Gold(III) Bis(di-N-heterocyclic carbene) Square Planar Trication with Axial Ligand Interactions with Bromides from Ag/Br Counteranion Assemblies

The mononuclear tricationic bis(di-N-heterocyclic carbene) gold(III) complex 13+ of formula [Au(MeImCH2ImMe)2]3+ (Im = imidazol-2-ylidene) was successfully synthesized by transmetalation of the di(N-heterocyclic carbene) ligand from the corresponding silver(I) complex to KAuBr4. The counteranion of the gold(III) cationic complex depends on the synthetic procedure. The crude product, isolated by the transmetalation reaction, consists of infinite (Ag4Br7 3-)n ribbons. By adding a stoichiometric amount of AgPF6, the complete precipitation of AgBr is achieved, and the counteranions are PF6 -. If substoichiometric amounts of silver salts are added, then the (Ag4Br7 3-)n ribbon breaks, and silverbromides anionic aggregates of lower nuclearity are obtained, for example, Ag2Br6 4-. The X-ray crystal structures of 1-Ag4Br7, 1-PF6, and 1-Br,(Ag2Br6)0.5 were determined. The complexes with any type of bromide present interactions between the Au center and two bromides of the counteranions so that the geometry around gold is pseudo-octahedral. The gold-bromide interaction has been investigated via DFT calculations and is mainly electrostatic.