Gold(III) chloride

Gold(III) chloride, traditionally called auric chloride, is one of the most common compounds of gold. It has the formula AuCl₃. The Roman numerals in the name indicate that the gold has an oxidation state of +3, which is the most stable form for gold in its compounds. Gold also forms another chloride, gold(I) chloride (AuCl), which is less stable than AuCl₃. Also, chloroauric acid (HAuCl₄), the product formed when gold dissolves in aqua regia, is sometimes referred to rather loosely as "gold chloride", "acid gold trichloride" or even "gold(III) chloride trihydrate".

Gold(III) chloride is very hygroscopic and highly soluble in water and ethanol. It decomposes above 160 °C or in light, and it forms a variety of complexes with many ligands.

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Structure

AuCl₃ exists as a dimer both as a solid and as a vapour; the bromide AuBr₃ follows the same pattern. Each Au center is square planar. This structure is reminiscent of the bitetrahedral structures adopted by AlCl₃ and FeCl₃. The bonding in AuCl₃ is mainly covalent, reflecting the high oxidation state and relatively high electronegativity (for a metal) of gold.

Chemical properties

Anhydrous AuCl₃ begins to decompose to AuCl at around 160 °C; however, this will in turn undergo 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 which readily forms complexes. For example with hydrochloric acid, chloroauric acid (HAuCl₄) is formed:

HCl(aq) + AuCl₃(aq) → H⁺AuCl₄⁻(aq)

Ionic chlorides such as KCl will also form the AuCl₄⁻ ion with AuCl₃.

Aqueous solutions of AuCl₃ react with alkalis such as sodium hydroxide to form a precipitate of impure 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.^{[1][2][3][4][5][6]}

Preparation

Gold(III) chloride is most often prepared by direct chlorination of the metal at high temperatures:

2 Au + 3 Cl₂ → 2 AuCl₃

Uses

Gold(III) chloride is one of the most common gold compounds and it is therefore used as the starting point for the synthesis of many other gold compounds, for example the water-soluble cyanide complex KAu(CN)₄:

AuCl₃ + 4 KCN → KAu(CN)₄ + 3 KCl

Gold(III) salts, especially NaAuCl₄ (made from AuCl₃ + NaCl), provide a non-toxic alternative to mercury(II) salts as catalysts for alkyne reactions. One important reaction of this sort is the hydration of terminal alkynes to produce methyl ketones, for example:^[7]

Ketones are generally formed in over 90% yield under these conditions. Also useful is the related amination of alkynes which can use gold(III) catalysis.

In recent years AuCl₃ has begun to attract the interest of organic chemists as a mild acid catalyst for other reactions such as alkylation of aromatics and a conversion of furans to phenols (see below). Such reactions find use in organic synthesis and in the pharmaceutical industry. For example, 2-methylfuran (sylvan) undergoes smooth alkylation by methyl vinyl ketone at the 5-position:

The reaction gives a 91% yield in only 40 minutes at room temperature, using only 1 mole% of AuCl₃ in acetonitrile. This yield is noteworthy since both the furan and the ketone are normally very sensitive to side-reactions such as polymerisation under acidic conditions. In some cases where alkynes are present, a phenol may be formed:^[8]

The reaction undergoes a complex rearrangement that leads to formation of the new aromatic ring^[9]

Precautions

Gold(III) chloride should be handled wearing gloves and goggles; direct contact with the material should be avoided.

References

1. ^ N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997
2. ^ Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990
3. ^ The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960
4. ^ H. Nechamkin, The Chemistry of the Elements, McGraw-Hill, New York, 1968
5. ^ A. F. Wells, Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984
6. ^ 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
7. ^ Y. Fukuda, K. Utimoto, J. Org. Chem. 56, 3729-3731 (1991)
8. ^ A. S. K. Hashmi, T. M. Frost, J. W. Bats, J. Am. Chem. Soc. 122, 11553-11554 (2000)
9. ^ Hashmi, A. S. K.; Rudolph, M.; Weyrauch, J. P.; Wölfle, M.; Frey, W.; Bats, J. W. Angew. Chem. Int. Ed. 2005, 44, 2798-2801