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Ruthenium tetroxide
Ruthenium tetroxide
IUPAC name
Identifiers
CAS number 20427-56-9
Properties
Molecular formula RuO4
Molar mass 165.07 g/mol
Appearance colorless liquid
Density 3.29 g/cm3
Melting point

25.4 °C

Boiling point

40.0 °C

Solubility in water 2% w/v at 20°C
Solubility in other solvents Soluble in
Carbon tetrachloride
Chloroform
Structure
Molecular shape tetrahedral
Dipole moment zero
Hazards
MSDS external MSDS sheet
NFPA 704
NFPA 704.svg
0
2
0
 
Related compounds
Related compounds RuO2
RuCl3
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Ruthenium tetroxide (RuO4) is a yellow, diamagnetic tetrahedral ruthenium compound. As expected for a charge-neutral symmetrical oxide, it is quite volatile. The analogous OsO4 is more widely used and better known. One of the few solvents in which it forms stable solutions is CCl4.

Contents

Preparation

RuO4 is prepared by oxidation of ruthenium(III) chloride with NaIO4. In a typical reactions featuring RuO4 as the oxidant, many forms of ruthenium usefully serve as precursors to RuO4, such as oxide hydrates or hydrated chloride.

Because RuO4 will readily decompose explosively at slightly elevated temperatures, most laboratories do not synthesize it directly, nor is it commercially available through major chemical vendors. Most laboratories instead use an anionic derivative from a salt of "TPAP", or tetrapropylammonium perruthenate (Pr4N+ RuO4). TPAP is synthesized by oxidizing RuCl3 to RuO4 by NaBrO3 and countering it with the tetrapropylamine cation.

Properties and uses

RuO4 oxidizes virtually any hydrocarbon. For example, it will oxidize adamantane to 1-adamantanol. It is used in organic synthesis to oxidize alkynes to 1,2-diketones and primary alcohols to carboxylic acids. When used in this fashion, the ruthenium tetroxide is used in catalytic amounts and regenerated by the addition of sodium periodate to ruthenium(III) chloride and a solvent mixture of acetonitrile, water and carbon tetrachloride.

Because it is such an aggressive oxidant, reaction conditions are mild, generally room temperature. Although a strong oxidant, RuO4 oxidations do not perturb stereocenters that are not oxidized. Illustrative is the oxidation of the following diol to a carboxylic acid:

RuO4oxidation.png

Oxidation of epoxy alcohols also occurs without degradation of the epoxide ring:

RuO4epoxy.png

Under milder condition, oxidative reaction yields aldehydes instead.

RuO4 readily converts secondary alcohols into ketones. Although similar results can be achieved with other cheaper oxidants such as chromium(VI) based compounds or DMSO-based oxidants, RuO4 is ideal when a very vigorous oxidant is needed but a mild conditions must be maintained.

RuO4 readily cleaves double bonds to yield carbonyl products. Osmium tetroxide, a more familiar oxidant that is structurally similar to RuO4, does not cleave double bonds.

In terms of practical details, the substrate to be oxidized is typically dissolved in solvent such as CCl4, and acetonitrile is added as an aiding ligand to the catalytic cycle. Ether can then be added to precipitate and recover the ruthenium pre-catalyst.

Oxidative catalyst and mechanism

Although used as a direct oxidant, due to the relatively high cost of RuO4 it is also used catalytically with an associated re-oxidant. For an oxidation of cyclic alcohols with RuO4 as a catalyst and bromate as a base, RuO4 is first activated by hydroxide:

RuO4 + OH → HRuO5

Then HRuO5 complexes with the cyclic alcohol and form a metal coordination complex (denoted C1 here):

HRuO5 + (CH2CH2)nCHOH → C1 + OH

The Ru complex is then attracted by a bromate in which the oxidation of the coordinated alcohol take place:

C1 + BrO3 → (CH2CH2)nC=O + HRuO5

In which HRuO5 is reformed as the catalyst, and the cyclic alcohol is oxidized into a cyclic ketone.

Staining

Ruthenium tetroxide can be used as an even more aggressive form of staining for the study of polymers by Transmission Electron Microscopy than osmium tetroxide. It has the advantage of staining even polyethylene, the disadvantage is that it is not very selective in what it stains.

References

  • Cotton, S. A. "Chemistry of Precious Metals," Chapman and Hall (London): 1997. ISBN 0-7514-0413-6
  • Martin, V. S., Palazón, J. M., 'Ruthenium(VIII) Oxide', Encyclopedia of Reagents for Organic Synthesis 2001. (Article)
  • Farmer, V., Welton, T., The oxidation of alcohols in substituted imidazolium ionic liquids using ruthenium catalysts Royal Society of Chemistry, 2002.
  • Singh, B., Srivastava, S., Kinetics and Mechanism of Ruthenium tetroxide Catalysed Oxidation of Cyclic Alcohols by Bromate in a Base Transition Met. Chem., 1991, 16, 466-468.
  • Courtney, JL., Swansbor, KF. Ruthenium tetroxide Oxidation Reviews of Pure and Applied Chemistry 1972, 22, 47-
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