Osmium supported

On osmium supported samples, Guerra and Schulman (112) reported a sharp band at about 2000 cm with weak satellite bands at a lower frequency of 1900 cm" and a higher frequency of 2060 cm. ... 

Figure 8.40 The k ySk) extended X-ray absorption fine structure (EXAFS) signal, Fourier transformed and then retransformed after application of the filter window indicated, in (a) osmium metal and (b) a 1% osmium catalyst supported on silica. (Reproduced, with permission, Ifom Winnick, FI. and Doniach, S. (Eds), Synchrotron Radiation Research, p. 413, Plenum, New York, 1980)...

Rapoport s findings have been confirmed in the authors laboratory where the actions of carbon-supported catalysts (5% metal) derived from ruthenium, rhodium, palladium, osmium, iridium, and platinum, on pyridine, have been examined. At atmospheric pressure, at the boiling point of pyridine, and at a pyridine-to-catalyst ratio of 8 1, only palladium was active in bringing about the formation of 2,2 -bipyridine. It w as also found that different preparations of palladium-on-carbon varied widely in efficiency (yield 0.05-0.39 gm of 2,2 -bipyridine per gram of catalyst), but the factors responsible for this variation are not knowm. Palladium-on-alumina was found to be inferior to the carbon-supported preparations and gave only traces of bipyridine,... 

Rhodium-on-carbon has also been found to bring about the formation of 2,2 -biquinoline from quinoline, the yield and the percentage conversion being similar to that obtained with palladium-on-carbon. On the other hand, rhodium-on-carbon failed to produce 2,2 -bipyridine from pyridine, and it has not yet been tried with other bases. Experiments with carbon-supported catalysts prepared from ruthenium, osmium, iridium, and platinum have shown that none of these metals is capable of bringing about the formation of 2,2 -biquinoline from quinoline under the conditions used with palladium and rhodium. ... 

Reduction of unsaturated aldehydes seems more influenced by the catalyst than is that of unsaturated ketones, probably because of the less hindered nature of the aldehydic function. A variety of special catalysts, such as unsupported (96), or supported (SJ) platinum-iron-zinc, plalinum-nickel-iron (47), platinum-cobalt (90), nickel-cobalt-iron (42-44), osmium (<55), rhenium heptoxide (74), or iridium-on-carbon (49), have been developed for selective hydrogenation of the carbonyl group in unsaturated aldehydes. None of these catalysts appears to reduce an a,/3-unsaturated ketonic carbonyl selectively. 

This proton transfer reaction is not fast, and it is suggested that this may be a more complicated reaction than was anticipated, perhaps occurring by initial addition of OH or OR to the metal followed by H2O or ROH expulsion. In support of this is the isolation of a complex Os(CO)-(CNC6H4CH3)(PPh3)2(H)OR from an analogous reaction sequence. (This is the only reference yet to any osmium carbonyl-isocyanide chemistry.)... 

Figure 4. Contributions of nearest neighbor copper and osmium backscattering atoms (circles in fields B and C, respectively) to the EXAFS (solid line) associated with the osmium Ltjj absorption edge of a silica supported osmium-copper catalyst, me circles in field A represent the combined contributions resulting from the data analysis. Reproduced with permission from Ref. 12. Copyright 1981, American Institute of Physics.

The results of the EXAFS studies on osmium-copper clusters lead to conclusions similar to those derived for ruthenium-copper clusters. That is, an osmium-copper cluster Is viewed as a central core of osmium atoms with the copper present at the surface. The results of the EXAFS investigations have provided excellent support for the conclusions deduced earlier (21,23,24) from studies of the chemisorption and catalytic properties of the clusters. Although copper is immiscible with both ruthenium and osmium in the bulk, it exhibits significant interaction with either metal at an interface. 

Fig. 4 Osmium clusters supported on MgO(OOl) a OssC/MgisOs and b OS5C at a surface point Vs defect site [33] these were represented by density functional theory, and the samples were characterized by EXAFS spectroscopy, transmission electron microscopy, and other techniques [15]...

Dapprich, S., Ujaque, G., Maseras, F., Lledos, A., Musaev, D. G., Morokuma, K., 1996, Theory Does Not Support an Osmaoxetane Intermediate in the Osmium-Catalyzed Dihydroxylation of Olefins , J. Am, Chem. Soc., 118, 11660. 

Oxidative cleavage of alkenes using sodium periodate proceeds effectively in a monophasic solution of acetic acid, water, and THF with very low osmium content or osmium-free. The orders of reactivity of alkenes are as follows monosubstituted trisubstituted >1,2 disub-stituted > 1,1-disubstituted > tetrasubstituted alkynes.100 Cleavage with polymer-supported OSO4 catalyst combined with NaI04 allows the reuse of the catalyst.101... 

