Stable Au^III complexes with four N-heterocyclic carbene groups can be prepared in high yield directly from KAuCl₄

Gold(iii) N-heterocyclic carbene (NHC) complexes of form [Au(NHC)₄Cl₂]Cl were synthesized by reaction of KAuCl₄ with bis- and tetrakis(imidazolium) salts in the presence of a mild base. Treatment of these complexes with KPF₆ afforded four-coordinate Au^III complexes of form [Au(NHC)₄](PF₆)₃. X-Ray crystallography showed the [Au^III(NHC)₄]³⁺ cations in the hexafluorophosphate salts to have a square planar Au(NHC)₄ moiety [AuC_NHC 2.024(4)-2.082(7) Å]. In the [Au^III(NHC)₄Cl₂]⁺ cations in the chloride salts, coordination about Au was tetragonally-distorted octahedral, the axial Au-Cl bonds being substantially longer [AuCl 3.148(2)-3.693(1) Å] than the equatorial Au-C_NHC bonds [AuC 2.024(4)-2.082(7) Å]. NMR and conductance studies suggested that the structures of the complexes seen in the solid state persisted in DMSO solution, except in one case where a chlorido ligand dissociated from [Au^III(NHC)₄Cl₂]⁺ to form [Au^III(NHC)₄Cl]²⁺. The Au^III(NHC)₄ unit was surprisingly robust. An Au^III complex was found to undergo H/D exchange reactions in D₂O solution at 100 °C with no signs of decomposition detectable by ¹H NMR spectroscopy. ¹H NMR studies showed that various complexes containing Au^III(NHC)₄ moieties underwent little or no decomposition when heated at 120 °C in DMSO-d₆ for extended periods.