Osmium tetroxide, volatility

Chemical ingenuity in using the properties of the elements and their compounds has allowed analyses to be carried out by processes analogous to the generation of hydrides. Osmium tetroxide is very volatile and can be formed easily by oxidation of osmium compounds. Some metals form volatile acetylacetonates (acac), such as iron, zinc, cobalt, chromium, and manganese (Figure 15.4). Iodides can be oxidized easily to iodine (another volatile element in itself), and carbonates or bicarbonates can be examined as COj after reaction with acid. 

The residue, which contains Ir, Ru, and Os, is fused with sodium peroxide at 500°C, forming soluble sodium mthenate and sodium osmate. Reaction of these salts with chlorine produces volatile tetroxides, which are separated from the reaction medium by distillation and absorbed into hydrochloric acid. The osmium can then be separated from the mthenium by boiling the chloride solution with nitric acid. Osmium forms volatile osmium tetroxide mthenium remains in solution. Ruthenium and osmium can thus be separately purified and reduced to give the metals. 

The osmium-catalyzed dihydroxylation reaction, that is, the addition of osmium tetr-oxide to alkenes producing a vicinal diol, is one of the most selective and reliable of organic transformations. Work by Sharpless, Fokin, and coworkers has revealed that electron-deficient alkenes can be converted to the corresponding diols much more efficiently when the pH of the reaction medium is maintained on the acidic side [199]. One of the most useful additives in this context has proved to be citric acid (2 equivalents), which, in combination with 4-methylmorpholine N-oxide (NMO) as a reoxidant for osmium(VI) and potassium osmate [K20s02(0H)4] (0.2 mol%) as a stable, non-volatile substitute for osmium tetroxide, allows the conversion of many olefinic substrates to their corresponding diols at ambient temperatures. In specific cases, such as with extremely electron-deficient alkenes (Scheme 6.96), the reaction has to be carried out under microwave irradiation at 120 °C, to produce in the illustrated case an 81% isolated yield of the pure diol [199]. 

To separate osmium from ruthenium, the aqueous solution is acidified with nitric acid. While nitric acid oxidizes osmate ion to volatile osmium tetroxide, Os04, it converts ruthenium to a nitric oxide complex. Osmium tetroxide is removed from the solution by distillation in air and collected in an aqueous solution of caustic soda containing ethanol. Osmium tetroxide solution is heated with ammonium chloride, upon which osmium precipitates out as a complex chloride, 0s02(NH3)4Cl2. The precipitate is filtered, washed and decomposed by ignition with hydrogen to yield osmium metal. 

Osmium tetroxide is obtained as an intermediate during recovery of osmium metal from osmiridium or other noble metal minerals (See Osmium). In general, oxidation of an aqueous solution of an osmium salt or complex, such as sodium osmate with nitric acid, yields the volatile tetroxide which may be distilled out from the solution. In the laboratory, the compound can be prepared by oxidation of the osmium tetrachloride, OsCh, or other halide solutions with sodium hypochlorite followed by distdlation. 

Osmium Tetroxide, 0s04, frequently but incorrectly known as osmic acid, is the highest oxide of osmium known, and is formed in a variety of ways. Finely divided metallic osmium slowly oxidises in air to the tetroxide, and more rapidly on heating in air or, better, in oxygen.1 At high temperatures the compact metal yields vapours of the volatile tetroxide, and this affords a useful means of quantitatively separating osmium from its iridium alloy (p. S38). 

Detection.—Metallic osmium is characterised by its volatility at white heat without liquefaction, and by the production of volatile and excessively poisonous osmium tetroxide upon ignition of the finely divided metal in air or oxygen. The same vapour is evolved when any osmium compound is heated with concentrated nitric acid. 

It is probably in asymmetric dihydroxylation, where the use of ionic liquids appears to be most promising. The decreased acute toxicity of osmium tetroxide due to its suppressed volatility certainly represents a great benefit for those who work with this reagent and its derivatives. Furthermore, high cost of both the osmium catalyst, as well as the chiral ligand, make recycling of the catalyst-ionic liquid particularly attractive. On the other hand, disposal of osmium-contaminated ionic liquids in an environmentally benign manner has yet to be addressed. 

