Heterobimetallic N‑Heterocyclic Carbene Complexes: A Synthetic, Spectroscopic, and Theoretical Study

A new synthetic methodology has been developed for the preparation of heterobimetallic group 11 and group 12 complexes of a symmetrical bis-NHC “pincer” ligand. The synthetic route involved the initial preparation of a mononuclear [Au­(NHC)2]+ complex with pendent imidazole moieties on the NHC ligands. Subsequent alkylation of the imidazole groups with Et3OBF4 and metalation with a second metal ion (Ag­(I) or Hg­(II)) provided two heterobimetallic complexes. Four homobimetallic (Cu­(I)2, Ag­(I)2, Au­(I)2, and Hg­(II)2) complexes of the same bis-NHC “pincer” ligand were also prepared. The homobimetallic Cu­(I)2, Au­(I)2, and Hg­(II)2 complexes and heterobimetallic Au­(I)–Ag­(I) and Au­(I)–Hg­(II) complexes and the synthetic intermediates for the heterobimetallic complexes were characterized by X-ray crystallography. These X-ray structures show that the bimetallic complexes adopt “twisted” conformations in the solid state, supporting short M···M interactions. Crystalline samples of the homobimetallic Ag­(I)2 and Au­(I)2 and heterobimetallic Au­(I)–Ag­(I) and Au­(I)–Hg­(II) complexes were emissive at room temperature and at 77 K. The geometries of the synthesized complexes were optimized at the M06-L/def2-SVP level of theory, and the electronic nature of the M···M interactions for all synthesized complexes was investigated using natural bond orbital (NBO) calculations.