Ruthenium reactivity

Characterization by wide-angle X-ray scattering (WAXS) evidenced crystalline NPs displaying the expected hep structure of bulk ruthenium. Reactivity smdies were carried out in particular with CO [17]. It has been observed that there is an influence of the reaction time on the coordination mode of CO on the surface of the NPs and that CO is mobile. Short reaction times give rise to CO adsorption in the bridging mode, while longer reaction times allow adsorption of more CO molecules only in the linear or multicarbonyl modes. 

Ruthenium dissolves anodically in alkaline solutions, as predicted by Pourbaix but its corrosion resistance when made anodic in acid solutions is variable. Under some conditions the volatile and toxic tetroxide is evolved. Osmium is even more reactive anodically than ruthenium. 

High-valent ruthenium oxides (e. g., Ru04) are powerful oxidants and react readily with olefins, mostly resulting in cleavage of the double bond [132]. If reactions are performed with very short reaction times (0.5 min.) at 0 °C it is possible to control the reactivity better and thereby to obtain ds-diols. On the other hand, the use of less reactive, low-valent ruthenium complexes in combination with various terminal oxidants for the preparation of epoxides from simple olefins has been described [133]. In the more successful earlier cases, ruthenium porphyrins were used as catalysts, especially in combination with N-oxides as terminal oxidants [134, 135, 136]. Two examples are shown in Scheme 6.20, terminal olefins being oxidized in the presence of catalytic amounts of Ru-porphyrins 25 and 26 with the sterically hindered 2,6-dichloropyridine N-oxide (2,6-DCPNO) as oxidant. The use... 

In summary, the order of reactivity for the most commonly used ruthenium-based metathesis catalysts was found to be 56d>56c>9=7. This order of reactivity is based on IR thermography [39], determination of relative rate constants for the test reaction 58—>59 (Eq. 8) [40], and determination of turnover numbers for the self metathesis of methyl-10-undecenoate [43]. 

The two most commonly used single-site catalysts for ADMET today are (1) Schrock s alkylidene catalysts of the type M(CHR )(NAr )(OR)2 where M = W or Mo, AC = 2, 6-C6H3-/-Pr2, R = CMe2Ph, and R = CMe(CF3)2 (14)7 and (2) Grubbs ruthenium-based catalyst, RuCl2(=CHPh)(PCy3)2 (12) where Cy = cyclohexyl.9 While both catalysts meet the requirements to be successful in ADMET, they are markedly different in their reactivity and in die results each can produce. 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

The reaction of the coordinatively unsaturated ruthenium amidinates with [Cp RuCl]4 tetramer or [CpRufMeCNlsJPFg provides access to novel amidinate-bridged dinuclear ruthenium complexes (Scheme 146), which in turn can be transformed into cationic complexes or hydride derivatives. In these complexes, a bridging amidinate ligand perpendicular to the metal-metal axis effectively stabilizes the highly reactive cationic diruthenium species. 

The most significant and widely studied reactivity of the ruthenium and osmium porphyrin carbene complexes is their role in catalyzing both the decomposition of diazoesters to produce alkenes and the cyclopropanation of alkenes by diazoesters. Ethyl diazoacetate is used to prepare the carbene complex 0s(TTP)(=CHC02Et)... 

Dibenzenechromium was studied by Baumgartner et al. 15). They found that the yield of Cr(Ph)2 was 11.8%. [One cannot fail to be struck by the similarity in yields of FeCp2, RuCp2, and Cr(PhH)2, although it may well be merely coincidence.] On heating the irradiated samples to 110°C, they found the yield to increase to 19.4%. It was found that dissolution of the radioactive crystals in benzene yielded no further Cr(PhH)2. This stands in contrast to the above-mentioned results of Zahn and Harbottle which, though not strictly comparable, show Cp to be quite reactive toward ruthenium atoms. Dibenzenechromium was also formed in low yield 14) from neutron irradiation of PhHCr(CO)3, as will be discussed in more detail later. 

Methane is a stable molecule and therefore hard to activate. As a result the sticking probability for dissociative chemisorption is small, of the order of 10 only, and ruthenium is more reactive than nickel. However, a stretched overlayer of nickel is significantly more active than nickel in its common form, in agreement with expectation. 

Many late transition metals such as Pd, Pt, Ru, Rh, and Ir can be used as catalysts for steam reforming, but nickel-based catalysts are, economically, the most feasible. More reactive metals such as iron and cobalt are in principle active but they oxidize easily under process conditions. Ruthenium, rhodium and other noble metals are more active than nickel, but are less attractive due to their costs. A typical catalyst consists of relatively large Ni particles dispersed on an AI2O3 or an AlMg04 spinel. The active metal area is relatively low, of the order of only a few m g . ... 

In the previous sections we have dealt mainly with the catalytic activity of pure substances such as metallic iron, ruthenium, copper, platinum, etc. Real catalyst, however, are often much more complex materials that have been optimized by adding remote amounts of other elements that may have a profound impact on the overall reactivity or selectivity of the catalyst. Here we shall deal with a few prominent examples of such effects. 

The coordination of ligands at the surface of metal nanoparticles has to influence the reactivity of these particles. However, only a few examples of asymmetric heterogeneous catalysis have been reported, the most popular ones using a platinum cinchonidine system [65,66]. In order to demonstrate the directing effect of asymmetric ligands, we have studied their coordination on ruthenium, palladium, and platinum nanoparticles and the influence of their presence on selected catalytic transformations. 

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