C2-Symmetric normal and mesoionic bis-N-heterocyclic carbenes with biphenyl backbone. A comparison of bis(1,2,3-triazol-5-ylidene) and bis(imidazol-2-ylidene) ligands

Abstract

C2-Symmetric normal and mesoionic bis-N-heterocyclic carbenes (NHCs) derived from 1,1′-((1,1′-biphenyl)-2,2′-diylbis(methylene))bis(3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium) diiodide (5) and 3,3′-((1,1′-biphenyl)-2,2′-diylbis(methylene))bis(1-phenyl-1H-imidazol-3-ium) dibromide (6) were used as ligands for the synthesis of the corresponding Pd(II) complexes. 2,2′-Disubstituted 1,1′-biphenyl moiety was used as the C2-symmetric backbone for the synthesis. 2,2′-Bis(bromomethyl)-1,1′-biphenyl was used as a common precursor for the synthesis of both 5 and 6. These salts were treated with Ag2O for the in-situ generation of the corresponding NHC–Ag(I) complexes which were transmetallated to the corresponding Pd(II) chloro and acetate complexes. The bis(1,2,3-triazol-5-ylidene) derivative gave structurally well-defined mono nuclear chelate complexes that were characterized by spectroscopic and single crystal XRD data. The bis(imidazol-3-ylidene) derivative gave polymeric complex and not the expected mono nuclear chelate complex. These complexes were compared for their reactivity in Suzuki–Miyaura coupling reaction.