Gold(III) chloride

Gold(III) chloride


Crystal structure of AuCl3
Names
IUPAC name
Gold(III) trichloride
Other names
Auric chloride
Gold trichloride
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.033.280
RTECS number
  • MD5420000
UNII
  • InChI=1S/Au.3ClH/h;3*1H/q+3;;;/p-3 Y
    Key: RJHLTVSLYWWTEF-UHFFFAOYSA-K Y
  • InChI=1/Au.3ClH/h;3*1H/q+3;;;/p-3
    Key: RJHLTVSLYWWTEF-DFZHHIFOAC
  • Cl[Au-]1(Cl)[Cl+][Au-]([Cl+]1)(Cl)Cl
Properties
AuCl3
(exists as Au2Cl6)
Molar mass 606.6511 g/mol
Appearance Red crystals (anhydrous); golden, yellow crystals (monohydrate)[1]
Density 4.7 g/cm3
Melting point 254 °C (489 °F; 527 K) (decomposes)
68 g/100 ml (cold)
Solubility soluble in ether and ethanol, slightly soluble in liquid ammonia
−112·10−6 cm3/mol
Structure
monoclinic
P21/C
a = 6.57 Å, b = 11.04 Å, c = 6.44 Å
α = 90°, β = 113.3°, γ = 90°[2]
Square planar
Thermochemistry
−117.6 kJ/mol[3]
Hazards[4]
Occupational safety and health (OHS/OSH):
Main hazards
 Irritant
GHS labelling:
Warning
H315, H319, H335
P261, P305+P351+P338
Related compounds
Other anions
 Gold(III) fluoride
Gold(III) bromide
Gold(III) nitrate
Other cations
 Gold(I) chloride
Silver(I) chloride
Platinum(II) chloride
Mercury(II) chloride
Supplementary data page
Gold(III) chloride (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references
This article is about the trichloride. It is not to be confused with chloroauric acid.

Gold(III) chloride, traditionally called auric chloride, is a compound of gold and chlorine with the molecular formula Au2Cl6. The "III" in the name indicates that the gold has an oxidation state of +3, typical for many gold compounds. Gold(III) chloride is hygroscopic and decomposes in visible light. This compound is a dimer of AuCl3. This compound has few uses, although it catalyzes various organic reactions.

Structure

AuCl3 exists as a chloride-bridged dimer both as a solid and vapour, at least at low temperatures.[2] Gold(III) bromide behaves analogously.[1] The structure is similar to that of iodine(III) chloride.

Each gold center is square planar in gold(III) chloride,[1] which is typical of a metal complex with a d8 electron count. The bonding in AuCl3 is considered somewhat covalent.

Preparation

Gold(III) chloride is most often prepared by passing chlorine gas over gold powder at 180 °C (356 °F):[1]

2 Au + 3 Cl2 → Au2Cl6

The chlorination reaction can be conducted in the presence of tetrabutylammonium chloride, the product being the lipophilic salt tetrabutylammonium tetrachloraurate.[5]

Another method of preparation is via chloroauric acid, which is obtained by first dissolving the gold powder in aqua regia to give chloroauric acid:[6]

Au + HNO3 + 4 HCl → H[AuCl4] + 2 H2O + NO

The resulting chloroauric acid is subsequently heated to give Au2Cl6:

2 H[AuCl4] → Au2Cl6 + 2 HCl

Reactions
Concentrated aqueous solution of gold(III) chloride

On contact with water, AuCl3 forms acidic hydrates and the conjugate base [AuCl3(OH)]−. An Fe2+ ion may reduce it, causing elemental gold to be precipitated from the solution.[1]

Anhydrous AuCl3 begins to decompose to AuCl at around 160 °C (320 °F), however, this, in turn, undergoes disproportionation at higher temperatures to give gold metal and AuCl3:

AuCl3 → AuCl + Cl2 (>160 °C)
3 AuCl → AuCl3 + 2 Au (>420 °C)

AuCl3 is a lewis acid and readily forms complexes. For example, it reacts with hydrochloric acid to form chloroauric acid (H[AuCl4]):

HCl + AuCl3 → H+ + [AuCl4]−

Chloroauric acid is the product formed when gold dissolves in aqua regia.

Other chloride sources, such as KCl, also convert AuCl3 into [AuCl4]−. Aqueous solutions of AuCl3 react with an aqueous base such as sodium hydroxide to form a precipitate of Au(OH)3, which will dissolve in excess NaOH to form sodium aurate (NaAuO2). If gently heated, Au(OH)3 decomposes to gold(III) oxide, Au2O3, and then to gold metal.[7][8][9][10][11]

Gold(III) chloride is the starting point for the chemical synthesis of many other gold compounds. For example, the reaction with potassium cyanide produces the water-soluble complex, K[Au(CN)4]:

AuCl3 + 4 KCN → K[Au(CN)4] + 3 KCl

Gold(III) chloride reacts with benzene (and a variety of other arenes) under mild conditions (reaction times of a few minutes at room temperature) to produce the dimeric phenylgold(III) dichloride:[12]

