From the City of Gold to the Golden Fleece, from the Californian gold rush to the earliest 'al'chemists, gold has captured our imagination like few other elements have. Unlike most elements, gold is typically found in nature in its pure elemental form. In complexes, its common oxidation states are −I, +I and +III, with the odd rare compound featuring gold +II, +IV and +V. Apart from the elemental form, however, the zero oxidation state of gold has generally remained elusive to chemists. There have been some examples of mixed gold(I)/gold(0) complexes and mixed-metal clusters where an oxidation state of zero was assigned to gold. But simple mononuclear (LnAu), binuclear (LAu-AuL) and polynuclear [(LAu)n] neutral complexes, with atoms of gold coordinated end-on by ligands, have not been isolated.
A team from the USA and Germany, led by Guy Bertrand at the University of California, San Diego, have now managed to isolate metal–ligand complexes — containing either one or two gold atoms — in which the formal oxidation state of the metal atoms is zero. Each complex features two cyclic (alkyl)(amino)carbene (CAAC) ligands, forming a linear L–Au–L structure in the case of the mononuclear complex and slightly bent L–Au–Au L dinuclear compound. CAACs are good π-electron acceptors, which helps them to stabilize the electron-rich gold(0) centres; the carbene ligands are capable of forming strong bonds with the gold atoms. The green (mononuclear) and brown (dinuclear) complexes are stable for days at room temperature in solution and in the solid state, and the team were able to determine their structures (pictured) using X-ray crystallography.
Phosphines are currently used as ligands to stabilize gold clusters of various sizes, and these have found a wide range of applications as organic photovoltaics and therapeutic agents and in catalysis. However, gold particles stabilized by phosphines have all been positively charged. Thus, the ability of these CAAC ligands to stabilize gold in the formal zero oxidation state could open the door to the synthesis of neutral, differently sized gold clusters. These tiny 'nuggets' of neutral gold could provide a wealth of future opportunities in catalysis and other applications.
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Doherty, R. 24-carat chemistry. Nature Chem 5, 728 (2013). https://doi.org/10.1038/nchem.1741