Gold(I) Alkynyls Supported by Mono- and Bidentate NHC Ligands: Luminescence and Isolation of Unprecedented Ionic Complexes

Reactions of NHC·HX (NHC = 1-benzyl-3-methylbenzimidazol-2-ylidene, X = Br-, PF6-) and (AuC≡CR)n (R = Ph, C3H6OH) in the presence of Cs2CO3 initially afford compounds of the general formula [(NHC)2Au]2[(RC2)2Au]X, which can be isolated by crystallization. With increased reaction time, only the expected mononuclear complexes of the type [NHCAuC≡CR] are produced. The crystal structure of [(NHC)2Au]2[(PhC2)2Au]PF6 reveals an unprecedented triple-decker array upheld by a remarkably short (2.9375(7) Å) unsupported Au···Au···Au contact. The mononuclear complex [NHCAuC≡CPh] was found to crystallize as three distinct polymorphs and a pseudopolymorph, which depending on the intermolecular Au···Au distances emit blue, green, or yellow light. Two synthetic approaches were employed for the preparation of a series of dinuclear NHC-ligated Au(I) alkynyl complexes of the general formula [NHC-(CH2)n-NHC(AuC≡CR)2], where NHC = N-benzylbenzimidazol-2-ylidene, R = Ph, C3H6OH, C6H10OH, and n = 1-3. In solution, the complexes with aliphatic substituents on the alkynyl fragment are nonemissive, whereas their phenyl-bearing congeners demonstrate characteristic metal-perturbed 3[IL(C≡CPh)] emission. In the solid state, a clear correlation between intermolecular aurophilic interactions and luminescence was established, including their role in the luminescent thermochromism of the phenylalkynyl complexes. The relationship between the Au···Au distance and emission energy was found to be inverse: i.e., the shorter the aurophilic contact, the higher the emission energy. We tentatively attribute this behavior to a smaller extent of excited-state distortion for a structure with a shorter Au···Au separation.