Silver(I), nickel(II) N-heterocyclic carbene complexes based on bidentate bis-imidazolium salt with a quinoxaline linker: syntheses, structures, and characterization

Abstract

Quinoxaline-bridged bidentate bis-imidazolium dicarbene ligand 1,1′-(quinoxaline-2,3-diyl)bis(3-methyl-1H-imidazol-3-ium) hexafluorophosphate salt H₂L·2PF₆ (3) was prepared by a two-step reaction based on 2,3-bis(imidazol-1-yl)quinoxaline (1). First, the 2,3-bis(imidazol-1-yl)quinoxaline reacted with CH₃I resulting in the 1,1′-(quinoxaline-2,3-diyl)bis(3-methyl-1H-imidazol-3-ium) iodide salt H₂L·2I (2), then through anion exchange reactions with NH₄PF₆ in water produced the desired bis-imidazolium bidentate ligand H₂L·2PF₆ (3). Reaction of the bidentate bis-imidazolium ligands H₂L·2PF₆ (3) with Ag₂O in acetonitrile gave the macrocyclic binuclear silver(I) carbene complex [Ag₂(L)₂]·2PF₆·CH₃CN (4). Nickel carbene complex [Ni(L)PPh₃Cl]·PF₆·2DMSO (5) was obtained via transmetalation of 4 with Ni(PPh₃)₂Cl₂ in DMSO. The bidentate carbene ligand is a chelating ligand in 5, while bridging in 4. The imidazolium ligand H₂L·2PF₆ (3) and transition metal carbene complexes 4 and 5 have been fully characterized by elemental analysis, NMR, ESI-MS spectroscopy, and X-ray diffraction analyses. Furthermore, the UV and luminescent properties of 3–5 were also studied.

Keywords:

• N-Heterocyclic carbene
• silver
• nickel
• quinoxaline
• crystal structure