Gold(III) chloride, traditionally called auric chloride, is an inorganic compoundofgold and chlorine with the molecular formulaAu2Cl6. The "III" in the name indicates that the gold has an oxidation state of +3, typical for many gold compounds. It has two forms, the monohydrate (AuCl3·H2O) and the anhydrous form, which are both hygroscopic and light-sensitive solids. This compound is a dimerofAuCl3. This compound has a few uses, such as an oxidizing agent and for catalyzing various organic reactions.
Each gold center is square planar in gold(III) chloride, which is typical of a metal complex with a d8 electron count. The bonding in AuCl3 is considered somewhat covalent.[1]
Gold(III) chloride is a diamagnetic light-sensitive red crystalline solid that forms the orange monohydrate, AuCl3 · H2O; the anhydrous and monohydrate are both hygroscopic. The anhydrous form absorbs moisture from the air to form the monohydrate which can be reversed by the addition of thionyl chloride.[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:[9]
Au + HNO3 + 4 HCl → H[AuCl4] + 2 H2O + NO
The resulting chloroauric acid is subsequently heated in an inert atmosphere at around 100 °C to give Au2Cl6:[10][11]
2 H[AuCl4] → Au2Cl6 + 2 HCl
Reactions
[edit]Concentrated aqueous solution of gold(III) chloride
AnhydrousAuCl3 begins to decompose to AuCl (gold(I) chloride) at around 160 °C (320 °F), however, this, in turn, undergoes disproportionation at higher temperatures to give gold metal and AuCl3:[5][10]
AuCl3 → AuCl + Cl2 (160 °C)
3 AuCl → AuCl3 + 2 Au (>210 °C)
Due to the disproportionation of AuCl, above 210 °C, most of the gold is in the form of elemental gold.[12][11]
Gold(III) chloride is more stable in a chlorine atmosphere and can sublime at around 200 °C without any decomposition. In a chlorine atmosphere, AuCl3 decomposes at 254 °C yielding AuCl which in turn decomposes at 282 °C to elemental gold.[2][13] This fact that no gold chlorides can exist above 400 °C is used in the Miller process.[14]
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.[15][17][18][19]
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]:[20]
Gold(III) chloride reacts with benzene under mild conditions (reaction times of a few minutes at room temperature) to produce the dimeric phenylgold(III) dichloride; a variety of other arenes undergo a similar reaction:[21]
2 PhH + Au2Cl6 → [PhAuCl2]2 + 2 HCl
Gold(III) chloride reacts with carbon monoxide in a variety of ways. For example, the reaction of anhydrous AuCl3 and carbon monoxide under SOCl2 produces gold(I,III) chloride with Au(CO)Cl as an intermediate:[22][23]
2 AuCl3 + 2 CO → Au4Cl8 + 2 COCl2
If carbon monoxide is in excess, Au(CO)Cl is produced instead.[24][25]
However, under tetrachloroethylene and at 120 °C, gold(III) chloride is first reduced to gold(I) chloride, which further reacts to form Au(CO)Cl. AuCl3 is also known to catalyze the production of phosgene.[25][26]
Since 2003, AuCl3 has attracted the interest of organic chemists as a mild acid catalyst for various reactions,[27] 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.[28]
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):[29]
This reaction involves a rearrangement that gives a new aromatic ring.[30]
This reaction is pH sensitive, requiring a mildly acidic pH to proceed, however, it does not require any additional steps.[5]
In the production of organogold(III) compounds, AuCl3 is used as a source of gold. A main example of this is the production of monoarylgold(III) complexes, which are produced by direct electrophilic auration of arenes by gold(III) chloride.[32]
Gold(III) chloride is used in the synthesis of gold nanoparticles, which are extensively studied for their unique size-dependent properties and applications in fields such as electronics, optics, and biomedicine. Gold nanoparticles can be prepared by reducing gold(III) chloride with a reducing agent such as sodium tetrafluoroborate, followed by stabilization with a capping agent.[33]
Gold(III) chloride has been used historically in the photography industry as a sensitizer in the production of photographic films and papers. However, with the advent of digital photography, its use in this field has diminished.[34]
This compound does not occur naturally; however, a similar compound with the formula AuO(OH,Cl)·nH2O is known as a product of natural gold oxidation.[35][36]
^Haynes, William M.; Lide, David R.; Bruno, Thomas J., eds. (2016). CRC Handbook of Chemistry and Physics: A Ready-reference Book of Chemical and Physical Data (95th ed.). Boca Raton, Florida. p. 5-5. ISBN978-1-4987-5428-6. OCLC930681942.{{cite book}}: CS1 maint: location missing publisher (link)
^ abcdefMichael J. Coghlan; Rene-Viet Nguyen; Chao-Jun Li; Daniel Pflästerer; A. Stephen K. Hashmi (2015). "Gold(III) Chloride". Encyclopedia of Reagents for Organic Synthesis: 1–24. doi:10.1002/047084289X.rn00325.pub3. ISBN9780470842898.
