What properties does osmium tetroxide have?
Osmium tetroxide consists of monoclinic yellowish crystals with an osmium content of 74-76 percent. Osmium tetroxide is a corrosive, very toxic compound which, due to its hazard class and volatility (sublimates at room temperature), is most commonly sold in sealed glass ampoules.
Osmium(VIII)-oxide has an acrid, chlorine-like odor. The osmium tetroxide molecule is tetrahedral and therefore non-polar. This non-polarity helps OsO4 to penetrate charged cell membranes. OsO4 is approximately 500 times more soluble in carbon tetrachloride than it is in water.
OsO4 is formed slowly when osmium powder reacts with oxygen at room temperature. To produce large quantities, osmium powder is heated to 400 °C under oxygen supply.
The chemical equation is as follows: Os + 2 O2 -> OsO4
Osmium tetroxide is an effective catalyst which is used for its reliability and efficiency in the dihydroxylation of alkenes to produce diols, i.e. organic compounds with two hydroxy groups.
Due to its value and toxicity, only small amounts of OsO4 are usually applied in combination with a stoichiometric oxidant. Osmium tetroxide is most commonly used in Upjohn and Sharpless dihydroxylation reactions.
Upjohn vs. Sharpless dihydroxylation
Both dihydroxylations are similar in their reaction sequence and produce diols. However, in Sharpless dihydroxylation, enantioselectivity is achieved by the addition of chiral ligands: enantiomerically pure diols are produced, which are required for producing active pharmaceutical ingredients.
Due to its catalytic properties, osmium tetroxide is used as a homogeneous catalyst mainly in the production of pharmaceutical and agrochemical active ingredients. With the help of OsO4 new potent pesticides and drugs could be developed.
Besides its catalytic function, osmium tetroxide is also used in microscopy. Thanks to its low polarity, osmium tetroxide can penetrate the cell membrane and fix the lipids. The high electron density of osmium enables high-contrast electron microscopic images of biological samples.
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