It has been known for decades that osmium tetroxide catalyzes the H2O2 oxidation of olefins to c -l,2-diols but the cost, toxicity and volatility of OSO4 have limited its use to the organic research laboratory. Industrial interest was aroused in the 1990 s by i) the invention of a system for vicinal hydro-xylation using an electrochemical device as the ultimate oxidant and ii) the discovery of conditions to carry out the reaction enantioselectively, mainly by Sharpless and his group. The reaction is currently carried out by Chirex to... 

In a typical example, sodium periodate (18.2 g, 85 mmol) was added in small portions over a 45 min period to l,4-dioxa-6-acetyl-6-allylspiro[4.5]decane (8.9 g, 40 mmol) and osmium tetroxide (0.10 g, 0.39 mmol) in a solution of THF (126 mL) and water (42 mL) at room temperature. The mixture was stirred for 2 h at this temperature during which time the black slurry turned brown. Water (600 mL) was introduced, and the mixture was extracted with ether. The extract was dried over anhydrous magnesium sulfate and stripped of solvent to give 7.4 g of crude aldehyde. (Because osmium tetroxide is a toxic and volatile irritant, all preparations should be carried out in a fume hood with use of adequate personal protection, gloves and safety glasses.) Other examples of the use of this reagent have b n summarized in Table 5. 

Hydroxylation by hydrogen peroxide in t-butyl alcohol solution is catalyzed by osmium tetroxide. The catalyst is volatile and dangerous to handle but is conveniently used in a solution of the tertiary alcohol. The yields of diols are usually low (30-60%), and the process has not been adapted to large-scale preparations. In contrast to hydroxylation by performic acid, this procedure leads to cis addition of the two hydroxyl groups to the double bond. An extensive study of other catalysts has been made. Some catalysts, e.g., selenium dioxide and pertungstic acid, catalyze addition in the trans direction. Hydroxylation of cyclopentadiene takes place in the 1,4-positions to give 2-cyclopenten-l,4-diol. ... 

The dihydroxylation of olefins with OSO4 (a very toxic and volatile reagent that must be handled with care) provides a reliable method for the preparation of cw-l,2-diols. Although osmium tetroxide is expensive, it is the reagent of choice for j yn-dihydrox-ylation because yields of diols obtained are usually high. 

Pershina [103] believes that a better way to compare the volatility of analogous compounds is to take the ratio of the experimental adsorption equilibrium constants, rather than to evaluate the adsorption enthalpies. The ratio should be corrected for the involved half-lives, and the contribution from the difference in internal entropy of the compared molecules (owing to unequal masses of the metal atoms and different bond lengths) can estimated by calculations from first principles. The approach was used for analysis of the experimental data on hassium and osmium tetroxides, which were reported in [104],... 

Osmium tetroxide is a highly toxic reagent. It is volatile and can cause blindness. 

Osmium tetroxide is volatile and toxic and therefore should be used only in a well-ventilated hood. On a 1-mole scale, osmium tetroxide was added as a solid. On a smaller scale, It was added as a solution (ca. 0.5 M) in toluene. ... 

OSmium(IV) oxide (osmium tetroxide OSO4) A volatile yellow crystalline solid with a penetrating odor, used as an oxidizing agent and, in aqueous solution, as a catalyst for organic reactions. 

Recycling of Osmium by Immobilization of Osmium Tetroxide. Several groups have been actively searching for immobilized forms of osmium tetroxide in order to overcome the problems of toxicity and volatility associated with this reagent. The following are some representative examples of immobilization methods for a catalytic system (OSO4 and/or cinchona alkaloid ligand). 

CAUTION Osmium tetroxide, 0s04, a volatile, extremely poisonous substance, is formed from almost any osmium compound under acid conditions in the presence of air. Reaction with corn oil or powdered milk will destroy it... 

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