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

•Biphenyl based C2-symmetric normal and mesoionic (NHC)–Pd(II) complexes are reported.•Mesoionic bis(triazolylidene) NHC gave chelating mono nuclear Pd(II) complexes.•Bis(triazolylidene)-Pd(II) complexes have trans geometry according to XRD data.•Normal NHC, bis(imidazolylidene), gave polymeric Pd(II) complexes.•Reactivity of these complexes were compared in Suzuki–Miyaura coupling reaction.

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.

Graphical abstractBiphenyl based C2-symmetric normal and mesoionic bis-N-heterocyclic carbenes were generated in situ for the synthesis of corresponding Pd(II) complexes. Mesoionic NHC, bis(triazolylidene), gave mono nuclear chelate complexes. Normal NHC, bis(imidazolylidene), gave polymeric complexes. These complexes are compared for Suzuki–Miyaura coupling.Figure optionsDownload full-size imageDownload as PowerPoint slide