Gold complexes have been investigated for use in catalysis and as anti-cancer compounds. N-heterocyclic carbenes (NHCs) are useful ligands for such applications. Most reported gold complexes are gold(I)-based, since gold(III) ions are highly oxidizing and can easily cause side reactions. In addition, most Au^III–NHC complexes have only one or two NHC ligands, with only a single example of an Au(NHC)₄-type complex to date.

Murray V. Baker, University of Western Australia, Perth, and colleagues have synthesized six new complexes of the Au^III(NHC)₄ type (general structure pictured). The team treated bis- or tetrakis(imidazolium) salts with KAuCl₄ and acetate as a base to prepare three different complexes of the type [Au(NHC)₄Cl₂]Cl. These could be further reacted with KPF₆ to give square-planar [Au(NHC)₄](PF₆)₃ complexes.

The reactions proceed in good yields and the [Au^III(NHC)₄] groups are remarkably stable for gold(III) complexes. The compounds show no signs of decomposition in ¹H-NMR experiments during prolonged heating in DMSO-d₆ (deuterated dimethyl sulfoxide) at 120 °C or in D₂O at 100 °C.

• Stable Au^III complexes with four N-heterocyclic carbene groups can be prepared in high yield directly from KAuCl₄,
  Ahmed H. Mageed, Brian W. Skeltona, Murray V. Baker,
  Dalton Trans. 2017.
  DOI: 10.1039/c7dt01272a