gold(III) chloride

{{chem2|AuCl3}} exists as a chloride-bridged dimer both as a solid and vapour, at least at low temperatures.{{cite journal | author = E. S. Clark | author2 = D. H. Templeton | author3 = C. H. MacGillavry | year = 1958 | title = The crystal structure of gold(III) chloride | journal = Acta Crystallogr. | volume = 11 | issue = 4| pages = 284–288 | doi = 10.1107/S0365110X58000694 | url = http://scripts.iucr.org/cgi-bin/paper?S0365110X58000694 | access-date = 2010-05-21| doi-access = free }} Gold(III) bromide behaves analogously. The structure is similar to that of iodine(III) chloride.

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 {{chem2|AuCl3}} is considered somewhat covalent.

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.

Gold(III) chloride was first prepared in 1666 by Robert Boyle by the reaction of metallic gold and chlorine gas at 180 °C:{{Cite book

| author1 = Egon Wiberg | author2 = Nils Wiberg | author3 = A. F. Holleman | year = 2001 | title = Inorganic Chemistry| edition = 101 | publisher = Academic Press| isbn = 978-0-12-352651-9| pages = 1286–1287

}}{{cite book |author1=Robert Boyle |author1-link=Robert Boyle |title=The origine of formes and qualities |date=1666 |page=370 |url=http://name.umdl.umich.edu/A29017.0001.001 |language=en}}{{cite journal |author1=Thomas Kirke Rose |title=The dissociation of chloride of gold |journal=Journal of the Chemical Society, Transactions |date=1895 |volume=67 |pages=881–904 |doi=10.1039/CT8956700881 |url=https://zenodo.org/record/1784163 |language=en}}

:{{chem2|2 Au + 3 Cl2 → Au2Cl6}}

This method is the most common method of preparing gold(III) chloride. It can also be prepared by reacting gold powder with iodine monochloride:

:2 Au + 6 ICl → 2 AuCl₃ + 3 I₂

The chlorination reaction can be conducted in the presence of tetrabutylammonium chloride, the product being the lipophilic salt tetrabutylammonium tetrachloraurate.{{cite journal |doi=10.1071/C97029|title=Reduction of [NBu4][AuCl4] to [NBu4][AuCl2] with Sodium Acetylacetonate|year=1997|last1=Buckley|first1=Robbie W.|last2=Healy|first2=Peter C.|last3=Loughlin|first3=Wendy A.|journal=Australian Journal of Chemistry|volume=50|issue=7|page=775}}

Another method of preparation is via chloroauric acid, which is obtained by first dissolving the gold powder in aqua regia to give chloroauric acid:{{cite book |doi=10.1002/9780470132357.ch4|chapter=Gold Powder and Potassium Tetrabromoaurate(III)|series=Inorganic Syntheses|year=1953|volume=4|last1=Block|first1=B. P.|title=Inorganic Syntheses |pages=14–17|isbn=9780470132357}}

:{{chem2|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 {{chem2|Au2Cl6}}:{{cite journal |author1=Ya-jie Zheng |author2=Wei Guo |author3=Meng Bai |author4=Xing-wen Yang |title=Preparation of chloroauric acid and its thermal decomposition |journal=The Chinese Journal of Nonferrous Metals |date=2006 |volume=16 |issue=11 |pages=1976–1982 |url=http://ysxb.csu.edu.cn/previewFile?id=36231978&type=pdf&lang=en |archive-url= https://web.archive.org/web/20240327045836/http://ysxb.csu.edu.cn/previewFile?id=36231978&type=pdf&lang=en|archive-date= March 27, 2024|language=chinese}}

:{{chem2|2 H[AuCl4] → Au2Cl6 + 2 HCl}}

File:Gold(III) chloride solution.jpg

Anhydrous {{chem2|AuCl3}} begins to decompose to AuCl (gold(I) chloride) at around {{convert|160|°C}}; however, this, in turn, undergoes disproportionation at higher temperatures to give gold metal and AuCl₃:{{cite journal |author1=Michael J. Coghlan |author2=Rene-Viet Nguyen |author3=Chao-Jun Li |author4=Daniel Pflästerer |author5=A. Stephen K. Hashmi |title=Gold(III) Chloride |journal=Encyclopedia of Reagents for Organic Synthesis |date=2015 |pages=1–24 |doi=10.1002/047084289X.rn00325.pub3|isbn=9780470842898 }}

:{{chem2|AuCl3 → AuCl + Cl2}} (160 °C)

:{{chem2|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.{{cite journal |author1=Yiqin Chen |author2=Xuezeng Tian |author3=Wei Zeng |author4=Xupeng Zhu |author5=Hailong Hu |author6=Huigao Duan |title=Vapor-phase preparation of gold nanocrystals by chloroauric acid pyrolysis |journal=Journal of Colloid and Interface Science |date=2015 |volume=439 |pages=21–27 |doi=10.1016/j.jcis.2014.10.017 |publisher=Elsevier |pmid=25463171 |bibcode=2015JCIS..439...21C |language=en}}{{cite journal |author1=Robert G. Palgrave |author2=Ivan P. Parkin |title=Aerosol Assisted Chemical Vapor Deposition of Gold and Nanocomposite Thin Films from Hydrogen Tetrachloroaurate(III) |journal=Chemistry of Materials |date=2007 |volume=19 |issue=19 |pages=4639–4647 |doi=10.1021/cm0629006 |publisher=ACS Publications |language=en}}