Other metals on silica supports have been investigated less extensively than platinum and nickel, and average particle diameters have only been estimated by gas adsorption methods, supported in a few cases by X-ray line broadening data. Thus, rhodium, iridium, osmium, and ruthenium (44, 45) and palladium (46) have all been prepared with average metal particle diameters <40 A or so, after hydrogen reduction at 400°-500°C. 

The co-condensation reactions described above have led to the formation of interesting new compounds and sometimes very unexpected products. The nature of the products formed for example in the osmium atom experiments indicate high degrees of specificity can be achieved. However, the detailed mechanisms of the co-condensation reactions are not known. It seems most likely that in all cases the initial products formed at the co-condensation temperature are simple ligand-addition products and that the insertion of the metal into the carbon-hydrogen bond occurs at some point during the warming up process. In support of this hypothesis we note the virtual absence of any... 

Scheme 8.2 Mechanism for the hydrogenation of ethylene catalyzed by silica-supported osmium clusters (CO ligands omitted for clarity).

Polymer-supported [e.g. 8, 9] and silica-supported [10] cinchona alkaloids have been used in the asymmetric dihydroxylation of alkenes using osmium tetroxide. Enantiomeric excesses >90% have been achieved for diols derived from styrene derivatives. 

Initially, it was thought more likely that the electron poor metal atom would be involved in the electrophilic attack at the alkene and also the metal-carbon bond would bring the alkene closer to the chiral metal-ligand environment. This mechanism is analogous to alkene metathesis in which a metallacyclobutane is formed. Later work, though, has shown that for osmium the actual mechanism is the 3+2 addition. Molecular modelling lends support to the 3+2 mechanism, but also kinetic isotope effects support this (KIEs for 13C in substrate at high conversion). Oxetane formation should lead to a different KIE for the two alkene carbon atoms involved. Both experimentally and theoretically an equal KIE was found for both carbon atoms and thus it was concluded that an effectively symmetric addition, such as the 3+2 addition, is the actual mechanism [22] for osmium. 

Fischer-Tropsch catalysis, 34 71, 38 331-335 C2 oxygenate formation, 38 338 oxide-supported osmium clusters, 38 335 product selectivites, 38 333-334 proton-induced reduction of CO, 38 332-333... 

Ravizza G, Norris RN, Blusztajn J, Aubry MP (2001) An osmium isotope excursion associated with the late Paleocene thermal maximum Evidence of intensified chemical weathering. Paleoceanogr 16 155-163 Raymo ME (1991) Geochemical evidence supporting Chamberlain, T. C., theory of glaciation. Geology 19 ... 

Further supporting evidence for this type of a mechanism comes from the consideration of an analogous reaction, the oxidation of alkenes by osmium tetroxide, which also produces cis-diols. In this reaction the... 

Cyclohexane dehydrogenates rather rapidly to benzene. Its rearrangement has not been reported over pure metals until now. Cg Ring opening is negligible over platinum and palladium 48, 5i) slight hexane formation was reported over carbon supported rhodium, iridium, and, especially, osmium and ruthenium (702), as well as over nickel on alumina (99). 

Other functionalized supports that are able to serve in the asymmetric dihydroxylation of alkenes were reported by the groups of Sharpless (catalyst 25) [88], Sal-vadori (catalyst 26) [89-91] and Cmdden (catalyst 27) (Scheme 4.13) [92]. Commonly, the oxidations were carried out using K3Fe(CN)g as secondary oxidant in acetone/water or tert-butyl alcohol/water as solvents. For reasons of comparison, the dihydroxylation of trons-stilbene is depicted in Scheme 4.13. The polymeric catalysts could be reused but had to be regenerated after each experiment by treatment with small amounts of osmium tetroxide. A systematic study on the role of the polymeric support and the influence of the alkoxy or aryloxy group in the C-9 position of the immobilized cinchona alkaloids was conducted by Salvadori and coworkers [89-91]. Co-polymerization of a dihydroquinidine phthalazine derivative with hydroxyethylmethacrylate and ethylene glycol dimethacrylate afforded a functionalized polymer (26) with better swelling properties in polar solvents and hence improved performance in the dihydroxylation process [90]. 

Scheme 1.4 Catalysis with a silica supported grafted osmium cluster (while keeping the molecular cluster intact).

Sections 8.3.1-8.3.3 present the use of iron, mthenium and osmium carbonyls, respectively, in the preparation of supported catalysts. Over non-inert supports, besides the characteristics of carbonyl compounds, the reactivity of the surface and that of the specific element, mainly related with its redox properties, will be covered for each metal. 

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