Gold Complexes with Tridentate Cyclometalating and NHC Ligands: A Search for New Photoluminescent Gold(III) Compounds

Many square-planar Pt­(II) complexes are strongly photoluminescent, particularly when they contain cyclometalating and/or NHC ligands which produce strong ligand fields. In this work we investigated the possibility of obtaining isoelectronic Au­(III) complexes with favorable luminescence properties. Toward this end, the coordination chemistry of gold with three different (potentially) tridentate ligands was explored: 1,3-bis­(1-hexyl-2′-benzimidazoyl)­benzene (L¹H), 2,6-bis­(3-butylimidazol-1-ium)­pyridine (L²H₂²⁺), and 2,6-bis­(3-hexylbenzimidazol-1-ium)­pyridine (L³H₂²⁺). Pt­(II) or Pd­(II) complexes are known for all of these ligands or closely related analogues, and hence we anticipated that similar Au­(III) complexes would be synthetically accessible as well. This turned out to be the case only for L¹, despite exploration of different synthetic routes including (i) direct complexation of Au­(III), (ii) transmetalation from Ag­(I) precursor complexes, and (iii) transmetalation from suitable Hg­(II) precursors. Only the mercury procedure was successful (at least in the case of L¹); transmetalation from silver or direct complexation yields other reaction products containing Au­(I) in most cases. Likewise, with ligands L² and L³ mainly complexes of Au­(I) were obtained: i.e., the propensity of gold for low oxidation states is a major obstacle to obtaining the desired Au­(III) compounds. Several new complexes of Au­(I) and Au­(III) were characterized crystallographically. Our results provide insight into the differences between the coordination chemistry of isoelectronic Pt­(II) and Au­(III) with tridentate cyclometalating and NHC ligands