Organogold chemistry

Organogold chemistry is the study of compounds containing gold–carbon bonds. They are studied in academic research, but have not received widespread use otherwise. The dominant oxidation states for organogold compounds are I with coordination number 2 and a linear molecular geometry and III with CN = 4 and a square planar molecular geometry.

Gold(I)

Gold(I) complexes are 2-coordinate, linear, diamagnetic, 14 electron species. Many have the formula LAuR, with L = triphenylphosphine or an isocyanide. Gold(I) can also exist as the aurate M[AuR2] (the ate complex) whereby the cation is usually fitted with a complexing agent to improve stability. The AuR2− anion is also linear just as other M(d10) species such as Hg(Me)2 and Pd(Me)22+. Gold forms acetylides (capable of forming polymeric structures), carbenes and carbynes. The classic method for the preparation of LAuR compounds is by reaction of a Grignard reagent with a gold(I) halide. A subsequent reaction with an organolithium R-Li forms the ate complex. The cyclic pentamer (MesAu)5 is formed by a reaction between Au(CO)Cl and the mesityl Grignard reagent. Gold cyanide compounds (MAu(CN)2) are of some importance to gold cyanidation, a process for the extraction of gold from low-grade ore. The carbon to metal bond in metal cyanides is usually ionic but evidence exists that the C-Au bonding in the gold cyanide ion is covalent.

Alkene and alkyne complexes

Isolated and characterized examples of gold(I) complexes of alkenes and alkynes are relatively rare, despite these species often being invoked as intermediates. A two-coordinate complex [(R3P)Au(C2H4)]+ has been crystallized (as its hexafluoroantimonate salt) for the very bulky phosphine ligand (R = 4,4′-di-tert-butylbiphenylyl). The tris(ethylene) complex is also known.

Gold(III) Gold(III) complexes are 4 coordinate, square planar, diamagnetic, toxic, 16 electron species. When the formal coordination number is less than 4, ligands such as chlorine can make up for it by forming a bridging ligand. Intramolecular chelation is another strategy. In general gold(III) compounds are toxic and therefore less studied than gold(I). Monoarylgold(III) complexes are one well-studied class of complexes. They are often prepared by direct electrophilic auration of arenes by AuCl3. Homoleptic tetraalkylaurate(III) complexes (e.g. Li[AuMe4]) are also well-characterized.

Bonding The bonding in gold complexes is subject to normal and some exceptional factors, which have been described as aurophilicity. The Dewar–Chatt–Duncanson model applies to gold complexes of alkenes and alkynes, although isolated examples of such complexes are rare, in part due to the relatively weak degree of backbonding of late transition metals. When compared to analogous complexes based on lighter congeners copper and silver, the computed difference in magnitude between π→M bonding and M→π* backbonding was found to be the most significant for a complex based on gold, in line with the high degree of electrophilicity of gold complexes observed catalytically. Relativistic effects are significant in organogold chemistry due to the large nuclear charge of the metal (Z = 79). As a consequence of relativistically expanded 5d orbitals, the LAu fragment can stabilize a neighboring carbocation via electron donation into the empty p-type orbital. Thus, in addition to their expected carbocation-like reactivity, these cations also exhibit significant carbene character, a property that has been exploited in catalytic transformations such as cyclopropanation and C-H insertion.

Gold catalysis Gold(I) chloride, gold(III) chloride, and chloroauric acid function as homogeneous catalysta, but they quickly deactivate or form precipitates. Phosphine- or NHC-ligated gold(I) complexes are more robust. These complex are typically prepared and stored as the bench-stable (but unreactive) chlorides, LAuCl, e.g., chloro(triphenylphosphine)gold(I), which are typically activated via halide abstraction with silver salts like AgOTf, AgBF4, or AgSbF6 to generate a cationic gold(I) species. Although the coordinatively unsaturated complex "LAu+" is notionally generated from a LAuCl/AgX mixture, the exact nature of the cationic gold species and the role of the silver salt remains somewhat contentious. The para-nitrobenzoate, bistriflimide, and certain nitrile complexes represent catalytically active yet isolable silver-free precatalysts. Cationic gold(I) forms π-complexes with alkene or alkynes. These complexes are similar to those of mercury(II) and platinum(II). Electrophilic ions and complexes such as these with a strong propensity to form π-complexes are generally known as pi(π)-acids (see also: cation–pi interaction). Gold(I)-alkene and -alkyne complexes are susceptible to nucleophilic attack. In oxymercuration the resultant organomercurial species is generated stoichiometrically, and requires an additional step to liberate the product. In the case of gold, protonolysis of the Au-C bond closes the catalytic cycle, allowing the coordination of another substrate. Some practical advantages of gold(I) catalysis include: 1) air stability (due to the high oxidation potential of Au(I)), 2) tolerance towards adventitious moisture (due its low oxophilicity), and 3) relatively low toxicity compared to other pi-acids (e.g., Pt(II) and Hg(II)). Chemically, Au(I) complexes typically do not undergo oxidation to higher oxidation states, and Au(I)-alkyls and -vinyls are not susceptible to β hydride elimination.

Historical development The hydration of phenylacetylene to acetophenone using tetrachloroauric acid in a 37% yield was reported in 1976. An analogous mercury(II)-promote reaction was known. This same study lists a published yield >150%, indicating catalysis that perhaps was not acknowledged by the chemists. The reaction of the gold(III) salt NaAuCl4) with alkynes and water was reported in 1991. A major drawback of this method as Au(III) is rapidly reduced to catalytically inactive metallic gold and in 1998 returned to the theme of ligand supported Au(I) for the same transformation:

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