Gold(III) bromide
5024105
195316889
2008-03-02T13:26:42Z
Benjah-bmm27
126395
/* Preparation */ decapitalisation
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| ImageFile = Gold-tribromide-dimer-3D-balls.png
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| ImageName = Ball-and-stick model of gold(III) bromide
| ImageFile1 = Gold-tribromide-dimer-3D-vdW.png
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| ImageName1 = Space-filling model of gold(III) bromide
| IUPACName = gold(III) bromide
| OtherNames = Auric bromide<br />Gold bromide<br />Gold(III) bromide<br />Gold tribromide <br /> Digold hexabromide
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| CASNo = [10294-28-7] ;[11092-53-8]
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| Section2 = {{Chembox Properties
| Formula = AuBr<sub>3</sub>
| MolarMass = 436.69 g/mol
| Appearance = dark red to black crystalline
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| MeltingPt = 97.5 °C
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| Section7 = {{Chembox Hazards
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Gold(III) bromide is a dark-red to black crystalline solid. <ref name="Macintyre">Macintyre, J. E. (ed.) ''Dictionary of Inorganic Compounds''; Chapman & Hall: London, 1992; vol. 1, pp. 121</ref> <ref name="Greenwood">Greenwood, N.N.; Earnshaw, A. ''Chemistry of the Elements''; Butterworth-Heineman: Oxford,1997; pp. 1183-1185</ref> <ref name="Cotton">Cotton, F.A.; Wilkinson, G.; Murillo, C.A.; Bochmann, M. ''Advanced Inorganic Chemistry''; John Wiley & Sons: New York, 1999; pp. 1101-1102</ref> It has the [[empirical formula]] AuBr<sub>3</sub>, but exists primarily as a dimer with the [[molecular formula]] Au<sub>2</sub>Br<sub>6</sub> in which two gold atoms are bridged by two bromine atoms.<ref name="Greenwood"/> <ref name="Cotton"/> <ref name="Schulz">Schulz, A.; Hargittai, M. ''Chem. Eur. J.'' '''2001''', vol. 7, pp. 3657-3670</ref> It is commonly referred to as gold(III) bromide, gold tribomide, and rarely but traditionally auric bromide, and sometimes as digold hexabromide. As is similar with the other gold halides, this compound is unique for being a coordination complex of a [[group 11]] transition metal that is stable in an [[oxidation state]] of three whereas copper or silver complexes persist in oxidation states of one or two.<ref name="Schwerdtferger1">Schwerdtfeger, P.'' J. Am. Chem. Soc.'' '''1989''', vol. 111, pp. 7261-7262</ref>
==History==
The first mention of any research or study of the gold halides dates back to the early to mid 1800s, and there are three primary researchers associated with the extensive investigation of this particular area of chemistry: Thomsen, Schottländer, and Krüss.<ref name="Lengefield">Lengefield, F. ''American Chemical Journal'' '''1901''', vol. 26, pp. 324</ref> <ref name="Thomsen">Thomsen, J.'' J. prakt. Chem.'' '''1876''', vol. 13, pp. 337</ref> <ref name="Schottlander">
Schottländer, ''Ann. Chem. (Liebig)'', vol. 217, pp. 312</ref> <ref name="Kruss">Krüss, G. ''Ber. d. chem. Ges.'' '''1887''', vol. 20, pp. 2634</ref>
==Structure==
The dimer, digold hexabromide, has structural properties similar to those of the other gold trihalide dimeric complounds, such as [[gold(III) chloride]]. The gold centers exhibit [[square planar]] coordination with bond angles of roughly 90 degrees.<ref name="Cotton"/> <ref name="Schulz"/><br />
Calculations indicate that in the hypothetical monomeric forms of the gold trihalides, the [[Jahn-Teller effect]] causes differences to arise in the structures of the gold halide complexes. For instance, gold(III) bromide contains one long and two short gold-bromine bonds whereas gold(III) chloride and gold(III) fluoride consist of two long and one short gold-halogen bonds.<ref name="Schulz"/> Moreover, gold tribromide does not exhibit the same coordination around the central gold atom as gold trichloride or gold trifluoride. In the latter complexes, the coordination exhibits a T-conformation, but in gold tribromide the coordination exists as more of a dynamic balance between a Y-conformation and a T-conformation. This coordination difference can be attributed to the [[Jahn-Teller effect]] but more so to the decrease in π-back bonding of the gold atoms with the bromine ligands compared to the π-back bonding found with fluorine and chlorine ligands. It is also this decrease in π-back bonding which explains why gold tribromide is less stable than its trifluoride and trichloride counterparts.<ref name="Schulz"/>