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.{{cite journal |author1=E.M.W. Janssen |author2=J.C.W. Folmer |author3=G.A. Wiegers |title=The preparation and crystal structure of gold monochloride, AuCl |journal=Journal of the Less Common Metals |date=1974 |volume=38 |issue=1 |pages=71–76 |doi=10.1016/0022-5088(74)90204-5 |language=en}} This fact that no gold chlorides can exist above 400 °C is used in the Miller process.{{Cite book|title=Ullmann's Encyclopedia of Industrial Chemistry|author1=Hermann Renner |author2=Günther Schlamp|year=2000|isbn=978-3-527-30673-2|chapter=Gold, Gold Alloys, and Gold Compounds|pages=106–107 |doi=10.1002/14356007.a12_499}}

{{chem2|AuCl3}} is a Lewis acid and readily forms complexes. For example, it reacts with hydrochloric acid to form chloroauric acid ({{chem2|H[AuCl4]}}):

:{{chem2|HCl + AuCl3 → H+ + [AuCl4]−}}

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

On contact with water, {{chem2|AuCl3}} forms acidic hydrates and the conjugate base {{chem2|[AuCl3(OH)]−}}. A {{chem2|Fe(2+)}} ion may reduce it, causing elemental gold to be precipitated from the solution.Cotton, F.A.; Wilkinson, G.; Murillo, C.A.; Bochmann, M. Advanced Inorganic Chemistry; John Wiley & Sons: New York, 1999; pp. 1101-1102

Other chloride sources, such as KCl, also convert {{chem2|AuCl3}} into {{chem2|[AuCl4]−}}. Aqueous solutions of {{chem2|AuCl3}} react with an aqueous base such as sodium hydroxide to form a precipitate of {{chem2|Au(OH)3}}, which will dissolve in excess NaOH to form sodium aurate ({{chem2|NaAuO2}}). If gently heated, {{chem2|Au(OH)3}} decomposes to gold(III) oxide, {{chem2|Au2O3}}, and then to gold metal.{{cite book |author1=N. N. Greenwood |author2=A. Earnshaw |title=Chemistry of the Elements |date=1997 |publisher=Butterworth-Heinemann |location=Oxford, UK |isbn=9780750633659 |pages=1184–1185 |edition=2 |language=en}}The Merck Index. An Encyclopaedia of Chemicals, Drugs and Biologicals. 14. Ed., 2006, p. 780, {{ISBN|978-0-911910-00-1}}.H. Nechamkin, The Chemistry of the Elements, McGraw-Hill, New York, 1968, p. 222A. F. Wells, Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984, p. 909

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, {{chem2|K[Au(CN)4]}}:{{cite journal |author1=Henry K. Lutz |title=Synthesis and Analyses of KAu(CN)4 |journal=Honors Theses. |date=1961 |url=https://digitalworks.union.edu/theses/1986 |publisher=Union Digital Works |language=en}}

:{{chem2|AuCl3 + 4 KCN → K[Au(CN)4] + 3 KCl}}

Gold(III) fluoride can be also produced from gold(III) chloride by reacting it with bromine trifluoride.

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:{{Cite journal|last1=Li|first1=Zigang|last2=Brouwer|first2=Chad|last3=He|first3=Chuan|date=2008-08-01|title=Gold-Catalyzed Organic Transformations|journal=Chemical Reviews|volume=108|issue=8|pages=3239–3265|doi=10.1021/cr068434l|pmid=18613729|issn=0009-2665}}

:{{chem2|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:{{cite journal |author1=Daniela Belli Dell'Amico |author2=Fausto Calderazzo |author3=Fabio Marchetti |author4=Stefano Merlino |author5=Giovanni Perego |title=X-Ray crystal and molecular structure of Au4Cl8, the product of the reduction of Au2Cl6 by Au(CO)Cl |journal=Journal of the Chemical Society, Chemical Communications |date=1977 |pages=31–32 |doi=10.1039/C39770000031 |language=en}}{{cite journal |author1=Daniela Belli Dell'Amico |author2=Fausto Calderazzo |author3=Fabio Marchetti |author4=Stefano Merlino |title=Synthesis and molecular structure of [Au4Cl8], and the isolation of [Pt(CO)Cl5]– in thionyl chloride |journal=Journal of the Chemical Society, Dalton Transactions |date=1982 |issue=11 |pages=2257–2260 |doi=10.1039/DT9820002257 |language=en}}

