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

ABSTRACT

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 ¹H-nuclear magnetic resonance (¹H-NMR), ¹³C-NMR and fourier-transform infrared (FTIR) spectroscopy. The molecular structures of compounds 3·PF₆, 4·PF₆, 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.

Graphical Abstract

KEYWORDS:

• Benzimidazolium salts
• silver(I)–NHC
• thermotropic liquid crystal
• polarised optical microscope
• differential scanning calorimetry
• smectic A

Acknowledgments

The authors would like to thank Universiti Sains Malaysia for the research facilities and financial support through the RUI research grant 1001/PKIMIA/811346. C. W. Yeap wishes to thank the Ministry of Higher Education, Malaysia for the MyPhd sponsorship.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental material

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Additional information

Funding

The authors would like to thank Universiti Sains Malaysia for financial support through the RUI research grant 1001/PKIMIA/811346. C. W. Yeap wishes to thank the Ministry of Higher Education, Malaysia for the MyPhd sponsorship.