Ruthenium tetroxide

Ruthenium tetroxide is the inorganic compound with the formula RuO₄. It is a yellow volatile solid that melts near room temperature. It has the odor of ozone. Samples are typically black due to impurities. The analogous OsO₄ is more widely used and better known. It is also the anhydride of hyperruthenic acid (H₂RuO₅). One of the few solvents in which RuO₄ forms stable solutions is CCl₄.

Preparation

RuO₄ is prepared by oxidation of ruthenium(III) chloride with NaIO₄. The reaction initially produces sodium diperiodo­dihydroxo­ruthenate(VI), which then decomposes in acid solution to the tetroxide:

2 Ru³⁺(aq) + 5 IO₄^−(aq) + 3 H₂O(l) → 2 RuO₄(s) + 5 IO₃^−(aq) + 6 H⁺(aq)

Due to its challenging reactivity, RuO₄ is always generated in situ and used in catalytic quantities, at least in organic reactions.

Structure

RuO₄ forms two crystal structures, one with cubic symmetry and another with monoclinic symmetry, isotypic to OsO₄. The molecule adopts a tetrahedral geometry, with the Ru–O distances ranging from 169 to 170 pm.

Uses

Isolation of ruthenium from ores

The main commercial value of RuO₄ is as an intermediate in the production of ruthenium compounds and metal from ores. Like other platinum group metals (PGMs), ruthenium occurs at low concentrations and often mixed with other PGMs. Together with OsO₄, it is separated from other PGMs by distillation of a chlorine-oxidized extract. Ruthenium is separated from OsO₄ by reducing RuO₄ with hydrochloric acid, a process that exploits the highly positive reduction potential for the [RuO₄]^0/- couple.

Organic chemistry

RuO₄ is of specialized value in organic chemistry because it oxidizes virtually any hydrocarbon. For example, it will oxidize adamantane to 1-adamantanol. Because it is such an aggressive oxidant, reaction conditions must be 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 conditions, 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. It is used in organic synthesis to oxidize internal alkynes to 1,2-diketones, and terminal alkynes along with primary alcohols to carboxylic acids. When used in this fashion, the ruthenium(VIII) oxide 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. RuO₄ readily cleaves double bonds to yield carbonyl products, in a manner similar to ozonolysis. OsO₄, a more familiar oxidant that is structurally similar to RuO₄, does not cleave double bonds, instead producing vicinal diol products. However, with short reaction times and carefully controlled conditions, RuO₄ can also be used for dihydroxylation.

Because RuO₄ degrades the "double bonds" of arenes (especially electron-rich ones) by dihydroxylation and cleavage of the C-C bond in a way few other reagents can, it is useful as a "deprotection" reagent for carboxylic acids that are masked as aryl groups (typically phenyl or p-methoxyphenyl). Because the fragments formed are themselves readily oxidizable by RuO₄, a substantial fraction of the arene carbon atoms undergo exhaustive oxidation to form carbon dioxide. Consequently, multiple equivalents of the terminal oxidant (often in excess of 10 equivalents per aryl ring) are required to achieve complete conversion to the carboxylic acid, limiting the practicality of the transformation.

Although used as a direct oxidant, due to the relatively high cost of RuO₄ it is also used catalytically with a cooxidant. For an oxidation of cyclic alcohols with RuO₄ as a catalyst and bromate as oxidant under basic conditions, RuO₄ is first activated by hydroxide, turning into the hyperruthenate anion:

RuO₄ + OH^− → HRuO₅^−

The reaction proceeds via a glycolate complex.

Other uses

Ruthenium tetroxide is a potential staining agent. It is used to expose latent fingerprints by turning to the brown/black ruthenium dioxide when in contact with fatty oils or fats contained in sebaceous contaminants of the print.

Gaseous release by nuclear accidents

Because of the very high volatility of ruthenium tetroxide () ruthenium radioactive isotopes with their relative short half-life are considered as the second most hazardous gaseous isotopes after iodine-131 in case of release by a nuclear accident. The two most important radioactive isotopes of ruthenium are ¹⁰³Ru and ¹⁰⁶Ru. They have half-lives of 39.6 days and 373.6 days, respectively.

References

Further reading