Perrhenic acid

Chembox new
ImageFile = ChemicalStructureOfPerrhenicAcid.png ImageSize = 150px
ImageName = Perrhenic acid
ImageFile1 = Perrhenic-acid-3D-balls.png ImageSize1 = 150px
ImageName1 = Ball-and-stick model of the perrhenic acid molecule
IUPACName = Perrhenic(VII) acid
OtherNames = Hydrated rhenium(VII) oxide
Section1 = Chembox Identifiers
CASNo = 13768-11-1
RTECS = TT4550000
Section2 = Chembox Properties
Formula = H₄O₉Re₂ (solid)
HO₄Re (gas)
MolarMass = 251.2055 g/mol
Appearance = Pale yellow solid
Density = ?
Solubility = Soluble
MeltingPt = °C (? K)
BoilingPt = sublimes
pKa = -1.25Fact|date=February 2007
Section3 = Chembox Structure
Coordination = octahedral-tetrahedral (solid)
tetrahedral (gas)
Section7 = Chembox Hazards
ExternalMSDS =
MainHazards = Corrosive
NFPA-H = 3
NFPA-F =
NFPA-R =
FlashPt = Non-flammable
RPhrases = R34
SPhrases = S26, S36/37, S39, S45
Section8 = Chembox Related
OtherCpds = Re₂O₇, Mn₂O₇

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. [cite journal | author = Glemser, O.; Müller, A.; Schwarzkopf, H. | title = Gasförmige Hydroxide. IX. Über ein Gasförmiges Hydroxid des Rheniums | journal = Zeitschrift für anorganische und allgemeine Chemie | year = 1964 | volume = 334 | pages = 21–26 | doi = 10.1002/zaac.19643340105.] When a solution of Re₂O₇ is kept for a period of months, it breaks down and crystals of HReO₄.H₂O are formed, which contain tetrahedral ReO₄⁻ [Greenwood&Earnshaw] For most purposes, perrhenic acid and rhenium(VII) oxide are used interchangeably.

Properties

The structure of solid perrhenic acid is [O₃Re-O-ReO₃(H₂O)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.] 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)₂Gaseous perrhenic acid is tetrahedral, as suggested by its formula HOReO₃.

Reactions

Perrhenic acid or the related anhydrous oxide Re₂O₇ converts to dirhenium heptasulfide upon treatment with hydrogen sulfide::Re₂O₇+ 7 H₂S → Re₂S₇ + 7 H₂OThe heptasulfide, which has a complex structure, [cite journal | author = Schwarz, D. E.; Frenkel, A. I.; Nuzzo, R. G.; Rauchfuss, T. B.; Vairavamurthy, A. | title = Electrosynthesis of ReS₄. XAS Analysis of ReS₂, Re₂S₇, and ReS₄ | journal = Chemistry of Materials | year = 2004 | volume = 16 | pages = 151–158 | doi = 10.1021/cm034467v] 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. [cite journal | title = Phosphine Complexes of Rhenium | author = Parshall, G. W. |journal = Inorganic Syntheses | year = 1997 | volume = 17 | pages = 110–112
doi = 10.1002/9780470132487.ch31]

Perrhenic acid combined with platinum on a support gives rise to a useful hydrogenation and hydrocracking catalyst for the petroleum industry. [Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.] 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.

Catalysis

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. [cite journal|author=van Vliet, M. C. A.; Arends, I. W. C. E.; Sheldon, R. A.|year=1999|title=Rhenium Catalysed Epoxidations with Hydrogen Peroxide: Tertiary Arsines as Effective Cocatalysts|journal=J. Chem. Soc., Perkin Trans. 1|pages=377–80 | doi = 10.1039/a907975k ] Perrhenic acid catalyses the dehydration of oximes to nitriles. [cite journal | author = Ishihara, K.; Furuya, Y.; Yamamoto, H. | title = Rhenium(VII) Oxo Complexes as Extremely Active Catalysts in the Dehydration of Primary Amides and Aldoximes to Nitriles | journal = Angewandte Chemie, International Edition | volume = 41 | pages = 2983–2986 | year = 2002 | doi = 10.1002/1521-3773 | doi_brokendate = 2008-06-23]
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Other uses

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

References