Ruthenium tetroxide

Ruthenium tetroxide
IUPAC name Ruthenium(VIII) oxide
Identifiers
CAS number	20427-56-9 Y
PubChem	119079
Properties
Molecular formula	RuO4
Molar mass	165.07 g/mol
Appearance	colorless liquid
Odor	pungent
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	0 2 0
Related compounds
Related compounds	RuO2 RuCl3
Y (verify) (what is: Y/N?) 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 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.

Preparation

RuO₄ is prepared by oxidation of ruthenium(III) chloride with NaIO₄.

8 Ru³⁺ + 5 IO₄⁻ + 12 H₂O → 8 RuO₄ + 5 I⁻ + 24 H⁺

In 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" (tetrapropylammonium perruthenate), [N(C₃H₇)₄]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 terminal 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:

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

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 PCC- or DMSO-based oxidants, RuO₄ is ideal when a very vigorous oxidant is needed but mild conditions must be maintained.

RuO₄ readily cleaves double bonds to yield carbonyl products, in a manner similar to ozonolysis. Osmium tetroxide, a more familiar oxidant that is structurally similar to RuO₄, does not cleave double bonds, instead producing vicinal diol products.

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. (1997). Chemistry of Precious Metals. London: Chapman and Hall. ISBN 9780751404135. 
• Martín, V. S.; Palazón, J. M.; Rodríguez, C. M.; Nevill, C. R. (2006). "Ruthenium(VIII) Oxide". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rr009.pub2. ISBN 0471936235.  edit
• Farmer, V.; Welton, T. (2002). "The oxidation of alcohols in substituted imidazolium ionic liquids using ruthenium catalysts". Green Chemistry 4 (2): 97. doi:10.1039/B109851A.  edit
• Singh, B.; Srivastava, S. (1991). "Kinetics and mechanism of ruthenium tetroxide catalysed oxidation of cyclic alcohols by bromate in a base". Transition Metal Chemistry 16 (4): 466. doi:10.1007/BF01129466.  edit
• Courtney, J.L.; Swansbor, K.F. (1972). "Ruthenium tetroxide oxidation". Reviews of Pure and Applied Chemistry 22: 47.