The first mesomorphic benzimidazolium-based silver(I)-N-heterocyclic dicarbene complexes: Synthesis, characterization and phase properties

Abstract The first mesomorphic benzimidazolium based mononuclear silver (I) N -heterocyclic dicarbene complexes, Ag(I)-NHC were prepared via in situ deprotonation of 1-octadecyl-3-alkyl/arylbenzimidazolium salts with silver oxide (Ag 2 O). Both series of Ag (I)-NHC complexes contained a C18 alkyl chain on one N atom and differed in term of the substituent (alkyl and substituted benzyl moiety) on the other N atom in the benzimidazolium ring. All the synthesized compounds were characterized by elemental analysis, Fourier-transform infrared, 1 H and 13 C NMR spectroscopy. Cation geometry, anion and alkyl chain length were found to be essential parameters for mesophase formation in the salts and Ag(I)-NHC complexes in this study. All dialkylated salts except the one with the shortest chain, n = 10 were smectogenic. On the other hand, a large width-to-length ratio of the cationic core in the benzyl analogues had a negative effect on the formation and stability of mesophase. No mesophases were observed for these compounds. Wider mesophase range was seen with decrease in anion size and elongation of alkyl chains. A metastable SmA phase was observed in two Ag(I)-NHC complexes. The minimum chain length required for compounds with different anions to exhibit liquid crystal (LC) properties was addressed. Mesophase was observed for bromide salts with C12 onwards while C14 is the minimum chain length required for the hexaflurophosphate containing Ag(I)-NHC complexes to display LC properties. All Ag(I)-NHC complexes except the one bearing a cyano group could be considered ionic liquids with melting/clearing temperatures below 100 °C.