Ruthenium tetroxide

Preparation

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

8 Ru³⁺(aq) + 5 IO₄⁻(aq) + 12 H₂O(l) → 8 RuO₄(s) + 5 I⁻(aq) + 24 H⁺(aq)

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.

Structure

The molecule adopts a tetrahedral geometry, with the Ru-O distances ranging from 169 to 170 pm.

Isolation of ruthenium from ores

The main 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 only 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 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. 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. Osmium(VIII) oxide, a more familiar oxidant that is structurally similar to RuO₄, does not cleave double bonds, instead producing vicinal diol products.

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 a base, RuO₄ is first activated by hydroxide:

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.

Related ruthenium compounds

Because RuO₄ will readily decompose explosively at slightly elevated temperatures, most laboratories do not synthesize it directly, nor is it commercially available. A related reagent is the anionic Ru(VII) derivative in the form of the salt of "TPAP" (tetrapropylammonium perruthenate), [N(C₃H₇)₄]RuO₄. TPAP is synthesized by oxidizing RuCl₃ to RuO₄⁻ by NaBrO₃ and isolated as the tetrapropylamine cation, which allows the salt to be used in organic solvents.