Gold(I) chloride is a compound of gold and chlorine with the chemical formula AuCl. It is a metastable yellow solid that hydrolyses in warm water and decomposes to gold and chlorine gas at elevated temperatures. It has limited uses in organic chemistry as a Lewis acid.
History Gold(I) chloride was first reported by Louis Jacques Thénard in 1814, who had carefully heated gold(III) chloride and isolated a yellow solid, claiming that it was the proto-muriate of gold. Joseph Proust disputed his claim, along with Thénard's other erroneous claims, such as of gold(I) oxide, later in the year. However, gold(I) chloride was found to be a distinct compound from gold(III) chloride, as the synthesis was repeated by others, such as Hans Peter Jørgen Julius Thomsen. The structure was elucidated in 1974 by X-ray diffraction.
Preparation A modern synthesis entails the decomposition of gold(III) chloride under a chlorine atmosphere at 247 °C:
AuCl3 → AuCl + Cl2 Gold(III) chloride can also be heated in air at 185 °C to produce gold(I) chloride with careful temperature control.
Structure The structure of gold(I) chloride, which crystallizes in the tetragonal crystal system, consists of zig-zag chains of gold and chlorine, similar to gold(I) bromide and gold(I) iodide. The Au-Cl bond length is 2.36 Å, and the chain bends at the Cl with a bond angle of 92°.
Reactions Gold(I) chloride decomposes to gold metal and chlorine gas at around 210 °C:
2AuCl → 2Au + Cl2 Although a region of stability exists at higher temperatures and appropriate chlorine vapour pressures (254 to 282 °C at PCl2 = 1 atm), the compound remains metastable at ambient conditions. In warm water, the compound dispropotionates to metallic gold and gold(III) chloride in an autoredox reaction:
3 AuCl → 2 Au + AuCl3 At still higher temperatures, around 500 °C, all gold chlorides convert to gold even under a chlorine atmosphere. This conversion is key to the Miller process, which is widely used for the purification of gold. In hydrochloric acid, gold(I) chloride dissolves to form the colorless dichloroaurate(I) (AuCl2–) ion. At higher pH, this complex hydrolyses to gold(I)-hydroxo complexes and disproportionates to gold and gold(III). However, the salts of the dichloroaurate(I) ion, such as the tetrabutylammonium salt, are prepared from the respective tetrachloroaurate(III) ion instead from gold(I) chloride. Reaction with potassium bromide yields potassium auric bromide and potassium chloride with separation of metallic gold:
3 AuCl + 4 KBr → KAuBr4 + 2 Au + 3 KCl Carbon monoxide reduces gold(I) chloride at 110 °C to form phosgene:
2AuCl + CO → 2Au + COCl2 However, under benzene, it forms carbonylchlorogold(I) (COAuCl) instead.
Organogold(I) complexes Various gold(I) chloride complexes are known, such as Ph3PAuCl and Me2SAuCl. However, the synthesis of these does not utilize gold(I) chloride, but instead goes through a direct reduction from Au(III) in the presence of the ligand. These complexes are the major gateways to other organogold(I) complexes.
Applications Gold(I) chloride mostly only has applications in organogold catalyzed organic reactions. It is used as a Lewis acid to activate alkenes, alkynes, and allenes. For example, acetylenic acids cycloisomerize in the presence of gold(I) chloride:
Safety Generally, gold(I) compounds are less toxic than gold(III) compounds. However, gold(I) chloride is a skin sensitizer that may irritate the skin and eyes.
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