Activation of Os

The catalytic activity of Os(VIII) is more than that of Ru(III) in the catalysed oxidation of paracetamol with diperiodatoargentate(III) (DPA). The active species of catalysts and oxidant have been identified and a mechanism has been proposed.55 The oxidation of aspirin, both uncatalysed56 and catalysed by Os(VIII) and Ru(III)57 with DPA in alkaline solution is of first order in DPA and has less than unit order in both aspirin and alkali. The order with respect to the catalyst is one. A decrease in the dielectric constant of the medium increases the rate of the reaction. A mechanism has been proposed. 

The chemical behaviour of 133 is a result of two factors the high basicity of the metallic center, as a consequence of the presence of the strong donor phosphine and the chloride ligands in the complex, and the large steric hindrance experienced by the triisopropylphosphine groups, which are mutually cis disposed. This mixture allows access to reactive points on the osmium center by activation of Os-P and Os-Cl bonds. 

Several different coordination modes have been found in dinitrogen metal compounds (Fig. 2) and they are strongly influenced by the identity of the metal atom(s), their oxidation state(s), and the ligand environment. This results in different degrees of reduction of the N=N bond, which is often also referred to as activation . For monomeric complexes, the end-on coordination mode A is most frequently encountered. However, in very rare cases side-on coordination B has also been observed, eg, a metastable Tj -bound Nz hgand can be trapped and structurally characterized upon photochemical activation of [Os(NH3)5(r -N2)][PF6]2. The large overlap between the osmium d-orbitals with the 2p orbitals of the bound Nz confers stability to this species. 

Activation of the pyrrole heteronucleus with respect to electrophilic substitution is enhanced by coordination of the [Os(NH3)5] " " unit. Species Os(NH3)5(OTf )3 is reduced in the presence of pyrroles to yield a series 61 (R = =... 

CMAW Festen, JFG Siegers, CH Van Os. (1983). Intracellular activities of chloride, potassium, and sodium ions in rabbit corneal epithelium. Biochim Biophys Acta 732 394-404. 

The formation of these compounds has been rationalized according to Scheme 6. The reaction of Os (E )-CH=C 11 Ph C1 (C())( P Pr3)2 with n-BuLi involves replacement of the chloride anion by a butyl group to afford the intermediate Os (/i> CH=CHPh ( -Bu)(CO)(P Pr3)2, which by subsequent hydrogen (3 elimination gives OsH ( >CI I=CHPh (CO)( P Pr3)2. The intramolecular reductive elimination of styrene from this compound followed by the C—H activation of the o-aryl proton leads to the hydride-aryl species via the styrene-osmium(O) intermediate Os r 2-CH2=CHPh (CO)(P Pr3)2. In spite of the fact that the hydride-aryl complex is the only species detected in solution, the formation of OsH ( )-CH=CHPh L(CO)(P Pr3)2 and 0s ( )-CH=CHPh (K2-02CH)(C0)(P,Pr3)2 suggests that in solution the hydride-aryl complex is in equilibrium with undetectable concentrations of OsH ( )-CH=CHPh (CO)(P,Pr3)2. This implies that the olehn-osmium(O) intermediate is easily accessible and can give rise to activation reactions at both the olefinic and the ortho phenyl C—H bonds of the... 

Furthermore, the utilization of preformed films of polypyrrole functionalized by suitable monomeric ruthenium complexes allows the circumvention of problems due to the moderate stability of these complexes to aerial oxidation when free in solution. A similar CO/HCOO-selectivity with regards to the substitution of the V-pyrrole-bpy ligand by an electron-with-drawing group is retained in those composite materials.98 The related osmium-based redox-active polymer [Os°(bpy)(CO)2] was prepared, and is also an excellent electrocatalyst for the reduction of C02 in aqueous media.99 However, the selectivity toward CO vs. HCOO- production is lower. 

