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

A series of tetrakisimidazolium salts bearing two di(phenylimidazolium)amine groups linked by differently substituted anthracenes has been prepared. These are H₄-1a(PF₆)₄ (anthracene bridge), H₄-1b(PF₆)₄ (phenyl-anthracene-phenyl bridge), H₄-1c(PF₆)₄ (anthracene-phenyl bridge), and H₄-1d(PF₆)₄ (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_anthracene bond. Depending on their flexibility, the reaction of the tetrakisimidazolium salts with Ag₂O followed by transmetalation with [AuCl(THT)] yielded octanuclear ([Au₈(1a)₄](PF₆)₈), tetranuclear ([Au₄(1b)₂](PF₆)₄), hexanuclear ([Au₆(1c)₃](PF₆)₆), or octanuclear ([Au₈(1d)₄](PF₆)₈) assemblies, demonstrating the direct bonding strategy can be employed for the selective synthesis of polynuclear poly-NHC (NHC = N-heterocyclic carbene) metallosupramolecular assemblies.