Perrhenic acid

Perrhenic acid is the chemical compound with the formula Re₂O₇(OH₂)₂. It is obtained by evaporating aqueous solutions of Re₂O₇. Conventionally, perrhenic acid is considered to have the formula HReO₄, and a species of this formula forms when rhenium(VII) oxide sublimes in the presence of water or steam.^[1] For most purposes, perrhenic acid and rhenium(VII) oxide are used interchangeably.

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Properties

The structure of solid perrhenic acid is [O₃Re-O-ReO₃(H₂O)₂].^[2] This species is a rare example of a metal oxide coordinated to water - most often metal-oxo-aquo species are unstable with respect to the corresponding hydroxides:

M(O)(H₂O) → M(OH)₂

Gaseous perrhenic acid is tetrahedral, as suggested by its formula HOReO₃.

Principal reactions

Perrhenic acid or the related anhydrous oxide Re₂O₇ converts to dirhenium heptasulfide upon treatment with hydrogen sulfide:

Re₂O₇+ 7 H₂S → Re₂S₇ + 7 H₂O

The heptasulfide, which has a complex structure,^[3] catalyses the hydrogenation of double bonds and is useful because it tolerates sulfur compounds, which poison noble metal catalysts. Re₂S₇ also catalyses the reduction of nitric oxide to N₂O.

Perrhenic acid in the presence of HCl undergoes reduction in the presence of thioethers and tertiary phosphines to give Re(V) complexes with the formula ReOCl₃L₂.^[4]

Perrhenic acid combined with platinum on a support gives rise to a useful hydrogenation and hydrocracking catalyst for the petroleum industry.^[5] For example, silica impregnated with a solution of perrhenic acid is reduction with hydrogen at 500 °C. This catalyst is used in the dehydrogenation of alcohols and also promotes the decomposition of ammonia.

Other catalytic reactions

Perrhenic acid is a precursor to a variety of homogeneous catalysts, some of which are promising in niche applications that can justify the high cost of rhenium. In combination with tertiary arsines, perrhenic acid gives a catalyst for the epoxidation of alkenes with hydrogen peroxide.^[6] Perrhenic acid catalyses the dehydration of oximes to nitriles.^[7]

Other uses

Perrhenic acid is also used in the manufacture of x-ray targets.

References

1. ^ Glemser, O.; Müller, A.; Schwarzkopf, H. “Gasförmige Hydroxide. IX. Über ein Gasförmiges Hydroxid des Rheniums” Zeitschrifft fur anorganische und allgemeine Chemie 1964, volume 334, pages 21-26. doi:10.1002/zaac.19643340105.
2. ^ Beyer, H.; Glemser, O.; Krebs, B. “Dirhenium Dihydratoheptoxide Re₂O₇(OH₂)₂ - New Type of Water Bonding in an Aquoxide” Angewandte Chemie, International Edition English 1968, Volume 7, Pages 295 - 296. doi:10.1002/anie.196802951.
3. ^ Schwarz, D. E.; Frenkel, A. I.; Nuzzo, R. G.; Rauchfuss, T. B.; Vairavamurthy, A., "Electrosynthesis of ReS₄. XAS Analysis of ReS₂, Re₂S₇, and ReS₄", Chemistry of Materials, 2004, volume 16, pages 151-158.doi:10.1021/cm034467v
4. ^ Parshall, G. W. Inorganic Syntheses, 1977, volume XVII, pages 110-112. ISBN 0-07-044327-0.
5. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
6. ^ ^  van Vliet, M. C. A.; Arends, I. W. C. E.; Sheldon, R. A. (1999). "Rhenium Catalysed Epoxidations with Hydrogen Peroxide: Tertiary Arsines as Effective Cocatalysts". J. Chem. Soc., Perkin Trans. 1: 377–80.
7. ^ Ishihara, K.; Furuya, Y.; Yamamoto, H. ”Rhenium(VII) Oxo Complexes as Extremely Active Catalysts in the Dehydration of Primary Amides and Aldoximes to Nitriles” [[Angewandte Chemie, International Edition, volume 41, 2983-2986; 2002. doi:10.1002/1521-3773

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