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Osmium tetroxide (also osmium(VIII) oxide) is the chemical compound with the Chemical formula OsO4. The compound is noteworthy for its many uses, despite its toxicity and the rarity of osmium. It also has a number of unusual properties, one being that the solid is volatile. The compound is colourless, but most samples appear yellow. This is most likely due to the presence of the impurity Osmium dioxide, which is yellow-brown in colour.Cotton and Wilkinson, Advanced Inorganic Chemistry, p.1002 In biology, its property of binding to lipids has made it a widely used stain in electron microscopy.
OsO4 is expensive and highly toxic, making it an unappealing reagent to use in stoichiometric amounts. However, its reactions are made Catalysis by adding to reoxidise the Os(VI) by-product back to Os(VIII). Typical reagents include H2O2 (Milas hydroxylation), N-methylmorpholine N-oxide (Upjohn dihydroxylation) and K3Fe(CN)6/water. These reoxidants do not react with the alkenes on their own. Other osmium compounds can be used as catalysts, including osmate(VI) salts (OsO2(OH)4)2−, and osmium trichloride hydrate (OsCl3· xH2O). These species oxidise to osmium(VIII) in the presence of such oxidants.
Lewis bases such as tertiary and increase the rate of dihydroxylation. This "ligand-acceleration" arises via the formation of adduct OsO4L, which adds more rapidly to the alkene. If the amine is chiral, then the dihydroxylation can proceed with enantioselectivity (see Sharpless asymmetric dihydroxylation). OsO4 does not react with most carbohydrates.
The process can be extended to give two in the Lemieux–Johnson oxidation, which uses periodate to achieve diol cleavage and to regenerate the catalytic loading of OsO4. This process is equivalent to that of ozonolysis.
When the Lewis base is an amine, adducts are also formed. Thus OsO4 can be stored in the form of osmeth, in which OsO4 is complexation with hexamine. Osmeth can be dissolved in tetrahydrofuran (THF) and diluted in an aqueous buffer solution to make a dilute (0.25%) working solution of OsO4.
With tert-Butylamine, the imido derivative is produced:
OsO4 is very soluble in tert-butyl alcohol. In solution, it is readily reduced by hydrogen to osmium metal. The suspended osmium metal can be used to catalysis hydrogenation a wide variety of organic chemicals containing double or triple bonds.
OsO4 undergoes "reductive carbonylation" with carbon monoxide in methanol at 400 K and 200 bar to produce the triangular cluster Os3(CO)12:
OsO4 also reacts with F2 to form yellow OsO3F2:
OsO4 reacts with one equivalent of Me4NF at 298 K and 2 equivalents at 253 K:
Osmium(VIII) oxide is also used in catalytic amounts in the Sharpless oxyamination to give vicinal amino-alcohols.
In combination with sodium periodate, OsO4 is used for the oxidative cleavage of (Lemieux-Johnson oxidation) when the periodate serves both to cleave the diol formed by dihydroxylation, and to regenerate OsO4. The net transformation is identical to that produced by ozonolysis. Below an example from the total synthesis of Isosteviol.
Coordination chemistry
4 is a Lewis acid and a mild oxidant. It reacts with alkaline aqueous solution to give the perosmate anion . This species is easily reduced to potassium osmate anion, .
Similarly, with ammonia one obtains the nitrido complex:
The − anion is isoelectronic and isostructural with OsO4.
Oxofluorides
Osmium forms several oxofluorides, all of which are very sensitive to moisture.
Purple cis-OsO2F4 forms at 77 K in an anhydrous HF solution:
Uses
Organic synthesis
In organic synthesis OsO4 is widely used to oxidize to the vicinal diols, adding two hydroxyl groups at the same side (syn addition). See reaction and mechanism above. This reaction has been made both catalytic (Upjohn dihydroxylation) and asymmetric (Sharpless asymmetric dihydroxylation).
Biological staining
OsO4 is a widely used staining agent used in transmission electron microscopy (TEM) to provide contrast to the image. This staining method may also be known in the literature as the OTO (osmium-thiocarbohydrazide-osmium) method, or osmium impregnation technique or simply as osmium staining. As a lipid stain, it is also useful in scanning electron microscopy (SEM) as an alternative to sputter coating. It embeds a heavy metal directly into cell membranes, creating a high electron scattering rate without the need for coating the membrane with a layer of metal, which can obscure details of the cell membrane. In the staining of the plasma membrane, osmium(VIII) oxide binds phospholipid head regions, thus creating contrast with the neighbouring protoplasm (cytoplasm). Additionally, osmium(VIII) oxide is also used for fixing biological samples in conjunction with HgCl2. Its rapid killing abilities are used to quickly kill live specimens such as protozoa. OsO4 stabilizes many proteins by transforming them into gels without destroying structural features. Tissue proteins that are stabilized by OsO4 are not coagulated by alcohols during dehydration. Osmium(VIII) oxide is also used as a stain for lipids in optical microscopy. OsO4 also stains the human cornea (see safety considerations).
Polymer staining
It is also used to stain copolymers preferentially, the best known example being block copolymers where one phase can be stained so as to show the microstructure of the material. For example, styrene-butadiene block copolymers have a central polybutadiene chain with polystyrene end caps. When treated with OsO4, the butadiene matrix reacts preferentially and so absorbs the oxide. The presence of a heavy metal is sufficient to block the electron beam, so the polystyrene domains are seen clearly in thin films in TEM.
Osmium ore refining
OsO4 is an intermediate in the extraction of osmium from its ores. Osmium-containing residues are treated with sodium peroxide (Na2O2) forming Na2OsO4(OH)2, which is soluble. When exposed to chlorine, this salt gives OsO4. In the final stages of refining, crude OsO4 is dissolved in alcoholic NaOH forming Na2OsO2(OH)4, which, when treated with NH4Cl, to give . This salt is reduced under hydrogen to give osmium.
Buckminsterfullerene adduct
OsO4 allowed for the confirmation of the soccer ball model of buckminsterfullerene, a 60-atom carbon allotrope. The adduct, formed from a derivative of OsO4, was C60(OsO4)(4- tert-butylpyridine)2. The adduct broke the fullerene's symmetry, allowing for crystallization and confirmation of the structure of C60 by X-ray crystallography.
Medicine
The only known clinical use of osmium tetroxide is for the treatment of arthritis. The lack of reports of long-term side effects from the local administration of osmium tetroxide (OsO4) suggest that osmium itself can be biocompatible, though this depends on the osmium compound administered.
Safety considerations
OsO4 will irreversibly stain the human cornea, which can lead to blindness. The permissible exposure limit for osmium(VIII) oxide (8 hour time-weighted average) is 2 μg/m3. Osmium(VIII) oxide can penetrate plastics and food packaging, and therefore must be stored in glass under refrigeration.
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