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

Abstract

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