The Flexibility of Tetra(N-Heterocyclic Carbene) Ligands Controls the Nuclearity and Geometry of Polynuclear M‒NHC Assemblies.

A series of tetrakisimidazolium salts bearing two di(phenylimidazolium)amine groups linked by differently substituted anthracenes has been prepared. These are H-1a(PF) (anthracene bridge), H-1b(PF) (phenyl-anthracene-phenyl bridge), H-1c(PF) (anthracene-phenyl bridge), and H-1d(PF) (anthracene-phenyl-anthracene bridge). X-ray crystallography showed that those ligand precursors having the di(phenylimidazolium)amine connected directly to the anthracene experience restricted rotation about the N─C bond. Depending on their flexibility, the reaction of the tetrakisimidazolium salts with AgO followed by transmetalation with [AuCl(THT)] yielded octanuclear (Au(1a)), tetranuclear (Au(1b)), hexanuclear (Au(1c)), or octanuclear (Au(1d)) assemblies, demonstrating the direct bonding strategy can be employed for the selective synthesis of polynuclear poly-NHC (NHC = N-heterocyclic carbene) metallosupramolecular assemblies.