Asymmetric N-heterocyclic carbene benzimidazolium salts and their silver(I) complexes: potential as ionic liquid crystals

The synthesis and characterisation of a homologous series of monodentate benzimidazolium salts, 1-4 and their mononuclear silver(I)-NHC (where NHC=N-heterocyclic carbene) complexes, 5-8, are reported. The benzimidazolium salts were prepared from the N-alkylation of 1-methyl-benzimidazole with alkyl halides of varying carbon chain lengths. The mono silver(I)-NHC complexes, 5-8, were prepared by the reaction of the benzimidazolium salts with Ag2O. All the synthesised compounds were fully characterised by H-1-nuclear magnetic resonance (H-1-NMR), C-13-NMR and fourier-transform infrared (FTIR) spectroscopy. The molecular structures of compounds 3PF(6), 4PF(6), 7 and 8 were elucidated through single-crystal X-ray diffraction analyses. We postulate that the attachment of long alkyl chains to the heterocyclic core of 1-methyl benzimidazole could induce mesophase formation. The liquid crystalline behaviour of the benzimidazolium salts was investigated by polarised optical microscope and differential scanning calorimetry. Salts 3 and 4 were found to be thermotropic liquid crystals which exhibited a smectic A phase. However, upon complexation with silver(I) ions, all the Ag(I)-NHC complexes are found to be non-mesogenic.