:2 AuCl₃ + 2 CO → Au₄Cl₈ + 2 COCl₂

If carbon monoxide is in excess, Au(CO)Cl is produced instead.{{cite book |chapter=Carbonylchlorogold(I) |date=1986 |volume=24 |pages=236–238 |doi=10.1002/9780470132555.ch66 |title=Inorganic Syntheses |last1=Dell'Amico |first1=D. Belli |last2=Calderazzo |first2=F. |last3=Murray |first3=H. H. |author4-link=John P. Fackler Jr. |last4=Fackler |first4=J. P. |isbn=9780470132555 }}

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.{{cite book |author1=T.A. Ryan |author2=E.A. Seddon |author3=K.R. Seddon |author4=C. Ryan |title=Phosgene And Related Carbonyl Halides |date=1996 |publisher=Elsevier Science |isbn=9780080538808 |pages=242–243 |language=en}}{{cite journal |author1=M. S. Kharasch |author2=H. S. Isbell |title=The Chemistry of Organic Gold Compounds. I. Aurous Chloride Carbonyl and a Method of Linking Carbon to Carbon |journal=Journal of the American Chemical Society |date=1930 |volume=52 |issue=7 |pages=2919–2927 |doi=10.1021/ja01370a052 |language=en}}

Gold(III) chloride has many uses in the laboratory, and primarily thrives in this environment.

Since 2003, {{chem2|AuCl3}} has attracted the interest of organic chemists as a mild acid catalyst for various reactions,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 although no transformations have been commercialised. Gold(III) salts, especially sodium tetrachloroaurate, 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.{{cite journal |author1=Y. Fukuda |author2=K. Utimoto | title = Effective transformation of unactivated alkynes into ketones or acetals with a gold(III) catalyst | journal = J. Org. Chem. | year = 1991 | volume = 56 | issue = 11 | doi = 10.1021/jo00011a058 | page = 3729}}

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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:

:450px

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):{{cite journal |author1=A. S. K. Hashmi |author2=T. M. Frost |author3=J. W. Bats | title = Highly Selective Gold-Catalyzed Arene Synthesis | journal = J. Am. Chem. Soc. | year = 2000 | volume = 122 | issue = 46 | doi = 10.1021/ja005570d | pages = 11553}}

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This reaction involves a rearrangement that gives a new aromatic ring.{{cite journal |author1=A. Stephen |author2=K. Hashmi |author3=M. Rudolph |author4=J. P. Weyrauch |author5=M. Wölfle |author6=W. Frey |author7=J. W. Bats | title = Gold Catalysis: Proof of Arene Oxides as Intermediates in the Phenol Synthesis | journal = Angewandte Chemie International Edition| year = 2005 | volume = 44 | issue = 18 | doi = 10.1002/anie.200462672 | pages = 2798–801 | pmid = 15806608}}

Another example of an AuCl₃ catalyzed reaction is a hydroarylation, which is basically a Friedel-Crafts reaction using metal-alkyne complexes. Example, the reaction of mesitylene with phenylacetylene:{{Cite journal | doi = 10.1002/ejoc.200300260| title = Gold-Catalyzed Hydroarylation of Alkynes| journal = European Journal of Organic Chemistry| volume = 2003| issue = 18| pages = 3485–3496| year = 2003| last1 = Reetz | first1 = M. T. | last2 = Sommer | first2 = K. }}

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Gold(III) chloride can be used for the direct oxidation of primary amines into ketones, such as the oxidation of cyclohexylamine to cyclohexanone.

:300px

This reaction is pH sensitive, requiring a mildly acidic pH to proceed, however, it does not require any additional steps.

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.{{Cite journal|last1=Kharasch|first1=M. S.|last2=Isbell|first2=Horace S.|title=The Chemistry of Organic Gold Compounds. III. Direct Introduction of Gold into the Aromatic Nucleus (Preliminary Communication)|date=1931-08-01|journal=Journal of the American Chemical Society|volume=53|issue=8|pages=3053–3059|doi=10.1021/ja01359a030|issn=0002-7863}}

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.{{cite journal |author1=M. Lin |author2=C. M. Sorensen |author3=K. J. Klabunde |title=Ligand-Induced Gold Nanocrystal Superlattice Formation in Colloidal Solution |journal=Chemistry of Materials |date=1999 |volume=11 |issue=2 |pages=198–202 |doi=10.1021/cm980665o |language=en}}

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.{{cite journal |author1=Philip Ellis |title=Gold in photography |journal=Gold Bulletin |date=1975 |volume=8 |pages=7–12 |doi=10.1007/BF03215055 |s2cid=136538890 |language=en|doi-access=free }}

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.{{cite web |title=UM1995-16-O:AuClH |url=https://www.mindat.org/min-53581.html |website=mindat.org |access-date=27 April 2023}}{{cite journal |author1=John L. Jambor |author2=Nikolai N. Pertsev |author3=Andrew C. Roberts |title=New Mineral Names |journal=American Mineralogist |date=1996 |volume=81 |page=768 |url=http://www.minsocam.org/msa/ammin/toc/Articles_Free/1996/Jambor_p766-770_96.pdf |language=en}}

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Category:Photographic chemicals

Category:Gold–halogen compounds