2 PhH + Au2Cl6 → [PhAuCl2]2 + 2 HCl

Applications

Organic synthesis

As of 2003, AuCl3 has attracted the interest of organic chemists as a mild acid catalyst for various reactions, [13] although no transformations have been commercialised. Gold(III) salts, especially Na[AuCl4], provide an alternative to mercury(II) salts as catalysts for reactions involving alkynes. An illustrative reaction is the hydration of terminal alkynes to produce acetyl compounds.[14]

Gold catalyses the alkylation of certain aromatic rings and the conversion of furans to phenols. Some alkynes undergo amination in the presence of gold(III) catalysts. For example, a mixture of acetonitrile and gold(III) chloride catalyses the alkylation of 2-methylfuran by methyl vinyl ketone at the 5-position:

The efficiency of this organogold reaction is noteworthy because both the furan and the ketone are sensitive to side reactions such as polymerisation under acidic conditions. In some cases where alkynes are present, phenols sometimes form (Ts is an abbreviation for tosyl):[15]

This reaction involves a rearrangement that gives a new aromatic ring.[16]

Production of gold nanoparticles

Gold(III) chloride is used in producing gold nanoparticles. Gold nanoparticles can be formed by the reaction of gold(III) chloride and sodium tetrafluoroborate and then coating with didodecyldimethylammonium bromide. Then washing with 1-dodecanethiol and ethanol proved to be the most effective method for forming nanoparticles. However, other methods work such as replacing the 1-dodecanethiol with dioctyl sulfide.[17] The gold(III) chloride is the source of gold in this production.

References
  1. Egon Wiberg; Nils Wiberg; A. F. Holleman (2001). Inorganic Chemistry (101 ed.). Academic Press. pp. 1286–1287. ISBN 978-0-12-352651-9.
  2. E. S. Clark; D. H. Templeton; C. H. MacGillavry (1958). "The crystal structure of gold(III) chloride". Acta Crystallogr. 11 (4): 284–288. doi:10.1107/S0365110X58000694. Retrieved 2010-05-21.
  3. CRC handbook of chemistry and physics : a ready-reference book of chemical and physical data. William M. Haynes, David R. Lide, Thomas J. Bruno (2016-2017, 97th ed.). Boca Raton, Florida. 2016. ISBN 978-1-4987-5428-6. OCLC 930681942.{{cite book}}: CS1 maint: others (link)
  4. "Gold Chloride". American Elements. Retrieved July 22, 2019.
  5. Buckley, Robbie W.; Healy, Peter C.; Loughlin, Wendy A. (1997). "Reduction of [NBu4][AuCl4] to [NBu4][AuCl2] with Sodium Acetylacetonate". Australian Journal of Chemistry. 50 (7): 775. doi:10.1071/C97029.
  6. Block, B. P. (1953). "Gold Powder and Potassium Tetrabromoaurate(III)". Inorganic Syntheses. 4: 14–17. doi:10.1002/9780470132357.ch4. ISBN 9780470132357.
  7. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997
  8. Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990
  9. The Merck Index. An Encyclopaedia of Chemicals, Drugs and Biologicals. 14. Ed., 2006, p. 780, ISBN 978-0-911910-00-1.
  10. H. Nechamkin, The Chemistry of the Elements, McGraw-Hill, New York, 1968
  11. A. F. Wells, Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984
  12. Li, Zigang; Brouwer, Chad; He, Chuan (2008-08-01). "Gold-Catalyzed Organic Transformations". Chemical Reviews. 108 (8): 3239–3265. doi:10.1021/cr068434l. ISSN 0009-2665. PMID 18613729.
  13. G. Dyker, An Eldorado for Homogeneous Catalysis?, in Organic Synthesis Highlights V, H.-G. Schmaltz, T. Wirth (eds.), pp 48–55, Wiley-VCH, Weinheim, 2003
  14. Y. Fukuda; K. Utimoto (1991). "Effective transformation of unactivated alkynes into ketones or acetals with a gold(III) catalyst". J. Org. Chem. 56 (11): 3729. doi:10.1021/jo00011a058.
  15. A. S. K. Hashmi; T. M. Frost; J. W. Bats (2000). "Highly Selective Gold-Catalyzed Arene Synthesis". J. Am. Chem. Soc. 122 (46): 11553. doi:10.1021/ja005570d.
  16. A. Stephen; K. Hashmi; M. Rudolph; J. P. Weyrauch; M. Wölfle; W. Frey; J. W. Bats (2005). "Gold Catalysis: Proof of Arene Oxides as Intermediates in the Phenol Synthesis". Angewandte Chemie International Edition. 44 (18): 2798–801. doi:10.1002/anie.200462672. PMID 15806608.
  17. M. Lin; C. M. Sorensen; K. J. Klabunde (1999). "Ligand-Induced Gold Nanocrystal Superlattice Formation in Colloidal Solution". Chemistry of Materials. 11 (2): 198–202. doi:10.1021/cm980665o.
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