall yield of ethanol is limited. The same catalyst systems have also been shown to be active in the homologation/carbonylation of ethers and esters. 

A new preparation of catalytically active ruthenium-carbene complexes can avoid difficulty by using accessible starting materials such as (PhjPjjRuClj and carbene precursors (e.g., diazoalkanes and diphenylcyclopropenes). It involves reduction of RuCl3 3H20 in THF Mg/ClCHjCHjCl in the presence of tricyclohexylphosphine under hydrogen at 60-85° which is followed by cooling to -40° and addition of a 1-alkyne along with a small amount of water. 

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