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Rhenium(VII) oxide

Rhenium(VII) oxide is the inorganic compound with the formula Re<sub>2</sub>O<sub>7</sub>. This yellowish solid is the anhydride of HOReO<sub>3</sub>. Perrhenic acid, Re<sub>2</sub>O<sub>7</sub>·2H<sub>2</sub>O, is closely related to Re<sub>2</sub>O<sub>7</sub>. Re<sub>2</sub>O<sub>7</sub> is the raw material for all rhenium compounds, being the volatile fraction obtained upon roasting the host ore.

Structure

Solid Re<sub>2</sub>O<sub>7</sub> consists of alternating octahedral and tetrahedral Re centres. Upon heating, the polymer cracks to give molecular (nonpolymeric) Re<sub>2</sub>O<sub>7</sub>. This molecular species closely resembles manganese heptoxide, consisting of a pair of ReO<sub>4</sub> tetrahedra that share a vertex, i.e., O<sub>3</sub>Re–O–ReO<sub>3</sub>.

Synthesis and reactions

Rhenium(VII) oxide is formed when metallic rhenium or its oxides or sulfides are oxidized at in air.

Re<sub>2</sub>O<sub>7</sub> dissolves in water to give perrhenic acid.

Heating Re<sub>2</sub>O<sub>7</sub> gives rhenium dioxide, a reaction signalled by the appearance of the dark blue coloration:

2Re<sub>2</sub>O<sub>7</sub> → 4ReO<sub>2</sub> + 3O<sub>2</sub>

Using tetramethyltin, it converts to methylrhenium trioxide ("MTO"), a catalyst for oxidations:

Re<sub>2</sub>O<sub>7</sub> + 2Sn(CH<sub>3</sub>)<sub>4</sub> → CH<sub>3</sub>ReO<sub>3</sub> + (CH<sub>3</sub>)<sub>3</sub>SnOReO<sub>3</sub>

In a related reaction, it reacts with hexamethyldisiloxane to give the siloxide:

Re<sub>2</sub>O<sub>7</sub> + 2O(Si(CH<sub>3</sub>)<sub>3</sub>)<sub>2</sub> → 2(CH<sub>3</sub>)<sub>3</sub>SiOReO<sub>3</sub>

Uses

Hydrogenation catalyst

Rhenium(VII) oxide finds some use in organic synthesis as a catalyst for ethenolysis, carbonyl reduction and amide reduction.

References