[[Image:Gold_Bromide.png|350px]]
==Preparation==
The most common synthesis method of gold(III) bromide is heating gold and excess liquid [[bromine]] at 140 °C: <ref name="Macintyre"/>
:2Au + 3Br<sub>2</sub> (140°C) → Au<sub>2</sub>Br<sub>6</sub>
Alternatively, the halide-exchange reaction of [[gold(III) chloride]] with [[hydrobromic acid]] has also been proven successful in synthesizing gold(III) bromide:
:Au<sub>2</sub>Cl<sub>6</sub> + 6HBr → 6HCl + Au<sub>2</sub>Br<sub>6</sub>
This reaction is driven by the production of the relatively more stable [[hydrochloric acid]] compared with [[hydrobromic acid]].<ref name="Dell">
Dell'Amico, D.B.; Calderazzo, F.; Morvillo, A.; Pelizzi, G; Robino, P. ''J. Chem. Soc. Dalton Trans.'' '''1991,''' pp. 3009-3016</ref>
==Chemical Properties==
The neutral monomer AuBr<sub>3</sub>, as well as the other neutral gold trihalide species, has not been isolated in the gas phase which indicates the [[coordination number]] three is not favored.<ref name="Schwerdtferger1"/> <ref name="Schwerdtfeger2">Schwerdtfeger, P.; Boyd, P.D.W.; Brienne, S.; Burrell, K. ''Inorg. Chem.'' '''1992''', vol. 31, pp. 3411-3422</ref> Predominantly, gold(III) displays square planar coordination corresponding to a preferred coordination number of four.<ref name="Cotton"/><br />
Specifically, in solution gold(III) trihalides have the tendency to add a fourth ligand to form the more preferred four-coordinate complex.<ref name="Schwerdtferger1"/> <ref name="Schwerdtfeger2"/> With respect to gold tribromide, it is common to purchase gold(III) bromide hydrate, AuBr<sub>3</sub> ⋅ H<sub>2</sub>O, where the central gold atom exhibits a coordination number of four, rather than the [[anhydrous]] form of the compound, which exhibits a coordination number of three.<br />
Alternatively, if there is no addition of a fourth ligand, gold tribromide will oligomerize to form the halogen-bridged dimer complex mentioned previously.<ref name="Schwerdtferger1"/>
:2AuBr<sub>3</sub> → Au<sub>2</sub>Br<sub>6</sub>
Furthermore, like [[gold(III) chloride]], gold tribromide is a lewis acid and can form several complexes.<ref name="Schwerdtfeger2"/> For example, in the presence of hydrobromic acid, the dimer dissolves and bromoauric acid is formed.<ref name="Cotton"/>
:[[Hydrobromic acid|HBr]]([[Aqueous|aq]]) + AuBr<sub>3</sub>(aq) → H<sup>+</sup>AuBr<sub>4</sub><sup>−</sup>(aq)
The dimer also undergoes [[hydrolysis]] rapidly in moist air.<ref name="Macintyre"/> <ref name="Cotton"/>
==Uses==
Gold(III) Bromide is used as a catalyst in a variety of reactions, but one of its most interesting uses is found in the [[Diels-Alder reaction]]. Specifically, the compound catalyzes the reaction between an enynal unit and [[carbonyl]] compounds to form a six-membered cyclic compound.<ref name="Asao"> Asao, N.; Aikawa, H.; Yamamoto, Y. ''J. Am. Chem. Soc.'' '''2004''', vol. 126, pp. 7458-7459</ref>
[[Image:Gold_Bromide_in_Diels-Alder_Reaction.png|400px]]
Another catalytic use of gold tribromide is in the nucleophilic substitution reaction of propargylic alcohols. In this reaction, the gold complex acts as an alcohol-activating agent to facilitate the substitution.<ref name="Georgy">Georgy, M.; Boucard, V.; Campagne, J.'' J. Am. Chem. Soc.'' '''2005''', vol. 127, pp. 14180-14181</ref>
[[Image:Propargylic_Alcohol_Nucleophilic_Substitution.png|400px]]
{{commonscat|gold tribromide}}
==References==
<references/>
==External links==
[[http://www.sigmaaldrich.com/catalog/search/ProductDetail/ALDRICH/398470]] Sigma Alrich Product info for Gold(III) Bromide
[[http://www.webelements.com/webelements/compounds/text/Au/Au2Br6-10294287.html]] Web Elements info page for Gold(III) Bromide
[[Category:Bromides]]
[[Category:Gold compounds]]
[[Category:Inorganic compound stubs]]
[[ar:بروميد ذهب ثلاثي]]