^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.
^ abRobert G. Palgrave; Ivan P. Parkin (2007). "Aerosol Assisted Chemical Vapor Deposition of Gold and Nanocomposite Thin Films from Hydrogen Tetrachloroaurate(III)". Chemistry of Materials. 19 (19). ACS Publications: 4639–4647. doi:10.1021/cm0629006.
^Yiqin Chen; Xuezeng Tian; Wei Zeng; Xupeng Zhu; Hailong Hu; Huigao Duan (2015). "Vapor-phase preparation of gold nanocrystals by chloroauric acid pyrolysis". Journal of Colloid and Interface Science. 439. Elsevier: 21–27. Bibcode:2015JCIS..439...21C. doi:10.1016/j.jcis.2014.10.017. PMID25463171.
^E.M.W. Janssen; J.C.W. Folmer; G.A. Wiegers (1974). "The preparation and crystal structure of gold monochloride, AuCl". Journal of the Less Common Metals. 38 (1): 71–76. doi:10.1016/0022-5088(74)90204-5.
^Daniela Belli Dell'Amico; Fausto Calderazzo; Fabio Marchetti; Stefano Merlino; Giovanni Perego (1977). "X-Ray crystal and molecular structure of Au4Cl8, the product of the reduction of Au2Cl6 by Au(CO)Cl". Journal of the Chemical Society, Chemical Communications: 31–32. doi:10.1039/C39770000031.
^Daniela Belli Dell'Amico; Fausto Calderazzo; Fabio Marchetti; Stefano Merlino (1982). "Synthesis and molecular structure of [Au4Cl8], and the isolation of [Pt(CO)Cl5]– in thionyl chloride". Journal of the Chemical Society, Dalton Transactions (11): 2257–2260. doi:10.1039/DT9820002257.
^ abT.A. Ryan; E.A. Seddon; K.R. Seddon; C. Ryan (1996). Phosgene And Related Carbonyl Halides. Elsevier Science. pp. 242–243. ISBN9780080538808.
^M. S. Kharasch; H. S. Isbell (1930). "The Chemistry of Organic Gold Compounds. I. Aurous Chloride Carbonyl and a Method of Linking Carbon to Carbon". Journal of the American Chemical Society. 52 (7): 2919–2927. doi:10.1021/ja01370a052.
^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
^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.
^ abA. 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.
^Reetz, M. T.; Sommer, K. (2003). "Gold-Catalyzed Hydroarylation of Alkynes". European Journal of Organic Chemistry. 2003 (18): 3485–3496. doi:10.1002/ejoc.200300260.
^Kharasch, M. S.; Isbell, Horace S. (1931-08-01). "The Chemistry of Organic Gold Compounds. III. Direct Introduction of Gold into the Aromatic Nucleus (Preliminary Communication)". Journal of the American Chemical Society. 53 (8): 3053–3059. doi:10.1021/ja01359a030. ISSN0002-7863.
^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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