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.