We have recently extended our interest to the analogous halfsandwich osmium-arene complexes and are exploring the chemical and biological properties of [Os(r 6-arene)(XY)Z]ra 1 complexes (Fig. 25) (105). Both the aqueous chemistry and the biological activity of osmium complexes have been little studied. Third-row transition metals are usually considered to be more inert than those of the first and second rows. Similar to the five orders of magnitude decrease in substitution rates of Pt(II) complexes compared to Pd(II), the [Os(ri6-arene)(L)X]"+ complexes were expected to display rather different kinetics than their Ru(II)-arene analogs. A few other reports on the anticancer activity of osmium-arene complexes have also appeared recently (106-108). 

Two points are emphasized (i) zeolites can be successfully operated at the same or higher severities (with respect to P/O (feed alkane/alkene) ratio and OS V (alkene space velocity)) than the liquid acids (ii) the productivities of zeolite catalysts (i.e., the total amount of alkylate produced per mass of catalyst) are roughly the same as of that of sulfuric acid. If the intrinsic activities of zeolites (which have 0.5-3 mmol of acid sites per gram) are compared with that of sulfuric acid (which has 20 mmol of acid sites per gram), zeolites outperform sulfuric acid. Nevertheless, the price of a zeolite catalyst and the high costs of... 

Grenoble and coworkers229 reported an important influence of the support on the water-gas shift activity of various metal catalysts. For example, the rate increased an order of magnitude when Pt was supported on alumina versus silica. Turnover numbers for alumina-supported metal catalysts decreased in the order Cu, Re, Co, Ru, Ni, Pt, Os, Au, Fe, Pd, Rh, and Ir, whereby the activity varied by 3 orders of magnitude, suggesting a correlation between activity of the metal and the heat of adsorption. To describe these differences in activity, the authors used a bifunctional model, involving chemisorption of water on alumina and CO on the metal, followed by association of the CO with the water to form a formic acid-like formate species, with subsequent decomposition via dehydrogenation on the metal sites (Scheme 55). 

Figure 11-15 shows the corrosion rate observed for a metallic nickel electrode in aerated aqueous sulfate solutions as a function of pH. In addic solutions, nickel corrodes in the active state at a rate which is controlled by the diffusion of hydrated oi en molecules (oxidants). In solutions more basic than pH 6, however, nickel spontaneously passivates by hydrated oiQ n molecules and corrosion is negligible. As shown in the inserted sub-figures in Fig. 11-15, the maximum current of anodic nickel dissolution in the active state is greater in the range of addic pH however, the Tnaximnm current of anodic nickel dissolution is smaller in the range of basic pH than the current of cathodic reduction of os en molecules (dashed curve) which is controlled by the diffusion of hydrated oiQ gen molecules. Consequently, metallic nickel remains in the active state in addic solutions but is spontaneously passivated by hydrated ojQ n molecules in basic solutions. It... 

The 19S RC recognizes the polyubiquitinated substrate and channels the substrate into the catalytic 2 OS core of the proteasome. It also has the capacity to regulate the activity of the catalytic core and determine the nature of the degradation process. Usually one 19S RC is attached to either end of the catalytic core. The subunits of the 19S RC are highly conserved across evolution. ° ° Two subcomplexes can be recognized within the 19S RC called the base and the lid (Figure 6(a)). 

Figure 2 IL-6-inducing activity of QM fractions (a) and GL4 and OS-4H (b) in human peripheral whole-blood cells. The data present one of several independent experiments with similar results. The IL-6 induction by un-stimulated cells was less than 50 pg/mL. Each bar represents the means SD. The blood donor was JY.

Royer, S Duprez, D Kaliaguine, S. Role of bulk and grain boundary oxygen mobility in the catalytic oxidation activity of LaCoi. Fe Os, J. Catal, 2005, Volume 234, Issue 2, 364-375. 

Further experience has demonstrated that it was not solely the choice of assay but the great good fortune to have received a group of cultures from the Kitasato Institute and to have made the decision to screen these cultures in the dubius assay. One of these cultures, OS-3153, was active. The screening of several tens of thousands of soil isolates in this assay has failed to detect any remotely similar anthelmintic activity. Of the fermentation products discussed, this is the only one where the activity for which the product was eventually marketed was found by direct screening. (Zeranol might be considered to be another example, but it was not discovered by screening.)... 

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